The closure of Palaeo-Tethys in the Sanjiang area, southwestern China: New insights from zircon U-Pb and Hf isotope data of granitoids

There are several granitoids associated with two sub-parallel suture zones in the Sanjiang area, southwestern China, which were interpreted to represent the magmatic expression of the tectonic evolution of Palaeo-Tethys during Late Palaeozoic to Early Mesozoic times. The granitoids are mainly peraluminous, high-K calc-alkaline with sub-parallel REE patterns and strong negative Eu anomalies, showing S-type characteristics. Recent geochronological and geochemical data of granotoids in the belts are adding some evidences on the Palaeo-Tethys evolution. Here we present new LA-ICP-MS ziron U-Pb ages and Hf isotopic data in the S type granitic intrusions from Lincang to Yacha to constrain on the times of the tectonic evolution. Measured ages of 200Ma - 230 Ma in the Lincang granite are equitant of the Wandashan granite (219Ma) and Yacha granite (220Ma) to the northern extension and granitic intrusions in eastern Myanmar (published ages of 210-220Ma) to the southern extension. There are also some I type granitoids (282-284 Ma) to the east in Jinghong and granodiorite (239 Ma) to the north in Baomaxueshan, which suggest the existence of island arc system and subduction setting. The occurrence of bimodal volcanism (Xiaodingxi Fm.) in Jinghong and middle Triassic andesite (Manghuai Fm.) with ages of 236-248 Ma supports this proposal. Thus, it has been confirmed that the S type granitoids from Lincang to Yacha formed under collsional setting during late Triassic period and the formation process lasted for 30 Ma approximately. Further, the new zircon Hf isotope data indicated that the granitoids should be formed by underlain Palaeoproterozoic crust with ɛHf model ages of 1900-2400 Ma. Therefore, the S type granitoids from Lincang to Yacha were formed during continental collision between the Baoshan-Gengma Terrane and the Lanping-Simao Terrane after subduction of the Paleo-Tethyan Oceanic Plate, which is suggesting the closure of the Paleo-Tethyan Ocean.