LA-ICP-MS zircon U-Pb ages of porphyritic granite in Machangqing complex of Yunnan Province and their geological significance

Abstract：U-Pb ages and trace elements of zircons from porphyritic granite of Machangqing complex were analyzed by the LA-ICP-MS method. As revealed by the cathodoluminescence (CL) images, all the zircons from porphyritic granite have the oscillatory zoning. REE patterns of the analyzed zircons are characterized by significantly high heavy rare earth elements (HREE)，depleted light rare earth elements (LREE), obviously positive Ce anomaly and moderately negative Eu anomaly. In addition, nearly all the zircons have high Th/U ratios. These characteristics suggest a magmatic genesis of the analyzed zircons. The zircons from the porphyritic granite of Machangqing complex yield the U-Pb weighted average age of (33.78±0.21)Ma (MSWD＝0.71), whereas zircons from syenite porphyry and granitic porphyry yield the U-Th-Pb ages of (35.6±0.3)Ma and (35.0±0.2)Ma by cumulative probability statistics. Molybdenites from the Baoxingchang ore block yield the Re-Os isochron ages of (35.8±1.6)Ma and (33.9±1.1)Ma. Muscovites of contact metasomatic gold mineralization from the Luandongshan ore block yield the 40Ar/39Ar age of (35.25±0.36)Ma. Muscovites of hydrothermal vein gold mineralization from the Rentouqin-Jinchangqin ore block yield the 40Ar/39Ar age of (35.35 ±0.32)Ma. These ages show that porphyry type copper-molybdenum mineralization, contact metasomatic gold mineralization and hydrothermal vein gold mineralization are products of the same tectonic-magmatic-hydrothermal ore-forming system，and that theⅡperiod magmatic combination (33～37 Ma) of syenite porphyry+monozonite porphyry+granitic porphyry+ porphyritic granite seems to have been the ore-forming geological body, which supplied materials, fluid and thermodynamic conditions for the ore-forming process. The duration of the porphyry-hydrothermal fluid-ore forming system is about 4 Ma. The ore-forming process of copper, molybdenum and gold occurred in the early-middle stage.