Archean surface crystalline rocks of the Canadian Shield differ chemically from those of Proterozoic age. The younger rocks are higher in K2O, TiO2, U, and Th and lower in Na2O, Cr, Ni, and possibly MgO and CaO. This may be a secondary result of vertical chemical zoning of the crust as a result of metamorphism accompanied by anatectic melting. The erosion of a zoned crust would result in the enrichment of elements such as K, Na, Si, U, and Th in younger sedimentary basins. The crystalline crust subsequently evolved from the sediments of these basins would be enriched in these elements relative to older rocks exposed as a result of erosion. The secular chemical evolution of the shield may also reflect a progressive, though irregular, shift in the composition of new sial being added to the crust. Evidence for this progressive change is found in the increase in the abundance of potassium of successive dike swarms that were feeders of new sial to the upper part of the crust.

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