MULTIPLE SOURCES OF THE UPPER TRIASSIC FLYSCH IN EASTERN HIMALAYA OROGEN, TIBET, CHINA: IMPLICATIONS TO PALEOGEOGRAPHY AND PALEOTECTONIC EVOLUTION

The Upper Triassic flysch----Langjiexue Group (tentatively named the “Shannan Terrane”) of the eastern Himalaya Orogen has been tectonically assigned either to the Tethys Himalaya or the Yarlung Zangbo Suture Zone (YZSZ). In this work, geochronology of detrital zircon U-Pb isotope shows that the Shannan Terrane is characterized by the population of ~260-200 Ma (peak ca. 240 Ma), strongly supporting the view of no affinity to the Tethys Himalaya. The detrital zircons dated as ~400-280 Ma display relatively positive εHf(t) values of -5.0 to +15.0 with T_DM^C ages of 2.6–1.3 Ga for the Shannan Terrane, whereas highly negative are of -20.0 to -5.0 for the Lhasa Terrane, indicating the two terranes have different Devonian-Carboniferous sources. Numerous Cr-spinels found in the Shannan Terrane but not in the Lhasa Terrane, exhibit contents in Cr₂O₃ and Cr# of mainly 44-100% and 48-95%, in TiO₂ of 0.01-1.0%, and in Al₂O₃ of 5-257%, respectively, denoting several parent lithologies. These differences suggested that the Shannan Terrane has multiple sources, not only from the Lhasa Terrane, but also from oceanic (island) arc/seamount and mid-ocean ridge areas as well as likely from Greater India and Australia. Considering the Early Cretaceous diabase dykes within the Upper Triassic flysch defined to the Comei-Bunbury Large Igneous Province, we propose that the Langjiexue Group could have been deposited on the ocean between India and Australia, and would have not stopped until the Lhasa Terrane was separated from Australia during the terminal Triassic. According to the Cenozoic deformation and metamorphic history and paleogeography of the Langjiexue Group, we postulate that the Shannan Terrane could have been loaded onto the Greater India during the middle Early Cretaceous, and subsequently drifted northward to the collision zone of India and Asia, implying that it does not represent an accretionary prism within the YZSZ.