Comparative petrography of deformed and undeformed migmatites from the Grampian Highlands of Scotland

Abstract

SummaryFabrics in migmatites from Portsoy in the NE Dalradian are compared with those in paratectonic and pretectonic migmatites from deeper structural levels of the Northern Grampain Caledonides. Paratectonic leucosomes, from Portsoy and Mid-Strathspey, have weak biotite fabrics. They lack the biotite foliation which is strongly developed in pretectonic leucosomes from the Upper Findhorn area. In conjunction with the sequence of deformation, intrusion and metamorphism in Banffshire, the fabric data indicate that migmatization at Portsoy was paratectonic with respect to the local F, folding. Partial melts generated in the aureole of the pre-F3 basic intrusions were, in some rocks, still mobile during F3 and completely crystalline only after the formation of small F3 folds. Granitic leucosomes in Mid-Strathspey were formed within the time-span of a deformation episode. Petrographic measurements show that these leucosomes have approximately minimum-melting compositions in Qz-Ab-An-Or, despite some deformation and retrograde alteration. They are comparable with the Portsoy migmatites in grain size and in complexity of leucosome shapes, and they are tentatively interpreted as products of incipient partial melting without dehydration of biotite.