The Archean of the Canadian Shield: A Résumé

The Archean of the Canadian Shield occurs as islandlike areas of supracrustal rocks in a sea of invasive granite—the latter having been emplaced no less than 2.5 b.y. ago. These “islands” consist of metavolcanic rocks that are mainly andesitic and basaltic greenstones with a much lesser volume of felsites, and also metasediments that in some places are altered graywacke and slate with locally interbedded conglomerates and iron-bearing formation and elsewhere are much more extensive quartz mica schists and paragneisses, at times heavily migmatized. A Shield-wide stratigraphy has not yet been established either by geological field studies or by geochronologic methods. Archean time is long enough to encompass several magmatic and orogenic cycles. Geologic evidence supports this view, but geochronology has generally failed to discriminate between Archean rocks thought to be significantly different from one another in age. Archean sediments belong to the eugeosynclinal suite. These are apparently deep-water, probably marine, deposits of the flysch turbidite facies. Shelf carbonates and orthoquartzites are missing. There is no basic difference in bulk chemical composition between the Archean graywackes and slates and those of later times. A sialic source for the sands is indicated by the quartz budget and the granitoid clasts in the associated conglomerates. The Archean strata have been massively invaded by granites. The greenstones are cut by domelike diapiric stocks and batholiths of granitoid rocks, mainly tonalites and granodiorites. The sediments outside of the greenstone belts are intimately interleaved with granite sheets and are in places much granitized and engulfed. Excluding the relatively small areas of infolded metasediments, and ignoring the extensive invasion and disruption by granites, the greenstones are organized into broad belts, traceable across the southern part of the Shield. These major belts are separated by equally broad and continuous belts of sediments that were apparently deposited in protogeosynclinal tracts. The conglomerates and iron formations are confined to the outlying infolds of sediments or to the marginal areas of the geosynclinal belts. Unlike younger geosynclines, these seem to contain only the flysch facies and seemingly were filled from both sides and hence, unlike younger geosynclines, have no polarity.