Ion microprobe U–Pb ages for Neoproterozoic basaltic magmatism in south-central Australia and implications for the breakup of Rodinia

Abstract

Ion microprobe U–Th–Pb analyses of baddeleyite and zircon yield precise estimates of the age of Neoproterozoic mafic magmatism associated with initial rifting in the Adelaide Geosyncline and adjacent platforms in southern and central Australia. An age of 827±6 Ma is inferred for emplacement of the Gairdner Dyke Swarm (GDS), based on 137 207Pb/ 206Pb analyses of 66 baddeleyite crystals from a drill-core sample of GDS dolerite. It has been proposed elsewhere that the GDS, together with ∼780 Ma mafic intrusive suites in three widely separated areas of western North America, represent sectors of a giant radiating dyke swarm fragmented by the separation of Australia and Laurentia during breakup of the Rodinia supercontinent. Our results show that the Australian dykes are at least 40 Ma older than the intrusions of North America and therefore cannot have been emplaced during the same event. The Little Broken Hill gabbro and associated dykes, situated close to the inferred Neoproterozoic margin of Australia, have been linked previously with extension related to breakup between Australia and Laurentia. If correct, then a zircon 207Pb/ 206Pb age of 827±9 Ma obtained for the gabbro, coupled with evidence that the GDS is plume-related rather than breakup-related, indicates that breakup commenced soon after emplacement of the GDS at 827 Ma and that the mantle plume is likely to have played an active role in triggering the process. Breakdown of primary baddeleyite in the Little Broken Hill gabbro to form overgrowths of polycrystalline zircon occurred at 500±7 Ma, thereby providing a precise estimate of the age of the thermal event associated with the Delamerian Orogeny in this region. Our results show the GDS and related rocks in Australia to be roughly coeval with mafic igneous rocks in similar stratigraphic position in the Lower Sinian System and may lend support to the hypothesis that parts of South China were situated between Australia and Laurentia prior to the breakup of Rodinia.