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Record of mid-Archaean subduction from metamorphism in the Barberton terrain, South Africa

Nature volume 442, pages 559–562 (2006)Cite this article

Abstract

Although plate tectonics is the central geological process of the modern Earth, its form and existence during the Archaean era (4.0–2.5 Gyr ago) are disputed1,2. The existence of subduction during this time is particularly controversial because characteristic subduction-related mineral assemblages, typically documenting apparent geothermal gradients of 15 °C km-1 or less3, have not yet been recorded from in situ Archaean rocks (the lowest recorded apparent geothermal gradients4 are greater than 25 °C km-1). Despite this absence from the rock record, low Archaean geothermal gradients are suggested by eclogitic nodules in kimberlites5,6 and circumstantial evidence for subduction processes, including possible accretion-related structures2, has been reported in Archaean terrains. The lack of spatially and temporally well-constrained high-pressure, low-temperature metamorphism continues, however, to cast doubt on the relevance of subduction-driven tectonics during the first 1.5 Gyr of the Earth's history7. Here we report garnet–albite-bearing mineral assemblages that record pressures of 1.2–1.5 GPa at temperatures of 600–650 °C from supracrustal amphibolites from the mid-Archaean Barberton granitoid-greenstone terrain. These conditions point to apparent geothermal gradients of 12–15 °C—similar to those found in recent subduction zones—that coincided with the main phase of terrane accretion in the structurally overlying Barberton greenstone belt8. These high-pressure, low-temperature conditions represent metamorphic evidence for cold and strong lithosphere, as well as subduction-driven tectonic processes, during the evolution of the early Earth.

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Figure 1: Location of the Inyoni shear zone and studied samples in the southern Barberton terrane14.
Figure 2: Metamorphic textures associated with garnet growth or breakdown (plane-polarized light).
Figure 3: THERMOCALC P – T estimates for garnet growth and breakdown sites in the studied samples.
Figure 4: Geodynamic sketch summarizing the inferred tectonic evolution of the southern Barberton terrane, and the associated metamorphic evolution.

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Acknowledgements

J.-F.M.'s post-doctoral stay at Stellenbosch university is funded by the South African National Research Fundation (NRF) and by a bursary from the Department of Geology, Stellenbosch University. Running costs were provided by the NRF. We thank G. Droop and J. Bédard for reviews of earlier versions of this manuscript. Author Contributions J.-F.M. and G.S. contributed equally to the metamorphic and petrologic analysis. All authors contributed to the interpretation of these results within the Barberton geodynamic framework.

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  1. Department of Geology, University of Stellenbosch, South Africa Private Bag X-1, 7602, Matieland, South Africa

    Jean-François Moyen, Gary Stevens & Alexander Kisters

Authors
  1. Jean-François Moyen
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Correspondence to Jean-François Moyen or Gary Stevens.

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Supplementary information

Supplementary Table 1

This table shows the data collection and refinement statistics. (XLS 69 kb)

Supplementary Notes

This file contains Supplementary Discussions, Supplementary Methods, Supplementary Figure Legends and additional references. (DOC 133 kb)

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Moyen, JF., Stevens, G. & Kisters, A. Record of mid-Archaean subduction from metamorphism in the Barberton terrain, South Africa. Nature 442, 559–562 (2006). https://doi.org/10.1038/nature04972

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