Multi-stage exhumation of young UHP-HP rocks: Timescales of melt crystallization in the D'Entrecasteaux Islands, southeastern Papua New Guinea

On the D'Entrecasteaux Islands, southeastern Papua New Guinea (PNG), a series of domes expose the youngest known ultrahigh-pressure (UHP) to high-pressure eclogites and associated gneisses. Abundant evidence of partial melting is preserved throughout the D'Entrecasteaux domes in the form of variably deformed leucosomes, dikes, and plutonic bodies. In order to understand (1) the timing of melt emplacement relative to the UHP metamorphism and (2) the complete exhumation and deformational history of the eclogites and surrounding gneisses, a series of leucosomes, dikes, and eclogites from the Mailolo dome in western Fergusson Island were analyzed by U-Pb ID-TIMS zircon geochronology. Zircons extracted from the eclogite at the UHP locality yield ²⁰⁶Pb/ ²³⁸U TIMS dates from 5.60±0.22 to 4.61±0.18Ma. Zircons from three foliation-parallel leucosomes are interpreted to record melt crystallization from 3.49±0.01Ma to 2.98±0.02Ma. In comparison, weakly deformed granitic-granodioritic dikes that cut the leucosomes yield zircon ²⁰⁶Pb/ ²³⁸U dates of 2.48±0.01Ma-2.420±0.002Ma. Based on these new and previously published results, the eclogites may have undergone UHP metamorphism for several million years, from 7.1±0.7Ma to 4.61±0.18Ma; based on TIMS dates alone, a more restricted interval of 5.6-4.6Ma is proposed. Following UHP metamorphism, the eclogite and gneisses were exhumed to lower crustal depths, intruded by foliation-parallel leucosomes by ca. 3.5-3.0Ma and by ca. 2.45Ma late discordant dikes. Multiple generations of leucosomes and dikes allow a timeline for the exhumation of the PNG UHP terrane to be constructed: mantle (~100km depths) to the base of the crust in ~1my, storage in the lower crust for ~1my, and final emplacement into the shallow crust ~1my later.