U-Pb zircon ages for the Loch Borralan and Flannan syenites, and their relationship to the Caledonian orogenic front [abstract]

Abstract

The Caledonian Front has, for many years, been considered to be mid-Silurian in age. An important piece of evidence for this is the age of the Loch Borralan Pluton (430 ± 4 Ma; van Breemen et al., 1979), which lies within the Moine Thrust Zone. The Loch Borralan Pluton comprises early and late magmatic suites; the early suite is poorly exposed and its structural relationships are equivocal, but there is little doubt that the later suite post-dates thrust movement (Parsons and McKirdy, 1983, confirmed by recent BGS mapping). Rb-Sr and K-Ar dating of mylonites in the Moine Thrust Zone has yielded ages between c. 440 and c. 410 Ma (Freeman et al., 1998; Dallmeyer et al., 2001), suggesting that deformation at the Caledonian Front may extend into the Devonian. This evidence is in direct conflict with the accepted age and structural position of the Loch Borralan Pluton. We therefore set out to re-examine the age of both early and late magmatic suites. To date, we have analysed zircons from a sample of the late (postthrusting) suite, which has given a U-Pb age of 428.4 ± 0.4 Ma. A further sample, from Bad na h- Achlaise where syenites cut quartzites belonging to a thrust klippe (Parsons and McKirdy,1983), contains inherited zircon and has given only highly discordant zircon data. A third sample, from the early magmatic suite, awaits analysis. The new age of 428.4 ± 0.4 Ma represents a minimum age for movement on thrusts within the Moine Thrust Zone. In parallel, we have carried out a U-Pb zircon study of an alkaline igneous intrusion, the Flannan syenite, which intrudes crystalline basement 50 km to the west of the Outer Hebrides. This intrusion has been modelled geophysically as saucer-shaped, with a maximum width of 9 km and depth extent of the order of 1 km. Its U-Pb age (427.7 ± 0.3 Ma) and geochemistry indicate close similarities to the Loch Borralan leucosyenite. The location of the Flannan syenite is enigmatic, as it lies 135 km to the NW of the Moine Thrust Zone (after allowing for post-Caledonian extension), in an area normally considered to lie well to the west of the Caledonian Front. This suggests that the influence of the Caledonian Orogeny may extend significantly further to the NW than previously recognised