trondhjemites and a norite pluton from the SW

ODD NILSEN, BJ0RNSUNDVOLL,DAVIDROBERTS

s

U-Pb geochronology and geochemistry of

trondhjemites and a norite pluton from the SW

Trondheim Region, Central Norwegian Caledonides

ODD NILSEN,BJ0RN SUNDVOLL,DAVIDROBERTS&FERNANDOCORFU

Nilsen,0., Sundvoll,B.,Rob ert s,D.&Corfu,F.2003:U-Pb geochron olog yandgeochemist ryof trondhjemites anda norit epluton from the SWTrondh eim Region, Cent ral Nor wegianCaledonid es.Norgesgeologiske undersekelse Bulletin441,5-16.

Zircon analysesfromtwotron dhj emite s anda biotite noritefrom a partof the sout hwestern Trondheim Region have yieldedalmost identi calEarly Silurianages.Thebiotit enorite andoneof the trondhjemites,fromaquarryin theInnset massifin the Ste ren Napp e,produ ced analyticaldatawhich defin e conco rdiaages of435.8±0.9Ma and 434.8±0.5Ma,respecti vely.Zircons from atrondhj emite bodyat Nyvollen,Innerdalen,in thewesternGulaComplex, yieldedanupper-interceptage of433.8±0.8Ma.Allthreedates areinte rpreted torepresentthe ages of emplace- ment of thesebodi es.

Geochemi calsignat ures fromthese,andot her trondhjemitesintrudin gthe Koli Napp es of theTrondh eimRegion show themto be of high-alumina, continental-margin type.Fieldand microscopicst udiesindicate that theInnset massif,and other composite gabbr o-tr ondhjemite bodiesin thisregion,pre-dat e Scandian orogenicdeform ation and metamorphi sm.The ages reportedhereaccordwit haknown pattern of gabb ro-diorit e-tron dhj emit emagm a- tismthatisrecord edin theKiiliNappesinseveral parts of the ScandinavianCaledo nidesinlatestOrdov ician/Early Silurian time.This sporadicmagmati smiscoevalwit h the earlystagesofob liq ue,Baltica-Laurent ia platecollision, whereinsho rt-lived,transtensional crustalsegment s developedinabroadlytranspressive-convergent system,thus allowinglocalised penetration of thesemagmas.Theserift-related,localtranstension alsett ingswere then quickly succeeded byprogr ade-metam orph ic,sinist ral-t ranspressive,Scandianoroge nesis,which isdat edto approx imate ly 430-425Main thisparti cularpartofcent ral Nor way.

OddNilsen&Fernando Corfu,Instituttfor qeoloqi,UniversitetiOslo,Postboks 1047,N-0316 Oslo,Norway.Bje mSundvoll, GeologiskMuseum,NHM,Universitet iOslo, Postboks1172,N-0318Oslo,Norway. DavidRoberts,Norgesgeologiske undersekelse.N-7491Trondheim, Norway.

Introduction

Large areas of the CaledonidesofCentr alNor way aredomi- nated by metasedim ent ary and metavo lcanic rocks of the Trondh eim Nappe Comp lex,a tripartite element formin g part of the K61i Nappes whichconstitute theupp er half of the Upp er Allochth on of Caledo nide tecton ostratigraphy.

No less promi nentona geolog icalmap, evenat1:1million scale(Sigmo nd et al.1984 ),arethe large bodi es of trond- hj emit e,up to200km' inoutcro psize,that occurscatte red through ou tthis region,insomecases coexisti ngwit h diorit e or gabbro. Tron dhj em it es also occur quit e commo nly as dykesin many areas.

Definition of the term trond hjemite derives from Goldschmidt's (1916)pioneering st udies on themagm atic rocks of centra land sout hernNorway. Incurrent classifica- tion schemes it isformally definedas aleucocrat icton alit e, wit h <10% of mafic mineral constit uents (Barker 1979, Le Mait re1989),but it has alsobeen regarded as synony mo us wit h plagiogranit e andrelated sodicgranitoidrocks.Thisis also the case in Norwegian geolog ical literatu re,and on most geolog ical maps, and itis therefore impo rtantfor pur- poses ofinterpretation andtectonic modelling totryto dis- tingui sh between thetwo variants,eitherby geolog icalaffil- iationor by geochemicalsignat ure.

Tron dhjem itessensu lata,and granitic rocks in general, are favouritetargets forgeochro nologica linvestiga tio n.This islargelybecause ofthegreater likelih ood offinding suffi- cientquanti tiesofeuhedra lzircongrainsin theserocks,for U-Pb dat ing,thanin maficpluton ic bodies.In recentyears, severalzircondati ngstudiesincent ralNorwayhave concen- trated more on the plagiogra nitic variety oftrondhjem ite that occurshereand thereinassociationwit hophio litesand related rock assemb lages (Dunning 1987, Dunning &

Pedersen1988,Bjerkqard&Bjorl ykke1994,Roberts&Tucker 1998,Roberts et al.2002).By comparison,the ages or age rangeof non-ophi olitictrondh jemite bodiesanddykes are less well defi ned (Dunning & Grenne 2000),even though theserocks are pot enti ally impo rta nttimemarkersin help- ingtoprovid ebett er const raintson the geologica levolution of anyone part icular area.In this contr ib ut ion,we try to redressthe balance, to someexte nt,inpresentin g U-Pbzir- condatesfrom twotrondhj emit e bodies andanassociated bioti te nori te pluto n from the sout hweste rn part of the Tron dh eim Regio n.

Regional setting

The geologyof theTron dheim Region isdominated by rocks of K61i Nappe affini ty,essentially three separate tectonic

NGU-BUL L 441,2003 - PAGE 6

units,the Storen,Gula andMeraker Nappes,which together form the Trondhe im Nappe Comp lex (Guezou 1978,Wolff 1979,Gee et al. 1985).Rocks of Seve Nappe affinity occur directlybelowthe KoliNappes in most placesand,together, these two elements constitute the Upper Allochthon (Roberts&Gee 1985) (Fig. 1).An updated description ofthe geology and Caledonide tectonostratigraphy of central Norwayand adjacentareasin Swede nhas been presented by Roberts&Stephens(2000).

The geology of southwestern and southern parts ofthe TrondheimRegion has been documented by,e.g.,Rohr-Torp (1972, 1974),Nilsen (1974, 1978),Guezou (1978), Krill (1980, 1983,1985)andGee etal.(1985).Thebedrockgeology ofthe distr ict is also covered by a comparative ly modern map comp ilation at 1:250,000 scale (Nilsen & Wolff (1978).In order to understand the regional context of the plutoni c bodies thatwehave dated(Fig. 2),it isnecessaryto consider

Caledonian plutonicrocks KbliNappes }

Upper Allochthon Sevenappes

MiddleAllo chth on Lower Allocht hon

Autochthon/paraut ochthon Precam brian basem ent

Fig.1.Simplifiedtectonostratigraphyof the southwestern part of the Trondhe im Region, showing the location of the area(box) depictedin Figure 2.Some of the principaltrondhjemiteor gab- bro-diorite-trondhjemitemassifsare also indicated.

ODD NILSEN,BJ(ZiRN SUNDVOLL,DAVIDROBERTS

s

briefly thesit uat ion a littl e farthernort h,in what isgenerally termed thewesternTrondheim Region. lntheStor endistrict (Fig. 2),afragment edophiolite represented by maficvolcan- ites and dykes oftheSterenGroup lies withtectoniccontact upon rocksofthe GulaCom p lex (Gula Nappe).The'Steren ophio lite'isconsidered to have been obducted upon the Gula rocksinEarlyOrdo vician time;at whichtime these two rock unit s were initially deformed and metamorphosed (Lagerblad 1984,Pannemans & Roberts 2000). Follow ing uplift and erosion,sedimentary and subordinate volcanic rocks ofthe Hovin and HorgGroups(Mid Arenig to possibly L1andovery age) (Fig.2)accum ulat edin a margina lbasinset- ting(Roberts etal. 1984);andwere event ually deformedand metamorphos ed at comparat ively low grade during the Siluro-Devon ian, Scandian orogen y. In the liter atu re, the Storen ophiol it ic rockstogether with the unconformabl y overlying Hovin and Horg Gro upshave been termed the Store n Nappe (Gale & Robert s 1974).Alth oug h this is an unfortunate term in many ways,itis ingrai nedinthelitera- ture and isretai ned here,purely forconvenie nce.

TheSterenophiolitic unit and theunconfor mab ly over- lying Hovin Group metasedimentary rocks extend south- wardsinto theareaconsidered here,between Ulsberg and Oppdal. In this area,mappe d and described by Rohr-Torp (1972,1974),the HovinGroup metasedimentaryrocks(also known as the 'Kro kstad sediments'; Rohr-Torp, 1972) are int ruded by the Innset massif(Naumann 1824,Gold schmidt 1916, Rohr-Torp 1974),a com posite,elongate plutoncover- inganarea ofabout180 km²(Fig. 2)and ranging in composi- tionfrom noritetotron dhj emite.The contact-metamorphic effects seen around this massif were first noted byKeilhau (1850),and describ ed in some detail by Rohr-Torp (1974) who concluded that the intru sion must havepredated the main Scandiandeformat ion andregional metamorphismof theKrokstad(Hovin)metasediment ary unit.Trondhjem ites cutting diffe rent lit hologi es and tect onostratigraphic units are common in the Ulsberg- Op pdal-Kviknearea. IntheGula Nappe,they occuras separate,minor, weaklyor negl igibly foliated,massivebodies,some ofwhic hcover areasof up to 30 km²inthe Kvikne -Inne rdalen district(Rohr-Torp&Nilsen 1979) (Fig.2).

The only previous,attempted dating of roc ksfrom the Innset massif is recorded in an unpublished report (Peterman & Barker,1976),which was later amended and revised by Barker & Millard (1979).In this Rb-Srstu dy,the authors analysed a bioti te diorite and a leucogranodior ite from the Inn set bod y,toget her wit h three trondhjemites from different areas30-40km north of Innset (wellout side thelnnset massif),andobtain edanisochron age of 548±35 Ma.Such anisochron age,derivedfrom wide lyseparate and geochem ically differen t plutonic rocks,has little meaning;

and itsinvalid ity hassince been provenbythe U-Pb dating of zirco n and titanite from one of these sampled trend- hj em ite s,fromthetype localityatFollst ad(Fig.2).Thisgavea crystallisation ageof 432±3Ma(Dunning&Grenne 2000).

ODDNILSEN,BJ0RN SUNDVOLL,DAVID ROBERTS

s

.-('

25

"-

20

Hulsj oen

6

ha

N

ST0RENNAPPE

CD

1 Hovingroup

====

___I

1 -

=====1

- --- -1- ^{Blih ;}

-- --- -- ---

Soknedal melange

Trondhjemite Opdalite

• •

10km

• •

Majorfault

Fig.2.Geolog ical map of theUlsberg-Kvikne-Soknedaldistrictshowingthelocations ofthe sam ples(1-5)investigatedinthis geochronological/geo- chemicalstudy.

NGU- BULL441,2003 - PAGE8

Inaddit iontothe above,thereisone recorded mineral datefrom adior it enearUlsberg.Thisisa K-Ar determinati on on biotite, which gave an original date of 355 Ma(Broch 1964).Recalculatin g this,using new constants(Dalrymple 1979),yields arevised date of362Ma; whichmayrelatein some wayto verylate-stage,5candiancooling.

Field relationships at the sampling localities

Two of the rocks sampled for our U-Pbdatinginvesti gation arefrom theInnset massif,taken from alarge quarry for- merly operated by NSB(the Norwegian State Railway) at Stut haugen (1:50,000 map-sheet 'Innset' 152011;UTM 496 537), where their mutual relationship can easily be seen.

ODDNILSEN,BJ0RN SUNDVOLL,DAVID ROBERTS&FERNANDOCORFU

There,amassive, weakly foliatedtrondhjemitebody(sample DR-1) (Fig. 3a) and sporadic trond hjemite dykes(Fig. 3b) transect oneofthedominant,intermediate-to-m aficlith olo- gies of the Innset massif,in thiscase a medium-grained, greenish-grey,biotite norite(sampleDR-2)whichwas origi - nallytermed'opdalite'byGoldschmidt(1916).Thebiotite,in thiscase,isprimary.In places,there is apervasive auto brec- ciation ofthewallrocks, andthere arespectacularexamples of the main trondhjemit e cutting through and isolating large andsmallxenolit hsof biot it enorite(Fig.3c).

The thirdsamplesuccessfullydated in thisinvestigation (sample DR-5) comes from a trondhjemite quarry at Nyvollen in Inn erdalen, in the western part of the Gula Complex (Figs.2& 3d) (1:50,000 map-sheet'Innset' 152011;

Fig. 3.Field photograph s of someof thetron dhj emit es.(a) Trondh jemitecutti ngthroug handenclosingxenoli t hsof biot it e norite,Stuthaugen quarry,south ofUlsberg;lookin gnort h.(b) Trond hj emite dyketransect ing bioti tenorite,Stu t haugen quarry. (cl Trond hjem itecuttingthroughandisolati ngxenolithsofbiotite norite;surfaceof a large block,Stu tha uge n quarry. (d) The sout h- eastern contact zoneof thetrondhjem it e at Nyvollenquarry, Innerdalen;lookin gnortheast.The host rocks are schists/phyllites andmetasiltstone s of the UndalForm ati on(Fig. 2).(e)Gentlydip - pingtrondhj emite dykecuttingmetag reywa ckesand phyllitesof the'Krokst ad unit: Road-cut alongtheE6 justnorthof Ulsberg,look- ing south-southeast.

000 NILSEN,BJ0RN SUNOVOLL,OAVIOROBERTS

s

Table1. Modalcompositio nsof trond hje mi tesamplesDR4and DR5and biot ite no rite DR2.

The Innerd alen trondhj em it e (sam p le DR-5) is a medium-grain ed,het erogr anul ar rock wit ha grainsize of 1- 3mm,and is compose d princip ally of more or lessunaltered plagiocl ase and quartz (c. 90vol.% of the mode;Table 1).

Although intergranular microcline and biotite are minor mineralconsti t uents,it isthe bioti te thatmakestherock par- ticularly at t ract iveas anorna me nta lsto ne.Accessoryminer- als aremuscovite,zircon, clinozo isiteandapat it e.

Thebiotite noritefro m theInnsetmassif (samp le DR-2;

Tabl e 1)is amassive,het erogranul ar,med iu m- grained rock wit hagrain sizeof 1-3mm.Plagiocl ase (An40)andort hopy- roxene are the chiefmineral const it uents(app roxi mate ly80 vol.%),and c1inop yr oxene, biot ite and opaqueore minerals are subo rdinate . Microclin e-perth it e,quart z, apati te,zirco n and chloriteareaccessory con stitu ent s.The ort ho pyroxe ne (hyperst hene) occur s as weakly chlo rit ised sub hed ral prisms,usually asevenl y distributed aggregates,togeth er wit h non-pl eochr oi c, colourlessclino pyroxe neand brown biot ite .

Geochemical traits

Major and trace eleme nt analysesof thefive samplescol- lect ed forthisgeochronol ogical st udy werecarried out at theGeologicalSurveyof Norw ay,Tron dh eim ,princip allyfor comparison wit hanalyti cal dataderivedfromtron dhj em it es investigated in other part s oftheTrondheim Regi on.The rock samples were analysed using an automatic Phil ips PW1480 XRF spect ro mete r. Major elements were deter- mined on glassdiscs,prepared by fusing one part of pre- ignited(1000°C) rock powd er wit h 7part s of Li2B 407.Trace elements were determ in ed on pressedpowd erpellet s,using wax as a binder. The analyti cal data and CIPW norm ati ve compositions are presented in Tabl e 2.The REE analyses were determined by ICP-MS at the Afd.Fysico-ch emi sche geologie,Universityof Leuven,Belgium .

Normative and modal datafrom the five sam plesare depi ctedin Figs.4a &b. Theseplot sclearly show thetro nd - hjemitic nature of the leucocrat ic rocks. The chem icaland mineralogic alcompo sition of thebiotitenoritetakenfrom UTM 536421).In the In nerda len-Kvikne district,many such

trondhj emites carry distinct flakes of biotite,making the rock suita ble as an orna me nt al st one. In Innerd alen ,the trondhj em it e is quarried under the comm ercial nam e 'CrystalWhite'. At Nyvollen , the tron dhj emite occursas a compos itedyke morethan100met res across trendingatc.

035° and dipping at just over30° tothe sout heast(Fig.3d).

This is more or less concorda nt wit h the schistosi ty in the host-rock Undal Form ati on (Nilsen 1978, Nilsen & Wolff 1989), here repr esented by dark grey to black,graph itic metapelit es wit hinte rcalationsofgrey met asiltst on e.

As wellasthethree samp les menti on edabove,tw ofiner - grained trondhj emitedykes werealsosam p led,from north of Ulsberg andwestofBerkak(Fig. 2).The c.1.3m -thi ckdyke west of Berkak (1:50,000 map-sheet 'Rennebu';UTM 481 678) (sam ple DR-3)lies subparallel totheESE-dippingfoli a- tion inschistose met abasalt s wit h mafic dykes,part of the Sto renophio litic unit. Justtwo fraction s of zirconfrom this trondhj em it edyke were analysed; both are discord ant and theappare ntagesobta ined pointtoinheritance and wall- rock contamin at ion.Thesecond trondhjemitedyke,inout- cropsalong theE6roadjust north of Ulsberg (1:50,000 map- sheet'Rennebu; UTM 505 602) (sam ple DR-4;Fig.3e), was found to contain too few and poorer-qualityzircons, and wasnot analysed.ThisparticularENE-WSW-trendingdyke is c. 1.5 m thick and cuts stee p ly dipping, greeni sh-gr ey greyw ackes and pelit esof the 'Krokstad unit'.Itslocat ion is wit hin the oute r part of thecontact aureole of the Innset massif (Rohr-Torp 1974).

Petrographic features

Samples DR-1, DR-2 and DR-4 are comparatively fine- grain ed,leucocrati c rocks wit h a heterogranular, sub-per- phyritic texture.A faint foliation isdefined bysheet -silicate minerals,principally muscovite , which constituteup to c.15 vol.% of the mode (Tab le 1).Quart z and plagi ocl ase (An_o-1s) const it ute 70-90vol.% of therock sand occuras ananhedral ground mass wit hagrain size of 0.05-0.1 mm.lnafewcases, thereare anhedr alplagi oclasephenocry sts wit ha grainsize of 1-2 mm.The feldspars are st rong ly saussurit ised,serici- tised and partly chlorit ised.Theprincipalaccessoryminerals are clinozoisite,titanite,zi rcon ,apatite andseco ndary calcite, allwit hgrainsizes ofc.0.1mm.

Gold schmidt (1916) conside red the presence of mus- coviteand chloritein thisparticulartypeof trondhjemite to be a repl acement feature arising from late-m agmatic, hydr oth erm al-pneum atolyti c act ivit y. How ever, the mus- covite, chlo rite and clinozoisite are clearly met am orphic repl acem ents aft erK-feldsparand biotite,andinall prob a- bility related to the low-grade Scandian overpr int of the contactaureolearound theInnsetmassif.Thefield and pet- rograp hiccharacterof dykesDR-3 and DR-4are remin iscent of the 'greenish-white tron dhj emite s' describ ed by Pann em ans &Rob ert s (2000) fromthe Sto ren-Gaulda len dis- trict fart her tothe north.

Mineral/Sample plagioclas e quartz microcline orthopyroxene c1inopyroxene biot ite chlori te muscovite clinozo isite sphene/zirco n apatite carbonate ore minerals Su m

DR4 41.8 34.0 0.0 0.0 0.0 0.0 2.2 15.6 1.4 1.0 0.0 4.1 0.0 100.1

DRS 60.4 29.4 2.4 0.0 0.0 4.3 0.0 2.3 1.0 0.1 0.1 0.0 0.0 100.0

DR2 65.4 1.8 1.2 13.2 10.6 6.9 0.0 0.0 0.0 0.0 0.1 0.0 0.9 100.1

NGU -BU LL 44 1,2003 - PAGE 10 ODDNILSEN,BJ(oRN SUNDVOLL,DAVIDROBERTS&FERNANDOCORFU

Tab le 2.Chemicaland normative compositions of trondhjemites and biotitenorite from the Berkak-lnnset-Innerdalenarea.

SiO, TiO, AbO, FeO MnO MgO CaO Na,O K,O P,O, CO, H,O Sum

Zr Y Sr Rb Zn Ni Cr Ba Nb V

Qz Cor Or Ab An Ne Lc

Ac

Na-met Cpx Wo Opx 01 11 Mt Hm Ap

OR1 69.49 0.21 15.50 1.41 0.02 0.66 2.70 5.51 1.32 0.07 0.00 0.86 97.75

82 3.6 580 27 29 6 7 440

<5 22 26.02 0.27 8.05 48.10 13.35 0.00 0.00 0.00 0.00 0.00 0.00 3.05 0.00 0.41 0.59 0.00 0.17 100

OR3 67.83 0.28 16.80 1.50 0.03 0.76 3.63 6.00 1.32 0.10 0.00 1.74 99.99

90 3.9 681 18 33 9 10 271 6 17 9.492 0.00 7.94 51.65 15.27 0.00 0.00 0.00 0.00 1.95 0.00 2.31 0.00 0.54 0.61 0.00 0.24 100

OR4 68.04 0.26 15.89 1.58 0.02 0.82 2.51 5.49 2.06 0.08 0.00 2.72 99.47

96 3.2 302 62 36 9 11 354

<5 33 1.90 0.27 12.58 47.99 12.33 0.00 0.00 0.00 0.00 0.00 0.00 3.58 0.00 0.5 1 0.66 0.00 0.19 100

ORS 69.32 0.18 16.26 1.19 0.02 0.54 2.37 6.08 1.60 0.06 0.00 0.46 98.08

84 3.4 868 32 23 6 8 304

<5 19 22.13 0.37 9.68 52.68 11.64 0.00 0.00 0.00 0.00 0.00 0.00 2.51 0.00 0.35 0.49 0.00 0.14 100

OR2 54.83 1.12 16.46 7.46 0.11 6.90 7.22 3.51 1.22 0.26 0.00 0.27 99.36

90 18.2 443 26 62 125 264 300 6 120 3.44 0.00 7.26 29.91 25.74 0.00 0.00 0.00 0.00 6.94 0.00 20.93 0.00 2.14 3.03 0.00 0.61 100

REE analyt ical data (Tab le 3) for three of the trend- hjemites(samples 1,3 and 4)are comparablein almost all respect s with those reported from the Gauldalen distr ict, wit h moderately highLREEenrichmentsand low HREEcon- tents,and avery small, positiveEu anomaly(Fig. 6).Thesam- plefromInnerdalen(DR-5)sho w s a slig htupwardconvexity and somewh at low er content s of LREEin itschond rite-nor- malisedpatte rn.From the modal,normative and other geo- chem ical data,this disparity in just the LREEis difficultto exp lain,but a likelycause is that of variationin the composi- tionof themafic source rock material.

U-Pb isotopic data

Analytical technique

The U-Pb analyses were carried out by isotope dilution (Krog h 1973).The zircon crystals were pickedund er a binoc- ular microscope,abraded (Krogh 1982),and dissolved at 185°Cin teflon bom bs after addition of a 2osPb/^ll5U spike.

After dissolution , the U and Pb were purifie d on anion exchangeresin with HCIand then loaded(together) on out- gassed Re-filament s with Si-g el and phosphoric acid.

Measurem ent swerecarried outwit h a MAT262mass spec- tromete r in static mode,or,for small amounts of Pb and all

207Pbl'⁰⁴Pb ratios,in dynamic mode using an ion-counti ng

ETP-multip lier.Pb data were corrected for 0.1%/amu frac- tionation and the U for 0.12%/amu, with an additiona l adjustment to correct for an expo nential SEM bias deter- mined from regularrunsof theNBS 982 Pbstand ard. Blan k correction was2pgPb and 0.1 pg U.Theini t ial Pb was cor- rected using Pb compositionscalculated withthe Stacey&

Krame rs (1975) model. The results were plotted and regressed using the software of Ludwig (1999).The decay constantsare those of Jaffeyet al.(1971).Uncertainties in the isotopi cratiosand the ages are given and plottedat 2 sigma.

Table3.Rareeartheleme ntabundances(ppm) of fourtro nd hj em it es and a biotitenor it efro mthe Berkak-lnn set-Innerd alen area.

theInnset massif(DR-2)correspondswell with the analyses presented by Goldschmidt(1916).

The four trondhjemitesof the present study are very similar in almost allrespectsto the diverse trondhjem ites from the Gauldalen dist rict described by Pannemans &

Rob erts(2000) and Roberts & Sundvoll (2000),and also to thetype-localit yFollstad trond hjemite(Size 1979, Dunning

&Grenne 2000).All theserocks,incl udingourown sam ples, arecharacteristicofhigh-AIP 3trondhj emitesreportedfrom contine nta l margin environments.This can be seen quite clearly from the Rb/Srvariation diagram (Fig.5a),and the same featuresareevident from aplot ofytt erbium against AlP3(Fig. 5b).

Y La Ce Pr Nd Sm

Eu

Gd Tb Dy Ho Er Tm Yb Lu

OR1 3.6 8 6.6 1.85 6,87 1.35 0.43 0.99 0.13 .7 0.13 0,.33 0.046 0.31 0.048

OR3 3.9 8.6 18.6 2.24 8,89 1.77 0.61 1.26 0.16 0.82 0.14 0.35 0.046 0.29 0.044

OR4 3.2 8.8 19.4 2.3 8,86 1.68 0.54 1.1 0.14 0.67 0.12 0.29 0.04 0.25 0.04

ORS 3.4 2.43 7.5 0.97 4,03 1.06 0.42 0.86 0.13 0.65 0.12 0.31 0.045 0.29 0.04 6

OR2 18.2 18.4 40.2 4.8 19.4 4.18 1.47 3.61 0.56 3.39 0.68 1.82 0.25 1.69 0.26

ODDNILSEN,BJ0RN SUNDVOLL,DAVID ROBERTS&FERNANDO CORFU

An

(a)

Q

NGU-BU L L441, 200 3 - PAGE 11

Ab A P

CJ

CJ

•

•

CJ

CJ

•

Fig 4. (a)Normat ive (An-A n-Or)and(b)modal(Q-A-P)diagram sof tro nhj emites andassociate d rocks from thewest-central Tro ndheim Regio n.The An-Ab -Ordiagr am is from O'Conn or s (l965).Gr-granite;Grd- granodiorite;Ton-tonalite;Qg b - quart zgabbro;Gb-gab b ro/d iori te.The field s of'Follsta d trondhj em it es'and'Green ish-w hi tetrondhj em ites' arefrom Pann em an s&Ro bert s(2000)(no rn at ive) and Size(1979)(mo da l);and the'Snoan trondhjemites'fro mRo bert s & Sundvo ll(2000).Thefield for'Gulatron dhj em it es' (12 sam ples)encompassesanalysesby Goldschmidt (1916)andO.Nilsen(unpub lished data).QBN-Onequart z-biotitenoritesamplefromtheInnsetmassif,taken fro mGold schmidt(1916)(mo dal analysis notavailable).

1000

E

~ ^D

a..

A

-So .c0:::

10

(a) 1

( [D

10 100 1000 Sr(ppm)10 000

10 Low AIP3(oceanic)

E

I f-

0.5l-

• • • •

0.1f-

(b) ^HighAJ20₃(ccntinental)

0.05 ^{I I I}

12 13 14 15 16 17 18

D

D

•

D

·

Results

Five analyses have beencarried outonzirconsfromsample DR-' using selections of varioustyp esof grains,including broken prismsand tips,in anatt empt to avo id grainswit h xenocryst ic compo nents(Table4,Fig.7).Allfive analyses are concordantwithinerror but oneof them plots atasome- what younger age suggesting that thesezircons have lost some Pb.Thetwo uppermost datapoint s overlapanddefine a concordia age of 434.8±0.5Ma.

SampleDR-2containsa largerpopulation of good-qual- ity zirconwith no opticalevidence for xenocryst ic compo- nents. Four analysesof broken euhedralcrystalsploton or nearly on the concordia curve, yielding overlapping 2°'Pb/206Pb ages.Calculation of a reg ression lineanchored at a lowerintercept of 50±50 Ma yields an upperinterceptage of 435.8±0.9Ma (Fig.7).

Sample DR-5 had a rather sparseand heterogeneous population of zircons, whichshowed clear evidencefor the probable presence of inherited grains.Indeed,one of the analyses provides a Mesoproterozoic apparent age.Four

Fig.5.The tro ndh jem ite and no rit e analyses(DR1-5)plotted on(a) Rb- Srand (b)Yb vs.AbO,diagr am s.In(a), Arep resentsthefield of continen tal tro ndhj emites,and B the field of plag iog ranites/- oceanictrondhjemites.Field s forot her trond hjem ites(cf.Panne- mans& Roberts2000) arealso show n.

NGU-BU LL441,20 03 - PAGE12 ODDNILSEN,BJ0RN SUNDVOLL,DAVIDROBERTS

s

Fig. 6. Chondrite-normalisedREE plots ofthe trond hjem ites andbiotite noritefrom theBer kak-lnn set-tnnerd alarea. Element normalisationvaluesfromTaylor&

McClellan(1985).

La Ce Pr Nd PmSrr Eu Gd Tb Oy Ho Er Trr Yb Lu

othe r analyses. carried out on parts of zircon crystals deemedto befreeof cores,are concordant,but they exhib it a distinct dispersion in age.The oldest age was obtained fro m the tip ofa sing le zircon.Asomew hat youngeranalysis obtained from 3flat crystals isim precise and also overlap s the two younger data points(Fig. 7).Because the latter were obtainedfrom well abrade d,good-qualitymaterial,theyare not suspected of being discordant, i.e., having lost Pb.

Calculation of a concor di a age using the three younge st analyses provid esan age of433±0.8 Ma,whichis conside redto bethe bestestimate forthe age ofint ru sion ofthistrondhjemite.This int erp retation implies thattheabove-mentioned zircontipcontained axenocrysti c component.

Discussion

The zirco nagesrepor tedhere,andtheir interpre- tatio n as ages ofemplacementof therespective bodies,denotethat themagmat ism represented at these three locali ties was broadl y coeval at around 435 Ma.On recent abso lutetim e-scales for the Early Palaeozoic (Tucker & McKerrow 1995,McKerro w&vanStaal 2000),this wo uld cor- respond to aroughly Mid L1andoveryage forthis pluton ic and hypabyssalevent. Thedatedtrend- hj emites and biot it e noriteoccur in tw o separate, adjacent, tecton ostratigraphic units of Koli affini ty - the Storenand GulaNap pes- partsof whichare consid eredto have been tect onically juxtapo sed in app roxim ately Mid Aren igtime(Grenne & Roberts 1981,Sturt & Roberts1991, Robert s etal.2002).

Geochemical signat ures fo r the dated trond hj ernites, one fro m the Innset massif and the other from Nyvollen, Innerdalen,are similar.The similar, high-alumina,continental

- - - - e o OR:

OR~

OR4 OR1

OR~

10C

(j)

2 5C

.;::

"0 C

0 2C

s:

0

ID_0. 1C Ero 5

Cl)

Table 4.U-Pb data on zirconsfromthethree investigatedsampl es.

zircon Weig ht U Th/U Pbc ""Pb/"Pb "'Pb/"U 2a ""Pb/"'U 2a rho ,.,Pb/""Pb 2a "'Pbl "'Pbl "'Pbl

characte ristics n·

U ^2l5U "'Pb

[>lg][ppm] [pg] [abs] [abs] [abs] [Ma]

(01) (1) (2) (3) (4) (5) (6) (6) (6) (6) (6) (6) (6) (6) (6)

DR-l

86 17 282 0.29 31.5 681 0.5321 0.0031 0.06964 0.00018 0.55 0.0554 1 0.00027 434.0 433.2 428.9 B20 10 289 0.53 33.4 397 0.5338 0.0055 0.06984 0.00029 0.44 0.05542 0.00051 435.2 434.3 429.7 fr 61 342 0.56 2.5 36411 0.5356 0.0013 0.06989 0.00015 0.96 0.05558 0.00004 435.5 435.5 435.5 eusp 1gr 28 50 0.29 2.5 2464 0.5320 0.0022 0.06966 0.00019 0.76 0.05539 0.00015 434.1 433.2 428.0 85 15 223 0.33 3.2 4545 0.5280 0.002 1 0.06913 0.00025 0.88 0.05540 0.00011 430.9 430.5 428.4

DR-2

fr 220 265 0.54 37.5 6803 0.5336 0.0016 0.0696 1 0.00019 0.96 0.05560 0.00004 433.8 434.2 436.5 fr 336 170 0.56 52.6 4774 0.5346 0.0013 0.06979 0.00014 0.92 0.05555 0.00005 343.9 434.8 434.5 fr 354 218 0.56 6.3 53907 0.5348 0.0015 0.06976 0.00018 0.97 0.05560 0.00003 434.7 435.0 436.2 eu broken 279 233 0.57 17.0 1670 1 0.5322 0.0013 0.06947 0.00015 0.96 0.05556 0.00004 433.0 433.3 434.8

DR-5

tip1gr 1 558 0.60 0.8 3286 0.5392 0.0022 0.07034 0.00022 0.78 0.05560 0.00014 438.2 437.9 436.4 flat crystals 3 gr 1 725 0.49 5.1 643 0.5354 0.0039 0.06987 0.00027 0.63 0.05558 0.00031 435.4 435.4 435.6 fr(Ipcrystals) 11 199 0.47 2.1 4652 0.5317 0.0021 0.069 56 0.00022 0.88 0.05544 0.00010 433.5 433.0 430.1 fr 2gr 37 191 0.45 3.8 8070 0.5319 0.00 17 0.06956 0.00019 0.89 0.05546 0.00008 433.5 433.1 430.9 823 18 56 0.33 12.3 821 1.8558 0.0195 0.15705 0.00156 0.95 0.08570 0,.00028 940.4 1065.5 1331.5 (1) allzirco n fractions con sisted ofclear,transparent grains;allwere abraded;fr

=

=

2gr=indicatesnumberof grainsanalyzes(noindication meansmorethan 10grains)

(2,4)weightand concent rationsareknownto bettertha n 10%.except thos e below2!1gwhich are known to+1- 50%

(3) Th/Umodel ratio inferredfrom 208/206ratioandageofsample (4) Pbc=total common Pb insample(initial+blank)

(5) rawdatacorrectedforfractio natio n andblan k

(6) correct ed forfractiona tion,spike,blankand initialcommonPb;error calculated bypro pagating the mainsourcesof uncertainty

ODD NILSEN,BJ0RNSUNDVOLL,DAVID ROBERTS

s

DR-1

0.52 4

margin-type chemist ry is also evident in the ot her twotrondh jemitedyke samples fromwhichwe wereunableto obtainreli- able zircon ages.The same geochemical trait s arealso ref lected in diverse trond- hjemite bodies from the Gauldalen dis- trict (Pannemans&Robert s 2000), includ- ing the Foll stad trond hjemite which has yieldeda U-Pb zircon/titaniteage of 432± 3 Ma (Dunning & Grenne 2000)- again, indi stingu ishable temp orally from the Innset and Nyvoll entron dhj emites.Thus, there is a strong case for believi ng that emplacement of high-alumina,continen- tal-margintrondhjemiteand related bod- ies occurredwidelyin the Koli rocks of the Trond heim Region in earliest Silurian tim e.

Contact-meta morp hic features around the Innset massif have been described in some detail by Rohr-Torp (1974). An innerzone of hornfels,1-2 km in widt h, is foll owed (to the southeast) by a 3-4 km-widezone ofspotted rocks.

The spots consist of c.60% sericite and, in places,prismatic outlines show that they are pseudo morphs after cordierite porph yob lasts (Rohr-Torp, op. cit.). An import ant aspectof thesefield andmicro- scopic studies isthatthe earliest schist o- sity recorded in the Krokstad metasedi- mentsisdeflect ed aroundthe Innsetmas- sif,and also around the pseudomorphic spots.This led Roh r-Torpto conclude that the Innset massif intrud ed the Mid/Late Ordovician Krokstad sedimentarysucces- sion at some time before the main, Scandian deformation.The ages of the norite and trond hjemi te reported here thus suppo rt Roh r-Torp's (1974)observa- tion s and predict ion s.

The precise age ofthe Scandian defor- mati on and metamorphism within the Trondheim Napp e Complexhasnot been accurate ly determin ed.From a biostrati- graphicstand poi nt,sedi men tatio n in,e.g., the Meraker Nappe continued into Early/Mi d L1andoverytime, based on finds ofmonogra pt ids(Getz 1890).K-Ar data on phyll itic schists in theGula Complexhave given (recalculated) agesof425-426Ma, 435.8 ± 0.9Ma

MSWD=1.5 4points

ConcordiaAge=434.8 ±0.5Ma

MSWD(ofconcordance)=0.01

lineconstrained at 50^±50 Ma

3points

ConcordiaAge=433.8± 0.8 Ma

MSWD (ofconcordance) =5.5

0.532 0.540

0.532 0.540

DR-5 DR-2

, /

0.0704 0.06 88

0.524 0.0688

0.0688 0.0704 0.0704

eo

'"

N-..0

a..

~ 0.0696

N

eo

'"

N-..0

a..

~ 0.069 6

N

co

'"

N-..0

a..

~ 0.0696

N

0.524 0.540

Fig. 7. Con cordi a diagrams for the zircon s analysed fromsamplesDR',DR2andDRS.

See maintextforexplanatio n.

NGU-BUL L441,2003 - PAG E14

wit h horn blend e datesup to 432 Ma (Wilson et al.1973).

4°Ar/39Ar analytica l data from the Meraker Nappe yield ed hornblende plateau agesof 427-420 Ma;and fromtheGula, hornblendegaveplateau xof432-424 Ma(Dallmeyer1990).

Plateau ages forbiotiteandmuscovit e from Gula rockswere in the ranges 433-427 and 424-418 Ma, respective ly (Dallmeyer,op. cit.).Although allthesedates perta intopost- metamorphic cooli ng,the higher-T horn blendeswoul d be registering plateau x closest to peak metamorph ism,and suggest that thisoccurr ed during the period 430-425 Ma.

This corresponds to a lat est L1andovery to Wenlock age.

Int rusionof the widespreadtrondhjemiteand related mafic bodies inthe Gula andStoren Nappes wouldthusappear to have taken placejustshortlybefore the atta inmentof peak Scandian metamorphism.

Therelati on ship betwee n the conti nenta l-margi ntrend- hj emites and Scand ian meta mo rphism/deforma tion, with magm ati sm pre-dati ng thelatter,has also been described and illustrated by Size(1979), Dunning & Grenne (2000), Pann emans&Roberts(2000)and Roberts&Sundvoll(2000).

Tron d hj emites in the Gula Complexcut an earlierfoli ationof inferred Early Ordovicianage,but many of these bod iesand dykesarefolded, andalsoaff ected by a variab lydeveloped schisto sit y or cleavage.These folds and schistosit y are undoubtedlyScandian. lna fewcases,especially in thecen- tral partsof larger trond hje m itebodies,only the faint esthint of a foliate structurecan be discerned .The trondhj em it es dated in the presentstudysho w mineral paragenesestyp i- cal of reg ional-m etamorph ic replacement,as well as syn- chronous or late r sericiti sati on and chlorit isatio n,features which provide evide nce of Scandianoverprint.

In the southern Trondhe im Region,the onlyradiometric dating study on trondhjemitesof high-aluminatype isthat of Berthomieretal.(1972a,b),from the Hoq-Giamassif(Fig.

2),approximately 30km southof lnnerdalen. Threetrend- hjemi tesam ples from this compositegabbro-diorite -t rond - hj emite massif in the Gula Complex gave a Rb-Sr mineral/whole-rockisochron age ofc.443±8 Ma(recalcu- lated using adecay constan tof 1.42,thiswould be 468 Ma).

Eventhou ghthis particularageislessprecise thanour own U-Pbzircondates,Berthomier et al.(1972a) consideredthat theHog-Gia massifintruded in Ordoviciantime, well before the main phase of Caledonian (Scandian) metamorphism.

Clearly,a U-Pb ag e would be desirable from this massif, before valid comparisons can be made wit h other trend- hjem itesdatedby zircon chrono log y.

The zircon ages for the trondh jem it es and nori te reportedherefit intoa known pattern ofmagmati sminthe Koli Nappes documented fro m other part s of the Upper Allochthon.Over an int erva l of 10-15 milli on yearsin Late Ordovician/Early Silurian time, many composit e gabbro- diorite-trondhjem it e pluton s penetrated these Koli vol- canosedime ntary assem b lages.Most U-Pband Srn-Nd ages for these majorpluton iccomp lexesrange betw een440and 430 Ma; e.g., those at Sulitjelma, Rana, Artfjallet and

ODDNILSEN,BJ0RNSUNDVOLL,DAVID ROBERTS&FERNANDOCORFU

Krutfje llet (Senior&Andriessen 1990,Pedersen et al. 1991, Mork etal. 1997).lnthe easternTrondheim Region,a late, fel- sic diffe rentiate of the Fong en-Hyll ingen complex has yield eda U-Pb zircon age of 426-8f-,Ma(Wilson et al. 1983).

This major body,cutting the Fundsjo Group,is known to have intru dedshortlybeforepeak Scandian metam or p hism, butitalsopenet rated an already foliated , volcanosedim en- tary succession.Descriptionsof the andalusite -bea ringcon- tact aureoles ofthe Hyllingengabbro and nearby0yungen gabbro (Fig.1) (Birkeland &Nilsen 1972)clearly show that these mafic plut onspre-dated the Scandian regional meta- morph ism.

Thepalaeot ectonic scenario forthe composite plutonic bodieshasbeendiscussedbyDunni ng&Grenne(2000) and Pannemans & Roberts(2000).In shor t,the mafic magmas were probablyderi vedfroma mantle source,and thetre nd- hj emitesby part ialmeltingunder garnet-amph ibolit e con- ditionsat dept hsin the range 20-30 km.The conti nental margin character ofthefelsic and maficintrusionsand their emplacement immediately prior to the earliest stagesof ob lique plate collision between Balticaand Laurentia were conside red to stem from a short-lived,rift-related,exten- sional or transtensional regime (Grenne et al. 1999).In such anoblique,continent-coll isiona lsetting,in a broadlytrans- pressive-convergent system (Dunning & Grenne 2000) of sinistralcharacter(Robert s1983,Braathenet al.2002),locally develo ped areas of relat ivecrustaltranstension ,akinto sur- ficialpull-apart basins,could be envisaged ashaving facili- tatedthe emplacement of these maficto felsic intrusions. This stage of collision -related,tectonomagmat ic develop - ment was then quickly succeeded by the polypha se, Scand ian,transpressive regime in prograde poT metamor- phic condit ion s,and ultimately by late-Scand ian,sinistr al transt ensio n coeval with orogenic collapse and Devonian basinal sedimentation .

Conclusions

Precise U-Pb zircon ages are presented from tw o tron d- hjemites and a biot ite norit e from the Gula and Storen Nappes in apart of the sout hwestern Trond heim Region, central Norway.A trondh j emite anda biotite norite (t radi- tionally term ed'opdalite')from a quarry inthe Innsetmassif, northeast of Oppdal, gave almost identical ages,thoug h fieldrelation ships showthe trondhjem it eto be theyounger of the two intrusions.Zirconsfrom the biotite norit edefin e an average2°'Pb/²⁰⁶Pbageof 435.5±1.5Ma,whereas a con- corda nt analysis alonehas a concordiaage of435.8±0.9Ma.

Three analyses ofzircons from the trond hje m iteareconcor - dantwithinerror and defi nea conco rdi a ageof 434.8±0.5 Ma.

Zirco ns from a second trondhjem ite,sam ple d fro m a worki ng quarry at Nyvollen,in Innerdalen, in the western partof theGula Complex, yielded an upper int ercept age of 433±0.8Ma. Allthree dates,from the Nyvo llen and Innset massif occurrences,are interp reted to represent the ages of

ODDNILSEN,BJ0R N SUNDVOLL,DAVIDROBERTS&FERNANDOCORFU

emplacement of thesebodies,ataround 435Ma;which is Early Silurian(approximate ly Mid L1andovery)on current ly accepted time-scales.

Geochemical signatures fromthedatedtron dhjemit es, and from two ot her, unsuccessfu lly dated trondhjemite dykes,show themto beof high-alumina,continental-mar- ginal type and, thus,similar to many ot her trondhjemite bodies that intrude the K61i Napp es of the Trondheim Region. Field and microscopic st udies indicate that the Innset massif,and compo sit e trond hj emite-ga bbro bodies within the GulaComplex,predateScandianorogenicdefor- mation which,it hasbeen suggested,occurred at around 430-425 Ma inthispart oftheCaledonides.

The ages reporte d herefit into a know n patt ern of com - posite, gabbro-diorite-trondhjemite magmatism that occurredwidel yin theK61iNappes ofthe UpperAllochth on in different partsofthe Scandina vianCaledo nidesin latest Ordovician/EarlySilurian tim e.Ina palaeotecto niccontext, this cont inental margi n magmati sm dates to the early stages of oblique,Balt ica-Laurent ia plat e collision,wherein short-lived, rift-related, transtensional crustal segments developed in a broadly transpressive-conv ergent system and thus allowed ingress of these magm as.These local transtension alsett ingswere thenquicklysucceeded bythe prograde-metamorphic, sinistra l-tr anspressive regim e whichcharacterisedtheScand ianorogeny.

Acknowledg ements

We aregratefu ltoTorGrenneand Arne Rahe im forcommentsonan earlyversio n oftheman uscript, andtoTGfor generaldiscussionsrelat- ing to trondhjemitesin theTrond hei mReg io n.Helpfulandconstructive comm ent s andsuggest io ns bytherevi ew ers,BillSize(At lanta)and Jan Hert ogen (Leuve n), led to improvements in the final manu script.

ProfessorHert og enisalsothanked forcarryingoutthe REEanalysesat hislabo rat or y inBelg ium,and fordiscu ssion s(with DR)in thefield in theFolldal-Dom basarea.

References

Barker,F.1979:Trondhjemite:d efinition,environment and hyp otheses of origin.In Barker,F.(ed.)Trondhjemites,dacit es,and rela tedrocks.

Elsevier,Amste rda m,1-12.

Barker,F.&Millard,Jr.H.T.1979: Geochemistryof thetype tro nd hje m ite and three associated ro cks.InBarker, F.(ed.) Trondhjemites,dacites, andrelatedrocks.Elsevier,Amsterd am ,S17-S29.

Berthomier,C, Maill ot,J.&Roll et,M.1972a:Etude dumassifdegabb ro et dio rit esdu Hog-Gia (No rd -Follda l,Trond heimmeridional,Nor veq e centrale).Sciencesdela Terre17, 329-340.

Bert ho m ier,C,Lacour, A.,Leutw ein,F.,Maillot,J.& Sonet,J.1972b: Sur quelqu es tro ndhjemitesdeNo rveq e:et ude qeoch ronoloqiq ueet qeochi mique.Soencesdela Terre17,343-3S1.

Birkeland ,T.&Nilsen,O.1972:Conta ctmetamorphi sm associate dwit h gabb ro sin theTrond heim Reg ion .Norgesgeol ogiskeund ersokelse 274,13-22.

Bjerkqard, T.&Bjorlykke,A.1994: Geologyof theFolldalarea,sout hern Trondh eim Region Caledonides,Norway.Norgesgeologiske under- sekelseBulletin426, 53-75.

Braathen,A.,Osmu ndsen,P.T.,Nordgulen,0.,Ro berts,D.&Meyer,G.B.

2002:Orog en-parall el exte nsio nof the Caled onidesin northern CentralNor w ay: anoverview.Nor w egi an Jaurna lof Geol ogy82,225- 241.

NGU-BUL L 441, 200 3 - PAGE15

Broch,O.A.1964:Agedet ermination ofNorwe gian mineralsuptoMarch 1964.Norgesgeologiskeundersokelse228, 84-113.

Dallmeyer,RD.1990:"Ar/"Ar mineral ag erecord ofapol yorog enic evo- lutio n within the Seve and Koli Napp es, Trondelag, Norway.

Tectonophysics179,199-226.

Dalrymple,G.B.1979:Criticaltab lesfo r conversionofK-Aragesfro m old tonew const ants.Geology7,558-560.

Dunning,G.R.1987:U-Pbzirconages ofCaledoni anophiol itesandarc sequences:im plicationsfor tectonicsetting.(Abstract) Terra Cognita 7,179.

Dun nin g,G.R. & Pedersen,R.B. 1988:U/Pbages of op hio lites and arc- related plutons ofthe Norwegian Caledonides:implica tio nsfor the developmentof lapetus.Contributiansto Mineralogy and Petrology 98,13-23.

Dunnin g,G.R.&Grenne,T.2000:U-Pbage dating and palaeot ect oni c sig- nificance of tron dhjemite from thetype locality in the Central Norwegian Caled on id es.Norgesgealogiske undersokelse Bulletin 437,57-65.

Gale,G.H.&Roberts,D.1974:Traceelemen tgeochemistryof Norwegian Lower Palaeozoic basicvolcanics andits tectonic implication s.Earth andPla netaryScience Letters22,380- 390.

Gee,D.G.,Guezou,

re,

Sons,Chicheste r, 109-133.

Getz, A.1890:Grapt o llt ferende skifer zoner I det tron dh j emske.Nyt MagazinforNaturvidenskap31,31-42.

Gol dschmi dt, V.M. 1916: Geolo gi sch-Petr ographi sche Studien im Hochgebirgedes sudliche Nor w eg en s.IV.Obersicht der Eruptiv- geste ine im Kaledon ischen Gebirge zwische n Stavange r und Trondhj em.Videnskapsselskape tsSkrifter I,Mat.-Nat. Klasse 19 16.

No.2,140 pp.

Grenne,T. & Roberts, D. 1981: Frag m ent ed ophiolite seq uences in Trond elag, central Norway. Guide to Excursion B12, Uppsala Caledo nianSympos ium,40pp.

Grenne,T.,Ihlen,P.M.&Vok es,F.M.1999:ScandinavianCaledo nidemetal- logenyinaplate tectonicperspective.MineraliumDep osita34,422- 471.

Guezo u,J.e.1978:Geology and structu reof the Dombas -Lesja area, southe rn TrondheimReg ion , South-centralNorway.Norgesqeol o- giskeundersokelse 340,1-34.

Jaffey,A.H.,Flynn,K.F.,Glendenin, L.E.,Bentley,W.e.&Essling,A.M.1971:

Precision measurem ent of half-lives andspecificactivitiesofmU and238U.PhysicalReview,NuclearPhysics4,1889-1906.

Keilhau,B.M.1850:Ueb erdenBauder Felsenmasse No rw egen s.(Pt.2).ln:

Keilhau,B.M.(ed.) Gaea Norvegica.VoU,Johann Dahl,Christiania (OsI0),341-462.

Krill,A.G.1980:Tecton ics of theOppd al area, centralNorway.Geologi ska For eninqensi Stockh olmFom andlinqar102,523-530.

Krill,A.G.1983:Rb-Srst udy ofrapaki vi grani teandauge n gneiss of the Risberget Nap pe,Oppdal,Norw ay.Norgesgeologiske undersokelse 380,51-6 5.

Krill,A.G.1985:Relation ship s betweentheWestern GneissRegion and theTro nd heim Reg ion: Stockwerk-tecton icsreconsidered.In Gee, D.G. & St u rt , BA (eds.)The Caledo nideOrogen - Scandinaviaand rela ted areas.John Wiley & Sons,Chichester,475-483.

Krogh,T.E.1973:Alow contam inat io n methodforhydrotherm aldeco m - posit ionof zirco nandext ractionof Uand Pb forisot opic age dete r- mination s.GeochimicaetCosmochim icaActa37,485-494.

Krogh ,T.E. 1982:Improved accu racyof U-Pbzirconages bythe creation of more con cor dant syste ms using an air abrasion technique.

Geochim ica et Cosmochimica Acta46, 637-649.

Lagerblad,B.1984:Tect o no-m et am orphic relati on ship s of the Gula- Fundsjo Groupcontactzonein thelnndal-Feerenarea,Trondelag, centralNorwegianCaledonides.Geolo gi skaForeninqensiStock ho lm Forhandlinqar 105,131-153.

Le Maitre,RW.1989:A classifica tio nofigneous rocks and glossary of terms.Recommendationsof thelUGS Subcommissiononthesystem - aticsofigneo us rocks.Blackwell Science Pub licati on s, Oxford, 193 pp.