Grenvillian-aged reworking in the North Australian Craton, central Australia: Constraints from geochronology and modelled phase equilibria

Abstract

In situ LA-ICP-MS U–Pb monazite geochronology and garnet Sm–Nd geochronology, combined with calculated metamorphic phase diagrams, indicate that the eastern Warumpi Province along the southern margin of the North Australian Craton has undergone Grenvillian-aged reworking. Rocks in the eastern Warumpi Province are characterised by a pervasive flat lying fabric, with a NE trending lineation that is usually associated with top to the north shear sense. This flat lying fabric has been overprinted by regional-scale E-W trending upright folds. Calculated metamorphic phase diagrams based on garnet-staurolite-andalusite-bearing metapelites indicate a clockwise pressure–temperature ( P– T) evolution involving decompression and heating from maximum pressures of 4 to 4.5 kbar at 530 °C to maximum temperatures of ∼570 °C at 3.5 kbar. Sm–Nd isotopic analyses of prograde-zoned garnet within these metapelites yield ages of 1141 ± 22 Ma and 1100 ± 22 Ma. Monazite within the fabric that encloses these garnets yield LA-ICP-MS U–Pb ages in the range 1140–1109 Ma. The age data suggest that the flat lying fabric and regional folding in the eastern Warumpi Province is associated with Grenvillian-aged reworking of the late Paleoproterozoic protoliths. Dolerite dykes belonging to the 1080 Ma Stuart Dyke Swarm cross cut the upright folding, constraining the deformation to the interval 1140–1080 Ma. The late Mesoproterozoic timing of deformation and metamorphism contrasts with previous work, which attributes the tectonic architecture in the eastern Warumpi Province to early Mesoproterzoic (ca. 1590 Ma) tectonism. The metamorphic conditions indicate a high geothermal gradient regime, associated with the formation of a regional, low-angle fabric and decompression which is suggestive of an extensional setting. The timing of the deformation in the eastern Warumpi Province is synchronous with the latter stages of high- T magmatism and metamorphism in the Musgrave Province, several hundred kilometres to the south. The similarity in age of high geothermal-gradient metamorphism in both regions implies that the effects of the Grenvillian-aged tectonism in central Australia are more extensive than previously understood. It also suggests that much of the basement to the intracratonic Amadeus Basin, which now separates the two provinces, also underwent deformation and metamorphism during this event.