Glaucophane schists and associated rocks from Sifnos (Cyclades, Greece): new constraints on the P-T evolution from oxidized systems

The island of Sifnos (Cyclades, Greece) is famous for its spectacular blueschist-facies rocks and eclogites. Their very well preserved high-pressure assemblages are characterized by the occurrence of Fe+3-rich minerals such as aegirine-rich pyroxene, riebeckite-rich amphibole, magnetite and deerite. Therefore, the common model system NCFMASH (Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O), in which all iron is assumed to be ferrous, is not sufficient to completely model these highly oxidized assemblages. We propose new constraints on the P–T evolution of the glaucophane schists and associated rocks from Vroulidia Bay (northern Sifnos) using P–T isochemical diagrams (pseudosections) modelled for the oxidized N(K) CFMASHO system [Na2O–(K2O)–CaO–FeO–Fe2O3–MgO–Al2O3–SiO2–H2O], and calculated using solid solution models involving Fe+3-end-members. The resulting P–T conditions could be fitted by a simple trajectory consisting of a smooth clockwise P–T loop, with two distinct high-pressure events at T=450–500 °C, PN2.0 GPa (assemblage A) and T=525–565 °C, PN2.1 GPa (assemblage B), respectively, followed by cooling and decompression. Geospeedometry based on 40Ar/39Ar data, however, allows constraint as to the duration of individual thermal excursions associated with these events. Should these limits be taken into account, the P–T envelope derived in this paper encompasses a more complex P–T history, with individual excursions that potentially involve relatively short periods of temperature increase±pressure variation.