Isotopic ages of Proterozoic acid volcanics and associated granitic intrusions in the northeast and southwest part of The Granites-Tanami region are reported and discussed. In the northwest, within The Granites-Tanami Block, the Mount Winnecke Formation, which includes acid volcanics that were probably erupted underwater, is intruded and weakly metamorphosed by the high-level Winnecke Granophyre. The acid volcanics and intrusives have similar major element compositions, and this, together with the field evidence, indicates that they are probably comagmatic. Rb-Sr total rock and mineral data for the Winnecke Granophyre yield an isochron indicating an age of 1802 ± 15 m.y. and an initial Sr87/Sr86 of 0.7074 ± 0.0036. Isotopic data for the lavas of the Mount Winnecke Formation give a preferred age of 1808 ± 15 m.y. and an initial Sr87 /Sr86 of 0.7052 ± 0.0038. In the southwest, at the western extremity of the Arunta Block, another acid volcanic suite, belonging to the Pollock Hills Formation, is intruded and weakly metamorphosed by the Mount Webb Granite. As in the northeast, the volcanics and intrusives have similar major element compositions, and are considered to be comagmatic. Combining Rb-Sr isotopic data from both the volcanics and intrusives yields an isochron indicating an age of 1526 ± 25 m.y. and an initial Sr87/Sr86 of 0.711 ± 0.004. These rocks are appreciably younger than those in the northeast. The acid igneous rocks in the southwest also postdate three other newly dated granitic units in The Granites-Tanami Block - the Lewis Granite, 1720 ± 8 m.y.; the Slatey Creek Granite, 1770 ± 55 m.y.; and The Granites Granite, 1780 ± 24 m.y. - and are younger than the Gardiner Sandstone, the basal unit of the Birrindudu Group, which unconformably overlies The Granites-Tanami Block. K-Ar and Rb-Sr ages determined on glauconite from the Gardiner Sandstone show little consistency, due evidently to partial loss of radiogenic daughter products, but a preferred K-Ar age of 1560 ± 20 m.y. is considered a reasonable minimum estimate for the age of sedimentation and diagenesis in the unit.
