Petrogenesis of granites in Dachen Island, Zhejiang Province: constraints from geochemistry, zircon U-Pb ages and Sr-Nd-Hf isotope

Dachen Island is located in Zhejiang-Fujian Late Mesozoic tectonic magmatic belt.K-feldspar granites, mafic dike and volcanic rock are wide distribution in the Dachen Island.In this study, petrography, major and trace elements, Sr-Nd-Hf isotopes and zircon U-Pb geochronology of the granites are studied systematically.The zircon U-Pb weighted average age shows the magmatic event at 95.9±1.8 Ma, representing the Early Late-Cretaceous.The Dachen Island granite is mainly composed of K-feldspar, quartz, plagioclase and few of biotite.The major and trace elements show that the granite has high SiO₂, K₂O and Ga/Al ratios(2.6~2.8), low CaO, MgO and weakly peraluminous(1.0~1.1 of A/CNK)and highly heterogeneous(96.0~96.9 in DI), belonging to high-K calc-alkaline series.The rare earth element is obvious with a rightward REE distribution pattern and negative Eu anomaly(0.32~0.39).The granite enriches in LILEs(Rb, K, Th, U)at different degress, and depletes relatively to Sr, Ti, Nb and Ta.The I_Sr(0.7092~0.7094), low Nb/U and Ta/U values, and ε_Nd(t)= -7.07~-6.93 of whole rock, ε_Hf(t)= -11.1~-6.7 of zircons indicate that the granite is mainly derived from aluminum enrichment crust in the southeast Zhejiang, and mantle materials inputting to the magmatic process.Combining with the previous study, the Dachen granites were generated by the subduction of the Paleo-Pacific plate in the Late Mesozoic, which resulted in partial melting of the crust materials to form felsic magmas and mixing proportion of mafic material about 35%.Aluminosilicate magmas experienced strong crystallization differentiation and then formed the Dachen Island A-type granite.