Johildo Salomão Figueiredo Barbosa

The Paleoproterozoic Eastern Bahia Orogenic Domain

The Paleoproterozoic Eastern Bahia orogenic domain occupies an approximately 200 km-wide area in the northern Sao Francisco craton, located between the Atlantic coast and the interior plateau of Bahia state known as the Chapada Diamantina. Together with its African counterpart, the West Central African belt of Gabon, the Eastern Bahia orogenic domain form a complete segment of a collisional orogen developed during the transition between the Rhyacian and Orosirian periods. Exposed in the level of its roots, the Eastern Bahia orogenic domain involves mainly amphibolite to granulite facies rocks that characterize three distinct Archean basement blocks (the Gaviao, Jequie and Serrinha blocks) and a Neoarchean magmatic arc (the Itabuna-Salvador-Curaca belt, ISAC belt). The Paleoproterozoic rock record of the domain includes volcanic, sedimentary and granitic assemblages deposited and emplaced during its pre-, syn- and post-collisional evolutionary stages. The supracrustal rocks are represented by continental to marine metasedimentary successions of presumed Siderian to Rhyacian ages, as well as volcano-sedimentary accumulated in intra- and back-arc basins settings. The granitic rocks comprise five distinct groups of relatively small plutons emplaced between 2320 and 1960 Ma, mainly in the Gaviao and Serrinha blocks. The structural framework of the domain involves sinuous NS-trending and E-dipping reverse, reverse-sinistral to sinistral strike-slip faults and ductile shear zones. These structures nucleated during a second and syn-metamorphic deformation phase dated at ca. 2080 Ma, which was preceded by the convergence and collision of the various basement blocks of the domain at around 2100 Ma. This chapter contains a synthesis on the stratigraphy, overall structure and Paleoproterozoic evolutionary history of the domain.

Applications and limitations of thermobarometry in migmatites and granulites using as an example rocks of the Araçuaí Orogen in southern Bahia, including a discussion on the tectonic meaning of the current results

In southern Bahia, there are outcrops of migmatites and granulites in the Jequitinhonha Complex, which is part of the northern portion of the Aracuai Orogen. Migmatites (garnet-cordierite diatexite) dominate the metamorphic rocks and host lenses and layers of felsic garnet granulite. The conditions of temperature and pressure of metamorphism were calculated using conventional thermobarometry and the software THERMOCALC. Values around 850 °C and 7 kbar were obtained with THERMOCALC. The calculations for the garnet-cordierite diatexite were made considering aH2O equal to 1, but the best results of calculations for the granulites are obtained with aH2O values of 0.3. Pressure values obtained with GAPES resulted in consistent values with THERMOCALC, but the pair garnet-orthopyroxene always produces low values for temperature and high ones for pressure. The results are consistent with the presence of the pair garnet and cordierite in diatexite and orthopyroxene in felsic granulite. From the tectonic point of view, the setting in which metamorphism of these rocks occurred requires high heat flow with a thermal anomaly in mid continental crust, as indicated by values of 7 kbar. Recent studies have favored the closure of a back-arc basin for this tectonic setting, but it does not solve the problem that the time span between metamorphic peak and the end of granite intrusions, involving large bodies of charnockite, is more than 80 million years. The model of tectonic switching is suggested here as it can explain the maintenance of high temperatures for a more extended interval of time.

Teores de potássio, urânio, tório e taxa de produção de calor radiogênico no embasamento adjacente às bacias sedimentares de Camamu e Almada, Bahia, Brasil

The bedrock adjacent to Camamu and Almada sedimentary basins is characterized mainly by rocks of granulite and amphibolite facies, with archean and paleoproterozoic ages, which belong to orogen Itabuna-Salvador-Curaca. The units in major proportion in this context are the metatonalites associated with basic and metamonzonites belonging to Itabuna belt. In smaller area occur the Teolândia granite and the Moenda granodiorite associated with the Ipiau band amphibolites, the charnockites and charnoenderbites of Jequie bloc, the neoproterozoic sienites and the mafic dikes. The K, U and Th contents of the rocks vary from 0,02 to 6,33% for K, from < 0,2 to 9,10 ppm for U and from < 0,4 to 64,38 ppm for Th. These contents are higher in the charnockites, Moenda granodiorite, Teolândia granite and sienites, intermediate in the metatonalites and metamonzonites and lower in the basic granulites. The heat production rates are higher in the lithologies where K, U and Th are also higher, varying from 0,58 to 5,57 µW m-3. The coverage areas of such lithologies are, however, small compared with that of the metatonalitic granulites, metamonzonitic granulites and sienites where the rates vary from 0,10 to 1,44 µW m-3, 0,23 to 5,55 µW m-3 and 0,60 to 2,24 µW m-3, respectively. In this case, the heat production rates vary from 0,10 to 1,44 µW m-3. The basic granulites have the smaller rates, from 0,06 to 0,36 µW m-3. The observation of the lithologies in the margins of the two basins suggest that, in the bedrock under the younger sediments, may predominate the metatonalites, followed by the metamonzonites, with some significant participation of sienites in the Almada basin. In those lithologies, the volumetric heat production rates, with one standard deviation range, are 0,41 ± 0,30 µW m-3 for metatonalites, 0,71 ± 0,57 µW m-3 for metamonzonites and 1,20 ± 0,51 µW m-3 for sienites.

Lead isotope evidence for recent uranium mobility in geological formations of Brazil: implications for radioactive waste disposal

Abstract Lead–lead isotope data from whole rock samples are used to investigate the recent (last few million years) mobility of U and Th. The method is based on the comparison of the calculated present day U and Th concentrations required to yield the Pb isotope composition in the samples with the actual present day concentrations of U and Th obtained by direct measurement. The geological formations studied include the Neoproterozoic carbonate sediments of the Bambui Group, Archean/Paleoproterozoic granite–greenstone terrain of the Contendas–Mirante Complex and a Proterozoic ortho-gneisses hosting U deposit in Lagoa Real. All these formations are in the Sao Francisco Craton, Brazil. The data show high U mobility in the carbonate sediments and in the deformed ortho-gneisses set in a ductile shear zone. Infiltration of groundwater through fault zones seems to have facilitated the U mobility. The Pb isotope approach is a useful technique complementing U-series disequilibrium studies and may be included for site characterization studies for radioactive waste disposal.

Low Density Carbonic Fluids in the Archean Granulite Facies Terrain of the Jequié Complex, Bahia, Brazil

The 2.7 Ga old high-grade metamorphic rocks of the Jequi Complex of Bahia include charnockites, enderbites, and late pink potassic granites. Mineralogic P-T determinations for the granite terrain yield 4-5 kb/750-850C for the peak of metamorphism. Microthermometry of fluid inclusions in quartz was performed on different rock types. Early

Stable Isotope Investigations on the Granulite Facies Terrains of Brazil: A Review

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Study of nucleolar behavior during spermatogenesis in Martarega brasiliensis (Heteroptera, Notonectidae).

-rich fluids have densities around 0.85