Neoproterozoic peraluminous granitoids in the Jiangnan Fold Belt: Implications for lithospheric differentiation and crustal growth

Neoproterozoic magmatism in the Jiangnan Fold Belt of South China is characterized by voluminous ca. 820 Ma peraluminous granitoids which show a large range of chemical compositions. The Meixian dioritic and the Lantian granitic intrusions from the central Jiangnan Fold Belt were emplaced at 821 ± 6 Ma and 823 ± 7 Ma, respectively. The Meixian diorites are mainly composed of amphibole (10-40%), biotite (5-30%), plagioclase (30-60%) and quartz (5-10%). They have low SiO2 (59.42-63.15 wt%), moderate MgO (2.96-3.56 wt%) and Na2O (3.51-4.39 wt%), and relatively high V (102-137 ppm) and Cr (47.1-58.7 ppm). They show nearly zero εNd (−0.6 to −0.1) and high εHf values (+2.3 to +10.6). These mineralogical and geochemical features suggest that mantle-derived melts were involved in their petrogenesis. Modeling calculations reveal that the Meixian diorites were formed by fractional crystallization of mantle-derived melts associated with 10-30% contamination by the country rocks. However, the Lantian granites are fractionated I-type granites that are composed of plagioclase (10-25%), quartz (30-40%), K-feldspar (15-20%), biotite (5-15%) and amphibole (5%). They have high SiO2 (72.3-80.2 wt%), moderate K2O (1.58-4.11 wt%) and Na2O (2.78-3.94 wt%), and low MgO (0.50-0.80 wt%) and CaO (0.16-0.50 wt%). Their negative εNd (−2.3 to −1.4) and positive εHf values (+0.4 to +9.9) are similar to those of the Neoproterozoic arc-affinity mafic rocks in the region, suggesting that the Lantian granites were derived from the mafic juvenile crust. The Neoproterozoic igneous rocks in the Jiangnan Fold Belt are mainly composed of peraluminous granites with minor diorites. The large volume of peraluminous granitoids are generally partial melts of the continental crust, whereas the limited diorites are differentiation products of the basaltic magmas. The petrogenesis of the granitoids in the Jiangnan Fold Belt suggests that basaltic magma underplating triggered crustal anatexis during or after collision between the Yangtze and Cathaysia Blocks.