244-5 Constraining the Nature and Significance of an Exhumed Blueschist-Eclogite Transition, Sifnos Island, Greece.

Session: Subduction Zone Processes: Insights from Geology, Geochemistry, and Petrochronology

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Abstract:

Subduction zones are dynamic tectonic boundaries where oceanic lithosphere descends beneath an overriding plate, ultimately recycling into Earth’s interior. As subduction progresses, the basaltic slab undergoes metamorphism to mafic blueschist and, at sub-arc depths, to eclogite—transformations that coincide with key rheological and density changes along the plate interface and may influence convergence rates, interplate coupling, and megathrust earthquake cycles. To investigate these processes, we examine an exhumed section of the blueschist-eclogite transition in the Cycladic Blueschist Unit (CBU) at Vroulidia Beach, Sifnos Island, Greece which was part of the Hellenic subduction zone. The CBU experienced peak metamorphic conditions in the Eocene and retrogression during exhumation in the Oligo-Miocene. Our study combines detailed microstructural analysis with new petrologic constraints to: (1) reassess the pressure-temperature (P–T) evolution of the CBU on Sifnos using modern thermodynamic modeling (e.g., pseudosections), and (2) evaluate how deformation, metamorphic reactions, and metasomatism interact at this blueschist-eclogite transition during subduction and exhumation.

Field and petrographic observations and electron probe microanalysis (EPMA) reveal a coherent section of exhumed oceanic lithosphere with mineral assemblages ranging from blueschist to eclogite facies over sub-meter scales. Bulk-rock geochemistry shows minimal major and trace element variation across the transition, suggesting a common calc-alkaline basaltic protolith. Garnet inclusions in the blueschists contain eclogitic phases (e.g., omphacite) aligned in a relict prograde foliation oblique to the dominant matrix blueschist deformation fabric, implying overprinting during retrograde deformation. In contrast, the eclogitic samples exhibit weak or indistinct foliation. Pseudosection modeling indicates that both blueschist and eclogite samples experienced peak metamorphism under eclogite-facies conditions, followed by fluid-assisted retrogression and deformation under blueschist-facies conditions during early and late stages of syn-subduction exhumation. These findings support a model in which deformation localized during hydration-driven metamorphic overprinting of eclogites to blueschists, occurring only in select domains within the oceanic lithosphere. Overall, our results highlight the dynamic interplay of metamorphism, metasomatism, and deformation during subduction and exhumation in controlling the expression of metamorphic facies with important implications for the rheological behavior of the subduction interface. These findings from the CBU inform models of strength, slip, and material cycling in active subduction zones.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6536

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