Space-time relation of post-collisional granitic magmatism in Santa Catarina, southern Brazil: U-Pb LA-MC-ICP-MS zircon geochronology of coeval mafic-felsic magmatism related to the Major Gercino Shear Zone

LA-MC-ICP-MS U-Pb zircon dating was performed on syntectonic, early post-collisional granitic and associated mafic rocks that are intrusive in the Brusque Metamorphic Complex and in the Florianopolis Batholith, major tectonic domains separated by the Neoproterozoic Major Gercino Shear Zone (MGSZ) in south Brazil. The inferred ages of magmatic crystallization are consistent with field relationships, and show that the syntectonic granites from both domains are similar, with ages around 630-620 Ma for high-K calc-alkaline metaluminous granites and ca. 610 Ma for slightly peraluminous granites. Although ca. 650 Ma inherited zircon components are identified in granites from both domains, important contrasts on the crustal architecture in each domain are revealed by the patterns of zircon inheritance, indicating different crustal sources for the granites in each domain. The granites from the southern domain (Floriandpolis Batholith) have essentially Neoproterozoic (650-700 Ma and 900-950 Ma) inheritance; with a single 2.0-2.2 Ga inherited age obtained in the peraluminous Mariscal Granite. In the northern Brusque Metamorphic Complex, the metaluminous Rio Pequeno Granite and associated mafic rocks have scarce inherited cores with ages around 1.65 Ga, whereas the slightly peraluminous Serra dos Macacos Granite has abundant Paleoproterozoic (1.8-2.2 Ga) and Archean (2.9-3.4 Ga) inherited zircons. Our results are consistent with the hypothesis that the MGSZ separates domains with distinct geologic evolution; however, the contemporaneity of 630-610 Ma granitic magmatism with similar structural and geochemical patterns on both sides of this major shear zone indicates that these domains were already part of a single continental mass at 630 Ma, reinforcing the post-collisional character of these granites. (C) 2012 Elsevier B.V. All rights reserved.

Time constraints on magmatism along the Major Gercino Shear Zone, southern Brazil: Implications for West Gondwana reconstruction

The Dom Feliciano Belt, situated in southernmost Brazil and Uruguay, contains a large mass of granite-gneissic rocks (also known as Florianopolis/Pelotas Batholith) formed during the pre-, syn- and post-orogenic phases of the Brasiliano/Pan-African cycle. In the NE extreme of this granitic mass, pre-, syn- and post-tectonic granites associated with the Major Gercino Shear Zone (MGSZ) are exposed. The granitic manifestation along the MGSZ can be divided into pre-kinematic tonalitic gneisses, peraluminous high-K calcalkaline early kinematic shoshonitic, and metaluminous post-kinematic granites. U-Pb zircon data suggest an age of 649 +/- 10 Ma for the pre-tectonic gneisses, and a time span from 623 +/- 6 Ma to 588 +/- 3 Ma for the early to post-tectonic magmatism. Negative epsilon Hf (t) values ranging from -4.6 to -14.6 and Hf model ages ranging from 1.64 to 2.39 Ga for magmatic zircons coupled with whole rock Nd model ages ranging from 1.24 to 2.05 Ga and epsilon Nd (t) values ranging from -3.84 to -7.50, point to a crustal derivation for the granitic magmatism. The geochemical and isotope data support a continental magmatic arc generated from melting of dominant Paleoproterozoic crust, and a similar evolution for the granitic batholiths of the eastern Dom Feliciano Belt and western Kaoko Belt. (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Ages (U-Pb SHRIMP and LA ICPMS) and stratigraphic evolution of the Neoproterozoic volcano-sedimentary successions from the extensional Camaqua Basin, Southern Brazil

During the Ediacaran, southern Brazil was the site of multiple episodes of volcanism and sedimentation, which are best preserved in the 3000 km(2) Camaqua Basin. The interlayered sedimentary and volcanic rocks record tectonic events and paleoenvironmental changes in a more than 10 km-thick succession. In this contribution, we report new U-Pb and Sm-Nd geochronological constraints for the 605 to 580 Ma Born Jardim Group, the 570 Ma Acampamento Velho Formation, and a newly-recognized 544 Ma volcanism. Depositional patterns of these units reveal the transition from a restricted, fault-bounded basin into a wide, shallow basin. The expansion of the basin and diminished subsidence rates are demonstrated by increasing areal distribution and compressed isopachs and increasing onlap of sediments onto the basement to the west. The Sm-Nd isotopic composition of the volcanic rocks indicates mixed sources, including crustal rocks from the adjacent basement. Both Neoproterozoic and Paleoproterozoic sources are indicated for the western part of the basin, whereas only the older Paleoproterozoic signature can be discerned in the eastern part of the basin. (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Using AMS combined with mineral shape preferred orientation analysis to understand the emplacement fabrics of the Apiai gabbro-norite (Ribeira Belt, SE Brazil)

The Apiai gabbro-norite is a massive fine-grained Neoproterozoic intrusion emplaced in a core of synformal structure that deforms low-grade marine metasedimentary rocks of the Ribeira Belt of south-eastern Brazil. The lack of visible magmatic layering or any internal fabric has been a major limitation in deciding whether the emplacement occurred before or after the regional folding. To assist in the tectonic interpretations, we combine low-field anisotropy of magnetic susceptibility (AMS) and silicate shape preferred orientation (SPO) to reveal the internal structure of the mafic intrusion. Magnetic data indicate a mean susceptibility of about 10(-2) SI and a mean anisotropy degree (P) of about 1.08, essentially yielded by titanomagnetite. The magnetic and silicate foliations for P >= 1.10 are parallel to each other, while the lineations tend to scatter on the foliation plane, in agreement with the dominant oblate symmetry of the AMS and SPO ellipsoids. For lower P values, the magnetic and silicate fabrics vary from coaxial to oblique, and for P <= 1.05, their shapes and orientations can be quite distinct. The crystal size distribution (CSD) of plagioclase for P > 1.05 is log linear, in agreement with a bulk simple crystallisation history. These results combined show that for a strong SPO, corresponding to a magnetic anisotropy above 1.10, AMS is a reliable indicator of the magmatic fabric. They indicate that the Apiai gabbro-norite consists of sill-like body that was inclined gently to the north by the regional folding.

Contribution to the stratigraphy of the onshore Paraíba Basin, Brazil

Several publications have contributed to improve the stratigraphy of the Paraíba Basin in northeastern Brazil. However, the characterization and distribution of sedimentary units in onshore areas of this basin are still incomplete, despite their significance for reconstructing the tectono-sedimentary evolution of the South American passive margin. This work provides new information to differentiate among lithologically similar strata, otherwise entirely unrelated in time. This approach included morphological, sedimentological and stratigraphic descriptions based on surface and sub-surface data integrated with remote sensing, optically stimulated luminescence dating, U+Th/He dating of weathered goethite, and heavy mineral analysis. Based on this study, it was possible to show that Cretaceous units are constrained to the eastern part of the onshore Paraíba Basin. Except for a few outcrops of carbonatic rocks nearby the modern coastline, deposits of this age are not exposed to the surface in the study area. Instead, the sedimentary cover throughout the basin is constituted by mineralogically and chronologically distinctive deposits, inserted in the Barreiras Formation and mostly in the Post-Barreiras Sediments, of early/middle Miocene and Late Pleistocene-Holocene ages, respectively. The data presented in this work support tectonic deformation as a factor of great relevance to the distribution of the sedimentary units of the Paraíba Basin.

Recurring extensional and strike-slip tectonics after the Neoproterozoic collisional events in the southern Mantiqueira province

In Eastern South America, a series of fault-bounded sedimentary basins that crop out from Southern Uruguay to Southeastern Brazil were formed after the main collisional deformation of the Brasiliano Orogeny and record the tectonic events that affected the region from the Middle Ediacaran onwards. We address the problem of discerning the basin-forming tectonics from the later deformational events through paleostress analysis of more than 600 fault-slip data, mainly from the Camaquã Basin (Southern Brazil), sorted by stratigraphic level and cross-cutting relationships of superposed striations, and integrated with available stratigraphic and geochronological data. Our results show that the Camaquã Basin was formed by at least two distinct extensional events, and that rapid paleostress changes took place in the region a few tens of million years after the major collision (c.a. 630 Ma), probably due to the interplay between local active extensional tectonics and the distal effects of the continued amalgamation of plates and terranes at the margins of the still-forming Gondwana Plate. Preliminary paleostress data from the Castro Basin and published data from the Itajaí Basin suggest that these events had a regional nature.

Petrology of Gameleira potassic lamprophyres, São Francisco Craton

Gameleira lamprophyres are dykes and mafic microgranular enclaves associated with the shoshonitic Gameleira monzonite. This association belongs to the Paleoproterozoic alkaline magmatism from Serrinha nucleus, northeast Brazil. The liquidus paragenesis is diopside, pargasite, apatite and mica. Reverse zoning was identified in the groundmass alkali feldspar and was related to the undercooling of lamprophyric magma during the emplacement, with high growth rate of pargasite/edenite inducing disequilibrium between feldspars and liquid. Chemical data indicate that the lamprophyres are basic rocks (SiO2 < 48 wt%), with alkaline character (Na2O + K2O > 3 wt%) and potassic signature (K2O/Na2O ≈ 2). High contents of MgO and Cr are consistent with a signature of a primary liquid, and such concentrations, as well as Al, K, P, Ba, Ni- and light rare earth elements, are consistent with an olivine-free metasomatic mantle source enriched in amphibole, clinopyroxene and apatite. By contrast, the ultrapotassic lamprophyres from Morro do Afonso, contemporaneous alkaline ultrapotassic magmatism in Serrinha nucleus, were probably produced by melting of a clinopyroxene-phlogopite-apatite enriched-source. The identification of different mineral paragenesis in the source of potassic and ultrapotassic lamprophyres from Serrinha nucleus can contribute to the understanding of the mantle heterogeneities and tectonic evolution of this region.

Fotopedologia, espectroscopia e sistema de informação geográfica na caracterização de solos desenvolvidos do Grupo Barreiras no Amapá

O presente trabalho utilizou ferramentas de sensoriamento remoto e sistema de informação geográfica, aliadas às informações espectrais do solo e quantitativas e qualitativas do relevo, para a caracterização e discriminação das classes de solos desenvolvidos do Grupo Barreiras na região do município de Porto Grande, Amapá. Após o mapeamento semidetalhado de solos, realizou-se a caracterização pontual das amostras dos perfis por meio de espectroscopia de visível e infravermelho (400-2500 nm), e a análise espacial das redes de drenagem e relevo a partir de fotos aéreas e imagens de radar. As informações quantitativas do relevo foram mais eficientes na caracterização e discriminação dos solos estudados do que as qualitativas, enquanto as informações espectrais permitiram caracterizar os solos em nível pontual. Métodos espaciais utilizando redes de drenagem e relevo e métodos espectrais utilizando amostras pontuais complementam-se na caracterização e discriminação de solos na paisagem.

O Recuo Brutalista

Em conformidade com a dúvida de seu título, o difundido livro The New Brutalism: Ethic or Aesthetic? de Reyner Banham não é possível explicar o Brutalismo como manifestação artística coesa, dotada de consistência e reprodutibilidade formal. A citação de arquitetos famosos, porém divergentes, parece associar e comparar obras ásperas com a pretensão de reerguer uma arquitetura moderna considerada ascética, monótona e insuficiente e gera um teoricismo da aparência crua e do moralismo dos objetos. Discurso que oculta, ou desvia a atenção do retorno artístico à sublimidade e construção artesanal. O Brutalismo é aceito como evolução natural dos estágios modernos anteriores e sanciona artefatos toscos, pesados e inacabados como se fossem filiados ao processo moderno desinfestado. Esconde contradições e disfarça seu rompimento com o moderno para prolongar a expressão Movimento moderno. Mas o objeto claro, econômico e preciso é repudiado pelo consumidor e, por ser pouco representativo, o artista faz sua maquiagem com episódios contrastantes e monumentais na informalidade das cidades espontâneas. No entanto, parece possível suspender a noção positiva e corretiva do Brutalismo para entendê-lo como um recuo artístico vulgarizador que despreza aperfeiçoamento e afronta a atitude moderna com banalização conceptiva, exagero, figuralidade, musculação estrutural, grandeza tectônica, rudimento e rudeza. Assim, moralismo, retorno rústico e originalidade desqualificam a expressão International Style entendida como a culminação da arquitetura moderna do pós-guerra, ao depreciá-la como decadente, como produto imobiliário, comercial e corporativo a serviço do capital. Essa interpretação desvela uma crítica anti-industrial, portanto antimodernista e diversa da pós-modernidade, porém contestadora e realista para fornecer imagens à cultura e aos insensíveis à estrutura da forma moderna. Diverso da pós-modernidade pela dependência ao moderno e ausência de apelo popular. Tornada insignificante a configuração oportuna do artefato, o arquiteto tenta reter sua notabilidade artística, ou o prestígio que parece enfraquecer na aparência símile da especificação de catálogo, no rigor modular. Indispõe-se e repudia componentes, Standards e acabamentos impessoais da indústria da construção para insistir em autoria e inspiração, mas repete cacoetes estilísticos de época e o inexplicável uso intensivo de concreto bruto e aparente para sentir-se engajado e atualizado. Porém, é necessário distinguir obras de aparência severa concebidas pela atitude moderna mais autêntica das de concreto aparente em tipos ou configurações aberrantes. Para avançar na discussão do Brutalismo propõe-se entender este fenômeno com a substituição do juízo estético moderno de sentido visual postulado por Immanuel Kant (1724-1804) por um sentimento estético fácil e relacionado com a sensação da empatia, com a Einfühlung de Robert Vischer (1847-1933). Na época da cultura de massas, admite-se o rebaixamento das exigências no artefato e a adaptação brutalista com a transfiguração dos processos de arquitetura moderna. Assim, a forma é substituída pela figura ou pelo resumo material; a estrutura formal subjacente pelo ritmo e exposição da estrutura física; o reconhecimento visual pelo entusiasmo psicológico ou pelo impulso dionisíaco; a concepção substituída pelo partido, ou, ainda, pelo conceito; a sistematização e a ordem pela moldagem e a organização; a abstração e síntese pela originalidade e essencialidade, o sentido construtivo pela honestidade material; a identidade das partes pela fundição ou pela unicidade objetal e a residência pela cabana primitiva.

Morphology and surface sedimentation on the upper and middle continental slope off Uruguay

This paper presents for the first time a morphological and surface sediment characterization of the Uruguayan outer continental shelf and slope. The study is based on a high-resolution coverage using hydrographical, geomorphological and sedimentological sampling and several textural and productivity proxies. Along slope terraces and an important canyon system characterizes continental slope morphology, indicating that across- and down-slope sedimentary processes control large-scale sedimentation. Terraces represent the prolongation of the Argentinean Contouritic Depositional System that vanishes in the study area, presumably as a result of the dynamic of the Brazil-Malvinas confluence. Canyons incised in the upper slope are likely related to low-stand sea level conditions. At the outer shelf and shallow upper slope (170-250 m depth), off-shelf sand transport is inferred from the distribution of relict sand and reworked biogenic gravel. In the upper continental slope, the northern region is characterized by an erosive environment controlled by a steep slope and the southward flowing Brazil current. In the south, a depositional environment is enhanced by the presence of a gentler slope and seaward incised canyons and is mainly controlled by hemipelagic processes associated with nutrient-rich Sub-Antarctic Waters (SAW), by its confluence with South Atlantic Central Waters (SACW) and by the Rio de la Plata’s (RdlP) influence. Additionally, within the upper slope, the occurrence of igneous-metamorphic cobbles and pebbles in canyon and mound lag deposits suggests the influence of glacial fluvial discharge and/or iceberg transport processes. In the middle slope, sedimentation is controlled by thermohaline-induced deep-water bottom currents. The decreasing influence of the erosive Antarctic Intermediate Water (AAIW) is evident in a northward diminution in grain size. The variety of transport and sedimentary processes identified reflect the control of the Brazil-Malvinas confluence zone and the Rio de la Plata’s discharge.

Testing the influence of far-field topographic forcing on subduction initiation at a passive margin

Despite favourable gravitational instability and ridge-push, elastic and frictional forces prevent subduction initiation fromarising spontaneously at passive margins. Here,we argue that forces arising fromlarge continental topographic gradients are required to initiate subduction at passivemargins. In order to test this hypothesis,we use 2Dnumerical models to assess the influence of the Andean Plateau on stressmagnitudes and deformation patterns at the Brazilian passive margin. The numerical results indicate that “plateau-push” in this region is a necessary additional force to initiate subduction. As the SE Brazilianmargin currently shows no signs of self-sustained subduction, we examined geological and geophysical data to determine if themargin is in the preliminary stages of subduction initiation. The compiled data indicate that the margin is presently undergoing tectonic inversion, which we infer as part of the continental–oceanic overthrusting stage of subduction initiation. We refer to this early subduction stage as the “Brazilian Stage”, which is characterized by N10 kmdeep reverse fault seismicity at themargin, recent topographic uplift on the continental side, thick continental crust at themargin, and bulging on the oceanic side due to loading by the overthrusting continent. The combined results of the numerical simulations and passivemargin analysis indicate that the SE Brazilian margin is a prototype candidate for subduction initiation.