U-Pb Zircon Age of a Dolerite Dyke from Near Hamningberg, Varanger
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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.