Heavy mineral compositions of the Changjiang (Yangtze River) sediments and their provenance-tracing implication

Abstract

A total of 71 floodplain sediments were collected from the mainstream and major tributaries of the Changjiang for heavy mineral examinations. The upper Changjiang samples contain more heavy minerals in the very fine sand fraction than the middle-lower samples which are relatively enriched in polygenetic heavy minerals. The heavy mineral assemblages are characterized by Fe–Ti oxide minerals, calcic amphibole, epidote, garnet, biotite, and zircon, which exhibit large variations in percent between different tributaries and the mainstream. The widely distributed granitoids and Permian Emeishan Basalt in the drainage basins account for the high contents of zircon and hypersthene in the Xiangjiang and Daduhe samples, respectively. Most of the detrital magnetite grains are homogeneous in typomorphic feature and almost stoichiometric in chemical composition. The discrimination plot of TiO 2 + V 2O 3 versus MgO/(MgO + Al 2O 3) suggests that most of the detrital magnetite grains are sourced from felsic plutonic/volcanic and metamorphic parent rocks, and those from different tributaries have distinct chemical compositions. Our study suggests that the combination of transparent heavy mineral assemblages and varietal study of individual heavy minerals sheds new light on the provenance discrimination of the Changjiang sediments and contributes to the mineralogical determination of ancient source rocks within a large drainage basin, despite the sediment recycling and complex source rock types.