Coupled U-Pb Dating And Hf Isotopic Analysis Of Detrital Zircon Of Modern Sand From The Yalu River (Yarlung Tsangpo) System In Southern Tibet

In order to define provenance signatures of the modern Yalu River system and the effects of transport processes on these provenance signatures, we carried out coupled U-Pb dating and Hf isotopic analysis on ten river-sand samples from the Yalu River and its second-order rivers covering distinct provenance domains of the Lhasa terrane. We also quoted four samples from our early work to complement our results in this study.Detailed comparison of U-Pb age spectra and theεHf(t) values between the Lhasa River sand and published bedrocks exposed in its drainage basin confirms the important roles of differential weathering and burial history for second-order rivers to pick up some certain signatures in the source areas as well as bedrock types and the relevant exposure areas.Although some parts of the northern Lhasa terrane are located within the Yalu River catchment areas in southern Tibet, our results indicate the detritus carried by the Yalu River are dominantly contributed by the southern Lhasa terrane and the Himalayan orogen, and the U-Pb age spectra and the correspondingεHf(t) values vary laterally for the southern Lhasa terrane. In addition, sediment-transport processes along the Yalu River have systematically diluted the strength of provenance signatures as it becomes farther away from the relevant source areas. Therefore, we believe laterally varying provenance and sediment-transport processes together determine the signatures of detritus finally delivered to the Himalayan foreland basin.Combined with those earlier results for the Neogene sandstone in the eastern Himalayan foreland basin, we found the bottom Dafla Fm in the Itanagar area and the Subansiri Fm in the Bhalukpong area had involved materials from both the southern Lhasa terrane between Gonggar and Gyaca and yonger Himalayan igneous rocks near the Yalaxiangbo, whereas the detritus for the top Subansiri and Kimin Fms in the Itanagar area resemble those carried by the present Siang River. Considering the effect of sediment-transport processes together, we incline to believer prior to the establishment of the present Yalu-Brahmaputra drainage configuration, there existed short period of connection between the paleo-Yalu and paleo-Brahmaputra Rivers via the paleo-Subansiri river, possibly due to the blockage by glacial dam or north-trending normal faults along the Yalu River in southern Tibet.