Provenance of Neogene sandstones in western Taiwan traced with garnet geochemistry and zircon geochronology

In this study, we use provenance analysis to shed light on the Neogene evolution of river drainage and sediment dispersal in eastern Asia as a consequence of the Himalayan orogeny and topographic rise of the Tibetan Plateau. As an attempt to envisage how sediment-routing systems operated in the past, and to identify the ultimate source of siliciclastic detritus originally generated in mainland China and now incorporated in, and recycled from, the western Taiwan accretionary prism, we have used Raman spectroscopy of detrital garnet in modern river sands and upper Neogene sandstones, complemented by detrital-zircon geochronology and by petrographic and heavy-mineral analysis. Garnet grains are relatively abundant and largely represented by almandine with various percentages of pyrope molecules in Yangtze sand as in western Taiwan sandstones and modern river sand derived from them. Instead, they are rare and largely represented by grossular or andradite in the sand of the Pearl River and of coastal rivers of SE China facing the Taiwan Strait. This finding supports the result of detrital geochronology, which documents a sharp difference between U-Pb age spectra of Pearl River and coastal SE China zircons (characterised by a Paleozoic-Mesozoic triple peak) and Yangtze and western Taiwan zircons (characterised by prominent Neoproterozoic, Paleoproterozoic, and latest Archean clusters). The Pearl River and minor rivers facing the Taiwan Strait are thus ruled out as ultimate sources of upper Neogene sandstones exposed today along the western front of the Taiwan fold-thrust belt. In the envisaged scenario, sand mostly supplied by the paleo-Yangtze River was entrained for ~ca.1,000 km southward by longshore currents and deposited on the Chinese passive margin before being accreted along the front of the Taiwan orogen since ~6 Ma. Littoral sand drift represents a major factor of long-distance sediment transport in modern shelves and needs to be taken into full account in source-to-sink studies and paleogeographic reconstructions.