The base of the Fortescue Group, Western Australia: further galena lead isotope evidence on its age

Abstract

Traces of galena have been found within amygdales of the Kylena Basalt, third unit of the Fortescue Group which is, by definition, the oldest sequence within the Proterozoic Hamersley Basin of the NW of Western Australia. Mutually-supporting Pb-isotope data from two laboratories yield a model age 2.76 ± 0.03 Ga, in good agreement with Rb-Sr and U-Pb datings of stratigraphically equivalent material. The low indicated source U/Pb is in accord with the observed basalt association, and contrasts with some other, fluorite-associated, samples of comparable model age, for which a ‘granite’ association is postulated. Lead ratios from a stratiform deposit within the Whim Creek Group, defined as of Late Archaean age, yield a model age 2.95 ± 0.01 Ga, in accord with a published Rb-Sr mineral age. Hence the base of the Fortescue Group, generally regarded as initiating the Early Proterozoic Era in Australia, cannot be younger than 2.8 Ga. This date has implications for future definitions of the Archaean-Proterozoic bo...