Baltic Shield

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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. It is composed mostly of Archean and Proterozoic gneisses and greenstones which have undergone numerous deformations through tectonic activity (see Geology of Fennoscandia map [9]). 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[edit]

Geological map of Fennoscandia.
  Archean rocks of the Karelia and Kola domains
  Proterozoic rocks of the Karelia and Kola domains

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 (billion years). 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,[1] 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 Transscandinavian 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.

Belomorian and Karelian provinces[edit]

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 andesitedacite 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."[2]

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 (garnetbiotite 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."[3]

Kola province[edit]

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 (komatiitetholeiite), 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.[4]

The geochronological data document a long and complicated evolution of the belt:[4]

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.[5]

Northeastern Baltic Shield[edit]

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.[6]

According to a study by Bayanova and Zozulya (1999),[6] 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.[7]

Southeastern Baltic Shield[edit]

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.

Vodla Block[edit]

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.[8]

Denudation chronology[edit]

Mountains that existed in Precambrian time were eroded into a subdued terrain terrain already during the Late Mesoproterozoic when the rapakivi granites intruded.[9] Further erosion made the terrain rather flat at the time of the deposition of Jotnian sediments.[9][10] With Proterozoic erosion amounting to tens of kilometers[11] many of the Precambrian rocks seen today in Finland are the "roots" of ancient massifs.[9] The last major leveling event resulted in the formation of the Sub-Cambrian peneplain in Late Neoproterozoic time.[9][12]

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.[13][14] 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.[15] While Finland has remained buried[15] 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.[9]

Luosto, an inselberg in Finnish Lapland.

Denudation in the Mesozoic is counted at most in hundreds of meters.[16] 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.[17] 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.[18]

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.[18][19] The uplifts events were concurrent with the uplift of Eastern Greenland [14] 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).[20][21] The uplift of the Scandinavian Mountains resulted in the progressive tilt of northern Sweden contributing to create the parallel drainage pattern of northern Sweden.[22] 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.[19]

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.[16] The southern coast of Finland, Åland and the Stockholm archipelago were subject to considerable glacial erosion in the form of scraping during the Quaternary.[23] 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.[9][24] Fractures in the bedrock were particularly affected by weathering and erosion, leaving as result straight sea and lake inlets.[9]

References[edit]

  1. ^ Swedish Museum of Natural History. (2006) "Geology of Fennoscandia." [1]
  2. ^ Bibikova, Elena V., Svetlana Bogdanova, Stefan Claesson and Torbjorn Skiold. (1999) "NORDSIM Ages on Provenance and Metamorphic Zircon Material in Belomorian Metasediments of the Baltic Shield." Journal of Conference Abstracts, Vol. 4, No. 1, Symposium A08, Early Evolution of the Continental Crust.[2]
  3. ^ Slabunov, Alexander. (1999) "Late Archaean Subduction and Collision Events in the Crustal Evolution of the Eastern Fennoscandian/Baltic Shield." Journal of Conference Abstracts, Vol. 4, No. 1, Symposium A08, Early Evolution of the Continental Crust.[3]
  4. ^ a b Kudryashov, Nikolay. (1999) "Archean Kolmozero-Voronja Greenstone Belt: U-Pb Zircon and Sphene Data." Journal of Conference Abstracts, Vol. 4, No. 1, Symposium A08, Early Evolution of the Continental Crust.[4]
  5. ^ Turkina, Olga M. and Valeri R. Vetrin. (1999) "Composition of Tonalite - Trondhjemite Protoliths from the Kola Superdeep Borehole (KSDB) and on the Surface." Journal of Conference Abstracts, Vol. 4, No. 1, Symposium A08, Early Evolution of the Continental Crust.[5]
  6. ^ a b Bayanova, Tamara, and Dmitry Zozulya (1999) "Discrimination Between Plume and Plate Tectonics in Late Archaean of NE Baltic Shield from the Study of the Keivy A-Granite Complex." Journal of Conference Abstracts, Vol. 4, No. 1, Symposium A08, Early Evolution of the Continental Crust.[6]
  7. ^ Kozlov, Nikolai Ye, and Natalia Ye Kozlova. (1999) "Relation between Garnet Plagiogranitoids and Acid Granulites in Laplandian Granulite Belt." Journal of Conference Abstracts, Vol. 4, No. 1, Symposium A08, Early Evolution of the Continental Crust.[7]
  8. ^ Puchtel, Igor, Albrecht Hofmann, Alexander Samsonov, and Andrey Shchipansky. (1999) "Combined Hotspot-Island Arc Model for the Formation of the 2.9 Ga Sumozero-Kenozero Greenstone Belt, SE Baltic Shield." Journal of Conference Abstracts, Vol. 4, No. 1, Symposium A08, Early Evolution of the Continental Crust.[8]
  9. ^ a b c d e f g Lindberg, Johan (April 4, 2016). "berggrund och ytformer". Uppslagsverket Finland (in Swedish). Retrieved November 30, 2017. 
  10. ^ Lundmark, Anders Mattias; Lamminen, Jarkko (2016). "The provenance and setting of the Mesoproterozoic Dala Sandstone, western Sweden, and paleogeographic implications for southwestern Fennoscandia". Precambrian Research. 275: 197–208. 
  11. ^ Lidmar-Bergström, Karna (1997). "A long-term perspective on glacial erosion". Earth Surface Processes and Landforms. 22: 297–306. 
  12. ^ Japsen, Peter; Green, Paul F.; Bonow, Johan M.; Erlström, Mikael (2016). "Episodic burial and exhumation of the southern Baltic Shield: Epeirogenic uplifts during and after break-up of Pangaea". Gondwana Research. 35: 357–377. 
  13. ^ Gabrielsen, Roy H.; Faleide, Jan Inge; Pascal, Christophe; Braathen, Alvar; Nystuen, Johan Petter; Etzelmuller, Bernd; O'Donnel, Sejal (2010). "Latest Caledonian to Present tectonomorphological development of southern Norway". Marine and Petroleum Geology. 27: 709–723. doi:10.1016/j.marpetgeo.2009.06.004. 
  14. ^ a b Green, Paul F.; Lidmar-Bergström, Karna; Japsen, Peter; Bonow, Johan M.; Chalmers, James A. (2013). "Stratigraphic landscape analysis, thermochronology and the episodic development of elevated, passive continental margins". Geological Survey of Denmark and Greenland Bulletin. 30: 18. Retrieved 30 April 2015. 
  15. ^ a b Murrell, G.R.; Andriessen, P.A.M. (2004). "Unravelling a long-term multi-event thermal record in the cratonic interior of southern Finland through apatite fission track thermochronology". Physics and Chemistry of the Earth, Parts A/B/C. 29 (10): 695–706. Retrieved December 10, 2017. 
  16. ^ a b Lidmar-Bergström, Karna (1997). "A long-term perspective on glacial erosion". Earth Surface Processes and Landforms. 22: 297–306. 
  17. ^ Kaitanen, Veijo (1985). "Problems concerning the origin of inselbergs in Finnish Lapland". Fennia. 163 (2): 359–364. 
  18. ^ a b Lidmar-Bergström, K.; Näslund, J.O. (2002). "Landforms and uplift in Scandinavia". In Doré, A.G.; Cartwright, J.A.; Stoker, M.S.; Turner, J.P.; White, N. Exhumation of the North Atlantic Margin: Timing, Mechanisms and Implications for Petroleum Exploration. Geological Society, London, Special Publications. The Geological Societ y of London. pp. 103–116. 
  19. ^ a b Lidmar-Bergström, Karna; Olvmo, Mats; Bonow, Johan M. (2017). "The South Swedish Dome: a key structure for identification of peneplains and conclusions on Phanerozoic tectonics of an ancient shield". GFF. 
  20. ^ Japsen, Peter; Chalmers, James A.; Green, Paul F.; Bonow, Johan M. (2012). "Elevated, passive continental margins: Not rift shoulders, but expressions of episodic, post-rift burial and exhumation". Global and Planetary Change. 90-91: 73–86. 
  21. ^ Løseth and Hendriksen 2005
  22. ^ Redfied, T.F.; Osmundsen, P.T. (2013). "The long-term topographic response of a continent adjacent to a hyperextended margin: A case study from Scandinavia". GSA Bulletin. 125 (1): 184–200. doi:10.1130/B30691.1. 
  23. ^ Kleman, J.; Stroeven, A.P.; Lundqvist, Jan (2008). "Patterns of Quaternary ice sheet erosion and deposition in Fennoscandia and a theoretical framework for explanation". Geomorphology. 97 (1–2): 73–90. 
  24. ^ Lidmar-Bergström, K.; Olsson, S.; Roaldset, E. (1999). "Relief features and palaeoweathering remnants in formerly glaciated Scandinavian basement areas". In Thiry, Médard; Simon-Coinçon, Régine. Palaeoweathering, Palaeosurfaces and Related Continental Deposits. Special publication of the International Association of Sedimentologists. 27. Blackwell Science Ltd. pp. 275–301. ISBN 0-632 -05311-9.