An analysis of the tectonic setting of Xujiajuan Formation of Middle Ordovician Xiangshan Group in Ningxia

Abstract：The Xiangshan Group in Ningxia is dominated by deep-water turbidity current deposits which are mainly a set of low metamorphic terrigenous clastic rocks intercalated with minor carbonate rocks and cherts. The Xiangshan Group is divided into three formations, which are from bottom to top Xujiajuan Formation, Langzuizi Formation and Mopanjing Formation. As relatively much research work was devoted to the formation epoch while the study of other aspects was insufficient in the past, the authors have made an integrated study of tectonic settings from the framework modes, major elements and trace elements of terrigenous sandstones in Xujiajuan Formation, which comprises the lower part of Xiangshan Group. Some conclusions have been reached： ① the framework modes suggest a typical passive margin; ② the discrimination diagrams for major elements suggest a result of uncertainty; and ③ the discrimination diagrams for trace elements suggest a transition tectonic setting. Thus, the tectonic setting during the deposition of Xujiajuan Formation of Xiangshan Group was not a simple passive margin, but most probably a transition source area among the passive margin, the active margin and the continental island arc.