Magmatic modification and metasomatism of the subcontinental mantle beneath the Vitim volcanic field (East Siberia): evidence from trace element dataon pyroxenite and peridotite xenoliths from Miocene picrobasalt

The genesis of several groups of pyroxenite xenoliths from Miocene picrobas alt of the Vitim volcanic field are considered on the basis of petrology an d mineral chemical data, including trace element analyses of minerals by io n probe. The pyroxenites and related xenoliths can be classified into three groups consisting of 10 sets of xenoliths: Group I (Cr-diopside series) is made up by three subdivisions; (Ia) common Cr-diopside garnet and spinel w ebsterites, (Ib) Al-poor Cr-diopside websterites, (Ic) modally metasomatize d Cr-diopside Iherzolites. Group II (Al-augite series) consists of four fur ther xenolith types; (IIa) Cr-rich and (IIb) Cr-poor megacrystic pyroxenite assemblages, (IIc) Al-rich garnet clinopyroxenites, and (IId) amphibole an d/or phlogopite-bearing rocks and associated orthopyroxenites. Group III co nsists of xenoliths all characterized by Ca-rich clinopyroxene (garnet clin opyroxenite and gabbros, garnet granulites and spinel websterites) thought to have a similar depth of origin near the crust-mantle boundary. Trace element data and textural criteria allow the distinction of three mel t types operating in the mantle: (1) Cr-rich melt migrating along small fra ctures and forming Cr-diopside pyroxenite veins as the result of percolatio n fractionation; (2) picrobasaltic to alkaline basaltic melt fractionating in large hydraulic fractures or magma chambers and producing host volcanics , clinopyroxene megacrysts and megacrystic pyroxenites (Groups IIa-IIb); an d (3) high field strength element (HFSE)-enriched smaller volume melts, whi ch formed the hydrous mineral-bearing xenoliths of Groups I and II (Ic and IId). The megacrystic pyroxenites (IIa-IIb) were crystallized in magma chambers o r large fractures at various depths during multistage polybaric fractionati on. The high-temperature Cr-rich websterites (Group IIa) correspond to the deepest level at near 30 kbar. The garnet websterites and megacrystic clino pyroxenites crystallized at intermediate levels, whereas the most different iated megacrystic clinopyroxenites with ilmenite and phlogopite inclusions are from the shallowest levels at 15-11 kbar. The HFSE-rich melt resulted i n amphibole- and phlogopite-rich assemblages crystallized in veins and inte rstitially in peridotites, and the development of ilmenite-phlogopite-beari ng orthopyroxenites in a reaction zone between these. Pyroxenites of Groups I and II represent samples of an intensely veined sub-Vitim lithosphere: t he preservation of textural and chemical disequilibrium indicates that the enrichment immediately preceded xenolith sampling and is related to the upw ard and outward migration of the asthenosphere-lithosphere boundary beneath the developing Baikal Rift. The formation of lower pressure xenoliths (Group III) is related to both of the first two melt types mentioned above. Spinel websterites of Group III are genetically related to the Group Ia melt crystallized near the crust-ma ntle boundary, and the mafic garnet granulites of Group III to intermediate fractionation products (gabbro-norites) of similar melts. Garnet gabbros a re associated with the most fractionated megacrystic pyroxenites. (C) 2000 Elsevier Science B.V. All rights reserved.