PROTOLITHS OF SUBDUCTION ZONE METAMORPHIC ROCKS FROM THE CYCLADIC ISLAND OF SYROS

The Greek island of Syros contains principally marbles and micaceous schists, with lesser quartzites, metabasites, metacomglomerates, calc-mica schists and ophiolite mélange. These rocks have undergone two metamorphic events. The first was a subduction-related, high-pressure, low-temperature blueschist to eclogite facies event that took place about 45 Ma. The second was a medium-pressure, greenschist to amphibolite facies, regional metamorphic overprinting event about 20-25 Ma.

Based upon mineralogy, the rocks of Syros are commonly interpreted as metamorphosed volcanics interlayered with shallow carbonates or as metamorphosed ocean-floor igneous rocks. Recent geochemical studies have further refined the protolith interpretations of these rocks. Kepler (2004) has concluded that glaucophane schists and eclogites have arc to MORB affinities and thus have possible back-arc basin protoliths. Similarly, Walton's (2001) data support MORB protoliths for the “Airport Ophiolite” sequence. Coarse-grained omphacite-bearing rocks and eclogites are metamorphosed gabbros whereas associated felsic rocks may have formed from residual melts of these gabbros (Schumacher, 2004). The mica schists of Syros have rhyolitic to andesitic compositions. Calc-alkaline protoliths have been identified for some of theses rocks. Mica schists depleted in Nb and Ti also appear to have arc-related protoliths, (Kepler, 2004). In contrast, the blueschists from Katergaki and greenschists from southernmost Syros are interpreted as having MORB and terrigenous sedimentary protoliths that were metamorphosed and altered by seawater (Prinkey, 2001).

Much work has been done to determine protoliths for these rocks. However conflicting interpretations can be resolved and genetic linkages evaluated by synthesizing existing whole rock and REE analyses and by supplementing this data with new analyses focused upon the white mica schists of Syros.