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Biotite from granite and gneiss in the southwestern part of the Yilgarn Craton near Perth has been dated by the Rb-Sr technique at about 430-500 Ma in a belt at the western edge of the craton. The belt widens southward from 30 km near Perth to 55 km east of Harvey. The eastern boundary of the belt passes along the east margin of the Saddleback greenstone belt. A transition zone 15-40 km wide separates the belt of young dates from an eastern chronological plateau where biotite dates, mainly 2300-2600 Ma, are marginally younger than regional Rb-Sr whole rock dates which average about 2550 Ma. In contrast to biotite dates, whole rock dates greater than 2500 Ma persist to the western edge of the craton. The 430-500 Ma dates probably represent resetting during uplift in the Early Palaeozoic.

New Rb-Sr, Sm-Nd and Pb-Pb data are presented from banded gneisses in the Mt Narryer region of the Western Gneiss Terrain, together with Rb-Sr and Sm-Nd data from cross-cutting granite sheets. The Meeberrie Gneiss, which previously gave a Rb-Sr isochron age of 3348 + 43 Ma and an initial ratio of 0.7037 + 0.0005, has given a Pb-Pb age of 3357 ± 70 Ma, whilst an additional suite of the same gneisses collected to the SW of the original samples gives a Rb-Sr age of c. 3100 Ma with an initial ratio of c. 0.705. Deformed sheets of recrystallized granite which intrude the Meeberrie Gneiss give a Rb-Sr age of 2579 + 122 Ma with an initial ratio of 0.7143 ± 0.0092. Sm-Nd analyses on samples of the Meeberrie Gneiss give TCHUR ages of 3710 ±30 Ma and 3620 + 40 Ma, which support the previously determined model age of 3630 ± 40 Ma. An additional TCHUR of 3540 ± 30 Ma for a sheet of Dugel Gneiss within the Meeberrie Gneiss coincides with a previously reported value of 3510 ± 50 Ma for another sample of the Dugel Gneiss. Two cross-cutting granite sheets give TCHUR ages of 3120 ± 30 Ma and 3070 ± 40 Ma. The geochronological data in this paper support and extend the isotopic evidence previously reported from the banded gneisses of the Mt Narryer region, and confirm that the Western Gneiss Terrain is the oldest known major segment of sialic crust in the Yilgarn Block, Western Australia.

This paper continues a survey of Sm-Nd age relationships at the margins of the Yilgarn Block, Australia's largest Archaean craton. Model ages have been determined along an irregular transect extending approximately 120 km from the extreme SW of the Western Gneiss Terrain of the Yilgarn Block through the western extremity of the Albany-Fraser Province, a major Proterozoic mobile belt. The ages demonstrate the significance of the Manjimup and Pemberton Lineaments as major crustal discontinuities. The sequence of ages across the lineaments strongly supports accretionary models of Precambrian crustal evolution, although some aspects of the age sequence can also be interpreted as mixing trends. Within the Western Gneiss Terrain, the Manjimup Lineament marks a change from older (c. 3.1 Ga) to younger (c. 2.7 Ga) Archaean gneisses. Further south the Pemberton Lineament, marking the northern tectonic boundary of the Albany-Fraser Province, defines a change from Archaean (c. 2.7 Ga) to Proterozoic (c. 2.1 Ga) mantle differentiation ages. The one exception within the mobile belt is a sample of granulite from Windy Harbour, the age of which (3.1 Ga) implies that the Proterozoic crustal material of the Albany-Fraser Province is sharply terminated to the west as well as to the north. The ages determined within the Albany-Fraser Province, together with those previously reported for the Gascoyne Province, point to the existence of a major crust-forming event surrounding a considerable portion of the Yilgarn Block about 2 Ga ago.

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.