Geochronology, Geochemistry And Petrogenesis Of Yunshan Bimodal Volcanic Rocks, SE China

Yongtai volcanic region is located in the middle of Zhejiang-Fujian costal NNE Mesozoic magmatic belt, SE China. Where outcrops abundant late Cretaceous bimodal volcanics, and which are emplaced in back-arc extensional settings triggered by the subduction of the Late Mesozoic Palaeo-Pacific Plate beneath the Eurasian continent. The Yunshan volcano is a representative late Cretaceous volcano in the Yongtai volcanic region, and the deposits of it are mainly composed of tuffs, pyroclastic deposits, ash flows and lave flows. These deposits have been divided in three cycles. They rang in compositions from basalt and basaltic andesite to quartz trachyte and aluminous (metaluminous to weakly peraluminous) rhyolite, or to weakly peralkaline rhyolite (comendite), and lack intermediate component (Daly Gap). In this work, various rocks have collected for zircon U-Pb dating and zircon Lu-Hf isotopic, petrologic, mineralogical, geochemical studies. Through these works, we discuss the age and petrogenesis of the Yunshan bimodal volcanics, and also undertaken to elucidate the role of crust-mantle interaction and the mechanisms for the involvement of mantle components during petrogenesis of silicic volcanics.Zircon U-Pb dating provides crystallization ages of100.6-103.1Ma,100.3-96.4Ma and96.4Ma-92.2Ma for Ⅰ-, Ⅱ-, Ⅲ-cycle volcanics, respectively. Age of mafic lavas are about3Ma old than silicic volcanics. Voluminous silicic volcanics erupted in96±1Ma which is the peak time of the Cretaceous eruption. The emplacement of subvolcanic beschtauite indicated the end of Yanshanian magmatism, and its age is89.4±0.9Ma. In comparison with age data of different methods, we consider the previous40Ar-39Ar ages are unfaithful. Because the minerals or whole rocks K-Ar system should be reset in reheating event which occurred with the emplacement of subvolcanic rocks.In the Yunshan bimodal volcanics, volume of mafic lavas are far small than silicic volcanics. These mafic lavas show apparent Nb, Ta negative anomalous, and have high silica, low MgO, Mg#, Cr and Ni. These mafic magmas are derived from partial melting of enriched metasomatized mantle wedge, and have undergone different degree fractional crystallization and crustal contamination. Geochemical differences of cycle-Ⅱ(100.3±0.7Ma) and cycle-Ⅰ (103.1±0.1Ma) mafic lavas indicate that the subcontinental lithospheric mantle (SCLM) of the region is depleted with time.Geologic, geochemical, zircon Lu-Hf isotopic studies all suggest that the silicic magmas are not the product of mafic magma differentiation. The opposite, they are derived from partial melting of crustal materials. Different kinds of silica volcanics have different source and petrogenesis. The silicic volcanics of cycle-I (IR,-100Ma) contain amphibole, biotite phenocrysts and micro-enclaves in rocks, and have low silica,104*Ga/Al ratios, HFSE and high Ba, CaO, A12O3contents. These features are similar to the calc-alkaline I-type granites. They have similar Sr-Nd-Hf isotope and REE pattern to coeval mafic lavas. The parental magmas of these silicic volcanics are derived from the mixing of mantle-derived basaltic magmas and silicic magmas from anatexis of ancient crust, then through fractional crystallization in crustal reservoir to generate cycle-I silicic lavas.Silicic volcanics from cycle-Ⅱ,-Ⅲ, including aluminous (metaluminous to weakly peraluminous) and peralkaline silicic volcanics (AR and PA,96.5-92.2Ma), show the uniform compositions. Their geochemical features are similar to the A-type granites. These volcanics have high104*Ga/Al ratios, SiO2, HFSE contents and very low Ba, Sr, A12O3and CaO contents. Compared to the cycle-I silicic volcanics and mafic lavas, they show the less radiogenic Nd-Hf isotopic compositions. On the basis of their petrographic, geochemical characteristics and comparison with experimental melts and the rhyolites compositions, it is suggested that these rhyolite magmas are derived from partial melting of H2O-poor (meta) felsic igneous rocks in the deep crust. Geochemical variations of major and trace elements indicate the possible fractionation of K-feldspar, calcium-rich pyroxene, Fe-Ti oxides and minor chevkinite, zircon during the mag ma evolution. In these silicic volcanics, aluminous and peralkaline silicic magmas have distinct pre-emptive conditions, the former are "high temperature","H20-poor" and "high fO2", in opposite, the peralkaline silicic magmas are "low temperature","H2O-rich" and "low fO2". The F-bearing fluid phase could help achieve a peralkaline melt during the source rocks partial melting.In PA, the rhyolite sample11YS40has a unique mineral assemblage and lack Na-amphibole phenocrysts. We found that the pre-existing Fe-Ti oxide and hedenbergite phenocrysts had been transformed into aegirine+oxide and aegirine+oxide+fluorite assemblages, respectively. Obviously, the assemblages are the products of the subsolidus reaction of pre-existing phenocrysts and extraneous Na-F-rich fluids. The fluids may be derived from the degassing of the subvolcanic rocks. The reactions indicate that the peralkaline rhyolites could be generated through the subsolidus reaction of highly fractionated aluminous silica magmas and the Na-F-rich fluid.Thus, in the petrogenesis of Yunshan silicic volcanics, underplated mafic magmas play the complex roles. The generations of different type of silicic volcanics are controlled by tectonics and crustal compositions.