Radioactive ages from the Precambrian rocks in Australia

Abstract

The central nucleus of the Australian Precambrian shield lies in the western half of the continent and is characterized by striking similarities in structure and age over a vast area. Ages in the vicinity of 2,700 m.y. have been determined at many localities by the Rb/Sr and K/A methods. Flanking and cutting the shield are much younger belts of Precambrian rocks which have been fused onto the older shield. On the south‐eastern side, the deep‐seated Fraser Fault separates the Goldfields rocks (2,700 m.y.) from pegmatized basic charnockites (1,300 m.y.). In this region the shield rocks also show a metamorphism at 2,400 m.y. A similar structure separates the nucleus on the southern side from the east‐trending rocks of the south coast which were pegmatized about 1,400 m.y. ago. The western margin of the shield bounded by the Darling Fault shows evidence of vigorous recrystallization (about 650 m.y.) and regional magmatic activity (900 to 1,100 m.y.). A long E.‐W. 1,000 m.y. belt in central Australia may continue westward to divide the central nucleus of the shield. So far no basement rocks with ages in excess of 2,000 rn.y. have been found on this continent outside Western Australia. Granites in northern Australia appear to be at least 1,650 m.y. old, with a uranium mineralization at 500 m.y. In South Australia two periods of uranium mineralization have been recognized at 500 and 1,500 m.y. The 500 m.y. event is particularly well documented at Myponga, where there is agreement between the U/Pb, K/A and Rb/Sr ages of minerals from the main uranium lode channel. Lead mineralization at Broken Hill and Mount Isa also appears to have occurred in the vicinity of 1,500 m.y. ago. The major age divisions in Canada and Australia appear to be closely comparable.