bluuebunny/crossref_metadata_2025_split huggingface dataset:
Structural relations of charnockites of the Archaean Dharwar craton, southern India
Authors: NAHA K. , SRNIVASAN R. , JAYARAM S. 
Published: May 2007

AbstractTwo varieties of charnockites are recognized in the Dharwar craton of southern India. The style and sequence of structures in one charnockite variety, and related intermediate to basic granulites, are similar to those in the supracrustal rocks of the Dharwar Supergroup and the adjacent Peninsular Gneiss. This style has isoclinal folds with long limbs and sharp hinges with an axial planar fabric in some instances. Additional evidence of flattening is provided by pinch‐and‐swell and boudinage structures, with basic granulites forming boudins in the more ductile charnockites/enderbites in the limbs of isoclinal folds. These folds are involved in near‐coaxial upright folding resulting in the bending of the axial planes of the isoclinal folds and the associated boudins. All these structures are overprinted by non‐coaxial upright folds with axial planes striking nearly N–S. The map pattern of charnockites suggests that this sequence of structures is present not only on a mesoscopic scale, but also on a macroscopic scale. Charnockites of this variety provide, in some instances, evidence of having been migmatized to give rise to hornblende–biotite gneiss and biotite gneiss, which form a part of the Peninsular Gneiss terrane.The second variety comprises charnockitesensu strictowith an entirely different structural style. This type occurs in the tensional domains of the hinge zones of the later buckle folds, in the necks of foliation boudinage, in shear zones and in release joints parallel to the axial planes of the later folds in the Peninsular Gneiss. Because the non‐coaxial later folds are associated with a strain pattern different from, and later than, that of the isoclinal folds of the first generation, it follows that charnockites of the Dharwar craton have evolved in at least two distinct phases, separate both in time and in process.
Petrology of lawsonite‐, pumpellyite‐ and sodic amphibole‐bearing metabasites from north‐east Oman
Authors: EL‐SHAZLY ALEY EL‐DIN K. 
Published: May 2007

ABSTRACTThe assemblages chlorite‐pumpellyite‐lawsonite‐albite‐quartz, chlorite‐lawsonite‐quartz‐epidote and chlorite‐epidote‐albite‐quartz occur in metabasaltic blocks and veins in a metamorphosed tectonic mélange in the structurally highest unit of the autochthonous and parautochthonous section underlying the Semail ophiolite in Saih Hatat, north‐east Oman. The pre‐Permian basement of this section contains mafic units characterized by the assemblage crossite‐epidote‐chlorite‐quartz‐albite /el actinolite. These assemblages indicate a down‐section increase in metamorphic grade from ‘lawsonite‐albite facies' conditions in the mélange to ‘epidote‐blueschist’ facies conditions in the basement.Application of empirically and experimentally based thermobarometers as well as petrogenetic grids calculated for a model basaltic system indicates that the P‐T conditions of metamorphism ranged from 3 to 6 kbar and 250 to 300d̀ C for the mélange and P > 6.8 kbar, T > 310d̀ C for the basement units. Textural relations interpreted in the context of petrogenetic grids indicate that these units followed clockwise P‐T paths of evolution. The estimated P‐T conditions and down‐section increase in metamorphic grade in central, western and northern Saih Hatat are consistent with the hypothesis relating metamorphism to the Late Cretaceous tectonic loading of the continental margin by an ophiolite slab < 18 km in thickness. These results contrast with field and petrological observations documented for blueschists and eclogites exposed along the eastern coast of Saih Hatat which may have formed at an earlier stage in response to an Early Cretaceous collisional event.
Controls on the mechanisms of fluid infiltration and front advection during regional metamorphism: a stable isotope and textural study of retrograde Dalradian rocks of the SW Scottish Highlands
Authors: FEIN J. B. , GRAHAM C. M. , HOLNESS M. B. , FALLICK A. E. , SKELTON A. D. L. 
Published: May 2007

ABSTRACTVein‐controlled retrograde infiltration of H2O‐CO2 fluids into Dalradian epidote amphibolite facies rocks of the SW Scottish Highlands under greenschist facies conditions resulted in alteration of calcite‐rich marble bands to dolomite and spatially associated 18O enrichment of about 10%. on a scale of metres. Fluid inclusion data indicate that the retrograde fluid was an H2O‐salt mixture with a low CO2 content, and that the temperature of the fluid was about 400d̀ C. Detailed petrographic and textural (backscattered electron imaging) studies at one garnet‐grade locality show that advection of fluid into marbles proceeded by a calcite‐calcite grain edge flow mechanism, while alteration of non‐carbonate wall‐rock is associated with veinlets and microcracks.Stable isotopic analysis of carbonates from marble bands provides evidence for advection of isotopic fronts through carbonate wall‐rocks perpendicular to dolomite veins, and fluid fluxes in the range 2.4–28.6 m3/m2 have been computed from measured advection distances. Coincidence of isotope and reaction fronts is considered to result from reaction‐enhanced kinetics of isotope exchange at the reaction front. Front advection distances are related to the proportion of calcite to quartz in each marble band, with the largest advection distance occurring in nearly pure calcite matrix. This relationship indicates that fluid flow in carbonates is only possible along fluid‐calcite‐calcite grain edges. However, experimental constraints on dihedral angles in calcite‐fluid systems require that pervasive infiltration occurred in response to calcite dissolution initiated at calcite‐calcite grain junctions rather than to an open calcite pore geometry.The regional extent of the retrograde infiltration event has been documented from the high δ18O of dolomite‐ankerite carbonates from veins and host‐rocks over an area of least 50 × 50 km in the SW Scottish Highlands. Isotopically exotic 18O‐rich retrograde fluids have moved rapidly upwards through the crust, inducing isotopic exchange and mineral reaction in wall‐rocks only where lithology, pore geometry or mineral solubilities, pressure and temperature have been appropriate for pervasive infiltration to occur.
Contrasting retrograde oxygen isotope exchange behaviour and implications: examples from the Langtang Valley, Nepal
Authors: MASSEY J. A. , HARMON R. S. , HARRIS N. B. W. 
Published: May 2007

ABSTRACTQuantitative modelling of oxygen exchange by diffusion during slow cooling has been compared to the observed oxygen isotope distributions from high‐grade metamorphic and granitic rocks of the High Himalayan Crystallines, Langtang Valley, central Nepal, in order to investigate the effect of retrograde diffusional exchange on the preservation of high‐temperature, oxygen isotope systematics.Modelled fractionations, using water‐present diffusion data reported in the literature, predict quartz‐mica fractionations to be much larger than those at peak metamorphic and igneous conditions due to low closure temperatures for micas. Quartz‐feldspar fractionations may be less than those at peak conditions, and in some samples may even be slightly negative.The observed oxygen isotope fractionations in the metamorphic rocks are small and largely appear to record equilibrations close to peak conditions determined by other methods. Hence these rocks clearly do not conform to predictions of fluid‐present diffusional retrograde exchange. It is suggested that their retrograde history was therefore within an anhydrous closed system in which diffusion was slow and hence mineral closure temperatures were high. The granitic rocks record rather larger quartz‐biotite fractionations, approaching those predicted by the diffusion modelling. However, quartz‐feldspar fractionations are large and hence, although significant retrograde exchange has clearly occurred, simple diffusion alone is not sufficient to explain the observed data and open‐system exchange may be required. The presence of fluids during the retrograde history of this part of the section is supported by petrographic evidence.The different retrograde oxygen exchange histories recorded between the regional metamorphic and magmatic regimes of the Langtang section would appear to support the importance of water on the kinetics of such exchange, and suggests that in its absence, diffusional exchange may become insignificant, allowing oxygen isotope thermometry to record meaningful high‐temperature data.
Stable isotopic signatures of superposed fluid events in granulite facies marbles of the Rauer Group, East Antarctica
Authors: BUICK I. S. , HARLEY S. L. , CARTWRIGHT I. , MATTEY D. 
Published: May 2007

ABSTRACTThe role of volatiles in the stabilization of the lower (granulite facies) crust is contentious. Opposing models invoke infiltration of CO2‐rich fluids or generally vapour‐absent conditions during granulite facies metamorphism. Stable isotope and petrological studies of granulite facies metacarbonates can provide constraints on these models. In this study data are presented from metre‐scale forsteritic marble boudins within Archaean intermediate to felsic orthogneisses from the Rauer Group, East Antarctica.Forsteritic marble layers and associated calcsilicates preserve a range of 13C‐ and 18O‐depleted calcite isotope values (δ13C= ‐9.9 to ‐3.0% PDB, δ18O = 4.0 to 12.1% SMOW). A coupled trend of 13C and 18O depletion (∼2%, ∼5%, respectively) from core to rim across one marble layer is inconsistent with pervasive CO2 infiltration during granulite facies metamorphism, but does indicate localized fluid‐rock interaction. At another locality, more pervasive fluid infiltration has resulted in calcite having uniformly low, carbonatite‐like δ18O and δ13C values. A favoured mechanism for the low δ18O and δ13C values of the marbles is infiltration by fluids that were derived from, or equilibrated with, a magmatic source. It is likely that this fluid‐rock interaction occurred prior to high‐grade metamorphism; other fluid‐rock histories are not, however, ruled out by the available data. Coupled trends of 13C and 18O depletion are modified to even lower values by the superposed development of small‐scale metasomatic reaction zones between marbles and internally folded mafic (?) interlayers. The timing of development of these layers is uncertain, but may be related to Archaean high‐temperature (>1000d̀C) granulite facies metamorphism.
Isotopic constraints on fluid infiltration from an eclogite facies shear zone, Holsenøy, Norway
Authors: MATTEY D. , JACKSON D. H. , HARRIS N. B. W. , KELLEY S. 
Published: May 2007

ABSTRACTGranulite facies anorthosites on Holsenøy Island in the Bergen Arcs region of western Norway are transected by shear zones 0.1–100 m wide characterized by eclogite facies assemblages. Eclogite formation is related to influx of fluid along the shears at temperatures of c. 700d̀C and pressures in excess of 1.7 GPa. Combined carbon and nitrogen stable isotope, 40Ar/36Ar, trace‐element and petrological data have been used to determine the nature and distribution of fluids across the anorthosite‐eclogite transition.A metre‐wide drilled section traverses the eclogitic centre of the shear into undeformed granulite facies garnet‐clinopyroxene anorthosite. Clinozoisite occurs along grain boundaries and microcracks in undeformed anorthosite up to 1 m from the centre of the shear and clinozoisite increases in abundance as the edge of the shear zone is approached. The eclogite‐granulite transition, marked by the appearance of sodic pyroxene and loss of albite, occurs within the most highly sheared section of the traverse. The jadeite‐in reaction coincides with increased paragonite activity in mica. The separation between paragonite and clinozoisite reaction fronts can be semiquantitatively modelled assuming advective fluid flow perpendicular to the shear zone. The inner section of the traverse (0.25 m wide) is marked by retrogressive replacement of omphacite by plagioclase + paragonite accompanied by veins of quartz‐phengite‐plagioclase.C‐N‐Ar characteristics of fluid inclusions in garnet show that fluids associated with precursor granulite, eclogite and retrogressed eclogite are isotopically distinct. The granulite‐eclogite transition coincides with a marked change in CO2 abundance and δ13C (<36ppm, δ13C=‐2% in the granulite; <180 ppm, δ13C=‐10% in the eclogite). The distribution of Ar indicates mixing between influxed fluid (40Ar/36Ar > 25 times 103) and pre‐existing Ar in the granulite (40Ar/36Ar < 8 times 103). δ15N values decrease from +6% in the anorthosite to +3% within the eclogite shear. The central zone of retrogressed eclogite post‐dates shearing and is characterised by substantial enrichment of Si, K, Ba and Rb. Fluids are CO2‐rich (δ13C ∼ ‐5%) with variable N2 and Ar abundances and isotopic compositions.Both Ar and H2O have penetrated the underformed granulite fabric more than 0.5m beyond the granulite/eclogite transition during eclogite formation. Argon isotopes show a mixing profile consistent with diffusion through an interconnecting H2O‐rich fluid network. In contrast, a carbon‐isotope front coincides with the deformation boundary layer, indicating that the underformed anorthosite was impervious to CO2‐rich fluids. This is consistent with the high dihedral angle of carbonic fluids, and may be interpreted in terms of evolving fluid compositions within the shear zone.
Metamorphic evolution of granulites in the Minto Block, northern Québec: extraction of peak P‐T conditions taking account of late Fe‐Mg exchange
Authors: BéGIN N. J. , PATTISON D. R. M. 
Published: May 2007

ABSTRACTIn the central Minto Block of northern Québec, the Lake Minto and Goudalie domains are dominated, respectively, by orthopyroxene‐bearing plutonic suites (granite‐granodiorite and diatexite) and a tonalitic gneiss complex, both of which contain scattered remnant paragneisses. Two main granulite‐grade mineral assemblages are observed in the paragneiss: garnet (Grt)‐orthopyroxene (Opx)‐plagioclase‐quartz (GOPQ) and garnet (Grt)‐cordierite (Crd)‐sillimanite‐plagioclase‐quartz (GCSPQ). These show distinct lithological associations, with the GCSPQ assemblages occurring exclusively within the diatexite in the Lake Minto domain. Petrogenetic grid considerations demonstrate that the GOPQ rocks are higher grade than the GCSPQ rocks. Maximum temperatures for GOPQ rocks, obtained from equilibria based on Al solubility in orthopyroxene in equilibrium with garnet, range from 950 to 1000d̀ C, significantly higher than garnet‐orthopyroxene Fe‐Mg exchange temperatures of 700 ± 50d̀ C, the latter probably representing a closure temperature below peak conditions. The Al temperatures were corrected for late cation exchange by adjusting the Fe/(Fe + Mg) ratios in garnet and orthopyroxene, to achieve internal consistency between the GOPQ thermometers and barometers. Grt‐Crd thermometry records temperatures of 750±50d̀ C. Peak P‐T conditions range from 5‐6 kbar and 750‐800d̀ C in the Goudalie and eastern Lake Minto domains, to 7‐10 kbar and 950‐1000d̀ C in the western and central Lake Minto domain. This variability contrasts with the uniform crustal pressures of 5 ± 1 kbar recorded by the GCSPQ assemblages in the diatexites and the hornblende granodiorites (c. 4‐5 kbar) across the same area. The GOPQ rocks are inferred to record earlier P‐T conditions that prevailed before the formation of GCSPQ assemblages and the intrusion of the granodiorites. Partial P‐T paths in GOPQ rocks from both domains, based on net transfer equilibria corrected for Fe‐Mg resetting, document cooling of 100‐250d̀ C from thermal‐peak conditions, concomitant with a modest pressure decrease of 2‐3 kbar. Although textures diagnostic of isobaric cooling are not developed, the paths are consistent with a tectonic model in which granulite metamorphism and crustal thickening in the Minto Block were consequences of magmatic underplating. The progression from higher P‐T conditions recorded by GOPQ assemblages to lower P‐T conditions recorded by GCSPQ assemblages is attributed to variable amounts of synmagmatic uplift and cooling in a single, continuous thermal event in the Minto crust, associated with protracted crustal magmatism. In the Goudalie and eastern Lake Minto domains, where GOPQ and GCSPQ rocks and Hbl granodiorites have similar P‐T conditions of equilibration, the crust may not have been thickened as much as further west, where GOPQ P‐T conditions are significantly higher than those of the hornblende granodiorites and the GCSPQ rocks.
Stable isotopic and fluid inclusion evidence for meteoric fluid penetration into an active mountain belt; Alpine Schist, New Zealand
Authors: JENKIN G. R. T. , CRAW D. , FALLICK A. E. 
Published: May 2007

ABSTRACTCalcite and quartz veins have formed, and are forming, in steeply dipping fissures in the actively rising Alpine Schist metamorphic belt of New Zealand. The fluids that deposited these minerals were mostly under hydrostatic pressure almost down to the brittle‐ductile transition, which has been raised to 5‐6 km depth by rapid uplift. Some fluids were trapped under lithostatic pressures. Fluids in the fissure veins were immiscible H2O + NaCl‐CO2 mixtures at 200‐350d̀ C. Bulk fluid composition is 15‐20 mol% CO2 and <4.3 total mol CH4+ N2+ Ar/100mol H2O. Water hydrogen isotopic ratio δDH2O in the fissure veins spans ‐29 to ‐68‰, δ18OH2O ‐0.7 to 8.5‰, and bulk carbon isotopic ratio δ13C ranges from ‐3.7 to ‐11.7‰. The oxygen and hydrogen isotopic data suggest that the water has a predominantly meteoric source, and has undergone an oxygen isotope shift as a result of interaction with the host metamorphic rock. Similar fluids were present during cooling and uplift. Dissolved carbon is not wholly derived from residual metamorphic fluids; part may be generated by oxidation of graphite.
Geothermobarometry of mafic granulites and metapelite from the Palghat Gap, South India: petrological evidence for isothermal uplift and rapid cooling
Authors: KUMAR C. R. RAVINDRA , CHACKO T. 
Published: May 2007

ABSTRACTThe Palghat Gap region is located near the centre of the large southern Indian granulite terrane. at the northern edge of the Kodaikanal charnockite massif. The dominant rock types in the region are hornblende‐biotite ± orthopyroxene gneisses and charnockites along with minor amounts of intercalated mafic granulite, metapelite and calc‐silicate. TheP‐Testimates from garnetiferous mafic granulites and metapelite samples are generally in the range 9‐10 kbar and 800‐900d̀ C using both conventional thermobarometric methods and the TWEEQU thermobarometry program. TheseP‐Testimates, which should be taken as minimum values, are among the highest yet reported for South Indian and Sri Lankan granulites. The occurrence of orthopyroxene + plagioclase symplectites around embayed garnet grains in the mafic granulites and cordierite rims around garnet grains in metapelite suggest an isothermal decompression‐type path. Similarly, a core‐rimP‐Ttrajectory indicatesc.3 and 7 kbar decompression at high temperature in the mafic granulites and metapelite, respectively. In both rock types, the key to the determination of the retrogradeP‐Tpath was the recognition of small amounts of second generation plagioclase with a more anorthitic composition than the matrix plagioclase. The preservation of high garnet‐pyroxene temperatures in the mafic granulites (despite small garnet grain size) suggests rapid cooling of the terrane. Calculated minimum cooling rates range from 8 to 80d̀ C Ma‐1. Such cooling rates are more rapid than those associated with normal isostatic processes and suggest that the terrane was tectonically exhumed at high temperature.
The association of poorly ordered graphite, coke and bitumens in greenschist facies rocks of the Poniklá Group, Lugicum, Czech Republic: the result of graphitization of various types of carbonaceous matter
Authors: KRÍBEK B. , HRABAL J. , LANDAIS P. , HLADÍKOVÁ J. 
Published: May 2007

ABSTRACTGreenschist facies rocks of the Poniklá Group (Ordovician‐Silurian), Czech Republic, contain several types of carbonaceous matter that differ in their morphology, texture, reflectance and Raman characteristics.The first type consists of large (up to 3 mm) irregularly bound particles of low reflectance (Romin = 0.9%; Romax = 5.6%). The area ratio of the 1585 cm‐1 to 1350 cm‐1 Raman peaks (1.08–1.17) indicates an intermediate degree of graphitization. The formation of this type of highly porous particle, displaying a texture reminiscent of regular or needle coke, is attributed to the thermal alteration of the amorphous (structureless) kerogen of the precursor sediments.The second type consists of lamellar particles up to 30 μm thick, which can be associated with the latter or can occur independently in white mica‐rich laminae. This type is characterized by high bireflectance (Romin = 0.6%; Romax = 11.9%) and by lower ratios (0.70–0.82) of the Raman peak areas. These particles are interpreted as the product of solid‐state, diffusion‐controlled graphitization of a chemically homogeneous organic material, e.g. of graptolite periderms.The third type consists of isometric, up to 2 mm large, commonly fractured grains and fragments which mainly occur in quartz‐rich laminae. In reflected light, the texture is either homogeneous or consists of various types of anisotropic mozaics. The Raman peak area ratios (0.75–1.14) indicate a highly variable degree of structural ordering. These particles are considered as the remains of metamorphosed bitumens, accumulated in the sandy laminae of the original sediments.The fourth type consists of small particles of carbonaceous matter (maximum length 25 μm, thickness 1‐2 μm), which occur adjacent to crystal faces of white micas. This type is probably the product of epitaxial growth of graphite from the gaseous phase.The results of this work indicate that the differences in the degree of graphitization of the carbonaceous matter in low‐grade metamorphosed rocks can be mainly related to the initial nature of the sedimentary organic matter and to its premetamorphic history.
The three‐dimensional geometry of simulated porphyroblast inclusion trails: inert‐marker, viscous‐flow models
Authors: GRAY N. H. , BUSA M. D. 
Published: May 2007

ABSTRACTA fluid dynamic model for a rigid spherical porphyroblast growing in a viscous fluid matrix undergoing simple shear deformation predicts an arrangement of captured inert inclusions that is remarkably similar to the spiral pattern observed in synkinematic ‘rolled’ garnets. The model assumes only creeping (Stokes) flow of the matrix and a kinetic model for the growth of the porphyroblast of the form rm=kmΩ, where r is the crystal radius, Ω is the amount of shear‐induced rotation, and m and k are kinetic parameters. For rotation less than 18d̀, J. B. Thompson and J. L. Rosenfeld's simple ring‐model provides a surprisingly accurate description of the three‐dimensional geometry of the central inclusion surface in the simulated porphyroblasts. Although marker planes, originally parallel to the shear plane, are deflected around the porphyroblast, their intersections with its surface remain approximately circular for much larger amounts of rotation than anticipated by Rosenfeld. The polar coordinate equation, T(r, θ), of the double spiral formed by the trace of the central inclusion surface in a section through the centre of simulated porphyroblasts, is also surprisingly simple, as follows: rm∼ (6/5k)mθ. This relationship was observed over the complete range of rotation angles investigated, 0‐720d̀, and could form a reasonable basis for estimating the growth‐rotation history of rolled garnets from the shape of their spiral inclusion trails. Two‐dimensional sections through the simulated porphyroblasts, parallel to the rotation axis, yield ‘clamshell’ (Rosenfeld's term) inclusion geometries similar to the controversial ‘millipede’ patterns observed in many natural porphyroblasts.