Regional metamorphism in the Ballachulish area, SW Highlands, Scotland: new perspectives on a famous old debate, with regional implications

The Ballachulish region of the SW Highlands of Scotland was the focal point of a famous debate in the 1920s between Sir Edward Battersby Bailey and Professor Cecil Edgar Tilley concerning the identity, geometry and timing of development of metamorphic zones in the SW Highlands. New mineral assemblage data, microstructural observations, and rock and mineral compositional data are combined with phase equilibrium modelling to reassess the metamorphism in this area. The newly revised chlorite, biotite and garnet zones are similar to those of Bailey. Compositional difference between lithologies is the primary cause of the irregular distribution of mineral assemblages in the region. Microstructural observations suggest that metamorphism occurred in the latter stages of the deformation history, in agreement with previous studies. Pressure–temperature conditions of the garnet isograd are estimated to be c. 7 kbar, c. 500 °C. These are compared with P–T estimates of the garnet zone elsewhere in Scotland.