Paleoproterozoic S- and A-type granites in southwestern Zhejiang: Magmatism, metamorphism and implications for the crustal evolution of the Cathaysia basement

摘要

Paleoproterozoic granites and metamorphic rocks of the Badu complex, the oldest rocks found in Cathaysia Block, provide a window for investigating the crustal evolution of the Cathaysia basement. SHRIMP and LA-ICP-MS zircon U-Pb ages of five representative granite samples from Jinluohou (Lizhuang) and Jingju granitic complexes indicate that the magmatism took place during Paleoproterozoic, and overprinted by early Mesozoic thermal event. The Jinluohou garnet-bearing biotite granite (1878±28Ma) has high A/CNK (1.28-1.42), low contents of P_2O_5 and low FeO~T/(FeO~T+MgO). It contains a large amount of biotites and garnets with minor inherited zircons but no primary muscovites, showing the S-type granite affinity. It was formed at low water fugacity and high Zr saturation temperature (792-809°C). The Jingju medium-coarse grained K-feldspar granite (1861±35Ma), Jingju medium-fine grained K-feldspar granite (1849±30Ma) and Jinluohou gneissic granodiorite (1877±10Ma) can reach strongly peraluminous (A/CNK=0.97-1.26), are enriched in alkalis and HFSE (Zr, Nb, Ce, Y) and have high Ga/Al and FeO~T/(FeO~T+MgO) ratios. Their high Zr saturation temperatures (840-854°C, most of them are higher than 830°C) together with high Rb/Nb and Y/Nb ratios suggest that they belong to the A_2 subgroup of A-type granites. The ages of inherited zircon cores from Jinluohou garnet-bearing biotite granite (2088-2929Ma) reveal the existence of the Archean basement, and the Hf and Nd isotopes and chemical compositions suggest that these S- and A-type granites were originated from the Paleoproterozoic and Archean crustal sources with different proportions of input from juvenile crust materials. The Paleoproterozoic S- and A-type granites intruded simultaneously, and formed under intraplate extensional geodynamic setting, which is an important event of episodic crustal growth and reworking. The age of Jingju porphyritic quartz monzonite (226.2±1.4Ma) and the lower intercept age of Jinluohou gneissic granodiorite, Jingju K-feldspar granite (224-231Ma) may indicate another important thermal overprinting event in this region.