The Petrogenesis Of Shizigang Mineralized Metasomatite In Zhenghe, Fujian Province

The Shizigang metasomatite,the southwestern part of tieshan pluton,is located in the Zhenghe county of northern Fujian.Tieshan pluton is stretching along the dongfeng-tieshan-zhangyuanan and controlled by the NE Zhenghe-Dapu fault.Shizigang metasomatite is one of the two Indosinian syenites in the southeastern China.The Shizigang pluton was regarded as syenite.But after our recent field investigation and drilling,we find that it undergone intense copper-lead-zinc mineralization. Exposeing cryptoexplosive breccia and subvolcanic rocks,the pluton experienced intense fluid metasomatism(including propylitization, beresitization, potassic alteration and silicification),but there are not typical syenite. Therefore, the petrogenesis of the ore bearing rocks in Shizigang is still a controversy between syenite and syenite metasomatic rock caused by potassic fluid.Through detailed minerals,geochemistry and geochronology study,we find that the "pluton" breccias,the light colored parts(the potassic veins and the controversial syenites) show similar geochemistry and geochronology characteristics,as well as same dark minerals.The K-feldspars in the light colored parts(including potassic veins and the controversial syenites) are all microcline with the temperature of structure of 200-370? which is lower than the solidus temperature of the intermediate-acid rocks(650?-925?).Therefore,this is an important evidence that the Shizigang metasomatite is formed by metasomatism.The trace elements characteristics of zircons in different types of rocks in the Shizigang metasomatite are similar to that of granite.The zircon LA-MC-ICP-MS ages are 250-255Ma, indicating that there are Indosinian magmatic events in Shizigang area.The ?Hf(t) of zircons in Shizigang metasomatite change in a big range, so that the zircon of the primary magmas are formed by magma mixing.However,the positive ?Hf(t) indicates that the mantle components participated in the magmatic events. The zircon two stage model age ranges from 1.2Ga to 1.86Ga,which shows that the source are mainly meso-paleoproterozoic strata.As most of the ages are younger than that of the basement of Cathaysia block,so the mantle components participated in the process of magmatic activity.