Geochemistry of siltstones of the Early Cretaceous Laiyang Group in Taolin area, Shandong Province, Eastern China: Implications for provenance, source weathering, palaeo‐environment, and tectonic setting

Abstract

The depositional environment and tectonic setting of the newly identified Ri‐Qing‐Wei Basin are the focus of controversy. In order to solve these questions, the geochemical characteristics of siltstones from Laiyang Group in Taolin area are analysed in detail. The siltstones present similar contents of SiO2 and Al2O3 compared with upper continental crust (UCC) and post‐Archean Australian Shale (PAAS) and are classified as wacke on chemical classification plot of clastic rocks. The trace elements are characterized by the depletion of transition elements (Sc, V, Cr, Ni, and Cu). The chondrite‐normalized REE patterns exhibit the enrichment of LREE and depletion of HREE, as well as significant negative Eu anomalies. Relative high Index of Compositional Variability (ICV) values suggest the siltstones are compositionally immature and the sediments of first‐cycle. The low Chemical Index of Alteration (CIA) and Plagioclase Index of Alteration (PIA) values of the samples indicate the weak source weathering condition. Moreover, the characteristics of A‐CN‐K plot suggest K‐metasomatism occurred after burial. A series of palaeo‐environment indexes imply that the Laiyang Group were deposited in a reducing marine environment with hot and arid climate. Major‐ and trace‐based tectonic setting discriminant diagrams all indicate that the Ri‐Qing‐Wei Basin was formed in an active continental margin setting; this conclusion is also supported by the analysis of the coeval igneous rocks. Based on the investigations of the subduction of the Palaeo‐Pacific plate and the development of Cretaceous basins on the eastern Eurasia, we propose that Ri‐Qing‐Wei Basin was formed in response to the delamination of the North China Craton and the subduction of Izanagi plate.