A metamorphic overprint on the pseudostratigraphy of ophiolite complexes in southern Chile shows an extremely steep vertical metamorphic gradient passing downward from zeolite to amphibolite facies in 2 km, followed by a transition to fresh gabbros. Burial metamorphism does not explain either the steep metamorphic gradient or the abrupt termination of this metamorphic effect. A combination of hydrothermal and contact metamorphism associated with circulation of seawater and igneous and tectonic activity at a spreading center can better explain these observations. Such a model is supported by 18O/16O isotope data. Disequilibrium textures indicate that spreading rapidly removes amphibolized metagabbros from the zone of effective metamorphism, while overlying rocks undero greenschist metamorphism in a less restricted region in the vicinity of the spreading center. This model is consistent with metamorphic features displayed by other ophiolites and ocean floor metamorphism. The metamorphism produces a zone of metagabbros (amphiobolites) underlain by fresh gabbros, which may account for the transition from seismic layer 3A to 3B. The amphibolites in the Chilean ophiolites contain abundant skeletal networks of secondary opaque minerals similar to those observed in ocean floor amphibolites from dredge hauls, which preserve higher-remnant magnetism than either fresh gabbros or the gabbros, dikes, and pillow lavas in the greenschist facies. A zone of amphibolitized metagabbros with high natural remnant magnetism could make a significant contribution to magnetic anomalies, particularly away from the ridge, where the remnant magnetism of the quenched lava of layer 2 is progressively reduced by metamorphism from below and weathering from above.