Chapter 12 - Brazilian Equatorial Margin: magmatic genesis and evolution

Abstract

The Brazilian Equatorial Margin (BEM) is composed of several seamounts, banks, and guyots, mainly contouring the 1200-m isobath through all its northeast borders. The BEM is composed of R-T fracture zones associated with Riedel shear structures in the NW–SE direction where volcanic rocks, such as the São Luís Guyot and Belém Seamount, were emplaced. Moreover, important fractures are observed parallel to the Mesoceanic ridge (Romanche, Saint Paul, 4N, and Chain) with Romanche and Saint Paul being an active pair of transform faults configuring the largest directional offset in the Equatorial Atlantic with related volcanism in the E–W transform faults. The analyzed samples in this chapter are related to the Cenozoic onshore volcanism (Rio Grande do Norte Basalts, Fortaleza Alkaline Volcanic Province, Macau-Queimadas Alignment, and Fernando de Noronha Archipelago) and for three seamounts: Ceará and Belém Seamounts and São Luís Guyot with estimated ages younger than <38Ma. They are composed of mugearite, and alkali basalts enriched in REEL and with a steep REE curve indicating residual garnet in the mantle source. BEM seamounts present a Dy/Yb ratio lower than the garnet-lherzolite melting curve, implying a larger contribution from the spinel-lherzolite mantle source in relation with a pure garnet-lherzolite mantle source. As noted in the Sr-Nd-Pb isotopic ratios diagrams, the Belém Seamount plots away from the Ceará Seamount and São Luís Guyot and the Rio Grande do Norte Basalts and the Fernando de Noronha basanites and nephelinites cluster, which may be indicative of a long-term enriched mantle source. As observed in almost all offshore Brazilian basalts, these ocean island basalts with EMI-EMII signatures in the South Atlantic Ocean may be related to the Brazilian Neoproterozoic continental lithosphere which was delaminated and contaminated a zone in the South Atlantic asthenosphere.