Cover--Intrusive relationships in the Sessera Valley, Ivrea-Verbano Zone. Photograph by 

Silvana Sinigoi. 
IVREA-VERBANO ZONE 

WORKSHOP, 1992 

ABSTRACTS 

VARALLO, ITALY 

JULY 20-25,1992 

Edited by 

James E. Quick, U.S. Geological Survey, Denver, CO 

Silvano Sinigoi, Universita di Trieste, Italia 

Sponsored by 

Consiglio N azionale delle Ricerche 

U.S. Geological Survey 

Comune di Varallo 

U.S. GEOLOGICAL SURVEY CIRCULAR 1089 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

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 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1992 

Free on application to the 

Book and Open-File Report Sales 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 
CONTENTS 

Introduction 1 

Acknowledgments 2 

Abstracts 3 

Authors' Address List 25 

Contents iii 

IVREA-VERBANO ZONE WORKSHOP, 1992 

James E. Quick and Silvano Sinigoi, Editors 

INTRODUCTION 

Our understanding of the continental crust-mantle 

transition is based primarily on indirect observation. 

Geophysical data provide constraints on large 

structures and regional variations, and xenolith 

populations and magma compositions provide insight 

into the local chemistry and petrology of this vast 

region. However, there are few exposed terranes 

where this transition may be studied directly. 

The Ivrea-Verbano Zone (IVZ), northern Italy (fig. 

1), is one of the best exposed and most accessible 

lower crustal terranes in the world. It is characterized 

by pronounced Bouguer gravity and magnetic 

anomalies, and seismic ·and potential-field data 

suggest that it dips steeply to the southeast and 

merges with the lower crust beneath the Po basin (for 

example, Lanza, 1982; Wagner, 1984; Kissling, 1984; 

Nicholas and others, 1990). Rocks of the IVZ crop out 

in a northeast-trending, tectonically bounded sliver 

that is more than 100 km long and as much as 15 km 

wide. To the northwest, they are separated from preTriassic basement, ophiolitic rocks, and high-pressure 

metamorphic rocks of the central Alps by the 

Canavese segment of the Insubric Line (Gansser, 1968; 

Schmid and others, 1987). To the southeast, they are 

separated from amphibolite-facies gneiss and late 

Paleozoic granites of the Strona-Ceneri Zone by the 

Cossato-Mergozzo-Brissago and Pogallo Unes 

(Boriani and Sacchi, 1973; Zingg, 1983; Handy, 1987). 

Although the precise relationship between the StronaCeneri and Ivrea-Verbano Zones remains to be 

unequivocally demonstrated, several investigators (for 

example, Mehnert, 1975; Fountain, 1976) have 

proposed that the Ivrea-Verbano and Strona-Ceneri 

Zones collectively represent an exposed cross section 

of continental crust. 

The IVZ displays a possible continental crustmantle transition that is cited as one of the best 

exposed examples of magmatic underplating in the 

world. Three principal lithologic components are 

recognized: (1) mantle peridotite (Shervais, 1979; 

Sinigoi and others, 1980; Rivalenti and others, 1981) (2) 

the Mafic Complex (Rivalenti and others, 1975, 1981, 

1984), which consists of predominantly gabbro and 

lesser amounts of dioritic and felsic rocks; and (3) 

amphibolite- to granulite-facies paragneiss grouped as 

the Kinzigite Formation (Schmid and Wood, 1976; 

Zingg, 1983). Some investigators have concluded that 

the Mafic Complex was magmatically underplated 

beneath continental crust (for example, Rivalenti and 

others, 1981, 1984; Fountain, 1989; Voshage and others, 

1990). According to this view, the lower continental 

crust is represented by the Kinzigite Formation, and 

mantle peridotites near the western margin of the IVZ 

preserve contacts that may be deformed relics of the 

intrusion floor. Geobarometry and mineral 

assemblages suggest that underplating, followed by 

isobaric cooling, occurred beneath a relatively thin 

(<25 km) crust (Sills, 1984; Bohlen and Mezger, 1989). 

This event, best dated by zircon and monazite ages, is 

thought to have occurred between 290 and 250 Ma 

(Voshage and others, 1990). Final rotation of the zone 

into a subvertical orientation and emplacement into 

the upper crust resulted from Alpine transpression 

and lithospheric wedging (Nicolas and others, 1990; 

Zingg and others, 1990). 

The potential of the IVZ as a natural laboratory has 

provoked considerable scientific interest in recent 

years. More than 350 publications have addressed the 

IVZ since 1950, and work in progress includes new 

field mapping and geochronologic, geochemical and 

petrologic, and geophysical investigations. The 

following abstracts present results of some of the most 

recent investigations on the Ivrea-Verbano and 

adjacent Strona-Ceneri Zones. The structure and 

evolution of the underplated Mafic Complex are 

addressed by Brodie and others, Quick and others, 

Meyer and others, and Sinigoi and others, and Evans 

and Burlini report on structure between the Pogollo 

and Cannobina Valleys in the northern IVZ. Boriani 

and others consider the relationship between and 

possible pre-Alpine tilting of the Ivrea-Verbano 

Strona-Ceneri zones in light of geologic, geochemical, 

and radiometric-age data. Ferrario and others and 

Hofmann and others discuss chemical and isotopic 

evidence for crustal contamination and metasomatism 

of IVZ peridotites and the Mafic Complex. New 

isotopic age data are reported by Gebauer and others, 

Ivrea-Verbano Zone Workshop, 1992 1 
Ragettli and others, and von Quadt and others. 

Experimentally determined seismic velocities for 

Ivrea-Verbano rocks are discussed by Barruol and 

Kern, Burlini, and Fountain and others. Zappone 

reports seismic velocity calculations and structural 

aspects of the Balmucda peridotite and, with 

colleague, describes efforts to geophysically probe the 

deep geometry of the Ivrea-Verbano Zone. Barker and 

others, Piccardo and others, Snoke and Fountain, and 

Thornber describe geologic and petrochemical 

characteristics of selected convergent and extensional 

terranes to provide a basis for comparison with the 

IVZ. 

\ 

J 

\ 

\ 

\ 

N 

I 

ACKNOWLEDGMENTS 

We gratefully acknowledge the skill and effort of 

Marsha Simpkins (USGS/Denver), who provided 

invaluable assistance in the preparation of this 

volume. Reviews were supplied by Carl Thornber and 

John Pallister. 

~PERMIAN 

~ VOLCANIC ROCKS 

~PERMIAN 

~ INTRUSIVE ROCKS 

[[[]GNEISS AND SCHIST 

IVREA-VERBANO ZONE 

E ; : :1 MAFIC COMPLEX 

1~/~~~~~q KINZIGITE FORMATION 

1~1 PERIDOTITE 

Figure 1. Geology of southern Alps in vicinity of Ivrea-Verbano Zone based on Zingg (1983). IL, Insubric Une; CMBL, Cossate-MergozzoBrissago Une; PL, Poggallo Une; 8, Balmuccia peridotite; F, Pinero peridotite; A, Aosta Valley; R, Sessera Valley; S, Sesia Valley; 0, Ossola Valley. 

2 Ivrea-Verbano Zone Workshop, 1992 
ABSTRACTS 

THE ACCRETIONARY PRISM AND 

MAGMATISM OF THE GULF OF ALASKA 

Fred Barker, U. S. Geological Survey, Denver, CO, G.L. 

Farmer, University of Colorado, Boulder, CO, George 

Plafker, U. S. Geological Survey, Menlo Park, CA, R. A. 

Ayuso, U. S. Geological Survey, Reston, VA 

Accretionary prisms are important in generation of 

continental crust (1) they are emplaced from surface to 

Moho, so any mantle-derived magma intrudes them; 

and (2) they are fertile in granitic melt--especially those 

prisms rich in graywacke-and all their sediments are 

easily assimilated by gabbroic liquid. Prisms are found 

in the Archean (for example, Quetico Belt) and are 

abundant in the Proterozoic and younger eras. 

The Gulf of Alaska contains a large (100-200 X 2,100 

km), composite prism of Cretaceous to Eocene rocks 

emplaced about 65-50 Ma. This prism was formed by (1) 

accretion of the Wrangellia-Alexander composite terrane 

to southeastern Alaska and British Columbia about 110-

100 Ma, with crustal overthickening; (2) growth of an 

Andean arc in this collage; (3) uplift of this arc of 20-30 

km and outflow of turbidites onto the north-moving 

Kula plate(s); (4) magmatism of arc and MORB--

seamount-transform types into the Kula-with-turbidite 

crust; and (5) offscraping against the 

s 

continental margin with continued basaltic magmatism. 

The Cretaceous part of the prism consists of graywacke, 

pelite, basalt, and an island-arc fragment. The Paleocene 

to Eocene part-the Orca Group-consists of 

quartzofeldspathic graywacke, 15-20% basalt, and minor 

pelagic sediment. 

Granitic rocks formed at 63-53 Ma in the western Gulf 

of Alaska, and at 53-50 Ma in the eastern Gulf. The Orca 

Group rocks, near Cordova, eastern Gulf, contain 

scattered plutons of granodiorite of 5-150 km2 area. 

Rare coeval gabbro plutons and trondhjemite dikes also 

occur. Three granodiorite plutons studied here show 

Si02=66.3-71.3%, Na20=2.8-3.6%, K20=1.8-3.0%, 

eNd=+2.1 to -3.3, 87sr;86sr=0.7051-0.7067, 

206p},;204Pb= 19.04-19.20, 207pb;204Pb=15.60-15.66, 

and 208Pb;204Pb=38.59-38.85. One pluton generally 

shows slightly lower K20, higher Al20J, higher eNd' 

and lower 87sr ;86sr ratios. All three plutons, however, 

have similar, well-defined minor and trace-element 

abundances characterized by relative enrichment in light 

rare earth elements and depletion in high field strength 

elements. 

Schematic present-day section through accretionary prism at Cordova. Overall control by seismic methods, but note that clear signals were 

received only from that part of the Orca Group where reflectors (=r) are shown. Present-day surface about 4-7 km below that at 50 Ma. 

Ivrea-Verbano Zone Workshop.l992 3 
The granodiorite plutons and flysch of the Orca 

Group show overlapping elemental and isotopic 

compositions. The only clearly defined chemical 

differences between the flysch and the plutons are weak 

negative Eu anomalies in the plutons and slightly lower 

Ca and higher Na contents in the flysch. The Nd and Sr 

isotopic compositions of two plutons completely overlap 

those of the flysch. The third pluton, however, shows 

discretely higher eNd and slightly lower 87 Sr ;86sr 

values than those of the flysch. Pb isotopic compositions 

of the flysch and this pluton also overlap, but Pb of the 

other two plutons is slightly less radiogenic. 

Our chemical data, modeling, and comparison with 

melting experiments (Conrad and others 1988) of 

graywacke indicate that the granodiorite originated by 

large fractions (65-90%) of melting of the Orca Group 

graywacke and argillite. Plagioclase, pyroxene(s), and 

biotite(?) were residual to melting at about sso·-9so·c 

and at low H20 activities. The trondhjemitic dikes have 

elemental and isotopic compositions (for example, 

£Nd=+7.9, Sri=0.7036) appropriate to origin from a 

basaltic protolith. 

These granodiorite plutons were generated where the 

prism was 15-20 km thick and while the prism was still 

being deformed. They are wholly flysch derived, do not 

contain a gabbroic component, yet were coeval with 

gabbro and trondhjemite. This magmatism is best 

explained by (1) introduction of basaltic liquid from the 

Kula plate during subduction; (2) pooling of basaltic 

liquid in lower regions of the prism; (3) growth of 

melting-precipitating cells in flysch over convecting, 

crystallizing lenses of gabbroic magma (as in Huppert 

and Sparks, 1988); and (4) requiring lenses of layered 

gabbro 1-3 km thick and thinner layers of flysch-melt 

residua. 

P AND S WAVE VELOCITIES AND SHEAR 

WAVE SPLITTING OF ROCKS FROM 

LOWER CRUST/UPPER MANTLE 

TRANSITION (IVREA ZONE): NEW 

EXPERIMENTAL AND CALCULATED 

DATA. 

G. Barruol, LAboratoire de Tectonophysique, France, and H. 

Kern, lnstitut der Universetiit Kiel, Germany 

To better constrain lower crustal seismic properties 

and particularly the relationships between anisotropy, S 

wave splitting, and kinematics, the seismic properties of 

rocks from the Ivrea lower crust and upper mantle 

(metagabbros, pyriclasites, amphibolites, pyroxenites) 

have been determined. P and S wave seismic velocities 

4 Ivrea-Verbano Zone Workshop, 1992 

have been obtained either by direct velocity 

measurements (performed in laboratory on cubic 

samples at pressure as great as 600 MPa and 

temperature as great as 600"C), and(or) by calculations 

based on the modal composition of the rock, the lattice 

preferred orientations (LPOs), and single crystal stiffness 

coefficients. The 21 elastic constants characterizing each 

sample allow calculation of the complete threedimensional seismic properties (Vp, Vs1, Vs2, Vs1-Vs2 

or seismic birefringence and the polarization planes of 

the shear waves). The two complementary techniques 

have been applied to a number of samples, and 

quantitative comparisons have been made between 

measured and calculated properties. 

The measured P and S wave velocities (6.0-7.5 km/s 

and 3.6-4.2 km/ s) are typical of lower crust. The P wave 

anisotropy is in the range 0-10% and is mainly controlled 

by the number of mineral phases. Seismic birefringence 

is in the range 0.0-0.6 km/s, but typical values are in the 

range 0.0-0.2 km/ s. In many cases, the birefringence is 

clearly related to inherent fabric elements (foliation, 

lineation). 

Mafic rocks such as anorthosite or pyroxene-bearing 

gabbro exhibit low P wave anisotropies ( <5%) and 

complex seismic properties particularly for the S waves; 

that is, shear wave birefringence is low (0.1 km/s) and 

without clear relationships between the fast shear wave 

polarization plane orientations and the fabric. 

In contrast, the seismic properties of felsic rocks such 

as biotite-bearing gneiss or of mafic rocks such as 

amphibolite display strong birefringence (0.3 km/ s) and 

high V p anisotropy (10%) and exhibit clear relationships 

to the structure. Biotite and amphibole preferred 

orientations clearly dominate seismic anisotropy, 

particularly in shear wave splitting. There is a strong 

correlation between the orientation of the foliation and 

the amplitude of the delay time. Maximum delay time is 

often observed for waves propagating parallel to the 

foliation. The fast shear wave is furthermore polarized 

parallel to the foliation plane. The lineation seems to be 

invisible by shear wave splitting measurements. 

The seismic anisotropy of the exposed lithologic 

sequence in the Ivrea Zone affects the P wave reflectivity 

in a very complex manner. Compared to the isotropic 

case, anisotropy enhances the reflection coefficient for 60 

percent of possible lithological interfaces. For 40 percent 

of lithologic contacts, anisotropy has a destructive effect 

on the P wave reflectivity. 
RELATIONSHIPS BETWEEN THE 

GRANITES OF SERlE DEI LAGHI AND 

THE IVREA MAFIC MAGMA: 

GEOLOGICAL, GEOCHEMICAL, AND 

ISOTOPIC EVIDENCE 

Attilio Boriani, Universita di Milano, Italia, Laura 

Pinarelli, Istituto di Geocronologia e Geochimica Isotopica del 

CNR, Italia, Luigi Burlini, Universita di Milano, Italia, 

Evelina Giobbi Origoni, Universita di Milano, Italia 

A possible genetic link between the Ivrea-Verbano 

(IV) mafic magma and the Lower Permian granites of 

Serle dei Laghi (SdL) arises from the recent 

demonstration that the mafic IV body is also of Lower 

Permian age (Voshage and others, 1990). Much of new 

geochemical and isotopic data (Boriani and others, 1992, 

Pinarelli and others, in press) speak in favour of such a 

common origin. The Lower Permian geological setting 

of IV and SdL that is now possible to reconstruct enables 

us to describe a scenario in which the origin and 

evolution of magmas can reasonably be envisaged. 

Two different Lower Paleozoic accretionary terranes, 

the IV and SdL (the latter with Ordovician acidic 

intrusives), were involved in the Hercynian orogeny. IV 

played the role of lower crust after the Hercynian 

thickening. At the end of the orogeny, large-scale strikeslip faulting, with remarkable vertical component, 

favours local thinning of the crust and intrusion of mafic 

magma. Induced crustal melting and granulitization 

were accompanied by complex interaction between 

mantle- and crust-derived melts. 

Early (around 280 Ma) tapping of hydrous maficintermediate magma produced the Appinite suite, a 

dyke swarm that was emplaced along the tectonic 

contact between IV and SdL, the CMB Line, but not 

along the Pogallo Line. Because the Appinites were not 

affected by the Pogallo deformation, this tectonic line 

must be coeval with their emplacement. 

More evolved, viscous, acidic magmas rose slowly in 

the crust and were emplaced at about 275 Ma in SdL at 

shallow depth or gave rise to a volcanic belt. Intrusion 

and extrusion were again guided by Lower Permian 

tectonic lineaments. 

This possible evolution conflicts with that proposed 

by many authors in the last years because it implies that 

IV and SdL are two segments of respectively lower and 

upper crust merely juxtaposed by the late-Hercynian 

tectonics. Therefore, the section through IV and SdL is 

not a tilted crustal section; this in turn implies that below 

SdL there are rocks similar to those of the IV. This 

scenario is far from satisfactory in many respects. Much 

research is still to be done in order to understand this 

fascinating geological mystery. 

HORNBLENDE 40 AR/39 AR AGE 

TRAVERSES OF VAL CANNOBINA AND 

VAL STRONA D'OMEGNA 

AMPHIBOLITES 

Attilio Boriani, Universita di Milano, Italia, and Igor M. 

Villa, Istituto di Geocronologia CNR and Isotopengeologie, 

Switzerland 

We report a dozen 40 Ar I 39 Ar dating experiments 

on homblends from the upper amphibolite fades rocks 

of the northernmost tip of the Ivrea Zone and from the 

lower amphibolite fades metabasites of the StronaCeneri Boundary Zone. 

The first observation of the Ar data is the ubiquitous 

presence of excess Ar (Arxs>· All samples have saddleshaped age spectra. K/ Ar ages from total gas release are 

always greater than saddle minima or plateaus. Few or 

no indications of alpine overprint are seen, but Arxs in 

the early gas release might be masking small losses. 

The release of Ca-derived isotopes indicates a 

protracted interaction with fluids and . formation of 

rather complex microstructures. 

The age pattern, despite the difficulties of 

unambiguously correcting for Ar XS' gives Triassic ages 

(three samples between 196 and 226 Ma from Cursola, 

higher epidotization correlating with higher 

rejuvenation) close to the Insubric line (IL), gradually 

reaching 308 Main Falmenta, older than the appinites' 

Pb/Sr ages by Pinarelli and others (1988). 

Comparison with lower-temperature chronometers is 

unsatisfactory. Zingg (1983, fig. 1) implies that biotite 

cooling ages show no trend with distance from the IL. 

On the contrary, Biirgi and Klotzli (1990) state that their 

biotite ages become older away from the IL, but the 

presence of chlorite and a rather irregular pattern 

somewhat limit the usefulness of their K/ Ar biotite ages, 

whereas their three muscovite ages are uniform from 

east to west. 

Following Zingg's figure 1, it can be observed that the 

change from a marked west-east trend to no trend is 

constrained to have taken place between the youngest 

hornblende cooling ages and the uniform biotite ages, 

that is between uppermost Triassic and middle Jurassic. 

In other words, there already existed a Schrllgstellung in 

pre-Alpine times during the Jurassic. 

Ivrea-Verbano Zone Worlcshop, 1992 5 
THE STRUCTURAL GEOMETRY OF A 

LOWER CRUSTAL SECTION: IVREAVERBANO ZONE, SOUTHERN ALPS 

K. H. Brodie, E. H. Rutter, and P. Evans, University of 

Manchester, United Kingdom 

The Ivrea-Verbano zone is believed to represent an 

upended cross section through the lower continental 

crust as it existed at the end of the Hercynian orogeny in 

the Southern Alps. We have carried out structurally 

oriented geological mapping in the central and northern 

parts of the Ivrea-Verbano zone, a region some 40 km 

along strike. 

The geology of the southern half of the zone is 

dominated by a large mafic-ultramafic complex (the 

Mafic Formation), which contacts a strongly banded 

metasedimentary and metabasic sequence to the north. 

We have paid particular attention to (1) the structural 

relationships between the rocks of the mafic formation 

and their envelope of high-grade metasedimentary and 

metabasic rocks, (2) the geometrical configuration 

throughout the Ivrea-Verbano zone of high-temperature 

shear zones, which accommodate post-Hercynian crustal 

extension, and (3) the geometry of late (Alpine?) 

faulting, the effects of which have been removed in 

order to produce a restoration of the structure as it 

existed during the late Hercynian extensional phase. 

The intrusion of large volumes of basic magma 

(about 50 percent of the outcrop area) to form the rocks 

of the Mafic Formation appears to be coeval with the 

onset of extension. The main basic body has a laccolithic 

form, originally about 10 km thick. Overfolding 

developed at the northern margin of the laccolith and is 

interpreted in terms of the gravitational collapse of the 

hot, immediately subsolidus body, incorporating its 

envelope of hot metasediments into a large, originally 

recumbent fold. A geometrical association with a hightemperature low-angle fault zone suggests that faulting 

was localized by the several kilometers of uplift 

associated with the laccolithic intrusion. The IvreaVerbano zone may therefore demonstrate at least one 

particular geometry of lower crustal magmatic 

underplating, which may aid interpretation of presentday deep seismic profiles. It also demonstrates the 

geometry of a network of conjugate, high-temperature, 

low-angle shear zones in a well layered lower crustal 

section. 

6 Ivrea-Verbano Zone Workshop. 1992 

ELASTIC PROPERTIES OF THE IVREAVERBANO ZONE AND SERlE DEI LAG HI 

MET APELITES 

Luigi Burlini, Universita di Milano, Italia 

Metapelite is the dominant rock type of the whole 

Massiccio dei Laghi (that is Ivrea-Verbano Zone and 

Serle dei Laghi). The metamorphic condition of the Serle 

dei Laghi (SdL) metapelites falls in the staurolite-kyanite 

subzone, whereas that of the Ivrea-Verbano Zone (IVZ) 

ranges from the staurolite-sillimanite subzone up to the 

granulite fades with inferred partial melting. The fabric 

of metapelites from the two adjacent zones is 

significantly different. These differences cause 

variations in the seismic properties: the SdL metapelites 

are typically transversely isotropic for the compressional 

wave propagation whereas the IVZ metapelites show an 

orthorhombic anistropy. The slowest compressional 

wave velocity, measured in laboratory up to 600 MPa 

confining pressure and room temperature, is normal to 

the foliation, whereas the fastest is parallel to the 

lineation (when present). The average Vp of IVZ 

metapelites is about 6.75 km/ s, with a maximum of 8.35 

km/s; the average VJ) of SdL metapelites is 6.48 km/s, 

with a maximum of 7.58 km/ s. The slowest shear wave 

velocity is again normal to the foliation, whereas the 

fastest propagation occurs in two directions, i.e, at 45 • to . 

the lineation within the foliation plane (VS1) and in a 

plane normal to the lineation and at 45 • between the 

foliation plane and the normal to the foliation (VS2). 

Because foliation and lineation are almost constant along 

the IVZ, a strong seismic anisotropy is to be expected in 

the intermediate to lower crust, with a maximum VP. 

parallel to the dominant foliation and minimum normal 

to it. The total anisotropy (as much as 28 percent) means 

that if the foliation within the continental crust is mainly 

horizontal, the compressional waves travelling vertically 

in a thick layer of metapelites will be as much as 28 

percent slower than those travelling horizontally. In 

such a case near-vertical-reflection and wide-anglereflection correlation will be difficult. 

Within the foliation plane the anisotropy is as much 

as 15 percent, which indicates that a compressional wave 

travelling horizontally and parallel to the lineation (the 

stretching direction) will be much faster than the wave 

travelling horizontally in the direction normal to the 

lineation. 

The comparison between samples collected at 

different distances from a post-metamorphic 

gabbrodioritic stock shows that the total anisotropy 

decreases from 20 percent to 2 percent approaching the 

stock. This decrease may indicate that if a great amount 
of mafic magma is intruded in the lower crust, as in the 

IVZ, un?er static ~nditions, the total anisotropy of host 

rocks w1ll be drastically reduced by annealing and static 

recrystallization. 

DEFORMATION WITHIN THE IVREAVERBANO ZONE BETWEEN VALLE 

POGALLO AND VALLE CANNOBINA (N. 

ITALY) 

Paul Evans, University of Manchester, United Kingdom and 

Luigi Burlini, Universita di Milano, Italia 

. The Ivrea-Verbano Zone (IVZ) of northern Italy is a 

sbce of lower crustal granulite- to amphibolite-facies 

metapelites, and metabasic and ultrabasic rocks that 

have been rotated into a vertical orientation. It has a 

northeast-southwest orientation, and it is bounded to the 

northwest by the Insubric Line and to the southeast by 

the Pogallo Line (POC) and the Cossato-MergozzoBissago (CMB) Line. At the northwest end of the IVZ, 

between Valle Pogallo and Valle Cannobina, the zone is 

relatively thin, having a maximum thickness ranging 

from 3 to 4.5 km. A traverse from southeast to 

n~r~w.est pas~s from predominantly metapelite schists 

(kinztgltes) wtth marble lense, into predominantly 

hornblende-plagioclase-garnet metabasic rocks, 

containing peridotite bodies (for example, the Pinero 

peridotite). 

Mapping of this part of the IVZ has shown the 

presence of high-temperature mylonites (at least 

amphi~lite facies) in the hornblende-plagioclase 

metabasic rocks, as much as 40-100m thick southeast of 

the contact with the Pinero peridotite. The mylonites 

sh~w. an. intense grain-size reduction. The mylonitic 

fobation 1s at a low angle to the metamorphic banding, 

and the lineation plunges 20°-40° northeast. These zones 

have a dextral movement sense and extend the 

~etamorphic banding. Outside of the mylonites less 

Intense recrystallization results in the formation of a 

lineation which also plunges towards the NE. 

Within the metapelites the shear zones are more 

diffuse, with some grain size reduction. At La Piota a 

shear zone is some hundreds of metres thick and 

contains shear bands with both dextral and sinistral 

kinematics, although are dextral. The mylonites at La 

Piota trend 040• and dip 70·-85· northwest. To the 

northeast of La Piota the mylonites appear to form 

several separate movement horizons. The most 

southern of these is the CMB, which juxtaposes the Serle 

dei Laghi metapelites, eclogitic amphibolites, and 

orthogneisses within the kinzigites. The deformation is 

generally diffuse, with limited grain-size reduction and 

dextral shear bands. However, this horizon contains a 

mylonite a few metres thick, with intense grain-size 

reduction and dextral kinematics. The other shear zones 

within the kinzigites north of the CMB are of 

amphibolite to greenschist facies. These are also diffuse 

zones within which are thin mylonites, and they contain 

both dextral and sinistral mylonites. East of Torrente 

Cannobino both the metamorphic banding and the 

mylonitic foliation trend 070· and dip steeply northwest. 

The shear-band lineation plunges northest about 40·. 

Between the most southerly and the most northerly of 

the metapelite mylonites are many granodioritic dykes, 

from a few tens of centimetres to 150 m thick. Most of 

these are intruded parallel to the mylonitic foliation but 

are not deformed by the mylonitization. The thickness 

and frequency of the dykes diminishes towards the 

southeast, disappearing completely between Gurro and 

La Piota, where the CMB and POG meet 

Tight to isoclinal folds are common within the 

kinzigites with wavelengths of a few centimetres to 10 

metres. Open to isoclinal folds also occur within the 

metabasic rocks, although they are not so common. 

These folds can be broadly divided into three groups. 

The oldest folds have axial planar fabric, defined by the 

metamorphic banding. The second are the most 

common and folds the metamorphic banding. Thirdly, 

there are late folds that fold greenschist-facies fabrics. 

The first two groups plunge at 20·-3s· northwest. The 

overall "Z" asymmetry of these folds when viewed 

toward the northwest suggests a major antiform toward 

the southeast. 

Associated with the Insurbric Line is a large 

greenschist-facies thrust that dips at a shallow angle 

toward the northwest. A stretching lineation and 

chlorite-rich shear bands suggest a dextral extensional 

movement sense. This extensional movement results in 

the reappearance of kinzigites next to the Insubric Line. 

Within the thrust slice is a greenschist-facies synform, 

although it may be tightening an earlier fold. This fold 

has a low angle of plunge toward the northwest. The 

Insubric Line mylonites in contact with the IVZ show 

this same dextral extensional movement. 

Ivrea-Verbano Zone Workshop, 1992 7 
MINERAL, TRACE, AND RARE EARTH 

ELEMENT SIGNATURE: RECORD OF A 

METASOMATIC EVENT IN THE FINERO 

ULTRAMAFIC COMPLEX 

A. Ferrario, Universita di Milano, ltalia, A. Tunesi, 

Universita di Milano, ltalia, V. Diella, Centro di Studio per 

Ia Stratigrafia e Petrografia delle Alpi Centrali, CNR, ltalia, 

A. Galletti, Universita di Milano, Italia, L. Bailo, Universita 

di Milano, Italia 

The mafic-ultramafic complex of Pinero is in the 

northeastern part of the Ivrea-Verbano Zone, which is 

made up of a volcano-sedimentary sequence 

metamorphosed under amphibolite- to granulite-fades 

conditions. The complex shows ellipsoidal shape with a 

northeast-southwest trending long axis. A phlogopiterich peridotite (PP) core is rimmed by three outer units: 

layered internal zone, amphibole peridotite (AP), and 

external gabbro. Other peridotite complexes (for 

example, Balmucda, Baldissero) are found in the IvreaVerbano Zone close to the lnsubric lineament and are 

considered, as is the Pinero complex, to be fragments of 

subcontinental mantle uplifted into high crustal levels. 

If compared with the Balmucda and Baldissero 

bodies, the peculiar feature of the Pinero complex is the 

widespread occurrence of hydrate phases (for example, 

phlogopite and pargasitic amphibole) and the unusual 

presence of zircon, apatite, chromite, base-metal sulfides 

(BMS), and platinum group minerals (PGM) in the AP 

andPP. 

Textural relationships, mineral chemistry, and traceREE patterns show that these minerals were formed in 

two main structural settings: marginal zones in the PP 

and shear zones in the AP. The former is characterized 

by the presence of chromite, Cr-diopside, olivine, zircon, 

and minor phases such as BMS and PGM that are 

concentrated in patches, layers, and seams. The shear 

zones are similar to those found by Brodie and others 

(1989) in the metapelitic sequence (kinzigitic serie) that 

represents the host rocks of the Pinero complex. The 

shear zones are characterized by a low-temperature 

stage developed in a brittle regime superimposed over a 

high temperature stage linked to a ductile regime. Crspinel, ilmenite, pentlandite, zircon, apatite, phlogopite, 

pargasite, and BMS were formed during the hightemperature event. The low-temperature event is 

characterized by serpentine, chlorite, talc, awaruite, 

native Cu, valleriite, heazlewoodite, djerfisherite, pyrite, 

marcasite, bravoite, violarite, mackinawite, and Cu 

sulfides formed in a hydrothermal regime. 

Minerals such as zircon, apatite, phlogopite, and 

pargasite constrain the REE pattern. The rock-chondrite 

8 lvrea-Velbano Zone Workshop, 1992 

spidergrams show enriched REE profiles unusual for 

normal subcontinental peridotites. The peridotites of 

Pinero display pronounced LREE (LaN /~ - 5.00), 

Ti02, K20, and Zr enrichment consistent with alkalirich fluid metasomatism. The alkaline fluid addition 

interacted by local diffusion along the marginal zone of 

the PP and by penetrative circulation along the 

extensional shears in the AP. 

The metasomatic event took place in a time span 

ranging from 200 to 230 Ma (v. Quadt and others, this 

volume) when the ascending mantle slab experienced 

partial melting. This event can be related to an 

extensional dynamic phase and relative crustal thinning. 

During this event, local and irregular input of hightemperature K-alkaline fluids formed minerals such as 

zircon, apatite, phlogopite, pargasite, and concentrated 

PGE-bearing chromite. The low-temperature 

assemblage has to be related to hydrothermal 

phenomena during alpine compressional tectonics. 

COMPRESSIONAL WAVE VELOCITIES OF 

LOWER CRUSTAL MYLONITES FROM 

THE IVREA-VERBANO ZONE, NORTHERN 

ITALY 

D.M. Fountain, University of Wyoming, Wyoming, K.H. 

Brodie, University of Manchester, United Kingdom and 

E.H. Rutter, University of Manchester, United Kingdom 

One popular theory, among many, to explain the 

pronounced seismic reflectivity of the continental lower 

crust is that reflections may be caused by a contrast of 

seismic properties between deep crustal mylonites and 

their undeformed adjacent protolith. To explore this 

hypothesis we measured compressional wave velocities 

(V 0> at high confining pressures of high-grade mylonites 

ana bounding protoliths collected from hightemperature shear zones exposed in Valle d'Ossola in 

the Ivrea-Verbano Zone. Oriented samples were 

collected from mylonites developed in metapelites 

(stronalites), an upper amphibolite-facies metabasic rock 

(Anzola metagabbro), and mafic garnet granulites 

(pyriclasites). Measurements were made on three 

orthogonal cores for each sample, and the cores were cut 

parallel to the major fabric elements (for example, 

lineation, foliation). The maximum confining pressure 

for each run was 600 MPa. 

The upper amphibolite-fades metabasic protolith 

collected at the quarry at Anzola is an equigranular rock 

with a granoblastic texture and is composed of calcic 

amphibole, plagioclase, minor oxides, and minor 

clinopyroxene (Brodie and others, 1989). The 
mylonitized equivalent is a fine-grained mylonite with 

porphyroblasts of amphibole. Brodie and others (1989) 

recognized that the formation of orthopyroxene by the 

breakdown of amphibole and plagioclase in this zone 

indicated that deformation occurred during prograde 

metamorphism into the granulite fades. The mean V0 of the proto lith is high (7 .5 kml s) and the ~mple fs 

modestly anisotropic. In contrast, mean velOCity of the 

mylonite is lower (7 kml s) and its anisotropy is reduced. 

A similar pattern was observed for the mafic garnet 

granulites (pyriclasites). Samples of these rocks were 

collected along Rio del Ponte above Premosello where a 

nearly complete 20-m-thick fabric transition from 

relatively undeformed to ultramylonitic pyriclasite is 

exposed (Brodie and others, 1991). A garnetclinopyroxene-orthopyroxene-plagioclase gra~~fels is 

progressively deformed from rocks that exhibit w~ak 

protomylonitic textures to mylonites and ultramylomtes 

that contain recrystallized orthopyroxene and 

plagioclase in the shear zone; plagioclase and 

orthopyroxene increase at the expense of garnet and 

clinopyroxene. Protomylonitic rocks are characterized 

by high average V (7.5 ~Is), low anisotropy, and 

densities of about 31& kglm . Average VP. decreases to 

about 7 kmls, density decreases to about 2900 kglm3, 

and anisotropy is very low in the ultramylonite sample. 

These results could, in part, be explained by 

randomization of the fabric due to high strain, but the 

occurrence of small pseudotachyli te veinlets and 

somewhat higher quartz content, as betrayed by the 

lower density, suggest that compositional changes in the 

shear zone may be responsible for the dramatic change 

in properties. 

Granulite-fades metapelites (stronalites) from the 

Ivrea Zone are typified by high velocities and high 

seismic anisotropy (Fountain, 1976; Burke and Fountain, 

1990). In marked contrast, mylonitic stronalites are 

characterized by relatively low average velocities (6.8 to 

6.9 kml s) and low anisotropy. These samples also 

contain minor amounts of pseudotachylite. 

These results suggest that deep crustal shear zones 

may have lower compressional wave velocities ~h~n 

surrounding undeformed rocks. The reduced velOCities 

are not a consequence of anisotropic effects as 

commonly thought because anisotropy is diminished in 

the mylonites. Instead, there are compositional changes 

that cause velocity and density decreases in the zones. 

In the case of the stronalites and pyriclasites, these 

compositional changes may be partly caused by the 

introduction of pseudotachylite into the mylonites. The 

pseudotachylite could be due to later reactivation of 

these shear zones under upper crustal conditions. But, if 

pseudotachylite can form under deep crustal conditions 

(Hobbs and others, 1986), the physical property changes 

associated with these shear zones may also occur in 

lower crust that shared the same type of strain history as 

the Ivrea Zone. 

EVOLUTION OF GABBRODIORITIC 

ROCKS OF THE BALMUCCIA PERIDOTITE 

AND OF THE LOWER LAYERED GROUP 

IN THE IVREA ZONE 

Dieter Gebauer, Rolf Schmid, Friedrich Heller, Albrecht 

von Quadt, and Peter Ulmer, ETH Ziirich, Switzerland 

U-Pb dating (SHRIMP) of magmatic zircons from a 

meta-gabbrodiorite dike within the Balmucda .~dotite 

yields an Oligocene age (31.1 +I- 1.5 Ma). Similarly, a 

meta-gabbrodiorite within the Lower Layered Group 

(LLG), adjacent to the Balmuccia peridotite, yields an 

age of 26.0 +I- 0.8 Ma. Based on petrological data and 

microstructural observations, both rocks must have been 

emplaced into, and equilibrated within, deep crustal 

levels, at ambient conditions of> 7oo·c and> 700 MPa. 

Sm-Nd whole-rock data of these two rocks plot on the 

600 Ma isochron of Pin and Sills (1986) established for 

similar rocks of the LLG. This isochron must thus be 

reinterpreted as an inherited mantle mix.ing line po~s~~ly 

resulting from Panafrican mantle melting. The Initial 

ENd values of +7.7 and +11.1, as well as the geochemical 

data, suggest a heterogeneous depleted mantle source 

for the two gabbroic rocks, possibly created by eastsoutheast subduction of Mesozoic oceanic crust (V alais 

trough?). Such a model is in line with south-directed 

Oligocene subduction of similar oceanic crust of the 

Valais trough inferred for the Lepontine Alps (Gebauer 

and others, 1992). 

The remanent magnetization direction (after thermal 

and alternating magnetic field demagnetization) of the 

meta-gabbrodiorite within the LLG plunges northeast 

(2361-38). This is more than 40• apart from the direction 

of the magnetic field of "stable Europe" during the 

Oligocene or from all known remanent ma~etization 

directions of Ivrea rocks (Heller and Schmid, 197 4; 

Schmid and others, 1989). The reason for this deviation 

is not yet known. 

A Panafrican magmatic event at 670+ I -36 Ma, 

probably related to mantle melting as well as a Permian 

high-temperature event (265+4 I -5 Ma), interpreted to be 

the result of underplating during successive PermoTriassic rifting episodes, was detected in zircons of a 

meta-pyroxenite layer from the LLG. 

Ivrea-Verbano Zone Workshop,l992 9 
CONTRASTING STYLES OF CRUSTAL 

CONTAMINATION IN IVREA MAGMA 

CHAMBERS AND PERIDOTITES 

Albrecht W. Hofmann, Max-Planck-Institut fiir Chemie, 

Germany, Meihua Lu, Max-Planck-Institut fiir Chemie, 

Germany, Walter Obermiller, Max-Planck-Institut fiir 

Chemie, Germany, Giorgio Rivalenti, Universita di Modena, 

Modena, Italy, Maurizio Mazzucchelli, Universita di 

Modena, Modena, Italy 

The peridotites and mafic complexes of the Ivrea 

Zone provide nearly ideal geological settings for "in situ" 

(or at least in-outcrop) studies of crust-mantle 

interaction. Our previous isotope and chemical work 

(Voshage and others, 1990) has shown that the lower 

crustal magma chamber of the Sesia Valley has been 

extensively contaminated by in situ assimilation of 

crustal material. The earliest magma injections have 

ENd(270) values very similar to those of the underlying 

peridotite. They could, therefore, have been directly 

derived from the peridotite. This similarity is hidden in 

subsequent magma injections because they were 

progressively contaminated by an irregular APC-like 

process, until a thermal balance between magmatic heat 

input and anatexis of crustal material was reached. This 

balance means that each successive injection of magma 

into the chamber caused an approximately proportional 

amount of crust to be melted and mixed into the 

chamber. Consequently, the hybrid magma maintained 

an approximately uniform isotopic composition of Nd 

and Sr. At the same time, the chemistry of the magma 

changed to become progressively enriched in 

incompatible elements and silica, roughly as predicted 

by the magma chamber model of O'Hara and Mathews 

(1981). 

New data for the underlying Balmuccia peridotite 

and the Baldissero peridotite show that both bodies are 

isotopically much more uniform than, for example, the 

Ronda Peridotite. Most of the more extreme 

heterogeneities found in the Ivrea peridotites are 

derived from pyroxenite dikes. Incompatible trace 

elements in these bodies show progressive depletion of 

LREE and extreme depletion in Th, U, and Nb. There is 

relatively little evidence for metasomatic or crustderived enrichment of trace elements in these bodies. 

The situation in Pinero is the reverse of that of 

Balmuccia. All the gabbros have rather uniform, 

positive ENd values with no sign of crustal 

contamination. A series of six cumulate amphibole 

peridotites within the mafic body yields an apparent 

isochron of 449 +I -52 Ma, the significance of which is 

10 Ivrea-VerbanoZoneWorkshop, 1992 

uncertain. We are currently attempting to extract 

zircons from the gabbros for an independent age 

determination. Three internal Sm-Nd isochrons (garamph, gar-cpx-plag) yield ages of 215 to 227 Ma, which 

probably represent the age of cooling through a -600"C 

isotherm. In contrast, the mantle peridotite at Pinero is 

strongly contaminated by metasomatism probably 

y~~ved .. Q-om (subducted?) continental crust. 

Sm/l~d ratios range from 0.10 to 0.14, much like 

typical continental crust, and ENd(270) values range 

from -3 to -4, only slightly higher than typical Hercynian 

continental crust. Nearly ubiquitous phlogopite and 

amphibole provide additional evidence for extensive 

metasomatism of the otherwise highly refractory dunitic 

to harzburgitic peridotite. The isotope contrast between 

mantle peridotite and mafic intrusion precludes any 

direct genetic connection between the two units, and 

their contact must be tectonic. 

A comparison between the Baldissero, Balmuccia, 

and Pinero peridotites shows a general, inverse 

relationship between the major-element and 

mineralogical fertility and the degree of metasomatic 

contamination, with Baldissero having nearly pyrolitic, 

that is primitive major-element composition, combined 

with severe depletion of incompatible trace elements, 

and Pinero being the most refractory peridotite, 

combined with the most pronounced metasomatic 

enrichment of incompatible elements. 

GEOLOGY AND GEOCHEMISTRY OF THE 

IVREA MAFIC COMPLEX ON THE SOUTH 

OF SESIA VALLEY: PRELIMINARY DATA 

ON THE VALMALA SECTION 

Adriano Mayer, Universitd di Trieste, Italy, Silvano 

Sinigoi, Universitd di Trieste, Italy, James E.Quick, U.S. 

Geological Survey, Denver, CO 

A small peridotite lens crops out in Comba di 

Valmala, south of the Sesia Valley, and is roughly along 

the strike with the Balmuccia peridotite body. The 

Valmala peridotite is composed of protogranular to 

tabular lherzolite and harzburgite and is characterized 

by the presence of Cr-diopside and A 1-augite dikes. 

Bulk-rock major-element composition and preliminary 

Sr isotopic ratios are indistinguishable from those of the 

Balmuccia peridotite. On the basis of these 

considerations, the Valmala peridotite is interpreted to 

be a mantle tectonite similar to, although smaller than, 

the Balmuccia body. 

On the east of the V almala peridotite, the 

stratigraphic sequence of the Mafic Complex is similar to 
that of the Sesia Valley, although the thicknesses of the 

various units are different. As in the Basal Zone in the 

Sesia Valley and near the Premosello peridotite in Val 

d'Ossola, a pegmatoidal clinopyroxenite occurs close to 

the mantle peridotite. At higher levels in the section, 

cumulus pyroxenites with minor peridotites, possibly 

corresponding to the Intermediate Zone in Sesia Valley, 

are interlayered with gabbros and metasedimentary 

septa. Charnockites and que:trtz-bearing leuconorites are 

closely associated with the metasedimentary septa. In 

this sequence, Sr isotope initial ratios are highly variable 

as in the corresponding stratigraphic units in Sesia 

Valley (Voshage and others, 1990). Sr isotopic 

compositions are positively correlated with Si~ (fig. 1), 

indicating large ranges in the ratios of mantle and 

crustal components of the hybrid i~';!us ggcks. In 

general, an increase in the value of ( SrI Sr)270 in 

gabbroic rocks is observed approaching charnokites 

and(or) metasedimentary septa. This observation 

suggests that the contamination of the mafic magma 

occurred in situ. Gabbros east of the Sella Boera fault 

are geochemically and petrographically akin to those in 

the Upper Zone of the Sesia Valley. It is still uncertain if 

this sudden variation is related to the offset of the fault 

or to the last appearance of metasedimentary septa. 

West of the Valmala peridotite, the sequence consists 

mainly of garnet- and(or) amphibole-bearing gabbros, 

and metasedimentary-charnockitic septum borders the 

western side of the ~ant~ peridotite. Gabbroic rocks 

show a decrease in ( SrI Srh70 values away from the 

septa. All gabbros on the western side, ~~th og'f one 

exception, are characterized by high ( SrI Srh70 

values, suggesting that crustal contamination affected 

also this part of the Mafic Complex. Nevertheless, their 

trace-element contents are distinct from those of the 

gabbros on the east of the peridotite. This fact suggests 

that they may not be tectonically repeated but may 

represent a deeper part of the intrusion. If so, the 

Valmala and Balmuccia peridotites do not represent the 

base of the mafic intrusion but, more likely, are 

fragments of mantle material mingled with deep crustal 

rocks prior to the intrusive event. Fieldwork is in 

progress with the aim of resolving this problem. 

0.716 

$> 0.712 co 

...... ~ 

o::> 

- 0.708 * 

0.704 

Figure 1. Plot of Si<>:z versus Sr isotopic oomposition at 270 Ma. 

Stars=Ultramafic rocks. Filled circles=gabbros. Open circles=quartzbearing nonites. Open triangles=chamockites. Filled triangles=restitic 

metasediments and granatites. Solid lines coonect samples that were collected 

at short distance from each other across metasediment-chamokite-gabbro 

transitions. 

EVOLUTION OF MANTLE PERIDOTITES 

DURING PASSIVE CONTINENTAL 

RIFTING: EXAMPLES FROM ANCIENT 

AND PRESENT SITUATIONS 

Giovanni B. Piccardo, Universita di Genova, Italy, 

Elisabetta Rampone, Universita di Genova, Italy, and 

Riccardo Vannucci, Universita di Pavia, Italy 

This work aims to revise petrological and 

geochemical knowledge of the tectonic, metamorphic, 

and magmatic processes that characterize the evolution 

of the lithosphere-asthenosphere system during passive 

rifting and the inception of an oceanic basin. 

The pre-oceanic rifts and the early stages of opening, 

connected to passive extension of the lithosphere, expose 

at the sea floor large masses of upper mantle rocks. 

Most of these mantle peridotites show relatively 

fertile compositions, incompatible with refractory 

residua after mid-oceanic-type partial melting. They 

preserve chemical, isotopic, and geochronologic 

signatures related to the accretion to the lithosphere of 

deeper asthenospheric material. 

Age constraints, in particular, show that the 

evolution of the accreted material was older than, or 

coeval with, the main tectono-metamorphic and 

Ivrea-Velbano Zone Workshop, 1992 11 
magmatic events recorded by the associated continental 

crust. Peridotite bodies from the Alpine-Apennine 

system show Proterozoic to Variscan ages, whereas 

mantle ultramafics from the Northern Red Sea (that is, 

Zabargad peridotites and Arabian mantle xenoliths) 

have Proterozoic to late-Precambrian ages. The latter are 

consistent with the Pan-African events recorded in the 

Arabian-Nubian Shield. 

After emplacement within the lithosphere and 

complete equilibration under spinel-facies conditions at 

intermediate geothermal gradients (that is, pressure in 

the range 10-22Kb and temperature not exceeding 1000-

11000C), the new lithospheric mantle underwent various 

metasomatic processes. Both Alpine-Apennine and Red 

Sea bodies show a widespread modal metasomatism, 

characterized by the growth of kaersutite-Ti-rich 

pargasite amphiboles in equilibrium with the spinalbearing assemblage. Trace-element compositions of 

coexisting clinopyroxenes and Ti-rich amphiboles 

indicate that Ti-pargasite crystallized during the upward 

migration of low-density H20-rich fluids equilibrated 

with deeper (garnet-bearing) mantle levels. These data 

are consistent with experimental results on aqueous 

fluid compositions in equilibrium with upper mantle 

rocks at different pressures. This process, well recorded 

by other sub-continental lithospheric mantle rocks (that 

is, Eastern Pyrenean massifs), could be related to 

degassing of deeper levels during the initial stages of 

extension. 

Lherzolitic rocks of sub-continental lithospheric 

origin underwent large structural-textural and 

paragenetic changes from the beginning of extension 

(passive rifting), which probably started during the 

Triassic in the Ligure-Piemontese system and during the 

Oligocene in the Northern Red Sea system. These 

changes denote a progressive exhumation from the 

lower lithosphere along nonadiabatic decompressional 

trajectories, at almost subsolidus conditions. The 

exhumation led to emplacement at shallow levels. 

The above trajectories are significantly different from 

those assumed for upwelling asthenospheric mantle 

beneath active rifts, as, for instance, mid-oceanic ridges. 

They could, however, be compatible with uplift along 

lithospheric shear zones during whole lithosphere 

extension. 

The pressure-temperature trajectories of the subcontinental lithospheric lherzolites from Zabargad 

(Northern Red Sea) and the External Ligurides (EL) 

(Northern Apennines) are consistent with a subsolidus 

evolution as footwalls of lithospheric shear zones, where 

progressive thermal relaxation can be caused by the 

juxtaposition with a progressively cooler hanging wall. 

The late evolutionary stages of the pre-oceanic rift 

mechanisms were characterized by basaltic dyke 

12 Ivrea-Verbano Zone Workshop, 1992 

intrusions, probably related to the inception of partial 

melting by decompression on the underlying 

asthenosphere, which began to rise following the passive 

extension of the overlying lithosphere. 

In the Jurassic Ligure-Piemontese Basin, 

asthenosphere-derived mantle rocks were first depleted 

by partial melting during the inception of rifting and 

then emplaced at the sea-floor. Their geochemical 

character is typical of melting residua from MORB 

sources and ages consistent with the rifting stages. 

The primary melts of the MORB-type basalts of the 

Internal (that is, more oceanic) Liguride Units (IL) of the 

Jurassic Basin appear to have been generated by 

fractional melting processes in an asthenospheric MORB 

source, closely similar to the parental asthenospheric 

mantle of the IL oceanic depleted peridotites. The early 

basaltic melts (for example, the parental liquids for some 

of the EL basalts) appear to derive from lower partial 

melting degrees with respect to the parent melts for the 

IL basalts. 

The data presented and discussed in this paper can 

be useful to identify the petrogenetic processes typical of 

pre-oceanic rifting and passive lithospheric extension, as 

contrasted with those dominated by active 

asthenospheric upwelling (active rifts). 

DYNAMIC EVOLUTION OF THE IVREAVERBANO ZONE MAFIC COMPLEX: PART 

I: GEOLOGIC CONSTRAINTS 

James E. Quick, U.S. Geological Survey, Denver, CO. 

Silvano Sinigoi, Universita di Trieste, Italy, Gabriella 

Demarchi, Universita di Trieste, Italy, Luisa Negrini, 

Universita di Trieste, Italy, Adriano Mayer, Universita di 

Trieste, Italy 

The Ivrea-Verbano Zone (IVZ) of northern Italy is one 

of the few exposed lower-crustal sections in which the 

effects of magmatic underplating may be studied 

directly. The steeply dipping section of the IVZ is 

composed of mantle peridotite; an igneous complex as 

much as 10 km thick, referred to as the Mafic Complex, 

and amphibolite- to granulite-facies paragneiss grouped 

as the Kinzigite Formation. Most investigators now 

agree that the Mafic Complex was magmatically 

underplated beneath continental crust represented by 

the Kinzigite Formation. We present new data, which 

indicate that the Mafic Complex is concentrically foliated 

and layered and that it contains evidence of widespread 

deformation under hypersolidus conditions. 

New measurements of foliation and layering 

orientations reveal an "onion-like" gross structure in the 
Mafic Complex (fig. 1). South of the Sesia River, 

foliation, layering, and metasedimentary septa define a 

northwardly concave, arcuate structural grain. In terms 

of the inferred primary orientation of the complex, 

attitudes steepen gradually up section and locally 

overturn near the overlying Kinzigite Formation. North 

of the Sesia River, an opposing, southwardly concave 

structural grain is defined by foliation, layering, and the 

contact between the Mafic Complex and the Kinzigite 

Formation. South of the Sesia Valley, the Mafic Complex 

has been profoundly affected by high-temperature 

deformation. Rocks are foliated and lineated, and 

isoclinal folds and boudinage are evident. Many dikes 

and layers are disrupted by small-scale normal faults. 

Most of this high-temperature deformation appears to have 

occurred under hypersolidus conditions. Locally, 

undeformed, poikilitic hornblende has grown across the 

foliation, suggesting that several percent of interstitial 

melt was present during and after penetrative 

deformation. Many of the small-scale normal faults are 

healed by undeformed veins crystallized from late-stage 

melts, and late-stage melt has crystallized as patches of 

undeformed leucogabbro that crosscut foliation and fill 

pressure shadows at the ends of boudins. This 

deformation is not uniform throughout the Mafic 

Complex. Primary igneous textures are more commonly 

preserved near the roof of the complex, and foliation is 

0 

" >70 ~ 50-70 

""- 30-50 ° ~ 1 0-30 ° 

FOLIATION I LAYERING 

/..X>85° l"/70-85° (/.. 55-70° 

weak to absent in a zone a few kilometres in diameter 

centered about the Sesia Valley and corresponding to the 

relatively undeformed diorite and Main Gabbro of 

Rivalenti and others (1981). 

Features similar to the IVZ "onion" and the 

synmagmatic deformation in the mafic complex are also 

characteristic of ophiolitic gabbros (Nicolas, 1989). In 

the latter case, these are symptomatic of hightemperature ductile deformation in an extensional 

environment. On the basis of an analogy with ophiolitic 

gabbros, we interpret the IVZ "onion" to be a primary 

igneous feature, and suggest that the "onion" and the 

synmagmatic deformation in the IVZ could be 

interpreted as additional evidence that underplating 

occurred beneath rifting, albeit continental, crust. 

5 KM 

N 

/ 

Figure 1. Structure of the southern Mafic Complex (Q. Balmuccia peridotite, black; Insubric Line, IL; Cossta-Mergozzo-Brissago Line, CMBL; Sesia 

River, S; Sessera River, R. 

Ivrea-Verbano Zone Workshop, 1992 13 
LARGE-SCALE DEFORMATION AND 

EVOLUTION OF THE UNDERPLATED 

IGNEOUS COMPLEX OF THE IVREAVERBANO ZONE 

James E. Quick, U.S. Geological Suroey, Denver, CO, 

Silvano Sinigoi, Universita di Trieste, Italy, and Roger 

Denlinger, U.S. Geological Suroey, Denver, CO 

The Ivrea-Verbano Zone (IVZ) exposes a slice 

through a concentrically foliated and layered, 

synmagmatically deformed igneous complex (Quick and 

others, 1992). In terms of the presumed primary 

orientation of the igneous complex, the foliation, 

layering, and metasedimentary septa south of the Sesia 

River steepen gradually up section and locally overturn 

near the top of the complex. Isoclinal folds, boudinage, 

and high-temperature normal faults appear to have 

developed while the complex was still partially molten. 

Analogous features are described in ophiolite 

gabbros. Upward steepening layering and foliation 

occur in the Bay of Islands and Samail ophiolites (for 

example, Casey and Karson, 1982; Smewing, 1981). 

Nicolas and others (1988) suggested that the regionally 

consistent foliation and lineation in ophiolitic gabbro are 

products of pervasive, viscous, synmagmatic ~ow .. As a 

result of this flow, gabbro layers are locally 1sochnally 

folded, and layers and foliation are cut in many places 

by small, high-temperature normal faults similar in scale 

and appearance to those in the IVZ. 

14 Ivrea-Verbano Zone Workshop, 1992 

Only two types of models address both the regionally 

consistent, high-temperature deformation and the 

upwardly concave layering and foliation in ophiolitic 

gabbros. Subsiding-floor models (for example, Dewey 

and Kidd, 1977) envision a small, thin magma chamber 

resting on a thick zone of crys~l mush benea~ 

spreading centers. During sp~eadtng, ~gmatic 

deformation and rotation of layenng and fobation occur 

as the crystal mush subsides, moving downward and 

outward from the magma chamber. Nicolas and others 

(1988) proposed another model in which layers 

crystallize on the roof and hanging walls of a large, thick 

magma chamber with initial orientati~ns dipping .awa.y 

from the spreading axis; synmagmatic deformation 1s 

induced by large amounts of mantle underflow. These 

models are difficult to test in ophiolites, which have no 

unequivocal indicators of spreading direction ~r initial 

layer orientation. In the IVZ, however, metasedimentary 

septa must have been incorporated from the root as t~e 

intrusion migrated upward. Also the concentric 

structure of the complex preserves the "spreading 

center," and the maximum magma chamber dimensions 

are indicated by the size of the Main Gabbro-the last 

intrusion into the complex (Sinigoi and others, this 

volume). 

To demonstrate how underplating during extension 

could produce the gross structure of the Mafic Complex 

south of the Sesia River, we present results of a model 

that approximates the effects of deformation due to 

extension of a broad lens of crystal cumulates. The 

actual geometry utilized in the calculation is for the right 

side of a symmetric oceanic spreading center proposed 
by Sinton and Detrick (1992). This is clearly only a firstorder approximation for the IVZ Mafic Complex, but we 

suggest that the left side of figure 1, which would 

represent the spreading axis in an oceanic spreading 

center, could be thought of as corresponding 

approximately to the Sesia Valley, which passes through 

the core of the Mafic Complex. In the model, cumulates, 

consisting of crystals and interstitial melt, are overlain 

by a small magma chamber ( <2 km wide). Flow is 

induced in the partially molten cumulates by a pull on 

the right boundary, and resistance to flow along the base 

and left boundary of the model is assumed to be 

negligible. Addition to the cumulates from the magma 

chamber balances removal along the right boundary, 

and the flow is calculated assuming a temperature- and 

strain-rate-dependent visco-plastic rheology consistent 

with the observed mechanical behavior of many 

mixtures of solid particles and fluid. 

Figure 1 illustrates results of this calculation. 

Cumulates move downward and outward from the 

small magma chamber, and layers are stretched and 

rotated into upwardly concave shapes. The similarity of 

layering orientations produced by this model observed 

in the Mafic Complex south of the Sesia Valley suggests 

that the gross structure of the complex may indeed be 

consistent with underplating during extension. In 

contrast to ophiolites, however, the geometry of the 

Mafic Complex north of the Sesia River suggests that 

subcontinental extension in this instance was 

asymmetric. We suggest that the addition of cumulates 

by underplating was compensated in part by symmetric 

rifting and that the Mafic Complex may be thought of as 

filling a giant "tension gash" near the base of the crust. 

U-PB ZIRCON AND MONAZITE AGES 

FROM THE IVREA ZONE AND THE 

ADJOINING SERlE DEI LAGHI 

R. Ragettli, W. Hansmann, V. Koppel, Eidgen6ssische 

Technische Hochschule Ziirich, Switzerland 

To obtain more detailed knowledge about the 

metamorphic history of the Ivrea zone and the adjacent 

Serle dei Laghi (Strona-Ceneri) U-Pb ages-on zircons and 

monazites have been determined (Koppel, 1974). 

Samples were collected along a profile crossing the 

Cossato-Mergozzo-Brissago (CMB) fault which 

separates the above tectonic units (Boriani and others, 

1981). The observed decrease of the monazite ages from 

about 450 Ma to 300 Ma was also confirmed by lower 

intercept ages on discordant zircons (Koppel, 1974). In 

this study we present U-Pb zircon and monazite results 

from additional samples collected along, as well as 

across, the CMB-fault and discuss them together with 

the previously obtained age data. 

Monazites from rocks of the Strona-Ceneri zone 

northeast of the Valle d'Ossola either show only minor 

lead loss and yield U-Pb ages of 460 Ma to 440 Ma (Val 

Grande: Alpe Ruspesso, Ponte Casletto) or they are 

strongly discordant indicating intercept ages of 440 Ma 

and 270 Ma, respectively (Valle Cannobina, Lago 

d'Orta). Zircon data of metasediments (Ceneri gneiss) 

from Ponte Casletto are discordant. They form a linear 

array showing considerable scatter. However, two size 

fractions yield overlapping concordant data points of 

458Ma. 

A late- to post-metamorphic diorite crops out 

between the CMB-fault and the Pogallo-line near 

Mergozzo and Candoglia. Monazite data partly show a 

weak lead loss and point to an intrusion age of 300-305 

Ma. Monazites from a paragneiss immediately 

adjoining yield slightly older ages. Zircon data points 

from the diorite scatter due to the combined effects of 

post-intrusive lead loss and presence of inherited lead 

components. 

Zircon data from the Mte. Orfano granite reflect the 

presence of premagmatic zircon components as well as 

post-intrusive lead loss, which was mainly observed in 

uranium-rich size fractions. U-Pb monazite data plot on 

the concordia curve or slightly below it, indicating 

minor lead loss. An estimated age of 280 Ma is 

considered to be a minimum age. 

Among the samples from the Scisti dei Laghi was one 

taken from the immediate contact to the Mte. Orfano 

granite. The monazites separated yielded nearly 

concordant U-Pb ages of about 310 Ma. Slightly older 

monazite ages were determined on samples more 

distant from the contact. All the monazite data points 

from the Scisti dei Laghi lie above the discordia 

intersecting at 440 Ma and 270 Ma, respectively, which 

are defined by some of the monazites from the StronaCeneri zone. We therefore conclude that the main 

metamorphic overprint of the Scisti dei Laghi took place 

during the Hercynian. Contrastingly, monazite 

formation ages and lower intercept ages of zircons from 

the Strona-Ceneri zone indicate for this unit a highgrade metamorphic event of Caledonian age. 

Monazites of kinzingitic gneisses from Candoglia 

show ages of about 280 Ma, and a concordant monazite 

fraction from a stronalite near Anzola yields an age of 

271 Ma. These results thus indicate that northwest of the 

CMB fault, high -grade metamorphic conditions 

prevailed during the Permian. 

Ivrea-Verbano Zone Worltshop, 1992 15 
DYNAMIC EVOLUTION OF THE IVREAVERBANO ZONE MAFIC COMPLEX: PART 

II, GEOCHEMICAL CONSTRAINTS 

S. Sinigoi, Universita di Trieste, Italy, J. Quick, U.S. 

Geological Survey, Denver, CO, D. Clemens-Knott, 

California Institute ofT echnology, Pasadena, CA, G. 

Demarchi, Universita di Trieste, Italy, A. Mayer, Universita 

di Trieste, Italy, M. Mazzucchelli, Universita di Modena, 

Italy, L. Negrini, Universita di Trieste, Italy, G. Rivalenti, 

Universita di Modena, Italy 

Recent investigations have shown that the 

underplated Mafic Complex of the Ivrea-Verbano Zone 

(IVZ) consists mainly of hybrid rocks formed by mixing 

of mantle melt(s) with a significant amount of crustal 

component(s) (Voshage and others, 1990). The thickest 

part of the complex, which crops out in the southern 

part of IVZ, is concentrically layered and foliated as a 

result of hypersolidus deformation of crystal mush in an 

extensional tectonic environment (Quick, and others, 

1992). 

Gabbroic rocks collected along the Sesia Valley 

section show significant geochemical variations that 

correlate with height in the Mafic Complex. A large 

variation in isotopic compositions shown by the Basal 

and Intermediate Zones is followed by a relative 

homogeneity in the Upper Zone, Main Gabbro, and 

"Diorites" (Voshage and others, 1990). Nevertheless, 

from the Upper Zone to the roof, a U-shaped. profile is 

defined by isotopic compositions of strontium and 

oxygen (fig. 1). Similar patterns are defined by Ba and 

Zr absolute concentrations. These profiles, accompanied 

by field evidence, suggest that the Main Gabbro 

crystallized from the last magma pulses injected into the 

core of the magma chamber and that these last melts 

were slightly less contaminated than the previous 

batches. 

Isotopically uncontaminated gabbros from the Basal 

and Intermediate Zones are depleted in incompatible 

elements and LREE. Hybrid gabbros show LREE 

enrichment accompanied by high Ba but low K and Rb 

concentrations. Therefore, the contaminating melts may 

have been produced by a garnet- and feldspar-rich 

crustal source, which has been previously depleted in K 

and Rb with respect to Ba. Metasedimentary septa 

enclosed in the lower part of the Mafic Complex and the 

associated charnockites have the appropriate 

compositions to represent examples of the 

contaminating crustal source and anatectic melt, 

respectively. In contrast, leucosomes in the roof 

migmatites have high K and Rb and comparatively low 

Ba abundances and, therefore, cannot represent a 

16 Ivrea-Verbano Zone Workshop. 1992 

suitable contaminant. Similar K- and Rb-rich 

compositions are shown by the Permian granites of the 

Strona-Ceneri Zone. On this basis, it is suggested that 

the incompatible elements composition of hybrid rocks 

is strongly affected by the degree of depletion of the 

crustal source. It may be noteworthy that rocks of the 

Mafic Complex have chemical compositions close to 

estimates for the deep crust, whereas the roof rocks, as 

well as the coeval granites of the Strona-Ceneri zone, 

show compositions appropriate for the middle and 

upper crust. . 

Considering the geochemical constraints collectively 

and the structural and field evidence, we suggest that 

the Mafic Complex was contaminated in situ by the 

enclosed, and previously depleted, deep crustal rocks, 

and that the intrusion occurred along a deep detachment 

fault, possibly close to the mantle-crust transition. 

Displacement of the fault concomitant with continuous 

magma injection, crystallization, and deformation 

brought an allocthonous roof of amphibolite-facies rocks 

into contact with the still partially molten Mafic 

Complex. Heat provided by the Mafic Complex on the 

roof rocks, accompanied by similar, although smaller, 

mafic intrusions at higher crustal levels induced melting 

of fertile middle to upper crust and consequent 

production of large volumes of granitic hybrid melts. 

r- o PERIDOTITE I 0 9 

I • GABBRO I 

I. 

r- -INCLUSION 0 

o DIORITE 

lo o c . 

• STRONALITE ( 

• 

5000 

• • AMPHIBOLITE 0 • 

• 0 ~ 

• .... 0 rj •:' 

l.6. "· __._ 4 ~ ... ........ !-& .• ~ • ·- Ill 

1000 

• ~ 10 • I ••Ell 

~~ ·' I G~ 0 0 

• • • "" ~~ ~ 0 8 - • • • t••n ! • 

0.7 tt I ~ 

~ I 

I 

... 

J I 

I :,) .o, oo 08~ tJ. • • 0 0 

eo • • ~ 

• • 

~ 0 

ro 

04 ~0 ~ 0 

BZIIZ uz MG 

0.7 

0.7 

Figure 1. BZ, Basal Zone; IZ, Intermediate Zone; UZ, Upper Zone; 

MG, Main Gabbro; D, Diorite. 
A left-lateral sense of shear is suggested by the 

megascopic shape of the mafic body and is consistent 

with the sense of shear of widespread mylonites of 

Permo-Triassic age in the area. Lubrication by 

underplated magma and consequent detachment along a 

large listric fault may have resulted in Permo-Triassic 

crustal thinning as an event precursory to the opening of 

the Tethyan ocean. 

THE IVREA-VERBANO ZONE, NORTHERN 

ITALY, AS A LOWER CRUSTAL ANALOG 

TO THE DEEP CRUST OF THE CENOZOIC 

BASIN AND RANGE PROVINCE, 

WESTERN U. S. CORDILLERA: 

PERSPECI1VE AND PROBLEMS 

Arthur W. Snoke, University of Wyoming, Laramie, WY, 

and David M. Fountain, University of Wyoming, Laramie, 

WY 

The lower crust of the Late Cenozoic Basin and Range 

Province (BRP), western U.S. Cordillera, has been cited 

by several authors (for example, Shervais, 1979; 

Fountain, 1989, Brodie and others, 1991) as a possible 

actualistic analog to the inferred late Paleozoic 

tectonothermal regime of the Ivrea-Verbano Zone (IVZ), 

northern Italy. Several distinctive aspects of BRP 

evolution are commonly cited in support of this model: 

steep geothermal gradients; inferred underplating by 

mafic magmas based on petrologic, geochemical, and 

geophysical studies; and widespread low-angle, normalsense, plastic-to-brittle shear zones. The BRP is a broad 

zone (about 800 km wide in the northern BRP) of normal 

faulting and active seismicity, high heat flow, 

widespread Cenozoic volcanism, thinned crust and 

lithosphere, but high elevation and is considered a 

classic example of continental extension. The Holocene 

BRP is part of a protracted history of extension that 

began at least in the mid-Tertiary and perhaps even 

earlier. Although the present extensional regime lies 

essentially behind an active transform fault (San 

Andreas fault system), the earlier extensional history 

occurred during back-arc and intra-arc rifting. 

Although principally upper crustal rocks are exposed 

at the surface in the BRP, in highly extended areas, 

middle crustal rocks form domiform footwall below 

low-angle, normal-sense faults (that is, Cordilleran 

metamorphic core complexes). These high-grade, 

metamorphic footwalls were exhumed during Tertiary 

crustal extension and uplift associated with deeply 

penetrating normal-fault systems. The Ruby-East 

Humboldt metamorphic core complex, northeastern 

Nevada, provides an exceptionally deep "window" into 

northern BRP crust, and seismic reflection data provide 

additional constraints on the character of the lower crust 

in this area. A cross section for the exposed rocks of the 

Ruby-East Humboldt core complex includes (shallow to 

deep) (1) upper crustal synextensional Tertiary volcanic 

and sedimentary rocks above subjacent Paleozoic to 

early Mesozoic cover rocks; (2) a transitional zone of 

greenschist- to amphibolite-fades metamorphic rocks 

intruded by Mesozoic granitic rocks; and (3) a mylonitic 

to migmatitic upper amphibolite-fades zone including 

abundant Tertiary granitic rocks (orthogneisses), 

synextensional plastic deformation, and hightemperature synextensional metamorphism (sillimanitegrade). Tertiary synextensional magmatism, 

deformation, and metamorphism apparently increase 

with structural depth. Normal-incident seismic 

reflection data indicate a reflective middle and lower 

crust perhaps related to extensional flow, whereas wideangle seismic reflection data suggest significant 

increases in velocity with depth, probably related to 

extensional granulite-facies metamorphism and 

magmatic underplating (Valasek and others, 1989). 

On a superficial level, most, if not all, of the above 

general characteristics can be found in the IVZ and the 

structurally higher parts of the Southern Alps. 

However, before a BRP analog can be explored in more 

depth, certain key questions must be addressed in 

regard to the IVZ. 

1. In the eastern BRP, supracrustal rocks were 

deposited on an older Precambrian crystalline basement. 

What is the basement for the supracrustal rocks that 

compose part of the IVZ? 

2. In the BRP, low angle, normal-sense, plastic-tobrittle fault systems are widely developed and well 

exposed. Were the conjugate extensional shear zones, 

now well documented in the IVZ (Brodie and Rutter, 

1987), truly developed at a low-angle and subsequently 

rotated to their present subvertical orientations? 

3. Synextensional, Tertiary (about 40 to 29 Ma) 

quartz dioritic to granitic orthogneisses are widespread 

in the migmatitic footwall of the Ruby-East Humboldt 

core complex and broadly coeval volcanic rocks also 

compose part of the Tertiary cover rocks. Can consistent 

age and geochemical relationships be demonstrated 

between possible synextensional magmatic rocks of the 

IVZ and structurally higher levels (for example, StronaCeneri zone and the late Paleozoic volcanic rocks of the 

Southern Alps)? 

4. Granulite-facies metamorphism has been inferred 

by several authors (for example, Sandiford and Powell, 

1986) for the deep crust of the BRP based on the high 

"reduced" heat flow for many parts of the region. Wideangle seismic reflection data also suggest granuliteIvrea-Verbano Zone Workshop, 1992 17 
fades compositions and velocities in the lower crust of 

the BRP. This inferred deep crustal metamorphism 

would be synextensional and presumably related to 

magmatic underplating. What is the age of 

metamorphism in the wallrocks of the mafic complex in 

the NZ? Are multiple metamorphic events preserved in 

these rocks? Is the granulite-facies metamorphism in the 

IVZ related to the emplacement of the mafic complex? 

5. The BRP developed across a complex regional 

geologic framework that involved widespread Mesozoic 

shortening, magmatism, and metamorphism prior to 

Cenozoic crustal extension. Can a regional geologic 

framework be devised that places the magmatism, 

metamorphism, and inferred extension of the NZ within 

a specific tectonic sequence as well as a plate-tectonic 

setting? 

The BRP is an unusually superb area in which crustal 

extension and associated magmatism and 

metamorphism can be studied at contrasting structural 

levels. This extensional history was widespread and 

long lived, but subsequently has not been overprinted 

by a contractile deformation. Therefore, the lower crust 

of the BRP provides a provocative present-day 

geological-geophysical analog for the commonly 

inferred late Paleozoic tectonothermal evolution of the 

IVZ. However, the detailed documentation of truly 

analogous. relationships shared by both regions await 

ongoing and future studies. 

LITHOSPHERIC-MANTLE MAGMATISM 

ASSOCIATED WITH RIFfiNG OF 

CONTINENTAL TERRANES: A CASE 

STUDY OF ULTRAMAFIC INCLUSIONS 

AND MAFIC ALKALINE MAGMAS FROM 

HARRAT HUT A YMAH, SAUDI ARABIA 

Carl R. Thornber, U.S. Geological Suroey, Denver, CO 

Mafic and ultramafic components of the IvreaVerbano Zone (NZ) have been interpreted by numerous 

investigators as products of rift-related uppermantle/ deep-crustal magmatism (for example, Shervais, 

1979; Fountain, 1989 and Quick and others, 1992). 

Petrologic, geochemical, and isotopic systematics of 

continental-rift-related magmas and entrained xenoliths 

of mantle and deep-crustal rocks document the genetic 

link between mantle-magmatism and regional crustal 

extension. Detailed studies of such occurrences provide 

information which is relevant to evaluating rift-related 

characteristics of NZ subcrustal evolution. 

Harrat Hutaymah is an \solated, diatremedominated, and xenolith-rich Tertiary /Quaternary 

18 Ivrea-Verbano Zone Workshop, 1992 

volcanic field made up of basaltic magmas erupted 

through the continental lithosphere on the east flank of 

the Red Sea rift. To the extent that magmatic 

underplating in the NZ is related to nascent rifting, 

Harrat Hutaymah may provide a Tertiary /Quaternary 

analog. Thorough investigation of an extensive suite of 

Hutaymah volcanics and ultramafic inclusions provides 

a basis for establishing general petrographic and 

chemical criteria for identifying rift-related 

heterogeneities among upper-mantle and deep-crustal 

rock types. Pressure-temperature-composition and time 

constraints on the origin of xenoliths, megacrysts, and 

host magmas are evaluated by combining petrographic 

and mineral chemical observations with available 

experimental phase-equilibria and thermobarometry, 

along with trace-element and Nd and Sr isotopic 

compositions. 

Twenty-six ultramafic rock types are distinguished 

among an extensive suite of 709 xenoliths and 

megacrysts. Distinctive aspects of texture, modal 

mineralogy, and mineral chemistry allow division of this 

suite into categories of Mg-Cr-group and Al-Fe-Ti-group 

igneous pyroxenite, Mg-Cr-group and Al-Fe-Ti-group 

metamorphic pyroxenite, and associated Mg-Cr-group 

mantle peridotites, with hydrous equivalents of each 

category. Igneous pyroxenite and partial-melt-bearing 

peridotite inclusions have relatively high pyroxene 

equilibration temperatures (990-1160.C), and their 

textures, modal mineralogy, and mineral compositions 

reflect primary igneous crystallization, high-temperature 

deformation, or partial melting. These samples show the 

effects of injection and polybaric crystallization of 

volcanic-host-related mafic alkaline magma and the 

complementary effects of heating, fluid infiltration, and 

partial melting of surrounding mantle lherzolite. 

Metamorphic pyroxenites and nonmelted peridotites 

have relatively low pyroxene equilibration temperatures 

(<700-950.C), and textures, modal mineralogy, and 

mineral chemistry that reflect varying degrees of 

deformation, recrystallization, and metamorphic 

annealing. Similar bulk-rock and mineral compositional 

trends and gradational textural and modal 

characteristics between ultramafic rocks of igneous 

affinity and their metamorphic counterparts reflect a 

similar but older magmatic episode within the deep 

crust and upper mantle. The maximum depth of origin 

of Mg-Cr-group xenoliths is broadly restricted by that of 

pristine spinel lherzolite (protolith mantle) at -70 km. 

Available experimental data and thermo barometric 

calculations indicate that all Al-Fe-Ti-group inclusions 

(igneous and metamorphic) may have been sampled 

from within +I -10 km of the base of the 40-km-thick 

crust. This interpretation is consistent with seismic data 

that show a heterogeneous crust-mantle transition zone 
in this region and contributes to an understanding of 

how magmatic underplating affects deep crustal 

evolution in continental terranes. 

Petrogenetic relations between Hutaymah magmas, 

xenoliths, and megacrysts made on the basis of 

petrography and mineral chemistry are supported by 

trace-element and Nd and Sr isotopic data. Mantle 

inclusions that have equilibrated texturally at relatively 

low temperatures sample the Arabian lithospheric 

mantle of Proterozoic age. Inclusions of relatively high 

temperature affinity reflect upper-mantle processes that 

are cogenetic with Red Sea rift-related magmas erupted 

at Harrat Hutaymah over Late Tertiary to Quaternary 

time. 

Trace-element and isotopic characteristics of igneous 

ultramafic inclusions record subcrustal syn-rift processes 

related to crystallization of mafic alkaline magmas. 

Partial-melt-bearing peridotite xenoliths are variably 

enriched in alkaline magmatic components and have 

isotopic signatures overlapping those of Hutaymah 

volcanics and cogenetic igneous inclusions. These 

isotopic signatures may have been acquired during 

recent magmatic enrichment in magma/mantle-wallrock reaction zones. Alternatively, anhydrous and 

hydrous melt-bearing lherzolites could represent the 

source mantle for host magmas. 

Trace-element and isotopic characteristics of 

unmelted spinel lherzolite and metamorphic pyroxenite 

xenoliths reveal time-integrated REE enrichment and 

depletion that are attributed to magmatism and 

subsequent metamorphism within the Arabian 

lithospheric mantle and associated with Proterozoic 

crustal formation during oceanic-arc and continentalarc-margin accretion. 

Correlation and trace-element and isotopic 

characteristics in both syn-rift mantle-magma samples 

and pre-rift upper-mantle rocks suggest that Hutaymah 

magmatism resulted from predominantly lithospheric as 

opposed to asthenospheric mantle melting. In general, 

these results suggest that it is important to consider the 

effects of subcrustal lithospheric magmatism in addition 

to asthenospheric melting during early stages of 

continental rifting. 

GEOCHEMISTRY OF METAPELITES OF 

SOUTHERN ALPS FROM LAKE 

MAGGIORE TO LAKE COMO 

(NORTHERN ITALY): PRELIMINARY 

RESULTS 

Annalisa Tunesi, Universitii di Milano, ltalia, Luigi 

Burlini, Universitii di Milano, ltalia, Annita Colombo, 

Universitii di Milano, ltalia 

Metapelites are the most abundant lithology of the 

Southalpine basement and crop out continuously from 

the Ivrea Zone to the Orobic Alps. The regional 

metamorphism is mainly under amphibolite-fades 

conditions (Variscan). The granulite-fades conditions 

(stronalites) are localized in the north westernmost IvreaVerbano Zone (IVZ). A high-temperature and lowpressure condition, with sillimanite and andalusite 

growth, overprints the amphibolite-fades assemblage of 

the Serie dei Laghi (SOL) and Lake Como region (LC). 

The IVZ is composed of a mafic-ultramafic complex 

associated with metapelites and minor marble lenses. 

Southwest of the IVZ, the Serie dei Laghi occurs; it is 

represented by metapelites (Scisti dei Laghi) and 

metarenaceous sequences (Strona-Ceneri). The whole 

SOL is intruded by Ordovician metagranitoids and 

Permian granites (Graniti dei Laghi). The Southalpine 

Orobic basement is mostly represented by metapelites 

(sdsti di Edolo; Filladi di Ambria; Gneiss di Morbegno 

p.p.) with minor occurrences of orthogneisses, 

amphibolites, quartzites and marbles. 

Major-, trace-element and REE chemical analyses 

were performed on metapelites from the IVZ, SOL, and 

the eastern LC region. Al203 content ranges from 25 to 

15 wt percent and Si02 from 55 to 70 wt percent. The 

most aluminiferous samples were from the LC whereas 

the most siliceous were from the SOL schists. The 

chemical data indicate a nearly uniform composition 

with respect to the major elements, except for the LC 

metapelites, which exhibit lower CaO and MgO contents 

than IVZ and SOL. Trace elements point out slight 

differences among the groups. Particularly, the LC and 

IVZ pelites show similar Ba and Sr contents, which are 

lower than those of the SOL. The low K/Rb ratios of all 

metapelites (250-500) indicate no significant depletion in 

LILE (DGT trend show K/Rb between 500 and 5000), as 

is also indicated by a Rb/Sr ratio about 1. REE patterns 

are homogeneous with a slightly negative Eu anomaly 

(Eu/Eu*=.0.5-0.7) as well as LREE/HREE fractionation 

(LaN /~ about 10 for SOL and LC, whereas for IVZ 

the ratios are 7-8). The overall trace-element and REE 

patterns are consistent with those of averaged shales of 

the upper crust (PAAS), and no significant depletion is 

Ivrea-Verbano Zone Woibhop, 1992 19 
observed as regards those elements. The strong ULE 

enrichment with respect to mantle abundance by factors 

of 1o2 is again typical of the upper crust. 

We compared the studied rocks with available data 

from stronalites because the latter are considered to be 

the corresponding lithotype re-equilibrated under highgrade conditions. In a normalized rock-P AAS 

spidergram all the rocks show a similar trend with low 

depletion in K20, Rb, and Zr and more pronounced Cr 

depletion for the stronalites samples. Moreover, REE 

patterns display the same trends and are comparable 

with those of upper crust. Therefore, the amphibolite 

and granulite facies metapelites cannot be distinguished 

chemically with the available data. 

U-PB AGES OF ZIRCONS FROM 

CHROMITITES OF THE PHLOGOPITE 

PERIDOTITE OF FINERO, IVREA ZONE, 

NORTHERN ITALY 

A. von Quadt, ETH Ziirich, Switzerland, A. Ferrario, 

Universita di Milano, Italia, V. Diella, Centro di Studio per 

Ia Stratigrafia e Petrografia delle Alpi Centrali, CNR, Italia, 

W. Hansmann, ETH Ziirich, Switzerland, G. Vavra, ETH 

Ziirich, Switzerland, V. Koppel, ETH Ziirich, Switzerland 

Ferrario and Garuti (1990) reported the unusual 

presence of zircons in chromitites that occur as small 

and discontinuous stratiform to podiform bodies within 

the outer zone of the phlogopite peridotite of Pinero. 

The zircons are irregularly distributed within the 

chromitites. In polished sections 10-25 grains, many 

with dimensions as reat as 400 J.lm, are observed in 

areas of < 100 mm . Euhedral inclusions occur in 

chromite, olivine and serpentinized olivine. Most 

zircons are subhedral to anhedral and occur between 

olivine and chromite. The zircons show no optical 

zoning and are virtually free of inclusions. However, 

cathode luminescence revealed sharp and often curved 

zoning. Microprobe investigation yielded variable 

concentrations of U, Hf, and Y of as much as 4000 ppm, 

1.5 percent, and 700 ppm, respectively. 

Four grain-size fractions, which included also 

fragments of coarser crystals, were analyzed. The U 

concentrations range between 1100 and 1240 ppm. One 

fraction yielded concordant ages of 204 .±4 Ma, whereas 

the others are slightly discordant. The data points define 

a discordia with an upper intercept at 207 ±5 Ma and a 

lower intercept at zero Ma. No evidence of an older 

inherited zircon component was detected. 

When trying to assess the significance of the zircon 

ages, one has to consider the following points: 

20 Ivrea-Verbano Zone Workshop, 1992 

-The irregular zircon distribution suggests that Zr 

was introduced relatively late into a magma and did not 

homogenize. The zircons are not accompanied by other 

minerals indicative of a highly differentiated melt. 

-Textural evidence indicates that zircon 

crystallization started relatively early, but the majority of 

the crystals formed after chromite and olivine. 

-The presence of curved crystal faces is indicative of 

growth at high temperatures (>7500C), and the 

preservation of sharp zoning boundaries argues against 

a prolonged period of annealing at high temperatures. 

-Zircons have until now been observed only in the 

phlogopite peridotite, where they seem to be 

preferentially associated with chromite. 

-According to Ferrario and Garuti (1990) the 

mineralogy and mineral chemistry of the platinumgroup minerals, the base-metal sulfides, and alloys, as 

well as the presence of Mg-rich carbonates and 

phlogopite is best explained by the presence of an alkalirich fluid during crystallization. 

The presence of zircons is also considered to have 

resulted from this alkali-metasomatism. Textural 

relationships and the irregular zircon distribution 

suggest that the metasomatism probably occurred 

during the ascent of the magma and/or during 

crystallization. 

Stahle and others (1990) reported 207pb;206Pb zircon 

ages from syenite pegmatite dykes that crosscut the 

phlogopite peridotite of 225 ±13 Ma. K-Ar ages of 206 

Ma were obtained by Hunzike~ and 40 Ar ;39 Ar plateau 

ages of 206.5 ±2.5 Ma by Friedrichsen (personal 

communication). 

The Triassic event reported here coincides with the 

opening of the Paleotethys which was accompanied in 

the Eastern and Southern Alps by a widespread alkalic 

magmatism. 

28~------~------~--~~-., 

30 

32 

34-

0.0500 0.0503 0.0505 0.0508 

238u t206pb vs 207Pb/206Pb plot for zircon fractions from a chromite 

layer within the phlogopite peridotite of Pinero. 
SEISMIC VELOCITY CALCULATIONS 

AND STRUCTURAL ASPECTS OF THE 

BALMUCCIA PERIDOTITE 

Alba Zappone, CNR-IGL/Universita di Milano, ltalia 

The Ivrea-Verbano Zone (IVZ) is thought to be a 

cross section through the continental crust-mantle 

transition. It is not a continuous sequence because 

important shear zones shorten the total thickness and 

juxtapose different lithological units. Published 

geobaromebic calculations indicate that the deepest part 

of the IVZ is exposed in Val Sesia. 

The peridotitic body and its surroundings are 

exposed near the village of Balmuccia. An area of about 

3 Jan2 between Balmuccia and Isola was selected for 

detailed structural fieldwork. In this area, the contact 

between the Balmuccia peridotite and layered gabbroic 

rocks is exposed. In the same area, representative 

samples of the peridotite body were collected. The 

attitudes of compositional banding and foliation in the 

peridotite and gabbroic rocks east of the peridotite are 

subvertical, and strike northwest in the eastern part of 

the area, whereas in the western part, near the Canavese 

Line, a more recent foliation striking northeast is 

superposed. Two main high-temperature shear zones 

(at least amphibotite facies) were recognized in the basic 

complex east of the peridotite body. These join together 

on the lsola-Cima Lavaggio ridge. One of these shear 

zones shows right-lateral movement, whereas the other 

is a left-lateral shear zone, stretching the banding toward 

the southwest. The high-temperature shear zones are 

cut by a northeast-trending system of very narrow and 

abundant low-temperature shear zones (greenschist 

facies), with a right-lateral movement generally of a few 

centimeters. 

About twenty samples of peridotite, showing 

different degrees of deformation, were collected. The 

crystallographic orientations of about 100 grains in each 

sample were measured in order to characterize the fabric 

dependence of seismic behaviour of the peridotite 

through the application of the Bunge (1985) method. 

THE IGL SEISMIC NETWORK IN THE 

IVREA-VERBANO ZONE 

A. Zappone, CNR-IGL/Universita di Milano, Italia, A. 

Morrone, M. Maistrello, M. Demartin, R. de Franco, A. 

Corsi, G. Boniolo, and G. Biella, lstituto per Ia GeofisiCil 

della Litosfera, Milano, Italia 

Since December 1991 the Istituto per Ia Geofisica 

della Litosfera (IGL) has been operating a local seismic 

network in the Northern part of the Ivrea-Verbano Zone. 

The IGL network is composed of five recording 

stations (three components MARS88) with automatic 

trigger; the stations work independently and record data 

on floppy disks. The mean distance between two 

recording units is about 20 km. The seismic stations are 

set up to record regional earthquakes and teleseisms. 

Rays coming from great distances are supposed to be 

near vertical at the station site, crossing vertically the 

geological bodies. 

The locations of the recording stations were chosen in 

order to have a sampling of the seismic behavior of the 

most representative lithologies of the Ivrea Zone, which 

are kinzigites (Cannobina Valley), stronalites (Strona 

Valley), and gabbros (Mastallone Valley). The other two 

stations were placed on Serle dei Laghi metapelites and 

along the Insubric Line (Fobello and Rimella schists). 

The main objectives are to investigate, as far as 

possible, the deep geometries of the Ivrea Body and to 

clarify the relationships between the geophysical and the 

geological Ivrea Body. These new data can be 

considered an integration of the data previously 

recorded by wide-angle and near-vertical experiments in 

this area (CROP-ECORS 1985 and CROP 1986). 

During the five months in which the IGL network has 

been operating, about 25 seismic events were recorded 

by more than three stations. 

Ivrea-Verbano Zone Workshop, 1992 21 
Figure1. TheiGLnetwork: === Mesozoicsediments, -_-_-

Serie dei Laghi, =--= Ivrea-Verbano Zone, Austroalpine 

and Pennine Units, '\7 3-components recording stations, - 100-

Bouguer anomaly contur lines of 10 mgal (after A. Guillarme and 5. 

Guillarme). 

-~~"" U l LLLU IIJ LLLil.U_LLU .LI.Ul . [W_LULLLil.li.LLLLLLLLLLLLUJ.JJ.ll ..... .29 .JQ "" ~" 

Figure 2. Seismograms of a regional earthquake recorded by four 

recording units (vertical component of the IGL network). 

22 Ivrea-Verbano Zone Workshop.1992 
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Ivrea-Verbano Zone Workshop, 1992 23 
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Ivrea-Verbano Zone Workshop, 1992 25 
AUTHOR'S ADDRESS LIST 

Robert A. Ayuso 

U.S. Geological Survey 

954 National Center 

12201 Sunrise Valley Drive 

Reston, VA 22092 

(703) 648-6347 

L.Bailo 

Dipartimento di Scienze della terra 

Universita di Milano 

Via Botticelli 23 

I-20133 Milano 

IT ALIA 

FAX: (39)-270-638-681 

Fred Barker 

U.S. Geological Survey 

Box 25046, MS 913 

Denver Federal Center 

Denver, CO 80225 

(303) 236-1248 

FAX: (303) 236-

Guilhem Barruol 

Laboratoire de Tectonophysique 

Place Eugene Batallion 

34060 Montpellier Cedex 5 

FRANCE 

(33) 67-14-30-30 

FAX: (33) 67-14-36-03 

Giancarlo Biella 

CNR Istituto per Ia Geofisica della litosfera 

Via Bassini 15 

I-20133 Milano 

IT ALIA 

(39)-2-266-5508 

FAX: (39)-2-236-2946 

G. Boniolo 

CNR Istituto per Ia Geofisica della litosfera 

Via Bassini 15 

I-20133 Milano 

IT ALIA 

(39)-2-266-5508 

FAX: (39)-2-236-2946 

26 Ivrea-Verbano Zone Wotkshop, 1992 

Attilio Boriani 

Departimento di Scienze della Terra 

Universita di Milano 

Via Botticelli 23 

I-20133 Milano 

ITAUA 

(39)-223-698-310 

FAX: (39)-270-638-681 

K. H. Brodie 

Department of Geology 

University of Manchester 

Manchester M13 9PL 

UNITED KINGDOM 

FAX: (44)-61-275-3947 

Luigi Burlini 

Dipartimento di Scienze della terra 

Universita di Milano 

Via Botticelli 23 

I-20133 Milano 

IT ALIA 

(39)-223-698-312 

FAX: (39)-270-638-681 

Diane Oemens-Knott 

Division of Geological &: Planetary Sciences 

California Institute of Technology 

Pasadena, CA 92235 

(818) 356-6123 

FAX: (818) 568-0935 

Annita Colombo 

Dipartimento di Scienze della terra 

Universita di Milano 

Via Botticelli 23 

I-20133 Milano 

ITAUA 

(39)-223-698-310 

FAX: (39)-270-638-681 

A. Corsi 

CNR Istituto per Ia Geofisica della Litosfera 

Via Bassini 15 

1-20133 Milano 

ITAUA 

FAX: (39)-2-236-2946 
Roberto de Franco 

CNR Istituto per Ia Geofisica della Litosfera 

Via Bassini 15 

1-20133 Milano 

IT ALIA 

FAX: (39)-2-236-2946 

Gabriella Demarchi 

Istituto di Mineralogia e Petrografia 

Universit\ degli Studi di Trieste 

Piazzale Europa 1 

1-34100 Trieste 

IT ALIA 

(39)-40-676-3250 

FAX: (39)40-676-3176 

M. Demartin 

CNR Istituto per Ia Geofisica della litosfera 

Via Bassini 15 

1-20133 Milano 

IT ALIA 

FAX: (39)-2-236-2946 

Roger Denlinger 

U.S. Geological Survey 

Hawaiian Volcano Observatory 

P.O. Box 51 

Hawaiian National Park, HI 96718 

(808) 967-8825 

FAX: (808) 967-8890 

V. Diella 

Centro di Studio per Ia Stratigrafia 

e Petrografia delle Alpi Centrali 

C.N.R. 

I-20133 Milano 

IT ALIA 

FAX: 

Paul Evans 

Department of Geology 

University of Manchester 

Manchester M13 9PL 

UNITED KINGDOM 

FAX: (44)-61-275-3947 

G. Lang Farmer 

Univeristy of Colorado 

Center for Geochronological Research 

Boulder, CO 80309-0450 

(303) 492-6387 

FAX: (303) 492-6388 

A. Ferrario 

Dipartimento di Scienze della terra 

Universit\ di Milano 

Via Botticelli 23 

1-20133 Milano 

ITAUA 

(39)-2-23-63-83-02 

FAX: (39)-270-638-681 

David M. Fountain 

University of Wyoming 

Department of Geology and Geophysics 

P.O. Box 3006, University Station 

Laramie WY 82071 

(307) 766-6299 

FAX: (307) 766-6679 

A. Galletti 

Dipartimento di Scienze della terra 

Universit\ di Milano 

Via Botticelli 23 

1-20133 Milano 

ITAUA 

FAX: (39)-270-638-681 

Dieter Gebauer 

Institut fiir Kristallographie und Petrographie 

ETH-Zentrum 

CH-8092 Ziirich 

SWITZERLAND 

(41)-12-56-36-55 

FAX: (41)-12-12-62-52-41 

Werner Hansmann 

Eidgenossische Technische Hochschule 

Isotope Geochemistry 

NOC61 

CH-8092 Ziirich 

SWITZERLAND 

FAX: (41)-1-262-5241 

Albrecht W. Hofmann 

Max-Planck-Institut fiir Chemie 

Strasse 23, Postfach 3060 

D-6500 Mainz 

FEDERAL REPUBLIC OF GERMANY 

(49)-61-31-30-52-81 

FAX: (49)-61-31-37-10-52 

Ivrea-Verbano Zone Workshop, 1992 27 
HartmutKem 

Mineralogisch-Petrographisches 

Institut der Universetat Kiel 

Olshausen Str. 40 

2300Kiel 

GERMANY 

FAX: (49)-431-880-4457 

v. Koppel 

Eidgenossische Technische Hochschule 

Isotope Geochemistry · 

NOC61 

CH-8092 Zurich 

SWI1ZERLAND 

(41)-1-256-3833 

FAX: (41)-1-262-5241 

Meihua Lu 

Max-Planck-Institut fiir Chemie 

Strasse 23, Postfach 3060 

D-6500 Mainz 

FEDERAL REPUBUC OF GERMANY 

FAX: (49)-61-31-37-10-52 

M. Maistrello 

CNR Istituto per Ia Geofisica della litosfera 

Via Bassini 15 

1-20133 Milano 

IT ALIA 

FAX: (39)-2-236-2946 

Adriano Mayer 

Istituto di Mineralogia e Petrografia 

UniversitA degli Studi di Trieste 

Piazzale Europa 1 

1-34100 Trieste 

IT ALIA 

(39)-40-676-3250 

FAX: (39)-40-676-3176 

Maurizio Mazzucchelli 

Istituto di Mineralogia e Petrologia 

UniversitA di Modena 

ViaS. Eufemia 19 

1-41100 Modena 

IT ALIA 

(39)-59-21-80-62 

FAX: (39)-59-22-36-05 

28 Ivrea-Verbano Zone Workshop, 1992 

A. Morrone 

CNR lstituto per Ia Geofisica della litosfera 

Via Bassini 15 

1-20133 Milano 

ITAUA 

FAX: (39)-2-236-2946 

Luisa Negrini 

Istituto di Mineralogia e Petrografia 

UniversitA degli Studi di Trieste 

Piazzale Europa 1 

1-34100 Trieste 

ITAUA 

(39)-40-676-3250 

FAX: (39)-40-676-3176 

Walter Obermiller 

Max-Planck-Institut fiir Chemie 

Strasse 23, Postfach 3060 

D-6500 Mainz 

FEDERAL REPUBUC OF GERMANY 

FAX: (49)-61-31-37-10-52 

Evelina Giobbi Origoni 

Departimento di Sdenze della Terra 

UniversitA di Milano 

Via Botticelli 23 

1-20133 Milano 

ITAUA 

FAX: (39)-270-638-681 

Laura Pinarelli 

Laboratorio di Geocronologia del C.N.R. 

Via Cardinale Maffi 36 

1-56100 Pisa 

ITAUA 

(39)-50-54-24-08 

FAX: 

George Plafker 

U.S. Geological Survey 

Mail Stop 904 

345 Middlefield Road 

Menlo Park, CA 94025 

(415) 329-5689 
Giovanni Piccardo 

Departimento di Scienze della Terra 

Universita di Genova 

Via le Benedetto 15 

1-16100 Genova 

IT ALIA 

(39)-10-353-83-08 

FAX: (39)-10-352-169 

James E. Quick 

U.S. Geological Survey 

Box 25046, MS 903 

Denver Federal Center 

Denver, CO 80225 

(303) 236-0085 

FAX: (303) 236-1414 

Elisabetta Rampone 

Departimento di Scienze della Terra 

Universita di Genova 

Via le Benedetto 15 

1-16100 Genova 

IT ALIA 

(39)-10-353-83-15 

FAX: (39)-10-352-169 

Riccarda Ragettli 

Eidgenossische Technische Hochschule 

Isotope Geochemistry 

NOC61 

CH-8092 Zurich 

SWITZERLAND 

( 41 )-1-256-3833 

FAX: (41)-1-262-5241 

Giorgio Ri valenti 

Istituto di Mineralogia e Petrologia 

Universita di Modena 

Via S. Eufemia 19 

1-41100 Modena 

IT ALIA 

(39)-59-21-80-62 

FAX: (39)-59-22-36-05 

E. H. Rutter 

Department of Geology 

University of Manchester 

Manchester M13 9PL 

UNITED KINGDOM 

FAX: (44)-61-275-3947 

Rolf Schmid 

Institut fiir Mineralogie 

ETH-Zentrum 

CH-8092 Ziirich 

SWITZERLAND 

(41)-12-56-37-91 

FAX: (41)-12-52-7~ 

Silvano Sinigoi 

Istituto di Mineralogia e Petrografia 

Universita degli Studi di Trieste 

Piazzale Europa 1 

1-34100 Trieste 

ITAUA 

(39)-40-676-3250 

FAX: (39)-40-676-3176 

Arthur Snoke 

University of Wyoming 

Department of Geology and Geophysics 

P.O. Box 3006, University Station 

Laramie WY 82071 

(307) 766-6299 

FAX: (307) 766-6679 

Carl R.Thomber 

U.S. Geological Survey 

Box 25046, MS 903 

Denver Federal Center 

Denver, CO 80225 

(303) 236-1022 

FAX: (303) 236-1414 

Annalisa Tunesi 

Dipartimento di Scienze della terra 

UniversiUI di Milano 

Via Botticelli 23 

1-20133 Milano 

ITAUA 

FAX: (39)-270-638-681 

Peter Ulmer 

Institut fiir Mineralogie 

ETH-Zentrum 

CH-8092 Zurich 

SWITZERLAND 

FAX: (41)-12-52-52-41 

Ivrea-Verbano Zone Workshop, 1992 29 
G. Vavra 

Eidgenossische Technische Hochschule 

Isotope Geochemistry 

NOC61 

CH-8092 Ziirich 

SWI1ZERLAND 

(41 )-1-256-3833 

FAX: (41)-1-262-5241 

Riccardo Vannucci 

Dipartimento Scienze della Terra 

Universita di Pavia 

Via A. Bassi 4 

27100 Pavia PV 

IT ALIA 

(39)-3-82-392-251 

FAX: (39)-3-82-392-252 

Igor M. Villa 

Laboratorio di Geocronologia del C.N .R. 

Via Cardinale Maffi 36 

1-56100 Pisa 

IT ALIA 

(39)-50-54-24-08 

or I Igor M. Villa 

Isotopengeologie 

Erlachstrasse 9a 

3012 Bern 

SWITZERLAND 

FAX: (41)-31-65-49-88 

Albrecht von Quadt 

Institut fiir Mineralogie 

ETH-Zentrum 

CH-8092 Zurich 

SWITZERLAND 

(41)-12-56-3724 

FAX: (41)-12-52-5241 

Alba Zappone 

CNR Istituto per Ia Geofisica della litosfera 

Via Bassini 15 

I-20133 Milano 

IT ALIA 

FAX: (39)-2-236-2946 

30 lvrea-Velbano Zone Workshop, 1992 *U.S. GOVERNMENT PRINTING OFFICE: 1992-673-049/46051 



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