Podiform chromitites in the Luobusa ophiolite (Southern Tibet): Implications for melt-rock interaction and chromite segregation in the upper mantle

Abstract

The Luobusa ophiolite in the Indus-Yarlung Zangbo suture of southern Tibet hosts the largest known chromite deposit in China. The podiform chromitites occur in a well-preserved mantle sequence consisting of harzburgite with abundant lenses of dunite. The harzburgites have relatively uniform bulk-rock compositions with mg-numbers [100Mg/(Mg + Fe)] ranging from 89 to 91 and show flat, unfractionated, chonarite-normalised platinum group element (PGE) patterns. Their accessory chromite varies widely in cr-number [100Cr/(Cr +Al)] (18-66). These rocks are essentially residua left after extraction of mid-ocean ridge basalt (MORB)-type magmas. The podiform chromitites display nodular, massive, disseminated and banded textures and typically have dunite envelopes that grade into the surrounding harzburgite and diopsidic harzburgite with increasing pyroxene contents. They consist of relatively uniform chromite with high cr-numbers (74-82), have strongly fractionated, chondrite-normalized PGE patterns with enrichment in Os, Ir and Ru relative to Rh, Pt and Pt, and are believed to have formed from a boninitic magma produced by a second stage of melting. Dunites contain accessory chromite intermediate in composition between those of harzburgite and chromitite and are believed to be the products of reaction between new boninitic magmas and old MORB-type peridotites. The melt-rock reaction removed pyroxene from the peridotites and precipitated olimne, forming dunite envelopes around the chromitite pods. The melts thus became more boninitic in composition and chromite saturated, leading to precipitation of chromite alone. The interplay of melt-rock interaction, chromite fractionation and magma mixing should lead to many fluctuations in melt composition, producing both massive and disseminated chromitites and phase layering within individual podiform bodies observed in the Luobusa ophiolite.