Petrogenesis and Tectonic Setting of High-Mg Volcanic Rock Xenoliths in Jianshui Area, Southeast Yunnan, China

It reports for the first time the presence of a suite of high-Mg volcanic xenoliths in Jianshui area of Southeast Yunnan, western Yangtze platform, China, which provides new insights into mantle plume activity in the platform during the Permian. Zircon U-Pb dating, geochemistry and petrology of the xenoliths were studied to assess the petrogenetic origin and geodynamic setting of these high-Mg volcanic rocks in this paper. The high-Mg volcanic xenoliths are porphyritic texture and contain only large phenocrysts of olivine, which show as dense mass, develop as lenticulars in the Permian Emeishan basalts in Jianshui area, Southeast Yunnan. The igneous zircons from the volcanic xenoliths yield a weighted age of ca. 259±2 Ma that is interpreted to be the formation age of the magmatic protolith, which is the same as the host rocks of the Emeishan basalts. The volcanic xenoliths are characterized by low SiO₂, moderate TiO₂ and high Mg^#. All the volcanic xenoliths are enriched in LREE but depleted in HREE. The geochemical characteristics of the xenoliths show that they belong to sub-alkali basalts and intra-plate tholeiitic basalts, suggesting that the primary magma of the high-Mg volcanic rock is likely produced by low partial melting of garnet lherzolite. The original magma may have undergone the process of fractional crystallization of olivines and clinopyroxenes. The original magma has not been affected obviously by crustal material contamination in the emplacement and uplifting. The high-Mg volcanic xenoliths may origin from riched-mantle and may be the product of the main stage of the mantle plume activity. All above indicates that there was a mantle branch developed in the study area in Late Permian. It is proposed that several mantle branches have not only ascending motion but also lateral movement while the thermal melting mantle plume arrives at the boundary of mantle and crust.