Geochemistry and petrogenesis of trondhjemites and granodiorite from Gauldalen, Central Norwegian Caledonides

BA RT PANNEMANNS& DAVID ROBE RTS NG U-BU L L 43 7,2000 - PAGE 43

Geochemistry and petrogenesis of trondhjemites and granodiorite from Gauldalen, Central Norwegian Caledonides

SART PANNEMANS&DAVID ROSERTS

Pannemans,B. & Robert s,D.2000: Geochemist ry and pet rogenesis of trondhjemitesand granodiorite from Gauldalen,Cent ralNorwegian Caledonides.Norgesgeo/ogiskeundersekelse Bulletin437,43-56.

IntheGauldalendistrict ofCentralNorway,three variet iesof trondhjemite cuttingrocks of theGula Complexhave beendisting uishedon thebasisof nuancesofcolourand fieldrelationships.Thehigh A1₂0₃,Rband5r,andlowYb conten tsofallthree typesof tron dhje mit e signify theirgenerationina cont inentalmarginpalaeotecto nicsett ing.

Theyare considered to have formedas melting productsof a garnet-bearingamphibol ite.Lesscommon, spessar- tine-garnetgranod iorit esin this same area arecalc-alkaline and peraluminous,andmayhavederivedfro man AI-richsedimentary source.Fieldrelationsdenotethatgreenish-white trondhjemitedykesandsheetsarethe oldest intrusion s.One of thesedykeshasprovided a Rb-Sr who le-rockisochron (minim um)ageof465+11Ma.The garnet iferousgranodio rites transectthese earlydykes andarethemselves cut by dykes and largerbodiesofwhite trondhjemite,includingthe type-localityFollstadtrondhjemiteU-Pb-dated(zirconandtitanite)to 432+3 Ma.

Allthetrond hjemites andthegranodiorite postdateanexisting foliationinthehostGularocks,butarethemselves weakly foliated,foldedandinplaces cutby alaterc1eavage.The overall,local andregional isotopic dataand biostrati- graph icconstraintssuggestthat anearly Caledoniantecton othermal eventaffectedtheGularocks befo re465 Ma, andprobablyinEarly toMid Arenig time.Thefoliation,foldsand cleavage inthetrondhjemites and granod iorit e,on theother hand,mustpostdat e432Ma andareconsidered tohaveformed duringthemainScandia n tecton ometa- morphic event datedisotopically to around 425 Ma.

Bart Pannemans,DepartementFysico-ChemischeGealogie,KULeuven,Celestijnenlaan200C,2001Leuven,Belgium.

DavidRoberts,GeologicalSurveyofNorway,7491Trondheim,Norway.

Introduction

Trondhjemitesconstitu teone ofthemore commonvarieties of plutonic or hypabyssalrocksin theUpperAllochth on of theNorwegianCaledonid es, occurring principall y in theK6/i Nappes,Le.inthe out boardor suspect terraneslying tecto- nostratigr aph ically above the cont inent (Baltica)-ocean (Iapetus)transitionzone amphibolitesandschists thatcom- pose the generally higher grade Skj 0t ingenNappe(=Seve Nappes).Thedefinition of thetermtrondh jemite stems from Goldschmidt's (1916) st udies in centr al and sout hern Norway, inthe Caledonides,andnotably in the't ype locality' of this rock-type at Follstad, just south of Trondh eim. Subsequent classificat ion schemes categ ori setrondhjem- ites as leucocratic tonalit es,wit h <10% of mafic mineral components(Barker1979,Le Maitre1989).

In Cent ral Nor way,trondhjemites are distincti ve rock- types wit hin the Koli Nappes of the Trondh eim Nappe Complex (Loeschke1976,Size1979) (Fig. 1).In manyplaces theystandout quite clearly,wit h theirwhite to palegreyor greenish-grey colour, against the darker heterogeneous host rocks,either asdykesor as largerbodies. Insome areas, they appearto coexist wit hsubordinate dioritesand gab- bros(Nilsen&Wolff 1989).Based on relation shipswit hthe principalschistositi esand cleavages, ithasbeensuggested thatthere were perhapsthree orfourpulses of trondhje m- itic intrusioninthisregion (Roberts 1978,Size 1979).Inthis

contrib ut ion,we give brief descriptions of trondhjemites andassociatedleucogranito idrocksfrom, and in theneigh- bourh ood of, the valley Gauldal en - one of three maj or, transversevalleyscuttingthroughthemetamorphi calloch- thon oftheTrond heim Region- and presentgeochem ical data andapetro geneticmod elforthesesamerocks.

Fig.1.Principaltectonostratigraph ic unitsin theCaledonidesofthe cen- tralpartoftheTrondh eimRegio n.Central blankarea-GulaNappe;dia- gonal lined area- Steren Napp e;hori zonta l lines- Meraker Nappe.

Boxedarea-Figure2.

NGU-BULL 437,20 0 0 - PAGE 44

Regional setting

In the liter at ure, the Tro ndheimNappe Co m plex (TNC)has been subdivide dinto three principaltecto nic unit s,eachof Kbli Nappe affin it y-- the MerakerNappein the east,the cen- tralGula Nappeand a western unit,the Sto renNappe(Fig. 1) (Wolff 1979). In general term s, the Meraker and St o ren Nappes comprisemainly low-g rad e metasedime ntary sue- cessions,butalsoincludesignifica ntvolumesof ophiolitic or immature volcanic arc rocks in thelow er and midd leparts of their lit host rati g raphies.The Gula Nap pe is aheterogen eous and in some ways enigmatic unit of rocks of generally, thou gh not ent irely, higher metamorphic grade.Detailed descript io ns of lith ol og iesof these unit s are contained in Nilson(1978),Wolff(1979) and Gee et al. (1985).

The Steren Nappe co m prisesabasal ophio litic complex overlainunconformablyby aLate Arenigto Ashgillvolcano- sedimentary succession.The ophiolitic rocks are considered to have been deformed and metamorpho sed,and obd uc- ted upontheGulainearliest Ordoviciantime.The unconfor- mabl yoverlying Ordovicia nrocks oftheHovin Group were subsequentl y first metamorphosed and deformed during the Scandian orogeny, in Silurian to earliest Devonian time.

In the Meraker Nappe, a bimodal magmatic complex (Fundsj o Group) wasinit ially deformed and met amo rpho- sedprior to upliftanderosion,and then overlain unco nfor- mably by mainly sedimenta ry rocks, wit h some volcanic mem bers, ofinferred Ordovician to Early Silurian age.This volcanosedimentaryassemblage was thendeformed during the Scandian orogeny.The tectonotherma l historiesof the StorenandMeraker Nappes arethus quite similar.

r r

r:

BAR T PANNEMANNS&DAVID ROBERTS

Thereis evidenc e tosugg est that the rocks of the GuJa Napp e (termed theGula Gro upor,more recen tly,the Gula Complex) were invo lved in both theEarlyOrdo vician and the Silu ro-Devo nian tect o no m et amo rphic events (Guezo u 1978,Lagerb lad 1984,Sturt&Ro ber ts 1991).In this part of the Gula,threeformation shave been distinguished --from west toeast,the Undal,Sinqs as and AsliFormations(Nilsen 1983).lnthe west,the contact betwee ntheUndalFor m at io n and thebasal volcani tesoftheSto ren Nappeis tectonicand mylonitic. The status of the easte rn contact, against the Fund sj o Group, is contr oversial; either tectonic (Guezou 1978, Lagerb lad 1984,McClellan 1994)or primary(Rui,1972, Rui&Bakke1975,Bje rkqard &Bjorlykke1994). lnthis eastern area,a Dietyonem a(Rhabdinop ora)-beari ngphylliteprovides our only evide nce ofbiost rati g rap hi c age in the Gula, i.e., Trem adoc(Vogt1941).

Inapalaeotect on ic context, theSt o ren ophiol iteis consi- dered to have been obd ucted upon GulaGro up rocks in Earlyto Mid Arenigtim e(Gren ne & Roberts1981, Roberts et al. 1984, Sturt&Rob erts 1991).Thebim odal magmaticrocks of the Fun d sj oGro up may have experienceda sim ilarfate.At any rate,thesethree unitswerethen deformed,metamor- phosed and uplift ed,and unco nfo rmab ly overlain by the Ordovician,immature basinalsequences now occurring to the westand tothe east. The weste rnbasin,at least,relates to back-arcspread in g above a subduction zone(Robert s et al. 1984,Grenne &Roberts 1998).Subseq uent ly,the basinal assem blages plus theirophio lit ic and Gula substrates were involvedin Scand ian, southeastwardthrust translat ion and imbricatio n,for min gthe Trond heim Nappe Complex.

§

" " SterenGroup UndalFo rmation Singsas Formation

}

ST0REN NAPPE

} GULA NAPPE

c=J

Fig.2.Simplified geologicalmap of the Gauldalen district.F - Fo llst ad,R- Reitstoa .

BARTPANNEMA N NS &DAVID ROBERTS NGU-BUL L43 7, 2000 - PAGE 45

Fig.3.(a)Thequarry at Follstad, showing the massive natureofthe white trondhjemite.Phot otakenlooking c. NNW,inAug ust 1999.(b)Close-upofthe trondhj emiteattheFollstad quarry.Althoughthisisthemostmassive variantofthetrondhjemite,therockshowsahypidiom orphic-granu lar text ure anda very faintfoliati on.Thicknessof pencil,7 mm.

In this general,overall scenario,tw o principal types of trondhj emite arerecognisedinthis dist rict; (1)sills anddy- kes associatedwiththeearly ophiolites or with MORB-type mafic volcanite units.Thesehave oceanic geochemical trait s, akin to plag iogra nit es,and have yielded U-Pb zirco nagesin therange 495-480 Ma(Dunning 1987,Dunning &Grenne, in prep.).(2) Bodies ranging from dyke swarms to major plu- tons.These arepart icul arly commo nin theGula and carr y continental -margi ngeochemica lsignatures.The onlyrelia- bleisotopic date so far isthatfrom the type-loca lityFollstad tro ndhjem it e, withaU-Pb zirco nandtita nite age of 432± 3 Ma (Dunni ng&Grenn e 2000 -- thisvolume).How ever,it has been suggested that these particular trondhj em ites may have intr uded over a wide inter valof time,througho ut the Ordovi cian and Silurian period s (Size 1979, Roberts &

Sundvoll 1996).The trondhj emitesdescribedin this account all belongto the continental margin category.

Plutonic and dyke rocks in Gauldalen

Several trondhjemite bodies and associated plut on icrocks and dykes intrude the Gula Com plex alo ng Gauldalen (Nilsen & Wolff1988).Here, we consider select ed bodies in two areas;(1) in western Gauldal, at and in the neighbour- hood of Follsta d, near Steren: and (2) in the vicinity of Reltstoa.between Sinqsas and Haltdalen, some 40 km sout h- east of Steren in the central part of the Gula Complex(Fig.2).

We also tou ch upon the geoche m istry of tron d hj em it e dykes from an area 6 kmsout h ofSteren:these dykes are the subjectof anotherpaper in this volume,citedlater.

Western Gauldalen

The Follstad trondhjemite and associated dykes

Thetrondhjemiteat Foll stad, north ofthe Gaulariverc.2 km sout heast of Steren, served as a type localit y for Gold-

schmidt (1916) and was the subject of a petrological and maj or-elem entgeoch emi calst udy by Size(1979).Thetrend- hj em ite is excellentlyexposed in alarge quarry,whereitis workedunderthecommercialname'Storengranite:

Field relationships and petrography

The FolIstadtrondhj emite is ac.7 km-long and up to 400m- wide, NNE-SSW-trending body intrudin g phyllites and schists ofthe Undal Fo rmat ion.In thevicin ity of themain quarry(Fig.3a)this white to very pale greytrondhjemitehas athickn ess of c.375 metres,and dips atc.70°to theWNW (Size1979). Althoug h the body is fairlyregularin shape,there isa prominent dyke apophysis in a smaller quarry in the southe rn part ofthe weste rncontac tshowingpartlyassimi- lated andhornfelsedxenoliths of schistoseGularocks upto 4 m across.In general,the contact zone of the trondhjemite doesnotdisplay anyparticula rlyclear, chilled margins,even thoughhornfelsictexture is evident in theadjacentcountry- rock schistsand phyllites (Size 1979).Thinnerdykesof sim ilar 'w hite'trondhj em it e occurwithi ntheGula phyllitesoutside of themain body,andsomeareboudinagedandaffectedby laterfolds.

Thepet rography and textu res oftheFollstad trondhjem- itehavebeendescrib ed insomeconside rabledetailby Size (1979). Here,we presentonly a brief account,combining the main aspectsof Size'sdescription and our own field and thin-secti on observatio ns.Thetron dhjem it e isamassive and homogeneous rock,carrying a weak foliation defined by biotite and muscovite.No visible differences in charact er could bedet ected over the entire,7 km length ofthe body.

The presenceof biot it e gives the medium-grained rock a speckled appearance(Fig.3b).Plagioclase(An" - calcic oli- goclase) forms almost 60% ofthe mod e, and quart z 26%.

Tog et her, theseminerals showa hypidiomorphic-granular

NGU-BU L L 43 7,2000 - PA GE46 BA RT PANN EMANNS&DAVIDROBERT S

• Follstad trondhjemite

o

.6.

o

<>

\l

Fig.4.Normative compositions ofthe trondhjem it es,garnet- bearing grano- diorites andmaficrocks plottedon a QAPdiagram(Streckeisen 1976). Field s:

1- granite ;2-granodiorite;3- tona- lite;4 - gabbroand diorit e.

Fig.5.The samesamples asin Fig.4plot ted on the An-Ab-Ordiagram of O'Connors(1965). Fields:Gr-granite;grd-granodi orite;ton-tonalite;

tr-trondhjemite.

dykesand into the country rocks. Field relationshipsthus sugg estthatthe reddishdykesmay represent a slightlyearli- erstageofint rusionthan themain trond hj emite body.The reddi sh-white dykes are fine grained,consisting mainly of quart z,plagioclase and some biotite,and contain conspi- cuous phenocryst s of zonedplagioclase.lnthe leachedparts ofthe rock,biotite has been replacedby epidote,in some casesin oneand the samecrystal, and a fibrous c1inozoisite hasbeen precipita tedin theassociatedmicrof ract ures.

In manypart s of thedistrict,both Gulaand Storenrocks (butnot the overlying HovinGroup)arecut by fine-grained, grey-w hite to greenish-whitetrondhjemite dykes trending texture,altho ugh in the more foliate parts the text ure is

closeto granoblastic.Most plagioclaselathsdisplayrimsof untw inn ed and unzoned albite. The ot her main minerals presentaremuscovite(6%),K-feldspar(c.3%),epidote(>4%) and biotite (2%).Accessory minerals include magnetit e, titanit e,zircon,apatite,chlorit e andcalcit e.

Despit e its massivecharacter,thetrond hj emitehas clear- ly been affected byregional metamorphism.Thechange- over to more grano blastic and allotriom orph ic-g ranular textures and a moreconspicuous,though weak,foliation correspond s with increasedconten ts of biotite andepidote;

and the general mineral assemblage muscovite- biot ite- quartz -oligoclase-epidoteis indicative of uppergreenschist facies(Goldschmidt 1916,Size1979).Thisgrade of metamor- phism is slight lylowerthanthealmandine-amphibolit e faci- es assemblages generally encounteredin themetasedimen- taryrocks oftheGulaGrou p.

Other trondhjemite dykes

Alongpartof theeasterncontactof theFol lstad body,there isa40 cm-thi ck,pale reddish-white to greytrondhjemite dyketrending parallelto themainbody,witha sharpcon- tact against the latter.The dykecontains xenolit hs of the Undal schists.Simi lardykes,also trending c.NNE-SSW, are present afew metresawayfrom themain contact.For des- criptive purposes,these are referredtohereafter as thered- dish-whitetrondhj emites(dykes).

Anastom osing fractures roughly perpendicular to the dykescoincide with a pattern of leaching and alteration, where the reddi shhuehasbeenreplacedbya greenish-whi- te colou r.Thisclearly indicates thatfluidshave beenpercola- ting fromthe mainFollstad body through thereddi sh-white

Ab

An

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BART PANNEMANNS& DAVID RO BERT S NGU - BULL 43 7,2000 - PAGE47

Fig. 6.Contactrelation ship sbetw eendiversetrondhjemites or granodi orites andcount ry-rockschists,etc., of theGulaCompl ex,Gauld alen; allloca li- tiesonmap-sheetHaltdalen1620I(4-NORedition coordinates).(a)Trondhjemitedyke cuttingvariab lydeformedmetased imentaryrocks;looking ENE. Road-cut exposureon theR30road,c.600mSSEofGil lset;03708150.(b)Apo physesfromatron dhjemite dyke cutti ng micaschists(+garnet )and felsic gneisses;looking north.Road outcrop onR30,c.99408335.(c) Granodi oritedyke,part lypegmatit icandweakly foliate,cuttingthepenetr ativ e schisto- sit yin theGulahost-rocksat a lowangle;lookingNNW. R30 outcrop,c.00108300. (d) Granodiorite, partlypegmatitic(muscoviteflakes upto 2 cm), cut ti ngand enclosingschistose,Gulacount ryrock;lookin g SE.R30 road-cut ,00158295.

NGU- B ULL 43 7 ,20 0 0 - PAG E 48 BART PANNEMAN NS

s

Table1.Major and trace elementdata fortheFollstad(white),reddish-wh it eand greeni sh- whitetrondhjemites,Gauldalen. Major elementsinwt.%,traceeleme ntsinppm.

Follst adtrondhjem it e Reddish- wh it etrondhje m it e Greenish-wh it etrondhjem ite

B1 B11 B16 B22 B2 B7 B10 B12 BB B15 B4 B5 B6 B20

SiO, 70.90 72.21 72.93 73.46 72.71 71.20 72.92 72.68 72.83 66.62 66.80 67.63 66.39 66.80

AI,O, 16.5 7 16.16 15.47 15.33 16.16 16.70 15.85 15.86 16.10 17.80 17.18 16.29 16.33 18.19

Fe,O, 1.32 1.15 1.34 0.49 1.24 1.53 1.04 1.39 1.03 1.74 1.95 1.60 1.79 2.00

TiO, 0.22 0.16 0.21 0.07 0.20 0.24 0.17 0.21 0.20 0.32 0.28 0.24 0.24 0.30

MgO 0.47 0.35 0.45 0.15 0.44 0.60 0.30 0.54 0.51 0.96 1.24 1.05 1.07 1.29

CaO 2.92 2.36 2.50 1.62 2.95 2.86 1.96 2.94 3.03 4.09 4.55 3.12 3.64 5.36

Nap 5.52 5.42 5.21 5.94 5.18 5.70 5.49 5.62 5.36 5.36 4.61 4.78 4.13 3.97

Kp 1.19 1.62 1.38 1.32 0.84 0.90 1.25 0.64 0.59 1.07 0.80 1.84 2.07 1.03

MnO 0.02 0.02 0.02 0.01 0.02 0.02 0.02 <0.0 1 0.01 0.Q2 0.02 0.02 0.02 0.03

pp, 0.07 0.07 0.06 0.03 0.06 0.08 0.05 0.07 0.07 0.10 0.08 0.08 0.07 0.09

LOt 0.84 1.08 0.84 0.37 0.51 0.50 0.69 0.55 0.53 0.89 2.29 3.40 4.04 0.72

Total 100.03 100.61 100.43 98.79 100.31 100.33 99.74 100.51 100.24 98.97 99.80 100.05 99.79 99.78

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

Zr 94 72 91 68 95 86 108 85 96 81 78 89 78 51

y <5 <5 5 5 7 5 <5 <5 8 5 <5 <5 6 <5

Sr 633 505 532 757 688 750 610 694 650 621 581 485 416 626

Rb 22 34 25 34 15 18 22 10 10 22 31 64 73 29

8a 346 382 383 519 332 284 424 283 256 295 263 362 320 144

U 0.6 0.6 0.8 1.0 1.1 <10 1.1 1.1 <10 0.9 <10 1.4 1.2 1.4

Th 1.8 1.S 2.6 0.6 1.8 <10 1.9 1.4 <10 1.3 <10 1.9 2.1 0.7

V 16 12 16 <5 17 23 12 19 18 23 33 27 29 36

Cr 3.1 2.5 4.1 2.4 4.4 <5 2.4 8.0 <5 5.0 12.0 12.3 11.7 6.4

Se 1.80 1.76 <10 1.24 1.86 <10 1.20 <10 <10 <10 <10 3.44 4.16 3.80

Hf 2.10 1.80 2.25 1.93 1.93 2.17 1.84 1.91 1.99 1.67 1.04

Ta 0.09 0.60 0.13 0.24 0.11 0.11 0.10 0.12 0.12 0.14 0.08

Co 2.0 1.5 1.9 0.5 1.9 1.0 2.5 4.4 4.7 4.0 6.4

Ga 14 15 16 16 14 16 14 16 15 12 16 14 14 15

Zn 26 24 27 12 16 35 33 8 <5 23 37 25 26 35

between 060°and070°.For convenienceof descript ion these dykes,whichare up to 2 min thickness,arehere referred to as the greenish-white trondhjem ites. The mineralogy is roughly the same as for the other trondhjemites,exceptthat conten ts of epidote (c1 inozoisite), chlorite and calcite are high er,thus explainingthe slightlygreenishhueof most of these dykes. Four modalanalyses showthat thesesamples st raddle theline separating thefields of trondhjemit e and granod iorite(Fig.4),while the normativecompositions sig- nifythatthey aretrond hj emites(Fig.5).These particulardy- kes are thesubjectof aseparate contribution in this same volume(Roberts&Sundvoll2000).

In thepresent study,greenish-w hitetrondhjemitedykes were sampled along a forestry road c.3 km northeast of Follstad.There,the dykes arefoldedand schistose,but they also cut the earliestfoliation present in the polydeformed rocks of the Gula Complex.Although themutualrelation - shipbetweenthe whiteandthegreenish-w hitetrond hjem- ites has not been observed,thereisisotopic datingevidence to suggest thatthegreenish-white trond hjemitedykesare older than the whit e, Follstad-type trondhjemites (ot her papersin thisvolume).

Reitstoa

In this cent ral part ofGauldalen,severalplutonic bodies and dykesvarying from mafic to felsicint rude the high-grade

rocks of theSinqsasForm ation(Nilsen 1978,Nilsen&Wolff 1989).On hisfirstmapcompilation,Nilsen(1978)indi cated just onecompositepluton intheReitstoaarea - an opdalite, orhypersph ene-bearing dior ite - but on the1:250,000 scale bedrock map this was separatedinto two units,trondh jem - ite-granite-monzonite and gabbro-diorite, without giving the specificrock namesforthisparticulararea.Our st udy has indicated,how ever,thatplutonic rocksrangingfrom granite andtrondhjemite to gabbro are present inthissmall area.

We refer to thismagm aticassemblage,quit e informally,as theReitst0amagmaticcomplex.Ourgeochemicaldata,how- ever,appeartoshow thatthe different plutonic rocks origi- nated in slightlydifferent geolog ical settin gs. Inourinvesti - gations,att entionwasfocused on astr etch of road outcrop 10 km west of Reit stea where the mutua l relat ionsh ipsof thedifferent mem bersofthecomplexare moreeasily seen.

Tron dhj em ites

Dykesof boththe whiteand the greenish-w hitetrondhjem- ites alsooccurin thisdistrict,aswell as larger bodiesof a white, medium-grained, Follstad-type trondhjemite. The mineralparageneses of thesetrondhjemitevarieti es are the sameasinthosedescrib edfrom weste rn Gauldalen;mostly quartz, plagioclaseand biotite,withminorconcent rations of epidote.Thesedykescutacross the regionalschistosity(Fig.

6a,b).Greenish-wh it edykes arealsofound in thisparticular

BART PANNEMANNS& DAVID RO BERTS

area,butthefieldrelation ships are toodiff usetopermitany definite conclusions to be made.

Garnet-bearing granodiorite

In the Reitstoa area,white leucogranitoid rocks containing garnets form a significantelement of the local geology(Fig.

6c,d).Thebodies are very irregular bothin formand in text u- re.Althoughmost are medium grained and weaklyto mode- rately foliated,others areof finer grain size. Pegmatitic varie- ties are also present locally, with megacrysts of microcline up to30 cmacross.Insomeexposures,the different varieties of granitoid can be seentoget her, with their slightly diffe- rent grain size,texture andcolou randavaryingcont entof garnet.In many cases,thereis amarked concentration and alignment of the wine-red garnets parallel to the contac t zone between appa rent ly differe nt granitoid intr usions,a featu re which isconsidered to relateto flow differenti ati on in the intruding magma.

The principal minerals are plagioclase, quartz, microcline, biotit e and garnet.Basedon theirmode (QAP diagram :Fig.

4),four samples fall inthe field for granodiorites and the other two in the granite field.The An-Ab-Or normative diagram,on the other hand,indi cate sthatthe roc ks sam pled are mostly granites, going over into the trondhjemite field (Fig. 5).As the primary classification and naming of plutonic rocks shouldbe basedonmineralcontent(Le Maitre 1989), we will hereaft er refer to these rocks as

granodiorites.

sensustricto.

The heterogeneity in textu re and grain sizedoes allow for such a possibility.Onthe other hand,K-metasomatism may possibly account for this hig her potassium content, even though, asyet,no directevide nce for thisprocess hasbeen detect ed.

Fieldrelationshipsshow thatthese varitex t ured,garneti- ferou s grano dio ritesare cut by dykes andlarge rsheets of the white, Follstad-type trondhj em ite. Although cross-cut- ting relationships with the greenish-white trondhjemites have not been observed,in one localit y a fine-grained, grey trondhjemite dyke which is chemically very similar to the greenish varietyis cut by a garnet-bear ing granitoid body.

These mutual relationships would suggest that the garnet- bearing granodior ites intruded at some stage in time bet- ween the early, greenish-whitetrondhjem itedykes and the somewhat later, white,Follst ad-t ype trondhjemites.

Mafic bodies

Although mafic plutonic rocks form a substantial part ofthe Reitstoa magmatic com plex (Nilsen & Wolff 1989),they were not a targetof this particular investigation.Just two samples of gabbro were collectedforchem ical analysis (Fig.4).

Geochemistry and petrogenesis

All rock samplesinvestiga te dinthis study were analysedon fusedglassbeads (maj or elements) or pressedpowd erpel-

NGU-BULL 437, 2000 - PAG E49

letsusing anautomati c Philips PW1480 XRFspectrom eter, at the Geological Survey of Norway, Trondheim. Sam ples were first heated to 1000°Cand (for the major elements) melted with 7 partsof Li₂B40,on a glass plate; for thetrace elements,9/2parts of Hoechst C-wax was added. REE,U,Th, Ta and Hf were determined by INAA at the Chemico-Physical Laboratory, University of Leuven, Belgium. Sam ples were measuredafter1 and3 weeks with a 75ern'Ge(Li)-d etector and a 16 rnm-x 7 mm LEPS.

Trondhjemites

The results of the chemi calanalyses are presented in Tables 1-3.From Table 1it can be seen that oneoftheprim ediffe- rencesbetweenthe white and thegreeni sh-wh itetrondhjem- itesis thatthe latt erhave lower Si02contents,compensated by slightly higher AI_{20 3} values. Plots of the normative comp osition sin aStreckeisendiagram andaAn-Ab -Ordia- gram according toO'Connors (Barker1979)clearly show the samples to be trondhjemites (Fig.5),and by definition they are hiqh-Al.O,trondhje mites. Harker diagrams (Fig.7) show no significant variations over the small range of Si02

contents.The reddish-whitetrondhjemites have geochemi- cal signatures that stro ngly resemble those of the white trondhjemites.Alt hough their K₂₀contents are,if anything , just marginally lower(d ue to minor alteration of biotite,in

Table2. Majorand traceelementdatafor thegarnet-b earing grano- diorit es and two gabbros,Gauldalen.Major elements inwt%,trace eleme ntin ppm.

Garnet-bear inggranodiorite Gabbro B17 B18 B19 B2 1 B2 3 B38c B41b B44 B41a SiO, 74.73 72.27 73.06 77.37 72.46 72.88 74.99 48.55 50.84 AlP, 15.24 15.00 15.05 14.94 16.39 14.89 14.85 19.57 16.22

Fe,O , 0.30 1.11 0.50 0.21 0.96 0.58 0.33 10.91 8.53

TiO, 0.02 0.10 0.02 0.01 0.13 0.06 0.03 1.72 1.16

MgO 0.08 0.27 0.08 0.01 0.57 0.25 0.06 3.65 7.18

CaO 1.08 1.39 1.24 1.34 2.90 1.48 1.75 7.49 10.34

Nap 3.82 4.65 4.47 4.96 5.50 3.61 4.65 4.34 3.30

K,O 4.90 3.52 3.16 2.63 0.57 5.09 2.82 1.49 0.75

MnO 0.01 0.11 0.02 0.03 0.04 0.05 0.01 0.18 0.16

P,O, 0.03 0.05 0.03 0.03 0.04 0.08 0.03 0.80 0.18

LOI 0.38 0.37 0.53 0.35 0.46 0.22 0.31 0.55 0.81

Tot al 100.59 98.84 98.16 101.89 100.03 99.19 99.83 99.25 99.47

Nb 21 39 46 20 <5 13 <5 20 9

Zr 22 52 37 35 73 31 48 317 111

Y 6 20 16 15 11 20 10 45 29

5r 177 166 160 159 441 155 297 983 611

Rb 100 85 78 66 17 131 72 44 19

Ba 624 438 431 200 462 649 740 603 168

U 13.0 16.6 16.4 <10 1.8 4.0 1.8 <10 <10

Th <10 5.9 6.2 <10 2.6 2.0 1.4 <10 <10

V <5 7 <5 <5 12 6 <5 87 161

Cr <5 3.3 2.4 <5 10.2 3.2 3.0 <5 197

Se <10 2.31 <10 <10 <10 <10 1.21 22 34

Hf 2.09 1.65 2.13 1.15 1.66

Ta 4.6 1 4.68 0.18 1.63 0.48

Co 0.9 0.1 2.4 0.6 0.4 15 32

Ga 12 18 17 13 14 11 12 22 16

Zn <5 24 <5 10 13 <5 90 59

NGU-BU L L437,20 00 - PAGE50

Table 3.Rare-earthelementconte nts(ppm) of diversetrondhjemit es andgarnet-b earinggranodiorit es,Gauldalen.

Follstadtrondhj emite Reddish -whitetrondhj em it e

B1 B1 1 B16 B22 B2 B10 B12 B15

La 8.3 6.5 11.5 1.9 7.7 9.3 7.1 7.2

Ce 15.7 12.2 22.0 3.5 14.5 16.2 12.8 14.2

Nd 6.1 4.7 9.1 1.7 5.9 6.2 5.4 7.2

Srn 1.21 1.04 1.67 0.58 1.19 1.21 1.06 1.46 Eu 0.44 0.36 0.44 0.20 0.39 0.36 0.38 0.55 Tb 0.103 0.103 0.143 0.101 0.102 0.103 0.096 0.131 Yb 0.25 0.25 0.29 0.31 0.25 0.25 0.22 0.19

Lu 0.04 0.038 0.044 0.044 0.04 0.037 0.033 0.027

Greenish-wh it etrondhj em it e Garnet-bearinggranod ior it e

B5 B6 02 B2 0 B18 B19 B23 B38c B4 1b

La 8.6 7.4 6.7 4.0 10.4 10.4 8.9 6.3 3.9

Ce 16.0 13.2 13.6 8.6 15.7 14.8 16.7 12.1 8.0

Nd 6.6 5.1 6.1 3.9 3.9 4.5 7.1 4.3 2.6

Srn 1.31 1.11 1.25 0.99 1.53 1.52 1.73 1.67 0.9 1

Eu 0.41 0.41 0.42 0.42 0.31 0.31 0.60 0.64 0.35 Tb 0.111 0.124 0.120 0.101 0.415 0.351 0.256 0.469 0.189 Yb 0.27 0.25 0.24 0.21 1.90 1.22 0.98 1.40 0.64 Lu 0.039 0.043 0.040 0.033 0.264 0.197 0.132 0.182 0.091

certai n samp les),they arelikelyto be broadly coeval wit h the white trondhjemites.

Rb and Sr abundances and the Rb:Srratio areknown to beparticularly sensit ive indi cator sindiscrimin at ingbetw een continental trondhjemites thatfo rmed by partialmeltingof K-poor rocks (Helz 1976, Rapp et al. 1991).and plagiogra- nites generate d bydifferent iat ion of K-poor tho leiit icmelts (Dixo n & Rutherford 1979, Pedersen & Malpas 1984).

Discrimin at io n basedon these particularelements andratios (Fig.8)clearlyshow sthat our Gauldalentro nd hj em it es have meanRb and Sr values,and Rb:Srratios,far exceed ingthose typifyingthe oceanic plagiogranite type oftro nd hj em ite.A Yb/AI,03plot (Fig.9)alsohelps to highlight the continental margin affiliat ionof these rocks.

BA RT PANN EMANNS&DAVID RO BERTS

The chondrite-normalisedREEpattern sof allthreetypes of trondhjemite are strong ly fractionated,withmoderat ely hig h enri chme nts forthe LREE and relative ly low conte nt s fortheHREE(Fig.1Oa.b.cand Table3);andinthe HREEsector thereis just a hintofupwardconcavit y.With oneexceptio n, LaNlTbNratios fall intherange 6to 14.La andCecontent s are 10-20times cho nd rites,andYb and Luare only1-2 tim es. In most ofthe samples thereis avery smallpositiveEu anomaly.

The REE data are sim ilar to those present ed by Barker &

Millard (1979) from the Follstad tro ndh j emite.Size (1979) also report eddatafo r5rare-earth elements,but hissama r- iumvaluesare4-5timesgreater thanthoserepo rted here (or by Barker & Millar d 1979);thesehig h valuesprobably relate toanalyticalerror.

It follow sfro mthe above thattheGauld alentrondh j em - it esarelikely tohave formedasmelting productsofa K-poor rock with a mineralogy thatcausedtherare-earthelement s to stro ng lyfract ionate.Amath emat icalmodel was usedto calculatethetheo ret ical change inthe REE,startingfromdif- ferent rock typ es,withdiffe rent degreesofpartialmelting and involvement of miner als.Becauseof the manyparame - tersinvolved,a preliminary processof elimi nationwasnec- essaryinorder to track down the mostlikelycandidat e.The low K,O contents areinterpreted toindi catethatthetrend- hjemites havea primar y origin, whichis alsosupported by the rather low 86SrfB'Sr rati o of0.70794±9 forthe greenish trondhje m it edykes(Robert s&Sund vo ll2000).This exclu des a gener at io n byrecyclingof oldcrustalmaterial,and the ori- ginof the magma shasto be so ug ht in the mantl e,either directl y or indi rectly. Productio n of tro nd hjemit ic melt directl yfrom the mantle isdo ubtful. Martin(1987) calcu la- ted,for Archaeantrondhj emites,that themelt ing of olivine andpyroxenecanno t exp lainthe markedREEfractionation . It wou ld also involve very low degrees of melt ing which could not acco unt for the vast volume sof tro nd hj em ites that occurin theArchaean.Themineralogyof theGauldalen

Fig.7.Samplesofthewhite and greenish-white trend- hjemitesplotted onHarker diagrams,intheSiO,range 70-74%.

16.8 32

0 04

0

•

•

164 24

• •

•

16.0 ₀₀ 16

•

^{o ·}

1.2

•

15 6 0.8

• •

•

152 00 00

70 71 72 73 74 70 71 72 73 74 70 71 72 73 74

08 6.0

•

0 MgO Na₂0 ₁₆ • K₂₀

0

0 5.6 ⁰

• •

~ ⁰

• •

•

04

•

•

_•

0 0

•

00 4.8 00

70 71 72 73 74 70 71 72 73 74 70 71 72 73 74

BARTPANNEMA NNS

s

Fig.8.Hb-Sr diag ram showing thedistributi on of thesam ples fro m Gaulda len.Symbo lsas in Fig.4.A- fieldofcont inent al trondhjem it es;

B-field of plag iog ranites/oceani ctrondhj emites.

Table4.Valuesusedin theREEcalculat io ns.

La Ce Nd Eu Srn Tb Vb Lu

Enriched MORB 3.7 11.5 10 3.3 1.3 0.87 5.1 0.56 DepletedMORB 1.7 5.34 4.88 1.88 0.77 0.57 2.93 0.46 Ocean-islandbasalt 37 80 38.5 10 3 1.05 2.16 0.30 Archaeantholeiite 9.86 24 15 3.76 1.37 0.73 2.32 0.34

fe/ogite Arnphibo fit e Garnet

Xi Pi Xi Pi Xi Pi

Clinopyroxene 0.35 0.35 0.35 0.35

Hornblende 0.35 0.55 0.35 0.35

Garnet 0.35 0.55 0.35 0.55

Plagioclase 0.3 0.1 0.3 0.1 0.3 0.1

trond hjemitesalsoprecludes a mantle origin;bot h the white and the greenish-white trondhjemites containbiotite,point- ing to a water-rich sourceregion somewhere inthecrust.

The alternat ive to a primary origin is the melting of a tholeiiticbasalt.This was theint erpret at ion favoured by Size (1979)for the Foll stad intrusion .Becauseof the widespread presence of metabasalt s in the Caledonian allochthon of Central Norway, their occurrencedeeperin the sourcereg- ion isvery likely.The originalbasaltic rockswill have been metamorphosed,grading from basalt through greenschist- faciesequivalentsto amphiboliteandeclogit e.These diver- se rocks were used as starting points forthe different REE calculations.For the modelling, the formulae used were those described by Shaw(1970); and theKd-valuesfor the differentmineralswere takenfrom Martin(1987).The use of Shaw'sformulaeassumes an equilibriumsituation, toallow diffusio nto redistribute the elements through the system;

the diffusion time,however,was calculated to 8,000years and this is far below the expected duration of the Early Ordovicianorogeny.Fracti onal crystallisat ionwillalso influ- ence thepatterns, but the Kd-valuesofquartz,plagioclase and biotite are too small to erase the influence of the meltingevent.However,a smallportionof fractionalcrysta l- lisation wasevidentl yinvolved to explainthemin or,positive, europ ium anomaly;and thisis inaccordwiththe presence

- -

18

o

17

HighAI₂0₃(continental) -

14 15 16

A1_{20 3}•wt%

12 13

,,,,

10 I- LowAI20 3(oceanic)

i , ,

,, , ,

,

,

___________ ____ ___ _ _____J

,,

,_,

,

0.1 -

of plagioclasepheno crystsinthe rocksand thin-sections.

The different REE patterns that were obtained by 20%

fractiona lcrystallisationafter thepart ialmelt in g of a tho leii- te that was metamorphosedto amphiboliteor eclogiteare shown in Fig.11(see also Table 4).Bothan enriched and a depletedtholeiitewereused in the calculatio ns.For a deple- tedtholeiite,in all cases it was difficult to obtain a good fit.

Simi larly,an ocean islandbasalt,being richinLa and Ce, is an improbablecandidate;so it is most likely that the source rock was enriched.Using an enriched tholeiite as source, different grades of metamorphism were employed in the calculations.Aneclogiteprovides a good explanationfor the HREE,but shows deficienciesat theLREEsideof the pattern becauseof thelow Kd-va luesof omphacite and garnet. A garnet-free amphibo lite,on theotherhand,does not fit at the HREE side of the pattern.The best fit,it would seem,is obtained by a combi natio nof garnetand hornblende,or a garnet-bearing amphibolite. This accords well wit h the mineralogy and geochemistry of the trond hjemites.There are low contents ofY(generally<5 ppm),an eleme ntwith a high affinity for garnet,and low abundancesof the HREE.

The presenceofbiot it e and thelack of perthit ictext ures in the plagioclasedo provide evidence of ahigh water pressure, so water-rich minerals would have to have been presentin the source rock. Biotite is a possibility,but hornblende is more likely in view of the low Kd-values of biotite.

Hydrot hermal activity,low-grade metamorphism andfrac- tional crysta llisatio nwill hardly haveinfluencedthe REEcon- tentsand ratios.Thus,as the greenish-whitetrondhjem it es have sim ilar REEpatterns tothose ofthe white,Foll stad-type trondh j emit es, a similarsourcerock could be assumed for their origin.

05

Fig. 9.The whi te, greenish-w hit e and reddish-white trondhj emites plottedonaYb vs.AlP,diagram (Art h 1979).The vertica ldashedline separates low-Al and high-AItypes,andthe horizon tallineseparat es ro cksdepletedin HREE fromthoseund epl et ed.Symbols asin Fig.4.

0.05 ' --:-':-_ _ -:-':-_ _-:-':----'-_-:'::-_ _-:-':-_ _-:-':-_ _ -:-':---1

Ea.

a. l e-

.ri>-

10000 1000

Sr(ppm) 100

0

e

1 "-

.. ~A

~

) ---

1 10 10 100 1000

E

-&

.cc::

NGU -BULL 43 7,2000 - PAG E52 BARTPAN N EMANNS

s

Fig.10. Chondrite-normalised REE 100 100

patt ern sof the (A)reddis h-white, (B)white, (Clgreenish-whitetrend-

hjemite sand (D) the garnet-bear-

A B

ing granodiorites.

10 10

YbLu

d

Eu

0.1-'---'--_ -'--_'---'_---l. ....L...J

La Ce 0.1-'---'----'---'---'----'---'--'

La Nd Sm Tb

Ce Eu

100-,--- - - ,

c o

10 10

d Sm Tb

Eu

o

La Nd Sm Tb Yb La

Ce Eu Lu Ce

Afterthe formationof themelt,otherprocesses will have influencedits composition.Inthe magma,plagioclasestar- tedto crystallise and becameconcent rated near the topof the magma chamber.This would explain why the reddish- white dykes, which predat edtheFollstadbody,are relatively rich in plagioclase phenocr ysts,an enrichment which can also be deducedfrom the more prom ine nt Eu anom aliesin theREE patt erns.Becausethe phenocrystsshowno resorp- tion str uct ures, this fractio nati on couldbeassumed to have occurredat shallow depths. Afterthe crystallisationand cool- ing of thereddish dykes,theirgeochemistry waschanged to some degree by percolationof fluidsacrossthe dykeswhich gave riseto ananasto mosing pattern of4 cm-widezones

wherethe reddishhueis replacedby a green colorat ion,the biotitebeing replacedby epidote-c1inozoisite.Atthe centre ofthese zones,microfra ctures(lined byfibrous zoisite)facili- tated thechannelling offluidsthroughthe rock.These frac- turesprobably formed when fluids wereexpelled from the cooling Follstad body,taking up calcium from plagioclase and recyclin git intothelattices of epidoteandcalcite.

A pheno menonthatisprobably relatedtothis autometa- som atic processisthe occurren ceof a sulphide dissemi na- tion,mainly pyrite,whichisfound in allrocksexcept for the Follstadintrusion.In somethin-sectio ns the pyrite is seen as aninfillin voids in the Gula schists;in othe rs,it hasaninter- granular location.Alt hough it is generally claimed that all

BART PAN NE MAN N S& DAVIDRO BERTS ^{NGU-BULL 43 7, 2000 - PAGE 53}

Fig.11. Calculatedchondrite- normalised REE patt erns for (al eclogite, (b) garnet-f ree amphibolite and (c) garnet- bearingamphibolite.The col- umnsto the left start from a depleted tholeiit e,and those tothe right from anenriched tholeiit e.The bold line repre- sents the pattern of sample BP1,and the others apartial meltof 10%, 20%and30%.

Yb

Yb

Nd Sm Eu

Ce Eu

1

1 0 10

100

Yb

Tb Nd

B

1 La Nd Sm

Ce Eu

A

10 10

O.1--'La---'---'---:-'---L..---'---YL-

b-' O. 1 Ce

Ce Lu La

100

1 00

1 00 - - . - - - ,

I00

c

Yb

Nd Sm Eu

10

1 00 - , - - - --

Y b 0.1

Lu Ce

Tb

1 0

0.1

Ce Eu

NGU-BULL 437 , 2000- PAGE 54

these sulphide mineralisations are likely to be of regional metamorphic origin, the forceful emplacement of the large Foll stad body is believed to have triggeredsome relocation of metal ions.The Ca-rich fluidsthat were expelledfrom the Follst ad body upon cooling probably played a rolehere, and had the potential to carry metal ionsin solution.The fluid pressures caused fracturing in the host rocks and along grainbounda rieswherethe sulp hidescouldbe precipitated.

Garnet-bearinggranodiorite

Chemicaldata for these garnetiferous granitoids,presented in Tables 2 and 3,show that the rocks are calc-alkalic and peraluminousand, followingthetermino logyof Chappell &

White (1974), can be classified as S-type granites.In the An-Ab-Or diag ram(Fig. S),the granitoidsplot far outside the TTG fields,and thustheirgenesis requiresa differentexp la- nationfrom that of the trondhjemite s.Uand Th contentsare higher in the granodiorites, though with highly variable UfTh ratios.NbandYcontents are also higherthan in the trond hje mites;in thecase ofY,this indicatesthat garnetwas lessinvol vedin the formationof the melt.

The REE patterns of theserocks are somewhatcomplex (Fig. 10d) wit h moderate LREE enrichment,fairl yflat HREE, and negativeEu and Nd anomaliesinsome,but not all,of the samples.LaNfTbNrat iosfall inthe range 2 to 5.

The source rock for thesegranitoidscould have been an AI-rich sediment that had undergone a comparatively low - to medium-grade metamorphism, as is indicated by the hydrous nature of the melt.Thegarnets were investigated by semi-quanti tative SEManalysis, which showed them to be Mn-rich(>15%spessart ine).Thiscontrasts wit h themore Ca-rich,almandine garnets from the schists of the Gula Complexcountry rocks.A hig hspessartinecontent istypical forgarnets of magmaticorig in (d u Bray 1988); and also the fact that mostmag mat ic garnets arefound in AI-richaplitic to pegmatit ic rocks points to a magmatic orig in. Sim ilar speassart ine -garnet granitoids have been reported from Nevada,USA(Kist leret al.1981),and Saudi Arab ia (du Bray 1988).These calc-alkalic rocks were interprete d to have formedfrom themelting ofsediment s,yieldinga melt rich in alum in ium which facilitatedthe crystall isation of garnet.The REEpatterns reported by Kistler et al.(1981)also show a negativeNd anomaly, which theseauthorsascribedeit her to the role played by apatite,zirconor titan ite in theresidu e, or to melting of REE-poorArchaeansediments.

Mafic bodies

As noted earlier,the mafic bodies were notexami nedin any detail in thisinvest igat ion. However,the analyticaldatafrom just two samples were added to the diagrams solely for comparativepurpo ses.

BA RT PANN EMANN S

s

Discussion

Field relationshi psofthree typesorcolour varietiesof trond- hjem it edescribedherefrom diffe rentpart s of theGauldalen district show that, in relat ive terms, the greenish-white trond hjem it edykes arethe oldest,and the white,Follstad- typ e trondhj emit es the youngest intrusions.The reddish- white dykesarebeliev ed to have been emplaced at about the same tim e as,or just slight ly earlier than the white, Follstad-t yp etro nd hj emites.The leucocratic,garnet-bearing granod iorit es occup y an inter mediate temporal position, since they transectthe early trond hj em it edykes in one area and are themselves cutbydykes and larger sheets of white trondhjemite.

Alt hou gh only two of these four varieties of felsicpluto- nic or hypaby ssal rock have so far been dated by isotopic methods,theresults do appear to support the overall field relationshi ps- a U-Pb age of 432±3 Ma for the Follstad trondhjemit e(Grenne &Dunning ,this volume),and an inter- preted,minimum, Rb-Sr isochron age of 465± 11 Ma for greenish-wh it etrond hjem it edykes from Snoan (Roberts &

Sund voll,this volume).Thus, continen tal margintype,trond- hjemiteint rusiveactivi tyin the Gula Complexspans a time int ervalofmorethan30million years.

A feature common to all these trondhjemites and the granodiorite isthat they cut an early foliat io n inthe Gula metasedimenta rycomplex (Fig.6);yet,they are themselves variabl y foliate andin places deformed by foldsand a later crenul at ion cleavage.As noted earlier,theolderSnoan-type dykes are known tocut both Gula and Storen rocks,but

not

the unconform abl y overlying, Late Arenig and younger, Hovin Gro up succession.Their true age may therefore be closetoMid Arenig(Roberts &Sundvoll 2000),i.e.,int ruding shortl y afte r the Early Ordovician defor mation,metamor- phism and ophioliteobduc tion.

Theweak foliation , fold structuresand cleavages obser- ved at different placesin the trondhjemitesand granodior ite arelikely to haveformed during the later,Sil uro-Devonian, Scand ian orogeni c event.The U-Pbdate of 432±3 Mafor the Follstadtrond hj em it ethus provides a maximumage for the upper greenschist-faciesmetamorphism of this body,and for the Scandian regionalmetamorphismin the rocksof the GulaNappe.Therehavebeentw o attem ptsto date the regi- onal metamorphic fabrics in theGula,initially employingthe K-Ar met hod (Wilsonet al. 1973)and later using conventi- onaI⁴°Ar-^{l 9Ar}techn iqu es(Dallmeyer 1990). After firstconver- tingthe datesreporte din Wilson etal.(1973)usingthe'new' decay consta nts doc umented by Dalrym ple(1979),in both studiesall muscovit esand bioti te s,and most hornblendes, recorddates in therang ec.432 to 416 Ma,whichwereinter- pret ed to represent post-m et amor ph ic cooling ages.K-Ar analytical data on phyllitic schists gave ages of around 425-426 Ma, whi le two sampl es of biot it e from the Follstad trond hje m it e yieldedrecalcula ted,K-Ar cooling ages of 411 and 415 Ma.Thus,Scand ian peak metamorphi smin the Gula Nap perocksoccurred in LateL1andovery to Wenlock time,