Geochemistry of the Early Cretaceous saline lacustrine shales on the Lingshan Island, East China: implications for provenance, tectonic setting, and paleoweathering

Early Cretaceous saline lacustrine black shales in the Fajiaying Formation were deposited under hothouse conditions and have a complex structure and history of sedimentary evolution. In this study, integrated petrographic and geochemical investigations were utilized to determine the provenance, tectonic setting, and paleoweathering conditions of these shales, therefore revealing their depositional history. Shale in the area under investigation has not been subjected to any post-depositional alteration, mechanical sorting, or recycling, hence its geochemistry has not been affected. The chondrite normalized rare earth elements (REE) pattern of shales displayed typical characteristics, including an enrichment in light REE (LREE), a flat pattern for heavy REE (HREE), and a negative europium (Eu) anomaly. Specific trace element ratios suggested that the sediments were primarily derived from felsic source rocks. Furthermore, there was no discernible shift in the sediment source between the Qiancengya and the Laohuzui sections. The discrimination diagrams of major and trace elements used to understand the tectonic history showed that the majority of the source rocks originated from the active continental margin (ACM) and continental island-arc (CIArc). The bivariate plots and low average values of CIW (chemical index of weathering) and CIA (chemical index of alteration) for the examined shale suggested that chemical weathering in the source terrain under arid conditions was extremely weak. Importantly, with the change of climate from dry-hot to warmhumid, an increase in CIA and CIW was observed which indicated a gradual increase in chemical weathering.