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Sawakuchi A.O.,University of Sao Paulo | Blair M.W.,Los Alamos National Laboratory | DeWitt R.,Oklahoma State University | Faleiros F.M.,CPRM Servico Geologico do Brazil | And 2 more authors.
Quaternary Geochronology

The optically stimulated luminescence (OSL) sensitivity of quartz has a significant influence on luminescence dating procedures. Furthermore, identifying the natural controls of quartz OSL sensitivity is an important step towards new applications of OSL in geology such as provenance tracing. We evaluate the OSL sensitivity (total and the proportion of the informally assigned fast, medium and slow components) of single grains of quartz extracted from 10 different igneous and metamorphic rocks with known formation conditions; and from fluvial and coastal sediments with different sedimentary histories and known source rocks. This sample suite allows assessment of the variability of the OSL sensitivity of single quartz grains with respect to their primary origin and sedimentary history. We observed significant variability in the OSL sensitivity of grains within all studied rock and sediment samples, with the brightest grains of each sample being those dominated by the fast component. Quartz from rocks formed under high temperature (>500 °C) conditions, such as rhyolites and metamorphic rocks from the amphibolite facies, display higher OSL sensitivity. The OSL sensitivity of fluvial sediments which have experienced only a short transport distance is relatively low. These sediments show a small increase in OSL sensitivity downstream, mainly due to a decreasing fraction of "dim" grains. The quartz grains from coastal sands present very high sensitivity and variability, which is consistent with their long sedimentary history. The high variability of the OSL sensitivity of quartz from coastal sands is attributed more to the mixture of grains with distinct sedimentary histories than to the provenance from many types of source rocks. The temperature of crystallization and the number of cycles of burial and solar exposure are suggested as the main natural factors controlling the OSL sensitivity of quartz grains. The increase in OSL sensitivity due to cycles of erosion and deposition surpasses the sensitivity inherited from the source rock, with this increase being mainly related to the sensitization of fast OSL components. The discrimination of grains with different sedimentary histories through their OSL sensitivities can allow the development of quantitative provenance methods based on quartz. © 2010 Elsevier B.V. Source

Santos L.C.M.D.L.,University of Brasilia | Santos L.C.M.D.L.,Federal University of Campina Grande | Dantas E.L.,University of Brasilia | Santos E.J.D.,CPRM Servico Geologico do Brazil | And 2 more authors.
Journal of South American Earth Sciences

The Alto Moxotó Terrane is a Paleoproterozoic inlier within the Transversal Domain of the Neoproterozoic Borborema Province (NE Brazil). An isotopic and whole-rock geochemistry study has been performed in the Sucuru region (Paraiba State, NE Brazil) which revealed a long-lived evolution for this terrane. The first event is Siderian-aged, dated on 2.44Ga, being represented by granitic to granodioritic banded orthogneisses and migmatites of the basement. They correspond to meta to peraluminous high-K calc-alkaline series, where geochemical patterns besides zircon features and Nd isotopic data indicate that they were formed in a convergent tectonic environment with reworking of an older Archean continental crust. This basement was intruded by different magmatic suites through two distinct tectono-magmatic events. The older one is Rhyacian-aged recorded by emplacement of the Carmo mafic-ultramafic suite and Pedra d'Água granitic suite, with ages varying from 2.15 to 2.0Ga. The Carmo Suite shows compositions similar to tholeiitic and minor calc-alkaline series and geochemical patterns of a depleted source. These general chemical characteristics are compatible with an arc-related magmatism in early stages of subduction. The Pedra d'Água suite corresponds to middle to peraluminous high-K calc-alkaline magmatism which presents a typical magmatic arc geochemical signature. The negative εNd (. t) values suggest a strong continental component for genesis of these magmas. The last tectonomagmatic episode occurred in the Statherian-Calymmian boundary and is represented by bimodal magmatic association of the Serra da Barra Suite, dated around 1.6Ga. The dominant felsic rocks present an evolved composition and correspond to typical metaluminous sub-alkaline suite. The trace-element and REE patterns of both mafic and mainly felsic rocks suggest a within-plate setting. The attributed source is of crustal derivation, which is supported by the negative εNd (. t) values. A mantle plume can be invoked for mechanism of generation of the Serra da Barra magmatism. This polycyclic Paleoproterozoic evolution observed at Alto Moxotó terrane is also well documented in orogenic terranes worldwide, mainly those related to Atlantica supercontinent amalgamation. On the other hand, Statherian-Calymmian extensional event is also coherent with worldwide descriptions and are commonly referred to early break-up stage of the large Paleoproterozoic land masses. © 2014 Elsevier Ltd. Source

Barreto C.J.S.,Federal University of Para | Lafon J.M.,Federal University of Para | da Rosa Costa L.T.,CPRM Servico Geologico do Brazil | Dantas E.L.,University of Brasilia
Journal of South American Earth Sciences

Central Amapá, northern Brazil is located at the boundary between: (a) a northern Paleoproterozoic domain, consisting mainly of granite-greenstones terrains and (b) a southern Archean continental block(Amapá block), including an Archean basement reworked during the Transamazonian orogeny (2.26-1.95Ga). Field investigations, Pb-Pb zircon and Sm-Nd whole rock geochronology supported by geochemical data on granitoids brought further constraints on Paleoproterozoic crustal growth in the southeastern Guyana Shield. A first magmatic episode, dated at 2.26Ga, is marked by the crystallization of metaluminous low-K tholeiitic tonalites and quartz-diorites, which geochemical affinity with volcanic arc and association with T-MORB amphibolites suggest that they formed in a back-arc basin - island arc system. This event is coeval to the oceanic stage registered in French Guyana during the Eorhyacian (2.26-2.02Ga). A second magmatic episode is represented by peraluminous, medium- to high-K calc-alkaline tonalite and granodiorite, which revealed some similarities with Mesorhyacian TTG rocks of French Guyana. For granitoids of both episodes, TDM and εNd values indicate the contribution of some Archean crustal component, probably by assimilation or contamination. This second magmatic episode occurred at 2.10Ga, indicating that the period of successive calc-alkaline magmatic arcs formation may have extended until the Neorhyacian. Meanwhile, during this time, tectonic accretion by collision of the newly formed continental landmass was the prevailing process in French Guyana. The latter magmatic episode, even though poorly constrained, was registered around 2.08-2.02Ga in central Amapá. It corresponds to the emplacement and solidification of high-K collisional granitoids, produced by partial melting of the Archean continental crust, as testified by the Archean TDM, inherited Pb-Pb zircon ages and strongly negative εNd values. Our results point toward the existence of a protracted episode of crustal growth during the Neorhyacian in the southeastern Guyana Shield. This episode has been predominantly driven by magmatic arc accretion during, at least, 160 My, along the period of 2.26-2.10Ga. This cycle ended with diachronic closure of the oceanic basins and arc-continent collision. © 2013 Elsevier Ltd. Source

Do Nascimento M.A.L.,Federal University of Rio Grande do Norte | Galindo A.C.,Federal University of Rio Grande do Norte | de Medeiros V.C.,CPRM Servico Geologico do Brazil
Journal of South American Earth Sciences

The Ediacaran-Cambrian plutonic activity is one of the most important geological features of the Rio Grande do Norte Domain (Borborema Province, NE Brazil). It is represented by several batholiths, stocks and dykes. Based on the petrographic, geochemical and geochronological characteristic of different rocks, this plutonic activity can be grouped in six separate suites: shoshonitic (Shos), porphyritic high-K calc-alkaline (PHKCalcAlk), equigranular high K calc-alkaline (EHKCalcAlk), calc-alkaline (CalcAlk), alkaline (Alk) and charnockitic alkaline (ChAlk). Geochemically, the Shos, CalcAlk and Alk suites are differentiated from the others, while ChAlc can be distinguished from the others in some diagrams. The greatest difficulty lies in distinguishing between the chemically similar PHKCalcAlk and EHKCalcAlk. To this end, existing geochronological data as well as related petrographic and textural field aspects may be used to distinguish the two mentioned suites (PHKCalcAlk and EHKCalcAlk).Petrographically, the Shos suite has composition between gabbro/diorite and quartz monzonite. Monzogranites (with subordinate granodiorites and quartz monzonites) predominate in both PHKCalcAlk and EHKCalcAlk. Calc is composed of granodiorites to tonalites. Alc is formed by alkali feldspar granites (with subordinate alkali feldspar quartz syenites and syenogranites), whereas ChAlc has quartz mangerites and charnockites.The suites were emplaced between the Ediacaran (635-541Ma) and Cambrian (541-485Ma), predominantly in the Ediacaran, based on 34 U-Pb datings (zircon, titanite, monazite and columbite-tantalite), 17 Rb-Sr (whole rock) and 1 Sm-Nd (total rock and mineral) internal isochrons. The Shos suite has U-Pb ages varying from 599±16 (Poço Verde pluton) to 579±7 (Acari and São João do Sabugi plutons), slightly older than those of the PHKCalcAlk suite, which ranges between 591±4Ma (Totoró pluton) and 544±7Ma (São José de Espinharas pluton). The CalcAlk Suite has only one dated body, aged 598±3Ma (Serra da Garganta pluton). The EHKCalcAlk suite U-Pb dating is younger than the aforementioned suites, ranging from 582±5Ma (dykes in Dona Inês pluton) and 527±8 (Cerro Corá pluton). The Alk suite was dated at 578±14Ma (Caxexa pluton, Sm-Nd internal isochron) and at 597±4Ma (Japi pluton, U-Pb zircon), while ChAlc U-Pb zircon ages of 601±10Ma and 593±5Ma (Umarizal pluton). The magmatism of Cambrian age in the Rio Grande do Norte Domain is represented by pegmatite dykes with U-Pb dating between 515 and 510Ma. A atividade plutÔnica ediacarana-cambriana constitui uma das mais importantes feiçoes geológicas encontrada no Domínio Rio Grande do Norte (Província Borborema, NE do Brasil). Ela é representada por diversos batólitos, stocks e diques. Com base em características petrográficos, geoquímicas e geocronológicas de diferentes rochas é possível agrupar esta atividade plutÔnica em seis suítes distintas, são elas: Shoshonítica (Shos), Cálcio-alcalina de alto K Porfirítica (CalcAlcAKP), Cálcio-alcalina de alto K Equigranular (CalcAlcAKE), Cálcio-alcalina (CalcAlc), Alcalina (Alc) e Alcalina Charnoquítica (AlcCh). Geoquimicamente, as suítes Shos, CalcAlc e Alc são individualizadas das demais, enquanto que a AlcCh pode ser distinguida das outras em alguns diagramas. A dificuldade maior está na distinção entre as CalcAlcAKP e CalcAlcAKE, sendo elas quimicamente similares. Os aspectos de campo aliados aos petrográficos e texturais podem ser utilizados para distinção entre as suítes CalcAlcAKP e CalcAlcAKE, bem como os dados geocronológicos existentes.Petrograficamente, a Suíte Shos possui composição entre gabros/dioritos e quartzo monzonitos. Na CalcAlcAKP predomina monzogranitos (com granodioritos e quartzo monzonitos, subordinados), semelhante a CalcAlcAKE. A CalcAlc é formada por granodioritos a tonalitos. A Alc é formada por álcali-feldspato granitos (com quartzo álcali-feldspato sienitos e sienogranitos, subordinados), enquanto que a AlcCh tem quartzo mangeritos e charnoquitos.As suítes foram alojadas entre o Ediacarano (635-541Ma) e o Cambriano (541-485Ma), predominando no Ediacarano, com base em 34 idades U-Pb (zircão, titanita, monazita e columbita-tantalita), 17 Rb-Sr (rocha total) e 1 isócrona interna Sm-Nd (rocha total e mineral). A Suíte Shos tem idades U-Pb variando de 599±16 (Plúton Poço Verde) a 579±7 (plútons Acari e São João do Sabugi). Esses valores são um pouco mais velhos do que as idades da Suíte CalcAlcAKP que possui valores entre 591±4Ma (Plúton Totoró) e 544±7Ma (Plúton São José de Espinharas). A Suíte CalcAlc possui apenas um corpo datado, com idade de 598±3Ma (Plúton Serra da Garganta). A Suíte CalcAlcAKE possui idades U-Pb mais jovens que as suítes anteriores, com valores entre 582±5Ma (diques no Plúton Dona Inês) e 527±8 (Plúton Cerro Corá). A Suíte Alc foi datada em 578±14Ma (Plúton Caxexa, isócrona interna Sm-Nd) e 597±4Ma (Plúton Japi, U-Pb em zircão), enquanto que a AlcCh foi datada, U-Pb em zircão, em 601±10Ma e 593±5Ma (Plúton Umarizal). O magmatismo de idade cambriana no Domínio Rio Grande do Norte está representado pelos diques de pegmatito com idades U-Pb variando entre 515 e 510Ma. © 2014 Elsevier Ltd. Source

Rodrigues J.B.,CPRM Servico Geologico do Brazil | Rodrigues J.B.,University of Brasilia | Pimentel M.M.,University of Brasilia | Pimentel M.M.,Federal University of Rio Grande do Sul | And 5 more authors.
Gondwana Research

The Vazante Group show varied U-Pb provenance patterns along the basin. Zircon ages range from 936 to 3409Ma, but Paleo- and Mesoproterozoic terrains constitute the main sources of the original sediments. The youngest population (~930Ma) establishes the maximum depositional age of the group. Sm-Nd T DM data show the predominance of Paleoproterozoic ages (1.90-2.08Ga) and also indicate some input from younger sources in rocks of the Lapa Formation (1.67 to 2.0Ga) in the upper part of the group, whereas rocks of the Serra do Garrote Formation present the oldest model ages (2.03 to 2.76Ga). Hf isotopic compositions of the detrital zircons indicate that they were derived mainly from recycled Paleoproterozoic crust with a minor Mesoproteroic juvenile component. Terranes within the São Francisco Craton represent the main sources of detrital sediments of this group and reinforce the interpretation that it may be a passive margin sequence developed along the western margin of the original continent. However, the origin of Mesoproterozoic grains remains uncertain. Slightly younger Sm-Nd model ages in the Lapa Formation, however, are not entirely consistent with derivation solely from the craton and may indicate contribution from younger sources, such as the Neoproterozoic Goiás Magmatic Arc. © 2011 International Association for Gondwana Research. Source

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