REINHARDT FUCK E RUDOLPH JOHANNES TROUWThis granulite from Carvalhos, Minas, contains cyanite (yellow): evidence of high pressureREINHARDT FUCK E RUDOLPH JOHANNES TROUW
The gaze has nowhere to fall in the middle of such flat lands. Sugarcane or soya plantations or pasture land with scattered cattle for kilometer after kilometer in the central region of Brazil, whose monotony is only broken by a few twisted trees, typical of the Cerrado (savanna). Much higher and more solemn, rising here and there are palm trees with their well spread leaves and their heavy bunches of coconuts – these are the buriti palms, the tree that is the symbol of the country’s capital. Millions of years ago, however, the landscape around there was inhospitable. Where today stands this plateau there was an extensive mountain chain of up to 8,000 meters in height, made of rocks covered only in lichens or snow. These were the Brazilian Himalayas that stretched out for almost 1,500 kilometers, from the south of the current state of Tocantins to the south of the state of Minas Gerais.
Anyone who journeys through these states will only find hills of a few hundred meters: rain, wind and natural fracturing consumed the immense mass of granite throughout 630 million years. But of the massive rocky mass there are traces from which geologists from São Paulo and Brasilia are reconstructing the geological history of the Central West of the country. The solid blocks that composed this imposing chain of mountains long before the continents assumed their current form have found themselves pulverized along the hundreds of kilometers in Minas Gerais and Goiás, mixed into the red earth that tinges the sky chestnut brown before storms.
Not everything turned to dust. The team led by the geologists Renato Moraes and Mario da Costa Campos Neto, from the University of Sao Paulo (USP), and that of Reinhardt Fuck, from the University of Brasilia (UnB), have found in Goiás and Minas testimony of these remote Himalayas of the New World. These are the granulites, rocks whose color varies from cream to bluish-green, speckled with dark caramel grains. Under the microscope the crystals of the four minerals can be seen, namely – quartz, feldspar, granite and pyroxene. The granulites were formed in regions below the earth’s surface subject to high temperatures, in the order of 800°C and extremely high pressure, thousands of times greater than human beings support in their day to day life. According to Campos Neto, who since 1995 has been studying the granulites in the south of Minas, these conditions of high pressure and temperature generally occur in very deep trenches of the earth’s crust. “These rocks were at the root of this mountain chain”, he comments.
Mountains in growth
The peculiarities of their mineral composition reveal that the granulites from the Central-West were formed between 40 and 60 kilometers beneath the surface. They resulted from a recombination of their essential components, such as silica, calcium, potassium, magnesium and iron, which regrouped in different proportions, giving rise to compounds that were more stable. They only flourished in the north of Goiás and the south of Minas Gerais because of the constant and slow movement of the gigantic rocky plates that make up the continents and the floor of the ocean beds. The continental plates collide on sliding over the mantle, the layer that is hotter and more viscous than the crust. As a consequence, a plate can bring about the wrinkling of another with which it has collided.
In a relatively short time, estimated at a few dozen million years, in the way in which a plate can squeeze another and amplify the wrinkling, a chain of mountains such as those of the current Himalayas can come about, the 2,500 kilometer mountain chain in the southwest of Asia that houses the highest peak in the world – mount Everest, of height 8,848 meters, and K2 with 8.611 meters. A continental plate can also pressurize the floor of an ocean. In this case the rocky layer under the sea normally dives under the plate, lifting up mountains such as the Andes, a very young mountain chain, which runs along the west coast of South America.
It was probably this second mechanism – the diving of the oceanic floor under a plate, also called subduction that began the rising of the Brazilian Himalayas some 700 million years ago. In that era the continents did not exist as they are known today: they were all joined together in a mega-continent – Rodinia or Mother-Earth, in Russian – that found itself close to the South Pole and at that time had began to break up. In this separation, the solid continental block upon which currently part of the Brazilian Northeast was sitting, the San Francisco craton, was connected to what today is Namibia and the Kalahari Desert, in the south of Africa. The separation of these plates pushed the San Francisco craton against the floor of a primitive ocean called Goianides, described by geologist Campos Neto and the French geologist Renaud Caby, from the Montpelier II University, in articles published in Precambrian Research in 1999 and in Tectonics in 2000. In accordance with their penetration under the edges of the craton, the ocean rocks reached regions close to the mantle, became viscous and their chemical composition went on to recombine itself into more stable compounds, forming new minerals.
In this dive towards the center of the earth part of these minerals were dragged around in the direction of the surface or expelled between the fractures in the rocks to less hotter regions or those under less pressure. In this manner they were rapidly cooled and preserved in the form of crystal registrations of the depths to which they had reached. Crystals of cyanite – a mineral formed from aluminum and silicon – revealed to Campos Neto that the granulites found with ease between the towns of Três Pontas and Pouso Alto in Minas Gerais state were probably formed in a very deep region of the crust. Non-colored under the microscope and clear blue in macroscopic quantities, the cyanite of the granulites must have been formed at almost 60 kilometer depths below the surface, under a pressure of 13,000 to 17,000 times that of atmospheric pressure and at temperatures that could have varied from 750oºC to 900ºC.
LUCIANA PASCARELI SANTOS, MÁRIO DA COSTA NETO and CARLOS HENRIQUE GROMANN / RENATO MORAESGranulite from the region of Goiania with tourmaline (green): temperatures above 1,000oCLUCIANA PASCARELI SANTOS, MÁRIO DA COSTA NETO and CARLOS HENRIQUE GROMANN / RENATO MORAES
Intriguing rock
Over in the central region of Goiás the granulites contain blue tourmaline, a mineral of exuberant aqua marine, formed by silicon, aluminum and magnesium. The granulites here were formed at 40 kilometers of depth, but under temperatures that were very much higher: between 1,000ºC and 1,100ºC, considered abnormal even by the geologists themselves. “This discovery is troubling”, says Moraes, who described the finding in the Journal of Petrology in 2002 and in two articles for the Journal of Metamorphic Geology, the most recent of them published last year. “We still don’t know how to explain with certainty why these rocks were formed at such high temperatures.”
Moraes began to investigate this type of rock in 1995 during his doctorate degree under the supervision of professor Reinhardt Fuck, of the UnB. During treks through the region of Goianesia he found outcrops well spread out, dozens of kilometers of distance from each other, some the size of a room, 5 meters wide, 5 meters long by 3 meters in height. Strikes with his geology hammer extracted hundreds of samples of the approximate size of a closed fist, similar to those found years later in the region of Inhumas, now much closer to Goiania.
During a season in which he worked with a team led by Rudolph Trouw, from the Federal University of Rio de Janeiro, Moraes got to know the granulite outcrops in the south of Minas, over which professor Campos Neto had already trodden. Hired by USP in 2003, Moraes worked closely with Campos Neto. A few times per year both have gone off with their students on excursions to collect more granulite samples in Minas and Goiás. “We want to better know the structure of this chain of mountains”, affirmed geologist Moraes. The Goiás-Minas Gerais Himalayas may not have been the only one that snaked across the lands that today form Brazil. “Long before that, around 2 billion years ago, there apparently were mountainous chains of high elevation in the current region of the Amazon and the Northeast”, explains Campos Neto. But this is another story.
The Project
The role of liquid silicates in the evolution of high grain rocks in the Brasilia strip (04/09682-8); Modality Regular Line of Research Assistance; Coordinator Renato de Moraes – USP; Investment R$ 114,356.50 (FAPESP)
