Petrogenesis of the Late Mesozoic Magnesian and Ferroan Granites in Northwest Zhejiang, Southeast China, and Their Implications

Northwest Zhejiang Province (NWZJ) is located in the southeastern Lower Yangtze River Belt, southeastern China. Here we document the occurrence of both magnesian (149-131 Ma) and ferroan (162-121 Ma) granitoids in NWZJ. The magnesian granitoids are calc-alkalic peraluminous in composition, with a wide range of SiO2 (58-72 wt%) contents. They have high K2O/Na2O, Sr/Y, and (La/Yb)(N) ratios, with insignificant Eu anomalies, whereas the calc-alkalic peraluminous ferroan granites have high SiO2 (76-77 wt%) contents, Fe indices (FeO*/(FeO*+MgO)), and Ga/Al ratios. The ferroan granites also have low Ce/Pb and Nb/U ratios, with strong Ba, Sr, and Eu negative anomalies. Most of the rocks have similar zircon Lu-Hf isotopes (epsilon Hf(t)=-6.0 to -0.7). However, rocks from two ferroan granitic bodies (Huangshitan and Jiuligang) have more depleted Hf isotopes, with epsilon(Hf)(t) ranging from -1.9 to 5.9. The whole-rock Nd isotopes of the ferroan granites (epsilon Nd(t)=-6.5 to -3.2) are slightly more depleted than those of magnesian granitoids (epsilon Nd(t)=-8.8 to -5.1). In addition, all ferroan granites show similar and high present-day whole-rock Pb isotopic ratios (18.3-18.8 for Pb-206/Pb-204, 15.6-15.7 for Pb-207/Pb-204, and 38.5-39.0 for Pb-208/Pb-204). On the basis of published data and our new results, we propose that the magnesian granitoids were generated by partial melting of lower-crustal materials, whereas the ferroan granites were derived from a similar source but some more-depleted materials were added into their source after approximate to 135 Ma. The water contents of the magma may have played an important role in determining the different geochemical affinities of the felsic magmatism. The felsic magmatism occurred under an extensional setting during the period 162-121 Ma. The extension of the lithosphere was further enhanced and followed by upwelling of asthenospheric mantle after approximate to 135 Ma. This study suggests that a change in the tectonic regime occurred at approximate to 135 Ma in NWZJ, which may have been triggered by the rollback of the subducted Paleo-Pacific Plate.