The island of Syros lies within the blueschist belt of the Attico-Cycladic metamorphic complex. The island is underlain by alternating, north dipping marbles, metapelites and acidic metavolcanics, which may be repeated in a greatly thickened sequence of stacked thrust sheets. Metabasite localities dot the island and a mélange zone exists to the north. Recent studies suggest that the island was metamorphosed under blueschist facies conditions at approximately 80 Ma, during the Alpine orogeny (Cheney et al., 2000; Bröcker and Enders, 1999). There is also local evidence of a greenschist overprint, widely accepted to be a late Oligocene/early Miocene event.

This study concerns the mineral assemblages of samples from an island-wide set of quartz-mica schists localities. In most samples, three texturally distinct assemblages have been recognized. The blueschist facies assemblages in these rocks contain quartz + white mica + glaucophane +/- garnet +/- chlorite +/- chloritoid +/- jadeite. A retrograde assemblage partially overprints the equilibrium assemblage with the minerals albite, chlorite, and/or biotite. Clinopyroxene inclusions in garnet suggest a pre-blueschist, eclogite facies assemblage, which is defined on the nearby island of Sifnos by the stability of jadeite + quartz and omphacite + garnet (Lister and Raouzaios, 1996). Chemical analysis and the study of textures and reactions on a NFM projection from quartz, zoisite and paragonite onto the plane NaALO2-FeAl2O4-MgAl2O4 are used to assess the causes of mineral assemblage variation within each facies. Metamorphic grade related variations in mineral assemblage across the island constrain the location of major faults and/or isograds. Using this information, the island can be divided into metamorphic terranes, each with a distinct P-T path.