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Abstract
Prolonged intraplate volcanism along the 4000 km-long East Australian margin for ca 100 Ma raises many genetic questions. Studies of the age-progressive pulses embedded in general basaltic activity have spawned a host of models. Zircon U–Pb dating of inland Queensland central volcanoes gives a stronger database to consider the structure and origin of Australian age-progressive volcanic chains. This assists appraisal of this volcanism in relation to plate motion and plate margin tectonic models. Inland Queensland central volcanoes progressed south-southeast from 34 to 31 Ma (∼5.4 cm/yr) until a surge in activity led to irregular southerly progression 31 to 28 Ma. A new inland southeastern Queensland central volcano line (25 to 22 Ma), from Bunya Mountains to North Main Range, followed 3 Ma behind the adjacent coastal progression. The Australian and Tasman Sea age-progressive chains are compared against recent plate motion modelling (Indian Ocean hotspots). The chain lines differ from general vector traces owing to west-facing swells and cessations in activity. Tectonic processes on the eastern plate margin may regulate these irregularities. These include subduction, rapid roll-back and progressive detachment of the Loyalty slab (43 to 15 Ma). West-flowing Pacific-type asthenosphere, related to perturbed mantle convection, may explain the west-facing volcanic surges. Such westward Pacific flow for over 28 Ma is known at the Australian–Antarctic Discordance, southeast of the present Australian plume sites under Bass Strait–West Tasman Sea. Most basaltic activity along eastern Australia marks asthenospheric melt injections into Tasman rift zone mantle and not lithospheric plate speed. The young (post-10 Ma) fields (Queensland, Victoria–South Australia) reflect new plate couplings, which altered mantle convection and stress regimes. These areas receive asthenospheric inputs from deep thermal zones off northeast Queensland and under Bass Strait.
Acknowledgements
This paper is dedicated to the memory of the late Philip Jon Stephenson, MA, a former Associate Professor of Geology, James Cook University of North Queensland, Townsville. He was involved in and encouraged many studies on Queensland basalt fields, including those of the first author in this paper.
Many samples for this study came from field trips associated with the Australian Museum, Sydney. Bentonite sampling at the Cedars Mine, Upper Yarraman, Queensland, was particularly helped by David Douglass, Mine Manager, and Leslie Stimpson, Mine Supervisor, in June 2010. Zircon concentrate separation was assisted by Gayle Sutherland, Australian Museum and Dr Kate Bromfield, Earth Sciences, University of Tasmania. The Australian Museum Trust provided funds towards zircon and whole-rock dating. Andrew Todd, CSIRO, is thanked for technical assistance and J. A. Wartho at JdL, Curtin University, is thanked for assistance with 40Ar–39Ar age-dating.
The study benefitted greatly from Dr Benjamin Cohen, Earth Sciences, Queensland University, for providing a copy of and permission to use data from his 2007 PhD thesis on 40Ar–39Ar dating of Cenozoic volcanic rocks. Maria Seton, University of Sydney, assisted with plate motion reconstructions. Script preparation involved Francesca Kelly, St Peters, Sydney; Zoe Hatzopoulos, Sydney, and Jacqueline Timms, School of Science, University of Western Sydney. The script was read by Dr Larry Barron, St Ives, Sydney. Constructive reviews were made by an anonymous referee and Dr Bill Birch, Museum Victoria.
