Magma Mixing Of Late-Triassic WuLong Pluton In Southen Qinling Orogenic Belt

This thesis report data of petrology, mineralchemistry and geochemistry for the Wulong pluton, and discuss the genesis, source region and geodynamic background of Wulong granite intrusion and its mafic microgranular enclaves (MMEs) respectively. The conclusion as following:(1) Mafic microgranular enclaves in Wulong pluton show disequilibrium mineralogy structure, its essential minerals are more basic than the host granodiorite, and show some feature of mixing of mantle and crustal magma. Plagioclase inclusions with corrosion border in host granodiorite indicate the host magma experienced transitory heating-up process. In the Harker diagram, there is a gap and display a linear trend correlation between mafic enclaves and host rocks. The above-mentioned phenomenon prove that the mafic microgranular enclaves in Wulong pluton were formed by mixing between mafic magma and felsic magma.(2) The Wulong pluton is mainly composed of monzonite granite with K-feldspar phenocrystal and granodiorite. They are weakly peraluminous, calc-alkaline to high-K calc-alkaline rocks, enrichment in LILE and LREE and depletion in HFSE and HREE. Geochemically, they have some characteristics of C-type adakites. They were derived from partial melting of lower crustal mafic magma which had not expericened metasomatism and contamination by mantle peridotite. The mineral in the source region are mainly garnet without plagioclase.(3) Mafic microgranular enclaves are dioritic and characterized by metaluminous and shoshonite. They have higher Mg, Fe, Ti, REE, LILE content and Mg# than host rocks. Their trace element content and ratio has wide range. All of these indicated that the dioritic magma were derived from an enriched mantle which have some connection with subduction fluids. The mantle magma had experenced crystal fractionation and mixed with crustal magma.In conclusion, the geodynamics cource of WuLong pluton genesis is as follows:the Orogen was converting from extension to extrusion at post-collisional setting between Qinling and Yangtze plate. The reduced pressure at the bottom of crust and the heat from underplated mantle magma induced by delamination of thickened crust render the melting of the lower crust, forming the mafic microgranular enclaves at contact part between the crustal and mantle magma and the adakite-like granitoids.