Southern Tianshan Mountains is a prominent intercontinental collision orogenic belt and key to understand the tectonic evolution of central Asia. This paper mainly focused on the mafic—ultramafic rocks located in the southwest of Kokshal, western margin of the Southern Tianshan Mountains, China. Petrological, geochronological, and geochemical studies were performed in order to reveal the petrogenesis and geochemical evolution of the mafic—ultramafic rocks, as well as the tectonic evolution of the Southern Tianshan Mountains. Geochemically, the Baleigong basic volcanic rocks are plotted as tholeiitic series, with high Ti, P, Fe contents and low Cr, Co and Ni concentrations; and also characterized by OIBtype trace element patterns, displaying significant enrichment of LILE, HFSE, LREE and MREE, and slight depletion of HREE, when normalized to NMORB. In addition, the positive correlation between Ni,Cr and MgO have implied that the fractionation of olivine and chromium spinel had occurred in the parent magmas. Lower Zr/Nb, Zr/Hf ratios suggest that these rocks were most likely to derive from partial melting of Garnetbearing enriched mantle. Ratios of high field strength element also indicate a hotspot or plume tail sources originated from the mixing of EM1 and EM2 components with a significant contribution from the melting metasomatised subducted oceanic lithosphere to the Early Paleozoic mantle. Zircon UPb dating of the diabase which, together with basalt, formed the interlayers in the ophiolitic mélange of Baleigong after early Carboniferous, yielded the forming age of 450±2Ma. Combined with the recently published geochronological data of the region, we postulate that the multiisland oceanic basin in the southern Tianshan Mountains have been formed in Early Paleozoic during Late Ordovician to Early Silurian.