Geochemical characteristics of Carboniferous-Permian Amushan Formation volcanic rocks in Ejin Banner and its vicinities, western Inner Mongolia

Based on an analysis of basalt geochemical characteristics of Amushan Formation （western Ganquan Formation） of Upper Carboniferous Xiaodushan Stage-Lower Permian Zisong Stage and the basalt type，this paper deals with the rock characteristics and tectonic environment. Previous studies have already revealed the basic type of Carboniferous-Permian basin. The Zr/TiO2-Nb/Y，SiO2-TFeO/MgO and Zr-Y-Nb diagrams show that the basalt in Ejin Banner and its vicinities is mainly of the sub-alkaline tholeiite series. Chondrite-normalized REE patterns are right-oblique curves; the relatively high (La/Yb）N and low (Gd/Lu）N indicate that light rare earth elements have experienced obvious fractionation，but heavy rare earth elements have been only subjected to week fractionation，and δEu=0.87~1.11，without remarkable Eu depletion；N-MORB-normalized spider diagram shows relatively high abundances of K2O，Rb， Ba，Th and low abundances of Hf，Nb，Y. Through an analysis of Zr-Zr/Y and Ta/Hf-Th/Hf diagrams，it is considered that the basalt in the study area should be of the sub-alkaline tholeiite series occurring in the rift valley，and Ejin Banner and its vicinities entered into an intracontinental evolution period, being a rift basin by tension.