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

Published on 2018-04-16T11:11:09Z (GMT) by
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 <i>c</i>. 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 <i>c</i>. 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 <i>c</i>. 900°C Zr-in-rutile temperatures. Garnet Lu–Hf ages of 138 Ma – produced by core–rim mixing – demonstrate that a component of Hf<sup>4+</sup> produced since <i>c</i>. 200 Ma was retained through the <i>c</i>. 16 Ma UHT event. Accordingly, UHT conditions must have been very short-lived and exhumation of the granulite complex very rapid.

Cite this collection

Pownall, J.M.; Armstrong, R.A.; Williams, I.S.; Thirlwall, M.F.; Manning, C.J.; Hall, R. (2018): Miocene UHT granulites from Seram, eastern Indonesia: a geochronological–REE study of zircon, monazite and garnet. Geological Society of London. Collection.