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

Abstract

The West Iberian Margin (WIM) preserves onshore testimonies of three Mesozoic magmatic cycles. In this paper, we present and discuss 40 Ar/ 39 Ar 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 SiO 2 -saturated and oversaturated rocks at "high" pressure. Magmas sampled a source of fairly homogenous composition characterized by Sr and Nd isotopic compositions (Nd i from +1.6 to +4.2), more enriched than the typical N-MORB 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 SiO 2 -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- A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT3 magmatic processes involving neighbouring Jurassic evaporite materials, leading to an increase in the Na 2 O content and 87 Sr/ 86 Sr ratio.

Interplay of tectonics and magmatism during post‐rift inversion on the central West Iberian Margin (Estremadura Spur)

“…The WIM was the locus of substantial alkaline magmatic activity during the Late Cretaceous and contrasts both in whereas the second cycle (140-148 Ma) is mildly alkaline, showing 1.6 < εNd i < 4.2 and initial Sr isotope ratios close to the CHUR 145 (Grange et al, 2008;Mata et al, 2015). Conversely, the Late Cretaceous magmatism is characterised by εNd i > 5 and initial Sr isotope ratios affine to a sublithospheric mantle source with a time-integrated evolution F I G U R E 2 Simplified regional stratigraphy of the West Iberian Margin (modif.…”

“…Conversely, the Late Cretaceous magmatism is characterised by εNd i > 5 and initial Sr isotope ratios affine to a sublithospheric mantle source with a time-integrated evolution F I G U R E 2 Simplified regional stratigraphy of the West Iberian Margin (modif. Pereira & Alves, 2012), associated magmatic cycles (Martins et al, 2008;Mata et al, 2015;Miranda et al, 2009) and major tectonic events controlling the evolution of the margin. LBS, Lithospheric Breakup Sequence EAGE PEREIRA Et Al.…”

“…Simplified regional stratigraphy of the West Iberian Margin (modif. Pereira & Alves, 2012), associated magmatic cycles (Martins et al., 2008; Mata et al., 2015; Miranda et al., 2009) and major tectonic events controlling the evolution of the margin. LBS, Lithospheric Breakup Sequence…”

“…Magmatic activity on passive margins is typically recorded at magma‐rich domains, contrasting with neighbouring segments where the evidence of magma is scarce both in time and space, that is, magma‐poor margins (Franke, 2013; Geoffroy, 2005). However, the magma‐poor WIM is in some respects unique, as three magmatic cycles are described associated with different stages of its evolution and depicting distinct geochemical affinities and tectonic settings (see Mata et al., 2015 for a review). The first two are associated with rifting extension (Martins et al., 2008; Mata et al., 2015), whereas the latter, corresponds to a post‐rift and syn‐tectonic event dissimilarly affecting the continental and oceanic domains of the margin (Grange et al., 2010; Merle et al., 2009; Miranda et al., 2009).…”

“…However, the magma‐poor WIM is in some respects unique, as three magmatic cycles are described associated with different stages of its evolution and depicting distinct geochemical affinities and tectonic settings (see Mata et al., 2015 for a review). The first two are associated with rifting extension (Martins et al., 2008; Mata et al., 2015), whereas the latter, corresponds to a post‐rift and syn‐tectonic event dissimilarly affecting the continental and oceanic domains of the margin (Grange et al., 2010; Merle et al., 2009; Miranda et al., 2009). Geophysical models have been recently applied for the first time on the proximal margin to help characterising in detail some of the outcropping alkaline massifs (Neres et al., 2014; Ribeiro et al., 2013; Terrinha et al., 2017; Figure 1b), and to carry out gravimetry‐magnetic modelling of conspicuous subsurface features off SW Iberia (Neres et al., 2018).…”