Out-of-Sequence Nucleation in a Blueschist From Syros, Greece

Two samples of garnet + glaucophane + chloritoid + phengite + paragonite + quartz + epidote + rutile schist from the Cycladic Blueschist Unit of Syros, Greece, have been examined in detail in order to constrain the paragenetic sequence and to infer the processes and time scales for metamorphic recrystallization. Petrographic evidence clearly demonstrates the sequence of porphyroblast growth is garnet ➔ glaucophane ➔ chloritoid in contrast to the sequence predicted by equilibrium calculations that is glaucophane ➔ garnet ➔ chloritoid. Most importantly, both garnet and glaucophane do not appear in the assemblage at conditions near the equilibrium phase-in reaction boundary and require considerable overstepping before nucleation occurs. These results reveal that equilibrium thermodynamic forward modelling (e.g., pseudosections) does not accurately predict or explain the observed paragenesis. Growth of glaucophane following nucleation has been modelled using a model of local equilibrium driven by excess reaction affinity and is shown to drive additional reactions among phases not directly in contact with the glaucophane. These reactions have resulted in the consumption of garnet in some locations and the simultaneous production of garnet in other locations, which is used to explain the formation of high-Mn overgrowths on many garnet crystals in these samples. Diffusion modelling of these high-Mn overgrowths suggests very rapid cooling, which must require extremely rapid exhumation subsequent to the rocks having reached peak metamorphic conditions.