PETROLOGIC AND GEOCHEMICAL DATA ON MANTLE XENOLITHS IN BASALTS FROM SOUTHERN SIBERIA AND MONGOLIA AND THEIR RELEVANCE TO GEODYNAMIC MODELS

D.A.IONOV, W.L. GRIFFIN, S.Y. O'REILLY, GEMOC Macquarie

Peridotite and pyroxenite xenoliths in alkali basaltic rocks in southern Siberia and Mongolia provide sampling of the upper mantle in the vicinity of the Baikal Rift and adjacent tectonic continental basins. Direct information on texture, modal and chemical composition and thermal state of the mantle at the time of eruption can be obtained from xenolith studies. A few volcanic fields in the region contain garnet-bearing xenoliths that also enable to estimate equilibration pressures and construct P-T-composition lithospheric cross-sections for the depth range 35-80 km. These results can be used in geodynamic modelling either as input data for geophysical models or for comparison with results of independent modelling on different scales.

On a regional scale xenolith studies have shown fundamental differences in the composition and thermal regime between the mantle beneath the ancient Siberian platform (sampled by kimberlites) and beneath younger mobile belts south of the platform. The mantle in southern Siberia and Mongolia has significantly higher contents of "basaltic" major elements (Ca, Al, Na) and iron, higher Fe/Si and Fe/Mg ratios than the mantle beneath the Siberian platform and shows higher T's at sub-Moho levels (1100-1150°C versus 800-850°C at 80 km). The combination of the moderately high geothermal gradient and the non-refractory compositions (Sobolev et al., 1997) in the mantle in southern Siberia and Mongolia may be responsible for lower seismic velocities right below the Moho in that region than in the Siberian craton.

Xenolith studies may provide information on geodynamic evolution of deep sections of tectonic continental basins. Mantle xenoliths found 30-50 km S-SE of rift basins in the southern Lake Baikal and the Tunka valley show no unequivocal evidence for unusual stress and deformation, solid state flow, magmatic activity or partial melting that could be indicative of an asthenospheric intrusion right below the Moho at the time of eruption (3-4 m.y.). There results are consistent with geophysical data that suggest that the zone of active extension in the Baikal rift is very narrow and is confined to restricted areas along the rift axis (Bocharova 1989). Comparisons between xenoliths from older and younger volcanic rocks and studies of phase transformations and mineral zoning in individual xenoliths have indicated mantle heating and possible asthenospheric upwelling in source regions of volcanic fields east of Lake Baikal (Ashchepkov et al., 1996).

References

Ashchepkov, I.V., Litasov, U.D., Litasov, K.D. (1996) Garnet peridotite xenoliths from melanefelinites of the Hentei Ridge (southern Baikal region): evidence for upwelling mantle diapir. Geology and Geophysics 37: 130-147 (in Russian).

Bocharova, N. (1989) A zone of active extention in the Baikal rift. Int. Geol. Rev. 31, 989-994.

Sobolev, S.V., Zeyen, H., Granet, M., Achauer, U., Bauer, C., Werling, F., Altherr, R., Fuchs, K. (1997) Upper mantle temperatures and lithosphere-asthenosphere system beneath the French Massif Central constrained by seismic, gravity, petrologic and thermal observations. Tectonophysics 275: 143-164.

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