Resumo em Inglês:

Abstract The Bacajá domain, southeastern Amazonian craton, comprises Mesoarchaean and Siderian terrains reworked during the Transamazonian cycle. Combined analyses of zircon LA-ICP-MS U-Pb and Lu-Hf with whole-rock Sm-Nd from the northeast portion of this domain made it possible to propose an evolutionary sequence between ca. 2.60 and 2.06 Ga. Gneisses with an igneous protolith age of 2630±15 Ma show negative signatures (ɛHft = -0.3 to -1.7 and ɛNdt = -3.08 to -2.98) and a Mesoarchaean formation (Hf-TDMC and Nd-TDM model ages range from 3.0 to 3.2 Ga). Rhyacian granite genesis lasted about 40 million years (2.10–2.06 Ga) and was divided into two magmatic periods. The first is represented by deformed granitoids with zircons yielding crystallization ages between 2.10 and 2.09 Ga and model ages (Hf-Nd) at about 2.5 Ga. The second event is represented by granitoids with preserved magmatic texture, crystallization ages of 2.06 Ga, and Siderian model ages (Hf-Nd) of around 2.3 Ga. The overall Hf isotopic analyses of this Rhyacian granite genesis exhibit a spread of ɛHft values between 1.8 and -2.9, which show a probably underestimated mantle-derived contribution in this period.

Resumo em Inglês:

Abstract Alkaline dikes from the Santos-Rio de Janeiro coast, SE Brazil, mainly from São Sebastião and Búzios islands, yielded a Late Cretaceous paleomagnetic pole (SR) located at 319.7°E 81.2°S (N = 44, A95 = 3.0°, k = 44). This pole includes some sites of alkaline stocks from São Sebastião Island of the same age and supersedes the previous pole for this island. To match the available radiometric ages and the prevailing normal polarity remanence of the rocks with the geomagnetic polarity time scale, the SR pole is placed at 84 Myr. Another group of alkaline rocks, dikes located mainly in Rio de Janeiro, was assigned an age of less than 70 Myr. The ages of the Poços de Caldas, Itatiaia, and Passa Quatro paleomagnetic poles are also discussed based on available radiometric data. Assuming a rigid plate, the SR pole indicates the southward movement of about 7° with virtually no rotation between 100 and 84 Myr. From approximately 84 to 70 Myr, a clockwise rotation of 8° is postulated, with slight variation in latitude.

Resumo em Inglês:

Abstract This article discusses the geochemical and petrological evolution of the São José do Campestre granite complex (SJCgr), the last Neoarchean plutonic event so far described in the São José do Campestre massif in NE Brazil. We report field, petrographic, zircon U-Pb dating, and whole rock and mineral chemistry for representative SJCgr samples. Laser ablation zircon U-Pb data indicate that the granite emplacement took place at 2664 ± 13 Ma. The rocks comprising the SJCgr have relatively well-preserved primary textures and fabrics and compositions varying from gabbro to syenogranite. Major and trace element contents reveal a metaluminous, calc-alkaline through transitional to alkaline signature, and LILE- and LREE-enriched series analogous to late Archean sanukitoid and modern arc granitoid. The evolution of the SJCgr is envisaged as follows: (1st) partial melting of a metasomatized mantle (2.5–3.0 GPa, ~85–102 km, 1,000–1,200°C), generating a basaltic to basaltic andesitic magma; (2nd) fractional crystallization (FC) of olivine at mantle or lower crustal depth, leading to the parental magmas of the magmatic series; and (3rd) 40–15% FC of olivine gabbro-norite and olivine monzonite cumulates (400–600 MPa, 15–23 km). The SJCgr shares similarities with post-collisional granitoids and, thus, would represent the last Neoarchean episode of mantle-derived magma in Northeastern Brazil.

Resumo em Inglês:

Abstract Santonian-Campanian tuffaceous siltstone, epiclastic siltstone, arkose, and fossiliferous mudstone occur in a 45-m-thick section of well 1-BRSA-37-RJS drilled in the southern Campos Basin, offshore SE Brazil. Well log data and petrographic and lithogeochemical data obtained from cutting samples were used to propose a schematic model of the volcano's sedimentary paleoenvironment. The volcaniclasts in the tuffaceous siltstone and epiclastic siltstone are scoria basalts, typical of spatter cones associated with monogenetic fields worldwide. The combination of these features with the petrographic ones of the arkoses found in the same well interval is likely to be related to volcaniclastic processes taking place on the continental shelf. Ratios between the immobile trace elements of the volcaniclastic rocks can be explained by mixing between sources in the upper continental crust adjacent to southern Campos and the scoria basalts extruded in the monogenetic fields. The little differentiated, olivine-rich basalts extruded from the scoria and spatter cones suggest a rising asthenosphere mantle extending from hundreds of kilometers eastwards till southern Campos in the Santonian-Campanian. This may have resulted in regional discordances in the Santonian, both in the Campos and Santos basins, and also places the petroleum systems in the southern Campos Basin under the thermal influence of such a rising asthenosphere in the Santonian-Campanian.

Resumo em Inglês:

Abstract The Paramirim Aulacogen was filled by the Espinhaço and São Francisco supergroups in the period from 1.75 to 0.69 Ga, and suffered partial inversion during the Ediacaran. In the northern sector of the intracontinental domain of the Araçuaí-WestCongo Orogen, six compressive deformational phases were interpreted and associated with the frontal inversion of the Paramirim Aulacogen. Three structural domains were identified: Jacaraci Fold-Thrust Belt, Caetité Nappe, and Transpressive Domain. The nucleation of deformational structures is related to a maximum stress field with general WSW-ENE orientation. The first deformational phase (D1), found in the Jacaraci Fold-Thrust Belt and the Transpressional Domain, is related to the evolution of the Rio Pardo Salient. A dextral transpressional system nucleated the Caetité Nappe. This system also juxtaposed amphibolite facies rocks over lower temperature facies, as well as Statherian over Tonian rocks. Furthermore, this system was responsible for deformation in the transpressional domains and late deformation in the Jacaraci Fold-Thrust Belt. The region presents a variety of structures and is, therefore, an excellent natural laboratory for studying positive inversion in aulacogens.

Resumo em Inglês:

Abstract Silicification is a diagenetic process commonly observed in sedimentary sections of the Aptian carbonate deposits of the Santos Basin that involves the replacement of carbonate minerals with silica. It is mostly associated with hydrothermal fluids percolating through faults and fractures. CO2 degassing favors an increase in acidity. Consequently, the fluids became subsaturated with calcite and supersaturated with silica. Given the increase in silica content, two distinct behaviors were identified: reduction and enhancement of permoporous properties. The aim of this work is to identify permoporous alteration due to silicification in the Barra Velha Formation. To accomplish this, a qualitative analysis was conducted correlating acoustic borehole images, nuclear magnetic resonance porosity and permeability, and elemental capture spectroscopy logs were performed, followed by a quantitative analysis based on crossplot evaluation. The Barra Velha Formation was divided into three intervals with respect to silica content. The common behavior observed is permoporosity reduction associated with silica increase, but locally, permoporosity enhancement associated with silica increase was also identified. Finally, we established a direct relationship between calcite dissolution and silica precipitation, a well-defined trend characterized by sonic log (DT) reduction and density increase, and an inverse relationship between acoustic impedance and permoporous properties. Finally, the quantitative analysis also favored improving the lack of reservoir qualities of the intrusive igneous interval.

Resumo em Inglês:

Abstract The terminal Ediacaran fossil record includes microbial mats and body fossils characterized by simple morphologies, which represents a challenge to understand several aspects related to the paleoecology of the emerging complex life. The marine siliciclastic deposits of the Cerro Negro Formation (~560–550 Ma) contain evidence of different styles of microbially induced sedimentary structures (MISS) and discoidal forms associated with them. Different types of MISS, such as Kinneyia and wrinkle structures, elephant skin, and Arumberia, are reported and related to shallow marine depositional environments. These morphologies are commonly associated with the high quality of preservation of body fossils in Ediacaran deposits. The preservation of Aspidella discoidal holdfast is related to biotic and abiotic processes involving substrate sealing by microbial mats, fluidization, and probably organic matter decay. Both abiotic factors (tidal currents and waves, added to liquefaction and sand injection) and biotic factors (substrate biostabilization by microbial mats favoring sediment cohesion) are considered the main ones responsible for the preservation style in the Cerro Negro Formation. This formation constitutes a remarkable example in SW-Gondwana of how preservation dynamics took place on a seabed sealed by microbial mats and is an important deposit conserving diverse Ediacaran forms of life in South America.

Resumo em Inglês:

Abstract The Parauapebas Formation and Igarapé Cigarra Formation constitute two Neoarchean extrusive units inserted in the metavolcanosedimentary sequence (~2.76 Ga) of the Grão Pará Group, Itacaiúnas Supergroup, being represented by basaltic seeps underlain and overlain by jaspillites, rhyolites, volcanoclastic rocks, and gabbro dikes/sills, thus being inserted in the Serra Sul district in the Mineral Province of Carajás, Pará. This study aimed to understand the igneous processes involved in the genesis of these basalts from both geological formations, supported by field, petrographic, and geochemical data. Inserted in the metavolcanosedimentary sequence of the Grão Pará Group (Carajás Domain), the basalts and basalt andesites, the most abundant types of the Parauapebas and Igarapé Cigarra Formations, occur as successions of massive and amygdaloidal lava flows. The study of core samples from nine stratigraphic holes that intersected basaltic rocks of the two geological formations in body S11D (Serra Sul) showed that these rocks reach thick layers with different characteristics, ranging from aphanitic to phaneritic. The basalts are dark green, aphanitic, phaneritic, thin to medium, and hypocrystalline. They show preserved igneous texture being commonly seriated as inequigranular, intersertal, ophitic, subophytic, intergranular, branching, and swallow tail. Its essential primary constituents are plagioclase, orthopyroxene, and clinopyroxene, and its accessory constituents are titanite and apatite. Chlorite, epidote, quartz, and calcite occur as secondary phases and are interpreted as the products of ocean-bottom hydrothermal alteration and/or incipient metamorphism. In general, the studied volcanic rocks stand out for SiO2 content between 47.00 and 56.04%, high alkali contents (4.70–7.50%), K2O contents between 0.40 and 3.97%, TiO2 (< 1.0%), and MgO between 4.90 and 8.16%. In the classification diagrams, the samples are situated in the fields of basalts, basalt andesites, trachyandesite, and trachybasalt basalts, in the transition of the toleitic and calc-alkaline series. The basalts of the Parauapebas and Igarapé Cigarra Formations were probably formed in an intraplate continental environment influenced by subduction zones. Therefore, the Grão-Pará Basin was probably formed in a convergent regime related to a subduction-type environment. The geochemical data indicate that the studied rocks of the Parauapebas Formation and the Igarapé Cigarra Formation correspond to basalts of calc-alkaline affinity. Two facies were identified in the Parauapebas Formation, namely, the Amendoim and Violão facies. In the Igarapé Cigarra Formation as well, the following facies were identified: Jacaré and Três Marias. Based on the geochemical interpretation, it is assumed that the rocks studied are arc-like continental basalts, which are formed in a subduction environment with influence from a possible asthenospheric source and modified by crustal contamination.

Resumo em Inglês:

Abstract The objectives of this study were the biostratigraphic and paleobiogeographic interpretations of the calcareous nannofossil records of the Romualdo Formation (Aptian-Albian) in the south-central portion of the Araripe Basin. The methods used to process the samples were decanting, smear slide, centrifugation, and ultra-thin section confection. The taxonomic and biostratigraphic interpretations were carried out through qualitative and quantitative investigations. A total of 11 genera and 17 species of calcareous nannofossils, namely, Biscutum constans, Biscutum sp., Braarudosphaera africana, Braarudosphaera sp., Calculites sp., Discorhabdus ignotus, Hayesites albiensis, Hayesites sp., Nannoconus bucheri, Nannoconus sp., Retecapsa surirella, Rhagodiscus sp., Thoracosphaera sp., Watznaueria barnesiae, Watznaueria sp., Zeughrabdotus erectus, and Zeughrabdotus noeliae were identified. The integrated analysis of three outcrops suggests two distinct marine phases: the first in the Aptian, recognized in two outcrops, and during the Albian age. The genera Hayesites and Nannoconus suggest a strong Tethyan affinity. Differently, other calcareous nannofossils such as Watznaueria, Thoracosphaera, and Braarudosphaera recorded here are considered more resistant, cosmopolitan, and/or opportunistic species. Biscutum and Zeughrabdotus are sensitive calcareous nannofossils because they are more susceptible to dissolution and cannot be surely used to define this paleobiogeography and biostratigraphic range.

Resumo em Inglês:

Abstract This study revises the early-magmatic, late-magmatic, and hydrothermal mineral assemblages occurring in rocks of agpaitic affinity, mainly represented by nepheline syenites and phonolites, from the Poços de Caldas alkaline massif. Mostly composed of different types of silicates, these rocks include rare minerals of of distinct groups, such as sulfides, oxides, carbonates, phosphates, sulfates, niobates, and notably halides, with villiaumite (NaF) as an important phase indicative of extreme alkali enrichment. Hydrothermal alteration products of nepheline and alkali feldspar typically consist of feldspathoids and zeolites. The rare minerals are of varied origin, from magmatic crystallization to hydrothermal and weathering processes. In general, they tend to occur interstitially or form euhedral crystals, especially when filling cavities or growing out from their walls. Petrological conditions involving the formation of the agpaitic rocks and their mineral assemblages are discussed.