Zircon xenocryst geochronology and implications for the Lhasa terrane evolution: Insights from Cenozoic volcanic rocks (Coqen, Tibet)

Abstract

The Cenozoic volcanic rocks of Coqen are located in the western central Lhasa subterrane, where knowledge of the regional lithospheric architecture and related magmatic processes is still lacking. To address this issue, we used in situ U–Pb dating, Lu–Hf isotopic and mineral inclusion analysis in zircon xenocrysts among trachyandesite. These zircons from volcanic samples yielded wide U–Pb ages from 18 Ma to 2424 Ma, showing three significant age peaks at ∼246 Ma, 193 Ma, and the most intense one at 128 Ma, which represent three major magmatic processes that developed in the area. The earliest zircon dates suggest the age of volcanic eruption in the Coqen area was not earlier than 18 Ma. zircon xenocrysts trace elements data point to the central Lhasa subterrane continental crust origin. We provide evidence for evolutive magmatism of the Lhasa terrane by calculating ΔFMQ, and show that the oxygen fugacity goes from reduced to oxidized (Cambrian to Miocene). The very low εHf(t) indicate a long-term magmatic event sequence, with the Zenong Group having the most vigorous magmatic activity. The old U–Pb ages and TDMC confirmed that an ancient basement of the Precambrian exist in the middle Lhasa terrane. Our work also restores the tectonic and crustal thickness evolutionary process of Coqen area even entire Lhasa terrane. In the back of a series of thickening and thinning processes, the Tibetan crust drastically thickened at ∼54 Ma, and gradually created what is now believed to be the thickest crust in the world.