The Formation Mechanism And Tectonic Setting Of Yanshanian Ore-Related Granite Porphyries In East Qinling Area

The East Qinling molybdenum belt located in southern margin of the North China Block and North Qinling belt, the molybdenum mineralization in this area is closely related to the hypabyssal intermediate-acidic granite porphyries, however, the formation ages, formation mechanism and tectonic setting of these granite porphyries have been controversial. In this paper, the multiple typically ore-related granite porphyries which located in southern margin of the North China Plate and North Qinling belt were selected to study:incuding Shijiawan, Heishancun, Mulonggou granite porphyries which located in southern margin of the North China Plate and Nantai, Taoguanping, Xigou, Gaogou, Niujiawan, Yaozhuangcun granite porphyries which located in the area to the west side of Mangling pluton in North Qinling belt, the detailed field geology, petrography, geochronology and geochemistry of these granite porphyries will be systematically studied.The LA-ICP-MS zircon U-Pb ages of the Shijiawan, Heishancun, Mulonggou granite porphyries which located in southern margin of the North China Plate are 140.9±1.6Ma, 148.6±1.2Ma,140.4±1.2Ma respectively, and Taoguanping, Xigou, Gaogou, Niujiawan, Yaozhuangcun granite porphyries which located in the area to the west side of Mangling pluton in North Qinling belt are 147.1±1.1Ma,147.2±1.3Ma,148.5±1.2Ma,143.95±0.91Ma, 147.1±1.1Ma respectively, the ages of granite porphyries are between 140.4±1.2Ma and 148.6±1.2Ma which belong to the Late Jurassic-Early Cretaceous, it is similar to the formation ages of Yanshanian granitoids in the East Qinling which is the product of tectono-magmatic activities.Geochemical studies suggested that the granite porphyries are characterized by high in SiO2 (64.70%-75.20%) and alkali (K2O+Na2O=6.78%-9.79%), showing that they are I-A type granite which belong to metaluminous or weakly peraluminous and high-K talc-alkaline or shoshonite series. Most of the REE patterns are characterized by enrichment of LREE, and they have slightly negative to positive Eu anomalies. The granite porphyries are enriched relatively in LILE (Cs, Rb, Ba, K, Sr), and depleted in HFSE (Nb、Ta、Zr、P、Ti). On the zircon Hf isotopic composition, the source of granite porphyries involve relatively complicated component which is derived from the continental crust and mix with the juvenile component, the source of magma exists in the deep crust which has a depth of more than 40Km.The comparative studies of the geochronology, geochemistry and magmatic source indicated that,1.The Yanshanian granite porphyries and the plutons in the East Qinling have the similar magmatic source and formation mechanism.2.The Yanshanian granitoids in the East Qinling and Dabie area have the similar tectonic setting of magmatic activity, these granitoids are the product of magmatic activity in different area.The comprehensive analysis indicated that, the tectonic setting of Yanshanian magmatic activity in Qinling-Dabie orogenic belt is related to the huge intra-continental subduction zone where the southern North China Block underthrust beneath the Qinling from north to south, along this east-west intra-continental subduction zone, a large scale of Yanshanian granitoids which are distributed from the east to the west formed in Qinling-Dabie orogenic belt accordingly.