A large-scale copper ore-forming event accompanying rapid uplift of the southern Tibetan Plateau: Evidence from zircon SHRIMP U-Pb dating and LA ICP-MS analysis

Cathodoluminescence imaging, combined with SHRIMP U-Pb dating, demonstrates that zircons from ore-bearing porphyries of the Gangdese porphyry copper belt on the southern Tibetan Plateau are composed of inherited, metamorphic and magmatic zircon with ages of 41.5 to 62.5 Ma, 17.69 to 26.0 Ma and 11.39 to 17.20 Ma, respectively. LA ICP-MS analysis also shows that the inherited zircons are characterized by high concentrations of Y (1121 ppm), HREE (641 ppm) and MIREE (182 ppm), and low concentrations of U (208 ppm), Th (171 ppm) and Hf (0.96%). Compared with the inherited and magmatic zircons, the metamorphic zircons have markedly lower Th/U ratios (average = 0.54). Although the three types of zircons all have negative Eu anomalies and positive Ce anomalies, the magmatic zircons display much larger variations in Ce anomalies. Importantly, these zircon types correspond to three events during the evolution of the Gangdese collisional orogen that allow us to propose a tectonic model. During the India-Asia collision (50-60 Ma), mantle-derived mafic magmas were underplated, forming a dense crustally-contaminated mafic to ultramafic layer at the base of the crust. At about 21 Ma, asthenospheric upwelling resulted in partial melting of the underplated mafic rocks under granulite facies conditions and rapid uplifting of the southern Plateau. Concomitantly with the extensional crustal collapse after the uplift at about 15 Ma, the ore-bearing magmas were emplaced, forming porphyry copper deposits.