Structural and geochronological constraints on the origin and evolution of rocks in the Ormiston Pound region of the Western MacDonnell Ranges, Northern Territory

Abstract

The Arunta Inlier preserves a complex structural history, subject to a series of igneous, metamorphic and deformational events from the Paleoproterozoic to the mid-Paleozoic. U-Pb detrital zircon ages from Paleoproterozoic and Neoproterozoic sediments at Ormiston Gorge coincide with the timing of various tectonic phases in the Arunta Inlier. First order interpretations suggest that the Northern Arunta Inlier was the source of the oldest zircons recorded at ~1820 Ma, coinciding with the timing of the Stafford Event. The Strangways Orogeny at ~1770 Ma and 1730 Ma is the earliest deformation preserved in the Central Arunta Inlier and is probably the source of zircons accumulated in these sediments. Zircons post-dating the Strangways Orogeny are likely sourced from the Southern Arunta Inlier, coinciding with the Argilke Tectonic Event at ~1680 Ma, the Chewings Orogeny at ~1600 Ma, the Anmatjira Uplift Phase at 1500-1400 Ma and the emplacement of the Teapot Granite Complex at ~1140 Ma. The Neoproterozoic Heavitree Range Quartzite sediments represent initial deposition in the Amadeus Basin, which forms the remnant of a once much larger intracratonic basin in central Australia known as the Centralian Superbasin. The Arunta Inlier was exhumed from beneath the Centralian Superbasin during the Devonian-Carboniferous Alice Springs Orogeny, forming a series of subbasins including the Amadeus, Ngalia and Georgina Basins. North-south crustal compression during this Orogeny reactivated a series of steep north-dipping Mesoproterozoic fault structures including the Redbank shear zone and the Ormiston thrust zone. The northern Amadeus Basin is characterised by coupled basement and cover deformation, producing a series of basement-rooted south-propagating thrusts, which penetrate the basal Heavitree Range Quartzite. Structural cross-sections constructed across the Ormiston region propose a series of splay thrusts within the Ormiston thrust zone, with the basement and Heavitree Quartzite heavily deformed. The conformably overlying Bittersprings Formation comprises salts and evaporates, interpreted as a detachment layer. 40Ar/39Ar muscovite dating of mylonitic shear zones at Ormiston Gorge have constrained ‘peak deformation’ conditions in the region to a minimum age of 350 ± 3 Ma. Mineral assemblages formed in the surrounding areas reflect greenschist to lower amphibolite facies metamorphism, with temperatures reaching at least 350°C.