Provenance, source weathering, and tectonic setting of the lower Cambrian Shuijingtuo Formation in the Middle Yangtze area, China

Abstract

The organic-rich shale of the Lower Cambrian Shuijingtuo Formation is an important hydrocarbon source rock in South China. The geochemical characteristics of this shale were investigated to determine the provenance, weathering conditions, sedimentary recycling, paleoclimate, and tectonic setting of the Shuijingtuo Formation in Yichang in the Middle Yangtze area. The shale is relatively enriched in CaO, P2O5, Mo, U, V, Cd, and Cu. The rare earth element (REE) patterns of the shale exhibit light rare earth element (LREE) enrichment and heavy rare earth element (HREE) depletion. It has weak negative Eu and Ce anomalies, which are similar to the characteristics of passive margin deposition. The analysis of the carbonate dilution effect conducted in this study reveals that carbonate dilution has a greater effect on the MgO, Ba, Sc, and Zr contents. The provenance discrimination diagrams suggest that the shale is primarily derived from a mixture of intermediate and felsic igneous source rocks, which were similar to Tonalite-trondhjemite-granodiorites (TTGs) in composition. The chemical index of alteration (CIA) values and the A–CN–K ternary diagram indicate that the source area of the shale experienced a low degree of chemical weathering under a cold and arid climate in the early period and a moderate degree of chemical weathering under a warm and humid climate in the later period. The relationship between Zr/Sc and Th/Sc indicates that the shale was deposited by the first cycle of sedimentation rather than via reworking. The discriminant function diagrams, major element ratios, immobile trace element ratios, and ternary relationships between La, Th, Sc, Zr, and Co indicate that the source rocks were deposited in a passive margin tectonic setting. It is suggested that the Ezhong old land, which is predominantly composed of the Huangling granite and the Kongling complex (composed of intermediate and felsic igneous rocks) was the source area.