Sm‐Nd model ages across the margins of the Archaean Yilgarn Block, Western Australia — III. The western margin

Abstract

Earlier studies of the nature and extent of Proterozoic activity around the Archaean Yilgarn Block, based on Sm‐Nd model ages, have now been extended by an examination of the western margin. New data from the western Yilgarn Block indicate that rocks of the Chittering Metamorphic Belt (2.8–2.9 Ga) are significantly younger than rocks in the other three metamorphic belts of the Western Gneiss Terrain (literature values ≥3.0 Ga). This suggests possible continental growth by marginal accretion during the Archaean. Data from Yilgarn Block granitoids indicate a complex and extended period of generation, ranging from 2.7 to 3.1 Ga. There is a major time break at the western margin of the Yilgarn Block, marked by the Darling Fault. All rocks sampled W of the fault have Proterozoic model ages. This confirms the existence of a Proterozoic mobile belt along the western edge of the Yilgarn Block and provides further evidence that the present Darling Fault follows a zone of Precambrian crustal discontinuity. The remarkably consistent age of 1.8 Ga for rocks occurring both within and below the Perth Basin is similar to the youngest values determined in the Gascoyne Province and western parts of the Albany‐Fraser Province. This suggests contemporaneous mobile belt activity around the entire western portion of the Yilgarn Block and strongly supports models of Proterozoic continental growth by marginal accretion. Although data are limited, a further westward younging is suggested by available isotopic data from the Leeuwin Block (age ca 0.6 Ga). This suggests a further westward, step‐like decrease in age away from the Archaean Yilgarn Block.