Differential geochemical features of clinopyroxene from the floodplain sediments in Yangtze and Hanjiang Rivers and its implication for detrital source identification

Abstract

The evolution history of the Yangtze River attracted increasing attention in recent years. Comparison of the properties of the floodplain sediments between the upper and lower reaches of the Three Gorges is of significance to identify the material detrital sources for the Yangtze River Basin. Clinopyroxene (Cpx) is the typical mineral widely distributed in the sediments of the Yangtze River Basin. Cpx was extracted from the fine sediments with the grain size ranging from 0.063 to 0.125 mm in the upper and lower reaches of the Three Gorges. The sediments, from which the Cpx was extracted, were taken from the floodplains of the catchments of both the Yangtze River and the Hanjiang River, as well as from the surface of the Zhoulao Core drilled in the center of the Jianghan Basin where the two rivers flow through. Geochemistry of these Cpx was investigated, concerning the contents of major elements measured by electron probe microanalysis (EPMA) and the contents of trace elements analyzed by laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS). The Cpx in the Yangtze River contains titaniferous augite which is sourced from the Panzhihua-Xichang region in South Sichuan Province, whereas the Cpx in the Hanjiang River is mainly from the surroundings without any titaniferous augite. The REE analysis infers that the titaniferous augite is the diagnostic mineral of the upper Yangtze River, and thus could be used as a tracer mineral to identify the detrital sources for the sediments formed during the evolution of the Yangtze River. The Cpx from the surface sediments of the Jianghan Plain (Zhoulao Core) shows comparable geochemical nature with that of the Yangtze River, but is different in geochemistry from that of the Hanjiang River, implying that the modern sediments in the Jianghan Plain are mainly sourced from the upper Yangtze River.