Late Triassic paleogeography of the northern Indian continental margin: Constraints from Nd isotopes of siliciclastic rocks (Tethys Himalaya, South Tibet)

A large volume of terrigenous detritus accumulated onto the northern Indian margin of the Neo-Tethys Ocean during the Late Triassic. The depositional process and provenance of these siliciclastic rocks have long been controversial. In particular, the deep-sea turbidites of the Langjiexue Group have been variously interpreted either as an in situ sedimentary sequence or as an exotic terrane. This study carries out a systematic whole-rock Nd isotopic analysis on Upper Triassic fine-grained siliciclastic rocks. The results reveal two independent sources of detritus. Whereas the coastal Tulong Group and Derirong Formation show enriched isotopic features [epsilon Nd(0) values from -18.1 to -13.8], indicating provenance exclusively from peninsular India, the Langjiexue Group yields less negative epsilon Nd(0) values (-9.8 to -5.4), pointing at detrital input mainly from eastern Gondwana (Australia and/or Antarctica). The shelfal Nieru and Qulonggongba formations yield intermediate epsilon Nd(0) values between -13.7 and -9.2, indicating mixing of sediments from both source-rock domains. The close genetic relationship with coeval Tethys Himalayan stratigraphic units testifies to the original deposition of the Langjiexue Group onto the northern Indian continental margin. Influx of terrigenous detritus onto the northern Indian margin during the Late Triassic was controlled by rifting of the Lhasa Block from northwestern Australia.