QUANTIFYING THE ROLE OF WATER IN THE MIGMATIZATION OF METAPELITIC UNITS OF THE BALLACHULISH IGNEOUS-METAMORPHIC COMPLEX USING PHASE DIAGRAMS

Localized anatectic migmatization of Appin Phyllite and Leven Schist units is observed within 400 meters of the intrusional contact of the Ballachulish Igneous Complex. The western flank migmatites are characterized by large degrees of partial melting and disrupted banding whereas the eastern flank migmatites are characterized by low degrees of localized partial melting. This difference has been attributed to the rheological and compositional differences between the Appin Phyllite and Leven Schist units, differing intrusion geometries at the contact with the two metapelitic units, and the availability of excess fluid derived from the intrusion. To quantify the role of water content versus whole-rock composition on partial melting reactions, iso-chemical phase diagrams were modeled using Theriak-DOMINO. The parameters for modeling include average compositions of Appin Phyllite and Leven Schist units and water contents between 0 to 10 mole percent.

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.