emplacement of ultramafic rocks (Aegean Sea, Greece): insights from diverse origins and modes of The geodynamic evolution of the Alpine orogen in the Cyclades Geological Society, London, Special Publications

The Alpine orogen in the Cyclades, wherein both high-pressure metamorphic rocks and ultramafic rocks co-occur, is a key area in studying the emplacement of mantle rocks into the crust. Within the Cyclades three distinct ultramafic associations occur: (1) HP–LT ophiolitic melanges of the Cycladic Blueschist Unit (CBU) on Evia and Syros; (2) meta-peridotites associated with migmatized leucogneisses on Naxos, which represent the deepest exposed levels of the CBU; (3) a greenschist-facies metamorphosed dismembered ophiolite juxtaposed on top of the CBU by an extensional detachment on Tinos. Most of the Cycladic ultramafic rocks were serpentinized prior to Alpine metamorphism, suggesting denudation prior to reburial. The Naxos metaperidotites preserve, however, relict mantle assemblage and mantle-like oxygen isotope ratios, and thus indicate direct emplacement from the mantle into an underthrust continent during collision and HP metamorphism (M1). Thus conditions for M1 in the Naxos leucogneiss core are constrained by ultramafic assemblages to 550–650 8C and 14 kbar. Mafic blocks of the ophiolitic melanges in the NW Cyclades span a wide range of chemical compositions indicating derivation from variable oceanic settings and sequential events of alteration and metasomatism. Given the comparable geochemical heterogeneity in the Syros and Evian melange intervals, the garnetbearing meta-basites of the Syros melange record higher M1 temperatures (450–500 8C) than the garnet-free epidote blueschists of the Evian melanges (400–430 8C). It follows that going southeastwards from Evia progressively deeper (i.e. hotter) levels of the subducted plate are exposed. Correspondingly, temperatures of the M2 overprint also increase from pumpellyitebearing assemblages on southern Evia, through greenschists on Syros to upper-amphibolite, sillimanite-bearing gneisses on Naxos. The diverse P–T paths of the CBU form an array wherein the deeper a rock sequence is buried, the ‘hotter’ is its exhumation path. Such a pattern is predicted by thermal modelling of tectonically thickened crust unroofed by either erosion or uniform extension. The occurrence of dense ultramafic rocks, peridotites, the prime constituent of the Earth’s mantle, at the surface of the continents requires significant vertical mobility. It is thus not surprising that orogenic belts where continents have collided and vast tectonic movements have taken place host most of the relatively rare peridotites. High-pressure metamorphic rocks best record the vertical movements involved in orogenesis: eclogites and blueschists mostly comprise surface-derived rocks, thus implying a full tectonic cycle of burial and exhumation. Orogenic segments where both high-pressure and ultramafic constituents occur in proximity are thus key areas in answering a fundamental question: how are mantlederived rocks incorporated into the subduction– exhumation sequence of surficial rocks? A partial answer to the puzzle of displacement of ultramafic rocks into the crust is given by ophiolite suites representing occasional portions of oceanic plates that escaped destruction at subduction zones and were carried onto the foreland of an adjacent continent. Based on their tectonic setting two major types of ophiolites were distinguished (Moores 1982; Coleman 1984; Wakabayashi & Dilek 2003): (1) ophiolites that occur as thick thrust sheets that rest upon passive margin substrate and are commonly associated with high-temperature metamorphic aureoles at their base (e.g. the Semail ophiolite, Oman; the Pindos ophiolite, Greece); (2) ophiolite bodies that occur as blocks within tectonic blueschist melange as part of an accretionary prism (e.g. the Franciscan melange). The differences in the manner of occurrence and tectonic context of the two types reflect their origin and mode of emplacement: Tethyan-type ophiolites formed by thrusting of an oceanic lithosphere slab upon passive continental margin sequences whereas upheaval of oceanic fragments within the accretionary prism of an active margin gave rise to Cordilleran-type ophiolites. Within the Hellenic segment of the Alpine orogenic belt a major Tethyan-type ophiolite From: TAYMAZ, T., YILMAZ, Y. & DILEK, Y. (eds) The Geodynamics of the Aegean and Anatolia. Geological Society, London, Special Publications, 291, 17–40. DOI: 10.1144/SP291.2 0305-8719/07/$15.00 # The Geological Society of London 2007. emplacement occurred in midto late Jurassic times. The ‘Eohellenic’ ophiolites are interpreted as originating from a Mesozoic Neo-Tethyan oceanic basin, the Pindos Ocean, and subsequently thrust northeastwards onto the Pelagonian passive continental margin (Fig. 1; Robertson et al. 1991; Smith 1993). Deep-water sedimentation continued, however, in the Pindos basin until its final closure in the early Tertiary (Jones & Robertson 1991). Within the Cycladic Massif of the Aegean Sea (Fig. 1), a Tertiary high-pressure orogenic segment that lies to the SE of the Hellenides, thin remnants of the Eohellenic ophiolites occur on the island of Paros (Papanikolaou 1980). However, most of the ophiolites in the Cyclades are regionally metamorphosed at variable conditions, they are highly attenuated and dismembered, and are bounded and dissected by low-angle tectonic contacts. The Cycladic ultramafic rocks are associated with a great variety of country rocks including leucogneisses of continental basement origin (the Main Ultramafic Horizon on Naxos; see below), thus raising questions concerning the provenance of peridotites. The diversity in field relations, metamorphic grade and tectonic position of ultramafic rocks makes the Alpine orogen in the Cyclades an attractive terrain to address the questions of their origin and emplacement. Moreover, in a complicated poly-metamorphosed orogenic segment such as the Cyclades, the relative sluggishness of metamorphic reactions in ultramafic rocks turns them into potential preservers of preand early metamorphic evolution invariably effaced by later events in other rocks. In this paper we review the tectonic position and field relations of major ultramafic occurrences in the Cyclades and examine in detail the petrography and chemical compositions of ultramafic and associated rocks. Thus, their origin and mode of emplacement are unveiled and new constraints on the orogenic evolution of the Cyclades are set. Regional geological setting