Mylonitic rocks of the Moine Thrust-zone in the Assynt Region, North-West Scotland

Abstract

The terminology of mylonitic rocks is reviewed. The mylonitic rocks in the thrust-zone are divided on the basis of textures into two groups, distinguished as Primary and Secondary Mylonitic Rocks. The primary mylonitic rocks are characterised by crystalloblastic textures and lack of post-crystalline strain in the constituent grains; texturally, they grade, with increasing grain size, into typical Moine Schists above the thrust-zone. The writer is in agreement with the views originally expressed by Lapworth and Peach as to the origin of these mylonitic rocks: namely, that they were formed in a movement-zone between the Moine Nappe and the Foreland during the regional metamorphism of the Moine Schists. The secondary mylonitic rocks are characterised by cataclastic breakdown of all the minerals; they are formed from primary mylonitic rocks and low-grade Moine Schists, indicating that they originated in a later phase of deformation. The secondary mylonitic rocks are not noticeably diaphthoritic, a feature which is probably due to the low grade of progressive metamorphism in the primary mylonitic rocks and Moine Schists in the vicinity of the thrust-zone. The study indicates at least two periods of mylonitization in the thrust-zone, the earlier of which was contemporaneous with the regional metamorphism of the Moine Schists.