PITFALLS AND NEW APPROACHES IN GRANULITE CHRONOMETRY - AN EXAMPLE FROM THE LIMPOPO BELT, ZIMBABWE

Timing and duration of metamorphism are particularly difficult to deci pher in granulites because the metamorphic temperature commonly experi enced by such rocks exceeds the closure temperature of all geochronome ters 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 st udy was conducted on a single hand specimen of an exceptional late Arc haean granulite from the Northern Marginal Zone of the Limpopo Belt, Z imbabwe. This rock not only contains zircon, monazite, and xenotime as accessory phases but is also depleted in Pb to such an extent that da ting of orthopyroxene was possible. In situ (207/206)Pb SIMS zircon, monazite, and xenotime ages, conventional U-Pb zircon and orthopyroxen e 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 a n interstitial monazite grain. Interstitial xenotime, however, retains a more than 55 Ma younger age than the cordierite shielded grain. The se ages are incompatible with a monazite/xenotime U-Pb closure tempera ture of 725 +/- 25 degrees C but suggest that grains which were shield ed from processes leading to additional Pb loss are closed to signific ant Pb volume diffusion at temperatures greater than or equal to 800 d egrees C. Zircon ages reflect partial protolith inheritance, total res etting during peak temperature conditions, and disturbance during mino r hydrothermal overprint and are therefore not useful, on their own, t o constrain the timing of granulite facies metamorphism. U-Pb and Pb-P b step leach systematics of orthopyroxene yielded ages identical to th e in situ phosphate ages, well within the independent age bracket. How ever, the step leach experiment showed that these ages merely date mon azite and zircon inclusions rather than the host orthopyroxene itself. Simple model calculations are used to demonstrate that even minute co ntamination of low mu minerals with high mu inclusions will have signi ficant effects on the obtained mixed age and may lead to erroneous con clusions regarding the U-Pb closure temperature of e.g. pyroxene and g arnet. These pitfalls in granulite geochronology will usually result i n an overall apparent age dispersion and time scales derived from fiel d studies should therefore be viewed as upper limits. (C) Published by 1998 Elsevier Science B.V.