Petrography and geochemistry of clastic rocks and their constraints on the tectonic setting of the Middle Triassic Pingxiang basin, Southern Part of Guangxi

Petrographic characteristics of Pingxiang basin show that the conglomerates can be divided into clast-support and matrix-support types, which consist of bioclast-dominated limestones, sandstones and mudstones. Petrographic features show that the sandstones, which have low compositional maturity and textural maturity, were derived from recycled orogen or collisional orogen. The geochemistry of sandstones and mudstones are characterized by moderate SiO2 values (61.71%~74.85%) , close to the value of the upper crust; in addition, they have high K2O/Na2O ratios (6.50~0.51) and high TFe2O3+MgO values (7.29%~10.31%). The TFe2O3/K2O ratios (2.05%~5.54%) show that the stability of the minerals is low. The REE patterns, which are similar to patterns of the upper crust and typical Archean shales, are characterized by LREE enrichment, flat HREE and obvious Eu and Ce anomalies, suggesting marine sediments. CIA indexes (generally 71~88) and Th/U ratios (3.68~9.53) imply that all sediments have undergone high degree of weathering. Various diagrams for sedimentary provenance using major and trace elements show that all the sediments studied are obviously associated with felsic volcanic rocks. The sediments were mainly derived from accretionary prism or active continental margin. Geochemical data of the clastic rocks suggest an active continental margin setting. The results achieved by the authors suggest that Pingxiang basin, considered to be a synchronal product with Early Triassic arc-type rhyolite around the basin, is an intensively modified sutural forearc basin that resulted from the closure of the Paleo-Tethys ocean.