Isotopic evidence on the age and origin of the Fraser Complex, Western Australia: a sample of Mid-Proterozoic lower crust

Abstract

SmNd, RbSr and Pb isotopic data have been obtained for a variety of samples from within and around the Fraser Complex, a large (> 400 × 30 km) composite body of metagabbro which is a major component of the Proterozoic Albany-Fraser Orogen.

Granites to the north and east of the complex and a granite sheet near its centre have SmNd model ages (t_DM) of ∼2000 to ∼ 2300 Ma, similar to values recorded in the western portion of the orogen. SmNd data for Fraser Complex gabbros scatter significantly on an isochron plot, but well-preserved igneous minerals extracted from one sample give 1291 ± 21 Ma with initial ϵ_Nd= -1.3 ± 0.2. RbSr for the biotite-rock pair gives a cooling age of 1268 ± 20 Ma with initial ⁸⁷Sr/⁸⁶Sr≈0.706. Slices cut from a mixed gneiss/ amphibolite zone of ductile deformation and retrogression along the western margin of the complex give a ∼ 1300-Ma Pb/Pb date for final tectonic emplacement. Intrusion, granulite-facies metamorphism and upper-crustal tectonic emplacement of the complex were probably all achieved within ≤ 30 Ma.

SmNd and RbSr characteristics of the western deformation zone suggest flushing by large volumes of fluid derived from a moderately depleted mantle source (ϵ_Nd ≈ + 2.4; ϵ_Sr, ≈ − 5 at 1300 Ma). The precursors to the Fraser Complex could have been derived from the same (or a similar) source at ∼ 1900 Ma.