K/Ar and40Ar/39Ar analyses of meltrock from the Acraman impact structure, Gawler Ranges, South Australia

S. L. Baldwin, I. McDougall, G. E. Williams

Research output: Contribution to journalArticlepeer-review

13 Scopus citations


The Acraman structure, located in the Gawler Range Volcanics, Gawler Craton, South Australia, is the largest probable meteorite impact structure known in Australia; it is notable also as the likely source of volcaniclastic ejecta within the Late Proterozoic Bunyeroo Formation located ˜ 300 km east of the impact site and within the correlative Rodda Beds in the Officer Basin ˜ 450 km to the northwest of the impact site. Conventional K/Ar analyses and40Ar/39Ar total fusion and step heat experiments were performed on two samples of meltrock from the centre of the Acraman structure in an attempt to directly constrain the age of the impact event. The two samples of devitrified meltrock consist primarily of albite microlites in a hematitic devitrified matrix of cloudy K-feldspar and quartz. Electron microprobe analyses of the samples reveal that the albite + potassium feldspar have nearly pure end-member compositions (Ab99 and Or97) indicating they are likely low-temperature authigenic phases. K/Ar,40Ar/39Ar total fusion and integrated apparent ages for both samples agree within analytical uncertainty. Results of step heat experiments show relatively complex patterns likely due to the degassing of multiple phases. Based on apparent ages that are least affected by diffusional loss, a minimum estimate for the age of the meltrock is ˜450 Ma, Late Ordovician. Because the feldspar in the meltrock may have formed as a result of authigenic replacement and devitrification, the apparent age is regarded as a minimum age for the impact event. The K/Ar and40Ar/39Ar data presented here indicate the Acraman impact event occurred at least ˜ 450 Ma ago; however, these results do not provide an unequivocal test of the hypothesis that the ejecta blanket in the Bunyeroo Formation and Rodda Beds is uniquely linked to the Acraman impact structure.

Original languageEnglish (US)
Pages (from-to)291-298
Number of pages8
JournalAustralian Journal of Earth Sciences
Issue number3
StatePublished - Jul 1991
Externally publishedYes


  • Acraman impact structure
  • Delamerian Orogeny
  • Gawler Craton
  • Gawler Ranges Volcanics
  • K/Ar andAr/Ar geochronology
  • Yardea Dacite

ASJC Scopus subject areas

  • Earth and Planetary Sciences (miscellaneous)
  • General Earth and Planetary Sciences


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