Regional Report: Luobusa ophiolite, Qusum (Qusong) Co., Shannan (Lhokha; Lhoka) Prefecture, Tibet Autonomous Region, China

The Luobusa ophiolite is situated along the Luobusa fault zone, also known as Yarlung Zangbo (or, Brahmaputra) suture zone. It consists mainly of mantle peridotite and dunite, overlying a mélange zone composed of pillow lavas, gabbros, pyroxenites and cherts in a serpentinite matrix. To the south, it is separated from Triassic flysch by a steep reverse fault; to the north, it is thrust over the Tertiary Luobusa Formation and Gangdese granitic batholith. The ophiolite extends 42 km in an east–west direction and has an outcrop area of approximately 70 km2. Podiform chromitite ores, included in a dunite-harzburgite assemblage, occur discontinuously in a band in the upper mantle peridotites. These are, from east to west, the Kangjinla, Xiangkashan and Luobusa deposits. The chromitites display massive, disseminated, brecciated and nodular textures, and are believed to have formed by precipitation from boninitic melts interacting with the host peridotites. These chromitites have an unusual mineral assemblage, including ultra-high pressure (UHP) minerals such as diamond and moissanite (SiC), as well as a number of highly reduced phases such as graphite, native chromium, iron, nickel, gold, copper and silicon identified by microprobe and X-ray diffraction studies. Some phases seem to have formed at the core/mantle boundary. Among the unusual phases found are alloys (Fe/Si-phases, Ni/Fe-phases, Fe/Cr-phases, Ir/Fe-phases, W/Co-phases, Cr/C-phases, Ti/N-phases, Si/Ca-phases, Si/C-phases, etc.), as well as numerous new oxides, sulphides and arsenides.

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