Spatial geochemistry variations of 130-135Ma A-type granites from the northern Zhejiang Province, South China: implication for crust-mantle interaction

Abstract:

Mesozoic granites are widely distributed in the Qin-Hang Belt in the Southeast China, but their petrogenesis and geodynamic settings are still matters of dispute. Three granite plutions from Machebu, Shenzhongwu and Daixi in the northern Zhejiang Province, South China were selected for analses of major and trace elements, Sr-Nd-Pb isotopic compositions and zircon U-Pb dating and Hf isotopic composition to reveal its petrogenesis. These plutons have a clear A-type granite geochemical signature (e.g., high SiO₂ (71 to 78 wt.%), total alkalis (Na₂O + K₂O = 7.57 to 9.12 wt.%), rare earth element (total REE = 174 to 519 ppm) and HFSE contents, and mostly high FeO^T/(FeO^T+MgO) ( 0.82 to 0.93) and Ga/Al ratios ( 2.49 to 5.07), suggesting they were genertaed in an extensional tectonic environment. Meanwhile, low CaO and Sr contents and remarkable negative Eu anomaly indicates fractionation crystallization of plagioclase.

Zircon U–Pb dating yieldes a weighted mean ²⁰⁶Pb/²³⁸U age of 131.7 ± 2.2Ma and 130.6 ± 1.2Ma for the Machebu granite, 134.4 ± 2.1 Ma for the Shenzhongwu granite and 131.9 ± 1.2 Ma for the Daixi granite. ε_Hf(t) values of zirons in these plutons gradually increase from south to north from -14.5 ~ -3.5 for the Machebu granite to -1.5 ~ -0.2 for the Shenzhongwu granite and to 5.1 ~ 8.6 for the Daixi granite, suggesting the important role of juvenile material in the magma source from south to north. Combination of variable zirocn ε_Hf(t) values and T_DM2(Hf) ages (1861 - 577 Ma), whole rock ε_Nd (t) (-4.3 ~ -0.5) and the estimated high temperatures (795 - 844 °C) suggests that these granites could have been formed by partial melting of Proterozoic metamorphic basement triggered by asthenosphere upwelling-related basaltic underplating.

Combining our data with the literature data, we suggest that the A-type granites along the zonal distribution from the northern Zhejiang Province maybe corresponding to subduction of the paleo-Pacific plate during 130-135Ma.