Chemical and textural equilibration of garnet during amphibolite facies metamorphism: The influence of coupled dissolution–reprecipitation

Abstract

Metamorphic equilibration requires chemical communication between minerals and may be inhibited through sluggish volume diffusion and or slow rates of dissolution in a fluid phase. Relatively slow diffusion and the perceived robust nature of chemical growth zoning may preclude garnet porphyroblasts from readily participating in low‐temperature amphibolite facies metamorphic reactions. Garnet is widely assumed to be a reactant in staurolite‐isograd reactions, and the evidence for this has been assessed in the Late Proterozoic Dalradian pelitic schists of the Scottish Highlands. The 3D imaging of garnet porphyroblasts in staurolite‐bearing schists reveals a good crystal shape and little evidence of marginal dissolution; however, there is also lack of evidence for the involvement of either chlorite or chloritoid in the reaction. Staurolite forms directly adjacent to the garnet, and its nucleation is strongly associated with deformation of the muscovite‐rich fabrics around the porphyroblasts. “Cloudy” fluid inclusion‐rich garnet forms in both marginal and internal parts of the garnet porphyroblast and is linked both to the production of staurolite and to the introduction of abundant quartz inclusions within the garnet. Such cloudy garnet typically has a Mg‐rich, Mn‐poor composition and is interpreted to have formed during a coupled dissolution–reprecipitation process, triggered by a local influx of fluid. All garnet in the muscovite‐bearing schists present in this area is potentially reactive, irrespective of the garnet composition, but very few of the schists contain staurolite. The staurolite‐producing reaction appears to be substantially overstepped during the relatively high‐pressure Barrovian regional metamorphism reflecting the limited permeability of the schists in peak metamorphic conditions. Fluid influx and hence reaction progress appear to be strongly controlled by subtle differences in deformation history. The remaining garnet fails to achieve chemical equilibrium during the reaction creating distinctive patchy compositional zoning. Such zoning in metamorphic garnet created during coupled dissolution–reprecipitation reactions may be difficult to recognize in higher grade pelites due to subsequent diffusive re‐equilibration. Fundamental assumptions about metamorphic processes are questioned by the lack of chemical equilibrium during this reaction and the restricted permeability of the regional metamorphic pelitic schists. In addition, the partial loss of prograde chemical and textural information from the garnet porphyroblasts cautions against their routine use as a reliable monitor of metamorphic history. However, the partial re‐equilibration of the porphyroblasts during coupled dissolution–reprecipitation opens possibilities of mapping reaction progress in garnet as a means of assessing fluid access during peak metamorphic conditions.

The partial equilibration of garnet porphyroblasts in pelitic schists and its control on prograde metamorphism, Glen Roy, Scotland

“…Many of the porphyroblasts in both samples have either been partially or completely modified by the processes forming cloudy garnet (Hames & Menard, ; Martin et al., ; Whitney, Mechum, Dilek, & Kuehner, ), as is reported in the Leven Schists towards the southwest (Dempster, La Piazza, et al., ). The cloudy garnet with crystallographically aligned fluid inclusions is thought to represent a patchy to complete alteration via a dissolution–reprecipitation mechanism (Dempster, La Piazza, et al., ; Erambert & Austrheim, ; Putnis & Putnis, ; Ruiz‐Agudo, Putnis, & Putnis, ) associated with the generation of large irregular quartz inclusions. This process causes partial re‐equilibration of garnet producing variable compositions both within and between individual porphyroblasts, and between adjacent rocks (Dempster, La Piazza, et al., ).…”

“…The cloudy garnet with crystallographically aligned fluid inclusions is thought to represent a patchy to complete alteration via a dissolution–reprecipitation mechanism (Dempster, La Piazza, et al., ; Erambert & Austrheim, ; Putnis & Putnis, ; Ruiz‐Agudo, Putnis, & Putnis, ) associated with the generation of large irregular quartz inclusions. This process causes partial re‐equilibration of garnet producing variable compositions both within and between individual porphyroblasts, and between adjacent rocks (Dempster, La Piazza, et al., ). Where patchy alteration occurs, the cloudy garnet inherits a partially modified chemistry from the local composition of the original porphyroblast.…”

“…No strong metamorphic field gradients have been reported from this area, although Dempster, La Piazza, et al. () highlighted local kinetic controls on the limited introduction of staurolite in the Leven Schists 8 km to the southwest. The age of regional metamorphism in these rocks is uncertain, as polymetamorphism is commonly reported in the Dalradian schists (Dempster & Harte, ; Dempster & Jess, ; Dempster et al., ).…”

“…Following the methods described in Dempster, La Piazza, et al. (), X‐ray maps and compositional traverses were generated using a Carl‐Zeiss Sigma VP electron microscope operated at 20 kV, with Oxford Instruments X‐Max 80 energy dispersive spectrometry typically with 2 μm spacing between points. Major element zoning profiles were processed using AZTEC Software 3.0.…”

“…Garnet porphyroblast compositions may also be reset by coupled dissolution–reprecipitation mechanisms (Ague & Axler, ; Dempster, La Piazza, Taylor, Beaudoin, & Chung, ; Martin et al., ; Pollok, Lloyd, Austrheim, & Putnis, ), although the effectiveness of such processes relative to the size and abundance of typical garnet is uncertain. However, because of the wide range of possible responses to changing conditions, garnet within mid‐amphibolite facies conditions offers the opportunity to investigate the kinetic controls on metamorphic reactions.…”