Significance of gabbronorite occurrence in the crustal section of the Semail ophiolite

The eastern end of the Haylayn massif exposes a complex paleoridge structur e interpreted as the tip of a northwestward propagating segment (Nicolas et al., this issue). The area, revisited from a petrostructural and geochemic al viewpoint, offers the most documented exposures of the association of ol ivine gabbros and gabbronorites in Oman (Juteau et al., 1988). Gabbronorite s were injected while the main gabbro unit was deforming in the magmatic st ate. Both units do not differ chemically, except for the SiO2 enrichment of the orthopyroxene-rich gabbros relative to olivine-gabbro. In addition, th ey display the same trace element signature, which implies the same parent magma for both units. The extension of the stability field of orthopyroxene is assigned to increase of oxygen fugacity due to hydration. The source of hydration is the ridge axis hydrothermal circulation, suggesting hydrother mal/magma interaction at temperatures above the gabbro solidus. The distrib ution of gabbronorites at the scale of the entire ophiolite suggests a rela tion with ridge tectonics where high-T conditions of hydrothermal-magmatic interaction are met. Such conditions are met when propagating segments rota te the structures of the dying magma chamber.