Subduction of continental lithosphere to mantle depths results in the recrystallization of crustal material during ultrahigh-pressure (UHP) metamorphism. Knowledge of the duration and pressure and temperature (P-T) conditions at which UHP rocks equilibrate is fundamental for assessing the rates and mechanisms involved in the transfer of crustal material to and from mantle depths. High-precision U-Pb zircon geochronology combined with thermobarometry, P-T pseudosection modeling, and Zr-in-rutile thermometry of the youngest known UHP eclogite from eastern Papua New Guinea (PNG) documents metamorphism at mantle depths on a sub-million year time scale. The P-T analyses based on garnet, omphacite, phengite, and rutile compositions show the crustal rocks were subducted to conditions of ~27–31 kbar and ~715 °C. Individual zircons containing inclusions of the peak UHP mineral assemblage, with identical composition as those used in P-T estimates, yield chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) 206Pb/238U (Th-corrected) dates of 6.0 ± 0.2 Ma to 5.2 ± 0.3 Ma. These results more precisely and accurately define the youngest UHP metamorphism on Earth and provide compelling evidence of rapid (3.2–2.3 cm/yr) exhumation of the Pliocene PNG terrane. Subsequent retrogression of the terrane near the base of the crust and final emplacement within the upper crust occurred in less than ~3 m.y.

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