A radiometric estimate of the duration of sedimentation in the Adelaide geosyncline, south Australia

Abstract

The Adelaide System forms the uppermost Precambrian sequence in South Australia and the Wooltana Volcanics lie near its base. Though affected by Palaeozoic metamorphism, the least‐altered samples give a minimum age of 850 ± 50 m.y., so that the base of the System is about 900 m.y. old or more. The unmetamorphbsed Roopena Volcanics of northeastern Eyre Peninsula are 1,345 ± 30 m.y. old and if correlated with the Wooltana Volcanics the base of the system becomes about 1,400 m.y. old. The data for the Wooltana Volcanics are consistent with this, provided that even the least‐altered total‐rock samples were open systems during the later metamorphism. Ages of basement in the Mount Painter and Olary districts (1,600 m.y.) and data for Willouran shales overlying the Wooltana Volcanics can fit both minimum and maximum estimates for the Volcanics. Lower Cambrian shales give a range of 530–690 m.y.; though some Palaeozoic isotopic movement occurred, the ages are approximately correct. Shales from the top of the Torrensian Series range from 660–840 m.y. (700 m.y. preferred value). If the base of the system is at 1,400 m.y., this is surprisingly young. It suggests either a hiatus between the Wooltana Volcanics and the Torrensian or that the correlation of the former with the Roopena Volcanics is wrong (and that the base is at about 900 m.y.). Alternatively, the shales may be abnormally updated. The Gawler Range Volcanics of Eyre Peninsula have been dated accurately at 1,535 ± 25 m.y. and illitic shale from the penecontemporaneous Corunna Conglomerate gives nearly the same value. These ages indirectly set a maximum for the age of the base of the system, as stratigraphy suggests that they are older. Granites underlying the Gawler Range Volcanics are about 1,600 m.y. old; some may be 1,800 m.y. old. Final Palaeozoic metamorphism in the northern Flinders Ranges was at 465 m.y. The ages of several post‐orogenic intrusions are given.