A discussion on Mesozoic volcanic stratigraphic sequence in Sangri County, Tibet: evidence from zircon U-Pb ages and geochemical compositions

The evolution of Neo-Tethys and associated magmatism during Mesozoic has become a topic of much debate in the basic study of the Tibetan Plateau. Voluminous Mesozoic volcano-sedimentary sequences corresponding to the subduction of the Neo-Tethys crop out on the southern margin of Lhasa Block. The systematic study of these volcano-sedimentary sequences is of great importance in understanding the evolution of the Neo-Tethys and the Lhasa Block. With the volcano-sedimentary sequences of the Yeba Formation, the Sangri Group and the Danshiting Formation exposed in Sangri County as the study object, the authors carried out zircon LA-ICP-MS and SHRIMP dating and geochemical analysis for the volcanic rocks. In combination with the data available, the authors hold that both the Bima Formation (189.0±3.0~195.0±3.0Ma) in the upper part of the Sangri Group and the Yeba Formation (174.2±3.6~192.7±1.3Ma) were formed in the early Jurassic. Furthermore, the volcanic rocks of the Bima and Yeba formations display similar geochemical characteristics, and thus might have been formed in the same tectonic setting. It is suggested that they are arc-related calc-alkaline volcanic rocks. Therefore, the Bima Formation could be separated from the Sangri Group, and correlated with the Yeba Formation. In contrast, the Mamuxia Formation (99.9±0.7~136.5±1.7Ma) in the upper part of the Sangri Group was much younger than the Bima Formation, and belongs to Lower Cretaceous strata. The Mamuxia Formation has geochemical characteristics of adakite, and should be regarded as a single independent formation. The Danshiting Formation was formed at 93.7±1.2Ma, belonging to Upper Cretaceous, and should also be classified as an independent formation.