Plume-Lithosphere Interaction during Migration of Cretaceous Alkaline Magmatism in SW Portugal: Evidence from U-Pb Ages and Pb-Sr-Hf Isotopes

Large massifs of alkaline rocks are exposed along similar to 250 km of the Atlantic coast of Portugal. Their origin is still poorly understood, including the precise timing of their emplacement and their relationships with the well-constrained alkaline magmatic rocks that occur 200-1000 km offshore. To elucidate the precise timing and origin of the alkaline magmatism in this region, magmatic rocks from the three major alkaline massifs (Sintra, Sines, and Monchique) and an isolated diorite intrusion (Ribamar, north of Sintra) have been dated by the U-Pb method on titanite and zircon and characterized based on their Pb, Sr, and Hf isotopic compositions obtained on feldspar and zircon. From north to south, the resulting ages are: 88 center dot 3 +/- 0 center dot 5 Ma (95% confidence level) for Ribamar, 83 center dot 4 +/- 0 center dot 7, 82 center dot 0 +/- 0 center dot 7, 81 center dot 7 +/- 0 center dot 4, and 80 center dot 1 +/- 1 center dot 0 Ma for the Sintra complex, 77 center dot 2 +/- 0 center dot 6, 77 center dot 2 +/- 0 center dot 4, and 76 center dot 1 +/- 1 center dot 3 Ma for the Sines massif, and 70 center dot 0 +/- 2 center dot 9 and 68 center dot 8 +/- 1 center dot 0 Ma for the Monchique complex. Initial isotopic compositions of Pb in feldspars are in the range of 18 center dot 522-19 center dot 299 for Pb-206/Pb-204, 15 center dot 555-16 center dot 007 for Pb-207/Pb-204, and 38 center dot 480-39 center dot 330 for Pb-208/Pb-204. Initial Sr-87/Sr-86 of feldspars varies between 0 center dot 70274 and 0 center dot 70481 and initial Hf isotope ratios yield epsilon Hf-i values between +3 center dot 7 and +9 center dot 6. These results, together with major, trace, and rare earth element analyses, show that the ages, (Pb-207/Pb-204)(i), and (Sr-87/Sr-86)(i) increase northward, whereas the alkaline affinity, (Pb-206/Pb-204)(i), and epsilon Hf-i increase southward, substantiating a north-south trend of geochemical and age variation. The isotopic composition of the studied rocks can be explained by partial melting of a sub-lithospheric mantle source with an enriched DMM (Depleted MORB Mantle) signature and subsequent contamination by the metasomatized Iberian subcontinental lithospheric mantle (SCLM). The north-south age trend is in agreement with the motion of the Iberian plate between 88 and 60 Ma. The spatial and temporal variations in the isotopic signatures are explained by differences in the contribution of the two source components. The sub-lithospheric mantle-derived magmas are more contaminated by the SCLM in the northern part of the alignment, compared with the southern part of the studied region, where the rocks have isotopic signatures closer to those of enriched sub-lithospheric mantle. Our data are incorporated into a geodynamical model that explains the overall distribution of alkaline magmatism in this part of the eastern Central Atlantic, and provide new constraints on the occurrence of alkaline magmatism along the Iberian margin and the NW African plate. The spatial distribution of the magmatism is directly correlated with the motion of the Iberian plate above a deep-rooted thermal anomaly (mantle plume) that has caused magmatism since the Cretaceous.