The activation and cessation of Late Cenozoic extension in the lithosphere at the margin of the Baikal Rift Zone: Alternating sources of volcanism in the Vitim Upland.

A comparative analysis of the concentrations of major oxides, trace elements, and the Nd/Nd ratios in representative sequences of volcanic and subvolcanic rocks in the western and eastern Vitim Upland has revealed petrogenetic groups with different relationships among components from lithosphere and sublithosphere sources. It is hypothesized that the initial 16-14-Ma eruptions of picrobasalts and Mg basanites in the east of the upland resulted from high-temperature melting, hence, the melting of sublithospheric peridotite and lithospheric Mg-pyroxenite mantle material with mildly and strongly depleted isotope compositions of Nd relative to the value in the primitive mantle (0.512638). The broad range of varying lava compositions in the 14-9 Ma time span was caused by 'passive' rifting in the west of the upland and by 'active' rifting in the east. The 'passive' rifting manifested itself in the melting of lithospheric material with some admixture of material from the underlying asthenosphere, while the 'active' rifting lifted deep-lying mantle material. The structural rearrangement that has been occurring in the Baikal Rift System during the last 9 Ma resulted in stopping the rifting in the area of study. Relaxation, flattening and thinning of the lithosphere beneath the east part of the system during the 1.1-0.6 Ma time span caused magma effusion with values of Nd/Nd that are typical of a moderately depleted asthenospheric source contaminated with deeper mildly depleted mantle material. [ABSTRACT FROM AUTHOR]