Abstract

U-Pb analyses of zircons from 4 intrusions of the Loch Borrolan alkaline igneous complex in the Moine Thrust Zone closely define a discordia corresponding to an upper intersection age of 430 ± 4 Ma. No evidence has been found for an inherited radiogenic lead component. Structural relationships show that the error limits on the age bracket a late period of movement along one of the splays of the Moine Thrust. Alkaline igneous activity (documented back to 456 ± 5 Ma) along the western margin of the Caledonian Orogen in Scotland is related to crustal arching, directly and/or indirectly in response to orogenic compression. Synchronous thrusts directed towards the foreland appear to overtake the alkaline centres—thus resulting in their burial, metamorphism, deformation and/or transport. The change from alkaline to the more widespread (late Caledonian) granite magmatism appears to coincide with the end of thrust movement and the beginning of post-orogenic uplift.