NORWEGIAN JOURNAL OF GEOLOGY Zircon U-Pb ages and Lu-Hf isotopes from basement rocks 35

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

Jarkko Lamminen, Tom Andersen & Johan Petter Nystuen

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

Introduction several rift basins at the Baltic margin (e.g. Kumpulai- nen & Nystuen 1985; Siedlecka et al. 2004; Nystuen et al. The rock record in present-day South Norway has a his- 2008). During the Caledonian Orogeny these basins and tory extending from Palaeoproterozoic times and con- detached parts of the crystalline basement were thrust tains evidence of major crustal building/reworking and transported up to several hundred kilometres east- events. The Transscandinavian Igneous Belt (TIB), the and southeastward to their present position on denu- Gothian domain with Sveconorwegian imprint and the dated Precambrian basement and its thin cover succes- Caledonian nappe pile represent events that have created sion of Ediacaran to lower Palaeozoic sedimentary rocks. the basement of South Norway as we know it today. A These basement slices can be considered as direct sam- considerable number of geochemical analyses and geo- ples of the outer Baltic margin, which does no longer chronological data already exist from basement rocks exist in situ. In this study we present U-Pb and Lu-Hf in southern Norway and Sweden. However, much of the data from zircon from basement slices in three alloch- western margin was destroyed during the Caledonian thonous basins within the Sparagmite Region of South Orogeny, and it is unclear how the basement geology was Norway: the Hedmark, Valdres and Engerdalen basins, arranged at the margin of the craton. which are now located in the Osen-Røa, Valdres and Kvitvola nappe complexes, respectively. From zircon The break-up of Rodinia with formation of the continent Lu-Hf isotopes the petrogenetic evolution of the rocks Baltica in the Neoproterozoic resulted in the creation of can be traced to provide information that is necessary 36 J. Lamminen et al NORWEGIAN JOURNAL OF GEOLOGY for evaluating the way in which the rocks in the nappes sub­sequent to the break-up of Rodinia (e.g. Dalziel 1991) are related to the present autochthonous basement, and and prior to the Caledonian Orogeny. how the crust of western Baltica prior to the Caledonian Orogeny might have been organised. Zircon U-Pb ages Summaries of the regional Precambrian geology are given and Lu-Hf data thus also provide background informa- by Gaàl & Gorbatschev (1987), Gorbatschev & Bogda- tion on potential provenances for the sedimentary suc- nova (1993) and Bogdanova et al. (2008). The South- cessions of the Neoproterozoic basins of the Sparagmite west Scandinavian Domain (SSD, Gaàl & Gorbatschev­ Region.Tables containing our U-Pb and Lu-Hf data from 1987) is the youngest part of the Fenno­scandian Shield, individual samples can be found in appendix 1-3, in the and is built up of rocks of late Palaeoproterozoic to late electronic supplement that accompanies the online ver- Mesoproterozoic age. The SSD comprises several differ- sion of this article. ent blocks with distinct tectonometamorphic histories. It was later influenced by Sveco­norwegian deformation and This study supplements previous work that has been metamorphism at c.1.14-0.90 Ga (Bingen et al. 2008a). done on the basement rocks in the Caledonides. We The SSD is separated from the Palaeoproterozoic arc and also present new data from the autochthonous base- granite terranes of the Sveco­fennian Domain in central ment immediately bordering the allochthonous Hed- Fennoscandia by the 1.85-1.65 Ga Trans­scandinavian mark Basin (Hedmark Group). A compilation of base- Igneous Belt (TIB), consisting mainly of granitoids and ment ages by Bingen & Solli (2009) is used as a reference their volcanic equivalents, and minor mafic rocks (Hög- base. It shows a gap in the Sparagmite Region, which will dahl et al. 2004) (Fig. 1). hopefully be filled by this work. During the Caledonian Orogeny (c.490-390 Ma) the Lau- rentian continental plate collided with the Baltic plate. Geological Setting The western margin of Baltica, including crystalline base- ment rocks and overlying Neoproterozoic to Palaeo- zoic sedimentary successions, was divided into a series The present Fennoscandian Shield is composed of of nappes and thrust sheets that were transported to the an Archaean core in the northeast and progressively east and south-east and assembled in a nappe pile on the younger Proterozoic crustal domains towards the south- autochthonous basement and its cover rocks in the fron- west. In this paper we apply the term Baltica for both tal zone of the orogen. The thrust sheets are traditionally the continent and craton, and for the time interval subdivided into Lower, Middle, Upper and Uppermost

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

Allochthon (Fig. 1), each consisting of several nappe com- Neoproterozoic and associated basement rocks in South plexes. The rocks of the three lowermost allochthons are Norway: The Sparagmite Region considered to have originated at the western Baltican mar- gin (also termed the Baltoscandian margin), including The Sparagmite Region in South Norway (Fig. 2), includ- structural and stratigraphic elements related to the Iapetus ing adjacent parts of Härjedalen in Sweden, is named Ocean, whereas the Uppermost Allochthon is considered after the old petrographic term “sparagmite” for the to have Laurentian affinity (Roberts & Gee 1985). coarse-grained feldspathic sandstones that dominate the region (Esmark 1829). The term “sparagmite basin” was The nomenclature used for the same nappes in Norway applied informally to the entire depositional basinal suc- and Sweden varies. For example, the Osen-Røa Nappe cession which includes the sparagmite sandstones. The Complex in Norway corresponds to the Vemdalen nappe basin was mostly thought to be autochthonous or par- (also termed the Hede nappe) of the Lower Allochthon of autochthonous (e.g. Schiøtz 1902; Holtedahl 1953; Skjes- Härjedalen in Sweden, and the Kvitvola Nappe Complex eth 1963; Englund 1973; Bjørlykke et al. 1976; Nystuen in Norway is correlative with the Tännäs Augen Gneiss 1976). An allochthonous origin for the “sparagmites” Nappe and the Särv Nappe of Härjedalen (Bockelie & (e.g. proposed by Oftedahl 1949, 1954) was subsequently Nystuen 1985; Gee et al. 1985; Kumpulainen & Nystuen supported by regional stratigraphic and structural stud- 1985). The Valdres Nappe Complex is also referred to the ies (e.g. Nystuen 1981). The Sparagmite Region also Middle Allochthon, and is located in the western part of includes Neoproterozoic successions within the Kvitvola the Sparagmite Region. This study is from the classical and Valdes nappe complexes which belong to the Enger- Sparagmite Region located north of Oslo in central-east- dalen and Valdres groups, respectively (Kumpulainen & ern Norway (study area in Fig. 1). Nystuen 1985; Siedlecka et al. 2004).

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 38 J. Lamminen et al NORWEGIAN JOURNAL OF GEOLOGY

Neoproterozoic sedimentary successions and associated Basement windows basement rocks in the Sparagmite Region occur in the The window structures in the northern part of the Spar- following tectonostratigraphic units, major lithostrati- agmite Region have traditionally been inferred to be graphic units and related palaeobasins: dome-shaped outcrops along a basement ridge (e.g. Hossack 1978; Nystuen 1981). Holtedahl (1921, 1930) 1. A several thousand metre thick sedimentary succes- and Skjeseth (1954, 1963) thought the basement ridge sion in the Osen-Røa Nappe Complex, representing formed an elongated geomorphologic high during the the Hedmark Group derived from the Hedmark Basin Neoproterozoic to Cambrian sedimentation, separat- with tectonic slices of the crystalline substrate. ing an autochthonous “sparagmite basin” on the south- 2. A several thousand metres thick sedimentary succes- ern side of the “window high” from an open sea (“eugeo- sion in the Kvitvola Nappe Complex, comprising the syncline”) at the northern side of the ridge. However, a Engerdalen Group from the Engerdalen Basin, with marked thrust separates the basement and its thin cover slices of basement rocks. rocks in the windows from the several thousand metre 3. A several thousand metres thick sedimentary succes- thick sandstone successions above. This important struc- sion, the Valdres Group, in the Valdres Nappe Com- tural relationship was also emphasised by Strand (1960). plex, derived from the Valdres Basin together with thrust sheets of basement rocks. Studies in northern Jämtland and Västerbotten have 4. A thin sedimentary cover in an autochthonous posi- revealed that window structures within the outcrop area tion on the crystalline basement along the erosional of the Lower Allochthon, similar to those in the rear of front of the Caledonian nappes. the Osen-Røa Nappe Complex, are allochthonous (Zach- 5. A thin sedimentary cover on Precambrian crystalline risson & Greiling 1993). Similar conclusions can be basement rocks in tectonic windows. drawn from an E-W seismic reflection section extending from Jämtland to Trøndelag (Juhojuntti et al. 2001) and The Hedmark Group in the Osen-Røa Nappe Complex magnetotelluric studies (Korja et al. 2008). In the present is the type and reference succession of Neoproterozoic study it is also suggested that the Atnsjøen, Spekedalen, strata in South Norway. The Hedmark and Valdres basins Øversjødalen, Tufsingdalen and Steinfjellet windows have been interpreted as continental rift basins, whereas (Fig. 2) are allochthonous as well, an interpretation in the Engerdalen basin is considered to represent a passive accordance with that of Morley (1986) and Rice (2005). continental shelf or pericratonic basin (Siedlecka et al. This interpretation implies that the Osen-Røa Thrust in 2004; Nystuen et al. 2008). the northern part of the Sparagmite Region is ramping up from the Caledonian sole thrust system north of the The Caledonides in the Sparagmite Region include a window structures. series of thrust sheets now referred to the Lower Alloch- thon with the Osen-Røa Nappe Complex, and to the Distance of thrusting in the Osen-Røa Nappe Complex Middle Allochthon with the Valdres Nappe Complex and Kvitvola Nappe Complex. The Valdres Nappe Com- Kjerulf (1879) was the first to carry out measurements of plex and Kvitvola Nappe Complex are overlain by the crustal shortening within the Osen-Røa Nappe Complex Jotun Nappe Complex of the Middle Allochthon and by calculating shortening of folded strata along sections the Trondheim Nappe Complex of the Upper Alloch- within the Oslo Region Decollement (being the south- thon. Törnebohm (1896) showed overthrust positions of ern part of the Osen-Røa Nappe Complex, Nystuen 1981; the rock succession at Lake Femunden (Rendalen For- Morley 1986). Oftedahl (1943) calculated the shortening mation) and called this unit the ‘Røa nappe’. This thrust within the Oslo Region Décollement to be 150 km for the unit passes southwards into the ‘Osen nappe’ at the ero- Cambro-Silurian succession in the northern part of the sional nappe front, and the combined nappe unit was lake Mjøsa area and estimated the total thrust distance termed the Osen-Røa Nappe Complex (Nystuen 1981). for the “sparagmites” to be 300 km. Strand (1960) found Törnebohm (1896) also pointed out the thrust posi- the minimum shortening of strata in the Oslo Region tion of Precambrian granite above sparagmite sand- Décollement to be in the order of about 150 km. The dis- stone (Rendalen Formation) in the mountain Sålekinna tance of 150 km corresponds to the distance from the west of Femunden. Basement thrust sheets occur in sev- erosional front of the Osen-Røa Nappe Complex in the eral places at the base of the Osen-Røa Nappe Complex, Elverum-Trysil area to the northern side of the basement Kvitvola Nappe Complex and the Valdres Nappe Com- windows in the northern part of the Sparagmite Region plex (Siedlecka et al. 1987, 2004). The structural position (Nystuen 1981). Morley (1986) also restored the thrust for some of these basement sheets, as for example those distance of the Cambro-Silurian strata in the Mjøsa area east of Lake Femunden, is the result of out-of-sequence to a position about 150 km to the northwest. A simi- thrusting post-dating the emplacement of the Kvitvola lar distance of tectonic transport was also suggested by Nappe Complex (Nystuen 1983). 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 NORWEGIAN JOURNAL OF GEOLOGY Zircon U-Pb ages and Lu-Hf isotopes from basement rocks 39 estimates of Caledonian nappe translations for the Lower concluded that the augen formation was related to the Allochthon put forward by Gee (1975, 1978). Rice (2005) main Caledonian thrusting. restored branch-lines of the Caledonian thrust sheets in South Norway and, like Morley (1986), suggested restor- Spectacular augen gneisses can be found east of Lake ing the position of the Hedmark Basin to the southeast- Storsjøen, east and west of Lake Lomnessjøen and north- ern side of the original position of the structural win- west of the town of Koppang (Fig. 2). The augen gneisses dows in the northern part of the Sparagmite Region. and associated igneous rocks were described by Holm- Rice (2005) put the northwestern margin of the Osen- sen & Oftedahl (1956). Holmsen (1953) suggested that a Røa Nappe Complex to be 615 km northwest of the pres- body of anorthosite west of Lake Lomnessjøen in Renda- ent position of the window structures. However, accord- len, being a member of this rock suite, might represent ing to the tectonostratigraphy in the window region, an outlier of the Jotun Nappe. The augen gneisses are of the basinal succession of the Osen-Røa Nappe Complex two types: one with very large augen (up to 15 cm across) must have been located west or northwest of the original and one with smaller augen. From a distance these rocks position of the rocks exposed in the basement windows. resemble a poorly sorted conglomerate. The augen are both angular and rounded, and rotational structures are The Kvitvola and Valdres nappe complexes (Middle commonly seen. Allochthon) Similar gneisses found in Sweden in the Tännäs augen Crystalline basement rocks and Precambrian sedimen- gneiss nappe have been dated by Rb-Sr and U-Pb zir- tary rocks dominate the Middle Allochthon. The Kvit- con methods (Claesson 1980) giving a Rb-Sr age of 1610 vola Nappe Complex consists of the Upper Proterozoic ± 85 Ma and a zircon upper intercept age of 1685 ± 20 Engerdalen Group which was deposited in fluvial to Ma with Caledonian lower intercept at 405 ± 90 Ma. shallow marine settings at the western Baltican margin The Risberget nappe in western Norway is also known (Kumpulainen & Nystuen 1985; Siedlecka et al. 2004). for its augen gneisses (Krill 1983; Handke 1995). Rosen- The basement rocks are dominantly augen gneisses, but qvist (1941) suggested Caledonian K-metasomatism for also gabbros, granites and anorthosites occur. The crys- the augen gneisses in the Oppdal area. Krill (1983) dated talline rocks occur as tectonic slices beneath and within rapakivi granite and associated augen gneiss with the the sedimentary succession; primary depositional con- Rb-Sr method to 1618 ± 44 Ma, which was interpreted tacts between basement rocks and sedimentary cover as an intrusion age. He suggested that the augen gneisses rocks are preserved both in the Kvitvola Nappe Com- are in fact cataclastically deformed rapakivi granites. plex and in the Valdres Nappe Complex (Nickelsen et Handke (1995) found that the rapakivi granites can be al. 1985). The Kvitvola Nappe Complex lies discordantly classified into two age groups, 1659 to 1642 Ma and 1190 upon the Osen-Røa Nappe Complex and also overlies to 1180 Ma. Rapakivi textures have not been found in the the autochthonous basement and its sedimentary cover augen gneiss samples of this study. rocks in the east. Thrust distances must have been c.300- 400 km (e.g. Kumpulainen & Nystuen 1985). Hossack et Autochthonous basement al. (1985) calculated the shortening of the Valdres Nappe Complex to have been of the order of c.315 km. Meta- The autochthonous basement rocks in the study area morphic grade in the Kvitvola Nappe Complex is in the belong to the Transscandinavian Igneous Belt (TIB) greenschist facies and deformation intensity increases in the east and southeast, and the Gothian domain in towards the northwest. In the study area, the deforma- the southwest. The TIB has been subdivided into three tion is moderate. magmatic phases (TIB 1 to 3; Larson & Berglund 1992) and generally becomesyounger from SE to WNW, being The augen gneisses in the Middle Allochthon 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 Augen gneisses in the Caledonides have sparked inter- extensively in the region east and southeast of the Spar- est for decades. Their origin was explained by graniti- agmite Region and continues to the west as far as Lake sation in the first half of the 20th century (Rosenqvist Mjøsa, where it is separated from the Gothian domain 1944). Even as late as in the 70s Strand (1972) favoured by a mylonitic zone (the Mylonite Zone). Most of the this explanation for many of the augen gneisses in the U-Pb work done on the TIB comes from Sweden. Heim study area. In places they apparently grade into sur- et al. (1996) dated a “tricolour” granite in the Trysil area rounding arkosic sandstones. Törnebohm (1896) consid- and obtained 1673 ±8 Ma. Andersen et al. (2002a) dated ered the augen gneisses in the Caledonides of Norway as the Odal granite north of the Mylonite Zone to1681 ±6 deformed Precambrian porphyritic granites. The age of Ma. Similar ages from this area have also been reported the augen formation has not yet been determined. Early by Bingen et al. (2008a). The Brustad augen gneiss at workers suggested a Caledonian age. The augen gneisses the Mylonite Zone south of the Odal granite is 1674 in the Kvitvola Nappe Complex can be correlated with ±10 Ma (Alm et al. 2002). These ages are all similar the augen gneisses in the Tännäs augen gneiss nappe in and can be referred to the TIB 3 phase of Larson & Ber- Sweden (Röshoff 1978; Claesson 1980). Röshoff (1978) glund (1992). 40 J. Lamminen et al NORWEGIAN JOURNAL OF GEOLOGY

Sample JL-09-3 was collected from the Mylonite Zone The Randsfjord-Lygnern and Kongsberg-Marstrand at a lay-by along the E6 east of Lake Mjøsa at Morsko- blocks west of the Mylonite Zone (Fig. 1) are built up by gen. It consists of an augen gneiss. Sample JL-06-39 was various Mesoproterozoic (1.75-1.50 Ga) meta-igneous collected from a granitic gneiss near Elverum at Oppset- and metasedimentary rocks, formed in a long-lived and grenda along the road to Rena. Sample JL-06-45 was col- possibly complex subduction zone environment along lected on the eastern shore of Lake Storsjøen near the the western margin of the Fennoscandian shield (Gor- Caledonian front. batschev 1980; Åhäll & Gower 1997; Åhäll & Connelly 2008; Andersen et al. 2004). This “Gothian” orogeny Allochthonous basement samples overlaps in time with the TIB 2/3 phases of the Transs- candinavian Igneous Belt (Åhäll & Larson 2000). The The samples in the Osen-Røa Nappe Complex start with rocks immediately south of the Mylonite Zone consist sample JL-08-14, a granite from the Atnsjøen window of migmatitic biotite gneisses of probable supracrustal (Fig. 2). Further to the northeast a coarse-grained gran- origin called the Romerike Complex (Berthelsen et al. ite (sample JL-07-11) was sampled from the Tufsing- 1996). They are a continuation of the Göteborg-Åmål dalen window where the depositional contact with the belt in Sweden (Åhäll & Connelly 2008). Further to Moelv Formation (Moelv tillite) is exposed. A monzo- the west, the rest of southwest Norway is characterised nite was sampled (sample JL-06-59) on the eastern side by gneisses not older than Gothian in age and various of Lake Femunden, at Litlesjøberget. This rock has depo- supracrustal suites. The ages are mainly Mesoproterozoic sitional contact with the Rendalen Formation and a pos- - early Neoproterozoic c.1555-914 Ma, while the young- sible weathering crust, and is localised within one of the est rock is the Egersund dyke complex of c.616 Ma (Bin- large basement thrust sheets in this area. Sample JL-09- gen et al. 1998). A more detailed discussion of this area is 6, a diorite, was collected from the river Mistra, north of outside the scope of this article. Lake Storsjøen. The diorite occurs within a thrust sheet dominated by granite at the sole of the Osen-Røa Nappe During the Sveconorwegian Orogeny, southwest Norway Complex (Holmsen & Oftedahl 1956; Sæther 1979; was intruded by several generations of mafic to granitic Siedlecka et al. 1987). intrusive rocks, which form a belt of intrusions extend- ing from southernmost Norway to the Western Gneiss Samples from the Middle Allochthon comprise one Region (WGR). Recent geochronological and geochem- alkali-feldspar granite sample from the Valdres Nappe ical studies of these intrusions include those of Ander- Complex from Ormtjernkampen (sample JL-08-36) sen et al. (2001, 2002a, 2009a) and Vander Auwera et al. and several samples from the Kvitvola Nappe Com- (2003). One of the largest plutons is the Flå granite. It is plex. From the Kvitvola Nappe Complex, sample JL-08- a large megacrystic granite body intruding the Gothian 10 was collected from an augen gneiss at Rosten, west gneisses in the Begna sector (Bingen et al. 2005), which of Høvringen near the mountain Storkuven (Fig. 2). is a continuation of the Gothian domain on the north- The gneisses at Høvringen were called “the Høvrin- west side of the Oslo Rift. It has previously been dated by gen Gneiss” and dated to 1180 ±1 Ma by Handke et al. Nordgulen (1999), but was included in this study for Hf (1995). The gneisses in the Høvringen area, including analysis. 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 Material studied southwest (Strand 1951; Englund 1973; Siedlecka et al. 1987). This thrust sheet, informally termed the “swan The sampling sites are marked in Fig. 2 and a sample sheet” due to its outcrop pattern, occurs in the lower summary with co-ordinates is presented in Table 1. part of the Kvitvola Nappe Complex and forms the base- ment of the Rosten conglome­rate and other sedimen- Autochthonous basement samples tary successions of the Engerdalen Group (Siedlecka et al. 1987). In three papers, Sturt et al. (1997), Nilsson et Along a transect from south to north the sampling sites al. (1997) and Sturt & Ramsay (1997), the augen gneiss span from the Gothian domain near the Mylonite Zone at Høvringen­ was erroneously correlated with gneisses of to north of Lake Storsjøen where the westernmost TIB the Rudihø complex; the latter gneiss complex is located granites are exposed in front of the Caledonian nappes. in the Trondheim Nappe Complex of the Upper Alloch- On the northwest side of the Oslo Rift, the Flå granite thon, overlying the Kvitvola Nappe Complex and the was sampled at Høgfjellet (sample JL-07-1). A granitic gneisses at Høvringen (Siedlecka et al. 1987). In the pres- gneiss was sampled east of Lake Sperillen (Randsfjord ent paper we include all gneissic basement rocks beneath Complex, Nordgulen 1999), at Lake Vestre Bjonevatnet conglomerate­ and sandstones within the Kvitvola Nappe (JL-07-2). On the southeast side of the Oslo rift, sample Complex in the Rosten-Høvringen area (Siedlecka et al. JL-09-2 was collected along the E6 road north of Gard- 1987) in the Høvringen gneiss complex. Since the previ- ermoen airport near Eidsvoll. This rock is a migmatitic ous dating results from the Høvringen Gneiss were never diorite, from which only the palaeosome was analysed. properly documented and the exact sampling locality is NORWEGIAN JOURNAL OF GEOLOGY Zircon U-Pb ages and Lu-Hf isotopes from basement rocks 41

Table 1. Summary of samples analysed in this study. Weighted Average Tectonostratigraphic UTM coordinates Concordia 207Pb/206Pb age Sample Name Locality position* (WGS 84, zone 32V) age (95% conf.) Notes JL-07-1 granite Høgfjellet, Autochthonous base- 556972 6690512 932 ± 8 Ma 932 ± 11 Ma Ringerike­ ment JL-07-2 granitic gneiss Vestre Bjonevatnet Autochthonous base- 561929 6711996 1592 ± 10 Ma 1592 ± 9 Ma ment JL-09-2 migmatitic Eidsvoll Autochthonous base- 619193 6683565 1276 ± 8 Ma 1276 ± 14 Ma zircon over- diorite ment growths at 1013 ± 40 Ma JL-09-3 augen gneiss Morskogen Autochthonous base- 623197 6713647 1670 ± 5 Ma 1671 ± 5 Ma ment JL-06-39 granite Oppsetgrenda Autochthonous base- 631228 6764603 1676 ± 6 Ma 1678 ± 6 Ma ment JL-06-45 granitic gneiss Eastern Lake Autochthonous base- 620177 6821825 1676 ± 5 Ma 1675 ± 6 Ma Storsjøen­ ment JL-06-59 monzonite Litlesjøberget Autochthonous base- 656391 6887251 1680 ± 7 Ma 1682 ± 7 Ma ment JL-08-14 granite Atnsjøen window ORNC/Lower 569393 6858277 1659 ± 5 Ma 1659 ± 5 Ma at Atndalen Allochthon JL-07-11 granite Tufsingda- ORNC/Lower 640642 6908939 1655 ± 5 Ma 1656 ± 5 Ma len window­ at Allochthon Høgåsen­ JL-09-6 diorite River Mistra near ORNC/Lower 618520 6844030,01 1675 ± 10 Ma 1678 ± 10 Ma Åkrestrømmen Allochthon JL-07-29 granitic gneissRostein, along road KNC/Middle 520610 6862653 1620 ± 8 Ma 1621 ± 8 Ma to Horgen Allochthon JL-08-10 augen gneiss Road to KNC/Middle 520181 6865353 1183 ± 7 Ma 1185 ± 8 Ma Nordsætrin­ Allochthon JL-08-36 alkali-feldspar Ormtjernkampen VNC/Middle 542788 6784440 1476 ± 9 Ma includes granite Allochthon coronitic garnets­ JL-09-8 augen gneiss Koppang KNC/Middle 605215 6830630 1632 ± 8 Ma 1631 ± 8 Ma Allochthon JL-09-4 augen gneiss road to Otnes KNC/Middle 616210 6846638 1193 ± 15 Ma 1198 ± 21 Ma Allochthon JL-09-5 mylonitic Åkrestrømmen, KNC/Middle 618858 6831742 1627 ± 9 Ma 1628 ± 9 Ma augen gneiss eastern Lake Stor- Allochthon sjøen JL-09-7 anorthosite west of Otnes KNC/Middle 613931 6848841 1187 ± 18 Ma 1191 ± 23 Ma Allochthon

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

also unknown, we decided to resample the augen gneiss exceptionally large augen. North of Lake Storsjøen, on at Høvringen for redating and for Lu-Hf isotopes (sample the eastern side of Lake Lomnessjøen, sample JL-09-4 JL-08-10). Another basement sample, JL-07-29, was col- was collected from an augen gneiss of considerably dif- lected from a road section a few kilometres south from ferent nature than sample JL-09-5 and with smaller the sampling site of sample JL-08-10. This sample is a augen. Another sample was taken from this basement highly sheared granitic-mylonitic gneiss, unlike sample unit, sample JL-09-7, which is an anorthosite lens within JL-08-10. the augen gneiss (cf. Holmsen 1953). Sample JL-09-8 was collected northwest of the town of Koppang, on the west- Sample JL-09-5 on the eastern side of Lake Storsjøen, ern side of the river Glomma, and is also an augen gneiss at Valsjøberget, consists of an augen gneiss with with large augen, resembling sample JL-09-5. 42 J. Lamminen et al NORWEGIAN JOURNAL OF GEOLOGY

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. NORWEGIAN JOURNAL OF GEOLOGY Zircon U-Pb ages and Lu-Hf isotopes from basement rocks 43

Western Gneiss Region calculated from the signal at mass 238 using a natural Two granite samples were included from the WGR for 238U/235U=137.88. Mass number 204 was used as a moni­ Lu-Hf analysis and for comparison purposes. Sample C-99- tor for common 204Pb. Analyses which yielded peak/ 55 comes from the Hustad Igneous Complex, and has background ratios at mass 204 of less than 1 + 3RSDB already been dated by Austrheim et al. (2003) to 1653 ± 2 (where RSDB is the observed relative standard deviation Ma. The Hustad Igneous Complex is a concentrically zoned of the on-peak background measurement), were consid- high-K calc-alkaline complex with a quartz-monzonitic ered to have common Pb below the detection limit. and granitic core surrounded by monzodiorites and pyrox- enites (Austrheim et al. 2003). The complex is described in The data were regressed using mainly two (occasion- Austrheim et al. (2003) and the sample used in this study ally up to four) calibration standards. Standard zircons is also described there. The Hustad Igneous Complex is a used for calibration were: 91500 (1065 ± 1 Ma; Wieden- low-strain enclave in an otherwise highly deformed part beck et al. 1995), GJ-01 (609 ± 1 Ma; Jackson et al. 2004), of the WGR, where Caledonian deformation was intense. Plešovice (337 ± 0.4 Ma; Sláma et al. 2008), and an in- The second sample from the WGR comes from the Jølster house Palaeoproterozoic standard A382, which is a zir- granite, which is the largest post-orogenic Sveconorwegian con separated from a hypersthene granite from Voin- intrusion in the WGR and has been dated to 966 ± 2 Ma by salmi, Finland, that has an average age of 1876 ± 2 Ma Skår & Pedersen (2003). The importance of this sample is by isotope dilution (ID)-TIMS U-Pb (H. Huhma, pers. its location in the WGR. Previous Lu-Hf work on post-oro- comm.). In our laboratory the GJ standard has shown genic granitoids comes from South Norway (e.g. Andersen to be c.601 Ma (concordia age) and the 91500 c.1059 Ma et al. 2009a). Some post-orogenic granitoids in South Nor- (concordia age). These values were used in calculations. way have been shown to carry a strongly crustal contam- The calculations were done off-line, using an in-house inated signature that could reflect crustal composition in interactive Microsoft Excel spreadsheet program allow- the deeper crustal levels; Hf isotopes from the Jølster gran- ing interactive choice of isotopically homogeneous inte- ite may reveal similar information about the WGR. gration intervals. The estimated uncertainties in isotope ratios incorporate error terms from counting statistics on signals and backgrounds for the relevant masses mea- Analytical methods sured on standards and unknowns, the standard error of the regression line determined from standards, and the The main method in this study focuses on zircon U-Pb published uncertainty of the calibration standards. The and Lu-Hf isotopes. The Lu-Hf isotope system adds a terms have been propagated through, using standard valuable possibility to obtain petrogenetic information error propagation algorithms (e.g. Taylor 1997). from the host rock in which the zircon crystallised (cf. Howard et al. 2009) and can be used like the more com- IsoplotEx 3.34 (Ludwig 2003) was used to plot concordia monly used Sm-Nd system. diagrams and for calculating the concordia ages (Ludwig 1998) and weighted averages. Zircon standard 91500 was Zircon was separated from samples by standard meth- run as unknown for checking the accuracy of the method ods including crushing, milling, heavy liquid and Frantz and outlier calibration standard analyses were rejected magnetic separation. Grains were mounted in epoxy and based on the outcome of the standard as unknown. Calibra- exposed by polishing. Before analysis, all grains were tion standards chosen for each day and session and the out- imaged by cathodoluminescence (CL) in a JEOL JSM come of standard run as unknown are listed in Appendix 1. 6460LV scanning electron microscope to detect internal Most of the time accuracy is not off by more than 5 Myrs. structures such as magmatic zoning, inherited cores and metamorphically disturbed and metamict zones. Lu-Hf

U-Pb For Lu-Hf isotope analysis of zircon, masses 172 to 179 were measured. The isobaric interferences on 176Hf by 176Lu U-Pb dating and isotope analysis were performed using and 176Yb were corrected by an empirical procedure using a Nu Plasma HR multicollector ICPMS and a New Wave/ reference zircons with homogeneous isotopic composition Merchantek LUV-213 laser microprobe at the Depart- and variable Yb/Hf ratio (Andersen et al. 2009). A value for ment of Geosciences, University of Oslo. U-Pb analyses the decay constant of 176Lu of 1.867 x 10-11 year-1 has been were made according to analytical protocols described by used in all calculations (Söderlund et al. 2004; Scherer et Rosa et al. (2009). A single U-Pb measurement included al. 2001, 2007). We have adopted the CHUR curve of Bou- 30 s of on-mass background measurement, followed by vier et al. (2008) and the depleted mantle model of Griffin 60 s of ablation with a stationary beam. Laser condi- et al. (2000), modified to the decay constant and chondritic tions for U-Pb analysis were: beam diameter: 40 μm; parameters used. Reference zircon Mud Tank (Black & pulse frequency: 10 Hz; fluence: ca. 0.06 J/cm2. Masses Gulson 1978; Griffin et al. 2004; Woodhead & Hergt 2005) 204, 206 and 207 were measured in secondary electron and Temora-2 (Black et al. 2004) were used as monitoring multipliers, and 238 in an extra high mass Faraday col- standards. Laser settings used were: c.50 % power (fluence: lector of the Nu Plasma U-Pb collector block. 235U was 1-2 J/cm2), 55 μm spotsize and 5 Hz repetition rate. 44 J. Lamminen et al NORWEGIAN JOURNAL OF GEOLOGY

Results U-Pb The U-Pb data are given in Appendix 2 and summarised Zircon in all samples shows mostly oscillatory zoning in Table 1. Concordia diagrams are presented in Figs. 3-7 in CL images, which is commonly thought to represent and a summary map in Fig. 8. Most of the analyses show magmatic origin. Only sample JL-09-2 showed distinct good concordance in the U-Pb system. Some grains show CL-bright overgrowths. 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

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. NORWEGIAN JOURNAL OF GEOLOGY Zircon U-Pb ages and Lu-Hf isotopes from basement rocks 45

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.

in age calculations. Concordia diagrams show only those from Flå granite (Bingen et al. 2008a). Sample JL-07-2, analyses that are < 2% discordant and without common granitic gneiss from Lake Vestre Bjonevatnet, has a wide Pb (except in Fig. 4a). Weighted average 207Pb/206Pb age spread of discordant analyses. Although few analyses was calculated from < 2% discordant grains and some allow a concordia age to be calculated to 1592 ± 10 Ma, outliers were rejected based on a statistical test in Isoplot this rock is most likely of sedimentary origin and con- (Ludwig 2003). The only sample showing inheritance is tains an indication of zircon as old as c.1800 Ma. Most of sample JL-07-2 (Fig. 4a). the zircon in this sample has suffered lead-loss (Fig. 4a).

Samples from the autochthonous basement on the east- Samples from the Lower Allochthon are similar to the ern side of the Mylonite Zone all have similar 1670−1676 Autochthonous basement east of the Mylonite Zone. Ma ages. On the western side of the Mylonite Zone, Diorite from Mistra is dated at 1675 ± 10 Ma and thus migma­tite sample JL-09-2 from Eidsvoll is 1276 ± 8 Ma identical in age to the autochthonous basement at Lake and has zircon overgrowth at 1013 ± 39 Ma. The Flå Storsjøen. Monzonite from Litlesjøberget at Lake Femun- granite­ from the western side of the Oslo Rift is 932 ± den is 1680 ± 6 Ma. Samples from the Atnsjøen and 8 Ma and corresponds well to previous dates obtained Tufsingdalen windows are younger than other samples ­ 46 J. Lamminen et al NORWEGIAN JOURNAL OF GEOLOGY

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 Figure 7. Sample JL-08-36 has considerable spread in concord­ 0.28188 and is thus also very similar to TIB rocks and ant ages, which does not allow calculation of its concordia age. samples from the Lower Allochthon. Sample JL-08-10, Weighted average is the best estimate for this sample. 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- Figure 8. Map of the study area showing the distribution of ues is rather narrow compared to other samples in this ages obtained in this study. Legend same as in Fig. 2. The study. Sample JL-06-45varies between 0.28165 and autochthonous basement on the eastern side of the Mylo- 0.28186 showing more crustal contamination than sam- nite Zone has rather uniform ages of c.1676 Ma. Ages in the ple JL-06-39. These values are typical moderately juve- Lower Allochthon are slightly younger in the Atnsjøen and nile TIB values (Andersen et al. 2009b). The Flå granite, Tufsingdalen windows. Ages in the Middle Allochthon are sample JL-07-1, on the southwestern side of the Mylonite significantly younger and show an association of ages c.1620 Zone, shows a wide range in Hf isotopes ranging from Ma and 1190 Ma in two places (Høvringen and Lake Stor- 0.28193 to 0.28234. Migmatitic diorite, sample JL-09-2, sjøen). NORWEGIAN JOURNAL OF GEOLOGY Zircon U-Pb ages and Lu-Hf isotopes from basement rocks 47

JL-09-7, anorthosite, from the Kvitvola Nappe Complex The plot in Fig. 9 contains three additional samples for on the eastern side of Lake Storsjøen is clearly juvenile comparison. Sample C-99-55 from the Hustad Igneous with 176Hf/177Hf from 0.28214 to 0.28230, while sample Complex in the WGR shows very similar Hf composi- JL-09-5, an augen gneiss, shows a TIB-like composition tion to the TIB, ranging from 0.28176 to 0.28189. The ranging from 0.28173 to 0.28184. Similar augen gneiss, Jølster granite from the WGR ranges from 0.28205 to sample JL-09-8, northwest of the town of Koppang shows 0.28218 and is similar to the Flå granite, but shows less similar 176Hf/177Hf values from 0.28175 to 0.28181. Sample spread. The sample from the Karlshamn pluton in South JL-08-36 obtained from the Valdres Basin is moderately Sweden is included in the plot for illustrating how Hf juvenile with values ranging from 0.28188 to 0.28200. isotopes would evolve in anatexis. These data are as yet

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. 48 J. Lamminen et al NORWEGIAN JOURNAL OF GEOLOGY unpublished. The pluton is situated next to c.1.77 Ga TIB rocks are very much the same as in the autochthonous rocks in Blekinge and has retained the Hf composition of basement below: Moelv Formation (tillite), Vangsås For- its country rocks. 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 Discussion original depositional basement of the several thousand metre thick Rendalen Formation and the younger forma- The ages obtained from the autochthonous basement tions of the basin-derived Hedmark Group. These base- east of the Mylonite Zone belong to the 1.68-1.66 Ga ment thrust sheets represent the original basement of TIB 3 phase of the Transscandinavian Igneous Belt as the Hedmark Basin and have been transported together defined by Larson & Berglund (1992). The ages are uni- with the sedimentary succession of the Osen-Røa Nappe form along the transect from the Mylonite Zone to Lake Complex. Storsjøen and are also within error to the 1673 ± 8 Ma Trysil “tricolor” granite dated by Heim et al. (1996). On Although all of the basement slices in the Lower Alloch- the northwestern side of the Oslo Rift, sample JL-07-2 thon in the northeasterly Hedmark Group must have in the Begna Sector of Bingen et al. (2005) belongs to been moved some distance, the ages are very similar to the c.1590 Ma Stora Le Marstrand stage of the Gothian what is currently the autochthonous basement east and Orogeny (Åhäll & Connelly 2008), while the c.1270 Ma south of the outcrop area of the Osen-Røa Nappe Com- diorite in the Romerike Complex falls into the interoro- plex. The basement rock within the Mistra thrust sheet genic period (see below). The CL-bright overgrowths at is in fact identical in age to the autochthon south of it at 1013 ± 39 Ma probably reflect Sveconorwegian defor- the eastern side of Lake Storsjøen. A monzonite base- mation of the Romerike Complex. A similar age has ment slice east of Lake Femunden (sample JL-06-59) is been reported from South Sweden, from the Åmål Suite also within the TIB 3 period. Granitic rocks in the win- (Scherstén et al. 2004). dow structures at Tufsingdalen and Atnsjøen are notice- ably younger. Granitic gneisses in the WGR have similar ages to the basement windows in the Lower Allochthon. Ages in According to current Caledonian thrust models, rocks of this area range from c.1680 to 390 Ma (Tucker et al. 1990; the Middle Allochthon originated even further away in Bingen & Solli 2009). There is almost a continuous range the present west than the Lower Allochthon. However, it of ages in the interval 1680-1630 Ma. An example is the is not demonstrated that this rule applies to all units clas- Hustad Igneous Complex in the northern WGR (Aus- sified as Middle Allochthon. For example, the Valdres trheim et al. 2003), which includes c.1650 Ma gneisses Nappe Complex could be related to the Lower Alloch- and 1250 Ma mafic rocks. In the WGR, the ages seem to thon due to similarity in deformation style. On the other get younger to the southwest (Gorbatschev 1980; Skår & hand, the ages obtained in this study demonstrate that Pedersen 2003). Sveconorwegian rocks are also present the Kvitvola Nappe Complex is indeed far travelled. The in the WGR, mainly as intrusions. The WGR has been c.1620 Ma age from the basement slice near Høvringen correlated with the autochthonous basement in south- is coeval with the Gothian Orogeny, whereas the c.1180 ern Fennoscandia (Gorbatschev 1980), though the actual Ma Høvringen Gneiss can be considered early Sveconor- structural position of the WGR could also be allochtho- wegian. These ages are within error to the augen gneisses nous (e.g. Rice 2005) . in the Lake Storsjøen area. Also the rock units seem to be spatially associated both in the Høvringen area and sev- From the data presented in this study, it seems that rocks eral hundred kilometres to the east in the Lake Storsjøen in allochthonous basement slices in the Middle Alloch- area. A similar association was also noted by Handke thon are noticeably younger than those in the Lower et al. (1995) from the Risberget Nappe further to the Allochthon. Whereas basement slices are no older than north from the Sparagmite Region. They were probably c.1650 Ma in the Osen-Røa Nappe Complex, those in also associated in their pre-thrust positions. In the Lake the Middle Allochthon can be as young as c.1180 Ma. Storsjøen area, there is a sharp age-contrast with nearby The nappes in the Lower Allochthon have a minimum autochthonous basement rocks (Fig. 8), which reflects displacement of at least 150 km (Oftedahl 1943; Mor- the long transport of the Middle Allochthon. ley 1986). However, it is not certain that all the base- ment slices have been displaced the same distance as the Basement slices in the Jotun Nappe have been stud- basin-derived Hedmark Group of the Osen-Røa Nappe ied before by several workers (e.g. Schärer 1980; Emmet Complex. Out-of-sequence thrusting, as has been dem- 1996; Lundmark et al. 2008) who concluded, mainly on onstrated in the Lower Allochthon (Nystuen 1983), may U-Pb grounds, that there was a clear Fennoscandian explain for example the structural position of the Mis- Shield signature for these rocks. Our results support tra thrust sheet (sample JL-09-6) and similar small thrust these findings. An age of c.1180 Ma was not reported sheets east of Lake Storsjøen in the lower part of the by these workers, but c.1630 Ma ages seem to be abun- Osen-Røa Nappe Complex. In these thrust sheets, the dant in the Jotun Nappe Complex. Corfu & Heim (2009) sedimentary succession above the crystalline basement reported Gothian, c.1520 Ma ages from the Espedalen NORWEGIAN JOURNAL OF GEOLOGY Zircon U-Pb ages and Lu-Hf isotopes from basement rocks 49

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.

valley in the Valdres Nappe Complex (Fig. 2). They also for the younger TIB rocks and the Karlshamn pluton. dated the same rock unit in Ormtjernkampen (sample JL-08-36 of this study), but obtained a slightly older age Most of the samples from the autochthonous basement of 1506 ±9 Ma. and Lower Allochthon follow the TIB trend. The c.1620 Ma samples from the Middle Allochthon also follow Hf isotopes this trend, but the c.1180 Ma and 1476 Ma samples are more juvenile and plot above the trend line (red line in Due to the higher amount of parent Lu in the depleted Fig. 9). The most juvenile samples are the anorthosite in mantle compared to the crust, juvenile magmas from a the Kvitvola Nappe Complex west of Lake Lomnes­sjøen depleted mantle source carry more radiogenic hafnium and the diorite from the Romerike Complex.­ The sample ­ than crust at any given time. This allows the Hf isotope from the Valdres Nappe Complex from Ormtjern­kampen composition of zircon in a rock to be used to deduce the shows a clear juvenile signature in Hf isotopes (Fig. 9). nature of its source, or the proportion of mantle- and Bingen et al. (2008b) called the 1.52-1.48 Ga period in crustal-derived components. Input of mantle-derived the Hardangervidda-Rogaland and Telemark blocks as magmas to the deep crust will also transfer heat to the the “Telemarkian accretion”. Previous studies have shown crust, and potentially cause anatectic melting. Gran- a rather juvenile character for the Mesoproterozoic mag- ites that are anatectic without new mantle input pre- matic rocks in this part of Norway (Andersen et al. 2002a serve their hafnium compositions and the radiogenic Hf & 2002b). The Tinn granite in the Telemark Block has a evolves only very slowly due to the low amount of parent matching age of 1476 ± 13 Ma (Andersen­ et al. 2002b) Lu in the rock. and also a juvenile character, which is comparable to sample JL-08-36 from Ormtjernkampen. Although the Previous hafnium work done on TIB rocks shows a Flå granite is hosted in the juvenile Gothian domain, rather distinct pattern in 176Hf/177Hf versus age plots (Fig. it shows a wide range in initial Hf composition that 9) (Andersen et al. 2009b). The age-corrected 176Hf/177Hf suggests­ the presence of older crust at depth. The same from the TIB is rather restricted, c.0.2816-0.2818, and has also been observed by Andersen et al. (2001), who shows a trend (red line in Fig. 9) without significant designated this granite as “Group 1” based on variations input of mantle-derived material during the three TIB in Sr, Nd and Pb isotopes. This observation has impli- events. In Fig. 9 the slope of the TIB evolution line corre- cations regarding the nature of the Mylonite Zone and sponds to a 176Lu/177Hf whole-rock value of 0.010, which whether the TIB continues underneath it. is obtained by linear regression from TIB zircon analy- ses and has an average TDM of c.2.1 Ga. Andersen et al. The Hustad Igneous Complex sample from the WGR (2009b) used a 176Lu/177Hf value of 0.015, which gives a shows a remarkable match with the TIB trend (Figs. 9a slightly higher angle on the evolution line, but a value of & b). This is strong evidence that the TIB continues into 0.010, corresponding to an average granitic composition the WGR and also that the magmatism probably lasted is used in this study, since it seems to give a better match until 1620 Ma, which is longer than previously thought 50 J. Lamminen et al NORWEGIAN JOURNAL OF GEOLOGY

(Högdahl et al. 2004). This adds to the already enigmatic Caledonian nappes, similar rocks with the same age and nature of the Gothian magmatism. It also ties the base- petrogenetic history may have occurred further to the ment windows in the Lower Allochthon and the c.1620 northwest of Telemark. Ma augen gneisses in the Middle Allochthon to the WGR or its continuation further to the northwest. The Hf com- The western margin of Baltica position of the Jølster granite is similar to that of the Flå granite, except in having a narrower compositional A database of published isotopic ages from Norway com- range. It does not show extremely crustal contaminated piled by Bingen & Solli (2009) shows a drastic difference values as some post-orogenic granites do from south- of ages from rocks of the Middle Allochthon in southern west Norway as reported by Andersen et al. (2009a). Norway compared to the Middle Allochthon in north- More analyses are needed from the WGR to see if there is ern Norway. Data from northern Norway come from the actually a difference in the crustal structure between the Akkajaure Nappe and are generally 1730-1880 Ma (Bin- WGR and SW Norway. gen & Solli 2009; Kirkland et al. 2011) whereas the ages of basement rocks in the Middle Allochthon in southern The interorogenic period in south Scandinavia Norway are mainly younger than 1650 Ma. A wide range of ages is present in southern Norway and some of them Although the youngest Gothian rock in Scandinavia coincide with the results of this study. is the Hisingen suite in South Sweden (1522 ± 10 Ma; Åhäll 1990), there is no clear evidence for the end of the Hafnium isotopes from the basement rocks indicate that Gothian Orogeny. Only the subsequent magmatism in the TIB magmatism may have lasted as long as until 1620 South Norway changed its nature from intracratonic to Ma, which would then largely overlap with the Gothian episodic (Åhäll & Connelly 2008). On both sides of the event. Roberts et al. (1999) reached similar conclusions Mylonite Zone the crust was penetrated by c.1.46 Ga from the 1633.2 ± 2.9 Ma Blåfjellhatten granite in the mafic dykes (Åhäll & Connelly 1998; Rämö et al. 2001). Grong-Olden Culmination in Central Norway. The age A typical intracratonic volcanic-sedimentary suite is of augen formation in the augen gneisses is still unclear, represented­ by the Telemark Supracrustals in southwest but it is most likely Sveconorwegian. For example the Norway, which started to accumulate at c.1.5 Ga. The c.1180 Ma Høvringen Gneiss has a depositional contact Hallandian event or Danopolonian Orogeny (Bogdanova­ above to just slightly deformed sedimentary rocks. If this et al. 2001) was active at c.1.46 - 1.40 Ga, and left an deformation was Caledonian, it would be unlikely that imprint in the south of Scandinavia. The nature of this the sedimentary strata escaped strong deformation. event still remains controversial. Hafnium isotopes obtained from the Mesoproterozoic The c.1.34-1.14 Ga period in Fennoscandia is commonly rocks in Telemark show a rather juvenile characteristic regarded as pre-Sveconorwegian (Bingen et al. 2008a) (Andersen et al. 2002a; Andersen et al. 2007; Pedersen or interorogenic between the Gothian and Sveconor- et al. 2009), except the Late Sveconorwegian post-oro- wegian orogenies. There is little crustal building in this genic granites which show Palaeoproterozoic inheritance period, but mostly local mafic magmatism. The cen- and have a wide range of Hf isotope composition from tral Scandinavian Dolerite Group (Söderlund et al. juvenile to strongly crustal contaminated. The juvenile 2006) formed in three pulses in the 1.27-1.25 Ga inter- Høvringen Gneiss and the granite from Ormtjernkam- val. The Iveland-Gautestad metagabbro was emplaced pen could originate from the same juvenile domain, in southern Telemark­ at 1280-1260 Ma (Pedersen & that most likely hosts similar basement rock types as in Konnerup-­Madsen 2000; Pedersen et al. 2009). The the Hardangervidda-Rogaland and Telemark blocks of 1.20-1.18 Ga Tromøy gabbro-tonalite complex (Knud- southwest Norway. Andersen et al. (2001) attributed the sen & Andersen 1999) in southernmost Norway is a westwards increasing juvenile character to an increasing possible­ island arc with a tectonic contact to the main- involvement of a mafic underplate in magma genesis. land. The younger Telemark supracrustal rocks in Tele- mark consist of bimodal volcanic rocks,1170-1140 Ma The results presented in this study allow the construction in age (Bingen­ et al., 2003). The 1184 +7/-5 Ma Flåvatn of a simple model for the crustal architecture of the west- granite gneiss (Dahlgren et al. 1990) and the 1187 ±2 ern margin of Baltica prior to the Caledonian Orogeny Ma Gjerstad augen gneiss (Heaman & Smalley 1994) are (Fig. 11). The presumed original locations of the base- coeval with the augen gneisses at Høvringen and Kop- ment slices are marked in Fig. 11. The TIB probably con- pang of this study. Figure 10 shows the distribution of tinued to the west until at least 1620 Ma, creating a zoned magmatic rocks at key age intervals in South Norway. It cratonic margin. A sharp boundary connected the TIB is notable that the WGR does not contain major granitic domain to a more juvenile crustal block in the south- magmatism after c.1620 Ma and before the Sveconorwe- west, which could be the continuation of the present-day gian post-orogenic granitoids were intruded, which sup- southwest Norway. The Mylonite Zone in South Nor- ports the derivation of c.1200 Ma granitic gneisses from way and Sweden is a major Sveconorwegian shear zone elsewhere than the present WGR. Although southern (Johansson & Johansson 1993; Andersson et al. 2002). It Telemark is not located along the transport path of the has been traced to the Caledonian nappe front, but may NORWEGIAN JOURNAL OF GEOLOGY Zircon U-Pb ages and Lu-Hf isotopes from basement rocks 51

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.

have continued all the way to the continental margin Allochthon), Kvitvola Nappe Complex and Valdres prior to Caledonian tectonism. It is possible that south- Nappe Complex (Middle Allochthon) shows typical Fen- west Norway is juxtaposed against the TIB belt by the noscandian Shield ages. Ages get seemingly younger Mylonite Zone. Several models have been presented in from the autochthonous basement to the Middle Alloch- the past including strike-slip movements (Torske 1985) thon. Ages in the Osen-Røa Nappe Complex are close to and collision of an exotic micro-continent (Bingen et al. the autochthonous TIB basement and may suggest a rela- 2005). One of the problematic questions is the continua- tively short transport distance. Augen gneisses belonging tion of the Gothian belt on the western side of the Cale- to the Kvitvola Nappe Complex are significantly younger donides. The Gothian belt typically has highly deformed than the surrounding autochthonous basement, which is gneisses and migmatites, but these rock types are con- in agreement with the long transport distance suggested spicuously scarce in the Caledonian nappes. The Hustad for the Middle Allochthon. The younging trend reflects Igneous Complex in the WGR was assigned as Gothian a westwards younging crustal structure, which is typical by Austrheim et al. (2003), but new Hf data reported in for the Fennoscandian Shield on a larger scale and has this study suggest a TIB origin for these granites. It seems also been observed in the WGR. This trend is truncated that the Gothian Orogeny still remains as a unique fea- by the appearance of c.1180 Ma and 1476 Ma basement ture of south Scandinavian geology. slices in the Middle Allochthon that has counterparts in the Hardangervidda-Rogaland-Telemark part of South Norway. Conclusions Hafnium isotopes show a genetic link between the TIB U-Pb age determination of allochthonous base- in Sweden and the autochthonous TIB 3 aged basement ment rocks in the Osen-Røa Nappe Complex (Lower immediately neighbouring the Hedmark Group. Samples 52 J. Lamminen et al NORWEGIAN JOURNAL OF GEOLOGY from the western side of the Mylonite Zone are clearly 158, 253– 267. more juvenile than the TIB. All the allochthonous base- Andersen, T., Griffin, W.L. & Pearson, N.J. 2002a: Crustal evolution in the SW part of the Baltic Shield: the Hf isotope evidence. Journal of ment rocks in the Osen-Røa Nappe Complex follow Petrology 43, 1725–1747. the TIB trend and the same TIB origin can be linked to Andersen, T., Griffin, W.L. & Sylvester, A.G. 2007: Sveconorwegian some of the augen gneisses in the Kvitvola Nappe Com- crustal underplating in southwestern Fennoscandia: LAM-ICPMS plex, which suggests that the TIB magmatism continued U–Pb and Lu–Hf isotope evidence from granites and gneisses in to c.1620 Ma. However, the c.1180 Ma basement in the Telemark, southern Norway. Lithos 93, 273-287. Kvitvola Nappe Complex and the c.1476 Ma basement in Andersen, T., Graham, S. & Sylvester, A.G. 2009a: The geochemistry, the Valdres Nappe Complex have more juvenile compo- Lu–Hf isotope systematics, and petrogenesis of late Mesoprotero- zoic a-type granites in southwestern Fennoscandia. The Canadian sitions, similar to rocks in Telemark, South Norway. Two Mineralogist 47, 1399-1422. spatially associated gneisses in the Kvitvola Nappe Com- Andersen, T., Sylvester, A.G. & Andresen, A. 2002b: Age and petro- plex, one having a crustal contaminated (TIB-like) Hf genesis of the Tinn granite,Telemark, South Norway, and its signature and the other having a distinctly more juvenile geochemical­ relationship to metarhyolite of the Rjukan Group. composition, can be explained by derivation from two NGU Bulletin 440, 19-26. different crustal domains that were probably tectonically Andersen, T., Andersson, U.B., Graham, S., Åberg, G. & Simonsen, juxtaposed before the Caledonian Orogeny. These could S.L. 2009b: Granitic magmatism by melting of juvenile continent­al crust: new constraints on the source of Palaeoproterozoic be the old Fennoscandian Shield and more juvenile rocks granitoids­ in Fennoscandia from Hf isotopes in zircon. Journal of of the Hardangervidda-Rogaland and Telemark blocks. the Geological ­Society, London 166, 233-247. The data support the idea that before the Caledonian Andersen, T., Griffin, W.L., Jackson, S.E., Knudsen, T.-L. & Pearson, Orogeny, the TIB belt and possibly also southwest Nor- N.J. 2004: Mid-Proterozoic magmatic arc evolution at the south- way continued further to the northwest from their pres- west margin of the Baltic shield. Lithos 73, 289–318. ent-day locations. Andersson, J., Möller, C. & Johansson, L. 2002: Zircon geochronology of migmatite gneisses along the Mylonite Zone (S Sweden): a major Sveconorwegian terrane boundary in the Baltic Shield. Precam- brian Research 114, 121–147. Acknowledgements – Siri Simonsen is thanked for analytical help with Austrheim, H., Corfu, F., Bryhni, I. & Andersen, T.B. 2003: The the ICPMS. Bernard Bingen is thanked for providing digital maps and Proterozoic­ Hustad igneous complex: a low strain enclave with data. Fernando Corfu is thanked for the Hustad Igneous Complex sample. Øyvind Skår is thanked for the Jølster granite sample. Audrius a key to the history of the Western Gneiss Region of Norway. Čečys is thanked for the Karlshamn pluton sample. Discussions with Precambrian­ Research 120, 149-175. Mattias Lundmark helped to improve the manuscript. Adrian Read Berthelsen, A., Olerud, S., Sigmond, E.M.O., Sundvoll, B., 1996: Geo- is thanked for correcting the language. Reviews by Stefan Claesson, logisk kart over Norge, berggrunnskart (bedrock map); Oslo; Øystein Nordgulen, Trond Slagstad and Kåre Kullerud improved the 1:250,000. Norges Geologiske Undersøkelse, Trondheim. manuscript greatly. This study is part of the corresponding author’s Bingen, B. & Solli, A. 2009: Geochronology of magmatism in the Cale- doctoral thesis on Proterozoic basins in southern Norway. This is donian and Sveconorwegian belts of Baltica: synopsis for detrital Contribution No. 19 from the Isotope Geology Laboratory at the zircon provenance studies. 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