Detrital Garnet Chemistry of the Changjiang (Yangtze River) Sediments and Their Provenance Implication

Abstract:

This study presents the compositions of major oxides including SiO2, FeO, Al2O3, MnO, CaO, MgO and TiO2 in the detrital garnet grains separated from the sediments of the Changjiang River and its major tributaries. The main garnet assemblage consists of almandine, grossularite, Grossular and spessartine, with the mean contents of 65.4%, 13.8%, 9.4% and 11.5%, respectively. The typical garnet assemblage of Changjiang sediments on the GPAS triangular diagram is characterized by low Mg and high Mn contents, which suggests they are mainly sourced from intermediate and lowgrade metasedimentary rocks. These garnet compositions are primarily determined by the widely distributed schist, phyllite, marble and the lowtemperature and lowpressure contact metamorphic rocks in the Changjiang catchment. Our data also suggests that the Jinshajiang River as the largest tributary in the upper drainage is the major supply of fine sands to the Changjiang mainstream. Similar to the average composition of the whole Changjiang sediments, the garnets of Jinshajiang are characterized by low Mg and high Mn contents, but much different to those of the middle and lower reaches. Therefore, the garnet assemblage of the Jinshajiang can indicate the contribution of Changjiang fine sand in the sediments provenance study of East China marginal seas.

Key words:Changjiang rivers; sediments; detrital garnet; geochemical composition; provenance identification