Detrital zircon U-Pb dating of Dahalajunshan Formation in Wusun Mountain region, western Tianshan, and its geological implications

Abstract: Dahalajunshan Formation volcanic-sedimentary rocks, widespread in Yili area, western Tianshan, are the most direct carrier in the study of Early Carboniferous palaeogeography of western Tianshan and Paleozoic orogenic evolution of the Tianshan. The age distribution of detrital zircons of sandstone in Dahalajunshan Formation was studied by using LA-ICP-MS U-Pb isotopic ages of zircons. The results show that the detrital zircons have a wide range of zircon ages from (321±2) Ma to (435±2) Ma, which can be divided into 2 groups: 321-372 Ma and 395-435 Ma, according to the age and frequency distribution characteristics. Based on these dating data, combined with the mineralogical characteristics and CL images of zircons as well as the regional geological data of the Wusun Mountain, the authors have reached the following conclusions: (1) The age of the deposition of Dahalajunshan Formation is not later than the later Early Carboniferous; (2) the detrital materials of the studied sandstone were mainly derived from volcanic and magmatic rocks related to the Wusun Mountain and Nalati Mountain; (3) during middle-late Devonian, western Tianshan orogenic belt experienced significant ocean-land transition events and entered into the post-collision rift extensional stage. Therefore, the formation of volcanic-sedimentary rocks of Dahalajunshan Formation was in a post-collision extensional setting.