Tectonic evolution of north-eastern Tethyan Himalaya: Evidence from U–Pb geochronology and Hf isotopic geochemistry of detrital zircons

It is well established that the Himalayan Orogen was formed by successive amalgamation of continental slices to the Eurasian continent, with the final collision of the Indian continent. The Upper Triassic Langjiexue Group on the north-eastern margin of the Tethyan Himalaya has been central to debates on the provenance with diverse models linking it with the northern India, Lhasa terrane, or multiple sources from surrounding terranes including Australia. In order to address this debate, here, we present U–Pb ages, trace element characteristics, and Hf isotope data of detrital zircons. The trace element data suggest that analysed zircons are mostly of igneous origin, with a broad affinity to mafic source rock, and some of the grains showing evidence for hydrothermal alteration. The zircon age spectra show distinct Permian to Triassic age peaks at 200–280 Ma with εHf(t) between −6.1 and 13.4, Neoproterozoic to Cambrian ages at 480–750 Ma with εHf(t) ranging from −24.7 to 8.5, and a broad Meso- to Neoproterozoic age range of 850–1,150 Ma with εHf(t) values of −8.1 to 10.1. The age spectra from the Langjiexue Group are in contrast with that of the Lhasa terrane which has pronounced age peaks of 300–325, 550–600, 1,150–1,350, and 1,750–1,900 Ma, suggesting that Lhasa might not be the source of Langjiexue Group detritus. In a similar way, north-western Australia and the Banda Arc are excluded as sources of the Langjiexue Group. The Hf model age spectra show distinct peaks of 750–800 and 1,200–1,300 Ma, and the second peak agrees with that of the Tethyan Himalaya. We infer that the basin of Langjiexue Group formed within the basement of Tethyan Himalaya in the Triassic and conclude that the sediments were deposited at the passive margin of northern Indian continent. Based on the results, we also propose the palaeogeographic evolution of the Himalayan Orogen.