The structural history of the Lewisian rocks of Gairloch, Wester Ross, Scotland

Abstract

The Lewisian rocks of the Gairloch area comprise a small outcrop of older granulite-facies gneisses (the Ialltaig gneisses), the deformation and metamorphism of which is provisionally assigned to the Scourian period, and a much more extensive group of younger rocks (the Gairloch schists and gneisses) considered to represent a series of sediments (deposited unconformably on the older gneisses) and basic igneous rocks, both extrusive and intrusive. The deformation and metamorphism of the younger group, provisionally assigned to the Laxfordian period, fall into three principal phases designated early, main, and late. During the early phase, a foliation was induced during metamorphism, probably of low grade, and large-scale folds were formed that were completely re-oriented or obliterated during the succeeding phase. The main phase was characterized by major folds trending nw-se with steep axial planes and later by smaller-scale nw-se folds and steep axial-plane foliation slightly oblique to the major folds. Several dolerite dykes were intruded at South Sithean Mor after the main-phase folding. Epidote-amphibolite-facies metamorphism accompanied the formation of the main-phase foliation and continued in the late phase, which commenced with paracrystalline folds and concluded with brittle structures produced during retrogressive metamorphism. The formation of the gneisses is assigned to the initial stages of the late phase.