Mantle Domains and Crust-Mantle Coupling in SE Siberia (Russia) and Mongolia

D. A. Ionov1, S. Y. O'Reilly1 and W. L. Griffin1, 1GEMOC, School of Earth Sciences, Macquarie University (Sydney), N.S.W. 2109, Australia (dmitri.ionov@mq.edu.au).

Mantle xenoliths occur in Cenozoic alkali basalts in southern and SE Siberia and in central and SE Mongolia. Together with xenoliths in Yakutian kimberlites they provide a 'cross-section' of subcontinental mantle in a traverse from the Precambrian Siberian platform through Baikal Rift Zone, Paleozoic-Mesozoic fold belts in Mongolia and Siberia to the Pacific continental margin of Siberia. These xenoliths sample upper mantle from different tectonic environments and provide evidence for major and trace element and isotope differences between these mantle domains and on thickness and thermal structure of the lithosphere.

Average major element compositions of the upper mantle sampled by peridotite xenoliths are different in major tectonic units in the area. The mantle beneath the Siberian Platform is most depleted and magnesian. In contrast, peridotite xenolith suites in areas east and south of Lake Baikal (Hamar-Daban, Bartoy, Vitim and Udokan) and in central Mongolia (Tariat) have fertile average compositions (rich in basaltic components, e.g. CaO 2.7-2.9%, Al2O3 3.3-3.5%; MgO ²40%). The combination of the non-refractory compositions and the moderately high geothermal gradient (established from studies of garnet-bearing xenoliths) is consistent with low seismic velocities right below the Moho in that region. Peridotite xenoliths found further east and south (Dariganga in SE Mongolia, Tok-Stanovik in southern Aldan Shield) have more refractory average compositions (1.6-2.0% CaO; 1.9-2.3% Al2O3) and generally lower equilibration temperatures. LREE-depleted peridotite xenoliths from that area also differ in Sr-Nd isotope compositions from those in and around the Baikal Rift zone (the 87Sr/86Sr ratios in the former commonly are higher). Peridotite xenolith suites from the southern Russian Far East near the Japan Sea coast usually have moderately fertile major element compositions (av. CaO 2.6%, Al2O3 3.1%), but some (i.e. those located within Precambrian lithospheric blocks) are more refractory.

Chemical and isotopic composition and T-P estimates of peridotite xenoliths define a few large-scale domains in the lithospheric mantle between the Siberian platform and the Pacific coast of Siberia that appear to be consistent with their tectonic setting, age and geologic history of the overlying crust as well as seismic data. These results are in line with the secular variation in the composition of subcontinental lithospheric mantle (Pearson et al., 1995; Griffin et al. 1997) and indicate long-term crust-mantle coupling.

References:

Griffin, W.L., O'Reilly, S.Y., Ryan, C.G., Gaul, O. and Ionov, D.A. (1997) Secular variation in the composition of subcontinental lithospheric mantle. In: AGU Monograph (in press).

Pearson, D.G., Carlson, R.W., Shirey, S.B., Boyd, F.R., Nixon, P.H. (1995) Stabilisation of Archaen lithospheric mantle: A Re-Os isotope study of peridotite xenoliths from the Kaapvaal craton. Earth Planet. Sci. Lett. 134, 341-357.

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