Silica-alkaline metasomatism and granitoid magmatism in the Major Sayan Fault zone

We have studied endogenic processes in the southeastern part of the Major S ayan Fault zone at the boundary of the Siberian Platform and Sayan-Baikal f olded region. This is a zone of repeated occurrence of deformations, silica -alkaline metasomatism, and granite formation, which proceeded under modera te pressures no higher than 5.5-6 kbar. Some silica-alkaline metasomatites replacing each other with time include products of high-temperature graniti zation and quartz-albite-microcline (+/-magnetite, biotite, riebeckite, aeg erine-augite) metasomatites and albitites. Early granitization took place u nder normal alkalinity with participation of rather reduced fluids, and pos tmagmatic fluid systems are carbon-enriched. Quartz-albite-microcline metas omatites formed at elevated alkalinity and F and f(o2) activity. There is a clear difference between granitoids of the early tectonometamorphic cycle and the latest granites of the stage of marginal-suture activity, which are characterized by elevated concentrations of Nb, Y, Be, Sn, Pb, Th, and U.