Early Palaeozoic mafic magmatism associated with intracontinental rifting in the Harts Range, Central Australia

Abstract

Mafic sills intrude the Upper Stanovos Gneiss in the southeastem Arunta Inlier, central Australia referred to here as the Stanovos Igneous Suite (SIS). The host rock Upper Stanovos Gneiss was deposited in a localised depo-centre within the Amadeus Basin at 545Ma from sediments derived from the Musgrave Block. The SIS intruded the Upper Stanovos Gneiss in a deep (approximately 16km) localised strike­slip basin (the Irindina sub-basin) formed during intracratonic rifting. LA-ICPMS U/Pb zircon dating of granites associated with the mafic rocks gives 524Ma ± 8Ma. The intrusions occurred at 5.3 kbars at temperatures > 1150°C. The mafic sills are extensively boudinaged due to the extensional Larapinta Event that occurred during the early Ordovician 480-460Ma. The event metamorphosed the Irindina sub-basin sediments and mafic volcanics to amphibolite and granulite grade. The extent of rifting was insufficient to have enabled partial melting of the lithosphere to occur via passive rifting. This combined with coeval widespread occurrences of mafic magmatism throughout Australia, including the continental flood basalts with the same characteristic low Ba/Rb, Nb/Th and TiN geochemical features as the SIS, indicate the presence of a mantle plume beneath the continent during the mid to late Cambrian. The SIS has tholeiitic affinities and a N-MORB like mantle signature altered by fractional crystallisation and crustal assimilation. Two component mixing isotopic ratios indicate that the mafic rocks have a crustal influence and the granites have a mantle input. Geochemistry and isotopic signatures of the mafic rocks provide insights into the evolution of the mantle reservoir beneath a newly recognised early Palaeozoic intracratonic rift in central Australia.