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^sub (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 ± 12 Ma (Brown 1975), c. 240 Ma (Halliday et al. 1977), 288 ± 9 Ma (Shelling, reported by Mykura 1976), and 249-268 Ma with a preferred age of 252 ± 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 …