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

Pre-tectonic metagabbroic rocks emplaced into the Glenfinnan and Loch Eil g roups 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 ass ociated with the Glen Doe body of the West Highland granite gneiss, and wer e emplaced soon after the granite gneiss protolith. The metagabbros have ch emical 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 rid ge basalt, albeit modified by interaction with metamorphic fluids. The pres ence of abundant MORE-like basaltic dykes, coupled with the lack of major c ompressional structures associated with the intrusive events, suggests that the c. 873 Ma event may have been dominated by extensional tectonics.