ASTHENOSPHERIC DIAPIR BENEATH THE BAIKAL RIFT - PETROLOGICAL CONSTRAINTS

Presence of partially molten material at the base of the crust is one of the key elements in models advanced for the deep structure of the Baikal rift zone. This upper mantle anomaly can be interpreted either as a discontinuous 0-50 km thick layer, which is connected with the asthenosphere via narrow conduits (asthenolith, tensional failure model), or as a 200-300 km wide asthenospheric bulge which is in contact with the base of the crust (mantle plume model). Petrological data on mantle xenoliths from Cenozoic basalts do not agree with the simplistic model of complete mechanical replacement of the subrustal lithosphere by hot asthenospheric material. Spinel and garnet lherzolite xenoliths represent lithospheric mantle material, derived from depths of 45-75 km; they display the compositional and thermal heterogeneity of the lithosphere which is related to its metasomatism and heating and also its cooling. It is suggested that in the area of the Baikal rift zone, tectonothermal activity of the lithosphere, reflecting its permeability, was caused by its extension in conjunction with the India-Eurasia collision. Lithospheric stretching induced decompression, partial melting and upwelling of the asthenosphere and intrusion of melts and fluids into the mantle lithosphere, and, perhaps, into the lower crust. Thus, the upper part of the asthenospheric bulge, which is characterized by low velocities at the Moho, low mantle densities and a highly uneven thermal field, probably corresponds to a zone of partially destroyed lithospheric mantle, that is impregnated with melts and fluids.