The Origin of Granites and 

Related Rocks 

U.S. GEOLOGICAL SURVEY CIRCULAR 11 29 
Cover. Boudins on several scales within layered amphibolite, Tolstik Peninsula, Karelia, 

Russia. Anatectic melt represented by leucosome has migrated preferentially parallel to 

the tectonic foliation to the boudin necks, at each of the scales, in response to applied 

differential stress. Width of field of view about 1 m. Photograph taken by Michael 

Brown. Cover design by Jane B. Russell. 
TH'EOB.Y 

OJ THE 

E A R T H, 

WIT1\ 

PR.OOFS AND ILLUSTRATIONS. 

IN FOUR. PAR. TS. 

Br J.AMMJ HUTTON, M.D. ~F. R. 8. E. 

VOL. 1. 

EDINBURGH: 

PRINTED FOR MESSRS CADEl-L, JUNIOR, AND DA\"1[_;, 

J,.ONJ.)Olf; A·ftD WILLIAM CREECH, EDINBURGH. 

J 7 9 S• 

Frontispiece. Facsimile of the title page of James Hutton's seminal work, Theory of the 

Earth, two volumes, Edinburgh, 1795. The geological writings of James Hutton (1726-

1797) provide the foundation of modem geology. Hutton recognized the length of 

geological time, the uniformity of geological processes through time, and the role of 

igneous activity, sedimentation, and erosion in forming the Earth as we know it. It is 

fitting that this Third Hutton Symposium on the Origin of Granites and Related Rocks 

should occur during the bicentenary of the publication of this great work, which records 

among its content an important stage in the development of ideas about igneous rocks. 
THE ORIGIN OF GRANITES 

AND RELATED ROCKS 
The Origin of Granites and 

Related Rocks 

THIRD HUTTON SYMPOSIUM 

A B S T R A C T S 

College Park, Maryland 

August 26-September 2, 1995 

Edited by Michael Brown and 

Philip M. Piccoli 

U.S. GEOLOGICAL SURVEY CIRCULAR 1129 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

GORDON P. EATON, Director 

For sale by U.S. Geological Survey, Information Services 

Box 25286, Federal Center, Denver, CO 80225 

This report has been prepared by the University of Maryland in cooperation 

with the U.S. Geological Survey; the report has not been reviewed for conformity 

with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. 

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government. 

Published in the Eastern Region, Reston, Va 

Manuscript approved for publication May 25, 1995. 
CONTENTS 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Program of Oral Presentations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

Abstracts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

Author Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 

v 

THE ORIGIN OF GRANITES AND RELATED ROCKS 

Michael Brown and Philip M. Piccoli, Editors 

INTRODUCTION 

Towards the end of his life, James Hutton (1726-1797) 

set forth his "Theory of the Earth" in which he recognized 

the length of geological time, the uniformity of geological 

processes through time, and the role of igneous activity, 

sedimentation, and erosion in forming the Earth as we 

know it. These ideas had been formulated as a result of 

much thought and discussion, and were based on a long 

and critical study of rocks in the field. Hutton's four part 

two-volume book was preceded by a public presentation of 

these ideas in 1785, in a paper on "Examination of the 

System of the Habitable Earth with Regard to its Duration 

and Stability" read to members of the Royal Society of 

Edinburgh on March 7 and April 4. Subsequently, the 

paper was published in Volume I of the Transactions of 

the Royal Society of Edinburgh in 1788. In the ten years 

between presentation of the paper to the Royal Society of 

Edinburgh and publication of his book, Hutton occupied 

himself in accumulating the "proofs and illustrations" of 

his Theory of the Earth which enhance the fuller exposition 

of the Theory as presented in the book. Indeed, for the 

three summers that followed his 1785 presentations to the 

Royal Society of Edinburgh, Hutton examined key field 

relationships along Glentilt in the Grampians (1785), 

around Loch Ken in Galloway (1786) and on Goatfell in 

the north of Arran (1787), because he concluded, that if 

his account in 1785 was correct, some confirmation of it 

must appear at those places where granite and stratified 

rock are in contact. The development of Hutton's views on 

the origin of igneous rocks was presented to members of 

the Royal Society of Edinburgh on January 4, 1790, and 

subsequently published in Volume III of the Transactions 

in 1794. The questions in Hutton's mind in 1785 concerned whether granites represented material "transfused 

from the subterraneous regions, and made to break and 

invade the strata". During his field work, Hutton was able 

to find unequivocal evidence to show that molten granite 

had invaded the surrounding stratified rock, that is to say 

it was an intrusive igneous rock of younger age than the 

country rocks. This represents the first demonstration of 

the class of rocks that we now refer to as intrusive igneous 

rocks. The conclusions of his short paper "Observations on 

Granite" are important to all of us, and were: 

"We are now fully assured that granite has been 

made to break, displace and invade the Alpine 

schistus or primary strata having been previously 

forced to flow in the bowels of the earth, and reduced into a state of fusion. From this too we are to 

draw the following conclusion: Granite, which has 

been hitherto considered by naturalists as being the 

original or primitive part of the earth, is now found 

to be posterior to the Alpine schistus; which schistus, 

being stratified, is not itself original; though it may 

be considered, perhaps, as primary, in relation to 

other strata, which are evidently of a later date." 

The subject of the origin of granite and related rocks has 

remained of intense topical interest since the time of 

Hutton, and the Hutton Symposium Series began as a 

result of the success of a symposium on "The Origin of 

Granites" organized jointly by the Royal Society of 

Edinburgh and the Royal Society of London in 1987 to 

celebrate the bicentenary of the work of James Hutton. 

Implicit in Hutton's conclusions concerning granite are the 

ideas of crustal anatexis, melt segregation, magma transfer 

and granite emplacement into lower-grade upper-crustal 

rocks. How far have we progressed in our understanding 

of these processes during the following 200 years? 

Detailed field studies, such as those completed by 

Hutton 200 years ago, remain the foundation of our 

understanding of granites and related rocks. Since Hutton's 

time, field work has provided a necessary framework for 

the interpretation of studies of, for example: granite 

crystallization; coeval and cogenetic felsic volcanic 

systems; pegmatite genesis; and granite-related hydrothermal systems (which may represent the culmination of the 

granite crystallization process). After several decades of 

experimental investigation, we have a firm understanding 

of the melt production of common crustal protoliths as a 

function of pressure, temperature and activity of water. 

1 
2 The Origin of Granites and Related Rocks 

The thermal budget of continental collision belts that 

involve thickening, thermal relaxation and exhumation has 

been modeled, and in many cases extension and/or 

delamination of part of the lithosphere to increase the heat 

flux across the Moho is necessary to explain observed 

features of granite magmatism in orogenic belts. Nonetheless, it remains unclear exactly how the thermal requirements for melting crustal protoliths are met in individual 

cases. Additionally, the role of mantle-derived magma in 

crustal anatexis remains a matter of debate, although 

hybridization and assimilation-fractional crystallization are 

well-established processes and in many cases the isotopic 

evidence seems unambiguous. Our understanding of 

controls on the geochemistry of crustal melts has improved 

significantly with increases in experimental data and better 

understanding of rates of, for example, diffusion and 

mineral dissolution. In contrast, our knowledge of pressure, temperature, fugacity of oxygen and fugacity of 

water in magmas during ascent and emplacement remains 

sketchy at best. Since 1980 our understanding of and our 

ability to model the processes that cause change in magma 

composition during ascent, emplacement and final solidification has advanced considerably, but the relative importance of these processes, such as fractional crystallization, 

assimilation-fractional crystallization, restite entrainment 

and unmixing, and hybridization remains uncertain. The 

past decade has seen a return to experimental work which 

is producing data to advance our understanding of the 

physics of magma, a renewed interest in textural interpretation and a quantitative approach to rates of, for example, 

melt segregation, ascent and emplacement; there is much 

progress still to be made in these areas of research. 

The whole concept of melt segregation is changing. The 

current view is that melt segregation is governed by the 

type of melting reaction, the rate of melting, the depth of 

melting, and the rate of deformation. In most circumstances, there will be no specific rheological critical melt 

percentage at which melt automatically will begin to 

segregate, although before any melt will segregate from 

the crustal protolith, connectivity of the melt-filled porosity must be achieved. Models for melt segregation include 

compaction, convection, filter-pressing, dilatancy-pumping 

and magma fracture, each of which may operate cyclically 

and at different strain rates in active tectonic environments 

undergoing syn-anatectic deformation. In some circumstances melt may be generated and accumulate until bulk 

magma mobility facilitates transfer of the whole system 

(i.e. melt plus residual material) but in other circumstances 

melt at fractions clearly below the rheological critical melt 

percentage has transferred from the melting environment 

to the upper crust. Thus, segregation of low melt fractions 

from residual solid can be an efficient process. Studies in 

migmatite complexes, in which at least part of the product 

of the melting reaction remains in situ, provide information about plumbing in the crust and melt transfer pathways. The relationship between lower crust that exhibits 

depleted geochemistry and fragments of apparently 

undepleted lower crustal material entrained as xenoliths in 

basalts, remains unresolved. 

There has been considerable debate concerning the 

mechanisms of ascent of granitic magma through the 

continental crust. During the past decade support for the 

traditionally assumed mechanism of diapirism has been 

waning as emphasis has been increasingly placed on the 

relationship between plutons and shear zones. More 

recently, magma transfer through the crust in dikes has 

become popular. Rates of melt segregation in response to 

applied differential stress and melt ascent in dikes or major 

shear zones have been modeled, and in some circumstances are thought to be fast. Both the processes themselves and the rates at which they proceed have important 

implications for the geochemistry of granites and for the 

potential to advect heat into higher levels of the crust. 

Additionally, the rheological consequences of partially 

molten crust are significant. For example, if major crustal 

shear zones initiate in partially molten regions subjected to 

applied differential stress, then melt will migrate to the 

lower-pressure shear zone. In this way the shear zone may 

localize the ascent path and emplacement site, but the 

driving forces both for ascent and emplacement will be 

some combination of the regional applied differential stress 

and buoyancy forces of gravity. At the emplacement site, 

the amount of strain may relate to the interaction between 

the rate of space creation by regional deformation and the 

rate at which magma arrives if the ascent mechanism is 

fast. During magma emplacement crust must be displaced 

at a rate equal to the growth of the pluton. Further, the 

evidence cited by Hutton and many others subsequently 

demonstrates a variety of small-scale mechanisms that may 

act in combination to enhance the space available for 

magma accumulation at the emplacement site. Multiple 

horizontal and vertical material transfer processes acting 

in different proportions from pluton to pluton may well be 

more realistic than the extremes implied by a single model 

either of ascent or of emplacement. 

The importance of mass transfer in dikes was recognized by Hutton during field work at Water of Leith, 

Scotland, where he examined many whinstone (diabase) 

dikes. Since that time, several questions have been 

answered concerning the origin, evolution and structure of 

simple and rare-metal granitic dikes (pegmatites and 

aplites), particularly concerning the role of volatiles in the 

generation of these highly evolved liquids, and yet many 

questions still remain unanswered. Detailed experimental 

studies provide an understanding of the phase equilibria, 

the role of trace metals (fluxes) on those equilibria (e.g., 
B, Li, F, and Cl), textures (including nucleation and 

crystal growth rates, and grain size), and the diverse 

mineralogies and structural complexities (e.g., zoning) of 

these systems. Our knowledge of the internal structure of 

granite-related pegmatites and controls on processes such 

as fractional crystallization, including the pressuretemperature-time conditions under which such structures 

form, is good in some systems. However, our knowledge 

of various pegmatite groups, and the ultimate origin of 

these granite-related pegmatite melts, is less clear. 

The role of volatiles (e.g., H20 and C02), both as 

structural constituents of granite melts and as a discrete 

volatile phase or phases, has been realized for some time. 

Evidence from rhyolites, the erupted counterpart of 

granites, with dissolved water of between 3 and 7 wt % 

(upon eruption), suggests magma degassing during ascent 

and decompression of H20-rich phases may be prevalent 

at depths less than c. 10 km. The magmatic volatile phase 

may be responsible for: modification of geochemical 

characteristics of source regions, in addition to modifying 

granite phase equilibria; alteration and mineralization of 

granite and country rock upon cooling; and, controlling 

fugacity of oxygen in some systems. Detailed, combined 

chemical-physical modeling still needs to be done to obtain 

a better understanding of these degassing processes. 

Degassing leads to hydrofracturing, which is an important 

control on the formation of well developed hydrothermal 

and ore systems. Granite-related hydrothermal systems 

and mineral deposits (e.g. porphyry-type deposits and 

granite skams), develop from the synergistic effects of a 

multitude of processes and conditions. All mineral 

deposits have a common requirement - a process whereby 

an element is brought from a dispersed state to a state of 

higher concentration. There has been considerable debate 

over the ultimate origin of metals in granite related oresystems, but several conditions including emplacement 

level within the crust, initial water concentration in the 

melt, fugacity of oxygen, and halide concentration in the 

melt play a role in both the final chemistry of granite 

bodies and the ore deposits they sometimes form. The 

close spatial and temporal relationships of some oredeposits with granitic systems suggest a genetic affinity; 

observed ore element ratios in some systems (e.g. eastern 

Australia) exhibit correlation with composition of the 

related granites at the present level of erosion. Granite 

porphyry systems (e.g. Cu, Mo, Pb, and Zn, with minor 

Au and Ag) often consist of several distinct periods of 

mineralization within a system, in which magmatic volatile 

phases associated with granites (sensu lato) appear to play 

a significant role in scavenging ore-metals from granite 

melts. Granite skams form when volatile phases (vapor 

and vapor-brine mixtures) are exsolved and interact with 

county rocks. Understanding these processes which 

Introduction 3 

produce economically important mineral deposits is 

fundamental if geology is to continue to support the 

economic development of humankind. 

The Third Hutton Symposium has been structured to 

promote discussion. A number of oral presentations 

(abstracts of these are identified as such) will provide an 

overview of recent developments in our understanding of 

one of several themes identified as foci for this symposium 

(crustal anatexis and pressure-temperature-composition 

relations in granites; physics and chemistry of melts; 

segregation, ascent and emplacement of granite magmas; 

volcano-plutonic systems; granite-related hydrothermal 

systems, mineralization and pegmatites; and, the contribution from isotope geochemistry). However, the majority of 

papers will be presented as posters during more informal 

afternoon and evening sessions to facilitate discussion with 

the evidence on hand. Furthermore, two evening discussions will be organized on "Current Debates and Future 

Research Directions", which will provide both a stimulus 

to research in areas where we need a better understanding 

as well as setting up some topics for possible elaboration 

at the Fourth Hutton Symposium in 1999. 

This Symposium has been organized by a committee 

that comprised Prof. Michael Brown (University of 

Mary land at College Park, General Chair of the Third 

Hutton Symposium), Prof. Philip Candela (University of 

Maryland at College Park, Technical Program Chair), Dr. 

Judy Ehlen (U.S. Army Topographic Engineering Center, 

Registration), Jane Hammarstrom (U.S. Geological 

Survey, Poster Session Chair), Prof. E-an Zen (University 

of Maryland at College Park, Field Excursions Chair), 

Prof. Eirik Krogstad (University of Maryland at College 

Park), Prof. Eileen McLellan (University of Maryland at 

College Park), Dr. Dallas Peck (U.S. Geological Survey), 

Dr. Philip Piccoli (University of Maryland at College 

Park), Mr. Kent Ratajeski (University of Maryland at 

College Park), Dr. Steven Shirey (Department of Terrestrial Magnetism, Carnegie Institution of Washington), Mr. 

Paul Tomascak (University of Maryland at College Park), 

and Prof. Richard Walker (University of Maryland at 

College Park). The Symposium is co-sponsored by the 

Department of Geology, University of Maryland at 

College Park, the U.S. Geological Survey, Reston, 

Virginia, and the Department of Terrestrial Magnetism & 

Geophysical Laboratory, Carnegie Institution of Washington. The Organizing Committee acknowledges a grant 

from the National Science Foundation to subsidize the cost 

of travel to the meeting by U .S.-based graduate students, 

and the assistance of staff in the Department of Geology 

at UMCP. The editors thank the BETR for support in 

producing this Abstracts Volume as a U.S. Geological 

Survey Circular, and acknowledge the professional help of 

Dona Brizzi, Jane Russell, and Connie Schafer. 
4 The Origin of Granites and Related Rocks 

PROGRAM OF ORAL PRESENTATIONS 

(Speaker identified in bold) 

MONDAY, AUGUST 28, 1995 

Alan Bruce Thompson 

Fertility of Crustal Rocks During Anatexis 

Alberto E. Patino Douce and James S. Beard, 

Effects of Pressure, H20 Activity, and Fe/Mg Ratio on the Compositions of Anatectic Melts 

Fernando Bea 

Controls on the Trace Element Chemistry of Crustal Melts 

Francois Holtz and Bruno Scaillet 

Effects of Dissolved C-0-H Species on Chemical and Physical Properties of Ascending Granitic Magmas 

D.B. Dingwell, K. Hess, R. Knoche and S.L. Webb 

New Experimental Constraints on the Physical Properties of Granitic Melts 

J. Lawford Anderson 

Status of Thermo barometry in Granitic Batholiths 

Hanna Nekvasil, Charles Jensen and William Carroll 

Experimental and Theoretical Constraints on the Differentiation of High Temperature Crustal Magmas 

Malcolm P. Roberts and John D. Clemens 

The Perils of AFC Modelling 

TUESDAY, AUGUST 29, 1995 

E.B. Watson and J.M. Hanchar 

Zircon Dissolution and Overgrowth in Granitic Melts: Effects of Thermal History and Local Melt Volume, 

with Implications for Chemical Zoning and Relative Core Size in Natural Zircons Showing an Inherited 

Isotopic Component 

J.P. Hogan 

Insights From Igneous Reaction Space: A Holistic Approach to Granite Crystallization 
Program of Oral Presentations 5 

Tracy Rushmer 

Application of Rock Deformation Experiments to Melt Segregation in the Lower Crust 

E.W. Sawyer 

Melt Segregation or Magma Mobility - The Difference Between Migmatites and Granites 

G. W. Bergantz 

Linking Phase Relations, Thermal Budgets and Deformation During Partial Melting: Two Contrasting 

Styles of Basalt-Crust Interaction 

Jean Louis Vigneresse 

Granites From Source to Emplacement: The View from Geophysics 

Roberto F. Weinberg 

Diapirism of Crustal Magmas 

Nick Petford 

Granite Ascent: Dykes or Diapirs? 

Scott R. Paterson, T. Kenneth Fowler, Jr. and Robert B. Miller 

Pluton Emplacement in Arcs: A Crustal-Scale Recycling Process 

WEDNESDAY, AUGUST 30, 1995 

Mid-Conference Excursions 

THURSDAY, AUGUST 31, 1995 

Takashi Nakajima 

Cretaceous Granitoids in Southwest Japan and Their Bearing on Crust-Forming Processes in the 

Eastern Eurasian Margin 

Bruce W. Chappell 

Causes of Compositional Variation Within Granite Suites 

W .J. Collins 

S- and I-Type Granitoids of the Eastern Lachlan Fold Belt: Three-Component Mixing, Not Restite Unmixing 

C.W. Rapela and R.J. Pankhurst 

Monzonite Suites: The Innermost Cordilleran Plutonism of Patagonia 
6 The Origin of Granite and Related Rocks 

Mark S. Drummond, Marc J. Defant and Pavel K. Kepezhinskas 

The Petrogenesis of Slab-Derived Trondhjemite-Tonalite-Dacite/ Adakite Magmas 

G. Poll, G.R. Davies and S. Tommasini 

Trace Element and Isotopic Exchange During Acid-Basic Magma Interaction 

R.A. Wiebe 

Mafic-Silicic Layered Intrusions 

Leslie Baker and Malcolm J. Rutherford 

Crystallization of Pinatubo Magma 

R. Brooks Hanson 

Hydrodynamics of Magmatic and Meteoric Fluids in the Vicinity of Granite Plutons 

FRIDAY, SEPTEMBER 1, 1995 

Mark D. Barton 

Phanerozoic Granitoids and Associated Mineral Deposits in Southwestern North America 

Phillip L. Blevin, Charlotte M. Allen, Colleen J. Bryant and Bruce W. Chappell 

A Metallogenic Template for Eastern Australia Based on Granite Source and Evolution 

Jacob B. Lowenstem 

Intrusive Degassing at the Magma/Wallrock Interface 

David London 

Pegmatites 

Robert A. Creaser 

Radiogenic Isotopes in Granitic Systems: Studies of Melting and Mixing at the Source 

M. Pichavant and T. Hammouda 

Control of Redox State and Sr Isotopic Composition of Magmas: Source or Processes? 

Richard J. Walker and Eirik J. Krogstad 

Heterogeneous Sources for Granitic Rocks, Black Hills, South Dakota: Do Granites Really 

Serve as Interpretable Crustal Probes? 

Clark Johnson, Katlin M. Barovich and Steven B. Shirey 

New Approaches to Crustal Evolution Studies Through the Lu-Hf and Re-Os Isotope Systems 
ABSTRACTS 

Petrology and Geochemistry of the 

Polly Wright Cove Pluton, 

Blue Ridge Province, Virginia: 

Perspectives on Late Neoproterozoic 

Anorogenic Magmatism, 

Central Laurentian Appalachians 

1

Abbey, W.J., 1Tollo, R.P. 

and 2 Aleinikoff, J. N. 1

Department of Geology, George Washington University, 

Washington, DC 20052, USA 2U.S. Geological Survey, Denver, CO 80225, USA 

The Late Neoproterozoic Polly Wright Cove Pluton 

(PWCP) intruded Grenville-age (1.1-1.0 Ga) basement 

rocks of the Blue Ridge anticlinorium in central Virginia. 

The PWCP is part of a regional Late Neoproterozoic (760-

700 Ma) anorogenic supersuite comprised of more than 

twenty granite plutons exposed throughout the Laurentian 

terrains of Virginia and North Carolina. These plutons 

were emplaced during an early period of crustal extension 

that preceded a younger period (at about 600 Ma) during 

which rifting of Laurentia (ancestral North America) 

produced the Iapetus ocean basin. The PWCP includes 

four major lithologic phases (from oldest to youngest): 1) 

fme-grained biotite granite (present only as xenoliths); 2) 

medium-grained biotite granite (the most areally extensive 

lithologic unit); 3) medium-grained biotite leucogranite; 

and, 4) fine- to medium-grained biotite granite (occurs 

only as dikes). The PWCP granites are metaluminous in 

composition and have geochemical signatures characteristic 

of A-type affinity, including: 1) high Ga/Al and Fe/Mg; 

2) marked enrichment (relative to other granite types) of 

Y, Nb, Zr, Ce, and Ga; and, 3) low Ba and Sr. These 

geochemical characteristics are shared by other members 

of the anorogenic supersuite, as well as by anorogenic 

granites worldwide. The broad range in Si02 values (67 .1 

- 76.5 wt %), FeO/MgO (12.6- 41.6), and Ba!Rb (0.34-

3. 50) indicates that fractionation of major minerals was 

responsible for developing significant compositional 

variation within the PWCP. The late-stage granites 

(leucogranites and dikes) of the pluton show marked 

depletion in Ba and Sr, as well as an increase in Gal Al, 

relative to the older units suggesting that fractionation of 

alkali feldspar and plagioclase was a major factor controlling differentiation of the PWCP magmatic system. 

Abstracts 7 

Removal of ferromagnesian silicate minerals is suggested 

by the range in FeO/MgO and by development of latestage leucogranite. Additionally, a marked decrease in 

Ce/Nb relative to Y /Nb in the late stage granites suggest 

fractionation of allanite. U-Pb isotopic data for zircons 

from the medium-grained biotite granite yield an upper 

intercept age of 706 ± 4 Ma, indicating that most of the 

PWCP was emplaced near the end of the first period of 

regional anorogenic magmatism. A 207Pbf206Pb age of 972 

Ma from a single xenocrystic zircon preserves isotopic 

evidence of an inherited component of possible Middle 

Proterozoic, post-Grenville age. Emplacement of anorogenic plutons in this region has been linked only 

indirectly to rift-related faulting. However, detailed 

structural studies by other workers have demonstrated that 

the PWCP was intruded syn-tectonically as a series of 

sheeted dikes into actively extending country rock. For 

this reason, the age of the PWCP has important implications for the timing of extensional tectonics in the region. 

The PWCP was one of the last plutons to be emplaced 

during the earlier period of Laurentian crustal extension 

and overlaps in age with an interval of peralkaline plutonism in the nearby anorogenic Robertson River batholith. 

Marked differences in related trace element ratios suggest 

that the metaluminous PWCP and coeval peralkaline 

magmas of the Robertson River may have been derived 

from different sources. 

Melting and Generation of Leucogranites 

Associated with the 

Emplacement of Ronda Peridotites 

Acosta, Antonio and Menendez, Luis G. 

Department of Mineralogy and Petrology, 

Campus Fuentenueva, University of Granada, 

18002 Granada, Spain 

The Ronda peridotite is a tectonic slab emplaced during 

the Alpine orogeny within high-grade crustal metamorphic 

rocks. The peridotite body has sheet-like geometry with a 

maximum thickness of about 4.5 km. It overlies a 

tectonically inverted metasedimentary sequence, composed 

from top to bottom of a low-P peraluminous migmatites of 

probable Hercynian age and Permo-Triassic sequences 

(marble and micaschist) that include meta-evaporites. The 

peridotite is crowded with subvertical leucogranite dikes 
8 The Origin of Granites and Related Rocks 

(thickness -10 em - 10 m) with predominant NW-SE 

direction. In the lower zone of the peridotite body, dikes 

consist of an agmatitic breccia formed by metapelitic 

enclaves of uniform size (c. 2-3 em) within a leucogranitic 

groundmass. Within the same dike, the relative proportions 

of enclaves and groundmass are highly variable. In the 

uppermost zone of the peridotite body the dikes have no 

metapelitic enclaves, have complex geometrical relations 

between medium-grained granites, fine-grained granites, 

aplites, and pegmatites, and some of them are internally 

differentiated. Miarolitic cavities are abundant. 

Luxullianitic textures sometimes occur. In some cases, the 

material of border zones and apophyses of dikes is a 

leucodiorite with a color index lower than 1. Available 

Rb/Sr data gave an age of 22 ± 4 Ma with (

87Sr/86Sr)i = 

0.7219. The Ronda leucogranites are formed of quartz, 

microcline, and albitic plagioclase as essential minerals, 

with tourmaline, minor cordierite, muscovite and rare 

biotite as varietal minerals. The accessory mineral assemblage is composed of apatite, zircon, monazite, xenotime, 

ilmenite, titanite, uraninite, brannerite, uranospherite, 

Th-orthosilicate (huttonite?), columbotantalite, rutile, 

pyrite, arsenopyrite, galena, barite, and occasional native 

gold. The chemical composition of the Ronda leucogranites 

is very unusual. Boron contents are in the range 500 -

1250 ppm. The aluminium saturation index, ASI = mol 

A120 3/(Ca0 + Na20 + K20), ranges from 1.40 to 0.92, 

with an average of 1.06. ASI values show an excellent 

negative correlation with Si02, Ti02, and FeOT, such that 

most leucocratic dikes are metaluminous. Fe/(Fe+Mg) 

ratios are uncommonly low for granites, ranging from 0. 75 

to 0.22, with an average of 0.48. Na20 and K20 show a 

clear antipathetic relation. K-rich facies are less silicic, 

more aluminous, richer in Th, with higher Fe/(Fe+Mg), 

lower K/Rb ratios and negative Eu anomalies, and may 

contain native gold. In contrast, Na-rich facies are more 

silicic, less aluminous, richer in U, with positive Eu 

anomalies, "gabbroic" Fe/(Fe+Mg) ratios, and abnormally 

high K/Rb values ( -500- 1300). Remarkably, Na20 and 

K/Rb have an excellent positive correlation (R - 0.85). 

Leucodiorites share many features of Na-rich facies with 

Fe/(Fe+Mg) <0.1, K/Rb >800 and high Ni, and 

occasionally, high Cr contents. The Ronda leucogranites 

were generated by partial melting of underlying migmatites 

due to the emplacement of the peridotite. Heat for melting 

was mostly provided by frictional heating along the thrust 

plane. This plane also acted as a trap for a B-rich volatile 

phase released from Permo-Triassic sequences, so easing 

the partial melting of the tectonic melange and producing 

a low-viscosity melt. Pods of this partially molten material 

were emplaced as dikes along fractures in the peridotite 

body and formed agmatitic dikes. The low viscosity of 

B-rich leucogranite melt as well as sustained high temperature (the temperature of peridotite was higher than 600°C 

during dike emplacement) favored restite unmixing, 

differentiation, and some contamination of leucogranitic 

melt with material from peridotite. Restite-rich high 

melt-fraction leucogranites are K-rich, whereas restitepoor, low melt-fraction leucogranites are Na-rich, formed 

by melting of quartz plus feldspars in a volatile-rich 

environment, with no participation of biotite or other 

Fe-Mg minerals. This explains why they have such high 

K/Rb ratios and positive Eu anomalies. Due to the very 

low Fe and Mg contents of these melts, just a little 

contamination from peridotitic material produced their 

abnormally low Fe/(Fe+Mg) ratios. Geological position of 

leucodiorites and the fact that they have simultaneous 

leucogranitic and peridotitic geochemical features support 

the idea that the leucodiorites are the result of an interaction between leucogranitic magma and ultramafic wall 

rock. 

Migmatite Sheaths as Indicators 

of the Emplacement Mechanisms 

of Granitic Plutons: 

A Case Study from the Al-Amar Suture, 

Eastern Arabian Shield, 

Kingdom of Saudi Arabia 

Al-Saleh, Ahmad 

Geology Department, King Saud University, 

Riyadh, Saudi Arabia 

The Al-Amar Suture is made up essentially of a thick 

and monotonous sequence of sericite-chlorite schists known 

as the Abt schist. This unit was derived from graywackes 

that were deposited in a short-lived (15 Ma) back-arc basin 

which separated the Afif microcontinent in the west from 

the Ar Rayn island arc in the east, and was closed at 

around 680 Ma. The Abt Schist is intruded by granitic 

bodies of various sizes, the majority of which are thought 

to have been produced by the partial melting of the Abt 

graywackes during a period of crustal thickening that 

followed basin closure. Radiometric dating of these 

granites, however, has revealed that most of them post-
date basin inversion by 60-80 Ma. The thermal anomaly 

that existed beneath the back -arc basin largely would have 

decayed during that time, and it is, therefore, obvious that 

a later phase of tectonic rejuvenation was responsible for 

the widespread granitic magmatism within the Al-Amar 

Suture. 

Using the 40 ArP9 Ar step-heating technique it was 

possible to recognize the effects of two episodes of 

tectonism. A first period coincided with the juxtaposition 

of the Ar Rayn island arc against the Afif block (c. 680 

Ma). A second phase took place in the period 610-600 

Ma, and is thought to be the outcome of the collision 

between the eastern part of the Arabian Shield and a large 

continental mass now concealed beneath the sedimentary 

cover of the Arabian Platform. This final collisional 

orogeny was contemporaneous with the development of the 

strike-slip Najd Fault System and its associated syntectonic 

plutonism. It is widely believed that this zone of 

transcurrent displacement was induced by oblique collision 

with a large continental mass near the end of the 

cratonization process of the Arabian Shield. 

Many of the Al-Amar Suture granites, especially near 

its southern end, are surrounded by sheaths of high-grade 

migmatites, and lie mainly near or within large-scale 

transpressive shear zones belonging to the Najd system. It 

is believed that these metamorphites represent thermal 

envelopes that have risen with the granites as ductile 

sheaths from lower crustal levels to their present positions. 

Evidence for the rapid (diapiric) emplacement of the 

granites come from the coincidence of biotite and 

hornblende 40 ArP9 Ar ages obtained from their associated 

migmatites. P-T estimates from the same rocks (using the 

GRIPS, biotite - garnet, amphibole - plagioclase and 

THERMOCALC techniques) indicate near isothermal 

decompression, a feature that is also consistent with the 

rapid ascent of a hot ballooning granite. The compressive 

stresses that accompanied transcurrent displacement along 

the Najd Fault System have produced similar and welldocumented granitic diapirs in other parts of the shield that 

are coeval with those of the Al-Amar Suture. 

Abstracts 9 

ORAL PRESENTATION 

Status of Thermobarometry in 

Granitic Batholiths 

Anderson, J. Lawford 

Department of Earth Sciences, 

University of Southern California, 

Los Angeles, CA 90089-0740, USA 

Knowledge of the intensive parameters that characterize 

formation and emplacement of granitic rocks lags behind 

that of metamorphic systems. In many cases this is due to 

the presence of high-variance phase assemblages, resetting 

during slow cooling, or the lack of well characterized, 

experimentally calibrated reactions. Yet portions of most 

granitic batholiths contain low-variance plutons that are 

amenable to detailed quantification of P - T - fo2 - kp 

conditions acquired during ascent and emplacement. 

Several igneous mineral phases resist subsolidus exchange 

and critical magmatic compositions can be retrieved 

through careful analysis. Detailed quantification of 

intensive parameters, however, is often not rigorously 

attempted and/or it is assumed that granitic magmas 

crystallize on or near a wet solidus. 

Thermometry. Based either on cation exchange or 

solvus relations, there are several thermometers applicable 

to granitic rocks. Many granites contain two ternary 

feldspars and/or two Fe-Ti oxides, but both systems are 

known to undergo subsolidus re-equilibration. Other more 

robust thermometers, such as hornblende - plagioclase, 

hornblende- clinopyroxene, pyroxene- ilmenite, pyroxene 

-biotite, garnet- hornblende, garnet -clinopyroxene, and 

garnet- biotite, are applicable. Some are well calibrated, 

others less so. A major challenge resides in the large 

range of liquidus to solidus crystallization temperature 

incompletely preserved in mineral profiles coupled with the 

significant addition of components that affect K0 relations 

between non-ideal solutions. Liquidus temperatures, based 

on the solubility of early crystallizing apatite or zircon, are 

often in excess of 900°C, whereas estimation of solidus 

and near solidus conditions often yield results > 74ooc, 

far above that expected on experimental grounds for 

crystallization in the presence of a H20-saturated fluid. 

Fugacity of Volatile Components. Volatile composition 

and the degree of fluid saturation dramatically affect 

solidus temperature and the stability of solid phases that 

incorporate volatile components. The observation from 

thermometry that many granitic plutons crystallize at 
10 The Origin of Granites and Related Rocks 

temperatures in excess of a water-saturated solidus argues 

for fluid undersaturation and/or dilution of H20 with C02 

or other volatiles. In contrast, fluid-saturated granitic 

melts containing substantial F or B can be expected to have 

solidus temperatures < 600°C. Hence, well grounded 

knowledge of crystallization temperature occupies a 

fundamental role in estimation of volatile contents of 

magmas. Equilibria including sillimanite (or andalusite) -

biotite - garnet - alkali feldspar, hypersthene - biotite -

alkali feldspar - quartz and biotite - alkali feldspar -

magnetite allow estimation of fH 0 • Estimates by the latter 2 

assemblage, however, are highly dependent on fa . 

Oxygen fugacity varies widely and can have 2a great 

affect on mafic phase composition. Granitic magmas are 

known to have / 0 ranging from two or more log units 2 

below the QFM buffer to a few log units below the HM 

buffer, a range that spans more than five orders of magnitude. Ilmenite - magnetite, quartz - ulvospinel - ilmenite 

- fayalite (QUILF), biotite - alkali feldspar - magnetite, 

biotite - almandine - muscovite - magnetite (BAMM), and 

titanite - magnetite - quartz (TM Q) are well characterized 

equilibria that provide a basis for calculation of fa . 

2 

Pressure. Granite barometry plays a critical role in 

constraining tectonic history. Barometric estimates for 

multiply intruded crustal sections has allowed successful 

tracking of the tectonic ascent of metamorphic core 

complexes and the deep residence periods of several 

allochthonous terranes. Thus, plutons can serve as 

"tectonic nails" in constraining P-T-t paths. 

Peraluminous granites often contain mineral assemblages 

that enable pressure determinations, including garnet -

aluminosilicate - silica - plagioclase (GASP), garnet -

biotite -muscovite -plagioclase (GBMP), and muscovite 

-biotite- alkali feldspar -quartz (MBAQ). Metaluminous 

granites likewise offer a range of barometers. Most well 

calibrated are barometers based on the assemblages garnet 

- hornblende - plagioclase - quartz (GHBQ) and garnet -

ferrosilite - plagioclase - quartz (GFPQ). The Al-inhornblende (AH) barometer remains at a developmental 

stage but potentially could serve as the basis for the most 

widely applied barometer for granitic batholiths. Recent 

work has incorporated the effects of temperature, but the 

complete reaction remains uncalibrated. Limiting pressures can be obtained from the presence of magmatic 

epidote and for low-Ca pegmatites or aplites, the presence 

of subsolvus versus hypersolvus alkali feldspars. 

As with all barometers, the influence of temperature, 

/ 02 , and choice of activity model are critical factors. 

Moreover, the nature and quality of calibration is variable. 

Foremost is the fact that batholiths are not static features. 

Mineral compositions imperfectly record conditions 

acquired during ascent and over a range of temperature 

and great care must be taken in properly quantifying 

intensive parameters. 

An Experimental Method for 

Determining Compositions of Aqueous 

Fluids Coexisting with Silicate Melts 

Anovitz, Lawrence M., Blencoe, James G., 

Horita, Juske and Cole, David R. 

Chemical and Analytical Sciences Division, 

Oak Ridge National Laboratory, P.O. Box 2008, 

Building 4500-S, Oak Ridge, TN 37831-6110, USA 

There have been numerous field, experimental and 

theoretical studies of the genesis and evolution of granitic 

systems. However, the chemistry of aqueous fluids in 

equilibrium with granitic melts is poorly known. Such 

data would be of extreme utility, as an understanding of 

the solubility of components of both molten and subsolidus 

igneous rocks in aqueous fluids is critical to understanding 

and modeling the chemistry of geothermal and other 

magma-hydrothermal systems. 

A number of factors have limited the ability of previous 

researchers to determine the composition of aqueous fluids 

coexisting with haplogranitic melts. The most important 

is that the high P-T composition of a silica-saturated 

aqueous fluid is not retained on quenching. Silicate 

materials precipitate out of the fluid as films and "quench 

roe" (amorphous silica-rich spheres). This prevents direct 

analysis of the fluid extant at high P-T. The tendency of 

the silicate to precipitate on the glass further complicates 

the analysis. 

We have developed a method that overcomes many of 

these difficulties, and we are currently investigating the 

compositions of aqueous fluids in the haplogranite system. 

These experiments will determine the Na, K, Si, and AI 

composition, as well as oD and o180, of a vapor coexisting 

with haplogranitic melt. Hydrous haplogranitic glasses of 

known chemical and isotopic composition are synthesized 

to serve as starting materials for the experiments. Nine 

such compositions have been prepared, five in the 

haplogranite system, three on the bounding binaries (2.5 

kbar minimum melt compositions), and albite. In each 
case, water-oversaturated and undersaturated glasses are 

prepared to allow reversal of the equilibrium water content 

of the melt. 

Several experimental capsule configurations have been 

evaluated. The successful design satisfied three criteria. 

The melt had to be confmed within a section of the capsule 

to permit its separation from the precipitate after the run. 

It was also necessary, however, that it remain in contact 

with the aqueous fluid during the experiment. In addition, 

the volume of fluid in contact with the melt at any time 

had to be minimized to limit the amount of precipitate on 

the glass after the quench. Such precipitate is inseparable 

from the melt and would therefore reduce the calculated 

solubility. This had to be done, however, without reducing the area of fluid/melt contact to the extent that the time 

to equilibrium would extend inordinately. Finally, both 

the composition and amount of precipitate had to be 

analyzable. 

Our approach is to load two inner 3 mm x 1 em 

capsules into a 5 mm x 3.75 em outer capsule. The lower 

end of each inner capsule is welded shut. The capsule is 

then partially filled with glass and its top plugged with a 

ball of Pt mesh. Both inner capsules and a water of 

known oD and o180 different from the glass are then sealed 

in the outer capsule, and the capsule is run vertically to 

prevent the melt from flowing out of the inner capsules. 

Two capsule assemblies with different waters are run to 

reverse the isotopic fractionation. The Pt mesh permits 

contact between the melt and fluid while minimizing the 

amount of fluid actually present at the interface during the 

quench. 

The first step in analyzing the run products is to remove 

the water, leaving the precipitate and any remaining solute 

behind. The capsule is frozen in liquid nitrogen, punctured under vacuum and then allowed to warm slowly, 

which prevents escape of precipitate from the capsule. 

The water is vacuum distilled into a break-seal tube for 

isotopic analysis. Next, the capsule is opened and the 

inner capsules removed for analysis of water content, 

chemical, and isotopic composition. The precipitateencrusted outer capsule is then placed in a Pt crucible, to 

which a known amount of Mg and Ca are added. This is 

then fused in a Li-borate flux, which is re-dissolved in 

nitric acid for ICPS analysis. This procedure yields the 

composition of the precipitate. The original amount of 

precipitate is then determined by ratioing the analyses to 

the original weight of Mg and/or Ca. 

Experiments have been performed at 2. 5 kbar and 

950°C, for periods of time ranging from 1 to 11 days, to 

Abstracts 11 

constrain the reaction kinetics and determine the time 

required to achieve equilibrium. Additional experiments 

will reverse the equilibrium water contents of the melts. 

These results will constrain the equilibrium and kinetics of 

major element and isotopic exchange between melts and 

aqueous fluids. 

Research sponsored by the Geothennal Technology Program of the Office 

of Energy Efficiency and Renewable Energy, U.S. Department of 

Energy, under contract number DE-AC05-840R21400 with Martin 

Marietta Energy Systems, Inc. 

Compositional Variations in Granitic Rocks 

of the Northern Appalachians, USA 

1

Ayuso, R.A. and 2Arth, J.G. 1 U.S. Geological Survey, MS 954, 

Reston, VA 22092, USA 2U.S. Geological Survey, MS 937, 

Menlo Park, CA 94025, USA 

Silurian and Devonian granitic plutons of the Northern 

Appalachians in the U.S.A. were used to assess the 

contributions of juvenile magmas and the extent of crustal 

recycling during terrane accretion. Petrologic, chemical, 

and isotopic-tracer studies were done on a total of 32 

plutons from the Brompton-Cameron and Central Maine 

terranes in western New England (WNE), and from the 

Peri-Gondwanan Merrimack-Harpswell, and St. Croix 

terranes in eastern New England (ENE). Remarkable 

heterogeneity exists in field (e.g., range of rock types, 

mode of emplacement, and types of enclaves) and petrographic features (e.g., hornblende/biotite ratios, presence 

of aluminous minerals, fabrics, accessory minerals, and 

magnetite/ilmenite ratios) even in temporally and geographically related groups of granitic rocks within single 

terranes. No consistent trends in most petrographic 

features have been identified, although in ENE, composite 

igneous intrusions commonly contain significant volumes 

of quartz gabbro and diorite associated with leucogranite. 

Intrusive rocks in WNE, in contrast, consist predominantly 

of granodiorite and granite, although gabbro may be 

present. 

Notable chemical differences exist between WNE (e.g., 

Northeast Kingdom batholith, Vermont) and ENE (e.g., 

Spruce Head pluton and the South Penobscot Intrusive 

Series in coastal Maine). For example, in WNE the 
12 The Origin of Granites and Related Rocks 

granitic rocks are largely calc-alkaline and at a given Si02 

content have lower A/CNK and K20/Na20 values, as well 

as lower contents of alkalies, Rb, and HFS elements than 

granitic rocks in ENE (see Table for ranges of selected 

trace element contents). Higher Sr and Ba contents are 

also found in granitic rocks of WNE. Granitic rocks of 

the St. Croix terrane (coastal Maine) in ENE, are more 

alkaline in character. An additional distinction is that 

plutons from WNE have associated porphyry Cu-Mo 

deposits whereas in ENE the plutons contain occurrences 

of Sn and W skarns. 

The table shows the range of selected trace elements and 

calculated isotopic values at 400 Ma for granitic rocks 

from ENE and WNE. The granitic rocks show the same 

range of initial Nd isotopic ratios regardless of their 

position relative to the North American craton or within 

the accreted terranes. Thus, underlying basement to the 

terranes cannot be differentiated on the basis of the initial 

Nd isotopic values of the plutons. This is in contrast to Pb 

isotopic values which distinguish between the relatively 

unradiogenic granites from WNE (Brompton-Cameron 

terrane) and the generally more radiogenic granites from 

ENE (St. Croix terrane). 

Table 

(trace elements in ppm; initial isotopic ratios at 400 Ma) 

WNE ENE 

Rb 70-300 50-400 

Sr 200-800 50-350 

Ta 0.4-1.1 0.2-2.1 

143Ndf144Nd 0.51177-0.51215 0.51181-0.51221 

20'Pbf204Pb 15.53-15.61 15.59-15.75 

(s'Sr/s6Sr)i 0.7041-0.7125 0.7045-0.7150 

cS 180 +6 to +12%c +9 to +11 %c 

Isotopic and chemical variations in the granitic rocks 

indicate significant diversity in the contribution of crustal 

and mantle rocks in all the terranes. Nd and Sr initial 

ratios of Grenville basement rocks during the Paleozoic 

overlap the range in isotopic values in granitic rocks from 

both WNE and ENE. More importantly, source regions 

of the mafic plutons have chemical features that resemble 

those of magmatic compositions in island arcs. The 

largest volume of granitic magma in the terranes is 

represented by quartz monzodiorite, granodiorite, and 

granite; the smallest volume of magma is represented by 

two-mica granites, and they indicate a source region 

dominated by metasediments (upper crustal) as suggested 

by the high initial Sr and o180 values. 

Terrane amalgamation during the Paleozoic was associated with granitic magma generation which involved a 

substantial amount of recycling of older crust. The most 

inboard terranes (Brompton-Cameron) contain granitic 

rocks that were generated from remobilization of 

Grenville-like basement; those from the most outboard 

terranes (St. Croix) were produced from non-Grenvillian 

(Avalon-like) basement. Source region characteristics in 

intermediate terranes (Central Maine, MerrimackHarpswell) are non-unique and cannot be distinguished 

from the other basement types - the granitic rocks may 

reflect mixtures of Grenville- and Avalon-like sources. 

ORAL PRESENTATION 

Crystallization of Pinatubo Magma 

Baker, Leslie and Rutherford, Malcolm J. 

Depanment of Geological Sciences, 

Brown University, Providence, RI 02912, USA 

The dacite magma erupted by Mount Pinatubo in June 

of 1991 was unusual for several reasons. At a temperature 

of 7800C, it was sulfur-rich enough to contain almost 1 vol 

% phenocrystic anhydrite, and it had a very high oxidation 

state (3 log units above NNO). It was extremely water-rich (6.4 wt %) and probably volatile-saturated, but 

almost too crystalline (up to 50 vol %) to erupt. The 

conditions in the pre-eruptive magma as deduced from 

existing phase equilibria have been corroborated by 

experiments which show these exact conditions are required to crystallize cummingtonite, a prominent overgrowth on hornblende in the Pinatubo magma. Unanswered questions still remain regarding the origin of the 

sulfur-rich and highly oxidized character of the magma and 

the amount of excess volatiles present prior to eruption. 

Experiments in the H -0-S system to determine the composition of the pre-eruptive volatile phase have shown that it 

contains approximately 1 mol % S02 at 800°C and 2 kbar. 

The highly crystalline Pinatubo dacite was essentially 

forced to erupt by the intrusion of basalt into its magma 

chamber. Had this not taken place, the dacite would 

presumably have continued to cool and solidify into a 

pluton. An interesting set of unanswered questions 

concerns the solid-liquid-vapor reactions that would take 
place as the magma crystallized. In fact, the dacite may 

at one time have been more crystalline, as suggested by 

partially resorbed quartz and biotite phenocrysts, and by 

plagioclase phenocrysts whose cores are more albitic than 

their rims. Probable reactions in the cooling magma 

include production of orthoclase and the regrowth of 

biotite and quartz. 

A question of particular interest concerns the fate of the 

anhydrite phenocrysts. While anhydrite was a stable phase 

in the pre-eruptive magma, it has not to our knowledge 

been reported in plutonic equivalents of this dacite. While 

it is possible that anhydrite is dissolved into the H20-rich 

vapor exsolved from the magma during crystallization, 

limited data on the temperature dependence of the vapor 

composition coexisting with anhydrite suggest that the 

efficiency of this process will drop as the magma cools. 

It may be that a minor and unexpected anhydrite component has been overlooked in some arc plutons. 

We are conducting experiments to determine the 

crystallization path of the Pinatubo dacite. The experiments place particular emphasis on the fate of anhydrite 

phenocrysts, in order to determine whether they might be 

preserved to some extent in plutonic rocks. If ancient 

anhydrite-bearing magmatic systems can be identified in 

the rock record, then this will yield more insight into the 

frequency and evolution of sulfur-rich eruptions than can 

be obtained from studying only modem, active volcanoes. 

The Enclave Swarms in Some Granitoids 

of the Sierra Nevada Batholith, 

California: Indicators of Magma 

Interactions and Evolution 

Barbarin, Bernard 

Universite de Paris-Sud et URA 1369 CNRS, 

Laboratoire de Petrographie-Volcanologie, 

Batiment 504, F-91405 Orsay Cedex, France 

Enclave swarms are abundant in the granitoids of the 

Sierra Nevada batholith. In these swarms are associated 

enclaves of various natures, shapes and sizes. Three 

groups of swarms can be distinguished. 

Enclave swarms of the first group consist of many 

different types of mafic microgranular enclaves (MME) 

and of granitoid or hornfels xenoliths, generally enclosed 

in a coarse-grained and mafic-mineral-rich matrix. LoAbstracts 13 

cally, two MME can touch without any deformation of 

their globular shapes. Contacts between the matrix of these 

"polygenic" swarms and the host granitoids are either 

sharp or progressive. These swarms are generally located 

in the margins of the normally-zoned plutons where they 

are frequently associated with magmatic layering. 

Enclaves swarms of the second group consist of a single 

type of fine-grained, isogranular MME, enclosed in a 

fine-grained relatively felsic matrix. The MME are also 

globular but they can deform each other when they are 

touching. Contacts between these "monogenic" swarms and 

the host granitoids are invariably sharp. These swarms 

occur locally in the areas where plutons are crosscut by 

swarms of mafic or composite dikes. 

Enclaves swarms of the third group also consist of a 

single type of fme-grained, isogranular MME, enclosed in 

a granitic matrix. The MME generally present lobate 

contacts, frequently underlined by extremely fine-grained 

margins. Their shape can pass in a very narrow distance 

from globular to angular. In these swarms, MME frequently display planar contacts on one side and lobate 

contacts on the other. These swarms, roughly described as 

''magmatic breccias'', surround the large (hectometric to 

kilo metric) dioritic or gabbroic bodies isolated in the 

granitoid plutons. 

The two last groups of enclave swarms differ from the 

first one because they result from in situ mingling and 

hybridization between mafic magma and felsic magma at 

the emplacement level. The MME are of a single type 

because they are all formed at the same time and in a 

relatively defmed mixing system. In the largely crystallized 

granitic plutons, lately injected mafic magmas can only 

mix with the residual granitic magmas channelled in the 

early fractures to form composite dikes. The "monogenic" 

enclave swarms are obtained after disruption of the earliest 

composite dikes in granitic magmas which could be 

affected by some late magmatic displacements. "Magmatic 

breccias" result from disruption at emplacement level of 

the margins of large mafic bodies. Depending on the 

importance of the rheology contrasts between the mafic 

and felsic magmas, the mafic blobs can either occur as 

globular enclaves, frequently with lobated limits, or be 

broken into angular fragments. 

Genesis of the "polygenic" enclave swarms is more 

complex. They represent aggregations and accumulation, 

on the rheologic interface between the magma and the 

relatively viscous margins of the plutons, of the many 

relatively solid elements present in the magma (i.e. 

phenocrysts of mafic and accessory minerals, K-feldspar 
14 The Origin of Granites and Related Rocks 

phenocrysts where present, xenoliths, and MME). The 

various types of MME result from mingling and hybridization between contrasted mafic and felsic magmas, in many 

small and limited mixing systems, at depth in the magma 

chamber or in the feeding conduits during ascent. 

Pipe-, funnel- or vortex-shapes displayed by the "polygenic" enclave swarms and some associated magmatic 

layerings also indicate that close to the contacts with the 

metamorphic screens, the margins of the magmatic body 

can be affected by magmatic movements that witness 

displacements of volatiles obtained through dehydration of 

the close metamorphic host rocks. 

The Dynamics of Dehydration Melting and 

Implications for Melt Extraction in the 

Lower Crust Following Underplating: 

An Example from the Ivrea-Verbano Zone, 

Northern Italy 

Barboza, Scott A. 

and Bergantz,George W. 

Department of Geological Sciences, AJ-20, 

University of Washington, Seattle, WA 98195, USA 

We have developed a thermo-mechanical model for the 

generation and flow of melts during the dehydration 

melting of pelitic rocks. Melt movement is modeled as a 

fully coupled two regime system of percolative flow 

through a variable permeability mush and convective flow 

in regions where the volume melt fraction (MF) exceeds 

some critical melt fraction (CMF) for viscous flow. The 

model also includes latent heat effects, temperaturecomposition-MF dependent viscosity, and both thermal and 

compositional buoyancy. The effects of compaction are 

not considered. The model phase relations include two 

solid solution loops and two invariant points. As our 

model composition, we chose the pelite of Vielzeuf and 

Holloway (1988) which, during melting, exhibits large 

jumps in MF at the reaction temperatures of the hydrous 

phases (muscovite, biotite). The major element composition of this pelite closely corresponds to the average 

composition of the roof rocks in the Ivrea-Verbano Zone 

(IVZ). 

For a constant temperature boundary condition, the 

amount of melt generated was found to be a maximum 

when the contact between the country rock and the intrusion was horizontal. Increasing contact dip angles decreased the total amount of melt that was generated, 

particularly shortly after emplacement. The total amount 

of melt generated by different contact angles tended to 

converge after more time had passed. For calculations in 

which convection initiated, convective velocities in the 

region of viscous flow were found to be small (lx10-3 m 

yr-1), but high enough to exceed a Peclet number of one. 

Convection strongly affected the position of the biotite 

isograd relative to that predicted by conduction (500 m 

versus 250 m after 12,700 years), but the position of the 

muscovite isograd remained relatively unaffected. With a 

constant temperature boundary condition, the model over 

predicts the location of the muscovite-out isograd at the 

IVZ. This indicates that there was probably not a continuous supply of magma to the Mafic Complex but rather a 

periodic supply punctuated by long intervals of stagnation. 

In the region of porous flow, the relative potential for 

a melt to be extracted under a given pressure field can be 

estimated by the permeability of the porous medium 

divided by the viscosity of the melt (K/p.). A region of 

higher K/ p. is found to occur at low melt fractions (between 5 and 15%) due to the high water content (near 10 

wt %) of these liquids. With respect to the extractability 

of these melts, the drop in viscosity far outweighs the 

effect of the low permeability. A peak in the magnitude 

of the velocities was observed to correspond with the 

region of high K/ p.. The compositions of the melts in this 

region were that of a high silica (74-77% ), peraluminous 

rhyolitic liquid. Interestingly, with few exceptions, the 

predicted major element composition of the liquids in the 

high K/ p. region corresponds reasonably well to average 

compositions of some of the late-Hercynian granites in 

regions near the IVZ. 

Bulk convection of the molten country rock did not 

occur until well above the CMF. Peclet numbers near one 

exist only when the MF in the pelite exceeds 0.6 for 

models in which the CMF was 0.5. This required a floor 

temperature of over 1 ooooc. Convection is suppressed 

until higher MF because the large amount of solids in the 

melt increases the effective viscosity enough to inhibit 

convection. Bulk convection of melt in the country rock 

therefore does not occur unless the initial temperature of 

the country rock is high or the intruding magma is unusually hot. Thermobarometry in the IVZ has shown that 

peak contact temperatures were never sufficient to create 

the high MF requited for viscous flow. 

We found that buoyancy forces alone were not sufficient 

to drive crystal liquid segregation within the chosen model 
parameters. The pressure field generated by buoyant 

convection was not sufficient to drive enough melt by 

porous flow to effect the bulk composition over geologically reasonable times. Maximum melt velocities observed 

in the region of porous flow were found to be on the order 

of 1x10-10m yr-1

• We submit that some other mechanism 

such as hydro fracturing, shear, or diking must occur to 

achieve significant crystal-liquid segregation. We also 

suggest that the melts most likely to be extracted from the 

source region are the high water content melts that exist at 

low melt fractions. 

Vielzeuf, D. and Holloway, J.R., 1988. Experimental determination of 

the fluid-absent melting relations in the pelitic system. Contrib. 

Mineral. Petrol., 98, 257-276. 

Grenville-Age A-Type 

Granitic Rocks in Texas 

1

Bames, C.G., 1Shannon, W.M., 

2

Smith, D.R. and 3James, E. 1

Depanment of Geosciences, Texas Tech University, 

Lubbock, TX 79409, USA 2

Depanment of Geosciences, Trinity University, 

San Antonio, TX 78212, USA 3

Depanment of Geological Sciences, 

University of Texas, Austin, TX 78712, USA 

The Red Bluff granitic suite (RBGS) in the Franklin 

Mountains of west Texas was emplaced inboard of the 

Grenville suture into a Middle Proterozoic shelf sequence 

at - 1.135 Ga. It lies - 290 km west of a subsurface 

occurrence of a 1.15 Ga layered intrusion interpreted as an 

extension of the Mid-Continent Rift (Adams and Keller, 

1994). Early workers proposed a subduction-related origin 

for the RBGS. However, recognition of widespread and 

coeval extension in the region provides an incentive to 

reevaluate the origin of the RBGS. 

The order of intrusion of the RBGS was: granitic to 

quartz syenitic sills, alkali feldspar granite, alkali feldspar 

syenite, leucogranitic dikes, and pegmatitic peralkaline 

arfvedsonite granite. Late-stage, transitional ferrobasaltic 

dikes are also present. Compositional variation is continuous from quartz syenite to leucogranite. Ferromagnesian 

silicates are ferrohedenbergite, ferroedenite, annite and 

relict fayalite, with accessory ilmenite and magnetite. 

Estimated / 0 was near FMQ for the entire suite. Initial 2 

Abstracts 15 

emplacement temperature was near 1050°C for the 

syenites; T decreased to -725°C among late-stage 

leucogranites. Initial H20 content was -0.3 wt % and 

J;fFifno was -10-2

·

5 , consistent with the presence of 2 

magmatic fluorite. The suite is predominantly metaluminous and is characterized by high concentrations of 

HFSE and REE typical of A-type, "within-plate" granites. 

eNd(l.13 Ga) values average - +3.0; measured Pb 

isotope ratios show the following ranges: 206PbP04Pb from 

16.76 to 17.43, 207Pbf204Pb from 15.36 to 15.44, and 208Pbf04Pb from 36.40 to 37 .11. Although hydrothermally 

"disturbed", (

87Sr/86Sr)i can be shown to be low (-

0.7034). o180 of quartz separates ranges from +7.7 to 

+8.9%o. 

The isotope values are consistent with an origin either 

by fractional crystallization of a mantle-derived OIB-like 

basaltic parent or by partial melting of young lower crustal 

rocks. However, partial melting models that use basement 

rocks as parental compositions fail to reproduce the 

observed trace element patterns. Models that assume other 

crustal compositions can produce good fits to the trace 

element patterns, but fail to explain the continuous 

compositional variation of the suite. In contrast, trace 

element data are consistent with mass balance fractionation 

models if the assumption is made that crystal separation 

was inefficient (i.e., in situ crystallization). Assimilation 

of < 10% crustal material can account for the slightly 

elevated o180 values. 

Petrologic and geochemical characteristics of the RBGS 

are virtually identical to those of the -1.08 Ga Pikes Peak 

batholith (PPB) of central Colorado. Late stage, alkaline 

intrusions emplaced within the PPB include a sodic and a 

potassic series. The PPB sodic series and the RBGS share 

characteristics typical of A-type, "within-plate" granites. 

They are suites of cogenetic syenites, quartz syenites, and 

granites with iron- and alkali-rich silicates, mineral 

assemblages consistent with water-poor magmas, high 

FeOT/MgO, and high abundances of Na20+K20, HFSE 

and REE. Both suites are associated with coeval mafic 

rocks and are interpreted to arise by fractional crystallization ( ± crustal assimilation) of basaltic magmas derived 

from OIB-like mantle sources. 

Earlier workers correlated the RBGS with 1.07-1.13 Ga 

high-K granites in the Llano uplift to the east, but recent 

work has shown the Llano granites to be geochemically 

distinct with generally higher P20 5 and Sr, lower Na20, 

FeOTfMgO, Zr, Y, and REE, and much lower Ta and Nb 

compared to the RBGS and PPB sodic suites. Geochemical characteristics of Uano granites are consistent with an 
16 The Origin of Granites and Related Rocks 

origin involving partial melting of slightly older 

subduction-related tonalitic crust, but a mantle component 

cannot be ruled out, as suggested by Nd data (Patchett and 

Ruiz, 1989) and the presence of microgranular magmatic 

enclaves. 

Derivation of the RBGS and PPB sodic series from 

basaltic parental magmas indicates that they represent 

additions of juvenile material to continental crust during 

Grenville time. The RBGS and PPB apparently developed 

in a broad regional zone of mild extension; both lay to the 

west (modem coordinates) of continental rift zones. In 

contrast, the Llano granites were late-stage intrusions 

associated with the Grenville orogenic belt. 

Adams, D.C. and Keller, G.R., 1994. Possible extension of the 

Midcrontinent Rift in west Texas and eastern New Mexico. Can. J. 

Earth Sci., 31, 709-720. 

Patchett, P.J. and Ruiz, J., 1989. Nd isotopes and the origin of 

Grenville-age rocks in Texas: Implications for Proterozoic evolution 

of the United States, Mid-continental region. J. Geol., 97, 685-695. 

Petrochemistry and Origin of 

Silurian - Devonian Plutons in 

the 11 Central Mobile Belt 11 of 

Cape Breton Island, Nova Scotia, Canada, 

in Comparison with 

Newfoundland and New Brunswick 

Barr, Sandra M. and O'Neill, Michael 

Department of Geology, 

Acadia University, Wolfville, 

Nova Scotia, BOP JXO, Canada 

Cape Breton Island is interpreted to preserve a condensed cross-section of the northern Appalachian orogen, 

analogous to the Humber - Avalon section preserved in 

Newfoundland. The Aspy terrane in central Cape Breton 

Island occurs between Laurentian Grenvillian basement 

(Humber Zone) to the north and Late Proterozoic former 

peri-Gondwanan terranes (Bras d'Or and Mira) to the 

south. It consists of Ordovician to Devonian rocks 

comparable to those in the eastern and southern parts of 

the Central Mobile Belt of Newfoundland and New 

Brunswick. As in those other parts of the Central Mobile 

Belt, one of the unresolved questions about the Aspy 

terrane is the nature of the underlying crust. This question 

is closely related to the nature of the "Central Crustal 

Block" identified on seismic reflection profiles - is it really 

a separate crustal entity or is it a variety of either 

Grenvillian or Avalonian crust, or a mixture of both? 

Evidence about the nature of the crust underlying central 

Cape Breton Island is provided by granitoid plutons which 

form more than half of the present level of exposure of the 

Aspy terrane. They are mainly of granodioritic and 

granitic compositions and Silurian through Devonian ages, 

but vary widely in depth of emplacement and petrochemical character. Based on standard parameters, their tectonic 

settings ranged from subduction through collision to 

extension, but overall, their petrochemical characteristics 

and Pb, Nd, and 0 isotopic signatures are consistent with 

derivation from mid-Proterozoic and older continental 

crust. Comparison with plutons in the Humber Zone and 

Avalon terrane indicates that the Aspy terrane plutons were 

not derived from either a Grenvillian or Avalonian basement. Their source may have been a complex crustal 

package similar to that underlying the now-adjacent Bras 

d'Or terrane, a peri-Gondwanan terrane isotopically and 

tectonically distinct from A val on terrane sensu stricto. 

Recently published petrochemical and isotopic data from 

Silurian and Devonian plutons in the Central Mobile Belt 

of both Newfoundland and New Brunswick confirm earlier 

preliminary interpretations of the postulated similarity in 

pluton type and magmatic evolution between these areas 

and the Aspy terrane. However, early Silurian plutons in 

the Aspy terrane are mainly gneissic, and both Silurian and 

Devonian plutons are more deformed than their counterparts in Newfoundland and New Brunswick - this is 

attributed to the location of the area on a Laurentian 

promontory during terrane assembly. The Aspy terrane 

plutons are also petrochemically distinct from plutons in 

the Meguma terrane in southern Nova Scotia, although 

they overlap in age. 
ORAL PRESENTATION 

Phanerozoic Granitoids and Associated 

Mineral Deposits in Southwestern 

North America 

Barton, Mark D. 

Department of Geosciences, University of Arizona, 

Tucson, AZ 85721, USA 

Southwestern North America (California - Colorado 

through Mexico) has a complex history of granitic 

magmatism from the early Proterozoic through the Cenozoic. A broad mix of hydrothermal systems and contact 

metamorphism formed with these granitoids during the late 

Phanerozoic (Mesozoic-Cenozoic convergent margin) 

portion of this record. Interpretation of these granitoids 

and their related rocks must be made through the tripartite 

filter of provincial, exposure level, and process controls. 

Voluminous Cordilleran magmatism began with development of one or more Permo-Triassic arcs along the 

western margin of Pangaea, spanning both Laurentian and 

Gonda wan basement and their marginal sequences. 

Plutonism continued with few major interruptions through 

the Mesozoic and Tertiary, reaching maximum fluxes in 

the Middle Jurassic, Late Cretaceous, and Oligocene. 

Two types of suites are present: the first type is associated with relatively stable convergence in the late Mesozoic and early-middle Cenozoic, and the second type is 

associated with variably extensional tectonics in the 

middle-Mesozoic and middle-late Cenozoic. Diachronous 

plutonic suites, 20-50 Ma in duration, vary systematically 

from older mafic-intermediate to younger felsic 

compositional ranges along major portions of the late 

Mesozoic and early Tertiary Andean margin (California-NW Mexico Cretaceous, Great Basin Cretaceous, 

Arizona-New Mexico Laramide, Colorado-north-central 

Mexico Tertiary). Compositional suites vary from 

calc-alkaline in the western regions ( dioritic to 

granodioritic, commonly strongly peraluminous) to 

moderately alkalic (monzonitic to alkali granitic) in eastern 

areas. In contrast, coeval multi-modal magmatism characterizes middle-Tertiary and portions of the middle-Mesozoic events, particularly in the Basin and Range 

province of the US through northern Mexico. Higher 

crustal levels are typically preserved with these multimodal events, consistent with the extensional interpretation. Some of the overall variability is intepretable in 

Abstracts 17 

terms of contrasting lithosphere; for example, basement 

signatures are clearly transmitted in isotopic systematics. 

Nevertheless, many of the suites of intrusive centers are 

built upon common lithospheric columns indicating that 

variability reflects process control (thermal structure, 

mantle flux, stress regime) linked to tectonic drives. 

Simple thermal and mechanical models of limits on 

assimilation and magma uprise, when constrained by 

regional tectonics, are broadly consistent with the general 

petrogenetic patterns, predicting the observed time variations in composition. In tum, these compositional variations help govern the characteristics of igneous-related 

mineralization and metamorphism. 

Igneous-driven processes including hydrothermal activity 

and contact metamorphism vary through time, reflecting 

exposure and province, as well as process controls. Major 

types of mineralization associated with upper crustal 

plutonic and hypabyssal suites of Phanerozoic granitoids 

include hydrothermal porphyry, skarn, epithermal, replacement, and syngenetic deposits of widely varying styles and 

metal contents. Orthomagmatic deposits are apparently 

absent. Late Mesozoic and early to middle-Cenozoic 

magmatic series show metallogenic progressions from 

oldest Cu (-Au-Fe) mineralization associated with intermediate composition intrusive centers through polymetallic 

suites (Zn-Pb-Ag-W-Cu-Au) associated with broadly 

granodioritic centers, and finally to lithophile-elementdominated suites (F-Mo-Be-W-Zn) associated with young 

granitic centers (alkalic to strongly peraluminous). 

Extensional magmatic series (Jurassic, middle-late Cenozoic) are metallogenically varied, with a predominant 

Cu-Fe-Au metal suite, but with common coeval base-metal 

(Pb-Zn-Ag) and lithophile-element (Mo-F-Be) suites in 

more felsic centers. The first-order filter on the modem 

distribution of mineralization and other features is level of 

exposure, a consequence of burial/erosional history. 

Simple isostatic-diffusional erosional models, again 

constrained by tectonic and geochemical evidence, help 

rationalize the temporal and spatial distributions of hydrothermal, metamorphic, and igneous features: at shallow 

levels ( < 3 km, epithermal, syngenetic deposits; hydrothermal metamorphism, volcanic-dominated suites), at intermediate levels (2-8 km, porphyry, skarn; defined thermal 

aureoles; variably porphyritic intrusions), and at deeper 

levels ( > 8-10 km; unmineralized; regionally extensive 

metamorphism; equigranular intrusions). Time-space 

distributions are consistent with erosional half-lives on the 

order of a few lOs of millions of years. Systematic 

regional variations in metal ratios suggest a basement 
18 The Origin of Granites and Related Rocks 

control in some regions (e.g. Arizona), but the overall 

variability within single provinces points to the importance 

of magmatic controls. The strong correlations between 

lithology and associated alteration and metal suites indicate 

that magmatic compositions provide the fundamental 

control on type of mineralization. Compositional patterns 

are broadly interpretable in terms of metal inventories, 

redox controls, and major element variations. Aluminosity (activity of alumina) and alkalinity (activity ratio of 

soda to lime) help quantitatively rationalize the variations 

in hydrothermal alteration with composition. Open 

questions include sources and controls on sulfur, redox, 

and halogens. 

Time-Space Evolution of a 

Two-Mica Granite: 

The Birch Creek Pluton, 

California, USA 

1

Barton, Mark, 1Ghidotti, Gregory, 2

Holden, Peter, 3Goodwin, Laurel 

and 3

Heizler, Matt 1

Department of Geosciences, University of Arizona, 

Tucson, AZ 85721, USA 2

Department of Earth and Space Sciences, UCLA, 

Los Angeles, CA 90024, USA 3

Department of Geoscience, New Mexico Tech, 

Socorro, NM 87801, USA 

Detailed field, petrographic, and geochemical studies of 

the composite Birch Creek pluton and its aureole reveal 

complex concurrent development of the magma chamber, 

deformation, contact metamorphism, and hydrothermal 

activity. The strongly peraluminous Birch Creek granite 

intruded and deformed Late Proterozoic and Early Cambrian sedimentary rocks in the White Mountains of 

east-central California at -82 Ma (Rb-Sr, Ar-Ar, K-Ar). 

It belongs to a large suite of Late Cretaceous strongly 

peraluminous granitoids in the hinterland of the Cordilleran 

batholithic belt which commonly have lithophile-element 

mineralization and strongly deformed margins. 

The intrusion consists of equigranular to porphyritic 

biotite-muscovite granodiorite to monzogranite formed 

from two isotopically distinct but coeval felsic magmas and 

accompanied by minor ( < 1 ppm by volume) mafic 

magma. All three magmas were emplaced broadly 

synchronously over at least three pulses. Episodic formation of aplite dike swarms and associated metasomatism 

provides time lines that allow evolution of the magma 

chamber to be tied to deformation and metasomatism in the 

periphery. Large subsolidus strains ( > 100%, mainly 

flattening?) accumulated during emplacement by high- and 

moderate-temperature ductile mechanisms in both the outer 

portions of the pluton and the inner portions of the contact 

aureole. These fabrics are crosscut by slightly younger 

undeformed phases of the intrusion, confirming early 

interpretations of dominantly synintrusive deformation 

(Nelson and Sylvester, 1971). Overall, intrusive mechanisms evolved from early diking with orientations controlled by regional stresses, through an intrusion-centered 

stress regime (radial dikes, concentric foliation), to a late 

set of regionally consistent dikes and brittle veins. 

Thermal constraints and Ar-Ar dates on igneous and 

metamorphic muscovites indicate that emplacement took 

< 1 Ma, probably < 100 ka. 

Isotopic (Nd, Pb, Sr, 0), whole rock (XRFS, INAA), 

and electron microprobe analyses combined with the field 

relationships indicate that crystal fractionation dominated 

chemical evolution on small scales (20-200 m, by sidewall 

crystallization and filter pressing), but that the intrusion 

underwent at most minor overall fractionation. Fluid 

evolution was nearly continuous as evidenced by the 

distribution of aplitic, pegmatitic and quartz-bearing dikes 

and veins, but it reached distinct maxima during major 

diking episodes. An antithetic spatial distribution of 

K-feldspar megacrysts and immediately surrounding 

dike/vein events suggests that fluid evolution operated on 

scales of 2-5 times that of major and trace element fractionation. Thermal metamorphism, deformation, and 

metasomatism in the metasedimentary rocks relate directly 

to particular emplacement and fluid release episodes. 

Ductile fabrics in the host carbonate and clastic rocks of 

the inner aureole (hornblende hornfels) occur as much as 

1 km from the intrusion on its western side. These fabrics 

both overprint and are cut by fluorine-rich metasomatic 

features that formed from infiltration of magmatic fluids. 

Brittle deformation in the distal (lower greenschist) 

portions of the aureole may accommodate some of the 

space for the intrusion, and it hosts intrusion-related 

hydrothermal veins. The lack of economically significant 

mineralization in this system probably reflects a combination of the dilution effect of high water contents, the lack 

of effective internal fractionation, and the relatively deep 

level of exposure. 
Overall, the Birch Creek system had a complex physical 

evolution and a modest chemical evolution that are best 

interpretable in terms of the progressive (though geologically rapid) emplacement of a viscous, water-rich magma. 

Recognition of internal structures allows documentation of 

the interplay between magmatic, deformational, and 

metasomatic processes. This pluton-wall rock system is 

representative of peraluminous granitoids in the Great 

Basin, and has many common features with other such 

systems worldwide. 

Nelson, C.A. and Sylvester, A.G., 1971. Wall rock decarbonation and 

forcible emplacement of the Birch Creek pluton, southern White 

Mountains, California. Geol. Soc. Am. Bull., 82, 2891-2904. 

ORAL PRESENTATION 

Controls on the Trace Element 

Chemistry of Crustal Melts 

Bea, Fernando 

Department of Mineralogy and Petrology, 

Campus Fuentenueva, University of Granada, 

18002 Granada, Spain 

The behavior of trace elements during crustal anatexis 

depends primarily on whether they reside 1) as trace 

elements in major phases (e.g. Li, Rb, Cs, Sr, Ba, Eu, 

Ga, and Ge), 2) as essential structural components in 

accessory phases (e.g. Zr, Y, Th, U, and REE other than 

Eu), or 3) as trace elements in accessories (e.g. Nb, Ta, 

V, Cr, Cu, Zn, Ni, and Co). 

Melt/solid partitioning of elements residing as traces in 

major phases is, in principle, controlled by crystal/melt 

partition coefficients (K), the extent of melting, modal 

mineralogy of source rock, and the proportions of minerals 

that melt. However, at P-T conditions of crustal anatexis, 

kinetic effects related to the balance between melting rate 

and rate of intracrystalline diffusion within restitic phases 

strongly affect trace-element partitioning. If melting is 

faster than diffusion, all Keffective tend to be equal to 1, 

regardless of their Kequilibrium values. Some experimentally 

determined Arrenius relations consistently indicate that 

diffusion coefficients at P-T conditions of crustal anatexis 

are D:::;; 10-16 cm2 s-1

• With this value, all Keffective for trace 

elements residing for example within 2 mm-sized crystals 

that melt faster than 10-13 - 10-14 em s-1 are equal to 1, and 

Abstracts 19 

Keffective become equal to K equilibrium only when the melting 

rate is lower than 10-17 -10-18 em s-1

• When Keffective and 

Kequilibrium are equal, the rate of melt segregation determines 

whether partial melting is equilibrium or Rayleigh type. 

ForD = I0-16 cm2 s-1

, 2 mm-sized restitic crystals need 

about 1.6 x 106 years to equilibrate with the melt. When 

Keffective and Kequilibrium are different, disequilibrium melting 

occurs. If Keffective = 1, the concentration of a given trace 

element in melt is simply given by: 

where Cm is the concentration of trace elements in source 

minerals, en is the concentration of elements in newly 

formed phases, zm is the weight fraction of source minerals that melt, and P n is the melt fraction of each new 

phase. Melts produced in these conditions tend to inherit 

the patterns of those trace elements which reside essentially within major minerals of the source rock. 

Trace elements which are essential structural components in accessory phases do not obey Henry's law and 

their concentrations in melt are ruled by accessories 

solubility and solution kinetics instead of partition coefficients. For a given temperature, the solubility of accessories depends on the melt bulk chemistry and volatile 

contents. The textural position of accessories controls 

whether they are available to the melt. Included accessories may be physically isolated from the melt and retained 

in restites together with their host. Biotite is especially 

prone to this effect because it usually includes about 85% 

mass fraction of monazite, xenotime, zircon, Thorthosilicates, etc., present in common crustal protoliths. 

Isolation of accessories produces leucosomes with low 

contents of Zr, Y, Th, U, LREE, HREE, etc. and positive 

Eu anomalies, simply due to the fact that feldspars are the 

only REE-bearing minerals that melt. Since dissolution 

rates of accessories are slow, fast segregation of melt 

could eventually produce the same effect. Sometimes, this 

process is also referred to as disequilibrium melting (but 

with a different sense than above) and leucosomes with the 

above composition are interpreted as proof of fast melt 

extraction. However, since the segregation of tiny crystals 

( cf>max ::= 50-1 microns) from a highly viscous melt is a 

very inefficient process, one would expect that accessories, 

once available for the melt at any time during anatexis, 

cannot be removed and will therefore dissolve either 

completely or, if they were in excess, until they saturate 

the melt, regardless of how fast melt is extracted from 

bulk restites. Accessories may also be entrained in the melt 
20 The Origin of Granites and Related Rocks 

as inclusions within restitic crystals, especially in biotite. 

This effect produces leucosomes with spurious positive 

correlations between such elements as Ti, Li, and Rb 

(characteristic of the host) with LREE, HREE, Y, and Th 

(characteristic of inclusions). 

Lastly, elements which occur as abundant traces within 

accessories are affected by both the kinetics of 

intracrystalline diffusion and the effects of accessories. 

The Residence of REE, Y, Th, and U 

in Granites and Crustal Protoliths. 

Implications for the Chemistry 

of Crustal Melts 

Bea, Fernando 

Depanment of Mineralogy and Petrology, 

Campus Fuentenueva, University of Granada, 

18002 Granada, Spain 

A systematic study (SEM, LA-ICP-MS, electron 

microprobe analyses) on unmineralized granites, 

migmatites, and granulites from Iberia, the Urals, Kola 

and Transbaikalia revealed 25 (REE, Y, Th, U)-rich 

mineral species. Nineteen species - monazite, cheralite, 

xenotime, huttonite, thorite, allanite, cerianite, uraninite, 

betafite, pyroclore, brannerite, uranosferite, bastnaesite, 

parisite, samarskite, aeschinite, fergusonite, zirkelite, and 

fluocerite - have at least one REE, Y, Th, or U as an 

essential structural component. Another six species -

zircon, baddeleyite, epidote, apatite, titanite, and fluorite 

-occasionally contain these elements as abundant impurities. The diameter of all primary (REE, Y, Th, U)-rich 

minerals (except fluorite, epidote, titanite, apatite, and 

occasionally allanite), usually is smaller than a few tens of 

microns, and they tend to occur included within other 

major or accessory phases, showing a definite preference 

for biotite, garnet, zircon, and apatite. The accessory 

assemblage changes with the rock bulk-chemistry and 

always accounts for an elevated fraction of REE, Y, Th, 

and U. Eu, however, resides almost completely within 

plagioclase, K-feldspar, and (in subaluminous granites) 

amphibole. Low-Ca peraluminous leucogranites have 

monazite + xenotime + apatite + Th-orthosilicate + 

zircon ± uraninite ± betafite ± uranosferite ± 

brannerite, which account for more than 90% of LREE, 

HREE, Th, and U. Peraluminous granites and 

granodiorites have monazite + apatite + zircon + 

Th-orthosilicate (huttonite?) ± allanite, also accounting for 

more than 90% of total LREE, HREE, Y, Th, and U. 

Metaluminous granites have allanite + apatite + zircon ± 

Th-orthosilicate (thorite?) ± titanite ± monazite ± epidote 

± REB-carbonates, which account for 70% LREE, 

70-60% HREE and Y, 80-85 % Th, and 95% U, although 

amphibole contains about 30% LREE, 40% HREE, and 

15% Th. Peralkaline granites have complex associations 

of niobotantalates, allanite, monazite, xenotime, 

bastnaesite, thorite, fluocerite, zircon, apatite, titanite, 

fluorite, zirkelite and baddeleyite, accounting for c. 90% 

of total LREE, HREE, Y, Th, and U. Migmatites and 

high-grade metamorphic rocks have practically the same 

accessory assemblage as granites with similar bulk composition. In garnet-bearing rocks, garnet rarely accounts for 

more than 15% HREE and Y, whereas xenotime + 

zircon + Th-orthosilicate usually account for more than 

80%. 

The fact that REE, Y, Th, and U are essential structural 

components in at least one accessory of every crystallizing 

granite and melting protolith implies that these elements 

will not obey Henry's law, and that their concentration in 

partial melts is ruled by accessory solubility and dissolution kinetics. During anatexis, the mass fraction of accessories located at major-phase grain boundaries (and 

therefore available to the melt) depends on the nature of 

the mineral and decreases with grain size. In common 

crustal protoliths from Iberia, for example, about 90% of 

the mass of monazite and xenotime, and 65% of the mass 

of zircon are included within biotite, which thus indirectly 

controls the behavior of REE, Y, Th, and U during melt 

segregation. 

Crystal settling of (REE, Y, Th, U)-rich accessories in 

granite melts is physically impossible, due to their small 

grain size. Crystallized accessories must remain in suspension until a growing major mineral includes them. Selective inclusion is caused by heterogeneous nucleation 

coupled to local saturation adjacent to a growing crystal. 

High surface/volume ratios and almost zero contents of 

REE, Y, Th, and U make biotite an ideal host for (REE, 

Y, Th, U)-rich accessories. Due to selective inclusion, 

host minerals may exert an indirect yet significant control 

on the geochemistry of REE, Y, Th, and U. The melt 

bulk -chemistry determines the nature of crystallized 

accessories, their appearance within the paragenetic 

sequence, and, hence, the partitioning of REE, Y, Th, and 

U between melt and crystals. During magmatic differentiation, the contrasting behavior of REE, Y, Th, and U in 
peraluminous granites (extreme depletion) as opposed to 

subaluminous granites (moderate enrichment) is caused by 

the low solubility of monazite and xenotime in 

peraluminous melts. The low-Ca activity related to high 

aluminosity inhibits apatite precipitation, thus increasing 

phosphorus activity and easing the precipitation of (REE, 

Y) phosphates. The elevated REE fraction contained in 

accessory minerals, non-Henry's law behavior, indirect 

control by major minerals, the effects of textural position 

of accessories during anatexis, and the controls of melt 

bulk-chemistry on the stability of accessories, mean that 

REE-based modeling for deciphering the behavior of major 

minerals during melting and crystallization is of little, if 

any, use in granite rocks. 

Tonalites as Barometers 

and Hygrometers: 

An Example from the 

Southern Virginia Appalachians 

Beard, James S. 

Virginia Museum of Natural History, 

Martinsville, VA 24112, USA 

Both total pressure and water pressure have profound 

effects on the phase equilibria of amphibolite melting and 

on the compositions of the tonalitic liquids so derived. 

High total pressure favors the formation of garnet and 

dissolution of plagioclase. Little is known about the effect 

of PH 0 on garnet stability per se, but, by analogy with 2 

other non-tectosilicate phases, it is likely to be less 

strongly affected than plagioclase. Even at PRo as low as 2 

2-3 kbar, the stability limits of plagioclase are greatly 

reduced. Thus, amphibolite melting at high PRo in the 2 

middle crust may yield a garnet-rich, plagioclase-absent 

residual assemblage similar to that produced by 

dehydration-melting at significantly higher pressure. 

Under these circumstances, the concentrations of major 

elements in the derivative melts may be useful in determining whether melting occurred in a system with or without 

excess water. Alumina content should be particularly 

useful in that high PH 0 melting at moderate total pressures 2 

simultaneously promotes the dissolution of plagioclase and 

the formation of metaluminous, Ca-bearing minerals such 

as amphibole and clinopyroxene. 

Abstracts 21 

Tonalite dikes within the Smith River Allochthon (SRA) 

in south-central Virginia have high Sr (average 710 ppm), 

positive Eu anomalies, and are depleted in HREE andY 

(LaN/YbN average 25, Y average 7.4 ppm). These 

characteristics are the signature of tonalite melts generated 

by partial melting of amphibolite under conditions where 

garnet is stable and plagioclase is not. Tonalites having 

these characteristics are generally interpreted as having 

formed either below the crust or at or near its base. The 

SRA tonalites, however, are, in addition, aluminous (Al20 3 

= 17% at 70% Si02), peraluminous, and Fe-poor (most 

< 2% FeOT at 70% SiOz). The tonalites appear to be too 

aluminous and Fe-poor to have formed by dehydration 

melting, even at pressures > 30 kbar. The chemistry of 

the tonalites is consistent with their formation by waterexcess melting of basalt or amphibolite in the garnet 

stability field and above plagioclase stability at Ptotal > 8-

10 kbar and PH 0 between 1 and 3 kbar. Trace element 2 

modeling suggests that the tonalites can be generated by 

partial melting of a local crustal source (Bassett 

Amphibolite). The models require a high proportion of 

residual amphibole ( 40-85% of the residuum mode) and 

large (25-40%) melt fractions, consistent with melting 

under relatively wet conditions. Melting may have been 

induced by influx of water from the cool, upper portion of 

the lower plate into the hot, lower portion of the SRA 

during or after emplacement of the thrust sheet. 

ORAL PRESENTATION 

Linking Phase Relations, 

Thermal Budgets and Deformation 

During Partial Melting: 

Two Contrasting Styles of 

Basalt-Crust Interaction 

Bergantz, G. W. 

Department of Geological Sciences, AJ-20, 

University of Washington, Seattle, WA 98195, USA 

The importance of basaltic input in granite generation 

has been well established and the evidence is discussed 

extensively elsewhere (Bergantz and Dawes, 1994; Kay et 

al., 1992). What isn't well understood are the physical 

processes and the attendant time and length scales of 

basalt-crust interaction. In this presentation, we will focus 
22 The Origin of Granites and Related Rocks 

only on the partial melting regime, we do not consider 

magma mixing in high level chambers. 

Two geologically motivated endmembers for basalt-crust 

interaction are presented. The flrst is the "Ivrea" type, the 

second, the "Chelan" type; these are named for their type 

locations in northern Italy and Washington State, respectively. The Ivrea type is the more commonly cartooned 

endmember. It is characterized by a maflc system with 

dimensions on the order of kilometers, discrete (or 

regionally mappable) contacts with broadly horizontal 

upper boundaries, and a compositional variation largely 

consistent with fractionation with contamination by crustal 

material. These systems typically originate by incremental 

ssembly into chambers which may be largely crystal liquid 

mush, like mid-ocean ridge chambers, as suggested by 

Quick et al. (1994). Heat and mass transfer between the 

country rock and the maflc magma takes place across a 

conjugate interface, and geologic evidence indicates that 

the enthalpy exchange is largely conductive. The dominant means of mass transfer is the wholesale incorporation 

of roof blocks or septa into the largely mushy, mafic 

magma (Sinigoi et al., 1994). Extensional deformation 

plays a key role in crystal-liquid organization and movement, both in the mafic complex and near the contact. 

The Chelan type endmember has been documented and 

discussed by Hopson and Mattinson (1994). The distinctive feature of this endmember is that the basalt intrudes 

regionally as sequences of dikes. This yields a temporal 

and spatial sequence of partial melting with an intimate 

association between basalt and partial melt. It produces an 

efficient means for basalt-melt interaction and a variety of 

mingling and mixing features have been documented. The 

type region as a whole has undergone incipient diapiric 

upwelling. Petrologic diversity in the Chelan type is 

readily generated at all scales. 

Thermo-mechanical models for partial melting and melt 

migration have been developed for both endmembers. The 

Ivrea type model is discussed by Barboza and Bergantz 

(this volume). One key point is that buoyancy alone, 

whether driven by thermal or compositional density 

contrasts, does not lead to appreciable amounts of melt 

segregation. This is in contrast to some laboratory models 

that suggest large amounts of melting (above the contiguity 

limit) will occur at the contact driven only by density 

differences. Our model predictions agree remarkably well 

with geological observations, and suggest a key role for 

shearing or diking in melt migration. 

Models for the Chelan type are more difficult to 

constrain as the volume-time interval-dike spacing relationships for the intrusion of the basalt have yet to be determined from the field area. In addition, our model 

protolith is different than that at the Chelan (we are 

currently working up the appropriate protolith composition). Nonetheless, the essential dynamic features of the 

Chelan type have been obtained: melting at many scales, 

and a regional change in bulk rheology that would initiate 

a "proto-diapir" type of structure (after Hopson and 

Mattinson, 1994). Note however, that this is not a simple 

diapir of material heated from below. The buoyancy 

originates from a region that is effectively heated internally, and thus the usual types of time dependent behavior 

and kinematic indicators associated with a classic RayleighTaylor instability, or "hot Stokes" diapir, do not obtain. 

The geometry and rise time of the proto-diapir depends on 

the basalt intrusion rate, distribution, and the rheological 

model chosen for the unheated crust. 

Despite the conceptual popularity of the Ivrea type, we 

propose (following Hopson) that the Chelan type may be 

more important in the development of granitoids which 

have a substantial mantle signature and metaluminous 

composition. The Chelan type may be difficult to recognize, or may be commonly misinterpreted in regional 

geological mapping. Shearing of some type is a fundamental feature for substantial melt migration in both types. 

In the Ivrea type, such shearing may originate from far 

fleld stresses combined with intrusion. In the Chelan type, 

it is a natural consequence of the thermal maturing and 

bulk buoyancy, following the basalt intrusion episode. 

Bergantz, G.W. and Dawes, R., 1994. Aspects of magma generation and 

ascent in continental lithosphere. In Ryan, M.P. (ed), Magmatic 

Systems, Academic Press, 291-317. 

Hopson, C.A. and Mattinson, J., 1994. Chelan migmatite complex, 

Washington: Field evidence for mafic magmatism, crustal anatexis, 

mixing and protodiapiric emplacement. In Swanson, D.A. and 

Haugerud, R.A. (eds), Geological Field Trips in the Pacific Northwest. Department of Geological Sciences, University of Washington, 

2K, 1-21. 

Kay, R.W., Mahlburg Kay, S. and Arculus, R.J., 1992. Magma genesis 

and crustal processing. In Fountain, D.M., Arculus, R.J. and Kay, 

R.W. (eds), Continental Lower Crust, Elsevier, 423-441. 

Quick, J.E., Sinigoi, S. and Mayer, A., 1994. Emplacement dynamics 

of a large mafic intrusion in the lower crust, Ivrea-Verbano Zone, 

northern Italy. J. Geophys. Res., 99, 21,559-21,573. 

Sinigoi, S., Quick, J.E., Clemens-Knot, D., Mayer, A., Demarchi, G., 

Mazzuchelli, M., Negrini, L. and Rivalenti, G., 1994. Chemical 

evolution of a large mafic intrusion in the lower crust, Ivrea-Verbano 

Zone, northern Italy. J. Geophys. Res., 99, 21,575-21,590. 
Mineralogy and Geochemistry of Tertiary 

A-Type Granitoids at a Continental Rift 

Margin, Southern Red Sea. 

Blakey, Suzanne, Thirlwall, M.F. 

and Menzies, M.A. 

Geology Department, Royal Holloway College, 

University of London, Egham, Surrey, 

TW200EX, UK 

Oligo-Miocene continental rifting of the Red Sea was 

associated with intense magmatic activity in Yemen, 

expressed as a thick sequence ( > 3500 m) of basaltrhyolite flows. Associated granitoids are up to 25 km 

across, located along a linear trend parallel to the Red Sea. 

The granitoids are smaller in the south and further inland 

than along the coastal escarpment and to the north. This 

could be a function of uplift and erosion, with the southern 

granitoids being unroofed to a lesser extent than those in 

the north. 

The granitoids are intruded late in the history of the 

volcanic margin, they intrude the Precambrian basement, 

the Mesozoic sedimentary cover and the Tertiary lavas. 

An ·Rb-Sr whole rock isochron for a southern granitoid 

gave 24.5 ± 1.4 Ma. Despite the altered nature of whole 

rock samples, fresh amphibole and biotite separates yield 

an identical age of 23.1 ± 0.6 Ma. 

The mineralogy and texture of the granitoids offer 

insight into magmatic processes. Quartz displays undulose 

extinction resulting from transformation of {3-quartz to exquartz, and may exhibit granophyric intergrowths with 

orthoclase as a result of eutectic crystallization. Feldspars 

occur as orthoclase, plagioclase and perthite. Perthite is 

the most common texture, suggesting sub-solidus cooling. 

Both hypersolvus and subsolvus granitoids are present, 

reflecting different pressures and temperatures of emplacement. Amphibole occurs as clusters of arfvedsonite, with 

minor riebeckite, these alkali amphiboles are typical of Atype granites. Minor amounts of eckermannite and 

katophorite occur in more mafic samples. Pyroxene is 

rare, occurring as optically continuous interstitial aegirineaugite, representing orthocumulate textures forming 

between the liquidus and solidus temperatures. Biotite is 

annite-rich, and often results from alteration. There are 

many accessory minerals, e.g. fluorapatite, titanite, 

magnetite, ilmenite, rutile, monazite, fluorite, uralite and 

epidote. Apart from apatite and Ti-magnetite, these do not 

Abstracts 23 

appear to affect the chemistry of the rocks and may be the 

result of subsolidus/in situ crystallization. 

The granitoids of Yemen have an unusual alkaline 

chemistry which led them to be classified as A-type 

granitoids, they are characterised by high Si02, Na20, 

K20, Zr (> 1000 ppm), Nb, Ga, Y, F, Sn, REE (except 

Eu), low AI, Mg, Ca, Ba, V and Sr. Typical 

spiderdiagrams display strong negative anomalies in K20, 

Ba, Sr, Eu, P and Ti. The Sr anomalies are caused by 

either plagioclase fractionation, or a crustal source for the 

granitoids. High 143Ndfl44Nd ratios (c. 0.512887) and 

Ce/Pb ratios close to mantle values ( - 25 - 30) suggest the 

granitoids are not crustal melts and that plagioclase 

fractionation causes the Sr anomalies. P and Ti anomalies 

are due to the fractionation of apatite and Ti-magnetite 

respectively. 

The granitoids plot in the "within plate granite" field of 

Pearce et al. (1984), with the exception of a few samples 

which plot in the field of volcanic arc granites (VAG), 

possibly due to plagioclase accumulation. The granitoids 

contain very little ammonium, 1-4 ppm, contrasting with 

high ammonium contents of VAG granites. 

Evolution of the textures observed in the granites is 

largely a result of subsolidus reactions. A-type magmas 

have high halogen contents and movement of the halogens 

through the magma depresses the melt viscosity and also 

overprints the original texture of the granites, e.g. in the 

Yemen granitoids a large proportion of the original 

mineralogy has been pseudomorphed by quartz, with only 

ghost structures of minerals remaining. 

Initial isotope data display a range of compositions from 

(

87Sr/86Sr)i = 0.7039 - 0.7083, 206PbJ204Pb = 18.15 -

19.17, 207PbP04Pb = 15.59- 15.67, 208Pbf204Pb = 38.53-

39.03, 143Nd/144Nd = 0.512591 - 0.512887. There is a 

great variation in Sr, Nd and Pb isotopes between individual granitoids. For instance the Jabal Sabir granitoid 

displays no isotopic variation, (

87Sr/86Sr)i is high, 0.705, 

143Nd/144Nd is low, 0.512596, therefore there must have 

been crustal involvement and homogenisation of the 

isotopic composition before emplacement. In Jabal Dubas, 

143Ndfl44Nd is identical to the Yemen basalts, therefore 

little bulk crust can have been assimilated, but (

87Sr/86Sr)i 

is high, 0.7068. The high (

87Sr/86Sr)i occurs because Sr 

has been removed by feldspar fractionation at high Si02, 

this is therefore very sensitive to a small crustal input. A 

good relationship between Sr isotopic compositions and Sr 

content indicates AFC-type processes. 

Pb isotopic data vary markedly between each granitoid 

and to a lesser extent within individual intrusions. Provin-
24 The Origin of Granites and Related Rocks 

ciality in Pb isotopic data reflect the Pb isotopic heterogeneity of the Pan-African lithosphere. Pb isotopic data for 

granitoids in central Yemen overlap those of coeval and 

spatially related silicic pyroclastic units. The A-type 

granitoids may be the exposed roots of a chain of caldera 

centers that fed the ignimbrites. 

Pearce J.A., Harris, N.B.W. and Tindle, A.G., 1984. Trace element 

discrimination diagrams for the tectonic interpretation of granitic 

rocks. J. Petrol., 25, 956-983. 

Methods for Determining the 

Thermodynamic Mixing Properties 

of Hydrous Haplogranitic Melts: 

Progress, Problems and Prospects 

Blencoe, James G. and 

Anovitz, Lawrence M. 

Chemical and Analytical Sciences Division, 

Oak Ridge National Laboratory, P. 0. Box 2008, 

Building 4500S, Oak Ridge, TN 37831, USA 

Quantitative modeling of phase equilibria in granitic 

systems would be greatly facilitated if the thermodynamic 

mixing properties of NaA1Si30 8 - KA1Si30 8 - Si40s- H20 

melts were known accurately. Recognizing this, numerous 

investigators have attempted to measure these properties 

experimentally. However, these efforts have been hindered by difficulties encountered in performing experiments on hydrous haplogranitic melts. High pressures and 

temperatures are required to stabilize melt compositions 

that correspond to natural granitic magmas, and it can be 

difficult to achieve complete equilibrium during experimentation. Moreover, H20-rich haplogranitic melts tend 

to exsolve water during quenching, and hydrous 

haplogranitic glasses lose dissolved water readily when 

heated at one atmosphere. Owing to these and other 

difficulties, very few calorimetric and volumetric data have 

been obtained for hydrous haplogranitic melts and glasses. 

On the other hand, some very useful phase-equilibrium 

and spectroscopic data have been obtained for hydrous 

haplogranitic melts and glasses. These data have been 

used to elucidatd melt - crystal - fluid reactions, water 

solubility, and water speciation, and to constrain thermodynamic models. However, almost all of the phaseequilibrium data for haplogranitic melts are unreversed, 

and spectroscopic data on water speciation in haplogranitic 

glasses must be interpreted with caution because the 

relative proportions of the two principal types of water 

species (OH groups and molecular water) change when a 

haplogranitic melt is quenched. Furthermore, because data 

on melt - crystal - fluid phase relations and water 

speciation do not provide direct information on the activities of the components and species in haplogranitic melts, 

supplementary thermochemical data are required for 

accurate thermodynamic modeling. 

Another difficulty is that the standard-state free energies 

of fusion of high albite, sanidine and quartz are poorly 

known. This is so not only because relevant 1-atm 

thermochemical data are either absent or of low accuracy, 

but also because fusion temperatures are not well known 

at high pressures. If the standard-state free energies of 

fusion of high albite, sanidine and quartz were known 

accurately in P-T regions where haplogranitic melts are 

stable, accurate activities for these components could be 

calculated from tightly reversed melt-crystal-fluid 

phase-equilibrium data. 

Because conventional experimental and thermodynamic 

approaches have failed to yield reliable equations for the 

thermodynamic mixing properties of hydrous haplogranitic 

melts, researchers have developed ad hoc thermodynamic 

models based on one or more key assumptions. For 

example, in speciation models for hydrous haplogranitic 

melts it is commonly assumed that the species either mix 

ideally, or obey Henry's law. However, all speciation 

models for hydrous haplogranitic melts are inconsistent 

with one or more sets of high-quality phase-equilibrium, 

calorimetric or spectroscopic data. We suggest that these 

inconsistencies arise partly because both fictive and real 

melt species do not interact thermodynamically in accordance with any simple mixing law. 

An alternative method for determining the thermodynamic mixing properties of hydrous haplogranitic melts is 

to measure the chemical potential of H20 (JLH2

o) in these 

melts. With these data, activities for the stoichiometric 

silicate components (NaA1Si30 8, KA1Si30s and Si40s) can 

be quantified by fitting calculated chemical potentials for 

H20 to a formulation which is consistent with the 

Gibbs-Duhem relation. Currently, we are exploring the 

possibility that reliable J1H20-XH2

o data for haplogranitic 

melts can be obtained from: 1) fluid-saturated, 

mixed-volatile experiments with C02, N2 or Xe; and, 2) 

fluid-undersaturated experiments at fixed hydrogen and 

oxygen fugacity (controlled:fH 0 experiments). Also, we 2 d . 

are collecting reversed experimental data on the boun ar1es 

of hypersolidus phase fields in hydrous haplogranitic 
systems. These data will constrain melt-crystal-fluid phase 

relations in these systems, and permit evaluations of the 

standard-state free energies of fusion of high albite, 

sanidine and quartz. 

Finally, binary and ternary haplogranitic systems should 

be investigated thoroughly before collecting large quantities 

of llH:zO-XH:zO and other phase equilibrium data for quaternary haplogranitic melts. This is advisable because it is 

possible that no quaternary mixing parameters will be 

required to develop an accurate excess free energy equation for NaA1Si30 8 - KA1Si30 8 - Si40 8 - H20 melts. 

Research sponsored by the Geothennal Technology Program of the Office 

of Energy Efficiency and Renewable Energy, U.S. Department of 

Energy, under contract number DE-AC05-840R21400 with Martin 

Marietta Energy Systems, Inc. 

ORAL PRESENTATION 

A Metallogenic Template for 

Eastern Australia Based on 

Granite Source and Evolution 

Blevin, Phillip L., Allen, Charlotte M., 

Bryant, Colleen J. and Chappell, Bruce W. 

Department of Geology, 

Australian National University, 

Canberra, 0200, ACT, Australia 

Phanerozoic granite-related mineralization in eastern 

Australia is heterogeneously distributed both temporally 

and spatially. Three broad mineralization categories have 

been recognized: porphyry Cu-Mo-Au deposits associated 

with continental margin type, dominantly low- to medium-K granites in the northern New England Orogen 

(NEO); Cu-Au associated with isotopically primitive, 

medium- to high-K and shoshonitic Ordovician magmatism 

in the central Lachlan Fold Belt (LFB); and W, W-Sn, and 

Mo-W mineralization associated with high-K, felsic, I- and 

S-type Siluro-Devonian, and Carboniferous to Triassic 

magmatism in Tasmania, the southern NEO, LFB, and 

northern Queensland. A -type granites are not associated 

with significant mineralization. The ore element associations found in granite-related mineralization; Cu-Au, W, 

Mo-W, Sn-W, and complex polymetallic (Pb-Zn-Ag-SnAs, etc.), are typical of intermediate to felsic granite 

magmatism worldwide. A widespread, though often 

Abstracts 25 

genetically tenuous, relationship between granite 

magmatism and Au mineralization is also present in 

eastern Australia. 

The ore element associations, in combination with age, 

compositional and source features for the associated 

granites allow recognition of several distinct granitemetallogenic provinces in eastern Australia. Granitemetallogenic provinces defmed by S-type granites are 

wholly restricted within individual terranes and fold belts. 

In contrast, some granite-metallogenic provinces defmed 

by I -type granites cross the boundaries between terranes 

and fold belts. These contrasting distributions probably 

reflect that S-type magma sources were located within the 

mid crust, whereas !-types were derived from lower 

crustal sources whose compositions and origins are not 

reflected in the exposed geology. 

Copper mineralizing systems are associated with 

granites that are chemically and isotopically primitive; 

these granites were derived through the rapid igneous 

reworking of juvenile igneous crust. Relatively isotopically primitive ((87Sr/86Sr)i - 0.705), but highly fractionated high-K granites, such as those in the southern NEO, 

are associated with substantial Sn mineralization. Elsewhere in eastern Australia Sn mineralization is genetically 

related to S- and I-type granites that are both isotopically 

((87Sr/86Sr)i > 0.710) and chemically evolved. Tungsten 

mineralization occurs in scheelite skams associated with 

moderately chemically evolved I-type granites, or as 

wolframite with Moor Sn in highly fractionated granites 

that are oxidized and reduced, respectively. Molybdenum 

mineralization is widespread through eastern Australia and 

is associated with porphyry Cu systems, but also occurs as 

Mo-rich greisen and pipe hosted deposits in highly fractionated K-rich granites. Climax-type porphyry Mo 

systems are rare and small in eastern Australia despite the 

presence of large volumes of compositionally similar 

granites in the southern NEO and north Queensland. 

The observed granite-metallogenic relationships in 

eastern Australia display a strong correlation between the 

ore element ratios in mineral deposits and the composition 

of the related granites. This is mirrored in the evolutionary trend from Cu dominant. deposits a<~sociated with less 

evolved granite suites through W, to Mo- and Sn-dominant 

deposits associated with highly fractionated K-rich granite 

suites. Elsewhere, these relationships may be affected by 

the action of granite composition modifying processes such 

as assimilation. While thousands of small deposits and 

showings are associated with the granites of eastern 

Australia, the number of world class deposits is remark-
26 The Origin of Granites and Related Rocks 

ably restricted given the volume of granite. In some cases 

this may reflect the relative degree of erosion and subsequent removal of epizonal ore deposits. However, most of 

these unmineralized granites, as typified by the LFB I- and 

S-type granites, define a compositional field based on 

fractionation and oxidation state parameters that is distinct 

from granites associated elsewhere with significant mineralization. Granites formed by the remagmatization of 

evolved, older continental crust appear to be capable of 

generating Mo and Sn mineralization only if they have an 

appropriate oxidation state and have undergone substantial 

crystal fractionation to even more evolved compositions. 

The association of certain metal commodities with certain 

tectonic settings is in part a function of these relationships, 

as tectonic processes affects the range of protolith materials, and heat and volatile fluxes available for magma 

generation. 

Intraplate A-Type Bimodal Magmatism: 

Why and How? 

Bonin, Bernard 

CNRS-URA 1369, Departement des Sciences de la Terre, 

Batiment 504 Universite de Paris-Sud, 

F-91405 Orsay Cedex, France 

Intraplate post -orogenic to anorogenic igneous suites are 

markedly bimodal and made up of large but varying 

amounts of basic and felsic rocks with scarce intermediate 

rocks. The volcanic (plutonic) series display liquid lines 

of descent contrasting from Ne-normative basalt (gabbro) 

to: 1) Q-normative rhyolite (granite); 2) saturated trachyte 

(syenite); and, 3) Ne-normative phonolite (nepheline 

syenite). Differentiation paths from the metaluminous 

basic end-member to the three possible A-type 

(per)alkaline felsic end-members are chiefly controlled by 

fractionation of anhydrous (olivine, pyroxene, plagioclase) 

and hydrous (amphibole, more rarely mica) silicate 

minerals in mantle and crustal magma chambers. 

The silica oversaturated igneous complexes are commonly characterized by huge volumes of rhyolite (granite) 

and comparatively minor volumes of basalt (gabbro). On 

the contrary, in the mixed and silica undersaturated 

igneous complexes, basalt (gabbro) compositions predominate largely over trachyte (syenite) and phonolite 

(nepheline syenite) intrusions. 

In continental areas, the distribution of rock types 

changes as a function of time elapsed since the end of the 

last orogenic event. Beginning at the very end of the 

late-orogenic episode and lasting less than 25 Ma, the 

onset of the post-orogenic episode is marked by rhyolite 

(granite) complexes predominating over scarce basic 

formations, and evidence exists for availability of large 

amounts of water (subsolvus granites). Then, during the 

500 Ma-long post-orogenic to anorogenic episode, 

subsolvus granites are progressively replaced by 

hypersolvus granites, occasionally altered by aqueous 

fluids. After this long period, mixed and silica 

undersaturated complexes are emplaced, where low-silica 

basic rocks predominate over rare nepheline syenitic 

differentiates, frequently altered by mixed (H20 + C02) 

fluids. 

In oceanic islands, a systematic evolution in time from 

tholeiitic, largely submarine, to transitional to alkaline, 

mainly aerial, magmatism has been documented and 

interpreted in terms of either decreasing partial melting of 

the same mantle source, or of mantle sources varying from 

depleted MORB types to variously enriched OIB 

(plume-like) types. The younger alkaline volcanic (plutonic) complexes display the typical twofold evolution 

toward silica oversaturated trachyte ± rhyolite (syenite ± 

granite) and undersaturated phonolite (nepheline syenite) 

differentiates. 

Whether the liquid line of descent yields ultimate silica 

oversaturation or undersaturation, calcic amphibole, a 

silica-deficient hydrous mineral, plays a critical role. If the 

amount of water dissolved into the melt exceeds approximately 2 wt %, which corresponds to a vapour pressure of 

at least 50 MPa, magmatic amphibole can crystallize near 

the liquidus. As melt viscosities and solidus temperatures 

are significantly lowered, diffusion processes are enhanced 

and promote efficient liquid-mineral separation. Massive 

and fast precipitation of early calcic amphibole favours 

nearly complete consumption of basalt (gabbro) melts and 

generation of large volumes of highly silicic rhyolite 

(granite) residual liquids. 

In water-deficient and/or very shallow environments, 

amphibole can no longer crystallize and early crystals 

dissolve ultimately into the melt. Melt viscosities and 

solidus temperatures increase, so that liquid-mineral 

separation becomes sluggish. Late in situ resorption of 

early calcic amphibole can generate strongly silica 

undersaturated phonolite (nepheline syenite) residual 

liquids but in small volumes only, due to inefficient 

liquid-mineral separation within basalt (gabbro) melts. 
Thus, magma differentiation is chiefly governed by the 

water molecule which combines two separate chemical 

effects: 1) calcic amphibole upper stability limit by filling 

its hydroxyl site; and, 2) catalysis of liquid-mineral 

reactions by lowering melt viscosities and solidus temperatures. Water dissolved into the melts at depths may come 

from at least two distinct reservoirs: 1) rock-forming 

hydrous minerals, which can release water by dehydration 

melting reactions; and, 2) aqueous fluids, which can 

percolate through permeable host rocks at the magma 

chamber level. 

Rapid Ascent Rates of Granitoid Magmas 

Determined from Epidote 

Dissolution Kinetics 

1

Brandon, Alan D., 2Creaser, Robert A. 

and 2Chacko, T. 1

Department of Terrestrial Magnetism, 

Carnegie Institution of Washington, 

5241 Broad Branch Road, N. W., 

Washington, DC 20015, USA 2

Department of Geology, University of Alberta, 

Edmonton, Alberta, T6G 2E3 Canada 

Time scales for ascent of silicic magmas from the lower 

to upper crust are important in controlling their compositions. During rapid ascent, little modification occurs and 

magma compositions likely are controlled by source 

characteristics, extent of melting, and separation processes. 

During slow ascent, more modification can occur, and 

resultant magma compositions may not accurately reflect 

the nature of the source rock. In addition, magma ascent 

times are a function of the method of migration, either 

diapirism ( -1 m yr-1

), or dike transport ( > > 1 m yr1). 

Numerical models suggest that dike transport of granitoid 

magma is viable, and such dikes can fill large silicic 

plutons in < 1000 years (Clemens and Mawer, 1992). 

Here, we use direct petrologic constraints to evaluate the 

transport rates and mechanisms of some granitoid magmas. 

Epidote is stable at > 6 kbar in granitoid magmas. 

Magmatic epidote formed at - 8 kbar is known from 

dacite dikes in the Colorado Front Range emplaced at 2 -

4 kbar, and is present because of the rapid ascent rate 

(Dawes and Evans, 1991). Magmatic epidote is present in 

the mid-Cretaceous White Creek batholith in SE British 

Abstracts 27 

Columbia (Brandon and Lambert, 1994), and has 

compositional characteristics typical for magmatic epidote 

reported elsewhere. Very low Ti02 ( < 0.1 wt %) and 

pistacite contents (Ps = [Fe3+/(Fe3+ + Al)]) of Ps27 to Ps29 

are diagnostic of magmatic epidote. The White Creek 

batholith was emplaced into metapelites and the mineral 

assemblage of its contact aureole consists of quartz, 

muscovite, sillimanite, and staurolite, with an emplacement 

level of 2.3 to 3.8 kbar (7 to 11 km depth). Emplacement 

pressures determined using two calibrations of the Al-inhornblende geobarometer for White Creek hornblende rims 

range from 2.2 to 4.0 kbar, overlapping with the range of 

pressures indicated by the contact aureole assemblage. 

Occurrence of epidote in this shallow-level pluton suggests 

epidote was preserved by rapid magma transport from the 

source region. 

We have quantitatively constrained magma transport 

rates from the deep crust by calibrating the low-pressure 

dissolution of epidote in granitoid magmas experimentally. 

The experiments show that epidote in the White Creek 

granodiorites are stable at pressures greater than 8 kbar, 

but breaks down at less than 6 kbar (temperatures of 7 50 

to 7800C). Reaction rims in the low pressure experiments 

consist of quenched melt, plagioclase and magnetite. 

Reaction rim growth is modeled by a parabolic rate law, 

and is controlled by diffusivity of major elements across 

the reaction rim. Measured reaction rim widths in the 

experiments produce effective diffusion coefficients from 

2.3 x 10-15 to 6.1 x 10-15 m2 s-1 at 750 to 780°C. Using 

these coefficients, at 780°C a 2 mm diameter epidote 

crystal will dissolve in < 100 years with reaction rim melt 

fractions of > 10%. At 700°C, just above the 3 kbar 

water-saturated granite solidus, a 2 mm diameter epidote 

will dissolve in < 1200 years. 

Epidote phenocrysts within the dacite dikes have highly 

corroded rims indicative of reaction and dissolution. 

Epidote resorption begins at 6 kbar during ascent and 

continues until fmal solidification at 2 kbar, equating to at 

least 14 km of epidote-reactive transport. The phenocryst 

assemblage in the dikes equilibrated at 800-880°C at 7 to 

12 kbar (Dawes and Evans, 1991). If the dikes were at 

800°C upon emplacement at 2 kbar, a minimum rate of 

350 m yr-1 is calculated using the reaction rim growth 

data. For epidote within the White Creek batholith, a 

minimum ascent rate of 1750 m yr-1 is obtained at 780°C. 

These rates suggest that rapid magma ascent is required 

for epidote preservation in low pressure granitoid rocks 

consistent with models for dike transport, but not 

diapirism. In conclusion, the petrological and experimen-
28 The Origin of Granites and Related Rocks 

tal evidence presented here support a model where magmatic epidote in shallow-level granitoid plutons results 

from rapid magma transport from the deep crust, likely via 

dikes, precluding large amounts of interaction with the 

surrounding country rocks. 

Brandon, A.D. and Lambert, R.St.J., 1994. Crustal melting in the 

Cordilleran interior: the Mid-Cretaceous White Creek batholith in the 

southern Canadian Cordillera. J. Petrol., 35, 239-269. 

Clemens, J.D. and Mawer, C.K., 1992. Granitic magma transport by 

fracture propagation. Tectonophysics, 204, 339-360. 

Dawes, R.L. and Evans, B.W., 1991. Mineralogy and geothermobarometry of magmatic epidote-bearing dikes, Front Range, Colorado. 

Geol. Soc. Am. Bull., 103, 1017-1031. 

Generation of Leucogranites at Granulite 

Facies Conditions: A Case Study from the 

Kerala Khondalite Belt in Southern India 

Braun, I. 

Mineralogisch-Petrologisches Institut, Universitiit Bonn, 

Poppelsdorfer Schloss, 53115 Bonn, Germany 

Leptynitic garnet-biotite gneisses in the central and 

northern part of the Neoproterozoic Kerala Khondalite Belt 

(KKB) in southern India were regarded as the main source 

rocks for garnet-bearing leucogranites (Braun and Raith, 

1994). From geochemical and petrological work it was 

suggested that granulite facies metamorphism (P = 5.5 -

6 kbar, T = 800 - 850°C, an 0 - 0.3) during the PanAfrican orogeny - 550 M~ ago led to fluid-absent 

dehydration-melting of biotite-bearing gneisses according 

to the reaction biotite + plagioclase + quartz = garnet ( + 

potassic feldspar) + granitic melt. This which gave rise 

to the development of garnet-bearing in-situ leucosomes in 

the leptynitic gneisses and the generation of leucogranitic 

melts. Although this model may explain the relationship 

between partial anatexis of the leptynitic gneisses and the 

generation of the leucogranites in general, some details of 

this process are still poorly known. Principally, a more 

precise knowledge of the source rock composition is 

required, the mechanisms of melt segregation at the 

assumed low melting degree ( < 10%) are not known, and 

the question of participation of melt intrusions from greater 

distances and short-range melt segregation from the 

adjacent gneisses in the formation of leucogranite dykes, 

eeds to be addressed. 

This study presents Rb and Sr isotope and REE data of 

selected leucogranite and leptynitic gneiss samples and first 

results of major and trace element modeling, which were 

carried out to achieve a more detailed understanding of the 

process that led to the generation of the leucogranites. 

In feldspar mineralogy and CaO and K20 abundances 

the leptynitic gneisses are highly variable. Gneisses which 

exclusively contain plagioclase ( - An25) are rich in CaO 

( - 3 wt %) and poor in K20 ( - 1 wt %) . They contrast 

with mesoperthite-bearing gneisses with CaO and K20 

abundances of 2 and 3 wt %, respectively. Rocks that 

contain both plagioclase and mesoperthite are less abundant 

and yield intermediate CaO and K20 values. 

Leucogranites form small dykes and veins in 

mesoperthite-bearing leptynitic gneisses and occur 

subparallel or discordant to the foliation. The granoblastic 

texture consists of euhedral garnet (Al~Pyr30 •••• ), quartz 

and mesoperthite (Ab380r57Aflo5), plagioclase is rare or 

absent. Accessory phases and opaque minerals are almost 

absent and only in very few cases gradations into 

fluorapatite-rich rocks are recognized. The chemical 

composition is characteriZed by high K20 and low FeO, 

MgO and Zr contents. REE abundances are low and yield 

flat chondrite-normalized patterns, which hardly show any 

Eu-anomaly. Compared to the leptynitic gneisses they are 

enriched in K20, Rb and Ba, while Na20, CaO, FeO, Sr, 

Zr and LREE are strongly depleted. 

REE and Rb-Sr isotope investigations were carried out 

on whole-rock samples of leptynitic gneisses, their in-situ 

leucosomes and gneiss domains and of leucogranites. RbSr isotope ratios of mesoperthite-bearing gneisses and their 

leucosome and gneiss domains plot in the same range and 

are weakly fractionated. REE patterns of these samples 

show a strong enrichment of HREE in the leucosomes, 

while LREE remain unfractionated. This suggests that 

dehydration-melting of the gneisses principally resulted in 

the formation of garnet which controlled the REE distribution between leucosome and gneiss domain. Furthermore 

these Rb-Sr and REE systematics preclude a high melting 

degree and significant melt extraction from the 

leucosomes. Leucogranites display HREE patterns similar 

to those of the leptynitic gneisses, while LREE are 

significantly depleted. Rb-Sr ratios of a leucogranite vein 

in a mesoperthite-bearing leptynitic gneiss are significantly 

higher than those obtained for the gneiss and do not plot 

on an isochron calculated for the gneiss and its both 

domains. This supports the idea that leucogranites intruded 

the gneisses from greater distances and that short-range 
melt segregation from the adjacent gneisses largely did not 

contribute to their generation. 

The source rock composition of the leucogranites was 

estimated from major and trace element mass balance 

calculations. It became obvious, that if equilibrium melting 

and low melting degrees are required, mesoperthitebearing leptynitic gneisses may not serve as source rocks. 

Instead, such rocks should be more sodic and less potassic 

and silicic in composition and also should have higher 

REE abundances. At the present stage plagioclase-bearing 

leptynitic gneisses provide the best results and thus are 

considered to approximate the source rock composition of 

the leucogranites. 

Braun, I. and Raith, M., 1994. The leptynitic gneisses of the Kerala 

Khondalite Belt, southern India: phenomena and mechanism of 

dehydration/melting at granulite facies conditions. Mineral. Mag., 

SSA, 119-120. 

The Taku Transect and its Granitic Rocks, 

Coast Mountains Complex, 

Southeastern Alaska 

Brew, David A., Drinkwater, James L., 

Ford, Arthur B. and Himmelberg, Glen R. 

U.S. Geological Survey, Branch of Alaskan Geology, 

MS 904, 345 Middlefield Road, 

Menlo Park, CA 94025, USA 

The Taku transect near Juneau, Alaska, is one of four 

transects that are currently used, together with reconnaissance field mapping, to define the Coast Mountains 

Complex. This complex has previously been informally 

called the Coast Mountains plutonic-metamorphic complex 

and by other names. The other transects are the Ketchikan 

and Skagway transects studied by Barker and Arth (1990) 

and the Stikine River transect studied in part by the present 

authors. The Taku transect is the only one that contains all 

of the magmatic, lithostratigraphic-terrane, metamorphic, 

and structural features that make up the Coast Mountains 

Complex. 

The Taku transect is about 40 km wide and 80 km long, 

and extends in a NE direction from Douglas and Admiralty 

Islands on the west to the Alaska-British Columbia boundary on the east. It is made up of these major units and 

structural features, with their lithotectonic terrane assignments: 1) low-pressure, low-temperature, mafic-composiAbstracts 29 

tion metavolcanic and lesser amounts of metapelitic 

sedimentary rocks assigned to the Douglas Island 

Volcanics of the Stephens Passage Group of the Gravina 

overlap assemblage; they are intruded by the 95 Ma 

granodiorite, tonalite, and quartz diorite plutons of the 

Admiralty-Revillagigedo belt; 2) the Coast Range 

megalineament, which is here the high-angle Gastineau 

Channel fault; 3) low- to medium-pressure, low- to 

medium-temperature, mafic-composition metavolcanic and 

some metapelitic and metacarbonate sedimentary rocks 

belonging to the Wrangellia terrane; 4) medium- to highpressure, medium- to high-temperature, Barrovian series 

pelitic schists together with minor amphibolitic schist and 

marble that are presently assigned to the Behm Canal 

structural zone; they are intruded by Late Cretaceous 

tonalite and granodiorite sills of the Mount Juneau pluton; 

5) well-foliated and locally lineated and mylonitized 

intermediate-composition granitic rocks of the 69 - 56 Ma 

Great tonalite sill composite batholith, which was emplaced 

at or close to the contact between the Behm Canal structural zone to the west and the metamorphic rocks of the 

Nisling terrane to the east; 6) layered biotite-hornblende 

gneiss, amphibole gneiss, quartz- and feldspar-rich schist, 

and multicomponent migmatite intruded by a series of 60 -

55 Ma hornblende-biotite granodiorite sills related to the 

Great tonalite sill; 7) generally unfoliated and homogeneous 50 Ma sphene-biotite-hornblende granodiorite and 

granite of the Turner Lake batholith, which contains 

sporadic screens of metamorphic rocks and local migmatite 

zones; and, 8) locally hornfelsed intermediate-composition 

metavolcanic rocks of the Stikine terrane and pelitic and 

semipelitic schists of the Nisling terrane at and near the 

international boundary. 

The 95 Ma Admiralty-Revillagigedo belt plutons are 

discrete and discordant, locally well foliated, mediumgrained, magnetite-free porphyritic biotite and hornblende 

quartz diorite and tonalite that contain primary epidote and 

garnet. The Mount Juneau pluton and the Great tonalite sill 

both are thick composite sill- or sheet-like bodies of 

foliated and locally well lineated medium- to coarsegrained tonalite. The Late Cretaceous Mount Juneau pluton 

is magnetite-free epidote-bearing biotite-hornblende tonalite 

and the 69 - 56 Ma Great tonalite sill is magnetite-bearing 

biotite-hornblende tonalite. The 60- 55 Ma sills to the east 

of and related to the Great tonalite sill are heterogeneous 

hornblende-biotite leucotonalite and granodiorite. The 50 

Ma Turner Lake batholith consists of homogeneous, 

massive, porphyritic to equigranular magnetite- bearing 

sphene- hornblende - biotite granodiorite to the west and 
30 The Origin of Granites and Related Rocks 

homogeneous, massive, porphyritic, coarse-grained 

allanite-bearing hornblende -biotite granite to the east. 

The metamorphic rocks that occur between these belts 

are assigned to several lithotectonic terranes and resulted 

from multiple metamorphic episodes both older than and 

about the same age as the plutonic events. Taken all 

together, the rocks of the Coast Mountains Complex 

record a long history of magmatism and metamorphism 

first near the converging plate margin between the Insular 

and Intermontane superterranes and then in a subsequent 

transtensional to extensional regime. 

Barker, F. and Arth, J.G., 1990. Two traverses across the Coast 

batholith, southeastern Alaska. In Anderson, J .L. (ed), The Nature 

and Origin of Cordilleran Magmatism, Geological Society of America 

Memoir 174, 395-405. 

Prosecuting the Connection Between 

Migmatites and Granites 

Brown, Michael 

Department of Geology, University of Maryland at 

College Park, MD 20742, USA 

" ...... the intensity of the conviction that a hypothesis 

is true has no bearing on whether it is true or not". 

P.B. Medawar 

Advice to a Young Scientist 

Migmatites are mixed rocks comprising granite in a 

metamorphic host. Regional migmatites typically occur in 

the higher-grade parts of orogenic belts, belts that exhibit 

polyphase and heterogeneous deformation at all scales 

(Brown, 1994). Many granites are located in orogenic 

belts: associated with high-grade metamorphic rocks; 

emplaced along shear zones associated with uplift and 

exhumation; and, emplaced in lower-grade metamorphic 

rocks at higher structural levels. Q.E.D. the crust is 

migmatitic at all scales! The plumbing by which melt 

segregates and magma transfers through the crust is 

marked by former melt (or its cumulate product) and 

magma that became stuck in the system. Outcrop-scale 

information in migmatite terranes indicates the importance 

of deformation during anatexis, both to drive melt segregation and to provide sites for accumulation. The same 

features occur at map-scale in orogenic belts, where 

regional tectonic features may control location of granite 

plutons. 

Static models of melt segregation by gravity-driven 

porous medium flow based on the dihedral angle model are 

unlikely to apply during most natural crustal melting. The 

application of differential stress to an anisotropic body 

such as the crust, in particular when it contains some 

volume of melt, leads to heterogeneous deformation and 

the development of sites for melt accumulation, as well as 

a driving force for melt movement. Melt may migrate 

through structurally-created pathways, such as fractures 

and shear zones, accumulate in a variety of dilatant sites, 

such as boudin necks, or immigrate as discordant bodies 

of leucosome. Melting at high an.o involves a volume 2 

reduction, segregation may be by convection and filter 

pressing, and melt retention is expected, since the system 

tends to "suck in" the melt (Brown et al., 1995a,b). Melt 

fraction may increase until the contiguity of the solid 

framework breaks down and bulk magma mobility with 

restite entrainment may occur; unless deformation drives 

out the melt at lower melt fractions. Major crustal melting 

involves hydrate breakdown at low aH 0 under volatile 2 

phase-absent conditions, which leads to an increase in 

volume, and will promote segregation by convection and 

filter pressing. Increasing volume may lead to pore fluid 

pressures high enough to facilitate melt-enhanced 

embrittlement, cataclastic flow and fracture to allow melt 

escape; effectively the system "blows out" the melt (Brown 

et al., 1995a,b). Field evidence is consistent with this -

excess melt in leucosomes commonly has been frozen 

while draining through an interconnected network of 

structurally-created pathways and dilatant sites to a higher 

structural level. Anatexis has a significant implication for 

crustal rheology. It is clear from studies of orogenic belts 

that there are feedback mechanisms between deformation 

and melting. Melting can lead to a concentration of 

deformation which not only facilitates melt segregation and 

ascent but partitions deformation preferentially back into 

the zones in which partial melting has occurred. 

Generally there are considered to be three end-member 

models by which melt might ascend through the crust to its 

level of emplacement, viz. as a diapir, in a fracture as a 

dike or along a shear zone (Brown, 1994; Brown et al., 

1995a,b). Magma will flow to low-pressure regions and 

migration of melt along major crustal shear zones appears 

fundamental in some orogenic settings, particularly 

transcurrent shear zones. The buoyancy force of the 

magma then will interact with regional stresses in the most 

convenient way to facilitate ascent. Zones dominated by 
buoyant magma flow may be characterized by a steep 

magmatic lineation, whereas zones characterized by strikeslip shearing may record sub-horizontal magmatic 

lineations. One consequence of an anisotropically layered 

lower crust is that melting in one zone may be capped by 

an unmolten layer above. If the partially molten zone 

within the crust did develop instabilities due to gravity, 

then melt might be expected to migrate parallel to the 

foliation and accumulate at dome culminations produced by 

the instability. If shear zones preferentially nucleate at 

these sites, then collection of melt followed by ascent 

within a shear zone would occur. 

An example of a middle crustal zone that preserves 

evidence for ductile transpression synchronous with 

melting is the St. Malo migmatite belt (Brown, 1995). 

Here melt has frozen during transfer along an interconnected system of shallow-oriented shear zones within the 

migmatite belt, and marginal to the migmatite belt larger 

volumes of magma have been frozen during transfer 

upward within major strike-slip shear zones. In this case, 

magma ascent through the crust from the zone of generation to the level of emplacement is inferred to have been 

channelized, having occurred through the strike-slip shear 

zones. Adjacent to the migmatite belt at a higher structural 

level the Mancellian granites are interpreted to represent 

emplacement into the upper crust of fugitive magma from 

migmatitic rocks analogous to the St. Malo migmatite belt 

at deeper structural levels (D'Lemos et al., 1992; Brown, 

1994, 1995). Ascent is inferred to have been channelized 

along steeply-oriented strike-slip shear zones with emplacement into tensile bridges developed between left-stepping 

segments of a major transcurrent fault zone. Along 

individual granite pluton/country rock contacts, however, 

local space creation by stoping clearly was the method by 

which emplacement was completed. 

Brown, M., 1994. The generation, segregation, ascent and emplacement 

of granite magma: The migmatite-to-crustally-derived granite 

connection in thickened orogens. Earth-Sci. Rev., 36, 83-130. 

Brown, M., 1995. The late-Precambrian geodynamic evolution of the 

Amorican segment of the Cadomian belt (France): Distortion of an 

active continental margin during south-west directed convergence and 

subduction of a bathymetric high. Geologie de la France, in press. 

Brown, M., Averkin. Y.A., McLellan, E.M. and Sawyer, E.W., 1995a. 

Melt segregation in migmatites. J. Geophys. Res., 100, in press. 

Brown, M., Rushmer, T. and Sawyer, E.W., 1995b. Mechanisms and 

consequences of melt segregation from crustal protoliths. J. Geophys. 

Res., 100, in press. 

D'Lemos, R.S., Brown, M. and Strachan, R.A., 1992. Granite magma 

generation, ascent and emplacement within a transpressional orogen. 

J. Geol. Soc. London, 149, 487-490. 

Abstracts 31 

Optimal Conditions for Porphyry Copper 

Ore Genesis Based on Experimental, 

Theoretical and Field Observations: 

An Overview 

Candela, P.A. 

Laboratory for Mineral Deposits Research, 

Department of Geology, University of Maryland at 

College Park, MD 20742, USA 

Extensive experimental, theoretical and field investigations by the Laboratory for Mineral Deposits Research, 

have shed light on the optimal conditions for the generation of Porphyry Copper Deposits, and other graniterelated mineral deposits. 

Candela and Holland (1984) demonstrated that copper 

partitions strongly into a chloride-bearing supercritical gas 

phase from a rhyolitic melt at 1.4 kbar and 750°C. Williams et al. (1995) confirmed those results and showed, 

further, that 1) copper partitioning is enhanced at 1 kbar 

and soooc relative to 1.4 kbar and 750°C; and, 2) copper 

partitioning is not further enhanced by changing the P and 

T to 0.5 kbar and 850°C. Further, whereas copper 

partitions strongly into a brine, as expected, the inferred 

exchange constant for the equilibrium: 

Cu(vapor) + Na(brine) = Cu(brine) + Na(vapor) 

was found to be on the order of unity. 

Regarding the partitioning of copper into crystalline 

phases, the univalent copper that prevails during magmatic 

processes does not have an affinity for ferromagnesian 

minerals. The uptake of copper by magnetite, biotite and 

amphibole can be related to subsolidus alteration (Simon, 

1995). Lynton et al. (1993) demonstrated experimentally 

that copper partitions strongly into pyrrhotite from a 

rhyolitic melt at 1 kbar and 800°C with D(po/rn) =775 

(averaged over oxygen fugacities of graphite - methane -

(water) and nickel-nickel oxide). These results indicate 

that magmatic sulfides are important, potentially, in 

sequestering copper at the magmatic stage. Whereas these 

data are necessary to demonstrate that sulfides are important hosts for copper in intermediate to felsic magmas, 

they are not sufficient. In a discussion of the conditions 

for the generation of giant ore systems, Candela and 

Blevin (1995) have shown that the variation in the copper 

concentration in the units of the Boggy Plain Suite of the 

Lachlan Fold Belt is consistent with the removal of copper 
32 The Origin of Granites and Related Rocks 

by pyrrhotite crystallization. Given the pyrrhotite/melt 

partition coefficient of Lynton et al. (1993) and the data on 

pyrrhotite solubility in high K melts from Luhr (1990) a 

bulk partition coefficient for copper on the order of 1.5-2 

is predicted. Further, given the Rayleigh fractionation 

formalism, treating the units of the Boggy Plain Suite as 

aliquots of melt, and assuming that Th is a perfectly 

incompatible element, yields a bulk partition coefficient for 

copper equal to 1.83 ± 0.17 (1o). These results do not 

falsify the hypothesis that copper sequestration in intermediate to felsic systems is controlled by the crystallization 

of pyrrhotite. 

The models of Candela and Holland (1986) and Candela 

(1989) showed that the critical variable for controlling the 

relative partitioning of a crystal-compatible element 

between crystalline phases and the magmatic volatile phase 

(MVP) is c!v'0

/C ;s , the ratio of the initial water concentration in the melt to the solubility of water in the melt. 

High values of c!v'0

/C ;s induce water saturation early, 

providing the MVP early access to copper dissolved in the 

melt. C~' /C~s is maximized when hot, wet magma is 

emplaced at high levels in crust. Further, the physical 

model of Candela ( 1991) suggests that interconnected 

volumes of the MVP are favored at upper crustal (P < 2 

kbar) conditions, facilitating the efficient removal of the 

MVP through spanning clusters of volumes of the MVP. 

Piccoli and Candela (1994) showed, from an analysis of 

apatite, that the Cl/H20 in the Tuolumne Intrusive Suite of 

the Sierra Nevada Batholith, USA, decreases with increasing (

87Sr/86Sr)i, suggesting that Cl/H20 is higher in more 

primitive melts in arc environments. Further, high oxygen 

fugacities ( > QFM) promote sulfide destruction after 

volatile saturation, and oxgen fugacities higher than NNO 

+ 3 log units may destabilize sulfides altogether. 

In summary, we can now understand, in a preliminary 

manner, the conditions that affect the probability of 

Porphyry Copper ore formation. According to the theoretical hypothesis outlined herein, emplacement of primitive, 

oxidized, hot, and wet melts at shallow levels of Earth's 

crust with devolatilization at approximately 1 kbar (with 

ore deposition at shailower levels) is optimal, in accord 

with observations. 

Candela, P.A., 1989. Magmatic ore-forming fluids: thermodynamic and 

mass transfer calculations of metal concentrations. In Whitney, J .A. 

and Naldrett, A.J. (eds), Reviews in Economic Geology, v. 4; Ore 

Deposition Associated with Magmas, Society of Economic Geologists 

Publishing Company, 203-221. 

Candela, P.A., 1991. Physics of aqueous phase exsolution in plutonic 

environments. Am. Mineral., 76, 1081-1091. 

Candela, P.A. and Holland, H.D., 1984. The partitioning of copper and 

molybdenum between silicate melts and aqueous fluids. Geochim. 

Cosmochim. Acta, 48, 373-388. 

Candela, P.A. and Holland, H.D., 1986. A mass transfer model for 

copper and molybdenum in magmatic hydrothermal systems: the 

origin of porphyry-type ore deposits. Econ. Geol., 81, 1-19. 

Candela, P.A. and Blevin, P.L., 1995. Physical and chemical magmatic 

controls on the size of magmatic-hydrothermal ore deposits. In Clark, 

A.H. (ed), Giant Ore Deposits-If, QminEx Associates and Queen's 

University, 2-37. 

Luhr, J.F.,1990. Experimental phase relations of water and sulfur 

saturated arc magmas and the 1982 eruptions of El Chichon volcano. 

J. Petrol., 31, 1071-1114. 

Lynton, S.J., Candela, P.A. and Piccoli, P.M., 1993. Experimental study 

of the partitioning of copper between pyrrhotite and a high-silica 

rhyolitic melt. Econ. Geol., 88, 901-915. 

Piccoli, P.M. and Candela, P.A., 1994. Apatite in felsic rocks: a model 

for the estimation of initial halogen concentrations in the Bishop Tuff 

(Long Valley) and Tuolumne Intrusive Suite (Sierra Nevada Batholith) 

magmas. Am. J. Sci., 294, 92-135. 

Simon, A., 1995. Copper in the Tuolumne Intrusive Suite: Is its behavior 

controlled by ferromagnesian phases. Unpublished B.S. thesis, 

University of Maryland at College Park. 

Williams, T.J., Candela, P.A. and Piccoli, P.M., 1995. The partitioning 

of copper between silicate melts and two-phase aqueous fluids: an 

experimental investigation at 1 kilobar, 800" C and 0.5 kilobar, 

850"C. Contrib. Mineral. Petrol., in press. 

Hercynite-Bearing Granites and Associated 

Metasedimentary Enclaves from the 

Taltson Magmatic Zone, Alberta, Canada: 

A Natural Example of High-Temperature 

Pelite Melting. 

Chacko, Thomas and Creaser, Robert A. 

Department of Geology, University of Alberta, 

Edmonton, Alberta, T6G 2E3, Canada 

The Taltson Magmatic Zone (TMZ) is the product of 

the 1.9-2.0 Ga collision between the Buffalo Head Terrane 

and the Churchill Craton (Ross et al., 1991). This 

collision produced a large amount of strongly peraluminous 

granitoid (Goff et al., 1986; Theriault, 1992). The present 

study focuses on a spectacular outcrop of one such 

granitoid and its associated metasedimentary enclaves. 

The rocks found at this outcrop (and in other parts of the 

TMZ) are remarkable in that the enclaves and some of the 

granites contain the high temperature mineral assemblage 

hercynitic spinel -quartz- garnet- cordierite ±sillimanite. 

Below, we argue that these enclaves are refractory vestiges 
of the pelitic composition protolith that gave rise to the 

granitoids. Furthermore, we suggest that the granites have 

not travelled very far from their source region. As such, 

this locality is an excellent natural laboratory for studying 

high-temperature, peraluminous magmas at their point of 

origin. 

The rocks in question are from Pelican Rapids (PR), a 

locality in northeast Alberta that is part of the regionally 

extensive Slave Batholith. The outcrop comprises two 

phases (I and II) of medium-grained, leucocratic, alkali 

feldspar granite. The sharp but sinuous contact between 

the two granite phases suggests that Phase I granite was 

still at high temperature and behaving plastically when 

intruded by Phase II. The two granites are very similar in 

field appearance and are distinguished from each other by 

relatively subtle mineralogical and textural differences. 

Phase I contains garnet, sparse biotite and mesoperthite as 

the only feldspar. Phase II granite lacks a primary mica 

but contains a small amount of plagioclase, hercynitic 

spinel, garnet and cordierite. Both phases of granite 

contain large (up to 60 meters across), lozenge-shaped 

enclaves of pelitic to psammitic composition paragneiss. 

The pelitic enclaves have migmatitic textures with variable 

amounts of quartz-alkali feldspar or quartz-rich leucosome. 

Interestingly, the accessory mineralogy of Phase II granite 

is identical to the mineral assemblage present in the 

enclaves. Thus, the enclaves and Phase II granite, 

although radically different in appearance, differ primarily 

in their modal proportions of minerals and their mineral 

textures. 

Both granite phases are characterized by high Si02 

(>73 wt %) and low FeO+MgO ( <2 wt %) but can be 

distinguished from each other on the basis of the higher 

K20/Na20, A/CNK, REE and Th of Phase II. The pelitic 

enclaves show a large range in chemical compositions 

depending on the amount of migmatitic leucosome remaining in the enclave; however, enclaves with little leucosome 

are characterized by a very refractory composition ( 49% 

Si02 , 29% Al20 3). This suggests that the enclaves have 

been extensively melted and that some have experienced a 

large degree of melt extraction. This observation is 

consistent with the extreme temperatures (900-1000°C) 

derived for these samples using TWEEQU thermobarometry and quartz-garnet oxygen isotope thermometry 

(Farquhar et al., 1994). 

The high temperatures, the mineralogical similarities, 

the peraluminous composition of the granites and the 

refractory nature of the enclaves all point to a melt-restite 

relationship between the granites and pelitic enclaves. If 

Abstracts 33 

this is correct then first order Stokes law calculations 

indicate that enclaves as large as those at PR could not 

have been transported a great distance from their point of 

origin by the granite magma. Therefore, the PR outcrop 

represents the root zone of a regionally extensive, S-type 

granite batholith. 

We suggest the proto lith for the granites at PR was a 

high-Al pelite with a relatively low plagioclase content. 

The early-formed melts derived from this protolith (i.e., 

Phase I) incorporated the plagioclase component leaving 

behind a residue rich in quartz, K-feldspar, sillimanite, 

and ferromagnesian minerals. Continued, higher temperature melting produced more potassic melts that were also 

richer in REE and Th (Phase II). These higher temperature melts, however, remained very leucocratic. This 

somewhat surprising result is consistent with recent 

experimental melting studies of true pelite bulk compositions (Patino Douce and Johnston, 1991) which indicate 

that melts produced from such protoliths remain 

leucocratic to T> 1000°C. Therefore, the term leucogranite should not necessarily be taken as synonymous 

with "low-temperature" granite. 

Many questions remain regarding the PR outcrop. In 

particular, how were the high temperature conditions 

recorded at the outcrop generated during a collisional 

orogen? This question is especially enigmatic because 

field and geophysical data do not indicate the presence of 

any contemporaneous basaltic magmatism or underplating. 

Farquhar, J., Chacko, T. and Ellis, D.J., 1994. High-temperature oxygen 

isotope thermometry in two contrasting terranes, the Taltson Magmatic Zone, Canada and the Napier Complex, Antarctica. Mineral. 

Mag., SSA, 265-266. 

Goff, S.P., Godfry, J.D. and Holland, J.D., 1986. Petrology and 

geochemistiy of the Canadian Shield of northeastern Alberta. Alberta 

Res. Counc. Bull., 51, 1-60. 

Patifio Douce, A.E. and Johnson, A.D., 1991. Phase equilibria and melt 

productivity in the pelitic system: Implications for the origin of 

peraluminous granitoids and aluminous granulites. Contrib. Mineral. 

Pertol., 107, 202-218. 

Ross, G.M., Parrish, R.R., Villeneuve, M.E. and Bowring, S.A., 1991. 

Geophysics and geochronology of the crystalline basement of the 

Alberta Basin, western Canada. Can. J. Earth Sci., 28, 512-522. 

Theriault, R.J., 1992. Nd isotopic evolution of the Taltson Magmatic 

Zone, Northwest Territories, Canada: insights into early Proterozoic 

accretion along the western margin of the Churchill Province. J. 

Geol., 100, 465-475. 
34 The Origin of Granites and Related Rocks 

ORAL PRESENTATION 

Causes of Compositional Variation Within 

Granite Suites 

Chappell, Bruce W. 

Department of Geology, 

Australian National University, 

Canberra, 0200, ACT, Australia 

Variation within granite suites potentially can be 

produced in several ways, which will be discussed with 

particular reference to the granites of the Lachlan Fold 

Belt (LFB). 

Magma mixing and/or mingling is generally proposed in 

terms of blending a crustal melt and the mafic material 

from the mantle which caused that melting. The Bega 

Batholith is the largest I -type granite complex in the LFB 

and shows many features that have elsewhere been interpreted as supporting magma mixing, such as the development of local hybrids, common mafic enclaves in the more 

mafic granites, linear patterns on variation diagrams, and 

systematic changes in isotopic composition across the 

batholith. However, when suites from the batholith are 

examined in detail, there are features of the chemical 

variation that are not compatible with mixing on a scale 

that would have produced the major variations in those 

suites. Any difference in compositions at either end of the 

range in composition of a suite is also seen at the other 

end, so that both the most mafic and felsic rocks show 

similar relative abundances of particular elements. Since 

su~h ~bser:ations are not consistent with magma mixing or 

mmglmg, It cannot have produced the major compositional 

variations. This observation is more widely applicable to 

the granites of the LFB, both I- and S-type. 

Assimilation of country rock had no detectable role in 

producing variations within granite suites of the LFB. 

While such processes would be accompanied by fractional 

crystallization and therefore be more complex than the 

variations expected with magma mixing or mingling, the 

within-suite correlations between isotopic and chemical 

compositions that would be expected are not seen. For the 

Bega Batholith, the variations in isotopic compositions 

correlate with differences in chemical composition between 

suites, but not within suites, so that the incorporation of 

sedimentary material occurred before the partial melting of 

the source rock. It has been argued that the variations in 

isotopic compositions of the LFB granites resulted from 

mixing of basaltic material and granitic melt derived from 

the melting of the Ordovician sedimentary rocks. However, the chemical compositions of many isotopically 

evolved I-type granites are not consistent with the addition 

of the amounts of sedimentary material that would have 

been required. Likewise, the chemical compositions of the 

S-type granites are not consistent with derivation from a 

mixture of basalt and the exposed sedimentary rocks. 

Fractional crystallization played a dominant role in 

producing the variation in the Boggy Plain Supersuite, 

which shows textural and compositional features which 

clearly indicate that the rocks, which range from 50% to 

75% Si02, formed progressively as cumulates from a 

liquid or largely liquid magma. Most of the granites of the 

LFB area are so distinct from the Boggy Plain rocks that 

an origin by fractional crystallization is certainly not self 

evident. Moreover, there are specific patterns of chemical 

variation that either make an origin by fractional crystallization unlikely, or in some cases preclude it, except at the 

most felsic compositions. Many of the most felsic granites 

of the LFB, particularly the S-types, have trace element 

abundances that resulted from extended fractional crystallization of a magma that had approached or reached minimum temperature compositions through the loss of restite 

(below), although not all such granites evolved further in 

that way. 

The restite model proposes that many granite suites 

contain crystals residual from melting, or restite, and that 

it is differences in the proportion of this restite that 

accounts for variation in composition. Melting within the 

crust will produce a magma comprising granitic melt in 

equilibrium with restite. The composition of a granite that 

results will be a function of the degree of separation 

between those two components. The difficulties of generating most of the granite suites of the LFB by the three 

processes discussed above are generally resolved with this 

model. 

Hydrothermal alteration has the potential to greatly alter 

the composition of a granite. Destruction of feldspar has 

chemical effects analogous to those of feldspar fractionation at higher temperatures. The primary evidence for 

alteration is generally petrographic; however chemical data 

show that its effects are overrated. Many rocks in which 

there are petrographic signs of alteration plot in a very 

tight field or array on chemical diagrams, and for felsic 

granites the compositions are very close to minimumtemperature compositions, so that alteration occurred over 

a scale much smaller than that at which the rocks are 

sampled for chemical analysis. 
Plutons Emplaced in Dextral Strike-Slip 

Deformation Zones in the Abitibi 

Subprovince, Canada 

Chown, E.H. 

Sciences de la terre, Universite du Quebec a Chicoutimi, 

Chicoutimi, Quebec, G7H 2Bl, Canada 

Numerous plutons in the Archean Abitibi Subprovince 

fall in the 2696 - 2692 Ma age bracket. The Colombourg 

(2696 Ma) and Franquet (2692 Ma) intrusions are representative of the two principal petrologic suites 

(monzodiorite and tonalite-granodiorite), and both occur 

adjacent to major southeast-trending dextral strike-slip 

faults. The strike-slip faulting is the final episode of 

regional deformation in the area. 

The Colombourg pluton is an elongate southeasttrending mass bounded on the north and south by dextral 

strike-slip faults. The southern fault dies out near where 

it encounters the buttress of a large pre-existing tonalitediorite complex just south of the intrusion site. The pluton 

is composed of quartz monzodiorite and granodiorite which 

grade almost imperceptibly into one another. The rocks 

are calc-alkaline, with LREE enrichment patterns similar 

to other intrusions of the monzodiorite suite. Magmatic 

foliation within the pluton defines a shallow northwest 

plunging trough, with steep sides and a prominent NW 

plunging lineation. Magmatic foliation is defined by 

alignment of xenoliths and hornblende and plagioclase 

grains. A later submagmatic foliation is subvertical and 

parallel to the foliation in the bounding faults. This 

foliation is defined by polygonized quartz, bent and broken 

biotite and plagioclase. Pressure estimates using the Al-inhornblende barometer give two signatures at 2. 2 kbar and 

3. 2 kbar suggesting that the lower value indicates the 

emplacement and the higher denotes earlier crystallization 

in a subjacent magma chamber. The unique shape and 

position of the intrusion suggest that it was emplaced 

where the principal displacement on the strike-slip faults 

shifted from south to north forming a left -stepping dextral 

pull-apart. 

The Franquet stock, although outwardly appearing to be 

a standard subcircular pipe, is a composite stock formed of 

an inner L-shaped dyke of granodiorite enclosed in a later 

fine-grained leucogranodiorite phase, occurring between 

two branches of dextral strike-slip fault which splits around 

the resistant mass of an earlier tonalite batholith. The 

central dyke is north-south parallel to the local tensional 

Abstracts 35 

direction, with the L-extension along the north-bounding 

fault. The outer partial rim of leucogranodiorite fills the 

angle of the L. This calc-alkaline series evolves from 

metaluminous to peraluminous. Pressure estimates using 

the Al-in-hornblende barometer indicate an emplacement 

pressure of 1. 75 kbar with an early-crystallization signature of 2.3 kbar. The reverse-zoned stock formed in a 

right-stepping dextral pull-apart structure, and is possibly 

a pipe feeding a higher level intrusion, but more likely a 

cupola rising from a differentiating body at depth. 

The study shows that deformation-related plutons may 

take on a variety of forms in response to local conditions, 

and that the late strike-slip faulting episode of regional 

deformation exercised considerable control on pluton 

emplacement. 

Granites, Subduction, and Extension in the 

Basin and Range Province, Western USA 

Christiansen, Eric H. 

Department of Geology, Brigham Young University, 

Provo, Utah 84602, USA 

Two plutons emplaced only 15 km apart and separated 

in time by about 13 Ma illustrate the contrasting 

compositional effects of subduction and extension on 

granite geochemistry. The Desert Mountain granite (34. 8 

Ma) and the Sheeprock granite (21.4 Ma) are both small, 

shallowly emplaced plutons. The older Desert Mountain 

pluton formed above a gently dipping slab of subducting 

oceanic lithosphere; it was emplaced in a southward 

migrating zone of magmatism that transected the Great 

Basin during the middle Tertiary. Late Tertiary normal 

faults cut the pluton and may have tilted it. The pluton is 

slightly peraluminous and comprised mostly of biotite 

monzogranite with titanite, magnetite, ilmenite, zircon, and 

apatite. Magnetic susceptibilities of the pluton average 

0.015 Sl units - typical of magnetite-series granites. The 

Desert Mountain granite appears to have crystallized at a 

relatively high oxygen fugacity - based on its mineral 

assemblage and low Fe/Mg ratios in the mafic silicates. 

It has trace element concentrations and patterns typical of 

subduction-related granites on continents, including deep 

Nb and Ti anomalies. Mineralization related to the granite 

consists of partially oxidized Cu deposits and F- and 

metal-poor greisen. Isotopic ratios are typical of middle 

Tertiary granites from this region ((87Sr/86Sr)i = 0.709 and 
36 The Origin of Granites and Related Rocks 

eNd = -18) and consistent with a major contribution from 

felsic crust of Proterozoic age known to underlie the 

eastern Great Basin. Heat for crustal melting was 

apparently derived from mafic magmas generated in the 

subduction zone. Contemporaneous andesites are apparently mixtures of crust and mantle components. 

The younger Sheeprock granite was emplaced after the 

subducting slab had slipped away to be replaced by warm 

asthenosphere. This is also near the beginning of the 

episode of late Tertiary extension that formed the Basin 

and Range province. The pluton consists of monzogranite 

with zircon, fluorite, monazite, thorite, uraninite, complex 

Ta-Nb-Sn-W-Fe-Ti oxides and local topaz, beryl, 

zinnwaldite, and muscovite. It is also slightly 

peraluminous, but appears to have crystallized at a low 

oxygen fugacity based on the Fe-rich character of the 

biotites and low magnetic susceptibilities (0.001 SI units -

typical of ilmenite-series granites). The Sheeprock granite 

has high concentrations of incompatible trace elements and 

fluorine and is similar in composition and mineralogy to 

topaz-bearing rhyolites erupted contemporaneously in the 

region. Mineralization consists of scattered clots of beryl 

- albite - zinnwaldite - muscovite and thin veins of W- and 

Sn-rich greisen with F-rich muscovite. The trace element 

pattern of the Sheeprock granite is typical of other highly 

evolved granites and, in many respects, to those of 

anorogenic granites. Nb concentrations approach 100 ppm 

and Rb concentrations 1000 ppm. However, contributions 

from old felsic continental crust are limited by the isotopic 

composition of the pluton ((87Sr/86Sr)i = 0.706 and eNd = 

-6). Perhaps, the magma was derived by partial melting 

of young mafic crust underplated during the middle 

Tertiary, but it is difficult to obtain fluorine-rich magmas 

with low oxygen fugacities by this process. Alternatively, 

the source may be granulite-facies Proterozoic rocks in the 

lower crust. The elevated concentrations of fluorine and 

low oxygen fugacity are more easily explained by the latter 

hypothesis. In either case, lithospheric extension appears 

to have been important because it allowed very small melt 

fractions (less than 10%) to accumulate and migrate 

upwards without mixing with contemporaneous mafic 

magmas. Heat for partial melting was probably provided 

by: 1) replacement of the cold lithospheric slab by hot 

asthenosphere when subduction ceased; 2) consequent 

extension, uplift, and crustal thinning; and, 3) emplacement of hot but dense mafic magma at the base of the 

crust. 

Respective Roles of Source Composition and 

Melting Conditions in the High-K 

Content of Some Magmas: 

Geochemical Constraints 

Cocherie, Alain and Rossi, Philippe 

BRGM, B.P. 6009, 

F-45060 Orleans Cedex 2, France 

The origin of potassic to ultrapotassic rocks, which are 

also Mg-enriched, is still not clear. Such rocks occur in 

the Variscan batholiths of Europe (Bohemia, Vosges, Alps, 

Corsica), and we have studied the respective roles of 

source composition and melting conditions in the formation 

of acid and intermediate components emplaced contemporaneously with mafic components. 

Special attention was paid to occurrences in Corsica and 

Bohemia. Major- and trace-element data indicate similar 

geochemical characteristics for all rocks of the Mg-K 

associations, i.e. high Mg, Co, Cr and Ni concentrations, 

high contents of incompatible elements (K20, Rb, Cs, 

LREE, Th, U, Zr, Hf, etc.), and strong REE fractionation 

without large Eu anomalies. Mineral composition varies 

only slightly from mafic to felsic rock -types. These points 

indicate a genetic link between rocks found in the same 

area, but isotope data rule out such a link. A common 

origin for granites ranging from monzodiorite to 

leucosyenogranite is possible, as indicated by trace-element 

isotope data, but mafic rocks show a wide range of 

variation for Sr and Nd initial ratios, suggesting a mixing 

process between two end-members consisting of mantlederived material and a crustal component. According to 

the isotope data, the crustal component appears to be 87Sr-

. h d d 143 ennc e an Nd-depleted, compared to the associated 

granite source. The most contaminated mafic rocks show 

REE, Sr and Nd evidence of a crustal end-member 

resembling that of normal detrital sedimentary rock, with 

a flatter HREE pattern, and higher 87Sr/86Sr and lower 

143Nd/144Nd ratios than Mg-K granite. The K20/Na20 ratio 

also decreases as a function of the importance of the 

crustal component. In other words, the high K20 and 

LREE contents seem characteristic of a mantle-derived 

magma, but not of the associated Mg-K granites, nor of 

any other crustal component. 

Thus, based on the geochemical data of mafic Mg-K 

and associated acid rocks of Corsica, at least three endmembers are involved. 
A mantle end-member shows isotope compositions of 

syenomonzonite: (

87Sr/86Sr)i = 0. 7046; eNd(t) = +0.7 

and o1SO = +6 %o. Several models have been proposed 

for the origin of a highly potassic mantle-derived magma: 

1) an extremely small degree of partial melting of garnetperidotite mantle material; 2) fusion of a metasomatized 

mantle source; 3) zone refining; 4) leaching of wall rock 

leading to an enrichment of incompatible elements during 

ascent of the melt; and, 5) separation of very small melt 

fractions from a thick mantle layer and accumulation in a 

metasomatized layer, which, upon remelting, would yield 

a strongly potassic magma. In our opinion, if metasomatic 

fluids are involved, their main role would be to favour the 

melting process, which could take place as a two-stagemodel: a phlogopite- (±garnet) bearing source melts at 

- 200 km depth; and the rising magma induces a zonerefining process in the traversed mantle, leading to a 

significant increase of incompatible elements and LREE, 

whereas the isotope characteristics of the melt retain their 

mantle affinity, despite the increase in Sr and Nd concentrations. 

A second end-member is typical detrital material. Its 

interaction with the H20-enriched mafic mantle-derived 

magma will lead to flatter LREE patterns of the most 

contaminated syenomonzonites. 

A third source has the isotope composition of granitic 

rocks: (

87Sr/86Sr)i = 0.706; eNd(t) = -3 and o180 = +8 

%o. Using trace elements and especially REE, we found 

that partial-melting of a non-modal batch (F = 0.3) of dry 

granulite with a graywacke composition fits monzodiorite 

data. Under wet melting conditions, this protolith could 

yield a calc-alkalin~ monzogranitic magma, characterized 

by the same isotopic feature and a distinct chemical 

composition compared to high-Mg-K granites. High Mg-K 

granites from Bohemia and the Vosges show similar 

chemical compositions as the same rock types from 

Corsica, but with another isotopic signature (i.e. Pb). 

Abstracts 37 

ORAL PRESENTATION 

S-and I-Type Granitoids of the 

Eastern Lachlan Fold Belt: 

Three-Component Mixing, 

Not Restite Unmixing 

Collins, W.J. 

Department of Geology, University of Newcastle, 

Newcastle, NSW 2308, Australia 

425-400 Ma old !-type granite suites of the eastern 

Lachlan Fold Belt (LFB) show strong linear chemical 

variation, which is considered to reflect crystal fractionation from parent magmas with -60% Si02 , rather than 

restite unmixing from intermediate-composition source 

rocks. The characteristic chemistry of I -type suites is 

evident in coeval gabbroic complexes, indicating an 

intrinsic link. The !-types are a mix of subordinate 

gabbroid, representing mantle-derived melts, and a 

dominant metaigneous, 500 - 600 Ma old lower crustal 

component, identified from N d isotopic data and the ages 

of inherited zircons in granites and mafic enclaves. Mafic 

microgranular enclaves are considered to be the final 

vestiges of the mixing process, reflecting mingling of 

contaminated, mantle-derived melts with granite magma in 

high-level, almost-solidified plutons. 

S-type granite suites of the eastern LFB are not chemically imaged by any metasediment of Early Palaeozoic or 

Proterozoic age in eastern or central Australia, but the 

zircon populations from S-types and Ordovician LFB 

sediments fall into similar patterns of age groups. Furthermore, gneissic metasedimentary inclusions in S-type 

granites closely approximate the Ordovician sediments, 

implying a genetic link. The Cooma granite is "remobilized" Ordovician sediment and represents one of the few 

restite-controlled granites in the LFB. It formed at 3.5 -

4 kbar, whereas most S-types were generated at 4 - 5 

kbar. Chemical tie-lines between Cooma granite and 

mafic !-type Jindabyne tonalite encompass almost all mafic 

S-type granites of the vast Bullenbalong suite, for all 

elements, consistently in the proportion Jindabyne:Cooma, 

30:70. This 30:70 mix corresponds to the Bullenbalong 

parent magma, and chemical variation in the suite is a 

response to crystal fractionation. 

A three-component mixing model is presented, whereby 

mantle-derived basaltic magmas intruded, melted, and 

mixed with metaigneous lower-crust to produce !-type 
38 The Origin of Granites and Related Rocks 

granites in the LFB. Continued underplating and intraplating of mantle material expanded the elevated geotherms 

into the mid-crust ( -15 km depth), where locally 

underthrust Ordovician metasediment was remobilized and 

mixed with the hybrid I-type magmas to produce S-type 

granites. Therefore, S-type granites of the LFB are 

heavily contaminated I-types. The model is consistent with 

the simple Nd-Sr-0-Pb isotope mixing arrays defined for 

LFB S- and I-type granites. The remarkable east-west, 

chemical and isotopic asymmetry across the Bega Batholith 

demonstrates a progressive westward increase of Ordovician metasediment that mixed with the I-type granitoid 

magmas, as the thickened crustal welt generated by the 

-430 Ma old "Benambran Orogeny" is approached. The 

model explains the paucity of I -types in areas dominated 

by S-types, and their later emplacement. Finally, the 

mixing model implies that Proterozoic continental basement does not exist, which is consistent with the known 

tectono-stratigraphy of the LFB. 

Pervasive Magma Transfer Through the 

Lower Middle Crust During Non-Coaxial 

Compressional Deformation: 

an Alternative to Diking 

1

Collins W.J. and 2Sawyer, E.W. 1

Depanment of Geology, University of Newcastle, 

Newcastle, NSW 2308, Australia 2

Sciences de la Terre, Universite du Quebec a 

Chicoutimi, Chicoutimi, Quebec, G7H 2Bl, Canada 

Theoretical studies of diapirism and diking suggest that 

these are efficient mechanisms of magma transfer through 

the crust. We examine the structural location of 

leucosomes and granitoids in the deep crust of the Arunta 

Inlier in central Australia to test whether these were the 

actual mechanisms of magma transfer. 

At Mt. Hay, granitoid magmas migrated through the 

middle-lower crust during a period (Dl - D4) of WSWdirected thrusting. The magmas utilized whatever channels 

were locally available, i.e. opportunistic use of existing 

folds, boudins and shear zones, or created new channels 

by magmatic fracture either parallel to layering or, much 

less commonly, in irregular arrays. Because the channelways were principally fold hinges, or controlled by the 

orientation of layering anisotropy (SO or Sl), the melt 

could not migrate vertically, but rose buoyantly parallel to 

the regional elongation direction, which is defined by 

coaxial folds and a strong elongation lineation. As a result 

of these factors, melt transfer in the Mt. Hay area was not 

focussed along a few conduits (i.e. dikes), but was 

distributed over a wide area and occurred through a 

diverse network of various structurally-linked channelways. 

The upper middle crust of the Anmatjira - Reynolds 

Range area contains abundant granitoid sheets that are of 

similar age, geochemistry and structure to granite 

leucosomes in the deeper Mt. Hay region. Thus, granitic 

melts formed at the base of the continental crust are 

believed to have passed through the middle-lower crustal 

level (25-30 km) exposed at Mt. Hay and accumulated, in 

batholithic proportions, at shallower crustal levels (12-20 

km) represented by the Anmatjira - Reynolds Range area. 

The observations demonstrate that magmas are capable 

of pervasive migration through, and accumulation in, the 

crust during a major compressive, non-coaxial shear 

deformation because the crust is anisotropic and, therefore, 

deforms heterogeneously to create innumerable interconnected dilatant sites. Diking is expected to be the principal 

magma transfer mechanism in comparatively isotropic 

crust. 

ORAL PRESENTATION 

Radiogenic Isotopes in Granitic Systems: 

Studies of Melting and Mixing at the Source 

Creaser, Robert A. 

Depanment of Geology, University of Alberta, 

Edmonton, Albena, T6G 2E3, Canada 

Radiogenic isotope systems (Sr, Nd, Pb) play a critical 

role in many aspects of granite petrology, such as evaluating petrogenetic models, interpreting deep crustal characteristics and tectonic studies. Isotopic studies of granites 

are, by nature, removed from the mechanisms by which 

magmas are produced; we typically only see (and analyze) 

the products of larger-scale tectonothermal processes once 

granitic magmas move from their formation region. 

Intuitively, this does not present any limitations, as 

radiogenic isotopic systems should directly record the 

characteristics of source region(s) or component(s) of 
formation; granites, after all, are "images of their 

sources". However, model studies of Pb isotopes in 

granites have advocated large Pb isotopic shifts between 

melt and source as a result of differential mineral solubility. Models of crustal heating by basalt intrusion suggest 

melting on very short timescales ( < 0.01 Ma); far less 

than timescales required for (diffusional) isotopic equilibration of a protolith. To better constrain the meaning of 

isotopic signatures carried by granites, studies of granites 

at their source have been undertaken. The localities 

reported on here cover two popular models for granite 

origins, namely partial melting of crustal rocks and magma 

mixing in a mafic-felsic magma system. 

Granite and associated high-grade metasedimentary 

enclaves from Pelican Rapids (PR) in NE Alberta are part 

of the 1. 93 Ga Slave Batholith from the Taltson Magmatic 

Zone. The outcrop at PR comprises three phases of 

leucocratic granite (migmatitic leucosome, Phase I, Phase 

II) containing large (up to 60 m), refractory enclaves of 

pelitic to psammitic paragneiss showing evidence of 

extensive partial melting and granite production. P-T 

estimates are 6-8 kbar, 900-1 000°C for granites and 

enclaves. When compared with experimental melt compositions, granites from leucosome to Phase I to Phase II 

appear to represent sequentially higher degrees of partial 

melting from a pelite source. Geochemical data support 

this hypothesis, particularly REE and Th contents which 

systematically increase with degree of melting reflecting 

increasing monazite solubility. Refractory pelitic xenoliths 

yield eNd values of -4 to -7. Leucosome and Phase I 

granites yield eNd values of -10 to -11, and Phase II granite 

-5 to -6. toM values for these granites differ by > 1 Ga. 

At face value, these data indicate that the leucosome and 

Phase I granites are unrelated to the refractory enclaves, 

despite intimate field and geochemical relationships. 

Alternatively, these systematics can be explained by partial 

melting of a pelite where Nd isotopic budget of a melt is 

controlled by the melting behavior (solubility) of two 

mineral phases (garnet and monazite) with vastly different 

Sm/Nd and Nd isotopic compositions at the time of 

melting. Significant isotopic contrasts can be produced 

between a (total-rock) protolith and the initial partial melts. 

Later, higher-temperature, melts approach the source 

(total-rock) isotopic composition as diffusion- and 

recrystallization-driven isotopic equilibration occurs. 

Field evidence of mafic-felsic magma interaction occurs 

in the Campanas Igneous Complex (CIC), Argentina. The 

CIC comprises the Campanas Granite (CG) which is 

intruded by mafic dikes and zoned mafic intrusions (ZMI). 

Abstracts 39 

The ZMI, which consist of gabbroic cores surrounded by 

concentric rings of diorite, tonalite and granodiorite, 

extend in an approximately 6 km long N -S band along the 

central axis of the CIC. Major and trace element abundances vary systematically from the core of each ZMI to 

the surrounding CG. Samples of CG close to the ZMI 

have lower abundances of Si02 and incompatible elements 

than samples far from the ZMI, reflecting chemical 

interaction with the ZMI. The ZMI are heterogeneous in 

composition and contain mineral disequilibrium features 

such as reverse zoned plagioclase and quartz ocelli. Sr 

and Nd isotope analyses along a profile from the gabbro to 

the granite define a mixing curve in Nd-Sr isotopic space 

With eNd variatiOnS from +4 tO -3 and esr VariatiOnS from 

-60 to +60; the Sr data likely reflect, in part, secondary 

processes after crystallization of the 468 ± 3 Ma CIC. 

This wide isotopic variation occurs in samples separated 

from each other by only a few meters, highlighting the 

absence of diffusion-controlled isotopic equilibration in this 

mixing system. Petrographic, geochemical and isotopic 

data suggest that the evolution of the complex was controlled by fractional crystallization and replenishment of 

mafic magma inside a granite magma chamber, followed 

by extensive magma mixing between the CG and the 

fractionated end-members of the ZMI to produce intermediate calc-alkaline rocks. 

Granitoid Magmatism and Crustal Evolution 

in the Southwest Baltic Shield. 

1

Darbyshire, D.P.F., 2Brewer, T.S., 

3 0 3 0 .. Larson, S.-A. and Ahall, K.-1. 1 NERC Isotope Geosciences Laboratory, 

Kingsley Dunham Centre, Nottingham NG12 5GG, UK 2

Borehole Research Group, Department of Geology, 

Leicester University, Leicester LEI 7RH, UK 3

/nstitute of Geology, University of GOteborg, 

S-412 96 GOteborg, Sweden 

The Baltic Shield forms the major exposure of Precambrian crust in northern Europe and is the principal component of Fennoscandia, one of the crustal segments of the 

East European Craton. Crust in the Baltic Shield defines 

a simple age pattern, with the Archaean in the north east 

and progressively younger Proterozoic; terranes to the 

south and west. The northern Archaean nuclei provided a 
40 The Origin of Granites and Related Rocks 

cratonic, region onto which the Palaeoproterozoic 

Svecofennian juvenile arc terranes were accreted between 

1.9- 1.8 Ga. This accretionary event represents a major 

episode of juvenile crustal growth, which was followed by 

the intrusion of granitoids in the Transscandinavian 

Igneous Belt (Tffi) and by rapakivi magmatism in Finland, 

Estonia, Russia and Sweden, between 1.8 and 1.65 Ga. 

The final stages of Proterozoic crustal growth in the Baltic 

Shield are recorded in the southwest Scandinavian Domain. 

Here Palaeoproterozoic juvenile arc accretion and crustal 

growth occurred between 1.76- 1.61 Ga and is post dated 

by granitoid intrusions of the Goteborg Batholith. Subsequent crustal development in the southwest Scandinavian 

Domain involved extensive reworking during the Mesoand Neoproterozoic. The last event prior to cratonisation 

of the crust in this domain was the intrusion of mafic and 

felsic plutons at 1.56 Ga. 

The Transscandinavian Igneous Belt (TIB) is a c.1600 

km long by 20-150 km wide belt composed predominantly 

of granitoid intrusions emplaced on the western margin of 

the Svecofennian Province. Published U-Pb zircon 

geochronology indicates magmatism in the interval 1.81-

1.65 Ga, although within the belt there appear to be three 

discrete magmatic episodes termed Tffi 1 (1.81 - 1.77 Ga), 

TIB 2 (1. 7 Ga) and TIB 3 (1.68 - 1.65 Ga). In SW 

Sweden TIB 1 magmatism appears to predominate on the 

eastern side of the belt, whereas TIB 2 and 3 plutons 

outcrop on the western side. The granitoids range in 

composition from quartz monzonites to granites, having lor transitional A -type geochemical affinities. TIB 3 

plutonism is synchronous with the earliest rapakivi 

magmatism in Finland. 

The Goteborg Batholith (GB) comprises c. 1.59 Ga 

granitoids which outcrop in the western part of SW 

Sweden and in the Western Gneiss Region of Norway. 

The granitoids range from tonalites to granites, although 

granodiorites are the most abundant, and all have 1-type 

affinities. Both the TIB and GB granitoids formed after a 

hiatus of juvenile Proterozoic crustal accretion, but they 

display contrasting geochemical signatures. 

This study seeks to provide a better understanding of the 

origin of the magmatism and its relationship to crustal 

evolution in the southwest Baltic Shield. Any genetic 

model for this region must accommodate the differing 

styles of plutonism and in particular, the rapakivi 

magmatism. 

The Tffi is characterised by eNd values of 0. 7 to 2. 8 and 

depleted mantle model ages (toM) of 1.94 - 2.11 Ga. 

There appears to be a trend towards more depleted 

signatures with time which is consistent with the whole 

rock geochemistry. The Nd-isotope systematics suggest 

derivation from a juvenile source, such as the 

Svecofennian crust, with the addition of a mantle component. In SW Sweden there is no evidence for a significant 

Archaean contribution to the TIB. The 1.59 Ga GB 

plutons yield more depleted e values of 1.6 to 4.2 and toM 

of 1. 71 - 1.89 Ga, indicating a larger mantle component 

than is seen in the TIB granitoids. 

The final episode at 1.56 Ga produced granites with eNd 

of 2.4 to 3.6 and toM 1.74 - 1.82 Ga. These Nd isotope 

signatures are more depleted than those displayed by the 

synchronous rapakivi plutons in Finland which would 

suggest either derivation from a different source or melting 

at different crustal levels. 

Arc accretion processes have significantly influenced the 

whole rock and isotope geochemistry of Proterozoic 

granitoids in the southwest Baltic Shield. The 

Svecofennian terrane, formed by the amalgamation and 

accretion of 2.1 - 1.9 Ga juvenile arcs, was the major 

source component for the TIB plutons. In the southwest 

Scandinavian Domain, a later episode of arc accretion 1. 76 

- 1. 61 Ga provided a more juvenile source for the 

Goteborg Batholith. 

S-type Granites in the Northern Slave 

Province, Northwest Territories, Canada? 

Davis, A.M. and Krogstad, E.J. 

Isotope Geochemistry Laboratory, 

Department of Geology, University of Maryland 

at College Park, MD 20742, USA 

The Archean Slave Province, located in the Northwest 

Territories, Canada, comprises c. 2700 Ma supracrustal 

belts, which include gneiss fragments older than 2900 Ma 

in the western Slave, separated and intruded by c. 2600 

Ma granitoid rocks (lsachsen and Bowring, 1994). Davis 

and Hegner (1992) found eNd(2600 Ma) = -6 to 0 for late 

granites in the western central Slave Province and + 1 to 

+4 for late granites in the east, indicating that long term 

LREE enriched basement is present in the western Slave 

Province and not in the east. In the northern Slave Province, late Archean (c. 2600 Ma) granites of the Kangguyak 

gneiss, Anialik River, High Lake, Torp Lake, and Hood 

River supracrustal belts, have eNd(2600 Ma) = -2 to + 1. 
These data can be interpreted to mean that the granites had 

isotopically homogeneous source materials that were 

somewhat evolved compared to "depleted mantle". A 

petrogenetic model for the granites involves mixing of 

magmas from long term LREE depleted (mantle) and long 

term LREE enriched (continental crust) sources. Alternatively, the late Archean granite magmas may have been 

derived exclusively from supracrustal sources. Two groups 

of granite REE patterns can be explained by a model that 

involves melting of materials chemically similar to 

supracrustal rocks exposed in the Slave Province. Country 

rocks include metavolcanic units with BNi2600 Ma) = -1 

to + 2, and heterogeneous gneisses with BNi2600 Ma) = 

-8 to -7 and t0 M = 3200 Ma. Some granites with CeN/YbN 

= 4 to 8 (e.g., Chin Lake Stock of the Anialik River Belt) 

may have been derived by 10-20% partial melting of 

metavolcanic rocks, whereas other granites (e.g., Tough 

Run and Torp Lake granites) with steeper patterns 

(CeN/YbN > 11) may have been derived by melting of 

metasedimentary rocks. Long term LREE enriched 

continental crust was not necessarily a component of the 

sources of c. 2600 Ma granite. 

Davis, W.J. and Hegner, E., 1992. Neodymium isotopic evidence for the 

tectonic assembly of Late Archean crust in the Slave Province, 

northwest Canada. Contrib. Mineral. Petrol., 111, 493-504. 

Isachsen, C.E. and Bowring, S.A., 1994. Evolution of the Slave craton. 

Geology, 22, 917-920. 

Microgranitoid Enclaves and Associated 

Intrusives: A Comparative Petrographic 

and Geochemical Study 

Dean, A.A. 

Department of Earth Sciences, Monash University, 

Clayton 3168, Australia 

Rame Head and Petrel Point are prominent rocky 

headlands, within Croajingolong National Park, 25 km 

southwest of Mallacoota, East Gippsland, Victoria, 

Australia. The headlands comprise granitic outcrop that 

contains abundant microgranitoid enclaves and closely 

related dyke-like minor intrusive bodies. This plutonic 

complex intrudes folded Late Ordovician age 

metasediments of the Bega basement terrane, and is 

considered a component of the eastern arm of the composite "1-type" Bega batholith (390-400 Ma), within the 

Abstracts 41 

Palaeozoic Lachlan Fold Belt of southeastern Australia. 

Sr-Nd-Pb isotopic systematics indicate a more primitive 

character for some intrusives of the Bega batholith than 

those elsewhere in the Lachlan Fold Belt (e.g. (

87Sr/86Sr)i 

- 0.7041, eNd - +3.0; McCulloch and Chappell, 1982) 

It has been suggested that the eastern arm intrusives, 

close to the supposed margin of the Australian continental 

plate, exhibit isotopic evidence for limited mixing associated with underplating and late stage intrusion into the 

crust of mantle-derived magmas. (McCulloch and 

Woodhead, 1993). 

Investigations of field and microstructural relationships 

have given insights into physical and chemical evolution 

within the Croajingolong plutonic complex. Four granitoid 

phases that exhibit distinct lithological features were 

determined in the field. Petrographic and geochemical 

evidence confirms these initial distinctions. The lack of 

chilled margins and sharp contact relationships indicate 

simultaneous intrusion of several fluid magmas that may 

have interacted. 

The predominant granitic phase is a weakly foliated 

coarse grained biotite granodiorite. The generally fme 

grained ovoid enclaves (usually between 10 mm- 1 min 

diameter) are of biotite microgranodiorite to microdiorite 

composition and have a lower quartz and higher mafic 

content (±hornblende) relative to the host granodiorite. 

There is considerable variation in enclave abundance. 

Where densely packed swarms occur enclaves exceed 70% 

of the total rock volume. In other areas they comprise < 

20%. The enclaves are typically ellipsoidal and preferential alignment of some minerals defines an incipient 

magmatic foliation. Within the microgranitoid enclaves 

there is a gradation from homogeneous fine grained to 

coarser grained with megacrysts of zoned plagioclase. 

Feldspar megacrysts also accumulate around enclaves 

within swarms. Schlieren swirls are common. 

Enclave/host granodiorite contacts are generally sharp 

and featureless, marked by a change in crystal size and 

corresponding increase in mafic content which in some 

enclaves forms a concentrated rim. Textural analysis 

shows primary igneous quench textures. Acicular apatite 

is abundant throughout enclaves. Megacrysts within 

enclaves show clear evidence of textural and chemical 

disequilibrium with their surroundings. Electron microprobe analyses distinguish complex compositional zoning. 

Plagioclase megacrysts have overgrowth rims more calcic 

than most core regions. 

Microgranitoid enclaves and the mafic dike-like 

intrusives have similar petrographic and geochemical 
42 The Origin of Granites and Related Rocks 

characteristics. However, the hornblende present within the 

dyke-like bodies is usually prismatic and megacrysts are 

rare. Dyke-like bodies have convoluted margins and 

disaggregate into pillow-like enclaves indicating a genetic, 

spatial and temporal relationship between the two. 

This study represents the initial stages of a research 

program that will incorporate data from several areas 

within the Lachlan Fold Belt that also contain spatially and 

temporally related mafic and felsic intrusives. Trace 

element abundances and isotope systematics will be used 

to support observations and to indicate likely cogenetic 

relationships. 

McCulloch, M.T. and Chappell, B.W., 1982. Nd isotopic characteristics 

of S-and I-type granites. Earth Planet. Sci. Let., 58, 51-64. 

McCulloch, M.T. and Woodhead, J.D., 1993. Lead isotopic evidence for 

deep crustal-scale fluid transport during granite petrogenesis. 

Geochim. Cosmochim. Acta, 51, 659-674. 

ORAL PRESENTATION 

New Experimental Constraints on the 

Physical Properties of Granitic Melts 

Dingwell, D.B., Hess, K., 

Knoche, R. and Webb, S.L. 

Bayerisches Geoinstitut, Universitiit Bayreuth, 

95440 Bayreuth, Germany 

The determination of physical properties of granitic and 

pegmatitic melts under the conditions of pressure and 

temperature relative to their emplacement in the Earth's 

crust provide valuable physical constraints on the behavior 

of such magmatic systems during igneous petrogenesis. 

The superliquidus properties of melts of relatively low 

viscosity are reasonably well-constrained by experimental 

geochemical investigations where methods borrowed from 

the metallurgical and glass sciences have been applied to 

geologically relevant compositions. The low viscosities 

that are a prerequisite for most of these conventional 

methods preclude the direct determination of the physical 

properties of relatively viscous granitic and pegmatitic 

melts using such conventional means. 

Recently, we have begun to employ methods more 

commonly used for glass-ceramics and extremely viscous 

glassmelts in order to obtain much new data for the 

properties of granitic and pegmatitic melt compositions at 

the low temperatures relevant to their intrusion and 

crystallization. We are in the process of applying these 

methods to hydrothermal pressures. 

The data for the supercooled liquids, at temperatures 

just above the glass transition, are complementary to those 

obtained at superliquidus temperatures on the same 

compositions. Such metastable liquids generate, in 

general, values of expansivity and activation energy of 

viscosity that are considerably higher than the values 

obtained from the superliquidus determinations. Nevertheless, both data sets (supercooled and superliquidus) are 

completely reconcilable with each other through a temperature-dependence of the expansivity and of the activation 

energy of viscosity. These complementary data sets 

provide strong constraints on the interpolation of temperature- dependence property data to the intermediate temperature range relevant to crystallization of granitic and 

pegmatitic melts. With the examples of viscosity and 

density we illustrate the development of multicomponent 

studies of the temperature-dependence of the viscosity of 

melts and the temperature-dependent density of melts in 

the multicomponent system Si02 - Al20 3 - N a20 - K20 -

Rb20 - Cs20- Li20- BaO - SrO - CaO - MgO - Ti02 -

B20 3 - P20 5 - F20_ 1 - Ta20s- Nb205 - W03• 

The systematics of the viscosity-temperature relationships are discussed in terms of the structural contribution 

of each added oxide to the granitic melt base. The density 

data are inverted to partial molar volumes for each 

component. The development of a database including the 

effects of water on these properties using the methods 

outlined above is in progress. 

Generation, Ascent and Emplacement 

of Granitic Magma in the 

Northeastern Gander Zone, 

Newfoundland Appalachians 

D'Lemos, R.S., King, T.R., 

Pembroke, J.W. and Tribe, I.R. 

Geology and Cartography Division, 

Oxford Brookes University, Oxford, OX3 OBP, UK 

Ordovician to Devonian deformation, migmatization, 

and granite emplacement within the Gander· Zone of 

northeastern Newfoundland resulted from the closure of 

Iapetus and transcurrent docking of A val on (Gondwana) 
with North America. From west to east, Gander Zone 

rocks pass transitionally from low greenschist facies 

Gander Group metasediment to upper amphibolite facies 

migmatite, coincident with an increase in structural 

complexity and intensity of deformation and volume of 

granite intruded. The migmatitic region is characterized 

by the c. 425 Ma Hare Bay Gneiss and occupies a c. 20 

km wide zone of syn-migmatization sinistral shear adjacent 

to the A val on - Gander boundary. Sillimanite is the 

dominant aluminosilicate within the migmatites, although 

andalusite rimmed by muscovite and sillimanite occurs 

rarely. Mineral reactions, amphibole - plagioclase 

geothermometry and XM8 cordierite, indicate high-temperature and low-pressure anatexis (c. 640-740°C, 4-5 kbar) 

requiring high geothermal gradients. 

Sm-Nd isotopic characteristics of metasedimentary 

rocks, gneisses and intrusive granitoids place some 

constraints upon their origins. A marked similarity in eNd 

( -8 - -6) for low grade Gander Group metasediments and 

sillimanite paragneisses of the Hare Bay Gneiss are most 

consistent with derivation from the same protolith. 

Despite some open system behavior these isotopic signatures can be traced into zones of local (contact) and 

regional migmatization and partial melting fingerprinting 

the possible contribution to granite magmatism. Voluminous Silurian and Devonian granites (eNd = -5 - 0) were 

not derived from such a source, nor were they derived 

exclusively from the mantle. The most likely source was 

lower crust and mantle, variably contaminated by mid- to 

upper crust, the range of eNd resulting from systematic 

changes in relative contributions through time. 

Silurian (c. 425-415 Ma) plutons are characterized by 

foliated K-feldspar megacrystic granite. Fabric elements 

within plutons parallel those developed in country rocks 

and show a broad continuum of down-temperature magmatic through solid-state microstructures resulting from 

emplacement synchronous with sinistral transpression. 

The plutons from elongate sheets often several km in width 

which parallel the regional structural grain. Contactgenerated migmatites are common along margins demonstrating considerable convective heat. Ambient regional 

metamorphic conditions and pluton microstructures indicate 

emplacement at mid-crustal levels. These plutons record 

the ascent routes for large volumes of granitic magma 

transported through the crust during the Silurian. 

Devonian (c. 385 Ma) plutons are characterized by 

extensive bodies of K-feldspar megacrystic granite markedly discordant to the regional tectonic grain and metamorphic isograds. Contacts are typically sharp and stoped 

Abstracts 43 

blocks of country rock occur near margins. These features 

demonstrate emplacement at upper crustal levels and after 

the main ductile (Silurian) deformation consistent with 

rapid regional exhumation. The plutons exhibit internal, 

often cryptic, contacts and heterogeneities indicative of 

assembly by batch filling, although differences between 

magma batches were small. Evidence for mixing between 

batches includes numerous grain-scale disequilibrium 

textures, outcrop heterogeneity and subtle pluton-wide 

lithological variation. Weak to moderate fabrics formed 

prior to extensive crystallization, and record flow during 

emplacement of magma batches. Pre-existing country rock 

fabrics on the northern side of the N -S trending Newport 

Granite are rotated anti-clockwise of the regional trend 

indicating emplacement into a tear structure initiated by 

dextral movements along a major, NE-SW trending 

crustal-deep transcurrent structure, the Dover Fault, 

located at the Avalon - Gander boundary. Passive emplacement during regionally important brittle dextral 

movements is further supported by local vein arrays at 

pluton contacts and by the orientation of internal contacts. 

The Newport Granite is itself displaced by, and cataclased 

within the Dover Fault system indicating that fault movements continued after emplacement. 

Our ongoing studies in the Gander Zone indicate that 

voluminous Silurian to Devonian megacrystic granites were 

generated during transpressional thickening followed by 

rapid exhumation of the region, possibly in response to 

delamination, leading to adiabatic decompressive melting 

of unexposed (amphibolitic?) lower crust. High temperature - low pressure migmatization recorded in the region 

resulted from a combination of ambient regional metamorphic conditions due to initial crustal thickening, coupled 

with advected heat from voluminous, syntectonic granites 

generated at deeper levels. The granites ascended through 

the mid-crust with relatively narrow zones (kms) of 

transcurrent shear to be emplaced passively as broadened, 

fault controlled bodies in the upper crust. Throughout the 

region there is clear evidence for a tectonic control on the 

location of granite plutonism. In tum, magmas may have 

controlled the local focusing of strain, thus creating a 

positive feedback loop. 
44 The Origin of Granites and Related Rocks 

ORAL PRESENTATION 

The Petrogenesis of Slab-Derived 

Trondhjemite-Tonalite-Dacite/ Adakite 

Magmas 

1

Drummond, MarkS., 2Defant, Marc J. 

and 2Kepezhinskas, Pavel K. 1 Geology Department, University of Alabama 

at Birmingham, Birmingham, AL 35294, USA 2

Geology Department, University of South Florida, 

Tampa, FL 33620, USA 

The focus of II granite II petrology has leaned towards the 

study of granite and rhyolites and their relevance to the 

generation and growth of continental crust through time. 

This pursuit has allowed us to find that a significant 

proportion of granites (senso stricto) represent recycling of 

continental crustal components and contribute sparingly to 

new net addition for continental crustal growth. In order 

to understand the birth of continental crust and its subsequent growth we should refocus our studies towards the 

sodic end of the "granite II spectrum, the trondhjemitetonalite-dacite suite (TTD). Most continental growth 

models indicate that over three quarters of the present -day 

continental crust was produced in the Archean with the 

majority of this Archean crust represented by TTD. 

(#) 

y 

Yb 

Sr 

Cs 

u 

Ni 

Mg# 

La/Yb 

Sr/Y 

Zr/Y 

Adakite high-AI TID CARC IARC low-A1 TID Plagiogranite 

(131) 

10 

0.93 

858 

1.2 

0.9 

41 

48 

18 

107 

14 

(386) 

9 

0.64 

659 

2.0 

0.9 

26 

43 

35 

103 

24 

(825) (476) 

27 28 

2.42 4.26 

428 229 

9.2 0.8 

3.6 0.7 

19 9 

41 34 

12 

19 

7 

9 

5 

(124) 

45 

4.03 

133 

0.8 

0.9 

18 

33 

4 

(53) 

83 

7.19 

150 

0.1 

1.4 

6 

31 

2 

TTD may be subdivided into high-Al TTD and low-Al 

TTD (> 15% and < 15% Al20 3 at 70% Si02 level), and 

ophiolitic plagiogranites. Defant and Drummond (1990) 

referred to some specific Cenozoic high-Al TTD as 

adakites. The table above indicates that the high-Al 

TTD/adakite contain lowerY, Yb and higher Sr, Sr/Y, 

Zr/Y, La/Yb relative to the other TTD subtypes. This 

compositional signature for the high-Al TTD/adakite 

results from stabilizing garnet (grt), hornblende (hbl), and 

clinopyroxene (cpx) at the site of partial melting and 

general lack of plagioclase (pl) in a hbl eclogite to eclogite 

restite In comparison to high-Al TTD/adakite, andesitedacite-rhyolite suites from continental (CARC) and island 

(IARC) arcs contains higher Y, Yb and lower Sr, Sr/Y, 

Zr/Y, La/Yb due to the lack of grt and presence of pl 

during magma genesis. 

Experimental studies have shown that high-Al 

TTD/adakite major element compositions can be produced 

by high-pressure(> 15- 16 kbar, > 50 km depth) partial 

melting of basalt leaving grt + cpx + hbl restite. We 

have suggested that the partial melting of subducted 

oceanic crust at 23- 26 kbar (75 - 85 km depth) and 700-

8000C (9°C km-1 geotherm) may defme the minimal 

physical conditions for slab melting. This P-T regime 

coincides with a number of potential dehydration reactions 

in the slab, such as clinohumite, paragonite, zoisite, 

serpentine, tremolite, talc+phengite. Recently, Thompson 

and Ellis (1994) have shown that at 800°C and 26 kbar 

partial dehydration melting of a basaltic source could 

follow, zoisite + hbl + quartz = cpx + pyrope + highA! TTD melt. 

Martin (1993) has suggested that the widespread 

Archean high-Al TTD is the product of slab dehydration 

melting under hbl eclogite conditions associated with a 

elevated geothermal gradient. Thus, a potentially higher 

slab geotherm may have been responsible for the voluminous high-Al TTD produced in the Archean. Cenozoic 

adakite localities are commonly associated with subduction 

of young ( < 25 Ma), hot oceanic crust. Approximately 

20% of the modem ocean floor is comprised of Miocene 

or younger crust, implying that adakite generation may not 

be as rare as originally thought. Viable alternative or 

supporting tectonic effects that may enhance slab melting 

include highly oblique convergence and resultant slow 

subduction and incipient subduction into pristine hot 

mantle. An alternative hypothesis to slab melting for highAl TTD/adakite genesis would involve partial melting 

over-thickened continental arc crust. The CARC compositions exhibit elevated Cs and U values relative to high-Al 

TTD/adakite due to incorporation of or derivation from 

continental crust. Alternatively, the high-Al TTD and 

Cenozoic adakite, in particular, exhibit elevated Ni and 

Mg# relative to CARC and IARC due to minor (5-10%) 

mantle - adakite mixing. Localities that best exemplify 

mantle - adakite mixing include: Setouchi belt, Japan 

(sanukitoids); Baja California (bajaites); Adak Island, 

Aleutians; Komandorsky Island, western Aleutians; and 
northern Kamchatka. Given our recent detailed studies in 

northern Kamchatka, we will use this locality as a case 

example for slab melting and adakite -mantle mixing. 

Defant, M.J. and Drummond, M.S., 1990. Derivation of some modem 

arc magmas by melting of young subducted lithosphere. Nature, 347. 

662-665. 

Martin, H., 1993. The mechanics of petrogenesis of the Archean 

continental crust-comparison with modem processes. Lithos, 30, 

373-388. 

Thompson, A.B. and Ellis, D.J., 1994. CaO + MgO + Al20 3 + Si02 

+ H20 to 35 kb: amphibole, talc and zoisite dehydration and melting 

reactions in the silica-excess part of the system and their possible 

significance in subduction zones, amphibolite melting, and magma 

fractionation. Am. J. Sci., 294, 1229-1289. 

Tismana (Neoproterozoic) Granitoid 

Pluton from the Danubian Basement 

(South Carpathians - Romania): 

A Geochemical Approach 

1

Duchesne, J.C., 2Berza, T., 

3

Liegeois, J.P. and 2Tatu, M. 1 University of Liege, Belgium 2

Geological Institute of Romania, Bucharest, Romania 3

Royal Museum of Central Africa, Tervuren, Belgium 

The Tismana Massif holds a special position between 

the granitoid bodies associated with the crystalline schists 

from the basement of South Carpathian's lower Alpine 

units - the Danubian Nappes. So, from the 15 major 

plutons known here, only Tismana and a smaller twin 

display geochemical, mineralogical, petrographical and 

structural patterns pointing to a different origin than the 

crustal anatexis generally advocated for this igneous 

province. The Tismana Massif is exposed on 300 km2 at 

the southern border of the western South Carpathians, but 

below Mesozoic and Cenozoic covers its area must be at 

least double. The body is emplaced in a high temperature -

low pressure metamorphosed sedimentary sequence. Both 

at small and medium scale, the contacts of the body are 

concordant with the structural elements of the surrounding 

crystalline schists. No apophyses or proper migmatites do 

exist and only a few roof pendants were mapped. The age 

of the Tismana massif is, from geological relations, 

certainly pre-Silurian. Twenty K-Ar ages range between 90 

and 540 Ma, reflecting Alpine overprinting related to 

nappe tectonics. An Ar-Ar plateau age of c. 593 Ma (R.D. 

Abstracts 45 

Dallmeyer, personal communication, 1994) and a U-Pb 

age of 567 ± 3 Ma (Liegeois et al., 1995) were interpreted as dating the intrusion of the pluton. 

The Tismana granitoid body is made up of porphyritic 

granites, which cover around 80% of the exposed area, 

and equigranular darker granitoids, grading from 

granodiorites to diorite-gabbros; hectometric pods of 

peridotite are also present. The areal distribution of the 

equigranular granitoids is asymmetric in the uncovered 

area: they are dominant near the western border, common 

in the central area and are missing in the southern part. 

There is also a southwards directed trend in the reduction 

of both the size of the lenses (from kilometric to 

centimetric) and of the grain size of the rocks (from 1 - 4 

mm to 0.3- 0.5 mm). The smallest bodies of equigranular 

granitoids can be described as mafic microgranular 

enclaves and are generally disposed in swarms. The MME 

have either rounded contours, when they are enclosed in 

hybrids, or planar borders, when they are hosted by 

porphyritic granites. The position of both the borders of 

the enclaves and of the foliations expressed by biotite 

flakes in the MME and by K-feldspar megacrysts in 

granites are parallel, producing a few symmetrical folds. 

The MME evidence the process of magma mingling and 

can be trapped either by hybrid rocks, or by porphyritic 

granite. The presence of porphyritic granodiorite, containing both centimetrically spaced K-feldspar megacrysts, 

typical for the granites, and of amphibole ± clinopyroxene, minerals characteristic for the equigranular 

granitoids, was frequently observed as metrical transition 

zones between the two main lithologies. Even in the 

"clean" porphyritic granite, there are hectometrical zones 

where amphibole is added to biotite and plagioclase is 

richer in anorthite. These aspects point to the existence of 

large scale mixing (hybridization) in the Tismana pluton. 

Tismana displays an alkali-calcic trend in Peacock diagram, with the intersection near the alkaline field. Potash 

content is increasing from 2% to 4% in the 48 - 52% Si02 

range and from 4% to 6% in the 52 to 70% Si02 range. 

The slight increase of K20 from 52% Si02 upwards 

reflects the higher increase of K-feldspar in respect with 

concomitant decrease of biotite. The Tismana trend lies 

entirely within the shoshonitic or alkaline field. In the (Zr 

+ Nb + Ce + Y) vs. (K20 + (Na20/Ca0)) diagram, 

Tismana has A-type affinities, shown only by trace 

elements due to relatively low N a20 abundances (2 - 3% 

for the whole silica range). This points to a clear potassic 

character (K20/Na20 ratio up to 2) for the massif. The 

geochemical data show that the genesis of this massif has 
46 The Origin of Granites and Related Rocks 

been greatly controlled by fractional crystallization and 

mixing processes leading to a composite series (following 

Rossi and Cocherie, 1991). 

Liegeois, J.P., Berza, T., Tatu, M. and Duchesne, J.C., 1995. The 

Neoproterozoic Pan-African basement from the Alpine Lower 

Danubian nappe system (South Carpathians, Romania). Precamb. 

Res., in press. 

Neubauer, F., Mocanu, V. and Fritz, H., 1994. 40ArP9

Ar mineral ages 

for the pre-Alpine and Alpine evolution of nappe complexes in the 

southern Carpathians. Romanian J. Teet. Region. Geol., 75, 77-86. 

Rossi, P. and Cocherie, A., 1991. Genesis of a Variscan batholith. Field, 

petrological and mineralogical evidence from the Corsica-Sardinia 

batholith. In Freeman, R., Huch, M. And Mueller, S. (eds), The 

European Geotraverse, 319-346. 

Late Variscan Amphibole-Bearing 

Granitoids in Lusatia 

(Northern Part of Bohemian Massif) 

as Melting Products of Metasomatically 

Overprinted Mafic Lower Crustal Rocks 

1

Eidam, J. and 1

•

2

Hammer, J. 1 FR Geowissenschaften, Universitiit Greifswald, 

Jahn-Str. 17a, 17489 Greifswald, Germany 2

Geochemisches lnstitut, Universitiit Gottingen, 

Goldschmidt-Str. 1, 37077 GOttingen, Germany 

The Lusatian Anticlinal Zone (LAZ; Southeast Germany; Eastern Saxothuringicum) is composed of Cadomian 

and Late Variscan granitoids and anchimetamorphic 

Brioverian greywacke-pelite successions (Kroner et al., 

1994). Variscan granitoid magmatism played rather a 

subordinate role in LAZ when compared with the adjacent 

Saxothuringian regions (Erzgebirge). The LAZ represents 

a tectonic block, which was already stabilized in the 

Cadomian era. Younger magmatic activity (early Variscan 

and Mesozoic mafic dykes, late Variscan granitoids) are 

related to mobile fault zones bounding the Cadomian 

Granodiorite Complex. 

The late Variscan granitoids were dated by single 

zircon evaporation method (304 ± 14 Ma; Kroner et al., 

1994). They range in modal composition from quartz 

diorites through tonalites and granodiorites to 

monzogranites. Occurrences of weakly postmagmatically 

altered (o180Q-Fsp = 0.6 - 1.0 o/oo) granitoids with Si02 

content down to 60.2 wt % are atypical for the eastern 

part of Saxothuringian Zone. The metaluminous rocks with 

increasing molar Al20/(Ca0 + Na20 + K20) ratios at 

increasing Si02 content are petrographically characterized 

by occurrences of magnesia-hornblende (up to 8 vol %) 

and plagioclase cores with up to 56 mol % anorthite. 

Relatively high Ca and Sr contents in Lusatian rocks 

correspond to values of 1-type granites. High (La/Yb)N 

ratios from 5 to 20 and negative Eu-anomalies in 

chondrite-normalized REE patterns indicate restitic garnet 

and plagioclase in their source rocks. 

In comparison to average granodiorite the Lusatian 

granodiorites (with MgO content between 1.6 and 2.3 wt 

%) and monzogranites (1.5 up to 2.3 wt % MgO) are 

characterized by high concentrations of Ba, Ce, Hf, K, Li, 

Nb, Pb, Rb, REE, Sr, Ta, Th, U and Zr. Enrichments of 

these elements are caused either by high contents in source 

rocks or by specific conditions of partial melting. Most 

probably are metasomatic overprints of upper mantle rocks 

above a subduction zone and element mobilization from 

upper mantle wedge into lower crustal mafic source rocks 

by dehydrated fluids from the subducted oceanic crust. 

According to results of partial melting experiments of 

mafic lower crustal rocks (e.g. Springer, 1992; Wolf and 

Wyllie, 1991, 1994) and to calculation of source rock 

composition using the batch melting model (Shaw, 1970) 

the Lusatian granitoids represent melting products of 

metasomatically overprinted mafic lower crustal rocks at 

T = 900-950°C, aHa< 0.5, P = 10-15 kbar and various 2 

degrees of partial melting (15-20 % for monzogranites; 20 

% for granodiorites and 30 % for quartz diorites). This is 

documented also by relatively low o180 whole rock values 

(quartz diorites, 7 .1-7. 3 %o; granodiorites, 7. 6-7.8 %o; 

monzogranites, 8.1-10.3 %o). The homogeneous Sr and 

Nd isotopic compositions ((87Sr/86Sr)i = 0.7055 - 0.7063; 

eNd(304 Ma) of -3.1 - -3.3 for quartz diorites; -2.3 - -2.5 

for granodiorites and -1.6 - -4.4 for monzogranites) 

correspond to mafic source rocks and homogenized effect 

of metasomatic fluids. 

Geophysical data, like gravimetric and magnetic 

anomalies, suggest a larger distribution of this granitoid 

type in marginal zones of the Cadomian Lusatian 

Granodiorite Complex. 

Kroner, A., Hegner, E., Hammer, J., Haase, G., Bielicki, K.-H., 

Krauss, M. and Eidam, J., 1994. Geochronology and Nd-Sr 

systematics of Lusatian granitoids: significance for the evolution of 

the Variscan orogen in east-central Europe. Geol. Rundsch., 83, 357-

376. 

Shaw, D.M., 1970. Trace element fractionation during anatexis. 

Geochim. Cosmochim. Acta, 34, 237-243. 
Springer, W., 1992. Entstehung granitoider Magmen durch partille 

Aufschmelzung bassischer Unterkruste: Eine experimentelle Studie. 

Unpubl. Diss., Universitiit Koln, 116 pp. 

Wolf, M.B. and Wyllie, P.J., 1991. Dehydration melting of solid 

amphibolite at 10 kbar: textural development, liquid interconnectivity 

and applications to the segregation of magmas. Mineral. Petrol., 44, 

151-179. 

Wolf, M.B. and Wyllie, P.J., 1994. Dehydration melting of amphibolite 

at 10 kbar: the effects of temperature and time. Contrib. Mineral. 

Petrol., 115, 369-383. 

Zircon Inheritance in an S-type Granite 

and its Microgranitoid Enclaves, 

Lachlan Fold Belt, Australia 

Elburg, Marlina A. 

Department of Earth Sciences, Monash University, 

Clayton 3168, Australia 

Microgranitoid enclaves (ME) in granites have been 

alternatively interpreted as restite, chilled margins of the 

granite itself, and chilled globules of a more mafic 

magma. For ME in the S-type Wilson's Promontory 

Batholith (WPB), Lachlan Fold Belt, Australia, the chilled 

margin interpretation is least tenable because of significant 

differences in initial isotopic composition between ME and 

host granite. Field and petrographic evidence points 

towards a magma mingling origin for the ME. To further 

assess this interpretation, zircons from the host granites 

and enclaves were dated by the SHRIMP ion microprobe. 

TI1e host was expected to be rich in inherited zircons, in 

keeping with its S-type geochemical signature. Restitic 

enclaves (which should be dominantly high-grade 

metasediments) might be expected to contain a larger 

proportion of inherited zircons than the host. However, if 

enclaves represent chilled globules of a mafic magma, 

inheritance might be expected to be less prominent, 

although mixing with the host magma might have introduced older zircons. 

Zirconium contents of the granites decrease with 

decreasing whole rock FeOT content. Zr concentrations in 

ME vary from 250 to 730 ppm irrespective of whole rock 

FeOT content. Zircons from the granitic samples are 

generally euhedral and large (up to 200 microns). Inheritance is not as prominent as expected, despite attempts to 

specifically target "cored" crystals. Only 8 out of 69 

analyses gave ages older than the granite crystallization 

age of 395 Ma. Zircons in ME are generally small and 

Abstracts 4 7 

anhedral. In thin section many zircons can be seen to have 

needle-like shapes, but these may have been broken during 

separation of the crystals. These small crystals typically 

yield ages around 395 Ma, although their small size makes 

reliable SHRIMP dating difficult. 

Two enclaves (bulk Zr contents 570 and 280 ppm 

respectively) with quartz and plagioclase megacrysts 

contain few large zircons, of which a significant proportion 

contains inherited cores. Some of these have thin 

overgrowths of magmatic zircon; others have not developed overgrowths. A megacryst-poor enclave with high 

Zr contents (730 ppm) contains very few euhedral zircons; 

all other zircons are small and anhedral. No dates indicating inheritance were found. 

Dates given by inherited zircons of granites and ME are 

similar. A "young" cluster gives ages of 500-650 Ma; 

three analyses give 1100-1200 Ma ages, two 1500-1600 

Ma. An age population of 750-830 Ma (3 analyses) is 

present only in the granite, whereas an enclave contains a 

2700 Ma old core. 

The small grain size, anhedral shape and apparent 

magmatic crystallization ages of most zircons in the ME is 

in accordance with these enclaves representing globules of 

a chilled mafic magma. The larger zircons which show 

inheritance are thought to have been derived from the 

granitic magma, like the quartz and plagioclase 

megacrysts. The higher proportion of inherited cores in 

the large zircons from megacryst-bearing ME, compared 

to those in the host granites, may reflect the timing of 

mixing between granitic and enclave magma. This may 

have taken place when relatively few zircons had 

crystallised from the granitic magma. All inherited zircons 

were already present in the granite magma, and their 

relative over-representation at the time of magma mixing 

has been translated into the "large" zircon population of 

the ME. 

Small zircons which are typical of the ME tend to be 

lost during separation, mounting and polishing prior to 

SHRIMP analysis, and are often too small to be analysed 

by SHRIMP. This, together with the introduction of 

inherited zircons from the granitic magma during mixing, 

can easily lead to a serious sampling bias and an overestimation of inherited (restitic) crystals in enclaves. 

The age groups for zircons analysed in this study are 

similar to those recognised by Williams et al. (1990) for 

I and S-type granites in the NE part of the Lachlan Fold 

Belt. Compared to these S-type granites the WPB is 

relatively poor in inherited zircons. It also has less 

negative eNd values compared to other Lachlan Fold Belt 
48 The Origin of Granites and Related Rocks 

S-type granites (-4 versus < -6). This could suggest a 

larger role for juvenile mantle derived melts in the 

petrogenesis of the WPB. The microgranitoid enclaves 

may represent heavily contaminated globules of this mafic 

magma. 

Williams, I.S., Compston, W., Chappell, B.W., Crook, K.A.W. and 

Chen, Y.D., 1990. Untangling the sources of granites by dating 

inherited zircons. Geol. Soc. Aust. Abstr., 27, 112. 

Could Some Regional Metamorphism 

be Induced by Flowing of Magma? 

The Case of Hercynian Metamorphic 

Domes and Plutonic Intrusions of 

the Eastern Pyrenees 

Enrique, P. 

Departament de Geoquimica, Petrologia i 

Prospecis Geologica, Facultat de Geologia, 

Universitat de Barcelona, Zona Universitaria de 

Pedralbes, 08028-Barcelona, Spain 

The Hercynian regional metamorphism in the Pyrenees 

is characterized by tightly concentric isograds around 

metamorphic domes. The intensity of metamorphism 

increases rapidly from low-grade (chlorite-muscovite) 

zones to high-grade (sillimanite and anatexis) zones, giving 

rise to typical low-P and high-T metamorphism. This 

metamorphism postdates the main orogenic deformation 

but is synchronous to a second group of less intense 

tectonic phases. Stephanian sediments lie unconformable 

over folded Namurian deposits, which narrowly constrains 

the timing of deformation, metamorphism and plutonic 

emplacement. 

In addition to the old dome cores (Lower Palaeozoic 

and Neoproterozoic schists and orthogneiss) and the 

younger Silurian to Wesphalian synforms located between 

them, about 20% of the area consists of large calc-alkaline 

plutonic intrusions (up to 600 km2

) which produce a 

contact metamorphic aureole in the Palaeozoic host rocks. 

Intrusions clearly cut the main Hercynian folds and 

foliations but they are not usually significantly deformed. 

The abundant apophyses and xenoliths of the enclosing 

rocks near the contacts suggest a moderate or high fraction 

of melt in the magmas during their emplacement. 

A volcanic calc-alkaline suite, ranging from basaltic 

andesites to rhyolites, is interbedded in the post-orogenic 

Stephano-Permian sequence. Thick layers of basaltic 

magma underlying each metamorphic dome, in an 

extensional context, have been postulated by Wickham and 

Oxburgh ( 1987) to account the high thermal gradients 

observed. Nevertheless there are some features that 

should be taken into consideration: a) existence of small 

stocks and dykes of calc-alkaline plutonic rocks associated 

with migmatites inside the thermal domes (e.g. Cap de 

Creus), these rocks range in composition from 

quartz-hornblende gabbros and diorites to leucogranites, 

and they display several degrees of mixing with anatectic 

melts; b) the stocks are slightly older than the 

perianatectic pegmatites; c) a subvertical foliation is 

present in these plutonic rocks, and to a lesser extent in 

the pegmatites, which was produced by the late-Hercynian 

compressive phases; d) the overlap in time of the 

calc-alkaline magmatism and low-P - high-T metamorphism. 

Therefore, it would appear probable that the source of 

heat for this metamorphism was the huge volume of 

calc-alkaline magmas that fed the upper crustal batholiths 

and, perhaps, volcanism. The small, compositionally 

expanded, calc-alkaline stocks might represent the last 

fractions of magma trapped in the conduits on their way to 

the upper levels of the crust. These magmas may have 

crystallized only when magmatic activity, and, consequently, temperature, decreased. Therefore, the intensity 

of metamorphism cannot be explained by present -day relict 

volumes of magma in high grade metamorphic zones since 

almost all of it would have been squeezed out by the late 

compressive phases. 

If this hypothesis is correct the isograds must have a 

subvertical arrangement (following more or less cylindric 

or conic surfaces) and the extent of the metamorphism 

would be limited around the magmatic feeders. On the 

other hand, the whole metamorphic area would have a 

lower density than the cold surrounding rocks, which 

might have originated the domes by diapiric ascent. 

Finally, the compositional zoning and temporal sequence, 

from dioritic to leucogranitic, observed in many plutons of 

the area, might have been originated by progressive 

contamination of the deep andesitic magmas by pegmatitic 

and granitic anatectic melts during their ascent through the 

narrow conduits. 

Wickham, S.M. and Oxburgh, E.R., 1987. Low-pressure regional 

metamorphism in the Pyrenees and its implications for the thermal 

evolution of rifted continental crust. Trans. Roy. Soc. London, 

A231, 219-242. 
Amphibole-Rich Clots in Calc-Alkalic 

Granitoids and in Their Mafic 

Microgranular Enclaves from 

Two Foldbelts, NE Brazil 

1

Ferreira, V.P., 1Sial, A.N., 

2

Fallick, A.E. and 3Cruz, M.J.M. 1

Nucleus for Granite Studies (NEG), Dept. Geol. UFPE, 

P. 0. Box 7852, Recife, 50732-970, Brazil 2 Scottish Universities Research and Reactor Centre, 

East Kilbride, Glasgow, G75 OQU, Scotland 3

Department of Geology, UFBA, Salvador, 

Bahia, 40170-290, Brazil 

Metaluminous granodiorite/tonalite plutons (Concei~aotype) intruded low-grade metaturbidites of the 

Cachoeirinha-Salgueiro Foldbelt (CSF) in the central states 

of Panu'ba and Pernambuco and metasediments of the 

Macurure Group (Coronel Joao Sa-type} in the Sergipano 

Foldbelt (MSF, states of Sergipe and Bahia) further south, 

around 620 Ma ago. Amphibole-rich polycrystalline clots 

(ARC), subrounded to angular, from millimetric size up to 

20 em long, are relatively common in these plutons. They 

are found within microgranular diorite/tonalite enclaves or 

in their granodiorite hosts. ARC from five plutons in the 

CSF and from two in the MSF have been studied in detail. 

Clots hosted by the granodiorites are usually angular, 

have granoblastic texture, being composed essentially of 

zoned amphibole, which has patchy actinolite cores and 

magnesian hornblende margins, in polygonal packing 

where grains display near-120° triple junctions (type I 

ARC). Other phases present are interstitial biotite, Caclinopyroxene, sphene and apatite. Clots are usually 

armored by an external layer of hornblende and biotite, 

crystallized from the host magma. Type II ARC (amphibole cumulates) and type III ARC (pseudomorphs after 

clinopyroxene-rich restites or early-crystallized 

clinopyroxene aggregates) are less common and were 

observed only in Concei~ao-type granitoids. 

In a (N a + Ca + K) vs Si diagram, compositions for 

most amphibole cores in type I ARC in the CSF (e.g. Boa 

Ventura pluton) plot in the metamorphic field, suggesting 

ARC are of metamorphic origin. Compositions of amphibole margins from either the clots or from the 

granodiorite/tonalite hosts plot in the field common to 

metamorphic and igneous amphiboles. 

Although these plutons are oxidized I-type granitoids, 

those from the Concei~ao-type ones display quartz-corAbstracts 49 

rected whole-rock o180 values typical for S-type granites 

( + 11 to + 13 %o sMow). Amphibole-rich clots are usually 

1. 5 %o lower than their hosts ( + 10 to + 11.5 %o sMow). 

Values of (

87Sr/86)i for plutons and their microgranular 

mafic enclaves is 0.70598 in the CSF. eNd(0.6 Ga) varies 

from -2.0--1.0 (Van Schmus et al., in press) and o34S 

from +1.0 to +9.0%o CDT for the ARC host plutons. 

Whole-rock D/H values for granodiorite hosts, mafic 

microgranular enclaves and ARC vary from -69 to -114 %o 

sMow (with no correlation with H20}, covering a significant 

proportion of the oD variation for most terrestrial rocks. 

The oD variation is much broader than that for the o180. 

Basalts probably filled a major rift and floored the 

furrow (at 1.0-1.4 Ga, as indicated by Nd toM model age 

in ARC granodiorite hosts; Van Schmus et al., in press), 

that later accommodated CSF marine turbidites. Partial 

fusion of this basaltic floor, hydrothermally altered at low 

temperature, generated the calc-alkalic ARC-host magmas. 

Type I ARC represent fragments or restites of such a 

fusion. Temperatures obtained for this type in the CSF 

( 670-690°C) are lower than those for host granodiorites 

(700-850°C), as expected if they are fragments from the 

source. 

If the source for the CSF magmas is a low temperaturealtered oceanic crust, as supported by oxygen isotopes, one 

would expect oD values above -70 %o, not below as 

attested. In one pluton, however, a very homogenous 

behavior of o1

SO (average + 11.8%o ± 0.2, for 5 analyses) 

show a broad variation of oD values. Such observation 

attests that H and 0 isotopes probably had decoupled 

behaviors. Actinolite pseudomorphs after clinopyroxene in 

some ARC resulted from reaction between clinopyroxene 

and hydrogen, which requires extraneous hydrogen to be 

incorporated into the magma, perhaps causing a wide oD 

isotopic range. 

Van Schmus, W.R., Brito Neves, B.B. de, Hackspacher, P. and 

Babinski, M., 1995. U-Pb and Sm-Nd geochronologic studies of 

eastern Borborema province, northeastern Brazil: initial conclusions. 

J. South Amer. Earth Sci. , in press. 
50 The Origin of Granites and Related Rocks 

Hercynian Evolution from Calc-Alkaline to 

Peraluminous A-Type Leucogranites, 

Catalonian Coastal Ranges, NE Spain 

1

Ferres, M., 2Enrique, P., 

1

Delaloye, M. and 2Sole, J. 1

Department de Mineralogie, 13 Rue des Maraichers, 

1211 Geneve-4, Switzerland 2

Facultat de Geologia, Zona Universitaria de Pedralbes, 

08028-Barcelona, Spain 

A fragment of the Hercynian Orogen is exposed over 

2000 km2 in the Catalonian Coastal Ranges. Magmatism 

evolved from a predominantly high-K calc-alkaline association to minor peraluminous A-type granites. Three types 

of high-silica (74 - 77 wt % Si02) biotite-leucogranites 

can be distinguished: 1) late plutonic calc-alkaline differentiates; 2) peraluminous A-type leucogranites post-dating 

the calc-alkaline magmatism; and, 3) an intermediate 

between types 1 and 2. The latter may represent a transition between calc-alkaline and peraluminous A-type 

granitoids. 

High-K calc-alkaline association. Coarse to fine 

grained plutons of different shapes and sizes form a 

composite batholith of 1500 km2

• Granodiorite is the most 

common rock type followed by granites and tonalites. The 

evolving plutonic sequence starts with more basic rocks 

represented by a few diorites, gabbros and hornblendites 

and ends with leucogranites of type 1. This magmatic 

evolution is also expressed in systematic changes in the 

biotite composition trending from phlogopite toward 

siderophyllite, with F content varying from 5 to 10% OH 

substitution. In addition to ubiquitous biotite, hornblende 

occurs in some granodiorites or less differentiated rocks. 

Major and trace element patterns of whole-rock analyses 

show linear trends for diorites to leucogranites indicating 

that they may be related by relatively simple differentiation 

processes. 

K-Ar, 40ArP9Ar and Rb-Sr data give cooling ages from 

about 300 to 270 Ma for the whole suite. Values for 

(

87Sr/86Sr)i range from 0.711 for acidic and intermediate 

rocks to 0. 709 for gabbros. These values suggest an 

important crustal component. A NE-SW calc-alkaline 

dioritic to granitic dyke-swarm intruded the calc-alkaline 

plutonic sequence. The dykes show well-developed chilled 

margins indicating already cooled host-rocks. 

Peraluminous A-type leucogranites. Slightly peraluminous A -type leucogranites crop out in the batholith 

over an area of about 70 km2 • These consist of major 

coarse to fine grained intrusions, and several small, very 

fme grained stocks. The latter post -date the calc-alkaline 

dyke swarm and contain miarolitic cavities plus 

granophyric intergrowths indicating a shallow level of 

emplacement. All these leucogranites are cut by associated 

granophyric and felsitic dykes with spherulitic textures 

indicating a devitrification of rhyolitic glass matrices. 

Biotite (the only mafic mineral) has a composition close to 

annite, with high F contents (up to 25% OH substitution). 

Plagioclase of albite to oligoclase composition and perthite 

are present in similar amounts (about 30%) of the modal 

composition. Whole-rock analyses show low CaO content 

( < 0.5 wt % ), a depletion in Ba, and high Ga concentrations peraluminous A-type leucogranites plot as WPG in 

the Y-Nb diagram after Pearce et al. (1984). Their REE 

concentrations are low with a strong negative Eu anomaly. 

Intermediate biotite-leucogranites. These consist of 

several medium to fine grained intrusions with miarolitic 

cavities. 

They occur widespread in the batholith covering areas 

of about several meters to 10 km2• They are slightly 

peraluminous. Biotite composition is intermediate between 

siderophyllite and annite, with F contents up to 15% OH 

substitution. Whole-rock major and trace element analyses 

show features of both leucogranite types 1 and 2. Namely, 

high CaO contents ( > 0. 5 wt %) and low Ga concentrations (14 - 17 ppm) but Ba depletion, low REE concentrations with strong negative Eu anomaly, and they plot as 

WPG. Timing of their emplacement is unclear. 

According to the chemistry, leucogranites of type 3 

may represent a transition from calc-alkaline 2 to 

peraluminous A-type 1 granites. Work underway is 

focused on establishing the timing and petrogenetic links 

between these three types of leucogranites. The lack of 

peralkaline granites already suggests an incomplete 

development (or a non-preservation) of the alkaline event 

that typically follows calc-alkaline activity, notably in the 

neighbouring Hercynian Island of Corsica. 

Pearce J.A., Harris, N.B.W. and Tindle, A.G., 1984. Trace element 

discrimination diagrams for the tectonic interpretation of granitic 

rocks. J. Petrol., 25, 956-983. 
Granitoids from Oceanic to 

Continental Environments: 

The Example of the Urals (Russia) 

1

Fershtater, G.B. and 2Bea, F. 1

/nstitute of Geology and Geochemistry, 

Ekaterinburg, Russia 2

Department of Mineralogy and Petrology, 

Campus Fuentenueva, University of Granada, 

18002 Granada, Spain 

Granitoid magmatism of the Ural mobile belt reflects 

main features of its tectonic structure and evolution. All 

geodynamic settings from oceanic to continental collision 

experienced by the Urals have their own magmatic events. 

The following megablocks in the W-E cross-section of 

the Ural mobile belt have been distinguished: 1) passive 

continental margin; 2) All-Ural suture with oceanic type of 

the crust; 3) and 4), NW and SE island-arc -continental 

megablocks with the following zones: island - arc (a), 

active continental margin (b) and continental (c); and, 5) 

Trans-Ural megablock as a transition zone to Kazakh 

structures. 

The magmatism of these zones is different in age and 

composition. The suture megablock is the area of 

ultramafic-mafic magmatism of middle-late Ordovician 

(harzburgite-type ophiolites) and early Devonian 

(lherzolite-type ophiolites) age which is accompanied by 

oceanic type diorites and plagiogranites. 

Magmatism of the island arc zones is represented by 

differentiated gabbro-granitoid and comagmatic basalt-rhyolite rock series of Silurian age in NW megablock 

and of middle-late Devonian age in the SE megablock. 

They are low-K tholeiitic at the early stage and K-Na calcalkaline to subalkaline at the late stage of evolution. All 

granitoids in oceanic and island arc zones have basic in 

composition magmatic source. Some of them are produced 

by fractionation of basic magma, some have an anatectic 

origin. The last are very poor in all incompatible rare 

elements and have the positive Eu anomaly. 

In continental marginal zone of the NW megablock two 

types of granitic rocks can be observed. Late Silurian-Middle Devonian magmatic rocks belong to 

calc-alkaline gabbro-granitic rock series with different K 

contents increasing from older to younger series. Early 

Carboniferous tonalite-granodiorite plutons mark the 

subduction of oceanic and island arc crust under the 

Eastern continent. They have clear geochemical lateral 

Abstracts 51 

zonation very similar to that in modem island arc. Good 

correlation between contents of K, Rb, Ba, Ga, Th, U, 

Mo, Nb, LREE, in granodiorites on the one hand, and the 

distance from suture being the track of paleosubduction 

zone, on the other hand, shows that it was the subduction 

that controlled the continental crust thickness and granite 

composition in continental marginal zone. The common 

geochemical feature of the Ural above-subduction 

granitoids is the absence of Eu anomaly, probably caused 

by high water contents in the initial magma and therefore 

early crystallization of allanite, controlling REE behaviour. 

In continental zones granitic magmatism is the most 

widely distributed. The early stage of magmatism fixing 

the beginning of collision is represented by different types 

of tonalite-granodiorite rock series derived from andesitic 

magma. They have the same age as above- mentioned 

continental margin orogenic rock series being close to 

them in composition. At the late stage of collision (late 

Carboniferous-Permian) anatectic granites appear. In their 

formation one can see three main episodes: migmatitic 

(synchronous with the main stage of regional amphibolitic 

stage metamorphism), granitic and leucogranitic. In 

vertical cross-section the rocks of these three episodes are 

placed from the bottom (migmatite roots) upwards. In the 

NW megablock granites have (

87Sr/86Sr)i = 0.712. The 

source rocks for them were Archean and Low Proterozoic 

metamorphic rocks of the basement of the Urals. In the 

South Urals anatectic crustal derived granites have the 

mantle 87Sr/86Sr ratio = 0.704463. It means that source 

material for anatexis was represented by products of 

erosion of island arc magmatic rocks and therefore South 

continental zone of the Urals was probably formed in the 

process of its development. 

The change of type of magmatic rock series, zoning in 

composition and age of the rocks in the W-E cross-section 

of the Urals reflect the sequence of geological events in 

the Urals mobile belt evolution from oceanic to continental 

environments. 

This work was supported by RFFR grant 95-05-142-80. 
52 The Origin of Granites and Related Rocks 

Non-linear Dynamics and the Distribution 

and Compositions of Enclaves in 

Granitoid Magmas 

1

Flinders, James and 2Clemens, John D. 1

Depanment of Geology, The University, 

Manchester, M13 9PL, UK 2

School of Geological Sciences, Kingston University, 

Kingston-upon-Thames, KTJ 2EE, Surrey, UK 

Physics attempts to quantify the non-linear, real world 

by the solution of equations that have been constructed 

principally on the basis that they are capable of solution. 

A hundred years ago, scientists were aware that deterministic systems can behave in unpredictable ways but, in 

order to learn anything about these complex systems, with 

numerous degrees of freedom, variables and parameters, 

they had to simplify their analyses. Simplification of 

imperfect information still dominates the analysis of 

complex natural systems. The equations that most nearly 

represent how a chaotic natural system behaves are 

insoluble. Thus, it is assumed that, because the higherorder discarded terms are small (which is true), the 

difference between the solution of the linearized and the 

"true" equation must also be small (which is not necessarily true). It is assumed that the discarded terms are 

unimportant. This is dangerous practice because the 

importance of such terms is both unknown and unknowable. Linear equations are poor descriptors of nature, and 

are incapable of predicting the evolution of any natural 

system beyond very narrow time limits. If only linearized 

equations are used, then a self-propagating self-censorship 

is established, producing innumerable triumphs of linear 

analysis that are far removed from reality. 

Some real chaotic systems have mostly predictable time 

evolutions, albeit with some noise that cannot be quantified 

beyond a certain limiting period. Although chaos is 

inherent in the system, the characteristic time period for 

non-linear dynamics to become the dominating factor is 

long enough to enable mathematical models to give usable 

results (e.g. planetary motion [several millions of years] 

and weather forecasting [several days]). Unfortunately, 

however, the relationship between most real systems and 

their models remains obscure. Such models, whilst 

intrinsically interesting, are unable to answer many 

questions posed by natural systems, since sensitive dependence on initial conditions makes them inaccessible to 

analysis or prediction beyond a very short time interval. 

The onset of hydrodynamic turbulence and convection in 

high-viscosity magmas, and mingling and mixing of 

partially miscible magmas are examples of natural chaotic 

systems. 

Patterns produced by folding and stretching provide 

visual analogues of the deterministic chaos within a 

dynamic fluid system, where two initially spatially juxtaposed points may be driven arbitrarily and exponentially 

far apart. It is impossible to predict or calculate the time 

evolution vectors of points within partially miscible fluids. 

However, the creation of interfacial area and the reduction 

of length scales produces reduced striation thickness and 

eventual disaggregation of a lower viscosity component. 

The amount of break-up is much less within "islands", 

associated with periodic elliptical points that hinder 

efficient mixing, than it is within "horseshoes", associated 

with periodic hyperbolic points that promote efficient 

mixing due to chaotic flow. 

We show some examples of the widely different 

morphologies of small, intermediate to mafic magma 

bodies suspended within felsic magmas, and conclude that 

these represent different stages in the temporal evolution 

of a fluid system driven by non-linear dynamics. 

Microgranitoid enclaves display a chaotic distribution 

of their Nd isotope systematics. This could be modelled as: 

several individual coeval magma fractions modified by 

internal differentiation and/or diffusion; a single magma 

whose composition has been modified by chaotic stretching 

and folding during mixing; the result of late-stage coalescence of enclaves; or a combination of these. The first of 

these possibilities may be capable of detailed analysis and 

solution, the others are not. 

Since we have no way of deciding between the alternative models, this could mean that the study of the compositions and distributions of such enclaves will yield no data 

that can adequately constrain their origin, original composition or their role in petrogenetic processes operating in 

a granitoid magma. The study of enclaves in granitoids 

might therefore be an irresolvable, time-consuming, 

petrological cul-de-sac. 

Ottino, J. M., 1989. The Mixing of Fluids. Sci. Am., 260, 40-49. 

Ottino, J. M., Leong C. W., Rising H. and Swanson P. D., 1988. 

Morphological structures produced by mixing in chaotic flows. 

Nature, 333, 419-425. 

Ruelle, D., 1994. Where can one hope to profitably apply the ideas of 

chaos? Physics Today, 47, 24-30. 
Granites and Microgranitoid Enclaves: 

The Importance of Cumulates 

Flood, R.H. and Shaw, S.E. 

Centre for Petrology and Lithospheric Studies, 

School of Earth Sciences, Macquarie University, 

NSW 2109, Australia 

Many models presently used to account for the range of 

granite compositions within individual plutons involve 

either magma mixing or restite separation. Yet the 

traditional view of magma chamber crystallization (Turner, 

1981) involves formation of crystal-rich cumulates along 

the sides and floor of the magma chamber and the accumulation of complementary residual melts near the roof of the 

pluton. Much of the chemical variation in both granite 

plutons and microgranitoid enclaves that most granites 

contain, can be explained using a cumulate model. 

We accept that chemical and textural zoning in many 

granitic plutons is the result of the crystallization of water 

undersaturated magmas, with most crystallization taking 

place as cumulates along the sides and floor of the developing pluton. Recent papers that demonstrate that granitic 

magma can be transported very rapidly along dykes means 

that the volume of magma present at any time within the 

magma chamber may be only a small fraction of the final 

pluton volume but if this is so, expansion of the pluton 

must be possible without necessarily deforming wall rocks 

or early-formed cumulates. 

Whatever the emplacement mechanism of granite 

magma, crystallization of relatively anhydrous cumulates 

will eventually produce residual magmas near the roof that 

are water saturated. With the crystallization of these 

magmas, excess water will escape episodically as vapour 

generated overpressure fractures the roof thus inducing 

pressure quench crystallization in the upper parts of the 

magma chamber. Water vapour collecting along the roof 

of the magma chamber during venting may prevent the 

magma from nucleating on the solid roof of the pluton. 

Under these pressure quench conditions, nucleation may 

well form around pre-quench phenocrysts. Build up of 

heat will also slow the growth of what initially are aggregates (rafts) of crystals, and for rafts of crystals too small 

to sink, this may limit the crystallision to the minerals on 

the liquidus. We suggest that in some hornblende-biotite 

1-type plutons, this would produce small microgranitoid 

enclaves rich in hornblende. Rafts that grow larger, 

perhaps by the amalgamation of several smaller rafts of 

crystals are able to sink and leave behind the hot depleted 

Abstracts 53 

magma produced by quench crystallization. The continued 

growth of these crystal rafts as they sink will form 

microgranitoid enclaves that are cumulates as a consequence of being depleted in the near solidus minerals of 

the host magma (typically K-feldspar and/or quartz). The 

extent to which the compositional zoning in the minerals 

of microgranitoid enclaves reflects different magma 

compositions, crystallization under different water pressure 

and crystallization under quench conditions needs to be 

more fully evaluated but we believe that crystallization in 

a quench environment is an important part of the explanation. The variety of microgranitoid enclaves in swarms 

and the common association of a small proportion of 

wall-rock xenoliths support the view that most concentrations of micro granitoid enclaves are II death assemblages II 

formed away from the site of crystallization. 

Microgranitoid enclaves that are not in isotopic equilibrium with the host pluton indicates that they formed from 

different (but we would argue, commonly only slightly 

different) magmas. Typically the isotopic composition of 

the enclaves overlaps that of the host pluton but the 

enclaves are less evolved. The pressure quench cumulate 

model of enclave formation requires that magma at depths 

in the magma chamber where the microgranitoid enclaves 

collect is commonly isotopically more evolved than magma 

near the roof. This difference may result from the 

addition of a small melt component from stoped blocks 

that have partially melted as they sank in the magma. 

Turner, J.S., 1980. A fluid-dynamical model of differentiation and 

layering in magma chambers. Nature, 285, 213-215. 

Significance of Magmatic Epidote in 

the Zeljin Pluton, Serbia 

1

Forizs, I., 2Vukov, M. and 2Jovic, V. 1

Laboratory for Geochemical Research 

of the Hungarian Academy of Sciences, 

1112 Budapest, Budaorsi ut 45, Hungary 2

Faculty of Mining and Geology, University of Belgrade, 

Djusina 7, 11000 Belgrade, Yugoslavia 

Magmatic epidote (mEP) can be a good indicator for 

the pressure, temperature and original water content of a 

granitoid body. A lot of work has been done to determine 

the stability field of mEP, but the borders of this field are 

uncertain even now. This paper contributes to the descrip-
54 The Origin of Granites and Related Rocks 

tion and explanation of the texture, and pressure of 

formation of magmatic epidote in the Zeljin pluton, Serbia 

(Yugoslavia). 

The Zeljin pluton is an 1-type granitoid body of Miocene age (24-22 Ma), lies in the Inner Dinarides and 

belongs to the Late Alpine granitoid zone. The mineral 

assemblage of the granitoid rock is hornblende, biotite, 

plagioclase, quartz, epidote, titanite, orthoclase, magnetite, 

and accessory apatite, zircon and some rare minerals. 

Secondary minerals are mainly chlorite, some titanite and 

rarely epidote. 

Two geobarometers have been applied to determine the 

pressure of solidification. One is the Al-in-hornblende 

barometer, which has been criticized since the moment it 

was published first by Hammarstrom and Zen (1986). The 

average AI content of rim of hornblende is 2.12 atom per 

formula unit. It indicates 7.2 kbar pressure using the 

formula P (± 1 kbar) = -4.76 + 5.64 · AI (Hollister et 

al., 1987). The calibration of this barometer has been 

improved recently, but in this case there is no big difference in the calculated pressure. To determining the 

pressure we use rim compositions of hornblende, which 

crystallized at the lowest temperature. A part of the 

hornblende is resorbed and has epidote precipitated during 

the narrow temperature interval where hornblende and 

epidote coexist (Naney, 1983). So the epidote and the rim 

of hornblende crystallized at the same conditions. The 

other barometer we have applied to these rocks is the 

biotite-amphibole thermobarometer (Perchuk and Fedkin, 

1976), which gives conditions ofT = 700°C - (650°C), 

total P = 7 kbar. 

Epidote crystals up to 2 mm are euhedral against 

biotite, and the same crystals have highly embayed, wormy 

contacts with plagioclase, quartz and K-feldspar. There is 

textural evidence that the epidote precipitated after resorption of hornblende. Rounded hornblendes are enclosed in 

epidote, which is the textural criterion for magmatic 

epidote described by Zen and Hammarstrom ( 1984). 

Some epidotes have zoned allanitic cores due to light rare 

earth element contents. Generally the epidote crystals are 

homogeneous, rarely showing a slight zonation caused by 

LREE. Their compositions range between 26% - 28% 

pistacite content. Quartz inclusions are the most frequent 

with some apatite and zircon in epidote. Synchronously 

with or after the crystallization of epidote, biotite surrounded a part of some epidote crystals. At a later stage 

of differentiation epidote would have become unstable 

against the residual melt in the rock and a part of it was 

resorbed, while plagioclase, orthoclase and quartz formed. 

This is our interpretation for the highly embayed, wormy 

contact between epidote and plagioclase, orthoclase and 

quartz. Where an epidote crystal was surrounded by 

biotite, it was protected from resorption and so its euhedral 

form has remained. The stable oxygen isotope composition of the epidote is 5.2 ± 0.1 %o vs. SMOW indicating 

magmatic origin. 

Magmatic epidote (mEP) of two kinds of textural 

characteristics ( euhedral and wormy) can be found in the 

Zeljin pluton, Serbia. It formed under conditions of about 

7 kbar pressure and 6750C temperature in equilibrium with 

melt. Probably as a result of ascent, at lower P-T conditions the mEP became unstable with the melt and a part of 

it resorbed. At the last stage plagioclase, K-feldspar and 

quartz formed showing wormy contact with magmatic 

epidote. 

Hammarstrom, J.M. and Zen, E-an, 1986. Aluminum in hornblende: An 

empirical igneous geobarometer. Am. Mineral., 71,1297-1313. 

Hollister, L.S., Grissom, G.C., Peters, E.K., Stowel, H.H., and Sisson, 

V. B., 1987, Confirmation of the empirical correlation of AI in 

hornblende with pressure of solidification of calc-alkaline plutons. 

Am. Mineral., 12, 231-239. 

Naney, M.T., 1983. Phase equilibria of rock-forming ferromagnesian 

silicates in granitic systems. Am. J. Sci., 283, 993-1033. 

Perchuk, L.L. and Fedkin, V.V., 1976. Temperature and gas regime in 

granitoid formation. Thermodynamic Regime of Metamorphism. 

Nauka Publishers, Leningrad, USSR (in Russian). 

Zen, E-an and Hammarstrom, J.M., 1984. Magmatic epidote and its 

petrologic significance. Geology, 12, 515-518. 

Extreme Compositional Variability 

in Granitic Monazites 

Forster, H.-J. and Rhede, D. 

GeoForschungsZentrum Potsdam, 

14473 Potsdam, Germany 

Electron microprobe analyses of more than 800 

monazites from geochemically diverse Hercynian granites 

of the Erzgebirge (Forster and Tischendorf, 1994) and 

Fichtelgebirge (Germany) revealed broad compositional 

variabilities and extraordinary non-essential element 

enrichments (Th, Ca, Si, U, Y) and LREE depletions not 

previously reported for granitic rocks. Particularly in 

multi-phase plutons, both monazites and other accessory 

phases show strong compositional differences at a variety 

of scales (Forster, 1993). Monazites ranging in size from 

a few microns to more than 200 JLm may exhibit delicate 
zonation patterns; however, the zonation is often complex 

and non-monotonic. Monazite displays contrasting compositions with respect to the abundance of non-essential 

elements and their substitution reactions between 

metaluminous and peraluminous granites in the studied 

area. Evolution trends of monazite composition and wholerock geochemistry in composite plutons are not always 

consistent, except for the ubiquitous trend of LREE 

depletion with magmatic differentiation. 

Monazites from a strongly peraluminous, multi-phase 

pluton composed of Li-Fe mica- and topaz-bearing monzoto syenogranites high in F, Li, Rb, Cs, Sn and P20 5 

(Eibenstock massif, W Erzgebirge), are distinguished by 

an extremely broad range in terms of Th (3 - 40.8 wt %) , 

U (<0.05- 4.6 wt %), Ca (0.7 -7.1 wt %}, andY (0.6-

4.1 wt %) concentrations. The strongest enrichment in 

these elements is found in monazites from late differentiates which have remarkably low total LREE (11 ppm), Th 

(4.4 ppm} andY (7.2 ppm) whole-rock contents. Substitution of the REEs by thorium and uranium is extensive; the 

mole fraction of the isomorphous cheralite molecule 

(Th,U,Ca}P04 varies between 0.09 and 0.81 indicating that 

the REEs do not always predominate. Even at the thinsection scale, the cheralite mole fraction may vary over a 

range of more than 50%, expressing that only part of the 

monazites is actually in equilibrium with the host-granite 

REE composition. Huttonite (Th, U)Si04 substitution is 

generally insignificant. With ongoing magmatic differentiation, the monazite La/Nd ratio decreases from 1.4 to 0.5 

corresponding to the observed progressive flattening of the 

whole-rock LREE pattern. Nd content does not exceed 

10.2 wt %. Monazite HREE abundances (sum Gd-Er: 0.9 

- 3. 7 wt %) and patterns vary non-systematically. 

Monazites from the geochemically similar 04 granite in 

the Fichtelgebirge, currently interpreted as a single 

intrusion, closely resemble those of the Eibenstock composite pluton (Th: 7.3 - 35.6 wt %; U: 0.1 - 6.3 wt %; 

Ca: 1 - 6.3 wt %; Y: 0.6- 3.1 wt %; La/Nd: 1.3- 0.7; 

sum Gd-Tb: 1.5- 4.1 wt %). 

By comparison, the monazites from metaluminous to 

slightly peraluminous biotite monzogranites low in F, Li 

and P20 5 , exemplified by the multi-phase Kirchberg massif 

(W Erzgebirge), display a more restricted compositional 

span with respect to Th (1.3 - 18.3 wt %}, U ( <0.05 -

2.3 wt %}, andY ( 0.2- 3.5 wt %) concentrations. They 

are usually characterized by a high degree of huttonite 

substitution reaching 20 mol % . Local extreme Th abundances up to 40 wt % may represent fine monazite-thorite 

intergrowths. Relative to monazites from the Eibenstock 

Abstracts 55 

pluton, the range in the La/Nd ratio (2. 7 - 0.8) is even 

more pronounced. 

Particularly impressive are the compositional characteristics of the monazites from the various, geochemically 

distinct fine-grained aplitic dikes which cross-cut the 

Kirchberg pluton. Both the absolute elemental abundances 

as well as the shape of the REE patterns show a strong 

compositional diversity within and between the individual 

aplites (Th: 2- 24.7 wt %; Y: 0.3- 2.8 wt %, La/Nd: 1.4 

- 0.4, sum Gd-Tb: 1.0- 4.5 wt %). Compared with the 

host pluton, unusual features include local extreme enrichment in cheralite component (45 mol %), Nd (max. 14.6 

wt %}, Sm (4.6 wt %), and Gd (3 wt %). Nd-rich 

monazites possessing relatively shallow and steep HREE 

profiles occur within the same thin section. 

The most outstanding composition (Th: 0.14 - 0.4 wt 

%; Nd: 17.6 - 18.5 wt %; La/Nd: 0.3; Dy and heavier 

REE below the electron microprobe analysis detection 

limit) was found in a large, anhedral monazite from a finegrained biotite monzogranite of Schlema-Alberoda (W 

Erzgebirge). This monazite formed presumably a very latecrystallizing phase. 

The abnormal variability and evolution of monazite 

compositions in the studied granite plutons must reflect 

strong spatial and temporal changes in melt composition 

during progressive differentiation and highlight the operation of different processes as described by W ark and 

Miller (1993). The great extent of non-essential element 

substitutions requires further experimental studies on 

monazite stability relations in terms of miscibility with 

huttonite and cheralite. 

Forster, H.-J., 1993. Th-Y-REB-bearing accessory minerals in Hercynian 

granites of the Erzgebirge (Ore Mountains), Germany. Geol. Soc. 

Am. Abst. Prog., 25, 42-43. 

Forster, H.-J. and Tischendorf, G., 1994. Evolution of the Hercynian 

granite magmatism in the Erzgebirge metallogenic provinces. 

Mineral. Mag., SSA, 284-285. 

Wark, D.A. and Miller, C.F., 1993. Accessory mineral behavior during 

differentiation of a granite suite: monazite, xenotime, and zircon in 

the Sweetwater Wash pluton, southeastern California. Chem. Geol., 

110, 49-67. 
56 The Origin of Granites and Related Rocks 

Geochronology of the Idaho-Bitterroot 

Batholith and Bitterroot Dome: 

An Example of Magmatism 

Preceding and Contemporaneous 

with Core Complex Extension 

1

Foster, D.A. and 2Fanning, C.M. 1 Victorian Institute of Earth and Planetary Sciences, 

School of Earth Sciences, LaTrobe University, 

Bundoora, Victoria 3083, Australia 2

Research School of Earth Sciences, The Australian 

National University, Canberra, ACT 0200, Australia 

Granitic plutonism and extension are broadly contemporaneous in many metamorphic core complexes. However, 

the relationship between magmatism and extension is 

rarely unambiguous, because in many cases it is not clear 

whether magmatism caused or was the product of extension. Furthermore, the apparent lack of synkinematic 

plutons in some core complexes has led to the suggestion 

that there is no relationship between magmatism and 

extension. Both of these problems are exacerbated by 

insufficient high precision geochronologic and thermochronologie data. 

The Idaho-Bitterroot batholith, Montana and Idaho, 

forms the infrastructure for most of the Bitterroot Dome 

metamorphic core complex and thus is an ideal area for 

assessing the relationships between magmatism and 

extension. Previous U /Pb zircon and monazite studies of 

main-phase granitoids from the Bitterroot batholith give a 

range of lower discordia intercepts from -73 to 48 Ma, 

with the youngest ages obtained from granite within the 

Bitterroot mylonite. These studies reveal complex inherited zircon populations and some radiogenic Pb-loss, so the 

significance of lower intercept ages is unclear for some 

samples. We have analyzed zircon from 6 samples of 

Bitterroot batholith granite using the SHRIMP ion microprobe. Three mylonitic granite samples from the 

Bitterroot mylonite in Lost Horse Canyon give a weighted 

mean 206PbP38U age of 54 ± 1 Ma. A protomylonitic 

granite from the central part of the Bitterroot Dome also 

gives a 206 PbP38U age of 53 ± 1 Ma. Mylonitic 

megacrystic granite from Sweathouse Canyon yields an age 

of 65 ± 1 Ma. Finally, a granite sample from the Lochsa 

Canyon, in the central Bitterroot batholith, gives 57 ± 1 

Ma. Inherited zircon from these samples ranges in age 

from 1460 to 1820 Ma. Paleoproterozoic crust dominates 

the source region of the Bitterroot batholith because the 

majority of the inherited grains formed between 17 50 and 

1800 Ma. 

These SHRIMP U-Pb zircon cystallization ages are 

consistent with the conventional zircon and monazite ages, 

though with higher precision and tighter range. 

Hornblende 40 ArP9 Ar ages for synplutonic mafic dikes in 

the central Bitterroot batholith also suggest crystallization 

of the main-phase plutons between 57 and 53 Ma. New 

and previously published 40 ArP9 Ar and Kl Ar apparent ages 

of biotite and muscovite from the Lochsa area and the 

western and central Bitterroot Dome are 50 - 48 Ma. 

Younger mica ages ( 46 - 43 Ma) are restricted to the 

vicinity of the mylonite zone. These results indicate that 

the cessation of main-phase magmatism migrated east with 

time, and that most of the plutons in the Bitterroot Dome 

were intruded during the Paleocene and early Eocene. If 

the - 48 Ma conventional ages actually record crystallization, these younger granites appear to occur only in and 

adjacent to the Bitterroot mylonite. 

Emplacement of magma during the period 65 to about 

50 Ma reduced the strength of the crust in the Bitterroot 

batholith region. When the regional tectonic setting 

changed from compressional to extensional at -50 Ma, 

mid-crustal plutonism was localized in what is now the 

Bitterroot Dome. Extensional deformation was focussed in 

the Bitterroot Dome, and formed the thick mylonite zone 

as a consequence of rheological contrasts with cooler areas 

to the east and west. The progression of granite crystallization ages and 40 ArP9 Ar cooling ages from west to east 

are consistent with top-to-the-east shear indicators in the 

mylonite zone. Thermochronology indicates that the 

western part of the Bitterroot Dome was below - 350°C at 

about the same time as the last stage of granite emplacement and metamorphism in the east. Therefore, the 

transition from mylonitization to brittle deformation to 

inactivity of the shear zone was progressive from west to 

east across the Dome from - 48 - 44 Ma. This explains 

the previously enigmatic occurrence of amphibolite facies 

ductile deformation in the eastern part of the Dome 

coincident with epizonal syenogranite magmatism along the 

western border. 
Pluton Emplacement Mechanisms: 

A View from the Roof 

Fowler, T. K. Jr., Paterson, S.R., 

Crossland, A. and Yoshinobu, A. 

Department, of Earth Sciences, 

University of Southern California, 

Los Angeles, CA 90089-0740, USA 

Exposures from tilted plutons and regions of steep 

topography reveal that the upper portions of many plutons 

in continental arcs are piston-shaped in three dimensions, 

being elliptical in plan view with steeply-dipping walls that 

abruptly roll over into gently-dipping or domal roofs. 

Such pluton roof exposures allow powerful tests of 

emplacement mechanisms that simply are not available 

from more commonly exposed map-plane sections through 

pluton walls. For example, lateral extensional models 

predict that host rocks above the pluton roof must be either 

strongly extended, structurally detached, or both. On the 

other hand, diapir models predict intense, roof-parallel 

flattening strains. At the roof, one can view directly the 

effects of doming, ring faulting, and cauldron subsidence. 

Because pluton roofs are directly in the path of magma 

ascent, they may represent a "process zone" preserving 

early portions of the ascent and emplacement history. 

We have mapped partially preserved roofs and walls of 

ten plutons: the Mt. Powell batholith, MT; the Guadalupe, 

Castle Creek, Emerald Lake, Mitchell Peak and Hall 

Canyon plutons, CA; the Yerington batholith, NV; the 

Mt. Stuart batholith, W A; and the Chita and Agua Negra 

plutons, San Juan Province, Argentina. Estimated emplacement depths range from 2 to 10 kilometers, whereas 

host rock types include fine- to coarse-grained clastic 

rocks, calcsilicate, carbonate and volcanic rocks, schist, 

granite and granodiorite, offering a varied panorama of 

emplacement conditions. 

Observed pluton/host rock relations share many traits 

in common. Pluton contacts are irregular, step-like (em to 

100m) and discordantly truncate host-rock structure and 

fabric at all scales. With respect to the host rocks, 

intrusive contacts are brittle fractures, although thin section 

analysis indicates some grain-scale modification by nearand/or sub-solidus processes. Adjacent to roofs, host 

rocks show only weak emplacement-related ductile strains 

( < few % ). Margin-parallel dikes are locally common. 

Emplacement-related faulting is uncommon and/or of small 

displacement (generally < 10 m). Host rocks above the 

Abstracts 57 

pluton roofs are non- to weakly-extended and are not 

detached. Roof lifting is relatively minor (generally < 200 

m). At Yerington, Guadalupe and Mt. Stuart, narrow (lOs 

to lOOs m) deformed aureoles occur along the pluton walls 

and are characterized by moderate contact-normal host 

rock ductile shortening (few to several lOs %). The overall 

pluton/host structural relationship is similar to that produced by an ice-cream scoop; the contact represents a 

"surface of truncation" along which host rock material has 

been removed. Although not all arc plutons exhibit these 

relationships, a variety of published geological and geophysical data suggest that such three-dimensionally discordant plutons are common in the upper crust of arcs. 

Within the plutons, spatially complex compositional 

zoning and fabric geometries show little relationship to 

host rock structure. Some of the plutons show large-scale 

( > lOOs m) vertical zonation. Locally, roof contacts have 

served as ponding sites for late aplite, pegmatite and 

schlieren. Magmatic foliation patterns vary from concentric and contact-parallel, to planar and contact-oblique, to 

chaotic, to isotropic. In some cases, oblique foliations 

only deflect into contact-parallelism within a few em of the 

contact. Host-rock xenoliths in the plutons occur from 

hand-sample to lOOs of meters in size, but are rarely 

abundant. 

Pluton emplacement destroys structural information; 

late, brittle processes eliminate kinematic evidence and 

host-rock material is "lost". Late magmatic and tectonic 

processes may overprint emplacement-related magmatic 

fabrics. Nevertheless, the data permit the following 

conclusions: 1) most host-rock material removed during 

emplacement has not been displaced upwards or laterally, 

and thus, must have been displaced downwards; 2) stoping 

is the final dominant emplacement process, however it may 

obliterate evidence for earlier downward host-rock displacement (e.g. downfolding, hot-Stokes flow); and, 3) 

pluton magmatic fabrics form extremely late and late-stage 

host rock and pluton strains are commonly mechanically 

decoupled. 
58 The Origin of Granites and Related Rocks 

Sr-Nd Isotopic Record of Multi-Stage 

Interactions Between Mantle-Derived 

Magmas and Crustal Components 

in a Collision Context: 

Ultramafic-Granitoid Association 

from Vivero, NW Spain 

1

Galan, Gumer, 2Pin, Christian 

and 2

Duthou, Jean-Louis 1

Departamento de Geologfa, 

Universidad Autonoma de Barcelona, 

Edificio C(S) 08193 Bellaterra-Barcelona, Spain 2

Departement de Geologie, URA 10 CNRS, 

Universite Blaise Pascal, 5 Rue Kessler, 

63038 Clermont-Ferrand, France 

One of the main characteristics of the Hercynian orogen 

is the abundance of orogenic granites. A pure crustal 

origin is widely accepted for the aluminous leucogranites 

but a mantle contribution is often invoked for granitoids of 

calc-alkaline affinity. However, the involvement of 

mantle-derived magmas in these granitoids is often controversial since 1) most available isotopic and other geochemical data favor crustal sources of meta-igneous and/or 

metasedimentary type, 2) isotopic compositions of basicintermediate rocks associated with them cannot be usually 

related to a depleted mantle. In some few examples of the 

Hercynian orogen, basic-intermediate rocks related to 

calc-alkaline granitoids include olivine-amphibole bearing 

ultramafic rocks (i.e. cortlandtites) which provide further 

insight into the mantle contribution to the composition of 

these granitoids and into the interaction processes between 

mantle-derived magmas and crustal components. 

The Vivero granitic body is situated at the 

north-western part of the Iberian Hercynides and displays 

such an association of ultramafic through melanocratic and 

calc-alkaline granitoids. These are syntectonic granitoids 

intruded into the middle crust soon after the Hercynian 

collision (c. 330 Ma). Ultramafic rocks are cumulates of 

peridotite, pyroxenite and homblendite type. Melanomesocratic rocks are biotite-amphibole diorites, quartzdiorites and tonalites. Calc-alkaline granitoids comprise 

fewer melanocratic amphibole-biotite tonalites, biotite 

granodiorites and monzonitic granites. 

Petrological, geochemical data and radiogenic isotopes 

serve to evaluate (i) the interaction processes operating 

between mantle-derived magmas and crustal components, 

and (ii) the nature of the involved sources. Scattered 

values for major and trace elements and heterogeneity for 

Sr-Nd isotope results preclude a common origin for the 

association as a whole, and even for each rock type. eNd 

and (

87Sr/86Sr)i of ultramafic-mesocratic rocks (eNd = 0.0 -

-3.8; (

87Sr/86Sr)i = 0.7058 - 0.7080) overlap those of 

granitoids (eNd = -1.9 - -6.0; (

87Sr/86Sr)i = 0.7061 -

0. 7082) but granitoids follow a divergent evolutionary 

trend with respect to the former rocks. Positive correlation between the degree of differentiation and the degree 

of contamination for melano-mesocratic rocks supports an 

origin from a tholeiitic basalt contaminated by a wall-rock 

assimilation (WRA) process, in a first stage, during ascent 

to the middle crust. Such a process followed by in situ 

fractional crystallization and interaction with 87Sr-enriched 

fluids would also account for the ultramafic cumulates. 

Among the granitoids, eNd decreases while (

87Sr/86Sr)i 

increases from tonalites through granodiorites and 

monzonitic granites reflecting an increasing contribution of 

crustal components. A dominant AFC process, including 

two-component mixing, would account for the divergent 

evolutionary trend of these granites. This AFC process 

would contaminate the mafic magmas at deeper crust levels 

producing hybrid tonalites and related MME-type enclaves. 

In a second stage, the tonalitic hybrid magmas intruded the 

ultramafic-mesocratic rocks and were followed by 

granodiorites and monzogranites. The monzogranites are 

interpreted as pure crustal melts; their HREE fractionated 

pattern ((Gd/Yb )N = 4 - 5) suggests equilibrium with a 

garnet-rich residuum. Intermediate granodiorites would 

result from mixing between tonalites and monzogranitic 

magmas. The isotopic composition of the monzonitic 

granites favors a moderate time-integrated Rb/Sr protolith 

such as metasediments of greywacke type and/or intermediate-felsic meta-igneous not exposed at present. Taking 

into account the isotopic characteristics of ultramaficmesocratic rocks and the operating contamination processes, the mafic end-member would have been extracted 

from mildly depleted domains of the lithospheric mantle. 

Partial melting of these fertile domains could have been 

triggered along shear zones of lithospheric scale during or 

soon after the Hercynian collision. 
Amphibole Mineral Chemistry of the 

Anorogenic Robertson River Igneous Suite, 

Blue Ridge Province, Virginia 

Gray, Karen J. 

U.S. Geological Survey, MS 954 National Center, 

Reston VA 22092 

The late Proterozoic Robertson River Igneous Suite 

(RRIS) is part of a compositionally distinct group of 

intrusives in the central and southern Appalachians exposed in the core of the Blue Ridge Anticlinorium. The 

elongate batholith intruded high-grade middle Proterozoic 

(1.2- 0.9 Ga) gneisses into the Robertson River rift zone, 

which trends N25°E and cuts across the Blue Ridge 

Anticlinorium from southeast to northwest. The RRIS is 

the largest known body preserving evidence of the plutonic 

and hypabyssal felsic magmatism associated with the initial 

stages of the late Proterozoic rifting event that led to the 

development of the Iapetus Ocean. It is composed of eight 

units that extend over 100 km, listed below from oldest 

(735 Ma) to youngest (702 Ma): 1) Rivanna Granite; 2) 

Laurel Mills Granite; 3) White Oak Alkali Feldspar 

Granite; 4) Arrington Mountain Alkali Feldspar Granite; 

5) Cobbler Mountain Alkali Feldspar Quartz Syenite; 6) 

Hitt Mountain Alkali Feldspar Syenite; 7) Amissville 

Alkali Feldspar Granite; and, 8) Battle Mountain Alkali 

Feldspar Granite-Felsite complex. Catoctin Formation 

(600-570 Ma) greenstone dikes, which were associated 

with subsequent stages of the rifting event, intruded the 

RRIS. The RRIS and the Catoctin Formation indicate that 

there were two distinct extensional stages, an earlier stage 

that resulted in extensive felsic A-type magmatism and a 

later stage that produced predominantly basalt with minor 

rhyolite. All metaluminous units (except the Rivanna 

Granite) contain zoned amphiboles and have early primary 

textures; the Laurel Mills Granite exhibits synneusis 

texture. The mildly peralkaline (A.I. = 1) Amissville 

Granite contains evidence for an intersertal primary 

amphibole, as well as a subsolidus/hydrothermal amphibole. The alkaline/peralkaline units (Amissville Granite, 

Battle Mountain Granite/Felsite) which are chemically 

similar to the Younger Granites of Nigeria (A .I. ~ 1, 

Ga/Al ~ 6), contain riebeckite and aegirine or aegirineaugite. The metaluminous rocks (all except the Amissville 

and Battle Mountain granites) contain calcic amphiboles 

and are similar in bulk composition to anorogenic rocks 

from other localities worldwide (for example the TopsailsAbstracts 59 

St. Lawrence complex in Newfoundland). All RRIS units 

contain high abundances of high field strength elements 

(Nb, Y, Zr, Ga, and Zn) and all contain fluorite (except 

the Laurel Mills Granite). The peralkaline rocks, 

characterized by higher Si02 (- 76 wt %), correlate with 

the higher Si02 contents of riebeckite amphiboles, whereas 

the metaluminous rocks exhibit a range in bulk rock Si02 

(59 - 74 wt %) that is not reflected in the Si02 range of 

the amphiboles. 

Detailed amphibole chemistry on seven units (no 

amphibole in the Rivanna Granite) allows identification of 

complex zoning and great variation within a single grain, 

as well as some variation between different localities 

within some units. The range of amphibole compositions 

for all metaluminous units is generally controlled by the 

edenite substitution and a combination of the Fe-, Ti-, or 

Al-tschermakite substitutions and aNa + Ti for Ca + Al 

substitution. The analyzed amphiboles are so Fe-rich that 

their compositional differences can best be illustrated in 

plots which compare Ti, Mn, Al, and Mg. In comparison 

to other alkaline suites, the amphiboles of the RRIS are 

unique in their high iron contents over the entire 

compositional range from calcic to sodic. Compositions of 

alkali and plagioclase feldspars and Fe-Ti oxides suggest 

that these minerals re-equilibrated during greenschist facies 

metamorphism. Late-stage magmatic and/or subsolidus 

reactions partially affected the hastingsitic to ferro-edenitic 

amphiboles and resulted in crystallization of biotite and 

later stilpnomelane. In spite of these postmagmatic and 

metamorphic reactions, the remaining amphiboles retain 

an igneous chemical signature (Si < 7.3 cations) and 

complex zonation. 

Experimental data indicate that magnesio-hastingsite is 

stable below lOOOOC at an oxygen fugacity of to-n bars at 

the Q FM buffer, hastingsite is stable at 60ooc at an 

oxygen fugacity of - l0-19 bars at the QFM buffer, and 

that ferro-pargasite has a stability field very similar to 

hastingsite. These experimental data generally are applicable to the range of calcic amphiboles in the RRIS; however, the high iron contents indicates that the temperature 

would be in the lower range. The evolutionary trend in 

the bulk chemistry as indicated by the range in Si02 (59 -

76 wt % ) is not reflected in the compositions of the calcic 

amphiboles. The change in amphibole composition from 

calcic to sodic is abrupt and represents a sudden change in 

temperature (lower) and oxygen fugacity and alkalinity 

(higher). The rocks bearing sodic amphibole were 

emplaced about 8 Ma after the last metaluminous unit, and 

then the magmatism in the area ceased. 
60 The Origin of Granites and Related Rocks 

Shoshonitic Granitoids of the 

Borborema Province, NE Brazil 

Guimaraes, I.P. and DaSilva Filho, A.F. 

DGEO - CT, UFPE CEP 50732-970, 

Cidade Universitaria, Recife PE, Brazil 

The Borborema Province (BP) has been interpreted by 

Van Schmus et al. ( 1994) as the central part of a major 

Pan-African/Brasiliano orogenic belt, that formed during 

the collision of the Sao Francisco(SFC)-Congo-Kasai 

Cratons and the Sao Luis/West Africa Craton. The BP is 

divided into three domains: Paleoproterozoic to Archean 

Rio Piranhas and Caldas Brandao Massifs; Paleoproterozoic to Archean SFC basement to the South; and a 

domain, which contains a large amount of Mesoproterozoic 

fold belts and small amount of reworked Paleoproterozoic 

to Archean blocks. 

The shoshonitic granitoids in the BP have a particular 

distribution. They have always been described close to the 

contacts between the Mesoproterozoic Fold Belts and the 

Paleoproterozoic-Archean blocks. The Rb-Sr data available, point out to an age of 600Ma for them. 

The shoshonitic granitoids can be divided into two 

groups: 1) oversaturated (OG), such as Curituba and Serra 

do Catu Complexes (North boundary of the Sergipano Fold 

Belt), Solidao and Texeira Complexes (NE Boundary of 

the Pajeu-Paraiba Fold Belt (PPFB); Sial and Ferreira, 

1988), Floresta area (SW boundary of PPFB; Valgueiro et 

al., 1992); and, 2) saturated (SG), such as Born Jardim 

and Toritama Complexes (SE boundary of the PPFB; 

Guimaraes, 1989), Terra Nova Complex (S boundary of 

the Pianco Alto Brigida Fold Belt, Silva Filho et al., 

1993), and Quixada and Prado Complexes (NW boundary 

of the Serido Fold Belt, Galindo et al., 1992). 

The OG are monzo- to syenogranites containing small 

enclaves, which are mainly amphibole clots. The SG are 

monzonites and syenites which contain large amount of 

mafic enclaves, with composition similar to the less 

evolved major facies. Low angle foliations recorded in 

some SG complexes, point out intrusions related to 

tangential tectonism (Guimaraes, 1989). The more mafic 

major facies of the SG show very high MgO (5 - 7 wt % ), 

Cr (200 - 500 ppm) and Ni ( 100 - 250 ppm) which point 

to a mantle contribution in the genesis of these rocks. 

Both groups are peraluminous to metaluminous and 

show high contents of Sr, Ba, LREE, medium content of 

Rb and Zr, low content of Ti02, P20 5 and Nb and, 

K20/N20 ratios > 1. These chemical characteristics, 

associated with the ubiquitous presence of calcic amphibole 

(Mg-homblende to edenite), clinopyroxene showing no Feenrichment trend and K-feldspar surrounded by 

plagioclase, fit with those of shoshonitic rocks associations 

(Morrison, 1980). The calc-alkali ratio - Si02 trends for 

the shoshonitic granitoids of the BP are similar to that of 

New Guinea Continental Arc (Brown, 1982). 

Shoshonitic rocks are produced above a steepening 

subduction zone when a flip in the direction of subduction 

occurs and uplift and block faulting occur within the arc 

(Morrison, 1980). The distribution in space and time of 

the shoshonitic granitoids within the BP, suggest that: the 

Archean to Paleoproterozoic blocks probably represent (or 

behaved as) continental microplates during the Pan-African 

collisional event in the BP, and an age of 600 Ma is a 

good estimation for the end of the compressive regime 

within the BP. The Shoshonitic magmatism the tectonic 

model (Caby, 1987; Guimaraes, 1989; Caby et al. 1991; 

J ardim de Sa, 1994) in which collision of continental 

microplates and arc took place during the Brasiliano 

orogeny (850 to 600 Ma), followed by transcurrent 

intracontinental reworking 

Brown, G.C., 1982. Calc-alkaline intrusive rocks: their diversity, 

evolution and relation to volcanic arcs. In Thorpe, R. (ed), Andesites, 

J. Wiley and Sons, p. 437-461. 

Caby, R., 1987. The Pan-African belt of West Africa from the Sahara 

desert to the Gulf of Benin. In Schaer, J. and Rodgers, J. (eds), The 

Anatomy of Mountain Ranges, Princeton University Press, p. 129-

170. 

Caby, R., Sial, A., Arthaud, M. and Vouchez, A., 1991. Crustal 

evolution and the Brasiliano Orogeny in northeast Brazil. In 

Dallmeyer, R. and Lecorche, J. (eds), The West African Orogen and 

Circum-Atlantic Correlatives, p. 373-397. 

Galindo, A.C., Dall'agnol, R., Mcreath, 1., Leterrier, J.,1993. Quimica 

mineral de anfibolios e biotitas de alguns granitoides Brasilianos do 

oeste do Rio Grande do Notre, nordeste Brasil. MAGMA-Academia 

Brasileira de ciencias, Rio de Janeiro, Brasil, Extended Abstracts, 

66-68. 

Guimaraes, I., 1989. The petrologic evolution and tectonic association of 

the Bom Jardim Complex, Pernambuco State, NE Brazil. PhD thesis, 

University of London, 424 p. 

Jardim de Sa, E.F. ,1994. A Faixa Serido (Providencia da Broborema, 

NE do Brasil) e seu significado geodinizmico na cadeia 

Brasiliana/Pan-Africana. Doctor Thesis, Universidade de Brasilia, 

803 p. 

Morrison, G., 1980. Characteristics and tectonic setting of the shoshonite 

rock association. lithos, 13, 97-10. 

Sial, A. and Ferreira,V., 1988. Brasiliano age peralkaline plutonic rocks 

of the central structural domain, NE Brazil. Rend. Soc. ltalliana., 43-

2, 307-342. 

Silva Filho, A.F., Guimaraes, J.P. and Thompson, R. 1993. Shoshonitic 

and ultrapotassic Proterozoic intrusive suites in the CachoeirinhaSalgueiro Fold Belt, NE Brazil: a transition from collisional to post-
collisional magmatism. Precambrian. Res., 62, 323-342. 

Valgueiro, C., Ferreira, V., Sial, A., Sa,L., 1992. Rochas shoshoniticas 

e peralcalinas a noroeste de Floresta, Pernambuco. 37 Cong. Bras. 

Geo., SBG, Sao Paulo-SP, Extended Abstracts, 2, 69. 

Van Schmus, W., Brito Neves, B.B. and Babisnski, M., 1994. 

Mesoproterozoic crustal domains in the Borborema Province: 

Evidence from U/Pb and Sm/Nd geochronology. 38 Cong. Bras. 

Geo., SBG, Camboriu-SC, Extended Abstracts, l, 403-404. 

Miocene Rapakivi-Like Granites of Southern 

Newberry Mountains, Nevada, USA: 

Comparison to the Proterozoic Rapakivi 

Granites of Finland 

1

Haapala, I., 1Ram6, O.T. and 2Volborth, A. 1

Department of Geology, P.O. Box 11, 

FIN-00014 University of Helsinki, Finland 2

Montana College of Mineral Science and Technology, 

Butte, MT 59701, USA 

Rapakivi granites occur on all continents and are 

commonly characterized by a Proterozoic age (1. 75 to 1.0 

Ga, with a few exceptions), bimodal magmatic association, 

extensional tectonic setting, A-type geochemical and 

mineralogical characteristics, dominantly crustal source, 

and Sn-W-Be-Zn mineralization associated with topaz-bearing late-stage intrusive phases. All these features 

are well-documented in southern Finland and adjacent 

Russian Karelia where 1.65 to 1.54 Ga rapakivi granite 

plutons intrude Paleoproterozoic or Archean metamorphic 

crust (Ramo and Haapala, 1995). 

The Tertiary extension-related magmatic association of 

the southwestern United States (Basin and Range Province) 

shows marked similarities, but also some dissimilarities, 

with the Proterozoic rapakivi magmatism of Finland. In 

both areas, extensional tectonics is indicated by swarms of 

parallel rhyolite and basalt dikes, normal and listric 

faulting, and by thinning of the crust (Gans et al., 1989; 

Haapala and Ramo, 1992; Korja and Heikkinen, 1995). 

The synextensional bimodal magmatism of the Basin and 

Range Province comprises basalts and rhyolites, in lesser 

amount plutonic equivalents, whereas in Finland the 

plutonic members prevail. In both areas, the felsic igneous 

rocks show commonly A-type or within plate granite 

characteristics. The Be and Sn deposits locally associated 

with the Tertiary topaz rhyolites of southwestern U.S.A. 

are probably volcanic - subvolcanic analogues of the 

greisen and skarn-type Sn-Be-W-Zn mineralization related 

Abstracts 61 

to the topaz-bearing granites of the Finnish and Russian 

Karelian rapakivi complexes. 

The Miocene Spirit Mountain pluton (SMP; Howard et 

al., 1994) in the southern Newberry Mountains, southern 

Nevada, is one of the few epizonal-subvolcanic 

synextensional granite plutons of the southern Basin and 

Range Province. Numerous NNW-trending dikes composed of rhyolite or basalt or their mingling products 

transect the pluton and the surrounding, mainly Proterozoic, crust. SMP is mainly composed of various textural 

types of light-coloured biotite-bearing or two-mica granites; hornblende is rarely present. Overall, the Finnish 

rapakivi granites are more mafic, often showing 

hornblende as a typical mafic silicate. Titanite and magnetite are characteristic accessory minerals of SMP, whereas 

in the Finnish rapakivi granites ilmenite and anatase are 

the Ti-bearing accessory minerals. Some of the alkali 

feldspar crystals in SMP are mantled by plagioclase; in 

Finland, plagioclase-mantled alkali feldspar ovoids are 

typical in the larger rapakivi granite batholiths. 

The Si02 content of the SMP granites ranges from 66.6 

to 77.4 wt %, (Na20 + K20) from 7.5 to 9.6 wt %, and 

molecular Al20 3/(Ca0 + K20 + Na20) from 0.98 to 

1.04; these values coincide with those measured for the 

Finnish rapakivi granites. A-type and within plate granite 

characteristics of the granites of SMP are not so well 

expressed as in the case of the Finnish rapakivi granites; 

some of the former plot into the field of volcanic arc 

granites or fractionated granites. FeOT /(FeOT + MgO) 

varies in SMP granites from 0. 7 4 to 0. 90, whereas in the 

Finnish rapakivis it is usually between 0.85 and 1.00 

(average 0.92). The F content (0.03 to 0.09 wt %) of the 

SMP granites is very low compared to the average of 0.35 

wt % Fin the rapakivi granites of Finland. 

Nd isotopic data on the Precambrian country rocks of 

SMP (BNil.4 Ga) c. - 3.5, toM 1.93 - 2.03 Ga) show that 

they conform to the evolution of the Paleoproterozoic 

Mojave province (cf. Gleason et al., 1994). The Nd 

isotopic composition of the granites of SMP (BNi15 Ma) 

- 9.6 - - 11.6, toM 1.14 to 1.34 Ga) is indicative of a 

substantially younger source. This contrasts with the 

Finnish rapakivi granites that have been interpreted to have 

derived from the Paleoproterozoic crust hosting the 

granites (Haapala and Ramo, 1992). 

Gans, P.B., Mahood, G.A. and Schermer, E.R., 1989. Synextensional 

magmatism in the Basin and Range Province: A case study from the 

eastern Great Basin. Geol. Soc. Am. Spec. Paper, 223, 53 p. 

Gleason, J.D., Miller, C.F., Wooden, J.L. and Bennet, V.C., 1994. 

Petrogenesis of the highly potassic 1.42 Ga Barrell Spring pluton, 
62 The Origin of Granites and Related Rocks 

southeastern California, with implications for mid-Proterozoic magma 

genesis in the southwestern USA. Contrib. Mineral. Petrol., 118 

182-197. 

Haapala, I. and Ramo, O.T., 1992. Tectonic setting and origin of the 

Proterozoic rapakivi granites of southeastern Ferroscandanavia. 

Trans. Royal Soc. Edinburgh: Earth Sci., 83, 165-171. 

Howard, K.A., John, B.E., Davis, G.A., Anderson, J.L., and Gans, 

P.B., 1994. A guide to Miocene Extension and magmatism in the 

Lower Colorado River Region, Nevada, Arizona, and California. 

U.S. Geological Survey, Open-File Report 94-246, 54 p. 

Korja, A. and Heikkinen, P.A., 1995. Proterozoic extensional tectonics 

of the central Fennoscandian Shield: Results from the Baltic and 

Bothnian Echoes from the Lithosphere experiment. Tectonics, 14, 

504-517. 

Ramo, O.T. and Haapala, 1., 1995. One hundred years of rapakivi 

granite. Mineral. Petrol., 52, 129-185. 

Geochemistry and Petrogenesis of 

Lusatian Cadomian Granitoids 

(Northern Margin of Bohemian Massif) 

1

•

2

Hammer, J. and 1Eidam, J. 1 FR Geowissenschaften, Universitiit Greifswald, 

Jahn-Str. 17a, 17489 Greifswald, Germany 2

Geochemisches lnstitut, Universitiit GOttingen, 

Goldschmidt-Str. I, 37077 GOttingen, Germany 

Within the Lusatian Anticlinal Zone (LAZ; Southeast 

Germany, Eastern Saxothuringicum) Cadomian (587 ± 17 

Ma to 542 ± 9 Ma) and subordinate late Variscan (304 ± 

14 Ma) granitoids occur (Kroner et al. , 1994). As 

documented by microscopic observations and low differences in o1

SO values of coexisting quartz and feldspars, the 

Cadomian granitoids of the western part of LAZ are only 

weakly overprinted by Caledonian and Variscan metamorphism and hydrothermal activity. Most probably, the 

intensive Caledonian tectogenesis along the western margin 

of East-European platform in connection with the closure 

of the Tornquist Ocean (Oliver et al., 1993; Oliver and 

Kelley, 1993) has slowly affected only slightly the LAZ 

rocks. 

Traditionally, based on macroscopic observations, three 

main petrographic rock types have been distinguished 

within the Cadomian Lusatian Granodiorite Complex 

(LGC): two-mica granodiorites (tmgd), muscovite-bearing 

biotite granodiorites (mbgd) and biotite granodiorites 

(bgd). According to the Streckeisen classification, the 

rocks represent predominantly granodiorites and tonalites. 

Rarely, small stocks of monzogranites/ syeno-granites 

occur as the youngest intrusive phases. The rock association displays a large range of chemical compositions, 

reaching Si02 contents between 64 and 77 wt %. Concentrations of such elements as Ba, La, Hf, Rb, Sr, Th and 

Zr are nearly constant over the whole compositional range 

of LGC. This means that fractional crystallization did not 

greatly modify the compositions of granitoid magmas and 

modifications of whole rock chemistry are interpreted to 

have been caused by restite unmixing. 

There are clearly differences in geochemical characteristics between tmgd and bgd, with the mbgd as a transitional rock type. The tmgd broadly fits the classical 

definition of S-type granites (metasedimentary inclusions; 

high occurrences of rounded inherited zircons; 

peraluminous whole-rock chemistry; particularly high 

modal proportions of cordierite, garnet, sillimanite, 

monazite/xenotime, muscovite; low Ca and Sr contents; 

(

87Sr/86Sr)i = 0.7067 - 0.7101; eNd = -8.8 - -4.9; o180 

whole rock values of 9.0- 10.5 %o and o34S < 0). The 

bgd are clearly richer in CaO, Ti02 and P20 5 than the 

tmgd. Particularly low o180 values (7.5 - 9.5 %o), mostly 

metaluminous whole-rock chemistry, (

87Sr/86Sr)i between 

0.7041 and 0.7063; eNd = -6.7- -2.1 and high anorthite 

content in plagioclase cores (up to 55 mol % An) are not 

compatible with derivation from metasedimentary source 

rocks. 

Calculated source rock composition for the bgd (on the 

basis of 30 % restite-free partial melt and 70 % restite) 

corresponds to the average chemistry of tonalites. In 

comparison to worldwide averages of tonalite (K.H. 

Wedepohl, personal communication, 1994) the source 

rocks of bgd are richer in Ti02, K20, Cs, Cr, Ni, Hf, Nb, 

Zr, REE and poorer in CaO. Most probably, partial 

melting processes occur in a heterogeneously composed 

intermediate to mafic lower crust with Nd model ages 

between 1.13 and 1.86 Ga. Melting may be attributed to 

tectonothermal activity in the hanging wall of a subduction 

zone and may be induced by subduction-related, mantlederived underplated mafic magma. Coincidence of 

Cadomian granitoids and of the gravimetric anomaly of 

Bemsdorf-Kamenz can be used as an argument for intrusion of large amounts of mafic melts from the mantle. 

Mixing processes between mantle-derived mafic magmas 

and lower crustal granitoid melts are possible. Increasing 

(

87Sr/86Sr)i and decreasing eNd in the mbgd and tmgd in 

comparison to the bgd may be interpreted either as due to 

an increasing contribution of reworked metasedimentary 

source rocks or by relatively independent melting processes in metasediments, probably in higher crustal horizons. 
The geological evolution in LAZ region is comparable 

with the late Proterozoic/early Palaeozoic tectonomagmatic 

development of Northern Armorican Massif (Brown and 

D'Lemos, 1991) and with Cadomian orogenic activity in 

other circum-Atlantic, late pre-Cambrian Gondwanan 

marginal terranes, including southern portions of the 

Iberian Massif (Ochsner et al., 1993) and various sectors 

of North African orogens (Nance et al., 1991). 

Brown, M. and D'Lemos, R.S., 1991. The Cadomian granites of 

Mancellia, northeast Armorican Massif of France: relationship to the 

St. Malo migmatite belt, petrogenesis and tectonic setting. Precambrian. Res., 51, 393-427. 

Kroner, A., Hegner, E., Hammer, J., Haase, G., Bielicki, K.-H., 

Krauss, M. and Eidam, J., 1994. Geochronology and Nd-Sr 

systematics of Lusatian granitoids: significance for the evolution of 

the Variscan orogen in east-central Europe. Geol. Rundsch., 83, 357-

376. 

Nance, R.D., Murphy, J.B. and Strachan, R.A., 1991. Late Proterozoic 

tectonostratigraphic evolution of the Avalonian and Cadomian 

terranes. Precambrian Res., 53, 41-78. 

Ochsner, A., Schafer, H.-J., Gebauer, D. and Friedl, G., 1993, The 

timing and nature of the Cadomian geodynamic evolution. within the 

Ossa Morena Zone (SW Spain) on the basis of new Upb dating of 

various intrusive rocks. Te"a abstracts, supplement 1 to Terra nova, 

5, 320. 

Oliver, G.J.H., Corfu, F. and Krough, T.E., 1993. U-Pb ages from SW 

Poland: Evidence for a Caledonian suture zone between Baltica and 

Gondwana. J. Geol. Soc. London, 150, 355-369. 

Oliver, G.J.H. and Kelley, S., 1993. 40ArP9Ar fusion ages from the 

Polish Sudetes: Variscan tectonothermal reworking of Caledonian 

protoliths. N. Jb. Geol. Paliiont. Mh., 321-334. 

ORAL PRESENTATION 

Hydrodynamics of Magmatic and Meteoric 

Fluids in the Vicinity of Granite Plutons 

Hanson, R. Brooks 

Science, 1333 H Street, N. W., 

Washington, DC 20005, USA 

During emplacement and cooling of granitic magmas in 

the crust, fluid production from magma and wall rocks, 

thermal expansion of fluid, thermal and chemical buoyancy, topography, and thermal and mechanical deformation 

of rocks all interact to drive fluid flow. Mixing of fluids 

with rocks and other fluids produces ore deposits and 

alters rocks, and variation of fluid pressure, particularly 

related to degassing of a magma leading to hydrofractAbstracts 63 

uring, is a dominant control on permeability. Numerical 

models that account for the dynamics of these basic 

processes following emplacement of a granitic magma in 

the upper crust reveal the evolution of the hydrothermal 

system. In general, the hydrothermal system evolves from 

lithostatic fluid pressures and expulsion of internally 

generated magmatic and metamorphic fluids to hydrostatic 

fluid pressures and circulation of externally derived 

(meteoric) fluids with time; high permeability, low rates of 

fluid production, and shallow depth accelerate the transition. The relative importance of these processes and thus 

the locations where distinct fluids mix or react with rocks 

to produce ore deposits depends most on 1) permeability 

and its evolution, and 2) the history of fluid generation, 

particularly from the magma (figure). Fluid production in 

the magma and wallrocks is the dominant mechanism 

elevating fluid pressures to lithostatic values. For granitic 

intrusions, about 3 to 5 times as much magmatic fluids are 

produced as metamorphic fluids. Permeabilities of < 1 

microdarcy are required for generation of lithostatic 

pressures for reasonable rates of fluid production in 

granites with dimensions of several kilometers. Lower 

permeabilities, <0.1 microdarcy are required around 

small systems or stocks. Such low permeabilities are 

unlikely to be maintained once thermal fracturing ensues; 

the resulting drop in fluid pressure allows rapid influx of 

meteoric fluids. The tight interaction of fluid production 

and permeability in controlling fluid pressures means that 

near the surface and around small stocks dramatic changes 

in fluid pressure, flow regime, permeability, and interaction of rocks with magmatic and meteoric fluids are likely. 

Buoyancy, and the strong pressure gradient from 

lithostatic pressures in the intrusion to near-hydrostatic 

parts closer to the surface drive magmatic fluids upward. 

The continued production of magmatic fluids from the 

interior of the chamber as the magma crystallizes further 

isolates magmatic fluids from wallrocks. Near the surface, 

topography and hydrothermal circulation can act to 

transport magmatic fluids laterally. At depth, in the 

metamorphic aureole, elevated fluid pressures in the 

aureole induced by metamorphic fluid production and low 

permeabilities hinder lateral influx of magmatic fluids. 

Extensive lateral flow of magmatic fluids at depth requires 

creation and maintenance of a laterally enhanced permeability, sublithostatic fluid pressures in the aureole -

produced either by low rates of metamorphic 

devolatilization or high permeability ( > 1 microdarcy) -

and early release of fluids from the magma (figure). 

Extensive lateral flow thus becomes unlikely with increas-
64 The Origin of Granites and Related Rocks 

ing depth. Together, these patterns are consistent with the 

distribution and evolution of skarns and hydrothermal ore 

deposits around granitic magmas. 

•·•·w.,. •. ,...,..,,;·;:::~, 

200 ·: 

~( •[ :J 

~ 

I 

0.5rnd 

Figure: Evolution of the boundary between magmatic 

fluids and predominantly meteoric fluids (and some 

residual pore fluids) around a granitic intrusion in a model 

(numbers give time in 1000 years after emplacement). 

The magma, on the left, contained 5% water by weight, 

which was released linearly with cooling in this model. 

The intrusion was 4 km wide and was emplaced at 800°C 

at a depth of 5. 5 km. There was no metamorphic fluid 

production; low fluid pressures in wallrocks allowed 

extensive lateral influx of magmatic fluids. Upper crustal 

layer has a permeability of 0.5 millidarcy; lower layer has 

a permeability of 1 microdarcy. 

Experimental Study of the Distribution 

of H20 Between Cordierite and 

Peraluminous Melt, and its Bearing on 

the Generation and Evolution of 

Migmatites and Granites 

Harley, S.L. and Carrington, D. 

Department of Geology and Geophysics, 

University of Edinburgh, Edinburgh, Scotland 

The occurrence of cordierite in migmatites and S-type 

granitoids testifies to its significance in melt generation and 

crystallization under upper-crustal conditions. As 

cordierite is able to incorporate both C02 and H20 within 

its structure it is potentially an indicator of the proportions 

and contents of these volatile species in any coexisting 

fluid or melt. Specifically, the volatile contents of melts 

present in high-grade metamorphism may be estimated 

using cordierite as a monitor providing the distribution of 

H20 between melt and coexisting cordierite, Dw [ =H20 melt 

I H20crd1, is known. 

Estimates of Dw for H20-saturated conditions, derived 

from maximum solubility data for Mg -cordierite (Mirwald 

et al., 1979) and granitic melts (Holtz and Johannes, 

1994), are in the range 5 - 7. Experiments aimed at 

defining Dw for melt-cordierite over a range of melt H20 

contents from saturated (i.e. cordierite + melt + vapour) 

to highly undersaturated have been conducted at 5.0 kbar 

and 800-1000°C, and at 900°C for a range of pressures. 

Several cordierite-melt pairs have been run in an 

internally-heated gas apparatus at identical pressuretemperature conditions but with differing bulk H20. 

Natural cordierite (XMg = 0.66) and a gel of peraluminous 

granite composition have been equilibrated, forward 

experiments involving evacuated (volatile-free) cordierite 

and reversals using H20-saturated cordierite. H20 contents in product cordierite and melt have been determined 

using SIMS, with 1HP0Si isotopic ratios converted to wt % 

H20 through calibration curves derived from standards. 

These reversed experiments demonstrate that cordierite 

in equilibrium with melt can have a range of H20 contents 

at a single P-T condition. Hence, volatile-undersaturated 

cordierite coexists with H20-undersaturated melt. For 

example, at 5.0 kbar and 900°C cordierite varies from 0.5 

wt % to 1.8 wt % H20, the latter saturation value being in 

good agreement with extrapolations from Mirwald et al. 

(1979). Under these P-T conditions the melts vary from 

a minimum of 2 wt% H20 to maximum H20 contents (10 
wt %) that agree well with previous solubility studies 

(Holtz and Johannes, 1994). Dw is near-constant with a 

value between 3.5- 4.5 for melt H20 contents of less than 

5 - 6 wt % . At melt H20 contents above this critical 

range D w increases up to 6 - 7 at saturation, a behaviour 

ascribed to melt H20-solubility mechanism effects. Dw at 

saturation also increases with temperature. Experiments 

using starting cordierites with differing XMg show that Dw 

is independent of this parameter. 

These experimental results can be used to infer the 

volatile contents of the fugitive melts that formerly 

equilibrated with cordierite in migmatites and granites. 

SIMS analysis of cordierites in patch leucosomes from 

several terranes (e.g. Arunta Complex, Cooma, Rauer 

Group, Kerala, Madagascar, Willyama) reveals low H20 

and C02 contents of 0.6- 1.2 wt % and 0.1 - 0.7 wt % 

respectively (Harley, 1994). Melt H20 contents calculated 

from low-C02 cordierites using the experimental Dw data 

are in the range 2.8- 4.4 wt %, consistent with generation 

of the migmatites through dehydration-melting reactions 

involving biotite ( ± sillimanite). Cordierite has also been 

used to compare migmatites and S-type granitoids in 

selected terranes. At Mt. Stafford in the Arunta Complex, 

C02-poor cordierites in segregated S-type granite 

leucosomes are clearly distinct in their volatiles from 

cordierites in locally-developed patch migmatites, which 

contain mixed H20-C02 fluids. It appears that extensive 

melting in this terrane has been triggered by the influx of 

fluids probably derived from subjacent crystallising 

plutons. Cordierites in otherS-type granites and pegmatites 

are also H20-rich and close to volatile-saturated and 

signify formation either during wet melting or from a melt 

approaching saturation in H20. 

Harley, S.L., 1994. Cordierite as a sensor of fluid and melt distribution 

in crustal metamorphism. Mineral. Mag., 58A, 374-375. 

Holtz, F. and Johannes, W., 1994. Maximum and minimum water 

contents of granitic melts: implications for chemical and physical 

properties of ascending magmas. Lithos, 32, 149-159. 

Mirwald, P.W., Maresch, W.V. and Schreyer, W., 1979. Der 

Wassergehalt von Mg-Corderierit zwischen 500°C und soooc sowie 

0.5 und 11 kbar. Fortschr. Mineralogie, 51, 101-102. 

Abstracts 65 

A Detailed Sr-Pb Isotope Study of the 

Naivasha Comendites, Kenya 

Heumann, A., Davies, G.R. and Staudigel, H. 

Faculteit der Aardwetenschappen, Vrije Universiteit, 

Amsterdam, The Netherlands 

Quantification of the time scales over which silicic 

magmatic systems evolve is crucial to the understanding of 

how fast batches of silicic magma are produced and how 

long they survive in magma chambers prior to eruption. 

Recent comendites of the bimodal Naivasha (Olkaria) 

volcanic complex in the Kenya Rift Valley have elevated 

Rb/Sr ratios (Sr = 1.3 - 2 ppm; Rb = 521 - 819 ppm) 

that potentially allows the resolution of time differences on 

the order of 1 ka by conventional Sr isotope determination. For our study we used pristine obsidian glasses and 

minerals from the most chemically evolved comendites, 

which comprise two out of seven chemostratigraphic 

groups with distinct trace element and isotope systematics 

(Group III and VI, the latter being morphologically the 

younger). Individual lava groups have total eruptive 

volumes of < 5 cubic km. Ar-Ar isotope dating of 

sanidines from both groups cannot resolve an age and 

hence implies young eruption ages of < 10 ka. This 

conclusion is supported by Carbon-14 ages from overlying 

lake sediments. 

The comenditic glasses have relatively radiogenic Sr 

isotope ratios between 0. 707355 and 0. 707950. Rb-Sr 

glass isochrons define ages older than the eruption ages. 

The data do not defme mixing relationships on diagrams of 

Sr isotopes against 1/Sr content. The Pb isotope ratios of 

glasses from an individual group are indistinguishable but 

there is a small difference between Group III and IV 

<_206Pbf04Pb of 19.87 and 19.73). These data establish that 

the Sr isochrons are not a consequence of mixing and 

suggest magma production at 50 ka for the morphological 

younger Group VI and 20 ka for the older Group III. In 

order for an isochron to be preserved in a magma chamber, the magmas of each group must form rapidly relative 

to the error in the isochron age (10 and 5 ka respectively). 

Subsequent to formation, mixing within the magma 

chamber must be limited, because homogenisation of 

Rb/Sr will destroy an isochron relationship. Crustal 

contribution from wall rocks can also be ruled out since it 

would change the Rb-Sr systematics of the magma due to 

the low Sr contents of the comendites. Together, these 

data imply that the comenditic magmas had long residence 

times of up to 40 ka. 
66 The Origin of Granites and Related Rocks 

Over time the Rb-Sr systematics of a mineral in a 

magma chamber will reflect a combination of the growth 

of radiogenic Sr and diffusive re-equilibration with the 

surrounding melt. The Sr diffusion coefficient of 

K-feldspar are low such that < < 1 % of the Sr in the 

feldspar will be exchanged with the host magma over a 

time period of 50 ka. An amphibole separate from Group 

VI has Sr isotope systematics that lie on the 20 ka isochron 

possibly implying that this phase remained an isotopically 

closed system in magma chamber for about 20 ka. The Pb 

and Sr systematics of the feldspars, however, are not 

consistent with growth from the magma and subsequent 

long residence times. Euhedral grains from Group VI 

have Sr isotope ratios that plot above the Group VI glass 

isochron and this implies that either, they crystallized at 

- 13 ka or they have a xenocrystic origin. Feldspars from 

Group III have different morphologies, some samples 

contain euhedral grains while others are· rounded and 

appear resorbed. Sr isotope ratios are very variable, up to 

0. 727. The feldspars also have very variable Pb isotope 

ratios e06PbJ204Pb from 19.7 to 20.2) which suggests that 

they are either xenocrystic or restitic in origin. The 

regional basement is Archaean and has very unradiogenic 

Pb isotope ratios e06PbJ204Pb = 17 .0) so we are able to 

conclude that the feldspars are derived from the Miocene 

to Recent volcanics that form the majority of the upper 6 

km of the Rift Valley. The important conclusion from this 

study is that phenocrysts in an individual magma batch 

may have multiple sources and in order to use the Sr 

isotope systems for age information other isotope systems 

(Pb-Nd-Th) are required to prove minerals are co genetic 

with the host magma. 

Geochemical Constraints on 

Magma Sources of Mesozoic 

Continental Arc Plutonic Complexes, 

Andean Plate Boundary Zone, 

North Chile 

1

•

2

Hodkinson, D., 1Krogstad, E.J. 

and 1

Brown, M. 1

Department of Geology, University of Maryland 

at College Park, MD 20742, USA 2

Now at: Department of Geosciences, 

University of Arizona, Tucson, AZ 85721, USA 

During the late Triassic, Jurassic and early Cretaceous, 

an extensional magmatic arc was formed in the Andean 

Margin of North Chile. The crust into which arc magmas 

were emplaced is composed of Upper Paleozoic 

metasedimentary rocks, part of an accretionary complex, 

intruded by plutonic complexes of a broad PermianTriassic magmatic arc. Plutons were emplaced at ramps 

within a hinterland-propagating extensional duplex, likely 

fed by dikes that transferred magma through the crust 

( Grocott et al., 1994). We present geochemical data, 

including Nd and Pb isotope data, for one Early Jurassic 

plutonic complex, and Nd isotope data for one Early 

Cretaceous plutonic complex. These data allow assesment 

of source components and temporal change in magma 

sources during the evolution of this extentional continental 

margin. 

The Caldera-Pajonales Plutonic Complex (CPPC) is 

part of the Early Jurassic magmatic arc. The Complex is 

composed of hornblende-biotite-two pyroxene gabbros, 

with subvertical pre-full-crystallization fabric and magmatic layering, clinopyroxene-hornblende diorites, 

hornblende-biotite tonalites and granodiorites, and 

microgranites. It has yielded 40 Arf39 Ar hornblende plateau 

isotope correlation ages in the range c. 194 to c. 188 Ma, 

and a 40Arf39Ar plateau age on hornfels muscovite adjacent 

to the complex of c. 194 Ma (Dallmeyer et al., 1995); for 

modelling isotope evolution we take the age of the complex to be c. 190 Ma. Whole-rock geochemical data 

demonstrate calc-alkaline affinity of the complex, with 

characteristic features such as relative depletion in Ta, Nb, 

P and Ti and relative enrichment in Rb, Th, K and LREE. 

Although REE abundances are variable, and a gabbro 

sample exhibits a small positive Eu anomaly, 

tonalite/granodiorite and microgranite samples from the 

CPPC exhibit negative Eu anomalies consistent with an 
evolution that includes fractional crystallization of 

plagioclase. Normalized HREE values suggest hornblende 

is stable in the source rather than garnet. Ten samples, 

selected to cover the range of bulk rock compositions 

within the CPPC, have been analyzed for Nd isotopic 

compositions. The samples exhibit a range of eNd(190) 

from c. +6 to c. -3 without covariation withf(Sm/Nd). 

These data require a minimum of three reservoir components to satisfy the range of values within the complex (see 

figure below), as follows: depleted mantle ("DM"); shortterm LREE-enriched mantle or juvenile crust ("EM"); and, 

c. 1 Ga continental crust ( "CC "). 

0_

2 • Granitic 

• Granodioritic to 

0.1 Quartz Monzodioritic 

• Gabbroic 

.-.. 0.0 

"'0 

z -0.1 .......... 

E -0.2 CIJ 

'l:::"" -0.3 

-0.4 

-0.5 

-6.0 -3.0 6.0 9.0 

Figure: fi:Sm/Nd) vs. eNit) for ten samples from the Early 

Jurassic Caldera-Pajonales Plutonic Complex. "DM" is 

depleted mantle, "EM" is short-term LREE-enriched 

mantle or juvenile crust, and, "CC" is c. 1 Ga continental 

crust; K is the field defined by six samples from the Early 

Cretaceous Las Tazas Plutonic Complex. 

The age of the continental crust component is consistent 

with basement ages inferred from published U-Pb zircon 

(inheritance) and Rb-Sr isotopic studies of other plutonic 

complexes in the region. Whole-rock Pb compositions of 

samples from the CPPC have a large spread, >0.1, in 

inferred 207Pbf204Pb. Rocks with eNd from c. +6 to -0.3 

have 207Pb/204Pb near or below the Stacey-Kramers and 

Doe-Zartman (orogene) growth curves, except one granite 

(eNd = +0.3) that lies well above the curves. The lowest 

207Pbf204Pb samples have both the highest and lowest eNd 

(toM = 400 and 1200 Ma). Metasedimentary country rocks 

have strongly negative eNd (-9.5), but 207Pb/204Pb above the 

growth curves. These data require a minimum of two 

crustal sources: one high 207Pbf204Pb and the other low 

Abstracts 67 

207PbJ204Pb. Separation of these two sources must have 

occurred by at least the middle Proterozoic, to explain how 

their 207Pbf04Pb can be so different. The isotopic composition of the low 207PbJ204Pb source resembles that of 

distinctive eastern Laurentian Pb. Such a correlation would 

be consistent with recent tectonic models for South 

America that would give this area a Laurentian basement 

by the middle Paleozoic. The high 207Pbf204Pb, low eNd' 

crustal source from which the metasediments were derived 

may have had an Archean component. These Pb data are 

not amenable to simple interpretation with standard Pb 

growth curves, requiring instead a recognition of the 

isotopic variations that occur within the region, and within 

the individual plutonic complexes. 

Six samples from the early Early Cretaceous Las Tazas 

plutonic complex, which occurs inboard to the east from 

the CPPC, have eNd(130) = +5.1 to +6.4 andf(Sm/Nd) 

= -0.30 to -0.41. The magmatic source(s) of these 

samples is similar to the LREE-enriched mantle or juvenile 

crust component ("EM") required by the Nd data from the 

CPPC (see figure above). In contrast to the Early Jurassic 

magmatic arc, no older crust is suggested to have been 

involved in the Early Cretaceous magmatism. 

Dallmeyer, R.D., Brown, M., Grocott, J., Taylor, G.K. and Treloar, 

P.J., 1995. Mesozoic Magmatic and Tectonic Events Within the 

Andean Plate Boundary Zone, 26°-27°30'5, North Chile: Constraints from 40 Arf39 Ar Mineral Ages, in review. 

GrocottJ., Brown, M., Dallmeyer, R.D., Taylor, G.K. and P.J. Treloar, 

1994. Mechanisms of continental growth in extensional arcs: An 

example from the Andean Plate Boundary Zone, Geology, 22, 391-

394. 

ORAL PRESENTATION 

Insights From Igneous Reaction Space: A 

Holistic Approach to Granite Crystallization 

Hogan, J.P. 

School of Geology and Geophysics, 

University of Oklahoma, Norman, OK 73019, USA 

Petrologic investigations of granite commonly reveal 

multiple periods of mineral growth punctuated by resorption as well as compositional heterogeneity within constituent minerals. Such reaction textures and compositional 

variability are indicative of complex crystallization histories which ultimately must reflect changes in intensive 
68 The Origin of Granites and Related Rocks 

parameters during petrogenesis and solidification of these 

melts. Traditional petrologic tools, such as comparison 

with experimentally determined phase diagrams or 

Schreinemakers analysis, are effective for analyzing 

relationships among reduced subsets of the crystallizing 

assemblage. In contrast Reaction Space (Thompson, 1982) 

provides a holistic approach that can simultaneously model 

mineral reaction relationships, compositional variations, 

and changes in intensive variables for the complete mineral 

assemblage during crystallization. A general reaction space 

is presented for crystallization of peraluminous biotite-muscovite-garnet granites using the Northport pluton 

Maine, USA as an example. 

Distinct periods of crystallization, resorption, ± 

renewed crystallization for biotite, muscovite, garnet and 

feldspar in Northport granite are recognized from petrography and compositional differences. Compositional variation 

in these minerals can be correlated with relative position 

in the order of crystallization and are critical to constructing permissible crystallization paths within the reaction 

space. The following observations were used in the 

reaction space analysis of the Northport pluton. Plagioclase 

glomerocrysts, formed during the reaction PI + melt --->-

Ksp + melt (e.g., Hogan, 1993), was used to subdivide 

crystallization into an earlier (Ksp absent) and later (Ksp 

present) periods. Earlier and later crystallization was 

modeled by seperate reaction space volumes in order to 

facilitate visual representation. The initial melt composition 

is represented by whole rock compositions and later 

adjusted for fractional crystallization. Early crystallizing 

garnets are typically euhedral, 0.2 - 0. 75 mm in size, 

unzoned to reversely zoned, with rare inclusions of oxide, 

zircon, or monazite and are preserved as II armored 11 

inclusions in plagioclase. These garnets react with the melt 

and replaced by crystallization of bioite and muscovite. 

Late crystallizing garnets are euhedral to anhedral, 0.15 -

1.50 mm, display both reverse and normal zonation (even 

within the same crystal), and contain inclusions of muscovite, biotite, alkali - feldspar, and quartz. Compositional 

zonation in garnet corresponds to changes in the crystalliz-

. ing assemblage. Earlier and later periods of muscovite 

crystallization separated by a period of resorption, are also 

recognized. Anhedral embayed muscovite partially rimmed 

by quartz included in alkali-feldspar is interpreted as 

evidence for resorption. Fm 2

·s; ·Al_:vAI_~ and Fm 2Ti4+ AI_~ 

components (Fm 2

• = Fe, Mg, Mn ) in muscovite systematically decrease with progressive crystallization. A hiatus in 

this compositional trend is interpreted to correspond to the 

period of non-crystallization and resorption. Biotite 

exhibits similar compositional changes with crystallization 

and is absent from the fmal crystallizing assemblage which 

yielded minor muscovite-garnet granite. 

The following general insights concerning crystallization 

of peraluminous granites can be made from the topology 

of the linearly independent set of reactions that define this 

reaction space and the reconstructed crystallization path of 

the Northport granite: 1) there is no one unique reaction 

that produces or consumes aluminous minerals such as 

garnet; 2) mineral phases can alternate as reactants or 

products in different reactions accounting for textures 

indicating multiple periods of crystallization separated by 

resorption; 3) increasing J; 0 favors crystallization of H2 

biotite and muscovite at the expense of garnet yielding 

biotite-muscovite granites; and, 4) garnet-forming and 

biotite-consuming reactions near the solidus can give rise 

to muscovite-garnet granites, without necessarily requiring 

an increase in Mn activity. 

Hogan, J.P., 1993, Monomineralic glomerocrysts: Textural evidence for 

mineral resorption during crystallization of igneous rocks. J. Geol., 

101, 531-540. 

Hogan, J.P. and Dickenson, M.P., 1986, Application of reaction space 

to the crystallization of muscovite - biotite ± garnet granites: Geol. 

Soc. Amer. Abstr. Progs., 18, 638. 

Thompson, J.B., Jr., 1982, Reaction Space: An algebraic and geometric 

approach. In Ferry, J .M. (ed) Characterization of Metamorphism 

through Mineral Equilibria, Reviews in Mineralogy 10, Mineralogical 

Society of America, Washington, D.C., p. 33-52. 

Petrogenesis of A-type Sheet-Granites 

from an Ancient Rift 

1

Hogan, J.P., 1Gilbert, M.C., 1Price, J.D. 

and 2Wright, J.E. 1 School of Geology and Geophysics, 

University of Oklahoma, Norman, OK 73019, USA 2

Department of Geology and Geophysics, 

Rice University, Houston, TX 77005, USA 

The Southern Oklahoma Aulacogen (SOA) is one of 

several aulacogens that formed in the North American 

craton during breakup of the Laurentian Supercontinent in 

late Proterozoic to Cambrian time. Lithospheric extension 

was accompanied by voluminous bimodal igneous activity 

that can be subdivided into two distinct magmatic episodes 

separated by a period of uplift and erosion (Fig. 1). 

Evidence for coeval mafic and felsic magmatism within the 
rift is scarce and spatially restricted. Late Precambrian-Cambrian layered anorthositic gabbro intruded by 

small ( < 10 km2

) plutons of hydrous gabbro (primary 

biotite+amphibole) forms an extensive substrate ( > 1000 

km2

, 3-4 km thick) to the rift. Unconformably deposited 

on this gabbroic floor is a voluminous sequence ( -44,000 

km2 , - 1.4 km thick) of middle-late Cambrian subaerial 

A-type rhyolite flows with minor tuffs and agglomerates. 

Intrusive into the base of this volcanic pile are a series of 

-coeval thin ( -0.5 km thick) but laterally extensive 

(- 20-55 km in length) A-type granite sheets of which the 

Mount Scott Granite sheet is typical. Numerous basaltic 

dikes and rare porphyritic "rhyolite" dikes represent the 

final stages of rift-related igneous activity. 

Felsic igneous rocks associated with the SOA are clearly 

A-type. Plutonic rocks are typically leucocratic alkalifeldspar amphibole-biotite granites. Both hypersolvus and 

subsolvus granites are present. Zircon, titanite, ilmenite 

and magnetite occur as accessory minerals. The presence 

of magmatic fluorite is indicative of high fluorine contents 

for the melt. They are characterized by high Si02 (71 - 78 

wt %), low CaO ( < 1.5 wt %) and are meta-luminous to 

weakly peraluminous and subalkaline. They are enriched 

in Ba (e.g., Mount Scott, 799 - 1180 ppm), high field 

strength elements (e.g., Mount Scott, Zr, 415 - 554 ppm) 

and have high Ga/ Al ratios with minor /trace element 

abundance that have a "within-plate" signature. The 

granites crystallized from high temperature (Tzr- 900 -

950°C), H20-poor magmas emplaced non- explosively at 

low pressures (1Q2 bar range) high in the crust beneath an 

-coeval volcanic pile. 

Geochemical characteristics of SOA A-type granite/ 

rhyolite are in marked contrast with typical crustal melts 

and require derivation from mantle-like sources with minor 

crustal involvement. The absence of a Ta-Nb anomaly and 

Yb/Ta, Y /Nb and Ce/Nb ratios of the granites are more 

typical of Ocean Island Basalt (Fig. 2). In addition low 

initial 87 Sr/86Sr (0.7030- 0.7043) and positive eNd ( +3.3 

- + 3. 7) for Mount Scott Granite indicate a primitive 

source. Initial 87Sr/86Sr (0.7033 - 0.7041) and positive eNd 

( +2.8 - +4.3) for Late Diabase Dikes are indistinguishable from the granite/rhyolite suggesting a closely related 

origin for rift-related mafic and felsic rocks. The extreme 

scarcity of igneous rocks of intermediate composition and 

lack of a significant Eu anomaly in Mount Scott Granite or 

phenocryst-poor rhyolite argues against derivation of these 

felsic liquids by extended fractional crystallization of 

basaltic magma. We suggest A-type felsic melts of the 

SOA formed by partial melting at depth of mafic rocks 

Abstracts 69 

that were intruded early during the initial stages of rifting. 

The large volume of mafic and felsic igneous rocks, both 

with primitive sources, associated with the SOA indicate 

a substantial amount of new material can be added to the 

crust during lithospheric extension and should have 

important consequences for modeling the time-integrated 

geochemical evolution of crust and mantle reservoirs. 

WICHITA MOUNTAINS IGNEOUS COMPLEX 

Figure 1. Schematic Cross-section of Cambrian Igneous 

Stratigraphy for the SOA. 

i 

- 100 

z 

~ 

• i 

:& 

iii 10 

=e 

0 

E 

"C 

a. 

I Southern Oklahoma Aulacogen I 

Figure 2. Comparative "Spidergram" for Mount Scott 

Granite and Late Diabase, SOA 
70 The Origin of Granites and Related Rocks 

ORAL PRESENTATION 

Effects of Dissolved C-0-H Species on 

Chemical and Physical Properties of 

Ascending Granitic Magmas 

Holtz, Francois and Scaillet, Bruno 

Centre de Recherche sur la 

Synthese et la Chimie des Mineraux, 

CNRS, IA, rue de la Ferollerie, 

45071 Orleans Cedex 02, France 

The most abundant volatile species dissolved in common 

granitic melts belong to the system C-0-H (apart from 

highly differentiated melts). Pressure is known to influence 

strongly the solubility of volatile species in aluminosilicate 

melts. However, both individual (H-0, C-0) and combined effects of dissolved volatiles on physical and chemical properties of ascending magmas are still not well 

constrained. New experimental data show that previous 

available experimental datasets and calculation models are 

not always suited to predict physical (viscosity) and 

chemical properties (phase relations) of ascending magmas. 

The viscosity of hydrous granitic melts is commonly 

calculated using the empirical model of Shaw (1972). New 

experimental data for haplogranitic compositions with 0 to 

8 wt % water show significant discrepancies with calculated viscosities. For the investigated haplogranitic composition Qz28Ab380r34 (normative proportions) the model 

overestimates the viscosity by up to 1.5 log units at low 

water contents ( < 2 wt % H20) while it underestimates 

the viscosity at high water contents (less than 1 log unit). 

In addition, the predicted increase in viscosity with 

increasing Qz content is not verified for compositions that 

contain less than 30% normative Qz. The viscosity of 

Qz28Ab380r34 and Ab melts is not significantly different 

(for water contents higher than 2 wt % H20). Viscosity 

measurements also were carried out on a natural 

peraluminous leucogranitic composition. A comparison 

with the experimental data obtained for a metaluminous 

obsidian (Shaw, 1963) shows that the excess of aluminium 

has no significant effect on the viscosity. 

Water solubility was shown to increase significantly up 

to 3-4 kbar by Oxtoby and Hamilton (1978). At higher 

pressure, the water solubility was found to remain almost 

constant. This last finding has to be reconsidered in the 

light of new experimental data showing that water solubility still increases strongly up to at least 8 kbar (typical 

increase for a haplogranitic composition from 3 to 8 kbar 

is + 1.3 wt % H20/ kbar). In addition, water solubility in 

haplogranitic (quartzofeldspathic) melts is almost constant 

for a given Na/K ratio in the compositional range Qzo to 

Qz35 (normative proportions), while at higher Qz contents 

water solubility decreases. This observation, together with 

viscosity data, suggest that two incorporation mechanisms 

of water are involved in granitic melts. A "feldspar-HzO" 

mechanism is dominantly involved in melts with less than 

30-35% normative Qz, explaining the same effect of water 

on properties of Ab and QZz8Ab380r34melts. At higher Qz 

contents, the interplay between a "feldspar-H20" and a 

"Si02-H20" solubility mechanisms must be taken into 

account. It is worth pointing out that different water 

solubility mechanisms in haplogranitic melts are not taken 

into account in current solubility models used to predict 

phase relations under both H20-saturated and -

undersaturated conditions (e.g., Burnham and Nekavsil, 

1986). In addition, important parameters such as the alkali 

effect (Na/K) or redox conditions are not included in these 

models. 

Phase relations carried out on a metaluminous granite 

under different oxygen fugacities (NNO + 0.5 and FMQ0.5) and aH 0 , coexisting with a C-0-H fluid phase, show 

that fa has 2

a strong influence on the stability of both ironbearir{g (e.g., amphibole, biotite, oxides) and iron-free 

(tectosilicates) phases. Under reduced conditions (FMQ -

0.5) the thermal stabilities of all major phases are depressed by c. 40-50°C relative to oxidized conditions 

(NNO + 0.5). The solidus is however not affected by 

changes in redox conditions. Therefore, at fixed temperature and melt water content, the melt fraction is lower 

under reduced conditions than under oxidized ones. One 

major implication is that physical properties, such as the 

viscosity, of granitic magmas undergoing oxidation change 

significantly due to the decrease in melt fraction. In 

particular, the redox state may control in part the emplacement level of magmas. 

Burnham, C.W. and Nekavsil, H., 1986. Equilibrium properties of 

granite pegmatite magmas. Am. Mineral., 11, 239-263. 

Oxtoby, S. and Hamilton, D.L., 1978. The discrete association of water 

with Na20 and SiOz in NaAl silicate melts. Contrib. Mineral. Petrol., 

66, 185-188. 

Shaw, H.R., 1972. Viscosities of magmatic silicate liquids: an empirical 

method of prediction. Am. J. Sci. , 272, 870-893. 

Shaw, H.R., 1963. Obsidian-Hp viscosities at 1000 and 2000 bars in the 

temperature range 700-900"C. J. Geophys. Res., 68, 6337-6343. 
Comparative Granitoid Geochemistry 

and Lateral Repetition 

of Magmatic Arcs 

1

Horak, Jana M. and 2Gibbons, Wes 1 Department of Geology, 

National Museum of Wales, 

Cardiff, Wales, CFJ 3NP, UK 2

Department of Earth Sciences, 

University of Wales Cardiff, P.O. Box 914, 

Cardiff, Wales, CFJ 3YE, UK 

Lateral repetition of magmatic arc and forearc sequences during oblique plate convergence is a well documented phenomenon. Initiation of this process in a young 

arc, such as Sumatra, may lead to substantial strike slip 

dispersal and consequent juxtaposition of coeval, but 

originally widely separated arc segments, such as the 

Salinian sliver of the Sierra Nevada arc in California. In 

older orogens the recognition of such dispersed slivers may 

be obscured by modifications of primary geochemical 

signatures, tectonic overprinting and poor exposure. An 

excellent example of the problems associated with interpretation of ancient arc-related slivers is provided by the 

Neoproterozoic rocks of Southern Britain. Here a poorly 

exposed magmatic arc complex [Avalonia] is suspected of 

being riddled with intra-arc strike slip faults. The margin 

of this arc (exposed in Anglesey, Wales) preserves 

blueschists within a steep sinistral ductile fault zone, 

beyond which are further exposures of granitoid rocks 

(Coedana Granite). Previous interpretations of the rocks 

outboard of this ductile fault zone have suggested that they 

are suspect terranes with a non-Avalonian affinity. 

Granitoid geochemistry has been used as a tool in 

terrane characterisation in several published studies (e.g. 

Barr, 1990) either to compare directly the chemical 

signature of juxtaposed granitoid intrusions or to determine 

the nature of basement terranes by granites derived from 

them. To more closely determine the affinity of the 

granitoid component of the rocks outboard of the 

blueschist belt on Anglesey, a study was undertaken on the 

Coedana Granite, which proved to be an evolved mildly 

peraluminous, calc-alkaline, arc-related, biotite 

monzogranite pluton. It has three main facies, of which the 

most evolved (leucomonzogranite) is spessartine garnetbearing. Various of geochemical parameters such as a 

trend of increasing P20 5 with fractionation and eNd values 

of- 3.6 to 6.1 (at 600 Ma) suggest that the granite was 

Abstracts 71 

derived from a metaluminous crustal source with a crustal 

residency age of 1.3-1.5 Ga., rather than from the adjacent 

metapelitic migmatites as proposed by some previous 

workers. 

A U-Pb zircon age of 604 ± 4 Ma for the Coedana 

Granite falls within the main phase of Avalonian arc 

magmatism as defined by O'Brien et al. (1992). These 

data indicate that the outboard Coedana Granite rocks 

belong to the Avalonian. Furthermore they are associated 

with marine siliciclastic metasediments containing abundant 

arc material and a chaotic melange unit bearing Coedana 

Granite clasts. The Coedana Granite and the arc plutonic 

complexes inboard of the ductile fault zone are exactly 

coeval in age and have similar Sm-Nd isotopic signatures 

(e.g. 1.3-1.5 Ga for the Coedana Granite compared to 1.2 

-1.6 Ga for the most NW expression of the Avalonian arc 

in Southern Britain), however, no direct correlation 

between the Coedana Granite and any specific plutons in 

the Avalonian of Southern Britain can be made. It is 

concluded that the Coedana Granite was derived from 

Avalonian lower crust closely resembling that in Southern 

Britain, but has been transported along the edge of the arc 

by sinistral faulting. The distance travelled by this forearc 

sliver, fortuitously preserved within the younger 

Caledonian Orogen, is unknown, but is likely to be 

considerable. 

Barr, S.M., 1990. Granitoid geochemistry and terrane characterisation: 

an example from the northern Appalachian Orogen. Geol. J., 25, 295-

304. 

O'Brien S.J., O'Driscoll, C.F., Tucker, R.D. and Dunning, G.R., 1992. 

Four-fold subdivision of the Late Precambrian magmatic record of the 

Avalon Zone type area (east Newfoundland): nature and significance. 

GAC-MAC Prog. Abst., 17, A85. 
72 The Origin of Granites and Related Rocks 

Late Neoproterozoic A-Type Granites 

and Rhyolites Associated with Rift-Related 

Clastic Rocks Within the 

Grenvillian Basement of Virginia 

1

Hutson, F .E. and 2Tollo, R.P. 1

Department of Geological Sciences, 

University of Texas,Austin, TX 78712, USA 2

Department of Geology, George Washington University, 

Washington, DC 20052, USA 

Integrated sedimentologic and petrologic studies of late 

Neoproterozoic clastic sediments and anorogenic granitoids 

and rhyolites exposed within 1.1-1.0 Ga basement rocks of 

the Blue Ridge anticlinorium (BRA) of central Virginia 

demonstrate a clear tectonic relationship between 

anorogenic igneous activity and rift basin sedimentation. 

A combination of field, petrographic, geochemical, and 

isotopic data have enabled us to determine the spatial and 

temporal relationship of rift-related plutonism, volcanism, 

and sedimentation. 

Late Neoproterozoic metasedimentary rocks of the 

Mechum River Formation (MRF) nonconformably overlie 

basement rocks of Grenvillian age and c. 0. 73 Ga 

anorogenic granitoids of the Robertson River Igneous Suite 

(RRIS). The MRF and RRIS are temporally correlative 

with rift sequences exposed throughout the southern 

Appalachians and, on a much larger scale, to sequences 

exposed on all margins of Laurentia. These sedimentary 

and igneous rocks are interpreted as part of a c. 0. 7 Ga 

extensional event which preceded a c. 0.6 Ga rift event 

that led to opening of the Iapetus Ocean. 

The MRF is composed of conglomerates, sandstones, 

and mudstones that are variably deformed and metamorphosed under upper greenschist facies conditions. The 

metasediments are exposed in a 100 km long by 0.5-3 km 

wide belt that has been down-faulted into the core of the 

BRA. The MRF experienced the effects of multiple 

Paleozoic tectonic events; nevertheless, it retains identifiable features associated with many modem rift basins. 

These include areal dimensions, grain size trends, facies 

patterns, lithologic associations, and structural style. For 

example, the present length and structurally restored width 

of the MRF outcrop belt is broadly equivalent in dimensions to many East African rift basins. Additional evidence includes increasing grain size toward the margins of 

the MRF belt and depositional environments that range 

from alluvial fan to lacustrine. A major identifying 

characteristic of many but not all rift basins is the presence 

of magmatic rocks, commonly volcanics, which are 

associated with the sediments. Prior to our studies, 

volcanic rocks had not been found within the MRF and the 

relationship of the c. 0. 73 Ga anorogenic granites of the 

RRIS to the MRF sediments was unclear. 

Field, petrographic, and geochemical studies of rocks 

from the RRIS, MRF, and a limited number of samples 

from the Grenvillian basement enabled us to identify the 

source of some clasts within conglomerates of the MRF 

and demonstrate that rocks previously mapped as MRF 

clastic rocks are felsic volcanic rocks. Clast count data 

indicate a large proportion of the clasts are composed of 

relatively undeformed granitoids, which is in direct 

contrast to the lithologic character of the presently exposed 

Grenvillian basement that is dominated by orthogneisses. 

Point count data from the clasts overlap data from the 

RRIS but these data are not clearly distinguishable from 

some Grenvillian granitoids. However, elevated concentrations of high field strength elements (HFSE) and light 

rare earth elements (LREE) were found for clasts that are 

similar in petrography to units of the RRIS. This same 

pattern of geochemical enrichment is typical of the RRIS. 

In addition, a local nonconformable relationship was 

recognized between a specific unit (U-Pb zircon age of 729 

Ma) of the RRIS and the MRF. Clasts from this unit are 

present within the basal portion of the MRF above the 

nonconformity. The combined field, petrographic, and 

geochemical evidence strongly supports the derivation of 

many of the MRF clasts from the RRIS. A detailed study 

of a MRF sub-basin that contains rocks previously mapped 

as sediments revealed the presence of rhyolites and 

associated volcaniclastic rocks. The evidence of a volcanic 

origin for these presumed clastic rocks includes: 1) the 

presence of euhedral grains of feldspar and quartz, 

volcanic rock fragments, and spherulites contained within 

a very fine-grained, white matrix and 2.) whole-rock 

geochemical data that display highly elevated concentrations in HFSE and LREE. These rhyolites can be clearly 

distinguished from sedimentary rocks, including samples 

of the MRF. More importantly, there is a strong geochemical correlation between these rhyolites and nearby 

mildly peralkaline subvolcanic rocks (U-Pb zircon age of 

702 Ma) of the RRIS. 

U-Pb isotopic dating of specific units of the RRIS 

combined with the data outlined above, make it possible to 

demonstrate a direct link between anorogenic magmatism 

and rift-related sedimentation. In addition, we are able to 
constrain the maximum depositional age of the MRF to 

0. 73 Ga and further demonstrate that sedimentation and 

volcanism were occurring at 0. 7 Ga. These integrated 

studies provide a unique perspective on the coeval relationship of anorogenic plutonism/volcanism and extension-related sedimentation. 

A Mechanism for the 

Segregation and Collection 

of Crustal Melts 

Jackson, M.D., Cheadle, M.J. 

and Atherton, M.P. 

Department of Earth Sciences, 

University of Liverpool, 

Jane Herdman Laboratories, PO Box 147, 

Brownlow St., Liverpool, L69 3BX, UK 

Partial melting of the lower crust can be caused by the 

intrusion of mantle-derived basalt, which acts as a heat 

source. Melting begins along grain boundaries and on 

grain faces, quickly forming an interconnected network of 

melt. Initially, the distribution of melt will range from a 

maximum next to the heat source, to zero at the position 

of the solidus isotherm. As melt migrates upwards, and 

the source region heats up, both the spatial distribution of 

melt and the position of the solidus isotherm change with 

time. Porosity falls to zero at the solidus isotherm, so 

melt migrating along grain edges cannot move beyond this 

point. If melt moves through an anastomosing network of 

small fractures, these fractures may propagate beyond the 

solidus isotherm, but the melt will quickly freeze in the 

cold region above. So effectively, the solidus isotherm 

acts like a "lid" on the top of the source region. Intuitively, if the melt migrates upwards faster than the "lid" 

migrates upwards, then melt will pond beneath the lid at 

the top of the source region. 

We present a dynamic model of the physical processes 

which govern crustal melting, to test this intuitive hypothesis. We assume that the lower crust is heated by the 

intrusion of sills of mantle-derived basalt, and that melt 

migrates by buoyancy driven porous flow and compaction 

of the matrix. The effect of melt moving along grain 

boundaries, or through a fracture network, is simulated by 

varying the permeability function. Both porous flow 

networks and small fracture networks are efficient heat 

Abstracts 73 

exchangers, so the melt is always in thermal equilibrium 

with the matrix. Hence the temperature at any point is 

described by one temperature variable. The basalt heat 

source is represented in the model as a "hot plate" at the 

spatial origin, which is switched on at time t = 0. The 

boundary condition on flow is that melt velocity = matrix 

velocity = 0 at the spatial origin. With these initial and 

boundary conditions, the 1-D equations that govern heat 

transport, melting, and resulting gravity-driven porous 

flow and compaction are solved numerically. 

We have applied this model to the generation of granitic 

melts from a basaltic protolith. Our results suggest that, 

with a maximum of 50 vol % melting at the hot plate, 

buoyant melt migrates upwards faster than the solidus 

isotherm. The melt collects below the solidus isotherm, 

increasing the porosity to >50% near the top of the 

source region, and decreasing the porosity to < 10% at the 

base of the source region. The high porosity region, 

effectively a "melt lens", is of the order 1km thick and 

forms in < lMa; a realistic timescale for a crustal melting 

event. As melt moves up through the source region it 

constantly thermally re-equilibrates with cooler matrix. 

Consequently, the melt in the "lens" near the top of the 

source region has thermally equilibrated with relatively 

cool matrix, and its composition reflects this; i.e. the melt 

in the lens has a composition corresponding to a 20% melt 

of the proto lith, but volumetrically occupies >50% of the 

host. Our results therefore differ from those of Fountain 

et al. (1989), primarily because we take into account the 

equilibration of melt and matrix temperatures as each 

move. We envisage that it is this melt, in the pool at the 

top of the source region, which is free to ascend to upper 

crustal levels and is emplaced to form a pluton. 

Fountain, J.C., Hodge, D.S. and Shaw, R.P., 1989. Melt segregation in 

anatectic granites: A thermomechanical model. J. Vole. Geotherm. 

Res., 39, 279-296. 
7 4 The Origin of Granites and Related Rocks 

Interaction of Tectonic-Related Strain Fields 

During Pluton Construction; Argyll Suite, 

SW Grampian Highlands, Scotland 

1

Jacques, J.M. and 2Reavy, R.J. 1

Department of Geological Sciences, University of 

Durham, South Road,. Durham, DH1 3LE, UK 2 Department of Geology, University College Cork, Eire 

Within the Argyll Suite, SW Grampian Highlands, 

integration of data show that these plutons have been 

emplaced into crust within which it is possible to distinguish tectonically induced strain fields, established by the 

interaction of shear zones, from strain fields associated 

with pluton expansion. The imposition of strain fields by 

NE-SW -trending shear zones and faults related to the 

Caledonian transpressional collision are recognised as 

being distinct from strain fields imposed by an intersecting 

set of NW -SE-trending pre-Caledonian crustal lineaments 

which were reactivated during Caledonian orogenesis. The 

combination of data, such as the overall shape, distribution 

of intrusive phases, fabric intensities, strain distribution 

and local emplacement phenomena, can be used to determine the dominant controlling structure during pluton 

construction, relative to depth of crystallization. This 

fundamental distinction has led to the recognition that 

pluton construction at relative depths of approximately 10 

km or greater is dominantly controlled by NW -SE-trending 

structures, whereas pluton emplacement near to the free 

surface and up to approximately 6 km in depth is dominantly controlled by NE-SW -trending faults and shear 

zones. 

It is envisaged that transpressive motion along the 

Caledonian NE-SW -trending shear zones is transferred to 

non-transpressive motion at lower crustal levels, the 

possible transfer zone corresponding to the upper detachment surface which bounds the lower crustal reflectivity 

zone (LCRZ). It is suggested that the lower crust inherits 

the dominant structural characteristics of the crust above, 

producing within the LCRZ a series of blocks bounded by 

anastomosing shear zones. Within the LCRZ it is foreseen 

that non-transpressive deformation is regionally dominant, 

allowing the rotation and interaction of the low strain 

mega-augen, leading to the development of a structural 

framework which may help to explain the location of 

anatectic zones, siting and type of ascent pathways and 

subsequent emplacement phenomena in orogenic belts. 

Evolution of Granites and Continental 

Crust by Dehydration-Melting of 

Amphibolites and Tonalites 

1

Johannes, Wilhelm and 2Singh, Jagmohan 1 University of Hannover, 30167 Hannover, Germany 2

G.N.D. University, Amritsar 143005, India 

Granitic rocks constitute a major proportion of the 

upper continental crust, and granite magmatism is an 

important phenomenon in the crustal differentiation leading 

to a granulitic lower and granitic upper continental crust. 

The evolution of the continental crust may have begun 

with the generation of a thick pile of basaltic material 

including amphibolites, granulites and eclogites. This first 

episode is mantle related and is followed by three 

intracrustal processes: 1) generation of tonalitic magmas 

by partial melting of amphibolites, eclogites, etc.; 2) 

subduction, overthrusting, etc. of tonalites and their 

sedimentary equivalents; and, 3) formation of 1-type and 

S-type granitic magmas by dehydration-melting of 

tonalites, metagreywackes, etc. This model is based on 

geochemical characteristics of upper mantle and crustal 

rocks (Wedepohl, 1991). 

Tonalitic magmas may be mainly generated by 

dehydration-melting of amphibolites. This process is often 

believed to take place at high P-T conditions. However, 

Wyllie and Wolf (1993) demonstrated that dehydrationmelting of amphibolites may begin already at 650°C at 

high pressures (above 10 kbar) and at > 900°C below 10 

kbar. Our investigations below pressures of 10 kbar 

yielded beginning of dehydration-melting temperatures as 

low as 850°C. The compositions of the partial melts are 

identical to those of natural tonalites. Garnet forms at 10 

kbar (at 900°C) and indicates that generation of tonalitic 

melts depleted in HREE, typical for most Archean 

trondhjemites and tonalites, can be an intracrustal process. 

The composition of partial melts seems to be mainly 

controlled by the composition of the parent rock. 

Dehydration-melting of tonalite (quartz-plagioclasebiotite assemblages), depending upon the composition of 

biotite, begins between 710 and 760°C. Orthopyroxene, 

clinopyroxene, potassium feldspar and anorthite-rich 

plagioclase are the main residual products. The composition of melt ranges from granitic to granodioritic, and is 

controlled by the degree of partial melting (increasing with 

1) and the starting composition. 
Thus, the dehydration-melting of amphibolites leads to 

an increase of Si02, Na20 and K20 in the melt and FeO, 

MgO and CaO in the residual minerals. Dehydrationmelting of tonalites contributes to a further increase of 

Na20 and K20 in the melt and concentration of FeO, MgO 

and CaO in the residuum. The increase in Si02 here is less 

pronounced. The experimental investigations support the 

model of evolution of the continental crust into a granitic 

upper and granulitic lower crust by dehydration-melting of 

amphibolites and tonalites. 

Wedepohl, K.H., 1991. Chemical composition and fractionation of the 

continental crust. Geol. Rundsch., 80, 207-223. 

Wyllie, P.J. and Wolf, M.B., 1993. Amphibolite dehydration-melting: 

Sorting out the solidus. In Prichard, H.M., Alabaster, T., Harris, 

N.B.W. and Neary, C.R. (eds), Magmatic Processes and Plate 

Tectonics. Geol. Soc. London Spec. Publ., 76, 405-416. 

Evidence for Magmatic Fracturing and 

Small-Scale Melt Segregation During 

Late Stage Pluton Emplacement 

1

John, Barbara E., 2Stunitz, Holger and 3

Blundy, Jonathan D. 1 Department of Geology and Geophysics, 

University of Wyoming, Laramie, WY 82071, USA 2

Geologisches-Paleontologisches Institut, 

Universitat Basel, Basel, Switzerland 3

Department of Geology, University of Bristol, 

BS8 JRJ, Bristol, UK 

Geologic evidence for the generation, segregation, 

ascent and emplacement of silicic magmas is fragmentary 

at best. Experiments and/or natural examples documenting 

the processes of silicic melt segregation during emplacement are rarely observed. Superb natural examples of 

residual melt distribution in a silicic magmatic system are 

preserved in the southern Adamello Massif, Italy, and 

provide insight into the processes of magmatic fracturing 

and melt segregation. 

Hydrous, calc-alkaline magmas that crystallized to form 

plutonic rocks ranging in composition from quartz diorite, 

through tonalite to granodiorite, crop out in the Lago della 

Vacca Suite, southern Adamello Massif. The suite is 

exposed as a small ( - 10 km2

), compositionally and 

temporally zoned pluton, emplaced at -3.5 kbar pressure. Textural evidence for melt segregation associated 

Abstracts 7 5 

with emplacement of the suite correlates with the intensity 

of foliation development and 3-D enclave strain estimates 

determined in the field. The most intensely foliated rocks 

occur in the older marginal units, as anastomosing, sinuous 

circumferential zones that wrap around lenses of less 

intensely foliated granitoid. These zones vary in width 

from millimeters to hundreds of meters, and extend for 

tens to hundreds of meters along strike. Contacts between 

the zones of intense deformation and their host are remarkably abrupt without strain gradient or intermediate grain 

size development, and are observed on the scale of < 1-2 

mm. This relationship implies an extremely steep strain 

gradient across the contact, resulting from migration and 

accumulation of interstitial melt in small dike-like features. 

Amphibole-plagioclase thermometry indicates deformation 

temperatures in the magmatic or "sub magmatic" state 

( -780 - 650°C). 

The deformed marginal units are finer-grained with 

documented increases in magmatic deformation. Macroscopically, these zones show a strong shape preferred 

orientation of plagioclase and hornblende parallel to their 

margins. Mafic enclaves in these zones have oblate shapes 

whose aspect ratios correspond to shortening values in the 

adjacent country rocks. Zones of the most intense foliation development are typically very fine grained (subequant 

grains ~50p.m), and have a strong crystallographic 

preferred orientation of hornblende and plagioclase. 

Microscopic evidence for melt migration and segregation comes from these zones of intense foliation and fine 

grain size, and comprises intra- and intercrystalline 

fractures and veinlets cutting marginal units of the suite. 

In regions of small volume melt segregation, and relatively 

low measured flattening strains, fringing discrete grains 

( ~ 50 p.m) of plagioclase and quartz are dispersed along 

grain boundaries. Angular intragranular cracks (20-30 p.m 

wide) cutting oriented, zoned plagioclase grains are filled 

either with slightly more sodic plagioclase or quartz. With 

an increase in volume percent melt, both intra- and 

intergranular fractures coalesce to form through-going 

veinlets or anastomosing bands of fine-grained material. 

The veinlets ar~ filled with non-eutectic composition melt, 

comprising the same mineralogy as their host; biotite often 

has a greater modal abundance (often at the expense of 

amphibole), and plagioclase a homogenous composition. 

Their texture is "aplitic", with polygonal boundaries 

between grains that are free of chemical zoning and optical 

strain. 

Documented shape fabrics and measured enclave strains 

indicate substantial shortening normal to the walls of the 
7 6 The Origin of Granites and Related Rocks 

veinlets while they were still in the magmatic state. 

Deformation was clearly concentrated in the zones of 

weakness, that is the veinlets of segregated melt. We 

suggest that deformation of the older, marginal units of the 

Lago della Vacca Suite was associated with non-hydrostatic 

stress (i.e. intrusion of subsequent magma pulses), which 

led to deformation assisted melt segregation, a potentially 

important deformation process in late stage pluton emplacement. 

ORAL PRESENTATION 

New Approaches to Crustal Evolution 

Studies and the Origin of Granitoids 

Through the Lu-Hf and Re-Os 

Isotope Systems 

1

Johnson, Clark M., 1Barovich, Karin, M. 

and 2

Shirey, Steven B. 1

Department of Geology and Geophysics, 

University of Wisconsin, Madison, WI 53706, USA 2

Department of Terrestrial Magnetism, 

Carnegie Institution of Washington, 

Washington DC, USA 

Several decades of Sr, Nd, and Pb isotope studies of 

granitoid rocks have highlighted the diverse origins of 

crustal igneous terranes, have supported models for crustal 

melting and differentiation of mafic magmas, and quantified the proportions of crust and mantle inputs to crustal 

magmatic systems. Recent N d isotope work on Western 

North America Cordilleran batholiths and volcanic fields 

have identified large mantle-derived basalt fluxes that may 

accompany magmatism that is associated with orogenic and 

extensional terranes. These basalt fluxes result in significant hybridization of ancient crust and can constitute 

addition of new crust at depth. Such processes are 

difficult to identify using Sr and Pb isotope ratios due to 

large uncertainties in the Sr isotope compositions of the 

crust and the minimal Pb flux from the mantle during 

injection of basaltic magmas into the crust. Geophysical 

and crustal xenolith studies indicate that the lower crust 

beneath the Cordillera may be mafic and garnet-rich, 

reflecting high-pressure crystallization of mafic magmas 

and/or metamorphism. Mafic, garnet-rich lower crust has 

Lu/Hf and Re/Os ratios that are distinct from those of 

silicic crust, or mafic crust at intermediate to shallow 

depths (non-garnet bearing), and therefore Hf and Os 

isotope ratios offer a new opportunity to evaluate the 

depth, chemical composition, and mineralogy of this type 

of crust if it is involved in granitoid petrogenesis. 

Because garnet is highly influential in Lu/Hf partitioning 

during melting or metasomatic processes in the mantle and 

crust, distinct Hf isotope compositions may be produced 

that are dependent upon the role of garnet and thus the 

depth and extent of melting in the crust. Orogenic 

Proterozoic and Archean granitoids have distinctly 

non-radiogenic Hf isotope compositions as compared to 

II enriched II (low eNd) sub-oceanic or sub-continental 

mantle. This indicates derivation from depleted garnet 

peridotite or garnet -bearing lower crust that was directly 

derived from the mantle with minimal prior residence in 

the crust. Granitoids that have these unique Nd-Hf isotope 

variations represent the only known Hf isotope mass 

balance to the depleted mantle that has radiogenic Nd and 

Hf isotope compositions (i.e. high eHf MORB). In 

contrast, mid-Proterozoic anorogenic granitoids from 

Western North America have anomalously radiogenic Hf 

isotope compositions for their non-radiogenic Nd isotope 

ratios that are likely to reflect intra-crustal melting and 

garnet growth during earlier, orogenically-related crustal 

hybridization and metamorphism. Although Sr, Nd, and 

Pb isotope compositions of the anorogenic granitoids 

indicate a major crustal component, these isotope systems 

cannot place constraints on the mineralogy of the deep 

crust that was involved in the genesis of the rocks. 

Although most work on terrestrial samples using the 

Re-Os isotope system has focused on mantle-derived rocks, 

new work on Proterozoic granitoids from the Penokean 

orogenic belt in the north-central USA and the Mesozoic 

Sierra Nevada batholith indicates that the Re-Os isotope 

system promises to provide important insights into evolution of the continental crust. Re/Os ratios of mafic rocks 

are well known to be exceptionally high, which has lead to 

the assumption that most continental crust has highly 

radiogenic Os isotope compositions and very low Os 

contents; these characteristics would be formidable obstacles to obtaining meaningful initial Os isotope ratios in 

ancient crustal rocks. New work on oxide minerals in 

granitoids indicate that Os contents in these minerals are 

enriched by up to several orders of magnitude relative to 

whole rocks and that they have relatively low Re/Os 

ratios, allowing precise calculation of initial Os isotope 

compositions. Moreover, Re/Os ratios apparently decrease 

strongly in oxide minerals from silicic rocks, suggesting 
that silicic Precambrian continental crust may have 

relatively non-radiogenic Os isotope compositions. Such 

a conclusion contrasts with previous estimates for the 

average Os isotope composition of the continental crust 

based on oceanic sediments, which are highly radiogenic. 

Future work lies in combining Nd, Hf, and Os isotope 

studies on granitoid rocks as a probe of the mineralogy 

and bulk chemical composition of the crust involved in 

generation of silicic crustal magma systems. 

Anatectic Melt Generation and Migration 

in a Shear Zone Associated with a 

Lower Crustal Mafic Intrusion, 

Ivrea-Verbano Zone, Italy 

Kalakay, Thomas J. and Snoke, Arthur W. 

Department of Geology and Geophysics, 

University of Wyoming, Laramie, WY 82071-3006, USA 

The Ivrea-Verbano zone, northern Italy, is a sample of 

lower continental crust that was upthrusted and exhumed 

during the Alpine orogeny. Uplift and tilting has resulted 

in a natural cross-section that exposes metasedimentary and 

metabasic rocks that were intruded at lower crustal levels 

in the late Paleozoic by a heterogeneous mafic to intermediate igneous complex. Amphibolite- to granulite-facies 

metamorphism, widespread anatexis, and intense deformation accompanied the underplating or intraplating of 

mantle-derived mafic magmas. 

Detailed geologic mapping indicates that the contact 

between the igneous complex and its wallrocks is a 

deformed, intrusive contact. A kilometer-scale zone of 

intense, high-temperature deformation, traced for at least 

20 km, is localized in the wallrocks. This amphibolitegrade shear zone, now steeply dipping, is characterized by 

highly strained pelitic paragneiss, scattered mafic and 

calcareous layers, and locally abundant leucosome. Shear 

zone fabrics include: transposition of earlier foliation(s), 

isoclinal folding, and widespread boudinage of competent 

layers. Qualitative assessment of strain intensity across the 

shear zone suggests a strain increase toward the intrusive 

contact. A mineral lineation, interpreted to have developed during magmatic emplacement, is defined by northeast-plunging bundles of sillimanite. In thin-section, the 

strongly oriented sillimanite is intergrown with other peak 

metamorphic phases such as garnet. Individual garnet 

Abstracts 77 

grains have an elongate shape-fabric subparallel to the 

sillimanite bundles. Crystal-plastic mylonitic fabrics are 

scarce, isolated, and overprint the high-temperature, 

crystalloblastic fabrics. The coincident timing of wallrock 

deformation with magmatic emplacement is indicated by 

cross-cutting mafic dikes that are deformed in the aureole. 

Sense-of-shear data indicate an overall oblique-sinistral, 

non-coaxial deformation in their present steep orientation. 

Mesoscopic shear-sense indicators include: asymmetric 

mafic boudins, shear folds, and rotated mafic enclaves. At 

the micro-scale, asymmetric pressure shadows around 

garnet demonstrate sinistral shear and are therefore 

consistent with the mesoscopic features. Within leucosome-rich portions of the shear zone, there are equal 

numbers of rotated enclaves for either sense of shear. 

This suggests that kinematic interpretation may be difficult 

in regions where overall rock viscosities have been 

substantially lowered by large melt volumes present during 

deformation. 

Metasedimentary wallrocks are mostly stromatic 

migmatites with leucosomes occurring both as discontinuous lenses and in more continuous sheets interlayered with 

biotite+garnet-rich selvages. Lithologic banding occurs 

on a centimeter to decimeter scale. Melting and deformation were coeval as evidenced by leucosome (frozen melt) 

concentrations found in boudin necks and extensional shear 

fractures. 

Leucosome concentrations vary within the shear zone. 

However, a consistent gradient exists with lowest ( < 20%) 

leucosome concentrations at or near the contact. Highest 

concentrations ( > 45%, with local concentrations > 65%) 

exist some 1-1.5 km away from the contact, coincident 

with the outermost limit of shear zone fabrics. Near the 

highly deformed interface with the pluton, wallrocks rich 

in garnet and sillimanite are interpreted as restites from 

which melt has been extracted. Leucosome compositions 

from the structurally higher, melt-rich zone, do not always 

reflect a minimum melt composition of the rocks in which 

they reside. This suggests that the melts have migrated 

from the point of generation to their present position, by 

vertical melt migration within the shear zone. and that it 

was the melts that migrated and not the melting zone. 

North of the Mastellone River, the eastern contact of the 

mafic complex curves westward into a large antiform, 

forming the flank or "sidewall" of the intrusion. In this 

region, the intense non-coaxial strain, characteristic of the 

eastern contact zone, grades into a domain of relatively 

low strain. For a distance of up to 2 km from the contact, 

the wallrocks are characterized by alterations of 
78 The Origin of Granites and Related Rocks 

quartzo-feldspathic leucosome with garnet- and 

sillimanite-rich gneiss. In the field, leucocratic layers 

commonly display coarse-grained igneous textures and 

locally comprise greater than 60% of a total outcrop. 

Based on the compositions of the leucosomes and those of 

the surrounding granulite-facies country rocks, it is not 

likely that these rocks represent locally-derived melts. 

Instead, they may correspond with crustal melts generated 

along the eastern contact. If so, the "sidewall" region may 

be viewed as a large pressure shadow or dilational zone 

adjacent to the intrusion. Such a low-pressure domain, 

positioned next to a melt-filled, low-angle shear zone, 

would result in a sub-horizontally oriented pressure 

gradient driving layer-parallel migration of melts within 

the shear zone. 

Neogene Granitoids in 

North Fossa Magna, 

Central Japan 

Kawano, Yoshinobu 

Department of Geology, 

Faculty of Education, 

Saga University, Honjo 1, 

Saga 840, Japan 

The Fossa Magna is one of the green tuff regions in 

Japan; it exposes the contact between the Northeast and 

Southwest Japan Arcs, which are different geological units. 

In this area, many Neogene granitoid rocks occur. These 

granitoids are characterized by low K20/Na20 ratios and 

low tin contents, and were considered to have been derived 

from an upper mantle source (Ishihara et al., 1976). 

However, granitoids in the South Fossa Magna have been 

investigated in detail, whereas those in the North Fossa 

Magna have not. New geochemical and isotopical data for 

granitoids in the North Fossa Magna will be discussed. 

The granitoids of the North Fossa Magna are divided 

into three groups; Utsukushigahara-Kirigamine (UK), 

Susaka-Ueda (SU) and Makihata-Tanigawa (MT) which 

locate from southwest to northeast in the North Fossa 

Magna region. They consist of quartz diorite, tonalite and 

granodiorite. K-Ar biotite and fission track zircon ages of 

the UK range from 8.5 Ma to 13.5 Ma (Kawano and 

Ueda, 1966; Shibata et al., 1976; Koshimizu and 

Yamagishi, 1987) and those of the MT from 3.1 Ma to 5.9 

Ma (Kawano and Ueda, 1966; Ganzawa and Kubota, 

1987; Kawano et al., 1992). Radiometric ages for the SU 

are not determined. However, ages of intrusion are 

inferred from stratigraphic sequence, they may be the 

same as those of the UK. 

NCNK ratios of the UK, SU and MT are from 0. 79 to 

1.03, 0.62 to 1.01 and 0.71 to 1.09, respectively, and 

( 87Sr/86Sr)i of the UK, SU and MT are from 0.7035 to 

0.7056, 0.7041 to 0.7046 and 0.7044 to 0.7047, respectively. These values suggest that granitoids are mostly 

classified as metaluminous rocks that belong to the I- or 

M-type granitoids, and it has been considered that source 

magmas were not affected by crustal material. Furthermore, on the Bsr - eNd diagram, granitoids in the North 

Fossa Magna plot in the field of Tertiary volcanic rocks 

(basalt and andesite) of the Northeast Japan Arc. It 

appears from these data that source magmas for granitoids 

in the North Fossa Magna are derived by partial melting 

of subducted oceanic crust or fractional crystallization of 

basalt. 

In comparison with Cretaceous granitoids in Northeast 

Japan, Neogene granitoids in North Fossa Magna are 

widely exposed. This may be due to a close association 

with the tectonic situation, the contact between the Northeast and Southwest Japan Arcs; the tectonic setting that 

caused generation of the source magmas will be discussed. 

Ganzawa, Y. and Kubota, Y., 1987. Geological setting of the Tanigawaquartz diorite and its cooling history. Abst. 94th Ann. Mtg. Geol. 

Soc. Japan, 194. 

Ishihara, S., Kanaya, H. and Terashima, S., 1976. Genesis of the 

Neogene granitoids in the Fossa Magna Region in Japan. Ocean Sci., 

8, 19-24. 

Kawano, Y. and Ueda, Y., 1966. K-Ar dating of the igneous rocks in 

Japan (N)- Granitic rocks in northeast Japan. Jour. Min. Petr. Econ. 

Geol., 56, 191-211. 

Kawano, Y., Shibata, K., Uchiumi, S. and Ohira, H., 1992. K-Ar age 

of the Tanigawadake Pliocene plutonic bosy, North Fossa Magna, 

central Japan. Jour. Min. Petr. Econ. Geol., 87, 221-225. 

Koshimizu, S. and Yamagishi, 1., 1987. Fission track dating of the 

Neogene intrusive rocks in the northern Fossa Magna, central Japan. 

Jour. Geol. Soc. Japan, 93, 773-776. 

Shibata, K., Aoki, M., Kawachi, S., Yamazaki, T. and Kobayashi, T., 

1976. Description and K-Ar age of pegmatite in the Tertiary 

quartzdiorite from Wada-mura, Nagano Prefecture, central Japan. 

Bull. Geol. Surv. Japan, 21, 509-516. 
Crust-Mantle Interaction at Bingie Bingie 

Point: Implications for 1-Type Granite 

Petrogenesis in the Lachlan Fold Belt 

Keay, Sue and Collins, Bill 

Geology Department, University of Newcastle, 

NSW 2308, Australia 

The Moruya Suite was used by White and Chappell 

(1977) as the type example of an 1-type minimum melt 

granitoid exhibiting restite-controlled chemical variation. 

It is well exposed at Bingie Bingie Point on the coast of 

eastern Australia, and represents the most primitive 

end-member of the Sr-Nd granitoid spectrum in the 

Lachlan Fold Belt (LFB) (McCulloch and Woodhead, 

1993). 

Excellent mingling relations between Moruya Suite 

tonalites and contemporaneous diorite-gabbros of the 

Bingie Suite are visible at Bingie Bingie Point. Transitional, 10 metre-wide contacts between the two intrusive 

bodies comprise abundant ( > 80%) pillows of diorite in 

tonalite, preserving striking evidence of in situ 

microgranular enclave formation. The enclaves are lobate, 

commonly with crenulate margins, and some have chilled 

contacts. Others show evidence of spalling and/or 

net-veining by the host tonalite magma. The proportion of 

enclaves decreases markedly only a few hundred metres 

away from these contacts, becoming sparse over several 

kilometres to background levels ( < 5 vol %) throughout 

the Moruya Suite. 

Fine-grained, hornblende-bearing microdiorite dykes of 

similar composition to the gabbroids intrude the tonalite 

and display sharp contacts. In one area a microdiorite 

dyke terminates in the tonalite as a spray of enclaves, but 

elsewhere the dyke disaggregated into blocks similar to 

those formed by chocolate tablet boudinage. These 

features indicate intrusion of the gabbroids and their 

associated dykes late in the solidification history of the 

tonalite. 

The gabbroids form a sublinear chemical array between 

hornblende and plagioclase, reflecting a two-component 

fractionation/accumulation system. The granitoids define 

a different chemical trend, which can be extended to 

encompass the compositions of those gabbroids which have 

not undergone significant fractionation. The microgranular 

enclave compositions plot in a triangle between the 

chemical trends of the two suites, indicating chemical 

hybridism. 

Abstracts 79 

N d model ages for the tonalite are Neoproterozoic 

(600-700 Ma), the microdiorite dykes have younger model 

ages (350-500 Ma), with enclaves and gabbroids overlapping both ranges. The diverse rock-types displayed at 

Bingie Bingie Point form a Sr-Nd isotopic array with 

tonalite and juvenile mantle of basaltic composition as 

end-members. These results suggest thaf the Moruya Suite 

was derived by partial melting of 600-700 Ma old, basaltic 

lower crust, which mixed with mantle-derived, hydrous 

basaltic magmas represented by the microdiorite dykes. 

The enclaves are interpreted as reflecting upper crustal 

mingling of hybrid magmas, which formed by deeper 

crustal mixing between Bingie Suite magmas of variable 

composition and the Moruya Suite. 

Field, chemical and isotopic evidence from the 

gabbroids, granitoids and enclaves at Bingie Bingie Point 

suggests that the Moruya Suite is a product of crust-mantle 

interaction. The data indicate that the enclaves can not be 

restite and casts serious doubts about the validity of 

applying the restite model to explain the evolution of I -type 

granitoids. Instead, the Sr-Nd isotopic array encompasses 

the isotopically primitive granitoids of the LFB indicating 

that crust-mantle mixing was significant in granite 

petrogenesis in the Lachlan Fold Belt. 

McCulloch, M.T. and Woodhead, J.D., 1993. Lead isotopic evidence for 

deep crustal-scale fluid transport during granite petrogenesis. 

Geochim. Cosmochim. Acta, 51, 659-674 

White, A.J.R. and Chappell, B.W., 1977. Ultrametamorphism and 

granitoid genesis. Tectonophysics, 43, 7-22. 

Rb-Sr and Sm-Nd Study of 

the Sao Rafael Batholith, 

Rio Grande do Norte, Northeast Brazil 

1

Ketcham, D.H., 1Long, L.E. and 2Sial, A.N. 1 Department of Geological Sciences, 

University of Texas, Austin, Texas 78712, USA 2

Department of Geology, Federal University 

Pernambuco, Recife, PE 50732-970, Brazil 

During the Brasiliano (Pan-African) orogeny from 7 to 

5. 5 Ga, the Borborema geologic province in northeast 

Brazil was intruded by more than 80 granitoid bodies, one 

of them the primary epidote-bearing Sao Rafael Batholith 

located in the Serid6 foldbelt. This unfoliated quartz 

monzonite pluton is thought to be of very late Brasiliano 
80 The Origin of Granites and Related Rocks 

age, perhaps between 6 and 5. 5 Ga based on regional 

considerations and local field relations. 

Twenty-four samples were analyzed isotopically, 

including 4 from a subtly different sub-facies of the pluton 

("Serra-type"), and 2 mafic enclaves. In general, the 

rocks have high Sr (655 ± 175 ppm) and low Rb (61 ± 

9 ppm), with consequent low enrichment in radiogenic Sr. 

Three mineral-whole rock isochrons provide cooling ages 

of 0.49 ± 0.01 Ga, consistent with the inferred age of 

emplacement. Data of 18 main-facies whole rocks plot as 

an amorphous "blob" on a Rb-Sr isochron diagram. 

'-

en 

co 

0.72 

~ 0.71 

en 

...... 

IX) 

• 0 • 

• &0 0 

0 • 

•• • =~· :.~., 

• Epidote-bearing facies 

o Serra-type facies 

• Mafic enclave 

0.70L-------------~------------~ 

0.0 0.3 0.6 

B7RbJB6Sr 

Inhomogeneous initial Sr ratios preclude the determination of a Rb-Sr age of crystallization. Somewhat more 

systematic data from the Serra-type samples suggest an age 

of about 0.6 Ga. Sr ratios of epidote-bearing facies rocks, 

projected back to 0.6 Ga, are high (0.713 ± 0.001), 

pointing to an ancient radiogenic crustal source. 

Six whole-rock samples provide a Sm-Nd isochron 

apparent age of about 1.1 Ga, which is inconsistent with 

field evidence for late Brasiliano emplacement. Anomalous Sm-Nd data indicate that initial 143Nd/144Nd ratios 

were also inhomogeneous, just as Sr initial ratios were. 

Strongly negative values of eNd(0.6 Ga) of -18 - -21 imply 

a source relatively enriched in Nd, a characteristic of 

continental crust. Crust-formation ages calculated with 

reference to the depleted mantle curve are consistent with 

minimum values of 2. 7 ± 0. 3 Ga. 

Limpid euhedral zircon crystals are optically faintly 

zoned, suggesting multi-stage growth. Extremely discordant U-Pb data of entire-crystal fractions are consistent 

with presence of an inherited component with 2 Ga 

minimum age. 

A precise age of emplacement of the Sao Rafael pluton 

is yet undetermined. U-Pb data, negative eNd(0.6 Ga), 

Sm-Nd depleted-mantle model ages, and high initial Sr 

(0. 6 Ga) all suggest that the source rock was ancient 

continental crust. Poorly understood Precambrian basement 

in northeastern Brazil has been complexly reworked, and 

there is independent evidence for relict Archean crust in 

the region. 

The only "normal" isotope data from Sao Rafael are 

sharply delimited Whole-rock 0180sMOW = +7.8 tO 

+8.1 %o, and consistent mineral cooling ages of about 0.49 

Ga. Formed at about 0.6 Ga (?) by partial melting of 

Archean crust, the Sao Rafael magma mixed sufficiently 

to attain a distinctively uniform physical appearance and 

chemical-mineralogical composition, but its parent-daughter isotope systems failed to homogenize isotopically . 

Thus, the Sao Rafael batholith consists neither of simply 

remobilized source rock nor of completely re-equilibrated 

material. 

Origin of the Shir-Kuh Granite, 

South-West Y azd, Central Iran 

Khalili, M. 

Department of Geology, University of Isfahan, 

Isfahan, Iran 

Within the Shir-Kuh batholith four granite types have 

been identified by means of field observations, mineralogy, 

and major and trace element geochemistry. Monzogranite 

covers the major part of the batholith area, and is assumed 

to be the dominant granite unit. These granites are 

characterized chemically by Si02 (66 - 70 % ), Al20 3 (14 

- 15 %), Na20 (2 - 3.82 %), K20 (3.5 - 4 %), and CaO 

(2 - 3 %) . The second unit, composed of mafic granodiorite grading into monzogranite and tonalite is probably 

an earlier consolidated portion of the main unit. These 

rocks are recognized by abundant biotite, plagioclase and 

some accessories (apatite, zircon and rutile). Chemically, 

they are lower in Si02, and higher in Al20 3 relative to the 

monzogranite rocks. However, these two units display 

large overlaps for most major and trace elements. The 

youngest intrusive units are leucogranitic and trondhjemitic 

rocks that generally are characterized by decreased Al20 3 

(12- 13 %), high Si02 (74- 78 %), and minor CaO (0.3 -
0.5 %) concentrations. In comparison with other units, Rb 

is high and Sr is much lower in the leucogranites. 

All of the Shir-Kuh granitoids are S-type granites 

according to their Ni!Mg ratio, ASI ((Al20i(Ca0 + Na20 

+ K20)) = 1.1 - 2. 7), and presence of muscovite and 

their lack of hornblende. The strongly peraluminous 

character of the Shir-Kuh granitoids is shown by high 

normative corundum and high molar A/CNK; which is 

reflected by the occasional presence of garnet and 

cordierite. In the Rb versus (Y + Nb) discrimination 

diagram, most data plot close to or within the field of synCOLG, and in the Nb versus Y discrimination diagram 

within the VAG - syn-COLG field. Hence, the Shir-Kuh 

granitic melts were presumably derived from 

metasedimentary crustal sources during the collisional 

tectonic regime of the Late Kimmerian orogenesis. 

Judging from many of the major and some of the trace 

element plots, these granites are interpreted to be oogenetic 

with the leucogranitic rocks derived from monzogranitic 

melts by crystal fractionation involving mafic minerals, 

plagioclase and some accessory phases. 

Hercynian Granitic Rocks of the 

Western Carpathians: 

Products of Crustal Reactivation 

1

Kohut, M., 2Kotov, A.B., 2Salnikova, E.B., 

2

Kovach, V. P. and 1 Michalko, J. 1 Dionyz Stur Institute of Geology, 

Bratislava, Slovakia 2

/nstitute of Precambrian Geology and Geochronology, 

St. Petersburg, Russian Federation 

Granitic rocks form numerous plutons in the Western 

Carpathians (WC). The majority of the granitoid plutons 

are metaluminous to peraluminous and are composed of 

several rock types ranging from tonalite to leucocratic 

granite. Silica contents of granitic rocks vary in the range 

c. 60 - 75 wt %, documenting the increase of alkalinity 

from the tonalites to the leucogranites. The prevalence of 

N~O over K20 is a common geochemical feature, except 

in the porphyritic granite of "Pra8iva" type, in which K20 

> Na20. Carpathians granitoids represent low- to 

high-potassium calk-alkaline (trondhjemite and monzonite) 

series of magmatic rocks. Biotite is the dominant Fe-Mg 

mafic mineral, and hornblende occurs only rarely in the 

Abstracts 81 

WC granitoids. Accessory minerals (magnetite + allanite, 

and monazite + ilmenite) show in some plutons (Tribec) 

dichotomy and/or antagonism, which permits distinguishing two principal granite groups (Petn'k and Broska, 

1994). The occurence of the mafic microgranular enclaves 

(MME) in the magnetite-bearing granites and presence of 

the host (metamorphic) rocks xenoliths in the 'magnetite-free granites support this division in the Western 

Carpathians. In some plutons (the Tatry Mts., the Mala 

Fatra Mts.) one can observe both enclaves and xenoliths in 

one place. REE are typically LREE enriched in the WC 

granitoids. Chondrite-normalized patterns of the REE 

exhibit no or slight negative Eu anomalies and uniform 

nonfractionated distribution trends for various granite 

types. (

87Sr/86Sr)i in the granites are low (0.703 - 0.708), 

suggesting a mixed lower crustal and mantle component 

and/or Rb-poor crustal source. Rarely is (

87Sr/86Sr)i higher 

than 0.715 documenting crustal origin only. Rb/Sr = 0.1 

- 0.5 for common tonalite - granite types, but for some 

leucogranites extending from 0.6 to 2.5; eNiO) = 50 - 130 

and 150 - 250, respectively. The eNd(O) values vary from 

-2. 3 to -9.4 and are comparable with data of Liew and 

Hofmann (1988). Apparent crustal residence ages indicated by the Nd model ages (toM = 1.1 - 3.8 Ga and toM = 

1.1 - 1. 7 Ga) support the authors concept that Hercynian 

Europe comprises mainly recycled Proterozoic components 

with significant new Paleozoic addition. The crustal 

contribution to the WC granitoids is essential and exhibit 

a Neodymium Crustal Index (DePaolo et al., 1992), NCI, 

of 0.49 - 1 (majority 0.6 - 0.8). 

All geochemical features suggest that the WC 

Hercynian granitoids are analogous to VAG (CAG) 

granites related to the subduction processes. However, 

during the late Devonian and early Carboniferous, metamorphic, sedimentary and structural data rule out this 

scenario as similar to almost everywhere in Europe and 

suggest continental collision processes. These collisional 

processes with overthrusting of the deep crustal nappes 

were juxtaposed with crustal reactivation and granitoid 

production. Field evidence together with P-T-t paths 

(generally clockwise) suggest in the Tatra Mts. tectonic 

inversion of metamorphism, documented by thrusting of 

hotter, higher-metamorphosed slab (migmatites, gneisses, 

amphibolites) over cooler parautochthon (mica schists). A 

comparable situation occurs in the Mala Fatra Mts. and in 

the Nizke Tatry Mts. During the climax of the Hercynian 

collisional tectonic event (350-340 Ma) older igneous 

material was partially melted in the Western Carpathians. 

The new anatectic granitic rocks inherited VAG geochemi-
82 The Origin of Granites and Related Rocks 

cal characteristics from older (Caledonian or 

Panafrican-Cadomian?) igneous rocks. Nowadays these 

granitoids show a low degree of magma fractionation and 

represent common products of the crustal reactivation and 

remagmatisation. 

DePaolo, D.J., Perry, F.V. and Baldridge, S.W., 1992. Crustal versus 

mantle sources of granitic magmas: a two-parameter model based on 

Nd isotopic studies. Trans. Royal Soc. Edinburgh: Earth Sci., 83, 

439-446. 

Liew, T.C. and Hofmann, A.W., 1988. Precambrian crustal components, 

plutonic associations, plate environment of the Hercynian Fold Belt 

of central Europe: Indications from a Nd and Sr isotopic study. 

Contrib. Mineral. Petrol., 98, 129-138. 

Petrik, I. and Broska, 1., 1994. Petrology of two granite types from the 

Tribec Mountains, Western Carpathians: an example of allanite ( + 

magnetite) versus monazite dichotomy. Geol. J., 29, 59-78. 

Remnants of Lower Crustal Mineral 

Assemblages in Granitoid Rocks: Examples 

from the South Bohemian Pluton, Austria 

Koller, F. and Kloetzli, U.S. 

University of Vienna, Austria 

The South Bohemian pluton is formed by a complex, 

6000 km2 sized batholith and belongs to the granitic 

intrusives of the Variscan orogeny in Central Europe. The 

late-syntectonic to post-tectonic magmatites were intruded 

into the polymetamorphic pre-Variscan basement rocks of 

the Southern Bohemian Massif. The widespread coarsegrained granitoids of the Weinsberg Granite and the more 

easterly situated Rastenberg Granodiorite exhibit 

K-feldspar megacrysts up to 20 em in length. The 

granitoids show a metaluminous to slightly peraluminous 

I-type composition. Sr- and Nd-isotope investigations 

seem to prove that no mantle component or only a strongly 

suppressed mantle component, contributed to the magma 

generation. 

Dark colored varieties of the Weinsberg granite are 

found in the vicinity of Sarleinsbach over an area of 10 -

15 km2

• These rocks show a close structural relationships 

to the normal type of Weinsberg Granite but a distinct 

quartz- monzonitic composition. The variation in Si02 

resembles closely the composition of the diorites found in 

the South Bohemian Pluton although significantly lower 

MgO, CaO and Cr, and higher K20, Zr and Ba concentrations are observed. Overall, an evolutionary trend to the 

normal Weinsberg Granite can be observed. In these 

rocks, two different mineral assemblages, which are not in 

mutual equilibrium, are found. The definitely younger one 

is clearly of magmatic origin (quartz - oligoclase -

orthoclase - biotite - ilmenite), having crystallized from a 

granitic magma at pressure of about 3 - 4 kbar. The 

granitic evolution of these rocks is best demonstrated by 

the pronounced zonation of the K-feldspar. Noteworthy is 

the considerable amount of celsian component of - 2 mol 

% with a normal chemical zonation. More elevated, 

erratic celsian components of up to 8 mol % are found 

with no visible correlation to magmatic zonation. This is 

thought to represent relicts of old K-feldspar. The formation of the granitic mineral assemblage took place at 355 

± 8 Ma (zircon Pb/Pb). The older, equigranular mineral 

assemblage is formed by plagioclase - orthopyroxene -

clinopyroxene ± ferrous tschermakite ± alkali feldspar 

(plagioclase with An >40 mol %, XMg for Opx 0.35 -

0.42, XMg for Cpx 0.50 - 0.54). A symplectic reaction 

zone with quartz and biotite is found between the two 

mineral assemblages, defining the breakdown reaction Opx 

+ Kfs + H20 + Bt + Qtz. Pyroxene is replaced by 

amphibole and, with continuing breakdown, by biotite. 

The first breakdown of pyroxene occurs without the 

interaction of a melt. Only in a later stage of breakdown, 

the reaction took place with the participation of a granitic 

magma. In the older, pyroxene bearing mineral assemblage granulite facies metamorphic conditions are found 

with pressures around 8 kbar and equilibrium temperature 

of 755 ± 26°C (two-pyroxene- thermometer). Single 

zircon ages of 529 ± 22 Ma provide good evidence, that 

this charnockitic relict assemblage is of Cadomian age. 

In the Rastenberger Granodiorite ( - 40 km2

) Precambrian zircons with ages around 623 ± 22 Ma were found. 

The growth of these zircons is thought to have taken place 

during a magmatic or a high grade metamorphic event. In 

monzonitic to dioritic enclaves, partly also with large 

K-feldspars, remnants of relictic Cpx (XMg between 0.82 

- 0.67) are found. The Cpx is normally replaced by 

amphibole. Biotite inclusions within the cpx provide some 

evidence, that the Cpx enclaves form relicts of a prograde, 

high-grade metamorphic rock at pressure ~6 kbar, 

according to the Ca-tschermacitic substitution in Cpx. 

Dating of zircons in large K-feldspar megacryst and in the 

finer grained matrix has shown a marked age difference 

between the two occurrences: a) in the K-feldspar, ages 

around 353 ± 9 Ma and the 623 ± 22 Ma age are found; 

and b) in the matrix, additionally an age group of 338 ± 

2 Ma is present, which is formed primarily by zircons 
with distinct different typology than found in the other age 

groups. The age of 338 ± 2 Ma is thought to represent 

the time of intrusion of the granodiorite magma. At least 

parts of the K-feldspar megacrysts were not formed during 

the 338 Ma event but distinctly earlier. 

The preliminary results lead to the assumption that in 

both investigated complexes at least some of large 

K-feldspar crystals and/or parts of the mineral assemblages 

and/or the above mentioned enclaves were possibly formed 

in a lower crustal level with distinct higher pressure than 

present in the granite melt system. 

Emplacement of the Abukuma Pluton, 

Northeast Japan, Inferred from the 

Subsurface Distribution of Gabbroic 

Xenoblocks and Structure of the Wall-Rocks 

Kubo, Kazuya 

Geological Survey of Japan, 1-1-3 Higashi, 

Tsukuba, 1baraki 305, Japan 

The Abukuma pluton is a large-scale Cretaceous 

granitic intrusion located in the northeast Japan. It is 160 

km long with a width of 50 km, trending in a NNW - SSE 

direction. Eastern and western margins of the Abukuma 

pluton are bounded by left-lateral shear zones. Many 

small-scale gabbroic masses are exposed aligned, showing 

the S-shaped trend in the Abukuma pluton. 

Based on the petrologic and gravitational studies, 

three-dimensional structures of the gabbroic masses and 

the uppermost part of Abukuma pluton, and structure of 

sedimentary wall-rocks were described. Results are as 

follows. (1) The Abukuma pluton was intruded into a 

zone between the upper sedimentary rocks and the lower 

regional metamotphic rocks. (2) Before the emplacement 

of the granitic pluton, the metamorphic wall-rocks were 

under the condition of left-lateral shear stress and deformed. After the emplacement, the same condition 

occurred, resulting in two shear zones bounding the 

eastern and western ends of the pluton. This physical 

condition may produce the enough space for the emplacement of the pluton. (3) Linearly aligned basic plutonism 

represented by the distribution of gabbroic masses occurred before the granitic plutonism. This basic plutonism 

may corresponds to the root zone of the pre-Cretaceous 

volcanic front. 

Abstracts 83 

Geochemical Constraints of Enclaves and 

Pre-Himalayan Champawat Granitoids of 

Kumaun Lesser Himalaya, India 

Kumar, Santosh 

Department of Geology, Banaras Hindu University, 

Varanasi-221 005, U.P., India 

The Champawat granitoids (CG) (560 ± 20 Ma) occur 

as an elongated body roughly trending east-west close to 

the Main Boundary Fault (MBF), and have intruded into 

Precambrian metapelite country rocks. On the basis of 

mode of occurrence, distribution, shape, size, colour index 

and microtexture, the enclaves hosted in CG can broadly 

be classified as country rock xenoliths, surmicaceous (dark 

coloured Bt-schist) enclaves (SE), limited mobilized mafic 

(Cpx-Hbd-Bt) enclaves (MME) and less abundant ( ~ 1 vol 

% on exposed level) light and dark colored igneous 

looking fine-grained micro granitoid enclave (ME). 

The CG show leucocratic (Bt-poor) and mesocratic 

(Bt-rich) medium- to coarse-grained hypidiomorphic 

textures and belong to granodiorite-granite-adamellite 

calc-alkaline medium-K series with Bt (±Hbd±Ms)-

Kf-Pl-Qtz and minor amount of zircon, Fe-Ti oxides, 

rutile, sphene and apatite. The ME (up to 0.5 m across) 

occur as rounded, ellipsoidal, discoidal, stretched and 

elongated shapes with occasjonal mineral foliation, which 

are suggestive of magma mingling and flow within the 

CG. The ME bear fine-grained porphyritic and nonporphyritic hypidiomorphic textures with dominant quartz, 

plagioclase and biotite, although K-feldspar is uncommon. 

The ME belong to the calc-alkaline -trondhjemite (quartz 

diorite to tonalite) low-K series. 

The CG broadly can be characterized as S-type 

(peraluminous; high Si02 = 65 - 75 wt %; mol A/CNK 

> 1.1; K20/Na20 > 1.0; Na20 < 3.2 wt %; Fe20 3/Fe0 

< 1.0; normative corundum > 1.0%; elevated (

87Sr/86Sr)i 

> 0.709). Moreover, CG also have limited affinity with 

A-type (anorogenic) granitoids (occurrence of tourmaline 

pegmatite; selective enrichment of Sc, V, Th, U, Sr, Ba 

and Pb relative to alkaline magma) and H-type (hybrid) 

granitoids (occurrence of enclaves; patchy zoned 

plagioclase; K-feldspar megacrysts and Bt (± Hbd) - Grt 

assemblage). The MME, restricted to the Khark region, 

are nepheline normative and enriched in Th, U, Nb, Y and 

LREE which are more akin to an alkaline rock. The ME, 

however, show relatively higher content of Sc, V, Cr, Ni, 

Co, Cu, Zn, Ga, Rb, Y, Nb and Cs compared to those of 
84 The Origin of Granites and Related Rocks 

the host CG, whereas Pb, Th and U are lower in the CG 

than in the ME, suggesting the more basic composition of 

ME than the CG. 

On the basis of HFSE, REE-patterns, LaN/YbN and 

EuN/Eu *, the ME can be grouped into three which are 

more likely resulted by an internal differentiation controlled by fractional crystallization of a moderately 

LREE-enriched ME without significant Eu-anomaly 

(EuiEu* = 0.92) giving rise to ME having partial cumulate with +ve Eu-anomaly (EuN/Eu* = 1.28 and 1.30) and 

subsequently evolved ME with -ve Eu-anomaly (EuN/Eu* 

= 0.24 to 0.64). The LREE patterns, however, may 

correspond to bulk crust (EuN/Eu* = 0.97), lower continental crust (EuN/Eu* = 1.12) and upper continental crust 

(EuN/Eu* = 0.63) respectively, demonstrating the crustal 

evolution phenomena, but the HREE patterns contradict 

the view. The CG show the different REE patterns with 

-ve Eu-anomaly (EuN/Eu* = 0.28 to 0.59) and relatively 

enriched LREE patterns compared to those of the ME, 

implying that the CG have either already experienced the 

feldspar fractionation or derived from feldspar depleted 

source. Comparing the chondrite normalized REE patterns 

of individual pairs of ME and respective host CG suggest 

their local partial equilibrium, although the ME still 

maintains a separate entity. The higher Th and U contents 

of the CG than the ME suggest the evolved nature of the 

CG but incomparable to those of upper and lower continental crust, respectively. Hence, the ME neither represents the cognate enclave of the host CG nor the older 

tonalitic crust. 

The ORG (Ocean Ridge Granite) normalized LILE and 

HFSE patterns of ME and CG are typical of calc-alkaline 

granitoid plutons. Some of the equivocal LILE concentration of the ME and the host CG can be attributed to 

equilibrium but cannot be characteristics of a common 

source. The major cations and HFSE (Nb, Y, Ta and Yb) 

discriminate the CG largely as syn-collisional granites 

whereas a majority of the ME and a few of the CG show 

affinity with within-plate granite. The diverse nature of 

the ME and the host CG is not surprising and appears to 

be a common coeval bimodal (mafic/basic and felsic) 

magma association in an orogenic belt. The ME represents the mingled and undercooled (hybrid) magma into the 

host CG leading to chemical exchange through volatile 

transfer. The lower mass fraction ( ~ 1 %) and cessation 

of basic ME magma injection into the CG promoted the 

mingling and inhibited the homogenization, however, the 

felsic melt (CG) continued to produce anatectic crustal 

melt increasing the large volume of the CG. 

Petrology and Geochemistry of a Calcic, 

Peraluminous and Ferrous Granitoid Pluton: 

the Mitsuhashi Granite in the Ryoke Belt, 

Southwest Japan 

Kutsukake, Toshio 

Aichi University, Toyohashi, 441 Japan 

The Mitsuhashi Granite pluton, 8 x 8 km in extent, was 

emplaced within the Ryoke metamorphic rocks. It belongs 

to the younger Ryoke granitoids, which constitute a 

continental-margin magmatic arc of Cretaceous age in 

Pacific Asia before the opening of the Japan Sea. This 

pluton yields a radiometric age of c. 80 Ma. 

The main rock types are coarse-grained hornblendebiotite tonalite and low-K20 granodiorite, with subordinate 

quartz diorite which bears cummingtonite or almandine 

garnet. At the margin of the pluton, wallrock assimilation 

produced garnet-biotite granite and granodiorite. 

Geochemically the rocks are unusually calcic (alkali-lime 

index = 65), peraluminous and low-potassic, and also 

have characteristically high Fe/Mg ratios. In the 

AFM -diagram the mafic enclaves and intermediate rock 

types plot in the tholeiite field. They have moderate (c. 

450 ppm) Sr and low ( < 50 ppm) Rb concentrations. 

Mineralogic equilibria show that crystallization took 

place at a depth of c. 20 km. Absence of clinopyroxene, 

expected to crystallize from the whole-rock chemistry, and 

the early crystallization of hornblende suggest a high 

water-vapour pressure condition. Very low ferric/ferrous 

ratios, absence of magnetite and occurrence of almandine 

garnet coexisting with hornblende imply a low oxygen 

fugacity. Trace element variations and REE patterns 

indicate strong plagioclase and moderate hornblende 

fractionation. Modal and geochemical variations are 

modelled principally by varying the ratios of cumulus 

plagioclase and inter-cumulus liquid, which precipitated 

quartz and alkali feldspar. 

Although the high XALK values ( > 1. 30) of the 

Mitsuhashi granite appear to relate to S-type granite in 

Pitcher's geotectonic classification of granites, the petrographic features suggest that it is !-type. A high 

(

87Sr/86Sr)i (> 0.708) indicates its source to be mafic rocks 

of old continental lower crust beneath the Ryoke Belt. 
Geochemistry and Evolution of the 

1.89-1.81 Ga Granitoids in the TampereHameenlinna Area, Southern Finland 

Lahtinen, Raimo 

Geological Survey of Finland, 

SF-02150 Espoo, Finland 

The study area is situated in southern Finland, in the 

central part of the Fennoscandian shield. The region lies 

entirely within the Paleoproterozoic Svecofennian domain 

and a number of distinct, broadly E-W trending tectonic 

units have been recognized, each having distinctive 

magmatic and metamorphic signatures. The northernmost 

unit is the Central Finland Granitoid Complex (CFGC) 

with the Tampere Schist Belt occurring along its southern 

margin. A thrust separates these from the Mica 

gneiss-migmatite Belt (MB) to the south which is characterized by tonalitic LPHT migmatites and abundant 

granitoids. An inferred suture in the southern part of the 

MB separates it from the Hameenlinna Schist Belt (HSB) 

which, together with the Microcline granite Complex 

(M C), form the southernmost sub-areas in this study. 

Granitoids in the study area have syn-tectonic 

(1.89-1.88 Ga) to late- to post-tectonic ages (1.88-1.86 Ga) 

in the CFGC, MB, and HSB. The gneissic granitoids in 

the MC belong to these age groups, but the dominant 

microcline granites belong to a SW-NE trending belt of 

1.84-1.81 Ga granites and granitic LPHT migmatites, and 

1. 81-1.79 Ga granitoids related to the 1. 86-1.84 Ga 

collision and subsequent collapse. 

Syn-tectonic granitoids are dominated by high-K 

calc-alkaline metaluminous to slightly peraluminous 

granodiorites with abundant mafic enclaves and local 

comingling features producing hybrid rocks. At least three 

end-members in different proportions are needed to 

account for the observed compositional variations; arc 

related calc-alkaline magma, WPB-affinity tholeiitic 

magma and crustal-derived partial melt. The complex 

interaction and mixing relations between different 

end-members complicates the situation, but melting of a 

mafic-intermediate source in the presence of a vapour 

phase is proposed for the silicic granodiorites, with an 

additional sedimentary component in the MB. 

Late- to post-tectonic granites in the CFGC coincide 

with a regional magnetic high and are divided into high-K 

and very high-K groups. Mafic enclaves and comingling 

features are rare. The silicic high-K granites are slightly 

Abstracts 85 

peraluminous and have low combined FeO + MgO + 

Ti02 ( < 3%) and high FeO/MgO. Vapour-phase absent 

melting of a tonalitic to granodioritic source is proposed. 

The very high-K granites are porphyritic to coarse-grained 

rocks with local rapakivi-like textures. They have a wide 

range of silica contents and are slightly peraluminous, 

alkaline to nearly alkaline rocks with high FeO/MgO. The 

intermediate variants have high Zr, Ba, K/Rb and Nb/Th 

indicating an affinity with WPB in the CFGC. The lack 

of rocks in the silica range 54-66% favors high temperature (high Zr) vapour-phase absent melting. The more 

silicic granites have lower Ba, higher Rb and F, and 

variable Th, Y and Zr. Although two feldspar fractionation can explain some features, the participation of more 

of the crustal component is also probable. 

The abundant MB high-K granitoids are of tholeiitic 

affinity and at least partly belong to the 1. 88-1.86 Ga 

stage. They comprise quartz diorites to predominantly 

porphyritic granodiorites with both mafic and intermediate 

mafic enclaves. They are peraluminous over a wide silica 

range with high S, Te, As, Bi and local graphite derived 

from the assimilated sedimentary component. These 

granitoids have high Ti02, P20 5, Ba, Nb, Y, Zr and La 

and they follow trends from WPB to strongly peraluminous 

granodiorites. The high Th/U ratios in the intermediate 

granodiorites show that the assimilation/interaction with 

sediments have occurred under disequilibrium conditions, 

leaving restitic zircon. 

Late pegmatite dykes and small granite bodies within 

the MB schists and migmatites are peraluminous S-type 

granites. S-type microcline granites (1.84-1.81 Ga) and 

granitic migmatites in the MC have lower level of Au, As, 

Bi, Sb and Cs compared to similar rocks in the MB 

indicating the loss of fluid phase before the onset of 

vapour-phase absent melting in the sedimentary source. 

Low Ba indicates the occurrence of K-feldspar in the 

source residue. 

Thrusting of hydrous rocks and the ponding of water-rich subduction related magmas in the lower and 

middle crust during crustal thickening and subsequent 

magmatic underplating (WPB magmas) produced 

vapour-phase present melting and migmatization at 

1.89-1.88 Ga. The continuous magmatic activity and high 

heat flow from the mantle, combined with more anhydrous 

conditions, promoted vapour-phase absent melting at 

higher temperatures at 1. 88-1.86 Ga. The prolonged high 

heat flow and anhydrous source rocks (escaped fluid 

phase) contributed to large scale vapour-phase absent 
86 The Origin of Granites and Related Rocks 

melting in the middle crust forming the 1. 84-1.81 Ga 

granites and migmatites. 

S-type Granites of the Hillgrove Plutonic 

Suite, New England Batholith, Eastern 

Australia: Products of Partial Melting 

of an Intermediate Greywacke Source 

Landenberger, B. and Collins, W.J. 

Department of Geology, University of Newcastle, 

Callaghan, New South Wales, Australia 

Late Carboniferous S-type granites of the Hillgrove 

Plutonic Suite (New England Batholith, eastern Australia) 

were emplaced into the Devonian - Carboniferous subduction - accretion complex of the southern New England 

Fold Belt. The suite was largely generated by partial 

melting of the accretionary prism during late Carboniferous arc migration. Volumetrically minor, primitive arc 

tholeiites (gabbros) also intruded the accretion complex at 

this time, and are considered to have been part of the heat 

source responsible for partial melting and local 

high-T/low-P metamorphism. 

The suite is isotopically primitive ((87Sr/86Sr)i = 0.7040 

- 0.7065, eNd = -1 - +4) compared to most S-types suites 

world-wide, and some members of the suite have a bulk 

rock chemistry which is transitional between I- and 

S-types. Compositionally the suite ranges from 67 - 77% 

Si02 , is mildly peraluminous (ASI 1.00 - 1.15), and is 

relatively high in Na20 (3 - 4 %) compared to most 

S-types. These differences are attributed to the chemical 

and isotopic immaturity of the source rocks involved in 

magma generation, and to minor contamination processes. 

A wide variety of potential S-type granite source rocks 

were available for melting in the subduction - accretion 

complex. These include pelites ((87Sr/86Sr)i = 0. 7080 -

0.7170, eNd= -2- -8), metabasalts ((87Sr/86Sr)i = 0.7045 -

0.7080, eNd= +6 - +4.5), and a range of mafic to felsic 

greywackes ((87Sr/86Sr)i = 0.7042- 0.7080, eNd= +6-

-3). Only the greywackes of intermediate composition 

( - 65% SiO~ overlap with the isotopic composition of the 

granitoids. Major and trace element modelling indicate 

that the parental magmas (68 - 70 % Si02) of the bulk of 

the suite, were produced by -48% partial melting of a 

dominantly intermediate greywacke source, via dehydration 

partial melting reactions involving biotite breakdown. 

The isotopic and chemical variability within the mafic 

granites of Hillgrove Suite demands that two additional 

magma sources have contributed to some members the 

suite. The most isotopically and chemically primitive 

granites ((87Sr/86Sr)i = 0. 7040, eNd= +4.0), which form 

-5% of the Hillgrove Suite, have a bulk chemistry which 

deviates from the main trend of the suite, with higher 

CaO, Al20 3 , Ti02 and lower K20, FeO contents. These 

granites plot on an isotopic mixing curve between intermediate greywacke ((87Sr/86Sr)i = 0.7048- 0.7070, eNd= +2 

- -1) and the coeval gabbros ((87Sr/86Sr)i = 0.7027, eNd = 

+9.5). Accordingly, mantle-derived magmas are considered to have been a contributor to the most primitive 

granites. Another possible minor magma source are the 

seawater-altered metabasalts, which are common in the 

deeper parts of the accretion complex. These metabasalts 

are likely to have undergone small degrees of partial 

melting (via amphibole breakdown), contributing a minor 

melt component to the primary S-types, causing an isotopic 

shift towards higher eNd and higher (

87Sr/86Sr)i. 

Himalaya and Karakorum Leucogranites: 

A Comparison 

LeFort, Patrick 

Geodynamique des Chaines Alpines (EPR 69, CNRS), 

Institut Dolomieu, Grenoble 38031, France 

Two remarkable collision belts in South-Asia contain 

leucogranites of similar age: Himalaya and Karakorum. 

But the two groups strongly differ by the characteristics of 

their setting, extent and volume, petrofabrics, chemistry of 

trace elements and isotope systematics. Each group also 

presents a typical lateral variation within the orogenic unit. 

The study of the determining parameters underlines the 

fundamental parts played by the acquisition of the temperature in the source region, the nature and the abundance of 

the melting fluids, and the composition of the protolith, in 

the production of the melt. The global tectonic setting is 

directly responsible for the (segregation?), mode of ascent, 

and level of emplacement of the leucogranitic bodies. The 

timing may vary in a given mountain belt and even be 

repeated as shown in the Nanga Parbat and Hunza regions 

of the Himalaya and Karakorum, respectively. The very 

different geodynamic framework of the two belts is clearly 
reflected in the singularity of the production and emplacement of the two types of leucogranite. 

Oxygen Isotope Mapping of Young 

Anatectic Migmatites From the 

Nanga Parbat Massif, Pakistan 

1

Leslie, Karen, 1Mattey, David 

and 2

Harris, Nigel 1 Geology Department, Royal Holloway College, 

University of London, Egham, Surrey, .TW20 OEX, UK 2

Department of Earth Sciences, Open University, 

Walton Hall, Milton Keynes, MK7 6AA, UK 

Rapid uplift of the Nanga Parbat-Haramosh Massif 

(NPHM) has exposed an unusually young suite of anatectic 

migmatites and leucogranites. Field and geochemical data 

show that anatexis of pelitic and semipelitic sillimanite 

grade gneisses was coeval with rapid exhumation of the 

NPHM over the past 10 Ma (Zeitler et al., 1993). Two 

phases of melting have been recognized, an earlier phase 

resulting in sheets and small plutons of leucogranites, 

followed by cordierite-bearing granitic pods and veins. 

Structurally, the youngest magmatism is found in the 

interior of the massif, at least 5 km from unequivocal 

exhumation structures. These cross-cut all units within the 

massif and petrographic studies suggest that the younger 

cordierite granite pods result from biotite breakdown in 

metasedimentary gneisses forming peritectic cordierite and 

granite melt. Preliminary thermobarometric studies of the 

metapelites indicate conditions of P = 3.5 ± 0.5 kbar, T 

= 690 ± 40°C (Whittington and Harris, 1995). 

Chamberlain et al. (1995) have reported evidence of 

hydrothermal activity in the area and identified o180 

depleted gneisses and migmatites and marked oxygen 

isotope disequilibrium between mineral pairs that is 

indicative of fluid infiltration on a regional scale. Local 

chemical and isotopic equilibria between melt and restites, 

normally vulnerable to the effects of later metamorphic 

reequilibration, may preserve evidence of the mechanism 

and kinetics of melt formation. The high uplift rates 

within this area (up to 7 mm yr-1

) and inferred rapid 

cooling rates ( > 100°C Ma-1

) minimize the effects of 

post -anatectic metamorphic reequilibration and provides an 

ideal opportunity to examine in detail the kinetics of 

melting and the role of fluids during anatexis. 

Abstracts 87 

Samples displaying melt-restite relationships have been 

selected for detailed oxygen isotope mapping by laser 

fluorination techniques at Royal Holloway College, 

University of London. Preliminary mapping has been 

undertaken on a centimeter scale by separating grains for 

analysis from domains sawn from leucocratic and 

melanocratic horizons. Oxygen isotope data were obtained 

using a Nd-Y AG laser fluorination system (Mattey and 

Macpherson, 1993) employing a fluxing technique for 

quartz and aluminosilicates (precision - 0.2 %o compared 

to < 0.1 %o for the normal analysis of ferromagnesian 

minerals). The o180 of NBS 28 quartz is 9.62 ± 0.15 %o, 

n = 14) using the fluxing technique. Quartz from all 

samples possesses o180 values that are typical of 

metasediments and range from 10 %o to 14 %o . Quartzbiotite and quartz-feldspar mineral fractionations are 

variable but generally record temperatures in the range 

550-650°C. Minerals from migmatites can be heterogeneous with o180 values showing up to 1 %o variation on a 

hand specimen scale. The preliminary data suggest that 

o180 heterogeneity resides in biotite-rich restite zones, 

whereas melt layers are isotopically more homogenous. 

Oxygen isotopic data confirm the existence of isotope 

disequilibrium phenomena on a variety of scales. A 

migmatitic gneiss from a locality near Fairy Meadows 

possesses extreme oxygen isotope disequilibrium of over 

10%o between quartz and biotite, consistent with exchange 

with a low o1

SO fluid (Chamberlain et al., 1995). Quartzbiotite and quartz-feldspar mineral fractionations for the 

gneisses are variable but generally record peak metamorphic temperatures; oxygen isotope temperatures for the 

young cordierite migmatites seem to be in agreement with 

preliminary thermo barometric studies of the metapelites. 

The data obtained so far confirm that some gneisses have 

interacted with low o180 fluids, but significant isotopic 

heterogeneity on a em scale suggests local disequilibrium 

may also be related to the process of melt formation and 

extraction. Detailed oxygen-isotope mapping of leucosomerestite relationships and selected cordierite granites is 

underway to determine the geometry and magnitude of 

equilibrium on an intergranular scale. These data will be 

used to determine the extent to which early formed granitic 

melts are in isotopic equilibrium with source and restite 

materials and constrain the cause and timing of disequilibrium among mineral pairs in these rocks. 

Chamberlain, C. P., Zeitler, P.K., Barnett, D.E., Winslow, D., Poulson, 

S.R., Leahy, T. and Hammer, J., 1995. Active hydrothermal 

systems during the recent uplift of Nanga Parbat, Pakistan Himalaya. 

J. Geophys. Res., 100, 439-453. 
88 The Origin of Granites and Related Rocks 

Mattey, D. and Macpherson, C., 1993. High-precision oxygen isotope 

microanalysis of ferromagnesian minerals by laser fluorination. 

Chern. Geol., 105, 305-318. 

Whittington, A., and Harris, N., 1995. Abs., 8th Himalayan Conf., 

ETH, Zurich 

Zeitler, P.K. and Chamberlain, C.P., 1993. Synchronous anatexis, 

metamorphism, and rapid denudation at Nanga Parbat (Pakistan 

Himalaya). Geology, 21, 347-350. 

Origin of Syenite Magmas in Anorogenic 

Granitoid Series: Field, Geochemical and 

Experimental Data from East Central Asia 

1

Litvinovsky, B.A., 2Wickham, S.M., 

1

Zanvilevich, A.N. and 2Newton, R.C. 1

Russian Academy of Sciences, Ulan-Ude 670047, Russia 2

Department of the Geophysical Sciences, 

University of Chicago, Chicago, IL 60637, USA 

Syenites and quartz syenites are generally abundant in 

anorogenic (A-type) granitoid series. Usually, syenites are 

far exceeded in volume by granites, which is probably why 

their origin attracts less attention. However, in some areas 

it can be shown that syenitic magmas are parental to the 

whole syenite-granite series, so the origin of syenites can 

be considered as a key to the origin of A-type granitoid 

magmas. We approached the syenite problem by making 

a petrogenetic study of Late Paleozoic alkaline and 

peralkaline granitoid series in East Central Asia. Here a 

complex system of long-lived granitoid belts is superimposed on a vast region of about 5000 by 2000 km. The 

largest of these belts (the Mongolian-Transbaikalian) 

stretches for > 2000 km. Within this belt four successive 

syenite-granite series have been formed over about 200 

Ma from the Devonian to the Triassic. Each series ' 

comprises several hundreds of individual plutons, the 

largest of them as big as 3000 km2 • In the Devonian, the 

tectonic setting of the belt was a broad area of diffuse 

postcollisional extension; beginning from the Early 

Permian, it evolved as a transcontinental rift zone. 

The following important features are characteristic of 

the syenite-granite series. 1) Both syenites and granites of 

each of the four suites are very similar in mineralogy and 

composition along the length and across the width of the 

entire belt, over distances of thousands of kilometers; the 

uniformity of individual suites contrasts with clear differences between suites. 2) In terms of 180/160 and 87Sr/86Sr, 

the magmas become progressively more mantle-like with 

time. Magmatic o180 values of granites and syenites 

decrease from - + 9 in the Devonian suite to - + 6 in 

the youngest Permo-Triassic suite. 3) In plutons of all 

suites coeval mafic igneous rocks are present, in each case 

mainly associated with the earlier syenitic intrusive phase. 

4) Both in granitoids and in the coeval mafic rocks, there 

is a progressive increase in the abundance of alkalis and 

high-field strength elements with time. 5) There is clear 

textural evidence for the mingling and mixing of mafic 

magmas with coexisting silicic liquids, leading to the 

generation of hybrid monzonitic magmas. 6) Mineralogical and chemical evidence suggests that syenite magmas 

were produced by crystallization-differentiation of hybrid 

monzonite. 

These data provide strong constraints on the origin of 

syenite magma. In addition, two more pieces of evidence 

should be taken into consideration. The first, is our new 

experimental results demonstrating that partial melting of 

typical mid- to deep-crustal acid-intermediate rocks 

(charnockite-enderbite) cannot form syenite liquid, even at 

very high pressure (15, 20 and 25 kbar); in all our runs 

(exhibiting < 5% to > 50% melting), the melt composition was rather similar to that of average A-type granite. 

The second is evidence for a very high (about 1 OOOoC) 

temperature for some of the A -type silicic magmas from 

Transbaikalia, obtained by study of melt microinclusions 

in quartz and sanidine phenocrysts from the volcanic 

counterparts to some of the granitoids. Taking into 

account the enormous dimensions of the K-rich granitoid 

belts, these high temperatures suggest that the generation 

of the silicic magmas (especially the syenites) took place 

at deep crustal levels and probably involved significant 

quantities of high-temperature mafic magma. 

These data clearly suggest that both crust- and mantlederived magmas were involved in the petrogenesis of the 

syenites, and by implication the entire anorogenic granitoid 

suite in Transbaikalia. The essence of our proposed 

crustal-scale hybridization model is as follows. Potassiumrich alkali basalt magma underplated and intruded the 

lower crust causing extensive melting of lower and midcrustal material; this resulted in the production of voluminous K-rich silicic melts. Juxtaposition and interlayering 

of silicic and basaltic magmas inevitably caused mixing 

and the formation of hybrid monzonitic magmas of 

intermediate composition. Crystal/liquid fractionation of 

this hybrid magma caused the formation of 50 - 60 wt % 

syenitic residual liquid. Further differentiation of syenite 

magmas led to the production of the K-rich granites which 
form the other major lithological component of this 

anorogenic granitoid province. 

ORAL PRESENTATION 

Pegmatites 

London, David 

School of Geology and Geophysics, 

University of Oklahoma, Norman, OK 73019, USA 

Nearing the end of a century of study, advances have 

been made in understanding the internal features of 

pegmatites, but less on their ultimate origins and zonation 

within a given group. This paper constitutes a review and 

summary of recent investigations. 

Compositions. The vast majority of pegmatites are 

granitic in composition, with affinities to S-, 1-, and A-type 

sources. As a generality, S-types have a characteristic 

trace-element (TE) signature enrichment of Li-Cs-Ta; Atypes carry a Nb-Y-F TE signature; !-types may carry 

either TE suite, and crossovers among source types are 

common. Pegmatites inherit the geochemical signature of 

the granitic source, but TE abundances and ratios are 

extensively modified by differentiation among pegmatite 

bodies. Pegmatites derived from S-type sources fractionate 

toward strongly peraluminous compositions. 

Relations to granites. Links between pegmatite groups 

and larger masses of granite have been confirmed by field 

and TE chemistry. Pegmatite dikes emanate from the 

cupolas of granites and manifest no entrained phenocrysts. 

Pegmatite magmas appear to represent crystal-free liquids 

derived from the most fractionated magmas that accumulate at the tops of larger plutonic bodies. 

Zonation within groups. The chemical zonation of 

pegmatite groups resembles the chemical zonation of 

rhyolites that is inverted by eruption; both apparently stem 

from zoned magma chambers. Pegmatite dikes span large 

gradients in P and T; refmement within this gradient by 

crystal fractionation probably dominates the chemical 

zoning patterns. 

Zonation within individual bodies. The textural and 

chemical zonation within individual pegmatites is most 

consistent with sequential crystallization, essentially from 

margins to (not necessarily central) pools of residual melt, 

rather than simultaneous crystallization of all marginal and 

internal units. The process originates from pronounced 

Abstracts 89 

undercooling of magma and low crystal nucleation rates in 

volatile-bearing siliceous melts; textural transition from 

granite to pegmatite marks the point of T-X conditions 

where crystal nucleation is highly suppressed, but growth 

rates are not. The more these processes operate (together), 

then the more sharply zoned the pegmatite mineralogy, and 

the more unidirectional the mineral fabric (comb, graphic, 

and radial growth habits dominate). 

P-T conditions. Phase relations among the Lialuminosilicates (petalite, spodumene, eucryptite, and Bphases) represent the most widely applicable petrogenetic 

grid. Liquidus and solidus T can only be inferred from 

estimates of bulk chemistry, but recent inclusion and 

experimental studies put solidi near 450°C for the most 

fractionated pegmatites. 

Advances in mineralogy. Much new data on crystal 

chemistry and zonation trends have been obtained for 

feldspars, micas, phosphates, complex oxides, tourmaline, 

and Cs-, Be-, and LiAl-silicates. These data have industrial 

as well as geologic applications. Mapping patterns of 

chemical zonation within a single phase has led to improved understanding of crystallization sequence within 

individual bodies. 

Internal subsolidus alteration. A myriad of secondary 

phase assemblages result from initially alkaline followed 

by acidic vapor (sensu lato) - solid reactions. Reaction 

fronts are most intense at the boundaries between zones or 

even between giant crystals. Alkalis, alkaline earths, F, 

and P tend to be redistributed during subsolidus alteration, 

but not high field-strength elements (e.g., Ti-Nb-Ta-Sn). 

Pegmatite-wallrock interactions. Metasomatic reactions 

between pegmatite-derived vapor and host rocks have 

provided insight into the timing and chemistry of pegmatite 

vapor exsolution. Wallrock alteration spatially mimics 

pegmatite zonation; all primary internal zones are crystallized prior to vapor loss from pegmatite. A large mass 

fraction of rare alkalis, B, and Fare lost from pegmatites 

and partly conserved by reactions in hosts. The chemical 

signature of wallrock alteration provides useful clues for 

exploration of internal zones, but few if any pegmatite 

mining activities have been initiated solely on the basis of 

the chemistry of wallrock metasomatism. 

Experimentation. Petrologic experiments under a wide 

range of conditions have added immensely to the understanding of likely processes in pegmatites. Most chemical, 

zonal, and textural features of pegmatites have been 

replicated experimentally. The effects of added components of Li, B, F, and P in addition to H20 on granite 

phase relations have been elucidated. High silica contents 
90 The Origin of Granites and Related Rocks 

plus these added components flux pegmatite melts to low 

temperatures, inhibit crystal nucleation, enhance element 

diffusion, forestall vapor saturation, and cause melt 

fractionation to initially peraluminous and eventually 

alkaline, silica-depleted melt compositions. Pegmatites 

associated with basic and alkaline rocks may be less 

common than granitic ones because the (less polymerized) 

liquid compositions of basic and alkaline magmas do not 

inhibit crystal nucleation and equilibration with melt, as is 

widespread in granitic systems. 

The Phosphorus Cycle in 

Peraluminous Granitic Magmas 

1

London, David, 1Wolf, Michael B., 

1

Morgan, G.B. VI, and 2Gallego, Marcos 1 School of Geology and Geophysics, 

University of Oklahoma, Norman, OK 73019, USA 2

Nuevo Baztan, 28809 Alcala de Henares, Madrid, Spain 

A distinctive feature of (mostly S-type) peraluminous 

granites, pegmatites, and rhyolites and their hydrothermal 

aureoles is an elevated P content compared to metaluminous and peraluminous I- or A-types. Whole-rock 

(WR) values > 0.2 wt % P20 5 are typical, and concentrations > 0.5 wt % are common. 

Reactions at the source. Apatite (Ap) and monazite 

(Mnz) are likely sources of P in metasedimentary 

protoliths. The solubility of Ap increases linearly with ASI 

(molar Al20/ M20 + MO) of melt from 0.1 wt % at ASI 

= 1 to 0.7 wt % P20 5 at ASI = 1.3. First-formed hydrous 

melts (Ms + Ab + Qtz + H20 = L + Als) possess an 

ASI between 1.3- 1.4, so that melts may contain up to 0.7 

wt % P2 0 5 contributed by Ap. The solubility of Ap 

increases with T, but for peraluminous magmas, initial 

low-T melting is likely to exhaust P in the source. The 

solubility of Ap is an activity product; increasing Ca ot F 

decrease the P content of melt at Ap saturation. REE (and 

Sr)-rich Ap melts incongruently to Mnz + P (and Sr)-rich 

melt; variations in Mnz solubility with ASI, if they occur, 

are below detection by electron microprobe analysis. 

Magmatic crystal fractionation. S-type granites are low 

inCa, and the crystallization of PI decreases the Ca:P of 

melt below that of Ap stoichiometry; this in part limits Ap 

saturation. In the absence of Ap, equilibrium among Bt, 

Fe-Mg-Mn (FM) phosphate (FMP, e.g., triplite), and melt 

buffers Pin peraluminous melt near 0,7 - 0.8 wt % P20s. 

The central facies of the S-type Albuquerque batholith, 

Spain, contains Bt-FMP assemblages at WR P20 5 of 

0.79 wt %. 

P is nearly compatible in alkali feldspars (At) via the 

exchange AIPSi_2; this substitution is not extensive in other 

silicates. As S-type melts fractionate toward increasing ASI 

(1.3 - 1.4), P partitions increasingly into Af. The partition 

coefficient, D/f1m, increases from 0.3 at ASI = 1 to 1.0 

at ASI = 1.35. Thus, as ASI increases, the Af become 

important reservoirs of P. In total, melt concentrations of 

p increase, but more slowly than for other more incompatible fluxing components (e.g., F, B). 

The combination of the experimental Dlflm with WR 

ASI provides a means of monitoring the P content of melt 

at various stages of Af crystallization. For the central 

facies of the Albuquerque granite, this method yields a 

calculated 0.8 wt % P20 5• Natural Af contain up to 2.6 wt 

% P20 5 (marginal layered aplite with WR P20 5 = 1.15 wt 

%, Albuquerque batholith) in rocks with WR ASI = 1.31; 

these values record P contents of melt well above the BtFMP and the Ap saturation limits (Bt and Ap are absent) 

and reveal the extent to which Pis not conserved in WR. 

Effects on liquidus relations. Increasing P drives melt 

compositions away from Qtz along an Or isopleth (Or28 at 

200 MPa[H20]). The association in melt of P with AI 

initially augments melt fractionation toward increasing 

ASI, though the limiting ASI of melt is 1.3 - 1.4 regardless of the P content or the Al-saturating phase (Ms, Als, 

C, Spl, etc.). Melt may eventually fractionate toward 

alkaline, sodic, and silica-depleted (but Qtz-saturated) 

compositions by the creation of an N aP03 component of 

melt. The solidus of the metaluminous haplogranite granite 

system (Ab-Or-Qtz-H20) plus P is unknown, but lies near 

or below 500°C at 200 MPa(H20). P may increase the 

solubility limits of H20 in melt. 

Late-stage reactions in melt. Evolved S-type magmas 

commonly attain saturation in the lithium aluminosilicates 

(spodumene, petalite and eucryptite in granites and 

pegmatites, B-phases in some rhyolites) and the phosphates 

moritebrasite-amblygonite [LiAlP04(0H,F)]; these are 

related by reactions such as LiA1Si40 10 (petalite) + 

Po2(0H) = LiAIPOiOH) (montebrasite) + 4 Si02• This 

P-buffering equilibrium is presently under investigation. 

Melt - vapor relations. P is substantially nonvolatile: 

Dp vaporlmett varies from 0. 3 for peraluminous to < 0.1 for 

metaluminous compositions. Only in alkaline compositions 

does P become soluble in vapor as NaP03• 
Subsolidus reactions. P appears to be incompatible in 

micas and clays, so that sericitic or argillic alteration of Af 

liberates P. Thus, magmas and WR do not hold their P for 

two reasons: 1) the NaP03 component in residual melt or 

magmatic vapor is highly soluble and, 2) alteration of Af 

is pervasive in many S-type granites (e.g., kaolin deposits 

of the UK, France). Hydrothermal veins containing a 

myriad of secondary phosphates manifest the near- to 

subsolidus flood of P from igneous Af and phosphates to 

endogenic or exomorphic hydrothermal aureoles. 

Ore deposits. P represents a likely ligand for high 

field-strength elements in melt or aqueous vapor. Preliminary experiments indicate that P enhances the solubilities 

of Sn (cassiterite) and Ta (manganotantalite) in melt, 

though the solubilities of these ore minerals are high even 

in simple haplogranitic melts. There is also evidence for 

stable W-P aqueous complexes, which may promote the 

formation of hydrothermal W-P deposits (e.g., 

Panasquiera, Portugal). 

Some Pampean Pre-Taconic 

and Taconic (Famatinian) 

Granites of 

Southwestern South America 

1

L6pez de Luchi, M.G. and 2Dalla Salda, L.H. 1

Centro de Investigaciones en Recursos Geologicos, 

J. Ramirez de Velasco 847, 

1414 Buenos Aires, 

Argentina 2

Centro de Investigaciones GeolOgicas, 

Calle 1 n 644, 1900 La Plata, 

Argentina 

Late Cambrian to Devonian granitoids are conspicuous 

rocks in the Famatinian Orogen, today the Pampean 

Ranges in Central Argentina. This orogen is considered 

as a Taconic collisional belt, developed by the interaction 

of Laurentia and South America, following the break-up 

of Rodinia. A precollisional subduction Andino-type 

regime along the Cambrian South American active margin 

produced the Pampean granitoids. Taconian granitoids are 

collisional rocks usually named as Famatinian granitoids. 

These plutonites may be used as tracers of the geodynamic 

evolution of the orogenic belt. 

Abstracts 91 

Three groups of granitoids are distinguished on the basis 

of their geochemical signatures, ages and their relationships to deformational events: 1) Pre-Taconic (Pampean); 

2) Taconic (Famatinian); and, 3) Late Taconic (Acadian ?) 

granitoids. The Pre-Taconic Group rocks (mostly 520-460 

Ma) are represented by batholiths and minor plutons 

showing slightly discordant to concordant contacts, 

moderate to strong deformation, extensive mylonitization 

and recrystallization. Granitoids are mainly biotite and 

biotite-hornblende tonalites and granodiorites with minor 

monzogranites, gabbros and diorites. Calcic to calcalkaline, metaluminous to moderately peraluminous 

compositions are typical. Sequences have a magmatic arc 

signature with differences arisen from their relative 

geological setting and erosion level. 

The Taconic Group rocks (mostly 460-420 Ma) are 

concordant plutons associated with migmatites and 

pegmatites or minor bodies intruded in the Pre-Taconic 

granitoids. They are chiefly biotite-muscovite monzogranites with minor syenogranites and granodiorites with 

metasedimentary xenolites. Apatite, garnet, cordierite and 

epidote are common accessories. Calc-alkaline to alkalicalcic peraluminous compositions are the rule. All of 

them share a collisional signature, that means crustal 

protoliths and a restricted compositional range. 

The Late Taconic Group rocks (mostly 420-360 Ma) are 

batholithic-sized plutons of syenogranitic composition. A 

NNE fault controlled emplacement and contact aureoles 

are characteristic. These rocks are alkali-calcic, peraluminous to slightly peralkaline, MgO poor crustal 

granites. Geotectonic diagrams show their collisional to 

late tectonic affinities. The youngest plutons may also be 

related to the Acadian tectonic event. 

These three groups might suggest a similar geotectonic 

setting to the contemporaneous granitoids in the Appalachian orogen. 

This paper is a contribution to IGCP Projects 376, 345 and 319. 
92 The Origin of Granites and Related Rocks 

ORAL PRESENTATION 

Intrusive Degassing at the 

Magma/Wallrock Interface 

Lowenstem, Jacob B. 

U.S. Geological Survey, M.S. 910, 

Menlo Park, CA 94025, USA 

Evidence from a wide variety of sources indicates that 

most erupted rhyolites contained 3 to 7 wt % dissolved 

H20 prior to their eruption. As yet, there is no substantive evidence that granitic magmas contain less H20 than 

their volcanic counterparts. The presence of sulfur and 

C02 in rhyolitic magmas requires that they, and by 

analogy granitic magmas, are vapor-saturated at crustal 

depths less than 10-12 km. For example, a granitic 

magma at 750°C, containing 5 wt % H20 and 1000 ppm 

C02 would be saturated with vapor at any pressure less 

than about 300 MPa ( - 10 km). At that pressure, the 

equilibrium vapor would contain -60 mol % H20, the 

rest being C02. Ascent to the shallow depths at which 

ore-forming porphyries are emplaced (3 to 4 km) must 

involve further magma vesiculation, with the vapor 

becoming more H20-rich as decompression progresses. 

Upon reaching a depth of 3 km, the magma would contain 

-10% vesicles by volume. The combined processes of 

magma convection, Stokes bubble ascent and percolation 

should allow exsolved fluid to coalesce at the roof of the 

magma reservoir, prior to intermittent fluid-loss events 

associated with fracturing of wallrock. Granites rarely if 

ever contain evidence for the former presence of tens of 

vol % of exsolved fluid, implying that mechanisms exist 

for magma to degas. 

One piece of evidence for the former presence of 

exsolved fluid at the magma/wallrock interface is comblayering and other types of coarse-grained textures composed of quartz and alkali feldspar. These textures are 

found near the tops of Mo porphyries, W-skarns, and Cu 

porphyries (Shannon et al., 1982). Layers of coarsegrained "pegmatitic" quartz alternate with fine-grained 

~plites that often contain b-form (high-temperature) quartz, 

mterpreted to be early-grown magmatic phenocrysts. The 

coarse layers contain a-form quartz, which grows at 

temperature less than 600°C at 100 MPa. I suggest two 

possibilities for growth of the low-temperature quartz 

form: 1) the magma solidus temperature was reduced 125° 

below that of the H20-saturated haplogranite system; or, 

2) the crystals grew from a hydrothermal fluid in equilibrium (and in contact) with a supercooled silicate liquid. 

The first scenario could occur if the solidus temperature 

were lowered by addition of F, B, Li, or some other flux. 

However, this should force the melt toward more albitic 

compositions, whereas the bulk compositions of most 

comb-layered rocks (including the aplite interlayers) have 

very little normative or modal albite/plagioclase. Potentially, an albite-rich melt could crystallize coarse-grained 

quartz and orthoclase due to surface-energetic effects; i.e. , 

the addition of B, Li, F, etc. could poison albite nucleation 

and growth. This would result in a residual liquid extremely rich in albite-component. Residual liquid formed 

in such a manner would have to escape the system without 

crystallizing or causing sodic alteration, as there is no 

evidence for either albite-rich pods or sodic metasomatism 

in or near the comb-layered rocks. 

Instead, comb-layering may form at the interface 

between supercooled magma and exsolved magmatic fluid, 

similar to the suggestions of Kirkham and Sinclair ( 1988). 

Heat loss due to conduction, exsolution of volatiles, and 

adiabatic ascent of melt/fluid mixtures will all cause 

cooling of the magma during its intrusion as a high-level 

porphyry. Because crystallization rates for degassed silicic 

magmas are extremely slow, they can remain at subsolidus 

temperatures for years without solidifying. Pockets of 

exsolved fluids that accumulate at the roof of the intrusion 

would be expected to form crystals larger and more wellformed than those from coexisting supercooled melt. 

Increases in fluid pressure would fracture the wallrock 

thereby quenching adjacent silicate melt to form aplite, and 

altering wallrocks by released magmatic fluids. Fracture 

sealing and accumulation of magmatic fluid would reinitiate growth of the comb layering. 

Kirkham, R.V. and Sinclair, W.D., 1988. Comb quartz layers in felsic 

intrusions and their relationship to porphyry deposits. In Taylor, R.P. 

and Strong, D.P. (eds), Can. Inst. Mining Metal. Sp. Iss., 39, 50-71. 

Shannon, J.R., Walker, B.M., Carten, R.B. and Geraghty, E.P., 1982. 

Unidirectional solidification textures and their significance in 

determining relative ages of intrusions at the Henderson Mine 

Colorado. Geology, 10, 293-297. ' 
Some Thermal Constraints on Crustal 

Assimilation in Calc-Alkaline Batholiths 

Matile, Lusius, Thompson, Alan Bruce 

and Ulmer, Peter 

Departement fur Erdwissenschaften, ETH Zurich, 

CH-8092 Zurich, Switzerland 

Simple thermal models constrain the extent of assimilation of adjacent rocks by an ascending magma, independently of geochemical considerations. We have examined 

the degree of melting of crustal materials of differing 

fertility by a rising hydrous mantle magma, halted at 

various crustal depths. Fractional crystallization causes the 

melt to evolve from gabbro to tonalite-granodiorite and is 

accompanied by increasing water content in the progressively smaller melt volumes. A pseudobinary phasediagram for fractional crystallization has been constructed 

along a pseudocotectic line, based on geochemical modelling of calc-alkaline rock suites and experimental data of 

liquidus temperatures. The AFC-process has been modelled by mixing of assimilate and slightly supercooled basic 

melt with subsequent crystallization until the hybrid melt 

reached the liquidus composition corresponding to the 

lowered temperature. The heat balance involves heating 

up and partial melting of assimilate, cooling down and 

crystallization of the basic melt. Not included in the heat 

balance was the time dependent conductive heatflux to the 

country rock and the heat of mixing of the two melts. 

The composition of the assimilated melt mainly affects 

the ratio of the assimilated mass to the crystallized mass. 

Approximately the same amount of granodioritic and 

granitic melt can be assimilated but after mixing with 

granite less crystallization of the mantle magma is possible 

than after mixing with a granodioritic melt (because the 

composition of the "hybrid" liquid is shifted much closer 

to the liquidus composition). The influence of the composition of the country rock on the degree of melting is very 

critical for the possible amount of assimilation. 

Because the liquidus is much flatter (e.g. large change 

in composition for small change in temperature) in the 

fractionation range gabbro-tonalite, than for more differentiated compositions, greater amounts of heat of crystallization are released at high temperature at the beginning of 

the AFC-process. On the other hand much more energy 

is required to heat up the assimilate to the high temperatures of the gabbro liquidus than to temperatures of the 

granodiorite liquidus and, hence, more assimilation is 

Abstracts 93 

possible in the intermediate than in the undifferentiated 

fractionation range. This conclusion is in apparent 

contradiction to much geochemical evidence, which 

suggests that the principal contamination of calc-alkaline 

rock suites occurs at the beginning of the AFC-process. 

This means either that if assimilation has occurred in the 

generation of calc alkaline magmas, then the primary 

magmas contributing heat to the assimilation are high-Mg 

melts, e.g. picritic or boninitic, or that other physical 

factors such as rheology of mixing control the extent of 

assimilation for intermediate to evolved compositions. 

It is true that assimilation of adjacent rocks could be 

easier if they were already at temperatures above their 

solidi, but then it is likely that they would have already 

lost their low-melting fractions. The ambient temperature 

of the crust controls the amount of assimilation and the 

ratio of assimilation to crystallization, obviously both have 

the greatest values at high temperatures (e.g. at the base of 

the crust). In reality the effect of the ambient temperature 

is even bigger, as the neglected conductive heat flux to the 

country rock increases with decreasing ambient temperature. 

The approaches developed here enable us to begin to 

address the fundamental problems of the relation of size 

and shape of magma bodies to the melt-temperature 

evolution during cooling at specific crustal depths, and also 

to the tectonic environment permitting intrusion. 

Isotopic Constraints on Petrogenesis of the 

Jurassic Eagle Mountain Intrusion, 

Southeastern California 

1

Mayo, David P., 1Anderson, J. Lawford 

and 2

Wooden, Joseph L. 1

Department of Earth Sciences, University of Southern 

California, Los Angeles, CA 90089-0740, USA 2U.S. Geological Survey, 345 Middlefield Rd., 

Menlo Park, CA 94025, USA 

The 165 Ma Eagle Mountain intrusion is a 

metaluminous, calc-alkaline pluton intruding Proterozoic 

crystalline rocks and Paleozoic supracrustal rocks. The 

heterogeneous main phase of the intrusion grades 

compositionally from diorite through tonalite and 

granodiorite to monzogranite, and is intruded by small 

leucogranite bodies. Thermobarometric calculations 
94 The Origin of Granites and Related Rocks 

indicate main phase emplacement at about 2.5 kbar and 

solidus temperatures between 700° and 7500C. Main phase 

rocks, in spite of diverse bulk compositions (52 to 68 wt 

% Si02), have relatively uniform (

87Sr/86Sr)i (0.7083 to 

0.7094), eNi165) (-8.9 - -9.8), and feldspar Pb ratios 

f<>6Pbf04Pb = 18.8, 207Pbf04Pb = 15.7, and 208Pbf04Pb = 

39.1). 

In coordinates of eNd vs. 87Sr/86Sr, the Eagle Mountain 

intrusion and other Jurassic plutons in southeastern 

California (data from Miller and Wooden, 1994, and 

Young, 1992) form a curve extending from the evolved 

end of the mantle array (eNd = -7.0, 87Sr/86Sr = 0.7065) 

into a field defined by Proterozoic country rocks (eNd < 

-15, 87Sr/86Sr > 0.71) exposed in the region. In spite of 

similar bulk compositions, each individual Jurassic intrusion occupies a relatively limited segment of the Sr-Nd 

curve. All of the Jurassic intrusions have similar feldspar 

Pb isotopic compositions that resemble Proterozoic country 

rocks. 

The Sr and Nd data require the involvement of more 

than one isotopic reservoir in the petrogenesis of the Eagle 

Mountain intrusion, and elemental compositions require the 

source to be relatively mafic and Sr-rich. Variably 

enriched mantle components and Proterozoic country rocks 

were used as endmembers to fit mixing and AFC curves 

to the Sr-Nd array defined by the Jurassic plutons, with 

solutions specifically anchored to the Eagle Mountain 

intrusion. Although these models are non-mechanistic, 

they do provide some constraints on the primitive 

endmember composition and the amount of crustal recycling recorded in the intrusion. The Eagle Mountain data 

can be reproduced by mixing 81 to 93 % enriched mantle 

component (eNd = -7.0, Nd = 25 - 29 ppm, 87Sr/86Sr = 

0.7065, Sr = 665-749 ppm) with 19 to 7 % Proterozoic 

crust (eNd = -17.5, Nd = 45- 125 ppm, 87Sr/86Sr = 0.72 

- 0.83, Sr = 145 - 425 ppm), or by mixing 60 to 86 % 

bulk earth-like mantle component (eNd = 0.0, Nd = 27 -

24 ppm, 87Sr/86Sr = 0.7043, Sr = 680- 947 ppm) with 40 

to 14 % Proterozoic crust (eNd = -17.5, Nd = 45- 125 

ppm, 87Sr/86Sr = 0.72 - 0.75, Sr = 185 - 425 ppm). A 

MORB-like depleted mantle component can be ruled out 

because of the high Sr concentrations ( > 690 ppm) 

required by the data. AFC calculations (r = 0.4, Dsr = 

1, DNd = 0.35) require 9 to 23 % crystallization of an 

enriched mantle component or 19 to 41 % crystallization 

of a bulk earth-like component during assimilation of 

Proterozoic crust to reproduce the Eagle Mountain data. 

The Sr and Nd data indicate that most of the "new" 

crust added as magma in the Eagle Mountain intrusion was 

juvenile, and probably derived from relatively enriched 

mantle. This conclusion is masked in the Pb data, where 

the isotopic compositions of mantle-crust mixtures or 

mantle-derived magmas that have assimilated crustal rocks 

are strongly levered toward Pb-rich crust. The small 

isotopic variations observed within the Eagle Mountain 

intrusion are interpreted as 1) reflections of small 

endmember compositional heterogeneities or 2) minor local 

assimilation during emplacement. These results are 

consistent with Miller and Wooden's (1994) hypothesis 

that mantle-derived magmas intruded the crust during the 

Jurassic and were strongly contaminated to form hybridized lower crust. 

Miller, C.F. and Wooden, J.L., 1994. Anatexis, hybridization and the 

modification of ancient crust: Mesozoic plutonism in the Old Woman 

Mountains area, California. lithos, 32, 111-133. 

Sheet-like Assembly of the Jackass Lakes 

Pluton, Central Sierra Nevada, California: 

Multiple Mode Emplacement 

McNulty, Brendan A., Tong, Weixing and 

Tobisch, Othmar T. 

Earth Sciences Department, University of California, 

Santa Cruz, CA 95064, USA 

The Jackass Lakes pluton/wall rock system, central 

Sierra Nevada, California, presents an exceptional case 

study of the many physical processes that may operate 

during the emplacement of magma. Pervasive N- to 

NNW -striking structural and petrologic fabrics in the 

pluton/wall rock system indicate that the pluton formed via 

in-situ assembly of a shallow magma chamber fed by 

mafic and felsic injections. These N- to NNW-striking 

fabrics include: 1) the pluton's straight eastern and western 

contacts; 2) mafic microgranular enclave swarms, 

synplutonic dioritic dikes, and aplitic and coarse-grained 

granodioritic dikes within the pluton; 3) leucogranitic to 

granodioritic dikes which cut the country rock; 4) magmatic foliation in the pluton and dikes; 5) narrow elongate 

volcanic septa and pluton/wall rock interfingering from the 

meter- to kilometer-scale; and, 6) solid state foliation in 

the septa and marginal wall rock. 
Disrupted synplutonic dikes and mafic microgranular 

enclave swarms within the pluton document intrusion and 

mingling of mafic and felsic magma at the final emplacement site. We infer the orientation, distribution and nature 

of syn-plutonic dikes and enclave swarms (e.g. their 

presence or absence in long linear belts) to represent the 

former presence of mafic and possibly felsic NNWoriented magma feeder conduits. Pluton/wall rock 

interfmgering east of the pluton may represent an arrested 

nascent stage of the magma chamber. Within the pluton, 

transverse east -to-west fluctuations in magmatic strain and 

fabric intensity, and in enclave population (presence/absence), further support the presence of linear, 

although somewhat cryptic, NNW-striking magmatic belts. 

Cryptic sheeting, or the coalescence of sheets, may be the 

result of compositionally and thermally indistinct magma 

pulses, and/or a rapid intrusion rate. U-Pb zircon dating 

from different parts of the pluton indicates that assembly 

via multiple pulse injection occurred during the time frame 

-97.1 ± 0.7 - -98.5 ± 0.3 Ma. We propose that 

fracture initiation for magma sheet emplacement was 

generated within a weak dextral tectonic regime that 

produced N- to NNW -striking tension gashes. 

The Jackass Lakes pluton is a classic example of 

multiple physical processes acting together to accommodate 

the emplacement of magma. The wall rock exhibits 

rheological criteria for both fracture and ductile flow, and 

therefore we propose a hybrid emplacement model involving viscoelastic behavior of the wall rock ( cf Rubin, 

1993). Evidence of brittle wall rock behavior includes the 

presence of dikes (implied fractures), formation of the 

overlying and roughly penecontemporaneous Minarets 

Caldera (Peck, 1980; Fiske and Tobisch, 1994), and 

stoping in the subvolcanic part of the magma chamber. 

Evidence for ductile wall rock behavior includes 

well-developed solid state fabrics, strongly flattened 

volcanic clasts and ptygmatically folded dikes emanating 

from the pluton. Although we document that emplacement 

of the pluton was facilitated by multiple processes, the 

bulk of the evidence strongly suggests that episodic magma 

transport via fracture(s) was the primary mechanism 

through which it was assembled. 

Fiske, R. S. and Tobisch, 0. T., 1994. Middle Cretaceous volcanic 

rocks in the Minarets Caldera, east-central Sierra Nevada, California. 

Geol. Soc. Am. Bull., 106, 582-593. 

Peck, D., 1980. Geologic map of the Merced Peak quadrangle, central 

Sierra Nevada, California. U.S.G.S. Geol. Quad. Map GQ-1531. 

Rubin, A. M., 1993. Dikes vs. diapirs in viscoelastic rock. Earth and 

Planet. Sci. Lett., 117, 653-670. 

Abstracts 95 

Plutonism in a Rapidly Evolving 

Continental Rift System, 

Eldorado Mountains, Southern Nevada 

1

Miller, C.F., 1Patrick, D.W., 2Miller, J.S., 

3

Heizler, M.T., 4Wooden, J.L., 5Hook, S.J., 

1

Unkefer, J., 1Lee, Y.F.S. and 1Falkner, C.M. 1

Department of Geology, Vanderbilt University, 

Nashville, TN 37235, USA 2

Department of Geology, University of North Carolina, 

Chapel Hill, NC 27599-3315, USA 3 New Mexico Geochronology Research Lab, 

New Mexico Institute of Mining and Technology, 

Socorro, NM 87801, USA 4 US Geological Survey, Menlo Park, CA 94025, USA 5 Jet Propulsion Laboratory, 

California Institute of Technology, 

Pasadena, CA 91109, USA 

Middle Miocene intrusive activity in the Eldorado 

Mountains spanned a brief but eventful period, beginning 

during early stages of tectonism that preceded rifting and 

terminating during waning extension. In the northern part 

of the range, an 18.5-13 Ma sequence of mafic, intermediate, and felsic volcanic rocks is imbricated by rotated 

normal faults. To the south, deeper levels of the Miocene 

crust are exposed. Hypabyssal to mid-crustal plutons and 

numerous dikes intrude Proterozoic basement and Mesozoic granites and comprise more than 60% of the subvolcanic crust. The plutons were emplaced between 16.2 

and 15.7 Ma and therefore preceded peak extension ( cf 

Gans et al., 1994). Much smaller volumes of magma 

were emplaced as dikes through 14.2 Ma, spanning the 

interval of peak extension. 

Six plutons with a total area of - 180 km2 are exposed 

beneath the volcanic sequence. They include, from north 

to south (

40 Arf39 Ar ages): 1) Keyhole Canyon, 15.5 Ma, 

very felsic (granite-leucogranite); 2) Nelson, 16.2 Ma 

(with a 15.8 Ma lobe), dominantly intermediate (quartz 

monzodiorite, quartz monzonite); 3) Aztec Wash, 15.7 

Ma, bimodal (granite, diorite); 4) Searchlight, 16.2 Ma, 

bimodal; 5) Rockefeller Mine, not yet well dated (U-Pb 

zircon lower intercept confirms mid-Miocene age; appears 

to be cut by Searchlight), intermediate (granodiorite, 

quartz monzodiorite); and, 6) Copper Mountain, undated, 

felsic (granite, leucogranite). Dikes range in age from 

synchronous with Nelson pluton to 14.2 Ma. Earlier dikes 

are mostly intermediate; those synchronous with Aztec 
96 The Origin of Granites and Related Rocks 

Wash pluton include both mafic and felsic; and post-Aztec 

Wash dikes are mostly mafic. Fabrics and hornblende 

barometry suggest that all intrusive rocks except the 

Rockefeller Mine pluton are hypabyssal, emplaced at < 5 

km. The fabric of the Rockefeller Mine pluton, which 

was emplaced at - 10 km, reflects much slower cooling 

and syn- to post-magmatic ductile deformation. 

With the exception of the Nelson pluton, the plutons are 

consistent with the regional pattern of northward younging 

of magmatic activity in the Colorado River region. The 

Nelson pluton is also anomalous in its emplacement style: 

it intruded in dike-on-dike fashion along an E-W active 

fault. This fault can be interpreted as either a sinistral 

strike-slip fault or a rotated, down-to-the-north dip-slip 

fault; neither interpretation is consistent with the E-W 

extension that occurred slightly later. N -S dike orientations and structures within plutons (e.g. N-S oriented 

mafic injections into felsic magma chambers) elsewhere in 

the Eldorado Mountains document the E-W extension that 

characterizes rifting here. The Keyhole Canyon and 

Nelson plutons and volcanic strata to the north were 

imbricated and tilted steeply eastward along rotating N -S 

normal faults; Aztec Wash pluton remained largely intact 

and untilted; and the Searchlight, Rockefeller Mine, and 

probably Copper Mountain plutons were tilted westward. 

Structures in the relatively deep-seated Rockefeller Mine 

pluton suggest that it was undergoing E-W stretching 

during and immediately after emplacement, and that it 

underwent subsequent rapid unroofing, probably during 

westward tilting. 

The Keyhole Canyon, Aztec Wash, and Searchlight 

plutons are bimodal, dominated by granite (- 72 wt % 

Si02 ) and mafic trachyandesite (- 54 wt % Si02) with 

spectacular mingling textures but limited mixing. Olivinerich cumulates from the trachyandesite in Aztec Wash 

pluton have as little as 44 wt % Si02 and up to 22 wt % 

MgO. Aztec Wash pluton experienced repeated injections 

of both felsic and mafic magma; what we interpret to be 

the lower portion of the magma chamber is dominated by 

mafic rock, and the upper portion is uniformly granite. 

The Nelson and Rockefeller Mine plutons also span a 

broad compositional range (- 50-70 wt % Si02), but they 

are dominantly intermediate (- 57-65 wt % Si02). 

Although all intrusives are enriched in K and all 

incompatible trace elements, as expected for rift-related 

magmas, they are calc-alkaline and plot with arc-related 

rocks on discrimination plots. We suspect that this 

geochemical signature reflects the ancient lithospheric crust 

and mantle beneath southern Nevada rather than the extant 

tectonic setting. Lead isotopic compositions of mafic to 

felsic rocks match the distinctive range of ancient regional 

(Mojave Province) crust and inferred lithospheric mantle 

(high 208PbJ204Pb (- 39); high 207Pbf204Pb (- 15.58) 

relative to 206PbP04Pb (- 18.2)). Initial eNd and (

87Sr/86Sr)i 

span a range that lies between the composition of regional 

crust and that of depleted mantle (-5 to -11, 0.7075 -

0.71 05). The isotopic compositions correlate well with 

composition among the bimodal plutons (more felsic rocks 

are more evolved isotopically). Higher incompatible 

element concentrations in mafic than in felsic rocks is not 

explained by mixing between depleted mantle-derived 

melts and crust; rather, a source for the mafic magmas in 

ancient enriched mantle is suggested ( cf. Ormerod et al. , 

1988; Farmer et al., 1989; Beard and Glazner, 1995). 

Additional complexities in sources are indicated by 

compositions of late mafic dikes, which are among the 

most isotopically evolved, and samples from the Nelson 

pluton, which are displaced toward higher eNd at a given 

87Sr/86Sr compared to the main trend. 

The Nelson pluton intruded while the crust was undergoing tectonic activation, but before the onset of active EW rifting. Its early appearance and intermediate composition are consistent with generalizations by Gans et al. 

(1989) about tectonomagmatic sequences in the Basin and 

Range. The other plutons constitute a major, rapid 

addition to the upper crust synchronous with the onset of 

rapid extension, suggesting the possibility that they 

accommodated extension and facilitated collapse of the 

overlying uppermost crust (cf Gans et al., 1989). 

However, there is no direct evidence that they facilitated 

such a collapse: they predate by several hundred thousand 

years imbrication of the volcanic sequence that is exposed 

to the north. 

Beard, B.L. and Glazner, A.F., 1995. Trace element and Sr and Nd 

isotopic composition of mantle xenoliths from the Big Pine volcanic 

field, California. J. Geophys. Res., 100, 4169-4179. 

Farmer, G.L., Percy, F.V., Semken, S., Crowe, B., Curtis, D. And 

DePaolo, D.J., 1989. Isotopic evidence on the structure and origin 

of subcontinental lithospheric mantle in southern Nevada. J. Geophys. 

Res., 94, 7885-7898. 

Gans, P.B., Mahood, G.A. and Schermer, E.R., 1989. Synextensional 

magmatism in the Basin and Range Province: A case study from the 

eastern Great Basin. Geol. Soc. Am. Spec. Paper, 223, 53 p. 

Ormerod, D.S., Hawkesworth, C.J., Rogers, N.W., Leeman, W.P.and 

Menzies, M.A., 1988. Tectonic and magmatic transition in the 

Western Great Basin, USA. Nature, 333, 349-353. 
Construction of Mid-Crustal Magma 

Chambers During Regional Contraction, 

North Cascades, Washington 

1

Miller, Robert B. and 2Paterson, Scott R. 1 Department of Geology, San Jose State University, 

San Jose, CA 95192-0102, USA 2

Department of Earth Sciences, University of Southern 

California, Los Angeles, CA 90089, USA 

Many recent models for pluton emplacement emphasize 

extension as a dominant space-making mechanism, either 

in dilational jogs within strike-slip zones or in normal fault 

systems. Conversely, the mid-crustal levels of many active 

arcs are constructed while undergoing contraction, and 

magma emplacement during contraction is well-documented in several ancient arcs. One such arc is the > 

1500-km-long Coast Belt of the North American Cordillera. Cretaceous plutonism in the south-east extension of 

this belt, the Cascades core, was synchronous with 

amphibolite-facies metamorphism and major crustal 

shortening. Markedly different styles of syn-contraction 

emplacement are illustrated by relatively elliptical 96-90 

Ma plutons and deeper, sheet-like, 75-65 Ma plutons. 

These observations raise the question of how magmas were 

emplaced during contraction. 

The 96-93 Ma Mt. Stuart batholith forms two main 

bodies, the largest of which is - 50 x 20 km, and crystallized at 2-4 kbar. Petrological variations in this dominantly 

tonalitic intrusion define complex internal zoning, elliptically shaped internal bodies, dikes and petrographic 

banding. Structural and petrologic data suggest that the 

batholith was constructed from small to moderate sized 

(100s to 1000s m3

) batches of fractionated and in places 

mingled magmas that were emplaced as crystal poor melts 

( < 50% crystals). The igneous structures described above 

are overprinted by magmatic foliation and lineation, which 

defme small- and large-scale folds, batholith-scale sigmoidal patterns, and lobate margin-parallel patterns. These 

patterns reflect simultaneously operating body forces and 

regional deformation. 

Space was made during emplacement by multiple 

country-rock material-transfer processes that include: 1) 

regional folding and faulting during SW-NE contraction; 

2) regional horizontal NW-SE extension; 3) vertical and 

horizontal ductile flow in a narrow (0.1 to 0.2 body radii) 

structural and thermal aureole that accounts for ~ 30% of 

the space, and possibly by stretching of roof rocks; 4) 

Abstracts 97 

possibly large-scale stoping and minor assimilation; and, 

5) minor fracturing associated with dike injection. 

A zone of 75-65 Ma, sheet-like plutons that crystallized 

at 6-8 kbar lies NE of the Mt. Stuart batholith. Regional 

structures in these plutons and country rocks indicate that 

NE-SW contraction with a component of SW -directed flow 

and NW -SE sub horizontal extension occurred during 

emplacement. We have mapped the dominantly tonalitic, 

Entiat and Cardinal Peak plutons, which are highly 

elongate (length: width > 10: 1) and contain widespread 

internal sheets that range from > 2 km wide and > 6 km 

long to - 10 em wide and decameters long. The 

compositionally heterogeneous north-west ends of these 

plutons consist of hundreds of sharply bounded sheets that 

range from hornblendite to granodiorite. To the SE the 

Cardinal Peak pluton consists of more homogeneous 

tonalite bounded by meter- to decameter-long rafts of 

country rock and slightly more mafic intrusive rocks. In 

the Entiat pluton, sheets are also less distinct to the SE and 

are marked by differences in texture and modal abundances. Sheets mainly strike NW and dip steeply, but in 

places are folded around N-S to NW-SE plunging axes. 

Thus, lateral compositional and structural changes in these 

chambers are as large as vertical changes proposed in 

other chambers. 

Well-developed magmatic foliation and lineation in both 

plutons are parallel to or crosscut sheets, bands and pluton 

contacts, defme large and small-scale folds, and along with 

axes of folded sheets have similar orientations to regional 

country rock structures. Magmatic structures thus formed 

after construction of the magma chamber by injection of 

multiple sheets and during regional folding. 

Both the sheet-like and Mt. Stuart plutons are elongate 

perpendicular to the regional contraction direction, presumably at high angles to the far-field a 1 axis. This is 

puzzling for the sheeted plutons, as the widespread rafts 

imply that sheets were repeatedly injected along fractures 

somewhat analogous to a crack-seal process. 

We conclude that regional contraction strongly influenced magma chamber dynamics of the sheet-like bodies 

by controlling chamber construction (sheeting), lateral 

compositional variations, later flow (and thus strain) during 

crystallization, and subsolidus deformation. In contrast, the 

effects of regional contraction on construction of the Mt. 

Stuart batholith are less obvious and contraction mainly 

served as a relatively minor ( < 20%) space-making 

mechanism. These differences may reflect the shallower 

crustal level (7-13 km) of the Mt. Stuart versus the sheeted 

plutons (20-25 km), and the sizes of individual batches of 
98 The Origin of Granites and Related Rocks 

magma. The emplacement of all three plutons was apparently not strongly controlled by fault zones, in contrast to 

models for other plutons emplaced during contraction. 

The Ardara Pluton, Co Donegal, NW 

Ireland: A Straightforward Example 

of a Ballooning Pluton 

Molyneux, S.J. and Hutton, D.H.W. 

Department of Geological Sciences, University of 

Durham, South Road, Durham, DHJ 3LE, UK 

The Ardara pluton occurs at the SW margin of the 

Donegal Batholith, NW Ireland and is a 2/3 stage 

sub-circular pluton, with a concordant aureole of deformed 

and thermally metamorphosed Neoproterozoic metasedimentary rocks. This pluton has been the subject of 

extensive study in recent years, it has variously been 

interpreted as: an intrusive diapir (Akaad, 1956a, 1956b); 

a ballooning pluton (Berger and Pitcher, 1972; Holder, 

1979); a pluton combining ballooning and stoping (Vernon 

and Paterson, 1993). 

The project has involved a remapping and reexamination of the granitic phases and the contact features, 

measurement of strain from mafic enclave sizes and Fry 

crystal spacing, together with timing and kinematic 

indicators from pre-full crystallization (PFC); and solid 

state fabrics. In particular, the Fry method has been 

extensively applied to the pluton, in order to examine the 

strained distributions of different mineral phases, using the 

grain size normalised method. Fry strains are consistent 

with (initial-shape-adjusted) mafic enclave strains, allowing 

extension of strain analysis into areas of low mafic enclave 

density. 

The general conclusions are: a) the pluton appears to 

have been emplaced from a point offset to the south west 

of its centre, expanding in concordance with the country 

rocks, preferentially in a northwards direction; b) during 

emplacement, deformation was confined almost entirely to 

the magmatic state, solid state deformation occurring only 

at the northern rim of the pluton in a metres-wide region; 

c) there is no evidence that the pervasive fabric within the 

pluton was formed by an superimposed regional strain, 

except in the tail where it is crossed by the Main Donegal 

Granite shear zone; d) mafic enclaves and Fry spacing 

show that strains are flattening in nature (K < < 1), a 

feature consistent with a balloon model for emplacement; 

e) kinematic indicators associated with PFC fabrics are 

consistent with emplacement directed towards the north, 

the regional shear zone to the south perhaps acting as a 

barrier to emplacement. 

Northward directed ballooning is consistent with earlier 

structural work in the aureole (Menielly, 1982), which 

showed circumferential stretching around the northern part 

of the pluton together with local enhancement of the 

regional D4 deformation. Such ballooning in a preferred 

direction is seen in other well-known plutons around the 

world e.g. Flamanville, France and Criffell, Scotland. 

Akaad, M.K., 1956a. The Northern Aureole of the Ardara Pluton, Co. 

Donegal. Geol. Mag., 93, 337-392. 

Akaad, M.K., 1956b. The Ardara granite diapir of Co. Donegal, 

Ireland. Quart. J. Geol. Soc., 112, 263-287. 

Berger, G. and Pitcher, W.S., 1972. Examples of diapiric intrusion. In 

Pitcher, W.S. and Berger, A.R. (eds), The Donegal Granites, a Study 

in Emplacement and Unroofing, J. Wiley and Sons. Chapter 8. 

Holder, R.A., 1979. An emplacement mechanism for post-tectonic 

Granites and its implications for the geochemical features. In 

Atherton, M.P. and Tamey, J. (eds.), Origins of Batholiths, 

Geochemical Features, Shiva Publishing Limited, p. 117-126. 

Meneilly, A. W., 1982. Regional structure and syn-tectonic granite 

intrusion in the Dalradian of the Gweebarra Bay area, Donegal. J. 

Geol. Soc. London, 139, 633-646. 

Vernon, R.H. and Paterson, S.R., 1993. The Ardara pluton: deflating an 

expanded intrusion. Lithos, 31, 17-32. 

Accumulation of Rare Earth and High Field 

Strength Elements in Peralkaline Granitoids: 

The Galifieiro Pluton, NW Spain 

Montero, Pilar G. 

Department of Mineralogy and Petrology, 

Campus Fuentenueva, University of Granada, 

18002 Granada, Spain 

The Galifieiro Pluton is a composite body formed by Atype peralkaline granitoids with very _l;ligh contents of rare 

earth and high field strength elements. Rb-Sr (whole-rock) 

and U-Pb (zircons) isotopic dating suggest an age of 460 

Ma, with later recrystallization at 320 Ma during the peak 

of Hercynian metamorphism. The Galifieiro Pluton is 

formed by three rock types: amphibole-biotite 

orthogneisses, riebeckite-aegirine orthogneisses, and 

metasomatic rocks. Amphibole-biotite orthogneisses have 

quartz + oligoclase + microcline + biotite + hastingsitic 
hornblende as major minerals, and zircon, titanite, apatite 

and fluorite as accessories. Riebeckite-aegirine 

orthogneisses have microcline + albite + riebeckitearfvedsonite + aegirine ± annitic biotite as main minerals. 

The accessory mineral assemblage is composed of zircon, 

magnetite, ilmenite, fluorite, astrophyllite, titanite, pyrite, 

chalcopyrite, sphalerite, and an overabundance of REBsaturated minerals such as fergusonite, samarskite, 

aeschynite, unknown Yb- and Y- silicates and niobotantalites, bastnaesite, allanite, monazite, xenotime, thorite, 

and uraninite, which appear either as single crystals or, 

more frequently, forming aggregates with complex 

intergrowths. Metasomatic rocks - customarily called 

"radioactive gneisses"- are composed by quartz + albite 

+ microcline + low-Al biotite ± gruneritic amphibole, 

with no riebeckite or aegirine. They have the same 

accessory mineral assemblage as riebeckite-aegirine 

orthogneisses, but the abundance of REB-saturated minerals is much higher. 

Amphibole-biotite gneisses have mol (Na20 + 

K20)/ Al20 3 - 0.8, Fe/(Fe + Mg) - 0.68, (Na20 + 

K20) - 8.5 wt % and (Na20/K20) - 1.13. Riebeckiteaegirine orthogneises have very low Al and Ca contents, 

mol. (Na20 + K20)/Al20 3 ;-;::: 1, Fe/(Fe + Mg) - 0.96, 

Na20 + K20 - 8 wt %, and Na20/K20 - 1.18. Radioactive gneisses have (Na20 + K20)/Al20 3 - 0.85, Fe/(Fe 

+ Mg) - 0.9, and low K20 contents with Na20/K20 -

6.5. All Galifieiro rocks have high REE and HFSE 

contents, which may occasionally reach economic levels in 

radioactive gneisses. The average composition of 

amphibole-biotite rocks is - 40 ppm Y, 66 ppm Nb, 10 

ppm Ta, 270 ppm Zr, 20 ppm Ga, 3 ppm U, 13 ppm Th, 

1850 ppm F, 53 ppm La, 116 ppm Ce, 8 ppm Dy, 4 ppm 

Yb, with LaN/LuN - 14.2 and Eu*/Eu - 0.75. 

Riebeckite-aegirine orthogneisses have - 303 ppm Y, 350 

ppm Nb, 35 ppm Ta, 1500 ppm Zr, 40 ppm Ga, 45 ppm 

U, 260 ppm Th, 2220 ppm F, 241 ppm La, 495 ppm Ce, 

43 ppm Dy, 20 ppm Yb, with LaN/LuN - 10.2 and 

Eu*/Eu - 0.11. Radioactive gneisses have - 1790 ppm 

Y, 1850 ppm Nb, 155 ppm Ta, 5865 ppm Zr, 45 ppm Ga, 

125 ppm U, 826 ppm Th, 6500 ppm F, 482 ppm La, 1444 

ppm Ce, 178 ppm Dy, 68 ppm Yb, with LaN/LuN - 12.4 

and Eu*/Eu - 0.15. Some peralkaline orthogneisses are 

extremely enriched in HREE, Y, and Be. 

Values of eNd - + 2.2 and the geochemical features of 

Galifieiro rocks suggest they were produced by differentiation of a mantle-derived magma. Original magmas were 

already REE and HFSE-rich, likely generated by melting 

of a strongly metasomatized area of the mantle. These 

Abstracts 99 

magmas were emplaced at high crustal levels where they 

underwent extensive fractionation. Residual melts were 

enriched in F, which stabilized RE and HFS elements due 

to the formation of complexes, probably alkali-Zr-fluorides. At the end of magmatic crystallization, a fluid phase 

rich in REE and HFSE complexes was released, causing 

intense local metasomatism either on peralkaline or country 

rocks. The decrease in fluoride activity as a consequence 

of lower temperatures and the formation of F-rich mica 

and amphibole produced the breakdown of the fluoride 

complexes and so the massive precipitation of REE and 

HFS, forming aggregates of REB-saturated minerals. The 

magmatic and metasomatic parageneses, due to their 

richness in F, are easily reactivated by inputs of energy 

and water, thus causing remobilizations and a total 

recrystallization during the Hercynian metamorphism, as 

reflected in U-Pb ages of hydrothermal zircons. 

Melt Inclusion, Matrix Glass and 

Mineral Compositions From a Zoned 

S-Type Peraluminous Rhyolite Suite, 

Morococala Volcanic Field, Bolivia 

1

Morgan, VI, G.B., 1London, D. 

and 2

Luedke, R.G. 1 University of Oklahoma, School of Geology 

and Geophysics, Norman, OK 73019, USA 2U.S. Geological Survey, National Center MS-959, 

Reston, VA 22092, USA 

The Miocene peraluminous tuffs and domal flows of the 

Morococala volcanic field, Bolivia, record a temporal 

sequence of early (basal) andalusite ± biotite ± muscovite 

rhyolites (AR), middle cordierite + biotite rhyolites (CR), 

and latest biotite quartz latites (QL). Quartz phenocrysts 

in all three rock types contain abundant euhedral (negative 

crystal-shaped) melt inclusions. Melt inclusions in the QL 

and CR are glassy, whereas those in the AR are typically 

devitrified; prior to analysis, the solids in the latter 

inclusions were homogenized to glass in R-41 cold-seal 

reaction vessels (750°C, PH 0 = 0.5 kbar). Melt inclu- 2 

sions are homogeneous within and between two samples of 

the QL (ASI = 1.15 - 1.23, Na/K = 0.92 - 1.08, F = 

0.21 - 0.29, Cl = 0.15- 0.19), but have higher AI and Na 

versus lower Si and Fe than perlitic glass in the matrix of 

one sample. Similarly, melt inclusion compositions are 
100 The Origin of Granites and Related Rocks 

fairly homogeneous within and between four samples of 

the CR (ASI = 1.21 - 1.27, Na/K = 0.80 - 1.09, F = 

0.35- 0.42, Cl = 0.16- 0.20), and with the exception of 

slightly higher F, are essentially identical to matrix glasses 

in all samples. Among four samples of the AR, melt 

inclusion compositions show considerable scatter both 

within and between samples (ASI = 1.21 - 1.40, Na/K = 

1.05 - 1.80, F = 0.20 - 0.60, Cl = 0.04 - 0.09), even 

among different inclusions within a single phenocryst. 

EMPA totals for all the melt inclusions are high (average 

wt %: QL = 97.4, CR = 97.9, AR = 98.1), suggesting 

low H20 contents. All melt inclusions show lower 

contents (by 0.5 - 0.1x) of Ca, Mg, and Fe than do their 

whole rocks. 

Sanidine (-Ab27) shows oscillatory zonation in barium 

throughout the QL and CR, but is unzoned in the AR; the 

molar celsian component of sanidine decreases from 1.4 -

2.2% in the QL, to 0.6% in the CR, and 0.2% in the AR. 

Plagioclase compositions become more evolved from the 

QL (cores = An2747 0r2_5 , rims = An29_31 Or4) through the 

CR (cores = An34420r2_

3 , rims = An220r5), and finally to 

the AR (cores = An13_21 Or4 ; rims = An11_

20 0r4-6). The 

Mg# for biotite decreases from QL (0.43 - 0.49), through 

CR (0.36 - 0.37), to AR (0.22 - 0.32), whereas F contents 

increase. Biotite displays zonation only in F, and in 

almost all sampl~s increases sharply from core to rim (wt 

%averages: QLcore = 1.16; QLrim = 1.74; CRcore = 1.41; 

CRrim = 1. 72; ARcore = 2.65; ARrim = 3.57); a similar 

increase is noted for F in muscovite from the AR (core = 

2 .19, rim = 3. 65 in wt %) . Biotites are unzoned with 

respect to Cl. The Cl contents of biotites increase slightly 

from QL (0.21 - 0.25 wt %) to CR (0.25 - 0.26 wt %), 

but are lower and more variable (0. 09 - 0.14 wt %) in the 

AR. Biotite appears to have become unstable (is altered 

and/or resorbed) in two of four samples of the AR. 

Muscovite was identified in only two of four samples of 

the AR, and its presence does not correlate with the 

apparent instability of biotite. 

The compositions of melt inclusions, matrix glasses, 

whole-rocks, and phenocrysts are all consistent with the 

origin of this sequence from a single zoned rhyolitic 

magma chamber in which the AR represent the apical most 

evolved portion. Core-rim increases of F in biotite could 

be explained by increased F activity in melt accompanying 

either progressive melt fractionation or devolatilization, but 

a lack of Cl zonation in biotite is inconsistent with the 

latter. The lower average Cl contents of both melt 

inclusions and biotite in the AR suggests a greater degree 

of melt devolatilization in the apical portion of the magma 

chamber. Homogeneity of melt inclusions in the QL and 

CR suggest little lateral heterogeneity within the lower 

regions of the magma chamber. Melt inclusions within the 

AR apparently record a quite different scenario for the 

apical portion of the chamber. Variations in average 

compositions between individual samples suggest the 

operation of processes giving rise to heterogeneity at 

length scales of tens to thousands of meters, and could 

involve gradients in temperature or volatile contents, or 

variable degrees of country rock (roof pendant) assimilation. Variability of melt inclusions in individual samples 

and phenocrysts, and variability in AI contents of quartz 

phenocrysts also suggest fine-scale disequilibrium in the 

AR. 

Kilometer Scale Vertical Translation, 

Concordancy, and Nonrotation of 

Porphyroblasts Surrounding the Papoose 

Flat Pluton: Inclusion Trails and AMS 

Reveal the Kinematics of Emplacement 

1 Morgan, Sven, 1 Law, Richard, 

2

Saint Blanquat de, Michel 

and 2

Bouchez, Jean-Luc 1

Department of Geological Sciences, Virginia Tech, 

Blacksburg, VA 24061, USA 2

Laboratoire de Petrophysique et Tectonique, 

Universitie Paul Sabatier, 38 Rue des 36 Ponts, 

F-31400 Toulouse, France 

Consistent inclusion trail geometry from andalusite 

porphyroblasts within the contact aureole of the Papoose 

Flat pluton reveals that wall rocks were translated vertically out of the Inyo Mt. Anticline during intrusion, and 

that porphyroblasts have not rotated, with respect to 

compositional layering, even though the wall rocks have 

been thinned to less than 10% of their original thickness. 

Restoring the wall rocks back into the Inyo Mt. anticline, 

by assuming a vertical motion out of the anticline's 

southwest dipping limb, returns the planar inclusion trails 

into a position parallel with the regionally developed slatey 

cleavage, consistent with our palinspastic restoration, and 

also explains the orientation of the well developed stretching lineation. Restoring the wall rocks back to the east in 

a horizontal movement, which is suggested by examination 

of the map pattern deflection of aureole rocks, produces an 
inconsistent inclusion trail geometry from the north side of 

the pluton to the south side, and does not explain the 

orientation of the lineation. 

The Harkless Formation has been deformed into 

concordancy with the western margin of the pluton for 

over 16 km, and defines a domal shape around the pluton 

at the present level of exposure, dipping away from the 

pluton on the north, west, and south sides. If the Harkless 

Formation is "undomed" from concordancy and placed into 

a planar reference frame, not only do inclusion trails have 

a constant angular relationship to the foliation, but all 

inclusion trails become aligned parallel to one another 

indicating that porphyroblasts grew when the Harkless 

Formation was originally planar, pre-dating the doming 

event and present emplacement position of the pluton. 

Andalusite porphyroblasts exhibit a two stage growth 

history; an initial period of static growth where andalusite 

overgrows the regional slatey cleavage (pre-doming/ 

inflation) and a second period of growth synchronous with 

the extreme attenuation and translation, where inclusion 

trails curve into the matrix foliation, representing the 

rotation of the matrix into parallelism with compositional 

layering. 

Our model also simplifies the "space problem" by 

including wall rock translation in strain/volume analysis. 

Concordant aureoles cannot be explained by simple, in situ 

shortening, and previous studies of strain in pluton-wall 

rock systems consistently calculate volumes of granite 

much greater than wall rock strains can provide space for. 

We argue that this space problem arises because translation of wall rocks has not been taken into account. 

Assuming, 1) a homoclinal structure for the pre-pluton 

emplacement position within the western limb of the Inyo 

Mt. anticline, the wall rocks surrounding the western 

margin of the pluton have been translated approximately 

2.4 km vertically in order to attain their present structural 

position, which is out of place with respect to the 

homocline. Also assuming that, 2) the 90 % shortening is 

related to stretching of the aureole rocks around the pluton 

as the pluton inflated vertically, we calculate the shape of 

the pluton, and therefore it's area at depth that the surrounding aureole rocks must have translated around in 

order to account for the 90% thinning. Our volume 

compatible model predicts a flat bottomed pluton and is 

inconsistent with diapiric transport. 

Detailed AMS and microstructural analysis reveals that 

magmatic fabrics are parallel to solid state fabrics. In the 

western portion of the pluton-aureole system, where the 

pluton has uplifted the furthest out of the southwest 

Abstracts 101 

dipping limb of the anticline, the grain shape and AMS 

lineation is best developed and has a constant trend from 

the deformed aureole, through the gneissic border of the 

pluton, and into the magmatic interior. Solid state and 

magmatic foliation's are subparallel to contacts. The 

parallelism of solid state and magmatic foliation's indicates 

that they developed synchronously and are related to the 

same event. 

Our analysis reveals a two stage intrusive history. An 

initial intrusion, possibly as a sill, statically metamorphosed the Harkless Formation and caused porphyroblasts 

to overgrow the regionally developed slatey cleavage. A 

second pulse of magma caused the sill to inflate vertically, 

translating and attenuating the immediate aureole, which 

had been thermally weakened. Extreme attenuation of the 

aureole rocks does not rotate porphyroblasts, allowing 

them to be used as "fossil directions" (Fyson, 1980). In 

this model, aureole rocks which have been translated the 

most vertically, should exhibit the most intense attenuation. 

The increasing amount of strain in the aureole to the west, 

where the wall rocks have been translated the most to 

attain their present structural position, is consistent with 

the idea that aureole strain is related to the amount of 

vertical translation. This emplacement model is consistent 

with the inclusion trail analysis, orientation of the stretching lineation, AMS data, increasing amount of strain to the 

west, and is volume/strain compatible. 

Fyson, W.K., 1980. Fold fabrics and emplacement of an Archean 

granitoid pluton, Cleft Lake, Northwest Territories. Can. J. Earth 

Sci., 17, 325-332. 

Geochemistry and Structural 

Petrology of the Father Pluton, 

Abitibi Subprovince, Canada 

Moukhsil, Abdelali 

Sciences de la Terre, Universite du Quebec a 

Chicoutimi, Chicoutimi, Quebec Canada G7H 2Bl 

The NW -SE elongated Father Pluton is a syntectonic 

intrusion in the Northern Volcanic Zone of the Abitibi 

Subprovince. The pluton ranges from an early mafic 

tonalite to a late porphyritic granodiorite phase. 

Geochemically the rocks are calc-alkaline, meta-aluminous 

to peraluminous, and related by a process of fractional 

crystallization involving hornblende, biotite, oligoclase, 
102 The Origin of Granites and Related Rocks 

apatite, epidote and sphene. The fractionated granodiorites 

are enriched in sphene and magnetite and show positive Ta 

anomalies. Rare earth patterns show LREE enrichment 

compared to HREE. Changes in rare earth abundance and 

shape of REE patterns with pluton evolution may be 

correlated to the crystal fractionation. Amphibole fractionation is also suggested by a diminishing Sc content with 

increasing Si02• Mineral analyses (biotite, hornblende and 

plagioclase) suggest that the pluton was emplaced at a 

temperature of 700-800°C and a pressure of 1. 6 to 2. 9 

kbar. 

The pluton occurs between a major E-trending fault 

with a dextral strike-slip component (Doda fault) in the 

north and several lesser faults in the south. The principal 

magmatic foliation is oriented NE-SW and is subvertical. 

A NW -SE foliation is associated with early phases of the 

pluton and appears to be cut by the principal foliation. 

The mineral lineation lying in the foliation plane is 

subhorizontal. Local ductile deformation zones occur 

parallel to both foliation directions. It is believed that the 

early foliation reflects mechanical conditions controlling 

the onset of pluton injection, which subsequently changed 

to those governing the main emplacement. 

Field measurements of foliation and lineation are based 

on xenolith, phenocryst and mafic mineral alignment. 

Further detailed laboratory analysis of oriented sections 

revealed two types of alignment; one characterized by 

alignment of biotite and hornblende masses, plagioclase 

and some potassium feldspar phenocrysts, and the other by 

potassium feldspar phenocrysts, isolated biotite and 

hornblende grains and some quartz. Analysis on the XZ 

plane shows a number of kinematic markers, chiefly 

crystal rotation and tiling indicating a dextral movement. 

The two principal foliation directions may be related to 

subsidiary directions (P and R) of the Doda fault, and the 

pluton is interpreted to have intruded while this fault was 

active. Pluton injection was principally subhorizontal, as 

indicated by the magmatic lineation. 

Sodic Granitoid Magmas Formed by 

Melting of Mafic Lithosphere: 

The Separation Point Batholith, 

New Zealand 

Muir, R.J. and Weaver, S.D. 

Department of Geology, University of Canterbury, 

Christchurch, New Zealand 

The Early Cretaceous Separation Point Batholith of the 

South Island, New Zealand, represents the final magmatic 

stage of an extensive arc system located on the SW Pacific 

margin of Gondwana during the Mesozoic. The batholith 

consists of Na-rich, alkali-calcic diorite to biotitehornblende monzogranite. The rocks have a distinctive 

chemistry, atypical of calc-alkaline subduction related 

granitoids, but comparable with those of adakites and 

Archaean trondhjemite-tonalite-dacite suites. 

All of the granitoids are characterized by high Al20 3 

(16 - 23 wt %), high alkalis (Na20 + K20 = 8 wt %; 

with Na20/K20 = 1.5 - 2.0), high LIL/HFS element 

ratios, low Y ( <20 ppm) and unusually high Sr (typically 

> 1000 ppm). REE abundance patterns are strongly 

fractionated ( (La/Yb )N > 20) with negligible Eu anomalies. Low concentrations of the heavy REE (down to 3 x 

chondritic abundances) suggest that garnet was present in 

the source region. Primitive Sr ((87Sr/86Sr)i = 0.7042) and 

Nd isotopic ratios (t:Nd(120 Ma) = + 1.2 - + 1.8) and the 

absence of inherited zircon, indicate that Separation Point 

magmas experienced little, if any, interaction with felsic 

crust. The distinctive chemical and isotopic features of the 

batholith, including high Na20 and Al20 3 , low heavy REE 

andY contents together with high Sr/Y ratios, suggest that 

these rocks were derived from the partial melting of a 

mafic source with residual garnet and amphibole. The 

lack of Eu anomalies and the unusually high Sr content 

preclude significant plagioclase in the source residue. On 

the basis of this observation, two likely sources for the 

origin of the Separation Point magmas can be envisaged; 

young, hot, subducted oceanic lithosphere or newly 

underplated basaltic crust beneath a thickened continental 

arc. The latter model is preferred because Separation 

Point rocks do not possess MORB isotopic characteristics, 

and cannot be explained as mixtures of MORB-melt and 

continental crust. 

Melting of amphibolitic underplate must have occurred 

at depths of >40 km (P > 1.3 GPa) under water 

undersaturated conditions. This took place either as a 
consequence of magmatic underplating and uplift at the 

base of the thickened arc, or more speculatively, was 

perhaps triggered by back-arc basin closure. A similar 

model has recently been proposed for the generation of the 

Cordillera Blanca Batholith of Peru. We note that in both 

the Peruvian and New Zealand cases, the adakitic/ 

trondhjemite - tonalite - dacite magmas were emplaced 

inboard of older calc-alkaline batholiths. 

REE-Depleted Leucogranites as a 

Consequence of Disequilibrium Melting 

of Monazite-bearing Schists, 

The Black Hills, South Dakota 

Nabelek, Peter I. and Glascock, Michael D. 

Department of Geological Sciences and 

MU Research Reactor, University of Missouri, 

Columbia, MO 65203, USA 

The Black Hills, South Dakota, include one of the 

finest examples of leucogranite-pegmatite complexes 

generated by partial melting of metasedimentary protoliths. 

The granites formed as a result of the collision of the 

Wyoming and Superior Archean cratons 1.7 Ga ago 

(Redden et al., 1990). The Harney Peak Granite and its 

satellite intrusions formed by emplacement of hundreds of 

dikes and sills. They comprises two spatially and isotopically distinct suites. Both can be related to melting of 

protoliths equivalent to the metasedimentary rocks into 

which the granites were intruded. The dikes and sills in 

the core of the Harney Peak granite are two-mica granites 

with average o180 value of 11.5 , whereas the peripheral 

tourmaline granites have relatively elevated o180 values 

which average 13.2%o (Nabelek et al., 1992). 207PbP04Pb 

ratios inK-feldspars from the core intrusions are higher 

than inK-feldspars from the peripheral intrusions, with the 

former indicating a crustal history of the protolith since the 

Archean and the latter indicating a short Proterozoic 

history (Krogstad et al., 1993; Walker and Krogstad, this 

volume). Leucosomes in wall-rocks that have undergone 

partial melting have compositions similar to the high-o180 

granites. 

The B and Ti02 concentrations in the two suites are for 

the most part distinct. The peripheral high-o180 granites 

have high B/Ti02 ratios (0.1-30.0) which indicates 

dehydration-melting of muscovite, the major source for B, 

Abstracts 103 

but at temperatures below the breakdown of biotite, the 

major source for Ti in the protolith metasedimentary 

rocks. In contrast, the B/Ti02 ratios in most (but not all!) 

samples of the low-01SO suite are low (0.01-0.07), indicating dehydration melting of biotite in the generation of 

these melts. Dehydration melting temperatures are also 

indicated by oxygen isotopic equilibration temperatures 

among minerals in excess of 750°C. 

The chondrite-normalized REE patterns in samples with 

low B/Ti02 ratios are LREE-enriched with negative Eu 

anomalies. The patterns are similar to those in the 

Archean and Proterozoic metasedimentary rocks in the 

Black Hills and most crustal sediments. In contrast, the 

patterns of samples with high B/Ti02 ratios are LREEdepleted. LREE depletions, analogous to those in the 

LREE-depleted granites, are also found in the leucosomes 

of the migmatites. The LREE-depleted patterns also 

correlate with very low Th concentrations in the samples. 

It is proposed that the LREE and Th depletions are the 

consequence of disequilibrium melting of schists involving 

monazite. Petrographic observations, back-scatter electron 

images and X-ray maps of migmatized schists that have 

undergone small extents of melting without the breakdown 

of biotite reveal that monazite remained armored by biotite 

(some also by garnet) in melanosomes. This prevented the 

equilibration of monazite with the melt. Application of the 

monazite saturation model of Montel (1993) demonstrates 

that REE concentrations in the LREE-depleted samples are 

below those necessary for monazite saturation even at 

temperatures as low as 6()()0C. In contrast, REE concentrations in the LREE-enriched granites yield 700-800°C 

monazite-saturation temperatures that are reasonable for 

biotite-dehydration melting reactions. The breakdown of 

biotite permitted the exposure of monazite in the protolith 

to equilibration with the partial melts, yielding normal 

crustal REE patterns. 

This study demonstrates that LREE-depletion in 

granites may say little about the relative rates of melt 

extraction and melt equilibration with accessory minerals 

in the residue, inasmuch as monazite may remain armored 

by biotite at melting conditions below biotite breakdown . 

Instead, the concentrations of LREE's indirectly reflect the 

different dehydration reactions that can lead to leucogranite 

melts. 

Redden, J.A., Peterman, Z.E., Zartman, R.E., and DeWitt, E., 1990. 

U-Th-Pb geochronology and preliminary interpretation of Precambrian tectonic events in the Black Hills, South Dakota. In Lewry, J .F. 

and Stauffer, M.R. (eds), The Early Proterozoic Trans-Hudson 

Orogen, Geol. Assoc. Can. Spec. Paper, 37, 229-251. 
104 The Origin of Granites and Related Rocks 

Nabelek, P.l., Russ-Nabelek, C., and Haeusser, G.T., 1992. Stable 

isotope evidence for the petrogenesis and fluid evolution in the 

Proterozoic Harney Peak leucogranite, Black Hills, South Dakota. 

Geochim. Cosmochim. Acta, 56, 403-417. 

Krogstad, E.J., Walker, R.J., Nabelek, P.I., and Russ-Nabelek, C., 

1993. Lead isotopic evidence for mixed sources . of Proterozoic 

granites and pegmatites, Black Hills, South Dakota USA. Geochim. 

Cosmochim. Acta, 51, 4677-4686. 

Montel, J.M., 1993. A model for monazite/melt equilibrium and 

application to the generation of granitic magmas. Chem. Geol., 110, 

127-146. 

ORAL PRESENTATION 

Cretaceous Granitoids in 

Southwest Japan and 

Their Bearing on 

Crust-Forming Processes in the 

Eastern Eurasian Margin 

Nakajima, Takashi 

Geological Survey of Japan, 1-1-3 Higashi, 

Tsukuba, Jbarald 305, Japan 

The Cretaceous granitic rocks and associated regional 

metamorphic rocks in Southwest Japan were formed by a 

Cordilleran-type orogeny. Southwest Japan was regarded 

as a hypothetical cross-section of upper to middle crust of 

the Eurasian continental margin in the Cretaceous, comprising high-level granitoids (called San-yo type) and 

weakly- to unmetamorphosed accretionary complexes 

which are exposed on the back-arc side, and low-level 

(Ryoke type) granitoids with high-grade metamorphites up 

to migmatitic gneisses on the fore-arc side. This granitic 

magmatism and metamorphism is an essential part of the 

crust-forming process at the continental margin. The 

lowest-level granitoids are foliated concordantly with 

associated gneisses and migmatites, and higher-level 

granitoids intrude them cross-cutting those foliations and 

gneissosities. The lower-level granitoids never intrude into 

the high-level ones. 

All these granitoids are of ilmenite series and predominantly !-type with subordinate amount of garnet- or 

muscovite-bearing varieties in some Ryoke type ones, but 

none of them contains cordierite. These mineralogical 

variations are likely to depend more on their slightly 

peraluminous chemistry rather than the difference of 

pressure during crystallization. 

Granitoids of both levels have similar cooling ages 

(given by K-Ar and Rb-Sr biotite ages) and show a 

systematic along-arc variation of eastward younging but 

the magmatic ages (given by SHRIMP U-Pb zircon ages) 

are slightly different between different levels. In the 

eastern part of Southwest Japan, the granitoids of the both 

levels give K-Ar biotite ages of approximately 65 Ma, 

while the magmatic age of high-level granitoids are 

approximately 70 Ma and are 15 Ma younger than the 

lower-level ones of nearly 85 Ma. It implies that the 

formation of the middle crust had started approximately 15 

Ma prior to the upper crust. Generally, the Cretaceous 

granitoids in Southwest Japan have a similar cooling rate 

of 20-40°C/Ma during cooling from 500°C to 300°C, 

judging from K-Ar hornblende and biotite ages. This 

cooling rate is too large to spend 20 Ma from crystallization to 300°C. The middle crust material was kept over 

500°C for 15-20 Ma after solidification, presumably 

foliated and recrystallized at deep beneath, and then it 

cooled down together with the upper crust to 300°C 6-7 

Ma after the formation of the upper crust. The coincidence of cooling history below 500°C of the upper and 

middle crust might reflect the regional uplift of the crust. 

The (

87Sr/86Sr)i ratios of the Cretaceous granitoids in 

southwest Japan have an along-arc lateral variation; 

relatively low (0. 705 - 0. 708) in the western part and high 

(0.707 - 0.711) in the eastern part of Southwest Japan. 

Also their neodymium isotopic ratios have an along-arc 

variation correlative to the strontium. These isotopic 

variations could mean that the granitic magma have been 

generated through the interaction with some crustal 

materials. The lateral variation patterns of (

87Sr/86Sr)i of 

the low-level granitoids and the high-level granitoids are 

slightly different. The low-level granitoids have higher 

( 87Sr/86Sr)i than those of high-level ones in the middlewestern part (Chugoku district) but it seems opposite in the 

eastern part. It may imply that they might have formed by 

separate magmatic pulses of c.15 Ma interval, even though 

they are not independent and are all in a big single geological event. 
Panafrican Stratoid Granites 

of Madagascar: 

A New Post-Collisional 

Alkaline Province 

1

Nedelec, A., 2Stephens, W .E. 

and 1

Bouchez, J.L. 1 Petrophysique, URA 67 CNRS, 

Universite Paul-Sabatier, Toulouse, France 2

Department of Geology, 

University of St Andrews, Scotland 

A major alkali granite province of late Panafrican age 

(around 585 Ma) occupies central Madagascar. It takes 

the form of sheets interlayered in gneissic and migmatitic 

basement. The thickness of the granitic sheets ranges from 

< 0. 5 m to > 500 m. Their total volume is around one 

quarter of the c. 7 km-thick outcropping midcrustal section 

in the studied area (NNW of Antananarivo). 

All granitic rocks are metaluminous and display A-type 

characteristics. Two distinct suites have been recognized: 

a mildly alkaline suite, including subsolvus granites; and, 

a strongly alkaline suite, including alkaline hypersolvus 

granites and quartz-syenites. LILE-enriched monzodioritic 

enclaves are ubiquitous. The strongly alkaline suite is 

characterized by higher Fe, Mn, Zr, Zn and REE contents 

and lower Al, Ca and Sr than the mildly alkaline suite. 

Major and trace element geochemistry and oxygen isotope 

data suggest that the mildly alkaline suite was derived from 

a granodioritic crustal protolith, whereas the strongly 

alkaline suite was derived from a LILE-enriched mantle 

source (Nedelec et al., 1995). 

A detailed structural study was carried out using the 

AMS (anisotropy of magnetic susceptibility) technique 

(Nedelec et al., 1994). The foliations follow a N-S strike 

and are gently dipping to the west. The lineations trend 

mostly to the WSW. The structures are mainly magmatic 

in the granitic rocks and result from a pronounced annealing in the gneisses. In the absence of obvious shear sense 

indicators in the field, quartz c-axis fabrics display either 

symmetrical patterns or asymmetrical patterns giving 

opposite shear senses. It is therefore concluded that the 

stratoid granites were emplaced under a roughly coaxial 

strain regime. 

Madagascar in its late-Proterozoic configuration was 

involved in the Panafrican orogeny of eastern Africa. 

Abstracts 105 

Emplacement of the stratoid granites was contemporaneous 

with high-T- low-P metamorphism. Underplating of a 

large volume of basic magmas is thought to be responsible 

for this high geothermal gradient and for partial melting of 

the continental crust. The late-Panafrican age and the 

structural and thermal data are consistent with postcollisional extensional tectonics as a result of lithospheric 

delamination in the Mozambique belt. 

Nedelec A., Paquette J.L., Bouchez J.L., Olivier P. and Ralison B., 

1994. Stratoid granites of Madagascar: structure and position in the 

Panafrican orogeny. Geodinamica Acta, 7, 48-56. 

Nedelec A., Stephens W.E. and Fallick A.E., 1995. The Panafrican 

stratoid granites of Madagascar: alkaline magmatism in a postcollisional extensional setting. J. Petrol., in press. 

Geochemistry of Granitic Rocks 

and Their Minerals from 

the Kavala Pluton, 

Northern Greece 

1

Neiva, A.M.R., 2Christofides, G., 

2

Eleftheriadis, G. and 2Soldatos, T. 1

Department of Earth Sciences, Coimbra University, 

3000 Coimbra, Portugal 2

Department of Mineralogy, Petrology, 

Economic Geology, Thessaloniki University, 

GR 5406 Thessaloniki, Greece 

The Kavala pluton mainly is composed of metaluminous 

granodiorite, with predominance of a medium- to coarsegrained gneissic granodiorite over a coarse-grained 

granodiorite. Both are cut by aplitic and mafic dykes. 

Metaluminous diorite and tonalite and metaluminous-toperaluminous granite occur locally. Diorite, tonalite and 

part of each granodiorite have magnesian hastingsitic 

hornblende > Mg-biotite, while other parts of each 

granodiorite have Mg-biotite > hornblende, and granite 

contains only Mg-biotite. There are neither mineralogical 

nor chemical distinctions between the gneissic and the nongneissic granodiorites. They contain numerous enclaves of 

metaluminous magnesio-hornblende + Mg-biotite diorite 

and tonalite. All contain magnetite. 

For bulk-rock compositions, variation diagrams of most 

major and trace elements indicate fractionation trends from 

diorite to granite, but enclaves fall outside the trends in the 

diagrams of Al, Fe, Mg, K, V, La, Nb, Rb and Sr/Rb, 
106 The Origin of Granites and Related Rocks 

and also in diagrams of trace elements of hornblende and 

biotite, and of Ca and Mn of magmatic epidote (23 - 29 % 

pistacite content). The variation diagrams of Nb of rocks, 

Zr, Ce, La, Nd, Rb and Rb/K of hornblende, Ca and Mn 

of epidote and Mn and Mn/Fe2+ of allanite indicate that 

the diorite does not belong to this sequence. This is 

supported by the fact that its plagioclase has an anorthite 

content similar to that of tonalite plagioclase, and because 

its REE pattern is cut in the HREE by the subparallel REE 

patterns from tonalite to granite. Therefore the sequence is 

of tonalite, hornblende-biotite granodiorite, biotitehornblende granodiorite and granite. Least square analysis 

of major elements and modelling of trace elements indicate 

that hornblende > biotite granodiorite, biotite > 

hornblende granodiorite and 1-type granite were derived 

from tonalite magma by fractional crystallization of 

plagioclase, hornblende, biotite and quartz. However K0 

values [(X~/Xi/e )1(X:::;xF~b)] are > 1 in diorite, tonalite and 

granodiorite, higher than normally observed in igneous 

rocks, but is < 1 in enclaves. So biotite of all rocks, 

except enclaves, could have been formed by replacement 

of hornblende, which is interpreted as a magmatic process, 

because there is increase in F, Rb, Rb/K and decrease in 

Cr, V, Sc, Nd, Ni, Ni/Fe2

+, Ni/Mg in biotite of the rock 

sequence. 

Based on hornblende composition, a pressure up to 6 

kbar for tonalite and granodiorite and a temperature up to 

790°C for tonalite and an average temperature of 750°C for 

granodiorite were calculated. Based on biotite composition 

and its equilibrium with alkali feldspar and iron oxides, a 

pressure of / 0 - 10-13 bar for tonalite and granodiorite 2 

magmas and PH 0 - 4 kbar for tonalite magma and - 6 2 

kbar for granodiorite magma were estimated. Log 

( !Hp/ fHF), log (In 0 / !He) and log (!Hell JHF) are similar 

in tonalite and grctnodiorite magmas. The tonalite and 

granodiorite were completely crystallized about 615-505°C 

(estimated from feldspar compositions). 

ORAL PRESENTATION 

Experimental and Theoretical Constraints 

on the Differentiation of 

High Temperature Crustal Magmas 

Nekvasil, Hanna, Jensen, Charles 

and Carroll, William 

Department of Earth and Space Sciences, 

State University of New York, 

Stony Brook, NY 11794-2100, USA 

Calculated phase equilibria in the granite system (i.e., 

the system Ab-Or-Qz-An) indicate major differences 

between the compositional evolution of water-poor, high 

temperature magmas (e.g. trachytic and syenitic liquids) 

and water-rich magmas (granitic liquids). Such calculated 

phase relations can provide important insights into possible 

crystallization trends in regions difficult to determine 

experimentally. For these trends to be meaningful, 

however, they must be pinned by experimental data. For 

high temperature magmas, the effect of water on the 

cotectic compositions in the bounding binaries of the 

granite system provides important constraints, since during 

differentiation high temperature magmas will likely 

traverse much of the water-undersaturated region towards 

water-saturation. Experiments were, therefore, conducted 

to determine the effect of water on the eutectics in the 

systems An-Or, Ab-Qz and Or-Qz. These data constrain 

the haplogranite cotectic surface in the system 

Ab-Or-:Qz-H20 and the 4-phase surface in the granite-H20 

system. Because of the importance of determining the 

effect of water alone, these experiments were designed to 

determine the location of the anhydrous eutectic and 

water-saturated eutectic at high enough pressure to permit 

an enhancement of the effects of water without the use of 

H20-C02 fluids. Nekvasil and Carroll (1993) obtained 

reversed experimental data in the system An-Or at 11.3 

kbar and showed that with increasing water content, the 

Or content of the melt also increases but to a much lesser 

extent than that determined by Ai and Green (1989). The 

new data imply that the maximum Or enrichment (in 

projection) in the melt as melts evolve along the 4-phase 

surface (i.e., during the coprecipitation of plagioclase, 

alkali feldspar, and quartz in the granite- water system) is 

10 wt % instead of 20 wt %. These data further imply 

that coprecipitation of plagioclase and alkali feldspar will 

result in Or- enrichment of the magma. The extent of 
this, however, will decrease with larger bulk water 

contents and higher bulk Ab contents. Experiments in the 

system Ab-Qz(-H20) were also conducted under dry and 

H20-saturated conditions. At 10 kbar there is an unequivocal shift of the eutectic towards Qz with increasing water 

content. This shift may, however, be as small as 3 wt % 

(from dry to water-saturated) towards Qz at this pressure. 

In contrast, experiments in the system Or-Qz indicate a 

very strong shift of the eutectic towards Qz with increased 

water content. At 10 kbar this shift is as much as 23 - 25 

wt % (from dry to water-saturated conditions). These data 

imply that unlike the inference of Pichavant et al. (1992) 

the projected quartz + alkali feldspar + L surface in the 

haplogranite-H20 system does not twist in three dimensional space and thus cannot project as a point at the 

minimum in the water-free haplogranite system. Instead, 

the surface narrows as the Ab-Qz sideline is approached. 

This in turn implies that Qz-enrichment of the melt will 

always occur during precipitation of quartz and feldspar 

from an H20-undersaturated, H20-unbuffered magma. 

The more albitic the melt is when it reaches this surface 

(at the onset of coprecipitation of alkali feldspar and 

quartz) or when it reaches the 2 feldspar + quartz + L 

surface in the granite system the less the extent of Qz 

enrichment. Importantly, it is possible that albitic melts 

may not show this Qz-enrichment trend if H20-C02 fluids 

are present. Experimental data of Holtz et al. (1992), in 

which the effect of water was determined using H20-C02 

fluids, indicate that increasing water content in the system 

Ab-Qz results (in projection) in a decrease in Qz content 

of the liquid in contrast to the increase seen in the system 

Or-Qz. The differences between the results with and 

without C02- bearing fluids suggests that there may be 

(perhaps unquenchable) alkali carbonate complexing in the 

fluid which renders the silicate melt peraluminous. In this 

case, higher C02 contents of the vapor (and hence lower 

water contents) would result in increased peraluminocity. 

Voigt and Joyce (1991) have shown that increasing 

peraluminocity shifts the projected quartz-albite eutectic to 

higher Qz contents. Thus, an increase of the activity of 

water in H20-C02 fluids would result in two competing 

effects. First, the effect of increased water in the melt 

would be to shift the eutectic towards Qz. Second, the 

effect of decreasing C02 activity in the fluid would be to 

decrease the peraluminocity of the melt and shift the 

eutectic towards the feldspar (Ab or Or). Since the effect 

of water alone is so small at the Ab + Qz eutectic (and 

the alkali feldspar quartz + L surface so narrow near the 

Ab-Qz sideline) C02 can readily cause a net shift of the 

Abstracts 107 

eutectic away from quartz with increasing activity of water 

in the fluid. Near the Or- Qz sideline, however, the effect 

of C02 cannot outweigh the effect of water and the shift in 

eutectic composition with increasing water activity in the 

presence of a H20-C02 fluid remains in the same direction, 

albeit smaller in magnitude, as for increasing water content 

alone. 

Ai, Y. and Green, D.H., 1989. Phase relations in the system anorthitepotassium feldspar at 10 kbar with emphasis on their solid solutions. 

Mineral. Mag., 53, 337-345. 

Holtz, F., Johannes, W. and Pichavant, M., 1992. Peraluminous 

granites: the effect of alumina on melt composition and coexisting 

minerals. Trans. Royal. Soc. Edinburgh: Earth Sci., 83, 409-416. 

Nekvasil, H. and Carroll, W.J., 1993. Experimental constraints on the 

high-temperature termination of the anhydrous 2 feldspar + L curve 

in the feldspar system at 11.3 kbar. Am. Mineral., 78, 601-606. 

Pichavant, M., Holtz, F. and McMillan, P.F., 1992. Phase relations and 

compositional dependence of H20 solubility in quartz-feldspar melts. 

Chern. Geol., 96, 303-319. 

Voight, D.E and Joyce, D.B.,1991. Depression of the granite minimum 

by the addition of sillimanite. EOS Trans. Am. Geophys. Union, 12, 

304. 

The "Mantle-Like" Component 

in Felsic Complexes of 

the Lachlan Fold Belt 

Nicholls, Ian and Bagaric, Steve 

Department of Earth Sciences, 

Monash University, 

Clayton 3168, Australia 

A number of studies of Silurian-Devonian granitic and 

felsic volcanic rocks of the Lachlan Fold Belt, eastern 

Australia (most recently e.g. Woodhead and McCulloch, 

1993) have presented Sr-Nd-Pb isotopic evidence for the 

involvement of "mantle-like" source components, representing end-members of apparent mixing trajectories to 

contrasted "crustal" components. The simplest interpretation of these "mantle-like" components is that they reflect 

the characteristics of mafic magmas which underplated and 

intruded the crust of the Lachlan Fold Belt (LFB), and 

which helped provide and transport the heat necessary for 

crustal melting. 

Most LFB felsic complexes which are dominated by 

hornblende-bearing metaluminous to weakly peraluminous 

("I-type") rocks include one or more candidates for 

representatives of "mantle-like" components. These 
108 The Origin of Granites and Related Rocks 

typically take the form of mafic-intermediate (gabbro-tonalite) minor plutons, basaltic-andesitic dykes and 

micro granitoid (microtonalite-microgranodiorite) enclaves 

within felsic host rocks. Recent detailed studies of the 

petrography and trace element and isotope geochemistry of 

these more mafic rock types and their hosts have provided 

evidence for the nature of processes of granitic magma 

evolution and emplacement, and also for the characteristics 

of "mantle-like" components associated with LFB 

magmatism. 

The 405 Ma Swifts Creek hornblende granodiorite 

pluton, eastern Victoria (Eberz et al., 1990) contains up to 

40 vol % of hornblende microtonalite enclaves, with 

diameters ranging from a few centimetres to tens of 

metres. These are typically rounded and often have 

fine-grained, scalloped margins -features which are most 

compatible with emplacement as globules of a hot fluid 

crystal-poor magma into a partially crystallized felsic host 

magma rich in coarse quartz and feldspar crystals. 

"Trails" of the latter crystals within some enclaves provide 

strong evidence for physical mingling and incomplete 

chemical mixing between enclave and host magmas. This 

interpretation is supported by observed gradations between 

the trace element and isotopic compositions of enclaves 

and host, which together define apparent two-component 

mixing trajectories between "mantle-like" and "crustal" 

end-members. Enclaves which lack quartz and feldspar 

megacrysts (( 87Sr/86Sr)i - 0. 7060; eNd - -3.5) plot near 

possible "mantle-like" end-members. Mixing models most 

compatible with both trace element and isotope 

systematics, involving "crustal" end-members with 

(

87Sr/86Sr)i > 0.710 and eNd - -9.0, indicate a "mantle-like" end-member with (

87Sr/86Sr)i - 0. 705 and eNd -

-2.0, which has significantly higher incompatible trace 

element abundances than possible "depleted mantle" 

components (McCulloch and Chappell, 1982). A component with isotopic and trace element compositions similar 

to "depleted mantle" requires mixing with unrealistically 

large proportions of complementary "crustal" components 

to meet trace element abundance constraints. 

The 385 Ma Mt. Stirling-Mt. Buller complex, 

east-central Victoria, includes the major hornblendebearing Mt. Stirling Granodiorite, which contains abundant 

microtonalite enclaves, and several gabbro-diorite stocks. 

Below the summit of Mt. Buller, at a level near the roof 

of the pluton, a complex zone of interfingering between 

the granodiorite and a variably megacrystic, apparently 

hybrid fine-grained microtonalite phase is exposed. Srand Nd-isotopic data for whole rock samples from the 

largest gabbroic stock, the microtonalites of the complex 

zone and enclaves in the granodiorite show a narrow range 

of isotopically primitive compositions ((87Sr/86Sr)i = 

0.7038-0.7045, eNd - +4.0- 4.3). These compositions 

are more primitive than those of some tonalitic representatives of the 400 Ma "!-type" Bega Batholith ((87Sr/86Sr)i 

-0.7041 and eNd - +3.0, McCulloch and Chappell, 

1982; Woodhead and McCulloch, 1993) and they are 

isotopically almost identical with the "depleted mantle" 

component used by the former authors. The host Mt. 

Stirling Granodiorite is only slightly more radiogenic 

((87Sr/86Sr)i -0.7043 and eNd - +2.6). This raises the 

possibility that the entire Mt. Stirling-Mt. Buller gabbro-granodiorite suite was directly related by crystal 

fractionation to parental basaltic magmas with strong 

"mantle" signatures, with a significant crustal input 

(probably due to crustal assimilation/fractional crystallization) apparent only in the large-volume Mt. Stirling 

Granodiorite. The more mafic members of this suite may 

therefore give a strong indication of the geochemical 

characteristics of mantle-derived magmas involved in LFB 

magmatism, minimally modified by interaction with felsic 

igneous or metasedimentary material or its partial melts 

during transport through the crust and magma mingling/mixing during emplacement. Further trace element, 

isotopic and related model-age data are required to assess 

this interpretation. Further representatives of "mantle-like" components may be sought amongst Devonian 

basaltic to high-Mg andesitic dykes which are widespread 

in the southeastern Lachlan Fold Belt in Victoria. These 

occurrences include a number of prominent dike 

"swarms", e.g. the Wood's Point, Tabberabbera and 

Ensay swarms, eastern Victoria. Dikes of this type are 

often associated with Devonian granitic complexes, but 

they occur throughout the region, and clearly reflect 

significant mantle magmatism at or near the time of 

maximum intensity of granitic batholith emplacement. 

Eben, G.W., Nicholls, I.A., Maas, R., McCulloch, M.T. and Whitford, 

D.J., 1990. The Nd- and Sr-isotopic composition of 1-type 

microgranitoid enclaves and their host rocks from the Swifts Creek 

Pluton, southeast Australia. Chem. Geol., 85, 119-134. 

McCulloch, M.T. and Chappell, 1982. Nd isotopic characteristics of Sand 1-type granites. Earth Plan. Sci. Lett., 58, 51-64. 

McCulloch, M.T. and Woodhead, J.D., 1993. Lead isotopic evidence for 

deep crustal-scale fluid transport during granite petrogenesis. 

Geochim. Cosmochim. Acta, 51, 659-674. 
Petrogenesis of a 1.87 Ga 

Post-Tectonic Granite Suite in the 

Svecofennian of Finland: 

General and Nd Isotope Geochemistry 

1

Nironen, M. and 2Ramo, 0. Tapani 1

Geological Survey of Finland, 

Betonimiehenkuja 4, 

FIN-02150 Espoo, Finland 2 Department of Geology, 

P.O. Box 11, FIN-00014 

University of Helsinki, Finland 

A considerable part of the Paleoproterozoic 

Svecofennian crust of southern and central Finland is 

occupied by the 40,000 km2 Central Finland Granitoid 

Complex (CFGC) that comprises mainly granites and 

granodiorites. Most of the granitoids are syntectonic and 

yield U-Pb zircon ages around 1.89-1.88 Ga, which was 

the main crust-forming and tectonometamorphic (synorogenic) period in the Svecofennian. On the southern 

flank of CFGC is a suite of post-tectonic, K-rich granitoids 

that yield U- Pb zircon ages around 1. 87 Ga and have 

characteristics similar to the 1.65-1.54 Ga anorogenic 

rapakivi granites further to the south. We report petrographic, general geochemical and Nd isotopic data on the 

1. 87 Ga granite suite and compare the results to those 

previously reported for the syntectonic 1.89-1.88 Ga 

granitoids and the anorogenic 1.65-1.54 Ga rapakivi 

granites. 

The 1.87 Ga granitoids are felsic, weakly to 

non-foliated, medium- to coarse-grained, and typically 

porphyritic with angular and rounded K-feldspar 

megacrysts. At least one of the plutons comprises a 

bimodal suite of granites and tholeiitic gabbroic rocks. In 

modal composition, the granitoids vary from quartz 

monzonite to syenogranite. The main mafic silicate is 

biotite that typically occurs in the interstices of feldspars; 

hornblende is occasionally present. The granites in the 

western part of the suite contain ubiquitous accessory 

fluorite. 

The rocks of the 1. 87 Ga suite are weakly 

peraluminous (molecular Al20/(Ca0 + Na20 + K20) 1.0 

to 1.15) whereas the two other groups exhibit a shift from 

metaluminous to marginally peraluminous compositions 

with increasing Si02 • A characteristic feature of the 

1.65-1.54 Ga suite is very high Fe/Mg (up to 50) whereas 

the 1. 89-1.88 Ga granites show uniformly low Fe/Mg (on 

Abstracts 109 

the order of 4 to 5); the 1.87 Ga granitoids plot between 

these extremes and exhibit increasing Fe/Mg with increased Si02 • In a P versus Rb diagram the three groups 

show strong depletion of P with increased Rb conforming 

to I -type granite evolution. In the tectonomagmatic diagrams (Nb versus Y, Rb versus Y + Nb), the granite suite 

is transitional between the WPG and VAG fields whereas 

the rapakivi granites fall in the WPG field. In general, the 

1.89-1.88 Ga group shows higher CaO, Na20, and Ba!Rb 

and lower K20 than the 1.65-1.54 Ga rapakivi granites; 

the 1.87 Ga granites plot between these two extremes. 

This suggests that the 1.89-1.88 Ga granites were probably 

derived from a plagioclase-dominated source and the 1. 87 

Ga and 1.65-1.54 Ga granitoids from a two-feldspar 

protolith. Moreover, the relatively high zircon contents (up 

to 600 ppm) in the 1.87 Ga granites point to hightemperature, vapor-absent melting of the source. 

On the average, the granitoids of the 1.87 Ga group 

show a slightly less radiogenic Nd isotopic composition 

(BNit) values range from -1.2 to +0.2) than the granitoids 

of the 1.89-1.88 Ga group (eNit) values -0.9 to + 1.2). 

Comparison of the initial N d isotopic composition of the 

1.87 Ga granitoids to those of the 1.65-1.54 Ga rapakivi 

granites (BNit) c. -1 to -3) indicates, in terms of Nd 

isotopes, a roughly similar Paleoproterozoic source for the 

two groups. 

The 1.65-1.54 Ga rapakivi granites have been interpreted as the results of vapor-absent melting of an intermediate to acid metaigneous protolith in the lower parts of the 

cratonized Svecofennian crust. The anatexis probably 

commenced in response to partial melting in the upper 

mantle and subsequent mafic underplating, resulting in the 

emplacement of the rapakivi granites and related mafic 

rocks in an extensional tectonic setting. The similarities 

between the 1.87 Ga granitoids and the rapakivi granites 

suggest that conditions for magmatism in an extensional 

regime were attained shortly after the 1.89-1.88 Ga 

synorogenic stage in the southern part of CFGC. 
110 The Origin of Granites and Related Rocks 

Midcrustal Pluton Emplacement 

in a Contractional Setting: 

c. 1400 Ma Synkinematic Granites 

of the Southwestern United States 

Nyman, M.W. and Karlstrom, K.E. 

Department of Earth and Planetary Sciences, 

University of New Mexico, 

Albuquerque, NM 87131, USA 

From a 3-D strain perspective, the "room problem" for 

syntectonic pluton emplacement is easily solved since any 

contractional deformation will involve local extension. 

Recognition of the importance of local extension in 

contractional settings has been utilized to explain pluton 

emplacement in transpressional settings, along fault ramps 

and during reactivation of crustal anisotropies. In some 

cases, processes such as magma wedging at pressures 

exceeding mean normal stresses may be required to 

overcome wall-rock stress during pluton emplacement. 

In this paper we present data describing variation of 

emplacement mechanisms for midcrustal Mesoproterozoic 

"anorogenic" granite plutons in southwestern U.S. which 

exemplify the role of local extension within a contractional 

tectonic setting. Mesoproterozoic magmatic activity from 

1500-1300 Ma resulted in emplacement of igneous complexes along the southern margin of Laurentia. In southwestern U.S. this event is marked by emplacement of 

midcrustal (2-4 kbar) granitic plutons. Previous workers 

have referred to this event as "anorogenic" citing lack of 

significant deformation and metamorphism. However, 

detailed kinematic, structural and metamorphic studies of 

plutons in AZ and NM indicate that many of these plutons 

record locally significant deformation and metamorphism. 

Synmagmatic and high temperature kinematic and structural features associated with widely separate 1400 Ma 

plutons record a consistent kinematic framework involving 

contraction, not bulk crustal thinning and extension as 

implied by earlier models. These plutons and aureoles, 

therefore, provide a midcrustal record of emplacement 

mechanisms inboard of a contractional margin. 

Although kinematic and structural features· are all 

consistent with northwest-southeast contraction, processes 

of pluton emplacement for Mesoproterozoic plutons vary 

across the orogen. In NM, the 1420 Ma Sandia pluton 

consists of a 5-7 km thick series of northwest-dipping 

granite sheets which are subparallel with a 1-2 km wide 

extensional shear zone at the base of the pluton. Fabrics 

in the northwest aureole are both cut by the pluton and 

reactivated during high temperature contact metamorphism. 

Kinematic and structural data in the pluton and wall rocks 

are consistent with a 3-D strain field involving 

subhorizontal southeast-northwest contraction and 

north-south local extension, which facilitated pluton 

emplacement. The presence of evolved phases of the 

Sandia granite within the extensional shear zone attests to 

the interaction between shear zone deformation and pluton 

emplacement. 

In central AZ, emplacement of 1420 Ma plutons was 

facilitated by Paleoproterozoic structures, some of which 

show reactivation during pluton emplacement. The Signal 

batholith is cut by a northeast-striking contractional shear 

zone that is parallel with magmatic foliations and moderately southeast-dipping Paleoproterozoic fabrics. Meltfilled shear bands, transition from magmatic to solid state 

foliation and high temperature K-feldspar microstructures 

indicate synmagmatic to high temperature deformation. 

Kinematic elements record southeast-northwest shortening 

during pluton emplacement. Southeast -directed extensional 

movement is also recorded along shallow dipping 

melt-filled shear zones which are both contemporaneous 

with and postdate contraction. In contrast to the Signal 

batholith, the Lawler Peak granite has an intrusive contact 

with only minor evidence for solid state deformation along 

the western margin. Stringers of Lawler Peak granite 

intrude parallel to country rock foliation and show varying 

degrees of boudinage and folding. Magmatic foliation dips 

steeply and parallel to the Mountain Springs fault, a 

Paleoproterozoic fault with a component of sinistral 

strike-slip movement. Orientation of dikes, shear bands in 

deformed granite and structures and kinematic features in 

the aureole are consistent with northwest -southeast contraction. The contrast in deformation behavior between the 

Signal batholith and Lawler Peak granite may reflect 

preferential synmagmatic reactivation of shallow dipping 

older fabrics adjacent to the Signal batholith whereas the 

bulk of strain around the Lawler Peak granite was partitioned into soften wall rocks and along the Mountain 

Springs fault. Synmagmatic sinistral strike-slip movement 

along the Mountain Springs fault may have facilitated 

accumulation of the Lawler Peak granite, consistent with 

a northeast-southwest extensional component during 

southeast-northwest contraction. 
Mesozoic-Miocene Granitoid Evolution and 

Petrogenesis in Southern Chile 

1

Pankhurst, R.J., 2Weaver, S.D. 

and 3Herve, F. 1 British Antarctic Survey, High Cross, 

Madingley Road, Cambridge CB3 OET, UK 2 University of Canterbury, Christchurch, New Zealand 3 Universidad de Chile, Santiago, Chile 

The main Cordilleran batholith of Northern Patagonia 

( 44-4 7°S) consists of Cretaceous granodiorites and 

leucogranites (90-125 Ma) of typical Andino-type 

((87Sr/86Sr)i = 0.7040 - 0.7045). To the west of this, there 

is a more extended intrusion history in the Chonos Archipelago. Here, early Cretaceous (125-130 Ma) quartz-rich 

granites with (

87Sr/86Sr)i ratios were emplaced directly into 

the Palaeozoic-Mesozoic fore-arc accretionary complex. 

Plutonism then switched to the Cordilleran belt, but 

subsequently moved back into the archipelago in Eocene 

(c. 40 Ma) times. Both Early (c. 20 Ma) and Late (c. 10 

Ma) Miocene granitoids are closely associated with the 

Liquine-Ofqui strike-slip fault zone, where small bodies 

crop out between and around the main branches of the 

fault. Late Miocene stocks also occur sporadically 

throughout the Cordilleran belt. 

The Miocene rocks are tonalitic to granitic and have 

major element compositions typical of calc-alkaline 

1-types; they are strikingly similar to the Cretaceous 

subduction-related granitoids. At given Si02, the Miocene 

rocks tend to have slightly higher Ti, Fe3

+, P, Na, Zr and 

K/Rb, but lower Rb. However, they are much more 

primitive in terms of isotopes, with (

87Sr/86Sr)i of 0.7034 

- 0.7041 and eNd of +2.6 to +5.9. These values are 

similar to those exhibited by Southern Volcanic Zone 

volcanic rocks and the Miocene granitoids are seen as the 

first expression of the present subduction-related magmatic 

regime, which began with a significant switch in the plate 

convergence angle c. 25 Ma ago. They were most 

probably generated by remelting of early Tertiary basaltic 

lavas within the crust - the absence of Yb- and 

HREE-depletion characteristics indicates that garnet was 

not involved in their petrogenesis. 

The close chemical and lithological comparability 

between the Miocene and earlier granitoids requires that 

they were generated by essentially similar processes, i.e. 

remelting of mafic material within the crust. There is no 

clear evidence that upper crustal contamination of primitive 

Abstracts 111 

magmas was a significant factor. The· more evolved 

isotopic character of the Cordilleran granitoids thus 

requires that their mafic sources included some that were 

isotopically mature, and depleted- mantle Nd model ages 

suggest that mafic crust as old as 1500 Ma was involved. 

ORAL PRESENTATION 

Pluton Emplacement in Arcs: A 

Crustal-Scale Recycling Process 

1

Paterson, Scott, R., 1Fowler, T. Kenneth, Jr. 

and 2

Miller, Robert B. 1

Department of Earth Sciences, 

University of Southern California, 

Los Angeles, CA 90089, USA 

2

Department of Geology, San Jose State University, 

San Jose, CA 95192, USA 

During magma ascent and emplacement, crust must be 

displaced at a rate equal to the growth or ascent of the 

magma chamber, emphasizing the mechanically linked 

nature of this system. We predict that in the county rock 

multiple material transfer processes (MTPs) will operate 

during magma chamber growth given the large horizontal, 

vertical; and temporal gradients in physical conditions. 

Thus our research has attempted to determine the importance of various MTPs, and whether MTPs largely 

transport material vertically or horizontally, rather than 

determine a single model of ascent or emplacement. 

We have now mapped partially preserved roofs of 10 

plutons with emplacement depths of 2 to 10 km and 

country rock hosts of plutonic rock, weakly metamorphosed clastic, calcsilicate, carbonate, and volcanic rock, 

and amphibolite-grade schist and gneiss. During emplacement, these roofs were not, in general, ductilely deformed, 

detached or extended by faults, or significantly uplifted. 

Instead, sharp, irregular, discordant contacts are the rule 

with stoped blocks often preserved immediately below the 

roof, even in the deeper plutons. Upper portions of these 

magma chambers are varied, sometimes preserving crests 

of more evolved magmas or local volatile-rich phases and 

sometimes complex zones of diking and magma mingling. 

Magmatic foliations also display a wide variety of patterns 

and consistently are associated with evidence that they 

formed after emplacement. 
112 The Origin of Granites and Related Rocks 

In a few cases we have been able to map roof-wall 

transitions. At shallow crustal levels steep wall contacts 

have sharp, discordant, stepped contacts, and sometimes 

nearby stoped blocks. Around some plutons, a relatively 

abrupt transition (sometimes within 100s of meters) occurs 

from discordant, brittle roofs to moderately concordant, 

ductilely deformed walls with narrow aureoles. We have 

not found brittle or ductile faults at roof-wall joins. 

Aureole width and ductile strains in aureoles increase 

slightly with depth. Igneous layering and magmatic 

foliations/lineations are common near these wall contacts. 

Around shallow to mid-crustal, steep wall contacts (5 

to 15 km), deflections of pre-emplacement markers (e.g., 

bedding, faults, dikes) and ductile strains in narrow 

aureoles (0.1 to 0.3 body radii) give a complete range of 

bulk shortening values from 0% to 100% of needed space, 

but average around 30%, or less, particularly for larger 

plutons and batholiths. Lack of far-field deflection of these 

same markers rules out significant horizontal displacement 

outside the aureoles and requires that any near-field lateral 

shortening is translated into vertical flow. Lateral variations from ductile to brittle MTPs are sometimes observed, 

but are difficult to decipher in the outer parts of aureoles. 

Compositional zoning is common within magma bodies, 

although this zoning commonly reflects separately evolved 

pulses that traveled up the same magma plumbing system. 

Magmatic foliations and lineations largely form after 

emplacement and reflect strain caused by late magma flow 

and regional deformation. 

Wall rocks near mid- to deep-crustal walls (10-25 km) 

are extensively deformed, with locally discordant contacts, 

but the distinction between regional and emplacement-related deformation is less clear. Internal sheeting is 

increasingly common in these plutons, although elliptical 

masses are still present. Lateral compositional variations 

are as large as vertical variations at shallower depths and 

occur over shorter distances. Magmatic foliations and 

lineations often reflect regional deformation. 

The lack of evidence for horizontal displacement 

outside the narrow, shallow to mid-crustal aureoles and the 

lack of lateral or upwards displacement of pluton roofs 

indicate that during emplacement most country rock is 

transported downwards in the region now occupied by the 

magma body and its aureole. The internal sheeting and 

zoning indicates that during downwards flow of country 

rock, multiple pulses of magma traveled up the same 

magma system. Thus, if these relationships are widespread 

in arcs, then magma emplacement in arcs is the driving 

mechanism of a crustal-scale recycling process. 

ORAL PRESENTATION 

Effects of Pressure, H20 Activity, 

and Fe/Mg Ratio on the 

Compositions of Anatectic Melts 

1

Patifio Douce, Alberto E. 

and 2

Beard, James S. 1

Department of Geology, University of Georgia, 

Athens, GA 30602, USA 2Virginia Museum of Natural History, 

Martinsville, VA 24112, USA 

New melting experiments on crustal protoliths (including some with varying amounts of added H20), coupled to 

a synthesis of previous experimental results, allow us to 

derive general conclusions about the effects of P, T, ana, 2 

Fe/Mg ratio, and .J;y on melting relations and melt compo- 2 

sitions during crustal anatexis. 

Orthopyroxene and garnet are two of the chief mafic 

residual phases formed by incongruent melting of biotiterich protoliths ( cordierite becomes important at lower P 

and/or in compositions more magnesian than those discussed here). Crystallization of either garnet or 

orthopyroxene affects the compositions of coexisting melts, 

and is controlled by several factors besides the obvious one 

of Al20 3 activity. Thus, in the assemblage biotite + 

plagioclase + quartz (with no added H20, and at .J;y ~ 2 

QFM) increasing Fe/Mg ratio at constant T and P, and 

increasing P at constant T and Fe/Mg, cause garnet to join 

orthopyroxene in the restitic assemblage, and eventually to 

replace it entirely as the dominant residual mafic phase. AI 

and Ca required for garnet crystallization are provided by 

incongruent breakdown of plagioclase. This in tum raises 

the Na!K ratio of the melt but has only a minor effect on 

its Si02 content. Increasing .J;y suppresses both mafic 2 

silicates phases in favor of magnetite, but the effect is 

stronger on garnet. For example, in an Fe-rich model 

graywacke at 850°C and 5 kbar, garnet disappears between QFM and Ni-NiO, whereas orthopyroxene is present 

at / 02 ~ Ni-NiO. Differences in / 02 within ± 2-3 orders 

of magnitude of the QFM buffer have only slight effects 

on melt compositions and melting temperatures, but can 

have stronger effects on restite assemblages, and hence on 

trace element characteristics of the melts. 

H20-poor melts formed by dehydration-melting of 

biotite + plagioclase + quartz at / 0 ~ QFM, whether 2 

saturated with orthopyroxene or garnet, have Si02 > 70 
wt% up to Tof at least 1000°C. However, FeO and MgO 

contents of melts saturated with orthopyroxene + quartz 

increase more strongly with T than those of melts saturated 

with garnet + aluminosilicate + quartz. At constant T 

(and / 02 ~ QFM), FeO concentrations in silicic melts 

saturated with either orthopyroxene or garnet ( ± 

aluminosilicate) vary inversely with P and directly with 

aHp. These relationships are also true in amphibolitederived melts saturated with orthopyroxene + 

clinopyroxene. If bulk H20 content is fixed (e.g., in the 

case. of dehydration melting), then a~0 at constant melt 

fractiOn decreases with increasing P. The effects of P and a 

fi ~a 

on rna c phase saturation act together to cause all dehydration melts to become more leucocratic with increasing p. 

Increasing aH2a at constant T and P favors congruent 

dissolution of plagioclase + quartz, which raises both the 

N aiK and CalK ratios of the melt and, depending on the 

bulk composition of the protolith, may either raise or 

lower the melt Si02 content. Isothermal CaiNa partitioning 

between plagioclase and melt is sensitive to both p and 

aH p. Increasing aHp at constant P and T causes the CaiN a 

ratio of melt to decrease relative to that of coexisting 

plagioclase. Increasing Pat constant a and T has the . . Hp 

oppos1te effect; plagmclase becomes more sodic relative to 

coexisting melt. 

Fe/Mg partitioning between garnet and melt, and 

between biotite and melt, are strong functions of both p 

and T and, in the case of biotite, insensitive to fa . Our 

results confirm those of previous workers (Green, 21977; 

Ellis, 1986) that garnet-melt Fe/Mg partitioning reverses 

at magmatic temperatures, and the same is also probably 

true of biotite-melt Fe/Mg partitioning. In contrast, 

orthopyroxene-melt and clinopyroxene-melt Fe/Mg 

partitioning appear to be rather insensitive to P and T. 

These results show that there is no exclusive explanation for the series tonalite-granodiorite-granite. Different 

magma compositions within this spectrum may reflect 

different melting temperatures, different source compositions (perhaps differing only in Fe/Mg ratio), different 

H20 activities, or any combination of these factors. For 

example, melting of a biotite- and plagioclase-rich gneiss 

(metagraywacke?) at high P and ana could produce a 

granodiorite or tonalite melt and ~ garnet pyroxenite 

residue, whereas H20-starved dehydration-melting of the 

same rock at lower P would produce a granite melt and a 

pyroxene granulite (noritic?) residue. 

Abstracts 113 

Ellis, D.J., 1986. Garnet-liquid Fe-Mg equilibria and implications for the 

beginning of melting in the crust and subduction zones. Am. J. Sci., 

286, 765-791. 

Green, T.H., 1977. Garnet in silicate liquids and its possible use as aP-T 

indicator. Contrib. Mineral. Petrol., 34, 59-67. 

Recognition and Significance 

of Mixing in Granites 

Pembroke, J.W. and D'Lemos, R.S. 

Geology and Cartography Division, 

Oxford Brookes University, Oxford OX3 OBP, UK 

Models in which plutons are emplaced during 

extensional space creation require the quasi -continuous 

injection of magma batches through time as the pluton 

develops. Evidence for the repeated addition of magmas 

should therefore be widespread in granites. Sheeted (dike) 

architectures would only develop if resident magma had 

largely solidified prior to subsequent injections. Magma 

addition prior to extensive crystallization would result in 

mingling and mixing. Some evidence for this style of 

sequential magma addition is clearly demonstrated by 

mafic synplutonic dykes and mafic enclaves in granites, 

where contrasting mineralogies and colors, and large initial 

thermal contrasts, render features both striking and 

permanent. However, apart from where obvious internal 

contacts occur, or gross variations in mode or texture 

exist, granite plutons are often perceived as homogeneous. 

Detailed observations of granites from numerous localities 

reveal subtle heterogeneities and disequilibrium features on 

a mineral to pluton scale. These features indicate to us 

that mixing of intruding and resident granite magma within 

evolving plutons may be a widespread but often overlooked phenomenon due the subtlety of the evidence. We 

present data from three well exposed case studies in 

Scotland and the Channel Islands (UK) and from Newfoundland. 

Heterogeneities within plutons picked out at outcrop 

scale range from abrupt boundaries to diffuse, transitional 

zones defined by gradual changes in mode or texture. 

These boundaries record intrusive contacts between 

original magma pulses which have been variably overprinted by post-intrusive processes although some may 

represent the result of in-situ crystallization processes (e.g. 

crystallization fronts). Plutons examined also exhibit subtle 

modal heterogeneity on the scale of mms to tens of metres. 
114 The Origin of Granites and Related Rocks 

Modelling of phase distributions using spatial analysis 

(SPANS) and Geographic Information System (GIS) 

software has enabled quantification of heterogeneity. 

Digitised data for given phases are divided into areas of 

attraction or cells to create a "voronoi" map. 

A simple entropy equation (Shannon Formula) using the 

relative proportions of each cell provides a numerical 

expression of the phase distribution. These data allow 

direct comparison of heterogeneity at different localities or 

in different samples. Such modelling should enable 

differentiation between heterogeneities which result from 

incomplete mixing from those that result from random 

crystallization processes. Textural heterogeneity, where the 

same mineral phases exhibit contrasting textural features 

over short distances, often within the same thin section, is 

commonly observed. For example, the coexistence of 

contrasting zonation patterns in plagioclases, or the 

presence of both phenocrystic and interstitial quartz or 

alkali feldspar within a single thin section are difficult to 

reconcile with standard crystallization models and it is 

more probable that the grains grew at different times 

and/or in places. 

Many, if not most, granites exhibit mineralogical and 

textural disequilibria. Rapakivi feldspar, cellular feldspars, truncation of growth zoning, acicular apatite, 

skeletal biotite, embayments and a variety of overgrowths, 

have been widely reported where magma mixing has been 

demonstrated (e.g. in association with mafic enclaves). 

However, we have found such obvious disequilibrium 

textures, and many more subtle features, widely in granites 

even where there is no mineralogical or geochemical 

evidence for local mixing with mafic magma, suggesting 

that mixing between differing granitic magmas has taken 

place. Interaction between granitic magmas with even 

small differences in temperature and melt composition can 

result in a shift from equilibrium conditions and resultant 

rapid growth, non-growth or resorption features. However, 

because thermal and chemical differences between the 

coexisting magmas are small, and subsequent cooling 

prolonged, the features are not visually striking. 

Our field and petrographic observations indicate 

widespread, albeit subtle, heterogeneity and disequilibrium 

in granites. While recognising that certain features can 

result from several processes (e.g. in-situ progressive 

crystallization) we would interpret many as having resulted 

from mixing between coexisting batches of granite magma 

during pluton assembly. 

ORAL PRESENTATION 

Granite Ascent: Dykes or Diapirs? 

Petford, Nick 

School of Geological Sciences, Kingston University, 

Kingston-Upon-Thames, Surrey KTJ 2EE, UK 

Until the last few years, diapirism reigned supreme 

among granitoid ascent mechanisms. Granitoid masses in 

a variety of material states, from pure melt through semimolten crystal mushes to solid rock, were believed to have 

risen forcefully through the continental crust to their final 

emplacement levels in a way analogous to salt domes. The 

structural analogy between granite plutons and salt diapirs, 

which gained acceptance in the 1930s (Wegmann, 1930) 

has clearly been an attractive one. Unfortunately, on 

closer inspection such comparative reasoning appears 

deeply flawed. Nonetheless the belief in diapiric ascent of 

granitoid magmas still persists, despite the pessimistic 

outcomes of thermal models and, at best, ambiguous field 

evidence. Until very recently, dyke ascent due to active 

fracture propagation (Clemens and Mawer, 1992), or more 

passive fault-fracture exploitation (Petford et al., 1993), 

was dismissed on the grounds that granitic melts were too 

viscous (due mainly to low intrinsic temperatures and high 

crystal contents) for dyke-controlled ascent to occur (eg. 

Miller et al., 1988). However, field evidence for the 

fluidity of granitiod melts is common, and the rejection of 

dyke transport mechanisms appears to have been based 

more on received wisdom than hard fact. The realisation 

that granitic melts generally have viscosities in the range 

105-107 Pa s effectively opened the way for the development of dyke ascent models. 

Thermal and fluid dynamical models for dyke transport 

of granitoid magmas have a number of important implications for the geochemistry and emplacement of granites 

that have yet to be fully explored. For example, rapid 

ascent rates of - 1 o-2 m s-1 predicted for granite melts in 

dykes (cf. -m yr1 for diapirs) means that granitoid melts 

may rise adiabatically from their source regions. As a 

consequence, any restite material present initially in the 

ascending melt will most probably be resorbed. Thermal 

balance calculations suggest that adiabatic ascent with a 

pressure drop of - 10 kbar will produce enough heat to 

resorb approximately 30 wt % restite. 

The ascending melts will have little time to assimilate 

country rock during ascent, and if source extraction is 

sufficiently rapid, significant magmatic contamination may 
occur only at the site of emplacement. However, as 

predicted pluton filling rates (c. 102-104 years) are orders 

of magnitude faster than estimated fault opening rates 

based on plate tectonic motions (Paterson and Tobisch, 

1992), there is serious doubt as to whether space can be 

made fast enough to accommodate large, upper crustal 

granitic magma "chambers". An important future goal is 

to reconcile these differences in ascent and emplacement 

rates with time scales for melt segregation at source. The 

trinity of ascent, segregation and emplacement still remain 

to be united. 

Clemens, J.D. and Mawer, C.K., 1992. Granitic magma transport by 

fracture propagation. Tectonophysics, 204, 339-360. 

Miller, C.F., Watson, E.B. and Harrison, M.T., 1988. Perspectives on 

the source, segregation and transport of granitoid magmas. Trans. 

Royal Soc. Edinburgh: Earth Sci., 79, 135-156. 

Paterson, S.R. and Tobisch, O.T., 1992. Rates of processes in magmatic 

arcs: implications for the timing and nature of pluton emplacement. 

J. Struct. Geol., 14, 291-300. 

Petford, N., Kerr, R.C. and Lister, J.R., 1993. Dyke transport of 

granitoid magmas. Geology, 21, 845-848. 

Wegmann, C.E., 1930. Uber Diapirismus. (Besonders im Grundgebirge), 

Bull. Comm. Geol. Finlande, 92, 58-76. 

Metal Partitioning from Melts 

into Vapor and Vapor/Brine Mixtures 

in Shallow Granitic Systems 

Piccoli, Philip M. and Candela, Philip A. 

Laboratory for Mineral Deposits Research, 

Department of Geology, University of Maryland 

at College Park, MD 20742-4211, USA 

Experimental and computational studies of element 

partitioning between granitic melts and vapor and 

vapor± brine are important in our understanding of the 

mass transfer that occurs in and around crystallizing 

plutons. Experimental studies provide the necessary data 

on the equilibria that control melt-vapor and melt-vaporbrine compositions. Theoretical modeling suggests factors 

that maximize the. probability of metal removal from 

granitic melts, and can also suggest specific tools for 

evaluating magmatic-hydrothermal ore potential. 

We have written an algorithm, MVP (Magmatic 

Volatile Phases), which can be used to calculate the 

composition of the magmatic volatile phase(s), vapor or 

vapor+ brine, that are expelled isobarically from granitic 

Abstracts 115 

(s.l.) magmas. This phenomenon of crystallization-driven 

volatile exsolution in plutonic environments has been 

referred to as second boiling (e.g. Bowen, 1928). Rather 

than perform simple mass-balance calculations for each 

element individually, MVP considers the simultaneous 

equilibria (among melt, crystals, and volatile phases 

including brine) that must be accounted for to model the 

efficiencies with which ore-metals are removed from 

magmas into the volatile phases. Briefly, this algorithm 

calculates the partitioning of elements between melt and 

volatile phase(s) accounting for crystal-melt partitioning 

both before and after volatile saturation. Implicitly, the 

melt composition is modelled as "near minimum", and 

does not change in major element composition with 

crystallization. Crystallization is modelled implicitly 

through the use of bulk crystal/melt partition coefficients 

(D) for each element; Ds are treated as constants throughout magma evolution for most elements. Once the composition of the melt is calculated at volatile saturation (which 

is equivalent to water saturation in this algorithm: saturation with a C02-free vapor, or a C02-free vapor + brine 

mixture can be modelled), the composition of successive 

aliquots of vapor, or vapor+brine can be calculated, until 

the volatile-saturated melt has crystallized. Both the initial 

and saturation water concentration (which is a function of 

pressure) can be set as input parameters. Sufficient data 

do not exist yet for the composition of the brine to be 

calculated with great accuracy; however, until such data 

are procured, we believe that our results represent the 

most rigorous treatment possible at this time, and are 

illustrative of the effects of brine saturation. The algorithm routinely considers Cu, Mn, Zn, Ca, Mg, Yb, Ce, 

K, Na, Eu, H (chloride complexed), and Mo, W, and B 

(non-chloride complexed), and can easily be modified for 

other elements for which partitioning behavior has been 

experimentally defmed (e.g. Sn2+, Sn4

+, Li, Be, Rb, Sr, 

Cs, Ba, U, Th and other REE: see Candela and Piccoli, 

1995). 

The importance of chloride in granite-volatile systems 

(melt±vapor±brine) has been discussed elsewhere (e.g. 

Candela and Piccoli, 1995). Phase rule constraints limit 

the variability in the Cl concentration in melts saturated 

with both vapor and brine at a given temperature and 

pressure. Variations in temperature and pressure during 

vapor+ brine saturation are rather restricted, and variations 

in the Cl concentration of the associated melt phase are apt 

to be small. For a system of a given major-element 

composition, the Cl concentration of the coexisting melt, 

vapor, and brine will be a function of temperature and 
116 The Origin of Granites and Related Rocks 

pressure only, and will not allow the free variation of an 

element, as in, e.g., Rayleigh fractionation. Rayleigh 

fractionation requires a system with a variance greater than 

zero. 

Results of this model suggest the following: 1) The 

probability of mineralization with respect to Cl-complexed 

ore-metals increases with increasing Cl!H20 of the associated magma. As brine can form at relatively at low 

magmatic Cl/H20, the presence of brine ± vapor-rich 

inclusions alone is not a good indicator of high Cl/H20 of 

the initial melt. The efficiency of removal of metals from 

a magma into exsolving volatile phases is a function of 

both the Cl/H20 ratio of the magma, and the extent to 

which the metals are taken up by crystallizing phases. For 

melts with low initial Cl/H20 ratios (0.01), brine saturation occurs relatively late if at all, and overall efficiencies 

of removal of chloride-complexed metals is low. 2) The 

Cl/H20 ratio in the melt is fixed at a given temperature 

and pressure when the system is saturated with respect to 

vapor and brine; further, the ratio of vapor to brine, and 

the composition of these phases remains constant throughout volatile exsolution. The concentration of unbuffered 

trace elements may vary in the system, and may follow 

Rayleigh-type fractionation patterns, whereas Cl cannot. 

Therefore, changes in magmatic Cl concentrations are not 

necessary consequences of crystal fractionation, even at 

low pressures. 3) At a given pressure of emplacement, the 

initial water concentration will determine the timing of 

volatile saturation. Initial water concentrations can be 

related to the timing of saturation of a magma with respect 

to a water-rich phase whether or not the melt is C02-

bearing. Other factors, suggested by quantitative modeling, that may increase Cu partitioning into "ore fluids" 

are: higher fa in mt-saturated systems because of both the 2 

destabilization of magmatic sulfides, and the suppression 

of aqueous FeC12; lower ASI in the melt (for 

metaluminous to peraluminous compositions) because 

hydrogen competes with other cations for chloride, and 

also increases FeC12 in mt-saturated systems; and lower 

Na20 and K20 in the melt (at a given melt ASI), because 

of competition for chloride in the volatile phases. 

Bowen, N.L., 1928. The Evolution of Igneous Rocks. Dover Publications 

Inc., New York. ' 

Candela, P.A. and Piccoli, P.M., 1995. Model ore-metal partitioning 

from melts into vapor and vapor/brine mixtures. In Thompson, J .J .F. 

(ed), Granites, Fluids, and Ore Deposits, Geological Association of 

Canada Short Course Volume 23, 101-127. 

ORAL PRESENTATION 

Control of Redox State and 

Sr Isotopic Composition of Magmas: 

Source or Processes? 

Pichavant, M. and Hammouda, T. 

Centre de Recherche sur la Synthese et la Chimie des 

Mineraux, CNRS, JA rue de la Ferollerie, 

45071 Orleans Cedex 02, France 

That granitic magmas reflect their source regions is 

now used as a basic underlying assumption in most 

geochemical studies, and has proven useful in rationalizing 

many characteristics of granitic rocks. However, the 

mechanisms by which source rocks control the intensive 

and compositional parameters of the magmas remain 

poorly known. In addition, processes that occur at the 

level of crystallization of the magmas (fractionation, 

degassing, ... )may modify the geochemical characteristics 

acquired in the source region. Here, we use recent experimental data obtained at Orleans to challenge the "source 

rock model" and discuss control of 1) redox states, and 2) 

Sr isotopic compositions of granitic magmas. 

Redox state. It has been suggested that the redox state 

of granitic magmas is controlled by that of the rocks in the 

source region. However, several other studies have 

demonstrated that shallow level processes such as degassing or diffusive loss of H2 can lead to a marked oxidation 

of the magmas. Experimental studies have been performed 

in parallel on biotite-muscovite (Manaslu) and tourmalinemuscovite (Gangotri) leucogranites from the High Himalayas. Results under reducing conditions (log fr = FMQ - 02 

0.5) at 4 kbar and variable aH. 0 show that biotite is a near 

liquidus phase in both magmas. Yet, biotite is nearly 

absent (except in cores of tourmaline crystals) and reacted 

out at some stage of evolution of the Gangotri magma. The 

latter granite is characterized by bulk rock Fe20 3 /FeO 

values much higher (1.9) than the Manaslu granite (0.2). 

These results suggest that the Gangotri granite evolved 

under progressively more oxidizing conditions during 

crystallization, up to fa values more than 3 log units 2 

above the FMQ buffer. Fe20 3 can be efficiently incorporated in muscovite and also possibly in tourmaline. Their 

crystallization (and also the crystallization of magmatic 

cassiterite) could be used as mineralogical indications of a 

magmatic oxidation process. Leucogranite magmas thus 

provide evidence of a control of redox conditions by 
shallow level processes rather than by their source regions. 

These processes may obliterate the geochemical signature 

acquired at the source level. 

Sr isotopic composition of partial melts. Experiments 

were designed to test mechanisms of isotopic equilibration 

of Sr during partial melting of a model crustal assemblage 

made of synthetic mica doped in Sr (

87Sr/86Sr = 4.0) and 

natural plagioclase (

87Sr/86Sr = 0. 70). Glasses and crystals 

(residual mica and plagioclase, newly-formed plagioclase) 

were analysed for Sr isotopes and concentrations using an 

ion microprobe. The Sr data were obtained for experimental conditions corresponding to melting above the critical 

temperature, i.e. , when melting is kinetically controlled by 

atomic diffusion in the liquid. The experimental liquids are 

chemically and isotopically zoned as long as the two 

starting minerals persist in the charge. The residual solids 

always keep their starting Sr concentration and isotopic 

composition unchanged. Therefore, Sr in the liquid is 

controlled by the proportion of phases participating to the 

melting reaction. Because of the faster melting rate of 

mica compared to plagioclase, early liquids are characterized by elevated 87Sr/86Sr and evolve progressively with 

time toward lower values of 87Sr/86Sr. However, the Sr 

isotopic composition of the liquid is always higher than 

that of the bulk starting assemblage, implying that isotopic 

disequilibrium persists between liquid and residual minerals in the experiments. Under our experimental conditions 

(i.e., above the critical temperature), crystal dissolution is 

faster than liquid-crystal equilibration, assumed to be 

controlled by diffusion in the crystals. For example, total 

equilibration of Sr isotopes by diffusion between melt and 

a plagioclase of 1 mm in size would require 104 years at 

900°C. Such time-scales are in the same range as crustal 

melting episodes (heat provided by mafic magmas) and 

processes of magma segregation, extraction and ascent 

(dikes). In the case of source rocks not previously homogenized during an earlier metamorphic event, partial melts 

(if formed above the critical temperature) may not necessarily reflect the Sr isotopic composition of source rocks 

if magma ascent takes place before liquid-crystal homogenization has occurred in the source region. 

Abstracts 117 

ORAL PRESENTATION 

Trace Element and Isotopic Exchange 

During Acid-Basic Magma Interaction 

1

Poli, G., 2Davies, G.R. and 2Tommasini, S. 1

Department of Earth Sciences, Perugia, Italy 2 Faculteit der Aardwetenschappen, Vrije Universiteit, 

Amsterdam, The Netherlands 

Physico-chemical interactions between acid and basic 

magmas produce diverse magma types and the process is 

now becoming accepted as an important mechanism in the 

genesis of composite batholiths. We report studies from 

two plutonic environments, Elba and Sardinia (Italy), 

designed to constrain the principal physical processes that 

control the chemical compositions of mixed lithologies. 

The Sardinia-Corsica Batholith is petrologically variable 

with granodiorite-monzogranite as the dominant lithologies. 

Gabbroic complexes occur throughout the batholith. Most 

granitoid lithologies contain evidence of mixing which is 

manifested by the presence of clots of mafic minerals 

( -0.01 m), mafic microgranular enclaves (MME, < lm) 

and basic septa (BS, > lm). MME often comprise 3-5% 

of surface outcrop. Interdigitating contacts between the BS 

and granites indicate that basic and acid magmas were 

liquid at the same time. Chemical compositions of 

gabbroic complexes in the batholith indicate formation by 

amphibole + clinopyroxene + plagioclase fractionation 

and accumulation. In contrast the centers of the BS have 

not undergone cumulus processes and probably represent 

liquids that were parental to the gabbroic complexes. The 

major and trace element contents of MME with "tonalitic" 

compositions can be modelled in terms of a process that 

involves granite contamination and fractional crystallization 

of amphibole + biotite + plagioclase + apatite and 

subordinate amounts of titanite and zircon (r = 0. 3 in 

AFC model). MME with "gabbroic" compositions and the 

margins of the BS have a cumulate geochemical signature. 

These rocks appear to have lost a liqui~ phase during 

crystallization (filter pressing) while undergoing fractional 

crystallization and assimilation. 

Together these field and geochemical observations 

establish that mixing has involved three physical processes: 

1) mechanical magma mingling of basaltic and granitic 

magmas- this process occurs when there are large temperature and viscosity contrasts between the magmas at an 

early stage of interaction; 2) contamination of basaltic 

magmas by granitic magmas (AFC): this process produces 
118 The Origin of Granites and Related Rocks 

an evolved "basaltic" magma that is thermally equilibrated 

with the granitic magma; and, 3) filter pressing of 

residual liquids from the evolved "basaltic" magma: this 

process may lead to mixing between the evolved "basaltic" 

liquid and granitic magmas. 

In order to better constrain how the above processes 

work and assess the possible role of diffusive processes in 

mixing systems we are undertaking a study of a young 

plutonic system on the island of Elba (Tuscany, Italy). 

Due to the 6 Ma age of the granitoids, insignificant age 

correction is required for Sr-Pb-Nd isotopes, hence 

relatively small isotope variations can studied. We have 

sampled a 0.8 m profile across an enclave (0.5m) and host 

granite and then determined the Sr-Pb isotope composition 

of plagioclase, K-feldspar and biotite (Nd isotope analyses 

in progress). At temperatures of c. 800°C the Sr isotope 

diffusion in plagioclase feldspar is relatively slow ( < 10 

-15 cm2 s-1

) so that plagioclase may retain the Sr isotope 

composition of the magma from which they crystallized. 

In contrast K-feldspar and biotite have diffusion coefficients that are orders of magnitude faster. Consequently 

these phase will undergo significantly greater degrees of 

diffusive exchange with any enclosing magma during 

crystallization and cooling. The Sr and Pb isotope ratios 

of plagioclase feldspars from the centre of the enclave 

(s7Srf86Sr = 0. 7116, 208Pbf204 Pb = 38.96) are distinct 

from the granite (

87Sr/86Sr = 0.715, 208Pbf204 Pb = 38.89). 

Plagioclase from the margin of the enclave have isotopic 

ratios intermediate between the granite and the centre of 

the enclave. Sr-Pb isotope ratios of the biotite and 

K-feldspar record smaller isotope differences. 

From these preliminary results we conclude that the 

Sr-Nd-Pb isotope composition of whole rock samples from 

mixed magmas cannot be used to constrain the extent of 

bulk mixing during magma interaction. The different 

diffusion coefficients for Sr-Nd-Pb in each mineral that 

form a mixed rock is liable to result in different isotopic 

compositions. Each mineral will therefore have different 

isotopic ratios and the whole rock isotopic composition 

will be controlled by the rock mineralogy not the physical 

extent of magma interaction. Therefore to examine the 

nature of mixing processes isotopic analyses must be 

performed on minerals and the effects of diffusion modelled in detail. 

Discordant Cores in Zircons 

and Granite Genesis 

Pup in, Jean-Pierre 

Z.I.R. C., Laboratoire de Petrologie-Mineralogie, 

lnstitut de Geologie, Faculte des Sciences, 

Pare Valrose, F-061 08 Nice Cedex 2, France 

(and URA CNRS 17-63) 

In magmatic rocks, zircon morphology (morphological 

or typologie diagram- Pupin 1976,1980) gives the chro~ological argument (order of appearance of succe~sive 

morphological subtypes in a single complete zucon 

population during a more or less long crystallization peri~d 

over the magmatic history of the host-rock). The mam 

factors that seem to influence the prismatic and pyramidal 

relative rate of growth of the crystals are yet considered to 

be temperature, aluminium-alkali antagonism and more 

indirectly activity of H20 in the magma. This last influences compatible or incompatible behaviour of Zr. 

A systematic study of zircon geochemistry in granitic 

and associated rocks from various origins reveals a clear 

relationship of geochemical parameters with zircon 

morphology. Electron microprobe analyses on polished 

crystals of the different crystallization ~eriods 

(premagmatic, magmatic and late magmatic = 

overgrowths) reveal the magmatic history of the rock. 

The Y20 3 versus Hf02 variation diagram in zircons 

(Pupin 1992, 1994) is used to infer zircon origin and 

granite petrogenesis. Hf02 content of zircon magmatic 

phase is generally < 1.2% in mantle deriv~d gran~tes, 

partly < 1.2% (early magmatic phase) in hybnd granites, 

and usually > 1.2% in anatectic granites and migmatites. 

The importance of the late magmatic zircon crystallization, always enriched in Hf and/or Y, is related to the ro~e 

of the fluid phase. The diagram OTU using U and Th m 

zircons shows the dominance of Th in mantle derived 

crystals, of U in crustal anatectic and late magmatic 

zircons. Cumulate curves (U02 + Th02) underline the role 

played by the fluid phase in melts with increasing contents 

of hygromagmatophile elements. 

Internal structures of zircon crystals are revealed by 

defocusing the beam in the electron microprobe (backscattered electron pictures). They are characterized by a 

more or less complex zoning (Van Breemen et al., 1986; 

Varva, 1990,1994) but also often by discordancies which 

correspond to fundamental features of magma genesis or 

later history. Among them, inherited cores from crustal 

melting in migmatites and anatectic granites are particu-
larly interesting for petrologists and geochronologists. If 

clearly discordant shaped cores (corroded ones or fragments) are easily detected, they are frequently associated 

with idiomorphic or sub-idiomorphic cores whose zoning 

is apparently in continuity with the zoning of the surrounding host-zircon. Chemical discordancies can reveal the 

presence of such idiomorphic cores and lead to a better 

interpretation of the chemistry of the different generations 

of zircon. 

A coupled study of morphological data with trace 

element geochemistry and internal structures leads to 

interpret the different origins of cores (or nuclei) in 

zircons. Three fundamental different origins are recognized: 1) relict cores inherited from the protolith that 

underwent the crustal partial melting (migmatites, anatectic 

granites of crustal origin); 2) cores corresponding to a 

more or less late contamination during the emplacement of 

the granitic melt; and, 3) cores corresponding to a mixing/mingling between two magmas of different origins 

(crystallizing directly or after remelting). The arguments 

used to decide each of these different cases are presented 

and discussed on the basis of some examples. Geochemical 

and structural data obtained on the cores are used to 

constrain the nature of the protolith, the contaminant or the 

hybridized rocks. Morphological data are used to determine at what period this event happened. 

The recognition and interpretation of cores in zircons 

are of a fundamental interest for the understanding of 

numerous magmatic rocks genesis, especially granitic 

ones. Zircon is a very powerful tool and the unlimited 

material offered by zircon populations in granites permits 

to obtain a lot of available and precise data - which can be 

separated if polygenic - associated with the primary events 

of the main magmatic period. The whole rock geochemistry is generally more difficult to be interpreted because 

inheritance, contamination, mixing/mingling or deuteric 

phenomena effects are not always easily separated. 

Pupin, J.P., 1976. Signification des caracteres morphologiques du zircon 

commun en petrologie. Base de la methode typologiue. Applications. 

These, Universire de Nice, 394 p. 

Pupin, J.P., 1980. Zircon and granite morphology. Contrib. Mineral. 

Petrol., 73, 207-220. 

Pupin, J.P., 1992. Les zircons des granites oceaniques et continentaux: 

couplage typologie-geochimie des elements en traces. Bull. Soc. geol. 

France, 163, 495-507. 

Pupin, J.P., 1994. Characterization of sources for migmatites and crustal 

anatectic granites from zircon studies. C. R. Acad. Sci. Paris, 319, 

1191-1197. 

Van Breeman, 0. and Parrish, R.R., 1986. Zircons record ancient 

geological processes. Geos, 15, 18-21. 

Abstracts 119 

Varva, G., 1990. On the kinematics of zircon growth and its petrogenetic 

significance: a cathodoluminesence study. Contrib. Mineral. Petrol., 

117, 331-344. 

ORAL PRESENTATION 

Monzonite Suites: The Innermost 

Cordilleran Plutonism of Patagonia 

1

Rapela, C.W. and 2Pankhurst, R.J. 1 Universidad Nacional de La Plata, Argentina 2

British Antarctic Survey, High Cross Madingley Road, 

Cambridge CB3 OET, UK 

The development of a wide early Mesozoic Cordilleran 

interior magmatic belt in Patagonia contrasts sharply with 

the typical Andino-type continental-margin arc that prevailed from Late Jurassic to Tertiary times along the 

Pacific margin of South America. This extra-Andean 

province contains Late Triassic - Early Jurassic granitoids 

in the North Patagonian Massif and the extensive Middle 

Jurassic Volcanic Province, one of the largest known 

silicic volcanic fields. 

The easternmost exposures of plutonic rocks occur in 

the Deseado Massif, more than 750 km east of the present 

oceanic trench, where two Early Jurassic monzonite suites 

crop out in erosional windows through the Jurassic 

volcanic rocks. The predominant rocks are hornblendebiotite granitoids, of quartz monzodiorite composition at 

La Leona and quartz monzonite at La Calandria; both 

show transitions to biotite monzogranite. Lenses of 

leucosyenogranite and aplite-monzogranite intrude the main 

facies in both areas, and are abundant at La Leona. A 

marginal facies of fine grained gabbro and basic quartz 

monzodiorite appears only at La Calandria. 

These two monzonite suites are metaluminous, ASI 

increasing from 0.7 to 1.1 as Si02 increases from 56 to 72 

wt %; only the felsic syenogranites and aplites are marginally peraluminous (ASI = 1.1 - 1.2). Compared with the 

typical metaluminous Cordilleran tonalite-granite series, 

they show higher P20 5 , K20, and MgO/FeOT ratios, 

signifying higher -temperature melts. Associated mineralization is of Mo-Cu type. 

Lithological and geochemical variations in both districts 

are the result of extensive, high level, fractional crystallization. Each suite precisely defines a whole-rock Rb-Sr 

isochron of 203 Ma, and each has a uniform eNit) value. 
120 The Origin of Granites and Related Rocks 

Internal co-sanguineous differentiation models, using actual 

modal proportions of essential minerals, closely account 

for observed trends in Rb, Sr, Ba and Th. REE models 

require accessory minerals (titanite, apatite and zircon) to 

be considered. During the early stages, the parent liquids 

(Si02 = 65- 69 wt %, K20 = 3.0- 3.8%, Rb = 100-

150 ppm, Sr = 470 - 610 ppm, (La!Yb)N = 16 - 23, 

Eu/Eu * = 0. 7 - 1.0) evolved by precipitation of basic 

plagioclase + hornblende + biotite + quartz + accessory 

minerals. Ubiquitous micro granular enclaves in both 

suites (Si02 = 60 - 61.5 wt %, K20 = 1.7 wt %) represent crystal accumulation, and the gabbros and basic quartz 

monzodiorites at La Calandria (Si02 = 55 - 58 wt %, K20 

= 0. 9 - 1.5 wt %, Rb = 25 - 60 ppm, Sr = 800 - 960 

ppm, (La!Yb)N = 10, Eu/Eu* = 0.95) were also formed 

by accumulation during this stage. K-feldspar and acid 

plagioclase dominate crystallization paths in the interval 69 

- 76 wt % Si02, where trends in both suites are similar 

and show a progressive depletion in Sr, Sc, Zr, MREE 

and HREE, and concomitant increases in Rb, Th, U, Ta 

and the negative Eu anomaly. Syenogranites and aplites 

represent the extreme development of these trends (Si02 = 

74.5 - 76.5 wt %, K20 = 4.7- 5.2 wt %, Rb = 190-

240 ppm, Sr = 20 - 60 ppm, (La!Yb )N = 26 - 28, Eu/Eu* 

= 0.40- 0.70). The parent magmas of these suites were 

derived from melting of more mafic sources, probably 

deep crust rocks of basaltic composition. 

However, there are small but significant differences 

between the two suites, especially in their isotopic compositions, which suggest independent evolution from similar 

but not identical parent magmas. The (

87Sr/86Sr)i ratios, 

average eNi203 Ma) and mean depleted-mantle Nd model 

ages are respectively 0. 70487, -0.5 and 950 Ma for La 

Leona, and 0.70508, -1.5 and 1000 Ma for La Calandria. 

Both suites are thus isotopically more primitive than the 

Jurassic silicic volcanic rocks ((87Sr/86Sr)i ratios c. 0.7068 

and eNi175 Ma) c. -4.0), which have attributed to partial 

melting of a mafic lower crust first formed in Grenvillian 

times (1150-1600 Ma). Sr-Nd isotopic variations within 

the Triassic-Jurassic extra-Andean province may be 

generally described by mixing curves, including a hypothetical mafic end-member within the mantle field and a 

CrUStal end-member at esr = + 75 and eNd = -10 (much 

less radiogenic and equivalent, respectively, than the 

crustal end-member recently proposed for such variations 

in Jurassic metaluminous-peraluminous granitoids of the 

Antarctic Peninsula). It is not possible to discriminate 

between variable mixing between end-member magmas and 

continuous variation in a zoned crust, although the perfect 

homogeneity of the small magma batches represented by 

the monzonite suites seems to favour the latter interpretation. The unconformable Jurassic volcanic cover of the 

monzonite outcrops indicates that there was significant 

uplift in the interval 200-170 Ma, after which a different 

crustal zone was involved in magma generation. 

Cordieritites in S-type Granites: 

Restites Following Low Pressure, 

High Degree Partial Melting of Metapelites 

1

Rapela, C.W., 2Pankhurst, R.J., 3

Baldo, E. and 4Saavedra, J. 1 Universidad Nacional de La Plata, Argentina 2

British Antarctic Survey, High Cross, 

Madingley Road, Cambridge CB3 OET, UK 3 Universidad de Cordoba, Argentina 4

/nstituto de Recursos Naturales y Agrobiologia, 

Salamanca, Spain 

Large amounts of very pure cordierite rock associated 

with S-type granites at El Pilon, near Soto in the eastern 

Sierras Pampeanas, are interpreted as residues from the 

partial melting of regional phyllites that produced the 

granite magmas. 

The Soto area belongs to the Famatinian orogenic belt, 

characterized by Lower to Middle Paleozoic granitoids 

emplaced in a polymetamorphic basement. The 

cordieritites and coeval cordierite-bearing granitoids were 

emplaced during the second metamorphic event, which 

reached temperatures high enough to provoke widespread 

anatexis of the metasedimentary sequences (680-730°C; 

4.5-6.7 kbar). 

The majority of the outcrop of the El Pilon granite is 

composed of a sillimanite-cordierite-biotite porphyritic 

granodiorite or monzogranite (Si02 = 69 - 71%, ASI = 

1.26- 1.38, Alk = 6.7- 7.2%, Mg# = 46- 49, K/Rb = 

180 - 220, (La!Yb)N = 8.7 - 10.0). A minor facies 

consists of Sill-Crd-Bt-Grt medium-grained leucomonzogranite (Si02 = 71 - 76%, ASI = 1.22 - 1.35, 

Alk = 7.5- 8.3%, Mg# = 25- 49, K/Rb = 180- 315, 

(La/Yb)N = 4.1 - 8.5). Four ovoid outcrops of cordieritite, each up to 140 m in length, are enclosed within 

this second, equigranular, phase. They display sharp 

magmatic-style contacts against both the granite and 

screen-like exposures of schist (Bt-Qtz-Pl-Kfs), and 0.5 
m-wide dykes of massive cordieritite (see below) intrude 

both the latter rock types. Xenoliths of the porphyritic 

granite are found within both the massive cordieritite and 

the leucogranite, whereas the porphyritic granite itself 

contains deformed enclaves of the schists. 

Two main textural varieties of the cordieritites occur 

-orbicular and massive. The orbicular facies consists of 

5-20 em egg-shaped orbicules, comprising an outer 

aggregate of polygonal, fresh cordierite with fibrolite 

between crystals and minor quartz and plagioclase 

(An2o.zJ, and a dark nucleus of oriented biotite, sillimanite, 

and cordierite. Space between orbicules is occupied by a 

pale mosaic of quartz, perthitic K-feldspar and plagioclase 

(Anl6-zo). The orbicules typically have Al20 3 = 28%, Alk 

= 3%, Mg# = 63, and (La/Yb)N = 5 - 12. The massive 

cordieritite approaches pure cordierite (95 %) , with a 

composition Al20 3 = 29- 31.5%, Alk = 0.5- 2%, Mg# 

= 61 - 68, (La/Yb)N = 3 - 6. There is a gradual transition to darker varieties of the massive facies with up to 

40% of modal biotite. The cordierite occurs as 1-2 em 

crystals set in a groundmass of small crystals (0.3-0.5 

em), both with fibrolite and biotite at the crystal edges. 

Staurolite appears in the more biotitic varieties, and 

common accessories are apatite, zircon and tourmaline. 

Rb-Sr and Sm-Nd isotope analyses of both granites, 

cordieritite and schist indicate that all these rock-types 

were genetically linked. In particular, the massive 

cordieritite and equigranular granite analyses together 

define a single Rb-Sr isochron (520 ± 5 Ma, MSWD = 

2.9), with a wide range in Rb/Sr ratios (3-20 in the 

cordieritites due to variation in the modal amount of 

biotite; 1.5-5.5 in the granite). Three samples of the local 

schist plot close to the lower end of the same isochron ' 

which has (

87Sr/86Sr)i of 0.714. eNi520 Ma) varies from 

-5.7 to -6.0 in the cordieritites, from -5.1 to -5.5 in the 

granites, and from -3.0 to -7.7 in schists, suggesting that 

the metapelites did not undergo complete isotopic 

re-equilibration during anatexis. 

It is suggested that the cordieritites and the leucogranite 

were formed from a large metasedimentary raft in the roof 

zone of the porphyritic granite, which was itself formed by 

anatexis of similar source rocks at greater depth. The 

relict metamorphic minerals in the core of the cordieritite 

orbicules suggest that melting of metapelites was accompanied by a reaction of the kind: 

Bt(ss) + Sill + Pl + Qtz = Crd + Kfs + melt 

Gamet is absent in both cordieritite facies, indicating 

that melting took place at very low pressure, probably well 

below 3 kbar. The newly formed Mg-rich cordierite from 

Abstracts 121 

this reaction grew as limpid large crystals until Pl and Qtz 

were exhausted (leaving the Bt -Sill cores of the orbicular 

facies), while the melt migrated and coalesced nearby 

forming the equigranular leucocratic facies of the El Pilon 

granite. The massive cordieritite represents an extreme 

situation where the original composition allowed complete 

reaction of the products. Water saturation, fluids and high 

temperature increased the mobility of the massive 

cordieritite so that it become sufficiently ductile to intrude 

the older, porphyritic, facies of the host granite. 

Estimation of Initial and Saturation Water 

Contents of Three Mesozoic Granitic Plutons 

in the North-central Great Basin, Nevada 

Ratajeski, K. and Candela, P.A. 

Laboratory for Mineral Deposits Research, 

Department of Geology, University of Maryland 

at College Park, MD 20742, USA 

According to the Magmatic-Hydrothermal Theory for 

the genesis of granite-related ore mineralization, the ratio 

of initial melt water content ( C~p) to saturation melt water 

content ( C~p) influences the timing of volatile phase 

saturation relative to the crystallization history of the 

magma, the relative amounts of crystal-compatible and 

crystal-incompatible ore-forming elements in the exsolved 

magmatic volatile phases, and the physical mode of egress 

of magmatic volatiles from the crystallizing magma 

(Candela, 1991, 1992). Three Mesozoic plutons from the 

north-central Great Basin in Nevada were studied to 

determine how well c~;a and cj;P can be constrained. 

These plutons are located in Lander and Eureka Counties, 

NV and include the Jurassic Mill Canyon stock (MCS), the 

Jurassic McCoy pluton (MP), and the Cretaceous Trenton 

Canyon pluton (TCP). Selected samples from this study 

include: MCS-63: a medium-grained biotite granite to 

granodiorite (68.5 wt % Si02); MCS-15, a fine- mediumgrained, porphyritic biotite-hornblende tonalite (66.4); MP43, a medium-grained biotite-hornblende granite to 

granodiorite (66.1); and TCP-41, a medium-grained 

biotite-hornblende granodiorite (65.1). Rare porphyritic 

samples (MP-75 and TCP-35) with high phenocryst 

contents ( -70 vol %) were collected from outcrops 

otheiWise similar to hypidiomorphic-equigranular samples 

MP-43 and TCP-41. 
122 The Origin of Granites and Related Rocks 

Estimates of C~oo were attempted by inferring crystalli- 2 

zation sequences from petrographic observations and 

comparing the inferred sequences with experimentally -

determined crystallization paths as depicted on published 

T-X//j;em diagrams for granites and granodiorites at 2 kbar. 

Except for MCS-63, all samples contain hornblende and 

texturally-early biotite (based on the presence of euhedral 

to subhedral biotite inclusions in plagioclase, quartz 

phenocrysts, and K-feldspar), implying that C~p >4 wt % 

if Naney's (1983) synthetic granodiorite R5+ 10M1 is used 

as a model. However, this conclusion is contradicted by 

porphyritic samples MP-75 and TCP-35, which contain 

hornblende phenocrysts but show textural evidence of H20-

undersaturated conditions to high crystallinities ( -70 vol 

%) in the form of resorbed quartz and/or K-feldspar 

phenocrysts. Assuming saturation with a water-rich 

volatile phase at 70% crystallization, a maximum c~o 0 - 2 2 

wt % is suggested if Al-in-hornblende geobarometry 

(presented below) is accurate. MCS-63 has texturallyearly biotite but no hornblende, which only suggests that 

c~oo >2 wt % (Naney, 1983). 2 

The Al-in-hornblende geobarometer was used on rocks 

with the appropriate mineral assemblage, except for 

titanite. Pressures were calculated with the Johnson and 

Rutherford (1989) calibration for a sample from the MCS 

with low (- 15 vol %) phenocryst content (high quench 

temperature), and with the Schmidt (1992) calibration for 

a sample with high phenocryst content ( -70 vol %) from 

the MP and a hypidiomorphic-equigranular sample from 

the TCP. Calculated pressures (± 1o) are 1.7 ±0.6 kbar 

for the MCS (12 rims analyzed), 2.3 ± 1.0 kbar for the MP 

(7 rims), and 2.9±1.2 kbar for the TCP (12 rims). 

Pressure for the MCS is best constrained, but results 

generally indicate moderately-shallow emplacement depths 

for all of the plutons. c}:p (± 1u) was estimated from 

pressure and water solubility data of Holtz et al. (1993) as 

follows: 5.2±1.6 wt % H20 (MCS); 6.3±2.7 wt % 

(MP); and, 7.3±3.3 wt.% (TCP). The uncertainty in the 

estimates of C~ 0 I C~s 0 for plutonic rocks of the M CS, MP, 2 2 

and TCP is due to limitations of hornblende barometry, 

probable alteration of hornblende in the TCP, and by 

limitations of textural criteria to provide reliable and 

constrained information on initial water contents. Using 

these estimates for saturation water concentrations and 2 Of • ~ I 0 • Ids I 0 I l,r 0 3 wt 10 maxnna 10r CJ[20 , y1e CJip Cnp < . ±0.1 for 

both MP-75 and TCP-35. These low values suggest that 

these magmas were relatively dry for their inferred level 

of emplacement (Candela, 1991), which is and are consistent with their lack of significant associated mineralization. 

Candela, P.A., 1991. Physics of aqueous phas\! evolution in plutonic 

environments. Am. Mineral., 76, 1081-1091. 

Candela, P.A., 1992. Controls on ore metal ratios in granite-related ore 

systems: an experimental and computational approach. Trans. Royal 

Soc. Edinburgh: Earth Sci., 83, 317-326. 

Holtz, F., Dingwell, D.B. and Behrens, H., 1993, Effects ofF, B20 3 and 

P20 5 on the solubility of water in haplogranitic melts compared to 

natural silicate melts. Contrib. Mineral. Petrol., 113, 492-501. 

Johnson, M.C. and Rutherford, M.J., 1989, Experimental calibration of 

the aluminum-in-hornblende geobarometer with application to Long 

Valley caldera (California) volcanic rocks. Geology, 17, 837-841. 

Naney, M.T., 1983, Phase equilibria of rock-forming ferromagnesian 

silicates in granitic systems. Am. J. Sci., 283, 993-1033. 

Schmidt, M.W., 1992. Amphibole composition in tonalite as a function 

of pressure: an experimental calibration of the At-in-hornblende 

barometer. Contrib. Mineral. Petrol., 110, 304-310. 

Experimental Determination of 

Rare Earth Element Distribution Between 

Chloride-Bearing Volatile Phase 

and Granitic Melt 

Reed, Marshall J. 

Laboratory for Mineral Deposits Research, 

Department of Geology, University of Maryland 

at College Park, MD 20742, USA 

Rare earth elements (REE) show systematic variation 

during most chemical and geologic processes, and the REE 

distribution patterns of granite-related mineral deposits can 

be used to monitor the -progress of mineralization through 

the stages of hydrothermal alteration. In this experimental 

study, the partitioning of rare earth elements between 

slightly-peraluminous granitic melts and chloride-bearing 

aqueous phases was investigated at conditions representing 

a magma chamber crystallizing under lithostatic pressure 

at a depth of c. 7 km in the crust (2 kbar) and 800°C. At 

these conditions, the aqueous phase is neither a liquid nor 

a vapor but is supercritical in low-C02 systems. Anhydrous monzogranite glass (34.0 wt % KA1Si30 8, 31.7 wt 

% NaA1Si30 8, 7.4 wt % CaA12Si20 8 , and 26.9 wt % Si02) 

was reacted with aqueous chloride solutions containing 

systematically varied concentrations of Na+, K+, H+ and 

ten REE of interest (La, Ce, Nd, Sm, Eu, Gd, Tb, Ho, 

Yb, and Lu). Starting solutions contained 0.007 to 0.03 

m REE, 0.44 to 3.5 m chloride, and in 4x104 to 0.8 m 

H+. Instrumental neutron activation analysis was used to 

determine trace-element concentrations (there are few 

interferences with the rare earth elements). To maintain 
a uniformly high accuracy in analyses, the concentrations 

of the REE were in proportion to their limits of detection. 

For a sample mass of 30 mg, gamma-ray counting statistics for 3% error limits required concentrations of 500 

mg/kg for Gd, Ho, and Nd; 200 mg/kg forCe and Yb; 

and 50 mg/kg for La, Sm, Eu, Tb, and Lu. 

To assure that samples were representative of run 

conditions, the experiments were rapid-quenched (800°C 

to 100°C within 3 seconds) at run pressure. The pH of 

each quenched solution was determined by dilution to the 

pH range 1.68 to 4.01, and measurement of samples and 

standards. Chloride analysis of each quenched solution 

was made using a digital chloridometer. A sample of each 

aqueous solution was analyzed for the major elements Na, 

AI, Si, K, Ca, and Fe by inductively coupled plasmaatomic emission spectrometry. Part of the glass recovered 

from each experiment was analyzed by electron probe 

microanalysis, and instrumental neutron activation analysis 

(INAA) was used to determine the trace element composition of both glass and quenched solution from each 

experiment. 

Equilibrium between granitic melt and the magmatic 

volatile phase at experimental conditions of 2 kbar and 

800°C was examined as a function of the partitioning 

coefficient of individual elements and as the apparent 

equilibrium constant for exchange reactions between Na 

and the other cations. The partition coefficient (D) of an 

element is the ratio of the concentrations in the two phases 

(aqueous/melt), for example: 

A 

D A,M _ CLa 

La - eM 

La 

but this parameter varies significantly with changes in 

phase composition and changes in experimental conditions. 

If the assumption is made that the activity coefficient is 

constant, or nearly constant, over the range of experimental conditions, then the apparent equilibrium constant, K', 

with respect to sodium can be calculated from the elemental concentration (C) in each phase rather than from the 

activities: 

Since this parameter is based on the exchange reaction, it 

is expected that the variations in the partition coefficient of 

a rare earth element will be compensated by changes in the 

Abstracts 123 

cube of the sodium partition coefficient (K' Eu,Na calculated 

with the square of DNa). Mean values and 1 u deviations 

are given for groups of experiments with three different 

starting solutions to test this assumption: 

La 

Ce 

1.1 mECl 

0.05 mH+ 

D K' 

0.037 0.157 

±0.0043 ±0.0047 

0.032 0.136 

3.5 mECl 

0.07 mH+ 

D 

0.043 

±0.047 

0.39 

K' 

O.Q74 

±0.013 

0.067 

1.1 mECl 

0.7 mH+ 

D 

2.02 

±0.28 

1.97 

K' 

0.41 

±0.08 

0.4 

±0.0049 ±0.014 ±0.043 ±0.0089 ±0.32 ±0.089 

Nd 0.022 0.096 0.34 0.058 1.96 0.39 

±0.0031 ±0.0091 ±0.029 ±0.006 ±0.3 ±0.082 

Sm 0.021 0.09 0.28 0.047 1.61 0.32 

±0.0031 ±0.006 ±0.037 ±0.0028 ±0.29 ±0.076 

Eu 0.033 0.087 0.48 0.016 1.08 0.37 

±0.0058 ±0.0098 ±0.077 ±0.0168 ±0.08 ±0.029 

Gd 0.021 0.089 0.23 0.039 1.38 0.28 

±0.0023 ±0.0005 ±0.052 ±0.0016 ±0.29 ±0.073 

Tb 0.019 0.079 0.21 0.035 1.23 0.25 

±0.0028 ±0.0044 ±0.033 ±0.0017 ±0.2 ±0.055 

Ho 0.017 0.07 0.16 0.033 0.95 0.19 

±0.003 ±0.003 ±0.028 ±0.0033 ±0.14 ±0.039 

Yb 0.014 0.036 0.12 0.021 0.62 0.13 

±0.0016 ±0.003 ±0.019 ±0.0046 ± 0.13 ±0.031 

Lu 0.013 0.058 0.11 0.019 0.53 0.11 

±0.0015 ±0.0032 ±0.02 ±0.0047 ±0.093 ±0.024 

Changes in the K' between experiments show there is a 

more complex relationship between K' and the concentrations than was assumed. The activity coefficients of the 

components appear to be a function of both ci- and H+ in 

solution. The aluminum saturation index (ASI = mole 

ratio Al/[Na + K + 2Ca]) shows a direct response to the H + 

concentration in the starting solutions. For the three 

groups of experiments, the mean ASI and deviation are: 

for 1.1 m Cl, 0.05 m H+, ASI = 1.02 ± 0.02; for 3.5 m 

Cl, 0.07 m H+, ASI = 1.02 ± 0.03; and for 3.5 m Cl, 

0.7 m H+, ASI = 1.16 ± 0.04. From these results, it 

appears that hydrogen ions from the aqueous solution 

exchange with both sodium and potassium in the melt. 

Increasing hydrogen ion concentration in the system 

contributes to greater partitioning of sodium and potassium 

from the melt into the volatile phase, and this exchange 

drives the melt toward peraluminosity. 
124 The Origin of Granites and Related Rocks 

A Complex Strain and Intrusion Fabric 

Related to Trans-Solidus Deformation: 

The Wolf Mountain Intrusion, 

Llano Uplift, Texas 

Reed, Robert M. 

Department of Geological Sciences, 

University of Texas, Austin, TX 78712, USA 

The Wolf Mountain Intrusion (WMI) is a chevronshaped (phacolithic) granite body which occupies an 

upright synform that plunges shallowly to the southeast. 

The WMI is one of many Proterozoic porphyritic granite 

intrusions in the Grenvillian Llano Uplift of central Texas 

that have previously been considered post-orogenic to 

anorogenic. Deformation during and after the emplacement of the granitic magma has produced a complex 

internal fabric including foliations with magmatic and 

tectonic components, multiple diking events, and shearing. 

This internal fabric is most complex in the hinge region of 

the intrusion where the history is as follows. 

1) Intrusion of a magma (coarse-grained porphyritic 

granite) as a roughly planar mass at or near the contact 

between the two major lithological units in the uplift. 

2) Development of a mostly linear magmatic alignment 

(Smag) of microcline megacrysts and xenoliths. 

3) Addition of biotite-rich magmatic enclaves, many of 

which occur in planar swarms suggesting intrusion into the 

host magma. 

4) Regional folding (the SE-plunging synform). Folding 

may have begun earlier, but evidence for deformation is 

observed after this point. 

5) Aplitic and pegmatitic dikes, such intrusions continued 

throughout crystallization. 

6) A second phase of coarse-grained porphyritic granite 

with convolute margins is also present. This phase is 

differentiated by having no mafic enclaves, less biotite, a 

more reddish color, and more-equant microcline 

megacrysts. Lack of elongate feldspars gives this phase a 

less-foliated appearance. 

7) Intrusion of SE-trending dikes that are later foliated 

(St). Dikes come in four varieties: aplitic with disseminated biotite, aplitic with planar biotite segregations, 

phenocrystic granite with a fine-grained matrix, and dikes 

of the second granitic phase. Much of the foliation (St) in 

these dikes was formed during solid-state deformation with 

accompanying recrystallization. The dikes are generally 

not boudinaged, indicating that they were less competent 

than the host granites at the time of deformation. Many 

dikes are foliated parallel to their margins. However, a 

significant number; which trend more ESE, have oblique 

foliations trending SE. 

8) Some of these dikes are cut by minor "healed" shears 

(Ses) with small components of motion ( <2.5 m.). Ses 

development in the granite varies from absent to moderate, 

indicating Ses formed over a period of time as the granite 

solidified. These shears come in two orientations, sinistral 

with ESE trends and dextral with ENE trends. Ses shears 

seem to form a conjugate pair indicating SW-NE compression, consistent with the axial plane of the main fold. 

9) Formation of a SE-trending foliation (St). St foliates 

earlier dikes and forms zones in the granite meters to tens 

of meters wide that cut or overprint Smag. St has erased 

Ses in some places, but is cut by it in others. St has a 

strong component of solid-state deformation. Quartz and 

feldspar have undergone high temperature recrystallization. 

Small xenoliths and elongate magmatic enclaves are 

oriented subparallel to this foliation where it occurs. St 

cuts across boundaries between the two porphyritic granite 

phases. Whereas microfabrics in the main granite are 

ambiguous, some of the sheared phenocrystic dikes contain 

sinistral indicators. Displacement of an enclave zone by 

an St zone also shows apparent sinistral motion. 

10) Lastly, intrusion of fine-grained pink granite dikes and 

bodies. These granite bodies have irregular shapes and 

poor to non-existent foliation. Dikes with SE and WSW 

trends radiate from these bodies. WSW -trending 

unfoliated pegmatites are also common at this stage. 

Although these dikes are themselves undeformed, the 

WSW dikes are intruded perpendicular to the inferred 

compression direction of the folding. This orientation 

suggests that these dikes are related to the deformation. 

Some of these dikes also offset earlier features with meterscale right-lateral offsets, suggesting transtensional intrusion. 

The internal structure of the granite is much simpler in 

the limbs of the intrusion, suggesting concentration of 

strain in the hinge zone during regional folding of the 

crystallizing intrusion. 

The WMI provides evidence for late synorogenic 

intrusion of granite in the Llano Uplift, tying what were 

considered to be "anorogenic" granites to an orogenic 

event. The WMI also provides an example of the complexities that arise from deformation occurring synchronous with solidification. Deformation is not just concentrated at the pluton margin but within an area in the 

interior. The localization of deformation in one area with 
reduced deformation elsewhere points out the need for 

caution in labeling a pluton "undeformed" based on an 

examination of only part of its area. 

The Albite Granites of the Central Eastern 

Desert (Egypt): The Evolution of a 

Hot and Dry Granulite-Derived Melt 

to a Pegmatoid Intrusion 

Renno, A.D. 

Institute of Mineralogy, Freiberg University 

of Mining and Technology, Germany 

In the Pan African consolidated basement of the Central 

Eastern Desert, three albite granite bodies are situated: 

Nuweibi, Abu Dabbab and lgla. The remarkable features 

of these granites have interpretation of part of the late- to 

post-Pan African evolution of this part of the Arabian 

Nubian Shield. The Abu Dabbab and Nuweibi granites are 

sheet-like intrusions along preintrusive structures in the 

wallrocks. The lgla granite forms a stockwork of felsic 

dikes and mineralized quartz -mica -beryl veins. Clear 

intrusive contacts, e.g., intrusive breccias, exclude the 

possible metasomatic formation of these rocks. The 

present level of erosion is different. The Igla granite is 

least eroded (only the top of the intrusion is uncovered), 

the Nuweibi granite has eroded down to the roof zone, and 

the Abu Dabbab granite has eroded the most. Felsic dikes 

are closely associated with the granites. 

The geochemical characteristics of the peraluminous 

granites are: Si02 > 75 %; molar Na20 I K20 ratio of > 

1, (Na20 + K20) > 8 %; extreme low values for CaO 

(0.1 %), MgO (0.02 %), P20 5 (0.01 %), and Sr (5 ppm); 

extreme high values for Ga ( > 100 ppm), Ta 

( > 150 ppm), and Zn ( > 400 ppm); moderate values for 

Li (10 - 100 ppm) and F- (400 - 700 ppm); and, Nb/Ta 

ratio < 1. 

The felsic dikes show big inhomogeneities and patterns 

that are not typical for magmatic rocks. These rocks 

cannot be defmed as Li-F granites, but as low-phosphorus, 

low-Ca Ta-granites. The mineralogical composition of the 

albite- alkali feldspar- quartz - (muscovite) rocks is very 

monotonous. However, they bear an uncommon assemblage of accessory minerals: orthopyroxene, garnet, 

gahnite, annite, Mn-Ta minerals, cassiterite and Hf-rich 

zircons (Hf02 10 - 25%). The microstructure of the 

Abstracts 125 

rocks ranges from medium grained equigranular over 

porphyritic (monzonitic feldspar or "snowball-quartz") to 

felsitic structures (dikes and margins of the intrusions). 

The age of the intrusion of the albite granites is - 590 

Ma. Even though there is no time gap between the 

postorogenic A-type granites and the albite granites, they 

are not related. There is also no geochemical evidence for 

the formation of the albite granites by fractionation of 

basaltic magmas, as proved by Coleman et al. (1992) for 

the A-type granites related to the Red Sea opening in Saudi 

Arabia and Yemen. 

I suggest the following petrogenetic model. At the base 

of the lower crust, melting of the (garnet) pyroxene 

granulites took place at 950 - 11 OOOC, 10 kbar and aHP < 

0.3 and yielded to Ab-rich melts. Extensional tectonic 

processes (Greiling et al., 1994) promoted the extraction 

of the small melt amounts out of the source rock. Due to 

the relatively low fluorine content in the albite granites, 

there was no "metasomatic" input of halogen-rich fluids to 

support the melt generation in the protolith. The quick 

ascent of the magma did not prevent fractionation processes, but residual garnets are preserved. In the hot and 

dry haplogranitic melt orthopyroxene, albite, alkalifeldspar, and later quartz and annite crystallized. During 

the intrusion of the magma in the upper crust, the melt 

absorbed water from the "wet" wall rocks. This turned the 

character of the melt into a pegmatoidal melt, which 

crystallized quickly. In the fine grained albite-quartz 

(muscovite) matrix typical pegmatite minerals like gahnite, 

manganotantalite and Hf-rich zircon were formed. The 

isotopic system of Sr was completely disturbed (measured 

(

87Sr/86Sr)i - 1.1 - 5.1). The felsic dikes, originating 

from coexisting silicate melts and fluid phases, concluded 

the magmatic activities. Circulating fluids formed belt-like 

bodies of amazonitisized or sericitisized albite granite, 

marginal greisen bodies and the quartz cap of the Nuweibi 

granite. 

The formation and intrusion of the albite granites was 

a local process. This process fits into a continuous 

magmatic evolution of the area from the Red Sea Hills 

(Sudan) to the Sinai Peninsula (Egypt) in the period from 

the end of the Pan African accretion to the beginning of 

the Red Sea rift. The impulses and conditions for this 

magmatic evolution are not yet completely understood. 

Coleman, R.G., DeBari, S. and Peterman, Z., 1992. A-type granite and 

the Red Sea opening. Tectonophysics, 204, 27-40. 

Greiling, R.O., Abdeen, M.M., Dardir, A.A., El Akhal, H., El Ramley, 

M.F., Kamal ElDin, G.M., Osman, A.F., Rashwan, A.A., Rice, 

A.H.N. and Sadek, M.F., 1994. A structural synthesis of the 
126 The Origin of Granites and Related Rocks 

Proterozoic Arabian-Nubian Shield in Egypt. Geol. Rundsch., 83, 

484-501. 

ORAL PRESENTATION 

The Perils of AFC Modelling 

1

Roberts, Malcolm P. and 2Clemens, John D. 1

Department of Geology, The University, 

Manchester M13 9PL, UK 2

School of Geological Sciences, Kingston University, 

Kingston-upon-Thames, KTJ 2EE, UK 

A major use of whole-rock and isotope geochemical 

analyses of granitoid rocks is to constrain the nature of 

magma source regions (protoliths), as well as possible 

processes by which the magmas evolved. The need to 

explain elemental variation on bivariate plots has led to the 

development of mathematical models of processes that may 

have produced the variation. 

Of the published geochemical models, one of the most 

popular is that of combined wall-rock assimilation and 

fractional-crystallization (the AFC model of DePaolo, 

1981). In this model, mathematical expressions are used to 

calculate isotopic ratios and elemental compositions of 

hybrids formed as the assimilating or original magma 

undergoes fractional crystallization, while simultaneously 

mixing with another magma or assimilating its wall-rocks 

(the assimilant). Trace-element and isotope concentrations, 

obtained from the equations, define curves on bivariate 

isotope-isotope and isotope-element plots. 

This model has been very effective in reproducing 

trends commonly seen on plots of Nd and Sr isotope data. 

The governing equations allow assumptions to be made on 

the ratio (r) of the rate of assimilation to the rate of 

fractional crystallization (an arbitrary fraction) and the 

bulk distribution coefficient (D) for the element under 

investigation. Choices of the values for r and D are 

typically based on those that produce the best fit to the 

data set (usually within a broad spectrum of what is 

considered possible) rather than being grounded in any 

solid geological, mineralogical or experimental constraints. 

For such a model to be effective, considerable differences 

must exist between the elemental and isotopic compositions 

of the assimilant and the assimilating magma (Powell, 

1984). 

The reason for the popularity of this model is readily 

apparent. It is an extremely powerful, yet simple, mathematical formalism that, on the face of it, permits erection 

of a viable petrogenetic model for essentially any group of 

related rocks. Herein also lies its Achilles heel, for it 

permits a pattern-matching kind of modelling, essentially 

unconstrained by the realities of magma physics, geological observation or major-element geochemical data. 

Recently, the AFC equations have been extended to 

allow independent estimation of p, the ratio of the mass of 

assimilant to the mass of original magma (Aitcheson and 

Forrest, 1994). Thus, a sensitive criterion of model 

feasibility is that calculated p versus r curves ought to be 

the same, within analytical error, for all elements tested. 

If not, the simple model is invalid. 

Modelling of variations among granitoid rocks from the 

Querigut Massif in the Pyrenees, a continental calcalkaline complex, typical of such bodies world-wide, 

shows that AFC approaches successfully match the 

variations in Sr and Nd isotopic systems. However, the 

models fail, catastrophically, to satisfy any of the other 

major- and trace-element constraints (e.g., Si02 , MgO, 

K20, Rb, Sr, Zr) for the hybrids. Magmas with appropriate Sr and Nd isotope ratios can be produced using an 

enormous and poorly constrained range of model parameters, but such an approach has little petrogenetic value. 

Curves generated in this fashion, on bivariate isotopeisotope and isotope-element plots, will always form arrays 

between the two end-member compositions, even if they 

are genetically unrelated. This is an artefact of the mathematics governing two-component mixing, as used by the 

AFC equations, and may explain why such modelling 

commonly makes use of isotope data in preference to 

major- or trace-element data. 

We do not contend that AFC modelling has no role to 

play in the formulation of petrogenetic hypotheses. However, it should be undertaken with an eye to what is 

geologically and geochemically reasonable (for values of 

r, D and p). If possible, there should be some hard 

evidence for the existence of the proposed assimilant, and 

any apparent solution should be tested and evaluated 

against a number of major- and trace-element constraints, 

as well as constraints imposed by physics and thermodynamics. Without these precautions, we are in danger of 

self-delusion. 

Aitcheson, S. J. and Forrest, A. H., 1994. Quantification of crustal 

contamination in open magmatic systems. J. Petrol., 35, 461-488. 
DePaolo, D. J., 1981. Trace element and isotopic effects of combined 

wallrock assimilation and fractional crystallization. Earth Planet. Sci. 

Lett., 53, 189-202. 

Powell, R., 1984. Inversion of the assimilation and fractional crystallization (AFC) equations; characterisation of contaminants from isotope 

and trace element relationships in volcanic suites. J. Geol. Soc. 

London, 141, 447-452. 

Are Enclaves Significant in 

Granite Petrogenesis? 

Roberts, Malcolm P. and Clemens, John D. 1 Department of Geology, The University, 

Manchester M13 9PL, UK 2

School of Geological Sciences, Kingston University, 

Kingston-upon-Thames, Surrey KTJ 2EE, UK 

The study of enclaves in granites has long been popular. This is undoubtedly due to the fact that they are a 

common feature of granitoid rocks, worldwide. Macroand microscopic textural features of many enclaves 

suggest that they crystallised from magmas following 

incorporation into the felsic host. It is these enclaves that 

attract most interest. There have been various models 

advanced for the petrogenetic relationships between 

enclaves and their hosts, with an emphasis on their 

potential importance in the chemical and mineralogical 

evolution of the rock body within which they are enclosed. Thus, enclaves are commonly viewed as being of 

fundamental importance in granitoid petrogenesis. 

The Querigut Massif in the French Pyrenees is a much 

studied Variscan age (285 Ma), metaluminous, calcalkaline complex renowned for a spectacular mafic-felsic 

association, including abundant enclaves. The host granitic 

rocks include hornblende-bearing granodiorites and 

tonalites ((87Sr/86Sr)i = 0.709 to 0.710; eNd = -5.5 - -6.0), 

monzogranites ((87Sr/86Sr)i = 0.709 to 0.710; eNd = -5.5 

- -6.0) and biotite granite ((87Sr/86Sr)i = 0.712 to 0.716; 

eNd = -6.0- -8.0). Associated with these are a number of 

diorite bodies with olivine hornblendite and hornblendite 

cumulates ((87Sr/86Sr)i = 0.706 to 0.710; eNd = -1.5 to 

-6.5). These intermediate and ultramafic rocks are among 

the least isotopically evolved of the main Querigut suite. 

Enclaves are distributed throughout the felsic rocks of 

the Massif, although they are most abundant within the 

granodiorites and tonalites. Most have a finer grain-size 

than their host, and have textures that indicate magmatic 

Abstracts 127 

crystallization. They vary widely in the modal abundances 

of hornblende, plagioclase and biotite phenocrysts. The 

presence of corroded calcic plagioclase overgrown by 

more sodic plagioclase rims, suggests that magma-mixing 

may have been involved in the formation and subsequent 

modification of some enclaves, although other features 

such as armoured quartz ocelli are generally absent, and 

there are alternative explanations for such zoning patterns. 

Many of the textural and mineralogical features of the 

enclaves are similar to those of the main diorites. Therefore, it could be hypothesised that the enclaves and the 

diorites are genetically related, with the enclaves representing dispersed blobs of the same magma that forme~ 

the larger diorite intrusions. However, it is particularly 

important, that there are assemblages of contrasting 

enclave types within centimetres of each other. Thus, 

enclave formation by some simple diorite-felsic magma 

mingling process is difficult to visualise . 

The geochemical and isotopic characteristics of the 

enclaves are of particular interest. On variation diagrams 

of all kinds, the enclaves do not form coherent arrays 

between any of the main Querigut magmas. Instead, they 

show a wide dispersion. This is most apparent for 

trace-element concentrations, and highlights the impact 

that simple variations in modal mineralogy can have on 

compositions. The wide dispersion of enclave data on 

isotope-isotope plots highlights the polymagmatic character 

of the Querigut enclaves, that does not fit with any 

simple, one- or two-magma model for their genesis. For 

those enclaves that do fall between potential 

end-members, the data cannot be used to limit their 

possible origins beyond a minimum of two different 

models. Another important observation is that some mafic 

enclaves have Sr and Nd isotopes more crustal than 

neighbouring intermediate enclaves. This lack of expected 

parity between enclave mineralogy, geochemistry and 

isotope characteristics makes interpretation difficult by 

any of the popular models for the origin and significance 

of enclaves (e.g. mingling, hybridisation, restite-unmixing 

or crystal accumulation). While, in some cases, they may 

be comagmatic with their hosts, the bulk of the enclaves 

seem to be no more than dispersed fragments of unrelated 

accessory, though broadly cogenetic, magmas caught up in 

the pluton plumbing system. 

Although the presence of enclaves in granitoid rocks is 

undoubtedly meaningful at some level, there is probably 

no general statement that can be made about their significance. It seems that the Querigut enclaves are of little 

significance in the overall petrogenetic evolution of any of 
128 The Origin of Granites and Related Rocks 

the granitic units. For most of the enclaves, geochemical 

and isotope data are unable to uniquely constrain their 

origins . If this is more generally the case, it would seem 

that, whilst intrinsically interesting, enclave studies may 

be of very little actual use in unravelling the mysteries of 

granite petrogenesis. 

The Plutonic Zone: Significance of the 

Lower Crust Between the Conrad and 

Mohorovicic Discontinuities 

Roddick, James A. 

Geological Survey of Canada, 100 West Pender St., 

Vancouver, B.C., V6B JR8 Canada 

In most places on all continents the lower crust is 

markedly seismically reflective and electrically conductive. 

The combined geophysical effect is most easily accounted 

for by hydrothermal fluids (saline solutions) resulting from 

the inevitable breakdown of hydrous minerals. Depth to 

the top of the zone is less in most Phanerozoic regions 

(10-20 km), because of higher heat flow, than in the 

shield regions (20-35 km), and corresponds to the temperature range of 350-400°C. That depth also marks the 

transition zone where plastic deformation begins to 

supplant brittle fracture. The principal physical effect of 

the tenuous hydrothermal fluid (rarely > 1 %) is a slight 

reduction in mass viscosity of the lower crust to perhaps 

1018 from normal crustal values of about 1023 • Structural 

integrity is preserved, but plastic creep (essentially 

horizontal) in response to load stress is enabled. The 

chemical effects, however, are much more profound, and 

over long periods inevitably rearrange the components of 

the lower crust and change its texture. This process 

constitutes plutonism, and its region of activity is best 

encompassed by the term plutonic zone. 

The temperature pattern across the plutonic zone drives 

metasomatic reactions in the direction of chemically 

stabilizing the heterogeneous lower crust. In a dry 

environment the main plutonic minerals are metastable 

over the normal temperature range of the crust but in the 

presence of free hydrous fluid they become temperature 

sensitive. Acting over long periods, plutonism basifies the 

lower part of the zone, at the expense of the upper part. 

Plutonism can be halted only by loss of hydrous fluid (into 

fractures, or into hydrous minerals) or by fusion which 

destroys both the hydrous medium and the structural 

integrity of the affected region. 

Substantial volumes of the lower crust are homogenized 

by plutonism, but mobilization and intrusion as plutons is 

dependent on stress being great enough to exploit the small 

viscosity difference between this material and its envelope. 

Such stress is thought to be commonly transmitted from 

converging plate margins. Because of horizontal inflow, 

the Moho does not rise in response to pluton emplacement, 

which, regionally, is a crustal thinning process. 

Can batholithic-size magma chambers exist anywhere 

in the crust, especially in the lower crust? The mechanism 

by which a large volume of low viscosity material can 

support the crust above, is not apparent. Nor is the 

mechanism intuitive by which such material can be brought 

up through many kilometers of heterogeneous crust in the 

shape and size of common plutons. The unmelted envelope 

is certainly not homogeneous and without irregularities. 

In such an environment the concentration of enormous 

stresses at certain points cannot be prevented, or resisted; 

magma will be forced into relatively narrow fractures, 

which, if they reach the surface will result in volcanism. 

Does this mean that the concept of a large magma chambers in the crust is a "veritable warren of untruth, in fact, 

a scarcely propped vacuity"? Probably, and we fake the 

rest. 

A Belt of High-K Plutonic Rocks 

Related to Crustal Extension: 

Evidence from the European Variscan 

Rossi, Philippe and Cocherie, Alain 

BRGM, B.P. 6009, 45060 Orleans, France 

Strongly potassic mafic magmas of calc-alkaline 

affinity, i.e. lamprophyres such as minettes, vogesites, 

kersantites and spessartites, occur as dyke swarms or sills 

that intersect country rocks in continental areas. They can 

be associated with trachydacites and rhyolites, and could 

be related to crustal extension and heating. 

In the Variscan belt of western and central Europe, 

stocks of ultrapotassic to high-potassic (1.5 < K20/Na20 

< 3) mafic plutonic rocks commonly are associated with 

strongly foliated, acidic and K-rich granitoid plutons. 

Ultrapotassic mafic rocks were recognized quite early 

and described with exotic names, such as: vaugnerites in 
the Lyonnais Massif, France; stavrites, fraidornites, 

redzvizites in the Schwarzwald; or mafic durbachites in the 

Bohemian Massif. Such rocks display several common 

characteristics throughout the different massifs: "primary 

minerals" are Cr-diopside to augite clinopyroxene, Caplagioclase, Ba-rich K-feldspar, and foxy-red Mg-Ti biotite 

with rare relics of phlogopitic mica. Ilmenite, generally 

surrounded by titanite, is the most abundant oxide. Of 

particular interest is the occurrence of chromium spinel in 

micaceous aggregates, which could be relics of olivine. 

Apatite and zircon are the earliest minerals. The chemical 

composition of such ultra-K to high-K rocks is characterized by a strong enrichment in HFSE elements, and can be 

compared to that of shoshonite or lamprophyre. 

Associated granitoids, ranging in composition from 

monzonite to monzogranodiorite, show great variety, but 

generally are medium- to coarse-grained and very commonly mega-K-feldspar bearing. The systematic occurrence of actinolitic hornblende is characteristic of both 

acidic and mafic rocks, and is interpreted as the result of 

magmatic crystallization. Such plutonic rocks are quite 

common and occur as banded-like intrusions, marked by 

K-feldspar megacrysts, lens- or cigar-shaped enclaves of 

mafic rock that range from very small to about 1 m in 

size, and layers of biotite. 

Such intrusions occur throughout the European Variscan 

N-S branch, from Corsica to Central Europe, e.g. the 

Bohemian massif, as a discontinuous belt of about 

1500 km width through some of the external massifs of the 

Alps (e.g. part of the Belledonne and Aar-Gotthard 

massifs) and in the north of the French Massif Central, 

and then in an E-W direction through parts of the VosgesSchwarzwald massifs into the Central Pluton of the 

Bohemian Massif. 

It has been shown in Corsica that this early plutonism, 

which postdates continental collision, is of Early Carboniferous age and was emplaced in an amphibolic environment 

during anatexis of the uplifting basement after a crustal 

shortening episode. 

The ages of different intrusions were investigated along 

such a mobile belt using the Pb-zircon evaporation method, 

taking into account that it occurs in both mafic and silicic 

rocks. The results indicate that most potassic intrusions 

were emplaced around 340 Ma (e.g. Corsica: 

339 ± 11 Ma; the external crystalline massifs of the Alps 

332 ± 13 Ma; the Bohemian Massif 346 ± 6 and 

343 ± 8 Ma). Other published U-Pb data fully agree with 

previous these ages. 

Abstracts 129 

This high-K magmatism thus took place within a 

relatively short period within pull-apart structures along a 

crustal mega-shearzone. This structural feature formed the 

southern margin of the Variscan orogen and affected the 

Gondwana palaeo-continent after the crustal-collision 

processes. 

Contrasting Fe-Enrichment Trends in 

Calc-Alkaline Granitoid Plutons From 

Cima d' Asta and Monte Croce, 

Southern Alps, Italy 

1

Rottura, A., 2Del Moro, A., 

3

Caggianelli, A., 4Bargossi, G.M., 

1

Gasparotto, G. and 2Pinarelli, L. 1 Dipartimento Scienze Mineralogiche, 

Universitil di Bologna, I-40126 Bologna, Italy 2

Istituto di Geocronologia e Geochimica Isotopica 

C.N.R., I-56100 Pisa, Italy 3

DiTEc, Universitil della Basilicata, 

I-85100 Potenza, Italy 4

Istituto di Petrografia e Giacimenti Minerari, 

I-43100 Parma, Italy 

The Cima d' Asta and Monte Croce granitoid intrusions 

(eastern Southern Alps) belong to a large calc-alkaline 

ilmenite-bearing volcano-plutonic association of Early 

Permian age. The granitoids were emplaced at shallow 

levels at the end of the Hercynian orogeny (274-285 Ma) 

in a post-collisional (trans)tensional setting. 

The Cima d'Asta complex consists of a multipulse main 

pluton ( - 200 km2

) made of biotite monzogranites with 

subordinate biotite ± hornblende tonalites and granodiorites. The Monte Croce pluton ( - 25 km2

) consists of 

biotite ± hornblende-bearing granodiorites. Fine-grained 

intermediate rock types (biotite-hornblende ± pyroxenebearing quartz no rite to melagranodiorite) occur as small 

satellite bodies (CM, PG, AR, see figure) around the Cima 

d' Asta main pluton (MP), and as mafic microgranular 

enclaves (MME) hosted in all the granitoid facies of both 

plutons. Magma mingling relationships can be observed in 

both granitoid complexes. 

The granitoids form a high-K, metaluminous to 

subaluminous, calc-alkaline 1-type suite, as suggested by: 

QAP, K20-Si02 , AFM, R1-R2 and Na20-K20-Ca0 

diagrams; LREE-enriched patterns (CeN/YbN = 5 - 12) 
130 The Origin of Granites and Related Rocks 

with moderate negative Eu anomalies; LILE enrichment 

and Nb, Sr, P and Ti negative spikes in the normalized 

multi-element diagrams. The plutonics have highly variable 

Sr and Nd isotopic compositions ((87Sr/86Sr)i from 0. 7057 

to 0.7111; eNd = -2.7- -7.5) indicative of an open system 

evolution. A single-stage AFC process is ruled out by the 

lack of increasingly crustal isotopic signatures with rock 

acidity. Field, petrographic, geochemical and isotopic 

evidence favors the hypothesis that the granitoid suite 

originated through complex hybridization and fractionation 

processes involving both mantle and crustal derived 

magmas, in accordance with the genetic models inferred 

for the associated basaltic andesitic to rhyolitic volcanics. 

CM and PG rocks, and some MME from the Monte 

Croce pluton have Fe-rich biotite (FeOT up to 29 wt %) 

and amphibole (Fe-Ts to Fe-Hbl) compositions, with Si02 

rock content ranging between 54 and 64 wt %. Despite 

their abundance in iron, Fe-Mg minerals have calc-alkaline 

affinity. However, the above rocks defme a steeper Feenrichment trend with respect to the other plutonics, 

similar to a tholeiite-type evolution (Fig. a, b). They also 

show evidence of chilling, as indicated by the shapes of 

biotite and amphibole crystals, with length/width ratio up 

to 30. This suggests that the iron-rich mineral compositions reflect a former chemical feature of the hybrid 

intermediate mafic magmas, unrelated to any later comingling with the host granitoids. The distinct Fe-enrichment trends indicate that at least two pulses of variably 

hybridized intermediate magmas were involved in the 

granitoid genesis. This might reflect the complex nature 

of the post-orogenic high-K calc-alkaline magmatism of 

this sector of the eastern Southern Alps and, more generally, of other 1-type granitoid suites. The contrasting Feenrichment trends could represent the product of: 1) two 

distinct magma sources; or, 2) two related magma batches 

which evolved under different redox conditions. 

~ 

0 

e 0.8 0 

C) 

~ 

+ 

0.7 

b 

Q) 0.6 !:!:. 

;--

0 

Q) 0.5 u. 

m o.7 

c; 

~ 0.6 

+ CD 

~ 0.5 

LL. 

0.4 

50 55 60 65 70 

Si02 rock 

Cimad'Asta: e=CM-PG Q=AR-MP O=MME 

Monte Croce: 0 =GP + =MME 

Polynomial curves calculated only for CM-PG and 

AR-MP datapoints, respectively 

ORAL PRESENTATION 

Application of Rock Deformation 

Experiments to Melt Segregation 

in the Lower Crust 

Rushmer, Tracy 

Department of Geology, University of Vermont, 

Burlington, vr 05405-0122, USA 

75 

The Third Hutton Symposium comes during a time of 

major change in our thinking in regard to melt segregation 

and melt migration in the lower crust. Current ideas on 

how the partially molten lower crust behaves, both rheologically and chemically, are just now being incorporated 

into crustal evolution models. One way in which we have 

gained further insight into melt segregation has come from 

the results of several recent experimental studies on rock 

behaviour during partial melting. Experiments performed 

on crustal rock cores under both hydrostatic conditions and 

during deformation have led, in part, to the conclusions 

that (1) the dihedral angle model which suggests that melt 

migration is largely or solely controlled by the interfacial 

energy between melt and mineral cannot dominate melt 
segregation processes in the lower crust, and (2) the 

"critical melt fraction" model which requires granitic melt 

to remain in the source until melt fractions reach > 25 vol 

% is not a reliable model for melt segregation. The most 

recent experimental results on crustal rock cores which 

have helped advance our understanding of melt segregation 

processes, including the potential geochemical consequences, are discussed below. 

Although not all have been able to produce equilibrium 

textures readily, hydrostatic experiments on cores of 

amphibolite by Hacker (1990) and Wolf and Wyllie 

(1995}, and on quartz + muscovite cores by Brearley and 

Rubie (1990) and on quartz aggregates by Laporte (1994}, 

the experiments have provided results for understanding 

the textures and reaction mechanisms at the beginning of 

melting. Melt (1-5 vol %) in the amphibolite samples was 

found both in pools at hbd/plag/qtz triple junctions and 

along hbd/plag grain boundaries. The melt distribution is 

initially crystallographically controlled in the hornblendedominated assemblage, and melt interconnectivity is 

achieved at low melt fractions. The presence of many 

diverse and anisotropic minerals in the lower crust, such 

as hornblende, precludes predicting melt interconnectivity 

which is a requirement for melt segregation by this model. 

Non-hydrostatic or rock deformation experiments on 

amphibolite by Hacker and Christie (1990) and Rushmer 

(1995), and on granite by Rutter and Neumann (1995) 

have shown that melting reactions induced fracture and 

cataclastic deformation, even in previously ductile rock, 

and formed pathways for melt migration. As melt fraction 

increased deformation became viscous and the rock flowed 

homogeneously. A sharp drop in effective viscosity was 

not observed. Interpretation of these observations suggests 

that melt may to be able to migrate out of the system by 

either fracture or flow depending on type of melting 

reaction (vapor phase-present versus vapor phase-absent), 

depth of melting reaction (brittle or ductile regime) and 

strain rate (e.g., see Dell' Angelo and Tullis, 1987), among 

other factors. This is in contrast to a critical melt fraction 

model in which viscous melt can only separate from its 

source at a pre-determined, specific melt fraction interval. 

Future work must tie in geochemical data with physical 

melt segregation models. With ion probe technology and 

specially designed experiments, we are now able to better 

measure partitioning of trace and rare earth elements 

during hydrostatic melting (see current work by Cavallini 

et al., 1995) and during deformation. Recent ion probe 

results from the deformed partially molten amphibolite 

samples show that major trends, such as a depletion in Y 

Abstracts 131 

and HREE when garnet is present, are as expected. 

However, even though local major element chemical 

equilibrium appears to have been achieved in the melted 

rock cores, HFSE such as Zr are not fully equilibrated 

(Rushmer et al. , 1994). This suggests that the slow 

dissolution rates of accessory phases such as apatite and 

titanite may easily result in extracted melt compositions 

which have lower abundances of these elements than 

predicted by partition coefficients. In addition, future melt 

segregation models must include permeability /porosity 

changes that occur during melting. The rate of melting 

may be a major factor in increasing permeability during 

reaction. These variables need also to be quantified by 

specific key experiments. 

As we continue to update our geochemical and physical 

models of melt segregation and migration, our overall 

understanding of the diverse possibilities of granite 

generation and potential emplacement mechanisms in the 

Earth's crust will hopefully advance dramatically as well. 

Brearley, A. J. and Rubie, D.C., 1990. Effects of H20 on the disequilibrium breakdown of muscovite + quartz. J. Petrol., 31, 925-956. 

Cavallini, M., Vielzeuf, D., Bottazzi, P., Mazzucchelli, M. and Ottolini, 

L., 1995. Direct measurements of rare earth contents in partial melts 

from metapelites. Terra Abstracts, 6, 344-345. 

Dell'Angelo, L.N. and Tullis, J., 1987. Experimental deformation of 

partially melted granitic aggregates. J. metamorphic Geol., 6, 

495-516. 

Hacker, B., 1990. Amphibolite- facies- to- granulite- facies reactions 

in experimentally defonned, unpowdered amphibolite. Am. Mineral., 

75, 1349-1361. 

Hacker, B. and Christie, J.M., 1990. Brittle/ductile and plastic/cataclastic 

transitions in experimentally deformed and metamorphosed 

amphibolite. In Durham, W., Duba, A., Handin, J. and Wang, H. 

(eds), Brittle-Ductile Transitions: The Heard Volume, Geophysical 

Monograph Series, AGU, 20, 127-147. 

Laporte, D., 1994. Wetting behavior of partial melts during crustal 

anatexis: The distribution of hydrous silicic melts in polycrystalline 

aggregates of quartz. Contrib. Mineral. Petrol., 116, 489-499. 

Rushmer, T., 1995. An experimental deformation study of partially 

molten amphibolite: Application to low melt fraction melt segregation. J. Geophys. Res. 100, in press. 

Rushmer, T., Pearce, J.A., Ottolini, L. and P. Bottazzi, P., 1994.Trace 

element behavior during slab melting: Experimental evidence. EOS 

Trans. Am. Geophys. Union, 15, 746. 

Rutter, E., and Neumann, D., 1995. Experimental deformation of 

partially molten Westerly Granite under fluid-absent conditions, with 

implications for the extraction of granitic magmas. J. Geophys. Res. 

100, in press. 

Wolf, M.B. and Wyllie, P.J., 1995. Liquid segregation parameters from 

amphibolite dehydration-melting experiments. J. Geophys. Res., 100, 

in press. 
132 The Origin of Granites and Related Rocks 

Melt Segregation and Melt Geochemistry: 

What Can Experiments Tell us? 

Rushmer, Tracy 

Department of Geology, University of Vermont, 

Burlington, VT 05405-0122, USA 

One of the important directions in the study of crustal 

evolution is linking theoretical models of granite source 

regions and melt segregation with the geochemical data 

extracted from the granites themselves. Recently several 

studies in granite and leucogranite-bearing terranes such as 

the Himalaya have shown that the geochemical trace and 

rare earth element behavior in these granites did not occur 

as predicted by equilibrium geochemical models. These 

geochemical data have increased suspicions that melt does 

not reside long enough in the source region to reach full 

equilibration. Major questions are now being raised about 

the tools we use to study granites. In particular, how we 

can best use the geochemistry data to interpret the nature 

of the lower crust? One method is to perform partial 

melting experiments on crustal rocks to better understand 

the partitioning of trace and rare earth elements during 

melting, both under hydrostatic conditions and with 

deformation. Early work on dissolution rates of accessory 

phases in granitic melt (e.g., Harrison and Watson, 1983) 

has shown that kinetics will play an important role in the 

resulting melt trace and rare earth element abundances. 

Data have been recently collected from partially molten 

amphibolite experiments by electron microscope and 

SIMS. The elements K, Sc, Ti, Cr, Rb, Sr, Y, Zr, Nb, 

Ba and the REE have been determined in two experiments; 

one partially melted and deformed at 0.8 GPa, and other 

at 1.8 GPa (Rushmer et al., 1994). In the 0.8 GPa 

experiment, analyses were taken of minerals and melt in 

the coexisting assemblage, clinopyroxene + hornblende + 

plagioclase + tonalitic melt. The resulting REE pattern of 

the melt is similar to that of amphibole (e.g. La/Sm = 

1.7-3.1; Sm/Yb = 0.8-1.1) but with lower absolute 

abundances due to residual mafic phases and titanite 

(which is still present in the sample and did not dissolve in 

the melt, as observed below). At the higher pressure (1.8 

GPa), analyses were taken of the mineral and melt assemblage, garnet + clinopyroxene + plagioclase + 

trondhjemitic melt. The accessory phases, titanite and 

zoisite were also analyzed. The REE pattern of the 

trondhjemitic melt is steep (La/Sm = 4.8-7.5; Sm/Yb = 

16-20) as expected owing to the presence of garnet, but the 

high field strength elements (HFSE) have unpredicted bulk 

distributions. Mass balance between the experimental 

charges and the starting composition shows that the 

abundances of some elements are less than unity for the 

major residual assemblages in the experiments. This 

means, theoretically, the melt phase should be enriched in 

these elements relative to the starting composition, but the 

data show that the melt is not enriched in important 

elements, such as the HFSE. This behavior is because the 

minor phases, in particular titanite, have not completely 

dissolved and have retained many of the elements which 

should be in the melt. 

During deformation-enhanced melt segregation in the 

lower crust, trace and rare earth elements might behave 

very similarly. Data are also currently being collected on 

partial melts in the felsic system under hydrostatic conditions by Cavallini et al., (1995). Together, these experimental results provide new information which will help in 

the interpretation of granite geochemical data and assist us 

in coming to a more complete understanding of the 

potential geochemical trace and rare earth element abundances and REE patterns produced during melt segregation. 

Cavallini, M., Vielzeuf, D., Bottazzi, P., Mazzucchelli, M. and Ottolini, 

L., 1995. Direct measurements of rare earth contents in partial melts 

from metapelites. Terra Abstracts, 6, 344-345. 

Harrison, T.M. and Watson, E.B., 1983. Kinetics of zircon dissolution 

and Zr diffusion in granitic melts of variable water content. Contrib. 

Mineral. Petrol., 84, 66-74. 

Rushmer, T., Pearce, J.A., Ottolini, L. and Bottazzi, P., 1994. Trace 

element behavior during slab melting: Experimental evidence. EOS 

Trans. Am. Geophys. Union, 15, 746. 

Geochemical and Nd-Sr Isotopic 

Composition of Alleghanian Granites of the 

Southern Appalachians: Origin, Tectonic 

Setting, and Source Characterization 

1

Samson, S.D., 1Coler, D.G. and 2Speer, J.A. 1

Department of Earth Sciences, Syracuse University, 

Syracuse, NY 13244-1070, USA 

2Mineralogical Society of America, 1130 171

h St., N. W., 

Suite 330, Washington, D.C. 20036, USA 

One of the major manifestations of the late Paleozoic 

Alleghanian orogenic event was the formation of numerous high potassium (K20 - 3-4% ), metaluminous granites 

that were emplaced into many of the major lithotectonic 
belts in the southern Appalachians. The range in initial 

eNd values, depleted mantle model ages, and (

87Sr/86Sr)b 

respectively, of these granites is -8.2 to -3.4, 940-1020 

Ma, and 0.70642- 0.72798 for the Inner Piedmont; -2.3 

to +2.0, 660-1130 Ma and 0.70421 - 0.71035 for the 

Kiokee belt; -2.7 to +2.4, 660-870 Ma and 

0.70353-0.70816 for the Eastern slate belt; -6.7 to + 1.9, 

690-1140 Ma and 0.70391 - 0.70739 for the Carolina 

terrane; and, -4.4 to +3.0, 600-1070 Ma and 0.7033 -

0.7078 for the Raleigh belt. 

The chemical and isotopic compositions of the granites 

are most consistent with their formation by anatexis of 

continental crust, rather than derivation from a depleted 

mantle source. The pre-Alleghanian rocks of the Carolina 

terrane, the Raleigh belt, and Eastern slate belt have 

BNi300 Ma) values indistinguishable from the values of 

the Alleghanian granites emplaced within these terranes, 

suggesting that these terranes are a likely source of many 

of the granites. Some Alleghanian plutons within the 

Carolina terrane, however, require a more evolved source 

than the exposed crust of the Carolina terrane. Either the 

Carolina terrane is isotopically heterogeneous with depth, 

or more likely, Grenville basement occurs structurally 

below, or tectonically interleaved within, the terrane. The 

granites within the Inner Piedmont have higher (

87Sr/86Sr)i 

and lower 143Nd/144Nd ratios than most of the other 

granites, possibly indicating that the Inner Piedmont is not 

as juvenile as the other major Appalachian terranes. 

Although it has been suggested that the granites were 

generated by east-dipping subduction beneath Laurentia, 

the lack of Alleghanian plutons with tonalitic to 

granodioritic compositions, the small volume of the 

granites, and the absence of geographic trends in major 

element and isotopic composition across the orogen, makes 

a subduction origin for the granites extremely unlikely. 

The chemical and isotopic data are more consistent with 

the formation of the granites by wholesale melting of 

continental crust. The evidence for the production of these 

granites by crustal anatexis, taken together with the 

observation that they were intruded synchronously with 

Alleghanian thrusting in the Valley and Ridge Province, 

suggests strongly that they are collisional in origin. The 

crustal heating events that produced the granites might 

have been caused by delamination of the mantle lithosphere 

during terminal collision of Laurentia and Gondwana, as 

suggested by Sacks and Secor (1990) and Nelson (1992), 

or alternatively adequate heating may have occurred by 

crustal thickening during emplacement of thrust sheets ( cf. 

Patino Douce et al., 1990). 

Abstracts 133 

Nelson, K.D., 1992. Are crustal thickness variations in old mountain 

belts like the Appalachians a consequence of lithospheric 

delamination? Geology, 20, 498-502. 

Patino Douce, A.B., Humphreys, E.D. and Johnston, A.D., 1990. 

Anatexis and metamorphism in tectonically thickened continental 

crust exemplified by the Sevier hinterland, western North America. 

Earth Planet. Sci. Lett., 97, 290-315. 

Sacks, P.E. and Secor, D.T., 1990. Delamination in collisional orogens. 

Geology, 18, 999-1002. 

ORAL PRESENTATION 

Melt Segregation or Magma Mobility - the 

Difference Between Migmatites and Granites 

Sawyer, E.W. 

Sciences de la Terre, Universite du Quebec a 

Chicoutimi, Chicoutimi, Quebec, G7H 2Bl Canada 

Melt segregation is the separation of melt from its 

residuum, whereas magma mobility is the movement of 

melt plus entrained crystals. For either to occur requires 

a differential movement between the melt and the residuum, or between the melt plus crystals and some 

pre-melting reference frame. This can be considered in 

terms of the velocity of the melt or magma (V m) and the 

residual solids (V5). Four cases are considered, and their 

relation to crustal melting is crucial in understanding how 

granite magmas form. 

1) V m = Vs = 0. For this end member case the melt 

fraction and solid residuum remain at the site of melting 

and there is no melt segregation or magma mobility. 

Melting under such conditions forms small-scale nebulitic, 

patch and flecky migmatites and mantle-core structures 

(melt rims on a core of garnet or orthopyroxene) typical of 

some granulites. At larger scales V m = V5 = 0 probably 

cannot be maintained, unless the melt fraction is very 

small (less than that required for melt interconnectivity), 

because deformation and/or melt buoyancy, will generate 

an instability that initiates melt segregation or magma 

mobility. 

2) V m > Vs = 0. Here, the restite remains in place, 

but the melt fraction moves. This condition applies to 

those migmatites containing leucosomes (melt) with 

associated melanosomes (restite), and to some depleted 

granulites. Estimates of the melt fraction suggest that melt 

segregation can begin with 5 to 9% melt present, but that 

in most migmatites 20 to 35% melt is needed. However, 
134 The Origin of Granites and Related Rocks 

the melt from this type of migmatite (metatexite) is not, in 

general, the source of granite plutons for two reasons. A) 

Composition - data from migmatites with leucosomes and 

melanosomes shows that, regardless of whether they come 

from the amphibolite or granulite facies, the migmatite 

melts have lower (FeOT + MgO), Zr, Y, and REE, but 

higher Si02, than parental granite magmas. The only 

granites for which migmatite leucosomes are a plausible 

source are the Manaslu-type leucogranites - a point already 

made by LeFort. The low (FeOT + MgO) of migmatite 

leucosomes indicates that the segregation process is very 

effective at separating melt from restite. In contrast, the 

presence of a significant restite component in many 

granites implies that granite magmas formed by a different 

and less efficient segregation process. B) Rheology - the 

outcrop appearance of metatexite migmatites shows no 

magma-like flow, i.e., their rheology is that of solid rock 

and not a magma. Thus, it is difficult to imagine how the 

restite component can be added to the almost pure melt 

fraction to make parental granite magmas. 

3) V m = V s > 0. This is magma mobility without 

segregation. Diatexite migmatites that exhibit flow 

structures, but have compositions identical to their 

palaeo some, are examples of this behaviour. Magma 

mobility can occur if there is enough melt to breakdown 

the matrix framework, such a critical melt fraction (CMF) 

depends upon many factors, e.g., size and shape of matrix 

crystals, matrix mineralogy, and on applied stresses, but 

is typically around 20 to 50% melt. However, there is 

evidence that mobility can occur with melt fractions below 

the CMF once sufficient melt has formed to coat the 

grains, perhaps other deformation mechanisms, such as 

grain boundary sliding or melt enhanced solution-precipitation processes occur. 

4) V m > Vs > 0. Both the melt and the restite move 

(magma mobility), but because the melt fraction has the 

greater velocity melt segregation also occurs; the melting 

region has the rheology of a magma. Diatexite migmatites 

are examples of melting under these conditions. In most 

diatexites the bulk of the restite does not move very far, 

but a small portion remains in the magma. As with case 

(3) uncertainty remains as to the proportion of melt 

required for mobility. Some Opatica Belt diatexites were 

mobile with only 19% melt, but in most mobility occurred 

when 34 to 70% melt was present, in agreement with the 

expected CMF for a leucotonalite source. The rheological 

and compositional properties of diatexite migmatites and 

the fact that they occur at the same scale as plutons and 

plutonic belts, indicates that this type of migmatite is the 

connection to granites. 

Melt Segregation or Magma Mobility and 

the Migmatite-Granite Connection: 

The Field and Geochemical Evid~nce 

Sawyer, E.W. 

Sciences de la Terre, Universite du Quebec a 

Chicoutimi, Chicoutimi, Quebec, G7H 2Bl Canada 

The idea that there is a transition through migmatites to 

granites is an attractive one, but in recent years several 

contrary opinions have been expressed. Migmatites 

occupy a fringe position in petrology, they are obviously 

not regarded as metamorphic rocks in the sense that 

greenschists or granulites are; probably because they have 

undergone melting, severe textural modification and 

material transport. Neither are migmatites treated as 

igneous rocks because they typically have a lot of unmelted 

rock and are structurally far more complex than most 

igneous petrologists like their rocks to be. There are 

basically two types of migmatite: 1) metatexites where 

melting is minor and original pre-melting fabrics or 

layering is preserved largely intact; and, 2) diatexites 

where the original textures and fabrics have been destroyed 

and flow structures dominate. Metatexites have been 

widely studied and typically have small leucosomes that 

are associated with mafic selvedges. Diatexites have not 

been studied much and exhibit complex leucosome-restite 

relationships, typically melt and restite show all degrees of 

separation. When the geochemistry of migmatites and 

granites is compared several interesting facts emerge. The 

leucosomes from metatexite migmatites are too Si02-rich 

but too poor in FeOT, MgO, Sc, Cr, Zr, Hf, REE, and Y 

to be considered as the source magma for most granite 

plutons, the exceptions are the Manaslu-type leucogranites. 

However, the melt-rich parts of diatexite migmatites 

overlap considerably the compositional field of granites, 

and could be suitable sources for granite magmas. The 

geochemical differences between metatexites and granites 

implies that segregation process was different - that 

operating in metatexites being far more efficient than that 

in granites or diatexites. Consider rheology. Metatexite 

migmatites preserve pre-migmatization structural integrity, 

implying a rock-like rheology. In contrast the structure of 
both diatexites and granites indicates a magma rheology. 

Nonetheless the existence of diatexites has been denied by 

some, simply sweeping this group of rocks into granites as 

"dirty granites" . This is not justified by the field evidence 

from the Opatica, St Malo, Trois Seigneurs, Ashuanipi, 

Quetico and Higo migmatite terranes. In some cases 

diatexite migmatites grade from metatexites, have the same 

composition as the source (Ashuanipi and Quetico) and are 

in situ. Which means that diatexites are an up-grade 

progression from metatexites or in heterodox parlance 

metatexites are the down grade equivalent of "dirty 

granites". In the Opatica Belt migmatites and granites are 

observed at their place of formation in 6 kbar rocks and 

the anatectic products can be traced without break right 

through the crust to emplacement in 2 kbar rocks. In this 

case structural and geochemical continuity can be demonstrated across the migmatite-granite transition and to the 

upper crust. Re-examination of published diatexite data 

from other terranes shows the same geochemical trends, 

and in at least two cases where details are provided the 

same structural continuity also. However, not all 

diatexites terranes produced significant volumes of granite 

plutons. 

Effects of / 0 on Phase Equilibria 2 

in Granitic Systems. 

Conditions of Stability and 

Compositional Variation of 

Biotite and Amphibole 

Scaillet, Bruno, Pichavant, Michel 

and Dall' Agnol, Roberto 

Centre de Recherche sur la Synthese et la Chimie des 

Mineraux, CNRS, JA rue de la Ferollerie, 

45071 Orleans Cedex 02, France 

Crystallization experiments have been carried out on a 

Proterozoic granite of N Brazil of bulk composition (in wt 

%): Si02 = 72.1, Al20 3 = 13.4, FeOT = 3.7, MgO = 

0.7, CaO = 2.2, Na20 = 3.8, K20 = 3.4, Ti02 = 0.8. 

Dry glasses were used as starting products, obtained by 

two steps of melting at 1400°C of the crushed granite. 

Experiments were performed at a constant pressure of 3 

kbar in two internally heated vessels pressurized with Ar 

(oxidized) and Ar-H2 mixtures (reduced). Temperatures 

ranged from 900 to 700°C. The Cln 0 was controlled by 2 

Abstracts 135 

H20- C02 mixtures with fluid/silicate ratios ranging from 

10 to 20% (X(Hp)i =starting H20/(H20+C02), between 

1 and 0.2). Ag70Pd30 and Au capsules were used. For 

oxidizing conditions, the fa was fixed by the intrinsic 2 

hydrogen fugacity of the vessels, evaluated to be at NNO 

+ 0.5 by using the H2 sensor technique of Taylor et al. 

(1992). For reduced conditions, fo. was fixed around 2 

FMQ-0.5 with PH read with a Shaw membrane. Run 2 

durations varied between 1023 (700°C) and 166 hours 

(900 ·C). After optical inspection, run products were 

characterized by X-ray and EMPA, SEM techniques. 

Water contents of quenched glasses were determined with 

the "by-difference" method using a specific calibration 

procedure of the EMPA. 

The major phases identified are: plagioclase (Pl), alkali 

feldspar (At), quartz (Qtz), orthopyroxene (Opx), Ca-rich 

pyroxene (CaPx), amphibole (Amph), biotite (Bi). Sphene 

(Sph), magnetite (Mt) and Ti-hematite (Th) are present at 

NNO + 0.5, while at FMQ - 0.5 only ilmenite (11) is 

present. The phase relations obtained under oxidized and 

reduced conditions have important features in common: 1) 

Pl is the liquidus tectosilicate phase followed by Qtz and 

Af at decreasing aH 0 for a given T; 2) Amph is the stable 2 

hydrous phase at high water content (melt water content > 

4-5 wt %), Bi being stable at low fa , and, 3) the solidus 2 

temperature does not change with fa . Differences between 2 

the two phase diagrams include: 1) at a given water 

content, the thermal stabilities of Pl, Qtz, Af, Amph and 

Opx are depressed by c. 40-50°C when fa is reduced; 2) 2 

there exists a significant overlap between Bi and Amph 

stability fields at FMQ - 0.5; 3) Amph is stable down to 

solidus conditions at FMQ- 0.5, whereas at NNO + 0.5, 

Amph is never stable at the solidus. 

When decreasing fa from NNO + 0.5 to QFM- 0.5 2 

at 700°C, Amph Fe/Fe+Mg ratio increases from 0.36 to 

0. 71. In contrast, v1Al remains constant (1.4 pfu for 23 

0), suggesting minor effects of T, aH 0 and fo. on this 2 2 

Amph compositional parameter. Bi composition increases 

in Fe/Fe+Mg from 0.43 (NNO+ 1.5) to 0.73 (QFM-0.5) 

at 7000C. Pl composition is strongly dependent on T, aH 0 2 

and fa . At a given fa , An content increases with T, and 2 2 

decreases with aH 0 • For a given temperature and aH 0 , an 2 2 

increase in fo. is marked by an increase in the An content. 2 

For melt compositions, the major chemical evolution with 

decreasing T is characterized in the AFM projection by a 

shift toward the A apex at increasing F /M ratio, under 

both redox states investigated. However, at NNO + 0.5, 

the residual melt composition is more leucocratic than at 
136 The Origin of Granites and Related Rocks 

FMQ - 0.5, reflecting the higher Fe content of granitic 

liquids crystallizing under reduced conditions. 

This work shows that fa has an extremely important 2 

effect on the phase relations of granitic systems by depressing the liquidus temperatures of Pl, Qtz, Af, Amph. 

Under oxidized conditions, Bi and Amph are mutually 

exclusive, whereas at low fa they coexist down to solidus 2 

conditions. It is important to stress that not only the 

stability of ferro-magnesian phases are affected but also 

that of all tecto silicates. In particular, the change in 

plagioclase composition under fixed T- aH.a but variable 2 

/ 0 , clearly indicates that activities of Pl forming compo- 2 

nents are fa dependent. Thus variation in redox state of 2 

crystallizing granitic magmas must also be recorded by 

plagioclase zoning. 

Taylor, J.R., Wall, V.J. and Pownceby, M.I., 1992. The calibration and 

application of accurate redox sensors. Am. Mineral., 77, 284-295. 

Quantifying Spatial Relationships 

Between Faults and Plutons 

Schmidt, K.L., Paterson, S.R. and Lund, S.P. 

Department of Earth Sciences, 

University of Southern California, 

Los Angeles, CA 90089-0740, USA 

Faults and plutons of similar age commonly occur in 

the same region. This observation is often used to argue 

that there is a causative relationship between faults and the 

generation, ascent, or emplacement of plutons. To 

establish a causative relationship the following must be 

demonstrated: 1) a close spatial relationship between faults 

and plutons; 2) a close geometrical relationship between 

the shapes and orientations of plutons and the orientations 

of faults; 3) a close temporal relationship between faults 

and plutons; 4) that the rates of magma generation, ascent, 

and emplacement are compatible with rates of fault 

displacement; and, 5) the specific mechanism(s) by which 

the magma is generated, ascended, or emplaced. Here we 

evaluate only the first requirement, that of quantifying the 

spatial relationship between faults and plutons, paying 

particular attention to how scale affects the degree of 

spatial overlap between the two. This is particularly 

important because the scale at which a close spatial 

relationship is established may correspond to the scale of 

the process( es) causing this overlap. 

A variety of processes may result in a close spatial 

relationship between faults and plutons. For example: 1) 

the ascent and emplacement of plutons may operate 

independently of faulting, and spatial overlap may be 

fortuitous; 2) fault systems may develop in crustal regions 

thermally softened by plutonism, or alternatively plutonism 

may occur in zones of active deformation due to changes 

in the stress or thermal structure of the crust; and, 3) 

individual faults may control the ascent and emplacement 

of plutons, or alternatively faults may nucleate on plutons. 

What then constitutes a close spatial relationship at the 

scale of individual faults and plutons or at the scale of 

fault systems and magmatic belts? Our approach to this 

problem has been to evaluate a set of spatial parameters at 

different scales that can be statistically analyzed in order 

to determine random versus nonrandom distributions. 

Parameters used at the scale of individual faults and 

plutons include: 1) pluton spacing relative to fault spacing; 

2) distance from pluton margin to nearest fault; 3) pluton 

diameters relative to fault spacing; 4) area covered by 

plutons relative to the total map area; and, 5) the amount 

of overlap between separate plutons in nested plutonic 

complexes. Parameters at the scale of fault systems and 

magmatic belts include: 1) distance between the centers of 

mass of fault systems and magmatic belts normalized to 

the total area of the zones; and, 2) area of the overlapping 

region between fault systems and magmatic belts normalized to the total area covered by these features. A computer program is used to create artificial maps of faults 

and plutons and produce statistical histograms for each of 

the above parameters. Random number generator algorithms allow us to examine a random distribution with 

respect to any given parameter. The program also generates nonrandom distributions, allowing us to define the 

characteristics of both random and non-random spatial 

relationships between faults and plutons. Once these have 

been examined, we have a statistical means for evaluating 

the degree of spatial overlap between single faults and 

plutons or fault systems and magmatic belts using data 

from geologic maps. 

Initial results from analysis of maps of the Cascades 

Mountains, Washington, British Caledonides, and southern 

Appalachians indicate that, at the scale of individual faults 

and plutons (tens of km), little spatial overlap occurs 

between faults and plutons. For example, in the Cretaceous Cascades arc the average distance from pluton 

centers to faults is 6.4 km, the largest distance is 16.6 km 

and the smallest is 0.0 km; fault spacing averages 8. 7 km, 

and pluton spacing averages 7.0 km. However, at the 
scale of orogens (hundreds to thousands of km) spatial 

overlap may occur between active fault systems and active 

magmatic belts. 

The Crooks Canyon Intrusive Complex: 

Bimodal Magmatism in a Proterozoic 

Island Arc Setting, Central Arizona 

Seaman, S.J. and Williams, M.L. 

Depanment of Geology and Geography, University of 

Massachusetts, Amherst, MA OJ 003, USA 

The Crooks Canyon intrusive complex (CCIC) is one 

of several 1750-1700 Ma apparently arc-related plutonic 

complexes in central Arizona (DeWitt, 1989; Anderson, 

1989). The CCIC is part of the Yavapai province, characterized by mostly juvenile volcanic and plutonic crustal 

materials (Nelson and DePaolo, 1984; Wooden and 

DeWitt, 1991), and interpreted to have been accreted to 

North America during the - 1700 Ma Yavapai orogeny 

(Bowring and Karlstrom, 1990; Karlstrom and Bowring, 

1991). Textural and compositional relationships in the 

well-exposed CCIC provide an unusual opportunity to 

assess the influence of infusions of basaltic magma on the 

evolution of felsic magmas occupying mid-crustal-level 

magma chambers in island arcs. 

The CCIC hosts three types of granodiorite, moderately 

high-K basalt, an array of chilled magmatic enclave 

compositions, and a suite of gabbros. The gabbroic suite 

ranges in composition from high-K to tholeiitic island 

arc-like basalts and seems to have crystallized prior to the 

development of the bimodal complex. It is not described 

further here. The three granodiorites volumetrically 

dominate the complex. The first is a highly siliceous, 

fine-grained biotite granodiorite that occurs in the northernmost exposures of the complex. To the south, it 

becomes coarser grained, less silicic, less potassic, and 

more aluminous. This coarser grained granodiorite grades 

to the south into a potassium feldspar megacrystic biotite 

granodiorite. The megacrystic granodiorite is slightly 

depleted in both LREE and HREE compared to the 

fine-grained Crooks Canyon granodiorite. All three of the 

granodiorites have sharply negative Nb and Ti spikes on 

trace element abundance diagrams, and have low concentrations of Rb and Y, supporting an island arc ancestry. 

The megacrystic granodiorite magma was repeatedly 

invaded by large volumes of basaltic magma. Several tens 

of square kilometers of exposure in the central portion of 

Abstracts 13 7 

the CCIC are dominated by complex basaltic/ granodioritic 

magma interaction textures resembling those in the Pleasant Bay magmatic complex of Maine (Wiebe, 1993). 

Resulting magma mingling textures include the preservation of pipes of silicic magma which buoyantly rose 

through blankets of basaltic magma, resorption of potassium feldspars in the megacrystic granite near bodies of 

hot basaltic magma, and the development of hybrid 

compositions which resulted from mechanical mixing of 

the basaltic and granodioritic end members. 

The largest volumes of basaltic magma occur in the 

southeastern portion of the northern lobe of the CCIC, and 

the felsic/mafic magma ratio increases to the north and 

west, suggesting that the lower reaches of the magma 

chamber may be preserved to the southeast. The basalt 

has the LREE-enriched REE pattern of typical high-K 

calcalkaline basalts of modem island arcs. High BalLa 

and low La/Sm ratios support this interpretation. True 

basaltic (rather than microgranitoid) enclaves, as well as 

enclaves which are basalt/granodiorite hybrids, are 

common in the megacrystic granodiorite. On the larger 

scale, regional variations in the compositions of 

granodiorite may have resulted from the addition of 

varying amounts of the high-K basalt to the northernmost, 

most siliceous Crooks Canyon granodiorite. Simple 

mixing between the two end members can explain differences in most trace and REE concentrations in the three 

granodiorites. 

The origin of the granodiorites, in view of their 

moderately peraluminous nature and KIN a near or slightly 

less than 1, may have resulted from anatexis involving the 

breakdown of biotite or possibly hornblende (Whitney, 

1988). The basalt may represent a contribution to the heat 

source that facilitated the melt-producing reaction. Local 

textural complexity and possibly the regional compositional 

variation in the felsic rocks of the complex resulted from 

mingling and mixing of the mafic and felsic magma end 

members. 

Anderson, P., 1989. Stratigraphic framework, volcano-plutonic evolution, and vertical deformation of the Proterozoic volcanic belts of 

central Arizona. In Jenney, J.P. and Reynolds, S.J. (eds) Geologic 

Evolution of Arizona, Arizona Geological Society Digest 17, 57-148. 

Bowring, S. and Karlstrom, K., 1990. Growth and stabilization of 

Proterozoic lithosphere in the southwestern United States. Geology, 

18, 1203-1206. 

DeWitt, E. 1989. Geochemistry and tectonic polarity of early Proterozoic 

(1700-1750-Ma) plutonic rocks, north-central Arizona. In Jenney, 

J.P. and Reynolds, S.J., (eds) Geologic Evolution of Arizona, 

Arizona Geological Society Digest 17, 149-163. 
138 The Origin of Granites and Related Rocks 

Karlstrom, K., and Bowring, S., 1988. Early Proterozoic assembly of 

tectonostratigraphic terranes in southwestern North America. J. 

Geol., 96, 561-576. 

Karlstrom, K. and Bowring, S., 1991. Styles and timing of Early 

Proterozoic deformation in Arizona: Constraints on tectonic models. 

In Karlstrom, K.E. (ed), Proterozoic Geology and Ore Deposits of 

Arizona, Arizona Geological Society Digest 19, 1-10. 

Nelson, B.K. and DePaolo, D.J., 1984. 1,700-Myr greenstone volcanic 

succession in southwestern North America and isotopic evolution of 

Proterozoic mantle. Nature, 312, 143-146. 

Whitney, J., 1988. The origin of granite: The role and source of water 

in the evolution of granitic magmas. Geol. Soc. Am. Bull. 100, 

1886-1897. 

Wiebe, R., 1993. The Pleasant Bay layered gabbro-diorite, coastal 

Maine: Ponding and crystallization of basaltic injections into a silicic 

magma chamber. J. Petrol. 34, 461-489. 

Wooden, J. and DeWitt, E., 1991. Pb isotopic evidence for the boundary 

between the Early Proterozoic Mojave and Central Arizona crystal 

province in western Arizona. In Karlstrom, K.E. (ed), Proterozoic 

Geology and Ore Deposits of Arizona, Arizona Geological Society 

Digest 19, 27-50. 

Sr and Nd Isotope Geochemistry of Mesozoic 

Plutonic Rocks from Hong Kong: 

Implications for Granite Petrogenesis, 

Regional Structure, and Crustal 

Evolution in SE China 

1

Sewell, R.J. and 2Darbyshire, D.P.F. 1

Hong Kong Geological Survey, 11 IF Civil Engineering 

Building, 1 OJ Princess Margaret Road, Hong Kong 2

NERC Isotope Geosciences Laboratory, 

Kingsley Dunham Centre, Nottingham NG12 5GG, UK 

Mesozoic granites of Hong Kong may be grouped into 

three NE-trending domains on the basis of Sr-Nd isotope 

and model age characteristics. Plutonic rocks of Zone I in 

the northwestern part of the Territory have BNit) < -9, 

Nd depleted-mantle model ages (t0M) of between 1.67 and 

2.02 Ga, and (

87Sr/86Sr)i > 0.710. They include 1-type 

granites of the Lamma Suite, A-type granites of the Lion 

Rock Suite subgroup I, and a previously undescribed 

S-type granite, here defined as the Deep Bay Granite. 

Granites in the central Zone II exhibit a broad spectrum of 

isotope characteristics with eNd(t) values of between -4.2 

and -7 .0, toM of between 1.30 and 1.51 Ga, and (

87Sr/86Sr)i 

in the range 0.7055- 0.7152. They include 1-type granites 

of the Lion Rock Suite subgroup II, 1-type granite porphyry dykes, syenite stocks and dykes, and A-type granite 

dykes and mafic dykes. Granites of Zone III in the 

southeastern part of the Territory dominantly comprise 

1-type compositions of the Lion Rock Suite subgroup III. 

These rocks have eNd(t) between -5.5 and -6.5, toM of 

between 1.39 and 1.47 Ga and (

87Sr/86Sr)i in the range 

0.7071 - 0.7109. 

Based on a recent report detailing three episodes of 

crustal growth at 2.5, Ga, 1.8 Ga and 1.2- 1.4 Ga in 

Southeast China (Li et al., 1992), the strongly contrasting 

isotope signatures and model ages between Zone I and 

Zone III rocks indicate the genetic involvement of late 

Archaean felsic crust and mid-Proterozoic mafic crust 

respectively together with a mantle-derived component. 

The boundary between these two crustal sources representing separate basement terranes is marked by Zone II in 

which isotope characteristics are more variable and a 

stronger mantle-derived influence is preserved. It is 

uncertain when the two basement terranes were juxtaposed. 

Comparisons with Sr-Nd isotope data reported for 

Mesozoic granites from Fujian Province (Huang et al., 

1986) show a similar NW-SE geographic isotope zonation. 

In Fujian, the boundary separating the two isotopically 

distinct sources is thought to mark the eastern limit of 

Precambrian crystalline basement. It corresponds to a 

deep crustal divide conventionally drawn between the 

South China Fold Belt and the Southeast Maritime Fold 

Belt. This tectonic lineament is thought to continue south 

through Guangdong Province to intersect the coast immediately north of Hong Kong. The results presented here 

suggest that this deep crustal discontinuity transects Hong 

Kong from northeast to southwest. 

Whole-rock geochemistry and Rb-Sr age data for 

Mesozoic plutonic rocks of Hong Kong indicate a transition from compressional to tensional tectonics. The climax 

of tensional conditions is marked by the emplacement of 

bimodal A-type granites within Zone II. These rocks have 

the strongest mantle signatures, suggesting that the boundary zone provided a pathway for upwelling of mantle-derived magmas which interacted with both Archaean 

and Mesoproterozoic basement rocks to produce the 

observed range of compositions. 

Li, X.H., Zhao, Z., Gui, X. and Yu, J., 1992. Sm-Nd and zircon U-Pb 

isotopic constraints on the age of formation of the Precambrian crust 

in Southeast China. Chin. J. Geoch., 11, 111-120. 

Huang, X., Sun, S.H., DePaolo, D.J. and Wu, K.L., 1986. Nd-Sr 

isotope study of Cretaceous magmatic rocks from Fujian Province. 

Acta Petrologica Sinica, 2, 50-63. 
Origin of Granites at an 

Active Plate Margin 

1

Shaw, S.E. and 2Todd, V.R. 1

Macquarie University, N.S. W. 2109, Australia 2 U.S. Geological Survey, U. C. Riverside, 

Riverside, CA 92521, USA 

The 1 ,600-km-long Peninsular Ranges batholith (PRB) 

of southern California, U.S. and Baja California, Mexico 

is a major element of the Mesozoic Cordilleran continental 

margin. The PRB, emplaced across the lithospheric 

boundary between North America and Pacific oceanic 

plates in the Jurassic and Cretaceous, is a classic 

continental-margin arc batholith. The tectonic setting of the 

magmatic arc at an active plate margin provided a variety 

of protoliths for granite formation, as reflected in the great 

diversity of lithologic, geochemical, structural, and 

geophysical characteristics of PRB plutons. In a 120-kmwide segment of the PRB across San Diego County, 

Cenozoic tectonism has resulted in approximately 14 km 

of west-to-east structural relief thus providing a superb 

section of a batholith emplaced within the middle to upper 

crust. Emplacement of the batholith was accompanied by 

synkinematic low-PI high-T metamorphism and intra-arc 

ductile defonnation. 

Granites of the PRB were emplaced in Jurassic, Early 

Cretaceous, and mid- to Late Cretaceous pulses above a 

west-facing subduction zone. The calcic compositions of 

the granites reflect a strong oceanic source component 

whereas, geochemical and isotopic characteristics of 

individual granite suites vary with age and geographic 

position within the batholith. Middle and Late Jurassic 

metagranodiorites originated by partial melting at midcrustal depths within a Triassic-Jurassic turbidite wedge 

(S-type melts) and also by incorporation of sedimentary 

material into primitive mafic magma (I-S-type transitional 

melts). Steep geophysical gradients that bound the 

Triassic-Jurassic crustal block on the west mark the trace 

of a regional east-dipping reverse fault, or suture, that 

extends to mid-crustal depths and juxtaposed the block 

against Late Jurassic island arc metavolcanic rocks. The 

Jurassic suture divides the batholith into a western zone of 

oceanic crust and an eastern zone underlain by continentally derived crust. 

Complex spatial and temporal variations among 

Cretaceous plutons whose compositions range from gabbro 

to granite reflect source differences west and east of the 

Jurassic suture as well as changes in the geometry of 

Abstracts 139 

Cretaceous convergence. Early Cretaceous 1-type granite 

suites intruded their volcanic cover and older metavolcanic 

rocks at depths from 1 to 10 km in the western zone. 

Synintrusive deformation culminated in a ductile shear 

zone located near the Jurassic suture. Systematic variations in composition and position within the batholith 

among the Early Cretaceous granite suites indicate that the 

partial melting of chemically inhomogeneous, PHP -variant 

older metaigneous rocks in the lower crust played a 

significant role in their origin. Mantle-derived mafic 

magmas rose through the crust and mingled with granitic 

melts to form spectacular sheeted gabbro-granite diapirs in 

the western zone. A mid- to Late Cretaceous eastward 

shift in the locus of magmatism made available new 

sources, such as continental basaltic crust, for the voluminous trondhjemitic plutons of the eastern zone. These 

geochemically distinctive plutons were emplaced at depths 

of 11 to 15 km after regional deformation had waned. 

Common Pb isotopic compositions of feldspars from 

Jurassic and Cretaceous granites show that Jurassic 

granites are more radiogenic as are Early Cretaceous 

granites emplaced east of the suture. Cretaceous granite 

suites that were emplaced across the suture show enrichments in K, radiogenic Sr, and whole-rock o180 values east 

of the suture, a reflection of interaction of magmas with 

thicker, continentally derived crust. Lead isotopic ratios 

of Early Cretaceous granites of the western zone are too 

radiogenic for a direct mantle origin and probably reflect 

derivation from older 1-type crustal rocks whose source 

may have been mantle melts. 

An Assimilation Role in the Crystallization 

of S-type Tonalite Magma at Granulite 

Facies Conditions, Hidaka Metamorphic 

Belt, Hokkaido, Northern Japan 

1

Shimura, Toshiaki and 2Komatsu, Masayuki 1

Graduate School of Science & Technology, 

Niigata University, Niigata, 950-21 Japan 2

Department of Earth Sciences, Ehime University, 

Matsuyama, 790 Japan 

The Hidaka Metamorphic Belt (HMB) is a high dT/dP 

type metamorphic belt, and represents a tilted crustal 

section of a magmatic arc of Tertiary age in Hokkaido, 

northern Japan (Komatsu et al., 1989). The Belt is 
140 The Origin of Granites and Related Rocks 

divided into four zones: (I) very low grade 

metasedimentary rocks; (II) greenschist to lower 

amphibolite facies rocks; (III) upper amphibolite facies 

rocks; and, (IV) of granulite facies rocks. Abundant 

gabbro, diorite and granite are intruded into these metamorphic rocks. 

The granitic rocks are mainly tonalitic and granodioritic 

in composition and classify into peraluminous (S-type) and 

metaluminous (1-type) granitoids. Both types generally 

coexist in the same intrusive mass, although S-types by far 

predominate. The S- and 1-type granitic magmas were 

generated by the crustal anatexis of pelitic granulite and 

pyroxene amphibolite, respectively, in the lowest crust. 

These granitic rocks are grouped into four depth types 

according to the intrusive metamorphic zones: 1) upper 

granite - granodiorite in zone I; 2) middle tonalite -

granodiorite in zone I and II; 3) lower tonalite in zone III; 

and, 4) basal tonalite in zone IV. Each type of granitic 

rocks has a characteristic mineral assemblage and chemical 

composition that change gradually from higher to lower 

level types: for S-types these are, 1) muscovite, 2) 

cordierite ± muscovite, 3) garnet + cordierite or garnet 

+ orthopyroxene, and 4) garnet + orthopyroxene ± 

cordierite, including biotite in each type. 

In the basal tonalite intruded into granulite facies zone, 

various types of metamorphic rocks are abundantly 

included, such as pelitic-psammitic (Grt-Opx-Bt granulite), 

mafic (amphibolite, Opx-amphibolite and gabbroic gneiss) 

and ultramafic (meta-harzburgite and -dunite) rocks. The 

basal tonalite is, therefore, very heterogeneous lithofacies 

because not only of varied-sized abundant enclaves but also 

of assimilation and reaction of the tonalitic magma with 

metamorphic enclaves. Based on these lithofacies and 

considering effects of the reaction and assimilation, the 

basal tonalite is further classified into three sub-types: 

S1-type tonalite with homogeneous tonalite matrix, completely free of metamorphic enclaves, S2-type tonalite with 

very heterogeneous lithofacies, occurring as a marginal 

facies around visible metamorphic enclaves, and S3-type 

tonalite with heterogeneously distributed small melanocratic clots consisting of orthopyroxene, biotite, garnet, 

plagioclase and quartz. 

Among them S 1-type tonalite represents a primary rock 

crystallized from the original tonalitic magma, although 

this type of tonalite occurs only. All the mineral grains of 

S1-type tonalite have normal compositional zoning, 

indicating a simple cooling process. Based on the textural 

and compositional evidence, the crystallization sequence of 

the S1-type tonalite is Grt-Pl-Opx-Bt-Qtz-Crd-Kfs. The 

range of crystallizing temperature is from 900°C to 700°C, 

and the magma contained 3-4 wt % H20, according to 

experimental study (Shimura et al., 1992). 

On the other hand, S2-type tonalite occurs widely, 

intimately associated with metamorphic enclaves. The 

lithology of S2-type tonalite differs, depending on the 

lithology of metamorphic enclaves which the tonalite 

surrounds. The S2-type tonalite, occurring around pelitic 

enclaves, is very wide up to several meters. This zone 

consists of normal zoned large garnet grains (0.5-3cm), 

hypersthene and biotite clots, and large K-feldspar pools 

that include corroded plagioclase grains. The latter 

represents anatectic melts issued from the pelitic enclaves. 

The other S2-type is orthopyroxene bearing leucocratic 

tonalite that surrounds mafic enclaves. Plagioclase An% 

in this tonalite is relatively high compared with those of 

S1-type tonalite. This tonalite was formed by the reaction, 

Hbl + primary magma (Si02) = 

Opx + secondary magma (rich inCa, Aland H20). 

The reactions of the tonalite magma with ultramafic 

enclaves produce peculiar rocks consisting of bronzite, 

phlogopite and Mg-cordierite, with a few em to 20cm thick 

around the enclaves. The ultramafic enclaves are originally serpentinite with relic olivine and chromite spinel, 

and were trapped as one of ophiolite blocks in the accretionary complex that is the precursor of the Hidaka 

metamorphic rocks. 

The S3-type tonalite is also heterogeneous because of 

random distribution of dark clots. The bulk chemical 

composition of the tonalite plots within an intermediate 

part between those of Grt-Opx granulite (restite) and of 

S1-type tonalite. The S3-type tonalite is poorer in LIL than 

S1-type tonalite. The dark clots are restite phases and the 

S3-type tonalite is a mixture of restites and tonalitic melts. 

The primary tonalitic magma generated by crustal 

anatexis in the deep crust crystallized and differentiated 

with changing P-T conditions during ascent to produce 

essentially the sequence of granitic rocks from basal to 

upper types, but locally the assimilation and reaction with 

metamorphic enclaves produced a variety of granitic rocks. 

Komatsu, J., Osani, Y, Toyoshima, T. And Miyashita, S., 1989. 

Composition and structure of the Hidaka metamorphic belt, northern 

Japan. In Daly, J.S., Cliff, R.A., and Yardley, B.W.D. (eds), 

Evolution of Metamorphic Belts. Geol. Soc. London Spec. Pub. 43, 

487-493. 

Shimura, T., Komatsu, M. and Iiyama, J.T., 1992. Genesis of the lower 

crustal Grt-Opx tonalite (S-type) in the Hidaka Metamorphic Belt, 
northern Japan. Trans. Royal Soc. Edinburgh: Earth Sci., 83, 259-

268. 

Magmatic Epidote-Bearing Granitoids from 

NW Argentina and NE Brazil 

1

Sial, A.N., 2Toselli, A.J., 3Saavedra J., 

1

Ferreira, V.P. and 2Rossi de Toselli, J.N. 1

UFPE, Dept. of Geology, Recife, PE, 50732-970, Brazil 2

/nst. Sup. Correl. Geologica, UNT, Miguel, 

Lillo 205-4000 Tucuman, Argentina 3 IRAN, CSIC, Apartado 257, 37080 Salamanca, Spain 

The occurrence of magmatic epidote-bearing (mEp) 

granitoids in South America has been recorded in five 

Neoproterozoic (Brasiliano Cycle) fold belts in NE Brazil 

(Serido- SFB, Cachoeirinha-Salgueiro - CSF, Riacho do 

Pontal - RPF, Pajeu-Paraiba - PPF and the Macurure 

Domain of the Sergipano Fold belt - MSF) and in the Early 

Paleozoic (Famatinian Cycle) Pampean Ranges (FamatinaLos Llanos plutonic belt - FIL, and along the Tafi 

Megafracture - TMF, separated by a belt of cordierite 

granites) in NW Argentina. 

In Argentina, mEp-bearing granitoids vary from 

tonalites to granites (with minor qz monzonites, qz 

diorites, gabbros and trondhjemites). In Brazil, they are 

mostly tonalites and granodiorites, commonly with 

microgranular qz diorite enclaves, and amphibole-rich 

clots. Plutons from the CSF, RPF, MSF have similar 

mineralogy and chemistry. In both countries, epidote is 

found in four textural relationships: included in plag; with 

euhedral, allanite core; rimmed by biotite; or along 

boundaries of hbl in contact with plag, or included in 

primary muscovite in some of Argentinean two-mica 

plutons. Brazilian mEp show 27 - 29% Ps (CSF) (fa at, 2 

or slightly below, NB buffer) and 20 - 24% Ps (SFB) 

compositions (!0 between HM and NB buffers). Optical 2 

measurements for Famatinian epidotes anticipate 

compositional range equivalent to the Brazilian ones. In 

Argentina, epidote is, in some cases, in paragenesis with 

garnet, while in Brazil with Ca-cpx. 

Granitoids in Brazil are essentially metaluminous (with 

subordinate peraluminous), whereas in Argentina they are 

distinctly peraluminous (TMF), or metaluminous (with 

subordinate peraluminous) (FIL) granitoids. In both 

countries they are essentially calc-alkalic or trondhjemitic, 

but tholeiitic members are present in Argentina (e.g. Cerro 

Abstracts 141 

Toro and San Agustin granitoids) and shoshonitic ones in 

Brazil (e.g. Teixeira pluton). 

Most mEp-bearing plutons in Brazil intruded 

intermediate-grade metamorphic rocks, except those in the 

CSF and RPF, which pierced low-grade metaturbidites, 

locally developing unusual ky-bearing thermal aureoles, as 

they intruded mostly cold, middle crust, at about 6 kbar 

(Al-in-hbl barometry). In Argentina, same barometry 

allowed pressures of 6. 6 kbar in tonalites which intruded 

syn-tectonically gneisses (e.g. Cerro Toro pluton), although other granitoids were clearly emplaced at shallower 

depth. Likely epidote, in the Famatinian granites, crystallized directly from a magma at P > 3 kbar (primary 

muscovite), though < 6 kbar ( 4. 7 kbar in Cerro Blanco 

and 5.7 kbar in Paganzo; hbl barometry). 

Rb-Sr isochron data from CSF microgranular mafic 

enclaves and host granodiorites (620 Ma) provides a highly 

linear regression ((87Sr/86Sr)i = 0. 70598), indicating that 

enclave and host magmas are cogenetic or had been 

isotopically equilibrated during cooling. Equivalent plutons 

in the MSF (e.g. Coronel Joao Sa pluton) display higher 

(

87Sr/86Sr)i (0.7080 - 0.7100). In Argentina, peraluminous 

Cafayate and Lorna Pelada plutons exhibit low (

87Sr/86Sr)i 

(0.7043 and 0.7069, respectively), and metaluminous ones 

of Famatina Ranges show, sometimes, higher values 

((87Sr/86Sr)i = 0.7097 at Cerro Toro and 0.7046 at 

Paiman). For the CSF plutons in Brazil, eNd (0.6 Ga) 

varies from -1.0 to -2.0, with Nd model age from 1.2 to 

1.4 Ga (t0M). For the TMF granites in Argentina, eNd (0.4 

- 0.5 Ga) varies from -1.0 to -4.0 and Nd model age is in 

the range observed for the CSF granitoids. 

Oxygen isotopes for the CSF plutons range from + 11 

to + 13sMow; their amphibole-rich clots being usually 

1.5%o lower. o34S values varies from + 1 to +9.0%ocoT· 

For the SFB plutons, o180 values are lower ( + 7 to 

+ 10 %o). Isotopic data for the CSF granitoids support the 

hypothesis that amphibole-rich clots represent fragments or 

restites of melting of oceanic floor basalts, hydrothermally 

altered at low T. There is no unique petrogenetic model 

for the granitoids in the PPF and SFB. 

In Argentina, low Sr isotopic ratio in the TMF mu-gtep-sph-bearing suite vs high Sr initial ratio in the 

metaluminous FIL one, challenge the application of the SI-M-A classification system. The various calc-alkalic and 

subalkalic tendencies can be explained by fractional 

crystallization with variable degree of assimilation and 

separation of cumulates and intercumulus melts. 

Peraluminous nature was partially achieved by selective 

leaching of B, Na and K to the volatile phase. Pegmatites 
142 The Origin of Granites and Related Rocks 

present in almost every pluton are the witness of such an 

effective process. 

Over 50 plutons have been studied, leading to the 

conclusion that mEp-bearing granitoids originate from 

more than one kind of source material and evolve as 

several magma series. Processes leading to their formation 

are repeated through geological time, and magmas intrude 

different crustal levels at different tectonic settings. 

Contrasting Migmatite Terrains, 

Tumbledown Mountain Area, 

West-Central Maine 

Solar, Gary S. and Brown, Michael 

Department of Geology, University of Maryland 

at College Park, MD 20742, USA 

Relationships between development of migmatites and 

regional tectonic histories in the Northern Appalachians are 

poorly constrained. It has been argued that some anatectic 

migmatites were associated with granitic pluton emplacement and with magma ascent; in effect the migmatites 

record the passage of the granitic pluton from the source. 

However, the deformation and timing of these migmatites 

have not been studied in detail. Two types of migmatites 

occur in the Kearsarge-Central Maine belt in west-central 

Maine: 1) migmatites found in proximity to granitic 

plutons; and, 2) migmatites that occur away from exposed 

granite bodies. The two types of migmatites appear to 

have been produced by different mechanisms of 

migmatization (granitic magma injection versus anatexis). 

Some mappable boundaries between the different 

migmatites coincide with discontinuities in strain and in the 

stratigraphy of the host rocks. A coordinated study of the 

structural, petrological, geochemical, and geochronological 

characteristics of different migmatite regions should enable 

evaluation of whether these regions may represent discrete 

tectonometamorphic terranes, juxtaposed after separate 

histories. 

The Tumbledown Mountain area (7 km north of 

Rumford, Maine) is an example within the KearsargeCentral Maine belt where a boundary between terrains 

characterized by two different types of migmatite occurs. 

Previous geologic mapping (Moench and Hildreth, 1976) 

has suggested that: 1) the border of the Tumbledown 

dome is coincident with a zone of cataclasis (significance 

remained open); 2) a regional stratigraphy is traceable 

into the dome from the north despite the lack of primary 

structures within the dome; and, 3) the border of the 

Tumbledown dome is sub-parallel to regional isograd 

surfaces. Interpretations of tectonic history based on this 

mapping have been narrowly focused on pre-pluton 

isoclinal folding (based on regional stratigraphy) and 

extensive contact metamorphism (e.g. Moench and 

Zartman, 1976; Moench and Pankiwskyj, 1988; Smith and 

Barreiro, 1990). 

These interpretations do not consider regional or 

orogen-scale ductile structures, styles of migmatization or 

timing relationships, or the influence of shear zone 

kinematics on migmatite development. Modem techniques 

of field geology have not been applied to the Tumbledown 

Mountain area. As a result, geochemical and geochronologic studies have been restricted to poorly constrained 

isotopic age data, despite well constrained geothermobarometry and exceptional petrologic observations (e.g. 

Smith and Barreiro, 1990). More modem study of the 

significance of regional migmatites (e.g. Allen, 1994) is 

necessary to make correlations among tectonic history, 

metamorphism and granite emplacement; in effect to 

investigate whether shear zones acted as conduits for 

granitic magma transport through the crust. 

Ongoing field work has led to the production of 

structural maps that allow an interpretation of the spatial 

relations of migmatites. These data suggest that the two 

regions of migmatite in the Tumbledown Mountain area 

contrast in stratigraphy, amount and style of deformation, 

nature of migmatite-forming processes (magma injection 

versus anatexis), and abundance and nature of exposed 

granitic plutons. These field relations suggest that the 

different migmatite types and contrasts in strain and 

stratigraphy may indicate that a major tectonic boundary is 

coincident with the border of the "Tumbledown dome". 

Comparisons of migmatite characteristics across the 

"Tumbledown dome" border and boundary between 

migmatite types indicate juxtaposition of discrete 

tectonometamorphic terranes, rather than in situ modification of protoliths by plutonic intrusion and doming. 

Field data will be integrated with laboratory studies to 

explore the significance and the timing of these migmatite 

regions and the boundary between them. The integration 

of structural (mapping, meso- and microstructural kinematic analysis, fabric trajectories, transpositional relations, 

etc.), petrological (P-T-t paths, metamorphic and 

deformational conditions, etc.), and geochemical (REE 

provenance studies, isotopic signatures to trace origin(s) of 
granitic material, etc.) characteristics, and high precision 

geochronology (titanite and monazite [U-Pb system], 

biotite and muscovite [Rb-Sr system], and multi-mineral 

separates from selected samples [Sm-Nd system]) will 

allow quantitative and qualitative comparison. If successful, this integrated approach should be applied to other 

areas of the Appalachians, and other mountain belts which 

feature polygenetic migmatites. 

Allen, T., 1994. New Hampshire migmatites: oxygen isotope fractionation during partial melting, and suggestions of structurally enhanced 

magma migration. EOS Trans. Am. Geophys. Union, 15, 360. 

Moench, R.H. and Hildreth, C.T., 1976. Geologic map of the Rumford 

quadrangle, Oxford and Franklin Counties, Maine: U.S.G.S. Geol. 

Quad. Map GQ-1272. 

Moench, R.H. and Pankiwskyj, K.A., 1988. Definition, problems, and 

reintetpretation of early premetamotphic faults in western Maine and 

northeastern New Hampshire. In Tucker, R.D. and Marvinney, R.G. 

(eds), Studies in Maine Geology, Maine Geol. Surv., 1, 35-50. 

Moench, R.H. and Zartman, R.E., 1976. Chronology and styles of 

multiple deformation, plutonism, and polymetamotphism in the 

Merrimack Synclinorium of western Maine. Geol. Soc. Am., Mem., 

146, 203-238. 

Smith, H.A. and Barreiro, B., 1990. Monazite U-Pb dating of staurolite 

grade metamotphism of pelitic schists. Contrib. Mineral. Petrol., 

105, 602-615. 

Petrology and Geochemistry of 

Mafic Enclaves in Granitoids of the 

Robertson River Igneous Suite, 

Blue Ridge Province, Central Virginia 

Tegeler, J. L. 

Department of Geology, University of Maryland 

at College Park, MD 20742, USA 

The late Neoproterozoic Robertson River Igneous Suite 

(RRIS) is an elongate batholith that intrudes Grenville age 

(1.1-1.0 Ga) basement within the Blue Ridge anticlinorium 

in central Virginia. The RRIS includes eight lithologic 

units, which were emplaced episodically over a 30 Ma 

period (730-700 Ma) during an early episode of crustal 

extension that preceded rifting of Laurentia by approximately 130 Ma. The RRIS is part of a regional supersuite 

of anorogenic plutons emplaced throughout the Laurentian 

terrane in Virginia and North Carolina. 

Variably sized (0.1-2 meters), dark colored enclaves 

occur locally in leuocratic granite at several sites in the 

main batholith of the RRIS. Petrographic and geochemical 

Abstracts 143 

evidence indicates that the host rock at two sites is part of 

the Laurel Mills Granite (LMG), which is one of the 

oldest and least chemically evolved plutons of the suite. 

The contact between the enclaves and the granite is very 

sharp and marked with a concentration of epidote and 

biotite. The enclaves are fme-grained and display a 

similar mineralogy to the granite, although the proportions 

of the minerals are different and the enclaves classify as 

quartz diorite. 

Detailed geochemical analyses of major and trace 

elements indicate local variation in granite composition 

immediately adjacent to the enclaves. The granite near the 

enclaves has a lower Si02 content than the LMG, although 

normative plots indicate that the granite is otherwise very 

similar to the LMG. Rb, U, Ga, and Nb are lower andY 

and Th are higher in the granite adjacent to the enclaves 

relative to the LMG. Geochemical data for the enclaves 

indicate broadly andesitic compositions. The data further 

indicate that the enclaves are alkaline, whereas the majority of mafic rocks in the central Blue Ridge are tholeiitic. 

The enclaves are enriched in Rb, Sr, Ga, Nb, Zr, and Zn 

in comparison to both the adjacent granite and the LMG. 

Trace element compositions of the LMG and the enclaves 

yield similar plots on discrimination diagrams using Nb-Y 

and Rb-Y-Nb, falling in the "within-plate granite" field. 

A comparison of samples taken across a contact between 

the enclave and the granite shows that from the granite to 

the enclave, Rb, Sr, Y, Zr, and Zn are enriched. This 

pattern suggests chemical exchange between the granite 

and the enclaves. The variation of data between the LMG 

and the granite adjacent to the enclaves could result from 

interaction between the LM G and the enclaves, but also 

may indicate that the host rocks are separate from the 

LMG. 

The RRIS has been shown to have been emplaced in an 

extensional environment related to the failed rifting of 

Laurentia. A curious aspect of the early period of regional 

crustal extension is the paucity of associated mafic rocks. 

Furthermore all of the mafic rocks documented to date are 

tholeiitic in composition. However the enclaves in this 

study represent the first discovery of mafic rocks of 

alkaline affinity in this region. These are also the first 

alkaline mafic rocks known to be at least 730 Ma. The 

alkaline nature of the enclaves is consistent with the 

tectonic environment of emplacement and with the association of anorogenic granites and may represent the first 

evidence of a previously unrecognized period of mafic 

magmatism associated with early crustal extension. 
144 The Origin of Granites and Related Rocks 

The Partitioning of Fe and Co Between 

Plagioclase and Biotite and the Eu2

+ /Eu3+ 

Ratio for Plagioclase in Granitic Rocks 

from the Inner Zone of Southwest Japan 

1

Terakado, Yasutaka and 2Fujitani, Tatsuya 1

Division of Earth and Environmental Sciences, 

Faculty of Human Development, Kobe University, 

Tsurukabuto 3-11, Kobe 657, Japan 2

Marine Technical College, Nishikura-cyo, 

Ashiya 659, Japan 

Abundances of rare earth elements (REE) and other 

trace elements were determined for granitic rocks and their 

mineral separates from the Sanin, Sanyo and Ryoke belts 

in the Inner Zone of southwest Japan. Among those data, 

the Fe and Co partition coefficients between plagioclase 

and biotite show interesting features which may be related 

to magnetite- or ilmenite-series designation. Moreover, 

we calculated Eu2+ /Eu3+ ratios in plagioclase, and will 

discuss these ratios with relevance to redox states of 

granite formation processes. 

The Inner Zone of southwest Japan is characterized by 

widespread exposure of Cretaceous to Paleogene granitic 

rocks which intrude late Paleozoic to Jurassic sedimentary 

and metamorphic rocks. The Ryoke metamorphic belt, 

which lies in the Pacific Ocean side of the Inner Zone, is 

distinguished from the Sanyo belt by the dominant occurrence of gneissose granitic rocks associated with mylonite 

and metamorphic rocks. The Sanin belt, which lies along 

the margin of the Japan Sea, is characterized by dominant 

occurrence of magnetite-series granitoids and of Paleogene 

rocks. The granitic rocks from the Ryoke belt are mainly 

ilmenite-series, and the Sanyo belt has intermediate feature 

with respect to magnetite/ilmenite-series classification. 

The plagioclase/biotite partition coefficients for Fe are 

negatively correlated with those for Co, and the values for 

the Sanin belt granites are clearly separated from those for 

the Ryoke belt granites. The Sanin granites have relatively 

high Fe and low Co partition coefficients, whereas the 

Ryoke belt granites show relatively low Fe and high Co 

values. Moreover, the Sanin granites are magnetite-series 

and the Ryoke belt granites are ilmenite-series. Since the 

mineral assemblages imply a higher oxygen fugacity in the 

magnetite-series granites than in the ilmenite-series 

granites during solidification of the granite magma, the 

above variations in Fe and Co partitioning may be attributed to the difference in redox states during solidification. 

Several other factors, such as effect of inclusion or 

disequilibration, were considered, but the redox state 

seems to have a primary importance. The Sanyo belt 

samples, which include both types of granites, fall within 

the range of the Ryoke ilmenite-series granites. The 

Sanyo belt data suggest that the ilmenite-series granites 

were converted from a magma with magnetite-series 

affinity during later solidification stages. 

The Eu2

+ /EuH ratios in minerals were calculated by the 

Phil potts' method in which the equivalence of Eu2+ and 

Sr2+ partitioning is assumed. Although no correlation 

between Eu2+ /EuH and Fe2+ /FeH ratios has been pointed 

out, the Eu2+ /EuH ratio in plagioclase from the Sanyo and 

Ryoke belt granites seems to be correlated with the 

whole-rock Fe2+ /Fe3+. On the other hand, the Sanin belt 

granites tend to show high plagioclase Eu2+ /Eu3+ ratios, 

which deviation is probably due to a disequilibration 

caused by rapid solidification. This suggests the possibility that the kinetic effect may more or less affect the 

magnetite-series granite formation. Furthermore, effect of 

redox states on the size of the Eu anomaly in 

plagioclase/melt partition coefficient was examined from 

the plagioclase Eu2

+ /Eu3+ data of the granites. 

Fluid Inclusion Evidence of the Evolution of 

Magmatic Fluids in the Harney Peak 

Leucogranite, Black Hills, South Dakota 

Ternes, Kim and Nabelek, Peter I. 

Department of Geological Sciences, University of 

Missouri, Columbia, MO 65211, USA 

Experimental and theoretical studies have shown that 

magmatic fluids have strong influence on the phase 

equilibria of granitic systems. The speciation of magmatic 

fluids can be determined by microthermometric fluid 

inclusion analyses; however, to date most studies have 

focused on altered granites where the bulk of the magmatic 

fluid stage has been overprinted by externally derived 

fluids. This study has been undertaken to obtain direct 

fluid inclusion evidence for the composition and evolution 

of fluids in a crustally-derived magmatic system. The 

Proterozoic Harney Peak peraluminous leucogranite (HPG) 

of the Black Hills, South Dakota, was emplaced at 3-4 

kbar as multiple sills, dikes and small intrusions. It is also 

thought to be genetically related to a large pegmatite field 
which surrounds it. The influence of magmatic fluids on 

crystallization of the granite is demonstrated by prominent 

local granite-pegmatite layering and chemical heterogeneities. Previous stable isotope study of the granite has 

shown that the granite has not interacted with non-magmatic fluids since its crystallization (Nabelek et al., 1992). 

Therefore, any inclusions found in the HPG are of magmatic origin and offer a direct sampling of magmatic fluids 

associated with a naturally occurring granitic system. 

Fluid inclusions were analyzed in tourmaline and 

quartz. Primary and secondary inclusions were identified. 

The primary inclusions were identified by their isolated 

positions within crystals and negative shapes of the 

enclosing crystals. They consist of variable H20-C02-

Salt-(CH4) mixtures. The C02-rich inclusions are less 

salty and yield higher isochore temperatures (calculated 

using the model of Brown and Lamb, 1989). The range 

of primary fluid inclusion compositions is ascribed to 

Rayleigh exsolution of the magmatic fluid which should 

lead to early C02-rich fluids due to the low solubility of 

C02 in high-silica magmas. The calculated isochore 

temperatures of the most water-rich primary inclusions are 

below the granite solidus. However, they may be in error 

because it was assumed that the only solute is NaCl. Ice 

melting temperatures, birefringent daughter crystals, 

phase-equilibria considerations, and alpha-track mapping 

of B and Li distribution in thin sections, all indicate that 

K, Li, B and other solutes must be additional constituents 

in the inclusions. 

Using published solubility models for H20 and C02 in 

granitic melts (Burnham and Davis, 1974; Stolper et al., 

1987) and the average composition of the primary inclusions (XH 0 = 0.70), the initial volatile content of the HPG 2 

of - 5.3 wt % H20 (X;0 = 0.45) and 650 ppm C02, 2 

was calculated using the computer program DEGAS 

(Holloway and Blank, 1994). At the estimated pressures 

of melting (5-7 kbar, (Nabelek et al., 1994)), the liquidus 

temperatures of the HPG melt having 5.3 wt % H20 are 

consistent with the dehydration-melting reactions that lead 

to the melts and 750-800°C oxygen isotope equilibration 

temperatures among minerals (Nabelek et al., 1992). It is 

suggested that saturation of the HPG melt at 3 .5 kbar with 

a C02-rich fluid, that is represented by inclusions with the 

lowest XH 0 of 0.20, increased the liquidus temperatures 2 

and partially quenched the melt, producing the aplitic 

segregations of the sills which comprise much of the 

granite. With further degassing, Rayleigh fractionation 

produced more H20-rich fluids which resulted in lowering 

of the solidus and crystallization of pegmatitic 

Abstracts 145 

segregations. At subsolidus temperatures the magmatic 

fluid unmixed into H20-Salt and C02-CH4 fluids which 

were trapped as secondary inclusions. This study demonstrates the strong influence of mixed fluid-species on phase 

equilibria of granitic magmas and provides the first direct 

evidence for the solubility of volatile species in a natural 

granite system. 

Brown, P.E. and Lamb, W.M., 1989. P-V-T properties of fluids in the 

system H20±C02±NaCl: New graphical presentations and implications for fluid inclusion srudies. Geochim. Cosmochim. Acta, 53, 

1209-1221. 

Burnham, C.W. and Davis, N.F., 1974. The role of H20 in silicate 

melts. II. Thermodynamcis and phase relations in the system 

NaA1Si30 8-H20, to 10 kbars, 700°C to l100°C. Am. J. Sci., 274, 

902-940. 

Holloway, J.R. and Blank, J.G., 1994. Application of experimental 

results to C-0-H species in narural melts. In Carroll, M.R. and 

Holloway, J.R. (eds), Volatiles in Magmas: Rev. Mineral., 30, 187-

230. 

Nabelek, P.l., Terry, M.P. and Friberg, L.M., 1994. The petrogenesis 

of the Harney Peak granite, Black Hills, South Dakota, as a consequence of differential uplift of a partially molten crust. EOS Trans. 

Am. Geophys. Union, 15, 363. 

Nabelek, P.l., Russ-Nabelek, C. and Haeussler, G.T., 1992. Stable 

isotope evidence for the petrogenesis and fluid evolution in the 

Proterozoic Hamety Peak leucogranite, Black Hills, South Dakota. 

Geochim. Cosmochim. Acta, 56, 403-417. 

Stolper, E.M., Fine, G.J., Johnson, T. and Newman, S., 1987. The 

solubility of carbon dioxide in albitic melt. Am. Mineral., 72, 1071-

1085. 

Assessment of Water Content in Granitic 

Melts Using Melt Inclusion Homogenization 

Data: Method - Results - Problems 

Thomas, R. 

GeoForschungsZentrum Potsdam, 

14473 Potsdam, Germany 

This contribution presents results of melt inclusion 

studies of Variscan granites of the Erzgebirge and 

Fichtelgebirge in Germany, including the highly evolved 

tin-granites of the Erzgebirge. The primary minerals of 

rhyolites, granites and pegmatites formed from silicate 

melts and may trap minute quantities of these melts as 

inclusions. Much attention has been paid in the past to 

glass inclusions in volcanic rocks and it is less well known 

that melt inclusions are also abundant in plutonic rocks. 

In contrast to melt inclusions in volcanic rocks, which 

consist of glass and a shrinkage bubble and have diameters 
146 The Origin of Granites and Related Rocks 

often greater than 20 to 50 p.m, the melt inclusions in 

plutonic rocks are generally very small, average diameters 

less than 10 p.m, and they are typically crystallized; thus 

they are often overlooked or misidentified as solid inclusions. Silicate melt inclusions in intrusive rocks frequently 

contain a separate fluid phase with a vapour bubble. This 

fluid phase results from crystallization of the mostly 

anhydrous minerals of the melt in the inclusions. 

Using a new time and diffusion-controlled quenching 

technique (Thomas, 1994) water contents and characteristic 

temperatures (solidus, liquidus, homogenization) of silicate 

melts can be estimated. With this method we obtain 

information about P-T conditions and fluid composition of 

melts which is otherwise very hard to obtain, particularly 

for plutonic rocks. In brief, inclusions are heated in 

evacuated quartz tubes at successive temperatures, 

quenched and examined microscopically. Because of the 

slow rates of melting and dissolution, the homogenization 

temperature (TH) depends on inclusion size. After quenching, smaller melt inclusions may be completely homogenized, larger ones not, and some of intermediate size are 

just on the verge of homogenization. The latter have the 

"critical" diameter (dK), which is measured with a micrometer ocular. Using an algorithm based on the StokesEinstein equation relating diffusion and viscosity of some 

liquids, we derive viscosity values from inclusion diameter 

(dK), run time (t) and temperature (T) and plot these in the 

coordinates In h vs. 104/T. In the next step, viscosity 

values at various H20 contents ( Cw) are calculated for the 

host rock composition using Shaw's viscosity model, and 

these values are plotted as H20 isopleths on the same 

diagram. The water contents of melt inclusions are then 

assessed by interpolation between the Cw-isopleths. This 

method eliminates the problematic quantities of the StokesEinstein equation. 

The water contents estimated by this method are "eq.-

wt % water" and the effect of other fluid phases is not 

explicitly determined. However, we can obtain information about the fluorine content from the solidus shift to low 

temperatures compared with haplogranite. According to 

microprobe investigations on melt inclusions the melt of 

the tin-granites of the Erzgebirge can contain greater than 

5 wt % fluorine. By comparison, the highest value 

determined from bulk rock analyses is about 1 wt % . 

It is interesting to note that nearly all high-level granites 

of the Erzgebirge and Fichtelgebirge, emplaced at shallow 

levels (1-5 km) in the crust, show evidence of melt 

evolution to highly fractionated compositions. Regardless 

of bulk composition, we find H20- and F -rich and sometimes P-rich melt inclusions at the end of each intrusive 

phase. The presence of such inclusions demonstrates an 

extended period of crystallization, in some cases over 

150°C. Understanding the formation of such highly 

evolved melts is of great importance for the concentration 

process of ore-forming elements such as Sn, W, Nb, Ta, 

Be, Mo, U, REE. For example, the so-called 

"stockscheider" -pegmatites of the Erzgebirge granites are 

produced by extensive fractionation of the granitic source 

and efficient separation of the silicate liquid by squeezing 

out ("filter pressing") of interstitial melt. The pegmatitic 

magma was a suspension of a F- and P-rich silicate melt 

with minor immiscible phases of extremely P-rich melt, 

and crystals of quartz, feldspar, apatite, triplite, berlinite, 

topaz and other solids. Moreover, during crystallization, 

a KCl- and NaCl-rich aqueous fluid was exsolved. This 

complex history of magmatic evolution is recorded in the 

minute melt- and fluid inclusions. 

Thomas, R., 1994. Estimation of the viscosity and the water content of 

silicate melts from melt inclusion data. Eur. J. Mineral., 6, 511-535. 

ORAL PRESENTATION 

Fertility of Crustal Rocks During Anatexis 

Thompson, Alan Bruce 

Department fur Erdwissenschaften, ETH Zurich, 

CH-8092, Switzerland 

After many years of systematic experimental investigations, it is now possible to quantify the degree of melting 

of most common crustal rock types to about 3 GPa, as 

functions of temperature and aH 0 • Quartzo-feldspathic 2 

melting produces steady increases in melt proportion with 

increasing temperature. The migration of the eutectic/minima radially from quartz in Ab-Or-Qz projections 

with increasing pressure at various ana , is reflected in 2 

quite different rock fertilities to melt production in lower 

and thickened continental crust. The cotectics including 

CaA12Si20 8 in plagioclase show displacements away from 

Or in Ab-An-Or projections that are large at high Pna, but 2 

strikingly less at lower pressure and lower aH 0 • These 2 

displacements are particularly important when muscovite 

or biotite proxy for KA1Si30 8 • The dehydration-melting of 

micas releases H20 over narrow temperature intervals 

resulting in rapid increases in melt quantity, directly 
proportional to the amount of hydrate. Mica melting also 

results in the consumption of Si02 from residual quartz or 

melt, during the formation of refractory Al2Si05, 

orthopyroxene, garnet or cordierite. 

Pelitic migmatites may be formed during decompression 

melting in continental collision belts relaxing even from 

perturbed "normal" geotherms. While anatexis of gneisses 

could reflect collision of "hot" continents, it is not easy to 

distinguish enhanced internal heat generation from increased mantle heat supply. Mafic migmatites are diagnostic of an enhanced mantle heat signature. The 

systematics that have emerged from examination of 

experimental data should permit deduction of the 

depth-temperature- aH 0 conditions of anatexis of all crustal 2 

rock types, simply on the basis of modal mineralogy in 

migmatites. Deduction of P-T-aH 0 conditions of residual 2 

assemblages should permit constraints on the degree of 

melt extraction from field observations. 

Tectonic Setting and Geochemistry of 

Granitoids: the Potential and Problems 

of Discrimination Diagrams 

Tischendorf, G., Forster, H.-J. 

and Trumbull, R.B. 

GeoForschungsZentrum Potsdam, 

14473 Potsdam, Germany 

Few would argue that there is some degree of correlation between chemical composition and tectonic setting of 

granitic rocks, but what are the limits of applicability of 

tectonic discrimination diagrams? Many workers have 

used discrimination diagrams suggested by Pearce et al. 

(1984); others have questioned the validity of the method 

as such. We present here a performance test of Pearce 

discrimination diagrams based on a compilation of 5000 

published analyses of granitic rocks (s.l., volcanic and 

plutonic) from well-constrained tectonic settings. 

Being products of melting, granites must reflect the 

composition of their source. However, the processes of 

melting, mobilization, ascent through crustal rocks and 

final crystallization are manifold, and granitic magma is 

generally not an isolated system but may interact with wall 

rocks, mix or mingle with accompanying mafic magmas, 

and actively adapt to changing P-T conditions by crystallization or further melting. Two consequences of this for 

Abstracts 14 7 

discrimination are that one must expect transitional types 

in any discrimination scheme, and granites may be poorly 

represented by "average" or "representative" chemical 

analyses; one should consider the compositional spectrum 

in a given pluton. 

As pointed out also by Pearce et al. (1984), granite 

composition depends on source rock composition primarily; tectonic setting is important to the degree that it 

controls the type of source rocks available and/or the P-T 

conditions and extent of melting. Of course, crustal 

tectonic regimes are far from simple and many orogens are 

polycyclic. Operation of the Wilson cycle ensures that 

extension can be superimposed on compressional orogeny 

and vice versa, leading to overlap of process and mixing 

of source regions. 

With these complexities and caveats in the background, 

results of our study can be summarized as follows: 

ORG- ocean ridge granitoids (10 localities). Two groups 

are distinguishable; those generated by differentiation of 

basaltic magma ("true M-types") plot within the ORG field 

but those which formed by partial melting of basaltic 

sources overlap into the VAG field. 

WPG - within-plate granitoids (36 localities). These 

granites are discriminated best and only 4 localities plot 

with some overlap into the VAG or SyncolG fields; there 

is no overlap with ORG. 

VAG - volcanic arc granitoids (70 localities). These 

granites show virtually no overlap with either the SyncolG 

or ORG fields, but there are many cases of overlap into 

the WPG field. This is exemplified in particular by the 

late Mesozoic and Tertiary rocks of the N. American 

Cordillera, where extension directly followed arc 

magmatism (an extreme example is the Trans Pecos, Texas 

locality). The same situation is found in an oceanic setting 

in the arcs of Papua New Guinea, New Zealand and Marie 

Byrd, Antarctica, all of which contain elements of back -arc 

spreading with which the "WPG-type" samples are 

associated. 

SyncolG - collisional granitoids (71 localities). Himalayan 

granites and granites from some parts of the European 

Variscides plot within the SyncolG field but many other 

examples from collisional orogens tend to cluster around 

the triple point of SyncolG, VAG and WPG fields. Good 

examples of the spread in compositions encountered are 

the Tasman fold belt, SE-Asia and the Rheno-Hercynian 

granites of Germany. 

We conclude that only granitoids from within-plate 

settings, whether oceanic (ORG) or continental (WPG), 

can be distinguished geochemically with some confidence. 
148 The Origin of Granites and Related Rocks 

Granitoids from active margins (arc settings) may give 

data arrays which span the VAG and WPG fields, probably because of the close time and space association with 

extension. Collisional settings are inherently the most 

complex in terms of source rocks and process, and do not 

produce characteristic granitoids in terms of the trace 

elements used here. We have tested a number of alternative elements or element ratios for discrimination but the 

compositional overlaps remain because the fundamental 

problem is rooted in the nature of granite formation, and 

orogenic processes. 

Pearce J.A., Harris, N.B.W. and Tindle, A.G., 1984. Trace element 

discrimination diagrams for the tectonic interpretation of granitic 

rocks. J. Petrol., 25, 956-983. 

Polygenetic Microgranitoid Enclave 

Swarms in Granitic Rocks, 

Central Sierra Nevada, California 

1

Tobisch, O.T., 2Vemon, R.H. 

and 1McNulty, B.A. 1 Earth Science Department, University of California, 

Santa Cruz, CA 95064, USA 2

School of Earth Sciences, Macquarie University, 

Sydney, NSW 2109, Australia 

Microgranitoid enclaves in granitic rocks of the central 

Sierra Nevada batholith have been studied extensively 

(e.g., Pabst, 1928; Reid et al., 1983; Frost and Mahood, 

1987; Dorais et al., 1990: Barbarin, 1991a). Less studied 

are enclave swarms which have previously been classified 

compositionally as monogenic or polygenic (Barbarin, 

1991b). Composition distributions of enclaves in swarms, 

however, can vary from nearly homogeneous (i.e., where 

modal % of mafics, feldspar phenocrysts, etc., varies 

· slightly) to swarms in which the modes of all enclaves 

vary markedly. The textures/compositions of the swarm 

matrix varies from matching the host to consisting mostly 

of coarse-grained aggregates rich in mafic minerals 

(inferred by us to be cumulates) and accompanied by 

schlieren layering. 

Field observations on swarms in the Dinkey Creek 

(DCP), Jackass Lakes (JLP) and Mt Givens (MGP) plutons 

in the central Sierra Nevada indicate they are not necessarily related to pluton boundaries as suggested for some 

plutons (e.g., Barbarin, 1991a,b; Knopf and Thelan, 

1905). Swarms are reasonably common and distributed 

over much of the DCP, although they tend to concentrate 

near proposed linear fracture feeding zones within the 

pluton. Swarms are also abundant in the JLP and are 

distributed throughout the pluton in zones lying parallel to 

proposed fracture-controlled pluton boundaries and emplacement sheets. Swarms in the MGP (work in progress) 

occur at scattered localities, and are much less common 

than in the other two plutons. 

Dimensional range of individual swarms typically is 1-

100 m. Swarms in the DCP and MGP are usually < 1 m 

to 4 m wide and several meters in longest dimension, with 

the maximum length observed - 30 m. The largest and 

most laterally continuous swarms occur in the JLP, where 

they may be tens of meters wide and extend - 100 m 

along strike. Multiple swarms also occur within complex 

mingle/mix zones approaching 1-2 km in width and -5 

km in length. Some swarm boundaries can be very sharp 

and straight and can contain enclaves abruptly truncated by 

the host magma, as if controlled by a discrete laminar flow 

surface; other swarm boundaries have gradational boundaries or appear partly dispersed. 

The two-dimensional shape of swarms can appear as 

straight "dikes", meandering layers or "trains", a folded 

lensoid layer with enclaves aligned -parallel to the fold 

axial plane and magmatic foliation, near-circular (pipelike?) shapes, or irregular masses confined by straight or 

angular boundaries, among numerous variations. Their 

three-dimensional geometry is often difficult to document; 

however in several places we have observed them as 

lensoid sheets, commonly but not always steeply dipping 

(plunging?). Two-dimensional axial ratio of enclaves in 

swarms is generally low to moderate (i.e., <3). 

Five possible models, with some overlap (cf also refs. 

Barbarin, 1991b; Barbarin and Didier, 1992), for producing polygenetic swarms, are: 1) the "log jam" model, in 

which rafts form by accumulation of enclaves from 

disparate sources due to blockages in the magma flow 

(e.g., viscosity "walls", channels too narrow to allow 

rapid passage, etc.); 2) gravitational sorting, whereby 

enclaves from disparate sources undergo density and/or 

size stratification; 3) flow sorting of enclaves and related 

cumulates from disparate sources (e.g., collection in eddies 

or zones parallel to flow gradients in host, etc.); and, 4) 

conduit mix/mingle model, whereby a) during ascent, 

"mafic" magma incorporates droplets or small batches of 

felsic magma from the host to produce domains of hybrid 

magma (repeating this process produces a variety of 

related compositions), which eventually mingle with the 
host and accumulate into swarms by processes 1-3, or b) 

multiple pulses of evolved hybrid magmas injected near or 

at the final emplacement site mingle with the host and 

accumulate via processes 1-3. 

Barbarin, B.,1991a. Enclaves of the Mesozoic calc-alkaline granitoids of 

the Sierra Nevada batholith, California. In Didier, J. and Barbarin, 

B. (eds), Enclaves and Granite Petrology. Elsevier, Amsterdam, 

135-153. 

Barbarin, B., 1991b. Contrasted origins for the "polygenic" and 

"monogenic" enclave swarms in some granitoids of the Sierra Nevada 

batholith, California. TERRA Abstracts, 3, 32. 

Barbarin, B. and Didier, J., 1992. Genesis and evolution of mafic 

microgranular enclaves through various types of interaction between 

coexisting felsic and mafic magmas. Trans. Royal Soc. Edinburgh: 

Earth Sci., 83, 145-153. 

Didier, J. and Barbarin, B. (eds.), 1991. Granites and their enclaves. The 

bearing of enclaves on the origin of granites. Dev. Petrol., 3. 

Elsevier, Amsterdam. 

Dorais, M.J., Whitney, J.A. and Roden, M.P., 1990. Origin of the 

mafic enclaves in the Dinkey Creek pluton, central Sierra Nervada 

batholith, California. J. Petrol., 31, 853-881. 

Frost, T. and Mahood, G., 1987. Field, chemical and physical constraints on mafic-felsic magma interaction in the Lamarck 

Granodiorite, Sierra Nevada, California. Geol. Soc. Am. Bull., 99, 

272-291. 

Knopf, A. and Thelan, P., 1905. Sketch of the geology of Mineral 

King, California. Univ. Calif. Pub. Bull., 4, 227-262. 

Pabst, A., 1928. Observations on inclusions in the granitic rocks of the 

Sierra Nevada. Univ. Calif. Pub., Dept. Geol. Sci. Bull., 17, 325-

386. 

Reid, J.B., Jr., Evans, O.C. and Fates, D.G., 1983. Magma mixing in 

granitic rocks of the central Sierra Nevada, California. Earth. Planet. 

Sci. Lett., 66, 243-261. 

Petrology and Geochemistry of Late 

Neoproterozoic Anorogenic Granitoids, 

Laurentian Appalachians, 

Virginia and North Carolina: 

Implications for Extension-Related 

Encratonic Magmatism 

Tallo, R.P., Hutson, F.E. and Aleinikoff, J.N. 1

Department of Geology, George Washington University, 

Washington, DC 20052, USA 2

Department of Geological Sciences, 

University of Texas, Austin, TX 78712, USA 3

U.S. Geological Survey, Denver, CO 80225, USA 

Magmatic activity associated with Late Neoproterozoic 

regional extension produced more than twenty granitoid 

Abstracts 149 

plutons within the Laurentian Appalachians of Virginia and 

North Carolina. The plutons and associated volcanic rocks 

form a supersuite that spans an age range of at least 760-

700 Ma and defines a period of magmatism that preceded 

development of an oceanic basin by about 130 Ma. 

Plutons constituting the supersuite, which includes 

granitoids of the Crossnore Complex (Virginia and North 

Carolina) and Robertson River Igneous Suite (central 

Virginia), among others, intruded Grenville-age (1.2-1.0 

Ga) basement of the Blue Ridge province and were 

metamorphosed at greenschist facies conditions during the 

Paleozoic. 

The plutons are variable in size, typically elongate, and 

range from monzogranite to alkali feldspar syenite. Most 

plutons record a sequence of multiple intrusions that, in 

the large Robertson River batholith (390 km2

), spanned at 

least 35 Ma. The plutonic and associated volcanic rocks 

of the supersuite display many petrologic characteristics of 

A-type granitoids, including: 1) metaluminous to (relatively rare) mildly peralkaline bulk compositions; 2) high 

(relative to well-documented I-, S-, and M-types) Zr, Nb, 

Ga, Y, REE (not Eu), Fe/Mg, and Ga/ Al; and, 3) common occurrence of hypersolvus feldspar assemblages, Perich ferromagnesian silicates, fluorite, and REB-bearing 

accessory phases including allanite. High Gal Al (9 .0 -

2. 7) is particularly diagnostic and serves as an effective 

discriminant between originally metaluminous ( < 6) and 

peralkaline ( > 6) bulk compositions. Intrapluton 

compositional variation is locally significant and reflects 

both source- and fractionation- (dominantly feldspar and/or 

accessory minerals) related mechanisms, locally modified 

by fluid-driven, late- and post-magmatic processes. Trace 

element data from the most thoroughly studied plutons 

indicate that the peralkaline rocks are unlikely petrologic 

derivatives of the more abundant metaluminous types. 

Moreover, comparison with other well documented suites 

suggests that the peralkaline rocks exhibit source-related 

characteristics similar to ocean-island basalts whereas 

metaluminous units display characteristics similar to bulk 

continental crust. Results of experimental investigations 

by other workers in which A-type metaluminous granite 

melts were produced through melting of charnockitic rocks 

representative of the Blue Ridge basement support derivation of the metaluminous magmas from local sources. SmN d isotopic data for the Robertson River Igneous Suite 

have eNd values (calculated for crystallization ages of 730 

Ma as indicated by U-Pb isotopic analyses of zircon) 

ranging from -0.1 to + 2.1 and model ages from 1.1 to 1.3 

Ga. eNd values increase with decreasing age within the 
150 The Origin of Granites and Related Rocks 

suite. Older metaluminous units have eNd values indicative 

of interaction with evolved crust ( -0.1 to + 1.4) but the 

younger peralkaline units show more positive eNd values 

( + 1.8 to +2.1) that suggest either a change in source 

characteristics or degree of crustal contamination with 

time. Collectively, these data indicate that abundant 

metaluminous magmas associated with the earliest stages 

of Laurentian extension were produced dominantly through 

melting of local continental crust and that such melts 

diversified in composition largely through crystal fractionation. Peralkaline magmas produced during the waning 

stages of regional magmatic activity were generated 

through melting of more geochemically primitive sources 

but the magmas evolved compositionally through similar 

fractionation mechanisms. 

The mode of occurrence, type and range of compositions, and inferred petrogenesis of the supersuite are 

similar to those of anorogenic, encratonic granitoids 

documented worldwide. The Laurentian Appalachian 

supersuite represents a significant, long-lived, temporally 

episodic period of magmatism that ended with encratonic 

rifting but not development of oceanic crust. As such, the 

rocks are indicative of processes operative during the 

earliest, pre-drift stages of Laurentian extension. A 

second period of anorogenic magmatism occurred at about 

600 Ma and involved abundant mafic and subordinate 

felsic volcanism. Relatively rare anorogenic granitoids 

emplaced in Grenville-age basement of the New York 

promontory during this period are chemically indistinguishable from the supersuite, suggesting that compositional 

characteristics cannot be used to discriminate between 

granitoids intruded during the two periods of extension. 

The more voluminous felsic rocks of the earlier period 

preserve an otherwise rare record of petrogenetically 

related plutonic-volcanic sequences and coeval clastic rift 

deposits associated with encratonic extension and thus 

provide a unique three-dimensional perspective of crustal 

processes preceding development of Iapetus. 

Geochemistry and Geochronology of Evolved 

Granitic Rocks in Southwestern Maine: 

Tectonic Implications 

Tomascak, Paul B., Walker, Richard J. 

and Krogstad, Eirik J. 

Isotope Geochemistry Laboratory, 

Department of Geology, University of Maryland 

at College Park, MD 20742, USA 

The emphasis of this work is the critical assessment of 

granite generation adjacent to a major shear zone system 

using comparative elemental and isotope geochemistry and 

high precision geochronology. The study area is in southwestern Maine and deals with a suite of granitic rocks: the 

Sebago batholith, small granitic bodies at Brunswick and 

granitic pegmatites around Topsham. . . 

The Sebago batholith is the largest exposed granite m 2 • New England, with an area of c. 2600 km. It compnses 

two-mica granite with varying proportions of the micas, 

locally contains garnet, and in places displays textures 

common to pegmatitic leucogranites. It crystallized at 

293 ± 1 Ma (concordant U-Pb monazite). Samples have 

rare earth element (REE) patterns with similar shapes: 

LREE enriched, gently flattened in the HREE with 

moderate to strong negative Eu anomalies. Nevertheless, 

samples fall into three spatially-distinct groups based on 

REE patterns. Patterns of samples from within the southwestern and central portions of the batholith (Group 1) 

overlap strongly with a (Ce!Yb)cN = 10 - 20. The bulk of 

samples peripheral to this region (Group 2) have a greater 

range in (Ce/Yb)cN (5 - 60) with negative kinks in Nd, 

yielding higher Sm/Nd. Pegmatitic and aplitic s~ples 

from Group 2 show strongly kinked REE patterns w1th low 

overall abundances. Samples from the northern extremity 

of the batholith (Group 3) have steep REE patterns 

((Ce/Yb)cN - 60) and in some cases negligible Eu 

anomalies. The REE patterns of samples with granitic 

textures are typical of evolved granites derived from 

anatexis of pelites leaving garnet in the residue. Initial eNd 

are as follows: Group 1 = -3.4 to -2.2, Group 2 = -5.6 

to -2.8, and Group 3 = -5.2 to -3.9. Depleted mantle Nd 

model ages are as follows: Group 1 = 0.9-1.2 Ga, Group 

2 = 1.2-2.7 Ga, and Group 3 = 1.0-1.2 Ga. The REE 

and Nd data from Group 1 samples permit their derivation 

from the same source, potentially by slightly different 

degrees of melting of long-term LREE-enriched crust. The 

data permit the origin of Group 2 and 3 samples through 
either variability of source materials or contamination of 

original, somewhat less-evolved magma with material with 

longer crustal history. In contrast to the Nd isotopic 

heterogeneity, the batholith displays a homogeneous Pb 

isotope signature. Severely leached K-feldspars taken from 

the same suite of whole rock samples have 206Pb/204Pb = 

18.41 - 18.75, 207Pbf204Pb = 15.60 - 15.65 and 208Pbf204Pb 

= 38.21 - 38.37. These Pb isotopic compositions lack the 

spatial variation seen in the Nd data and overlap the Pb 

isotopic compositions of felsic igneous rocks from coastal 

Maine (Ayuso and Bevier, 1991). The data suggest that the 

Sebago batholith was produced by melting of Proterozoic 

sediments of variable provenance. The high 206Pbf204Pb and 207Pbf204Pb of the Sebago batholith common Pb, however, 

rule out the involvement of North American basement 

sources (Grenville). 

Granitic bodies at Brunswick occur as homogeneous 

masses of 10-100 m diameter, approximately 7 km from 

the eastern margin of the Sebago batholith. The granite at 

Brunswick contains two primary micas and garnet is 

common. The granite crystallized at 278±1 Ma (U-Pb 

monazite). Spatially-related evolved granitic pegmatites 

around Topsham (7-13 km NE) have monazites that record 

younger crystallization ages of 274-269 Ma. The monazite 

ages are corroborated by isochron ages of pegmatite 

apatites (c. 270 Ma) and one uraninite age (258±14 Ma). 

The correspondence in ages of granite in Brunswick and 

pegmatites in Topsham permit the rocks to be related to 

the same magmatic system. In spite of a documented 

widespread Permian thermal event, these rocks are the 

first record of Permian granitic magmatism north of New 

Hampshire in the Appalachians. The granite at Brunswick 

has similar bulk chemical composition to the Sebago 

batholith. REE patterns are kinked in a similar fashion to 

the Group 2 Sebago batholith granites and samples exhibit 

a similar range of eNd (-6.6 to -3.9) and t0 M (1.4-2.3 Ga). 

The Sebago batholith is not related to the granite 

magmatism in the Brunswick area. The granite at Brunswick and Topsham pegmatites postdate high-temperature 

ductile deformation in the Brunswick area, constraining 

deformation to ;;?: 280 Ma. These Permian granitic rocks 

in southwestern Maine were evidently generated during a 

spatially isolated thermal event. The limit on the age of 

ductile deformation in southwestern Maine suggests that 

final assembly of Avalonian crustal blocks into their 

current positions may have been diachronous from north 

to south over c. 250 km in the northern Appalachians 

Abstracts 151 

Ayuso, R.A. and Bevier, M.L., 1991. Regional differences in Pb 

isotopic compositions of feldspars in plutonic rocks of the northern 

Appalachian mountains, U.S.A., and Canada: A geochemical method 

of terrane correlation. Tectonics, 10, 191-212. 

Li Isotope Geochemistry of the 

Tin Mountain Pegmatite, 

Black Hills, South Dakota 

Tomascak, Paul B., Lynton, Stephen J., 

Walker, Richard J. and Krogstad, Eirik J. 

Isotope Geochemistry Laboratory, 

Department of Geology, University of Maryland 

at College Park, MD 20742, USA 

Using newly-established techniques for the separation 

and analysis of Li isotopes in natural materials, we have 

analyzed mineral separates from the highly evolved Tin 

Mountain pegmatite of the Black Hills granite-pegmatite 

system. Our goals are: 1) to assess the nature of Li isotope 

variability within the pegmatite; 2) to examine Li isotope 

fractionation among coprecipitating minerals within interior 

zones of the pegmatite; and, 3) to compare the Li isotopic 

compositions of minerals with their oxygen isotopic 

signature, to determine if criteria might be defined for Li 

isotope geothermometry. 

Based on field relations, geochronology and geochemistry, the Tin Mountain pegmatite is considered to be related 

to the 1715 ±3 Ma Harney Peak granite (Walker et al., 

1989; Redden et al., 1991; Krogstad and Walker, 1994). 

The pegmatite constitutes five mineralogically and 

texturally distinct zones: wall zone (albite + quartz + 

muscovite); first intermediate zone (perthite + quartz + 

albite + muscovite); second intermediate zone (perthite + 

quartz + albite; minor muscovite, spodumene); third 

intermediate zone (albite + quartz + spodumene + 

muscovite; minor beryl, amblygonite); core (quartz + 

spodumene + muscovite; minor albite, beryl, 

amblygonite). On the basis of field evidence, geochemistry 

and intermineral fractionation of oxygen isotopes, Walker 

et al. (1986) determined that the wall zone was first to 

crystallize among the zones, followed by the first intermediate zone. The remaining intermediate zones and the core 

then crystallized simultaneously. 

Lithium was separated from albite samples via cation 

exchange chromatography, using a mixture of HCl and 

ethanol as an eluant. Li isotope ratios were measured via 
152 The Origin of Granites and Related Rocks 

thermal ionization mass spectrometry, volatilizing Li2B40 7 

from a Ta side filament and measuring Li +. The 

reproducibility of measurements on the NBS L-SVEC 

(Li2C03) standard at present permits uncertainties on o6Li 

determinations of ±0.8o/'oo. 

The Li isotope data gathered to date are summarized on 

the diagram below. The data show the overall isotopic 

variability of albites is restricted, underscoring the need 

for high precision isotope ratio measurements in order to 

distinguish small-scale differences in isotopic composition. 

The wall zone albite (o6Li = -11.1 o/'oo) is depleted in light 

Li compared to the first intermediate zone (-8.0%o). The 

frrst intermediate zone albite has overlapping o6Li with that 

of second intermediate zone ( -7.1 %o ), within analytical 

uncertainty. The third intermediate zone albite is light Li 

depleted ( -9.8 %o), within uncertainty of the wall zone 

albite. The isotopic composition of albite from the core is 

between those of the second and third intermediate zones 

(-8.8o/'oo). The datum for the wall zone albite is identical to 

the whole rock value determined by Vocke et al. (1990), 

suggesting Li isotope equilibrium during crystallization of 

the exterior part of the pegmatite or subsequent 

reequilibration. The isotopic variations among albites may 

reflect differences in Li fractionation between different 

crystallizing assemblages. Changing temperature during 

pegmatite consolidation could also play a role in the 

variations demonstrated here. More thorough treatment of 

these data, however, can only be accomplished with 

further isotopic measurements on other coexisting minerals 

(e.g. muscovite, quartz, spodumene). 

-4f- Tin Mountain pegmatite albites 

-5 r- -

-6 - -

-7 r- Lnd -

-8 I""' L. core 

~ f 

-

-9 ,... - intermediate zones f 

-10 - 3rd 

f -11 - -

-12 f- - wall zone 

-13 f- -

Krogstad, E.J. and Walker, R.J., 1994. High closure temperatures of the 

U-Pb system in large apatites from the Tin Mountain pegmatite, 

Black Hills, South Dakota. Geochim. Cosmochim. Acta, 58, 3845-

3853. 

Redden, J.A., Peterman, Z.E., Zartman, R.E. and DeWitt, E., 1991. UTh-Pb geochronology and preliminary interpretation of Precambrian 

tectonic events in the Black Hills, South Dakota. In Lewry, J.F. and 

Stauffer, M.R. (eds), Geol. Assoc. Can. Spec. Paper 37, 229-251. 

Vocke, R.D., Beary, B.S. and Walker, R.J., 1990. High precision Li 

isotope ratio measurements of samples from a variety of natural 

sources. V.M. Goldschmidt Conf Prog. Abst., p.89. 

Walker, R.J., Hanson, G.N., Papike, J.J., O'Neil, J.R. and Laul, J.C., 

1986. Internal evolution of the Tin Mountain Pegmatite, Black Hills, 

South Dakota. Am. Mineral., 71, 440-459. 

Walker, R.J., Hanson, G.N., and Papike, J.J., 1989. Trace element 

constraints on pegmatite genesis: Tin Mountain Pegmatite, Black 

Hills, South Dakota. Contrib. Mineral. Petrol., 101, 290-300. 

The Carboniferous - Ordovician Connection 

in the Sardinia-Corsica Batholith: 

Constraints from the Carboniferous 

Calc-Alkaline Granitoids and 

Ordovician Metavolcanics 

1

Tommasini, S., 2Poli, G., 3

Halliday, A.N. and 4Manetti, P. 1

Faculteit der Aardwetenschappen, 

Amsterdam, The Netherlands 2

Department of Earth Sciences, Perugia, Italy 3

Department of Geological Sciences, 

University of Michigan, Ann Arbor, MI 48109, USA 4

Department of Earth Sciences, Firenze, Italy 

Isotopic and trace element signatures of granitoids can 

yield valuable information to assess the geochemical 

characteristics of source reservoirs and the importance of 

new mantle additions to the continental crust during 

orogenic processes. Also, inferences on the geodynamic 

setting can be achieved from isotopic signatures of source 

reservoirs. In spite of this powerful tool, care must be 

paid when dealing with granitoids because their composition can be affected by open system differentiation processes that could have significantly altered their potential 

bearings on source characteristics. In the case of the 

Sardinia-Corsica Batholith, several interaction processes 

between crust- and mantle-derived magmas formed a wide 

variety of hybrid magmas ranging from homogeneous 

tonalitic intrusions to heterogeneous, mafic enclave-bearing 

granitoids. As such, isotopic and geochemical characteris-
tics of these hybrid magmas cannot be used to assess the 

nature of source reservoirs during the Hercynian orogeny 

in Sardinia and Corsica. It is necessary to take into 

account just those magmas directly originated from source 

regions to accomplish this task. 

Isotopic and trace element data are presented for 

selected gabbros and I-type granitoids representative of the 

parental mantle- and crust-derived magmas, respectively. 

The gabbros belong to normal calcalkaline suites and have 

marked relative enrichments in Rb, Ba, K and Pb in a 

primitive mantle-normalized diagram. The granitoids 

belong to high-K calcalkaline suites and have fairly 

uniform trace element compositions resembling Volcanic 

Arc Granitoids (VAG). A significant overlap in Sr and 

Nd isotope compositions is observed between gabbros and 

granitoids. 

Geochemical and isotopic data provide evidence for the 

origin of the gabbros from mantle sources enriched in 

incompatible trace elements through recycling of sediments 

via subduction zones, whereas the granitoids were derived 

from crustal sources composed mainly by igneous 

protoliths with relatively homogeneous composition. Sr 

and Nd isotope compositions of gabbros and granitoids are 

consistent with both the mantle enrichment process and the 

formation of the igneous crustal sources occurring at -

450 Ma, during the earlier calc-alkaline igneous activity. 

Major and trace element analyses of the Ordovician 

basic-intermediate metavolcanic rocks provide further 

evidence for the formation of the igneous crustal sources 

during this calc-alkaline igneous activity. A partial melting 

model using mass balance calculations for major elements, 

direct and inverse estimates for trace elements, and 

experimental petrology data is consistent with the 

metavolcanic rocks (or their underplated equivalent) being 

potential source rocks of the Hercynian I -type granites. 

The connection between Hercynian and Ordovician 

igneous activity has important and new implications for the 

Paleozoic evolution of the Sardinia and Corsica lithosphere, and permits the Hercynian orogeny to be placed in 

a wider geodynamic setting, consisting of three main 

phases. The Ordovician precollisional phase was characterized by a NNE dipping subduction of an oceanic plate 

under a continental plate with emplacement of acid and 

subordinate basic-intermediate volcanic and intrusive 

rocks. The subcontinental mantle underneath Sardinia and 

Corsica experienced an enrichment event in incompatible 

trace elements through recycling of sediments via subduction zones. Major crustal accretion also occurred with 

underplating of basaltic magmas. The Devonian collisional 

Abstracts 153 

phase was characterized by the collision of two continental 

plates after the total consumption of the oceanic plate. 

Crustal thickening processes took place together with 

regional metamorphic events that recorded a clockwise PT-t path. The Carboniferous postcollisional phase was 

characterized by isostatic and thermal readjustments 

following crustal thickening that caused extensive partial 

melting processes. Large quantities of I-type granitoids 

and subordinate gabbroic complexes were emplaced in the 

middle-upper crust and formed the main frame of the 

Sardinia-Corsica Batholith. 

This geodynamic model is consistent with the Paleozoic 

evolution of other sectors of Western Europe suggested on 

the basis of geological, geochronological, and 

paleomagnetic data. The paleomagnetic restoration of the 

Late Paleozoic position of Sardinia and Corsica close to 

Southern France suggests that Sardinia and Corsica could 

have been portions of the southern edge of the Armorican 

plate that, during the Siluro-Devonian, collided with the 

lbero-Aquitanian plate after the total consumption of the 

Late Cambro-Ordovician South Armorican and/or Massif 

Central Ocean. 

Mesozoic Plutonism in Western 

New Zealand - a 50 Ma Transition from 

Subduction to Continental Rifting 

on the Gondwana Margin 

1

Tulloch, A.J. and 2Kimbrough, D.L. 1

/nstitute of Geological and Nuclear Sciences, 

Private Bag 1930, Dunedin, New Zealand 2

Department of Geological Sciences, San Diego State 

University, San Diego, CA 92182, USA 

Mesozoic subduction along the New Zealand margin of 

Gondwana is clearly recorded in the 248-131 Ma calcalkaline mafic to felsic plutonic and volcanic rocks of the 

Median Tectonic Zone (MTZ). Silica contents range from 

48 - 74 wt % although intermediate compositions dominate. Nd-Sr isotope compositions indicate derivation from 

mildly depleted mantle. 

No more than 5-10 Ma then elapsed before the initiation of magmatism of the alkali-calcic I-type Separation 

Point Suite (SPS), and its deeper level equivalent, the 

granulite facies Western Fiordland Orthogneiss (WFO), 

which stitched the MTZ to the continental margin (126-105 
154 The Origin of Granites and Related Rocks 

Ma). The SPS/WFO granitoids (56- 74 wt % Si02) have 

high AI, Na, Sr and HREE depletions with insignificant 

Eu anomalies, indicative of derivation from an eclogitic or 

garnet amphibole source. 

Towards the end of SPS/WFO magmatism, at about 

105-100 Ma, several metamorphic core complexes formed 

within the Gondwana margin to the west of the boundary 

with the recently amalgamated MTZ. Development of 

these extensional features preceded final rifting of New 

Zealand from Australia and Antarctica by some 20 Ma . 

The core complexes appear to form the loci for a further 

suite of plutons (the transitional liS-type Rahu Suite; 65 -

75 wt % Si02) which exhibit chemical and isotopic 

features indicative of significant continental source components. Zircon and monazite U-Pb ages for the Rahu Suite 

are concentrated in the range 115-107 Ma. In their 

chemistry, variable isotopic composition, associated 

appinitic rocks and probable post closure tectonic setting 

the Rahu Suite has much in common with the Caledonian 

granites of Scotland. Most of these plutons were emplaced 

at 10-20 km depth, but were rapidly uplifted on detachment faults which trended subparallel to the continental 

margin to the east and to the eventual spreading centre of 

the Tasman Sea to the west. 

Although the Early Cretaceous plutonic rocks in the 

footwalls of the core complexes are dominated by the Rahu 

Suite, some rocks with SPS/WFO and intermediate 

characteristics are also present in all complexes. Resetting 

of Cenozoic displacement on the Alpine Fault strongly 

suggests that large volumes of WFO underlie the Rahu 

Suite. Nd-Sr isotopic data suggest that the Rahu Suite was 

derived from a combination of the SPS/WFO source 

together with a continental crustal component. Assimilation of old crust is also indicated by Proterozoic zircon 

inheritance, particularly in the Rahu Suite, but it is also 

clearly present in some SPS plutons. Traces of Proterozoic inheritance also appear in the younger parts of the 

MTZ arc rocks, suggesting that in its fmal stages the locus 

of MTZ magmatism overlapped the craton margin. 

Despite a wide range in initial Nd and Sr for the 

SPS/WFO and Rahu Suites, feldspars in both suites exhibit 

extremely limited variation in Pb isotopic composition, 

consistent with crustal scale Pb isotopic homogenisation 

during, or immediately prior to, melting to produce these 

granitoid suites. 

A likely protolith for the SPS/WFO magmas is the 

slightly older MTZ arc, which may have achieved the 

appropriate metamorphic grade within a thick underplate. 

In detail, models which derive the SPS/WFO from an 

MTZ arc protolith (magmatic or tectonic underplating) 

must account for the -20 Ma time span of SPS/WFO 

magmatism and its geographic span from the Permian 

accreted terrane east (oceanward) of the MTZ, westward 

across the MTZ and Gondwana margin at least to the 

present west coast of New Zealand ( - 140 km across the 

margin in present configuration), and the observed pattern 

of Proterozoic zircon inheritance. 

Plutonic rocks similar to SPS/WFO in several other 

parts of the Pacific rim occur inboard of slightly older arcrelated batholiths (e.g. Eastern Peninsular Ranges Batholith), where they also appear to precede continental 

extension further inboard and intrude along major sutures 

between craton and accreted terranes. 

Potential derivation of the SPS/WFO suites from an 

MTZ protolith means that arc characteristics may have 

been inherited from this source, and thus such features of 

the SPS/WFO do not necessarily indicate subduction 

contemporaneous with the SPS. Thus the youngest time at 

which Mesozoic subduction beneath the New Zealand 

margin of Gondwana can be shown unambiguously to 

have occurred using geochemical/magmatic criteria is 140-

130 Ma. 

Further inboard from the convergent margin a series of 

increasingly alkaline rocks ranging from gabbro (100 Ma) 

to A-type granite (83 Ma) and camptonitic lamprophyres 

(- 84 Ma) clearly herald the culmination of extension and 

the opening of the Tasman Sea between NZ and Australia 

at, or slightly before, 84 Ma. 

ORAL PRESENTATION 

Granites from Source to Emplacement: 

The View from Geophysics 

Vigneresse, Jean Louis 

CREGU, and ENS Geologie, Nancy, France 

Two examples are used to show how the transfer of 

ideas between disciplines is realized. In the first example, 

the depth of a pluton floor is estimated from gravity data 

inversion and correlated to structural measurements 

registered by the magma when being emplaced. The 

second example refers to two granitic complexes, very 

close in time, but very contrasted in their shape at depth, 

as well as in their petrological pattern. 
The pluton of Cabeza de Araya, Estramadura, Spain, 

intruded late Precambrian micaschists, c. 300 Ma, in an 

extensional zone due to regional shear. The pluton presents 

a normal petrographic zoning. On its periphery, facies A 

is porphyritic, gradually passing to facies B, a coarsegrained two-mica granite which is cross-cut by facies C, 

a leucocratic two-mica variant. 760 gravity stations have 

been realized, which result in a Bouguer anomaly map. 

After data inversion and tests on the sensitivity of the 

results to the density values, the depth of the pluton floor 

is mapped with an estimated error of 15 %. The pluton is 

thin with 75% of its volume within 4.5 km of the present 

erosion level. Its walls dip inward by 30° to 70°. 

Structural measurements, using the orientation of Kfeldspars, and the anisotropy of magnetic susceptibility 

provide the orientation and dip of the foliation planes and 

the lineation. The lineations show a bimodal distribution. 

Subvertical lineations ( > 60°) are located in the southern 

part of the pluton, in facies C. They also cluster in the 

north of the pluton. The subset with low plunges ( < 30°), 

located principally in facies A and B, trend with the same 

azimuth as the pluton. The regions of deep floor and 

vertical lineations are interpreted as the magma feeding 

zones. 

Two small and unconnected tension cracks, between 6 

km and 13 km in depth, served as feeding zones for the 

magma. Simultaneous infilling by magma during opening 

was followed by an interruption at the A/B transition 

before a new stage of progressive opening/infilling led to 

facies B (gradational contacts). A longer period of quiescence separated the intrusion of facies C (sharp contacts). 

The magma flow was continuously guided by incremental 

opening of the walls. 

The Hercynian Fichtelgebirge granite, Bayern, Germany, is subdivided into two heterochronous complexes, 

hereafter designed as the Older Intrusive Complex (OIC) 

and the Younger Intrusive Complex (YIC), and simplified 

to equigranular and porphyritic facies. All granitic types 

have a pronounced peraluminous character. 

The porphyritic (OICp) facies is biotite-rich, whereas 

the equigranular (OICe) facies is mainly medium to fine 

grained and muscovite rich. The latter is much more 

peraluminous than the OICp facies, with a compositional 

gap and distinct chemical variation. The OICe is also more 

Mg-rich than would be expected from simple biotite 

fractionation from the OICp. The YICp facies shows a fine 

grained groundmass contammg several different 

phenocrysts compared to the YICe facies. Both share a 

typical fractionation trend, reflected by the increase of the 

Abstracts 155 

peraluminous character and no discernible gap in the 

fractionation process. 

After gravity data inversion, the OIC is modeled as a 

thin body, c. 1. 9 km thick on average. An eastward 

deepening of the massif to 6 km that coincides with the 

more evolved facies is interpreted to reflect the root zone. 

The YIC averages 6 to 7 km in thickness with a nearly flat 

floor and steeply dipping walls which trend N160°. The 

western wall, dipping at 70° to the east, is related to the 

Franconian Line, a major dextral shear zone. The two 

smaller massifs of the YIC, north of OIC, are connected 

to the southern unit of the YIC, beneath the OIC unit. 

The OIC forms one unique massif on the field and all 

granitic facies belong to a more or less homogeneously 

evolving magma. The most fractionated magmas are close 

to the root zone, whereas the less evolved magma travelled 

away from its root zone. So, the bulk trend is a higher 

degree of fractionation with decreasing distance to the root 

zone. The YIC outcrops in three massifs, each one shows 

an imbricated disposition of facies. All samples located 

near the root zone are the least fractionated, with low 

values of differentiation index and low Th content. Fractionation develops in samples located far from the root 

zone. These contrasted patterns between OIC and YIC are 

interpreted as reflecting differences in the magmatic 

evolution during the emplacement of the two massifs. 

In the OIC, two separate magma pulses are defined 

according to the porphyritic/non-porphyritic character of 

the rocks, the gap within the fractionation trend, and 

fmally the sharp borders at facies changes. The bulk shape 

of the floor, with only one root, suggests that this unique 

root delivers all the magma undergoing an evolving 

fractionation. Because of the gap in the fractionation 

pattern, two major pulses of magma have been delivered. 

The first one invades the region from the east and gives 

rise to the OICp along an east-west elongated trend. A 

second pulse of magma immediately follows. It is more 

evolved and fractionated than the first, and therefore 

probably more viscous and resistant to flow. It pushes 

aside the formerly emplaced OICp magma and solidifies as 

it reaches the uppermost levels to form the present OICe. 

In the YIC, the very steep walls and parallel is in to the 

Franconian line, suggest a tectonic control on emplacement. The geochemical variations within the two main 

facies are almost identical and show a nearly identical 

fractionation trend. One single magma generated two 

facies during successive major steps of continuously 

opening space filled with granite. During each step of 

intrusion, the marginal part of the magma crystallized first, 
156 The Origin of Granites and Related Rocks 

so the remaining magma appears more evolved and forms 

part of the YICe. Another sequence follows, with lower 

differentiation index, but with a slope of evolution similar 

to the first. It also shows a porphyritic and an equigranular 

facies. 

Mid-Cretaceous Plutonism Associated 

with Crustal Extension: 

The Hohonu Batholith, New Zealand 

Waight, Tod 

Department of Geological Sciences, 

University of Canterbury, Christchurch, New Zealand 

The middle Cretaceous tectonic history of the New 

Zealand portion of the Gondwana margin was complex, 

involving subduction, terrane accretion, crustal extension 

and eventual continental breakup. This history includes 

emplacement of several distinct pulses of granitoid 

magmatism. Combined field and petrological investigations 

have enabled identification of four suites in the Hohonu 

Batholith, a - 600 km2 region of granitoids outcropping 

adjacent to the Alpine Fault on the West Coast of the 

South Island of New Zealand. A single Devonian pluton 

represents the oldest magmatism in the batholith, whereas 

a Late Cretaceous A-type pluton, closely associated with 

an alkaline mafic dike swarm and the opening of the 

Tasman Sea at 82 Ma, represents the youngest activity. 

The bulk of the batholith comprises the Hohonu 

supersuite (114-109 Ma) which is characterized by relatively restricted radiogenic isotopic compositions 

((87Sr/86Sr)ll10) = 0.7062 to 0.7085 and eNd(llO) = -4.4 

- -6.1). The Hohonu supersuite is considered to represent 

melting of a complicated source combining depleted 

mantle-derived material and a complex, heterogeneous and 

largely unconstrained lower continental crustal component. 

Two constituent suites are recognized within the Hohonu 

supersuite, the Te Kinga suite and the Deutgam suite. 

Geochemical contrasts between these two suites are 

attributed to melting at differing crustal depths, at varying 

water activities, and in equilibrium with different residual 

assemblages. The relatively mafic, metaluminous, 1-type 

compositions of the Deutgam suite are ascribed to volatile 

phase-absent melting in equilibrium with an amphibolitic 

(plagioclase + amphibole) residue. Residual plagioclase 

retained Sr, Al20 3, Na20 and Eu and resulted in the low 

concentrations of these elements which characterize this 

suite. In contrast, the peraluminous high silica compositions of the Te Kinga suite are attributed to water-saturated 

to undersaturated melting in equilibrium with an eclogitic 

(garnet + amphibole residue) at greater depths in the 

crust. Residual garnet produced the HREE-depleted nature 

of the suite, and a lack of residual plagioclase contributed 

to the characteristically higher Sr, Al20 3, Na20 and Eu 

contents of the Te Kinga suite. 

The Hohonu supersuite was emplaced during a period 

of rapid tectonic transition along the Gondwana margin. It 

post-dated crustal thickening and compression associated 

with subduction, collision and the generation of the 

II adakitic II Separation Point suite and was also emplaced 

contemporaneous with the cessation of subduction and the 

initiation of crustal extension in the Western Province of 

New Zealand. 

ORAL PRESENTATION 

Heterogeneous Sources for Granitic 

Rocks, Black Hills, South Dakota: 

Do Granites Really Serve as 

Interpretable Crustal Probes? 

Walker, Richard J. and Krogstad, Eirik J. 

Isotope Geochemistry Laboratory, 

Department of Geology, University of Maryland 

at College Park, MD 20742, USA 

The Early Proterozoic (1715 Ma) Harney Peak Granite 

(Black Hills, SD) consists of a complex of hundreds of 

dikes and sills. In addition, the granite complex is interwoven with, and surrounded by a compositionally diverse 

pegmatite field. Earlier studies of neodymium (Walker et 

al. , 1986), oxygen (N abelek et al., 1992) and lead 

(Krogstad et al., 1993) isotope variations indicated that the 

Harney Peak Granite complex was not derived from a 

single source, or even different sources of a single age. 

New Nd and Pb isotopic results are consistent with a 

complex source for the Harney Peak Granite. For example, the Harney Peak Granite has eNd values (1715 Ma) 

that range from -2.0, indicating an Early Proterozoic (2.2 

to 2.3 Ga) crustal source, to -13.4, indicating a Middle 

Archean (3.1 to 3.2 Ga) crustal source. The Nd isotopic 

compositions of spatially associated metasedimentary rocks 
are consistent with the interpretation that the Early Proterozoic source included local metasedimentary rocks. The 

Late Archean sources might have included rocks like those 

exposed on the periphery of the Black Hills. 

The N d and Pb isotopic data for granitic pegmatites 

display a range similar to that of the Harney Peak Granite 

suite. For example, eNd values for apatites from the Tin 

Mountain, Hugo and Peerless pegmatites range from +0.7 

to -10.0. There is no obvious correlation between pegmatite type and Nd isotopic composition. However, the 

interpretation of Nd isotopic data on small, presumably 

water-rich igneous systems is problematic. The pegmatites 

evidently exchanged significant Nd with country rocks 

during their crystallization, and the pegmatites have 

moderately inhomogeneous Nd isotopic compositions. This 

observation illustrates potential problems in the study of 

trace element/isotopic systematics of any fluid-rich granite 

system. 

The range of model ages calculated for the granitepegmatite system brings into question the viability of using 

N d model ages of an evolved granite system to quickly 

fingerprint the II average II age of the crust in a given 

region. Because such heterogeneity is present in the 

magmatic compositions of the Harney Peak Granite, it can 

be inferred that at least as much heterogeneity was present 

in the sources. Melts were evidently derived from isolated, heterogeneous zones, and did not have the opportunity to coalesce into large, isotopically homogeneous 

magma bodies. In this instance, the evidence for source 

complexity was not lost. Such complexities, however, are 

likely lost in some systems. This study highlights the 

danger of drawing inferences from the isotopic data for a 

few samples of plutonic rocks in studies of regional crustal 

evolution. 

Krogstad, E.J., Walker, R.J., Nabelek, P.l. and Russ-Nabelek, C., 1993. 

Lead isotopic evidence for mixed sources of Proterozoic granites and 

pegmatites, Black Hills, South Dakota, USA. Geochim. Cosmochim. 

Acta, 57, 4677-4685. 

Nabelek, P.I., Russ-Nabelek, C. and Haeussler, G.T., 1992. Stable 

isotope evidence for the petrogenesis and fluid evolution in the 

Proterozoic Harney Peak leucogranite, Black Hills, South Dakota. 

Geochim. Cosmochim. Acta, 56, 403-417. 

Walker, R.J., Hanson, G.N., Papike, J.J. and O'Neil, J.R., 1986. Nd, 

O, and Sr isotopic constraints on the origin of Precambrian rocks, 

southern Black Hills, South Dakota. Geochim. Cosmochim. Acta, 50, 

2833-2846. 

Abstracts 157 

Crustal Evolution and the Generation of 

Magmas Akin to Adakites in Western 

Palmer Land, Antarctic Peninsula 

tWareham, Christopher D. and 2Millar, Ian L. 1

British Antarctic Survey, High Cross, Madingley Road, 

Cambridge CB3 OET, UK 2

British Antarctic Survey, c/o NERC Isotope Geosciences 

Laboratory, Keyworth, Nottingham, NG5 5GG, UK 

Antarctic Peninsula Mesozoic-Cainozoic magmatism 

forms part of a belt of subduction-related magmatism that 

extended along the Pacific margin of Gondwana prior to 

continental break -up. The east -directed subduction of 

proto-Pacific and Pacific oceanic plate produced granite 

magmatism in western Palmer Land between c. 230 Ma 

and 60 Ma. Subduction in the region ceased between c. 

50 and 20 Ma with the formation of a passive margin as a 

result of ridge-trench collision. 

Plutonic magmatism in western Palmer Land ranges in 

composition from gabbro to granite (s.s.), and is locally 

overlain by volcanic rocks and intruded by mafic dykes. 

These plutons intrude basement paragneiss of unknown 

age. Elsewhere in the peninsula, basement gneiss has 

yielded whole-rock Rb-Sr and zircon U-Pb ages of up to 

c. 400 Ma. Intrusions with Proterozoic Nd model ages 

(toM) and intrusions containing zircons with inherited cores 

are present throughout the peninsula and suggest that 

Proterozoic basement may be present. 

Many of the granites are LREE-enriched (LaN/YbN up 

to c. 30), sodic (Na/K > 1), have A120 3 > 15 wt %, have 

high Sr/Y ratios (up to c. 100), and are low in Sc, HFSE 

and HREE. They have many major element characteristics 

that are typical of experimental partial melts of metabasalt, 

leaving a pyroxene - amphibole - garnet restite. These 

chemical features are also characteristic of Archean 

tonalite - trondhjemite - dacite suites and Phanerozoic 

adakites, which are believed to be the products of slab 

melting. However, the major element composition of 

many Palmer Land granitoids, such as their Na20 and 

MgO contents, lies outside the range produced experimentally for melts of metabasalt. Simple trace element modeling suggests that the high Sr content, low Y content, and 

consequently the high Sr/Y ratios that are characteristic of 

some of these magmas can not be derived by fluid absent 

melting of a N-MORB protolith. 

t:Nit) = -7 - +5), common Pb e06Pbf204Pb = 18.638 -

18. 796; 207Pbf204Pb = 15.612 - 15.677; 208Pbf204Pb = 
158 The Origin of Granites and Related Rocks 

38.469 - 38.649) and Sr ((87Sr/86Sr)i) c. 0.704 - 0.714) 

isotope studies indicate that magma components were 

derived from the mantle wedge, mantle lithosphere and 

sub-arc crust. There is an overall increase in eNd and 

decrease in 2-stage Nd model age (toM = 400 - 1300 Ma) 

with decreasing stratigraphic age. Isotope and trace 

element data are comparable to those obtained on contemporaneous arc basalts in western Palmer Land. These data 

suggest that the granites were in part generated by the 

melting of mafic underplate. Major element data and the 

presence of mafic inclusions in many of the granites 

indicate that a component of primary mafic magma is also 

present. The role of mafic magma in the genesis of the 

Palmer Land granites is two-fold: it is a component of the 

magmas, whether as a primary melt component or as 

melted underplate, and it provided the heat necessary for 

fusion of mafic lower crust. 

Juvenile continental crust was produced in western 

Palmer Land, and elsewhere in the peninsula, during the 

Mesozoic and Cenozoic. This occurred by underplating, 

by intraplating and by lateral accretion of magmas in shear 

zones. Granite magmas in western Palmer Land were 

largely derived by the melting of underplated mafic 

magma. Their origin is essentially intra-crustal and 

consequently their production will have contributed little to 

the growth of new continental crust. The generation of 

these magmas will have simply redistributed previously 

accreted lower crust. The net flux of material into the 

crust, whether above subduction zones or in 

intra-continental rift settings, is basaltic in composition. 

Despite this, the continental crust has a broadly andesitic 

composition. The melting of underplated basaltic material 

at the base of the crust can produce a spectrum of 

granitoid magma compositions and a dense garnet-eclogite 

residuum. This residuum may detach and sink into the 

mantle convective system, for example, during continental 

collision. Therefore, although the generation of granitoids 

by melting of basaltic underplate may contribute little 

towards the growth of new continental crust, their production will define the composition of the crust. Thus, 

subduction zones are not only areas in which new continental crust is formed, but also areas in which the average 

composition of this crust is refined. 

ORAL PRESENTATION 

Zircon Dissolution and Overgrowth 

in Granitic Melts: 

Effects of Thermal History 

and Local Melt Volume, 

with Implications for Chemical 

Zoning and Relative Core Size 

in Natural Zircons Showing 

an Inherited Isotopic Component 

Watson, E.B. and Hanchar, J.M. 

Department of Earth and Environmental Sciences, 

Rensselaer Polytechnic Institute, 

Troy, NY 12180, USA 

When imaged with cathodoluminescence or backscattered electrons, individual zircon crystals in granitic 

rocks commonly exhibit a rounded (sometimes embayed) 

core overgrown by at least one euhedral, chemically-zoned 

rim. This multistage record of dissolution and growth 

suggests a complex history, including survival of zircon 

during crustal melting-a phenomenon that is not only well 

documented by U /Pb-isotope studies but also consistent 

with known systematics of zircon solubility and Zr diffusion in granitic melts. The complexity of the dissolution/growth process is further underscored by the fact that 

core/rim volume ratios vary widely among individual 

zircons separated from a single rock. 

In an effort to understand the process of zircon dissolution in melts followed by chemically-zoned overgrowth, 

and to place constraints on the time scales of these processes, we have undertaken a series of finite-difference 

numerical simulations using existing experimental data on 

zircon solubility and Zr diffusion in granitic melts containing 3 wt.% dissolved H20 (Watson and Harrison, 1983; 

Harrison and Watson, 1983). The simplest models involve 

linear cooling from an assumed initial temperature at 

which the melt is saturated in zircon. Cooling from 7 50 o 

- 650°C over 105 years produces an overgrowth rim 5.6 

pm thick on a preexisting 100-pm crystal (assuming a 

physically static system in which no new zircons nucleate). 

The radial growth rate averages 1.8x10-16 em s-1

, but is 

highly non-linear in time, peaking at - 30K years. The 

non-linear growth rate could lead to marked chemical 

zoning in slow-diffusing minor constituents of zircon such 

as Y, HREE and Hf. A 100° cooling interval extended 

over 1 Ma results in 31 pm of new zircon growth on an 
original 100-~-tm core. Lower cooling rates yield still 

thicker overgrowth rims, as do higher initial temperatures. 

Our model can incorporate almost any degree of 

assumed geological complexity, and we are now exploring 

the consequences to zircon behavior of a heating-cooling 

cycle appropriate to crustal anatexis. Under such circumstances, protolith zircons can dissolve partially over the 

heating interval and precipitate new rims upon cooling in 

what amounts to highly local processing of isotope systems 

controlled by zircon. For a given melting episode (t-T 

path) and a melt reservoir infinitely large relative to the 

diffusion field around the dissolving (or growing) zircon, 

the volume ratio of old core to new overgrowth depends 

only upon the initial size of the zircon. If, however, the 

melt reservoir is of finite volume in relation to the diffusion field (e.g., a melt pocket on the order of 1 mm in 

size), the core/rim volume ratio is critically dependent 

upon melt reservoir volume. A time-dependent reservoir 

volume (due to melting or crystallization of major mineral 

phases) leads to additional complexities in both core/rim 

volume ratios and minor-element zoning in zircons. 

Several general conclusions can be drawn from the 

numerical simulations: 1) zircon growth (and dissolution) 

rates in granitic melts are very low (- 10-16 em s-1

) at 

plausible crustal melting conditions due to the combination 

of slow Zr diffusion and low zircon solubility in the melt 

with the high demand for Zr to form zircon; 2) zircon 

growth and dissolution rates are likely to be highly nonlinear in time even for very simple t-T melting histories; 

and, 3) during anatexis, "finite reservoir" effects are likely 

to play a major role in determining the relative core sizes 

of zircons containing an inherited isotopic component. In 

the light of these conclusions, the observed complexity and 

intrapopulation variability of internal structures in natural 

zircons is not surprising. 

Harrison, T.M. and Watson, E.B., 1983. Kinetics of zircon dissolution 

and diffusion of zirconium in granitic melts of variable water content. 

Contrib. Mineral. Petrol., 84, 66-72. 

Watson, E.B. and Harrison, T.M., 1983. Zircon saturation revisited: 

temperature and composition effects in a variety of crustal magma 

types. Earth Planet. Sci. Lett., 64, 295-304. 

Abstracts 159 

Geochemical Evidence for Deformation 

Enhanced Melt Segregation: 

An Example from the 

Kirtomy Migmatite Suite, 

Sutherland, Scotland 

Watt, Gordon R. and Bums, Ian M. 

Geology and Cartography Division, 

Oxford Brookes University, 

Headington, Oxford, OX3 OBP, UK 

The role of accessory phases such as zircon and 

monazite in controlling the trace and rare-earth element 

chemistry of granites (Hinton and Paterson, 1994; 

Montel, 1993), leucogranites (Sevigny et al., 1993) and 

migmatite leucosomes (Sawyer, 1991; Watt and Harley, 

1993) is now well documented. Restricted monazite and 

zircon dissolution during anatexis, coupled with disequilibrium melting of feldspars (Carrington and Watt, 1995), 

may give rise to melts with characteristic low LREE and 

HREE contents and positive Eu anomalies. Dissolution 

may be prevented by armouring of accessories or by 

removal of melt before equilibration with the source 

lithology (Brown, 1994; Sawyer, 1994). Compilations of 

melt segregation rates show that melt percolation velocities 

are more rapid than diffusion velocities in silicates, and if 

segregation occurs by channelised migration along veins or 

high-porosity pathways then melt migration may be more 

rapid still (Bedard, 1989). Disequilibrium melt compositions may therefore be generated and extracted during 

water saturated melting, if melt removal is rapid. Evidence 

for channelised flow and deformation enhanced melt 

segregation on a hand specimen and outcrop scale (boudin 

necks, veins, melt migration into micro-shears) is present 

in semi-pelitic migmatitic gneisses of the Kirtomy 

Migmatite Suite (KMS) in Sutherland. These amphibolite 

facies semi-pelitic and pelitic migmatites are interpreted to 

have formed during evolution of the Caledonian Moine 

Nappe pile and record metamorphic conditions of 

680-700°C, 4-6 kbar. Melt migration distances are on an 

outcrop to lOOm scale. Leucosomes in the KMS have low 

Zr contents and positive Eu anomalies, similar to those of 

granitic sheets in the Scourian Complex (Rollinson, 1994) 

and mafic migmatites described by Sawyer ( 1991). REE 

patterns of this type can be produced by removal of 

leucosome before complete equilibration with source due 

to the inhibited dissolution of LREE bearing accessory 

phases in water-undersaturated melts. Melting in the KMS, 
160 The Origin of Granites and Related Rocks 

however, occurred at or near the wet granite solidus, 

forming biotite as a restitic phase. Experimental work 

shows that during water-saturated melting conditions 

accessory phase dissolution is near instantaneous, and 

therefore slow accessory phase dissolution because of low 

melt H20 contents cannot be responsible for the geochemical signatures witnessed in the KMS. Furthermore, 

detailed back-scattered electron imaging shows that 

accessory phases were not armoured during melting. We 

suggest that deformation enhanced melt extraction was 

responsible for the REE and trace element chemistry of the 

KMS leucosomes - rapid migration of melt from the 

melting site on a timescale of less than 10000 years 

prevented leucosomes from equilibrating with zircon and 

monazite before extraction. 

Barbero, L. and Villaseca, C., 1992. The Layos Granite, Hercynian 

Complex of Toledo (Spain): an example of a parautochthonous 

restite-rich granite in a granulitic area. Trans. Royal Soc. Edinburgh: 

Earth Sci., 83, 127-138. 

Bedard, J. H., 1989. Disequilibrium mantle melting. Earth Planet. Sci. 

Lett.' 91, 359-66. 

Brown, M., 1994. Melt segregation mechanism controls on the geochemistry of crustal melts. Mineral. Mag. SSA, 124-125. 

Carrington D.P. and Watt, G. R., 1995. A geochemical and experimental study of the role of K-Feldspar during water-undersaturated 

melting of metapelites. Chem. Geol., in press. 

Hinton, R. W. and Paterson, B. A., 1994. Crystallisation history of 

granitic magma: evidence from trace element zoning. Mineral. Mag., 

SSA, 416-417. 

Montel, J. M., 1993. A model for monazite/melt equilibrium and 

application to the generation of magmas. Chem. Geol., 110, 127-146. 

Rollinson, H., 1994. Origin of felsic sheets in the Scourian granulites: 

new evidence from rare earth elements. Scott. J. Geol., 30, 121-129. 

Sawyer, E. W., 1991. Disequilibrium melting and the rate of meltresiduum separation during migmatisation of mafic rocks from the 

Grenville Front, Quebec. J. Petrol., 32, 701-738. 

Sawyer, E. W., 1994. Melt segregation in the continental crust. Geology, 

22, 1019-1022. 

Sevigny, J. H., Parrish, R. R. and Ghent, E. D. 1993. Petrogenesis of 

peraluminous granites, Monashee Mountains, south-eastern Canadian 

Cordillera. J. Petrol., 30, 557-581. 

Watt G. R. and Harley, S. L. 1993. Accessory phase controls on the 

geochemistry of crustal melts and restites produced by dehydration 

melting. Contrib. Mineral. Petrol., 114, 550-566. 

Early Exsolution of Chlorine-Enriched 

"Fluids" From Felsic Magmas: 

A Crucial But Poorly-Understood Process 

Webster, James D. 

Department of Earth and Planetary Sciences, 

American Museum of Natural History, 

Central Park West at 79th Street, 

NY, NY 10024-5192, USA 

The exsolution and subsequent movement of aqueous 

fluids within aluminosilicate magmas can profoundly affect 

heat and mass transfer in and around magma reservoirs, 

alter magma composition, and may also control late-stage 

processes such as hydrothermal brecciation-mineral 

deposition and volcanic eruptions. Experimental studies 

have shown that fluid exsolution from silicate melt is a 

function of the solubilities of all volatiles present, and 

hence aqueous fluids will exsolve from magma that is 

nominally water undersaturated if comparatively small 

quantities of other volatiles such as C02 or S02 are also 

dissolved in the magma. In this context, new experiments 

have been conducted at 0.5 and 2 kbar to determine the 

solubility of chlorine and water in a variety of F -deficient 

and F-enriched felsic melts. The results can be used to 

predict the exsolution of supercritical Cl-bearing aqueous 

fluids, coexisting brine plus vapor, and/or marginally 

hydrous alkali chloride melts from silicate melt. This 

combination of volatiles is particularly significant for 

mineralizing magmatic systems because it pairs the most 

abundant magmatic volatile (water) with one of the most 

important complexing anions in hydrothermal fluids (the 

chloride ion). 

The solubility of H20 plus Cl in silicate melt is a strong 

function of pressure. For example, the reciprocal solubilities of these volatiles in fluid saturated haplogranite melt 

were determined at two pressures by equilibrating alkali 

chloride melts with silicate melt or by equilibrating Clbearing fluids with silicate melt. The following figure 

shows a reduction in solubility for both volatiles with 

decreasing pressure; 2 kbar data are shown as squares and 

0.5 kbar as circles. 
... ..... Q,l 6 

~ 

.e 

0 

N 

:c 

= 

O.Skbar 

~ --- Q,l 

~ 

... 

..... 

~ Q,l 

~ 0 0.1 0.2 0.3 

Weight Percent Cl in Melt 

Consequently, highly-evolved (F-poor) felsic magmas at 2 

kbar will exsolve a fluid phase (i.e., an alkali chloride 

melt) with as little as 1 wt % H20 in melt if at least 0.28 

wt % Cl is also present. These results are consistent with 

recent studies of glass inclusions from barren and mineralized felsic volcanic systems which indicate that many 

rhyolite magmas contain from 0.2 to greater than 0.5 wt 

% Cl prior to eruption. 

The reciprocal solubilities of H20 and Cl are also a 

strong function ofF, Ca, Na, Mg, and A1 in melt. For 

example, H20-deficient, F-bearing topaz rhyolite melts 

(crosses) dissolve significantly more Cl than anhydrous 

haplogranite melt; the solubility of Cl in haplogranite melt 

is one-fourth that of topaz rhyolite melt containing > 7 wt 

% F. Chloride solubilities at 2 kbar and 800 - 1075°C are 

shown in the following figure. 

'ii 

::e 0.04-----------. 

cu 7.8 wt% Fin "B topaz rhyolite ..... = 0.03 en 

~ 

0 

{I} 0.02 l 

0.01 

0.00 -1-----.------,,..--.......----1 

0.20 0.30 0.40 0.50 0.60 

Moles (Al+Na+Ca+Mg/Si) in Melt 

Abstracts 161 

In addition, H20-deficient, Ca- and Mg-rich (F-poor) melts 

(triangles) also exhibit enhanced Cl solubility relative to 

haplogranite melt; Cl solubility in latite melt is about 4 

times that in haplogranite melt under similar conditions. 

Extrapolation of these experimentally-determined Cl 

and H20 solubilities to the higher pressures required for 

the generation of felsic magmas by partial melting in the 

lower crust suggests that the solubilities of both volatiles 

in such melts will be quite high. Hence, it follows that 

Cl-enriched fluids may not be stable in the presence of 

felsic silicate melt at these conditions. Conversely, as 

felsic magmas begin their ascent toward the surface and 

subsequently undergo magmatic differentiation, supercritical Cl-bearing aqueous fluids, coexisting brine plus 

vapor, and/or marginally hydrous alkali chloride melts 

may exsolve even if the melts contain only minimal H20 

contents. Thus, exsolution Cl-enriched fluids may occur 

early, after either minimal fractional crystallization of 

H20- and Cl-poor minerals or after minimal ascent and 

pressure reduction. 

These results are also applicable to processes of 

magmatic-hydrothermal ore deposition because they are 

compatible with the presence of salt-rich fluid inclusions 

(exhibiting magmatic homogenization temperatures) in 

many molybdenum- and copper- mineralized porphyritic 

magmas. Such inclusions imply that highly saline fluids 

do exsolve directly from mineralizing, felsic magmas; 

saline fluids are not generated solely as a result of fluid 

boiling 

ORAL PRESENTATION 

Diapirism of Crustal Magmas 

Weinberg, Roberto F. 

Research School of Earth Sciences, 

Australian National University, 

Canberra, ACT 0200, Australia 

There has been considerable discussion surrounding the 

mechanisms of transport of felsic magma through the 

continental crust. This work questions the recent tendency 

in the literature to attribute magma ascent mostly to dykes, 

and the emphasis given to their relation to plutons and 

shear zones. It first discusses the difficulties involved in 

the propagation of felsic dykes away from the source into 
162 The Origin of Granites and Related Rocks 

cold crust, and then describes recent results which suggest 

that diapirism may be an efficient mechanism of magma 

and heat transport through the lower crust. 

Several segregation mechanisms may operate within the 

magma source region: a) upwards porous flow; b) magma 

overpressure and cataclasis when external strain rates are 

faster than the rate with which the melt can flow out of the 

system (Dell' Angelo and Tullis, 1988); c) flow of magma 

towards low pressure sites (Robin, 1979; Stevenson, 

1989). These mechanisms give rise to melt pockets, veins 

and magma-filled fractures oriented mainly parallel to the 

direction of maximum compressional stress. Although 

melt segregation may occur within the source region, 

Rubin (1993) suggested that crustal (felsic) melts might be 

unable to leave the source region as dykes because the 

high viscosity magma freezes faster than dykes propagate. 

If this is the case, the source becomes increasingly buoyant 

and may start to dome. Doming initiates preferentially in 

the low pressure areas where melt has accumulated, and as 

it proceeds melt flows preferentially to the top of the dome 

to form a granitic cap underlain by restites and low-melt 

migmatites. It is important to stress that diapirism does 

not require the source to reach the rheological critical melt 

percentage before it is able to start rising, in fact even a 

solid buoyant body may rise diapirically. Shear zones are 

generally weak areas softened by strain. If major crustal 

shear zones (active or inactive) cut across a partially 

molten region submitted to regional stresses, they will 

attract melt from the surroundings because of their low 

pressure. In this way shear zones may localise the sites of 

diapir initiation and provide a preferential path for magma 

ascent, thereby controlling the ascent without necessarily 

having any influence on the ascent mechanism or on the 

creation of space for the magma. 

It has been considered that diapirs rise too slowly to 

travel a significant distance through the crust before 

freezing. However, assuming power-law rheology of 

crustal rocks, low effective crustal viscosity allows much 

faster ascent than that predicted for Newtonian crust, thus 

allowing diapirs to rise to mid-crustal levels prior to 

solidification (Weinberg and Podladchikov, 1994 and in 

press). Fast diapirs are able to pull the lower crust and 

the isotherms upwards. A dry lower crust may then melt 

due to diapir-driven decompression. The buoyancy of the 

wall rocks due to partial melting will further enhance the 

diapir's velocity and its ability to penetrate the crust. A 

combination of power-law crustal rheology and decompression melting of the surrounding might greatly enhance the 

effectiveness of diapirs in transporting magmas through the 

crust. Despite recent developments in the theory of 

diapirism, field evidence has yet to identify their ascent 

path. The difficulty in determining the pathways may be 

due to a combination of relatively rare exposure of lower 

crustal sections with the often observed complexity of 

structures in these terranes and the interaction between 

structures related to the passage of diapirs and syn- and 

post-emplacement regional structures. The steeply plunging kilometre-scale sheath folds mapped in the granulite 

terrane of central Australia (Goscombe, 1992) may 

conceivably represent such structures. 

Dell' Angelo, L. N. and J. Tullis, 1988. Experimental deformation of 

partially melted granitic aggregates, J. metamorphic Geol., 6, 495-

515. 

Goscombe, B., 1992. High-grade reworking of central Australian 

granulites. Part 1: Structural evolution. Tectonophysics, 204, 361-

399. 

Robin, P.-Y., 1979. Theory of metamorphic segregation and related 

processes, Geochim. Cosmochim. Acta, 43, 1587-1600. 

Rubin, A. M., 1993. On the thermal viability of dikes leaving magma 

chambers, Geophys. Res. Lett., 20, 257-260. 

Stevenson, D. J., 1989. Spontaneous small-scale melt segregation in 

partial melts undergoing deformation, Geophys. Res. Lett., 16, 1067-

1070. 

Weinberg, R. F., and Podladchikov, Y., 1994. Diapiric ascent of 

magmas through power-law crust and mantle, J. Geophys. Res., 99, 

9543-9559. 

Weinberg, R. F., and Podladchikov, Y. The rise of solid-state diapirs, 

J. Struct. Geol., in press. 

Diapirism and Decompression Melting 

Weinberg, Roberto F. 

Research School of Earth Sciences, 

Australian National University, 

Canberra, ACT 0200, Australia 

In a dry crust, rapid decompression caused by the 

upward drag of a rising diapir may lead to partial melting 

of the surrounding rocks. These rocks become buoyant 

and accelerate the original diapir, further enhancing 

decompression melting in a "runaway" process, that ends 

only when the diapir reaches cold crust and decelerates due 

to loss of buoyancy caused by freezing and high crustal 

viscosity. Decompression melting depends on the buoyancy of the diapir, the thermal structure of the crust and 

the solidus curve of the wall rocks, and considerably 

enhances diapir penetration through the crust. In contrast 

with the hot-Stokes models, in which thermal softening of 
the wall rocks accelerates the diapir but leads also to rapid 

freezing of the diapiric magma, decompression melting 

accelerates the diapir without rapidly consuming its 

thermal energy. In fact the diapir may regain some of the 

thermal and mechanical energy lost during ascent when the 

surrounding rocks become buoyant due to melting. 

Diapirism and decompression melting are studied here 

numerically, and assuming an initial warm, molten diapir 

within solid lower crust (Fig. 1a). The model crust has 

Newtonian viscosity, which increases upwards by four 

orders of magnitude due to the geothermal gradient. The 

temperature at the base of the crust equals the solidus 

temperature of the model crust and decreases upwards 

slower than the crustal temperature. This means that, with 

the exception of the initial diapir, the whole crust is solid. 

The volume of partially molten crust and the depth at 

which the diapir freezes are presented in dimensionless 

parameters based on the buoyancy of the diapir, the 

rheology of the crust, the geothermal gradient and solidus 

curve, and absolute temperature at the base of the crust. 

It is shown that a diapir may treble its volume by partially 

melting the wall rocks (Fig. 1b) and that the increased 

buoyancy, rather than the decreased viscosity of the wall 

rocks, allows diapirs to travel much further through the 

crust before freezing. Most melting occurs below the 

diapir, a region that is heated during the passage of the 

diapir, but more importantly, where hot lower crustal 

rocks are dragged upwards by the diapir. If the partially 

molten crust is denser than the diapir, as would be the case 

for most granitoid diapirs, the partially molten crust 

remains below the diapir (Fig. 1b), and only if segregation 

occurs will the newly formed melt rise to join the original 

diapir. If, on the other hand, the partially molten crust is 

lighter than the diapir (e.g. if there are large amounts of 

melt in the wall rocks of a relatively dense tonalite/diorite 

diapir), the partially molten crust will invade the diapir 

from below and occupy its core. The results suggest that 

the buoyancy added to the diapir by decompression melting 

of the surroundings may be much more important in 

enhancing diapiric ascent of magmas than thermal softening of the wall rocks. Furthermore, the results show how 

the volume of crustal melt produced may depend not only 

on the thermal structure of the crust but also on the 

dynamics of magma emplacement. 

Abstracts 163 

a) 

[ J 

b) 

Figure 1. a) The starting geometry of the numerical 

model. The black circle is the initial diapir, the arrow 

shows the ascent direction and the lines indicate isotherms 

controlling the upward increase in viscosity. b) The diapir 

causes partial melting of the crust along its tail (grey area) 

partly due to heat transferred to its wall rocks during 

passage, but mainly due to upward drag of the hot lower 

crust. The partially molten crust in this model is lighter 

than the solid rocks but denser than the diapir. The lateral 

spread of the original diapir is due to the upward increase 

in viscosity and to the addition of the buoyant partially 

molten crust to its tail. 
164 The Origin of Granites and Related Rocks 

ORAL PRESENTATION 

Mafic-Silicic Layered Intrusions 

Wiebe, R.A. 

Department of Geosciences, 

Franklin and Marshall College, 

Lancaster, PA 17604, USA 

Plutonic complexes that consist of interlayered mafic 

and silicic rocks, "mafic-silicic layered intrusions" 

(MASLI), are common and occur in a wide range of 

tectonic settings. Recent work on the coast of Maine and 

a review of earlier work indicate that these complexes 

have generally formed by multiple injections of basaltic 

magma into floored silicic magma chambers rather than by 

multiple sill-like injections of silicic magma into preexisting, partly-solidified mafic rocks. Some MASLI are 

basin-form - up to 3 km thick and 30 km in diameter. 

Most appear to have been emplaced at shallow crustal 

levels. Field relations, petrography and geochemistry 

indicate that the silicic layers tend to be cumulates and that 

the basal parts of many mafic layers are quenched basaltic 

liquids. MASLI appear to be the plutonic expression of 

composite magma chambers that have long been inferred 

from the study of silicic volcanic rocks. 

MASLI typically contain macrorhythmic units, from 

less than one to several 1 Os of meters thick, characterized 

by a chilled gabbroic base that grades upward to mediumgrained gabbro, diorite or highly evolved silicic cumulates. 

Each chilled base records the introduction of new basaltic 

magma into the magma chamber. The top of each unit is 

truncated by the chilled base of the overlying unit. 

Cumulates beneath the chilled base of a macrorhythmic 

unit vary widely in composition, suggesting that the level 

at which the basaltic magmas ponded was controlled by a 

rapid inward drop in crystallinity and viscosity rather than 

by neutral buoyancy. Density inversion is demonstrated 

by load-cast structures which occur at the bases of many 

units and silicic pipes and veins which extend from the 

underlying unit into the base of the overlying unit. Where 

the input of new mafic magma was small, gabbro typically 

occurs as discontinuous zones of chilled pillows. 

Thick macrorhythmic units that grade upward from 

chilled gabbro to highly silicic cumulates preserve a record 

of double-diffusive convection with stably stratified, hotter, 

denser mafic magma beneath silicic magma. The lower 

gabbroic rocks are commonly hornblende-rich with large 

corroded biotite crystals; the upper silicic rocks of many 

units are relatively anhydrous. Intermediate rocks provide 

evidence for mechanical mixing of crystals and selective 

exchange of H20, alkalies and isotopes. Crystallization of 

biotite and hornblende in mafic magma at the boundary 

contributed to exchange of H20 and alkalies. Although 

diffusion and turbulent mixing were probably restricted to 

a thin boundary layer, convection away from that boundary 

transferred the effects of hybridization into the interior of 

both magmas. Systematic variation in (

87Sr/86Sr)i occurs 

over tens of meters in some units. The apparent lack of 

comparable variation in Nd isotopes is consistent with the 

lower diffusivity of Nd. When mafic infusions established 

a long-lasting basal layer of mafic magma, mixing and 

fractional crystallization contributed to producing an 

intermediate layer of magma highly enriched in incompatible elements. 

The compositional and thermal effects of repeated 

infusions of basaltic magma may greatly affect the character of granite that crystallizes from overlying silicic 

magma. In Maine, processes that operated in MASLI 

have had profound and varying effects on the evolution of 

overlying granites. In the Cadillac Mountain granite, 

basaltic infusions converted an 1-type, subsolvus, biotite 

hornblende granite into a hypersolvus, A-type granite. 

The characteristic trace-element enrichments shown by Atype granites were produced by interactions with incompatible element-rich intermediate magmas that evolved near 

the base of the chamber. In the Gouldsboro granite 

repeated injections of basaltic magma mainly promoted 

exchange of alkalies and led to the development of a low-K 

(trondhjemitic) magma layer beneath normal granitic 

magma. The contrasting responses of these two silicic 

systems may be largely related to differences in the rates 

of influx of mafic magma. 

MASLI demonstrate that basaltic infusions into silicic 

magma chambers can significantly effect the thermal and 

chemical character of granites even in shallow-level 

chambers. If comparable interactions occur at deeper 

crustal levels, it is possible that selective thermal, chemical 

and isotopic exchange may be even more efficient. 

Because the mafic magmas crystallize first and relatively 

rapidly, silicic magmas that rise away from deep composite chambers may eventually show little obvious record of 

their prior involvement with mafic magma. In addition, 

heat transfer from mafic to silicic magma may play an 

important role in permitting crystal-poor silicic magmas to 

reach high crustal levels. 
Enclave-Rich Zones in the 

Gouldsboro Granite, Maine: 

A Record of Eruption and 

Compositional Stratification in a 

Silicic Magma Chamber 

Wiebe, R. A. and Adams, S. D. 

Department of Geosciences, 

Franklin and Marshall College, 

Lancaster, PA 17604, USA 

The Gouldsboro granite is a shallow-level Silurian 

intrusive complex, roughly 350 km2 in area, located on the 

Maine coast. Except for a heterogeneous mixture of 

granitic, hybrid, and mafic rocks near the base of the 

complex, it consists of leucocratic, biotite hornblende 

granite. In the highest-level exposures of the granite there 

are zones up to 300 meters in width and at least 1 km in 

length that contain roughly 30 to 50 percent of chilled 

intermediate to silicic globular enclaves (ranging in 

diameter from tens of ems to several meters) in a matrix 

of granite. In the largest of these zones, there are two 

different types of enclaves (high-K20 and low-K20) with 

identical Si02• For most major and trace elements plotted 

against Si02, both enclaves and host granite produce 

coherent trends with Si02 ranging from 65 and 71 % in the 

globules and from 71 to 78% in the granite. The 

compositional variation of these enclaves ·and the host 

granite strongly suggest they evolved within the same 

chamber. The low-K enclaves are concentrated in the 

central axis of the zone, while the high-K enclaves are 

dominant in the margins. Scarce chilled pillows and 

angular fragments of basalt occur only in the low-K 

enclaves. Some of these are unusually rich in K20. Most 

enclaves contain some amygdules filled with feldspar and 

quartz, and the basaltic inclusions are weakly vesicular. 

Some enclaves are composite, consisting of both high-K 

and low-K types separated by a sharp but highly irregular, 

crenulate boundary. This zone of enclaves appears to be 

the locus of an eruption from within the chamber through 

its partly crystallized granitic roof, and the enclaves 

probably preserve a record of magma compositions within 

the chamber at the time of eruption. On the basis of 

models for magma withdrawal, the distribution of the two 

types of enclaves suggests that the chamber was 

compositionally stratified with low-K magma below high-K 

magma. The restriction of scarce chilled basalt inclusions 

to the low-K type suggests that basaltic magma entered the 

Abstracts 165 

base of the chamber shortly before the eruption and may 

have triggered it. 

The main compositional differences between the two 

types of silicic magmas are inK, Na, Rb, and Ba concentrations. Each group shows a fairly wide range in alkalis, 

and the compositional gap between the two groups is 

nearly bridged by a few samples. The high-K enclaves 

have alkali contents that are comparable to the dominant 

host granite. The low-K (trondhjemitic) enclaves have 

alkali contents that are matched only by a small volume of 

high-Si02 , very low-K trondhjemite that seems only to 

occur in the vicinity of enclaves. Basaltic inclusions in the 

low-K enclaves are commonly K-rich. These relations 

suggest that the low-K magma could have been derived 

from the high-K magma by selective exchange of alkalies 

between mafic and silicic magmas. 

More mafic, chilled enclaves dominate another zone of 

similar size in the granite. These have Si02 that varies 

from about 54 to 59%; their major- and trace-element 

compositions form very tight trends against Si02 except for 

alkalies which vary widely at a constant value of Si02 

(e.g. K20 varies from 0.4 to 1.6% at about 56% Si02). 

In terms of most elements, these mafic enclaves have 

compositions which suggest they evolved largely by 

fractionation from a basaltic parent. The host granite near 

these enclaves has highly variable K20 and ranges from 

normal granite to trondhjemite with very low K20. The 

compositional variation of alkalies in both enclaves and 

granitic host can best be explained by selective exchange 

during commingling. 

The layer of low-K (trondhjemitic) silicic magma that 

developed near the base of chamber was probably generated by alkali-exchange between resident silicic magma 

with normal K20 and multiple injections of mafic magma. 

On a small scale, comparable exchange of alkalies is 

commonly observed at the margins of chilled pillows in 

granite. With many replenishments of basaltic magma into 

a silicic chamber, it may be possible to generate a hotter, 

drier (and, hence, denser) layer of low-K silicic magma 

between stratified mafic and granitic magmas. This 

process may play an important role in the origin of some 

trondhjemites. 
166 The Origin of Granites and Related Rocks 

Testing Fault-Controlled Magma 

Emplacement Mechanisms 

Yoshinobu, A.S., Okaya, D.A., 

Paterson, S.R. and Fowler, T.K. 

Department of Earth Sciences, 

University of Southern California, 

Los Angeles, CA 90089-0740, USA 

When shear zones are mapped in or around granitoids, 

is it reasonable to conclude that these faults produced the 

space now occupied by the pluton? The preponderance of 

fault -controlled magma emplacement models in the current 

literature indicates that this model is accepted as a viable 

mechanism for magma emplacement in arcs. However, 

no adequate tests have been carried out to establish the 

validity of the above hypothesis. In order for a fault or set 

of faults to create space for magma emplacement, a 

number of structural conditions must be met which include 

the following: 1) faults should exist that bound the plutons; 

2) these faults should have significant displacements 

relative to the size of the pluton and continue into the 

country rocks far beyond the structural aureoles of the 

intrusions; 3) it should be possible to "balance" the map by 

removing the pluton and restoring the country rocks along 

the faults back to their initial pre-emplacement configuration; 4) where exposed, the roofs (and floors) of the 

magma chambers should be highly extended or detached 

by sub-horizontal shear zones; 5) the pluton orientation 

should have a predictable relationship with arc-scale faults; 

and, 6) the rates of processes must be compatible with slip 

rates in modem arcs. 

We can also test the fault-controlled hypothesis 

utilizing 1-D and 2-D finite difference thermal modeling 

which constrains the thermal requirements needed to create 

and maintain a chamber during the successive emplacement 

of magma into space created by the simultaneous displacement of country rock along faults. The results of our 

preliminary modeling indicate that early crystallized 

sheeted complexes should form along the margins of 

plutons emplaced by dilation within fault zones, consistent 

with published modeling. 

Do real plutons meet these thermal and structural 

conditions? We are currently examining the spatial and 

temporal relations of proposed space-making faults in the 

Late Cretaceous Sierra Nevada batholith, California and 

the Permian Colanguil batholith, northwestern Argentina, 

using both field data and thermomechanical modeling. 

Voluminous granitoid intrusive suites along the eastern 

edge of the Sierra Nevada in the age range of 80-90 Ma. 

have been hypothesized to be emplaced into a) tensional 

cracks and dilational jogs within arc-parallel strike slip 

faults, or b) zones of dilation bridging en echelon p-shear 

fault arrays within the upper crust. However, based on 

published mapping and our own reconnaissance field work, 

we conclude the following: 1) plutons within these 

intrusive suites are locally to regionally discordant to 

country rock markers; 2) the plutons are irregularly 

shaped; 3) the proposed space-making faults generally do 

not completely bound the plutons, nor do they show 

evidence of large displacements; 4) locally preserved roof 

rocks are not extended nor detached; 5) the granitoids do 

not have significant sheeted border phases; and, 6) country 

rock markers cannot be reconstructed by removing the 

plutons and restoring the faults. 

Plutons within the Colanguil batholith have been 

postulated to be emplaced during Permian trench-parallel 

arc extension. We have examined two of these plutons in 

detail, and a third in reconnaissance fashion; all were 

emplaced into an older fold-and-thrust belt at shallow 

depths. Although these plutons locally contain 

syn-magmatic dikes parallel to the long dimension of the 

magma chamber ( - 3 % of the total map area in one 

pluton), our mapping indicates that: 1) country rock roofs 

capping the plutons are neither detached nor extended; 2) 

no significant faults of the appropriate age exist in the field 

area; 3) no marginal sheeted phase was recognized as 

predicted by the thermal modeling; and, 4) country rock 

markers cannot be reconstructed. 

The discordant nature of these batholiths indicates that 

stoping has played an important role at least during the 

final emplacement of the magmas into the upper crust. 

Although it is permissible that stoping by younger intrusive 

pulses could effectively "erase" earlier evidence of marginal sheeting, we note that the inability to reconstruct 

regional country rock markers and the lack of significant 

pluton-bounding faults with large displacements suggest to 

us that faults did not produce the space for these magmas. 

We contend that the relationships described above are 

widespread in arcs. The resulting implication is that the 

majority of country rock material displaced by the plutons 

moved downwards rather than laterally along intra-arc 

faults. 
One-Dimensional Thermal Modelling of 

Crustal Anatexis in an Extensional Regime 

Zen, E-an 

Department of Geology, University of Maryland 

at College Park, Maryland 20742, U.S.A. 

One-dimensional thermal modelling using the 1 DT 

program (Haugerud, 1986) was used to test the idea that 

mid-crustal anatexis of fertile sedimentary material in an 

extensional environment could produce large volumes of 

granitic melt. The actual Neogene-to-Recent crustal, 

tectonic, and thermal conditions of the Basin and Range 

Province (B&R) of western United States guided the 

modelling. Table 1 compares the model parameters with 

data for B&R (Zen, 1995). 

Table 1. Model Parameters and Basin and Range Data 

MODEL B&R 

Thermal Conductivity 2.0 W/rn/K 

Heat Capacity 900 J/K/Kg 

Rock Specific Gravity 2780 kg/m3 

Depth to mid-crustal 22, 26, or 32 15-25 

discontinuity, km 

Duration of Extension, Ma 30 >20 

Extensional Strain Rate 3%/Ma -3-10%/Ma 

(lx10-15/sec) 

Heat Productivity, A, 2or3 <3, mostly -2 

uW/m3 

Productivity /Depth linearly decreasing 

Relations to 0 at, or constant 

to, 20/30 km 

Subcrustal Flux, mW/m2 

Initial 30 

Maximum 60 or 75 60-80 

Surface Heat Flow at 80 to 165 mW/m2 c. 90 mW/m2

; 

Maximum Flux Battle Mt. High, 

120-160 mW/m2 

Time Interval for Adjusting 2.0Ma 

Thermal Profile 

Final Monitor Depth 30 km or rises 

with deletion 

Extension is simulated by 15 iterations of deletion, 

mostly at 2 Ma intervals, of a 2-km cell from an initial 

60-km thickness. The deleted material is accounted for by 

linear extension mostly at a strain rate of 1x10-15s-1• The 

heat productivity, A, specified at the surface, either 

remains constant to a depth Z' (Z' = 20 or 30 km) or 

tapers linearly to 0 at that depth (nearly all the runs used 

tapered A). The subcrustal flux is increased by a constant 

amount with every deletion to attain its peak value, 

Abstracts 167 

corresponding to the effect of mafic underplating and 

thinning of the subcrustal mantle. The monitor depth 

(-base of the crust) either is stationary or rises with 

deletion; after termination of deletion the flux either 

remains at the peak value or drops back toward the 

pre-extension value. 

Melting is stepwise, with the parametric heat of 

fusion, hr defined as .!lHrfCp = 300°C, i.e., 1% melting 

equals 3 degrees of virtual cooling. The incremental 

melting scheme is: 700°C (solidus), 15%; 740°C, 20%; 

780°C, 15%; 820°C, 10%; 860°C and 900°C, 5% each. 

The volumes of melts generated for 40 runs are given in 

Table 2; the magma volumes would be augmented by 

lateral pooling, residual material, and melt formed below 

the deletion zone. 

Table 2. Percentages of Melt Volume, in km3

/km2, 

Formed Above Deletion Depth 

Volume 0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 

Range 

End of 32.5 22.5 17.5 17.5 5.0 5.0 0 0 0 

Deletion 

18 Ma later 17.5 12.5 25.0 10.0 7.5 10.0 7.5 5.0 5.0 

Prolonged extension using thermal, tectonic, and 

melting properties that are geologically realistic can lead 

to voluminous anatexis of fertile material at pressures as 

low as 4 kbar provided that fertile source material is 

available at the proper depths. 

I applied the results to the granitic plutons of the 

Lachlan Belt (LB), southeastern Australia. There, the 

strongly peraluminous, S-type granite, granodiorite, and 

rare tonalite must have been derived from fertile sedimentary source rocks. The An contents of the residual cores 

("restitic") and magmatic overgrowth of plagioclase are 

demarked at about An50 and suggest melting within a 

temperature range of c. 1500C; rare An80 cores could mean 

that the source layer was as much as 3 km thick. A 

mid-Paleozoic extensional episode lasting a few tens of Ma 

for LB is controversial but permissive. If the process did 

happen, then the model-predicted steep average thermal 

gradient of c. 50°Cikm would explain the in situ anatexis 

at Cooma (pressure above the Al-silicate triple point). The 

model also provides an alternative explanation for the I-S 

line as the mid-crustal boundary of the limit of extension. 

In the model, the upper crust is passive and stationary 

and the pressure-temperature-time paths are isobaric lines. 

Results of many model runs predict progressive 

low-pressure, high-temperature metamorphism for the 

upper crust as the P-T-t paths traverse through the 
168 The Origin of Granites and Related Rocks 

andalusite field into the sillimanite field, and may even 

reach the breakdown of muscovite+quartz in equilibrium 

with plagioclase (the second sillimanite isograd). 

Haugerud, R.A., 1986. lDT- an interactive, screen-oriented microcomputer program for simulation of !-dimensional geothermal histories: 

U.S. Geological Survey, Open-file Report 86-511. 

Zen, E-an, 1995. Crustal magma generation and low-pressure, 

high-temperature regional metamorphism in an extensional environment: possible application to the Lachlan Belt, Australia. Am. J. 

Sci., 295, in press. 

Anatomy and Genesis of 

Shuiximiao 

Asymmetrically Layered 

Pegmatite-Aplite Dikes 

Zhu, Jinchu, Rao, Bing and Xiong, Xiaolin 

Department of Earth Science, 

Nanjing University, 

Nanjing 210008, China 

The Sn-Ta bearing Shuiximiao granite of Guangxi 

Autonomous Region, South China, is a concealed granite 

cupola of Early Jurassic age. It is hosted by a Carboniferous limestone sequence and characterized by a well 

expressed vertical zonation. From the exocontact downwards to the endocontact within a distance of some 

400-500 m, the following zones are successively developed: 1) lepidolite -fluorite stringers; 2) Sn/W bearing (K 

feldspar-) quartz veins; 3) Sn/Ta bearing pegmatite - sodic 

aplite dikes and pegmatoid stockscheider; 4) Sn/Ta bearing 

albite-rich topaz granite; 5) albite granite; and, 6) 

microcline-albite granite. Because the zones 2, 3 and 4 

are mined for Sn, W, Ta and Nb by underground works, 

the relationship between the dikes and granite cupola is 

very clear. 

Several subparallel NS trending pegmatite-sodic aplite 

dikes are rooted in the roof part of the albite-rich topaz 

granite and pegmatoid stockscheider of the cupola. They 

are gently to steeply dipping, tabular in shape, >200 m 

long along the strike, > 100 m deep and 2-20 m thick. 

The dikes are mostly asymmetrically layered. The 

footwall portions are usually the albite-rich granitic aplite 

consisting of Ab, Qz, Kf, Top and mica; the hangingwall 

portions are usually the granitic pegmatite consisting of 

Qz, Kf, Ab, mica and Top; the middle portions are usually 

the alternating bands of sodic aplite and pegmatite up to 

about one hundred layers. Sometimes porphyritic finegrained aplite with Kf phenocrysts are also developed. 

The average aplite versus pegmatite volume ratio is about 

1:1. 

Multiphase juxtaposed composite intrusives, igneous 

gravitational crystal settling, metasomatism, magmatic 

flowage and stressed dynamic metamorphism all may be 

considered as the main controlling mechanism for formation of this kind of dikes. A combination of the following 

factors is considered to be responsible for their genesis: 1) 

the parental magma of the dikes is derived from the Abrich topaz granite melt at the top of the cupola which is the 

late differentiate of granite fractionation, rich in N a, H20, 

F, Li, Ta, Nb, Sn, W, but not necessarily be wateroversaturated; 2) At certain degree of undercooling and a 

range of water content, the Ab + Q assemblage is usually 

the first liquidus phases of granite crystallization yielding 

sodic aplite at the footwall portions for gentle to normal 

dikes; 3) fluctuation of F content in the melt and episodic 

crystallization of topaz would result in consequent change 

of eutectic composition of the moving crystallization front. 

Disequilibrium crystallization of the residual granitic melt 

with changing composition is responsible for the altemating layering; and, 4) the above-mentioned hypothesis can 

be used only to interpret the compositional layering. The 

frequently existing textural layering is believed to be 

related to the crystallization kinetics of the residual granite 

melt which is poorly understood at the present. Further 

experiments at proper temperatures, pressures, water 

contents and initial compositions are critically needed. 
Author Index 169 

AUTHOR INDEX Clemens, John D. 52,126,127 Guimaraes, I. P. 60 

Cocherie, Alain 36,128 Haapala, I. 61 

Abbey, W.J. 7 Cole, David R. 10 Halliday, A.N. 152 

Acosta, Antonio 7 Coler, D.G. 132 Hammer, J. 46,62 

Adams, S.D. 165 Collins, W. 37,38,79,86 Hammouda, T. 116 

Ahall, K.-1. 39 Creaser, Robert A. 27,32,38 Hanchar, J.M. 158 

Al-Saleh, Ahmad 8 Crossland, A. 57 Hanson, R. Brooks 63 

Aleinikoff, J .N. 7,149 Cruz, M.J.M. 49 Harley, S.L. 64 

Allen, Charlotte M. 25 Dall'Agnol, Roberto 135 Harris, Nigel 86 

Anderson, J. Lawford 9,93 Dalla Salda, L.H. 91 Heizler, M.T. 18,95 

Anovitz, Lawrence M. 10,24 Darbyshire, D.P.F. 39,138 Herve, F. 111 

Arth, J.G. 11 DaSilva Filho, A.F. 60 Hess, K. 42 

Atherton, M.P. 73 Davies, G.R. 65,117 Heumann, A. 65 

Ayuso, R.A. 11 Davis, A. 40 Himmelberg, Glen R. 29 

Bagaric, Steve 107 Dean, A.A. 41 Hodkinson, D. 66 

Baker, Leslie 12 Defant, Marc J. 44 Hogan, J.P. 67,68 

Baldo, E. 120 Delaloye, M. 50 Holden, Peter 18 

Barbarin, Bernard 13 Del Moro, A. 129 Holtz, Francois 70 

Barboza, Scott A. 14 Dingwell, D.B. 42 Hook, S.J., 95 

Bargossi, G.M. 129 D'Lemos, R.S. 42,113 Horak, Jana M. 71 

Barnes, C.G. 15 Drinkwater, James L. 29 Horita, Juske 10 

Barovich, Karin M. 76 Drummond, MarkS. 44 Hutson, F.E. 72,149 

Barr, Sandra M. 16 Duchesne, J. C. 45 Hutton, D.H.W. 98 

Barton, Mark D. 17,18 Duthou, Jean-Louis 58 Jackson, M.D. 73 

Bea, Fernando 19,20,51 Eidam, J. 46,62 Jacques, J.M. 74 

Beard, James S. 21,112 Elburg, Marlina A. 47 James, E. 15 

Bergantz, George W. 14,21 Eleftheriadis, G. 105 Jensen, Charles 106 

Berza, T. 45 Enrique, P. 48,50 Johannes, Wilhelm 74 

Blakey, Suzanne 23 Falkner, C.M. 95 John, Barbara E. 75 

Blencoe, James G. 10,24 Fallick, A.E. 49 Johnson, Clark M. 76 

Blevin, Phillip L. 25 Fanning, C.M. 56 Jovic, V. 53 

Blundy, Jonathan D. 75 Ferreira, V.P. 49,141 Kalakay, Thomas J. 77 

Bonin, Bernard 26 Ferres, M. 50 Karlstrom, K.E. 110 

Bouchez, Jean-Luc 100,105 Fershtater, G.B. 51 Kawano, Yoshinobu 78 

Brandon, Alan D. 27 Flinders, James 52 Keay, S. 79 

Braun, I. 28 Flood, R.H. 53 Kepezhinskas, Pavel K. 44 

Brew, David A. 29 Ford, Arthur B. 29 Ketcham, D. H. 79 

Brewer, T.S. 39 Forizs, I. 53 Khalili, M. 80 

Brown, Michael 30,66,142 Forster, H. -J. 54,147 Kimbrough, D.L. 153 

Bryant, Colleen J. 25 Foster, D.A. 56 King, T.R. 42 

Bums, Ian M. 159 Fowler, T. K., Jr. 57,111,166 Kloetzli, U.S. 82 

Caggianelli, A. 129 Fujitani, Tatsuya 144 Knoche, R. 42 

Candela, P.A. 31,115,121 Galan, Gumer 58 Kohut, M. 81 

Carrington, D. 64 Gallego, Marcos 90 Koller, F. 82 

Carroll, William 106 Gasparotto, G. 129 Komatsu, Masayuki 139 

Chacko, Thomas 27,32 Ghidotti, Gregory 18 Kotov, A.B. 81 

Chappell, Bruce W. 25,34 Gibbons, Wes 71 Kovach, V. P. 80 

Cheadle, M.J. 73 Gilbert, M.C. 68 Krogstad, E.J. 40,66, 150,151' 156 

Chown, E.H. 35 Glascock, Michael D. 103 Kubo, Kazuya 83 

Christiansen, Eric H. 35 Goodwin, Laurel 18 Kumar, Santosh 83 

Christofides, G. 105 Gray, Karen J. 59 Kutsukake, Toshio 84 
170 The Origin of Granites and Related Rocks 

Lahtinen, Raimo 85 Piccoli, P.M. 115 Thirlwall, M.F. 23 

Landenberger, B. 86 Pichavant, Michel 116,135 Thomas, R. 145 

Larson, S.-A. 39 Pin, Christian 58 Thompson, Alan Bruce 93,146 

Law, Richard 100 Pinarelli, L. 129 Tischendorf, G. 147 

Lee, Y.F.S. 95 Poli, G. 117,152 Tobisch, Othmar T. 94,148 

LeFort, Patrick 86 Price, J.D. 68 Todd, V.R. 139 

Leslie, Karen 87 Pupin, Jean-Pierre 118 Tollo, R.P. 7,72,149 

Liegeois, J.P. 45 Ramo, o. Tapani 61,109 Tomascak, Paul B. 150,151 

Litvinovsky, B.A. 88 Rao, B. 168 Tommasini, S. 117,152 

London, David 89,90,99 Rapela, C.W. 119,120 Tong, Weixing 94 

Long, L.E. 79 Ratajeski, K. 121 Toselli, A.J. 141 

LOpez de Luchi, M.G. 91 Reavy, R.J. 74 Tribe, I.R. 42 

Lowenstern, Jacob B. 92 Reed, Marshall 122 Trumbull, R.B. 147 

Luedke, R.G. 99 Reed, Robert M. 124 Tulloch, A.J. 153 

Lund, S.P. 136 Renno, A.D. 125 Ulmer, Peter 93 

Lynton, Stephen J. 151 Rhede, D. 54 Unkefer, J. 95 

Manetti, P. 152 Roberts, Malcolm P. 126,127 Vernon, R.H. 148 

Matile, Lusius 93 Roddick, James A. 128 Vigneresse, Jean Louis 154 

Mattey, David 86 Rossi, Philippe 36,128 Volborth, A. 61 

Mayo, David P. 93 Rossi de Toselli, J.N. 141 Vukov, M. 53 

McNulty, Brendan A. 94,148 Rottura, A. 129 Waight, T. 156 

Menendez, Luis G. 7 Rushmer, Tracy 130,132 Walker, Richard J. 150,151,156 

Menzies, M.A. 23 Rutherford, Malcolm J. 12 Wareham, Christopher D. 157 

Michalko, J. 81 Saavedra, J. 120,141 Watson, E.B. 158 

Millar, Ian L. 157 Saint Blanquat de, Michel 100 Watt, Gordon R. 159 

Miller, C.F. 95 Salnikova, E.B. 81 Weaver, S.D. 102,111 

Miller, J.S. 95 Sampson, S. 132 Webb, S.L. 42 

Miller, Robert B. 97,111 Sawyer, E.W. 38,133,134 Webster, J.D. 160 

Molyneux, S .J. 98 Scaillet, Bruno 70,135 Weinberg, Roberto F. 161' 162 

Montero, Pilar G. 98 Schmidt, K.L. 136 Wickham, S.M. 88 

Morgan, G.B. VI 90,100 Seaman, S.J. 137 Wiebe, R.A. 164,165 

Morgan, Sven 100 Sewell, R.J. 138 Williams, M.L. 137 

Moukhsil, Abdelali 101 Shannon, W.M. 15 Wolf, Michael B. 90 

Muir, R.J. 102 Shaw, S.E. 53,139 Wooden, Joseph L. 93,95 

Nabelek, Peter I. 103,144 Shimura, Toshiaki 139 Wright, J.E. 68 

Nakajima, Takashi 104 Shirey, Steven B. 76 Xiong, X. 168 

Nedelec, A. 105 Sial, A.N. 49,79,141 Yoshinobu, A. 57,166 

Neiva, A.M.R. 105 Singh, J agmohan 74 Zanvilevich, A.N. 88 

N ekvasil, Hanna 106 Smith, D.R. 15 Zen, E-an 167 

Newton, R.C. 88 Snoke, Arthur W. 77 Zhu, J. 168 

Nicholls, Ian 107 Solar, Gary S. 142 

Nironen, M. 109 Sol datos, T. 105 

Nyman, M.W. 110 Sole, J. 50 

0 'Neill, Michael 16 Speer, J.A. 132 

Okaya, D.A. 166 Staudigel, H. 65 

Pankhurst, R.J. 111' 119,120 Stephens, W.E. 105 

Paterson, S.R. 57,97,111,136,166 Stunitz, Holger 75 

Patino Douce, Alberto E. 112 Tatu, M. 45 

Patrick, D.W. 95 Tegeler, J.L. 143 

Pembroke, J.W. 42,113 Terakado, Yasutaka 144 

Petford, Nick 114 Ternes, Kim 144 

*U.S. G.P.D.:1995-387-030:94