Baltic Shield
Encyclopedia
The Baltic 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
gneiss
es and greenstones which have undergone numerous deformations through tectonic
activity (see Geology of Fennoscandia map http://www.nrm.se/theswedishmuseumofnaturalhistory/researchandcollections/geology/laboratoryforisotopegeology/geologyoffennoscandia.291_en.html). The Baltic Shield contains the oldest rocks of the Europe
an continent
. The lithospheric thickness is about 200-300 km. During the Pleistocene
epoch, great continental ice sheet
s 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 glacier
s during the Quaternary Period.
s 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 granite
s 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 nappe
s, 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
.
glaciations and subsequent retreats, the Baltic Shield has been scoured clean of its overlying sediments, leaving expansive areas (most within Scandinavia) exposed. It is therefore of importance to geophysicists
studying the geologic history and dynamics of eastern Europe.
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 esker
s. 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 fossil
s.
-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 xenolith
s 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." (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 xenolith
s of ultramafic to andesite-dacite composition that occur in it. The CBMZ is dominated by metabasalts (amphibolite
s) with widespread metaultrabasic rocks (metaperidotite
s, serpentinite
s and apoultramafic amphibolite
s), and extremely rare acid metavolcanics. The chemical composition of metabasalts in the CBMZ is similar to that of mid-oceanic ridge basalt
s (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, metakomatiite
s 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 plagioamphibolite
s are present among schist
ose plagioamphibolite
s 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 tonalite
s and trondhjemite
s (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 massif
s here. (Turkina and Vetrin, 1999).
and some nepheline
syenite
fault-type intrusion
s 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. (Kozlov and Kozlova, 1999).
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 highNd(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 rock
s of the upper unit are enriched in LREE, depleted in HFSE, but have positiveNd(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.
, represented by the lower mafic-ultramafic volcanic sequence, and also displays the products of subduction-related magma
tism. 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. (Hofmann et al., 1999)
s and ore
s, such as those of iron
, nickel
, copper
and platinum group
metals. Because of its similarity to the Canadian Shield
and cratons of southern Africa
and Western Australia
, the Baltic Shield had long been a suspected source of diamond
s and gold
. Currently, especially the Central Lapland Greenstone Belt
in the north is considered to be an unexplored area that has the potential to hold exploitable gold deposits.
Recent exploration by De Beers
and others has revealed a significant number of diamond-bearing kimberlite
s 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.
in any craton is the area of exposed crystalline crust
while the other part of a craton is a “platform
” where the crystalline crust or basement
is overlaid by younger sedimentary cover. Thus the crustal segments comprise both the shield areas and parts of the platform basement. As such, the Baltic Shield is not the same as the Baltic Plate
. The Baltic, Ukrainian shield
s and the Voronezh Massif are the present-day geomorphological highs which were formed much later, even in the Cenozoic
. Geomorphologically
the Baltic Shield is only one segment/region of the East European craton
. The East European craton
as a whole was a part of Neoproterozoic
-Early Palaeozoic Baltica
.
Fennoscandia
Fennoscandia and Fenno-Scandinavia are geographic and geological terms used to describe the Scandinavian Peninsula, the Kola Peninsula, Karelia and Finland...
(Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
, Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
and Finland
Finland
Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...
), northwest Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
and under the Baltic Sea
Baltic Sea
The Baltic Sea is a brackish mediterranean sea located in Northern Europe, from 53°N to 66°N latitude and from 20°E to 26°E longitude. It is bounded by the Scandinavian Peninsula, the mainland of Europe, and the Danish islands. It drains into the Kattegat by way of the Øresund, the Great Belt and...
. The Baltic Shield is defined as the exposed Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
northwest segment of the East European Craton
East European craton
The East European craton is the core of the Baltica proto-plate and consists of three crustal regions/segments: Fennoscandia to the northwest, Volgo-Uralia to the east, and Sarmatia to the south...
. It is composed mostly of Archean
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...
and Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
es and greenstones which have undergone numerous deformations through tectonic
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
activity (see Geology of Fennoscandia map http://www.nrm.se/theswedishmuseumofnaturalhistory/researchandcollections/geology/laboratoryforisotopegeology/geologyoffennoscandia.291_en.html). The Baltic Shield contains the oldest rocks of the Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
an continent
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
. The lithospheric thickness is about 200-300 km. During the Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
epoch, great continental ice sheet
Ice sheet
An ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² , thus also known as continental glacier...
s 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
Post-glacial rebound
Post-glacial rebound is the rise of land masses that were depressed by the huge weight of ice sheets during the last glacial period, through a process known as isostasy...
today following the melting of the thick glacier
Glacier
A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
s during the Quaternary Period.
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/cratonCraton
A craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by...
s 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
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
.
According to the Swedish Museum of Natural History
Swedish Museum of Natural History
The Swedish Museum of Natural History , in Stockholm, is one of two major museums of natural history in Sweden, the other one being located in Gothenburg....
(2006), the oldest rocks of the Fennoscandian Shield are found in the northeast, in the Kola peninsula
Kola Peninsula
The Kola Peninsula is a peninsula in the far northwest of Russia. Constituting the bulk of the territory of Murmansk Oblast, it lies almost completely to the north of the Arctic Circle and is washed by the Barents Sea in the north and the White Sea in the east and southeast...
, Karelia
Karelia
Karelia , the land of the Karelian peoples, is an area in Northern Europe of historical significance for Finland, Russia, and Sweden...
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
Granulite
Granulites are medium to coarse–grained metamorphic rocks that have experienced high temperature metamorphism, composed mainly of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure...
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
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
. 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 granite
Rapakivi granite
Rapakivi granite is a hornblende-biotite granite containing large rounded crystals of orthoclase mantled with oligoclase. The name has come to be used most frequently as a textural term where it implies plagioclase rims around orthoclase in plutonic rocks...
s 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
Småland
' is a historical province in southern Sweden.Småland borders Blekinge, Scania or Skåne, Halland, Västergötland, Östergötland and the island Öland in the Baltic Sea. The name Småland literally means Small Lands. . The latinized form Smolandia has been used in other languages...
in southern Sweden through Värmland
Värmland
' is a historical province or landskap in the west of middle Sweden. It borders Västergötland, Dalsland, Dalarna, Västmanland and Närke. It is also bounded by Norway in the west. Latin name versions are Vermelandia and Wermelandia. Although the province's land originally was Götaland, the...
and western Dalarna
Dalarna
', English exonym: Dalecarlia, is a historical province or landskap in central Sweden. Another English language form established in literature is the Dales. Places involving the element Dalecarlia exist in the United States....
(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
Bohuslän
' is a Swedish traditional province, or landskap, situated in Götaland on the northernmost part of the country's west coast. It is bordered by Dalsland to the northeast, Västergötland to the southeast, the Skagerrak arm of the North Sea to the west, and the county of Østfold in Norway to the north...
granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
, 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
Caledonian orogeny
The Caledonian orogeny is a mountain building era recorded in the northern parts of the British Isles, the Scandinavian Mountains, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that occurred from the Ordovician to Early Devonian, roughly...
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
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
metasedimentary and metavolcanic rocks, deposited in the Iapetus Ocean
Iapetus Ocean
The Iapetus Ocean was an ocean that existed in the Neoproterozoic and Paleozoic eras of the geologic timescale . The Iapetus Ocean was situated in the southern hemisphere, between the paleocontinents of Laurentia, Baltica and Avalonia...
(the predecessor of the present-day Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
) c. 700 to 400 Ma ago. Together with slices of older basement, these rocks were thrust
Thrust fault
A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...
several 100 km eastwards over the edge of the Fennoscandian Shield in several large thrust sheets known as nappe
Nappe
In geology, a nappe is a large sheetlike body of rock that has been moved more than or 5 km from its original position. Nappes form during continental plate collisions, when folds are sheared so much that they fold back over on themselves and break apart. The resulting structure is a...
s, when North America and Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
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
Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...
. Similar Phanerozoic rocks also cover the Baltic republics, Poland and northern Germany. The magmatic rocks of the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
(c. 250 Ma) Oslo Graben
Oslo Graben
The Oslo Graben or Oslo Rift is a graben formed during a geologic rifting event in Permian time, the last phase of the Variscan orogeny. The main graben forming period began in the late Carboniferous, which culminated with rift formation and volcanism, with associated rhomb porphyry lava flows...
formed in a failed rift system
Aulacogen
In geology, an aulacogen is a failed arm of a triple junction of a plate tectonics rift system. A triple junction beneath a continental plate initiates a three way breakup of the continental plate. As the continental break-up develops one of the three spreading ridges typically fails or stops...
that continues into the Skagerrak
Skagerrak
The Skagerrak is a strait running between Norway and the southwest coast of Sweden and the Jutland peninsula of Denmark, connecting the North Sea and the Kattegat sea area, which leads to the Baltic Sea.-Name:...
and the North Sea
North Sea
In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...
.
Regolith
Thought to be formerly part of an ancient continent, the Baltic Shield grew in size through collisions with neighbouring crustal fragments. The mountains created by these tectonic processes have since been eroded to their bases, the region being largely flat today. Through five successive PleistocenePleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
glaciations and subsequent retreats, the Baltic Shield has been scoured clean of its overlying sediments, leaving expansive areas (most within Scandinavia) exposed. It is therefore of importance to geophysicists
Geophysics
Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...
studying the geologic history and dynamics of eastern Europe.
The scouring and compression of the Baltic Shield by glacial
Glacier
A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
movements created the area's many lakes and streams, the land retaining only a thin layer of sandy sediment collected in depressions and esker
Esker
An esker is a long winding ridge of stratified sand and gravel, examples of which occur in glaciated and formerly glaciated regions of Europe and North America...
s. Most soil consists of moraine
Moraine
A moraine is any glacially formed accumulation of unconsolidated glacial debris which can occur in currently glaciated and formerly glaciated regions, such as those areas acted upon by a past glacial maximum. This debris may have been plucked off a valley floor as a glacier advanced or it may have...
, 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
Pine
Pines are trees in the genus Pinus ,in the family Pinaceae. They make up the monotypic subfamily Pinoideae. There are about 115 species of pine, although different authorities accept between 105 and 125 species.-Etymology:...
, spruce
Spruce
A spruce is a tree of the genus Picea , a genus of about 35 species of coniferous evergreen trees in the Family Pinaceae, found in the northern temperate and boreal regions of the earth. Spruces are large trees, from tall when mature, and can be distinguished by their whorled branches and conical...
and birch
Birch
Birch is a tree or shrub of the genus Betula , in the family Betulaceae, closely related to the beech/oak family, Fagaceae. The Betula genus contains 30–60 known taxa...
, dominate the landscape, clearly demarcating its boundaries. The soil is acidic and has next to no carbonates such as limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
. The scouring by the ancient glaciers and the acidity of the soil have destroyed all palaeontologically interesting materials, such as fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s.
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 graniteGranite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
-greenstone
Greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies....
province reworked during the Proterozoic. The North Karelian greenstone belt sequence (NKGBS) is dominated by volcanics of calc-alkaline
Calc-alkaline
The calc-alkaline magma series is one of two main magma series in igneous rocks, the other magma series being the tholeiitic. A magma series is a series of compositions that describes the evolution of a mafic magma, which is high in magnesium and iron and produces basalt or gabbro, as it...
series, diorite
Diorite
Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory...
-plagiogranitic
Trondhjemite
Trondhjemite is a leucocratic intrusive igneous rock. It is a variety of tonalite in which the plagioclase is mostly in the form of oligoclase. Trondhjemites are sometimes known as plagiogranites....
batholith
Batholith
A batholith is a large emplacement of igneous intrusive rock that forms from cooled magma deep in the Earth's crust...
, and xenolith
Xenolith
A xenolith is a rock fragment which becomes enveloped in a larger rock during the latter's development and hardening. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption...
s of ultramafic to andesite
Andesite
Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...
-dacite
Dacite
Dacite is an igneous, volcanic rock. It has an aphanitic to porphyritic texture and is intermediate in composition between andesite and rhyolite. The relative proportions of feldspars and quartz in dacite, and in many other volcanic rocks, are illustrated in the QAPF diagram...
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
Kyanite
Kyanite, whose name derives from the Greek word kuanos sometimes referred to as "kyanos", meaning deep blue, is a typically blue silicate mineral, commonly found in aluminium-rich metamorphic pegmatites and/or sedimentary rock. Kyanite in metamorphic rocks generally indicates pressures higher than...
-facies
Metamorphic facies
The metamorphic facies are groups of mineral compositions in metamorphic rocks, that are typical for a certain field in pressure-temperature space...
metamorphism
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
, granitoid
Granitoid
A granitoid or granitic rock is a variety of coarse grained plutonic rock similar to granite which mineralogically are composed predominately of feldspar and quartz. Examples of granitoid rocks include granite, quartz monzonite, quartz diorite, syenite, granodiorite and trondhjemite. Many are...
magmatism
Magmatism
The formation of igneous rocks from magma is known as Magmatism.Magmatism is a process responsible for mountain formation. The process of magmatism produces an additional mass and volume to the Earth’s surface. For example the formation of volcanoes or island arcs at convergent plate boundaries...
, and the formation of folded nappe
Nappe
In geology, a nappe is a large sheetlike body of rock that has been moved more than or 5 km from its original position. Nappes form during continental plate collisions, when folds are sheared so much that they fold back over on themselves and break apart. The resulting structure is a...
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
Samarium-neodymium dating
Samarium-neodymium dating is useful for determining the age relationships of rocks and meteorites, based on decay of a long-lived samarium isotope to a radiogenic neodymium isotope. Nd isotope ratios are used to provide information on the source of igneous melts as well as to provide age data...
isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
studies of whole rock metasediment samples have constrained their mean protolith
Protolith
Protolith refers to the precursor lithology of a metamorphic rock.For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.Sedimentary...
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
Tectonostratigraphy
In geology, tectonostratigraphy refers either to rock sequences in which large-scale layering is caused by the stacking of thrust sheets or nappes in areas of thrust tectonics or the effects of tectonics on lithostratigraphy.-Tectonically formed stratigraphy:...
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
Accretion (geology)
Accretion is a process by which material is added to a tectonic plate or a landmass. This material may be sediment, volcanic arcs, seamounts or other igneous features.-Description:...
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
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
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
Diorite
Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory...
-plagiogranitic batholith (NKB) and xenolith
Xenolith
A xenolith is a rock fragment which becomes enveloped in a larger rock during the latter's development and hardening. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption...
s of ultramafic to andesite-dacite composition that occur in it. The CBMZ is dominated by metabasalts (amphibolite
Amphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
s) with widespread metaultrabasic rocks (metaperidotite
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
s, serpentinite
Serpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
s and apoultramafic amphibolite
Amphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
s), and extremely rare acid metavolcanics. The chemical composition of metabasalts in the CBMZ is similar to that of mid-oceanic ridge basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
s (MORB). The isotopic
Isotopic
The word isotopic has a number of different meanings, including:* In the physical sciences, to do with chemical isotopes;* In mathematics, to do with a relation called isotopy.* In geometry, isotopic refers to facet-transitivity....
age of the rhyolite
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
-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, metakomatiite
Komatiite
Komatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content...
s and felsic
Felsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
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
Garnet
The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
-biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
kyanite
Kyanite
Kyanite, whose name derives from the Greek word kuanos sometimes referred to as "kyanos", meaning deep blue, is a typically blue silicate mineral, commonly found in aluminium-rich metamorphic pegmatites and/or sedimentary rock. Kyanite in metamorphic rocks generally indicates pressures higher than...
-bearing gneisses. This lateral series indicates the Late Archean (3.0–2.8 Ga) subduction
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
stages in the evolution of the eastern Baltic shield. During the first stage, the oceanic lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's 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)
Kola province
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
Komatiite
Komatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content...
-tholeiite
Tholeiite
The tholeiitic magma series is one of two main magma series in igneous rocks, the other magma series being the calc–alkaline. A magma series is a series of compositions that describes the evolution of a mafic magma, which is high in magnesium and iron and produces basalt or gabbro, as it...
), Voronjatundrovskya (basalt-andesite-dacite) and Chervurtskya (an upper terrigenous formation). Zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
in the quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of 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
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
porphyry
Porphyry (geology)
Porphyry is a variety of igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in a fine-grained feldspathic matrix or groundmass. The larger crystals are called phenocrysts...
during the final stage of the belt development. Ovoid plagioamphibolite
Amphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
s are present among schist
Schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...
ose plagioamphibolite
Amphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
s of the Polmostundrovsky suite and have been dated at ~2.6 Ga, indicative of andalusite
Andalusite
Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5.The variety chiastolite commonly contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section....
-sillimanite
Sillimanite
Sillimanite is an alumino-silicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman . It was first described in 1824 for an occurrence in Chester, Middlesex County, Connecticut, USA....
facies
Facies
In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment....
metamorphism
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
. 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:
- 3.0–2.9 Ga formation of a mafic volcanic sequence and its probable magmatic analogueAnalog (chemistry)In chemistry, a structural analog , also known as chemical analog or simply analog, is a compound having a structure similar to that of another one, but differing from it in respect of a certain component. It can differ in one or more atoms, functional groups, or substructures, which are replaced...
, a ~2.9 Ga old gabbroGabbroGabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
, - 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 granodioriteGranodioriteGranodiorite is an intrusive igneous rock similar to granite, but containing more plagioclase than orthoclase-type feldspar. Officially, it is defined as a phaneritic igneous rock with greater than 20% quartz by volume where at least 65% of the feldspar is plagioclase. It usually contains abundant...
intrusionIntrusionAn intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
s and their vein analogues, - and 2.6–2.5 Ga intrusion of postkinematic tourmalineTourmalineTourmaline is a crystal boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone and the gem comes in a wide variety of colors...
and microclineMicroclineMicrocline is an important igneous rock-forming tectosilicate mineral. It is a potassium-rich alkali feldspar. Microcline typically contains minor amounts of sodium. It is common in granite and pegmatites. Microcline forms during slow cooling of orthoclase; it is more stable at lower temperatures...
granites. (Kudryashov, 2001).
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 tonalite
Tonalite
Tonalite is an igneous, plutonic rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase with 10% or less alkali feldspar. Quartz is present as more than 20% of the rock. Amphiboles and pyroxenes are common accessory minerals.In older references tonalite is...
s and trondhjemite
Trondhjemite
Trondhjemite is a leucocratic intrusive igneous rock. It is a variety of tonalite in which the plagioclase is mostly in the form of oligoclase. Trondhjemites are sometimes known as plagiogranites....
s (TT) is most likely the result of partial melting of mafic sources. The increase of alkalinity
Alkalinity
Alkalinity or AT measures the ability of a solution to neutralize acids to the equivalence point of carbonate or bicarbonate. The alkalinity is equal to the stoichiometric sum of the bases in solution...
in the protoliths of TT-gneisses correlates with the abundance of the Late Archean peralkaline (2750±50 Ma) and alkaline (2760±60 Ma) granite massif
Massif
In geology, a massif is a section of a planet's crust that is demarcated by faults or flexures. In the movement of the crust, a massif tends to retain its internal structure while being displaced as a whole...
s here. (Turkina and Vetrin, 1999).
Northeastern Baltic Shield
The Keivy complex in the NE Baltic shield consists mainly of sheet-like peralkaline granite bodies, granosyenite dykesDike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
and some nepheline
Nepheline
Nepheline, also called nephelite , is a feldspathoid: a silica-undersaturated aluminosilicate, Na3KAl4Si4O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites...
syenite
Syenite
Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or...
fault-type intrusion
Intrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
s 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
Massif
In geology, a massif is a section of a planet's crust that is demarcated by faults or flexures. In the movement of the crust, a massif tends to retain its internal structure while being displaced as a whole...
to 2670 Ma for the Western Keivy massif
Massif
In geology, a massif is a section of a planet's crust that is demarcated by faults or flexures. In the movement of the crust, a massif tends to retain its internal structure while being displaced as a whole...
and are spatially confined to voluminious gabbro
Gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
-anorthosite
Anorthosite
Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...
magmatism
Magmatism
The formation of igneous rocks from magma is known as Magmatism.Magmatism is a process responsible for mountain formation. The process of magmatism produces an additional mass and volume to the Earth’s surface. For example the formation of volcanoes or island arcs at convergent plate boundaries...
of 2.66–2.68 Ga. The predominantly "juvenile" Sm-Nd isotopic
Isotopic
The word isotopic has a number of different meanings, including:* In the physical sciences, to do with chemical isotopes;* In mathematics, to do with a relation called isotopy.* In geometry, isotopic refers to facet-transitivity....
signatures from most suites of Keivy complex suggest that they must be of mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
derivation or else have has short-lived crustal precursors. "The granites are petrologically and geochemically similar to Phanerozoic
Phanerozoic
The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...
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
Magmatism
The formation of igneous rocks from magma is known as Magmatism.Magmatism is a process responsible for mountain formation. The process of magmatism produces an additional mass and volume to the Earth’s surface. For example the formation of volcanoes or island arcs at convergent plate boundaries...
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
Mantle plume
A mantle plume is a hypothetical thermal diapir of abnormally hot rock that nucleates at the core-mantle boundary and rises through the Earth's mantle. Such plumes were invoked in 1971 to explain volcanic regions that were not thought to be explicable by the then-new theory of plate tectonics. Some...
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).
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 plateauPlateau
In geology and earth science, a plateau , also called a high plain or tableland, is an area of highland, usually consisting of relatively flat terrain. A highly eroded plateau is called a dissected plateau...
sequence of submarine komatiite-basalt lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
and volcanic sediments. The belt is intruded and overlain by an island arc
Island arc
An island arc is a type of archipelago composed of a chain of volcanoes which alignment is arc-shaped, and which are situated parallel and close to a boundary between two converging tectonic plates....
-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
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
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
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
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
Crust
Crust may refer to:* Crust * The Crust, television seriesPhysical sciences:* Crust , at least continent-wide structure* Soil crust, local biology-sensitive structureFood:* Crust, dense surface layer of bread...
with low Nb/U ratios. The intermediate-felsic volcanic and subvolcanic rock
Subvolcanic rock
A subvolcanic rock, also known as a hypabyssal rock, is an igneous rock that originates at medium to shallow depths within the crust and contain intermediate grain size and often porphyritic texture. They have textures between volcanic and plutonic rocks. Subvolcanic rocks include diabase and...
s of the upper unit are enriched in LREE, depleted in HFSE, but have positive
Adakite
Adakite is a petrologic term for volcanic or intrusive igneous rocks that are interpreted to form in subduction zones from the mixing of mantle material with felsic partial melts of descending slabs of oceanic crust basalt...
) melts, erupted in the inner and frontal parts of an intraoceanic island arc
Island arc
An island arc is a type of archipelago composed of a chain of volcanoes which alignment is arc-shaped, and which are situated parallel and close to a boundary between two converging tectonic plates....
." 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
The Sumozero-Kenozero greenstone belt displays fragments of unsubductable oceanic crustCrust
Crust may refer to:* Crust * The Crust, television seriesPhysical sciences:* Crust , at least continent-wide structure* Soil crust, local biology-sensitive structureFood:* Crust, dense surface layer of bread...
, represented by the lower mafic-ultramafic volcanic sequence, and also displays the products of subduction-related magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
tism. This implies that the thick plume-derived oceanic crust
Crust
Crust may refer to:* Crust * The Crust, television seriesPhysical sciences:* Crust , at least continent-wide structure* Soil crust, local biology-sensitive structureFood:* Crust, dense surface layer of bread...
reached an intraoceanic convergent plate boundary
Convergent boundary
In plate tectonics, a convergent boundary, also known as a destructive plate boundary , is an actively deforming region where two tectonic plates or fragments of lithosphere move toward one another and collide...
and was intruded and overlain by felsic melts coming from both a subducting slab
Slab
-Physical materials:* Slab , a length of metal* Concrete slab, a flat plate used in construction* A piece of stone or concrete used to pave sidewalks or road surfaces* Slab : That portion of a tectonic plate that is subducting...
and an overlying mantle wedge. Later, the oceanic plateau
Plateau
In geology and earth science, a plateau , also called a high plain or tableland, is an area of highland, usually consisting of relatively flat terrain. A highly eroded plateau is called a dissected plateau...
, together with the volcanic arc complex built on top of it, were accreted
Accretion (geology)
Accretion is a process by which material is added to a tectonic plate or a landmass. This material may be sediment, volcanic arcs, seamounts or other igneous features.-Description:...
to and obducted onto the continental crust of the Vodla block. (Hofmann et al., 1999)
Economic geology
The Baltic Shield yields important industrial mineralMineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s and ore
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....
s, such as those of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
, copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
and platinum group
Platinum group
The platinum group metals is a term used sometimes to collectively refer to six metallic elements clustered together in the periodic table.These elements are all transition metals, lying in the d-block .The six...
metals. Because of its similarity to the Canadian Shield
Canadian Shield
The Canadian Shield, also called the Laurentian Plateau, or Bouclier Canadien , is a vast geological shield covered by a thin layer of soil that forms the nucleus of the North American or Laurentia craton. It is an area mostly composed of igneous rock which relates to its long volcanic history...
and cratons of southern Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
and Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
, the Baltic Shield had long been a suspected source of diamond
Diamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...
s and gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
. Currently, especially the Central Lapland Greenstone Belt
Central Lapland Greenstone Belt
The Central Lapland Greenstone Belt is a greenstone belt located in the northern part of the Fennoscandian Shield. The region belongs to Lapland, northern Finland...
in the north is considered to be an unexplored area that has the potential to hold exploitable gold deposits.
Recent exploration by De Beers
De Beers
De Beers is a family of companies that dominate the diamond, diamond mining, diamond trading and industrial diamond manufacturing sectors. De Beers is active in every category of industrial diamond mining: open-pit, underground, large-scale alluvial, coastal and deep sea...
and others has revealed a significant number of diamond-bearing kimberlite
Kimberlite
Kimberlite is a type of potassic volcanic rock best known for sometimes containing diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an diamond in 1871 spawned a diamond rush, eventually creating the Big Hole....
s in the Kola Peninsula
Kola Peninsula
The Kola Peninsula is a peninsula in the far northwest of Russia. Constituting the bulk of the territory of Murmansk Oblast, it lies almost completely to the north of the Arctic Circle and is washed by the Barents Sea in the north and the White Sea in the east and southeast...
, 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.
Baltic Shield relation to Baltic plate
There is considerable misunderstanding in the literature with the use of the term "Baltic Shield." A shieldShield (geology)
A shield is generally a large area of exposed Precambrian crystalline igneous and high-grade metamorphic rocks that form tectonically stable areas. In all cases, the age of these rocks is greater than 570 million years and sometimes dates back 2 to 3.5 billion years...
in any craton is the area of exposed crystalline crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
while the other part of a craton is a “platform
Platform (geology)
In geology, a platform is a continental area covered by relatively flat or gently tilted, mainly sedimentary strata, which overlie a basement of consolidated igneous or metamorphic rocks of an earlier deformation...
” where the crystalline crust or basement
Basement (geology)
In geology, the terms basement and crystalline basement are used to define the rocks below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin...
is overlaid by younger sedimentary cover. Thus the crustal segments comprise both the shield areas and parts of the platform basement. As such, the Baltic Shield is not the same as the Baltic Plate
Baltic Plate
The Baltic Plate was an ancient tectonic plate that existed from the Cambrian period to the Carboniferous period. The Baltic Plate collided against Siberia, to form the Ural Mountains about 500 million years ago. The Baltic Plate, however, fused onto the Eurasian Plate when the Baltic Plate...
. The Baltic, Ukrainian shield
Ukrainian shield
The Ukrainian Shield is the southwest shield of the East European craton. The Ukrainian Shield and the Voronezh Massif consist of 3.2-3.8 Ga Archaean crust in the southwest and east, and 2.3-2.1 Ga Early Proterozoic orogenic belts....
s and the Voronezh Massif are the present-day geomorphological highs which were formed much later, even in the Cenozoic
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
. Geomorphologically
Geomorphology
Geomorphology is the scientific study of landforms and the processes that shape them...
the Baltic Shield is only one segment/region of the East European craton
East European craton
The East European craton is the core of the Baltica proto-plate and consists of three crustal regions/segments: Fennoscandia to the northwest, Volgo-Uralia to the east, and Sarmatia to the south...
. The East European craton
East European craton
The East European craton is the core of the Baltica proto-plate and consists of three crustal regions/segments: Fennoscandia to the northwest, Volgo-Uralia to the east, and Sarmatia to the south...
as a whole was a part of Neoproterozoic
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...
-Early Palaeozoic Baltica
Baltica
Baltica is a name applied by geologists to a late-Proterozoic, early-Palaeozoic continent that now includes the East European craton of northwestern Eurasia. Baltica was created as an entity not earlier than 1.8 billion years ago. Before this time, the three segments/continents that now comprise...
.
External links
- Rb-Sr Data for Diamond-Bearing Kimberlite and Alkaline Massives in NE Fennoscandia
- Precambrian evolution of the major Archaean blocks of the Baltic Shield (archive.org link)
- Geology of Fennoscandia - from Swedish Museum of Natural History - retrieved 2010-07-05.
- Why does the East European Platform has (sic) 3 km of sediments? Insights from lithospheric structure, thermal regime, and composition (PDF file)