Diverse P-T paths for the Cycladic ultramafic rock associations: Constraints on their origin, emplacement and exhumation

Abstract

Within the Alpine orogen of the Cyclades ultramafic rocks occur in diverse tectonic positions, are associated with a variety of rock sequences and are regionally metamorphosed at variable conditions. Here this diversity is interpreted in terms of variable mechanisms for incorporation of mantle rocks into orogenic belts and used to better constrain spatial heterogeneity of P-T burial and exhumation paths in the Cyclades. The Cycladic Blueschist Unit (CBU) was metamorphosed first at high pressure conditions in the Eocene (M1) and variably overprinted during exhumation in the Early Miocene (M2). On the islands of Syros and Evia the CBU includes relatively thin HP-LT ophiolitic mélange intervals hosted by meta-sedimentary sequences. Meta-serpentinites are associated with eclogitized metagabbro and metabasites on Syros, but with epidote blueschists on Evia. Given the wide and comparable geochemical heterogeneity in the Syros and Evian mélanges, the Syros metabasites record higher M1 temperatures (450–500°C) relative to their Evian counterparts (400–430°C). The M2 overprint is manifested in Syros by greenschist facies assemblages (~450°C), whereas pumpellyite-actinolite assemblage (~350°C) overgrew blueschists in Evia. Within the M2 migmatized leucogneiss core of the Naxos dome (~700°C) meta-peridotite lenses are the sole preservers of pre-M2 history. The Naxos meta-peridotites possess relict mantle assemblage, fertile spinel lherzolite chemical composition and mantle-like oxygen isotope ratios. Thus unlike ophiolite-associated ultramafics in the Cyclades that were serpentinized prior to metamorphism, the Naxos metaperidotites were directly emplaced from the subcontinental mantle into an underthrust continent during collision and HP metamorphism. Ultramafic assemblages constrain M1 temperatures on Naxos to 550–650°C. It follows that going southeastwards from Evia through Syros to Naxos progressively deeper levels of the subducted plate are exposed. Correspondingly temperatures of the M2 overprint also increase. 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 modeling of tectonically thickened crust unroofed by either erosion or uniform extension.