The origin of banding in the Main Donegal Granite, N.W. Ireland

This Caledonian pluton exhibits a regular, sub‐vertical banding in which layers of dark, relatively fine grained trondhjemite alternate with layers of light, coarse grained granite. The textures and the presence of cross‐cutting bands of granite indicate that the light bands were derived by potash feldspathization of the trondhjemite. The geometry of the banding indicates that it formed during the “synplutonic” deformation of the pluton, largely in situ. The bands formed by segregation of alkalies into dilatant zones of movement produced during the deformation of partly consolidated magma.