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The nature of the basement to Archean volcanic-sedimentary (greens tone) belts was obscured by the Keno1·an Orogeny 2600 m. y. ago and it is now controversial as to whether it was simatic or sialic. Types of evidence observed in the Canadian Shield that suppo1·t a sialic nature for the early crust are: 1. Unconformities; in six widely-separated localities unconformities between the volcanic-sedimentary assemblages and underlying granitoid rocks are evident in outcrop or interpretable from the geology. 2. Source of sedimentary materials; sedimentary components of many, or most, greenstone belts have, in part, a granitic provenance marked by the p1·esence of granite-bearing conglomerates and quartz-rich greywackes. While both are in much greater abundance in the upper parts of the successions they may, nevertheless, be present at almost any level. 3. Radiometric ages. Ages determined on granitoid rocks in five separate localities of the Superior and Slave Provinces exceed 2900 m. y ; ages markedly older than any yet obtained from the greenstone belts. The early sialic crust thus indicated was evidently a soda-rich granitoid rock; a tonalite, trondhjemite, or sodic granodiorite. A model proposed for development of the greenstone belts envisions a thin, continuous, sialic crust upon which a volcanic pile accumulates. Initially, both are below sea level and erosion is minimal. Withdrawal of material as magma from below the crust plus the density inversion of basalt over sial causes downwarp and outward spreading of the crust. At the base of the downwarp crustal melting begins and feeds plutons which subsequently rise through the crust and eventually into the volcanic pile. Crustal swelling and upflexing adjacent to the volcanic trough, marking the lateral transfer of material in crust and mantle, becomes a source of granitic sediment as it rises to sea level. Ultimately, as volcanism wanes, it becomes the major contributor to the "greenstone" succession.
DOI: 10.4095/102622