Geochronology (Ar/Ar and K-Ar) of the South Atlantic post-break-up magmatism

This work integrates the available geological information and geochronology data for the Cretaceous-Recent magmatism in the South Atlantic, represented by onshore and offshore magmatic events, including the oceanic islands along the transform faults and near the mid-ocean ridge. The analysis of the igneous rocks and their tectonic settings allows new insights into the evolution of the African and Brazilian continental margins during the South Atlantic opening. Following the abundant volcanism in the Early Cretaceous, the magmatic quiescence during the Aptian-Albian times is a common characteristic of almost all Brazilian and West African marginal basins. However, rocks ascribed to the Cabo Granite (104 Ma) are observed in NE Brazil. In West Africa, sparse Aptian-Albian ages are observed in a few coastal igneous centers. In the SE Brazilian margin, an east-west alkaline magmatic trend is observed from Pocos de Caldas to Cabo Frio, comprising igneous intrusions dated from 87 to 64 Ma. Mafic dike swarms trending NW also occur in the region extending from the Cabo Frio Province towards the Central Brazilian Craton. On the West African side, Early Cretaceous-Recent volcanism is observed in the Walvis Ridge (139 Ma), the St Helena Ridge (81 Ma) and the Cameroon Volcanic Line (early Tertiary-Recent). Volcanic islands such as Ascencion (1.0-0.65 Ma), Tristao da Cunha (2.5-0.13 Ma) and the St Helena Islands (12 Ma) most probably correspond to mantle plumes or hot spots presently located near the mid-Atlantic spreading center. Within the South America Platform and deep oceanic regions, the following volcanic islands are observed: the Rio Grande Rise (88-86 Ma), Abrolhos (54-44 Ma), the Vitoria-Trindade Chain (no age), Trindade (2.8-1.2 Ma) and Fernando de Noronha (12-1.5 Ma). There are several volcanic features along the NW-SE-trending Cruzeiro do Sul Lineament from Cabo Frio to the Rio Grande Rise, but they have not been dated. The only known occurrence of serpentinized mantle rocks in the South Atlantic margin is associated with the Saint Peter and Saint Paul Rocks located along the Sao Paulo fracture zone. The Cameroon Volcanic Line in NW Africa is related to the magmatism that started in the Late Cretaceous and shows local manifestations up to the Present. The compilation of all available magmatic ages suggests an asymmetrical evolution between the African and South America Platforms with more pre-break-up and post-break-up magmatism observed in the Brazilian margin. This is most likely to have resulted from the different geological processes operating during the South Atlantic Ocean opening, shifts in the spreading centre, and, possibly, the rising and waning of mantle plumes. Supplementary material: A complete table with radiometric dates that have been obtained by universities, government agencies and research groups is available at: www.geolsoc.org.uk/SUP18596