Lithosphere–asthenosphere interaction and the origin of Cretaceous tholeiitic magmatism in Northeastern Brazil: Sr–Nd–Pb isotopic evidence

Rio Ceará-Mirim Swarm forms a 400 km long, mainly E–W trending dykes which have been emplaced in Northeastern Brazil during the opening of South and Equatorial Atlantic oceans at Early Cretaceous. This swarm is dominated by high- and low-Ti tholeiitic dykes showing EM-type chemical characteristics, such as: (i) strong to moderate enrichment in large-ion lithophile and high-field strength elements, and (ii) radiogenic Sr (0.70713 to 0.70473) and non-radiogenic Nd (0.512494 to 0.512) and Pb (206Pb/204Pb 18.77 to 18.34) isotopic compositions. They are compatible with the melting of an old (TDM ages ca. 1.0 Ga) enriched lithospheric mantle source, indicating that both Proterozoic- and Mesozoic-related mantle processes accounted for the genesis of continental tholeiitic magmatism. Conversely, a distinct chemical component, comparable to HIMU-type, has been identified in local alkaline sub-swarm. Such dykes have depleted Sr (0.70367–0.7031) and Nd (0.5129-0.512772), and strongly radiogenic 206Pb/204Pb (19.95) ratios. The source of the Rio Ceará-Mirim basalts appears to be related to melting of an enriched lithospheric mantle triggered by a long-lived thermal anomaly. It could be related either to deep (St. Helena) mantle plumes or convective cells induced by lithospheric discontinuities.