40Ar/39Ar constraints on the timing of Oligocene intraplate volcanism in southeast Queensland*

Abstract

Laser 40Ar/39Ar analyses were undertaken on Oligocene bimodal intraplate volcanic rocks from five volcanic areas spanning 320 km along a north-northeast-trending belt in southeast Queensland. Sixty-one mineral grains and groundmass fragments from 21 samples were analysed by incremental heating, while 99 mineral grains from 10 samples were analysed by total fusion. Fourteen samples are from localities dated previously by the K – Ar method. The 40Ar/39Ar ages are reproducible among different aliquots from the same sample and different samples from the same location, and correlate with latitude, ranging from 30.6 ± 0.3 Ma on Fraser Island in the north (25°S) to 25.8 ± 0.2 Ma at Flinders Peak in the south (27.7°S). This age progression is not evident from K – Ar ages compiled for the same region. The decrease of 40Ar/39Ar ages with latitude is consistent with previous suggestions for a hotspot origin for east Australian central volcanoes and yields an Australian Plate velocity, relative to the postulated fixed hotspot reference frame, of 71 (+7, −4) mm/y at an azimuth of N10°E for the period of ca 31 – 26 Ma. This velocity agrees within error with the values of about 65 ± 3 mm/y obtained from previously reported K – Ar analyses of east Australian central volcano provinces active in the last 35 million years. These results demonstrate that the improved accuracy and precision of the 40Ar/39Ar method permit resolution of age versus latitude relationships for narrower time windows, which may potentially provide constraints on changing plate velocities with time. This improved temporal resolution may also contribute to resolving current debate over the existence and the stationary versus mutable position of postulated mantle plumes and hotspots.