Potassium‐argon ages for some Australian Mesozoic igneous rocks

Abstract

Potassium‐argon dating shows that Late Triassic to Late Jurassic mainly alkaline igneous activity was widespread in New South Wales and Victoria; it overlapped the Middle Jurassic tholeiitic basaltic activity in Tasmania and South Australia. The more accurate new ages include 205 ± 3 m.y. for microgranodiorites from the Middle Brother in the Lorne Basin, 190 to 177 m.y. for intrusions near Mittagong, 163 ± 8 m.y. for the Myalla Road Syenite near Cooma, 207 ± 4 m.y. for syenitic intrusions near Benambra, Victoria, and 170 ± 5 m.y. for basalt on Kangaroo Island in South Australia. The tholeiitic Bunbury Basalt of Western Australia is at least 90 m.y. old.

Much of this magmatism is typical of that associated with the final stages of stabilization of an orogenic zone. The magmatism is thought to be related to the initiation of rifting in this part of Gondwanaland, preceding the separation of the New Zealand‐Lord Howe Rise and Antarctic continental blocks from Australia.