Neoproterozoic extensional basic magmatism associated with the West Highland granite gneiss in the Moine Supergroup of NW Scotland

Abstract

Pre-tectonic metagabbroic rocks emplaced into the Glenfinnan and Loch Eil groups of the Moine Supergroup give a U–-Pb zircon age of 873 ± 6 Ma. This new age for the metagabbros confirms the absence of Grenvillian ( c. 1.0 Ga) tectonic events in the Moine assemblage. The metagabbros are spatially associated with the Glen Doe body of the West Highland granite gneiss, and were emplaced soon after the granite gneiss protolith. The metagabbros have chemical characteristics indicating contamination with local country rocks. A regionally developed suite of tholeiitic metadolerite dykes post-dates the metagabbros. These dykes are geochemically similar to modern mid-ocean ridge basalt, albeit modified by interaction with metamorphic fluids. The presence of abundant MORB-like basaltic dykes, coupled with the lack of major compressional structures associated with the intrusive events, suggests that the c. 873 Ma event may have been dominated by extensional tectonics.