Petrology and geochemistry of the Xiugugabu ophiolitic massif, western Yarlung Zangbo suture zone, Tibet

Abstract: The Yarlung Zangbo Suture Zone (YZSZ), southern Tibet, is a discontinuous belt that is more than 2000km long, composed of the remnants of Neo-Tethyan Mesozoic ocean. One of these relicts is the Xiugugabu ophiolitic massif which is a mantle thrust sheet of more than 260km2 overlying the Cretaceous tectonic mélange south of the YZSZ in SW Tibet. The massif is composed of harzburgites and clinopyroxene–harzburgites with porphyroclastic and porphyromylonitic textures. In the southern part of the massif, peridotites were intruded by amphibole-bearing microgabbro and microgabbronorite sills. A diabase unit which is overlaid by a sedimentary sequence crops out on the NE flank of the massif. Mineral chemistry in harzburgites and clinopyroxene–harzburgites indicates compositions similar to abyssal and forearc peridotites. Peridotites are slightly LREE depleted to enriched with [La/Yb]CN 0.06–2.8 and [La/Sm]CN 0.34–2.64. These ultramafic rocks are inferred to be the residues of 5–25% of partial melting of a depleted mantle that has been enriched by percolating metasomatic melts in a suprasubduction environment. Amphibole–microgabbro and amphibole–microgabbronorite sills are mostly composed of brown to green amphibole, calcic plagioclase, clinopyroxene, ilmenite and orthopyroxene in gabbronorite. Textures and compositions of the brown amphiboles indicate a near-solidus high temperature hydrothermal origin (>800°C). These intrusive rocks are tholeiitic and show N-MORB type REE patterns ([La/Yb]NC 0.35–0.90), a LILE (mainly Th) enrichment and noticeable Nb, Ta and Ti negative anomalies. They have a suprasubduction affinity and were formed in a back-arc basin setting. The diabase unit outcropping to the NE of the massif is not directly related to the ultramafic and mafic ophiolitic rocks. The diabase shows LREE enriched patterns ([La/Yb]NC 8–8.9) and slight Nb, Ta and Ti negative anomalies. The diabase has an intraplate affinity and could have been derived from a mantle source enriched by subduction-related fluids. The absence of continental crustal assimilation indicates that these rocks were probably emplaced in the Jurassic, in an oceanic environment after the Triassic disaggregation of the Indian plate. The data are consistent with the recent geodynamic model proposed for the central part of the suture for the closure of the Neo-Tethys and suggest that the geodynamic evolution of the western part of the basin was comparable to the central part. [Copyright &y& Elsevier]