Petrological and LuHf age constraints for eclogitic rocks from the Pam Peninsula, New Caledonia

Abstract We report new petrological and geochronological data for eclogitic rocks of the Pouebo Eclogite Melange (PEM) in the New Caledonian high-pressure/low-temperature (HP/LT) belt. Pseudosections were calculated to link newly determined Lu Hf dates with P-T conditions corresponding to garnet growth, which started on the prograde path and continued into the eclogite facies to approximately 560 °C and 2.05 GPa, coinciding with omphacite formation. The Lu Hf garnet dates of 38.27 ± 0.14, 36.5 ± 0.7, and 38.42 ± 0.15 Ma are weighted towards the early part of the garnet growth interval, and thus represent maximum ages of eclogite-facies conditions. These dates are indistinguishable from those reported earlier from Ar Ar in white mica, which had been interpreted as cooling ages, and they are about 6 Myr younger than those from U Pb in metamorphic zircon, which had been previously considered to date peak eclogite-facies metamorphism at 44 Ma. The new Lu Hf dates overlap with Rb Sr white mica ages of nearby metasomatic veins related to the blueschist-to-eclogite transition. The Lu Hf and Rb Sr data imply that eclogite-facies metamorphism in parts of the PEM lasted at least until 38 Ma and is thus younger than previously suggested. This prograde, near-peak event cannot be reconciled with concurrent (41–36 Ma) cooling previously inferred from the Ar Ar dates. We conclude that both the 38.4–36.5 Ma Lu Hf and Rb Sr dates as well as similar Ar Ar results record near-peak conditions, while the 44 Ma U–Pb zircon rims probably are related to prograde mineral reactions. The HP stage is followed by rapid exhumation and cooling at minimum rates of ~0.7 cm/yr and ~ 50 °C/Myr.