Geochemical characteristics and tectonic setting of cherts in the Langzuizi Formation of the Xiangshan Group

Abstract：The age assignment of the Xiangshan Group has long been controversial since its establishment. Regional geological data indicate that the Xiangshan Group is Middle-Late Cambrian in age. In order to discuss the sedimentary environments and tectonic setting of the Xiangshan Group on the southwestern margin of the Ordos basin, the petrological and geochemical characteristics of cherts in the Langzuizi Formation of the Xiangshan Group are analyzed. The analysis of cherts suggests that: the SiO2 content is 82.44%-98.13%, the Al/(Fe+Mn+Al) ratio ranges from 0.46 to 0.69 and Al2O3/(Al2O3+Fe2O3) is 0.53-0.77; the fractionation between LREE and HREE is strong with LREE/HREE=3.57-8.45, LaN/CeN is 0.66-1.15 and Ce/Ce* is 0.66-0.93. These characteristics show that the cherts formed in a continental-margin environment and that the diagenetic processes included hydrothermal deposition and normal deposition. The geochemical characteristics of clastic rocks and diabase sills in cherts indicate that: clastic rocks were also deposited on the passive continental margins and diabase belongs to the oceanic tholeiite series formed in an extensional setting. An integration of geological data suggests that an environment of divergent continental-margin nature occurred on the southwestern margin of the Ordos block and the southern margin of the Alxa block during the Mid-Late Cambrian.