Geochemistry of Furongshan Granitic Porphyry and Mafic Microgranular Enclaves in Central Zhejiang Province
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    Abstract:

    Objectives: By comparing the geochemistry and isotopic compositions of Furongshan granitic porphyry and mafic microgranular enclaves from central Zhejiang province, the genesis of Furongshan granitic porphyry,the characteristic of source area and the relationship between granitic porphyry and mafic microgranular enclaves are able to be explored,through which further constraints of the tectonic setting, and the geodynamic process of the Late Mesozoic crust-mantle magma interaction in the South China Plate could be revealed.Methods: Based on the field work, through the microscope observation,the whole rock chemical analysis,LA ICP-MS zircon U-Pb dating,Ti-thermometer,Sr-Nd isotopic compositions and Hf isotopes of Mesozoic host granitic porphyry and their mafic microgranular enclaves (MMEs) from Furongshan pluton.Results: Zircon U-Pb dating results show that the weighted mean n(206Pb)/ n(238U) age of Furongshan granitic porphyry is 133.6±1.6 Ma (MSWD=2.1,2σ),and the weighted mean n(206Pb)/ n(238U) age of mafic microgranular enclaves is 130.3±4.2 Ma (MSWD=5.8, 2σ). Their diagenetic ages are basically the same and formed in the early Cretaceous.The Furongshan granitic porphyry are?sub-alkaline with low Ga/Al(<2.6)(10000*Ga/Al=2.19-2.48)and high Na2O(2.75-4.5%),enriched in alkalis (Na2O+K2O=7.93-8.75%).And calculated results based on zircon Ti-thermometry indicate a relatively low magmatic temperature(631-690℃).The Furongshan granitic porphyry has a large range of zircon εHf(t) values (-10.9~-1.1) and exhibits initial [n(87Sr)/n(86Sr)]i ratios of 0.7062 to 0.7078 and εNd(t) values of-5.6 to -4.7,the enclaves have relatively uniform initial [n(87Sr)/ n(86Sr)]i ratios of 0.7071 to 0.7079 and εNd(t) values of -3.8 to -2.8.Conclusions: These petrographical and geochemical characteristics of the Furongshan granitic porphyry suggest I-type granitic affinities rather than A-type granitic affinities.Field and petrography and geochemistry characteristics suggesting Furongshan mafic microgranular enclaves and host granitic porphyry were formed by magma mixing. The source of mafic microgranular enclaves may be derived from the subduction metasomatic mantle,and the Furongshan granitic porphyry was formed by the fractional crystallization after the mixing of ancient K-rich crustal melt and metasomatic mantle melt.The modeling results indicate that the hybrid magma may have been formed by~80% basaltic and ~20% felsic melts.The Furongshan K-rich I type granitic porphyry and the enclaves in the central Zhejiang province may indicate that they were formed in the tectonic setting of weak extension at the earlier stage of the roll-back of paleo-Pacific plate subduction. Acknowledgements: This study was supported by the Deep Resources Exploration and Mining,a Special Project in the Framework of National Key R&D Program of China (No.2016YFC0600404) and the National Science Foundation of China (No. 41873034).

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DU Xiuling, WANG Fangyue, YAN Haiyang, GU Haiou, SUN He, GE Can.2021. Geochemistry of Furongshan Granitic Porphyry and Mafic Microgranular Enclaves in Central Zhejiang Province[J]. Geological Review,67(5):67060002.

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  • Online: September 20,2021
  • Published: September 15,2021