Petrology and geochemistry of lower-crustal xenoliths from alkali basalts of the Vitim Plateau

Lower-crustal xenoliths from alkali basalts of the Vitim Plateau are subdivided into three groups: 1) garnet granulites; 2) garnet-bearing clinopyroxenites and gabbros; and 3) spinel websterites. According to PT-data, the xenoliths form large intrusive bodies, lenses, and veins in the lower crust along the Moho at depths of 35-45 km and are under-lied by spinel lherzolites. The temperatures of cumulate formation range from 800 to 1020 degrees C. A xenolith-derived geotherm corresponds to a "hot" geotherm for pyroxenites of megacryst assemblages from picrobasalts and coincides well with geotherms for similar rocks of Central Mongolia and Eastern Australia. Most of xenoliths have a magmatic cumulative microstructure; they resulted from the underplating of mantle melts. It Is suggested that the garnet clinopyroxenites and gabbros resulted from the following processes: a) crystallization of residual melts after the megacryst series formation (group 2); b) crystallization of melts comagmatic to the green-series pyroxenites (group 3); and c) both of these processes (group 1). The differences between xenolith assemblages from Miocene picrobasalts and Pliocene basanites are related to either differences in magmatic-rock structure beneath volcanoes or upwelling of the roof of asthenospheric diapir to the uppermost mantle during the Late Miocene-Pliocene.