P-T Conditions and the Influence of Graphite on Pelitic Phase Relations in the Ballachulish Aureole, Scotland

Abstract

Pressure-temperature conditions of pelites in the Ballachulish aureole, Scotland, have been determined from a calibrated petrogenetic grid and from published geothermometers and geobarometers. To calibrate the mineral reactions in the grid, thermodynamic data for appropriate end members of Ms, Chi, Qtz, And, Sil, Ky, Crn, Crd, Kfs, and Bt were derived from experimental data. This approach was hampered by the unknown compositions of many of the minerals used in the experiments, and by apparent inconsistency between the experiments. A best compromise grid that satisfies most of the data was obtained, which is applicable to the Ballachulish and other contact aureoles. In this grid, the first development of sillimanite is constrained to lie between the Richardson et al. (1969) and Holdaway (1971) andalusite-sillimanite boundaries. A pressure estimate of 3.0 + 0.5 kb is obtained from the calibrated grid, within 0.3 kb of estimates from geobarometry and from two other independent petrological studies. Temperatures ranged from 560±20°C at the first development of cordierite in the assemblage Ms+Qtz+Chl+Crd+Bt to 750–800°C in Grt+Crd+Hy assemblages in pelitic screens within the igneous complex. In graphitic slates, in contrast to non-graphitic pelites, an entire andalusite-bearing subzone is developed, and initial cordierite development occurs further from the igneous contacts. The presence of graphite lowered aH2o in the slaters, expanding the stability field of the andalusite-bearing assemblage And+Qtz+Bt+Ms+Crd relative to the assemblage Kfs+Qtz+Bt+Ms+Crd developed in non-graphitic units. Initial development of cordierite in the assemblage Ms+Qtz+Chl+Crd+Bt was also promoted by reduced aH2o in graphitic slates. The regular sequence and spacing of mineral zones in the aureole suggests that gross equilibrium was attained during contact metamorphism, even though the thermal metamorphic pulse is estimated to have been less than 0.2 Ma (Buntebarth, in press). There is no evidence for reaction overstepping in cordierite-producing reactions.