Journal of the Geological Survey of Brazil

Deep crustal structure of the Sergipano Belt, NE-Brazil, revealed by integrated modeling of gravity, magnetic, and geological data

2022-08-17T09:28:48+00:00

The Sergipano Belt is located in the Southern Subprovince of Borborema Province in the Northeast of Brazil. Its tectonic framework was consolidated in the Pan-African-Brasiliano Orogeny at the end of the Neoproterozoic. The most recent geological models indicate that its evolution occurred over a complete Wilson Cycle. Gravity and magnetic data profiles that crossed the Sergipano Belt from south to north were modeled jointly by the forward method to provide a 2D view of the deep crustal structure. The modeling process was linked and supported by the use of geological data and models. The result revealed the deep structure of the crust and identified the geometry of the main geological domains to the depth of Moho discontinuity. The folds and thrusts toward the São Francisco Craton are a persistent and deep feature in the Southern crust of the Sergipano Belt. The general tectonic context of the models is compatible with the subduction and collision of the São Francisco Paleoplate under the Pernambuco-Alagoas Superterrane, sutured in the São Miguel do Aleixo Shear Zone. The presence of dense blocks at the base of the crust was interpreted as layers of ophiolites placed by obduction. The metasediments of the Vaza Barris and Macururé domains are tabular bodies with thicknesses lower than 5 km, which dip horizontally or at a low angle on the flanks of the shear zones. Small vertical bodies of lower density within the Macururé Domain have a correlation with granitic intrusions. In the Canindé Domain there is a dense layer in the lower crust that was interpreted as the relicts of the oceanic crust that based a back-arc basin. The granitic bodies modeled north of the Canindé Domain and correlated with the Serra do Catu batholith in Pernambuco-Alagoas Superterrane, may be the record of subduction of this crust to the north. The data and models revealed geophysical differences between the Jirau do Ponciano and Rio Coruripe domains, and the crust north of the Palmeira dos Índios Shear Zone. These terranes are separated by shear zones with evident expression in gravity and magnetic data. However, the existence of Neoproterozoic metasedimentary supracrustal rocks partially covering both the domains and zones and the boundary shear indicates that the junction among these blocks occurred before the deposition of Neoproterozoic sediments.

New zircon U-Pb dating and review of geological and geochronological data with implications for lithostratigraphy and crustal evolution of North Borborema Province, northeastern Brazil

2022-11-21T18:54:36+00:00

The northern portion of the Borborema Province, located north of the Patos Shear Zone (northeast of Brazil) is formed by litho-stratigraphic units dating from the Archean to the Neoproterozoic, with complex evolution resulting from several orogenic and extensional events occurring in this interval. In this work, the litho-stratigraphic units of the Northern Borborema Province were reviewed and ordered here, regarding the rock associations and their chronological positioning. Zircon U-Pb dating of three samples of migmatitic orthogneisses from the Rio Piranhas-Seridó Domain, defined crystallization ages of 561 ± 2 Ma, 574 ± 6 Ma and 547 ± 5 Ma for their protoliths. Thus, the origin of the orthogneisses is related to the syn- to late-tectonic magmatism in relation to the Brasiliano Orogeny. From the Jaguaretama Complex, U-Pb dating of detrital zircons of a migmatitic schist indicates Paleoproterozoic sources, with a maximum deposition age of 2144 Ma. The protolith of a migmatitic orthogneiss has crystallization age of 2186 ± 10 Ma, therefore, contemporary compared to other litho-stratigraphic units of the Borborema Province, which are interpreted as having been developed in the Rhyacian Orogeny.

Review of the lithostratigraphy of the Mesozoic basins of the Borborema Province, Northeastern Brazil

2022-09-08T21:51:23+00:00

Several Phanerozoic sedimentary basins developed over the rocks of Borborema Province. The formation and opening of the Atlantic Ocean influenced tectonic events that reactivated large internal lineaments of the province (Transbrasiliano, Senador Pompeu, Patos, and Pernambuco). This event culminated with the production of several structures, including the grabens, where intra-continental sediments were deposited. This deformation played a fundamental role in the development of the continental margin basins that form the WNW-ESE and N-S segments of the Brazilian northeast. In this work we address the terrestrial portion of the Mesozoic basins of the Borborema Province and provide a lithostratigraphic review in addition to unpublished data of the interior basins and the Jatobá Basin. The descriptions of the marginal basins were subdivided by the following structural domains: dextral equatorial, orthogonal, and sinistral south branching. The Ceará and Potiguar basins are part of the equatorial margin. The orthogonal domain encompasses the Pernambuco and Paraíba basins and comprises the crustal block limited by the Pernambuco and Patos lineaments. Finally, the Sergipe-Alagoas basins are located in the sinistral branch. The intra-continental basins are equivalent to the failed rift arms, which are composed of aborted rifts and half-grabens of the interior basins of northeastern Brazil. The Recôncavo-Tucano-Jatobá rift system was aborted in the Lower Cretaceous and represents the North Tucano and Jatobá basins in Borborema. The interior basins are concentrated in the Transversal Zone. However, to the north of the Patos Lineament, important basins occur, such as Iguatu and Rio do Peixe. The basins of the orthogonal domain correspond to the final stretching rift in a late rupture in Aptian due to the rigidity of the basement. Due to the potential of oil exploration, the marginal basins are the ones that present the data available in the literature the most. Both marginal basins with lower oil potential (such as the Pernambuco and Paraíba basins) and the interior basins of the northeast, covering the Jatobá, need a more consistent lithostratigraphic update to consolidate and validate newly proposed units.

The Zona Transversal Domain of the Borborema Province, northeastern Brazil: Synthesis of the Archean to Cambrian evolution, and new tectono-stratigraphic interpretation

2022-11-01T10:30:35+00:00

The Transversal Zone Domain (TZD) is the central segment of the Borborema Province, with its limits given by the Patos and Pernambuco shear zones, to the north and south respectively. This part of the province presents Archean nuclei surrounded by Paleoproterozoic units that together correspond to the basement of belts of Neoproterozoic metasupracrustal rocks. All this set was intruded by syn to post-tectonic granitoids related to the Brasiliano Orogeny (c. 640 – 500 Ma). The TZD records important shear zones of regional scale, dextral (Patos and Pernambuco), and sinistral (Serra do Caboclo, Afogados da Ingazeira and Congo), being the dextral used to delimit the TZD of adjacent domains and the sinistral ones for internal compartmentalization in four subdomains: Piancó-Alto Brígida, Alto Pajeú, Alto Moxotó and Rio Capibaribe. The lithostratigraphic units of this domain were described and organized in a way that would be regionally presentable. An unprecedented dating is presented for a metagranitoid of the Serra de Jabitaca Complex, with maximum crystallization age at 1790 ± 40 Ma and metamorphism at 615 ± 3 Ma. The tectonic evolution of the domain begins with an orogenic event and development of magmatic arches at the beginning of the Paleoproterozoic (involving Archean portions), in addition to an extensional event at the end of this period, already in the transition with the Mesoproterozoic. The Neoproterozoic begins with two extensional events (Cariris Velhos and pre-Brasiliano) until the formation of the magmatic arches related to Brasiliano Orogeny.

The arsenic contamination event of Santana, Amapá State, Brazil, and the high values of arsenic in the brownish Amazon waters

2022-10-21T11:00:44+00:00

The contamination with arsenic in Santana led to the perception that the brownish waters of the Amazon River carry arsenic, at a rate higher than 235,000 tonnes per year, and it seems important that this arsenic should be studied in detail. The anomaly was caused by the discharge of tailings of a pellet plant in an excavated sump. These tailings were composed of iron- and arsenic-rich minerals, crystallized at above 900°C in the metallurgical process, which become unstable when submersed in fresh water at pH 5.0-6.0. In the sump, iron, arsenic, manganese, and other metals were slowly solubilized from the minerals, entering the groundwater. When iron got in contact with the water organic compounds, it formed a gel, absorbing As, Mn, and other metals, as it normally happens during weathering in the Amazon. The sump has been built in practically impermeable clayey soil, which made the gel move slowly, giving time to form hard concretions of limonite, characteristic of the Amazonian soil. Iron, manganese, arsenic and other metals were retained in the concretions, and the arsenic-bearing iron gels went for a maximum of some hundreds of meters away from the sump. Soon after the identification of the contamination, the water of the sump was, under close control, discharged into the Amazon River, and the tailings were removed to high ground. In sequence, as it was impossible to properly isolate the tailings, they were sold and shipped away. In the meantime, there were complaints that persons have been affected by arsenic-contaminated water that flowed from a broken dam (that never existed) into a small creek that crossed a community, and that people that consumed the water had cancer. A large medical team was called by the local government, thousands of persons were examined, diseases were found, and none related to arsenic. Analysis of blood and hair presented arsenic in limits worldwide accepted as normal. As samples of the Amazon showed a conspicuous presence of arsenic, they led to the study of the river waters for arsenic. Information is presented to correct misunderstandings present in the literature. Two important points are commented on to understand the event. One is the geological affinity of arsenic with iron. The other is that the toxicity of a solid is determined by how much of the toxic element is liberated for contamination; the standards used for the proper definition of toxicity of a solid are presented.

Overview of the geological mapping in Brazil: historical analysis until 2022

2023-01-23T11:22:01+00:00

The purpose of this work is to contextualize the evolution of the geological mapping in Brazil since the creation of Company of Research of Mineral Resources (CPRM-Companhia de Pesquisa de Recursos Minerais), in 1969, as part of a set of governmental measures that aimed to boost the Brazilian mineral sector. Three mapping cycles were individualized (1969-1993, 1994-2002, and 2003-2022) over such 53 years, which time limits were defined by historic milestones, and which differ between them especially due to the methodological approach and to the support tools available. The analysis was carried out strictly considering 1:250,000 and 1:100,000 scales, which represent the main reference scales of geological cartography in Brazil. The assessing of the performance of the mapping shows that the level of the geological knowledge of the country is still incompatible with its mineral potentialities, with its strength in other natural resources, and with its importance in the world scenario. Only 48% of the Brazilian territory are mapped at the 1:250,000 scale, and 27% in 1:100,000 scale. Amazon represents the territory with lower geological knowledge, since less than 40% of such territory have mapping coverage in 1:250,000 scale. In contrast, more than 50% of the non-Amazon areas are mapped at the 1:100,000 scale. Over the period under analysis, emphasis was given to the Precambrian shields in relation to the sedimentary basins, which configure large cartographic gaps, as well as critical Precambrian areas of the Amazon, for example, a large part of the Guiana Shield, in the northern margin of Amazon River. In addition to the challenge of mapping a country of continental dimensions and with heterogeneous geoeconomic contexts, other factors contribute to this scenario, among them, the discontinuity of governmental initiatives and programs, which directly affect the planning of the mapping, the budget for investments in projects, human resources, infrastructure, and on the articulation of productive partnerships. This review shows that the periods of greater productivity in the geological mapping were a consequence of well-defined governmental guidelines, proper investments, and sharing of responsibilities with State governments and Brazilian universities, for example, such as happened in countries that had the perception of the importance of basic geological surveys as a strategy for development.