Geochemistry and U–Pb geochronology of K‐bentonites from the Pingliang Formation of the Upper Ordovician in Gansu, North China, and their tectonic implications

The Late Ordovician Pingliang Formation in the Guanzhuang section, located in the south‐western Ordos Basin, North China, contains several K‐bentonite layers, which are important for understanding the tectonic evolution of the south‐western Ordos Basin. This study provides new geochemical and SIMS U–Pb geochronological data for the K‐bentonites to establish the Ordovician high‐resolution chronostratigraphic framework for the Ordos Basin and to provide temporal constraints of major volcanic–magmatic activity. K‐bentonites in the succession yield U–Pb zircon ages of 457.6 ± 3.8 Ma and 454.8 ± 3.8 Ma (2σ analytical uncertainty), showing a depositional age in the Sandbian Stage (Late Ordovician). The result is consistent with the biostratigraphic and astrochronologic framework, achieving the intercalibration of different dating methods. The discrimination diagrams of whole‐rock Na2O+K2O against SiO2 and Zr/TiO2 against Nb/Y and Th/Yb against Nb/Yb and Th‐Hf‐Ta indicate that the K‐bentonites have obvious arc affinities. Based on the geological background, we conclude that the K‐bentonites in the south‐western Ordos Basin were mainly deposited during the development of the North Qilian‐North Qinling volcanic arc in the Sandbian.