Proterozoic tectonic reworking in the Rudall complex, Western Australia

Abstract

Metasedimentary gneisses and schists in the northern part of the Rudall Complex are divided into the Yandagooge and Tjingkulatjatjarra Formations. The Yandagooge Formation occurs as inclusions in, and is intimately interlayered with, a composite orthogneiss, which is the dominant rock type of the Rudall Complex. The Tjingkulatjatjarra Formation is not disrupted by the orthogneiss, and is interpreted as being younger than both the orthogneiss and Yandagooge Formation, possibly forming a cover sequence to these rocks. The Yandagooge Formation and orthogneiss record a tectonic event, D₁, that pre‐dates the Tjingkulatjatjarra Formation, but all rock units were intensely deformed and metamorphosed during the Watrara Orogeny, which comprises events D_2–3 and began at ∼ 1330 Ma. The recognition of marker horizons in the metasedimentary rocks enables regional structural analysis, and the identification of mesoscopic to macroscopic recumbent folds produced during D₂, which were redeformed by upright D₃ folding. Metapelitic assemblages produced during D₂ include kyanite + staurolite + biotite and garnet + staurolite + biotite, and suggest that the Watrara Orogeny was a consequence of the burial of the Rudall Complex to depths of ∼ 20 km. These Barrovian‐style assemblages are unusual for the mid‐Proterozoic orogenic belts of Australia, and the Watrara Orogeny could be equivalent to the Capricorn Orogeny, which was consequent to the juxtaposition of the Pilbara and Yilgarn Cratons. Unconformably overlying low‐grade Late Proterozoic cover rocks, together with the basement rocks, were mildly affected by a regional folding and thrusting event during the Paterson Orogeny at ∼ 700 Ma.