Zircon U-Pb Geochronology, Geochemistry and Petrogenesis of Miocene Syenite in Chazi Area, Tibet

The Chazi syenite mass is a newly discovered Miocene potassic-ultrapotassic intrusive rock in the middle Lhasa block, and its lithology is mainly composed of medium-coarse grained quartz hornblende syenite and porphyraceous quartz syenite. Mafic magmatic Enclaves were found in the syenite body. Petrography, zircon U-Pb dating and geochemical analysis were done, in order to study the petrogenesis of the Chazi syenite. Zircon U-Pb ages of 10.37±0.24 Ma and 11.06±0.39 Ma were obtained for these two rock types respectively, which represent that they were formed in the Miocene Epoch and product of post-collision magmatism in the Lhasa block. The Chazi syenite rocks have relatively high K2O (6.75%-7.39%) and low MgO (1.44%-2.97%), with K2O/Na2O>1, so they are potassic rocks. They have similar characteristics of trace elements to the ultrapotassic rocks: strongly enriched in large ion lithophile elements (LILE) such as Rb, Th, U and K, and light rare earth elements (LREE),relatively depleted in high field strength elements (HFSE) such as Nb, Ta, Zr, Hf, Ti and P, and heavy rare earth elements (HREE), but the contents of Cr (22.7×10-6-64.6×10-6) and Ni (18.9×10-6-46.6×10-6) of the syenite are obviously lower. Compared with porphyraceous quartz syenite rocks, the quartz hornblende syenite rocks have relatively higher SiO2, and lower elements sucn as TiO2, FeOT, MgO, K2O, Cr, Ni, REE and Y. By comprehensive analyses, it is proposed that the Chazi syenite was mainly formed by magma mixing, and produced by the mixing between the ultrapotassic magma that originated from the lithosphere mantle partial melting, and acid magma that originated from the continental crust partial melting. The chemical difference of two rock types is mainly due to the different degrees of magma mixing and ratio of mixed magma. The Chazi syenite possibly resulted from convective thinning or delamination of over thickened mantle lithosphere. © 2020, Editorial Department of Earth Science. All right reserved.