Resumo em Inglês:

Abstract Fascinating circular hydrothermal bowls 0.5 m in diameter occur in paleodunes of the 1.2 million km2 Botucatu paleoerg in the Fronteira Oeste of Brazil, Rio Grande do Sul, and were exposed upon erosion of presumed 2,000 m of basalt and rhyodacite cover of the Paraná Volcanic Province (1 million km2) since the Cretaceous (135 Ma). The tops of the paleodunes display low-temperature (150-50°C) hydrothermal structures comparable to those in basalts and rhyodacites. The source beds of sand injectites display evidence of sequential fault-valve action. The bowls represent a new structure in continental paleodunes. They present circular rims sealed by quartz and a flat horizontal bottom with a central sealed vent. The inclined internal rims of the bowls exhibit a radial set of linear fractures filled with quartz, forming prominent, sharp, positive structures. Their origin is ascribed to the work of water vapor originated in the heated freshwater aquifer. The concave ceiling was lifted a few millimeters along circular fractures long enough for quartz to be deposited. Erosional exhumation of the buried Botucatu hyperdry sand sea has revealed the effects, including the new structures described here, of hydrothermalism associated with a major igneous province upon a giant paleoerg turned-aquifer.

Resumo em Inglês:

Abstract Records of Statherian intracontinental ruptural tectonics, magmatism and sedimentation events are recognized all over the South American Platform as an immediate tectonic response to the formation of a large continental domain during Rhyacian-Orosirian orogenic collage. In this paper, we present a preliminary synthesis of the variety of tectonic-sedimentary and magmatic occurrences of these taphrogenic processes along the Statherian Period, which are recorded in all cratons, as well as in the basement of most Brasiliano orogenic belts. The Statherian rock units are better preserved over the cratons, and within the Brasiliano structural provinces they are reduced by erosional processes and discretely overprinted by Tonian and Cryogenian structures, and strongly by the Ediacaran-Brasiliano Orogeny. Clastic sediments are dominantly mature, primary structures clearly pointing to the environmental setting. Anorogenic bimodal magmatism, represented by volcanic and plutonic rocks is common, where felsic rocks are largely dominant. Their geochemical signature indicates within-plate settings and dike swarms are common within the cratons. The magmatism was very intense locally, suggesting LIP occurrences, although in need of additional geophysical and petrological support. The taphrogenic and magmatic events took place within Orosirian and Rhyacian basement, in which main tectonic-magmatic events ceased just before the Late Statherian, occasionally extending into the Calymmian.

Resumo em Inglês:

Abstract A section investigated in the region of Paraopeba, Minas Gerais, provided detailed sedimentologic, stratigraphic and chemostratigraphic data from the Ediacaran Lagoa do Jacaré Formation, Bambuí Group, Southeast Brazil. This information allowed interpretation of tempestite facies in transgressive-regressive cycles, reinforcing the previously proposed storm-influenced sedimentation model and clarifying how it is associated with unusually high C-isotope values related to the Middle Bambuí Excursion (MIBE). Facies analysis of nine lithofacies identified at the GMD quarry based on field and petrographic descriptions showed distal tempestite facies grading upwards to a shallower, oncoidal/ooidal carbonate marine environment, then to basinal shales. The entire succession comprises a low-order transgressive hemicycle, recording the transition from a storm-influenced carbonate ramp to a siliciclastic-dominated platform. Chemostratigraphic data yielded high δ¹³C values ranging between +11.11‰ and +13.94‰. Our data contribute to the revision of the previously proposed interpretation and as well as to the understanding of paleoenvironmental conditions and C isotope signatures across the MIBE, near the Ediacaran-Cambrian boundary in the Bambuí Basin.

Resumo em Inglês:

Abstract The Seis Lagos Carbonatite Complex hosts a lateritic deposit developed over a siderite carbonatite. In the 1980’s, it was estimated to contain 2.89 Bt @ 2.81 wt% Nb2O5. Recent studies have produced new data allowing re-evaluation of the area’s mineral potential. In this work, a geological model has been constructed and new estimates of grades and tonnages are presented. The volume of cavities that occur in the lateritic crust is still unknown, and it was then segmented in possible scenarios ranging from 0 to 63%, impacting estimates of tonnage. The estimations were divided into two domains that vary according to the available data. In Domain 1, it has been estimated between 10.5-28.3 Mt @ 2.97 wt% Nb2O5 and 0.71 wt% REE2O3, and in Domain 2, between 1.40-3.78 Bt @ 1.17 wt% Nb2O5 and 0.46 wt% REE2O3. High grades of Ti, Sn and W were also detected for the lateritic crust. It is estimated that sediments in the Esperança Basin contain 11.8 Mt @ 1.76 wt% REE2O3 and 2.36 wt% P2O5, whereas the siderite carbonatite shows high grades of Nb, REE, Sc and Zn, further increasing the area’s mineral potential. Soil grids on neighboring hills do not show similarly high Nb grades, but do present high grades of REE and P.

Resumo em Inglês:

Abstract New petrographic, lithogeochemical and structural analyses were performed on the Upper Cretaceous lamprophyres emplaced in the Proterozoic basement rocks of the Resende sedimentary basin in SE Brazil. Field characteristics and petrographic data confirm their identification as alkaline lamprophyres. Geochemical variations were mainly controlled by fractional crystallization of clinopyroxene and olivine. Liquid immiscibility, rapid cooling and crustal assimilation may have occurred subordinately during the evolution of the Resende lamprophyres. Mantle and chondrite-normalized diagrams point to an enriched mantle as the lamprophyre’s magma source. Th/Yb and Ta/Yb ratios confirm a within-plate enriched mantle source with residual carbonate and phlogopite/amphibole as suggested by LILE enrichment and the presence of carbonate and phlogopite/amphibole as primary phases. The predominance of an ENE-trend of the dykes is related to a prevalent NNW-SSW orientation of the minimum principal stress (σ3) orientation in an extensional regime during the emplacement of the Resende lamprophyres’ dyke swarm. Frequent offsets produced by local dilation direction are parallel to the NNW-oriented extension of the main segments. WNW to NW secondary sets are controlled by previous discontinuities that were favored as pathways for the highly volatile lamprophyre magma. An NNW-SSE distension related to the Resende lamprophyre dyke swarm emplacement is consistent with the early stages of evolution of the Resende basin and uplift of its basement during the Late Cretaceous to the Paleocene.

Resumo em Inglês:

Abstract Tectonic rejuvenation has been widely described in ancient rifting areas of intraplate settings, despite extensive active tectonics being discontinued long ago. Various models have been invoked to justify such tectonic rejuvenation, suggesting that the long-term deformation in intraplate interiors may be more complex than once supposed. Inherited weakness zones, e.g., pre-existing faults are stress concentrators. However, not all weakness zones in plate interiors show tectonic rejuvenation and recognizing intraplate fault reactivation remains puzzling. Bedrock channels play an essential role in tectonically unstable landscapes and therefore they may unveil which pre-existing structures have been rejuvenated. Here we explore the Cinzas Catchment in order to investigate the interactions between tectonic rejuvenation, bedrock inheritance and geomorphic river signatures in an important Brazilian rifting feature, the Ponta Grossa Arch. We perform a geomorphic analysis combined with fieldwork, dating techniques and existing data to evaluate whether the geomorphic responses of the major rivers show perturbations related to the bedrock inheritance and post-rift rejuvenation. We also show that the interactions between river signatures and the bedrock fault zones are responses to post-rifting rejuvenation coupled with bedrock inheritance. This post-rifting rejuvenation is consistent with a mosaic of superimposed mechanisms which have been invoked by previous studies.

Resumo em Inglês:

Abstract The Cr-rich Jacurici Complex is located at the limit between the Serrinha Block and the Salvador-Curaçá Belt, Northeastern São Francisco Craton. In order to better understand the age span of magmatism and tectonism, we have studied a basement high-grade charnockite orthogneiss, a metanorite of the Complex and a crosscuting undeformed quartz-feldspar pegmatite. For the orthogneiss, SHRIMP U-Pb ages of zircon cores and overgrowth rims yielded, 2966 ± 9 Ma for crystallization and 2091 ± 10 Ma for metamorphism, respectively. Zircons from the metanorite show internal textures consistent with high grade metamorphism. The U-Pb LA-ICP-MS age determination provided a 2099 ± 6 Ma metamorphic age for the Jacurici Complex, indicating it was already emplaced during amalgamation around 2.1 Ga. Zircons from the undeformed quartz-feldspar pegmatite provided a LA-ICP-MS U-Pb age of 2083 ± 11Ma, marking the end of the N-S transcurrence. The magmatic Jacurici Complex age remains undefined, but is equal to, or older than, 2.1 Ga. Therefore, possible correlations with other mafic-ultramafic intrusions, such as the Cu-rich Caraíba Complex, should be reassessed. Our results indicate that the N-S transcurrence ended earlier in the Jacurici Complex region than in the northwestern area of the Salvador-Curaçá belt.

Resumo em Inglês:

Abstract Understanding the variability in synthetic aperture radar (SAR) backscatter as a function of the phenological variability of vegetation and tropical environmental conditions is still a challenge. Through quantitative and qualitative analyses, this study addresses how both environmental and precipitation conditions, tidal range, and vegetation density affect the backscatter coefficients of coastal environments based on the analysis of four C-band, HH-Polarization, RADARSAT-1 Wide 1 images, in descending orbit, acquired in the wet and dry seasons. The canopy structural variations were analyzed using the enhanced vegetation index (EVI), and correlated with the backscatter values of SAR images, in order to define the predominant backscatter mechanisms, making it possible to distinguish the mangroves from the brackish marshes in the easternmost Amazon coastal plain (Caranguejos Island). The C-band, HH-Polarization RADARSAT-1 backscatter shows the highest correlation with average EVI values in the dry season. The results show that mangroves and brackish marshes can be distinguished in images acquired under low precipitation, especially in the dry season. Variations in the tidal conditions have no effect on the backscatter values of RADARSAT-1 images. We conclude, therefore, that multitemporal RADARSAT-1 images are adequate for monitoring and discriminating wetland environments in a humid tropical climate.

Resumo em Inglês:

Abstract The Borborema Province, NE Brazil, is marked by several Ediacaran granitic plutons that generated high-temperature metamorphic aureoles in the country rock. However, information about magma emplacement, age of plutonism, and metamorphic conditions are necessary to understand this scenario. To this end, we present field, petrographic, and zircon U-Pb geochronological data for migmatized hornfels and intrusive Umarizal and Tourão-Caraúbas plutons. The field features allowed the construction of a structural evolution model starting with high-temperature sinistral strike-slip shear zones followed by dextral strike-slip movement associated with the magmatic emplacement. Mineral paragenesis of andalusite/sillimanite, garnet, scapolite, and phlogopite in country rocks within the metamorphic aureole indicate temperatures of at least 700-800°C and pressures lower than 4.5 kbar corresponding to the pyroxene hornfels facies. Zircon U-Pb ages of 563.7 ± 6.2 for the Umarizal granite, 589 ± 4.4 Ma for the Tourão-Caraúbas granite, and 580.5 ± 4 Ma for the neosome from contact aureole were obtained. The results show that magma emplacement and HT/LP (high-T/low-P) contact metamorphism were synchronous with a transtensional event. These features suggest a late- to post-tectonic context, following the collapse of the Brasiliano/Pan-African orogenetic chain, that favored Late Ediacaran plutonism and synchronous HT/LP metamorphism.

Resumo em Inglês:

Abstract Chonetoidea corresponds to one of the most abundant superfamilies throughout the Malvinokaffric Realm (Argentina, Uruguay, Bolivia, Paraguay, Antarctica, Peru and South-Central Brazil). In this paper we provided a better understanding of the diversity of this superfamily during the Devonian in southern Gondwana. For Bolivian specimens, we corroborate both the occurrence of Pleurochonetes anteloi in the Huamampampa Formation, that is, this species may have occurred at least in the Givetian, and the occurrence of Australostrophia mesembria in the Pragian-Emsian of Bolivia. We further identify both Aseptonetes isaacsoni in the Icla/Gamoneda Formation, which represents the oldest occurrence of this species, and Chonostrophia truyolsae in the Belén Formation/Bolivian Altiplano, thereby expanding its geographical distribution. We also provide the first description and illustration of a specimen of Chonetoidea from Antarctica and record the first occurrence of Australostrophia mesembria, Devonochonetinae indet. and Eodevonaria sp. in the Cabanillas Formation of Peru. Because A. mesembria is a typical Malvinokaffric species, this corroborates the extension of this realm into southern Peru.