Mantle-plume model for the origin of Archaean greenstone belts based on trace element distributions

TRACE element studies of Archaean greenstone volcanics indicate the existence of two major types of tholeiite^1-3: depleted Archaean tholeiite (DAT) showing flat rare earth element (REE) patterns similar to modern rise tholeiites and enriched Archaean tholeiite (EAT) exhibiting slight light REE enrichment and generally greater concentrations of LIL (large-ion lithophile) elements than DAT. Included in the DAT category are komatiitic tholeiites. DAT and associated ultramafic flows and sills seem to make up from 50 to 90% of Archaean greenstone successions. EAT becomes relatively more abundant at stratigraphically higher levels and in the upper parts of minor volcanic cycles within greenstone successions. Although overall EAT is generally less abundant than DAT, it accounts for up to 50% of some greenstone sections^2,4. DAT seems to be absent in the upper 10-20% of most greenstone successions.