Geochemistry characteristics and geological significance of clinopyroxene phenocrysts in the 4.8 Ma shoshonite from in Quanshuigou area of West Kunlun Mountains, NW Tibetan Plateau

Cenozoic post-collisional potassic igneous rocks are widely distributed across the Tibetan Plateau, and have been used to constrain the timing and mechanism of plateau uplift and geodynamic evolution. Although these post-collisional magmatic rocks have broadly been studied, their magma source and petrogenesis still remain controversial, and works in the West Kunlun Mountains (Mts.) is rare. There is a suite of 4.8 Ma shoshonite in Quanshuigou area of West Kunlun Mts. The phenocrysts and matrix in this suite are composed of clinopyroxenes, K-feldspar, plagioclase, and olivine. We research the major compositions of the clinopyroxenes phenocrysts by means of Electron microprobe. The clinopyroxene phenocrysts have low TiO₂ and Al₂O₃,but high FeO_T and CaO contents (TiO₂ =0.62-1.07 wt.%, Al₂O₃ =2.8-5.26 wt.%, FeO_T =5.45-8.37 wt.%, CaO= 20.39-22.65 wt.%), and thus are classified as Quad pyroxenes and diopside. These clinopyroxenes have relatively high Mg^# (Mg/(Mg+Fe²⁺) =0.711-0.837). CaO contents are positively correlated with Mg^#, whereas negative correlations exist between FeO content and Mg^#, indicating that the clinopyroxene evolves from Mg-rich toward Fe-rich during magma evolution. Crystals from the shoshonite fall within the field of HKS+KS clinopyroxene, and pyroxene chemistry is used to reveal the nature of the original magma. All analyzed pyroxenes define an alkali basalt as the original magma on the Ca+Na (atomic) vs. Ti diagram. This is confirmed by whole-rock chemistry of igneous clinopyroxene-bearing shoshonite.The clinopyroxene phenocryst plots within the volcanic arc basalt/ocean floor basalt fields on the F1 versus F2 diagram and in the volcanic arc basalt (VAB), within-plate tholeiitic basalt (WPT), and within-plate alkalic basalt (WPA) fields on the TiO₂-MnO-Na₂O diagram. Thus, it appears that shoshonite could be produced in volcanic arc-related environments. Combining that there was an intracontinental environment in the Miocene western Kunlun Mts., the island arc tectonic environment is likely to suggest that the primary magmas of shoshoniteare derived from ancient subducted ocean.