Basalts, underplated gabbros and pyroxenites record the rifting process ofthe West Iberian margin

New petrological data on magmatic rocks obtained from the Iberia Abyssal Pl ain and from the Gorringe Bank, combined with those already known on the Ga licia Bank, allow to better constrain the formation of the West Iberian Mar gin. These three zones were sampled along East-West transects of the ocean- continent transition, immediately West or at the foot of the last tilted co ntinental block of the margin. These sections expose similar lithological successions including scarce bas alts, locally pillowed, resting directly on peridotites (predominant) which themselves include layers of pyroxenites and rare lenses of gabbros and ga bbroic differentiates. The latter are locally strongly sheared and metamorp hosed e.g. to chlorite schists. At the Galicia Bank the exposed undepleted lherzolites are considered to be typical of a sub-continental lithospheric mantle environment. The mantle s ections exposed at both the Iberia Abyssal Plain and the Gorringe Bank are clearly different. The presence of plagioclase-bearing websterite lenses an d of small alkaline pyroxenites within the predominant harzburgites, is unk nown in pure oceanic environments and supports their lithospheric sub-conti nental origin. Along each transect, the mineralogy of the gabbroic series documents variou s depths of emplacement and deformation. The highest pressures near 0.8 GPa ( similar to 24 km depth) at Galicia Bank and Iberia Abyssal Plain are est imated from Al-rich pyroxenes and from the occurrence of metamorphic spinel s in the sheared,gabbros (Iberia Abyssal Plain). These gabbros are interpre ted as deriving from underplated magmas at the base of a slightly thinned c ontinental crust. The lower pressures ( less than or equal to 0.6 GPa) regi stered in the Gorringe Bank gabbroic rocks and in some reexamined gabbros f rom the Galicia Bank, suggest the existence of successive intrusions during the stretching process. The few basalts sampled on top of the exposed peridotites of the sea floor are inferred to be among the first post-rift extrusive magmas. They differ slightly from the North to the South of the margin: enriched and transition al tholeiites occur at Galicia Bank and in the Iberia Abyssal Plain, transi tional and depleted tholeiites at the Gorringe Bank. The trace elements and the isotope data available show that the basalts and the gabbros derived f rom similar heterogeneous mantle sources (10.3 greater than or equal to eps ilon Nd-T greater than or equal to 3.6). The behavior of some incompatible elements (Nb in particular) documents locally either a possible fractionati on of rutile in shallow lithospheric mantle or some contamination by the co ntinental crust. A very slow lithospheric stretching, assisted by the intru sion, underplating, and the shearing of a few gabbroic sills, followed by t he unroofing of heterogeneous subcontinental mantle can account for the for mation of the entire West Iberian margin.