Baltic Shield

a major structural uplift in the northwestern part of the Eastern European Platform, where its Pre-cambrian folded basement emerges on the surface, while the rock of the younger sedimentary mantle is almost completely absent. The Baltic Shield occupies the Karelian ASSR, Murmansk Oblast, and part of Leningrad Oblast in the USSR and Finland and Sweden. The highly metamorphosed and complexly dislocated Precambrian rock of the basement, which has been pierced by intrusions of an acid and basic composition, can be divided into a number of variously aged complexes which form the Precambrian folded systems. The Archean gneisses, the crystalline schists and intrusions (White Sea, Kola, and other complexes) have an absolute age of from 3.5 billion to 2.7–2.5 billion years. The younger Karelian and Svecofennian complexes of the Proteiozoic are widely developed; they were formed 1.85–1.75 billion years ago. Rapakivi granites also developed 1.64–1.62 billion years ago. On small areas, Upper Protero-zoic rock of the Jotnian age (1.4–1.3 billion years ago) has remained, and this occurs as the platform mantle.

On the Baltic Shield there are iron ore deposits (Kiruna and the Grangesberg region in Sweden, the Olenegorsk and Kovdor regions in the USSR), as well as copper-nickel ores (Monchetundra, the Pechenga region of the USSR); apatite deposits are associated with the Paleozoic alkali intrusions (Khibiny in the USSR).