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-Mg O-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, P > 2.0 GPa (assemblage A) and T = 525-565 °C, P > 2.1 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.