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.