Xainxa ultramafic rocks, central Tibet, China: Tectonic environment and geodynamic significance

The Xainxa ultramafic rocks, 200 km south of the Donqiao ophiolite, Bangong Nu Jiang suture zone, central Tibet, form a 1-km-thick sheet in tectonic contact with Paleozoic sediments to the south and with Mesozoic sediments to the north through roughly east-west, south-dipping reverse faults. The ultramafic rocks consist of depleted harzburgites with minor dunite patches and 1-m-sized deposits of podiform chromite. Mafic rocks, represented by a few isotropic gabbros and dolerites, occur in only minor amounts. These rocks, which may have underlined a suture zone within the Lhasa block, have lithologies and tectonics very similar to those occurring in the Donqiao area along the Bangong-Nu Jiang suture zone and therefore can be considered as a klippe of this ophiolite. The Xainxa ultramafic rocks were emplaced in Early Cretaceous time. The main tectonic event observed in that area (northward folding and overthrusting) occurred after late Early Cretaceous time.