40Ar/39Ar ages and petrogenesis of the West Iberian Margin onshore magmatism at the Jurassic-Cretaceous transition: Geodynamic implications and assessment of open-system processes involving saline materials

The West Iberian Margin (WIM) preserves onshore testimonies of three Mesozoic magmatic cycles. In this paper, we present and discuss 40Ar/39Ar ages and geochemical data for the second cycle, which occurred at least from 148 Ma to 140 Ma, at the Jurassic-Cretaceous transition, during the late stages of an important extensional event associated with the Iberia-Newfoundland rifting. The related lithospheric stretching induced magma genesis by adiabatic decompression. Primitive rocks are fairly alkaline but evolved to SiO2-saturated and oversaturated rocks at “high” pressure. Magmas sampled a source of fairly homogenous composition characterized by Sr and Nd isotopic compositions (Ndi from +1.6 to +4.2), more enriched than the typical NMORB source. Magmas were generated at the top of the garnet zone. Considering the thickness of the lithosphere and the geochemical constraints, an origin by melting of a metasomatized domain of the lithosphere is favoured. The composition of these onshore magmas is somewhat distinct from the quasi coeval magmas emplaced offshore, which is interpreted as a result of the less important onshore lithospheric stretching, leading to lower degrees of partial melting. This favoured the contribution of lithospheric metasomatized domains to onshore magmas. Rocks intruded two sectors of the Lusitanian Basin separated by the Nazaré Fault and characterized by distinct subsidence rates during the Jurassic. The fact that the rocks to the north of the Nazaré Fault are significantly more evolved indicates the more important development of magma chambers in the north, suggesting distinct thermal profiles for those two sectors. Such magma chambers enabled the “high-pressure” fractionation necessary to drive magma compositions from Ne-normative to SiO2- saturated and -over-saturated. The rocks cropping out south of the Nazaré fault are clearly less evolved, and its variability is mostly due to different partial melting events. Some rocks present evidence of post magmatic processes involving neighbouring Jurassic evaporite materials, leading to an increase in the Na2O content and 87Sr/86Sr ratio.