Abstract
THE Archaean–Proterozoic boundary, it has been suggested, marks a fundamental change in the behaviour of the Earth, ending an early phase of cratonisation and the beginning of a period characterised by stable shelf sedimentation and linear orogens1,2,3. Almost all Archaean terrains, other than those dominated by granulites and migmatites, seem to be characterised by a variety of granitoid rocks intimately associated with ultramafic through mafic to calc-alkalic sequences, and their sedimentary associations. These greenstone belt assemblages are remarkably similar from one craton to another; they are usually folded into keel-shaped troughs, subjected to greenchist facies metamorphism and usually show similar structural styles which suggest that they were acted on by predominantly vertical forces. Their alleged antiquity (2,000 Myr or more) and widespread distribution have been attributed to either a change in the geothermal gradient as the Earth became colder3 or the existence of a relatively thin, Archaean, continental crust4,5.
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BELL, K., BLENKINSOP, J. & MOORE, J. Evidence for a Proterozoic greenstone belt from Snow Lake, Manitoba. Nature 258, 698–701 (1975). https://doi.org/10.1038/258698a0
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DOI: https://doi.org/10.1038/258698a0
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