Generation of Late Mesozoic Qianlishan A 2 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 9.04 wt. high Ga/Al ratios (10,000*Ga/Al 2.6) and high Zr Nb Y Ce concentrations 350 ppm) 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 158 157 Ma, to 158 155 Ma and to 154 Ma 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 eHf(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 8690 ppm) and W (7 158 ppm) and Sn (6 51 ppm) 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.