Serra Do Bueno
Serra do Bueno consists of a series of small necks and dykes intruding low grade schists. The rocks are 'alkali olivine basalts', containing perovskite and zeolite.
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Brazilian alkaline rocks and carbonatites has been given by Ulbrich and Gomes (1981) and J. G. Valenca has written an unpublished review paper.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).
Serra do Bueno consists of a series of small necks and dykes intruding low grade schists. The rocks are 'alkali olivine basalts', containing perovskite and zeolite.
At least 12 of a much larger number of radioactive anomalies in the Bambui area prove to be pipes up to about 100 km in diameter, which are usually filled with breccia.
A circular complex 4.5 km in diameter, Araxa cuts and domes quartzites and schists of the Proteroxoic Araxa Group.
Tapira is an almost circular intrusion about 6 km in diameter emplaced in Precambrian quartzites of the Canastra Group, which strike parallel to and dip away from the contact. The complex is deeply weathered for between 30 and 200 m.
The Sacramento complex consists of a sequence of alkali basaltic flows, tuffs, breccias, dykes and plugs of Cretaceous age.
The small intrusion of Jaboticabal is emplaced in Cretaceous basalts. To the west and northwest near Aparecida de Monte Alto, Taiuva and Piranji alkaline lavas up to 15 m thick intercalated in sandstones of the Bauru Group have been encountered in drill holes.
This intrusion forms prominent steep-sided hills on either side of the Paraguay River (see 130-00-002 Cerro Boggiani). It is surrounded by Recent sediments, but has not been mapped in detail.
An oval-shaped intrusion 4x2.5 km in diameter, Ubari forms a hill cutting Precambrian biotite gneisses. The predominant rock type is a syenite of perthite, aegirine-augite and biotite. There is a little quartz together with apatite, sphene and magnetite.
Morro de Sao Joao is an approximately circular stock 3.5 km in diameter, forming a conical hill reaching 800 m. It intrudes Precambrian granitic and granodioritic gneisses which fringe the hill and then pass beneath Quaternary alluvial deposits.
This is an area of alkaline rocks which includes much of the island of Cabo Frio (6.3 km2) together with the smaller intrusions of Morro do Forno and Atalaia on the adjacent mainland; there is an abundant dyke suite.