Eclogite-Blueschist Relationships as Evidenced by Mineral Equilibria in the High-Pressure Metabasic Rocks of Sifnos (Cycladic Islands), Greece

The northern part of the Cycladic island of Sifnos (Greece) is formed by a coherent sequence of interlayered acid and basic metavolcanic rocks and metasediments, which underwent a high-pressure blueschist facies metamorphism during the Eocene. The metabasic rocks, including eclogites, blueschists, and actinolite-bearing rocks, are discussed in terms of their mineral assemblages, and bulk-rock and mineral chemistries. Metamorphic conditions of 470 ± 30 °C and 15 ± 3 kb are indicated by garnet-omphacite geothermometry and by the development of deerite in meta-ironstones and jadeite +quartz in meta-acidites. Mineral textures and systematic element distributions between coexisting minerals suggest attainment of chemical equilibrium. A new projection from garnet, epidote, quartz and vapour onto the NaAlO2-Al2O3-CaMgO2 plane is used to illustrate equilibrium phase relations between omphacite, glaucophane, actinolite, paragonite, and chloritoid. It is demonstrated that eclogites, blueschists, and actinolite-bearing metabasites represent different bulk-rock compositions that recrystallized under the same fluid pressure and temperature conditions. Eclogites containing hydrous minerals such as glaucophane, actinolite, phengite, or paragonite in equilibrium with garnet and omphacite can occur together with blueschists in high-pressure terranes without indicating different metamorphic conditions.