DAVID ROBERTS s NICHOLAS WALKER NGU - BULL 4 3 2,1 9 9 7 - PAGE 9 5

U-Pb zircon age of a dolerite dyke from near Hamningberg,Varanger Peninsula, North Norway, and its regional significance

DAVID ROBERTS & NICHOLASWALKER

Robert s, D.&Walker,N. 1997: U-Pb zircon age of a do lerite dyke from near Hamningberg,Varanger Peninsula, Nort h Norway,and its regional significance.Norgesgeologiskeundetsekelse Bulletin 432,95-102.

A dolerite dyke cutting very low-grade metasedimentary rocks of the Lekvikfjellet Group near Hamningberg, NE Varanger Peninsula, Finnmark, has yielded a U-Pb zircon upp er intercept age of 567.1 +3°/ ,23 Ma.This ENE-WSW-tre n­ ding dyke, and others of comparable field character, petrography and geochemistry in th is north eastern part of Varanger, have previo usly given Late Devoni an/ Early Carboniferous, K-Ar whole-rock ages. Accepting 567 Ma as th e appro ximate age of int rusion, th e K-Ar dates are considered to reflect a th erm al resett ing associated with either a late-Scandian pulse or a slightly later, regional crustal extension and rifting event know n from borehole data in the Barent s Sea.A Late Devoni an th ermal overprint has also affected comp arable dole rite dykes on th e nearby Rybachi Penin sula of Northwest Russia. The Late Vendian age of the Hamningb erg dyke is in good accord with regional geo logical development,involving a Late Vendian, Baikalian deforma tion and metamorphism affecting rocks on Rybachi and probably also NEVaranger, characterised by a NW-SE (to e N-S) fold axial trend and cleavage strike. The Hamningberg dyke and related dykes in this NE Varang er/Rybachi regio n cut th e folds and cleavage at hig h angles, and probably penetrated along exte nsion al master fractu res at a late stage in the Baikalian deformation episode. It is suggested that many of the dykes in the Batsfjo rden, Kongsfjorden and Berlevag swarm s in th e north and northwest of t he Varanger Peninsula may also be of Vendi an (to possibly Camb rian) age; th ese dykes were subsequently affected to varying degrees by Caledon ian deforma tio n and metamorphism.

David Roberts.Norgesgeologiske undersekelse, Post Box 3006-Lade, 7002 Trondheim, Norway. Nicholas Walker, Department ofGeological Sciences, Brown University,Providence, RI02912, USA.

Introduction Indirect evidence which also has a bearing on th e relative Varanger Peninsula in Finnmark, northern Norway, is under­ ages of parts of the lithostratigraphical successions has lain by weak ly metamorphosed sedimentary rocks of come from a K-Ar dating investigation of mafic dykes Neoproterozoic to Early Cambrian age. In term s of (Beckinsale et al. 1976). From th ese K-Ar determinations, Caledon ide tectonostratigraphy (Roberts & Gee 1985), the three age groups were defined: (A) Ages of c.360 Ma for peninsula exposes the transition from autochthonous form­ compa ratively unaltered dolerite dykes from the eastern ations lying unconformably upon Palaeoprot erozoic crystal­ parts ofVaranger Peninsula,in both th e BSR and th eTVR;(B) line rocks, th rough the weakly fold ed sequences of th e Ages of c.650 Ma for weakly cleaved metadolerites from the Para utochthon, into more penetratively fol ded and cleaved Batsfjo rden area of th e BSR;(C) A group of ages ranging successions of the Lower Allochthon in westernmost areas. from 945 to 1945 Ma, from schistose metadolerites in the In addition to th e Caledonian structures, it has been sugges­ Kongsfjorden area of the BSR. All K-Ar ages given here, and ted that fold s and cleavage of Late Vendian age (the in th e subsequent discussion, are recalculated according to Baikalian deformation of NW Russia) may be represented in Dalrymple (1979). A palaeomagnetic study of one of the the nort heasternmo st parts of Varanger Peninsula (Roberts group A dykes in the easternmost part of the TVR yielded 1993, 1996). A complex, polyphase, NW-SE-trending fault data which suggested a probable Vendian to Cambrian age zone, the Trollfjorden-Komagelva Fault Zone (TKFZ), divides (Torsvik et al. 1995), rather than the Late Devonian age indi­ the peninsula into two halves - the Tanafjorden­ cated by th e K-Ar analyses of Beckinsale et al.(1976). Results Varangerfjorden Region (TVR) to th e southwest, and the of a simila r study on dolerite dykes from th e BSR were not Barent s Sea Region (BSR) to the northe ast (Siedlecka & definitive, but also tended to favou r a latest Neop rote rozoic Siedlecki 1967) (Fig.1). to Ea rly Palaeozoic age (Knutsen 1995). Finally, there is an The chronostratigraphy of th e successions on th e penin­ unpublished Srn -Nd date of 550 Ma, noted in Andersen & sula has been based largely on diverse faunal evidence ­ Sundvoll (1995), for a mafic dyke from the Kongsfjord area acritarchs (VidaI 1981,Vidal &Siedlecka 1983),stroma tolites (locality not given). (Bertrand-Sa rfati & Siedlecka 1980) and Ediacaran fossils In view of the se discrepancies in age, there is clearly a (Farmer et al. 1992) - but th ere are also Rb-Sr whole-rock iso­ n eed for a m ore precise isotopic d ating of m any of these chron ages for two formations in the TVR (Sturt et al. 1975). mafic dykes. As the 4°Ar_39Ar and Rb-Sr methods had been N G U -BU LL 4 3 2,199 7 -PAGE 96 DAVID ROBERTS& NICHOLASWALKER

Barents Sea

sokm

0 Berlevaq Fm. D Vestertana Gr. ~ D Tanafjorden Gr. N illIill L0kvikfjellet Gr. D- Vadse Gr. ~ Tyvjofjellet Fm. § Batsfjord Fm. ~ Precambrian crystalline basement \ EJ....- BasnCE ringen Fm. • Kongsfjord Fm. Fig. 1. Si mplified geological map of the Va ranger Peni nsula, showing the location of the Hamningberg dolerite dyke (circled H). BSR - Barents Sea Region; TVR - Ta nafjorden-Va rangerfjorden Region; GNC - Ga issa Nappe Com plex; TN- Tanahorn Nappe;TKFZ - Trollfjorden-Komagelva Fault Zone; SRFZ - Sredni-R ybachi Fault Zone.

t ried wi t hout m uch success (Robert s et al. 1995 and unpu b­ Hamnin gberg area (Fig. 1) (Siedlecka 1984). Microfossils re­ lished data), w e d elive red a 3.5 kg sam ple of dolerit e from a covered fro m the l.ekvikfjellet Group yielded a less defin it i­ dyke nea r Hamningbe rg (Fig.1) to NGU's m ineral separati on ve,Vend ian age;and Vid al & Sied lecka (1983) specula te d that laborato ry, in the hope that badd eleyit e or some few zirco ns th e highest format ions cou ld be as young as Late Vend ian . co uld be recovered. A lim ited num ber of zirco ns w ere, An attempt to date the pr omin en t cleavage in in fact, found. In th is paper w e present th e results of U-Pb Kongsfjor d For mation pelit es south of Ham ningberg by the analyses of zirco n fractions ext racte d from th is dyke. Rb-Sr me thod yielded a w hole -rock isoch ro n age of 520±47 Ma (Taylor & Pickerin g 198 1), interpreted by t hese auth ors as dating a fo lding and cleavag e event at ca.520 Ma. Rice et al. Aspects of regional geology (1989) questioned this interp retat ion, arguing for a Scand ian The geology of Varanger Pen in sula has been describ ed in age for the me tamorphism in the BSR. many publications over t he past 20 years. Reviews or ac­ The Tanaljorden-Varangerljorden Region is underlain by counts of various kinds includ e th ose of Sied lecka (1975, a 4 km -th ick succession comprising three groups, the Vadso, 1985), Johnson et al. (1978), Rice et al. (1989a), Siedlecka & Tanafjorde n and Vestert ana Groups, w hi ch range in age Robe rt s (1992) and Karpuz et al. (1993). Lithostratigraphic from Late Riphean to Early Cam b rian. Tillites markin g th e correlat ion s w it h successions occurring on the Ryba chi and Varangerian Ice Age occur in the basal part s of the Sredni Peninsula s in nearb y Nort hw est Russia can be fo und Vester tana Group (Edw ard s 1984). In general, most of the in Siedlecka (1975) and Siedl ecka et al. (1995a,b). rocks of th e TVR are in diagen esis grade, formi ng part of th e On Varang er, th e Baren ts Sea Region is do mi nated by tw o Caled onian Aut ochthon and Parautochth on , but in th e w est principal, epizo ne-grade lithostratigraph ic m ega-un it s, the metamorphic grade increases to upper anchi zon e in and im­ 9 km-thick Barents Sea Gro up and the unconformabl y over­ med iate ly be neat h the Gaissa Nappe Com plex (Rice et al. lying 5.7 km -thi ck l.ekvikfj ellet Group (Siedlecka & Siedlecki 1989b). The younge st fossiliferous ro cks in th is allochtho­ 197 1, Siedlecki & Leve ll 1978 )(Fig.1). ln th e extreme north­ nou s un it are found on the Digermul Peninsula, and are of w est of t he regi on, t he l.ekvikfj ellet Group is overthrust by Tremadoc age. In on e small area imm edi at ely southwest of th e Tanahorn Nap pe (Sied lecka & Robert s 1992) which com­ the Trollfjorde n-Koma ge lva Fault Zone, roc ks of the upper prises a m ixed ph yllite-meta sand stone succession, the part of the Vadso Gro up are lying un conformably upon a for­ Berlevaq Formati on (Levell & Robert s 1977).The Barents Sea mation of the Barent s Sea Group (Rice 1994), a rela tionship Group shows a shal lowing-upward succession of 4 forma­ w hich ma rks an im po rtant link between the TVR and the tions com mencin g wi th subm arine-fan tu rbidites of th e BSR. Kong sfjord Formation (Siedlecka 1972, Pickering 198 1). The The m etadolerite dykes of gr oups B and C (p.95) occur as age of t his group, based on microfossils, is Late Riphean swarms in the Bat sfjorden and Kongsfj or den areas of the (Vidal & Siedlecka 1983). The terrig enous l.e kvikfje llet BSR, respective ly, tr en ding mainly ENE-WSW to NE-SW Group has been divid ed into 5 formations and it is the oldest (Robert s 1972, Rice & Reiz 1994 ). A swa rm ofdykes of sim ilar unit, the Sandfjorde n Form at ion, w hi ch occurs in t he trend also occ urs in th e northern parts of t he Berlevaq DAVIDROBERTS& NICHOLASWALKER NGU-BULL 43 2 ,1 9 9 7 - PAGE 97

Fig.2. The Hamningberg dyke, cutting med­ ium- bedde d sandstones of the Sandfj orden Form ati on, Lokvikfjell Group; looking appro­ ximate lyeast-no rtheast.

Format ion in the Tanahorn Nappe.Group A dykes,on th e oth­ ped stee ply to the SSE-5E. er hand, are few in number and occur in comparative isolati ­ The dark brown to almost black, medium - to coarse­ on in th e easte rn part s of th e Barents Sea Region; and also in grained do lerite shows orange-brow n colou rs on weat hered eastern coastal tracts of th e Tanafjorden-Varangerfjorden surfaces.A feat ure of the dyke, best seen along its margins, is Region (Roberts 1972, 1975, Beckinsale et al. 1976) where th at it disp lays well deve loped, pseudo hexagonal, columnar th ey generally t rend between NE-SW (BSR) and N-S (TVR). jointing ( Fig.3). The minera logy of th e dyke is dominated by Dol erite dy kes also occur in t he no rt hwestern part of plagioclase (An....,,) and c1inopyroxene (pigeonite,with som e Rybachi Peninsula, alo ng t he no rt hern coast of t he Kola augite in outer marginal zones). Lath-shaped plagioclase ex­ Peninsula, j ust 60 km sout heast of Varanger Peninsula.The hibits minor sericitisation, and the c1 ino pyroxene is ty pically dy kes th ere are comparable in petrography, geochemistry twinned. There are minor amounts « 1%) of a primary and weathering colours to thos e ofgroup A on Varanger and yellow-green amphibole and secondary chlorite; and mag­ th ey also trend NE-SW, cut ting NW-SE-tre nding folds and an netite is the opaque phase.A subophitic textu re prevails , but associated penet rative cleavage almost at right-angles there are also glomerophyric clots of plagi oclase + clinopy­ (Roberts & Onstott 1995). 4°Arf'9Ar laser microprobe analy­ roxene and jus t c1inopyroxene. ses of py roxenes and feldspars from th ese dykes yielded onl y Mid Ordovician-Silu rian and Late Devon ian over print date s, w hereas palaeom agn et ic data acquired from th ese same dy kes have suggested a Vendi an to Cambrian age of intrusion (Torsvik et al. 1995).

The Hamningberg dolerite dyke The sample is from a 13.5 m-thick dy ke located j ust south­ east of t he fo rm er fishing sett lement and whaling stat ion of Hamn ingb erg, in t he no rt heastern part of th e Barents Sea Regio n on Varanger Peninsula (grid.ref. 1125 2695, 1:50,000 map-sheet Syltefjord 2436-2). The samp le was taken from the cent ral part of t he dy ke along a road-cut. The dolerite dyke trends between NE-SW and ENE-WSW and dips at c.700 to t he NNWlt cuts very low-grade, th ick-bedded sandstones of the Sandfjorden Formation (Siedlecka 1984) of the Lokvikfjellet Group (Fig. 2) which dip at cAO° to the SE. Thin , silty pelite intercalations carry a weakly developed, steep, ESE-dipping, spaced cleavage whic h is axial planar to th e op en, upri gh t, ca. N-S-trending folds which characte rise this Fig. 3. Pseudohe xagon al, columnar jointing in th e Hamn ingberg dyke. part of the BSR (Siedlecki 1980). Rotating the bedding back The photo was taken looking NNW, perpendicular to th e struc turally lo­ to t he hori zontal would indica te tha t the dyke orig inally dip- wer margin of the dyke. NGU-BULL 4 32,1 997 - PA GE 98 DAVID ROBERTS & NICHOLASWALKER

Analytical methods and results FigA . U-Pb systemat ics are normally discord ant for the three fract ions .A best-fit d iscord ia tr ajectory through the­

U-Pb zircon analytical methods se data defines an upper inter cept of 567.1 '30/.23 Ma and a Zircon separates were cleaned in successive solutions of lower intercept of 392 '''/.36 Ma; and with a mean square

2NHN03, 3N HCI and disti lled Hp to remove contaminant of weight ed deviate of 17. The rat her large inte rcept un­ sulphides and surface impurities. Zircon concentrates thus certainties are not due to th e inhere nt qu ality of the ana­ treated were th en hand picked into three 3-grain fracti ons lytical data . The uncertainties are th e result of apparently on the basis of mor phological similarity, employing visual non -simple isotopic systematics (a com bination of Pb loss discriminants such as size, form , colour, clarity and aspect and variable but small amo unt s of inheritance? ) cou ple d ratio . In each case the zircon grains appea red to be mag­ wi th the low ang le of the discord ia traj ectory to concor­ matic in origi n (as j udged by aspect ratio and preservation dia, a problem th at befalls all geolog ically young zircons of crystal terminatio ns and crystal edges) wit h no macro­ th at are discordant due to Pb loss relatively shortly after scopic evidence of mag matic resorption or mechan ical crystallisation . rounding as might be observed if these zircon s were xeno­ If we assume that these zircons crystallised from the crystic. We note, however, that there are no ind ependent same melt as the dyke, the upper and lower intercepts data to confirm whether or not these zircons are co-ge ne­ place some im portan t const raints on the emplacement tic wi th th e melt th at crystallised to form th e dyke . age.We interpret th e upper disco rdia inter cept of 567 Ma The handpicked fract ion s were air abraded in a device to give th e approximate crystallisation age of the dyke similar to that described by Krogh (1982). Zircon dissolu­ wi thin th e num erical uncer taint ies expressed above. The tion and ion-exchange pro cedures were compa rable to lowe r interce pt is inter pre ted to reflec t a Devonian isoto­ those described by Krogh (1973) and Parrish et al. (1987). A pic distu rbanc e related, most likely, to a Caledonian (late mixe d 2os Pb_m U_2JS U tr acer was em ployed. Pb was loaded Scandian ; possib ly Ea rly Devonian) tecto nothermal event; on W fil ament s and U was loaded to Re fil ament s and or, given th e error s for th e inter cept, conc eivably to th e analysed on the Finnigan MAT 261 mu lticollector mass initial, Late Devo nian stages of a regional, Devono­ spectrometer at Brown University. Pb wasanalysed in static Carboniferous rifting and crustaI extens ion w hich affec­ mu lticollector mo de employing Faraday cup collection of ted wi de areas of the southern Barents Sea and probably masses 208,207,206 and 205, while simultaneously collec­ also the coastal areas of no rthern Finnmar k and the Kola ting mass 204 in a secondary electr on multip lier. Uranium Peninsula. was analysed in static multi collecto r mode em ployi ng Faraday collectors on ly. Add itional analyti cal details are gi­ Discussion ven in th e footnotes to Table 1. The U-Pb zircon crystallisation age for the Hamningberg dyke differs appreciably from the Late Devon ian/ Ea rly Data and age interpretation Carboniferous (c.360 Ma) K-Ar date of Beckinsale et al. Three zircon fractions from sam ple HMBG were analysed. (1976).On most modern time-scales an age of 567 Ma falls in The data are shown in Table 1 and graphically displayed in th e Late Vendian, taking the Vendian-Camb rian boundary at

Fraction Amount Concentranon- Pb isotop ic compositio ns Radiog enic ratios@ Age and uncerteinty • • prop erties" analysed (ppm) (Ma) (grains) Pb U 206/208 206/20 7 2061204 206Pb'/ 238U 207Pb'/235 U 207Pb'/206 Pb' 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%) 0.05842 (0.18%) 528.5 (z 1.5) 531.7 (± 1.8) 545.8 (± 1.0)

ar=2:1, pale yellow 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, pale brown 64.3 855.3 14.433 16.350 3,515 0.0773 (0.23%) 0.6111 (0.26%) 0.05721 (0.11%) 48 1.1 (± 1.1 ) 484.3 (± 1.3) 499.5 (± 0.6)

f Zirconsmechanically abraded for 6 · 12 hours;ar= aspect ratio prior to abrasion;colourasobservedwith grainsin alcohol. +Concentration is total Pb and inclu des blank Pb, common Pb in zircon, and radiogenic Pb. Total procedu ral blanks are -2 picograms for U and - 10 picograms for Pb. Because weightsof grainsare estimated and a mixed U·Pb tracer wasemployed.uncertainty in estimatesof grainweight affectsonly concentration data,not calculatedU-Pb or Pb/Pb ages. #Measured isotopic ratios priorto correction for mass fractiona tion of -0.11 % per ato mic mass uni t based on replicate analyses of NISTSRM981 and 982 and adjus ted forsmall amount of 206Pb in tracer. @Number in parenthesesispercent uncertainty in the calculated ratio, stated at the 2-sigma level. " Decay constant s: 238U ; I .5S13 E-lO/yr.; 235U = 9.8485 E-l 0/yr. Atom ratio 238U1235U = 137.88. Uncer tainty in the calcula ted ages is stated at the two -sigma level and estimated from combined uncertaintiesin calibrationsof mixed 20SPb- 233U - 235U tracer, measurement of isotopic ratiosof Pb and U, common and laboratory blankPb isotopic ratios.Pband U mass fractionation corr ect ions,and reproducibility in measurement of NIST Pb and U standards.

Table 1. Zircon data, Hamning berg dyke DAVIDROBERTS& NICHOLASWALKER NGU -BU LL 432, 1997 -PAGE 99

.10

% .09

::::> co .08 (') ' C\!

.0 --n, <0 0 .07 C\!

Intercepts at .06 567.1 +39.-- and 392 +2§..- Ma /-23 / -36

.05 '___ __'__--'-__~_--'-_~__.l.__~_ ___J'___ __'__ __'__ ___' .40 .48 .72 .80 Fig.4. U-Pb concordia diagram for th e three zircon analyses from the Hamningberg dyke.The data are presented in Table 1.

545 Ma (Tucker & McKerrow 1995).Asa consequence of this, dy kes on Rybachi which clearly cut across the NW-SE folds it is necessary to reassess certain aspects of the geol ogical and cleavage, at high angles. Based on 4OAr-J9Ar mineral data, history ofthe region. the dykes are evidently older than Mi d Ordovician; and are Although in general field appearance the dated dyke se­ more likely to have intruded at some stage during the ems to be unmetamorphosed, and as such should postdate Vendi an-Cambrian time interval judging from the results of t he spaced cleavage seen in silty pelite interbeds, a defini­ the paleomagnetic investigations (Torsvik et al. 1995). tive mutual relationship cannot be demonstrated at this loca­ It has been argued that the Baikalian structures occur­ lity. Some 2 km to the southeast,however, near Finvik,a similar rin g on Rybachi and Sredni can also be recognised in the NE-SW trending, unmetamorphosed dolerite dyke with a eastern parts ofVaranger Peninsula,especially in the eastern comparable K-Ar age cuts an ESE-dipping cleavage which is BSR (Roberts 1995,1996).This is, in fact, an idea first presen­ axial planar to mesoscopic and mappable c.N-S folds. The ted almost a century ago (Ramsay 1899,Tchernyschev 1901), host rocks here are part of the Kongsfjord Formation.The in­ i.e., tha t the 'Timanian moun tain chain ' could be followed dicat ions are, therefore, t hat t he earliest folding, cleavage from th e Timan s along the northern coastline of the Kola and very low-grade metamor phi sm in this particular area of Peninsu la, and possibly into th e Varanger Penin sula in north ­ Varang er Peninsula are pre-567 Ma, Le. pre-Caledonian. In eastern Norway, following th e same t rase as th e pre-existing this regard, t he Rb-Sr date of 520±47 Ma for th e cleavage in Riphean to Vendian rift basin along the northern and north ­ Kongsfjord Formation pelites reported by Taylor & Pickering eastern margins of th e Russian Platform. (1981) takes on renewed interest. Accepting th e error of ±47 As the arguments for an extension of Baikalian fo lds and Ma, a maximu m age for the cleavage would thus be put at cleavage into NEVaranger are thus quite strong, th en th e U­ 567 Ma. What we clearly need is a more accurate dating of Pb zirco n age present ed here should be viewed in a mo re re­ th is cleavage-forming event, perha ps by the 4OAr-39Ar meth­ gional context . Considering th e group A do lerites, they oc­ od ( a 4O Ar-39Ar cleavage dating project has, in fact, just com ­ cur almos t exclusively in t he nor theastern part s of Varanger menced, on Rybachi Peninsula, with the possibility of future Peninsula,and t rend between NE-SWand c.N-S.The only ex­ exten sion onto the Varanger Peninsula). cepti on is a dyke which intrudes the rocks of the Tanahorn On the Rybachi Peninsula (Fig.1), the turbiditic Upper Nappe in the northwestern BSR.The do lerites occurring in Riphean succession is qu ite pervasively folded and cleaved NW Rybachi also fall into this same category. If the ca.567 Ma along a consistent NW-SE trend (Negrutsa 1971, Roberts zircon age can also be applied generally to this group of rna­ 1995, Roberts &Karpuz 1995). These structures were genera­ fic dykes (but they need not all be of precisely the same age), ted during the Baikalian deformation, an event recognised th en it is tempting to interpret these particular dykes as hav­ also southeast of Kola in the Timans. Biostratigraphic and ra­ ing int ruded either in the terminal stages of, or shortly after diometric dating evidence on Rybachi, and on the nearby the peak ofthe Baikalian tectonothermal event. As the dykes Sredni Peninsula , have constra ined th is deformation and strike at high angles to, and clearly t ransect the Baikalian low-grade metamorphic event to c.580-560 Ma (Roberts folds and cleavage, they can be readily envisaged as having 1995). As noted earlier, th ere are NE-SW trending dolerite been emplaced along major 'ac' tensi onal fractures or faults NGU-BULL 432 . 1997 - PAGE 100 DAVID ROBERTSs NICHOLASWALKER

which developed in assoc iation w ith th e regional folds. It is parabl e Late Vend ian age. Since they intr ude rocks of the in such an extensional sit uation, antithetic to the c1eavage­ Vendian Lokvikfj elle t Group, this is indeed likely. Some dyke and fold-generating compressive stress, t hat one would ex­ intrusion may have exten de d into Cambrian t ime, but the pect rising mafic magma to have met least resistan ce to very ol dest of the K-Ar dates of group B may be the mos t penetration of the uppermost crust. questio nable in view of the biostratig raphic con straint. The 567 Ma age for th e Hamningberg dyke thus fit s qu i­ Although dy ke/fo ld relationsh ip s are not th e subject of te neatly into this general story, w hereby the dyke em place­ this cont ribu t ion , it can be noted th at a fold- related spaced ment would appear to have occurred during the later stages cleavage isoriented subparallel to the ENE-WSW-striking dy­ of the Baikalian tec tonothermal event. The U-Pb age also kes in the Batsfjord district. In several cases it can be seen falls within t he 'age range' for t hese gr oup A dolerit e dykes t hat the regional cleavage bends into perfect parallelism w hich is in di cated by palaeom agne tic data, some of w hich wi t h th e dyke margin, and the dykes, especially the thin ner are yet unpublished (Knutsen, Torsvik & Robert s, in prep.). on es, are we akly cleaved . The preferred interpretati on here Another pi ece of evidence which provides indirect sup port is th at the dykes are pre-c1ea vage/fold ing.This cleavage and for this general reinterpretation of the structural develop­ these folds can be traced southwestwards into t he ment of NEVaranger Peninsula is that of Rb-Sr dating of illite Kongsfjord area where they are more intensively developed fractions from formations in the centr al and eastern parts of and metamorphic grade is sligh tly higher (Roberts 1972). the Tanafjorden-Vara nge rfj orde n Region (Gorokho v et al. In th e Kongsfj ord area, metadolerite dykes (group C) are 1996). Data fro m the Nyborg and Stappogiedde Formations also pro fuse,wi t h fo lds and axial pl anar cleavage subparalle­ of the Vestertana Group indicate an age of burial diagenesis ling the now strongly schistose, ENE-WSW-t rend ing dykes in these very low-grade rocks of ca.560 ± 10 Ma. This was in­ (Roberts 1972, 1975, Rice & Reiz 1994). Although Rober ts terpreted to relate in some way to a regional compaction of (1972) initially favoured a syn-tectonic emplacement age, the succession arising from the effe cts of th e approaching subsequent geochemical data (Roberts 1975), addi ti on al fi­ Baikalian deformation front to th e northeast (Gorokhov et eld studies, and more recent ly a detailed investi gation by al. 1996). Rice & Reiz (1994), has steered interpretat ion in favour of a The U-Pb dating reported here naturally questions the pre-c1eavage/folding origin for the majority of these dykes validity, or rather the interpretation of the K-Ar ages (Late (but see Siedlecka & Roberts 1992). Moreover, the situation Devonian- Early Carboniferous) of the group A dykes of in t he western BSR is such t hat the folds and penetrative cle­ Beckinsale et al. (1976). The 4°Ar_39 Ar laser m icrop robe analy­ avage there - from t he Berle vaq area eastwards via t ical data reported by Rober ts & On stott (1995) from the Kongs fjorden to Batsfjord - are alm ost certainly Caledonian Rybachi dykes also indicated a Late Devonian 'age; but in (Roberts 1996),and probab ly Scandian. Thus, the maj ori ty of this case the data clearly showed that the dyke emplace­ the dykes also in the Kongfjord and Berlevaq swarms could ment age was older t han Mid Ordovician. These authors did, well be of Late Vendian to Cam brian age, and relate to a ten­ in fact, interpret the ca.376 Ma isoch ron age to relate to a sional /transtensional scenario situated close to the j un ction th erm al resetting event associated w it h th e Late Devon ian of t he lapetu s Ocean and Varange r-Timan rift basin to Carboniferous rifting which is known to have occurred ex­ (Siedlecka 1975, Rob erts & Gale 1978, Rice & Reiz 1994). tensively in the southern par ts of the Barents Sea. It does, Whereas the Rybach i and NE Varanger dykes appear to re­ the refore, seem more reasonable that the K-Ar dates derived present magma emplaced in a tens ional situation associa­ for the group A dykes are likely to be a reflection of a weak ted wi th the tail-end of a compressional fo ld belt, the th ermal inpu t arising from th e regi onal, Devon o-Carbon­ swar m-like character of the broadly coeval dykes fur ther to iferous, crusta l exte nsion . the northwest, and also in the higher nappes, may rather be The K-Ar dates of the group C dykes of Beckinsale et ind icative of the more reg ional, upper crustaI extension al.(1976) are kno wn to be highly suspect in view of the ex­ wh ich is kno wn to have affected the Baltos cand ian margin tremely low potassium contents; and some of the ages re­ in the Neoproterozoic rifting phase pr ior to lapetus opening ported exceed th e biostratigraphic age of the metasedi­ (Roberts & Gale 1978, Gayer et al. 1985, Kumpulainen & me nts. However, a recent unpubl ished Srn-Nd date for one Nystuen 1985). of these dykes points to an age of ca.550 Ma (Andersen & Sundvoll 1995). Group B dykes, on the other hand, do app­ ear to have K-Ar ages which, geologically speak ing , are not Conclusions unreasonable. Although ca.650 Ma was said to be «a proba­ A 13.5 m-thick dolerite dyke cutting the lower part of the ble true age», the dates repo rte d do in fact range from Lokvikfjellet Group near Hamningberg in NE Varang er ca.660 to ca.550 Ma, wi t h an error bar of ± 19 Ma (Beckinsale Peni nsula, has yie lded a U-Pb zircon cry stallisation age of et al.1976). These dates, taken at face value , thus overlap 567.1 '30/ " Ma. It is argued that dykes of comparable fiel d with the U-Pb zircon age of the Hamningberg dyke. This character, petrography and chemistry occurring in th is may mean that many, though not necessarily all of the dykes northeastern part of Varanger Peninsula and in north­ in the ENE-WSW-trending Batsfjord swarm may be of a corn - western Rybachi Peninsula, NW Russia,wh ich have previously DAVID ROBERTS & NICHOLAS WALKER NGU-BULL 4 32,1 9 97 - PAGE 101

terazo ic shales, Varanger Peninsula, North Norway. Norge s geologis ­ given K-Ar or 4<>Ar-39Ar, Late Devon ian to Early Carbon iferous ke undersokelse Report 96-154, 28 pp. dates,may be of broadly the same age, i.e. LateVendian. The Johnson, HoO., Levell, B.K.& Siedl ecki, S.1978:Late Precambrian sedimen­ Devono-Carbon iferous dates probably signify a therm al re­ tary rocks in East Finn mar k, North Norway, and their relationship to setting associated with either late-Scandian cooling or a the Trollfjo rd -Kom agelv fault. Journal of the Geological Society of slightly later, regional crustal extension and rifting event. London 135, 517-533. Karpuz , M.R., Robert s, D.,Olesen, 0 .,Gab rielsen, R.H. & Herrevold,T. 1993: Taking into account the regional geological develop­ Application of multiple data sets to struc tural stud ies on Varange r ment, which involves a Late Vendian, Baikalian deformation Peninsul a, northern Nor w ay.InternationalJournal ofRemote Sensing and low-grade metamorphic event on Rybachi and Sredni, 14,979-1003. Knutsen, J.O. 1995: Kompleks flerkomponent magnetisering i and probably also in NEVaranger, then the 567 Ma dyke age Senprekambriske doleritt/ metadoleritt ganger poVarang erhalvoyen. can be seen as relating eithe r to a very late-stage Baikalian Cand.Scient . hovedo ppgave, Universitet i Bergen ,85 pp . event, or an immediately post-Baikalian emplacement. Kragh ,T.E. 1973: A low contamination method for hydrothermal decom­ Following the regional development of NW-SE trending po sit ion of zircon and extracti on of U and Pb for isotopic age deter­ folds and cleavage, mafic magma is envisaged as having mination.Geochimica Comoc himica Acta 37,485-494 . Krogh, T.E. 1982: Im proved accuracy of U-Pb ages by th e creation of penetrated along extensional master fractures and fault s more concordant systems using an air abrasion techni qu e. trending c.NE-SW, i.e.,almost perpendicular to the Baikalian Geochimica Cosmochimica Acta 46, 560-567. fold axes. Kumpulain en, R & Nystuen, J.P. 1985: Late Proterozoic basin evoluti o n Dykes occurring in a swarm in the Batsfjord district may and sedimentation in th e we stern most part of Balto scandi a.l n Gee, D.G.&Sturt, BA (eds.) The Cafedonide orogen - Scandinavia and rela­ be of approximately the same age as the Hamningberg ted areas. John Wi ley & Sons,Chich ester, 213- 232. dyke. Similarly, the dyke swarms in the Kongsfjord and Levell, B.K.& Rob ert s, D. 1977:A re-inv estigatio n of the geology of north­ Berlevaq areasare here suggested to be of broadly the same west Varanger Pen in sula, Finnmark, Nor th Norwa y. Norges geolog­ age. In contrast to the Hamningberg dyke and the dykes on iske uridersekelse 334, 83-90. Negrutsa, V.Z. 1971: Stratigraphy of hyp erbo rean deposits in the Sredni Rybachi, however, these dyke swarms were probabl y intru ­ and Rybac hi Peninsu la and in the Kildi n Island. VSEGEI 175, 153-186. ded in a more regionall y extensile regime and were also (In Russian) subsequently affected by penetrative Caledonian deforma­ Parrish, R.R, Roddic k, J.e., Loveridge, WOO. & Sullivan, R W. 1987: Uranium ti on and metamorphism. lead analytical techniques at th e geochranology labora tory. Geolog ical Survey of Canada Paper 87-2, 3-7. Pickering, K.T. 1981: The Kongsfjord Form ation - a Late Precam bria n submarin e fan in nor theast Finnm ark, North Norway. Norges geolo­ Acknowledgements giske underseketse 367, 77-104. We are gratefu l to Ste phen Lippard, Hug h Rice, Brian Sturt and Bob Ramsay,W.1899: Neue Beitraqe zur Geologie der Haalb in sel Kola.Fennia Tucker fo r their critical and con structi ve comments on either earlier or 15,4, 1-15. revised versio ns of the manuscript. Irene Lundqvist kindly drafted th e Rice,A.H.N.& Reil,W. 1994:The struct ural relati ons and regional te ctonic fig ures. implicat ions of met adolerit e dykes in the Kongsfjord Forma tio n, North Varang er Reg ion , Finnmark, N.Norway. Norsk Geologisk Tidsskrift 74, 152-165. Rice, A.H.N., Gayer, RA ., Robinson , D. & Bevins, R.E. 1989a: Str ike-slip re­ References storation of th e Barent s Sea Caledonid es terrane, Finnmark, North Andersen,T.&Sundvoll, B. 1995: Neodymium isotope systematics of th e Nor way. Tectonic s 8, 247-264. mantle beneath the Baltic Shield : evid ence for depl eted mantle Rice, A.H.N., Bevin s, R.E., Robinson, D. & Rob erts, D. 1989b: Evolution of evo lution since th e Archaean. Lithos 35, 235-243 . low-grade metamorphic zon es in th e Caledonides of Finnmark, Beckinsale, RoO., Readi ng, H.G.&Rex, D.e. 1976:Pot assium -argon ages for N.Norway. In Gayer,RA (ed.) The Caledonide geofog y ofScandinavia. basic dykes from East Finnm ark: stratigraphic and struct ural im pli­ Graham & Trotman, Londo n, 177-191. cat ions.Scottish Journal of Geology 12,51-65. Roberts, D. 1972: Tecton ic defor mation in th e Barent s Sea Reg ion of Bertrand-Sarfati, J. &Siedlecka, A. 1980: Columnar stromatolites of th e Varanger Peninsula, Finnm ark. Norges geologiske undersekelse 282, term inal Precambrian Porsanger Dolo mite and Grasdal Formation 1-39. of Finnmark, North Nor way.Norsk Geologisk Tidsskrift 60,1 -27. Roberts, D. 1975:Geochemistry of doler it e and metadol erit e dy kes from Dalrym ple, G.B. 1979: Crit ical tabl es for conversion of K-Ar ages from old Varanger Peninsul a, Finnmark, north Norway. Norges geologiske un­ to new constants.Geology 7, 558-560. dersokelse 322, 55-72. Edwards, M.B. 1984: Sedim ent ology of the Upper Proterozoic glacial re­ Robert s, D. 1993: Aspect s of th e structural geology of Rybachi and cord , Vestertana Group, Finnmark, Nort h Norway. Norges geologiske Sred ni Peninsulas, and some comparisons with Varanger Peninsula . undersokelse 394, 76 pp . (Abst ract). First International Barents Symposium , Kirkenes, Nor way, Farmer,J.,Vidal,G., Moczyckowsk a, M., Strauss,H., Ahlberg, P.&Siedle cka, October 1993, Abstract Volume. Norges geologiske undersekelse. A. 1992: Ediacaran fossils fro m the Innerelv Member (Late Robert s, D. 1995: Principal features of the structu ral geology of the Prot erozo ic) of th e Tanafjorden area, northeastern Finnmark. Ryb achi and Sred ni Peninsul as, Nor thwest Russia, and some com­ Geological Magazine 129, 181-195. paris ons w ith Varang er Pen insu la, Nort h Norway.Norges geologiske Gayer, RA, Humphreys, RJ., Binns, RE. Chapman, T.J. 1985: Tectonic & undersekelse Special Publication 7, 247-258. modelling of the Finnmark and Tram s Caledonid es based on high ­ Rob ert s, D. 1996: Caledonian and Baikalian te cto nic stru ctures on level igneous rock chemistry. In Gee, D.G. & Sturt , BA (ed s.) The Varang er Pen insula, Finnmark. Nor th Nor way, and coastal areas of Caledonide orogen - Scandinavia and related areas. John Wiley & Kola Peninsula, NW Russia. Norges geolo giske undersokelse Bull etin Sons, Chich ester, 931-951. 431, 59-65. Gorokhov, I.M., Sied lecka, A., Robert s, D., Melnik ov, N.N.,Turchenko,T. N., Roberts, D. & Gale. G.H. 1978: The Caledonian-Appala chian lapetus Konstantino va,G.V., Kuty avin, E.P.& Soch ava,A.V. 1996:ARb-Sr inve ­ Ocean .In Tarling, D.H.(ed.) Evolution of the Earth s crust. Academic stigation of iIIite-forming even ts in diage nesis-g rade Neopro- Press, London, 255-342. NGU - BU L L 43 2 , 199 7 - PA GE 102 DA VID ROBERTS s NICHOLAS WA LKER

Roberts, D. & Gee, D.G. 1985: An introducti on to the struc ture of th e Peninsula and Kildin Island in the northern coastal area of Kola Scand inavian Caledonides. In Gee, D.G. & Sturt, BA (eds.) The Peninsula in Russia. Norges geo log iske undersoketse Special Caledonide orogen - Scandinavia and relat ed areas. Joh n WHey & Publication 7, 217-232. Sons, Chichester, 55-68. Siedlecka, A.,Negrutsa, V.Z. & Pickeri ng, K.T. 1995b: Upper Prot erozoic Roberts, D.& Karpuz, M.R. 1995: Structura l features of the Rybachi and turbidi te system of the Rybachi Peninsula, northe rn Russia - a po s­ Sredni Peninsulas, Northwest Russia, as interpreted fro m Land sat­ sible stratigraphic counterpart of the Kongsfjo rd Submar ine Fan of TM imagery. No rges geologiske undersokelse Special Publication 7, the Varanger Peninsula, nor thern Norway. Norges geo log iske under­ 145-150. sokelse Special Publica tion 7, 201-216. Roberts, D. & Onstott, T.e. 1995: " Arl"Ar laser mi croprobe analyses and Siedlecki, S. 1980:Geologisk kart over Norge, bergg runnskart Vadso M ­ geo chemistry of dolerite dyk es from th e Rybachi and Sredn i 1:250 000.Norges geologiske undersokelse. Peninsulas, NW Kola, Russia. Nor ges geo log iske undersokelse Special Siedl ecki, S. & Levell, B.K. 1978: Lithostratigraphy of the Late Publication 7,307 -314. Precambrian Lokvikfjell Group on Varanger Peninsula, East Roberts, D., Mitch ell,J.G.& Ande rsen,T.B. 1991:A post-Caledonian do leri­ Finnmark, Nor th Norway.Norges geologiske undersekelse 343,73 -85. te dyke from Mage roy, North Norway :age and geochemi stry.Nor sk Stacey, J.5. & Kramers, J.D. 1975: Approximation of terres tr ial Pb isotope Geolog isk Tidsskriit 71, 289-294. evol ution by a two-stage model. Earth and Planetary Science Letters Roberts, D., Rex, D.e. & Guise, P.G. 1995: " Ar-"Ar analyses of pyroxenes 26,207-221. fro m dol erite dykes from southeastern Varanger Peninsula, Sturt, B.A., Pringle, I.R..& Roberts, D. 1975:Caledonian nappe sequence of Finnmark.Norges geologiske undersekelse Report 95.158,14 pp. Finnmark, northern Norway, and th e timing of the orogenic defor­ Siedlecka, A. 1972: Kongsfjord Formation - a late Precambrian f1ysch se­ mation and metamorphism. Bullet in of the Geolog ical Society of quence from th e Varanger Penin sula, Finnmark. Norges geologiske America 86, 710-718. undersekelse 278,41 -80 . Taylor, P.N. & Pickering, K.T. 1981: Rb-Sr isotopic age determination on Siedlecka, A. 1975: Late Precambrian st ratigraphy and structure of the the Late Precamb rian Kongsfjo rd Formation, and the timi ng of north eastern marg in of the Fenn oscand ian Shield (East Finnmark ­ compressional defor mation in the Barents Sea Group, East Timan Regio n). Norges geolog iske undersokelse 316, 313-348. Finnmark.No rges geologiske undersokelse 367, 105-110. 5iedlecka, A. 1984: Syltefjo rd, fo relopig berggrunnsgeologisk kart 2436 Torsvik, T.H., Rob erts, D. & Siedlecka, A. 1995: Palaeomagnetic data from 11 , 1:50 000.Norges geologiske undersekelse. sedimentary rocks and dolerite dykes, Kild in Island, Rybachi, Sredni Siedlecka, A. 1985: Development of the Upper Proter ozoic sedimentary and Varanger Peninsulas, NW Russia and NENorway: a review. basins of the Varanger Penin sula, East Finnmark, Nor th Norway. Norges geologiske undersokelse Specia l Publicatian 7,315-326. Bulletin ofthe Gealo gical Survey of Finland 331, 175-185. Tschernyschev,T. 1901:On geological structure of the Timans and on re­ Siedlecka, A.& Siedlecki, S. 1967: Some new aspects of the geology of lation of the Timan Fault to other region s of nor thern Europe. th e Varanger Peninsula (Northern Nor way). Norges geolo giske un­ Zapiski Min. Obstwa 34, 29-33. (in Russian) dersokelse247, 288-306. Tucker, R.D.& McKerrow,W.5. 1995: Early Paleozoic chronology: a review Siedl ecka, A.& 5iedlecki, S. 1971:Late Precambrian sedimentary rock s of in light of new U-Pb zircon ages from Newfoundland and Britain. th e Tanafjord-Varange rfjord regi on ofVarange r Peninsula, northern Canadian Journal ofEarth Sciences 32, 368-379. Norw ay.Norges geo logiske undersekelse 269, 246-294. Vidal, G. 1981: Micropalaeontology and biostratigr aph y of the Upper Siedlecka, A. & Robert s, D. 1992: The bedrock geology of Varanger Protero zoic and Lower Cambrian sequence in East Finnmark, nor­ Peninsu la, Finnmark, North Norway:an excu rsion gu ide.Nor ges qeo­ thern Norway.Norges geolog iske undersokelse 362, 63 pp. lo giske unde rsokelse Specia l Publi catio n 5,45 pp. Vidal, G.& Siedlecka, A. 1983: Planktonic, acid-res istant m ic r 0 f 0 s s i I s Siedlecka, A., Lyub tsov, VV. & Negrutsa, V.Z. 1995a:Correlation bet ween from the Upper Protero zoic strata of the Barent s Sea Region of the Upp er Proterozoic successions in th e Tanafjorden-Varangerfjorden Varang er Peninsula, East Finnmark, nor thern Norway. Norges qeo lo­ Region of Varang er Peninsula, northern Norway, and on 5redni giske unde rsokel se 382, 145-179.

Manuscript received February 1997; revised manuscrip t accepted May 1997.