Abstract

Dr A. L. Harris asked if it were possible to distinguish between progressive and retrogressive mineral assemblages in the calc-silicates. Where studied by him at Loch Quoich and in River Loyne, the Quoich granitic gneiss and the adjacent Moine metasediments clearly share a common history of penetrative deformation followed by thorough annealing recrystallization. The Quoich granitic gneiss must originate either by migmatization involving potash enrichment in a zone of high strain (cf. Johnstone et al. 1969) or by the introduction of slices of granitic basement into the Moine cover, again presumably along zones of high strain. Discordance of these zones to the metamorphic isograds traced by Dr Winchester is an important demonstration that the pattern of metamorphic grade in this district was established later than the main penetrative espisodes to affect the rocks. If it can be demonstrated conclusively by textural evidence that the calc-silicate assemblages, on which the isograds are based, are retrogressive assemblages then the theory of the origin of the gneisses by migmatization may still be valid. On the other hand, if the assemblages are progressive, it should be recognized that the migmatization, which is unlikely to have taken place during progressive metamorphism at garnet grade, cannot be solely responsible for the presence and nature of the gneiss. In this event, the granitic gneiss must in all probability consist of basement slices, but of a completely different lithological type from those recognized along the Sgurr Beag Slide and analogous structures elsewhere in the Northern Highlands.

The Author

First Page Preview

First page PDF preview
You do not currently have access to this article.