MANTLE COMPONENTS IN LATE CENOZOIC VOLCANICS OF THE EAST SAYAN (FROM PB, SR, AND ND ISOTOPES)

Abstract

Space and time variations in Pb, Sr, and Nd isotope ratios in Late Cenozoic alkali-basalt lavas from the vast East Sayan territory allowed an interpretation of the volcanism evolution in the context of interaction of a plume-like mantle thermal anomaly with a moving lithospheric plate. Volcanism initiated 22-20 Ma ago in the northeastern Riphean Tuva-Mongolian massif (Urik segment), propagated westward through its northwestern part, and focused within the Caledonian East Tuva zone in the past 2 Ma. From 22 to 12 Ma ago, basaltic lavas contained a predominant common component of convecting mantle (probably, rising from greater depths) with high Pb and Nd and low Sr isotope ratios. The interaction of this material with the lithosphere produced a shallower lens with lower Pb and Nd and higher Sr ratios. This component first appeared 12-9.5 Ma ago and became part of all later volcanic products. The role of lithospheric material varied in space and time: Δ8/4Pb = 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 massif.