Miguel Tupinambá

Correlation of Neoproterozoic terranes between the Ribeira Belt, SE Brazil and its African counterpart: comparative tectonic evolution and open questions

Abstract Four main classes of tectonic entities may be considered for the Ribeira Belt and southwest African counterparts: (1) cratonic fragments older than 1.8 Ga and their passive margin successions, (2) reworked basement terranes with Mesoproterozoic and/or Neoproterozoic deformed cover, (3) magmatic arc associations, (4) terranes with Palaeoproterozoic basement and deformed Neoproterozoic back-arc successions. Based on comparative investigation, a tectonic model of polyphase amalgamation is proposed with c. 790 and 630–610 Ma major episodes of intra-oceanic and cordilleran arc magmatism along both sides of the Adamastor Ocean. Subsequent diachronous collision of the arc terranes and small plates followed at c. 630, 600, 580 and 530 Ma. The tectonic complexity reflects an accretionary evolution from Cryogenian to Cambrian times. The São Francisco–Congo and Angola palaeo-continents did probably not behave as one consolidated block, but rather may have accommodated considerable convergence during the Brasiliano/Pan-African episodes. The final docking of Cabo Frio and Kalahari in the Cambrian was coeval with the arrival of Amazonia on the opposite side, resulting in lateral reactivation and displacement between the previously amalgamated pieces. The transition between the Cambrian and the Ordovician is marked by the extensional collapse of the metamorphic core zones of the orogens.

Poorly crystalline Pd–Hg–Au intermetallic compounds from Córrego Bom Sucesso, southern Serra do Espinhaço, Brazil

Potarite, ideally PdHg, is reported in the literature to have compositions varying from PdHg or Pd(Hg,Au) to Pd3Hg2. Such a Pd3Hg2 phase is unknown in the synthetic Pd–Hg binary system. For the first time, Pd–Hg grains recovered from the historical Bom Sucesso alluvium, regarded as the type locality of Pd, are shown to consist of arborescent and lamellar intergrowths of two intermetallic compounds, compositionally close to empirical Pd(Hg,Au), i.e. auriferous potarite, and (Pd,Au)3Hg2. The Pd–Hg–Au grains have a rim of palladiferous Pt. The otherwise sharp Pd–Hg–Au intergrowths become diffuse at the contact with the palladiferous Pt rim. Both the Pd–Hg–Au compounds and the palladiferous Pt rim did not diffract using the electron-backscattered diffraction (EBSD) and powder X-ray microdiffraction techniques, indicating that they are poorly crystalline. Their poor crystallinity and the diffuse zone between the Pd–Hg–Au core and the Pt-rich overgrowth are suggestive of electrochemical metal precipitation from dilute solutions within the alluvium.

Rio de Janeiro: A Metropolis Between Granite-Gneiss Massifs

The city of Rio de Janeiro is well-known for its outstanding landscape, particularly the several steep rocky domes that emerge close to the coast. Such a wonderful scene results from a combination of differential weathering controlled by the presence of a variety of Neoproterozoic gneisses and Cambrian granites and strong valley incision along subvertical faults and fractures. In this chapter we present the geological and geomorphological aspects that influence the daily life of more than 12 million inhabitants of the Rio de Janeiro metropolitan region, in two areas around Guanabara Bay: Coastal Massifs and the Serra dos Orgaos escarpment. The combination of such topographic constraints, intense summer rainstorms, and dense occupation frequently creates a scenario with a high potential for landslide-derived hazards.

The Guanabara Bay, a Giant Body of Water Surrounded by Mountains in the Rio de Janeiro Metropolitan Area

The Guanabara Bay in Rio de Janeiro is one of the most important Brazilian morphological features, both in terms of its geological and geomorphological evolution as well as considering its historical importance to the development of Brazil. The existing morphological contrast, varying from mountain escarpments , hills, fluvial, and marine coastal plain s, beaches, and lagoons, results from a complex evolution that imprinted in the landscape an intriguing diverse morphology which constitutes a highly attractive touristic natural scenario that is well known internationally. Its geological and geomorphological history is related to the Paleogene extensional faults and alkaline magmatism within the Guanabara Graben that was filled by Cenozoic continental and/or fluviomarine sediments. In this landscape, the use of natural resources has constrained human occupation for more than 500 years. It was only in the 2000s that improved environmental consciousness promoted measures toward minimizing environmental degradation processes.

Evolução tectónica e magmática da faixa ribeira entre o neoproterozoico e o paleozoico inferior na região serrana do estado do Rio de Janeiro, Brasil

The Highlands of the Rio de Janeiro (RSF) are composed of granitic gneiss, paragneiss and granite that are part of the Ribeira Belt. Their geological features were studied for over 50 years by authors who described orthogneisses and granites arranged in large overlapping tectonic sheaths. Tectonic domains and terranes were proposed for the Ribeira Belt, and the rocks of the RSF were included in the Oriental Terrane. The tectonic and magmatic evolution of the RSF begins in the Cryogenian with the installation of the magmatic arc of the Rio Negro Complex and sedimentation of the Italva, St. Fidelis and Bom Jesus do Itabapoana groups. During the Ediacaran collision of this arc with a passive margin the migmatitic leucogneisses of the Cordeiro Suite were generated. At the collisional ending the calc-alkaline magmatic Serra dos Orgaos Suite was produced. At the end of evolution, tectonic exhumation and tectonic collapse was coeval with the intrusion of Cambro-Ordovician granites of the Nova Friburgo Suite.

The Ribeira Belt

The generation of the NW-verging Ribeira belt that fringes the Sao Francisco craton to the south resulted from collisional episodes dated at 620–605, 605–565, and 535–510 Ma. During these collisional events, micro-continents and magmatic arcs converged and accreted to a continental margin system formed along the previously amalgamated Sao Francisco-Paranapanema landmass. The Sao Francisco-Paranapanema collision at around 640–620 Ma led to the development of the Southern Brasilia belt. The Ribeira orogenic front overprinted the pre-existent southern end of east-verging Brasilia belt, thereby creating a tectonically very complex interference zone. The Ribeira belt as whole comprises four tectono-stratigraphic terranes: the Occidental, Paraiba do Sul, Oriental, and Cabo Frio terranes. The Occidental terrane, representing the external sector of the belt, involves the reworked cratonic basement (Archean and Paleoproterozoic orthogneisses and orthogranulites), Mesoproterozoic intra-cratonic basins and a Neoproterozoic passive margin unit (Andrelândia Group). The uppermost unit of the Andrelândia Group is regarded to be deposited in an active margin setting and sourced by a magmatic arc installed in the Paranapanema plate. The Paraiba do Sul terrane, composed of an Archean-Paleoproterozoic basement, Neoproterozoic metasedimentary units, and a continental magmatic arc (the 640–595 Ma Serra da Bolivia complex), accreted to southeastern sector of the craton margin between 620 and 605 Ma. Afterwards, the juvenile to immature magmatic arc of the Oriental terrane (860–620 Ma Rio Negro and Serra da Prata complexes) collided to proto-Ribeira belt. The crustal thickening resulting from these collisions gave rise to widespread generation of I, S and hybrid granites that intrudes both basement and cover units in the most deformed sectors of the belt. The Cambrian (535–510 Ma) docking of the Cabo Frio terrane (an Angola craton fragment) reworked the previous accreted terranes, generating large scale folds and dextral transpresional shear zones that reached the SFC margin. Finally, a vigorous bimodal magmatic event associated with transtensional deformation episodes characterizes the orogenic collapse of the belt and marks its stabilization in the interior of the Gondwana supercontinent.

Modelado gravimétrico de una sección entre el margen continental del sudeste de Brasil y el área emergida adyacente

This study was conducted in a gravimetric section between the continental margin and the onshore area of southeastern Brazil, encompassing the northern portion of the Santos Basin and the Neoproterozoic Brasilia and Ribeira mobile belts. Free-Air gravimetric terrestrial and satellite data have been fixed and calculated for Free-Air anomaly and used as the basis for the 2D gravity modeling. Constraints imposed to the 2D gravity model were based on magnetotelluric model 2D inversion for the terrestrial part of the section and model of joint inversion and 2D seismic reflection migrated in time for the Santos Basin. Residual data were calculated from the Free-Air anomaly subtracted from the regional field and were used to associate the shallow part of the model to the geophysical models. In the area of the continental margin, crustal and lithospheric thinning was observed below shallow shelf and in the proximal part of the Santos Basin. Gravity data of the regional field were calculated from a polynomial fit to most Free-Air anomaly data were used to model and fix the lower crust and lithospheric mantle. The fit of the model to regional data was done only it was infered keels at the base of the lithosphere, interpreted as subducted paleoplates situated below suture zones in Brasilia and Ribeira belts. This model fits the observed gravity data with error less than 8% of the total gradient.

A corrida de massa do rio Vieira em Teresópolis, sudeste do Brasil: caracterização da área-fonte dos sedimentos transportados

This work has presents the geology of the bedrock and the sedimentary deposits of the upper course of the Vieira river where a debris flow killed 23 people on January 2011. The upper course of rio Vieira has drawn a straight line and is embedded in a N25E trending fracture and fault system. Over the almost 2 km of exposed bedrock, there is a predominance of gneissic Cordeiro and Serra dos Orgaos suites. The granite of the Nova Friburgo suite occurs only in the river head and in a narrow strip in the middle section of the channel. Three types of sedimentary remnants were observed: a) talus deposit with granite boulders in the river head; b) gneissic rock fall deposits near the the channel walls; c) granite boulders and gneissic blocks in the floodplain. The presence of granite boulders at the head and the scarcity of the granite in the bedrock indicates that the bulk transported solids comes from the erosion of talus deposits in the headwaters of the river. The blocks of gneiss comes from rock-fall deposits along the channel walls.

Spessartine in compact-hematite rock, southern Serra do Espinhaco, Minas Gerais, Brazil, and genesis of compact hematite

Abstract Fine-grained garnet (grains <50 μm across) is an accessory component of compact hematite, a rock consisting essentially of hematite; compact hematite is a variety of high-grade Fe ore. The garnet is characterized compositionally as spessartine (81-86 mol.%) with subordinate, but significant, amounts of calderite (5-11 mol.%) and ‘‘blythite’’ (up to ~5 mol.%), as well as andradite (4-7 mol.%); pyrope and almandine endmembers are ≤~2 and 1 mol.%, respectively. The recognition of spessartine in compact hematite indicates that oxidation state, rather than whole-rock chemical composition, controlled the garnet composition. The spessartine has a positive Eu anomaly, and a low Th/U ratio (0.13-0.65) compared to the average upper continental crust, Th/U = 3.9, as well as a positive linear correlation of U vs. Li. The spessartine-hosting compact-hematite rock is a high-grade hematite ore that similarly shows a low Th/U ratio, but a convex, tetrad-like segment between Gd and Dy. Decoupling of Eu from other rare-earth elements and of U from Th in the spessartine, together with the anomalous geochemical behaviour of Gd, Tb and Dy in the compact hematite, and its low Th/U ratio, could have been achieved under oxidizing conditions at greenschist-facies metamorphic temperatures. The U and Li present in the spessartine could have been sourced from metamorphic fluids of continental, possibly evaporitic, origin. These interpretations are underpinned by the regional hematitization and tourmalinization observed in the southern Serra do Espinhaço.