Abstract:The Batamayineishan Formation from Kalamaili area of East Junggar is built up of a set of basalt-andesite-dacite-rhyolite. The age and tectonic background of its formation have been disputed for long. In this paper, base on field investigations and detailed petrologic and geochronologic researches of Batamayineishan Formation in Shuangjingzi area and LA-ICP-MS zircon U-Pb dating and petrological geochemical analysis of basalts from the lowermost of this formation were carried out. The results show that the formation age of basalts collected is 335.1±9.3Ma, which belong to Late Carlboniferous epoch. Combined with the previous chronological data and the results of this paper, it is suggests that the Batamayineishan Formation is a diachronic lithostratigraphic unit from Early Carboniferous to Late Carboniferous. Geochemically, basalts samples contains SiO2 of 46.49%~52.19%, they have relatively high Na2O+K2O (4.07%~8.55%), K2O/Na2O (0.35~1.31) and A12O3 (15.62%~17.97%). Their Rittman indices are greater than 3.3, and alkaline minerals are enriched in phenocryst and matrix, with the typical characteristics of alkaline basalt. The REE patterns of the alkaline basalts are characterized by strongly fractionated REE patterns (La/Yb)N=3.08~5.25 and without Eu anomalies. The concentrations of incompatible trace elements such as Rb, Ba, Sr are enriched, whereas the high field strength element (eg. Nb, Ta and Ti) contents are depleted. Combined with discrimination diagrams, the authors hold that basalts from the Batamayineishan Formation were derived from the partial melting of a depleted mantle that had been metasomatized by subduction-related fluids, possibly with a few additions of continental materia, then fractional crystallization of mafic-minerals and Fe-Ti oxides occurred during magmatic evolution. Alkaline basalts have the characters of intra-plate basalt, post-collision and arc volcanic rock. In combination with the regional geological background, alkaline basalts of Batamayineishan Formation are formed in post-collision extensional period after the closure of Karamaili Ocean. It shows that in Late Carlboniferous the northeastern Junggar terrane entered a key stage of structural transformation from collisional extrusion to extension.