Variety, age and origin of zircons in the mid‐Cenozoic Westonia Formation, southwestern Yilgarn Craton, Western Australia

Abstract

The Westonia Formation comprises clayey quartz sand commonly exposed in interfluve breakaways in the southwestern Yilgarn Craton, Western Australia. As is typical of Cenozoic sediments in the region, it is texturally immature, poorly stratified and unfossiliferous, making it difficult to distinguish from the underlying deeply weathered Archaean granite. In order to verify the difference in origin and elucidate provenance, the varietal properties and U–Pb ages of zircons from the Westonia Formation and underlying granitic saprolite were compared. Zircons from the Westonia Formation comprise 11 distinct morphological varieties, seven of which are not present within the granitic saprolite. SHRIMP U–Pb ages of zircons from the granitic saprolite range from 2657 ± 13 to 2554 ± 10 Ma, corresponding to published age determinations of zircons from the Darling Range Batholith. Angular zircons from the Westonia Formation range in age from 2898 ± 12 to 2631 ± 11 Ma, reflecting a mixed provenance including local granitic rocks, and possibly greenstones in adjacent parts of the Yilgarn Craton. Rounded zircons from the Westonia Formation range in age from 1144 ± 38 to 467 ± 16 Ma (Late Mesoproterozoic – Early Palaeozoic), similar to detrital zircons from the Jarrahdale bauxite deposit and various stratigraphic levels in the Perth Basin. The Late Mesoproterozoic – Early Palaeozoic zircons at East Yornaning and Jarrahdale most likely resulted from burial of the southwestern Yilgarn Craton by sediment sourced from the Proterozoic Albany–Fraser–Wilkes and Pinjarra Orogens during the Early Permian glaciation, or during an earlier episode of mountain building associated with the final stages of the Pinjarra Orogeny. Most of the sedimentary cover was then reworked into the evolving Perth Basin during the Late Permian – Early Cretaceous. Remaining pockets on the craton were repeatedly recycled, leaving only the most chemically and physically resistant components, such as rounded zircons, in the Westonia Formation.