Metamorphosed and deformed pegmatites and basic dykes in the Lewisian complex of the Outer Hebrides and their geological significance

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Abstract
The central zone of the Outer Hebrides is composed of a gneissic (‘grey-gneiss’) complex containing evidence of the earliest as well as many of the later events in the history of the Lewisian. In northern South Uist, an old metamorphic complex is intruded by pegmatites which were strongly deformed by a tectogenesis equated here with the Scourian of more than 2600 m.y. ago. It is concluded that much of the original material of the greygneiss may represent the products of a pre-Scourian orogenic cycle of possible Katarchaean (>3000 m.y.) age. Widespread post-Scourian basic (tholeiitic) dykes which cut the grey gneiss are thought to be equivalent to the Scourie dykes of the Scottish Mainland. These intrusions were subjected to granulite-facies metamorphism, regarded as Early Laxfordian, followed by retrogressive amphibolite-facies metamorphism with local folding and migmatization, all of Late Laxfordian date (1500 to 1600 m.y.). A number of dykes described from Benbecula and South Uist illustrate stages of Laxfordian deformation in which perfectly preserved cross-cutting relationships are progressively obliterated, the final products in the southern zone of South Uist being highly deformed and disrupted amphibolite masses with concordant margins in mobilized Laxfordian migmatitic gneiss.