Zircon U–Pb age for the Orkney lamprophyre dyke swarm, Scotland, and relations to Permo-Carboniferous magmatism in northwestern Europe

Abstract: Magmatic zircon in the syenite (bostonite) part of a composite NE–SW-trending cogenetic bostonite–camptonite dyke in Orkney, Scotland, yields a laser ablation inductively coupled plasma mass spectrometry age of 313 ± 4 Ma and εHf(313 Ma) values of +6 to +11. This suggests that the NE–SW-, east–west- and NW–SE-trending Scottish lamprophyre dyke swarms were emplaced during one late Carboniferous magmatic event, contrasting with published K–Ar dates ranging between c. 325 and 250 Ma. This magmatism is interpreted as a response to late Variscan regional extension or an early response to the Skagerrak mantle plume. Lamprophyre magmatism was initiated some 10 Ma earlier in Scotland than in the Oslo Rift. Supplementary material: Thin-section images of the investigated rocks, geochemical data and an REE plot, U–Pb and Hf isotopic data, and a Hf v. time diagram are available at www.geolsoc.org.uk/SUP18501.

Alkaline (lamprophyre) dyke swarms in Scotland are part of widespread Permo-Carboniferous magmatism in NW Europe that can be interpreted as the result of extension in the Variscan foreland, or of a mantle plume centred in the Skagerrak area (Upton et al. 2004). The Scottish lamprophyre dyke swarms comprise .3000 dykes, mainly occurring NW of the Great Glen Fault. The dykes can be divided into three groups with preferred east-west, NW-SE and NE-SW strike directions, the last group being centred on the Orkney Islands (Rock 1983 ; Fig. 1). The three groups have been dated with the K-Ar method to c. 325 Ma, 290 Ma and 250 Ma, respectively (Speight & Mitchell 1979;Baxter & Mitchell 1984), and interpreted to represent separate tectonomagmatic events implying c. 70 Ma of discontinuous alkaline magmatism (e.g. Baxter & Mitchell 1984). However, Smythe et al. (1995) pointed out that the strikes of the lamprophyre dyke swarms conform to an arcuate trend defined by c. 300 Ma (Monaghan & Parrish 2006) tholeiitic quartz dolerite dykes in northern Britain and the North Sea, and argued for roughly coeval emplacement under one stress regime.
In the literature the age of the Orkney lamprophyre dyke swarm is typically quoted as c. 250 Ma, but four reported K-Ar studies have yielded ages of 245 AE 12 Ma (Brown 1975), c. 240 Ma (Halliday et al. 1977), 288 AE 9 Ma (Shelling, reported by Mykura 1976), and 249-268 Ma with a preferred age of 252 AE 10 Ma obtained from three dykes in the Thurso area considered to belong to the Orkney lamprophyres (Baxter & Mitchell 1984). Ages above published before 1977 were recalculated with the decay constants of Steiger & Jäger (1977) by Baxter & Mitchell (1984). Although the age spread may reflect magmatic pulses of different ages, the apparent difficulty in isolating isotopically undisturbed material for the analyses makes the age of the Orkney dyke swarm suspect.
In the present study we report zircon in situ U-Pb and Hf data from a bostonite in a composite bostonite-camptonite dyke on West Mainland, Orkney, and the geochemistry of the composite dyke. The new data are used to assess the relation to the Orkney lamprophyre dyke swarm, and the age and tectonic setting of the Scottish lamprophyre dyke swarms.
The Orkney lamprophyres and the Garthna Geo composite dyke. The Orkney camptonite-monchiquite (mafic alkaline lamprophyre) suite comprises .200 dykes and sills (Flett 1935). The general NE-SW strike of the dykes may reflect the regional Caledonian grain (Baxter & Mitchell 1984), or, alternatively, a regional palaeo-stress field (Smythe et al. 1995). The monchiquites are the most primitive of the alkaline rocks, whereas the camptonites have undergone crystal fractionation. More evolved compositions (i.e. bostonite (syenite lamprophyre)) are present on Orkney (Flett 1935), but are volumetrically insignificant on a regional scale (Baxter 1987). The lamprophyres probably reflect low-degree melting of a heterogeneous, enriched mantle source related to decompression melting during a phase of regional extension (Kirstein et al. 2006).
At Garthna Geo (59800'58''N, 03821'52''W), Yesnaby, a subvertical, ENE-WSW-striking, c. 3 m wide composite dyke consisting of bostonite margins with sharp contacts to a c. 1 m wide central camptonite dyke is exposed in the palaeo-escarpment of a pre-Caledonian granitic basement. The dyke cuts the oldest of the Orcadian deposits, the Early Devonian coarsegrained Hara Ebb sediments, but no contacts to overlying sediments are exposed. The fine-grained bostonite is dominated by flow-banded potassic feldspar and albite laths, with interstitial carbonates (Fe-dolomite . calcite) and quartz. Accessory minerals include apatite, Fe-oxides and zircon. The feldspars have locally undergone minor illite alteration.
The camptonite is an augite-phyric rock with a matrix dominated by plagioclase, kaersutite, augite and Fe-Ti oxides and accessory apatite. Both the phenocrysts and the matrix minerals tend to be subhedral. Oval millimetre-sized ocelli are common and define flow banding. The ocelli consist of potassic feldspar surrounding and intergrown with a carbonate core, with subordinate albite, kaersutite and biotite, and accessory quartz and Yb-rich zircon. Feldspar crystals are (sub) parallel, suggesting flow orientation. Octahedral porphyric pseudomorphs made up of calcite, Fe-Mg carbonate, quartz and an unidentified clay mineral, and rimmed by Ti-Fe oxides, are common, and interpreted as the remains of olivine crystals. Olivine phenocrysts at various stages of breakdown have previously been reported from the camptonites (Flett 1935).
Analytical techniques. To aid classification and petrogenetic interpretation, bostonite and camptonite from the Garthna Geo composite dyke were analysed for major and trace elements by whole-rock inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS), respectively, at Canadian Activation Laboratories Ltd.
Polished zircon grains mounted in epoxy were imaged in cathodoluminescence on a JEOL-JSM6460LV scanning electron microscope at the University of Oslo. U-Pb and Lu-Hf isotopic compositions were determined separately by LA-ICP-MS using a Nu Plasma HR mass spectrometer and a NewWave LUV213 laser microprobe at the Department of Geosciences, University of Oslo. The analytical protocols described in detail by Andersen et al. (2009) and Rosa et al. (2009) were used for U-Pb zircon geochronology and those of Heinonen et al. (2010) for Lu-Hf analyses.
During the Lu-Hf run standard zircons Temora-2 and Mud Tank were run as unknowns at intervals. Because of the small size of most zircons in the sample several Hf analyses were cut short owing to lack of material, resulting in slightly low precision in four analyses.
The U-Pb data from the mass spectrometer were calibrated to reference zircons GJ-01 and 91500. The U-Pb ages were calculated from data uncorrected for common lead using ISO-PLOT version 3 (Ludwig 2003 ; Fig. 2). Fourteen analyses of the reference zircon Plešovice (Sláma et al. 2008) were run as unknowns at intervals. Of these, 13 yield a concordant 337.6 AE 1.4 Ma age (MSWD of concordance 0.7 at 2ó confidence limit including decay constant errors). The remaining analysis was excluded from the age calculation owing to its anomalously low 206 Pb/ 204 Pb ratio (c. 2800) and high degree of discordance (23%).
Results. Both parts of the composite dyke are alkaline (Irvine & Baragar 1971) and belong to the potassic series of Middlemost (1975). The felsic part is a quartz syenite or trachyte in the QAPF classification (Le Maitre 2002), but lacks mafic minerals apart from oxides. It contains interstitial carbonate, and the feldspar laths form an interlocking texture. Comparable dykes on Orkney are traditionally referred to as bostonites (Flett 1935; compare the definition by Le Maitre 2002). The mafic dyke is an alkaline lamprophyre based on the K 2 O v. SiO 2 ratio (Rock 1987), and is assigned to the camptonite group based on mineralogical criteria (Le Maitre 2002). The camptonite chemistry conforms to previous analyses from the Orkney dyke swarm, with trace element abundances showing similarities to  ocean island basalt (Fig. 3). The bostonite is comparatively enriched in incompatible elements such as K and Rb, and depleted in Sr, P and Ti. Both rocks have similar Nb/Ta (14.7 and 15) and Zr/Hf (43 and 42) ratios. The chondrite-normalized (Nakamura 1974) REE patterns display negative slopes with (La/ Lu) N ratios of 19 and 20 for the camptonite and bostonite, respectively, and lack Eu anomalies.
Zircons in the bostonite are typically ,100 ìm, colourless, euhedral to subhedral, and elongated with square cross-sections and pyramidal terminations. Cathodoluminescence imaging reveals oscillatory zoning and partial (late magmatic) resorption features. Of 31 grains analysed for U-Pb, 17 form a concordant group with a concordia age of 312.6 AE 1.4 Ma (Fig. 2). Their weighted mean 206 Pb/ 238 U age is 312.6 AE 3.8 Ma, which is interpreted as the best age estimate. Two inherited zircons yield 207 Pb/ 206 Pb ages of c. 1.8 and 2.5 Ga. The remaining analyses are variably discordant and yield lead ages that predate the Early Devonian sediments cut by the composite dyke ( 207 Pb/ 206 Pb . 0.0552), reflecting high levels of common lead, coupled with Pb loss in analyses yielding 206 Pb/ 238 U ages ,300 Ma.
An attempt at extracting zircons from c. 2 kg of the camptonite part of the composite dyke failed, probably because of the small size of zircons in the ocelli.
Sixteen Lu-Hf analyses were performed on 13 U-Pb dated grains. The åHf (313 Ma) values range from +6 to +11, which compared with a depleted mantle åHf (313 Ma) value of c. +16 point to a mantle source. The inherited c. 2.5 Ga grain yields an åHf (2:5Ga) value of c. À5, corresponding to a depleted mantle extraction model age of c. 3 Ga.
Discussion. Syenitic and mafic lamprophyres are commonly associated, with the former interpreted to reflect fractionation and crustal contamination of the latter (Rock 1987;Litvinovsky et al. 2002). Flett (1935) proposed a gradual transition from (genetically related) camptonite to bostonite on Orkney. The differentiated signature of the Garthna Geo bostonite compared with the camptonite (Fig. 3), their similar Nb/Ta and Zr/Hf ratios and REE patterns, their field association, and the juvenile but variable Hf isotopic character of the bostonite coupled with the presence of (Lewisian) inherited zircon grains, support this interpretation.
The similar geochemistry (Fig. 3), geographical occurrence and strike directions of the Garthna Geo composite dyke and the camptonite-monchiquite dyke swarm on Orkney indicate that they represent one alkaline magmatic suite, dated by the bostonite to 313 AE 4 Ma.
The new U-Pb age of the bostonite is some 20-60 Ma older than the previously reported K-Ar ages. The age spread indicates partial resetting of the K-Ar isotopic system, suggesting that some of the alteration in the dykes is post-magmatic. Alternatively, but less likely, the composite dyke represents a hitherto unknown magmatic event, predating the main lamprophyre magmatism on Orkney.
The presence of ,30 ìm zircons in the camptonite ocelli suggests that in situ U-Pb ion microprobe dating of the rock is possible, and if zircon saturation is typically achieved in ocelli, this may be applicable to a large number of lamprophyre dyke swarms.
Changes in far-field stresses in NW Europe accompanying the Variscan orogeny, and potentially the Ural orogeny (Coward 1993), were accommodated by adjustments between the crustal blocks that make up northern Britain, leading to a complex pattern of strike-slip, extensional and compressional tectonics (Timmerman et al. 2009), and transitional to alkaline (Smedley 1986) episodic magmatism (Monaghan & Parrish 2006). The short duration and geographical extent of one episode of intense magmatism, particularly voluminous in the Oslo Rift and northern Germany, and represented by tholeiitic quartz dolerite dykes and sills in northern Britain and the North Sea, has been proposed to reflect a 297 AE 4 Ma mantle plume centred in the Skagerrak Sea ( Fig. 1; Torsvik et al. 2008). Other researchers have argued that the lack of evidence for a hotspot trace and initial doming (Pedersen & van der Beek 1994), and geochemical characteristics of the various magmas (Kirstein et al. 2006), are more compatible with a passive extensional setting The 313 AE 4 Ma age of the Orkney lamprophyre dyke swarm is intermediate between the suggested K-Ar ages of 291 AE 5 Ma (Speight & Mitchell 1979), and 326 AE 8 and 323 AE 9 Ma (Baxter & Mitchell 1984) for the NW-SE-striking and the east-weststriking Scottish lamprophyre dyke swarms, respectively. Given the analytical errors of the ages and the potential for resetting of the K-Ar system (see Smythe et al. 1995), the age data are compatible with emplacement during a relatively brief period. This suggests one tectonomagmatic event, as opposed to 70 Ma of discontinuous alkaline magmatism, and fits with the structural coincidence (Smythe et al. 1995) of the regionally arcuate strikes of the lamprophyre dyke swarms and the c. 300 Ma tholeiitic quartz dolerite dykes in northern Britain and the North Sea.
The quartz dolerites are generally viewed in the context of the intense magmatic episode at the Permo-Carboniferous boundary, and Smythe et al. (1995) argued that they, along with (some of) the Scottish lamprophyres and the 300-260 Ma (Corfu & Dahlgren 2008) magmatism in the Oslo Rift, reflect regional rifting in NW Europe.
The magmatic stage of the Oslo Rift was initiated by lamprophyre dykes and sills, including camptonite and 300 AE 1 Ma (Corfu & Dahlgren 2008) syenitic lamprophyre. We propose that the Scottish lamprophyres reflect a similar but slightly earlier response to incipient regional extension dated by the Orkney bostonite to 313 AE 4 Ma. The magmatism thus coincides with the Moscovian (312-307 Ma; Gradstein et al. 2004) proto-rift stage of the Oslo Rift (Olaussen et al. 1994). With progressive extension the Scottish lamprophyre magmatism was succeeded by tholeiitic magmatism in northern Britain and the North Sea at 308 AE 5 Ma (Midland Valley Complex; Monaghan & Parrish 2006).
The arcuate pattern of the lamprophyres and quartz dolerites diverges from the radial pattern expected from a Skagerrak Fig. 3. Partial geochemistry of the Garthna Geo composite dyke with ocean island basalt (OIB; Sun & McDonough 1989) and Orkney camptonite analyses for comparison. Chondrite normalization after Thompson (1982). centred mantle plume. However, if the lamprophyres are related to the proposed mantle plume, the new age of the Orkney lamprophyres suggests that it affected northern Britain some 10 Ma earlier than previously proposed. The early appearance and the non-radial strikes of the Scottish lamprophyre dyke swarms could reflect the interaction of the plume with a preexisting extensional stress field.