Origin, HP/LT metamorphism and cooling of ophiolitic melanges in southern Evia (NW Cyclades), Greece

Basic and ultrabasic blocks within ophiolitic melanges of the Cycladic Blueschist Unit in southern Evia provide a detailed insight into its ocean floor igneous and hydrothermal evolution, as well as the regional poly-metamorphism occurring during Alpine orogenesis. The upper structural levels (Mt. Ochi exposures) are dominated by metamorphosed wehrlites, gabbros and highly light rare earth element (LREE)-enriched pillow basalts, whereas the underlying Tsaki melange consists of basic protoliths with much less fractionated REE patterns as well as mantle harzburgites. Most of the metabasites show Nb anomalies, indicative of derivation from a subduction-affected mantle. The igneous bodies were juxtaposed and incorporated into the enclosing sedimentary sequences prior to high-pressure/low-temperature (HP/LT) metamorphism (M1). Glaucophane, epidote, sodic clinopyroxene and high-Si phengite constitute the Eocene M1 assemblage, which is estimated to have formed at > 11 kbar and 400-450 °C. High δ¹⁸O values of M1 minerals in Ochi metagabbros indicate that the formation of the high-pressure assemblage was controlled by infiltration of fluids from the dehydrating host sediments. Cooling during decompression is indicated by an overprinting (M2, Early Miocene) pumpellyite-actinolite facies assemblage in metabasic rocks, calculated to have developed at P < 8 kbar and T < 350 °C. Possible mechanisms for such cooling include: exhumation from shallower burial levels relative to the eclogites of the NW Cyclades, accretion of colder rocks from below and extensional unroofing by low-angle normal faults and detachments. The occurrence of sodic augite in the M2 assemblage of Tsaki metagabbros indicates that rocks at the base of the Blueschist Unit cooled faster or longer than their higher level Ochi counterparts. This suggests that differential cooling of the blueschists was enhanced by the underthrusting of colder rock units.