Eur. J. Mineral. 2001, 13, 901–920

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

FABIEN TROTET1*,OLIVIER VIDAL1 and LAURENT JOLIVET2

1Laboratoire de G´eologie de l’Ecole Normale Sup´erieure, 24 rue Lhomond, F-75005, Paris, France *Corresponding author, e-mail: [email protected] 2D´epartement de G´eotectonique, URA Tectonique, Universit´ePierreetMarieCurie, T26-0 E1, case 129, 4 place Jussieu, F-75252 Paris Cedex 05, France

Abstract: 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. Key-words: exhumation, Cyclades, Sifnos, Syros, multi-equilibrium thermobarometry, P-T paths.

1 Introduction first one assumes that the greenschists, blueschists and eclogites were derived from a single structural The Cyclades area is a classic example of a high- level, and that the greenschist-facies overprint re- pressure belt in a back-arc environment. In many sults either from pervasive fluid infiltration (Matt- islands of the Aegean domain, well-preserved hews & Schliestedt, 1984; Schliestedt & Matthews, high-pressure rocks occur below greenschists or 1987) or from a temperature increase (Wijbrans et amphibolites associated with post-orogenic shear al., 1990) at the base of the metamorphic pile. An- zones (Lister et al., 1984; Avigad & Garfunkel, other interpretation has been proposed by Avigad 1991; Gautier & Brun, 1994a and b). However, it is (1993) and Lister & Raouzaios (1996), who sug- noteworthy that on Sifnos and Syros, the eclogites gest that the blueschists and greenschists were de- and blueschists generally occur at the top of the rived from different structural levels and were jux- metamorphic pile and are overprinted by green- taposed during the post-orogenic history. Accord- schist-facies assemblages at their base (Avigad, ing to Avigad (1993), this juxtaposition is the result 1993). Contrasting interpretations based on struc- of extensional tectonics whereas Lister & Raouzai- tural, petrographic or geochemical arguments have os (1996) argue for thrusting. These contrasting in- been proposed for the occurrence of these eclogites terpretations have contrasting implications on the and blueschists rocks overlying greenschists. The understanding of blueschist-exhumation processes

0935-1221/01/0013/0901 $ 5.00 DOI: 10.1127/0935-1221/2001/0013/0901 ˇ 2001 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart