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Abstract Late Archaean orthogneisses and aluminous and iron-rich metasedimentary rocks intruded by anorthosite and a ferrodiorite-granite suite were completely recrystallized during Proterozoic granulite facies metamorphism. Geobarometry and geothermometry indicate P-T conditions of around 7.5kbar. 700°C, with a CO2 -rich fluid phase and logfO2 at or below -16. A two-stage high-grade history of near isochemical corona growth is preserved in metasediments with the reaction cycle opx + plag + H2 O → hbl+gar+SiO2 → opx+plag+H2 O. End product compositions resemble those of the initial phases, and the only mobile components were SiO2 and/or H2 O. The coronas reflect shortlived fluctuations in chemical activity at essentially constant P and T, contrary to simple progressive change in equilibrium parameters recorded by most corona-bearing textures. 相似文献
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Within the Isortoq Complex of central West Greenland, doleritedykes have been metamorphosed in upper amphibolite to garnetgranulite facies. Detailed sampling and petrographic study ofthese dykes demonstrate that the entire sequence of assemblagesmay occur in individual dykes, with the amphibolite facies assemblagesnormally restricted to dyke margins, garnet granulite faciesrocks occurring in the dyke cores, and pyroxene granulite faciesrocks intervening between these two. Electron microprobe analysisof all coexisting phases demonstrates that the progression fromamphibolite facies to granulite facies assemblages results inthe extraction of edenite and tschermakite components from theamphibole in approximately a 2: 1 ratio. As amphibole breakdownproceeds, systematic changes in element abundance within amphiboleoccur when reactions are crossed, due to the development ofnew phases. The algebraic removal of small but systematic compositionalvariations within the dykes allows derivation of six equilibria,which provide a general model for metamorphism of basaltic rocksunder high grade metamorphic conditions. This model takes theform of the idealized assemblage sequences which will developalong (regional) gradients in XH2O, temperature and/or solidpressure. Analysis of these equilibria demonstrates that variablesilica activity and Fe2+–Mg2+ exchange will modify reactionstoichiometry, will affect the ratio of edenite to tschermakitein amphibole breakdown reactions, and will modify the compositionof the product plagioclase. These variations together with variationsin water fugacity will locally modify the ideal assemblage sequences,and will explain the complex assemblage relationships observedin metabasic rocks of natural high grade terranes.
*Present address: Instituttet for Teknisk Geologi, Danish Technical University, Building 204, DK–2800 Lyngby, Denmark 相似文献
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