a b s t r a c tThe onshore sector of the West Iberian Margin (WIM) was the locus of several cycles of magmatic activity during the Mesozoic, the most voluminous of which was of alkaline nature and occurred between 70 and 100 Ma. This cycle took place in a post-rift environment, during the 35 counter-clockwise rotation of Iberia and initiation of the alpine compression. It includes the subvolcanic complexes of Sintra, Sines, and Monchique, the volcanic complex of Lisbon and several other minor intrusions, covering an area of approximately 325 km 2 . Previous cycles were tholeiitic and transitional in nature, occuring around 200 Ma and 130-135 Ma, respectively. New LA-ICP-MS U-Pb, 40 Ar/ 39 Ar, K-Ar and Rb-Sr ages on several intrusions distributed along the onshore WIM are presented, which combined with previously published data allows us to constrain the duration of the Late Cretaceous alkaline cycle to circa 22 Ma (94-72 Ma) and define two pulses of magmatic activity. The first one (94-88 Ma) occurred during the opening of the Bay of Biscay and consequent rotation of Iberia and clusters above N38 20 0 . The second pulse (75-72 Ma) has a wider geographical distribution, from N37 to N39 . This final pulse occurred during the initial stages of the Alpine orogeny in Iberia that led to the formation of the Pyrenees and Betics and to tectonic inversion of the Mesozoic basins. Isotope and trace element geochemistry point to a sublithospheric source for the alkaline magmatism that clearly distinguishes it from the previous cycles which had an important lithospheric mantle component. Also, it allows the discrimination between the two different alkaline pulses in terms of trace element abundance and residual mantle minerology. It is speculated that these differences might be the result of distinct magma ascent rates due to either more or less favourable tectonic settings that avoided or allowed the interaction with metasomatized lithosphere and equilibration with K rich minerals like amphibole and/or phlogopite.

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.

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