QUANTIFYING THE ROLE OF WATER IN THE MIGMATIZATION OF METAPELITIC UNITS OF THE BALLACHULISH IGNEOUS-METAMORPHIC COMPLEX USING PHASE DIAGRAMS
The Leven Schist bulk rock composition produced the observed migmatite assemblages for the eastern flank at 850°C with <1.75 mole percent H2O, whereas the Appin Phyllite bulk rock composition produced the observed assemblages at 750°C, but only when up to 5 mole percent H2O was present. The western flank migmatite assemblages were only calculated using the Leven Schist bulk rock composition at temperatures between 650 and 675°C with 2.5 to 10 mole percent H2O.
The absence of partial melting in the eastern flank or the occurrence of the reaction only at significantly higher temperatures at much lower H2O abundances suggest that these systems were H2O under-saturated, and there may have been little to no fluid transfer between the igneous complex and the surrounding host rock. In contrast, the abundance of H2O required to cause partial melting in the western flank is greater than the number of moles H2O in the hydrous components of the assemblage suggesting that partial melting occurred in a water-saturated system. The protolith to partial melted rocks were regionally metamorphosed pelites with relatively low water contents. Therefore, the excess H2O required to create water-saturated systems was introduced during metamorphism, with the most logical source being the igneous complex.