Lithospheric Evolution in Circum-Cratonic Settings: A Re-Os Isotope Study of Peridotite Xenoliths from the Vitim Region, Siberia

Abstract

The lithospheric mantle beneath most cratonic regions appears to be of comparable age to the overlying crust and is tectonically robust. Less clear is the age and robustness of the lithospheric mantle beneath circum-cratonic regions. Questions of particular interest are (1) whether the cratonic mantle extends beyond the craton cover for significant distances, (2) the nature of circumcratonic lithospheric mantle and its relationship to the age and geologic history of the overlying crust, and (3) the relationship of the lithospheric mantle to the modem tectonic setting, particularly continental rifting. One of the regions that these questions can be addressed is Siberia. The Siberian craton is intruded by numerous kimberlites that carry mantle peridotite xenoliths of Archaean age (Pearson et al., 1995), which are petrologically similar to Archaean lithospheric mantle elsewhere (Boyd et al., 1997). The poly-stage Sayan-Baikal fold belt originated in the Early Palaeozoic from closure of the palaeoAsian ocean and collision of several Precambrian micro-continents with the Siberian craton to the north. It experienced repeated orogenic and intracontinental magmatic episodes; the last of them producing the Cenozoic Vitim volcanic field, in which Miocene and younger alkali basalts and tufts contain xenoliths of spineland garnet-bearing peridotites that provide a valuable insight of the lithospheric mantle (Ionov et al., 1993). The Vitim Highland region that the xenoliths are erupted through is not thought to have undergone significant modification of lithospheric mantle due to recent rifting in the Baikal rift zone, some 100-200 km to the west, and so the xenoliths should provide a record of pre-rift lithospheric evolution (Ionov et al., 1993). We have analysed a suite of these xenoliths, described by Ionov et al. (1993), plus new samples, for Re-Os isotopic composition, with the aim of constraining the age and evolution of the lithospheric mantle beneath the region.