DR A. L. HARRIS asked if it were possible to distinguish betweenprogressive and retrogressive mineral assemblages in the calc-silicates.Where studied by him at Loch Quoich and in River Loyne, theQuoich granitic gneiss and the adjacent Moine metasedimentsclearly share a common history of penetrative deformation followedby thorough annealing recrystallization. The Quoich graniticgneiss must originate either by migmatization involving potashenrichment in a zone of high strain (cf. Johnstone et al. 1969)or by the introduction of slices of granitic basement into theMoine cover, again presumably along zones of high strain. Discordanceof these zones to the metamorphic isograds traced by Dr Winchesteris an important demonstration that the pattern of metamorphicgrade in this district was established later than the main penetrativeespisodes to affect the rocks. If it can be demonstrated conclusivelyby textural evidence that the calc-silicate assemblages, onwhich the isograds are based, are retrogressive assemblagesthen the theory of the origin of the gneisses by migmatizationmay still be valid. On the other hand, if the assemblages areprogressive, it should be recognized that the migmatization,which is unlikely to have taken place during progressive metamorphismat garnet grade, cannot be solely responsible for the presenceand nature of the gneiss. In this event, the granitic gneissmust in all probability consist of basement slices, but of acompletely different lithological type from those recognizedalong the Sgurr Beag Slide and analogous structures elsewherein the Northern Highlands.
THE AUTHOR
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