SHRIMP U-Pb zircon geochronology of the Narryer Gneiss Complex, Western Australia

Abstract

Abstract SHRIMP U-Pb zircon geochronology of the Narryer Gneiss Complex of the Yilgarn Craton, Australia, has confirmed that its eastern half contains abundant early Archaean gneisses (3300–3730 Ma), but has found gneisses with ages of only 3000 Ma or less in its western half. In the more intensively-studied southern part of the Narryer Gneiss Complex, the early Archaean gneisses are divided into the Eurada Gneiss Association (oldest rocks 3490 Ma) and the Nookawarra Gneiss Association (oldest rocks 3730 Ma). In both associations, only the oldest components are tonalitic, and all younger components are granodioritic or granitic in composition. These associations show different histories until emplacement of granites and pegmatites at 3280–3300 Ma, when they might have been tectonically juxtaposed. These two gneiss associations are found intercalated with the Narryer Supracrustal Association, which contains abundant detrital, quartz-rich sediments. The largest exposures of this association form Mount Narryer and the Jack Hills, where a small proportion of the detrital zircons within sediments are >4000 Ma old. Detrital zircon age populations vary from sample to sample in the Narryer Supracrustal Association. Analysis of these age populations shows that they could have been derived from Nookawarra and Eurada Gneiss Association rocks in varying proportions, apart from 3800 to 4280 Ma zircons, whose source remains unknown. The original relationships between the early Archaean gneiss associations have been obliterated by intrusion of several generations of granite sheets, intercalation with the Narryer Supracrustal Association and folding between 2750 Ma and 2620 Ma, followed by movement on subvertical shear zones either at the end of the Archaean or in the Proterozoic. The late Archaean events coincide with similar ones throughout the Yilgarn Craton, and are thought to be concomitant with growth of the craton by terrane assembly, probably in several distinct episodes.