Moine and Lewisian near the Great Glen Fault in Easter Ross

Abstract

Synopsis Two inliers of metamorphic rock—the Cromarty and Rosemarkie inliers—occur adjacent to the Great Glen Fault and within the Devonian sediments of Easter Ross. The Cromarty Inlier consists of striped psammitic Moine rocks, probably Glenfinnan Division, which have suffered high grade regional metamorphism and which have probably been locally transformed into mobilised granite. In the Rosemarkie Inlier, which corresponds to a well marked positive anomaly on the aeromagnetic map, Moine metasediments are accompanied by Lewisian hornblendic gneisses. The earliest of at least four episodes of deformation (D1–D4) is inferred to have interleaved the Moine and Lewisian while the Moine/Lewisian boundary was folded during three subsequent episodes. Granitic veins, including pegmatite, were emplaced in the Rosemarkie Inlier during or before the second deformation episode and carry intense D2 LS fabrics involving strains of 18:2.5:1 which have locally been modified during the third episode. Pegmatites and microgranites which occur in the Cromarty Inlier have not suffered comparable ductile deformation and postdate garnetiferous metabasic intrusions which are a feature of the Cromarty Inlier. Details of lithology and igneous history serve so to distinguish the two inliers that it is possible that, pre-D3, they were much further apart than at present.