GRANITOIDS OF DERBINA COMPLEX (EAST SAYAN ) - GEOCHEMISTRY AND MELT SOURCES

Proterozoic granitoids intruding the metasedimentary cover of the Derb ina block (microcontinent) include rocks of granitic and tonalite-tron dhjemitic series. Characteristic features of the Derbina granitoids ar e steep chondrite-normalized rare-earth element patterns ((La/Yb)(n)=9 -17), depletion in rare lithophile elements (Th, Rb, LREE), as well as high Sr (400-1000 ppm) and Ba contents. Geochemical modeling shows th at the Derbina granite melts could be formed by dehydration melting (2 0-30%) of plagiogneisses with removal of garnet-orthopyroxene-quartz-p lagioclase restite. According to experimental data the melt origin cor responds to P > 10 kbar and T > 980 degrees C. The reconstructed plagi ogneiss source of sialic melts is depleted in rare lithophile elements relative to the average composition of gray gneisses and is similar t o tonalite-trondhjemitic gneisses of the Arzybei block. According to r ock associations and their geochemical features the Derbina block crys talline basement is supposed to be analogous to island are-related com plexes of the Arzybei block.