A roughly circular structure 2.5 km in diameter occurs in dense rain forest. It is situated on a very large fault trending northwest-southeast, extending south into Brazil and lying on the eastern edge of the Roraima structure.
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With over 100 complexes, more than 20 of which include carbonatite, Brazil is notably well endowed with alkaline rocks. Vast areas, particularly in the Amazon Basin, are not yet thoroughly explored or are obscured by a thick lateritic cover, so that further occurrences are certain to be found. Many intrusions have been exploited economically, particularly those with carbonatite, making Brazil the world's foremost producer of Nb and a major producer of phosphate, vermiculite and rare earths.
A very full review of Brazilian alkaline rocks and carbonatites has been given by Ulbrich and Gomes (1981) and J. G. Valenca has written an unpublished review paper. Rodrigues and Lima (1984) give a brief account of the carbonatites of Brazil and have tabulated the main features of 21 intrusions; they also list all occurrences of alkaline rocks, including circular structures, detected from the air, which may prove to be alkaline intrusions. Particularly useful compilations for the Rio de Janeiro-Sao Paulo area are to be found in Lima (1976) and Liu et al. (1976). Herz (1977 and 1978) has reviewed the ages of the alkaline rocks of southern Brazil and related them to associated basaltic volcanism, the opening of the Atlantic Ocean, and movement of the South American Plate. Asmus (1978) lists the ages of many alkaline complexes and discusses them in terms of oceanic and continental fracture systems, while the plate tectonic setting of alkaline igneous occurrences south of latitude 15°S are discussed by Almeida (1983). Numerous ages are given by Amaral et al. (1966).
A lava flow with a maximum thickness of 20 m outcrops over a semicircular area less than 1 km in diameter, 4.5 km southeast of Volta Redonda. It is associated with sediments of the Resenda Formation.
A series of widely spaced, scattered dykes, up to a metre thick, are aligned northeast-southwest and cut variable Precambrian gneisses. Rock types represented include nepheline-phyric tinguaite, porphyritic shonkinite and monchiquite.
Itanhaem is a series of biotite tinguaite dykes cutting shield rocks. In the same area dykes of olivine sovite are reported.
This is a small dyke of porphyritic shonkinite with phenocrysts of biotite, aegirine-augite, apatite and orthoclase, in a groundmass of the same minerals plus nepheline. Weathered syenitic rocks also outcrop.
Dykes exposed in a railway cutting south of Tatui are shonkinites similar to those of Piedade (025-00-097).
Barra do Teixeira is a small phonolite plug intruding the Precambrian Tres Corregos batholith. Sete Quedas is a similar phonolite plug some 10 km to the east.
Small phonolite plugs are found a few km north and northeast of Cerro Azul.
An oval-shaped complex of about 1 km diameter. Olivine gabbro and phonolite are present but little information is available.
A stock of less than 1 km2 forms the hill of Sao Joao and is surrounded by Recent sediments which obscure the contacts. The principal rocks are biotite pulaskite and nordmarkite, the latter containing quartz; both rock types include amphibole and a diopsidic pyroxene.