Geochemistry of marine black shale of the Cambrian Qiongzhusi Formation, Yangtze Plate, SW China: implications for provenance and paleoweathering

Abstract

Marine black shale in the Lower Cambrian Qiongzhusi (QZS) Formation in the southwestern Yangtze Plate, SW China, is the key target for shale gas development. The paleoenvironment plays an important role in the formation of organic-rich shale. Based on the analysis of major, trace and rare earth elements (REE), the authors discussed element composition, paleoweathering and provenance of Qiongzhusi black shale (QZS shale). The results show that the main components of the Qiongzhusi Formation sample are SiO 2 , Al 2 O 3 and total Fe 2 O 3 (TFe 2 O 3 ), with the average values of 64.08 wt%, 15.00 wt% and 5.39 wt%, respectively. Redox-sensitive elements, such as V, Cr, Ni, Zn and U, are richer in QZS shale compared to the upper continental crust (UCC). The total concentration of REE (∑REE) of QZS shale is 174.58 ppm on average, which is higher than that of UCC (average 146.37 ppm) and the North American Shale Composite (NASC) (average 173.21 ppm). The ratios of w(SiO 2 )/w(Al 2 O 3 ) and w(Al 2 O 3 )/w(TiO 2 ), the Zr-TiO 2 diagram, the Th/Sc vs Zr/Sc plot, the discriminant function of F1 vs F2 and F3 vs F4, as well as the discrimination diagram of ∑REE vs La/Yb indicated that the main provenances of QZS shale are sedimentary and felsic igneous rocks. The values of a chemical weathering index, the chemical index of alteration (CIA), of the Lower QZS Formation (Stage 1) range from 51.84 to 64.33, indicating a low degree of chemical weathering and a cold and dry climate. The CIA values of the Upper QZS Formation (Stage 2) range from 66.58 to 82.42, being indicative of a medium degree of chemical weathering, probably in a humid climate.