Relations of movement and metamorphism in the Dalradians of Banffshire

Abstract

The Dalradian rocks of Banffshire have been affected by at least four periods of folding. The earliest period resulted in the formation of isoclinal folds which are presumed to be related to large-scale recumbent folds, such as the recumbent anticline of Banffshire and Aberdeenshire postulated mainly on stratigraphical grounds by Professor H. H. Read. The Boyndie Syncline, first described by Sutton and Watson and the only major fold recognised in the Banffshire coast section, belongs to the third fold period. Textural evidence is given for polymetamorphism in the Banffshire Dalradian. The movements responsible for the primary recumbent folds were probably accompanied by only a low-grade metamorphism. The main phases of constructive metamorphism occurred before, during and after the second fold movements. The metamorphic climax, during which both Buchan and Barrovian types of metamorphic mineral assemblages developed, is shown to have occurred after the second fold movements. The andalusite-cordierite-staurolite assemblages (Buchan type) apparently formed at a higher structural level than the garnet-staurolite-(kyanite) (Barrovian type of assemblages). During the third and fourth fold movements lower grade metamorphic conditions prevailed; local retrograde metamorphism occurred. By reference to the work of Rast, Sturt and Harris a comparison is made between the available time-scales of structural and metamorphic events in Banffshire and Perthshire. Many points of similarity are revealed; in particular it is found that the climax of metamorphism (indicated by the Buchan metamorphism in Banffshire and the main Barrovian type metamorphism in Perthshire) is co-eval in the two regions, which represent different structural levels.