José de Araújo Nogueira Neto

Evidence for Neoproterozoic Continental Arc Magmatism in the Santa Quitéria Batholith of Ceará State, NW Borborema Province, NE Brazil: Implications for the Assembly of West Gondwana

Abstract Recent field investigations and geochronological studies of Neoproterozoic rocks in the northwestern part of the Borborema Province, Ceara State, NE Brazil provide important clues pertaining to the nature of convergence between the Borborema Province and the West African-Sao Luis craton during the assembly of West Gondwana. U-Pb zircon data indicate that the earliest evidence of convergent magmatism along the northwest margin of the Borborema Province occurred around 777 Ma, and was followed by the development of a large continental arc batholith (Santa Quiteria batholith) between ca. 665 and 591 Ma within the central part of Ceara State. These findings, along with supporting geophysical data, suggest that convergence between the Borborema Province and the West African-Sao Luis craton involved closure of an oceanic realm with subduction polarity to the southeast beneath the northwestern part of the province. Consequently, it seems likely that the Pharusian Ocean was continuous from the Hoggar Province in West Africa into South America during the late Neoproterozoic and additional data suggests that it may have even been connected with the Goianides Ocean of the Brasilia Belt farther to the southwest.

Comparisons between the northwestern Borborema Province, NE Brazil, and the southwestern Pharusian Dahomey Belt, SW Central Africa

Abstract Geological and geochronological data for the northwestern part of the Brasiliano Borborema Province are described and compared with their counterparts in the Pan-African Dahomey (Pharusian) belt that flanks the southeastern margin of the West African Craton, where outcrops are sufficiently continuous to discern the nature of the collision during West Gondwana assembly. In the Médio Coreaú domain, NW Borborema Province, U–Pb and Sm–Nd data have revealed unusual basement rocks representing 2.35–2.30 Ga juvenile crust, along with large tracts of 2.15–2.10 Ga juvenile gneisses in the Ceará Central domain. These basement blocks were affected by two pulses of intracratonic extension at 1785 and 775 Ma. Prior to West Gondwana collision, a continental arc (the Santa Quitéria batholith) developed between 665 Ma and 620 Ma. The presence of this arc strengthens the hypothesis that convergence between the Borborema Province and the São Luis craton involved closure of an oceanic basin. New geochronological data are presented showing that Palaeoproterozoic orthogneisses (U–Pb upper intercept 2288±2 Ma) were affected by a major late Neoproterozoic event (554±4 Ma U–Pb lower intercept, 558±3 Ma Sm–Nd whole-rock and mineral isochron). Exhumation and cooling of granulite rocks between 568 and 550 Ma in the Médio Coreaú domain and between c. 587 and 576 in the West African Dahomey Belt indicate that the final tectonic phase was not simultaneous along this front of the orogen.

Microtomography-Based Pore Structure Modelling of Geologic Materials Used as Building and Dimension Stones

In this paper, an initial X-ray Computed Tomography study of sulphate salt degradation of two Portuguese Dimension Stones (“Semi-rijo” and “Mocacreme”) is presented, based on the Standard EN 12370 (1999). This study was performed using a high resolution X-ray Computed Tomography, a 3D X-ray microscopy non-destructive technique, in order to get representative digital information on 3D texture of the external surface and of the interior of visible light opaque objects for imaging and eventually measuring them, at resolutions in the m range. This will produce spatial quantitative information map on the density distribution of the scanned samples, usually associated to different mineral phases and voids’ textures constituting the natural materials. Both types of stones are also being investigated by the combined application of classic methods: Optical Microscopy, Scanning Electron Microscopy and Mercury Injection Porosimetry (MICP). Most of those classic studies use mainly traditional 2D imaging techniques, none of these being able to produce the 3D resolution details that X-ray micro-tomography enables. In order to achieve future better qualitative and quantitative integrated models, it will be important to combine its non destructive and 3D characteristics results with those of 3D MICP models obtained for such complex materials. This enables to qualitatively and/or quantitatively assess the evolution and decay potential of different phases and voids (pores + fissures) textures in different environmental interaction conditions. So, in order to set-up more efficient forecasts of their engineering properties behaviour in a given environment this study is an essential initial complementary step to compare and integrate in near future studies all the advantages and disadvantages of the application of these classic and new methods, based respectively upon visual examination of the samples’ exteriors and on micro-tomographic image models of the samples’ interiors and surfaces, to these geologic materials.

Petrografia e química mineral do Stock Granítico Serra do Barriga (Sobral, CE, nordeste do Brasil)

Estudos sobre aspectos petrograficos e de quimica mineral no Stock Granitico Serra do Barriga (SGSB) mostram tipos faciologicos que variam de sienogranitos e monzogranitos inequigranulares a megaporfiriticos diferenciados pela coloracao, aspectos composicionais e texturais, alguns sao utilizados como rochas ornamentais. A despeito da similaridade mineralogica entre os facies, existem algumas diferencas na composicao quimica dos minerais, a qual permitiu a clasificacao dos mesmos atraves de diagramas. O feldspato potassico e representado pelo ortoclasio, os plagioclasios mostram transicao de albita para oligoclasio. As biotitas posicionam-se no campo da annita com tendencia a siderofilita, enquanto que as cloritas correspondem a dafinita e os anfibolios a ferro-edenita. As transformacoes minerais pos-magmaticas ou hidrotermais como exsolucao, potassificacao, albitizacao, cloritizacao, sericitizacao e neoformacao de fluorita sao comuns no SGSB e convergem para a ideia de que os facies foram formados por um mesmo evento magmatico, sendo que os sienogranitos (RI, RO e BCQ) corresponderiam a faciologias originadas por fracionamento de um mesmo magma parental, acrescidas por novo pulso magmatico mais mafico representado pelo monzogranito megaporfiritico (RO).

Caracterização geoquímica, mineralógica, termogravimétrica, e por espectrometria de infravermelho de argilominerais representativos da porção central da Província Borborema

Todas as rochas expostas na superficie sofrem intemperismo. Esse processo leva a formacao de minerais secundarios, entre eles os argilominerais, que sao de larga aplicacao em diversos setores produtivos. Os argilominerais sao silicatos de aluminio hidratados, placoides, do grupo da caulinita, montmorillonita, illita, entre outros, constituidos por particulas de diâmetro inferior a 0,002 mm, depositados como sedimentos fluviais, marinhos, lacustres ou eolicos. Nesse trabalho, foram estudadas caulinita branca e cinza, montmorilonita e nontronita oriundas de depositos localizados na Paraiba e no Rio Grande do Norte, provincia Borborema, nordeste do Brasil. Estes materiais apresentam elevado potencial tecnologico, servindo de base de estudo para a producao de adsorventes de contaminantes oriundos de derivados de petroleo, por exemplo. As amostras foram caracterizadas por difratometria de raios X (DRX), analise termogravimetrica (ATG), capacidade de troca cationica (CTC), espectrometria de infravermelho (IR), e microscopia eletronica de varredura (MEV). As analises quimicas das argilas estudadas revelaram elevados teores de SiO 2 e Al 2 O 3 e teores < 10% para TiO 2 , CaO, MgO, Na 2 O e BaO; montmorilonita apresenta altos teores de Fe 2 O 3 (~13%). A caulinita analisada possui, como impurezas, oxidos e hidroxidos de ferro. A montmorilonita e o principal mineral contido nas argilas bentoniticas da regiao. As amostras de montmorilonita sao argilas naturais e possuem elevada capacidade de adsorcao cationica e, portanto, provavelmente eficiente em posteriores ensaios com essa finalidade. Palavras-chave : Argilominerais, Provincia Borborema; Caracterizacao de minerais.