Geochemistry and Sr-Nd-Pb-Hf isotopic composition of the Donggou Mo-bearing granite porphyry, Qinling orogenic belt, central China

摘要

The eastern Qinling belt is characterized by widespread Mesozoic post-orogenic magmatism and abundant Mo-(Au-Ag) polymetallic mineralization. Most Mo deposits in this belt are genetically related to Mesozoic granitoids. The tectonic context of this close spatial and temporal relationship is still debated. This study reports U-Pb ages and Hf isotopic composition of zircons, major and trace element and Sr-Nd-Pb isotopic composition of the Donggou granite porphyry, host rock to one of the important Mesozoic Mo deposits in this orogen. Based on geochemical results, the Donggou granite porphyry is a silica-supersaturated, high-K metaluminous A-type granite showing enrichment in light REEs, depletion in middle REEs and significant negative Eu, Ba, Nb, Sr, P, and Ti anomalies. Negative initial ε_(ND) values of — 17.0 to —13.2 for whole-rock and negative initial ε_(Hf) values of — 19.9 to —7.8 for zircon suggest that the magma was derived from a mixture of Archaean/Proterozoic crastal rocks and mantle-derived or newly added crust. Its Pb isotopic composition is similar to the lower crust of the North China block, but different from superjacent country rocks (Xiong'er and Taihua Groups). Zircon U-Pb dating yields a late Mesozoic emplacement age of 118-117 Ma, identical with the third episode of Mo mineralization in the eastern Qinling-Dabie belt. We postulate that the Donggou Mo-related porphyry granite formed by reworking of North China lower crust with significant input of juvenile material. The magmas formed in an extensional tectonic setting, induced by lithospheric thinning and asthenospheric upwelling beneath eastern China during Cretaceous time.