Canadian Shield

Area of Precambrian rocks in eastern and northern Canada. It is comprised of the hard crystalline remains of a heavily eroded mountain system, between 300 and 600 million years old, forming the North American craton, one of the three major structural provinces of North America. Extending over 2.5 million sq km/970,000 sq mi, the craton occupies the major part of the north of the continent, tapering towards the south in an inverted triangle.

It contains rich mineral-bearing belts. Repeated glaciation has swept away most surface debris, exposing bare rock over extensive areas, with innumerable lake-filled hollows. Sufficient soil for tree cover is found only in moraines (glacial debris), mainly in the southern Laurentian Highlands, which lie north of the St Lawrence River, stretching westwards from Ottawa to the Saguenay River.

The highlands provide timber for Canada's enormous pulp and paper industry, and a focus for tourist activities. The Laurentides conservation area, covering an area over 10,500 sq km/4,000 sq mi, lies north of Québec city.

The Canadian Shield can be divided into a number of structural provinces, each representing a mobile area active during a different part of Precambrian time; the three main provinces are the Superior, the Churchill, and the Grenville. The oldest Precambrian rocks, formed more than 3,000 million years ago, occur as small isolated masses within larger areas of younger rocks. All the provinces represent fragments of formerly far greater complexes.

Superior province The Superior province is a huge area of rocks of middle Precambrian age. It is made up of a basement of granites, gneisses, and magmatites, containing elongated greenstone belts which have been less highly metamorphosed and preserve many primary features. The belts have an east-west trend and incorporate metamorphosed volcanics and sediments. These thick accumulations of sediment in unstable basins of deposition carry deposits of gold and other metal ores, and form some of Canada's major mining resources. The cycle of Precambrian activity which created this province reached a climax in the Kenoran orogeny, 2,400-2,600 million years ago, during which time there was widespread regional metamorphism and deformation, and granites were intruded. After this orogenic (mountain-building) phase there was a relatively stable phase, during which an extensive dyke swarm was intruded.

Churchill province The Churchill province, which adjoins the north margin of the Superior province, is made up of rocks formed during a later Precambrian orogenic cycle, the Hudsonian cycle. The Hudsonian zones of mobile belt activity occur to the north (in the Churchill province), northeast (in the Nain province), and south (in the Southern province) of the stable Superior province. The rocks in these belts are volcanics and sediments, and contain thick banded iron formations, one of the main sources of iron ore in North America; other important mineral deposits include uranium, copper, lead, zinc, and silver. Within the mobile belts the rocks have suffered intense deformation and regional metamorphism during the Hudsonian orogeny, 1,650-1,800 million years ago, but the sediments deposited on the foreland outside the mobile belts are often only gently folded. The end of the Hudsonian cycle was followed by the welding of the new and stabilized belts onto the old Superior craton to form a much larger shield area. A thick succession of plateau basalts (the Keweenawan succession) was poured out onto the southern part of the craton 1,050-1,300 million years ago; this succession is up to 15 km/9 mi thick, and is characterized by copper mineralization.

Grenville province The Grenville province is the youngest province of the Canadian Shield. It is a clearly defined belt lying in the east of the Canadian Shield and extending southwest through the central plains of the USA to the Mexican border. It is largely covered by later Phanerozoic sediments. The Grenville mobile belt is characterized by a high grade of regional metamorphism, and it is therefore difficult to distinguish the original rock types occurring in the belt. This belt suffered its major metamorphism and folding in the late Precambrian, 900-1,100 million years ago.