Rb-Sr isotope systems of granitoids from Ol'khon Island and the Ol'khon region

Study of the Rb-Sr isotope systems of granitization and magmatism products in the Ol'khon metamorphic complex has shown that the granitoids formed ove r a long period at various stages of evolution of the continental crust. Th e stage of bland-are magmatism (570+/-88 Ma ago) was completed with the int rusion of diorites, granodiorites, and granites of the Khaidai complex (sou th of the Ol'khon region). Formation of migmatites and autochthonous granit es of the Sharanur complex proceeded synchronously with zonal metamorphism (522+/-20 Ma ago) at the final stages of collisions on the southern framing of the Siberian Platform. Later on (463-449 Ma ago), during the shift tect ogenesis, veins of leucogranites that formed in local magma chambers in gra nite-gneiss domes intruded into the metamorphic rocks. The increase in prim ary Sr isotope ratio from 0.7039 in granitoids of the Khaidai complex to 0. 7054 in granites of the Sharanur complex and 0.7089 in shales (altered duri ng the late cataclasis) reflects the regular evolution of the crust in the study region from intermediate (between oceanic and continental) to contine ntal. The older age of shales and granites of Cape Kaltygei (1866-1880 Ma) indicates an autonomous development of this block, which Is suggested to be part of the basement of the Siberian Platform.