The Eastern Gneisses of South Uist

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Abstract
Synopsis: The Eastern Gneisses of South Uist which overlie the Outer Isles Thrust consist of three units; the Corodale Gneiss mass which is interpreted as an igneous body; the Mashed Gneiss; a group of quartzo-feldspathic gneisses and an intervening zone of phyllonites, the Usinish Mylonites. The phyllonites appear to have been formed by intense deformation at the margin of the igneous body. Three phases of deformation have affected the Corodale Gneiss and are tentatively correlated with Laxfordian deformation phases affecting the rocks below the thrust. The igneous body is cut by dykes and is considered to be a member of a widespread group of basic to intermediate intrusions which post-date the Scourian gneiss-forming events, but pre-date the emplacement of Scourie dykes. There is evidence for an eastward increase in metamorphic grade during an early Laxfordian episode, prior to the main deformation, and an eastward decrease in metamorphic grade during the three main Laxfordian deformation-phases.