The lower portions of the volcanic sequence of some Archaean greenstone belts include members with crystallized from ultramafic liquids extruded at the earth's surface at 1600-1650°C. These liquids are interpreted as products of 60-80% melting of their mantle source composition which implies more catastrophic conditions of mantle melting than obtained in Palaeozoic, Mesozoic or Recent crust-mantle dynamics. Such conditions may be a consequence of major impacts on the surface of the primitive earth. It is suggested that the production of the lunar maria basins was accompanied by similar impacts on the earth and that such terrestrial maria played an important role in early stages of chemical differentiation of the crust and upper mantle. An hypothesis is presented in which some Archaean greenstone belts are interpreted as very large impact scars, initially filled with impact-triggered melts of ultramafic to mafic composition and thereafter evolving with further magmatism, deformation and metamorphism to the present Archaean greenstone belts. © 1972.
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