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

∗Appendix 2 [indicated by an asterisk (∗) in the text and listed at the end of the paper] is a Supplementary Paper; copies may be obtained from the Geological Society of Australia's website (www.gsa.org.au) or from the National Library of Australia's Pandora archive (http://nla.gov.au/nla.arc-25194).

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.

Key words:

• argon-argon dating
• Cenozoic
• geochronology
• hotspots
• plate migration rates
• volcanism

Acknowledgements

We would like to thank Tony Ewart for his discussions and assistance in setting up the project, the staff at the Queensland Museum for their help on finding archived rocks, the Queensland National Parks and Wildlife Service for providing access to National Park land, and David Thiede and Robert Bolhar for helpful comments on the manuscript. The assistance provided by David Thiede during the analytical work is also much appreciated. Lin Sutherland and Ian McDougall are thanked for providing detailed and helpful reviews of the manuscript. Construction of the ⁴⁰Ar/³⁹Ar laboratory at the University of Queensland was partially funded by ARC Equipment Grant A39531815, and the ⁴⁰Ar/³⁹Ar analyses in this study were funded by UQ-AGES.

Notes

∗Appendix 2 [indicated by an asterisk (∗) in the text and listed at the end of the paper] is a Supplementary Paper; copies may be obtained from the Geological Society of Australia's website (www.gsa.org.au) or from the National Library of Australia's Pandora archive (http://nla.gov.au/nla.arc-25194).