Age and composition of crystalline basement rocks on the Norwegian continental margin: offshore extension and continuity of the Caledonian–Appalachian orogenic belt

Abstract: Twenty-two wells on the Norwegian continental margin have penetrated underlying basement. We present U–Pb zircon, whole-rock geochemical, and Sm–Nd and Rb–Sr isotopic data from nine wells in the North Sea and Norwegian Sea with relevance to the offshore continuation of the Norwegian Caledonides, and their correlation throughout the Caledonian–Appalachian orogenic belt. Palaeozoic magmatism in the North Sea can be divided into two groups. The older group consists of 460 Ma calc-alkaline granites with evolved isotopic compositions, correlative with similar rocks in the Uppermost Allochthon. The younger group consists of a 430 Ma dacite and a 421 Ma leucogabbro, with less evolved isotopic compositions. In the Norwegian Sea, isotopically evolved granitic magmatism at 437 Ma and more juvenile dioritic magmatism at 447 Ma are correlative with magmatism in the Bindal and Smøla–Hitra Batholiths in the Uppermost Allochthon. Metasedimentary basement rocks from the North Sea and Norwegian Sea, dominated by Late Palaeoproterozoic and Mesoproterozoic grains, resemble rocks found in the Caledonides of Scotland, Greenland and Svalbard. The new data, along with studies elsewhere along the belt, suggest that similar rocks may exist along much of the orogen. Supplementary material: U–Pb isotopic data are available at http://www.geolsoc.org.uk/SUP18471.

. Simplified geological map of south and central Norway, modified after Solli & Nordgulen (2006), with locations and ages of samples discussed in the text. lithium tetraborate discs for the major and minor elements, whereas the trace elements were determined on pressed powder pellets. REE and several trace elements (Nb, Hf, Ta, Th, U) were analysed by LA-ICP-MS following Flem et al. (2005), using the same glass discs from which XRF major element data were obtained. Whole-rock major-and trace-element data are presented in Table 1. Sm-Nd and Rb-Sr isotopic data (Table 2) were obtained at University College Dublin.

Geochronology
Zircons were separated from nine samples from the North Sea and Norwegian Sea, using standard techniques including Wilfley table, heavy liquids, magnetic separation and final hand picking under a binocular microscope. Zircons from eight samples were analysed by secondary ion mass spectrometry (SIMS) on the Cameca IMS 1270 system at the Nordsim laboratory in Stockholm, and one sample of metasediment was analysed by LA-ICP-MS at NGU. Prior to SIMS analysis, the grains were mounted in epoxy and polished to approximately half grain thickness. Cathodoluminescence (CL) images were obtained using a scanning electron microscope. LA-ICP-MS age determinations were performed on whole grains mounted on double adhesive tape. These grains were ablated twice, first at low energy removing potential rims, followed by a second ablation with an energy chosen to achieve optimal counting statistics. The age obtained from the latter ablation is taken as the age of the grain. Isoplot 3.00 (Ludwig 2003) was used to calculate and plot the U-Pb isotopic data from both SIMS and LA-ICP-MS analyses.
SIMS. The analytical method, data reduction and error propagation of the results have been outlined by Whitehouse et al. (1999) and Whitehouse & Kamber (2005). The analyses were conducted with an O 2 beam of c. 2-4 nA and a spot size of 15-20 ìm, and U-Pb ratios were calibrated to the Geostandard 91500 reference zircon with an age of 1065 Ma (Wiedenbeck et al. 1995). The error on the U/Pb ratio includes propagation of the error on the day-to-day calibration curve obtained by regular analysis of the reference zircon. A common-Pb correction was applied using the measured 204 Pb counts and a present-day isotopic composition (Stacey & Kramers 1975), where significant 204 Pb counts were recorded. This procedure avoids negative common Pb corrections where background counts are greater than low levels of 204 Pb.
LA-ICP-MS. The instrumentation used at NGU consists of a Finnigan MAT Element 1 single collector high-resolution sector ICP-MS system, in this case supplied by a Finnigan MAT 266 nm (Nd:YAG) UV-laser. During analysis the laser is operated in Q-mode, at a frequency of 10 Hz. Depending on the sample, the energy used on samples typically varies from about 0.01 mJ to 1.0 mJ, tuned to give the optimum counting statistics. To minimize elemental fractionation, the zircon crystals are ablated in a 80 ìm 3 60 ìm raster. The sample aerosol is transported from the sample chamber in He gas, and introduced in the ICP-MS instrument as a mixture of He and Ar gas. The data are acquired in a time-resolved counting scanning mode for 60 s. Masses 202 Hg, 204 (Hg + Pb), 206 Pb,207 Pb,208 Pb,232 Th and 238 U are measured. Monitoring 202 Hg and assuming a 202 Hg/ 204 Hg ratio of 4.36 (natural abundance) corrects the interference of 204 Hg on 204 Pb. A 60 s delay is performed after each zircon analysis, and a 60 s gas blank is acquired at regular intervals. The measured isotope ratios are corrected for element-and mass-bias effects using the Geostandard 91500 reference zircon (Wiedenbeck et al. 1995), normally based on 15-30 analyses during one analytical session. The data reduction is performed using MS Excel spreadsheets with Visual Basic macros developed in-house.

Geochronological, geochemical and isotopic data
North Sea 16/1-4 (1937 m), leucogabbro. Sample 16/1-4 (1937 m) is a medium-grained leucogabbro (Fig. 3a), consisting mainly of strongly saussuritized plagioclase, amphibole and biotite, with accessory, secondary calcite in thin veins or as small crystal aggregates. The only evidence of deformation is a slight kinking observed in many biotite laths. The leucogabbro is calc-alkaline with a moderately fractionated REE pattern (La/Yb N ¼ 26) and small negative Eu anomaly (Eu/Eu* N ¼ 0.88) (Fig. 4). The sample displays a strong negative Nb anomaly in the primitivemantle normalized diagram, and plots in the volcanic arc field in the Yb-Ta tectonic classification diagram of Pearce et al. (1984) (Fig. 4d).
The zircons from sample 16/1-4 vary from stubby to elongate biprismatic, 100-300 ìm, with well-developed crystal faces. They display internal oscillatory zoning, and in some cases sector zoning (Fig. 5a), typical of magmatic zircons (see Corfu et al. 2003). Some grains display a narrow CL-bright rim (0-5 ìm), and occasionally CL-bright patches may occur within the grains. The zircons are transparent to pink to light brown. Inclusions and fractures are common.
Nine SIMS analyses from nine grains yield a concordia age of 421 AE 3 Ma, interpreted to represent the crystallization age of the leucogabbroic magma (Fig. 6a). One analysis yields a 206 Pb/ 238 U age of 374 Ma and is interpreted to have lost radiogenic Pb. This analysis was excluded from the calculation.  The leucogabbro yields a Nd depleted mantle model age (T DM ) of 1008 Ma (å Nd ¼ À1.25) and an initial 87 Sr/ 86 Sr ratio of 0.707 (Table 2, Fig. 4e).
16/6-1 (2059.7 m), dacite. Sample 16/6-1 (2059.7 m) is a finegrained, undeformed, grey volcanic rock with abundant phenocrysts of plagioclase (Fig. 3b). The plagioclase phenocrysts are ,1-2 mm and are blocky to angular prismatic, typically with albite twinning. The phenocrysts are strongly saussuritized, but many of the larger crystals contain less altered cores. The matrix consists of fine-grained plagioclase and slightly larger biotite, with small amounts of quartz. The phenocrysts are unoriented and the rock does not appear deformed. The sample also contains a small (c. 4 cm) xenolith of slightly coarser grey rock. The dacite is calc-alkaline, with a moderately fractionated REE pattern (La/Yb N ¼ 13) and a small, negative Eu anomaly (Eu/ Eu* N ¼ 0.90) (Fig. 4). In the primitive mantle-normalized diagram, the dacite displays a marked negative Nb anomaly and small, negative P and Ti anomalies, and plots in the volcanic arc field in tectonic discrimination diagrams (Fig. 4d).
The zircons from sample 16/6-1 are biprismatic, 75-200 ìm, with well-developed, slightly rounded crystal faces. They are transparent and colourless to pink. Inclusions occur in some grains. The zircons have a well-developed oscillatory zoning (Fig. 5b), and a few have brighter cores and/or brighter growth zones in the core. One crystal (number 5) has a core with an internal diffuse zoning that is truncated by the surrounding rim.
Ten SIMS analyses of oscillatory-zoned zircons yield a concordia age of 430 AE 6 Ma (Fig. 6b), interpreted to represent the crystallization age of the porphyritic magma. Two zircon cores yield 206 Pb/ 238 U ages of 491 and 1100 Ma. We interpret the oscillatory-zoned zircons to have crystallized from the porphyritic magma or lava at 430 Ma, whereas the cores may be inherited from the magma source or represent grains entrained in the magma during ascent and emplacement.
16/3-2 (2017.7 m), granite. Sample 16/3-2 (2017.7 m) is a medium-grained, unfoliated granite ( Fig. 3c) consisting of plagioclase, K-feldspar and quartz, locally with development of myrmekite. Irregular, small grains of biotite and amphibole are the only mafic minerals, and the granite also contains some muscovite and minor epidote. The granite is calc-alkaline, with a moderately fractionated REE pattern (La/Yb N ¼ 18) and nearly negligible Eu anomaly (Eu/Eu* N ¼ 0.91) (Fig. 4). In the primitive mantle-normalized diagram, the granite displays marked negative Nb, P and Ti anomalies, and plots in the volcanic arc field in the tectonic discrimination diagrams of Pearce et al. (1984).
The zircons from sample 16/3-2 are stubby biprismatic, 100-300 ìm, with well-developed crystal faces. They are pink to brownish, and may have an iron-oxide stained surface. The grains locally contain inclusions and fractures. Internally, the zircons typically contain large, CL-bright, rounded cores with (locally truncated) oscillatory or irregular zoning, surrounded by thick, oscillatory-zoned rims (Fig. 5c). Given the high abundance of cores in the zircons, we consider the cores most likely to be inherited, whereas the rims represent zircon crystallized from the magma.
(e) å Nd v. initial 87 Sr/ 86 Sr ratio for samples presented here, with fields indicating isotopic compositions for major granitoid batholiths in the Norwegian Caledonides. Data presented in Table 2; references to other data are presented in the text. 6d), interpreted to reflect inheritance from the source or assimilated wall rocks. The granite yields a Nd depleted mantle model age (T DM ) of 1664 Ma (å Nd ¼ À10.67), and an initial 87 Sr/ 86 Sr ratio of 0.709 (Table 2, Fig. 4e).
Overlying the granite is a fine-grained sedimentary rock consisting of alternating layers of immature quartzite and dark grey siltstone. The granite clearly cuts the layering in the overlying sedimentary rock (Fig. 3c inset). Like the granite, the sedimentary rock appears undeformed except for undulose extinction in quartz. The granite is calc-alkaline, with a moderately fractionated REE pattern (La/Yb N ¼ 16) and small, negative Eu anomaly (Eu/Eu* N ¼ 0.82) (Fig. 4). In the primitive mantlenormalized diagram, the granite displays marked negative Nb, P and Ti anomalies, and plots in the volcanic arc field in the tectonic discrimination diagrams of Pearce et al. (1984).
The zircons from sample 16/4-1 are rather similar to those found in sample 16/3-2; they are pink to brown, stubby biprismatic, 100-300 ìm, with well-developed crystal faces, and typically contain large (often CL-bright) cores surrounded by  thick, oscillatory-zoned rims (Fig. 5d). Some of the cores themselves have oscillatory zoning that is commonly truncated, locally by a CL-brighter layer. As above, the rims most probably represent zircon crystallized from the magma, whereas the cores could be inherited from the source of the granitic magma or represent assimilated zircons during ascent and/or emplacement.
Eight SIMS analyses of eight oscillatory-zoned rims yield a concordia age of 460 AE 8 Ma, interpreted to date the crystallization of the granitic magma (Fig. 6e). Seven analyses of zircon cores yield 207 Pb/ 206 Pb ages ranging from 942 to 2719 Ma (Fig. 6f).
16/5-1 (1929.3 m), granite. Sample 16/5-1 (1929.3 m) is a finegrained, unfoliated, dark red granite (Fig. 3e), which consists mainly of K-feldspar, plagioclase and quartz. Biotite is the only mafic mineral and the rock contains minor amounts of muscovite and accessory epidote. The granite appears undeformed, except for moderately undulating extinction in quartz. The granite is calc-alkaline, with a moderately strongly fractionated REE pattern (La/Yb N ¼ 22) and no Eu anomaly (Eu/Eu* N ¼ 1) (Fig.  4). In the primitive mantle-normalized diagram, the granite displays marked negative Nb, P and Ti anomalies, and plots in the volcanic arc field in the tectonic discrimination diagrams of Pearce et al. (1984).
The zircons from sample 16/5-1 are similar to those found in samples 16/3-2 and 16/4-1, albeit slightly smaller. The zircons are prismatic, 100-200 ìm, with well-developed external crystal faces, and commonly contain large, rounded CL-bright cores, surrounded by thick, oscillatory-zoned rims (Fig. 5e). We interpret the cores to be inherited from the source of the granitic magma or assimilated during ascent and/or emplacement, whereas the rims most probably represent zircon crystallized from the magma.
Twelve SIMS analyses of oscillatory-zoned rims yield a concordia age of 463 AE 6 Ma, interpreted to date the crystallization age of the granitic magma (Fig. 6g). Six analyses of zircon cores yield 207 Pb/ 206 Pb ages ranging from 1040 to 1877 Ma (Fig. 6h).
The granite yields a T DM of 1590 Ma (å Nd ¼ À10), and an initial 87 Sr/ 86 Sr ratio of 0.709 (Table 2 is a faintly banded, grey metasandstone (Fig. 3f) consisting of fine-grained quartz with minor plagioclase and K-feldspar layers alternating with more feldspar-rich, very fine-grained layers. Muscovite is abundant as oriented laths, up to a few millimetres long. Subhedral pyrite is rather common and appears to be associated with the feldspar-rich layers. The metasandstone is criss-crossed by numerous calcareous veins.
The zircons from sample 25/7-1S are slightly to moderately abraded or rounded, with shapes ranging from imperfect biprismatic (some are multifaceted), via rounded (abraded) biprismatic to rounded elongate grains. The crystals are colourless to faint yellowish, and commonly have fractures or contain inclusions. The zircons range in size from 90 to 200 ìm and are typically oscillatory-zoned (Fig. 5f), and in some cases also display sector zoning (see Corfu et al. 2003). The zoning suggests a magmatic origin for the zircons. The internal zoning is cut by the grain surface in some cases, demonstrating sedimentary transport and abrasion. Some grains contain homogeneous zones or patchy, convoluted domains, possibly owing to recrystallization. A thin (typically 2-10 ìm), CL-bright rim is present in about half the grains. Figure 6k shows a cumulative probability plot based on 93 LA-ICP-MS analyses of zircon grains that are ,10% discordant. The main age group, ranging from 1000 to 1800 Ma, comprises several populations, at 1000-1150 Ma, c. 1350 Ma and 1600-1650 Ma, with a subordinate peak at 1500 Ma. In addition there are a few data points at c. 1900 and c. 2800 Ma.
Norwegian Sea 6407/10-3 (2972.1 m), granite. Sample 6407/10-3 is a finegrained granite (Fig. 3g), consisting mainly of K-feldspar, plagioclase and quartz. Biotite is the only mafic mineral, occurring as small (,1 mm), dispersed grains. Muscovite is an accessory phase. The granite appears undeformed except for undulose extinction in quartz. The granite is calc-alkaline, with a strongly fractionated REE pattern (La/Yb N ¼ 137) and negative Eu anomaly (Eu/Eu* N ¼ 0.52) (Fig. 4). In the primitive mantlenormalized diagram, the granite displays marked negative Nb and P anomalies, and plots in the syncollisional and volcanic arc field in the tectonic discrimination diagrams of Pearce et al. (1984).
The zircons from sample 6407/10-3 are commonly stubby biprismatic, 100-300 ìm, in some cases multifaceted and in others with irregular or poorly developed crystal faces that give the grains a rounded appearance. The grains are clear, colourless to yellowish, commonly with an iron-oxide stained surface. A few grains contain inclusions. The zircons are oscillatory-zoned and some of the grains have CL-bright cores or CL-brighter domains within the grains (Fig. 5g); however, such cores are not nearly as ubiquitous as in the granite samples from the North Sea. The cores typically display oscillatory zoning that in some cases appears to be in crystallographic continuity with the surrounding oscillatory-zoned rim. The cores and rims of the zircons may therefore represent episodic growth in a magma of varying composition, or alternatively the cores could represent grains entrained in the magma during ascent and/or emplacement. The two analysed cores yield ages indistinguishable from the darker, oscillatory-zoned margins in other grains, lending support to the first interpretation.
Six SIMS analyses of oscillatory-zoned zircon (including the two 'cores') yield a concordia age of 437 AE 4 Ma (Fig. 6i), interpreted as the crystallization age of the granitic magma. One analysis, with excessive common Pb (12% f 206 ), was excluded.
6306/10-1 (3158.5 + 3159.2 m), diorite. Sample 6306/10-1 is a medium-grained diorite (Fig. 3h), mainly consisting of strongly saussuritized plagioclase and amphibole, with accessory, secondary calcite in thin veins. The rock appears undeformed apart from a slight kinking observed in some plagioclase grains where albite twinning is preserved. The diorite is calc-alkaline, with a moderately fractionated REE pattern (La/Yb N ¼ 12-15) and no Eu anomaly (Eu/Eu* N ¼ 0.97-1.02) (Fig. 4). In the primitive mantle-normalized diagram, the diorite displays a marked negative Nb anomaly, and plots in the volcanic arc field in the tectonic discrimination diagrams.
The zircons from sample 6306/10-1 are mostly stubby biprismatic, 100-200 ìm, with well-developed crystal faces, and internal oscillatory or parallel zoning, typically combined with sector zoning (Fig. 5h). The grains are colourless to smoke coloured and clear. Inclusions may occur locally, whereas fractures are more abundant.
Eight SIMS analyses from eight grains yield a concordia age of 447 AE 4 Ma (Fig. 6j), interpreted as the crystallization age of the dioritic magma. The diorite yields a T DM of 870 Ma (å Nd ¼ 1.42), and an initial 87 Sr/ 86 Sr ratio of 0.705 (Table 2, Fig. 4e). 6609/7-1 (1945.8 m), metasandstone or metasiltstone. Sample 6609/7-1 is a laminated metasediment consisting of ,1 cm, light orange layers dominated by very fine-grained quartz (sandstone or quartzite) with some muscovite, and thinner grey layers dominated by very fine-grained quartz and abundant muscovite (siltstone) (Fig. 3i). Weak undulatory extinction of quartz attests to some deformation. The rock is jointed with calcite-filled veins, and also contains dispersed calcite within the quartzitic matrix.
The zircons from sample 6609/7-1 are rather small, typically between 50 and 100 ìm. The majority of the grains are rounded equidimensional to elongate, but sub-rounded biprismatic (in some cases multifaceted) grains also occur. In general, the zircons display oscillatory zoning (Fig. 5i), in some cases combined with sector zoning. Many grains represent abraded fragments of larger crystals, reflecting sedimentary transport and abrasion. A few grains have a distinct core, but there is no evidence of metamorphic growth of zircon in the rock.
Twenty-seven SIMS analyses yield 207 Pb/ 206 Pb ages between 1056 and 2127 Ma. Figure 6l inset shows a cumulative probability plot of analyses that are ,10% discordant. The dominant populations are 1600-1750 Ma, 1500 Ma and 1100-1200 Ma.

Discussion
The Palaeozoic Caledonian-Appalachian orogenic belt formed in response to closure of the Iapetus ocean and Siluro-Devonian collision between Baltica, Laurentia and Avalonia (e.g. van Staal et al. 1998). The belt thus incorporates the pre-collisional histories of three continents, of which Baltica and Laurentia are the most important here. The Laurentian margin was active during the Late Cambrian-Ordovician, with development several arc-back-arc and ophiolite complexes that were accreted to the Laurentian margin during the Mid-Ordovician Taconian orogeny (e.g. Zagorevski et al. 2006). In contrast, the Baltican margin appears to have been largely passive prior to onset of the Late Silurian Scandian orogeny (e.g. Roberts 2003). As discussed in the introduction, the rocks discussed here most probably represent vestiges of magmatic complexes formed on or near the Laurentian margin. To place the studied samples in a wider geological context, we present a summary of the pre-collisional evolution of the exotic Upper and Uppermost Allochthons of the Scandinavian Caledonides, and the larger Caledonian-Appalachian orogenic belt. Figure 7 shows a compilation of published U-Pb zircon ages, interpreted to be crystallization ages, from the Norwegian Caledonides (see also Bingen & Solli 2010, for a comprehensive compilation and discussion of the available geochronological data). It is generally accepted that the Upper and Uppermost Allochthons record a Neoproterozoic to Ordovician magmatic and tectonometamorphic history, along both the Laurentian and Baltican margin (e.g. Meyer et al. 2003;Barnes et al. 2007;Roberts et al. 2007).

Magmatic and tectonic evolution of the Upper and Uppermost Allochthons, Norwegian Caledonides
The Bindal Batholith intruded between 482 and 424 Ma Yoshinobu et al. 2002;Nissen et al. 2006;Barnes et al. 2007). Lithologically, the batholith spans the compositional range from gabbro to granite, and isotopic studies have shown that the rocks formed by partial melting of a variety of crustal sources with variable mantle and crustal contributions (Nordgulen & Sundvoll 1992;Birkeland et al. 1993;Barnes et al. 2004Barnes et al. , 2005. Nordgulen & Sundvoll (1992) and Birkeland et al. (1993) documented highly variable isotopic compositions for the Bindal Batholith granitoids, with å Nd ranging from À3 to À9 and initial 87 Sr/ 86 Sr ratios varying between 0.705 and .0.715 (Fig. 4e). Nordgulen & Sundvoll (1992) identified a consistent lithological and geographical distribution in initial 87 Sr/ 86 Sr ratios, with granitoids to the SE yielding the lowest ratios and tourmaline granities and anatectic granitoids to the west yielding the highest ratios. The available isotopic and geochemical data suggest that the plutons constituting the Bindal Batholith formed from variable and heterogeneous sources, including upper mantle, lower and upper crustal rocks in an evolving active-margin setting (Nordgulen & Sundvoll 1992;Birkeland et al. 1993;Barnes et al. 2003Barnes et al. , 2004Barnes et al. , 2007. The Nesåa Batholith is located in the Upper Allochthon Gjersvik Nappe, near the southeastern margin of the Uppermost Allochthon (Meyer et al. 2003). The Nesåa Batholith consists of two intrusive complexes, the largely gabbroic Grøndalsfjell Intrusive Complex, dated at 458 AE 3 Ma (Meyer et al. 2003), and the dominantly quartz monzodioritic Møklevatnet Complex, dated at 456 AE 2 Ma (Roberts & Tucker 1991). The Nesåa Batholith is characterized by an arc-like geochemistry, and a relatively restricted range of Nd and Sr isotopic compositions with å Nd between 2 and 3.5 (one outlier at À3.5) and initial 87 Sr/ 86 Sr ratios of 0.704 (Meyer et al. 2003) (Fig. 4e). The isotopic data suggest more juvenile compositions than those observed in the Bindal Batholith, and Meyer et al. (2003) interpreted the Nesåa Batholith as an early phase of the more voluminous Bindal Batholith. They argued for formation of the batholiths in an active continental margin, intruding the Gjersvik Nappe after accretion of the latter onto the Laurentian margin.
In SW Norway, only the Middle and Upper Allochthons are exposed and most of the reported magmatic U-Pb ages are older than c. 470 Ma and related to ophiolite complexes. There are, however, several notable exceptions that have been dated by only the Rb-Sr or Sm-Nd method. These include the gabbroic to granitic Sunnhordland Batholith, south of Bergen, and the related Krossnes Granite, which have been dated at 430 AE 10 and 430 AE 6 Ma, respectively (Rb/Sr whole-rock isochrons, Andersen & Jansen 1987;Fossen & Austrheim 1988); the Kattnakken volcanic series (Lippard 1976) dated at 445 AE 5 Ma (Rb/Sr whole-rock isochron, Priem & Torske 1973) and cut by a dolerite dyke swarm at 435 AE 5 Ma (Ar-Ar maximum age, Lippard & Mitchell 1980); and the bimodal Siggjo Group (Nordås et al. 1985), including andesitic and rhyolitic lavas, dated at 468 AE 23 Ma and 464 AE 16 Ma, respectively (Rb/Sr whole-rock isochron, Furnes et al. 1983). These data suggest that the absence of rocks younger than c. 470 Ma in West Norway may be more apparent than real, but require confirmation from U-Pb zircon analyses. The Sunnhordland Batholith and related Krossnes Granite yield relatively low initial 87 Sr/ 86 Sr of 0.7056 and 0.7066 (Fig. 4e), respectively, and the structural, geochemical and isotopic investigations of the Sunnhordland Batholith are compatible with formation in a continental magmatic arc (Andersen & Jansen 1987).

Tectonomagmatic evolution of the Caledonian-Appalachian orogen
There is ample evidence that tectonically and temporally similar processes, including Early and Middle Ordovician arc-back-arc magmatism and accretion, took place along the length of what later became the Caledonian-Appalachian orogen (e.g. van Staal et al. 1998;Roberts 2003). Although along-strike events, such as Taconian and Grampian orogenesis, may not be direct correlatives (e.g. van Staal et al. 1998), a brief discussion of these processes is helpful for placing the rocks described here in a wider tectonic and magmatic context.
Early Ordovician ophiolites and ophiolite fragments are strung out along the entire length of the Caledonian-Appalachian orogen, and are generally considered an integral part of the Taconian and time-equivalent Grampian orogenic events along the SE margin of Laurentia Cawood & Suhr 1992;van Staal et al. 1998). The ophiolites are commonly interpreted to have formed by rifting along the Laurentian margin to form peri-continental microcontinents separated from Laurentia by small, ophiolite-floored ocean basins (Cawood et al. 1995;van Staal et al. 1998;Waldron & van Staal 2001;Hibbard et al. 2007). Closure of these ocean basins resulted in arcs forming on the various microcontinent blocks, ophiolite obduction and Taconian orogenesis, recorded in different segments of the Caledonian-Appalachian orogen, such as New England (Karabinos et al. 1998), Newfoundland (Waldron & van Staal 2001), Scotland (Kinny et al. 1999;Friend et al. 2000) and the Uppermost Allochthon of the Norwegian Caledonides (Barnes et al. 2007;Roberts et al. 2007). In contrast, the Greenland Caledonides has no record of Taconian orogenesis, and in the northern parts carbonate sedimentation persisted into Early Silurian times (Higgins et al. 2004), just prior to Scandian orogenesis. Figure 8 shows probability density plots of Caledonian magmatism in Scandinavia, East Greenland and Svalbard, the British Isles and the Newfoundland Appalachians. All segments show extensive Late Ordovician to Early Silurian magmatism between c. 445 and 425 Ma, related to final closing of the Iapetus ocean and initiation of continent-continent collision. In contrast, the Greenland-Svalbard segment lacks the Early Ordovician (c. 480-460 Ma) magmatic activity that is seen in the British and Irish, Newfoundland, and Norwegian segments, although less pronounced in the latter. It is possible that studies (and thus available data) from Greenland are biased towards processes related to crustal anatexis and associated magmatism, which took place at c. 435-425 Ma (e.g. Kalsbeek et al. 2001); however, considering the lack of ocean-derived rocks in the Greenland Caledonides (e.g. Gee et al. 2008), the absence of this largely oceanic magmatic event is not surprising. Observations from the various segments constituting the Caledonian-Appalachian orogen suggest that late orogenic, Siluro-Devonian extension was accompanied by sinistral megashear (e.g. Soper et al. 1992;Strachan et al. 1992;Dewey & Strachan 2003). Thus, as discussed by Roberts et al. (2007), restoring the Scandinavian Caledonides southwestward to their likely pre-megashear position would place Norway and the Upper and Uppermost Allochthons closer to the northern Appalachians than Greenland.
In Newfoundland, the Andean-type Notre Dame arc formed on Laurentian continental crust between c. 490 and 455 Ma (van Staal et al. 1998. The arc-related rocks range in composition from tonalite to granite, and zircon xenocrysts and Sm-Nd isotopes (with å Nd values between 2.6 and À13.5) suggest significant contributions from old crustal and/or subcontinental lithosphere material (Whalen et al. 1997;van Staal et al. 1998van Staal et al. , 2007. Simultaneously with formation of the Notre Dame Arc, the Annieopsquotch accretionary tract developed. Zagorevski et al. (2006) described the Annieopsquotch accretionary tract as a series of west-dipping, eastward-younging thrust slices containing remnants of ophiolitic and arc-back-arc complexes that developed intermittently in response to eastward retreat of a single, west-dipping subduction zone just outboard of the Dashwoods microcontinent. They interpreted these complexes to represent peri-Laurentian terranes accreted to the Laurentian margin during Ordovician closure of the Iapetus ocean. Geochronological, geochemical and isotopic data show that several arcs formed and were accreted to the continental margin between c. 473 and 460 Ma. The arc rocks range from basaltic to rhyolitic, are tholeiitic to calc-alkaline, and have variable Sm-Nd isotopic compositions with å Nd ranging from +9 to À10. The isotopic compositions reflect variable input from Laurentian continental crust, indicating that the arcs formed on a substrate of previously rifted continental crust (Dashwoods ribbon continent). Likely equivalents of the Notre Dame Arc and Annieopsquotch accretionary tract in the British Isles have been described from the Midland Valley (Midland Valley arc, Bluck 1984) and possibly the Northern Belt of the Southern Uplands (van Staal et al. 1998). Flowerdew et al. (2005Flowerdew et al. ( , 2009) discussed the Sm-Nd and Rb-Sr whole-rock, and Hf-in-zircon isotopic compositions of c. 470 Ma tonalitic to granitic rocks intruding metasedimentary rocks of the Slishwood Division in NW Ireland. å Nd values range from À6 to À9 and initial 87 Sr/ 86 Sr from 0.715 to 0.720, and å Hf is À7.7. The magmatic rocks are interpreted to have formed by subduction at or outboard of the Laurentian margin, and the isotopic compositions suggest that Palaeoproterozoic crust represents a significant magma source. Based on these data, Flowerdew et al. interpreted the Slishwood Division to represent a microcontinental block situated outboard of the Laurentian margin.

Timing and nature of magmatism in the North Sea and Norwegian Sea
The samples from the North Sea fall into two groups; an older group consisting of three granites (16/3-2, 16/4-1 and 16/5-1) yielding ages between 456 and 463 Ma, and a younger group consisting of leucogabbro and dacite (16/1-4 and 16/6-1) yielding ages of 421 and 430 Ma, respectively. The older granite group displays arc-like geochemical characteristics, and a narrow range of Palaeoproterozoic T DM between 1590 and 1831 Ma (å Nd of À10) and an initial 87 Sr/ 86 Sr ratio of 0.709. The Palaeoproterozoic model ages are consistent with abundant inherited zircon cores of Mesoproterozoic, Palaeoproterozoic and Archaean age, and the combined geochemical and isotopic data point towards formation in an arc situated on Proterozoic crust at c. 460 Ma. As discussed above, a similar setting as been proposed for numerous Ordovician rock complexes elsewhere in the Caledonian-Appalachian orogen, from Newfoundland to Norway, that formed outboard of the Laurentian margin. In particular, the ages, geochemical and isotopic compositions of this group of rocks are similar to those displayed by the Bindal Batholith in the Uppermost Allochthon. Interestingly, the oldest inherited core, dated at 3605 Ma (from 16/3-1), is older than the oldest known rock in Baltica (Mutanen & Huhma 2003), consistent with a Laurentian affinity. These data suggest that rocks similar, and probably correlative, to the Uppermost Allochthon underlie the continental margin as far south as the North Sea.
Although the younger samples (leucogabbro and dacite) also display arc-like compositions, their T DM values are younger and their å Nd values are higher than those of the older plutonic rocks. Initial 87 Sr/ 86 Sr ratios are also lower, at 0.706-0.707. The ages of the leucogabbro and dacite are indistinguishable from the Rb/ Sr ages of the Sunnhordland Batholith and Krossnes granite, and the initial 87 Sr/ 86 Sr ratios are identical. Andersen & Jansen (1987) interpreted the Sunnhordland Batholith to have formed in a continental arc at c. 430 Ma. The new data from the North Sea are consistent with this interpretation. The leucogabbro (421 AE 3 Ma) and dacite (430 AE 6 Ma) overlap in age with the main phase of the Scandian orogeny elsewhere in the Scandinavian Caledonides. It is therefore uncertain whether this magmatism reflects the last stages of oceanic subduction prior to ocean closure and continental collision in this part of the Caledonides or represents melting of older crust during orogenesis. Although inherited zircon is absent from the leucogabbro and sparse in the dacite, an inherited grain from the latter, dated at 491 Ma, suggests input of Palaeozoic material. The age of this inherited grain is similar to the ages of ophiolite complexes in the Scandinavian Caledonides (see compilation by Slagstad 2003), British and Irish Caledonides (Shetland ophiolite, 492 Ma, Spray & Dunning 1991) and Newfoundland (Bay of Islands ophiolite, 484 Ma, Jenner et al. 1991). A likely interpretation is that the 420-430 Ma magmatism is related to the early stages of the Scandian orogenic event, following earlier, Taconian thrusting that emplaced the ophiolite fragments (including arc-back-arc assemblages) onto older Laurentian crust. The isotopic data from the 420-430 Ma rocks, indicating a continental component, are also consistent with such an interpretation. This situation might be analogous to that inferred from the Bindal Batholith in central Norway, where Barnes et al. (2007) interpreted mafic to felsic magmatism of this age to have a previously strongly reworked Palaeozoic source.
The two samples of granite (6407/10-3) and diorite (6306/10-1) from the Norwegian Sea are intermediate in age (437 and 447 Ma, respectively) between the two age groups identified in the North Sea. Although their geochemical composition is rather similar to that of the samples from the North Sea, their isotopic compositions are not. The granite yields a Mesoproterozoic T DM of 1215 Ma (å Nd of À5) and initial 87 Sr/ 86 Sr ratio of 0.707, whereas the diorite is more juvenile with a T DM of 870 Ma (å Nd of 1.5) and initial 87 Sr/ 86 Sr ratio of 0.705. The ages and isotopic compositions overlap with major magmatic complexes in the Upper and Uppermost Allochthons, including the Bindal and Hitra-Smøla Batholiths, suggesting that rock units correlative with these Allochthons underlie much of the mid-Norwegian margin.

Sedimentary successions within the Caledonian orogenic belt
Samples of metasedimentary rock were obtained from well 25/7-1S in the North Sea and well 6609/7-1 in the Norwegian Sea (Fig. 1). The samples display very similar age distributions characterized by dominantly Late Palaeoproterozoic and Mesoproterozoic zircons, with rare Palaeoproterozoic grains. The age distributions observed in these two samples strongly resemble those observed in samples from the Kalak Nappe Complex in Finnmark, North Norway (Kirkland et al. 2007), the Northwestern Terrane of the Svalbard Caledonides (Pettersson et al. 2009), the Moine Supergroup in the Scottish Caledonides Cawood et al. 2004;Kirkland et al. 2008a) and the Krummedal-Smallefjord sequence in the East Greenland Caledonides (Kalsbeek et al. 2000;Watt et al. 2000). The latter probably correlates with the Brennevinsfjorden Group in the Svalbard Caledonides (Johansson et al. 2005). Cawood et al. (2007) and Kirkland et al. (2007) discussed the similarities between these and other sedimentary successions within the Caledonian orogen. In Finnmark, Kirkland et al. (2007) identified two major metasedimentary successions: the Svaerholt Succession, deposited between 980 and 1030 Ma, and the Sørøy Succession deposited between 840 and 910 Ma. The zircon populations in these successions are dominated by Late Palaeoproterozoic and Mesoproterozoic grains, similar to the samples investigated here, but the Sørøy Succession contains a significant Neoproterozoic population, which is not observed in our samples. Kirkland et al. (2007) argued that the metasediments comprising the two successions were deposited in successor basins, located in the developing Grenville(-Sveconorwegian) Orogen. Cawood et al. (2010) presented an alternative model in which these metasedimentary rocks, and correlative sedimentary rocks from East Laurentia and Baltica, were deposited along the Laurentian margin facing the Late Mesoproterozoic Asgard Sea. Cawood et al. (2010) hypothesized that the Asgard Sea developed in response to southward movement and clockwise rotation of Baltica with respect to Laurentia. In contrast to Kirkland et al. (2007) and Cawood et al. (2010), Roberts (2007) argued, based on palaeocurrent data from northern Finnmark, that the sediments were derived from a south to southeasterly, Fennoscandian source. Although no potential sources of the Mesoproterozoic zircons are known from northern Fennoscandia, Roberts (2007) suggested that the concealed basement underneath the Caledonian nappes in NW Finnmark might contain such sources.
The new data presented from the Norwegian and North Seas suggest that these sediments, sourced from the same area as several other metasedimentary units in the Caledonian orogen, may be found along much of the length of the orogen, from Scotland to northern Norway and Svalbard. These two samples therefore strengthen the impression that geological units, whether magmatic or sedimentary, within tectonostratigraphically higher nappes of the Scandinavian Caledonides can be correlated with similar units in other parts of the Caledonian-Appalachian orogenic belt. Sample 25/7-1S (metasediment) from the North Sea is located in the same general area as seemingly voluminous c. 460 Ma arc-related granites (the latter c. 60 km farther south). The 460 Ma granites are a feature that appears diagnostic of the Laurentian margin, and, although the relationship to the metasediment is unconstrained, the data from the North Sea appear to support a Laurentian affinity. However, the possibility of tectonic juxtaposition of originally unrelated assemblages clearly cannot be ruled out.

Conclusions
The new geochronological data from basement samples from the North Sea and Norwegian Sea provide a unique glimpse into the constitution of the Norwegian continental margin. The crystalline basement consists of magmatic and metasedimentary rocks that can be correlated with rocks in tectonostratigraphically high (i.e. Upper and Uppermost Allochthons) nappe complexes within the Scandinavian Caledonides. The data also provide an improved correlation between the Scandinavian Caledonides and more southerly segments of the Caledonian-Appalachian orogenic belt. In particular, metasedimentary rocks that have detrital zircon populations that resemble metasediments in the northern Caledonides in Finnmark and the Moine Supergroup in Scotland can be found continuously along the length of the orogen. The new data also show that the Ordovician Taconian magmatic event, a major orogenic event along the Laurentian Iapetus margin, is more widespread in the Norwegian Caledonides than hitherto recognized. D. Bering and C. Magnus from the Norwegian Petroleum Directorate helped obtain the investigated samples. Ø. Skår and T. Røhr are thanked for technical assistance and discussion of the data. We thank Michael Murphy (University College Dublin) for assistance with Sm-Nd and Rb-Sr analyses. We thank C. Kirkland and an anonymous reviewer for constructive comments. The work formed part of the Statoil-sponsored KONTIKI project (NGU Project 306600). The Nordsim facility is operated under an agreement between the research funding agencies of Denmark, Norway and Sweden, the Geological Survey of Finland and the Swedish Museum of Natural History. This is NORDSIM Contribution 273.