Sr-isotopic evidence for Late Neoproterozoic rifting in the Adelaide Geosyncline at 586 Ma: implications for a Cu ore forming fluid flux

Abstract

Analyses of shales and basalts from all stratigraphic levels below the Wonoka formation in the Neoproterozoic Adelaide Geosyncline combine to define a 586±30 Ma Rb–Sr isochron with an initial 87Sr/86Sr value of 0.7180. The Sr-isotopic composition of most carbonate units deposited in this basin before this time are also altered and have Sr-isotopic compositions that fall between their primary contemporary seawater values and those of the clastic basin fill. By contrast the Neoproterozoic carbonates from strata including and younger than the late Vendian Wonoka formation conform to internationally correlated chemostratigraphic variations for both 87Sr/86Sr and δ13C. These data, together with S- and Sr-isotopic evidence from barite veins which cut late Marinoan shales, are interpreted to indicate that the Adelaidean succession experienced a phase of intrabasinal fluid flow; the fluid had an 87Sr/86Sr value of 0.7180 when convective circulation was terminated at 586 Ma. This timing is synchronous with down-cutting of basin fill to form canyons beneath the Wonoka formation and also coincides with the production of rift-affiliated alkaline lavas to the east of the Curnamona craton in western NSW. The interpreted changed fluid flux regime, together with the other geological features listed, may indicate the onset of a new phase of extension and rifting of East Gondwana's eastern margin at 586±30 Ma, contributing to a growing body of evidence that suggests that the main phase of proto-Pacific opening may have been Vendian rather than Sturtian in age. The proposed basinal fluid flow event is of economic importance as it mobilised Cu and other base metals, and may have carried them to depositional sites in the thin platformal western marginal succession of the basin on the Stuart shelf.