Characteristics And Petrogenesis Of The Dalongshan Granitoids In Anqing Area, Anhui Province

Two alkali-rich granitoid belts are distributed to the north and south of the Yangtze River,respectively.A detailed study on the alkali-rich granitoids would provide important evidence for understanding the Mesozoic magmatism in the Middle–Lower Yangtze River Valley.Dalongshan intrusion,located in the southern end of the alkali-rich granite belt to the north of the Yangtze River,is one of the representative alkali-rich granitoids.so the detailed petrographical,mineralogical and petrochemical study has been carried out on the Dalongshan granitoids in this thesis.The following are major achievements obtained in this thesis:1.Lithologies of the Dalongshan granitoids mainly include syenite,quartz syenite,and alkali feldspar granite.They are similar in assemblage of major minerals.Salic mineral is dominated by alkli-feldspar(>85%of feldspar),plagioclase and quartz,and mafic mineral by amphibole and biotite,its changing from amphibole in syenite to amphibole and biotite in quartz syenite,and then to biotite in alkali feldspar granite.Accessory minerals include zircon,titanite,and magnetite.2.Electron microprobe analysis results show that amphibole includes richerite and ferro richerite,and biotite is magnesian biotite in the Dalongshan granitoids.3.The Dalongshan granitoids are rich in alkali,REE and LILE(e.g.,Rb,Th and K),but poor in HFSE(e.g.,Nb,Ta,Ti and Zr),Sr and P,and have higher ratios of10~4*Ga to Al,with strongly negative Eu anomalies.The features indicates that the Dalongshan granitoids have a pronounced A-type affinity.4.The petrographic,mineralogical and geochemical data presented in this thesis support the formation of the syenite and quartz syenite through slight crustal material contamination combined with intensive fractional crystallization of basaltic magma derived from an enriched lithospheric mantle,and the formation of the alkali feldspar granite through fractional crystallization of syenite.