The Great Eucrite intrusion of Ardnamurchan, Scotland: Reevaluating the ring-dike concept

Ring-dikes are cylindrical sheet intrusions that develop at a subvolcanic level due to ascent of magma along steep outward-dipping ring fractures. Magma ascent is triggered by central block subsidence, and fully formed ring-dikes are composed of a flat-lying sill-like roof as well as steeply outward-dipping walls on all sides. The Great Eucrite of the Ardnamurchan Paleocene igneous complex, NW Scotland, is a spectacular gabbro intrusion that has been cited as one of the classic examples of a ring-dike for the past 70 yr. We combine field observations, detailed structural measurements of primary magmatic features, and anisotropy of magnetic susceptibility data in a reinvestigation of this intrusive body. Magmatic layering and macroscopic planar crystal arrangements dip inward, and magnetic lineations plunge consistently toward the center of the intrusion, in contrast to what would be expected for a ring-dike. We propose that the Great Eucrite ring-dike is in fact a lopolithic intrusion with an overall funnel-shape geometry. This conclusion brings into question the presence of three individual foci of activity in Ardnamurchan, purported to have shifted throughout the development of the complex. It also has significant implications for the status and structural evolution of other igneous complexes of the British Paleocene igneous province, which contain layered mafic intrusions currently regarded as ring-dikes.