NORWEGIAN JOURNAl OF GEOLOGY Ar-Ar doting of Coledonion ond Grenvillion rocks, Svalbard 263
Ar-Ar dating of Caledonian and Grenvillian rocks from northeasternmost Svalbard - evidence of two stages of Caledonian tectonothermal activity in the high Arctic?
Åke Johansson, Henri Maluski & Da vid G. Gee
Johansson, Å., Maluski,H. & Gee, D.G. Ar-Ar dating of Caledonian and Grenvillian rocks from northeasternmost Svalbard - evidence of two sta ges of Caledonian tectonothermal activity in the high Arctic ? Norsk Geologisk Tidsskrift,Vol. 81, pp. 263-281. Trondheim 200 l. ISSN 029-196X.
Ar-Ar analyses have been carried out on Caledonian and Grenvillian rocks from the northernmost Barents Shelf (approx. 80"N) on Nordaustlan det and Kvitøya, northeast Svalbard. Five muscovites and one biotite from Caledonian granitoids, three muscovites and one biotite from Grenvil lian granitoids, and two samples of hornblende from mafic rocks of uncertain age, were analysed using stepwise laser heating of single grains. The Grenvillian granitoids, as well as the mafic rocks, yield ages between 410 and 425 Ma, interpreted to date cooling afterCaledonian regional meta morphism; they show no traces of a Grenvillian argon component. Thus, Caledonian metamorphism in central and eastern Nordaustlandet must have reached above the closure temperature for argon in these minerals. In the Caledonian granitoids, two age groups may be discerned: one with Ar-Ar ages in muscovite between 415 and 430 Ma, and another with muscovite and biotite ages around 400 Ma. The first group is related to grani tic and aplitic rocks having Late Ordovician to Early Silurian U-Pb monazite ages (ca. 440-460 Ma); the second group includes the large Rijpfjor den granite with an Early Devonian U-Pb monazite age of 412 Ma. Thus, prolonged tectonothermal activity during the Caledonian orogeny may be discerned in northeasternmost Svalbard, possibly divisible into two stages: one stage with regional metamorphism and syn-tectonic magma tism occurring in the Late Ordovician to Early Silurian, and a second stage with late- or post-tectonic magmatism in the Early Devonian. For each stage, the Ar-Ar age follows the U-Pb age with a delay of 10-20 Ma, corresponding to rapid cooling of 20-45 °C per million years from near-mag matic temperatures to the closure temperature of argon in these minerals.
Ake Johansson, Laboratory fo r Isotope Geology, Swedish Museum of Natura/ History, Box 50 007, S-104 05 Stockholm, Sweden (e-mail: ake.johans [email protected]). Hen ri Maluski, Laboratoire de Geochronologie, Institut des Sciences de la Terre, de l'Eau et de l'Espace de Montpellier, Universite Mont pellier II, Place Eugene Bataillon, F-340 95 Montpellier Cedex05, France ( e-mail: [email protected]). David G. Gee, Department of Earth Sciences, Uppsala University,Villavagen 16, S-752 36 Uppsala, Sweden (e-mail: [email protected]).
lntroduction (1994), indicated that upliftand cooling afterpeak meta morphism took place in the Late Silurian to Early Devo The Svalbard archipelago is located in the northwestern nian, at around 420-410 Ma. Conventional U-Pb dating corner of the Barents Sea Shelf and the Eurasian Plate, in of titanites from the sheared Palaeoproterozoic grani a key area for restoring Proterozoic and Palaeozoic oro toids indicated a slightly younger, early Devonian (ca. gens in the North Atlantic and Arctic regions. The Sval 410 Ma) lower intercept age, interpreted to be related to bard Caledonides were most likely assembled during metamorphic recrystallization and/or lead loss at a late mid-Palaeozoic orogeny from a variety of terranes, with stage of the Caledonian metamorphism (Johansson et different pre-orogenic evolutions (Harland 1997). Sval al. 1995), and similar lower intercept ages were also bard's Eastern Terrane (Fig. l) is composite and may be recorded for zircon in some samples (Larionov et al. subdivided in to a West Ny Friesland Te rrane and a Nord 1995; Johansson & Gee 1999). austlandet Te rrane (Gee et al. 1995; Witt-Nilsson 1998). The Nordaustlandet Terrane is characterized by a We stern Ny Friesland is characterized by a Caledonian Grenville-age basement complex, overlain by Neoprote thrust pile, the AtomfjellaComplex, exposed in the N-S rozoic to Ordovician platformal sediments and intruded trending Atomfjella Antiform, with intercalated base by Caledonian granitoids. Separation of Grenvillian and ment gneisses and granitoids, mainly oflate Palaeoprote Caledonian granitic magmatism has only been possible rozoic age, and Mesoproterozoic or younger cover rocks through recent U-Pb dating (Gee et al. 1995; Gee et al. (Witt-Nilsson et al. 1998, and references therein). These 1999; Johansson et al. 2000; Johansson et al., submitted, rocks were metamorphosed in amphibolite facies and in prep.). Caledonian metamorphism on Nordaustlandet pervasively deformed during Caledonian transpression. is generally in greenschist facies, except in some areas in 40Af-39Ar ages on hornblendes and muscovites from the central and eastern parts where the grade increases northwestern Ny Friesland, presented by Gee & Page and Caledonian migmatization influences the deeper 264 Å. Johanssonet al. NORWEGIAN JOURNAL OF GEOLOGY structural levels (Johansson & Larionov 1999; Tebenkov The oldest exposed rock unit is the Mesoproterozoic et al. 1999; Tebenkov et al., submitted). Caledonian BrennevinsfjordenGro up, composed mainly of phyllitic deformation is generally also less pervasive than in Ny turbidites and sandstones, deposited some time between Friesland, mainly resulting in upright to W-vergent anti 1100 Ma (age of youngest detrital zircon grains; A. Lari fo rms and synforms. onov, unpublished Pb-Pb data) and 960 Ma (age of Nordaustlandet's Early Palaeozoic successions and unconformably overlying Kapp Hansteen Group volca faunas, underlain by characteristic Neoproterozoic tilli nics; Johansson et al. 2000). The Kapp Hansteen Group tes, carbonates and clastic formations, compose a strati is composed of andesitic volcanic and volcaniclastic graphy that closely correlates with the Laurentian mar rocks with related intrusive quartz porphyry stocks, gin of East Greenland. Even the Grenville-age basement dated to 940-960 Ma (Johansson et al. 2000), and intru of late Mesoproterozoic metasediments, intruded by ca. ded by granites of similar age (Kontaktberget and Lapo 950 Ma granites, is present in central East Greenland niafjellet granites on the Laponiahalvøya peninsula, Gee (Steiger et al. 1993; Strachan et al. 1995; Watt et al. 2000). et al. 1995; augen gneisses of central Nordaustlandet, The evidence fo r Laurentian affinities fo r Nordaustlan Johansson et al. 2000). Geological relations in central det, taken together with W-vergent fo lding and increase Nordaustlandet suggest that their transformation to of Caledonian metamorphic grade towards the east, sug augen gneisses occurred during Grenvillian deformation gests that the axial zone of the Caledonides lies furtherto (Tebenkov et al., submitted). the east in the Barents Shelf. A major zone of Iapetus This Grenvillian basement complex is unconforma suturing can thus be expected to separate the Nordaust bly overlain by the Neoproterozoic Murchisonfjorden landet Te rrane fromthe continental margin of Baltica as Supergroup and the Vendian to mid-Ordovician Hinlo it is represented in Scandinavia, the Urals and Novaya penstretet Supergroup. Caledonian granites are difficult Zemlya. to separate from Grenvillian granitoids in the field, since In this paper, we present new 40Af_39Ar results on they rarely show cross-cutting relations to the Neoprote muscovites, biotites and hornblendes from Caledonian rozoic and younger sedimentary rocks, and the degree of and Grenvillian rocks (according to U-Ph age determi deformation is variable. U-Ph isotopic dating yields nations) from the Nordaustlandet Terrane, as a comple Caledonian ages of 410-420 Ma fo r the anatectic Rijp ment to previous and on-going U-Ph studies. The aim fjorden granite batholith in Prins Oscars Land (Johans has been two-fold: (l) to study the Caledonian tectono son et al., submitted) and the high-magnetic Djupkil thermal evolution of Nordaustlandet and relate it to that sodden pluton in southern Duvefjorden ( Gee et al. of western Ny Friesland, already documented by Ar-Ar 1999). Similar or slightly higher ages are also indicated studies, and (2) to investigate whether traces of the ear by U-Ph studies on the Nordkapp granite on Laponia lier, Grenvillian, evolution have been preserved in the Ar halvøya, the Winsnesbreen granite in central Nordaust Ar system in the less pervasively deformed and meta landet, and other smaller massifs and dykes of relatively morphosed Nordaustlandet Te rrane. undeformed granite and aplite in the central and eastern areas (Johansson et al., submitted, in prep.). Preliminary Ph-Ph data suggest that the tectonothermal activity in the east may have occurred slightly earlier, with migma Geology of Nordaustlandet tization at 440-450 Ma, and cross-cutting aplitic dykes intruding at c. 430 Ma (Johansson & Larionov 1999, in Nordaustlandet fo rms the eastern part of Svalbard's Eas prep.). tern Caledonian Terrane (Fig. 1). Caledonian and older Mafic rocks are sparse. Disregarding the Mesozoic rocks are exposed along the north coast of Nordaustlan dolerites along Hinlopenstretet and on the northern det, in an ice-free strip in central Nordaustlandet, and in tip of Botniahalvøya, mafic rocks mainly occur in the scattered outcrops in easternmost Nordaustlandet and fo rm of metagabbros and subordinate amphibolites in on the smaller islands towards the north and east, most the easternmost areas: eastern Orvin Land, Nord notably Kvitøya (Fig. l); the remaining part of Nordaust marka, Isispynten, Storøya, and Kræmerpynten on landet is covered by two large ice-caps (Vestfonna and Kvitøya (Hjelle et al. 1978; Ohta 1978). Based on their Austfonna). Towards the south, the Caledonian base generally little deformed nature, Ohta (1978) conside ment is overlain by Carboniferous and younger sedi red the gabbros Caledonian, but their ages are not mentary rocks. Detailed accounts of the Caledonian geo known with certainty. logy of Nordaustlandet are given by Flood et al. (1969) Caledonian folding along N-S-trending axes gave rise and Ohta (1982), with shorter summaries in the map to the present outcrop pattern of the rocks, with the description by Hjelle & Lauritzen (1982) and Lauritzen Grenvillian complex as well as the Caledonian granites & Ohta (1984). More recent descriptions, with a fo cus being exposed in broad antiforms in Botniahalvøya - on structural geology and/or isotopic dating, are fo und Laponiahalvøya, central Nordaustlandet, and the fa r in Gee et al. (1995), Gee & Tebenkov (1996), Gee et al. east, and the Neoproterozoic and younger fo rmations (1999) and Johansson et al. (2000); only a short sum being preserved in the intervening synforms, most nota mary is given below. bly the Hinlopenstretet Syncline (Fig. 1). A major NORWEGIAN JOURNAl OF GEOlOGY Ar-Ar dating of Caledonian and Grenvillian rocks, Svalbard 265
18 E 20E � 22E 24E 26E 28E 30E 32E
<;\l� o 20 40 60 80 lOOkm
NORDAUSTLANDET Austfonna Mesozoic doler ites
g carboniferous-Jurassic sediments
!!!!]Cal edonian granites
- Gab bros of uncertain age
� Hinlop_enstretet S_upergroup � (Vendtan-Ordovtctan) � Murchisonfjorqen Supergroup � (Neoproterozotc)
Caledonian granitoids :•:•:• Grenvillian Kontaktberget Gran i te Fig. l. Simplified geologicalmap ofNordaustlandet and adjacentislands, based on Flood et al. (1969), Hjelle & Lauritzen (1982), Lauritzen & Ohta (1984) and recent SWEDARCTICmapping, with sample locations indicated. Inset map shows the Caledonian terranes of Svalbard (from Gee 1986). Caledonian granitoids on inset map: H = Hornemantoppen batholith, N = Newtontoppen granite, R = Rijpfjorden granite; major Caledonian fa ult lines: BP= BillefjordenFault, BBF = Breibogen-BockfjordenFault, RF= RaudfjordenFault. unconformity separates the Neoproterozoic sequence vites from mica schists (2 samples). These yielded pla from the underlying units in central Nordaustlandet teau or "near plateau" ages in the 413 to 425 Ma range, (Gee & Tebenkov 1996). Subsequently, the whole com with one older hornblende age of 447 ± 5 Ma. plex became slightly tilted towards the south, perhaps in connection with Te rtiary upliftrelated to the opening of the North Atlantic and Arctic Oceans; the late Palaeozoic and Mesozoic cover rocks are still preserved on southern lnvestigated rocks Nordaustlandet, but have been eroded away from the northern part. Te n rock samples were used fo r Ar-Ar analysis: fivesam Early K-Ar dating of rocks from Nordaustlandet, ples of Caledonian granitic rocks, three samples of Gren summarized by Gayer et al. (1966) and recalculated by villian granitoids transformed into augen gneisses, and Ohta (1994) to the decay constants recommended by two samples of gabbro and amphibolite from eastern Steiger and ]iiger (1977), gave Caledonian ages in the 345 most Nordaustlandet and Kvitøya (Fig. l). Both the - 445 Ma range. From northern Ny Friesland, Gee & Page Caledonian and Grenvillian granitoids are anatectic two (1994) presented modem stepwise heating Ar-Ar data on mica granites, derived from crustal precursors, lacking horn blendes fromamphibolites ( 6 samples) and musco- hornblende or other amphiboles (see Johansson et al. 266 Å. Johansson et al. NORWEGIAN JOURNAL OF GEOlOGY 2000, fo r a discussion of the geochemistry and origin of containing quartz, K-feldspar and plagioclase in a relati the Grenville granites). From all eight granite samples, vely even-grained mosaic with grain sizes up to 2 mm. muscovite was analysed, whereas biotite was analysed Muscovite and biotite occur in about equal amounts as from only one each of the Caledonian and Grenvillian grains less than l mm, showing no preferred orientation, granitoids. From the two mafic rock samples, only horn both appearing fresh and unaltered, except fo r the pleo blende was analysed. chroic halos surrounding possible zircon grains within the biotites. Quite often, muscovite and biotite occur intergrown as parallel laths within the same aggregate. The muscovite grains recovered during mineral separa Caledonian granitoids tion were substantially larger than those observed in the Sample 28-1 is derived fromVi ndbukta dose to the nor thin section, with grain sizes of several millimetres. thern margin of the Rijpfjorden granite massif (Fig. 1). Sample G95:030 is from a red, undeformed sheet of The Rijpfjorden granite is a generally undeformed, pink, granite cutting migmatites and augen gneisses south of crustal anatectic two-mica granite containing abundant Innvika in inner Duvefjorden; it is possibly related to the xenoliths (Hjelle 1966). It also contains abundant inheri nearby Caledonian Rijpfjorden granite. However, U-Pb ted zircons, and conventional U-Pb multigrain dating analysis of monazite suggests a slightly higher age: two has proved difficult; however, single-zircon Pb-evapora discordant fractions yield 207Pbf206Pb ages of 440-450 tion and ion microprobe spat analyses indicate ages of Ma, and if regressed together, an upper intercept of 438 400-430 Ma fo r granite emplacement, and monazite has ± 5 Ma (Johansson et al., submitted). The sample con yielded a concordant conventional U-Pb age of 412.5 ± sists of a mosaic of up to 4 mm large grains of K-feld 0.5 Ma (Johansson et al., submitted). Sample 28-1 itself spar, plagioclase and quartz, with muscovite occurring as consists of an even- and medium-grained mosaic of up to 2 mm large hypidiomorphic flakes, and biotite as quartz, K-feldspar and plagiodase, with muscovite scat smaller irregular brown flakes, sometimes intergrown tered around as ca. l mm large flakes without any prefer with muscovite. Neither of the micas show any preferred red orientation. Biotite was also recovered during mine orientation in thin section. ral separation, and was used alongside with muscovite Sample G95:051 is from an aplite dyke cross-cutting fo r Ar-Aranalys is. However, in thin section, no freshbia the augen gneiss in the southern part of Nordmarka, a tite could be seen; only altered grains containing a mix small ice-free area in easternmost Nordaustlandet (Fig. ture of almost colourless mica and opaque material, pro 1). U-Pb analysis of three discordant monazite fractions bably iron oxides and hydroxides, were observed. yielded zo7pbfZ06Pb ages between 464 and 484 Ma and an Sample 94047 is derived from the Winsnesbreen gra upper intercept age of 463 ± 9 Ma (Johansson et al., in nite, a smaller massif of granite of presumed Caledonian prep.). Single-zircon Pb-evaporation analyses on similar age occurring in the southern part of central Nordaust ap litie dykes from Andreeneset on Kvitøya suggest an age landet, apparently as a southern extension of the Rijp of ca. 430 Ma (Johansson & Larionov 1999, in prep.). fjorden granite (Fig. 1). U-Pb ion microprobe spot ana The investigated sample consists of a medium- and lyses of zircons have produced a wide array of ages, while even-grained mosaic of K-feldspar, plagioclase and conventional U-Pb analyses of monazite yielded one quartz, lacking any fabric or foliation. Biotite occurs as concordant point at 422 Ma, and two discordant points thin laths up to 2 mm long without orientation, ftlling with 207Pbf206Pb ages of 440-460 Ma (Johansson et al., the interstices between the feldspar crystals, while mus submitted). The sample consists of medium-grained, covite occurs as a few more scattered flakes, less than l light pink to grey granite with a marked foliation defined mm, also lacking any preferred orientation. by the mica. In thin section, this fabric is also seen as mm-wide lenses of quartz and K-feldspar, set in a finer matrix of quartz, K-feldspar and plagioclase. Muscovite occurs as flakes of varying size, up to 3 mm large, weakly Grenvillian granitoids oriented after the gneissic fabric. No biotite was observed Samples G95:049 and G95:050 are both augen gneisses in thin section, and opaque minerals occur in very sub from the southern part of the above-mentioned Nord ordinate amounts. marka area of easternmost Nordaustlandet. A geological Sample 94048 is derived from the Nordkapp granite description of Nordmarka and the augen gneiss (porphy on northern Laponiahalvøya, where associated pegmati ritic granite) is fo und in Hjelle et al. (1978). U-Pb zircon tes cross-cut the Grenvillian Laponiafjellet augen gra ion microprobe dating and single-zircon Pb-evaporation nite. AI; with the Rijpfjorden granite, it contains abun dating suggest a protolith age of ca. 950 Ma (Johansson et dant inherited zircons, but single-zircon Pb-evaporation al., in prep.), similar to the age of the Fonndalen and dating suggests an emplacement age of 424 ± 14 Ma, Ringåsvatnet augen gneisses of central Nordaustlandet whereas discordant monazite yields a somewhat older U (Johansson et al. 2000). Sample G95:049 contains quartz Pb upper intercept age of 440 ± 3 Ma (Johansson et al., and K-feldspar, with the fo rmer mineral fo rming elonga submitted). Sample 94048 from Gryteberget is a light ted lenses outlining the gneissic fabric, as well as musea grey, medium-grained, massive and leucocratic granite, vite and biotite as elongated crystals that occur together NORWEGIAN JOURNAL OF GEOLOGY Ar-Ar dating of Caledonian and Grenvillian rocks, Svalbard 267 fo rming narrow hands along the fo liation. Sample Sample 598:129 from Andreeneset on Kvitøya is a G95:050 consists of quartz and partly megacrystic (cm fo liated, medium-grained amphibolite, consisting of sized) K-feldspar and plagioclase, but lacks strong fo lia fresh hornblende (about 50 %), plagioclase and K-feld tion or fabric. Biotite occurs as up to l mm long crystals spar, with minor olivine and opaque minerals. fo rming oriented aggregates outlining a weak foliation. Muscovite occurs as up to l mm flakesin aggregates lac king any preferred orientation, but also as small mica inclusions (coarser than normal sericite) following diffe Analytical procedures rent crystallographic directions in the feldspar mega crysts. The muscovite and biotite in both samples are Muscovite, biotite and hornblende were separated at the fresh and unaltered, except fo r dark pleochroic halos sur Laboratory fo r Isotope Geology, Swedish Museum of rounding zircon inclusions in the biotite. A few garnet Natura! History, Stockholm, using standard mineral crystals were observed in the thin section in sample separation techniques (Frantz isodynamic magnet sepa G95:049; in addition, apatite is a common accessory rator, heavy liquids, and vibrating "mica table"). Final phase in both samples and zircon occurs in small selection of grains sent fo r irradiation took place at the amounts. From sample G95:050, both muscovite and Laboratoire de Geochronologie, Universite Montpellier biotite were analysed; from G95:049 only muscovite was Il, where the subsequent analytical work was also done. analysed. Irradiation took place at the McMaster reactor in Sample G95:031 is from a similar augen gneiss, south Canada, with an irradiation time of 2.5 days. As monitor, of Innvika, in central Nordaustlandet. It consists of up to the MMHb-1 hornblende standard was used (Alexander 5 mm 'ong K-feldspar augen in a medium-grained et al. 1978), with an accepted age of 520.4 ± 1.7 Ma mosaic Jf quartz and plagioclase with a weak gneissic (Samson & Alexander 1987). Analysis of single mineral fabric. Muscovite occurs as elongated small crystals fo r grains, about 1-3 mm in diameter, took place using an ming narrow hands that define the fo liation. Biotite OptiLas Lexel 3500 continuous argon laser fo r stepwise sometimes occurs together with the muscovite, but also heating and a mass spectrometer of model 215-50 from fo rms separate irregular aggregates of anhedral brown Mass Analyser Products equipped with an electron crystals lacking clear orientation. multiplier fo r the mass analysis. For each heating step, lO scans of the masses 40, 39, 38, 37 and 36 with back grounds in between were made. The initial intensity fo r Mafic rocks of uncertain age each mass at the time when the mass spectrometer was Metagabbroic rocks occur in the eastern part of Orvin equilibrated with the inlet section was computed using Land, in Nordmarka, on Isispynten in easternmost linear regression, and used fo r furthercalculat ion. Every Nordaustlandet and on Storøya and Kv itøya (Kræmer fo urth step, a blank analysis was made. The obtained pynten) east thereof (Hjelle et al. 1978; Ohta 1978). The values were corrected fo r blank and atmospheric argon, large layered gabbro on Storøya (more than 7 x 11 km, as well as Ca- and Cl-derived neutron-induced argon the size of the island) was described by Ohta (1978), who using the masses 3 7 and 38, befare calculation of argon considered it Caledonian due to its well-preserved isotope ratios and ages. The reported errors are l stan nature, but independent isotopic evidence concerning dard deviation, and include uncertainties in the J-factor the age of these rocks is still lacking. For this study, a gab (radiation). A detailed description of the analytical bro fromIsisp ynten containing the most unaltered horn methods used is fo und in Monie et al. (1994). blende was selected fo r analysis, whereas hornblende in gabbro samples from Storøya and Kræmerpynten on Kv itøya was either more altered or intergrown with other minerals, and therefore not analysed. In addition, horn Results blende from a foliated amphibolite occurring as a lens within the migmatitic gneisses on Andreeneset on the The isotope ratios and ages obtained fromeach step are west tip of Kvitøya, was also analysed. reported in Appendix l, and resulting plateau ages, total Sample 94062c from Isispynten is a medium- grained, gas ages and isochron ages fo r each sample are summari massive gabbro, dominated by hornblende occurring as zed in Table l. The age spectra are illustrated in Figs. 2-4. mm-sized pale green crystals intergrown with brown For most samples, two plateau ages have been calculated, biotite. Plagioclase and opaque minerals occur in subor one broad plateau with contiguous steps corresponding dinate amounts. The biotite ranges from completely to more than 90 o/o of the released 39Ar, and o ne more fresh to relatively altered; the large amphibole crystals restricted, but more well-defined,plateau, encompassing show some incipient alteration along the partings. Bet between 40 and 70 o/o of the released 39 Ar. However, the ween the larger crystals, there are zones of fine-grained differences between the two plateau ages, or between the amphibole, biotite, plagioclase, and possibly olivine, plateau ages and the total gas ages, are not significant. which appear cataclastic and altered, with dark rusty The isochron ages, both normal (40Arf36Ar vs. 39Arf36Ar material along grain boundaries. using the regression calculation of Yo rk 1969) and inver- 268 Å. Johanssonel al. NORWEGIAN JOURNAL OF GEOLOGY ted (36Arf40Ar vs. 39Arf40Ar following Turner 1971, Rod Ma, and the aplite from No rdmarka (sample G95:051) dick 1978, and Roddick et al. 1980) have been calculated yields a 417 ± 5 Ma muscovite age. The U-Pb monazite using the same steps as the "broad plateau age': but may ages available from these rocks are uncertain because of still deviate somewhat due to outlying points. In the case discordant analyses, but suggest similar or even higher of the inverted isochrons, most analyses p lot in a duster ages than fo r the Nordkapp granite. dose to the intercepts with the x-axis, so that an isochron age can be determined with quite good precision. Because of this dustering, the slopes of the isochrons Grenvillian granitoids and the y-intercepts, however, become badly defined, In none of the analysed Grenvillian granitoids are any tra irrespective of MSWD value. The y-intercept ideally ces of Precambrian Ar-Ar ages preserved; they all yield should fall at the composition of atmospheric argon relatively well-definedCaledonian plateau ages, indicating ( 40Arf36Ar = 295.5), but in many cases shows large devia that Caledonian metamorphism was strong enough to tions (values in brackets in the last column of Table 1), reset the K-Ar system in both biotite and muscovite (i.e. due to the small contributions of atmospheric argon in reached temperatures above about 300-350 °C). Augen the analyses. Thus, the main emphasis is put on the pla gneiss sample G95:050 from No rdmarka yields plateau ages teau ages when interpreting the data. of 425 ± 5 Ma fo r muscovite, and 422 ± 5 Ma fo r biotite, The hornblendes from the mafic rocks did not pro respectively (Table l, Fig. 3). As with the biotite in the duce any well-defined plateaux, due to the small Rijpfjorden granite, the biotite in the Nordmarka augen amounts of argon present and the limited number of gneiss shows a tendency fo r Ar loss in the first few steps, steps during which it was released. The "plateau ages" which could be related to heating during Mesozoic burial reported in Tab le l and illustrated in Fig. 4 are thus more and/or mafic magmatism in the area (cf. Lauritzen & loosely defined, based on only 1-3 steps, and have to be Ohta 1984). The much higher inverse isochron age fo r this interpreted with caution. biotite, 458 Ma, is an artefact of the dustering of all points dose to the x-axis of the isochron diagram (not shown); the corresponding negative y-intercept (40Arf36Ar ratio) Caledonian granitoids shows its lack of reliability. From the Rijpjjorden granite ( sample 28-1), both musco Muscovite from the other augen gneiss sample from vite and biotite give similar plateau ages of 399 ± 5 and No rdmarka (G95:049) in eastern Nordaustlandet, and 405 ± 11 Ma, respectively, encompassing more than 90 from the augen gneiss south of Innvika ( G95:031) in cen %of the released Ar (Table l, Fig. 2). Although biotite is tral Nordaustlandet, yields slightly lower plateau ages of supposed to have a lower closure temperature fo r Ar 415 ± 6 and 411 ± 11 Ma, respectively. However, within than muscovite (Purdy & Jager 1976; Harrison et al. the margin of error, these ages are identical to the ages 1985; McDougall & Harrisorrt988; Hames & Bowring fromsample G95:050, all being related to heating during 1995), it gives the older age; however, within the margin Caledonian regional metamorphism and magmatism. of error, the ages are identical. A slight tendency fo r a resetting of the biotite at a lower age may be discerned in the first two steps; a possible cause fo r such a resetting Mafic rocks of uncertain age would be heating related to the intrusion of Mesozoic As discussed above, the hornblende from the maficrocks dolerites in eastern Svalbard (cf. Lauritzen & Ohta 1984). did not produce any well-definedp la tea u ages, due to the The Ar-Ar ages are ca. 10 Ma younger than the U-Pb much smaller amounts of argon present. From amphibo monazite age of 412 Ma (Johansson et al., submitted), an lite sample 598:129 from Andreeneset on Kvitøya, the first offset that probably is related to post-magmatic cooling. analysis of hornblende released about 80 % of the argon The Winsnesbreen granite (sample 94047) gives a in one step, corresponding to an age of 418 ± 8 Ma (Table muscovite Ar-Ar age similar to that of the Rijpfjorden l, Fig. 4A). The total gas age of that sample is slightly granite, with a plateau at 406 ± 5 Ma, whereas the No rd lower, 411 ± 8 Ma. A second hornblende crystal from the kapp granite (sample 94048) yields a significantly higher same sample was then analysed. This yielded an age spec muscovite plateau age of 428 ± 12 Ma. The higher age of trum with ages starting at ca. 460 Ma ( disregarding the the Nordkapp granite muscovite is also supported by two first steps with less than l o/o of the gas) and decrea total gas and normal and inverse isochron ages in the sing to ca. 350 Ma, but with three steps encompassing 57 range 427 to 434 Ma, and would agree with the U-Pb % of the released argon yielding a plateau at 406 ± 7 Ma monazite age of 440 ± 3 Ma fo r this granite, with a simi (Tables l and 2, Fig. 4B). The total gas age is 399 ± 7 Ma. lar offsetfo r cooling of about lO Ma as seen in the Rijp From the ga bbro at Isispynten (sample 94062c), a fjorden granite. A single spot analysis in another musco spectrum with slightly increasing ages was obtained (Fig. vite grain from the same sample gave a slightly lower age 4C). Two possible plateaux may be discerned: one at ca. of 422.5 ± 1.4 Ma (Appendix l); however, still within the 390 Ma encompassing steps S-7, and one at ca. 417 Ma margin of error fo r the result of the step analysis. being the ave rage of steps 8, l O and 11 (the deviating and The red granite from south of Innvika (sample much lower step 9 was disregarded). The total gas age is G95:030) yields a plateau age fo r muscovite at 420 ± 12 397 ± 7 Ma. NORWEGIAN JOURNAL OF GEOLOGY Ar-Ar dating of Caledonian and Grenvillion rocks, Svalbard 269 �st Table-- l Summa-· ryofJ\r-Ar- results from -- Svalbard Samp le number, Rock type, Pla tea u age ' Steps, o/o39Ar Total gas age ' Normal isochron l nverse isochron 40Aff36Ar mineral location age (MSWD)' age (MSWD)4 intercept Caledonian granitoids 28-1 muscovite Rijpfjorden granite, 398.6± 4.9 Ma 3-14, 90.5 o/o 396.2 ± 4.9 Ma 404.7 ± 5.1Ma (0.99) 403.8± 4.9 Ma (8.26) (21 ± 7) Vindbukta 401.1 ± 4.9 Ma S-8, 61.8 o/o 28-1 biotite Rijpfjorden granite, 404.7 ±!I.l Ma 3-18, 96.3 o/o 402.3 ±!I.l Ma 412.4 ±11.4 Ma (1.33) 412.8 ±11.3 Ma (1.69) (36± 10) Vindbukta 403.5± 11.2 Ma 3-11, 63.9% 94047 muscovite Winsnesbreen granite 405.6 ± S.OMa 4-15, 98.6 o/o 405.4 ± S.OMa 417.0± 5.3 Ma (1.33) 414.2± 5.0Ma (1.16) (-47±14) 406.1± S.OMa S-8, 46.1 o/o 94048 muscovite Nordkapp granite, 428.2 ±11.7 Ma 9-19, 96.2 o/o 427.1±11.7Ma 434.4 ±12.5 Ma (7.09) 427.2± 11.7 Ma (5.72) 309 ± 94 Gryteberget 425.5 ±11.7 Ma 13-17,45,4% G95:030 muscovite Red granite, 420.1 ± !l.S Ma 6-21, 98.8 o/o 420.1 ±11.5 Ma 421.8 ±11.6 Ma (3.55) 420.2± !l.S Ma (2.38) 345± 28 Innvika 418.7 ± !l.S Ma 10-18,66.5 o/o G95:051 muscovite Ap lite, 416.7 ± 5.4 Ma 4-27, 99.5 o/o 416.1 ± 5.4Ma 424.9± 5.9 Ma (18.8) 418.1± 5.5 Ma (ILO) 279 ± 40 Nordmarka 416.8± 5.4Ma 10-23,66.9 o/o Grenvillian granitoids G95:050 muscovite Augen gneiss, 425.1 ± 5.2Ma 11-23,93.9 o/o 424.6 ± 5.2Ma 429.3 ± 5.4 Ma (1.49) 427.0± 5.2Ma (0.62) (61 ±23) Nordmarka G95:050 biotite Augen gneiss, 422.4 ± S.!Ma 5-18, 92.2 o/o 419.5 ± 5.1 Ma 426.4 ± 5.7Ma (5.34) 458.4± 10.1Ma (2.61) (-3435 ± Nordmarka 425.3± 5.1Ma 7-15, 59.7% 2268) G95:049 muscovite Augen gneiss, 415.2± S.SMa 9-26, 96.7% 414.8± 5.4Ma 418.1 ± 5.6 Ma (1.56) 416.3± 5.4Ma (1.68) 268 ±56 Nordmarka G95:031 muscovite Augen gneiss, 411.3± 11.2 Ma 9-16, 93.0% 411.2 ±11.2 Ma 417.1 ± 11.5 Ma (0.62) 415.5± 11.3 Ma (0.33) (71 ± 41) Innvika 410.3± 11.2 Ma 9-11. 56.0% Mafic rocks of uncertain age S98:129 hornblende l Amphibolite, 418.5± 8.5 Ma Il only, 82.6 o/< 411.4 ± 8.3Ma 423.7 ± 8.6Ma (0.74) 412.4 ±29.3 Ma (9.64) 236± 383 Andn!eneset,Kvi tøya S98:129 hornblende 2 Amphibolite, 406.4 ± 6.8Ma 4-6, 56.8 o/o 398.8± 7.1 Ma 420.6 ±11.7 Ma (7.21) 401.6 ± 7.4 Ma (3.87) 280 ±68 Andreeneset,Kvitøya 94062c hornblende Gabbro, 391.5± 7.3Ma 5-7, 49.9% 396.9 ± 7.0Ma 429.3 ± 7.9Ma (1.63) 428.4± 5.9 Ma (1.22) (104± 35) Isispynten c. 417Ma 8,10-11,36.0 o/o l. Two plateau ages are normally reported, the first one containing more than 90 % of the released 39 Ar,the second derived from a more restricted but well-definedplateau encompassing 40-70 % of the released 39Ar. From the hornblendes in the mafic rocks, no well-defined plateaux were obtained,and the ages reported are more loosely defined < In summary, with the possible exception of the first dan (Llanvirn) platform successions (Hinlopenstretet steps in 598:129 hornblende 2, there are no traces of pre Supergroup). Only in the uppermost unit (Valhallfonna Caledonian ages. The hornblende ages of ca. 418 Ma are Formation in eastern Ny Friesland, Fortey & Bruton comparable to the mica ages, indicating resetting of horn 1973) does the carbonate platform facies give way blende during Caledonian metarnorphism (if the mafic upwards into basinal graptolitic shales. Younger strata rocks themselves are older), or cooling after Caledonian may be present in the hinge of the Hinlopenstretet Syne intrusive magmatism (if the rocks are Caledonian). The line, beneath the waters of the Hinlopen Strait. Thus the plateaux at ca. 390 and 405 Ma, respectively, may possibly upright to W-vergent folding and associated thrusting of be related to incipient alteration of the hornblendes. the Neoproterozoic and Palaeozoic successions of the Nordaustlandet Terrane occurred at some stage after the Llanvirn (ca. 460 Ma, Tucker & McKerrow 1995) and before the intrusion of late to post-tectonic Caledonian Discussion granites. U-Ph and Ph-Ph zircon data indicate that most of these peraluminous granites were intruded in the The Neoproterozoic strata (Murchinsonfjorden Super Early Devonian, or perhaps the latest Silurian (accepting group) deposited on the Grenville-age basement of an age of 417 Ma for the Silurian-Devonian boundary, Nordaustlandet are overlain by Vendian to mid-Ordovi- Tucker & McKerrow 1995). 270 Å. Johansson et al. NORWEGIAN JOURNAL OF GEOLOGY 500 5oo r--.---,--r=:::::t:==:::::�==:;-.----, Rijpfjorden gran/te: Rijpfjorden gronffe: 28-1 muscovlte 28-1 blot/te 450 450 4----- 398.6 ± 4. 9 Mo ---J> 404.7 ± 11.1 Mo 4----- 401.1 ± 4.9M:J --i> 403.5 ± 11.2M:J O' � � 400 h '-'- 400 n IT � o ...... L � Totolgas age: 396.2 ± 4. 9 Mo Totolgas age: 402.3 ± 11.1 Mo 350 350 Normalisochron age : 404.7 ± 5.1 Mo Norrrx:�lisochron age: 412.4 ± 11.4 Mo Inverseisochron age: 403.B ± 4. 9 Mo Inverse isochron age: 412.B ± 11.3 Mo 300 300 �--��--�--�--�--�------�----� o 20 40 60 BO 100 o 20 40 60 BO 100 39 39 % Arcumu/ ottve % Arcumu/ ottve 500 r---.---,--��====�====�.----, Wlnsnesbreen gronlte: Nordkapp gran/te: 94047musco vffe 94048 muscovffe 450 450 405.6 ± 5.0Mo 428.2 ± 11.7 Mo Totolgas age : 405.4 ± 5.O Mo Totolgosage: 427.1 ± 11.7Mo 350 350 Normalisochron age: 417.0 ± 5. 3 Mo Normalisochron age: 434.4 ± 12.5Mo Inverse isochron age: 414.2 ± 5.0 Mo Inverseisochron age: 427.2 ± 11.7 Mo 300 L---�--�---��----�--�--�-----� 300 L-----4--�-�-�-�-�-�----� o 20 40 60 80 100 o 20 40 60 BO 100 39 39 % Arcumu/ ottve % Ar cumulattve Red gran/te, lnnvfko: Ap/lte. Nordmarka: G95:030 muscovtte G95:051 muscovffe 450 450 t>cf---- 420.1 ± 11.5 Mo 416.7 ± 5.4 fv1o � ._...... __ � � '-'-� 400 �'-'- 400 70tolgas age: 420.1 ± 11.5 Ma Totolgosage: 416.1 ± 5.4Ma 350 350 Normo/ isochron age: 421. B ± 11.6 Mo Normalisochron age: 424.9 ± 5. 9 Mo Inverseisochron age: 420.2 ± 11.5Mo Inverseisochron age: 41B.1 ± 5.5Mo 300 �----��----�------�------�------J �0 �----�------�------�------�----� o 20 40 60 BO 100 o 20 40 60 BO 100 39 % Arcumu /affve % 39 Arcumulatlve Fig. 2. Ar-Ar spectra on muscovite and biotite from Caledonian granitoidsfrom northeast Svalbard. Height of boxes corresponds to l sigma uncertainty of analyses. NORWEGIAN jOURNAl OF GEOLOGY Ar-Ar dating of Caledonian and Grenvillian rocks, Svalbarda 271 Tåble2 Comparisonof lt-Pb monazile ages and lv-Ar rnUkovite dhd biotite ages fbrCaledohiat granitoids fl&n northeast SValbard U-Ph monazite age l Ar-Ar muscovite/biotite age 2 Rijpfjorden granite 4125 ±0.5 Ma (4 conc. points) 399± 5 Mal 405 ± 11 Ma Winsnesbreen granite c. 420 Ma (l concordant point) 406±5 Ma Nordkapp granite 440 ± 3 Ma (upper intercept) 428± 12 Ma Red granite, Innvika c. 440 Ma (upper intercept) 420± 12 Ma Aplite, Nordmarka 463± 9 Ma (upper intercept) 417±5 Ma l. Concordant analyses or upper intercept age of discordia (Johansson et al., submitted; Johansson et al., in prep.). 2. Plateau ages (this paper). In none of the Grenville-age rocks are any traces of a al. 1985; McDougall & Harrison 1988; Hames & Bowring pre-Caledonian argon component preserved. Caledo 1995) during the Silurian and into the Early Devonian. nian plateau ages in micas (ca. 410-425 Ma) provide evi Temperatures at the base of the Murchisonfjorden dence of regional heating above the Ar-Ar closure tem Supergroup increase from west to east across Nordaust perature (300-350 °C, Purdy & Jager 1976; Harrison et landet. Hornblende Ar-Ar data are particularly impor- 500 Augen gneiss, Nordmarka: Augen gneiss, Nordmarka: G95:050 muscovite G95:050 biotite 450 450 l 422.4 ± 5.1 Ma <1--- 425.1 ± 5.2 Ma ----i> r o � 425.3 ± 5.1 Ma l=! - 400 --i> � Totalgas age: 424.6 ± 5.2 Ma Total gas age: 419.5 ± 5.1 Ma 350 350 Normal isochron age: 429.3 ± 5.4 Ma Namd isochron age: 426.4 ± 5.7 Ma Inverseisochron age: 427.0 ± 5.2 Ma lnverseisochron age: 458.4 ± 10.1 Ma 300 300 �----�------�------L-----�------� o 20 40 60 80 100 o 20 40 60 80 100 39 39 % Ar cumulatrve % Arcumulative 500 Augen gneiss, Nordmarka: Augen gneiss, lnnvika: G95:049 muscov!te G95:031 muscovite 450 450 - l ± ---l> 415.2 ± 5.5 Ma .,...._ 411 .3 11 .2 Ma 410.3 ± 11.2 Ma --i> o n Total gas age: ± Ma 414.8 5.4 Totalgas age: 411.2 ± 11.2 Ma 350 350 NormaliOClchron age: 418.1 ± 5.6 Ma Namallsochron age: 41 7 .l ± 11 .5 Ma Inverse isochron age: 41 6.3 ± 5.4 Ma InverseiOClchron age : 415.5 ± 11.3 Ma 300 �----�------�------�----�------� 300 o 20 40 60 80 100 o 20 40 60 80 100 39 39 % Ar cumulative % Arcumulative Fig. 3. Ar-Ar spectra on muscovite and biotite from Grenvillian granitoids (augen gneisses) from northeast Svalbard. Height of boxes corres pondsto I sigma uncertaintyof analyses. 272 A. Johansson et al. NORWEGIAN JOURNAL OF GEOLOG Y tant in this context and it is indeed unfortunate that this tible with recently acquired zircon data (Johansson & mineral is extremely scarce and, if present, usually Larionov 1999; Johansson et al., in prep.) that Caledo altered to actinolite in Nordaustlandet. The two horn nian migmatization of the Grenvillian basement occur blendes analysed here indicate that at least the eastern red in the latest Ordovician (ca. 440-450 Ma) in eastern areas were subject to Caledonian temperatures above Nordaustlandet. 500-550 °C (Ar-Ar closure temperature of hornblende, In the Caledonian granitoids, two groups of Ar-Ar McDougall & Harrison 1988). This evidence is compa- ages may be discerned, one at 415-430 Ma, similar to the ages in the Grenvillian rocks, and another at 400-405 Ma. In Table 2 and Fig. 5 compare the U-Pb monazite and Ar-Ar muscovite ages for the Caledonian granitoids. 500 The first age group is related to granites having U-Ph Amphibolite, Andreeneset: ages around 440 Ma (Nordkapp granite and red granite 598:129 homblende l l from Innvika), the second group to granites with U-Pb 450