Baltic shield

BALTIC SHIELD (RU: Балтийский щит) is the protrusion of the pre-Cambrian basement at the north-west of the East European tectonic plate. To the north-west, it borders the folded structures of the Caledonides of Scandinavia, which are thrusted onto the crystalline rocks of the shield; to the southern and south-eastern direction, the metamorphic rocks submerge under the cover of the sedimentary rocks of the Russian tectonic plate. It occupies the south-eastern half of the Scandinavian peninsula, the Kola Peninsula, and Karelia. There are distinguished within the structure of the shield the three large parts, namely, the geoblocks (segments): the eastern one (Kola-Karelian), the central one (Svecofennian), and the western one (South Scandinavian). Kola-Karelian geoblock is almost entirely situated on the territory of the USSR, the Svecofennian one on the territory of Finland and Sweden, the South Scandinavian one at the south of Sweden and Norway. There are distinguished within the structure of the Kola-Karelian segment several large blocks of the north-western orientation (Murmansk, Central Kola, White sea, Karelia), which are separated by the large faults. The Murmansk block is formed by the massifs of the Archean granite-gneisses (2.7-2.8 billion years), the Central Kola block is formed by the Lower Archean Kola series of the gneisses, amphibolites, magnetitic quartzites, high-alumina crystalline schists (3.0-3.5 billion years). The Upper Archean high-alumina schists, quartzites, and metavolcanites form the complicated synclinoria. Within the southern part of the block, there is situated the system of the graben-synclines, which are filled with the thick stratum of the Lower Proterozoic metasedimentary and metavolcanic rocks. There play the important role the Lower Proterozoic basic nickel-bearing intrusives of the Pechenga region, the alkaline intrusions of the Keivy tundras, and also the Paleozoic alkaline massifs (Lovozero, Khibiny). The White sea block is formed by the Archean White sea series, which is reminiscent of the Kola series in thickness, direction of the change of the substances, composition, degree of metamorphism, types of the folded forms. There have been determined the three stages of metamorphism: more than 3 billion years, 2.5-2.6 and 1.9-2.0 billion years. Within the Karelian block, there are distinguished several narrow complicately built near-fault synclinoria zones, which are formed by the rocks of the lower part of the Proterozoic (clastic, carbonatic, volcanic rocks, quartzites), and are separated by the anticlinorial zones with outcrops of the pre-Karelian foundation. The upper part of the Lower Proterozoic (Jatulian - sub-Iotnian) is embedded unconformably, forming large depressions, which are filled with the clastic, carbonatic, volcanogenic strata. The most late magmatic manifestations within Karelia correspond to the rapakivi porphyritic granites, namely, the Vyborg massif (approximately 1600 million years), and the dikes of the gabbro-diabases (approximately 1.2 billion years), which are breaching the quartzites, rapakivi granites, and Riphean sandstones.

There are developed within the Svecofennian segment the strata of the Proterozoic of the geosynclinal type. The complicated tight folds form the arcuate shape as the projection onto the horizontal plane. The Svecofennian granites have the age range of 1.9-1.6 billion years. There are developed along the western, southern, and south-eastern periphery of the segment the middle volcanites of the sub-Iotnian, which are overlain by the Iotnian quartzites. The folded structures, which are formed by the sub-Iotnian, and are traced along the western border of the segment, have got the name of the Gothides; the granites of the Småland massif, which are breaching them, have the age of 1750 million years (the Karelian folding).

The South Scandinavian segment is divided into the two (east and west) blocks by the Oslo graben, which is meridionally oriented, and is filled with the Permian alkaline effusives. The Archean gneissic basement of the western block within the province of the Telemark plateau is overlain by the thick (approximately 4 kilometres) stratum of the Proterozoic metamorphic rocks of the green-schist facies of the metamorphism (1000 million years). There are embedded within the eastern block on the Archean gneisses the analogs of the Upper Karelian; the granites, which are breaching them, have the age of 1000 million years. There are confined to the Lower Proterozoic the numerous iron ore deposits, including the "Kiruna" within Sweden, copper-nickel deposits of the Kola peninsula, the deposits of the micas, ceramic and high-alumina raw materials. There are associated with the Archean the deposits of the ores of iron, and with the Paleozoic alkaline complexes the deposits of the nepheline, apatite, and rare earth elements.