CHALLENGES IN PHASE DIAGRAM MODELING, FOCUSING ON METAPELITES

The Ballachulish area of the southwest Highlands of Scotland was the site of a famous debate in the 1920s between two of the giants of British geology: Sir Edward Battersby Bailey, a field geologist with of the Geological Survey of Great Britain and University of Glasgow, and Professor Cecil Edgar Tilley, a petrologist at the University of Cambridge. At issue were contrasting interpretations of the identity, geometry and timing of regional metamorphic zones in the southwest Highlands, and the implications these carried for the tectonic evolution of the region. The point of contention in the Ballachulish area was the cause of the abrupt disappearance of garnet, in the direction of increasing metamorphic grade, across a major fault, the Ballachulish Slide. Bailey attributed the abrupt disappearance of garnet to differing bulk composition of rocks of the same metamorphic grade on either side of the slide, interpreting the slide to be pre-metamorphic, whereas Tilley and later his Cambridge colleague Gertrude Elles and he attributed the abrupt disappearance of garnet to mechanical juxtaposition of rocks of different metamorphic grade, implying that the slide was post-metamorphic. New mineral assemblage data, microstructural observations and rock and mineral compositional data are combined with phase equilibrium modeling to re-assess the regional metamorphism in Ballachulish area. The newly revised chlorite, biotite and garnet zones are more similar to those of Bailey than Tilley. Compositional difference between lithologies accounts for the irregular distribution of mineral assemblages in the region, especially garnet. Isograd patterns and microstructural observations indicate that metamorphism occurred in the latter stages of the deformation history, after development of the major slides (faults), in agreement with other studies. Pressure-temperature conditions of the garnet isograd are estimated to be ~7 kbar, ~500 °C.