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2. The Geology of the Baltic Shield

The Baltic Shield is characterized as the Precambrian part of Fennoscandia (northern part of the East European Craton in fig. 2.1). It covers Norway, Sweden, Finland and the western parts of Russia. Estonia, Latvia, Lithuania and parts of Poland and Norway are covered with Phanerozoic rocks.

The East European Craton was created by the collision of Fennoscandia, Volga-Uralia and Sarmatia. Sarmatia and Volga-Uralia were already joined at ca. 2.05 - 2.0 Ga with the final amalgamation with Fennoscandia occurring at ca. 1.7 Ga (Bogdanova et al. 2001).

The Baltic Shield was created by the accretion of different micro-continents onto the Archaean core of Fennoscandia during the late Archaean and early Proterozoic. The age of the Baltic Shield decreases from the northeast to the southwest. Fig. 2.1 gives an overview on the major tectono-stratigraphic units of the Baltic Shield.

Crustal accretion happened in four main phases and are known as:

Saamian Orogeny (≥ 3.1 - 2.9 Ga)
Lopian Orogeny (2.9 - 2.6 Ga)
Svecofennian Orogeny (1.9 - 1.75 Ga)
Gothian Orogeny (1.75 - 1.5 Ga)

During the late Proterozoic Sveconorwegian-Grenvillian Orogeny (1.05 - 0.9 Ga) previously formed crust was mainly reworked.

The Baltic Shield is divided into four main domains, which are the Archaean Domain, the Svecofennian Domain, the Transscandinavian Igneous Belt and the Southwest Scandinavian Domain. They will be described in the following section.

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2.1 The Archaean Domain

Fig 2.1  

The Archaean Domain is located in the northeastern part of the Baltic Shield and comprises the Kola, Belomorian and Karelian Provinces, separated by early Proterozoic thrust faults (cf. fig. 2.1). The oldest preserved rocks on the Baltic Shield, which can be found in the Karelian Province, have been referred to the Saamian Orogeny between 3.1 and 2.9 Ga. In the southwestern part of the Karelian Province Saamian rocks mainly consist of granitoids with a tonalite-trondhjemite-granodiorite composition and intermediary granulite belts. Gaál & Gorbatschev (1987) assumed that these granitoids can also be found within the basement of the other provinces. The eastern part of the Karelian Province consists of numerous greenstone belts, formed during the Lopian Orogeny. They contain a substantial amount of komatiites and are intruded by Lopian granitoids (Gorbatschev & Bogdanova 1993).

The Belomorian borders the Karelian Province to the northeast. It mainly consists of high-grade gneisses and mafic volcanic rocks (2.9 - 2.7 Ga). There also exist tonalitic-granodioritic plutonites. Greenstone belts from the Karelian Province have been detected to continue into the Belomorian Belt (Gorbatschev & Bogdanova 1993). The Kola Peninsula north of the Belomorian Province is separated by the Lapland Granulite Belt. Like the Belomorian, the Kola Peninsula Province is a high-grade gneiss terrain where exposed terranes of older granitoids are surrounded by Archaean gneiss belts (Gaál & Gorbatschev 1987, Gorbatschev & Bogdanova 1993).

The Archaean core of Fennoscandia was formed from several protocontinental terranes. This seems rather clear for the Kola Peninsula. For the Saamian crust it is still discussed whether it was formed from a continuous continental block or resulted from several micro-continents. Gaál & Gorbatschev (1987) and Ruotoistenmäki (1996) believe that the greenstone magmatism about 2.79 Ga resulted from intracontinental rifting with an ophiolitic character than from thrusting tectonics between small terranes. Ruotoistenmäki (1996) additionally assumes an amalgamation of the Svecobaltian and Karelian continent around 2.97 Ga, which can be correlated with the Saamian Orogeny.

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2.2 The Svecofennian Domain

Fig 2.2  

The rocks of the Svecofennian Domain were mainly formed during the Svecofennian Orogeny. The area is divided into three provinces, that are the Northern, Southern and Central Svecofennian Subprovince (cf. fig. 2.1). The Northern Svecofennian Subprovince contains volcanic and plutonic rocks with an age of about 1.89 - 1.87 Ga as well as metasedimentary rocks. The volcanic rocks of the Southern Svecofennian Subprovince are dated around 1.90 - 1.88 Ga, whereas the granitoids in this area show younger ages of 1.87 - 1.85 Ga. Both the Northern and Southern Subprovince are assumed to represent the remnants of island arcs. The Central Svecofennian Subprovince contains mainly graywackes of the Bothnian Basin with an approximate thickness of 10 km and intercalations of mafic volcanics (Nironen 1997). Nironen (1997) suggested the existence of at least two volcanic arcs, separated by a large sedimentary basin. He has drawn a model for the development of the Svecofennian Domain between 1.91 and 1.82 Ga, where a microplate consisting of an arc complex, accreted to the Archaean craton about 1.91 Ga ago. Because of the missing subduction-related rocks on the cratonic side, the subduction is considered to have occurred in a SW direction beneath the arc complex. The Bothnian Basin started closing when the southern arc complex collided against the northern one at about 1.89 Ga. The rocks of the Svecofennian Domain experienced low-pressure-type metamorphism (cf. fig. 2.2), with the peak of metamorphism decreasing in age from 1.88 - 1.82 Ga in Finland to 1.85 - 1.80 Ga in Sweden.

U-Pb ages have been selected from detrial zircons. They resulted in two age groups for the Svecofennian sedimentary rocks. Only a minor part of the zircons turned out to be of Archaean age, the major contribution is of 2.1 - 1.9 Ga age (Sultan et al. 2004). The source of these two fractions is still unclear, however, Nironen (1997) assumed that the Archaean contribution is evidence of sediments from the Archaean craton in the present northeast. Claesson et al. (1993) concluded that the Archaean zircons in the sediments of southern Sweden may have a source area in the southwest.

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2.3 The Transscandinavian Igneous Belt

The Transscandinavian Igneous Belt (TIB) accrued during a phase of magmatic activity from 1.83 to 1.65 Ga in the western part of the Svecofennian Domain (Gorbatschev & Bogdanova 1993). This igneous belt is about 1600 km long, up to 200 km wide, nearly NS trending and mainly consists of granitoids and felsic volcanics (Gaál & Gorbatschev 1987). It extends beneath the Caledonides where it crops out in several windows (cf. fig. 2.1). To the north and northeast TIB granitoids show intrusive relations to Svecofennian rocks (Lindh 2002). Older TIB rocks seem to be coeval with younger Svecofennian granitoids (Romer & Nisca 1995). The TIB rocks of the Västervik area adjoin to the Svecofennian Domain separated by the Loftahammar-Linköping-Deformation-Zone (LLDZ) that is assumed to represent a plate boundary. In the west the TIB rocks are bounded by the Protogine Zone (cf. fig. 2.1). The Transscandinavian Igneous Belt comprises the Småland-Värmland granitoids in the south, followed by the Dala and Rätan granitoids in the north (Lind 2002). Within the southern part of the TIB the rocks of the Oskarshamn-Jönköping-Belt (OJB), which has been studied by Mansfeld (1996), are slightly older than the Småland-Värmland granitoids and are considered to represent a separate terrane that was formed before the emplacement of the TIB granitoids.

TIB rocks are divided into two age groups by Åhäll & Larson (2000). The first one is aged 1.81 - 1.77 Ga and is called TIB1. Rocks formed at this age are widely spread in Småland. The second group is aged 1.72 - 1.66 Ga, formerly known as TIB2 and 3 according to Larson & Berglund (1992). The origin of the Transscandinavian Igneous Belt is not yet clarified. Andinotype and extensional formation have been discussed as two possible scenarios. Nyström (1982) considered an andinotype setting and noted that the TIB rocks intruded into the only slightly older Svecofennian rocks. A series of batholiths intruded in a linear arrangement, which is similar to other subduction related belts. TIB rocks are normally of an I-type character. Nyström (1982), Gorbatschev & Gaál (1987) and Gorbatschev & Bogdanova (1993) ascribed the occurrence of the TIB to an eastward subduction zone. Johansson (1982) preferred an ensialic rift setting for the formation of the TIB.

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2.4 The Southwest Scandinavian Domain

Fig 2.3  

The Southwest Scandinavian Domain was mainly built by crustal accretion during the Gothian Orogeny (1.75 - 1.5 Ga) and reworking by the Sveconorwegian-Grenvillian (1.05 - 0.9 Ga) and Caledonian Orogeny (0.44 - 0.4 Ga). During this period shallow and deep marine sediments were deposited. This domain also includes different generations of granitoids and metamorphic rocks (Gaál & Gorbatschev 1987).

The Southwest Scandinavian Domain, consisting of the Western and Eastern Segment, is separated from the Transscandinavian Igneous Belt by the Protogine Zone (cf. fig. 2.1). The Protogine Zone was formed by several episodes of faulting and shearing, accompanied by syenitic, granitic and mafic intrusions. To the west strongly foliated orthogneisses of the Mylonite Zone are attached to the Protogine Zone (Gaál & Gorbatschev 1987). Newly formed Gothian crust was following affected by the Hallandian Orogeny (1.5 - 1.4 Ga). This period is characterized by granulite-facies metamorphism, amphibolite facies in southwestern Sweden (cf. fig. 2.3), associated with deformation, emplacements of anatectic charnockites and mafic intrusions (Hubbard 1975 in Gaál & Gorbatschev 1987). After a period of relative quiescence the Sveconorwegian-Grenvillian Orogeny (1.05 - 0.9 Ga) followed and the crust in the western part of the Baltic Shield was mainly reworked (Gaál & Gorbatschev 1987).

During the Caledonian Orogeny (0.44 - 0.4 Ga) Laurentia and Baltica collided and allochthonous nappes were thrusted in a southeast to easterly direction onto the Precambrian basement. The degree of metamorphism decreases from the northwest to the southeast. Today the Caledonides cover large areas of Norway.

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