Post-collisional potassic magmatism in the Southern Awulale Mountain, western Tianshan Orogen: Petrogenetic and tectonic implications

Geochronological, elemental and Sr–Nd isotopic data are presented for the syenites, trachyandesites and trachybasalts from the Southern Awulale Mountain, Xinjiang. The syenites were emplaced and crystallized in the late Carboniferous (312 Ma), and are characterized by high positive εNd(t) values and high initial 87Sr/86Sr ratios, Th and Th/Ce ratios. They were thought to have generated dominantly by the partial melting of the underplated juvenile basaltic crust that originated from the mantle previously modified by subducted sediments. The trachybasalts have high positive εNd(t) values, medium initial 87Sr/86Sr ratios and high Rb, Ba, Sr contents, and are interpreted as the derivatives of a mantle source modified by subduction zone fluids. The trachyandesites also exhibit high positive εNd(t) values, medium initial 87Sr/86Sr ratios, but relatively low Th, Rb, Ba, Sr contents, and as such are considered as the products of crystal fractionation of a basaltic magma that was originated from the mantle slightly modified by subducted sediments. The systematic geochemical variations of the Carboniferous magmatism indicate that the western Tianshan Orogen experienced a transformation from convergence to extension in the late Carboniferous, probably in response to the upwelling of an asthenospheric mantle triggered by the detachment of an orogenic root zone.