Late Indosinian extensional metallogenic events in South China: Evidence from Triassic granites of the Zhilingtou gold deposit in Zhejiang province

The Zhilingtou gold deposit represents the sole large–scale gold deposit in Zhejiang Province, China. The gold orebodies are hosted within the Paleoproterozoic Badu Group biotite–plagioclase gneiss without penetrating the overlying Cretaceous volcanic cover. Notably, the deposit has a well–defined spatial relationship with the concealed biotite monzogranite at a depth of 400 m. Here, we report zircon U–Pb ages, geochemical and Sr–Nd–Hf isotope data for the biotite monzogranite and K–feldspar granite of the Zhilingtou area. LA–ICP–MS U–Pb zircon analyses provide weighted mean ages of 230 ± 1 Ma and 229 ± 1 Ma for the biotite monzogranite and the K–feldspar granite, respectively. Mineralogical and geochemical data regarding zircon saturation temperatures indicate that these two rock types can be classified as metaluminous to peraluminous and are categorized within the high–K calc–alkaline–shoshonite A–type granites, which are associated with an extensional tectonic environment. The biotite monzogranite exhibits lower ε_Hf(t) values and older two-stage model ages, higher ε_Nd(t) values and younger T_DM2 (Nd) ages than the K–feldspar granite. These findings suggest that both rock types likely originated from mixed sources, primarily from the partial melting of a late Paleoproterozoic basement in the lower crust. The A-type granites in the region formed in a post-collisional extensional tectonic environment, emerged after the collision of the South China, Indosinian, and North China blocks during the Triassic period. The resultant crustal extension and thinning facilitated partial melting, which was instrumental in the formation of the granites and established conducive conditions for the evolution of the gold metallogenic system.