Lithos 226 (2015) 147–168

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Lithos

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Exhumation of an eclogite terrane as a hot migmatitic nappe, Sveconorwegian orogen

Charlotte Möller a,⁎, Jenny Andersson b, Brendan Dyck a,1, Ildiko Antal Lundin b a Department of Geology, Lund University, SE-223 62 Lund, Sweden b Geological Survey of Sweden, Box 670, SE-751 28 Uppsala, Sweden article info abstract

Article history: We demonstrate a case of eclogite exhumation in a partially molten, low-viscosity fold nappe within high-grade Received 23 May 2014 metamorphosed crust in the Eastern Segment of the Sveconorwegian orogen. The nappe formed during Accepted 13 December 2014 tectonic extrusion, melt-weakening assisted exhumation and foreland-directed translation of eclogitized crust, Available online 31 December 2014 and stalled at 35–40 km depth within the collisional belt. The eclogites are structurally restricted to a regional recumbent fold in which stromatic orthogneiss with pods of amphibolitized eclogite make up the core. High- Keywords: N Eclogite temperature mylonitic gneiss with remnants of kyanite eclogite (P 15 kbar) composes a basal shear zone b Exhumation 50 km long and 4 km wide. Heterogeneously sheared and partly migmatized augen gneiss forms a Extrusion tectonostratigraphic marker in front of and beneath the nappe, and is in turn structurally enveloped by a compos- Partial melting ite sequence of orthogneisses and metabasites. The entire tectonostratigraphic pile underwent near-pervasive Migmatite deformation and recrystallization under high-pressure granulite and upper amphibolite conditions. U–Pb SIMS Precambrian metamorphic zircon ages of eclogite and stromatic orthogneiss constrain the time of eclogitization at 988 ± 6 Ma and 978 ± 7 Ma. Migmatization, concomitant deformation, and exhumation are dated at 976 ± 6 Ma, and crystallization of post-kinematic melt at 956 ± 7 Ma. Orthogneiss protoliths are dated at 1733 ± 11 and 1677 ± 10 Ma (stromatic gneiss) and 1388 ± 7 Ma (augen gneiss in footwall), demonstrating origins indigenous to the Eastern Segment. Eclogitization and exhumation were coeval with the Rigolet phase of the Grenvillian orogeny, reflecting the late stage of continental collision during construction of the supercontinent Rodinia. © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction bearing units have been unknown. This paper presents the first documentation of the structure of a large eclogite-bearing nappe, and Eclogite-bearing metamorphic terranes are present in former deep demonstrates that it was emplaced at orogenic mid-crustal levels crustal sections of ancient and eroded mountain belts, some of which (35–40 km) as a partly molten recumbent fold nappe. We document are located several hundreds of kilometres inboard of the inferred the metamorphic and deformational relations and constrain the timing continental margin. Examples are found in the Grenville orogen in of eclogite metamorphism and tectonic exhumation to mid-crustal levels. Canada and the Sveconorwegian orogen in Scandinavia, both of which The fold nappe provides an example of eclogite exhumation by formed part of supercontinent Rodinia 1.1–0.9 billion years ago (Dalziel, means of extrusion of hot, partially molten material within deep conti- 1997; Li et al., 2008). These eclogite occurrences testify to deep tectonic nental crust. Our case differs from the melt-filled channel exposed in the burial far from the inferred former continental margin, rendering the pro- Himalayas (Searle et al., 2010) in that it represents a hot nappe in a deep cesses involved in their formation and exhumation enigmatic. section which did not reach the surface until a post-orogenic stage. Our The presence of 1.0 Ga eclogites in rocks derived from Baltic crust in case demonstrates that foreland-directed ductile flow of partially mol- the Sveconorwegian orogen demonstrates burial to depths in excess of ten crust in a deep thrust stack is a viable mechanism for emplacement 55 km and subsequent exhumation at a late stage of the orogeny. of eclogite-bearing crust at mid-crustal levels in large hot orogens. Hitherto, the distribution of eclogites and the structure of eclogite-

2. Regional geological setting ⁎ Corresponding author at: Department of Geology, Lund University, Sölvegatan 12, SE- 22362Lund,Sweden.Tel.:+46462227878;fax:+46462224830. 2.1. The Sveconorwegian Province E-mail addresses: [email protected] (C. Möller), [email protected] (J. Andersson), [email protected] (B. Dyck), [email protected] (I. Antal N Lundin). The Sveconorwegian Province is a 500 km wide orogenic belt 1 Present address: Department of Earth Sciences, South Parks Road, Oxford OX1 3AN, UK. extending across the Baltic Shield in southern Scandinavia (Fig. 1). It

http://dx.doi.org/10.1016/j.lithos.2014.12.013 0024-4937/© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 148 C. Möller et al. / Lithos 226 (2015) 147–168

Fig. 1. (a) Plate reconstruction at the end of the Grenvillian–Sveconorwegian orogeny (unpublished compilation by B. Bingen, partly based on Cawood et al., 2007). (b) The Sveconorwegian Province. Terrane divisions after Bingen et al. (2008c); Telemarkia magmatism after Slagstad et al. (2013). (c) Tectonic subdivisions of the Eastern Segment. Figure based on 1:5 M Fennoscandian map database, and Geological Survey of Sweden 1:1 M Bedrock map database. (d) Schematic vertical E–W section across the Eastern Segment.

exposes deeply eroded continental crust involved in mountain building The Sveconorwegian Front defines the eastern boundary of at 1.14–0.90 Ga, during formation of supercontinent Rodinia. The large- Sveconorwegian ductile deformation, delimiting the Sveconorwegian scale architecture of the Sveconorwegian Province is similar to that of Province (Fig. 1; Berthelsen, 1980). The pre-Sveconorwegian foreland the Grenville Province in North America: wide allochthonous belts in comprises metamorphic and igneous rocks of the 2.0–1.8 Ga the orogenic hinterland were metamorphosed at 1.09–1.02 Ga, follow- Svecokarelian orogen, partly overlain by supracrustal rocks and intrud- ed by metamorphism at 1.00–0.98 Ga in the belt closest to the cratonic ed by voluminous 1.7 Ga igneous rocks (Högdahl et al., 2004). Southern foreland (Fig. 1a; Bingen et al., 2008c; Rivers et al., 2012). areas were metamorphosed and intruded by granites during the 1.5– C. Möller et al. / Lithos 226 (2015) 147–168 149

1.4 Ga Hallandian orogeny (Fig. 1; Zarins and Johansson, 2009; Ulmius granite and dolerite (0.95–0.92 Ga), and lamprophyre (0.93–0.91 Ga; et al., in review). Wahlgren and Kähr, 1977; Eliasson and Schöberg, 1991; Årebäck and The Sveconorwegian Province is made up of five lithotectonic seg- Stigh, 2000; Hellström et al., 2004; Årebäck et al., 2008). ments separated by high-strain zones (Fig. 1b; Bingen et al., 2008c, Exceptions to this large-scale magmatic zoning in the orogen are and references therein). Lithologies in the Eastern Segment are semi- mafic dykes. An orogen-parallel mafic dyke swarm intruded along and continuous with those in the Baltica foreland. The Mylonite Zone, a east of the Sveconorwegian Front at 0.97–0.94 Ga (Blekinge-Dalarna prominent b10 km wide and N450 km long ductile high-strain zone, Dolerites, Fig. 1c; Söderlund et al., 2005; one older age at 978 ± 2 Ma separates the Eastern Segment from allochthonous units to the west recorded at the Caledonian Front). (Fig. 1b, c). The Mylonite Zone dips gently west or is subvertical, and ac- commodated top-to-the-ESE displacement followed by top-to-the-W 2.2. The Eastern Segment extension in the late Sveconorwegian (Berglund, 1997; Stephens et al., 1996; Viola et al., 2011). Migmatization in the southern Mylonite Zone The Eastern Segment is a 50–100 km wide belt consisting mainly of took place at 0.98–0.97 Ga (Andersson et al., 2002), synchronous with orthogneisses. Their protolith ages and compositions suggest they are high-grade metamorphism in the Eastern Segment (Möller et al., equivalent to 1.81–1.66 Ga granitic–monzonitic intrusions east of the 2007); U–Pb ages of 0.92 Ga (titanite and zircon) record a younger Sveconorwegian Front (Fig. 1; e.g. Söderlund et al., 1999, 2002; Möller stage of metamorphism and ductile deformation (Johansson and et al., 2007; Brander et al., 2012; Petersson et al., 2013). Based on meta- Johansson, 1993; Scherstén et al., 2004). 40Ar/39Ar cooling ages of the morphic grade and structural relationships, the Eastern Segment is boundary range from 0.92 Ga (hornblende, Page et al., 1996)to subdivided into three compartments (Fig. 1c, d). In the frontal wedge, 0.88 Ga (biotite and muscovite, Viola et al., 2011). the bedrock is non-penetratively deformed and exhibits a fan structure The units west of the Mylonite Zone have been displaced from their across strike (Wahlgren et al., 1994). Sveconorwegian metamorphism original tectonic position and cannot be directly linked to the pre- grades from greenschist-facies in the east to amphibolite-facies in the Sveconorwegian shield to the east. Their Pre-Sveconorwegian origins west. The transitional section exposes penetratively gneissic but not are variable, as are the timing and conditions of Sveconorwegian migmatitic rocks, deformed under Sveconorwegian amphibolite-facies deformation and metamorphism (Andersson et al., 2002; Bingen et al., conditions (c. 10 kbar, Söderlund et al., 2004) and folded into km- 2008c). Conceptually, the Mylonite Zone is equivalent to the Allochthon scale folds that plunge shallowly to the east. Mafic rocks are amphibolite Boundary Zone of the Grenville Province (cf. Rivers et al., 2012). and garnet amphibolite. Relict igneous textures are preserved in boudins of competent coarse-grained felsic and mafic intrusions; 2.1.1. Sveconorwegian metamorphism metagabbro commonly exhibits garnet-rich coronas between relict Sveconorwegian orogenesis included four metamorphic events. igneous minerals. Early (likely pre-collisional) 1.14–1.08 Ga high-temperature metamor- The internal section consists of high-grade orthogneisses. The phism reaching granulite-facies conditions, at low–moderate pressures, protoliths are granitic–monzonitic intrusions 1.74–1.66 Ga (N85%, includ- is found in central parts of the province (Bamble and Kongsberg, Fig. 1; ing minor dioritoid components), and 1.42–1.38 Ga and 1.22–1.18 Ga Bingen et al., 2008c). The Idefjorden and Telemarkia Terranes record (Andersson et al., 1999; Christoffel et al., 1999; Hubbard, 1975, 1989; mid-Sveconorwegian 1.05–0.99 Ga greenschist–upper amphibolite- Petersson et al., 2013). The orthogneisses host lenses and layers of facies conditions, distributed in a composite patchwork where mig- garnet amphibolite and high-pressure mafic granulite. Partial melting matitic complexes are juxtaposed against rocks with little or no is common in both felsic and mafic rocks; relics of primary igneous Sveconorwegian imprint. High-pressure amphibolite and granulite- textures are rare. The high-grade gneissic layering has been folded facies metamorphism at 1.05–1.02 Ga is found in the Idefjorden Terrane, along E–W trending axes and reoriented by regional-scale open folding immediately west of the Mylonite Zone (Bingen et al., 2008b; Söderlund along N–S trending axes giving rise to fold interference patterns (Fig. 2). et al., 2008a). Metamorphic conditions reached high-pressure granulite- and upper Late Sveconorwegian high-pressure metamorphism affected the amphibolite-facies; geothermobarometry of mafic rocks yields temper- Eastern Segment at 0.99–0.98 Ga. Temperatures reached high- atures between 680 and 795 °C and pressures of 8–12 kbar (Johansson pressure granulite-facies and remnants of eclogites occur (Johansson et al., 1991; Möller, 1998; Wang and Lindh, 1996). et al., 1991, 2001; Möller, 1998, 1999). Partial melting took place at Retrogressed eclogite occurs locally with assemblages in the least 0.98–0.96 Ga, and granitic pegmatite dykes intruded at 0.96–0.94 Ga retrogressed varieties demonstrating pressures N 15 kbar (Hegardt (e.g. Möller et al., 2007 and references therein). Records of early- and et al., 2005; Möller, 1998, 1999). The eclogite-facies metamorphism mid-Sveconorwegian metamorphism are lacking in the Eastern was dated at 972 ± 14 Ma (zircon inclusions in prograde garnet), and Segment. resetting of titanite at 945 ± 4 Ma age (titanite inclusion in garnet; Apart from low-pressure and high- to ultrahigh-temperature meta- Johansson et al., 2001). The geographical and structural distribution of morphism at 0.93–0.92 Ga associated with high-temperature AMCG in- retroeclogites has hitherto been unknown, but our mapping (detailed trusions in the westernmost part of the orogen (e.g. Bingen et al., 2008c; below) shows that retro-eclogite bodies are confined to the core of a Vander Auwera et al., 2011), there is a younging of Sveconorwegian recumbent fold structure (area shaded white in Figs. 2–3). This fold metamorphism towards the foreland in the east. Final relative motion structure is situated in the internal Eastern Segment, with its western between the Sveconorwegian terranes is postulated to have occurred boundary at or close to the Mylonite Zone. at 0.92–0.91 Ga (Bingen et al., 2008a). Some metadolerites, usually less than a few metres wide (marked on the Geological Survey bedrock maps), cross-cut the regional gneissic 2.1.2. Sveconorwegian magmatism layering, but are metamorphosed and commonly show a weak defor- Sveconorwegian felsic magmatism is confined to terranes west of mation fabric. They occur from the Sveconorwegian Front and across the Mylonite Zone and increases in volume towards the hinterland the Eastern Segment showing westwards progressively higher meta- (Fig. 1). Formation of 1.05–1.02 Ga granites (syn-collisional) in morphic grades, reaching high-pressure granulite-facies. The intrusion Telemarkia was followed by 0.98–0.92 Ga ferroan, granitic magmatism age of one metadolerite was dated at 0.96 Ga by zircon in contact across the allochthons, and terminating with intrusion of 0.93–0.92 Ga melt veins (Söderlund et al., 2008b); another gave a c. 0.93 Ga Sm–Nd anorthosite, norite, mangerite and charnockite (Fig. 1; Andersen et al., whole rock model age (Johansson and Kullerud, 1993). Thus, available 2001; Bingen et al., 2008c; Vander Auwera et al., 2011). In the data suggest that some metadolerites correlate with the 0.97–0.94 Ga Idefjorden Terrane, late- to post-orogenic magmatism produced granite Blekinge–Dalarna Dolerite swarm. The youngest rocks in the Eastern sheets (0.92 Ga), and minor E–W-trending bodies of anorthosite–norite, Segment are undeformed, discordant 0.96–0.94 Ga pegmatitic–granitic 150 C. Möller et al. / Lithos 226 (2015) 147–168

Fig. 2. Airborne magnetic anomaly map of south-westernmost Sweden (data source: Geological Survey of Sweden). Eclogite-bearing terrane = shaded white (provisional northern boundaries); augen gneiss = black.

dykes (Andersson et al., 1999; Christoffel et al., 1999; Möller and 3. Architecture and setting of the eclogite-bearing nappe Söderlund, 1997; Möller et al., 2007). The Eastern Segment also preserves a pre-Sveconorwegian high- The architecture and setting of the eclogite-bearing terrane have grade metamorphic evolution that primarily involved migmatization been documented in detail in one northern and one southern study at 1.47–1.38 Ga (Hallandian orogeny; Hubbard, 1975; Christoffel area (Fig. 2). The northern area illustrates a transition from the Mylonite et al., 1999; Söderlund et al., 2002; Möller et al., 2007). Hallandian Zone, across the contact between the Idefjorden Terrane and the Eastern metamorphic conditions are largely poorly known due to the Segment, to the eclogite-bearing nappe. The southern area shows the Sveconorwegian overprint (cf. however Ulmius et al., in review). The transition from inner parts of the eclogite-bearing nappe, across a robust U–Pb zircon isotope system nevertheless preserves age records basal shear zone, and into underlying gneisses of the Eastern Segment that can be linked to partial melting during this event (Möller et al., (Fig. 3, Table 1). 2007). The late- to post-orogenic Hallandian stage at 1.42–1.38 Ga in- Most of the bedrock mapping that forms the basis for this paper was volved intrusion of granite–monzonite and subordinate charnockite performed during 2003–2011, partly during mapping programmes and mafic rocks (Åhäll et al., 1997; Andersson et al., 1999; Christoffel conducted by the Geological Survey (at scales 1:250,000 and et al., 1999; Hubbard, 1989), locally associated with charnockitization 1:50,000). Detailed mapping in targeted areas was done at scales of the host gneisses (Harlov et al., 2006; Hubbard, 1989; Rimsa et al., 1:10,000 and 1: 12,500. Coordinates of key localities are given in 2007). supplement (Table S1). C. Möller et al. / Lithos 226 (2015) 147–168 151

Fig. 3. E–Wprofile constructed by down-plunge projection of the southern study area (Fig. 2), including magnetic anomaly pattern (a), and structural interpretation (b).

3.1. Overview 3.2.3. Eclogite nappe, inner part Most of the eclogite-bearing nappe is composed of extensively Retro-eclogite bodies are exposed in an irregularly shaped area migmatitic, stromatic gneiss, with garnet- and clinopyroxene-rich approximately 75 × 25 km large (shaded white in Fig. 2). Most of this mafic boudins in various states of amphibolitization (Figs. 4–6). This domain consists of stromatic migmatitic orthogneiss, with eclogite distinct association can be traced continuously for several tens of occurring as pods, lenses and layers. The shape of the eclogite-bearing kilometres. terrane in the horizontal section, and the fold pattern seen in the mag- The stromatic gneisses are penetratively deformed with leucosome netic anomaly map, are the result of superposed folds with axes that veins defining a gneissic layering (Fig. 5a–c). They are texturally and are subhorizontal or moderately plunging. The eclogite terrane is compositionally homogeneous suggesting a magmatic origin. Relict enclosed within, and concordant with, the regional structures of the protolith textures (uneven grain size, K-feldspar augen) are rare. Eastern Segment, but its western boundary is located within the Greyish varieties dominate (granitic and quartz-monzonitic–quartz- Mylonite Zone. monzodioritic compositions). Protolith zircon has been dated at c. 1.69 Ga (Andersson et al., 2002; Hegardt et al., 2005). Migmatization, fi 3.2. Lithotectonic components associated with amphibolitization of ma c boudins, was dated at 968 ± 13 Ma (Andersson et al., 2002; Fig. 5b). Leucosome in stromatic Six main lithotectonic components, comprising a crustal stack, have migmatite is typically tightly folded (Fig. 5d), but minerals in the been distinguished (Figs. 2, 3, Table 1). Transecting down from the leucosome are unstrained, exhibit uneven grain size, and form a char- Idefjorden terrane into the internal section of the Eastern Segment acteristic coarsely sugary texture. Patch leucosomes cross-cut the these components are: 1) Idefjorden orthogneiss 2) Eastern Segment folded stromatic foliation (Fig. 5e). Well-developed stretching linea- orthogneiss, above the eclogite nappe, 3) Eclogite nappe, inner part, tions are rare overall, but more common towards the basal shear 4) Eclogite nappe, basal shear zone, 5) a distinctive augen gneiss zone (below). fi b N fi (black in Fig. 2), and 6) Eastern Segment orthogneiss, enveloping the Ma c boudins range in size from 1to 50 m. Ma crockswerecom- eclogite nappe. monly competent during deformation, as evidenced by boudinage and a low degree of internal strain. The stromatic banding in the host rock en- velops concordantly around boudins demonstrating high strain (Fig. 5a). 3.2.1. Idefjorden orthogneisses The least retrogressed boudins preserve coarse-grained eclogite assem- The northern study area (Figs. 2, 4) covers a part of the Idefjorden blages of clinopyroxene, garnet, quartz and rutile (Fig. 5f). Conspicuous Terrane composed of pervasively schistose meta-granitoids. Feldspar 1–3 cm long clinopyroxene with micro-scale plagioclase inclusions is porphyroclastic and only partly recrystallized; metamorphic mineral forms a characteristic texture. This high-pressure mineral assemblage parageneses are typically rich in biotite, muscovite, titanite and epidote. has been amphibolitized at contacts with the surrounding gneiss and Migmatitic rocks are absent. Metabasic rocks are amphibolites with along fractures, in places penetrating far into the boudins. amphibole, plagioclase, biotite, epidote, and titanite. The eclogite terrane differs from other parts of the Eastern Segment in its pervasive stromatic migmatite layering with higher melt propor- 3.2.2. Eastern Segment orthogneisses, above the eclogite nappe tions and the presence of retro-eclogite. The stromatic migmatites Above the eclogite-bearing nappe are penetratively gneissic and within the eclogite nappe are also more retrogressed than surrounding folded meta-intrusions of dominantly granitic–quartz-monzonitic orthogneisses (Section 3.2.6 below). They typically occur with tartan- composition. They record deformation under amphibolite-facies condi- twinned microcline, pale-brown biotite, green to bluish-green horn- tions and incipient partial melting. Relict igneous textures (uneven blende, titanite and epidote. Similarly, many mafic boudins have been grain size, porphyritic texture) occur in places. Feldspar augen (micro- completely amphibolitized and consist of hornblende, plagioclase, bio- cline) are recrystallized to fine-grained strung-out aggregates. Mafic tite and titanite. Fine-grained greenschist-facies minerals (chlorite, rocks are amphibolites, in places garnet-bearing. muscovite) occur locally. 152 C. Möller et al. / Lithos 226 (2015) 147–168

Table 1 Tectonostratigraphy (top to bottom).

Segment/terrane Compartment Lithotectonic component Maps Photographs

Idefjorden Terrane 1 Idefjorden orthogneisses Figs. 2; 4 Eastern Segment Transitional-internal section 2 Eastern Segment orthogneisses, above the eclogite nappe Figs. 2; 4 Internal section 3 Eclogite nappe, inner part Figs. 2; 4; 6 Figs. 5, 8, 9 Internal section 4 Eclogite nappe, basal shear zone Figs. 2; 6 Figs. 7a–d, h–i Internal section 5 Augen gneiss, footwall of the eclogite nappe Figs. 2; 6 Figs. 7e–g, j; 10 Internal section 6 Eastern Segment orthogneisses, wrapping the eclogite nappe Figs. 2; 6

3.2.4. Eclogite nappe, basal shear zone The basal shear zone contains abundant layers and lens-shaped bodies A highly ductile basal shear zone, ranging in width from a few hun- of eclogite and kyanite-bearing eclogite, the largest 2 × 1.5 km (Ammås, dred metres to 4 km has been traced from the coast and eastwards along Fig. 6b). The margins are strongly deformed and amphibolitized, where- strike for c. 50 km (Fig. 6). as central parts expose better-preserved low-strain domains. Medium- The shear zone is defined by high-temperature mylonitic gneisses to coarse-grained Mg–Al-rich kyanite eclogite and Fe–Ti-rich quartz- alternating with mafic rocks and strongly strained orthogneiss. The bearing eclogite, the latter rich in rutile and garnet, occur in the large mylonitic gneisses are dominantly felsic and finely banded with lens at Ammås. Both types form dm- to m-thick layers (Fig. 7c) within mafic layers and lenses of variable size (Fig. 7a). Mylonitic gneisses kyanite-free and commonly quartz-bearing retro-eclogite. Locally, gar- are characterized by quartz-ribbons b 1 mm-wide and tens of mm net crystals are ≥2 cm. Coarse (2 cm) kyanite occur in quartz-rich long, and stretched domains of recrystallized feldspar a few mm wide. veins, locally N1 dm wide, and rich in garnet and rutile (Fig. 7d). Matrix Porphyroclasts include feldspar, garnet, or hornblende (c. 5 mm, locally clinopyroxene is dull greenish grey with expelled blebs of plagioclase larger). Mafic layers are finely banded with porphyroclasts of garnet, typical of decompressed omphacite. Smaller eclogite bodies are variably hornblende, plagioclase, and, locally, clinopyroxene. Some retro- retrogressed to amphibolite. The metamorphic evolution of the Ammås eclogitic tectonites are rich in porphyroclastic garnet, b30% (Fig. 7b). eclogite and host gneisses was summarized as (Möller, 1998, 1999): Highly strained 1.7 Ga orthogneiss and 1.46–1.40 Ga leucocratic granitic dykes are present (Söderlund et al., 2002). The high-temperature defor- 1. Prograde high-P metamorphism with peak pressure in the kyanite mation structures and fabrics formed under granulite- and upper eclogite-facies. This stage was dated by zircon in prograde garnet at amphibolite-facies conditions (Möller, 1999; Möller et al., 1997). 972 ± 14 Ma by Johansson et al. (2001).

Fig. 4. Northern study area. Background is magnetic anomaly map (total field). Lithotectonic units (west to east): shaded yellow = Idefjorden orthogneisses; shaded white = eclogite nappe. Symbols mark gneissic foliation/layering and stretching lineation; M = mylonitic zones (s.s.). C. Möller et al. / Lithos 226 (2015) 147–168 153

Fig. 5. Inner parts of the eclogite nappe. (a) Stromatic migmatite gneiss with mafic boudins. (b) Close-up of stromatic gneiss (dated, Andersson et al., 2002) with small amphibolite boudin. (c) Stromatic migmatite (dated, this study). (d) Folded (F2) stromatic gneiss with unstrained leucosome texture. (e) Cross-cutting patch leucosome with indistinct boundaries (same lo- cality as (d)). (f) Quartz-bearing eclogite (dated, this study) with coarse green clinopyroxene, fine-grained red garnet and black hornblende.

2. High-T decompression and deformation at high- to medium-P The granite–quartz-monzonite is heterogeneously deformed, with granulite-facies (10.5 ± 2 kbar and 770 ± 50 °C) characterized transitions over short distances between undeformed (Fig. 7e), recrystal- by symplectitic intergrowths. Sapphirine + corundum + anor- lized augen gneiss (Fig. 7f), proto-mylonitic–mylonitic gneiss (Fig. 7g), thite formed after kyanite. Omphacite expelled plagioclase, and and migmatite gneiss. The heterogeneous response to deformation symplectitic orthopyroxene + plagioclase formed along rims. Garnet reflects the strong competence of the protolith, with coarse (3–8cm) was weakly resorbed and mantled by thin plagioclase rims. tabular orthoclase megacrysts forming an interlocking texture, and 3. Amphibolite-facies retrogression and deformation. Associated low contents of micas and quartz (Fig. 6e). partial melting in felsic and maficrocks. The mineral assemblage of the granitic rocks represents amphibolite- 4. End of penetrative ductile deformation and intrusion of post- facies with hornblende, biotite and garnet; clinopyroxene occurs in deformational pegmatite at 0.96–0.94 Ma (Andersson et al., 1999; quartz-monzonitic compositions (Andersson et al., 1999). Enclosed Möller and Söderlund, 1997). Cooling through the amphibolite- metadolerite boudins show a relict igneous texture, recrystallized into facies. granoblastic garnet, clinopyroxene, plagioclase, biotite, hornblende, 5. Local micro-scale greenschist-facies imprint, e.g. fine-grained chlo- ilmenite, and quartz, representing high-pressure granulite–upper am- rite and muscovite. phibolite conditions.

3.2.5. Augen gneiss 3.2.6. Eastern Segment orthogneisses, enveloping the eclogite nappe Two large bodies of 1.42–1.38 Ga orthoclase megacrystic granite– The orthogneiss complex structurally below augen gneiss consists of quartz-monzonite are located at the coast (Torpa granite, Hubbard, a number of components characteristic of the internal section of the 1975) and 50 km east thereof (Tjärnesjö granite, Andersson et al., Eastern Segment (Section 2.2). The main lithology is moderately- 1999; Fig. 6a). The western body occurs beneath the eclogite-bearing discretely migmatitic, granitic orthogneiss. Relict igneous protolith tex- terrane and connects (semi-continuously) along strike with the eastern tures are commonly recognizable (porphyritic or coarse uneven- body. grained texture). Felsic dykes occur locally. Strain is variable; leucosome 154 C. Möller et al. / Lithos 226 (2015) 147–168 C. Möller et al. / Lithos 226 (2015) 147–168 155 is commonly deformed, parallel with the gneissic layering, and locally refold mylonitic and gneissic D2 structures (Fig. 7j). F3 folds are isoclinally folded. As a rule, pre-Sveconorwegian leucosome is deformed commonly gently plunging but variably orientated, partly de- with stretched and flattened mineral domains while Sveconorwegian pending on the orientation of earlier structures. The map pattern leucosome is coarsely sugary and uneven-grained. Concordant with and measured foliation data indicate dominantly E–SE-vergence, the felsic orthogneisses are lenses and sheets, up to 2 km thick, of although S-vergent inclined folding of S2 mylonitic foliation has migmatitic and garnet-rich amphibolite. There are also local occur- been recorded. rences of garnet-rich metadolerite and metagabbro with relict igneous D4 The latest fold phase produced gentle wavy folds on undulating, texture and well-equilibrated high-pressure granulite-facies assem- dominantly N–S trending, axes. Distinction between F3 and F4 blage, similar to metadolerite in augen gneiss (Section 3.2.5). is locally difficult. Orthogneisses underlying the eclogite nappe have a better- preserved high-grade metamorphic mineralogy than the pervasively migmatitic gneisses in the inner part of the eclogite nappe. Feldspar is The first two deformation phases were near-penetrative and pro- perthitic orthoclase and antipertitic plagioclase. Fe–Mg phases are duced the layering and rootless fold hinges seen in outcrop and in the brownish hornblende, garnet, dark red biotite and oxides, locally meta- airborne magnetic map pattern (Fig. 2). Tight similar folds, locally morphic clinopyroxene and occasionally orthopyroxene. with markedly angular hinges, are present throughout the internal Eastern Segment. The eastward plunge of F2 axis at Ätran (Fig. 6a, 3.3. Structure b) suggests that the eclogite terrane dips down beneath the present exposure level, and that it is present beneath the Tjärnesjö augen gneiss 3.3.1. General structure (Figs. 2, 6) and the orthogneisses immediately east thereof. We Along its length, the southern contact zone of the eclogite nappe dips interpret the large-scale recumbent fold nappe as generally non- north (Fig. 6a; N–NW west of Kungsäter, east thereof N–NE–E). These cylindrical; the location of its eastern tip beneath the present erosion measured surface observations concur with seismic reflectors dipping surface is however undetermined. Augen gneiss is interpreted to have shallowly north (Lundberg and Juhlin, 2011; 14 km across-strike profile enveloped the eclogite nappe above the present erosion level. Subse- over Gällared, Fig. 6b). Fig. 3 illustrates the geometry in E–W section, quent deformation (SE-vergent D3 and gentle D4) gave rise to the fold using down-plunge projection of Fig. 6a (along F4 fold axis plunging interference pattern seen on map scale (Fig. 2). D4 folds have wave- 18° N). The augen gneiss outlines a 50 km long, refolded recumbent lengths of tens of kilometres and include the gentle N–Strending east-vergent fold, enclosing the migmatitic eclogite-bearing nappe. antiform along the contact with the Idefjorden Terrane and the corre- A four-phase deformation sequence, identified from the map pattern sponding synform at Tjärnesjö (Figs. 2–3, 6). The variation in plunge and outcrop-scale structures, is summarized below (Fig. 3b): of L1–L2 lineations and F2 fold axes along the basal shear zone (gently WNW west of Kungsäter (Fig. 6), gently ESE–Eeastthereof)reflects D1 Formation of a structural succession including the eclogite ter- late folding. rane, augen gneiss, and surrounding orthogneisses. This event took place under high-pressure granulite- to upper amphibolite- facies conditions at 35–40 km depth. Pre-Sveconorwegian 3.3.2. Northern study area migmatite structures and cross-cutting structural relations The northern study area transects the Mylonite Zone, from the base (e.g. 1.46–1.40 Ga granitic dykes) were transposed into near- of the Idefjorden Terrane, across the terrane boundary, and into the parallelism with the gneissic layering (S1). Competent rocks Eastern Segment (Fig. 4). (eclogite, metagabbro and megacrystic granite) were dismem- Rocks in the Idefjorden Terrane have penetrative north–south- bered to form lenses or attenuated to layers. Top-to-the-east striking, subhorizontal or shallowly (10–20°) west-dipping schistose shear localized along the basal shear zone and in the augen foliations. Stretching lineations plunge shallowly WNW (Fig. 4b). C–S- gneiss footwall (Fig. 7f–g). The sequence with an eclogite- fabrics are well developed; together with asymmetric wings on K- bearing unit overlying eclogite-free equivalents requires that feldspar augen these fabrics indicate top-to-the-west sense of shear, the tectonic contact between them is a thrust. i.e. normal-sense, down-dip, westward transport of Idefjorden units. D2 Recumbent F2 folding of the layered tectonic pile produced The youngest ductile structures are discrete mm-wide, brittle–ductile outcrop-scale tight–isoclinal folds; open–gentle folds, plunging deformation zones that are parallel or slightly discordant to the schis- gently E or W, characterize some areas (Fig. 4). The km-scale tosity. These late discrete deformation zones are commonly chlorite- hinge zone at the southeasternmost exposed tip of the nappe rich with a crenulation lineation. Increasing metamorphic grade (Ätran, Fig. 6), represents an F2 fold. F2 folding was contempora- towards the contact to the Eastern Segment is shown by the transition neous with partial melting in the eclogite nappe (Fig. 5d), pro- to dominantly gneissose foliations and sparse veins formed by incipient duced similar-style flow folds or, in high-strain domain, folds partial melting. At the terrane boundary are discrete (bcm-wide), late with markedly angular hinges (dependent on rock competence; phyllonites, which are parallel or low-angle discordant to the gneissic Fig. 7h–i). D2 fabrics were produced in strongly strained do- foliation. mains, notably in the basal shear zone. These include stretching The westernmost part of the Eastern Segment (structurally part of lineation (L2) parallel to F2 fold hinges, axial-planar foliation the Mylonite Zone) consists of strongly deformed orthogneisses with (S2). Shear fabrics in deformed coarse white leucosome (Fig. 7i; shallowly west-dipping ductile foliation. A few kilometres east of the in moderately strained domains locally cutting F2 folds) demon- contact with the Idefjorden Terrane, gneissic (S1) foliations outline strate that shear continued top-to-the-east throughout D2. kilometre-scale east–west trending folds (inferred F2) with shallowly Metamorphic conditions were similar to D1. west-plunging axes (Fig. 4). Eclogite-bearing stromatic grey gneiss D3 The D1–D2 structures were refolded on map scale by open– appears 6 km east of the terrane boundary where it occurs in a gentle gentle F3 folds (Figs. 2–3, 6). Outcrop-scale, parallel (class 1B) synform with a shallowly west-plunging fold axis. Eclogite boudins and concentric, generally inclined–upright, open–tight folds appear evenly distributed in the fold hinge and the limbs. Mineral

Fig. 6. Southern study area. Background is magnetic anomaly map (total field). Shaded light white = eclogite nappe, inner part, denser white = eclogite nappe, basal shear zone, Black = augen gneiss, shaded beige = Eastern Segment orthogneisses, enveloping the eclogite nappe. Symbols mark gneissic foliation/layering and stretching lineation. (a) Structures across the basal contact of the eclogite nappe. Lineations marked with plunge. KL = Klosterfjorden, TO = Torpa granite; K = Kungsäter, A = Ammås TJ = Tjärnesjö granite, ÄT = Ätran. (b) Close-up of the basal shear zone with augen gneiss lenses around F2 hinge. 156 C. Möller et al. / Lithos 226 (2015) 147–168 C. Möller et al. / Lithos 226 (2015) 147–168 157 aggregate stretching lineations and small-scale folds are parallel to the making up c. 25 vol.% of the rock, and some quartz. Thin sections synform axis. (Fig. 8a) show garnet grains scattered in a matrix of larger-sized clinopyroxene crystals with abundant plagioclase inclusions, horn- 3.3.3. Southern study area blende (some of which are partial replacements of clinopyroxene) and Augen gneiss forms the footwall and eastern border of the eclogite quartz. Single scattered biotite flakes are present and are in textural nappe; it is continuously exposed eastwards from the coast, outlining equilibrium with matrix hornblende. Anhedral opaque grains, domi- an antiform (Fig. 6a, Kungsäter), but east of the antiformal crest nantly ilmenite, are present throughout the sample and commonly displaced 2 km southwards by a late NNE-trending fault. East of the surrounded by a thin rim of plagioclase. Zircon grains occur in the ma- fault, the augen gneiss forms a continuous sheet for 14 km. It disappears trix and as inclusions in garnet. Compositional zoning in garnet shows in the eastern limb, being either pinched out or soil-covered, and is slightly increasing pyrope-contents from core to rim, implying prograde missing to the south and east. Sheared layers of augen gneiss, a few growth. The shape of garnet grain boundaries indicates resorption and dm thick, occur at two outcrops two kilometres south of the large the garnet grains are surrounded by b0.1 mm-wide plagioclase collars eclogite lens at Ammås (Fig. 6b). Well-preserved orthoclase mega- (Fig. 8b). Along the outermost 50 μm wide rim of garnet, there is a con- crystic granite exposed in a two-metre thick lens 7 km farther southeast trasting chemical zonation with drop in the contents of grossular and is interleaved with mylonitic gneisses in the basal shear zone. Five pyrope. Clinopyroxene includes blebs or rods of plagioclase and tiny kilometres farther east, two augen gneiss lenses (10–100 m) appear in grains of hornblende and quartz. the lower limb of the F2 fold at Ätran (Figs. 6b, 7g). A third lens is Sample JAN050326A is dominated by fine-grained greyish green located in the hinge, proving structural connection with Tjärnesjö clinopyroxene, black amphibole, up to mm-sized crystals of red garnet augen gneiss. (c. 20 vol.%), and a few scattered, 1–2 mm quartz grains. Scattered Gneissic layering (S1) dips generally north, but vertical and southern clinopyroxene megacrysts (b8 mm) contain bleb-like inclusions of dips are present due to F2 tight folding along E–W fold axes. Variations plagioclase and lesser amounts of small grains of hornblende and in the orientation of gneissic layering are also due to F3–F4 cross folding. quartz. Zircon occurs as small grains in the matrix and as inclusions in Gneissic S1 layering outlines the km-scale F2 hinge at Ätran (Fig. 6b): garnet. Compositional zoning in garnet is similar to JAN050261C. poles to S1 define a β (fold) axis identical with the orientation of outcrop-scale F2 folds. Meso-scale folds are cylindrical with fold axes 4.1.1.1. Interpretation. The clinopyroxene–plagioclase intergrowth, plunging shallowly east, and fold axes are parallel to the L1–L2 with subordinate amounts of hornblende and quartz, are interpreted stretching direction. as pseudomorphs after omphacite. Area analyses of clinopyroxene In the basal shear zone (Fig. 6b), F2 folds are dominantly south- with inclusions yield apparent compositions with 24–32 mol% jadeite. vergent (Tual et al., in review). Asymmetric structures, mainly winged The intergrowth formed by breakdown of jadeitic–tschermackitic com- boudins and porphyroclasts, suggest top-to-the-east shear with dextral ponents of pyroxene during decompression. shear in steeply dipping layers. Chemical zoning of garnet cores (decreasing spessartine and Fe/Fe + Mg-ratio) reflects prograde growth; the small size of individual garnet 4. Geochronology and petrography crystals and the small amplitude of the zoning suggest, however, that original growth zoning has been modified by diffusional relaxation. Aims of the geochronology were to obtain data from unknown parts Zoning at outermost rims (decreasing grossular, increasing Fe/Fe + of the eclogite-bearing terrane and to provide tighter age constraints for Mg-ratio and spessartine) suggest decompression, cooling and resorp- the eclogite-facies metamorphism. Four localities were chosen: two tion of garnet. eclogite boudins and a stromatic migmatite gneiss inside the eclogite The textures suggest an original eclogite-facies assemblage compris- terrane, and augen gneiss adjacent to the basal shear zone; sample ing omphacite, garnet, quartz, rutile, and possibly hornblende, and locations are marked in Figs. 2, 4 and 6b. Below is a summary of the that all present plagioclase formed by decompression after the peak petrography, zircon characteristics, and U–Th–Pb analytical results of metamorphic pressure. There are no indications of ultrahigh pressure each sample. Coordinates of sample locations, detailed documentation (i.e. radial fracture pattern or palisade texture around quartz inclu- of the petrography and geochronology, and a description of analytical sions). In addition to recording decompression, the rocks were partially methods are provided in Supplementary files (Text-file S1, Fig. S1, retrogressed to the assemblage hornblende + andesine plagioclase + Tables S1–S3). ilmenite + biotite at upper amphibolite-facies. Field relations suggest that amphibolitization was coeval with partial melting in the surround- 4.1. Eclogite boudins in stromatic gneiss ing orthogneisses, and local partial melting in the retroeclogites them- selves. The local replacement of garnet by coarsely symplectitic biotite, Two different eclogite boudins in the northern part of the eclogite plagioclase and quartz probably reflects partial melting. fold nappe were sampled (Figs. 2, 4): a near isotropic non-migmatitic boudin (sample JAN050261, Fig. 5f) and a homogeneous non- 4.1.2. U–Pb geochronology migmatitic layer in a large (N50 m) partly veined and compositionally Samples JAN050261 and JAN050326 are both rich in zircon and the layered boudin (JAN050326). Both boudins are set in stromatic morphology, and internal textures of the zircons are nearly identical. migmatite gneiss. The zircon is subhedral to near euhedral, colourless, translucent, and generally devoid of cracks and inclusions. Grain size and shape vary, 4.1.1. Petrography with c-axes 20–200 μm long and length:width ratios of 1–3. Long Sample JAN050261C is composed of medium-grained greyish green prismatic grains are generally large; short prismatic grains are found clinopyroxene, black amphibole, mm-sized crystals of red garnet, in all sizes. Polished sections show texturally simple zircon and crystals

Fig. 7. The basal shear zone and augen gneiss. (a) Felsic mylonitic gneiss with amphibolitized eclogite lenses partly attenuated to thin bands, stretched leucosome, and porphyroclasts (garnet, feldspar, hornblende). Lineation-parallel, subvertical surface. (b) Garnet-rich mylonitic retro-eclogite. Lineation-parallel, vertical surface. (c) Compositional layering: garnet- rich, quartz-bearing eclogite (left); kyanite-rich eclogite (right). (d) Kyanite- and garnet-rich vein in coarse-grained kyanite-bearing eclogite. (e) Orthoclase-megacrystic quartz- monzonite. (f) Granitic augen gneiss. Recrystallized K-feldspar augen, quartz-ribbons, and asymmetric shear bands (top-to-the-east). Lineation-parallel, steep surface. (g) Mylonitic augen gneiss in lower F2 limb (Fig. 6b). Subvertical E–W-striking foliation, subhorizontal surface. (h) South-vergent angular fold (F2) outlined by thin mafic layers and quartz-feldspar veins in felsic mylonitic gneiss. White, originally coarse-grained leucosome (arrow) cross-cuts subparallel to the axial-plane. Lineation-perpendicular, vertical surface. (i) Tight shear folds (F2) in mylonitic gneiss with amphibolite layers, and strongly sheared, coarse-grained white leucosome (arrow). Lineation-perpendicular, vertical surface. (j) Late folding (F3) of mylonitic augen gneiss (thin dashed line = mylonitic foliation marked). White pen marks fold axis. Discordant undeformed pegmatite below coarse dashed line. Subvertical section. 158 C. Möller et al. / Lithos 226 (2015) 147–168

Fig. 8. Petrography and zircon geochronology of eclogites. (a) Photomicrograph of sample JAN050261; coarse clinopyroxene with plagioclase inclusions, garnet, quartz, and secondary hornblende and biotite. (b) Back-scattered electron (BSE) image [at central quartz grain in (a)]. Exsolution-like plagioclase in clinopyroxene and resorbed outer rims of garnet. (c) BSE (left) and CL (right) images of representative zircons showing analytical spots and 206Pb/238U-age. Analytical ID refers to Table S3. (d) and (e) Inverse concordia diagrams. Filled symbols = data used for age calculation. Coarse ellipses = concordia age. with core–rim textures. The latter have CL-bright cores, commonly Nineteen analyses of zircon in layered eclogite (JAN050326) define a with diffuse sector or irregular zonation (Fig. 8c). The rims are unzoned concordia age of 978 ± 7 Ma (MSDW = 1.7; Fig. 8e). Only one rim (CL- CL-dark, either with diffuse irregular zoning or sector zoning. Simple dark) was thick enough to analyse (n3473-03a). The other analyses hit grains are CL-dark with a distinct sector zonation, or unzoned to texturally coherent crystal sections precluding further subdivision. irregularly-zoned CL-bright. Twenty-two concordant analyses of zircon in isotropic eclogite 4.1.2.1. Interpretation. Concordia ages of zircon in the eclogites are (JAN050261) define a concordia age of 985 ± 4 Ma (MSWD = 1.2; 985 ± 4 and 978 ± 7 Ma. The internal textures and low Th/U-ratios of Fig. 8d). Spots in central parts of zircon crystals yield a concordia age zircon indicate a metamorphic origin. The eclogite protoliths were of 988 ± 6 Ma (MSWD = 0.89, N = 11), whereas analyses in marginal basic igneous rocks (gabbro, dolerite) probably devoid of free quartz. parts define a concordia age of 981 ± 7 Ma (MSWD = 1.2, N = 6). Th/U- It is inferred the zirconium was hosted by igneous minerals ratios are 0.01–0.05, with no distinct difference in U–Th chemistry (baddeleyite, ilmenite, pyroxene), and released to form zircon during between the central and marginal parts of the crystals. metamorphic reaction (cp. Bingen et al., 2001; Söderlund et al., 2004; C. Möller et al. / Lithos 226 (2015) 147–168 159

Morisset and Scoates, 2008; Beckman et al., 2014). The mineralogy and conf.; weighted average 207Pb/206Pb age = 1740 ± 10 Ma, MSWD = texture of the eclogites demonstrate complete metamorphic consump- 1.4). Fifteen analyses in discordant rims and simple crystal sections tion of igneous minerals. Zircon inclusions in zoned garnet suggest that with low Th/U-ratios ≤ 0.03 define a concordia age of 976 ± 6 Ma zircon growth was initiated during prograde metamorphism. Together, (MSWD = 1.6, 95% conf.; weighted average 206Pb/238U-age = 981 ± the data suggest that the dated zircon formed by the first pervasive 8 Ma, MSWD = 0.8), including unzoned U-rich (1000–4000 ppm) inter- metamorphic recrystallization of the protolith. mediate rims and U-poor (≤20 ppm) inner/outer rims and simple All eclogite zircons are late Sveconorwegian in age. Some grains are grains with sector zonation (Fig. 9e). texturally complex with distinct CL-bright cores overgrown by CL-dark rims (Fig. 8c). Zonation and CL-properties vary also in texturally simple grains. This suggests different stages of zircon growth at different 4.2.2.1. Interpretation. Oscillatory-zoned zircon cores in thinly and physicochemical conditions with progressive crystallization of zircon coarsely banded stromatic gneiss are dated at 1677 ± 10 Ma, and from centre towards margin. Central parts of zircon (CL-brighter) in 1733 ± 11 Ma, respectively. These ages represent crystallization of the the isotropic eclogite are dated at 988 ± 6 Ma, while rims (CL-darker) igneous protolith and are typical for orthogneiss of the Eastern Segment are 981 ± 7 Ma, supporting prolonged zircon growth. In situ zircon (Section 2.2). analysis and trace element data are required to ascertain the petrologi- Discordant rims and simple grains are dated at 969 ± 10 Ma (thinly cal context, e.g. linkage to garnet growth and plagioclase breakdown banded migmatite) and 976 ± 6 Ma (coarsely banded migmatite). The – (ongoing study). Meanwhile, it can be concluded that the texturally core rim textures, the CL/BSE-characteristics, and the U-contents, indi- and isotopically oldest zircon in the eclogite is dated at 988 ± 6 Ma, cates three stages of secondary zircon growth, suggesting successive setting the maximum age for the eclogite-facies metamorphism. new zircon crystallization at evolving physicochemical conditions, but the data do not allow further chronological subdivision (large analytical 4.2. Stromatic migmatite uncertainties for spots in low-U zircon). In summary, all types of secondary zircon grew more voluminous in Two varieties of stromatic migmatite gneiss were sampled for dating thick leucosome, while oscillatory-zoned zircon dominates in the of partial melting in the northern part of the eclogite nappe (Figs. 2, 5c): leucosome-poorer thinly banded variety. These relations demonstrate fi thinly banded (b5 mm, sample DLL051429A) and coarsely banded (cm- a rm link between the volume of partial melt and crystallization of wide, DLL051429B). secondary zircon. Melt crystallization associated with partial melting is thereby dated at 976 ± 6 Ma. 4.2.1. Petrography The stromatic gneiss is light grey with a slight reddish tint (due to 4.3. Augen gneiss hematite staining). Gneissic layering is defined by leucosome layers – – varying in thickness from 2 4 mm in thinly banded parts, to 1 3cm Granitic augen gneiss was sampled at Ätran (Fig. 6b), to test correla- in coarser banded parts (Fig. 9a). Both varieties are rich in microcline, tion with 1.4 Ga granite, and investigate if secondary zircon had formed quartz and plagioclase, with biotite aligned in streaks parallel to the in mylonitic and migmatitic varieties. Three types were investigated: layering. The coarser variety has more distinct biotite selvages augen gneiss with orthoclase megacrysts (BD26-0), mylonitic augen b – ( 1 2 mm thick). The texture is unstrained (Fig. 9b). The amount of gneiss (BD26-1), and augen gneiss with sugary granitic leucosome – leucosome is visually estimated at 10 20% (Fig. 5c). (BD26-4).

4.2.2. U–Pb geochronology The external zircon morphology is similar in the two samples with 4.3.1. Petrography stubby to long prismatic subhedral crystals with slightly rounded termi- Augen gneiss at Ätran is greyish red, variably deformed and recrys- nations (length:width ratios of 1:1–2:1). They are mostly translucent tallized, and has a relict coarse-grained texture. Most orthoclase is re- and colourless, some with cracks, inclusions and/or yellowish interiors. placed by fine-grained red K-feldspar aggregates. Elongated domains Zircon in thinly banded stromatic migmatite (DLL051429A) shows of fine-grained aggregates define the foliation: red K-feldspar, whitish distinct oscillatory zoning in places cut by thin (b5 μm) envelopes of plagioclase, grey quartz, and dark hornblende. Fine-grained garnet and unzoned CL-bright (inner) and/or CL-dark (outer) rims (Fig. 9c, grain biotite occur in small amounts. The state of deformation and recrystalli- n5119-10a). CL-bright fragments or anhedral to euhedral simple grains zation varies over short distances (metres). that are unzoned, or with sector or fir-tree zonation, are also present The least strained variety (BD26-0) is foliated and recrystallized, and (Fig. 9c, grains n5119-01 and 13). preserves scattered (ca. 5%) grey Carlsbad-twinned orthoclase mega- Nine concordant analyses in oscillatory-zoned cores define a crysts, locally N4cmlong(Fig. 10a). Single strained remnants (b1cm) concordia age of 1677 ± 10 Ma (MSWD = 1.7, 95% conf.; weighted av- of igneous plagioclase are present. Dark mineral aggregates are erage 207Pb/206Pb age = 1680 ± 7 Ma, MSWD = 1.7). Seven concordant hornblende with lesser amounts of opaque minerals and biotite. Garnet analyses in discordant rim domains, or texturally simple crystal sec- is scattered, small (b0.3 mm), and rounded–anhedral. tions, define a concordia age of 969 ± 10 Ma (Fig. 9d; MSWD = 0.7; Textures are similar in mylonitic augen gneiss (BD26-1), but average weighted average 206Pb/238Uage=972±11Ma,MSWD=0.1). matrix grain size is finer (0.2–0.5 mm) and maficaggregatesformdis- Zircon in coarsely banded stromatic migmatite (DLL051429B) has tinct foliae, b1 mm thick and b5 cm long. Orthoclase and plagioclase oscillatory-zoned cores discordantly overgrown by thick (N50 μm) porphyroclasts are rounded and typically 3–5 mm across, locally rims (Fig. 9c). The oscillatory zonation is more altered and the discor- reaching 1 cm. Quartz-ribbons (coarsely recrystallized) envelope the dant rims more voluminous than in zircon in DLL051429A. The rims in- porphyroclasts. clude up to three growth zones showing large variation in luminescence The migmatitic augen gneiss (BD26-4) has a coarse gneissic foliation and U-content (≤100 times relative enrichment in U). The CL-bright with b1.5 cm-wide strain-free leucosome with irregularly intergrown inner rim, typically ≤ 10 μm wide, is mantled by a thick CL-dark microcline, quartz and plagioclase, and sparse, anhedral grains of horn- (U-rich) unzoned rim, in turn overgrown by a CL-bright, U-poor, blende (Fig. 10b). The mesosome consists of feldspar, quartz and ≤5 μm, often euhedral, rim (Fig. 9c). The U-rich rims are commonly biotite-rich foliae, hornblende and titanite. Pink discordant and unde- unzoned while the U-poor rims show sector zonation (in CL). formed granitic leucosome consists of sugary K-feldspar, plagioclase, Sixteen concordant analyses in oscillatory-zoned core domains quartz, and minor biotite (1–10 mm), similar to discordant patch define a concordia age of 1733 ± 11 Ma (Fig. 9e, MSWD = 1.7, 95% leucosome in other places (Fig. 5e). 160 C. Möller et al. / Lithos 226 (2015) 147–168

Fig. 9. Petrography and zircon geochronology of stromatic migmatite (Fig. 5c). (a) Coarsely banded stromatic migmatite DLL0501429B. (b) Photomicrograph (crossed polars) of leucosome, illustrating unstrained (except undulose extinction in quartz) and uneven-sized grains (microcline, plagioclase and quartz) with rounded–sutured grain boundaries. (c) BSE (left) and CL (right) images of representative zircons showing analytical spots and 207Pb/206Pb-age. Analytical ID refers to Table S3. (d) and (e) Inverse concordia diagrams. Filled symbols = data used for age calculation. Coarse ellipses = concordia age.

4.3.2. U–Pb geochronology (sample 26-0). Six concordant spots define a weighted average Zircon grains in augen gneiss (sample BD26-0) are clear, 207Pb/206Pb age of 1375 ± 30 Ma (MSWD = 3.6). colourless, subhedral, generally without fractures and inclusions, c. In migmatitic augen gneiss (sample BD26-4) the size and external 100–200 μm long, and with length:width ratios of 3:1. BSE-images zircon morphology are similar to those in non-migmatitic varieties show a weak, commonly diffuse, oscillatory zonation, locally with (26-0, 26-1). Internal textures are, however, different, including simple unzoned 10–N50 μm wide mantles (Fig. 10c). Seven analyses in unzoned and irregularly sector-zoned zircon, and oscillatory-zoned oscillatory-zoned or unzoned zircon define a concordia age of 1388 ± zircon cores with distinct 10–20 μm wide BSE-bright cross-cutting 7Ma(MSWD=1.2;Fig. 10d). Zircon crystals in mylonitic augen gneiss rims (Fig. 10c). Oscillatory-zoned cores define a concordia age of (sample BD26-1) are similar to those in the least strained augen gneiss 1387 ± 10 Ma (95% conf., MSWD = 0.17, N = 6; Fig. 10e). Unzoned C. Möller et al. / Lithos 226 (2015) 147–168 161 or sector-zoned rims and simple crystals with distinctly lower Th/U- Segment generally remain isotopically undisturbed in the absence of ratios (c. 0.01) define a concordia age of 956 ± 7 Ma (MSWD = 1.9, partial melting (Andersson et al., 1999, 2002; Möller et al., 2007). N=4). 5. Interpretation and discussion 4.3.2.1. Interpretation. Igneous crystallization of the granite protolith is constrained at 1388 ± 7 Ma by zircon in the best preserved augen gneiss 5.1. Protolith setting, character and timing of eclogite metamorphism (BD26-0). This age is within error identical to published ages for orthoclase megacrystic meta-intrusions (Torpa-Tjärnesjö-type) in the The eclogite terrane in the Eastern Segment of the Sveconorwegian Eastern Segment (Åhäll et al., 1997; Andersson et al., 1999, 2002; Province underwent a clockwise P–T evolution with peak-pressures in Christoffel et al., 1999; Petersson et al., 2013). the kyanite eclogite-facies and subsequent decompression through Rim overgrowths and newly crystallized simple zircon (low Th/U) in granulite- and upper amphibolite-facies (Möller, 1998, 1999). The max- the migmatitic variety (BD26-4) are dated at 956 ± 7 Ma. This zircon is imum pressure is unknown, but the conservative estimate of 15 kbar associated with leucosome, parts of which is discordant to the gneissic corresponds to a minimum depth of 55 km. There are no indications foliation, and dates melt crystallization after partial melting of augen of UHP conditions. gneiss. The homogeneity, protolith ages and bulk compositions of stromatic Secondary zircon is lacking in non-migmatitic varieties of augen gneisses in the eclogite nappe suggest that they originated from gneiss (BD26-0, BD26-1) despite advanced dynamic recrystallization 1.74–1.66 Ga protoliths similar to the surrounding Eastern Segment at high temperature. This is in accordance with other evidence that zir- orthogneisses. This implies that prior to eclogite-facies metamorphism, con in strongly deformed and high-grade felsic rocks in the Eastern the protoliths now comprising the eclogite nappe originated as mid- to

Fig. 10. Petrography and zircon geochronology of augen gneiss. (a) Augen gneiss (BD26-0), section oblique to the foliation. Grey orthoclase megacryst (arrow) is preserved in a recrystallized K-feldspar-rich matrix. (b) Photomicrograph (crossed polars) of leucosome in migmatized augen gneiss, demonstrating an unstrained texture. (c) BSE-images of representative zircon with analytical spots and 207Pb/206Pb-ages. Analytical ID refers to spot-ID in Table S3. (d) and (e) Inverse concordia diagrams. Filled symbols = data used for age calculation. Coarse ellipses = concordia age. 162 C. Möller et al. / Lithos 226 (2015) 147–168 upper-crustal levels of the same crustal block as the Eastern Segment. therefore be expected to exhibit relative extensional movement, The presence of 1.40–1.46 Ga granitic dykes supports this interpreta- accommodating top-to the-west shear. The roof zone is largely eroded tion, and the presence of 1.40 Ga granite dykes in eclogite (Johansson or poorly exposed; however, southern parts of the Mylonite Zone et al., 2001) pins the eclogite protoliths to their country rocks. There is record top-to-the-west kinematics (e.g., Berglund, 1997; Viola and no evidence of an “exotic” origin of the eclogites, i.e. tectonic incorpora- Henderson, 2010). Many of these structures are greenschist grade tion of subducted oceanic slab- or mantle-material. (Figs. 2, 4), but the lowermost level of the Idefjorden Terrane The clockwise path at high pressures and temperatures and the lith- (Klosterfjorden, Fig. 2), demonstrates syn-migmatitic shear structures ological association (high-grade orthogneisses with dismembered with top-to-the-west vergence (Andersson et al., 2002). This syn- eclogite bodies) are typical of eclogitized continental crust involved in deformational migmatization at Klosterfjorden was dated at 980 ± continental collision. These two features are comparable to e.g. high- 13 Ma, indicating a synchroneity with migmatization in the eclogite temperature northwestern parts of the Western Gneiss Region in the nappe, although more precise age data are needed to confirm this. Scandinavian Caledonides (subducted continental margin; e.g., Griffin The envelope of the augen gneiss around the southern part of the et al., 1985; Cuthbert et al., 2000; Hacker et al., 2010) or the high- eclogite-bearing fold nappe is an intriguing structure. Even where the pressure belts in the Grenville Province of eastern Canada (large hot augen gneiss has been pinched out, small boudins of augen gneiss, in orogen; e.g., Davidson, 1990; Indares et al., 1998; Rivers, 2009; Rivers places b 10 m long, occur in predicted locations along the tectonic con- et al., 2012). tact (Fig. 6b). The 1388 ± 7 Ma igneous crystallization age of augen A maximum age of 988 ± 6 Ma of the eclogite-facies metamorphism gneiss in the hinge of the recumbent fold confirms the correlation is set by a U–Pb concordia age of early zircon in isotropic eclogite, within with larger bodies of 1.42–1.38 Ga augen gneiss (Fig. 6a). The augen error similar to the 978 ± 7 Ma pooled age of zircon in layered eclogite. gneiss envelope may be explained by localization of the basal thrust of Both ages are slightly older, but more precise, than previous ages of the extruding eclogite nappe along a pre-existing zone of weakness, at eclogite metamorphism in the Eastern Segment (Hegardt et al., 2005; the contact of a sheet-like intrusion. The 1.42–1.38 Ga intrusions are Johansson et al., 2001). The new data shifts the interval for the late late to post-orogenic relative to the 1.47–1.38 Ga Hallandian orogeny, Sveconorwegian high-pressure and high-temperature metamorphism and may have injected along late-Hallandian extensional fault zones. (Falkenberg phase of Bingen et al., 2008c) to 0.99–0.98 Ga, indicating an overlap with the preceding Sveconorwegian metamorphism in 5.3. Exhumation and P–T–t–devolution allochthonous units (1.05–0.98 Ga Agder phase, op. cit.). The age of late Sveconorwegian orogenesis in the Eastern Segment, including the The exhumation history of the eclogite terrane involved two high-pressure metamorphism, correlates with the 1.00–0.98 Ga Rigolet distinctly different stages: the emplacement of the eclogite terrane at phase in the Parautochthonous Belt of the Grenville orogen in North 35–40 km during high-pressure granulite- to upper amphibolite-facies America (e.g., Rivers, 2009; Rivers et al., 2012). conditions, and subsequent exhumation of the entire southern (high- grade) Eastern Segment. The latter stage involved cooling through 5.2. The structure: a hot fold nappe amphibolite- and greenschist-facies. The P–T–t–d evolution is schemat- ically summarized in a pressure–time diagram (Fig. 11). The semi-continuous sheet of augen gneiss, and its contact to the The exhumation of the eclogite terrane up to mid-crustal levels of eclogite-bearing terrane, defines a 50 km long, refolded recumbent, the thickened orogenic belt (35–40 km) involved foreland-directed east-vergent fold, which encloses the migmatitic eclogite-bearing translation, recumbent folding and shearing (D1 + D2, Fig. 3b). The rocks. There is a systematic difference in metamorphism, where eclogite main part of this exhumation was related to tectonic extrusion, occurrences are restricted to the core of the fold, demarcated by the con- associated with extensive partial melting within the eclogite nappe, tact with augen gneiss. Another difference is the amount and character and moderate to discrete partial melting in the surrounding units. of leucosome, with inner portions of the eclogite-bearing nappe being Inclined–upright, southeast-vergent folding (F3, Fig. 3b) followed extensively migmatized (N15% leucosome) with pervasive stromatic, exhumation. Metamorphic deformation fabrics attest to high-pressure folded, coarsely sugary, and commonly uneven-grained leucosome granulite and upper amphibolite conditions. Amphibolite-facies partial that locally grades into undeformed pods of granitic pegmatite. This is melting has previously been constrained in the interval 0.98–0.95 Ga; in striking contrast to most of the surrounding orthogneisses of the in this study stromatic migmatization in the northern part of the Eastern Segment, in which Sveconorwegian migmatite veins are more nappe is dated at 976 ± 6 Ma and post-deformational partial melt in discrete. the southern fold hinge at 956 ± 7 Ma. These data demonstrate that The eclogite-bearing nappe is an exhumed, migmatized and largely partial melting prevailed for an extended time period (Fig. 11), retrogressed eclogite-facies orthogneiss complex with an up to 4 km and that a significant part of the melting post-dated the eclogite meta- wide basal shear zone. It is surrounded by a structurally concordant morphism. The 956 ± 7 Ma migmatization age of the augen gneiss is sequence of high-pressure granulite- to upper amphibolite-facies identical to that of a cross-cutting pegmatite in the basal shear zone orthogneisses and metabasic rocks which did not reach eclogite-facies (Möller and Söderlund, 1997), suggesting that the post-kinematic pressures. Partial preservation of eclogite assemblages inside the pegmatites were generated by the same partial melting event. nappe demonstrates the existence of different tectonometamorphic The deformation structures demonstrate protracted east-vergent units formed in the deepest part of the orogen. It also demonstrates transport during D1–D3, until about 0.96–0.94 Ga (onset of pegmatite that it is possible to identify different metamorphic terranes despite a dyke intrusion). Tectonic extrusion and emplacement of the eclogite general reworking at high temperature; the key is provided by meta- terrane on top of a higher level (lower pressure) crustal footwall (D1) morphic assemblages in domains that escaped pervasive dynamic must have been accomplished under plate convergence. Subsequent recrystallization. stages (D3, possibly D2) may have involved the transition from conver- The eclogite-bearing nappe was tectonically exhumed from N55 km gence to E–W extension, still during east-vergent translation, leading to depth and stalled at 35–40 km. Shear structures in the mylonitic further extensional faulting of the Idefjorden Terrane in the hanging- gneisses of the basal shear zone, and in the augen gneiss footwall, wall. demonstrate foreland-directed translation. The metamorphic relations Late exhumation involved the Eastern Segment in its entirety, across the tectonic contact, with higher pressure rocks overlying lower including the eclogite nappe, and led to cooling and uplift of the deep pressure equivalents, suggest that the basal shear zone is a thrust and crustal section. The onset is manifested as steep fracturing and intrusion that the fold nappe was exhumed by east-vergent extrusion. The of discordant granite, pegmatite and dolerite dykes. Zircon ages of bounding shear zone along the roof of the eclogite nappe would granitic pegmatites pin-point this event at 0.96–0.94 Ga. The rate of C. Möller et al. / Lithos 226 (2015) 147–168 163

Fig. 11. Schematic pressure–time diagram summarizing the P–T–t–d evolution of the eclogite-bearing nappe. Eclogite stage: blue box (this study), with conceptual sketch of the collisional stage. Exhumation to orogenic mid-crustal levels: conceptual sketch of nappe extrusion. Melt crystallization: red boxes = this study; white box with red frame = previous studies. Cooling: green and black frames (Page et al., 1996).

exhumation and cooling of the Eastern Segment was slow, with temper- clinopyroxene in leucosome in eclogitic rocks does indicate dehydration atures reaching 350 °C first at 0.9 Ga (Fig. 11). melting at relatively high pressures, possibly during early exhumation Two contrasting structural models have been proposed for the stages, but these leucosomes have not yet been thoroughly investigated. exhumation of the high-grade metamorphic Eastern Segment relative Decompression melting or melting due to fluid infiltration at mid- to its eastern foreland: a core complex model related to crustal-scale crustal levels enhanced and accompanied exhumation, but this melting E–W extension, and a transpression model related to crustal-scale did not demonstrably initiate exhumation at eclogite-facies conditions. E–W convergence. The concept of the southern Eastern Segment as a The source of the hydrous fluids is unknown, but deformation should large metamorphic core complex was first suggested by Andréasson have been important in creating pathways for fluid transport during and Rodhe (1990). It was based on down-dip east-vergent shear struc- the exhumation (possibly also during tectonic burial; cp. Jolivet et al., tures at the transition from high to lower metamorphic grades at 2005). the boundary between the transitional section and frontal wedge of The effects of partial melting on rock viscosity and strength is dra- the Eastern Segment (Fig. 1c–d). Later, Viola et al. (2011) outlined a matic (Rosenberg and Handy, 2005). Even at moderate melt fractions, similar core complex model integrating also the down-dip west- up to 7%, melt along grain boundaries reduces the rock strength by vergent shear structures along the southern Mylonite Zone. A contrast- several tens of %. This has severe implications for the dynamics of moun- ing, transpressional, tectonic model has been proposed by Wahlgren tain building, as partially molten crust will flow even under low stress. et al. (1994), based on the fan-like geometry of structures at the Melt-weakening can also trigger detachment of high-pressure terranes Sveconorwegian Front (transitional section–frontal wedge, Fig. 1c–d). if partial melting takes place at depth (e.g. Labrousse et al., 2011). Most Coincident with the eastward gradual decrease in metamorphic grade stromatic migmatite gneisses in the Eastern Segment eclogite nappe and the transition from penetrative to spaced deformation, the orienta- contain well above 7 vol.% leucosome (Fig. 5), suggesting the nappe tion of the foliation changes gradually from dipping east, through was weak and responsive to tectonic forces during east-vergent vertical, to west. The sense of shear is the same: top-to-the-east, sug- exhumation (D1–D2). The timing of onset of partial melting remains, gesting an eastwards squeezing up and out of the Sveconorwegian however, to be constrained. metamorphic crust. Hydrous fluids associated with melts promoted further retrogression Precise data on the timing of deformation are required to test the into epidote–amphibolite assemblages in the nappe. This is in contrast late exhumation models. One key question is whether the west- to surrounding orthogneisses (e.g. south of the nappe) where high- vergent structures along the southern Mylonite Zone represents the temperature assemblages are overall better-preserved. Sveconorwegian extensional roof fault of the extruding eclogite nappe (i.e. coeval with migmatization was less pervasive outside the eclogite-bearing terrane, D1 thrust structures in the eclogite-bearing nappe: 0.98–0.97 Ga), or and included also dehydration melting. Examples beneath the eclogite the extensional roof zone of an Eastern Segment core complex (regional nappe are mafic rocks in which leucosome carries coarse peritectic extension: 0.96 Ga or younger), or both. Another related question con- orthopyroxene or clinopyroxene. cerns the timing of normal- and thrust-sense deformation next to the Sveconorwegian Front. 5.5. Maficdykeintrusion

5.4. Character and effect of partial melting The orogen-parallel Blekinge–Dalarna Dolerite dyke swarm (Fig. 1c) intruded simultaneously with synorogenic high-temperature metamor- Amphibolite-facies water-saturated melting characterizes the phism in the Eastern Segment, at 0.97–0.94 Ga. N–S trending, cross- eclogite-bearing nappe. The field relations suggest that partial melting cutting dolerites are present across the southern Eastern Segment, was associated with extensive amphibolitization (locally granulite- have been metamorphosed at high grades, and record moderate defor- facies conditions) during decompression. Most leucosomes are devoid mation (D2–D4, Pinan-Llamas et al., submitted). The Eastern Segment of peritectic minerals, although leucosome in quartz-monzodioritic metadolerites that correlate with the Blekinge–Dalarna swarm must gneisses may contain hornblende. Locally, however, peritectic have been metamorphosed very shortly after intrusion. The regional 164 C. Möller et al. / Lithos 226 (2015) 147–168 extent of mafic dyking demonstrates that melting operated at mantle The flow pattern in Himalaya–Tibet provides an important concep- depth beneath the orogen, suggesting that mantle heat could have tual template for large collisional belts. Lower crustal flow may be partly also contributed to the budget for high-temperature metamorphism in decoupled from surface movements, but it is directed towards the fore- the Eastern Segment. Theoretically, the dykes could have formed during land under the Himalayas, and is inferred as orogen-parallel in response convergence, but the N–S vertical orientation of the dykes supports a to the convergence under the central Tibetan plateau, (e.g., Clark and case of post-convergent E–W extension, thereby constraining the set- Royden, 2000; Searle et al., 2011). In northernmost parts of the Tibetan ting for the late exhumation. plateau surface deformation is characterized by a combination of strike- slip and crustal thickening (e.g. McKenzie et al., 2014; Searle et al., 2011). 5.6. Collisional scenario The 100 km wide belt of foreland-directed transport in the Eastern Segment is in accordance with that under an orogenic front such as The cause of eclogite metamorphism c. 500 km inboard of the the Himalayas (Fig. 12). The foreland-directed E–W structural grain present western margin of the Sveconorwegian Province can be and flow folding in the Eastern Segment suggest that transport was discussed in terms of four broadly defined settings within a continental accommodated by highly ductile flow. Due to the lack of large-scale collision zone of Himalayan–Tibetan type: 1) continental subduction strike-slip zones the setting is not likely equivalent to the intra-plate (s.s.), 2) tectonic burial beneath the frontal part of an orogenic plateau, setting immediately north of the Tibetan plateau (south of the Tarim 3) intracratonic tectonic burial beneath the internal or distal part of a Basin). The doubly vergent kinematics in the Sveconorwegian orogen plateau, and 4) intracratonic tectonic burial beneath the lateral part of (Viola et al., 2011) precludes a setting at a lateral part of a plateau a plateau (Figs. 12, 13). (Shichuan basin, e.g., Cook and Royden, 2008). Neither is the setting In the first scenario, the Eastern Segment would represent a compatible with central parts under the Tibetan plateau. Sveconorwegian continental margin of Baltica that was subducted fol- Based on the near-foreland setting of the Eastern Segment, foreland- lowing the descent of a west-dipping oceanic slab beneath a colliding directed extrusion and flow, and absence of ultrahigh-pressure eclogite, continent (1 in Figs. 12, 13a). This is the scenario related to buoyancy- we suggest crustal thickening beneath the frontal parts of a plateau, driven exhumation of ultrahigh-pressure terranes (e.g., Western Gneiss similar to the present-day situation beneath the southern margin of Region in the Scandinavian Caledonides; Hacker et al., 2010; Warren, Tibet (star 2, Fig. 13b). We find no discriminating evidence for (nor 2013). The lack of ultrahigh-pressure metamorphism in the Eastern against) a true suture at the boundary between the Idefjorden Terrane Segment might speak against this setting, and a higher geothermal gra- and the Eastern Segment, but we regard the present data supportive dient (thereby weaker crust) in Precambrian times may have of it. counteracted subduction to extreme depths (Brown, 2014; Sizova et al., 2014). The second scenario pictures eclogite metamorphism by effective 5.7. Comparison with the Grenville orogen doubling of crustal thickness beneath the frontal part of a plateau, sim- ilar to the present-day situation beneath southern Tibet (2, Figs. 12, The Grenville orogen forms the core of supercontinent Rodinia in 13b). Also in this model Baltica is underthrusting, but it is the crustal classical plate reconstructions (Dalziel, 1997; Li et al., 2008), and the thickening itself that causes eclogite metamorphism. This setting may Sveconorwegian orogen was part of the same assembling system be envisaged for high-temperature overprinted 15 Ma eclogites (Fig. 1a). Comparison between the Grenvillian and Sveconorwegian exhumed immediately below the South Tibetan Detachment (Grujic orogens reveals striking similarities regarding the first-order et al., 2011; Warren et al., 2011), and it is the setting envisaged for the tectonometamorphic architecture and timing of orogenic stages. In the Grenville Province (e.g., Rivers, 2008). Convergent push-from-behind Grenville orogen, the main (Ottawan) orogenic phase affected the is needed for tectonic extrusion of the eclogitized crust in order to Allochthonous Belt at 1.09–1.02 Ga, after which metamorphism and de- bring it up towards the orogenic front. formation progressed into the Parautochthonous Belt at 1.00–0.98 Ga The margin of Baltica subducts in scenario 1 (Fig. 13a), with major (Rigolet phase, Rivers, 2009; Rivers et al., 2012); high-pressure meta- implications for the Sveconorwegian orogen: the Mylonite Zone morphism in the Allochthonous Belt was succeeded by high-pressure represents a collisional suture and the Sveconorwegian terranes west metamorphism in the Parautochthonous Belt. This progression is iden- thereof were parts of a micro-continent or a colliding continent, and tical in the Sveconorwegian orogen, where high-pressure metamor- not a coherent part of Baltica. Subduction polarity is less clear in scenar- phism in the allochthon (Idefjorden) is recorded at 1.05–1.02 Ga, io 2 (Fig. 13b). followed by high-pressure metamorphism at 0.99–0.98 Ga in the The third scenario is intra-plate subduction or thickening beneath Eastern Segment. In both orogens, high-pressure units occur in the the distal part of a plateau (3; Figs. 12, 13c). The Eastern Segment frontal regions, while low and medium pressure metamorphism is would be part of the overriding continent (Baltica), and the collision is characteristic of the hinterland. Also, the magmatic zoning is similar, driven by eastward subduction of another plate, N1000 km west. The with syn- and post-collisional magmatism confined to allochthonous setting corresponds to that beneath the northern parts of the Tibetan units and increasing in volume towards the hinterland (Fig. 1b). plateau (south of the Tarim Basin), and the Greenland Caledonides; The Grenville orogen is envisaged as a large, long-lived, hot orogen both are characterized by significant strike-slip (e.g. Gilotti and in which prolonged convergence gave rise to a wide orogenic plateau McClelland, 2007; Tapponier et al., 2001). and heating of thickened crust (Beaumont et al., 2006; Rivers, One critical question is whether or not the contact between the 2008). Coupled thermo-mechanical modelling, simulating the melt- Eastern Segment and the Idefjorden Terrane represents a collisional weakened orogenic belt, has reproduced its gross structure (GO model suture. There are no remnants of oceanic or arc material in this contact of Jamieson et al., 2007, 2010; Jamieson and Beaumont, 2011). The zone, but as envisaged by Brueckner (2009) westward subduction into model assumes a variably strong crust, weaker in its upper part, and a the mantle of the Baltica margin (Fig. 13a) would not only explain the lower crust weakest at the continental margin and successively stronger eclogite metamorphism in the Eastern Segment but also create a source into the craton. Ductile deformation is controlled by a combination of for the post-orogenic granite and AMCG magmatism west of the melt-weakening and incoming competent crust acting as tectonic Mylonite Zone. In this case the Sveconorwegian terranes west of the drivers. Outward flow of the orogen takes places by stacking of nappes, Mylonite Zone (coined “Sveconorwegia” by Brueckner, 2009) would progressing towards the foreland. In the deep crust, this results in large- represent a micro-continent or pieces of a larger continent with which scale thrust and fold nappes tectonically forced towards the foreland Baltica collided. and partially exhumed above crustal ramps. C. Möller et al. / Lithos 226 (2015) 147–168 165

Fig. 12. Schematic representation of continent Baltica (shade white), with the Sveconorwegian orogen (red), at alternative locations (scenarios marked 1–4) around a Tibetan-type plateau. See text for discussion. Composed satellite photograph of Asia in orthographic projection from Wikimedia Commons (source: NASA).

The model offers an explanation for foreland-directed extrusion of competent crustal ramp, the top of which is presently partly exposed melt-weakened crust. It is based on an extended period of high- as the footwall (Varberg area, Fig. 3). temperature metamorphism, with elevated temperatures deep under an orogenic plateau during several tens of millions of years, and repro- 5.8. Eclogite exhumation at depth in collisional orogens duces the long duration of Ottawan metamorphism (1.09–1.02 Ga) in the Allochthonous Belt, at high temperatures and intermediate to high Study of deeply eroded Precambrian shield areas can be thought pressures. of as lifting off the lid to currently inaccessible parts of modern The Rigolet phase is shorter (1.00–0.98 Ga), and in the model of orogens (e.g., St-Onge et al., 2006). Indication of melting at depth Jamieson et al. (2010) is perceived as post-convergent lateral spreading. beneath the Tibetan plateau (Unsworth et al., 2005) invites comparison Rivers (2008) questioned whether gravity-driven thrusting could drive to migmatized crustal sections such as those exposed in the the exhumation of high-pressure rocks in the Parautochthonous Belt Sveconorwegian and Grenville Provinces. Sections as that investigated and proposed continued convergence with active migration into the in this study (35–40 km) represent deep interiors not yet exposed in foreland after extensional collapse of the hinterland. There are thus un- young mountain belts like Himalaya–Tibet. The information on crustal certainties whether the GO model serves as an accurate representation sections at different depth can be compared with quantitative model- of the Rigolet phase and the P–T–t evolution of the Parautochthonous ling (e.g., Jamieson and Beaumont, 2013). high-pressure belt. The eclogite-bearing fold nappe in the Eastern Segment is to our The short time span for eclogite metamorphism and foreland- knowledge the first described deep section with a partially molten directed exhumation at high temperatures in the Eastern Segment fold nappe exhuming eclogitized continental crust. There may be (0.99–0.98 Ma) is virtually identical to that of the Rigolet phase. Direct similar structures in the exposed high-grade crust of ancient and comparison with the GO model is difficult as it refers to the long- modern collisional orogens not yet fully described or proposed as duration Ottawan phase; still we find the similarity striking between eclogite-exhuming low-viscosity nappes. For example, Robinson et al. the deep nappe structures in the GO model and observed structure in (2014) emphasized the importance of foreland-vergent recumbent the Eastern Segment. We envisage the weak and partially molten folding with bounding thrust and extensional zones in the eclogite- eclogite nappe be exhumed by push-from-behind, up over and onto a bearing Western Gneiss Region of Norway and in Trollheimen east 166 C. Möller et al. / Lithos 226 (2015) 147–168

Fig. 13. Schematic lithospheric sections illustrating alternative collisional scenarios and three fundamentally different settings (a–c) of eclogite metamorphism (at stars marked 1–3; see text for discussion). Figure modified from Tapponier et al. (2001) for the Himalayan–Tibetan orogen.

thereof, structures that in the Western Gneiss Region are partially Supplementary data to this article can be found online at http://dx. masked by later lateral shear and upright folding. Highly ductile fold doi.org/10.1016/j.lithos.2014.12.013. structures (e.g. Culshaw et al., 1994) and eclogite-bearing thrust nappes are also present in the Grenville orogen (Rivers et al., 2012). Acknowledgements In order to understand initial exhumation mechanisms at depth we need evidence from deeply eroded orogens. The eclogite nappe in the We acknowledge the many persons who have taken part in regular Sveconorwegian orogen demonstrates that foreland-directed flow of field mapping for the Geological Survey of Sweden (SGU) during partially molten crust within continental crust is a viable mechanism 2003–2011; M. Ekdahl and. F. Hellström provided significant contribu- for exhumation and emplacement of eclogite-bearing crust in deep tions. L. Lundqvist provided sample DLL051429, Fig. 5c, and unpub- sections of large hot orogens. lished map data (Kinna). Discussions with D. Gee, I. Lundqvist, L. Lundqvist, A. Pinan-Llamas, M. Stephens, and L. Tual are greatly acknowledged. Martin Whitehouse and his staff at the Nordsim- 6. Conclusions laboratory in Stockholm are thanked for technical guidance. The Nordsim facility is operated under a contract between the research 1. Eclogite-bearing crust in the Sveconorwegian orogen is structurally funding agencies of Denmark, Iceland, Norway, and Sweden, the Geo- bound inside a migmatitic recumbent fold nappe. The structure logical Survey of Finland, and the Swedish Museum of Natural History. provides an example of eclogite exhumation as a partially molten, This paper is Nordsim publication No 387. R. Litzell edited Fig. 1;B.Bing- low-viscosity nappe within deep crust in a collisional belt. en is acknowledged for sharing unpublished template for Fig. 1A, L. 2. The nappe records tectonic extrusion, melt-weakening-assisted ex- Johansson for comments to the manuscript. Careful reviews by W. humation and foreland-directed translation of previously eclogitized McClelland and T. Rivers improved the manuscript. Research was crust, stalling at 35–40 km depth. The case described in this paper funded by grants from SGU (35160 (CM), 80023 (JA), 60-1655/2009 demonstrates that extrusion of melt-weakened crust is a viable (CM)) and the Crafoord Foundation (20110817, CM). exhumation mechanism within hot and deep collision zones. 3. The maximum age of eclogite metamorphism is dated at 988 ± 7 Ma, References syn-deformational partial melting at amphibolite-facies conditions at 976 ± 6 Ma, and post-kinematic partial melting at 956 ± 7 Ma. Åhäll, K.-I., Samuelsson, L., Persson, P.-O., 1997. Geochronology and structural setting of the The high-pressure and high-temperature metamorphic event took 1.38 Ga Torpa granite; implications for charnockite formation in SW Sweden. GFF 119, 37–43. place at a late stage of the Sveconorwegian orogeny, synchronous Andersen, T., Andresen, A., Sylvester, A.G., 2001. Nature and distribution of deep crustal with the Rigolet phase of the Grenvillian orogeny in North America. reservoirs in the southwestern part of the Baltic Shield: evidence from Nd, Sr and C. Möller et al. / Lithos 226 (2015) 147–168 167

Pb isotope data on late Sveconorwegian granites. Journal of the Geological Society of Harlov, D.E., Johansson, L., van den Kerkhof, A., Förster, H.-J., 2006. The role of advective London 158, 253–267. fluid flow and diffusion during localized, solidstate dehydration: Söndrum Andersson, J., Söderlund, U., Johansson, L., Möller, C., 1999. Sveconorwegian (−Grenvillian) Stenhuggeriet, Halmstad, SW Sweden. Journal of Petrology 47, 3–33. deformation, metamorphism and leucosome formation in SW Sweden, SW Baltic Hegardt, E.A., Cornell, D., Claesson, L., Simakov, S., Stein, H., Hannah, J., 2005. Eclogites in Shield: constraints from a Mesoproterozoic granite intrusion. Precambrian Research the central part of the Sveconorwegian Eastern Segment of the Baltic Shield: support 98, 151–171. for an extensive eclogite terrane. GFF 127, 221–232. Andersson, J., Möller, C., Johansson, L., 2002. Zircon chronology of migmatite gneisses Hellström, F.A., Johansson, Å., Larson, S.Å., 2004. Age and emplacement of late along the Mylonite Zone (S. Sweden): a major Sveconorwegian terrane boundary Sveconorwegian monzogabbroic dykes, SW Sweden. Precambrian Research 128, in the Baltic Shield. Precambrian Research 114, 121–147. 39–55. Andréasson, P.-G., Rodhe, A., 1990. Geology of the Protogine zone south of Lake Vättern, Högdahl, K., Andersson, U.B., Eklund, O. (Eds.), 2004. The Transscandinavian Igneous Belt southern Sweden; a reinterpretation. Geologiska Föreningens i Stockholm (TIB) in Sweden: a review of its character and evolution. Geological Survey of Finland, Förhandlingar 112, 107–125. Special Paper 37. Geological Survey of Finland, Espoo, pp. 1–125. Årebäck, H., Stigh, J., 2000. The nature and origin of an anorthosite associated ilmenite- Hubbard, F.D., 1975. The Precambrian crystalline complex of south-western Sweden. The rich leuconorite, Hakefjorden Complex, south-west Sweden. Lithos 51, 247–267. geology and petrogenetic development of the Varberg Region. Geologiska Föreningens Årebäck, H., Andersson, U.B., Petersson, J., 2008. Petrological evidence for crustal melting, i Stockholm Förhandlingar 97, 223–236. unmixing, and undercooling in an alkali-calcic, high-level intrusion: the late Hubbard, F.H., 1989. The geochemistry of Proterozoic lower crustal depletion in southwest Sveconorwegian Vinga intrusion, SW Sweden. Mineralogy and Petrology 93, 1–46. Sweden. Lithos 23, 101–103. Beaumont, C., Nguyen, M.H., Jamieson, R.A., Ellis, S., 2006. Crustal flow modes in large hot Indares, A., Dunning, G., Cox, R., Gale, D., Connelly, J., 1998. High-pressure, high- orogens. In: Law, R.D., Searle, M.P., Godin, L. (Eds.), Channel Flow, Ductile Extrusion temperature rocks from the base of thick continental crust: geology and age con- and Exhumation in Continental Collision Zones. Geological Society of London Special straints from the Manicouagan Imbricate Zone, eastern Grenville Province. Tectonics Publications 268, pp. 91–145. 17, 426–440. Beckman, V., Möller, C., Söderlund, U., Corfu, F., Pallon, J., Chamberlain, K.R., 2014. Jamieson, R.A., Beaumont, C., 2011. Coeval thrusting and extension during lower crustal Metamorphic zircon formation at the transition from gabbro to eclogite in ductile flow — implications for exhumation of high-grade metamorphic rocks. Journal Trollheimen–Surnadalen, Norwegian Caledonides. Geological Society of London of Metamorphic Geology 29, 33–51. Special Publications 390. http://dx.doi.org/10.1144/SP390.26. Jamieson, R.A., Beaumont, C., 2013. On the origin of orogens. Geological Society of Berglund, J., 1997. Mid-Proterozoic evolution in southwestern Sweden. (Ph.D. thesis). America Bulletin 125, 1671–1702. A16. Earth Sciences Centre, Göteborg University, pp. 1–132. Jamieson, R.A., Beaumont, C., Nguyen, M.H., Culshaw, N.G., 2007. Synconvergent ductile Berthelsen, A., 1980. Towards a palinspastic analysis of the Baltic Shield. In: Cogné, J., flow in variable-strength continental crust: numerical models with application to the Slansky, M. (Eds.), Geology of Europe from Precambrian to post-Hercynian Sedimen- western Grenville orogen. Tectonics 26. http://dx.doi.org/10.1029/2006TC002036 tary Basins. International Geological Congress Colloque C6, pp. 5–21. (TC5005, 23 pp.). Bingen, B., Austrheim, H., Whitehouse, M., 2001. Ilmenite as a source for zirconium during Jamieson, R.A., Beaumont, C., Warren, C.J., Nguyen, M.H., 2010. The Grenville Orogen high-grade metamorphism? Textural evidence from the Caledonides of western explained? Applications and limitations of integrating numerical models with Norway and implications for zircon geochronology. Journal of Petrology 42, 355–375. geological and geophysical data. Canadian Journal of Earth Sciences 47, 517–539. Bingen, B., Andersson, J., Söderlund, U., Möller, C., 2008a. The Mesoproterozoic in the Johansson, L., Johansson, Å., 1993. U–Pb age of titanite in the Mylonite Zone, southwestern Nordic countries. Episodes 31, 29–34. Sweden. Geologiska Föreningens i Stockholm Förhandlingar 11, 1–7. Bingen, B., Davis, W.J., Hamilton, M.A., Engvik, A., Stein, H.J., Skår, Ø., Nordgulen, Ø., 2008b. Johansson, L., Kullerud, L., 1993. Late Sveconorwegian metamorphism and deformation in Geochronology of high-grade metamorphism in the Sveconorwegian belt, S. Norway: southwestern Sweden. Precambrian Research 64, 347–360. U–Pb, Th–Pb and Re–Os data. Norwegian Journal of Geology 88, 13–42. Johansson, L., Lindh, A., Möller, C., 1991. Late Sveconorwegian (Grenville) high-pressure Bingen, B., Nordgulen, Ø., Viola, G., 2008c. A four-phase model for the Sveconorwegian granulite facies metamorphism in southwest Sweden. Journal of Metamorphic orogeny, SW Scandinavia. Norwegian Journal of Geology 88, 43–72. Geology 9, 283–292. Brander, L., Appelqvist, K., Cornell, D., Andersson, U.B., 2012. Igneous and metamorphic Johansson, L., Möller, C., Söderlund, U., 2001. Geochronology of eclogite facies metamor- geochronologic evolution of granitoids in the central Eastern Segment, southern phism in SW Sweden. Precambrian Research 106, 261–275. Sweden. International Geology Review 54, 505–546. Jolivet, L., Raimbourg, H., Labrousse, L., Avigad, D., Leroy, Y., Austrheim, H., Andersen, T.B., Brown, M., 2014. The contribution of metamorphic petrology to understanding litho- 2005. Softening triggered by eclogitization, the first step toward exhumation during sphere evolution and geodynamics. Geoscience Frontiers 5, 553–569. continental subduction. Earth and Planetary Science Letters 237, 532–547. Brueckner, H., 2009. Subduction of continental crust, the origin of post-orogenic granit- Labrousse, L., Prouteau, G., Ganzhorn, A.-C., 2011. Continental exhumation triggered by oids (and anorthosites?) and the evolution of Fennoscandia. Journal of the Geological partial melting at ultrahigh pressure. Geology 39, 1171–1174. Society of London 166, 753–762. Li, Z.X., Bogdanova, S.V., Collins, A.S., Davidson, A., De Waele, B., Ernst, R.E., Fitzsimons, Cawood, P.A., Nemchin, A.A., Strachan, R., Prave, T., Krabbendam, M., 2007. Sedimen- I.C.W., Fuck, R.A., Gladkochub, D.P., Jacobs, J., Karlstrom, K.E., Lu, S., Natapov, L.M., tary basin and detrital zircon record along East Laurentia and Baltica during as- Pease, V., Pisarevsky, S.A., Thrane, K., Vernikovsky, V., 2008. Assembly, configuration, sembly and breakup of Rodinia. Journal of the Geological Society of London and break-up history of Rodinia: a synthesis. Precambrian Research 160, 179–210. 164, 257–275. Lundberg, E., Juhlin, C., 2011. High resolution reflection seismic imaging of the Ullared Christoffel, C., Connelly, J.N., Åhäll, K.-I., 1999. Timing and characterization of recurrent Deformation Zone, southern Sweden. Precambrian Research 190, 25–34. pre-Sveconorwegian metamorphism and deformation in the Varberg-Halmstad McKenzie, D., Yi, W., Rummel, R., 2014. Estimates of Te from GOCE data. Earth and region of SW Sweden. Precambrian Research 98, 173–195. Planetary Science Letters 399, 116–127. Clark, M.K., Royden, L.H., 2000. Topographic ooze: Building the eastern margin of Tibet by Möller, C., 1998. Decompressed eclogites in the Sveconorwegian (−Grenvillian) orogen lower crustal flow. Geology 28, 703–706. of SW Sweden: petrology and tectonic implications. Journal of Metamorphic Geology Cook, K.L., Royden, L.H., 2008. The role of crustal strength variations in shaping orogenic 16, 641–656. plateaus, with application to Tibet. Journal of Geophysical Research 113. http://dx. Möller, C., 1999. Sapphirine in SW Sweden; a record of Sveconorwegian (−Grenvillian) doi.org/10.1029/2007JB005457 (B08407). late-orogenic tectonic exhumation. Journal of Metamorphic Geology 17, 127–141. Culshaw, N.G., Ketchum, J.W.F., Wodicka, N., Wallace, P., 1994. Deep crustal ductile Möller, C., Söderlund, U., 1997. Age constraints on the regional deformation within the extension following thrusting in the southwestern Grenville Province, Ontario. Eastern Segment, S. Sweden: late Sveconorwegian granite dyke intrusion and Canadian Journal of Earth Sciences 31, 160–175. metamorphic-deformational relations. GFF 119, 1–12. Cuthbert, S.J., Carswell, D.A., Krogh-Ravna, E.J., Wain, A., 2000. Eclogites and eclogites in Möller, C., Andersson, J., Söderlund, U., Johansson, L., 1997. A Sveconorwegian deforma- the Western Gneiss Region, Norwegian Caledonides. Lithos 52, 165–195. tion zone (system?) within the Eastern Segment, Sveconorwegian Orogen of SW Dalziel, I.W.D., 1997. Neoproterozoic–Paleozoic geography and tectonics: review hypothesis, Sweden — a first report. GFF 119, 73–78. environmental speculation. Geological Society of America Bulletin 109, 16–42. Möller, C., Andersson, J., Lundqvist, I., Hellström, F., 2007. Linking deformation, migmatite Davidson, A., 1990. Evidence for eclogite metamorphism in the southwestern Grenville formation and zircon U–Pb geochronology in polymetamorphic orthogneisses, Province. Current research, part C. Geological Survey of Canada, Paper 90-1C. Sveconorwegian Province, Sweden. Journal of Metamorphic Geology 25, 727–750. pp. 113–118. Morisset, C.-E., Scoates, J.S., 2008. Origin of zircon rims around ilmenite in maficplutonic Eliasson, T., Schöberg, H., 1991. U–Pb dating of the post-kinematic Sveconorwegian rocks of Proterozoic anorthosite suites. The Canadian Mineralogist 46, 289–304. (Grenvillian) Bohus granite, SW Sweden: evidence of restitic zircon. Precambrian Page, L.M., Möller, C., Johansson, L., 1996. 40Ar/39Ar geochronology across the Mylonite Research 51, 337–350. Zone and the Southwestern Granulite province in the Sveconorwegian Orogen of S Gilotti, J.A., McClelland, W.C., 2007. Characteristics of, and a tectonic model for, ultrahigh- Sweden. Precambrian Research 79, 239–259. pressure metamorphism in the overriding plate of the Caledonian orogen. Interna- Petersson, A., Scherstén, A., Andersson, J., Möller, C., 2013. Zircon U–Pb and Hf — isotopes tional Geology Review 49, 777–797. from the eastern part of the Sveconorwegian Orogen, SW Sweden: implications for Griffin, W.L., Austrheim, H., Brastad, K., Bryhni, I., Krill, A.G., Krogh, E.J., Mørk, M.B.E., the growth of Fennoscandia. Geological Society of London, Special Publications 389. Qvale, H., Tørudbakken, B., 1985. High-pressure metamorphism in the Scandinavian http://dx.doi.org/10.1144/SP389.2. Caledonides. In: Gee, D.G., Sturt, B.A. (Eds.), The Caledonide Orogen — Scandinavia Pinan-Llamas, A., Andersson, J., Möller, C., 2014. Polyphasal foreland-vergent deformation and Related Areas. Wiley, Chichester, pp. 783–801. in a deep section of the 1 Ga Sveconorwegian orogen. In: Roberts, N., Viola, G., Grujic, D., Warren, C.J., Wooden, J.L., 2011. Rapid synconvergent exhumation of Miocene- Slagstad, T. (Eds.), The structural, metamorphic and magmatic evolution of aged lower orogenic crust in the eastern Himalaya. Lithosphere 3, 346–366. Mesoproterozoic orogens [in review, Submitted September 2014 to Special issue of Hacker, B.R., Andersen, T.B., Johnston, S., Kylander-Clark, A.R.C., Peterman, E.M., Walsh, Precambrian Research]. E.O., Young, D., 2010. High-temperature deformation during continental-margin Rimsa, A., Johansson, L., Whitehouse, M.J., 2007. Constraints of incipient charnockite subduction & exhumation: the ultrahigh-pressure Western Gneiss Region of Norway. formation from zircon geochronology and rare earth element characteristics. Contri- Tectonophysics 480, 149–171. butions to Mineralogy and Petrology 154, 357–369. 168 C. Möller et al. / Lithos 226 (2015) 147–168

Rivers, T., 2008. Assembly and preservation of lower, mid and upper orogenic crust in the Stephens, M.B., Wahlgren, C.-H., Weijermars, R., Cruden, A.R., 1996. Left-lateral Grenville Province — implications for the evolution of large hot long-duration transpressive deformation and its tectonic implications, Sveconorwegian orogen, Bal- orogens. Precambrian Research 167, 237–259. tic Shield, southwestern Sweden. Precambrian Research 79, 261–279. Rivers, T., 2009. The Grenville Province as a large hot long-duration collisional orogen — St-Onge, M.R., Searle, M.P., Wodicka, N., 2006. Trans-Hudson Orogen of North America insights from the spatial and thermal evolution of its orogenic fronts. In: Murphy, and Himalaya – Karakoram–Tibetan Orogen of Asia: structural and thermal charac- J.B., Keppie, J.D., Hynes, A.J. (Eds.), Ancient Orogens and Modern Analogues. Geologi- teristics of the lower and upper plates. Tectonics 25. http://dx.doi.org/10.1029/ cal Society of London Special Publications 327, pp. 405–444. 2005TC001907 (TC4006). Rivers, T., Culshaw, N., Hynes, A., Indares, A., Jamieson, R., Martignole, J., 2012. The Tapponier, P., Zhiqin, X., Roger, F., Meyer, B., Arnaud, N., Wittlinger, G., Jingsui, Y., 2001. Grenville orogen — a post-Lithoprobe perspective. Chapter 3. In: Percival, J.A., Cook, Oblique stepwise rise and growth of the Tibet Plateau. Science 294, 1671–1677. F.A., Clowes, R.M. (Eds.), Tectonic Styles in Canada: The LITHOPROBE perspective. Tual, L., Möller, C., Pinan-Llamas, A., 2014. High-temperature deformation in the basal Geological Association of Canada Special Paper 49, pp. 97–236. shear zone of an eclogite-bearing fold nappe, Sveconorwegian orogen, Sweden. In: Robinson, P., Roberts, D., Gee, D.G., Solli, A., 2014. A major synmetamorphic Early Devoni- Roberts, N., Viola, G., Slagstad, T. (Eds.), The structural, metamorphic and magmatic an thrust and extensional fault system in the Mid Norway Caledonides: relevance to evolution of Mesoproterozoic orogens [in review, Submitted August 2014 to Special exhumation of HP and UHP rocks. In: Corfu, F., Gasser, D., Chew, D.M. (Eds.), New issue of Precambrian Research]. Perspectives on the Caledonides of Scandinavia and Related Areas. Geological Society, Ulmius, J., Andersson, J., Möller, C., 2014. Hallandian high temperature metamorphism at London, Special Publications 390 (http://dx.doi.org/10.1144/SP390.24). 1.45 Ga in continent Baltica: P–T evolution and SIMS U–Pb zircon ages of aluminous Rosenberg, C.L., Handy, M.R., 2005. Experimental deformation of partially melted granite gneisses, SW Sweden. In: Roberts, N., Viola, G., Slagstad, T. (Eds.), The structural, revisited: implications for the continental crust. Journal of Metamorphic Geology 23, metamorphic and magmatic evolution of Mesoproterozoic orogens [in review, 19–28. Submitted July 2014 to Special issue of Precambrian Research]. Scherstén, A., Larson, S.-Å., Cornell, D.H., Stigh, J., 2004. Ion probe dating of a migmatite in Unsworth, M.J., Jones, A.G., Wei, W., Marquis, G., Gokarn, S.G., Spratt, J.E., 2005. Crustal SW Sweden: the fate of zircon in crustal processes. Precambrian Research 130, rheology of the Himalaya and southern Tibet inferred from magnetotelluric data. 251–266. Nature 438, 78–81. Searle, M.P., Cottle, J.M., Streule, M.J., Waters, D.J., 2010. Crustal melt granites and Vander Auwera, J., Bolle, O., Bingen, B., Liégeois, J.-P., Bogaerts, M., Duchesne, J.-C., De migmatites along the Himalaya: melt source, segregation, transport and granite em- Waele, B., Longhi, J., 2011. Sveconorwegian massif-type anorthosites and related placement mechanisms. Transactions of the Royal Society of Edinburgh 100, granitoids result from post-collisional melting of a continental arc root. Earth Science 219–233. Reviews 107, 375–397. Searle, M.P., Elliott, J.R., Phillips, R.J., Chung, S.L., 2011. Crustal-lithospheric structure and Viola, G., Henderson, I.H.C., 2010. Inclined transpression at the toe of an arcuate thrust: an continental extrusion of Tibet. Journal of the Geological Society 168, 633–672. example from the Precambrian “Mylonite Zone” of the Sveconorwegian orogen. In: Sizova, E., Gerya, T., Brown, M., 2014. Contrasting styles of Phanerozoic and Precambrian Law, R.D., Butler, R.W.H., Holdsworth, R.E., Krabbendam, M., Strachan, R.A. (Eds.), continental collision. Gondwana Research. Gondwana Research 25, 522–545. Continental Tectonics and Mountain Building: The Legacy of Peach and Horne. Slagstad, T., Roberts, N.M.W., Marker, M., Røhr, T.S., Schiellerup, H., 2013. A non- Geological Society, London, Special Publications 335, pp. 707–727. collisional, accretionary Sveconorwegian orogen. Terra Nova 25, 30–37. Viola, G., Henderson, I.H.C., Bingen, B., Hendriks, B.W.H., 2011. The Grenvillian– Söderlund, U., Jarl, L.G., Persson, P.O., Stephens, M.B., Wahlgren, C.-H., 1999. Protolith ages Sveconorwegian orogeny in Fennoscandia: back thrusting and extensional shearing and timing of deformation in the eastern, marginal part of the Sveconorwegian along the “Mylonite Zone”. Precambrian Research 189, 368–388. orogen, southwestern Sweden. Precambrian Research 94, 29–48. Wahlgren, C.-H., Kähr, A.-M., 1977. Micro-lamprophyres in western Värmland, south- Söderlund, U., Möller, C., Andersson, J., Johansson, L., Whitehouse, M., 2002. Zircon western Sweden. Geologiska Föreningens i Stockholm Förhandlingar 99, 291–295. geochronology in polymetamorphic gneisses in the Sveconorwegian orogen, SW Wahlgren, C.-H., Cruden, A.R., Stephens, M.B., 1994. Kinematics of a major fan-like struc- Sweden: ion microprobe evidence for 1.46–1.42 and 0.98–0.96 Ga reworking. ture in the eastern part of the Sveconorwegian Orogen, Baltic Shield, south-central Precambrian Research 113, 193–225. Sweden. Precambrian Research 70, 67–91. Söderlund, P., Söderlund, U., Möller, C., Gorbatschev, R., Rodhe, A., 2004. Petrology and ion Wang, X.D., Lindh, A., 1996. Temperature–pressure investigation of the southern part of microprobe U–Pb chronology applied to a metabasic intrusion in southern Sweden: a the Southwest Swedish Granulite Region. European Journal of Mineralogy 8, 51–67. study on zircon formation during metamorphism and deformation. Tectonics 23, Warren, C.J., 2013. Exhumation of (ultra)-high-pressure terranes: concepts and mecha- 1–16. nisms. Solid Earth 4, 75–92. Söderlund, U., Isachsen, C.A., Bylund, G., Heaman, L., Patchett, P.J., Vervoort, J.D., Warren, C.J., Grujic, D., Kellett, D.A., Cottle, J., Jamieson, R.A., Ghalley, K.S., 2011. Probing Andersson, U.B., 2005. U–Pb baddeleyite ages sand Hf–Nd isotope chemistry the depths of the India–Asia collision: U–Th–Pb monazite chronology of granulites constraining repeated mafic magmatism in the Fennoscandian Shield from 1.6 to from NW Bhutan. Tectonics 30. http://dx.doi.org/10.1029/2010TC002738. 0.9 Ga. Contributions to Mineralogy and Petrology 150, 174–194. Zarins, K., Johansson, Å., 2009. U–Pb geochronology of gneisses and granitoids from the Söderlund, U., Hellström, F.A., Kamo, S.L., 2008a. Geochronology of high-pressure mafic Danish island of Bornholm: new evidence for 1.47–1.45 Ga magmatism at the south- granulite dykes in SW Sweden: tracking the P–T–t path of metamorphism using Hf western margin of the East European Craton. International Journal of Earth Sciences isotopes in zircon and baddeleyite. Journal of Metamorphic Geology 26, 539–560. 98, 1561–1580. Söderlund, U., Karlsson, C., Johansson, L., Larsson, K., 2008b. The Kullaberg peninsula — a glimpse of the Proterozoic evolution of SW Fennoscandia. GFF 130, 1–10.