GEOCHRONOLOGICAL CONSTRAINTS ON OROGENIC EVENTS IN THE ARUNTA-INLIER - A REVIEW

Early workers subdivided the Arunta Inlier, central Australia, into th ree major tectonic provinces, classified the principal tectonostratigr aphic units into three ''Divisions'', and separated the tectonomagmati c history into five discrete ''Events'', ranging in age from similar t o 1800 Ma to similar 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 provinc es, 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 canno t be time equivalents, Division 1 rocks grade into Division 2 in the n orthern region, and Division 3 of the northern region may be equivalen t to Division 1 in the central region. Rocks originally assigned to di visions are better grouped into a larger number of lithological assemb lages until better stratigraphic and isotopic correlations can be made . Compilation of Rb-Sr and Ar-40/Ar-39 age determinations obtained bef ore 1984 indicate an almost complete age spectrum from 1800 Ma to 1000 Ma, but age clusters exist at similar to 1400 Ma, similar to 1100 Ma and 400-300 Ma, which have geological significance. Subdivision of old er ages into discrete tectonic events was not justified from this data alone, but more recent U-Pb isotopic analysis of zircons, in rocks de signated to particular structural/metamorphic events, has revealed a c omplex 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 similar to 1600 Ma. The revised tectonic history nomenclature is proposed for the Arun ta Inlier: following deposition of a widespread turbiditic unit at or before 1880 Ma, possibly on thinned Archaean crust, the northern and c entral provinces underwent localized tectonism/magmatism at similar to 1880 Ma (Yuendumu tectonic event) and 1820 Ma (Stafford tectonic even t), prior to the widespread Strangways orogeny, which is subdivided in to an early phase at 1780-1770 Ma and late phase at 1745-1730 Ma. It i s not clear whether the Strangways orogeny is a single, Arunta-wide ev ent or a series of separate short-lived, smaller-scale events, althoug h the major deformation in all areas appears to have involved W- to SW -directed thrusting. In the southern province, the major events occurr ed at similar to 1680 Ma (Argilke tectonic event) and similar to 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. T he 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 Ar- 40-Ar-39 systems in hornblende from mafic granulite; outcrop-scale Rb- Sr total rock isotopic systems in mylonitized granitoids; and Sm-Nd ga rnet-hornblende pairs in mylonitized amphibolites. A similar to 1150 M a thermal event in the southern part of the Inlier (Teapot magmatic ev ent) is recorded by intrusion of granite, pegmatites, and the alkaline Mordor Igneous Complex, and by isotopic resetting of Rb-Sr mineral sy stems. Dolerite dyke injection at similar to 1050 Ma reflects limited extensional tectonism that may have related to formation of the Amadeu s Basin. The second major uplift event was the Alice Springs orogeny a t 400-300 Ma, an unusual intracratonic, thick- and thin-skinned thrust event that involved limited retrograde amphibolite facies, but extens ive greenschist facies metamorphism, and restricted pegmatite intrusio n.