Magma mixing in the Yandangshan volcanic-intrusive complex, Zhejiang Province: Evidence from feldspar zoning of the mafic microgranular enclave

The Yandangshan volcanic-intrusive complex in Zhejiang Province is an important component part of the immense Late Yanshanian volcanic-intrusive complex zone in southeastern China. In this study, the authors conducted a detailed investigation of the interior texture and complex zoning patterns of the phenocryst and groundmass plagioclase grains of the mafic microgranular enclave in the central intrusive quartz porphyritic syenite, which is used to constrain the formation of the complexly zoned plagioclase and the corresponding magmatic processes. The plagioclase phenocryst consists of resorbed core, clear mantle and is rimed by K-feldspar. The rounded or embayed core is characterized by sieve texture with An content generally lower than the content of the mantle, which represents plagioclase xenocrysts captured from the host felsic magma. The plagioclase mantle and plagioclase in the groundmass show similar An values and contain inclusions of acicular apatite, which should crystallize from the mafic magma related to the mafic microgranular enclave. The complex feldspar zoning recorded the magma mixing and evolution processes during the generation of the Yandangshan volcanic-intrusive complex. Subsequent to the magma mixing, the eruption induced abrupt decreasing of pressure of the magma chamber and, as a result, the crystallization path moved to the alkali feldspar + liquid field. K-feldspar mantle formed along the phenocryst plagioclase of the quartz porphyritic syenite and the phenocryst and groundmass plagioclase of the mafic microgranular enclave, yielding an anti-rapakivi texture.