DAVID ROBERTS
s
NICHOLAS WALKER NGU-BULL 432 ,1 9 97- PAG E95U-Pb zircon age of a dolerite dyke from near
Hamningberg, Varanger Peninsula, North Norway, and its regional significance
DAVID ROBERTS&NICHOLASWALKER
Roberts,D.& Walker, N.1997: U-Pbzircon ageofa doleritedykefrom nearHamningberg,Varang erPeninsula,Nort h Norway,anditsregionalsig nifica nce.NorgesgeologiskeundetsekelseBulletin432,95-102.
A doleritedyke cutting verylow-grademetasedim entary rocksof theLekvikfjellet Group nearHamning berg, NE Varange rPeninsula,Finnmark,has yieldedaU-Pbzirconupperinterceptage of 567.1+3° /,23Ma.ThisENE-WSW-tre n- dingdyke,andothersof comparablefield character, pet rographyandgeochem istryinthisnortheasternpart of Varanger,haveprevio uslygiven LateDevonian/EarlyCarb oniferou s,K-Arwho le-rock ages.Accepting567Maasthe approximateageof intrusion,theK-Ardatesare considered to ref lectatherm alresettin gassociate dwit heit hera late-Scandianpulseora slightlylater,regionalcrustalextensionand riftingeventknownfromboreholedatain the Barents Sea. ALateDevoniantherm al overprint hasalso affecte d comparabledoleritedykesonthenearbyRybachi PeninsulaofNort hwest Russia.
TheLateVend ianageoftheHamnin gberg dyke isin goodaccord with region algeologicaldevelopment, involving a LateVendian,Baikalian deforma tionandmetamorphismaffectingrockson Rybachi andprobablyalsoNEVaranger, character ised byaNW-SE(to eN-S)fold axialtrend andcleavage strike.TheHamning bergdykeand relate ddykesin this NEVarang er/Rybachi regio ncut the folds and cleavageat high ang les,and probably penetr ated along extension almaster fracturesat a latestageinthe Baikaliandeformationepisode.Itis suggested thatmanyofthe dykesin theBatsfjorden,KongsfjordenandBerlevagswarmsinthenorth and northwest oftheVarang erPeninsula may alsobeof Vendian(to possibly Camb rian)age;thesedykeswere subsequentlyaffect ed to varyingdeg rees by Caledon ian deforma tio nandmetamorphism.
David Roberts.Norgesgeologiskeundersekelse,PostBox3006-Lade, 7002Trondheim,Norway.
NicholasWalker,DepartmentofGeological Sciences,Brown University,Providence,RI02912, USA.
Introduction
Varanger Peninsula in Finnmark,northernNorway,is under- lain by weakly metamorphosed sedimentary rocks of Neoproterozoic to Early Cambrian age. In term s of Caledon idetectonostratigraphy (Roberts &Gee 1985), the peninsula exposes the transition from autochthonou sform- ations lying unconformably upon Palaeoproterozoiccrystal- line rocks,through the weakly folded sequ ences of the Parautochthon,into more penetrativelyfolded and cleaved successionsof the Lower Allochthon in westernmost areas. In addit ionto theCaledonianstruct ures,ithasbeensugges- ted that folds and cleavage of Late Vend ian age (t he Baikalian deformationof NW Russia)may berepresented in the nort heasternmostpart s of Varange rPeninsu la (Roberts 1993,1996). Acom plex,polyphase,NW-SE-trend ing fault zone,the Trollfjo rden-KomagelvaFaultZone(TKFZ),divides the penin sula into two halves - the Tanafjorden- Varang erfjorden Region (TVR) to the southwest, and the Barent s Sea Region (BSR) to the northeast (Siedlecka &
Siedlecki1967)(Fig.1).
The chron ostratigraphyof the successionson thepenin- sula hasbeen based largely on diversefaunal evidence- acritarchs(VidaI1981,Vida l&Siedlec ka1983),stroma t olit es (Bert rand-Sa rfat i & Siedlecka 1980) and Ediacaran fossils (Farmeret al. 1992) - but therearealsoRb-Srwhole-rockiso- chronages for two formations in theTVR(Sturtet al.1975).
Indirect evidence which also hasa bearing on therelative ages of parts of the lithostratig raph ical successions has come from a K-Ar dating investigation of mafic dykes (Beckinsale et al. 1976).From theseK-Ar determinations, three age groups were defined: (A) Agesof c.360 Ma for comparatively unaltered dolerite dykes from the eastern partsofVarangerPeninsula,in both theBSRand theTVR; (B) Agesof c.650 Ma for weakly cleavedmetadoleritesfrom the Batsfjorden areaof the BSR; (C) A group ofages ranging from 945 to 1945 Ma,from schist ose metadolerites in the Kongsfjorde n areaof the BSR. AllK-Ar ages givenhere,and inthe subsequent discussion,arerecalculated according to Dalrympl e (1979).A palaeom agn etic st udy of one of the group Adykes in theeasternm ost part oftheTVR yielded data whichsuggest ed a probabl eVendianto Cambrianage (Torsvik etal.1995),rathe rthantheLateDevonian ageindi - cated bytheK-Ar analyses of Beckinsale etal.(1976).Results of a simila rst udy on doleritedykesfrom theBSR werenot definit ive,butalso tended to favoura latestNeop rote rozoic to EarlyPalaeozoicage(Knut sen 1995). Fin ally,there is an unpublished Srn-Nd date of 550 Ma,noted in Andersen &
Sundvoll (1995),for a mafic dyke from the Kongsfjord area (localit y not given).
In view of thesediscrepanciesin age, there isclearlya ne ed for a more preci se isotop ic dating of manyof the se mafic dykes.Asthe 4°Ar_39Ar and Rb-Srmethod shad been
NGU-BULL 43 2,199 7- PAGE 96 DAVIDROBERTS&NICHOLASWALKER
Bare nts Sea
sokm
0
BerlevaqFm.D
VestertanaGr.~
D
Tanafjorden Gr.N
illIill
L0kvikfjelletGr.D
Vadse Gr.\
~ TyvjofjelletFm. -§
Batsfjord Fm. ~ PrecambrianEJ
....- BasnCEringenFm. crystallinebasement•
Kongsfjord Fm.Fig.1. Simplifiedgeologicalmap ofthe VarangerPeninsula, showingthelocation of the Hamningberg doleritedyke(circled H). BSR-Barents Sea Region;TVR-Tanafjorden-Varangerfjorden Region; GNC-GaissaNappeComplex;TN -Tanahorn Nappe;TKFZ-Trollfjorden-KomagelvaFaultZone;
SRFZ-Sredni-RybachiFault Zone.
triedwitho ut much success(Rob ertsetal. 1995 and un pu b- lished data),we delive reda 3.5 kg sampleof dolerit e from a dykenearHamningberg(Fig.1) to NGU's mineral sep arati o n lab o rato ry,inthe ho pethat badd eleyit e or some fewzirco ns cou ld be recovered. A limit ed nu m ber of zirco ns were, in fact,found.In thispaper wepresent theresultsof U-Pb analysesofzirco n fractionsext racte d from thisdy ke.
Aspects of regional geology
The geology of Varanger Pen in sula hasbeen descr ib ed in many pu b lications over the past 20 years.Rev iews or ac- countsof variouskinds includethose of Sied lecka (1975, 1985),Johnson et al.(1978),Riceet al.(1989a),Siedlecka&
Robert s(1992) and Karpuzet al.(1993). Lithostratigraph ic correlat ion swit hsuccessions occurring on the Rybachiand SredniPeninsula s in nearb y No rt hwest Russiacan be fou nd inSiedlecka(1975) and Siedlecka et al.(1995a,b).
On Varang er,theBaren tsSeaReg ionis domi na tedbytwo principal,ep izo ne-g rade lithostr atig raph ic meg a-un it s,the 9 km-thickBarent sSeaGrou p andtheunconformabl yover- lying5.7km -thi ckl.e kvikfj elletGro up(Siedlecka&Siedlecki 1971,Siedlecki& Levell 1978) (Fig.1). ln the extreme north- west ofthe regi on, thel.e kv ikfjellet Group is overth rust by the Tanaho rn Nappe(Sied lecka&Roberts1992)which com- prises a mixed phyll ite-metasandstone successio n, the Berlev aq Forma ti o n(Levell&Roberts1977).The BarentsSea Gro up shows a shallowing -upward successio n of 4 forma- tio ns co m m encin g wit h su b m arine-fan turbid ites of the Kongsfjord Formation(Sied lecka1972,Picker ing 1981).The age of this group, based on microfossils, is Late Riph ean (Vid al & Siedlecka 1983). The terrig en o us l.ekvikfje lle t Gro u p hasbeendiv id edint o5 fo rma tio nsanditis the oldest unit, the Sandfjorden Form atio n, which occurs in the
Hamn ingberg area (Fig.1)(Siedlecka1984).Microfossilsre- cove redfro m the l.ekvikfjel letGroup yieldedaless defin it i- ve,Vend ian ag e;and Vid al &Sied lec ka(1983)spe cula te dthat thehighestform at io ns could be as young as Late Vendian.
An attempt to date the prominen t cleavag e in Ko ngsfj or dFor ma tio n pelitessouth ofHam ni ngbergbythe Rb-Srmethodyieldedawhole -rockiso ch ro nag eof 520±47 Ma(Taylo r&Pickering 1981),interpreted by these autho rs as dat ing a foldingand cleavag eevent at ca.520 Ma. Riceet al.
(1989) questioned thisinterp ret atio n, arguing fo r aScandian age for themet amorphismin theBSR.
TheTanaljorden-VarangerljordenRegion is underla in by a4 km-thic k succession comprisingth ree groups,the Vadso, Tanafjorde n and Vestert ana Gro ups, which range in age from Late Riphean to Early Cam b rian.Tillites marking the Varangerian Ice Age occu r in the basal part s of the Vestertana Gro up (Edward s 1984).In general,most of the rocksof theTVR areindiagen esisgrad e, fo rmi ng par tofthe Caled o n ian Aut ochth o n and Parauto chth on,but inthewest metamor phicgrade increasesto upper anchi zon einandim- med iate ly beneat h the Gaissa Nappe Co m p lex (Rice et al.
1989b).The younge st fossiliferous rocks in thisallochtho- nousunit are found on the DigermulPen in sula,and areof Tremad oc age.In onesmallarea im m edi ately southwestof theTro llfj o rde n-Ko ma gelva Fault Zone,rocksof the up p er partoftheVad so Gro u pare lyingunconformablyupon a for- mation oftheBarent s SeaGroup(Rice 1994),a relationsh ip which marksan im po rt ant link between the TVR and the BSR.
The metado leritedyke s of groupsBandC(p.95)occuras swarms in the Bat sfjorden and Kongsfj or den areas ofthe BSR, respectively, trending mainly ENE-WSW to NE-SW (Ro b ert s1972,Rice& Reiz1994).A swa rmofdykes of sim ilar trend also occurs in the northern parts of the Berlevaq
DAVID ROBERTS&NICHOLASWALKER NGU-BULL 432 ,1 9 9 7- PAGE 97
Fig. 2.TheHamningbergdyke,cutting med- iu m- bedde dsandst onesof theSandfj orden Form ati on,LokvikfjellGroup;lookingap pro- ximatelyeast-no rtheast.
Fo rm at io n in the TanahornNappe.Group A dykes,on theoth- er hand, are few in number and occurin comparati ve iso lati- on in the easte rn part sof theBarentsSeaReg ion; and also in eastern coastal tracts of the Tanafjorden-Varangerfjord en Reg ion (Ro berts 1972, 1975, Beckin sale et al. 1976) where theygenerallytrend bet ween NE-SW(BSR)and N-S(TVR).
Dolerite dykes also occur in the nort hwestern part of Rybachi Pen insu la, along the nort hern coast of the Kola Penin sula, just60km southeastof VarangerPeninsula.The dykesthere arecomparab le in petrograp hy,geochemistry andweathe ri ng co lours tothoseof groupAonVaranger and theyalso trend NE-SW, cut t in g NW-SE-tre ndi ngfoldsandan associated penet rat ive cleavage almost at right-angles (Ro bert s&Onstott 1995). 4°Arf'9Ar lasermicropro be analy- sesof pyroxen es and feldsparsfrom these dykes yielded only Mid Ordovi cian-Silu rian and Late Devon ian overprint dates, whereas palaeom agn et ic data acq uired fro m these samedykeshave suggestedaVendianto Cambrian age of intrusion (Torsviketal. 1995).
The Hamningberg dolerite dyke
The sam p le isfroma13.5m-thickdykelocated just south- eastof theform er fishingset t leme ntandwhalingstatio nof Hamn ingb erg,in thenort heastern part of the Barents Sea Reg io n on VarangerPeninsula(g rid.ref. 11252695, 1:50,000 map-sheet Syltefjord 2436-2).The sample was taken from the cent ral part ofthe dyke alo ng aroad-cut. The dolerite dyke trends between NE-SWand ENE-WSW and dips at c.700 to the NNWlt cuts very low-grade,thick-beddedsandstones of the Sandfjorden Formation (Sied lecka 1984) of the Lo kvikfj elletGro up(Fig. 2) whi ch dipat cAO°to the SE. Thin, silt y pe lite intercalat io ns carry aweakly developed,steep, ESE-d ip pin g, spaced cleavage whic h isaxial planar to the open, upri gh t,ca.N-S-trend ingfolds which characte rise thi s part ofthe BSR(Sied lecki 1980).Rotating the beddingback to the horizontalwould indicatethatthe dyke originallydip-
ped stee ply to theSSE-5E.
The dark brown to almost black, med ium- to coarse- grained dolerite shows orange-brow n colou rsonweat he red surfaces.A feat ure of the dyke, bestseenalo ngitsmargins,is that itdisplayswell develo ped,pseudo hexago nal,co lum nar joint ing(Fig.3).Themineralogyofthe dyke isdominated by plagioclase(An....,,)and c1inopyroxene (pigeo nite, with some aug it einouter marginal zones).Lath-shaped plag ioclase ex- hibitsminor sericitisatio n,andthec1ino py roxene is typically twinned. There are minor amounts « 1%) of a primary yellow-green amphibole and secondary chlorite; and mag- neti teis the opaquephase.A subophitictextu reprevails,but the reare also glomerophyric clot s of plagioclase+cli no py- roxeneand justc1inopyroxene .
Fig.3. Pseudohexagon al,columnar jointingintheHamn ingbergdyke.
The photowastaken looki ng NNW,perpendiculartothe struct ura lly lo- wermargin ofthe dyke.
NGU-BULL 432,1 997 - PAG E98
Analytical methods and results
U-Pbzirconanalyticalmethods
Zircon separates were cleaned in successive solutions of 2N HN03,3NHCI anddisti lledHp to remove cont aminant sulphides andsurfaceimpurit ies.Zircon con cent ratesthus treated were then handpicked into three3-g rain fracti ons on the basis ofmorphological similarity,employing visual discriminant s such as size,form,colour,clarity and aspect ratio. In each casethe zircon grains appeared to be mag- matic inorigi n(as judged by aspect ratioandpreservat ion of crystalterminatio ns and crystal edges)wit h nomacro- scopic evide nce of magmatic resorp tion or mechan ical rounding as mightbe observed if these zircon s werexeno- crystic.We note,however,that there are no independ ent data to confirm whether or not thesezircons are co-gene- ticwit h the melt thatcryst allisedto formthe dyke.
Thehandpickedfract ion s wereairabra dedin a device similar to that described by Krogh (1982).Zircon dissolu- tion and ion-exchange procedures were comparable to those described by Krogh (1973)and Parrish et al.(1987). A
mixed 2osPb_mU_2JSU tracer was employed.Pb was loaded
on W filament s and U was loaded to Re filament s and analysed on the Finnigan MAT 261 mult icollect or mass spectrometerat Brown University.Pbwas analysed in static multicollectormode employing Faraday cup collectio nof masses 208,207,206and205,while simultaneously collec- ting mass 204ina secondary electr on multip lier.Uranium was analysed in static multi collecto r mode employi ng Faraday collectors only.Additional analyti caldetailsare gi- ven in thefootnotesto Table1.
Data and age interpretation
Three zirco nfract io nsfromsam pleHMBGwere analysed.
Thedat a areshown inTable1and grap hicallydisplayedin
DAVIDROBERTS& NICHOLASWALKER
FigA.U-Pb system at icsare norm all y discord ant for the threefractions.Abest- fi t discord iatraj ectorythrough the- sedata defines anupp erinter cept of567.1'30/.23Ma anda lowerinte rcept of 392'''/.36Ma;and with a mean sq uare ofweig ht ed dev iateof 17.The rat her large inte rcept un- certain t iesarenotdue to the inhere nt quality of the ana- lyticaldata.The uncertainti es are theresult ofap parently non-simpl e isot o pi c system at ics(a combination ofPbloss and variabl e but small amo unt s of inheritance?)couple d wit h the low angleof thediscord ia trajectory to conco r- dia,aproblem that befall sallgeo log ically youngzirco ns that are discordant due to Pblossrelatively shortly aft er crystallisation .
If we assume that thesezirco ns crystallised from the sam e melt as the dy ke,the up per and low er int ercepts place som e impo rtan t const raints on the empla cement age.We interpret theupperdiscord ia inter ceptof 567 Ma to give the approximate cryst alli sation age of the dyke withi n thenum ericaluncer t aint ies expressedabove.The lowe rinterce ptisinter pre te d to reflectaDevo nianisoto- picdistu rbanc e relate d,most likely, to aCaledo nian(late Scandian;possib lyEarlyDevo nian)tecto not hermal event;
or,given theerror sfor the inter cept, conceivably to the initi al, Lat e Devonian stages of a regional, Devo no- Carbon ife rous riftin g and crustaI extens ion which affec- ted wideareas of thesouthernBarent sSea and pro bably also the coastalareas of nort hern Finnmar k and the Kola Peninsula.
Discussion
The U-Pb zircon crystallisation age for the Hamningberg dyke differs appreciably from the Late Devon ian/Early Carboniferou s (c.360 Ma) K-Ar date of Beckinsale et al.
(1976).On most modern tim e-scales anage of 567 Ma falls in theLate Vendian,takingtheVendian-Camb rian boundaryat
Fraction Amount Concent ranon- Pbisot op iccompositio ns Radiog enic ratios@ Age anduncertei nty••
prop erties" analysed (ppm) (M a)
(g rains) Pb U 206/208 206/207 2061204 206Pb'/238U 207Pb'/235U 207Pb'/206Pb' 206Pb'/238U 207Pb'/235U 207Pb'/206Pb'
ar= 3:1,
pale brown 73.5 835.2 7.207 15.629 2.473 0.0854 (0.29%) 0.6882 (0.35%) 0.0584 2(0.18%) 528.5 (z1.5) 531.7(± 1.8) 545.8 (±1.0)
ar= 2:1,
paleyello w 82.3 996.9 9.266 15.930 2,940 0.0852(0.29%) 0.6823(0.34%) 0.05809 (0.17%) 527.1(± 1.5) 528.2(±1.8) 533.2(± 0.9)
ar=3:1,
palebrown 64.3 855.3 14.433 16.350 3,5 15 0.0773(0.23%) 0.6111 (0.26%) 0.05721(0.11%) 481.1(±1.1) 484.3(±1.3) 499.5(±0.6)
fZirconsmechanicallyabradedfor6· 12 hours;ar=aspect ratiopriorto abrasion;colourasobservedwith grainsin alcohol.
+Concentr at io nistotalPb andinclu des blankPb,common Pbinzircon , andradiogenic Pb. Totalprocedu ralblanks are-2 picograms for U and-10 picogramsfor Pb.
Becauseweightsofgrainsareestimated and amixedU·Pbtracerwas employed.uncertaintyin estimatesof grainweightaffectsonly concentrationdata,not calculatedU-Pbor Pb/Pbages.
#Measuredisotopi crati ospriortocorrec tionformass fractionation of -0.11%perato mic mass unitbased onrep licat e analyses ofNISTSRM 981 and 982 and adjustedforsmallamount of 206Pbintracer.
@Number in parenthesesispercentuncertainty inthecalculated ratio,statedat the 2-sigma level.
"Decay constan ts:238U ; I.5S13E- lO/yr.;235U=9.8485 E- l 0/yr.Ato mratio238U1235U = 137.88.Uncertaintyin the calculatedagesis st at ed at thetwo -sig m a leveland estimatedfrom combineduncertaintiesin calibrationsof mixed20SPb- 233U- 235Utracer,measurementofisotopicratiosof Pb and U,common andlaboratory blankPbisotopic ratios.Pb and U mass fractionationcorr ect ions,andreproducibility inmeasurement of NISTPbandU standards.
Table 1. Zircon data,Hamning bergdyke
DAVID ROBERTS&NICHOLAS WALKER NGU-BULL 432, 1997 - PAGE 99
Intercept s at
567.1+/-2339.-- and392 +2§..-/ -36 Ma .06
.10
%
.09
::::>
co .08
(')'
--
C\!.0n,<0
0 .07
C\!
Fig.4.U-Pbconcordia diagramfor thethree zircon analyses from the Hamningberg dyke.The data are presented in Table1.
.72 .80 .48
.05 '___ __'__--'-_ _~ _ - - ' - _ ~_ _.l.__~_ ___J' _ __ _ _ ' __ _ _ ' __ _ _ _ '
.40
545 Ma(Tucker & McKerrow 1995).Asaco nseq uence of this, it is necessary to reassess certainaspect s of the geological historyof the region.
Although in general field appearancethe dated dyke se- em s to be unmetamorphosed, and as such should postdate the spacedcleavage seen in silty pelite interbeds,a defini- tive mutual relationship cannot be demo nst rated at this loca- lity. Some2 km to the southeast,however, near Finvik,a similar NE-SWtrending, unmetamorphosed dolerite dyke with a comparable K-Arage cuts anESE-d ip ping cleavage which is axial planar to mesoscopic and mappable c.N-Sfolds. The hostrocks here are part of the KongsfjordFormation. The in- dicat io ns are, therefore, that the earliest folding,cleavage and very low -g rade metamor phi smin this particular areaof Varang er Penin sula are pre-567 Ma, Le. pre-Caledon ian. In this regard, the Rb-Srdateof 520±47Ma for thecleavagein KongsfjordFo rmat io n pelites repo rt ed by Taylor&Pickering (1981)takeson renew edinterest.Acceptingthe erro r of±47 Ma,a maximu m age fo r thecleavagewould thusbeputat 567 Ma. What we clearly need isamore accurate dat ingof thiscleavage-fo rm ing event, perhaps by the 4OAr-39Ar meth- od( a4OAr-39Ar cleavage dating proj ecthas,in fact, just com- menced,on Rybachi Peninsula, with the possibility of future extensiononto the Varanger Peninsula).
On the Rybachi Peninsula (Fig.1), the turbiditic Upper Riphean succession is quite pervasively folded and cleaved along a consistent NW-SE trend (Negrut sa 1971,Roberts 1995,Roberts&Karpuz 1995).These structureswere genera- ted during the Baikaliandeformation,an event recognised also southeast of Kolain the Timans.Biostratigraphicand ra- diometric dating evidenceon Rybachi,and on the nearby Sred ni Peninsula , have constrained this deformation and low-grade metamorphic event to c.580-560 Ma (Robert s 1995).As noted earlier, there are NE-SWtrending dolerite
dykeson Ryb achi which clearly cut acrosstheNW-SEfolds and cleavage,at high angles. Basedon4OAr-J9Ar mineral data, thedykes are evidently olderthan Mid Ordovician; and are more likely to have intruded at so me stage during the Vendi an-Cambriantime interval judging from the results of the paleomagneticinvestigations (To rsvik et al. 1995).
It hasbeen argued that the Baikalian structures occur- ring on Rybachi and Sredni can also be recognised in the easternparts ofVaranger Peninsula,especially in the eastern BSR(Roberts 1995,1996).This is,in fact,an ideafirst presen- ted almos t a century ago (Ramsay 1899,Tchernyschev 1901), i.e., that the'Tim anian moun tain chain'could be followed from theTiman s along the nor thern coastline ofthe Kola Peninsu la, and possiblyinto theVarangerPenin sula in north - eastern Norway,following thesam etraseas thepre-existing Riphean to Vendian rift basin alo ng the northern and north- eastern marginsof theRussian Platform.
Asthe argumentsfor anextensionof Baikalianfoldsand cleavage into NEVaranger are thus quite strong,thentheU- Pb zirco nagepresent edhere sho uld be viewedinamore re- gionalcontext .Considering thegroup A dolerites, they oc- cur almos t exclusi vely inthenortheastern part sof Varanger Peninsula, andtrendbet w een NE-SWand c.N-S. The only ex- cepti on is a dyke which intrudes the rocks of the Tanahorn Nappe in the northwestern BSR. Thedolerit es occurring in NW Rybachi also fall into this same category.If the ca.567 Ma zircon age can also be applied generally to this group of rna- fic dykes(b ut they need not all beof
precisely
the same age), thenit istempting to interpret these particular dykes as hav- ing int rud ed either in the terminal stages of,or shortly after the peak of the Baikalian tectonothermal event. Asthe dykes strike at high angles to, and clearly transect the Baikalian folds and cleavage,they can be readilyenvisaged as having been emplacedalong major'ac' tension alfractures or faultsNGU-BULL 432.1997- PAGE 100
which develop ed in assoc iation wit h the regiona l folds. It is in such an extensional sit uat io n, antithetic to the c1eavage- and fold-generating compressivest ress,thatone wouldex- pect rising mafic magma to have met least resistan ce to penetrationof theuppermostcrust.
The 567 Ma age for theHamningbergdyke thusfit squi- te neatlyint othis generalstory,whe reby thedyke emp lace- ment would appear to have occurredduring the laterst ages of the Baikalian tect onothermal eve nt. The U-Pb age also falls within the'age range'for these gro up A dolerit edykes whichis indi cated bypalaeom agne t ic data, som e of whi ch are yet unpublished (Knut sen,Torsvik & Robert s,in prep.).
Another pieceof evidencewhich providesindirect support for this general reinterpretation of the structural develop- ment of NEVaranger Peninsula is that of Rb-Srdating ofilli t e fractionsfrom formationsin the centr alandeasternparts of the Tanafjorden-Vara nge rfj o rde n Region (Go ro kho v et al.
1996).Data fromthe Nyborgand Stap pogiedde Form at io n s of the Vestertana Group indicatean ageof burial diagenesis in these verylow-graderocks of ca.560
±
10 Ma.This was in- terpreted to relate in some way toa regional compactionof the succession arising from the effe ctsofthe approaching Baikali an deformatio n fronttothenortheast (Gorokhov et al. 1996).The U-Pb dating reported here naturally questions the validity,or rather the interpretation of the K-Ar ages (Lat e Devonian-Early Carboniferous) of the group A dykes of Beckinsaleetal.(1976).The4°Ar_39Arlasermicrop ro be analy- tical data reported by Roberts& Onstott (1995) from the Ryba chi dykes also indicated aLat e Devo nian'age; but in this case the data clearly showed that the dyke emplace- ment age was olderthan Mid Ordovician.These authors did, in fact, interpret the ca.376 Ma isoch ro nageto relate to a therm al resetti ngeve nt associated wit h theLat eDevon ian to Carbo nifero us riftingwhich is knownto have occurred ex- tensively in the southern partsof theBarent sSea. It does, therefore,seem more reasonable that the K-Ardates derived for the group A dykes are likely to be a reflection of a weak ther mal inpu t arising from the regi o na l, Devon o-Carbo n- ifero us, crustalextension.
The K-Ar dates of the group Cdykes of Beckinsale et al.(1976 )are known to be highlysuspectin view of the ex- tremely low potassium contents; and some of the ages re- ported exceed the biostratigraphic age of the metasedi- ments. However, a recent unpubl ished Srn-Nddate for one of these dykes points to an age of ca.550Ma (Andersen &
Sundvoll 1995).Gro up B dykes,on the otherhand, do app- ear to have K-Arages which,geologically speaking ,are not unreasonable.Although ca.650 Mawas said to be«a proba- ble true age»,the dates repo rte d do in fact range from ca.660to ca.550Ma, wit han error bar of
±
19 Ma(Beckinsale et al. 1976).These dates,taken at face value,thus overlap with the U-Pbzircon age of the Hamningberg dyke. This may mean that many,though notnecessarily all of the dykes in the ENE-WSW-trendingBatsfjordswarm may be of a corn-DAVID ROBERTS
s
NICHOLASWALKERparabl e LateVend ian age. Since they intr ude rocks of the VendianLokvikfj elle t Group,this is inde ed likely.Some dyke intrusion may have exten de d into Cam b riantime,but the very oldest ofthe K-Ar dates of group B may be the most questio nab le in view of thebiostratig rap hiccon stra int.
Although dyke/fo ld relationsh ip s are notthesubjectof thisco nt ribu t ion ,it can be notedthat afo ld- relat ed spaced cleavageis orient edsubparalleltotheENE-WSW-st riking dy- kes in the Batsfjord district.In several casesit can be seen that the regional cleavage bends into perfect parallelism wit h the dyke marg in,and thedykes, especially the thin ner ones, areweakly cleaved .The preferred int erpret ati on here isthatthe dykes arepre-c1ea vage/fold ing.This cleavage and these folds can be traced so ut hwest w ard s int o the Kongsfjord area where they are moreintensively developed and metamorphicgrade is slightlyhigher (Ro berts 1972).
In the Kong sfj o rd area,met adol erit e dykes(group C) are alsoprofuse,wit hfoldsandaxialplanar cleavagesubpar all e- ling the now strongly schistose,ENE-WSW-trend ing dykes (Ro berts 1972, 1975,Rice & Reiz 1994). Although Robert s (1972) initially favoured a syn-tectoni c emplacement age, subsequent geochemi caldata (Ro bert s 1975),addi ti on alfi- eld studies,and more recent lyadet ail ed investi gatio n by Rice & Reiz(1994),has steered interpret atio nin favour ofa pre-c1eavage/fold ing origin for the majority ofthese dykes (but seeSiedlecka&Roberts 1992).Moreover,the situation inthe western BSR issuchthat the folds and penetrativecle- avage there - fro m the Berle vaq area eastwards via Kongsfjordento Batsfjord- are almost cert ainly Caled o nian (Ro b ert s 1996),and probab ly Scandian .Thus,the majori ty of the dykes alsoin the Kongfjord and Berlevaq swarms could well be of Late Vendian toCam brian age,and relate to a ten- sional/tr anstensionalscenario sit uat ed closetothe jun ction of the lap etu s Ocean and Varange r-Timan rift basin (Sied lecka 1975, Rob erts & Gale 1978, Rice & Reiz 1994).
Whereas the Rybachiand NE Varanger dykes appearto re- present magma emplaced in a tensional situation associa- ted with the tail-end of a compressional fold belt , the swarm-likechara cter of the broadlycoeval dykes fur ther to the northwest,andalso in the higher nappes,may ratherbe indicative of the more regional, upper crust aI extension which is known to have affected the Baltoscandian margin in theNeoproterozoicriftingphase priorto lapetus opening (Ro bert s & Gale 1978, Gayer et al. 1985, Kumpulainen &
Nystuen 1985).
Conclusions
A 13.5 m-thick dolerite dyke cutting the lower part of the Lo kvikfj ellet Gro up near Hamningberg in NE Varang er Peni nsula,has yielded a U-Pb zircon crystallisatio n age of 567.1'30/" Ma. It is argued that dykes of comparable field character, petrography and chemistry occurring in this northeastern part of Varanger Peninsula and in north- western RybachiPeninsula,NW Russia,whichhave previously
DAVID RO BERTS&NICHOLAS WALKER
givenK-Ar or4<>Ar-39Ar,LateDevon ianto EarlyCarbon iferous dates,may be of broadly thesame age,i.e.Late Vendian.The Devono-Carboniferous datesprobably signifya thermal re- settin g associated with either late-Scandian cooling or a slight lylater,regionalcrustal extensionand rifting event.
Taking into account the regional geological develop- ment, which involves a LateVendian,Baikalian deformation and low-grade metamorphicevent on Rybachi and Sredni, and probably also in NEVaranger,then the 567 Ma dyke age can be seen as relatingeithe rto avery late-stageBaikalian event, or an immediatel y post-Baikalian emplacemen t.
Following the regional development of NW-SE trend ing folds and cleavage,mafic magma is envisaged ashaving penet rated alo ng extensional master fractures and faults trending c.NE-SW, i.e.,almost perpendi cular to theBaikalian foldaxes.
Dykes occurring ina swarmin theBatsfjorddistri ct may be of approximately the same age asthe Hamningberg dyke. Similarly, the dyke swarms in the Kongsfjord and Berlevaq areasarehere suggestedto be of broadly the same age.In contrast tothe Hamningberg dyke and the dykeson Rybachi,however,thesedyke swarmswere probabl y intru- ded in a more regionallyextensile regime and werealso subseq uent ly affected by penetrativeCaledonian deforma- tionand metamorphism.
Acknowledgements
Weare grat efu lto Ste p henLippard,Hug hRice,Brian Sturt and Bob Tuckerfor the ircritical and con st ructi ve commentson eitherearlier or revised versions ofthe manuscript. IreneLundqvistkindlydraftedthe figures.
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Manuscrip t receivedFebruary 1997;revised manuscriptaccepted May1997.
