Partial melting of an ancient sub-continental lithospheric mantle in the early Paleozoic intracontinental regime and its contribution to petrogenesis of the coeval peraluminous granites in South China

The appinite–granite association has been found in various tectonic regimes related to recent subduction, arc-continent or continent–continent collision and post-collision (orogen), and appinites generally originate from recently subduction-modified lithospheric mantle. We conducted a study on a rarely reported appinite–granite association formed in an intracontinental regime, the Zhangjiafang–Qinglongshan complex (ZQC), in which the appinites were derived from an ancient sub-continental lithospheric mantle (SCLM). The ZQC is located in the western Wugongshan domain, and consists of basaltic to intermediate appinites and granitoids. Ten dated samples (including massive and gneissoid granitoids, hornblende gabbro, and diorite) give zircon ²⁰⁶Pb/²³⁸U ages ranging from 444 ± 3 Ma to 452 ± 4 Ma, indicating that these various lithologies were emplaced synchronously. The basaltic appinites show radiogenic ⁸⁷Sr/⁸⁶Sr_i (0.71016–0.71431) and negative ε_Nd(t) (− 6.1 to − 8.9) and zircon ε_Hf(t) (− 4.2 to − 7.5) values. Combined with regional geological background, an origin from the Neoproterozoic metasomatised SCLM can be inferred for the appinites in the Wugongshan domain. The granitoids are peraluminous and almost high-K calc–alkaline to shoshonitic. They exhibit a wide range of isotopic compositions (⁸⁷Sr/⁸⁶Sr_i = 0.70828–0.71857, ε_Nd(t) = − 6.2 to − 10.5, zircon ε_Hf(t) = − 9.5 to − 26.6). Some of the granitoids display the most evolved Sr–Nd isotopic signatures among all the studied lithologies, which are consistent with those of the middle to upper crust, suggesting a pure crustal origin. Other granitoids show relatively mafic composition and less evolved isotopic signature. The intermediate appinites have intermediate chemical compositions between those of the basaltic appinites and granitoids, and similar Sr–Nd isotopic compositions to those of the basaltic appinites that have relatively evolved composition and isotopic signature. The petrographical and the geochemical studies, as well as magma mixing and mingling between mafic magma and felsic magma observed in field work, indicate that the ZQC was formed by several complex magmatic processes including assimilation and fractional crystallization (AFC) of a basaltic magma derived from an enriched mantle, magma mixing and mingling, fractional crystallization of a crust-derived felsic magma. The contribution of the Neopreoterozoic subduction-modified lithospheric mantle to the early Paleozoic granitoids in South China has been poorly constrained due to its evolved isotopic signatures (such as negative ε_Nd(t) and ε_Hf(t) values, high initial ratio of ⁸⁷Sr/⁸⁶Sr) and few studies on the early Paleozoic basaltic–felsic rock association. Our work here suggests that the Wugongshan appinite–granite association was formed in an intracontinental orogen, and the ancient enriched mantle may have had a significant chemical contribution to the formation of some of the Paleozoic peraluminous granites in South China.