Multistage metamorphic events in granulitized eclogites from the North Dabie complex zone, central China: Evidence from zircon U-Pb age, trace element and mineral inclusion

摘要

The studied ultrahigh-pressure (UHP) eclogites, located in the southwestern part of the North Dabie complex zone (NDZ) in central China, represent deeply subducted mafic lower continental crust of the South China Block and display a multiple metamorphic evolution. However, the exact timing of the UHP metamorphism in the NDZ is poorly constrained, and thus impedes our understanding of the tectonic evolution of this area. In order to constrain the ages of peak UHP metamorphism and subsequent retrogression during continental subduction and exhumation, zircon from the eclogites in the NDZ has been investigated by a combined petrological, trace element and U-Pb isotopic study. In combination with petrological data, the present zircon SHRIMP and LA-ICPMS U-Pb dating provides precise constraints on the timing of multistage metamorphic events on the eclogites in the region. The U-Pb isotope in zircon from the eclogites records the times of multiple discrete events in the history of the rock such as Neoproterozoic, 238 ±2 Ma, 222 ±4-227 ±2 Ma, 210 ±4-215 ±2 Ma, 199 ±2 Ma and 176 ±2-188 ±2 Ma. Neoproterozoic ages defined by relic igneous cores represent their protolith time and the other age-groups reflect their metamorphic records by CL images, low Th/U ratios and mineral inclusions. By U-Pb age, trace element and mineral inclusion of zircon, at least two episodes of eclogite-facies metamorphism have been identified from the eclogites, best estimated at 226 ± 3 and 214 ± 3 Ma, respectively. The younger ages of 199 ± 2 and 176-188 Ma most likely record the granulite-facies overprinting and amphibolite-facies retrogression occurring during exhumation whereas the age of 238 ± 2 Ma probably reflects the prograde metamorphic timing prior to the UHP metamorphism. Thus, Neoproterozoic mafic lower continental crust was subducted to depths greater than 120 km (corresponding to the lowest pressure for diamond formation) and suffered from UHP metamorphism at 226 ±3 Ma. Then these UHP metamorphic rocks were exhumed to about 60 km depth at 214 ±3 Ma and experienced high-pressure quartz eclogite-facies retrograde metamorphism, and subsequently to lower- and upper-crustal levels and overprinted by granulite- and amphibolite-facies metamorphism, respectively.