Mafic dykes in the Leksdal Nappe at S0rli, Central Norwegian Caledonides: geochemistry and

NGU-BULL435 ,199 9 -PAGE 59

Mafic dykes in the Leksdal Nappe at S0rli, Central Norwegian Caledonides: geochemistry and

palaeotectonic implications

MONICA BECKHOLMEN&DAVID ROBERTS

Beckh ol m en, M. & Roberts,D.1999:Mafi cdykes in theLeksdalNapp e atSorli, CentralNo rwegian Caledo nides:geo- che m ist ryand palaeo tec to nicimplicat io ns.Norg esgeol ogiskeundersekelseBulle tin435,59-67.

Maficdykesintruding feldspathicmetasandstonesoftheLek sdal(Sarv) Nappe in the Serlidistric tofNord -Trond elag, Central Norway ,varyfrom weakly fol iat ed,porp hy riti ctypes tomo re com mon,st ro ng lyschistose,sparsely phyricto ap hy ric bio t ite-a m p hiboleor biotite schists. Analyt icaldat a,how ever, show that thedyk es arechem ically fairly hom o gen eou s.The geoche m icalsig nat ureof the dykes istran siti on al,i.e.betwe enthol eiitic andalkalic, and points indisputed lyto a within-p late setting.Rare-eart heleme nt data showclearLREE-enr iched,mildl y alkaline tend en cies.

The geoche mist ryofthe Sorlidykes,based on trace and rare-earth eleme ntabunda nces,is, infact,quite simila r to that of the'mildly alkaline' group of met ad ol erit es disti ng uished byearlie rworkersfrom diversepartsof theSarv Nappe, andequivalent nap pes, overwideareasofthe Scand inav ianCaled o nides.

Althou ghthe emp laceme ntage of the Serli dykesisnot known it isinferr edtobeVendian,based on dat a availa b le fro mcomparabl e dole ritedykesin ot her partsof the Sarv andimmediate ly overl yin g SeveNapp es.Dolerite dykes of T- andN-MORBaffinit iesinsomeofthehighe stthr ust-sheetsof theBalt o scandi an marg insuccessio ns farth ernorth have yieldedMid toLat eVendian ages,andare inferred tohaveintrud ed im med iate ly priortothe act ual inceptio n of lapetu s sea-floo rsp read ing.The LREE-enri ch ed,mildly alkaline dykesfrom Serll,cutting afluvi almet asedimentary successio nat the Midd le Allochth on tect on ostrati gr aphiclevel,are thusassum ed to haveintruded in amor e inboard,conti nentalsett ing,and mostlikely ata justslightlyearl ier stageof lat est Neop rote rozo iccrusta lexte nsion and riftin g, perhapsin EarlyVend iantim e.

Monica Beckho lmen,Institution enfor Geovetenskape r,UppsalaUniversit et,Villaviigen16,S-75236Uppsala,Sweden.

Dav idRoberts,Nor gesgeologiske underse kelse,N-7491 Trondh eim , Norway.

Introduction

In the Caledonides of Scandinavia,the'sandst one nappes'of the MiddleAllochthon constitute a distinctive though some- what discontinuous element in the tectonostratigraphy of the mountain chain. Forming part of the original Neoproter- ozoic to Early Palaeozoic miogeoclin eof the Baltoscandian margin of the palaeocontinent Baltica,thesethick, arkosic sandstone successions were, in places,hosts to widely occur- ring dolerite dykes. In the higher thrust sheets (Sarv Nappes) of the Middle Allochthon,and also in different partsof the immediately overlying Seve Nappe Complex (Seve Nappes) of the basal part of the Upper Allochthon,such mafic dykes are particularly abundant. In the contextoftectonomagmatic evolution ofthe Caledonide orogen, the profuse dyke activity is considered to relateto aphase of Vendian to Early Cam- brian crustal extensionand rift magmatism, priorto incipient sea-floor spreading and the birth of the lapetus Ocean (Gee 1975, Roberts& Gale 1978,Kumpulainen & Nystuen 1985, Stephens etal. 1985,Andreasson 1994).

In several areas, and in diverse thrust sheets,many of these mafic dykes,or dyke swarms,have been the subject of geochemical investigations (e.g., Roberts 1975, 1990, Andreasson et al. 1979, Solyom et al 1979, 1985, Stolen 1994a, b, Svenningsen 1994), the results of which aredis- cussed briefly later (p. 65).There have also been a few attempts at isotopicdatingthat,basedon the more reliable U-Pb,Sm-Ndand 4oAr-³⁹Ar methods, are pointing to latest

Riphean to Vendian ages of dyke emplacement (rangi ng from c.665 toc.573 Ma)(Claesson&Roddick 1983,Zwaan&

van Roermund 1990,Svenningsen 1994, 1996).In one area in Southwest Norway, in the Sveconorwegian crystalline base- ment of Baltica southeast of the Caledonianfront,a dolerite from the Egersund dyke swarm has also yieldeda Vendian emplacement age(U-Pb, baddeleyite,616

±

3 Ma;Bingenet al. 1998).

In the Caledonidesof Central Norway, mafic dykes occur- ring in the arkose-dominated Leksdal Nappe on the south- western flank of the Ternrneras Antiform,east of Steinkjer, have beenstudied by Andreassonet al.(1979). Analyses of dykesoccurring to the northeastof Ternrnerasand also in the Orkanger district, southwest of Trondheim, were incorpora- ted inalater statisticalinvestigation (Solyomet al. 1985),but there was no discussion of these particular dykes or their geochemistry.In this short contribution we describe, and present geochemical data from, mafic dykes occurring in the Leksdal Nappein the Serlidistrictof the Grong region,in the county of Nord-Trendelaq.

Regional setting

Serli liesin the eastern part of the area covered by the 1:250,000 bedrock map-sheet'Grong'(Roberts 1997),an area which contains most of the element s of Caledonide tecto- nostratigraphyfrom the parautochthonous Olden Nappe in

NGU-BULL435,1999-PAGE 60

I I I I

MON/CA BECKHOLMEN&DAV/D ROBERTS

SKJfJTINGEN NAPPE Garnet-mica schist, amphibolite

OFFERDAL & LEKSDAL NAPPES Metasandstone, quartz sc h is t

FORMOFOSS NAPPE COMPLEX F oliated gran ite

Porphyritic rhyolite and trachyte

OLDEN NAPPE

Metasedimentary 'cove r roc ks M etagabbro , a mphibolite B lafjellhatten granite O lden granite

P o rphyritic rhyolite and trachyte

Fig.1. Simplifiedgeological/tectonost ratig raphicmapofthe Serfdistrict.In the Offerdal &Leksdalbox,0-Offerdal(=Dearka) NappeandL-Leksdal Nappe.

the core of the Grong-Olden Culminat ion through the several nappes of the Lower,Middleand Upper Allochthons(Robert s

& Gee 1985) into the Helgeland Nappe Complex of the UppermostAllochthon.TheMiddleAllochth o n is onlymod- estly repre sented,andmainlyassmall,lensoidthrust sheets of meta-arenites inthe lower Dearka(=Off erdal) and upper Leksdal(=Sarv) Nappes.

The greenschist-facies metasandstones, quartz schists and thinquartzitesof thelow er, dyke-freepart of the Middl e Allochthon (Dearka/Offerdal Nappe) are inferred to be of Neoproterozoicage.Some20km southof S0r1i,fluvial sand- stonesoftheOfferdal Nap pearelocally highl y st raine dand transform ed into thin-band ed flagstones of commercial quality.Farther southeastinto Jarntland,Sweden,theOffer- dal Nappe is thicker and more extens ive, and there the metasandstones have formed the basisof an important flag- stone industrywit hlong tradit ions .

The overlying Leksdal Nappe occurs mainlyto the east and southeastof Sorli,but also asa small,2 x0.5km lens just westofthe lakeUlen(Fig.1) (Beckholmen&Robert s1989).

Lithologies vary from greenschist-facies,medium-to thick- bedded, feldspathic sandstones to mica-rich sandstones, semi-pel itesand thin quartzitesandmica schists.Crossbed- ding is quite common in many sandstonebeds,especially in low-strainzones;and thesuccession as awhol eis conside red to be of fluvial orig in.Flagstones are less well develop ed, occurring ina few,rest ricted,duct ilehigh -strain zones and mor eparticularly close tothe nappe boundaries.Maficdykes are fairlycommon feat uresoftheLeksdal Nappe.Depend ing on thenatureof the orig inalhost lithologyand the degreeof strain, the dykes vary from massive and porphyritic to stronglyschistose.Comparable variation s in dyke character, dependent upon the degreeof ductile shear deformation , have been described from the Turtbakktjerna and Svart - tj ernalenses, on the southwestern sideoftheGron g-Olden Culminat io n(Kautsky 1978, Gilotti1989).

The mafic dykes

Field relationships and petrography

The mafic dykesoccurthroughout the fol dedand schistose psammiticrocks of theLeksdal Nap pe but theyarenever suf- ficient lynumerou stowarrant thedesig nat io n'dyke swarm', asin otherpartsofthe mountain beltin the Sarv and Seve Nappes. On Gunnarfj ell, where exposure is comparative ly good,oneof severalsubparalleldykescan be followedalo ng strike over severalhundred metres.

Infieldappe arance(Fig. 2),the mafic dykes are quitehet- erogeneous,varyi ng from comparat ively massive, weakly foliated,porphyritictypesto stronglyschisto se,finer grained, aphyric or sparsely phyric, biot it e-am p hibole or biotite schists. Ingeneral, this changein character is clearly a reflec- tio nof thedegr eeof deform at ion ofthe rocksin different partsof thenapp e,as well asthe dyke thickness.Thicknesses vary from thecentimet rescale up to 2-3 m; in these cases, the very thindykes aregenerally fine-grained and strongly schistose, and concorda nt wit h the tecto nised layering, whereasthe thickest dykestend to be variably porph yrit ic and may lie at a small to moderate angle to the compost- tio nal layering.

Mapping has shownthattherocks ofthe Leksdal Nappe, in thisarea,have been affect ed by polyphase Caledonia n defo rmat io n.Inthe multilayered,psammite-pelite parts of the successio nthe earlie st,syn-metamorphicfold saretight toisoclinal(Beckholme n 1987), wit haxial trendsand stretc h- ing lineationsvaryingfromE-Wto ESE-WNW.Dykesare par- ticularlythin,andmaybe excisedalong the attenuate dlong lim bs of early,asymme tri cfoldsandinlocalised shear zones.

On the contrary,the thicker,more massive dykes occur in zones ofmarked ly lower strain and in fold short lim bs and composit e hinges,wherecross-bedding can alsobe recog- nised infavour able localit ies. There are alsoexamples show - ing that someof the early folds developed initiallyadjacent

MON/CA BECKHOLMEN&DAV/DROBERTS NGU-BULL435 ,1 999-PAGE61

Fig.2. (A) Mafiedykesinmet asandst ones,bothinvolvedin synme ta mo rphic, tightfolding alo ng WNW-ESE axes;phot o,looking nort hwest.Riverside exposure,c.150 m NE of thebridgeoverthe lnderdalsaa,c.3km WSW ofJule. Grid ref.41151570.(B)Shear-bande d mafi edyke inarkosiemetasand- st ones; photo,lookingnorthw est.Tight tonear-isoclinalfold sin themeta sand ston esboth aboveand belo w the dyke.Roadsid e expo surealo ngthe road from BrasimestangentoBakken.Grid -ref.5120 1470.

to the dykes-- the dykes having acted as a buffer to the imposed contractional deformation(Robert s 1989).In these casesthe dykes probablylay at fairly highangles to bedding and were progressivelyrotated into subparallelism wit hthe transposed layering as the simple-sheardeformation pro- ceeded (cf.Gayer et al. 1978,Krill 1986).

In addition to the fold deformation and thinning,a later tectonic feature of the dykes is that of boudinage.This flattening deformation also affected packages of folded metasedimentarylayers,and most likelyrelatesto a fairly late but ductile component of strainof pureshear character.The lensoid nature of the small 'out liers' of the Leksdal Nappe in this region (see above) can probably be ascribed to this episode of deformation.

In thin -section,compo sit e nematoblastic-Iepidoblasticto who lly lepido blast ic textures areprevalent amongthenon- po rph yriti cdyke rocks, which is a direct reflection of the degre e of st ra in an d acco m pa n y i n g metam orphi ctran sfor- mation .In the most extre me cases, the retrogr ession has yieldedbioti te schists,inoneexamp lewit haFe-rich bioti te groun dm ass surroundi ngscattered , 1-2mm-longmuscovit e metacrysts.lngeneral, however, the schistosedykes are com- posed of biot ite, act ino lite and epidote/c1inozoisite, with lesser amountsofrecrystallised plagioc lase,quartz and cal- cite,and accessory apatite, spheneand titanomagnetite. Rel- ict 2-3mm plagioclase crysts,wherepresent,are transformed to small recrystallised grains.In some cases,recrystallisedpla- gioclaseis associated wit h profusesmalllathsand grainsof biotite,epidoteand calcite.These particularsecondarymin- erals may have been derived from original glomerophyric assemblages of c1inopyroxeneand plagioclase.

The porphyriticdykes are inte nsely diaphthorit ised.Orig- inal plagiocl aselaths up to 5-6 mmin sizehavebeenreplaced by a dense mat of sericite surround ed by epl dote/d ino- zoisite.The original pyroxene (?augite), where present, is uralit ised and has partly (margi nally) or entirely been

replaced by actinolite,which shows up asdark greento black laths up to 1 cm in length.In the groundmass,the ret rogres- sion has produced afiner grained assemblage of epi dote, biotite,quartz,chlorite,sericiteand some magnet it e.

Geochemistry

A selection of fourteen samples of maficdykesof varying character,from porphyriticto strongly schistose,were taken from differ ent parts of the nappe.Majorand trace element s were analysed on fused glass beads and pressedpowd erpel- lets,respectively,using an automatic Philip s 1450120 XRF spectrometer,at the Geological Survey of Norway, Trond- heim .Calibration curveswere made with int ernat ional stand- ards.Ferrous iron,H20+,H20' and CO2weredeterminedby wet chemica lmethods.Four ofthe samples havebeenana- lysed byINAA for eightrare earthelements(REE) and the ele- ments Se,Co,Hf,Ta,Thand U, at theUniversity of Leuven,Bel- gi um, foll owin g the method described an d evaluated by Pedersen & Hert ogen (1990). The 'in hou se' silicate rock sta ndards used have been repeated ly calibrat ed against interna ti onal reference sta ndards.The analyt ical data are show ninTables1and2.

Thevariat ion incharacter ofthe dykesrecognised in the field, from markedly porphyritic tonon-porphyrit ic and schis- tose, does not appear to be reflectedin the generalmajor ele- mentchemist ry (Table 1), except perhaps for the alkaliesand for justone or two isolated dykes.K20,for example,shows two unusually high and low valuesof 6.58 and 0.96wt.%, respect ively, in the former case associated with the excep- tionaloccurrenceof porphyroblasticmuscovite;and in three other dykes,Kp contents exceed5 wt.%.Na20values,onthe otherhand,are in some cases quitelow.Some of these dis- crepant values may possibly be ascribed to wall-rock con- tamination. Si02values range from 45.5 to 49.5%, and Ti02 from 1.4 to2.8%.In aclassical alkalies-silica diagram (Fig. 3),

NGU-BULL435,1999- PAGE62 ^{MON/CA BECKHOLMEN}

s

^DAV/DROBERTS

Table 1: Maficdykesfromthe LeksdalNappe,Serli:major and trace ele me ntcompositio ns.

Sample 41-87 42-87 57-87 58-87 60-87 71-87 75-87 106-87 112-87 113-87 15-88 17-88 18-88 19-88 Si02 45.46 46.38 47.56 47.23 49.45 46.71 45.68 45.64 47.92 48.54 45.45 48.08 45.68 49.20

Ti02 1.64 2.18 2.45 2.44 2.78 1.36 2.29 1.70 1.58 1.62 2.23 2.30 2.02 2.70

AI₂03 14.29 14.07 14.33 14.57 13.90 14.57 14.37 17.06 13.38 16.65 14.29 14.53 14.46 14.87

Fe203 2.48 2.51 2.51 2.67 4.03 3.25 3.61 3.14 2.23 2.64 4.18 2.85 2.42 3.23

FeO 8.61 9.54 8.71 8.76 8.66 5.99 10.06 6.86 8.43 6.42 7.16 8.95 8.82 9.45

MnO 0.20 0.21 0.17 0.17 0.19 0.15 0.30 0.17 0.18 0.14 0.35 0.19 0.18 0.19

MgO 6.48 5.07 5.10 5.19 4.77 9.85 5.63 6.67 7.38 6.33 4.67 5.49 5.46 4.30

CaO 9.26 8.30 8.95 8.65 8.32 11.75 8.92 12.55 10.49 11.30 7.10 8.73 8.90 7.18

Nap 0.23 0.83 0.79 0.87 0.22 1.49 2.23 2.25 2.58 1.77 0.45 0.10 1.25 1.22

K₂0 5.03 5.09 4.64 4.71 4.20 1.71 2.68 0.96 1.80 1.39 6.58 4.73 5.03 4.43

P20S 0.29 0.47 0.84 0.82 0.75 0.38 1.23 0.57 0.32 0.28 0.44 0.40 0.46 0.74

H2O+ 2.76 2.73 2.43 2.51 2.14 2.33 2.05 1.91 2.06 2.22 3.00 2.32 2.36 1.89

Hp ' 0.14 0.07 0.06 0.07 0.08 0.08 0.12 0.08 0.13 0.04 0.04 0.03 0.04 0.02

CO

₂ 3.10 2.70 1.60 1.40 0.00 0.10 0.00 0.00 1.20 4.30 4.30 1.44 3.71 0.15

Sum 99.97 100.15 100.14 100.06 99.49 99.72 99.17 99.56 99.68 100.24 100.24 100.14 100.79 99.57

Zr 119 228 213 196 275 173 217 171 149 158 276 252 209 321

Y 31 34 40 36 43 24 44 25 28 29 34 38 28 35

Sr 313 367 466 457 500 577 404 855 365 411 219 366 358 517

Rb 211 249 223 224 154 73 89 28 51 45 306 232 240 195

Zn 133 125 133 135 115 76 131 78 84 82 118 151 123 121

Cu ~5 7 ~5 ~5 46 73 24 30 14 45 18 15 11 54

Ni 90 46 49 51 38 183 43 87 85 62 19 24 29 18

Cr 114 64 89 107 81 356 81 154 194 149 12 20 18 15

Ba 71S 815 606 640 933 278 794 713 380 381 773 663 635 788

Nb 26 35 37 30 40 26 59 69 23 20 31 27 33 56

V 241 270 204 208 282 188 314 246 255 220 218 233 207 232

thesam p lesst ra d dlethe dividin glin e betwee n the subalka - linean dalka line fields an d exte n dwellintoth elatterfie ld.

Sincetheeleme n t sKandNaare kn ownto beparti c ul arl y mobile during metam orp hism,the analytica ldata we re plot - ted in ot he r diag rams fe atu rin g more immob ile eleme nts su c has P,Ti,Zr,Nb an d Y(Floyd &Winc he ster 1975);an d in all the se cases the data againrevealalkalinetenden c ie s. The avera geNb/Yrati o of 1.1,how ever,(whic h include son e ana -

10

lys iswithanexcept io nal ratio of2.7 6)is only slightlyabove the su g gesteddivide betweenalkali basalts(>1.0)and tholei- ites« 1.0) (Pe a rce &Ca n n 1973,Solyometal. 1985). On this ba s is,theSerlimetadoleritescanth u s be classifiedas mild ly alkaline.

In th e V-Till000 diagramofSh e rv a is(19 8 2) the dyke sam- ple s fall equallyon eitherside of the dividing lin e between alkalinebasalts and MORS(Fig.4),denotingsome measure of tra n sitio n algeochemical character, irre s p e cti v e of theirpal-

25 20

10 15

T i/1000

5

Fig. 4.The Sorlimafic dyke samples plotted on the V-Tidiagramof 5her- vais(1982). The TiNratiofieldbetween 20 and 50coversMORB,back- arc basin andcontinental basalts;thatbetween50 and 100is for alkali andocean-islandbasalts.

55 60

45 50

Si0₂

600

Alkaline ^{10 20}

•

⁵⁰⁰

• ....

^{40 0}

• • • _• >

³⁰⁰

•

Subalkaline ₂₀₀ 100

10 0

40 8

0

'"

⁶

::.:::

+

0

'"

4

Z1Il

2

0

35

Fig.3.Alkalies-silicadiagramshowingthe distributi on of the Serli mafic dyke samples(filled circle s).

MON/CA BECKHOLMEN&DAV/DROBERTS NGU-BULL 435,1999-PAGE 63

1000 A

Zr

100

1L-_-'----'---J...--i-.JL-J....Jw...L_--''--....L.-....L...J'--'--'-'-'-'

10 10

£

N

•

• •

• • •

• •

• • ,

•

Fig.5.ThesameSorli mafic dykesamplesplottedon the TiOr Kp-P₂0_S diagr amofPearceetal.(1975).

Fig. 7. Zr/Y-Zr plotof the same dyke sam ples;diagr amfromPearce &

Norry(1979).Field A-Wit hin-platebasalts;B-Volcanic arc basalts;C- Mid-oceanrid ge basalts.

Ti/100

2 Nb

Fig.6.Ti-Zr-Ydiscri mi nant diagram of Pearce&(an n(1973)showi ngthe distribution of the mafic dyke sam p lesfromSerli.FieldA- low-Ktholei- ites;B -ocean-floor tholeiites;( - calc-a lkalinebasalt s;D - wit hin-p late tholeiites.

Zr Y*3

Fig.8.The sameSerli dyke samplesplot ted on theNb-Zr-Ydiagram of

Mesched e(1986).FieldA- wit hi n- platebasalts(AI- within-platealkali basalts;All- within-platealkali basalt sandtholeiites):B- E-ty pe MORB; ( - within-plate tholeiitesand volc anicarcbasalts;D-N-type MORBand vo lcanicarc basalts.

aeoenvironmental setting. We do know, of course, that the dykes were emplaced into fluvial feldspathic sandst ones,and this non-oceanic affi liat ion is also confirmed in the Ti02-K20-

P₂₀sdiagramof Pearceetal.(1975)(Fig.5).

Using the incompatible trace elements Y, Nb and Zr, partly in association with Tl,the Ti-Zr-Ydiag ramof Pearce &

(ann(1973),for exam ple(Fig.6),indicatesalargely wit hi n- plate character for theSerli dykes, and thisisconfi rmed on theZr/Y-Zrplot(Fig.7)of Pearce &Norry(1979).On the Nb- Zr-Y diagramof Meschede (1986),there is atendencyfor the samples to fall in thetholeiitic (rat her than alkaline)sub-field of the within-plateassociation (Fig.8).How ever,thetransi- tional, within-plate,geochem icalcharacterof the Serlidykes

isalsodepicted quite clearlyin theTi/Y-Nb/Ylog-log plot of Pearce (1982)(Fig. 9).

The rare earth element(REE) contentsof threefairly rep- resentativesamplesand anothercharact erisedby high erFe, Pand Nb contents wereanalysedby INAA.The data are given in Table 2.The chondrite-normalis edpatterns show a clear LREE enrichment,and no trace of any Euanomaly (Fig.10).

Lanthanum enrichment varies fromc.90to 145timeschon- dritic abundancesin themain group of threesamples (Fig.

10),but reaches close to 300in thecase ofsample 75-87.La_N/

LU_Nratios varyfrom 7.7 to 11.4for the three comparable sam- ples,but for sample 75-87this sameratio is 20.Apart from thisanomalous,high est-LREE sam ple,the otheranalysesand ratios,and chondrit e-normalis edpatterns,are verysim ilarto those of a'mildly alkaline' group of mafic dykes reported

NGU-BULL435,199 9-PAGE64 MON/CABECKHOLMEN

s

DAV/D ROBERTS

Sample 57-87 75-87 15-88 17-88

La

^49.7 101 36.9 31

Ce 108 212 82 70

Nd 52.4 94 42.4 37.1

Sm 9.8 15.6 8.3 7.74

E u

2.84 4.50 2.38 2.31

Tb

1.25 1.71 1.15 1.13

Y b

2.9 3.6 3.3 2.9

Lu

.44 .52 .49 .39

Ta

2.57 3.52 2.0 1 1.69

Hf 5.5 5.3 6.7 6.1

Th

5.3 10.1 3.7 2.9

U 1.3 2.4 1.4 .84

Sc 27.1 24.5 30.0 32.4

Table2:Rare-eart h elementandTa,Hf,Th,U andSc contents(pp m) of the representativesamles of mafic dykes fromthe LeksdalNappe,So rli.

10

•

alkalic

1 NblY within-plate

transitional., ^., tholeiitic : . "

.: ":eo

•• • •

oceanfloor 500

100' - - _---'_ - ' ----'--'-..L...L...L..l.-'--_ ---''--- ' ----'-- ' --'-'-...LU

0.1

Fig.9.The Serlidykes plotted on the TiIY-NbIYdiagra mof Pearce(1982).

where the'w it hin-p late'field is subdividedinto tholeiit ic,transition al and alkaline types.

Fig.10. Chondrit e-no rm al- ised REE pro fi les for three representative samples and one othe r sample of the Sorlimafic dy kes.Sour ces of chond ritic values for nor- malisation: Masuda et al.

(1973), Nakamura (1974) and Evensen et al. (1978).

- .

Yb Lu + 75 - 87

o 57 - 87

o 15- 88

• 17 - 88

300 200

C/)

2 ¹⁰⁰

~_c:

..c:0 50

U

---

^ID

0..

Eca 20

Cf)

10

5

La Ce Nd

S m

Eu Tb

from theSarvNappe(So lyo metal.1985). This alsocomesout quite clearly in a cho nd rite-no rm alised La/Sm -LalYb varia- tionplot (Fig.11).

, I

In sum mary,the trace andrare-earthelement geochemi- cal sig nat ures of the S0r1 idykesare denoting transitiona l, within-pla te feat ureswith clear,LREE-enri ched, mildlyalka- line tende ncies. The continental, wit hin-plate signature is consistentwit h theirfield occurrence in continental,fluvia- tile,feld spath ic sand st o nes.

o Discussion

The occurrenceof mafic dykes and dykeswarmsthroughou t thesuccessionsof the Sarv and Seve Nappes whichoriginally formed partsof theBalt oscand ian passive marg inand Bal- tica-Iapet us,continent -ocean transit ion zone is now gener- ally accep tedto relate to a majorepisodeof Late Neoproter- ozoic, rift-related magm atism. Review s of this phase of mioge oclinal basin development, essentially a precursor stageof exten sion inthe break-up of the palaeoco nt ine nt Rodinia,can befo undin Roberts &Gale(1978) andKurnpu- lainen & Nystuen(1985).

Constraints on the tim ing and durati on of rifting and empla cem ent ofindividualdykes in the variousCaledonian nappesandthrust sheets are comparat ivelyfew. From the typearea of the Ottfjalletdoleritedykesinthe SarvNappe, in 10

1 (La/Yb)N

0.1'--_ _ -'--_'----'---'---.L....l-LJ'--'---_ _---'---_'--'---'----'--'-..L..I....LJ 0.1

E

z en

Fig.11.Chondrite-no rma lisedplotof rat iosLa/Sm vs.La/Yb. Circle - Sorli average(n=3); opentriangle- SarvNap pe mildl yalkaline dole ritedykes (m ean); filledtriangle-Sarv Nappetho leiit ic do leritedykes (m ean)(Soly- ometal. 1985);two dots - dolerite dykesfro mthe Corrov arre Nap pe, Troms(Robert s1990); elongatelinedarea -Palaeogene T-MOR8tho lei- iticlavas and dykes from the VorinqPlateau(Viereck et al. 1988).

MON/CA8ECKHOLMEN&DAV/D R0 8ERTS

Jamtland,CentralSwed en(some 150 km SSW ofSerli), Claes- son& Roddick(1983) obtained a4oAr_39Arint rusionage of 665

±

10 Ma. Similardykesfrom the Seetra(=Sarv) Nappe of theOppdal area,south-cent ral Norway,yielded a Rb-Sriso- chron ageof 745±37Ma(Krill1983).In more recentyears, Sm-Nd andU-Pbzircon dating of mafic dykes from theSeve/

Kaiak and Sarv Nappesof Troms, northern Norway, and neighbo uringareasin northernSw eden,has yielded Vend ian ages- 582

±

30 Ma(Srn-Nd,Zwaan&van Roermund 1990);

573

±

74Ma (Srn-Nd, Svenningsen1994); and608+1Ma(U- Pb zircon, Svenningsen 1996).The comparabl e geological and tectonostratigraphic situations of thesedykesmakeit reasonabl e to assumethat most, though not necessarilyall of this rift-relateddyke intrusionalong theBaltoscandianmar- ginoccurred duringtheVendian period.TheSerlimetadoler- ite dykes areprobab ly too deformed and metamorphosedto yield any meaningfulisotopicintrusion age.Based on the evidencefrom other areas, however, weconsiderthat these particular dykes are also likely to have int ruded during Vendian time.

Clues asto the relati veages ofthe abun dantmafic dykes in theSarv and SeveNappes may beobt ain edfromtheirgeo- chem icalsignatu resas wellas fromthe nat ure of the host- rock lit hologi es and their posit ionsinCaledoni de tecton o- strat igraphy .A bipartite divisionof SarvNapp e dolerit e dykes into(1) alarge and characteristictholeiiticgroup and (2)a subordinat ebut quite sign ificant mildly alkaline group was demonstrated by Solyom et al.(1985) basedon a statistical discriminantanalysisof samplesover a 600 km-longby200 km-widebelt.The Serli mafic dykes thus appeartobelong to the second,mildly alkaline group.

Farther north, in Trorns,tholeiitic dolerit e dykesin the Corrovarre Nappe show homogeneous T-MORB affinities (Robert s 1990), comparable to those of Tertiary dykesand lavas from the North Atl ant icVoring Plateau(Viereck et al.

1988).TheCorrovarreNappe occurs in the uppermost part of the KaiakNappe Complex,whichis believedto bea northern equivalent of the Seve Nappes(Robert s 1988, Andreassonet al. 1998).Studies by Andreasson et al.(1992), Stolen(1994a,b, 1997)and Svenningsen (1994, 1995), in Troms and Norrbot- ten,have also shown that the mafic dykesthere, in thrust sheetsof theSeve Nappe Complex, aremainlyof T-MORBto locally N-MORB character. These particula r dykes occur mainly asswarmsand in the high ernappes(derived from a setting in theoutermo stparts of thecontinent -oceantransi- tionzone)they cut calcareousarenit esof marineorig in.The youngestisotopic datesso farrecorded(seeabove)are from theseT-MORBtholeiitic dolerites, whichaccordswith their interpretation as dykes intr uded immediately prior to the inception of lapetus sea-floor spreadi ng (Roberts 1990, Stolen1994b).

Up to now,therearenoreliabledates for the mildlyalka- line maficdykesfrom theSarv Nappe.However,inview of their transit ional , wit hin-plate geochemical traits and em p la cemen t in amore in b o a rd,fluvi al , continen ta lsetti ng, they should be expectedtorepresent arelat ively slightly ear- lier phaseof dyke intrusiondating from the initial stages of crustalextension and rifting in,perhaps,Early Vendian time.

NGU-BUL L435,1999 -PAGE65

Thedoleritedyke in theEgersund swarm that providedthec.

616 Ma baddeleyiteage belongs to an alkaline subsuite(Bin- gen etal.1998,Bingen&Dem aiffe 1999). Althoug hthese par- ticulardykes arenot situa tedin the Caledonianalloeht hon they dorepresenthypaby ssalmag matis mfrom theLateNeo- prot erozoicpassivemarginof Baltiea,and thus suggest that the timegap between intrusi on of thedistinct ive alkaline/

mild lyalkalineand N-IT-MORBmafiedykeswarm smay have been comparati vely short.

Conclusions

Mafic dykes intruding feldspathic metasandstones of the LeksdalNappe in the Serli district of Nord-Trondelag ,Central Norway, vary from massive porphyritic types to more com- mon,st rongly schistose,aphyricbiotit e-amphibo leor biotite sehists.Despite these differencesin appearance, andthe fact that metamorphic grade reached greenschist facies, qeo- chemicaldata from vari-t extureddykesfrom different parts of thenappeshowa reasonable homogeneit y,withjust one or twoexception s,and the samplescluster fairlywell on most trace-elementdiscriminantdiagrams.

The geochem icalsignatu reofthedykes is transitional, i.e.,between tholeiiticand alkalie,and points indi sputed ly to a wit hin-plate setting. Rare-earth element datashow clear LRE E-enriched, mildly alkalinetendencies.Thegeochemistry oftheSerlidykes,based on themorereliable trace and rare- eart heleme nts, is, in fact,quit e similar to that of the 'mildly alkaline'group of(met a)dolerit es distingu ishedby Solyom et al.(1985)from diversepart sof theSarvNapp e,and equiva- lent nappes,over wide areas of theSeand inavian Caledo- nides.

Although the emplacementageof theSorlidykes is not known it isinferred to be Vendian,based on the data availa- ble fromcomparable doleritedykes in other parts ofthe Sarv and immediately overlyingSeveNappes.Doleritedykes ofT- and N-MORBaffinities in some ofthe highest thrust-sheetsof the Baltoscandian margin successionsfarther north have yielded Mid to LateVendian ages,andare inferred to have intr uded immediately priorto the act ual inceptionof lapetus sea-floo r spreading .The LREE-enrich ed,mildly alkaline dykes from Serli,cutting a fluvial metasedim entary succession in theMiddle Allochthon tectonostratigraphic level, are thus assumed to haveintrudedinamoreinboard,continentalset- ting,and most likely at a slightly earlierstageof latest Prot er- ozoic crustal extensionand rifting,perhapsin EarlyVendian tim e.

Acknowledgements

We are gratefulto the reviewe rs,HaraldFurnes,BernardBingenand Per- Gunnar Andr easson,for their crit icalcomment s and help ful suggesti ons thatledto improvementsinthemanuscript.Thanks are due to Irene Lund qvist for her digitaldraftingof thefigures.

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MON/CA BECKHOLMEN&DAV/DROBERTS NGU- BULL435,1999-PAGE67

Appendix ^71-87 52301555 Green-grey,foliate amphibolite, Grid references and brief descriptions of the dyke samples with5 mm chl-amph.crysts.

analysed.All sam ples are from 1:50,000 map-sheet 'Serli', 75-87 49851805 Med-grained,foliate amphib.,actin- 1923-2 (3-Nor edition, blue coordinates, 33W VM). Dyke bio-clinozo:actin 2 mm.

thicknessesvary from a fewdecimet resto 2-3metr es. 106-87 4920 2410 Green-grey,schistoseamphib.,wit h small-scale crenu lation fol ds.

Sample no. Grid ref. Briefsamp le descrip tion 112-87 4865 1680 Porp h.metadol.,uralit isedpx 41-87 53651510 Green-grey,foliate,aphyricto « 1cm)+sericit. plag(5 mm),

sparsely phyric,blo-hbl foliate.

amphib/schist. 113-87 49901735 Porph.metadol.,sauss.plagcrysts 42-87 53501510 Similarto above.Lepidoblast ic, (4 mm)+ someretrogr. px., foliate.

sparselyphyric.Chlorite+c1inozo. 15-88 41101570 Green-grey,aphyric,schistose

present. biotite (+musc)amphibo lite .

57-87 50851475 Green-grey,fine-qrn.,bio. 17-88 50901470 Green-g rey,fine-qrn .,aphyric,

amphib/schist.Abundant biotit e schistose bio-amph ibolite.

and epidote. 18-88 50901470 Similar to above, aphyric, foliate 58-87 50851475 Similarto above.Bio-amph ib.schist, metadolerite/amphibolite.

aphyric,lepidoblastic. 19-88 50401430 Green-grey,aphyric,biotite-rich 60-87 50251425 Green-grey,foliate,aphyric,blo-hbl amphibolite.

amphibo lit e.

Manuscrip treceivedMay1999;revisedmanuscriptaccepted August 1999.