Most previously dated zircons in high-pressure mélange blocks and associated matrix rocks from Tinos and Syros (Cyclades), Greece, yielded ion microprobe (SHRIMP II) U-Pb ages of ca. 80 Ma. In many cases it remains unclear whether the zircons are igneous or metamorphic/hydrothermal in origin. Oxygen and hafnium isotope ratios in the dated zircons have been determined to further constrain their mode of formation. Spot analysis of zircons from metagabbro, eclogite, glaucophanite, jadeitite and chlorite schists by ion microprobe (CAMECA IMS-1280) yields a large range in δ18O, varying from 2.0 ‰ to 7.6 ‰ VSMOW. The average δ18O values for most zircons in 11 samples however fall within a relatively small compositional range between 4.7 ‰ and 5.5 ‰, which is consistent with an igneous origin. These values suggest a relationship to magmas typical for modern oceanic crust or to precursors that had been in equilibrium with primitive magma compositions or the mantle (5.3 ± 0.6 ‰, 2 SD). The δ18O (mantle-like) and initial epsilon hafnium values [ɛHf(t) = +10 to +24] suggest that the 80 Ma old zircons are igneous in origin and crystallised from magmas that were derived from depleted mantle. Scanning Electron Microscope cathodoluminescence imaging indicates that lower-δ18O zircons (grains or domains; down to 2.0 ‰), mostly from one exceptional eclogite sample (average 4.3 ± 0.8 ‰, 2 SD, n = 23), either represent cauliflower-like internal structures or weakly zoned (or porous) features in outer rims. Previous age dating of zircon domains with cauliflower-like internal structures indicated apparent ages that are considerably younger (ca. 54 Ma and ca. 57 Ma) than the Cretaceous age determined for the majority of the zircon population. Taken together, these observations suggest that the low-δ18O zircons (and three high-δ18O zircons, > 6.0 ‰) are genetically linked to a well-documented Eocene high-pressure metamorphic event (ca. 53–40 Ma), but post-magmatic seafloor hydrothermal alteration cannot be completely ruled out.