UHT metamorphism on Seram, eastern Indonesia: reaction microstructures and P-T evolution of spinel-bearing garnet-sillimanite granulites from the Kobipoto Complex.

Abstract

The island of Seram, part of the northern limb of the Banda Arc in eastern Indonesia, exposes an extensive Mio-Pliocene granulite facies migmatite complex (the Kobipoto Complex) comprising voluminous leucosome-rich diatexites and scarcer Al–Fe-rich residual granulites. The migmatites are intimately associated with ultramafic rocks of predominantly lherzolitic composition that were exhumed by substantial lithospheric extension beneath low-angle detachment faults; heat supplied by the lherzolites was evidently a major driver for the granulite facies metamorphism and accompanying anatexis. Residual garnet–sillimanite granulites sampled from the Kobipoto Mountains, central Seram, contain scarce garnet-hosted inclusions of hercynite spinel (~1.5 wt% ZnO) + quartz (± ilmenite) in direct grain-boundary contact – an assemblage potentially indicative of metamorphism under ultrahigh-temperature (UHT) conditions. thermocalc ‘Average P–T’ reactions and melanosome-specific thermocalc inline image, T–MO, and P–T pseudosections in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) chemical system, supported by Ti-in-garnet thermobarometry, are permissive of the rock having experienced a clockwise P–T path peaking at 925 °C and 9 kbar – thus narrowly reaching UHT conditions – before undergoing near-isothermal decompression to ~750 °C and ~4 kbar. Spinel + quartz assemblages are interpreted to have formed at or just after the metamorphic peak from localized reactions between sillimanite, ilmenite and surrounding garnet. Further decompression of the rock resulted in the formation of complex reaction microstructures comprising cordierite ± plagioclase coronae around garnet, and symplectic intergrowths of cordierite + spinel + ilmenite around sillimanite. Small grains of sapphirine + corundum developed subsequently within spinel by localized quartz-absent reactions. The post-peak evolution of the granulites may be related to previously published U–Pb zircon and 40Ar/39Ar ages of c. 16 Ma, further substantiating the claim for the Kobipoto Complex granulites having recorded Earth's youngest-identified episode of UHT metamorphism, albeit at slightly lower temperature and higher pressure than previously inferred. The Kobipoto Complex granulites demonstrate how UHT conditions may be achieved in the ‘modern’ Earth by extreme lithospheric extension, which, in this instance, was driven by slab rollback of the Banda Arc.