Table 1.

Evolution of the Rum Central Igneous Complex.

Lewisian gneisses (Amphibolite-facies) overlain by >2.5 km of mid-Proterozoic Torridon Group sandstones and siltstones, a thin covering of lower Jurassic Broadford Beds (limestone, sandstone and siltstone) and Paleocene basaltic lavas of the Eigg Lava Formation, all intruded by dykes of the Muck swarm and the Rum sub-swarm. The Long Loch Fault was probably active well before the Palaeogene.
Palaeogene Rum Central Igneous Complex: Stage 1
Initiation of the MRF accompanied by major central uplift, with subsequent subsidence leading to caldera formation, the collapse of caldera walls and the formation of debris avalanche deposits, with concomitant intrusion and effusion of acidic and mixed acid/basic magmas and emplacement of the Am Mam intrusive breccia. Severe distortion of country rocks adjoining the MRF and collapse of major masses of country rocks off the rising dome, as at Welshmans Rock, etc. Emplacement of the Western Granite. Possible further central uplift on the MRF.
Palaeogene Rum Central Igneous Complex: Stage 2
Change to basaltic and ultrabasic magmatism, heralded by the intrusion of basaltic cone-sheets and dykes of the Rum sub-swarm, followed by emplacement of ultrabasic rocks and gabbros of the Central Intrusion, with construction of the Layered Suite (the Eastern and Western layered intrusions) through successive sheet-like injections of ultramafic and basaltic magmas from the Central Intrusion but ultimately supplied by feeders on the Long Loch Fault. Intrusion of numerous gabbro and peridotite plugs pre- and postdating the MRF. Further central uplift probably accompanied emplacement of Stage 2 as the Central Complex developed beneath a cover of earlier uplifted rocks (Stage 1 and pre-Palaeocene) and (contemporaneous?) lavas.
Post Central Complex
Immediately following the formation of the Central Complex, a succession of deep valleys developed during vigorous erosion of the Rum Volcano, becoming rapidly filled with coarse clastic debris (conglomerates, breccias) derived from the Central Complex and surroundings; these are interbedded with predominantly basaltic lavas belonging to the Canna Lava Formation (Palaeocene Skye Lava Group) which ponded against the flanks of the volcano. Some north to northwest-trending basalt dykes post-date the Canna Lava Formation but regional dykes were probably intruded throughout the Paleocene and a subsequent heating event (45 Ma) indicates that activity may have been prolonged.
Significant right-lateral movement on the Long Loch Fault postdates the Central Complex and Rum underwent further deep erosion by local glaciers and by ice from the Scottish mainland during the Pleistocene.