Scourian-Laxfordian relationships in the Barra Isles

Abstract

The Lewisian rocks of Barra are a Laxfordian supra-structure of amphibolite facies gneisses and a Scourian infra-structure characterised by orthopyroxene bearing gneisses. Both units contain representatives of the Scourie Dyke suite; in the supra-structure they are highly deformed and folded; in the infra-structure they are relatively undeformed, unfolded, and retain original discordant relationships. The infra-structure also has several suites of intrusive igneous rocks earlier than the Scourie dyke suite, the most widespread being dykes of dioritic composition. The same sequence of Laxfordian deformation may be recognised in both units. Folding occurred in the supra-structure under amphibolite facies conditions, which continued after deformation ceased. In the infra-structure, evidence for a pre-Scourie dyke pyroxene granulite facies metamorphism is preserved. The Scourie dykes of the infra-structure have pyroxene granulite facies mineral assemblages resulting from intrusion into hot or dry country rock gneisses. Probably a very old cover to basement relationship is preserved in the area, which was first deformed in the Scourian orogeny, and then subsequent Laxfordian deformation of the interface between the two major units produced overall structure of the area.