Establishment of Middle-Upper Ordovician Dawashan Formation in central Qiangtang, Tibetan Plateau, and its significance

In this paper, a set of flysch sediments were firstly discovered in central Qiangtang, northern Tibet. Their thickness is more than 3000 m, and the top and bottom were not observed. Rock association consists of metamorphic sandstone, greywacke and phyllite, intercalated with bimodel volcanic rocks. The main lithology of the volcanic rocks includes rhyolite and basalt. The results of zircon LA-ICP-MS U-Pb dating indicate that the rhyolite rocks formed at 454Ma and 458Ma, indicating Middle-Late Ordovician Darriwilian-Sandbian age, which represents the formation age of Dawanshan Formation. The Middle-Upper Ordovician Dawashan Formation was established based on the section measurement and regional correlation, which shows that the abyssal-bathyal flysch was formed in the rift of northern Gondwana continent margin in the Middle-Late Ordovician. The establishment of Dawashan Formation contributes to the improvement of Ordovician stratigraphic system and also confirms the existence of Middle-Late Ordovician Ocean in this area. The results obtained by the authors are of great importance for the study of the Middle-Late Ordovician lithofacies paleogeograghy on the northern margin of Gondwana continent.