Early Paleozoic crust–mantle interaction and lithosphere delamination in South China Block: Evidence from geochronology, geochemistry, and Sr–Nd–Hf isotopes of granites

Abstract

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.