Geochronological constraints on orogenic events in the Arunta Inlier: a review

Abstract

Early workers subdivided the Arunta Inlier, central Australia, into three major tectonic provinces, classified the principal tectonostratigraphic units into three “Divisions”, and separated the tectonomagmatic history into five discrete “Events”, ranging in age from ∼ 1800 Ma to ∼ 300 Ma. A review of the evidence for tectonic boundaries within the Arunta Inlier suggests that no major structural discontinuity exists between the northern and central tectonic provinces, but that the Redbank Thrust Zone is, at least in part, a province boundary. The tectonostratigraphic “Division” concept also requires revision as Division 3 rocks of the northern and southern Arunta cannot be time equivalents, Division 1 rocks grade into Division 2 in the northern region, and Division 3 of the northern region may be equivalent to Division 1 in the central region. Rocks originally assigned to divisions are better grouped into a larger number of lithological assemblages until better stratigraphic and isotopic correlations can be made. Compilation of RbSr and 40 Ar 39 Ar age determinations obtained before 1984 indicate an almost complete age spectrum from 1800 Ma to 1000 Ma, but age clusters exist at ∼ 1400 Ma, ∼ 1100 Ma and 400-300 Ma, which have geological significance. Subdivision of older ages into discrete tectonic events was not justified from this data alone, but more recent UPb isotopic analysis of zircons, in rocks designated to particular structural/metamorphic events, has revealed a complex tectonic history in the 1880-1600 Ma interval. The northern and central provinces underwent major tectonism between 1780-1730 Ma, but much of the southern province was not deformed until ∼ 1600 Ma. The revised tectonic history nomenclature is proposed for the Arunta Inlier: following deposition of a widespread turbiditic unit at or before 1880 Ma, possibly on thinned Archaean crust, the northern and central provinces underwent localized tectonism/magmatism at ∼ 1880 Ma ( Yuendumu tectonic event) and 1820 Ma ( Stafford tectonic event), prior to the widespread Strangways orogeny, which is subdivided into an early phase at 1780-1770 Ma and late phase at 1745-1730 Ma. It is not clear whether the Strangways orogeny is a single, Arunta-wide event or a series of separate short-lived, smaller-scale events, although the major deformation in all areas appears to have involved W- to SW-directed thrusting. In the southern province, the major events occurred at ∼ 1680 Ma ( Argilke tectonic event) and ∼ 1600 Ma ( Chewings orogeny), the latter involving N-directed thrusting that was penecontemporaneous with post-tectonic pegmatite intrusion in the northern province, suggesting linkage of both regions at this stage. The younger history of the Inlier is characterised by several stages of uplift and isolated thermal (magmatic) events. Major Mesoproterozoic crustal exhumation ( Anmatjira uplift phase) has been interpreted from widespread 1500-1400 Ma ages, including: regional-scale closure of 40Ar 39Ar systems in hornblende from mafic granulite; outcrop-scale RbSr total rock isotopic systems in mylonitized granitoids; and SmNd garnet-hornblende pairs in mylonitized amphibolites. A ∼ 1150 Ma thermal event in the southern part of the Inlier ( Teapot magmatic event) is recorded by intrusion of granite, pegmatites, and the alkaline Mordor Igneous Complex, and by isotopic resetting of RbSr mineral systems. Dolerite dyke injection at ∼ 1050 Ma reflects limited extensional tectonism that may have related to formation of the Amadeus Basin. The second major uplift event was the Alice Springs orogeny at 400-300 Ma, an unusual intracratonic, thick- and thin-skinned thrust event that involved limited retrograde amphibolite facies, but extensive greenschist facies metamorphism, and restricted pegmatite intrusion.