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,