Early Carboniferous volcanic rocks from north Zhaosu in western Tianshan Mountains are composed of alkaline olivine basalts and minor tholeiitic basalts. These rocks are characterized with moderate TiO2 (1.10% similar to 1.99%) and P, 0, (0.22% similar to 0.70%) contents, enriched large-ion-lithophile elements (LILE) such as Rb, Ba and K, strongly depleted high-field-strength elements (HFSE) such as Nb, Ta and Ti ((Nb/La)(N) = 0.20 similar to 0.36), as well as fractionated rare earth elements (REE). The geochemical characteristics in general resemble that of the Cenozoic (before middle Miocene) volcanic rocks from Basin and Range, western America, and that of the back-are "transitional" mafic rocks in Central and South America, but are some different from that of typical volcanic arc magma. Abundant inherited zircon of Proterozoic age, and major and trace elemental and neodymium isotopic geochemistry together prove that the magma experienced extensive fractional crystallization and crustal contamination (AFC) before eruption. The tholeiites are likely to be derived from the parental alkaline magma through higher degree of AFC processes, as is inferred from their higher SiO2 contents and K2O/Na2O ratios and lower epsilon(Nd) (t), MgO and Mg-# values relative to the alkaline basalts. Fe-Ti basalts, typically titanium- and iron-rich and silica-poor, occur as thin layer within the alkaline basalts, and are interpreted to be produced from the parental alkaline basalts through moderate to high degree of Fenner trend differentiation. Elemental geoehemistry of the volcanic rocks is attributed to a lithospheric mantle source that had been enriched by subduction fluids. However, occurrence of the Fe-Ti basalts and regularly temporal geochemical variation of the whole volcanic stratum are consistent with a rifting tectonic environment. It is proposed that the volcanic rocks from north Zhaosu were formed in a back-are extensional environment of active continental margin with Proterozoic continental basement. This conclusion also argues against the previous suggestion that the Dahalajunshan group volcanic rocks in western Tianshan Mountains represent continental rift magmatism induced by mantle plume.
