1Geological Survey Of Qld, Brisbane, Australia
The migration of zircon geochronology data collected by Geoscience Australia (GA) and Geological Survey of Queensland(GSQ) from the Mount Isa Province into the Online Geochron Delivery System, an important repository maintained by GA, provided an opportunity to review the data and replot it using a consistent approach. This included data for which only preliminary plots of had been available to GSQ and never published.
The review highlighted that the main magmatic events that would have contributed zircon to the Eastern Succession sedimentary rocks occurred at 1850–1870 Ma, 1790–1800 Ma, 1780 Ma, 1760 Ma, 1735–1745 Ma, 1725 Ma, 1705–1715 Ma and 1670–1680 Ma and volumetrically smaller events at 1770 Ma, 1755 Ma, 1655–1660 Ma and 1650 Ma.
The Soldiers Cap Group in the easternmost part of the Mount Isa Province and extending under cover to the east is younger than most of the eastern succession. It consists of Llewellyn Formation, Mount Norna Quartzite and Toole Creek Volcanics in ascending stratigraphic order. The Kuridala Group comprises the Starcross Formation and Hampden Slate.
Samples of the two lowermost units of the Soldiers Cap Group and Starcross Formation have similar maximum depositional ages. A closer comparison has been made of their respective provenances by pooling analyses for units in each group. These provenances are similar, indicating a minor, very old source around the Archean–Proterozoic boundary and then almost none up to ~1900 Ma (the Barramundi Orogeny). Except for minor components from the Kalkadoon–Leichhardt basement (1850–1870 Ma ) and Argylla Formation (1780 Ma), by far the major sources appear to be the Wonga–Burstall–Gin Creek plutonic suites at ~1740 Ma and Fiery Creek Volcanics or Weberra Granite at ~1710 Ma. They also both have a significant younger component (slightly older in the Soldiers Cap Group at ~1685 Ma, and ~1675 Ma in the Starcross Formation). Pooling analyses from the Hampden Slate indicates that apart from the youngest component being ~1655 Ma, other components are almost identical to those in the Starcross Formation.
By contrast the provenance of the Toole Creek Volcanics is dissimilar to the other units. It shows an isolated, almost unimodal population at ~1658 Ma, with small populations at ~1795Ma, ~1850Ma, and ~ 2680Ma.
Comparing the provenance spectra of the lower part of the Soldiers Cap and Kuridala Groups with those of the lower part of the Etheridge Group in the Etheridge Province (Georgetown region) suggests that they were probably deposited at about the same time, but the provenance patterns are strikingly different. The Etheridge Group shows a large Archean component as well as almost continuous spread of data points throughout the Paleoproterozoic including peaks around 1900–2000 Ma. This dissimilarity has been cited as evidence that the Georgetown rocks were not distal to Mount Isa and were part of Laurentia until welded to the Australian craton during the assembly of Nuna. The provenance of the upper part of the Etheridge Group, however, is like that of the Toole Creek Volcanics.
Ian Withnall spent 42 years with GSQ in regional studies. He was principal compiler of the Queensland Geology 1:2M-scale map and a major contributor to the Geology of Queensland volume, before retiring in 2014. He continues at GSQ voluntarily, working on NW Queensland geology and assisting with map publishing.