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.