Miocene UHT granulites from Seram, eastern Indonesia: a geochronological–REE study of zircon, monazite and garnet
Published:April 17, 2019
J. M. Pownall, R. A. Armstrong, I. S. Williams, M. F. Thirlwall, C. J. Manning, R. Hall, 2019. "Miocene UHT granulites from Seram, eastern Indonesia: a geochronological–REE study of zircon, monazite and garnet", Metamorphic Geology: Microscale to Mountain Belts, Silvio Ferrero, Pierre Lanari, Philippe Goncalves, Eugene G. Grosch
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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, garnet-consuming (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 cores were overgrown by peritectic garnet rims during UHT metamorphism, 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 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.
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Metamorphic Geology: Microscale to Mountain Belts
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In Earth evolution, mountain belts are the loci of crustal growth, reworking and recycling. These crustal-scale processes are unravelled through microscale investigations of textures and mineral assemblages of metamorphic rocks. Multiple episodes of metamorphism, re-equilibration and deformation, however, generally produce a complex and tightly interwoven pattern of microstructures and assemblages. Over the last two decades, the combination of advanced computing and technological capabilities with new concepts has provided a vast array of novel petrological tools and high-resolution/high-sensitivity techniques for microanalysis and imaging. Such novel approaches are proving fundamental to untangling the enigma represented by metamorphism with an unprecedented level of detail and confidence. As a result, the first decade and a half of this century has already seen the tumultuous development of new research avenues in metamorphic petrology. This book aims to provide a timely overview of the state of the art of this field, of newly developed petrological techniques, future advancements and significant new case studies.