Isotope studies of granitoids from the BangenhukFormation, Ny Friesland Caledonides, Svalbard
A. JOHANSSON*, D. G. GEEf, L. BJORKLUNDJ & P. WITT-NILSSON§
' Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50 007, S-104 05 Stockholm, Swedenf Department of Geophysics, Uppsala University, Box 556, S-751 22, Uppsala, Sweden
\ Department of Geology, University of Goteborg, S-412 96 Goteborg, Sweden§ Department of Geology, University of Lund, Solvegatan 13, S-223 62 Lund, Sweden
(Received 28 July 1994; accepted 19 December 1994)
Abstract-The Caledonian Hecla Hoek succession in Ny Friesland, eastern Svalbard has beeninterpreted, for many decades, to be a continuous stratigraphic sequence. Early Palaeozoic andNeoproterozoic strata in its upper parts pass more or less conformably down into amphibolite faciesrocks (Stubendorffbreen Supergroup) at depth. Recent isotopic age-determination and structuralstudies have indicated that the Stubendorffbreen succession is tectonostratigraphic and made up ofat least three major thrust sheets. This paper provides new data from two meta-igneous units withinthe succession, the Bangenhuk and Instrumentberget gneisses. Both are granitoid sheets, consistingmainly of red, strongly lineated gneisses of monzogranitic composition; the Bangenhuk unit alsocontains some lenses of little deformed granitoids, as well as cross-cutting aplite dykes, amphibolitizeddolerites and subordinate metasedimentary rocks. The latter are locally cut by granitoids. U-Pbzircon dating of six samples of variably deformed Bangenhuk granitoids, including one cross-cuttingaplitic dyke, has yielded ages between 1720 and 1770 Ma, the higher values generally from the lessdeformed samples. The Instrumentberget gneissic granite yielded an age of 1737 +J5 Ma. These agesare interpreted to date the time of intrusion of the granitoids at around 1750 Ma; the younger agesmay have been slightly lowered by Caledonian deformation, particularly those from specimenslocated close to a major fracture (the Billefjorden Fault Zone) in Wijdefjorden-Austfjorden. U-Pbdating of titanite from the least deformed granitoid also yields comparable Palaeoproterozoic ages;in the more deformed rocks, however, titanites give evidence of Caledonian ductile deformation atc. 410 Ma. The Rb-Sr system of the corresponding whole rock samples has been disturbed and yieldsan errorchron age of about 1650 Ma and, for some samples, an impossibly low initial Sr ratio. TheSm-Nd system may be more intact and yields initial eNd values of - 2 to - 3 , suggesting somecontribution from older crustal material to the granitoid magmas. The results indicate the presenceof extensive units of Palaeoproterozoic granitic basement within the Lower Hecla Hoek successionof Ny Friesland, supporting the hypothesis that the latter is composed of tectonically intercalatedbasement and cover units.
1. Introduction
The pre-Devonian rocks of Svalbard (Fig. 1), exposedin the northwestern corner of the Barents Shelf,provide evidence essential for interpretation of theevolution of the North Atlantic Caledonides. Duringthe last decade, a wide range of pre-Caledonianbasement protoliths have been recognized within theSvalbard Caledonides; their discovery influencesinterpretation of Caledonian stratigraphy and struc-ture and places further constraints on the relationshipsof the various Caledonian terranes on Svalbard toeach other and neighbouring Arctic regions. Recently,Palaeoproterozoic crystalline rocks have beenidentified within the lower part of the Caledoniansuccession in Ny Friesland in the northeastern part ofthe archipelago. This paper presents new evidencefrom Ny Friesland of Palaeoproterozoic rock units atdifferent structural levels, and infers that their presentposition in the structure is controlled by Caledonianthrusting.
The Caledonian rocks of northeastern Svalbardcompose the so-called Hecla Hoek Complex. A nearly20 km thick, more or less conformable, succession hasbeen described (Harland, 1959), that includes littlemetamorphosed Early Palaeozoic and Neoproterozoicsuccessions (the Hinlopenstretet and Lomfjordensupergroups), which pass down, with rapid increasein metamorphic grade, into a variety of meta-sedimentary and meta-igneous rocks (Harland, Wallis& Gayer, 1966; Manby, 1990; Harland et al. 1992).The latter, the Stubendorffbreen Supergroup, has beeninferred by most authors to compose a part of theCaledonian geosyncline and to be of Neoproterozoic,or perhaps Mesoproterozoic, age. However, someauthors (Sokolov, Krasil'scikov & Livshits, 1968;Krasil'scikov, 1973, 1979) have regarded the lowerpart of the Stubendorffbreen Supergroup to be ofPalaeoproterozoic or older age, composing a crys-talline complex, referred to as the Atomfjella Series.This interpretation supports the view of the earlySpitsbergen explorers (Nordenskiold, 1863, 1866;
http://journals.cambridge.org Downloaded: 10 Oct 2014 IP address: 130.241.16.16
304 A. JOHANSSON AND OTHERS
mm\ Devonian (O.R.S.)
South-
western
Terrane
Figure 1. Svalbard's Caledonian Terranes and youngersedimentary cover, based on Harland (1985) and Gee(1986). B, Billefjorden Fault; BB, Breibogen-BockfjordenFault; L, Lomfjorden Fault; R, Raudfjorden Fault.
Blomstrand, 1864; de Geer, 1909; Nathorst, 1910)who considered the high-grade schists and graniticgneisses in the lower structural levels to be part of anancient basement.
2. Western Ny Friesland Caledonides
The Lower Hecla Hoek rocks of the StubendorffbreenSupergroup occur throughout the western part of NyFriesland (Fig. 2), within a major longitudinal fold,the Atomfjella Antiform (Fig. 3) (Harland, 1941).This major structure ' arches a great series of alreadyrecumbently folded rocks' (Harland, 1959, p. 321).Mapping of Ny Friesland by Harland et al. (1992) andS. A. Abakumov (unpub. geological map, 1991),defined the essential geometry of this antiform, notedthe occurrence and correlation of most of theformations in both limbs of the structure and showedthat two of the units (the Vassfaret and SorbreenFormations) are cut out, locally at least, in the easternlimb.
The Stubendorffbreen Supergroup succession iscomposed of a variety of metasedimentary and meta-igneous rocks, mostly in high amphibolite fades.Primary sedimentary and igneous structures (Gayer &Wallis, 1966; Gayer, 1969; Gee, Bjorklund & Stolen,1994) are locally well preserved, but are generallyobscured by a regional penetrative foliation. Harland(1959, p. 326) commented on the gross stratificationof the rock units and the problems of structural inter-
pretation, 'Some of the difficulty in interpretationstems... from the nature of the deformation... Closeexamination shows no evidence of fault structures,and it is assumed that a series of nappe structures wereformed with basal thrusts which at great depths werelater obscured by recrystallization and flow defor-mation'. Nevertheless, he concluded then and morerecently (Harland et al. 1992) that Ny Friesland'sHecla Hoek succession was essentially an uninter-rupted geosynclinal assemblage. We challenge thisinterpretation on the basis that Palaeoproterozoicgranitic gneisses of plutonic (not metasomatic) originoccur and that they are distributed at at least threelevels in the 'stratigraphy'. The StubendorffbreenSupergroup is a tectonostratigraphic succession, withthrust intercalation of basement and cover rocks.
The published stratigraphy of the Lower HeclaHoek in Ny Friesland (Table 1) shows the presence oftwo levels of granitic gneisses, referred to as theEskolabreen and Bangenhuk formations. Reconnais-sance observations by the early investigators identifiedthese to be deformed igneous rocks (Fairbairn, 1933),either of' primary magmatic or refusion' origin (Bayly,1957, p. 390). Sedimentary intercalation occurs andtransitional relationships, e.g. from quartzites tofeldspathic gneisses, were thought to result frommetasomatic processes (Harland, 1959). Detailedstudies of the Bangenhuk Formation in the type areaof northern Ny Friesland (Gayer & Wallis, 1966)emphasized the interstratification of sedimentary andigneous rocks and concluded that a pyroclastic originwas most probable ('The sedimentary origin of theformation is shown by the compositional banding ofthe lithologies, by the interbanding of psammite ofunquestionable sedimentary origin and by the presenceof amphibolites thought to represent basic pyroclasticdeposits'; Gayer & Wallis, 1966, p. 19). Nevertheless,the presence within the Bangenhuk Formation ofintrusive igneous rocks was well established; Gayer(1969) described a dolerite dyke swarm partly de-formed and metamorphosed to amphibolites and,locally, a granodioritic boss with associated aplites.
In northern Ny Friesland, the granitic gneisses haveall (including those of Blomstrand, 1864) beenpreviously thought to be part of the BangenhukFormation (Harland, 1959; Gayer & Wallis, 1966).Our mapping (unpublished reports) in recent yearshas shown that this interpretation is unlikely; thosecropping out in the area southwest of Verlegenhukendirectly underlie the Polhem Formation (Gee et al.1992). Likewise, on Instrumentberget, southeast ofMosselbukta, gneissic granite immediately underliesthe Polhem Formation at a structural level below theBangenhuk Formation and above the EskolabreenFormation. U-Pb zircon age-determination datapresented here on both the Bangenhuk andInstrumentberget granitic gneiss sheets have yieldedlate Palaeoproterozoic ages, as have some of the
Instrumentberget - Flatan(granitic gneiss c. 1750 Ma)
Smutsbreen
Eskolabreen(granitic gneiss c. 1750 Maand 2400 Ma)
Figure 2. Geology of Ny Friesland, based on Bayly (1957), Harland (1959), Gayer & Wallis (1966), Gayer (1969), Wallis (1969),Manby (1990), S. A. Abakumov (unpub. map, 1991) and Harland et al. (1992), with revisions in northern areas. Samplelocations (1-7), as well as the Flatan locality (8), are indicated.
http://journals.cambridge.org Downloaded: 10 Oct 2014 IP address: 130.241.16.16
306
Atomfjella Antiform
A Wijdefjorden Sorgfjorden Hecla Hoekfjellet Hinlopenstretet
A. JOHANSSON AND OTHERS
Hinlopenstretet Syncline
NORDAUSTLANDET
I B
C Austfjorden
Smutsbreen
Sea.level
Chydeniusbreengranite
Figure 3. Schematic profiles through northern (A-B) and southern (C-D) Ny Friesland (location and legend on Fig. 2) fromGee, Bjorklund & Stolen (1994), with revised structural interpretation based on recent mapping.
Eskolabreen Formation gneisses (Larionov et al. inpress).
The gneissic granite at Flatan southwest ofVerlegenhuken, first referred to by Blomstrand (1864)and later studied by A. M. Tebenkov and A. N.Sitotkin (unpub. map, 1990) was investigated by theU-Pb zircon method (Gee et al. 1992) and yielded anage of 1778 tfh Ma. From within the BangenhukFormation, fine-grained granite from the 'grano-dioritic boss' on Brennkollen (Gayer, 1969), referredto above, yielded an age of 1809 t\f2 Ma (Gee et al.1992). These authors also reported 207Pb/206Pb singlezircon evaporation ages (Kober method) of c.1670-1700 Ma for these two rocks.
More recently, Balashov et al. (1993) have reporteda U-Pb zircon age from the Eskolabreen Formationof 2.4 Ga. This age was derived from one of twopopulations of zircon morphotypes separated from agneiss; its significance is not clear. In addition, Manby& Lyberis (1991) reported a preliminary Sm-Nd ageof 1757 + 90 Ma from Harkerbreen Group (includesthe Bangenhuk Formation; cf. Table 1) amphibolitesof undefined location.
The published data indicate that the gneissicgranitoids at two of the three 'stratigraphic' levelshave very similar ages. The plutonic origin of thesesimilarly aged units is indicated by structural evidencein low strain areas (Gee, Bjorklund & Stolen, 1994).Together with a variety of other more or less foliatedgranitic rocks from the Bangenhuk Formation, thesamples dated in the present study have been analysedfor a standard range of major, minor, trace and rareearth elements (Carlsson, Johansson & Gee, inpress) and shown to be part of a geochemicallycoherent magmatic suite of granodioritic to graniticcomposition and A-type affinity. They arecharacterized by high alkali contents, high FeO/MgOratios and high contents of Zr, Zn, Y, Nb, Ga andREE (except Eu). Manby (1990) also providedgeochemical data on some of the Harkerbreen Group(including Bangenhuk Formation; cf. Table 1) acidgneisses; he concluded that they were of igneousorigin, had rhyolitic to rhyodacitic compositions andprobably had originated in an attenuated 'withinplate' setting.
Contact relationships between the various sheets of
http://journals.cambridge.org Downloaded: 10 Oct 2014 IP address: 130.241.16.16
Isotope studies of Svalbard granitoids 307
Table 1. The Hecla Hock succession in Ny Friesland, from Harland(1985) with revision of lower parts based on this paper
Hinlopenstretet Supergroup (Upper HeclaHoek)
Valhallfonna Fm (limestone) Arenig- OslobreenLlanvirn Group (1.2 km)
Kirtonryggen Fm (limestone and dolomite)Arenig
Tokommane Fm (dolomite and sandstone)Caerfai
Drakoisen Fm (shale) PolarisbreenWilsonbreen Fm *(tillite) late Varangian Group (0.8 km)Elbobreen Fm (shale) early Varangian
Lomfjorden Supergroup (Middle Hecla Hoek)Backlundtoppen Fm (dolomite and shale) AkademikerbreenDraken Conglomerate Fm (early Sturtian) Group (2 km)Svanbergfjellet Fm (limestone anddolomite)
Grusdievbreen Fm (limestone)
Oxfordbreen Fm (shales) VeteranenGlasgowbreen Fm (greywacke and Group (3.8 km)quartzite)
Kingbreen Fm (quartzite, shale, greywacke,carbonate)
Kortbreen Fm (quartzite and limestone)
VETERANEN LINE - SORGFJORDEN FAULT
Stubendorffbreen Supergroup (Lower HeclaHoek)
Vildadalen Fm (semipelite, psammite and Planetfjella Groupmarble) (Mossel Group)
Flaen Fm (semipelite, psammite with acidpyroclastics (?))
MAJOR THRUST
Sorbreen Fm (quartzite, meta-tuffs and Harkerbreenamphibolite) Group (4.1 km)
Vassfaret Fm (semipelite, psammite andamphibolite)
Bangenhuk Fm (granitic gneiss c. 1750 Ma,psammite and amphibolite)
MAJOR THRUST
Rittervatnet Fm (psammite, semipelite, Harkerbreenmarble, amphibolite) Group (4.1 km)
Polhem Fm (quartzite and amphibolite)Instrumentberget/Flatan unit (graniticgneiss c. 1750 Ma)
MAJOR THRUST
Smutsbreen Fm (semipelite and marble) FinnlandveggenEskolabreen Fm (granitic gneiss c. Group (2.7 km)
1750 Ma, metaseds., amphibolite)
granitic gneisses and their underlying and overlying'formations' have generally been described to be'transitional' and 'gradational', hence an interpret-ation favouring stratigraphic continuity. At leastlocally, these concordant relationships can be shownto result from high ductile deformation, usuallyfollowed by textural recrystallization. In the case ofthe Flatan gneissic granite (Gee et al. 1992), the lowercontact to underlying schists is unambiguouslymylonitic; the upper contact to overlying Polhemquartzites is generally mylonitic, but appears to beprimary in local low-strain areas. With regard to theInstrumentberget gneissic granite, the lower contact is
not exposed and the upper contact is overlain byconglomerates and sandstones of the PolhemFormation (F. Hellman, unpub. B.A. thesis, Univ.Lund, 1994). In the case of the Bangenhuk Formation(Gayer & Wallis, 1966; Gayer, 1969), the lowercontact shows an intercalation of the gneiss withunderlying Rittervatnet Formation metasedimentaryrocks and the upper contact, though highly strained,locally preserves intrusive relationships to theVassfaret Formation (Gee, Bjorklund & Stolen, 1994).Thus, the present structural evidence from northernNy Friesland indicates that the similarly aged Flatan,Instrumentberget and Bangenhuk gneissic granitoidsare probably related to the same Palaeoproterozoiccrystalline basement and were emplaced at differentstructural levels by thrusting.
Examination of cross-sections through the Atom-fjella Antiform further south in Ny Friesland hasprovided support for this tectonic interpretation. Oneof us (L. B.) has mapped the Reinsbukkdalen-Reinsbukkbreen section through the core of the anti-form and identified sheets of gneissic granitoidsseparating the Polhem Formation quartzites from theunderlying Smutsbreen Formation marbles in boththe fold limbs. Furthermore, in the case of theBangenhuk granitic gneiss unit, exposed in the westernlimb of the Atomfjella Antiform, the basal partproved to be extensively sheared and mylonitic andthe underlying Rittervatnet (?) Formation meta-sedimentary rocks highly deformed and attenuated.
The Bangenhuk Formation thus forms a large,2 km thick, tectonic sheet, with its main outcrop areaalong the west limb of the Atomfjella Antiform inwestern Ny Friesland; outcrops are also found in theeastern limb of the antiform on the nunataks of innerNy Friesland. It is mainly composed of reddish,coarse- to medium-grained, strongly foliated orlineated quartzo-feldspathic gneisses of granitic com-position (Femmilsjoen member of Gayer & Wallis,1966). These were referred to as 'feldspathites' byGayer & Wallis (1966), and interpreted to represent'coarse acid tuff/agglomerate'. A plutonic originis preferred here, based on coarse igneous texturesand presence of enclaves in low-strain parts ofthe gneisses. Underlying, more inhomogeneous andcoarsely banded gneissose varieties were referred to asthe Flat0yrdalen member by Gayer & Wallis (1966),and thought to represent reworked pyroclastics; thesehave not been studied here. The strongly lineatedgneisses are often rich in attenuated mafic enclaves.Locally, up to 100 m thick lenses of virtuallyundeformed grey granite or granodiorite arepreserved, such as in the Brennkollen area (Gayer,1969;Geee/a/. 1992; Gee, Bjorklund & Stolen, 1994),passing outwards into the normal, deformed varietyof Bangenhuk granitic gneiss. The Brennkollen granitecontains a variety of xenoliths and is intruded byaplitic dykes. Aplitic dykes are also found at
Bangenhuken, where they cut both the normalBangenhuk granite and adjacent metasediments. Nu-merous amphibolitic dykes, sometimes with well-preserved doleritic texture, also intrude the BangenhukFormation gneisses.
3. New isotopic age-determination studies
In this paper, we report the results of a more extensivestudy of the age of granitoids within the BangenhukFormation, in order to ascertain whether the agesobtained by Gee et al. (1992) from Brenkollen andFlatan are representative for the whole suite, includingboth deformed and undeformed varieties and cross-cutting aplitic dykes. U-Pb zircon analyses have beencomplemented by U-Pb investigations of titanite, andby Rb-Sr and Sm-Nd studies of the correspondingwhole rock samples. The investigated rocks (locatedon Fig. 2, with latitude/longitude and short de-scriptions of mineralogy and zircon appearance givenin the Appendix) include an undeformed granite fromthe Brennkollen locality (sample L90:13), a littledeformed granodiorite from north of Einsteinoddenin Austfjorden (LP 91:16), two samples of the normalvariety of reddish, lineated, gneissic Bangenhukgranite, one from the western limb (Reinsbukkdalen,J 91:017) and one from the eastern limb(Reinsbukkbreen, J 91:013) of the Atomfjella Anti-form, one sample of strongly schistose granitic gneisstaken on a small island (the northern of Gyllenskold-holmane, J 91:006) in Austfjorden, where theBillefjorden Fault Zone runs, and one sample from anaplitic dyke cutting the normal Bangenhuk granite atBangenhuken itself. Also reported here is a U-Pbzircon age determination of a red gneissic granitefrom Instrumentberget, southeast of Mosselbukta innorthern Ny Friesland, which occurs at a lowerstructural level than the Bangenhuk granitoids proper.In the QAP triangle (Streckeisen, 1976), all samplesexcept granodiorite LP91:16 plot as monzogranite,and in the P-Q diagram of Debon & Le Fort (1982)all except LP91:16 plot as adamellite or granite(Carlsson, Johansson & Gee, in press).
3.a. Analytical methods
Samples weighing 17-36 kg were crushed and milledand zircons and titanite separated using standardtechniques. Some of the crushed material was milledinto whole rock powder used for Rb-Sr and Sm-Ndas well as chemical analyses. A few smaller samples(weighing a few kilograms) were taken from shearzones for U-Pb dating of titanite, but titanite wasonly recovered from one of these, and from three ofthe larger samples.
Zircons were dissolved in teflon bombs (Krogh,1973) with insets and microcapsules (Parrish, 1987)and spiked with 235U- or 233"235U-tracer and 208Pb-
http://journals.cambridge.org Downloaded: 10 Oct 2014 IP address: 130.241.16.16
Isotope studies of Svalbard granitoids 311
tracer. For sample L90:13, uranium and lead wereseparated by anion exchange in HC1, with the leadfurther purified by electroplating. For the remainingsamples HBr was used for this separation, followed bya second ion exchange in HC1 for uranium, but noelectroplating of lead. Uranium was analysed withdouble Re filaments and lead with single Re filamentson a Finnigan MAT 261 multicollector mass spec-trometer in static mode. Parallel blank runs gave Pbblanks between 0.03 and 0.3 ng. Calculations weremade following Ludwig (1991a, b), using the decayconstants recommended by Steiger & Jager (1977) andthe fractionation, blank, and common lead valueslisted in Table 2. For common lead correction, thevalues of Stacey & Kramers (1975) for 1800 Ma wereused. All errors reported are two standard deviations.
The largest titanite samples were dissolved directlyin the larger Krogh-capsules, while the two smallerfractions were dissolved in microcapsules. 233-235U-and 208Pb-tracers were used. For the larger fractions,U and Pb were separated using HC1 chemistry,followed by purification of Pb in HBr columns and byelectroplating, and purification of U in columns withHNO3. For the two smaller samples, separation of Uand Pb was done using HBr columns directly, followedby purification of U in HN03 columns. Massspectrometry and calculations were similar to thosedescribed for the zircons.
For Sr and Sm-Nd isotopic analyses, about100-150 mg of whole rock powder was dissolved inKrogh-capsules at 205 °C. Prior to dissolution, amixed l47Sm-150Nd tracer was added. The unspikedNd isotopic composition was calculated from thespiked Nd run. Rb and Sr contents and Rb/Sr ratioswere determined by XRF. After dissolution, Sr andREE as a group were separated using columns with2.5 M HC1 and 6 M HNO3. Sm and Nd werethen separated from each other using alpha-hydroisobutyric acid. Sr, Sm and Nd were loaded ondouble Re filaments and analysed as metal ions on theFinnigan MAT 261 multicollector mass spectrometerin static mode. Calculations and corrections weredone according to the footnotes to Tables 3 and 4.Isochron calculations were made with the program ofLudwig (19916), which is based on the method ofYork (1969), using decay constants recommended bySteiger & Jager (1977).
3.b. U-Pb zircon dating
3.b.l. Sample L 90:13 - undeformed granite, Brennkollen
Sample L 90:13 comes from the Brenkollen locality innorthern Ny Friesland (Fig. 2). As described and
illustrated by Gee, Bjorklund & Stolen (1994), a lensof virtually undeformed grey granite, about 100 mthick, and containing many randomly orientedxenoliths and cross-cutting aplitic dykes, is foundhere. Outwards, it gradually passes over into thenormally foliated and lineated type of Bangenhukgranites. Unlike the sample from Brennkollen datedby Gee et al. (1992), the present sample is from thisundeformed lens.
Zircon microphotographs are shown in Figure 4and the results of the analyses are reported in Table 2.The zircons are relatively poor in uranium (100 to370 ppm, increasing with decreasing grain size) andhence in radiogenic lead (30 to 110 ppm). Also thecommon lead contents are low (0.2 to 1.0 ppm). Thezircons are moderately discordant (7-24%). Duringthe first analysis, the coarsest fraction (1; > 210) wasthe most discordant (opposite to the normal pattern)and also fell off the discordia, but after repeating theanalysis (7) it fell on the discordia slightly abovefractions 2 and 3. Fraction 2 became significantlymore concordant after abrasion (analysis 8 in Table2), and fraction 3 slightly more concordant after HF-leaching (analysis 9). Excluding the presumablyerroneous analysis 1, the 207Pb/206Pb ages range from1730 to 1747 Ma. A similar range in 207Pb/206Pb ages(from 1735 to 1746 Ma) was obtained by direct Pbevaporation analysis (Kober, 1986) of two large zirconcrystals from the same sample.
Exclusion of point 1 produces an eight-pointdiscordia with an upper intercept of 1759 +{J Ma, alower intercept of 274 t\™ Ma, and a MSWD of 1.4(Fig. 5 a). Point 1 plots significantly to the right of thisline. In view of the simple magmatic appearance of thezircons and the undeformed nature of the rock, theupper intercept is interpreted to reflect thecrystallization age of the Brennkollen granite.
If a comparison is made with sample 89141, a moredeformed Brennkollen granite dated by Gee et al.(1992), it is seen that the 89141 zircons are muchricher in uranium and radiogenic lead and plot farmore discordantly, with an ill-defined upper interceptage of 1809 !}•* Ma (MSWD 44). If the zircons fromboth samples are combined in one regression, theyyield an upper intercept age which appears veryprecise (1764 + 8 Ma), but still has a large MSWD(18). Thus there is no perfect linear alignment, but therocks could well be comagmatic, with a common agearound 1760 Ma.
3.b.2. Sample LP 91:16 - granodiorite, north ofEinsteinodden
Sample L P 9 1 : 1 6 comes from a small (about 10 m
Figure 4. Zircon microphotographs, taken in glycerine. Length of photos 0.5 mm. (a) Sample L 90:13 Brennkollen; (b) SampleLP 91:16 N. Einsteinodden; (c) Sample J 91:006 Gyllenskoldsholmane; (d) Sample J 91:013 Reinsbukkbreen; (e) SampleJ 91:017 Reinsbukkdalen; (f) Sample J 92:011 Bangenhuken; (g) Sample J 92:010 Instrumentberget.
http://journals.cambridge.org Downloaded: 10 Oct 2014 IP address: 130.241.16.16
Isotope studies of Svalbard granitoids 313
thick) lens of grey granodiorite, outcropping alongthe shore of Austfjorden about 2 km north ofEinsteinodden (Fig. 2). The granodiorite lens is locatednear the contact between the Bangenhuk and under-lying Rittervatnet Formation. The rock is even- andmedium-grained, and virtually undeformed in theinterior of the small lens, from where the zirconsample was taken; on both sides it gradually passesthrough sheared granodiorite into dark myloniticgneiss.
The results of the analyses of four non-magneticsize fractions of zircons are reported in Table 2. Thezircons are relatively poor in uranium (150-190 ppm),radiogenic lead (45-60 ppm) and common lead(0.1-0.4 ppm), as is normal for zircons from rocks ofintermediate composition. They are thus also onlymoderately discordant (8-17%), and the 207Pb/206Pbages range between 1723 and 1737 Ma. A regressionthrough these four points produces a well-defineddiscordia (MSWD = 0.55) with an upper interceptage of 1748 + 8 Ma, and a lower intercept of236 + 96 Ma (Fig. 5 b). The age 1748 + 8 Ma isinterpreted as the crystallization age of this rock, andis similar to or only slightly younger than that of theBrennkollen granite (1759 t\\ Ma).
Sample J 91:006 comes from a strongly foliated andlineated, reddish Bangenhuk granite collected at thenorthern of the two small islands ofGyllenskoldholmane in Austfjorden (Fig. 2). Thissample may have been affected by strong shearingalong the Billefjorden Fault Zone, which followsWijdefjorden-Austfjorden and separates theCaledonian terrane of Ny Friesland from theDevonian Old Red Sandstone basin of Andree Land,west of the fiord.
The results of the analyses of four non-magneticand one magnetic zircon fractions are reported inTable 2 and shown in Figure 5 c. The dark and turbidzircons of the two coarsest fractions are markedlyricher than the others in uranium (1810-1970 ppm)and radiogenic lead (390-435 ppm) and plot mostdiscordantly (36%). Fraction 3, which containsrelatively clear zircons, shows the lowest contents ofuranium (264 ppm) and radiogenic lead (81 ppm) andplots least discordantly (11 %). Fractions 4 and 5 areintermediate (690-730 ppm U, about 180 ppmradiogenic Pb) and plot in between. 207Pb/206Pb agesrange between 1670 and 1716 Ma. A regression
through all points gives an upper intercept age of1728 !2* Ma, a lower intercept of 199 + 86 Ma, andMSWD 28 (Ludwig, 19916, model 2).
The high MSWD value may partly reflect increasedscatter of these U-rich, turbid and metamict zirconsthat are derived from a rock which has undergonestrong shearing in Caledonian time. However, thehigh MSWD value may also be partly an analyticalartefact, reflecting the smaller apparent analyticalerror resulting from a more precise 204Pb analysisusing electron multiplier. The obtained upper interceptage is 20-40 Ma younger than that of most otherBangenhuk granitoids. Plotting of sample J 91:006together with sample J 91:013 (see section 3.b.4)suggests that the ages (17281^ and 1766 ^i? Ma,respectively) are significantly different, with theJ 91:006 points being systematically shifted to the leftof those for J 91:013, although the error envelopesoverlap. Whether this is due to an original agedifference, or an effect of the more intense shearing ofsample J 91:006 in the Billefjorden Fault Zone, is notclear. Titanite from sample J 91:006 suggests an agefor this deformation of about 410 Ma (see section 3.c).
Sample J 91:013 comes from the eastern limb of theAtomfjella Antiform, collected on the northeasternside of the glacier Reinsbukkbreen (Fig. 2). Thesample comes from a relatively undeformed part ofthe normal reddish lineated Bangenhuk gneissicgranite. It contains some primary millimetre- tocentimetre-sized augen of fresh K-feldspar and plagio-clase, surrounded by a much more fine-grainedschistose mosaic of quartz, feldspar and brown biotite.There are also some strongly altered aggregates thatmay have been amphibole, but now appear to consistof a mixture of epidote, opaques and other secondaryminerals.
The results from the seven zircon analyses are listedin Table 2 and shown in Figure 5d. The dark turbidzircons of fraction 1 (NM > 150 A) and 6 (M > 106)have markedly higher contents of uranium (1227 and1643 ppm, respectively) and radiogenic lead (262 and351 ppm) and are also higher in common lead (2.3 ppmboth). They plot almost identically in the concordiadiagram and are the most discordant points (39%discordant). The others range in uranium from 510 to860 ppm and in radiogenic lead from 130 to 210 ppm,with 0.3-1.6 ppm common lead. The 2»'Pb/206Pbmodel ages range from 1708 to 1741 Ma.
Figure 5. Concordia diagrams. Numbering of points refers to Table 2, points with number in brackets have been excluded fromthe regression calculation, (a) Sample L 90:13 Brennkollen (Ti = titanite, not included in calculation); (b) Sample LP 91:16N. Einsteinodden; (c) Sample J 91:006 Gyllenskoldsholmane; (d) Sample J 91:013 Reinsbukkbreen; (e) Sample J 91:017Reinsbukkdalen; (0 Sample J 92:011 Bangenhuken; (g) Sample J 92:010 Instrumentberget (H18 and H29 are conglomerateclasts from the overlying conglomerate, not included in calculation).
http://journals.cambridge.org Downloaded: 10 Oct 2014 IP address: 130.241.16.16
314 A. JOHANSSON AND OTHERS
A discordia through all seven points yields an upperintercept age of 1766 I3J? Ma, a lower intercept of158 I!*7 Ma, a n d a MSWD of 90 (Ludwig, 19916,model 2). The high MSWD partly reflects some realscatter around the best-fit line, but may partly also bea consequence of the small analytical errors obtained,thanks to precise multiplier 204Pb determinations. Therelatively large error in the upper intercept age reflectsthe relatively large discordancy of the zircons,combined with their limited spread along thediscordia. It is hard to justify exclusion of any datapoints to improve the linear fit, and 1766 1" Ma istaken as the best estimate of the age. This would makegranite sample J 91:013 very similar in age to theundeformed granite from Brennkollen (1759 t" Ma).
Sample J 91:017 is derived from the normal type ofred, medium-grained foliated/lineated Bangenhukgranite occurring in the western limb of the AtomfjellaAntiform at Reinsbukkdalen (Fig. 2). The rock isdominated by quartz and feldspar, partly asmillimetre-sized irregular grains, partly in a more fine-grained recrystallized form. The dominating maficmineral is olive-green biotite. Fairly large amounts oftitanite occur, as well as a more brownish mineral thatmay be allanite, and a few relatively large opaquegrains. Apatite is a relatively common accessory.
Four non-magnetic and two magnetic zirconfractions were analysed (Table 2, Fig. 5e). The zirconsare relatively rich in uranium (780-1430 ppm),radiogenic lead (180-320 ppm), and common lead(1.5-7.1 ppm), and like J 91:013 and J 91:006 theyare highly discordant (29-43 %). The 207Pb/206Pb agesrange between 1670 and 1696 Ma. However, theyshow a good fit to the discordia line, and yield anupper intercept age of 1724+14 Ma and a lowerintercept age of 147 + 46 Ma, with a MSWD of 1.5(Fig. 5e).
The obtained age is significantly lower than that ofsample J 91:013 (1766 !« Ma), but similar to J 91:006(1728 +_\\ Ma). Although the sampled rock atReinsbukkdalen (J 91:017) does not show the strongshearing seen at Gyllenskoldholmane (J 91:006), thelocality is situated within a kilometre of the coast, andis thus fairly close to the Billefjorden Fault Zone; itmay have been influenced by disturbances associatedwith this faulting. On the other hand, the low MSWDvalue speaks against such disturbances, and wouldsuggest that the age is primary. In this case,samples J 91:006 and J 91:017 would represent asomewhat younger pulse of magmatic activity, com-pared to J 91:013 (Reinsbukkbreen) and L90:13(Brennkollen).
Sample J 92:011 comes from an aplitic dyke, a fewmetres thick, cutting the normal red foliatedBangenhuk granitic gneisses 1 km south ofBangenhuken (Fig. 2). Similar fine-grained felsicintrusions in the area have also been observed cuttingadjacent metasedimentary rocks (Gee, Bjorklund &Stolen, 1994). The sampled aplite is red, relativelyfine-grained and homogeneous, and displays only aweak foliation. However, in thin section the rockshows a marked foliation, outlined by small biotiteflakes, which are strongly altered to almost opaquematerial. A few larger (about 1 mm) subhedral opaquegrains are also present. The foliation is also clearlyseen in the mosaic of fine- to medium-grained quartzand feldspar which dominates the rock totally. Thedyke follows the same geochemical trends as thegranites which it is intruding (Carlsson, Johansson &Gee, in press).
Four non-magnetic and two magnetic zirconfractions were analysed (Table 2, Fig. 5 f). The uraniumcontents range from 400 to 2270 ppm, radiogenic leadfrom 80 to almost 400 ppm, and common lead from1.3 to 15.2 ppm. The highest contents are found in thesmallest size fraction ('NM < 45'), which stands outas containing much more impure zircons than theothers, and also plots above the discordia line. Asmight be expected, the zircons are highly discordant(43-62%). The 207Pb/206Pb ages range from 1583 to1670 Ma. If all points are included in the regressioncalculation, a discordia with a very ill-defined upperintercept (1741 «j» Ma) and high MSWD (38) isproduced. Excluding the clearly deviating point 4gives a discordia with an upper intercept age of1739 i^ Ma, a lower intercept of 215 I7,7 Ma, andMSWD 8.2 (Fig. 5f). There is thus little change in theupper intercept age itself, but some improvement inthe error and MSWD; this is the preferred calculation.Though still rather imprecise, it nevertheless indicatesthat the aplites at Bangenhuken are of similar age tothe Bangenhuk Formation granitoids, and wereintruded during the same general magmatic event.
3.b.7. Sample J 92:010 - red gneissic granite,Instrumentberget
Sample J 92:010 comes from a red, muscovite-rich,gneissic granite at Instrumentberget southeast ofMosselbukta in northern Ny Friesland (Fig. 2). Thisgranite is situated between the Smutsbreen and PolhemFormations, at a lower tectonostratigraphic level thanthe Bangenhuk Formation granitoids proper, andmay in that respect be equivalent to the Flatan granitefurther north. It is overlain by conglomerates pre-viously interpreted to belong to the RittervatnetFormation (Gayer & Wallis, 1966; Gayer, 1969), butnow interpreted to belong to the base of the Polhem
http://journals.cambridge.org Downloaded: 10 Oct 2014 IP address: 130.241.16.16
Isotope studies of Svalbard granitoids 315
Formation (P. Witt-Nilsson and F. Hellman, unpub.mapping). These conglomerates have been investi-gated in detail by F. Hellman (unpub. B.A. thesis,Univ. Lund, 1994). The sampled rock is a red-coloured, muscovite-rich gneissic granite. It lacks thepervasive foliation/lineation typical for many of theBangenhuk granitoids, but instead shows signs of afaint migmatitic structure. However, the sample wastaken from an area of more homogeneous and massivegranite. A nearby sample analysed geochemically byCarlsson, Johansson & Gee (in press) has a monzo-granitic composition and seems to follow the samegeochemical trends as the Bangenhuk Formationgranitoids.
The analysed zircon fractions contain 470-680 ppmU, 85-100 ppm radiogenic Pb, and 2-4 ppm commonPb (Table 2). They plot highly discordantly (37-68 %)with 207Pb/206Pb model ages between 1547 and1679 Ma, and yield a rather imprecise upper interceptage of 1737 t\\ Ma, with lower intercept 204 !*7 Maand MSWD 4.5 (Fig. 5g). The upper intercept age,however, is supported by' Kober method' single grainevaporation analyses of six zircons from the samesample, where the maximum 207Pb/206Pb ages recorded(from two grains) are 1732 + 10 Ma and 1 744 +11 Ma,respectively (F. Hellman, unpub. B.A. thesis, Univ.Lund, 1994). The combined evidence would thussuggest a crystallization age around 1735 Ma for theInstrumentberget granite, similar to the ages obtainedfor the Bangenhuk Formation granitoids.
3.c. U-Pb titanite dating
Out of the seven rock samples investigated for zircon,titanite was recovered from three: the undeformedBrennkollen granite (L90:13), the strongly shearedgneissic granite from Gyllenskoldholmane (J 91:006),and the normally foliated gneissic granite fromReinsbukkdalen (J 91:017). In addition, titanite wasalso recovered from a smaller sample of 'ultra-mylonitic veined gneissic granite' taken at the thrustcontact between the Bangenhuk and underlyingRittervatnet Formation in Reinsbukkdalen (sampleL91:32). After the first analyses of one fraction ofbrownish titanite from each sample, additionalcolourless titanite was hand-picked and analysed fromsamples J 91:006 and J 91:017. The results arereported in Table 2 and illustrated in Figure 6.
The titanite from the undeformed Brennkollengranite contains 106 ppm U, 36.1 ppm radiogenic leadand 1.9 ppm common lead. It plots slightly to the leftof the zircon discordia (Fig. 5 a), being about asdiscordant as the abraded zircon fraction 8, with a207Pb/206Pb age of 1735 Ma. This suggests that thetitanite crystallized at about the same time, or onlyslightly later, than the zircons. It has undergone somelead loss (or uranium gain), but no majorrecrystallization in Caledonian time.
The brown-coloured titanites from samplesJ 91:006 and J 91:017 are unusually rich in uranium(290-300 ppm) and radiogenic lead (about 21 ppm)for titanites; the colourless fractions contain aboutone magnitude less of both elements (27-28 ppm U,2.4-2.9 ppm radiogenic lead, and almost equalamounts of common lead). The titanite from sampleL 91:32 falls in between: 94 ppm U, 7.8 ppm radio-genic lead, and 2.8 ppm common lead. All thesetitanites plot on a discordia close to its lower interceptat 410 Ma (88-97% discordant with respect to theupper intercept at 1750 Ma).
One would expect the choice of common leadvalues to be critical, especially for the two colourlessfractions with their low content of radiogenic lead.Choosing a Stacey & Kramers (1975) common leadvalue for 400 Ma instead of 1800 Ma has the effect ofmoving the points down the discordia, lowering theU/Pb and Pb/Pb model ages by 20-75 Ma, but haslittle influence (0-1 Ma) on the more meaningfulintercept ages, and thus on the final interpretation. Alldata presented here have been calculated with acommon lead value for 1800 Ma (2MPb/204Pb = 15.57,207pb/204pb = 1 5 i 2 8 > 208pb/204pb = 35 2Q).
A regression through the five highly discordanttitanite points produces a discordia with an ill-definedupper intercept of 1804 +"J Ma, a lower intercept of413 +_H Ma, and MSWD 3.5. If the far less discordanttitanite from the Brennkollen sample L90:13 isincluded, the precision improves and the upperintercept becomes 1749+12 Ma, the lower intercept408+ 9 Ma, and the MSWD 3.3 (Fig. 6). It is, ofcourse, debatable to plot titanite from several widelyspaced rock samples on the same discordia, and oneshould not place too much attention on the precisionof these intercepts. However, the titanites come fromigneous rocks of similar age and composition andtherefore the ages themselves may be of geologicalsignificance. The upper intercept agrees closely withthe upper intercept ages of the zircons from theserocks, and the lower intercept with the age ofCaledonian metamorphism and uplift as deducedfrom 40Ar/39Ar measurements on muscovites andhornblendes from northern Ny Friesland (Gee &Page, 1994). The result suggests that most of thetitanite recrystallized during Caledonian amphibolite-grade metamorphism and deformation about 410 Maago, but that it also retains a Palaeoproterozoiccomponent (about 1750 Ma old), similar in age to theprotoliths of the gneisses. In the undeformedBrennkollen sample, the titanite is dominated by thisolder component.
3.d. Rb-Sr and Sm-Nd isotopic results
Whole rock powders from the six zircon samples ofBangenhuk Formation granitoids have been analysedfor Rb-Sr and Sm-Nd (Tables 3 and 4). As could be
Figure 6. Concordia diagram for titanite from theBangenhuk granitoids.
expected, the samples do not form a well-definedisochron in the Rb-Sr system, but fall on an errorchronwith an age of around 1650 Ma, initial 87Sr/86Sr of0.703, and MSWD 67 (using 1.0% uncertainty in87Rb/86Sr, and 2 sigma errors from the 87Sr/86Srruns). The slope is very much controlled by the twoRb-rich and Sr-poor samples J 91:017 and J 92:011.This relationship between Rb-Sr whole rock andU-Pb zircon ages is similar to that found in manyother associations, e.g. in the Fennoscandian Shield(especially the Svecofennian Domain and theTransscandinavian Igneous Belt), with the Rb-Sr agebeing 100-150 Ma younger than the U-Pb age, andlacking geological significance (Welin, Kahr &Lundegardh, 1980; Welin, Vaasjoki & Suominen,1983; Welin, 1992). It is interesting to note, however,that there has been no additional lowering or resettingof the Rb-Sr age during the Caledonian orogeny,despite high amphibolite fades metamorphism (Bayly,1957).
Taken at face value, the low initial Sr ratio of 0.703would suggest a primitive, mantle-like origin for these
granitoids. However, since the isochron has beendisturbed, not too much faith should be placed in thisfigure. Calculation of initial Sr isotope compositionsfor the individual data points, using the obtainedU-Pb zircon ages, produces unreasonably low initial87Sr/86Sr ratios, even below 0.700 (eSr(i) around orbelow —400 for the two most Rb-rich samples). This,again, points to the Rb-Sr system being disturbed inthese samples; no firm conclusions can thus be basedon this system.
The Sm-Nd data plot along a line with a slopecorresponding to an age of 1850 + 310 Ma, with aninitial 143Nd/144Nd ratio of 0.51016 (eNd(i) =-1.5) .The Sm-Nd isochron age is thus similar or slightlyhigher than the U-Pb zircon ages of the samples, butit has a large error reflecting the limited spread in14?Sm/l44Nd (0.0933-0.1233). Using the newlyacquired U-Pb zircon ages in the calculation, theindividual samples will have initial eNd-values between— 2.1 and —3.3 (Table 4, Fig. 7). Extrapolation backto the chondritic mantle line (CHUR) yields TCHUR
model ages of 1.91-2.04 Ga, and extrapolation backto the Depleted Mantle curve of De Paolo (1981)yields TDM model ages of 2.17-2.33 Ga. The youngestmodel ages are given by the aplitic dyke fromBangenhuken, but the difference is not significant, andthe Sm-Nd data suggest a similar age and origin forall these rocks. The data are not compatible with adirect mantle origin for the magmas forming theBangenhuk granitoids. They would either suggest thatthese magmas were derived from crustal rocks a fewhundred million years older, or by a mixture of mantlematerial and even older crustal rocks.
4. Discussion and conclusions
The U—Pb zircon analyses of the Bangenhuk andInstrumentberget granitoids presented here prove theexistence of extensive units of Palaeoproterozoicgranitic basement within the Ny Friesland Caledo-nides. The ages obtained range between 1720 and
Table 3. Rb-Sr data for the Bangenhuk granitoids, Ny Friesland, Svalbard
(a) Rb and Sr contents and 87Rb/86Sr ratio from XRF. Estimated analytical uncertainty of the 87Rb/86Sr ratio is 1.0%.(b) 87Sr/86Sr ratios corrected for Rb interference and normalized to 86Sr/88Sr = 0.1194. Error given as 2 s.D. of the mean from the mass
spectrometer run in the last digits.(c) ESr-values and TUr Sr model ages according to McCulloch & Chappell (1982); present-day 87Rb/86Sr mantle ratio = 0.0827, present-
j l l^ 1 2.S£ * o>. H °rt jp O« feoc *C <=>.2 a ua .5 _><
O T J O
.2 « o
S 3 S
3 6 >'.
"2 3 „ '
u
T3
•aco
•o
c
oo
.2 £ "i 3
1 8 I V•» x o. -5. - p "
•a
a E?a 3
'•" £ ?
IXI
8
c.t;2.S
is
60 C JJ«> o a.
2 2i
I 3 "8
"5 n . r<i
J2
| 3
Bangenhuk granitoids:
ENd(i) = -2.110-3.3
TcilUR - 1.91- 2.04 0a
TDM - 2.17-2.33 0a
Figure 7. eNd vs. time for the Bangenhuk granitoids. CHUR,Chondritic Uniform Reservoir. Dashed lines areextrapolations of the Nd evolution lines back to the DepletedMantle Curve of De Paolo (1981).
1770 Ma, with those from the most undeformed rocksbeing between 1745 and 1760 Ma. Whether theyoungest ages of about 1720 to 1730 Ma reflect aslightly younger magmatic event, or have been lowereddue to later deformation, is still an open question.
It has been claimed (Gayer & Wallis, 1966; Manby,1990; Harland et al. 1992) that the BangenhukFormation gneisses were derived from volcanic (pyr-oclastic) rocks by Caledonian (or older, Krasil'scikov,1973) recrystallization and/or metasomatism. Noneof the evidence presented here supports these hy-potheses. Rocks of undoubted plutonic origin haveyielded suites of zircons that are notably uninfluencedby Caledonian recrystallization or overgrowths. Thezircon morphologies are typical of igneous bodies andthe coarse grain sizes do not favour a volcanic origin.Small euhedral prismatic zircons, typical of volcanicrocks, have been separated from meta-acid tuffs,occurring in the basal parts of the Serbreen Formation(Gayer & Wallis, 1966), about 600 m above thecontact to the Bangenhuk Formation gneisses. Theirexcellent preservation, despite Caledonian amphibo-lite facies metamorphism, also argues against anystrong recrystallization of the Bangenhuk zircons.
The Caledonian influence on the U-Pb system is,however, seen in the recrystallization ages of thetitanites. We conclude that the new age-determinationdata provide support for the field interpretation (Gee,Bjorklund & Stolen, 1994) that the intense penetrativeregional concordant foliation, characterizing mostrock units, is the result of Caledonian shearing undermiddle crustal P/T conditions. The similar age ofgneissic granitoids at more than one structural levelindicates that thrust repetition is an essentialcomponent of the Stubendorffbreen Supergroup. Ourongoing mapping and isotopic age work on thevarious igneous and sedimentary protoliths in thelower part of the Hecla Hoek succession will throw
http://journals.cambridge.org Downloaded: 10 Oct 2014 IP address: 130.241.16.16
318 A. JOHANSSON AND OTHERS
more light on these issues in the near future. However,there can no longer be any doubt that the lower partof the Hecla Hoek succession in Ny Friesland is acompositie packet of Caledonian cover and basementrocks.
Acknowledgements. The field work on Svalbard was fundedby the Swedish Polar Research Secretariat, and thelaboratory studies by the Swedish Natural Science ResearchCouncil (NFR). We also want to thank the NorwegianPolar Institute for excellent support and collaboration in thefield. This study would have been impossible without theassistance in the field of the other participants in theSWEDARCTIC Svalbard expeditions of 1990, 1991 and
1992 (see Cruise Report of Gee, 1994), especially thesampling assistance of Lars-Kristian Stolen (Lund) andLaurence Page (Lund). The zircons of sample L 90:13 wereseparated by Paula Allart and Sandeep Singh; the rest of thecrushing and mineral separation work was done by KarinHogdahl (Stockholm). Dan Holtstam (Stockholm) assistedwith XRD identification of titanite separates. Improvementsin chemical and mass spectrometric procedures during thecourse of this work were introduced by Hans Schoberg(Stockholm). A special grant from the Hasselblad Foun-dation enabled many of the improvements in zirconanalytical techniques. Figures 1-3 were modified after Gee,Bjorklund & Stolen (1994) and Figure 7 was drawn by IngerArnstrom (Stockholm). We also want to thank the twoanonymous reviewers for their comments and suggestions.
L 90:13 Brennkollen 79° 46' 30" N / 15° 49' 30" EGrey, even- and medium-grained, undeformed granite, with qz, kfsp, plag, and mafic aggregates of amph,bi, ep, opq, ti and zr.
Zircons light beige; semi-turbid to transparent; prismatic but often with uneven surfaces; no visible cores,overgrowths or zonations, few inclusions, but often fractured; fragments common.
LP91:16 2 km N Einsteinodden 79° 04'10" N / 16° 18'20" EGrey, even- and medium-grained, undeformed granodiorite, with qz, kfsp, plag, and mafic aggregates ofamph, cpx-relics, bi, opq, grt, minor ti and zr.
Zircons light beige; semi-transparent to transparent; prismatic with sharp edges; no visible cores orovergrowths, but dark inclusions common.
J 91:006 Gyllenskoldholmane 79° 00' 40" N / 16° 16' 55" ERed, medium-grained, strongly foliated and lineated granite from within the Billefjorden Fault Zone, withfine-grained recrystallized qz and fsp, plus amph (partly altered), bi, ep, opq, ti, ap and zr.
Zircons brownish, dominantly rounded and turbid; too dark to allow observation of internal structures;semi-transparent only in finer fractions.
J 91:013 Reinsbukkbreen 79° 12' 00" N / 16° 51' 30" ERed, medium-grained, foliated and lineated granite, with qz, kfsp, plag, bi, aggregates of ep, opq and othersecondary minerals, and zr.
Zircons brownish, semi-transparent to turbid, but often euhedral with sharp edges; occasional faint growthzoning observed, but no cores.
J 91:017 Reinsbukkdalen 79° 13'30" N/16° 11'10" ERed, medium-grained, foliated and lineated granite with qz, kfsp, plag, bi, ti, all, opq, zr.Zircons brownish, semi-transparent to turbid; euhedral to rounded or irregular; growth zoning sometimesvisible, occasional small possible cores, many cracks, dark inclusions common in magnetic fractions.
J 92:011 Bangenhuken 79'51'55" N/15° 39'00" ERed, fine-grained, foliated aplite from aplitic dyke, with qz, fsp, strongly altered bi, opq.Zircons small, rounded, brownish, semi-transparent to turbid, with occasional rusty staining on surfaces;no cores, overgrowths or zonations observed.
J 92:010 Instrumentberget 79° 50' 05" N / 16° 11' 30" ERed, medium-grained, muscovite-rich, gneissic granite, with qz, kfsp, plag, mu, bi, opq, ap, zr.Zircons brownish with occasional rusty staining; semi-transparent to turbid; prismatic but with roundededges and uneven surfaces; no cores, overgrowths or zonations observed.
http://journals.cambridge.org Downloaded: 10 Oct 2014 IP address: 130.241.16.16
Isotope studies of Svalbard granitoids 319
References
BALASHOV, YU. A., LARIONOV, A. N., GANNIBAL, L. F.,
SIROTKIN, A. N., TEBENKOV, A. M., RYUNGENEN, G. I.
& OHTA, Y. 1993. An Early Proterozoic U-Pb zirconage from an Eskolabreen Formation gneiss in southernNy Friesland, Spitsbergen. Polar Research 12, 147-52.
BAYLY, M. B. 1957. The Lower Hecla Hoek of Ny Friesland,Spitsbergen. Geological Magazine 94, 377-92.
BLOMSTRAND, C. W. 1864. Geognostiska iaktagelser underen resa till Spetsbergen ar 1861 (Geognosticalobservations during a voyage to Spitsbergen in theyear 1861). Kungliga Svenska VetenskapsakademiensHandlingar 4 (6), 1-46.
CARLSSON, P., JOHANSSON, A. & GEE, D. G. In press.
Geochemistry of the Paleoproterozoic Bangenhukgranitoids, Ny Friesland, Svalbard. Geologiska Fb'renin-gens i Stockholm Forhandlingar.
DEBON, F. & LE FORT, P. 1982. A chemical-mineralogical
classification of common plutonic rocks andassociations. Transactions of the Royal Society ofEdinburgh: Earth Sciences 73, 135-49.
DE GEER, G. 1909. Some leading lines of dislocation inSpitzbergen. Geologiska Foreningens i StockholmForhandlingar 31, 199-208.
DE PAOLO, D. J. 1981. Neodymium isotopes in the ColoradoFront Range and crust-mantle evolution in theProterozoic. Nature 291, 193-6.
FAIRBAIRN, P. E. 1933. The petrology of the Hecla HookFormation in central Spitsbergen. Geological Magazine70, 437-54.
GAYER, R. A. 1969. The geology of the FemilsJ0en region ofnorthwestern Ny Friesland, Spitsbergen. NorskPolarinstitutt Skrifter 145, 1-45.
GAYER, R. A. & WALLIS, R. H. 1966. The petrology of the
Harkerbreen Group of the Lower Hecla Hoek of NyFriesland and Olav V Land, Spitsbergen. NorskPolarinstitutt Skrifter 140, 1-32.
GEE, D. G. 1986. Svalbard's Caledonian terranes reviewed.Geologiska Foreningens i Stockholm Forhandlingar 108,284-6.
GEE, D. G. 1994. Svalbard's Caledonian Terranes. InSwedish Research in Svalbard- A Cruise Report (eds A.Karlqvist and M. Lonnroth Carlsson), pp. 92-117.Stockholm: Swedish Polar Research Secretariat.
GEE, D. G., BJORKLUND, L. & STOLEN, L.-K. 1994. Early
Proterozoic basement in Ny Friesland - implicationsfor the Caledonian tectonics of Svalbard.Tectonophysics 231, 171-82.
GEE, D. G. & PAGE, L. 1994. Caledonian terrane assemblyon Svalbard: new evidence from Ar/Ar dating in NyFriesland. American Journal of Science 294, 1166-86.
GEE, D. G., SCHOUENBORG, B., PEUCAT, J. J., ABAKUMOV,
S. A., KRASIL'SCIKOV, A. A. & TEBENKOV, A. M. 1992.
New evidence of basement in the Svalbard Caledonides:early Proterozoic zircon ages from Ny Frieslandgranites. Norsk Geologisk Tidsskrift 72, 181-90.
HARLAND, W. B. 1941. Geological notes on the StubendorffMountains, West Spitsbergen. Proceedings of the RoyalSociety of Edinburgh B51 (10), 119-29.
HARLAND, W. B. 1959. The Caledonian sequence in NyFriesland, Spitsbergen. Quarterly Journal of the Geo-logical Society of London 114, 307-43.
HARLAND, W. B. 1985. Caledonide Svalbard. In The
Caledonide Orogen - Scandinavia and Related Areas
(eds D. G. Gee and B. A. Sturt), pp. 999-1016.Chichester: Wiley.
HARLAND, W. B., SCOTT, R. A., AUCKLAND, K. A. & SNAPE,
I. 1992. The Ny Friesland orogen, Spitsbergen.Geological Magazine 129, 679-708.
HARLAND, W. B., WALLIS, R. H. & GAYER, R. A. 1966. A
revision of the lower Hecla Hoek succession in centralnorth Spitsbergen and correlation elsewhere. GeologicalMagazine 103, 70-97.
JACOBSEN, S. B. & WASSERBURG, G. J. 1984. Sm-Nd isotopicevolution of chondrites and achondrites. II. Earth andPlanetary Science Letters 67, 137-50.
KOBER, B. 1986. Whole-grain evaporation for 2O'Pb/2O6Pbage investigations on single zircons using a double-filament thermal ion source. Contributions to Min-eralogy and Petrology 93, 482-90.
KRASIL'SCIKOV, A. A. 1973. Stratigraphy andpalaeotectonics of the Precambrian and early Palaeo-zoic of Spitsbergen. Trudy Arkticheskogo Nauchno-IssledovateF skogo Instituta 172, 1-120 (in Russian).
KRASIL'SCIKOV, A. A. 1979. Stratigraphy and tectonics ofthe Precambrian of Svalbard. Norsk PolarinstituttSkrifter 167, 73-9.
KROGH, T. E. 1973. A low-contamination method forhydrothermal decomposition of zircon and extractionof U-Pb for isotopic age determination. GeochimicaCosmochimica Ada 37, 485-94.
LARIONOV, A. N., JOHANSSON, A., TEBENKOV, A. M. &
SIROTKIN, A. N. In press. U-Pb zircon ages fromthe Eskolabreen Formation, southern Ny Friesland,Svalbard. Norsk Geologisk Tidsskrift.
LUDWIG, K. R. 1991a. P B D A T - A computer program forprocessing Pb-U-Th isotope data, version 1.20. U.S.Geological Survey Open-file Report 88-542.
LUDWIG, K. R. 1991ft. ISOPLOT-A plotting and re-gression program for radiogenic-isotope data, version2.56. U.S. Geological Survey Open-file Report 91-445.
MANBY, G. M. 1990. The petrology of the HarkerbreenGroup, Ny Friesland, Svalbard: protoliths and tectonicsignificance. Geological Magazine 127, 129-46.
MANBY, G. M. & LYBERIS, N. 1991. Contrasting tectono-metamorphic terranes in the NE Svalbard:Sm/Nd-Rb/Sr isotopic and structural constraints.Terra Nova Abstracts Supplement 4, 22-3.
MCCULLOCH, M. T. & CHAPPELL, B. W. 1982. Nd isotopic
characteristics of S- and I-type granites. Earth andPlanetary Science Letters 58, 51-64.
NATHORST, A. G. 1910. Beitrage zur Geologie der BarenInsel, Spitzbergens und des Konig-Karls-Landes.Bulletin of the Geological Institution of the University ofUppsala 10, 261-415.
NORDENSKIOLD, A. E. 1863. Geografisk och geognostiskbeskrifning over de nordostra delarna af Spetsbergenoch Hinlopen Strait. Kungliga SvenskaVetenskapsakademiens Handlingar 4 (7), 1-25.
NORDENSKIOLD, A. E. 1866. Utkast till Spetsbergens geologi.Kungliga Svenska Vetenskapsakademiens Handlingar 6(7), 1-35.
PARRISH, R. R. 1987. An improved micro-capsule for zircondissolution in U-Pb geochronology. Chemical Geology(Isotope Geoscience Section) 66, 99-102.
SOKOLOV, V. N., KRASIL'SCIKOV, A. A. & LIVSHITS, Y . Y.
1968. The main features of the tectonic structure ofSpitsbergen. Geological Magazine 105, 95-115.
STACEY, J. S. & KRAMERS, J. D. 1975. Approximation of
http://journals.cambridge.org Downloaded: 10 Oct 2014 IP address: 130.241.16.16
320 Isotope studies of Svalbard granitoids
terrestrial lead isotope evolution by a two-stage model.Earth and Planetary Science Letters 26, 207-21.
STEIGER, R. H. & JAGER, E. 1977. Convention on the use of
decay constants in geo- and cosmochronology. Earthand Planetary Science Letters 36, 359-62.
STRECKEISEN, A. 1976. To each plutonic rock its propername. Earth Science Reviews 12, 1-33.
WALLIS, R. H. 1969. The Planetfjella Group of the LowerHecla Hoek of Ny Friesland, Spitsbergen. NorskPolarinstitutt Arbok 1967, 80-108.
WELIN, E. 1992. Isotopic results of the Proterozoic crustalevolution of south-central Sweden: review andconclusions. Geologiska Fbreningens i StockholmForhandlingar 114, 299-312.
WELIN, E., KAHR, A.-M. & LUNDEGARDH, P. H. 1980.
Rb-Sr isotope systematics at amphibolite faciesconditions, Uppsala region, Eastern Sweden.Precambrian Research 13, 87-101.
WELIN, E., VAASJOKI, M. & SUOMINEN, V. 1983. Age
differences between Rb-Sr whole rock and U-Pb zirconages of syn- and postorogenic Svecokarelian granitoidsin Sottunga, SW Finland. Lithos 16, 297-305.
YORK, D. 1969. Least squares fitting of a straight line withcorrelated errors. Earth and Planetary Science Letters5, 320-4.
