摘要:

AbstractThe 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–Tconditions 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 itsP–Tconditions are estimated to have been 550–700°C and 2 kbar.TheP–T–tpaths of the Hidaka metamorphism show a thickening–heating–uplifting process. The metamorphism is inferred to have taken place beneath an active is

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00421.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractPartitioning of Fe and Mg between garnet and phengitic muscovite was calibrated as a geothermometer by Green&Hellman (1982) using experimental data at 25–30 kbar. When the thermometer is applied to pelites regionally metamorphosed at pressures of between 3 and 7 kbar it yields temperatures much higher than those from the garnet–biotite thermometer. A new empirical calibration is proposed for use with such rocks, with particular application where garnet occurs at lower grades than biotite. The new calibration is whereKis given by: InK= InKdandXiiare mole fractions in the garnets.The calibration was derived from comparison with the garnet–biotite thermometer of Ferry&Spear (1978), assuming no pressure‐dependence for the partitioning between garnet and muscovite, no ferric iron partitioning, ideal mixing in muscovite, and the garnet mixing model of Ganguly&Saxena (1984) modified for a non‐linear Ca effect. This latter garnet mixing model was selected because it gave the geologically most reasonable results. It has not proved possible to distinguish a pressure effect from a ferric‐iron effect.Despite the simplifying assumptions used to derive the calibration, it yields temperatures generally within 15°C of those given by the garnet–biotite thermometer, and has been used to supply thermometric data in a low‐grade region of the

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00422.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractChloritoid and pyrophyllite occur together in all major goldfields of the Witwatersrand Basin and are widespread in virtually all rock types of the upper Witwatersrand Supergroup, including metaconglomeratic reefs and altered mafic rocks. Both minerals are particularly characteristic of the pelitic horizons intimately associated withreef packages, but they are also developed locally in theregionally persistentmetapelites that have basin‐wide extent. Pyrophyllite is particularly common in foliated zones, adjacent to quartz veins, and near unconformably overlying auriferous conglomerates.The wide distribution of chloritoid and pyrophyllite in metapelites of the Witwatersrand Basin is attributed to alteration of chlorite‐rich shales, rather than to unusual premetamorphic starting materials. This alteration event involved the redistribution of many elements, with up to 40% volume loss, mainly due to removal of silica. Removal of most of the Mg and some Fe accounts for the stabilization of chloritoid and pyrophyllite. Relatively immobile elements included Al, Ti, Nb, Cr, V, P, La and Ce, whereas Si, Fe, Mn, Zn, Co, Ni, Cu, Mg and Ca were lost, and K, Rb and Ba were introduced by an infiltrating fluid.The alteration event is inferred to have been within the chloritoid and pyrophyllite stability field (and thus syn‐metamorphic) as bulk chemical changes in metapelites are from chlorite directly towards chloritoid and then pyrophyllite, rather than to lower grade minerals such as kaolinite. Muscovite–chlorite–chloritoid and muscovite–chloritoid–pyrophyllite assemblages are attributed to fluid buffering along appropriate curves, as their production by metamorphism of lower grade mineral mixes is considered unlikely, based on the present bulk rock compositional data. A metamorphic timing for the alteration accounts for the correlation of strongly foliated areas with greater degrees of inferred alteration. The transitions from chlorite to chloritoid to pyrophyllite define zones of increasing alteration.Widespread infiltration as part of peak metamorphism is suggested by the distribution of chloritoid and pyrophyllite, quartz veining and textures. Fluid:rock ratios calculated from a silica budget in one metapelitic horizon exceed 100:1 over many square kilometres. These values need not imply multi‐pass fluid flow, as much of the silica migration may be redistribution on a scale of a few metres, from source rocks into veins. Although infiltration during metamorphism may have affected much of the upper Witwatersrand succession, channelized fluid flow within reef packages, along faults and unconformities and in certain metaconglomerates and metapeli

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00423.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractCrystal‐chemical relationships between coexisting sodic and calcic amphiboles have been studied in eclogitic metagabbros from the Aosta Valley, Western Alps. Textural analysis gives evidence of three successive high‐pressure parageneses:1. Pre‐kinematic high‐grade blueschist assemblages, preserved as polymineralic inclusions in garnet cores and made of glaucophane and actinolite (stage A).2. Synkinematic eclogite assemblages, composed of garnet + omphacite + glaucophane ± actinolite ± white mica ° Clinozoisite + quartz + rutile (stage B).3. Post‐kinematic epitactic overgrowths of barroisitic amphibole on glaucophane and actinolite (stage C).P–Tconditions of the eclogitic metamorphism have been estimated at around 500–550°C, 16 kbar.Glaucophane and actinolite coexist as discrete grains in stage A and B assemblages. This texture and the chemistry of the amphiboles unambiguously denotes the existence of a miscibility gap between sodic and calcic amphiboles (from NaM4= 0.80 in actinolite to NaM4= 1.70 in glaucophane atT= 500–550°C). A comparison with published analyses allows a new solvus along the glaucophane–actinolite join to be drawn.The later barroisitic amphibole (stage C) exhibits strong chemical zonation indicating disequilibrium growth. This amphibole cannot either be used to define a miscibility gap with glaucophane or actinolite or be considered as an intermediate stage between

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00424.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractGarnet granulites from Sri Lanka preserve textural and chemical evidence for prograde equilibration at temperatures of at least 700–750°C and pressures in the vicinity of 6–8 kbar. Associated strain patterns suggest prograde metamorphism occurred during and immediately following an episode of crustal thickening, with the progradeP–Tconditions probably reflecting a combination of the conductive and advective transport of heat at the mid‐levels of tectonically thickened crust. The occurrence of prograde wollastonite provides evidence for internally buffered fluid compositions, or fluid absent conditions, during peak metamorphism and precludes pervasive advection of a CO2‐rich fluid. The advective heat component is therefore likely to have been provided by the transport of silicate melt. Intricate symplectitic textures record partial re‐equilibration of the garnet granulites to lower pressures (˜ 4–6 kbar) at high temperatures (600–750°C), and testify either to the erosional denudation of the overthick crust prior to significant cooling (i.e. quasi‐isothermal decompression) or to a subsequent static heating possibly of early Palaeozoic age (Pan‐African). The metamorphic history of the Sri Lankan granulites is compared with high grade terrains in the neighbouring fragments of Gondwana, with the emphasis on similarities with Proterozoic granulites of the

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00425.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractMineral assemblages in different samples of amphibolite facies pelitic schists collected from two separate outcrops in the Moosilauke area, NH, record differences in the chemical potential of water during metamorphism. Mineralogical, petrological, and field relations indicate that mineral assemblages at both outcrops equilibrated at 520°C and 3.5–4.0 kbar. Thermodynamic analysis of the mineral assemblages demonstrates that maximum chemical potential differences at each outcrop were of the order of 150 calories, over distances of 10–20 m.The differences in the chemical potential of water recorded in both bed‐to‐bed and outcrop‐to‐outcrop relations are consistent with the following conclusions: (1) mineral assemblages on a specific outcrop did not equilibrate with an external reservoir of fluid of fixed composition, (2) the relatively small magnitude of the chemical potential differences suggests little or no infiltration of externally derived fluid, (3) these differences on the outcrop scale are probably related to initial compositional variations and the buffer capacity of the mineral assemblage, and (4) the different values of the chemical potential of water exhibited by the various mineral assemblages permits an understanding of the effects of variable μH2Ofor amphibolite facies pe

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00426.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

TECTONIC SETTINGS OF REGIONAL METAMORPHISM. Edited by E.R. Oxburgh, B.W.D. Yardley and P.C. England. Royal Society of London. 1987.

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00427.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00428.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY