Detrital zircon characteristics of the LowerCretaceous Isachsen Formation, Sverdrup Basin:source constraints from age and Hf isotope data

Torkil S. Røhr, Tom Andersen, Henning Dypvik, and Ashton F. Embry

Abstract: Detrital zircons from the Lower Cretaceous Isachsen Formation of the Sverdrup Basin, Canadian Arctic Archi-pelago, have been dated by the U–Pb method and analyzed for Hf isotopes by laser ablation microprobe – inductivelycoupled plasma – mass spectrometry (LAM–ICP–MS). Five samples from four locations on Ellesmere and Axel Heibergislands display similar ranges of U–Pb ages, with significant zircon populations at 2.8–2.6, 1.9–1.8, 1.7–1.6, and 1.2–1.0 Ga. Major hiatuses occur between 2.4 and 2.0 Ga and from 0.96 to 0.5 Ga. Low initial 3Hf values indicating recycledcrust components are significant in Palaeoproterozoic (1.9–1.8 Ga) and Neoarchaean (2.8–2.6 Ga) zircon populations.Other U–Pb age populations in the studied samples are dominated by zircon with positive 3Hf values, indicating a signifi-cant contribution from mantle-derived protoliths. The 3Hf values seen within a given U–Pb age population are generallyconsistent, with only minor scatter among the different samples. U–Pb and Hf data closely resemble previously publisheddata from Lower Cretaceous rocks in northern Greenland, suggesting they have the same origin. The data are also largelyconsistent with the East Greenland Caledonides and the Precambrian basement of Greenland and northern Canada as pre-dominant sources of zircon for the studied sandstones. However, based on the level of similarity between data from theWandel Sea Basin and Sverdrup Basin sediments and on previous Nd isotope work in the Sverdrup Basin, it is likely thatthe sediments represent redeposited lower and middle Palaeozoic sediments.

Resume : Des zircons detritiques de la Formation d’Isachsen (Cretace inferieur) du bassin de Sverdrup, dans l’archipelArctique, ont ete dates par U–Pb et analyses pour les isotopes Hf par ablation au laser et spectrometrie de masse a plasmainduit par haute frequence (LAM–ICP–MS). Cinq echantillons de quatre sites sur les ıles d’Ellesmere et d’Axel Heibergmontrent des plages similaires d’ages U–Pb avec des populations significatives de zircons a 2,8 a 2,6, 1,9 a 1,8, 1,7 a 1,6,et 1,2 a 1,0 Ga. Les principales lacunes se situent entre 2,4 a 2,0 et 0,96 a 0,5 Ga. Les faibles valeurs initiales eHf, indi-quant la presence de composantes de la croute recyclee, sont importantes dans les populations de zircons du Paleoprotero-zoıque (1,9–1,8 Ga) et du Neoarcheen (2,8–2,6 Ga). D’autres populations dont les ages ont ete determines par U–Pb dansles echantillons etudies sont dominees par des zircons a valeurs eHf positives, indiquant une contribution importante deprotolites provenant du manteau. Les valeurs eHf percues a l’interieur d’une plage donnee de population determinee par U–Pb sont generalement constantes, avec seulement une dispersion mineure entre les differents echantillons. Les donnees U–Pb et Hf sont tres similaires aux donnees publiees anterieurement concernant des roches du Cretace inferieur dans le norddu Groenland, suggerant une origine commune. Les donnees concordent aussi grandement avec les Caledonides de l’est duGroenland et le socle precambrien du Groenland et du nord du Canada en tant que sources predominantes de zircons pourles gres etudies. Toutefois, en se basant sur le niveau de similitude entre les donnees des sediments de la mer de Wandelet du bassin de Sverdrup et sur des travaux anterieurs sur les isotopes Nd dans le bassin de Sverdrup, il est probable queles sediments representent des sediments deposes de nouveau au Paleozoıque inferieur et moyen.

[Traduit par la Redaction]

IntroductionThe principal idea behind detrital zircon chronology is that

the ages of detrital zircons reflect the age of magmatic and

metamorphic rocks in their source terranes, thus making itpossible to locate the provenance of the sediment. With laserablation – inductively coupled plasma – mass spectrometry(LA–ICP–MS), it is possible to date large numbers of zirconsin a relatively short time and at relatively low cost. Addition-ally, an ICP–MS with a multicollector array enables analysisof Hf isotope data from the same zircon grains, providing ad-ditional data in the search for the source terrane (e.g.,Andersen et al. 2002; Griffin et al. 2004).

Røhr et al. (2008) suggested that Lower Cretaceous sedi-ments in the Wandel Sea Basin (Fig. 1) were sourced fromthe East Greenland Caledonides and Laurentian basement.Although the depositional ages of Cretaceous northernGreenland successions are poorly constrained, the bedsstudied by Røhr et al. (2008) are believed to be contempora-neous with the Hauterivian–Aptian Isachsen Formation in

Received 25 September 2009. Accepted 14 January 2010.Published on the NRC Research Press Web site at cjes.nrc.ca on18 March 2010.

Paper handled by Associate Editor W. Davis.

T.S. Røhr,1 T. Andersen, and H. Dypvik. Department ofGeosciences, University of Oslo, P.O. Box 1047 Blindern, NO-0316 Oslo, Norway.A.F. Embry. Geological Survey of Canada, 3303 33rd StreetNW, Calgary, AB T2L 2A7, Canada.

1Corresponding author (e-mail: torkil.rohr@ngu.no).

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the Sverdrup Basin (Figs. 1, 2). Unlike the Wandel Sea Ba-sin, the Sverdrup Basin of the Canadian Arctic Archipelagohas been extensively studied and the provenances of variousstratigraphic levels within this area are relatively wellknown (e.g., Patchett et al. 1999, 2004). This is also truefor the Isachsen Formation, for which a source area in theeast and south is supported by paleocurrent directions(Embry 1991a). Sediment composition and Nd isotope datasuggest that the Isachsen Formation received a mix of clas-tic material derived from the Caledonides and the Green-land – Canadian Shield (Patchett et al. 2004). Sedimentssourced from the Caledonides may have been supplied di-rectly from the eastern flank of this mountain belt but mayalso have taken alternative routes involving several stagesof redeposition. These stages would have included (1) depo-sition into Ordovician and Silurian sedimentary formationsof the Franklinian Basin, (2) erosion of these lower Palaeo-zoic formations and deposition into Middle and Upper Dev-onian clastic successions, and finally (3) erosion of theDevonian clastics and deposition into the Isachsen and otherMesozoic formations (Embry 1991a; Patchett et al. 2004).Considering that the Caledonides also include large volumes

of older sediment, the detrital zircon found in the IsachsenFormation has potentially been through several recyclingevents (e.g., Kalsbeek et al. 2000; Watt et al. 2000).

The work presented here is an integrated U–Pb and Lu–Hf isotope study of detrital zircons from the Isachsen For-mation of the Sverdrup Basin, which to a great extent illus-trates the complexity of detrital zircon studies involvingrecycled sediments. This study also provides insight into thedetrital zircon populations of rocks sourced from the Cale-donides and the Greenland – Canadian Shield and representsa valuable reference frame for comparison with the prove-nance of other Lower Cretaceous formations in the area(Røhr et al. 2008).

Geological setting

Basement geologyThe Precambrian geology of Canada and Greenland in-

cludes large Archaean crustal blocks such as the ChurchillProvince and Slave Craton (Fig. 1). These Archaean blocksare dominated by Neoarchaean rocks in addition to smallerMesoarchaean enclaves (Davis et al. 2006 and references

Fig. 1. Early Cretaceous palaeogeographic reconstruction of North Atlantic region based on Lawver et al. (2002) with modifications basedon Torsvik (T. Torsvik, personal communication, 2008). EIMB, Ellesmere–Inglefield mobile belt; FEFB, Franklinian–Ellesmerian Fold Belt;GO, Grenvillian Orogen; KMB, Ketilidian mobile belt; KS, northernmost exposure of the Krummedal supracrustals; NMB, Nagssugtoqidianmobile belt; RMB, Rinkian mobile belt; THO, Trans-Hudson Orogen; TMZ, Taltson–Thelon magmatic zone; WSB, Wandel Sea Basin. *,Devonian sediments include the so-called Devonian Clastic Wedge in Arctic Canada.

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therein). The Slave Craton also includes some of the oldestknown rocks on Earth, i.e., the Eoarchaean Acasta Gneiss(Iizuka et al. 2007 and references therein).

The Archaean blocks are in turn bordered by Palaeopro-terozoic orogenic belts, the largest of which is the 1.9–1.8 Ga Trans-Hudson Orogen (Jackson and Berman 2000;Peterson et al. 2002; St. Onge et al. 2007). The Nagssugto-qidian and Rinkian mobile belts in Greenland display manysimilarities to the Trans-Hudson Orogen; all three largelyconsist of Archaean crust reworked at approximately thesame time (Fig. 1; Thrane et al. 2005; Connelly et al. 2006;St. Onge et al. 2007). This has led some authors to correlatethese three terranes and suggest they form part of a largeHimalayan-type orogen (Zhao et al. 2002).

The Inglefield mobile belt is somewhat different from the

Trans-Hudson, Nagssugtoqidian, and Rinkian terranes. Itgenerally yields ages of 1.98–1.91 Ga and 1.78–1.74 Gaand predominantly consists of juvenile magmatic rocks; i.e.,it is a magmatic system that did not incorporate significantArchaean components (Frisch and Hunt 1988; Nutman et al.2008).

The East Greenland Caledonides extend along the easterncoast of Greenland from *698N to 808N. The northeastern-most part of this orogenic belt is covered by Carboniferous–Tertiary sediments of the Wandel Sea Basin. Cretaceoussediments within this basin are thought to have been sourcedfrom the Caledonides and from basement rocks in Greenland(Fig. 1; Røhr et al. 2008).

North of ca. 768N, the East Greenland Caledonides aredominated by Proterozoic sediments (the Hagen Fjord

Fig. 2. Simplified geological map of the Canadian Arctic Archipelago and northern Greenland, modified from Arne et al. (2002). Subseaoutline of the Sverdrup Basin is shown by broken lines. Sampling sites are indicated by encircled numbers: (1) C-072382, (2) C-82218,(3) C-86116, (4&5) C-100614 and C-100629. AHI, Axel Heiberg Island; NP, North Pole. Other abbreviations as in Fig. 1.

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Group) and 2.0–1.75 Ga gneisses (Kalsbeek et al. 1993;Pedersen et al. 2002). Parts of the latter went througheclogite-facies metamorphism during the Caledonian Orog-eny (at ca. 0.44 Ga; Brueckner et al. 1998). South of ca.768N, this orogenic belt is dominated by sediments of theNeoproterozoic Eleonore Bay Supergroup and the Mesopro-terozoic Krummedal Sequence, whereas the Palaeoprotero-zoic basement is less exposed (e.g., Kalsbeek et al. 2000;Watt et al. 2000). Caledonian age (0.44–0.42 Ga) granitoidsare common south of ca. 768N but do not cover large areas(Kalsbeek et al. 2001; Andresen et al. 2007). A Grenvillian-aged event recorded by 0.97–0.9 Ga granitoids and associ-ated metamorphism of the Krummedal sequence has beendocumented in the southern parts of the East Greenland Ca-ledonides.

The Franklinian mobile belt is a Palaeozoic orogenic zonethat extends from Prince Patrick Island through Devon andEllesmere islands to northern Greenland (FEFB; Figs. 1, 2).It has been affected by several stages of deformation, themost extensive being the Ellesmerian Orogeny of Late Dev-onian – Early Carboniferous age. This belt is dominated bylow-grade (up to lower amphibolite facies) lower Palaeozoicsediments, which were deposited in two main provinces, anorthern one characterized by deep water settings and asouthern shelf province. The shelf province was dominatedby carbonate sedimentation; whereas the deep water prov-ince was dominated by deposition of fine-grained mud andreworked carbonates, commonly in the form of turbidites(Higgins et al. 1991; Trettin et al. 1991). The oldest of thesePalaeozoic sedimentary formations, i.e., the Cambrian andOrdovician deposits, were generally sourced from theGreenland – Canadian Shield. The Silurian and Lower Dev-onian deposits, on the other hand, were derived from the Ca-ledonides (Patchett et al. 1999).

Large amounts of clastic sediments were deposited frommid- to Late Devonian times, forming the sequences knownas the ‘‘Devonian Clastic Wedge’’ (see Devonian sedimentsin Figs. 1, 2). Palaeodrainage patterns suggest that the Mid-dle Devonian clastic sequences were sourced from the east,mainly from the Caledonides (Embry 1991b; McNicoll et al.1995). The Upper Devonian deposits, on the other hand,were sourced from the north, more precisely from lower Pa-laeozoic formations that were uplifted during the Ellesmer-ian Orogeny (Embry 1991b).

The Sverdrup BasinThe Sverdrup Basin (Figs. 2, 3) was a major depocentre

from Carboniferous to Early Tertiary and contains a thickand generally complete Mesozoic succession. The locationof the Sverdrup Basin partly coincides with that of theFranklinian Orogen, and the basin is thought to have formedby the extensional collapse of this fold belt (Beauchamp etal. 1989). In this basin, rifting began in the mid-Carboniferous, continued until the Early Permian, and wassucceeded by a long period of thermal subsidence. Renewedrifting, probably related to the opening of the Amerasian Ba-sin, commenced in the Early Cretaceous and lasted until theearliest Late Cretaceous. A period of thermal subsidence fol-lowed this event and lasted until the latest Late Cretaceous.The northeastern part of the basin was subsequently com-pressed during the Early Tertiary Eurekan Orogeny. The

rift-bounded Carboniferous and Permian deposits includecarbonates and evaporites, in contrast to the succeeding pre-dominantly siliciclastic beds (Davies and Nassichuk 1991;Embry 1991a).

Mesozoic strata in the Sverdrup Basin were mainly de-rived from the south and east, but minor contributions werealso made from a northwestern source area from the Triassicto Late Cretaceous (Embry 1991a; Omma et al. 2007). Asource area to the south and east has also been inferred forthe sediments analyzed in this study, i.e., the Hauterivian–Barremian Isachsen Formation (Embry 1991a). The IsachsenFormation can be divided into three members, known as thePaterson Island, Rondon, and Walker Island members(Fig. 3). The Paterson Island Member is made up of deltafront and basin margin deposits in its lower parts, whereasthe bulk of this regressive member consists of delta plaindeposits. The Rondon Member is associated with a majormid-Barremian transgression and is dominated by shale andsiltstone deposits of offshore shelf origin. Prograding deltasediments are represented by the overlying Walker IslandMember (Embry 1991a). The Walker Island Member dis-plays similar sedimentary facies as the Paterson IslandMember, with delta front sandstones in the lower portion,and delta plain deposits dominating the upper section. Alongthe basin margins, where the Rondon Member is absent, theIsachsen Formation is consequently not subdivided. Two ofthe samples analyzed in this study were collected from lo-calities at the basin margin (samples C-072382 and C-86116); one from the Walker Island Member (sample C-82218); and two from the Paterson Island Member (samplesC-100614 and C-100629).

Detrital zircon data from the IsachsenFormation

Analytical methodsU–Pb and Lu–Hf analyses were for the most part made

according to analytical protocols given in Andersen et al.(2007) and Røhr et al. (2008). However, a new approach re-garding standard bracketing of U–Pb data has been imple-mented. The approach used in Andersen et al. (2007) andRøhr et al. (2008) assumes that the isotopic ratio is a linearfunction of signal ratio and time, i.e.,

½1� y ¼ xðaþ ctÞ

where y is the isotopic ratio to be determined, x is the ob-served voltage ratio, and t is the time since the start of theanalytical session. The coefficients a and c are determinedby linear regression of the calibration standards. This ap-proach is equivalent to that incorporated in the commercialsoftware package GLITTER (van Achterbergh et al. 2001).One or more standards can be used to determine a and c bylinear regression. At high counting rates on the ion counters(typically >100 000 cycles per second), effects of dead timeand deviations from detector linearity will affect the results.In the present study, the effects caused by high countingrates have been compensated by introducing a second-orderterm in the calibration equation:

½2� y ¼ xðaþ bxþ ctÞ

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The coefficients a, b, and c are determined by regression ofdata from two or more reference samples. The reference sam-ples used were GJ-01 (609 ± 1 Ma; Belousova et al. 2006),91500 (1065 ± 1 Ma; Wiedenbeck et al. 1995), and the in-house standard A382 (Voinsalmi, Finland) dated to 1876 ±5 Ma by isotope dilution – thermal ionization mass spectrom-etry (ID–TIMS; H. Huhma, personal communication, 2008).The nonlinear calibration is mainly relevant for zircons withelevated 206Pb/238U ratios, i.e., mid-Proterozoic or olderzircons. The Phanerozoic Temora-2 (ID–TIMS U–Pb age:416.8 ± 1.3 Ma; Black et al. 2003) and Plesovice (ID–TIMSU–Pb age: 337.1 ± 0.4 Ma; Slama et al. 2008) referencezircons were run as unknowns. Forty LA–ICP–MS runs onTemora-2 calculated using the nonlinear calibration gave anaverage age of 418.2 ± 1.1 Ma, whereas 58 runs on the Pleso-vice standard gave 339.3 ± 0.7 Ma. The long-term (>2 years)precision is <1% (2s) for 206Pb/238U and 207Pb/206Pb. Ura-nium and 206Pb concentrations were calculated from observedsignals at atomic masses 238 and 206, calibrated to standardsaccording to eq. [1].

Data reduction protocols for Lu–Hf were fully describedby Røhr et al. (2008) and Andersen et al. (2009). Based onrepeated runs of reference zircons Mud Tank, Temora-2, andGJ-1, the external reproducibility of the 176Hf/177Hf ratio canbe estimated to 2SD = ±0.0000050 (i.e., ±1.8 epsilon units)for zircons with low rare earth element (REE; 176Yb/177Hf <0.10) and 2SD = ±0.000065 (i.e., 2.3 epsilon units) for zir-cons with 176Yb/177Hf ‡ 0.10. The chondritic uniform reser-voir (CHUR) parameters of Bouvier et al. (2008) have beenused, and the depleted mantle model of Griffin et al. (2000)adapted to the new 176Hf/177Hf of CHUR (Andersen et al.2009).

U–Pb dataFive samples from the Isachsen Formation were analyzed

(Figs. 2, 3). Sample C-072382 is from a location near the east-ern basin margin in the Lake Hazen area of Ellesmere Island(80853’N/70850’W) and is likely of late Barremian to earlyAptian age. Sample C-86116, also from a basin margin local-ity, was sampled on the Bjorne Peninsula of Ellesmere Island(79857’N/85810’W) and is of Barremian age. Sample C-82218is from the Walker Island Member on the Fosheim Peninsulaof Ellesmere Island (79857’N/85810’W) and is of late Barre-mian age. Samples C-100614 and C-100629 are from the Pa-terson Island Member on southern Axel Heiberg Island(78837’N/89846’W) and are of Hauterivian and Barremianage, respectively. The number of zircons dated by U–Pb rangebetween 111 and 114 per sample, whereas the number of zir-cons analyzed for Lu–Hf isotopes range between 89 and 106per sample. Zircon grains that are small (<50 mm) or yieldhighly discordant ages (disc >30%) have not been analyzedfor Lu–Hf, resulting in different numbers of Lu–Hf and U–Pbanalyses. The isotope data are summarized in Fig. 4 but canalso be found in Supplementary Data.2 Figure 5a has been in-cluded to show the similarities between samples with respectto the U–Pb age distributions.

The analyzed samples produced a wide range of U–Pbages, ranging from Mesoarchaean (3.0 Ga) to Early Devon-ian (0.4 Ga). Most of the samples also contain a small num-ber of zircons in the 3.7–3.0 Ga age range. The mostsignificant age fractions include zircons aged 2.8–2.7 Ga,1.9–1.8 Ga, 1.7–1.6 Ga, and 1.1–1.0 Ga. These fractions aredominant in most samples and present in all (Fig. 5a). Zir-cons with ages in ranges of 2.4–2.0 Ga and 0.96–0.5 Ga arerare, leaving gaps in the age histograms. Additional common

Fig. 3. Simplified cross section of Lower Cretaceous strata from northeastern to southwestern parts of the Sverdrup Basin, modified fromEmbry (1991a). Sample C-86116 is from the southwestern margin of the basin where the Rondon Member is not present; therefore, thestratigraphic location of this sample is not indicated correctly. Numbers in parentheses (1–5) correspond to the sampling locations indicatedin Fig. 2.

2 Supplementary data for this article are available on the journal Web site (http://cjes.nrc.ca) or may be purchased from the Depository ofUnpublished Data, Document Delivery, CISTI, National Research Council Canada, Building M-55, 1200 Montreal Road, Ottawa, ON K1A0R6, Canada. DUD 5381. For more information on obtaining material refer to http://cisti-icist.nrc-cnrc.gc.ca/eng/ibp/cisti/collection/unpublished-data.html.

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minima appear at 2.63–2.58 Ga, 1.72–1.68 Ga, 1.57–1.51 Ga, and 1.27–1.22 Ga. All ages reported in this andthe following sections refer to data that are <10% discord-ant.

Sample C-82218, Fosheim Peninsula, Ellesmere IslandOf the 112 zircons analyzed form this sample, 91 are

<10% discordant. A probability density plot (PDP; Fig. 4a)reveals a large maximum at 1.83 Ga and subsidiary peaksat 1.92, 2.64, and 2.72 Ga. Including discordant data do notalter the plot significantly. Two major age fractions can beidentified from the PDP: (1) a Palaeoproterozoic fractionfrom 1.93 to 1.72 Ga containing 41% (n = 37) of all dataand (2) an Archaean fraction from 2.81 to 2.63 Ga contain-ing 29% (n = 26) of data. Small additional fractions arefound at 1.13–1.02 Ga (n = 5), 1.50–1.28 Ga (n = 9), and1.68–1.61 Ga (n = 5). A small number of the analyzed zir-cons yield ages between 2.6 and 2.0 Ga and between 3.8 and3.0 Ga. Additionally, there is one Silurian aged zircon at0.42 Ga.

Sample C-86116, Bjorne Peninsula, Ellesmere IslandA total of 114 zircons were analyzed from this sample, of

which 86 are <10% discordant. The PDP in Fig. 4a reveals amaximum at 1.83 Ga and subsidiary peaks at 1.05, 1.92,2.51, and 2.71 Ga. Including discordant zircons increase theproportion of Mesoarchaean to Neoarchaean zircons consid-erably. Three main age fractions can be identified: (1) aMesoproterozoic to Neoproterozoic fraction from 1.18 to0.98 Ga containing 16% (n = 14) of all data, (2) a Palaeo-proterozoic fraction from 1.96 to 1.73 Ga with 28% (n =24) of data, and (3) an Archaean component from 2.85 to2.62 Ga containing 22% (n = 19) of all data. Minor addi-tional age fractions are found in age ranges of 1.68–1.63 Ga(n = 6), 2.12–1.99 Ga (n = 6), and 2.56–2.47 Ga (n = 8).The remaining zircons yield isolated ages or form clustersof less than four zircons. Noteworthy among the latter arezircons aged 0.46–0.44 Ga (lower Palaeozoic) (n = 3) and1.6–1.2 Ga (mid-Proterozoic) (n = 4), as well as two Meso-archaean zircons.

Sample C-072382, Lake Hazen, Ellesmere IslandOf the 111 zircons analyzed from this sample, 104 are

<10% discordant. The PDP in Fig. 4a reveals significantpeaks at 1.07 and 1.19 Ga and subsidiary peaks at 0.97,1.14, 1.60, and 1.64 Ga. The PDP is not significantly alteredby including discordant zircons. Five main age fractions canbe identified: (1) a Mesoproterozoic fraction from 1.23 to1.00 Ga containing 32% (n = 33) of all data, (2) a Mesopro-terozoic fraction from 1.54 to 1.27 Ga containing 20% (n =

21) of all data, (3) two Palaeoproterozoic age fractions from1.91 to 1.71 Ga and from 1.67 to 1.58 Ga containing 12grains each. Additional smaller age fractions include Ordo-vician zircons aged 0.49–0.44 Ga (n = 5), Neoproterozoiczircons aged 0.98–0.96 Ga (n = 6), and a relatively smallassemblage of Archaean zircons containing a total of 13grains. None of the analyzed grains were >3.0 Ga.

Sample C-100614, Glacier Fjord, southern Axel HeibergIsland

Of 111 zircons analyzed from this sample, 91 are <10%discordant. A significant peak in the PDP (Fig. 4b) can beidentified at 1.87 Ga and subsidiary peaks are found at1.05, 1.08, 1.66, and 1.98 Ga. The inclusion of discordantdata does not alter the PDP significantly. Four main agefractions can be identified from the same plot: (1) a Meso-proterozoic fraction from 1.20 to 1.04 Ga containing 21%(n = 19) of all data, (2) a Mesoproterozoic fraction from1.58 to 1.29 Ga containing 12% (n = 11) of all data, (3) alate Palaeoproterozoic fraction from 1.69 to 1.63 Ga con-taining 9% (n = 8) of data, and (4) a Palaeoproterozoic frac-tion from 1.91 to 1.72 Ga containing 22% (n = 20) of alldata. Additional smaller age fractions include zircons aged1.99–1.96 Ga (n = 5) and 0.46–0.44 Ga (n = 4). The Arch-aean component is relatively small with a total of sevengrains yielding concordant ages between 3.0 and 2.5 Ga andtwo grains >3.2 Ga. This sample also yielded two late Palae-ozoic zircons aged 0.29 and 0.25 Ga.

Sample C-100629, Glacier Fjord, southern Axel HeibergIsland

A total of 113 zircons were analyzed from this sample, ofwhich 85 are <10% discordant. The PDP (Fig. 4b) reveals amaximum at 1.86 Ga and subsidiary peaks at 1.65, 1.92, and2.72 Ga. Including discordant data into the plot lead to en-hanced significance of the Palaeoproterozoic (1.98–1.73 Ga)and Archaean components. The PDP also reveals four mainage fractions: (1) a Mesoproterozoic fraction from 1.15 to1.05 Ga containing 13% (n = 11) of all data, (2) a Palaeo-proterozoic to Mesoproterzoic fraction from 1.66 to 1.57 Ga(divisible into two subfractions by a minimum range of1.63–1.58 Ga) containing 11% (n = 9) of all data, (3) a Pa-laeoproterozoic fraction from 1.98 to 1.73 Ga containing27% (n = 23) of data, and (4) a Mesoarchaean to Neoarch-aean fraction from 2.86 to 2.66 Ga containing 18% of thedata from this sample. Less significant components includea Palaeozoic fraction from 0.45 to 0.41 Ga (n = 5), a Meso-proterozoic fraction from 1.44 to 1.31 Ga (n = 8), and aNeoarchaean fraction from 2.58 to 2.52 Ga (n = 5).

Fig. 4. Graphic presentation of U–Pb and Hf data from the (a) Ellesmere Island samples C-82218, C-86116, and C-072382; and (b) AxelHeiberg Island samples C-100614 and C-100629. (i) Normal concordia diagrams with data plotted as 2s error ellipses. Inset figures areenlarged plots of data <2000 Ma. (ii) Combined histogram and probability density plots (PDP) of detrital U–Pb ages. Plotted data are 207Pb/206Pb ages for zircons with ages >700 Ma and 206Pb/238U ages for younger zircons. In relative probability diagrams, n = 93/112 denotes thenumber of grains that are <10% discordant relative to the total number of analyzed grains. Histograms only plot ages that are <10% discor-dant. PDPs in light grey present all data, whereas the superimposed plots in dark grey present data that are <10% discordant. See Fig. 5 formore detail. (iii) Plots of initial 3Hf versus U–Pb ages (time). U–Pb data are 207Pb/206Pb ages for zircons with ages >700 Ma and 206Pb/238Uages for younger zircons. Rods in the lower right corners of the plots approximate the external 2s precision (ca. 1.8 3Hf units). CHUR,chondritic uniform reservoir; DM, depleted mantle after the model of Griffin et al. (2000); Ref. line, 3Hf growth curves for rocks with a mid-crustal 176Lu/177Hf ratio of 0.014, anchored at 3.98 Ga.

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esearchPress

Hf isotope dataThe 3Hf notation used throughout this paper is a measure

of the difference in 176Hf/177Hf between a sample and the ata certain time (t). In this study, the samples are detrital zir-cons and the times t are the U–Pb ages of those zircons.Negative 3Hf values signify that 176Hf/177Hf ratios are lowerthan in CHUR, whereas positive values translate into 176Hf/177Hf ratios higher than CHUR. Melts and their crystallizedequivalents tend to have a lower Lu/Hf ratio than the reser-voir from which they were extracted. Given that 176Lubreaks down to 176Hf, the 176Hf/177Hf ratio of a magmaticrock increases at a slower pace and ultimately yield lower176Hf/177Hf ratios (and hence lower 3Hf values) than the pa-rent reservoir. The mantle and mantle-derived rocks thusyield high 3Hf values, as indicated by the depleted mantlelines in Fig. 4. Rocks derived from continental crust, on theother hand, yield lower 3Hf values. Just how low depends onthe degree of crustal influence and (or) on the age of thecrustal reservoir.

The results of Lu–Hf analyses are presented graphically inFigs. 4a and 4b but can also be found in SupplementaryData.2 The various age fractions presented in the previoussection tend to yield a restricted range in 3Hf values, i.e., zir-cons of a certain age tend to yield similar 3Hf values irre-spective of which sample the zircon comes from. Thefollowing section therefore focus on the range of 3Hf valuescharacterizing the various age fractions.

The Ordovician–Devonian aged zircons in the studiedsamples yield 3Hf values in the range from –10.1 to +5.6.Negative 3Hf values predominate in this fraction, with onlyfour zircons (all from sample C-100629) yielding positive3Hf values.

The small cluster of Neoproterozoic zircons aged 1.0–0.9 Ga is mainly represented by zircon in sample C-072382.It yields predominantly low 3Hf values, ranging from –4.9to +0.2, suggesting that their protosource had interactedwith crustal material during formation.

Zircons ranging in age from 1.2 to 1.0 Ga yield relatively

Fig. 4. (concluded).

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high 3Hf values, ranging from –4.0 to +11.3. Treating alldata in this category together, there appears to be a signifi-cant positive correlation between ages and 3Hf values. How-ever, the most juvenile proportion of this fraction, yielding3Hf values from +6.8 to +11.3, is dominated by sampleC-072382. If data from this sample are withdrawn from thefraction, the correlation coefficient drops from 0.24 to 0.17.The latter correlation coefficient is insignificant according toa t test of correlation (Davis 2002).

Zircons in the 1.68–1.27 Ga age range can be binned intotwo fractions, separated by a minimum between 1.57 and1.51 Ga. There appears, however, to be no significant differ-ence in the Hf data of these two fractions. Of the zircons inthese fractions, 80% yield positive 3Hf values that, apartfrom two outliers at –9 and –10, fall between –3.5and +10.0. There is no apparent correlation between agesand 3Hf values, although most zircons yielding negative val-ues are >1.57 Ga.

The 2.0–1.72 Ga zircon fraction yields 3Hf valuesfrom –22.6 to +9.4 and can be split into at least three sub-fractions based on dominant 3Hf values. The first fractionconsists of zircons aged 1.8–1.72 Ga and is dominated bygrains yielding positive 3Hf values; the second fraction in-cludes zircons aged 1.94–1.8 Ga, of which 77% yield nega-tive values; and the third fraction includes zircons aged 2.0–1.94 Ga, which predominantly yield positive values.

The main Archaean assemblage, from 2.9 to 2.4 Ga, can besplit into two fractions by a minimum between 2.63 and2.58 Ga. All zircons in the younger part of this assemblageyield negative 3Hf values, generally from –11.9 to –0.7. Zir-

Fig. 5. Comparison of U–Pb age data from this study with pub-lished U–Pb ages from possible source areas. (a) Summary ofprobability density plots of U–Pb age data from the Isachsen For-mation. Light grey broken bars indicate the most significant agefractions. From right to left (oldest to youngest): 2.8–2.7 Ga, 1.9–1.8 Ga, 1.7–1.6 Ga, and 1.1–1.0 Ga. (b) Overview of published U–Pb ages from possible source areas. Age spans dominated by detri-tal zircon and inherited ages are marked by a darker colour. Detritalzircon data from this study are shown as light grey shading behindthe other data and are otherwise plotted in Fig. 5(a). *, Backgrounddata from Scandinavia (N-Scandinavia) are from north of 658N andinclude the Kola Peninsula. **, Background data from northernNorth America (N. N. Am.) are from north of the Grenville terrane.References from Scandinavia: Levchenkov et al. (1995); Daly et al.(2001); Paulsson and Andreasson (2002); Corfu et al. (2003a,2003b); Bibikova et al. (2004); Corfu (2004a, 2004b); Rehnstromand Corfu (2004); Kirkland et al. (2005, 2007a); Myskova et al.(2005). From northern North America: Trettin et al. (1987); Frischand Hunt (1988); Ross et al. (1993, 2005); McNicoll et al. (1995);Rainbird et al. (1997); Gehrels and Ross (1998); Gehrels et al.(1999); McDonough et al. (2000); Hartlaub et al. (2005, 2007); Da-vis et al. (2006); St. Onge et al. (2006); Iizuka et al. (2007); vanBreemen et al. (2007). From Greenland: Tucker et al. (1993); Kals-beek et al. (1993, 1999); Nutman & Kalsbeek (1994); Strachan etal. (1995); Nutman et al. (1996); Amelin et al. (2000); Watt et al.(2000); Thrane (2002); Leslie & Nutman (2003); Gilotti et al.(2004); Gilotti and McClelland (2005); Thrane et al. (2005). FromSvalbard: Balashov et al. (1995); Peucat et al. (1989); Johansson etal. (2000, 2002, 2004); Johansson (2001); Hellman et al. (2001).

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cons within the older part of this assemblage yield 3Hf valuesfrom –15.6 to +6.7, and 75% of these grains yield negativevalues. There appears to be some correlation between U–Pbages and 3Hf values. However, this correlation is highly influ-enced by zircons that are >2.8 Ga, which may or may notform a separate group. If this ‘‘group’’ is disregarded, the cor-relation coefficient drops from 0.38 to an insignificant 0.05.

Discussion

Origins of detrital zirconsFigure 5b shows published ages from four possible source

areas: Greenland, northern North America, northern Scandi-navia, and Svalbard. Figure 5 is a useful visual aid to thediscussion, which compares the detrital zircon data from thestudied sandstones with ages typical of the prospectivesource areas.

The youngest zircons detected in the studied sandstonesare two grains aged 0.29–0.25 Ga. Possible sources for theyounger of these zircons include the Siberian Traps and theTaymir region (Campbell et al. 1992; Vernikovsky et al.2003), whereas the older of the two matches the age ofgranitoids within the Urals (Scarrow et al. 2002). Anotherpossible source of these zircons could be Permian volcanicson northern Axel Heiberg Island (Cameron and Muecke1996). These detrital zircon ages are commonly foundwithin Triassic strata of the Sverdrup Basin, suggesting thatthese two zircons may be redeposited remnants of olderstrata (J. Omma, personal communication, 2007).

Although zircons aged 0.5–0.4 Ga constitute a minor agecomponent in the studied samples, their presence may be ofsome significance. Possible sources for this component in-clude the East Greenland Caledonides (Watt et al. 2000;Gilotti et al. 2004; Gilotti and McClelland 2005), Svalbard(Johansson et al. 2002, 2004), and (or) the Pearya Terranein northern Ellesmere Island (Trettin et al. 1987). The low3Hf values yielded by this zircon age fraction are comparableto low 3Nd values of Caledonian units in both Svalbard andeast Greenland (Hansen et al. 1994; Johansson et al. 2002),although the less studied Pearya Terrane may also yield sim-ilar Hf and Nd isotope signatures. Caledonian age igneousrocks and metamorphic rocks dominated by Caledonian-agemetamorphic zircon are not particularly widespread withinthe East Greenland Caledonides. Caledonian-age detrital zir-con fractions in sediments derived from the East GreenlandCaledonides would therefore be modest. In light of this,there is no contradiction between the small amount of 0.5–0.4 Ga zircon in the studied sediments and an east Green-land provenance.

Metamorphic or magmatic terranes yielding ages from 1.7to 1.0 Ga are rare in northern parts of Canada and Green-land. The only representatives of some significance are asmall enclave of *1.04 Ga rocks in the Pearya Terrane and1.38 Ga basaltic dikes and extrusives in northern Greenland(Trettin 1987; Upton et al. 2005). It may therefore seem sur-prising that these age components make up as much as21%–63% of the detrital zircon data from the Isachsen For-mation and similar proportions of zircon data gathered fromLower Cretaceous formations in northern Greenland (Røhret al. 2008). The most likely explanation for the presence ofthese components in the studied sediments is that they were

recycled from variously metamorphosed sediments withinCaledonian thrust sheets.

Detrital zircon populations from the MesoproterozoicKrummedal Formation (MKF) and the Neoproterozoic Eleo-nore Bay Supergroup (EBS) in the East Greenland Caledo-nides are dominated by zircons aged 1.7–1.6 Ga but alsocontain significant amounts of zircons aged 1.2–1.0 Ga andsubordinate amounts aged 1.6–1.2 Ga (Strachan et al. 1995;Kalsbeek et al. 2000; Watt et al. 2000; Leslie and Nutman2003; Dhuime et al. 2007). These ages are also found ingranulite-facies metapsammite believed to be of similar ori-gin as the MKF (McClelland and Gilotti 2003) and as inher-ited zircons in S-type granites allegedly derived from thesame formation (Kalsbeek et al. 2001). The MKF and EBShave been correlated to sedimentary formations within theNorwegian and Scottish Caledonides due to similarities indetrital zircon patterns (Cawood et al. 2003; Friend et al.2003; Kirkland et al. 2007a). It is believed that these unitswere deposited in comparable regional settings, i.e., in theproximity of the Grenvillian–Sveconorwegian orogenic belt,and later incorporated as thrust sheets into the Caledonianfold belt (Dhuime et al. 2007; Kirkland et al. 2007a).

The relative importance of the 1.2–1.0 Ga age fraction isone difference between the age distribution of 1.7–1.0 Gadetrital zircons from the Isachsen Formation and the age dis-tribution of the Proterozoic strata mentioned previously.This component is generally more prominent in the IsachsenFormation and in Cretaceous sediments from northernGreenland than in the Proterozoic strata (Røhr et al. 2008).This discrepancy prompted Røhr et al. (2008) to suggestthat the Neoproterozoic Morænesø Formation had providedthis zircon component (Kirkland et al. 2007b, 2009). On theother hand, the detrital zircon patterns of the MKF and EBSmay not be representative for all sediments once presentwithin the East Greenland Caledonides, and it cannot beruled out that these and similar allochthonous units providedall of these zircons.

The Isachsen Formation contains large quantities of detri-tal zircon that are >1.8 Ga, suggesting input from cratonicsources in Greenland and (or) Canada. However, distin-guishing between cratonic sources in Greenland and north-ern Canada is difficult given the similarities in protolithages between the two regions, where large areas of both cra-tons are dominated by rocks aged 2.0–1.8 Ga such as theRinkian and Nagssugtoqidian mobile belts in Greenland andthe Trans-Hudson Orogen in Canada (e.g., Kalsbeek andNutman 1996; Theriault et al. 2001; Thrane et al. 2005).These Palaeoproterozoic terranes commonly yield low 3Ndvalues indicative of crustal protoliths; i.e., they were formedby metamorphism or partial remelting of older crustal rocks.These low 3Nd values are directly translatable into low 3Hfvalues, as is seen among 2.0–1.8 Ga zircons from the Isach-sen Formation. The Ellesmere–Inglefield mobile belt is atpresent the exposed part of the shield, which is most proxi-mal to the Sverdrup Basin (Figs. 1, 2). This area is domi-nated by various Archaean rocks and 1.98–1.91 GaPalaeoproterozoic rocks with a juvenile Nd isotope signature(Frisch and Hunt 1988; Nutman et al. 2008). The relativeproximity of this area to the Sverdrup Basin may suggestthat sands derived from a proximal shield source would bedominated by 1.98–1.91 Ga and Archaean zircons. Although

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large quantities of Archaean zircons are present in thestudied sandstones, zircons aged 1.98–1.91 are few; theycluster *1.92 Ga as opposed to 1.95 Ga, which would bethe case if they came from Inglefield land, and they predom-inantly yield low 3Hf values, which most likely would nothave been the case if they had been locally derived. Thismay imply that the Inglefield land area was not exposed inEarly Cretaceous times, that the 2.0–1.8 Ga zircon compo-nent was transported from a more distal source, and (or)that these zircons too are a product of redeposition.

Several terranes in Greenland and Canada should be men-tioned as possible sources for zircons of Archaean age such asthe Rinkian and Nagssugtoqidian mobile belts in Greenlandand the Trans-Hudson Orogen in Canada, which all containremnants of Archaean terranes (Figs. 1, 5b). More pristineArchaean terranes are also possible sources for these zircons,including the Archaean block in southern Greenland and thewestern Churchill Province and Slave Craton in Canada.

Shown within the 3Hf plots of Fig. 4 are reference linesanchored at 3.98 Ga. These lines represent growth curves ofprotoliths with a 176Lu/177Hf ratio of 0.014, corresponding tothe average composition of the middle crust (Rudnick andFountain 1995; Rudnick and Gao 2004). Provided that theU–Pb zircon ages are magmatic, these lines can be used to cal-culate model ages that are probably more accurate than nor-mal depleted mantle ages (in which case the 176Lu/177Hf ratioof zircon, i.e., *0.001, is used. See Supplementary Data2 forreference). The reference lines show that several zircons yieldmodel ages *4 Ga, similar to model ages calculated usingdata from the Amıtsoq gneisses in southwest Greenland andthe Acasta gneiss in the Slave Craton of Canada (Vervoortand Blichert-Toft 1999; Amelin et al. 2000). The sametendency was discovered by Røhr et al. (2008), among detritalzircons from Lower Cretaceous sediments in northern Green-land, and was used to infer that Archaean and Palaeoprotero-zoic tectonothermal events had reworked *4 Ga protoliths.Although this is fully compatible with an origin from Green-land basement, the occurrence of *3.8 Ga gneisses in theSlave Craton suggests that a Canadian source cannot be ruledout.

The arguments listed indicate that the Isachsen Formationsediments are sourced from the Caledonides and the Green-land – Canadian Shield. It should, however, be mentionedthat a similar distribution might arise with northern parts ofBaltica as a sediment source. Neoproterozoic sedimentswithin the northern Scandinavian Caledonides have beenshown to yield detrital zircon patterns, which are similar tothose of the Eleonore Bay Supergroup (Kirkland et al.2007a). Crystalline rocks aged 0.5–0.4 Ga are present withinCaledonian nappes of northern Scandinavia, and rocks ofArchaean and Palaeoproterozoic age form part of the base-ment across northern Scandinavia and the Kola Peninsula(e.g., Stephens et al. 1993; Levchenkov et al. 1995; Bibikovaet al. 1999; Corfu et al. 2003a, 2003b; Corfu 2004a; Kirklandet al. 2005). Preliminary data even suggest that 1.9–1.8 Gaunits within the Fennoscandian shield yield low 3Hf values,not unlike those yielded by 1.9–1.8 Ga zircons in the Isach-sen Formation samples (Patchett et al. 1982; Andersen et al.2009). Ages between 2.5 and 2.6 Ga constitute the only agefraction present in the Isachsen Formation samples with pos-sible equivalents in Greenland and Canada and no apparent

counterpart in northern parts of Baltica (Kalsbeek et al.1999; van Breemen et al. 2007; Martel et al. 2008). Never-theless, there is no reason to suggest that Baltica was an im-portant source, first and foremost because nearly all thediscovered detrital age components have counterparts in Lau-rentia. Another argument against a Baltica source includesthe more or less continuous sedimentary succession in theBarents Sea region. Large mass movements from northernBaltica across the proto Barents Sea to Laurentia is thereforeimprobable from Carboniferous time onward (Larssen et al.2005). Nd isotope signatures of Upper Devonian – UpperCretaceous rocks in the Sverdrup Basin also make such asource unlikely, since these data suggest no significant shiftsin provenance throughout these times (Patchett et al. 2004).We find it fairly unlikely that sediments delivered to theCanadian Arctic before and during Late Devonian timeswere derived from across the Caledonides and later redepos-ited into Mesozoic formations.

Comparison with Nd isotope dataPatchett et al. (2004) suggested that Devonian postoro-

genic sediments, originally derived from erosion of the Ca-ledonian and Franklinian orogenic belts, covered large areasof North America and Greenland during Mesozoic and latePalaeozoic times. They mainly based these assumptions onSm–Nd isotope data from Lower Carboniferous – UpperCretaceous strata in the Sverdrup Basin, which were remark-ably uniform throughout these periods and yielded higher3Nd values than what would be expected if cratonic base-ment rocks had been the main source.

A simple mixing model has been used to test whether the de-trital zircon data presented here are compatible with the Sm–Nddata of Patchett et al. (2004) and thus equivalent to whole rocksamples. The model uses depleted mantle 143Nd/144Nd at spe-cific ages and probable Sm and Nd concentrations (and hence147Sm/144Nd) of Archaean and Proterozoic rocks to calculatepresent day 143Nd/144Nd. Table 1 lists the three componentsused in the mixing model: the first component has Sm and Ndconcentrations typical for Archaean rocks (Taylor andMcLennan 1985) and a mantle extraction age of 2.78 Ga. Thesecond and third components have Sm and Nd concentrationssimilar to post-Archaean shale (PAAS of Nance and Taylor1976) and mantle extraction ages of 1.83 and 1.39 Ga, respec-tively. The mantle extraction ages have been found by averag-ing the 176Hf/177Hf depleted mantle ages of zircons yieldingU–Pb ages of 3.8–1.7 Ga, 1.7–1.2 Ga, and 1.2–0.2 Ga, respec-tively. The following equations, originally drawn up by Ander-sen and Laajoki (2003), have been used to estimate the Sm–Nddata for the various samples:

½3� CNdm ¼

Xn

i¼1

XiCNdi

½4� CSmm ¼

Xn

i¼1

XiCSmi

½5� ð143Nd=144NdÞm ¼1

CNdm

Xn

i¼1

XiCNdi ð143Nd=144NdÞi

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½6�Xn

i¼1

Xi ¼ 1

where Cm is the concentration of the element in the mixtureand Ci the concentration in the end members listed in Ta-ble 1. (143Nd/144Nd)m and (143Nd/144Nd)i are the correspond-ing present day 143Nd/144Nd, whereas Xi is the relativeproportion of the age fractions 3.8–1.7, 1.7–1.2, and 1.2–0.2 Ga within a sample. The results of the mixing test arelisted in Table 2.

The time-corrected 3Nd values presented by Patchett et al.(2004) from the Isachsen Formation are generally from –11.5to –16.0 with one outlier at –21.0, whereas the 3Nd valuesproduced by the mixing model are in the order of from –14.7to –21.1. In other words, there is a partial overlap between themodelled database on detrital zircons and the analytical databy Patchett et al. (2004), but the latter tend to cluster athigher 3Nd values.

Both the frequency of zircon and the concentration of Ndvary between lithologies. Different terranes or different partsof a terrane may therefore contribute differently to the Ndisotope and zircon budgets of a sediment. A good match be-tween Hf data from detrital zircons and Nd isotope datafrom whole rocks can therefore not be taken for granted.However, redeposition of clastic sediments will promotemixing of material and dilute the effects of particular unitsand (or) lithologies in the final deposit. Redeposition andsubsequent mixing of the analyzed sediments is supportedby the wide and relatively continuous detrital zircon agespectra, and the fact that most of the detrital zircon ages arerepresented in all analyzed samples from the Sverdrup andWandel Sea basins (Fig. 5a; Røhr et al. 2008). This interpre-tation is also supported, if not confirmed, by the mature

composition of the sandstones and the fact that all of the de-tected zircon were rounded or fragmented (fragmenting pre-sumably happened during crushing). Redeposition couldtherefore be the reason for the overlap between modelledand measured data. What this sedimentary precursor was isnot properly revealed by the detrital zircon data alone,although a Franklinian provenance could explain some ofthe differences between samples. Cambrian and Ordoviciandeposits in the Franklinian Basin have largely been sourcedfrom the Precambrian shield, whereas Silurian and LowerDevonian rocks were sourced from the Caledonides(Patchett et al. 1999). Sediments formed by eroding thelower levels of the Franklinian would therefore be domi-nated by Palaeoproterozoic and Archaean detrital zirconages and might resemble sample C-82218. On the otherhand, sediments sourced from upper levels of the Frankli-nian stratigraphy would inherit a ‘‘Caledonian’’ signatureand would probably be similar to sample C-072382. How-ever, these differences may also reflect the heterogeneity ofthe Devonian Clastic Wedge, which is partly derived fromFranklinian basin sediments. As a consequence, the Devon-ian clastic sediments would have inherited different zirconage spectra depending on which stratigraphic level of theFranklinian basin these were sourced from. The differencesbetween samples from the Isachsen Formation could there-fore be explained by local differences in the detrital zirconcomposition of their main source, i.e., the Devonian ClasticWedge.

ConclusionThe detrital zircon data presented here, perceived without

regard for previous provenance studies of the Isachsen For-mation, suggest the provenance of this sedimentary unit laywithin the Caledonian Orogenic Belt and the Greenland –

Table 1. Characteristics of reservoirs used to model 3Nd values.

Reservoir* Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd 3Nd(0.123 Ga) TDM{

Archaean crust(1) 4 20 0.1213 0.511362 –23.73 2.78post-Archaean shale(2) 5.6 32 0.1062 0.511743 –16.03 1.83post-Archaean shale(3) 5.6 32 0.1062 0.512056 –9.94 1.39

*Crustal reservoirs used to model "Nd values: (1), average Archaean upper crust from Taylor and McLennan (1985); (2), (3), post-Archaean average Australian shale from Nance and Taylor (1976).

{Assumed depleted mantle ages of the listed reservoirs.

Table 2. Nd isotope data resulting from mixing of crustal reservoirs.

Sample Mixing proportions* Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd 3Nd{ TDM

{ (Nd) TDM§ (Hf)

C-82218 0.80(1)/0.15(2)/0.05(3) 4.32 22.40 0.1170 0.511493 –21.09 2.44 2.60C-86116 0.70(1)/0.10(2)/0.20(3) 4.48 23.60 0.1152 0.511602 –18.94 2.22 2.50C-072382 0.25(1)/0.35(2)/0.40(3) 5.20 29.00 0.1088 0.511816 –14.67 1.77 1.87C-100614 0.47(1)/0.22(2)/0.31(3) 4.85 26.36 0.1116 0.511725 –16.48 1.95 2.06C-100629 0.61(1)/0.20(2)/0.19(3) 4.62 24.68 0.1137 0.511632 –18.33 2.14 2.41

*Relative mixing proportions (sums to one). Superscripts (1), (2), and (3) denote the reservoirs listed in Table 1, i.e., Archaean upper crust, post-Archaeanshale at 1.83 Ga, and post-Archaean shale at 1.39 Ga. See text for further explanations.

{"Nd at an approximate depositional age of 0.123 Ga.{Depleted mantle age from modelled Nd isotope data using the mantle evolution curve of DePaolo (1981).§The average depleted mantle age based on Hf isotope data from zircons and according to the mantle evolution curve of Griffin et al. (2000).

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Canadian Shield. However, this interpretation is complicatedby the possibility of recycling zircon from the FranklinianBasin and the Devonian Clastic Wedge. Sediments depositedin the Franklinian Basin were derived from the Greenland –Canadian Shield and the Caledonides, whereas sources ofDevonian clastics were the Caledonides and Franklinian Ba-sin sediments. This suggests that sediment sourced from theFranklinian terrane or from Devonian clastics would inherita detrital zircon pattern similar to sediment sourced directlyfrom the Caledonides and Greenland – Canadian Shield. It istherefore difficult to decide whether the studied Cretaceoussediments were derived from other sediments or weresourced directly from the Caledonides and the Greenland –Canadian Shield. However, wide and relatively continuousdetrital zircon age spectra, the fact that most of the detritalzircon ages are represented in all analyzed samples, andsimilarities between modelled and measured Nd isotopedata suggest the sediments were significantly mixed prior todeposition. We propose that this mixing is a result of rede-position and that a significant proportion of the IsachsenFormation was derived from older sedimentary strata. Suchan interpretation is fully compatible with previous sugges-tions stating that large proportions of the Mesozoic strata inthe Sverdrup Basin, including the Isachsen Formation, werederived from older sediments (Patchett et al. 2004).

AcknowledgementsThe authors would like to thank S.L. Simonsen, the ICP–

MS operator at the time of sample analysis. This workwould not have been possible without her skilful supervisionand careful nurturing of the mass spectrometer. T. Winjeand B.L. Berg are acknowledged for assistance during back-scattered electron and cathodoluminescence (CL) imaging ofzircons. Many thanks are also given to L.E. Haug for under-taking the mineral separation and to M. Erambert for help-ing to quantify the chemistry of zircons. This study wasfinanced by the Norwegian Research Council through thePETROMAKS programme. The Research Council is grate-fully acknowledged for giving the authors this opportunity.Many thanks are also given to J. Patchett and E. Miller forreviewing this paper and to Associate Editor E. Davis. Theircomments have significantly increased the quality of thismanuscript.

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