Geochemistry and Rb-Sr isochron age of trondhjemite dykes from the Gula Complex, near Snean,

DAVIDROBERTS&BJ0RN SUNDVO LL NG U- BULL437, 2000 - PAGE 67

Geochemistry and Rb-Sr isochron age of trondhjemite dykes from the Gula Complex, near Snean,

Central Norway

DAVID ROBERTS&BJI2lRNSUNDVOLL

Robert s,D.& Su ndvo ll, B.2000:Geochem ist ryand Rb-Srisoch ron age oftron dhj em it e dyke sfromthe Gu laComplex, near Snoan,Cent ral Nor way.Norgesgeologiskeundersekelse Bulletin437,67-74.

Trond hjem itedykestrendingc.ENE-WSW and transect ingearly meta morph icfabricsin rocksof theGula Complex, sout h-sout hw estof Ste ren,are themselvesweaklymeta m orphosed and foli ate.Geochemica ldatashow that the dykesare characteristicof hiqh-Al.O, trondhj emites,and typical of thosereported from continental margin enviro nments worldwide.

Rb-Srisotop ic analysisof aseries ofsamplesfro m one particu lar dykedidnotyield anyisochron.However, sam ples lyin g fart hest fromthe regressionline are excl usivelyfromthedykemargins;andifthesesamp les are excluded, then abest-fitisoch ron defines anage of 465±11Ma (MSWD2.2,i.r,0.70794± 9).This istaken asthemin imu m ageof intrusion,Le.basal L1anvi rn,and isint erp retedasreflect ing apost-emplacementtherm al resett ing age.Dykemargin samp les arelikelyto have beenaffec te d byan influxof fluids conta ini ng abundant ions of radi ogenic Sr, and conce ntra te d alon gthe dykemargi ns bythedecay of^87Rb durin gtherm al reset t ing.

Theisot opic age ofthe tron dhj em it e also provi d esaminimum agefor theEarly Ordoviciandeforma tionand meta- morp hismofthe juxta posed Sterenop hioliteandGulaComplex successio n.The gentle folding of the dykesand theirweakfoli atefabri c arethus likely to beof Scandian(LateSil urian-EarlyDevonian) age.

DavidRoberts,Norgesgeologiskeundetsekeise.N-7491Trondheim,Norway.

BjernSundvoll,Mineralogisk-GeologiskMuseum,Sars'gate 1,0532Oslo,Norway.

Introduction

Several, large, metamorphosedtrondhjemite bodies,reach- ing up to 20x 5 km in outcrop size, occur within the Gula Com p lex (fo rmer ly Gula Group) of theTro ndh eim Nappe Com pl ex (TNC),in theCaled onid es ofCent ral Nor w ay.These aredep ict ed on the 1:250,000 map-sheets 'Tro ndh eim ' and 'Reros'(Wolff 1976, Nilsen & Wolff1988),and some are also indicated on the 1:1 million bedrock map of Norway (Sig m o nd et al. 1984). lnsome areas, there are also swarms of finer-grained, variably foliate,trondhjemite dykes,many of wh ich cut an earlier fo li at io n. These dykes are therefore potenti ally valuab le- if their agescanbedet ermi ned,then they will provide usefu l tem po ral constrai nts on infer red earlyCaledo nian deformation.

In this brief contributionwe presentthe results of a study that was purposely concentrat edonjust 4 metatrondhjem- itedykes in road-cuts,whichwere newlyexposed in theear- ly 1980's,along theminor road betweenSnean and Budal, south of Ste ren (Fig.1).This local st udy began aspart of a mo re regiona l investiga tio n of greensto nesand associate d rocks intheTro nd heim Region,butone specifically hatched by an inspiringexcu rsionin the area, led by Odd Nilsen.More or lessat the same time,Nilsen(1983) describeda sedimen- tary melange,or olistostrome,from thissame general area.

The investigationencompassesan attempt at Rb-Srda- ting of oneofthe more betterexposed examplesof these tro nd hjem ite dykes,and a paralle lgeoche m ical investig at i-

on of thisand three othercomparabledykes.Itwascarried out over a period of severalyearsthrough a ste p-by-ste p analysis of the dat a available to us at different times.

Althou gh the dykes have been affected byScandian meta- morphi sm,in thisdescript io nwe will,in general,omit the 'meta' prefix.

Local geology

A general int ro d ucti o n to the geology of this part ofthe Tron dh eim Nappe Complexis presentedin tw ocompanion papers in th is same volu me (Du n ning & Grenne 2000, Pan nema nset al. 2000). Nilsen 's (1983) map ofthe district (Fig.1)shows the area arou nd Snoan to consist main lyof a grey andblackphyll it e memberofthe Undal Formation,part of the Gula Complex;and a unit ofsed im ent ary melang e.

Farther to the west, the Gula Complex(and Nappe)isst ruc- turally overlain by metabasalts,mafic dykesand asso ciate d rocks of thebasal,ophioli t icpart of the Steren Nappe(Gale

& Rober ts 1974, Nil sen 1983), a rock association that has trad it ion all y been term edtheSte ren Gro up.

The Undal Formation consists larg ely of fine-grain ed, graphit ic,grey to black,chlorite-sericitephylliteswith a vari- ablecontentof calcite.In places,thereare thininte rcalatio ns of ribbon chertand chlorite-bearingamp hibo lit ised green- stone.Ubiquitous sulphidemineralisations are seenasthin sch lierenparallellingthepervasiveschistos ity(Nilsen1978).

Part s of the successio n, incl ud ing exposed sections near

NGU -BULL 43 7,200 0 - PAGE 68 DA V ID ROB ER TS

s

BJ 0 R N SUNDVOLL

Snoan,are also reminiscent of a tectonic melang e wit h a high content of deformed clast material in a schistosema- trix.Thisparticu larmelange is comparable to that described by Home(1979) from the Selbusjoenareac.50km northeast of Snean,in what appearsto be a st ri ke exte nsion of the Und alForm at ion.

Thesediment ary melang e,as describ edby Nilsen(1983) andin thatpapernamed the Soknedalmelang e, is essential- lyanolisto strome formed as a debris flow with submarine slidi nginvolved.The clast material,includi ng one large block of greenstonecovering 400m',was considered byNilsen to have beenderived mainly from the Gula Complex,but also, in part,from the Steren ophiolite.He interpreted this me- lange asa comparatively young deposit,of possible Late Silurian age, and not a part of the surroundi ng Undal Format ion with its older,tectonic melange(Fig. 1).

Thetro ndhjem it edyk es ofthis stu dycut through both the GulaComplexandthe Ste renGrou procks,butnot the unconformably overlying, Ordovician Hovin Group which contains fossils as old as Late Arenig (St e rrner1932).In the UndalFormationthey transect the earlier tectonic melange.

Acceptingthatthere isjust one broad age for thetrondhj e- mitedyke swarm, then it is thus likely to bepre-LateArenig.

Although Nilsen (1983) reported "dykesof tro ndhjemitic typ e" cutti ng the Soknedal olistostrorne,they have sinc e beenreinterp reted by him as elongate megaclastsof trend- hjem it e(O.Nilsen, pers.comm. 2000).

The trondhjemite dykes

Field relationships

Of the fourtrondhj em ite dykesinvest igated,the one expo- sedinthe road-cut atgrid ref.634858, 1:50,000map-sheet Buda I 1620-4,is the best exposed ,andisfrom 80cm to1min thickness(Fig. 2).This isthe one thatwas chosen forthe Rb- Sr datingst ud y;and is hereaftercalled the 'main dyke'.The oth er dykes varyinthickness from 50cm to 2 m and strikeat c.060°,dipp ing eitherto thesout heast or to the nort hwest.

The maindykest rikes at 065°and alsovariesin dip,from 60°SSE to 55°NNWthroug h verti cal, in the road outcrop, defining agent le,inclined fold (Fig. 2).The dyke, which is fine-to mediu m-gr ained and massive,app earstotransect a prominentspaced cleavage in thecountry rocks that dips at 18°toward sESE.Thiscleavage maybe broad lyaxialplanarto the shallow ly ESE-plunging fold, and is it self defor mi ngan earlierschistosity.Thecountryrockhereconsists ofa matrix- supported,small-c1 astmelange where thematrix is mainlya grey to darkgrey, silty phyll it e.A few metres downhill,along the road,banded chert and phyllite are present. Thisunit carries small-scale folds wit h an axial planar crenulation cleavage which is paralleltothat in the melange.Alensoi d stru ct ureof probab le tect oni corigin presentin the melange dipsatupto 50°to the eastandis older than both thedyke and thegent ly dippingcleavage.

The trondhjemites are com paratively fine -gra ined and show a weak foliatestructure thatis broadly parallelto the

} Storen Nappe

~

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3km

Sings~s

Fm. Und al

Fm. Gula

appe

2,

o

Trondhjemite Soknedal melange (olistostrom e)

Metabasalt

Tuffite and ribbon chert withbasal mylonite

Calc-s ilicateschists ,

~garnet

Amphibolite

6 6

I I I

I

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I

( ( (\1

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Fig.1.Geologi cal map ofthe Steren-Snean-Scknedalarea,Ser-Trondelaq.reprod uced and modified from Nilsen(1983).Nilsenconsideredthe Soknedal melange(olistost rome) to be thevery youngest metasedimentar yunitin the area.This relationsh ip is retainedhere,even though the presentauthors believethatpartsor all of thismelange forman integral partof the UndalForm ati on.Theroad-sect ion with dykes,formingthe basisof thisstudy, is markedbya cross (just NNW of Snoan).

DAVID ROBERTS &BJ0RN SUNDVO LL

Fig.2.The'main'trond hj emite dykecutting a small-c1ast tecton ic me-

lange of theUndal Formatio n,looking c.north east.Thedykeisvery gent lyfolded,wit h theprom inent ,shallowlydipping,late crenulat ion cleavageinthemelangelyingrou ghl y axial planar tothefold.

spaced cleavage seenin the melange.The dykes thuspre- date the spaced cleavage noted above.In thin-section,the text ure is allotriomorphic-granular, in places granoblastic.

The principal minerals are quartz andplagioclase, with lesser andvariable amountsofsericiticmuscovite, chlorite,e pi do te and calcit e, andaccessoryapatite,titanite,zirconand mag- netite.This is aparagenesis which indicates that the dykes have been affected by a regional metamo rphis mof green- schist facies.Trondhjemite dykes of similar character and trend to those described here also occur wide ly in the Gauldalen distri ct(Pannema ns&Robert s 2000).

Geochemistry

Major and trace elements were analysed on rock powders using an automatic Philips 1450/20 XRF,at the Section for Analytical Chemistry,NGU,Trondheim.Calibration curves were made with international standards.For the determina- tion of majorelements the rock samples were melted with lithium tetra borate 1:7.Trace elements were determined on pressed rock powders. Ferrous iron,H20',H20'andCO2were determined by wetchemical met hod s.Rare eart heleme nts (REEl. Th, Ta, Hf and Se were analysed by INAA and Nb by mass spectrometry.

Major elements

Major element concentrations and ratios of the Snean trondhjemite dykes (Tables 1&2)fall largely within tho se

NGU-BULL 437,2000 - PAGE 69

specifiedunder Barker's(1979) definition of trondhjemites, except that the Si0₂content of the Snean dykes is slightly low,and lower than the mean value from the Follstad quarry trondhjemite (Size 1979,Pannemans & Roberts,this volu- me).This is compensated,however,by a highercontent of A1₂0₃, which is not abnormal for trondhemites worldwide and is, in fact, a measure of palaeotectonic setting (see below).The Snean trondhjemites are, by definition,high- AI_{20 3}trondhjemites.By comparison wit h the Follstad trend- hjemite, the Snean dykes showa lower Na20/K20rat io(2.3, as against 3.8),and the FeO'o/MgO rati ois also a good deal lower(l Avs 2.8).On an AFMplot theyare,not unexpectedly, calc-alkaline,at the low -T end of the gabbro-trondhjemite trend(Fig.3), butthey are slight lymoreFe- andMg-ric h than theFollstadtrondhjemite.

Trace elements

Rb andSr abundances and the Rb:Sr ratio are known to be particularly sensitive indicators in discriminating between trondhjemites and related plagiogranites generated in wi- dely different tectonic settings. In the case of the Snoan trondhjemite dykes,mean values of388ppm for Srand 70 ppm for Rb far exceed thosetypifying oceanictrondhjem- ites(c.100 and <5 pprn, respective ly)(Fig A). The Rb:Sr rat io of 0.18 for Snoan is also substant ially higher than that (0.0 15) for oceanic trondhj emit es(Coleman & Donato 1979).

Onthe Rb-Srplot(FigA)the Snean dykes mostly fall within thefieldfor conti nenta l trondhj em it es.

The high AI_{20 3}contentof trondhjemitesis also knownto providean indicationof their continentalaffinity,and nota- bly when seen in relatio nto depletion in heavy rare-earth elements.Contentsof Ybarelow, and on a Yb vs.AI_{20 3}plot (not shown here)(Art h 1979)the Snoan dykes clusterwell wit hin the contin enta l field (see Fig. 11 in Pannemans &

Roberts2000).

Chondrite-norm alised rare-earth element(REE) plots,as well asREEabunda ncesandratios,provide reliable pointers wit h regard to trondhjemitetecton ic set ti ng and genesis.

Tw o sampleswerecho sen from Snea n for REEanalysis,from differen tdykes.TheirREEcontents(Table 3)and patterns are extremely similar(Fig.5), with fairly low abundances,a frac- tionated pattern for the LREE with La c.20xchondritesand the HREEdown to 1xchondrites.TheLaJLu.ratio isjust un- der 20,and thereis just a hint of a smallpositiveEu anomaly.

These data are sim ilarto those presented by Barker&Millard (1979) from the St e ren district,but differslightly from the REE dat ainSize(1979) where the La toSm patterns are flat.

TheREE patterns andcontentsfromthe Snean dykes are almost identicalto thosereport ed fromconti nenta lmargin environme nts (Art h 1979).Trondhj emitesfrom continental interiorsshowamorepron ou nced LREEenrichment,wit h La about 70 x chondri t es.Oceanic or island-arctrondh em it es, onthe ot her hand,havefairlyflatREE,or eve n LREE-d ep let ed patte rn s, negative Euanomaliesand HREEconte nts <10 x chondrites(Fig.5).

NGU-BULL 437,2000 - PA GE70 DAVIDROBERTS &BJ0 RNSUND VO LL

Ta ble1.Majoran d traceelementcompos itionofthe Snoantrondhjemite dykes.Majorelem e ntsinwt.% ,trac e elements in ppm.

Sample 1 2 3 4 S 6 7 8 9 10

SiO, 66.04 66.50 64.16 64.08 68.30 67.95 66.30 66.77 67.41 66.40

TiO, 0.23 0.24 0.24 0.24 0.17 0.18 0.24 0.24 0.22 0.20

AlP, 16.98 17.12 17.11 16.92 16.87 17.28 17.05 17.11 16.64 17.24

Fe,O, 0.12 0.33 0.60 0.61 0.48 0.60 0.60 0.33 0.29 0.20

FeO 1.26 1.25 1.12 1.19 0.75 0.71 1.16 1.17 1.14 1.17

MnO 0.05 0.05 0.03 0.03 0.02 0.03 0.02 0.03 0.03 0.03

MgO 1.01 0.98 1.37 1.40 0.76 0.77 1.08 1.09 1.10 1.19

CaO 2.23 2.17 3.47 3.74 3.60 3.25 3.34 3.22 3.17 3.51

Na,O 4.50 5.10 5.00 4.80 5.20 5.00 5.33 5.30 5.60 4.80

K,O 2.86 2.51 2.08 2.07 0.97 1.49 1.54 1.50 1.44 1.68

P,O, om ^{0.07 0.07 0.06} 0.06 0.06 0.06 0.07 0.08 0.06

H,O' 1.95 2.02 1.88 1.91 1.15 1.56 1.89 1.67 1.65 1.85

H,O- 0.06 0.03 0.04 0.03 0.03 0.04 0.05

co, 1.61 1.50 1.44 1.60 0.40 0.52 1.52 1.47 1.31 1.37

Tot a l 98.97 99.84 98.60 98.69 98.73 99.43 100.16 100,01 100.13 99.70

Zr 80 80 73 71 74 80 75 72 71 65

y <5 <5 5 <5 <5 <5 <5 <5 <5 <5

Sr 210 241 284 299 548 498 521 522 533 568

Rb 92 80 80 80 34 54 49 50 46 61

Zn 28 28 23 24 26 26 33 34 36 32

Cu <5 <5 6 <5 <5 <5 7 <5 <5 12

Ni 7 6 11 11 <5 <5 7 8 6 8

Cr 15 15 26 27 6 7 18 18 13 11

Ba 454 394 380 367 278 302 345 297 292 282

Nb <5 <5 <5 <5 <5 6 <5 <5 <5 5

V 26 27 40 37 13 14 30 25 26 18

Sample 11 12 13 14 15 16 17 18 19 20

SiO, 66.53 65.14 65.34 66.35 65.97 65.80 66.24 65.30 65.89 64.91

TiO, 0.21 0.24 0.24 0.25 0.24 0.23 0.23 0.24 0.24 0.25

AI,O, 17.51 17.18 17.25 17.10 16.56 16.24 16.30 16.57 16.59 18.34

Fe,O, 0.29 0.42 0.48 0.49 0.54 0.56 0.55 0.56 0.55 0.50

Fe O 1.13 1.07 1.15 1.05 1.00 1.09 0.98 0.96 1.01 1.04

MnO 0.02 0.05 0.04 0.03 0.03 0.07 0.05 0.04 0.04 0.04

MgO 1.28 0.98 0.96 1.00 0.96 0.93 0.96 1.04 0.98 0.96

CaO 3.19 2.84 2.50 2.48 3.19 2.75 3.11 3.32 2.96 2.15

Nap 5.10 5.60 4.50 4.60 4.30 3.80 4.30 4.40 4.50 4.60

K,O 1.76 2.13 2.49 2.26 2.41 3.01 2.46 2.53 2.43 2.62

P,O, om 0.05 0.03 0.05 0.04 0.05 0.05 0.05 0.05 0.04

HP' 1.76 2.15 1.80 1.67 1.93 2.04 2.03 2.06 2.09 1.82

H,O- 0.05 0.06 0.05 0.04 0.05 0.06 0.05 0.03 0.05

eo, 1.18 1.78 1.42 1.22 1.74 1.96 2.04 2.12 2.00 1.41

Tota l 100.08 99.69 98.25 98.59 98.91 98.58 99.36 99.24 99.36 98.69

Zr 67 89 89 92 83 82 89 87 91 99

y <5 <5 <5 <5 <5 <5 <5 <5 <5 <5

5r 543 358 335 357 324 223 310 355 366 358

Rb 63 69 80 69 73 94 79 81 81 88

Zn 30 28 28 27 30 25 28 33 32 28

Cu <5 <5 <5 <5 <5 <5 <5 <5 6 <5

Ni 10 6 <5 <5 <5 <5 <5 7 7 6

Cr 17 17 17 16 17 15 15 20 16 18

Ba 380 306 373 357 347 439 337 321 353 400

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

V 24 24 25 27 29 27 24 26 26 31

Rb-Sr geochronology

exclu sively from themarginalparts ofthedyke.Ifthesedyke Thirteensamplesfrom the mainSnoan dyke were subj ect ed marg insamples areexcluded,then the remaind er(nos.1,2, to Rb-Srisotopi canalysis.Theresult sare report ed in Tab le4. 12, 17,96a,96cand96e;Tab le4)yielda reason ablyaccepta- Ascan beseenfrom Fig.6a,the completedatado not yield ble fit(MSWD =2.2),with an age of 465±11Ma andanini- any isochron.Itwasrealised,howeve r,thatthefivesamples tiaI^87Sr/^86Srrat io of 0.70794±9(Fig.6b).

lying farthest from the regression line in thisfigure were Areason forexcludingthe sam plestakenfromthe very

DAVID ROBERTS&BJ0RN SUNDVOLL

Snoan FolIstad Table2.Mean major element

SiO, 66.07 71047 and selected trace element

AI,Q, 16.99 16.20 contentsfortheSnea ntrond- TiO, .23 .23 hjemitedykesand theFollstad FeO 1.07 .78 trondhjemitepluton.TheFolI- Fe,Q, AS .58 stad major element data are MgO 1.04 048 takenfromSize(1979), and the CaO 3.01 2.72 trace element means from Na,O 4.82 5.33 Pannemans&Roberts(2000).

K,Q 2.11 1.39

MnO .03 .02

P,O, .06 .08

In) 20 15

Zr 80 81

Nb <S <5

y <S <5

Ba 3S0 407

Sr 38B 606

Rb 70 29

Cr 16 3

V 26 12

Rb/Sr .18 .05

In) 20 4

marginal parts of the dyke is not immediatelyevident if one considersjust the majorand trace element data(Tables 1 &

2). These do not show any significa nt variatio n between sam ples taken from thedy ke marg insand those from the centralparts of the int rusion,i.e.a bulkassimilationof wall- rock does not appea rto have occurred.How ever,transpo rt offluid s into,and admixtureof fluids fro mthe wall-rock are likely to have taken placealong the margins oftheintrusion. Such fluids may have contained abundant ions of radiogenic Sr,which are commonlyexpelledfrom the mineralswherei n they wereformed and concentrated by thedecay of87Rb.

Th is mechanism may provide an explanation for samples with excess radiogenic Sr (e.g., sample no.16).Experience fromRb-Srdating ofdykes and sillsin ot herareas ofNorway stro ng ly suggests that suchamech anism has been opera- tive (Su ndvo llet al.1992,Su ndv oll& Larsen 1993),not abl yin contac t zones thatdisplay otherevidence of fluid shaving been present during bothintrusionand subsequentregio- nal metamorphism.

Samplesshowing a deficiency in radiogenicSr generally reflect:(a) a variable degree of some post-intrusive altera- tio n process, or (b) reheating,which may have disturbed the isotopic eq uili b rat io n. Both mechanisms are governed by the general fact that radiogenic Sr is unstablein thelat t ices of minerals containi ng Rb and therebyeasily removab le.If mech ani sm (a) can be dismisse d,the n the dyke mayhave suffe red a post -em p laceme nt therm al resettin g eve nt, whichmayhave resu lted in varyingdeg rees of reequi lib ra- tio n of the Rb-Sr system in differ ent parts ofthe intrusion which had disparate physical properties, Le.the marginal versus the central parts.

Thus,the Rb-Sriso ch ro n age derived from the central

NGU-BULL43 7,20 00 -PAGE71

part of the mainSnoandykemay be taken,at best,asa mini- mum age for the intrusion.As noted earlier,thefossilrecord in the adjacent, overlying, Lower Hovin metasedimentary succession which is not cut by the dyke swarm , ext end s downinto theUp perAren ig.The geologica levidence thus allows fo r the possibi lity thatthedyke intrusionitselfmay be atleast 10 millionyears older thanindicated by theRb-Sr iso- chron age,i.e.moreor less equivalentto the upper erro r limit.

Accord ingly,the Rb-Srisochronagecan most probablybein- terpretedasreflecting a post-intrusivethermalreset ti ng age.

Discussion

Geological and structural relat io nships in th is part of the southwestern Trondheim Region invo lved an obduction of lap etu socean-floorvolcanites(d ismembered Storen ophio- lite)upon partof what has been termed theGula Complex, coevalwith a commonearly Caledo nia n deformationand metamorphism.The complexeswerethen uplifted and ero- ded,priorto depositionofthe Lower Hovin Group above a su rface of major unconformity.As theoldest fossils in the Lower Hovin are of Late Arenigage,then this placesa min i- mum age on theearly Caledon ian deformation.Since the Gula contain sTremadoc graptolites in one areafart he rto the east,then an Early to MidAren igag e has beenassumed forthis tectonotherma levent(St u rt & Roberts199 1).

The trondhjem ite dyke swarm in the Snean-Storen- Gauld alen district cuts both the Gu la seque nce and the Stere n metabasal ts,and theirearly deforma tion/metamor- phic fab rics, butnotthe low -g rad eHovin metasedimentary rocks.Our attemptat Rb-Sr dating of the'm ain'tro nd hj em it e dyke at Snoan has producedabest-fit isochronof c.465±11 Ma, interpreted asa min imum age fo rthe dykeswarm and

F

, ••~^{I · •}

....

M

Fig.3.AFM diagramshowing themeanvalue(circle)for t heSnean trend- hjemitedyke samples,thefieldfor theFollstadtrondhjemite(shaded), and thetrend lineforthe'gabbro-trondhjemite'suite(Size1979).The enlargementof thealkaliescorner of the diagramshowsaplot of all20 samples of theSneandykes.

NGU- B U LL 437,2000 - PAGE 72 DAVID ROBE RTS & BJ0RNSUN D VO LL

following inferred EarlyArenig conti nent/arc collision with coevalophiol ite obduction.Thereis no traceinthe REE data, how ever,ofaninheritedarc component.

Whilethere are evidentsimilaritiesin chemistrybetween the Snoan trond hj emit e dykes and the medium -grained trondhjemit efrom thetyp e locality at Follstad, some small differences are apparent (cf.Table 2; also; Pannemans &

Rob erts 2000).Three or possibly four phases or pulsesof trondh j emit e emplacement have been reported from this southwes tern Trondheim Region,seen in relation to fold- str uct uralepisodes. The U-Pbzircon and titaniteage of the Follstad trondhj emite, 432±3 Ma, is rep orted in another paperin this volume (Dunning &Grenne 2000).Rb-Srand U-Pb dating oftrondhjemitesof Early to Mid Ordovicianage have previously been reportedfrom Trondelag,though only in anabstract (Klingspor & Gee 1981).A small,hiqh -Al.O, trondhjemite pluto n from near Jonsvatne t, southeast of Trondheim,hasprovided theyoungestU-Pb zirconage so far - LateSilurian(Roberts,Solli&Walker,unpublished data).

Thesediverse agesthussupport the suggestion(Size 1979) that trond hjemite magma,generated by an early partial meltingof basalt,ultim atelyintrudedthrough the continen- tal margin crust at various stages throughout the Ordo- vician,andevenwell into Siluriantime.

Plagiog ranit e or trondhjemitedykes also occurin associ- ation with the Lokken (493 ± 10 and 487 ± 5 Ma) and Vassfje llet(480 ± 4 Ma)Ophiolit es(Dunning1987 and pers.

comm.1990). the U-Pb zircon dates pointing to a latest Cambrian to very Early Arenig age (Landing et al. 1997, David ek et al. 1998).Theseparticulartrondhjemitesdevelo- ped in an oceanic/arcsetting and aretherefore chem ically differentandunrelated to the continental-margintrondhjem - ite dykes reported here.At Lokken,however,there are also youngertrondhj em it icdykesthat cutthe basal greenstone cong lomerate above theobdu ctedophiolite,and one such dykehas yielded a LateArenig zircon age(T. Grenne &G.

Dunning,unpu blished dat a;in prep.).

1000 1"--_---'-_-'--'--'-J...J...L....I>"--...£.---'-_-'--'---'-.J...L..l....L.l...-_-'

10 100

Sr.ppm

Fig.4.Rb-Sr log/logvariation diagram with the 20 Snoan dyke analyses plot t ed.The fields of continental trond hj emites and oceanic plagio- granite/tron dhj emit esareindicated (afte r Coleman &Peterm an1975).

100

CONTINENTAL TRONDHJE MITES

one probably reflecting a measure of post-emplacement thermal resetting.This age is the age of the base of the L1anvirn on the latest time scale for the Ordovician (McKerrow&vanStaal 2000) - buttaking accountof the error bar,this would bringus closeto Mid Arenig .Thus,theRb-Sr dat ing does appear to verify thepalaeontologicalevide nce from the Lower Hovin Group, Le. sign ify a likely pre-Late Arenigage for thispart icu larswarmof trondhjem itedykes.

The geochemicaldata on the Snoan dykes add afurt her deta il to our understand ing ofthe geologicaldevelopment ofthis region.The datashow convincingly that the trondhjem- it e dykes were emplaced in acontinental margin environ- ment,through a crust which had presumabl y thickened E0-

0-

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20

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0

10

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5

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SN0 AN

DYK ES

^{• sample 2}x sample8

j

Fig. 5. Chondri te-normali- sed REEplots for represen- tative samples(nos.2 and 8)from two of the trend- hjemitedykes; road-secti on near Snoan.For comparati- ve purposes,theprofilesfor a typical ophiolite-rela ted trond hj em it e/p lagiog rani- te(circles) and anisland-arc trondhjem ite(triangles) are also shown (aft er Arth 1979).

La Ce Nd Sm Eu Gd Tb

Dy Er Yb Lu

DAVID ROBERTS

s

BJ0RN SUNDVOLL NGU-BULL 437,20 0 0 - PAGE 73

Table 3.Rar e -e arth elementan d Th, Ta,Hf,Uan dSc analyse sforthetrondhjemitedyke sam plesnos.2an d8(se eFig.5).

Sample La Ce Nd Srn Eu Tb Yb Lu Hr Ta Th U Se

2 6.7 13.6 6.1 1.25 .42 .12 .24 .04 2.07 .087 1.41 .68 3.11

8 6.2 12.3 5.3 1.18 .42 .11 .22 <.04 1.95 .077 1.21 .71 3.05

Ta bl e4.Snoa ntrondhje mitedykes,Rb-Srisot opic data.Rb andSr inppm.

Sampl e Rb Sr ^87Rb/86Sr 87Sr/86Sr

1 81.18 195.10 1.2048 .71596

2 74.30 233.26 .9222 .71411

12 63.81 323.09 .5717 .71189

13 73.43 314.67 .67S4 .71124

14 65.63 337.79 .5623 .71113

lS 71.90 328.86 .6328 .71125

16 87.11 211.44 1.1931 .71780

17 72.93 289.74 .7286 .71271

96a 31.32 388.17 .2335 .70953

96b 76.36 319.70 .6941 .71179

96e 61.81 389.35 .4594 .71088

96d 50.69 409.32 .3584 .71052

96e 84.30 246.85 .9870 .71452

SE

±13

±8

±10

±10

±10

±10

±10

±6

±3

±3

±3

±3

±3

Conclusions

Trondhjemite dykes,locallyofswarm proportionsand trend- ing c.ENE-WSW,transectearly metamorphicfabrics in rocks of the GulaComplex,nearSnoan,sout hw est of Steren.The Snean dykes are only weakly metamorphosed,in green- schist facies,and poorlyfoliate.Geochemicaldata show the dykesto be fairlyhomogeneousand quite characteristicof hiqh -Al.O, trondhjemites.Trace element abundances and ratios,including REEdata and chondrite-norma lisedpatterns (with la c. 20xchondritesand a laNfluNratio of justunder 20),indicate that the dykes are typical of those reported fromcontinentalmarginenvironments.

Rb-Sr isotopic analysisof thirteen samples from one particulardyke showed that the completedata set doesnot yield any definitive isochron.However,the samples lying fart hestfrom the regressionline are those taken from the

0719

•

0.717

0715

~ ^0.713

eo

~

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eo

...---_

....

0.709 0.707

lA 06 08 1.2

87Rb/86Sr

OA 02

0705-t-- - - r -- - ---,- - - ---,- - - -.-- - --,- - - ---,- - - j

a o

87Rb/86Sr

Fig.6. (a) Rb-Sr sca t t e rc h ro n diagram sho wing the completeanalyticaldatafor 13sa m plestak enfrom the maintrondhjemitedyk e,Sn ca n. The465 Marefe- rence line (cf. Fig. 6bl isalso show n. (b) Rb-Sr iso- chron diagram excl ud in g the five sam p les taken fromthe ve ry marginalpartsof thedyk e.Seete xtfor explana t io nan d discussion.

1.2 lA 0.6 0.8

0.4 0.2

0705+-- - - , - - ---,- - --,-- - - , -- - - , -- ----.- ---i

b o

0719 0.717

0715

en

₀₇₁₃

eo~ en^r-, 0711 eo

0709 0707

NGU - BULL 437,2000 - PAGE74

marginalparts of the dyke.Ifthese samplesare excluded, then a reasonable best-fit isochron definesan ageof 465± 11 Ma, wit h aMSWD of 2.2and an init ial87SrfB6Sr ratio of 0.70 794±9.Thisisochron ageis takentobe a minimumage for the dyke intr usion,and is provisional ly interpreted as reflecting a post-emplacement, thermal resetting age.

Samp lestaken from the dyke margins are likely to have beenaffected by anint roduction offluids,andad mixt ureof fluidsfrom the wallrock;and suchfluid smayhave contai- ned abundant ions of radiogenic Sr,expelledfromdiverse mineralsand concentrated bythedecayof87Rb duringther- malresettin g.

The isochron age for this setof trondhjemitedykesthus providesaminim umage fortheEarlyOrdoviciandeforma- tionand metamorphismofthe juxtaposed Steren ophiol ite and Gula succession,and of the Selbusj oen tectonic me- lange.Italso supportsan earlierpostulate(Size1979) that trondhjem itemagmaderived fromparti almeltingofbasalt may have intr uded throug h thecontinent al margi n crust, and the Gula Nappe in particular,throu ghout Ordovician time andwellinto theSilurian.Thegent lefolding of the dy- kesand theirweak foliate fabricarelikelyto be of Scandian (LateSilurian-Early Devon ian)age.

Acknowledgements

Wethank OddNilsen andTorGrenne for theirhelpful comments on an early draft of the manuscript.Const ructive suggestions by the revie- wers,Arne Raheim andWilliam Size, andfurther discussio n andcom- ments on therevisedmanuscrip tbyOddNilsen,are greatly apprecia- ted..TheINAA analyses(REE) wererunbyProfessorJanHertogen and histeam at theDepart me ntof Physico-Chemical Geology,Universityof Leuven,Belgium.

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