Mantle domains in southeastern Siberia (Russia) and Mongolia

IONOV, D.A. and O'REILLY, S.Y. (GEMOC, School of Earth Sciences, Macquarie University, Sydney, 2109 Australia)

Mantle xenoliths occur in Cenozoic alkali basalts in a large area in southern and southeastern Siberia and in central and southeastern Mongolia. They provide a cross-section of subcontinental mantle from the Baikal Rift Zone (BRZ) at the southern margin of the Precambrian Siberian platform through Mongolia (and adjacent NE China) to the Pacific continental margin of Siberia. These xenoliths sample upper mantle from different tectonic environments and provide evidence for major and trace element differences between these mantle domains and on thermal structure of the lithosphere.

A xenolith-derived geotherm has been defined for central Mongolia (Tariat locality). It passes through 12 kbar at 850°C and 20 kbar at 1220°C, with T values for spinel lherzolite xenoliths ranging from 860°C to 1070°C. 75% of these fall into a narrow interval of 900±25°C, and projection of the cut-off temperature for spinel lherzolites (870°C) on to the geotherm indicates a crustal thickness of ~45 km. The P-T data obtained for garnet-bearing lherzolite xenoliths from the Vitim Highland some 200 km east of Lake Baikal and 1100 km north east of the Tariat region define narrow P-T arrays as they are derived from a small depth range.

The uppermost mantle sampled by peridotite xenoliths east and south of Lake Baikal (Hamar-Daban, Bartoy, Vitim and Udokan localities) and in central Mongolia (Tariat) has a fertile average composition (enriched in basaltic components, e.g. CaO 2.7-2.9%, Al2O3 3.3-3.5%, MgO ~40%) providing a geochemical contrast with more depleted and more magnesian mantle beneath the Siberian Platform to the north sampled by kimberlites. The combination of the non-refractory compositions and the moderately high geothermal gradient is consistent with anomalously low seismic velocities right below the Moho in that region. Peridotite xenoliths in the areas east of the mantle zone with anomalous seismic velocities (e.g. Dariganga in southeastern Mongolia, Tok-Stanovik in southern Aldan Shield) have more refractory average compositions and generally lower equilibration temperatures. Peridotite xenolith suites from the southern Russian Far East inland from the Japan Sea coast usually have moderately fertile major element compositions (av. CaO 2.6%, Al2O3 3.1%) and T values ranging from 950 to 1000°C.

Major element compositions and T estimates of peridotite xenoliths define a few large-scale domains in the lithospheric mantle between the Siberian platform and Pacific coast of Siberia that appear to be consistent with seismic data.