Preservation of Blueschist Facies Minerals Along a Shear Zone By Fast Flowing CO2-Bearing Fluids – a Field Study from the Cycladic Blueschist Unit on Syros, Greece

Abstract:

Our study was undertaken at Fabrika Beach on the southeastern shore of Syros which belongs to the Greek Cycladic archipelago in the Aegean Sea. The island is situated within the Attic-Cycladic metamorphic core complex belt and is now located in the back-arc of the active Hellenic subduction zone.

At Fabrika Beach, blueschist facies minerals are observed in haloes fringing a shear zone within greenschist facies rocks. The approximately vertical shear zone cuts a near horizontal layer of greenschist facies rocks. The blue haloes are ca. 1 m wide, and are seen on both sides of the shear zone. The haloes consist of a carbonated blueschist facies mineral assemblage.

Based on petrological, geochemical and thermodynamic evidence we show that these haloes were preserved at greenschist facies conditions in response to fast flowing CO₂-bearing fluid. Furthermore, we use a simple mass balance to calculate the fluid flux within the shear zone which would be required to cause the observed preservation of blueschist facies minerals.

We constructed a simplified P-T vs. X_CO2 pseudosection using PerPlex 6.6.6 to confirm that preservation of carbonated blueschist can occur at greenschist facies conditions in the presence of CO₂-bearing fluid. The flux of CO₂-bearing fluid along the shear zone was rapid with respect to the fluid flux in the surrounding rocks. Mass balance calculations reveal that the fluid flux within the shear zone was at least 100 – 2000 times larger than the fluid flux within the surrounding rocks. Mineral textures show greenschist facies minerals partially replacing blueschist minerals in the haloes supporting our interpretation that blueschist facies minerals were preserved during greenschist facies retrogression.