Evolution of the Mull Tertiary volcano, western Scotland

The early Tertiary Mull volcano, western Scotland, is one of the most dissected and best exposed igneous complexes of the North Atlantic Province. The new and published geochemical data enable us to chart the magmatic evolution of the Mull volcano from the oldest lavas through the intrusive rocks of three overlapping igneous centres, to the youngest dykes. In this study, we identify four successive magma types within the remnant volcano. The earliest type-the Mull Plateau Group-comprises mildly alkaline basaltic rocks with steep chondrite-normalized rare earth element (REE) patterns. This type is succeeded, within the lava succession and dyke swarm, by the Coire Gorm magma type with essentially flat chondrite-normalized REE patterns. A third magma type represented within the lava and dykes-the Central Mull Tholeiites-is more depleted in incompatible trace elements than the preceding types and has flat to LREE-depleted chondrite-normalized patterns. The major intrusions and cone sheets of Mull Centre 1 and early Centre 2 belong to this magma type. Midway through the igneous activity associated with Centre 2, the magma type changed to become more alkalic and more enriched in incompatible trace elements. This magma type (the Late Mull type) is found to persist through the cone sheets and major intrusions of Centre 3, to the youngest dykes. These changes in magma composition were related to variations in the mantle source and depth of partial melting beneath. Mull, and/or differences in the efficiency of melt pooling before ascent through the lithosphere. With the exception of the early Staffa magma sub-type (part of the Mull Plateau Group), the location of magma chambers, in which the bulk of contamination occurred, changed with time from deep (lower-crustal Lewisian gneiss) to shallow (upper-crustal Moine schist). Intermediate members of the Plateau Group and the Late Mull magma type are enriched in Fe, Ti and P relative to the Central Mull Tholeiites. We attribute this difference to the more alkalic nature of these suites, lower fO(2), and the formation of Fe3+-P complexes in the magma. The intermediate rocks were important in magma mixing processes, with two types of mixing identified on Mull: (1) cryptic mixing between basalts and low-Fe intermediate magmas, typified by lavas and early basic cone sheets of the Central Mull Tholeiite magma type; (2) observable mingling between rhyolitic magmas and high-Fe intermediate magmas of the Late Mull type, shown by the mixed-magma bodies of the Glen More and Lock Ba ring dykes. The main factor in determining which type of mixing occurred appears to have been the density contrast between the various magmas.