Geology of the Folldal area, southern Trondheirr Region Caledonides, Norway

TERJE BJERKGA RD & ARNE BJ0RLYKK E

Bjerkqard ,T. & Bjor lykke. A. 1994: Geology of the Folldal area, southern Trondheim Region Caledonides, Norway. Nor. geol. unders. Bull. 426, 53-75.

The lowest stratigraphic unit in the Folldal area is the Gula Group, com prising turbidites and minor greenstones in the lower part (the Sinqsas Formation) and graphitic pelites with subordinate marble and conglome rate in the upper part (the Asli Formation). The overly ing Funds]o Group is bimodal with basalts and rhyodacit es, com agma­ tic gabbros and a large suovotcanlc trondhjem itic intrusion . The bimodality, high content of volcanlclastics, interca­ lations of continental sediments, geochemistry and lead isotopes of massive sulphides reveal that the Fundsje Gro up represents an island arc acc umulation formed close to a continent. The contact between the Fund sje and Gula Gro ups is interpreted to be primary, as shown by a grad ual lithological transition and no break in metamor­ phism and structural elements across the contact. The Folldal Trondhjemite in the Fundsjo Group has yielded a U­ Pb zircon age of 488 ± 2 Ma. Relationships between published ages from the Gula Group and rocks in the Seve Nappes suggest a possible tectonic and metamorphic link between the Seve and the lower part of the Gula Group . Either a primary unconformity or a thrus t marks the contac t between the Asli and Sinqsas Formations in the Gula Gro up.

T. BjerkgaJd & A. Bjartykke I), Department of Geology, University of Oslo, P.o.Box 1047, Blindern, N-0316 Oslo, Norway. 1) Present address: Norges geol ogiske unoersoketse, Postboks 3006 - Lade, 7002 Trondheim, Norway.

Introduction in relation to the volcanic and sedimentary Rocks of the Gula Group , which consist rocks of the Meraker and Steren Nappes, is mainly of metasediments, occupy the cen­ important to any tectonic model of the Cale­ tral part of the Caledonides in the Trond­ donides and has therefore been debated heim Region (Fig.1). The stratigraphic and since at least the beginning of this century tectono-stratigraphic position of this group, (see Gee et al. 1985 for a review).

Tcctonostratigraphy of the Trondhcim Region West East

Steren Nappc >r--oc: V/?/J r--o GulaN appc ~ Gula Nappe ~ ~ ~ •Meraker Nappe ~

Essandsje-Oyfjell Nappe

Setra-Risbcrgct r·.....:··:·.·\·::··:l Remsklepp Nappe ~ ~ Nappe :.;'::.:.:-,::.:.:-,::.:.;.: R~ ------:I: t"!"l ~ D Rondane Nappe

D ~:~;Rea

PA RAUTOCHTHON· AUTOCHTHON • Major ore deposits

Fig.1. Tectonostratigraphic map of the southern part of the Trondheim Region (modified from Nilsen 1988). The Folldal Area (fig.2) is indicated. 54 Terje Bjerkgard & Arne Bjorlykke GU• BULL 426. 1994

There is now a commo n opinion that the re of the contact between the Fundsjo and rocks of the Storen Group in the Storen Gula Groups and also the internal tectonic Nappe (west of the Gula Group) had a diffe­ structure of the Gula Group are critical for rent origin to that of the rocks of the Fundsjo the interpretation of the palaeotecton ic set­ Group in the Merake r Nappe. They probably ting of the Gula and Fundsjo Groups and for represent , respectively, an ocean floor and the existence of a separate Gula Nappe an island arc / marginal basin setting (Gale (Fig.1). & Roberts 1974, Grenne & Lagerblad 1985, Grenne 1988). The intervening sediments The Folldal area comprises both the Gula of the Gula Group have a more uncertain and the Fundsjo Groups, covering about origin. Based on lithology, sedimentology 150 km2 around the village of Folldal in the and geochemistry, Guezou (1978, p.14) county of Hedmark (Figs.1 & 2). Because of considered that the Gula metasediments in extensive mining activity, mapping has the Dornbas district are of epicontinental ori­ been carried out by several workers and gin and "are devoid of either turbidites or with different perspect ives (e.g. Bjorlykke rhythmic coarse clastic deposits ". Nilsen 1905, Marlow 1935, Page 1963, Heim 1968, (1978), however, reported that turbidites are 1972, Pedersen 1979 and by geologists at found in the central part of the group further Folldal Verk A/S). north, while graphitic phyllites with conglo ­ merates and marbles occur both in the east In this paper, data on petrography, structu­ and in the west. Both back-arc and fore-arc res and tectonics, petrology and geoche­ positions have been inferred, either for the mistry and one U-Pb zircon age determinati­ whole group or part of the group (Gale & on will be presented from the Folldal area. Roberts 1974, Grenne 1988, Stephens & The intention is to throw light on the nature Gee 1989). of the contact between the Gula and Fund­ sjo Groups; and further, to compare petro­ Major differences in metamorphic grade graphy, geochemistry and the age of the and deformation (Wolff 1979) and age rela­ volcanic rocks with work done elsewhere in tionships (see below) indicate that an the 'greenstone belt', especially in the unconfo rmity or thrust may exist internally Roros-Meraker district. These data also within the Gula Group. Therefore, the natu- form a basis for a discussion of the origin of

Tec tono stra tigrap hy and Tec tonic Co mplex Terrane (Stcphens & Gce 1989) Lithological Units (Mod ified (Gee et al. 1985) from Nilscn 1988) Metamorphic Gula , appc; Exotic tcrranes complexes Gula Group derived from outboard of Baltic, Meraker Nappc: Oceanic sequences UPPE R Fundsj o Group. SIIMmo Group ALLOCHTHON Suspect terranes: probably outer Essandsjo-Oyfjell Nappc; Seve Nappcs tcct. shonened Aursund Group margin of Baltica

Remsklepp Nappc: Platformal and Augen Gneiss Unit. Tynset Gro up Tectonically MIDDLE miogcoclinal shortened margin ALLOCHTHO sediments and Rondanc Nappe of Baltica Precambrian crystalline rocks LOWER Osen-Roa l appc ALLOCHTHON

Platformal sedimcnts PARAUTOCHTHON/AUTOCHTHON Table 1. The divisions in tectonostra­ Cratonic Baltic. tigraphy. terranes and litholoqical Prec, cryst. basement PRECAMBRIAN CRYST, BAS EM E1-·/T units in the Southern Trondheim Region. NGU - BULL 426. 1994 Terje Bjerkgard & Arne Bjarlykke 55

LEGElI:D TYNSET & AURSUND GRO UPS r::'::' ::' :'~ Mica schists and :';::'.:::'.:::'.::. quartzites. subordinate :.;::.;.,'..:::.;:. ultrarnafic rocks and greenstones

SULAMo GROUP

Micaschist s

o 0 0 0 Conglomerate ~0000

RJNDSJ0 GROUP

Tuffite

Metasediments, partly graphitic.

Metarhyodacite

Metaba salt

Metagabbro

Trondhjemite wl mali c xenoliths

GULAGROUP <=0_"_" Ii-:, ,". Asli Formation, pelites Singsas Forma tion. banded quartzites and mica schists

d - Sheeled dyke complex p - Pillow lavas

Strike and dip

Major fault

, Sulphide mine Sample locality * for dating I km I------l

Fig.2. Simplified geolog ical map of the Folldal Area. 56 Terje Bjerkgard & Ame Bjorlykke GU - BULL 426. 994 and connection between the Gula Group two sedimentary units, the Asli and Sinqsas and the Fundsjo Group and finally for the Formations, (Nilsen 1978) in the eastern interpretations and understa nding of the Trondheim Region. The Gula Greenstone, a setting of the ore deposits in Folldal, work metabasalt unit, is common ly found betwe­ which is currently in progress. en the two and also within the Sinqsas For­ mation (Nilsen & Mukherjee 1972). All units are represented in the Folldal area and Geological setting were described by Heim (1972) under the name Storho i Formation. The Gula Group The Caledonides of the southern Trond­ constitutes the Gula Nappe according to heim Region comprise tectonic and litho­ Roberts (1978) or Gula Nappe Complex logical units derived from several d i ff e r~ n t (Gee et al. 1985). Because of possible con­ environ ments, thrust upon a Precambnan fusion with the lithological term Gula Group , basement with a thin, mostly Cambrian, the terms Gula Nappe and Nappe Complex sedimentary cover. According to Gee et al. should really be avoided as names for this (1985), the tectonostratigraphy of this regi­ tectonic unit. They will, however, be used in on can be divided into three major alloch­ this paper because of their common usage thonous complexes; namely the Lower, in the literature. Middle and Upper Allochthons (Table 1, b e ~ n Fig.1). A new division of units has pr?­ The Sinqsas Formation consists of psam­ posed for the Scandinavian Caledonides In mitic to semipelitic quartz-plagioclase-m ica the last five years based on the terrane con­ schists. They show typically a rhythmical cept (e.g. Roberts 1988, Stephens 1988). variation in quartz and mica content as well Terranes are defined on the basis of their as grain size, on 0.5 - 2 m scale, from near­ overall geological features and histories, ly pure quartzites to mica schists. Porphyro­ which are distinctive from those of neigh­ blasts of garnet and c1 ino-amph ibole are bouring terranes. In the Scandinavian Cale­ common . No primary sedimentary structu­ donides, outside of the tectonically shorte­ res have been found in the Folldal area, but ned shelf and miogeoclinal sequences rela­ in other places, e.g. the Roros area, graded ted to Baltica there are several suspect and bedding, cross bedding and load casts are exotic terranes of uncertain palaeogeo­ commonly observed in this unit and the graphic origin (Table 1). rocks have been interpreted as turbidites (Rui 1972, Nilsen 1978). The Folldal area (Fig.2) comprises units occurring above the arkosic rocks of the The Asli Formation is dominated by semi­ Rondane Nappe (Bockelie & Nystuen 1985) pelitic garnet-chlorite-bearing mica sc ~ i .sts . in the Middle Allochthon . The structurally Based on a varying content of phyllosilica­ lower units in the area are dominated by tes and quartz the formation might be divi­ argillaceous sediments occurring both in the ded in a lower unit of quartz-rich schists and Remsklepp Nappe (Wolff 1979) of the Midd­ a higher unit of partly graphitic phyl.losilica­ le Allochthon and in the Essandsjo-0yfjell te-rich schists, a division also noticed by Nappe (Nilsen 1988) of the Upper Alloch­ Heim (1972). Quartz , biotite and muscovite thon. Most of the area is occupied by volca­ are main phases while chlorite, garnet and nic and volcaniclastic rocks of the Meraker in some cases c1 ino-amphibole are subordi­ Nappe and divers e sediments of the Gula nate. Apatite, tourma line and zircon are the Nappe, both units belonging to the Trend ­ most important accessory minerals in these heim Nappe Complex . Most workers agree schists. Benches of metasandstone, several that these two units show internal strati­ metres thick, are intercalated in the upper graphic inversion (Rui 1972, Nilsen 1988). unit. About 250-300 m below the Gula­ Fundsjo contact , horizons of about 20 m­ The Gula Group thick calcite marble are found at several localities at the same stratigraphic level. The Gula Group (Kjerulf 1876) comprises The marble is locally quite pure and coarse NGU - BULL 426.1 994 Terje Bjerkgard & Arne Bjorlykke 57

bekken in the northern part of the area (Fig.2).

A thin (15-20 m) horizon of fine-grained amphibol ite separates the Asli and Sinqsas Formations at one locality. It is at least 600 m in strike length and contains amphibole and epidote as main phases , while quartz, chlorite and ilmenite are subordinate. The mineralogy and position of this horizon bet­ ween the sedimentary units fits with the Gula Greenstone .

The Fundsje Group In the Folldal area, the Meraker Nappe (Wolff 1979) is represented by the domi­ nantly volcanic Fundsjo Group together with the stratigraphically overlying sedimentary Sularno Group. The volcanic unit can be fol­ lowed all the way from Meraker to Grimsda­ len east of Dornbas, and has a variety of local names (e.g. the Hersjo Formation is the name given to the group as a whole in the Roros area (Rui 1972, Grenne 1988)). To underline the continuity of the unit the Fig.3. L2 - lineation in calcareous conglomerate of the Gula term Fundsjo Group will be used here. Group. The c1asts are mostly pure marble. while the matrix is calcite-rich metasiltstone. The Fundsjo Group widens in outcrop from about 1 km in thickness west of Alvdal to 5­ grained , though at some localities it has a 8 km in the Folldal area (Fig.1). The reason high content of quartz and commonly fuch­ for this is probably a combination of folding sitic muscovite , giving it a green colour . and primary differences in thickness , and especially due to the presence of a large fel­ About 500 m below one of the marbles in sic intrusive body near the stratigraphic the stream Husombekken two lenses of base of the unit. The unit can be divided into conglomerate are found (the Husom Cong­ essentially three lithologically different units: lomerate , Heim 1972). They are at least 15 1) Metabasalts and gabbros, 2) tuffitic and and 25 m thick and compr ise mainly clasts metasedimentary rocks and 3) felsic intrusi­ of pure calcite marble and quartzite in a ve and extrusive rocks. matrix consisting mainly of pale green clino­ amphibo le and biotite (Fig.3). Clasts of Greenstones, interpreted as metabasalts, quartz-albite rock, greensch ist, gabbro and are found mostly near the base of the unit, biotite schist are also present. Quartz , mus­ especially in the northeastern part of the covite and calcite are subordinate matrix area. They occur below, intercalated within , minerals. Although no pre-pebble foliation and above the large felsic intrusion (Fig.:? has been seen in the clasts , this might be Thin horizons are also found intercalatec' In due to subsequent deformation and meta­ the overlying tuffites. The metabasa lts are morphism . The clasts are extremely elonga­ dark green, fine-grained and homogeneous ted and ruler-shaped with maximum long­ with a weak foliation. White mm-thick stre­ axes up to 1 m. A similar but less deformed aks of albite and minor quartz in a green conglome rate, also with calcite marble as matrix of chlorite and amphibole define the clast material, is found in the stream Lang- main texture of the rocks. 58 Terje Bjerkgard & Arne Bjorlykke NGU . BULL 426. 1994

Because of the high degree of deformation only the 'spotted' texture distinguishes them in the area primary textures in the basalts from the metabasalts in the field. are rare. Structures resembling pillows have been found in the mine area just north of the At two localities small lenses of ultramaf ic village (p in Fig.2). West of Bukletten a pos­ rocks have been found associated with a sible sheeted dyke complex has been found metagabbro and metabasalt, respectively. in a road cut (d in Fig.2). Numerous fine­ These contain talc, chlorite and up to cm­ grained and metre-thick dykes intrude one sized euhedral crysta ls of magnes ite. another within the same strike trend. Becau­ Growth of colourless c1ino-amphibole (tre­ se of deformation the dykes are now paral­ molite ?) is observed at the contact with the lel to the regional foliation in the area. In country rocks. Both lenses have been some places it is possible to distinguish the mined for talc. individual dykes by the presence or absen ­ ce of plagioclase phenocrysts, and some The felsic igneous rocks of the Funds]o dykes have chilled margins. Group , on grounds of texture , stratigraphic position and contact relationship to other Horizons of metabasalts heavily studded lithologies, are divided into an intrusion and with mm to cm plagiociase phenocrysts are severa l extrusive equivalents. The intrusion common both east of Bukletten and in hori­ occurs in the lower part of the Fundsje zons contained in the felsic intrusion. The Group. Small apophyses and aplitic dykes best exposure is probably in the mine area intrude the metabasalts (FigA) . Xenoliths of just north of the village: Here one can see a mafic rocks are found in the upper part of sharp contact between a basalt without phe­ the intrusion (Fig.5), occurring over a strike­ nocrysts and a basalt with 30-40 % white length of at least 2 km (shown with black phenocrysts of plagiociase (1-10 mm) in an dots in Fig.2). The first deformation phase amphibole-rich matrix. Eastwards from the recognised in the area , which produc ed the contact the content and size of the pheno­ regional foliation , has affected the intrusion, crysts gradually decrease to 1-5 mm, beco­ ming more dispers ed, until they finally dis­ appear. The thickness of this phenocryst­ bearing zone is about 2.5 m.

Clino-amphibole (actinolite and actinolitic hornblende), epidote, albite and chlorite are the main mineral constituents. Quartz and calcite are generally subordinate , while bio­ tite, sphene and/or rutile and haematite are the most common accessories. Epidote/cii­ nozoisite replace plagiociase to varying degrees.

The metagabbros have the same mineralo­ gy as the metabasalts, consisting of unori­ ented 0.1-1 mm grains of actinolite/actinoli­ tic hornb lende , porphyroblastic epidote a nd fine-grained recrystallised albite in mm-lar­ ge aggregates. These aggregates are pro­ bably a result of the breakdown of larger crystals of more Ca-rich plagioclase, Ca now taken up by epidote. Amph iboles might have formed from pyroxenes. Due to the FigA . Drawing after a photograph of trondhjemitic aplite intru­ extensive deformation and recrystallisation , ding metabasalt at an angle to the regional foliation S1 (shown no ophitic texture has been preserved and by strike-dip symbol). NGU - BULL 426. 1994 Terje Bjerkgard & Arne Bjorlykke 59

The intrusion has earlier been called 'granu­ lit' (Bjarlykk e 1905). Goldschmidt (1916) kept this name but classified it as a trond­ hjemite. The mineralogy allows it to be clas­ sified as a trondhjemite and this is suppor­ ted by geochemistry (see later). As it is very well exposed immediately east of the village the informal term Folldal Trondhjemite will be used for the intrusion.

It should be stated that trondhjemites in the Trondheim Region were intruded before, during and after the time of nappe formation and thrusting. According to Size (1979), four periods of trondhjem ite intrusive activity are distinguished in the Steren area, ending with the post-tectonic trondhjem ite s.s. Similarly, in the Tolga area, trondhjemites of two generations have been observed (Bjerkqard 1989). Trondhjemitic dykes of an early generation have been affected by the D2 deformation and are schistose , while the larger bodies in the area (the best example is the Tolga Trondhjem ite) cross-cut all Caledonian structures.

The felsic extrusive rocks occur as confor­ Fig.5. (a).Amphibolitic xenoliths in the trondhjemite intrusion below the contact to the stratigraphically overlying tuffites. The mable layers, mostly in the tuffites in the xenoliths are elongated parallel to the Sl-foliation. (b) Close-up stratigraphically upper part of the group of the xenoliths. showing the foliation (Sl) affecting the contact (Fig.2). These layers are 10-30 m thick whi­ between the xenoliths and the felsic host. le their lengths can be up to several kilo­ the dykes, as well as the xenoliths (FigA, 5 metres. The contacts to the surrounding tut­ b). This shows that the intrusion was empla­ fites are always very sharp. A matrix of very ced before the thrusting and nappe formati­ fine-grained quartz and albite is dominating , on. giving the rock a grey-white colour and a weak foliation. In this matrix partly idiomor­ The intrusion has a strike extension of more phic, sharply defined phenocrysts of albite than 20 km. It appears to consist of three (1-3 mm) and irregular aggregates of quartz individual sills with maximum thickness of occur and are evenly distributed. As in the 800 m, divided by metabasaltic horizons . trondhjemite, garnet porphyroblasts and Because of its resistance to erosion the needles of amphibole are common . Epidote intrusion is reasonably well exposed. In out­ and muscovite are subordinate , while chlori­ crop it is quite homogeneous, though te and apatite are accessory minerals. somewhat variable in grain-size. A faint foli­ ation is present in the rock where it is more The upper part of the group is dominated by fine-grained (1 -3 mm). It is dominated by pale greyish-green chlorite schists with a albite and quartz and is grey-white in colour. pronounced schistosity. Alternating calcite­ Red garnet (1-3 mm) and up to cm long quartz laminae, fine-grained lenses of needles of green amphibole are the other quartz-albite and thicker chlorite layers defi­ major minerals. Muscovite and epidote are ne a characteristic lamination on mm to cm­ SUbordinate, while chlorite and apatite are scale (Fig.6). Where deformation was less common accessories. severe , lenses of quartz-albite and chlorite­ amphibol e rocks occur and are interpreted 60 Terje Bjerkgard & Arne Bjorlykke NGU . BULL 426. 1994

Fig. 6. Laminated rnafic u ite. wi h alternating layers dominated by chlorite and calcite-quartz on cm scale. An Fr-tolc refolded by F2 can be seen near the pencil. as clasts derived from the volcanic rocks. the planes of schistosity, somet imes orien­ This observation, together with a high con­ ted. Garnet, biotite, magnetite, sphene and tent of calcite and chlorite and a lamination apatite are the most common accessories, which is possibly primary, strongly suggests garnet occurr ing in porphy roblasts up to 1 that these rocks are volcan iclastic or tuffitic cm in size. rocks. The contents of the main and subordinate Quartz, albite, chlorite, epidote/c1 inozoisite phases vary and the tuffites are classified and calcite are the main constituents whilst as mafic or felsic on the basis of the ratio muscovite and amphibole usually are sub­ between mafic phases such as chlorite, epi­ ordinate. Amphibole is commonly found on dote and amphibole and the felsic phases

30 m 20 10 o

225 200 175 150 125 100 75 50 25 Om

Chloritc-rnica schist! ~r~~il~ ~j~: :: ~h:'/ • Clastic scdimcnts D I·!··I Fclsic tuffite mctagrcywacke ~ I:=:=:=1 Metabasalt [::::::::::::::::::::::::1 Mafic tuffitc Shear sense Graph ite schist

Quartzite

Fig.? A stream profile (S orbekken) showing the diversity of tuffites and the contact with the trondhjemitic intrusion. Notice he mere­ ased degree of shear deformation towards the contact. NGU - BULL 426. 1994 Terje Bjerkgard & Arne Bjorlykke 61

quartz and albite . The tuffites are textura lly are recognised in the Folldal area in additi­ dominated by a fine-grained quartz-albite on to later fractures and fault zones. The matrix together with parallel-oriented phyllo­ most pronounced structure is a penetrative silicates. More or less rounded mm-sized westerly dipping foliation or schistosity c1 asts of albite are found in this matrix, pro­ affecting all units. The data from the Fund­ bably representing phenoc rysts derived sje Group are shown in Fig.8 a. There is a from the volcanic rocks which have been general trend of increasing dip of the foliati­ abraded during transport. on planes westwards, while the strike is nearly constant. On a micro-scale this struc­ Intercalated in the tuffites are quartzites, ture, here called S1, is due to a parallel graphitic pelites and more coarse-grained alignme nt of mica and chlorite. Recrystalli­ chlorite-mica schists (metagreywacke). sation of quartz to elongate grains and an These have thicknesses from less than 1 m increasing grain size are also important in to more than 25 m. Commonly the graphite the development of the foliation. schists grade into the mica schists with a corresponding decrease in the content of An earlier foliation or possibly the primary graphite. Quartz , muscovite and chlorite are SO-layering is seen to be isoclinally folded, the main mineral phases in the sedime ntary especially on the micro-scale. At the meso­ rocks while biotite , garnet and, in more scale, hinge-zones of these folds, which are coarse-grained varieties also albite, are F1 structures , are isolated and disrupted, subordinate. Fig.7 shows the typical diversi­ showing the S1-foliation to be partly a trans­ ty from felsic to mafic tuffites and interca lati­ position structure as well as the axial plane ons of clastic sediments in Storbekken east foliation. On the mega-scale, structures of of Folldal. this phase are developed as isoclinal and recumbent folds with flat-lying axes trending The Sularno Group ENE-WSW. The folds are responsibl e for a repetition of the different lithologies across A highly deformed conglomerate with c1 asts strike (Fig.2). of quartzite, greenschist, quartz-albite rock, chlorite-mica schist and marble in a chlorite­ The 02 deformation phase is represented amphibole matrix is situated at the contact by meso-scale folds (about 0.1-30 m in to the Funds]e Group. The greenschist and wavelength) of a more open style, small­ quartz-albite c1asts may have been derived scale crenulations and by mineral lineati­ from the Fundsje Group. Graphitic phyllites ons. F1-folds are also seen refolded by F2 and mica schists are folded into the conglo ­ (Fig.6). Crenulations are generally well merate and a thin horizon of calcite marble developed in the incompetent schists and occurs at the lower contact. These lithologi ­ tuffites . 0 2 fold axis orientations for the es constitute the upper part of the Randan Fundsje and Gula Groups are shown in Group of Heim (1972). The stratigraphic Fig.8 b. The axes in the Fundsje Group are position and lithologies represented in the quite well concentrated and have a mean 0 0 c1asts makes a correlation with the Sceter­ plunge of 41 towards 269 • Measurem ents sja Conglomerate in the Heres area (Rui of axial planes to some of these folds show 1972) and the Lille Fundsjo Conglomerate strikes between 020 0 and 1100 and dips in the Meraker area (Wolff 1967) probable . varying between 40 0 and 750 to the north These conglomerates are interpreted to and northwest. The fold axes and lineations mark a major unconfo rmity between the in the Gula Group show a larger spread, but Fundsje Group volcanites and the overlying are within the same range as the Fundsje sediments (Hardenby et al. 1981, Grenne & data. Lagerblad 1985). Other lineations in the Fundsjo Group have the same trend and plunge as the fold axes Structures and tectonics (Fig.8 b). The lineation s are due to recrys­ Structures ascribed to two phases of folding tallisation of quartz , growth of mica and 62 Terje Bjerkgard & Arne Bjorlykke NGU . BULL 426. 1994 a)

Sl -Foliatio ns in the Fundsjo Gro up

Contours per 1% area: o 1-2 % I 1 3-6 % h:::: :1 7- 10 %

I t : ~ t :l 11-14 % _ 15-18 %

_ 19-26 %

Schm idt equal area projection. 402 measurements

b)

c : i • •

[)2·structures in th e Fundsj o Gro up D2·structures in th e Gula Gro up • Lincations Lineations and fold axes } N = [46 = 22 o Fold axes

Schmidt Equal Area Projection

Fig.a. (a) St -Ioliations (contoured) of the Fundsjo Group. (b) F2·folds and lineations of the Fundsjo and Gula lirou~~ . especially amphibole parallel to the b-axis The conglomerates of the Asli Formation in of the 02-phase. Growth of garnet was also the Husombekken stream section were also important and accompanied biotite. These affected by this deformation phase as the minerals (garnet, biotite and amphibole) marble c1asts are extremely elongated and represent the highest grade of metamor­ ruler-shaped (Fig.3) parallel to the a-axis. phism, namely the transition from greens­ The c1asts have lengths (X-axes) up to 1 chist to amphibolite facies (see later), and metre and the ratio between the X and Z were formed during 0 2. axes is up to 60 : 1. NGU - BULL 426, 1994 Terje Bjerkgard & Arne Bjorlykke 63

Development of mylonitic rocks is mostly epidote , 'fishes' of quartz-albite and by a restricted to nappe boundaries, but has also strongly preferred orientation of amphibole. occurred internally within units, especially where there is a strong contrast in compe­ At Sivilvangen east of Savalen (Fig.1), tence between lithologies. An example is severa l intercalations up to 5 m-thick of both the contact between the Folldal Trondhjemi­ chloritic mica schists and dm-banded mafic­ te and the structurally underlying tuffites felsic tuffites are found up to 150 m into the (Fig.7): within a 50 m zone below the con­ graphit ic pelites of the Gula Group. The tact, the mafic tuffites are transformed into contact itself is gradual and thin intercalati­ fine-grained, mylonitic, banded schists. A ons of pelites are found at least 15 m into 12 m-thick layer of felsic tuffite or fine-grai­ the tuffites of the Fundsje Group. Both the ned trondhjem ite aplite has been transfor­ tuffites and the pelites at the contact consist med into a rock with a texture resembling of extremely fine-grained mylonitic quartz augengneiss with rounded quartz-feldspar and phyllosilicates. This deformation is tra­ clasts in a chlorite-amphibole-epidote ced up to 5 and 25 m away from the con­ matrix. Lenses or 'fishes' of less deformed tact, respectively . The deformation is also material show a top-to-the-SE movement. shown by dm-Iong and cm-thick lenses of The deformation can be traced to about 90 psammitic quartz- rich schist in the pelites. m below the contact , where mafic tuffites The intercalations might possibly be due to have been displaced along a reverse fault small-scale folding or thrusting but may also at the contact to underlying metabasalts, be relict primary features overprinted by a the movement sense shown by lenses / 'fis­ local development of mylonitic rocks. hes' of quartz. The trondhjemite has also been affected by this deformation. Usually it About 3 km north of Sivilvangen , a lens of is coarse grained with a very poorly develo­ metagabbro occurring about 100 m inside ped foliation , but near this contact it is very the Fundsje Group is strongly deformed and schistose and mafic xenoliths are strongly more or less transformed to a mylonitic epi­ flattened and elongated . This deformation dote-chlorite schist. This metagabbro is can be traced for at least 40 m into the intru­ surrounded by tuffites, and the deformation sion. is thought to be a result of a high competen ­ cy contrast. The contact between the Gula Contact between the Fundsje and and the Fundsje Group is found 100 m west Gula Groups of and structurally above this locality , where it seems to record a gradual transition from The contact between the Gula and Fundsje a felsic, tuffitic, quartz-albite rock of the Groups is exposed at several localities in Fundsje Group to graphitic pelite of the the Grimsdalen, Folldal and Savalen areas. Gula Group. In Grimsdalen there is a gradual change over about 50 m from tuffitic chlorite schists McClellan (1993) has described the contact belonging to the Fundsje Group to pelitic east of Savalen as a ductile shear zone with graphite-chlorite-mica schists of the Gula widespread mylonitisation textures in the Group . This is in contrast to the view of rocks of both groups. Pedersen (1979) who claimed there to be a tectonic contact on the basis of observati­ Metamorphism ons of mylonitic rocks in both groups. In the stream Husombekken in Folldal, the contact The metabasalts of the Fundsje Group have is between tuffite and slightly graphitic peli­ a typical prograde mineral assemblage of te. The contact is quite sharp, but marked hornblende + albite + epidote + chlorite + by an increasing content of muscovite and quartz ± calcite ± biotite, while the tuffites decreas ing content of chlorite in the tuffite have chlorite + albite + quartz + epidote + within a sequence of about 40 cm. Some calcite ± hornblende ± garnet ± muscovite ± differential movement in the tuffite is recor­ biotite. Amphibole and biotite are observed ded by the presence of rotated c1asts of to have formed at the expense of chlorite. 64 Terje Bjerkgard & Arne Bjor/ykke NGU . BULL 426. 1994

The assemblage of the felsic rocks is albite on between greenschist and amphibolite + quartz + garnet + hornblende ± muscovite facies according to Winkler (1979) and Tur- ± epidote. These assemblages and especi- ner (1981). ally the coexistence of albite and hornblen- de, place the metamorphism at the transiti- Garnets have commonly overgrown the S1-

Main elements (wt.%) Mafic rocks Si02 Ti02 AI20 3 FeO(t) ~1n O MgO CaO xszo K20 £>2 05 L.O.! Sum G7·mg 47.9 0.50 14.6 9.0 0.20 11.8 9.7 1.5 0.59 0.08 3.0 99.0 G8· mg 48.1 0.8 1 15.7 10.7 0.27 9.1 7.9 3.3 0.19 0.10 3.5 99.6 GI -mb 54.0 0.62 16.0 8.9 0.19 5.3 9.7 2.5 0.56 0.07 1.5 99.3 G2-mb 44.8 0.47 14,4 9.7 0.24 7.0 14.7 2.0 0.13 0.05 6.9 100 .3 G3-m b 46,4 0.64 15.2 11.0 0.26 8.0 15.1 0.9 0.22 0.09 1.2 99.1 Gd-rnb 49.5 0.28 14.2 9.5 0.2 1 8.9 9.9 2.9 0.08 0.06 4.1 99 .6 G5·mb 53.7 0.25 10.9 8.9 0.22 10.5 9,4 1.1 0.00 0.05 4.2 99.2 G6-l11b 52.6 0.26 12.6 9,4 0.20 10.1 7.8 3.7 0,41 0.04 2.6 99.8 G9-mb 52.9 0.88 15.7 11.0 0.27 4.8 10.8 I.3 0.01 0.09 l.7 99.5 GIO-mb 46.2 1.02 15.6 10,4 0.27 8.9 1l.7 2.6 0.20 0.08 2.3 99.3 GII -mb 49.2 1.50 13.8 12.9 0.25 6.9 11.5 2.0 0.15 0. 12 1.0 99.3 G l 2-mb 53.1 0.79 15.9 12.3 0.24 5.6 5.5 5.3 0. 15 0.08 0.9 99.7 Gl4-mb 49.0 0.74 14.8 9.2 0.23 6,4 13.8 1.9 0.05 0. 11 2.9 99.2

Fe/sic rocks Ol -lrh 77.3 0' - 12.0 3.1 0.14 0.3 1.9 4.0 0.66 0.05 0.3 99.8 0 5-trh 75.5 0.2 1 12.0 3.5 0.11 0.6 2.2 3.7 0.62 0.07 0,4 98.9 06-trh 74.5 0.20 12.5 2.7 0.09 0.5 2.0 4.9 0.22 0.05 0,4 98.2 0 7-trh 74.8 0.16 12.7 2.5 0.10 0.5 1.1 4.5 0.78 0.05 0.6 97.8 46-92-trh 74.7 0.27 12.5 2.9 0.04 0.6 2.7 4.9 0.3 1 0.06 0,4 99.2 03 -rh 80.7 0.22 10.5 1.6 0.09 0.3 2.3 3.6 0.13 0.06 0.3 99.8 04-rh 76.5 0.19 12.6 2.5 0.10 0,4 l.7 3.5 1.28 0.06 0.7 99,4 2462-rh 7&.4 0.2 1 12.1 2.1 0.09 0.3 0.9 5.0 0.28 0.07 0,4 99.9

Trace e lements (p p m) Mafic rocks ~1 o l'b Zr Y Sr Rb Pb Cu Zn Cr Co l'i G7-mg 13 4 40 11 177 11 0 0 61 690 57 194 G8-mg 12 4 58 23 133 5 6 0 56 372 5 1 152 G I-mb 14 3 4 1 19 128 12 0 6 60 300 38 58 G2'mb 14 3 43 16 109 5 2 54 78 463 42 117 G3-mb 16 3 53 18 69 6 9 78 609 64 170 G4-mb 14 4 44 1I 47 3 31 74 634 80 109 G5-l11b 15 3 40 7 83 2 30 67 1419 60 295 Go-mb 12 5 46 5 4 1 8 12 I 1315 88 277 G9 -mb 14 4 54 25 II I 3 2 197 70 73 50 18 G IO-l11b 14 3 45 30 324 4 18 0 54 358 58 125 G l l -mb 13 4 70 45 246 4 0 0 49 131 62 63 G l 2-mb 15 3 52 23 70 2 I 11 92 44 32 15 Gl4-mb 14 3 3 1 20 318 9 0 5 298 44 105

Fe/sic rocks DI-Irh 11 6 93 35 70 17 4 0 62 99 3 2 05 -lrh 1I 6 102 45 96 7 I 0 18 80 3 4 Do-trh 7 5 168 74 135 6 4 0 22 l OO 12 3 D7-tdl 10 6 113 27 79 18 6 0 25 93 I 3 46-92-trh I 3 69 29 125 9 8 2 14 126 5 6 D3-dl 13 5 64 24 155 4 3 0 29 86 2 I 0 4-dl 7 7 186 53 69 14 9 0 56 82 2 4 2462-rh 8 7 133 63 73 9 I I 0 34 97 9 2 Table 2. XRF analyses of ma fic and fe lsic rocks in the FoUdal a rea. mg - metagabbro; mb - metabasalt; trh - trondh jem ite; rh - rnyo- dacite . Sample G9 - mb represe nts the dyke complex. s a mples Gt t , G12 and G14 - mb are from the basalts intercalated in the trondhjemite . Ana lyses have been done at University of Oslo. NGU • BULL 426. 1994 Terje Bjerkgard & Arne Bjarlykke 65

foliation as defined by inclusions of epidote, Geochemistry of igneous rocks chlorite and quartz. The parallel alignment of amphiboles and simultaneous growth of The geochemical compositions of both fel­ biotite and garnet in F2-folds suggest that sic and mafic igneous rocks in the Fundsje the peak of metamorphism was reached Group (Table 2) show a clear chemical during this phase. Some later retrogression bimodality. The highest value of Si02 in the is shown by local chloritisation of the amphi­ metabasalts is 54 wt.% (average 50.2 %) , bole. while the lowest in the felsic rocks is 74.5 % (average is 76.8 %) . In the Asli Formation of the Gula Group, the typical prograde paragenesis is quartz + Mafie rocks muscovite + biotite + chlorite ± albite ± gar­ net. This is a typical assemblage of the Most samples of greenstones analysed, upper part of the greenschist facies, accor­ including two metagabbros , are from the ding to Winkler (1979) and shows that there thick unit in the Bukletten area (Fig.2), one was no apparent difference in metamorphic sample is from the dyke complex and three grade between the upper part of the Gula from the horizons intercalated in the trond­ and the Fundsje Groups. The metamorphic hjemite. Based on CIPW-norm calculations grade increases westwards and stratigrap­ (not shown) the rocks are classified as olivi­ hically lower in the Gula Group, as shown ne-tholeiites to quartz-tholeiites. Half of the by the appearance of kyanite and staurolite, samples from Folldal have a loss-of-ignition as well as andesine in the Sinqsas Formati­ (L.O.I.) of more than 2.5 % and should be on (Heim 1972, Pedersen 1979). considered as altered (Le Bas et al. 1986). By comparison with MORB and low-K tho-

a) 10 Rhyolite

Trachydaci te

r§< 8 ~ o {'l ::<: 6 +

o{'l Basalt Z"" 4 Dacite + Andesitc Basahi c 2 Picro- . ". andesitc basalt •

45 50 55 60 65 70 80 85 Si02wt%

ToJ IOO b) c) 400 ~ "lo~ Fig.9 (a) Classification of the volca- 350 nic rocks in Folldal from the Fundsjo -::- 300 Group by the total alkalis vs. Si02 wPB §: 250 diagram (after Le Sas et aI.1986). ~ '00 Filled circles· metabasalt; open cir- > ~ 5 0 cles - trondhjemite; crosses -rnyoda- . .. 100 cite.(b) The metabasalts plotted in •• KT. OFB. 50 the Ti-Zr-Y discrimant diagram of @•• • CAB' Pearce & Cann (1973). (c) The CAB 0 0 2000 4000 6000 8000 10000 metabasalts plotted in the Ti·V dis- Ti (ppm) criminant diagram of Shervais z,. Y"3 (1982). 66 Terje Bjerkgard & Arne Bjarlykke GU. BULL 426. 1994

Metabasalt Metagabbro Trondhjemite Extrus ive N-MORB LKT IAG Folldal' Folldal Folldal rocks Follda l SW Pacific Shich ito R. Si02 50. 2 (3.2) 47.9-48.1 74.5-77.3 76.6-80.7 50.4 5 50.73 70.0-74.6 Ti02 0.7 (0.4) 0.5-0.8 0.15-0.21 0.19-0.22 1.62 0.83 0.2 5-058 AI203 14.5 (1.5) 14.6-15.7 12.0-12.7 10.5-12.6 15.26 17.38 12.9-13.7 FeO(t) 10.3 (1.3) 9.0-10.7 2.5-3.5 1.6-2.5 10.4 3 9.38 1.8-3.7 MnO 0.23 (0.0.3) 0.2-0.3 0.09 -0.14 0.09-0.10 0.19 0.10-0.18 MgO 7.5 (1.9) 9.1-11.8 0.3-0.6 0.3-0.4 7.58 6.97 0.4-1.0 CaO 10.9 (2.8) 7.9-9.7 1.1-2.2 0.9-2.3 11.30 11.51 1.3-3.0 Na20 2.4 (1.2) 1.5-3.3 3.7-4.9 3.5-5.0 2.68 2.06 4.6-5.5 K20 0.2 (0.2) 0.2-0.6 0.2-0.8 0.1 -1.3 0.11 0.26 0.9-1.3 P205 0.08 (0.02) 0.1 0.05-0 .07 0.06-0.07 0.09 0.04-0.13 L.O.I 2.7 (1.7) 3.0-3.5 0.3-0.6 0.3-0.7

Mo 14 (1) 12-13 7-11 7-13 Nb 4 (0.5) 4 5-6 5-7 3.5 1.7 Zr 47 (10) 40-58 93 -168 64 186 104 30.8 171-253 Y 20 (11) 11-23 27-7 4 24-63 36 15.8 47-63 Sr 140 (100) 133-177 70-135 69 -155 113 224 45-176 Rb 4 (3) 5-11 6-18 4-14 1.26 4.1 10.6-20 Pb 3 (5) 0-5.6 1-6 3-11 0.49 4.0 3-4 Zn 69 (12) 56-61 18-62 29 -56 78 .2 57 - 88 Cr 513 (444) 372 -690 80-100 82-97 251 82 .4 o Co 56 (17) 51-57 1-12 2-9 47 35.7 3-38 Ni 122 (88) 152-194 2-4 1-4 150 35.5 0-7 V 258 (66) 165-239 6-13 6-9 286 0-14

K1 Rb 293 296 - 441 314 - 714 250 - 755 700 526 498 -666 TiN 15.0 18.2-20 .3 28-3 5' 17.4 SrlZ r 3.2 2.3-4.4 1.09 7.3 Rb/Sr 0.053 0.040-0.063 0.043-0.248 0.027-0.203 0.011 0.0 18 0.065-0.400 K20 /Na20 0.045 - 0.171 0.036 -0.37 1 0.169-0 .28 1 RblZ r 0.034-0.186 0.066 -0.076 0.059-0.103

Number of Analyses 11 2 4 3 26 43 4

Table 3. Geochemistry of the intrusive and volcanic rocks in Folldal. Values of MaRS (Hofmann 1988). Low-K Tholeiite (LKT) (Ewart 1982) and Island Arc Granite (IAG)(Ikeda & Yuasa 1989) are shown for comparison. 1Standard deviations in brackets . 2Ti/V from Hawkins et al. 1990. leiites (Table 3), the Folldal greenstones samples by Skyseth & Reitan (1992). This have normal Na20 and Sr contents while pattern is typical of low-K tholeiites from CaO is lower than both MORB and LKT, island arcs (IAT) and shows similarities to suggesting some alteration, probably by average SW Pacific IAT (Ewart 1982) with interaction with sea water (spilitisation). This respect to Zr, Ti and Y. The lower values of is probably also one reason why in the total Sr, K and Rb might be due to alteration, as alkali - silica diagram (Fig.9 a) the mafic discussed above .One important exception rocks spread out in the fields of basalt and is the extremely high values of Ni and Cr basaltic andesite. The close association (respectively, 1.5 and 3 times as high as and similarity in geochemistry between the IAT). These values suggest an influence metagabbros and the basalts (Tables 2 and from a primitive source (discussed below). 3) suggest that they belong to the same Compa red to the Folldal data, basalts from m agm atic system , w ith the gabbros as th e d estructive margins near co ntinents, such intrusive equivalents of the basalts. as the western part of America, are highly enriched in Nb, P, Zr, Y and Ti compared to The Folldal metabasalts are enriched in K MORB (Ewart 1982). and Rb and depleted in P, Zr, Y and Ti com­ pared to MORB (Fig.10 a). A similar pattern In the Ti-Zr-Y diagram (pearce & Cann was also found in Grimsdalen from drillcore 1973, Fig.9 b), the rocks plot as CAB, LKT NGU - BULL 426. 1994 Terje Bjerkgard & Ame Bjorlykke 67

a) on the geochemis try and field relationships • Metabasalts - Folldal • SW Pacific as described above it seems reasonable to ... Cascades- Aleutian s interpret the trondhjemite as a subvolcanic o WestemUS A intrusion related to the extrusive rocks. o South America

The high content of Na20 (> 4 "le) in the fel­ sic rocks suggests alteration by spilitic or hydrothermal processes , though the altera­ tion has probably not been pervasive. Com­ pared to both trondhjem ites from modern island arcs (Table 3) and the presuma bly unaltered post-tectonic trondhjemite from Sr K Rb Nb n05 Zr Ti02 Y Cr Ni near Steren (see Size 1979), the sodium content is similar.

b) The bimodal volcanism in the Fundsje 3.00 Group is a typical feature of both arc and 2.50 back-arc environments. Compared to felsic rocks of an arc back-arc setting (Gill & Stork 0 2.00 b 1979, Ikeda & Yuasa 1989, Fryer et al. 1.50 1990, see Table 3 for a comparison with 1.00 ..-:.:= ====";"-<::;::--- (AT-trend island arc granite from lzu-Bonin arc), the Folldal Trondhjemite and extrusive equiva­ 0.50 lents are similar with respect to most main 0.00 and trace elements. Most characteristic is a 0.50 1.50 2.00 2.50 3.00 low content of Ab03and Ti02 , as well as a FeOfMgO high K/Rb-ratio; but as in the metabasalts, Fig.l 0 (a) Average of the Folldal greenstones (13 analyses) the content of Cr is anomalously high com­ normalised to MORS (MORS-data from Pearce 1983). Data pared to felsic rocks in an arc setting (80­ from active continental margins and island arcs are shown for comparison (data from Ewart 1982). (b) Ti02 vs.FeO/MgO 100 ppm compared to < 10 ppm, op.cit.). trend diagram for the 13 Folldal samples. Domains of basalt types (A, S, C) distinguished by Grenne (1988) from the Roros Area are shown for comparison. An and OFB, while in the Ti-V diagram (Sher­ vais 1982, Fig.9 c) most samples plot above the line TiN= 20, which is typical of IAT or small scale back-arc basins (op.cit.). Regar­ ding the ratio K/Rb the Folldal basalts are quite similar to IAT (Table 3), while the Sr/Zr ratios is lower than that of IAT. The latter may be the result of Sr loss during alterati­ on. Similarly, in the Ti02 vs. FeO(t)/MgO dif­ ferentiation diagram, the Folldal samples follow an IAT trend (Fig.10 b). Fe/sic rocks As shown in Table 2, there is a striking simi­ larity in chemistry between the Folldal trond­ hjemite and the extrusive rocks. This is fur­ ther supported in the total alkali - silica dia­ Ab Or gram, both plotting as rhyodacites (Fig.9 a). Fig.l l . Classification of the felsic rocks in Folldal according to In an or-ab-an diagram (Fig.11) all the FolI­ the Or-Ab-An diagram of Barker (1979). Open circles - trend­ dal samples plot as trondhjemites. Based hjemite; crosses - rhyodacite. 68 Terje Bjerkgard & Arne Bjorlykke GU -BULL 426. 1994

0.080

492

0.079 =:l oc N '" Fig.12. Concordia diagram for the it-- analysed zircons from the Folldal ~ Trondhjemite: Three fractions of fair o N quality zircon were analysed from 484 0.078 one sample. The best estimate for the age of the trondhjemite can be estimated to be 488±2 Ma based on zircon fractions A and C which inter­ 480 sect concordia and each other. Fraction B is slightly discordant, which is probably a result of analyti­ cal problems. The amount of zircon 0.077 in this sample was very small and 0.600 0.615 0.630 0.645 therefore the amount of Pb analysed was quite small. 207 Pb / 235 U Age of the Folldal Tro ndhjemite work in the Heros-Me raker part of the Fund­ sjo Group: Grenne & Lagerblad (1985) and Four samples of trondhjemite and two sam­ Grenne (1988) distinguished between two ples of metarh yodacite, each around 30 kg types of basalt , one type with a typical lAT­ in weight , were collected. The samples signature (Iow Ti0 2 , Zr and high Rb) and the were crushed to 125-250 mesh in a sling other with similarities to MORB. As seen in mill and the heavy minerals collected with a Fig.10 b the Folldal samples are similar to 'Goldhound' (used for gold prospecting and type A of Grenne (1988), which display an easy to clean). AFrantz magnetic separator IAT trend . In the Roros and Meraker areas, was used to remove magn etic minerals. some samples intermediate between the The remaining concentrates (a few grams) two have also been found , as well as transi­ were sent to the Geochronology Division at tional calc-alkaline to within -plate type the Geological Survey of Canada. Clear zir­ metabasa lts (Grenne 1988). Based on this, cons were picked out and analysed follo­ Grenne & Lagerblad (1985) concl uded that wing the method outlined in Parrish et al. the metabasa lts represented immature arc (1987). U and Pb blanks were approximate­ to arc rifting volcanism, while Grenne (1988) ly 1 and 15 picograms, respectively. claimed a possible opening of a marginal basin above a mature subduction zone. Only one of the sampl es of trondhjemite contained enough zircons of fair quality to The Folldal basalts have anomalously high provide an age (sampl e locality marked by content of Ni and Cr. The contents of these the asterisk in Fig.2). The result is shown in elements are also high in the Fundsjo Fig.12. As can be seen, two of the three Group metabasalts in the Horos-Meraker fractions intersect the concordia and the area (Grenne & Lagerblad 1985, Grenne age is calcu lated to 488 ± 2 Ma. 1988), showing it to be a regional featu re. High values of these elements are also known from some of the Stekenjokk volcani­ Discussion tes in the Stikke and Gelvenakko Nappes in Origin of the volcanites northern Jamttand in Sweden (Stephens 1982) and the Gjersvik Group volcanites in The volcanic rocks in Folldal have most like­ the Gjersvik Nappe north of the Grong­ ly been formed in an island arc setting, as Olden Culmination (Lutro 1979). Both these discussed above. This is in agreement with units have been correlated with the Fundsjo NGU - BULL 426, 1994 Terje BjerkgiJrd & Arne Bjorlykke 69

Group (Stephens & Gee 1985). These ano­ cases the felsic intrusive rocks are included. malies suggest an influence from a primitive This is far too much to be generated by source. In fact , two of the samples from fractional crystallisation from a mafic mag­ Folldal might be classified as boninites, with ma. As seen in Table 2, the felsic volcanites 2 Mg/Mg+Fe + :2: 0.6 and Si02 :2: 53 wt.% generally have a slightly higher content of (Crawford et al. 1989) . In this regard it is incompatible elements, and especially L1L interesting that boninites have recently elements like Rb, K and Zr, compared to the been reported from the nearby Einunnfjell­ basalts, but also values as low as the avera­ Savalen area (McClellan 1993) . Boninites ge for the basalts. The increase is generally are either among the first melt products in a too little to conclude that the felsic rocks primitive island arc (Hawkins et al. 1984) , or have differentiated from the mafie magma. formed at a later stage because of subducti­ One would also expect to find intermediate on of an active spreading ridge (Crawford et compositions. On this basis , the mafic and al. 1989) , in both cases by partial melting of felsic rocks seem to have different origins. A the mantle wedge above the subducted possibility is that the felsics have formed by slab. Boninitic melts are known to be very partial melting from the downgoing slab and rich in elements such as Cr and Ni (Ni =70­ the mafics from the mantle above the slab. 450 ppm, Cr = 200-1800 ppm , Hawkins et al. 1984) and the two samples from Folldal have 277 and 295 ppm Ni and 1315 and Relationship between the Gula and 1419 ppm Cr, respectively. Fundsje Groups The contact between the Gula and Fundsje In the Horos-Meraker area volcanites with Groups has been variously described as of both IAT and MORB characteristics are either primary or tectonic origin. A tectonic found , while only IAT volcanic rocks have contact has been reported from the Dom­ been found in the Folldal area . Two possi­ bas-Grirnsdalen area (Guezou 1978 , Peder­ ble explanations for this are: 1) MORB vol­ sen 1979) , the area east of Savalen canites might be present also in Folldal, but (McClellan 1993) and in Tydal north of the not detected because of too few samples; Horos area (Horne 1979). In contrast, Rui or 2) there is a development from arc to rif­ (1972) and Rui & Bakke (1975) found no ted arc/back-arc northwards, the samples evidence for a tectonic contact between the with IAT signature in the Heros-Meraker units in the Heros area and claimed a con­ area representing disruption of the arc , as formable contact (see below), a conclusion envisaged by Grenne (1988) . A higher con­ also reached by Fox from work near the tent of Na20 in the northern part of the Hersje deposit (Fox et al. 1988) . group might support the latter interpretation because a higher degree of spilitisation In the Meraker area , a unit is distinguished, might be suspected in a rifting environment the Guda Formation (Wolff 1973) or Guda because of increased water depth and Group (Lagerblad (1983) , containing both increased heat flow. greenstones and graphitic schists with conglomerates and marble. The conglome­ Two possibilities exist for the origin of the rates usually contain quartzitic and calcare­ trondhjemite and associated volcanic rocks ous pebbles in a mafic matrix and are thus (see Barker 1979 for review): 1) part ial mel­ similar to the Husom Conglomerate in FolI­ ting of the downgoing slab; or 2) fractional dal (Heim 1972 and this work). Based on crystallisation from the basal tic magma. tectonostratigraphic position and lithologies Based on field observations and experimen­ the unit is correlated with the similar 'hetero­ tal studies, trondhjemitic melts have been geneous banded sequence' of Rui (1972) in shown to be generated by partial melting of the Heros area (Lagerblad 1983) . Accor­ amphibolites (Drummond & Defant 1990) . ding to Nilsen (1978) this sequence belongs Furthermore, the volume of felsie rocks in to the Asli Formation. Rui (1972) considered Folldal is around 30 % and in Meraker about this unit as transitional between the Gula 50 % (Grenne & Lagerblad 1985) , if in both and the Fundsje Groups. In the Meraker 70 Terje Bjerkgard & Arne Bjorlykke NGU • BULL 426. 1994 area the contact between the Guda Forma­ Guda Formation (see above) in the Tynset­ tion and the Fundsjo Group is similarly des­ Meraker area are found close to the Gula­ cribed as transitional, with decreasing Fundsjo contact. In this case the clastic amounts and thicknesses of sedimentary material might have been partly derived layers eastwards into the Fundsjo Group from the growing arc and partly from the (Wolff 1973). This corresponds to what we Gula rocks. have observed in the Savalen and Grimsda­ len areas (this work). The fossil locality at Nordaunevoll with the graptolite Dictyonema f1a belliforme sociale Further north, in northern Jamtland, the first described by Vogt (1889) is situated Remdalen Group in the Stikke Nappe struc­ very close to the Fundsjo Group (Rui 1972) turally overlies the Stekenjokk volcanites in the Asli Formation. The host rock is a with a conformab le contact (Stephens very conspicuous V, U and Mo-rich graphite 1982). The structural base of this group phyllite (Gee 1981). The age of the graptoli­ consists of quartzitic conglomerate and te is Tremado cian, that is to say 488-505 limestone in quartz-graphite phyllite follo­ Ma (Harland et al. 1989). The trondhjemite wed by a succession of quartz-graphite in Folldal, intruding slightly older basalts, is phyllites and greenschists (Remdalen Gre­ dated to 488 ± 2 Ma (p.68), which indicates enschist) together with quartz porphyry and a continuous stratigraphic sequence from at limestone and uppermost, calcareous least the Asli Formation and upwards quartz-graphite phyllites. The whole strati­ through the Fundsjo Group. A correspo n­ graphic succession is probably inverted as ding relationship appears to exist in the Ste­ in the Trondhe im Region (op.cit.).The asso­ kenjokk area in Jarntland, where similar U­ ciation of conglomerate and limestone is and Mo-rich graphite phyllites (but not con­ then in the same stratigraphic position and taining fossils) are intercalated in the gre­ is very similar to the upper part of the Gula enstones (Stephens 1982) and an age of Group. The occurrence of transitional units 492 ± 1 Ma has been obtained from a trend­ with intercalations of greenstones or tuffites hjemite (Claesson et al. 1987). between the Gula and Fundsjo Groups in the Savalen, Heres and Merake r areas sug­ Stephens et al. (1993) were able to date (U­ gests a close relationship between the Pb zircon) a trondhjemite c1ast in a conglo­ units.The development of mylonitic rocks merate from Storfjal let to 489 +10/-5 Ma seen at some localities might be due to and a trondhjemite from Gjersvik to 483 +5/­ mechanisms other than large-scale thrus­ 3 Ma, both in the Kbli Nappes of the Upper ting (cf. the contact between the trondhjemi­ Allochthon and probably correlatives of the te and tuffites in the Folldal area, described Fundsjo Group (see above). These rocks above). are within error margins identical in age to the Folldal trondhjemite. The authors The c1asts in the conglomerates in the Asli (op.cit.) use these ages to indicate that the Formation are from several sources. Clasts terrane containing these trondhjemites was of marble and quartzite are always domina­ considerably separated from Baltica at that ting and are derived from nearby lithologies time. This is probab ly not true (see discussi­ in the Gula Group. Igneous c1asts have on below). been recognised in Folldal (Heim 1972, see a bove ) a n d al so in the Tyn s e t-R oro s areas A h ig h c ontent of e r is typic al of the Fundsjo (Fasrden 1949, Ellevold 1976). These cong­ metabasalts, both in Folldal and partly at lomerates are overlain by sediments, mostly both Heres and Meraker (see above). graphitic pelites of considerable thickness Accord ing to data by Ellevold (1976) the (up to 500 m), making perhaps the volcani­ phyllites in the Asli Formation also have tes in the Gula Group (the Gula Greensto­ high contents of Cr and which seem to ne) the most probable source for these decrease away from the contact. This might c1 asts. In contrast, the conglomerates and be interpreted as a decreasing influence associated tuffites and greenstones of the from the volcanic rocks which may have NGU - BULL 426. 1994 Terje Bjerkgard & Arne Bjorlykke 7 1 been the source. A similar feature also Vagamo Ophiolite (B. Sturt, pers. seems to be displayed by Se and Co comm.1993 ). Even so, the Otta conglome­ (op.cit.). The regional extent of the conglo­ rates can be compared with conglomerates merates in the Asli Formation indicates a in the Sularno Group (see above), since major uplift event of the Gula rocks. This both are polymict and contain c1asts derived might have happened during a phase of from the underlying volcanites. These relati­ compression in connection with the deve­ onships and correlations point to a similar lopment of a subduction zone leading at last obduction and post-obduction history of the to formation of the arc-related Fundsje Vagamo ophiolite and the 'Fundsje arc'. Group. The marble horizons indicate shal­ low-marine conditions. Such conditions are Ensimatic vs. ensialic arc also indicated by the black schists in the Closely tied to the question of the connecti­ upper part of the Asli Formation, which con­ on between the Gula and Fundsie Groups tain the platformal graptolite Dictyonema at is whether the arc/back-arc system develo­ one locality. ped in an ensimatic or an ensialic setting. An ensimatic setting has been proposed by Correlations with Gudbrandsdalen Grenne & Lagerblad (1985) and Grenne Recently, the greenstones in the Otta-Dom­ (1988). This is also in accordance with what bas area have been identified to represent has been suggested for the Middle K61i and an ophiolite complex, the Vaqarno Ophiolite Gjersvik volcanic rocks (e.g. Stephens & (Sturt et al. 1991). These greenstones have Gee 1989).Indeed, the widespread occu r­ an N-MORB signatu re and have been inter­ rence of low-K tholeiites in the Folldal, preted to represent either the ancient crust Grimsdalen and Heros-Meraker areas (see of lapetus or oceanic crust from a marginal above) is in favour of an ensimatic setting basin (op.cit.). The ophiolite has a thrust for the volcanites. In ensialic environments boundary to the underlying sediments of the such rocks are rare (see for instance the Heidal Series (Sturt et al. 1991). The Heidal compilation by Thorpe 1982) and confined Series comprises mostly quartzitic schists to the oceanic side of the arc (Baker 1982). and garnet-mica schists and may be corre­ The Tonga-Kermadec-New Zealand arc lated with the Aursund Group (Table 1, system shows that with increasing influence Fig.1). Both the ophiolite and the Heidal of continental crust, the magmas change Series are overlain unconformably by the from low-K tholeiites to calc-alkaline basalts Sel Series , which generally commences (Ewart 1982). Further, the low content of with conglomerates (op.cit.), These conglo­ incompatible elements in the felsic rocks merates vary laterally but generally contain also supports an ensimatic setting. The c1 asts derived from the ophiolite as well as compilation by Ewart (1979) indicates that from the Heidal Series. Fossils in this cong­ low-K rhyolites are restricted to ensimatic lomerate give an Early L1anvirn age (:0: 470 arcs. Ma). The relationship between the Heidal Series, Vagamo Ophiolite and Sel Series On the other hand, as Grenne (1988) pro­ demonstrates a probable Early Ordovician posed, the presence of quartzites and obduction of the ophiolite onto the Fenno­ quartzofeldspathic wackes intimately rela­ scandian Shield (op.cit., Sturt & Roberts ted to the volcan ites supports a near-conti­ 1991). nent origin of the arc. This is also supported by lead isotopes, which indicate a continen­ tal component in the massive sulphides in Recent mapping by Sturt & Ramsay in the Fundsje Group (Bja rlykke et al. 1993 Grimsdalen shows that the major unconfor­ and unpublished data from Folldal). mity which oversteps the Vaqarno Ophiolite onto the subjacent Heidal Group (Sturt et al. The contact between the upper part of the 1991 ), passes stratigraphically beneath the Gula Group and the Fundsje Group in the Fundsje Group volcanites. This precludes a Folldal area seems to be primary and in pla­ correlation of these volcan ites with the ces gradual (see above), implying that the 72 Terje Bjerkgard & Arne Bjarlykke NGU • BULL 426. 1994 arc must have formed above the Gula Because of the marked difference in meta­ Group. One possible (and probable?) ans­ morphic grade between the Sinqsas and wer to the ensimatic signature of the arc is Asli Formations, the age difference between that it may overlie only the outer and platter­ these formations and the close relationsh ip mal Asli Formation of the Gula Group . The between the Asli Format ion and the Fund­ thickness of this unit is only a few hundred Sj0 Group, the Sinqsas-Asf contact must be metres at the most , which might be too thin either a primary unconformity or a thrust to influence the trace-element composition contact. of the volcan ic rocks. Because of the gene­ rally much higher content of Pb in sedi­ As discussed above , the Gjersvik and Fund­ ments than in volcanites, lead isotopes are Sj0 volcanites are likely correlatives . Step­ very sensitive to any influence from sedi­ hens et al. (1993) used ages from Gjersvik ments (Hawkesworth 1982). This might to indicate a considerable distance betwee n expla in why the lead isotopes of the sulphi­ Baltica and the Gjersvik Terrane (see abo­ des give an ensialic signature, in opposition ve). This is negated by the Gula-Funds jo to the trace elements. stratigraphy and particu larly by the cont i­ nental Pb-isotope signatures of the mass ive Of fundamental importance in this context is sulphide deposits in the Fundsje Group. the status of the centra l part of the Gula Group and the nature of the Asli-Sinqsas Fieldwork, geochemistry and dating in the contact. The Sinqsas Formation consists Folldal area have shown that there is proba­ most ly of outer marginal sediments such as bly a continuous stratigraphy from at least turbidites, but of importance are also gre­ the upper part of the Gula Group and enstones and associated chert (Gula Gre­ upwards through the Fundsje Group. This enstone, Nilsen 1978). The greenstones means that the nappe contact betwee n the have formed in an oceanic environment Meraker and Gula Nappes in this area eit­ (Nilsen 1974), and pillow lavas have been her does not exist or that the contact is fur­ identified near Savalen (Rui 1977). The ther to the west, i.e. betwee n the Asli and massive sulphide depos its at Flott urn and Sinqsas Formations. More work, such as Bukkhammer are very low in radiogenic more dating of obvious pre-tectonic igneous lead (206/204 Pb of 17.909 and 17.806, rocks in the Sinqsas Formation and detailed respectively, Bjarlykke et al. 1993), suppor­ fieldwork along the Asli-Sinqsas contact is ting the ensimatic character of the volcan ic needed to answer these questions . rocks .

The Sinqs as Formation has been metamor­ Summary and concluding phosed partly to upper amphibolite facies remarks and contains migmatitic gneisses in the Meraker area (Wolff 1979). A possible syn­ 1. The bimodal character, large volume of tecto nic trondhjemitic intrusion in this forma ­ volcaniclastic rocks and geochemistry of the tion at Vakkerlien has been dated by the U­ Fundsjo Group volcanites favour their for­ Pb method on zircons to 509 +5/-4 Ma mation in an island arc setting. Intercalati­ (Klingspor & Gee, referred to in Stephens et ons of cont inental sediments in the volcani­ al. 1985) The trondhjemite is cross-cutting a tes and the lead isotopes of the mass ive nickelife rous metagabbro (Boyd & Nixon su lphides show that the arc must have for­ 1985) possibl y comagmatic with the Gula med close to a continent. Green stones (Nilsen 1974), thus providing a minimum age for the unit. The age fits 2. Based on field relationships, mineralogy very well with the metamorphi c Sm/Nd age and geochemistry, the Folldal Trondhjem ite of 505 ± 18 Ma of the Seve eclogites (Merk and felsic extrusive rocks are interpreted to et al. 1988). Thus , it is possible that the be comagmatic. Seve and Gula units have been involved in the same tectonic event around 505 Ma. 3. The large volume of felsic volcanites and NGU • BULL 426. 1994 Terje Bjerkgard & Arne Bjorlykke 7 3

trondhjemites in the Fundsjo Group and References their trace element chemistry show that they cannot have been derived from the same Baker, P.E. 1982 : Evolution and class ification of orogenic vol­ magma as the mafic rocks. A possibility is can ic rocks . In Thorpe .R.S. (ed.) And esites. John Wiley & that the felsic rocks have been formed by Sons. Chicheste r. 11-23. partial melting from the downgoing slab and Barker, F. 1979: Trondh jemite: definition, environment and hypo­ theses of origin . In Barker, F. (ed.): Trondhjem ites, dacites the mafic rocks from the mantle wedge abo­ and retsted rocks. Elsevier, Amsterdam 1979, 1-12. ve the slab. Bjerkg1lfd, T. 1989: Regionalgeologiske undersokelser i Vinge­ lenfeltet, Nord-Osterdsten, i det sorlige Trondheims feltet, 4. There are no differences between the med henblikk pa sulfidmineraliseringene. Unpubl. Cand. Fundsje Group and the upper part of the scient. Thesis , Univ.Oslo , 191 pp. Gula Group regarding degree of metamor­ Bjo rlykke, K.O. 1905 : Det Centrale Norges fjeldbygning . Nor. phism or structures. The contact between geol . unoers. 39, 595 pp.

the Fundsje Group and the Asli Formation Bjo rlykke, A., Vokes, F.M., Birkeland, A. & Thorpe, R.1. 1993: of the Gula Group has been observed to be Lead isotope systematics of strata-bound sulfide deposits partially gradational and is interpreted to be in the Caledon ides of Norway . Econ.Geol. 88, 397-417 primary. Bockelie, J.F. & Nystuen,J.P. 1985: The southeastern part of the Scandinavian Caledonides. In Gee, D.G. & Sturt, BA (eds.): The Caledonide Orogen - Scandinavia and related 5. Age determination and palaeontological areas . John Wiley & Sons , Chichester, 69-88 . data are in accordance with a primary upper Gula-Fundsje contact: the Dictyonema Boyd, R. & Nixon, F. 1985: Norwegian nickel deposits : A review . Geol. SUN . Finland, Bull. 333, 363-394 . schist in the upper part of the Gula Group is Tremadocian in age (488-505 Ma), while Claesson, S., Stephens, M.J. & Klingspor, I. 1987: U-Pb zircon dating of felsic intrusions. Middle K61i Nappes, cen tral the age of the Folldal Trondhjemite intruding Scandinavian Caledonides. Nor. Geol. Tidsskr. 68, 189-97. slightly older basalts is 488 ± 2 Ma (U-Pb on Crawford, A.J., Falloon, T.J. & Green , D.H. 1989: Classification, zircons). petrogenesis and tecton ic setting of bon inites . In Crawford, A.J. (ed.) Boninites and related rocks . Unw in Hyman, Lon­ 6. The lower or central part of the Gula don , 1-49. Group, the Sinqsas Formation, consists of Drummond, M.S. & Defan l, M.J. 1990: A mode l for trondh jem i­ outer shelf-marginal sediments and minor te-tonalite-dac ite genesis and crus taI growth via slab mel­ ting : Archean to modern comparisons. J. Geoph. Res. 95, greenstones with MORS character. A cross­ B I 3, 21503-21521. cutting, probably syntectonic trondhjemite has been dated to 509 +5/-4 Ma, compara­ Ellevold, K. 1976: En pe trografisk og geokjemisk undersokelse gjennom et utvalgt profil i den kambro-siluriske lagrekken i ble to the metamorphic age of 505 ± 18 Ma Berostettet. Unpubl. Cand. real Thesis, Univ. Oslo, 151 pp. for the Seve eclogites in northern Sweden, Ewart , A. 1979: A review of the mineralogy and chemistry of and is considered to provide at least a tecto­ Tertiary- Recent dacitic, rhyolitic, and related salic volcanic nic and metamorphic link between the Seve rocks. In Barker, F. (ed.): Trondhjemites, dacites and rela­ Nappes and the lower part of the Gula ted rocks. Elsevier, Amsterdam 1979, 13-121. Group. Ewart, A. 1982: The mineralogy and petrology of Tertiary-R ecent orogenic volcanic rocks: with spec ial refere nce to the ande­ sitic-basaltic compositional range . In Thorpe,R.S. (ed.) Andesites. John Wiley & Sons, Chichester, 25-95.

Acknowledgements Fox , J.S., Farquhar, R., Rui, I.J. & Coo k, N. 1988: Genesis of The authors are grateful to Odd Nilsen and Brian A. Sturt for basa lt-hosted mass ive sulphide deposits from the Trend­ the ir constructive criticism and comments on the first draft of heim and Sulitjelma districts, Norway : ore lead isotop ic the manuscript. Brian A. Sturt , Elisabeth A. McClellan and cons iderations.Mineralium Deposita 23, 276-285. another anonymous referee are thank ed for constructive com­ ments on the manuscript at the reviewing stage. Brian A. Sturt Fryer, F., Taylor, B., Langmuir, C.H. & Hochstaedter,A.G. 1990: and Donald M. Ramsay are also than ked for valuable corn­ Petrology and geochemistry of lavas from the Sumisu and ments during a field trip. The first author thanks Follda l Verk Torishima backarc rifts. Earth Plan. Sci. Letters, 100,161 -178. NS for accommodation and econo mic support dur ing field· work. Johann G. Heirn, forme rly at NS Norsulfid. is thanked for Frerden , J. 1949: Petrografisk og tekton isk beskrivelse trs Kjo li valuable comm ents and for introd ucing the field area to the aut­ -Ovre Tydal. Unpub l. Cand . real. Thesis, Univ. Oslo, 59 pp . hors . Gale, G.H. & Robert s, D. 1974: Trace element geoc hemistry of Norwegian Lowe r Palaeozoic basic volcanics and its tec­ tonic implica tions. Earth Plan. Sci. Letters 22, 380 -390 . 74 Terje Bj erkgard & Arne Bjorlykke GU - BULL 426. 1994

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Manuscript received January 1994; final typescript accepted May 1994,