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Institute of Experimental Mineralogy, USSR Academy of Sciences, 142432 Chernogolovka, Moscow District, USSR
Petrological studies, including geothermobarometry, of metamorphic mineral parageneses together with isotopic data, reveal differences in the P-T-fluid regimes of Precambrian shields and surrounding fold belts. Typical features of shields are CO2-rich fluids and preservation of retrograde portions of P-T trajectories, while in fold belts, prograde as well as retrograde stages of metamorphism are recorded in the minerals and the CO2 contents of the fluids vary widely. Geothermobarometric estimates of prograde and retrograde stages in several metamorphosed fold-belt complexes are in accord with the distribution of Al2SiO5 polymorphs.
A model involving vertical redistribution is proposed for the granulite-facies Sharyzhalgay complex of Lake Baikal, which is interpreted to be the metamorphosed equivalent of the Karelian greenstone belt. Increased heat flow and burial leads to gravitational redistribution due to density differences, so that dome structures cored by granulite-facies gneiss occur within envelopes of down-sinking metabasites and meta-ultrabasites. The model is supported by the geochronology and by the P-T-fluid regimes deduced from geothermobarometry and fluid-inclusion studies of the polymetamorphic Sharyzhalgay complex. This model may apply to other granulite-facies complexes which appear to be the result of high-grade metamorphism of Archaean greenstone belts made up of tholeiitic and komatiitic volcanics. These rocks were originally underlain by a complex of acid volcanics (dacites and rhyolites).
