The geology and evolution of the Ballachulish Igneous Complex and Aureole

Synopsis The Ballachulish Igneous Complex and Aureole is one of the world’s most intensively studied plutonic-metamorphic complexes. The 412 ± 28 Ma igneous complex was emplaced at a depth of about 10 km in regionally deformed and metamorphosed mica schists, quartzites and siliceous carbonates belonging to the Dalradian Supergroup. The intrusion is exposed over an area of c. 7.5 × 4.5 km 2 and is roughly cylinder-shaped to about 4 km beneath the surface, with the exception of two areas where the intrusion lies shallowly beneath the country rocks. The intrusion consists of an outer orthopyroxene-bearing diorite shell (emplacement temperature c. 1100°C) surrounding a central body of granite (emplacement temperature c. 850°C), the latter emplaced when the central portion of the diorite was still partially molten. A small, late leucocratic body in the centre of the granite is associated with weak Cu–Mo mineralization. The different intrusive phases represent separate magma batches which underwent little physical or chemical interaction during emplacement. A well-developed contact aureole surrounds the intrusive complex. Isograds in pelitic rocks, the most abundant rock type in the aureole, can be mapped around the intrusion and range from development of cordierite ‘spots’ (c. 550°C) up to anatectic migmatization (700–800°C). Isograds in siliceous carbonates rocks range from development of talc (< 480°C) up to periclase formation (c. 750°C). In the quartzites, recrystallization and coarsening of clastic quartz occurs, and clastic feldspar grains develop a high-temperature structural state, as the contact is approached. The agreement of the sequence and spacing of isograds in pelites, siliceous carbonates and quartzites with equilibrium phase diagrams indicates no significant kinetic control on the positioning of isograds, although some metamorphic processes appear to have been kinetically controlled. The contact metamorphism was mainly caused by intrusion of the diorite phase, with the later granite having little effect. Variations in width of the aureole are mainly due to variations in shape of the intrusion, temperature of the different intrusive phases, and the relative proportions of quartzite and pelite in the country rocks. The duration of the contact metamorphic event, for temperatures above conditions of the cordierite isograd (c. 550°C), was about 500 ka, whereas rocks were hot enough to be partially molten (temperatures above ca. 660°C) for about 270 ka. With the exception of some extensively fluid-fluxed partial melting on the west flank of the complex (Chaotic Zone), fluid communication between the intrusion and aureole was generally limited. Fluid fluxes in siliceous carbonates from the inner aureole on the east flank ranged from 100–1000 moles fluid cm −2 . There is no evidence for the development of a large-scale hydrothermal circulation system.