Garnet Lu–Hf dating of retrograde fluid activity during ultrahigh-pressure metamorphic eclogites exhumation

Abstract

Previous studies on the atoll-shaped garnets in ultrahigh-pressure (UHP) metamorphic eclogites from the Dabie orogen, east-central China, suggest a fluid-enhanced overgrowth origin at the onset of exhumation. The atoll-garnets bearing eclogite place better constraints on the timing of the retrograde fluid activity and are a straightforward target to gain insight into the isotopic equilibrium and/or disequilibrium during exhumation. Comprehensive textural, chemical and Lu–Hf geochronological analyses on the atoll garnet-bearing eclogite show that the retrograde fluid activity event likely occurred at ca. 221 Ma. The Lu–Hf age of 221.0 ± 2.3 Ma marks the last garnet overgrowth episode during exhumation rather than prograde metamorphism. This somewhat restricted study suggests that dating the prograde-zoning-preserved garnets may bias results towards a particular metamorphic event rather than the prograde timing, as previously thought. The general assumption that larger garnet crystals in metamorphic rocks are older should be made with caution, and it is likely invalid in atoll garnet-bearing metamorphic eclogites because the preliminary garnet cores have been largely consumed. These observations highlight that linking textural and chemical analyses is crucial for interpreting geochronological data.