Journal of Metamorphic Geology | 2019
The partial equilibration of garnet porphyroblasts in pelitic schists and its control on prograde metamorphism, Glen Roy, Scotland
Abstract
Garnet porphyroblasts in sillimanite-bearing pelitic schists contain complex \ntextural and compositional zoning, with considerable variation both within and \nbetween adjacent samples. The sillimanite-bearing schists locally occur in \nregional Barrovian garnet zone assemblages and are indicative of a persistent \nlack of equilibrium during prograde metamorphism. Garnet in these Dalradian \nrocks from the Scottish Highlands preserves evidence of a range of metamorphic \nresponses including initial growth and patchy coupled dissolution- \nreprecipitation followed by partial dissolution. Individual porphyroblasts each \nhave a unique and variable response to prograde metamorphism and garnet with \nmainly flat compositional profiles co-exists with those containing largely \nunmodified characteristic bell-shaped Mn-profiles. This highlights the need for \ncaution in applying traditional interpretations of effective volume diffusion \neliminating compositional variation. Cloudy garnet with abundant fluid \ninclusions is produced during incomplete modification of the initial \nporphyroblasts and these porous garnet are then particularly prone to partial \nreplacement in sillimanite-producing reactions. The modification of garnet via a \ndissolution-reprecipitation process releases Ca into the effective whole rock \ncomposition, displacing the pressure-temperature positions of subsequent \nisograd reactions. This represents the first report of internal metasomatism \ncontrolling reaction pathways. The behaviour of garnet highlights the \nimportance of kinetic factors, especially deformation and fluids, in controlling \nreaction progress and how the resulting variability influences subsequent \nprograde history. The lack of a consistent metamorphic response, within and \nbetween adjacent schists, suggests that on both local and regional scales these \nrocks have largely not equilibrated at peak metamorphic conditions.