LATE PALEOZOIC BIMODAL VOLCANOPLUTONIC AS SOCIATIONS OF TRANSBAIKALIAAND GEODYNAMIC CONDITIONS OF THEIR FORMATION

Results of the geological, geochemical, and isotopic studies of the bi modal trachybasalt-trachyrhyolite-syenite-alkali-ganite) volcanopluton ic association widespread within Transbaikalia and Mongolia are presen ted. Detailed studies of the Tamir volcanotectonic structure support a geochemical relationship of the basic and salic members of the associ ation. The Rb-Sr isotopic characteristics of the rocks were used to pl ot the isochron e and determine their absolute ages, 253+/-5 Ma, which agrees with the geological data obtained for the Early Permian cordai tes. The initial ratio of Sr isotopes in the alkali-salic volcanogenic rocks (IoSr = 0.70469+/-36 and MSWD=0.99) supports their mantle origi n and precludes a marked participation of the upper-crust matter with high ratios of Sr isotopes in their genesis. The great manifestation o f intraplate magmatism on the active continental margin of the Siberia n continent is the result of the Late Paleozoic Pangea formation.