Pitfalls and new approaches in granulite chronometry: An example from the Limpopo Belt, Zimbabwe

Abstract

Timing and duration of metamorphism are particularly difficult to decipher in granulites because the metamorphic temperature commonly experienced by such rocks exceeds the closure temperature of all geochronometers other than the U–Th–Pb systems in a few minerals. Age constraints on granulite metamorphism therefore often rely on data from a single geochronometer and consistency tests are rarely possible. This case study was conducted on a single hand specimen of an exceptional late Archaean granulite from the Northern Marginal Zone of the Limpopo Belt, Zimbabwe. This rock not only contains zircon, monazite, and xenotime as accessory phases but is also depleted in Pb to such an extent that dating of orthopyroxene was possible. In situ 207/206∗Pb SIMS zircon, monazite, and xenotime ages, conventional U–Pb zircon and orthopyroxene ages, and Pb–Pb step leaching orthopyroxene ages were obtained. The in situ 207/206∗Pb SIMS ages of monazite and xenotime inclusions in cordierite are well within an independent age bracket as are ages of an interstitial monazite grain. Interstitial xenotime, however, retains a more than 55 Ma younger age than the cordierite shielded grain. These ages are incompatible with a monazite/xenotime U–Pb closure temperature of 725±25°C but suggest that grains which were shielded from processes leading to additional Pb loss are closed to significant Pb volume diffusion at temperatures ≥800°C. Zircon ages reflect partial protolith inheritance, total resetting during peak temperature conditions, and disturbance during minor hydrothermal overprint and are therefore not useful, on their own, to constrain the timing of granulite facies metamorphism. U–Pb and Pb–Pb step leach systematics of orthopyroxene yielded ages identical to the in situ phosphate ages, well within the independent age bracket. However, the step leach experiment showed that these ages merely date monazite and zircon inclusions rather than the host orthopyroxene itself. Simple model calculations are used to demonstrate that even minute contamination of low μ minerals with high μ inclusions will have significant effects on the obtained mixed age and may lead to erroneous conclusions regarding the U–Pb closure temperature of e.g. pyroxene and garnet. These pitfalls in granulite geochronology will usually result in an overall apparent age dispersion and time scales derived from field studies should therefore be viewed as upper limits.