Continental collisions and the creation of ultrahigh-pressure terranes: Petrology and thermochronology of nappes in the central Scandinavian Caledonides

Bradley R. Hacker† Philip B. Gans‡ Department of Geological Sciences, University of California, Santa Barbara, California 93106-9630, USA

ABSTRACT what extent is this paradigm correct? Conti- review the petrology and geochronology of nent-collision orogens form through a series of the major thrust sheets and then present new The formation of the vast Devonian stages involving (1) early arc ophiolite emplace- thermobarometry and thermochronology from ultrahigh-pressure terrane in the Western ment and continental passive margin contraction a key section inboard of the UHP terrane. We Gneiss Region of Norway was investigated by (typifi ed by the modern Australia-Banda arc conclude that the largest Norwegian UHP ter- determining the relationship between these collision) and subsequent relaxation (typifi ed by rane formed in the end stages of the continental ultrahigh-pressure rocks and the structur- Oman), (2) emplacement of oceanic sediments collision, following ophiolite emplacement and ally overlying oceanic and continental Köli and telescoping of the ophiolite-on-passive- passive margin subduction. Throughout, we use and Seve Nappes in the Trondelag-Jämtland margin assemblage, (3) emplacement of the the time scales of Tucker and McKerrow (1995) region. Thermobarometry and thermochro- upper-plate continent with its Andean-style arc, and Tucker et al. (1998). nology reveal that the oceanic Köli Nappes and (4) plateau formation and intracontinental reached peak conditions of 9–10 kbar and shortening (e.g., Tibet–Pamir). Can UHP ter- THE SCANDINAVIAN CALEDONIDES 550–650 °C prior to muscovite closure to ranes form during all of these stages, as implied Ar beginning at ca. 425 Ma. The continental by Searle et al. (2001)? The Scandinavian Caledonides are conven- Seve Nappes attained slightly higher pres- This question can be profi tably addressed in tionally subdivided into a number of structurally sures and temperatures (~11–12 kbar and the Scandinavian Caledonides, an archetype oro- defi ned units: the autochthon, Lower Alloch- 700–725 °C) and closed to Ar loss in musco- genic belt composed of thin, laterally extensive, thon, Middle Allochthon, Upper Allochthon, vite by 415 Ma in the east and by 400 Ma in far-traveled nappes or thrust sheets (Törnebohm, and Uppermost Allochthon (Roberts and Gee, the west. In contrast, the ultrahigh-pressure 1888) and three or four HP to UHP provinces of 1985) (Fig. 1). The autochthon consists of Pre- rocks were still deep in the mantle at eclogite- different ages (Brueckner and Roermund, 2004). cambrian Baltica basement overlain by Vendian facies pressures at 410–400 Ma. These data, We focus on the UHP province formed during through Upper Silurian sedimentary rocks. The in combination with structural, petrological, the 430–390 Ma Scandian orogeny—either the Lower Allochthon is composed of metasedi- and thermochronological data from else- largest or second largest on Earth (Ernst, 2001). mentary and crystalline rocks, compositionally where in the orogen, show that the ultrahigh- These UHP rocks might be a result of early arc similar to the autochthon, that have been thrust pressure metamorphism occurred in the ophiolite emplacement (case 1 above), but the east-southeastward over the autochthon. In the late stages of continental collision, after the latest ophiolite emplacement onto Baltica also more deformed and metamorphosed core of earlier stages of ophiolite emplacement and happened 10–20 m.y. before ultrahigh pressures the orogen, the Lower Allochthon contains the passive-margin subduction. were attained. They might be a result of passive UHP signature of the Scandian orogeny. These margin subduction during the initial stages of UHP rocks recrystallized at pressures as high Keywords: ultrahigh-pressure, Barrovian, continental collision (case 2 above), but sedi- as 3.6 GPa at ca. 410–400 Ma (Cuthbert et al., continental collision, argon geochronology, mentological (Soper et al., 1992) and paleomag- 2000; Terry et al., 2000a; Terry et al., 2000b; Scandinavian Caledonides. netic (Torsvik et al., 1996) studies suggest that Carswell, 2001; Krogh et al., 2003; Root et the collision between Baltica (Norway–Swe- al., 2004). The Middle Allochthon consists of INTRODUCTION den) and Laurentia (Greenland–eastern North crystalline and sedimentary rocks also inter- America) began as much as 20–35 m.y. before preted to have been derived from Baltica, but Ultrahigh-pressure (UHP) terranes, charac- the UHP metamorphism. Perhaps they formed from farther outboard than the autochthon. The terized by the presence of regional metamorphic as a result of intracontinental subduction, like Upper Allochthon consists of continental rocks coesite (pressures ≥27 kbar), are widely equated the Hindu Kush (case 4 above?). thought to represent the outermost margin of with the collisions between continents—but to The purpose of this paper is to examine pos- Baltica, plus ophiolitic rocks interpreted to sible cause-and-effect relationships between represent chiefl y Iapetus Ocean lithosphere. Scandian UHP tectonism and the emplace- The Upper Allochthon has been subdivided into †E-mail: [email protected]. ment of oceanic and continental thrust sheets many nappes; for the purposes of this study we ‡E-mail: [email protected]. onto the Baltica continental margin. We group them into two simplifi ed units: the Köli

GSA Bulletin; January/February 2005; v. 117; no. 1/2; p. 117–134; doi: 10.1130/B25549.1; 9 fi gures; 3 tables; Data Repository item 2005024.

For permission to copy, contact [email protected] © 2005 Geological Society of America 117 HACKER and GANS

Broken Formation Devonian–Carboniferous(?) sedimentary basins * Västerbotten

Uppermost Allochthon Gäddede Helgeland Nappe, Smøla terrane

Limingen Group * Upper Allochthon Nord- Käli: Meråker, Støren, Tännfors and Gula Nappes Trøndelag Seve, Blåhø, Surna, and Skjøtingen Nappes Jämtland Snåsa Vestranden Åre Middle Allochthon Risbergbet, Tännäs, Dalsfjord, Jotun, Tømmerås Tännfors Särv, Sætra, Offerdal, Valdres, and Kvitvola Nappes Inndalen Handöl Lower Allochthon Meråker Rondane, Åmotsdal, Bergsdalen, and Osen–Røa Nappes/Western Gneiss Complex Fongen– Baltica basement Hyllingen Selbusjøen Trondheim Røros plutonic rock Sør- Trøndelag Holonda

Figure 2 Høgstegia Hitra

Folldal Smøla

Otta Dombås Vågamo ophiolite W

intrusive/ e depositional s t

contact e

r fault n Jotun eclogite G nappe n e i s s ultrahigh- pressure R areas e g i o Hornelen area n Nordfjord– Sogn detachment zone Figure 1. Geologic map of southwestern Scandinavian Solund– Atløy Caledonides, highlighting the Western Gneiss Region Stavfjord ophiolite and nappes. Emplacement of the Uppermost, Upper, and N Solund Middle oceanic and continental-margin Allochthons is related to the ultrahigh-pressure metamorphism in the 100km Bergen Lindås Nappe core of the orogen.

118 Geological Society of America Bulletin, January/February 2005 CONTINENTAL COLLISIONS AND ULTRAHIGH-PRESSURE TERRANES and Seve Nappes (Stephens and Gee, 1985). the east; and (3) the Lower Köli Nappe locally rocks in the Gula Nappe are similar to the Upper The Uppermost Allochthon is lithologically grades lithologically and structurally downward Köli Nappe (Krutfjellet Nappe) in Västerbotten distinct from Baltica and is considered to be a into the Seve Nappes (Stephens, 1980; Stephens and Nordland (Stephens and Gee, 1985) and the fragment of Laurentia. This study focuses on the and Gee, 1985). Støren Nappe (Grenne et al., 1999). The Tänn- tectonic histories of the better-known nappes in Outcrops south of the study area near Otta fors Nappe (Fig. 2) has been correlated with the the Trondheim region (Fig. 2) but draws on rela- (Fig. 1) reveal that older parts of the Köli Nappes Lower Köli Nappe (Beckholmen, 1978). tionships across the Western Gneiss Region. were emplaced onto the Baltica margin prior to The youngest volcanoplutonic sections of the late Arenig (Sturt and Roberts, 1991) before the Köli Nappes are marginal-basin ophiolites NAPPE TECTONOSTRATIGRAPHY, younger parts of the Köli Nappes had even such as the Solund–Stavfjord (443 ± 3 Ma; PLUTONISM, DEFORMATION, AND formed. There, the MORB-affi nity Vågåmo Fig. 1) and Sulitjelma (437 ± 2 Ma; north of METAMORPHISM ophiolite lies in fault contact on psammites and Fig. 1) (Boyle, 1980; Dunning and Pedersen, crystalline rocks interpreted as part of Baltica 1988; Furnes et al., 1990; Pedersen et al., 1991). Uppermost Allochthon and is unconformably overlain by the Otta Con- Formation of these ophiolites was accompanied glomerate (Sturt and Roberts, 1991) that has a by the intrusion of widespread ca. 445–432 Ma The Uppermost Allochthon is considered late Arenig–early Llanvirn (485–464 Ma) fauna gabbroic to granitic, plutonic–hypabyssal bod- to be a fragment of Laurentia, based on C and of mixed Baltican–Laurentian affi nity (Bruton ies in the Upper Köli Nappe (Gee and Wilson, Sr isotopic chemostratigraphy (Melezhik et al., and Harper, 1981). This ophiolite-emplacement 1974; Senior and Andriessen, 1990; Pedersen 2002; Roberts et al., 2002a), early NW-directed event caused the appearance of detrital chromite et al., 1991; Stephens et al., 1993; Mørk et al., thrust faults (Roberts et al., 2001), and sedi- in upper Caradoc shales and limestones on the 1997), Middle Köli Nappe (Claesson et al., mentary successions that are distinctly different craton in the Oslo area (Bjørlykke, 1974). 1988; Tucker et al., 1990; Roberts and Tucker, from those of Baltica (Stephens and Gee, 1985). In the study area (Figs. 2 and 3), the Köli 1991), Støren, Meråker (Nilsen et al., 2003), It is extensively intruded by the Bindal batholith Nappes have been divided into four units: the and Gula (Berthomier et al., 1972; Dunning and and related plutons, which are inferred to have Støren, Meråker, Tännfors, and Gula Nappes. Grenne, 2000; Nilsen et al., 2003) Nappes. (We developed above a W-dipping subduction zone The Støren and Meråker Nappes begin with include the poorly dated 426 +8/–2 Ma Fongen– at 447–430 Ma by melting of diverse crustal Early Ordovician 493–480 Ma mafi c and felsic Hyllingen gabbro [Wilson, 1985] in this group.) and mantle rocks (Nordgulen et al., 1993). A igneous rocks (references in Fig. 3); rock asso- The ca. 450–442 Ma Smøla-Hitra batholith (zir- UHP eclogite developed in Cambrian volca- ciations and geochemistry suggest that these con ages of Tucker, 1988; Gautneb and Roberts, noplutonic arc rocks near Tromsø (Ravna and early rocks of the Støren and Meråker Nappes 1989) is slightly older. Sedimentary deposits that Roux, 2002) gave a 452 Ma zircon age (Corfu represent mid-ocean ridge and intraoceanic apparently postdate this widespread magmatism et al., 2002). arc rocks, respectively (Grenne et al., 1999). include the upper Llandovery (443–428 Ma) These rocks were deformed and unconform- Broken Formation (Bassett, 1985) in the Lower Upper Allochthon: Köli Nappes ably overlain (Bjerkgård and Bjørlykke, 1994) Köli Nappe, lacustrine deposits of the Limingen in the Meråker–Folldal area by turbidites and Group in the Middle Köli Nappe (Lutro, 1979), The Köli Nappes are the uppermost nappes conglomerates (Liafjellet, Slågån, Kjølhaugen, and possibly the Horg and Slågån Groups (Vogt, in the Trondheim region. Northeast of the study and Sulåmo Groups) that include early–middle 1945; Siedlecka, 1967) in the study area. area, in Jämtland-Västerbotten (Fig. 3), the Köli Llandovery (443–428 Ma) graptolites (Olesen Nappes are grouped into the Upper, Middle, and et al., 1973; Hardenby, 1980; Lagerblad, 1984; Upper Allochthon: Seve Nappes Lower Köli Nappes (Gee et al., 1985). There, all Bassett, 1985; Gee et al., 1985); in the Hølonda three nappes have foliated mafi c to felsic igne- area they are overlain by shoshonitic to calc- The Seve Nappes are traditionally inter- ous basement that has yielded Early Ordovician alkaline volcanic rocks intercalated with shales preted as late Precambrian to Cambrian rocks zircon ages of 492–476 Ma (see references and turbidites with late Arenig–early Llanvirn of the Baltoscandian continental margin to in Fig. 3). In the Middle Köli Nappe (locally, (485–464 Ma) fossils of mainly Laurentian ocean–continent transition. However, much the Stikke Nappe), this igneous suite postdates affi nity (Nilsen, 1978; Bruton and Bockelie, remains to be understood about their evolu- U-Mo-V-rich sedimentary rocks that have 1980), capped by Caradoc (458–449 Ma) black tion; for example, whether they were actually been correlated with Tremadoc (490–485 Ma) shales. The Gula Nappe consists of metasand- attached to the rest of Baltica or were a rifted sedimentary rocks on the Baltica craton (Sun- stone, pelite, migmatitic gneiss, and calcareous microcontinent is unknown. Their continental blad and Gee, 1984). The igneous rocks in all phyllite with minor conglomerate, mafi c volca- character is indicated by the dominance of mica three Köli Nappes are intercalated with and nic rock, and felsic volcanic rock, all intruded schist, amphibolite, and quartzofeldspathic depositionally overlain by calcareous turbidites, by trondhjemite-diorite-gabbro associations gneisses, and they have been suggested to be limestone, and volcanic rocks (Lutro, 1979; Ste- (Olesen et al., 1973; Nilsen, 1978; Size, 1979; higher-grade equivalents of rocks within the phens and Gee, 1985). In the Lower Köli Nappe, Grenne et al., 1999; Pannemans and Roberts, Middle Allochthon (Dallmeyer, 1988); this cor- the limestone is of Ashgill age (449–443 Ma), 2000). The clastic rocks have been interpreted to relation is reinforced by the presence of 608 Ma the same age as black shales exposed farther comprise turbidites (Singsås Formation; Nilsen, crosscutting mafi c dikes (Svenningsen, 2001), east on the autochthon (Stephens and Gee, 1978) and shallow marine deposits (Åsli For- similar to dikes of the Middle Allochthon. 1985). Three features have been interpreted to mation; Bjerkgård and Bjørlykke, 1994) from indicate that the Lower Köli Nappe formed on a continental margin or shelf (Grenne et al., Middle Allochthon or near the Baltica continental margin: (1) The 1999). Two features suggest an affi nity with the stratigraphy is similar to that of the Lower Baltica craton: Tremadoc (490–485 Ma) fossils The Middle Allochthon consists of crystalline Allochthon, except for the presence of volcanic of Baltican affi nity (Spjeldnes, 1985) in a U-V- and sedimentary rocks interpreted to have been rocks; (2) some turbidites were derived from Mo-rich graphitic phyllite (Gee, 1981). Volcanic derived from the continental margin of Baltica.

Geological Society of America Bulletin, January/February 2005 119 HACKER and GANS 20 50 45 40 20 20 15 50 Åre 35 Särv 30 30 40 30 40 40

Mullfjället window not xes of samples names are

50 40 Handäl 15 30 1B 10 Seve 40 Nappe 1C1,1C2 35 40 Tännfors Nappe “H160” prefi

30 25 30 ure 1, except that Middle and Lower Alloch- 1, except that Middle and Lower ure r of teeth (1–5) indicating the structural level r of teeth (1–5) indicating the structural level

45

Särv

al. (1973), Hardenby (1980), Roberts and Wolff Wolff al. (1973), Hardenby (1980), Roberts and

ära window ära 55 Skärd 30 20 25 40 Tånnås 40 30 2D1 30 25 35 35 65 Nappe 2E 30 Meråker Meråker 1D1 2F2–2F4 45 55 Fongen– intrusion Hyllingen 40 20 60 80 30 2C Meråker Røros Nappe(?) 65 31B1 10 65 35 55 1E1 60 30 2A1 30 25 2B1 3M1 3N3,4 20 65 Levanger 30 line of section 30 45 30 40 15 Gula Nappe 3P1 ondheim 60 Tr Støren Nappe 25 Høg-gia trondjhemite 45 40 25 Storen Gula Meråker Seve 10 30 45 60 Hølonda 25 50 4J1–4J3 Orkanger 70 60 25 40 70 50 3U

40 Offerdal/Leksdal 3R 30 3T Blåhø 3S Seve Nappe 1 70 km Ð10 45 15 25 10 km 20 75 10 km N 4E1–4E4 45 70 35 55 Hitra 10 Frøya 25 of the hanging wall. Strike and dip, trend and plunge of foliation and lineation shown. Cross section uses data from Olesen et section uses data from Cross and lineation shown. and plunge of foliation and dip, trend of the hanging wall. Strike et al. (1991), Gee (1994), and Hurich Roberts (1997). “H15” (1983), Gee et al. (1985), Sjöström (1981), Sjöström Figure 2. Geologic map and cross section of the study area, a key section through the Scandian allochthons. Symbols same as Fig section through a key section of the study area, 2. Geologic map and cross Figure with teeth, the numbe marked The hanging walls of gently dipping faults are section. in cross not differentiated thons are shown; e.g., “H1601C1” is shown as “1C1”. “H1601C1” is shown e.g., shown;

120 Geological Society of America Bulletin, January/February 2005 CONTINENTAL COLLISIONS AND ULTRAHIGH-PRESSURE TERRANES

Oppdal–Trondheim, Folldal, Meråker Nordland–Nord-Trøndelag–Jämtland–Västerbotten areas & Selbusjøen–Fongen areas Upper Jofjället Nappe Horg Group Liafjellet Group Käli Nappe Krutfjellet/Storfjället/Artjället Nappe Støren Slågån Group Meråker Laurentian affinity 489 Ma clast in conglomerate Nappe early–middle Llandovery fossils Nappe Upper Hovin Group Middle Limingen Group Kjølhaugen Group Käli Nappe Gjersvik Group 483 Ma (W) Lower Hovin Group (E) Sulåmo Group Caradoc black shale Leipikvatnet Nappe late Arenig–early Llanvirn (Whiterock) fossils of Laurentian affinity Orklumpen Nappe Lille Fundsjø conglomerate Stikke Nappe Tremadoc (?) U-Mo-V layer 492, 476 Ma (W) Støren Group (E) Fundsjø/Hersjø Group Lower Bjärkvattnet & Joesjä Nappes Ashgill limestone Støren, Bymarka, Grefstadfjell, Løkken, Vassfjellet Käli upper Llandovery Broken Formation ophiolites: 493, 488, 487, 482, 481, 480 Ma nappe 488 Ma Gula Nappe Skardshøi Conglomerate Gudå Conglomerate Gula (W) Undal Formation (E) Åsli Formation Nappe conglomerate felsic volcanic rock Gula Greenstone sandstone mafic volcanic rock Singsås Fm U-V-C black shale w/ Tremadoc fossils of Baltican affinity shale carbonate

Figure 3. Schematic sections of the pre-Llanvirn sections of the Köli Nappes. Zircon ages for older, generally felsic, intrusions are shown. Oppdal-Trondheim, Folldal, Meråker and Selbusjøen-Fongen areas after Vogt (1941, 1945), Siedlecka (1967), Siedlecka and Siedlecki (1967), Wolff (1967), Rui (1972), Olesen et al. (1973), Guezou (1978), Nilsen (1978), Bruton and Bockeliem (1980), Grenne (1980), Hardenby (1980), Lagerblad (1984), Roberts et al. (1984), Bassett (1985), Gee et al. (1985), Spjaeldnes (1985), Stephens and Gee (1985), Dunning and Pedersen (1988), Bjerkgård and Bjørlykke (1994), Grenne and Roberts (1998), Grenne et al. (1999), Roberts and Stephens (2000), and Rob- erts et al. (2002b). Jämtland-Västerbotten area after Sjöstrand (1978), Lutro (1979), Stephens (1980), Claesson et al. (1983), Sundblad and Gee (1984), Stephens and Gee (1985), Claesson et al. (1988), and Stephens et al. (1993).

In the study area, the Särv, Saetra, and Leksdal Lower Allochthon with the Middle to Upper in the Scandian orogen and in forming the Nappes are composed of sandstones intruded by Ordovician Gausdal Formation, continuing with UHP rocks. The Köli Nappes underwent both mafi c dikes (Gee et al., 1985; Roberts, 1988; the deltaic upper Llandovery–Wenlock Brufl at regional and contact metamorphism (Fig. 4). Greiling, 1989), while sandstones without dikes Sandstone and ending with the Ludlow and The Bymarka ophiolite of the Støren Nappe compose the Offerdal, Kvitvola, and Dearka younger, tidal to fl uvial, Ringerike Sandstone in underwent ~9 kbar epidote-blueschist facies Nappes (Gee et al., 1985). The Tännäs, Risber- the Oslo region (Bockelie and Nystuen, 1985). metamorphism in early Arenig time (ca. get, and Jotun Nappes include a variety of domi- 485–475 Ma) (Eide and Lardeaux, 2002). nantly alkalic plutonic rocks (Gee et al., 1985). Autochthon Regional Barrovian metamorphism in other Köli Nappes known to predate the 445–432 Ma Lower Allochthon The crystalline basement of the Fennoscan- intrusive event reached kyanite + staurolite + dian Shield is overlain by thin Vendian silici- garnet (Stephens and Gee, 1985) and garnet + The Lower Allochthon is composed of clastic rocks, Cambrian alum shale, Tremadoc– staurolite + biotite (Mørk, 1985) grade (Fig. 4). weakly metamorphosed (Andréasson, 1980; Ashgill graywacke and shale, lower Llandovery The 445–432 Ma intrusions then caused contact Arnbom, 1980) sedimentary and subordinate shallow marine sandstone derived from the metamorphism (Birkeland and Nilsen, 1972). crystalline rocks of the Baltica craton that were west, upper Llandovery limestone and black Scandian postintrusion regional metamorphism shortened and displaced east-southeastward shale, upper Llandovery–lower Wenlock gray- in the Köli is also Barrovian and spatially vari- over the autochthon (Gee et al., 1985; Gayer wacke, and lower(?) Wenlock fl uvial sandstone able in grade. In the study area, metamorphic and Greiling, 1989). The sedimentary rocks (Bassett, 1985; Gayer and Greiling, 1989). grade increases westward within the Meråker include Neoproterozoic and Lower Cambrian Nappe from greenschist to amphibolite facies sandstones overlain by Middle to Upper Cam- PREVIOUS METAMORPHIC (Siedlecka, 1967; Dudek et al., 1973; Olesen et brian black shales and local Lower Ordovician PETROLOGY al., 1973; Lagerblad, 1984). The Fongen–Hyl- carbonates, shales, and graywackes (Garfunkel lingen gabbro was foliated and metamorphosed and Greiling, 1998; Greiling et al., 1998). Understanding the P-T histories of the oce- to kyanite + garnet + staurolite + biotite (Wilson, Clastic deposition in the Lower Allochthon and anic and continental allochthons is central to 1985). The lowest unit in the Fundsjø Group, the autochthon migrated eastward, beginning in the reconstructing the role of these thrust sheets Gudå Conglomerate, shows garnet + staurolite

Geological Society of America Bulletin, January/February 2005 121 HACKER and GANS

muscovite and hornblende ages Norrbotten: M448-426 Ma Tännfors/Åre: M425-409 Ma Gäddede: M419-416 Ma H433-428 Ma Meråker: M424-418 Ma H435-420 Ma this study, W: M402±25 Ma this study, E: M415-416 Ma H423(?) Ma Käli

GBC KGB 14 1st regional GCC KGSB 12 metamorphism >445–432 Ma Figure 4. Pressure-temperature diagrams KSG 10 for Köli and Seve/Blåhø Nappes show early regional metamorphism older than ca. 440 Ma, followed by contact metamorphism 8 nd (kbar) 2 regional

P at ~3 kbar, and fi nal regional metamorphism metamorphism KSB SGB at ~9 or ~12 kbar, respectively (pre–440 Ma 6 < 432–443 Ma SSG metamorphism not shown). Stability fi elds GSB SGK of mineral assemblages reported previously 4 are shown in shades of gray (from the pro- SGC gram “Gibbs” by Spear and Menard, 1989). contact Calculated P-T conditions from this study 2 GSA metamorphism shown by ellipses depicting ±1σ absolute 445–432 Ma uncertainties and circles showing condi- 0 tions inferred from mineral assemblages; 350 450 550 650 750 calculated apparent P-T paths from this T (¡C) study shown by thin arrows. Dashed line shows hypothetical P-T path connecting 1st muscovite and hornblende ages regional metamorphism (defi ned by gray Norrbotten: M444-425 Ma H500-448 Ma fi elds) with contact metamorphism (defi ned Gäddede: M419-416 Ma H432-423 Ma by SGC and SGK fi elds) with 2nd regional Tømmerås M417-414 Ma H478, 435 Ma metamorphism (defi ned by ellipses). Diago- nal ruling shows approximate closure tem- Tännfors/Åre: B427-429 Ma H464-455 Ma peratures for hornblende and mica; corre- Meråker: M422-415 Ma sponding 40Ar/39Ar ages are shown at top of this study, W: M398-404 Ma each panel. Field labels: GBC—garnet-bio- this study, E: M411-416 Ma H422(?) Ma Seve/Blåhø tite-chlorite; GCC—garnet-chloritoid-chlo- 1st regional rite; GSA—garnet-sillimanite-andalusite; metamorphism GSB—garnet-staurolite-biotite; KGB—kya- 14 >445–432 Ma nite-garnet-biotite; KGSB—kyanite-garnet- staurolite-biotite; KSB—kyanite-staurolite- 12 biotite; KSG—kyanite-staurolite-garnet; 2nd regional SGB—sillimanite-garnet-biotite; SGC—sil- metamorphism limanite-garnet-cordierite; SGK—silliman- 10 KGB ite–garnet–K-feldspar; SSG—sillimanite- staurolite-garnet. 8 KGSB (kbar) P

6

4 SGC contact 2 metamorphism 445–432 Ma 0 350 450 550 650 750 T (¡C)

122 Geological Society of America Bulletin, January/February 2005 CONTINENTAL COLLISIONS AND ULTRAHIGH-PRESSURE TERRANES

+ biotite (Dudek et al., 1973) and kyanite + in Norrbotten yielded Sm/Nd isochrons of ca. Støren, Meråker, and Tännfors Nappes not only garnet + staurolite + biotite overprinted by silli- 503 Ma (Mørk et al., 1988), and titanite from lack pelites, but also garnet. manite (Lagerblad, 1984). The Tännfors Nappe calc-silicates in the same general area gave ages Mineral compositions were measured with is characterized by an inverted metamorphic of 495–480 Ma (Essex et al., 1997). The eclog- the University of California, Santa Barbara, gradient from greenschist to garnet-amphibolite ites and garnet peridotites in Jämtland gave an SX-50 electron microprobe operated at 15 kV facies, but the lowermost greenschist-facies unit age of ca. 450 Ma (Brueckner et al., 2004). and 15 nA using natural and synthetic mineral underwent prograde metamorphism at the base The Middle Seve Nappe in the Åre and standards (Table DR1, electronic supplement).1 to lower amphibolite facies (Dallmeyer et al., Handøl areas shows an early low-pressure We determined peak pressures and tempera- 1985; Bergman and Sjöström, 1997). Garnet + granulite-facies metamorphism in which silli- tures using THERMOCALC (Powell and Holland, chlorite + chloritoid grew in Jämtland, Väster- manite + garnet + cordierite were stable (Fig. 4) 1988) (Table 2). Where possible, we used the botten, and Nordland (north of Fig. 1) during (Arnbom, 1980; Sjöström, 1984), suggestive of intersections between the well-characterized nappe emplacement (Stephens, 1980; Mørk, contact metamorphism, large-scale extension, garnet-biotite (GARB in Table 2), garnet-bio- 1985). The Gula Nappe shows inward increases or rifting. This low-P metamorphism in the Åre tite-muscovite-plagioclase (GBMP in Table 2), in metamorphic grade from both the west and and Handøl areas is overprinted by Barrovian garnet-aluminumsilicate-silica-plagioclase the east, beginning at the lowest grade with gar- metamorphism that produced kyanite + garnet (GASP in Table 2), garnet-hornblende (GARH net + biotite + chlorite assemblages (Lagerblad, + staurolite + biotite in the Upper Seve Nappe, in Table 2), and garnet-hornblende-plagioclase- 1984; Bjerkgård and Bjørlykke, 1994) (Fig. 4). kyanite + garnet + biotite in the Middle Seve quartz (GHPQ in Table 2) reactions. Otherwise In a northern part of the Gula Nappe near Snåsa, Nappe, and created an inverted metamorphic we used THERMOCALC to calculate intersections kyanite + garnet + biotite, kyanite + staurolite gradient in the Lower Seve Nappe ranging down among as many reactions defi ned by well-known + biotite, garnet + staurolite + biotite, and kya- to greenschist facies (Arnbom, 1980; Sjöström, activities as possible. Generally, we fi nd that the nite + garnet + staurolite + biotite ± sillimanite 1984; Bergman and Sjöström, 1997). This Bar- garnet-aluminumsilicate-silica-plagioclase and assemblages (Lagerblad, 1984) are overgrown rovian metamorphism was widespread, also garnet-biotite-muscovite-plagioclase barometers by sillimanite + staurolite + garnet (Andréas- producing kyanite + garnet + biotite in Jämtland yield pressures that are statistically indistinguish- son and Johansson, 1982). Sillimanite + garnet (Sjöstrand, 1978) and kyanite + garnet + stauro- able; the pressure differences are <1 kbar, and the + biotite (Dudek et al., 1973) and sillimanite + lite + biotite in the Tømmerås area (Andréasson, pressures are well correlated, with a slope of 0.86 garnet + K-feldspar (Lagerblad, 1984) migma- 1980). It was accompanied by amphibolite- and χ2 = 0.05. Garnet-hornblende-plagioclase- tite in the Inndalen and Fongen areas are asso- facies mylonitization along internal nappe con- quartz and garnet-biotite-muscovite-plagioclase ciated with trondhjemite bodies (Dudek et al., tacts (Sjöström, 1984; Bergman and Sjöström, are similarly close, with a slope of 1.2 and χ2 1973; Olesen et al., 1973). In the southern part 1997), implying that the mylonitization and = 0.24. Pressures and temperatures calculated of the Gula Nappe (Folldal area), metamorphic metamorphism were coincident with construc- with THERMOCALC were checked for consistency grade reaches kyanite + staurolite + biotite(?) tion of the nappe stack. Coeval or subsequent with the petrogenetic grid constructed from the (Bjerkgård and Bjørlykke, 1994). Contact meta- greenschist-facies retrogression accompanied Holland and Powell (1998) database using Gibbs morphism around the Fongen–Hyllingen intru- motion along the Seve–Köli and Seve–Middle (ver. March 2001; Spear and Menard, 1989). sion reached garnet + sillimanite + cordierite Allochthon contacts (Sjöström, 1984; Bergman Pressure-temperature paths were modeled using grade and predated the growth of regional kya- and Sjöström, 1997). the differential thermodynamics program of nite + garnet + staurolite + biotite assemblages Gibbs (Spear and Menard, 1989). (Olesen et al., 1973); farther south Bøe (1974) NEW METAMORPHIC PETROLOGY Garnets in the Seve Nappes are typically 4–10 reported this same secondary mineral assem- mm in diameter and idioblastic; some show tex- blage replacing contact metamorphic andalusite. Because the extant metamorphic petrology tural evidence for two stages of growth. Those In the Dombås area, Guezou (1978) described includes few quantitative pressure determina- from hornblende-free samples show rimward kyanite + staurolite + biotite overprinting garnet tions—and yet such information is needed to increases in Mg# [Mg/(Mg + Fe)] of 3–9 percent- + staurolite + andalusite contact metamorphism. constrain depths of burial and exhumation—we age points and rimward increases in grossular Isograds within the Gula Nappe cut lithologic studied selected parts of the Köli and Seve content of 2–12 mol%. Garnets in hornblende- boundaries (Dudek et al., 1973; Olesen et al., Nappes in the Trondelag–Jämtland region bearing rocks are invariably signifi cantly more 1973; Lagerblad, 1984; McClellan, 1994) and (Figs. 4 and 5). Pelites, both aluminous and calcic. None of the garnets shows rimward Mn also cross into the Meråker Nappe, implying calcareous, are widespread and therefore enable increases indicative of resorption. Plagioclase a premetamorphic juxtaposition of these two an areally comprehensive assessment of the P-T in samples without hornblende shows rimward thrust sheets. evolution of these nappes. All the pelites studied decreases in anorthite content, whereas plagio- Evidence of a pre-Scandian orogeny in the (Tables 1 and 2) include mineral assemblages clase in samples with hornblende shows rim- Seve Nappes comes from geochronology and indicating a Barrovian metamorphic sequence, ward increases in anorthite content. Calculated metamorphic petrology. An early high-pressure but quantitative P-T determinations reveal pressures and temperatures for the Seve Nappes event is indicated by eclogites and garnet perido- development at pressures ~50% higher than range from ~645 °C and 10 kbar to 745 °C and tites in lower thrust sheets of the Seve Nappes in a classic Barrovian metamorphic sequence. 13 kbar (Figs. 4 and 5); the zoning described Gäddede and Norrbotten (north of Fig. 1) (Nich- These pelite assemblages range from garnet + above implies that those conditions were reached olson, 1984; van Roermund, 1985; Santallier, biotite + chlorite through staurolite + biotite + via heating and compression (Fig. 4). 1988; van Roermund, 1989; Kullerud et al., garnet or kyanite to kyanite + garnet + biotite; 1990); we calculate pressures of 18–21 kbar and sillimanite did not develop during this parage- 1GSA Data Repository item 2005024, electron temperatures of 500–600 °C for this event using netic sequence. Calcareous rocks, characterized probe data, is available on the Web at http:// THERMOCALC and mineral compositions from the by the presence of hornblende, typically lack www.geosociety.org/pubs/ft2005.htm. Requests may aforementioned studies. Two of these eclogites staurolite and kyanite. Nearly all outcrops of the also be sent to [email protected].

Geological Society of America Bulletin, January/February 2005 123 HACKER and GANS

N Levanger 10 km 13/677 8/625 Frøya Mullfjället window

Meråker 10/575 Hitra Nappe Trondheim 9/655 ?re Meråker Tännfors Nappe

Fongen– Offerdal/Leksdal Støren Hyllingen eclogite 9/625 intrusion Handäl Orkanger Nappe

eclogite 9/604 9/630 10/645 Seve Hølonda 11/712 Nappe 12/633 9/618 12/661 Seve eclogite Gula 12/724 Nappe 12/710 Nappe 9/650 Skärdära window 13/745 9/660 eclogite 13/715 Tånnås Meråker Nappe(?) Särv Høg-gia trondjhemite Røros

Figure 5. Pressure (kbar) / temperature (°C) determinations from the Trondelag-Jämtland region show temperatures of 604–660 °C at 30–35 km depth for the Gula Nappe and 645–745 °C at 40–50 km for the Seve Nappes.

Garnets in the Gula Nappe are typically <1 TABLE 1. SAMPLES AND UTM COORDINATE LOCATIONS mm in size (locally reaching 3 mm) and range Sample UTM easting UTM northing Unit Rock type from xenoblastic to idioblastic. Garnet from one H1531B1 629134 7038262 Gula Pelite sample (H1603P1) shows a rimward increase in H1601B 370955 7014712 Seve Amphibolite Mg# of 4 mol% and a decrease in Ca of 10 mol%; H1601C1 369988 7047900 Tännfors Calcareous pelite these changes are compatible with decreasing H1601C2 369988 7047900 Tännfors Calcareous metavolcanic pressure and increasing temperature. Garnets H1601D1 642868 7066811 Gula Pelite from four Gula Nappe samples show core–rim H1601E1 622316 7068535 Seve (Skjøtingen) Pelite decreases in Mg# of ≤8 percentage points and H1601E2 622316 7068535 Seve (Skjøtingen) Pelite core–rim increases of 2–7 mol% grossular. These H1602A1 612416 7004747 Gula Pelite changes are compatible with increasing pres- H1602B1 615154 6999644 Gula Pelite sure and decreasing temperature and are likely H1602C 624618 6994276 Fongen-Hyllingen Gabbro H1602D1 646431 7002166 Seve (Øyfell) Pelite the result of regional metamorphic overprinting H1602E 644244 6999848 Seve (Essandsjø) Amphibolite of a contact metamorphic mineral assemblage. H1602F2 638465 6986475 Seve over Saetra Amphibolite Calculated pressures and temperatures for the H1602F3 638465 6986475 Seve over Saetra Pelite Gula Nappe cluster in a restricted range— ~604– H1602F4 638465 6986475 Seve over Saetra Pelite 660 °C and ~9 kbar—distinctly lower than those H1603M1 612598 6971848 Gula Pelite in the Seve Nappes. In the eastern part of the Gula H1603N3 612396 6972866 Gula Pelite Nappe, this regional metamorphism overprints an H1603N4 612396 6972866 Gula Pelite earlier low-pressure contact metamorphism. One, H1603P1 573565 6987388 Gula Pelite possibly two, samples from the western part of H1603S1 532116 6973976 Seve (Blåhø) Pelite H1603T1B 532116 6973976 Seve (Blåhø) Pelite the Gula Nappe record heating and decompres- H1603T3 532116 6973976 Seve (Blåhø) Amphibolite sion. Thus, the Gula Nappe may have been H1603T7 532116 6973976 Seve (Blåhø) Pelite assembled at ~9 kbar from two distinct pieces. H1604E1 507598 6984251 Seve (Blåhø) Pelite The fault identifi ed by Bjerkgård and Bjørlykke H1604E2 507598 6984251 Seve (Blåhø) Amphibolite (1994) along the Singsås–Åsli contact is a poten- H1604E4 507598 6984251 Seve (Blåhø) Pelite tial candidate. H1604J1 05454xx 70225xx Seve (Blåhø) Amphibolite The difference in pressure between the H1604J2 05454xx 70225xx Seve (Blåhø) Pelite Gula and Seve Nappes implies different lev- H1604J3 05454xx 70225xx Seve (Blåhø) Pelite

124 Geological Society of America Bulletin, January/February 2005 CONTINENTAL COLLISIONS AND ULTRAHIGH-PRESSURE TERRANES els of burial and exhumation—~30–35 km a contact metamorphic event. In some cases, PREVIOUS THERMOCHRONOLOGY and 40–50 km, respectively. These are “lower the early regional metamorphism predated the crustal” metamorphic conditions, signifying contact metamorphism to such an extent that a Extensive thermochronologic work has been that these rocks represent either the exhumed cooling period between the two is likely; this is conducted in the allochthons in the area of Fig- base of a crustal section or a distinct layer shown in the hypothetical, long-term P-T paths ure 1. In addition to the (chiefl y U/Pb) intrusion buried beneath an overlying section of normal of Figure 4. We deduce that cooling must also ages mentioned above, 40Ar/39Ar ages indicate crustal thickness. The lower pressures recorded have followed the contact metamorphism for the existence of at least two major thermal in the Gula Nappe imply structural separation three reasons: (1) Contact metamorphic textures events (Fig. 4). Rather old hornblende ages of from the Seve Nappes. are locally preserved; (2) calculated P-T paths 500 Ma to 448 Ma come from the Seve Nappes In conjunction with the petrological observa- for the subsequent regional metamorphism of in the Norrbotten (n ≈ 15; north of Fig. 1), Tøm- tions of previous workers discussed above, it is many samples show heating from sub-contact- merås, and Åre areas (n = 4), with a cluster in clear that parts of each of the major composite metamorphic temperatures; and (3) calculated the 469–463 Ma time range (Dallmeyer et al., units—Köli Nappes and Seve Nappes—experi- P-T paths for the subsequent regional metamor- 1985; Dallmeyer and Gee, 1986; Dallmeyer, enced regional metamorphism before and after phism of other samples show cooling. 1990; Dallmeyer and Stephens, 1991; Page, 1992; Svenningsen, 2000). This range of older hornblende ages suggests that a major amphibo- lite-facies metamorphism in the Seve Nappes ended by ca. 469–463 Ma and that temperature TABLE 2. THERMOBAROMETRY RESULTS subsequently did not rise signifi cantly above Sample Minerals Thermometer Barometer T P (kbar) cor (°C) ~550 °C. Slightly younger hornblende ages of 464–455 Ma near Åre (Dallmeyer, 1990) may H1531B1 Ky St Grt Bt Pl Qtz (no Ms) GARB GASP 685 ± 50 9.6 ± 1.0 0.75 refl ect Ar loss from a subsequent metamor- “ KFMASH GASP ~655 9.5 ± 1.0 n/a H1601C1 Grt Bt Ms Hbl Pl Qtz GARB GBMP 577 ± 49 9.6 ± 0.9 0.89 phism or may indicate southward younging of “ GARH GHPQ 551 ± 39 9.6 ± 0.8 0.73 this major amphibolite-facies metamorphism. H1601D1 Grt Bt Ms Hbl Pl Qtz GARB GBMP 625 ± 63 8.2 ± 1.0 0.92 Muscovite 40Ar/39Ar ages from the Seve Nappes H1601E2 Grt Bt Ms Hbl Pl Qtz GARB GBMP 677 ± 59 12.8 ± 1.1 0.91 and Middle Allochthon in Norrbotten are “ GARH GHPQ 627 ± 86 12.7 ± 2.0 0.8 ~35 m.y. younger than the hornblende ages, H1602A1 Grt Bt Ms Chl Pl Qtz GARB GBMP 604 ± 46 9.5 ± 1.0 0.76 at 444–425 Ma (most are 434–425 Ma) (Dall- H1602B1 St Grt Bt Ms Pl Qtz GARB GBMP 574 ± 50 8.4 ± 1.0 0.87 “ KFMASH GBMP ~630 ± 20 9.4 ± 0.8 n/a meyer and Gee, 1986; Dallmeyer and Stephens, H1602D1 Ky St Grt Bt Ms Hbl Pl Qtz GARB GBMP 601 ± 51 9.8 ± 0.9 0.87 1991; Page, 1992; Svenningsen, 2000), indicat- “ KFMASH GBMP ~650 10.5 ± 0.9 n/a ing slow cooling rates of ~3 °C/m.y. “ GARB GASP 600 ± 50 10.0 ± 1.0 0.75 In contrast, 40Ar/39Ar hornblende ages from “ KFMASH GASP ~645 10.5 ± 1.0 n/a the Meråker Nappe in the study area form a “ GARH GHPQ 560 ± 37 7.6 ± 0.7 0.65 younger, fairly tight group at 435–420 Ma (Dall- H1602F3 Grt Bt Ms Pl Qtz GARB GBMP 661 ± 61 11.7 ± 1.2 0.88 H1602F4 Grt Bt Ms Pg Pl Qtz GARB GBMP 724 ± 65 12.4 ± 1.2 0.89 meyer et al., 1985; Dallmeyer, 1990). In the Gäd- H1603M1 Ky St Grt Bt Ms Pl Qtz GARB GBMP 610 ± 52 9.2 ± 1.0 0.84 dede area (Fig. 1), the same range, 433–423 Ma “ GARB GASP 610 ± 50 9.1 ± 1.0 0.75 (n = 4), is evident in 40Ar/39Ar hornblende ages “ KFMASH GBMP ~650 9.5 ± 0.9 n/a from the Lower Köli Nappe, the lowest nappe of “ KFMASH GASP ~650 9.2 ± 1.0 0.75 the Middle Köli Nappe, and the Seve Nappes. H1603N3 Ky St Grt Bt Ms Pl Qtz GARB GBMP 600 ± 52 8.3 ± 0.8 0.85 These hornblende ages suggest that a major “ KFMASH GBMP ~660 9.2 ± 0.7 n/a “ GARB GASP 575 ± 50 7.9 ± 1.0 0.75 amphibolite-facies metamorphism ended by ca. “ KFMASH GASP ~660 8.7 ± 1.0 n/a 435–420 Ma. Like the hornblende ages, musco- H1603P1 Grt Bt Ms Pl Qtz GARB GBMP 618 ± 57 8.7 ± 1.0 0.84 vite ages from the Seve and Köli Nappes from H1603S1 Ky Grt Bt Ms Pl Qtz GARB GBMP 742 ± 60 12.9 ± 1.1 0.86 the Gäddede area and the study area are slightly “ GARB GASP 745 ± 60 13.3 ± 1.1 0.79 younger, mostly 425–416 Ma (Dallmeyer et al., H1603T1B Ky Grt Bt Ms Pl Qtz GARB GBMP 713 ± 61 12.8 ± 1.0 0.9 1985; Dallmeyer, 1988, 1990), suggesting more “ GARB GASP 710 ± 61 11.7 ± 1.0 0.78 “ all reactions all reactions 701 ± 71 11.4 ± 1.5 0.78 rapid cooling rates of ~15 °C/m.y. Biotite ages H1603T7 Ky Grt Bt Ms Pl Qtz GARB GBMP 712 ± 60 12.7 ± 1.0 0.9 are similar but span a larger apparent age range, “ [St inclusions in Grt] GARB GASP 715 ± 60 13.2 ± 0.8 0.86 presumably because of undetected excess 40Ar. “ all reactions all reactions 680 ± 69 12.6 ± 1.5 0.86 H1604E1 Grt Hbl Pl Qtz GARH GARHB 633 ± 53 11.7 ± 1.2 0.86 NEW THERMOCHRONOLOGY H1604E4 Grt Bt Ms Pl Qtz GARB GBMP 712 ± 69 11.4 ± 1.3 0.89 H1604J2 Grt Bt Ms Pl Qtz GARB GBMP 621 ± 53 9.3 ± 0.9 0.84 “ KFMASH GBMP 625 ± 15 9.3 ± 0.9 n/a To tie the metamorphic history of the inboard oceanic and continental allochthons more Note: “KFMASH” refers to pelite phase diagram produced with Gibbs (Spear and Menard, 1989) from Holland and Powell (1998) database; all other calculations from THERMOCALC v3.1 with May 2001 database (Powell tightly to that of the UHP core of the orogen, we and Holland, 1988). Mineral formulae and activities were calculated with the program “A-X”, by Tim Holland and measured 40Ar/39Ar ages of eight hornblendes, 3+ Roger Powell; A-X calculates Fe in clinopyroxene using charge balance considerations, which Carswell et two biotites, and eight K-white micas (hence- al. (2000) demonstrated is a good approximation to Fe3+ measured by Mössbauer spectrometry. Uncertainties are ±1σ; “cor” is correlation coeffi cient from THERMOCALC. Mineral abbreviations after Kretz (1983). GARB— forth muscovite), using analytical procedures garnet-biotite; GARH—garnet-hornblende; GASP—garnet-aluminumsilicate-silica-plagioclase; GBMP—garnet- detailed by Calvert et al. (1999). Summaries of biotite-muscovite-platioclase; GHPQ—garnet-hornblende-plagioclase-quartz. the results are in Figures 6 and 7 and Table 3,

Geological Society of America Bulletin, January/February 2005 125 HACKER and GANS

A) 490 450 700 H1601E1 biotite Seve/Blåhø H1602F3 muscovite Seve/Käli H1603P1 muscovite Gula 440 480 600 430 WMPA= 416.5 ± 2.4 Ma WMPA= 402 ± 25 Ma 470 420 500 460 410 WMPA= 467.6 ± 3.4 Ma TFA= 418.1 ± 2.4 Ma 400 450 400 TFA= 465.3 ± 3.0 Ma 390 300 TFA= 401 ± 29 Ma apparent age (Ma) apparent age (Ma) apparent age (Ma) 440 380 430 370 200 0 cumulative 39Ar 1 01cumulative 39Ar 01cumulative 39Ar 420 450 430 H1601E1 muscovite Seve/Blåhø H1602D1 muscovite Seve H1602A1 muscovite Gula 416 WMPA= 412.2 ± 1.2 Ma 440 WMPA= 414.9 ± 2.4 Ma 420 412 430 WMPA= 411.3 ± 3.1 Ma 408 TFA= 411.2 ± 1.2 Ma 420 410 404 410 apparent age (Ma) apparent age (Ma) apparent age (Ma) TFA= 417.8 ± 2.4 Ma TFA= 414.8 ± 3.0 Ma 400 400 400 01cumulative 39Ar 01cumulative 39Ar 01cumulative 39Ar 420 500 440 H1604E4 muscovite Seve/Blåhø H1602F2 hornblende Seve/Saetra H1603N4 muscovite Seve/Gula 412 480 432 ~450 Ma 404 WMPA= 398.4 ± 1.2 Ma 460 424 WMPA= 415.0 ± 1.2 Ma 396 440 416 TFA= 400.0 ± 1.2 Ma ~422 Ma TFA= 443.6 ± 3.7 Ma 388 420 408 TFA= 415.4 ± 1.2 Ma apparent age (Ma) apparent age (Ma) apparent age (Ma) 380 400 400 01cumulative 39Ar 01cumulative 39Ar 01cumulative 39Ar

430 440 H1604J3 muscovite Seve/Blåhø H1531B biotite Seve/Gula 420 430 410 WMPA= 404.5 ± 1.2 Ma 420 400 TFA= 405.1 ± 1.2 Ma WMPA= 427.9 ± 2.3 Ma 390 410 TFA= 426.7 ± 2.6 Ma apparent age (Ma) apparent age (Ma)

380 39 400 01cumulative Ar 01cumulative 39Ar

Figure 6. 40Ar/39Ar data. (A) Well-behaved spectra. Step ages show uncertainties of ±1σ, and age uncertainties are ±2σ (continued on fol- lowing page).

which show age uncertainties of ±2σ. Most H1602C, from the Fongen–Hyllingen intrusion Four micas from the Gula Nappe were dated: of the hornblendes yielded crankshaft-shaped (Fig. 6D), gave a saddle-shaped or monotoni- Paragonite H1603P1 from the western half of spectra with isotopic ratios indicative of excess cally increasing age spectrum, with the bulk of the Gula Nappe gave an imprecise plateau age 40Ar and no well-fi t isochrons (Fig. 6B). Three the step ages ranging from 431 to 435 Ma; an of 402 ± 25 Ma, whereas muscovites H1602A1 hornblende samples, however, yielded inter- isochron from most of the steps gave 422.9 ± and H1603N4 from the eastern half are 415 pretable spectra. Hornblende H1602E from the 4.6 Ma. We provisionally accept the isochron ± 2 Ma and 415 ± 1 Ma, respectively. Biotite Seve Nappes gave a crankshaft-shaped spec- age of 423 Ma as the best age of the sample; H1531B1, like H1601E1, is anomalously old trum but a well-fi t isochron with an age of 865 an imprecise U/Pb zircon age of 426 +8/–2 Ma when compared to muscovite ages and is prob- ± 198 Ma for 24% of the gas; while not precise, (Wilson, 1985) from the same intrusion lends ably affected by excess 40Ar. the isochron suggests that this sample has not credence to this interpretation. The closure temperature for hornblende and been heated above ~500–600 °C since the late Four micas were dated from Seve Nappes out- muscovite in these rocks, with their mm-scale Precambrian (Fig. 6C). Six steps with low K/Ca crops in the southwestern half of the study area. grains and probable 15 °C/m.y. cooling rates, ratios yield an isochron age of 466 ± 28 Ma for The three muscovites, H1601E1, H1604E4, and are ~575 and 475 °C, respectively (Harrison, H1604E2. Hornblende H1602F2, from either H1604J3, gave plateau ages ranging from 398 to 1981; Kirschner et al., 1996). The high meta- the Seve or Saetra Nappes, gave a more-or-less 412 Ma; a biotite from H1601E1 yielded a pla- morphic temperatures documented for the study monotonically increasing age spectrum from ca. teau age that is much older and therefore must area exceeded muscovite closure everywhere 422 to ca. 450 Ma (Fig. 6A); we provisionally be contaminated by excess 40Ar. Muscovite and hornblende closure everywhere except per- interpret this spectrum to indicate initial closure H1602D1 from the Seve Nappes yielded a pla- haps in the Tännfors Nappe. This requires that at ca. 450 Ma, followed by Ar loss at ca. 422 Ma teau age of 411 Ma. Muscovite H1602F3 from the Barrovian metamorphism ended in the east- or slow cooling to ca. 422 Ma. Hornblende the Seve Nappes gave a plateau age of 416 Ma. ern part of the study area by ca. 425 Ma and in

126 Geological Society of America Bulletin, January/February 2005 CONTINENTAL COLLISIONS AND ULTRAHIGH-PRESSURE TERRANES

B) 2700

2400 H1602E hornblende

2100

1800 H1601B hornblende

1500

apparent age (Ma) 1200 H1604E2 hornblende 900 H1604J1 hornblende H1603T3 hornblende 600 400 H1601C2 hornblende 0 cumulative 39Ar 1

C) 0.0005 0.0001 H1602E hornblende H1604E2 hornblende MSWD = 0.38 MSWD = 0.15 Age =864 ± 197 Age =466 ± 28 Ma 40/36 intercept=6288 ± 851 Ar Ar 40 40 Ar/ Ar/ 36 36

0.000 0.000 0.000 39Ar/40Ar 0.018 0.000 39Ar/40Ar 0.052

D) 450 0.004 H1602C hornblende H1602C hornblende Atm. 440 Fongen–Hyllingen intrusion Age =422.9 ± 4.6 Ma WMA ~ 433 ± 2 Ma 0.003 MSWD = 0.78

430 Ar 40 0.002 420 Ar/ 40/36 intercept =701 ± 92 36 0.001 410 apparent age (Ma) TFA= 434.3 ± 1.4 Ma 400 0.000 01cumulative 39Ar 0.000 0.056 39Ar/40Ar

Figure 6 (continued). (B) Complex hornblende spectra. (C) Isochrons for H1602E and H1604E2. (D) Spectrum and isochron for Fongen– Hyllingen intrusion.

the western part by ca. 400 Ma. Central parts of in the southwestern Seve Nappes at 400 Ma. and 9) that can be used to address the question the study area may have cooled through musco- Cooling rates were ~15 °C/m.y. posed at the beginning of this article—when dur- vite closure at an intermediate time, ca. 415 Ma. ing continent collisions are UHP rocks created? In combination with the data reviewed above, DISCUSSION 1. The fi rst, still poorly understood, period the 40Ar/39Ar ages indicate major Köli Nappes, of tectonism relates only peripherally to the Seve Nappes, and Middle Allochthon imbrica- Our new data, combined with the existing Scandian UHP event. The Finnmarkian event tion at amphibolite facies prior to ca. 425 Ma. data reviewed above, allow us to create a more produced high-pressure metamorphism at ca. Subsequent differential imbrication/exhumation detailed and quantitative tectonic history of this 503 Ma in the Seve Nappes in Norrbotten (north of <10 km occurred prior to muscovite closure part of the Scandinavian Caledonides (Figs. 8 of Fig. 1); this orogeny has been attributed to

Geological Society of America Bulletin, January/February 2005 127 HACKER and GANS

N Levanger H429 M419 B427 10 km M412 → Frøya M415 B420 410 Bxs Mullfjället window H435 B436 M409 B413 M418 B416 Hxs B427,420 Hitra H414? B418 H457 Upper Hovin H458 B429 Trondheim H426 M418 Bxs H424 M424Bxs ?reH464, B428? H425,421,420 455 B428,427 B415 Fongen– erdal/Leksdal B444 Off Støren Hyllingen M425 intrusion OrkangerM404 Nappe Hxs M415 B397 M411 Seve Hølonda Hxs H403 Nappe H423 M398 M402±25 H422 Seve H466±28 Gula M416 Nappe

Skärdära window Hxs Nappe M415

Tånnås Meråker Nappe Särv Høg-gia trondjhemite Røros Figure 7. 40Ar/39Ar ages from the Trondelag-Jämtland region. Ages are clearly older in the east (micas ca. 425 Ma) compared to the west (micas ca. 400 Ma), separated by intermediate ages (micas ca. 415 Ma) in the center of the study area. B—biotite; H—hornblende; M—mus- covite; xs—excess 40Ar. Ages in italic are from Dallmeyer et al. (1985) and Dallmeyer (1990); westernmost two ages are from Root (2003).

TABLE 3. SUMMARY OF 40Ar/39Ar DATA Sample Mineral J TFA IA MSWD 40Ar/36Ar WMPA Steps %39Ar (Ma) (Ma) (Ma) (used) (used) H1531B1 bio 0.01338 426.7 ± 2.6 426.6 ± 5.6 0.08 531 ± 147 428.0 ± 3.2 3–16/16 94 H1601B hbl 0.01322 1164.6 ± 5.0 no good fi t n/a n/a n/a H1601C2 hbl 0.01339 528.9 ± 1.8 no good fi t n/a n/a n/a H1601E1 bio 0.01327 465.3 ± 3.0 477.9 ± 15 0.15 788 ± 809 466.6 ± 3.0 3–13/15 77 H1601E1 wm 0.01331 411.2 ± 1.2 406.5 ± 7.8 1.7 999 ± 922 412.2 ± 1.2 5–10/11 52 H1602A1 wm 0.01336 414.8 ± 3.1 415.0 ± 2.0 0.08 284.9 ± 1.8 414.9 ± 2.4 1–16/16 100 H1602D1 wm 0.01333 417.8 ± 2.4 441 ± 27 0.33 825 ± 526 413.9 ± 2.6 3–9/11 69 H1602C hbl 0.01323 434.3 ± 1.4 422.9 ± 4.6 0.78 701 ± 81 433 ± 2 2–13/13 99 H1602E hbl 0.01331 2162 ± 5.6 865 ± 197 0.38 6027 ± 888 n/a 4–12/20 24 H1602F2 hbl 0.01335 444 ± 3.6 no good fi t; see text for interpretation H1602F3 wm 0.01329 418.1 ± 2.4 417.9 ± 2.0 0.28 174 ± 30 416.5 ± 2.4 1–11/11 100 H1603P1 wm 0.01137 401 ± 30 395 ± 34 0.79 347 ± 81 402 ± 25 2–10/10 97 H1603N4 wm 0.01334 415.4 ± 1.2 415.1 ± 2.0 0.44 289 ± 31 415.0 ± 1.2 3–11/11 90 H1603T3 hbl 0.01326 615.8 ± 1.8 no good fi t n/a n/a n/a H1604E2 hbl 0.01339 570.6 ± 1.6 466 ± 28 0.15 6288 n/a 11–16/18 60 H1604E4 wm 0.01338 400.0 ± 1.2 397.5 ± 1.4 1.74 387 ± 14 398.7 ± 1.2 5–12/12 79 H1604J1 hbl 0.01336 671.1 ± 2.2 no good fi t n/a n/a n/a H1604J3 wm 0.01338 405.1 ± 1.2 403.2 ± 3.2 0.77 342 ± 115 404.5 ± 1.2 4–10/12 88 Note: J—irradiation fl ux parameter; TFA—total fusion age (uncertainty refl ects only analytical precision); IA—isochron age; MSWD—mean square weighted deviation (Wendt and Carl, 1991), which expresses the goodness of fi t of the isochron (Roddick, 1978); WMPA—weighted mean plateau age (italics indicate a “weighted mean age,” rather than plateau age, and the quoted uncertainty refl ects our assessment of the spectrum quality, which generally encompasses the range in ages of nearly concordant steps); hbl—hornblende; bi—biotite; wm—K-white mica; IA and WMPA are based on T steps and fraction of 39Ar listed in the last two columns. Preferred age is in boldface. Age uncertainties are ±2σ. Abbreviations: bio—biotite; hbl— hornblende; wm—white mica.

128 Geological Society of America Bulletin, January/February 2005 CONTINENTAL COLLISIONS AND ULTRAHIGH-PRESSURE TERRANES black shale ≈East sandstone Ringerike bentonite black shale foreland U-Mo-V shale U-Mo-V Bruflat sandst bentonite& detrital chromite mica K-feldspar idge basalt; VAB—volcanic VAB—volcanic idge basalt; Lower & Middle

Allochthons hons 18–21 kb mica Solund Lindås Nappe area area regional n Gneiss Complex of the Lower Allochthon n Gneiss Complex of the Lower 8 kb Solund metamorphism 15 kb Hornelen 15–22 kb Solund area Lower & Middle mica Atløy Allochthons Herland Group mica mica Norrbotten west of NSDZ

Hornelen emplacement of allocht of emplacement

(NSDZ) mica Åre contact metamorphism hornblende Åre & Norrbotten regional Seve ondelag ondheim event: 9–12 kb Nappes Tr metamorphism 18–21kb Gäddede orogen-scale plutonism & contact metamorphism Tr mica Trondheim Nappes ophiolite emplacement Ar age ranges. Ellipses indicate metamorphic pressures, and vertical and vertical pressures, Ar age ranges. Ellipses indicate metamorphic 39 U-Mo-V shale U-Mo-V hornblende Ar/ contact metamorphism ondheim Nappes 40 Tr blueschist Råna 12 kb ondheim ? ? 9 kb Köli ondelag Tr Nappes mica plutons black shale Nappes Tr Røragen basin

Nordfjord-Sogn Detachment Zone volcanics plutons & Broken Fm Upper Allochthon

Solund- hthons ophiolite Stavfjord & ? ? ? ? ? istein basin volcanics plutons & Stord basin Solund basin oceanic Tr Hornelen basin regional allochthons west of NSDZ metamorphism granodiorite Sogneskollen Hitra

batholith emplacement of alloc of emplacement ? contact metamorphism ? Smøla Hitra Hitra, Fosen, UHP eclogite UHP nity, such as the Blåhø Nappe (Terry and Robinson, 2004). References to data are given in the text of this paper given to data are and Robinson, 2004). References such as the Blåhø Nappe (Terry nity, titanite Skattøra titanite

Ørlandet basins Uppermost Allochthon Bindal Batholith 12 kb mantle Scandian Orogeny dow ? Central Norway Win Allochthon Basement Asenøya basin of Uppermost top-W stacking top-W titanite K-feldspar B in Meråker mica oldest volcanics are MORB in Støren, VA Jamtlandian orogeny: continental margin arc? ponding at Moho stern Gneiss Region

exhumation of slab thru stranden 14 kb We Ve hornblende Solund area

12 kb

west east subduction of continental margin to UHP to margin continental of subduction

UHP metamorphism UHP ≈West

EML EL EML 380 390 400 410 420 430 440 450 460 470 480 490 vian Pridoli enlock Arenig Ashgill Emsian Ludlow emadoc Eifelian Caradoc Lochko- Givetian Llanvirn Frasnian Praghian W

Tr Cambrian

Llandovery Famennian roiinSlra Devonian Silurian Ordovician Figure 8. Tectonostratigraphic event diagram with E-W order of units at ca. 410 Ma. “Western Gneiss Region” includes the Wester Gneiss Region” includes the diagram with E-W order of units at ca. 410 event Ma. “Western Tectonostratigraphic Figure 8. Allochthon affi nappes of Middle and Upper and overlying and in Hacker et al. (2003). “Titanite,” “hornblende,” “mica,” and “K-feldspar” indicate U/Pb “mica,” “hornblende,” et al. (2003). “Titanite,” and in Hacker ruling indicates regional metamorphic episodes. NSDZ—Nordfjord-Sogn Detachment Zone; UHP—ultrahigh-pressure; MORB—mid-oceanic-r episodes. NSDZ—Nordfjord-Sogn Detachment Zone; UHP—ultrahigh-pressure; metamorphic ruling indicates regional basalts. arc

Geological Society of America Bulletin, January/February 2005 129 HACKER and GANS

A.~480–470 Ma: subduction of Gula–Seve–Baltica(?) beneath pre-Llanvirn Köli Nappes (e.g., Støren); exhumation of subducted Gula–Seve–Baltica(?) & emplacement of Støren etc. future Lower Allochthon Støren Meråker Seve future Middle Gula Allochthon autochthon

B. ~450–445 Ma: subduction of Seve beneath pre-Llanvirn Köli Nappes; exhumation of subducted Seve; burial & exhumation of Høyvik Group (Middle Allochthon) future Lower Allochthon Støren Meråker Høyvik Gula Seve autochthon

C. 445–432 Ma: formation of marginal-basin ophiolites (e.g., Solund–Stavfjord) of Köli Nappes; rift magmatism in Köli Nappes; arc magmatism in Uppermost Allochthon Laurentian arc Sulitjelma Støren Meråker future Lower Allochthon ? Gula Seve future Middle Allochthon autochthon

D. 435–415 Ma: continental collision; formation of Laurentian plateau(?); burial & exhumation of Köli, Lindås, & Seve Nappes & Vestranden Middle Laurentian plateau(?) Åre Allochthon Lower Allochthon Uppermost Allochthon Köli Seve autochthon

Herland Solund area Råna Complex Vestranden

E. 415–400 Ma: continental subduction Laurentian plateau(?) Middle Seve Allochthon Lower Allochthon Uppermost Köli Allochthon autochthon ?? C o m p l e x i s s n e subducted G r n Köli & Seve? t e (e.g., Blåhø) e s W ultrahigh-pressure metamorphism 100 x 100 km Figure 9. Tectonic history. (A) Subduction of the Gula Nappe (microcontinent?)–Seve Nappes–Baltica(?) composite beneath those parts of the Köli Nappes that existed prior to the Llanvirn (e.g., the Støren Nappe), exhumation of the subducted Gula-Seve-Baltica(?) rocks, and emplacement of the pre-Llanvirn portions of the Köli Nappes onto the Baltoscandian continental margin. (B) Subduction of the Seve Nappes beneath those parts of the Köli Nappes that existed prior to the Llanvirn, burial of the Høyvik Group (Middle Allochthon) beneath the Seve Nappes, and exhumation of the Seve Nappes and Høyvik Group. (C) Formation of marginal-basin ophiolites (e.g., the Solund–Stavfjord ophiolite) of the Köli Nappes, rift magmatism in the Köli Nappes, and arc magmatism in the Uppermost Allochthon. (D) Collision of the active margin of Laurentia, emplacement of the newly created arcs, marginal basins, and their basement onto the Seve Nappes, and telescoping of all structurally lower units; deep burial of parts of the continental margin in the Solund, Råna, and Vestranden areas. (E) Subduction of Baltica to ultrahigh-pressure depths. (Precontractional confi guration of Baltica margin is modeled after modern Norwegian margin [Mosar, 2000].)

westward subduction of a Seve microcontinent were locally thrust onto Baltica(?) (e.g., the 2001; Yoshinobu et al., 2002). Studies of well- (Brueckner and Roermund, 2004) beneath an Otta area, Sturt et al., 1991) or onto a “Gula known ophiolites, such as Oman (e.g., Hacker older part of the Köli Nappes or to subduc- microcontinent” (Roberts and Stephens, 2000; and Gnos, 1997), show that ophiolite emplace- tion of Seve Baltoscandian continental margin Roberts et al., 2002b) during the Trondheim ment onto the passive margin of one continent rocks beneath an unnamed arc (Dallmeyer and event (Fig. 9A). On the other side of Iapetus precedes subduction of both beneath another Gee, 1986; Roberts, 2003). Somewhat later, the Uppermost Allochthon was imbricated by continent—a modern example is the imminent but prior to the late Arenig, Early Ordovician top-W thrusting beginning at 477–468 Ma, per- subduction beneath Iran of the Arabian passive MORB-type ophiolitic rocks of the Köli Nappes haps during the Taconic orogeny (Roberts et al., margin with its cargo of the Oman ophiolite.

130 Geological Society of America Bulletin, January/February 2005 CONTINENTAL COLLISIONS AND ULTRAHIGH-PRESSURE TERRANES

This ophiolite emplacement can lead to near- mafi c crustal melts are all related. In contrast, were juxtaposed prior to this regional metamor- UHP metamorphism of the continental margin the Bindal batholith intruding the Uppermost phism; muscovite ages indicate a pre-415 Ma (Searle et al., 2001). Emplacement of the pre- Allochthon formed in an arc setting (Nordgu- age for this juxtaposition. (f) The compression Llanvirn ophiolites of the Köli Nappes onto len and Sundvoll, 1992). There are no known + heating paths to ~12 kbar shown by the Seve the Gula Nappe microcontinent (the outboard plutons of this age structurally beneath the Köli Nappes (Fig. 4) likely formed in the footwall Baltoscandian continental margin) before both Nappes, implying that this magmatic event of the Köli Nappes. (g) Inverted metamorphic were overrun by the Uppermost Allochthon took place prior to the fi nal emplacement of gradients indicate thrusting of the Lower Seve (Laurentia) could refl ect a similar tectonic set- the Uppermost Allochthon and the Köli Nappes Nappe over the Middle Allochthon during ting, but high-pressure metamorphism has not (except the Early Ordovician elements of the regional metamorphism in the Åre, Handøl, been discovered in the Gula Nappe. Köli Nappes, e.g., the Vågåmo ophiolite) onto and Norrbotten areas (Arnbom, 1980; Sjöström, 2. The Trondheim event was followed the Seve Nappes and Baltica. In the Köli Nappes 1984; Greiling and Kumpulainen, 1989); this by another cryptic regional metamorphic/ this intrusive event postdates Ashgill limestone must have happened after or during hornblende deformation event, the Jämtlandian orogeny, and apparently predates the upper Llandovery closure at 464–455 Ma and before mica closure at ca. 450–445 Ma (Brueckner and Roermund, Broken Formation. This rifting event marks a at 429–427 Ma. (h) The youngest sedimentary 2004) (Fig. 9B). The Caradoc black shales in major interregnum in the contractional history rocks in the Lower Allochthon place a Wenlock the Köli Nappes provide an older bound to this of the orogen, implying that the Scandian UHP (Bassett, 1985) bound on the end of thrust- event. Indicators of this Baltoscandian margin metamorphism that followed is unrelated to the ing of this allochthon. (i) The transition from event include ca. 450 Ma eclogites and garnet pre-435 Ma contractional history. marine carbonate platform to continental fl uvial peridotites in the Seve Nappes (Brueckner et al., 4. The fourth major identifi able tectonic molasse sedimentation in the foreland in the lat- 2004) and the 447 ± 4 Ma muscovite ages that episode is the fi rst that relates directly to the est Wenlock likely marks the easternmost effect postdate amphibolite-facies deformation and UHP event (Figs. 9D and 9E). Piecing together of nappe emplacement. Active continental colli- metamorphism in the outboard Middle Alloch- the evidence that defi nes this event is pivotal sions, like the India–Asia collision, are typically thon on Atløy (Høyvik Group; Andersen and to reconstructing the origin of the UHP rocks. associated with high topography—even pla- Jamtveit, 1990; Andersen et al., 1998); coeval The evidence summarized in Figure 8 suggests teaus—such that the Laurentia-Baltica collision orogeny in Laurentia is indicated by 452 Ma that diachronous, eastward-propagating nappe could have been characterized by a high-altitude eclogite (Corfu et al., 2002) and 456 Ma titanite emplacement began at ca. 437 Ma in the west plateau. The overlying 35–45 km thick crustal (Selbekk et al., 2000) in Uppermost Allochthon and terminated by ca. 415 Ma in the east: (a) column required to produce the 9–12 kbar arc rocks. The regional metamorphism in the Regional metamorphism in the Vestranden metamorphism exhibited by the Vestranden Köli and Seve Nappes that predates the 445– gneiss (Fig. 1) (14 kbar at 435 Ma; Dallmeyer et gneiss, Råna intrusion, Seve Nappes, Gula 432 Ma intrusive suite might also have occurred al., 1992) and in the Råna complex of the Upper Nappe, Lindås Nappe, and Solund area supports at this time. Together these features suggest Allochthon (12 kbar at 432 Ma; Northrup, this idea. westward subduction of the Seve Nappes 1997) may refl ect tectonic burial beneath the Thus, an eastward-propagating sequence of beneath, perhaps, the Köli Nappes, followed by Uppermost Allochthon; hornblende in the nappe emplacements is permitted by diverse thickening, heating, and exhumation (Brueckner Vestranden gneiss did not close to Ar loss until data that span the western to eastern edges of and Roermund, 2004), but the presence of high- ca. 400 Ma (Dallmeyer et al., 1992). (b) The the orogen, corroborating the ideas of many pressure rocks in at least three major units—and youngest fossiliferous rocks in the Upper earlier workers. This Scandian deformation no identifi ed subduction-related magmatic Allochthon are upper Llandovery, and these began in the west with the emplacement of arc—demonstrates an as-yet-unraveled, more are overlain by a thick turbiditic succession that the Köli Nappes onto the Seve Nappes and the complicated tectonic setting. may stretch into the Wenlock (Bassett, 1985), emplacement of the Uppermost Allochthon 3. Third was an areally extensive magmatic requiring that the faults bounding the Upper (Laurentia) onto the Köli-Seve-Baltica amal- episode at ca. 445–432 Ma that included forma- Allochthon are younger than late Llandovery gam (although the stacking history is unclear); tion of signifi cant new oceanic crust (e.g., the (443–428 Ma). (c) The Wenlock Herland the combined Uppermost Allochthon–Upper Solund–Stavfjord ophiolite; Dunning and Ped- Group in the Middle Allochthon on Atløy was Allochthon–Middle Allochthon–Lower Alloch- ersen, 1988) and intrusion of plutons and dikes deposited during emplacement of the Upper thon stack subsequently reached its easternmost throughout the Uppermost Allochthon and Köli Allochthon (Andersen et al., 1990). (d) Top-E thermal infl uence on the Baltica margin ca. Nappes (Fig. 9C). This includes the plutons thrusting of the Upper Allochthon after 434 Ma 420 Ma. At that time, the peak of the UHP described above, plus others intruding the Köli probably caused the 15–22 kbar metamorphism metamorphism was still 10 m.y. in the future. Nappes such as the Sunnhordland batholith in the Solund area (Hacker et al., 2002) and What, then, caused the UHP metamorphism? (Andersen and Jansen, 1987; Fossen and Aus- may have caused the 423 Ma eclogite-facies, Why did the Western Gneiss Region sink so far trheim, 1988), the Bremanger granitoid com- 18–21 kbar metamorphism in the Lindås Nappe into the mantle? plex, the Gåsøy diorite (Hansen et al., 2002), (Bingen et al., 2003); the Solund area remained In general terms, continental crust can be and the Sogneskollen granodiorite (Skjerlie et above hornblende closure to Ar until ca. 400 Ma driven to UHP depths if it (1) becomes denser al., 2000). Some of the plutons within the Köli (Chauvet and Dallmeyer, 1992). (e) The Gula than the mantle and sinks under its own weight, Nappes were derived from melting of mantle Nappe was tectonically buried to 9 kbar and (2) is attached to sinking oceanic lithosphere, in a continental-rift setting, and others were then cooled to hornblende closure by 423 Ma (3) is overlain by denser, sinking rocks that push derived by melting of mafi c crustal rocks at (Fig. 4). Isograds within the Gula Nappe cut it downward, or (4) is attached to sinking conti- ~900 °C and 10–15 kbar (Dunning and Grenne, lithologic boundaries (Dudek et al., 1973; nental lithosphere. (1) The continental crust of 2000; Pannemans and Roberts, 2000; Hansen Olesen et al., 1973; Lagerblad, 1984; McClel- the Western Gneiss Complex is too felsic to have et al., 2002; Nilsen et al., 2003); it is plausible lan, 1994) and also cross into the Meråker and reached greater-than-mantle densities at UHP that the seafl oor-spreading, rift magmatism, and Støren Nappes, requiring that these Nappes conditions: Walsh and Hacker (2004) calculated

Geological Society of America Bulletin, January/February 2005 131 HACKER and GANS

3 on fi eld studies in the Sogn-Sunnfjord region of west- Brueckner, H.K., Van Roermund, H.L.M., and Pearson, N.J., maximum densities of 2.85–3.05 g/cm for the ern Norway: Tectonics, v. 9, p. 1097–1111. 2004, An Archean(?) to Paleozoic evolution for a garnet predominantly quartzofeldspathic gneisses of Andersen, T.B., and Jansen, Ø.J., 1987, The Sunnhordland peridotite lens with sub-Baltic Shield affi nity within the Western Gneiss Complex at 10–30 kbar. Batholith, W. Norway; regional setting and internal the Seve Nappe Complex of Jämtland, Sweden, central structures, with emphasis on the granitoid plutons: Scandinavian Caledonides: Journal of Petrology, v. 45, (2) The Western Gneiss Region cannot have Norsk Geologisk Tidsskrift, v. 67, p. 159–183. p. 415–437, doi: 10.1093/PETROLOGY/EGG088. been pulled deep into the mantle because it was Andersen, T.B., Skjerlie, K.P., and Furnes, H., 1990, The Bruton, D.L., and Bockelie, J.F., 1980, Geology and pale- Sunnfjord Mélange, evidence of Silurian ophiolite ontology of the Hølonda area, western Norway—A attached to subducting, old oceanic lithosphere accretion in the West Norwegian Caledonides: Geo- fragment of North America?, in Wones, D.R., ed., The as the emplacement of ophiolitic and continen- logical Society [London] Journal, v. 147, p. 59–68. Caledonides in the USA: Blacksburg, Virginia, Virginia tal margin rocks onto the Baltica margin just Andersen, T.B., Berry, H.N., Lux, D.R., and Andresen, A., Polytechnic Institute and State University, p. 41–47. 1998, The tectonic signifi cance of pre-Scandian 40Ar/ Bruton, D.L., and Harper, D.A.T., 1981, Brachiopods and 10–20 m.y. earlier requires large-scale thrusting 39Ar phengite cooling ages from the Caledonides of trilobites of the Early Ordovician serpentinite Otta between the two. (3) The Scandian thrust sheets western Norway: Geological Society [London] Jour- Conglomerate, south central Norway: Norsk Geologisk reached pressures of 12–14 kbar (Fig. 8). At nal, v. 155, p. 297–309. Tidsskrift, v. 61, p. 3–18. Andréasson, P.-G., 1980, Metamorphism in the Tømmerås Calvert, A.T., Gans, P.B., and Amato, J.M., 1999, Diapiric just a few kbar higher pressure, mafi c alloch- area, western Scandinavian Caledonides: Geologiska ascent and cooling of a sillimanite gneiss dome thons overlying the Western Gneiss Complex Föreningens i Stockholm Förhandlingar, v. 101, revealed by 40Ar/39Ar thermochronology: The Kigluaik p. 273–290. Mountains, Seward Peninsula, Alaska, in Ring, U., would have become eclogites dense enough to Andréasson, P.-G., and Johansson, L., 1982, The Snåsa et al., eds., Exhumation processes: Normal faulting, sink into the mantle (e.g., Fig. 8 of Hacker and mega-lens, west-central Scandinavian Caledonides: ductile fl ow, and erosion: Geological Society [London] Abers, 2004) and depress the underlying West- Geologiska Föreningens i Stockholm Förhandlingar, Special Publication 154, p. 205–232. v. 104, p. 305–326. Carswell, T., 2001, Petrological constraints on tectonic ern Gneiss Complex continental crust. Perhaps Arnbom, J.-O., 1980, Metamorphism of the Seve Nappes at models for the stabilization and exhumation of high the Blåhø Nappe, which is folded into the West- Åreskutan, Swedish Caledonides: Geologiska Förenin- and ultrahigh-P rocks: The case history of the Western ern Gneiss Complex in the core of the orogen gens i Stockholm Förhandlingar, v. 102, p. 359–371. Gneiss Region of Norway [abs.]: 6th International Bassett, M.G., 1985, Silurian stratigraphy and facies devel- Eclogite Conference, Ehime Prefectureal Science and consists of about half mafi c rocks that are opment in Scandinavia, in Gee, D.G., and Sturt, B.A., Museum, Niihama, Japan, p. 12–13. locally UHP eclogites (Terry and Robinson, eds., The Caledonide orogen—Scandinavia and related Carswell, D.A., Wilson, R.N., and Zhai, M., 2000, Metamor- areas: Chichester, John Wiley and Sons, p. 283–292. phic evolution, mineral chemistry and thermobarom- 2004; Walsh and Hacker, 2004), is the record Beckholmen, M., 1978, Geology of the Nordhallen- etry of schists and orthogneisses hosting ultra-high of this event. (4) Continental lithosphere capped Duved-Greningen area in Jämtland, central Swedish pressure eclogites in the Dabieshan of central China: by 20–30 km of crust with the 2.95–3.05 g/cm3 Caledonides: Geologiska Föreningens i Stockholm Lithos, v. 52, p. 121–155. Förhandlingar, v. 100, p. 335–347. Chauvet, A., and Dallmeyer, R.D., 1992, 40Ar/39Ar mineral density calculated above for the Western Gneiss Bergman, S., and Sjöström, H., 1997, Accretion and lateral dates related to Devonian extension in the southwest- Complex is negatively buoyant with respect to extension in an orogenic wedge: Evidence from a ern Scandinavian Caledonides: Tectonophysics, v. 210, the asthenosphere (Cloos, 1993), implying that segment of the Seve-Köli terrane boundary, central p. 155–177, doi: 10.1016/0040-1951(92)90133-Q. 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Nappes, Trøndelag, Norway: Tectonophysics, v. 179, ACKNOWLEDGMENTS Bøe, R., 1974, Petrography of the Gula Group in Hessdalen, p. 199–226, doi: 10.1016/0040-1951(90)90291-F. southeastern Trondheim region, with special reference Dallmeyer, R.D., and Gee, D.G., 1986, 40Ar/39Ar mineral Reviewed by Torgeir Andersen, Elizabeth Eide, to the paragonitization of andalusite pseudomorphs: dates from retrogressed eclogites within the Balto- Scott Johnston, David Roberts, Emily Walsh, Michael Norges Geologiske Undersøkelse Bulletin, v. 304, scandian miogeosyncline: Implications for a polyphase Wells, and Aaron Yoshinobu; David Roberts provided p. 33–46. Caledonian orogenic evolution: Geological Society of Boyle, A.P., 1980, The Sulitjelma amphibolites, Norway: America Bulletin, v. 97, p. 26–34. two exceptionally thorough and educational reviews. 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