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Relationship between Gangdese porphyry copper deposits and uplifting of southern Tibet plateau: Evidence from multistage zircon of ore-bearing porphyries

QU XiaoMing1, HOU ZengQian2, MO XuanXue3, DONG GuoChen3, XU WenYi1 and XIN HongBo1(1 Institute of Mineral Resources, CAGS, Beijing 100037, China; 2 Institute of Geology, CAGS, Beijing 100037, China; 3 China University of Geosciences, Beijing 100083, China)  
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 zircons with ages of (51.1±4.8) Ma, (21.1±2.6) Ma and (14.47±0.5)Ma, respectively. LA_ICP_MS analysis shows that the inherited zircons are characterized by high concentrations of Y (1121×10 -6), HREE (641×10 -6) and MREE (182×10 -6), and low abundances of U (207×10 -6), Th (171 ×10 -6) and Hf (0.96%). Compared with the inherited and magmatic zircons, the metamorphic zircons have markedly lower Th/U ratios (0.54 on average). Although the three types of zircons all have obvious negative Eu anomalies and positive Ce anomalies, the magmatic zircons show much larger variation in Ce anomaly.What is of significance is that these zircon types correspond to three important tectonic events in the evolution of the Gangdese collisional orogen, and this makes the authors advance the following tectonic model. During the India_Asia collision (50~60 Ma), mantle_derived mafic magmas were underplated, leading to the formation of the source of ore_bearing porphyries. Some 21 Ma ago asthenospheric upwelling resulted in partial melting of the underplated mafic rocks under the conditions of metamorphic garnet_amphibolite facies 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.
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