Geochemistry and geochronology of eclogites from the northern Dabie Mountains, central China

The Sm-Nd age of 212±4 Ma defined by garnet+omphacite is interpreted to represent a cooling age corresponding to in the retrograde stage of granulite-facies metamorphism during eclogite exhumation. The relationship between the δ18O values of the eclogitic minerals such as garnet and omphacite from an eclogite sample suggests oxygen isotope equilibrium, pointing to Sm-Nd isotope equilibrium between garnet and omphacite according to comparability between O and Sm-Nd diffusivities. However, the Sm-Nd 'isochrons' defined by whole+garnet or omphacite+zonal garnet yield a large age range of 187-266 Ma, which may be produced by Nd isotopic disequilibrium between garnet and retrograded minerals in whole-rocks. Therefore, the eclogites in the NDC also underwent Triassic UHP metamorphism like the UHP eclogites in the southern Dabie Mountains. The δ18O values of whole-rocks for the eclogites are 2.40 to 4.32‰, indicating that protolith of the eclogites in the NDC underwent hydrothermal alteration at high-temperature by surface water before subduction of the Yangtze continental crust during the Triassic period in a manner similar to protoliths of UHP rocks in the southern Dabie Mountains.