Major and trace element provenance signatures in stream sediments from the Kando River, San'in district, southwest Japan

摘要

Basement rocks in the catchment of the Kando River in southwest Japan can be divided into two main groups. Paleogene to Cretaceous felsic granitoids and volcanic rocks dominate in the upstream section, and more mafic, mostly Miocene volcanic and volcaniclastic rocks occur in the downstream reaches. Geochemically distinctive Mount Sambe adakitic volcanic products also crop out in the west. X-ray fluorescence analyses of major elements and 14 trace elements were made of two size fractions (< 180 and 180-2000 mu m) from 86 stream sediments collected within the catchment, to examine contrasts in composition between the fractions as a result of sorting and varying source lithotype. The < 180 mu m fractions are depleted in SiO2 and enriched in most other major and trace elements relative to the 180-2000 mu m fractions. Na2O, K2O, Ba, Rb and Sr are either depleted relative to the 180-2000 mu m fractions, or show little contrast in abundance. Sediments from granitoid-dominated catchments are distinguished by greater K2O, Th, Rb, Ba and Nb than those derived from the Miocene volcanic rocks. Granitoid-derived < 180 mu m fractions are also enriched in Zr, Ce and Y. Sediments derived from the Miocene volcanic rocks generally contain greater TiO2, Fe2O3*, Sc, V, MgO and P2O5, reflecting their more mafic source. Sediments containing Sambe volcanic rocks in their source are marked by higher Sr, CaO, Na2O and lower Y, reflecting an adakitic signature that persists into the lower main channel, where compositions become less variable as the bedload is homogenized. Normalization against source averages shows that compositions of the 180-2000 mu m fractions are less fractionated from their parents than are the < 180 mu m fractions, which are enriched for some elements. Contrast between the size fractions is greatest for the granitoid-derived sediments. Weathering indices of the sediments are relatively low, indicating source weathering is moderate, and typical of temperate climates. Some zircon concentration has occurred in granitoid-derived < 180 mu m fractions relative to 180-2000 mu m counterparts, but Th/Sc and Zr/Sc ratios overall closely reflect both provenance and homogenization in the lower reaches.