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

Abstract

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 on40Ar/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.