bluuebunny/crossref_metadata_2025 huggingface dataset:
Plagioclase porphyroblast growth and its relation to deformation in the Alhamilla unit (Sierra Alhamilla, Betic Cordilleras, SE Spain)
Authors: EECKHOUT B. VAN DEN , KONERT G.
Published: May 2007

Abstract. Plagioclase porphyroblasts from silvergrey schists belonging to the Nevado Filabride Complex in the Sierra Alhamilla (Betic Zone, SE Spain) are interpreted as having been formed preand synkinematically with respect to the second phase of deformation. Different types of inclusion patterns represent 'snap‐shots’(high growth‐rate/strain‐rate ratio features) of the formation of a diffentiated crenulation cleavage during this second phase of deformation, by the processes of kinking, crenulation and associated differentiation.

Regional considerations indicate an Alpine age for this tectono‐metamorphic event, which can be explained by the‘hot emplacement’of the higher Nevado Filabride units. The observed structural evolution is not consistent with a pre‐Alpine polyphase deformation history.
A garnet–hornblende geothermometer: calibration, testing, and application to the Pelona Schist, Southern California
Authors: GRAHAM COLIN M. , POWELL ROGER
Published: May 2007

Abstract
A garnet–hornblende Fe–Mg exchange geothermometer has been calibrated against the garnet–clinopyroxene geothermometer of Ellis & Green (1979) using data on coexisting garnet + hornblende + clinopyroxene in amphibolite and granulite facies metamorphic assemblages. Data for the Fe–Mg exchange reaction between garnet and hornblende have been fitted to the equation. In KD=Δ (XCa,g) where KD is the Fe–Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form: with very satisfactory agreement between garnet–hornblende and garnet–clinopyroxene temperatures. The thermometer is applicable below about 850°C to rocks with Mn‐poor garnet and common hornblende of widely varying chemistry metamorphosed at low aO2.

Application of the garnet–hornblende geothermometer to Dalradian garnet amphibolites gives temperatures in good agreement with those predicted by pelite petrogenetic grids, ranging from 520°C for the lower garnet zone to 565–610°C for the staurolite to kyanite zones. These results suggest that systematic errors introduced by closure temperature problems in the application of the garnet–clinopyroxene geothermometer to the ‘calibration’data set are not serious. Application to ‘eclogitic’garnet amphibolites suggests that garnet and hornblende seldom attain Fe–Mg exchange equilibrium in these rocks.

Quartzo‐feldspathic and mafic schists of the Pelona Schist on Sierra Pelona, Southern California, were metamorphosed under high pressure greenschist, epidote–amphibolite and (oligoclase) amphibolite facies beneath the Vincent Thrust at pressures deduced to be 10±1 kbar using the phengite geobarometer, and 8–9kbar using the jadeite content of clinopyroxene in equilibrium with oligoclase and quartz. Application of the garnet–hornblende thermometer gives temperatures ranging from about 480°C at the garnet isograd through 570°C at the oligoclase isograd to a maximum of 620–650°C near the thrust. Inverted thermal gradients beneath the Vincent Thrust were in the range 170 to 250°C per km close to the thrust.
Amphibolite facies metamorphism in shear zones in the Buchan area of NE Scotland
Authors: KNELLER B. C. , LESLIE A. G.
Published: May 2007

Abstract
A major system of steep Caledonian shear zones, of regional extent, has been identified in NE Scotland. The shear zones affect a wide range of lithologies, including Argyll and Southern Highland Group Dalradian, ‘Younger Basic’intrusives and their hornfelses, and also the earlier of the more acid intrusions. The observed fabrics and parageneses are consistent with low‐pressure amphibolite facies metamorphism. These shear zones represent a phase of movement which occurred in the 490‐465 Ma interval when ambient temperatures were still high, and it is concluded that this is the principal control on the metamorphic grade achieved within the shear zones, although local anomalies may exist.
Disequilibrium in co‐existing clinopyroxene and garnet from blueschist‐facies Fe‐rich eclogites, Voltri Group, Italy
Authors: MORTEN L. , BONDI M.
Published: May 2007

Abstract
Clinopyroxenes and garnets from 11 blueschist‐facies Fe‐rich eclogite samples from the Voltri Group show a wide range of chemical compositions. Detailed analyses of single pyroxene and garnet grains show wide and scattered chemical inhomogeneity, the KD(KD= (Fe2+/Mg)Gt/(Fe2+/Mg)Cpx) ranges from 20 to 87 based on rim analyses only. The data obtained indicate that the mineral pairs never attained equilibrium under uniform P‐T conditions and that the compositions of the metamorphic minerals were influenced mainly by the composition of the pre‐metamorphic minerals and by topotactical reactions.
Topological relations in multisystems of more than n+ 3 phases 1
Authors: GUO QITI
Published: May 2007

Multisystems of n+k (k > 3) phases are very complicated and knowledge of them has suffered as a result. The successful solution of the topological relationships in n+ 3 phase multisystems by Zen (1966, 1967) and Zen & Roseboom (1972) has aroused much interest regarding what will happen in a multisystem of more than n+ 3 phases. Since 1979, some important research results on this topic have been published. These results have expounded the substantial rules governing the appearance of phase relations in phase diagrams of n ‐ k (k > 3) phase multisystems. The most significant conclusions include: (1) It is impossible to incorporate all the possible phase relations in an n+k (k > 3) phase multisystem in a single closed net. Therefore, it is no longer enough to use only a single closed net to depict the topological relations involved in these types of multisystems. Instead, one or more groups of closed nets, namely the complete system(s) of closed nets are necessary for this purpose. (2) A principle called the Combination Principle has been proposed and proved. It states: Any closed net of one n+k (k > 3) phase multisystem must be a combination of two or more distinct n+ 3 order submultisystem closed nets belonging to the given n+k phase multisystem, if it is not one of the n+ 3 order submultisystem closed nets itself. The combination principle provides both a theoretical basis and a practical method for the construction of closed nets and, hence, for the derivation of the real phase diagrams for any n+k (k > 3) phase multisystem. (3) A theorem on divariant‐assemblage‐characteristic‐stability‐polygons is also important to our understanding of the n+k (k± 3) phase multisystem closed nets. This theorem can be stated as follows: A divariant assemblage of an n+k (k± 3) phase multisystem will be stable in an l‐polygon lacking diagonals in an appropriate set of closed‐net‐diagrams, and this l‐polygon may be at least a triangle, and at most a k‐polygon. In addition, the closed‐net‐diagrams of unary and binary n+ 4 phase multisystems derived respectively by Guo (1980b, 1980c, 1981a) and by Roseboom & Zen (1982) have also been summarized. The combination principle is applied to a practical petrological problem in this paper, dealing with 7 phases in the system FeO‐Fe2O3‐SiO2.
The trace element geochemistry of some high‐grade Bergen‐Jotun kindred gneisses from the Jotun Nappe around Gjendebu, central southern Norway
Authors: EMMETT TREVOR F.
Published: May 2007

Abstract
The high‐grade meta‐plutonic rocks of this study lie entirely within the Jotun Nappe of the southern Norwegian Caledonides. They are divisible on the basis of metamorphic grade and petrographic character into three units, the Storadalen Complex (SCX), the Svartdalen Gneiss (SG), and the Mjølkedøla Purple Gabbro (MPG). The SCX is a differentiated series of ultrabasic to intermediate rocks now showing only tectonite fabrics. It has been metamorphosed to spinel‐Iherzolite granulite facies grade. The broadly monzonitic SG is weakly tectonized and internally differentiated. Its metamorphic grade does not exceed plagioclase‐lherzolite granulite facies grade. The mis‐named MPG is also broadly of monzonitic composition but it retains a coarse ophitic texture, and is of amphibolite facies grade. A gradational boundary exists between the MPG and SG, but the contact between these two units and the SCX is the steeply dipping Tyin‐Gjende Fault. The three units represent a comagmatic body of mid‐Proterozoic age, metamorphosed during a Sveconorwegian event and finally dismembered and upthrust during the Caledonian Orogeny.

The new trace element analyses reported here show that the three rock units have remarkably similar trace element abundances and trends. K‐Rb covariation shows increasing K/Rb ratios with increasing K. These patterns were produced by magmatic fractionation processes acting at deep crustal levels, possibly in the presence of a non‐aqueous fluid phase. With the exception of K and Sr, close similarities exist between the rocks of this study and present‐day calc‐alkaline basalts and andesites from island arcs. The high K content is regarded as a primary magmatic feature, but the available data are insufficient to indicate its origin. The Sr contents are abnormally high and are ascribed to metasomatism which occurred during either high‐grade metamorphism or post‐climactic cooling. There are no systematic geochemical variations with metamorphic grade or degree of deformation.
Crystal chemistry of a Mg‐vesuvianite and implications of phase equilibria in the system CaO‐MgO‐Al2O3‐SiO2‐H2O‐CO21
Authors: VALLEY J. W. , PEACOR D. R. , BOWMAN J. R. , ESSENE E. J. , ALLARD M. J.
Published: May 2007

Abstract
Chemical analysis (including H2, F2, FeO, Fe2O3) of a Mg‐vesuvianite from Georgetown, Calif., USA, yields a formula, Ca18.92Mg1.88Fe3+0.40Al10.97Si17.81‐ O69.0.1(OH)8.84F0.14, in good agreement on a cation basis with the analysis reported by Pabst (1936). X‐ray and electron diffraction reveal sharp reflections violating the space group P4/nnc as consistent with domains having space groups P4/n and P4nc. Refinement of the average crystal structure in space group P4/nnc is consistent with occupancy of the A site with Al, of the half‐occupied B site by 0.8 Mg and 0.2 Fe, of the half‐occupied C site by Ca, of the Ca (1,2,3) sites by Ca, and the OH and O(10) sites by OH and O. We infer an idealized formula for Mg‐vesuvianite to be Ca19Mg(MgAl7)Al4Si18O69(OH)9, which is related to Fe3+‐vesuvianite by the substitutions Mg + OH = Fe3++ O in the B and O(10) sites and Fe3+= Al in the AlFe site.

Thermodynamic calculations using this formula for Mg‐vesuvianite are consistent with the phase equilibria of Hochella, Liou, Keskinen & Kim (1982) but inconsistent with those of Olesch (1978). Further work is needed in determining the composition and entropy of synthetic vs natural vesuvianite before quantitative phase equilibria can be dependably generated. A qualitative analysis of reactions in the system CaO‐MgO‐Al2O3‐SiO2‐H2O‐CO2 shows that assemblages with Mg‐vesuvianite are stable to high T in the absence of quartz and require water‐rich conditions (XH2O > 0.8). In the presence of wollastonite, Mg‐vesuvianite requires very water‐rich conditions (XH2O > 0.97).
Precambrian granulites of the Aldan shield, eastern Siberia, USSR
Authors: PERCHUK L. L. , ARANOVICH L. Ya. , PODLESSKII K. K. , LAVRANT'EVA I. V. , GERASIMOV V. Yu , FED'KIN V. V. , KITSUL V. I. , KARSAKOV L. P. , BERDNIKOV N. V.
Published: May 2007

Abstract
Precambrian granulites of the Aldan shield in southern Yakutia, USSR, form a massif of 200,000 km2 bounded by younger fold‐belts to the south, west and east. The massif consists of several blocks that reflect a primary heterogeneity of composition and differences in structural and thermodynamic evolution of different parts of the area. According to structural and petrological data the massif can be divided into two megablocks: eastern Aldan and western Aldan. They are separated by a narrow meridional fold‐belt. Structural evolution of this central zone was determined by the geodynamics of the mega‐blocks and was completed in the late Archaean. Towards the south, this central zone is ‘transformed’into the relatively small Sutam block adjoining the Stanovoy fold‐belt that bounds the Aldan shield on the south. The Sutam block is separated from the other structural units of the Aldan shield by a system of north trending grabens filled by post‐Archaean sediments.

The Aldan shield is composed of Archaean high‐grade granulites, while the Stanovoy fold‐belt, to the south, consists of highly foliated Proterozoic rocks metamorphosed under relatively lower‐grade conditions. However, relics of the granulites are mapped within the fold‐belt. They contain high‐grade assemblages (e.g. Opx + Sil + Qz, Sap + Qz, Opx + Gr + Sil, etc.). One of the relics, the Tokskii block, which is only slightly touched by diaphthoresis, is located in the southeastern part of the Stanovoy fold‐belt. Metamorphic conditions of the Tokskii block are compared with those of the Sutam block and a similar evolution of the units is revealed.

Mineral assemblages and mineral compositions do not vary within each unit, but they change in a north‐south direction. The Opx + Sil + Qz assemblage has been found only in Sutam and Tok, but not in eastern Aldan and western Aldan. The Sap + Qz assemblage has been found in the Tokskii block but has not yet been found in the Sutam block. The pyrope content in garnets, from metapelites of both blocks, is significantly higher than that from the Aldan (eastern and western blocks) rocks to the north. The most important assemblages from different units of the Aldan shield have been studied using the electron microprobe in order to unravel the metamorphic evolution of the granulites and thus to deduce the thermodynamic regime of this evolution. A geodynamic model for the Aldan shield is discussed in terms of Archaean island arc development.
P–T conditions for the Arendal granulites, southern Norway: implications for the roles of P, T and CO2 in deep crustal LILE‐depletion
Authors: LAMB R. C. , SMALLEY P. C. , FIELD D.
Published: May 2007

Abstract
The orthopyroxene‐clinopyroxene, garnet‐orthopyroxene and garnet‐clinopyroxene geothermometers, and the garnet‐orthopyroxene‐plagioclase, garnet‐clinopyroxene‐plagioclase and anorthite‐ferrosilite‐grossular‐almandine‐quartz geobarometers are applied to metabasites and the garnetplagioclase‐sillimanite‐quartz geobarometer is applied to a metapelite from the Proterozoic Arendal granulite terrain, Bamble sector, Norway. P–T conditions of metamorphism were 7.3 ± 0.5 kbar and 800 ± 60°C.

This terrain shows a regional gradation from the amphibolite facies, into normal LILE content granulite facies rocks and finally strongly LILE deficient granulite facies gneisses. Neither P nor T vary significantly across the entire transition zone. The change in ‘grade’parallels the increasing dominance of CO2 over H2O in the fluid phase.

LILE‐depletion is not a pre‐condition of granulite facies metamorphism: granulites may have either ‘depleted’or ‘normal’chemistries. The results presented herein show that LILE‐deficiency in granulite facies orthogneisses is not necessarily related to variations in either P or T. The important mechanisms in the Arendal terrain were (a) direct synmetamorphic crystallization from magma, with primary LILE‐poor mineralogies imposed by the prevailing fluid regime, and (b) metamorphic depletion, involving scavenging of LILEs during flushing by mantle‐derived CO2‐rich fluids. The latter process is constrained by U–Pb and Rb–Sr isotopic work to have occurred no later than 50 Ma after intrusion of the acid‐intermediate gneisses, and was probably associated with contemporary basic magmatism in a tectonic environment similar to a present day cordilleran continental margin.
A reconnaissance study of the distribution of Ba, Nb, Y, and Zr in some Jotun kindred gneisses from Central Jotunheimen, southern Norway
Authors: EMMETT T. F.
Published: May 2007

Abstract
The Bergen‐Jotun kindred rocks of this study, the Storådalen Complex (SCX), Svartdalen Gneiss (SG) and Mjølkedøla Purple Gabbro (MPG), have been shown to be a co‐magmatic series with calc‐alkaline affinities. The analyses of Ba, Nb, Y, and Zr presented here show no variation in these elements between the three rock units and are consistent with the calc‐alkaline character of the rocks. The lithophile elements Ba, K, and Sr are enriched relative to MORB and the high field strength elements Nb, Y, and Zr are depleted relative to MORB, Zr especially so.

The SCX contains rocks with low (>30) differentiation indices which are interpreted as plagioclase + pyroxene ± olivine ± amphibole cumulates. The remainder of the SCX, together with the MPG and SG, is regarded as the congealed liquid in equilibrium with these cumulates. The distribution of trace elements between these two components of the SCX can be adequately modelled using a Rayleigh fractionation process, measured ‘liquid’compositions, and calculated bulk distribution coefficients. It is thus concluded that the trace element geochemistry of the rocks of this study is consistent with subduction‐related, mantle‐derived magmas that fractionate within a continental or mature island arc environment. Subsequent high‐grade metamorphism and deformation of Sveconorwegian age have been essentially isochemical.
Sphalerite geobarometry in metamorphic terranes: an appraisal with implications for metamorphic pressure in the Otago Schist
Authors: JAMIESON R. A. , CRAW DAVID
Published: May 2007

Abstract
Sphalerite geobarometry has long been known to give poor results when applied to regional metamorphic terranes. Application of the sphalerite geobarometer to three low‐to medium‐grade sulphide deposits—the Moke Creek and Waitahuna deposits, Otago, and the Goose Cove deposit, Newfoundland—yields pressures up to 9 kbar, which appear to be too high when compared with other geological data. Textural and mineralogical relationships suggest that the Goose Cove and, possibly, the Moke Creek deposits lacked the required equilibrium assemblage (pyrite + hexagonal pyrrhotite + sphalerite) during peak metamorphic conditions, rendering the geobarometer inapplicable. In addition, all three deposits show evidence of re‐equilibration at T < 300°C, which has resulted in decreased FeS contents and high apparent pressures. Analyses of sphalerites from very low‐grade metachert from South Georgia Island, which contains the assemblage sphalerite + pyrite + monoclinic pyrrhotite + chalcopyrite, confirm that low‐temperature equilibration of this assemblage results in approximately 10–11 mol. % FeS in sphalerite. Comparison of these results with published descriptions of other deposits suggests that lack of the appropriate assemblage and retrograde re‐equilibration of sphalerite probably account for most anomalously high‐pressure estimates. Erratic compositions of sphalerites containing chalcopyrite inclusions may result from replacement of high‐temperature intermediate solid‐solution by chalcopyrite during cooling. Strain may enhance retrograde re‐equilibration of sphalerite by grain‐size reduction or dislocation‐assisted diffusion and/or nucleation. Re‐evaluation of the data from Moke Creek suggests that the sulphides experienced pervasive greenschist facies re‐equilibration at pressures of about 4.5 kbar, with late stage mobilization at about 2.8 kbar, and thus sphalerite compositions are not likely to reflect blueschist facies conditions. Pressure estimates based on sphalerite geobarometry should take into account at what stage in the history of a metamorphic terrane the sphalerite composition equilibrated.
Metagabbro and associated eclogites in the Lubrin area, Nevado‐Filabride Complex, Spain
Authors: MORTEN L. , BARGOSSI G. M. , MARTINEZ J. M. MARTINEZ , PUGA E. , de FEDERICO A. DIAZ
Published: May 2007

Abstract
Several small bodies of metabasite (maximum dimensions of 1000 m x 500 m) are included in the metamorphic rocks of the Nevado‐Filabride Complex in the Betic Cordilleras (Almeria Region). The body of 400 m x 100 m, located 200 m due west of the Lubrin village, contains troctolitic gabbro with well‐preserved igneous textures and mineral compositions, wholly amphibolitized gabbro, garnet‐bearing metagabbro eclogite. Along with the textural and mineral changes, sensible and regular geochemical variations can be observed, where the content of MgO decreases from 24% to 11%, while that of CaO and Na2O increases from 7% to 11% and from 2% to 3%, respectively. In addition, the content of some minor elements such as Sr, Y, Nb, Zr and Sc increases while that of Ni and Cr decreases from troctolitic gabbro to the eclogite. The amphibolitized gabbro shows values scattered around those of the troctolitic gabbro. These geochemical variations are ascribed to inherited differences in the pre‐metamorphic protolith, i.e. a fractionated gabbro which varies from olivine‐rich to clinopyroxene‐rich gabbro. Nevertheless, some metasomatism affected the Lubrin body without changing the main chemical trends, as documented by the significantly different 87Sr/86Sr ratios of each rock‐type. This points to a metasomatism which involved the introduction of crustal radiogenic strontium. The petrographical and mineral chemical features are interpreted to be the result of syn‐metamorphic fluid circulation possibly combined with deformation by shearing. The igneous texture and mineral chemistry have been retained wherever both fluid circulation and shearing were ineffective. On the contrary, where both events were effective, the formation of eclogite occurred. Later, the entire body underwent a retrogressive amphi‐bolitic stage under greenschist facies conditions, which was probably responsible for the formation of the amphibolitized gabbro portion and for the retrogression of the eclogite.
Metamorphism of the Witwatersrand gold fields: conditions during peak metamorphism
Authors: PHILLIPS G.N.
Published: May 2007

Abstract
Pelitic assemblages from all major Witwatersrand gold fields record metamorphic conditions of the greenschist facies, with minimal regional grade changes over at least 200 km strike length. Diagnostic metamorphic assemblages are less common in the volumetrically dominant quartzites, the actively‐exploited auriferous conglomerates and some of the regionally persistent metapelitic horizons. Bulk rock composition has been a major control on assemblage development.

Key metapelitic assemblages include pyrophyllite, chloritoid, chlorite and muscovite in each gold field, with less common metamorphic biotite. Accessory minerals are pyrite, tourmaline, rutile and zircon. The abundance of chloritoid and pyrophyllite in thin shaly units, together with their minor, but widespread, distribution in quartzites and conglomerates, indicate that metamorphic temperatures reached 350°C ± 50°C in all the gold fields. Pressures are less‐well constrained, 1–2 kbar being inferred. Outside the gold fields, higher grades are indicated by andalusite and kyanite near granitoid domes and later intrusions.

The temperatures during peak metamorphism and the abundance of pyrite provide ideal conditions to (re)mobilize gold and may explain its secondary textural features.
An internally consistent dataset with uncertainties and correlations: 3. Applications to geobarometry, worked examples and a computer program
Authors: POWELL R. , HOLLAND T.J.B.
Published: May 2007

Abstract
This paper provides methods and a description of a Pascal computer program, thermocalc, for various thermodynamic calculations using the thermodynamic dataset presented in earlier papers in this series (Holland & Powell, 1985; Powell & Holland, 1985). The dataset involves uncertainties on the thermodynamic parameters and therefore allows uncertainties to be calculated on results, for example in geothermometry and geobarometry. Recommendations are made for the uncertainties on activities to be used in calculations on rocks, particular emphasis being placed on preventing underestimates of these uncertainties at small mole fractions. Apposite examples of phase diagram and rock calculations are presented with ouput from thermocalc, demonstrating the utility of the program. Of the rock calculations, the most valuable are considered to be those involving simultaneous combination ‘least squares’of calculated conditions for a set of reactions applicable to a rock. This set of reactions involves the independent reactions which can be written between the end‐members in the minerals in a rock and in the thermodynamic dataset. In contrast to an approach based on specific geothermometers and geobarometers, this approach maximizes the benefit of having an internally consistent thermodynamic dataset. thermocalc is available in IBM PC and Mac versions, from Roger Powell for A$25 or Tim Holland for £10 per version.
Metamorphic evolution of the southern part of the Hidaka belt, Hokkaido, Japan
Authors: SHIBA M.
Published: May 2007

Abstract
The Hidaka metamorphic terrane in the Meguro‐Shoya area, Hokkaido, Japan is divided into four progressive metamorphic zones: A—biotite zone; B—cordierite zone; C—cordierite–K‐feldspar zone; and, D—sillimanite–K‐feldspar zone of the andalusite–sillimanite facies series type of metamorphism. The metamorphic grade ranges from the higher temperature part of the greenschist facies (zone A) through the amphibolite facies (zones B and C) to the lower temperature part of the granulite facies (zone D). The zone boundaries intersect the bedding planes at high angles. P–T conditions estimated are 450–550°C and 2 kbar for zone A, 550–600°C and 2–2.5 kbar for zone B, 600–650°C and 2.5–3 kbar for zone C and 650–750°C and 3–4 kbar for zone D. The metapelites of zone D were partially melted.

At the later stage of the regional metamorphism which is early Oligocene to early Miocene in age, cordierite tonalite and biotite tonalite intrusives associated with segments of the highest grade rocks (zone D) were emplaced into the lower temperature part of the regional metamorphic rocks, giving rise to a contact metamorphic aureole. The thermally metamorphosed terrain (zone C') belongs to the amphibolite facies and its P–T conditions are estimated to have been 550–700°C and 2 kbar.

The P–T–t paths of the Hidaka metamorphism show a thickening–heating–uplifting process. The metamorphism is inferred to have taken place beneath an active island arc accompanied by partial melting of the crust.
What controls partial melting in migmatites?
Authors: JOHANNES W.
Published: May 2007

Abstract
The layers of six stromatic migmatites from Northern, Western, and Central Europe display small but systematic chemical and mineralogical differences. At least five of these migmatites do not show any signs of largescale metamorphic differentiation, metasomatism, or segregation of melts. It is concluded, therefore, that the compositional layering observed in most of the investigated migmatites is due to compositional differences inherited from the parent rocks. Almost isochemical partial melting seems to be the most probable process transforming layered paragneisses, metavolcanics, or schists into migmatites.

The formation of neosomes is believed to be caused by higher amounts of partial melts formed due to higher amounts of water moving into these layers. The neosomes have less biotite and more K‐feldspar, if K‐feldspar is present at all, than the adjacent mesosomes. These differences are small but systematic and seem to control the access of different amounts of water to the various rock portions. Petrographical observations, chemical data, and theoretical considerations indicate a close relationship between rock composition, rock deformation, transport of water, partial melting, and formation of layered migmatites.
Experimental deformation of partially melted granitic aggregates
Authors: DELL'ANGELO L.N. , TULLIS J.
Published: May 2007

Abstract
The effects of varying amounts of partial melt on the deformation of granitic aggregates have been tested experimentally at conditions (900°C, 1500 MPa, 10‐4 to 10‐6/s) where melt‐free samples deform by dislocation creep, with microstructures approximately equivalent to those of upper greenschist facies. Experiments were performed on samples of various grain sizes, including an aplite (150 μm) and sintered aggregates of quartz‐albitemicrocline (10–50 and 2–10 μm). Water was added to the samples to obtain various amounts of melt (1–15% in the aplite, 1–5% in the sintered aggregates). Optical and TEM observations of the melt distribution in hydrostatically annealed samples show that the melt in the sintered aggregates is homogeneously distributed along an interconnected network of triple junction channels, while the melt in the aplites is inhomogeneously distributed.

The effect of partial melt on deformation depends an melt amount and distribution, grain size and strain rate. For samples deformed with ˜ 1% melt, all grain sizes exhibit microstructures indicative of dislocation creep. For samples deformed with 3–5% melt, the 150 μm and 10–50 μm grain size samples also exhibit dislocation creep microstructures, but the 2–10 μm grain size samples exhibit abundant TEM‐scale evidence of dissolution‐precipitation and little evidence of dislocation activity, suggesting a switch in deformation mechanism to predominantly melt‐enhanced diffusion creep. At natural strain rates melt‐enhanced diffusion creep would predominate at larger grain sizes, although probably not for most coarse‐grained granites.

The effects of melt percentage and strain rate have been studied for the 150 μm aplites. For samples with ˜ 5 and 10% melt, deformation at 10–6/s squeezes excess melt out of the central compressed region allowing predominantly dislocation creep. Conversely, deformation at 10‐5/s produces considerable cataclasis presumably because the excess melt cannot flow laterally fast enough and a high pore fluid pressure results. For samples with 15% melt, deformation at both strain rates produces cataclasis, presumably because the inhomogeneous melt distribution resulted in regions of decoupled grains, which would produce high stress concentrations at point contacts. At natural strain rates there should be little or no cataclasis if an equilibrium melt texture exists and if the melt can flow as fast as the imposed strain rate. However, if the melt is confined and cannot migrate, a high pore fluid pressure should promote brittle deformation.
Deformation, metamorphism and imbrication of the Indian plate, south of the Main Mantle Thrust, north Pakistan
Authors: TRELOAR P. J. , BROUGHTON R. D. , WILLIAMS M. P. , COWARD M. P. , WINDLEY B. F.
Published: May 2007

ABSTRACT South of the Main Mantle Thrust in north Pakistan, rocks of the northern edge of the Indian plate were deformed and metamorphosed during the main southward thrusting phase of the Himalayan orogeny. In the Hazara region, between the Indus and Kaghan Valleys, metamorphic grade increases northwards from chlorite zone to sillimanite zone rocks in a typically Barrovian sequence. Metamorphism was largely synchronous with early phases of the deformation. The metamorphic rocks were subsequently imbricated by late north‐dipping thrusts, each with higher grade rocks in the hanging wall than in the footwall, such that the metamorphic profile shows an overall tectonic inversion. The rocks of the Hazara region form one of a number of internally imbricated metamorphic blocks stacked, after the metamorphic peak, on top of each other during the late thrusting. This imbrication and stacking represents an early period of post‐Himalayan uplift.
Silicate and sulphide thermobarometry of low‐ to medium‐grade metamorphic rocks from Holkham Bay, south‐east Alaska
Authors: STOWELL H. H.
Published: May 2007

ABSTRACT The western metamorphic belt of the Coast Plutonic Complex, south‐east Alaska and adjacent British Columbia, contains strongly deformed rocks and a prominent topographic low: the Coast Range megalineament. Near Holkham Bay, south‐east Alaska, the lineament separates the western metamorphic belt into: a western low‐grade (greenschist facies) terrane, and an eastern medium‐grade (amphibolite facies) terrane.

Sphalerite compositions of grains in direct contact with pyrite and pyrrhotite in chlorite‐muscovite zone rocks in the low‐grade terrane give pressures of about 8 kbar; compatible with pressures of 8‐10 kbar at 500°C calculated from plagioclase‐biotite‐garnet‐muscovite assemblages adjacent to the Windham Bay pluton about 15 km away. A pressure of 4.8 ± 0.7 kbar was calculated from sphalerite compositions in staurolite zone rocks east of the Coast Range megalineament. This is indistinguishable from pressures of 4.8 ± 1 kbar at 585°C and 5.1 ± 1 kbar at 680°C (plagioclase‐garnet‐aluminum silicate‐quartz equilibria), and 4.1 ± 1 kbar at 585°C (plagioclase‐biotite‐garnet‐muscovite equilibrium) determined for the medium‐grade terrane. An identical pressure of 4.8 ± 0.7 kbar was calculated from sphalerite compositions in biotite zone rocks adjacent to the lineament; this is considerably higher than a pressure of 3.1 ± 1 kbar at 525°C obtained using plagioclase‐biotite‐garnet‐muscovite geobarometry from shear zones within the lineament. The discrepancy may be explained by later equilibration of mineral phases within the shear zones.

The geothermobarometry suggests relatively low temperatures and high pressures for the low‐grade terrane (6‐10 kbar), and intermediate temperatures and pressures for the medium‐grade terrane to the east (4‐6 kbar). Comparison of the barometers indicate that sphalerite can be used to estimate metamorphic pressures, similar to those estimated from silicate mineral chemistry when pyrrhotite‐sphalerite‐pyrite assemblages are used.
Metamorphic evolution of the Bunger Hills, East Antarctica: evidence for substantial post‐metamorphic peak compression with minimal cooling in a Proterozoic orogenic event
Authors: STÜWE K. , POWELL R.
Published: May 2007

ABSTRACT The Bunger Hills, East Antarctica, experienced a low‐pressure granulite facies orogenic event during the Proterozoic. The stable coexistence of the S1 foliation‐parallel M1 assemblages, garnet‐cordierite‐spinel‐ilmenite and garnet‐sillimanite‐spinel‐ilmenite‐rutile, in quartz‐bearing pelitic gneisses is evidence for metamorphic peak pressures of around 4 kbar during M1, at temperatures of about 800°C. The growth of massive reaction coronas of garnet and cordierite around hercynitic spinel and iron‐titanium oxides during M2 is evidence for the destabilization of the M1 assemblages during compression. Thermodynamic calculations on the M2 assemblages indicate formation pressures of 6–7 kbar at temperatures of about 750°C. Thus, the gneisses from the Bunger Hills indicate about 2 kbar or more of compression during minimal cooling. Such a P‐T path is different from that of many other Proterozoic terranes which are characterized by isobaric cooling or decompression. A large charnockite body, which is undeformed, was intruded at ∼950°C, towards the end of compression.

The low pressures during M1 can be best explained by metamorphism at mid‐crustal levels in thin continental crust in thin lithosphere above a thermal perturbation in the underlying asthenosphere. We suggest that the compression during cooling was a result of gravitational backflow in which the action of body forces between adjacent normal thickness crust and the thin crust of the Bunger Hills is 'switched on’by the thermal perturbation. Within such a model, the timing of intrusion of the charnockite exposed in the Bunger Hills is consistent with its generation by partial melting during the metamorphic maximum of the lowermost crust.
Statistical microanalysis applied to garnet growth zoning and to cordierite fluid content
Authors: PAQUET J. , DELAY F. , LEMAGUER D.
Published: May 2007

An analytical study to evaluate quantitatively weak zoning of a garnet from a high‐grade kinzigite has been performed with an electron microprobe. The technique consists of the reconstruction of a profile step‐by‐step by successive analyses performed during relatively long counting times (30 s), along a radial profile of 2,500 μm length. The successive analytical data along this profile are statistically treated by Fisher's test and compared with the χ2 values (Pearson's law). These statistical tests were applied to assess microprobe stability and analysis homogeneity, and as a consequence to assure high credibility of the radial variations of the garnet. From core to rim, and for each element, zoning appears as the radial juxtaposition of stationary Poissonian samples. These samples being associated, the garnet appears to be constituted of successive concentric domains with stationary compositions. Different substitutions between Mg, Fe, Mn and Ca are evidenced. Such an analytical approach to chemical zoning can be useful for understanding growth mechanisms, and the possible diffusion reactions with the environment at each growth step. In addition, such a procedure can be used to evaluate accurately the fluid content of cordierite, and to appreciate the nature of the fluids concerned. As an example, the fluid content of a cordierite from a similar high‐grade kinzigite has been evaluated.
Reviews
Authors: Brodie K.
Published: May 2007

INTRODUCTION TO METAMORPHIC TEXTURES AND MICROSTRUCTURES. By A. J. Barker. Blackie, Glasgow, 1989.

ECLOGITE FACIES ROCKS. Edited by D. A. Carswell. Blackie, Glasgow, 1990.

EVOLUTION OF METAMORPHIC BELTS. Edited by J. S. Daly, R. A. Cliff & B. W. D. Yardley.

AN INTRODUCTION TO METAMORPHIC PETROLOGY. By B. W. D. Yardley.
Assessment of the garnet–clinopyroxene Fe–Mg exchange thermometer using new experimental data
Authors: GREEN T. H. , ADAM J.
Published: May 2007

A detailed high‐pressure experimental study of two mafic xenoliths, in which coexisting garnet and clinopyroxene (± plagioclase, spinel and olivine) were crystallized over a P–T range of 10–30 kbar and 950–1200°C, has revealed significant differences in temperatures from those estimated for coexisting garnets and clinopyroxenes using the Ellis & Green Fe–Mg exchange thermometer. The results show perfect matching at 30 kbar, 1150–1200°C, but increasing deviation at lower pressure and lower temperature, with the Ellis & Green calibration reaching a ΔT (overestimate) of c. 145°C at 10–12 kbar and 950°C. The grossular content of the garnet increases from c. 21 mol.% at 10 kbar to 26–31 mol.% at 30 kbar. These results confirm other recent experimental studies that show that the pressure correction, and possibly to a lesser extent the correction for grossular content, applied by Ellis & Green, are not appropriate for lower pressure conditions, and give estimated temperatures that are significantly high when applied to granulitic terranes formed at c. 10 kbar. The new reconnaissance results allow a graphical interpolation of a garnet–clinopyroxene geothermometer based on the Fe–Mg exchange reaction which should be applicable to assemblages formed under lower crustal conditions.
Thermobarometry and P–T–t paths: the granulite to eclogite transition in lower crustal xenoliths from eastern Australia
Authors: PEARSON N. J. , O'REILLY S. Y.
Published: May 2007

‘Lower crustal’ suite xenoliths in basaltic and kimberlitic magmas are dominated by mafic granulites and may also include eclogites and garnet pyroxenites. Pressures of up to 25 kbar obtained from such xenoliths are well in excess of an upper value of c. 12 kbar for exposed granulite terranes. Palaeogeotherms constructed from xenoliths for the lower crust beneath the Phanerozoic fold belts of eastern Australia (SEA) and beneath the eastern margin of the Australian craton (EMAC) indicate two distinct thermal regimes. The two geotherms have similar form, with the EMAC curve displaced c. 150°C to lower temperatures. Reaction microstructures show the partial re‐equilibration of primary igneous assemblages to granulite and eclogite assemblages and are interpreted to reflect the cooling from magmatic temperatures. Variations in mineral compositions and zoning are used to constrain further the history of several EMAC xenoliths to near‐isobaric trajectories.

Detailed graphical models are constructed to predict compositional changes for isobaric P–T paths (at 7, 14 & 21 kbar) to transform an SEA‐type geotherm to a cratonic geotherm. The models show that for the assemblage grt + cpx ± ky + plag + qtz, the changes associated with falling temperature in Xgr, Xjd (increase) and Xan (decrease) will be greater at higher pressures. These results indicate that discernible zoning is more likely to be preserved in the higher pressure xenoliths. The zoning recorded in clinopyroxene from mafic granulite xenoliths over the pressure range c. 12–22 kbar suggests isobaric cooling of a large crustal thickness (30–35 km).

An isobaric cooling path is consistent with magma accretion models for the transition of a crust–mantle boundary from an SEA‐type geotherm to a cratonic geotherm. The coexistence of granulite and eclogite over the depth range 35–75 km beneath the EMAC indicates that the granulite to eclogite transition in the lower crust is controlled by P–T conditions, bulk chemistry and kinetic factors. At shallower crustal levels, typified by exposed granulite terranes, isobaric cooling may not result in the transition to eclogite.
Decompression‐induced growth of albite porphyroblasts, Fleur de Lys Supergroup, western Newfoundland
Authors: JAMIESON R. A. , O'BEIRNE‐RYAN A. M.
Published: May 2007

Abstract
Albite porphyroblasts are widely distributed in pelitic and semi‐pelitic schists of the Fleur de Lys Supergroup, western Newfoundland. Textures and mineral assemblages indicate that albite grew during nearly isothermal decompression from P‐T conditions of about 500° C, 9 kbar, to conditions of 550° C, 6.5 kbar. Three compositional varieties of albite‐bearing schists, here termed PMAQ (paragonite‐muscovite‐albite‐quartz), MMAQ (microcline‐muscovite‐albite‐quartz), and PMMQ (paragonite‐muscovite‐margarite‐quartz), can be distinguished on the basis of pre‐porphyroblast mineral assemblages. Analysis of these assemblages in terms of the composition of the coexisting fluid [log a(Na+/H+) versus log a(K+/H+)] suggests that, as pressure and temperature changed, the stability field of albite expanded at the expense of coexisting matrix phyllosilicates. This promoted growth of albite on pre‐existing or newly formed nuclei. Late oligoclase in PMAQ and PMMQ samples is associated with replacement of matrix garnet by plagioclase + mica ° Chlorite, particularly in strongly sheared samples.
Evolution of regional metamorphism in the Cape Smith Thrust Belt (northern Quebec, Canada): interaction of tectonic and thermal processes
Authors: ST‐ONGE M. R. , LUCAS S. B.
Published: May 2007

Abstract
Syn‐metamorphic re‐imbrication of the internal part of thrust belts can result in distinct pressure–temperature–time–deformation (P–T–t–d) pathways for different structural–metamorphic domains. In the early Proterozoic Cape Smith Thrust Belt (Canada), an external (piggyback‐sequence thrusting) domain is characterized by thermal peak metamorphism occurring after deformation. In contrast, thermal peak metamorphism in an internal domain occurred during re‐imbrication by out‐of‐sequence thrusting. The interactions of tectonic and thermal processes have been studied using three methods: (i) qualitative evaluation of the timing between mineral growth and deformation; (ii) analytical P–T paths from growth‐zoned garnet porphyroblasts; and (iii) numerical modelling of vertical heat conduction. Derived P–T–t–d pathways suggest that uplift in the external domain resulted in part from erosion and isostatic unloading. In contrast, paths for the internal domain indicate that the out‐of‐sequence portion of the thrust belt may have experienced faster unroofing relative to the external domain. This is attributed to thickening by out‐of‐sequence thrusting and possibly to extensional faulting at (now eroded) higher structural levels. Observations on the timing of metamorphism, coupled with numerical modelling, suggest that the thermal peak metamorphism documented in the external domain is a consequence of the emplacement of the out‐of‐sequence thrusts stack in the internal portion of the thrust belt.
Hercynian blueschist metamorphism in North Portugal: tectonothermal implications
Authors: GIL IBARGUCHI J. I. , DALLMEYER R. D.
Published: May 2007

Abstract
Aegirine–jadeite clinopyroxene (>60 mol% jadeite) locally occurs within blueschists of the ‘Lower Allochthon’exposed in the Trás‐os‐Montes region of northern Portugal. Peak conditions attained during blueschist facies metamorphism are estimated to have been c. 420° C and >11 kbar. Porphyroblastic white mica (paragonite/phengite) within the blueschist assemblage records a 36Ar/40Ar versus 39Ar/40Ar isotope correlation age of 329.4 ± 1.6 Ma. In view of the relatively low‐T nature of the metamorphism, the c. 330‐Ma age is interpreted to date closely the high‐P recrystallization. This tectonothermal activity is interpreted to have resulted from structural emplacement of a previously assembled crystalline nappe complex (‘Upper Allochthon/Ophiolite Nappe’) onto Iberian protoliths of the Lower Allochthon during terminal stages of the Hercynian orogeny.
Episodic metamorphic reactions during orogenesis: the control of deformation partitioning on reaction sites and reaction duration
Authors: BELL T. H. , HAYWARD N.
Published: May 2007

Abstract
Textural ‘unconformities’or truncations are common in porphyroblasts with complex inclusion trails. They reflect cycles of successive foliations that develop against competent porphyroblasts during orogenesis and are preserved by successive growth increments. Their truncational character results from shear and dissolution along a particular foliation generating a differentiated crenulation cleavage. The increment of porphyroblast growth that follows a textural ‘unconformity’may or may not mark a significant compositional change, depending on the amount of movement of the rock through P–T space between cleavage‐forming events.

Although historically interpreted to result from a significant metamorphic hiatus, most textural unconformities indicate that the reactions involved in the formation of these minerals are episodic during continuous prograde metamorphism, starting and stopping as a function of the stage of crenulation of the matrix foliation and the pattern of deformation partitioning. Such episodic reaction behaviour can only occur for multivariant reactions, or successive but different univariant reactions. The reason why garnet is the most common porphyroblast to exhibit evidence for episodic reactions is probably the fact that it grows by multivariant reactions over a much wider P–T range than most other common porphyroblast phases.

Porphyroblast growth is micrometasomatic. It is episodic because a significant reduction of strain occurs within domains of progressive shortening each time continuous progressive shearing domains form on their margins. This stops microfracture development across the progressive shortening domains, thereby preventing rapid access and interaction of fluid, ions and complexes with porphyroblast boundaries. Shifting patterns of deformation partitioning and resulting small‐scale juxtaposition of different compositional layers spreads the duration and location of multivariant reactions and causes differential timing of porphyroblast growth along a particular stratigraphic horizon. It may also locally preserve metastable metamorphic assemblages.

In regionally metamorphosing/deforming pelites, near‐simultaneous cessation of porphyroblast growth on all rims, once continuous differentiated progressive shearing domains have formed nearby, precludes fluid recirculation as a significant process for removal of material during cleavage development. Alternatively, diffusion of simple molecules and dissociated ions along actively shearing and micro‐gaped phyllosilicates, with recomplexing in fluid‐filled microfractures, readily explains the control of deformation partitioning on reaction site and reaction duration.