Multimineral fingerprinting of modern sand generated from the Tethys Himalaya (Nianchu River, Tibet)

The Nianchu, as the largest southern tributary of the middle Yarlung Tsangpo (upper Brahmaputra River in southern Tibet), drains mainly sedimentary rocks of the Tethys Himalaya and subordinately metamorphic and igneous rocks of the Greater Himalaya, Kangmar gneiss dome, and Indus-Yarlung ophiolitic suture zone. This study presents the first detailed compositional fingerprinting of detritus released from the Tethys Himalaya in southern Tibet. Sand carried by the Nianchu and by its major tributaries range from quartzo-lithic to litho-quartzose sedimentaclastic with a few metamorphic, volcanic, and ultramafic lithic grains, reflecting dominant erosion of Tethys Himalayan strata. Transparent heavy-mineral assemblages are moderately poor. They include a mixed recycled assemblage with amphibole, tourmaline and rounded zircon, together with first-cycle sillimanite and garnet grains from the Greater Himalaya, young titanite and monazite grains from either the Greater Himalaya or the Kangmar dome, chloritoid from the low-grade metapelites surrounding the Kangmar dome, and clinopyroxene, olivine and enstatite grains shed from forearc ophiolites. Forward mixing models indicate that ~80% of Nianchu sand is recycled from the Tethys Himalaya, which is roughly the same percentage as the corresponding exposure area. Based on previous geochronological and provenance studies, and on available suspended-load data from the Nianchu, Lhasa and Yarlung Tsangpo rivers, we assess the average erosion rate in the Nianchu catchment as ~0.10 mm a−1, about twice as that in the middle Yarlung Tsangpo catchment (0.05 mm a−1) and about five times as that of the Lhasa River catchment (0.02 mm a−1). This marked difference is principally ascribed to the higher erodibility of Tethys Himalayan sedimentary strata. Within the Nianchu basin, low precipitation in the rain shadow of the high Himalayan range may explain a lower erosion rate in the headwaters (0.07 mm a−1) than in the less steep lower reaches (0.14 mm a−1).