Navegando por Assunto "Geocronologia Pb-Pb"

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Geology, petrology and geochronology (Pb-Pb) of the Serra da Bocaina Formation: evidence of an Orosirian Amoguijá Magmatic Arc in the Rio Apa Terrane, south of the Amazonian Craton
(2013-03) BRITTES, Ana Flávia Nunes; SOUSA, Maria Zélia Aguiar de; RUIZ, Amarildo Salina; BATATA, Maria Elisa Fróes; LAFON, Jean Michel; PLENS, Dalila Pexe
This work presents the results of the geological mapping and the petrological characterization of the Serra da Bocaina Formation belonging to the Amoguijá Magmatic Arc of the Rio Apa Terrain. In the Bocaina hills, the Serra da Bocaina Formation consists of intermediately to predominately acidic volcanic rocks, classified as andesites and rhyolites and subdivided into five petrographic facies, which include four volcanoclastics and one effusive. In the studied rocks, tectonic structures are generated in two compressive ductile to ductile-brittle deformational phases. The first phase, which is more intense, presents along the entire studied area and is responsible for the Santa Rosa Shear Belt, whereas the second phase is more localized. The geochemical study indicates a calc-alkaline magmatism of medium to high K, peraluminous, and generated in a volcanic arc environment. These volcanic rocks formed in an explosive environment related to the Amoguijá Magmatic Arc, as indicated by their Pb-Pb zircon age of 1877.3 ± 3.9 Ma.
Acesso aberto (Open Access)
Geoquímica isotópica Sr e geocronologia Pb-Pb da capa carbonática neoproterozoica do Grupo Araras, Tangará da Serra, MT
(Universidade Federal do Pará, 2012-04-24) ROMERO, John Alexander Sandoval; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
The Neoproterozoic cap carbonates have been the subject of countless paleoenvironmental and chemicalstratigraphy studies in almost all Cratons around the world. Those were deposited after global glacial events deposits, are composed of dolostones and limestones, overlying glacial diamictites with specific features as stromatolits, tubes like structures, megaripples and crystal fans of calcite. In Brazil this ocucurrence, principal objetive of this work is exposed in Mirassol d ́Oeste and Tangará da Serra towns at Southeast of Mato Grosso State, Southeastern of Amazon Craton, recovering glacial diamictites with age of ≈ 630 Ma, The cap carbonate is part of base of Araras Group compoused by dolostone of Mirassol d ́Oeste and limestones of base of the Guia Formation. The main goal of this work is consolidate an Early Ediacaran age for the cap carbonate of the Tangará da Serra area by Pb-Pb geochronology and bring new Sr isotopic data to understand the paleoemviromental conditions as well as the increasing Sr marine evolution curve of during Neoproterozoic. Were collected 36 samples from top of Mirassol d ́Oeste (13 samples) and base of Guia Formation (23 samples), for petrography, X-ray difractometry, X- fluorescence as well as Sr isotopic analyses and Pb-Pb dating, using mass spectrometer TIMS and ICP-MS. The petrography allow recognize diagenetic features as stylolits, dolomite grains recristalized, spatic dolomite, iron oxides and autigenic quartz in dolostones of Mirassol d ́Oeste Formation. In Guia Formation, were observed partial substitution from crystal fans of calcite to spatic dolomite, stilolits as well as microfractures filled by calcite and secondary dolomite. The X-difractometry complemented the determination of mineralogical assemblage about carbonates, identificating the presence of secondary dolomite as well as quartz and feldspar in limestones of Guia Formation. The X-ray fluorescence allows determinate the teors of Fe, Mn, Ca and Sr, which were used to help the interpretation of Sr isotopic signatures. The Sr isotopic signature in 5 samples in dolostones of Mirassol d ́Oeste Formation and 6 limestones samples of base of Guia Formation, were determinated by sequential leaching with acetic acid. For dolostones the 87 Sr/ 86 Sr ratios ranging 0.708 to 0.709. For limestones the 87 Sr/ 86 Sr ratios ranging 0.707093 to 0.707289, after eliminate the leaching by probably terrigenous contribution for 87 Sr/ 86 Sr ratios until 0.7114. The lowest 87 Sr/ 86 Sr ratios were found in samples with lowest Mn/Sr ratios (0.31-0.45) and Fe/Sr (6.03-9.8). The differences in analytical procedure used to obtain the 87 Sr/ 86 Sr ratios, allow to explain the differences a out the isotopic Sr signatures in limestones from Tangará da Serra área (sequential leaching) with which, more radiogenic from limestones from Mirassol d ́Oeste área (total dissolution), previously published. The 87 Sr/ 86 Sr ratios ranging 0,70709 – 0,70729 in limestones from Tangará da Serra area, are positioned above from Sr marine curve in late Neoproterozoic, in transition from Cryogenian to Ediacaran, before the abrupt rise in early Ediacaran. Six dolostones samples and 17 limestones samples were analyzed, for determination of isotopic Pb composition, the samples were leached with HCl. The isotopic Pb signatures are homogeneous in dolostones (19.05< 206 Pb/ 204 Pb < 19.50; 15.69 < 207 Pb/ 204 Pb < 16.89) and were not used for elaborate the Pb-Pb isochron. From 17 limestones samples just 15 provided and age of 622 ± 33 Ma (2σ), despite de limited isotopic variations of the isotopic composition (18.77 < 206 Pb/ 204 Pb < 31.18; 15.71 < 207 Pb/ 204 Pb < 16.46). This result reinforces the Ediacaran early age to cap carbonate of Araras Group and check their association with the events after the last Criogenian glaciation at South of Amazon Craton.
Acesso aberto (Open Access)
Gnaisse Turvo: registro de magmatismo paleoproterozoico no Terreno Paraguá - sudoeste do Cráton Amazônico, Vila Bela da Santíssima Trindade, Mato Grosso
(2013-06) FIGUEIREDO, Fernando Lisboa Pinto de; RUIZ, Amarildo Salina; SOUSA, Maria Zélia Aguiar de; MACAMBIRA, Moacir José Buenano
The Turvo Gneiss, object of this work, corresponds to a polideformed orthogneiss exposed in the region of Vila Bela da Santíssima Trindade, southwestern of state of Mato Grosso. From the tectonic point of view, is inserted into the Amazon Craton and represents the Paleoproterozoic basement of the Paraguá Terrain, one of the crustal blocks that form the Province Rondonian-San Ignacio (1.55 - 1.3 Ga). Two facies were identified from petrographic study: garnet-amphibole-biotite gneiss composed of granodiorite and biotite-amphibole gneiss, more abundant, with granodioritic to syenogranitic composition. The metamorphic paragenesis identified characterizes as responsible for these gneisses of amphibolite facies. The structural analysis features two stages of deformation in ductil crustal level. The oldest (F1) is responsible for developing the gneissic banding, while the structures of phase (F2), the second oriented N30-60W indicate compressive stress with tectonic transport from SW to NE. The minimum age of crystallization of the Turvo Gneiss defined by Pb-Pb method on zircon evaporation corresponds to 1651 ± 4 Ma and is interpreted as emplacement age of igneous protolith. The data indicate that significant lithogeochemical calcium-alkaline magmatism of high K, metaluminous to peraluminous, associated with magmatic evolution in a subduction environment (Lomas Manechis Orogeny - 1.7 to 1.6 Ga), dominated the period Estaterian in the Paraguá Terrain. The orthogneissic unit studied was later reworked, metamorphic and tectonic (F2), during the San Ignacio Orogeny (1.4 to 1.3 Ga).
Acesso aberto (Open Access)
Implantação da metodologia Pb-Pb em rocha total: exemplos de aplicação na província mineral de Carajás (PA)
(Universidade Federal do Pará, 1992-12-12) RODRIGUES, Elizabeth Maria Soares; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
Acesso aberto (Open Access)
Proveniência das rochas metassedimentares do Cinturão Araguaia com base em datações Pb-Pb em zircão e idades-modelo Sm-Nd
(2011-06) PINHEIRO, Bruno Luís Silva; MOURA, Candido Augusto Veloso; GORAYEB, Paulo Sérgio de Sousa
This paper presents single zircon Pb-evaporation ages, obtained in detrital zircon grains, and Sm-Nd (TDM) for metassedimentary rocks of the Araguaia belt, to investigate the possible source areas of these rocks, and to contribute to understanding the geologic evolution of this belt in the context of the West Gondwana amalgamation. Quartzite of the Morro do Campo Formation, located in the north segment of the belt (Xambioá region) gave Archean 207Pb/206Pb zircon ages (2.65-3.0 Ga), while in the south segment (Paraíso do Tocantins region), the 207Pb/206Pb zircon ages were mainly Meso-Neoproterozoic (1.25-0.85 Ga), and, secondarily, Paleoproterozoic (1.85-1.70 Ga). These results suggest the existence of different sources areas for this quartzite. The Sm-Nd (TDM) model ages of metapelites (slate, phyllite and micaschist) present bimodal distribution with the main population occurring between 2.1 and 1.4 Ga (mode in 1.6-1.7 Ga), and a less abundant population between 2.7 and 2.4 Ga. These data have been interpreted as mixing of Paleoproterozoic (or even Archean) sources with younger, probably Meso-Neoproterozoic, sources. The main candidates to be the sources of the metassedimentary rocks of the Araguaia belt would be those crustal segments located in the southeastern (São Francisco Craton, Goiás Massif, Goiás Magmatic Arc). The sedimentary succession deposited in the Araguaia oceanic basin and the associated magmatic rocks were transported, later, towards the east margin of the Amazonian Craton, during the main tectonic that led to the structural development of the Araguaia belt as a result of the West Gondwana amalgamation.
Acesso aberto (Open Access)
Vulcanismo félsico paleoproterozóico do Grupo Iricoumé, NW do Pará, domínio Erepecuru-Trombetas, Província Amazônia Central: petrografia, geoquímica e geocronologia Pb-Pb em zircão e Sm-Nd em rocha total
(Universidade Federal do Pará, 2012-04-11) BARRETO, Carla Joana Santos; ROSA COSTA, Lúcia Travassos da; http://lattes.cnpq.br/3352314623448523; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
The Iricoumé Group is part of the Orosirian large volcano-plutonic event in the Amazonian Craton, which comprises pyroclastic volcanic rocks with well-preserved textures and structures. The study area is located in the southwestern part of the Erepecuru-Trombetas Domain in the Guyana Shield. This area is bounded by the Paleozoic cover of the Amazonas Basin to the south and by basement rocks to the north. Petrographical studies allowed the distinction of a dominant pyroclastic volcanism (ignimbrites, reoignimbrites, lapilli-tuff related to surge and fall co-ignimbrites), and subordinate hypabissal (spessartitic lamprophyres, trachyte, rhyolite) and effusive (andesitic lava flows). Pyroclastic rocks display features indicative of pyroclastic deposition at high temperatures, suggesting generation in caldera environments. Zircon Pb-Pb ages of 1888 ± 2.5 Ma e 1889 ± 2 Ma obtained in trachytic ignimbrites from the Iricoumé Group confirm that most rocks belong to this group. On the other hand, a zircon age of 1992 ± 3 Ma obtained in a trachyandesite sample allowed the identification of a subordinate late Orosirian volcanism, already identified to the South of the Amazonas Basin, in the Tapajós Domain. Less differentiated pyroclastic rocks (compositions trachyte and dacite) have metaluminous signature, Sr positive anomalies and weak to absent negative Eu anomalies, while the more differentiated pyroclastic rocks (compositions rhyolites) have slightly peraluminous signature, strong negative Sr, P and Ti anomalies, and moderate negative Eu anomalies. Hypabissal rocks display metaluminous to peraluminous characters, strong negative Sr, Ti and P anomalies in trachyte and rhyolite, and positive Sr anomalies for the lamprophyre. In addition, these hypabissal rocks show weak to absent negative Eu anomalies. The geochemical characteristics of these rocks suggest formation in a post-collisional tectonic setting. This interpretation is accepted for volcanoplutonic associations from other domains of the central portion of the Amazonian Craton. The trachyandesites display metaluminous character with positive Sr anomalies and absent Eu anomalies, as well as an affinity with a volcanic arc setting, suggesting that these rocks were formed in subduction-related geodynamic setting. Sm-Nd results yielded Nd values between - 3.04 and +2,35 and TDM ages ranging from 1.98 Ga to 2.39 Ga, which indicate dominantly Rhyacian to Siderian crustal sources for the parental magmas without Archean crustal contributions. These new results constitute additional evidence for the large extension of the Iricoumé volcanism and for the correlate volcanic rocks in the central portion of the Amazonian Craton, reinforcing the hypothesis of the presence of a large Felsic Igneous Province (SLIP), as already described by some authors.