High-precision P-T estimates for retrogressed kyanite eclogites from Thermes, central Rhodope (Greece)

The Rhodope massif in northern Greece and southern Bulgaria constitutes the hinterland of the Hellenide orogen. It exposes low- and high-grade metamorphic rocks and their sedimentary cover. Kyanite eclogites from Thermes-Rhodope (northern Greece) belonging to the structurally upper unit were studied in order to constrain their metamorphic conditions. The kyanite eclogites are boudins enclosed in quartzofelspathic gneisses. They experienced a polyphase metamorphic history involving equilibration at granulite-, amphibolite- and greenschist-facies conditions successively. Textural relations reveal the successive equilibrium mineral assemblages and provide constraints that very local, domainal equilibria were attained during metamorphic evolution. Omphacite formed symplectites of diopsidic pyroxene and plagioclase during decompression while garnet formed coronas of two amphiboles (ortho- and clino-), plagioclase and magnetite. The orthoamphibole is sodic gedrite with the most sodic composition found in the literature. Symplectites of plagioclase, spinel and corundum formed at the expense of kyanite suggesting some metasomatic process. During metasomatism the mineral chemistry and the local composition of the equilibration volume were modified by diffusion processes, thus nullifying any assumption that the system was closed. Conventional geothermobarometric methods and thermodynamic modelling were combined to decipher the evolution in the rock mineral assemblages as a response to P-T conditions. Modelling revealed that the formation of sapphirine, corundum, spinel and plagioclase symplectites after kyanite is only possible during decompression at pressures less than 0.8GPa.