Late Paleozoic calc-alkaline to shoshonitic magmatism and its geodynamic implications, Yuximolegai area, western Tianshan, Xinjiang

Continuous Late Paleozoic magmatic activity in the western Tianshan, Xinjiang, China, temporally overlapped the Late Carboniferous collision between the Tarim and the Kazakhstan–Yili plates. Isotopic dating, whole-rock geochemistry, and isotopic characteristics of a suite of calc-alkaline to shoshonitic rocks from Yuximolegai area, in the eastern end of the Awulale Mountains, help define the tectonic evolution of this part of the Tianshan. Zircon U–Pb ages of a quartz diorite and a quartz syenite porphyry are 310.8 ± 2.1 Ma and 284.4 ± 3.6 Ma, respectively. The geochemical and isotopic characteristics of Late Carboniferous igneous rocks, including the quartz diorite, as well as basaltic andesite and K-rich trachyandesite, indicate an evolution from calc-alkaline towards shoshonitic series. This was generated by the partial melting of the hydrated mantle wedge induced by changing subduction angles. The younger quartz syenite porphyry, however, was more likely derived from the partial melting of a thickened crust in a post-collisional setting or the partial melting of hot juvenile basalt in the lower crust during Early Permian delamination of the subcontinental lithospheric mantle. Therefore, the generation of the calc-alkaline to shoshonitic igneous rocks in Yuximolegai area is genetically linked to the geodynamic evolution of the western Tianshan from convergent subduction to collision during the Late Paleozoic.