Zircon U-Pb ages and geochemical characteristics of basalts in Zhongcang area, central Tibet: constraints on the evolution of Shiquan River-Namu Co back-arc basin

Tectonic attribute and evolution history of Shiquan River-Namu Co mélange zone have been controversial for a long time.This paper deals with the petrology, geochronology, and geochemistry of the basalts from the Zhongcang ophiolite mélange, in order to reveal its formation age, petrogenesis and tectonic significance.LA-ICP-MS zircon U-Pb age of Zhongcang basalt is 115.7±2.0 Ma, which is consistent with the formation age of gabbro in the region.The Zhongcang basalts have a relatively flat REE distribution curve, similar to the mid-ocean ridge basalt(MORB).Furthermore, these samples show Th enrichment and Nb, Ta depletion.It is proposed that the Zhongcang basalts were derived by partial melting of the spinel peridotite mantle, which was modified by subduction sediments, in the back-arc basin spreading center.Combined with the study of the regional gabbros, our research favors that the Shiquan River-Namu Co mélange zone represents the remnant of a Mesozoic back-arc basin, which was produced by the back-arc rifting in response to the southward oceanic subduction of Bangong-Nujiang Tethyan Ocean.