Resumo em Inglês:Abstract According to previous studies, the Palaeoproterozoic Minas-Itacolomi meta-sedimentary mega-sequence in the Quadrilátero Ferrífero metallogenic province records the transition of a continental rift to passive margin to foreland basin. From the perspective of sequence stratigraphy, this article identifies five sequences summarised in the Minas-Itacolomi Basin stratigraphic chart, providing information of the Palaeoproterozoic eustatic changes, such as the Great Oxidation Event. Interpretations were based on the systematic recognition of 41 facies and their associations, the identification of nine recurring depositional systems, nine system tracts, and five sequences (in that order). The Caraça and Itabira Groups comprise the first sequence, for which three system tracts were identified. The Piracicaba Group hosts two sequences and four system tracts. The Sabará and Itacolomi Groups are the two upper sequences, both related to a foreland basin, where the underfilled and overfilled system tracts were recognised. Sedimentary provenance was determined mainly from detrital zircon U-Pb age spectra of previous authors’ databases, as well as on palaeocurrent field data and thickness variations. All of the units of the Minas-Itacolomi mega-sequence share detrital zircons from Archaean sources (Rio das Velhas Supergroup and crystalline basement). Only the Sabará and Itacolomi Groups exhibited bimodal U-Pb age histograms, thus indicating both Archaean and Palaeoproterozoic (Mineiro Belt) provenances.
Resumo em Inglês:Abstract Orthopyroxene-bearing tonalites/trondhjemites with scarce quartz diorites comprise the Café enderbite that crops out in three Neoarchean plutons in the central portion of the Canaã dos Carajás domain, Carajás Province, northern Brazil. Intrinsic parameters based on the mineral chemistry of plagioclase, biotite, amphibole, and pyroxene constrain crystallization conditions to 1150–850°C and 750–600 MPa, moderate water content in the melt (4.8–5.6 wt.%) and relatively oxidizing conditions, between the fayalite–magnetite–quartz (FMQ) and nickel– nickel oxide (NNO) + 1.7 buffers. Geochemical modeling indicates that the Café enderbite evolved via at least two fractional crystallization stages – quartz diorite to orthopyroxene tonalite and orthopyroxene tonalite to orthopyroxene trondhjemite – with high crystal content (45–60%, or even higher). This high crystal content during fractional crystallization was the key factor to the preservation of orthopyroxene in the magmatic system as it left only a relatively small proportion of melt to react with early-formed orthopyroxene.
Resumo em Inglês:Abstract Studies involving the magnetic susceptibility (MS) of rocks are becoming increasingly frequent in geological surveys, mainly in those with petrogenetic approaches and in mineral prospecting. Portable devices have become a good alternative for data acquisition because of their practicality and low cost when compared to laboratory analyses. To assess the efficiency of data acquisition with such devices, 291 soil samples were measured with a Terra Plus KT-5 portable device in the field and with a double frequency Bartington MS2B sensor, with mass correction, in the laboratory. The data obtained allowed the characterization of four distinct populations, ranging from high to very low MS values. Statistical analysis showed good correlation between measurements with mass correction in the laboratory and those made with portable devices, resulting in a correlation coefficient (r) = 0.98. All samples obtained in the field and laboratory were utilized in this correlation. The populations identified by both means are in agreement, with subtle discrepancies, thereby demonstrating the efficiency of portable equipment for acquisition of MS data in geological studies.
Resumo em Inglês:Abstract The Cauê Aquifer is an important water supplier to the third largest urban agglomeration in Brazil. The Metropolitan Region of Belo Horizonte is in the Iron Quadrangle, site of intense iron ore mining, requiring large volumes of water and mine dewatering. In this context, we evaluate the regional impact on groundwater level between 2000 and 2019 based on a conceptual hydrogeological flow model on the eastern limb of the Moeda Syncline. This study includes the definition of boundary conditions and aquifer units, the calculation of hydrodynamic parameters, water balance, renewable reserve, and flow analysis. The Cauê, Gandarela, and Moeda aquifers are considered aquifer units, among which the Cauê Aquifer showed the highest hydraulic conductivity (9.45×10-7 m/s on average) as well as the highest recharge rate, with 38.67% rainfall. The average volume pumped was estimated at 41 million m³/year, equivalent to about 150% of the renewable reserve of the Cauê Aquifer, calculated at 27 million m³/year. Underground flow in the area tends southward, and the Cauê Aquifer acts as a water divide, which is less evident in the center of the area, where the greatest impact on groundwater level lowering occurs.
Resumo em Inglês:Abstract The Rio Jacaré Batholith (RJB; 617 ± 4 Ma) is inserted in the Poço Redondo Domain, Sergipano Orogenic System. This batholith is formed by monzodiorite, quartz monzodiorite, monzonite, and quartz monzonite, with abundant microgranular enclaves (MEs). The MEs vary from black to light gray and exhibit globular to slightly elongated shapes with clear-cut, crenulated, and cuspate, or, more rarely, diffuse contacts. They correspond to diorites, monzodiorites, quartz monzodiorites, and monzonites, and textural features indicate mixing of magmas, such as compositional zoning in plagioclase, inclusion zones in plagioclase phenocrysts, poikilitic alkali feldspar, acicular apatite, and ocellar quartz. Calculations of linear correlations of major elements showed that the smallest fraction of mafic magma involved in the mixing was 0.43. MEs represent the breakdown and cooling of a mafic magma that was injected into a cooler felsic magmatic chamber. Emplacement of this mafic magma occurred at different stages of crystallization of the RJB magmatic chamber. The MEs are magnesian and metaluminous, with affinity to the shoshonitic series. Ratios for Ba/Nb (> 23), Ba/La (> 15), and Nb/La (0.22–0.69) are characteristic of magmas generated from partial melting of an enriched lithospheric mantle source. Batch melting modeling suggests that source melting rates of less than 3% are necessary to generate magmas similar to those of the RJB MEs.
Resumo em Inglês:Abstract The Carajás (3.0–2.5 Ga) and Xingu-Iricoumé (1.99–1.86 Ga) blocks comprise the Central Amazonia Province (CAP) that is in contact with the Maroni-Itacaiúnas Province (MIP) within the Amazonian craton. The CAP is the oldest portion (Nd-TDM) of the craton and corresponds to an Archean nucleus bordered by younger Paleo-Mesoproterozoic mobile belts, including the MIP. Because the location and tectonic boundaries between these provinces are insufficiently known, we carried out a geological survey along the Middle Xingu River, cutting the WNW-ESE regional trend, to further understand cratonic evolution of the MIP and its southeastern boundary in this key area. Geochronologic results (Pb-evaporation and U-Pb SHRIMP in zircon and monazite), supported by petrographic and field observations, allowed identification of the following lithotypes and their ages: migmatitic gneisses (2859–2080 Ma), tonalitic gneisses (2554 ± 3 Ma, 2480 ± 9 Ma), enderbites (2114 ± 3 Ma), charnockites (2094 ± 4 Ma, 2084 ± 2 Ma), granodiorites (2079 ± 3 Ma), leucogranitic vein (2075 ± 2 Ma), and pelitic paragneisses (2062 ± 8 Ma). These ages are related to the reworking of Archean crust during Rhyacian magmatic arc amalgamation (2.22–2.13 Ga) and collision in the Transamazonian cycle (ca. 2.1 Ga).
Resumo em Inglês:Abstract The volcanic Trindade Island is a remote Brazilian offshore territory in the South Atlantic, located ca. 1.140 kilometers east of the southeast coast of Brazil. The island’s permanent exposure to geological hazards requires assessment. However, the lack of erosion and landslides temporal data impedes predictive geohazard analyses. Therefore, we compiled pre-existing data from nautical charts and surveyed the surface terrain on Trindade Island to generate Digital Terrain Models (DTMs) and comparative accuracy analyses. The DTM based on pre-existing data shows the lowest accuracy (root mean square error - RMSE: 12.3 m) yet is adequate for regional studies. In contrast, the DTM developed from real-time kinematic global navigation satellite systems (RTK-GNSS) has the highest vertical accuracy (RMSE: 0.48 m), but spatial variability of ground elements was underestimated and limited to meter-sized (and larger) elements. The DTM obtained using the unmanned aerial vehicle (UAV) with ground control points (GCP), on the other hand, presented lower accuracy (RMSE: 2.37 m) than the RTK-GNSS model but still allowed observation of centimetric (and larger) ground features. For geohazard assessment on Trindade Island, models that allow fine-scale studies are needed. A UAV with GCP provides such standards and proved to be the most viable option in remote and complex sites as well. Hence, this study, the first to allow multi-temporal analysis of geohazard assessment on Trindade Island, offers a viable solution for similar analyses in other remote locations.
Resumo em Inglês:Abstract The Umbuzeiro Doce skarn target in the Seridó Belt (NE Brazil) was studied using reflectance spectroscopy and petrography. Interpretation of the 61 reflectance spectra obtained in the study established the following zones: marble zone, with diagnostic absorption features at 2,340 and 2,475 nm (C-O); tremolite marble zone, with main absorption at 1,393 (OH−) and 2,313 nm (Mg-OH); garnet-vesuvianite zone, with the main absorption at 2,215 nm (OH−); diopside-hornblende zone, marked by a broad absorption at 1,153 nm (Fe2+); late wollastonite zone, which lacks a diagnostic spectral signature; and a local violet vesuvianite-bearing marble, characterized by potential gemological vesuvianite marked by Cr3+ absorption features (~548 and 680 nm). Centimeter-sized molybdenite aggregates occur at the interface of the marble and calc-silicate alteration zones. Spectral indices were produced to automate identification of potential zones to explore Mo, W, and gemological vesuvianite. Indices for the zones of marble, tremolite marble, garnet-vesuvianite, diopside-hornblende and violet vesuvianite were produced and tested. This methodology appears as a powerful exploratory guide for Mo, W, and violet vesuvianite occurrences or deposits that show a mineral zoning like the Umbuzeiro Doce target.
Resumo em Inglês:Abstract Turbidity current hydrodynamic properties are evaluated experimentally to understand the formation of turbidity current depositional lobes and the relationship with flow properties, in particular, flow rates. This study focused on the depositional behavior of unconfined turbidity currents through the analysis of three-dimensional experiments performed in a large-scale channel-basin tank without slope break. Three flow rates were simulated when flow velocities, both in longitudinal and transversal directions, were measured and resulting depositional features were evaluated. The three-dimensional physical experiments carried out in this work allowed the identification of two flow rate models with different hydrodynamic characteristics and two distinct lobes. Lower flow rates produced elongated lobate deposits, with characteristic lower flow regime plane bed on the surfaces, characteristic downstream sediment fining that resulted from lower flow velocities, and visibly less turbulent flows from less competent and waning turbidity currents. Higher flow rates showed a more characteristic radial and downstream fining sediment with lobe surfaces displaying ripples and dunes, generated by the higher flow velocities, presumably more turbulent, and more competent turbidity currents.
Resumo em Inglês:Abstract We present this manuscript as a methodological approach and general guidelines for geochemical mapping and background/baseline projects for environmental assessment in tropical areas. A case study was carried out in the Itacaiúnas River watershed (IRW), Eastern Amazon, to fill in a gap in knowledge on the distribution of chemical elements, particularly those potentially toxic, in the near-surface environment of the area. The high-impact results of this research project revealed the need for similar scientific investigation across the globe with the implementation of a systematic methodology. The study shows, for example, the importance of well-planned field activities, multi-medium sampling, analytical methods, laboratory procedures, database construction, and general aspects of data processing and statistical treatment. The importance of this contribution is that it can be used as a reference in support of geospatial analysis in research within the scope of geochemical mapping and background-baseline projects. The database is accessible through a web-based geographic information system front-end; a Geochemical Atlas of the IRW will be available as soon as possible.
Resumo em Inglês:Abstract The transition from the Late Paleozoic Ice Age (LPIA) to fully postglacial conditions in SW Gondwana is under increasing discussion due to either the radiometric ages of its boundary or the stratigraphic nature of this transition. The record of this transition in the Paraná Basin is found in the glacial and glacially influenced deposits of the upper Mafra and Rio do Sul Formations (upper Itararé Group) and postglacial strata of the Rio Bonito Formation (Guatá Group). Here we address the depositional architecture and stratigraphic evolution of these deposits in the Rio do Sul depocenter, eastern Paraná Basin, Brazil, the main area of subsidence in the basin during this transition in Pennsylvanian-Cisuralian time, bringing an opportunity to examine the characteristics of glacial to postglacial transition. Analyses of facies, stratigraphic logs, stratigraphic correlations, and paleocurrent dispersal trends allowed us to define three evolutionary stages. The first stage registers glacial advance from the south-southwest represented by an erosive surface and subglacial tillites. Gravitational deposits covered the tillites in response to ice retreat (upper Mafra Formation), and the Lontras Shales (lower Rio do Sul Formation) correspond to the marine maximum flooding. The second stage comprises co-genetic deepwater (Rio do Sul Formation) to shallow (Rio Bonito Formation, Triunfo Member) progradational deposits after the Lontras Shale maximum flooding. Paleocurrent data and glacially related features point to glaciated source areas located to the NE, E, and SE for the Rio do Sul depocenter during this stage. The third stage corresponds to retrogradational stacking pattern upon a fluvial subaerial unconformity (incised valley), starting with fluvio-deltaic beds (Triunfo Member), followed by fine-grained deposits of the Paraguaçu Member of Rio Bonito Formation. No features related to glacial influence characterize this third stage. As previously suggested, tectonic uplift likely drove the additional NE source and created the space that allowed the transitional contact between Rio do Sul and Rio Bonito formations in the Rio do Sul depocenter.