PYROXENITE XENOLITHS IN PICRITE BASALTS (VITIM PLATEAU): ORIGIN AND DIFFERENTIATION OF MANTLE MELTS

Abstract

The petrography and major- and trace-element chemistry of pyroxenite xenoliths from Miocene picrite basalts of the Vitim plateau suggest an origin by polybaric fractional crystallization of mantle melts. High-temperature giant-grained black pyroxenites and clinopyroxene megacrysts make a long trend (Fe# = 0.12 to 0.23%) corresponding to polybaric Cpx-Ga cotectic fractionation, possibly, associated with formation of a pre-eruption vein system. Low-temperature black pyroxenites are products of interaction of residual melts with the lower crust material. High-temperature hybrid websterites may have been produced by mixing of basaltic melts with fused (<5–10% partial melting) metasomatic mantle lherzolites or by AFC processes with Ga-Cpx dissolution. Anatectic Cr pyroxenites were formed mostly by segregation of partial melts from lherzolite material and remelting of former veins. Low-temperature hybrid garnet pyroxenites result from interaction of metasomatic lherzolites (or earlier Cr-diopside veins) with volatile-enriched differentiated basaltic melt. Remelting of earlier mineral generations, which occurs in all rising melt systems, is reflected by mirror-like peaks on spidergrams, spikes on TRE diagrams, and LREE and HFSE (and ore element) enrichment.