THE GEOLOGY AND EVOLUTION OF THE BALLACHULISH IGNEOUS COMPLEX AND AUREOLE

The Ballachulish Igneous Complex and Aureole is one of the world's mos t intensively studied plutonic-metamorphic complexes. The 412 +/- 28 M a igneous complex was emplaced at a depth of about 10 km in regionally deformed and metamorphosed mica schists, quartzites and siliceous car bonates belonging to the Dalradian Supergroup. The intrusion is expose d over an area of c. 7.5 x 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 degrees C) surrounding a central body of granite (emplacement temperature c. 850 degrees C), the latter emplaced when t he 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 s eparate magma batches which underwent little physical or chemical inte raction during emplacement. A well-developed contact aureole surrounds the intrusive complex. Isograds in pelitic rocks, the most abundant r ock type in the aureole, can be mapped around the intrusion and range from development of cordierite 'spots' (c. 550 degrees C) up to anatec tic migmatization (700-800 degrees C). Isograds in siliceous carbonate s rocks range from development of talc (< 480 degrees C) up to pericla se formation (c. 750 degrees C). In the quartzites, recrystallization and coarsening of elastic quartz occurs, and elastic feldspar grains d evelop a high-temperature structural state, as the contact is approach ed. The agreement of the sequence and spacing of isograds in pelites, siliceous carbonates and quartzites with equilibrium phase diagrams in dicates no significant kinetic control on the positioning of isograds, although some metamorphic processes appear to have been kinetically c ontrolled. The contact metamorphism was mainly caused by intrusion of the diorite phase, with the later granite having little effect. Variat ions 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 con ditions of the cordierite isograd (c. 550 degrees C), was about 500 ka , whereas rocks were hot enough to be partially molten (temperatures a bove ca. 660 degrees C) for about 270 ka. With the exception of some e xtensively fluid-fluxed partial melting on the west flank of the compl ex (Chaotic Zone), fluid communication between the intrusion and aureo le was generally limited. Fluid fluxes in siliceous carbonates from th e inner aureole on the east flank ranged from 100-1000 moles fluid cm( -2). There is no evidence far the development of a large-scale hydroth ermal circulation system.