Potassium‐argon ages in eastern Australia

Abstract

From the results of this preliminary survey it would appear that the bathyliths of the southern belt of the Tasman Geosyncline represent a history of continuous tectonic activity from Middle Silurian through to Middle Devonian, with a gradual movement eastwards of the axis of the intrusions. The gneissic “Ordovician‐type” granite is dominant in the Upper Silurian, just preceding the foliated “Silurian‐type” plus massive types which are dominant in the Lower Devonian, but with significant overlap in time between them to the extent that “rock type” is not a safe determinant of age. It is therefore recommended that this terminology be no longer used. A few cases of Carboniferous intrusion have been observed. These generally are transgressive to the main trend. Later intrusions occurred to the east and north, ranging from Permian in New England to Mesozoic further north, in good agreement with expectation. Smaller Mesozoic instrusions in the southern coastal region have ages different from those anticipated. Measurements on four drill cores from the Great Artesian Basin confirm trends in basement boundaries suggested by the new Tectonic Map of Australia. One sample of the Ferrar Dolerite of Antarctica confirms a suggested comparison with the Tasmanian dolerite. Three results from the Adelaide Geosynçline confirm current ideas of a tectonic event in the Cambrian‐Ordovician in that area. Examples are presented which illustrate the effect of subsequent tectonic heating and/or proximity to a fault on argon loss; confirm recent suggestions that hornblende is a suitable material for K‐Ar rock dating; and concur with recent suggestions that low‐potassium, high temperature feldspars may also be suitable material. New measurements, including some obtained in this work, are reviewed and suggest essential agreement with the recently published time scales of Holmes and Kulp with some minor adjustments, principally that the base of the Ordovician should be somewhat higher at 480 m.y.