U-Pb zircon ages of orthogneisses and supracrustal rocks of the Cariris Velhos belt: Onset of Neoproterozoic rifting in the Borborema Province, NE Brazil

New geochronological and geochemical constraints on Precambrian sedimentary and volcanic successions exposed in the western part of the Central Domain of the Borborema Province, NE Brazil, indicate the presence of two distinct tectono-stratigraphic complexes: Riacho Gravata and Sao Caetano. Both complexes and associated orthogneisses are referred in the literature as the Cariris Velhos belt, having depositional, extrusive, or intrusive ages within the interval 985-913 Ma. The Riacho Gravata complex consists of bimodal (but mostly felsic) volcanic and volcanoclastic rocks, muscovite+/-graphite schists, quartzites, and marble with local occurrences of banded-iron-formation. The Sao Caetano complex mainly consists of metagreywackes, marbles, calc-silicate rocks, and rare meta-mafic rocks. Meta-mafic rocks from both complexes have geochemical signatures similar to those of continental flood basalts, with epsilon Nd (1.0 Ga) values ranging from -1.0 to -2.8. Felsic volcanic rocks from the Riacho Gravata complex show epsilon Nd (1.0 Ga) values ranging from -1.0 to -7.4 and geochemical signatures similar to A(2)-type granitoids. New SHRIMP U-Pb zircon data from felsic volcanic rocks within the Riacho Gravata complex yielded ages of 1091 +/- 13 Ma and 996 +/- 13 Ma. In contrast, meta-graywackes from the Sao Caetano complex show a maximum deposition age of ca. 806 Ma in the northern part and ca. 862 Ma in the southern part of the outcrop area. The orthogneisses show epsilon Nd (1.0 Ga) values ranging from 1.0 to -4.2 with U/Pb TIMS and SHRIMP ages ranging from 960 to 926 Ma and geochemical signatures of A(2)-type granitoids. The data reported in this paper suggest at least two periods of extension within the Central Domain of the Borborema Province, the first starts ca. 1091 Ma with magmatism and deposition, creating the Riacho Gravata basin and continued intrusion of A-type granites to 920 Ma. A second rift event, which reactivated old faults, generated a basin with a maximum deposition age of ca. 806 Ma. Furthermore, the oldest granitoids cutting these metasedimentary rocks have crystallization ages of ca. 600 Ma. This suggests that the second rift event could be early Brasiliano in age. The resulting Sao Caetano basin received detritus from a variety of sources, although detritus from the Riacho Gravata complex dominated. Deposition ages of the Riacho Gravata and the Sao Caetano complexes are coeval with deposits in other basins of the Borborema Province (Riacho do Tigre in the Central Domain; Macurure and Maranco in the Sergipano Belt of the Southern domain). The Macaubas Group from SE Brazil and its counterparts in Africa, the Zadanian and Mayumbian Groups, in the western edge of the Congo Craton are also coeval. Closure of the Riacho Gravata and Sao Caetano basins occurred during the Brasiliano convergence (705-600 Ma). During the last stage of convergence, ca. 612 Ma, pull-apart basins were created and filled; final basin closure took place 605-592 Ma, after deposition ceased. (C) 2011 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.

U-Pb zircon in situ dating with LA-MC-ICP-MS using a mixed detector configuration

The LA-MC-ICP-MS method applied to U-Pb in situ dating is still rapidly evolving due to improvements in both lasers and ICP-MS. To test the validity and reproducibility of the method, 5 different zircon samples, including the standard Temora-2, ranging in age between 2.2 Ga and 246 Ma, were dated using both LA-MC-ICP-MS and SHRIMP. The selected zircons were dated by SHRIMP and, after gentle polishing, the laser spot was driven to the same site or on the same zircon phase with a 213 nm laser microprobe coupled to a multi-collector mixed system. The data were collected with a routine spot size of 25 μm and, in some cases, of 15 and 40 μm. A careful cross-calibration using a diluted U-Th-Pb solution to calculate the Faraday reading to counting rate conversion factors and the highly suitable GJ-1 standard zircon for external calibrations were of paramount importance for obtaining reliable results. All age results were concordant within the experimental errors. The assigned age errors using the LA-MC-ICP-MS technique were, in most cases, higher than those obtained by SHRIMP, but if we are not faced with a high resolution stratigraphy, the laser technique has certain advantages.

U-Pb dating by zircon dissolution method using chemical abrasion

Chemical abrasion was carried out on zircons grains of the Temora II standard for U-Pb dating prior to analyses using in situ Laser Ablation-MultiCollector Ion Coupled Plasma Mass Spectrometer (LA-ICPMS) followed by the Isotope Dissolution Thermal Ionization Mass Spectrometer (ID-TIMS) method. The proposed methodology was herein applied in order to reduce primarily the effects of secondary Pb loss, the presence of common lead and/or silicate impurities. Nine Temora II zircon grains were analyzed by the laser ablation method yielding an age of 418.3±4.3 Ma. Zircon grains of a same population were separated for chemical abrasion before dissolution and mass spectrometry analyses. Six fractions of them were separated for isotope dissolution using 235U-205Pb mixed spike after we have checked and assured the laboratory conditions of low blank values for total Pb of less than 2 pg/g. The obtained U-Pb zircon age by the ID-TIMS method was 415.7±1.8 Ma (error 0.43 %) based on four successful determinations. The results are consistent with the published ages for the Temora diorite (Temora I â 416.75±1.3 Ma; Temora II â 416.78±0.33 Ma) and established as 416±0.33 Ma. The technique is thus recommended for high precision U-Pb zircon analyses (error < 1 %), mainly for high resolution stratigraphic studies of Phanerozoic sequences.

U-topias (urbanas) do pensamento sociológico

O que os lugares produzidos conceitualmente no pensamento sociológico revelam sobre a relação da sociologia com a esperança? A esperança dos sociólogos se expressa, entre outros, em representações de lugares associados a possibilidades históricas de ordens sociais outras. Enfocar notadamente u-topias urbanas dos primeiros tempos da sociologia na Alemanha, na França, nos Estados Unidos e no Brasil, e seus desdobramentos mais recentes, evidencia o papel metodológico de representações de tempo em relação ao espaço (urbano), nos vínculos da sociologia com a esperança. Em particular, concepções históricas de tempo em relação às cidades parecem decisivas para uma sociologia "esperançosa".

Os objetivos da circunavegação da U.S. Exploring Expedition (1838-1842): longitude, mapeamento náutico e instituição das coordenadas geográficas modernas

Analisa o principal objetivo da viagem científica de circunavegação realizada pelos EUA entre 1838 e 1842. A tarefa de mapeamento destaca-se entre outras metas científico-estratégicas da viagem exploratória. A iniciativa da empreitada foi movida pela busca da exata localização em alto-mar após a instituição do sistema de longitudes, quando cartas náuticas e mapas de vários países foram conferidos, e outros, novos, desenhados. Os EUA participaram desse esforço internacional, dando início à constituição de um sistema cartográfico próprio.