The Ordovician Gronda lsfjell lntrusive Complex, Central Scandinavian Caledonides: f ield relat ions, petrography and emplacement

NGU-BULL435,1999- PAGE5

The Ordovician Gronda lsfjell lntrusive Complex, Central Scandinavian Caledonides: f ield relat ions, petrography and emplacement

GURUB.MEYER&TOR GRENNE

Meyer,G.B.&Grenne, T.1999: The OrdovicianGrondalsfje ll lnt rusiveComplex,Centra lScandinavianCaledo nides:

field relati on s,petr ogr aphy andemplacement.Norgesgeologiske undersekelse,Bulletin435,5-22.

The Grondalsfje ll Intrusive Complex (GIC)ispart of a major pluton icprovince located in the GjersvikNappeof the Upper Allochthoninthe CaledonidesofCent ral Norway.Fieldrelationships andpetrography suggestthat the mag- maticevolution of the pre-456 Ma complextook placeinthree main stages.It commencedwit h the intrusion of relativelyprimiti ve maficmagma and crystallisationof coarse-grainedolivine gabbro, presumablyin a macro-dyke or smallerintrusive body.This was followed by intrusionof more evolvedbasalticmelts to forma largermagma chamber, nowpreserved astheLayered Series,theemplacementofwhichwas characterisedby activestoping and incorp orati onofa large number of xenolit hs.Continuousinflux ofmagmaled todeposit ion ofathick pile of layered olivinegabbrocumulates which nowfor mtheLower Zone.Astheinflux of magma decreased,fraction alcrystallisa- tion causedanevolution towardsgabbronoritein theMiddleZoneandquartz diorite inthe Upper Zone.

Renewed magm atic act ivityledtothe formation ofa third magm a chamber representedbythehornb lende-diorite- gabbroseries. Thesemagmas werehydrou s andcharacterised byhornblendeas thepredominantmafic cumulus phase.Initialcrystallisationofa dioriticcrystal mushwas succeeded by conti nuousorinter mitte nt replenishmentof comparable,butmore basicmagmaandextensive minglingof maficand dioritic componen ts.Penecon temporane- ousinjecti on ofbasicmagm a orcrystalsuspensionstookplacealongdykes in theLayered Series.Fract ionation pro- ducedperidoti te,olivine gabbroand hornblend e-gabb roic 'cumulate-ty pe'dykesin theLower andMiddleZone,and this wasfollo wedupwardsby injec t ionof coevalbasicmelts into the part lycrysta llisedUpper Zone.

TheGICwas emplacedinto previouslydeformed andmetamorphosedvolcanicand related intrusive rocksof the Skoro vassComplex,andextensive partialmelting too kplace along the marginsof the intrus ivecomplex.Intense shearingwaslocalisedalongcontactzones of theintrusion s andmuch of thisdeform at ion wasevide nt lycontempo- raneous wit hthemagmaticevents.

Gurli B.Meyer&TorGrenne,Norgesgeologiskeutidersekelse,N-7497Trondheim,Norway.

Introduction

TheGrond alsfjell Int rusiveComplex(GIC) is situated in the GrongDistrictof the central Norwegian Caledo nides,250km northe ast of Trondhei m (Fig.1).The presence ofvolcano- genic massive sulp hideoreshasmadethe regionatt ractive from aneconomicpointofview, and regionaland det ailed geolog ical investi gation s havebeencarried out through out this cent ury(see,e.g.,Halls et al.1977,Kollung 1979,Lutro 1979 ,Reinsbakken 1980, 1986 , 1992, Reinsbakken & Halls 1987, Robert s 1979, 1997, Roberts & Tucker 1991 ). The presentstudy form spartof a multidiscipli naryprog ramm e of invest igat ions by theGeologicalSurvey of Norw ay wit h the aim ofinteg ratin g andim pro vi ng the geologica lbasis for fur- therexploratio n inthearea.Based on detailed studiesand remapping ofa ca.50km²area at ascale of1:10,000,the field relati ons and petrography of a cogenetic plut onic suite of ultra mafic to acidic rocks in the Grond alsfj ell area are describe d.The com plex is remarkably well exposed (>90- 95%)andin generallittleaffectedby metamorph ic recrysta l- lisation and laterdeform ation .Atalt itudesaboveca.700m in the centraland sout hern partsof theGIC,theintrusiverock s provide auniquerecord ofprimary magmaticrelationships.

Wor k isin progr ess on other aspectsof theGIC.The resultof investigat ions ontheprocessesgoverni ng theevolutionof the magma chambers,the chemicaland isot opiccharacter i-

st ies of the magmas and the mechani sm and time of emplacementwillbepublishedseparately.

Regional setting

TheGron g District is divi ded into two princ ipa l parts.The sout heaste rnpartincludes theOrklump and Bj 6 rkvat n Nap- pes ofthe K61iNappe Complex in the Caledo nian Upper Allocht hon (Roberts 1997), and consists mainly of various met asedimentaryrocks with localmet avolcanicunits.Tothe northwest,thesenapp es aretect onost rati graph ically over- lain by the Gj ersvikNap pe which hosts theGIC and is also regarded aspartof theUpperAllocht hon(e.g.,Stephens etal.

1985, Roberts 1997).The northwestern part ofthe Gj ersvik Nappebordersonequivalents of the Helg elandNappeCom- plex of the Uppermost Allochthon, which comprises migmatiti c gneisses, mica schists, marbl es and avariety of plutoniccomplexes,partlyof bat holithicdimensions(Nord- gulenetal.1993 ).

The Gjersvik Nap pe iscomposed of volcanicandsubvol- canic rocks of the Skorov assComplex (Roberts 1997;pre- vio usly referredto astheGjer svikGro up) and the overlying, pred om inantlysedime nta ry,Limin genGroup,inadditio nto a varietyof pluton icrocks(Fig.1).Accordin gto Reinsbakken (1992),the volcanic rocksof the Skorova ssCom pl excanbe

NGU-BULL435,199 9-PAGE6 GURU B.MEYER&TOR GRENNE

rrrrrrn

~ metasedimenta ry rocks TheSkorovass Complex

~ Moklevatngranodiorite (c.

456

~

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~

B.

Quartz diorite /g ra nod iorite , und ifferentiated

Gabbroicrocks

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20 km

Fig. 1.Geolog icalmapof theGj ersvik Nappe show ing the Skorovass Com- plexand thelocation oftheGIC.The area coveredbyFig.2isframed.

subdivided into three major stratigraphicunits: 1) a lower unit comprising massive and pillow ed lavas of thol eiitic basalt to basalti candesite;2)a very heterogeneous midd le unit, composed of differ ent iated tholeiit ic basalts,basalt ic andesitesand andesit esform ing massiveto pillow edflow s in additi on to abundant feldsp ar-ph yric rhyodacite flow sand pyroclastit es;and 3)anupperunit comprising prim itive,pi1- low edand massivetholeiitic basalts and localboninites alter- nating wit h quartz-phyric rhyoda cit eflows and tuffites.The lower unit isassumedtohaveformed duringthe early stages of construction of an ensim atic island arc, whereas the middle and upper units develop ed later inrespon seto rifting ofthe arc com plex(Reinsbakke n 1992).

Gabbroictotonal it icbodies are abu ndant and are inter- preted assub volcan ic intrusions relate dmain lyto the middle volcanicunit.Parts ofthe SkorovassCom p lexcontainrneta- gabbroswhich pass upwardsinto sheeted dyke complexes reminiscent of ophi olite fragments (Heim 1992 and ow n observations). Prelim inary int erpretat ions of geochemical data suggest a link between these dykecomplexesand the upper volcanicunit.

In addi tionto syn-vo lcanic intrusions, the metavolcanic rocks of the SkorovassCom p lex were intruded by a large number of plut oni c bodies with areal extentsof some10km² or more(Fig.1)rangingin composition fromult ramafic and

gabbroic, to trondhjem itic and granod io ritic (Halls et al.

1977, Lutro 1979).Fieldevidenceand prelim ina rygeochem i- cal data im pl y that many of these intrusionswere unrelated to the preserved,tholeiitic,metavolcan ic sequence of the Skorovass Complex,whereas others which cut across only the lower to middlevolcanicmembers may possib lybe coge- netic wit heffusive rocks in the upper parts ofthe volcanic seque nce.Atrondhjem it ebelong ing to the latter suite,near Bjerkvatn et in thenorth ern part of the region (Fig. 1), has yielded a U-Pb zircon age of 483+s/-3Ma (Stephens et al.

1993).

The southernpart of the region(Fig.1)is dominated by the Meklevatnet granodior itein addition to gabbro complexes such the GICdescribedin the presentaccount and the Heim- dalshaugen gabbro complexto the southwest (Reinsbakken

& Halls 1987,Roberts 1997).The Meklevat net granodior ite approaches dimensionsof 20x14kmand has been dated(U- Pb,zircon)at 456± 2 Ma(Robert s&Tucker 1991). Recent mapping shows that parts of the Moklevatn granodio rite int ruded sedimentaryrockswhich havebeen considered as belongingto the low er part of the LimingenGroup ,priorto a major uplifteventwit h subsequent deposit ionof arkosesand conglome rateslargelyderivedfromthe granodio ritecomplex.

Furthermore,the Meklevatngranodioriteshows int rusive rela- tionships to the Heimdalshaugen gabbro complex which

GURU B.MEYER

s

appearsto besimilar in ageto the GIC,implying that a mini- mumageforthe latte rwou ldbe 456Ma.

The rocksoftheGjersvikNappe are regionallyinvert ed,so thattheLimingenmetasedim entaryrocksarelocatedat the structuralbase along a marked thrustcontactto theunderly- ing sout heast ernnappes(Kollung1979,Lutro1979).Tothe nort hwe st, there is a less distinct tecto nic contact between themetavolcan itesof the 5korovassComp lex and equiva- lents of the tectono stratigraph ically overlying Helgelan d Nappe Complex.Thiscontact isapparent ly older than the southeasterncontact, being partly obliterated byabundant granitoid intrusions,someof which aredeformed by early folds(Kollung 1979,M.Heim pers.comm .1994).The rocks of theGjersvikNappehavebeen subj ected to polyphasedefor- mation under lower to uppergreenschist-faciesconditions, with the first tw o episodesof deformation being the most conspicuous (Halls et al. 1977, Lutro 1979, Roberts 1979).

According to Lutro (1979),the early phase wasrelated to coupling of the Gjersvik and Helgeland Nappes and was responsiblefor a regionalinversionof thesequ ence,as well asthe tight to isoclinal folding and development of the regionalschistosity.In theSkorovassComplex,thisstageis characterisedby a veryheterog eneousst yleofdeformation which was controlle d largely by contrast in rheology betw eendiffer ent rock units.Laterdeformation led to more open foldingaccompanied by further movementsalong pre- viouslyformed, low-angleshear zones.The heterogeneo us patternsof deformationhavetypically left the plutoniccom- plexesmoreorless intact withwell-preserved originaligne- ous fabrics and mineralogy, whereas penetrative tectonic fabrics are develop edalong theirmargins (Hallset al. 1977).

Geology of the Gr0ndalsfjell area

The GICis situatedinthe sout h-cent ralpartsof theSkorovass Complex(Fig.1).Other gabbroic rocks whichresembleparts of the GIC are exposedto the west and southwe st in the vici- nity of the mountain Heimdalshaugen, and a similar, but smaller,complexoccursat VestreLitlfjellet.The latter issepa- rated from the GIC by a belt of moderately to strongly sheared metavolcanitesandsubvolcanic intrusionsand local thrusts(Reinsbakken&Halls 1987).This shear-beltenvelops the GICandits shapewas evidentlycontro lled bythe rigid behaviour of the gabbro complexes(Fig.2). Similar, post- intrusive shear zones are observed locallynearthe eastern and southern contactsof the GIC,mainly affecting metavol- canic wall rocks but also deforming marginal partsof the intrusive complex.Despite this deformation,well-preserved intrusivecontactscan be observed at several placesalong the eastern margin and in the Murfjelletarea inthesout h- west (Fig.2).In the area betweenthemountainGrondalsfj el- let andthe river Grondalselva the rocks are metamorphosed to a degreewhere xenolit hs canno longer be mapped sepa- rately. In this area the zone boundariesare int erpolated from areas withless metamorphosed rocks.

The Grondalsfjellareawas earlierdescribed asazoned complexinwhich a core of metagabbro conta iningxenoliths oflayered olivinegabbrowas enclosed byhornblende diorite

NGU-BULL435,1999-PAGE7

(Halls et al. 1977).A new tripartitedivisionof the GIC is pro- posed inthispaper. The olde st part is foun dinxenoliths and large rafts of coarse-grained, massive to layered olivine gabbro.The olivine gabb ro is enti rely enclosed wit hin a youn ger intrusion,heretermedtheLayered Serieswhichcon- stitutesthe great er part oftheGIC.On the basisof phaselay- ering defined by the presenceor absence of olivine,Ca-poor pyroxeneand quartz,the Layered Seriescan be subdivided intothree main zones:theLower Zone(LZ)consisting of lay- eredolivinegabbroin thecent raland northernpart, grading sout hw ards into theMiddleZone(MZ) consist ing of partly- layeredgabbronoritewhichin turn grades intoquartz diorit e of the Upper Zone (UZ).Thethree zonesarelaterally persi- stentfrom thecentral part ofthearea towards the west and east. Thelayering intheLZ olivinegabbro and in parts of the MZ gabbronorite is approximately vertical in most of the area.A change in strike direction from E-W in the central north ernandweste rnparts,toESE-WNW in theeasternand sout hern part sof the area (Fig. 2) is due to minor post- magm aticdeformation, also revealedbya similar deflection of cross-cutting granodi orit ic dykes.The Layered Seriesis intruded by variou smafic dykes,and it is argued here(see below)thatthese are related toa third intrusive phaserepre- sentedbythe horn blende-diorite-gabbroseries whichis par- ticularly wellexpo sedalong the riverSkorovasselva.

Theent ire GIC is intr uded by numero us NE-SW striking, approximat ely vertical,granodioritedykes (Fig.2).Some of thesecan befollo wedcont inuouslyformore than 500 metres beforethey wedgeout,jumpor branchout inan enechelon patte rn.Thedykesvary in widthfrom 15cm(Fig.3)tomore than 10m,butaretyp ically 1.5to 2 m wide.Inadditi on to quartz, plagioclase and alkali feldspar they cont ain mino r muscovite, garnet, spheneandaccessory amounts ofmag- netite and sulphides.A fewof these cross-cutti ng dykes com- prisemingled mafic and granodioritic component s (Fig.2).

Field relations

Thefollow ingdescript ion s of the rocksoftheGIC are prima- rily based on mappingand detailed st udi es ofkeyfieldrela- tionships atwell-exposedlocalities,toget herwit hevidence obtained from texturaland mineralogical investig ation s(see below).Thedescriptive terms for primary magmatic struc- tures,types of layeringandcumulatesare used inaccordance with Irvine (1982). Terms such as adcumulate and ortho- cumulateareavoided,sinceavariety of differentprocesses can lead to thedevelopment of similar cumulate text ures, and the degree of post-cumulu s overgrowth on cumulus mineralsis often difficult to est imate. Accordin gly,we have chosento character isethevariousrocktypesby the cumu lus phasesand clearly ident ifiable intercumulu s phases they contain,assuggestedby Hunter (1996).Pyroxenes are gene- rally termedCa-richor Ca-poorbecausecomposit ion s cover fairlywid eranges.

Wall-rock xenoliths in the GIC

Xenolit hsofmetavolcanicwallrocks are veryabundant in the Layered Series.Thexenolit hs are found mainlyinthecentr al

NGU-BULL435,1999-PAGE8 GURU B.MEYER&TOR GRENNE

'00 '0 1 '02 '03 '04 '05 '06 '07

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Skorovass klumpen

GURU B.MEYER

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LEGEND

NGU-BULL 435,1999-PAGE9

Intrusiverocksof uncerta inage Cover

GJERSVIK NAPPE

Felsicintrusive rocks

Maf ic tointermediateint rusive rocks

Tunnsjaelv Thrust Sheet Wallrocks

Gneiss,micaschist and defor m ed granodiorite Post-GIC Intrusion

Metavolcanicrocks Dolerit icdyke complex

Gmndalsfjelllntrusive Complex (GIC) Thehorn bl endediorite-ga bb roseries

c::J - -

Granod ioriticdyke s

Compositedykes

Upper Zone(q uart z dior it e) MiddleZone (ga b bro nor ite) LowerZon e(olivine gabb ro) Gabbroicdomainsin Upper Zone

1

Layered

series

-

----

I· I'

Lit hologi calboun dary

Transitionalli thological bou nd ary Semi-ductileshear zone Mylonitic shearzone

Thrust zone(TunnsjoelvThrustSheet) Road

St rikeanddipofmod allayering Profile

N

Xeno lit hs ofcoarse-grainedoliv ine gabbro Xeno lit hsof met avolcanic wall rock

5

800 600 400 200

o

Fig.2.Geolog ical map of the Grondalsfje llarea andcrosssect ionA-A'.Forlocation of map, seeFig.1.

I'

and easternmost partsof the olivine gabbro(LZj, in mostof the gabbronorite (MZ) and in an area close the contact zone of the quartz diorite (UZ) insout h east (Fig.2).In the upper part of theMZ and the lowerpart of the UZ thereisan interval wit h few or no xenolit hs. In some partsof the LZ and MZ, xenolit hscompose more than 60%of the volume of the rock.

The wall-rockxenolit hs have an average size ofabout 5x 5 m but canrang efrom lessthan1 m(Fig.3)to morethan50 m across.Theyvary from angular to rounded, and are gene- rally difficult to distinguishfrom the host olivine gabbroor gabbronorite becauseofstr ong recrystallisation and diffuse contacts.Strongly recrystallisedmetavolcanic xenolit hs may only be recognisableby their lackof modal layeringand their granular texture, together wit h the reactions rims of pegmatitic material between 10-50 cm thick which form locally along theirmargin s(Fig. 3).

Xenolit hs of variably altered ano rt hositic rocks(Fig.3) have been observedat threelocalit ies in theLowerZoneof the LayeredSeries. The anorthositeis white or greyincolour due to saussurit isat ion of the plagioclase.The rock has a homogeneousappearance,but aweak magmatic lami natio n caused by a preferredorientation of tabularplagi oclasecry- stals can beseen locally in less alte red parts.Theoriginofthe anorthosite is uncertain.It can be considered to represent eit her a part of thepre-GIC,wall-rock litholog y, oranearly phase of crystallisation of thecoarse-grain edolivine gabbro (sebelow) equivalentto the anort hosit ic blocksin theSkaer- gaard Intrusion (Irvine 1987).

Xenoliths of coarse-grainedolivine gabbro The oldestplutonicrocksof the GICarecoarse-grained oli- vinegabbros which occur exclusively as angular toslight ly roundedinclusio ns in the Layered Series. Therearetwo very

NGU-BULL435,1999 - PAGE10 GURLlB. MEYER

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Fig.3. A.Granodio riti cdyke cutt ing deformedmetavolcanic wall rocksand gabbroin thecontact zo neof theGIC.Located 2 km east of the 944 m peak (Fig. 2).B. Xenolith of metavolcanicwallrockinolivine gabbro locatedin the centr al northern partoftheLower Zone. Apeg mat it ic reaction rim is partly develo ped alongthemargin of the xenolith(markedwith arrow). C.Xenolithsof anorthosite in LowerZoneolivine gabb ro.Locatedinthe centralnorth- ernpartoftheLowe rZone.D.Xenoli thofcoarse-grainedolivine gabbro in layered olivinegabbro in thecentraleastern part ofthe Lower Zone.Stra ti- graphicwayup istothe right.E.Layeredolivinegabb ro in thecentral northernpartof the Lower Zone.Stratigraphic way up is down to the right.A large xenolithof metavolcanic rockoccup iestheupperleft part of theoutcrop.Two mino runconfo rmitiesaremarkedwith arrows.F.Draping of modal layering alongthe flanks ofaxeno lit h inthecentralnorthernpartof theLower Zone.St rat ig raph icway up istothe top.

GURUB.MEYER&TORGRENNE

largeblocks in thenorth w estern partofthearea(Fig.2)and numerous xenolit hs(Fig.3) andseverallarge blocksfurther east. At onelocality ,an anorthosite xenolit h isincluded in a blockofcoarse-grained olivin e gabbro, the latter beingitself enclosed by layered olivine gabbro of the LZ.Inclusions of ot herwall rocks, suchas met avolcanic rocks which arevery abundantasxenolit hs in thelayeredolivinegabbro of theLZ, havenot beenfound wit hin the coarse-g rainedolivinegab- bro.

Typically,the coarse-g rained olivine gabbr o isisotropic wit habrowni sh-red,conspicuou slyknotty, weat heringsur- face and numerou sfractu res along which the gabb ro is alt ered forup toadistance of 50cm.Layeringis locally con- spicuous, wit h norm allayers20-50cm thick.The orientatio n of thelayering in two xenolit hic rafts is116/60° Sand260/

80° N,respectiv ely.Thisis slight ly different from that of the surrounding layeredolivine gabbrowhich is about 090/90°, indicating that the blocks rotated during descent through the magmabeforeimpact wit h thecont emporaryfloor ofthe magmachamb er.

The Layered Series

The layered olivine gabbro of the Lower Zone (LZ), the gabbronorite of the MiddleZone(MZ) and the quartzdiorite of the Upper Zone (UZ) constitutethe Layered Series,which has been mapped as one large intrusive body composing 65%of theGIC. The preserved part of the LZ is up to 3 km thickand occupies more than 35% of the Layered Series, passing sout h-w ards into gabbronoritic rocksof the MZas Ca-poor pyroxene takes over from olivine as a cumulus phase. This compositional change is accompanied by increasing contents of oikocrysticmagnetite/ ilmenite.The MZ is approximately1.5 km thick and formsabout 30%of the Layered Series.Farthersout h, around the 944 m peak (Fig.2), quartz and biotite gradually appearand the ca.2 km-thick UZ,which forms about 30% of the Layered Seriesin the so ut h, consistspredominantly of quartz diorite. Alkalifeld- sparis presentin certainvarieties of felsicveins and dykesin the contact zone, locally alsoin thecentral partsof the UZ (Fig. 2).It should be emphasisedthat the stratigraphy of the Layered Series given above is somewhat simplif ied; for exam- ple,olivine locallyreappears in the MZ and quartz and biotite are present intermittently over fifty to a hundred metres in the uppermostpart of the MZ.

Lower Zone

The LZ layeredolivine gabbrois easily distingui shed in the field by its reddish-bro wn weat hering. It ischaracterisedby alternating lightand dark layersin which the modal propor- tions of plagioclase and olivineand partl yalso Ca-richpyrox- enevary (Fig. 3).Compositions range from pure anort hosit ic to peridotitic layerson thescaleof a fewmillimetresto tens of centimetres.Primary magmatic features such as uncon- formities (Fig. 3),modally gradedlayering and drapingof lay- ersoverxenolit hs(Fig. 3) and depressionunderthem are par- ticularly commonin the olivine gabbro.In addition tomodal layering on the medium- to small-scale, othervarieties such as uniform layers,micro-rhythmiclayering and texturallayer-

NGU-BULL435, 1999-PAGE 11

ing occur inthe olivine gabbro.All these structures distin- guishthe layered olivinegabbroof the LZ from the xeno liths of massive ordm-scalelayered,coarse-graine d,olivine gab- bro.

The modally graded layering, impact str uct ures and slump str uct ures intheLZolivinegabbroprovideconsistent indicators of the way-up of the Layered Series which is tow ardsthe south.On the assumption that the modal layer- ingwas origi nallysub-horizo ntal,thereis evidencethatthe LayeredSerieshasbeentilt ed app roximate ly80°towards the sout heast. In some outcrops, however, the strike of the mod allayerin g changes byup to 90° fromthegeneral E-W orientat io n,show ingachaotic foldedpatt ern (Fig.4).lnmost casesthe presence ofxeno lit hsadjace nt tothese deformed areasshows thatthedisturb ance was dueto plastic deforma- tion when xeno lit hs sett led into the partly consolidated cumulates residing on thefloor ofthemagm a chamber.

Locally,theLZ olivinegabbro hasintruded intolarge rafts ofthe coarse-grainedolivine gabbro ina mannerreminiscent of stoping. Angular fragm ent s of coarse-grained olivine gabb rohavebeenbroken apartslig ht ly,eviden tlywit h littl e or no rotation, sothatthemelt s ofthe Layered Seriesseem to havepenetratedfracturesin the solidi fied raft sto form small pockets and dykes ofweakly layered, medium-grained, olivin e gabbro.

MiddleZone

In little-metamorphosedparts ofthe Layered Series,thetran- sitio nfrom LZolivine gabbro to MZ gabbronoriteis marked by a gradual change over lOOm from layered, reddish- coloured rocks toward smore massive, browni sh-coloured rocks.Thisisaccompanied by anincreasein the amo unt of Fe-Tioxides.The preferred orientat ionof plagioclase lath s and partlyalsoof prismsof Ca-poorpyroxene is commonin the gabbronorite giving rise to a magm atic lamin ation.

Modal layering isdeveloped insome areas (Fig. 4) but is less common thanin theLZ.Towardsthe sout h, themodal pro- portionof plagioclase gradually increases,and the colour of therock changesto alight browni sh-grey over100to 150m asquartz and biotite appears and form majorphases atthe baseof theUZ.

Upper Zone

The greater part of the UZ consistsof medium-g rainedquartz dior ite, wit h an increasing number of metavolcanic xeno lit hs towardsthe intrusivecontacts.Neither layering norlamina- tion hasbeen observed.Locally,there are transition s into gabbroic to dioriticdomainsup to 300 macross,apparent ly concentr ated along a roughly E-W zon etrendin gfro m the central to the eastern part of theUZ and locally along the wall-rock contact. Transitions into quartz monzodi orit e or granodiorite are also seen.Mafic dykes and enclavesare volumetrically subordinatebut forma sig nificantcompo nent of parts of the UZ. Back-veining of the quartz dioritic or dioritichost by the maficintrusions iscommon.Mafic dykes which locally passinto 't rains' of individu al subcircularor complex-shapedindividualbodies(Fig.4),provide evidence

NGU-BULL435,1999 - PAGE12 GURUB.MEYER&TOR GRENNE

Fig.4.A.Plasticallydeformed sequence ofmodally layered olivinegabbroin thecentraleasternpart ofthe Lower Zone.The general way upis to the left. The'normal'orientatio nofthelayeringis seen inthe left part ofthe outcrop.B.Layered olivine gabbrointhe centralnorthern part of the lower Zone,showing internalplasticdeform ation st ructureswit hin a seq uenceofthin olivine-rich layersand undist urbed,modally graded layers below and above.Stratigraphic way upis up totheright. C.Gabb ron orite in the eastern partof the Midd leZone withweak modal layeringand magmaticlamina- tion definedby a subparallel orientatio n ofplagioclaselaths.D.Mafic,fine-grainedintrusion inthecent ral easternpart of the Upper Zone quartz dior ite, withint ricate minglingofintrud ingmeltandpre-existingcrystal mush.E.Coeval int rusio nsin the south easte rn part ofthe Upper Zone.The dark,coarse- grained rock isalocal gabbro ic dom ain in the UZ,intruded bya netw or k of granodio ritic dykes andveins.A later cross-cutt ing mafic dyke shows partial min glin gwiththe granodio rite.F.Compositedyke inthe centraleastern part oftheUpperZone.Metadolerit ic 'pillows'(dark) in a lighter granodioritic to quartz dioriticmatrix.Weaklydeveloped,chilledmargin s are presentinthe pillows,butarepreservedonlylocally dueto extensivefracturing ofthe maficenclaves and back-veiningof the granod ioriti c material.

GURUB.MEYER&TOR GRENNE

thatthese dykes wereemplacedintoamelt oranunconsoli- dated crystal mush.

Rocks of quartzmonzodi orit ic and granodioritic compo- sitionare particul arly abundantin thecent ral-easternpart s of theUZ,in placesformingdyke-li kebodi esorveins wit h thick- nessesrangin gfrom afewmillim et resto several metres (Fig.

4).Som e of these are composit ebodi esinwhichevide nt ly penecont emp oraneous,fine-graineddykes,enclavesand pil- low-like st ruct ures of mafic compositi onarecontainedina leucocrat ic host (Fig.4).The mafic rock usually shows a chilled, finer-grained margin against the felsic host.Many enclaves have been fractu red, and back-veining of felsic mate rialinto the enclaves isquite common.Although the contactsareingeneralsharp,grad ual transiti ons can be seen, showing thatmixing of the mafic and theleucocratic compo- nent s occurred,at least onalocal scale.Someof these com- posite dykes exte nd throughthe contactzone betw eenthe UZ and the metavo lcanicwall rocks (Fig.2).

The hornblende-diorite-gabbro series

Asuiteof textu rallyvaried,hornbl ende-rich,intrusiverocks rangi ng in composit ion from dioriteto gabbro andsubo rdi- natequartzdiorite, occurs in thenorthernpart of theGren- dalsfjell area(Fig.2).The arealexte ntof thiscomplexhasnot beenmapped in detail,but itcoversat least5 km²along the riverSkoro vasselva and apparent ly 9-10km²of theNord re Grond alsfj ell area(Reinsbakken&Halls1987).In thefield,the contactwit h theLayered Series isdifficult to defin ein this areabecause of exte nsive metamorphism and the lack of good exposures,but in placesthedioriticrocksform a net- work ofrelat ivelycoarse-grained, partlypegm at iti cvein sin theLZ layered gabbro . Also, overazonein theLZapproxi- mately100metres wide tow ardsthe contactwit h thediorite, there seems tobe agradualincrease in brownhornbl end e at the expenseofolivi ne.Above (sout hof) the contact,a series of maficdykes and associat ed pegmatite shasintruded the Layered Series. These aredescribedseparate ly below.How- ever, petro logical data, includi ng geochemical st udies (in progress).leadto the conclusion thatthey are part of the hornblende-di orit e-gabbr o series.

The major body of the hornbl ende -di orite-gabb ro series consists mainl y of plagioclase and hornbl ende wit h varying , butgenerally minor, amountsof quart z,magn etite,apat ite andsulphides. Thetextur einwhich hornblend e crysta ls form subparallelprismswit h interstiti alplagioclaserange wide ly from equig ranular, med ium- or coarse-grained to fine- or medium -grained inequigranular. Metamorph osed varieties of horn blend ediorite can berecogn isedin thefieldbytheir yellowis h-b row nweat heringsurfaces,whereas thecolour on unmetamorph osedrock surfaces isdark grey tobrown.

Intrusivebod ies offine-to med ium-grained hornbl end e gabbroof variableshapeandsizeareabundant in thehorn - blendediorit e.Ingeneral, these bodies intrudethediorit e in very complex patte rns(Fig.5).Commo nly the gabbr o and hybr id enclavesare back-veinedbythediorit e.Hybridisati on in which thedioriti chostispresent as tinyveinsoras scat- tered remn ant crysta lsor compos ite grains of plagioclase and hornblende along the contacts of the intruding gabbro

NGU-BU LL435,199 9-PAGE 13

or gabbro -dom inatedhybrid magma is commo n. These rela- tion shipsdemon stratethatthehornbl ende-gabbroi c intr u- sionswere emplaced into the hornblende diorit e when a sig nif icant fraction of melt was st illpresentin thelatt er.This led to min glin g and hyb ridi sati on between the intruding gabb roic melt and the pre-existi ngcrysta l mush.

Mafic dykes in the Layered Series Peridotiteandolivine gabbro dyk es

The olivinegabbrosofthe LZhavebeen intrudedby anearly generat io nofsteep lydipping,N-S striki ng,perid ot it e dykes almost at rig ht ang lesto the modal layering. These were closely foll owed in tim ebymedium-grained olivi negabbro dykes (Fig.5). Thesedykeshave been observedonlyat a few localiti esin the cent ral-nort hern part ofthe LZ andseem to be arelativ elylocal feat ureof the GIC.(The dykes are not show nin Fig.2).Theyhave widthsof 0.5-1m andcan befol- low ed alongstrike forupto 100m.Theirmargins show no evidenceofchilli ng, butfeat uressuchasbrid ging and offset ofthe layersin the olivinegabbroindi catethatthehost rock was sufficie nt ly rigid to be fractu red at the time of dyke emplacement. Inplaces,thepatt ern of intrus ionofthe dykes is sinuous. Thereis also localfineinterdigit ation of veinsof dyke mate rial wit h the host rock. Locally, the peridot it ic dykes appear to have replaced or mechanically eroded plagioclase-richlayers intheLZ olivi negabbro.

Hornblende-gabbroicdyk es

Hornbl ende-gabbr oic dykes are numerousin bot h the LZ and theMZ,but thereis a gradualdecrease inthefrequ ency of dykestow ardstheupperpart ofthe MZ (notshow n inFig.

2).Wherecross-cutting relationship s can be observed,the hornbl end e-gabb roi cdykes are consistent ly younger than theperid otite andolivinegabbrodykes. How ever, closesimi- larit ies inorientat ionand theintrusion patte rnsuggest that theyhave a commo norigi n.This is part icularlyevidentwhere olivi ne gabb rodykes and then hornbl ende-gabb roic dykes have followed along the same sinuous path asperidot it e dykes (Fig. 5), and whereall these dykes show the same, partl y branching, patt ern of intrusion in the enclosing lay- ered olivine gabbr o.

Thecomposition of theserocksranges from magnetite- bearing hornbl endite through horn bl ende gabbro and locally into anort hosit ic varieties. The dykes are fine- to medium-grained, and generally have a lami nated texture caused by thepreferred orientat io nofhornbl ende andplagi- oclase parallel or sub-parallel to the contact. Commo nly, thesedykeshave afinelamin at ion orbandin gparallel with theirmargin s.Individuallamin aelbandsof 1-2mm thickness are commo nly fluentl yfold ed intoopenst ruct ures.Locally, a band ingreminiscent of modallayering occurs nearly atright angles tothedyke walls(Fig.5).

Thedykes are traceable alongstrikefor20to 50mbefore they wedgeout or merge wit hot her dykes of simi larcompo- sit ion.Their widt hisgenerallybetw een 2 and20 cm,but can belessthan acentim etre, exte nding alongstrike for several met res.Dykes widerthan ametr e onlyoccur locally. Chilled contactsare never observed,butwhere dykes intr ude larger

NGU-BULL435,199 9-PAGE 14 GURUB.MEYER &TORGRENNE

Fig. 5.A.Complex, mutual intrusiverelation shipsbetweenco-ming linghornblend ediorit e (light)and microgabbr o(dark).Back-veining ofthe horn- blendedioriteintothe microgabbroismarked wit h anarrow. Outcroplocated along the riverSkorovasselva.B.Close-upof hornblende microgabbro, forming complex bodies of variablethicknessin hornblende diorite.Outcrop located along theriver Skorovasselva.C.Coarse-grainedperidotite dykes (dark brown ,markedwit h Pl,succeededand partly cut by a subparallel,thinnerdyke ofolivine gabb ro (palebrownish-grey,weakly outlinedinwhite).

Thedykesintrude layered olivinegabbro of the LowerZone nearly atrightangles. Locatedinthe centralpart of the Lower Zone.D.Hornblende- gabbr oicdykein theeastern part of theMiddleZone gabbronorite.The dyke displaysa weakplastic foldingof the modal bandingwhich may be due tolateral magma flow.E. Hornblende-gabbroicdykeintheLowerZone, showing branchingapop hyses intothe host olivinegabbro.Located in the cen- tralpart of theLowerZone.F.Dykes of hornblende gabbro(dark) andsucceeding hybrid int rusions(medium grey)in gabbronorite (light grey)of the MiddleZone.Located inthe eastern part of the Middle Zone.The circulardark areais a wet spot.

GURUB.MEYER&TOR GRENNE

blocks of the coarse-grained olivi ne gabbro,they seem to follow angularfract ures inthe rock. In contrast, the pattern of intr usion into the layered olivine gabbro and the gabb ronorite is complex(Fig.5). andindividualdykescom- monly branch into a fine web of thin veins in thelayered gabbro.

The contact s of the dykes in the MZ are locally well defined.However,mixing and mingling is also quitecom- mon and obviously related to intrusion of the dykesinto the gabbronorite at a late-magma tic stage.This process has resultedin transit ional boundariesbetween thehornblende- gabbroic dykes andthegabbr onor itichostrockanda variety of complex,mut ual,intr usiverelations hips(Fig.5).

P egm atitic dyke s

The hornblende-gabbroic dykes in the Layered Seriesare commonly accompanied by pegmatitic dykes of asimilar composition(Fig. 6).These are found mainly in the LZ, but dykes ofthis type arealso observed morelocally in theMZ gabbronorite.Theyusually showamorerectilinear patte rnof intrusio nthan theirfiner grained counte rparts,butthetwo varieties were evidently closely related in time and space since some of the hornblende-gabbroic dykes are cut by

Fig.6. Composite, peg mati t ic to fine-g rained, hornblende -gabbroic dykesintrudin g oliv inegabbro.The preferred crystal growt hof horn- blend eperp end icul artothecontact s is welldevelop edbelowthe com- pass.Locatedin the easternmostpartoftheLower Zone.

NGU-BULL435,1 99 9-PAGE15

pegmatitewhereasotherscut the pegmatitic dykes them- selves. In some cases transitional boundaries have been observed,providi ngfurt herevidenceof their closerelation- ship.

The widt hofhornblende-gabbroicpegmatit e dykesvari- esfrom 20 cmto 1mand individu al dykes can be followedfor up to 300 m along strike.The textureis striking,wit h crystals of hornblende up to 10 cm growing perpend icular to the walls.Commonly they show conspicuous bilateral banding parallel to theircontacts in which hornblende-rich and pla- gioclase-rich, or fine- and coarse-grained layers alternate.

The euhed ralhornblende crysta lshavenucleated along the contactswit h thehost rock, alongsmallxenoliths ofhost rock in the dykes or along the contacts to hornblende-gabbroic dykes (Fig. 6). In hornblende-rich bands, plagioclase and minoramount sof magnetiteandsulphidesoccupytheint er- st it ialmatrix.

Relationships between the Layered Series and the wall rocks

The contact betweenthe MZ gabbronorite,theUZ quartz dioriteand themetavolcanic wall rocks islocally sharp and can be defined within centimetres. However, more com- monly thecontact isatransitionalzone 10-50 m thick,in whicheithergabbro noriteorquartz dioriteforma networkof veins alongcracks and fracturesin the wallrock(Fig.7).the veinsbeingthinnerand more scarceoutwards.Onthemap (Fig.2)the contactisdefined along a zone whereintr usive rocks predo minate overwall-rockmaterial.

The metavolcanicwallrocks nearthe easterncontact of the Layered Series haveapronouncedN-Sto NW-SEfoliation with westerlydips of 30-45° .The rocksvary from ast rongly sheared state in whichpillow structuresand dykecontacts arest ill recogn isable,to a completelypenetrative mylonit e.

In places,this zoneof deformationis cut bythe intr usivenet- work origin atingfromtheLayered Series(Fig.7).

Locally,the net of gabbronoritic or quartz dioritic vein s emanati ngfrom the intrusion is cut by,or has mingled wit h, very leucocraticquartz-feldspar-rich veins. Both at outcrop and in thin-sectio n,theseleucocraticveinsshowprogressive transition sincompositio n towards that of themetavolcanic host rock.Compared wit h typical rocks of the Skorovass Comp lex,thesemetavolcanic wallrocksappearto bemore recrystallisedand deplet ed infelsicmin erals,andit is likely that theleucocraticveins areleucosomesformed by partial melti ng of the metavolcanic rocks.Thick leucosome veins locally contain inclusions of gabbronorite and quartzdiorit e which appearto havebeenat least partly crystallised at the timeof emp lacement.

Alongthecontact zone,the foliat io n described aboveis locally overprinted by zonesof super-ducti le shearing in whichmigmati te-Iikerocksaredevelop ed.These migmatites consist of achaot ic mixture of highly sheared MZ gabbro- no rite or UZquartz dioriteand undeformed veinsof similar composition,and variably deformed wall rockswhichshow transiti onsinto sheared as well asundeformed leucosome veins(Fig.7).Inplaces,theleucosome veinsclearlycut the zones ofductiledeformation.

NGU-BULL 435,1999-PAGE16 GURU B.MEYER&TORGRENNE

Fig.7.A.Net-ve ining ofmetav ol canicwall rock byquartzdior it e atthesoutheas tern wall-rockcontactof the Layered Series.B.Fragments of part ly sheared met avo lcan icrock sina matr ix ofundeform edquartzdiorite.Eastern partofthe contac tzone of theLayered Series.C.Ductiledeform ati onof sheared met avolc an ic wall rock sinthecontact zoneof theLayeredSeri es.Theoutcropislocat ed 10 m fromthat of B.D.Leuco someand relicsof meta- volcanic ro cksinthe easte rncontac t zone of theLaye red Series.A markeddeformat ion fabric is presentin both the leucosom e and themet avolcan ic ro cks; however,thisislocally cut by und efo rmedleucosomeveins.

In the area southeast of the lake Fjellskarvtjerna (Fig.2), the highly ductile shear zone along theintrusive contactis cut by a semi-ductile shear zone2-3m wide. This separates the Layered Series and its contact zone from moderately deformed metavolcanicrocks.This later shear zone contin- ueswestwards along the southern margin of the GIC to the area of Murfjellet. It is cut by an even later,3-5m wide,sini- stral shear zonewit h a myloniticfabric running N-S alongthe eastern contact of the Layered Series(Fig. 2).To the north- west, a20to30m-wide thrustzoneseparates the rocks of the SkorovassComplexfrom a sheet of biotite- bearing gneiss, mica schistand deformed granodioritewhich form sa distinct subdivision wit hinthe Gjersvik Nappe.This was termed the Tunnsjc elvThrust Sheetby Reinsbakken&Halls (1987).

Pet rography

Wall- rock xenolit hs

Inthemetavolcanicxenolit hs, theoriginal wallrocks have been recrystallised so that they have a marked granulartex-

ture (Fig. 8).Thetextureandmineralogy ofthe reactio nrims mimic themin eralogy of thesurroun ding rocks of eith eroli- vinegabbroor gabbronorite.Themineralogyismadeup by plagiocla se(50-60%),Ca-rich pyroxene (10-20% )and Ca- poor pyroxene(15-25%),magnetit e(1-3%), minor ilmenite and pyrite« 1%),brow nhornbl end e(2-10%,inplaces>30%) and locally10-20mm-larg epoikiliticolivines(1-5%).

The anorthositic xenolit hs consistchiefl yof stron gly saus- suritised plagioclase.Someplagiocla se crystalsshow slight bending of albite twinlamell ae. Thistyp eof deform atio n is not observedin the surroundi ng layered olivine gabbro and was apparently imposed priorto emp laceme ntof theGIC.

Xenoliths of coarse-grained olivinegabbro Thecoarse-grai ned olivin egabbr o consist s mainl yof euhe- dralto rounded oliv ine (1-4 mm), which forms anadjoining networkof grains(Fig. 8).However,at both theoutcrop and thethin -section scale, euhedra l phenocrysts of plagioclase from2 to10mm indiamet er form the mostobvious compo- nent ofthe texture.A few small grains ofplag ioclase may be

GURU B.MEYER&TORGRENNE NGU-BULL435,1999 -PAGE17

Fig.8.Photom icrograph s ofthe text ur es in thin sectio ns.All photos are8m minwidthand heig ht. A.Metavolcanicwall rockxeno lit hin Lower Zone.B.

Coarse-g rained olivinegabbroinxeno lith inLower Zone.C.Troctolitein LowerZon e.D.Olivine gabbroinLower Zone.E.Gabb ronoritefrom Middle Zone.F.Quartzdiorite fromUpper Zone.Abbreviat ions:pi = plagiocla se,px =pyroxene, mt=magnetite±minor ilmenite, amph = am phi bole.01= olivine, qz=quartz and bt = biotite.

NGU-BULL435,1999-PAGE18

enclosed in thecoresoflargerolivines.The modal prop or- tionsofolivi neand plagioclase aretypically 45-60%and25- 40% respectively,but moredunitic and anorthositic layers occur.Ca-rich pyroxeneand locally brown hornblendeform oikocrysts 1-10cminsizewhichoccupy10-15%of the modal volume. In additio n, 0.5 to 1% of magnetite with lamellae of ilmenite occurs as interstitial, partly oikocrystic grains.

Ca-poorpyroxeneis present as coronas around olivine.

Serpentinisationand formationof magnetite fromolivine is commonalong thinfractu res runningacross grain boun- daries.Coronas duetoreaction between plagioclaseand oli- vine and alteration of Ca-rich pyroxene and brown horn- blende to green amphibole arecommon alongmafic dykes.

TheLayered Series

The Lower Zone

The LZ olivinegabbroconsistsdominantlyof plagioclase (60 - 70%)and olivine (15-25%),withvariableamounts ofCa-rich pyroxeneand minor magnetite(Fig. 8).Comparedwit hthe xenolit hic coarse-grained olivine gabbro, thetext ure ofthe rockis finer-g rainedand less poikilitic. Plagioclase is tabular wit h grainboundaries partlycontrolled by adjacent olivine, and there is a tendency for plagioclase to form laminate d aggregates.Thegrain sizevariesfrom 1 to 3mm;however, smallergrains «0.5mm)are enclosed in the cores of single olivinegrains.Olivine (0.5-1mm)is anhedral torou nded and locally has areacti on rim of Ca-poorpyroxene.

Ca-rlch pyroxeneis rarein the lower part of theLZ,but becomes more abundant upwards where it form s subhedra l grainsof 1 to 2 mm in size (Fig. 8).Locally,this pyroxene formsoikocrysts enclosing smallroundedchadacrystsof pla- gioclase.The Ca-richpyroxenecontains thin, densely spaced lamellae of Ca-poor pyroxene,in addition tohair-like lamel- lae of exsolved magnetit e. Brown hornblende occurs as small, inte rsti t ial grains «2%),but nearmetavolcanic xeno- liths, bigge r, oikocryst ic hornblende grains (1-5 mm) are abundant.Magnetitewith exsolvedilmeniteoccursas small oikocrysts, generally closely associated wit h Ca-rich pyrex- ene.

Coronasareformed locally aroundolivinein contactwit h plagioclase,especially alongmafic dykes in the gabbro. The corona sconsistof aninner rimofCa-poor pyroxeneandan outer rim of green amphibole with symplectitic spineI towardstheplagioclase.Along dykecontacts,olivinemaybe part lyto complete ly repl acedbyasympl ecti cintergrow th of Fe-Ti oxide and Ca-poorpyroxene.

The Middle Zone

Thegabb ronorite of theMZ containsvariable amou nt sof plagioclase (60-65%), Ca-rich pyroxene (20-25%) and Ca- poor pyro xene (15-20%) in addition to magnetite (2-4%), brownhornblende(0-4%)and locallyapatite«0.5%)(Fig. 8).

Plagioclaseand Ca-poorpyroxenearecommonly tabular and displaya clearmagmati c lamination whichis subp arallelto modallayering in the gabbronorite.TheCa-poor pyro xene contains small plates of more strongly pleochroic brown amphibole arranged along crystallographic planes.Ca-rich pyroxene occurs eit her as tabulargrainsorasmedi um-sized

GURU B.MEYER

s

(2-4mm)oikocrysts whichincluderounded grain s of plagio- clase.The Ca-rich pyroxeneis locally associated witha rimof pleochroic brown hornblende,probably representinga late- magmat ic overgrowth on the pyroxene.Brown hornblende forms oikocryst s with inclusion sof Ca-poor pyroxene and magnetite,particu larlynearmetavolcanicxenolit hs. Intersti - tialmagnetit e,partly withsmallergrainsof ilmenite,isclosely connected withCa-rich pyroxene.Apatiteformssmalleuhe- dral grains « 0.2 mm) which are partly or complete ly enclosed in plagioclase.

The Upper Zone

Thequartz dior iteof the UZ consistsmain lyof plagioclase (40-55%).brow n hornblende(15-29%),biotite(10-15%),Ca- rich and Ca-poor pyroxene (5- 15 %).quartz (5- 10%) and minor amoun tsofmagn etite(1-2%),apatiteand sulph ides (Fig. 8).From north to south,there is a general increasein the quartz andbiotitecontents atthe expenseof pyroxene. The plagioclase is mostly euhed ral, forming laths and tabular grainsof0.5to 3mm in size.Hornblendeforms large oiko- crysts(1.5cm)withchadacrysts of plagioclaseand pyroxene.

Biot it e is finer grained butotherw iseshowsa similartexture.

Ca-rich and Ca-poo r pyroxene bothform clustersof sub he- dral tabu lar or prismati cgrainsof relativelysmall size(0.2-1 mm). Ca-rich pyroxene is present throughout the UZ, whereas the occurrenceof Ca-poorpyro xene is restrict edto the lower part of the UZ.Quartz is clearly interstiti alto all othermineral phases.Magne titeis presentas small, rounded to oikocrysticgrains.

The gabb roic to dioritic domains in the UZ are text urally variablebut generally coarser grainedcomparedtothe'nor- mal'UZ quartz diorite.The amounts ofbothCa-richandCa- poor pyroxene are high er, whereas quartz, brown horn- blendeand biot it e are lessabundant(0-5%).The granodio- ritic to quartzmonzodioriticveinsand dyke-likebodies in the UZ are fine- to medium-grained and equigranu lar,and are characterisedby subhedral to euhedralgrainsof alkali feld- spar(- 10%)and plagio clase(25-30%).The content of quartz as inte rstitial grains varies between 40 and 50%. Minor amoun tsof calciteand biot iteare presentlocally.

The fine-graineddoleriticintrusions,presentbot hasindi- vidual dykes in the UZquartz diorite and in the composite dykes together with thegranod iorit icto quartz monzodior i- ticmaterial,consist mainly of partlysaussuritisedplagioclase and brow n hornblende(variably alteredto green amphibole) together with minor amounts ofc1inopy roxene and biot ite.

Highly porp hyrit ic, plagioclase-rich variet ies can be seen locally,andsome dykes cont ain conspicuousclot s or glome- rocrysts composed mainlyof hornblende.Up to 10%quartz can be present in the mafic portionsof composite dykes.

Interstit ial grains of opaque minerals, mainly magnetite, makeupfrom1 to 5%of the rock.

The hornblende-d ior ite -g abbro series

The hornblende-diorite-gabbro series is,in general, more alt ered by metamorphism than the Layered Series.Locally, unalteredor moderately altered parts composed mainly of plagioclase and brown hornblend e wit h subordinate mag-

GURU B.MEYER&TOR GRENNE

netit eand apatite are preserved.Quartz is present asaminor phase locally in the centralto northern parts of the intrusion.

The texture s,grain size(0.5-5 mm) and the modal propor- tions are highly variable. Plagioclase and primary brown hornblende are mostly subhedral. Rims of hornblende around coresof Ca-rich pyroxene were formed by syn- or late -magmat ic overgrowth on the pyroxene. Parts of the diorite show a conspicuous texture defined by growth of long, euhedral,hornblend ecrystalswit hint erst itia l plagio - clase.Thegrain sizevaries from pegmatitic (1-5 cm)to fine- grained « 1 mm).Quartz is presentassmall interstitial grains

« 0.2 mm).Magnetite is presentas rounded individual grains and as small poikil itic grains,whereas apat it e formssmall euhedralgrains mainlyincluded in hornblende.

The hornbl ende -gabb roic intr usionsin thediorite have mineralog ies and text uressimilarto thoseofthe hornblende- gabbroicdykes describ edbelow.

Mafic dykes in the Layered Series

Peridotite and olivineqabbro dykes

Theperidotite and olivine gabbrodykesbothhaveequig ra- nular,cumulate-liketextures.Theperidotitedykesarecom- posed of olivin ewit h interstit ialplagiocla seand mino rCa- rich pyroxene,theolivin egabbro dykes of plagioclase,oli- vineand Ca-rich pyroxene.Ca-poor pyroxeneoccurs as reac- tion products around grains of olivinein both types of rock, and oikocrysts of brown hornblende are common in the oli- vine gabbro dykes.

Theperidotite dykes are coarse-grained withagrain size of 5-10 mm. Partly serpentinised olivine forms a densely granularmatrixin which subhedralplagioclase phenocrysts and oikocrystic Ca-rich pyroxene arescat tered. Magnet it e occurs exclusivelyas a reaction product of the serpentinisa- tion of olivine. The olivine gabbro dykes arefine-grained(0.1 to 1 mm) and have a partly granular texture in which are com- bined rounded grains of olivine,rounded to subhedral grains of plagioclase, oikocrystic Ca-rich pyroxene (1-2 mm),and interstitial brown hornblende which coexists with varying amountsof subhedral magnetite and ilmenite.Plagioclase forms a continuous networkandis the dominantphase.

Hornblende-gabbroic dykes

These dykes consist of hornblende, plagioclase and magne- tite in variablemodal proportions.In general, hornblende is the dominant mineraland a few dykes arepurelyhornblen- dit ic.Hornblende and plagioclasehave a grain size of 0.1 to 0.5 mm and are largely euhedral. Magnetite is generally smaller than 0.1 mm and is almost exclusivelyassociated with brown hornblende.In most dykesthereis a marked preferred orientation of elongate plagioclase and hornblende grains subparallel to the contacts.

Contacts between the dykes and theolivine gabbro are characterised by symplect it ic int ergrow t hs of Fe-Ti-oxide and Ca-poorpyroxene,aswellas corona structuresbet ween plagioclaseand olivine (see also above) in the hostolivine gabbro.Thesefeatures are common over an intervalof a few centimetresalong the contact. Single grains of plagioclase from the host rock are enclosed and evenly distributed

NGU-BULL435 .19 99 -PAGE 19

wit hin thehornblend e-gabbroicdykes.Grainsof olivinealso occurin the dykes, but only along thecontactsand always partly or completely alteredto asymplect it ic intergrowth of Fe-Tioxid e and Ca-poor pyroxene.

Discussion

Fordescriptivepurpo ses,theGrond alsfjell lntrusiveComplex and surrou ndings havebeensubdivid ed into nine disti nct geolog icalunitsthat have beenmappedon the basis of field observat ions, petrography and mineralogy.The magmatic relationships and modes of emplacementof these units are described below.

Wall rocks

The oldest rocks recogn ised in the Grondalsfjell area are the metamorph ic volcanic and intrusiverocksof theSkorovass Complex,forming both the wall rocks to theGICandxeno- liths within theGIC The general absence of primary textures in the xenolit hs implies that theyrecrystallised and reacted wit h thegabbroicmeltto a completelynewmineralog y and texture.Thepresenceofa reacti onzone around mostxeno- lit hs inthe olivine gabbro and gabbron oriteisfurt her evi- denceof interact ion wit h the enclosing melt.Theorigin of theanorthositic xenolit hs ismore ambig uousand fromthe present observation sitisnot possibl eto deducetheirorigi n in any detail. They may be a part of the assemblages of pre- GIC subvolcanic intrusive rock, because they locally form inclusions within xenolit hs of the coarse-grained olivi ne gabbro,but theycouldalsorepresentanearlycrystallisati on phase from the same magmafrom which the coarse-graine d olivin egabbrocrystallised.

A penetrative foliationis observedthroughoutthemeta- volcanic sequence.Thisis not seen in the GIC, and thereis little doubt that the metavolcanic rocksof the Skorovass Complex were affected by regional deform ationand associ- ated metamorphism priortoemplacementoftheGICClose to the contact zone of the LayeredSeries,this early, regio nal foliation is transposed int o a compl ex, ductile and partl y mylonitic shearzone which is sub-parallel to the intrusive contact. Along this intrusivecontact,there iscompellingevi- dence of extensive parti almelting of the metavolcanic wall rocks.Within the temperature regime of at leastthe UZ, sig- nificant melting of basaltic rocks wit h formation ofst rongly quartzo-feldspathicmelts,canbeinterpret edonly in term sof meltingof a hydrousmineralassemblagesuch asgreenstone or amphibol it e (Helz 1976).The observedcomplexrelation- ships between leucosomes, intr usive rocks and shearing impliesthat theductiledeformation waspenecontempora- neous with the intrusiveemplacement of theGIC It isnot clear,however,whether the deformation wasa result of the intrusiveevent orif itwas relatedtoatectonic event of more regional signif icance.

Xenoliths of coarse-grained olivine gabbro The presence of coarse-grainedolivinegabbroonlyas xeno- liths implies that they had crystallisedprior to thegeneration of the Layered Series. However, no coarse-grained olivine