Dawn of Phanerozoic orogeny in the North Atlantic tract; Evidence from the Seve-Kalak Superterrane, Scandinavian Caledonides

GFF volume 120 (1998), pp. 159–172. Article

Dawn of Phanerozoic orogeny in the North Atlantic tract; Evidence from the Seve-Kalak Superterrane, Scandinavian CaledonidesPER-GUNNAR ANDRÉASSON, OLAF M. SVENNINGSEN, and LENA ALBRECHT

Andréasson, P.G., Svenningsen, O.M. & Albrecht, L., 1998: Dawn of Phanerozoic orogeny in the North Atlantic tract; Evidence from the Seve-Kalak Superterrane, Scandinavian Caledonides. GFF, Vol. 120 (Pt. 2, June), pp. 159–172. Stockholm. ISSN 1103-5897.

Abstract: Despite their Early Phanerozoic age, the Scandinavian Cal-edonides provide a remarkably rich record of the continental break-up and development of the continent–ocean transition as well as the en-suing subduction and imbrication of the pristine plate margin, all em-placed on land and accessible. We first organize the evidence for Bal-toscandian rift basin formation and magmatism, now scattered in two major nappe complexes (and by semantics) in terms of a superterrane, the Seve-Kalak Superterrane. Extensive (1000 km) mafic dyke swarms and partly sheeted dyke complexes are interpreted as fragments of a Large Igneous Province. While attempted break-up and some tholeiitic magmatism took place already at c. 800 Ma, successful rifting occurred first in connection with intense, partly alkaline dyking and emplacement of ultramafic complexes between c. 620 and 550 Ma. This late magma-tism is markedly enriched as compared to MORB, interpreted to reflect mixing between an enriched mantle source component and depleted mantle. The evidence for Early Ordovician imbrication and subduction of the rifted and intruded margin is then reviewed. In order to explain the preservation of parts of the continent–ocean transition and rift ba-sins, we suggest early detachment and thrusting towards the foreland, by analogy with the emplacement of the Semail Ophiolite. Intercalated garnet peridotites require incorporation of subcontinental mantle frag-ments during imbrication. Structures and fabrics of eclogites and their host rocks suggest that extensional tectonics was important during their Early Ordovician exhumation. If so, how much of Scandian extensional tectonics is instead inherited Finnmarkian? Keywords: Baltica, Baltoscandia, Scandinavian Caledonides, Seve, Kalak, terrane, rift magmatism, mantle source, eclogites, petrochemis-try, extensional tectonics, exhumation, Neoproterozoic, Late Cambrian, Early Ordovician.

P.G. Andréasson, O.M. Svenningsen, and L. Albrecht, Department of Min-eralogy and Petrology, Lund University, Sölvegatan 13, SE-223 62 Lund, Sweden, e-mail [email protected]. Manuscript received 8 September 1997. Revised manuscript accepted 23 March 1998.

Ancient mountain belts seldom preserve a coherent record of their early evolution because of severe overprint by later tecton-ic and thermal events. Despite their Early Phanerozoic age, the Scandinavian Caledonides provide a remarkably rich geological record of the initial continental break-up and development of the magmatic continent–ocean transition and ensuing subduction and imbrication of the pristine plate margin. This segment of the Caledonide-Appalachian orogenic belt therefore provides an ex-cellent opportunity to reconstruct earliest Phanerozoic orogeny in the North Atlantic region.

Two main occasions of subduction, ophiolite obduction, and arc construction have been recognised in the Scandinavian Cal-

edonides. An Early Ordovician event, named Finnmarkian and restricted to tectonic units derived from the Baltoscandian Mar-gin (Sturt et al. 1975; Dallmeyer & Gee 1986; Stephens et al. 1993; Gromet et al. 1996; Essex et al. 1997) was followed by a Mid-Silurian to Early Devonian (Scandian; Gee 1975) event related to the ultimate collision between Baltica and Laurentia. Since the Scandian event is much better preserved and con-strained in most aspects, there has been a tendency to overlook the Finnmarkian event. For long, palaeogeographers took little notice of the evidence for the Early Ordovician destruction of a margin of Baltica in their plate reconstructions of the Early Palaeozoic Iapetus Ocean. Geologists have applied new tectonic concepts to the Scandian evolution but have so far avoided to discuss the consequences if the same concepts be applied to the Finnmarkian event. For instance, current models propose a pro-found structural and thermal reorganisation of the overthickened crust by extensional collapse after Scandian collision. Preser-vation of high-pressure metamorphic rocks is regarded as an important evidence for extensional collapse (Platt 1987; Dewey 1988). In the Scandinavian Caledonides, there are two distinct generations of eclogites, c. 500 Ma (Mørk et al. 1988) and c. 425 Ma old (Griffin & Brueckner 1980). It is reasonable to assume that also the Finnmarkian subduction and eclogite formation re-sulted in an overthickened crust, which, concomitantly or subse-qently, adjusted to normal thickness by extensional tectonics. If so, how much of the evidence of Scandian extensional tectonics is instead inherited Finnmarkian?

The purpose of this contribution is to call attention to the evi-dence for early magmatic, metamorphic, and structural evolution of the Scandinavian Caledonides, in order to provide constraints on future plate reconstructions of the Early Palaeozoic Iapetus Ocean and models for the evolution of the Scandinavian Cal-edonides based on Scandian collision only. We first organize the evidence for Baltoscandian rift basin formation and magmatism, now scattered over two major nappe complexes and by seman-tics, in terms of a superterrane. Interpreting extensive mafic dyke swarms as fragments of a Large Igneous Province, we explore available geochemical data in order to constrain the magma source. The evidence for a Late Cambrian imbrication and sub-duction of the rifted and intruded margin is then reviewed. Fi-nally, structures and fabrics of eclogites and their host rocks are interpreted in terms of Early Ordovician extensional collapse of the imbricated prism.

TectonostratigraphyThe Baltoscandian Margin formed during Neoproterozoic su-percontinent fragmentation (Torsvik et al. 1996; Dalziel 1997). Slices of the passive margin and of the continent–ocean transi-tion now occur in the Särv Nappes and the overlying Seve Nappe Complex and in the Kalak Nappe Complex of Finnmark (Fig. 1; for review and extensive reference list, see Andréasson 1994).

160 Andréasson et al.: Dawn of Phanerozoic orogeny in the North Atlantic tract GFF 120 (1998)

The Seve Nappe Complex is a major allochthon of the Scandinavian Caledo-nides, extending for nearly 1000 km along the mountain belt and underlying most of Swedenʼs highest mountains. The ʻSeve ̓concept is old (Törnebohm 1896) and central in the development of ideas about the tectonic evolution of the Scandinavian Caledonides (for reviews, see Zachrisson 1973 and Sjöstrand 1978). Baltoscandian

and in Andréasson & Gee 1989) pointed out that the current confusion and some contradictions in recently published maps could have been avoided, had Särv been retained among the Seve units, where it originally belonged until it was treated as separate from the ʻreal ̓ Seve Nappe by Asklund (1960). Clearly, as the low-est tectonostratigraphic unit dominated by Baltoscandian rift basin infill and magma-tism, the Särv unit would be the logical and well defined basal unit of a redefined Seve Nappe Complex.

The extensive Kalak Nappe Complex (Sturt et al. 1975; Ramsay et al. 1985) in Finnmark belongs according to some maps (e.g. Gee et al. 1985) to the Mid-dle Allochthon, i.e., the same level as the Särv Nappes, while on other maps (e.g. Zachrisson 1986) it belongs to the Upper Allochthon. In addition to exten-sive psammites intruded by mafic dyke swarms, the Kalak Nappe Complex hosts the renowned Seiland Igneous Province (SIP) with intrusions and dyke swarms of tholeiitic and alkaline compositions, in-cluding picritic dykes and cumulates, car-bonatites and ultramafites (a.o. Sturt et al. 1980; Robins & Gardner 1975; Robins & Takla 1979; Daly et al. 1991; Reginiussen et al. 1995; Elvevold 1990). The most ex-tensive lithotectonic unit (Sørøy Group) has been interpreted as a transition from alluvial/shallow marine deposits to distal turbidites. This sequence of psammites, quartzites, biotite and muscovite schists, marble, and graphitic schists is similar to eclogite-bearing Seve lithologies in southern Norrbotten (Andréasson & Al-brecht 1995).

In this paper we upgrade all nappes characterized by Baltoscandian rift basin infill and rift magmatism to a super-ter-rane, the Seve-Kalak Superterrane (SKS; Fig. 2). Amalgamation of different ter-ranes began during the Early Ordovician (Dallmeyer & Gee 1986; Dallmeyer et al. 1991; Page 1992) and was completed dur-ing the Early Devonian. Docking was or-thogonal as well as oblique. As pointed out by Gibbons (1994), the term superterrane “has the advantage that it is analogous to the lithostratigraphic use of group and su-pergroup”. For individual units within the superterrane we retain, for sake of clarity with respect to existing maps, the names nappe or nappe complex. We adopt, with some modification, the definition given by Rodgers (1997) i.e.: a nappe is a body of rock that was translated laterally over other, allochthonous or autochthonous rocks. The cause of translation may be

Fig. 1. The Seve-Kalak Superterrane as defined in this paper. A, Abisko; C, Corrovare; F, Funäs-dalen; G, Grapesvare; K, Kebnekaise; M, Marsfjällen; R, Rohkunborri; S, Snasahögarna-Sy-larna-Essandsjö; SK, Sarek; SIP, Seiland Igneous Province.

rift basin infill and magmatism dominate both Särv and Seve lithologies. Such palaeogeographic similarity in combi-nation with numerous modifications of Törnebohmʼs original Seve concept, and the subdivision of the tectonostratigra-phy into the Lower, Middle, Upper, and Uppermost Allochthons have resulted in a confusing and sometimes inconsistent nomenclature. Gee (pers. comm. 1988,

GFF 120 (1998) Andréasson et al.: Dawn of Phanerozoic orogeny in the North Atlantic tract 161

Fig. 2. Nappes of the Seve and Kalak Nappe Complexes and the Särv Nappes interpreted in terms of a superterrane derived from the Baltoscandian Margin. Numbers of ʻUnits ̓refer to local names of terranes as follows: 1. Snasahögarna Nappe (Sjöström 1983); Åreskutan Nappe (Arn-bom 1980); Marsfjället Gneiss (Trouw 1973). 2. Central Seve Belt (Williams & Zwart 1977). 3. Särv Nappes (Strömberg 1961). 4. Blåhammarfjället Nappe (Sjöström 1983); Lower Seve Nappe (Arnbom 1980). 5. Sylarna unit. 6. Maddåive Nappe (Nordgren 1987). 7. Tsäkkok unit (Kullerud et al. 1990). 8. Sierkavagge Nappe (Svenningsen 1993, 1994a). 9. Vidja Assemblage (Page 1993). 10. Eastern Seve Belt (Williams & Zwart 1977); Grapesvare Nappe (Andréasson & Albrecht 1995); Aurek Assemblage (Page 1993). 11. Sarektjåkkå Nappe (Andréasson 1986). 12. Kebne Dyke Complex (Andréasson & Gee 1989). 13. Rohkunborri Nappe (Stølen 1994a, 1997); Vaivvanchockka Nappe (Kathol 1989). 14. Kalak Nappe Complex (Sturt et al. 1975). Question-marks indicate uncertain provenance of some paragneisses, and of the ages of some eclogites and their mafic protoliths.

compression and/or gravity. Strain and grade of metamorphism may vary within one and the same nappe.

Birth of a plate marginSupercontinent fragmentation Following initial break-up of Rodinia between Laurentia and East Gondwana before c. 725 Ma, rifting began along the Baltoscandian Margin at 650 Ma (Dalziel 1991; Torsvik et al. 1996). However, at-tempted break-up probably occurred from c. 800 Ma as evident from preserved roots of Baltoscandian rift-basins within the craton (Vättern Graben; Andréas-son 1994), numerous aulacogens (Vidal & Moczyd∏owska 1995; Nikishin et al. 1996) and magmatism (Reginiussen et al. 1995). The lithology and evolution of Baltoscandian rift basins are extensively treated elsewhere (e.g. Nystuen 1982; Kumpulainen & Nystuen 1985). Andréas-son & Albrecht (1995) presented an inter-pretation of eclogite-bearing lithologies of the SKS in terms of rift-basin succes-sions. Here, we only add that tentative bi-ostratigraphical correlations between Up-per Riphean successions of the Vis-ingsö Group (Vättern Graben) and successions within the large Volhynian Aulacogen in the East European Platform have been proposed (Vidal & Moczyd-∏owska 1995). In the southwestern extension of this aula-cogen, on the Lublin slope of Poland, an age of 551±4 Ma (U–Pb zircon) obtained from tuffs has been taken to indicate final, Iapetan rifting of Baltica (Compston et al. 1995; Vidal & Moczyd-∏owska 1995).

Evidence for rifting-related faulting is recorded from deep as well as shallow crustal levels. Reginiussen et al. (1995) interpreted the pervasive foliation of the Seiland intrusions and their granulite facies host as extensional and related to rift-ing. Dykes of the Corrovare dyke swarm (c. 580 Ma old) truncate high-grade mylonite zones, melted wall-rock, and foliated gab-broic and ultramafic rocks (Zwaan & Van Roermund 1990). Within the Särv Nap-pes (of much lower metamorphic grade as compared to Seiland and Corro-vare), mylonite zones cut by Ottfjäll dolerites (Röshoff 1978) may have a similar im-plication. Evidence for early extension abound in the Sarektjåkkå Nappe; angular unconformities within the metasedimen-tary rocks, caused by pervasive normal faulting, demonstrate extensional defor-mation already during the deposition of the sequence. Mesoscale, low-angle nor-

mal faulting and asymmetrical boudinage, affecting both the metasedimentary rocks and the earliest dykes, but not the later generations, indicate that the same exten-

sional deformation persisted into the dyke emplacement period (Svenningsen 1995 and unpubl. data).


Fig. 3. Mafic dyke swarms and intrusions of the Seve-Kalak Superterrane. A. Dolerite dyke swarm at Funäsdalen, central Sweden. Asymmetric peaks are dolerite dykes dipping west; in the foreground is another row of hills. B. Dolerite dyke swarm at Corrovare, 1000 km north of Funäsdalen. Red lines delineate dykes. Higher ground in the left background is a gabbro intrusion. Scale: herd of reindeers to the left on the snow. C. Dykes and screens of sandstone (with subvertical layering), Favoritkammen, Sarek Mts. The wall is about 150 m high. D. Rabotʼs glacier cutting into the Kebne Dyke Complex, Mt. Kebnekaise. E. David Gee taking notes in front of gabbro intrusions, Seiland Igneous Province. F. Tertiary dyke swarm at Fladø, East Greenland. The outcrop is of about the same size as that in C.


Fragments of a Large Igneous ProvinceMost of Swedenʼs highest mountains (Sylarna, Sarek, Kebne-kaise, and Abisko Mts.) owe their rugged nature to the predomi-nance of resistant mafic intrusions and contact metamorphosed country rocks. In their type area, the Ottfjäll Dolerite of the Särv Nappes make up sixty per cent of the bedrock. The partly sheet-ed dyke complexes of the Sarek, Kebnekaise, Abisko, and Indre Troms Mts. locally represent a dyke density of 100 per cent (Fig. 3); these complexes must have fed voluminous extrusive vol-canism. Extending for at least 1000 km along the Scandinavian segment of the Caledonides, the Baltoscandian dyke swarms and sheeted-dyke complexes most probably belonged to a larger sys-tem that fed a volcanic system of the size of a Large Igneous Province. Also further southwest along the Vendian margin of Baltica, volcanism was intense, including the 650–570 Ma old Volyn flood basalt province (Nikishin et al. 1996).

Table 1 is a compilation of radiometric ages of mafic rift mag-matism relevant to this study. The age of the extensive dyke swarms of the Särv Nappes is poorly constrained. Clearly, since the dykes cut a tillite, their age will depend on which age we pre-fer for the Varanger glaciation. Moreover, it is crucial whether the dykes cut the lower, >653±7 Ma old (Sturt et al. 1975) tillite or only the upper one, which in southern Norway (Moelv) is transitional into the Ekre Shale, dated at 612±18 Ma (Rankama 1973). If the 610–590 Ma age range for the Varanger glaciation proposed by Knoll & Walter (1992) based on microfossils and geochemistry is preferred, the age of the Särv dykes becomes <610 Ma. The Sparagmite Basin basalt underlying the Moelv Tillite is a continental tholeiite (Furnes et al. 1983), and therefore probably intruded at an earlier stage of continental break-up as compared to the MORB (see further below) dykes of the Särv Nappes. The Egersund dyke swarm, considered to be related to the initiation of rifting (Torsvik et al. 1996 and references therein) was recently dated at 616±3 Ma (Bingen et al. 1997). Based on these considerations, it appears that Baltoscandian rift magmatism was most intense and voluminous during a period of c. 30–40 m.y. and broadly coeval with rift magmatism along the Appalachian margin (615–550 Ma; Soper 1994).

Magma character and sourceThe following subdivision of the various mafic dyke swarms of the SKS is used in this paper: (a) alkaline dykes of Baltoscan-dian rift basins (ARB; Särv and equivalent Nappes); (b) tholeiitic dykes of Baltoscandian rift basins (TRB; Särv Nappes; Seve and Kalak Nappe Complexes); (c) tholeiitic dyke-complexes of the continent–ocean transition (TCO; Sarek, Kebne, Abisko, and In-dre Troms Mts.); (d) tholeiitic volcanics of the continent–ocean transition (VCO; Mt. Sylarna); (e) alkaline dyke swarms of the Seiland Igneous Province (ASIP). Geochemical characteristics have been described in detail elsewhere (Andréasson et al. 1979, 1992; Solyom et al. 1979a, 1979b, 1984; Furnes et al. 1983; Gayer et al. 1985; Rice 1987; Kathol 1989; Kullerud et al. 1990; Roberts 1990; Stølen 1994b; Svenningsen 1994a; Regi-niussen et al. 1995; Andréasson & Albrecht 1995); a review is found in Andréasson (1994). For precision and accuracy of analysis, the reader is referred to Solyom et al. (1992, table 1). For all groups except ASIP, values for Nb (analyzed by XRF) have been re-placed by values for Ta (analyzed by the more accurate INAA) multiplied by a factor of 17 (Condie 1997).

Figure 4A–D shows trace element concentrations normal-ized to those of the primitive mantle, and in order of increasing compatibility. Shown in all diagrams is normal type mid-ocean ridge basalt (N-MORB), which is depleted in the most incompat-ible elements because it derives from a mantle region depleted by extraction of continental crust. As evident from the figures, all SKS dyke swarms are strongly enriched in incompatible el-ements relative to N-MORB. This feature is characteristic of oceanic island basalts (OIB; a.o. Saunders et al. 1992; Hofmann 1997). In particular, the positive anomalies of Nb and Ta, and the mutually complementary negative anomaly of Th displayed by the VCO (Fig. 4A), the ASIP (Fig. 4D) and, though less dis-tinct, by the TRB (Fig. 4C) are characteristic features of OIB (a.o. Saunders et al. 1992; Hofmann 1997). Of the four most incompatible ʻlarge-ion-lithophile ̓elements (Rb, Ba, Th, K), Th is the most immobile and thus the most likely to retain the sig-nature of the magma source. Condie (1997) demonstrated that a graph of Th/Ta (where Ta is used instead of Nb because of higher accuracy of analytical results) versus La/Yb can be used to distinguish various tectonic settings; the latter ratio being a

Table 1. Radiometric ages of the mafic intrusive rocks within the SKS referred to in this study. Analyses 9–10: Plagioclase respectively apatite were excluded when isochrons were calculated.

Rock/unit Age Ma Method MSWD I εNd Source

1. Tholeiitic dykes, Corrovare Nappe 582±30 Sm–Nd w.r. + mineral isochron 3.96 0.51216±4 +5.3 Zwaan & Van Roermund (1990)2. = 1 578±64 Rb–Sr w.r. + mineral isochron 0.7 0.70291±6 Zwaan & Van Roermund (1990)3. Dyke complex, Sarek Mts. 573±74 Sm–Nd w.r. + mineral isochron 0.512170 +5.3 Svenningsen (1994a)4. Ditto 608±1 U–Pb zircon Svenningsen, unpubl.data5. Olivine cumulate gabbro, Kvalfjord, SIP 612±33 Sm–Nd w.r. + mineral isochron 0.1 0.512134±32 +5.5±0.6 Daly et al. (1991)6. Ditto, Storvik, SIP 604±44 Sm–Nd w.r. + mineral isochron 0.2 0.512013±36 +3.0±0.5 Daly et al. (1991)7. Ditto, Lillebukt Alkaline Complex, SIP 517±61 Sm–Nd w.r. + mineral isochron 1.64 0.512187±59 +4.0±1.2 Cadow (1993)8. = 7 534±8 Rb–Sr w.r. + mineral isochron 4.6 0.702892±19 Cadow (1993)9. Ultramafic rock, same complex 540±39 Sm–Nd w.r. + mineral isochron 0.2 0.512157±28 +4.0±0.6 Cadow (1993)10. Ditto 521±22 Rb–Sr w.r. + mineral isochron - 0.703096±24 Cadow (1993)11. Ultramafic rock, Øksfjord, SIP 550±34 Sm–Nd w.r. + mineral isochron 1.6 0.512118±34 +1.5–2.6 Mørk & Stabel (1990) Range of εSr –1 to –1012. Egersund dolerite dyke swarm 641±25- Rb–Sr isochron 0.70471±10 Miller et al. (1996) 668±28 645±20- Sm–Nd isochron +0.9–2.7 659±2013. Ditto 616±3 U–Pb baddeleyite Bingen et al. (1997) Range of initial Sr ratios 0.7038 to 0.7060 Range of εNd +1.0 to +3.1

measure of the slope of incompatible el-ement concentrations. Fig. 5A shows how the Baltoscandian dyke swarms plot on a line between depleted mantle and enriched mantle (EM), but closer to the EM. On a Th-Ta-Hf plot (not shown here; Wood 1980), all thol-eiitic dyke swarms except the TCO fall within the P-MORB field and alkaline dykes plot in the ʻalka-line within-plate ̓field. According to the same method, the TCO show ʻdestructive plate margin ̓ character but plot close to the boundary to P-MORB. Andréasson et al. (1992) interpreted the TCO of the Sarek Mts. as transitional to plume-type MORB (P-MORB).

REE concentrations of tholeiites (Fig. 4E) also show the enriched character of the SKS. Reginiussen et al. (1995) found similarities between ASIP and OIB from oceanic (St. Helena, Samoa, Gough) and


Fig. 4. Trace and rare earth element abundance patterns of mafic dykes swarms and the continent–ocean transition zone of the SKS. Primitive mantle, chondrite normalization factors and N-MORB from Sun & McDonough (1989). A. Tholeiitic volcanic rocks from Mt. Sylarna, VCO (Solyom et al. 1984). Data for Iceland oceanic island basalts (OIB) from (Wood 1978). Note distinct Th and Nb+Ta anomalies. B. Tholeiites of the continent–ocean transition, TCO (Sarek Mts.; Andréasson et al. 1992). OIB from the Kerguelen Plateau for comparison (Saunders et al. 1992 and further refer-ence therein). C. Tholeiites of the Baltoscandian rift basins, TRB (Solyom et al. 1984). OIB from the Ontong Java Plateau for comparison (Saun-ders et al. 1992 and further reference therein). D. Alkaline dykes from the Baltoscandian rift basins, ARB (Solyom et al. 1984) and from Øksfjord, Seiland Igneous Province, ASIP (Reginiussen et al. 1995). Note distinct Th and Nb+Ta anomalies of the ASIP. E. Rare earth element concentrations of the SKS dyke swarms (except the ASIP) compared to normal and enriched MORB. Data sources as in A–D. MORB patterns from Wilson (1989).

continental (Basin and Range) occur-rences. Another commonly used criterion for estimating the degree of enrichment is εNd. A basalt is enriched if it has lower εNd values than the expected value for MORB at the time of intrusion. Available εNd data for mafic dyke swarms of the SKS (+1 to +5.5; Table 1) are distinctly lower than values for the mantle beneath the Baltic Shield at 500–600 Ma (+7 for DePaoloʼs 1981 model; 7.5 for the model of DePaolo et al. 1991; cf. also Andersen & Sundvoll 1995), suggesting enrichment. Our study of the magma source of Baltoscandian rift magmatism is hampered by a shortage of combined (Sr, Nd, Pb) isotope data. How-ever, on a 143Nd/144Nd versus initial 87Sr/86Sr graph (Fig. 5B), available data plot on an array between MORB and enriched mantle (EM; Zindler & Hart 1986), partly within the field of OIB.

Fundamental to the above interpreta-tions is the criterion that crustal contami-nation is insignificant, because if magmas from a depleted mantle source become contaminated by continental crust, they aquire an ʻenriched ̓ geochemical signa-ture. The lack of negative Nb anomalies in mantle-normalized, trace element abun-dance patterns per se suggests negligible crustal contamination. The variation of Th/Yb versus Ta/Yb is often used to esti-mate the amount of crustal contamination (Pearce 1983; Wilson 1989). Pure mantle enrichment concentrates Th and Ta at the same rate. Both elements are sensitive to crustal contamination. A plot of their ra-tios with Yb (Fig. 5C) therefore defines a band with a slope of unity for pure mantle enrichment, while basalts contaminated by continental crust will plot above the band. Reginiussen et al. (1995) used the same method to demonstrate that con-tamination of the ASIP is negligible. All dyke swarms of the SKS plot along the pure enrichment array.

Initial destruction of the plate marginImbrication of the margin and intercala-tion of mantle fragments. – The dyke com-plexes of the Sarektjåkkå and correlative nappes, interpreted as the continent–ocean transition, are remarkably well preserved (Andréasson 1986; Andréasson & Gee 1989; Kathol 1989; Svenningsen 1994a, 1994b; Stølen 1994a, 1994b). Dallmeyer et al. (1991) proposed that the Sarek dyke complex was detached from the edge of the margin at an early stage of subduc-tion, thus escaping high-pressure meta-morphism and the intense deformation observed in the adjacent eclogite-bear-ing nappes (cf. below). Early selective detachment of the Baltoscandian conti-nent–ocean transition is supported by ra-diometric ages obtained from the Kebne and Sarek dyke complexes (Table 2, Nos. 6–8); however, more dating and kinemat-ic analyses of the thrust zones are needed. Also the Baltoscandian rift-basins must have been thrust at an early stage. Based on elaborate K–Ar experiments, Claes-son & Roddick (1983) demonstrated that the rift basins of the Särv Nappes never had been subjected to temperatures above 300°C after 665±10 Ma.

Van Roermund (1989) made the impor-tant recognition that some high-pressure ultramafic bodies of the SKS (Fig. 2, units 2 and 10) are garnet peridotites. A prima-


ry assemblage olivine + clinopyroxene + orthopyroxene + garnet was replaced during dynamic recrystallization by the assemblage olivine + clinopyroxene + or-thopyroxene + garnet + spinel + amphib-ole. Since the first assemblage is unstable at pressures below 18 kbar (Schmä-dicke & Evans 1997), i.e. those prevailing in crust of normal thickness (< c. 65 km), ultramafites with primary garnet-bearing

Fig. 6. PT-grid for SKS garnet peridotites and eclogites in Jämtland. Phase relations in the NCMASH system from Schmädicke & Evans (1997). Field for eclogites from Van Roer-mund (1985); data for garnet peridotites from Van Roermund (1989).

Table 2. Early Ordovician metamorphic ages within the SKS referred to in this study. 40Ar/39Ar ages are incremental heating plateau ages. For a more complete list of ages of metamorphism of Seve rocks, the reader is referred to Essex et al. (1997).

Rock/Unit Age Ma Method Interpretation Source

1. Mafic granulite, Seiland 502±28 Sm–Nd w.r. + min Metamorphic recrystallization Mørk et al. (1988)2. Eclogite, Seve 503±14 Sm–Nd w.r. + min Eclogite facies metamorphism Mørk & Stabel (1990)3. Ditto 505±18 Sm–Nd w.r. + min Ditto Mørk & Stabel (1990)4. Calc-silicate host rock of 2. 475–500 U–Pb titanite Prograde metamorphism Gromet et al. (1996), Essex et al. (1997)5. Retrogression of eclogite 2. 491±8 Ar–Ar hornblende Cooling below 500±25°C Dallmeyer & Gee (1986)6. Amphibolite, transition zone 495±8 Ar–Ar hornblende Ditto Page (1992)7. Ditto 486±1 Ar–Ar hornblende Ditto Page (1992)8. Basal thrust of transition zone dyke complex 500±2 Ar–Ar muscovite Cooling below c. 350°C Dallmeyer et al. (1991)

Fig. 5. A. Distribution of mafic rocks of the SKS on a Th/Ta vs. La/Yb graph for various mantle components after Condie (1997). Data for mantle sources were taken from Condie (1997) and references cited therein, e.g. Hart et al. (1992). DM, depleted mantle; EM, mantle enriched by recycling of young abyssal or ancient sediments and/or depleted lithosphere; FOZO, ʻfocal zone ̓lower man-tle component common in OIB. Abbrevia-tions of SKS rocks as in Fig. 4 and in the text. B. 143Nd/144Nd vs. 87Sr/86Sr diagram for mafic intrusions of the SKS for comparison with MORB (M), FOZO (F), OIB and EM (for acronyms, see A). Diamonds: Lille-bukt intrusions, Seiland Igneous Province (Cadow 1993). Empty circle beneath left diamond: tholeiitic dyke swarm of rift basins (Zwaan & Van Roermund 1990). Grey oval: Øksfjord intrusions of the Seiland Igneous Province (Mørk & Stabel 1990). Filled and empty circles: range of Egersund dyke swarm according to Bingen et al. (1997). Filled and empty triangles: range of Egersund dyke swarm according to Miller et al. (1996). For 1-2, horizontal lines show the level of 143Nd/144Nd (Sr isotopic data are not available). 1 = continent–ocean transi-tion (Svenningsen 1993); 2 = c. 600–610 Ma gabbros (Storvik and Kvalfjord) of the Seiland Igneous Province (Daly et al. 1991). Vertical line 3 is initial 87Sr/86Sr for tholei-ites of rift basins (Claesson 1976). Data for mantle end members and OIB from Wilson (1989) and Hofmann (1997). C. Th/Yb vs. Ta/Yb plot (Pearce 1983) of SKS dyke swarms. Abbreviations as in Fig. 4A and text. Shaded area is the zone of basalts from MORB and within-plate settings, i.e. enrich-ment without influence of crustal contamina-tion or subduction.

assemblages are considered as mantle fragments tectonically emplaced dur-ing deep-level imbrication of continental margins (Medaris 1984; Carswell 1986; Medaris & Carswell 1990). In a recent study of garnet-bearing ultramafic rocks from the Erzgebirge and the Bohemian Massif, Schmädicke & Evans (1997) ex-tended the phase diagram for the CMASH system to include sodium and thus to ac-

count also for amphibole in the highest-pressure assemblage (NCMASH; Fig. 6). According to their calculations, sodium stabilizes amphibole to pressures of up to c. 25 kbar. These authors also point out that even small amounts of chromium may stabilize spinel to pressures above the 18 kbar of the garnet/spinel peridot-ite transition. Amphiboles from the Jämt-land garnet-peridotites contain 2.14–2.46 wt.% Na2O (Van Roermund 1989). Spinel in kelyphitic spinel-amphibole replac-ing garnet contains up to 30 wt.% Cr2O3. Van Roermund (1989) reported spinel as inclusions in garnet, also indicating pres-sures above 18 kbar. Clearly, the garnet peridotites of the SKS represent anoma-lously high pressures and temperatures. Van Roermund (1989) inferred that they recrystallized at temperatures up to 150°C higher than those indicated by the sur-rounding eclogites.

Subduction and exhumation of rift facies dolerites. – Eclogites of the SKS have been described elsewhere (Van Roer-mund 1982; Andréasson et al. 1985; Kul-lerud et al. 1990; Andréasson & Albrecht 1995) and we focus here on the evidence that they formed and were exhumed dur-


ing the Late Cambrian to Early Ordovician. Table 2 lists radio-metric ages of metamorphism of the SKS relevant to this study.

Eclogites of the Grapesvare Nappes are retrogressed; however, peak equilibrium assemblages are found in the massive cores of larger boudins. Zoning of garnet is rare (Santallier 1988) but retrograde (Mørk et al. 1988). The 503–505 Ma Sm–Nd ages of eclogitization obtained by Mørk et al. (1988) is supported by 475–500 Ma U–Pb ages of titanite recently obtained from pro-grade assemblages in calc-silicate gneisses of the host rock (Es-sex et al. 1997). Retrogression of the eclogites first resulted in

break-down of garnet-omphacite into symplectitic intergrowth of plagioclase and clinopyroxene (the latter mineral showing decreasing sodium contents towards the periphery of the sym-plectite) and eventually plagioclase and amphibole. Deformation is first seen as a faint fabric defined by minute titanite crystals replacing rutile and ilmenite (Estifanos 1997). A new generation of Fe-rich garnet grew as idioblastic rims on older garnets and as aggregates of small idioblastic grains. With increasing defor-mation and coarsening of symplectite, hornblende and sphene became deflected around garnet porphyroblasts. Within the am-phibolitic selvage of eclogite boudins, deflection of hornblende-titanite fabrics around garnet porphyrblasts suggest flattening of at least 70%. Still in this fabric, titanite is replacing rutile, sug-gesting continued retrograde metamorphism due to decompres-sion (Estifanos 1997). 40Ar/39Ar cooling ages of 491±8 Ma for hornblende of this fabric were obtained by Dallmeyer & Gee (1986). This fabric is related to vertical shortening described be-low. During the last stage of retrogression including break-down of garnet, deformation had ceased.

Vertical shortening is a very conspicuous structural feature of the eclogite-bearing nappes. It transposed sedimentary layering into a regional, often flaggy foliation wrapping around eclogite boudins (S2 in Fig. 7A–B; for photographs, cf. Andréasson et al. 1985, fig. 5). Lenses between anastomosing shear zones may preserve primary sedimentary structures (Andréasson et al. 1985, fig. 5). Despite an overall dextral (top-to-the-east) transport ex-pressed by asymmetric folds and axes of tubular folds, the pure-shear component of the deformation allowed sinistral movement along some shear zones (Fig. 7B). While some sinistral isoclinal folds may represent parasitic folds of large recumbent folds, it can be demonstrated that others are conjugate to dextral ones. We regard fold structures indicating pure and simple shear as contemporaneous because their fabrics are of the same meta-morphic grade, ductility and style are the same, and they never intersect each other.

We interpret lenses at high angle to the pervasive foliation but with opposite shear sense (Fig. 7A) as a result of rotation, because they often occur along one and the same horizon;

Fig. 8. ENE–WSW trending lineations of inferred pre-Scandian age from the eclogite-bearing Grapesvare Nappe. A. Stretching lineations, quartz rodding, axes of intrafolial isoclinal folds (ʻF2ʼ) and tubular folds; this study. N = 43. B. 67 mesoscopic folds and 5 tubular folds (black dots); Skjernaa (1989). Contours at 2, 10 and 25 %.

Fig. 7. A. Schematic compilation of structures within the eclogite-bearing Grapesvare Nappe as viewed in section perpendicular to the pervasive (S2) foliation and approximately parallel to axes of tubular folds. Features shown derive from different areas; there is accordingly no interference between different domains. The figure is not to scale; however, long axes of eclogite boudins are <3 m. The pervasive folia-tion contains isoclinal folds with opposite vergence (2); rare lenses of preserved sedimentary layering (4, 5); concordant and symmetric eclog-ite boudins (6); asymmetric boudins (7), and boudins pinched at high angle to the pervasive foliation (1, 3). While lens 4 appears to escape to the left by pure shear; lens 5 displays dextral simple shear. B. Interpre-tation of kinematics of structures and overall stress regime within the eclogite-bearing Grapesvare Nappe (cf. text). Black and white arrows describe dextral and sinistral shear respectively. Net translation is top-to-the-east.

moreover, structures similar to those associated with rotated porphyroblasts have been observed (Albrecht in prep.). The lo-cally extreme eastward shearing resulted in spectacular tubular folds (Andréasson et al. 1985; Skjernaa 1989). At one locality, we observed how an eclogite boudin was connected to a thin, hornblende- and biotite-rich ring of a tubular fold, supporting the hypothesis that the formation of tubular folds was intimately related to the exhumation and retrogression of the eclogites.

In the Grapesvare area, tubular and ʻtransversal ̓ folds and lineations trend ENE–WSW (Fig. 8) and thus differ from the characteristic ESE–WNW lineation related to Scandian nappe emplacement. Skjernaa (1989) suggested large-scale, post-em-placement recumbent folding of the nappes to account for the unusual trend. However, post-emplacement folding of Scandian nappes is absent or very open in the area, and fabrics defining Scandian ʻtransversal ̓lineations are characterized by markedly lower metamorphic grade. We interpret the ENE–WSW linea-tion as a pre-Scandian structure.

Other evidence of an Early Ordovician event. – Mørk & Stabel (1990) obtained a Sm–Nd age of 502±28 Ma from a foliated me-tagabbro of the SIP, interpreted to date recrystallization during fo-liation-forming deformation under intermediate-pressure, granu-lite-facies conditions (Elvevold 1990). Mørk & Stabel (1990 p. 275) inferred that the deformation could represent “transport of the cooling palaeorift” followed by underthrusting with pressure increase (Elvevold 1990) within local shear zones. Early Ordo-vician ages have been obtained also from rift-magmatic dyke-complexes which escaped high-pressure metamorphism. Zircons from a granitic dyke cutting the Kebne Dyke Complex yielded an upper intercept age of 1217±11 Ma and a lower intercept age of 487±7 Ma, MSWD 0.9 (P.G. Andréasson, U. Goerke & H. Schöberg unpubl. results). The result is preliminary interpreted as evidence for anatexis of host rock sediments with Neoprot-erozoic provenance during an Early Ordovician thermal event which affected the Baltoscandian Margin.

DiscussionWhich margin of Baltica faced Laurentia when? Plate reconstruction of the late Neoproterozoic–early Phanero-zoic North Atlantic tract is no longer a matter of only two conti-nents (Laurentia and Baltica), two opposite margins, and a single ocean in between. Siberia has entered the scene from the right, southern margins of Laurentia and the proto-Andean margin in-terfere, and there are two Iapetan Oceans, and a Tornquist Sea. Palaeomagnetic reconstructions considering angular rotations of continental blocks (Torsvik et al. 1990) have turned the Early Or-dovician Baltica upside down (Torsvik et al. 1991) and created whirl-pools among traditional concepts. Of particular relevance to this study is the orientation of Baltica during rift magmatism and eclogite formation.

The classical view of the Baltoscandian and Laurentian mar-gins as a conjugate pair has recently been questioned (Soper 1994). Evidence for Iapetan rift magmatism appears to be lack-ing in Greenland, but dyke swarms of age similar to those of the Baltoscandian margin, though with deviating magmatectonic implication (Andréasson 1994), occur in the Appalachians. More important are the contrasting subsidence and depositional pat-terns between the Baltoscandian and East Greenland rift basins (see Soper 1994 for review). Based on such and other consid-

erations, Soper (1994; cf. also Torsvik et al. 1992) placed the Tornquist margin conjugate to East Greenland and Amazonia against the eastern Laurentian margin. We note that if so, the Baltoscandian Margin would face an empty hemisphere during break-up of Rodinia. Isotopic evidence indicates that the proto-liths of paragneisses of some terranes within the SKS (1–2 in Fig. 2, questionmarks) belonged to the Sveconorwegian–Grenvillian belt (Williams & Claesson 1987). With regard to Baltoscandian rift magmatism, available data are inconclusive. However, the lack of coeval magmatism, or dissimilar character of magmatism are not necessarily signs of misfit between two margins but may merely indicate asymmetric rifting. Moreover, evidence from East Greenland indicates that variations of magma chemistry may be as large within one and the same margin as between dif-ferent margins (Svenningsen unpubl. results).

Concepts about mantle plume influence on Baltoscandian continental break-upDespite their intangible nature, mantle plumes have been in-ferred to influence the rate of sea-floor spreading and transgres-sion, black shale deposition, and climate (a.o. Wilson 1993). Their potential existence in latest Precambrian–Cambrian time is, therefore, of broad geological interest. The evidence of vo-luminous emplacement of high-temperature (Sturt et al. 1980; Bennet et al. 1986) magmas in the SIP provoked discussions of the significance of mantle diapirism in this province (Ramsay 1973; Robins & Gardner 1975; Bergström & Gee 1985). On the basis of similarities with OIB, Reginiussen et al. (1995) dis-cussed whether the alkali basalts of the SIP dyke swarms could be related to plume activity. Also for the Egersund dyke swarm in southern Norway, a plume origin has been proposed (Miller & Barton 1992; Miller et al. 1996) and adopted in continental break-up models (Torsvik et al. 1996).

The anomalously high heat flow of mantle plumes allows gen-eration of melts with very high liquidus temperatures, crystalliz-ing rocks with high MgO contents. McKenzie & Bickle (1988) proposed that picrites with a MgO content of 17 wt.% require a melting temperature of 1480°C (after adiabatic decompression to 1 bar). Campbell & Griffiths (1992) concluded that picrites and komatiites provide “a simple criterion for the recognition of plume activity”. Bennet et al. (1986, p. 38) inferred that SIP magmatism involved magmas, “the most magnesian of which were probably picritic, with up to c. 20 wt.% MgO and emplaced at temperatures of c. 1450°C”.

Although oceanic island basalts are generally ascribed to plume activity, Reginiussen et al. (1995) rejected plume influence on SIP magmatism and instead favoured a model in which mag-matism was controlled by a periodically permissive lithosphere. They found the confined and long-lived character of SIP magma-tism difficult to explain in terms of large-scale, short-lived man-tle plume dynamics. However, considered in a regional context as carried out in the present paper, the SIP is not confined, but probably belonged to a Large Igneous Province. Late Vendian basalt volcanism was extensive also at the southwestern margin of the East European Craton, including the large Early Vendian (650–570 Ma) Volyn flood basalt province (Nikishin et al. 1996). Moreover, at modern margins (e.g. at Kangerdlugs-suaq on East Greenland), there is a distinct concentration of central volcanoes to the site of the inferred impact of the plume. With regard to the longevity of SIP magmatism, we agree that 300 m.y. is far too long a cycle, but are the early, i.e. 700–830 Ma old (see Regin-



iussen et al. 1995 for review) tholeiitic intrusions necessarily related to the subsequent magmatism? We admit that any visitor to the magnificant igneous landscape of Seiland would probably think so. However, the most voluminous magmatism in the SIP is represented by 604–612 (±44 and ±33) Ma old gabbroic intru-sions (Daly et al. 1991) overlapping with the most intense dyking of the margin (cf. above). On East Greenland, the most volumi-nous magmatism took place within a very short time (<5 m.y.) but magmatism continued over some 35 m.y. (c. 65 to 30 Ma; Tegner et al. 1996). Within some Phanerozoic continental plume provinces, magmatic activity was intermittent but lasted for to-tally up to a hundred m.y. In the Karoo province, magmatism began with basalts at c. 190 Ma and ended with small syenite intrusions at c. 130 Ma (Hill et al. 1992). Following long-lived, attempted continental break-up, plume impact could be coinci-dent with (and dependent of) lithospheric extension (White & McKenzie 1988).

The evidence summarized in this paper does not contradict plume influence on the rifting of the Baltica margin. Available geochemical data suggest that Baltoscandian rift magmatism could derive from mixing between an enriched mantle source component and the depleted mantle, which has existed beneath the Baltic Shield since the late Archaean (Andersen & Sundvoll 1995). However, since such an evolution would not be unique to mantle plumes, more petrological, geochemical and, in particu-lar, isotope data are needed; also timing of magmatism must be much better constrained.

The early collisional event Emplacement of the Semail Ophiolite and also Caledonian ophi-olites on Newfoundland was accompanied by imbrication of the rifted continental margin, and detachment and thrusting of parts of the rift facies sediments and continent–ocean transition, while other parts became overridden by the ophiolite slab (Cawood 1991). We infer that the continent–ocean transition preserved in the Sarek, Kebne, and Indre Troms Mts. as well as parts of the rift basins (Särv and correlative nappes in the Kalak Nappe Com-plex) were detached and transported in the same way. Correlation of the Eastern Seve Belt, hosting eclogites and garnet peridotites, with the Grapesvare Nappe supports Van Roermundʼs (1989, p. 216) argument that the garnet peridotites represent mantle slices sampled during Finnmarkian deep imbrication of the Baltoscan-dian Margin.

Ophiolite obduction and island arc development related to sub-duction of the Baltoscandian Margin have been the subject of in-tense debate (see review in Pedersen et al. 1992). Identifying ter-rane-linking successions, Sturt & Roberts (1991) and Sturt et al. (1995) inferred Early Ordovician obduction of ophiolite onto the Baltoscandian Margin and related this event and Tremadoc ensi-matic arc magmatism to the formation of the c. 500 Ma old Seve eclogites. However, Early Ordovician faunas of Laurentian affin-ity require that island-arc development and ophiolite obduction occurred instead adjacent to the Laurentian margin (Stephens & Gee 1985; Pedersen et al. 1992). Archaean isotopic signatures of granites cutting the Karmøy Ophiolite in southern Norway were interpreted to derive from subducted terrigenous sediments of an Archaean province (Pedersen et al. 1992). Since the Archaean province nearest to southern Norway is in NW Scotland, i.e., on the Laurentian side of the Caledonian suture, this radiometric result reinforced arguments based on faunal affinity. However, recent palaeogeographic reconstructions, according to which the

Baltoscandian Margin faced Siberia in Early Ordovician times may resolve this conflict both with regard to faunal affinities and isotopic signatures (Torsvik et al. 1996).

In the Trondheim Region, the Seve Nappe Complex is overlain by the Trondheim Nappe Complex (TNC). McClellan (1995) made the important recognition that some metavolcanic rocks within the TNC represent suprasubduction (boninitic) magma-tism (cf. also Nilsen 1974). The boninite-bearing Bangardsvola complex had previously been correlated partly with the Fundsjø Group of oceanic origin (Köli) and partly with the Essandsjøen Nappe, which in turn has been correlated with Seve. McClellan (1995) concluded that, if correct, the chain of correlation would suggest that the Seve is a composite unit of both Baltoscandian and oceanic terranes. However, as discussed elsewhere (An-dréasson 1998), the weak point in the correlation is the redefini-tion (Nilsen & Wolff 1989) of the continental Essandsjø Nappe to include a calcareous garben schist which previously (in fact, ever since Törnebohm 1896) has been correlated with the oce-anic terranes. The hornblende garben schist is probably in prox-imity to a major shear zone (Andréasson & Gorbatschev 1980, p. 345; Biermann 1977; Selverstone 1993, pp. 234–236), in this particular case the basal thrust of the TNC. Moreover, the trond-hjemite, keratophyre, coticule, ferruginous chert, and graphitic schist associated with the boninite-bearing metavolcanic rocks (McClellan 1994, tables 2–3) are characteristic rocks of the Gula Nappe which overlie the Seve with distinct thrust contact in the northern Trondheim Region (Andréasson & Johansson 1983). The evidence for suprasubduction magmatism reported by Mc-Clellan (1995) is nevertheless of fundamental interest. Fabrics of Gula rocks are rich in parageneses formed at various P/T ra-tios and useful for assessment of palaeotectonic environments and changes, and there are several generations of trondhjemites cutting or cut by these fabrics. A 509+5

–4 Ma U–Pb zircon age (Stephens et al. 1985) of a Gula trondhjemite has been compared to the 505±18 Ma age (Table 2) of eclogite and proposed as a pos-sible palaeotectonic link between Seve and Gula (Bjerkgård & Bjørlykke 1994). PTt-modelling fabrics could help to constrain such relations, if any. However, only faunal or isotopic prov-enance data can confidently link a TNC unit to Baltica. Inferred ʻFennoscandian Shield ̓lead-isotope signatures of sulphide de-posits hosted in the TNC (Bjørlykke et al. 1993; Bjerk-gård & Bjørlykke 1996) suggest that the “cryptic arc” (Stephens et al. 1993) above the early Caledonian subduction zone may, in fact, be hiding in the classical Trondheim Region.

The early extensional event During the 1970s, the pinch-and-swell structures so typical of the Seve Nappe Complex were modelled by mathematics and el-egant experiments, and interpreted as evidence for gravity-driv-en nappe displacement (Ramberg & Sjöström 1973; Ram-berg 1977, 1981). Estimates made on boudins suggest a minimum of 60% vertical shortening (Williams & Zwart 1977). Another school interpreted the same lenses also as evidence for gravita-tional collapse, but superimposed on the assembled nappe pile (Gee 1978). During the early 1980s, ramp-flat thrust models came into vogue and replaced gravity-driven nappes and fold nappes, and lenses became horses (Hossack & Cooper 1986; Gilotti 1989). The entire Seve Nappe Complex with nappes of varying age and intensity of deformation and metamorphism was interpreted as “an enormous horse” (Hossack 1983) and the alleged wedging-out of the belt towards the west (Zachrisson


1973) became a “trailing branch-line”. During the late 1980s, kinematic analysis came into use and revealed asymmetrical ex-tension and foreland- as well as hinterland-vergent displacement on extensional faults within the Seve Nappe Complex during Silurian–Devonian nappe emplacement (Sjöström & Bergman 1989; Bergman & Sjöström 1997). Thus, the early 1990s saw a rennaissance of gravity, but now in terms of extensional col-lapse and faulting according to an increasing variety of models (Nor-ton 1986; Andersen et al. 1992; Fossen & Rykkelid 1992; Gee et al. 1994; Milnes et al. 1997). However, this extension has been interpreted entirely as a response to Scandian collision and thickening, culminating in the Devonian. With regard to the Seve Nappe Complex in Jämtland, this interpretation is safely based on unambiguous radiometric evidence (Gromet et al. 1996) for Scandian lateral extension during accretion (Bergman & Sjö-ström 1997).

In the Scandian eclogite terrain of the Western Gneiss Region, deformation included a substantial component of vertical pure shear, both during eclogite facies metamorphism and during initial, extension-related exhumation and reworking by simple shear (Andersen & Jamtveit 1990). In the Ofoten area, North-rup (1996) estimated the relative proportions of simple and pure shear in a lower portion of the nappe stack by calculating vortic-ity from shear bands. He concluded that mid-crustal, simulta-neous simple and pure shear may account for foreland-directed nappe transport, subvertical shortening and transport-paral-lel elongation. As described above, structures preserved in the eclogite-bearing Grapesvare Nappe suggest a similar, though, pre-Scandian development. As evident from radiometric ages of the Grapesvare eclogites listed in Table 2, there is an overlap of prograde metamorphic ages of host rocks and cooling ages of ec-logites during their exhumation. While this may to some extent be due to limited resolution of radiometric methods, it could also reflect the rapid nature of the subduction-exhumation process according to theoretical models. Hynes et al. (1996) modelled mechanisms and rates during exhumation of high-pressure rocks following upon partial subduction of a rigid continental margin. They found (Hynes et al. 1996, p. 22), that upward migration of wedges of the subducting slab from lower crust levels occurs rapidly and “in direct response to the arrival of the thermally ma-ture continental lithosphere in the subduction zone”, i.e., during the early stage of collisional history of a mountain belt.

Structures of the Seve Nappe Complex are often subdivided into Group I structures, which predate or contribute to the per-vasive (ʻregionalʼ) foliation formed during the ʻmain ̓ nappe emplacement (terrane accretion), and Group II structures which fold the same foliation and related high strain zones (Williams & Zwart 1977; Sjöström 1983; Skjernaa 1989; Bergman & Sjö-ström 1997). Group I structures include characteristic lenticular structures and ʻtransversal ̓ (ESE–WNW) folds and lineations. This subdivision is based on the assumption that there was only one (i.e. Scandian) event of ʻmain ʼnappe emplacement (accre-tion). This may be valid for some units of the Seve Nappe Com-plex, but not for eclogite-bearing units such as the Grapesvare Nappe and the Eastern Seve Belt. As described above, the len-ticular structures of these nappes formed prior to c. 490 Ma. In the psammitic host rocks of the eclogites, ʻGroup I folds ̓fold a gneissosity with delicate decompression fabrics (such as biotite-plagioclase symplectite replacing phengite) unlikely to have sur-vived pervasive Scandian deformation.

Conclusions While attempted continental break-up and some Baltoscandian rift magmatism occurred from c. 800 Ma, successful rifting and voluminous magmatism took place between c. 620 and 550 Ma. We interpret this latter magmatism as related to a Large Igneous Province and at least partly derived by mixing of depleted mantle and an enriched mantle source component.

Early Ordovician subduction of the Baltoscandian Margin is manifest by detachment of the outermost margin, intercalation of slices of the mantle (now solitary garnet peridotites), and ec-logite metamorphism of rift facies dolerites. The corresponding arc system remains cryptic. However, recent palaeomagnetic and geochemical findings suggest that fragments may be hiding in northern Siberia, as well as in the Trondheim Region.

Early exhumation of eclogites was accompanied by vertical shortening. Structures of this pre-Scandian event of inferred ex-tensional tectonics can be separated from Scandian extensional structures by the age, higher metamorphic grade, and geometry of their fabrics.Acknowledgements. – This work is dedicated to David G. Gee, who initiated and led the rediscovery of the Swedish Caledonides in the 1970s, and ever since has supported our work in the highlands with never-ending energy and enthusiasm. The work was financed by grants from the Swedish Natural Science Research Council (NFR) to P.G.A. (G-AA/GU 01639-314) and O.S.M. (G-AA/GU 10050-311). We are indebted to Harald Furnes, Håkan Sjöström, and an anonymous referee for careful and constructive criticism, which moderated and considerably improved an early version of this paper.

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Documents

Dawn of Phanerozoic orogeny in the North Atlantic tract; Evidence from the Seve-Kalak Superterrane, Scandinavian Caledonides