Geochemistry and geochronology of eclogites from the northern Dabie Mountains, central China
Résumé
Recent discoveries of UHP eclogites from the northern Dabie Complex (NDC) in central China provide an opportunity to improve our understanding of the tectonic setting of the NDC and exhumation mechanism of UHPM rocks. The petrology, geochemistry, SrNdO isotopic compositions and SmNd isotopic chronology of some eclogites from the NDC were obtained in this study. The major elemental compositions and relatively low Nd (T) of about -10 for the eclogites suggest that the protoliths of most eclogites were ancient basaltic rocks developed in continental crust, and thus these eclogites were produced by subduction of the Yangtze continental crust. However, some of the eclogites within foliated peridotites with relatively high Nd (T) values up to +2.4 are interpreted as pieces of lithospheric mantle scraped off the overlying mantle by subducting continental crust and then subducted together into mantle depths. Therefore, the metamorphosed mafic-ultramafic belt containing eclogite and meta-peridotite blocks along the southern part of the MozitanXiaotian fault zone may represent the tectonic melange produced by the subduction of the Yangtze continental crust. The SmNd age of 2124 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 d18O values of the eclogitic minerals such as garnet and omphacite from an eclogite sample suggests oxygen isotope equilibrium, pointing to SmNd isotope equilibrium between garnet and omphacite according to comparability between O and SmNd diffusivities. However, the SmNd 'isochrons' defined by whole+garnet or omphacite+zonal garnet yield a large age range of 187266 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 d18O 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.