Transparent heavy minerals and magnetite geochemical composition of the Yangtze River sediments: Implication for provenance evolution of the Yangtze Delta

Abstract

Heavy mineral and detrital magnetite geochemistry were analyzed to extract sediment provenance indexes from different reaches of the modern Yangtze River which were used to trace sediment source of the Yangtze Delta and to speculate its geomorphology change since the Pliocene. Our results show that diagnostic heavy minerals of the upper Yangtze sediment are characterized by clinopyroxene (12% on average) and magnetite (7% on average); the middle reaches by ilmenite, zircon and tourmaline; and the local small rivers by fluorite. Detrital magnetite composition of Ti, Mg, V and Cr is high in the upper Yangtze from the underlying basalt. These diagnostic indexes are then used in the Pliocene sediment core to extract provenance signal of different Yangtze reaches. Analysis of core sediment of the Yangtze Delta reveals that sediment provenance of the Pliocene was from local small rivers. Since the beginning of the Pleistocene, core sediments provenance was similar to that of the middle Yangtze tributaries. After 1.2Ma, high content of pyroxene and magnetite grains that are rich in Ti, Mg, V, Cr imply sediment provenance signals from the upper Yangtze. Sediment provenance shift from short-distance sources to more distant sources indicates that the geomorphology of the Yangtze Delta region has undergone a great transformation since the Pliocene. This dramatic landform change is likely in response to continuous uplift of the Tibetan plateau and accelerated subsidence of the east China coast since the Pliocene.