Letter | Published:

Earth’s first stable continents did not form by subduction

Nature volume 543, pages 239242 (09 March 2017) | Download Citation

  • A Corrigendum to this article was published on 10 May 2017

Abstract

The geodynamic environment in which Earth’s first continents formed and were stabilized remains controversial1. Most exposed continental crust that can be dated back to the Archaean eon (4 billion to 2.5 billion years ago) comprises tonalite–trondhjemite–granodiorite rocks (TTGs) that were formed through partial melting of hydrated low-magnesium basaltic rocks2; notably, these TTGs have ‘arc-like’ signatures of trace elements and thus resemble the continental crust produced in modern subduction settings3. In the East Pilbara Terrane, Western Australia, low-magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup have trace-element compositions that are consistent with these being source rocks for TTGs. These basalts may be the remnants of a thick (more than 35 kilometres thick), ancient (more than 3.5 billion years old) basaltic crust4,5 that is predicted to have existed if Archaean mantle temperatures were much hotter than today’s6,7,8. Here, using phase equilibria modelling of the Coucal basalts, we confirm their suitability as TTG ‘parents’, and suggest that TTGs were produced by around 20 per cent to 30 per cent melting of the Coucal basalts along high geothermal gradients (of more than 700 degrees Celsius per gigapascal). We also analyse the trace-element composition of the Coucal basalts, and propose that these rocks were themselves derived from an earlier generation of high-magnesium basaltic rocks, suggesting that the arc-like signature in Archaean TTGs was inherited from an ancestral source lineage. This protracted, multistage process for the production and stabilization of the first continents—coupled with the high geothermal gradients—is incompatible with modern-style plate tectonics, and favours instead the formation of TTGs near the base of thick, plateau-like basaltic crust9. Thus subduction was not required to produce TTGs in the early Archaean eon.

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Acknowledgements

We acknowledge financial support from The Institute of Geoscience Research (TIGeR) at Curtin University. R.H.S. publishes with the permission of the Executive Director, Geological Survey of Western Australia.

Author information

Affiliations

  1. Department of Applied Geology, The Institute for Geoscience Research (TIGeR), Centre for Exploration Targeting – Curtin node, Australian Research Council Centre of Excellence for Core to Crust Fluid Systems, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia

    • Tim E. Johnson
    • , Nicholas J. Gardiner
    •  & Christopher L. Kirkland
  2. Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, Maryland 20742-4211, USA

    • Michael Brown
  3. Geological Survey of Western Australia, 100 Plain Street, East Perth, Western Australia 6004, Australia

    • R. Hugh Smithies

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Contributions

T.E.J. conceived the project and performed the phase equilibria calculations. N.J.G. undertook the trace-element modelling. All authors analysed the data and contributed to writing the paper.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Tim E. Johnson.

Reviewer Information Nature thanks J. Bédard, R. Rapp and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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https://doi.org/10.1038/nature21383

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