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 disputed^{1, 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 kimberlites^{5, 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.

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

Correspondence to: Jean-François Moyen¹Gary Stevens¹ Correspondence and requests for materials should be addressed to J.-F.M. (Email: moyen@sun.ac.za) or G.S. (Email: gs@sun.ac.za).

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