A possible basement to the Mesoproterozoic quartzites on Hardangervidda, South-central Norway:

ELLEN M.O.S/GMOND,ANNEB/RK ELAND

s

^{BERNARDBINGEN NGU-BUL L 437 ,2000 -PAGE 25}

A possible basement to the Mesoproterozoic quartzites on Hardangervidda, South-central Norway:

zircon U-Pb geochronology of a migmatitic gneiss

ELLE N M.O.5IGMOND,ANNEBIRKELAND&BERNARDBING EN

Sigmond,E.M. 0., Birk eland , A.& Bing en,B.2000 :A possiblebasem ent totheMesop rot erozoi c quart zites on Hardang ervid da, Sout h-ce nt ral Norw ay:zirco n U-Pb geoch ro nology of amigm atiticgneiss.Norges geologiske undersekeise Bulletin437,25 -32.

OnHardang ervidd a,westof theMand al-U staosetfaultzon einSout h Nor w ay, some80-90%of the crustconsistsof orthog neissesand foliat ed and massive granites. In thisplutoni cenvi ronmenttheMesoprot er ozoicquartzites oft he Hettefj or d enGroup are foun d asraft s alloverHardangervid da.AtViuv at net,amigmat iti c gneiss isint erlayered with andpossiblyunderli esthe qua rtz ites.Thismigmatit ic gneiss hasnoint rusive contac twith thequart zit es andcould possiblyrepr esenttheirbasem ent. Backscat te redelect ro n imag es and U-Pbionprob e datawereacquired on zircon fromthisgneiss.Themainpopulati on of zircon sgives a weig hte daverageageof1468±12Ma.Thisisinterp retedto probablyreflect a migm atisationevent(evide nce for whichis obse rv edin thefieldand in the sam p le)and pointsto ahigh-g rademetam orp hism occurring afterdeposition of theHettefj ord enGroup. Aminor zirco n population isre- presentedby a zo ne d zircon at1667±22Ma.Thisdat e couldcorrespo nd tothe ageofcrysta llisationof themag - mat icprotolithof thegneiss; andit is similar totheageof a foliatedgranitein theregi on ,theMarsbrotet granite.A featureless zirco noverg row t h at1167±104Maisbelieved toprovide evide nce fo r Svecono rweg ianmeta mo rp hic overp rint i ng in theregi on . Alte rn at ive interpretation s of thedat a setarepossible; and mor e geoch ro no lo g ica l dat a are necessaryinorde r tounderstandthereg ionalsignificance of the1.67and1.47Ga events.

EllenM.O.Sigmond,AnneBirke/and&BernardBingen,Geologica/ Survey ofNorway,7497Trondh eim,Norway.

Introduction

The Hardangervidd aplateau,Sout h Nor way, is situated in the Sveconorweg ian orogen,to the west of theMandal- Ustaosetfault zone(Fig.1; Sigmon d 1985).Most of the rocks in the regionare ofplutonicorigi nandofgranit iccompos i- tion.Different generatio nsofintrusionsrangefromstro ng ly foliated orthog neisses to massive pluto nic rocks.Within these domi nant lypluto nic rocks,there areraftsof amp hi- bolite-fac ies gneissesand migmatites of uncertain origin anda fewrocks ofcertainsupracr ustalorigi n.

Among the metasupracrustalrocks,the quartzitesofthe Hett efjorden Group are of special interest because they occur through out theHardangervidd a region (Fig.2),and alsobecause they unambigu ou sly representepicontine nta l sed iments oncedeposited on a(crystalline?) basement. To find and definethisbasement is ofgreatimpo rta ncefor our understandi ng of the geolog ical histor y of theHardanger- vidda area.Thishasturned out to be part icularlydiffic ult, becausein mostplacesa closeexaminat ion has shown that the quartzit eswere int ruded by the surround ing ort hog- neisses and foliated granites. However,near Viuvat net,a migmatiticgneiss interlayered with,and possiblyunderlying the quartzites, shows non-int rusive relationships to the quartzi tes,andthus could representthe basement tothese sedimentary rocks.Inthis paper new U-Pbionprobedata are reporte d from zircons extracted from this migmatit ic gneiss.

Pre-Sveconorwegian regional

geochronology of Hardangervidda

The Precambr ian crust west of the Mandal-Ustaoset fault zone displaysmainlyhigh-grade orthogneissesandfol iated granitesofdifferentages wit h larger and smaller inclusions of migmatites, gneisses and metamo rphic supracrust al rocks.All rocks are int ruded by massive Sveconorwegian granitepluto ns form in g aN-S trendi ngbeltextendi ngfrom Mandaltothe centra lpartof Hardangervidda (Fa lkum 1982, Sigmondetal. 1984, Sigmo nd 1985) (Figs.1 and2).

Inthe area under consideratio nin this paper(Figs.2 and 3), west of theMandal-Ustaoset faultzone and east of the granitebelt,regional mappi ngclearly reveals thattherocks have gonethroughalong andcomplicated history of mel- ting andremelti ng,anddifferentstagesofdefor matio nand metamor phism,prior tothe intr usionof the Sveconorwegian massive granitepluto ns(Sigmo nd 1998).The ort hogneisses, the migmat itesand thedeform ationresult in g in thedomi- nantE-Wfold ing and foliation (Sigmo nd 1988 and Fig.3)are thuspre-Sveconorwegia ninage (pre-1100 Ma).These obser- vat ions havebeenconfirm edby afew absoluteagedetermi- nation s ofort hog neisseswhichyieldedint rusiveages inthe range1.65-1 .50Ga.These data havefurther show n that an ArchaeantoPalaeopr ot erozoic crusta lcompo nent hasbeen recycled in thecrust westof theMandal- Ustaosetfault zone;

yet the data have sti ll failedtoprovethe existe nceofexposu- resof magmatic rockscrystallised before ca.1.7Ga(Ragn-

NGU-BUL L437,2000 -PAGE26 ELLEN M.O.SIGMOND,ANNE BIRKELAND

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,.,...Fault.shear zone Caledonid es.Oslo rift

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Anorthosile

~ ^Granite

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Supracrustal rocks

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Fig.1.Geo logical sketch map ofSouthNorwaysimplifiedfromSigm ond et al.(1984). Numbersrefertolocalities quotedin thetext .

hildstveit etal. 1994, Birkeland et al. 1997).

One of the light-colour ed foliated granites, the Mars-

brotet granite

(locati on1in Fig. 1),gave a U-Pbupper inter- cept zircon age of 1649+33/-19 Ma (Ragnhildstveit et al.

1994).This granite displays cross-cuttingrelationships relati- ve to thesurrounding orthogneisses,these gneissesshould thus be older than 1.65Ga.The

Skr ykkjevieren orthogneiss

(lo- cation 2 in Fig. 1) is now here in direct contact with the Marsbrotet foliated granite, and igneous looking zircons from this orthogneiss show ed a bimodal age distribution with frequency maxima at 1591

±

7 and 1500

±

7 Ma (Birkelandetal.1997).The firstdateisprobably recording the age of intrusion of the prot olith , whereas the second is thoughtto be relatedto aremelting with the formationof felsicveins. Aminor 2.84- 2.73 Ga zircon population in this gneissmay possibly be providing evidence for inherited Archaean material in thesource regionof the magma or of

contaminationbyArchaeanmateri al duringintru sion.

High-gradesupracrustalsaredivisible into the

Festnings- nutan

and

H ettefjorden Groups

(Sigmond 1998).TheFestn- ingsnutanGroupconsists of lig ht coloured fine-to medium - grained gneisses, interpret ed asmetamorphicsupracrustal rocks.Itincludessomehighlydeformedgneissicconglome- rat elayers.The Hettefjorden Group consistsof medium-to coarse-grained grey quartzit e wit h some layers of mica gneiss.The quart zitecoversalargeareaaround Hett efjorden, but occurs elsewhere on Hardang ervidda only assmaller raftsless than1km'insurfacearea(Fig.2).

TheFest ningsnutan and Hettefjord enGroupsareclosely associatedinthe fieldandhaveexperienced the same pha- ses offolding and metamorphism.The relativetim ingofde- positionof the twogroups is diff icultto establish in thefield, but they aremost probably more or lesscoeval.Detrital zir- conswere separated and analysed from asamp le ofmeta-

ELLEN M.O.SIGMOND,ANNE BIRKELAND&BERNARD BINGEN NGU- BU L L 437,2000 - PAGE 27

conglomeratetakenfrom theFestningsnutan Group(locat i- on 3 in Fig.1; Bingen et al. submitted manuscript).Twelve Prot erozoic grains range from 1.99 to 1.57 Ga and three grains give ages in the range2.76-2.24Ga.These data imply that thedeposition of the sediment isnot older than 1572± 10Ma.Detrital zirconswere also analysedfrom a sampleof quartzit e of the Hett efjo rden Group (location 4 in Fig.1;

Bingenetal. submittedmanuscript).Nine ofthe32analysed zircons are Archaean and range from 3.25 to 2.77 Ga.

Proterozoic zirconsrange from 2.42 to 1.52 Ga withafre- quencymaximumat1.90-1.85Ga.Theageof deposition of this quartzite is not well constrained asthe data on the youngestanalysed zirconscatt er between1539±21Ma and 1480±17Ma.A conservat iveesti mate of theageof this zir- conisgivenby themostconcordant analysesat1536± 24 Ma,indicating that the deposition of the sediment is not olderthan 1536±24Ma.

In the Roldalregion,to the southwestof the Hardanger- viddaplateau,variousgneisses,mainlyaugengneisses,have

given a Rb-Srwhole rock errorchron age of 1488 ±30 Ma, (location 5in Fig.1;Berg1977).This age was interpretedas probably reflecting the age ofcrysta llisatio n ofthegranite protolith(Berg1977).

U-Pb ion probe dating

The sample

Sampl e 5.95-157 was collected east of Viuvatnet on Hardangervidda(Fig.3)in a ca.200m-thickunitofst rom ati c migrnat it ic gneiss(Fig 4)structurally int eriayered wit h, and possiblyunderlyingthequartzite of theHett efjorden Group.

Theseunit s areaffectedby a commondeformationand me- tamor ph ism,and the original field relat ionshipsarenotpre- serve d.Ascross-cutting or intr usiverelat ionshipswith the overlying quartzitehavenotbeenobserved,themigmatit ic gneisscould therefore represent part of an original base- mentto thequartzites.

The sample isamedium- to coarse-grain ed migmatitic banded gneiss wit h 1 to 5 cm-thick, alternating, dark-

N

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Veivatnet -h....

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Plutonicrocks,mainlygranites Ouartzite

Fig.2.Occurrences ofquartzit eon Hardangervidda.Theallocht honousPrecambrianrocksinCaledonian nappesand the autocht ho no usandalloc h- thon ous Cambro-Silurianrocks arenot shown.

NGU-BULL 437,2000 - PAG E28

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Light-colouredred granite,fine-g rained,porphyritic

+ + +

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Graniticgneiss,palegrey,fine-grained,dominantly metamorph osed granite Granite,palepink,fine-to medium -grained,foliated(M~rsbrotetgranite) Fest ningsn ut an Group

Gneiss,light -colou red,locally richin muscovit e,assume dmetamorp hosedsupracrustalrock HettefjordenGroup

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Lithologicalboundary,certain/uncertain

x

Joint,crush zoneor possible fault

Samplelocality for isotopicage determ ination,S95- 157;UTM,X:4468,Y:66682

Fig,3,Geological mapfromtheViuvatnetarea,Hardangervidda,show ingthesamplingsite.

ELLEN M.O.SIGMOND, ANNE BIRKELAND&BERNARD BINGEN NGU-BULL 437,2000 -PAGE29

Fig.4.Stromatic,migmatiticgneisseast of Viuvatnet.Agranodioritic paleosome alternateswith granitic neosome.Theneosomepods and bands generallyhave concordant,butalso in placestransecting con- tacts (Photo,J.Ragnhildstveitl.

coloured and light-coloured layers. The dark-coloured granodioriticpaleosome contains saussuritised plagioclase, amphibole, greenish-brownbiotite,some quartz and micro- cline,with accessory epidote,allanite,titanite,apatite and zircon. In some darker layers amphibole is prominent. The granitic neosome forms bands, veins,schlieren and pods, which generally are concordant withthe fol iat io nbut may also transect it (Fig.4).This indicates a syn-to post tectonic partial melting of the protolith.The neosome consistsmain- ly of microcline,quartz,plagioclase,greenish-brown biotite and opaque minerals (m ag net it e) with accessory epidote, allanite, chlorite, apatite and zircon. Plagioclase grains common lyco ntainquar tzinc lusio nsand showamyrm ekit ic texture.Perthite and ant ipert hi te textures are common in thefeld sp ar.It isworth noti ng thatmyrmekite,perthite and ant ipe rt hite are notobserved inthefel d sp arsin thepaleo - some; these st ruc tures are probably a result of the partia l meltingprocess.

Fig.5.Backscattered electron imagesof zirconsfrom the migmatitic gneiss east ofViuvatnet.Thelocationsof the centreof theSIMSanalyti- calspotsare indicated.

(A)Thecrystalhas a somewhat diffuseprismatic zonation.At theupper end of thecrystalthezonation is clearlytruncated,indicatinga breakin the originalcrystal.There is alsoa transgressiveovergrowth into the coreattheupper end of thecrystal,indicatinga late new growthor recrystallisationduringahigh-grade event.

(B)Thecrystalhas aprismaticzoningpattern.The zonationistruncated at thepointedtip and overgrownby afeaturelessnewzircon.At the oppositeend thereisa transgressiveovergrowth of thesame gene- ration ofnew zircon.

(Cl The crystal has a small, unzonedroundedcore.The zircon over- growth has aregular,oscillating,zoning pattern consistentwith a prismatic,euhedralgrainwhichdeterminestheouter shapeof the crystal.The crystal isbroken, and there is sometransgressive,featu- relesszirconalong theupperlong edge.

(D)Thecrystalhas a core withadiffuseprismatic zoning pattern.Along thesideto the right,anew featurelesszircontransgressesintothe core and transectsthe zonation.Thenew generationof zirconforms tipsat bothendsof thecrystal.

(E)The coreof thecrystalhasan irregular zoningwhichisnot controlled by the crystallographicst ructure.The rimhas aprismatic zoning indicati ngan originaleuhedral,prismaticexternal shape.

Method

Zircon was purified from the whole rock sample using a Wilfley water table, heavy liquidsand a magneticseparator.

About 60 crystals were mountedin epoxy and polished to approximatelyhalfthickness.Backscatteredelectron ima ge s of these grains were tak en ,to characterisetheint ernal struc- ture and guide spot analysis(Fig . 5). Nineteen grains were analysed for Th,U and Pb bySIM S usinga CamecaIM S 1270 inst ru m ent at the Nordsim laboratory,Swedish Museum of Natura lHistory,Sto ckh o lm.Analyses were carried out follo- wing the methodoutlinedin Whitehouse et al.(1997,1999).

They were performed with a ca.4 nA

0 ,'

beam and a spot size of ca.40 urn, using the Geostandard 91500reference zir- con with an age of 1065 Ma(W ied en beck et al. 1995).Acom- mon Pb correctionwas applied with modern isotopic com- position (St acey & Kramers 1975)on the basisof the '04pb signa l.Theanalyses are listed in Table1.They were plottedin a Tera-Wasserbu rg concordia diagram (Fig. 6), and the weighted average age was calculatedat the 95%confiden- ce level using theISOPLOTprogram(Ludwig 1995).

Results

Zirconswere abundantin the sample and the extracted zir- cons represent both paleo- and neosome.Zircon s in the sampleare long and short prisms with rounded tips.Almost round grains are also present. Twenty analyseswere perfor- med in 18grains.The analyses are concorda ntto slightly re- versly discordantexceptfora fewnotablydiscorda ntpoint s.

The 207Pb/'06Pb age of 16 analyses range from 1484±24 to

1399±44 Ma(Fig . 6) with a weighted averagevalue of1459

± 13 Ma (M SWD =2.8).If the concordant analyses in crack- free zirco nareselected,abetter average of 1468±12 Ma (n

=

11;MSWD

=

1.8) is ob ta ine d.These ana lyses have a well- gro upedTh/Urat ioof 0.43±0.07,typicalfo r zirconcrysta lli- sedin a magmati cenv iro nme nt(Fig.6).The main zirconpo-

NGU-BULL 43 7,20 0 0- PAGE30 ELLEN M.O.S/GMO ND,ANNEB/RK ELAND

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207Pb 206Pb 0.10

1700

1600

_~ o

1667±22 Ma

S95- 157:gneissat Viuvatnet, Hard ange rvid da

0.09

10error ellipse

11concordantanalyses: 1468±12Ma

o

Fig.6.(A)Tera-Wasserburg concordiadiagram with51M5zircon analyses ofsample595-157. (B)Th/Uratio of the different typesof analy- ses asa functionofage.

1300

•

5.4

20error bar

~

5.0 4.6

1200

4.2 Metamorphic ov erg row ths

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207Pb/206Pb _{Age(M a)}

pulation thusprobablyhasgrowninamelt,eit her inamag- ma chamber,or in thepartialmelt(t hegraniti cneosome).

One grain, which cannot bedistinguishedon thebasisof morphologyand zoning,gives anage of 1667±22 Ma(Fig.

SA),and theunzonedcoreofanot her grain(Fig.5C)provides a discordantage of1642±36Ma.Overgrowthsof presuma- bly metamorp hic origin are commonly observed at the tips ofthe grains (Fig s. 56 and 50).They are generallytoonarro w to permit good analyseswit h the40 urn beam.Two over- growth analysesshowlow U contents(12 and 52 ppm)and lowTh/Uratios (Fig. 6).Oneof them isconco rdant at 1167± 104 Ma,butthispartiallycoverstheovergro wth-coreinter- face.

6ackscatteredelectron imagesof zircons revealthe dis- tribution of trace element s, mainlyHf,in the crystal latt ice.

The main populationof zircons (1468±12 Ma)showsan in- ternalpatt ern whichisnot thenarrow, regular, oscil latoryzo- nation typical of ideal magmatic crystal s.Thezonation is more diffuse and show sfading and broadening of the pri- mary patt ern(Figs.56 toSE),ormaybequiteirregular as,for

exampl e,the core of the zircon in Fig.SF.According to Pidgeonetal.(1998), suchst ruct uresmaybeduetomigrati- on of traceeleme nts in a closedsyste m duringslow cooling at hightemp eratu res.The crystalsmay be brokenorsome- whatresorbed(Figs.56 and50).

One ofthe older crystals (1667 ±22Ma)has a somewhat diffusezonation,whichisclearly transect ed by ayounger generat ionofzircon.This indicates thatthe crystal hasbeen brokenata lat ersta ge(Fig. 5A).Anot her crystal(Fig.5C)has a featureless round core(1642±36Ma)surro undedbyan irre- gularly zoned rim (1436 ± 34 Ma).Such round coresare found inanatecticand partiallymelted rocks.Thecore surfa- cemaybethepaleo-dissolutionsurfaceofthe originalcrys- tal(Vavra 1990).

Theyoung erzircon ages(1191±144 and1167±104Ma) have been measured only in featurelessand locallytrans- gressiverims surroundingtheolde rzircons(Figs.56and 50).

This patte rn,toget herwiththe lowTh/Uratio(Fig. 4),indica- testhatthe overgrow ths are ofmetamorphic origin.

ELLEN M.O.SIGMOND,AN NEBIRKELAND

s

BERNARD BINGEN NG U- BU LL437,2000 - PAGE 31

Discussion

Asnot ed earlier,the migmatit ic gneisseast of Viuvat net, from which sample 5.95-157was collected,isinterlayered wit h quartz ites of the Hettefjorden Group. The new geo- chronologicaldata on thissample,clusteri ngat 1468 ± 12 Ma(Fig. 6),and on quartzitesoftheHett efj orden Group gi- vinga depositionageyoungerthan1536 ± 24 Ma(Bingenet al.submittedmanuscript), areinconclusive and can beinter- preted in different ways.Three interpretations(a,b,c)aredis- cussed hereafte r,and allare open torevision when more re- gionaldatabecome available in the future.

a) In this inter pretatio n themain zirconpopul ation of 1468

± 12 Ma is related to the conspicuous migmatisation eventrecordedin the migmatitic gneiss.Thezircondate of 1667±22 Ma isattribut ed to the age of magmatic crystallisat ion of the protolith.lt is wellestablishedthat a migmat isation eventcanbe associatedwith zircon crys- tallisation,thezirconsthencommon lydisplaya prisma- tic habit (Vavraet al. 1999).The zircons of the migmatitic- gneiss of this age are prismatic. Thedistributionof trace- elements is primarily determined by crystal growth and diffusion.Solid-state diffusion at high temperatures in closedsyste ms mayhave resultedin the shadyandeven convultedzoning patt ern of some crystals(Fig.5 E).This interpretation wouldimplythat the regionwas affected by high-grademetamorphism at ca.1.47Ga,and that the migmatiticgneissand thequartzit es ofthe Hettefjorden Group were deformed and metamorphosed togeth er during this event.The idea ofaca.1.45 Garegionalmeta- morphic event in the Rogaland-Hardangervidda area was originally proposedbyVersteeve(1975) on the basis ofa 1453 ± 60 Ma Rb-Sr errorchroninmigmatit ic gneisses in Rogaland(locatio n6 in Fig.1),andis compatib lewith the str uct ural and geochro nolog ical con straints on

Ta bl e1.lon-pr obe analysesofzirco n insample595-157

Hardangervidda.The closecorrespo ndencebetween the ageof 1667±22 Ma andthe 1649+33/-19 Ma age for theMarsbrotet granit e is noteworthy.It indicatesthat thesetwo rocktypesmay belong to thesamemagmatic event.This firstinterpretationis in accordance wit hfield observations.However, it is somewhat speculative be- causeofthelow abundance of zircon giving theproto - lit hage(one concordantgrain at 1667±22 Maand one discordantcore at 1642 ± 36Ma).lnamigmatite,thepro- portion ofnewly grown zircon relat ive to inherited zir- con variesfrom aminority(Oliver etal. 1999; Watt etal.

2000)to a majority (Anderssonetal. submi tted manus- cript,Vavra et al. 1999).Thelow abundanceof zircon at 1.67Gain theViuvatnet-gneisscould be explained by the meltingof thezirconsin theprotolit h during mig ma- tisation,or by a an originalsmall grain size.In the last case the magmatic protolith waslikelyto have been a volcanicorsubvo lcanicrock.This interpretation (a)leads to the conclusionthat the migmatiticgneissrepresentsa 1.66Ga year oldbasement to the quartzite,and that the two rocks have later experienced acommon metamo r- phismataround 1.47Ga.

b) Thesecond possibility isthatthe migmati tic gneissisa supracrusta lrock originallyinterlayeredwiththe quartzi- te,and thus could be included in the Festnings nut an Group .lnthefield, theorigin ally supracrusta l rocks of the Festningsnutan Group and themigmatitic gneissoccur near to eachot her(Fig. 3),butthey are quitedifferent in appearance.Inthe supracrustra lstheoriginalcharacter can be seen in some places(folded pebbles,smallphe- nocrysts, possiblelayering,etc).The rocks are finetome- dium grained and never show a migmatitic characte r.

The migmatiticgneiss ismedium to coarse grained and itsorigina l character cannot be discerned,eitherinthe

Gra in W' Witb ZoningC U

Pb

^Th

'··Pb '· 'Pb

±cr

'·· Pb

±cr

Ispot

'·'-P b '·'Pb ^{2 3 SU}

(p m ) (p p m ) (ppm) (p p m) (%) (%)

01a 130 0,70 opz 200 72 102 8200 0,1023 0,6 0,2878 2,2

43b 8S 0040 nz 204 63 12S 11010 0,1010 1,0 0,2393 1,9

03a 90 0,65 opz 259 81 92 110 10 0,0924 0,7 0,2S84 2,0

04a 110 0,60 pz 373 121 3S4 930 0,0900 0,9 0,2277 1,3

05a 95 0045 opz 161 S2 75 21590 0,0917 0,7 0,2610 1,9

11a 80 0040 opz 163 Sl 55 4010 0,0916 0,9 0,2598 3,2

20a 90 0,50 opz 401 129 199 36820 0,0927 0,5 0,2575 3,2

29a 125 0,65 opz 98 31 40 6610 0,0888 1,1 0,2634 2,6

32a 110 0040 opz 236 80 127 19130 0,0928 0,6 0,2696 2,8

33a 126 0,50 pz 174 54 69 7060 0,0920 0,7 0,2570 204

34a 7S 0040 opz 223 66 93 18560 0,0900 0,7 0,2408 1,5

35a 10S 0045 opz 222 74 97 15480 0,0917 0,8 0,2735 304

36b 90 O,SS opz 208 65 86 1600 0,0892 1,0 0,2515 1,7

37a 100 0,50 opz 214 69 94 13690 0,0927 0,6 0,2623 1,7

38a 120 0,55 opz 46 15 17 3730 0,0925 1,9 0,2647 304

40a 100 0045 opz 159 55 88 9710 0,0906 0,9 0,2752 2,2

40b opz 205 68 100 12670 0,0906 0,7 0,2673 204

43a 85 0040 opz 132 42 52 11340 0,0905 0,9 0,26SS 2,6

44a 85 0,30 opz 472 117 189 1060 0,0891 2,1 0,1973 1,8

4Sa 105 0,45 opz 222 72 114 4740 0,0910 0,9 0,2587 3,1

31a 80 0,3S m 52 12 10 1770 0,0788 2,6 0,1927 5,3

33b 126 0,50 m 12 2 0 6220 0,0797 3,6 0,1676 6,9

Disc·

(%)

14 6

-3

-s

-2 -1 16

6

'· 'Pb '

±cr ²⁰⁶Pb@ ±cr

'·"P b ^{2 3SU}

(Ma) (Ma)

1667 11 1630 31

164 2 18 1383 24

1476 14 1482 27

142S 17 1322 16

1460 13 149S 2S

14S9 17 1489 42

1481 10 1477 42

1399 22 1507 36

1484 12 1539 38

1467 14 1474 31

1425 13 1391 19

146 1 1S 1558 46

1408 20 1446 22

1481 12 1S02 23

1477 36 1S14 46

1439 17 1567 31

1438 14 1527 32

1436 17 1518 35

1406 41 1161 19

1446 18 1483 41

1167 52 1136 5S

1191 72 999 64

a)Wid thof the grain.b)Width/lengt hrat io.c)Zonin gpattern:op z:oscillato ryprisma ti c zon ing, pz:prismatic zonin g,nz:novisiblezoni n g, m:over - gro wt h.d)Degre e ofdisco rdan cein%,calc ulatedat theclose st2slimit. Blan ksindica teconco rdanc e wit h in2aerro r.e)Age

NGU-BULL 437,200 0 - PAGE 32 ELLEN M.O.SIGMO N D,ANNEBIRK ELAND&BERN A RD BINGEN

field or in the microscope. Furt her, the mainzirconpopu- lat ion at 1.47 Ga inthe migmatiticgneiss does not cor- respondto any of the zircon ages found in the twolith o- strat igrap hic groups,something that should be expec- ted ifthe migmat it icgneiss was part of the sequence.

Thisinterp retation isthereforeconsidered to be less like- ly than the first.

(cl A third,alternative interpretation is to attribute the main magmatic zircon population in the migmatit icgneissto the intrusionof a granodioritic sill into the quartzites at 1468

±

12 Ma. The zircon grain dated to 1667

±

22Ma could reflect eitherinheritancefrom the source orcon- tamination during intrusion.Inthiscasethe strong me- tamorphism and syn-to posttectonic migmatisationob- served in the rocks is not reflected in the zircon ages.The metamo rphism should then be younger than 1.47 Ga (but pre-SveconorwegianJ,butno such ageshavebeen reported from any rocks from this part of Hardanger - vidda.This interpretationthereforeseems less likely.

Whatever the interpretation,the age of 1167 ±104Maona metamorphic overgrowthis consider edto provideevidenc e for a later Sveconor wegian metamorphism in the Hard- angervidda region. This ageis imprecise and thusdoesnot represent a quantitative est imatefor thetim ing of meta- morphism.

Conclusions

On Hardangervidd a,the quartzitesof the Hettefjorden Group occurgenerallyasrafts in orthogneissesand plutoni crocks.

However,a migmatitic gneiss east of Viuvatnetcouldbe part of the basement to thequartzit es.The oldest zirconin this gneissare1667±22 Ma while the mainpopulation ofzircons givesan average age of1468±12 Ma.Thefirst age probably reflects the magmaticcrystallisation of theprot olith,while the second age most likely relates to the migm atisat ion event, and points to a high-grademetamorphismatabout 1.47 Ga.Unzonedzircon overgrowthsformed at 1167± 104 Mapoint to a Sveconorwegianoverprintingin the region.

From this interpre tatio n it follow sthat the migmatitic gneissis partofthe crystalline basement to the quartzites, and that both rockshavebeen metamorphosed and defor- med together afterthedepositionof thequartzit es.

The present data allow foralternative int erpret ations of the migmatit icgneiss -quartzite relati onship.Moredataare evidently necessary before we can reachaclearer under- standing of the Proterozoic geological evolu tion in the region.

Ackn owledg ements

SIMSdata were acq uiredat theNORDSIMlaboratory,eo-funded bythe Nor wegianResearch Council.T.Sunde,J.Vestin and M.Whitehou se are thanked for superv isionduring dataacquisit ion and data reduction.

Thanksaredue totheNGUlaboratory forXRFdata,mineralseparat io n and technical assistance.We ackn owl edge P.Padgetand D. Rob ertsfor reading themanuscript,and G.-J.deHaas and T.Torskefor providing construct ive and helpfulreviews.Thisis NORDSIM contribution41.

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