TRACE ELEMENT COMPOSITIONS OF MINERALS IN GARNET AND SPINEL PERIDOTITE XENOLITHS FROM THE VITIM VOLCANIC FIELD, TRANSBAIKALIA, EASTERN SIBERIA

Abstract

Peridotite xenoliths from the Bereya alkali picrite tuff in the Vitim volcanic province of Transbaikalia consist of garnet lherzolite, garnet-spinel lherzolite and spinel lherzolite varieties. The volcanism is related to the Cenozoic Baikal Rift. All peridotites come from pressures of 20-23 kbar close to the garnet to spinel peridotite transition depth, and the presence of garnet can be attributed to cooling of spinel peridotites, probably during formation of the lithosphere. The peridotites show petrographic and mineral chemical evidence for infiltration by an alkaline silicate melt shortly before their transport to the Earth's surface. The melt infiltration event is indicated petrographically by clinopyroxenes which mimic melt morphologies, and post-dates outer kelyphitic rims on garnets which are attributed to an isochemical heating event within the mantle before transport to the Earth's surface. Single-mineral thermometry gives reasonable temperature estimates of 1050 ± 50°C, whereas two-mineral methods involving clinopyroxene are falsified by secondary components in clinopyroxene introduced during the melt infiltration event. Excimer Laser-ICP-MS analysis has been performed for an extensive palette of both incompatible and compatible trace elements, and manifests the most thorough dataset available for this rock type. Orthopyroxene and garnet show only partial equilibration of trace elements with the infiltrating melt, whereas clinopyroxene and amphibole are close to equilibration with the melt and with each other. The incompatible element composition of the infiltrating melt calculated from the clinopyroxene and amphibole analyses via experimental mineral/melt partition coefficients is similar to the host alkali picrite, and probably represents a low melt fraction from a similar source during rift propagation. The chemistry and chronology of the events recorded in the xenoliths delineates the series of events expected during the influence of an expanding rift region in the upper mantle, namely the progressive erosion of the lithosphere and the episodic upward and outward propagation of melts, resulting in the evolution of the Vitim volcanic field. © 1999 Elsevier Science B.V. All rights reserved.