Origin of ultrahigh pressure and highly reduced minerals in podiform chromitites and associated mantle peridotites of the Luobusa ophiolite, Tibet

Abstract

Podiform chromitites and their host peridotites in the Kangjinla mining district of the Luobusa ophiolite contain similar collections of ultrahigh pressure (UHP), highly reduced and crustal-type minerals. Abundant diamonds have been recovered from both lithologies and these are associated with a wide range of base metal alloys, native elements, carbides, oxides, silicates and others. The presence of UHP and highly reduced minerals in these rocks indicates that at least some of the chromite must have crystallized deep within the mantle as well as in a shallow mantle wedge in a suprasubduction zone (SSZ) environment. The unusual minerals were encapsulated in chromite grains and carried upward by mantle convection. The peridotite of Luobusa was trapped in the mantle wedge where it was modified by SSZ fluids and melts. Partial melting and mobilization of the chromite grains allowed them to be carried to shallow levels in melt channels and eventually deposited as chromitites near the crust mantle boundary. The unusual minerals were preserved during this process because they were encapsulated in chromite grains, either during crystallization or by later fluid fluxing.