QUANTIFYING THE ROLE OF FLUID AS RELATED TO MELT MOVEMENT IN THE CHAOTIC ZONE OF THE THERMAL AUREOLE IN THE BALLACHULISH IGNEOUS COMPLEX, SCOTLAND

Migmatites in the western flank (Chaotic Zone) of the thermal metamorphic aureole of the Ballachulish Igneous Complex display higher degrees of partial melting relative to elsewhere in the aureole. The migmatites are characterized by disrupted layering of the metapelites and persist up to 400 m beyond the contact. The greater degree of partial melting is proposed to have been caused by upwelling of fluids expelled from the underlying quartz diorite creating a locally water saturated system. This may be explored by investigating if partial melting in the study area results from water saturated or water undersaturated reactions. If partial melting reflects a water saturated system then it is assumed water was added to the system. If the partial melting reflects a water undersaturated system then it assumes partial melts result from dehydration reactions. Samples of Appin Phyllite – the migmatite protolith – were collected along an east-west transect from the contact down temperature. Samples were analyzed for major element oxides, and modeled using Theriak-DOMINO to identify component(s) that controlled partial melting reactions.

The equilibrium assemblage observed outside the Chaotic Zone suggest metamorphic conditions of 550 to 635°C in the presence of 0.4-4.3 mol % water. Petrography and modeling of mesosome compositions yield temperatures of 550-622°C and water abundances of 0.6 to 5.3 mol % for the stable mineral assemblage Qz + Kfs + Pl + Ms + Rt + Sil + Opq ± And. Based on measured water abundances defined by LOI, water binaries predict that water was present as stoichiometric rather than free water, and likely accommodated by muscovite. The observed partial melt assemblage yield model temperatures of >635°C, defined by the mineral assemblage Qz + Kfs + Pl + Sil + Crd + Opq. Modeling of partial melt volumes suggest that temperatures in the sample would need to reach temperatures in excess of 900°C to produce the observed volume of partial melt. This suggests that the observed volume of melt reflects melt accumulation in the sample rather than in situ melting. Modeling and petrography of the samples suggests that melting reactions on the Western flank of the Ballachulish Igneous Complex may not have required the addition of excess water from the pluton, but melt migration may have altered effective bulk composition.