北桐柏地区镁铁质麻粒岩锆石U-Pb年代学及变质作用

LA-ICP-MS U-Pb dating of metamorphic zircons from mafic granulite in northern Tongbai area yielded weighted mean ²⁰⁶PbZ²³⁸U concordia age of 418. 7 ±3. 2Ma. Detailed studies on petrology and mineralogy of the mafic granulites indicate that the peak metamorphic condition was at ca. 845°C and ca. 0. 85GPa. According to the Zr saturation model, Zr was not saturated in melts at peak metamorphic condition and would be saturated when cooling to 750∼800°C, calculated by partial melting modeling. Additionally, Ti-in-zircon geothermometer also gives the crystallization temperature of ca. 75°C. These are suggested that the age of 418. 7Ma is not the age of peak metamorphic condition, and probably is a record for the time when cooling to 750 ∼ 800 °C. The age of peak metamorphism could be around 430 ∼445Ma. Some zircon grains have obvious overgrown rims, with stronger cathodoluminescence and higher common Pb than the core. The rims and high common Pb zircons domain yield a age of 404. 7 ± 6. 5Ma, indicating that the mafic granulite suffered a retrograde metamorphism at about 404Ma We suggest that the high-temperature mafic granulites from north Tongbai were probably formed in residual overthickened crustal basement in continental subduction collision zones or arc continent collision belts.