Baltic Shield - Provinces and Blocks

Provinces and Blocks

The Baltic Shield is divided into five provinces: the Svecofennian and Sveconorwegian (or Southwestern gneiss) provinces in Fennoscandia, and the Karelian, Belomorian and Kola provinces/cratons in Russia. The latter three are divided further into several blocks and complexes and contain the oldest of the rocks, at 2.5–3.4 Ga. The Vodlozero block in south-eastern Karelia has been dated to 3.4 Ga. The youngest rocks belong to the Sveconorwegian province, at 900–1700 Ma old. Sometimes included as part of the Baltic Shield is the East European Platform (or Russian Platform), an area of western Russia covered by 3 km of sedimentary rock.

According to the Swedish Museum of Natural History (2006), the oldest rocks of the Fennoscandian Shield are found in the northeast, in the Kola peninsula, Karelia and northeastern Finland. These Archean rocks are mainly gneisses and greenstone belts, ca. 2.5-3.1 Ga. Within this area, there are also some Paleoproterozoic cover rocks (Karelian rocks), ca. 1.9-2.5 Ga, and the ca. 1.9 Ga collisional Lapland granulite belt. Some Archean rocks are also found in northernmost Sweden (Norrbotten county), and Archean crust probably underlies much of that area. Most of northern and central Sweden, however, belongs to the Svecofennian province, together with the southwestern part of Finland. The bedrock here formed 1.75-1.9 Ga during the Svecofennian (also known as Svecokarelian) orogeny. This bedrock includes both metasedimentary and metavolcanic rocks and several generations of granitoids, and hosts the Bergslagen ore deposits (iron and sulfide ores), the Skellefte (sulfides) and Norrbotten (iron and sulfide ores) districts. This area also contains some younger (ca. 1.5-1.65 Ga) Rapakivi granites as well as Jotnian sandstones (ca. 1.2-1.5 Ga). The Transscandinanavian igneous belt (TIB) consists of largely undeformed granitoids and associated porphyries formed in at least three different episodes between c. 1800 and 1650 Ma ago. It stretches from Småland in southern Sweden through Värmland and western Dalarna (where it is partly covered by Jotnian sandstone) and then continues under much of the Caledonian mountain chain up to northern Scandinavia. Southwest the TIB follows the Southwestern gneiss province (also known as the Sveconorwegian province), which has a long and complex evolution ranging from ca. 1.7 to 0.9 Ga ago. Most of the bedrock originally formed in the Gothian orogeny 1.7-1.55 Ga, but was later intruded by several generations of granitoids, the youngest in Sweden being the 900 Ma old Bohus granite, and metamorphosed and deformed again during the Sveconorwegian orogeny ca. 1.1-0.9 Ga. The Southwestern gneiss province is divided into several north-south-trending segments by Sveconorwegian deformation zones. In western Norway, these gneisses were again deformed during the Caledonian orogeny ca. 400 Ma. The Scandinavian Caledonides, which stretch through most of Norway and include adjacent parts of Sweden, are made up of Neoproterozoic to Silurian metasedimentary and metavolcanic rocks, deposited in the Iapetus Ocean (the predecessor of the present-day Atlantic Ocean) c. 700 to 400 Ma ago. Together with slices of older basement, these rocks were thrust several 100 km eastwards over the edge of the Fennoscandian Shield in several large thrust sheets known as nappes, when North America and Greenland collided with Scandinavia during the Caledonian orogeny ca. 400 Ma ago. Areas of Caledonian deformation, which also include the Precambrian gneisses of western Norway. Remains of Cambro-Silurian sedimentary cover (550-400 Ma old sandstones, shales and limestones) are found in some areas in southern Sweden, while Mesozoic and Tertiary sediments (younger than 250 Ma) are found in southernmost Sweden (Skåne) and in Denmark. Similar Phanerozoic rocks also cover the Baltic republics, Poland and northern Germany. The magmatic rocks of the Permian (c. 250 Ma) Oslo Graben formed in a failed rift system that continues into the Skagerrak and the North Sea.