Genesis of the Intrusive Rocks from the Tongling Area during the Yanshanian and Mixing of Three-end-member Magma

The Tongling area is one of the largest copper-gold mineralized areas in China. The genesis of the intrusive rocks in this area is still controversial. The intrusive magmatism in this area has been discussed in terms of petrology, mineralogy, petrochemistry and isotopic geochemistry. Mixing textures of the magma were first discovered in the intrusive rocks from the Tongling area. The intrusive rocks contain a few mafic microgranular enclaves. These features suggest that the magma mixes homogeneously, belonging primarily to chemical mixing. The Sr-Nd isotopic characteristics also reflect the magma mixing. The high-K calc-alkaline-shoshonite rock associations indicate that this area had a thickened continental crust or was an erogenic belt with a mountain root in the early Yanshanian period, and that the magma should be generated as deep as 55 km or below. The rnagmatism is related to the subduction of the Izanagi plate under Eurasia and lithospheric delamination. The delamination of continental lithospheric mantle resulted in adiabatic upwelling of hot asthenosphere and generation of basalt magma by decompressional melting. The basalt magma underplated and heated directly the lower crust to produce syenite magma. The basic magma involeved in the magma mixing is evolved basalt magma. Simulation of magma mixing has demonstrated that the intrusive rocks are primarily a product of mixing of three kinds of end-member magmas-basalt magma, syenite magma and granite magma. Mixing of basalt and syenite magmas in magma chambers of the lower crust can produce gabbro-syenite associations (monzogabbro, pyroxene diorite, pyroxene monzodiorite and monzonite).Diorite-monzonite magma formed in the lower crust ascends to the middle crust and high temperature of the diorite-monzonite magma can make the metamorphic rocks in the middle crust melt partially and generate granite magma. Similarly, mixing of diorite-monzonite and granite magma can produce monzodiorite-granodiorite associations (monzodiorite, monzonite, quartz monzonite, granodiorite).