Journal of the Geological Survey of Brazil

Pernambuco-Alagoas, Riacho do Pontal, and Sergipano domains of the Borborema Province, Northeastern Brazil: a geological and geochronological synthesis

Jocilene dos Santos Santana — 2024-10-02

Abstract

In recent years, the number of works seeking to advance the geological and geotectonic knowledge of the Borborema Province has been increasing. However, in the context of the Southern Borborema Province there are few studies aimed at understanding its tectonic evolution in an integrated way. This work presents a review of the lithostratigraphic and geochronological data available for the Pernambuco-Alagoas, Riacho do Pontal and Faixa Sergipana Domains, located in the southern portion of the Borborema Province (Northeast Brazil), in addition to a discussion on the tectonic evolution model. The Southern Borborema Province fully encompasses the state of Alagoas and partially the states of Pernambuco, Sergipe, Bahia and Piauí, and is delimited to the north by the Pernambuco Lineament and to the south by the São Francisco Craton. The methodology involved bibliographical survey, geological cartography, geochronological compilation, creation of detrital zircon histograms for metasedimentary rocks (with unconformity below 10%) and magmatic “barcodes” for ortho-derived rocks and preparation of regional geological profiles. The tectonic evolution of the Southern Borborema Province began with the breakup of the Rodínia Supercontinent (1000 Ma) during the Cariris Velhos Cycle. The processes involved in the interaction of fragments descending from Rodinia are diachronic, in a broad scenario of accretions and collisions that resulted in the merger of Gondwana. The Pernambuco-Alagoas Block is considered a complex tectonic domain, which was affected by the Cariris Velhos (1000 – 950 Ma) and Brasiliana (740 - 570 Ma) orogenies (Caxito & Alkmim 2023), preserving remnants of Archean rocks (Riacho Seco Fragment) and proterozoic. The Riacho do Pontal Domain and the Faixa Sergipana Domain are part of a large folding system formed from the collision of the Pernambuco-Alagoas Domain with the São Francisco Craton, in a tangential deformation, vergent towards the craton, during the Brasiliana Orogeny. From the ages of the syncollisional granites we can infer that the orogenic event that gave rise to the aforementioned folded bands occurred at very close time intervals, presenting a slight diachronism. The article presents an integrated approach to tectonic evolution, however, the need for new studies is quite evident, with an emphasis on isotopic geology, geochronology, geochemistry and geological mapping that can contribute to the understanding of the processes involved in the formation of tectonic domains, their space-time relationships and their implications for the evolution of the Gondwana Continent.

A synthesis of the National-scale Geochemical Survey in the last 50 years in Brazil

Marcelo Almeida — 2024-11-27

At the end of the 19th century, classical geochemistry arrived in Brazil through Henri Gorceix, being primarily used in his studies of mineralogy and petrology. Meanwhile, exploration geochemical arrives later, in the mid-20th century. But it was only in the late 1960s that systematic survey at the national level began, starting with the creation of CPRM - Geological Service of Brazil, culminating in over 50 years of uninterrupted activities, providing coverage of the entire country at various scales: global (7.3%), national (6.2%), regional (22.7%), and local (2.3%). Throughout this period, 681 projects were carried out and a variety of sample matrices were collected, such as stream sediment (226,792), soil (122,202), heavy-mineral pan concentrate (107,959), rock (56,807), among others (8,258), totaling 522,018 samples so far. All this collection has been progressively recovered, compiled, and organized into a robust public database (open access), developed by CPRM - Geological Service of Brazil. More recently, high-density geochemical surveys were conducted in partnership with the China Geological Survey with the aim of comparing analytical and sampling methodologies. For the future (until 2050), two main actions are planned: 1) an expectation of an increase in the coverage of high-density prospecting surveys, ranging from 951,386 km² (restrictive scenario: 11.2% of Brazilian territory) to 2,204,778 km² (favorable scenario: 25.9% of Brazilian territory); 2) a forecast of Brazil's participation in the Global Geochemical Baselines Program, where sampling will be subdivided by regions – North (736), Central-West (225), Northeast (305), and South-Southeast (343) – totaling 1,609 sampling cells.

Geodiversity of the coast of the state of Pernambuco, northeastern Brazil

Fernanda Soares de Miranda Torres — 2024-11-21

The study of geodiversity provides insight into the potential and limitations of an area's physical environment, making it highly valuable for public and private managers in supporting the development of appropriate land use and occupation proposals for a region. In this perspective, the Geodiversity Map of the coast of the state of Pernambuco was created by combining the Geodiversity Map of the Recife Metropolitan Region (RMR) and the Geodiversity Map of the Southern Coast of Pernambuco. Both were developed by the Geological Survey of Brazil through bibliographic studies and field surveys of geology, geomorphology, soils, and superficial formations, resulting in fifteen geological-environmental units composed of materials that significantly influence the suitability and usage constraints of the region. An example is the DCT unit, which covers a substantial area along the coastal zone of the northernmost municipalities of the RMR. Composed of sediments with varied composition (irregular alternation between layers of sandstone, siltstone, claystone, and gravel), it is widely used for urban occupation and mineral exploitation (borrow material in civil construction). However, on the slopes of the plateaus, particularly near the cliffs along the coastline, this area is unsuitable for housing due to the risk of landslides caused by slope gradients and the high susceptibility of these materials to erosion. Other information of significant importance and interest for public managers can be obtained as a result of this geodiversity study, such as mineral resources, groundwater potential and quality, geotechnical characterization, and areas prone to geological-geotechnical events that could impact land use, among others. All this knowledge is valuable for developing public policies and guiding actions by managers to generate employment and income, protect the environment, and promote health in the region.

Geochemistry of serpentinites from the central portion of the Rio Piranhas-Seridó Domain, Borborema Province, Northeastern Brazil: Implications for genesis and geotectonic environment

Frank Gurgel Santos — 2024-12-05

Serpentinites from the central portion of the Rio Piranhas-Seridó Domain (RPSD), in the northeast portion of the Borborema Province (BP), occur in association with the Caicó Complex gneisses, paragnaisses/schists of the Seridó Group. Serpentinites, which were sampled in two localities near the city of São Tomé-RN, are melanocratic rocks, fine-grained, greenish in color and foliation consistent with the regional trend. The mineralogical composition is primarily composed of serpentine, which occurs as a replacement for the iron and magnesium minerals of the protolith, with chlorite, muscovite, tremolite, calcite-dolomite, quartz, and opaque minerals as accessory phases. Chemical analyzes show SiO₂ values ranging from 35.72 to 43.48%, MgO> 28.88%, Fe₂O₃ between 7.08 and 11.29%, Al₂O₃ < 3.42% and very low TiO₂, CaO, K₂O and Na₂O, suggesting an ultrabasic composition. Graphs obtained with the major elements ratios indicate olivine as the primary mineral, and despite the influence of metamorphic processes, the rock still retains the chemistry predominant in the protolith. The high magnesium content points to a komatiitic signature, implying in high temperature, high pressure and high degree of partial melting in the protolith genesis. Data acquired in this study corroborate the bibliography and suggest a compatible scenario for the studied serpentinites to be part of a greenstone belt.

Geological Survey of Brazil Spectral Library v. 1.0: Rock reflectance signatures in the visible to shortwave infrared range

Monica Mazzini Perrotta — 2024-12-05

Reflectance spectral libraries are essential reference databases for the mineralogical characterization of geological samples analyzed using visible and infrared spectroscopy. Since 2011, the Geological Survey of Brazil has been actively developing a comprehensive database of reflectance spectral signatures, based on samples collected from systematic geological mapping and mineral resource projects across the Brazilian territory. This spectral library is now available on a digital platform hosted by the Geological Survey of Brazil's geoscientific database, providing quantitative data for the numerical characterization of lithological types and their alteration products, whether due to weathering or hydrothermal processes.