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Brazilian alkaline rocks and carbonatites has been given by Ulbrich and Gomes (1981) and J. G. Valenca has written an unpublished review paper.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).
This is a small intrusion of trachyte.
This is a multiple intrusion covering approximately 30 km2 which is related to the nearby Tangua and Rio Bonito (Fig. 1_206) intrusions.
Rio Bonito is a zoned stock of some 28 km2 emplaced in Precambrian gneisses and granulites near to the Tangua and Saorinho intrusions (Fig. 1_206).
This approximately circular intrusion 6-7 km in diameter is cut by plugs of intrusive breccia, and emplaced in Precambrian biotite gneisses and granulites, close to Saorinho and Rio Bonito (Fig. 1_206).
A small intrusion of volcanic breccia cuts Tertiary sediments of the Barreiras Formation. It contains small alkaline rock fragments 1-10 mm in diameter in a phonolitic matrix of orthoclase, nepheline, cancrinite, pyroxene and secondary calcite.
A small plug of porphyritic phonolite cuts Tertiary sediments of the Barreiras Formation. Phenocrysts of feldspar, aegirine, nepheline and mica occur and analcime is present in amygdales.
Itauna covers 6 km2 and comprises a number of incomplete but concentric rings and a central plug. It is emplaced into Precambrian gneisses. Pseudoleucite syenites and microsyenites are intruded by a central core of pseudoleucite phonolite.
This is a small phonolite plug.
Canaa is a 20 km2, approximately circular intrusion emplaced into Precambrian quartzofeldspathic gneisses and migmatites, in which foliated and banded litchfieldites surround a core of alkali syenite.