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

Abstract

Large massifs of alkaline rocks are exposed along ∼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: 88·3 ± 0·5 Ma (95% confidence level) for Ribamar, 83·4 ± 0·7, 82·0 ± 0·7, 81·7 ± 0·4, and 80·1 ± 1·0 Ma for the Sintra complex, 77·2 ± 0·6, 77·2 ± 0·4, and 76·1 ± 1·3 Ma for the Sines massif, and 70·0 ± 2·9 and 68·8 ± 1·0 Ma for the Monchique complex.Initial isotopic compositions of Pb in feldspars are in the range of 18·522–19·299 for 206Pb/204Pb, 15·555–16·007 for 207Pb/204Pb, and 38·480–39·330 for 208Pb/204Pb. Initial 87Sr/86Sr of feldspars varies between 0·70274 and 0·70481 and initial Hf isotope ratios yield εHfi values between +3·7 and +9·6. These results, together with major, trace, and rare earth element analyses, show that the ages, (207Pb/204Pb)i, and (87Sr/86Sr)i increase northward, whereas the alkaline affinity, (206Pb/204Pb)i, and εHfi 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.