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

Abstract

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 midocean 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 centres. In the SE Brazilian margin, an east-west alkaline magmatic trend is observed from Poços de Caldas to Cabo Frio, comprising igneous intrusions dated from 87 to 64 Ma. Mafic dyke 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), Tristão 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 centre. Within

Alkaline Complexes as Potential Areas for Installation of Repositories of Nuclear Waste: Cretaceous Intrusions in Brazil

“…In the Rio de Janeiro State and its adjacent areas, there are a dozen of felsic alkaline intrusive rock bodies forming WNW-ESE alignments (Amaral et al, 1967;Almeida, 1976), called Poços de Caldas -Cabo Frio alkaline rock alignment (Figure 1), such as Itatiaia, Morro Redondo, Tinguá, Mendanha, Itaúna, Tanguá, Rio Bonito, Soarinho, Morro dos Gatos, Morro de São João, Cabo Frio Island (Motoki, 1979;Brotzu et al, 1989Brotzu et al, , 2007Mota et al, 2009;Motoki et al, 2012;Geraldes et al, 2013).…”

“…These rocks are relatively common in the Rio de Janeiro State and extracted as highly qualified as ornamental construction material (Petrakis et al, 2009). Some of the intrusive complexes such as Itatiaia, Mendanha (Motoki et al, 2012), Itaúna (Motoki et al, 2013a), Morro dos Gatos and Cabo Frio Island (Geraldes et al, 2013) have subvolcanic pyroclastic conduit and fissures made up of pyroclastic rocks (Mota and Geraldes, 2006;Motoki et al, 2013b). Along the alignment, there is Cambrian nepheline syenite gneiss of different genesis called Canaã complex.…”

“…The main intrusive bodies have rare homogeneous form (Geraldes et al, 2013). That is, the horizontal section of upper level of the pluton is circular and that of lower level is elliptic.…”

“…The volcanic edifices and surface eruptive materials of that time have been completely eliminated by the regional uplift of 3 km that occurred up to 40 Ma (Geraldes et al, 2013).…”

Conjugate divergent margins: an introduction

“…Some of these features are discussed by Almeida et al (2013)'s paper, which focuses on the pre-and syn-rift volcanism that heralded the formation of the continental margins. Important volcanic and magmatic features are also recorded in the drift phase of the plates, and the paper by Geraldes et al (2013) analyses some of the mechanisms responsible for developing intraplate igneous structures. Both papers include electronic supplementary material that provides the readers with a revised table of available radiometric ages in the East Brazilian and West African margins.…”