Zircon U-Pb geochronology and metamorphism of mafic granulite from north Tongbai, central China

LA-ICP-MS U-Pb dating of metamorphic zircons from mafic granulite in northern Tongbai area yielded weighted mean Pb-206/U-238 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 degrees 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 similar to 800 degrees C, calculated by partial melting modeling. Additionally, Ti-in-zircon geothermometer also gives the crystallization temperature of ca. 750 degrees 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 similar to 800 degrees C. The age of peak metamorphism could be around 430 similar to 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.