The Baltic Shield, sometimes referred to as the Fennoscandian Shield, is located in Fennoscandia (Norway, Sweden and Finland), northwest Russia and under the Baltic Sea. The Baltic Shield is defined as the exposed Precambrian northwest segment of the East European Craton.
The Baltic Shield contains the oldest rocks of the European continent. The lithospheric thickness is about 200–300 km. During the Pleistocene epoch, great continental ice sheets scoured and depressed the shield's surface, leaving a thin covering of glacial material and innumerable lakes and streams. The Baltic Shield is still rebounding today following the melting of the thick glaciers during the Quaternary period.
Provinces and blocks
Belomorian and Karelian provinces
The Archean segment of the Baltic/Fennoscandian shield is divided into the Karelian, Belomorian and Kola provinces. The Karelian province is a granite-greenstone province reworked during the Proterozoic. The North Karelian greenstone belt sequence (NKGBS) is dominated by volcanics of calc-alkaline series, diorite-plagiogranitic batholith, and xenoliths of ultramafic to andesite–dacite composition.
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."
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."
The Archaean Kolmozero-Voronja greenstone belt is located on the Kola Peninsula between Murmansk, Central Kola and the Keivy terrains of Upper Archaean age. Four suites are distinguished in the greenstone belt:
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.
The geochronological data document a long and complicated evolution of the belt:
- 3.0–2.9 Ga formation of a mafic volcanic sequence and its probable magmatic analogue, a ~2.9 Ga old gabbro,
- 2.9–2.8 Ga formation of intermediate and felsic volcanic rocks with an upper age limit of ~2.83 Ga,
- 2.7–2.6 Ga granodiorite intrusions and their vein analogues,
- and 2.6–2.5 Ga intrusion of postkinematic tourmaline and microcline granites.
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.
Northeastern Baltic Shield
The Keivy complex in the NE Baltic shield consists mainly of sheet-like peralkaline granite bodies, granosyenite dykes and some nepheline syenite fault-type intrusions in the total exposed ~2500 km. square area.
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.
Southeastern Baltic Shield
The Sumozero-Kenozero greenstone belt in the southeastern section of the Baltic shield is ~400 km long and up to 50 km wide. It comprises a 5-km thick oceanic plateau sequence of submarine komatiite-basalt lava and volcanic sediments. The belt is intruded and overlain by an island arc-like sequence of intermediate-felsic volcanic rocks including andesitic basalts, andesites, dacites and rhyolites. According to a study by Puchtel et al. (1999): "The komatiites were derived from a liquid containing ~30% MgO. This liquid was initiated at depths of 300–400 km in a mantle plume that was some 250°C hotter than the ambient mantle. Both komatiites and basalts of the lower sequence are strongly depleted in LREE, have high εNd(T) of +2.7±0.3, relatively unradiogenic Pb isotope compositions (µ m1 = 8.7±0.2) and show Nb-maxima (Nb/Nb* = 1.2±0.2, Nb/U = 43±6)." These parameters are found in a number of other early Precambrian greenstones and in recent Pacific OFB. "They are regarded as plume source characteristics and provide further evidence for the existence of certain Nb-excess in the Archaean mantle due to the early extraction of large volumes of continental crust with low Nb/U ratios. The intermediate-felsic volcanic and subvolcanic rocks of the upper unit are enriched in LREE, depleted in HFSE, but have positive εNd(T) values of +2.5±1.2. They represent both mantle wedge-derived basalt-andesite-dacite-rhyolite (BADR), and slab-derived (adakite) melts, erupted in the inner and frontal parts of an intraoceanic island arc." U–Pb zircon ages for the felsic volcanic rocks are 2875±2 Ma, and Pb–Pb and Sm–Nd ages of 2892±130 and 2916±117 Ma for the komatiites-basalts.
The Sumozero-Kenozero greenstone belt displays fragments of unsubductable oceanic crust, represented by the lower mafic-ultramafic volcanic sequence, and also displays the products of subduction-related magmatism. This implies that the thick plume-derived oceanic crust reached an intraoceanic convergent plate boundary and was intruded and overlain by felsic melts coming from both a subducting slab and an overlying mantle wedge. Later, the oceanic plateau, together with the volcanic arc complex built on top of it, were accreted to and obducted onto the continental crust of the Vodla block.
Mountains that existed in Precambrian time were eroded into a subdued terrain terrain already during the Late Mesoproterozoic when the rapakivi granites intruded. Further erosion made the terrain rather flat at the time of the deposition of Jotnian sediments. With Proterozoic erosion amounting to tens of kilometers many of the Precambrian rocks seen today in Finland are the "roots" of ancient massifs. The last major leveling event resulted in the formation of the Sub-Cambrian peneplain in Late Neoproterozoic time.
Laurentia and Baltica collided in the Silurian and Devonian producing a Himalayas-sized mountain range named the Caledonian Mountains roughly over the same area as the present-day Scandinavian Mountains. During the Caledonian orogeny Finland was likely a sunken foreland basin covered by sediments, subsequent uplift and erosion would have eroded all of these sediments. While Finland has remained buried or very close to sea-level since the formation of the Sub-Cambrian peneplain some further relief was formed by a slight uplift resulting in the carving of valleys by rivers. The slight uplift does also means that at parts the uplifted peneplain can be traced as summit accordances.
Denudation in the Mesozoic is counted at most in hundreds of meters. The inselberg plain of Finnish Lapland it is estimated to have formed in Late Cretaceous or Paleogene time either by pediplanation or etchplanation. Any older Mesozoic surface in Finnish Lapland is unlikely to have survived erosion. Further west the Muddus plains and its inselbergs formed —also by etching and pediplanation— in connection to the uplift of the northern Scandinavian Mountains in the Paleogene. Parts of northern Fennoscandia have been though to have been below sea level during the Eocene, but this is disputed as evidence, built upon microfossil findings of Astrid Cleve, is tenuous.
The northern Scandinavian Mountains had their main uplift in the Paleogene while the southern Scandinavian Mountains and the South Swedish Dome were largely uplifted in the Neogene. The uplifts events were concurrent with the uplift of Eastern Greenland  All of these uplifts are thought to be related to far-field stresses in Earth’s lithosphere. The Scandinavian Mountains and the South Swedish Dome can according to this view be likened to a giant anticlinal lithospheric folds. Folding could have been caused by horizontal compression acting on a thin to thick crust transition zone (as are all passive margins). The uplift of the Scandinavian Mountains resulted in the progressive tilt of northern Sweden contributing to create the parallel drainage pattern of northern Sweden. As the South Swedish Dome uplifted this resulted in the formation of a piedmonttreppen and the obstruction of the Eridanos River diverting it to the south.
While being repeatedly covered by glaciers during the last 2.5 million years glacial erosion has had a limited effect in changing the topography of Fennoscandia. Denudation during this time is geographically highly variable but averages tens of meters. The southern coast of Finland, Åland and the Stockholm archipelago were subject to considerable glacial erosion in the form of scraping during the Quaternary. The Quaternary ice ages resulted in the glacier's erosion of irregularly distributed weak rock, weathered rock mantles, and loose materials. When the ice masses retreated eroded depressions turned into the many lakes seen now in Finland and Sweden. Fractures in the bedrock were particularly affected by weathering and erosion, leaving as result straight sea and lake inlets.
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