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Nature geoscience 5 (2012) 60-65
Modern-style plate subduction preserved in the Palaeoproterozoic West African craton
J. Ganne 1, V. De Andrade 2, 3, R. Weinberg 4, Olivier Vidal 5, B. Dubacq 6, N. Kagambega 7, S. Naba 7, L. Baratoux 1, M. Jessell 1, J. Allibon 8

The timing of onset of modern-style plate tectonics is debated. The apparent lack of blueschist metamorphism1--a key indicator of modern plate subduction2--in rocks aged more than about 1 billion years calls into question the existence of plate tectonics during the Archaean and Palaeoproterozoic eras3, 4. Instead, plate tectonics and subduction could have either not occurred at that time5, or could have proceeded differently6 owing to warmer conditions in the early Earth mantle7. Here we use thermodynamic models8, 9, 10 to investigate the formation conditions of metamorphic minerals in the 2.2-2.0 Gyr old West African metamorphic province. We find a record of blueschist metamorphism in these rocks. We show that minerals such as chlorite and phengite formed at high pressures of 10-12 kbar, low temperatures of 400-450 °C and under a geothermal gradient of 10-12 °C km−1. These conditions are typical of modern subduction zones. We therefore suggest that modern-style plate tectonics existed during the Palaeoproterozoic era. We conclude that ancient blueschist metamorphism may exist in other parts of the world, but the identification of these rocks has so far been hampered by methodological problems associated with deciphering their pressure and temperature evolution.
1 :  Géosciences Environnement Toulouse (GET)
CNRS : UMR5563 – Institut de recherche pour le développement [IRD] : UMR239 – Université Paul Sabatier (UPS) - Toulouse III – Observatoire Midi-Pyrénées
2 :  ID21 Beamline
3 :  NSLS II, Brookhaven National Laboratory
4 :  Australian Crustal Research Centre, School of Geosciences, Monash University
Monash University
5 :  Institut des sciences de la Terre (ISTerre)
CNRS : UMR5275 – IFSTTAR – Université de Savoie – Université Joseph Fourier - Grenoble I – INSU – OSUG – Institut de recherche pour le développement [IRD] : UR219
6 :  Department of Earth Sciences
Cambridge University
7 :  Département des Géosciences
Université de Ouagadougou
8 :  Institute of Mineralogy and Geochemistry, IMG-ANTHROPOLE
University of Lausanne
Minéralogie et environnements
Planète et Univers/Sciences de la Terre/Géomorphologie