Age of the Great Lake basalts, Tasmania, in relation to Australian Cainozoic volcanism

Abstract

K/Ar age determinations (23.6-21.8 m.y.) date the Great Lake basalts as latest Oligocene/earliest Miocene, a significantly older age than was previously assigned to them on physiographic grounds. Slight differences in the ages are consistent with the established field succession. Palaeomagnetic data determined on some of the dated rocks are consistent with such ages. The dates establish that Great Lake, its structural depression, and the subbasaltic Derwent tributary drainage are relatively old features belonging to the Lower Tertiary. They also suggest that similar basaltic sequences elsewhere down the Derwent drainage are older than previously supposed, leaving little young volcanism known in Tasmania. The concept of a strong Oligocene-Miocene peak in the Tasmanian volcanism can be matched with the New South Wales and Queensland patterns and contrasts with the Victorian pattern. The eastern Australian volcanic patterns, based on isotopic and other data, are compared with periods of known tectonism and adjacent sea-floor spreading episodes over the last 80 m.y. This suggests some correlations of volcanism with local tectonism. Although some volcanism accompanied initial spreading episodes, the great bulk was not directly linked to them, but was active after the main spreading between Australia and Antarctica began 55 m.y. ago.