Metamorphic events in the eastern Arunta Inlier, Part 2. NdSrAr isotopic constraints

Abstract

In the Harts Ranges, the Central Province of the eastern Arunta Inlier is characterised by upper amphibolite facies ortho- and paragneisses. Previously published UPb dating of zircons, as well as new SmNd and RbSr data indicate that the intrusive ages of both mafic and felsic orthogneisses are in the range 1780-1740 Ma (Strangways Orogeny). These rocks had initial ϵ Nd values of 0 to − 3 and depleted mantle model ages of 2.5-2.1 Ga and they intruded supracrustal rocks which are now preserved as paragneisses with initial ϵ Nd of about − 15. SmNd and RbSr whole-rock isotope data indicate that the hornblende-rich amphibolitic gneisses and mylonites in the Harts Range Detachment Zone (Ding and James, 1989) were equilibrated at ∼ 1473 Ma (± 133) when the rocks experienced hydration, isotopic homogenisation and thrust faulting. This is the Anmatjira Event described elsewhere in the Arunta Inlier and is a predominantly structural influence with thrust faults and hydrous fluid flux. Accordingly the record of this event by radiogenic geochronometers has been diffuse, a problem also exacerbated by the probable influence on these rocks, of least one more subsequent lower-temperature amphibolite facies metamorphic event. In spite of the indications of ∼ 1470 Ma isotopic homogenisation of amphibolites in the Harts Ranges, RbSr isotopic analyses of all minerals and 40Ar 39Ar from hornblendes yield ages in the range 450-300 Ma. Using the SmNd system, a few garnet-hornblende pairs give 1400-1250 Ma ages, but several of these pairs also yield early Palaeozoic ages. These data suggest that the Central Province of the eastern Arunta Inlier also experienced an early Palaeozoic thermal event, having cooled from temperatures > 550°C during and immediately before the Carboniferous Alice Springs Orogeny. In the Southern Province, south of the Illogwa Shear Zone, micas record Mesoproterozoic ages (1500-1250 Ma) for the RbSr, KAr and ArAr systems, equivalent to the whole-rock ages obtained from the Central Province, to the north of the Illogwa Shear Zone. Clearly the exposed southern basement cooled to <350°C, presumably as a result of uplift and exhumation during the Anmatjira Event, when basement now exposed in the Central Province remained at depth, undergoing one or more additional amphibolite facies events. The spectacular contrast in cooling histories across the Illogwa Shear Zone emphasises that this is an important crustal scale tectonic structure.