U-Pb age dating and paleotectonic significance of trondhjemite from the type locality in the Central Norwegian Caledonides

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GREG DUNNING

s

TOR GRENNE NG U-BU LL 43 7 ,20 0 0 -PAGE57

U-Pb age dating and paleotectonic significance of trondhjemite from the type locality in the Central Norwegian Caledonides

GREG DUNNING&TOR GRENNE

Dunning,G.R.&Grenne,T.2000:U-Pb agedat ing and paleotectoni c significanceof tron dhj emit efrom the typeloca- lity in theCent ralNorwegian Caledon ides.NorgesgeologiskeundersekelseBulletin437,57-65.

Zirconandtita niteU-Pb dataare reportedfrom thetrondhj emitetypelocality atFollstad,near Storen in Central Norway.Aconcordantfractionofacicularzirconandan overlappi ng fracti on oftit anit egive anage of 432±3 Ma fortheintr usion of the pluto n.Fractions ofprismatic zircondisplayinherit ance.TheFollstadtrondhj emit erepresent s a suiteofsimilarintrusionsoccurringinmetasedimentary and metavolcanic rocks of theUpperAllocht honinthe ScandinavianCaledonides.Trondhjemite emplacement,commonly wit hassociatedmafic magmatism, was broadly coevalwit h metamorphismand deform ationduringtheclimactic Caledo nian(Scandian) orogenic event, immedia- telybeforerapidupliftand cooling in LateSilurianandEarly Devoni antimes.A possiblescenariofor theformat ion of the Follstad andequivalent trondhj emitesisthat of partial melti ng of crustalrocks at garnet-amphibolit egrade, facilitate d by the emplacementofrift-related,mantl e-derived mafic intrusions.The latter could have formed,in Early Siluriantime,ina paleotectonicsetting characterisedbythedevelopm entofextensionalsegmentswithina predo- minantlytranspressionalregi me,due tohighlyobliquecollision betweenthe converging margins ofBalticaand Laurentia.

GregR.Dunning,Dept.ofEarth Sciences Research,MemorialUniversityofNewfoundland,St.John's,Ni=, Canada A 1B3X5.

TorGrenne,GeologicalSurveyofNorway,N-7497Trondheim,Norway.

Introduction

The term 'trondhjemite' was introduced by Goldschmidt (1916) in hiswork on Caledonian intrusive rocks in central andsout hern Norway.It is now strictly defined as a leucocra- tic varietyof tonalite(colour index

=

10)consistingessential- ly of sodi c plagiocla se and quartz with minor biotit e (Le Mait re etal.1989),but thetermhasbeenwidely used in the literature asa synonym for plagiogranitesand othersodic granitoidsirrespect ive of theirmodalcomposition .

Goldschmidt'stype localityfor trondhjemiteswas the in- trusivebody found in the Follstadquarryat Steren,some 50 kmsout h ofTrondheim(Fig.1),which has beenexploitedfor dimen sion stone under the commercial name 'Steren Granite' since 1956.This trondhjemite was the subj ect ofa detailed petrographic and geoch emic al study by Size (1979), and furtherfieldand traceelem entinvestigationsby Panneman s& Robert s (t hisvolu me).The purp ose of the present account is to present new age datafor the typ e trondhjemiteas abasisforregionalandpaleotect onicconsi- derations.

Regional setting

The Follstadtrondhjemite belongsto a suite of similarintru- sionswhichare particularly abundant in anexte nsive,essen- tiallymet asedimentar y,rockunit tradit ion allyreferred toas theGulaGrouporCompl ex (Wolff &Roberts1980).Thisis tect onically bou nd to the east and west by metavolcan ic and younger metasedimentary rocksof the Meraker and

Steren Nappes, respectively(Fig.2).Togetherwit h theGula Complex, these units co nst it ut e the Trondheim Nappe Complex(TNC),whichisequivalent in parttothe K61iNappe Complexand forms amaj or partof theCaledoni an Upp er Allochthon(Roberts &Gee1985,Gee etal.1985).TheTNCis composedofavariety of meta sedim entary and metavolcanic rocks that are generally conside red as being derived from original depos it ion al set tin gsout board ofeit herthe Balticanor theLaurentiancont inent.Amalgamationof these exoticterran esoccurred during thepolyphaseCaledonian orogeny (Stephens&Gee 1989).Theclimacticstageof this orogeny included the collision of the two continents in Silurian time(Scandian phase of orogeny), accom panied by regional metamorphism and subseq uent nappetranspor t to theeastabove the platemarg inof Baltic a.

The Steren and Meraker Nappes both contain thick metavolcanic unitsat their base.TheSteren Group to the west is composed largely of MORB-t ype met abasalts of inferred Late Cambrian or earliest Ordovician age and is commonly referred to as the 'Steren Ophiolite' (Furneset al.

1980),whereasthe Fundsje Group in the Meraker Nappe comprisesa bimodal volcanic and sedimentary succession of oceanic arc and marginal basin affinity (Grenne et al.

1999), atrondhjemite fromwhich has beendated at c.488 Ma (Bj erkqard &Bjerlykke1994).TheFundsj e arc/marginal basin seq uence is unconformably overlain by themetagrey- wacke-domi nated sequences of the Sularno Group and younger metasedime ntary rocks.Age constraint sfor the

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NGU -B U LL 43 7,2000 - PA GE 58 GREG DUNNING& TOR GRENN E

ScandinavianCaledonides

Fig. 1. Simplified tectonostratigraphic map of the Scandina vian Caledonides showingthe location of the Follstad trondhjemite in the UpperAllochth on (afterGrenn e et al.1999).Precambriancrystalline rocks areauto cht ho nous tothe east and sout h,and alloch t ho nous (mainlyLower Allocht hon)or ofuncertaintectonostratigraphicstatus inwindow s in the Caledonides andin northwesternpartsofNorw ay.

sedimentary units are limitedto Early Silurian (L1 andovery) grapto lites ina graphitic phyllite nearthe top of thesed i- mentarysuccession(Siedlecka1967).

TheStoren Ophiolitetothewest is unconformably over- lainbytheLow erHovi n Group,apred om inant ly greywacke- shale sequence which contains Late Ordovician (Caradoc) graptolit es in black shalesinit s upper parts and which is overlain by metasedimentary rocks and rhyolitic tuffs of latest Ordovicianandinfer red Early Silurianage(Chaloupsky 1970).Northwest ofthese sequences,Early Ordovician(c.485 Ma;Grenne & Dunning , in prep.)ophiolites inthe Vassfje ll- Lokken area (Fig.2) are consid ered asequivalents of the oph iol itic sequences of SW Norway (Grenne et al. 1999, Pedersen & Dunning 1997).They are unconformablyover- lain by a sequence of sedimentaryand volcanic(shoshonitic) rocksin the classical Holo nda district, which conta in a rich fossil fauna of clear Laurentian affinity (Bruton &

Bockelie1980)yieldingagesthatrange fromEarly to Middle Ordovician(M idd le Arenig to Early L1anvirn ).Thesesequen- ces have traditiona lly been correlated wit h the Storen and

Hovin Group s

sensustricto

to the southeast(e.g.,Vogt 1945, Chaloupsky 1970),but such a relationsh ip is highly uncer- tain in viewof the presence of an interveningtectonicboun- dary(Fig.2) and the significa nt differences in lithostratigra- phyand knownages(Grenne & Rober ts1998).

Whereas the Hovin, Sularno and younger sedim ent- domina te d successions were apparently affected only by deforma tion and metamorphi sm related to the Scandian continent-contin en t collisional orogeny, the underlying St e renand Fundsj o groupsboth witnessedan earliertecto- nicevent.This early deformation also affected rocksof the GulaComplexand has been tentative lyrelatedto obduction of the oceanic unit s upon Gula sequences in earliest Ordovician time(Furnes etal.1980,Lagerblad 1983).

The Gula Complex is a heterogeneousunit comprising predomi nantlymetasedimentary and subordinatemetavol- canicrocks(Nilsen 1978,Wol ff1976,Nilsen&Wolff 1989).The age is unknown ,but it has traditio nally beenconsideredas largely Camb rian with possible elements of Precambrian rocks.Itis divided into three formations (Fig. 2):the central Sinqsas Format ion ofpsamm it ic and semipeliticclasticmeta- sediment ary rocks and thin maficmetavolca nic unitswit h associate dribbon-chertsand graphiticschists,borderedto theeastbythe AsliFormat ion and to thewest bytheUnd al Formation of main lymetapelites.TheSinqsas and Aslifor- mati ons have been affected by medium- to high-grade metamorphism, whereas theUndalFormat ion is generally of low metamorp hic grade,ranging from chlorit e-sericit e phyllites in northern and central areas to garnet-biotite schistsfarth er south.

Trondhjemites and associated int rusive rocks in the TNC

TheGulaCom pl ex isintrud ed by numerous trond hjem ite bodies, as well as subord inat e granodiorite, diorite and gab bro. On the regi on al scale (Fig.2).there is an obvious spatial relationshipbetweenthe occurrence oflargermafic intrusionsand trondhjemite bodies as seen,e.g.,from the 1:250,000 map-sheet 'Reros' (Nilsen & Wolff 1989). The Reit stoa areashow s one exam ple of this,which has been descri bed in some detail by Pannemans& Roberts (t his volum e).Furt hersout h,trond hj emite-gabb ro complexes up to 30 km in length are found at Hoggia and Valasjoen.

Simi lar relati on shipshave beennoted at a smaller scale near Skj ae kerdaleninthenorthe rn part of the Trondheim District (Hildreth1997).

Com parab le trondhjemites,sometimes associated with gab bro, are found alsoin adj oining units eastand west of the GulaComplex (Wolff 1976,Nilsen & Wolff 1989).The most pro mine nt exam pleisseen in the Fongen-Hyllingen areato the east(Fig.2).The Fongen gabbro complex,dated at426.8/2^(U-Pb^{zircon age}^{on a late}differentiate;Wilson et al. 1983). int ruded met avolcanit es and metapelitesof the Fund sjo Gro up that had already been invol vedin an early phase ofdeforma tion and low-grade metamorp hism,pre- N

Precambrian crystallinerocks Uppermost Allochthon UpperAllochthon

Lowerand Middle Allochthons

•

D

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GREG DUN NI N G

s

^{TO R GRE}^{NN E}

dating the metamorphic peak.Trondhjemitesare very abundant particularly above the roof-zoneof the large gabbroic int rusion (Kisch 1962) and also postd ate an early tect onic fabri c.West of theGula Com plex, tron dhj emite dykesare rather scarcein theSterenGrou pin theGaul dalregi on (b utsee Rob ert s & Sundvoll, this volume).butarequitecom mo n farther sout h tow ard s Berkakand Innset .In that area, they are also fou nd abovethe Sterenmetavolc anit es,where met ase- dimentary rocksofinferred equivalentsof theHovinGroup have been intrudedby a large trondhjemitic-gabbroiccom- plex (t he'InnsetMassif';Goldschmidt 1916, Rohr-Torp1974).

The majorityof theint rusions in the Gula Complexform cross-cutti ng dykes or larger bodiesof varying size and shape.Most dykes cut the penetrati ve,Scandian orearlier, tectonic fabric in the surroundi ngschists(Fig.3a),butthere are alsoabundan tsimilardykesthat are parallel tothefoli a- tion.Examp lesofclearlysyn-k inemat ic multiple int rusio ns are seen locally (Fig.3b). In the cent ral part s of the Gula Comp lex,trondhjemites are generally almost und eformed or onlyweakly sheared and met amorp ho sed at low grade (Pannemans& Roberts, this volume).The relationshipsare morevariedin the Undal Formation,atleast where investi- gatedby us in theGauld al district.Here, some trondhjemites form swarms of dykes that lie,in general,parallel to the stro ng foliat ion in the Unda l phyllites but locally cut the fol iat ion atalow ang le.Along the seve ral hund redmetre s-

Fig.2.Simplifiedgeologicalmap showingmajor unit s andintrusions in the TrondheimNappe Complex (afterWolff1976 and Nilsen&Wolff 1989).

NGU-BULL437 ,2000 - PAGE59

widemyl onit ic contact to the Steren Group, all trondhjemites have been aligned parallel to the foliation and are them selv eshighly sheared,insome casesto protomy loni tes (Fig.3c). Asnot ed above,trond hje mi tesare compa ratively scarcein the Ste ren Group west of thetect on iccontactto theUndalFormat ion .

The FolIstad trondhjemite

The Follstadtrondhjemiteis located in the UndalFormation ofthe GulaCom plex, where it constitutes an elongate, strike- parallel, 7 km-longand nearly 400 m-wide body with a general orientation of N200-25°Eand a steepnorthwesterly dip (Size 1979).The dime nsion-stonequarry at Follstad is located inthecentral part of the intrusion.This part,as well as mostofthebodyelsewhere,comprisesa rather hom ogene- ousand massive,medium-grained, whiterock wit hspeck les of dark brown biot it e.Fluxion band ing related to magm a flow can be seen locally, wit h darker biot ite-rich zones com mo nlybeing aligned subparallelto eachother(Fig. 3d).

Furt her deta ilscan be found in a companion paper in this volume (Pannemans& Roberts 2000).

Size(1979)demonst rat ed that the Follstad rocksare true trondhj em it es interms of chemical and mineralogical com- posit ion (LeMaitreet aI.1989).Theprimarytextureis hypidio- morph ic-g ranu lar.Calcic oligoclase (An_21•3o) and interstitial quartz are themajorprim ary phases with average modal conte ntsof58 and 26%,respective ly.Theplagi oclaseform s sub hedra l,weaklyzoned lath s whicharegenerallycluste red in larger glo meroc rysticaggregates.Microcli ne is present as intersti tia lgrainsandmakes up c.3%of the rock on average.

Biot it e and muscovite form subhedral plates with modal contents ofabout 2%and 6%,respectively.Apatite,titanite and zircon are accessory phases,the latter two occurring part lyas inclusi onsin biotite.

The primary textureandmineralogy is variably overprin- ted bymetamorph ic recrystallisatio nandshearing,wit h the gradual developmentof anallotrio mo rp hic-granu lartexture which locally grades into a morta rstr uct ure. The mineral parage nesisindicates thatmetam orphi smtookplaceunder up per greenschistfacies con ditions(Size 1979).This is broad lyeq uivalent tothe metamorp hicgrade ofthe host phyllites ofthe UndalFo rmat ion (Nilsen1978).

The contactofthe Follstad trondhjemiteto the surroun- ding phyllites is sharp, with no evidence of chilling against the wall rocks which have undergone minor silicification and development of a hornfelstext ure.Thinner,but other- wiseidenti cal,dykesnearthe intrusio nare clearlydiscordant to the st rong folia t ion in the host phyllit es.The lack of chi llingevi denceeven in thindykes wastaken bySize(1979) to indicatethatthewall rocks were at hightemperatures at thetim e ofint rusio n. Inaminor quarrysout hofFollstad, the weste rncontac tof the intrusivebodyshowssome varietyin trondhjemitetypes, from earlybrownish-gre y,comparatively fine-grained dykes,to later, 'normal' medium-grained types. The latter may grade into coarser,almost pegmatiticvarie-

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NGU-BU L L 437,2000 -PAG E60

ties,and arein placessucceeded by a new generation of brow nish-g rey, fine-g rained trondhjemite dykes (Fig.3e).

Abundant evidence of sim ilar multiple intrusion is not ed alsofor othertrondhjemite bodies and dykesin theGula Complex.

Thegeochemistry ofthe Follstadtrondh j emite and rela- tedintrusions in theGulaComplexhasbeen st ud iedbySize (1979) and Pannemans & Roberts (thisvolume).All these trondhj em it esareof thehiqh-Al.O,typewit hrelatively low K,O(c.1.3

% l.

andare furthe r characterised byhig hlyfract io- natedREE patt erns andvery lowcontentsofY and theheavy REE.According to these authors, a possible petrogenetic

GREG DUNNING

s

^{TO R}^G^{REN NE}

Fig.3.Photog raphs of typical trondhjemit e intrusions in the Gula Complex.A)trondhjemite dykecutti ngregionaltectonicfabric in the SinqsasFormation,Gula Complex(locality: Sorungen,Selbu);B)Syn-tec- ton ictrondh jemit e dykesinthe SinqsasFormation,Gula Complex.A fol- deddyke is cutby a similar,but straigh tdyke whichis parallel to the foliat ionin the hostschist.Thedykes show no evidenceofinternaldeforma- tion.Dykemargins aremarked by stip pledline(locality:Reitstoa); Cl proto mylon itictrond hj emit eband s inthe mylonitezone separatingthe Gula Complex (Undal Formation) from the Storen Group (locality:

Hundberga, Storen):D)f1uxion banding in typical medium-grained trondhjemitefrom the Follstad body.Width of fieldc.1.3 m (locality:

abandonedquarrynearEgga,2km southof the Follstadquarry);E)fine- grainedtro ndhj emitedyke cutting normal,medium -grained trondhjemite,Follstad trondhjemitebody.The irregular contact indicates dykein- trusion intoatrondhjemit e that was only partly crystallised(locality:

sameasD).

mode lis thatofpartialmelting of a garnet- bearing amphibolite source rock withthecompo sition of potassiu m-poor tholeiiti c basalt, during crustalanatexis in anorogeniczone.

Subord inate granodiorites are interprete d to have formed from a metasedime nt ary source under similar cond it ions (Pannemans &Robert s,this volume).

Thecom posit ion ofthe trond hj emitesisfairly unifo rm throu ghout the Gula Complex (Table 1). It is argued by Roberts&Sundvoll (thisvolume)thatthereisasligh tdiffe- rence betw een the Follstad intrusion and a supposedly older generat ion of more foliated and metamorpho sed trondhj emites which occur afew kilom etr es to thesouth-

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GREG DUNNING &TO R GRENNE

west, near Snoan (Fig. 2).Dat a from several varieties of multiple trond hjemite intr usions within the Follstad body and in the SinqsasFormation,however,show a compositio- nal overlap wit h the Snoan dykes wit h the exception of Rb and Sr.Wit h respect tothese elements,the Snoan dykesare similar tothe highlysheared and partly mylonitictrondhjem- ites that occurwithin the tectonic contactzone between the Gula and Storen unit s.

Sample preparation and dating

Approximately20 kg offresh samplematerialwere collected for age datingfromone sing leoutcropin theFollstad trondhjemite quarry at Stere n (UTM-WGS84 32VNQ 566850 6989700,1:50,000 map-sheet 'Steren'1621 Ill).The sample was separated into min eral constituents using standard techniquesof crushingandmineral separationwithaWilfley table, Frantz isodynamicseparatorand heavyliquids under clean conditions.

Mineralfractionswere dissolved in concentratedHFand 8NHN03in Teflon bombsover5days and Uand Pbextrac- ted using standard procedures of ion exchange chemistry.

The isotopic ratios were measured using a Finnigan-MAT 262multi -collectorthermalionization mass spectrometer in stat ic mod e.TheFaraday cupswerecalibrated against NBS 981 and 204Pb was measured in the secondary electron multiplier in ion counting mode.

NG U - B U L L437 ,2000 - PAGE 61

Uncert aint ies inthe isotopic ratios and agesare reported attwosig ma inTable 2, and these tot al uncertaintiesconsi- der propagation of uncertaintiesfrom mass spect romet ry measurements,isotopicfractionation,Pb and U blanks,and the uncertai nty on the isotopic compositionof initialcom- mon Pb.Full details of the analyticalprocedure arereport ed in Dubeetal.(1996).

The sample yielded a large amount of clear to pale brown tita niteand zircon (Table2).Thezircon occurs as50 to 100 micron prismsand very thin elon gate needles.One large fraction of titanite analysed is concordant with 206Pbj238U and207Pbj206Pb agesof 429 and 435 Ma,respectively (Fig.4). Fou r fractionsofzirconwere analysed,and three of these were composed oflargerequant euhedral prisms,and all are discordant.The discordance is interpreted to result fromthepresenceof olderinheritedzircon presentas cores insome of the igneousgrains.

The fourthfraction,of much smaller sample weight,was composed entirely of fine-grained needles and yields a concordantdatapointwith206Pbj238U and207Pbj206Pb agesof 432 and437 Ma,respectively(Fig.4).The data from thispoint and the titanite, interpreted to be igneous,together indicate an age of 432 ±3 Ma, based on the 206Pbj238U age ofthe concordant zircon, wit h the uncertainty large enough to include the titaniteages.

Table 1.Majo rand tra ce eleme nt com posi t io ns of repre sent ati ve trondhj e m it e s fromtheUndal For mati o n of theGula Complex (Fo llstadqua rry, Store n),SinqsasFor m ati on oftheGulaCo mpl ex (Bjornklettenqua rry,Budal) and themylo n it e zonese p a rat in g the Un d a l FormationfromtheSteren Group(Hu ndberga, Ste re n).

Sampl e TGST97 TGST97 TGST97 TGST97 TGST97 TGST97 TGST97 TGST97 TGST97

No. 6S 69 67 122 124 126 S3 4S 49

Ro ck trondhjemite, trondhj emit e, trondhjemite, sheared mylonitic myloniti c early norma l trondhjemite,

ty pe earlydyke no rmaltype latedyke trondhjem ite tro ndhjemite trondhjem ite trond hjemite trondh jerni te latedyke

Loca lity Fol/stad(Unda l Formation) Hundberga(Steren-Undalcon tact) 8jornklett en(SingsQsForma tion)

SiO, 71.19 70.68 68.06 67.89 67.0S 6S.30 69.20 67.36 73.83

AI,O, 15.80 15.83 17.97 16.37 16.15 16.32 17.06 18.30 15.52

TiO, 0.20 0.38 0.33 0.17 0.16 0.26 0.18 0.17 0.14

Fe,O, 1.19 1.45 1.79 1.21 1.15 1.94 1.32 1.25 0.87

MnO 0.01 0.02 0.03 0.03 0.06 0.04 0.02 0.02 0.02

MgO 0.45 0.48 0.95 0.67 0.67 1.05 0.79 0.86 0.25

CaO 4.07 2.85 4.11 2.65 2.95 3.71 3.65 4.20 1.89

Nap 5.69 5.89 5.73 5.70 4.92 5.20 5.54 5.93 5.70

K,O 0.65 1.38 1.11 1.63 2.54 1.96 0.78 1.05 1.78

P,O, 0.07 0.07 0.10 0.07 0.07 0.08 0,07 0.07 0.04

LOI 0.50 0.73 0.96 2.90 2.79 3.84 0.22 0.32 0.29

Total 99.81 99.76 101.14 99.30 98.51 99.69 98.83 99.52 100.33

Nb <5 <5 <5 <5 <5 5 <5 <5 <5

Zr 93 95 78 79 83 65 83 68 89

Ce 33 31 32 21 23 28 27 23 26

Y <5 7 <5 7 6 8 7 <5 <5

Sr 641 601 687 438 280 447 573 702 635

Rb 15 20 22 63 82 68 28 28 37

Ba 285 329 284 326 350 258 330 305 337

Cr <5 22 <5 5 6 11 9 8 9

V 17 22 20 16 12 32 14 13 9

XRFanalysescarriedoutat the Geolog icalSurveyof Norway.Major eleme nts measuredonfused glassbeadsprepa redby1:7dilutio nwithlit hiumte tra borate,and traces on pressedpowderpellet s.Thesa mpleswereanalysed OnaPhilipsPW1480 seq ue ntialX-rayspectrometereq uippedwithaSclW-anode X-raytube,usingcommo ninter- nationalstandardsforcalib rat ion.

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NGU-BULL 437,200 0 - PAGE 62 GREG DU N NING&TO R GRENNE

Table 2.U-Pb isotopicagedata fortheFollstadtrondhjemit e.

Con e. Measured Corrected Atomic Ratios' Age(Ma)

Fract ionanddescription Weight U Pb rad T.C. ""Pb ""'Pb ""Pb ""Pb ""Pb ""Pb ""Pb ""Pb

Pb ""Pb ""Pb

mu

^±

mu

^± ^""^Pb ^± ~

mu

^""^Pb

(mg) (p pm) (pg)

Zl smallcleareuhprisms 0.095 616 42.7 11 15545 0.1020 0.06946 21 0.5431 25 0.05671 10 433 440 480

Z2 clear euh needle sabr 0.008 378 26.4 8 1640 0.1133 0.06931 28 0.5316 26 0.05562 20 432 433 437

Z3 cleareuhprism sabr 0.025 511 36.1 21 2685 0.1157 0.07008 24 0.5520 20 0.05712 10 437 446 496

Z4 cleareuh prismsabr 0.029 469 33.8 17 3524 0.1135 0.07150 44 0.5658 34 0.05739 8 445 455 507

T1 clearpale brownabr 1.035 57 3.8 2434 118 0.0757 0.06879 30 0.5270 57 0.05556 52 429 430 435

Note s:'Isotop icrat ios correct ed forfractio natio n,spike,labora toryblan k of2-10picogr am s of comm on lead and initialcom mo n lead atthe age ofthe sam plecalculated fromthemodel ofStacey &Kram ers(1975),and1 picogramU blank.2sig mauncert aintiesrepo rt edaft ertherat iosrefertothefin aldigi tsandarecalculate das outli ned in Dubeet al.(1996).T.C.Pb=tot alcom mo nlead,Z=zircon,T=titanite,euh=euhed ral,abr=abrade d(cf.Krog h 1982).

FOLL STADTRON DHJEMITE

Fig.4.U-Pbco ncordiadiagramshowin gzirco nand titanitedatafro m theFo llstadtron dhjemite,St o ren,The lett ers Z1-4and T1 refe rtozirco n an d titan itefraction s,respec t iv e ly,asdescri bed inTable2.

Discussion

Precise zircon dating ofCaledonian rocks overthe last two decadeshas demo nst rated the existe nce of fairly well-defined magmatic provinces andepochs of different paleotectonic significance.The one that is best documented is that representing various oceanic and conti nental-margi n arc sequences,which were developed in the early stages of plate convergence prior to closure of the lapetus Ocean.

Agesfrom these sequences range mostlyfrom 495 to 470 Ma(Early Ordovician),but also extend int o the Mid to Late Ordovician with ages of around 460 Ma for arc-related pluto ns(see Grenne etal.1999 for a review).

The 432 ±3 Ma crystallisationage of the Follstad trondhj emiteplacestheintrusion ina timeinte rvalwhere itis generally accepted that collision tectonics was active during the climactic,Scandian phase of the Caledonian orogeny (see,e.g.,Ste phensetal.1993,Grenneet al.1999, and referencestherein). A minimum age ofthe penetrativeScandian deformationand metamorphismin the Gula Nappeis provi-

ded by 4°Arj39Ar cooling ages for hornb lende of abo ut

425-430 Ma(Oallmeyer 1990),but the age of peakmetamor- phism is poorly known in the TNC part of the Upper

Alloch tho n.The Narvi k NappeComplex innorth ernNorway, which may possib lybe correlatab lewith theGulaComplex, has yieldedU-Pbagesof metamorphicmonaziteand zircon which showthatpeak met amorphismoccurredat c.432 Ma (Northrup1997).East ofthe TNC,rocks of the Seve Nappesin the centralSwed ishCaledonideshaveyieldedmetamorphic U-Pb ages from sphene,monazit eand zircon that range from 425to 440 Ma (Gromet et al.1996).Thus,metamor- phismseems to havebeen broadlycoevalwith trond hjemite emplacement,immediately before the rapid upliftand cooling that occurred in Late Silurian and Early Oevonian times.This is in accordance with the general absence of chilled marginsin the Follstad trondhjemite indicatingthat itwas emplacedintohot country rocks.

Similar arguments are valid for the Fongen-Hyllingen complex and adjacent trondhjemites.Alate,syenite diffe- renti ate of thismagmaticcomplexwas emplacedat426*8/.2

Ma,just prior to the O₂ deformation and accomp anying medium- to high-grademetamorphism at a crustal depth of 15-20km.Adjacent trondhjem itesaretemporally over- lapping with this tectonomag matic event (Olesen et al.

1973,Wilson etal.1983).

ElsewhereintheCaledonides,zircon ages from thisEarly Silurian period representessent iallythreedifferentmagma- tic setting s.The most prominentone is thatof thebatholit - hic int rusions which are foun d mainly in the Uppermost Allochthon of centralNorway(Fig.2),witha span ofcrystalli- sat ion ages of c.450-430 Ma. Bat holithicmagma tisminclu- des awiderange ofmagmatyp es of gabb roicto granitic compositions, but trondhjemitesareabsent or scarce(see, e.g.,Nordg ulenetal.1993and referencestherein).Thepale- otecton icsett ing has beeninterpretedintermsof an active continental marginabove awest-dipp ingsubd uction zone in Ordoviciantime,possibly along the margin ofLaurentia (e.g.,Stephens&Gee 1989),wit h continuedmagmatismin a continent-c ontinent collision zone during the Silurian, Scand ian orogeni c phase(e.g.,Grenneet al. 1999).

In the KoliNapp es oftheUpper Allochthon therearese- veral gabbro comp lexesthat have been dated at around 440-430 Ma. The Sulitj elmaand R<3na layered intrusionsin 85GD2 7

.577 207Pb 235U 455

~3

450

551 440

445

538

1432+/-3Ma

I

525 425 .0739

.0707

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GREG DUNNING& TO RGRENNE

Nord land have similar ages of 437 ± 2 and 437 +1(, Ma, respect ively(Pedersen et al.1991,Tucker et al. 1990).The Artfjallet gabbro in northernSwedenis434±5 Ma (Senior&

Andriessen 1990).Comparable, but less precise,ages of around435 Ma have beenobtained by Sm-Nddating from theKrutfj ellet gab bro(Merketal.1997).Some of these intr u- sions sharethe syn-tectonic and syn-metamorphiccharac- teristicsof the Fongenint rusion in the TNC. Moreover,they have, in gene ral,beeninterpret edto ref lectapaleotectonic regime characterised at least part ly by crustal extensio n (St urt& Roberts 199 1,Stephenset al. 1993).and some are closely associated with mafic dyke swa rmsand associated pillowlavas, suchas at Sulitjelma.ltispossiblethatalso the Solund -Stavfjord ophiolit ebelongs to thistecto nomagma- ticstagealthough it hasyieldedaslight ly olderzirconage of 443±3 Ma(Dunning&Ped ersen1988).

Trondhjemitesare concentrated in the K61i Nappes of the Upper Allochthon and,as noted above,some of these are spatiall y associated wit hgab bros of the typejustmenti- oned.Reliablezircon dates,howev er, exist only for afew of the trondhjemites.Within theTNC, two trondhjemitic peg- matites in high-gradeschists westofTrondheim,have been datedat about 431

±

2 and 423

±

2Ma by U-Pb on zircon (RD. Tucker, in Sollietal. 1997).Earlier,Rb-Srand U-Pbzircon datingoftrondhjemiteswithin the Gula Complexhavebeen reported in a preliminary form by Klingspor&Gee (1981).

According to these authors, zircon s from the Vakkerlien trondh jemitenearKviknedefin ed aU-Pb discord ia linewit h an upper intercept of 477+8( 5Ma,but the intrusionwaslater reinterpretedto have an age of 509 ^{+8( 4}Ma(I.Klingspor &

D.G.Gee,pers. com m. in Step hensetal. 1985).Rb-Sr whole- rock agedeterminations indi catedages in theorderof c.478 to447 Ma,with large uncertaint ies,for this and three other trondhjemites in the Gula Complex.Rb-Sr data from trond- hjemitedykesin the Snean area,just sout hw est of Follstad, havegivenanisochron ageof465± 11Ma, inte rpreted by Roberts&Sundvoll(t his volume)as aminimum ageofintr u- sion and reflecting a phase of post-emplacement thermal resetting.In our opinion,all theseages should be considered with caution until they are confirmed by precise zircon dat ing.

Stephens&Gee (1985)argued foracorrelationbetw een the Gula Complex and the Upper K61i Nappes in Vaster- botten,Sweden, where there isevidence that granitic and gabbroic magmat ism occurred in the Early Silur ian (Stephenset al.1993and refere nces therein),broad ly con- temporaneous wit h the Follstad trondhjemite.In northern Norway,Upper Allochthon metasedimentary sequences that canpossiblybecorrelat ed wit h the GulaComplex (the Narvik Nap peComplex) contain granit ic dykes that have yielded U-Pbzirconages of 437±1 Ma (Nort hrup1997).This age is identicalto that of the R~lnagabbro which islocated in the same unit.Farthernorth,a syn-tec to nicgraniteintheLyngen NappeComp lexofcentr alTromshasbeen date dat432±7 Ma bytheRb-Sr met hod (Lind strern&Andresen1995).

NGU- B U L L437,2000 - PAGE63

On aregionalscale, the age and field relation shipsofthe intrusions mayprovidesome info rm ation on thehistory of tectonic juxtaposition of the different nappe units within the TNC. Their occurrence in the Gula as well as in the Meraker and SterenNapp esseem stoimply that these units wereat least closeto each other, ifnotstacked togeth er,at the time ofint rusion.It is possible that this proximity was due to obduction of the Steren and Fundsje Groupson to Gula sequences,as suggested byprevious auth ors,but there is no unequivocal evi de nce of such an Early Ordovician amalgamati on of theterranes.Atanyrate, themyloni ti c contact zonebetw eentheunits asseen now in the Gauld alregi- on must bearesult,at leastin part,ofsubsequent napp e movem ents.Thisisbecause (1)trondhj em it es which,in our view, are sim ilar to the Follstad intr usio n are them selves mylonitisedalong thiszone, and (2) the trondhjemite s are much more abundant on one side of the contact in the vicin it yof Ster en.

Inapaleot ectoniccontext,availabledata indicate that the Follstad trondhjemitebelong sto an EarlySilurian tecto- nomagmaticepoch lasting perhapsovera timeinterval of 15 Ma or more and represented mainl y in the Upper Allochthon ofthe Caledo nid eorogen.Theperiod was cha- racter isedby orogenic,continentcollision-relat ed,deformation and metamorphism penecontemporaneou s wit h emplacement of felsic and mafic (rift -related) intrusion s. Moreover,existing zircon agesshow thatthis magma tism was coeval wit h the late stages of batholith develop ment farther west, now represented mainly in the uppermost parts of the nappe pile.Thisim p lies considerablevariation s in magmaticactivityacrossthecollisionzone.

It has been demonstrated elsew here (Size 1979, Pannemans & Roberts, thisvolume) that the Follstad and similar trondhjemites in the region wereprobably formed by partial melting of maficrocksunder garnet-amph ibol it e condit ions.Similarconclusionshave beendrawn forcompa- rable trondhj emitesworldw ide (e.g. Drummond et al.1996, Rollinson 1996,Martin 1999).Such rockshave ofte nbeen int erpret ed as the products of dehydration melting of a downgoing slab of young and hot oceanic crust in a sub- ductionzone.Such an origin,how ever,seemshigh lyunlikely forthe Caledoniantrondhjemitesunder consideration, since there isoverwhelming evidence that they formed during the interaction of two continental platesand that you ng oceanic crustsubductionwas not involv ed at this sta ge.

Unlessthe int imate spatial and temporal relat ionshi ps betweenmafic and felsic intrusionsin theUpperAllochthon arepure coincidence,which weconside r unlikely,this may be akeyto our understand ingof theorig inoftrondhjem it es in this partof the orogen.There is littledoubt thatthe mafic magm as,ranging in compositionfrom tholeiit icto alkali ne or calc-alkalin e (see Grenne et al. 1999 and references therein ),werederived fromamantle source.It is very likely thatthesehot maficmeltswould provide suffi cient heatfor part ialmelting of deep crust al rocks whichwereat garnet-

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NGU-BULL 437,200 0 - PAGE 64

amphibolite grade. It seem s less obviouswhy garnet amphi- bolites of basalticorigin should play such a dominatingrole inthe source region;there is evidence of more K-rich metasedime nta ry sources that gave rise to contempora neo us granodioritic to granitic magmas but theseappear to have been subordinate.

Thepaleotectonic setting of this magmatism is not unequivocal,but there is general consensus that the Scandian phase wascharacterisedbyhighly obliq uecollisionbetween the continents.This may have beendue in partto a rapid anticlockwise rotation of Baltica and alateral relative movement ofthe converging margins ofBalt icaand Laurent ia at the Ordovician-Silurian boundary (e.g., Torsvik et al. 1996).

Such a scenario would be broadly equivalent to the situa- tion for Late Cenozoic magm atism acrosstheArabia-Eurasia collision zoneinTurkey(Pearce et al. 1990)and mightallow the development ofexte nsional segments that facilitated episodi c rift magmatism and associated trondhjemite formation, wit hin a predominantly transpressionalregime.

Conclusions

The age ofint rusion of the trondhjem itebody at the type lo- calityat Foll stadis esti matedat432±3 Ma fromoverlapping concordant zircon and titaniteU-Pb analyses.The trondhje- miteis a representative of anext ensive suite of intrusions that are concent rated in the Kbli Nappes of the Upper Allochthon of the Scandinavian Caledonides.Trondhjemite emplacement was commonly associated with mafic magmatism,andthe magmatismwas broadly coeval with metamorphism and deformation during the early stages of the Scandian,climacticCaledo nian,orogenic event.The geoche- mical composition of the trondhjemites indicatesthat they were formed as products of part ialmelt ingof garnet-amphi- bolitegrade,deepcrustalrocks.We suggestthat thispart ial meltingwas facilitatedby the emplacement of the contem- poraneous rift-related,mantle-derivedmafic intrusions.

A possiblescenario is a paleotecto nicsetti ng characterised bythe development of ext ensional segments wit hin a predominantly transpressional regime, due to highly obli- que continentalcollision. This was probablya consequence of the rapid anticlock w iserotation of Balt icaand a lateral relative movement of the converging marginsofBalticaand Laurent iaat theOrdovician-Silu rianboundary.

Acknowledgements

The authors are grateful to Mike Stephensand Bernard Bingenfortheir helpfulrevie w sof the manuscript.

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