LA-ICP-MS zircon U-Pb age and geochemical characteristics of the granodiorite in the Riwanchaka area, central Qiangtang

This paper reports new zircon U-Pb age and whole-rock major and trace element data of the granodiorite body in the Riwanchaka area, central Qiangtang, northern Tibetan Plateau. The granodiorite lies in the western part of the Lungmu Co-Shuanghu-Lancang River suture zone. Its zircons show no euhedral crystals and have clear oscillatory zones as well as high Th/U ratios, suggesting a magmatic origin. Zircon LA-ICP-MS dating of the granodiorite yielded a weighted mean age of 211.0Ma±1.8Ma, implying that the crystallization of the granodiorite body took place in late Triassic. Petrological and geochemical studies show that it is characterized by high silicon and aluminum, abundant alkali, low TFe2O3 and TiO2, with A/CNK being 0.97-1.04. Hence the granodiorite body belongs to aluminous-peraluminous series. The REE model is characterized by enrichment of light REE with negative Eu anomaly. In addition, it is enriched in Rb, Th, U, K, Pb with negative anomalies of Nb, Ta, Ti, P. These features suggest that the granodiorite has the characteristics of collision volcanics. A preliminary analysis shows that it was probably generated from partial melting of ancient crust through underplating of the mantle-derived magma. The collision-type granodiorite provides important petrologic constraints for the Lungmu Co-Shuanghu-Lancang River Ocean and the data for the study of the evolution of the Lungmu Co-Shuanghu-Lancang River Ocean.