The time-space distributions of hotspot-derived volcanoes are widely utilised as a frame of reference for the motions of the Earth’s plates. In eastern Australia, hotspot-derived volcanoes of mid- to late-Cenozoic age (termed ‘central volcanoes’) are widespread, occurring over a north-south distance of nearly 2000 km. Previous geochronologic results from these central volcanoes indicate a fast northward velocity of the Australian plate of 65 ± 3 km.Ma-1 over the last 32 million years, relative to a fixed hotspot. In this project, 40Ar/39Ar geochronology is used to date 87 samples from central volcanoes in eastern Australia, revealing new details of Australia’s dynamic volcanic and tectonic history.
The 40Ar/39Ar ages confirm that mafic activity at individual central volcanoes typically occurs over a time span of three to five million years and reveal that the silicic rocks were emplaced within one million years or less towards the end of this lifespan. As such, the silicic rocks provide consistent markers in the history of central-volcano activity, allowing detailed age comparisons to be made between volcanoes. Using this approach, age progressions for east Australian volcanism can be resolved over time windows as short as a few million years. The time-space distribution of the silicic rocks indicates the northward velocity of the Australian plate has not been constant throughout the Cenozoic. A major reduction in northward plate velocity down to 26 +5_3 km.Ma-1 is indicated between 26-23 Ma from the ages of volcanoes in southeastern Queensland. This reduction in plate velocity was accompanied by a clockwise rotation of the plate, as shown by sigmoidal bends in the Tasmantid and Lord Howe hotspot chains in the Tasman Sea. An earlier period of slower plate velocity is also interpreted between 30-29 Ma from the ages of volcanoes in central Queensland.
Both the 30-29 and 26-23 Ma periods can be correlated to major collision events in New Guinea and Ontong Java, respectively, which changed the subduction tectonics along the northern plate boundary. These velocity changes also appear to have affected the characteristics of volcanism, with larger overlapping volcanoes constructed during periods of slower plate velocity. Crustally derived rhyolites were also produced from volcanoes erupted between 26-23 Ma, and possibly also 30-29 Ma. This is in contrast to volcanoes erupted during periods of fast plate velocity, where silicic rocks are the differentiation products of mantle-derived magmas, with only minor crustal contributions. The implication is that the slower plate velocity - and hence longer period of time over the mantle thermal anomaly - promoted the construction of larger, more voluminous volcanoes, and facilitated more efficient heating and melting of the crust.
40Ar/39Ar geochronology was also undertaken on two other styles of intraplate volcanism in eastern Australia: leucitites and lava fields. Ages for seven leucitites from southeastern Australia confirm that these potassic ultramafic rocks become younger to the south, at a similar time and rate to the nearby central volcanoes. As such, the 40Ar/39Ar results are consistent with a hotspot-related origin for the leucitites. Eighteen lava field samples were analysed, revealing that many of these rocks contain excess argon or have experienced argon loss, and indicating that at least some of the scatter in ages currently identified in these volcanoes is due to inaccurate K-Ar ages.