Archean bimodal volcanism

Field, petrographic, and chemical evidence from several areas of the Superior Province suggest that many Archean volcanic successions are bimodal, involving an early tholeiitic basaltic-andesitic magma and a later dacitic-rhyolitic magma. Field evidence also suggests the simultaneous existence of mafic and felsic magmas: felsic pumice is found in mafic feeder dykes, composite dykes without chilled margins between the rhyolite and basalt dyke components are found in the Abitibi Subprovince, and mafic pumice is found toward the top of a felsic welded ignimbrite in the Uchi Subprovince. Bimodal volcanism is most clearly defined by major element content, but sequences not obviously bimodal, based on major element chemistry, may be bimodal in trace element content. Subequal volumes of felsic and mafic magma are involved in Archean bimodal volcanism.