Search

Alkaline rocks of prevailing sodic composition form major ring complexes and stocks along a seismically active N–S structural lineament parallel to the Paraguay river course. All these occurrences are encompassed by the Alto Paraguay Province, their rocks corresponding to the oldest known alkaline magmatic activity in the Paraná Basin, with a Middle Triassic age of 241.5 ± 1.3 Ma. Intrusive rocks ranging from silica-undersaturated (nepheline and/or sodalite syenites) to silica-oversaturated (quartz syenites and alkali granites) and their fine-grained counterparts constitute the most abundant rock-types. Clinopyroxenes, associated amphiboles and varying amounts of biotite are the main mafic minerals present. Alkali feldspar (mainly mesoperthites and subordinate sanidine and anorthoclase) and feldspathoids (nepheline and/or sodalite) are widespread. A late-stage magmatic to metasomatic/deuteric mineral assemblage is mostly typical of miaskitic to agpaitic rocks of the Cerro Boggiani complex, whereas rare accessory silicate minerals are found in peralkaline syenites of the Cerro Siete Cabezas major intrusion. Positive Nb–Ta and Zr–Hf anomalies and negative Sr, P, and Ti spikes are common features to the whole province. REE behavior shows enrichment in LREE and strong LREE/HREE fractionation with various parallel subtrends. This paper addresses petrological aspects of the origin and evolution of these alkaline rocks. • The paper reviews knowledge about the Alto Paraguay Alkaline Province (border between Brazil and Paraguay). • The main rock types vary from undersaturated (nepheline and sodalite syenites) to silica-oversaturated (quartz syenites and alkali granites) and their fine-grained equivalents. • The intrusion of the alkaline bodies took place along an N–S-oriented structure that marks the boundary of the Pampia and Rio Apa cratonic blocks. • The rocks are dated from the Middle Triassic and constitute the oldest alkaline magmatic event on the edge of the Paraná Basin. [ABSTRACT FROM AUTHOR]

The Juiz de Fora Complex (JFC) in the Araçuaí-Ribeira orogenic system, southeastern Brazil, is a basement unit displaying a variety of rock types that is tectonically imbricated with the supracrustal rocks of the Raposos Group in the Upper Thrust System of the Occidental Terrane, Ribeira Belt. Here we report new petrological, geochemical, and zircon U–Pb and Lu–Hf isotopic data on mafic garnet-granulites from the JFC with a view to evaluate the timing and P-T conditions of the collisional event as well as protolith formation. This work presents the most comprehensive metamorphic study on the Juiz de Fora Complex rocks to date. Metamorphic P-T estimates for the metamorphic peak conditions of 800–900 °C and 8–9 kbar in basic garnet-bearing granulites were obtained for the JFC, indicating that metamorphism took place at deeper crustal levels. Geochemical, zircon U–Pb geochronology, whole-rock Sm–Nd, Rb–Sr isotopes, and Lu–Hf isotopic data of mafic granulites yielded U–Pb ages of ca. 2201 Ma, and moderately juvenile εNd(t) = −0.5 and were obtained from the JFC marking the timing of volcanic arc magmatism during the Rhyacian period. This arc-related stage was followed by the post-collisional period during the Early Paleoproterozoic with ca. 1.94 Ga E-MORB signature granulites that shows evidence of magma sourced from Neoarchean (∼2.8 Ga) crust with addition of juvenile components. The high-grade metamorphism is possibly related to the final collision of the Congo and São Francisco Cratons at 590–560 Ma, which placed the Neoproterozoic metasediments and the JFC rocks at the same level. The integrated information sheds light on two collisional episodes where the oldest is represented by arc-related granulites during the Rhyacian Orogeny and the youngest is the higher-pressure Neoproterozoic overprint during the Brasiliano Orogeny. • The garnet-bearing mafic granulites formed under 800–900 °C with pressures of 8–9 kbar. • Rhyacian and Brasiliano orogenic cycles imprinted the Juiz de Fora Complex rocks. • Zircon data and metamorphic textures indicate a complicated evolution over the two orogenic cycles. [ABSTRACT FROM AUTHOR]

Absolute dating of sedimentary rocks and fossils from the Mesozoic are still rare, mainly due to the difficulties arising from diagenetic changes. Diagenesis promotes changes in the primary structure and chemical composition of biomineralization, which should remain unchanged from the time of deposition and fossilization. Three bioapatite teeth extracted from a jaw fragment of Cladocyclus gardneri from the Romualdo Formation, Araripe Basin, Northeastern Brazil were dated by the U/Pb method in order to obtain an absolute age. The rocks that contain this fossil are dated to Aptian by biostratigraphy of palynomorphs (Sergipea variverrucata biozone, Petrobras P-270), ostracods (Damonella grandiensis biozone, Petrobras) and from the international biozoning of planktonic foraminifera Microhedbergella miniglobularis. The teeth were mounted in 25 mm epoxy resin rounds, imaged by optical microscope and SEM and dated with a Nu Instruments AttoM single-collector ICP-MS coupled to an NWR193nm UC (Ultra Compact) ArF excimer laser system. A Discordia U–Pb date obtained from three separate Cladocyclus gardneri dentine is 110.5 ± 7.4 Ma (2s, MSWD = 2.6), consistent with the biostratigraphic age. This date was based on 155 integrations of line scan analyses (spot size of 30 μm, scan moved at 2 μm per second/100 μm in total length). 238U/206Pb ratios that differed from the mean value by more than 2SD for each tooth were excluded based on the likelihood they reflect U-redistribution during fossilization/post-fossilization fluid alteration. Remaining variations in 238U/206Pb and 207Pb/206Pb ratios along the Discordia reflect variable contributions from common Pb, both within and between the three teeth. They reflect primary U/Pb compositions given that they date the expected age of fossilization. • Direct dating on fossils is still a challenge to be overcome in Geochronology. • Biomineralized parts are more likely to be preserved in the fossilization. • First absolute dating result for the Romualdo Formation, Araripe Basin. • The U–Pb dating corroborates biostratigraphy data. [ABSTRACT FROM AUTHOR]

Although the extinct Quaternary megafauna is widely recorded in northeastern Brazil, geochronological studies carried out on its fossiliferous deposits are still scarce. In this study, Electron Spin Resonance (ESR) was applied to date 12 tooth fragments of Toxodon platensis (Mammalia, Notoungulata) collected in the tank deposit of Lago do Rumo, Baixa Grande, state of Bahia, Brazil. The results obtained in the geochronological analysis performed here reveal ages from around 60 ka to 34 ka for T. platensis in Lagoa do Rumo; and, considering the integration data in the paleontological literature, our results evidence the largest time-averaging ever recorded for tank deposits: around 51 thousand years. • Twelve teeth of Toxodon platensis from Brazilian Northeast were dated using Electron Spin Resonance. • Absolute dating revealed fossils of Toxodon platensis ranging from around 60 ky to 34 ky. • Considering the literature, time-averaging in Lagoa do Rumo is around 51 thousand years. [ABSTRACT FROM AUTHOR]

Abstract: Models of fault growth propose that rift initiation starts with short fault segments. Knowledge of the growth of these segments and their interactions is important to understanding rift geometry and evolution. In the northern part of the Borborema Province, northeastern Brazil, a continental-scale, Cretaceous extensional system of faults has been observed to have reactivated ductile Precambrian shear zones. The faults form small grabens that represent the rift stage of the sedimentary basins. We integrated airborne radiometric and magnetic data with terrestrial gravity survey to investigate the influence of crustal heterogeneity on fault growth and the development of the extensional faults in one of these grabens, the Iguatu basin. Previous studies presented geophysical data, which provide evidence that the Iguatu basin contains a half-graben geometry. In our study, gravity and airborne geophysical data indicate that the basement of the Iguatu basin is part of a heterogeneous structural framework composed of two structural domains, is affected by several ductile shear zones and intruded by a few granite bodies. The gravity modeling reveals that this basin is composed of three right-bend en echelon fault segments. They form a sigmoid system of normal faults that accommodate the strong ∼90° bend of the Precambrian shear zones from E–W to roughly N–S. The growth of these segments led to the generation of two isolated depocenters. The overlapping fault segments link through relay ramps. Release faults that are nearly perpendicular or oblique to the three main fault segments form marginal strike ramps and horst structures in both depocenters. 3D-gravity modeling incorporates the presence of interfering sources of a heterogeneous structural framework. The modeling reveals a maximum sedimentary cover 1620m thick, which occurs at the bend of the reactivated shear zones. The gravity signature of a possible granite body, after removal of the gravity effect of the basin-filling deposits, indicates that this possible intrusion forms a horst oriented roughly parallel to the basement fabric. The relationship between maximum displacement and fault length indicates that the fault segments formed independently during the initiation of the half-graben. But the fault segments, linked by relay ramps, influence the development of one another. We interpret the segments to represent a pre-linkage, multisegmented stage of an aborted rift system. This pattern of fault growth may have also influenced the rift stage of other sedimentary basins in the region. [Copyright &y& Elsevier]

We applied the ESR (Electron Spin Resonance) dating method to fossils of Notiomastodon platensis (two teeth) and Toxodontinae (two teeth) found in the fossiliferous deposit of Lajedão do Patrício, João Dourado, State of Bahia, Brazil, to identify the period of formation of this fossil accumulation. Neutron Activation Analysis was applied to determine the concentrations of the main radioisotopes in enamel, dentine and sediment. The ages found for N. platensis are 16.8 ± 2.6 ka and 12.5 ± 2.3 ka, while the ages for Toxodontinae are 9.6 ± 1 ka and 9.1 ± 1 ka. The results for Notiomastodon. platensis and Toxodontinae are similar to other fossiliferous of Brazil. The estimated maximum time-averaging for Lajedão do Patrício is 11.3 ka, indicating a long period of accumulation of skeletal remains, attributed to resedimentation and reworking. The crossing between the period of formation of fossil assemblage Lajedão do Patrício and ages of climatic variations diponible in paleoclimatic curves produced for the Quaternary of northeastern Brazil indicates different climatic and environmental conditions during the formation of the deposit. • Dating Electron Spin Resonance (ESR) Notiomastodon platensis and Toxodontiane from the Quaternary of Northeast Brazil. • Notiomastodon platensis are 16.8 ± 2.6 ka and 12.5 ± 2.3 ka, and Toxodontinae 9.6 ± 1 ka and 9.1 ± 1 ka. • Maximum time-averaging for Lajedão do Patrício is 11.3 ka, indicating a long period of accumulation. • Comparison of Lajedão do Patrício and paleoclimatic curves different climatic and environmental conditions. [ABSTRACT FROM AUTHOR]