Transscandinavian Igneous Belt Research Articles

Cretaceous Depositional Systems in the Norwegian Sea: Heavy Mineral Constraints

Deep-water Cretaceous sandstones in the Norwegian Sea were deposited by a number of distinct sediment transport systems tapping different sediment source terrains. Three distinct sandstone types (K1, K2, and K3) have been identified, distinguished, and mapped on the basis of a combination of heavy mineral parameters. Sandstone type K1 occurs on the Trondelag Platform, Halten Terrace, and Nordland Ridge, but does not appear to be present in the deeper water Voring Basin. Sandstone type K1 was ultimately derived from the Scandinavian landmass with detritus sourced from metasediments of the Caledonian fold belt, intrusives of the Trans-Scandinavian Igneous belt, and, to a smaller extent, Svecofennian basement. K1 sandstones were deposited on an unstable slope by debris flows and slumps, with minor reworking by bottom currents. Sandstone type K3 was derived from the Western Gneiss region farther south on the Scandinavian landmass. Sandstone type K2 occurs in more basinal locations in the Norwegian Sea and was not supplied by the systems operating along the Scandinavian margin because its mineralogy contrasts with that of K1 and K3. K2 mineralogy is not consistent with a source in the Lofoten area or East Greenland; therefore, K2 is believed to represent the deposits of a separate axial transport system fed by a source that lay in northeast Greenland. K2 zircon ages indicate involvement of Early Proterozoic (approximately 2000 Ma) and Archean basement, together with metasediments of the Caledonian fold belt. Previous sedimentological models for the Cretaceous of the Norwegian Sea suggest that sand deposition occurred as slumps and debris flows along the continental slope, resulting in discontinuous and unpredictable sandstone units, unless they become amalgamated into thick reservoir sequences. The mineralogical evidence indicates that this model can be applied only to sandstone types K1 and K3. By contrast, sandstone type K2 represents sediment introduced from the conjugate margin of the basin and occurs up to 200 km from its detrital source region; therefore, K2 is likely to occur as more predictable, sheetlike bodies on the basin floor.

A precursory study of magnetic fabrics in the transition zone between magmatic and metamorphic textures in the Askersund granitoid, Sweden

The Askersund granitoid (age 1845–1850 Ma) is the oldest member of the Transscandinavian Igneous Belt which forms a major structure in the Scandinavian peninsula. The external, metamorphic and gneissic parts of this older generation have the same age as the more central and isotropic parts. A petrographical investigation of samples collected in two traverses from the macroscopically isotropic part to the gneissic part of the intrusion has been complemented with studies of the anisotropy of the magnetic susceptibility of the rocks. The mean degree of anisotropy (PJ ) is different from site to site and increases from the macroscopically isotropic part to the gneissic part. The shape of the susceptibility ellipsoid is slightly oblate in the former part and prolate in the latter. Under the microscope this change can be correlated with a change in textures indicating deformation in a partly crystallized magma to more solid state deformation structures. A major conclusion of this investigation with regard to the geological development of the area, is that the emplacement of the granite magma took place contemporaneously with regional deformation that continued after the intrusion. Wikström, A., Sjöberg, B. & Kapicka, A., 1997: A precursory study of magnetic fabrics in the transition zone between magmatic and metamorphic textures in the Askersund granitoid, Sweden. GFF, Vol. 119 (Pt. 4, December), pp. 285–290. Stockholm. ISSN 1103–5897.

Granitoids from the Äspö area, southeastern Sweden ‐ geochemical and geochronological data

The bedrock on and around Äspö island, north of Oskarshamn is predominated by Småland granitoids belonging to the Trans‐scandinavian Igneous Belt (TIB). A second magmatic event was the formation of anorogenic granites (e.g. the Götemar granite) 1.4 Ga ago, represented by massifs of coarse‐grained granite cut by fine‐grained granitic dikes. The Småland granitoids constitute a suite of medium‐grained quartz monzodiorites (Äspö diorite) to granites (Ävrö granite) penetrated by fine‐grained granites. They are classified as alkalic to alkali‐calcic, metaluminous to peraluminous I‐granites. A U‐Pb dating on zircon and titanite of the Äspö diorite yielded an age of 1804±3 Ma. Two darings of the fine‐grained Småland granite gave ages of 1794+16 ‐12 and 1808+33 ‐30 Ma. The fine‐grained, felsic Småland granites are distinguished chemically from the similar looking fine‐grained anorogenic granites through, e.g., differing Fe/Mg, alkali contents and HREE‐pat‐terns. Kornfält, K.‐A., Persson, P.‐O. & Wikman, H., 1997: Granitoids from the Äspö area, southeastern Sweden ‐ geochemical and geochronological data. GFF, Vol. 119 (Pt. 2, June), pp. 109–114. Stockholm. ISSN 1103–5897.

Proterozoic sutures and terranes in the southeastern Baltic Shield interpreted from BABEL deep seismic data

A hitherto unknown terrane and its bounding sutures have been revealed by a combined study of normal-incidence and wide-angle seismic data along the BABEL profile in the Baltic Sea. This Intermediate Terrane is situated between a Northern Terrane of Svecofennian age and a Southwestern Terrane of Gothian age. It is delimited upwards by two low-angle and oppositely dipping sutures and occupies mainly middle and lower crustal levels with a width of ∼ 300 km at Moho level. The ∼ 1.86 Ga suture against the Northern Terrane is imaged by a prominent almost continuous NE-dipping crustal reflection from 3.5 to 14 s twt over 175 km. Where it downlaps on the Moho, sub-Moho velocities change from 8.2 to 7.8 km/s (±0.2) over less than 25 km. A relatively strong, NE-dipping normal-incidence and wide-angle reflection at 19–23 s twt indicates that the suture extends into the upper mantle. The pervasive NE-dipping reflection fabric of the Intermediate Terrane is interpreted as shear zones that developed during collision and possibly were reactivated by later events. High Poisson's ratios suggest a mafic composition or high fluid content. The ∼ 1.86 Ga collision was probably succeeded by continental break-up and removal of an unknown continent, except for the Intermediate Terrane. Subsequent formation of an east-dipping subduction zone further to the west led to the emplacement of 1.81-1.77-Ga-old granitoids in the southern part of the Transscandinavian Igneous Belt. The ∼ 1.65-1.60 Ga suture against the Southwestern Terrane is defined by a semi-continuous band of strong SW-dipping reflections between 3 and 8 s twt over 65 km, which are interpreted as a low-angle thrust zone along which Gothian crust overrode the Intermediate Terrane. The identification of three individual seismic terranes in the southeastern part of the Baltic Shield provides new evidence for Palaeoproterozoic plate tectonic processes.

The Rymmen gabbro, a layered mafic intrusion from the Transscandinavian Igneous Belt, southern Sweden

The Rymmen gabbro, a layered mafic intrusion from the Transscandinavian Igneous Belt, southern Sweden

Polyphase tectonometamorphic reworking in the Transscandinavian Igneous Belt, east of lake Vänern ‐ regional tectonic implications for southern Sweden

Polyphase tectonometamorphic reworking in the Transscandinavian Igneous Belt, east of lake Vänern ‐ regional tectonic implications for southern Sweden

The Solstad Cu‐Co‐Au mineralization and its relation to post‐Svecofennian regional shear zones in southeastern Sweden

The Solstad Cu-Co-Au deposit, 21 km S of Vastervik in SE Sweden (Fig. 3:2), was mined for copper from the 17th century until 1919. Previous geological documentation of the Solstad mines is more than one hundred years old (e.g. Hoppe 1884). During the entire mining history, nothing was mentioned about other economic constituents at Solstad than copper (except a brief comment by Ramdohr 1950). The Vastervik quartzites are the oldest rocks in the region and represent a well-preserved Svecofennian sequence of Early Proterozoic fluvial sediments. The 1.85-1.77 Ga post-Svecofennian Smaland granitoids show intrusive contacts to the Vastervik quartzites and constitute a calcalkaline batholith complex within the Transscandinavian Igneous Belt (TIB). One part of the Smaland granitoids (the Vimmerby batholith) extends continuously from Lake Vattern to the Baltic Sea and is bordered by older and metamorphosed rocks in the south (Vetlanda region) and in the northeast (southernmost Bergslagen). The intrusion age of the Vimmerby batholith (1802±4 Ma) has been obtained by the U-Pb method at Ramnebo, 10 km southwest of Solstad (Mansfeld 1991). Regionally significant ductile shear zones (the Loftahammar-Linkoping Deformation Zone) have been identified along the northeastern margins of the Vimmerby batholith and are considered to have formed in the time interval 1805 to 1745 Ma (Beunk et al. 1995; Wijbrans et al. 1995), i.e., immediately after the emplacement of the batholith. The Solstad deposit is located in the northeastern margin of the Vim-

UPb baddeleyite dating of dolerite dykes in the eastern part of the Sveconorwegian orogen, south-central Sweden

Two dolerite dykes occurring in semi-penetratively deformed Transscandinavian Igneous Belt (TIB) granitoids, east of the traditional ‘Protogine Zone’ have a composite UPb baddeleyite age of 1568 +30 −8 Ma. Since the dolerites are deformed themselves, this age sets a maximum time constraint for the ductile deformation in the easternmost part of the Sveconorwegian orogen, northeast of Lake Vänern, west of the Sveconorwegian Frontal Deformation Zone (SFDZ; Wahlgren et al., 1994). The new age agrees, within the error limits, with previously reported but less precise UPb and SmNd ages for petrographically and geochemically indistinguishable dolerites in the orthogneisses west of the traditional ‘Protogine Zone’, and together provides abundant evidence that these dolerites belong to the same generation of mafic magmatism. Therefore, the dolerites straddle the boundary between the orthogneisses and the semi-penetratively deformed TIB, i.e. the traditional ‘Protogine Zone’. Apart from intracrustal deformation, the area north of Lake Vänern from the SFDZ in the east to the Mylonite Zone in the west has constituted a coherent segment of crust from the time of the TIB intrusions (∼ 1850-1650 Ma). The dolerites in this study have intruded as dykes into isotropic host rocks. The present dip and concordance to the schistosity in the surrounding rocks are due to subsequent progressive deformation in the eastern part of the Sveconorwegian orogen. It is suggested that the structural geometry and concordant relationships between the dolerites and the orthogneisses west of the traditional ‘Protogine Zone’ are also mainly a consequence of this subsequent progressive deformation that post-dates the dolerite emplacement.

Oblique Terrane Assembly in the Late Paleoproterozoic during the Labradorian-Gothian Orogeny in Southern Scandinavia

The earliest deformation of the curved Kongsberg-Bamble Belt in Norway produced a northwestward-directed tectonic wedge, suggesting oblique NW-SE collision during the Labradorian-Gothian Orogeny in southern Scandinavia. Oblique terrane assembly, with the outboard (western) units colliding last, explains the trapping of the Ostfold-Marstrand island arc (formed at c. 1760 Ma) between younger (1700-1600 Ma) terranes, the apparent lack of Svecofennian crust west of the Trans-Scandinavian Igneous Belt, and the apparent lack of Labradorian-Gothian age crust in the British Isles. The Labradorian of North America may have also resulted from oblique terrane assembly.

Geological, geochemical and geochronological evidence for a new palaeoproterozoic terrane in southeastern Sweden

The geological and geochronological relationships between a rhyolite, belonging to the Transscandinavian Igneous Belt (TIB), and an intrusive suite (the Bäckaby intrusion), southeast Sweden, have been investigated by means of bedrock mapping, geochemistry and radiometric age determinations (UPb on zircons). Field relations suggest that the rhyolite rests unconformably on the Bäckaby intrusion, which previously has been correlated with ∼ 1.9 Ga granitoids within the Svecofennian Domain. The rhyolite yielded an age of 1800 ± 8 Ma and a tonalite belonging to the Bäckaby intrusion basement yielded an age of 1834 ± 3 Ma. The age of the rhyolite is slightly older (10–15 Ma) than adjacent granitoids belonging to the Transscandinavian Igneous Belt, and is considered as the earliest magmatic expression of TIB in the area. The age and the geochemical trends for the Bäckaby intrusion is significantly different from that of the surrounding TIB granitoids and indicate a rock forming event at about 1830–1840 Ma in southeastern Sweden represented by the Oskarshamn-Jönköping Belt. This event is significantly younger than the older Svecofennian rocks within the Svecofennian Domain proper, indicating a hitherto unidentified geological terrane in southeastern Sweden.

Absolute gravity measurements at Nordic geodetic laboratories in Fennoscandia and Spitsbergen 1991–1993

Two dolerite dykes occurring in semi-penetratively deformed Transscandinavian Igneous Belt (TIB) granitoids, east of the traditional ‘Protogine Zone’ have a composite UPb baddeleyite age of 1568+30−8 Ma. Since the dolerites are deformed themselves, this age sets a maximum time constraint for the ductile deformation in the easternmost part of the Sveconorwegian orogen, northeast of Lake Vänern, west of the Sveconorwegian Frontal Deformation Zone (SFDZ; Wahlgren et al., 1994).The new age agrees, within the error limits, with previously reported but less precise UPb and SmNd ages for petrographically and geochemically indistinguishable dolerites in the orthogneisses west of the traditional ‘Protogine Zone’, and together provides abundant evidence that these dolerites belong to the same generation of mafic magmatism. Therefore, the dolerites straddle the boundary between the orthogneisses and the semi-penetratively deformed TIB, i.e. the traditional ‘Protogine Zone’. Apart from intracrustal deformation, the area north of Lake Vänern from the SFDZ in the east to the Mylonite Zone in the west has constituted a coherent segment of crust from the time of the TIB intrusions (∼ 1850-1650 Ma).The dolerites in this study have intruded as dykes into isotropic host rocks. The present dip and concordance to the schistosity in the surrounding rocks are due to subsequent progressive deformation in the eastern part of the Sveconorwegian orogen. It is suggested that the structural geometry and concordant relationships between the dolerites and the orthogneisses west of the traditional ‘Protogine Zone’ are also mainly a consequence of this subsequent progressive deformation that post-dates the dolerite emplacement.

Westward accretion of the Baltic Shield: implications from the 1.6 GaÅmål-Horred Belt, SW Sweden

Three new U-Pb zircon age determinations are reported from the Horred region, south-southeast of Göteborg, SW Sweden. This is a region of the Southwest Scandinavian Domain, within which a major N S trending shear zone and tectonic boundary, the Mylonite Zone, juxtaposes comparatively weakly migmatised lithologies in the west against more intensely migmatised gneisses in the east. West of the Mylonite Zone, a metavolcanic rock (the Mjösjödacite) yields an age of 1643 ± 29 Ma, whereas a cross-cutting plutonic rock (the Idala tonalite) has an age of 1584 ± 15 Ma. Together with a recent age for a volcanic rock from theÅmål region farther north (1.61 Ga, Lundqvist and Skiöld, 1992), these ages help to establish the existence of a coherent calc-alkaline igneous belt of 1.6 Ga age for which the nameÅmål-Horred Belt is proposed. East of the Mylonite Zone, a presumably metavolcanic rock (the Grimmared gneiss) yields an age of ∼ 1.61 Ga. The obtained age and the compositional similarity of rocks on each side of the Mylonite Zone indicate that more deformed and more strongly metamorphosed equivalents of the rocks in theÅmål-Horred Belt may occur also to the east of the Mylonite Zone in what is termed the Eastern Segment of the Southwest Scandinavian Domain. The new results establish theÅmål-Horred Belt as a major geological unit younger than most other crustal components in southern Sweden such as theÖstfold-Marstrand Belt (∼ 1.76 Ga), the Eastern Segment gneisses (> 1.66 Ga) and the three age groups of the Transscandinavian Igneous Belt (∼ 1.81 – 1.65 Ga; Larson and Berglund, 1992). The configuration of the crustal units in SW Sweden appears to necessitate more complex Proterozoic models than those with a persistent younging from the present east to the west. The present concept of the “Gothian orogeny” must be revised since at least two different orogenic episodes at ∼ 1.7 and 1.6 Ga can now be distinguished.

Kinematics of a major fan-like structure in the eastern part of the Sveconorwegian orogen, Baltic Shield, south-central Sweden

Abstract The N-S-trending so-called Protogine Zone in the Baltic Shield of south-central Sweden is usually considered to mark a tectonic boundary between the rocks of the Transscandinavian Igneous Belt (TIB) in the east and the Sveconorwegian orogen in the west. Detailed structural mapping in the Karlskoga-Kristinehamn area has shown that an anastomosing network of ductile deformation zones with generally N-S strike extends ca. 40 km east of the traditional “Protogine Zone”. Furthermore, the western boundary of this ductile deformation and the TIB is not constrained in the Kristinehamn area. Reconnaissance studies indicate that they both extend westwards towards the so-called Mylonite Zone. It is suggested that the eastern limit of the Sveconorwegian orogen is located some 40 km east of the present boundary and that the “Protogine Zone” concept is obsolete. The term Sveconorwegian Frontal Deformation Zone (SFDZ) is proposed as a more appropriate alternative in southern Sweden (south of lake Vattern) and to correspond to a younger set of oblique ductile deformation zones with reverse and right-lateral components of movement in the easternmost part of the orogen farther to the north. Ductile deformation zones older than the SFDZ in the Karlskoga-Kristinehamn area display a fan-like geometry in an E-W cross-section, with steep westerly dips in the eastern part of the section, vertical dips farther west and moderate easterly dips in the western part of the section. Kinematic analysis indicates that dip-slip movements predominate with a consistent top-to-the-east sense of movement across the entire fan-like structure. In their present orientation, deformation zones are characterized by reverse movements in the eastern part and normal movements in the western part of the structure. Between Kristinehamn and the Mylonite Zone, the main foliation is gently dipping to subhorizontal, indicating that the regional structure is strongly asymmetric, and that the fan-like structure occurs close to the foreland of the orogen. Deformation zones are spaced to semi-penetrative in the eastern part of the fan-like structure, whereas the deformation is more or less penetrative and the TIB rocks are transformed to orthogneisses west of Kristinehamn. This east to west increase in bulk strain is in accordance with an increase in syn-deformational metamorphic grade across the structure. Younger ductile deformation zones belonging to the SFDZ are responsible for a major change in orientation of the older deformation zones in the easternmost part of the structure. The fan-like structure is best explained by models involving the interference of two separate tectonic events. Deformation occurred after ∼ 1.57 Ga and prior to deposition of Neoproterozoic and younger cover sedimentary rocks. It is not yet clear whether the initial phase of deformation (early Sveconorwegian or older) was related to the build-up of an imbricate thrust stack in a compressional regime, as favoured here, or to regional E-W extension. The younger deformation phase was related to rotation of these older structures into the compressional, late Sveconorwegian SFDZ.

U‐Pb age of the Yxsjöberg Tungsten‐Skarn deposit, Sweden

Abstract The Yxsjoberg scheelite‐skam deposit of western Bergslagen (south‐central Sweden) formed in Palaeoproterozoic time when aqueous fluids migrating along lithologic discontinuities replaced limestones in acid volcanic rocks. The age of the mineralization and, thus, the source of the fluids are controversial. U‐Pb dating of titanite that formed during the skarnification of the limestones yields an age of 1789±2 Ma. This age is not compatible with an exhalati ve origin of the deposit or a genetic relation to Svecofennian arc magmatism, as has been suggested. Instead, the U‐Pb titanite age corresponds to the age of Y, Rb, F, and Nb‐rich post‐kinematic granitoids from the Bergslagen area and granitoids of the Transscandinavian Igneous Belt (TIB) farther to the west. As the TIB granitoids are not fertile with respect to W‐mineralizations, the U‐Pb titanite age indicates that the post‐kinematic granitoids were the source of heat, metals, and probably fluids for the formation of the Yxsjoberg scheelite‐ska...

Sm-Nd isotope systematics of 1.9-1.8-Ga granites from western Bergslagen, Sweden: inferences on a 2.1-2.0-Ga crustal precursor

The Bergslagen ore province forms part of the Svecofennian domain of the Baltic Shield. The initial ϵ Nd-values of least-altered and albitized granite samples of the 1.88-Ga Bergslagen Older Granite suite vary between −1.2 and +2.5. The granite samples of the 1.78-Ga Fellingsbro-Malingsbo suite vary between −0.3 and +7.7. The Rb-Sr system of the Older Granite suite yields an errorchron of 1.70±0.01 Ga, with an initial 87 Sr 86 Sr ratio of 0.7053, indicating tectonothermal disturbance, perhaps due to the intrusion of younger granites of the Transscandinavian Igneous Belt. The initial Nd isotopic composition of the 1.88-Ga Older Granite suite can be interpreted in two extreme end-member models: (a) interaction between depleted mantle-derived melts and Archaean crustal material at 1.88 Ga; and (b) remelting of Early Proterozoic (pre-1.88 Ga) crust. Previous studies in the Baltic Shield have advocated the former model, and discarded the latter, because of absence of 2.0–2.4-Ga crust. Likewise, in the case of western Bergslagen, no Archaean crust is present. It is hence argued that the Nd isotopic data may also be interpreted in terms of generation of the Older Granite suite through remelting of a crustal precursor. In this case the 2.1-2.0-Ga depleted mantle model ages of the Older Granite suite could give an indication of the time that such a precursor was extracted from the depleted mantle.

Disturbed radiometric ages and their bearing on interregional correlations in the SW Baltic Shield

RbSr whole-rock and zircon UPb isotopic work is reported from the northern part of the region between the Mylonite and Protogine Zones in the southern Baltic Shield. Three age determinations on polymetamorphic gneissic granites were carried out. Two of these yielded UPb upper-intercept ages of ∼1650 and ∼1675 Ma, respectively, which is approximately one hundred Ma less than expected from the combined field evidence and earlier isotopic age determinations. Although the new discordias appear well defined, various criteria suggest that the UPb isotope system was disturbed. Thus, the RbSr system had been opened, but nevertheless the RbSr errorchrones are consistent with the UPb upper-intercept ages, and an abraded zircon fraction suggests an older age than the unabraded fractions. The third UPb age determination resulted in a poorly constrained discordia. Its upper intercept ranges between 1575 and 1605 Ma, depending on the number of fractions considered. This paper tests a three-stage model, where the intrusion age is set identical to the 1780 Ma intrusion age of the undeformed assumed protolith. The first two analysed rocks are consistent with a model suggesting intrusion at 1780 Ma followed by partial opening of the isotopic systems early (∼1200 Ma) and late (∼950 Ma) during the Sveconorwegian orogeny. An abraded fraction together with the time of the suggested last opening of the system 950 Ma ago defines a two-point discordia with an upper-intercept age of ∼1745 Ma. This age is in close agreement with the assumed intrusion age. The isotope data from the third gneissic granite cannot be fitted to this simple model. It is concluded that the analysed rocks could belong to the granitoids of the Transscandinavian Igneous Belt, but if they do, they were reworked during the Sveconorwegian orogeny. As a corollary, it follows that the regions separated by the northern part of the Protogine Zone largely had a common pre-Sveconorwegain geological history. Another important implication is that the intrusion ages in the area between the Mylonite and Protogine Zones are older than those of the Trans-Labrador Batholith in Canada.

A U–Pb dating of the Askersund granite and its marginal augen gneiss

Abstract U–Pb zircon ages of 1848±15 and 1842+23 -13 Ma have been obtained for the coarse porphyritic Askersund granite (‘Filipstad type’) and for its marginal augen gneiss, respectively. The results indicate that the onset of magmatism in the Transscandinavian Igneous Belt (TIB) took place in connection with the late Svecofennian orogenic development. They also support a structural model implying that regionally deformed and folded marginal parts of older granitoid intrusions in the TIB have similar ages as the isotropic and discordant central parts. The deformation is reflected in the morphology of the zircons but does not affect the isotopic ages.

Frontiers in the Baltic Shield

Abstract Recent work in the Baltic Shield and its continuation in the basement of the East European Platform has created a new image of the Precambrian in the northeastern half of Europe. Towards the southeast, a continuation of the crust in the Shield can be traced to a Proterozoic system of palaeorifts in the East European Platform. These rifts apparently reproduce an earlier Precambrian crustal boundary that separates Fennoscandia, the northwestern crustal segment of the East European Craton, from its other two segments, Sarmatia and Volgo-Uralia. In the Archaean of Fennoscandia, the granite-greenstone province in Karelia is the only part that has ages in excess of 3 Ga. Interpretations of the Belomorian Belt along the White Sea and part of the Kola Peninsula as Early Archaean are therefore untenable. Rifting and break-up between 2.5 and 2.0 Ga strongly affected and partly dispersed the Archaean Domain. In the north, the resultant basins were closed by collisional orogeny between ∼ 1.95 and 1.82 Ga ago. Semi-simultaneously, the Svecofennian orogeny created the continental crust in the central Baltic Shield. The Svecofennian Orogen continues into the area southeast of the Baltic Sea where a system of beltiform structures extends to the fault boundary with central Europe, featuring westward younging from ∼ 2.0 to 1.8 Ga. In the west, the Svecofennian Orogen is truncated by the ensialic Transscandinavian Igneous Belt of ∼ 1.8-1.65 Ga age. The final stage of extensive crust formation in the Baltic Shield occurred in western Scandinavia between ∼ 1.75 and 1.55 Ga ago. Many of the granites in the interior of Fennoscandia are better explained as intracratonic manifestations of this process than as late- or post-orogenic products of the orogenies that created the immediately surrounding crust. After its formation, the continental crust of Fennoscandia underwent major reworking during the Sveconorwegian-Grenvillian and Caledonian orogenies ∼ 1.2-0.9 and 0.5-0.4 Ga ago. A previously unknown realm of crustal stacking and high-pressure Sveconorwegian metamorphism has recently been discovered in southwestern Sweden. While the formation of new and the reworking of preexisting continental crust in Fennoscandia was a consequence predominantly of processes at destructive plate margins, the geodynamics cannot be interpreted in terms of a succession of more or less uniform cyclic orogenies of similar duration. Although there were Late Archaean and Palaeoproterozoic activity maxima in the formation of new continental crust in the Baltic Shield, the definition of individual orogenies requires a new look.

A chronological subdivision of the Transscandinavian Igneous Belt — three magmatic episodes?

(1992). A chronological subdivision of the Transscandinavian Igneous Belt — three magmatic episodes? Geologiska Foreningen i Stockholm Forhandlingar: Vol. 114, No. 4, pp. 459-461.

Geochemistry of Middle Proterozoic mafic and composite mafic-felsic dykes in southeastern Sweden

Abstract Mafic and composite mafic-felsic dykes of Middle Proterozoic age occur within the general area of the Transscandinavian Igneous Belt in southeastern Sweden. The composite dykes consist of felsic, quartz-feldspar porphyritic cores and mafic margins. The mafic rocks are altered with pyroxene typically replaced by amphibole and plagioclase by sericite or epidote. Two subalkaline groups can be distinguished among the mafic dykes. A tholeiitic group comprises NE-trending mafic dykes in the southern part of the area, some NW-trending dykes further north, and all the mafic margins of the composite dykes irrespective of dyke trend. The calc-alkaline dyke geochemistry is constrained mainly to NE-trending dykes in the northern part of the studied area. Within both groups there is enrichment of LIL-elements to contents above those of normal within-plate tholeiites. This was caused by metasomatism across the dyke contacts during greenschist-facies metamorphism and to some extent probably also by crustal contamination. The REE contents are those of normal continental tholeiites. Geotectonic discrimination diagrams indicate oceanic or arc affinity which is in conflict with the ensialic setting of the dykes. This is in accordance with studies of continental tholeiites elsewhere. Field relationships suggest that the composite dykes were formed by semi-synchronous intrusion of mafic and felsic magma. However, trace elements and the REE distribution indicate that the two magmas were not derived from a single parental melt. A possible explanation is that the mafic magmatism caused anatectic melting in the lower crust. The two magmas then intruded more or less simultaneously.