Geochronology, Hf Isotope, Geochemistry and Petrogenesis of the Baimashan Granitic Complex in the Central Hunan Province

In order to thoroughly investigate the tectonic evolution, magmatic activities, and their effects on mineral resource formation during the Early Paleozoic and Early Mesozoic eras in South China, systematic field investigations were conducted in the Baimashan complex located in the central Hunan region. Additionally, zircon geochronological analysis, Hf isotope studies, and geochemical assessments were performed. Zircon LA-ICP-MS U-Pb dating illustrates that the weighted mean ages of the Shuiche mylonitized granite and the Longtan-Xiaoshajiang biotite granodiorite within the Baimashan granitic complex are 409±2 Ma and 211±1 Ma, respectively. These ages correspond to the Late Caledonian and Indosinian magmatic events, respectively. The Caledonian mylonitized granite is characterized by high silica, high alkali and strong peraluminosity, accompanied by a pronounced negative Eu anomaly. This, combined with its enriched Hf isotopic composition [ε_Hf(t)=-13.4 to -5.4], indicates that it is likely a S-type granite formed through partial melting of Paleoproterozoic sedimentary rocks. In contrast, the Indosinian biotite granodiorite exhibits low-silica, high-alkalinity and aluminous, displaying a moderate Eu-negative anomaly with a relatively negative ε_Hf(t) value (-10.3 to -7.4) and an ancient modal age (1.71 to 1.89 Ga). This granodiorite can be categorised as a Paleoproterozoic meta-igneous mixing of partially metasomatised sedimentary rocks remelted to form Ⅰ type granites. The Caledonian and Indosinian superunits of the Baimashan complex may have been formed in a tectonic environment of intra-plate orogeny. There is a strong genetic connection between the Caledonian rocks and the regional W mineralization, as well as between the Indosinian granitic rocks and the Au-Sb-W mineralization in the area. These rocks exhibit significant potential for Au-Sb-W mineralization.