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Russian Geology and Geophysics

2002 year, number 12

MANTLE SOURCES OF LATE CENOZOIC VOLCANICS (EAST SAYAN), AS INFERRED FROM Pb, Sr, AND Nd ISOTOPES

S. V. Rasskazov, E. V. Saranina, and E. I. Demonterova
Keywords: Late Cenozoic, alkali basalts, Pb, Nd, Sr isotopes, East Sayan.
Pages: 1065-1079

Abstract

Mantle sources of Late Cenozoic alkali-basalt lavas in the vast territory of East Sayan have been investigated based on space and time variations of Pb, Sr, and Nd isotopic ratios. The evolution of volcanism is interpreted in the context of interaction of a plume-like mantle thermal anomaly and a moving lithospheric plate. Volcanism initiated 22-20 myr ago in the northeastern part of the Riphean Tuva-Mongolia massif (Urik segment), propagated westward through its northwestern part, and focussed within the Caledonian East Tuva zone in the past 2 Ma. Between 22 and 12 Ma, basaltic lavas contained a predominant common component of a deep convecting mantle material (probably, rising from greater depths) with high Pb and Nd and low Sr isotopic 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 at 12-9.5 myr 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 basaltic massif.