Origin of Late Jurassic high‐K felsic volcanic rocks and related Au mineralization in the Dongyang deposit, central‐eastern Fujian, SE China, and its tectonic implications

The newly discovered Dongyang low-sulphidation epithermal Au deposit is located in central-eastern Fujian Province, coastal SE China. Ore bodies occur in Late Jurassic rhyolite porphyry, rhyolite, and dacitic crystal tuff lava. In this study, rhyolite and dacitic crystal tuff lava are dated to 151.9 +/- 0.4 Ma and 158.3 +/- 0.4 Ma, respectively, and belong to the second member of the Nanyuan Formation. They are high in K2O (6.91-3.81 wt%) and Na2O/K2O (0.55-0.02), strongly peraluminous (A/CNK = 2.11-1.67) and high-K calc-alkaline or shoshonitic. They contain low n-ary sumation REEs (248-125 ppm), are enriched in Rb, K, Au, and depleted in Ba, Sr, P, Zr, Hf, and have slightly negative Eu anomalies (delta Eu = 0.94-0.56), suggesting that they are weakly fractionated felsic volcanic rocks. The zircon epsilon Hf(t) values (-11.2 to -5.4; two-stage model ages = 2,650-2,128 Ma) suggest that they were mainly derived from Palaeoproterozoic crust. Lead isotope compositions also imply a dominantly lower crustal source. Detailed geological characteristics and elemental and isotopic data suggest that the oxidizing and weakly fractionated nature of the Dongyang high-K felsic magma may have played a key role in Au mineralization. Integrating new and published data on the tectonic evolution, we suggest that the Late Jurassic Au mineralization and its causative magmatism actually extended to the SE China coastal area. The Dongyang rhyolite and dacitic crystal tuff lava may have been generated from partial melting of the crust caused by underplating of mantle-derived magmas in an extensional environment. Regional extension may have been related to the NW-directed roll-back of the Palaeo-Pacific Plate beneath the South China Block.