Göta Älv shear zone

The GÄSZ, with a northerly to north-northeasterly trend, mainly follows the river valley with the same name. The zone extends from Kungsbacka in the south to Trollhättan and Vänersborg near to Vänern in the north (Fig. 16.2). Several splays branch off from the main structure SW of Vänern, continuing northwards to define the anastomosing network of deformation zones in the western part of the Idefjorden terrane. One of these splays trends in a northwesterly direction towards Uddevalla, providing a link to the Lerdal shear zone (Fig. 16.2 and see below). North of Trollhättan, several splays continue northwards (Park et al. 1991), connecting up with the deformation zone with a north–south strike passing through Årjäng (see below) and also providing a link to the ‘Dalsland boundary thrust’ with northwesterly strike (Fig. 16.2).

The GÄSZ constitutes a belt of more or less strongly deformed rocks, several kilometres in width, formed under variable amphibolite and greenschist facies (Samuelsson 1982, 1985). Banded mylonitic (striped) gneisses and augen-bearing rocks, characteristic of amphibolite facies ductile strain, form a prominent component in the exposed parts (Fig. 16.8). The ductile high-strain zones in this belt are also commonly reactivated in the brittle regime, resulting in a localization along topographical depressions.

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Fig. 16.8.

Ductile structures formed under amphibolite facies along the GÄSZ. (a) Striped gneiss, Älvängen area, c. 25 km north of Göteborg. A major part of the exposed rocks in the Göta Älv river valley are high-temperature mylonites occurring as banded and augen gneisses. Photographer: L. Lundqvist. (b) Tectonically banded section composed of deformed and metamorphosed 1.3 Ga granite (Kungsbacka suite) in the upper part, and c. 1.57 Ga granodioritic gneiss (Hisingen suite) with flattened and rotated mafic dykes in the lower part of the section. Kungsbackafjorden, c. 30 km south of Göteborg. Photographer: T. Eliasson.

Between Kungsbacka and north of Göteborg (Fig. 16.2), the zone generally dips moderately to the west (30–50°); associated stretching lineations show a mean plunge of 30° obliquely to the NW (Park et al. 1991), suggesting combined dip- and strike-slip displacement. In a similar manner to the MZ south of Vänern, kinematic indicators, mainly shear bands and augen tails, yield both reverse top-to-the-SE and normal top-to-the-NW components of movement, the normal movement apparently occurring after the reverse (Park et al. 1991).

Dextral rotation of an intense magnetic banding structure into the shear zone in the area between Älvängen and Göteborg (Fig. 16.5a) is consistent with the normal top-to-the-NW displacement. Furthermore, a marked metamorphic gradient across the main branch of the GÄSZ in the Trollhättan area (Fig. 16.2), from high-pressure granulite facies and migmatite to the east of the zone to greenschist or lower amphibolite facies to the west (Söderlund et al. 2008 and see below), can be explained by such movement along the shear zone. The main branch of the GÄSZ continues northeastwards beneath the southwesternmost part of Vänern inside the Median segment and, probably further to the NE, close to the western shoreline of the lake SE of Mellerud (Fig. 16.2).

Lerdal shear zone

The Lerdal shear zone is a ductile deformation zone striking north–south and extending from Uddevalla northwards towards the border to Norway; it is located partly along the boundary between the 1.6 Ga paragneisses in the SLM formation and the orthogneisses to the east in the Hisingen suite (Fig. 16.2). The shear zone merges together with the ‘Dalsland boundary thrust’ (Ørje shear zone) close to the border between Sweden and Norway (Fig. 16.2).

The zone is generally 1–3 km in width and dips c. 50–60°, or even more steeply, to the west; stretching lineations plunge mainly to the NW oblique to its strike (Park et al. 1991). Kinematic indicators combined with lineament orientation data suggest reverse top-to-the-SE and sinistral components of movement (Park et al. 1991). In its southern part, the zone narrows to c. 0.5 km, changes in orientation to gently east-dipping and displays normal movement (Park et al. 1991). South of Uddevalla, it connects to the GÄSZ along a zone with a northwesterly strike (see also above), along which a Sveconorwegian syn-orogenic granite emplaced at c. 1.03 Ga is exposed (see Fig. 16.2 and the presentation below).

Deformation zones in the Årjäng area

SE of Årjäng (Järnsjön area: Fig. 16.2), in the northern part of the Idefjorden terrane, a retrogressive shear zone that formed under greenschist facies splays in a northeasterly direction away from the main north–south zone; it dips gently to the SE and contains a stretching lineation plunging gently to the south (Wahlgren 1988). The zone constitutes a marked break in rock type (Fig. 16.2), in the degree of metamorphic reworking and in structural style. It also coincides with a change in the pattern on the magnetic anomaly map (Fig. 16.5a).

The lithological unit to the north of the shear zone consists of pervasively deformed 1.3 Ga gneissic granite (Kungsbacka suite: see the presentation below) that is partly migmatitic and comprises an LS-tectonite. This orthogneiss lies structurally beneath strongly lineated, gneissic 1.6–1.5 Ga granite (Hisingen suite) south of the zone, which was unaffected by migmatization. Calcareous phyllite occurs as tectonic lenses in spatial association with the protomylonitic to mylonitic rocks in the shear zone beneath the 1.6–1.5 Ga gneissic granite. The phyllite has been interpreted as representing a calcareous sedimentary protolith deposited after 1.3 Ga (Dalsland group: see the presentation below). Based solely on these tectonostratigraphic relationships, the shear zone has been interpreted as a result of thrusting with northerly vergence (Wahlgren 1988).

To the SW, the thrust zone links up with a deformation zone belonging to the anastomosing north–south network (Fig. 16.2). The structural relationships to the NE are not fully resolved. However, inspection of the magnetic anomaly map (Fig. 16.5a) suggests that a deformation zone with a similar north–south trend bounds the thrust along its northeastern margin.

Broader tectonic implications

The close spatial relationship between shear zones dipping moderately or steeply westwards with oblique reverse-sinistral displacement, a zone dipping gently eastwards showing normal movement and a zone dipping southeastwards with a significant thrust component indicates an overall sinistral transpressive tectonic regime for the development of the anastomosing shear-zone network (see also Park et al. 1991). It is proposed that the emplacement of the c. 1.03 Ga granite in the Uddevalla area took place along an extensional releasing bend with a northwesterly trend between the Lerdal and Göta Älv shear zones in this sinistral transpressive system. In a complementary manner, thrusting with northwards vergence south of Årjäng took place in a contractional restraining bend.

The kinematic data and major metamorphic break along and across the GÄSZ indicate later top-to-the-west movement along at least this deformation zone. The ductile zones were subsequently reactivated in the brittle regime. The tectonic evolution for the shear-zone network inside the Idefjorden terrane resembles that observed along the MZ along the eastern boundary of this lithotectonic unit.

South of Lake Vänern

High-pressure granulite facies metamorphism affected 1.3 Ga mafic dykes immediately east of the GÄSZ close to Trollhättan (Söderlund et al. 2008; Fig. 16.9). These metamorphosed dykes are surrounded by pervasively deformed and garnet-bearing migmatitic orthogneisses within which, to the south, a 1.6 Ga tonalitic protolith has been identified (Åhäll & Connelly 2008). A thermobarometric study applied to garnet and omphacitic clinopyroxene coronas in the mafic dykes indicates minimum estimates of peak metamorphism of c. 15 kbar and c. 740°C (Söderlund et al. 2008). The breakdown of baddeleyite to zircon, inferred to be consanguineous with the high-P granulite event, took place at c. 1.05 Ga; exhumation followed with metamorphism at c. 10 kbar and c. 700°C at around 1.03 Ga (Söderlund et al. 2008; Table 16.1; Fig. 16.4a). Crustal thickening apparently took place during the Agder tectonothermal phase of the Sveconorwegian orogeny (Bingen et al. 2008b).

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Fig. 16.9.

Detailed view of Figure 16.7a showing the form-line pattern, variation in metamorphic grade and some specific geological features in the Idefjorden terrane referred to in the text.

South of Trollhättan, the entire Median segment consists of orthogneisses that are commonly migmatitic. Between Göteborg and Lerum, characteristic augen-bearing gneissic bands (Samuelsson 1978; Ahlin et al. 2006) are folded on a regional scale around gently SSE-plunging fold axes (Figs 16.5a & 16.9). These folds are generally isoclinal, rotate an older complex gneissic structure and bend westwards in a dextral manner into the GÄSZ (Figs 16.5a & 16.9). Thermobarometric calculations from a garnet amphibolite located east of Göteborg yielded a pressure estimate of 9–10.5 kbar at temperatures of 700–750°C (Austin Hegardt & Cornell 2010). Metamorphic zircon rims in a veined granitic gneiss in the same area formed at c. 1.02 Ga (Austin Hegardt & Cornell 2010; see Table 16.1; Fig. 16.4a).

Geochronological data from metamorphic embayments and rims on inferred xenocrysts in veins injected into a migmatized granodioritic orthogneiss north of Lerum (Fig. 16.2) also provide evidence for a Sveconorwegian event during the Agder tectonothermal phase (Scherstén et al. 2004). Crystallization of the veins apparently occurred later at c. 0.92 Ga (Table 16.1) and was followed by folding, suggesting ductile strain coeval with late Sveconorwegian magmatism (Scherstén et al. 2004). No traces of a Gothian metamorphic event were identified.

West and north of Lake Vänern

The strip of Median segment close to Trollhättan, sandwiched between the GÄSZ and the boundary to the Western segment further west, is affected by metamorphism no higher than greenschist or lower amphibolite facies (Fig. 16.9). The spatial extent of rocks with this lower grade of metamorphism, compared to that east of the GÄSZ, widens northwards (Fig. 16.9). Such low-grade rocks are particularly prominent in the relatively well-preserved supracrustal rocks belonging to the 1.6 Ga Åmål and <1.3 Ga Dalsland groups (see Fig. 16.2 and the presentation below) north of Mellerud on the western side of Vänern. A metamorphic mineral assemblage containing plagioclase, epidote, chlorite and hornblende in mafic volcanic rocks is a characteristic component. The lower grade of metamorphism is also apparent in a gently to moderately dipping tectonic stack of 1.6 and 1.3 Ga magmatic units between Säffle and Arvika north of the lake (Glaskogen nappe complex in Gorbatschev 1988; Lindh et al. 1998; see Fig. 16.9). Amphibolite facies metamorphism without anatexis affected these rocks.

East of Säffle, at tectonostratigraphic levels beneath the Glaskogen nappe complex, and north of Arvika, the metamorphic grade increases as the MZ along the eastern boundary of the Idefjorden terrane is approached (Fig. 16.9). Migmatitic orthogneisses with protolith ages of around 1.6 and 1.3 Ga prevail (Figs 16.2 & 16.10a); mafic bands in the orthogneisses recrystallized to garnet-bearing amphibolite. It is apparent that deeper crustal levels in the Sveconorwegian orogenic evolution are exposed not only between the main branch of the GÄSZ and the MZ south of Vänern, but also in the eastern and northern parts of the Median segment close to the MZ north of the lake.

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Fig. 16.10.

Sveconorwegian amphibolite facies (including migmatitic) structures in the Median and Western segments, and relicts of older Gothian migmatitic structures in the latter. (a) Gneissic (S_SN1) and garnet-bearing 1.3 Ga granite (Segmon granite, Kungsbacka suite). Median segment, NW of Grums. Photographer: L. Lundqvist. (b) Folds with westwards vergence along axes with a northwesterly trend (F_SN2). The folds deformed a migmatitic structure and amphibolite boudin in migmatitic granodioritic gneiss (Göteborg suite). Median segment, Grums area. Photographer: K. Persson Nilsson. (c) Amphibolitic, c. 1.46 Ga mafic dyke cross-cutting migmatitic gneissic structure (S_G1) and later folds (F_G2) in paragneiss (SLM formation). Western segment, a small island SW of Orust. Photographer: T. Eliasson. (d) Close-up of the contact relationships in (c) showing some ductile shearing along the contact. Photographer: T. Eliasson. (e) A c. 1.59 Ga tonalite (Hisingen suite) with an intrusive relationship to migmatitic gneissic structure (S_G1) in paragneiss (SLM formation), both intruded by pegmatite veins. Western segment, Rönnäng on Tjörn. Photographer: T. Eliasson. (f) Strongly deformed amphibolite belonging to the c. 1.46 Ga mafic dyke suite inside a Sveconorwegian ductile deformation zone with a north-northwesterly trend. Western segment, northeastern part of the Koster archipelago. Photographer: C.-H. Wahlgren.

The early stage of the Sveconorwegian tectonothermal evolution, most apparent in the area south of Bengtsfors in both the Median and Western segments, is characterized by eastwards-directed thrusting that disturbed, for example, the stratigraphic relationships in the <1.3 Ga Dalsland group and placed 1.6–1.5 Ga orthogneisses on top of the Dalsland group (e.g. Törnebohm 1883; Park et al. 1991). Mylonites containing a gently westwards-plunging mineral stretching lineation occur along the tectonic contacts. Eastwards-directed thrusting was also proposed for the Glaskogen nappe complex north of Vänern to the east (Zeck & Malling 1976; Gorbatschev 1988). However, the presence of orthogneisses unaffected by anatexis lying structurally above migmatitic gneisses in this complex has not been explained. There is clearly a need for a detailed kinematic analysis of the internal and bounding shear zones, combined with geochronological studies, in order to unravel the tectonic evolution of this ‘nappe complex’.

Bearing in mind the timing of emplacement of the orthogneisses north of Vänern, it is apparent that the gneissic foliation in these rocks, referred to here as S_SN1, is related entirely (1.3 Ga protoliths) or at least partly (1.6 Ga protoliths) to the Sveconorwegian orogeny (Fig. 16.10a). However, the characteristic structural feature in this area is west-vergent folding (F_SN2) of this foliation around subhorizontal to gently north-plunging fold axes, suggesting westwards-directed tectonic transport after development of the pervasive gneissic structure (Fig. 16.10b). The polyphase ductile deformation inside the Median segment was completed by large-scale open folding with a north–south axial surface trend (F_SN3). The boundary between the rocks affected by metamorphism without anatexis and the underlying migmatitic gneisses to the east and north, as well as the mylonites in the tectonic stack in the Glaskogen nappe complex, are folded by a major open synform formed during the F_SN3 folding phase (Fig. 16.9).

Gothian polyphase ductile deformation and metamorphism

The emplacement of a c. 1.51–1.49 Ga bimodal plutonic suite (Stigfjorden suite: Fig. 16.3) and c. 1.46 Ga mafic dykes (Koster and Orust dykes: Fig. 16.3) took place after gneissic fabric development and migmatization in the SLM formation (Fig. 16.10c, d), thereby providing an upper age limit for the Gothian orogeny. The minimum age of detrital zircon grains in these siliciclastic metasedimentary rocks provides the lower age limit. Field and geochronological studies, summarized below, also provide constraints on its polyphase character.

A gneissic foliation, migmatization and intrafolial isoclinal folding of quartz–feldspar veins are preserved in xenoliths of siliciclastic metasedimentary rocks (SLM formation) inside a c. 1.59 Ga tonalite (Hisingen suite) near to Rönnäng on the island of Tjörn (Connelly & Åhäll 1996; see Figs 16.2 & 16.10e; Table 16.2). They constitute the oldest ductile structures and metamorphic event in the Western segment, and are referred to here as D_G1 and M_G1, respectively. In combination with provenance ages from detrital zircon grains, it is apparent that siliciclastic sedimentation, rapid burial of these rocks, early Gothian ductile deformation under high-grade metamorphic conditions and magmatic activity all occurred around 1.59 Ga in this part of the Idefjorden terrane (Åhäll & Connelly 2008).

Later Gothian deformation and metamorphism (D_G2 and M_G2) during the time span c. 1.56–1.55 Ga are regionally more significant and somewhat better constrained by combined field and geochronological studies (Park et al. 1987; Åhäll et al. 1990; Connelly & Åhäll 1996; Åhäll & Connelly 2008; Austin Hegardt et al. 2010; see Table 16.2; Fig. 16.4). This event gave rise to tight folding of the older structures with transposition of the S_G1 foliation into parallelism with S_G2 in the siliciclastic metasedimentary rocks of the SLM formation (Park et al. 1987) and a gneissic fabric development in the Hisingen suite. It corresponds to the pre-Sveconorwegian D₂ deformational phase identified in Park et al. (1987) and Austin Hegardt et al. (2010).

In the coastal area NW of Göteborg (Fig. 16.2), Park et al. (1991) described D_G2 as west-verging asymmetrical folding with an oblique top-to-the-SW sense of movement. Late D_G2 shear zones, as well as migmatitic M_G2 veins and neosomes, are sub-parallel to the axial surfaces of these folds. A close chronological connection between D_G2, M_G2 metamorphism with anatexis, and both mafic and felsic magmatism (Hisingen suite) is apparent (Table 16.2; Fig. 16.4). Using thermobarometry calculations from garnet–amphibole–plagioclase, garnet–amphibole and garnet–muscovite–biotite–plagioclase reactions on samples from one of the small islands south of Tjörn (Fig. 16.2), M_G2 metamorphic conditions at c. 8 kbar and 640°C have tentatively been inferred (Austin Hegardt et al. 2010).

Sveconorwegian polyphase ductile deformation and metamorphism

On a regional scale, the D_G1 and D_G2 Gothian structures constitute parallel structural form lines from which the post-D_G2 interference structures, inferred to have formed during the Sveconorwegian orogeny, may be studied (Park et al. 1987, 1991; Austin Hegardt et al. 2010). Two phases of Sveconorwegian ductile deformation, referred to here as D_SN1 and D_SN2 and associated with upper and lower amphibolite facies metamorphism, respectively, correspond to the D₃ and D₄ deformational events described in Park et al. (1987, 1991) and Austin Hegardt et al. (2010). Some more localized, post-D_SN2 ductile deformation is also described in these publications.

The D_SN1 event on one of the small islands south of Tjörn (Fig. 16.2) is characterized by isoclinal folding overturned to the east, containing a moderately dipping axial surface foliation with a general north–south trend (Austin Hegardt et al. 2010). Granitic veins are present along this foliation and are generally coarser-grained compared with the older M_G2 migmatite veins. Transposition into this foliation is common and garnet occurs locally in related shear zones. Metamorphic conditions at 11.5 kbar and 750°C during D_SN1 have been inferred using thermobarometry calculations from garnet–amphibole–plagioclase and garnet–amphibole reactions in samples of post-D_G2 amphibolite dykes hosted by paragneiss (SLM formation); Sm–Nd (garnet) geochronology yielded a c. 1.03 Ga age for this event (Austin Hegardt et al. 2010). Emplacement of post-D_SN1 pegmatite at around 994 Ma provides an upper age limit for the D_SN1 event in this area (Austin Hegardt et al. 2010).

Further north, on the islands of Tjörn and Orust (Park et al. 1987, 1991; Fig. 16.2), amphibolite facies ductile strain along shear zones with varied orientation is the first deformational event that affected the post-Gothian 1.51–1.49 Ga bimodal plutonic rocks (Stigfjorden suite). The D_SN1 event in this area also refolded earlier Gothian structures and resulted in the development of an axial surface foliation (S_SN1) in the paragneisses of the SLM formation.

A second phase of Sveconorwegian deformation (D_SN2) is a prominent structural feature in the Western segment. This phase is characterized by folds trending NE–SW with subhorizontal axial surfaces and a pronounced lineation development. In the area of most intense D_SN2 deformation on a small island south of Tjörn (Fig. 16.2), chevron-style folding and a crenulation cleavage are particularly well developed in the mica-rich layers in the paragneisses belonging to the SLM formation (Austin Hegardt et al. 2010). Based on ⁴⁰Ar–³⁹Ar (muscovite) geochronology, these authors suggested that the D_SN2 structures developed at c. 980 Ma. Folds formed during the D_SN2 phase also deformed the D_SN1 shear zones further north on the Tjörn and Orust islands (Park et al. 1987, 1991).

In the Koster archipelago, close to the border with Norway (Fig. 16.2), a swarm of post-Gothian c. 1.46 Ga mafic dykes shows a prominent increase in deformation and metamorphic overprint towards and into a ductile shear zone in the northeastern part of the archipelago (Hageskov 1985). This zone trends in a north-northwesterly direction and shows a sinistral strike-slip component of movement. The mafic dykes were converted to strongly deformed amphibolite sheets along the shear zone (Fig. 16.10f). Hageskov (1985) assigned movement along this zone to the D_SN2 deformation phase during the Sveconorwegian orogeny.

Supracrustal rocks deposited at c. 1.66 Ga (Horred formation)

Fine-grained gneissic rocks, showing a banding that is both compositional and textural in character (Fig. 16.11a), occur in the southeastern part of the Idefjorden terrane, close to and in the hanging wall to its eastern boundary zone (MZ: Fig. 16.2). These rocks, referred to as the Horred formation, have been interpreted to represent a predominantly volcanic protolith deposited around 1.66 Ga, with thin, quartz-rich, clastic sedimentary intercalations (Åhäll et al. 1995; Åhäll & Connelly 2008; Table 16.1). They form the oldest rock unit exposed in the Idefjorden terrane (Fig. 16.3). The volcanic rocks in this unit are locally porphyritic, vary in composition from basalt to rhyolite, and show an enrichment in the large ion lithophile elements (LILEs), a Nb trough and a Zr/Y ratio <3 (Brewer et al. 1998). Based on the geochemical characteristics, these authors suggested an island arc volcanic setting for the Horred formation.

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Fig. 16.11.

Character of the c. 1.66 Ga (Horred formation) and c. 1.64–1.61 Ga (Åmål group including 1.64 Ga Kappebo formation) supracrustal rocks in the Median segment. (a) Banded andesite (Horred formation). Photographer: L. Lundqvist. (b) Ignimbritic rhyolite (Åmål group) in the Tösse area. Photographer: P. Nysten. (c) Cross-bedding in arkosic sandstone (upper part of Åmål group), south of Tösse. Photographer: P. Nysten. (d) Conglomerate with granite clasts in a volcanosedimentary groundmass (Kappebo formation), west of Mellerud. Photographer: U. Bergström.

Supracrustal rocks deposited at c. 1.63–1.59 Ga (Åmål group)

Supracrustal rocks deposited in the time frame c. 1.63–1.59 Ga, including volcanic rocks erupted at c. 1.63–1.61 Ga (Table 16.1), form a major lithological component in the Median segment (Figs 16.2 & 16.3). Well-preserved varieties showing primary volcanic textures, facing directions and constraints on volcanic facies and lithostratigraphy, referred to as the Åmål group (e.g. Gorbatschev 1977; Lundqvist & Skiöld 1993), occur inside a large supracrustal inlier along the western shore of Vänern in the Tösse area, south of Åmål (Fig. 16.2). In this type area, the degree of ductile strain is low and metamorphism occurred under greenschist to lower amphibolite facies. The basal part of the succession includes coherent rhyolitic rocks and ignimbrites (Fig. 16.11b), while plagioclase-phyric dacite and mafic volcanic rocks occur at stratigraphically higher levels. The mafic volcanic rocks are intercalated with strata containing reworked volcanic debris. Cross-bedded arkosic sandstone (Fig. 16.11c) and conglomerate form the youngest part of the succession (Fig. 16.3).

Detrital zircon populations in the arkosic sandstone are dominated by grains with provenance ages in the time span c. 1.62–1.59 Ga (Åhäll & Connelly 2008). Although hindered by a small sample size, local derivation of debris from volcanic material in the Åmål group or from granitoids in the coeval plutonic rocks (Göteborg intrusive suite; see below) is indicated. Magmatic activity, rapid uplift and erosion of possibly both volcanic and plutonic components, and deposition of siliciclastic rocks all occurred close in time to each other during formation of the Åmål group; a strongly active tectonic setting during deposition is apparent (e.g. Åhäll & Connelly 2008). Detrital zircon grains with provenance ages of around 2.0 and 1.8 Ga, reminiscent of crustal components in the cratonic foreland east of the Sveconorwegian orogen, are also present in the arkosic sandstone; ages around 1.7 Ga typical of the Eastern Segment of the Sveconorwegian orogen (see Chapter 15, Stephens & Wahlgren 2020a) are absent (Åhäll & Connelly 2008).

Separate inliers of sandstone and conglomerate, metamorphosed under greenschist facies and containing volcanic debris (Kappebo formation: e.g. Gorbatschev 1977), are exposed west and SW of Mellerud, immediately east of a ductile shear zone belonging to the network of zones dividing the Idefjorden terrane into different crustal segments (Fig. 16.2). The conglomerates also contain granite clasts (Fig. 16.11d), including an augen-bearing variety which is exposed in close vicinity, was emplaced around 1.63 Ga and belongs to the Göteborg intrusive suite (see below and Table 16.1). In the stratigraphically upper part of the Kappebo formation, volcanic rocks dominate and a U–Pb (zircon) age determination of a coherent rhyolitic lava also indicates eruption at around 1.63 Ga (Table 16.1). For this reason, this formation has been interpreted to form the lowermost stratigraphic part of the Åmål group (Åhäll & Connelly 2008). A fine-grained gneiss of uncertain volcanogenic (Scherstén et al. 2000; Petersson et al. 2015) or sedimentary (Samuelsson 1978) origin in an inlier east of Göteborg (Fig. 16.2) contains zircons yielding magmatic ages of c. 1.64 Ga. These highly deformed and metamorphosed rocks may be an equivalent unit to the Kappebo formation.

The lithogeochemical dataset used here includes mafic and felsic volcanic rocks metamorphosed under lower amphibolite facies and with volcanic textures preserved. There is virtually no evidence for alteration involving the elements Na, K and Mg in the volcanic rocks (Fig. 16.12a). Compositions in the sub-alkaline field ranging from rhyolite to dacite and basaltic andesite dominate (Fig. 16.12b, c). REE data show discrete light REE (LREE) enrichment and negative Eu anomalies, except for the samples with the lowest SiO₂ content in the dataset (Fig. 16.12d). Spider plots with data normalized to primordial mantle (Fig. 16.12e) have a distinct spiked pattern, with a negative Nb–Ta trough for all compositions, and negative Sr, P and Ti anomalies in the samples where SiO₂ exceeds 57%, consistent with plagioclase, apatite and magnetite fractionation processes. Some mafic samples also show a Th–U trough, a feature also noted for the Göteborg and Hisingen suites (see below). Previous work indicated a high-K calc-alkaline composition and a Zr/Y ratio >3, inferred to represent deposition in an active continental magmatic arc setting (Brewer et al. 1998). Although care is needed with the interpretation of tectonic discriminant diagrams, the data used here, plotted on examples of such diagrams, are consistent with this interpretation (Fig. 16.12f, g).

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Fig. 16.12.

Geochemical characteristics of the c. 1.63–1.61 Ga metamorphosed volcanic rocks belonging to the Åmål group. Data from the type area south of Åmål extracted from the lithogeochemistry database at the Geological Survey of Sweden. (a) Modified alteration index after Ishikawa et al. (1976) and Allen et al. (2003). Samples in the field between the two horizontal lines do not show an alteration that affected the elements K, Na and Mg. (b) Geochemical classification using the (K₂O + Na₂O)–SiO₂ diagram after Le Bas et al. (1986). (c) Geochemical classification according to Winchester & Floyd (1977). (d) Chondrite-normalized REE diagram. Normalization after Boynton (1984). (e) Primitive mantle-normalized multi-element diagram. Normalization after Wood et al. (1979). (f) Zr–Ti/100–3 × Y diagram after Pearce & Cann (1973). CAB, calc-alkaline basalt; IAT, island arc tholeiite; MORB, mid-ocean ridge basalt; WPB, within-plate basalt. (g) Nb/La–La/Yb diagram after Hollocher et al. (2012).

Intrusive rocks emplaced at c. 1.63–1.59 Ga (Göteborg suite)

Large parts of the Median segment are composed of gneissic (Fig. 16.13a) or poorly preserved migmatitic (Fig. 16.13b) intrusive rocks belonging to the batholithic Göteborg suite (Fig. 16.2). Granite sensu stricto commonly occurs as late-stage stocks or dykes. This magmatic suite was emplaced at c. 1.63–1.59 Ga, coevally with the deposition of the supracrustal rocks in the Åmål group (Fig. 16.3; Table 16.1). In the well-preserved area south of Åmål, some plutons form sub-volcanic components to the volcanic rocks in the Åmål group.

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Fig. 16.13.

Character of gneissic plutonic rocks belonging to the c. 1.63–1.59 Ga (Göteborg) and c. 1.59–1.52 Ga (Hisingen) intrusive suites. (a) Gneissic granodiorite–quartz monzonite with igneous texture preserved, located c. 10 km SSW of Mellerud (Göteborg suite, Median segment). Photographer: U. Bergström. (b) Migmatitic granodiorite in the Mölndal area, south of Göteborg (Göteborg suite, Median segment). Photographer: L. Lundqvist. (c) Augen-bearing orthogneiss located c. 15 km SW of Mellerud. (Göteborg suite, Median segment). Photographer: U. Bergström. (d) Gneissic granodiorite with mafic enclaves located in the northern part of Göteborg (Hisingen suite, Western segment). Photographer: T. Eliasson.

Equigranular felsic varieties are most common, but porphyritic massifs occur, for example, in the Lerum area (Fig. 16.2; Welin & Samuelsson 1987). Mafic plutons with distinctive positive magnetic (Fig. 16.5a) and Bouguer gravity (Fig. 16.5b) anomalies are conspicuous close to Lidköping, in the hanging wall immediately above the MZ. A specific component of the Göteborg suite is an augen-bearing orthogneiss, interpreted as an originally coarsely porphyritic plutonic rock (Fig. 16.13c). This type of orthogneiss occurs in bands with variable width that can be traced along strike for several kilometres. Age determination studies of such rocks east of Göteborg (Ahlin et al. 2006) and west of Mellerud (Åhäll & Connelly 2008; Petersson et al. 2015) have confirmed their affiliation to the Göteborg intrusive suite. West of Mellerud, there is also a close temporal and spatial relationship to the siliciclastic rocks in the Kappebo formation (see above).

The lithogeochemical data for the Göteborg intrusive suite define a continuous compositional trend, ranging mainly from granite, through the granodiorite and tonalite fields, to quartz diorite and gabbro; quartz monzodioritic and monzogabbroic compositions are also present (Fig. 16.14a). The rocks are predominantly magnesian and calc-alkaline (Fig. 16.14b, c) with a marked increase of Fe-number for the most SiO₂-rich varieties, similar to Cordilleran granites (Frost et al. 2001). REE data show LREE enrichment and a discrete Eu anomaly, except for the samples with the highest SiO₂ content, which show a strong Eu anomaly (Fig. 16.15a). A negative Nb–Ta trough in all compositions, negative Sr, P and Ti anomalies in samples where SiO₂ exceeds 57%, suggesting the influence of significant fractionation processes, and a negative Th–U anomaly in a sample where the SiO₂ content is ≤57% are present in spider plots with data normalized to primordial mantle (Fig. 16.15b). The chronological and compositional similarities with the volcanic rocks in the Åmål group are consistent with a common origin in a continental magmatic arc; an interpretation consistent with the position of the samples on a tectonic discrimination plot (Fig. 16.14d).

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Fig. 16.14.

Geochemical characteristics of the c. 1.63–1.59 Ga (Göteborg) and c. 1.59–1.52 Ga (Hisingen) intrusive suites. Data selected and extracted from the lithogeochemistry database at the Geological Survey of Sweden. (a) Geochemical classification using the P–Q diagram after Debon & Le Fort (1983). (b) Geochemical classification using the Fe-number after Frost et al. (2001). (c) Geochemical classification using the modified alkali-lime index after Frost et al. (2001). (d) Tectonic discrimination diagram after Pearce et al. (1984).

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Fig. 16.15.

Geochemical characteristics of the c. 1.63–1.59 Ga (Göteborg) and c. 1.59–1.52 Ga (Hisingen) intrusive suites. Data source as in Figure 16.14. (a) Chondrite-normalized REE diagram for the Göteborg suite. Normalization after Boynton (1984). (b) Primitive mantle-normalized multi-element diagram for the Göteborg suite. Normalization after Wood et al. (1979). (c) & (d) are the same as (a) & (b), respectively, for the Hisingen suite.

Using combined U–Pb–Hf–Lu–O isotope data, Petersson et al. (2015) suggested that the magmas represented in the volcanic rocks in the Horred formation and Åmål group (see above), and in the Göteborg and younger Hisingen (see below) intrusive suites were produced from a mixed source. Reworked crustal material and an increasing proportion of a juvenile component with time were involved. These data were inferred to support a retreating volcanic arc system with back-arc extension, and shorter pulses of crustal shortening and accretion (Petersson et al. 2015).

Siliciclastic metasedimentary rocks and amphibolite (Stora Le-Marstrand formation)

The siliciclastic metasedimentary rocks belonging to the Stora Le-Marstrand formation (SLM formation: Figs 6.2 & 6.3) with a semi-pelitic composition (Daly 1978) correspond to a greywacke protolith. In better-preserved parts, the greywackes are transformed into mica schist and fine-grained gneiss (Bergström 1963); primary structures (e.g. graded bedding; Fig. 16.16a) are occasionally preserved. Thicker beds of massive psammite without grading are common, while pelitic varieties are rare, possibly due to the strong metamorphic overprint. With increasing metamorphic grade, the rocks are transformed into veined paragneisses (Fig. 16.16b) with several generations of leucosome, ultimately passing into diatexitic migmatite (Fig. 16.16c). Calc-silicate pods or bands in the migmatites indicate occasional carbonate deposition during formation of the greywackes.

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Fig. 16.16.

Character of siliciclastic metasedimentary rocks and amphibolite in the 1.6 Ga Stora Le-Marstrand formation, Western segment. (a) Well-preserved turbiditic metagreywacke on a small island south of Tjörn, with bedding and facing direction. (b) Folded mica schist and veined paragneiss, on a small island in Kungsbackafjorden c. 30 km south of Göteborg. (c) Diatexite with xenoliths of intercalated amphibolite, Koster island. (d) Garnet amphibolite showing strong S_G2 foliation on Tjörn. Photographs by T. Eliasson, except for (d) which is by U. Bergström.

Internal stratigraphic subdivision of this siliciclastic metasedimentary succession is not possible due to the structural and metamorphic overprint, but separate parts with apparently different depositional ages have been identified (Åhäll & Connelly 2008). The detrital zircon age population in the SLM formation is dominated by grains with an age of 1.6 Ga, inferred to represent material derived locally from the Åmål group and the Göteborg intrusive suite. Older detrital zircons include magmatic grains in the interval 1.8–1.6 Ga and some c. 2 Ga and Archean detritus (Cornell et al. 2000; Austin Hegardt et al. 2010), verified by data from the SLM formation in Norway (Andersen et al. 2004). However, different minimum provenance ages for detrital zircon grains in different areas have motivated the distinction into older eastern (SLM 1) and younger western (SLM 2) parts.

The SLM 1 siliciclastic rocks were deposited after 1591 ± 8 Ma (Åhäll et al. 1998) but prior to emplacement of a pluton belonging to the Hisingen intrusive suite (see below) at 1587 ± 3 Ma (Connelly & Åhäll 1996; Åhäll et al. 1998; see also Table 16.2). This pluton also intruded after early Gothian deformation and metamorphism of the siliciclastic rocks (see Fig. 16.10e). A strongly active tectonic setting prevailed around 1.59 Ga, with uplift and erosion of slightly older magmatic rocks, deposition of siliciclastic sedimentary rocks, and rapid burial preceding ductile deformation and metamorphism. By contrast, the spatially more restricted SLM 2 siliciclastic rocks, exposed in westernmost areas in the Koster archipelago and southwards (Fig. 16.2), contain detrital zircon grains showing provenance ages as young as c. 1.57–1.56 Ga (Åhäll & Connelly 2008). These rocks were deposited prior to late Gothian deformation and metamorphism during the time span c. 1.56–1.55 Ga (see earlier in the text) and the emplacement of c. 1.55 Ga plutons belonging to the Hisingen suite (Åhäll & Connelly 2008; see Table 16.2).

Amphibolite (Fig. 16.16d) constitutes a subordinate but widespread component in the paragneisses and migmatites of the SLM formation, occurring commonly as irregular lenses in diatexitic varieties (Fig. 16.16c). They are interpreted as basaltic relicts that were emplaced as sub-volcanic sills or erupted as lavas or rare ash deposits coeval with the siliciclastic sedimentary deposition. In the Hakefjorden area (Fig. 16.2), mafic metavolcanic rocks that erupted as pillowed and massive lavas dominate the stratigraphy. The amphibolites in the SLM formation have depleted LREE, show a Nb–Ta trough, a Zr/Y ratio <3 and are inferred to have been deposited in a back-arc basinal setting (Brewer et al. 1998; Scherstén et al. 2004). Åhäll & Daly (1989) reported ε_Nd values >4 and suggested a depleted mantle source for the amphibolites. The paragneisses in the SLM formation show lower ε_Nd values but generally >−2 (Brewer et al. 1998), some samples showing a mixed source of greywacke and basaltic material.

Plutonic rocks emplaced at c. 1.59–1.52 Ga (Hisingen suite)

Plutonic rocks emplaced at 1.59–1.52 Ga and referred to as the Hisingen suite occur together with, and intruded, the siliciclastic metasedimentary rocks of the SLM formation in the Western segment (Figs 6.2 & 6.3). Plutons of this age also occur to the east in the Median segment but contact relationships between them and the older intrusive rocks in the Göteborg suite are not commonly preserved (Samuelsson 1985). Based on critical field relationships to the SLM 1 rocks, the Hisingen suite intruded after the early Gothian deformational and metamorphic event (see text above and Fig. 16.10e). Similarly to the older Göteborg suite, the intrusive rocks in the Hisingen suite vary considerably in texture, composition and age of emplacement. The suite may be divided from east to west into two principal components based mainly on their crystallization age (Åhäll & Connelly 2008).

Plutons emplaced during the range of c. 1.59–1.52 Ga occur in virtually the entire area east of the coast. They show a continuous compositional trend from gabbro to granite, frequent evidence for magma mixing and mingling between the mafic and felsic components (Fig. 16.13d), and a conspicuous reddish granite unit. The oldest member is tonalitic in composition and occurs close to a large accumulation of amphibolites in the SLM formation in the Hakefjorden area; it is also spatially associated with granite and trondhjemite. Further west, along the coastline and in the Koster archipelago and southwards, the Hisingen suite is composed of a diverse variety of porphyritic and equigranular granites with frequent xenoliths of SLM rocks, including diatexitic migmatite. These plutons yield emplacement ages at c. 1.55–1.54 Ga restricted to the closing stages of the Gothian orogeny.

The intrusive rocks in the Hisingen suite show similar geochemical characteristics as the slightly older Göteborg suite (Figs 16.14 & 16.15). Using U–Pb–Hf–Lu–O isotope data, Petersson et al. (2015) suggested a higher juvenile proportion in the magma generation of the Hisingen suite compared to the Göteborg suite (see also above). Emplacement is inferred to have occurred in a continental magmatic arc setting where subduction-related processes prevailed (Åhäll & Connelly 2008; see Fig. 16.14d).

Bimodal plutonic suite emplaced at c. 1.51–1.49 Ga (Stigfjorden suite)

On the Tjörn and Orust islands north of Göteborg (Fig. 16.2), granites and spatially associated mafic intrusive rocks were emplaced at c. 1.51–1.49 Ga (Stigfjorden suite: Åhäll & Connelly 1998; see also Fig. 16.3; Table 16.2). The contacts of these bodies to the dominant country rocks belonging to the SLM formation are commonly obscured by ductile strain, but the wealth of migmatite xenoliths in granite indicate an intrusive origin into Gothian crust. The main component is a porphyritic, greyish red granite with a moderately developed Sveconorwegian gneissic structure; a more strongly deformed variety occurs as augen gneiss (Bergström 1963). A conspicuous feature is the mingling relationship with coeval mafic intrusive rocks; composite dykes are also locally present.

Supracrustal rocks (Skagerrak formation) intruded by mafic sills

Felsic volcaniclastic rocks and more mature quartz-rich sandstone beds (Skagerrak formation), intruded by mafic sills, are exposed on some of the islands offshore of the west coast of Sweden. They occur as large-scale xenoliths close to the western margin of a major 0.92 Ga granite batholith (Bohus granite: see Fig. 16.2 and below) and show a pervasive gneissosity with migmatization related to the Sveconorwegian orogeny. Bedding is only occasionally preserved in the supracrustal rocks. Apart from a single detrital zircon grain with a U–Pb age of c. 1.88 Ga, most grains from a volcaniclastic sample with dacitic composition yield concordant to near-concordant ages in the time range 1.56–1.44 Ga, and a depositional age after c. 1.48 Ga has been inferred (Åhäll et al. 1998; see Fig. 16.3). However, care is needed with this interpretation due to the small sample size.

Mafic dykes emplaced at c. 1.46 Ga (Koster and Orust dykes)

In the Koster archipelago (Fig. 16.2), a swarm of mafic dykes was emplaced into the Gothian crust, cross-cutting the structures formed during this orogeny (Fig. 16.10c, d). They form c. 15–20% of the total rock volume, are normally 2–10 m thick, and show steep dips and chilled margins (Hageskov 1987). The dykes trend in a north-northeasterly direction across the archipelago, being rotated into an amphibolite facies Sveconorwegian ductile deformation zone with north-northwesterly trend in its northernmost part (see above). They continue offshore along the coast to the north to join up with similar dykes (Kattsund dykes) in the Oslofjorden area (Hageskov 1987; Hageskov & Pedersen 1988). In texturally well-preserved varieties, the mafic dykes are fine- to medium-grained, and are dominated by plagioclase (An = 35–70), clinopyroxene, olivine and magnetite. With increasing metamorphic grade, they are transformed into schistose amphibolite containing predominantly hornblende and plagioclase (An = 25–35) without magnetite (Hageskov 1987).

A similar dyke swarm cross-cutting Gothian polyphase structures in migmatitic varieties of the SLM formation, but with some ductile shear along contacts (Fig. 16.10c, d), is situated on the western part of the island of Orust (Daly 1978; Daly et al. 1983; Fig. 16.2). Compared with the Koster archipelago, there are fewer dykes and the Sveconorwegian metamorphic overprint is generally more pronounced, with garnet-bearing amphibolite being common. However, on the basis of the field relationships to the country rocks, they bear a clear resemblance to the dykes in the Koster archipelago.

Geochemically, the dykes in both areas are tholeiitic, and show an enrichment in K₂O, Rb and Ba (Daly 1978; Hageskov 1987). Assimilation of material from the country rock is evident along dyke margins (Hageskov 1987), possibly explaining this enrichment. A fractionated, folded and metamorphosed garnet-bearing variety from Orust has yielded a U–Pb (zircon) age of c. 1.46 Ga, interpreted as the timing of dyke emplacement (Åhäll & Connelly 1998; see also Table 16.2). Regional intracratonic extension after the Gothian orogeny has been inferred (Hageskov & Pedersen 1988; see Fig. 16.3).

Bimodal plutonic suite emplaced at c. 1.34–1.30 Ga (Kungsbacka suite)

Individual intrusions in a bimodal plutonic suite emplaced at c. 1.34–1.30 Ga are referred to in the literature by a variety of local names (e.g. Askim, Hästefjorden, Segmon and Veddige: see Holme 1986; Andersson 2001; Austin Hegardt et al. 2007). They are addressed here simply as the Kungsbacka suite after the type occurrence south of Göteborg (see also Austin Hegardt et al. 2007). Although the plutonic rocks in this suite are usually better preserved compared with the surrounding older gneisses, the occurrence in several areas of strongly deformed or migmatitic orthogneiss and the recrystallization of porphyritic varieties into augen gneiss indicate the partly high-grade character of the Sveconorwegian tectonothermal overprint (Andersson 2001; Austin Hegardt et al. 2007; see also earlier in the text). Some examples of the plutonic rocks in the Kungsbacka suite are addressed here to illustrate their variation.

The Askim granite in the Göteborg area is a porphyritic, red to grey, medium-grained granodiorite to granite, with K-feldspar phenocrysts mainly <2 cm (Fig. 16.17a). More coarsely porphyritic varieties are present in some other plutonic bodies. By contrast, the Hästefjorden pluton is mainly an equigranular, red granite; other granites exhibit variable texture from porphyritic to equigranular. The granites in the Kungsbacka suite are normally quartz and K-feldspar rich with biotite as the principal mafic phase, and fluorite as a characteristic accessory mineral in the more fractionated varieties (Samuelsson 1985).

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Fig. 16.17.

Character of partly gneissic plutonic rocks in the c. 1.34–1.30 Ga Kungsbacka suite. (a) Gneissic porphyritic granite–granodiorite (Askim granite), Onsala peninsula south of Göteborg. Photographer: F. Hellström. (b) Magma mixing and mingling in mafic and felsic components, Onsala peninsula south of Göteborg. Photographer: T. Eliasson. (c) Spatial distribution of U in the uppermost part of the bedrock and in the cover drift in the Idefjorden terrane. Positive anomalies marked on this radiometric anomaly map, generated by Ildikó Antal Lundin (Geological Survey of Sweden), are discussed in the text.

The mafic rocks in the Kungsbacka suite occur either together with the granites in complex plutons or as separate bodies (Kiel et al. 2003), which may have originated as a dyke swarm. The gabbro–granite complexes have their type area south of Göteborg, which is characterized by a positive Bouguer gravity anomaly (Fig. 16.5b), several positive magnetic anomalies (Fig. 16.5a) and mixing–mingling relationships between the two components (Fig. 16.17b; Samuelsson 1982). The close spatial association between the felsic and mafic components, and isotope geochemical data, suggest that granitic melts may have formed in the lower Gothian crust, as a response to the thermal rise related to the emplacement of mantle-derived gabbroic melts (see also Petersson et al. 2015).

The bimodal character of the Kungsbacka suite is apparent in the geochemical data used here. Granitic to quartz syenitic compositions and gabbroic to quartz dioritic or quartz monzodioritic compositions plot in separate fields on a compositional classification diagram (Fig. 16.18a). A monzonitic composition was identified in the Segmon pluton (Holme 1986), west of Grums (see Fig. 16.2). The samples with SiO₂ >57% are ferroan (Fig. 16.18b) and alkali-calcic (Fig. 16.18c), consistent with the proposed fields for A-type granites (Frost et al. 2001). Fractionated granites close to Göteborg show anomalously high contents of U (Fig. 16.17c) and Th. REE data typically show rather flat patterns with pronounced negative Eu anomalies in the SiO₂-rich samples (Fig. 16.18d). Spider plots normalized to primordial mantle have very strong negative Ba, Sr, P and Ti anomalies in such samples, indicating advanced fractionation, and a Nb–Ta trough for virtually all the samples (Fig. 16.18e).

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Fig. 16.18.

Geochemical characteristics of the plutonic rocks in the c. 1.34–1.30 Ga Kungsbacka suite. Explanations to the figures correspond to those accompanying Figures 16.14a, b, c, 16.15a, b and 16.14d, respectively.

A single homogeneous source region for the granitic melts is suggested from the character of the Hf and O isotopes (Petersson et al. 2015), probably the arc crust represented by the Göteborg intrusive suite and the Åmål group rocks. The Nb–Ta trough pattern on the spider plots normalized to primordial mantle, for example, may have been inherited from the source crust (cf. Gothian magmatic suites discussed earlier). Geochemical similarities of the mafic component in the Kungsbacka suite to continental rift basalts has been discussed (Kiel et al. 2003), the rift scenario tentatively supported by the linear spatial distribution of these intrusions. Felsic varieties lie mainly in the within-plate granite field on a tectonic affinity discrimination diagram designed for such rocks (Fig. 16.18f).

Supracrustal rocks deposited between c. 1.33 and 1.05 Ga (Dalsland group)

Polymict conglomerate and poorly sorted red arkose (Fig. 16.19a), passing upwards into a >100 m-thick reddish quartz-rich sandstone with occasional intraformational conglomerates, form the stratigraphically lower part of the Dalsland group (Fig. 16.3). The basal conglomerate shows an irregular thickness, suggesting strong topographical relief during deposition, and the clasts in the conglomerate are similar in composition to the local granitic basement rocks. The quartz-rich sandstone shows abundant lamination and, occasionally, ripple structures and cross-bedding (Fig. 16.19b).

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Fig. 16.19.

The character of supracrustal rocks deposited after c. 1.33 Ga (Dalsland group). (a) Laminated, poorly sorted arkose from the base of the lower sandstone unit, c. 15 km NW of Mellerud. (b) Ripples in the lower sandstone unit, NW of Mellerud. (c) Amygdaloidal flow top in a basaltic lava flow occurring as an intercalation in argillite above the lower sandstone unit, c. 10 km NW of Mellerud. (d) Bedding and cross-cutting foliation in turbiditic greywacke in the uppermost part of the Dalsland group (Liane formation), c. 8 km NW of Mellerud. All photographs by U. Bergström.

The sandy rocks are overlain by dark grey argillites occurring as slates or phyllites. Sandstone intercalations, varying in thickness from centimetres to tens of metres, are impregnated with carbonates occasionally forming almost pure limestone layers; basaltic intercalations also form an important component (Fig. 16.3).

The basalts occur as prominent flows, mainly in the lower part of the succession with argillitic rocks; they are far less common to the north and NW. Some basalts show amygdaloidal, blocky flow tops and represent subaerial flows (Fig. 16.19c). The basalts are tholeiitic and LREE-enriched, have been correlated with c. 1.16 Ga basalts in the Bandak group in the Telemarkia terrane in southern Norway (see Chapter 14, Stephens et al. 2020), and were inferred to have formed by shallow melting of subcontinental lithospheric mantle during extension in a continental back-arc setting (Brewer et al. 2002). These authors also reported ε_Nd values of around +2, suggesting a minor but significant assimilation of older crustal material. Subduction of oceanic lithosphere further to the west (current geographical coordinates) was envisaged along the outer margin of the ancient continent represented in the Sveconorwegian cratonic foreland (Brewer et al. 2002).

The stratigraphically upper part of the Dalsland group (Fig. 16.3) is composed of a whitish, quartz-rich sandstone, occurring as quartzite, with only diffuse primary structures due to recrystallization of the quartz grains. This unit is overlain by dark grey, commonly laminated turbiditic greywackes (Liane formation: Fig. 16.19d). These two units are up to several hundred metres thick and are exposed in the core of two synforms in the central part of the Dalsland group.

Hypersthene–quartz monzodiorite to granite (c. 0.95 Ga)

A hypersthene-bearing quartz monzodiorite (quartz jotunite) to granite intrusion (Vinga intrusion), trending WNW–ESE and emplaced at c. 0.95 Ga (Table 16.2), is exposed on the outermost islands west of Göteborg (Årebäck et al. 2008; Fig. 16.2). This body contains complexly zoned plagioclase megacrysts, skeletal/acicular crystals of, for example, apatite, a wealth of xenoliths, restites and co-mingled enclaves, and high contents of Fe, Ti, P, K and F. Discrete magmatic layering parallel to the trend of the intrusive body occurs in a marginal zone in its northern part. Late-stage miarolitic cavities, in combination with thermobarometric studies, indicate emplacement at high crustal levels (≤7.5 km) and a crystallization temperature of around 750–800°C (Årebäck et al. 2008). Disequilibrium textures were explained by crystallization during under-cooling conditions. A heat source for melting related to mafic underplating and magma mingling has been envisaged (Årebäck et al. 2008).

Dolerite dykes (c. 0.95–0.93 Ga)

Subvertical dykes emplaced around 0.95–0.93 Ga (Hellström et al. 2004; Hellström & Persson 2009; Table 16.2) trend WNW–ESE and cross-cut all the Sveconorwegian ductile structures close to Göteborg (Figs 16.2 & 16.20a). The largest occurrence (Tuve dyke) shows a maximum thickness of 200 m and some dykes can be traced for several kilometres; they terminate abruptly eastwards towards the GÄSZ. Dolerites with a similar orientation also occur further north in the western part of the Idefjorden terrane (south of Uddevalla and near Bengtsfors: see Fig. 16.2). The dykes display chilled margins, peeled off wall-rock inclusions and primary igneous textures (Hellström et al. 2004; Hellström & Persson 2009).

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Fig. 16.20.

The character of intrusive rocks emplaced at c. 0.95–0.92 Ga during the late Sveconorwegian crustal extensional phase. (a) Chilled margin of a c. 20 m-wide dolerite dyke belonging to the c. 0.95–0.93 Ga dolerite swarm with a WNW–ESE trend NW of Göteborg. The dyke is discordant to migmatized paragneiss of the SLM formation. Photographer: F. Hellström. (b) Back-veining granite (pale colour in middle and lower-left parts of photograph) formed by partial melting of the country rock (SLM formation). The granite intruded and mingled with norite (dark colour), and was used to date the norite intrusion. The norite contains variably angular blocks of anorthosite (pale colour left of the hammer and in the lower-right part of the photograph). The anorthosite is dominated by homogeneous andesine–labradorite plagioclase, and the norite by orthopyroxene with some clinopyroxene and minor amphibole. Photographer: A. Scherstén. (c) Granite belonging to the c. 0.92 Ga granite batholith exposed between Lysekil and Strömstad (Bohus granite: see also Fig. 16.17c), containing xenolith of paragneiss (SLM formation) and pegmatite vein. The location is c. 18 km NW of Lysekil. Photographer: T. Eliasson. (d) Magmatic flow banding in the Bohus granite west of Kungshamn. The outcrop is approximately 2 m across. Photographer: A. Scherstén.

The dolerites are high Fe–Ti–K–P, low Ca, olivine- or subordinate quartz-bearing tholeiites with an evolved within-plate tectonic affinity. The occurrence of olivine tholeiites attests to an input of basaltic magma derived from the lithospheric mantle into the crust (Hellström et al. 2004). This process possibly served as a heat source for partial melting in the middle crust with the generation of quartz monzodioritic to granitic melts in the coeval and spatially associated Vinga intrusion (Hellström & Persson 2009 and see above).

Norite, anorthosite and granitic back veins (c. 0.92 Ga)

Plutons composed of hypersthene-bearing gabbro (norite), which are intrusive into paragneisses of the SLM formation, trend east–west on two small islands north of Göteborg along the southwestern coast of Sweden (Hakefjorden complex). Anorthosite inclusions up to 50 m across occur mainly in the western and eastern parts of this complex (Fig. 16.20b), and a hybrid rock with numerous mafic microgranular enclaves and rocks with plagioclase megacrysts are locally present (Årebäck & Stigh 1997). Crystallization from a melt enriched in Fe and Ti has been inferred for the noritic component (Årebäck & Stigh 2000).

Thermobarometric data in granulite facies contact metamorphic assemblages indicate emplacement of the complex at a maximum crustal depth of 21–22 km and at a temperature of c. 900°C (Årebäck & Andersson 2002). Granitic back veins in the norite–anorthosite complex dated at c. 0.92 Ga provide constraints on the timing of emplacement (Scherstén et al. 2000). One such dyke, inferred to have been derived from the SLM formation, contains xenocrystic zircons yielding Paleoproterozoic and even Archean ages (Cornell et al. 2000).

Granite and pegmatite (c. 0.92 Ga)

Granite plutons and spatially associated pegmatite, emplaced at c. 0.92 Ga during the waning stages of the Sveconorwegian orogeny (Eliasson & Schöberg 1991; Bingen et al. 2006; Table 16.2), form a significant component in the Idefjorden terrane west of (Bohus granite) or within (Blomskog granites: Lindh et al. 1998) the network of shear zones that includes the GÄSZ (Fig. 16.2). Anomalously high concentrations of U (Fig. 16.17c) and Th are a characteristic feature. Focus here is on the well-exposed and batholithic Bohus granite.

The Bohus granite is exposed along the coastline in southwestern Sweden between Lysekil and Strömstad (Fig. 16.2), continues northwards into Norway, and extends over a distance of more than 100 km. It forms a gently dipping sheet with a thickness of 4–8 km (Lind 1982) and corresponds to a variably positive anomaly pattern on the magnetic total field map (Fig. 16.5a). A positive Bouguer gravity anomaly is present in its northern part, close to Strömstad (Fig. 16.5b), possibly corresponding to a mafic component at depth.

The batholith was mainly emplaced into and contains abundant xenoliths of migmatitic paragneisses belonging to the SLM formation (Fig. 16.20c). Contact-metamorphic effects are locally present; synmagmatic pegmatitic and aplitic veins and pods are common (Eliasson et al. 2003; Fig. 16.20c). Internal contact relationships are irregular and undulating, suggesting repetitive pulses of granite magma (Eliasson et al. 2003). Restitic material dominates the zircon population and the age of c. 0.92 Ga (Table 16.2) comes from the U–Pb dating of monazite in spatially associated aplite–pegmatite (Eliasson & Schöberg 1991).

The batholith includes red and grey equigranular granites, and, in its central part, also porphyritic varieties. Most of the granites show a macroscopic flow foliation with a preferred orientation of minerals or flow banding (Fig. 16.20d). Garnets in the granite are low-Ca almandine–spessartines, in contrast to the generally Ca-rich almandine-type garnets in the surrounding metamorphic rocks. Thermobarometric studies indicate emplacement at c. 4.0 kbar, corresponding to a crustal depth of around 15 km, and crystallization temperatures of c. 700°C (Eliasson et al. 2003). The granite contains high primary K, Rb, U (Fig. 16.17c) and Th contents.

Secondary alteration in the central part of the batholith involved dissolution of magmatic quartz and albitization of plagioclase, giving rise to a vuggy rock, which facilitated fluid penetration and thereby further alteration (Petersson & Eliasson 1997). This alteration occurred during the Permo-Carboniferous after emplacement and cooling of the granite (Petersson & Eliasson 1997). Dykes with a north–south trend related to the formation of the Oslo Rift were emplaced into the batholith at this time. Zheng (1996) attempted to relate the fracture pattern inside the Bohus granite and its surroundings to the tectonic evolution during the latest part of the Sveconorwegian orogeny, and after this orogeny during the Neoproterozoic and Phanerozoic.

Lamprophyre dykes (c. 0.92 Ga)

Lamprophyre dykes, restricted to the northern part of the Median segment, also form part of the magmatic phase related to late Sveconorwegian extension. They were emplaced at c. 0.92 Ga (Wahlgren et al. 2015; Table 16.1). Similarly to the slightly older dolerites and coeval noritic to anorthositic plutons in the Western segment, the lamprophyre dykes trend approximately east–west and cross-cut Sveconorwegian ductile structures, including the mylonitic fabric along the eastern boundary shear zone (MZ) to the Idefjorden terrane (Wahlgren et al. 2015). The lamprophyres are characterized by a high magnetic susceptibility, chilled margins, biotite phenocrysts and deuteric alteration of the groundmass, and have been classified as kersantite or minette (Wahlgren et al. 2015).

Stratabound sediment-hosted, Cu–Fe sulphide deposits

Stratabound Cu–Fe sulphide deposits are hosted by the Dalsland group. Most of the mineralization is restricted to the stratigraphic break between the lower quartz-rich sandstone and the overlying phyllites in the western part of the area where the Dalsland group is exposed (Fig. 16.21).

The host rock to the sulphide deposits is normally fine-grained, carbonate-cemented sandstone in the uppermost part of the sandstone formation (Alm et al. 1997). The mineralization is composed of disseminated pyrite and chalcopyrite along thin fracture fillings and quartz stringers. The ore-bearing stratigraphic level is very persistent along strike and can be followed for many kilometres. Ore shoots with higher grades occur sporadically along strike, with grades averaging around 0.5% Cu with some additional Ag. The Dingelvik mineralization (Fig. 16.21) is the largest with c. 21 million tonnes (Mt) of ore with 0.83% Cu (Thelander & Hammergren 1985). The Stora Strand deposit (Johansson 1909) in the eastern part of the Dalsland group (Fig. 16.21), consisting of a thin, semi-massive sulphide horizon occurring stratigraphically higher in the phyllites, was mined during the early part of the twentieth century.

Structurally controlled, polymetallic quartz vein mineralization

Structurally controlled and polymetallic vein mineralization occurs in the north-central part of the Idefjorden terrane, most conspicuously between Mellerud and Åmål north of Vänern in the Median segment (Fig. 16.21). Mineralization commonly occurs in anastomosing breccia systems containing several veins, veinlets and intermediate wall-rock blocks. The deposits are generally metre-wide quartz veins containing patchy clusters of ore minerals. Some veins show syn-emplacement deformation, others are passive fracture fillings. Hydrothermally altered wall rock within 1 m of the contact with the quartz vein is common.

Alm & Sundblad (1994) grouped the quartz vein deposits into four types according to ore mineralogy, each group showing a characteristic trace element content; bornite–chalcopyrite; pyrite–chalcopyrite (–galena); galena–sphalerite; and sulphide–Mn oxide minerals. The fourth of these groups also includes Mn–Fe oxide deposits with only minor sulphide content. The largest deposit is the Kesebol mine (Fig. 16.21), active during 1917–44, which produced 0.04 Mt with c. 33% Mn; a chalcopyrite ore shoot is located adjacent to the Mn–Fe oxide ore.

Precious metal grades are generally low in the quartz vein mineralization. However, 0.071 Mt of ore with 4.6 g t⁻¹ Au was mined during 1993–97 at the Harnäs deposit (Hallberg 2012; Fig. 16.21). Re–Os dating of pyrite and chalcopyrite from the Harnäs and spatially associated vein deposits yields an age of 973 ± 34 Ma, with a possible second pyrite mineralization event at 616 ± 40 Ma related to the reopening of the vein system (Stein et al. 2000). Although poorly constrained, the c. 973 Ma mineralization event is inferred to have occurred during late Sveconorwegian regional extension, after the main phase of ductile deformation and metamorphism (Stein et al. 2000).