Characteristics And Origin Of The Jinziwei Pluton,Tongling,Anhui Province

The Jinziwei pluton,located in the eastern part of the Tongling area,is closely related to the molybdenum-tungsten mineralization,and its geological characteristics are special in this area.A detailed field investigation,petrology and mineralogy study has been carried out on the Jinziwei pluton,with focus on an analysis of the source and evolution of the pluton.Rocks of the Jinziwei pluton are porphyritic,containing5⁸%phenocrysts?mainly plagioclase,K-feldspar,quartz,hornblende and biotite?in a fine to medium-grained granular matrix of quartz?20²5%?,plagioclase?40⁴5%?,K-feldspar?15²0%?,hornblende?8¹0%?,and locally biotite?1³%?.Accessory minerals include sphene,zircon,apatite,and magnetite.Electron probe analysis shows that the plagioclase phenocrysts are oligoclase and andesine,whereas the plagioclases in the matrix are labradorite and bytownite.Hornblendes are magnesiohornblende,and biotites are magnesian biotite.Petrographic observation shows that there are a large number of inequilibrium textures in the Jinziwei pluton,such as rounded quartz phenocryst,poikilitic quartz and poikilitic K-feldspar,short prismatic and acicular apatite,and reverse zoning of plagioclase,indicating magma mixing occurred.On the hornblende Al₂O₃-TiO₂ and biotite?FeO)/??FeO+MgO?-MgO diagrams,all spots of the Jinziwei pluton lie in the area of mixture of crust-and mantle-derived magma,providing further support for the idea that the Jinziwei pluton was formed by magma mixing.These lines of evidence indicate that the Jinziwei pluton was probably generated by mixing of mantle-derived magmas and crust-derived melts.Additionally,the formation depth,temperature,oxygen fugacity,and water content of the Jinziwei pluton are 4.09km,751?,1.77??NNO value?,3.99%,respectively,estimated using electron probe data of plagioclase,hornblende and biotite.Compared with the granodiorite associated with copper-gold deposits in the Tongling area,the oxygen fugacity and water content of Jinziwei pluton are lower,which may be one of the reasons for the mineralization difference of granodiorite magma in the Tongling area.