Rb‐Sr isotopic systematics of an Archaean granite–gneiss terrain: The Mount Edgar Batholith, Pilbara Block, Western Australia

Abstract

Rb‐Sr whole rock analyses describe the isotopic system of the Mount Edgar Batholith in the Archaean Pilbara Block, Western Australia. The batholith comprises a complex gneiss terrain intruded by ‘older’ variably deformed granites, minor aplite, and a ‘younger’ post‐tectonic granite, the Moolyella Suite. The ‘older’ granites occur as six suites with isotopically indistinguishable individual isochrons; the pooled mean age is 3202 ± 17 Ma with initial ⁸⁷Sr/⁸⁶Sr ratios in the range 0.7010–0.7025. This age is taken to record timing of granite emplacement, although it may represent a metamorphic resetting. If the latter is correct, resetting took place in the greenschist to lower amphibolite facies, less than 100 Ma after batholith formation. Localized Rb‐Sr resetting and aplite dyke emplacement record a ∼3000 Ma event also detected in other parts of the Pilbara Block. The ‘younger’ Moolyella suite is dated at ∼2800 Ma, but appears to have undergone minor isotopic disturbance. The gneiss complex records a complex history of major isotopic disturbance at ∼ 3200 Ma, an earlier crustal history, and localized outcrop‐scale resetting at ∼3000 Ma. Most of the banded gneisses have Sr model ages in excess of 3400 Ma, similar to the adjacent, dominantly felsic volcanic Duffer Formation. Field evidence indicates that banded gneisses are the least modified components of the complex and resemble metamorphosed equivalents of the Duffer Formation. Major isotopic disturbance of the gneisses at 3200 Ma was caused either by high grade metamorphism, associated with extensive granitic magmatism, or by low grade metamorphism. The former interpretation is preferred because the complex is dominantly orthogneiss, similar in age to the ‘older’ foliated granites. If the 3200 Ma age records a major magmatic and high grade metamorphic event in the Mount Edgar Batholith, then the modelled source rock ages of ∼3475 Ma for the granite suites suggest that a substantial component of granitoid material was derived from Duffer Formation equivalents.