A high precision U-Pb age for the Ben Vuirich granite: implications for the evolution of the Scottish Dalradian Supergroup

Abstract

High precision/high accuracy U-Pb analyses of zircons from the Ben Vuirich granite in the Scottish Dalradian succession were performed on carefully picked, high integrity, abraded fractions of specific morphologies. Zircon needles, which grew during crystallization, give an age of 590 ± 2 Ma. Stubby prisms show inheritance with an upper concordia intercept of 1448 ± 7 Ma. The age of intrusion at 590 Ma contrasts strongly with a previous U-Pb zircon age of 514 +6 −7 Ma, determined on bulk-fraction unabraded grains, which, it is argued, must have suffered significant Pb loss. As the Ben Vuirich granite was emplaced between the regional D2 and D3 deformation events in the Dalradian block, the new data imply that all Dalradian sedimentation was Precambrian. Supposed Lower Cambrian fauna1 links between the Dalradian succession and Laurentia are thus discounted, and the possibility that the Dalradian Supergroup has non-Laurentian affinities must be considered. The early D1/D2 nappe-forming Grampian structures are also Precambrian, and may be associated with terrane docking along a tectonically active ocean margin. Later D3 structures and associated peak Barrovian metamorphism suggest a subsequent thickening event, possibly c . 520 Ma ago.