The early Paleozoic orogen in South China Block is an intracontinental orogen, and synchronous magmatism (440-390Ma) is mainly acidic with minor intermediate-mafic magmatism. Previous studies suggest that most of the early Paleozoic granites in South China belong to peraluminous S-type genesis while amphibole-bearing I-type granites are subordinate. However, our results indicate that considerable amounts of these early Paleozoic granites have characteristics of both S- and I-type granites. Thus, we propose to divide these granites into two groups: fewer of them are Group A with relatively high εHf(t) values (clustering within -3.0 to +9.0) and εNd(t) values (-5.2 to +1.3) as well as higher initial temperatures at 810-850°C, and most of them are Group B with relatively low εHf(t) values (clustering within -16.0 to -1.0) and εNd(t) values (-13.2 to -4.1) as well as relatively low initial temperatures at 700-830°C. The Xiawan monzogranite and Duntou granodiorite are typical Group A granitoids and yield zircon U-Pb ages of ca. 410Ma. These two granites are characterized by high SiO2 (between 67.59 and 74.87wt.%), metaluminous to peraluminous (A/CNK=0.96-1.48) compositions, and a negative correlation between P2O5 and SiO2. Their biotites belong to magnesium biotites, indicating that they have partial features of either I- or S-type granites. Duntou granodiorites exhibit higher εHf(t) values (clustering within +1 to +8) and εNd(t) values (-3.0 to +1.1) while Xiawan monzogranites show relatively low εHf(t) values (clustering within -1 to +5) and εNd(t) values (-5.0 to -3.7). Group B granitoids are represented by the Miao'ershan-Yuechengling batholith, which are characterized by high SiO2 (between 64.57 and 77.37wt.%), metaluminous compositions (A/CNK=0.90-1.24), and a negative correlation between P2O5 and SiO2. Yuechengling porphyritic amphibole-bearing biotite granites in this batholith contain abundant amphibole, indicating that they are I-type granites. Miao'ershan-Yuechengling batholith also exhibits relatively low εHf(t) values (-12.7 to -1.8) and εNd(t) values (-8.9 to -6.7).Geochemical and isotopic analyses on early Paleozoic granites, mafic enclaves and mafic to intermediate rocks demonstrate that the Group A granitoids including Xiawan and Duntou granites may be generated by AFC processes with interactions between asthenosphere-derived magma and metasedimentary rocks, and the Group B granitoids may be formed by AFC processes with interactions between synchronous basaltic magma and metamorphic basement. The post-collisional delamination and asthenospheric upwelling directly participate in the generation of Group A granitoids but indirectly induce the formation of Group B granitoids.
- Assimilation fractional crystallization (AFC) process
- Early Paleozoic granites
- South China
- Zircon U-Pb-Hf isotopes