The establishment and significance of the Upper Permian-Lower Triassic Tianquanshan Formation in central Qiangtang, northern Tibet：The constraint on the tectonic evolution of Lungmu Co-Sh- uanghu-Lancang River area

There exist extensive epimetamorphic rocks in Tianquanshan and Pingfengling of Gaize County. These rocks are mainly metamorphic complex sandstone, phyllite and some interlayered volcanic rocks which are metamorphic basalt and andesite. Due to the lack of fossils, their formation epoch is unknown. There exists interlayered andesite, and the authors obtained the 206Pb/238U age of andesite (251.4±2.4Ma) by LA-ICP-MS zircon U-Pb isotope dating. Combined with the 254Ma 206Pb/238U age obtained by Fan Ji?anjun in the region, it is confirmed that the epimetamorphic rocks in the region should belong to the Late Permian-Early Triassic rather than to the Zhangjin Formation held by the traditional opnion. Based on the profile measurement and regional correlation, the strata of Tianquanshan and Pingfengling along the shallow metamorphic rocks are redefined as the Upper Permian-Lower Triassic Tianquanshan Formation (P3T1t). The discovery and establishment of Tianquanshan Formation (P3T1t) not only further improve the regional stratigraphic system but also provide the first oceanic basin record for Late Permian-Early Triassic strata in Lungmu Co-Sh? uanghu-Lancang River area. The results obtained by the authors provide important evidence for furthe study of the evolution of Lungmu Co-Shuanghu-Lancang River suture zone.