The age of the Ormiston Pound Granite: implications for Mesoproterozoic evolution of the Arunta Inlier, central Australia

Abstract

The Ormiston Pound Granite, southern Arunta Inlier, intruded during the terminal stages (late- to post-D 2) of a major, N-directed overthrust event called the Chewings Orogeny, which deformed supracrustal rocks of the Simpsons and Chewings assemblages. Ion probe zircon UPb analysis of the granite indicates an emplacement age of 1603 ± 10 Ma, more than 500 Ma older than the RbSr isochron age. Timing of emplacement is confirmed by an age of 1575 ± 20 Ma for zircon mantles from a gneissic xenolith in the granite. Zircon cores from the xenolith yield an age of 1678 ± 14 Ma, which is taken to represent the protolith age of the surrounding gneisses. The SHRIMP age is similar to, but more precise than the 1586 ± 69 Ma RbSr isochron age for the Chewings Orogeny, determined by Marjoribanks and Black (1975). Penecontemporaneous ∼ 1600 Ma magmatism is evident as pegmatites and associated fluid flow zones in the Anmatjira and Reynolds ranges, northern Arunta Inlier. 1100-1030 Ma RbSr mica ages from the ∼ 1600 Ma Ormiston Pound Granite, coeval pegmatites and ∼ 1680 Ma host-rock gneisses are similar to a SmNd isochron age for the mafic Stuart Dyke Swarm of the southern Arunta Inlier, and the contemporaneous Alcarra Dyke Swarm in the Musgrave Inlier, some 500 km farther south. The RbSr mica ages reflect isotopic resetting by a thermal perturbation, either associated with regional crustal extension during ∼ 1080 Ma dyke intrusion or with the ∼ 1150 Ma magmatic event in the southern Arunta (cf. Black and Shaw, 1992). Irrespective, the 1050-900 Ma Ormiston Event, described by Shaw et al. (1984) as granite intrusion associated with high-grade metamorphism, migmatization and folding, needs to be re-defined or abandoned.