Australia: The Land Where Time Began

A biography of the Australian continent 

Neoproterozoic Basaltic Magmatism, South-Central Australia Ion Microprobe U-Pb Ages and Implications for Rodinia Breakup

Precise estimates of the age of mafic magmatism from the Neoproterozoic that was associated with the initial rifting that occurred in the Adelaide Geosyncline and adjacent platforms in central and southern Australia has been achieved by the use of ion microprobe U-Th-Pb analyses of baddeleyite and zircon. Based on 137 ²⁰⁷Pb-²⁰⁶Pb analyses of 66 baddeleyite samples from a drill core of dolerite from the Gairdiner Dyke Swarm (GDS) an age of 827 ± 6 Ma is inferred for the emplacement of the GDS. It has been suggested that the GDS, together with mafic intrusive suites from 3 areas of western North America that are widely separated, from about 780 Ma, represent sectors of a giant radiating swarm of dykes that was separated when Australia broke from Laurentia as the Rodinia supercontinent was breaking up. In this paper the authors¹ argue that the results they obtained show that the Australian dykes are at least 40 My older than the North American intrusions, ruling out their emplacement during the same event. Situated close to the inferred margin of Australia during the Neoproterozoic the Little Broken Hill gabbro and associated dykes have previously been linked to extension occurring as Australia was breaking from Laurentia. The authors¹ suggest that if this is correct a zircon age of 827 ± 9 Ma indicated by a  ²⁰⁷Pb-²⁰⁶Pb analysis obtained for the gabbro, together with evidence of the GDS being related to plume activity instead of being related to breakup, it would indicate that the breakup commenced soon after the emplacement of the GDS at 827 Ma and that the mantle plume probably had an active role in the triggering the process. In the Little Broken Hill gabbro breakdown of primary baddeleyite to form overgrowths of polycrystalline zircon occurred at 500 ± 7 Ma, which thereby provided a precise estimate for the age of the thermal event that was associated with the Delamerian Orogeny in this region. The GDS and related rocks in Australia have been shown by the results obtained by the authors¹ to be roughly coeval with mafic igneous rocks from a similar stratigraphic position in the Lower Sinian System and may support the hypothesis that sections of South China were located between Australia and Laurentia prior to the breakup of Rodinia

Sources & Further reading

1. Wingate, Michael T. D., Ian H. Campbell, William Compston, and George M. Gibson. "Ion Microprobe U–Pb Ages for Neoproterozoic Basaltic Magmatism in South-Central Australia and Implications for the Breakup of Rodinia." Precambrian Research 87, no. 3–4 (2// 1998): 135-59.