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Geology

Mountains of history

Rocks of up to 250 million years old tell us about the formation of Brazil and the movements of the earth's crust

On the journey between São Paulo and Rio de Janeiro, part of the landscape that stands out are the 2,000-meter high mountains in the vicinity of the towns of Engenheiro Passos and Itatiaia. What many travelers perhaps do not know is that these mountains of the Mantiqueira Ridge represent two of the main events surrounding alkaline magmatic rocks in Brazil. The result of the cooling of magma that flowed from the interior of the earth, these dark colored rocks were formed some millions of years ago and are rich in sodium and potassium and in minerals such as feldspar.

“Alkaline rocks are messengers of remote times”, explains Celso de Barros Gomes, a professor at the Geosciences Institute of the University of São Paulo (USP). “They reveal details about magmatism that occurred on our continent starting from the beginning of the Mesozoic Era, some 250 millions years ago, and they offer precious information about the geological formation of Brazilian territory as well as helping us to understand the continental plate movements upon the earth’s crust, including the moment at which Brazil and Africa separated.”

After almost twenty years of research, Gomes and his team have produced a map that indicates where the alkaline magmatic rocks appear in Brazil. According to this survey, which includes Paraguay, Uruguay and Bolivia, these rocks distribute themselves over 15 geographical provinces with dimensions that are highly variable. As well as their occurrence at Itatiaia, with 220 km2, and Passa Quatro, with 165 km2, their occurrence at Poços de Caldas stands out with the largest exposure in the country of nearly 800 km2. “In some parts of Paraguay this type of rock is found in the form of small outcrops, which cover only a few square meters”, says Gomes. The book entitled, Mesozoic to Cenozoic alkaline magmatism in the Brazilian platform, produced in partnership with the University of Trieste, in Italy, shows that the age of the rocks can also vary considerably  – from 50 million years in the proximity of Cabo Frio, in the state of  Rio de Janeiro, and around Asunción, the capital of Paraguay, to 250 million years in some regions along the Paraguay river.

The chemical analysis suggests that the rocks were formed starting from heterogeneous magma that had occupied the region of the mantle, the internal viscous layer inside the earth whose temperature can reach 1,200ºC. The depth would not have been so great: this magma would have been close to the transition zone of the lithosphere, the planet’s superficial structure, of almost 100 kilometers in depth, which includes the earth’s crust, which makes up the continents. The magma, viscous and incandescent, was enriched by fluid elements, mainly water, calcium and fluorine, as well as the alkaline earth metals such as magnesium, beryllium and barium. Even after cooling the alkaline magmatic rocks continue until today to be located at some dozens of kilometers of depth, although they can flourish on the surface. The state of Goiás  town of Santo Antônio da Barra is one of the locations where the formations are outstanding in the soil, mainly because of erosion (see Pesquisa FAPESP Nº 15).

Brusque and sudden ruptures
The conclusions about the origin of the magma reinforce the theory, as yet not consensual among geologists, that the incandescent material flowed upwards and  came close to the surface, stimulated by fractures occurring in the earth’s crust. As these fractures were brusque and sudden, the mantle’s pressure and temperature rose rapidly, pushing and expelling the magma that, distant from the zones of intensive heat, lodged in the base of the crust, cooling and giving origin to rocks.

This work seems to reinforce this evidence, since a large part of the material analyzed corresponds, in terms of age, to the moment of the fragmentation of the super continent, Gondwana, when various terrestrial plates, up until then united, had broken apart and moved away from each other – including the eastern portion of Brazil and the west coast of Africa. Teams from the Natural History Museum of London have already located in African countries, such as Angola and Namibia, alkaline magmatic rocks with compositions and ages similar to those of Brazil. This could be one more piece of evidence not only of the remote territorial union between Brazil and Africa, but also of the origin of rocks.

In spite of their importance, the alkaline rocks represent around 1% of the total of the planet’s magma. They come in second place in relation to their closest and popular relations: the basalts, with little potassium and lots of silicates, responsible for the formation of fertile soils known as rich red soils, and the granites, rich in quartz and mica, used as building paving stones, street paving and cemetery headstones.

Even being seen as a poor cousin to the basalts and granites, the alkaline magmatic rocks, especially the carbonatites, show high economic potential. In the regions of Jacupiranga, in the state of São Paulo, of Araxá and Tapira in Minas Gerais, and of Catalão in Goiás, these rocks appear associated with phosphates, widely used in the fertilizer industry. Araxá, as well as having phosphate reserves, concentrates the world’s largest deposit of niobium, a highly resistant metal, used in the manufacture of alloys by the aviation and space industries.

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