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.