Miocene UHT granulites from Seram, eastern Indonesia: a geochronological–REE study of zircon, monazite and garnet

The island of Seram, eastern Indonesia, incorporates Miocene ultrahigh-temperature (UHT;>900°C) garnet–sillimanite granulites that formed by extensional exhumation of hot mantle rocks behind therolling-back Banda Arc. UHT metamorphic conditions are supported by new Zr-in-rutile thermometry resultsand the Miocene age of the UHT event is confirmed by closely-matched heavy rare earth element (HREE) abundancesbetween garnet and c. 16 Ma zircon. Monazites also record identical U–Pb ages, within uncertainty.However, these geochronometers do not date peak UHT metamorphism; instead, they date retrograde, garnetconsuming(Zr- and rare earth element (REE)-liberating) reactions that produced the granulites’ post-peakcordierite + spinel reaction microstructures. Zircons shielded within garnet did not crystallize c. 16 Ma rimsand so were unaffected by the entire UHT event. Miocene UHT metamorphism overprinted a Late Triassic–Early Jurassic upper-amphibolite facies event that grew garnet cores and 216–173 Ma zircon. In the Miocene,these garnet coreswere overgrown by peritectic garnet rims duringUHTmetamorphism, with some rutiles recordingc. 900°C Zr-in-rutile temperatures. Garnet Lu–Hf ages of 138 Ma – produced by core–rim mixing – demonstratethat a component of Hf4+ produced since c. 200 Ma was retained through the c. 16 Ma UHT event. Accordingly,UHT conditions must have been very short-lived and exhumation of the granulite complex very rapid.