bluuebunny/crossref_metadata_2025_split huggingface dataset:
Porphyroblast-matrix microstructural relationships: recent approaches and problems
Authors: Vernon R. H. 
Published: September 2007

Summary
Careful observation and interpretation of suitably oriented sections indicates that many porphyroblasts with curved inclusion trails grow during the development of a crenulation foliation. Inclusion trails that are approximately parallel in separate porphyroblasts, or that outline axial surfaces of crenulations that are parallel in separate porphyroblasts, suggest that many porphyroblasts do not rotate, with respect to geographic coordinates, during later deformation. Moreover, where such rotation has not occurred, inclusion trails in porphyroblasts may indicate former orientations of S-surfaces that have been obliterated from the matrix.
In ideal situations, porphyroblast-matrix microstructural relationships can be used, with more confidence than before, to provide useful information on metamorphic/deformation histories and on the details of metamorphic reactions. Porphyroblast-matrix relationships are also useful in contact metamorphic situations and can provide evidence of deformation accompanying contact metamorphism. Though mostly applied to medium-grade metamorphic rocks, porphyroblast-matrix relationships can also be useful in some high-grade situations.

A model suggested by Bell & Rubenach and Bell
et al.
postulates that porphyroblasts grow in zones of relatively weak coaxial deformation between anastomosing zones of strong non-coaxial deformation during crenulation folding. Many observations appear to be consistent with this model, but it is difficult to verify, and much detailed observation and testing of critical aspects of the model are needed. Regardless of whether or not the model is accepted, curved inclusion trails may be used to relate growth of porphyroblasts to generations and stages of crenulation development.
Vertical movement in orogenic belts and the timing of metamorphism relative to deformation
Authors: Ridley John 
Published: September 2007

Summary

Any phase of deformation in an orogenic belt that causes relative vertical movements of bodies of rock by a few kilometres or more should have a recognizable imprint in the metamorphic history. The effect seen in a
P-T-t
path will depend on whether the rock body examined moved relatively up or down in the orogenic pile. An upthrown block is predicted to cool soon after the onset of movement, but the resulting
P-T
path will be insignificantly different to that resulting from a cycle of thickening and erosion. A downthrown block will undergo renewed heating after movement, and evidence of the earlier metamorphic history will probably be overprinted. It is suggested that the recognition of phases of relative vertical movement in an orogenic belt is best made by combined studies of deformation-metamorphism textural relationships, and
P-T-t
paths. The Lukmanier Pass area of the Alps illustrates the textural patterns predicted: here, a phase of deformation immediately post-dates the metamorphic peak, suggesting uplift of the area relative to the Helvetic nappes to the north.
Deformation and metamorphism in convergent orogens: a model for uplift and exhumation of metamorphic terrains
Authors: Jamieson R. A. , Beaumont C. 
Published: September 2007

Summary

A conceptual model is presented for deformation, uplift and exhumation in convergent orogens based on flexural models of foreland basin subsidence, the role of inherited rifted margins in convergent orogens, critical wedge theory and the use of
P-T-t
data to construct particle paths within the deforming orogen. Three states of overthrust orogens can be defined, based on the relative rates of mass loss by erosion and mass addition by accretion and underplating. Rapid exhumation of metamorphic terrains is most likely in the steady state, where erosion balances accretion; a constructive state, where mass addition outweighs erosion, initially results in deep burial and relatively little exhumation; a destructive state, where mass loss outweighs mass addition, results in slow exhumation or subsidence of metamorphic rocks, depending on whether mass is lost by erosion or redistributed during extension. In order to test these predictions, reliable
P-T-t
data are required on peak metamorphism, pluton emplacement, cooling, exhumation, and uplift, for a range of localities within an orogen.
The geochronology of metamorphic processes
Authors: Zeitler Peter K. 
Published: September 2007

Summary

Metamorphic processes are complex, and the geochronology of these processes will be complex as well. A primary requirement of metamorphic geochronology is adequate time resolution, if metamorphism is to be viewed as more than just an event. Currently, geochronologists can achieve time resolution of 10
6
years or better for Phanerozoic and some Proterozoic rocks. A second requirement of metamorphic geochronology is that the systematics of dating schemes be understood in the context of the geochemical or geodynamic environment being examined. The approach that has been used most successfully, at least in young rocks, exploits the temperature-dependent nature of	daughter-isotope diffusion to make measurements of the thermal history of metamorphic belts. Specifically, the fission-track and
40
Ar/
39
Ar methods can be used to trace the post-peak temperature
P-T
path of a metamorphic rock from 500 to 100°C. Some progress has also been made in using other systems such as U-Pb, Rb-Sr, and Sm-Nd to date minerals having higher closure temperatures. New technologies and techniques of interpretation, such as the use of
40
Ar/
39
Ar age spectra to determine diffusion profiles, or the ion microprobe to determine age zoning in zircon, are making available a great deal of additional geochronological information. In general, however, before tight constraints on the evolution of metamorphic belts can be obtained, many more diffusion data for systems of geochronological interest are needed, as are better constraints on the stability relationships under metamorphic conditions of accessory minerals such as zircon.
K-Ar and
40
Ar/
39
Ar dating of cleavage formed during very low-grade metamorphism: a review
Authors: Reuter A. , Dallmeyer R. D. 
Published: September 2007

Summary

Dating cleavage formed during very low-grade (anchizonal) metamorphism of pelitic rocks by K-Ar and/or
40
Ar/
39
Ar whole-rock analysis is complicated bypolymineralogical sample characteristics and potential argon inheritance within detrital grains. Complex mineralogical effects may be resolved by careful resolution of both K/Ca and
40
Ar/
39
Ar age spectra. However, detrital influence can be maintained even within penetratively cleaved metapelite samples (whole-rock and constituent white-mica-rich size fractions down to 0.4–0.63 μm) until metamorphic conditions of the anchizone/epizone boundary. The
40
Ar/
39
and K-Ar whole-rock dating of cleaved metatuffs interbedded with anchizonal metapelites has yielded results consistent with biostratigrapic constraints on the age of cleavage formation, indicating a lack of detritai influence. Constituent grains within white-mica-rich size fractions separated from upper anchizone metatuffs are characterized by well-defined margins and record mutually concordant K-Ar and
40
Ar/
39
Ar total-gas ages which display no systematic variations with grain size. Middle anchizone metatuff size fractions are composed of white mica grains with poorly defined edges. These consistently record
40
Ar/
39
Ar total-gas ages which are 10–15% older than corresponding K-Ar dates (whole-rock and size fractions). This discordance is attributed to recoil-loss of
39
Ar during irradiation, and implies that morphology of grain edges (affecting surface/volume ratios) can be a major factor controlling
39
Ar recoil.
Discordant relationship between degree of very low-grade metamorphism and the development of slaty cleavage
Authors: Kisch Hanan J. 
Published: September 2007

Summary
Definitive comparisons of data compiled on the grade of incipient metamorphism associated with the appearance of slaty cleavage in different areas are impeded by the lack of uniform characterization of the slaty cleavage. However, the grade associated with the appearance of a well-developed, closely spaced, continuous slaty cleavage is not uniform: though low- to mid-anchimetamorphic in many areas, it ranges from high diagenetic to ‘epimetamorphic’.
This discordant relationship may in part reflect differences in lithology of the preexisting rocks, taking place at lower grades in pelites and marls than in greywackes. It may also possibly reflect differences of fabric and preservation of pre-deformational illite ‘crystallinities’ during subsequent regional deformation and cleavage formation, for instance due to lack of synkinematic recrystallization of phyllosilicates in the initial stages of formation of crenulation cleavage in pelites with a strong pre-existing layer-parallel fabric.
Another explanation for the discordance is believed to lie in the different time relations between deformation and metamorphism in the various terrains: in many cases the assumed syncrystalline nature of the deformation has been insufficiently substantiated. More information—specifically including TEM/SEM studies on the deformation of the newly formed phyllosilicates—are needed to interpret the nature of the discordance adequately.
The influence of fluids on the thermal history of a metamorphic terrain: New Hampshire, USA
Authors: Chamberlain C. P. , Rumble Douglas 
Published: September 2007

Summary

The metamorphic terrain in central New Hampshire is characterized by over 9000 km
2
of primarily sillimanite — muscovite-bearing rocks. Superimposed on this large thermal high are a series of hot spots about 20 km
2
in area. Thermal models suggest that the large-scale thermal structure of central New Hampshire can be explained by conductive heat transfer processes. However, the hot spots can neither be explained by conductive heat transfer nor by intrusion of plutons. Isotopic and petrological studies demonstrate that the hot spots were formed by the focusing of hot metamorphic fluids into zones that had relatively high fracture permeability.

The infiltration of hot fluids has significantly affected the
P-T
paths of rocks in central New Hampshire. Within the region of abundant hot spots, the
P-T
history of the metamorphic rocks is different from that outside the hot spot region. The
P-T
paths in the central portion of New Hampshire, the area with the hot spots, are characterized by isobaric-heating paths. The rocks passed near aluminium silicate triple point pressures and temperatures and later equilibrated at similar pressures, around 4.0 kbars, and at higher temperatures, approximately 650–750°C. The absence of other mechanisms to produce the isobaric-heating paths and the evidence for intense hydrothermal activity suggest that fluids played an integral role in creating the thermal spikes along the
P-T
trajectory.

Analytical solutions to differential equations that describe focused fluid flow are used to estimate the duration of hydrothermal activity and the amount of fluid involved. The steep thermal gradients adjacent to the hot spots and the observed temperature increase within hot spots are consistent with large fluid fluxes, > 1 × 10
−9
m
3
fluid per m
2
s
−1
, over short periods of time, c. 100000 years. This indicates that
P-T
paths from rocks within hot spots record only small portions of the geological evolution of the metamorphic terrain.
Implications of thermal and baric structure for controls on metamorphism: northern New England, USA
Authors: Day Howard W. , Chamberlain C. Page 
Published: September 2007

Summary
Isograds and bathograds define an apparent thermal and baric structure of the Appalachian metamorphic belt in northern New England. Peak metamorphism occurred primarily during the Devonian Acadian orogeny at about 390 Ma. Kyanite-sillimanite metamorphism took place along heating-decompression paths and andalusite-sillimanite metamorphism is consistent with isobaric heating or limited decompression during heating. Isograds were deformed by early nappes and late domes, and dip steeply in some areas. The aluminium silicate bathograd dips gently east towards lower preserved pressures, intersects the regional trends of isograds, and is deformed by late cross-folds. Argon ages of micas as young as 250 Ma in the highest grade rocks record a long postmetamorphic cooling history, whereas lower grade rocks record much older argon ages. Comparison of these observations with simple thermal models suggests that differences of uplift history did not dominate the prograde metamorphic history and that spatial variations of the pre-tectonic thermal regime or of thermophysical characteristics of the lithosphere were important influences on the observed metamorphism.
Thermal evolution of the Kisseynew sedimentary gneiss belt, Manitoba: metamorphism at an early Proterozoic accretionary margin
Authors: Gordon T. M. 
Published: September 2007

Summary

The Kisseynew sedimentary gneiss belt forms part of an early Proterozoic accretionary margin preserved between two Archaean cratons in the south-west Canadian Shield. The sediments are volcanogenic turbidites deposited from flanking volcanic belts at 1910–1875 Ma. Two compressional tectonic events preceded 1815 Ma metamorphism, which reached conditions of 750°C at about 5.5 kbar. Subsequent uplift took the rocks through to 280°C at about 1705 Ma. Thermal modelling suggests that, over this time interval, the average uplift rate was 0.06 km Ma
−1
. The K, U and Th analyses of the metasediments, combined with the estimated burial depth, show that in situ radiogenic heat production was insufficient to cause the high-temperature, moderate-pressure metamorphism. A modern analogy in southern Alaska suggests that elevated thermal flux due to magmatic intrusion may be a common phenomenon during the late stages of evolution of accretionary environments.
An empirical model for metamorphic evolution of the Archaean Slave Province and adjacent Thelon Tectonic Zone, north-western Canadian Shield
Authors: Thompson Peter H. 
Published: September 2007

Summary

On the basis of
P-T
estimates derived from metamorphic assemblages and the geological setting before and after peak metamorphism, Archaean thermal evolution is attributed to overthickening of previously thinned sialic crust and overlying supracrustal basins. The extensional phase increases geothermal gradients and forms relatively ductile zones within which subsequent compression transports supracrustal rocks to metamorphic pressures, contributes to the rise of isotherms to high levels in the crust, and causes the moderate uplift and erosion required to bring low-
P
-high-
T
rocks to the surface of crust 35 km thick.

Proterozoic upper amphibolite- and granulite-facies metamorphism and plutonism in the western Thelon Tectonic Zone is attributed to a similar thermal evolution 500 Ma later. Continued convergence across a long linear belt of overthickened, partially melted crust caused hot upper and middle crustal-level rocks to be thrust on to the Slave Province and formation of a medium pressure overprint on the Archaean.

The empirical models derived for the region generate low-
P
-high-
T
metamorphism without excessive overthickening or major heat transfer by granitic magmatism. For both terrains, metamorphic data, considered in their geological context, are consistent with an ensialic intra-plate tectonic setting rather than an ensimatic, subduction-related interplate setting.
P-T
-time history and thermal modelling of an anorthosite-granulite interface, Eastern Ghats metamorphic belt, India
Authors: Mukherjee Amalbikash 
Published: September 2007

Summary

A pressure-temperature-time synthesis and a thermal model are presented for the anorthosite-massif-granulite interface near Bolangir, Orissa, India: a type area of the Eastern Ghats metamorphic belt. While the anorthosite pluton can be shown to have been forcefully intruded as a schlieren dome into the granulite cover, the calculated mineral equilibria of the interface suggest a
P-T
convergence at 620–740°C and 4.7–7 kbar and a nearly isobaric cooling in this pressure range under a cratonic overburden between 15 and 25 km. Parameterized model calculations suggest that a noritic anorthosite pluton of 7.5 and 10 km radius with 50% initial crystallinity and a lower crustal contact temperature in the region of 800°C would cool from the initial temperature of 1200°C to 850–900°C in about 20 Ma and then the cooling rate would fall dramatically to the order of 0.75°C Ma
−1
. Finite difference calculations of the heat conduction from the equilibrated anorthosite-granulite interface suggest that for some combinations of the thermal properties of the medium and the heat generation and uplift rates, boundary conditions based on an initial transient elevated geotherm produce a nearly isobaric cooling path. The initial prograde metamorphism of the granulites, the anorthosite invasion and the final cooling and
P-T
convergence of the anorthosite- granulite interface have resulted in a composite overturned (anticlockwise) Y-shaped
P-T-t
configuration. It is suggested that this configuration is characteristic of the Eastern Ghats type metamorphic belt, representatives of which seem to be widespread in the Archaean to Proterozoic continental segments of North America, Scandinavia and parts of Africa, Western Australia and Antarctica. In contrast to the Alpine type belts, the Eastern Ghats type belts are much wider and thickly cratonized with low isostatic resilience (low uplift/exhumation rate) and have anorthosite intrusive cores acting as ‘exotic’ heat sources, resulting in characteristic anticlockwise overturned and forked Y-shaped
P-T-t
loops.
P-T
-fluid regimes of metamorphism and related magmatism with specific reference to the granulite-facies Sharyzhalgay complex of Lake Baikal
Authors: Perchuk Leonid L. 
Published: September 2007

Summary

Petrological studies, including geothermobarometry, of metamorphic mineral parageneses together with isotopic data, reveal differences in the
P-T
-fluid regimes of Precambrian shields and surrounding fold belts. Typical features of shields are CO
2
-rich fluids and preservation of retrograde portions of
P-T
trajectories, while in fold belts, prograde as well as retrograde stages of metamorphism are recorded in the minerals and the CO
2
contents of the fluids vary widely. Geothermobarometric estimates of prograde and retrograde stages in several metamorphosed fold-belt complexes are in accord with the distribution of Al
2
SiO
5
polymorphs.

A model involving vertical redistribution is proposed for the granulite-facies Sharyzhalgay complex of Lake Baikal, which is interpreted to be the metamorphosed equivalent of the Karelian greenstone belt. Increased heat flow and burial leads to gravitational redistribution due to density differences, so that dome structures cored by granulite-facies gneiss occur within envelopes of down-sinking metabasites and meta-ultrabasites. The model is supported by the geochronology and by the
P-T
-fluid regimes deduced from geothermobarometry and fluid-inclusion studies of the polymetamorphic Sharyzhalgay complex. This model may apply to other granulite-facies complexes which appear to be the result of high-grade metamorphism of Archaean greenstone belts made up of tholeiitic and komatiitic volcanics. These rocks were originally underlain by a complex of acid volcanics (dacites and rhyolites).
The Precambrian mobile belt of southern Madagascar
Authors: Ackermand D. , Windley B. F. , Razafiniparany A. 
Published: September 2007

The Precambrian of southern Madagascar consists largely of medium- and high-grade paragneisses and metasediments that have a predominantly north-south regional trend, which is parallel to that of the Mozambique belt of eastern Africa. The Precambrian geology of Madagascar is synthesized by Besairie (1971) and Hottin (1976). Much mineralogical information of relevance to metamorphic petrology is given by Lacroix (1922–23). Nicollet (1985, 1986) gives good metamorphic and
P-T
data from Ihosy and Vohibory (Fig. 1).

Figure 1 shows the distribution of the main metamorphic belts in the south of the island. Included in the amphibolite-facies belts are low-, medium- and high-grade types, and the granulite-facies belt includes pyroxene- and hornblende-facies types as well as some high-grade amphibolite facies (Vachette 1979). Most abundant are gneisses with various combinations of cordierite, sillimanite, garnet and biotite. The widespread development of cordierite reaches a peak in cordieritites which form layers up to several tens of metres thick in the gneisses. In the gneisses, sillimanite is locally concentrated in sillimanitite layers up to several metres thick, and marble and quartzite layers reach a few metres in thickness. Diopsidites occur in the gneisses as conformable layers up to several metres thick and have pegmatitic pockets with phlogopite, diopside, scapolite and anhydrite; some phlogopite crystals are over 1 metre long and phlogopite is mined locally (Lacroix 1941). Pegmatites occur in most rocks.
Foliation and layering are predominantly parallel and steep to vertical. Isoclinal folds have axes which plunge shallowly north-south, but in general there is a notable paucity
Multiple granulite-facies events in the southern Appalachians, USA
Authors: McLellan Eileen , Linder Daniel , Thomas Jenny 
Published: September 2007

Summary

This paper summarizes present knowledge of an area in the southern Appalachians in which two granulite-grade events can be recognized in a complexly deformed suite of rocks. Metamorphic conditions in the first event exceeded 800°C, whereas those in the second attained only 700°C. Both events are shown to involve prograde dehydration; the rocks must therefore have experienced multiple
P-T-t
loops. Structurally interleaved with these rocks is a suite of less-deformed rocks which only experienced the second of these events and may represent cover lithologies to the polymetamorphic basement.
Inverted metamorphic zonation and Variscan thrust tectonics in the rouergue area (Massif Central, France):
P-T-t
record from mineral to regional scale
Authors: Burg J. P. , Leyreloup A. F. , Romney F. , Delor C. P. 
Published: September 2007

Summary

Field evidence together with microstructural analysis define three types of tectonically produced inverted metamorphic zonation in the Variscan terrains of Rouergue (southern French Massif Central). They are associated with large-scale thrusting of rock slabs metamorphosed prior to their southward tectonic emplacement. Mineral equilibria, garnet-biotite geothermometry and garnet-plagioclase geobarometry reveal that post-, syn- and pre-metamorphic deformations result in three different
P-T
patterns. Post-metamorphic deformations are responsible for inverted and contrasted zonations where metamorphic discontinuities are critical evidence for nappe translations. Synmetamorphic thrusting results in thrust-related metamorphic zones with inverted thermal profiles and normal baric profiles. Apparent pressure differences are artifacts of different
P-T
paths and variable uplift rates. Pressures at peak metamorphic conditions are lowest in zones of rapid uplift, such as the thrust sequences. Metamorphism may outlast all important structural events apart from late folding in cases where syntectonic intrusions are emplaced within thrust contacts. They are responsible for sharp inverted thermal profiles and they strongly modify the isograd distribution in the rocks underlying metamorphic thrust units.