Permian back-arc basin basalts in the Yushu area: New constrain on the Paleo-Tethyan evolution of the north-central Tibet

Abstract

The Paleo-Tethyan tectonic evolution of north-central Tibet remains controversial, particularly regarding the nature and extension of associated sutures. The Yushu mélange, located in the junction part of the western Jinshajiang, the southern Jinshajiang and the Ganzi–Litang sutures, is a key area to reveal the Paleo-Tethyan tectonics of this area. This study presents new geochronological and geochemical data for the Zhimenda volcanic suite of the Yushu Mélange. These rocks preserve a record of back-arc magmatism along the northern margin of the North Qiangtang Block. The Zhimenda volcanics are composed of ignimbrite, basalt, and porphyritic tuff intercalated with terrigenous clastic rocks. Zircon 206Pb/238U ages of the ignimbrites indicate they formed at ~254Ma. The basalts are relatively enriched in large-ion lithophile elements and depleted in high field strength elements. Their high MgO, Ni, and Cr contents, relatively high Sm/Yb and Rb/Nb values, and positive zircon εHf(t) and bulk εNd(t) values suggest they were derived from the partial melting of a depleted subcontinental lithospheric mantle source metasomatized by hydrous fluids. The Zhimenda basalts are geochemically similar to back-arc basin basalts in the Okinawa Trough. They were erupted related to subduction of the Longmuco–Shuanghu Paleo-Tethyan oceanic plate beneath the North Qiangtang Block. We propose that the Yushu Mélange converges with the Ganzi–Litang Suture Zone to the east, rather than with the Jinshajiang Suture Zone to the southeast, and that a huge Permian trench–arc–back-arc system developed in north-central Tibet.