The emplacement of a large, chemically zoned, rheomorphic, lava-like ignimbrite: the Sgurr of Eigg Pitchstone, NW Scotland

Abstract

The Sgurr of Eigg Pitchstone on the Isle of Eigg, NW Scotland, is a crystal-rich, trachydacitic, partially vitrophyric rock, which has previously been interpreted both as a lava and as a sill. We interpret this rock as a chemically zoned, rheomorphic, lava-like ignimbrite that formed during a sustained pyroclastic eruption. The Sgurr of Eigg Pitchstone can be subdivided into discrete emplacement units distinguished by the following features: (1) their present-day weathering characteristics; (2) the orientation, spacing and morphology of the columnar joints; (3) sharp, undulating boundaries with marked topographic breaks. The absence of weathered surfaces, palaeosols, pyroclastic fall deposits or sedimentary rocks at emplacement unit boundaries suggests deposition from a single eruption. The emplacement units, some of which display upper and basal vitrophyres, represent distinct depositional packages that record several rapidly emplaced ignimbrites, which welded, cooled and devitrified as a simple, essentially single, cooling unit, during eruption from a sustained, low pyroclastic column. The Sgurr of Eigg Pitchstone displays a pervasive base-parallel flow banding, which is folded into intrafolial recumbent isoclinal folds. The flow banding and folds indicate that rheomorphism occurred throughout deposition. The Sgurr of Eigg Pitchstone is interpreted as an erosional remnant of an extensive ignimbrite sheet, the first such unit recorded within the North Atlantic Igneous Province.