Isotopic age of the Tollu Volcanics, W.A.

The Tollu Volcanics have been dated by the Rb ‐ Sr isochron method at 1060 ± 140 million years (95% certainty limit). The folded volcanics have an initial 87Sr/86Sr ratio of 0.705 and the scatter of the total‐rock points about the isochron is ascribed to post‐crystallisation redistribution of Rb and Sr. A rhyolite specimen from the Warburton area to the west fits on the isochron without significantly increasing the statistical error. The geological setting of the volcanics is briefly described. The importance of this age to the dating of the Late Proterozoic sedimentary sequence of the region is pointed out.