Rb‐Sr isotopic evidence for Archaean‐Proterozoic crustal evolution of part of the central Yilgarn Block, Western Australia: Constraints on the age and source of the anorthositic Windimurra Gabbroid

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Abstract
The anorthositic Windimurra Gabbroid is the largest gabbroid body in the central Yilgarn Block of Western Australia, and is approximately equivalent in size to the Stillwater Complex. It contrasts with most other Yilgarn layered gabbroid bodies in its generally fresh mineralogy, anorthositic affinities, apparent lack of regional or static metamorphic effects, and unusual greenstone setting, wherein mafic and ultramafic volcanics are absent. The contacts with surrounding granitoids and greenstones are tectonic, and it has not been possible to establish direct age relationships. The age of the gabbroids cannot be directly determined using the Rb/Sr whole‐rock technique, because of the small range and low Rb/Sr ratios of the gabbroids, but a minimum age of 2669 ± 135 Ma is indicated from a Rb/Sr isochron for porphyry dykes—one of the three dyke sets herein dated that cut the complex; the other dykes give ages of about 2480 Ma. The Windimurra Gabbroid is thus Archaean in age. In contrast, a mineral isochron age of 1496 ±189 Ma, with an initial 87Sr/86Sr ratio of 0.7014 ± 0.0006, is obtained from gabbroid sample 66990. However, from the whole‐rock Sr‐isotopic data and constraints imposed by the ages of the dykes, an initial ratio range of 0.7007–0.7012 is indicated for the Windimurra Gabbroid. This is consistent with derivation of the gabbroid from an upper mantle source. The young mineral age probably results from isotopic re‐equilibration of Rb and Sr in the minerals, caused by a relatively mild thermal event. The old age and low initial 07Sr/86Sr ratio of the Windimurra Gabbroid set it apart from other large stratiform bodies (e.g. Bushveld, Sudbury, Dore Lake, Great Dyke, Stillwater); yet structurally it is dissimilar to the pre‐3000 Ma old ‘stratigraphic’ type anorthositic complexes (e.g. Fiskenaesset). The post‐gabbroid crustal history of the Windimurra region is similar to that of the Agnew‐Mt Keith region (about 190 km to the east), and a 2574 ± 74 Ma Rb‐Sr isotopic age of the granitoids is in agreement with the well‐documented period (2700–2550 Ma) of widespread emplacement and/or metamorphism of granitoid rocks elsewhere in the Yilgarn Block.