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 the rolling-back Banda Arc. UHT metamorphic conditions are supported by new Zr-in-rutile thermometry results and the Miocene age of the UHT event is confirmed by closely-matched heavy rare earth element (HREE) abundances between 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-peak cordierite + spinel reaction microstructures. Zircons shielded within garnet did not crystallize c. 16 Ma rims and 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 recording c. 900°C Zr-in-rutile temperatures. Garnet Lu-Hf ages of 138 Ma - produced by core-rim mixing – demonstrate that a component of Hf ⁴⁺ 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.