zircon u-pb ages and lu-hf isotopes from basement rocks … · 2016-02-03 · norway: the hedmark,...

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Introduction

The rock record in present-day South Norway has a his-tory extending from Palaeoproterozoic times and con-tains evidence of major crustal building/reworking events. The Transscandinavian Igneous Belt (TIB), the Gothian domain with Sveconorwegian imprint and the Caledonian nappe pile represent events that have created the basement of South Norway as we know it today. A considerable number of geochemical analyses and geo-chronological data already exist from basement rocks in southern Norway and Sweden. However, much of the western margin was destroyed during the Caledonian Orogeny, and it is unclear how the basement geology was arranged at the margin of the craton.

The break-up of Rodinia with formation of the continent Baltica in the Neoproterozoic resulted in the creation of

several rift basins at the Baltic margin (e.g. Kumpulai-nen & Nystuen 1985; Siedlecka et al. 2004; Nystuen et al. 2008). During the Caledonian Orogeny these basins and detached parts of the crystalline basement were thrust and transported up to several hundred kilometres east- and southeastward to their present position on denu-dated Precambrian basement and its thin cover succes-sion of Ediacaran to lower Palaeozoic sedimentary rocks. These basement slices can be considered as direct sam-ples of the outer Baltic margin, which does no longer exist in situ. In this study we present U-Pb and Lu-Hf data from zircon from basement slices in three alloch-thonous basins within the Sparagmite Region of South Norway: the Hedmark, Valdres and Engerdalen basins, which are now located in the Osen-Røa, Valdres and Kvitvola nappe complexes, respectively. From zircon Lu-Hf isotopes the petrogenetic evolution of the rocks can be traced to provide information that is necessary

Lamminen, J., Andersen, T. & Nystuen, J.P.: Zircon U-Pb ages and Lu-Hf isotopes from basement rocks associated with Neoproterozoic sedimen-tary successions in the Sparagmite Region and adjacent areas, South Norway: the crustal architecture of western Baltica Norsk Geologisk Tidsskrift, Vol 91, pp. 35-55 . Trondheim 2011, ISSN 029-196X.

The Neoproterozoic Hedmark Group (Lower Allochthon), Valdres Group (Middle Allochthon) and Engerdalen Group (Middle Allochthon) con-tain allochthonous basement slices with tectonic and depositional contacts to sedimentary rocks. The basement slices were transported from the western margin of the continent Baltica during the Caledonian Orogeny. Zircon U-Pb and Lu-Hf isotopes reveal that allochthonous basement rocks in the Osen-Røa Nappe Complex, forming the depositional substrate of the Hedmark Group, are related to the Transscandinavian Igneous Belt (TIB) in terms of age and petrogenetic history. The Kvitvola Nappe Complex of the Middle Allochthon, carrying the sedimentary Engerdalen Group, contains c.1620 Ma and 1180 Ma augen gneisses. The oldest augen gneiss has TIB-like Hf characteristics, while the younger gneiss is more juvenile. These basement rocks may originally have been separated by a major lithotectonic boundary. A sample from the basement slice, loca-ted stratigraphically beneath the Valdres Group in the Valdres Nappe Complex, is dated at c.1480 Ma and also has juvenile Hf composition. The apparent younging of basement ages from Lower to Middle Allochthon suggests age zonation at the SW Baltican margin. The juvenile 1180 Ma and 1480 Ma samples are probably derived from the same crust as what is today known as SW Norway or “Telemarkia”. Crustal rocks of present-day SW Norway must have continued further to the northwest (according to present geography) and were probably bordered by TIB rocks in the NE. We also present new U-Pb and Lu-Hf data from the autochthonous basement in front of and tectonostratigraphically beneath the Osen-Røa Nappe Complex, sampled along a transect from the Romerike Complex across the Mylonite Zone to Lake Storsjøen. Hf isotopes show the juvenile character of the gneisses on the western side of the Mylonite Zone and typical TIB 3 compositions on the eastern side. Hafnium isotopes from the Hustad Igneous Complex in the Western Gneiss Region (WGR) show strong similarity with the TIB, which supports a genetic connection between the TIB and WGR. Hafnium isotopes from the late Sveconorwegian Flå granite show crustal contamination, which supports models where the TIB continues underneath the Gothian domain.

Jarkko Lamminen ([email protected]), Tom Andersen ([email protected]) & Johan Petter Nystuen ([email protected]), Depart-ment of Geosciences, University of Oslo, P.O. Box 1047, Blindern, N-0316 Oslo, Norway

Jarkko Lamminen, Tom Andersen & Johan Petter Nystuen

NORWEGIAN JOURNAL OF GEOLOGY Zircon U-Pb ages and Lu-Hf isotopes from basement rocks

Zircon U-Pb ages and Lu-Hf isotopes from basement rocks associated with Neoproterozoic sedimentary successions in the Sparagmite Region and adjacent areas, South Norway: the crustal architecture of western Baltica

36 J. Lamminen et al NORWEGIAN JOURNAL OF GEOLOGY

sub sequent to the break-up of Rodinia (e.g. Dalziel 1991) and prior to the Caledonian Orogeny.

Summaries of the regional Precambrian geology are given by Gaàl & Gorbatschev (1987), Gorbatschev & Bogda-nova (1993) and Bogdanova et al. (2008). The South-west Scandinavian Domain (SSD, Gaàl & Gorbatschev 1987) is the youngest part of the Fenno scandian Shield, and is built up of rocks of late Palaeoproterozoic to late Mesoproterozoic age. The SSD comprises several differ-ent blocks with distinct tectonometamorphic histories. It was later influenced by Sveco norwegian deformation and metamorphism at c.1.14-0.90 Ga (Bingen et al. 2008a). The SSD is separated from the Palaeoproterozoic arc and granite terranes of the Sveco fennian Domain in central Fennoscandia by the 1.85-1.65 Ga Trans scandinavian Igneous Belt (TIB), consisting mainly of granitoids and their volcanic equivalents, and minor mafic rocks (Hög-dahl et al. 2004) (Fig. 1).

During the Caledonian Orogeny (c.490-390 Ma) the Lau-rentian continental plate collided with the Baltic plate. The western margin of Baltica, including crystalline base-ment rocks and overlying Neoproterozoic to Palaeo-zoic sedimentary successions, was divided into a series of nappes and thrust sheets that were transported to the east and south-east and assembled in a nappe pile on the autochthonous basement and its cover rocks in the fron-tal zone of the orogen. The thrust sheets are traditionally subdivided into Lower, Middle, Upper and Uppermost

for evaluating the way in which the rocks in the nappes are related to the present autochthonous basement, and how the crust of western Baltica prior to the Caledonian Orogeny might have been organised. Zircon U-Pb ages and Lu-Hf data thus also provide background informa-tion on potential provenances for the sedimentary suc-cessions of the Neoproterozoic basins of the Sparagmite Region.Tables containing our U-Pb and Lu-Hf data from individual samples can be found in appendix 1-3, in the electronic supplement that accompanies the online ver-sion of this article.

This study supplements previous work that has been done on the basement rocks in the Caledonides. We also present new data from the autochthonous base-ment immediately bordering the allochthonous Hed-mark Basin (Hedmark Group). A compilation of base-ment ages by Bingen & Solli (2009) is used as a reference base. It shows a gap in the Sparagmite Region, which will hopefully be filled by this work.

Geological Setting

The present Fennoscandian Shield is composed of an Archaean core in the northeast and progressively younger Proterozoic crustal domains towards the south-west. In this paper we apply the term Baltica for both the continent and craton, and for the time interval

Figure 1. Simplified geological map of southern Scandinavia showing the principal tectonic units and the study area. Thick lines mark major tectonic zones. The two samples from the Wes-tern Gneiss Region are marked on the map. Map modified from basemap courtesy of B. Bingen.

37NORWEGIAN JOURNAL OF GEOLOGY Zircon U-Pb ages and Lu-Hf isotopes from basement rocks

Neoproterozoic and associated basement rocks in South Norway: The Sparagmite Region

The Sparagmite Region in South Norway (Fig. 2), includ-ing adjacent parts of Härjedalen in Sweden, is named after the old petrographic term “sparagmite” for the coarse-grained feldspathic sandstones that dominate the region (Esmark 1829). The term “sparagmite basin” was applied informally to the entire depositional basinal suc-cession which includes the sparagmite sandstones. The basin was mostly thought to be autochthonous or par-autochthonous (e.g. Schiøtz 1902; Holtedahl 1953; Skjes-eth 1963; Englund 1973; Bjørlykke et al. 1976; Nystuen 1976). An allochthonous origin for the “sparagmites” (e.g. proposed by Oftedahl 1949, 1954) was subsequently supported by regional stratigraphic and structural stud-ies (e.g. Nystuen 1981). The Sparagmite Region also includes Neoproterozoic successions within the Kvitvola and Valdes nappe complexes which belong to the Enger-dalen and Valdres groups, respectively (Kumpulainen & Nystuen 1985; Siedlecka et al. 2004).

Allochthon (Fig. 1), each consisting of several nappe com-plexes. The rocks of the three lowermost allochthons are considered to have originated at the western Baltican mar-gin (also termed the Baltoscandian margin), including structural and stratigraphic elements related to the Iapetus Ocean, whereas the Uppermost Allochthon is considered to have Laurentian affinity (Roberts & Gee 1985).

The nomenclature used for the same nappes in Norway and Sweden varies. For example, the Osen-Røa Nappe Complex in Norway corresponds to the Vemdalen nappe (also termed the Hede nappe) of the Lower Allochthon of Härjedalen in Sweden, and the Kvitvola Nappe Complex in Norway is correlative with the Tännäs Augen Gneiss Nappe and the Särv Nappe of Härjedalen (Bockelie & Nystuen 1985; Gee et al. 1985; Kumpulainen & Nystuen 1985). The Valdres Nappe Complex is also referred to the Middle Allochthon, and is located in the western part of the Sparagmite Region. This study is from the classical Sparagmite Region located north of Oslo in central-east-ern Norway (study area in Fig. 1).

Figure 2. Simplified geological map of the study area showing the allochthonous basement slices in black and the sampling sites. For UTM co-ordinates, see Table 1. Map modified from NGU bedrock maps (scale 1:250 000) avai-lable online in 2010 at www.ngu.no


Basement windowsThe window structures in the northern part of the Spar-agmite Region have traditionally been inferred to be dome-shaped outcrops along a basement ridge (e.g. Hossack 1978; Nystuen 1981). Holtedahl (1921, 1930) and Skjeseth (1954, 1963) thought the basement ridge formed an elongated geomorphologic high during the Neoproterozoic to Cambrian sedimentation, separat-ing an autochthonous “sparagmite basin” on the south-ern side of the “window high” from an open sea (“eugeo-syncline”) at the northern side of the ridge. However, a marked thrust separates the basement and its thin cover rocks in the windows from the several thousand metre thick sandstone successions above. This important struc-tural relationship was also emphasised by Strand (1960).

Studies in northern Jämtland and Västerbotten have revealed that window structures within the outcrop area of the Lower Allochthon, similar to those in the rear of the Osen-Røa Nappe Complex, are allochthonous (Zach-risson & Greiling 1993). Similar conclusions can be drawn from an E-W seismic reflection section extending from Jämtland to Trøndelag (Juhojuntti et al. 2001) and magnetotelluric studies (Korja et al. 2008). In the present study it is also suggested that the Atnsjøen, Spekedalen, Øversjødalen, Tufsingdalen and Steinfjellet windows (Fig. 2) are allochthonous as well, an interpretation in accordance with that of Morley (1986) and Rice (2005). This interpretation implies that the Osen-Røa Thrust in the northern part of the Sparagmite Region is ramping up from the Caledonian sole thrust system north of the window structures. Distance of thrusting in the Osen-Røa Nappe Complex

Kjerulf (1879) was the first to carry out measurements of crustal shortening within the Osen-Røa Nappe Complex by calculating shortening of folded strata along sections within the Oslo Region Decollement (being the south-ern part of the Osen-Røa Nappe Complex, Nystuen 1981; Morley 1986). Oftedahl (1943) calculated the shortening within the Oslo Region Décollement to be 150 km for the Cambro-Silurian succession in the northern part of the lake Mjøsa area and estimated the total thrust distance for the “sparagmites” to be 300 km. Strand (1960) found the minimum shortening of strata in the Oslo Region Décollement to be in the order of about 150 km. The dis-tance of 150 km corresponds to the distance from the erosional front of the Osen-Røa Nappe Complex in the Elverum-Trysil area to the northern side of the basement windows in the northern part of the Sparagmite Region (Nystuen 1981). Morley (1986) also restored the thrust distance of the Cambro-Silurian strata in the Mjøsa area to a position about 150 km to the northwest. A simi-lar distance of tectonic transport was also suggested by Hossack (1978). Nystuen (1981) estimated a total dis-tance of tectonic transport of the order of 200-400 km to compensate for additional shortening within the Osen-Røa Nappe Complex. These figures correspond with

Neoproterozoic sedimentary successions and associated basement rocks in the Sparagmite Region occur in the following tectonostratigraphic units, major lithostrati-graphic units and related palaeobasins:

1. A several thousand metre thick sedimentary succes-sion in the Osen-Røa Nappe Complex, representing the Hedmark Group derived from the Hedmark Basin with tectonic slices of the crystalline substrate.

2. A several thousand metres thick sedimentary succes-sion in the Kvitvola Nappe Complex, comprising the Engerdalen Group from the Engerdalen Basin, with slices of basement rocks.

3. A several thousand metres thick sedimentary succes-sion, the Valdres Group, in the Valdres Nappe Com-plex, derived from the Valdres Basin together with thrust sheets of basement rocks.

4. A thin sedimentary cover in an autochthonous posi-tion on the crystalline basement along the erosional front of the Caledonian nappes.

5. A thin sedimentary cover on Precambrian crystalline basement rocks in tectonic windows.

The Hedmark Group in the Osen-Røa Nappe Complex is the type and reference succession of Neoproterozoic strata in South Norway. The Hedmark and Valdres basins have been interpreted as continental rift basins, whereas the Engerdalen basin is considered to represent a passive continental shelf or pericratonic basin (Siedlecka et al. 2004; Nystuen et al. 2008). The Caledonides in the Sparagmite Region include a series of thrust sheets now referred to the Lower Alloch-thon with the Osen-Røa Nappe Complex, and to the Middle Allochthon with the Valdres Nappe Complex and Kvitvola Nappe Complex. The Valdres Nappe Com-plex and Kvitvola Nappe Complex are overlain by the Jotun Nappe Complex of the Middle Allochthon and the Trondheim Nappe Complex of the Upper Alloch-thon. Törnebohm (1896) showed overthrust positions of the rock succession at Lake Femunden (Rendalen For-mation) and called this unit the ‘Røa nappe’. This thrust unit passes southwards into the ‘Osen nappe’ at the ero-sional nappe front, and the combined nappe unit was termed the Osen-Røa Nappe Complex (Nystuen 1981). Törnebohm (1896) also pointed out the thrust posi-tion of Precambrian granite above sparagmite sand-stone (Rendalen Formation) in the mountain Sålekinna west of Femunden. Basement thrust sheets occur in sev-eral places at the base of the Osen-Røa Nappe Complex, Kvitvola Nappe Complex and the Valdres Nappe Com-plex (Siedlecka et al. 1987, 2004). The structural position for some of these basement sheets, as for example those east of Lake Femunden, is the result of out-of-sequence thrusting post-dating the emplacement of the Kvitvola Nappe Complex (Nystuen 1983).


concluded that the augen formation was related to the main Caledonian thrusting.

Spectacular augen gneisses can be found east of Lake Storsjøen, east and west of Lake Lomnessjøen and north-west of the town of Koppang (Fig. 2). The augen gneisses and associated igneous rocks were described by Holm-sen & Oftedahl (1956). Holmsen (1953) suggested that a body of anorthosite west of Lake Lomnessjøen in Renda-len, being a member of this rock suite, might represent an outlier of the Jotun Nappe. The augen gneisses are of two types: one with very large augen (up to 15 cm across) and one with smaller augen. From a distance these rocks resemble a poorly sorted conglomerate. The augen are both angular and rounded, and rotational structures are commonly seen.

Similar gneisses found in Sweden in the Tännäs augen gneiss nappe have been dated by Rb-Sr and U-Pb zir-con methods (Claesson 1980) giving a Rb-Sr age of 1610 ± 85 Ma and a zircon upper intercept age of 1685 ± 20 Ma with Caledonian lower intercept at 405 ± 90 Ma. The Risberget nappe in western Norway is also known for its augen gneisses (Krill 1983; Handke 1995). Rosen-qvist (1941) suggested Caledonian K-metasomatism for the augen gneisses in the Oppdal area. Krill (1983) dated rapakivi granite and associated augen gneiss with the Rb-Sr method to 1618 ± 44 Ma, which was interpreted as an intrusion age. He suggested that the augen gneisses are in fact cataclastically deformed rapakivi granites. Handke (1995) found that the rapakivi granites can be classified into two age groups, 1659 to 1642 Ma and 1190 to 1180 Ma. Rapakivi textures have not been found in the augen gneiss samples of this study.

Autochthonous basement

The autochthonous basement rocks in the study area belong to the Transscandinavian Igneous Belt (TIB) in the east and southeast, and the Gothian domain in the southwest. The TIB has been subdivided into three magmatic phases (TIB 1 to 3; Larson & Berglund 1992) and generally becomesyounger from SE to WNW, being c.1.8 Ga in South Sweden and 1.67 Ga in eastern Nor-way (Heim et al. 1996; this study). The TIB crops out extensively in the region east and southeast of the Spar-agmite Region and continues to the west as far as Lake Mjøsa, where it is separated from the Gothian domain by a mylonitic zone (the Mylonite Zone). Most of the U-Pb work done on the TIB comes from Sweden. Heim et al. (1996) dated a “tricolour” granite in the Trysil area and obtained 1673 ±8 Ma. Andersen et al. (2002a) dated the Odal granite north of the Mylonite Zone to1681 ±6 Ma. Similar ages from this area have also been reported by Bingen et al. (2008a). The Brustad augen gneiss at the Mylonite Zone south of the Odal granite is 1674 ±10 Ma (Alm et al. 2002). These ages are all similar and can be referred to the TIB 3 phase of Larson & Ber-glund (1992).

estimates of Caledonian nappe translations for the Lower Allochthon put forward by Gee (1975, 1978). Rice (2005) restored branch-lines of the Caledonian thrust sheets in South Norway and, like Morley (1986), suggested restor-ing the position of the Hedmark Basin to the southeast-ern side of the original position of the structural win-dows in the northern part of the Sparagmite Region. Rice (2005) put the northwestern margin of the Osen-Røa Nappe Complex to be 615 km northwest of the pres-ent position of the window structures. However, accord-ing to the tectonostratigraphy in the window region, the basinal succession of the Osen-Røa Nappe Complex must have been located west or northwest of the original position of the rocks exposed in the basement windows.

The Kvitvola and Valdres nappe complexes (Middle Allochthon)

Crystalline basement rocks and Precambrian sedimen-tary rocks dominate the Middle Allochthon. The Kvit-vola Nappe Complex consists of the Upper Proterozoic Engerdalen Group which was deposited in fluvial to shallow marine settings at the western Baltican margin (Kumpulainen & Nystuen 1985; Siedlecka et al. 2004). The basement rocks are dominantly augen gneisses, but also gabbros, granites and anorthosites occur. The crys-talline rocks occur as tectonic slices beneath and within the sedimentary succession; primary depositional con-tacts between basement rocks and sedimentary cover rocks are preserved both in the Kvitvola Nappe Com-plex and in the Valdres Nappe Complex (Nickelsen et al. 1985). The Kvitvola Nappe Complex lies discordantly upon the Osen-Røa Nappe Complex and also overlies the autochthonous basement and its sedimentary cover rocks in the east. Thrust distances must have been c.300-400 km (e.g. Kumpulainen & Nystuen 1985). Hossack et al. (1985) calculated the shortening of the Valdres Nappe Complex to have been of the order of c.315 km. Meta-morphic grade in the Kvitvola Nappe Complex is in the greenschist facies and deformation intensity increases towards the northwest. In the study area, the deforma-tion is moderate.

The augen gneisses in the Middle Allochthon

Augen gneisses in the Caledonides have sparked inter-est for decades. Their origin was explained by graniti-sation in the first half of the 20th century (Rosenqvist 1944). Even as late as in the 70s Strand (1972) favoured this explanation for many of the augen gneisses in the study area. In places they apparently grade into sur-rounding arkosic sandstones. Törnebohm (1896) consid-ered the augen gneisses in the Caledonides of Norway as deformed Precambrian porphyritic granites. The age of the augen formation has not yet been determined. Early workers suggested a Caledonian age. The augen gneisses in the Kvitvola Nappe Complex can be correlated with the augen gneisses in the Tännäs augen gneiss nappe in Sweden (Röshoff 1978; Claesson 1980). Röshoff (1978)


Sample JL-09-3 was collected from the Mylonite Zone at a lay-by along the E6 east of Lake Mjøsa at Morsko-gen. It consists of an augen gneiss. Sample JL-06-39 was collected from a granitic gneiss near Elverum at Oppset-grenda along the road to Rena. Sample JL-06-45 was col-lected on the eastern shore of Lake Storsjøen near the Caledonian front.

Allochthonous basement samples

The samples in the Osen-Røa Nappe Complex start with sample JL-08-14, a granite from the Atnsjøen window (Fig. 2). Further to the northeast a coarse-grained gran-ite (sample JL-07-11) was sampled from the Tufsing-dalen window where the depositional contact with the Moelv Formation (Moelv tillite) is exposed. A monzo-nite was sampled (sample JL-06-59) on the eastern side of Lake Femunden, at Litlesjøberget. This rock has depo-sitional contact with the Rendalen Formation and a pos-sible weathering crust, and is localised within one of the large basement thrust sheets in this area. Sample JL-09-6, a diorite, was collected from the river Mistra, north of Lake Storsjøen. The diorite occurs within a thrust sheet dominated by granite at the sole of the Osen-Røa Nappe Complex (Holmsen & Oftedahl 1956; Sæther 1979; Siedlecka et al. 1987).

Samples from the Middle Allochthon comprise one alkali-feldspar granite sample from the Valdres Nappe Complex from Ormtjernkampen (sample JL-08-36) and several samples from the Kvitvola Nappe Com-plex. From the Kvitvola Nappe Complex, sample JL-08-10 was collected from an augen gneiss at Rosten, west of Høvringen near the mountain Storkuven (Fig. 2). The gneisses at Høvringen were called “the Høvrin-gen Gneiss” and dated to 1180 ±1 Ma by Handke et al. (1995). The gneisses in the Høvringen area, including the gneiss at the sampling site of JL-08-10, occur within a large thrust sheet extending about 60 km from the Blåhø area in the northwest to the Venabu mountains in the southwest (Strand 1951; Englund 1973; Siedlecka et al. 1987). This thrust sheet, informally termed the “swan sheet” due to its outcrop pattern, occurs in the lower part of the Kvitvola Nappe Complex and forms the base-ment of the Rosten conglome rate and other sedimen-tary successions of the Engerdalen Group (Siedlecka et al. 1987). In three papers, Sturt et al. (1997), Nilsson et al. (1997) and Sturt & Ramsay (1997), the augen gneiss at Høvringen was erroneously correlated with gneisses of the Rudihø complex; the latter gneiss complex is located in the Trondheim Nappe Complex of the Upper Alloch-thon, overlying the Kvitvola Nappe Complex and the gneisses at Høvringen (Siedlecka et al. 1987). In the pres-ent paper we include all gneissic basement rocks beneath conglomerate and sandstones within the Kvitvola Nappe Complex in the Rosten-Høvringen area (Siedlecka et al. 1987) in the Høvringen gneiss complex. Since the previ-ous dating results from the Høvringen Gneiss were never properly documented and the exact sampling locality is

The Randsfjord-Lygnern and Kongsberg-Marstrand blocks west of the Mylonite Zone (Fig. 1) are built up by various Mesoproterozoic (1.75-1.50 Ga) meta-igneous and metasedimentary rocks, formed in a long-lived and possibly complex subduction zone environment along the western margin of the Fennoscandian shield (Gor-batschev 1980; Åhäll & Gower 1997; Åhäll & Connelly 2008; Andersen et al. 2004). This “Gothian” orogeny overlaps in time with the TIB 2/3 phases of the Transs-candinavian Igneous Belt (Åhäll & Larson 2000). The rocks immediately south of the Mylonite Zone consist of migmatitic biotite gneisses of probable supracrustal origin called the Romerike Complex (Berthelsen et al. 1996). They are a continuation of the Göteborg-Åmål belt in Sweden (Åhäll & Connelly 2008). Further to the west, the rest of southwest Norway is characterised by gneisses not older than Gothian in age and various supracrustal suites. The ages are mainly Mesoproterozoic - early Neoproterozoic c.1555-914 Ma, while the young-est rock is the Egersund dyke complex of c.616 Ma (Bin-gen et al. 1998). A more detailed discussion of this area is outside the scope of this article.

During the Sveconorwegian Orogeny, southwest Norway was intruded by several generations of mafic to granitic intrusive rocks, which form a belt of intrusions extend-ing from southernmost Norway to the Western Gneiss Region (WGR). Recent geochronological and geochem-ical studies of these intrusions include those of Ander-sen et al. (2001, 2002a, 2009a) and Vander Auwera et al. (2003). One of the largest plutons is the Flå granite. It is a large megacrystic granite body intruding the Gothian gneisses in the Begna sector (Bingen et al. 2005), which is a continuation of the Gothian domain on the north-west side of the Oslo Rift. It has previously been dated by Nordgulen (1999), but was included in this study for Hf analysis.

Material studiedThe sampling sites are marked in Fig. 2 and a sample summary with co-ordinates is presented in Table 1.

Autochthonous basement samples

Along a transect from south to north the sampling sites span from the Gothian domain near the Mylonite Zone to north of Lake Storsjøen where the westernmost TIB granites are exposed in front of the Caledonian nappes. On the northwest side of the Oslo Rift, the Flå granite was sampled at Høgfjellet (sample JL-07-1). A granitic gneiss was sampled east of Lake Sperillen (Randsfjord Complex, Nordgulen 1999), at Lake Vestre Bjonevatnet (JL-07-2). On the southeast side of the Oslo rift, sample JL-09-2 was collected along the E6 road north of Gard-ermoen airport near Eidsvoll. This rock is a migmatitic diorite, from which only the palaeosome was analysed.


exceptionally large augen. North of Lake Storsjøen, on the eastern side of Lake Lomnessjøen, sample JL-09-4 was collected from an augen gneiss of considerably dif-ferent nature than sample JL-09-5 and with smaller augen. Another sample was taken from this basement unit, sample JL-09-7, which is an anorthosite lens within the augen gneiss (cf. Holmsen 1953). Sample JL-09-8 was collected northwest of the town of Koppang, on the west-ern side of the river Glomma, and is also an augen gneiss with large augen, resembling sample JL-09-5.

also unknown, we decided to resample the augen gneiss at Høvringen for redating and for Lu-Hf isotopes (sample JL-08-10). Another basement sample, JL-07-29, was col-lected from a road section a few kilometres south from the sampling site of sample JL-08-10. This sample is a highly sheared granitic-mylonitic gneiss, unlike sample JL-08-10.

Sample JL-09-5 on the eastern side of Lake Storsjøen, at Valsjøberget, consists of an augen gneiss with

Table 1. Summary of samples analysed in this study.

Sample Name LocalityTectonostratigraphic

position*UTM coordinates

(WGS 84, zone 32V)Concordia

age

Weighted Average 207Pb/206Pb age

(95% conf.) NotesJL-07-1 granite Høgfjellet,

Ringerike Autochthonous base-

ment556972 6690512 932 ± 8 Ma 932 ± 11 Ma

JL-07-2 granitic gneiss Vestre Bjonevatnet Autochthonous base-ment

561929 6711996 1592 ± 10 Ma 1592 ± 9 Ma

JL-09-2 migmatitic diorite

Eidsvoll Autochthonous base-ment

619193 6683565 1276 ± 8 Ma 1276 ± 14 Ma zircon over-growths at 1013 ± 40 Ma

JL-09-3 augen gneiss Morskogen Autochthonous base-ment

623197 6713647 1670 ± 5 Ma 1671 ± 5 Ma

JL-06-39 granite Oppsetgrenda Autochthonous base-ment

631228 6764603 1676 ± 6 Ma 1678 ± 6 Ma

JL-06-45 granitic gneiss Eastern Lake Storsjøen

Autochthonous base-ment

620177 6821825 1676 ± 5 Ma 1675 ± 6 Ma

JL-06-59 monzonite Litlesjøberget Autochthonous base-ment

656391 6887251 1680 ± 7 Ma 1682 ± 7 Ma

JL-08-14 granite Atnsjøen window at Atndalen

ORNC/Lower Allochthon

569393 6858277 1659 ± 5 Ma 1659 ± 5 Ma

JL-07-11 granite Tufsingda-len window at

Høgåsen


640642 6908939 1655 ± 5 Ma 1656 ± 5 Ma

JL-09-6 diorite River Mistra near Åkrestrømmen


618520 6844030,01 1675 ± 10 Ma 1678 ± 10 Ma

JL-07-29 granitic gneissRostein, along road to Horgen

KNC/Middle Allochthon

520610 6862653 1620 ± 8 Ma 1621 ± 8 Ma

JL-08-10 augen gneiss Road to Nordsætrin


520181 6865353 1183 ± 7 Ma 1185 ± 8 Ma

JL-08-36 alkali-feldspar granite

Ormtjernkampen VNC/Middle Allochthon

542788 6784440 1476 ± 9 Ma includes coronitic garnets

JL-09-8 augen gneiss Koppang KNC/Middle Allochthon

605215 6830630 1632 ± 8 Ma 1631 ± 8 Ma

JL-09-4 augen gneiss road to Otnes KNC/Middle Allochthon

616210 6846638 1193 ± 15 Ma 1198 ± 21 Ma

JL-09-5 mylonitic augen gneiss

Åkrestrømmen, eastern Lake Stor-

sjøen


618858 6831742 1627 ± 9 Ma 1628 ± 9 Ma

JL-09-7 anorthosite west of Otnes KNC/Middle Allochthon

613931 6848841 1187 ± 18 Ma 1191 ± 23 Ma

C-99-55 granite WGR for details see Austrheim et al. (2003)

Jølster granite WGR for details see Skår and Pedersen (2003)

* ORNC = Osen-Røa Nappe Complex, VNC = Valdres Nappe Complex, KNC = Kvitvola Nappe Complex, WGR = Western Gneiss Region


Figure 3. Concordia diagrams of samples from the autochthonous basement. Only analyses < 2% discordant are shown. Outliers were discarded based on weighted average 207Pb/206Pb age and statistical test in Isoplot software. Sample JL-09-3 has also one old inherited age, which is not shown.


calculated from the signal at mass 238 using a natural 238U/235U=137.88. Mass number 204 was used as a moni-tor for common 204Pb. Analyses which yielded peak/background ratios at mass 204 of less than 1 + 3RSDB (where RSDB is the observed relative standard deviation of the on-peak background measurement), were consid-ered to have common Pb below the detection limit. The data were regressed using mainly two (occasion-ally up to four) calibration standards. Standard zircons used for calibration were: 91500 (1065 ± 1 Ma; Wieden-beck et al. 1995), GJ-01 (609 ± 1 Ma; Jackson et al. 2004), Plešovice (337 ± 0.4 Ma; Sláma et al. 2008), and an in-house Palaeoproterozoic standard A382, which is a zir-con separated from a hypersthene granite from Voin-salmi, Finland, that has an average age of 1876 ± 2 Ma by isotope dilution (ID)-TIMS U-Pb (H. Huhma, pers. comm.). In our laboratory the GJ standard has shown to be c.601 Ma (concordia age) and the 91500 c.1059 Ma (concordia age). These values were used in calculations. The calculations were done off-line, using an in-house interactive Microsoft Excel spreadsheet program allow-ing interactive choice of isotopically homogeneous inte-gration intervals. The estimated uncertainties in isotope ratios incorporate error terms from counting statistics on signals and backgrounds for the relevant masses mea-sured on standards and unknowns, the standard error of the regression line determined from standards, and the published uncertainty of the calibration standards. The terms have been propagated through, using standard error propagation algorithms (e.g. Taylor 1997). IsoplotEx 3.34 (Ludwig 2003) was used to plot concordia diagrams and for calculating the concordia ages (Ludwig 1998) and weighted averages. Zircon standard 91500 was run as unknown for checking the accuracy of the method and outlier calibration standard analyses were rejected based on the outcome of the standard as unknown. Calibra-tion standards chosen for each day and session and the out-come of standard run as unknown are listed in Appendix 1. Most of the time accuracy is not off by more than 5 Myrs.

Lu-Hf

For Lu-Hf isotope analysis of zircon, masses 172 to 179 were measured. The isobaric interferences on 176Hf by 176Lu and 176Yb were corrected by an empirical procedure using reference zircons with homogeneous isotopic composition and variable Yb/Hf ratio (Andersen et al. 2009). A value for the decay constant of 176Lu of 1.867 x 10-11 year-1 has been used in all calculations (Söderlund et al. 2004; Scherer et al. 2001, 2007). We have adopted the CHUR curve of Bou-vier et al. (2008) and the depleted mantle model of Griffin et al. (2000), modified to the decay constant and chondritic parameters used. Reference zircon Mud Tank (Black & Gulson 1978; Griffin et al. 2004; Woodhead & Hergt 2005) and Temora-2 (Black et al. 2004) were used as monitoring standards. Laser settings used were: c.50 % power (fluence: 1-2 J/cm2), 55 μm spotsize and 5 Hz repetition rate.

Western Gneiss RegionTwo granite samples were included from the WGR for Lu-Hf analysis and for comparison purposes. Sample C-99-55 comes from the Hustad Igneous Complex, and has already been dated by Austrheim et al. (2003) to 1653 ± 2 Ma. The Hustad Igneous Complex is a concentrically zoned high-K calc-alkaline complex with a quartz-monzonitic and granitic core surrounded by monzodiorites and pyrox-enites (Austrheim et al. 2003). The complex is described in Austrheim et al. (2003) and the sample used in this study is also described there. The Hustad Igneous Complex is a low-strain enclave in an otherwise highly deformed part of the WGR, where Caledonian deformation was intense. The second sample from the WGR comes from the Jølster granite, which is the largest post-orogenic Sveconorwegian intrusion in the WGR and has been dated to 966 ± 2 Ma by Skår & Pedersen (2003). The importance of this sample is its location in the WGR. Previous Lu-Hf work on post-oro-genic granitoids comes from South Norway (e.g. Andersen et al. 2009a). Some post-orogenic granitoids in South Nor-way have been shown to carry a strongly crustal contam-inated signature that could reflect crustal composition in the deeper crustal levels; Hf isotopes from the Jølster gran-ite may reveal similar information about the WGR.

Analytical methodsThe main method in this study focuses on zircon U-Pb and Lu-Hf isotopes. The Lu-Hf isotope system adds a valuable possibility to obtain petrogenetic information from the host rock in which the zircon crystallised (cf. Howard et al. 2009) and can be used like the more com-monly used Sm-Nd system.

Zircon was separated from samples by standard meth-ods including crushing, milling, heavy liquid and Frantz magnetic separation. Grains were mounted in epoxy and exposed by polishing. Before analysis, all grains were imaged by cathodoluminescence (CL) in a JEOL JSM 6460LV scanning electron microscope to detect internal structures such as magmatic zoning, inherited cores and metamorphically disturbed and metamict zones.

U-Pb

U-Pb dating and isotope analysis were performed using a Nu Plasma HR multicollector ICPMS and a New Wave/Merchantek LUV-213 laser microprobe at the Depart-ment of Geosciences, University of Oslo. U-Pb analyses were made according to analytical protocols described by Rosa et al. (2009). A single U-Pb measurement included 30 s of on-mass background measurement, followed by 60 s of ablation with a stationary beam. Laser condi-tions for U-Pb analysis were: beam diameter: 40 μm; pulse frequency: 10 Hz; fluence: ca. 0.06 J/cm2. Masses 204, 206 and 207 were measured in secondary electron multipliers, and 238 in an extra high mass Faraday col-lector of the Nu Plasma U-Pb collector block. 235U was


U-PbThe U-Pb data are given in Appendix 2 and summarised in Table 1. Concordia diagrams are presented in Figs. 3-7 and a summary map in Fig. 8. Most of the analyses show good concordance in the U-Pb system. Some grains show Pb-loss and some grains have common Pb. No common Pb corrections were made to raw data. The grains having common Pb are highly discordant and are not included

ResultsZircon in all samples shows mostly oscillatory zoning in CL images, which is commonly thought to represent magmatic origin. Only sample JL-09-2 showed distinct CL-bright overgrowths.

Figure 4. Concordia diagrams of granitic gneiss from the Rands-fjord Complex. (A) Most of the analyses show Pb-loss on a Tera-Wasserburg diagram (Tera & Wasserburg 1972). Some analyses indicate inheritance of up to 1800 Ma and one analysis suggests Sve-conorwegian resetting. (B) The crystallisation age of the protolith is calculated from < 2% discordant analyses. This rock is probably supracrustal in origin.

Figure 5. Concordia diagrams of sam-ples from the Lower Allochthon. Only ana-lyses < 2% discordant are shown. Outliers were discarded based on weighted average 207Pb/206Pb age.


from Flå granite (Bingen et al. 2008a). Sample JL-07-2, granitic gneiss from Lake Vestre Bjonevatnet, has a wide spread of discordant analyses. Although few analyses allow a concordia age to be calculated to 1592 ± 10 Ma, this rock is most likely of sedimentary origin and con-tains an indication of zircon as old as c.1800 Ma. Most of the zircon in this sample has suffered lead-loss (Fig. 4a).

Samples from the Lower Allochthon are similar to the Autochthonous basement east of the Mylonite Zone. Diorite from Mistra is dated at 1675 ± 10 Ma and thus identical in age to the autochthonous basement at Lake Storsjøen. Monzonite from Litlesjøberget at Lake Femun-den is 1680 ± 6 Ma. Samples from the Atnsjøen and Tufsingdalen windows are younger than other samples

in age calculations. Concordia diagrams show only those analyses that are < 2% discordant and without common Pb (except in Fig. 4a). Weighted average 207Pb/206Pb age was calculated from < 2% discordant grains and some outliers were rejected based on a statistical test in Isoplot (Ludwig 2003). The only sample showing inheritance is sample JL-07-2 (Fig. 4a).

Samples from the autochthonous basement on the east-ern side of the Mylonite Zone all have similar 1670−1676 Ma ages. On the western side of the Mylonite Zone, migma tite sample JL-09-2 from Eidsvoll is 1276 ± 8 Ma and has zircon overgrowth at 1013 ± 39 Ma. The Flå granite from the western side of the Oslo Rift is 932 ± 8 Ma and corresponds well to previous dates obtained

Figure 6. Concordia diagrams of sam-ples from the Middle Allochthon. Only ana-lyses < 2% discordant are shown. Outliers were discarded based on weighted average 207Pb/206Pb age.


from the Romerike Complex is fairly juvenile and has 176Hf/177Hf between 0.28209 and 0.28235.

The three basement slices from the Lower Allochthon are very similar in their Hf isotopes. Sample JL-06-59 from Litlesjøberget ranges from 0.28166 to 0.28184, sample JL-07-11 from Tufsingdalen window is between 0.28177 and 0.28189, sample JL-08-14 from Atnsjøen window is between 0.28174 to 0.28186, and sample JL-09-6 from Mistra thrust-sheet is 0.28175-0.28188. In general, all these samples are similar to TIB rocks from the autoch-thonous basement.

Samples from the Middle Allochthon start with sample JL-07-29 from the Høvringen gneiss complex in the Kvit-vola Nappe Complex near Høvringen, which has an ini-tial 176Hf/177Hf composition within the range 0.28166 to 0.28188 and is thus also very similar to TIB rocks and samples from the Lower Allochthon. Sample JL-08-10, also from the Høvringen gneiss complex from nearby, has composition between 0.28201 and 0.28212. Sample

from the Lower Allochthon. Granite from Atnsjøen window is dated at 1659 ± 5 Ma and a granite from the Tufsingdalen window has a similar age of 1655 ± 4 Ma.

Samples from the Middle Allochthon are notice-ably younger than those from the Lower Allochthon. Mylonitic gneiss from Rosten is 1620 ± 7 Ma and the Høvringen Gneiss some kilometres to the north is 1183 ± 7 Ma. In the east, augen gneiss west of Lake Lomnes-sjøen is dated at 1193 ± 15 Ma and an anorthosite lens in the augen gneiss is almost identical at 1187 ± 18 Ma. An augen gneiss northwest of the town of Koppang is 1632 ± 8 Ma in age and a very similar augen gneiss east of Lake Storsjøen is 1627 ± 9 Ma. Sample JL-08-36 from the Valdres Nappe Complex shows a wide spread along the concordia (Fig. 7) and allows only a weighted average 207Pb/206Pb age of 1476 ± 8 Ma to be calculated.

Lu-Hf

Lu-Hf isotopes were analysed from most of the samples. Data are given in Appendix 3. The initial 176Hf/177Hf versus age is plotted in Fig. 9, which conveys the same information as more traditional ε-plots, but is graphi-cally easier to analyse.

From the Autochthonous basement, sample JL-06-39 varies between 0.28178 and 0.28188. The spread in val-ues is rather narrow compared to other samples in this study. Sample JL-06-45varies between 0.28165 and 0.28186 showing more crustal contamination than sam-ple JL-06-39. These values are typical moderately juve-nile TIB values (Andersen et al. 2009b). The Flå granite, sample JL-07-1, on the southwestern side of the Mylonite Zone, shows a wide range in Hf isotopes ranging from 0.28193 to 0.28234. Migmatitic diorite, sample JL-09-2,

Figure 7. Sample JL-08-36 has considerable spread in concord-ant ages, which does not allow calculation of its concordia age. Weighted average is the best estimate for this sample.

Figure 8. Map of the study area showing the distribution of ages obtained in this study. Legend same as in Fig. 2. The autochthonous basement on the eastern side of the Mylo-nite Zone has rather uniform ages of c.1676 Ma. Ages in the Lower Allochthon are slightly younger in the Atnsjøen and Tufsingdalen windows. Ages in the Middle Allochthon are significantly younger and show an association of ages c.1620 Ma and 1190 Ma in two places (Høvringen and Lake Stor-sjøen).


The plot in Fig. 9 contains three additional samples for comparison. Sample C-99-55 from the Hustad Igneous Complex in the WGR shows very similar Hf composi-tion to the TIB, ranging from 0.28176 to 0.28189. The Jølster granite from the WGR ranges from 0.28205 to 0.28218 and is similar to the Flå granite, but shows less spread. The sample from the Karlshamn pluton in South Sweden is included in the plot for illustrating how Hf isotopes would evolve in anatexis. These data are as yet

JL-09-7, anorthosite, from the Kvitvola Nappe Complex on the eastern side of Lake Storsjøen is clearly juvenile with 176Hf/177Hf from 0.28214 to 0.28230, while sample JL-09-5, an augen gneiss, shows a TIB-like composition ranging from 0.28173 to 0.28184. Similar augen gneiss, sample JL-09-8, northwest of the town of Koppang shows similar 176Hf/177Hf values from 0.28175 to 0.28181. Sample JL-08-36 obtained from the Valdres Basin is moderately juvenile with values ranging from 0.28188 to 0.28200.

Figure 9. Initial 176Hf/ Hf versus age plot of samples analysed in this study. A) Diagram shows all samples. The red line is evolution line for TIB granites when they undergo repeated anatexis. The evolution line of TIB corresponds roughly to 2.1 Ga model age. Samples post-dating the Gothian Orogeny are clearly juvenile and plot above the TIB trend, or above the CHUR line. The large spread in the Flå granite sample indicates old crust in the protosource. Possibly related to the TIB. B) This diagram shows the Palaeoproterozoic part of the diagram illustrating the similarity of Hf isotopes. All samples from the autochthonous basement on the eastern side of the Mylonite Zone, Lower Allochthon and c.1620 Ma sample from the Middle Allochthon follow the TIB trend. The sample from the Hustad Igneous Complex follows the TIB trend as well.


rocks are very much the same as in the autochthonous basement below: Moelv Formation (tillite), Vangsås For-mation and Cambrian shale (Holmsen & Oftedahl 1956; Nystuen 1982; Sæther 1979). The large granitic thrust sheets at Lake Femunden, on the other hand, form the original depositional basement of the several thousand metre thick Rendalen Formation and the younger forma-tions of the basin-derived Hedmark Group. These base-ment thrust sheets represent the original basement of the Hedmark Basin and have been transported together with the sedimentary succession of the Osen-Røa Nappe Complex.

Although all of the basement slices in the Lower Alloch-thon in the northeasterly Hedmark Group must have been moved some distance, the ages are very similar to what is currently the autochthonous basement east and south of the outcrop area of the Osen-Røa Nappe Com-plex. The basement rock within the Mistra thrust sheet is in fact identical in age to the autochthon south of it at the eastern side of Lake Storsjøen. A monzonite base-ment slice east of Lake Femunden (sample JL-06-59) is also within the TIB 3 period. Granitic rocks in the win-dow structures at Tufsingdalen and Atnsjøen are notice-ably younger.

According to current Caledonian thrust models, rocks of the Middle Allochthon originated even further away in the present west than the Lower Allochthon. However, it is not demonstrated that this rule applies to all units clas-sified as Middle Allochthon. For example, the Valdres Nappe Complex could be related to the Lower Alloch-thon due to similarity in deformation style. On the other hand, the ages obtained in this study demonstrate that the Kvitvola Nappe Complex is indeed far travelled. The c.1620 Ma age from the basement slice near Høvringen is coeval with the Gothian Orogeny, whereas the c.1180 Ma Høvringen Gneiss can be considered early Sveconor-wegian. These ages are within error to the augen gneisses in the Lake Storsjøen area. Also the rock units seem to be spatially associated both in the Høvringen area and sev-eral hundred kilometres to the east in the Lake Storsjøen area. A similar association was also noted by Handke et al. (1995) from the Risberget Nappe further to the north from the Sparagmite Region. They were probably also associated in their pre-thrust positions. In the Lake Storsjøen area, there is a sharp age-contrast with nearby autochthonous basement rocks (Fig. 8), which reflects the long transport of the Middle Allochthon.

Basement slices in the Jotun Nappe have been stud-ied before by several workers (e.g. Schärer 1980; Emmet 1996; Lundmark et al. 2008) who concluded, mainly on U-Pb grounds, that there was a clear Fennoscandian Shield signature for these rocks. Our results support these findings. An age of c.1180 Ma was not reported by these workers, but c.1630 Ma ages seem to be abun-dant in the Jotun Nappe Complex. Corfu & Heim (2009) reported Gothian, c.1520 Ma ages from the Espedalen

unpublished. The pluton is situated next to c.1.77 Ga TIB rocks in Blekinge and has retained the Hf composition of its country rocks.

DiscussionThe ages obtained from the autochthonous basement east of the Mylonite Zone belong to the 1.68-1.66 Ga TIB 3 phase of the Transscandinavian Igneous Belt as defined by Larson & Berglund (1992). The ages are uni-form along the transect from the Mylonite Zone to Lake Storsjøen and are also within error to the 1673 ± 8 Ma Trysil “tricolor” granite dated by Heim et al. (1996). On the northwestern side of the Oslo Rift, sample JL-07-2 in the Begna Sector of Bingen et al. (2005) belongs to the c.1590 Ma Stora Le Marstrand stage of the Gothian Orogeny (Åhäll & Connelly 2008), while the c.1270 Ma diorite in the Romerike Complex falls into the interoro-genic period (see below). The CL-bright overgrowths at 1013 ± 39 Ma probably reflect Sveconorwegian defor-mation of the Romerike Complex. A similar age has been reported from South Sweden, from the Åmål Suite (Scherstén et al. 2004).

Granitic gneisses in the WGR have similar ages to the basement windows in the Lower Allochthon. Ages in this area range from c.1680 to 390 Ma (Tucker et al. 1990; Bingen & Solli 2009). There is almost a continuous range of ages in the interval 1680-1630 Ma. An example is the Hustad Igneous Complex in the northern WGR (Aus-trheim et al. 2003), which includes c.1650 Ma gneisses and 1250 Ma mafic rocks. In the WGR, the ages seem to get younger to the southwest (Gorbatschev 1980; Skår & Pedersen 2003). Sveconorwegian rocks are also present in the WGR, mainly as intrusions. The WGR has been correlated with the autochthonous basement in south-ern Fennoscandia (Gorbatschev 1980), though the actual structural position of the WGR could also be allochtho-nous (e.g. Rice 2005) .

From the data presented in this study, it seems that rocks in allochthonous basement slices in the Middle Alloch-thon are noticeably younger than those in the Lower Allochthon. Whereas basement slices are no older than c.1650 Ma in the Osen-Røa Nappe Complex, those in the Middle Allochthon can be as young as c.1180 Ma. The nappes in the Lower Allochthon have a minimum displacement of at least 150 km (Oftedahl 1943; Mor-ley 1986). However, it is not certain that all the base-ment slices have been displaced the same distance as the basin-derived Hedmark Group of the Osen-Røa Nappe Complex. Out-of-sequence thrusting, as has been dem-onstrated in the Lower Allochthon (Nystuen 1983), may explain for example the structural position of the Mis-tra thrust sheet (sample JL-09-6) and similar small thrust sheets east of Lake Storsjøen in the lower part of the Osen-Røa Nappe Complex. In these thrust sheets, the sedimentary succession above the crystalline basement


for the younger TIB rocks and the Karlshamn pluton.

Most of the samples from the autochthonous basement and Lower Allochthon follow the TIB trend. The c.1620 Ma samples from the Middle Allochthon also follow this trend, but the c.1180 Ma and 1476 Ma samples are more juvenile and plot above the trend line (red line in Fig. 9). The most juvenile samples are the anorthosite in the Kvitvola Nappe Complex west of Lake Lomnes sjøen and the diorite from the Romerike Complex . The sample from the Valdres Nappe Complex from Ormtjern kampen shows a clear juvenile signature in Hf isotopes (Fig. 9). Bingen et al. (2008b) called the 1.52-1.48 Ga period in the Hardangervidda-Rogaland and Telemark blocks as the “Telemarkian accretion”. Previous studies have shown a rather juvenile character for the Mesoproterozoic mag-matic rocks in this part of Norway (Andersen et al. 2002a & 2002b). The Tinn granite in the Telemark Block has a matching age of 1476 ± 13 Ma (Andersen et al. 2002b) and also a juvenile character, which is comparable to sample JL-08-36 from Ormtjernkampen. Although the Flå granite is hosted in the juvenile Gothian domain, it shows a wide range in initial Hf composition that suggests the presence of older crust at depth. The same has also been observed by Andersen et al. (2001), who designated this granite as “Group 1” based on variations in Sr, Nd and Pb isotopes. This observation has impli-cations regarding the nature of the Mylonite Zone and whether the TIB continues underneath it.

The Hustad Igneous Complex sample from the WGR shows a remarkable match with the TIB trend (Figs. 9a & b). This is strong evidence that the TIB continues into the WGR and also that the magmatism probably lasted until 1620 Ma, which is longer than previously thought

valley in the Valdres Nappe Complex (Fig. 2). They also dated the same rock unit in Ormtjernkampen (sample JL-08-36 of this study), but obtained a slightly older age of 1506 ±9 Ma.

Hf isotopes

Due to the higher amount of parent Lu in the depleted mantle compared to the crust, juvenile magmas from a depleted mantle source carry more radiogenic hafnium than crust at any given time. This allows the Hf isotope composition of zircon in a rock to be used to deduce the nature of its source, or the proportion of mantle- and crustal-derived components. Input of mantle-derived magmas to the deep crust will also transfer heat to the crust, and potentially cause anatectic melting. Gran-ites that are anatectic without new mantle input pre-serve their hafnium compositions and the radiogenic Hf evolves only very slowly due to the low amount of parent Lu in the rock.

Previous hafnium work done on TIB rocks shows a rather distinct pattern in 176Hf/177Hf versus age plots (Fig. 9) (Andersen et al. 2009b). The age-corrected 176Hf/177Hf from the TIB is rather restricted, c.0.2816-0.2818, and shows a trend (red line in Fig. 9) without significant input of mantle-derived material during the three TIB events. In Fig. 9 the slope of the TIB evolution line corre-sponds to a 176Lu/177Hf whole-rock value of 0.010, which is obtained by linear regression from TIB zircon analy-ses and has an average TDM of c.2.1 Ga. Andersen et al. (2009b) used a 176Lu/177Hf value of 0.015, which gives a slightly higher angle on the evolution line, but a value of 0.010, corresponding to an average granitic composition is used in this study, since it seems to give a better match

Figure 10. Basement ages in South Norway from the U-Pb database of Bingen & Solli (2009). Ages in the Caledonides and the post-orogenic Sveco-norwegian granitoids are excluded. Also Rb-Sr ages are excluded. The WGR contains significant crustal building events until c.1620 Ma, after which magmatic events are episodic and mainly mafic. The southwest Norway (or “Telemarkia”) has a different history with abundant interorogenic bimodal magma-tism and sedimentation.


Caledonian nappes, similar rocks with the same age and petrogenetic history may have occurred further to the northwest of Telemark.

The western margin of Baltica

A database of published isotopic ages from Norway com-piled by Bingen & Solli (2009) shows a drastic difference of ages from rocks of the Middle Allochthon in southern Norway compared to the Middle Allochthon in north-ern Norway. Data from northern Norway come from the Akkajaure Nappe and are generally 1730-1880 Ma (Bin-gen & Solli 2009; Kirkland et al. 2011) whereas the ages of basement rocks in the Middle Allochthon in southern Norway are mainly younger than 1650 Ma. A wide range of ages is present in southern Norway and some of them coincide with the results of this study.

Hafnium isotopes from the basement rocks indicate that the TIB magmatism may have lasted as long as until 1620 Ma, which would then largely overlap with the Gothian event. Roberts et al. (1999) reached similar conclusions from the 1633.2 ± 2.9 Ma Blåfjellhatten granite in the Grong-Olden Culmination in Central Norway. The age of augen formation in the augen gneisses is still unclear, but it is most likely Sveconorwegian. For example the c.1180 Ma Høvringen Gneiss has a depositional contact above to just slightly deformed sedimentary rocks. If this deformation was Caledonian, it would be unlikely that the sedimentary strata escaped strong deformation.

Hafnium isotopes obtained from the Mesoproterozoic rocks in Telemark show a rather juvenile characteristic (Andersen et al. 2002a; Andersen et al. 2007; Pedersen et al. 2009), except the Late Sveconorwegian post-oro-genic granites which show Palaeoproterozoic inheritance and have a wide range of Hf isotope composition from juvenile to strongly crustal contaminated. The juvenile Høvringen Gneiss and the granite from Ormtjernkam-pen could originate from the same juvenile domain, that most likely hosts similar basement rock types as in the Hardangervidda-Rogaland and Telemark blocks of southwest Norway. Andersen et al. (2001) attributed the westwards increasing juvenile character to an increasing involvement of a mafic underplate in magma genesis.

The results presented in this study allow the construction of a simple model for the crustal architecture of the west-ern margin of Baltica prior to the Caledonian Orogeny (Fig. 11). The presumed original locations of the base-ment slices are marked in Fig. 11. The TIB probably con-tinued to the west until at least 1620 Ma, creating a zoned cratonic margin. A sharp boundary connected the TIB domain to a more juvenile crustal block in the south-west, which could be the continuation of the present-day southwest Norway. The Mylonite Zone in South Nor-way and Sweden is a major Sveconorwegian shear zone (Johansson & Johansson 1993; Andersson et al. 2002). It has been traced to the Caledonian nappe front, but may

(Högdahl et al. 2004). This adds to the already enigmatic nature of the Gothian magmatism. It also ties the base-ment windows in the Lower Allochthon and the c.1620 Ma augen gneisses in the Middle Allochthon to the WGR or its continuation further to the northwest. The Hf com-position of the Jølster granite is similar to that of the Flå granite, except in having a narrower compositional range. It does not show extremely crustal contaminated values as some post-orogenic granites do from south-west Norway as reported by Andersen et al. (2009a). More analyses are needed from the WGR to see if there is actually a difference in the crustal structure between the WGR and SW Norway.

The interorogenic period in south Scandinavia

Although the youngest Gothian rock in Scandinavia is the Hisingen suite in South Sweden (1522 ± 10 Ma; Åhäll 1990), there is no clear evidence for the end of the Gothian Orogeny. Only the subsequent magmatism in South Norway changed its nature from intracratonic to episodic (Åhäll & Connelly 2008). On both sides of the Mylonite Zone the crust was penetrated by c.1.46 Ga mafic dykes (Åhäll & Connelly 1998; Rämö et al. 2001). A typical intracratonic volcanic-sedimentary suite is represented by the Telemark Supracrustals in southwest Norway, which started to accumulate at c.1.5 Ga. The Hallandian event or Danopolonian Orogeny (Bogdanova et al. 2001) was active at c.1.46 - 1.40 Ga, and left an imprint in the south of Scandinavia. The nature of this event still remains controversial.

The c.1.34-1.14 Ga period in Fennoscandia is commonly regarded as pre-Sveconorwegian (Bingen et al. 2008a) or interorogenic between the Gothian and Sveconor-wegian orogenies. There is little crustal building in this period, but mostly local mafic magmatism. The cen-tral Scandinavian Dolerite Group (Söderlund et al. 2006) formed in three pulses in the 1.27-1.25 Ga inter-val. The Iveland-Gautestad metagabbro was emplaced in southern Telemark at 1280-1260 Ma (Pedersen & Konnerup- Madsen 2000; Pedersen et al. 2009). The 1.20-1.18 Ga Tromøy gabbro-tonalite complex (Knud-sen & Andersen 1999) in southernmost Norway is a possible island arc with a tectonic contact to the main-land. The younger Telemark supracrustal rocks in Tele-mark consist of bimodal volcanic rocks,1170-1140 Ma in age (Bingen et al., 2003). The 1184 +7/-5 Ma Flåvatn granite gneiss (Dahlgren et al. 1990) and the 1187 ±2 Ma Gjerstad augen gneiss (Heaman & Smalley 1994) are coeval with the augen gneisses at Høvringen and Kop-pang of this study. Figure 10 shows the distribution of magmatic rocks at key age intervals in South Norway. It is notable that the WGR does not contain major granitic magmatism after c.1620 Ma and before the Sveconorwe-gian post-orogenic granitoids were intruded, which sup-ports the derivation of c.1200 Ma granitic gneisses from elsewhere than the present WGR. Although southern Telemark is not located along the transport path of the


Allochthon), Kvitvola Nappe Complex and Valdres Nappe Complex (Middle Allochthon) shows typical Fen-noscandian Shield ages. Ages get seemingly younger from the autochthonous basement to the Middle Alloch-thon. Ages in the Osen-Røa Nappe Complex are close to the autochthonous TIB basement and may suggest a rela-tively short transport distance. Augen gneisses belonging to the Kvitvola Nappe Complex are significantly younger than the surrounding autochthonous basement, which is in agreement with the long transport distance suggested for the Middle Allochthon. The younging trend reflects a westwards younging crustal structure, which is typical for the Fennoscandian Shield on a larger scale and has also been observed in the WGR. This trend is truncated by the appearance of c.1180 Ma and 1476 Ma basement slices in the Middle Allochthon that has counterparts in the Hardangervidda-Rogaland-Telemark part of South Norway.

Hafnium isotopes show a genetic link between the TIB in Sweden and the autochthonous TIB 3 aged basement immediately neighbouring the Hedmark Group. Samples

have continued all the way to the continental margin prior to Caledonian tectonism. It is possible that south-west Norway is juxtaposed against the TIB belt by the Mylonite Zone. Several models have been presented in the past including strike-slip movements (Torske 1985) and collision of an exotic micro-continent (Bingen et al. 2005). One of the problematic questions is the continua-tion of the Gothian belt on the western side of the Cale-donides. The Gothian belt typically has highly deformed gneisses and migmatites, but these rock types are con-spicuously scarce in the Caledonian nappes. The Hustad Igneous Complex in the WGR was assigned as Gothian by Austrheim et al. (2003), but new Hf data reported in this study suggest a TIB origin for these granites. It seems that the Gothian Orogeny still remains as a unique fea-ture of south Scandinavian geology.

ConclusionsU-Pb age determination of allochthonous base-ment rocks in the Osen-Røa Nappe Complex (Lower

Figure 11. A proposed model for the western margin of Baltica before the Caledonian Orogeny. The rocks in the Middle Allochthon show a more juvenile character, which could be explained by the Telemarkia crustal block continuing further to northwest. The association of the ages c.1190 Ma and 1620 Ma in both the Høvringen and Lake Storsjøen areas must mean that these rocks were originally close to each other, probably separated by a shear zone that could be a continuation of the Mylonite Zone.


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from the western side of the Mylonite Zone are clearly more juvenile than the TIB. All the allochthonous base-ment rocks in the Osen-Røa Nappe Complex follow the TIB trend and the same TIB origin can be linked to some of the augen gneisses in the Kvitvola Nappe Com-plex, which suggests that the TIB magmatism continued to c.1620 Ma. However, the c.1180 Ma basement in the Kvitvola Nappe Complex and the c.1476 Ma basement in the Valdres Nappe Complex have more juvenile compo-sitions, similar to rocks in Telemark, South Norway. Two spatially associated gneisses in the Kvitvola Nappe Com-plex, one having a crustal contaminated (TIB-like) Hf signature and the other having a distinctly more juvenile composition, can be explained by derivation from two different crustal domains that were probably tectonically juxtaposed before the Caledonian Orogeny. These could be the old Fennoscandian Shield and more juvenile rocks of the Hardangervidda-Rogaland and Telemark blocks. The data support the idea that before the Caledonian Orogeny, the TIB belt and possibly also southwest Nor-way continued further to the northwest from their pres-ent-day locations.

Acknowledgements – Siri Simonsen is thanked for analytical help with the ICPMS. Bernard Bingen is thanked for providing digital maps and data. Fernando Corfu is thanked for the Hustad Igneous Complex sample. Øyvind Skår is thanked for the Jølster granite sample. Audrius Čečys is thanked for the Karlshamn pluton sample. Discussions with Mattias Lundmark helped to improve the manuscript. Adrian Read is thanked for correcting the language. Reviews by Stefan Claesson, Øystein Nordgulen, Trond Slagstad and Kåre Kullerud improved the manuscript greatly. This study is part of the corresponding author’s doctoral thesis on Proterozoic basins in southern Norway. This is Contribution No. 19 from the Isotope Geology Laboratory at the Department of Geosciences, University of Oslo.

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