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 Bergslagen ore deposits (iron and sulfide ores), 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.
The scouring and compression of the Baltic Shield by glacial movements created the area's many lakes and streams, the land retaining only a thin layer of sandy sediment collected in depressions and eskers. Most soil consists of moraine, a grayish yellow mixture of sand and rocks, with a thin layer of humus on top. Vast forests, featuring almost exclusively the three species pine, spruce and birch, dominate the landscape, clearly demarcating its boundaries. The soil is acidic and has next to no carbonates such as limestone. The scouring by the ancient glaciers and the acidity of the soil have destroyed all palaeontologically interesting materials, such as fossils.
According to a study by Slabunov (1999): "The Belomorian Province is a mobile belt that evolved in a polycyclic manner. A lateral sequence of Late Archean tectonic units has been revealed in the Belomorian Province and in the eastern part of the Karelian Province. The collision events in the Belomorian Province are represented by high pressure (6–12 kbar) and high temperature (500–700°C) kyanite-facies metamorphism, granitoid magmatism, and the formation of folded nappe structure and granite gneiss domes. The time of collision is estimated at 2.7–2.74 Ga. This stage of evolution in the NE part of Karelian Province is accompanied by the generation of North Karelian greenstone belt (NKGB).
The Belomorian Belt is a tectonic pile of metasedimentary, metavolcanic and metaplutonic rocks which has been folded and metamorphosed several times. According to a study by Bibikova et al. (1999), the earliest metamorphic event took place ca. 2.8 Ga. "Sm-Nd isotope studies of whole rock metasediment samples have constrained their mean protolith ages to between 3.00 and 2.86 Ga, indicating a short prehistory....Zircons were separated from different localities and at various levels of the Belomorian tectonostratigraphical column. We have recognized three age groups of ancient cores at 3.2–3.1 Ga, 3.00–2.97 Ga and 2.93–2.90 Ga. The plus 3.1 Ga cores were obtained solely from localities in the northern part of the Belt. It has also been possible to distinguish three groups of metamorphic grains and overgrowths which are 2.84–2.80 Ga, 2.72–2.68 Ga and ca. 2.61 Ga old. The data presented confirm the absence of detrital material older than 3.2 Ga in the Belomorian metasediments. This differs from the adjacent Karelian craton where crustal rocks of about 3.4 Ga have been recorded. If these ages are representative of the rocks discussed, our data suggest that the Belomorian Belt represents a Neoarchaean accretionary environment in the vicinity of the Karelian craton." (Bibikova et al., 1999).
From NE to SW the Baltic shield consists of the following structural-formational zones: 1) the Central Belomorian mafic zone (CBMZ) dominantly formed by mafic and ultramafic rocks, 2) the Chupa Paragneissic Belt (ChPB) composed of deep and repeatedly metamorphosed metagraywackes (mainly high-alumina gneisses), 3) the North Karelian system of greenstone belts (NKGB) dominated by volcanics of calc-alkaline series, and 4) the North Karelian diorite-plagiogranitic batholith (NKB) and xenoliths of ultramafic to andesite-dacite composition that occur in it. The CBMZ is dominated by metabasalts (amphibolites) with widespread metaultrabasic rocks (metaperidotites, serpentinites and apoultramafic amphibolites), and extremely rare acid metavolcanics. The chemical composition of metabasalts in the CBMZ is similar to that of mid-oceanic ridge basalts (MORB). The isotopic age of the rhyolite-dacites is 2.887 Ga. This association is interpreted as a fragment of a Late Archean ophiolitic complex. The CBMZ marks a collision suture. The supracrustal strata of NKGBS consist of metabasalts, metakomatiites and felsic to intermediate metavolcanics. Metaandesites-metarhyolites make up a considerable portion of the sequence. The age of these volcanics is estimated at 2.877–2.820 Ga. Between the NKGB and the CBMZ there lies the ChPB which consists of metagraywackes (garnet-biotite kyanite-bearing gneisses. This lateral series indicates the Late Archean (3.0–2.8 Ga) subduction stages in the evolution of the eastern Baltic shield. During the first stage, the oceanic lithosphere subducted from NE to SW under the subcontinental crust. In the second stage it subducted under the continental crust of the Karelian plate." (Slabunov, 1999)
Ljavozerskya (lower terrigenous formation), Polmostundrovskya (komatiite-tholeiite), Voronjatundrovskya (basalt-andesite-dacite) and Chervurtskya (an upper terrigenous formation). Zircon in the quartz of the upper part of the Voronjatundrovskya suite yield an U-Pb age of 2.8 Ga. This is interpreted as the intrusive emplacement of the quartz porphyry during the final stage of the belt development. Ovoid plagioamphibolites are present among schistose plagioamphibolites of the Polmostundrovsky suite and have been dated at ~2.6 Ga, indicative of andalusite-sillimanite facies metamorphism. Tourmaline granites are found all over the Kolmozero-Voronja belt occurring among volcanogenic sedimentary rocks with good correlation dating of 2520±70 Ma. (Kudryashov, 1999).
The geochronological data document a long and complicated evolution of the belt:
Within the Murmansk block, from west to east, there is an increase in the rare earth elements (REE) content in the initial melts, a change in composition of protoliths from tholeiite with the highest content of REE to subalkaline basalt, and in the eastern part of the Murmansk block the REE content is even higher. The origin of tonalites and trondhjemites (TT) is most likely the result of partial melting of mafic sources. The increase of alkalinity in the protoliths of TT-gneisses correlates with the abundance of the Late Archean peralkaline (2750±50 Ma) and alkaline (2760±60 Ma) granite massifs here. (Turkina and Vetrin, 1999).
According to a study by Bayanova and Zozulya (1999), the emplacement ages for peralkaline granite magmatic vary from 2610 Ma for the White Tundra massif to 2670 Ma for the Western Keivy massif and are spatially confined to voluminious gabbro-anorthosite magmatism of 2.66–2.68 Ga. The predominantly "juvenile" Sm-Nd isotopic signatures from most suites of Keivy complex suggest that they must be of mantle derivation or else have has short-lived crustal precursors. "The granites are petrologically and geochemically similar to Phanerozoic A-type granitoids, presumably emplaced into noncompressive or extensional environments. The distinct tectonic regime of such type of granites indicates that the Keivy peralkaline granite magmatism can be regarded as a consequence of post-collisional events. Collision in the region has possibly taken place earlier than 2.74 Ga. The granites studied were formed after the Late Archaean Keivy-Voronja greenstone belt evolution." (Bayanova, 1999). The above model suggests that the NE Archaean portion of the Baltic shield was dominated by plume tectonics.
The Laplandian Granulite Belt is in the central northeast section of the Baltic shield. Garnet plagiogranitoids occur in the northeastern part, crystallised from melting of host rock acid granulites. The absence of stratification in the north part of the Lapland Granulite Belt are related to the E-W extension at the final period of thrusting. This deformation stage was characterised by persistently high temperatures and increasing water activity. (Kozlov and Kozlova, 1999).
Recent exploration by De Beers and others has revealed a significant number of diamond-bearing kimberlites in the Kola Peninsula, and (possibly extensive) deposits of gold in Finland. In Sweden, Alcaston Diamond NL is presently conducting a diamond exploration project, claiming some 9,550 km².
Contrasting Petrogenesis of two Volcanic Suites in the Devonian Vrbno Group (Hrubý Jeseník Mts., Czech Republic)
Jan 01, 2006; The problem of determining palaeotectonic setting of old volcanic suites becomes quite challenging in metamorphosed terrains out...