Geochemistry of Neogene sedimentary rocks from the Jiyang basin, North China Block: The roles of grain size and clay minerals

Abstract

Major and trace element compositions of 83 fine-grained sedimentary rocks (including 14 fine-grained sandstones, 24 siltstones and 45 mudstones) from the Zhanhua Sag of Jiyang basin in the North China Block were analyzed to investigate the effects of sedimentary processes (e.g., source provenance, mineral assemblage, grain size and chemical weathering etc.) on the bulk chemical compositions. Compared with the average upper continental crust (UCC), these rocks have higher TiO₂, Al₂O₃, Fe₂O₃, Ba, Th, U, Nb, Ta, Zr, Hf, Y and REE, but lower SiO₂, MnO, MgO, CaO, K₂O, Na₂O, P₂O₅, Cr, Co, Ni and Sr. From sandstones, siltstones to mudstones, there exist systematic increase in Rb, Cs, V, Cr, Co, Ni, Nb, Ta, Th, U and REE, and decrease in SiO₂, K₂O, Na₂O and Ba. Such variation trends reflect a predominant role of grain size effect on the geochemistry of these sediments. K₂O, Na₂O, Ba, Zr and Hf positively correlate with SiO₂ and negatively correlate with Al₂O₃, whereas other elements generally show negative correlations with SiO₂ and positive correlations with Al₂O₃, suggesting that clay minerals are major hosts for most elements, while quartz and feldspar exert a dilution effect. The accessory heavy minerals exert a minor control on the bulk REE composition of the sediments, e.g., the contribution of heavy minerals (apatite, zircon, sphene, monazite etc.) to the bulk REE concentrations is less than 20%. Our results show that grain size and clay minerals are the two most important factors to control the bulk chemical compositions of the Zhanhua Sag sediments, and probably terrestrial fine-grained sediments in general.