Generation of Late Mesozoic Qianlishan A2-type granite in Nanling Range, South China: Implications for Shizhuyuan W–Sn mineralization and tectonic evolution

Abstract

The Late Mesozoic Qianlishan granitic complex in the western Nanling Range, South China is associated with the Shizhuyuan giant W–Sn–Mo–Bi polymetallic deposit. It mainly consists of three phases of intrusions, P-1 porphyritic biotite granite, P-2 equigranular biotite granite and P-3 granite porphyry. All three phases of granite contain quartz, plagioclase, K-feldspar and Fe-rich biotite. They have geochemical affinities of A-type granites, e.g., high FeOT/(FeOT+MgO) ratios (0.84–0.99), total alkali (Na2O+K2O, 7.50–9.04wt.%), high Ga/Al ratios (10,000*Ga/Al>2.6) and high Zr+Nb+Y+Ce concentrations (>350ppm). High Y/Nb ratios (>1.2) suggest that the Qianlishan complex belongs to A2-type granite. Zircon U–Pb ages indicate a short age interval decreasing from 158–157Ma, to 158–155Ma and to 154Ma for the P-1, P-2 and P-3 granites, respectively. These ages are similar to the mineralization age of the Shizhuyuan tungsten polymetallic deposit, within error. The Qianlishan granites were generated at low oxygen fugacity conditions based on the low values of zircon Ce4+/Ce3+ ratios (1.53–198) and significantly negative Eu anomalies (EuN/EuN*, 0.03–0.13) in apatite. New zircon εHf(t) values for the P-3 granite range from −13.0 to −4.4, similar to those previously obtained for the P-1 and P-2 granites. Both the granite and apatite grains therein are characterized by high F but low Cl concentrations, suggesting the influx of a high F/Cl component. The P-2 granites especially contain higher F contents (1840–8690ppm) and W (7–158ppm) and Sn (6–51ppm) concentrations and with stronger evolution features. Positive trends between F and W and Sn of Qianlishan complex indicate that high F source is crucial for mineralization of W and Sn. We consider that the lithospheric mantle source may have been metasomatized by subduction fluids in the far end of subduction zones to produce the A2 feature of the Qianlishan granite and the fluorine was introduced through breakdown of phengite in the oceanic slab. Partial melting of the lithospheric mantle was induced by upwelling of the asthenospheric mantle as a consequence of slab rollback. Tungsten and tin have been enriched in reduced granitic magmas through fractionation enhanced by high F contents, forming the giant polymetallic deposits.