Abundance of Radioactive Elements in Crystalline Shield Rocks

Abstract

DATA on the major element composition of the crystalline surface rocks of 200,000 square miles of the Canadian Shield in northern Quebec have been published recently¹. The rocks discussed are chiefly foliated Archaean granodiorite gneisses of amphibolite facies. Within the area, however, rocks of hornblende granulite facies are also widespread (Fig. 1). The work on major elements has provided the average composition of this large area of shield and indicated that, relative to rocks of amphibolite facies in this area, the gneisses of granulite facies are depleted in potassium, water and silica, and enriched in aluminium, total iron phosphorus, magnesium, calcium and titanium. These differences were thought to reflect a chemical zoning of the Earth's crust in response to regional metamorphism. Because this is an ancient terrain containing much gneissic material of high metamorphic grade it was suggested that it represents deep continental crust. For this reason the abundance of trace elements in these rocks, and in particular the abundances of the radioactive elements thorium, uranium and potassium, are of special interest.