Early Archaean gneisses from the Yilgarn Block, Western Australia

The Yilgarn Block is an ancient crustal block which extends over ∼650,000km2in the south-west of Western Australia and consists of high-grade gneiss and granite–greenstone terrains1. Being one of the largest segments of Archaean crust in the world, it is important to studies of early crustal evolution. We report here a geochronological study of banded gneisses near Mount Narryer, in the north-west of the Yilgarn Block, which has given a Rb–Sr whole-rock isochron age of 3,348±43 Myr with an initial 87Sr/86Sr ratio of 0.7037±0.0005. The data indicate that the rocks had a prior crustal residence time of ∼200 Myr. The validity of this early Archaean age is supported by model Sm–Nd ages of 3,510 Myr and 3,630 Myr for two of the samples. Taken together, these ages represent the oldest evolutionary sequence so far identified in the Yilgarn Block, and are comparable with old ages from other Archaean cratons elsewhere in the world.