Petrogenesis and Tectonic Setting of the Guantian and Shangbao Granodiorite, Southern Zhuguangshan Complex, China: Evidence from Zircon U-Pb-Hf Isotopes, and Whole-Rock Geochemistry and Sr-Nd Isotopes

The southern Zhuguangshan complex is one of the most important sources of granite-hosted uranium deposits in the Nanling region and consists predominantly of Caledonian (Cambrian–Silurian), Indosinian (Late Permian–Triassic), and Yanshanian (Jurassic–Cretaceous) granitoids. However, the Caledonian plutonic rocks have not yet been systematically investigated. In this study, we present new LA-ICP-MS zircon U–Pb ages and Hf isotopes, together with whole-rock elemental and Sr–Nd isotope data, for the Guantian and Shangbao granodiorites from the southern Zhuguangshan complex, in order to constrain their petrogenesis and tectonic setting. Zircon U–Pb dating yields emplacement ages of ca. 431.5 and 442.3 Ma for the Guantian and Shangbao plutons, respectively. Both plutons are characterized by high Al2O3 (14.35–14.77 wt.% and 13.55–14.38 wt.%) and total alkali (Na2O + K2O = 6.22–6.82 wt.% and 6.46–6.85 wt.%) contents, with A/CNK ratios <1.0, and are classified as metaluminous I-type granites. They show enrichment in light rare earth elements (LREE), K, Th, and U, coupled with marked depletion in Ba, Sr, Nb, Ta, P, and Ti, and exhibit pronounced negative Eu anomalies. These geochemical signatures indicate fractional crystallization of Ti-bearing minerals (e.g., rutile and ilmenite) and plagioclase. The granodiorites have initial 87Sr/86Sr ratios of 0.70595–0.71227, negative εNd(t) values (−7.8 to −4.1), and negative εHf(t) values (−8.7 to −0.4), pointing to a dominantly continental crustal source. Their average Nb/Ta (11.42) and Th/U (5.07) ratios are also consistent with typical continental crust. Based on the lithological assemblage and geochemical characteristics, we propose that the Guantian and Shangbao granodiorites were formed in a post-collisional extensional setting.