The geochronology of the igneous rocks of Eastern Queensland

Abstract

Isotopic dating of igneous rocks in eastern Queensland south of Bowen has indicated phases of granite emplacement in the Devonian (360 m.y.), Carboniferous (310 and 285 m.y.), Permian (265, 245 and 235 m.y.), Triassic (220 m.y.) and Cretaceous (125 and 110 m.y.). This activity moved generally eastwards with time.

The combination of Rb‐Sr and K‐Ar dating has produced a better understanding of the geological history in the complex area of the Connors Arch than would have resulted from the use of either method alone. Also, it was found that, contrary to predictions that the argon retentivity of hornblende is greater than that of biotite during metamorphism, the two minerals had a similar degree of retentivity under burial metamorphism.

The relationship between granite intrusion and tectonic phases is examined. The emplacement of Carboniferous granites in the Connors and western Auburn Arches was synchronous with the Kanimbla Movement, while episodes of igneous intrusion in the Late Permian, from Marlborough south to New England, can be correlated with phases of the Hunter‐Bowen Orogeny. Other intrusions appear to be unrelated to tectonism.

The granites have low initial ⁸⁷Sr/⁸⁶Sr ratios similar to those found in materials that are probably derived from the Earth's mantle. They contrast with the high initial ⁸⁷Sr/⁸⁶Sr ratios of granites adjacent to the Precambrian rocks in north Queensland, and are interpreted as indicating that if Precambrian crust underlies the Tasman Geosyncline in central and southern Queensland, it has not played a significant role in the genesis of the granites.