Features and Geological Significance of the Plagioclases from the Intrusive Body in Xiangride Hydrological Station, Qinghai

With mafic enclave developed, the intermediate-acidic magmatic rocks are widely distributed in Xiangride area, Qinghai province. They have high content of plagioclase, in which plagioclase zoning texture is common. Based on the field geological investigation and petrography study, the technology of EMPA is used to study the different types of plagioclases and the zoning texture of intrusive body in Xiangride Hydrometric Station for retrieving the magma evolution process, and then the genesis of plagioclase zoning has been discussed. The results show that the plagioclases have normal zoning and oscillation zoning textures. And the mineral crystallization depths of quartz diorite and enclave are both mainly 12km, whereas the granodiorite is 9km. The erosion velocity based on crystallization depths is 0.04km/Ma, and the uplift velocity during 20Ma is 0.6km/Ma with 3.0km/Ma during 3.6Ma. Besides, the zoning of plagioclase is caused by strong metasomatic replacement of Ca²⁺ ⇔ Na⁺, Al³⁺ ⇔ Si⁴⁺ and weak metasomatic replacement of Fe³⁺ ⇔ Si⁴⁺. It is concluded that the evolution of these rocks from the Xiangride Hydrometric Station is not a simple emplacement mechanism, but the alternative emplacement and repeated mix of mafic magma and felsic magma.