—We have estimated the P–T conditions of formation of basaltoid melts: P = 1.15–1.06 GPa and T = 1379–1293 ºC. Olivine pyroxenites (Ol + Cpx + Grt) are assumed to be a mantle source for nepheline-normative basanitic melts. During ascent, the melt trapped mantle xenoliths, which were disintegrated into olivine and augite xenocrysts. A decrease in pressure and temperature led to the crystallization of highly magnesian (Mg# = 86) olivine and diopside and Timgt + Ilm ± Pl phenocrysts. Then, the Ol + Cpx + Timgt + Pl microlite paragenesis formed. Alkaline aluminosilicates (acid plagioclase + nepheline + leucite) were the last to crystallize in the rock interstices. The presence of residual glass indicates that the last stage of crystallization took place under subsurface conditions. The volcanic area of southern Transbaikalia (Khentei Ridge) resulted, most likely, from the mantle plume impact on the lithosphere. The age of this area is estimated at 3.51 Ma. PREMA was the main mantle source for these volcanics, and the contribution of HIMU was strongly subordinate. In geochemical features the studied volcanics correspond to mafic rocks of oceanic islands. They are similar in composition to alkali basalts of the South Baikal volcanic area.