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Belo Horizonte, Brazil

De Brito Neves B.B.,University of Sao Paulo | Van Schmus W.R.,University of Kansas | Angelim L.A.A.,Servico Geologico do Brazil
Geologia USP - Serie Cientifica | Year: 2015

The Riacho do Pontal System (RPS) is just the central part of a long Neoproterozoic orogenic development placed at the northern periphery of the São Francisco Craton (SFC), from northeast (Sergipano belt) to southwest (Rio Preto belt). The basement of the foreland (SFC) and the backland ("Morro do Estreito-Icaiçara") is mostly formed by Archean rocks, with some insertions of supracrustal and orthogneisses of Tonian age (Cariris Velhos cycle?). RPS exhibits a remarkable tectonic-metamorphic zoning from north to the south: the backland; the internal zone (supracrustal terrigenous assemblages); the central zone (ocean floor rock assemblages); the external zone (supracrustal terrigenous assemblages); and the cratonic domain (SFC), remarked by the presence of a wide (> 200 km) foreland thrust and fold belt (to the south), formed by the Neoproterozoic pelitic-carbonatic cratonic cover. The processes of deformation were intense, presenting vergence towards south and southwest, resulting in reclined and overturned folds, reverse and thrust faults, nappes, and specially the occurrences of some klippen over the basement of the northern part of the CSF. The regional metamorphism was undertaken under amphibolite conditions, with some local retrogression to the greenschists facies. This paper presents a series of new geochronological determinations (Rb-Sr, Sm-Nd and U-Pb methods) of practically all rock types of the basement, supracrustals, and of the associated syncollisional granites. It was possible to discriminate some Archean nuclei (2740 - 2624 Ma), as basement (backland), and two different set of Neoproterozoic volcano-sedimentary rock units associated with granites, the first one of Eotonian age (980 - 940 Ma) and the second one positioned in the Cryogenian-Ediacaran boundary (ca. 635 Ma). A preliminary essay of "wilsonian" tectonic evolution is here introduced for this south-western portion of the Borborema Province (the regional branching system of orogens). © 2015, Universidade de Sao Paulo. All rights reserved. Source

The Iricoumé group consists of very well preserved pyroclastic and effusive volcanic rocks as a part of an extensive volcano-plutonic event, which marked the central part of the Amazonian craton during the Orosirian. Such rocks occur in the southern part of the southwestern of Erepecuru-Trombetas Domain (NW of the Pará state), south of the Guyana Shield. Petrographic study allows distinction of two volcanic types. A dominant explosive volcanism is characterized by pyroclastic rocks (ignimbrites, rheoignimbrites, lapilli-tuff related to surge deposit, and co-ignimbrite fall tuff) while a subordinated effusive volcanism is represented by coherent lavas flows and hypabissal rocks (andesites, lamprophyres and latites). Most of the pyroclastic rocks exhibit features of deposition under high temperature suggesting formation in a caldera-related environment. Pb-Pb zircon ages of 1888 ± 2.5 and 1889 ± 2 Ma have been obtained for the formation of trachydacitic ignimbrites and confirm that the dominant volcanism is part of the Iricoumé group. The Pb-Pb zircon age of 1992 ± 3 Ma yielded by an andesite points to the occurrence in the area of an older Orosirian magmatic episode, which has been previously recognized locally southward in the Tapajós domain. These new results constitute a further evidence of the wide extent of the Iricoumé group and correlated volcanic rocks in the central part of the Amazonian craton, and reinforce the assumption that this volcanic episode and correlated magmatism may be considered as a silicic large igneous province (SLIP), as previously proposed by some authors. Source

De Souza Marinho M.,Servico Geologico do Brazil | De Souza Marinho M.,Federal University of Ouro Preto | De Souza Gomes C.J.,Federal University of Ouro Preto
Brazilian Journal of Geology | Year: 2013

Structural and microstructural studies have been carried out to assess the structural evolution of Au, Ag and Pb lode occurrences that spread among Serrita and Parnamirim municipalities, Pernambuco, Brazil. These occurrences are hosted in metassediments, correlated to the Salgueiro Group, and granodiorites from Serrita Suite, in the Piancó-Alto Brígida Domain, southwestern Zona Transversal, Borborema Province. The metassediments underwent four deformation phases that switched from compressional to strike-slip tectonics. The first two phases developed a low-angle fabric and the third and fourth ones produced folds with NE-SW/ENE-WSW plane-axial crenulation cleavages and regional shear zones, respectively. In the metassediments, the veins grew by E-W and N-S/NNW-SSE branched fractures, associated with rapid propagation and dilatation. These veins underwent new mineral crystallization phases concomitant with deformation and mineralization, which mark the transition from the ruptile-ductile to the ruptile regime. The granodiorite hosted-veins show incipient deformation, as do theirs hosts. We conclude that the veins, in both host-rocks, formed by the same stress field attributed to the end of fourth phase, under a NW-SE maximum compressive tensor. The differences between the veins resulted from a fracture refraction in function of rheological contrasts in the host-rocks. This refraction induced the development of E-W/ESE-WNW and N-S/NNW-SSE shear fractures in the less competent lithology, the metassediments, and NW-SE tensile fractures in the more competent one, the granodiorite. Source

Barreto C.J.S.,Federal University of Para | Lafon J.M.,Federal University of Para | Da Rosa Costa L.T.,Servico Geologico do Brazil | Lima E.F.,Federal University of Rio Grande do Sul
International Geology Review | Year: 2014

Hypabyssal and explosive volcanic rocks of the Iricoumé Group crop out in the Western Erepecuru-Trombetas Domain, in the Central Amazonian Province, south of the Guyana Shield. They are part of an extensive volcano-plutonic event, which marked the central part of the Amazonian Craton during the Orosirian. These volcanic rocks, formed at approximately 1.89 Ga, consist of pyroclastic rocks (ignimbrites, rheoignimbrites, surge-related lapilli-tuff, co-ignimbrite fall tuff) and subordinate felsic hypabyssal rocks. An older episode, with an age of approximately 1.99 Ga, occurs locally and is represented by latitic lava flows and spessartitic lamprophyres. The geochemical characteristics of the Iricoumé volcanic rocks suggest that they formed in a post-orogenic tectonic setting, while those of the older Orosirian magmatism suggest a subduction-related geodynamic setting. TDM model ages ranging from 2.18 to 2.39 Ga and slightly negative εNd(T) values for most of the Iricoumé volcanic rocks indicate dominant Rhyacian crustal sources for the parental magmas. The positive εNd(T) values exhibited by some Iricoumé rhyolites (TDM: 1.98 and 2.06 Ga) and by the 1.99 Ga lava flows and lamprophyre (TDM: 2.13 and 2.21 Ga) suggest a juvenile mantle source, although a minor contribution of Palaeoproterozoic crust cannot be ruled out. These new results furnish further evidence of the wide extent of the Iricoumé Group and coeval volcano-plutonic associations in the central part of the Amazonian Craton. © 2014 © 2014 Taylor & Francis. Source

Antonietto L.S.,University of Brasilia | do Carmo D.A.,University of Brasilia | Viviers M.C.,Petrobras | Adorno R.R.,Servico Geologico do Brazil
Revista Brasileira de Paleontologia | Year: 2015

A biostratigraphic and paleozoogeographic revision of the upper Aptian-Albian ostracods of Riachuelo Formation, in the Sergipe-Alagoas Basin, Brazil, was conducted. The studied material comprised 336 samples from several outcrops, along with well cores drilled by Petróleo Brasileiro S.A. (PETROBRAS). After taxonomic analysis, including the description of Reticulocosta edrianae n. sp., a new biozonation scheme was obtained for the Albian interval of that formation, and the paleozoogeographic affinities of the Brazil-Central West Africa province were reviewed through comparison with results published in the current literature. Several changes were made to previously established ostracod zones. A new code for the Aracajuia benderi Zone (OSE-1), MSA-0, was created, with impact in all of its subzones. The names of the Pattersoncypris ex. gr. angulata (OSE-1.1), ‘Patellacythere’ sp. GA E 27 (OSE-1.3) and Veenia guianensis (OSE-1.4) subzones were changed to Harbinia sinuata? (MSA-0.1), Praebythoceratina amsittenensis (MSA-0.3) and Reticulocosta edrianae (MSA-0.4), respectively. The Sergipella viviersae Subzone (OSE-1.2) had its code changed for MSA-0.2; also, a stratotype was stablished for it, and the lower limit of the subzone was changed, from lower Albian in previous works to upper Aptian. The Metacytheropteon aff. Metacytheropteron sp. GA C 24 Subzone (OSE-1.5) was renamed MSA-0.5. The uppermost Albian Aracajuia antiqua Latest Occurrence Interval Zone (MSA-1) is herein proposed, following the reclassification Aracajuia benderi specimens of the Aracajuia benderi Subzone (OSE-1.6) to Aracajuia antiqua. Paleozoogeographic analyses showed some faunal interchange with Tethyan provinces, beginning in the latest Aptian, at both specific and generic levels. Limited northward marine influxes across the Walvis ridge were also observed during the Albian, while larger scale southward migrations occurred in the early Cenomanian. The presence of Brachycytherinae species in the Brazil-Central West Africa province as early as the Aptian indicates a new zoogeographic origin for this subfamily, while explaining the occurrence of the group in both the Austral realm and part of the Tethyan realm during the Albian. © 2015 by the Sociedade Brasileira de Paleontologia. Source

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