Zircon U-Pb dating and geological implications for the granodiorites from the Dazhuka region in Xizang

The precise dating for the granodiorites from the Gangdise zone in Xizang is of great importance to the research of the Tethyan evolution and Qinghai-Xizang Plateau uplift. The granodiorite masses south of Dazhuka Bridge occur in the central part of the Gangdise zone in Xizang. There is the large discrepancy between the preexisting K-Ar age determinations given by the previous workers. The present paper deals with the zircon U-Pb re-dating of the granodiorite masses south of the Dazhuka Bridge with the aid of LA-ICP-MS method. The selected zircons for dating show regular oscillatory zonal structures. The Th/U ratios vary from 0.20 to 1.69. These characteristics suggest a magmatic origin of the zircons. The weighted average age of 80.3±6 Ma (MSWD=3.2) may represent the crystallization age of the granodiorites. Since the granodiorite masses are very close to the southern Yarlung Zangbo suture zone, the granodiorites may well be originated from the magmatism caused by the northward subduction of the Yarlung Zangbo Neo-Tethys oceanic plate.