Proterozoic Evolution and Provenance of the High-Grade Jotun Nappe Complex, SW Norway: U–Pb Geochronology A.M

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Precambrian Research 159 (2007) 133–154

Proterozoic evolution and provenance of the high-grade Jotun Nappe Complex, SW Norway: U–Pb geochronology A.M. Lundmark a,∗, F. Corfu a,S.Spurgin¨ b, R.S. Selbekk b,c a Institute of Geosciences, Sem Sælands vei 1, 0371 Oslo University, Norway b Mineralogisch-Geochemisches Institut, Albert-Ludwigs-Universit¨at, Albertstraße 23b, 79104 Freiburg, Germany c Natural History Museum (Geology), Oslo University, PB 1172 Blindern, 0318 Oslo, Norway Received 14 June 2006; received in revised form 21 December 2006; accepted 25 December 2006

Abstract New U–Pb geochronological data are used to explore the Proterozoic history and provenance of the Jotun Nappe Complex in the Scandinavian Caledonides, SW Norway. Orthogneisses in the upper part of the complex, the high-grade Upper Jotun Nappe, yield protolith ages of 1.66, 1.63 and 1.26 Ga. During the Sveconorwegian orogeny, the rocks underwent penetrative deformation and metamorphism up to granulite facies conditions. Later shearing, hydration and retrogression under amphibolite-facies conditions is dated to 954 ± 3 Ma by local, syntectonic melting of granitic gneiss and 950 ± 1 Ma emplacement of syntectonic, syn-retrogression pegmatites, suggested to reflect decompression and partial exhumation of the lower crustal rocks. A second cycle of granulite- facies metamorphism is dated to 934 ± 1 Ma, coeval with 934 ± 3 Ma anatexis of granitic gneiss. We propose that the second cycle represents post-collisional elevated heat flow, correlated to granulite-facies metamorphism in the related Lindas˚ Nappe, and to anorthosite and granite plutonism at higher crustal levels in the western Baltic Shield, possibly in response to late-orogenic underplating or delamination. The tectonometamorphic record of far-travelled, high-grade crystalline rocks in the Jotun Nappe Complex, the Lindas˚ Nappe and the Dalsfjord Nappe confirms their origin in the Baltic Shield, and suggests that they provide a glimpse of a deeper segment of the Sveconorwegian orogen than presently exposed in the basement of SW Norway. © 2007 Elsevier B.V. All rights reserved.

Keywords: Caledonides; Southwest scandinavian domain; U–Pb geochronology; Zircon; High-grade metamorphism; Proterozoic; Sveconorwegian

1. Introduction ern Norway along with the allochthonous sedimentary cover of the Baltic Shield and exotic fragments of Iapetan As Baltica collided with Laurentia during the Cale- afﬁnities (Fig. 1). The crystalline nappes have tradition- donian orogeny, crustal shortening was partly accommo- ally been thought to represent the imbricated margin dated by nappe stacking, leading to the emplacement of the Baltic Shield (Roberts and Gee, 1985), but their of a series of thrust sheets on top of the Baltic Shield. enigmatic positions in the nappe sequence have also Far-travelled crystalline crustal rocks in the Jotun Nappe led to proposals that they may constitute exotic ter- Complex, the Lindas˚ Nappe and the Dalsfjord Nappe ranes (Andersen and Andresen, 1994; see also Emmett, are present in the Caledonian thrust sheets of southwest- 1996), and the validity of correlations between the crystalline nappes has been questioned (Milnes et al., 1997; Wennberg et al., 1998). ∗ Corresponding author. Fax: +47 228 54215. Comparisons of the tectonometamorphic histories E-mail address: [email protected] (A.M. Lundmark). of the nappe complexes and the Baltic Shield have

Fig. 1. Geology of southwestern Norway (modified after Gee et al., 1985; Lutro and Tveten, 1996; Fossen and Holst, 1995; Fossen and Dallmeyer, 1998; Sigmond, 1998; Tucker et al., 1990). The Valdres Sparagmite is interpreted to represent the cover sequence of the Lower Jotun Nappe in the area of the Jotun Nappe Complex (see text), the units are not shown separately on the map. Localities are: (I) Hurrungane; (II) Lake Galgebergstjernet; (III) Fannaraken;˚ (IV) Rambera. so far been hampered by a lack of age data from age, are present only in the Upper Jotun Nappe, thus the high-grade rocks of the Jotun Nappe Complex, separating the complex into two parts, juxtaposed dur- the largest of the crystalline units in the Caledonian ing the Caledonian orogeny (Lundmark and Corfu, allochthon of southwestern Norway. The matter has 2007). been further confused by the report of an erroneous In this study, new U–Pb geochronological data are Precambrian age of granitic dykes in the Jotun Nappe reported and used to examine the Proterozoic history of Complex. The granites, now known to be of Silurian the Upper Jotun Nappe, and to attempt to constrain its A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154 135 provenance and relation to the Lower Jotun Nappe, the 2.2. Caledonian nappes Lindas˚ Nappe and the Dalsfjord Nappe. The collisional phase of the Caledonian orogeny was 2. Geological setting accompanied by southeast-directed thrusting in southwestern Norway and emplacement of nappes of various 2.1. Western Gneiss Region origins on top of the Baltic Shield. The nappes have traditionally been divided into four levels according to The Proterozoic basement of the Western Gneiss their inferred origin; the Lower and Middle Allochthon Region (WGR), southwestern Norway, is overlain by represent parts of Baltica and its margin, the Upper a set of Caledonian thrust sheets and is itself inter- Allochthon consists of the outermost border of Baltica nally imbricated (e.g., Tucker et al., 2004)(Fig. 1). and exotic fragments of Iapetan affinities, and the Upper- The parautochthonous WGR is generally regarded as most Allochthon is comprised of rocks of Laurentian the continuation of the Baltic Shield, although its rela- affinity (Roberts and Gee, 1985; Gee et al., 1985). tion to the autochthonous basement is obscured by the The Jotun Nappe Complex, dominating the Cale- overlaying thrust sheets (e.g., Gaal´ and Gorbatschev, donides of southwestern Norway, and the smaller Lindas˚ 1987; Milnes et al., 1997). The WGR consists mainly Nappe and Dalsfjord Nappe, are allochthonous slices of juvenile crust formed at 1.7–1.55 Ga (Kullerud et al., of Proterozoic crystalline crust (Fig. 1). The high-grade 1986; Tucker et al., 1990). During the ca. 1.25–0.9 Ga gneisses in the nappes have been correlated by field geol- Sveconorwegian orogeny (Starmer, 1990), part of the ogists since they were first described by Goldschmidt in assembling of the supercontinent Rodinia, the southwest 1916 as the “Bergen-Jotun Stamm”. The origin of the margin of the Baltoscandian craton and the southwest- nappes is uncertain, but they are generally assigned to ern parts of the WGR (the Southwest Scandinavian the Middle Allochthon, implying an origin in the pre- Domain; Gaal´ and Gorbatschev, 1987) were extensively imbricated Baltoscandian margin. reworked (Tucker et al., 1990; Skar˚ and Pedersen, 2003). The heterogeneous Dalsfjord Nappe is dominated by In contrast, the northeastern parts of the WGR under- syeno-monzonites and minor gabbro and anorthosite. went no corresponding isotopic and tectonic disturbance Zircon U–Pb protolith ages of 1634 ± 3 and 1464 ± 6Ma at this time. This led to the proposal that the Pro- for monzonite (partially retrogressed mangerite) and togine Zone/Sveconorwegian Frontal Deformation Zone gabbro, respectively, were reported by Corfu and (PZ/SFDZ), which marks the boundary of Sveconor- Andersen (2002), while an imprecise lower intercept wegian influence in Sweden, resurfaces west of the age of 882 ± 29 Ma along with concordant titanite ages Caledonian allochthon in the WGR (Tucker et al., 1990). interpreted to reflect ≥960 and ≤920 Ma titanite growth, In the WGR southwest of the SFDZ, high-grade testify to a strong Sveconorwegian influence. The nappe metamorphism has been dated to 987 ± 10 Ma (Røhr displays a meta-sedimentary cover sequence, the Høyvik et al., 2004). Extensive, ca. 970–960 Ma migmatiza- Group, possibly a correlative of late Precambrian arkoses tion under amphibolite facies conditions was triggered and conglomerates that make up the Valdres Sparagmite by widespread emplacement of metaluminous gran- (Andersen et al., 1998), which in turn is interpreted as ites, dated at one locality to 966 ± 3Ma (Skar˚ and the allochthonous cover of the Baltic craton (Roberts Pedersen, 2003). Post-tectonic dykes were emplaced at and Gee, 1985). Phengite 40Ar/39Ar ages date metamor- ca. 950–940 Ma (Tucker et al., 1990), corresponding to phism of the sediments to ca. 447 Ma (Andersen et al., a monazite age of 949 ± 3 Ma interpreted to date the 1998). In the Silurian, the Høyvik Group was once more waning stages of metamorphism (Røhr et al., 2004). exposed at the surface and was unconformably over- The Sveconorwegian event in the WGR ended with the lain by the sediments of the Herland Group (Brekke and intrusion of granites dated to 932 ± 8Ma(Corfu, 1980). Solberg, 1987). Following the break-up of Rodinia and the formation The Lindas˚ Nappe, part of the Bergen Arc Complex, is of the Iapetus Ocean, orogenic activity resumed during mainly composed of Proterozoic anorthosites, charnock- the Ordovician-Silurian Caledonian orogeny. The colli- ites to mangerites and banded gneisses (Kolderup and sional phase involved subduction of the leading edge of Kolderup, 1940) that, in the field, are indistinguishable the Baltoscandian shield, during which the northwest- from rocks exposed in the Jotun Nappe Complex. The ern parts of the WGR were extensively deformed (Rice, only exception is the presence of Caledonian eclog- 2005) and exposed to high pressure/ultra high-pressure ites in the Lindas˚ Nappe, absent in the Jotun Nappe conditions (e.g., Cuthbert et al., 2000; Carswell et al., Complex. Discordant zircon data from a charnockite 2003). in the Lindas˚ Nappe yield a protolith (upper inter- 136 A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154 cept) age of 1237(+43/−35) Ma and date high-grade commonly exhibiting primary compositional layering, metamorphism (lower intercept) at 932(+28/−36) Ma. and (meta-) gabbro with subsidiary (meta-) troctolites A 951 ± 2 Ma jotunite-mangerite body, possibly part of (Bryhni et al., 1983). Small bodies of ultramafic rocks as an anorthosite mangerite-charnockite-granite (AMCG) well as thin quartz-rich layers (which may represent sed- suite, cross-cuts the anorthosite. New zircon growth at imentary rocks and/or sheared quartz veins) are locally 933 ± 2 and 929 ± 1 Ma date ensuing metamorphism up present, the latter commonly associated with highly to granulite facies conditions (Bingen et al., 2001). deformed contact zones between the anorthosite-gabbro- troctolite lithology and other rock types (Sigmond, 2.3. The Jotun Nappe Complex 1998). Despite extensive mapping carried out during the 1970s and 1980s, it is still unclear how the different rock The Jotun Nappe Complex consists of polymetamor- types relate to each other. Geochronological data on the phosed and polydeformed rocks transected by numerous Upper Jotun Nappe rocks are limited to an imprecise shear zones, including prominent extensional shear 1528(+123/−99) Ma U–Pb zircon upper intercept age zones associated with late- to post-Caledonian extension from a mafic gneiss (Scharer,¨ 1980b), a Rb–Sr whole (Fossen, 1992; Milnes et al., 1997; Andersen, 1998). A rock age of 1678 ± 36 Ma (Koestler, 1983) obtained layer of early Palaeozoic phyllitic and quartzitic rocks from four variably retrograded granulites, and a set of ascribed to the Lower Allochthon (the Vang and For- mostly Caledonian K–Ar ages (Battey and McRitchie, tun phyllites) separates the Jotun Nappe Complex from 1973). the basement and forms the detachment along which the nappes were emplaced (Milnes et al., 1997). The 2.4. Study area nappe complex can be subdivided into two units, the Lower and the Upper Jotun Nappe, juxtaposed during The main study area is situated along the northwestern the Caledonian orogeny (Lundmark and Corfu, 2007). margin of the Upper Jotun Nappe (I–III; Fig. 1). Here, The Lower Jotun Nappe crops out along the mar- three localities were chosen for sampling and age deter- gins of the complex and consists mainly of syenitic minations. A fourth locality in the south-central Upper to monzonitic (1694 ± 20 Ma) and gabbroic rocks Jotun Nappe was sampled for comparison (IV; Fig. 1). (1252 ± 28 Ma) metamorphosed to amphibolite-facies Locality I straddles a Caledonian shear zone on the conditions (909 ± 16 Ma) (Scharer,¨ 1980a). Preserved western slopes of the Hurrungane massif (Fig. 2). The in the Lower Jotun Nappe is a basement-cover contact, north-trending, steeply inclined, ca. 100–200 m wide linking late Precambrian Valdres Sparagmite (arkoses shear zone cross-cuts all other structures and has a green- and conglomerates) to the crystalline rocks. These schist facies mineralogy dominated by green hornblende, allochthonous sediments are commonly correlated with epidote, biotite and chlorite, typical of Caledonian shear the (par-) autochthonous cover of the Baltic craton, zones in the nappe complex (Milnes et al., 1997). East linking the Lower Jotun Nappe to the pre-Caledonian of the shear zone, two-pyroxene granulites of interme- Baltoscandian margin (Nickelsen et al., 1985; Hossack diate to felsic compositions dominate. The granulites et al., 1985; Milnes et al., 1997). The Lower Jotun Nappe are variably retrograded under amphibolite-facies con- is separated by mylonitic shear zones from the overlying ditions, and range from apparently massive or weakly rocks of the Upper Jotun Nappe, which occupies the cen- foliated to gneissic. A weakly foliated, medium- to fine- tral parts of the nappe complex (Battey and McRitchie, grained granitic two-pyroxene granulite was sampled for 1973; Lutro and Tveten, 1996). age determination (sample A; Fig. 2). Granitic, allanite- The Upper Jotun Nappe is dominated by variably ret- bearing pegmatites intrude the granulites both oblique rograded granulite-facies rocks intruded by 427 ± 1Ma and parallel to the main foliation. Similar pegmatites granitic dykes (Lundmark and Corfu, 2007) that were present in various parts of the northwestern Upper Jotun injected during top-to-southeast translation of the nappe Nappe have been interpreted to represent several gener- prior to its emplacement on top of the Lower Jotun ations of intrusions (Koestler, 1983). Nappe (Lundmark and Corfu, in review). The north- A north-trending lens-shaped body of garnet amphi- eastern part of the Upper Jotun Nappe is dominated by bolite adjacent to the two-pyroxene gneiss is delimited partially retrograded gneisses of granitic (charnockite), on both sides by steeply east-dipping shear zones. syenitic (hypersthene syenite), monzonitic (mangerite), Numerous sub-parallel north-northwest striking granitic dioritic (jotunite) and gabbroic compositions, whereas pegmatites, typically ca. 0.5–1.5 m wide, with allanite, the central and southwestern parts of the Upper Jotun tourmaline, beryl and various other accessory phases Nappe are dominated by high-grade (meta-) anorthosite, (Spurgin,¨ 2006) (sample B; Fig. 2), intrude the gar- A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154 137

Fig. 2. Detail of locality I, on the southwestern slope of Hurrungane (modiﬁed after Koestler, 1989; Spurgin,¨ 2006), sample locations A–F correspond to descriptions in text.

net amphibolite lens. Locally, the pegmatites are both ing garnet amphibolite. It was suggested by Battey and accommodated and deformed in shear zones, imply- McRitchie (1975) that the peridotites represent the pro- ing syntectonic magmatism. This was also proposed by toliths of the hornblende-rich garnet amphibolite. Spurgin¨ (2006), based on the observation of healed frac- To the west of the Caledonian shear zone, the road tures in beryl crystals in the pegmatites and dynamic from Ardal˚ to Turtagrø offers good exposure of granitic recrystallisation of quartz close to deformation zones, to monzonitic gneiss (sample C; Fig. 2B). The felsic implying temperatures in excess of the greenschist facies gneiss is closely associated with generally dioritic or conditions typically observed in Caledonian shear zones gabbroic gneiss, possibly reﬂecting a genetic relation- in the area. The garnet amphibolite retains ample relics ship. Remnants of granulite-facies mineral assemblages of a pre-hydration mineral assemblage in the form of are rare, but locally partially preserved in lilac coloured hornblende mantled orthopyroxene and clinopyroxene. gneiss. The lilac gneiss, owing its colour to the hue of Locally preserved coronas of plagioclase, garnet and its feldspar constituent, typically occurs as lenses mea- pyroxene (the latter commonly partially replaced by suring a few tens of meters across and grades into the hornblende) correspond to observations elsewhere in the surrounding amphibolite-facies gneisses. In lilac gneiss Upper Jotun Nappe by Battey and McRitchie (1975), of gabbroic composition sampled in the southeastern who deduced corona formation from originally coex- extension of the ﬁeld area, clinopyroxene is partially isting olivine and plagioclase. Hydration of the garnet replaced by hornblende growing along grain edges, amphibolite (i.e. retrogression of pyroxene to amphibole whereas cummingtonite (also partially replaced by horn- and the disappearance of garnet) seems to be pro- blende), is interpreted to have formed at the expense moted by proximity to both pegmatites and ubiquitous of orthopyroxene (cf. Austrheim and Robins, 1981), deformation zones. Small bodies of peridotite, com- now absent from the sample. The rock has a typical mon in high-grade gneisses in the northwestern Upper metamorphic granoblastic polygonal texture, and biotite Jotun Nappe and interpreted as an original magmatic laths in textural equilibrium with the high-grade assem- component of the high-grade assemblage (Battey and blage show no preferred orientation. In situ melting McRitchie, 1975), are also present in the garnet amphi- of granitic gneiss has locally produced banded ana- bolite. The medium- to coarse-grained peridotites are texite with pockets or layers of neosome (Fig. 3A). dominated by pyroxene, hornblende and olivine and do The neosomes (sample D; Fig. 2) typically lack fab- not appear to have tectonic contacts to the surround- ric and, locally, aggregates in synanatectic shear zones 138 A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154

Fig. 3. (A) Locality I: Granitic neosome (sample C) and gneissic paleosome in banded anatexite. Rock hammer for scale. (B) Locality I: Granitic neosome aggregating in coeval shear zone. (C) Locality I: Shear zone hosting a granitic pegmatite (sample F). The shear zone deforms banded anatexite. (D) Locality II: Mobilised anatectic granite intruding amphibolite gneiss. Outcrop is ca. 1 m wide.

(Fig. 3B). Three types of granitic pegmatites were A small body of peridotite, similar to those occur- distinguished in the area: allanite-bearing pegmatites ring east of the shear zone, is present in the gabbroic with white and/or lilac feldspar, a solitary zoned peg- gneiss. Other peridotite bodies have been mapped in matite with a core of white quartz surrounded by pink the continuation of both the felsic and gabbroic gneiss potassic feldspar that intrudes parallel to the gneissic north and south of the field area (Koestler, 1989; Maurer, foliation of the host rock (sample E; Fig. 2), and up 2005). to 2 dm thick pegmatites with large lilac feldspar that Locally, mylonitic shear zones associated with retro- locally cross-cut migmatites in the felsic gneiss (sample gression to greenschist facies mineral assemblages affect F; Figs. 2 and 3C). all rock types on both sides of the shear zone and are A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154 139 interpreted to represent Caledonian overprinting, also netic separators and heavy liquids. The minerals were evident in the disturbance of the isotopic systems in some hand-picked from the least magnetic fraction under a of the samples (see below). binocular microscope. For isotope dilution thermal ion- At locality II granitic gneiss is exposed along the road isation mass spectrometry zircons and titanites were air from Lom to Turtagrø at Lake Galgebergstjernet (Fig. 1). abraded (Krogh, 1982) and dissolved at 190 ◦C in teflon The granitic gneiss (sample G) shows ubiquitous signs bombs (zircon) or in Savillex vials on a hot-plate (titan- of incipient anatexis and contains pockets of anatectic ite) after addition of 205Pb/235Uor202Pb/205Pb/235U granite. Locally, the anatectic granite has been mobilised spike. Other details are given in Lundmark and Corfu into dykes and veins intruding the granitic gneiss and (2007) and references therein. The data were corrected adjacent intermediate hornblende gneiss (Fig. 3D). The with fractionation factors of 0.1%/amu for Pb and granitic gneiss is similar to and possibly correlated with 0.12%/amu for U or by factors determined with the granitic gneiss at locality I. 202Pb/205Pb spike. Blank correction was 2 pg Pb and Locality III is situated on the east-facing slope of 0.1 pg U. The initial Pb was corrected using Pb compo- the Fannaraken˚ massif (Fig. 1). Here, two-pyroxene sitions calculated with the Stacey and Kramers (1975) granulite is deformed by an anastomosing network of model. The results were plotted and regressed using the mylonitic shear zones, commonly controlled by the ori- software of Ludwig (2003). The decay constants are entation of pre-existing, locally allanite-bearing granitic those of Jaffey et al. (1971). Uncertainties in the isotopic pegmatites (Koestler, 1988). The undeformed centre of ratios and the ages are given and plotted at 2 sigma. a ca. 10 m wide lens of partially retrograded granulitic Cathodoluminescence (CL) imaging was performed gneiss surrounded by northwest trending, steeply dip- on grains mounted in epoxy on a JEOL-JSM6460LV ping dextral mylonitic shear zones was sampled (sample Scanning Electron Microscope at the University of Oslo. H). The lens also hosts dm-sized fine-grained mafic xenoliths and a felsic dyke that cross-cuts the gneissic 3.2. Sample A (locality I): granitic two-pyroxene fabric. granulite Locality IV, Rambera, lies in the south-central part of the regional high plateau defined by the Upper Jotun The sample is dominated by perthitic feldspar and Nappe granulites (Fig. 1). The area is dominated by quartz. The minor dark constituents are polygonal various gneisses that are intruded by pegmatites and grains of clinopyroxene and orthopyroxene in roughly deformed by mylonitic shear zones. Felsic gneiss (sam- equal amounts, and minor (secondary?) biotite. Zircons ple I) and intermediate to mafic gneiss (sample J) obtained from the sample fall into three morphologi- were sampled. Field relations indicate that the gneisses cal groups: slightly opaque, ca. 200–400 ␮m fragments have undergone penetrative deformation and high-grade of large crystals, clear, ca. 100 ␮m irregularly shaped metamorphism together, followed by the intrusion of crystals, and clear, subhedral, slightly rounded or granitic pegmatites (sample K). One generation of multi-faceted, ca. 100–300 ␮m crystals. In CL images, mylonites, post-dating formation of the gneissic fab- oscillatory growth zoning is present in zircons from all ric of the host rocks, is typically northeast-trending and groups (1–6; Fig. 4). Irregular CL-bright patches are southeast-dipping with a top-to-northeast sense of shear. interpreted as recrystallised domains. Regular growth The shear zones vary in size from centimeter- to meter- patterns dominate fragments and irregularly shaped scale, and locally host abundant ≤1 cm, post-kinematic grains (1–3; Fig. 4), while rounded to multi-faceted granoblastic garnets. The pegmatites locally cross-cut grains are dominated by rims surrounding rounded oscil- the mylonites and are in other instances deformed by latory zoned core domains (4–6; Fig. 4). them, suggesting that shearing and intrusion of the peg- Four multi-grain zircon analyses plot on a discor- matites were coeval. A younger generation of mylonite dia intercepting at 1633 ± 10 and 933 ± 10 Ma (A2–A5; zones that cross-cut all other structures is interpreted to Fig. 5A; Table 1). The CL images indicate that the discor- represent Caledonian deformation. dance reflects both zircon growth and lead-loss through recrystallisation at ca. 933 Ma. A fourth data point plots 3. Geochronology: samples and results below the curve indicating more recent lead-loss (A1; Fig. 5A). Two multi-grain analyses of monazite produce 3.1. Analytical methods a concordant age of 934.2 ± 1.4 Ma (A6–A7; Fig. 5A). Anchoring the zircon data at the monazite age produces Zircons were separated from the 11 samples using an upper intercept age of 1633.7 ± 3.6 Ma, interpreted standard methods: crushing, Wilfley table, Frantz mag- as the protolith age of the two-pyroxene granulite. 140 A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154

Fig. 4. Cathodoluminescence images from selected zircons. Numbers correspond to descriptions in text.

3.3. Samples B and B (locality I): allanite-bearing clear to opaque, partly resorbed zircons of variable mor- pegmatites phology. To date the intrusion of the pegmatite, U–Pb analyses were carried out on multi-faceted, pink grains. A sample from a ca. 30 cm wide subvertical pegmatite The single grain analyses plot on the Concordia curve, dyke (sample B) in garnet amphibolite yielded abundant yielding a concordant age of 949.6 ± 1.3 Ma (B1–B4; zircon, dominated by ca. 100–500 ␮m multi-faceted, Fig. 5B). A similar pegmatite present in the greenschist pink, sharp-edged crystals. A second group consists of facies sheared edge of the garnet amphibolite (sample B ) A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154 141 f Pb/ Pb 207 206 [Ma] f Pb/ U 207 235 [Ma] f Pb/ U 206 238 [Ma] f 2 sigma [abs] f Pb/ Pb 207 206 rho f 2 sigma [abs] f Pb/ U 206 238 f 2 sigma [abs] f Pb/ U 207 235 e Pb/ Pb 206 204 d Pbcom [pg] c [ppm] Th/U b U b g] ␮ Weight [ a : Hurrungane, allanite-bearing granitic pegmatite (M04-81) UTM68122N4327E B5B6B7 Z [2]B8 o frgt Z [2] o frgt Z [1] pm-fC1 Z 11 [1] pm-fC2 6C3 29 Z [4]C4 cl 35 euh 145 Z [1]C5 cl tp Z [1]C6 240 tp 642 Z [1] 7C7 frgt 0.58 287 Z [1]C8 cl tp 1 Z [3] 0.59C9 tp 1.6 0.44 Z [1]C10 1 frgt 417 0.45 0.6 1 Z [1] cl 4.1 tp 3 16443 Z 555 [1] frgt 3.2 TD1 [1] br 3.3917 anh 35482 6 0.44 189D2 5 44755 0.0088 344 3.0483D3 6 31501 0.41 1.5538 1.7 604 22 Z 0.0116 [4] 0.25530D4 9 euh, cr 1.5438 0.0038 Z 0.18 [4) 1.1 138 0.00063D5 euh, cr 0.17 0.23629 0.0035 428 Z 17840 [19] 0.15906 sbh-anh 0.92 0.40 20 0.00093 462 0.7 Z [12] 1.0 0.15830 272 0.00035 anh-sbh 1.6422 0.09635 16 299E1 0.91 Z 7 5231 0.40 [24] 0.6 0.00033 cl 0.0038 0.96 0.00010 rd 0.22 20E2 0.09356 1.6003 0.96 0.07085 0.54 2662 1465.8 2.7 0.00015 0.16317 293 3411 0.02 1.2 0.0053 0.07073 0.00005 Z 0.37 29904 [1] 1502.5 1.5696 15 cl 0.00034 1367.4 tp 1.5 1.5562 481 34.0 79 0.00005 Z [2] 0.16101 1.5484 0.0068 tp 234 951.6 1554.6 0.94 1.6 1419.8 0.0062 3029 16972 0.47 0.00049 0.0041 947.3 0.07300 1499.3 0.15981 952.0 18611 1.5446 0.15874 1.5377 10 0.90 0.00006 1656 0.41 370 0.37 8.8 0.15849 0.00055 948.0 16501 0.31 1.5342 0.0042 0.00057 0.0035 0.07209 953.1 13.6 1.4190 0.00040 0.81 974.3 1.4 0.0036 1 1.5314 0.88 0.00010 949.7 0.15776 0.0039 2.0 0.15738 0.90 0.07123 0.0036 0.07110 0.31 397 9904 986.6 0.15732 0.00035 0.00031 962.4 0.07086 0.00018 0.14894 0.00014 3621 12693 0.00033 0.15690 3.0374 1013.8 0.83 0.94 0.00008 4.3 2644 31473 0.00030 955.7 970.3 0.00033 2.9910 0.0091 2.7736 0.94 0.07101 949.8 0.07086 0.38 2.5500 0.82 948.4 0.0104 0.0062 0.07073 0.00011 0.93 0.00005 958.3 988.4 0.23624 0.0056 16802 0.06910 953.0 0.00006 0.07079 0.35 4.3 949.9 0.23291 0.00067 0.22322 944.3 0.00011 942.2 964.1 2.5246 0.00006 0.21110 960.3 0.00082 0.00043 941.8 0.96 0.7 953.3 0.0086 0.00041 895.0 948.3 945.6 939.5 0.90 0.09325 0.95 9190 944.2 0.96 0.20975 0.09314 0.00008 0.09012 896.9 957.7 953.4 34661 1.5290 943.0 0.08761 0.00069 0.00014 0.00007 949.6 1367.1 0.0058 0.00006 901.7 1.5231 0.97 951.3 1349.7 1298.8 1417.1 0.0040 1234.7 0.08729 0.15752 1405.3 1348.5 1493.0 0.00008 1286.4 0.00054 0.15665 1490.7 1428.1 1227.5 1373.9 0.93 0.00037 1279.1 0.07040 0.96 0.00010 1367.0 0.07051 0.00005 943.0 938.2 942.1 939.7 940.0 943.4 A1A2A3 Z [3]A4 cl euh sp Z [3]A5 fr Z [14]A6 cl 8 sbh sp Z [21]A7 cl sbh sp 26 Z [3] fr 14 M 35 [10] NAB1 134 M [7] NAB2 263B3 16 11 268 149 Z [1] 0.45B4 p m-f 12 Z [3] p 0.42 m-f 3.5 Z [1] p 0.33 m-f 250 1.10 238 Z 34 4.7 [1] p m-f 14.3 36 188 1.7 4415 16 31.0 20822 0.29 2.9124 23 223 31.1 28573 11.5 2.8641 5260 20.1 0.0115 460 2.4814 0.0077 2.9047 548 9.5 0.22989 0.0055 0.0128 0.38 490 0.22807 0.00090 3427 1639 0.43 0.20772 0.00054 0.23026 0.89 1.5121 2.3 2.3152 0.38 2323 0.00040 0.95 0.00093 0.09188 0.0036 1.8 0.0062 0.38 0.96 0.09108 1.5076 0.00017 0.94 2.9 32542 0.15608 0.08664 0.00008 0.0038 0.19879 1.6 0.09149 1333.9 92087 0.00030 1.5495 0.00006 0.00041 1324.4 0.00013 1385.1 0.15574 29771 1.5498 0.92 0.0042 1216.6 0.86 1372.5 1335.9 0.00030 70504 1465.0 1.5482 0.0039 0.07026 0.08447 1266.6 0.15872 1448.3 1383.1 0.87 1.5444 0.0065 0.00007 0.00012 0.15891 0.00040 1352.6 0.07020 1456.8 0.0038 1168.8 0.15878 0.00036 935.0 0.95 0.00009 0.15842 0.00066 1217.0 0.96 0.07081 935.3 933.1 0.00036 0.96 0.07073 0.00006 1303.4 0.95 0.07072 0.00005 936.0 933.4 949.6 0.07070 0.00009 950.7 934.3 0.00006 950.3 950.0 950.4 948.0 951.9 949.8 949.7 948.3 949.2 948.9 Sample C: Hurrungane, granitic neosome in ibanded anatexite (M05-38) UTM6812N4325E Sample D: Hurrungane, alkali granitic gneiss (M04-51) UTM68116N4322E Sample E: Hurrungane, zoned granitic pegmatite (M04-76) UTM6812N4325E Table 1 U–Pb isotopic data and ages No. in plot Fraction Sample A: Hurrungane, granitic two-pyroxene granulite (M04-09) UTM68118N4333E Sample B: Hurrungane, allanite-bearing granitic pegmatite (SS088) UTM68118N4327E Sample B 142 A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154 ˚ aken, quartz-dioritic (jotunite) gneiss (M04-54) UTM68207N4457E E3E4 Z [2] tp Z [1] clF1 tpF2F3 Z [1] clp 3 frgt 4F4 Z [1] clp frgtF5 Z 20 1048 [1] clp frgt 224 Z 17 [1] clp frgt Z 19 388 [1] 0.32 clpG1 frgt 20 0.33 550G2 1.4 539 0.37 ZG3 1 [2] cl sbh 1.7 677 0.27 ZG4 [1] cl tp 22278 1414 2.6 0.26 ZG5 [1] cl tp 5.0 1 0.27 5234 ZG6 [5] cl 1.5232 sbh 29279 1.5 0.29 ZG7 [2] cl 1 cr 1.4982 18077 7.1 0.0042 Z 231G8 [2] b 7 1.5087 10 tp 67579 1.6 ZG9 0.0056 [1] b 0.15646 1.4831 363 tp 18354 0.0055 ZG10 0.83 [1] b 1 1.4833 140 tp 216 0.15372 0.00038 0.0034 ZG11 8511 [4] b 1.4782 0.15586 3 tpZ 0.69 [1] 0.0037 b 1.0G12 0.00053 tp 0.96 0.15393 233 1 Z 0.66 0.64 [1] 1.4693 0.0036 b 0.00051G13 tp 3908 0.15380 0.5 1 T 0.95 [2] 0.07061 b 0.00031G14 12 anh 1346 4351 0.0046 0.15349 0.93 2.5 T 0.60 2.7 [1] b 0.00034G15 1 anh 0.07069 1557 0.08 13829 0.00006 0.96 T [7] 0.15261 1587 b 0.00033 0.07021 4.0228 1 anh 18 0.09 0.96 0.6 13537 T 0.00008 [1] 6709 3480 b 0.06988 3.9474 anh 13 0.9 0.00044 0.06 937.0 0.96 0.00009 0.0169 0.07 0.06995 15 3.6185 924 1.5 420 0.00005 3.6883 921.8 0.0140 139352 6538 0.91 0.06 0.06985 0.29003 0.8 939.8 122 933.7 0.00005 6 0.0254 4.4 0.0143 0.28547 164 0.06 0.06983 922.9 0.06 0.00005 1.6358 929.7 3.3849 0.00108 8908 2.7 19590 946.2 0.26841 933.9 922.2 0.01 0.26997 0.00103 140 43052 0.00009 50.5 0.0042 0.0158 0.92 4.9 1.5883 923.5 920.5 948.5 0.88 0.00191 12483 1.5631 38.7 0.00103 0.92 934.4 923.6 1.5384 0.16266 0.25509 0.10059 915.6 0.0042 0.32 49.1 924.8 1407 0.94 921.5 0.0039 1.5167 0.90 1842 0.10029 0.0045 0.00038 0.00118 0.00017 926.8 0.15982 20.1 394 917.8 0.09777 0.15837 1.4159 0.0039 0.09909 0.00015 923.8 1.5053 0.15744 0.96 0.89 1641.7 477 0.00037 0.00024 1.3949 0.00036 0.0041 0.15595 0.00017 923.3 1618.8 0.0047 0.00043 398 0.07293 0.09624 1638.8 0.93 1.3899 1532.7 0.0092 0.94 0.14841 0.00036 1540.6 1623.5 0.15544 0.98 0.00005 0.00021 1635.1 1.3829 0.07208 0.0109 1553.6 0.14686 0.07158 0.00029 0.96 1568.8 1629.4 0.00038 0.07087 1464.7 0.0089 0.00007 0.14634 971.6 1582.1 0.00031 0.00006 0.76 0.07054 1607.0 0.83 0.00005 1500.9 0.14564 0.00051 955.8 984.1 0.34 0.06919 0.00006 0.07023 947.7 1552.4 0.00033 0.51 942.6 1012.1 0.06889 0.00013 965.6 0.00012 934.2 955.7 0.37 0.06888 945.9 0.00043 892.1 988.2 931.4 0.06886 937.1 0.00047 974.1 953.6 883.3 0.00041 895.7 932.5 944.0 880.4 886.8 876.5 904.5 935.2 884.6 895.3 881.7 895.3 894.7 H1H2 ZH3 [4] euh Z [21] euh Z [5] euhI1 1I2 18I3 Z [2] 1186 cl euh 9 204 Z [3] cl m-f Z [3]J1 cl 0.43 frgt 243 7 0.43J2 12J3 7.7 Z [5] cl 0.46 4 1.9 m-fJ4 48 Z [3] cl 46 frgt 21.1 Z [1] 2804 cl 35120 euh 0 55 Z 0.57 [1] cl frgt 0.75 1 4.0525 4.0402 1892 1 8.1 0.86 72 0.0135 3.9 0.0090 9 4.0301 346 2.9 0.28950 110 0.28911 0.0103 0.56 501 1670 177 0.66 0.00084 0.00057 0.28852 0.59 2.0055 0.0 1.9739 853 0.93 1.4 0.55 0.96 0.00060 0.0095 2.9 0.0273 1.8489 0.10152 0.10135 6229 0.90 3.5 3017 0.18731 0.18559 0.0141 0.00012 0.00006 2.0298 0.10131 451 0.00045 2.0282 0.00255 4631 0.17695 1639.0 1637.1 0.0004 0.00011 0.54 1.9034 0.0125 0.96 0.00109 1.6231 1644.8 1642.3 0.18898 1634.1 0.07765 0.0173 0.18838 0.07714 0.81 0.0040 1652.2 1649.0 0.00040 1640.3 0.00031 0.18181 0.00119 0.00030 0.07578 0.16327 0.89 1648.2 1106.8 0.00087 0.86 1097.4 0.00034 0.00036 0.07790 1117.4 0.54 0.07808 1106.6 1050.3 0.88 0.00009 1138.0 0.07593 0.00026 1124.7 1063.0 0.07210 1115.9 0.00058 1112.6 1089.3 0.00009 1125.5 1076.8 1125.0 974.9 1144.2 1082.3 1149.0 979.2 1093.2 988.8 Sample F: Hurrungane, granitic pegmatite (M05-08) UTM6812N4325E Sample G: Lake Galgebergstjernet, granitic gneiss (M04-75) UTM6823N4389E Sample H: Fannar Sample I: Rambera, quartz-monzonitic gneiss (M04-67) UTM67754N3751E Sample J: Rambera, dioritic two pyroxene gneiss (M04-68) UTM67747N3757E A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154 143 l; f contains similar pink, multi-faceted zircons. Two grains Pb/ Pb were analysed and plot close to 950 Ma on Concordia 207 206 [Ma] (B7–B8; Fig. 5B). Two analyses of grains belonging to

f the second, morphologically variable population show a Pb/ U strong component of inheritance (B5–B6; Fig. 5B). 207 235 [Ma]

f 3.4. Samples C and D (locality I): granitic neosome Pb/ U and alkali granitic gneiss 206 238 [Ma] f Granitic neosome in banded anatexite with biotite as the only maﬁc phase was sampled along the 2 sigma [abs] Ardal–Turtagrø˚ road to determine the age of anatexis

f (sample C). To date the protolith age of the gneiss, a sec-

Pb/ Pb ond sample was collected in alkali granitic gneiss (sam- 207 206 ple D) further south along the road (Fig. 2), in an effort to avoid zircons directly affected by the anatectic melting. rho The medium-grained gneiss is dominated by quartz and f potassic feldspar and contains large perthitic feldspar crystals in addition to minor amounts of muscovite. The

2 sigma [abs] dark constituents are mainly biotite and garnet, while hornblende is almost completely replaced by biotite. f

Pb/ U Zircons from the neosome are generally small, clear

206 238 and either multi-faceted to subrounded, or euhedral and

f oblong. Cathodoluminescence images of the two types reveal similar features comprising generally faint pat-

2 sigma [abs] terns of growth zoning and small cores (8–10; Fig. 4). The cores are interpreted to represent zircons from the f

Pb/ U host rock protolith, surrounded by zircon grown during 235 207 anatexis. This is supported by the observation of cores e g and not better than 50% for small single grains. with well preserved oscillatory zoning, surrounded by ␮ Pb/ Pb CL dark rims (11; Fig. 4). In a few grains no cores were 206 204 detected (7; Fig. 4). To avoid cores, analyses of zircons

d from the neosome were performed on clear tips. How- ever, the scatter of the data indicates that most grains Pbcom [pg] contained minor inheritance as well as having under-

c gone some Caledonian lead-loss (Fig. 5C). The dataset is delimited on one side by the two data points C5 and C8, one of which (C5) is nearly concordant and has a 207Pb/206Pb age of 953 ± 2 Ma. A line through C5 and [ppm] Th/U

b C8 anchored at the time of Caledonian magmatism in the U ]: number of grains in fraction; NA: not abraded; cl: clear; p: pink; b: brown; r: rounded; frgt: fragment; euh: euhedral; sbh: subhedral; anh: anhedra ±

N Upper Jotun Nappe (430 10 Ma, see discussion in Sec-

b tion 4.1), yield an upper intercept age of 955.1 ± 2.3 Ma. 207 206

g] Treating the Pb/ Pb age as the minimum age and the ␮ Weight [ upper intercept age as the maximum age of anatexis, we deduce an age of 954 ± 3 Ma as the most likely age of

a the neosome.

) Zircon crystals extracted from the host gneiss (sample D) are ≤300 ␮m, and the generally clear crystals vary from euhedral to subhedral. Cathodoluminescence Continued imaging of the zircons shows oscillatory growth zoning

Z: Zircon; M: monazite; T: titanite; [ Weight and concentrations are known toTh/U about model 10% ratio for inferred samples from largerpbc: 208/206 than Total ratio 5 common and Pb age in of sample sample. Raw corrected data for corrected spike for and fractionation. fractionationCorrected (initial for + blank). fractionation, spike, blank and initial common Pb; error calculated by propagating the main sources of uncertainty. and CL-bright patches of recrystallised zircon, the latter K1K2K3 Z [1]K4 frgt euh Z [1]K5 frgt euh 16 Z [7] frgt euh Z 2 [7] frgt euh Z 1 [3] frgt euh 2 330 4 933 1907 0.99 524 0.91 575 5.8 0.76 4.1 0.94 5.1 8874 0.72 3.5 1.4881 4390 6.4 3610 0.0083 1.4692 1.4806 2883 0.0041 0.15458 3483 0.0046 1.4889 0.00082 0.15271 1.4791 0.0041 0.15367 0.93 0.00036 0.0039 0.00042 0.15435 0.06982 0.92 0.93 0.15357 0.00035 0.00007 0.06977 0.06988 0.00034 0.85 926.6 0.00008 0.00008 0.91 0.06996 916.1 925.6 921.5 0.06985 0.00010 917.8 0.00008 923.1 925.3 922.5 921.0 921.7 925.9 924.8 921.9 927.2 924.1 f a c e b d m-f: multi-facetted; sp: short prismatic; tp: tip; cr: core. Table 1 ( No. in plot Fraction Sample K: Rambera, granitic pegmatite (C-03-23) UTM6774N3750E typically concentrated in the central parts of the grains 144 A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154

Fig. 5. Concordia diagrams showing zircon, titanite and monazite data. All data point error ellipses are 2σ. Locality I: (A) two-pyroxene granulite; (B) allanite-bearing pegmatite, symbols are larger than corresponding error ellipses; (C) granite neosome (D) granitic gneiss; (E) zoned granitic pegmatite; (F) granitic pegmatite in shear zone. Locality II: (Ga) granitic gneiss: all zircon analyses, symbols are larger than corresponding error ellipses; (Gb) granitic gneiss—brown, secondary zircons; (Gc) granitic gness—titanite analyses. Locality III: (H) felsic granulite. Locality IV: (I+J) quartz-monzonitic gneiss and dioritic two-pyroxene gneiss; (K) granitic pegmatite. A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154 145

Fig. 5. (Continued).

(12–16; Fig. 4). The five zircon analyses show consid- of crystals will not decrease the degree of discordance. erable scatter in a Concordia diagram, likely reflecting The oldest data point yields a 206Pb/207Pb minimum pro- Sveconorwegian recrystallisation of the zircons com- tolith age of ca. 1493 Ma, whereas analysis D3 anchored bined with Caledonian lead-loss (Fig. 5D). The disturbed at the 954 ± 3 Ma age of anatexis yield a maximum pro- interiors of the zircons suggest that additional abrasion tolith age of 1651 ± 6 Ma. Thus, the real age should lie 146 A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154 within these limits. However, the semi-linear arrange- All 11 zircon analyses roughly define a discordia, ment of the data points suggests that the real age is close but without statistical significance. Four analyses of the to the ca. 1630–1660 ages of other granitic gneisses in clear, short prismatic zircons (G1–G4; Fig. 5Ga) plot the area (samples A and G). We estimate a conservative near the upper intercept. Tips of the brown, euhedral zir- protolith age of 1630 ± 30 Ma. cons were manually separated with tweezers to exclude cores, yielding six fractions (G6–G11; Fig. 5Ga and Gb) 3.5. Sample E (locality I): zoned granitic pegmatite that plot near the lower intercept. A single analysis of two cores (G5; Fig. 5Ga) plots in between the other two Zircons in the sample are typically subhedral and groups. short prismatic. To date the magmatic age of the peg- We interpret the zircon data to reflect crystallisation of matite clear tips with sharp edges were selected for the gneissic protolith (upper intercept), and Sveconorwe- analysis. One analysis plots on the Concordia, yielding gian incipient anatexis (lower intercept). The clear, short an age of 942.2 ± 2.7 Ma (E1; Fig. 5E). The remain- prismatic zircons are interpreted as being primary and ing three grains have younger 206Pb/238U ages and older igneous, variably affected by lead-loss during the Sve- 207Pb/206Pb ages than the concordant grain. This indi- conorwegian. Similarly, the clear cores are considered to cates a combination of minor inheritance and lead-loss be primary, with resorption and partial lead-loss reflect- following 942 Ma. The concordant age is interpreted as ing Sveconorwegian incipient anatexis, which also led the age of crystallisation of the pegmatite. to the crystallisation of the secondary, light brown zircon overgrowths. This is compatible with the observation in thin section of brown, secondary zircon associated 3.6. Sample F (locality I): granitic pegmatite with with metamorphic biotite in the host rock, suggesting lilac feldspar concurrent growth of secondary zircon and biotite. The discordance toward the upper intercept age (Fig. 5Gb) is An undeformed pegmatite intruding along a shear interpreted to reflect minor inheritance. zone that cross-cuts the banded anatexite was sampled The lack of statistical significance of the discordia ≤ (Fig. 3C). The pegmatite contains 5 mm pink, euhe- derived from the entirety of the zircon data suggests dral zircons, one of which was shattered and split into minor post-Sveconorwegian lead-loss. Four analyses several fractions. The fragments plot in a cluster around (G1, G2, G6 and G9; Fig. 5Ga and Gb) yield the best esti- 922 Ma (F2–F5; Fig. 5F), except for one data point plot- mate for the upper and lower intercept ages: 1633.6 ± 5.0 ± ting on the Concordia at 934 3 Ma (F1; Fig. 5F). This and 933.6 ± 2.9 Ma, respectively. The remaining analy- point either represents the true age of the dyke, or inher- ses plot to the right of this discordia, and, based on the itance. The most likely explanation for the age pattern regional geology as well as titanite data discussed below, is that the large zircon contained an inherited core that this is interpreted to reflect Caledonian lead-loss. was included among the fragments chosen for analysis Four analyses of anhedral titanite lie near the upper ± from the shattered crystal. The 934 3 Ma age is rep- intercept of a Sveconorwegian–Caledonian discordia resented in other rocks in the area (samples A and G). (G12–G15; Fig. 5Gc). Regression of these analyses in Also, the pattern of the analyses indicates that minor conjunction with one analysis of anhedral titanite from lead-loss has affected some of the zircons. This can be sample C (C10) yields intercept ages of 909 ± 14 and accounted for by projecting a discordia from a Caledo- 412 ± 460 Ma. Fixing the lower intercept to the age ± nian age (430 10 Ma), yielding an upper intercept age of Caledonian magmatism (430 ± 10 Ma) improves the ± of 927.3 3.2 Ma, deemed to be the most likely age of upper intercept age to 909.1 ± 4.0 Ma. the pegmatite. 3.8. Sample H (locality III): jotunite gneiss 3.7. Sample G (locality II): granitic gneiss The sample of medium-grained, approximately The sampled gneiss contains plagioclase, potassic quartz-dioritic (jotunite) gneiss contains minor potas- feldspar and quartz along with biotite, hornblende (par- sic feldspar and large poikilitic plagioclase grains tially replaced by biotite) and minor amounts of garnet. as well as remnants of perthitic feldspar. The mafic Analysed zircons fall into two categories: clear, ca. phases are poikilitic garnet, clinopyroxene, and to a 100 ␮m, short prismatic, euhedral to slightly rounded large extent replacing the pyroxene, fine-grained horn- crystals, and light brown ≤1 mm euhedral zircons which blende and biotite. Zircons in the sample are clear contain clear, round cores. and ca. 100–300 ␮m long, typically prismatic with a A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154 147 square cross section and blunt pyramidal terminations. 1 reflects mainly the deviation of analysis K1. Exclud- Cathodoluminescence images reveal patterns of oscil- ing this analysis and constraining the lower intercept latory growth zoning typical of igneous zircons (e.g., at 430 ± 10 Ma, the remaining four higher precision Pidgeon, 1992), sometimes with minor CL-bright recrys- analyses (K2–K5) yield an upper intercept age of tallised patches and thin overgrowths (17–21; Fig. 4). 926.6 ± 2.0 Ma, interpreted as the age of emplacement The data points plot close to the Concordia curve of the dyke. (Fig. 5H), and a discordia projected from a young Sve- conorwegian age (940 ± 10 Ma) yields a protolith age of 4. Discussion 1660.1 ± 2.1 Ma. 4.1. Proterozoic evolution of the Upper Jotun Nappe 3.9. Samples I and J (locality IV): quartz-monzonitic and dioritic gneiss The magmatic ages of four dated gneisses in the northwestern Upper Jotun Nappe are 1660 ± 2, 1634 ± 4, Two fine- to medium-grained gneisses were sam- 1634 ± 4 and 1630 ± 30 Ma. A second episode of mag- pled. In a felsic, approximately quartz-monzonitic gneiss matism is evidenced by 1257 ± 22 Ma pre- to early (sample I) the high-grade orthopyroxene/clinopyroxene Sveconorwegian protolith ages in the southwestern parts mineral assemblage has been completely replaced, first of the nappe. by hornblende, and subsequently by various low grade The 1257 ± 22 Ma age of two-pyroxene granulite alteration products, the latter probably related to Caledo- gneiss also sets an upper age limit on (one episode of) nian (?) deformation apparent in the sample. The second granulite-facies metamorphism and penetrative defor- sample, collected from an approximately dioritic gneiss mation. Polygonal granoblastic textures with ortho- (sample J), displays a polygonal granoblastic texture and and clinopyroxene in gneisses throughout the Upper is dominated by feldspar, orthopyroxene (partly retro- Jotun Nappe (Battey and McRitchie, 1973; Battey and gressed under low-grade conditions) and clinopyroxene, McRitchie, 1975; Qvale, 1982; this study) show that with minor reddish biotite, amphibole, apatite, zircon high-grade metamorphism either post-dates, or was and ilmenite. Zircons in the two gneisses have sim- coeval with and out-lasted formation of the gneissic ilar morphologies and fall into two groups. The first fabric. A lower age limit for penetrative deformation consists of ca. 100–300 ␮m, clear, sub-prismatic grains and high-grade temperature conditions is provided by with rounded terminations, length/width ratios of ca. 954 ± 3 Ma granite neosome (sample C: locality I) that 4:1, and square cross sections. The second group con- also dates local shearing. This contrasts to some previous sists of fragments of large grains. In CL images both interpretations that limited Sveconorwegian metamor- groups typically display core domains with oscillatory phism to amphibolite grade (Koestler, 1982, 1988). zoning, locally showing signs of partial resorption. The Local shearing is further constrained by 950 ± 1Ma cores are locally overprinted by irregular patchy zones syntectonic pegmatites in garnet amphibolite (sample of CL-bright, recrystallised zircon, and surrounded by B; locality I). The geometry of the (sub-) parallel peg- thin rims of CL-bright zircon (22–26; Fig. 4). Six anal- matites indicates that they were emplaced normal to yses of zircons from the two samples together define a the minimum compressive stress (σ3) during sinistral discordia with an upper intercept of 1257 ± 22 Ma and non-coaxial shearing of the amphibolite lens (Spurgin,¨ a lower intercept of 941 ± 10 Ma (Fig. 5I+J), whereas a 2006). Amphibolite-facies retrogression of the host rock seventh analysis (I3) plot below the discordia, interpreted lens was promoted both by proximity to the pegmatites to reflect post-Sveconorwegian lead-loss. and to synmagmatic shear zones. This implies that the shear zones acted as pathways for the metamorphic flu- 3.10. Sample K (locality IV): granitic pegmatite ids, dating retrogression to the time of emplacement of the pegmatites. Widespread hornblende-rich amphibo- The sample represents a massive and undeformed lites occurring in association with shear zones in the pegmatite consisting of up to 10 cm wide crystals of Upper Jotun Nappe have been reported by Battey and quartz, potassic feldspar, plagioclase, biotite and oxides. McRitchie (1975). We propose that the ca. 950 Ma age Pieces of zircon were broken off from large crystals visi- dates widespread, local shearing and hydration under ble in the hand-sample. Five analyses (K1–K5; Fig. 5K) amphibolite-facies conditions throughout the northwest- are concordant to slightly discordant and define a dis- ern Upper Jotun Nappe. The ages of inherited zircons cordia with an upper intercept age of 926.9 ± 4.7 Ma in the pegmatites and the presence of 954 ± 3Ma and a lower intercept of 496 ± 330 Ma. The MSWD of granitic neosome in the area suggest an anatectic ori- 148 A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154 gin of the pegmatites. Alternatively, the inherited zircons Helgedalen zone. Although we subscribe to this inter- could represent xenocrystic material picked up by the pretation, some mylonitic shear zones in the area display magma during its passage through surrounding gneisses. abundant post-kinematic garnet porphyroblasts, mirror- Emplacement of a granitic pegmatite at 942 ± 3Ma ing observations from Sveconorwegian shear zones (e.g., (sample E; locality I) parallel to the fabric of the host locality III), suggestive of a pre-Caledonian age. We rock may also reflect local melting. conclude that shearing is a widespread and important A second episode of in situ anatectic melting is dated component of the late-Sveconorwegian evolution of the to 934 ± 3 Ma (sample G; locality II). As at locality I, the nappe rocks. anatexite is associated with a thoroughly retrogressed The youngest Sveconorwegian age inferred from the protolith, pointing to a causal relation between hydra- U–Pb data is a 909 ± 4 Ma upper intercept from com- tion and anatexis. The age is identical within error to bined titanite data from granitic gneisses (samples C: the 934 ± 1 Ma lower intercept defined by zircon and locality I, and G: locality II). The age may reflect either concordant monazite data from two-pyroxene granitic cooling of the rocks through the ca. 700 ◦C closing tem- gneiss (sample A; locality I). Roberts and Finger (1997) perature of titanite (Frost et al., 2001 and references suggested that zircons in high-grade rocks commonly therein), or formation and/or recrystallisation of titan- date retrogression and partial melting related to decom- ite during a final metamorphic event that was not intense pression as opposed to peak metamorphic conditions, enough to affect the more resistant zircons (Cherniak et in which case the age from the high-grade rock might al., 1991; Mezger and Krogstad, 1997; Lee et al., 1997). also reflect anatexis. However, there is no petrographic The latter interpretation is supported by age data from evidence for in situ melting following recrystallisation the southern parts of the Upper Jotun Nappe (Lundmark and formation of the orthopyroxene/clinopyroxene high- and Corfu, in press). The titanite discordia projects grade assemblage. Alternatively and, in our view, more toward a Caledonian age. This disturbance, along with likely, the 934 ± 1 Ma age record high-grade metamor- widespread lead-loss apparent in the zircon data from phism and dehydration, in which case monazite growth several of the samples (Fig. 5A–K), reflect the emplace- likely was controlled by breakdown of hydrous phases ment of the Upper Jotun Nappe onto the Baltic Shield (biotite and/or amphibole) (Bingen and van Breemen, and final juxtaposition of the units studied at locality 1998). Thus, the similar ages of anatexis and high- I. Other data by Lundmark (2006) and Lundmark and grade metamorphism may reflect local devolatilisation Corfu (2007, in review, in press) support the use of in response to high heat flow, leading to a corresponding 430 ± 10 Ma to constrain Caledonian effects, effectively influx of fluids and anatexis in adjacent rocks (cf. Moser accounting for both Caledonian metamorphic resetting et al., 1996). Similarly, the 940 ± 10 Ma lower intercept and 427 ± 1 Ma magmatism, and allowing for a correct in two-pyroxene gneiss and partially retrograded gneiss interpretation of the U–Pb data in this study. at locality IV (samples I and J; locality IV), is also inter- The results testify to a complex Sveconorwegian evo- preted to date high-grade metamorphism, although the lution of the Upper Jotun Nappe, with at least two uncertainty of the age determination prevents more exact separate events of anatectic melting and possibly several comparisons. episodes of dehydration and (re-) hydration. This may The last phase of magmatism recorded is the reflect the juxtaposition of different segments of lower 927 ± 3 Ma (sample F; locality I) and 926 ± 2 Ma (sam- crustal rocks. However, despite the multitude of shear ple K; locality IV) emplacement of granitic pegmatites. zones of different ages, the different domains seem to The position of an undeformed pegmatite at local- form part of a common crustal unit, albeit with locally ity I in a shear zone that cross-cuts and deforms the different timing of late-Sveconorwegian events. This 954 ± 3 Ma neosome in banded migmatite, shows that implies a high degree of heterogeneity in the evolution local Sveconorwegian shearing took place both during of lower crustal rocks within a relatively restricted area. and after the anatexis, as well as during the intrusion of The geological processes that generate the Sveconor- allanite-bearing pegmatites. Similarly, field observations wegian ages in this study largely reflect fluid activity, at locality IV indicate that local shearing took place prior i.e. dehydration and fluid influx, the latter associated with to, during and after intrusion of the Sveconorwegian local anatexis, zircon growth and lead-loss. Fluid flux can pegmatites. be strongly localised to (transient) zones of relative high The area comprising locality III was the focus of a permeability, such as active deformation zones (e.g., study by Koestler (1988), who attributed abundant shear- Jamtveit et al., 1990; Oliver, 1996), whereas imbrication ing to Caledonian thrusting and identified the area as and thrusting may have led to repeated emplacement of part of the deformed base of the Upper Jotun Nappe, the hydrated rocks, with potential for devolatilisation, below A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154 149 previously dehydrated high-grade rocks. This allows for Dalsfjord Nappe, while the eastern parts of the WGR, complex age patterns and multiple episodes of metamor- exposing a higher crustal level (Skar˚ and Pedersen, phism and anatexis during an orogeny, and may partially 2003) and the Lower Jotun Nappe, were metamorphosed account for the age pattern observed. up to amphibolite facies (Table 2). Local 954 ± 3Ma On the other hand, it is appealing to interpret the data migmatization in the Upper Jotun Nappe is coeval with in the context of two separate high-grade events, the 951 ± 2 Ma monzodioritic magmatism in the Lindas˚ first ending with 954 ± 3 Ma anatexis and deformation Nappe, while, at this time, post-tectonic felsic pegmatites under amphibolite-facies conditions, and the second tak- were emplaced in the WGR (Skar˚ and Pedersen, 2003). ing place at 934 ± 3 Ma, involving coeval anatexis and A second cycle of high-grade metamorphism and high temperature metamorphism. The second cycle is anatectic melting in the Upper Jotun Nappe at ca. suggestive of post-collisional introduction of an exter- 934 Ma coincides with 933 ± 2 and 929 ± 1 Ma high- nal heat source, locally inducing high temperatures and grade metamorphism in the Lindas˚ Nappe (Bingen et al., anatexis. This will be discussed further in the next sec- 2001) and 932 ± 8 Ma granitic magmatism in the WGR tion. (Corfu, 1980). At the same time, the 932 ± 3 Ma Roga- land anorthosite was emplaced (Scharer¨ et al., 1996) 4.2. Provenance of the Jotun Nappe Complex and along with widespread ca. 950–920 Ma granites (e.g., other high-grade crystalline nappes in SW Norway: Andersen et al., 2002) in the Southwest Scandinavian a tectonometamorphic comparison Domain during lower and middle crustal uplift in southwestern Norway (Bingen et al., 2006). The pattern of southeast directed thrusting during the The age correlation and complementary nature Caledonian orogeny places the origin of the far-travelled of these phenomena, i.e. high temperature metamor- crystalline nappes in southwestern Norway, outboard of phism and anatexis in the nappes, and uplift and the presently exposed WGR (e.g., Milnes et al., 1997). A granitic/anorthositic plutonism in the shield, suggest comparison of available U–Pb ages from the nappes and that they represent different expressions of the same the WGR (Table 2) indicates a regional pattern with three event. Duchesne et al. (1999) proposed that post- distinct groups of pre-Sveconorwegian protolith ages. collisional high heat flow in the lower crust and The oldest protolith ages fall within the 1700–1550 Ma associated anorthosite plutonism at higher crustal levels Gothian time span, during which much of the Southwest in the SSD was the result of delamination along lin- Scandinavian Domain was formed. In particular, rocks ear Sveconorwegian terrane boundaries, while Andersen of ages close to 1635 Ma occur in all of the crystalline (1997) suggested late-Sveconorwegian underplating to units except for the Lindas˚ Nappe. A second genera- account for partially juvenile isotopic signatures of late- tion of protolith ages, so far recorded in the Dalsfjord Sveconorwegian granites in the SSD. Specifically, the Nappe, the WGR and in a gabbroic complex in the Lower age of the 940 ± 10 Ma juvenile Tovdal granite was inter- Jotun Nappe coincide at roughly 1450 Ma, an age cor- preted to date the presence of mafic magmas in the lower responding to sporadic magmatism recorded throughout crust (Andersen et al., 2002). Both scenarios imply ele- Baltoscandia (e.g., Ah˚ all¨ and Connelly, 1998). The third vated heat flow in lower parts of the crust at the time group of protolith ages, clustering around 1250 Ma, is of ca. 930 Ma high-grade metamorphism in the Upper represented in the Upper and the Lower Jotun Nappe, the Jotun Nappe and the Lindas˚ Nappe. Lindas˚ Nappe and the WGR. The magmatism was of pre- Late-Sveconorwegian titanite ages have so far been dominantly gabbroic composition and occurred close to recorded in the Upper Jotun Nappe, 909 ± 4 Ma (this the onset of the Sveconorwegian orogenic cycle. Similar study) and 913 ± 3Ma(Lundmark and Corfu, in press), ages have been reported in relation to bimodal magma- in the Lower Jotun Nappe, 907 ± 11 Ma (recalcu- tism from southern Norway and southwestern Sweden lated using three analyses overlapping the Concordia, (e.g., Brewer et al., 2004). data from Scharer¨ (1980b)) and the Dalsfjord Nappe, Late-Sveconorwegian high-grade metamorphism, ≤920 Ma (Corfu and Andersen, 2002). Hence, with the migmatization and granite plutonism in the western exception of the lack of high-grade assemblages in the parts of the WGR at ca. 990–960 Ma coincide with Lower Jotun Nappe, the Proterozoic histories of the crys- the suggested emplacement-age of a large, ca. 965 Ma talline nappes display distinct similarities, also evident gabbro-anorthosite complex in the southern parts of the in the tectonometamorphic history of the WGR. Upper Jotun Nappe (Lundmark and Corfu, in press). Sve- However, during the Caledonian orogeny the conorwegian high-grade metamorphism also affected tectonometamorphic histories diverge, as evidenced by the Upper Jotun Nappe, the Lindas˚ Nappe and, likely, the the presence of eclogites in the Lindas˚ Nappe and their 150 A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154 d d d d Atløy Stordalsvatn 3 Monzonite, 6 Gabbro, ± ± 960 Monzonite, Atløy [T] 920 Monzonite, Atløy [T] 1634 1464 ≥ ≤ l l l l l granulite, Radøy 35) Chamockite, Lygra 36) Chamockite, Lygra − − 2 Granulite, Radøy 2 Retrograded 1 Granulite, Holsnøy ˚ as Nappe Dalsfjord Nappe ± ± ± 1237(+43/ 951 933 932(+28/ 929 c k c c c ]: recalculated. [T], [R] 20 Syenite to monzonite, cs 26 qz-monzonite, Tyin 28 Gabbro, Tyin 11 Syenite to monzonite, cs 3 Gabbro, Leirungsmyran ± ± ± ± ± 907 1450 1694 1666 1252 b b b m b ˚ aki b b b b b h b b ˚ Ardal b b b Hurrungane Hurrungane Hurrungane 99) Mafic gneiss, − 24 Quartz-diorite, Fannar 4 Granite, Galgebergstjern 30 Granite, Hurrungane Granite, Hurrungane 22 qz-monzonite and gabbro, cs 4 Anorthosite, Hurrungane 3103 Anatexis, Hurrungane 1 qz-monzonite and gabbro, cs Anatexis, Hurrungane Granite, Hurrungane 41 Granite, cs [T] 3 Granitic pegmatite, 3 Granitic pegmatite, 2 Granitic pegmatite, Pegmatite, Fresvik ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 1660 1634 1634 1630 1528(+123/ 1257 965 954 941 934 934 909 950 942 927 926 e g a g a a f ˚ alsnes g g j n g j a e g g e Hisarøy Breiddalsvatn Hisarøy [M] 29) Migmatite gneiss, − 3) Granodiorite, Dypvatn 3) Granodiorite, Ornfjell − − 110 Quartz-monzonite, Hisarøy 28 Quartz diorite, Atløy 3 Quartz syenite, cs Gabbro, Lavikdal 24 Gabbro, Haram Gabbro, Flem 10 Quartz-monzodiorite, 5 Composite dyke, Hella 32 Granite, Jølster Granite, Hafslo 8 Anatexis, cs 3 Quartz-monzodiorite, 3 Monzonite dyke, M 5 Pegmatite, Breiddalsvatn ± ± ± ± ± ± ± ± ± ± ± ± ± ± 987 971 1662(+41/ 1646 1641 1633 1621 1466 1252 966 930 970–960969 Anatexis, cs 949 951 951(+5/ 943 942(+5/ . a a i a i . a . . a . a a . . i ...... Astfjord Damvatn Hustad 3) Leucogabbro, − 2 Tonalite, Tingvoll 2 Granite, Sagfjord 2 Migmatite gneiss, 2 Granite, Frei 12 Granite, Hustad Monzodiorite, 22 Granite, Ingdal Gabbro,3 Selsnes Dolerite, Hustad ¨ ¨ arer (1980a) arer (1980b) ± ± ± ± ± ± ± ± ± ˚ ˚ ar et al. (1994) ar and Pedersen (2003) This study. Sch Corfu and Andersen (2002) Tucker et al. (1990) Sch Sk Sk Austrheim et al. (2003) Krogh et al. (2004) Corfu and Emmett (1992) Røhr et al. (2004) Bingen et al. (2001) Lundmark and Corfu (in press) Corfu (1980) i j l f 1686 1661 1659 1658 1657(+5/ 1654 1653 1653 1462 1251 c a e b d h g k n m Table 2 U–Pb isotopic ages of protoliths andWGR of north of Sveconorwegian SFDZ anatexis, metamorphism andProtolith magmatism ages in crystalline crustal nappes and WGR, SW Norway WGR south of SFDZ Upper Jotun NappeSveconorwegian high-grade metamorphism and anatexis Lower Jotun NappeSveconorwegian amphibolite-facies metamorphism or unknown grade Lind Sveconorwegian pegmatites and dykes WGR: Western Gneiss Rregion; SFDZ: Sveconorwegian Frontal Deformation Zone; cs: several samples combined; [T]: titanite age; [M]: Monazite age; [R A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154 151 absence from the Jotun Nappe Complex and the Dals- gneiss and 950 ± 1 Ma emplacement of syntectonic, syn- fjord Nappe, and the intrusion of Silurian granites in the retrogression pegmatites. We propose that this reflects Upper Jotun Nappe and the Lindas˚ Nappe at a time when decompression and partial exhumation of the lower the Dalsfjord Nappe was exposed at the surface (Brekke crustal rocks following the high-grade event. and Solberg, 1987). It was pointed out by Lundmark and A second cycle of high-grade metamorphism is dated Corfu (2004) that while leucogranites of Silurian ages to 934 ± 1 Ma, coeval with local 934 ± 3 Ma anatexis in are absent from the basement of Baltica, they are rep- granitic gneiss. This event coincides with metamorphism resented in the East Greenland Caledonides (ranging in up to granulite facies in the Lindas˚ Nappe. The ages are ages from ca. 430 to 425 Ma; Andresen et al., in press). interpreted to reflect post-collisional elevated heat flow This, and the presence in the Scandinavian Caledonides in the nappes, and correspond in time to 932 ± 3Ma of thrust sheets originating in Laurentia (the Uppermost anorthosite magmatism in Rogaland and widespread Allocthon), warrants a comparison of the nappes to East granitic intrusions at higher crustal levels throughout Greenland. the Southwest Scandinavian Domain. The simultaneity Late-Sveconorwegian metamorphic ages are present is suggestive of a common underlying cause, possibly in the Caledonian fold belt of East Greenland underplating and/or delamination. (950–920 Ma; Kalsbeek et al., 2000). However, the Additional generations of granitic pegmatites in the nappes consist primarily of paragneisses, and are not Upper Jotun Nappe were dated to 942 ± 3, 927 ± 3 and possible correlatives of the orthogneisses in the investi- 926 ± 2 Ma. The pegmatites constrain the timing of some gated nappes on the Scandinavian side of the orogen. of the ubiquitous shear zones and mylonites in the Upper The autochthonous East Greenland basement is typi- Jotun Nappe to the Sveconorwegian and offer a reliable cally older than 1.75 Ga and lacks protolith ages between way of differentiating between Proterozoic and Caledo- <1.75 and 0.93 Ga (Kalsbeek et al., 1993, 1999). The nian deformation. Grenville orogen, stretching across southern Laurentia, We propose that similarities in the tectonometamor- is comparable to the southern Scandinavian region in phic record between the crystalline nappes in southwest- terms of protolith ages, but its metamorphic evolution is ern Norway and the WGR confirm derivation of the distinctly different. Late Grenvillian metamorphic and nappes from the Southwest Scandinavian Domain of the tectonic activity generally predated its Sveconorwegian Baltic Shield. We also suggest that the high-grade nappes counterparts and had all but ceased prior to plutonism and represent a vertical as well as a lateral extension of our migmatization in the WGR at ca. 970–960 Ma (Gower sampling of the Sveconorwegian orogen compared to and Krogh, 2002; Watt and Thrane, 2001). the presently exposed basement in the WGR, while the Thus, geochronological evidence distinctly favours amphibolite grade Lower Jotun Nappe represents sam- a correlation between the crystalline nappes of south- pling of a more shallow level of the same crustal unit. western Norway and the parautochthonous WGR, i.e. the Baltic Shield. The Sveconorwegian tectonometamorphic Acknowledgements histories of the nappes further suggest that the nappes, with the exception of the Lower Jotun Nappe, sample This study was financially supported by the Nor- a lower crustal segment of the Baltic Shield than that wegian Research Council (NFR). Laboratory assistance presently exposed in the WGR. was provided by Gunborg Bye-Fjeld. We would also like to thank reviewers B. Bingen and H. Rice for helpful 5. Conclusions comments and suggestions.

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