GARNET PERIDOTITE XENOLITHS FROM THE VITIM VOLCANIC FIELD, BAIKAL REGION - THE NATURE OF THE GARNET SPINEL PERIDOTITE TRANSITION ZONE IN THE CONTINENTAL MANTLE

A suite of large and fresh peridotite xenoliths from a picrite tuff deposit in the Cenozoic Vitim volcanic field, approximately 200 km east of Lake Baikal, shows a continuous gradation from protogranular spinel through garnet-spinel to very abundant garnet peridotites. This includes composite nodules in which all these lithologies coexist on the scale of a few centimeters. Garnet and many spinel lherzolites are remarkably fertile in terms of their 'basaltic' major element contents (CaO 3.0-3.7%, MgO 37-40%, Ca/Al = 1.1, Cr/Al < 0.13), whereas some garnet-spinel and spinel peridotites are moderately depleted (Cr/Al 0.14-0.45). T estimates are 850-880-degrees-C for the fertile spinel lherzolites apparently brought up from shallow depths of 40-50 km. This contrasts with 980-1030-degrees-C for depleted spinel peridotites and 1000-1150-degrees-C for the garnet-bearing peridotites for which equilibration pressures between 16 and 23 kbar are inferred. The data suggest that garnet and spinel peridotites coexist in the sub-Vitim mantle at a pressure of approximately 18 kbar over an interval of approximately 2 kbar, with the appearance of garnet, and with the garnet-to-spinel ratio in this transitional zone primarily being controlled by bulk rock contents of Ca, Al, Cr, and Cr/Al ratios, in addition to P-T conditions. The Vitim peridotites show little evidence for metasomatic enrichment: they commonly show depletion of LREE compared with intermediate REE; this includes also rare amphibole-bearing veins. The fertile spinel and garnet Iherzolites have very similar bulk rock major oxide contents and REE distribution patterns; these features indicate a lack of significant chemical vertical mantle stratification in that region. Garnet peridotites from Vitim show large differences in modal and chemical composition from garnet peridotite xenoliths from Yakutian and South African kimberlites, suggesting distinct lithospheric mantle structure and composition in Archean cratons and post-Archean mobile belts.