The Triassic age for oceanic eclogites in the Dabie orogen: Entrainment of oceanic fragments in the continental subduction

Low-temperature and high-pressure eclogites with an oceanic affinity in the western part of the Dabie orogen have been investigated with combined Lu-Hf and U-Pb geochronology. These eclogites formed over a range of temperatures (482-565 °C and 1.9-2.2. GPa). Three eclogites, which were sampled from the Gaoqiao country, yielded Lu-Hf ages of 240.7 ± 1.2. Ma, 243.3 ± 4.1. Ma and 238.3 ± 1.2. Ma, with a corresponding lower-intercept U-Pb zircon age of 232 ± 26. Ma. Despite the well-preserved prograde major- and trace-element zoning in garnets, these Lu-Hf ages mostly reflect the high-pressure eclogite-facies metamorphism instead of representing the early phase of garnet growth due to the occurrence of omphacite inclusions from core to rim and the shell effect. An upper-intercept zircon U-Pb age of 765 ± 24. Ma is defined for the Gaoqiao eclogite, which is consistent with the weighted-mean age of 768 ± 21. Ma for the country gneiss. However, the gneiss has not been subjected to successive high-pressure metamorphism. The new Triassic ages are likely an estimate of the involvement of oceanic fragments in the continental subduction.