Geochronology and geochemistry of intermediate−acid intrusive rocks in Xinlong area, Sichuan Province and its constraints on the evolution of Ganzi−Litang Ocean

The Yajiang residual basin is located on the southeastern margin of Tibetan Plateau and is an important part of the Yulong−Bayankala foreland basin in the Qiangtang−Sanjiang orogenic system. Based on detailed field investigations, we carried out petrology, geochemistry and zircon U−Pb isotope dating of different types of intrusive rocks in the Xinlong area of Sichuan Province, to discuss their petrogenesis and tectonic background. The results indicate that both the Gulongba granodiorite and the Riguo diorite belong to the metaluminous granites, with low silica, low alkaline, high Fe−Mg characteristics, intermediate REE total contents, and obvious Nb and Ta negative anomalies. They belong to the I−type granites, which are products of subduction magmatic activity. The Riguo diorite was formed at the Late Triassic with zircon U−Pb age of 217.1 ± 0.8 Ma. The Rililongba granodiorite and granite are the peraluminous rocks, with high silica, high alkaline, low Fe−Mg and weak Nb and Ta negative anomalies. Both of them belong to S−type granite, but the latter experienced higher differentiation degree. They were the products of collisional and post−collisional magmatic activity. The zircon U−Pb age of the Rilongba granodiorite is 203.6 ± 0.5 Ma, and its formation period is the end of the Late Triassic. It is believed that the Ganzi−Litang paleo−ocean basin was subducted westward in the Late Triassic, and a small amount of arc intrusions were located in the Yajiang residual basin on the passive continental margin. At the end of the Late Triassic, the arc−land collision occurred due to the extinction of oceanic plates, which caused partial melting of the crust to form strong peraluminous granites.