Monjolos
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.
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Brazilian alkaline rocks and carbonatites has been given by Ulbrich and Gomes (1981) and J. G. Valenca has written an unpublished review paper.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 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.
This is an approximately circular intrusion covering some 50 km2 and forming hills rising to 1600 m. It is emplaced into Precambrian biotite gneisses.
These form an east-west-trending group of major intrusions, volcanic vents, necks and dykes intrusive into Precambrian gneisses and granites.
Minor plugs and dykes of phonolite cut Precambrian gneisses; pseudoleucite-bearing rocks occur (J. G. Valenca, personal communication).
This is an approximately circular intrusion of about 5 km2 cutting Precambrian gneisses, but overlapped on the northern side by Tertiary sedimentary rocks.
An elliptical multiple intrusion 31x12 km and covering 330 km2, Itatiaia is emplaced in Precambrian gneisses, but at the southeastern end is in poorly defined contact with Tertiary clastic sediments.
This is an elliptical intrusion of 18x8 km, covering 110 km2 and lying only 1 km southwest of Itatiaia (Figs 2_204 and 2_210). It is probably a multiple intrusion but has not been mapped in detail.