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

J M Pownall,I S Williams,R Hall,R A Armstrong,C J Manning,M F Thirlwall

doi:10.1144/sp478.8

J M Pownall, I S Williams + Show 4 more

Open Access

https://doi.org/10.1144/sp478.8

Copy DOI

Abstract

AbstractThe 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.

Highlights

Small garnets with high heavy rare earth element (HREE) contents likely grew during this Late Jurassic–Early Cretaceous event, in order to account for elevated 176Hf contents that cannot be explained by a single garnet growth event at c. 16 Ma
As the garnets no longer preserve major element or light REE (LREE) core–rim zoning, in contrast to the more retentive HREEs, it is likely that these less retentive cations were homogenized under peak conditions
Diffusion of HREEs from core to rim may have occurred to a small extent, since R2 zircon which grew contemporaneously with the garnet cores at c. 200 Ma have higher HREE abundances

Summary

Introduction

As zircon and rutile grains occur in the same microtextural domains – both as inclusions within garnet and in the leucosomes of the Kobipoto Complex granulites (Figs 6 & 9) – it is reasonable to assume in this instance that rutile and zircon were in (or close to) equilibrium. Garnets analysed from UHT granulite sample KP11-588 yielded precise Lu–Hf ages of 138.6 ± 0.7 Ma and 137 ± 0.6 Ma calculated for grt-1 and grt-2 fractions and the whole rock (i.e. 2-point isochrons), respectively (Fig. 17a; Table 5).

Results

Conclusion