摘要:

Garnet lherzolite from the Lyonnais area (eastern French Massif Central) occurs as several lenses elongated within the regional foliation of garnet‐biotite‐sillimanite gneisses. Within the peridotites a mylonitic foliation can be observed which clearly is oblique to the regional foliation of the surrounding gneisses. Petrological and thermobarometric studies emphasize a tectonometamorphic re‐equilibration for both crustal and mantle rocks characterized by a prograde metamorphic stage followed by retrograde evolution. During the burial stage, interpreted as lithospheric subduction, the peridotites underwent their mylonitic deformation, under high‐pressure conditions (23–30 kbar). In contrast, the paragneisses have suffered their deformation during the retromorphic evolution under mesozonal conditions (6–8 kbar, 700°C). Our thermobarometric investigations allow us to interpret the granulitic/ultramafic association from the Monts du Lyonnais area as a lithospheric section buried into a Palaeozoic subduction zone, laminated during continental collision and uplifted by erosi

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1990.tb00481.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Garnet‐biotite gneisses, some of which contain sillimanite or hornblende, are widespread within the Otter Lake terrain, a portion of the Grenville Province of the Canadian Shield. The metamorphic grade is upper amphibolite to, locally, lower granulite facies.The atomic ratio Fe2+/(Fe2++ Fe3+) in biotite ranges from 0.79 to 0.89 (ferrous iron determinations in 10 highly pure separates), with a mean of 0.86. Mg and Fe2+atoms occupy 67–78% of the octahedral sites, the remainder are occupied by Fe3+, Ti, and Al, and some are vacant. Mg/(Mg + Fe2+), denotedX, in the analysed samples ranges from 0.32 to 0.65. Garnet contains 1–24% grossular, 1–12% spessartine andXranges from 0.07 to 0.34.Compositional variation in biotite and garnet is examined in relation to three mineral equilibria:(I) biotite + sillimanite + quartz = garnet + K‐feldspar + H2O;(II) pyrope + annite = almandine + phlogopite;(III) anorthite = grossular + sillimanite + quartz.Measurements ofX(biotite) andX(garnet) are used to construct an illustrative model for equilibrium (I) which relates the observed variation inXto a temperature range of 70°C or a range in H2O activity of 0.6; the latter interpretation is preferred.In sillimanite‐free gneisses, the distribution of Mg and Fe2+between garnet (low in Ca and Mn) and biotite is adequately described by a distribution coefficient (KD) of 4.1 (equilibrium II). The observed increase in the distribution coefficient with increasing Ca in garnet is lnKD= 1.3 + 2.5 × 10−2[Ca]where [Ca] = 100 Ca/(Mg + Fe2++ Mn + Ca). The distribution coefficient is apparently unaffected by the presence of up to 12% spessartine in garnet.In several specimens of garnet‐sillimanite‐plagioclase gneiss, the Ca contents of garnet and of plagioclase increase in unison, as required by equilibrium (III). The mean pressure calculated from these data (n= 17) is 5.9 kbar, and the 95% confidence l

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1990.tb00482.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

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 SiO2and 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 contai

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1990.tb00483.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

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 gr

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1990.tb00484.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Pumpellyite from four‐phase assemblages (pumpellyite + epidote + prehnite + chlorite; pumpellyite + epidote + actinolite + chlorite; pumpellyite + epidote + Na‐amphibole + chlorite, together with common excess phases), considered to be low variance in a CaO‐(MgO + FeO)‐Al2O3‐Fe2O3(+Na2O + SiO2+ H2O) system, have been examined in areas which underwent metamorphism in the prehnite‐pumpellyite, pumpellyite‐actinolite and low‐temperature blueschist facies respectively. The analysed mineral assemblages are compared for nearly constant (basaltic) chemical composition at varying metamorphic grade and for varying chemical composition (basic, intermediate, acidic) at constant metamorphic conditions (low‐temperature blueschist facies).In the studied mineral assemblages, coexisting phases approached near chemical equilibrium. At constant (basaltic) bulk rock composition the MgO content of pumpellyite increases, and the XFe3+of both pumpellyite and epidote decreases with increasing metamorphic grade, the Fe3+being preferentially concentrated in epidote. Both pumpellyite and epidote compositions vary with the bulk rock composition at isofacial conditions; pumpellyite becomes progressively enriched in Fe and depleted in Mg from basic to intermediate and acidic bulk rock compositions. The compositional comparison of pumpellyites from high‐variance (1–3 phases) assemblages in various bulk rock compositions (basic, intermediate, acidic rocks, greywackes, gabbros) shows that the compositional fields of both pumpellyite and epidote are wide and variable, broadly overlapping the compositional effects observed at varying metamorphic grade in low‐variance assemblages.The intrinsic stability of both Fe‐ and Al‐rich pumpellyites extends across the complete range of the considered metamorphic conditions. Element partitioning between coexisting phases is the main control on the mineral compositio

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1990.tb00485.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The Southern Brittany Migmatite Belt (SBMB), which evolved through the metamorphic peak betweenc.400 Ma andc.. 370 Ma ago, consists of a heterogeneous suite of high‐grade gneisses and anatectic migmatites, both metatexites and diatexites. Rare garnet‐cordierite gneiss layers record evidence of an early progradeP‐Tpath. In these rocks, growth‐zoned garnet cores and a sequence of included mineral assemblages in garnet, from core to rim, of Qtz + Ilm + Ky, Pl + Ky + St + Rt + Bt and Pl + Sil + St + Rt + Bt constrain a prograde evolution during which the reactions Ilm + Ky + Qtz→ Aim + Rt, Ms + Chl→ St + Bt + Qtz + V and St + Qtz→ Grt + Sil + V were crossed. Parts of this prograde evolution are preserved as inclusion assemblages in garnet in all other rock types. In all rock types, garnet has reverse zoned rims, and garnet replacement by cordierite and/or biotite and plagioclase suggests the following reactions have occurred: Grt + Sil + Qtz→ Crd → Hc ± Ilm, Bt + Sil + Qtz → Crd ± Hc → Ilm → Kfs + V and (Na + Ca + K + Ti) + Grt → Bt + Pl + Qtz. Microstructural analysis of reaction textures in conjunction with a petrogenetic grid has enabled the construction of a tightly constrained ‘clockwise’P–Tpath for the SBMB. The high‐temperature part of the path has a steepdT/dPslope characteristic of near isothermal decompression. It is proposed that theP‐Tpath followed by the SBMB is the result of the inversion, by overthrusting, of a back‐arc basin and that such a tectonic setting may be applicable to other high‐temperature migmatite terranes. The near isothermal decompression is at least partly driven by the upward (diapiric) movement of the dia

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1990.tb00486.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

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.

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1990.tb00487.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1990.tb00488.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1990.tb00489.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY