Volcanism in the rift-valley system that evolved into the western margin of Australia

The Mesozoic forerunner of the western margin of Australia has been regarded tectonically as an ancient analogue of the multiple rift-valley system of East Africa, which comprises two arms: volcanic on the E, and virtually non-volcanic on the W. The abundance of widespread volcanics recently dredged and cored along the outermost margin, which corresponds with the volcanic arm of the East African system, contrasts with the apparent scarcity of volcanics inshore, in the inner arm of the rift system. We tested the possibility that volcanogenic material has been overlooked inshore by a petrographic study of the Late Jurassic to earliest Cretaceous Yarra-gadee Formation of the Perth Basin; only rare possible pyroclasts of quartz and glass (probably emplaced by air-fall from the volcanic outer arm) were found, confirming the contrast in volcanism between the arms. This petrological evidence, together with the appropriate range of composition of the volcanism, from silicic to mafic, including alkaline and peralkaline members, reinforces the analogy with East Africa.