Petrogenesis and Metallogenic Implications of A-type Granites in the Mid–Late Jurassic Dayishan Complex, Southern Hunan Province, South China

The petrogenesis and genetic link to polymetallic mineralization of the granites in the Dayishan complex, southern Hunan province remain debated. Here, we present an integrated study on the petrology, zircon U-Pb ages and whole-rock geochemistry for this complex. Our findings indicate that the southern complex consists of (amphibole-bearing) biotite granites and muscovite granites emplaced at 153–151 Ma, and the central and northern complex consists of two-mica granites and tourmaline-bearing muscovite granites, respectively with the former emplaced at 164 Ma and the latter at 150 Ma. The (amphibole-bearing) biotite granites have SiO₂ contents of 68.0–73.8 wt% and are enriched in alkalis and rare earth elements and depleted in Sr and Ba. They display Zr + Y + Ce + Nb > 350 ppm and 10000 × Ga/Al > 2.6 along with high zircon saturation temperatures (821–883°C). The two-mica granites and (tourmaline-bearing) muscovite granites have high SiO₂ (74.4–77.3 wt%) and low Ga/Al, Zr + Nb + Ce + Y, K/Rb, Zr/Hf, and Nb/Ta along with low zircon saturation temperatures (709–817°C). Geochemical characteristics suggest that the (amphibole-bearing) biotite granites are A-type granites generated through shallow dehydration melting of early Paleozoic granitoids, and that the two-mica granites and (tourmaline-bearing) muscovite granites are fractionated A-type granites produced through fractionation crystallization from the (amphibole-bearing) biotite granites accompanied by fluid fractionation.