Portal de Periódicos da CAPES

Idade do vulcanismo no Oceano Atlântico Sul

1970; Volume: 1; Linguagem: Inglês

10.11606/issn.2316-9001.v1i0p09-75

2316-9001

Umberto G. Cordani,

Geography and Environmental Studies

About eighty K-Ar age determinations were carried out on samples from the Brazilian South Atlantic islands, and on samples of magmatic rocks associated to post-palaeozoic fractures of the Brazilian coast. 37 analyses were obtained from 27 rock samples of Trindade island. The results show that most of the intrusive rocks of the Trindade Complex (basic and ultrabasic dikes, and phonolitic plugs) were formed during a volcanic cycle between 2.9 and 2.3 m. y. Some older results, up to 3.6 m. y. were also found. The lava flows of the Desejado Sequence exibited ages close to 2.5 m. y. A maximum age of 0.17 m. y. was postulated for the Morro Vermelho Formation, but no significant results could be obtained for the younger formations. Two samples of rocks from Martin Vaz were analysed. One of them showed a 60 m. y. apparent age, regarded as anomalous. 28 determinations were carried out on 23 rocks from Fernando de Noronha Archipelago. The phonolitic rocks, intrusives into the Remédios Formation, exibited ages close to 9 m. y. Other intrusive bodies resulted older, up to 12 m. y. Apparently, the nepheline basanite of the São José island is synchronous with the Remédios volcanism, and the ankaratritic lava flows of the Quixaba Formation are younger, with results between 6.3 and 1.7 m. y. The islands of Abrolhos are the remnants above sea level of a volcanic cone on the continental shelf. Some of their basaltic rocks were dated, with results between 52 and 42 m. y. The analysed samples belong to the last phases of the volcano, which started at least in the Upper Cretaceous, based on fossil evidences . Post-palaeozoic anorogenic magmatic activities on the continental cratonic margin were also considered. Basaltic volcanism of the Paraná and Parnaiba basins took place in Lower Cretaceous times, but started somewhat earlier. The alkalic rocks of Southern Brazil, the igneous suite of Cabo, Pernambuco, and several intrusives of northeastern Brazil indicate that magmatic events were going on along the Brazilian coast until at least Miocene times. The oldest ages found for the volcanic cones of the South Atlantic are always consistent with the hypothesis' of ocean £oor spreading. The data here assembled suggest that the opening of the South Atlantic rift started in the Jurassic.