Evolution of structurally contrasting anatectic migmatites in the 3‐kbar Ballachulish aureole, Scotland

Abstract

Abstract Anatectic migmatites of contrasting structural style are found adjacent to the contacts of the Ballachulish Igneous Complex, Argyllshire, Scotland. On the east flank, evidence for migmatization is largely restricted to the local development of millimetre‐centimetre scale Kfs + Qtz‐rich leucocratic segregations, which accompany fragmentation of brittle hornfels layers and ductile deformation of mm‐cm scale semipelitic layers. Large volumes of semipelitic rock rich in feldspar and quartz on the east flank show no migmatitic features, and bedding is usually preserved undisturbed right up to the contact. On the west flank, in contrast, similar semipelitic rocks show widespread migmatitic features and disruption of layering is substantial and widespread over a 400 m wide zone. Within the west‐flank migmatites, 1–100 cm scale rigid bedding fragments (schollen) may be suspended and disoriented in a semipelitic matrix that underwent ductile deformation. The P‐T conditions on both flanks are in the same range: 3 kbar and 650–700°C.The contrast in gross structural style is believed to result from differences in the volumes of melt produced and differences in the proportion of rock in which the critical melt fraction of the rocks was exceeded. On the east flank, only on a mm‐cm scale was enough melt locally accumulated to cause disruption of some layers and segregation of melt. On the west flank, melting proceeded substantially in a broad tract of semipelitic rocks, resulting in larger scale contrasts in rheology that led to the present chaotic structures in this zone.Because migmatization occurred at a pressure too low for muscovite dehydration melting, and at temperatures too low for substantial biotite dehydration melting, the different amounts of melting on the east and west flanks most probably resulted from the introduction of differing amounts of externally derived water. On the east flank, and throughout most of the aureole, the absence of melting even in quartzofeldspathic protoliths indicates that there was no substantial movement of fluid towards or away from the igneous complex during migmatization. The contrasting situation on the west flank may have resulted from devolatilization of underlying quartz diorite magma (˜ 690–710°C), which released heat and fluids into the overlying quartz‐ and feldspar‐rich semipelites (solidus temperature ˜ 650–680°C).