The structure and petrology of the volcanic rocks of Eigg, muck and Canna, N. W. Scotland

Abstract

The early research and the more recent contributions are reviewed. The stratigraphical succession of lavas on each of the three islands is established and conclusions drawn concerning the relative ages of the lava piles. More than one source is indicated for the lavas of Eigg and Canna, and various localised eruptions occurred on Muck and Eigg. Basic minor intrusions occur on all three islands. Dykes are remarkably abundant on Muck, forming about 10% of the island, and are extremely rare on Canna, only sixteen being documented. Eigg contains sheets and sills in addition to dykes. The petrography of the lavas and the basic minor intrusions is described and comparisons are made between them. Six bodies of acidic rock outcrop on Eigg, including the famous Sgurr pitchstone, the origin of which was hotly debated at the beginning of this century. From extensive field studies, it was concluded that the Sgurr pitchstone is of an extrusive origin, with some auto-injection during cooling. The base of the pitchstone was mapped in detail and a plan of the pre-pitchstone topography constructed. Comparisons are made between the Sgurr and Gigh Sgeir (Hyskeir) pitchstones. The red beds of Eigg and Muck, and the various sedimentary deposits of Canna are described. The red beds are thought to be of a tuffaceous origin. The conglomerates of Canna are fluvial, while some of the finer grained, bedded deposits are probably of lacustrine origin. The Canna conglomerates include igneous pebbles, which imply deposition after the Central Intrusive Complex of Rhum was at least partially unroofed. An account of the mineralogy of the basic and acid rocks is included, and their geochemistry is described. A comparative study of the lavas and basic minor intrusions of Eigg, Cluck and Canna with other Hebridean suites (the lavas and dykes of Skye, the lavas of Rhum and Mull, and the cone sheets of Ardnamurchan) is included. Hypotheses on the petrogenesis of the acid rocks of Eigg are proposed. The Tertiary volcanic activity on Eigg, Muck and Canna is summarised and the petrogenesis of basic rocks assessed. The magmas are the result of a partial melting event at a depth in excess of 30 km (the estimated depth to the Moho in the area). This depth is reflected in the transitional nature of the basic rocks and the association of the basalts with the thermal divide in the plagioclase- olivine- clinopyroxene- nepheline/quartz system.