Exhumation of Syros and Sifnos metamorphic rocks (Cyclades, Greece). New constraints on the P-T paths

Three different levels are recognised in the metamorphic pile of Syros and Sifnos. Well-preserved eclogites and blueschists occur at the top, whereas they are overprinted in the albite-epidote blueschist facies below, and in greenschist facies at the base. A multi-equilibrium approach involving several end members and solid-solution models for chlorite, phengite, and garnet is used to estimate the stability P-T conditions of different parageneses. The combination of textural criteria with the results of P-T calculations is consistent with the assumption of local equilibrium. Contrasted P-T conditions are inferred for successive parageneses coexisting out of equilibrium in the same samples. When considering all the P-T estimates together, contrasted P-T trends can be distinguished for the three different levels of the metamorphic pile. These trends are interpreted as the signature of the P-T paths followed by the rocks during their exhumation. The preservation of eclogitic parageneses at the top of the structural pile is correlated with cooling during decompression. In contrast, the progressive retrogression observed in the underlying rocks, toward the base of the metamorphic pile, is correlated with an increasing isothermal decompression before cooling.