MASS TRANSFER DURING BLUESCHIST-ECLOGITE FACIES METAMORPHISM, TINOS AND SYROS ISLANDS, GREECE: IMPLICATIONS FOR SUBDUCTION ZONE FLUIDS

Blueschist-eclogite facies metacarbonate rocks from the exhumed Cretaceous-Eocene subduction complex exposed on Tinos and Syros Islands, Greece, were investigated to gain a better understanding of elemental mobility in subduction zone fluids. Metacarbonate rocks were chosen because they are sensitive monitors of fluid-rock interaction. High spatial resolution geochemical profiles were done across two representative metacarbonate layers cut by syn-metamorphic quartz veins on Tinos, and one folded and veined metacarbonate layer enclosed in mélange from Syros. Relatively unaltered parts of the layers consist mostly of calcite (originally aragonite)+ glaucophane+ phengite+ rutile+/- omphacite+/- quartz+/- epidote+/- chlorite. The modal abundances of epidote, glaucophane, and omphacite rise sharply adjacent to veins and in other areas infiltrated by fluids. Quantitative mass balance analysis reveals that fluid infiltration drove mass transfer of major, minor, and trace elements. Rocks in mélange and adjacent to veins gained considerable Si, Al, Fe, Mg, and Na, and lesser Ti. Conversely, Ca, K, Rb, Ba, Sr, and volatile species including CO₂ were lost to the fluid phase. Addition of P and Y was generally accompanied by addition of heavy REE, suggesting that P and Y complexes may transport REE in subduction zones. The loss of Sr and gain of Y resulted in smaller Sr/Y ratios for the altered rocks. The behavior of U and Th were strongly decoupled. Th mobility was limited, but U mass additions in excess of +200 % in vein and mélange zones produced strongly elevated U/Th ratios in some rocks. Geochemical modeling of the mass changes suggests that regions of chemical alteration were subjected to massive time-integrated fluid fluxes of ~10⁵ (m³ fluid)/(m² rock). The fluids were derived from devolatilization of subducted mafic crust, metapelitic rocks, and metamorphosed ultramafic rocks. This study demonstrates that channelization of fluids into veins or mélange in subduction zones can produce large fluid fluxes capable of significant major, minor, and trace element transport that can strongly alter the bulk chemical composition of subducted crust.