Constraints from zircon U-Pb ages, O and Hf isotopic compositions on the origin of Neoproterozoic peraluminous granitoids from the Jiangnan Fold Belt, South China

Zircon textures and micro-chemical compositions precisely record the origin and petrogenesis of granitoids, which are crucial for evaluating crustal growth and reworking, thermal and geodynamic evolution. Zircons in peraluminous granitoids from the three largest 820 Ma complexes (Guibei, Yueyang and Jiuling) in the Jiangnan Fold Belt in South China are used to constrain their sources and petrogenetic processes. Zircons in the Guibei granitoids have complex internal structures. Nearly all magmatic and inherited zircons have similar epsilon Hf (-6.8 to +5.6) and delta O-18 values (8.8-11.6 aEuro degrees) and dominantly lie between epsilon Hf evolution vectors for a crust created between 1.7 and 2.1 Ga, suggesting that the Guibei granitoids were produced by partial melting of recycled heterogeneous supracrustal material. However, the Yueyang granitoids contain zircons with high epsilon Hf (-0.5 to +9.7) and relatively low delta O-18 values (5.9-8.4 aEuro degrees) and two-stage model ages of 1.1-1.8 Ga, and thus may have been formed by melting of mafic rocks from the lower crust. The Jiuling granitoids and their enclaves contain more complex zircons with more variable epsilon Hf (-7.2 to +9.7) and delta O-18 values (7.0-10.6 aEuro degrees), and lie along the mixing trend between the above-proposed infracrustal and supracrustal granitoids. Therefore, the Neoproterozoic peraluminous granitoids in the Jiangnan Fold Belt were produced by melting and mixing of continental crust. Compared with extremely low (a parts per thousand currency sign4 aEuro degrees) and negative delta O-18 values of Neoproterozoic igneous zircons formed in its northern active continental margin, the high delta O-18 peraluminous granitoids in the southeastern Yangtze Block are considered to have been formed by melting of hydrothermally unaltered continental crust triggered by asthenosphere upwelling in the Nanhua back-arc basin.