Geochemistry and detrital zircon U-Pb geochronology of the Jurassic Ridang Formation in the southern Tibet and its tectonic implications

The Mesozoic-Cenozoic strata deposited in the passive continental margin of India in the eastern Himalayas are important carriers for the study of the evolution of the Neo-Tethyan Ocean. In this paper, detrital zircon U-Pb dating and whole-rock geochemical analysis of the Lower Jurassic Ridang Formation siltstone and shale in Quzhuomu Township, Cuona County, Shannan City, Tibet Autonomous Region have been carried out. The detrital zircons have two peak ages, of 496-546 Ma and 853-952 Ma, with the peak ages of 516 Ma and 926 Ma, respectively, and the youngest age of 292.5±5.5 Ma. The content of Al₂O₃ in the Ridang Formation is high, with an average of 20.8%, while the contents of CaO, Na₂O and K₂O are low (0.4%-4.0%), indicating that the Ridang Formation is rich in clay minerals. The source area has high compositional maturity and strong weathering, which represents the recycling of sediments and the tectonic quietness or cratonic environment. The Ridang shale is enriched in light rare earth elements (LREEs) and depleted in heavy rare earth elements (HREEs), and the negative Eu anomaly is obvious. Based on the paleogeographic environment, it is considered that the material of Ridang Formation is mainly derived from the Indian plate and the Australian plate on the northern margin of Gondwana continent, and the tectonic background is a stable passive continental margin.