Résumé
We present a review of structures in the Oman ophiolite that can be related to off-axis magmatism in the lower crust and uppermost mantle at fast spreading ridges. First, we show evidence that a ridge axis is preserved within the ophiolite. As this axis can be used as a reference frame, we distinguish three cases of off-axis magma injection near the Moho or in the lower crust. The first two cases, though partly situated off-axis, are related to excess magmatism caused by mantle diapirs rising beneath the ridge axis, while the third is linked to the emplacement of a diapir far from the axis. (1) The Moho transition zone (MTZ) beneath the ridge axis is normally a few meters thick, but increases up to several hundred meters thick above mantle diapirs, within a ten Km radius. It forms from underplating of successive melt pulses. Once melt ponds at the base of the MTZ, it transforms harzburgite into dunite; then most of the melt is compacted and injected into the crust, but a portion forms plurimetric gabbro lenses within the dunite. Eight to ten Km away from the axis, the MTZ thickness is greatly reduced, thus on-axis diapiric emplacement may not induce off-axis magmatism at this level further than 10 Km away from the axis. (2) Pseudo "wehrlites" that form up to 30% of the lower crust define a second type of off-axis melt body. These intrusions are rooted in the MTZ where dunite is turned into an olivine-rich mush upon melt delivery, so they may be considered as chunks of MTZ intruded into the crust. Though many of these intrusions are tectonically transposed by the magmatic flow that affects lower crustal gabbros, showing that they were injected within the magma chamber, others are undeformed intrusions, showing that it was emplaced in an off-axis melt-poor crust, probably away from the crustal seismic low velocity zone. (3) We mapped an off-axis diapir located 30 km from the ridge axis that may be the root of an incipient seamount. The overlying MTZ is up to a thousand meters thick and contains pyroxenite instead of gabbro lenses in dunite. Above, and at the diapir periphery, melt intrudes a cooled lithosphere and forms amphibole-rich-microgabbro intrusions. Low epsilon Nd may suggest that off-axis melting occurred because of a compositional anomaly such as enriched veins. However, at the MTZ we find low incompatible element contents and LREE depletion more marked than that of typical MORB. We explain this compositional paradox using geological evidence of assimilation and mixing between incoming melt and the pre-existing depleted and hydrated off-axis lithosphere that drifted from the rise axis.