Geochemistry Of Some Mo-bearing Granites In East Qinling-Dabie Mo Orogenic Belt

The East Qinling-Dabie orogenic belt is known as the most typical porphyry or porphyry-skarn-type Mo ore districts of China, systematic study of these Mo districts will help us know better about the mineralization events that developed in the middle part of China since the late Mesozoic. Detailed petrography,petrogeochemistry, isotope chronology and isotope geochemistry work was carried out on some granite of the typical Mo districts, which help us discuss the chronology framework, ore-forming materials and the geodynamic background of the Mo districts in the East Qinling-Dabie orogenic belt.Nannihu and Shangfanggou granites are characterized by high SiO2 and high alkaline contents which apt to shoshoritic,with A/CNK>1.1, considered as peraluminous.Shanggusi granite is also characterized by high SiO2 and high alkaline contents which apt to calc-alkaline and shoshoritic, with A/CNK=1.0-1.1, considered as weak peraluminous. Shapinggou granite is characterized by sub-high SiO2 and high alkaline contents which apt to shoshoritic, with A/CNK little low than 1 to 1.1, considered as Metaluminous to weak peraluminous. The amount of total rare earth elements(ΣREE) are low in four granites, with obvious fraction between light rare earth elements(LREE) and heavy rare earth elements(HREE). The negative of Eu anomaly showed slightly in Nannihu and Shangfanggou granites, while strongly in Shanggusi and Shapinggou granites. The distribution of trace elements in four granites are similar,which show high content of Rb and low content of Ba, Ti.The analysis results of zircon U-Pb show that the age of Nannihu granite is 144.3Ma, Shangfanggou granite is 147.5Ma, Shanggusi granite is 126Ma, Shapinggou granite is 111.3Ma. Combined with the predecessor’s research data of chronology in the East Qinling-Dabie orogenic belt, we can know that there existed two major periods of magmatism and mineralization during late Mesozoic. The former period took place from Late Jurassic to Early Cretaceous(148-138Ma), outcropped at the western part of East Qinling-Dabie orogenic belt including granites like Nannihu Shangfanggou Jinduicheng; the later period took place during Early Cretaceous(131-112Ma), including granites like Shanggusi Shapinggou Donggou, which mainly found in the North of Dabie area.Whole rock Sr-Nd isotope data of 4 granites shows an enrich signature with 87Sr/86Sr=0.707-0.713, εNd(t)=-10～-14, demonstrate that these granites are mainly generated from crust.The initial εHf(t) values of 4 granites show a wide variety range from-17.6 to 5, suggesting that there admixture of juvenile or mantle material in the magma.The values(average) of Ce4+/Ce3+ in zircon is 305.9,17.9,79.8,120.4, corresponging to granite Nannihu Shangfanggou Shanggusi and Shapinggou. Compared with the data form porphyry copper in other area, the Ce47Ce3+ values is low, suggesting that there might a lower oxygen fugacity during the mineralization in the East Qinling-Dabie orogenic belt.Mesozoic tectonic regime in eastern China could have great influenced in the building the East Qinling-Dabie orogenic belt. During Mid-Late Jurassic,magma that partial melting from thickened lower crust went upwelling through the weak tectonic belt to the shallow part and form the deposits like Nannihu and Shangfanggou,dued to the tectonic framework transformation in eastern China. During Late Jurassic to Early Cretaceous, large-scale lithospheric thinning prevailed in eastern China,lithosphere became thinning and mantle materials went upwelling, leading to strong crust -mantle interaction and stretching in lower crust, forming deposits like Shanggusi and Shapinggou.