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Fortaleza, Brazil

Walde D.H.G.,University of Brasilia | Do Carmo D.A.,University of Brasilia | Guimaraes E.M.,University of Brasilia | Vieira L.C.,University of Brasilia | And 4 more authors.
Annales de Paleontologie | Year: 2015

This study aims to update the knowledge about the importance of the late Ediacaran fossil record and geochemical data from Corumbá, State of Mato Grosso do Sul, at the Brazilian-Bolivian border. The Corumbá graben system is located near a triple junction developed above a hot spot of two young (545-480 Ma) Brasiliano provinces: the mostly Bolivian Chiquitos-Tucavaca aulacogen which cuts across the Amazon Craton/Rio Apa Block, and Paraguay Fold Belt. The Neoproterozoic sedimentary cover of South Paraguay Belt starts with the metasediments, diamictites and iron formation of the Jacadigo Group, now related to an end-Cryogenian age. Most remarkable geochemical and paleontological data come from the overlying Corumbá group, mainly from the dolostones with stromatolites of the Bocaina Formation, and limestones with shale and silty intercalations at this group's upper part, in the Tamengo Formation. This last unit contains a fossil assemblage correlated to late Ediacaran fauna. This fauna contains originally substrate-emergent tube-like Corumbella werneri, Cloudina lucianoi and microfossils. Furthermore, the fossils from the Corumbá Group in Brazil and Paraguay represent the most important witnesses for the occurrence of late Ediacaran fossils close to the basal Cambrian boundary in South America. Therefore, the Corumbá region is significant for paleogeographical reasons and, on the other hand, allows insights into the evolution of the oldest skeletonized metazoans. After new research results, the high degree of similarity of the geological facies evolution with other parts of the world (e.g. Yangtze Platform/Southern China, Siberia, Spain and Namíbia) can be demonstrated, where the fragmentation of the Rodinia super-continent and Neoproterozoic glaciations are also well-documented. The sharp top contact of the shallow marine Tamengo Formation with the laminated black shales (containing rare angular dropstones) of the discordantly overlying Guaicurus Formation indicates that the latter represents a new transgressive glacially influenced marine onlap succession. A Cambrian age of the Guaicurus Shales is not (yet) biostratigraphically verified, however, the underlying fossil record of cloudinids indicates a terminal Ediacaran age for the top of the Tamengo Formation. The microtubular cloudinids are interpreted as dysoxic analogues of recent tubeworms and are suggested to serve as first skeletonized worldwide "index fossils" to delineate the onset of a Phanerozoic-type body fossil vectorial evolutionary pathway. Based on the FAD of cloudinids as marker fossils, a revision of the Precambrian/Cambrian boundary is here advocated. This would avoid placing this important GSSP into the virtually worldwide Nomtsas-Baykonurian glacial hiatus. © 2015 Elsevier Masson SAS. All rights reserved. Source

Ganade de Araujo C.E.,Geological Survey of Brazil | Ganade de Araujo C.E.,University of Sao Paulo | Cordani U.G.,University of Sao Paulo | Weinberg R.F.,Monash University | And 3 more authors.
Lithos | Year: 2014

The Ceará Central Domain of the Borborema Province is a key tectonic domain within the 5000km-long West Gondwana Orogen, which extends from Algeria in Africa to Central Brazil. Igneous rocks of the Tamboril-Santa Quitéria Complex, investigated in this study, record a long-lived history of convergent magmatism and crustal anatexis. SHRIMP U-Pb dating and Hf-O isotope analyses of zircons from granitoids and migmatites, coupled with whole-rock Sr-Nd isotopes were used to constrain the evolution of this long-lived continental margin. Magmatism can be divided into three main periods: i) an early period comprising essentially juvenile arc magmatism at ca. 880-800Ma and continuing to 650Ma as evidenced indirectly by detrital zircons from syn-orogenic deposits, ii) a more mature arc period at ca. 660-630Ma characterized by hybrid mantle-crustal magmatic rocks, and iii) crustal anatexis at 625-618Ma continuing until ca. 600Ma. Detrital zircons with mantle values of δO18 (<5.7‰) in the range of 950 to 650Ma retrieved from fore-arc deposits indicate that juvenile input persisted throughout the evolution of the convergent magmatism. Juvenile and mature arc igneous rocks underwent anatexis that gave rise to extensive areas of diatexites within the complex. Anatexis overlap in time with the ages of (ultra)-high pressure (U)HP eclogitic metamorphism dated at 625-615Ma. In accordance with other continental collision zones, age of UHP/HP metamorphism is interpreted to mark the timing of continental collision and therefore indicate that the anatexis of arc rocks took place during continental subduction in a continent-continent collisional setting. Extensive migmatization continued until ca. 600Ma and are in part synchronous to the exhumation of the rocks to shallower crustal levels. Thus, the 350m.y. of magmatic activity in the Ceará Central Domain records the evolution of the West Gondwana margin of the Borborema Province from a juvenile arc setting through a mature arc and continental collision at around 625-600Ma. © 2014 Elsevier B.V. Source

Marques E.D.,Geological Survey of Brazil | Marques E.D.,Federal University of Fluminense | Tubbs D.,Rural University | Gomes O.V.O.,Federal University of Fluminense | And 2 more authors.
Journal of Geochemical Exploration | Year: 2012

Seropédica-Itaguaí sand mining district has an important role in the Rio de Janeiro Metropolitan Region, supplying approximately 70% of the sand for civil construction. The sand extraction process removes surface sedimentary layers (with quartz-feldspatic composition), increasing water table in the mined pit. However, this activity causes some chemical and physico-chemical changes in the water, such as the oxidation of reduced sediments lowering pH values (reaching values<3.5) and increasing SO 4 concentration (reaching more than 90mg.l -1). The relatively low pH values of those waters promote an increase of weathering rate, especially the silicate minerals and, as a consequence, high Al concentrations (>10mg.l -1) are obtained. Four sand pit lakes and four wells located close to each other were sampled from November 2005 to March 2007. The groundwater physico-chemical parameters and the dissolved species in the sand pit lakes are controlled by sand extraction process as well as the rainfall regime. During dry season the reduced sediments become exposed by the water table drawdown, promoting oxidation which releases H+ and other ions present in those lithologies. The increase of water table during wet season induces lithologies lixiviation, increasing the concentration of dissolved species in groundwater. However it also promotes a decreasing of pH values due to the diluting factor induced by rainwater infiltration. During dry season, the sand pit lakes have high pH values due to buffering action caused by the dissolved species present, leading to the ion shielding for H +, caused by evapoconcentration process (high ionic strength for this season). The evapoconcentration conditions along that season promote the precipitation of some complex salts of Na +, Mg 2+, Cl - and sulfate, decreasing the concentrations of major dissolved species. The opposite occurs during wet season, where pH values increase due the rainwater input, breaking the ion shielding (lower ionic strength) and dissolving some precipitated compounds (increasing the concentrations of major dissolved constituents). PHREEQC and WATEQ4F modeling were used for sand pit lakes and groundwater samples in order to show the Al behavior in these specific environments, which could be a limiting factor for an aquaculture activity towards the end of mining activities. The sand pit lakes showed Al 3+ as the main dissolved species in both dry and wet season, while groundwater presented Al(OH) 2+ and Al(OH) 2 +, which are the most toxic species for aquatic life. The dissolved aluminum concentration in the studied environments was considered by the minerals formation, which the SO 4 2- salts complex are the main Al scavengers, followed by the silicate minerals in both seasons. The information provided by this study gives an insight to the conditions of aquaculture in sand pit lakes, having peculiar characteristics. On the other hand, the dissolved aluminum should be considered in groundwater due to the possible water contamination, considering that wells used for water consumption are prevalent in the region. © 2011. Source

Ganade De Araujo C.E.,Geological Survey of Brazil | Ganade De Araujo C.E.,University of Sao Paulo | Rubatto D.,Australian National University | Hermann J.,Australian National University | And 3 more authors.
Nature Communications | Year: 2014

The deeply eroded West Gondwana Orogen is a major continental collision zone that exposes numerous occurrences of deeply subducted rocks, such as eclogites. The position of these eclogites marks the suture zone between colliding cratons, and the age of metamorphism constrains the transition from subduction-dominated tectonics to continental collision and mountain building. Here we investigate the metamorphic conditions and age of high-pressure and ultrahigh-pressure eclogites from Mali, Togo and NE-Brazil and demonstrate that continental subduction occurred within 20 million years over at least a 2,500-km-long section of the orogen during the Ediacaran. We consider this to be the earliest evidence of large-scale deep-continental subduction and consequent appearance of Himalayan-scale mountains in the geological record. The rise and subsequent erosion of such mountains in the Late Ediacaran is perfectly timed to deliver sediments and nutrients that are thought to have been necessary for the subsequent evolution of sustainable life on Earth. © 2014 Macmillan Publishers Limited. All rights reserved. Source

Nadeau S.,Guyana Geology and Mines Commission | Chen W.,State Key Laboratory of Mineral Deposit Research | Reece J.,Guyana Geology and Mines Commission | Lachhman D.,Guyana Geology and Mines Commission | And 5 more authors.
Brazilian Journal of Geology | Year: 2013

A Hadean zircon xenocryst with a U-Pb zircon age of 4,219 ±19 Ma, along with several zircon xenocrysts of Archean age (ca. 2,510 to 3,811 Ma) were found in a rock of the Paleoproterozoic Iwokrama Formation, which includes felsic volcanics and co-magmatic granitic intrusions. It demonstrates the existence of an underlying "Lost Hadean Crust", representing the oldest crustal component of the Guiana Shield. Detrital zircons of late Archean age, up to 2,700 Ma, are also present in the high-grade rocks of the Kanuku Complex, located to the south, and may be derived from the same Hadean-Archean crustal block. Source

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