Gawler Range magmatism—Further isotopic age data

Zircon U‐Pb age determinations have been made on Gawler Range type acid volcanics from Saint Francis Island, Great Australian Bight (1631±3 Ma), and the crosscutting Hiltaba Granite of the Gawler Ranges (1514± 32 24 Ma). The time of Gawler Range Volcanism is extended back by at least 100 Ma (140 Ma at St Francis Island) from earlier measurements, and now approaches that of volcanism in the Broken Hill, Willyama Sequence. Rb‐Sr total rock isochrons on such material give post‐crystallization ages, and initial 87Sr/86Sr ratios obtained from non‐perfect isochrons are misleading petrogenetic indicators. The St Francis Island acid volcanics appear to be I‐type magmas whereas associated, mildly alkaline granitoids show some A‐type character.