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This digital data represents an interpretation of the metamorphic geology of Geoscience Australia's Northern Australian Craton project (NAC) in the Northern Territory and Western Australia. It consists of linework, polygons and attributes for both metamorphosed and non-metamorphosed geological features.

Metamorphic map of Australia

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The Harts Range Metamorphic Complex (HRMC) in the eastern Arunta Province is part of the exhumed core of the intracratonic Alice Springs Orogen. SHRIMP U-Pb dating of intrusive rocks and metamorphism in the HRMC has constrained the timing and character of tectonism, showing that mutliphase deformation under high grade metamorphic conditions spanned at least 50 m.y. The age of a syn-tectonic pegmatite dyke in the northern HRMC records ductile shearing on E-W trending shear zones during or just after the ~380 Ma Pertnjara Movement, possibly related to extension after the main contractional phase. The age of a second syn- to late-deformational pegmatite dyke in the eastern Harts Range suggests that a SW-vergent fold and thrust system in the eastern HRMC formed during the ~360 Ma Brewer Movement. This deformation coincided with the intrusion of small granitic bodies in the northern HRMC, and the formation of rare metamorphic zircon in metasedimentary rocks during a period of crustal thickening. Metamorphic monazite and zircon reflect high-grade metamorphism during the ~330 Ma Mount Eclipse Movement, which coincided with the formation of a flat-lying, kyanite grade foliation in basement rocks of the HRMC. This foliation and later large-scale doming might reflect extensional collapse of the Alice Springs Orogen towards the end of the orogenic cycle. 40Ar-39Ar cooling ages indicate that much of the HRMC was exhumed at that time. Localisation of ASO tectonism in the eastern Arunta Province appears to be a result of thermal weakening associated with the ~480-460 Ma Larapinta Event, which partitioned plate boundary stresses into central Australia.

This study arose out of the regional geological mapping of the Cairns Hinterland which was begun in 1956 as a joint undertaking by the Bureau of Mineral Resources and the Geological Survey of Queensland. It is another unit in the series of dating surveys stemming from the co-operation between the Australian National University and the Bureau of Mineral Resources. One of us (D.A.W.), assisted by F. de Keyser, was responsible for the supervision of the field operations connected with this study; another (C.D.B.) was concerned with the general investigation of these acid igneous rocks; the others (J.R.R. and A.W.W.) were assisted by J. A. Cooper in the production of the K-Ar results. We are indebted to W. Compston and M. J. Vernon for permission to publish the Rb-Sr results on the Palaeozoic rocks, and for assistance with measurements on the Croydon Volcanics. Samples were chosen with the aim of defining more closely the time limits of the two periods of activity that were thought to have produced the granites and acid volcanic rocks now covering about half the area. Regional mapping evidence had already suggested that a Precambrian age for the older granites was a reasonable possibility, and that the second event took place in the late Palaeozoic. Fossils collected by the mapping parties suggested that these periods were pre-Upper Ordovician and late Devonian/Carboniferous to Permian/Triassic (White, 1961). The present set of age data offers a most gratifying confirmation of some of these conclusions and provides a basis for the selection of samples for further study.

Detrital zircon geochronology of high-grade metasedimentary rocks of the Harts Range Group (HRG) in central Australia indicates that its protoliths were deposited during the Neoproterozoic and Cambrian, coeval with sedimentation in the adjacent Amadeus and Georgina basins of the former Centralian Superbasin. The similar provenances of the HRG and basin successions imply that the HRG is the high-grade metamorphic correlative of the basin sequences. Metamorphic zircon formation at ~477 Ma and ~459 Ma appears to reflect peak and retrograde phases of the Early Ordovician Larapinta Event. Palaeogeographic reconstructions indicate that burial and metamorphism took place beneath an epicratonic sea, associated with the formation of a flat-lying foliation in the lower crust and tholeiitic magmatism, consistent with an extensional setting. Burial of the HRG to ~30 km appears to have taken place predominantly by sedimentary loading within an exceptionally deep intracratonic rift basin, the depth of which rivals those of the deepest basins in Earth history. This indicates that lower crustal high-grade metamorphism need not reflect compressional thickening of the crust.

The timing and duration of metamorphic events is commonly constrained by radiometric dating using the U-Pb or 40Ar-39Ar dating methods, or a combination of both. Each dating method can be applied to a different range of minerals, and a combination of the two methods can provide more complete timing constraints than either method on its own. Comparison of radiometric ages from different isotopic systems introduces the problem of systematic uncertainties arising from uncertainty in parameters such as decay constants and the age of method-specific reference materials. Over the past decade it has been increasingly recognized that the laboratory-based determinations of the 40K decay constants, on which the 40Ar-39Ar method is based, are relatively imprecise and that the values recommended by Steiger and Jager (1977) result in a systematic offset of 40Ar-39Ar ages relative to U-Pb-derived ages by up to ~1%. This problem has been addressed by several studies over the past decade, with the most recent study (Renne et al., 2010; 2011) providing refined estimates for the 40K decay constants, and very significant improvements in precision. Paleozoic and Paleoproterozoic examples will be presented which illustrate the improvements in the accuracy and precision of 40Ar-39Ar ages calculated using the revised decay constants, and discuss the implications for studies that use a combination of U-Pb and 40Ar-39Ar data to constrain the timing and duration of metamorphic, deformation, and mineralisation events. An Excel spreadsheet is available on request that allows recalculation of 40Ar-39Ar ages and uncertainties using the revised parameters of Renne et al. (2010; 2011), provided certain minimum information has been reported with the published ages.

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