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

摘要

New petrological data on magmatic rocks obtainedfrom the Iberia Abyssal Plain and from the Gorringe Bank,combined with those already known on the Galicia Band, allow tobetter constrain the formation of the West Iberian Margin. Thesethree zones were sampled along East-West transects of the ocean-continent transition, immediately West or at the foot of the lasttilted continental block of the margin.These sections expose similar lithological successionsincluding scarce basalts, locally pillowed, resting directly onperidotites (predominant) which themselves include layers ofpyroxenites and rare lenses of gabbros and gabbroic differentiates.The latter the locally strongly sheared and metamorphosed e.g. tochlorite schists.At the Galicia Bank the exposed undepleted lherzolites areconsidered to be typical of a sub-continental lithospheric mantleenvironment. The mantle sections exposed at both the IberiaAbyssal Plain and the Gorringe Bank are clearly different. Thepresence of plagioclase-bearing websterite lenses and of smallalkaline pyroxenites within the predominant harzburgites, isunknown in pure oceanic environments and supports theirlithospheric sub-continental origin.Along each transect, the mineralogy of the gabbroic seriesdocuments various depths of emplacement and deformation. Thehighest pressures near 0.8 GPa (~25km depth) at Galicia Bank andIberia Abyssal Plain are estimated from Al-rich pyroxenes andfrom the occurrence of metamorphic spinels in the sheared gabbros(Iberia Abyssal Plain). These gabbros are interpreted as derivingfrom underplated magmas at the base of a slightly thinnedcontinental crust. The lower pressures (≤0.6 GPa) registered inthe Gorringe Bank gabbroic rocks and in some reexamined gabbrosfrom the Galicia Bank, suggest the existence of successiveintrusions during the stretching process.The few basalts sampled on top of the exposed peridotites ofthe sea floor are inferred to be among the first post-rift extrusivemagmas. They differ slightly from the North to the South of themargin: enriched and transitional tholeiites occur at Galicia Bankand in the Iberia Abyssal Plain, transitional and depleted tholeiitesat the Gorringe Bank. The trace elements and the isotope dataavailable show that the basalts and the gabbros derived fromsimilar heterogeneous mantle sources (10.3≥?NdT≥3.6). Thebehavior of some incompatible elements (Nb in particular)documents locally either a possible fractionation of rutile inshallow lithospheric mantle or some contamination by thecontinental crust. A very slow lithospheric stretching, assisted bythe intrusion, underplating, and the shearing of a few gabbroic sills,followed by the unroofing of heterogeneous subcontinental mantlecan account for the formation of the entire West Iberian margin.