Radiometric ages of some precambrian rocks from the Northwest division of Western Australia

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Abstract Rb‐Sr age measurements are reported for acid volcanic rocks, a tuffaceous siltstone, and two granites, all of Precambrian age, from the northwest division of Western Australia. The lava samples are from four localities within the Woongarra Volcanics, a formation situated near the top of the “Proterozoic” Hamersley Group, formerly a part of the “Nullagine Beds”. On field evidence these samples are of the same age, so that the Rb‐Sr data have been interpreted as giving approximately 2,100 m.y. for this age, assuming a variation in initial Sr87Sr86 of 0.700 to 0.720. The tuffaceous siltstone, from the overlying Wyloo Group, has a maximum indicated age of 1,850 m.y. This value is subject to uncertainty since only one sample was analysed, but nevertheless it is significantly younger than the Woongarra Volcanics, in accordance with its strati‐graphic position, and it is likewise consistent with concordant total‐rock and mineral ages of 1,720 m.y. measured in the Boolaloo Granite, which is intrusive into the Wyloo Group. Owing to suspected later metamorphism only broad limits of 2,300 to 3,000 m.y. can be set for the age of an “Archaean” granite which is older than the oldest “Proterozoic” sediments. Two unpublished U‐Pb and Rb‐Sr ages from the same area communicated to the authors are also reported.