FLUID FLOW PATHS IN THE ATTIC-CYCLADIC BLUESCHIST BELT: IMPLICATIONS FOR ARC MAGMA GENESIS

The Attic-Cycladic blueschist belt represents the exhumed remnants of an extensive subduction zone. Devolatilization of igneous and sedimentary rocks during Cretaceous (?) through Eocene subduction was extensive, yet very little arc magmatism developed in the region. This study examines blueschist and eclogite facies metamorphism on Tinos and Syros islands (Greece; ~500 deg. C, ~1.5-2.0 GPa), to better constrain fluid flow paths and implications for arc magmatism. Geochemical mass balance and stable isotope relations were studied for metamorphosed mafic rocks, carbonate rocks, pelitic rocks, and ultramafic mélange matrix. O and C isotope results indicate limited cross-layer flow, as observed in previous studies in the Cyclades (e.g., Rye et al., 1976; Ganor et al., 1989, 1996; Bickle and Baker 1990). However, large fluid fluxes were channelized along lithologic contacts, fractures, fold hinges, and mélange matrix. Mass transfer in metamorphosed mafic rocks and sediments was extensive in these zones; the most common geochemical signatures involve increases in Na/K and mass loss of K, Rb, Ba, Cs, and Pb. The strong mobility of Na is consistent with recent experimental and theoretical studies of subduction zone fluids (e.g., Manning, 2004). Fluids were almost certainly derived from devolatilization of downgoing metamorphosed oceanic crust, metasediments, and ultramafic rocks. Geochemical modeling suggests that time-integrated fluid fluxes along conduits may have been as much as 10⁴ to 10⁵ m³ fluid per m² rock, but were considerably lower away from conduits. The results of this study indicate that fluid flow along mélange zones, through fractures, and parallel to lithologic contacts focused regional fluid movement along the décollement rather than vertically upward into the mantle wedge. Consequently, the amount of arc magma that could be generated during subduction was strongly limited because the mantle wedge was deprived of volatiles.