Defining tectonic boundaries using detrital zircon signatures of Precambrian metasediments from Svalbard’s Southwestern Caledonian Basement Province
Published:June 14, 2019
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Grzegorz Ziemniak, Karolina Kośmińska, David A. Schneider, Jarosław Majka, Henning Lorenz, William C. McClelland, Virginia T. Wala, Maciej Manecki, 2019. "Defining tectonic boundaries using detrital zircon signatures of Precambrian metasediments from Svalbard’s Southwestern Caledonian Basement Province", Circum-Arctic Structural Events: Tectonic Evolution of the Arctic Margins and Trans-Arctic Links with Adjacent Orogens, Karsten Piepjohn, Justin V. Strauss, Lutz Reinhardt, William C. McClelland
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Detrital zircon provenance studies of Precambrian metasedimentary rocks in Wedel Jarlsberg Land and Sørkapp Land, Svalbard’s Southwestern Caledonian Basement Province, were conducted to evaluate local stratigraphic correlations and the role of long-distance strike-slip displacements in assembling the basement of the Arctic Caledonides. The detrital zircon U-Pb age spectra of the late Mesoproterozoic to Neoproterozoic metasediments revealed mainly Mesoproterozoic to Paleoproterozoic age signatures characteristic for a Grenville–Sveconorwegian orogen provenance. These results confirmed a stratigraphic correlation between basement units of southern Sørkapp Land and the Isbjørnhamna Group of Wedel Jarlsberg Land and suggest relocation of the tectonic boundary between the Eimfjellet Complex and the Isbjørnhamna Group above the Eimfjellbreane Formation. Moreover, the results support the Vimsodden Kosibapasset Shear Zone (VKZ) as a major tectonic boundary and highlight the inhomogeneity in the Southwestern Caledonian Basement Province. The detrital zircon age signatures south of the VKZ bear similarities with coeval metasediments of the Northwestern Caledonian Basement Province of Svalbard and other localities in the Greenland and Scandinavian Caledonides. In contrast, the detrital zircon age spectra north of the VKZ are comparable with the high Arctic Neoproterozoic sediments of Baltican affinity. In conjunction with previous studies, the results suggest that the basement units may continue across the traditional boundaries of the Svalbard’s Caledonian basement provinces.
The tectonic evolution of the Arctic Ocean and its margins is a subject of continuous debate (Harland, 1997; Li et al., 2008; Pettersson et al., 2010; Cawood et al., 2010; Kirkland et al., 2011; Lorenz et al., 2012). Neoproterozoic sedimentary rocks are common on Ellesmere Island (e.g., Pearya Terrane), East Greenland, Svalbard, northernmost Scandinavia, and at several other localities in the Russian and Canadian Arctic. They reveal characteristic Mesoproterozoic to late Paleoproterozoic U-Pb detrital zircon age signatures that are typical for a Grenville–Sveconorwegian orogen provenance (Lorenz et al., 2012). Several tectonic reconstructions for the Neoproterozoic in the North Atlantic region were proposed to explain the source of these metasediments. Some authors favor long-distance river transport of the zircons northward (modern coordinates) from type areas of the Grenville–Sveconorwegian orogen (Li et al., 2008). In contrast, more proximal derivation from Tonian granites and gneisses currently exposed in the East Greenland Caledonides (Strachan et al., 1995; Leslie and Nutman, 2003), Scandinavian Caledonides (e.g., Kalak nappe; Kirkland et al., 2006), Svalbard (Nordaustlandet Complex; Gee et al., 1995; Johansson et al., 2005; Richarddalen Complex; Peucat et al., 1989; Berzeliuseggene unit; Majka et al., 2014, 2015), and the Pearya Terrane (Succesion I; Trettin, 1987; Malone et al., 2014, 2017) is emphasized by the others (e.g., Lorenz et al., 2012). The subordinate Tonian crust in the high Arctic was explained either by the existence of a separate accretionary orogen along the Laurentian margin (Valhalla orogen of Cawood et al., 2010; Malone et al., 2014, 2017) or by an extension of the Grenville–Sveconorwegian orogen northward below the younger Caledonian and Timanian domains of the continental shelves of the North Atlantic and the high Arctic (Lorenz et al., 2012). As the aforementioned crustal fragments were involved in the opening of the Iapetus Ocean and the Caledonian orogeny, it was also proposed that transport of the Arctic terranes, including Grenville–Sveconorwegian crust, is a result of long distance displacement along north-south trending transcurrent faults during the terminal stages of the Caledonian orogeny (Harland, 1997; Pettersson et al., 2010).
Unclear tectonic position and sparse localities of Mesoproterozoic to Neoproterozoic strata in the high Arctic has led to extensive provenance studies in order to determine their relative position to the Grenville–Sveconorwegian orogen (e.g., Pease and Scott, 2009; Pettersson et al., 2009; Lorenz et al., 2012; Malone et al., 2014). Detrital zircon age signatures of Baltica and Laurentia are similar in Mesoproterozoic to lower Neoproterozoic metasedimentary rocks, but in younger strata can be resolvable (Bingen et al., 2011). The most representative example of the Baltica-sourced sediment is Baltica’s outer margin, exposed as the Middle Allochthon in Scandinavian Caledonides (Bingen et al., 2011). Detrital zircon age signatures of eastern Laurentian margin are represented by metasedimentary rocks of the East Greenland and Scottish Caledonides (Cawood et al., 2007). Potential eastern Laurentian to Baltican affinity of Mesoproterozoic to Neoproterozoic strata has been reported in few locations across Svalbard (e.g., Krossfjorden Complex, Pettersson et al., 2009; St. Jonsfjorden Group, Gasser and Andresen, 2013). The Pearya Terrane of Ellesmere Island, commonly correlated with southwestern Svalbard (e.g., Gee and Teben’kov, 2004; Mazur et al., 2009; Kośmińska et al., 2014; Majka et al., 2015), also shares the Grenville–Sveconorwegian orogen imprint (e.g., Hadlari et al., 2014; Malone et al., 2014). Its exotic affinity in relation to the rest of the Canadian Arctic supports the later major strike-slip system (Malone et al., 2017).
The modern configuration of the high Arctic was largely determined by tectonic transport mechanisms of the Caledonian orogeny. Some authors support dominant sinistral transpression that led to the fragmentation of the Pearya and Svalbard terranes from Laurentia by major transcurrent faults (e.g., Harland, 1971; Soper et al., 1992). The traditional tectonic subdivision of the Caledonian basement of Svalbard by Harland (1997) includes the Eastern, Central, and Western terranes that are separated by major north-trending strike-slip faults (Fig. 1). A modification of this hypothesis proposes that transcurrent faulting was related to late-orogenic tectonic escape only (Gee, 1986; Gee and Page, 1994), with a subdivision of the Caledonian basement of Svalbard into three terranes (later called basement provinces; i.e., Dallmann, 2015): Eastern, Northwestern, and Southwestern. The Eastern and Northwestern basement provinces are thought to be the northern extension of the East Greenland Caledonides (Gee and Teben’kov, 2004), whereas the Southwestern Basement Province (SBP), at least in parts, represents an extension of the Pearya Terrane of northern Ellesmere Island (Gee and Teben’kov, 2004; Mazur et al., 2009; Kośmińska et al., 2014). This hypothesis does not require long-distance transcurrent faulting in order to explain basement affinities.
The bedrock of the SBP is subdivided into low-grade metasedimentary rock successions with minor components of mafic volcanic sequences and medium-to high-grade metamorphic rocks of both sedimentary and igneous origin. The former were studied using detrital zircon geochronology and revealed mainly, but not exclusively, non-Laurentian provenance interpreted as Baltican (Gasser and Andresen, 2013). However, rock complexes associated with Ordovician subduction and juxtaposed against the low-grade units suggest a link between the SBP and the Pearya Terrane (e.g., Labrousse et al., 2008; Majka et al., 2015). The high-grade units record ca. 640 Ma Torellian metamorphism and deformation (Majka et al., 2008), a potential result of the Timanian orogeny (Mazur et al., 2009; Rosa et al., 2016). The low-grade and medium-to high-grade units are juxtaposed by high-angle large-scale shear zones (Mazur et al., 2009; Majka et al., 2015).
We present new detrital zircon U-Pb ages (by secondary ion mass spectrometry [SIMS] and laser ablation–inductively coupled plasma–mass spectrometry [LA-ICP-MS]) from the metasedimentary rock successions of Wedel Jarlsberg and Sørkapp lands, both located in southwestern Svalbard. Detrital zircon geochronology proved its effectiveness in determining the rock units of Baltican provenance in southwestern Svalbard (Gasser and Andresen, 2013). Presented here, new geochronological data do not only provide the foundation for local stratigraphic correlations within the SBP, but also allow subprovinces of Laurentian and non-Laurentian affinity to be distinguished. Moreover, such provenance studies provide a potential test of large-scale tectonic models, including whether long-distance strike-slip displacements played a major role in assembling the basement of the Arctic Caledonides.
The SBP comprises the westernmost part of Spitsbergen to the south of Kongsfjorden and including Prins Karls Forland (Fig. 1; Harland, 1997). Exposed bedrock of the SBP was to a large extent involved in the Cenozoic West Spitsbergen Fold and Thrust Belt during the Eurekan deformation event (e.g., Dallmann et al., 1993a).
The Caledonian basement in the northern part of the SBP, in Oscar II Land and Prins Karls Forland, is dominated by sedimentary formations metamorphosed under greenschist facies conditions, comprised mostly of pelitic rocks, carbonate rocks, and thick, glacially influenced, matrix-supported conglomerate (diamictite) of latest Cryogenian–early Ediacaran age (Harland et al., 1993; Harland, 1997). In Oscar II Land, the detrital zircon age spectra of the latest Mesoproterozoic St. Jonsfjorden Group are comparable with other coeval strata in the North Atlantic region, whereas upper Neoproterozoic sedimentary rocks show similarities with the Baltican side of the Iapetus Ocean (Gasser and Andresen, 2013). On Prins Karls Forland, amphibolite facies rocks of the Pinkie unit are exposed as fault-bounded crustal blocks and juxtaposed with low-grade metamorphosed psammitic rocks, carbonate rocks, and slates of inferred Neoproterozoic and Paleozoic age (Harland and Wright, 1979; Hjelle et al., 1979; Manby, 1983, 1986; Harland, 1997). Th-U-total Pb monazite dating of the Pinkie unit revealed a ca. 360 Ma Ellesmerian age of metamorphism related to Svalbard’s collision with the Franklinian Basin of Laurentia (Kośmińska et al., 2017). In Oscar II Land, Ediacaran rock successions form the footwall to the Vestgötabreen Complex, a thrust sheet with greenschist, blueschist, and eclogite facies rocks (Kanat and Morris, 1988) that yield Ordovician (ca. 470 Ma) K-Ar and 40Ar/39Ar ages (Horsfield, 1972; Dallmeyer et al., 1990).
Farther south, in Nordenskiöld Land, a metasedimentary rock succession is developed as two levels of phyllites, carbonate rocks, and diamictites. The younger level is possibly a late Ediacaran glacially influenced diamictite. However, stratigraphic position and age of the succession is equivocal (Hjelle et al., 1986; Harland, 1997). The Vestgötabreen Complex is also exposed in Nordenskiöld Land in the form of metabasite lenses that were metamorphosed under greenschist to blueschist facies conditions (Kośmińska et al., 2014) and embedded in lower grade metasedimentary rocks (Hjelle et al., 1986; Harland, 1997).
Wedel Jarlsberg Land and Sørkapp Land form the southern part of the SBP (Fig. 1). The northern part of Wedel Jarlsberg Land (WJL) reveals a composite structure. To the east, the metasedimentary rocks of the Deilegga and Sofiebogen groups are juxtaposed with the polymetamorphic Berzeliuseggene unit along a high-strain shear zone reoriented within the Cenozoic West Spitsbergen Fold and Thrust Belt (Majka et al., 2015). Zircon U-Pb dating of augen gneisses of the Berzeliuseggene unit yielded a Tonian 950 ± 5 Ma age that is interpreted as the time of magmatic emplacement (Majka et al., 2014). Metamorphic rims on zircon from the Berzeliuseggene unit likely grew synchronously with metamorphic garnet and record an amphibolite facies metamorphic event at 635 ± 10 Ma (Majka et al., 2014). The Precambrian succession in this region was overprinted by greenschist facies metamorphism during the Caledonian orogeny (ca. 460 Ma; Dallmann et al., 1990).
The western part of the Caledonian basement of WJL consists of the Nordbukta Group pelitic, quartzitic, and carbonate rocks that are separated by a prominent angular unconformity, known as the Torellian unconformity, from the Dunderbukta and Recherchefjorden metasedimentary and metavolcanic rocks that include diamictites and greenstones (Birkenmajer, 1975; Bjørnerud, 1990). These rocks continue south to the Hornsund area, where the Deilegga and Sofiebogen groups are thought to be correlatives of the Nordbukta, Dunderbukta, and Recherchefjorden groups (Bjørnerud, 1990). In the northern WJL, the succession is covered by the Kapp Lyell Group, which is thought to be of glacial origin (Bjørnerud, 1990). The exact age and nature of the lower boundary of the Kapp Lyell Group remain unknown (Dallmann et al., 1990; Bjørnerud, 1990, 2010). The rocks below the Torellian unconformity record late Neoproterozoic low-grade metamorphism and deformation (Bjørnerud, 1990). During Caledonian orogeny, the Precambrian succession in this area was affected by low-grade metamorphism and deformation, recorded in the Sofiebogen Group by the muscovite 40Ar/39Ar age of ca. 432 Ma (Manecki et al., 1998).
In the southwestern part of WJL (Fig. 2), enigmatic higher-grade rocks are separated from the Sofiebogen and Deilegga groups by the NW-trending, high-angle Vimsodden Kosibapasset Shear Zone (VKZ). The latter records both dip- and strike-slip movement with a minimum of 600 km of sinistral displacement (Mazur et al., 2009). The enigmatic rocks comprise the Eimfjellet Complex and the Isbjørnhamna Group, with the former thrust onto the latter (Fig. 3). The lowermost units of the Eimfjellet Complex are the Eimfjellbreane and Skjerstranda formations, which comprise feldspathic quartzites and chloritic schists that underwent pervasive deformation and are thought to be located in the footwall of the Eimfjellet thrust (Dallmann, 2015). The Eimfjellbreane Formation is overlain by the Bratteggdalen and Skålfjellet formations, which comprise a suite of amphibolites, metagabbros, metagranitoids, metarhyolites, and metasandstones. Gabbros and granites of the Bratteggdalen and Skålfjellet formations yield U-Pb zircon ages of ca. 1.2 Ga (Balašov et al., 1996; Larionov et al., 2010). The Bratteggdalen Formation is overlain and laterally replaced by quartzites of the Gullichsenfjellet and Pyttholmen formations of metavolcanoclastic origin, including rhyolitic conglomerates (Birkenmajer, 1990; Czerny et al., 1993). Similar ca. 1.2 Ga U-Pb zircon ages were reported from the rhyolite clasts of the Pyttholmen Formation (Balašov et al., 1995). The underlying Isbjørnhamna Group consists of mica schists and metacarbonate rocks that, together with the Eimfjellet Complex, underwent Barrovian-type metamorphism during the ca. 640 Ma Torellian tectonothermal event resolved by Th-U-total Pb monazite and U-Pb zircon dating (Majka et al., 2008, 2010, 2012), and K/Ar and 40Ar/39Ar amphibolite and mica ages (Gayer et al., 1966; Manecki et al., 1998). The Isbjørnhamna Group and the Eimfjellet Complex underwent a low-grade metamorphism during the Caledonian orogeny (Manecki et al., 1998; Mazur et al., 2009; Majka et al., 2010).
The Deilegga and Sofiebogen groups reappear farther to the south in northwestern Sørkapp Land. However, their metamorphic grade is lower than in WJL. In the middle of Sørkapp Land, units of quartzite, garnet mica schist, and marbles are exposed. They resemble the Isbjørnhamna Group, and were similarly affected by multiple metamorphic events (Y. Ohta, 2008, personal commun.). Near Sørkapp, the southernmost tip of Svalbard, the Caledonian basement consists of unnamed units of phyllite with garnet-bearing mica schist (Unit 35), quartzites (Unit 36), and metacarbonate rocks (Unit 34; map units according to Dallmann et al., 1993b). The tectonostratigraphic position and the age of these rock units are unknown. They are, however, thought to be stratigraphic equivalents of the Deilegga or Isbjørnhamna groups (Harland, 1997).
SAMPLING STRATEGY AND METHODS
Eight samples of Proterozoic metasedimentary rocks from the SBP were collected and examined to help resolve the stratigraphic correlations across the region. Sampling was focused on the different siliciclastic successions of the southern part of the province, strategically targeting potentially corresponding units across the VKZ and Hornsund. Thus, two samples were collected from southern Sørkapp Land, four samples from southwestern subprovince of WJL, and two samples from northeastern subprovince of WJL. The sample locations are shown in Figure 2 and in the simplified tectonostratigraphic scheme of Figure 3. The sample coordinates and short descriptions are reported in the GSA Data Repository, Appendix A1.
Zircon grains were separated using standard rock crushing and heavy mineral separation procedures at the University of Ottawa (Ottawa, Canada). Seven samples (HL:XX-0XX) were analyzed using secondary ion mass spectrometry (SIMS) at the University of California Los Angeles (UCLA) on the Cameca ims 1270 following the techniques of Quidelleur et al. (1997) and Schmitt et al. (2003). Detrital zircon U-Pb analyses of sample SP 21/07 were conducted by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) at the Arizona LaserChron facility (Tucson, USA). Complete descriptions of the U-Th-Pb analytical protocol are reported in Gehrels et al. (2006, 2008) and Gehrels and Pecha (2014). U-Pb isotopic results for all samples are reported in the GSA Data Repository as Appendix B.
Detrital zircon U-Pb age results are presented in probability density plots with stacked histograms (Figs. 4 and 5) made with the Isoplot 4.15 Excel macro of Ludwig (2003). Analyses with >10% discordance or >5% reverse discordance for samples analyzed with LA-ICP-MS, and >10% discordance or >10% reverse discordance for samples HL:XX-XXX analyzed with SIMS were rejected. 207Pb/206Pb ages were selected for all analyses (since no grains are younger than 0.8 Ga). All single grain ages are given with 1σ uncertainties. The value “n” corresponds to a number of analyses matching the criteria out of the total number of analyses for each sample. For statistical comparison of detrital zircon ages, a Kolmogorov-Smirnov (K-S) statistical test was applied using the Excel macro developed by Arizona LaserChron Center.
Wedel Jarlsberg Land—Southwestern Subprovince
Gullichsenfjellet Fm. (Sample HL:12–017). Sample HL:12–017 (IGSN HOL000100) was collected from the distinctive green quartzite of the Gullichsenfjellet Formation. The zircon population consists of subequant to elongate (length:width ratio 1:1–3:1), well-rounded grains ranging in size from 80 to 140 µm. A set of 45 grains gave U-Pb ages ranging from ca. 1.72–3.10 Ga (Fig. 4A). A dominant population is present at ca. 2.70 Ga, and the youngest at ca. 1.75 Ga. Subordinate peaks are present at ca. 1.90 Ga and 2.50 Ga with few older (3.00–3.10 Ga) grains. The Th/U ratios vary from 0.07 to 1.57, with U concentrations between 53 and 754 ppm.
Eimfjellbreane Fm. (Sample HL:12–014). Sample HL:12–014 (IGSN HOL000097) is a meta-sandstone of the Eimfjellbreane Formation. Zircons exhibit rounded and elongated shapes (length:width ratio 1:1–2:1), varying in size from 60 to 150 µm.
The U-Pb ages obtained on the population of 23 zircons are from ca. 1.02 Ga to 1.90 Ga. The probability density plot reveals two dominant populations at ca. 1.50–1.60 Ga and 1.70–1.80 Ga with minor age clusters at ca. 1.10 Ga and 1.90 Ga (Fig. 4B). The U concentrations are from 59 to 757 ppm, with the Th/U ratios of 0.11–0.57.
Skjerstranda Fm. (Sample HL:12–012). The feldspathic quartzite HL:12–012 (IGSN HOL000095) was sampled from the Skjerstranda Formation. The zircons are typically elongated needle-shaped grains that are ~70 µm long (length:width ratio 2:1–4:1). A set of 43 zircon grains yielded ages between ca. 0.98 Ga to 2.68 Ga (Fig. 4C). The predominant population is 1.75–1.95 Ga, whereas the subordinate peaks occur at 1.20 Ga and 1.54 Ga. The Th/U ratios are 0.07–1.21, and the four zircons with values <0.15 yielded ages between 1400 Ma and 1600 Ma. The U concentrations are between 27 and 267 ppm with two grains >600 ppm.
Isbjørnhamna Group (Sample Sp21/08)
Sample Sp21/08 is coarse-grained quartzite from the Isbjørnhamna Group. The zircons are equant to elongated shaped grains that range from 60 to 200 µm in length (length:width ratio 1:1–3:1). Outer rims are rare, and all analyzed zircons are interpreted as detrital grains. U-Pb zircon ages range from ca. 1.05–2.68 Ga with a dominant population yielding ca. 1.63 Ga ages and the youngest at ca. 1.07 Ga (Fig. 4D). The minor peaks are at ca. 1.20 Ga, 1.46 Ma, and 1.75 Ga. The U concentrations have large variations (16–1454 ppm). A zircon population of 56 grains is characterized by Th/U ratios between 0.21 and 1.63; one zircon has a low value of 0.03.
Unit 35 (Sample HL:12–008 and Sample HL:12–010)
The quartzite sample HL:12–008 (IGSN HOL000091) contains 40 µm to 170 µm long, rounded to elongated grains of prismatic shape (length:width ratio 1:1–2:1). U-Pb data from 45 zircons yielded ages from ca. 1.01 Ga to 3.67 Ga, with a dominant zircon population at ca. 1.69 Ga and the youngest peak at ca. 1.09 Ga (Fig. 4E). Minor peaks are present at ca. 1.30 Ga, 1.87 Ga, and 2.40–2.50 Ga, with a few older zircons (>2.5 Ga). The zircons are characterized by 0.18–2.96 Th/U ratios with U concentrations varying from 29 to 779 ppm.
Sample HL:12–010 (IGSN HOL000093) is meta-sandstone with rounded zircons, some of which form triangular shapes, but with rounded edges. The grains are up to 170 µm long and have slightly elongated shape (length:width ratio 1:1–2:1). Analyses of the 48 detrital zircon grains yielded ages from ca. 0.82 Ga to 2.80 Ga, with a main population at 1.68 Ga, and the youngest age at ca. 1.05 Ga (Fig. 4F). Subordinate populations at ca. 1.40 Ga and 2.10 Ga with the oldest ages at 2.60–2.80 Ga completing the data set. The Th/U ratios are from 0.14 to 1.5, with two grains <0.05. Uranium concentrations are dispersed in between 35 and 1516 ppm.
Wedel Jarlsberg Land—Northeastern Subprovince
Sofiebogen Group (HL:12–019)
The conglomerate sample HL:12–019 (IGSN HOL000102) contains mainly quartzite clasts in a siliceous matrix. A set of 50 zircons is composed of rounded and elongated zircons (length:width ratio 1:1–3:1) that range from 50 to 100 µm in length. The ages range from ca. 0.84 Ga to 2.05 Ga. The U-Pb ages show a main age cluster at ca. 1.43–1.62 Ga, and subordinate peaks at ca. 1.05 Ga and 1.90 Ga (Fig. 5A). The zircons are characterized by the Th/U ratios between 0.12 and 0.71, with two grains <0.02. The U concentrations are between 26 and 1507 ppm.
Deilegga Group (HL:12–018)
Sample HL:12–018 (IGSN HOL000101) was collected from quartzite strata in the Deilegga Group. Zircons are rounded, elongated grains (length:width ratio 1:1–3:1), that range in size from 50 to 100 µm long. A population of 41 analyzed zircons yielded ages between ca. 0.81 and 1.77 Ga. The dominant populations are at ca. 1.45 Ga and 1.57 Ga, with a minor peak at ca. 0.90 Ga (Fig. 5B). Th/U ratios are from 0.02 to 1.12; five grains with the ratio <0.20 yielded dispersed ages. The U concentrations exhibit a large range from 55 to 2566 ppm.
Age Groups and Trends
All rock units, except the Gullichsenfjellet Formation quartzite, are dominated by Paleoproterozoic to Mesoproterozoic age populations with minor Archean and Neoproterozoic input, but the proportions and presence of specific populations vary between the samples. Therefore, the data can be divided into three groups: (A) the Gullichsenfjellet Formation, (B) Eimfjellbreane and Skjerstranda formations, Isbjørnhamna Group and unnamed unit of Sørkapp Land, and (C) Sofiebogen and Deilegga groups.
Group A displays distinctively older detrital zircon ages with dominant Archean (ca. 2.7 Ga) and late Paleoproterozoic (1.8–1.9 Ga) populations with no grains younger than <1.72 Ga. With a major Archean population and no Mesoproterozoic grains, it differs significantly from all other samples.
The samples in Group B exhibit various intracorrelations between the samples. The Eimfjellet Complex samples show distinct zircon populations at ca. 1.5 Ga and 1.8 Ga, with a notable gap between 1.6 and 1.7 Ga. Single Archean ages are present only in the Skjerstranda Formation sample; however, the relatively small number of analyses for the Eimfjellbreane Formation sample may explain this apparent absence of Archean ages. Samples representing the Isbjørnhamna Group and unnamed units of Sørkapp Land, respectively, are characterized by a main age population of ca. 1.6–1.7 Ga and by Early Paleoproterozoic to Archean ages of 2.4–2.8 Ga with two older grains at 3.1 Ga and 3.65 Ga. Mesoproterozoic populations are concentrated at ca. 1–1.2 Ga; however, the sample HL:12–010 unit has a peak at ca. 1.38 Ga, which is lacking in the remaining samples. Statistically it is very likely that the Isbjørnhamna Group sample (Sp21/08) and the unnamed units of Sørkapp Land sample (HL:12–010) have similar source (p = 0.559; Fig. 6).
Detrital zircon spectra of group C originate from the northeastern subprovince of WJL and show a high statistical probability for being a part of the same sedimentary basin (p = 0.967). The rock units are strongly dominated by the Mesoproterozoic detrital zircons with the main age populations in the range of 1.42–1.62 Ga. Smaller populations are present in the range of 0.9–1.05 Ga. The Sofiebogen Group is characterized by a subordinate Paleoproterozoic population of ca. 1.9 Ga. However, both samples lack evidence of Archean sources. The Sofiebogen Group sample reveals a high probability of having statistically the same source as the unnamed units of Sørkapp Land sample (HL:12–010; p = 0.582) and a relatively low correlation with the Isbjørnhamna Group sample (Sp21/08; p = 0.064).
The Gullichsenfjellet Formation
The Gullichsenfjellet Formation represents the oldest Mesoproterozoic metasedimentary rocks in the area, deposited before ca. 1.2 Ga, which is the age of the bimodal magmatic plutonism and volcanism within the Eimfjellet Complex. The gradational contact of the quartzite with the underlying volcanic succession of the Bratteggdalen Formation suggests deposition in the Late Mesoproterozoic, but prior to ca. 1.2 Ga magmatism. The youngest detrital zircon grain in the Gullichsenfjellet Formation was dated to 1.719 Ga ± 0.073 Ga. A time gap of ca. 500 m.y. between the youngest detrital zircon age and late Mesoproterozoic age of deposition is typical for extensional basins (Cawood et al., 2012). The deposition of the Gullichsenfjellet Formation and subsequent bimodal anorogenic magmatism suggests an extensional regime resulting from formation of an intra-continental or sag-basin.
No potential source rocks for detritus of the Gullichsenfjellet Formation are exposed in the SBP. In the Eastern Basement Province, Archean (Hellman et al., 2001) and ca. 1.75 Ga Paleoproterozoic granites and gneisses are exposed (Hellman et al., 1997). Moreover, detrital zircon spectra of the Sørbreen and Vassfaret units of the Atomfjella Complex (Ny Friesland, Eastern Basement Province) reveal similar age populations (Bazarnik et al., 2017) as the Gullichsenfjellet Formation. Paleoproterozoic granitoids of Ny Friesland are typically correlated with those in the Hekla Sund region in the East Greenland Caledonides (Pedersen et al., 2002; Gee and Teben’kov, 2004). Also, the detrital zircon age signatures of the Gullichsenfjellet Formation (Fig. 7A) resemble those present in Paleoproterozoic sandstones of the Hekla Sund area (e.g., Norreland and Lambert Land; Kalsbeek et al., 1999; Fig. 7B). Similar Paleoproterozoic to Archean detrital zircon ages in the Mesoproterozoic Inuiteq Sø formation of Peary Land in northern Greenland should be noted, as well (Kirkland et al., 2009; Fig. 7C). The ca. 1.75 Ga granites intruding Paleoproterozoic basement have been reported in the northernmost Scandinavian Caledonides in the Fagervik Complex (Kirkland et al., 2008a; Fig. 7D). In metaturbidites of Tanahorn Nappe overlying the Fagervik Complex, Archean and Paleoproterozoic zircons dominate the age signature (Kirkland et al., 2008b; Fig. 7E), resembling that of Gullichsenfjellet Formation. The basement that is characterized by ca. 1.2 Ga bimodal volcanic assemblages and that is underlying the Gullichsenfjellet Formation is not exposed in any of these regions. Nevertheless, the detrital zircon ages pattern is typical for Mesoproterozoic and younger metasedimentary rocks of northern and northeastern Greenland or northernmost Norway.
Southwestern Subprovince of WJL and Sørkapp Land
The depositional age constraints of group B lie between ca. 1 Ga from detrital ages and the ca. 0.64 Ga age of metamorphism. The Eimfjellbreane and Skjerstranda formations possess detrital zircon grains younger than the age of the overlying Bratteggdalen and Skålfjellet formations. Pervasive tectonic deformation apparent in the lower parts of the Eimfjellet Complex suggests that these young grains could have been added during tectonic transport. However, the age spectra of the Eimfjellbreane and Skjerstranda formations differ much from the one of the Gullichsenfjellet Formation, which suggests rather that the main detachment is located between the Eimfjellbreane and Skålfjellet formations (Fig. 3). Moreover, the Eimfjellbreane and Skjerstranda formations, containing more proximal feldspathic quartzites, might represent the final stages of the basin evolution primarily filled by the Isbjørnhamna Group. Unit 35 of Sørkapp Land yields detrital zircon ages similar to the Isbjørnhamna Group and Skjerstranda Formation and supports the hypothesis that these units can be stratigraphically correlated (Harland, 1997) and that the southwestern subprovince of WJL continues to the southernmost Sørkapp Land.
The Isbjørnhamna Group and Unit 35 of Sørkapp Land display characteristic Mesoproterozoic to Paleoproterozoic populations with a prominent peak at 1.6–1.7 Ga (Fig. 8A) that is commonly observed in upper Mesoproterozoic–lower Neoproterozoic rocks across Svalbard. Similar detrital zircon spectra were reported in the St. Jonsfjorden Group of the SBP (Gasser and Andresen, 2013; Fig. 8B), the Krossfjorden Complex of Northwestern Basement Province (Pettersson et al., 2009; Fig. 8C), the Mosselhalvøya Group and Rittervatnet unit of Atomfjella Complex (Ny Friesland; Bazarnik et al., 2017), and the Brennevinsfjorden and Helvetesflya groups of the Eastern Basement Province (Nordaustlandet; Johansson et al., 2005, Lorenz et al., 2012; Fig. 8D). Other localities in the North Atlantic region bear resemblance to the Isbjørnhamna Group and Unit 35 age spectra: the Middle Allochthon of the Scandinavian Caledonides (e.g., the Sværholt succession; Kirkland et al., 2007; Fig. 8E) and the Greenland Caledonides (e.g., the Krummedal/Smallefjord sequence; Strachan et al., 1995; Kalsbeek et al., 2000; Watt et al., 2000; Watt and Thrane, 2001; Leslie and Nutman, 2003; Fig. 8F). These upper Mesoproterozoic to lower Neoproterozoic metasedimentary rocks have been interpreted as the remnants of a successor basin deposited at, or on the border of, Rodinia (e.g., Cawood et al., 2004). Therefore, the Isbjørnhamna Group and Unit 35 of Sørkapp Land probably have the same source of Mesoproterozoic to Paleoproterozoic crust on Baltica and Laurentia, which was exhumed during the Grenville–Sveconorwegian Orogeny.
Similar source areas characterize the detritus of the Eimfjellbreane and Skjerstranda formations. However, in these metasedimentary rocks the population of 1.6–1.7 Ga zircon is lacking, which suggests that deposition of the Skjerstranda Formation is not an effect of recycling of the Isbjørnhamna Group. It would also indicate that the 1.6–1.7 Ga population, prevalent in the upper Mesoproterozoic–lower Neoproterozoic strata of the North Atlantic region, is connected to source areas that are located south of Svalbard. Nevertheless, the Eimfjellbreane and Skjerstranda formations are strongly deformed, and the primary contact with the Isbjørnhamna Group remains unclear. Therefore, interpretation of the detrital zircon spectra of the Eimfjellbreane and Skjerstranda formations in a wider context should be treated with caution.
Detrital zircon age distributions that have a high degree of similarity to the main age populations present in the Eimfjellbreane and Skjerstranda formations were reported from the Polhem Formation of Ny Friesland (Hellman et al., 1997; Bazarnik et al., 2017). Correlation of the Eastern Caledonian Basement Province with the Laurentian margin (Gee and Teben’kov, 2004) suggests that potential source regions for the lower Eimfjellet Complex and the Isbjørnhamna Group might be Laurentian as well. Evidence for the Timanide-age basement (0.57–0.63 Ga) present underneath Cambrian–Ordovician sediments in Peary Land, northern Greenland (Rosa et al., 2016), opened a discussion of whether a correlation of the southwestern subprovince of WJL (metamorphosed at ca. 0.64 Ga) with northern Laurentia is possible (cf. Gee and Teben’kov, 2004). However, the detrital zircon ages of the Mesoproterozoic to late Neoproterozoic Morænesø Formation in Peary Land are dominated by an 1.8 Ga to 2.1 Ga signature (Kirkland et al., 2009), which is almost absent in the southwestern subprovince of WJL.
Alternatively, detrital zircon age spectra with main populations at ca. 1.5 Ga and 1.75 Ga, resembling the Eimfjellbreane and Skjerstranda formations, were noted in the Middle Allochthon of Scandinavian Caledonides of northernmost Norway (e.g., the Laksefjord Nappe; Zhang et al., 2016; Gee et al., 2017; Fig. 8G). The northernmost parts of the Middle Allochthon have been recently interpreted to possess a Timanian to Baltoscandian provenance (Zhang et al., 2016). The detrital zircon U-Pb ages from the southwestern subprovince would support the correlation of the Torellian event known from southwestern Svalbard with the Timanide orogen of northern Baltica (Mazur et al., 2009).
Northeastern Subprovince of WJL
The Deilegga and Sofiebogen groups represent middle to upper Neoproterozoic strata with a maximum depositional age of between 0.87 Ga (age of the youngest detrital zircon) and 0.64 Ga (age of the Torellian unconformity). The Sofiebogen Group detrital zircon ages are similar to that of the underlying Deilegga Group, which suggests that the former might be derived mainly from the latter. The presence of Paleoproterozoic ages suggests additional input of detritus exhumed during the Torellian orogeny (Majka et al., 2008, 2010, 2014) even though no 0.64 Ga detrital grains provide a straightforward confirmation of the Torellian exhumation. The detrital zircon spectra of sedimentary rocks from the northeastern subprovince of WJL are limited to early Neoproterozoic–late Paleoproterozoic ages (Fig. 9A), indicating that the basin was either distant from any early Paleoproterozoic and Archean sources or was shielded from older cratonic sources. Detrital age signatures of samples from the northeastern subprovince differ from those of the southwestern subprovince, which suggests that the sediment sources were clearly distinct in the subprovinces, during middle and late Neoproterozoic time. These provenance differences support the idea of the VKZ as a significant tectonic boundary that separates two subprovinces of different affinities (Mazur et al., 2009).
Neoproterozoic rock units in Oscar II Land, the Daudmannsodden and Comfortlessbreen groups, contain 0.9–1.7 Ga detrital zircons and are potentially correlative with the Sofiebogen and Deilegga groups (Gasser and Andresen, 2013; Fig. 9B). The provenance of the rocks in the northeastern subprovince of WJL does not exclude a possible link between the Deilegga and Sofiebogen groups and coeval metasedimentary rocks of the Pearya Terrane (Fig. 9C). However, the low abundance of middle to late Mesoproterozoic zircons do not support a straightforward correlation. The ca. 1.42–1.62 Ga populations with the relatively low middle Paleoproterozoic to Archean input are somewhat similar to the Särv, Sätra (Be’eri-Shlevin et al., 2011), and Seve Nappes (Kirkland et al., 2011; Gee et al., 2014; Fig. 9D) of the Middle Allochthon in the Scandinavian Caledonides. The detrital zircon ages of the Deilegga and Sofiebogen groups reveal patterns less similar to the Neoproterozoic sedimentary rocks of Laurentian margin. This might suggest deposition of the northern subprovince metasedimentary rocks in a relatively distant position from Laurentia in the late Neoproterozoic or strong influences of another potential source.
Defining Tectonic Boundaries Using Detrital Zircon Age Signatures
Detrital zircon geochronology can be an important tool for unravelling the complexity of areas affected by multiple tectonic and metamorphic events (e.g., Cawood et al., 2004). The region of southern WJL and Sørkapp Land, even though relatively small, underwent a multitude of tectonometamorphic events spread unevenly across the area (Czerny et al., 1993). This makes stratigraphic relations between the units hard to define, and therefore any stratigraphic divisions in southern WJL should be taken with prudence. Thus, new detrital zircon age spectra presented herein may help to resolve local complexity, but not without confronting them with additional structural studies. Another restriction of this study is the relatively small number of analyzed detrital zircons per sample (n <60), which implies that small, but potentially relevant fractions may not be reflected in the results (Vermeesch, 2004). Still, the results allow us to draw significant conclusions concerning the development of the SBP of Svalbard.
Provenance studies of the Proterozoic rocks of southwestern subprovince of WJL and Sørkapp Land, which were dismembered by Paleozoic Caledonian and Cenozoic Eurekan deformation, support a pre-Caledonian connection between these areas. New geochronological data from the Precambrian rocks of Sørkapp Land allow us to strengthen correlations between Unit 35 and the Isbjørnhamna Group. Detailed field studies in Sørkapp Land to identify potential correlatives of the upper part of the Eimfjellet Complex will be required. In WJL, the Isbjørnhamna Group together with the Skjerstranda and Eimfjellbreane formations is overthrust by the upper part of the Eimfjellet Complex. The age of thrusting is thought to be Torellian due to the ca. 0.64 Ga amphibolite facies metamorphism, which affects the entire southwestern subprovince of WJL. This tectonometamorphic event is still not well understood, and more work directly aimed at understanding the Torellian event will be valuable for future correlations.
Detrital zircon U-Pb data suggest that the southwestern and northeastern subprovinces of WJL, beyond the contrasting structural and metamorphic history across the VKZ (Mazur et al., 2009), are also characterized by a different sediment age provenance. Similarities can be found between the provenance of the Deilegga and Sofiebogen groups and the provenance of the Middle Allochthon of the Scandinavian Caledonides, suggesting potential affinities with Baltican side of Iapetus. Straightforward correlation of the upper Neoproterozoic sedimentary rocks of the northeastern subprovince and the Pearya Terrane, taking into account previously proposed connections (Mazur et al., 2009; Kośmińska et al., 2014), cannot be fully confirmed by data presented herein. Nevertheless, provenance studies suggest that late Caledonian sinistral shear zones similar to the VKZ, as the one in northern WJL described by Majka et al. (2015), contributed substantially to the assembly of the SBP.
The Isbjørnhamna Group contains detrital zircon grains that can be correlated with upper Mesoproterozoic to lower Neoproterozoic strata of the Krossfjorden Group of the Northwestern Caledonian Basement of Svalbard Province (Pettersson et al., 2009). Both of these units are dominated by potentially coeval metapelitic rocks and characterized by similar ca. 1.0–1.9 Ga age signatures with a dominant 1.6–1.7 Ga peak. A similar connection between these provinces (Torellian basement rocks of the Berzeliuseggene unit in the SBP and the Richarddalen Complex of the Northwestern Caledonian Basement Province) has been postulated by Majka et al. (2015). The detrital zircon geochronological data presented here reveal another deviation from the traditional models that divide the Svalbard’s basement into three Caledonian basement provinces (Gee and Teben’kov, 2004) and suggest that the proposed divisions of the Caledonian basement provinces need to be revised.
The main tectonic discontinuity within the southwestern subprovince of Wedel Jarlsberg Land occurs between the Skålfjellet and Eimfjellbreane formations. The units underlying the Skålfjellet Formation should therefore be regarded as a part of the Isbjørnhamna Group.
The detrital zircon age signatures of the metasedimentary rocks of the southwestern subprovince of Wedel Jarlsberg Land resemble those of Unit 35 of southern Sørkapp Land. Thus, Unit 35 appears to be a correlative of the Isbjørnhamna Group (and not of the Deilegga Group).
Detrital zircon U-Pb data support the interpretation of the Vimsodden-Kosibapasset Zone as a major tectonic boundary that separates two subprovinces of different affinity (Mazur et al., 2009). It highlights the key role of late Caledonian sinistral shear zones in the assembly of the Caledonian basement of western Svalbard.
Detrital zircon spectra of the late Mesoproterozoic to Neoproterozoic metasedimentary rocks of Wedel Jarlsberg Land reveal similarities with coeval metasedimentary rocks in the Northwestern Caledonian Basement Province of Svalbard. This supports the idea that basement units may continue across the traditional boundaries of Svalbard’s Caledonian Basement provinces (cf. Majka et al., 2015).
Luke Beranek and Victoria Ershova are thanked for their constructive reviews and Karsten Piepjohn for the editorial handling. This research was supported by the National Science Centre (Poland) NAC project no. 2015/17B/ST10/03114 to Majka and a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery grant to Schneider. Fieldwork was supported by the Swedish Polar Research Secretariat in frame of the NOA-Svalbard project to Lorenz and Majka and by the AGH-UST statutory funds no. 220.127.116.11.
Figures & Tables
Circum-Arctic Structural Events: Tectonic Evolution of the Arctic Margins and Trans-Arctic Links with Adjacent Orogens
The circum-Arctic region has received considerable attention over the past several decades with vigorous debate focused on topics such as mechanisms for opening the Eurasian and Amerasian basins, the importance of plume-related magmatism in the development of the Arctic Ocean, and mechanisms for ancient terrane translation along the Arctic margins. In recognition of the 25th anniversary of the Circum-Arctic Structural Events (CASE) program, an international polar research effort organized and led by the Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) of Germany, this volume presents results from 18 major field expeditions involving over 100 international geoscientists from a broad spectrum of disciplines. The resulting publication focuses on the Proterozoic to Cenozoic tectonic evolution of the circum-Arctic region with correlations to adjacent orogens.