17 Dec 2021

17 Dec 2021

Review status: this preprint is currently under review for the journal GChron.

Complex 40Ar/39Ar age spectra from low metamorphic grade rocks: resolving the input of detrital and metamorphic components in a case study from the Delamerian Orogen

Anthony Reid1,2, Marnie Forster3, Wolfgang Preiss1,2, Alicia Caruso1, Stacey Curtis1,4, Tom Wise1, and Naina Goswami3 Anthony Reid et al.
  • 1Geological Survey of South Australia, Department of State Development, GPO Box 320, Adelaide, SA 5001, Australia
  • 2Department of Earth Sciences, School of Physical Sciences, University of Adelaide, SA 5005, Australia
  • 3Mineral Exploration Cooperative Research Centre, Research School of Earth Sciences, The Australian National University, Canberra, ACT 2601, Australia
  • 4Mineral Exploration Cooperative Research Centre, STEM, University of South Australia, Mawson Lakes, Australia

Abstract. Low metamorphic grade rocks contain both detrital minerals and minerals newly grown or partly recrystallised during diagenesis and metamorphism. However, rocks such as these typically yield complex 40Ar/39Ar age spectra that can be difficult to interpret. In this study, we have analysed a suite of variably deformed rocks from a region of low metamorphic grade within the c. 514–490 Ma Delamerian Orogen, South Australia. The samples analysed range from siltstone and shale to phyllite and all contain either muscovite or phengite determined by hyperspectral mineralogical characterisation. Furnace step heating 40Ar/39Ar analysis produced complex apparent age spectra with multiple age components. Using the concept of asymptotes that define minimum and maximum ages for different components, we interpret the age spectra to preserve a range of detrital mineral ages, along with younger components related to either cooling or deformation- induced recrystallisation. Two samples contain Mesoproterozoic detrital age components, up to c. 1170 Ma, while the c. 515 Ma Heatherdale Shale which has both c. 566 Ma and c. 530 Ma detrital components. All samples contain younger lower (younger) asymptotes in the age spectra defined from multiple heating steps that range from c. 476 to c. 460 Ma. One interpretation of these younger ages is that they are caused by post-metamorphic cooling. However, the shape of the age spectra and the degree of deformation in the phyllites suggest the ages may record recrystallisation of detrital minerals and/or new mica growth during deformation. Potentially these c. 476 to c. 460 Ma ages suggest deformation in the upper portion of the orogen was facilitated by movement along regional faults and shear zones up to around 20 million years after the cessation of deformation in the high-metamorphic grade regions of the Delamerian Orogen.

Anthony Reid et al.

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Anthony Reid et al.

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Complex 40Ar/39Ar age spectra from low metamorphic grade rocks, Delamerian Orogen, Reid et al, Anthony Reid, Marnie Forster

Anthony Reid et al.


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Short summary
Age dating of rocks in mountain belts is often undertaken using minerals that grow at relatively high temperature and pressure deep within these belts. However, the upper, cooler parts of mountain belts also contain information on the process by which they formed. Using the radioactive decay of potassium to argon, we investigate the upper part of an ancient mountain belt and discuss how to interpret the complex age signals that can be obtained by this method.