bluuebunny/crossref_metadata_2025_split huggingface dataset:
Pseudotachylite from the Main Zone of the Hidaka metamorphic belt, Hokkaido, northern Japan
Authors: TOYOSHIMA T. 
Published: May 2007

Pseudotachylite veins have been found in the mylonite zone of the Hidaka metamorphic belt, Hokkaido, northern Japan. They are associated with faults with WNW‐ESE to ENE‐WSW or NE‐SW trends which make a conjugate set, cutting foliations of the host mylonitic rocks with high obliquity. The mylonitic rocks comprise greenschist facies to prehnite‐pumpellyite facies mineral assemblages. The mode of occurrence of the pseudotachylite veins indicates that they were generated on surfaces of the faults and were intruded as injection veins along microfractures in the host rocks during brittle deformation in near‐surface environments. An analysis of the deformational and metamorphic history of the Hidaka Main Zone suggests that the ambient rock temperature was 200–300° C immediately before the formation of the Hidaka pseudotachylite.Three textural types of veins are distinguished: cryptocrystalline, microcrystalline and glassy. The cryptocrystalline or glassy type often occupies the marginal zones of the microcrystalline‐type veins. The microcrystalline type is largely made up of quench microlites of orthopyroxene, clinopyroxene, biotite, plagioclase and opaque minerals with small amounts of amphibole microlites. The interstices of these microlites are occupied by glassy and/or cryptocrystalline materials. The presence of microlites and glasses in the pseudotachylite veins suggests that the pseudotachylites are the products of rapid cooling of silicate melts at depths of less than 5 km.The bulk chemical composition of the pseudotachylite veins is characterized by low SiO2 and a high water content and is very close to that of the host mylonitic rocks. This indicates that the pseudotachylite was formed by virtual total melting of the host rocks with sufficient hydrous mineral phases. Local chemical variation in the glassy parts of the pseudotachylite veins may be due to either crystallization of quench microlites or the disequilibrium nature of melting of mineral fragments and incomplete mixing of the melts. Pyroxene microlites show a crystallization trend from hypersthene through pigeonite to subcalcic augite with unusually high Al contents. The presence of pigeonite and high‐Al pyroxene microlites, of hornblende and biotite microlites and rare plagioclase microlites may indicate the high temperature and high water content of the melt which formed the pseudotachylite veins. The melt temperatures were estimated to be up to 1100° C using a two‐pyroxene geothermometer. Using published data relating water solubilities in high‐temperature andesitic magmas to pressure, a depth estimate of about 4 km is inferred for the Hidaka pseudotachylites.Evidence derived from pseudotachylites in the Hidaka metamorphic belt supports the conclusion that pseudotachylite is formed by frictional melting along fault surfaces at shallow depths from rocks containing hydrous minerals.
Prograde–retrograde P–T–t–deformation path of Austroalpine micaschists during Variscan continental collision (Eastern Alps)
Authors: SCHULZ B. 
Published: May 2007

A complete prograde P–T path, defined by 10 calculated P–T fields in succession, is recognized from metapelites by using geothermobarometry on garnet‐bearing assemblages with microstructural control. Overstacking of several tectonic units during an early Variscan continental collision explains the complex prograde P–T history. Isostatic uplift and deformation controlled the retrograde P–T path.Deformation with changing character acted continuously during all stages of the evolution of the Austroalpine basement complex. After the intrusion of Caledonian granitoids, metapelites and magmatic rocks suffered a shearing deformation D1–D2, which produced sheath folds as well as the main foliation S2. Spessartine‐rich first‐generation garnets, situated in microlithons enclosed by S2, record the onset of shearing under increasing high‐pressure–low‐temperature conditions (7 kbar/380°C). Geothermobarometry on second‐generation garnets which have been rotated during growth indicates isothermal decompression from 9 kbar to 5 kbar/500°C and subsequent recompression/heating during continuing shearing. This is explained by overthrusting of a tectonic unit (unit 2) from NE to SW upon the micaschist unit (unit 1), followed by isostatic uplift and further overstocking of a third unit (unit 3). The resulting Pmax of 12 kbar at 650°C and further increasing temperatures up to 680°C accompanied by decompression have been calculated using a third generation of garnets. These high‐pressure–high‐temperature conditions may explain the occurrence of eclogitic metabasites in adjacent regions.Staurolite and kyanite first appeared under decreasing pressures at the last stage of prograde P–T evolution. Shortening deformation D3 and simultaneous growth of typical amphibolite facies minerals (staurolite 2, kyanite 2, sillimanite, andalusite) occurred during the retrograde path. A final step of Variscan evolution was marked by an oppositely directed shearing D4 (at T > 300°C and P > 3 kbar), possibly indicating backthrusting or extension.Apart from acid intrusions, no signs of a previous Caledonian thermotectonic history were found in the area to the south of the Defereggen–Antholz–Vals Line.
From granulites to eclogites in the Sesia zone (Italian Western Alps): a record of the opening and closure of the Piedmont ocean
Authors: LARDEAUX J. M. , SPALLA M. I. 
Published: May 2007

The Sesia zone (Italian Western Alps) offers one of the best preserved examples of pre‐Alpine basement reactivated, under eclogite facies conditions, during the Alpine orogenesis. A detailed mineralogical study of eclogitized acid and basic granulites, and related amphibolites, is presented. In these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished.(a) A medium‐ to low‐Pgranulite stage giving rise to the development of orthopyroxene + garnet + plagioclase + brown amphibole + ilmenite ± biotite in basic granulites and garnet + K‐feldspar + plagioclase + cordierite + sillimanite + biotite + ilmenite in acid granulites.(b) A post‐granulite re‐equilibration, associated with the development of shear zones, producing discrete amphibolitization of the basic granulites and widespread development of biotite + sillimanite + cordierite + spinel in the acid rocks.(c) An eo‐Alpine eclogite stage giving rise to the crystallization of high‐Pand low‐Tassemblages.In an effort to quantify this evolution, independent well‐calibrated thermobarometers were applied to basic and acid rocks. For the granulite event,P‐Testimates are 7–9 kbar and 700–800° C, and for subsequent retrograde evolution,P‐Twas 4–5 kbar and 600° C. For the eo‐Alpine eclogite metamorphism, pressure and temperature conditions were 14–16 kbar and 550° C.The inferredP‐Tpath is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks. In the light of the available geochronological constraints we propose to relate the pre‐Alpine granulite and post‐granulite retrograde evolution to the Permo‐Jurassic extensional regime. The complex granulite‐eclogite transition is thus regarded as a record of the opening and of the closure of the Piedmont ocean.
Structural and metamorphic controls on the distribution of zircon in an evolving quartzofeldspathic migmatite: an example from the Reynolds Range, central Australia
Authors: DIRKS P. , HAND M. 
Published: May 2007

Three types of zircon occur in a complexly deformed and variably migmatized quartzofeldspathic gneiss from the Reynolds Range, central Australia. The oldest type is inherited from the granitic precursor of the gneiss, and is overgrown by a second group of zircon grains that formed during prograde, granulite facies metamorphism. Partial melting of the gneiss resulted in solution of both the inherited and metamorphic zircon. No new zircon growth accompanied crystallization of the partial melt, suggesting loss of zirconium–rich residual fluids. Hydrous, amphibolite facies retrogression of the gneiss and its migmatized variants during late shearing produced new, idiomorphic zircon in both the shear zone and its wall rocks.Important implications of this study are that (i) zircon has a tendency to dissolve if it comes into direct contact with a melt produced from anhydrous biotite breakdown in a quartzofeldspathic granulite, (ii) melt crystallization is not necessarily accompanied by zircon growth, and (iii) euhedral zircon can grow from a hydrous fluid phase under subsolidus, amphibolite facies conditions, e.g. within shear zones.
Pressure–temperature evolution of metapelitic granulites in a polymetamorphic terrane: the Rauer Group, East Antarctica
Authors: HARLEY S. L. , FITZSIMONS I. C. W. 
Published: May 2007

Distinctive lithological associations and geological relationships, and initial geochronological results indicate the presence of an areally extensive region of reworked Archaean basement containing polymetamorphic granulites in the Rauer Group, East Antarctica.Structurally early metapelites from within this reworked region preserve complex and varied metamorphic histories which largely pre‐date and bear no relation to a Late Proterozoic metamorphism generally recognized in this part of East Antarctica. In particular, magnesian metapelite rafts from Long Point record extreme peak P–T conditions of 10–12 kbar and 100–1050°C, and an initial decompression to 8 kbar at temperatures of greater than 900°C. Initial garnet–orthopyroxene–sillimanite assemblages contain the most magnesian (and pyrope‐rich) garnets (XMg= 0.71) yet found in granulite facies rocks. A high‐temperature decompressional P–T history is consistent with reaction textures in which the phase assemblages produced through garnet breakdown vary systematically with the initial garnet XMg composition, reflecting the intersection of different divariant reactions in rocks of varied composition as pressures decreased. This history is thought to relate to Archaean events, whereas a lower‐temperature (c. 750–800°C) decompression to 5 kbar reflects Late Proterozoic reworking of these relict assemblages.The major Late Proterozoic (c. 1000 Ma) granulite facies metamorphism is recorded in a suite of younger Fe‐rich metapelites and associated paragneisses in which syn‐ to post‐deformational decompression, through 2–4 kbar from maximum recorded P–T conditions of 7–9 kbar and 800–850°C, is constrained by geothermobarometry and reaction textures. This P–T evolution is thought to reflect rapid tectonic collapse of crust previously thickened through collision.
Reaction textures in calc‐silicate granulites from the Bolingen Islands, Prydz Bay, East Antarctica: implications for the retrograde P–T path
Authors: MOTOYOSHI Y. , THOST D. E. , HENSEN B. J. 
Published: May 2007

Calc‐silicate granulites from the Bolingen Islands, Prydz Bay, East Antarctica, exhibit a sequence of reaction textures that have been used to elucidate their retrograde P–T path. The highest temperature recorded in the calc‐silicates is represented by the wollastonite‐ and scapolite‐bearing assemblages which yield at least 760°C at 6 kbar based on experimental results. The calc‐silicates have partially re‐equilibrated at lower temperatures (down to 450°C) as evidenced by the successive reactions: (1) wollastonite + scapolite + calcite = garnet + CO2, (2) wollastonite + CO2= calcite + quartz, (3) wollastonite + plagioclase = garnet + quartz, (4) scapolite = plagioclase + calcite + quartz, (5) garnet + CO2+ H2O = epidote + calcite + quartz, and (6) clinopyroxene + CO2+ H2O = tremolite + calcite + quartz.The reaction sequence observed indicates that aCO2 was relatively low in the wollastonite‐bearing rocks during peak metamorphic conditions, and may have been further lowered by local infiltration of H2O from the surrounding migmatitic gneisses on cooling. Fluid activities in the Bolingen calc‐silicates were probably locally variable during the granulite facies metamorphism, and large‐scale CO2 advection did not occur.A retrograde P–T path, from the sillimanite stability field (c. 760°C at 6 kbar) into the andalusite stability field (c. 450°C at <3 kbar), is suggested by the occurrence of secondary andalusite in an adjacent cordierite–sillimanite gneiss in which sillimanite occurs as inclusions in cordierite.
High‐pressure metamorphism in the Western Cordillera of North America
Authors: WHITNEY D. L. 
Published: May 2007

The Skagit Gneiss, a major component of the crystalline core of the North Cascades, was metamorphosed during a mid‐Cretaceous(?) to early Tertiary high‐P event driven by the collision of the Insular and Intermontane superterranes. Maximum pressures recorded by metapelitic rocks are 8–10 kbar at 650–725° C. High pressures are also indicated by coexisting staurolite and hornblende in amphibolites in the Skagit Gneiss and adjacent Cascade River Schist.Mineral reactions continued during nearly isothermal decompression from 8–10 kbar to c. 3–5 kbar. Early high‐P minerals (e.g. kyanite) are present as armoured relics in garnet in gneisses that contain sillimanite and cordierite in the groundmass. Skeletal relics of kyanite are also present in the groundmass of lower‐grade, staurolite‐bearing schists that contain texturally later cordierite. This matrix kyanite may have been preserved as a result of rapid uplift following initial decompression at high temperature.These results represent a revision of the metamorphic history of the Skagit Gneiss, which was formerly thought to have experienced only relatively low‐P Barrovian metamorphism. Qualitative estimates of metamorphic conditions based on stable matrix mineral assemblages result in an underestimation of maximum pressures because mineral reactions continued during decompression.Geobarometric results for the Skagit Gneiss are interpreted as evidence for major crustal thickening in the North Cascades. Recognition that pressures of c. 9 kbar were attained supports a contractional model for North Cascades orogenesis and requires that tectonic syntheses account for the burial of the Skagit Gneiss protoliths to a depth of c. 25–30 km.
Hydrodynamic modelling of some metamorphic processes
Authors: PERCHUK L. L. , PODLADCHIKOV Y. YU. , POLYAKOV A. N. 
Published: May 2007

The P–T paths for metamorphic complexes from the Precambrian shields and fold belts of different ages may result from advection, i.e. one‐cycle convective processes in the lithosphere. This conclusion has been exemplified by the metamorphic evolution of several well‐known complexes, for which an advective model can be successfully applied. Numerical simulations of the above processes in terms of Newtonian rheology by using a two‐dimensional finite element program have been conducted.Two representative models for intracontinental gravitational ordering initiated presumably by mantle activity are considered: (i) a thermally activated multi‐layered rhythmic sequence and (ii) huge rising diapiars causing circulation, in which crustal lithologies underwent high‐P metamorphism (above 10–15 kbar) and subsequent ascent toward the Earth's surface.
The (Na–Ca)amphibole–albite–chlorite–epidote–quartz geothermobarometer in the system S–A–F–M–C–N–H2O. 1. An empirical calibration
Authors: TRIBOULET C. 
Published: May 2007

ABSTRACTThe proposed geothermobarometer is based on an empirical calibration which takes account of two equilibria involving the tremolite, edenite, pargasite and hastingsite components in amphiboles. It has applications to assemblages found in metabasic rocks of widely different chemical compositions (magnesian to Fe‐rich metabasalts), and for metamorphism ranging from lower greenschist to highest amphibolite facies. Knowing the Si(T1), Aliv, Alvi, Fe3+, Fe2+, Mg, Ca, NaM4, NaAand A vacancy in an amphibole, and the Al3+andXMgin coexisting epidote and chlorite, it is possible to calculate two values of InKdfor this assemblage. These equilibria involve edenite‐tremolite and (pargasite/hastingsite)‐tremolite end‐members in amphibole (the calculation program is given). For these equilibria, the isopleths (iso‐values ofKd) have been calculated for 0.27 <XMg< 0.75 and 0 <XFe3+= Fe3+/(Fe3++ Alvi) < 0.8. It is then possible to determine pressure and temperature directly whenXMg,XFe3+, InKdfor tremoliteedenite and InKdfor (pargasite/hastingsite)‐tremolite are known. Application of this geothermobarometer is limited to Ca‐free plagioclase assemblages, and completeP–Tpaths can be drawn only if all the minerals are considered together. Phase relations at successive stages of crystallization can be constrained by studying the relationships between the coexisting minerals, their zoning and the metamorphic fabrics.
Sequential, syndeformational porphyroblast growth during Hercynian low‐pressure/high‐temperature metamorphism in the Canigou massif, Pyrenees
Authors: GIBSON R. L. 
Published: May 2007

ABSTRACTThe sequence of growth of garnet, staurolite and aluminosilicate in Fe‐rich metapelitic rocks from the Canigou massif, Pyrenees, is established using evidence of inclusion, reaction and pseudomorphing textures between the different minerals, compositional zoning patterns in garnet and staurolite (that can be related to the KFMASH reaction grid), and the geometric relations between inclusion trails in the porphyroblasts and the matrix microstructures. The evidence indicates that garnet and staurolite commenced growth before aluminosilicate in all cases, even where all three are in textural equilibrium. Interpretation of the reaction textures between the porphyroblasts and of the compositional zoning in garnet and staurolite in terms of the KFMASH reaction grid indicates the importance of continuous reactions in the development of these phases. Some garnet and staurolite porphyroblasts underwent renewed growth during breakdown, producing rims enriched in Mn and Zn respectively. The presence of aluminosilicate in these assemblages (i.e. the absence of a clear andalusite‐absent zone in the field) is attributed to a strong pressure‐dependence for the aluminosilicate‐producing reactions. Porphyroblast‐matrix microstructural relations indicate that Hercynian metamorphism in the massif was synchronous with the development of the regional subhorizontal foliation (S3).
Orthopyroxene–sillimanite–sapphirine granulites from the Bamble granulite terrane, southern Norway
Authors: KIHLE J. , BUCHER‐NURMINEN K. 
Published: May 2007

ABSTRACTSilica‐deficient sapphirine‐bearing rocks occur as an enclave within granulite facies Proterozoic gneisses and migmatites near Grimstad in the Bamble sector of south‐east Norway (Hasleholmen locality). The rocks contain peraluminous sapphirine, orthopyroxene, gedrite, anthophyllite, sillimanite, sapphirine, corundum, cordierite, spinel, quartz and biotite in a variety of assemblages. Feldspar is absent.Fe2+/(Fe2++ Mg) in the analysed minerals varies in the order: spinel > gedrite ≥ anthophyllite ≥ biotite > sapphirine>orthopyroxene > cordierite.Characteristic pseudomorph textures indicate coexistence of orthopyroxene and sillimanite during early stages of the reaction history. Assemblages containing orthopyroxene‐sillimanite‐sapphirine‐cordierite‐corundum developed during a high‐pressure phase of metamorphism and are consistent with equilibration pressures of about 9 kbar at temperatures of 750–800°C. Decompression towards medium‐pressure granulite facies generated various sapphirine‐bearing assemblages. The diagnostic assemblage of this stage is sapphirine‐cordierite. Sapphirine occurs in characteristic symplectite textures. The major mineralogical changes can be described by the discontinuous FMAS reaction: orthopyroxene + sillimanite → sapphirine + cordierite + corundum.The disequilibrium textures found in the Hasleholmen rocks are characteristic for reactions which have been in progress but then ceased before they run to completion. Textures such as reaction rims, symplectites, partial replacement, corrosion and dissolution of earlier minerals are characteristic of granulite facies rocks. They indicate that, despite relatively high temperatures (700–800° C), equilibrium domains were small and chemical communication and transport was hampered as a result of dry or H2O‐poor conditions.
Ionic association in H2O–CO2 fluids at mid‐crustal conditions
Authors: WALTHER J. V. 
Published: May 2007

The equilibrium constant, Ka, of the association reaction to form ion pairs from charged solute species in supercritical solutions can be calculated from a model based on published equations. Log Ka at constant pressure is a linear function of the inverse in the dielectric constant of the fluid times temperature. The dielectric properties of H2O and CO2 at supercritical pressures and temperatures can also be evaluated using the Kirkwood equation. Using Looyenga mixing rules, the dielectric constant of H2O–CO2 mixtures can be obtained and the change in log Ka with addition of CO2 in aqueous solutions evaluated. These changes in log Ka with addition of CO2 are consistent with measured changes of log Ka with addition of Ar in supercritical H2O–Ar solutions.Log Ka of KCl and NaCl increase to an increasing extent as the mole fraction of CO2 increases in H2O–CO2 solutions. For instance, at 2 kbar and constant temperature between 400 and 600° C, log Ka of KCl increases by about two orders of magnitude whilst that of NaCl increases by over four orders of magnitude as the CO2 mole fraction increases from 0.0 to 0.35. Such changes in log Ka will have dramatic effects on the solubility of minerals in CO2‐rich environments.
A garnet population in Yellowknife schist, Canada
Authors: KRETZ R. 
Published: May 2007

Abstract
Data are presented on a garnet population in a specimen of garnet‐biotite‐plagioclase‐quartz schist from the cordierite zone of an Archaean thermal dome in the Southern Slave Province of the Canadian Shield. Garnet crystals are bounded by planar dodecahedral faces and by trapezohedral faces which on the 10‐μm scale are corrugated. Crystal distribution, as revealed by dissection of a small cubic volume of rock, is random. The size distribution is normal, with a mean diameter of 0.81 mm and a standard deviation of 0.32 mm. In the largest crystal of the population (mean radius 0.83 mm), [Mn] = 100 Mn/(Fe + Mg + Mn + Ca) decreases from 14.5 at the centre to 7.5 and then increases in the outer margin to 8.5; [Fe] increases continuously from 67 at the centre to 77 at the surface; [Mg] increases from 12.5 to 13.5 and then falls sharply to 11; [Ca] remains unchanged at 4.0 and then drops to 3.3. Progressively smaller crystals have progressively lower [Mn] and higher [Fe] concentrations at their centres, while all crystals have the same margin composition. Growth vectors extending from given concentration contours to crystal surfaces are of equal length regardless of the size of the crystal in which the vector is located.A garnet‐forming model is presented in which reaction was initiated by a rise in temperature. Nucleation sites were randomly selected. The nucleation rate increased with time and then declined. Crystal faces advanced at a constant linear rate, which implies an increase in volume proportional to surface area. Initially, the composition of garnet deposited on crystal surfaces was determined by van Laar equations of equilibrium, which demanded the withdrawal of Mn and Fe from within chlorite crystals. This transfer reaction was then accompanied by an ion exchange reaction which moved Mn and Fe to garnet surfaces from biotite, in exchange for Mg. The exchange reaction provides an explanation for the high overall concentration of Mn and Fe in garnet and for the observed Mn and Mg reversals in the margins of crystals. The increase of garnet volume in the garnet population is found to be parabolic, i.e. Vαα5.