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GEOLOGY

The ancient volcanoes of Minas

Volcanic eruptions 600 million years ago inundated what is now Rio Doce Bay with lava

Sugarloaf mountain in inland Minas Gerais State: now exposed, the block of rock once formed part of the Rio Doce arc

Leonardo Gonçalves/UFOPSugarloaf mountain in inland Minas Gerais State: now exposed, the block of rock once formed part of the Rio Doce arcLeonardo Gonçalves/UFOP

Anyone viewing the mountainous landscape of the Brazilian Southeast would never suspect that this region was the site of a long chain of high, active volcanoes some 600 million years ago. During that time, the shape and position of the continents differed from what they are today, and the mountain range extended along the shores of a gulf at one extremity of a narrow sea, stretching for nearly 550 kilometers from the present-day city of Teófilo Otoni, in the state of Minas Gerais, nearly into Paraná State. At the height of this period, its peaks may have been as impressive as those of the Andes.

But we may never know the actual dimensions of this chain of mountains because, like the narrow sea and the ancient continent, it disappeared, consumed by erosion. What remains are fragments of the sea bed and of the rocks that once formed the deep roots of the chain of volcanoes, now exposed as outcroppings in Minas. These records are helping geologists understand the origin and evolutionary history of what is known as the Rio Doce volcanic arc. “We worked to reconstruct the history and topography of this group of mountains,” says geologist Antônio Carlos Pedrosa Soares of the Federal University of Minas Gerais (UFMG). In an article published in November 2015 in the journal South American Earth Sciences, Soares and 14 other researchers describe how the Rio Doce arc emerged and was formed.

Soares and his colleagues collected rocks from outcroppings on the outskirts of the towns of Governador Valadares and Teófilo Otoni, near the border between the states of Minas Gerais and Espírito Santo, and analyzed their chemical composition and the age of the constituent minerals. They combined those data with information from prior studies on the geology of the remnant blocks of the volcanic chain in order to reconstruct the region—the first signs of which were identified in the 1960s by geologist Fernando de Almeida of the University of São Paulo (USP).

The compiled data now confirm that there is some variation in the age of the rocks in the arc. The rocks farther to the east, near the Minas Gerais-Espírito Santo border, are older than those to the west, in the area between Teófilo Otoni and Governador Valadares. The ages were determined by analysis of the decay of radioactive chemical elements in the mineral zircon, conducted in laboratories at UFMG, USP, the Federal University of Ouro Preto (UFOP), the University of Brasília (UnB) and Rio de Janeiro State University (UERJ). “It’s a very consistent dataset,” Soares says. “Analyses from different laboratories, using different methods, arrived at the same conclusions.”

Geologia_241Determination of the ages of the rocks, combined with the study of the chemical composition and structure of their minerals, produced a history divided into three chapters. The Rio Doce arc probably began to form 630 million years ago, when the then-existing pieces of continents were joined into a supercontinent near the South Pole.

According to the analyses, one of the continents had a long peninsula that had formed from a very ancient continental embryo involving parts of what are now eastern Brazil and West Africa. Within that region, which geologists call the São Francisco-Congo Craton, there was a narrow sea known as the Adamastor Ocean, which was similar to the Red Sea and extended from the present-day state of São Paulo to Bahia State.

Between 630 million and 605 million years ago, the movement of tectonic plates likely caused the oceanic crust in the region of the Adamastor Ocean to submerge beneath the Congo Craton, triggering the formation of the volcanoes—a phenomenon similar to the forces now helping to form the Andes and causing volcanic activity in Chile (see infographic). Over the next 20 million years, as the Adamastor closed up, new mountains formed and vulcanism expanded towards the shores of that primitive ocean. Finally, between 585 million and 575 million years ago, when the ocean had practically disappeared, a large block of oceanic crust that was sinking beneath the continental plate is thought to have broken off and submerged beneath the Earth’s mantle. As a probable consequence, there was greater warming of the continental crust and a final episode of volcanic activity, more intense than before. “That was likely the final moment in the evolution of the Rio Doce arc,” explains Pedrosa Soares, “when the ocean closed up and the edges of the São Francisco and Western Congo cratons came together.”

Other groups have already identified remainders of volcanic chains from the same period in Brazil and other countries. None of them, however, has had a history as fascinating as that of the Rio Doce arc, formed between two continents at the edge of an inland sea. Because of that unique configuration, the rocks at the extreme north end of the arc have an unusual composition. They are formed from magma containing more melted continental crust than oceanic crust, and were described by geologist Leonardo Gonçalves of UFOP, one of Soares’ colleagues, in a recent paper published in the journal Gondwana Research.

According to geologist Mahyra Tedeschi, a PhD student of Pedrosa Soares and first author of the article in the Journal of South American Earth Sciences, we still need to clarify many details of the history of the Rio Doce arc. “There are several rock formations that may be part of the arc, but they still need further study,” she says.

Scientific articles
GONÇALVES, L. et al. Granites of the intracontinental termination of a magmatic arc: An example from the Ediacaran Araçuaí orogen, southeastern Brazil. Gondwana Research. August 29, 2015.
TEDESCHI, M. et al. The Ediacaran Rio Doce magmatic arc revisited (Araçuaí-Ribeira orogenic system, SE Brazil). Journal of South American Earth Sciences. November 26, 2015.

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