- Abstract Two spatial scales of fluid–rock interaction in an ophiolite suite are revealed by oxygen isotope and hydrogen isotope studies of metagabbros on the island of Tinos (Cyclades, Greece). Sequentially formed mineral generations in the metagabbros include relict igneous augite, hornblende of sub-seafloor hydrothermal origin, and actinolite and albite formed by regional greenschist-facies metamorphism during orogenesis. With the exception of augite (δ 18 O=4.4–5.6‰), the metagabbros are characterized by unusually high δ 18 O values: hornblende (5.8–7.4‰), actinolite (6.5–10.2‰), feldspar (14.6–14.9‰) and whole rocks (7.0–10.5‰). Hornblende δ D values range from −57 to −66‰. The high δ 18 O values and the δ D range of the hornblendes are compatible with interaction of oceanic gabbro with seawater that had previously been enriched in 18 O/ 16 O (δ 18 O=6.5–8‰) by isotopic exchange at moderate to high temperatures. The high degree of oceanic alteration in the layered gabbros, mass balance calculations of isotopic exchange, and field evidence for early oceanic thrusting suggest that seawater could have penetrated deeply into the ocean crust, becoming 18 O/ 16 O-enriched through isotopic exchange with gabbros at progressively increasing temperature. Upward, down-temperature flow of the high-δ 18 O water would be very effective in elevating the δ 18 O values of gabbros. The regional greenschist metamorphic overprint of the ophiolite, possibly the result of continued thrusting and piling up of nappes during obduction, is characterized by localized fluid–rock exchange. Actinolite in massive gabbroic layers has δ 18 O values (6.5–7.2‰) close to those of the hornblende, whereas in deformed meter-sized gabbroic blocks the amphiboles have significantly higher values (8.4–10.2‰). Likewise, albite in the gabbroic blocks has high δ 18 O values of ca. 15‰ that are ascribed to meter-scale exchange with 18 O-rich fluids derived from dehydration reactions in low-temperature hydrothermally altered basaltic host rock enclosing the blocks. Deformation-enhanced permeability facilitated fluid infiltration in gabbroic blocks, whereas the relatively undeformed, and therefore less permeable, massive gabbros experienced minor interaction with fluids. The orogenic fluid–rock interaction thus represents local-scale redistribution of hydrous mineral components introduced during seafloor hydrothermal exchange.