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The Foyers 'Newer Granite' is a composite pluton consisting of tonalite, granodiorite and adamellite emplaced in that order. The complex evolved by crystal-differentiation from a series of cooling magmas which were additionally modified by contamination with appinite, microdiorite and semi-pelite. A distensive foliation was progressively imparted to the units of the complex as they solidified, followed by post-magmatic alkali autometasomatism which extended into the envelope.
Parts of the roofing envelope remain and the outer, largely tonalitic mantle to the inclined funnel-shaped intrusion conforms structurally with the envelope. After some uplift the adamellite was emplaced under brittle conditions, but was also subjected to some distension. Intense movements along NE–SW faults before and after the deposition of the local Old Red Sandstone complicate relations on the north-western margin of the complex, which is truncated by the Great Glen fault. The respectively high and low structural levels of exposure of the Foyers and Strontian intrusions complicate comparison, but there are sufficient chemical and structural similarities to confirm their former unity before being displaced laterally and vertically by the Great Glen fault.
