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

Analyses of shales and basalts from all stratigraphic levels below the Wono ka formation in the Neoproterozoic Adelaide Geosyncline combine to define a 586 +/- 30 Ma Rb-Sr isochron with an initial Sr-87/Sr-86 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 f all between their primary contemporary seawater values and those of the ela stic basin fill. By contrast the Neoproterozoic carbonates from strata incl uding and younger than the late Vendian Wonoka formation conform to interna tionally correlated chemostratigraphic variations for both Sr-87/Sr-86 and delta C-13. These data, together with S- and Sr-isotopic evidence from bari te veins which cut late Marinoan shales, are interpreted to indicate that t he Adelaidean succession experienced a phase of intrabasinal fluid flow; th e fluid had an Sr-87/Sr-86 value of 0.7180 when convective circulation was terminated at similar to 586 Ma. This timing is synchronous with down-cutti ng of basin fill to form canyons beneath the Wonoka formation and also coin cides with the production of rift-affiliated alkaline lavas to the east of the Curnamona craton in western NSW. The interpreted changed fluid flux reg ime, together with the other geological features listed, may indicate the o nset of a new phase of extension and rifting of East Gondwanas eastern marg in at 586 +/- 30 Ma, contributing to a growing body of evidence that sugges ts that the main phase of proto-Pacific opening may have been Vendian rathe r than Sturtian in age. The proposed basinal fluid flow event is of economi c importance as it mobilised Cu and other base metals, and may have carried them to depositional sites in the thin platformal western marginal success ion of the basin on the Stuart shelf (C) 2001 Elsevier Science B.V. All rig hts reserved.