THERMODYNAMIC ANALYSIS OF BLUESCHIST LINKS GARNET GROWTH TO PROGRESSIVE SUBDUCTION ZONE DEHYDRATION IN THE CYCLADIC BLUESCHIST UNIT OF SYROS, GREECE

Using Perple_x, a thermodynamic modeling software, pseudosections and mineral/volatile isopleth diagrams were produced in order to constrain the pressure-temperature (P-T) stability of important mineral phases in a garnet lawsonite blueschist from the Cycladic Blueschist Unit in Syros, Greece. Petrographic observations, XRF data, the sample’s modal mineralogy, and knowledge of the island’s P-T history were all used to constrain the sample’s P-T trajectory during prograde subduction zone metamorphism. Along the calculated P-T path, garnet growth is correlated with the release of structurally bound water from the subducting rock.

The produced P-T evolution of the blueschist suggests peak metamorphic conditions of 2.4 GPa and 547 ˚C, which are hotter and deeper than the previously published data of 1.8 GPa (Ring et al., 2010) and 500 ˚C (Lagos et al., 2007). However, the pressure calculated here (2.4 GPa) is in agreement with the 2.2 GPa maximum pressures predicted by thermodynamic analysis from nearby Sifnos island (Dragovic et al., 2012). Syros and Sifnos experienced peak subduction zone metamorphic conditions at ~47 Ma and 53 Ma, respectively. The existing differences in pressure and temperature from study to study on Syros could suggest different rates of metamorphism along the Attic Cycladic Crystalline Complex and future work on zoned garnet geochronology of subduction zone lithologies from Syros could help explore this.

The P-T curve predicts 1.29 weight percent water loss from the subducting lithology during garnet growth. Epidote pseudomorphs after lawsonite occur both as inclusions in garnet and as porphyroblasts. Thus, garnet growth spans lawsonite stability and its growth correlates with the dehydration event associated with lawsonite breakdown.

Sm/Nd garnet geochronology can further constrain the progressive dehydration event. Preliminary attempts at garnet geochronology were unsuccessful, instead revealing artifacts of a Triassic age protolith within the garnet’s inherited inclusion population. This underscores the importance of removing inclusions to achieve pure garnet for accurate ages of garnet growth.