Geology of the Folldal area, southern TrondheirrRegion Caledonides, NorwayTERJE BJERKGARD & 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 inthe lower part (the Sinqsas Formation) and graphitic pelites with subordinate marble and conglome rate in theupper part (the Asli Formation). The overly ing Funds]o Group is bimodal with basalts and rhyodacit es, com agma­tic gabbros and a large suovotcanlc trondhjem itic intrusion . The bimodality, high content of volcanlclastics, interca­lations of continenta l sediments, geochemistry and lead isotopes of massive sulphides reveal that the FundsjeGroup represents an island arc accumulation formed close to a continent. The contact between the Fundsje andGula Groups is interpreted to be primary, as shown by a gradual lithological transition and no break in metamor­phism and structural elements across the contact. The Folldal Trondhjemite in the Fundsjo Group has yielded a U­Pb zircon age of 488 ± 2 Ma. Relationships between published ages from the Gula Group and rocks in the SeveNappes 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 GulaGroup.

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.

IntroductionRocks of the Gula Group , which consistmainly of metasediments, occupy the cen­tral part of the Caledonides in the Trond­heim Region (Fig.1). The strat igraphic andtectono-stratigraphic posit ion of this group,

in relation to the volcanic and sedimentaryrocks of the Meraker and Steren Nappes , isimportant to any tecton ic model of the Cale­donides and has therefore been debatedsince at least the beginning of this century(see Gee et al. 1985 for a review).

Tcctonostratigraphy of theTrondhcim Region

West East

Steren Nappc

> c:r--oV/?/J r--oGulaN appc~ Gula Nappe ~ ~

~• Meraker Nappe ~

Essandsje-OyfjellNappe

Setra-Risbcrgct r·.....:··:·.·\·::··:l Remsklepp Nappe~ ~Nappe :.;'::.:.:-,::.:.:-,::.:.;.: R~

- ----- :I: t"!"lD 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) isindicated.

54 Terje Bjerkgard & Arne Bjorlykke

There is now a common opinion that therocks of the Storen Group in the StorenNappe (west of the Gula Group) had a diffe­rent origin to that of the rocks of the FundsjoGroup in the Merake r Nappe. They probablyrepresent , respect ively, an ocean floor andan island arc / marginal basin setting (Gale& Roberts 1974, Grenne & Lagerblad 1985,Grenne 1988). The intervening sedimentsof the Gula Group have a more uncerta inorigin. Based on lithology, sedimentologyand geochemistry, Guezou (1978, p.14)considered that the Gula metasediments inthe Dornbas district are of epicontinental ori­gin and "are devoid of either turbidites orrhythmic coarse clastic deposits ". Nilsen(1978) , however, reported that turbidites arefound in the central part of the group furthernorth , while graphitic phyllites with conglo ­merates and marbles occur both in the eastand in the west. Both back-arc and fore-arcpositions have been inferred, either for thewhole group or part of the group (Gale &Roberts 1974, Grenne 1988, Stephens &Gee 1989).

Major differences in metamorphic gradeand deformation (Wolff 1979) and age rela­tionships (see below) indicate that anunconfo rmity or thrust may exist internallywithin the Gula Group. Therefore, the natu-

GU • BULL 426. 1994

re of the contact between the Fundsjo andGula Groups and also the internal tectonicstructure of the Gula Group are critical forthe interpretation of the palaeotecton ic set­ting of the Gula and Fundsjo Groups and forthe existence of a separate Gula Nappe(Fig.1).

The Folldal area comprises both the Gulaand the Fundsjo Groups, cover ing about150 km2 around the village of Folldal in thecounty of Hedmark (Figs.1 & 2). Because ofextensive mining activity, mapping hasbeen carried out by several workers andwith different perspect ives (e.g. Bjorlykke1905, Marlow 1935, Page 1963, Heim 1968,1972, Pedersen 1979 and by geolog ists atFolldal Verk A/S).

In this paper, data on petrography , structu ­res and tectonics, petrology and geoche­mistry and one U-Pb zircon age determ inati­on will be presented from the Folldal area.The intention is to throw light on the natureof the contact between the Gula and Fund­sjo Groups; and further , to compare petro­graphy, geochemistry and the age of thevolcanic rocks with work done elsewhere inthe 'greenstone belt', especially in theRoros-Meraker district. These data alsoform a basis for a discussion of the origin of

Tec tonic Co mplexTec tono stra tigrap hy and

Terrane (Stcphens & Gce 1989)(Gee e t al. 1985)

Lithological Uni ts (Mod ifiedfrom Nilscn 1988)

Metamorphic Gula , appc;Exotic tcrranes complexes Gula Groupderived fromoutboard of Baltic,

Oceanic sequencesMeraker Nappc:

UPPE R Fundsj o Group. SIIMmo GroupALLOCHTHON

Suspect terr anes:probably outer

Seve NappcsEssandsjo-Oyfjell Nappc;

tcc t. shonened Aursund Groupmargin of Baltica

Remsklepp Nappc:

TectonicallyPlatformal and MIDDLE Augen Gneiss Unit. Tynset Gro up

shortened marginmiogcoclinal ALLOCHTHO

of Balticasediments and Rondanc NappePrecambriancrystalline rocks LOWER Osen-Roa l appc

ALLOCHTHON

Platformal sedimcnts PARAUTOCHTHON/AUTOCHTHONCratonic Baltic.

Prec, cryst. basement PRECAMBRIAN CRYST, BAS EM E1-·/T

Table 1. The divisions in tectonostra­tigraphy. terranes and litholoqicalunits in the Southern TrondheimRegion.

NGU - BULL 426. 1994 Terje Bjerkgard & Arne Bjarlykke 55

GULAGROUP

Metarhyodacite

Metagabbro

Metaba salt

Metasediments,partly graphitic.

Tuffite

Trondhje mite

wl mali c xenolith s

Major fault

Strike and dip

Sample localityfor da ting

Sulphide mine,*

~Micaschist s

o 0 0 0 Conglomer ate0000

SULAMo GROUP

d - Sheeled dyke complexp - Pillow lavas

LEGElI:D

TYNSET & AURSUND GRO UPS

RJNDSJ0 GROUP

r::'::'::':'~ Mica schists and:';::'.:::'.:::'.::. quartzites. subordinate:.;::.;.,'..:::.;:. ultrarnafic rocks and

greenstones

Ii <=0_"_"-:, ,". Asli Formation, pelites

Singsas Forma tion. bandedquartzites and mica schists

I kmI------l

Fig.2. Simplified geolog ical map of the Folldal Area.

56 Terje Bjerkgard & Ame Bjorlykke

and connection between the Gula Groupand the Fundsjo Group and finally for theinterpretations and understanding of thesetting of the ore deposits in Folldal, workwhich is currently in progress.

Geological sett ingThe Caledonides of the southern Trond­heim Region comprise tectonic and litho­logical units derived from several d i ffe r~nt

environments, thrust upon a Precambnanbasement with a thin, mostly Cambrian,sedimentary cover . According to Gee et al.(1985), the tectonostratigraphy of this regi­on can be divided into three major alloch­thonous complexes; namely the Lower ,Middle and Upper Allochthons (Table 1,Fig.1). A new division of units has be~n pr?­posed for the Scandinavian Caledonides In

the last five years based on the terrane con­cept (e.g. Roberts 1988, Stephens 1988).Terranes are defined on the basis of theiroverall geological features and histories,which are distinctive from those of neigh­bouring terranes. In the Scandinavian Cale­donides, outside of the tectonically shorte­ned shelf and miogeoclinal sequences rela­ted to Baltica there are several suspect andexotic terranes of uncerta in palaeogeo­graphic origin (Table 1).

The Folldal area (Fig.2) comprises unitsoccurring above the arkosic rocks of theRondane Nappe (Bockelie & Nystuen 1985)in the Middle Allochthon . The structurallylower units in the area are dominated byargillaceous sediments occurring both in theRemsklepp Nappe (Wolff 1979) of the Midd­le Allochthon and in the Essandsjo-0yfjellNappe (Nilsen 1988) of the Upper Alloch­thon. Most of the area is occupied by volca­nic and volcaniclastic rocks of the MerakerNappe and diverse sediments of the GulaNappe, both units belonging to the Trend ­heim Nappe Complex . Most workers agreethat these two units show interna l strati­graphic inversion (Rui 1972, Nilsen 1988).

The Gula Group

The Gula Group (Kjerulf 1876) compr ises

GU - BULL 426. 994

two sedimentary units, the Asli and SinqsasFormations, (Nilsen 1978) in the easternTrondheim Region. The Gula Greenstone, ametabasalt unit, is common ly found betwe­en the two and also within the Sinqsas For­mation (Nilsen & Mukherjee 1972). All unitsare represented in the Folldal area andwere described by Heim (1972) under thename Storho i Formation. The Gula Groupconstitutes the Gula Nappe according toRoberts (1978) or Gula Nappe Complex(Gee et al. 1985). Because of possible con­fusion with the lithological term Gula Group ,the terms Gula Nappe and Nappe Complexshould really be avoided as names for thistecton ic unit. They will , however, be used inthis paper because of their common usagein the literature.

The Sinqsas Format ion consists of psam­mitic to semipelitic quartz-p lagioclase-m icaschists. They show typically a rhythmicalvariation in quartz and mica content as wellas grain size, on 0.5 - 2 m scale, from near­ly pure quartz ites to mica schists. Porphyro­blasts of garnet and c1 ino-amph ibole arecommon . No primary sedimentary structu­res have been found in the Folldal area, butin other places, e.g. the Roros area, gradedbedding, cross bedding and load casts arecommonly observed in this unit and therocks have been interpreted as turbidites(Rui 1972, Nilsen 1978).

The Asli Formation is dominated by semi­pelitic garnet-chlorite-bear ing mica sc~ i.sts .

Based on a varying content of phyllosilica­tes and quartz the formation might be divi­ded in a lower unit of quartz-rich schists anda higher unit of partly graphitic phyl.losilica­te-rich schists, a division also noticed byHeim (1972). Quartz , biotite and muscoviteare main phases while chlorite, garnet andin some cases c1 ino-amph ibole are subordi­nate. Apat ite, tourma line and zircon are themost important accessory minerals in theseschists. Benches of metasandstone, severalmetres thick, are intercalated in the upperunit. About 250-300 m below the Gula­Fundsjo contact , horizons of about 20 m­thick calcite marble are found at severallocalities at the same strat igraphic level.The marble is locally quite pure and coarse

NGU - BULL 426.1 994

Fig.3. L2 - lineation in calcareous conglomerate of the GulaGroup. The c1asts are mostly pure marble. while the matrix iscalcite-rich metasiltstone.

grained , though at some localities it has ahigh content of quartz and commonly fuch­sitic muscovite , giving it a green colour .

About 500 m below one of the marbles inthe stream Husombekken two lenses ofconglomerate are found (the Husom Cong­lomerate , Heim 1972). They are at least 15and 25 m thick and compr ise mainly clastsof pure calcite marble and quartzite in amatrix consisting mainly of pale green clino­amphibo le and biotite (Fig.3). Clasts ofquartz-albite rock, greensch ist, gabbro andbiotite schist are also present. Quartz , mus­covite and calcite are subordinate matrixminerals. Although no pre-pebble foliationhas been seen in the clasts , this might bedue to subsequent deformation and meta­morphism . The clasts are extremely elonga­ted and ruler-shaped with maximum long­axes up to 1 m. A similar but less deformedconglome rate, also with calcite marble asclast material, is found in the stream Lang-

Terje Bjerkgard & Arne Bjorlykke 57

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

A thin (15-20 m) horizon of fine-grainedamphibol ite separates the Asli and SinqsasFormations at one locality. It is at least 600m in strike length and contains amphiboleand epidote as main phases , while quartz,chlorite and ilmenite are subordinate. Themineralogy and position of this horizon bet­ween the sedimentary units fits with theGula Greenstone .

The Fundsje Group

In the Folldal area, the Meraker Nappe(Wolff 1979) is represented by the domi­nantly volcanic Fundsjo Group together withthe stratigraphically overlying sedimentarySularno Group. The volcanic unit can be fol­lowed all the way from Meraker to Grimsda­len east of Dornbas, and has a variety oflocal names (e.g. the Hersjo Formation isthe name given to the group as a whole inthe Roros area (Rui 1972, Grenne 1988)).To underline the continuity of the unit theterm Fundsjo Group will be used here.

The Fundsjo Group widens in outcrop fromabout 1 km in thickness west of Alvdal to 5­8 km in the Folldal area (Fig.1). The reasonfor this is probably a combination of foldingand primary differences in thickness , andespecially due to the presence of a large fel­sic intrusive body near the stratigraphicbase of the unit. The unit can be divided intoessentially three lithologically different units:1) Metabasalts and gabbros, 2) tuffitic andmetased imentary rocks and 3) felsic intrusi­ve and extrusive rocks.

Greenstones, interpreted as metabasalts ,are found mostly near the base of the unit,especially in the northeastern part of thearea. They occur below, intercalated within ,and above the large felsic intrusion (Fig.:?Thin horizons are also found intercalatec' In

the overlying tuffites. The metabasa lts aredark green, fine-grained and homogeneouswith a weak foliation. White mm-thick stre­aks of albite and minor quartz in a greenmatrix of chlorite and amphibole define themain texture of the rocks.

58 Terje Bjerkgard & Arne Bjorlykke

Because of the high degree of deformationin the area primary textures in the basaltsare rare. Structures resembling pillows havebeen found in the mine area just north of thevillage (p in Fig.2). West of Bukletten a pos­sible sheeted dyke complex has been foundin a road cut (d in Fig.2). Numerous fine­grained and metre-thick dykes intrude oneanother within the same strike trend. Becau­se of deformation the dykes are now paral­lel to the regional foliation in the area. Insome places it is possible to distinguish theindividual dykes by the presence or absen ­ce of plagioclase phenocrysts, and somedykes have chil led margins.

Horizons of metabasalts heavily studdedwith mm to cm plagiociase phenocrysts arecommon both east of Bukletten and in hori­zons contained in the felsic intrusion. Thebest exposure is probably in the mine areajust north of the village: Here one can see asharp contact between a basalt without phe­nocrysts and a basalt with 30-40 % whitephenocrysts of plagiociase (1-10 mm) in anamphibole-rich matrix. Eastwards from thecontact the content and size of the pheno­crysts gradually decrease to 1-5 mm, beco­ming more dispersed, until they finally dis­appear. The thickness of this phenocryst­bearing zone is about 2.5 m.

Clino-amphibole (actinolite and actinolitichornblende), epidote, albite and chlorite arethe main mineral const ituents. Quartz andcalcite are generally subordinate , while bio­tite, sphene and/or rutile and haematite arethe most common accessories. Epidote/cii­nozoisite replace plagiociase to varyingdegrees.

The metagabbros have the same mineralo­gy as the metabasalts, consisting of unori­ented 0.1-1 mm grains of actinolite/actinoli­t ic hornb lende , porphyroblastic epidote andfine-grained recrystall ised albite in mm-lar­ge aggregates. These aggregates are pro­bably a result of the breakdown of largercrystals of more Ca-rich plagioclase, Canow taken up by epidote. Amph iboles mighthave formed from pyroxenes. Due to theextensive deformation and recrystallisation ,no ophitic texture has been preserved and

NGU . BULL 426. 1994

only the 'spotted' texture distinguishes themfrom the metabasalts in the field.

At two localities small lenses of ultramaf icrocks have been found associated with ametagabbro and metabasalt, respectively.These conta in talc , chlorite and up to cm­sized euhedral crysta ls of magnes ite.Growth of colourless c1ino-amphibole (tre­molite ?) is observed at the contact with thecountry rocks. Both lenses have beenmined for talc.

The felsic igneous rocks of the Funds]oGroup , on grounds of texture , strat igraphicposition and contact relationship to otherlithologies, are divided into an intrusion andsevera l extrusive equivalents. The intrusionoccurs in the lower part of the FundsjeGroup. Small apophyses and aplit ic dykesintrude the metabasalts (FigA) . Xeno liths ofmafic rocks are found in the upper part ofthe intrusion (Fig.5), occurring over a str ike­length of at least 2 km (shown with blackdots in Fig.2). The first deformation phaserecognised in the area , which produced theregional foliation , has affected the intrusion,

FigA . Drawing after a photograph of trondhjemitic aplite intru­ding metabasalt at an angle to the regional foliation S1 (shownby strike-dip symbol).

NGU - BULL 426. 1994

Fig.5. (a). Amphibolitic xenoliths in the trondhjemite intrusionbelow the contact to the stratigraphically overlying tuffites. Thexenoliths are elongated parallel to the Sl -foliation. (b) Close-upof the xenoliths. showing the foliation (Sl) affecting the contactbetween the xenoliths and the felsic host.

the dykes, as well as the xenoliths (FigA, 5b). This shows that the intrusion was empla­ced before the thrusting and nappe formati­on.

The intrusion has a strike extension of morethan 20 km. It appears to consist of threeindividual sills with maximum thickness of800 m, divided by metabasaltic horizons .Because of its resistance to erosion theintrusion is reasonably well exposed . In out­crop it is quite homogeneous, thoughsomewhat variable in grain-size. A faint foli­ation is present in the rock where it is morefine-grained (1 -3 mm). It is dominated byalbite and quartz and is grey-white in colour.Red garnet (1-3 mm) and up to cm longneedles of green amphibole are the othermajor minerals. Muscovite and epidote areSUbordinate, while chlorite and apatite arecommon accessor ies.

Terje Bjerkgard & Arne Bjorlykke 59

The intrusion has earlier been called 'granu­lit' (Bjarlykke 1905). Goldschmidt (1916)kept this name but classified it as a trond­hjemite. The mineralogy allows it to be clas­sified as a trondhjemite and this is suppor­ted by geochemistry (see later). As it is verywell exposed immediately east of the villagethe informal term Folldal Trondhjemite willbe used for the intrusion.

It should be stated that trondhjemites in theTrondheim Region were intruded before,during and after the time of nappe formationand thrusting. According to Size (1979), fourperiods of trondhjem ite intrusive activity aredistinguished in the Steren area, endingwith the post-tectonic trondhjem ite s.s.Similarly, in the Tolga area, trondhjemites oftwo generations have been observed(Bjerkqard 1989). Trondhjemitic dykes of anearly generation have been affected by theD2 deformation and are schistose , while thelarger bodies in the area (the best exampleis the Tolga Trondhjem ite) cross-cut allCaledonian structures.

The felsic extrusive rocks occur as confor­mable layers, mostly in the tuffites in thestratigraphically upper part of the group(Fig.2). These layers are 10-30 m thick whi­le their lengths can be up to several kilo­metres. The contacts to the surrounding tut­fites are always very sharp. A matrix of veryfine-grained quartz and albite is dominating ,giving the rock a grey-white colour and aweak foliation. In this matrix partly idiomor­phic, sharply defined phenocrysts of albite(1-3 mm) and irregular aggregates of quartzoccur and are evenly distributed. As in thetrondhjemite, garnet porphyroblasts andneedles of amphibole are common . Epidoteand muscovite are subordinate , while chlori­te and apatite are accessory minerals.

The upper part of the group is dominated bypale greyish-green chlorite schists with apronounced schistos ity. Alternat ing calcite­quartz laminae, fine-grained lenses ofquartz-albite and thicker chlorite layers defi­ne a characteristic lamination on mm to cm­scale (Fig.6). Where deformation was lesssevere , lenses of quartz-albite and chlorite­amphibole rocks occur and are interpreted

60 Terje Bjerkgard & Arne Bjorlykke NGU . BULL 426. 1994

Fig. 6. Laminated rnafic u ite. wi halternating layers dominated bychlorite and calcite-quartz on cmscale. An Fr-tolc refolded by F2 canbe seen near the pencil.

as clasts derived from the volcanic rocks.This observation, together with a high con­tent of calcite and chlorite and a laminationwhich is possibly primary, strongly suggeststhat these rocks are volcan iclast ic or tuffiticrocks.

Quartz, albite, chlorite, epidote/c1 inozoisiteand calcite are the main constituents whilstmuscovite and amphibole usually are sub­ordinate. Amphibole is commonly found on

the planes of schistosity, somet imes orien­ted. Garnet, biotite, magnetite, sphene andapatite are the most common accessories,garnet occurr ing in porphy roblasts up to 1cm in size.

The contents of the main and subordinatephases vary and the tuffites are classifiedas mafic or felsic on the basis of the ratiobetween mafic phases such as chlorite, epi­dote and amphibole and the felsic phases

30 m

20

10

o

225 200 175 150 125 100 75 50 25 Om

D ~r~~il~~j~:::~h:'/I·!··I Fclsic tuffite

I:=:=:=1 Metabasalt [::::::::::::::::::::::::1 Mafic tuffitc

• Clastic scdimcnts

~ Shear sense

Chlori tc-rnica schist!mctagrcywacke

Graph ite schist

Quartzite

Fig.? A stream profile (S orbekken) showing the diversity of tuffites and the contact with the trondhjemitic intrusion. Notice he mere­ased degree of shear deformation towards the contact.

NGU - BULL 426. 1994

quartz and albite . The tuffites are textura llydominated by a fine-grained quartz-albitematrix together with parallel-oriented phyllo­silicates. More or less rounded mm-sizedc1asts of albite are found in this matrix, pro­bably representing phenoc rysts derivedfrom the volcanic rocks which have beenabraded during transport.

Intercalated in the tuffites are quartzites,graph itic pelites and more coarse-grainedchlorite-mica schists (metagreywacke).These have thicknesses from less than 1 mto more than 25 m. Commonly the graphiteschists grade into the mica schists with acorresponding decrease in the content ofgraphite . Quartz , muscovite and chlorite arethe main mineral phases in the sedime ntaryrocks while biotite , garnet and, in morecoarse-grained varieties also albite, aresubordinate. Fig.7 shows the typical divers i­ty from felsic to mafic tuff ites and interca lati­ons of clast ic sediments in Storbekken eastof Folldal.

The Sularno Group

A highly deformed conglomerate with c1astsof quartzite, greenschist, quartz-albite rock,chlor ite-mica schist and marble in a chlorite­amphibole matrix is situated at the contactto the Funds]e Group. The greenschist andquartz-albite c1asts may have been derivedfrom the Fundsje Group. Graphitic phyllitesand mica schists are folded into the conglo ­merate and a thin horizon of calcite marbleoccurs at the lower contact. These lithologi ­es constitute the upper part of the RandanGroup of Heim (1972). The stratigraphicposition and lithologies represented in thec1asts makes a correlation with the Sceter­sja Conglomerate in the Heres area (Rui1972) and the Lille Fundsjo Conglomeratein the Meraker area (Wolff 1967) probable .These conglomerates are interpreted tomark a major unconfo rmity between theFundsje Group volcan ites and the over lyingsediments (Hardenby et al. 1981, Grenne &Lagerblad 1985).

Structures and tectonicsStructures ascribed to two phases of folding

Terje Bjerkgard & Arne Bjorlykke 61

are recognised in the Folldal area in additi­on to later fractures and fault zones. Themost pronounced structure is a penetrativewesterly dipping foliation or schistosityaffect ing all units. The data from the Fund­sje Group are shown in Fig.8 a. There is ageneral trend of increasing dip of the foliati­on planes westwards, while the strike isnearly constant. On a micro-scale this struc­ture , here called S1, is due to a parallelalignme nt of mica and chlor ite. Recrystalli­sation of quartz to elongate grains and anincreasing grain size are also important inthe development of the foliation.

An earlier foliation or possibly the primarySO-layering is seen to be isoclinally folded,especially on the micro-scale. At the meso­scale, hinge-zones of these folds, which areF1 structures , are isolated and disrupted,showing the S1-foliation to be partly a trans­position structure as well as the axial planefoliation. On the mega-scale, structures ofthis phase are developed as isoclinal andrecumbent folds with flat-lying axes trend ingENE-WSW. The folds are responsibl e for arepetition of the different lithologies acrossstrike (Fig.2).

The 02 deformation phase is representedby meso-scale folds (about 0.1-30 m inwavelength) of a more open style, small­scale crenulations and by mineral lineati­ons. F1-folds are also seen refolded by F2(Fig.6). Crenulations are generally welldeveloped in the incompetent schists andtuffites . 0 2 fold axis orientations for theFundsje and Gula Groups are shown inFig.8 b. The axes in the Fundsje Group arequite well concentrated and have a meanplunge of 410 towards 2690

• Measurem entsof axial planes to some of these folds showstrikes between 020 0 and 1100 and dipsvarying between 400 and 750 to the northand northwest. The fold axes and lineationsin the Gula Group show a larger spread, butare within the same range as the Fundsjedata.

Other lineations in the Fundsjo Group havethe same trend and plunge as the fold axes(Fig.8 b). The lineation s are due to recrys­tallisation of quartz , growth of mica and

62 Terje Bjerkgard & Arne Bjorlykke

a)

NGU . BULL 426. 1994

Sl -Folia tio ns in theFundsjo Gro up

Contours per 1% area :

o 1-2 %

I 1 3-6 %

h:::: :1 7- 10 %

It : ~ 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 G ro up D2·structures in th e G ula G ro up

•o

Lincations

Fold axes} N = [46

Lineations and fold axes= 22

Schmidt Equal Area Projection

Fig.a. (a) S t -Ioliations (contoured) of the Fundsjo Group. (b) F2· folds and lineations of the Fundsjo and Gula lirou~~ .

especially amphibole parallel to the b-axisof the 02-phase. Growth of garnet was alsoimportant and accompanied biotite. Theseminerals (garnet, biotite and amphibole)represent the highest grade of metamor­phism, namely the transition from greens­chist to amphibolite facies (see later), andwere formed during 0 2.

The conglomerates of the Asli Formation inthe Husombekken stream section were alsoaffected by this deformation phase as themarble c1asts are extreme ly elongated andruler-shaped (Fig.3) parallel to the a-axis.The c1asts have lengths (X-axes) up to 1metre and the ratio between the X and Zaxes is up to 60 : 1.

NGU - BULL 426, 1994

Development of mylonitic rocks is mostlyrestricted to nappe boundaries, but has alsooccurred internally within units, especiallywhere there is a strong contrast in compe­tence between lithologies. An example isthe contact between the Folldal Trondhjemi­te and the structurally underlying tuffites(Fig.7): within a 50 m zone below the con­tact , the mafic tuffites are transformed intofine-gra ined, mylonitic, banded schists. A12 m-thick layer of felsic tuff ite or fine-grai­ned trondhjem ite aplite has been transfor­med into a rock with a texture resembl ingaugengneiss with rounded quartz-feldsparclasts in a chlorite-amphibole-epidotematrix. Lenses or 'fishes' of less deformedmaterial show a top-to-the-SE movement.The deformation can be traced to about 90m below the contact , where mafic tuff iteshave been displaced along a reverse faultat the contact to underly ing metabasalts,the movement sense shown by lenses / 'fis­hes' of quartz. The trondhjemite has alsobeen affected by this deformation. Usually itis coarse grained with a very poorly develo­ped foliation , but near this contact it is veryschistose and mafic xenoliths are stronglyflattened and elongated . This deformationcan be traced for at least 40 m into the intru­sion.

Contact between the Fundsje andGula Groups

The contact between the Gula and FundsjeGroups is exposed at several localit ies inthe Grimsdalen, Folldal and Savalen areas.In Grimsdalen there is a gradual changeover about 50 m from tuffitic chlorite schistsbelong ing to the Fundsje Group to peliticgraphite-chlorite-mica schists of the GulaGroup . This is in contrast to the view ofPedersen (1979) who claimed there to be atectonic contact on the basis of observati­ons of mylonitic rocks in both groups . In thestream Husombekken in Folldal , the contactis between tuffite and slightly graphitic peli­te. The contact is quite sharp, but markedby an increasing content of muscovite anddecreas ing content of chlorite in the tuffitewithin a sequence of about 40 cm. Somedifferential movement in the tuffite is recor­ded by the presence of rotated c1asts of

Terje Bjerkgard & Arne Bjorlykke 63

epidote , 'fishes' of quartz-albite and by astrongly preferred orientation of amphibole.

At Sivilvangen east of Savalen (Fig.1),severa l intercalations up to 5 m-thick of bothchloritic mica schists and dm-banded mafic­felsic tuffites are found up to 150 m into thegraphit ic pelites of the Gula Group. Thecontact itself is gradual and thin intercalati­ons of pelites are found at least 15 m intothe tuffites of the Fundsje Group. Both thetuffites and the pelites at the contact consistof extremely fine-grained mylonitic quartzand phyllosilicates. This deformation is tra­ced up to 5 and 25 m away from the con­tact, respectively . The deformation is alsoshown by dm-Iong and cm-thick lenses ofpsammitic quartz- rich schist in the pelites .The intercalations might possibly be due tosmall-scale folding or thrusting but may alsobe relict primary features overpr inted by alocal development of mylonitic rocks.

About 3 km north of Sivilvangen , a lens ofmetagabbro occurring about 100 m insidethe Fundsje Group is strongly deformed andmore or less transformed to a mylonitic epi­dote-chlorite schist. This metagabbro issurrounded by tuffites, and the deformationis thought to be a result of a high competen ­cy contrast. The contact between the Gulaand the Fundsje Group is found 100 m westof and structurally above this locality , whereit seems to record a gradual transition froma felsic, tuffitic, quartz-albite rock of theFundsje Group to graphitic pelite of theGula Group.

McClellan (1993) has described the contacteast of Savalen as a ductile shear zone withwidespread mylonitisation textures in therocks of both groups.

Metamorphism

The metabasalts of the Fundsje Group havea typical prograde mineral assemblage ofhornblende + albite + epidote + chlor ite +quartz ± calcite ± biotite , while the tuffiteshave chlorite + albite + quartz + epidote +calcite ± hornblende ± garnet ± muscovite ±biotite . Amphibole and biotite are observedto have formed at the expense of chlor ite.

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-

Garnets have commonly overgrown the S1-de, place the metamorphism at the transiti-

Main elements (wt.%)Mafic rocks

Si02 Ti02 AI20 3 FeO(t) ~1nO MgO CaO xszo K20 £>205 L.O.! SumG7·mg 47.9 0.50 14.6 9.0 0.20 11.8 9.7 1.5 0.59 0.08 3.0 99.0G8· mg 48.1 0.8 1 15.7 10.7 0.27 9.1 7.9 3.3 0.19 0.10 3.5 99.6GI -mb 54.0 0.62 16.0 8.9 0. 19 5.3 9.7 2.5 0.56 0.07 1.5 99.3G2-mb 44.8 0.47 14,4 9.7 0.24 7.0 14.7 2.0 0.13 0.05 6.9 100 .3G3-m b 46,4 0.64 15.2 11.0 0.26 8.0 15.1 0.9 0.22 0.09 1.2 99.1Gd-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 .6G5·mb 53.7 0.25 10.9 8.9 0.22 10.5 9,4 1.1 0.00 0.05 4.2 99.2G6-l11b 52.6 0.26 12.6 9,4 0.20 10.1 7.8 3.7 0,41 0.04 2.6 99.8G9-mb 52.9 0.88 15.7 11.0 0.27 4.8 10.8 I.3 0.01 0.09 l.7 99.5GIO-mb 46.2 1.02 15.6 10,4 0.27 8.9 1l.7 2.6 0.20 0.08 2.3 99.3GII -mb 49.2 1.50 13.8 12.9 0.25 6.9 11.5 2.0 0.15 0. 12 1.0 99.3G 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 99.7Gl4-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 rocksOl -lrh 77.3 0' - 12.0 3.1 0.14 0.3 1.9 4.0 0.66 0.05 0.3 99.80 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 98.906-trh 74.5 0.20 12.5 2.7 0.09 0.5 2.0 4.9 0.22 0.05 0,4 98.20 7-trh 74.8 0.16 12.7 2.5 0.10 0.5 1.1 4.5 0.78 0.05 0.6 97.846-92-trh 74.7 0.27 12.5 2.9 0.04 0.6 2.7 4.9 0.3 1 0.06 0,4 99.203 -rh 80.7 0.22 10.5 1.6 0.09 0.3 2.3 3.6 0.13 0.06 0.3 99.804-rh 76.5 0.19 12.6 2.5 0.10 0,4 l.7 3.5 1.28 0.06 0.7 99,42462-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

~1o l'b Zr Y Sr Rb Pb Cu Zn Cr Co l'iG7-mg 13 4 40 11 177 11 0 0 61 690 57 194G8-mg 12 4 58 23 133 5 6 0 56 372 5 1 152G I-mb 14 3 4 1 19 128 12 0 6 60 300 38 58G2'mb 14 3 43 16 109 5 2 54 78 463 42 117G3-mb 16 3 53 18 69 6 9 78 609 64 170G4-mb 14 4 44 1I 47 3 31 74 634 80 109G5-l11b 15 3 40 7 83 2 30 67 1419 60 295Go-mb 12 5 46 5 4 1 8 12 I 1315 88 277G9 -mb 14 4 54 25 II I 3 2 197 70 73 50 18G IO-l11b 14 3 45 30 324 4 18 0 54 358 58 125G l l -mb 13 4 70 45 246 4 0 0 49 131 62 63G l 2-mb 15 3 52 23 70 2 I 11 92 44 32 15Gl4-mb 14 3 3 1 20 318 9 0 5 298 44 105

Fe/sic rocksDI-Irh 11 6 93 35 70 17 4 0 62 99 3 205 -lrh 1I 6 102 45 96 7 I 0 18 80 3 4Do-trh 7 5 168 74 135 6 4 0 22 l OO 12 3D7-tdl 10 6 113 27 79 18 6 0 25 93 I 346-92-trh I 3 69 29 125 9 8 2 14 126 5 6D3-dl 13 5 64 24 155 4 3 0 29 86 2 I0 4-dl 7 7 186 53 69 14 9 0 56 82 2 42462-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 inte rca lated in thetrondhjemite . Ana lyses have been done at Univer sity of Oslo .

NGU • BULL 426. 1994

foliation as defined by inclusions of epidote,chlorite and quartz. The parallel alignmentof amphiboles and simultaneous growth ofbiotite and garnet in F2-folds suggest thatthe peak of metamorphism was reachedduring this phase. Some later retrogressionis shown by local chloritisation of the amphi­bole.

In the Asli Formation of the Gula Group, thetypical prograde paragenesis is quartz +muscovite + biotite + chlorite ± albite ± gar­net. This is a typical assemblage of theupper part of the greenschist facies, accor­ding to Winkler (1979) and shows that therewas no apparent difference in metamorphicgrade between the upper part of the Gulaand the Fundsje Groups. The metamorphicgrade increases westwards and stratigrap­hically lower in the Gula Group, as shownby the appearance of kyanite and staurolite,as well as andesine in the Sinqsas Formati­on (Heim 1972, Pedersen 1979).

Terje Bjerkgard & Arne Bjarlykke 65

Geochemistry of igneous rocks

The geochemical compositions of both fel­sic and mafic igneous rocks in the FundsjeGroup (Table 2) show a clear chemicalbimodality. The highest value of Si02 in themetabasalts is 54 wt.% (average 50.2 %) ,while the lowest in the felsic rocks is 74.5 %(average is 76.8 %) .

Mafie rocksMost samples of greenstones analysed,including two metagabbros , are from thethick unit in the Bukletten area (Fig.2), onesample is from the dyke complex and threefrom the horizons intercalated in the trond­hjemite. Based on CIPW-norm calculations(not shown) the rocks are classified as olivi­ne-tholei ites to quartz-tholeiites. Half of thesamples from Folldal have a loss-of-ignition(L.O.I.) of more than 2.5 % and should beconsidered as altered (Le Bas et al. 1986).By comparison with MORB and low-K tho-

a) 10

r§< 8~o{'l

::<: 6+o{'l

""Z 4

2 Picro-basalt

Basalt

. ".Basahi c

andesitc•

Trachydaci te

Dacite

Andesitc

Rhyolite

+

45 50 55 60 65 70 80 85Si02wt%

ToJ IOOb) c)

400 ~ "lo~

350 Fig.9 (a) Classification of the volca-

-::- 300nic rocks in Folldal from the Fundsjo

wPB §: 250Group by the total alkalis vs. Si02

~ '00diagram (after Le Sas et aI.1986).

@' > ~50Filled circles· metabasalt; open cir-cles - trondhjemite; crosses -rnyoda-

. .. 100 cite. (b) The metabasalts plotted in•• KT. OFB. 50 the Ti-Zr-Y discrimant diagram of

• • • CAB

CAB 0 Pearce & Cann (1973). (c) The

0 2000 4000 6000 8000 10000 metabasalts plotted in the Ti·V dis-

Ti (ppm) criminant diagram of Shervaisz,. Y"3 (1982).

66 Terje Bjerkgard & Arne Bjarlykke GU . BULL 426. 1994

Si02Ti02AI203FeO(t)MnOMgOCaONa20K20P205L.O.I

MoNbZrYSrRbPbZnCrCoNiV

K1RbTiNSrlZ rRb/SrK20 /Na20RblZ r

Number ofAnalyses

Metabasalt

Folldal '

50. 2 (3.2)0 .7 (0 .4)14.5 (1.5)10.3 (1.3)

0.23 (0 .0 .3)7 .5 (1.9)10.9 (2.8)2.4 (1.2)0.2 (0.2)

0 .08 (0 .02)2.7 (1.7)

14 (1)4 (0.5)47 (10)20 (11)

140 (100)4 (3)3 (5)

69 (12)513 (444)

56 (17)122 (88)258 (66)

29315.03.2

0.053

11

Metagabbro

Folldal47.9-48 .1

0.5-0.814.6-15 .79.0-10.70.2-0.3

9.1-11.87.9-9.71.5-3.30.2-0.6

0.13.0-3 .5

12-134

40-5811-23

133-1 775-110-5.656-61

372 -6905 1-57

152-194165-239

296 - 44118.2-20 .32.3-4.4

0.040-0.063

2

Trondhjemite

Folldal

74 .5-77.30.15-0 .2112.0-12.72.5-3.5

0.09 -0.140.3-0.61.1-2.23.7-4.90.2-0.8

0.05-0 .070.3-0 .6

7-115-6

93 -16827-7 4

70-1356-181-6

18-6280-100

1-122-4

6-13

3 14 - 714

0.043-0.2480.045 - 0 .1710.034-0.186

4

Extrus ive

rocks Follda l

76 .6-80.70.19-0.2210.5-12 .6

1.6-2.50.09 -0. 10

0.3-0.40.9-2.33 .5-5 .00.1 -1.3

0.06 -0.070.3-0.7

7-135-7

64 18624-63

69 -1554-143-11

29 -5682-97

2-91-46-9

250 - 755

0.027-0.2030.036 -0 .37 10.066 -0 .076

3

N-MORB

50.4 51.62

15.2610.4 3

7.5811.302.680.11

3.5104361131.260.49

25147150

70028-3 5'1.09

0.011

26

LKT

SW Pacific

50.730.8317.389.380.196.9711.5 12.060.260.09

1.730 .815.82244.14.0

78 .282 .435 .735.5286

52617.47.3

0.0 18

43

IAG

Shich ito R.

70.0-74.60.2 5-05812.9-13 .7

1.8-3 .70.10-0.18

0.4-1.01.3-3.04.6-5.50.9-1.3

0.04-0. 13

171-25347-63

45-17610.6-20

3-457 - 88

o3-380-7

0-14

498 -666

0.065-0.4000.169-0 .28 10.059-0.103

4

Table 3. Geochemistry of the intrusive and volcanic rocks in Folldal. Values of MaRS (Hofmann 1988). Low-K Tholeiite (LKT) (Ewart1982) and Island Arc Granite (IAG) (Ikeda & Yuasa 1989) are shown for comparison. 1Standard deviations in brackets . 2Ti/V fromHawkins et al. 1990.

leiites (Table 3), the Folldal greenstoneshave normal Na20 and Sr contents whileCaO is lower than both MORB and LKT,suggesting some alterat ion, probably byinteraction with sea water (spilitisation). Thisis probably also one reason why in the totalalkali - silica diagram (Fig.9 a) the maficrocks spread out in the fields of basalt andbasaltic andes ite. The close associationand similarity in geochemistry between themetagabbros and the basalts (Tables 2 and3) suggest that they belong to the samem agm atic system , w ith the gabbros as th eintrusive equivalents of the basalts.

The Folldal metabasalts are enriched in Kand Rb and depleted in P, Zr, Y and Ti com­pared to MORB (Fig.10 a). A similar patternwas also found in Grimsdalen from drillcore

samples by Skyseth & Reitan (1992). Thispattern is typical of low-K tholeiites fromisland arcs (IAT) and shows similarities toaverage SW Pacific IAT (Ewart 1982) withrespect to Zr, Ti and Y. The lower values ofSr, K and Rb might be due to alteration, asdiscussed above . One important exceptionis the extreme ly high values of Ni and Cr(respect ively, 1.5 and 3 times as high asIAT). These values suggest an influencefrom a primitive source (discussed below).Compa red to the Folldal data , basalts fromdestructive margins near co ntinents , suchas the western part of Amer ica, are highlyenriched in Nb, P, Zr, Y and Ti compared toMORB (Ewart 1982).

In the Ti-Zr-Y diagram (pearce & Cann1973, Fig.9 b), the rocks plot as CAB, LKT

NGU - BULL 426. 1994 Terje Bjerkgard & Ame Bjorlykke 67

a)

• Metabasalts - Folldal

• SW Pacific... Cascades- Aleutian s

o WestemUS A

o SouthAmerica

Sr K Rb Nb n05 Zr Ti02 Y Cr Ni

on the geochemis try and field relationshipsas described above it seems reasonable tointerpret the trondhjemite as a subvolcanicintrusion related to the extrusive rocks.

The high content of Na20 (> 4 "le) in the fel­sic rocks suggests alteration by spilitic orhydrothermal processes , though the altera­tion has probably not been pervas ive. Com­pared to both trondhjem ites from modernisland arcs (Table 3) and the presumablyunaltered post-tectonic trondhjemite fromnear Steren (see Size 1979), the sodiumcontent is similar.

Or

An

Fig.l l . Classification of the felsic rocks in Folldal according tothe Or-Ab-An diagram of Barker (1979). Open circles - trend­hjemite; crosses - rhyodacite.

The bimodal volcanism in the FundsjeGroup is a typical feature of both arc andback-arc environments. Compared to felsicrocks of an arc back-arc setting (Gill & Stork1979, Ikeda & Yuasa 1989, Fryer et al.1990, see Table 3 for a comparison withisland arc granite from lzu-Bonin arc), theFolldal Trondhjemite and extrusive equiva­lents are similar with respect to most mainand trace elements. Most characteristic is alow content of Ab03and Ti02 , as well as ahigh K/Rb-ratio; but as in the metabasalts,the content of Cr is anomalously high com­pared to felsic rocks in an arc setting (80­100 ppm compared to < 10 ppm, op.cit.).

Ab

..-:.:= ====";"-<::;::--- (AT-trend

b)

3.00

2.50

0 2.00

b 1.50

1.00

0.50

0.00

0.50

and OFB, while in the Ti-V diagram (Sher­vais 1982, Fig.9 c) most samples plot abovethe line TiN= 20, which is typical of IAT orsmall scale back-arc basins (op.cit.). Regar­ding the ratio K/Rb the Folldal basalts arequite similar to IAT (Table 3), while the Sr/Zrratios is lower than that of IAT. The lattermay be the result of Sr loss during alterati­on. Similarly, in the Ti02 vs. FeO(t)/MgO dif­ferentiation diagram, the Folldal samplesfollow an IAT trend (Fig.10 b).

Fe/sic rocksAs shown in Table 2, there is a striking simi­larity in chemistry between the Folldal trond­hjemite and the extrusive rocks. This is fur­ther supported in the total alkali - silica dia­gram, both plotting as rhyodacites (Fig.9 a).In an or-ab-an diagram (Fig.11) all the FolI­dal samples plot as trondhjemites. Based

1.50 2.00 2.50 3.00

FeOfMgO

Fig.l 0 (a) Average of the Folldal greenstones (13 analyses)normalised to MORS (MORS-data from Pearce 1983). Datafrom active continental margins and island arcs are shown forcomparison (data from Ewart 1982). (b) Ti02 vs. FeO/MgOtrend diagram for the 13 Folldal samples. Domains of basalttypes (A, S, C) distinguished by Grenne (1988) from the RorosArea are shown for comparison.

68 Terje Bjerkgard & Arne Bjorlykke

0.080

492

GU - BULL 426. 1994

0.079=:loc

'"N--it~oN

0.078

0.077

0.600

480

484

0.615 0.630

207 Pb / 235 U

Fig.12. Concordia diagram for theanalysed zircons from the FolldalTrondhjemite: Three fractions of fairquality zircon were analysed fromone sample. The best estimate forthe age of the trondhjemite can beestimated to be 488±2 Ma based onzircon fractions A and C which inter­sect concordia and each other.Fraction B is slightly discordant,which is probably a result of analyti­cal problems. The amount of zirconin this sample was very small and

0.645 therefore the amount of Pb analysedwas quite small.

Age of the Folldal Trondhjemite

Four samples of trondhjemite and two sam­ples of metarhyodacite, each around 30 kgin weight , were collected. The sampleswere crushed to 125-250 mesh in a slingmill and the heavy minerals collected with a'Goldhound' (used for gold prospecting andeasy to clean). A Frantz magnetic separatorwas used to remove magnetic minerals.The remaining concentrates (a few grams)were sent to the Geochronology Division atthe Geological Survey of Canada. Clear zir­cons were picked out and analysed follo­wing the method outlined in Parrish et al.(1987). U and Pb blanks were approximate­ly 1 and 15 picograms, respecti vely .

Only one of the samples of trondhjemitecontained enough zircons of fair quality toprovide an age (sample locality marked bythe aster isk in Fig.2). The result is shown inFig.12. As can be seen, two of the threefractions intersect the concordia and theage is calcu lated to 488 ± 2 Ma.

DiscussionOrigin of the volcanites

The volcanic rocks in Folldal have most like­ly been formed in an island arc setting, asdiscussed above. This is in agreement with

work in the Heros-Me raker part of the Fund­sjo Group: Grenne & Lagerb lad (1985) andGrenne (1988) distinguished between twotypes of basalt , one type with a typical lAT­signature (Iow Ti0 2 , Zr and high Rb) and theother with similarities to MORB. As seen inFig.10 b the Folldal samples are similar totype A of Grenne (1988), which display anIAT trend . In the Roros and Meraker areas,some samples intermediate between thetwo have also been found , as well as transi­tional calc-alkaline to within -plate typemetabasa lts (Grenne 1988). Based on this,Grenne & Lagerblad (1985) concl uded thatthe metabasa lts represented immature arcto arc rifting volcanism, while Grenne (1988)claimed a possible opening of a marginalbasin above a mature subduction zone.

The Folldal basalts have anomalously highcontent of Ni and Cr. The contents of theseelements are also high in the FundsjoGroup metabasalts in the Horos-Merakerarea (Grenne & Lagerb lad 1985, Grenne1988), showing it to be a regional featu re.High values of these elements are alsoknown from some of the Stekenjokk volcani­tes in the Stikke and Gelvenakko Nappes innorthern Jamttand in Sweden (Stephens1982) and the Gjersvik Group volcanites inthe Gjersvik Nappe north of the Grong­Olden Culmination (Lutro 1979). Both theseunits have been correlated with the Fundsjo

NGU - BULL 426, 1994

Group (Stephens & Gee 1985). These ano­mal ies suggest an influence from a primitivesource. In fact , two of the samples fromFolldal might be classified as boninites, withMg/Mg+Fe2

+ :2: 0 .6 and Si02 :2: 53 wt.%(Crawford et al. 1989) . In this regard it isinteresting that boninites have recentlybeen reported from the nearby Einunnfjell­Savalen area (McClellan 1993) . Boninitesare either among the first melt products in aprimitive island arc (Hawkins et al. 1984) , orformed at a later stage because of subducti­on of an active spreading ridge (Crawford etal. 1989) , in both cases by partial melting ofthe mantle wedge above the subductedslab. Boninitic melts are known to be veryrich in elements such as Cr and Ni (Ni =70­450 ppm, Cr = 200-1800 ppm , Hawkins etal. 1984) and the two samples from Folldalhave 277 and 295 ppm Ni and 1315 and1419 ppm Cr, respectively.

In the Horos-Meraker area volcanites withboth IAT and MORB characteristics arefound , while only IAT volcanic rocks havebeen found in the Folldal area . Two possi­ble explanations for this are: 1) MORB vol­canites might be present also in Folldal, butnot detected because of too few samples ;or 2) there is a development from arc to rif­ted arc/back-arc northwards, the sampleswith IAT signature in the Heros-Merakerarea representing disruption of the arc , asenvisaged by Grenne (1988) . A higher con­tent of Na20 in the northern part of thegroup might support the latter interpretationbecause a higher degree of spilitisationmight be suspected in a rifting environmentbecause of increased water depth andincreased heat flow.

Two possibilities exist for the origin of thetrondhjemite and associated vo lcanic rocks(see Barker 1979 for review): 1) part ial mel­ting of the downgoing slab; or 2) fractionalcrystallisation from the basal tic magma.Based on field observations and experimen­tal studies, trondhjemitic melts have beenshown to be generated by partial melting ofamphibolites (Drummond & Defant 1990) .Furthermore, the volume of felsie rocks inFolldal is around 30 % and in Meraker about50 % (Grenne & Lagerblad 1985) , if in both

Terje BjerkgiJrd & Arne Bjorlykke 69

cases the felsic intrusive rocks are included.This is far too much to be generated byfractional crystallisation from a mafic mag­ma. As seen in Table 2, the felsic volcanitesgenerally have a slightly higher content ofincompatible elements, and especially L1Lelements like Rb, K and Zr , compared to thebasalts , but also values as low as the avera­ge for the basalts. The increase is generallytoo little to conclude that the felsic rockshave differentiated from the mafie magma.One would also expect to find intermediatecompositions. On this basis , the mafic andfelsic rocks seem to have different origins. Apossibility is that the felsics have formed bypartial melting from the downgoing slab andthe mafics from the mantle above the slab.

Relationship between the Gula andFundsje GroupsThe contact between the Gula and FundsjeGroups has been variously described as ofeither primary or tectonic origin. A tectoniccontact has been reported from the Dom­bas-Grirnsdalen area (Guezou 1978 , Peder­sen 1979) , the area east of Savalen(McClellan 1993) and in Tydal north of theHoros area (Horne 1979). In contrast, Rui(1972) and Rui & Bakke (1975) found noevidence for a tectonic contact between theunits in the Heros area and claimed a con­formable contact (see below), a conclusionalso reached by Fox from work near theHersje deposit (Fox et al. 1988) .

In the Meraker area , a unit is distinguished,the Guda Formation (Wolff 1973) or GudaGroup (Lagerblad (1983) , containing bothgreenstones and graphitic schists withconglomerates and marble. The conglome­rates usually contain quartzitic and calcare­ous pebbles in a mafic matrix and are thussimilar to the Husom Conglomerate in FolI­dal (Heim 1972 and this work). Based ontectonostratigraphic position and lithologiesthe unit is correlated with the similar 'hetero­geneous banded sequence' of Rui (1972) inthe Heros area (Lagerblad 1983) . Accor­ding to Nilsen (1978) this sequence belongsto the Asli Formation. Rui (1972) consideredthis unit as transitional between the Gulaand the Fundsje Groups. In the Meraker

70 Terje Bjerkgard & Arne Bjorlykke

area the contact between the Guda Forma­tion and the Fundsjo Group is similarly des­cribed as trans itional, with decreasingamounts and thicknesses of sedimentarylayers eastwards into the Fundsjo Group(Wolff 1973). This corresponds to what wehave observed in the Savalen and Grimsda­len areas (this work).

Further north, in northern Jamtland, theRemdalen Group in the Stikke Nappe struc­turally overlies the Stekenjokk volcaniteswith a conformab le contact (Stephens1982). The structural base of this groupconsists of quartzitic conglomerate andlimestone in quartz-graphite phyllite follo­wed by a succession of quartz-graphitephyllites and greenschists (Remdalen Gre­enschist) together with quartz porphyry andlimestone and uppermost, calcareousquartz-graphite phyll ites. The whole strati­graphic succession is probably inverted asin the Trondhe im Region (op.cit.). The asso­ciation of conglomerate and limestone isthen in the same stratigraphic position andis very similar to the upper part of the GulaGroup. The occurrence of transitional unitswith intercalations of greenstones or tuffitesbetween the Gula and Fundsjo Groups inthe Savalen, Heres and Merake r areas sug­gests a close relationship between theunits . The development of mylonitic rocksseen at some localities might be due tomechanisms other than large-scale thrus­ting (cf. the contact between the trondhjemi­te and tuffites in the Folldal area, describedabove).

The c1asts in the conglomerates in the AsliFormation are from severa l sources. Clastsof marble and quartzite are always domina­ting and are derived from nearby lithologiesin the Gula Group. Igneous c1asts havebeen recognised in Folldal (Heim 1972, seea bove ) a n d al so in the Tyn s e t -R oro s areas(Fasrden 1949, Ellevold 1976). These cong­lomerates are overlain by sediments, mostlygraphitic pelites of considerable thickness(up to 500 m), making perhaps the volcani­tes in the Gula Group (the Gula Greensto­ne) the most probable source for thesec1asts. In contrast, the conglomerates andassociated tuffites and greenstones of the

NGU • BULL 426. 1994

Guda Formation (see above) in the Tynset­Meraker area are found close to the Gula­Fundsjo contact. In this case the clasticmaterial might have been partly derivedfrom the growing arc and partly from theGula rocks.

The fossil locality at Nordaunevoll with thegraptolite Dictyonema f1abelliforme socialefirst described by Vogt (1889) is situatedvery close to the Fundsjo Group (Rui 1972)in the Asli Formation. The host rock is avery consp icuous V, U and Mo-rich graphitephyllite (Gee 1981). The age of the grapto li­te is Tremado cian, that is to say 488-505Ma (Harland et al. 1989). The trondhjemitein Folldal, intruding slightly older basalts, isdated to 488 ± 2 Ma (p.68), which indicatesa continuous stratigraphic sequence from atleast the Asli Formation and upwardsthrough the Fundsjo Group. A correspo n­ding relationship appears to exist in the Ste­kenjokk area in Jarntland, where similar U­and Mo-rich graphite phyllites (but not con­taining fossils) are intercalated in the gre­enstones (Stephens 1982) and an age of492 ± 1 Ma has been obtained from a trend­hjemite (Claesson et al. 1987).

Stephens et al. (1993) were able to date (U­Pb zircon) a trondhjemite c1ast in a conglo­merate from Storfjal let to 489 +10/-5 Maand a trondhjemite from Gjersvik to 483 +5/­3 Ma, both in the Kbli Nappes of the UpperAllochthon and probably correlatives of theFundsjo Group (see above). These rocksare within error margins identical in age tothe Folldal trondhjemite. The authors(op.cit.) use these ages to indicate that theterrane containing these trondhjemites wasconsiderably separated from Baltica at thattime. This is probab ly not true (see discussi­on below).

A h ig h c ontent of e r is typic al of the Fundsjometabasalts, both in Folldal and partly atboth Heres and Meraker (see above).Accord ing to data by Ellevold (1976) thephyllites in the Asli Formation also havehigh contents of Cr and which seem todecrease away from the contact. This mightbe interpreted as a decreasing influencefrom the volcanic rocks which may have

NGU - BULL 426. 1994

been the source. A similar feature alsoseems to be displayed by Se and Co(op.cit.). The regional extent of the conglo­merates in the Asli Formation indicates amajor uplift event of the Gula rocks. Thismight have happened during a phase ofcompression in connection with the deve­lopment of a subduction zone leading at lastto formation of the arc-related FundsjeGroup. The marble horizons indicate shal­low-marine cond itions. Such conditions arealso indicated by the black schists in theupper part of the Asli Formation, which con­tain the platformal graptolite Dictyonema atone locality.

Correlations with Gudbrandsdalen

Recently, the greenstones in the Otta-Dom­bas area have been identified to representan ophiolite complex, the Vaqarno Ophiolite(Sturt et al. 1991). These greenstones havean N-MORB signatu re and have been inter­preted to represent either the ancient crustof lapetus or oceanic crust from a marginalbasin (op.cit.). The ophiolite has a thrustboundary to the underlying sediments of theHeidal Series (Sturt et al. 1991). The HeidalSeries comprises mostly quartzitic schistsand garnet-mica schists and may be corre­lated with the Aursund Group (Table 1,Fig.1). Both the ophiolite and the HeidalSeries are overlain unconformably by theSel Series , which generally commenceswith conglomerates (op.cit.), These conglo­merates vary laterally but generally containc1asts derived from the ophiolite as well asfrom the Heidal Series. Fossils in this cong­lomerate give an Early L1anvirn age (:0: 470Ma). The relationship between the HeidalSeries, Vagamo Ophiolite and Sel Seriesdemonstrates a probable Early Ordovicianobduction of the ophiol ite onto the Fenno­scandian Shield (op.cit., Sturt & Roberts1991).

Recent mapping by Sturt & Ramsay inGrimsdalen shows that the major unconfor­mity which oversteps the Vaqarno Ophioliteonto the subjacent Heidal Group (Sturt et al.1991 ), passes stratigraphically beneath theFundsje Group volcan ites. This precludes acorrelation of these volcan ites with the

Terje Bjerkgard & Arne Bjorlykke 7 1

Vagamo Ophiolite (B. Sturt , pers .comm.1993 ). Even so, the Otta conglome­rates can be compared with cong lomeratesin the Sularno Group (see above), sinceboth are polymict and contain c1asts derivedfrom the underlying volcanites. These relati­onships and correlations point to a similarobduction and post-obduction history of theVagamo ophiolite and the 'Fundsje arc'.

Ensimatic vs. ensialic arcClosely tied to the question of the connecti­on between the Gula and Fundsie Groupsis whether the arc/back-arc system develo­ped in an ensimatic or an ensialic sett ing.An ensimatic setting has been proposed byGrenne & Lagerblad (1985) and Grenne(1988). This is also in accordance with whathas been suggested for the Middle K61i andGjersvik volcanic rocks (e.g. Stephens &Gee 1989). Indeed, the widespread occur­rence of low-K tholeiites in the Folldal,Grimsdalen and Heros-Meraker areas (seeabove) is in favour of an ensimatic settingfor the volcan ites. In ensialic envi ronmentssuch rocks are rare (see for instance thecompilation by Thorpe 1982) and conf inedto the oceanic side of the arc (Baker 1982).The Tonga-Kermadec-New Zealand arcsystem shows that with increasing influenceof continen tal crust , the magmas changefrom low-K tholeiites to calc-alkaline basalts(Ewart 1982). Further, the low content ofincompatible elements in the felsic rocksalso supports an ensimatic setting. Thecompilation by Ewart (1979) indicates thatlow-K rhyolites are restricted to ensimaticarcs.

On the other hand, as Grenne (1988) pro­posed, the presence of quartz ites andquartzofeldspathic wackes intimately rela­ted to the volcan ites supports a near-conti­nent origin of the arc. This is also supportedby lead isotopes, which indicate a continen­tal component in the massive sulphides inthe Fundsje Group (Bja rlykke et al. 1993and unpublished data from Folldal).

The contact between the upper part of theGula Group and the Fundsje Group in theFolldal area seems to be primary and in pla­ces gradual (see above), implying that the

72 Terje Bjerkgard & Arne Bjarlykke

arc must have formed above the GulaGroup. One possible (and probable?) ans­wer to the ensimatic signature of the arc isthat it may over lie only the outer and platte r­mal Asli Formation of the Gula Group . Thethickness of this unit is only a few hundredmetres at the most , which might be too thinto influence the trace-element compositionof the volcan ic rocks. Because of the gene­rally much higher content of Pb in sedi­ments than in volcanites, lead isotopes arevery sensitive to any influence from sedi­ments (Hawkesworth 1982). This mightexpla in why the lead isotopes of the sulphi­des give an ensialic signature, in oppositionto the trace elements .

Of fundamental importance in this context isthe status of the centra l part of the GulaGroup and the nature of the Asli-Sinqsascontact. The Sinqsas Formation consistsmost ly of outer marginal sediments such asturbidites, but of importance are also gre­enstones and associated chert (Gula Gre­enstone, Nilsen 1978). The greenstoneshave formed in an oceanic environment(Nilsen 1974), and pillow lavas have beenidentified near Savalen (Rui 1977). Themassive sulphide depos its at Flott urn andBukkhammer are very low in radiogeniclead (206/204 Pb of 17.909 and 17.806,respective ly, Bjar lykke et al. 1993), suppor­ting the ensimatic character of the volcan icrocks .

The Sinqsas Formation has been metamor­phosed partly to upper amph ibolite faciesand contains migmatitic gneisses in theMeraker area (Wolff 1979). A possible syn­tecto nic trondhjemitic intrusion in this forma ­tion at Vakkerlien has been dated by the U­Pb method on zircons to 509 +5/-4 Ma(Klingspor & Gee, referred to in Stephens etal. 1985) The trondhjemite is cross-cutting anicke life rous metagabbro (Boyd & Nixon1985) possibl y comagmatic with the GulaGreen stones (Nilsen 1974) , thus prov idinga minimum age for the unit. The age fitsvery well with the metamorphi c Sm/Nd ageof 505 ± 18 Ma of the Seve eclog ites (Merket al. 1988). Thus , it is possible that theSeve and Gula units have been involved inthe same tectonic event around 505 Ma.

NGU • BULL 426. 1994

Because of the marked difference in meta­morphic grade between the Sinqsas andAsli Formations, the age difference betweenthese formations and the close relationsh ipbetween the Asli Format ion and the Fund­Sj0 Group, the Sinqsas-Asf contact must beeither a primary unconformity or a thrustcontact.

As discussed above , the Gjersvik and Fund­Sj0 volcanites are likely correlatives . Step­hens et al. (1993) used ages from Gjersvikto indicate a considerable distance betweenBaltica and the Gjersvik Terrane (see abo­ve). This is negated by the Gula-Funds jostratigraphy and particu larly by the cont i­nental Pb-isotope signatures of the mass ivesulphide deposits in the Fundsje Group.

Fieldwork, geochemistry and dating in theFolldal area have shown that there is proba­bly a continuous stratigraphy from at leastthe upper part of the Gula Group andupwards through the Fundsje Group. Thismeans that the nappe contact betwee n theMeraker and Gula Nappes in this area eit­her does not exist or that the contact is fur­ther to the west, i.e. between the Asli andSinqsas Format ions. More work , such asmore dating of obvious pre-tectonic igneousrocks in the Sinqsas Formation and detailedfieldwork along the Asli-Sinqsas contact isneeded to answer these questions .

Summary and concludingremarks

1. The bimodal characte r, large volume ofvolcaniclastic rocks and geochemistry of theFundsjo Group volcanites favour their for­mation in an island arc sett ing. Intercalati­ons of cont inental sed iments in the volcani­tes and the lead isotopes of the mass ivesu lphides show that the arc must have for­med close to a continent.

2. Based on field relationships, mineralogyand geochemistry , the Folldal Trondhjem iteand felsic extrusive rocks are interpreted tobe comagmatic.

3. The large volume of felsic volcan ites and

NGU • BULL 426. 1994

trondhjemites in the Fundsjo Group andtheir trace element chemistry show that theycannot have been derived from the samemagma as the mafic rocks. A possibility isthat the felsic rocks have been formed bypartial melting from the downgoing slab andthe mafic rocks from the mantle wedge abo­ve the slab.

4. There are no differences between theFundsje Group and the upper part of theGula Group regarding degree of metamor­phism or structures. The contact betweenthe Fundsje Group and the Asli Formationof the Gula Group has been observed to bepartially gradational and is interpreted to beprimary.

5. Age determination and palaeontologicaldata are in accordance with a primary upperGula-Fundsje contact: the Dictyonemaschist in the upper part of the Gula Group isTremadocian in age (488-505 Ma), whilethe age of the Folldal Trondhjemite intrudingslightly older basalts is 488 ± 2 Ma (U-Pb onzircons).

6. The lower or central part of the GulaGroup, the Sinqsas Formation, consists ofouter shelf-marginal sediments and minorgreenstones with MORS character. A cross­cutting, probably syntectonic trondhjemitehas been dated to 509 +5/-4 Ma, compara­ble to the metamorphic age of 505 ± 18 Mafor the Seve eclogites in northern Sweden,and is considered to provide at least a tecto­nic and metamorph ic link between the SeveNappes and the lower part of the GulaGroup.

AcknowledgementsThe authors are grateful to Odd Nilsen and Brian A. Sturt forthe ir constructive criticism and comments on the first draft ofthe manuscript. Brian A. Sturt , Elisabeth A. McClellan andanother anonymous referee are thank ed for constructive com­ments on the manuscript at the review ing stage . Brian A. Sturtand Dona ld M. Ramsay are also than ked for valuable corn­ments during a field trip. The first author thanks Follda l VerkNS for accommodation and economic support dur ing field·work. Johann G. Heirn, forme rly at NS Norsulfid. is thanked forvaluable comm ents and for introducing the field area to the aut ­hors .

Terje Bjerkgard & Arne Bjorlykke 7 3

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Manuscript received January 1994; final typescript accepted May 1994,