Glaucophane-bearing marbles on Syros, Greece

The occurrence of glaucophane-bearing marbles on Syros is noteworthy because reports of marbles that contain glaucophane are rare among descriptions of high-pressure marbles. On Syros, the marbles are composed primarily of calcite with or without dolomite and quartz. Much of the calcite in these marbles shows oriented columnar structures that are interpreted as pseudomorphs of prismatic aragonite. The columnar structure is particularly well developed in layers of pure CaCO(3) and is one indicator of the high-pressure history of these marbles. Metamorphosed admixtures of carbonate and mafic silicate material yielded minerals that are typical for eclogite facies and blueschist facies. These impure marbles are widespread and contain assemblages of various combinations of glaucophane/ferroglaucophane, Na-pyroxene (omphacite to jadeite), epidote, garnet, paragonite and phengitic white mica. Based on calculated mineral equilibria, the assemblages and mineral compositions in the marbles and associated rocks place narrow constraints on the metamorphic P-T path and the grain-boundary fluid composition of the marbles. The occurrence of glaucophane + CaCO(3) + dolomite + quartz suggests that the P-T trajectory that was followed by the rocks crossed a reaction such as albite/Na-pyroxene + dolomite + quartz -> glaucophane + CaCO(3), but did not exceed the P-T stability of the reaction dolomite + quartz -> tremolite CaCO(3). The P-T locations of these reactions are sensitive to fluid composition and indicate that the attending fluid phase was water-rich with X(CO2) constrained to be <0.03; a value of X(CO2) of 0.01 best fits the observed assemblages. Relict lawsonite + Al-rich epidote in schists associated with the glaucophane marbles also has a T-X(CO2) stability that is limited to fluids with X(CO2) <0.03. This observation suggests that the grain-boundary fluid of the whole subduction package of schist, blueschist and marble was rich in H(2)O over most of its metamorphic history. The P-T-X(CO2) stability of assemblages common in the schist and marble constrains the P and T maxima for these rocks to about 500 degrees C and 15-16 kbar. These P-T constraints, together with the tectonic fabric of the marbles, suggest that deformation and recrystallization occurred at or near the thermal maximum of metamorphism.