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

Abstract:

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)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 (εHf(t)=−6.0εHf(t)=−6.0 to −0.7). However, rocks from two ferroan granitic bodies (Huangshitan and Jiuligang) have more depleted Hf isotopes, with ε_Hf(t) ranging from −1.9 to 5.9. The whole-rock Nd isotopes of the ferroan granites (εNd(t)=−6.5εNd(t)=−6.5 to −3.2) are slightly more depleted than those of magnesian granitoids (εNd(t)=−8.8εNd(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 ²⁰⁶Pb/²⁰⁴Pb, 15.6–15.7 for ²⁰⁷Pb/²⁰⁴Pb, and 38.5–39.0 for ²⁰⁸Pb/²⁰⁴Pb). 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 ∼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 ∼135 Ma. This study suggests that a change in the tectonic regime occurred at ∼135 Ma in NWZJ, which may have been triggered by the rollback of the subducted Paleo-Pacific Plate.

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