TIMING OF LATE ARCHEAN GRANULITE-FACIES METAMORPHISM IN THE SOUTHWESTERN YILGARN CRATON OF WESTERN-AUSTRALIA - EVIDENCE FROM U-PB AGES OF ZIRCONS FROM MAFIC GRANULITES

Conventional single and multigrain zircon U-Pb ages were determined from six samples of mafic granulites from locations widely distributed throughout the southwestern Yilgarn Craton. Zircons interpreted to have formed during the granulite facies event record ages within the narrow time range of 2649-2640 Ma. Interpretation of this as the age of the granulite facies metamorphism is supported by an identical age on metamorphic titanite from one of the granulite samples. Zircons in two of the samples contained remnant euhedral zoning, interpreted to have been inherited from the igneous protolith. However, U-Pb ages of these zircons were not significantly different from the metamorphic age suggesting that emplacement of the parent rocks was coeval with metamorphism, or that the zircons were completely reset during the metamorphic event. A third sample of mafic granulite contains rounded zircons with complex forms which record an age of approximately 2798 Ma, indicating that these are inherited from an older protolith. At present there is no geochronological evidence for rocks of this age in greenstone belts in the Yilgarn Craton placing in doubt suggestions that all the mafic granulites are high-grade metamorphic equivalents of greenstone belts found elsewhere in the craton. The granulite facies metamorphism is significantly younger than the age of approximately 2670 Ma for lower-grade greenstones to the west, supporting the hypothesis that the western limit of granulite facies rocks marks the boundary between two terranes that accreted after the peak of metamorphism.