The core of Fennoscandia formed during Svecofennian 1.91–1.86
Ga orogenesis that included amalgamation of Archean micro-continents in the northeast and extensive arc accretionary growth toward the ...southwest. Post-Svecofennian crustal growth persisted southwestwards and later westwards, mainly by subduction-related magmatism that lasted another ∼330
m.y. By integrating mapping and geochemistry with new U–Pb geochronology from 31 igneous and 4 metasedimentary rocks in the Idefjorden terrane, we have defined major growth-related crustal units in this southwestern segment of Fennoscandia. Available constraints permit definition of nine lithotectonically distinct, semi-continuous stages of crustal growth between 1850 and 1522
Ma. The first stages included 1.85
Ga continental arc magmatism along the southwestern Svecofennian margin and recurring, 1.83–1.82
Ga growth further south. Voluminous continental arc magmatism at 1.81–1.77
Ga (TIB 1 rocks) and 1.72–1.66
Ga (TIB 2–3 rocks) resulted in large batholithic domains with a high proportion of rejuvenated material. The sixth stage (∼1.66
Ga) marks a transition in the south from continental arc magmatism to island arc magmatism that created the Horred supracrustal rocks. After accretion of the Horred sequence, a seventh magmatic stage (1.63–1.59
Ga) represents the return to a fully continental arc system. A short-lived eighth stage (∼1.59
Ga) returned to an oceanic arc system represented by an early package of volcano-sedimentary Stora Le-Marstrand formation rocks (SLM 1). Accreted SLM 1 rocks were intruded by stage nine rocks (1.59–1.52
Ga) that represent the last convergent arc magmatism in the Idefjorden terrane. A younger ca. 1.57–1.55
Ga package of volcano-sedimentary Stora Le-Marstrand formation rocks (SLM 2) is recognised in the Koster segment, demonstrating a return to an oceanic arc system in the north, coeval with continued continental arc magmatism in the south.
This near-continuous, oceanward-stepping growth of Fennoscandia over this 330
m.y. period represents a remarkably persistent convergent-margin system that permits evaluation of Proterozoic processes of continental growth and pre-Rodinian plate reconstructions. Several stages of arc magmatism were followed by intracratonic episodes of “A-type” magmatism up to 1500
km inboard of the evolving margin. These typically bimodal magmatic episodes also swept westward with time such that the oldest episode in the Idefjorden terrane (1502
Ma) provides a minimum age for this crustal segment to occupy an intracratonic position, inboard of an inferred
“proto-SW Norway”. The origin and final fate of such a pre-Sveconorwegian segment are speculative, but juxtaposition alongside the Idefjorden terrane during the 1.50–1.20
Ga interval is supported by independent evidence in SW Fennoscandia.
Between 1.8 and 1.0 Ga (Grenville-age), a series of subparallel accretionary orogens were added progressively to the southern edge of Laurentia. These belts now extend from Greenland/Labrador to ...southern California and are truncated at late Precambrian passive margins, suggesting that they once extended farther. We propose that Australia and Baltica contain their continuations. Together they comprise a long-lived orogenic system, >10 000 km long, that preserves a record of 800 million years of convergent margin tectonism. This tectonism culminated during Grenvillian continent–continent collisions in the assembly of the supercontinent Rodinia. Our reconstruction of the Australia–western US part of this assembly (AUSWUS) differs from the SWEAT reconstruction in that Australia is adjacent to the southwestern US rather than to northern Canada. The AUSWUS reconstruction is supported by a distinctive ‘fingerprint’ of geologic similarities between Australia and the southwestern US from 1.8 to 1.0 Ga, by numerous possible piercing points, and by an arguably better agreement between 1.45 and 1.0 Ga paleomagnetic poles between Australia and Laurentia. Geologic and paleomagnetic data suggest that separation between Laurentia and Australia took place ∼800–755 Ma and between Laurentia and Baltica ∼610 Ma. The proposed association of Australia, Laurentia, and Baltica, and the long-lived convergent margin they expose, provide a set of testable implications for the tectonic evolution of these cratons, and an important constraint for Proterozoic plate reconstructions.
The core of Fennoscandia formed during Svecofennian 1.91–1.86
Ga orogenesis that included amalgamation of Archean micro-continents in the northeast and extensive arc accretionary growth toward the ...southwest. Post-Svecofennian crustal growth persisted southwestwards and later westwards, mainly by subduction-related magmatism that lasted another ∼330
m.y. By integrating mapping and geochemistry with new U–Pb geochronology from 31 igneous and 4 metasedimentary rocks in the Idefjorden terrane, we have defined major growth-related crustal units in this southwestern segment of Fennoscandia. Available constraints permit definition of nine lithotectonically distinct, semi-continuous stages of crustal growth between 1850 and 1522
Ma. The first stages included 1.85
Ga continental arc magmatism along the southwestern Svecofennian margin and recurring, 1.83–1.82
Ga growth further south. Voluminous continental arc magmatism at 1.81–1.77
Ga (TIB 1 rocks) and 1.72–1.66
Ga (TIB 2–3 rocks) resulted in large batholithic domains with a high proportion of rejuvenated material. The sixth stage (∼1.66
Ga) marks a transition in the south from continental arc magmatism to island arc magmatism that created the Horred supracrustal rocks. After accretion of the Horred sequence, a seventh magmatic stage (1.63–1.59
Ga) represents the return to a fully continental arc system. A short-lived eighth stage (∼1.59
Ga) returned to an oceanic arc system represented by an early package of volcano-sedimentary Stora Le-Marstrand formation rocks (SLM 1). Accreted SLM 1 rocks were intruded by stage nine rocks (1.59–1.52
Ga) that represent the last convergent arc magmatism in the Idefjorden terrane. A younger ca. 1.57–1.55
Ga package of volcano-sedimentary Stora Le-Marstrand formation rocks (SLM 2) is recognised in the Koster segment, demonstrating a return to an oceanic arc system in the north, coeval with continued continental arc magmatism in the south.
This near-continuous, oceanward-stepping growth of Fennoscandia over this 330
m.y. period represents a remarkably persistent convergent-margin system that permits evaluation of Proterozoic processes of continental growth and pre-Rodinian plate reconstructions. Several stages of arc magmatism were followed by intracratonic episodes of “A-type” magmatism up to 1500
km inboard of the evolving margin. These typically bimodal magmatic episodes also swept westward with time such that the oldest episode in the Idefjorden terrane (1502
Ma) provides a minimum age for this crustal segment to occupy an intracratonic position, inboard of an inferred
“proto-SW Norway”. The origin and final fate of such a pre-Sveconorwegian segment are speculative, but juxtaposition alongside the Idefjorden terrane during the 1.50–1.20
Ga interval is supported by independent evidence in SW Fennoscandia.
The Ätran terrane of southwestern Sweden is a consequence of westward crustal growth away from an Archean core and records both Gothian (c. 1.75–1.55
Ga) and c. 1.0
Ga Sveconorwegian (Grenvillian) ...tectonism. In spite of widespread consensus on the first-order aspects of this westward growth model, there is a current debate as to whether the bulk of the deformation and migmatization in the Ätran terrane should be attributed to Gothian or Sveconorwegian events.
U–Pb zircon, titanite, monazite and rutile ages from gneiss samples and three tectonostratigraphic groups of dykes in the Varberg–Halmstad region constrain the timing of major tectonothermal events. A felsic orthogneiss yields a protolith age of 1664±7
Ma. Mafic dykes of a first dyke group (Steninge dykes) intruded after the first recognized deformation (D1) and were subsequently metamorphosed at 1654±9
Ma. The Steninge dykes constrain D1 to the 1664–1654
Ma interval and D2 at 1654±9
Ma, requiring the D1/D2 gneissosity to have formed during Gothian orogenesis. Steninge dykes and migmatitic D1/D2 structures in the host gneisses are cross-cut by less deformed dykes of a second dyke group, one regional granitic suite dated at 1426+9/−4
Ma and a pegmatite swarm at 1399+7/−6
Ma. The second group of dykes therefore confirms a pre-1.43
Ga age for the main D1/D2 gneiss-forming event(s) consistent with previous models of regional Gothian deformation in this terrane.
Group 2 magmatism also includes the Varberg Charnockite–Granite Association, from which a sample of charnockite is now dated at 1399+12/−10
Ma. In addition, metamorphism at 1438+12/−8
Ma resulted in new zircon growth in a mafic gneiss. Together, the Group 2 magmatic rocks and metamorphic zircons represent an inter-orogenic, 1.44–1.38
Ga thermo-magmatic episode in the Varberg–Halmstad region that has c. 1.4
Ga equivalents elsewhere in southern Sweden. Previous models of high heat flow and comprehensive metamorphism during a thermo-magmatic Hallandian event at c. 1.4
Ga is thus supported.
Sveconorwegian deformation and recrystallization (D3–D4) occurred before 946+6/−4
Ma, as constrained by a post-kinematic third group of dykes. U–Pb ages for monazite (948±9
Ma), titanite (935±7
Ma and 932±5
Ma) and rutile (878±9
Ma) reflect Sveconorwegian cooling after peak metamorphism. During this stage, U–Pb ages and closure temperatures suggest slow cooling rates of 5–11°C/m.y. from 948 to 932
Ma and 2.5–5°C/m.y. from 932 to 878
Ma, attributed to late-stage erosion and isostatic uplift. The lack of regional penetrative migmitization during the Sveconorwegian granulite conditions is consistent with metamorphism of previously dehydrated rocks (during Gothian and Hallandian times) during eastward Sveconorwegian thrusting of western segments over the Ätran terrane.
Intermittent, widespread and often bimodal magmatism characterized the Mesoproterozoic development in both western Baltica and eastern Laurentia. Interorogenic intrusions representing early episodes ...of post-Gothian and pre-Sveconorwegian/Grenvillian magmatism in SW Sweden, have yielded U–Pb zircon ages at 1502±2, 1503+3/-2 and 1457±6
Ma. Integration of these new ages with well-constrained U–Pb data for other 1.53–1.13
Ga interorogenic intrusions in western Baltica implies that crustal addition occurred in discrete magmatic episodes. The occurrence of temporally and petrologically similar rocks in the North Atlantic region supports models of a coherent Laurentia–Baltica supercontinent during the Mesoproterozoic. The prolonged interorogenic magmatism in Baltica
east of the Oslo Rift was typically manifested by mafic dyke swarms and gabbro–dolerite–granite complexes. This lithological association, absence of attendant regional deformation and lack of evidence of continental-margin processes, collectively imply an intracratonic position for this segment between 1.50 and 1.20
Ga. It further implies that some segment of Baltica
west of the Oslo Rift was attached prior to 1.50
Ga. These observations also imply that large-scale rifting, now dated at 1.46
Ga in western Baltica, did not lead to full-scale crustal separation and development of a new continental margin. During the same period, the evolution in eastern Laurentia included orogenic conditions at 1.51–1.45
Ga, and continental margin events between 1.45 and 1.19
Ga. This discrepancy in tectonic settings for eastern Laurentia and the area in Baltica east of the Oslo Rift indicates fundamental geodynamic differences along the southern margin of Mesoproterozoic Laurentia–Baltica or that the lesser-known segment west of the Oslo Rift may have been close enough to the proto-margin to experience an evolution more akin to that of eastern Laurentia.
During the late Palaeoproterozoic, western Baltica was characterised by convergent-margin tectonics that resulted in the formation of N-S trending crustal units: the Transscandinavian Igneous Belt ...(TIB) and the westward younging Gothian growth zones. Together, these occupy a 200-300 km wide belt between Svecofennian (c. 1.9 Ga) crust in the east and the Skagerrak Sea/Oslo Rift in the west. Spatial and temporal constraints for 83 U-Pb dated rocks previously included in the TIB allow recognition of age groups at 1850, 1813-1766, and 1723-1657 Ma. However, the c. 1.85 Ga rocks should preferably not be included in the TIB because they form a tectonically distinct crustal unit (the Askersund suite). The 1.81-1.77 Ga age group is the major contributor to the TIB and corresponds to TIB 1. Westward younging of the TIB magmatism is demonstrated by the 1.72-1.66 Ga age group. Integration of the petrogenetic and spatial aspects for these rocks suggests an older magmatic phase at c. 1.72-1.69 Ga (TIB 2) and a final phase at c. 1.68-1.66 Ga (TIB 3). Recent structural, geochemical and isotopic data provide spatial and temporal constraints for the Svecofennian and Gothian developments, and for a bridging, c. 1.81-1.75 Ga tectonic stage we term the Smålandian. This tectonic stage includes the N-S trending, 1.81-1.77 Ga TIB 1 magmatism and manifests a marked shift by 1.81 Ga from previous north-directed convergence during the Svecofennian arc-accretionary event to east-directed convergence that also characterised the following Gothian evolution (c. 1.75-1.55 Ga). The early, c. 1.75-1.70 Ga Gothian evolution is still enigmatic but the subsequent development was due to westward stepping orogenesis, resulting in distinct c. 1.69-1.65, 1.62-1.58, and 1.56-1.55 Ga igneous belts between the TIB rocks and the Oslo Rift. The prolonged period of semi-continuous crustal growth during the Svecofennian-Smålandian-Gothian course appears to have ceased after the postulated docking of pre-1.60 Ga crust ("proto-SW Norway") with Gothian crustal units of Baltica at c. 1.55 Ga. At the very least, the subsequent 1.50-1.20 Ga evolution lacks evidence for continental-margin processes east of the Oslo Rift.
Intermittent crustal growth characterised late Paleoproterozoic development in western Baltica during Gothian orogenesis, and in eastern Laurentia during Labradorian orogenesis. Both regions are ...inferred to have belonged to the same margin of a supercontinent, but they do not show identical tectonic histories. Long-lived convergent margin activity associated with successive, oceanward migrating stages of sub-duction characterized western Baltica during the late Paleoproterozoic, in contrast to the development of a pre-Labradorian, ca. 1.71 Ga sedimentary depocentre close to the margin of pre-Labradorian Laurentia that gave way to Labradorian 1.68-1.65 Ga calc-alkaline magmatism associated with subduction away from cratonic Laurentia. Continued Gothian, ca. 1.62-1.58 Ga continental-margin calc-alkaline magmatism and arc accretion has no recognized counterpart in eastern Laurentia, where collision of the outboard microcontinents/arcs resulted in voluminous granitoid magmatism caused by crustal thickening. Subduction either ceased at 1.65 Ga or northward subduction was initiated much farther south. The caveat to all interpretations is that some of the apparent differences may reflect inadequate geochronological databases of western Baltica and southeasternmost Laurentia.
Åhäll, K.-I. & Gower, C.F., 1997: The Gothian and Labradorian orogens: variations in accretionary tectonism along a late Paleoproterozoic Laurentia-Baltica margin. GFF, Vol. 119 (Pt. 2, June), pp. 181-191. Stockholm. ISSN 1103-5897.
Following the Svecofennian arc accretionary growth and extensive granitoid magmatism in the Transscandinavian Igneous Belt (TIB), new crustal growth occurred west of the TIB to form the Gothian ...orogen. An early stage is manifested by 1.69–1.65
Ga subduction-related magmatism in the Ätran Terrane. A second stage, forming an 140
km wide segment east of the Permian Oslo Rift, is recorded by three 1.66–1.59
Ga metamorphosed volcano-sedimentary units, exposed in the Horred, Åmål and Stora Le-Marstrand formations in the Idefjorden Terrane. The 1.66
Ga Horred Formation is dominated by felsic volcanics and has geochemical signatures indicative of formation in an island arc setting. In contrast, the lithologically similar volcanic sequences in the 1.61
Ga Åmål Formation have geochemical signatures consistent with a continental-margin setting. The 1.60–1.59
Ga Stora Le-Marstrand Formation is dominated by greywacke-type metasediments with subordinate metabasalts. These volcanics have markedly primitive trace element signatures and depleted Nd isotopic compositions, all consistent with their derivation in an oceanic island arc setting. The sediments document two provenances: a distal continental source and one with Nd isotopic compositions similar to the SLM volcanism. Many of the metasediments in the Stora Le-Marstrand formation have chemical signatures consistent with derivation from continental crust, suggesting that this volcanic arc developed in the vicinity of a continental massif, possibly in a setting similar to the Philippine Sea. Accretion of the Horred and Stora Le-Marstrand arc systems occurred prior to 1.61 and 1.59
Ga, respectively, and was followed by voluminous, ca 1.59
Ga calc-alkaline magmatism.
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.
In the Varberg region of the Southwest Scandinavian Domain in SW Sweden, the megacrystic Torpa granite forms a sheet-like, partly charnockitic intrusion in intensely migmatised gneisses. The granite ...has been interpreted as part of an igneous suite, termed the Varberg Charnockite-Granite Association. The gneisses are made up of ca. 1.75-1.55 Ga old crust that has been subjected to high-grade Sveconorwegian-Grenvillian metamorphism. A U-Pb zircon age of 1380±6 Ma for a charnockitic portion of the Torpa granite is interpreted as the emplacement age and is also suggested to represent the age for the entire Varberg Charnockite-Granite Association. Together, this age and the field relationships demonstrate that the regional gneiss-forming event(s) in the Varberg region is older than 1.38 Ga, and that the pervasive deformation of the Mylonite Zone, a major N-S trending shear zone and tectonic boundary in the region, is younger than 1.38 Ga and therefore probably Sveconorwegian in age (1.1-0.9 Ga). The similarity of the Torpa granite age to other megacrystic granites in SW Sweden confirms the existence of extensive anorogenic, bimodal magmatism in SW Sweden at 1.38-1.36 Ga. This magmatism is coeval with abundant anorogenic granite magmatism in SE Sweden and wide-spread volcanic-plutonic activity elsewhere in Laurentia-Baltica. The charnockitic assemblages of the 1.38 Ga Torpa granite are interpreted to be primary in origin, and Sveconorwegian metamorphism is not evident in the U-Pb systematics of the analysed zircon fractions. In contrast, granulite-facies rocks from several localities in the Varberg region, including rocks belonging to the Varberg Charnockite-Granite Association, have yielded Sveconorwegian isochron ages. Consequently, models based on two discrete events of charnockite-granulite formation must be considered for the Proterozoic evolution of SW Sweden.
Åhall, K.-I., Samuelsson, L. & Persson, P.-O., 1997: Geochronology and structural setting of the 1.38 Ga Torpa granite; implications for charnockite formation in SW Sweden. GFF, Vol. 119 (Pt. 1, March), pp. 37-43. Stockholm. ISSN 1103-5897.
