GEOCHEMISTRY OF PROGRADE AND RETROGRADE METAMORPHISM, BAIKAL REGION, USSR

Abstract

The distribution pattern of about 40 elements in zoned prograde metamorphic complexes of andalusite-sillimanite and kyanite—s sillimanite types has been studied in the Khamar-Daban region and Patom highlands, located along the southeastern margin of the Siberian platform. Mctapelitic layers of similar composition from the two areas are discussed as an example. The temperature of metamorphism ranged from 400 to 750�C for both complexes, and pressure was 3–6 kb for Khamar-Daban and 4–8 kb for Patom. The retrograde metamorphic trend was studied for the proto-ophiolite Baikal-Muya complex, North Baikal region (T = 850–360�C, high pressure). Metamorphism within the greenschist-amphibolite fades range was shown to have quasi-isochemical features, and contents of some elements varied either as a result of original variation in sedimentary fades or diversity of basic rocks. Given the genesis of rocks, the geodynamic and paleogeographic settings of their formation can be ascertained. Granitization, which causes a major change of rock composition at different pressures in the complexes, displays peculiar characteristics. Migmatitic complexes formed at high lithostatic pressure and high fluid pressure are characterized by vast areas of Na-migmatites, which are the products of metamorphic recrystallization (to produce leucosomes and mesosomes) with insignificant metasomatism. High-pressure migmatite complexes also contain undifferentiated water-saturated anatectic melts of potassic-stage granitization. The potassic-stage granitization, with anatexis in pelitic sequences and metasomatism and subsequently anatexis in basic sequences, is more typical of complexes with moderate and low lithostatic and fluid pressure. These complexes are remarkable for low-water differentiated series of granites and pegmatites with rare-metal mineralization.