305-3 Constraining the recorded P-T path of The Exhumed Blueschist-Eclogite unit of Sifnos Greece: Revisiting an Incomplete Origin Story

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

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Subduction zones host megathrust earthquakes and arc volcanism, processes influenced by rheological changes in subducting slabs. As hydrated basaltic oceanic crust is buried and metamorphosed, it transforms from blueschist to eclogite facies. This transition is  associated with dehydration reactions that release fluids. These reactions alter strength and influence the subduction interface rheologic behavior with implications for these geohazards. The exact depth where lithologic transitions occur is influenced by the bulk composition of a given rock and the pressure-temperature (P-T) path it takes. To understand the significance of this transformation, we must know where it occurs.  

The exhumed Cycladic Blueschist Unit (CBU) at Vroulidia Beach, Sifnos Island, Greece  was chosen for our study because it exposes well preserved m-scale intercalated blueschist and eclogite lithologies. Previous work suggests bulk compositional differences between eclogites and blueschist is evidence for different protoliths at identical peak P-T condition resulting in separate metamorphic facies. However, our new geochemical analysis of intimately-intermingled blueschist and eclogite samples show nearly identical bulk compositions, challenging these previous interpretations of P-T conditions recorded by each of these facies within the CBU on Sifnos. 

We combined bulk rock geochemical data, detailed petrography, electron microprobe mineral compositions, and thermobarometry established the (1) original protoliths and (2) equilibrium conditions of our  blueschists and eclogites. Applying garnet-phengite-omphacite thermobarometry to matrix and inclusion assemblages with textural evidence for quartz-after-coesite yields constraints on garnet growth to temperatures from ~430–560°C and a peak pressure of ~2 Gpa. We refine pressure estimates along this prograde path during garnet growth using Raman-spectroscopy based Quartz-in-garnet barometry. Analyzing inclusions and zoning profiles captures distinct metamorphic stages from prograde to peak eclogite-facies conditions to retrogression under blueschist-facies conditions. Our data suggests the examined blueschist-eclogite transition on Sifnos represents a coherent section of oceanic crust derived from a calc-alkaline basaltic protolith, with retrograde overprinting of earlier eclogites by fluid interactions during the early stages of exhumation. This is supported by preserved high-pressure mineral inclusions (e.g., omphacite) in garnet cores within the blueschist samples, and compositional shifts between glaucophane in the matrix and in garnet inclusions. Due to the CBU’s rich history of exhumation and retrogression, our work highlights the importance of a detailed analysis to establish peak P-T conditions. 

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

doi: 10.1130/abs/2025AM-9343

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