GEOCHEMISTRY OF THE LATE PAOLAEOZOIC DARK SHALES FROM EASTERN ORDOS BASIN, CHINA: IMPLICATION FOR PALEOENVIRONMENT

Late Paolaeozoic dark shales from eastern Ordos basin of north China were firstly analysed based on investigations of TOC (total organic carbon), minerals and elementary geochemistry to determine the paleo-environment conditions during their deposition. In this study, 26 samples of the Shanxi Formation and Taiyuan Formation were collected from Well LY4 and XY1, located in Shanxi province, eastern Ordos basin. The dark shales have high content of TOC (average on 2.77%), and Quartz (35.42%) and clay minerals (48.34%) are the dominant minerals in the samples. Trace elements, such as Li, Cs, Cu, V, Co, Cr, are commonly enriched in these formations compared to the upper continental crust. The value of CIA (chemical index of alteration) (77.92-98.36%) and CIW (chemical indices of weathering) (89.19-99.11%) indicate an intense chemical weathering in the source area, while the C-value varies between 0.2 and 4.5, together with CIW suggestting that the paleoclimate was semi-humid and humid in the late Palaeozoic of studied area. Trace elemental redox indices (e.g. V/Cr, Ni/Co, V/(V+Ni) and U/Th) indicate that the dark shales were deposited under the oxic and dysoxic conditions in a transitional sedimentary environment. PAAS-normalized REE in shale are enriched of LREE with weakly negative Ce and positive Eu anomalies, suggesting the felsic source rocks of sediments. The Ce anomalies indicate an oxic environment with terrigenous input during black shale deposition, while the LREE enrichment with negative Eu anomalies suggest that both formations are affected by the detrital input rather than hydrothermal fluid.