Geochemistry Of The Felsic Magmatic Rocks From The Northern Wenzhou Area,Zhejiang,and Implication For Petrogenesis

Since the Late Mesozoic,the large-scale magmatic-tectonic activities occurred in the Southeastern China with a wide distribution of igneous rocks.And the origin and geneses of igneous rocks in South China are controversial during several decades.In this study,the petrology,petrochemistry,zircon U-Pb dating,Sr-Nd-Hf isotopic data and trace element temperature calculation of zircon from the intermediate and acid magmatic rocks in northern Wenzhou are studied.The main conclusions in this study are obtained as follows.The zircon SHRIMP U-Pb dating of the Dongshan granites,acidic volcanic rocks,basalt-andesite and Hesheng granites in the Yandangshan area show that they are formed at 114±1 Ma,116±2Ma,108±5Ma,103±2 Ma,respectively.They are formed during the magmatic activity in the late Early Cretaceous,with granitoids widely generated in southeast Zhejiang.The characteristics of major and trace elements in rocks indicate that,the Dongshan granites have high Si O₂and total alkaline,with low Fe O^T,Mg O,and Ca O,belonging to the peraluminous series and magnesian granite.It is enriched in LREE with negative Eu anomalies,and depleted Ba,Nb and Ta.The Hesheng granite porphyries have high Si O₂,total alkaline and Fe O^T contents,and high A/CNK ratios with low in Mg O and Ca O contents.The Dongshan acidic volcanic rocks have high Si O₂,total alkaline and Fe O^T contents,belonging to the peraluminous series.It is enriched in LREE with negative Eu anomalies,and depleted Nb,Ta and Ti.The basalt-andesite has low Si O₂,high total alkaline,and Fe O^T contents,belonging to the metaluminous series.It is enriched in LREE with no obvious Eu anomalies,and depleted Nb,Ta and Ti,showing difference with acidic volcanic rocks.It means that the influence of mantle source materials during mixing maybe more enormous.The Dongshan acidic volcanic rocks and Hesheng granites have high Fe O^T/Mg O and Ga/Al ratios,similar to geochemical characteristics of the A₂-type granite,indicating that the extension tectonic setting of lithosphere.The values of the Sr-Nd isotopic data of the whole rocks and the Lu-Hf isotopic data of zircon in the studied samples show that they are similar,which are(⁸⁷Sr/⁸⁶Sr)_ivalue(0.7083?0.7094),_{(9)()value(-6.9?-5.5)and1}()value(-6.53?-5.41).The results of the Sr-Nd isotopic data of the whole rocks and the Lu-Hf isotopic data of zircon show that the main components of the Hesheng and Dongshan granites are derived from the partial melting of the ancient crustal source materials of the Proterozoic.In addition,the depleted isotopic components indicate that the proportion of the mantle source components gradually increased during the process of crust-mantle interaction.Zircon saturation thermometer(T_Zr)and Ti-in-zircon thermometer(T_Zircon)are used to estimate the temperature conditions of the evolved magma;and the results show that the magma temperature conditions of the Dongshan granite porphyries(T_Zr=798°C,T_Zircon=792°C,mean values)are lower than that of the Hesheng A-type granites(T_Zr=862°C,T_Zircon=859°C,mean values)and Dongshan acidic volcanic rocks(T_Zr=891°C,mean values).The oxygen fugacity of the Dongshan granite porphyries(?FMQ=1.16?3.47)is also higher than those of the Hesheng type granites(?FMQ=-0.41?1.14).The whole rock Sm/Yb values of every granitoids are low(0.72?2.27).It implies that the granitoids generated under low pressure and high magmatic temperature,and the influence in magma source of mantle source materials gradually increased with the decrease of oxygen fugacity in the meantime.It is proposed that the Cretaceous granites in the Southeastern Zhejiang are mainly derived from the partial melting of the Proterozoic crustal materials,with a mixing of the mantle source materials during the continuously mantle magma upwelling.Accompanied with the paleo-Pacific plate continued a subduction process and the subduction slab rolled back stage at ca.117 Ma,it was transformed into a back-arc extensional environment in the Southeastern Zhejiang.The lithosphere in the Southeastern Zhejiang entered into a new stage of strongly extension,crust thinning,and the upwelling of the asthenosphere material;and partial melting of the lower crust materials occurred,and formed the wide distribution of the Cretaceous intermediate-acidic magmatic rocks.