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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 CaCO3 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 + CaCO3 + 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 + CaCO3, but did not exceed the P–T stability of the reaction dolomite + quartz → tremolite + CaCO3. The P–T locations of these reactions are sensitive to fluid composition and indicate that the attending fluid phase was water-rich with XCO2 constrained to be XCO2 of 0·01 best fits the observed assemblages. Relict lawsonite + Al-rich epidote in schists associated with the glaucophane marbles also has a T–XCO2 stability that is limited to fluids with XCO2 2O over most of its metamorphic history. The P–T–XCO2 stability of assemblages common in the schist and marble constrains the P and T maxima for these rocks to about 500°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. [ABSTRACT FROM AUTHOR]