APPLICATION OF EPIDOTE AND QUARTZ INCLUSION RAMAN BAROMETRY: REEVALUATION OF THE P-T CONDITIONS OF GARNET BEARING BLUESCHISTS FROM SIFNOS, GREECE

Our increased knowledge of the nature and extent of overstepping of metamorphic reactions has challenged our understanding of the tectonic processes associated with metamorphism. The advent of mineral inclusion thermobaRamantry has allowed petrologists to better study metamorphic overstepping as thermobaRamantry relies on mechanical equilibrium and empirical equations of state as opposed to chemical equilibrium. In order to assess the role overstepping plays in subduction zone processes in particular we applied the new epidote in garnet (EpiG) Raman barometer, the quartz in garnet (QuiG) Raman barometer, and textural and mineral assemblage diagram analysis to 5 garnet-bearing blueschist samples from a Southeast-Northwest transect across the blueschist unit in Sifnos, Greece. All the samples are garnet-glaucophane blueschists with variable amounts of epidote/zoisite, quartz and other accessory phases, but their textures and bulk compositions vary. Garnet in samples from the southern and middle sections are typically ~2 cm large and display a honeycomb texture with ample quartz and rutile inclusions. Garnets in the northern most section, however, are generally 2-5 cm in diameter, and poikoblastic with inclusions of quartz and rutile along with occasional zircon and epidote. Preliminary EpiG and QuiG results suggests that in samples SPH99-1, SPH99-2 and SPH99-5, from the southern section of the range, garnets grew at a pressure from 12 to 16 Kbar. EpiG and QuiG analysis of samples SPH99-7 and SPH99-8 from the northern section, however, reveal pressures of 21-22 Kbar. Rigorous thermodynamic modelling is underway and may reveal more differences in the P-T evolution of rocks across this subduction zone. The ~6-8 Kbar difference across the unit may be explained by: 1) the rocks subducting as a coherent unit and then titling nearly 90° during exhumation so that both the deepest rocks (highest P) and shallowest rocks are visible at the surface; 2) tectonic thinning and accretion of a large HP unit as a result of post subduction extension and subsequent unroofing; or 3) variable amounts of overstepping of the garnet isograd which caused garnet to grow at different pressures, despite the possibility of a uniform peak P and T across the region. Further analysis will help identify the correct model.