Emplacement of alkali-dolerite sills relative to extrusive volcanism and sedimentary basins in the Carboniferous of Fife, Scotland

Abstract

ABSTRACTNamurian alkali-dolerites in W and central Fife comprise up to nine component sills. These have three-dimensional shapes which are directly related to penecontemporaneous structures in the sedimentary host rocks. Two forms of control are identified. Basinal control is apparent where restored sill geometry shows thickening towards the bottoms of saucer-shapes which mimic sediment disposition at the time of emplacement. Fault control is evidenced by sills which change horizon along extensive E–W fractures once assumed to be entirely post-Carboniferous but now confirmed as growth-faults.There are no associated dykes representing likely feeders comparable with those of the later tholeiitic Midland Valley Sill, but evidence is presented to suggest that at least some of the alkali-dolerite magma reached sill intrusion levels through volcanic pipes.