The origin and metamorphism of ultrabasic rocks within an alpine leucogneiss terrain, Naxos, Greece

Ultrabasic rocks commonly occur in high P/T orogenic terrains as blocks within deep-basin melanges, associated with other members of the oceanic suite. Eclogitic belts, however, also contain rarer ultrabasic associations, whereby peridotite lenses are enclosed within high-temperature leucocratic gneiss terrains, thus posing the question of the mechanism responsible for the displacement of mantle rocks into the upper continental crust. Ultrabasic bodies occur within the metamorphic complex of Naxos at the contact between a leucogneiss core and a pelite and marble envelope. This sequence has undergone intense metamorphism including an Eocene (M1) blueschist event, overprinted by an Early Miocene (M2) Barrovian-type event reaching amphibolite facies to migmatitic melting in the leucogneiss core. Two main phases of mineral growth are observed in the ultrabasics: (i) Synkinematic medium-grained tabular mosaics of olivine, orthopyroxene, Ca-amphibole, chlorite and spinel. Phase diagram calculations show that at 6-8 kbar (pressures estimated for M2 event) this assemblage is stable at 670-720 degrees C, temperatures that are within the range determined for the sillimanite-bearing pelitic country-rock. Cr- to Al- zoning in spinel is consistent with progressive metamorphism. The similarity in metamorphic temperatures and structural concordance with the country rock indicates syn-M2 phase. (ii) Pre-kinematic coarse-crystalline relics of olivine, green spinel and orthopyroxene, the latter containing exsolution-lamellae of spinel. Thermometry based on Al- and Ca-contents of orthopyroxene indicates temperatures of 1030-1070C, within the spinel-lherzolite stability field. Thus, deep peridotites cooled to below 700 degrees C and were incorporated into the Naxos continental section prior to M2 event. The major and trace element chemistry of the peridotites indicates that they were not serpentinized before M2. Deep-seated emplacement is thus favored, the most probable scenario being tectonic "sampling" of the overriding subcontinental mantle by underthrusting continental slab during Eocene collision and high P/T M1 metamorphism. The steep thermal gradient recorded in the relict hot peridotite suggests derivation from beneath a thinned crust.