Effects of retrograde-zoning of garnet on Sm-Nd isotopic dating of eclogite and oxygen isotopic disequilibrium between eclogitic minerals

The compositional zoning of the garnet in a strongly deformed eclogite from Raobazhai foliated peridotite has been recognized. The CaO concentrations of the garnet are decreased from the core to the rim, while its MnO concentrations are increased, suggesting the retrograde origin of such CaO—MnO zoning. The tie line of garnet + omphacite from this eclogite gives a Sm-Nd age of (187 ± 5) Ma, which is less significant than the Sm-Nd ages of (221±5)—(228 ± 3) Ma and (210 ± 6)—(214 ± 6) Ma for ultrahigh-pressure eclogites in the southern Dabie zone and in the northern Dabie zone, respectively. This younger Sm-Nd age could result from the¹⁴³Nd/¹⁴⁴ Nd ratio decrease of the retrograde zone in the garnet. The δ¹⁸O values of the garnet and omphacite show that their fractionation values are less than the equilibrium fractionation value between the garnet and omphacite at 500—900°C, which suggests an oxygen isotopic disequilibrium between them.