Pyroxenites and megacrysts from Vitim picrite-basalts (Russia): Polybaric fractionation of rising melts in the mantle?

Picrite basalt tuffs and lavas from the Miocene basalt plateau of Vitim (Trans Baikal, Russia) contain abundant megacrysts and varied pyroxenite and mantle lherzolite xenoliths (spinel facies and upper part of the garnet-facies) and crustal cumulates. Black pyroxenites and megacrysts show decreasing temperatures from 1350 to 900 degrees C, and range from high-T dark green websterites and clinopyroxenites, to low-T black megacrystalline garnet clinopyroxenites and phlogopite-ilmenite-bearing varieties. Garnet-bearing Cr-diopside veins and zoned veins with mica and rare amphiboles cross-cut peridotite xenoliths. Veins consisting of almost pure amphibole are more common in spinel lherzolite xenoliths. P-T calculations for pyroxenites yield pressure intervals at 3.3-2.3, 2.2-2.0, 1.9-1.5 and 1.3-1.0 GPa, probably corresponding to the locations of dense magmatic vein networks in mantle. Major and trace elements for clinopyroxenes from the black megacrystalline series can be modeled by fractional crystallization of a picrite-basalt melt. In contrast, green high-temperature pyroxenites and black giant-grained garnet pyroxenites with lower LREE-enrichment and variable LILE and HFSE concentrations probably result from AFC processes and mixing with partial melts derived from older pyroxenites and peridotites. Gray low-Cr garnet clinopyroxenites with highly fractionated and inflected trace element patterns may have been formed by remelting of metasomatic veins within peridotites. Multistage melting of metasomatic assemblages with selective removal of clinopyroxene in vein contacts produce the REE patterns with low MREE concentrations and usually with elevated HFSE contents. Cr-diopside veins were most probably formed by partial melting of phlogopite- and/or amphibole-bearing lherzolites. The trace element and Sr-Nd isotopic features of the megacrystalline pyroxenites suggest that they crystallized from magma volumes that evolved in separate systems during formation of pre-eruption vein networks and magma chambers, which together formed the feeding system for the host basalts. (C) 2011 Elsevier Ltd. All rights reserved.