bluuebunny/crossref_metadata_2025_split huggingface dataset:
The metamorphism in the Central Himalaya
Authors: PECHER A. 
Published: May 2007

ABSTRACTAll along the Himalayan chain an axis of crystalline rocks has been preserved, made of the Higher Himalaya crystalline and the crystalline nappes of the Lesser Himalaya. The salient points of the metamorphism, as deduced from data collected in central Himalaya (central Nepal and Kumaun), are:The Higher Himalaya crystalline, also called the Tibetan Slab, displays a polymetamorphic history with a first stage of Barrovian type overprinted by a lower pressure and/or higher temperature type metamorphism. The metamorphism is due to quick and quasi‐adiabatic uplift of the Tibetan Slab by transport along an MCT ramp, accompanied by thermal refraction effects in the contact zone between the gneisses and their sedimentary cover. The resulting metamorphic pattern is anapparent(diachronic) inverse zonation, with the sillimanite zone above the kyanite zone.Conversely, the famous inverted zonation of the Lesser Himalaya is basically aprimarypattern, acquired during a one‐stage prograde metamorphism. Its origin must be related to the thrusting along the MCT, with heat supplied from the overlying hot Tibetan Slab, as shown by synmetamorphic microstructures and the close geometrical relationships between the metamorphic isograds and the thrust.Thermal equilibrium is reached between units above and below the MCT. Far behind the thrust tip there is good agreement between the maximum temperature attained in the hanging wall and the temperature of the Tibetan Slab during the second metamorphic stage; but closer to the MCT front, the thermal accordance between both sides of the thrust is due to a retrogressive metamorphic episode in the basal part of the Tibetan Slab.
Low‐grade regional metamorphism in the Mesozoic‐Cenozoic volcanic sequences of the Central Andes
Authors: LEVI B. , AGUIRRE L. , NYSTRÖM J. O. , PADILLA H. , VERGARA M. 
Published: May 2007

ABSTRACT All the Mesozoic and Cenozoic volcanic rocks of the Central Andes (from southern Ecuador to central Chile), except Recent ones, have been affected by episodes of regional metamorphism, without change in texture and structure. The metamorphism, which ranges from low zeolite to greenschist facies, can be classified as burial metamorphism because there is an overall increase in metamorphic grade with stratigraphic depth in the individual volcanic sequences separated by regional unconformities. Some sequences display metamorphic patterns transitional to ocean‐floor and to geothermal field types, reflecting variations along and across the Andes in tectonic setting and thermal gradients.Volcanism was closely followed by metamorphism during each cycle characterizing the geological history of the Central Andes. The episodic nature of the metamorphism has led to breaks in metamorphic grade at regional unconformities and repetition of facies series, where strata of higher grade may even overlie those of lower grade. The existence of permeability‐controlled distribution patterns of secondary minerals within individual flows shows that gradients of chemical activity, rate of reaction and Pfluid were acting, in addition to temperature and P,tot overall gradients, during the regional metamorphism. The alteration is accompanied by chemical changes and disturbances of the K‐Ar and Rb‐Sr isotope systems. Similarities between Mesozoic facies series in the western and eastern flanks of the Andes are consistent with a mechanism of ensialic spreading‐subsidence.
A method for modelling mass balance in partial melting and anatectic leucosome segregation
Authors: BEA F. 
Published: May 2007

A method is proposed for adjusting the mass balance to characterize quantitatively the behaviour of minerals in anatexis. The method is based on an unconstrained simple mixing model that can be expressed as:imagewhereB,A0, andA1‐n, are compositional vectors of segregate, source rock and source minerals, respectively. The most important concepts are: (1) degree of partial fusion:FMM= 1/a0; (2) mineral fractionation index:imageand (3) plagioclase differentiation index:image.For a given mineral, theMFIvalues have the following meaning: (a)MFI<0: residual phase originated, at least partly, as a product of incongruent melting; (b) 0 >MFI<1: preferential retention in the residue; (c)MFI= 1: identical modal fraction in source and melt; (d)a0>MFI> 1: preferential incorporation into the segregate, and (e)MFI>a0: external contribution to the anatectic system defined bya0A0. To test the method and illustrate its use, it was applied to two real problems of partial melting in the Peña Negra Anatectic Complex (Central Spain). The first is a very simple case of segregation of a diktyonitic neosome from an orthogneiss through partial melting located in vertical shear zones. This process is characterized by: (1)FMM= 0.51; (2) active incorporation of K‐feldspar, plagioclase and biotite into the segregate; (3) disequilibrium melting of plagioclase; (4) residual behaviour of quartz and ilmenite. The second case concerns the formation of a cordierite‐bearing granite from granodioritoid diatexites through an anatectic process, whose most salient characteristics are: (1)FMM= 0.45; (2) incongruent melting of biotite; (3) residual behaviour of plagioclase, which melted with aPDIof 1.22; (4) preferential incorporation of quartz into the segregate; (5) total extraction of K‐feldspar from the residue.
Low‐pressure regional metamorphism in the Omeo Metamorphic Complex, Victoria, Australia
Authors: MORAND V. J. 
Published: May 2007

The Omeo Metamorphic Complex forms the southern end of the Wagga Metamorphic Belt, which is the main locus of Palaeozoic low‐pressure metamorphism in the Lachlan Fold Belt, south‐eastern Australia. It comprises metamorphosed Ordovician quartz‐rich turbidites originally derived from Precambrian cratonic rocks. Prograde regional metamorphism occurred in the early Silurian, very soon after sedimentation had ceased. The sequence of metamorphic zones, with increasing grade, is: chlorite, biotite, cordierite, andalusite–K‐feldspar and sillimanite–K‐feldspar. Migmatites occur in the sillimanite–K‐feldspar zone, but large bodies of S‐type granite were derived from rocks underlying the exposed Ordovician sequence. P and T estimates for the highest grade rocks are T= 700°C and P= 3.5 kbar, indicating a very high P–T gradient of 65°C/km.The high heat flow during prograde metamorphism probably resulted from a combination of a thermal anomaly persisting from a pre‐metamorphic back‐arc basin environment, and intrusion of hot, mantle‐derived magmas into the lower and middle crust.Regional retrograde metamorphism coincided with a general reheating of the crust in the Siluro‐Devonian, accompanied by intrusion of many I‐type plutons and resetting of the K–Ar dates of some earlier plutons. The Omeo Metamorphic Complex was exposed to erosion at this time.
An enlarged and updated internally consistent thermodynamic dataset with uncertainties and correlations: the system K2O–Na2O–CaO–MgO–MnO–FeO–Fe2O3–Al2O3–TiO2–SiO2–C–H2–O2
Authors: HOLLAND T. J. B. , POWELL R. 
Published: May 2007

We present, as a progress report, a revised and much enlarged version of the thermodynamic dataset given earlier (Holland & Powell, 1985). This new set includes data for 123 mineral and fluid end‐members made consistent with over 200 P–T–XCO2–fO2 phase equilibrium experiments. Several improvements and advances have been made, in addition to the increased coverage of mineral phases: the data are now presented in three groups ranked according to reliability; a large number of iron‐bearing phases has been included through experimental and, in some cases, natural Fe:Mg partitioning data; H2O and CO2 contents of cordierites are accounted for with the solution model of Kurepin (1985); simple Landau theory is used to model lambda anomalies in heat capacity and the Al/Si order–disorder behaviour in some silicates, and Tschermak‐substituted end‐members have been derived for iron and magnesium end‐members of chlorite, talc, muscovite, biotite, pyroxene and amphibole.For the subset of data which overlap those of Berman (1988), it is encouraging to find both (1) very substantial agreement between the two sets of thermodynamic data and (2) that the two sets reproduce the phase equilibrium experimental brackets to a very similar degree of accuracy. The main differences in the two datasets involve size (123 as compared to 67 end‐members), the methods used in data reduction (least squares as compared to linear programming), and the provision for estimation of uncertainties with this dataset. For calculations on mineral assemblages in rocks, we aim to maximize the information available from the dataset, by combining the equilibria from all the reactions which can be written between the end‐members in the minerals. For phase diagram calculations, we calculate the compositions of complex solid solutions (together with P and T) involved in invariant, univariant and divariant assemblages. Moreover we strongly believe in attempting to assess the probable uncertainties in calculated equilibria and hence provide a framework for performing simple error propagation in all calculations in thermocalc, the computer program we offer for an effective use of the dataset and the calculation methods we advocate.
Metamorphic and post‐metamorphic fluid flow in the low‐grade rocks of the Harlech Dome, north Wales
Authors: BOTTRELL S. H. , GREENWOOD P. B. , YARDLEY B. W. D. , SHEPHERD T. J. , SPIRO B. 
Published: May 2007

A combination of fluid inclusion, stable isotope and geochemical techniques has been used to study the nature of fluids present and their behaviour during Caledonian low‐grade metamorphism of the Harlech Dome, north Wales. Fluid inclusion studies show that in most of the metasedimentary sequence the peak metamorphic fluid was an aqueous Na–K–Cl brine but in the graphitic Clogau Formation and in parts of the overlying Maentwrog Formation immiscible H2O‐rich and CH4‐rich fluids coexisted.Late‐stage inclusions are of calcium‐rich brine and a dilute aqueous fluid. The chemical composition of chlorite in metamorphic veins and rocks varies between different formations and quartz‐oxygen isotopic compositions show considerable variation between different units. Both of these features are taken to indicate that there was little or no pervasive movement of fluid between different units at the peak of metamorphism. After the metamorphic peak there was focused flow of fluid upward through the sequence along fractures, in response to end‐Caledonian uplift and unloading. Where the migrating fluid crossed the graphitic shales, interaction between the fluid and the shales gave rise to the formation of the auriferous veins of the Dolgellau Gold Belt. Subsequent to this mineralizing event there was widespread development of 18O‐enriched calcites and micas.In the case of vein minerals it is possible that these crystallized directly from late‐stage fluids at lower temperature than the quartz in the same veins. Alternatively, the original vein minerals may have re‐equilibrated with later 18O‐enriched or cooler fluid. In the case of muscovites in the rock matrix it is proposed that the isotopically heavy compositions are the result of re‐equilibration of initially light grains with an introduced fluid, requiring considerable influx of fluid. This event may relate to either of two late‐stage fluids observed as inclusions.
Conditions of Archean granulite metamorphism in the Godthab‐Fiskenaesset region, southern West Greenland
Authors: RICIPUTI L. R. , VALLEY J. W. , McGREGOR V. R. 
Published: May 2007

The Nordlandet peninsula (Akia terrane) and the Tasiusarsuaq terrane in southern West Greenland were metamorphosed to granulite facies at 3.0 and 2.8 Ga respectively. Temperatures of metamorphism are estimated using magnetite + ilmenite, garnet + orthopyroxene, garnet + clinopyroxene and garnet + biotite thermometers. Barometry has been carried out in the two terranes using eight different garnet barometers. A uniform set of activity models for all minerals, including the garnet activity model of Newton et al. (1986), is applied to each barometer in order to permit comparison. Pressure estimates using the different barometers are generally quite consistent (±1.5 kbar). Use of the Newton et al. (1986) garnet activity data results in pressures similar to those obtained using other garnet activity models.Peak metamorphic conditions on the Nordlandet peninsula are estimated to have been 800 ± 50°C, 7.9 ± 1.0 kbar. Values of logfO2 are estimated to have been 1.1 to 2.0 above the quartz + magnetite + fayalite buffer from assemblages of magnetite + ilmenite and quartz + magnetite + ferrosilite. Peak metamorphic conditions in the Tasiusarsuaq terrane are estimated to have been 780 ± 50°C, 8.9 ± 1.0 kbar. Estimates of fH2O and fCO2 using biotite, amphibile, grossular + anorthite and grossular + scapolite equilibria are low in both terranes. These results suggest that granulite metamorphism was fluid absent in both terranes, and that the metamorphism in the Akia terrane and possibly also in the Tasiusarsuaq terrane was initiated by the injection of large volumes of magma into the lower crust.
Compositional characteristics and phase equilibria in manganiferous iron formations from a high‐grade terrain near Satnuru, Karnataka, India
Authors: BHATTACHARYA A. , SPIERING B. , SEN S. K. , NATARAJAN R. , MAZUMDAR A. C. 
Published: May 2007

Manganese‐rich and manganese‐poor iron formations which occur as thin layers in the Halaguru‐Satnuru area, south of Kabbaldurga, Karnataka, India are chemically intermediate between the ‘Algoma’and ‘Lake Superior’types, but higher in their MnO and TiO2contents. The rocks are of four petrographic varieties: (a) quartz‐magnetite‐orthopyroxene‐clinopyroxene, (b) quartz‐magnetite‐orthopyroxene‐clinopyroxene‐garnet, (c) quartz‐magnetite‐clinopyroxene‐garnet, and (d) quartz‐magnetite‐clinopyroxene‐garnet‐plagioclase.In the orthopyroxene‐clinopyroxene pairs, Mn‐Mg and Mn‐Fe exchange is ideal irrespective of the MnSiO3contents of orthopyroxenes (0.6–1.8 mol. % in Mn‐poor and 15–25 mol. % in Mn‐rich compositions). Mg‐Fe exchange in the same pair is however non‐ideal. Mn‐Fe exchange in orthopyroxene‐garnet pairs is ideal. The distribution patterns in the other binaries are inconclusive regarding ideality of exchange. Orthopyroxene‐garnet and clinopyroxene‐garnet geothermometers, modified for high spessartine contents, give temperatures of 800 ± 30° C. A modified version of the Harley (1984) geothermometer registers 740 ± 60° C, in agreement with the consensus temperature value.The equilibrium logffo2values in the iron formations, as calculated from the reaction 6FeSiO3+ O2= 2Fe3O4+ 6SiO2are in the range of −14.2 to −15.5. Algebraic analysis of variations offo2with composition of phases indicates buffering of O2in the rocks. The absence of grunerite in these assemblages is compatible withXH2Obeing less than 0.3 in the ambient fluid. Computations from volatile equilibria in the C‐O‐H system, however, predict high XH2Ovalues (>0.7) atac= 1.0, implying that the activity of graphite must have been greatly reduced—this is in accordance with the absence of graphite in these rocks.
Prograde and retrograde metamorphism of hematitebearing basic schists in the Sanbagawa belt in central Shikoku
Authors: OTSUKI M. , BANNO S. 
Published: May 2007

The mineral assemblages of hematite‐bearing basic schists in intermediate high‐pressure metamorphism are temperature dependent. For assemblages with excess hematite, albite, muscovite and quartz, the paragenetic relations can be dealt with in terms of a four‐component system, without omitting or grouping major components.In the Sanbagawa belt in central Shikoku, the dominant amphibole in the hematite‐bearing basic schists changes from winchite, via crossite and barroisite to hornblende. The stability of amphibole is described chemographically within a pseudoternary system with another excess phase, epidote. Many amphiboles are chemically heterogeneous owing to retrograde reactions which produced low‐T/P amphibole around the prograde amphibole. The examination of amphibole zoning makes it possible to draw a retrograde P‐T trajectory which passes on the lower pressure side of the prograde one.
Petrological constraints on the eclogite‐ and blueschistfacies metamorphism and P‐T‐t paths in the Western Alps*
Authors: POGNANTE U. 
Published: May 2007

Phase relationships in the model mafic system and geothermobarometry allow discrimination of four main groups of high‐P rocks in the nappes of the Western Alps: very high‐P eclogite‐facies (including kyanite eclogites and coesite‐pyrope assemblages), eclogite‐facies (paragonite‐zoisite eclogites), high‐T blueschist‐facies (glaucophane‐garnet ± lawsonite assemblages) and low‐T blueschist‐facies (glaucophane‐lawsonite ± pumpellyite assemblages). The blueschist‐facies‐eclogite‐facies transition is promoted chiefly by increasing T, low bulk XMg and relatively low μH2O. The variety of assemblages and the heterogeneous approach to equilibrium observed in the Alpine rocks are not only constrained by the intersection of the reaction surfaces in P‐T‐X space, but also by the effect and timing of the processes which control kinetics (i.e. pervasive deformation and fluid infiltration). The faster rate of dehydration reactions relative to hydration reactions along with the fact that different bulk compositions crossed the reaction curves at different temperatures (and times), all may have induced μH2O gradients and contributed to the heterogeneous distribution of deformation through a process of reaction‐enhanced ductility. Also mass‐transfer may have been an effective process in determining the type of high‐P assemblage in particular rock volumes.As regards the P‐T‐t paths, only the post‐climax histories are recorded well in the Alpine nappes. Post‐eclogitic exhumation paths at decreasing temperatures characterize structurally higher nappes which were first subducted during the early‐Alpine (Cretaceous) event. In contrast, more or less isothermal decompression paths characterize structurally deeper nappes formed by westward propagation of the underthrust surfaces during the early‐Alpine event and the subsequent meso‐Alpine (Palaeogene) collision between the ‘European’and ‘African’plates. In the Western Alps, prevalent eclogite‐facies conditions were attained during the metamorphic climax of the early‐Alpine subduction, while blueschist‐facies recrystallization characterizes the early‐Alpine exhumation of the eclogitized units and the subsequent intracontinental underthrusts linked to the meso‐Alpine continental collision.
Infiltration‐driven dehydration and anatexis in granulite facies metagabbro, Grenville Province, Ontario, Canada
Authors: PATTISON D. R. M. 
Published: May 2007

Dark hornblende + garnet‐rich, quartz‐absent metagabbro boudins from the Seguin subdomain, Ontario Grenville Province, are transected by anastomosing light‐coloured veins rich in orthopyroxene, clinopyroxene, plagioclase and sometimes quartz. The veins vary in texture from fine‐grained diffuse veins and patches that overprint the metagabbro, to coarse tonalitic leucosomes with sharp borders. The diffuse veins and patches are suggestive of channellized subsolidus dehydration of the metagabbro, while the tonalitic leucosomes are suggestive of local internally‐derived anatexis. All vein types grade smoothly into each other, with the tonalitic leucosomes being the latest.Relative to the host metagabbro, the veins have higher Si, Na, Ba & Sr, lower Fe, Mg, Ca & Ti, and similar Al. The coarser veins are enriched in K. Plagioclase becomes steadily enriched in Na in the transition from host metagabbro (An47) to the veins (An35), and in the coarsest veins it is antiperthitic. Differences in composition of the other minerals between host metagabbro and vein are minor. Pressure–temperature estimates are scattered, but indicate a minimum temperature during vein formation of 700°C at about 8 kbar.Mass balance constraints indicate that the veins formed from the metagabbro in an open system. The transecting veins are interpreted to represent pathways of Si + Na + Ba + Sr ± K ± Al‐enriched, low aH2O fluids that metasomatized the host metagabbro to form the anhydrous veins. An initial period of localized solid‐state dehydration of the metagabbro, represented by the diffuse veins, was followed by a transition to localized anatexis, represented by the tonalitic leucosomes. The change to anatexis may have been due to the addition of K to the infiltrating fluid. The source and delivery mechanism of the fluids is unknown.
Integrated TEM, XRD and electron microprobe investigation of mixed‐layer chlorite–smectite from the Point Sal ophiolite, California
Authors: BETTISON‐VARGA L. , MACKINNON I. D. R. , SCHIFFMAN P. 
Published: May 2007

Abstract
Five basalt samples from the Point Sal ophiolite, California, were examined using HRTEM and AEM in order to compare observations with interpretations of XRD patterns and microprobe analyses. XRD data from ethylene‐glycol‐saturated samples indicate the following percentages of chlorite in mixed‐layer chlorite–smectite identified for each specimen: (i) L2036 ± 50%, (ii) L2035 ± 70 and 20%, (iii) 1A‐13 ± 70%, (iv) 1B‐42 ± 70%, and (v) 1B‐55 = 100%. Detailed electron microprobe analyses show that ‘chlorite’analyses with high Si, K, Na and Ca contents are the result of interlayering with smectite‐like layers. The Fe/(Fe + Mg) ratios of mixed‐layer phyllosilicates from Point Sal samples are influenced by the bulk rock composition, not by the percentage of chlorite nor the structure of the phyllosilicate.Measurements of lattice‐fringe images indicate that both smectite and chlorite layers are present in the Point Sal samples in abundances similar to those predicted with XRD techniques and that regular alternation of chlorite and smectite occurs at the unit‐cell scale. Both 10‐ and 14‐Å layers were recorded with HRTEM and interpreted to be smectite and chlorite, respectively. Regular alternation of chlorite and smectite (24‐Å periodicity) occurs in upper lava samples L2036 and 1A‐13, and lower lava sample 1B‐42 for as many as seven alternations per crystallite with local layer mistakes. Sample L2035 shows disordered alternation of chlorite and smectite, with juxtaposition of smectite‐like layers, suggesting that randomly interlayered chlorite (<0.5)–smectite exists. Images of lower lava sample 1B‐55 show predominantly 14‐Å layers. Units of 24 Å tend to cluster in what may otherwise appear to be disordered mixtures, suggesting the existence of a corrensite end‐member having thermodynamic significance.