rocha alcalina, barita, Baia de Guanabara, magmatismo cenozóico
Abstract
New evidence of alkaline magmatism, probably of late Cretaceous-Cenozoic age, is described from Guanabara Bay, State of Rio de Janeiro. It is represented by rock occurrences covering very small áreas. The alkaline breccia of Jurubaíba Island has a phonolitic matrix with trachyte, phonolite and nepheline syenite pyroclasts. Barite of Governador Island occurs in a chalcedony breccia vein and its gênesis is related to hydrothermal fíuids of the same alkaline magmatic event. These occurrences are thought to be associated with normal faulting during the separation of South America and África.
