Tectonic Implications of Ultra-High Pressure Minerals in the Loubusa Ophiolite, Tibet

Abstract

An unusual collection of ultrahigh pressure (UHP) and associated minerals has been recovered from podiform chromitites of the Luobusa ophiolite, southern Tibet. The minerals were hand picked from mineral separates but many of the minerals are enclosed in, or attached to, chromite grains leaving now doubt as to their provenance. The mineral collection includes diamond, graphite, moissonite, coesite, CrC, SiFe, silicates, w�stite, PGE and base metal alloys, and a wide variety of native elements (Si, Fe, Ti, Ni, Cr, W, Au, Ag, Zn, Cu, Pb, Sn). Diamonds from Luobusa are clear, colorless octahedra with high Ni aggregation states confirming their natural origin and indicating a long residence time in the mantle. A few have dark inclusions of a Mg-Fe silicate, probably enstatite. Graphite occurs as grey, tabular prisms and irregular grains, many of which preserve a hexagonal morphology. Abundant moissonite forms small, euhedral or broken crystals ranging from dark blue to green to colorless. Some grains of moissonite and Fe-silicides contain inclusions of native Si. Coesite occurs with kyanite as rims on native Ti. Primary Os-Ir and Pt-Fe alloys, interpreted to be of UHP origin, are intergrown with chromite grains, whereas secondary PGE minerals and alloys occur along cracks where they are associated with a variety of sulfide minerals. Numerous octahedral Mg-Fe silicate grains have been pseudomorphed by serpentine. There is no evidence that the Luobusa ophiolite itself was formed at great depth, thus the UHP minerals are interpreted as xenocrysts incorporated into the chromitites during crystallization. Their preservation in this high-temperature, relatively oxidizing environment is difficult to explain. Our preferred model calls for rapid rise of deep mantle rocks to relatively shallow levels where they were picked up by boninitic melts and incorporated into the chromitites upon cooling and crystallization. Preservation of the UHP minerals in this environment may have been facilitated by inclusion in xenolithic blocks and by relatively rapid crystallization and cooling of the host chromitites.