EVALUATION OF OVERSTEPPING IN GARNET SCHISTS FROM SYROS, GREECE

Calculating peak metamorphic conditions has long relied on the assumption of chemical equilibrium. However, it is understood that a certain amount of overstepping is required in order to crystallize a mineral though the extent of this overstepping and its implications are not well understood. This study uses garnet mica schist samples from various locations from the Greek Cycladic Island of Syros to constrain pressures and temperatures at which garnet crystallized. Syros is a classic example of a back-arc environment that underwent high grade eclogite to blueschist-facies metamorphism between 40 and 80 Ma and was later overprinted by a greenschist-facies event. This project uses chloritoid-glaucophane-garnet-mica-schists from the northwest of the island, several garnet-mica-schists from various locations, and one garnet-mica-jadeite schist from the southeast to investigate overstepping in a well-studied metamorphic environment.

Raman Spectroscopy on quartz inclusions in garnet from is used as a barometer to constrain the pressure of crystallization. The electron microprobe was used to determine Zirconium concentration in rutile inclusions in garnet from these same samples. The zircon concentrations are used as a thermometer to constrain the temperature of crystallization. X-ray maps and EDS compositions of garnet will be used to investigate the variability of the overstepping and whether the garnet formed overtime at different pressures. This data is supplemented by compositions of paragonite, muscovite, chloritoid, glaucophanes and jadeite as appropriate. Compositional X-ray mapping of whole thin sections is used to determine the bulk composition of the samples. These will be used to calculate metamorphic mineral assemblage diagrams. These “overstepped” pressures and temperatures will be compared to pressures and temperatures calculated under the assumption of chemical equilibrium to determine the extent of overstepping. Preliminary results suggest significant overstepping of the garnet isograd in some samples which suggests the analyzed garnet crystallized out of equilibrium with the surrounding matrix.