Tertiary volcanism in the Cape Hillsborough area, North Queensland

Abstract

Potassium‐argon ages on alkali feldspar phenocrysts from trachyte lavas and on trachyandesites of the Cape Hillsborough Beds, which crop out north of Mackay, Queensland, yield concordant ages of 32.5 ± 0.4 m.y. This age is Early Oligocene and by correlation provides much firmer control than previously available on the age of the sediments deposited in the Hillsborough Basin. Isotopic ages on an altered biotite and on alkali feldspar from the Mount Jukes Syenite Complex, to the southwest of Cape Hillsborough, are indistinguishable from those measured on the Cape Hillsborough Beds, suggesting a possible genetic relationship between the lavas and the syenite intrusions. Present evidence indicates that Cainozoic volcanism in Queensland occurred in two main eruptive episodes. The earlier episode covered much of the Oligocene and its products are widespread in the southern half of the State. After a long hiatus of little or no volcanism there followed a later episode of volcanism in the Pliocene and Quaternary when widespread basaltic eruptions occurred, mainly in North Queensland.