Archean

ARCHEAN (from the Greek word "archaios" (original, ancient) * EN: archaean; DE: Archaikum; FR: archéen; ES: arcaico; RU: архей) is the lower of the two largest sub-divisions of the pre-Cambrian. The upper age limit is approximately 2.6 billion years ago, the duration is more than 1.5 billion years.

The rocks of the Archean form the basement of the ancient tectonic plates, and emerge onto the surface within the regions of their shields, namely, Baltic, Aldanian, Canadian and others, and also within the cores of the folded structures of the geosynclinal belts. Within the most regions, the Archean is represented mainly with granitoids, various gneisses, crystalline and graphitic schists, amphibolites, to a lesser extent with marbles and quartzites. The organic remains, namely, microscopic primitive unicellular plants (algae), have been found within the most ancient rocks, starting since the age of more than 3 billion years. It is sub-divided into the local stratigraphic units.

There are defined as the typical regional stratigraphic sub-divisions for the European part of the USSR the White Sea and Lopian complexes, for the Asian part the Aldanian and Subganian ones. The name "Archean" ("Archaeozoic Group") has been introduced by the American geologist J.Dana in the 1872. There are associated with the rocks of the Archean the deposits of the ores of chromites (Australia, North America, Africa), of the copper-nickel ores, gold, iron (Canadian, Baltic shields, Australia), the pyrite-copper-gold-silver mineralization, sillimanite, corundum, rare-metallic pegmatites.