MANTLE COMPONENTS IN LATE CENOZOIC VOLCANICS, EAST SAYAN (from Pb, Sr, and Nd isotopes)

The evolution of Late Cenozoic volcanism in the East Sayan region is interpreted in the context of a plume-like mantle thermal anomaly beneath a moving lithospheric plate on the basis of space and time variations of Pb, Sr, and Nd isotopic ratios in alkali basalts. The volcanic activity started 22–20 Ma ago in the northeast of the Riphean Tuva-Mongolia microcontinent (Urik segment), propagated westward through its northwestern part, and focussed within the Caledonian East Tuva zone in the past 2 Ma. The 22 to 12 Ma basaltic lavas contain a predominant common component of convecting mantle (probably, rising from greater depths) with high Pb and Nd and low Sr ratios. The interaction of this material with the lithosphere produced a shallower lens with lower Pb and Nd ratios and higher ⁸⁷Sr/⁸⁶Sr. The shallow component first appeared between 12 and 9.5 Ma and became part of all later volcanic products. The role of lithospheric material varied in space and time: ∆8/4Pb = 60–93 were in the range of the DUPAL anomaly in the Tuva-Mongolian basalts, below this range (20–54) in the East Tuva lavas, and intermediate (50–63) on the periphery of the microcontinent.