Determination of the formation sequence of Chaqupu Formation of Early-Middle Triassic in the Quesang area, Tibet, China, and its tectonic significance

Original Chaqupu Formation of Early-Middle Triassic in the Quesang area of Lhasa is composed by the lower part carbonate rocks which contain abundant Early-Middle Triassic fossils and the upper part volcanic rocks with no information on paleontology or chronology. Based on their stratigraphic overlying relationship, the volcanic rocks were determined as the upper part of Chaqupu Formation, and its age was determined to be Early-Middle Triassic, which has become a classical formation sequence locally and been widely used. Recently, SHRIMP U-Pb zircon dating was carried on the zircons of basaltic andesites of the volcanic rocks in the upper part of original Chaqupu Formation. The dating results show that these rocks were formed in Early Triassic (248Ma±4Ma), so the original Chaqupu Formation is a inversion tectonic, and the volcanic rocks used be under the carbonate rocks and contact conformably with Lielonggou Formation of Permian, which is a trustworthy evidence of the continuous deposition in this area from Permain to Triassic and further shows that this area existed magmatic activities in Early Triassic. The determination of the sequence of Chaqupu Formation has important significances in studying the evolution process of the Gangdise area.