The Geochemical Characteristics And Petrogenesis Of Cenozoic Volcanic Rocks In Tengchong Block

Located in the southeast margin of Qinghai-Tibet plateau,Tengchong block is one of the most important parts of the eastern Tethys tectonic belt.Tectonic activities and strong magmatism have widely taken place in Tengchong block since Cenozoic,and among them the Cenozoic volcanic rocks are great objects for studying the magmatism and the current mantle.In this paper,we have studied major elements,trace elements and Sr-Nd isotopic compositions of the Cenozoic volcanic rocks in Tengchong,the geochemical compositions of olivines and plagioclases in these rocks and their zircon O isotopes to find out their origin,evolution and characteristics of the mantle source in the Tengchong block.These basalts among them are enriched in LILE,LREE,Th,U,Pb,Zr and Hf,depleted in Sr,Eu,and they have relatively high 87Sr/86Sr(0.705964-0.709050)and low 143Nd/144Nd(0.512191-0.512467).Without being significantly assimilated and contaminated during the ascending progress,the basaltic magma indicates an enriched mantle,which possibly resulted from oceanic crust subduction.These olivines have fine crystallines and high CaO contents,low Fo,Ni and Fe/Mn ratios,resulted from the separation and fractional crystallization of olivine in the early stage.The FC3MS of basalts is 0.84-1.17,without significant fractional crystallization of pyroxene,suggests that their mantle source is mainly composed of pyroxenite.These basalts are enriched in Th and have high Th/Nb and Ti/Eu ratios,which related to ocean subduction and silicate melts metasomatism in their mantle source.The geochemical study shows that Tengchong andesites have good affinity to basalts,which mainly derived from basalts'fractional crystallization,with a degree of assimilation and magma mixing.And relatively enriched Sr-Nd-Pb isotope compositions and inhomogenous zircon O isotope compositions(5.22‰-7.42‰)indicated a complicated magma evolving process.