MINERALOGY, PETROLOGY AND MICROFABRIC ANALYSIS OF THE EILRIG SHEAR ZONE, FORT AUGUSTUS

Detailed mapping of the area between Loch Lochy and Loch Ness (Scotland), SE of the Great Glen, has identified a 1 to 1.2 km-wide belt of quartz-feldspar-mylonites, gatnetiferous phyllonites and relatively undeformed arkosic psammites underlying a sequence of metasediments correlated with the Neoproterozoic Grampian Group. The mylonites and garnet-phyllonites define a previously undescribed major ductile shear zone, here referred to as the Eilrig Shear Zone, which has an approximate strike length of at least 20 km. The shear zone is underlain by a sequence of pebbly arkosic psammites of unknown stratigraphical affinity. Kinematic indicators present within the various mylonitic rocks of the Eilrig Shear Zone yield a consistent NW-directed sense of overthrusting. Initial P-T estimates obtained for the garnet-bearing phyllonites indicate that ductile deformation resulting in mylonitization occurred during prograde amphibolite facies metamorphism. It is concluded that the Eilrig Shear Zone represents a major, possibly regional, ductile structure which resulted in the overthrusting of the Grampian Group towards the NW early in the structural history of the area.