Structure and metamorphism of the Lewisian of east Tiree, Inner Hebrides

Abstract

Synopsis A study of the Lewisian of east Tiree has shown the presence of previously unmapped psammitic, calcareous and semipelitic paragneisses which occur, often with basic gneisses, in the ‘grey gneiss’ of the area. Four main phases of folding have been determined: F 1 —isoclinal intrafolial folds; F 2 —tight similar folds; F 3 —close and open folds with NW.–SE. axial planes; and F 4 —open and gentle folds with NE.–SW. axial planes. Fold phases (F 1 –F 3 ) have been related to successive metamorphic episodes of granulite facies, upper amphibolite facies and lower amphibolite facies which have been assigned to the Scourian, Early and Late Laxfordian respectively on the basis of their structural relationship with intrusive basic bodies which are now amphibolites. The relations of the ‘grey gneiss’ to the paragneisses and basic gneisses suggest the existence of a pre-Scourian basement. The region is cut by late shears associated with the formation of pseudotachylite. The sequence in Tiree is related to the structural and metamorphic sequences given by other authors for the Outer Hebrides.