Volcanism in the Vitim Volcanic Field, Siberia: Geochemical Evidence for a Mantle Plume Beneath the Baikal Rift Zone

The Baikal Rift is a zone of active lithospheric extension adjacent to the Siberian Craton. The 6-16 Myr old Vitim Volcanic Field (VVF) lies approximately 200 km east of the rift axi's and consists of 5000 km3 of melanephelinites, basanites, alkali and tholeiitic basalts, and minor nephelinites. In the volcanic pile, 142 drill core samples were used to study temporal and spatial variations. Variations in major element abundances (e.g. MgO = 3.3-14.6 wt reflect polybaric fractional crystallization of olivine, clinopyroxene and plagioclase. (0.7039-0.7049), (0.5127-0.5129) and (0.2829-0.2830) ratios are similar to those for ocean island basalts and suggest that the magmas have not assimilated significant amounts of continental crust. Variable degrees of partial melting appear to be responsible for differences in Na2O, P2O5, K2O and incompatible trace element abundances in the most primitive (high-MgO) magmas. Fractionated heavy rare earth element (HREE) ratios (e.g. Gd/Lun > 2.5) indicate that the parental magmas of the Vitim lavas were predominantly generated within the garnet stability field. Forward major element and REE inversion models suggest that the tholeiitic and alkali basalts were generated by decompression melting of a fertile peridotite source within the convecting mantle beneath Vitim. Ba/Sr ratios and negative K anomalies in normalized multi-element plots suggest that phlogopite was a residual mantle phase during the genesis of the nephelinites and basanites. Relatively high light REE (LREE) abundances in the silica-undersaturated melts require a metasomatically enriched lithospheric mantle source. Results of forward major element modelling suggest that Melting of phlogopite-bearing pyroxenite veins could explain the major element composition of these melts. In support of this, pyroxenite xenoliths have been found in the VVF. High Cenozoic mantle potential temperatures (∼1450° C) predicted from geochemical modelling suggest the presence of a mantle plume beneath the Baika1, Rift Zone