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Mountains and volcanoes in the interior of Ceará

Mineral analysis used to reconstruct the landscape and movements of large blocks of rocks in the region around 600 million years ago

A wilderness in the municipality of Forquilha, northwestern Ceará, where geologists discovered rocks that offered insights into the tectonic-plate collisions that occurred in the region in the deep past

Ticiano J. S. Santos / IG-Unicamp

In early April, when recounting a research project he had started 36 years ago to reconstruct the movements of large blocks in parts of Brazil’s Northeast and Midwest around 600 million years ago, geologist Ticiano dos Santos recalled a student from the Institute of Geology at the University of Campinas (IG-UNICAMP), Michele Pitarello, currently at the Geological Survey of Brazil in Manaus.

“In 2012, she spent months examining dozens of rock slides, which I had preselected, under optical and scanning electron microscopes,” Ticiano recalls, preferring to go by his first name. “She said, ‘If there’s coesite, I’ll find it.’”

Finally, in one of the cut and polished rock slides, with a thickness of 30 micrometers (1 micrometer is equivalent to 1 thousandth of a millimeter), she finally found micrometric grains of what she thought could be coesite — a mineral that forms at depths of around 90 kilometers (km) under an ultra-high pressure of 2.5 gigapascals, roughly 25 thousand times greater than sea-level pressure, in rocks known as eclogites. Coesite is quite rare because it typically transforms into quartz as it rises to shallower depths and pressure decreases.

Ticiano J. S. Santos / IG-UnicampA sample of rock subjected to ultra-high pressure, collected to the south of Irauçuba, where coesite was identifiedTiciano J. S. Santos / IG-Unicamp

More advanced instruments at IG-UNICAMP, the UNICAMP Institute of Physics, and the National Synchrotron Light Laboratory (LNLS), also in Campinas, confirmed the identity of the coesite, found embedded in a rock collected by Santos and his team in Forquilha, a municipality with a population of 25,000 people in eastern Ceará. Characterized with the assistance of geologists from the University of Brasília (UnB), the first coesite ever found in Ceará and Brazil was reported in October 2015 in the journal Gondwana Research.

For nearly a year during his master’s degree program, also supervised by Santos, geologist Matheus Ancelmi later surveyed and cataloged over 40 outcrops subjected to high pressure, but found no coesite. They then moved to an area in the municipality of Irauçuba, 70 km from Forquilha, and five years later, they found another sample of the rare mineral. This time the microscope analysis was tasked to Nádia Borges Gomes, who at the time was also pursuing a master’s degree under Santos.

“The coesite discovery was huge,” says Benjamin Bley, a geologist from the University of São Paulo’s Institute of Geosciences (IGc-USP), who was not involved in the research. “This, coupled with the discovery of eclogites, which are also quite rare, further bolsters research surrounding the geological connection between northeastern Brazil and northeastern Africa, regions that were once joined together.”

Ticiano J. S. Santos/IG-UnicampJoão Paulo Pitombeira, of UFPE, Nádia Borges, and Ticiano Santos (wearing shorts) selecting rocks and minerals in the Irauçuba area, CearáTiciano J. S. Santos/IG-Unicamp

Punctuating decades of geological research within the area, these two coesite discoveries have enabled researchers to reconstruct the formation of Ceará’s now-flat landscape — with only a few remaining mountain ranges, such as the Baturité range south of Fortaleza and the Maranguape range, closer to the state capital — and the complex movements of massive rock formations, often referred to as microplates. These formations collided, fragmented, or fused at different points in time, ultimately shaping the South American continent.

“Around 640 million years ago, the Forquilha region used to be a mountain range comparable to the much more recent Himalayas, which are still in the process of formation,” Ticiano remarks. He explains that this mountain range likely formed in a subduction zone [an area where two tectonic plates meet, with one sliding beneath the other] in an ancient ocean, as a result of the collision of two separate continents, one situated to the east of the city of Sobral, and the other to the west. “The rock formations in the western continent differ geologically from those in the eastern continent, with each aging around 2.3 billion and 2.1 billion years, respectively.

This means people living to the east and west of Sobral, in Ceará, can now say the areas where they live once belonged to separate continents.” Between these continents there was once an ocean called Goianides, which cut through Brazil in a northeast-southwest direction, as documented by researchers from UnB in the late 1990s. The perimeters of this ocean were flanked by mountain ranges roughly 600 million years ago.

Whenever one tectonic plate sinks beneath another, a portion of the rock within the magma melts and can emerge as volcanic lava, thereby forming mountains. “The mountains and volcanoes in the northern regions of Brazil’s Northeast and Midwest have been entirely eroded away,” explains Ticiano. “All that remains is the root of what we term the magmatic arc, essentially a stretch of magmatic rocks that rose to the surface.” These magmatic arcs — also known as volcanic arcs, so called because they create a shape resembling an arc when observed from above — are sections of molten mantle that rise to the surface. Due to erosion, only the foundations — or roots — of these structures have endured in Brazil.

Santos’s research first began in 1987 when he made his first trip through the Caatinga (semiarid scrublands) of northwestern Ceará in search of oceanic bedrock, during the final year of his geology studies at the Federal University of Rio Grande do Norte (UFRN) in Natal. He had set out to test a hypothesis put forth by two UFRN professors, Peter Hackspacker (1952–2021) and Reinaldo Petta, which postulated potential linkages between the rock formations in Ceará and Africa.

Alexandre Affonso / Revista Pesquisa FAPESP

However, Santos returned empty-handed from his initial trips. As he pursued his doctoral studies, he widened his search area and ultimately found rocks that led to the identification of the Santa Quitéria continental magmatic arc, working alongside American geologist Allen Fetter, then affiliated with the University of Kansas, and Ebehard Wernick (1940–2019) from São Paulo State University (UNESP) in Rio Claro. He returned in 2003, a year subsequent to his appointment at UNICAMP, now with funding from FAPESP and the Brazilian National Council for Scientific and Technological Development (CNPq).

During these trips, Santos spent days, geological map in hand, scouring the terrain for outcrops and chipping away at stones. Santos knew that the presence of the newly identified arc — characterized by distinctive rock formations several kilometers long and wide — meant that there must have been a collision between tectonic plates composed of blocks of rock from the earth’s outermost layers.

“One day, I chanced upon a dense black rock that looked to be primitive, possibly from the time when the arc was formed,” he recalls. “Though intriguing, it initially didn’t cross my mind that it could be an eclogite, formed through compression deep within the earth.” But eclogite it was, although his colleagues initially met this conclusion with skepticism. Eventually, validation came from an Indian geologist at UNICAMP, Asit Choudhuri, after scrutinizing the rock under an optical microscope. To further clarify the region’s geological history, all that was lacking was to locate coesite, a mineral indicating the depth at which one tectonic plate has dipped beneath another.

Globally, only 24 occurrences of coesite have been unearthed thus far, including the two discoveries in Ceará — the first in Brazil — as reported in the June edition of Lithos. “We will no doubt find others in Brazil,” Ticiano predicts. His confidence is explained by the many research groups now studying the magmatic arcs that cut across the country, from Amazonas to Rio Grande do Sul, dating to as much as 930 million years.

“High-pressure rocks and rocks from magmatic arcs demarcate the region where tectonic plates collided,” explains Mônica Heilbron, a geologist at Rio de Janeiro State University (UERJ). “However, a tectonic plate can break off and attach to another one further away. A section of the Amazonian craton [a large block of rock], for example, used to be attached to another from North America.”

Having dedicated decades to studying the Ribeira-Aracuaí magmatic arc, stretching from the southern part of Bahia to the southwest of São Paulo, with an age of up to 840 million years and a width of up to 40 km, she explains that studies such as those by the UNICAMP group help reconstruct tectonic processes in the distant past and provide a better understanding of similar present-day phenomena. “Even though the rate of magma ascent may vary, the mechanisms are similar,” she says. “The subduction process is complete in Ceará, but is still active in California and Chile, as denoted by the frequent earthquakes there.”

East border of the Santa Quitéria magmatic arc: A new ultra-high pressure belt in the Ceará Central domain? (nº 16/08289-8); Grant Mechanism Regular Research Grant; Principal Investigator Ticiano José Saraiva dos Santos (UNICAMP); Investment R$183,454.53.

Scientific articles
GOMES, N. G. et al. P-T-t reconstruction of a coesite-bearing retroeclogite reveals a new UHP occurrence in the western Gondwana margin (NE-Brazil). Lithos. vol. 446–7, 107138. june 2023.
SANTOS, T. J. S. dos et al. U-Pb age of the coesite-bearing eclogite from NW Borborema Province, NE Brazil: Implications for western Gondwana assembly. Gondwana Research. vol. 28, no. 3, pp. 1183–1196. oct. 2015.