Navegando por Assunto "Geocronologia U-Pb"

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Acesso aberto (Open Access)
Geocronologia da capa carbonática e proveniência do intervalo Ediacarano Superior-Cambriano da bacia Araras-Alto Paraguai, sul do cráton Amazônico
(Universidade Federal do Pará, 2022-06-21) CARVALHO, Davi Ferreira de; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998; MACAMBIRA, Moacir José Buenano; http://lattes.cnpq.br/8489178778254136
The Araras-Alto Paraguai basin, on the Southern Amazonian craton includes Cryogenian-Ediacaran-Cambrian deposits related to the Marinoan glacial (ca. 635 Ma) and post-glacial events (ca. 635-600 Ma) and initial stages of Western Gondwana. The glaciogene diamictites of the Puga Formation are directly overlaid by the Neoproterozoic cap carbonate of the lower Araras Group, evidencing the rapid icehouse-greenhouse transition in the context of the Snowball Earth theory. The Araras Group is unconformably superimposed by the Cambrian siliciclastics of the Raizama Formation, lower Alto Paraguai Group. The succession is bounded by U-Pb maximum depositional ages of < 652 ± 5 Ma (basement - Cuiaba Group) and < 528 ± 9 Ma (upper Alto Paraguai Group). Carbonate Pb-Pb ages of 627 ± 32 Ma and 622 ± 33 and δ13C negative excursions in the cap carbonate are coherent with these time estimates. Correlation of the isotopic data of the Araras-Alto Paraguai basin with δ13C and 87Sr/86Sr global curves indicates an age of ca. 614 Ma for the eroded upper limit of the Araras Group. The presence of skolithos at the base of the Alto Paraguai Group confirms the Cambrian age (<530 Ma), implying a stratigraphic gap of ca. 80 x 106 yr between the groups. The available data are still sparse, allowing only imprecise timing of the geologic events in the succession. New δ13C isotopic data with the typical cap carbonate negative signature, with values oscillating between -7 and -4 ‰ complemented the δ13C curves available for the succession, extending stratigraphic correlations over the inner platform of the Araras Group. The LA-ICP-MS U-Pb dating of calcite crystal fans, interpreted as aragonite pseudomorphs, provided the first diagenetic age of 624 ± 6 Ma of the cap carbonate. Textural relations and heavier isotopic signal in the fans (δ13C = -6.80 a -5.85, δ18O = -7.0 a -5.80) compared to the micrite (δ13C = -7.25 a -5.85, δ18O = -8.9 a -7.5), indicates dissolution-reprecipitation under the influence of oxidizing fluids in the diagenetic environment. Such a change in the composition of the diagenetic fluids may be related to the sea level fall during deposition of the highstand system tract of the cap carbonate sequence. The diagenetic age demonstrates that the depositional and diagenetic processes of the cap limestone, including micrite and pyrite precipitation, inversion of aragonite to calcite, burial neomorphism, chemical compaction and calcite dissolution-reprecipitation within the crystal fans lasted ca. 11 ± 6 x 106 yr. Although a similar spectrum of detrital zircon ages indicates the same cratonic source areas for the studied succession, there is a general higher proportion of ages older than 1.25 Ga for the upper Araras Group compared to the lower Alto Paraguai Group. This result corroborates the erosive hiatus between the groups, indicating a relative increase of sources from the SW-SE (<1.25 Ga, Sunsás belt) in detriment of sources to the N-NW (> 1.25 Ga, other terrains of the Amazonian craton) of the basin during the Ediacaran-Cambrian transition. On the other hand, in the cratonic domain, absence of ages younger than 1.38 Ga and paleocurrents persistently to SE for the lower Alto Paraguai Group suggest intensification of the siliciclastic influxes from the NNW. Integration of the sedimentological, chemostratigraphic, geochronological and tectonic data allowed to propose five evolutionary steps for the Ediacaran-Cambrian succession of the Araras-Alto Paraguai Basin: 1) Ice melting with deposition of the cap dolostone (635-632 Ma); 2) Thermal expansion of the ocean with deposition of the cap limestone and highstand system tract of the cap carbonate sequence (ca. 632-615 Ma); 3) Regression with transference of accommodation space to the East (ca. 615- 600 Ma); 4) Complete exposition of Southern Amazonian craton (600 – 530 Ma); and 5) Sea level rise with implantation of a siliciclastic platform (ca. 530-520 Ma). The obtained geochronological and chemostratigraphic data allowed a better stratigraphic characterization of the Araras-Alto Paraguai sedimentation exposed in the southern Amazonian craton, revealing an important part of the geological evolution of the primordial epicontinental seas related to western Gondwana assembly.
Acesso aberto (Open Access)
Geocronologia U-Pb e geologia isotópica Sm-Nd do Granito Rio Verde, Neoproterozoico no Terreno Granjeiro -Várzea Alegre (CE)
(Universidade Federal do Pará, 2019-12-20) COELHO, Dayane do Nascimento; MOURA, Candido Augusto Veloso; http://lattes.cnpq.br/1035254156384979
Several magmatic events, from the late Archean to the beginning of the Phanerozoic, are recognized in Borborema Province. The plutonic Ediacaran-Cambrian activity (Brasiliano cycle) is one of the most important geological features of this Province since it generated a large number of granitic bodies of different ages and nature. In this context, there are several granitoids emplaced in the Neoarchean rocks of the Granjeiro Terrain, of the Rio Grande do Norte Domain. Undergraduate geology students of the UFPA mapped some granitic bodies hitherto not described in this terrain yet. Among the new occurrences, a small granitic pluton located southwest of the city of Várzea Alegre-CE is the object of geochronological investigation of this dissertation. It is being referred here as Rio Verde Granite. The pluton presents porphyritic texture with feldspar phenocrysts (1,0 to 5,0 cm in size) immersed in a medium-grained matrix. The Rio Verde Granite has three lithofacies. Titanite-biotite-hornblende granodiorite (TnBtHbGdr), biotite monzogranite (BtMzg), and enclaves of quartz diorite composition. It also shows evidence of magmas mingling. The rocks are deformed; however, magmatic structures are also observed. The U-Pb zircon dating of the BtMzg and associated intermediate enclaves by LA-MC-ICP-MS was carried out to correlate this magmatism with one of the magmatic events recognized in the Borborema Province. In parallel, the Sm-Nd isotopic systematics was performed to characterize the source of the granitic magma (crustal rework or juvenile crust). U-Pb zircon dating provided a concordant age of 592 ± 3.2 Ma (2σ, n = 5) for the BtMzg. In turn, zircon dating in two samples of the quartz diorite enclave was not good enough due to the degree of metamictization of zircon crystals, which resulted in poor preservation of primary igneous features. Nevertheless, it was possible to define, in one of the samples, the concordant age of 607 ± 4.8 Ma (2σ, n = 3), which is indicative of the timing of the intermediate rocks. Thus, the contemporaneity between the granitic and the more mafic magmatism is suggested. However, additional geochronological studies are needed to define the age of the mafic magmatism accurately. The use of the Sm-Nd isotopic system in whole-rock revealed negative εNd (590Ma) values of -18.26 and -19.42, indicating the reworking old crust as source of the granitic magma. The Nd-TDM ages, calculated in two-stage, of 2.48 and 2.56 Ga, show the contribution of the Archean crust, probably of the Granjeiro Complex, to the formation of the magma that originated the Rio Verde Granite. However, the mixing of Archean crust with younger crustal material cannot be discarded as the Nd-TDM ages are in the Archean-Paleoproterozoic boundary. Considering the petrographic and geochronological features the Rio Verde Granite, it may be associated with the 570-590Ma syn-transcurrent granitic magmatism that occurred in the Borborema Province. This magmatic event has the granitoids of the Itaporanga Intrusive Suit as one of its representatives.
Acesso aberto (Open Access)
Geocronologia U-Pb e geoquímica isotópica Hf-Nd dos ortognaisses e granitoides da região de Granjeiro-Várzea Alegre (CE), Domínio Rio Grande do Norte.
(Universidade Federal do Pará, 2019-07-08) VIEIRA, Daniella Soares Cavalcanti; GALARZA TORO, Marco Antonio
The Borborema Province extends for approximately 450,000 km2 in the northeastern portion of Brazil and represents a complex geotectonic unit, characterized by a collage of crustal blocks with different ages, origins and evolution. Comprises domains such as the Granjeiro Complex, whose highly deformed paleoproterozoic gneissic basement locally includes Archaean nuclei that alternate with remnants of supoprostal and paleoproterozoic supracrustal rocks, all intruded by numerous granite bodies of Brasilian age (ca. 0.60-0.50 Ga) placed along shear zones. In the region of Granjeiro-Várzea Alegre (CE), north of the Patos Lineament, a variety of rocks forms an imbricated system, Duplex type, with units of different ages. Geochronological studies of U-Pb and Lu-Hf in zircon by laser ablation mass spectrometry (LA-ICP-MS) were performed in 5 samples and provided crystallization ages for a Hornblende-Biotite tonalitic gneiss at 2549 ± 16 Ma (OG-07) and at 2532 ± 10 Ma (OG-03) of the Granjeiro Complex; a Hornblende tonalitic orthogneiss at 2354 ± 15 Ma (OG-05) of the Arrojado Complex; a Hornblende-Biotite monzogranitic orthogneiss at 2224 ± 12 Ma (OG-09) from the Várzea Alegre Suite and for a Biotita Monzogranite at 570 ± 6 Ma (G-02) from the Pereiro magmatic arc. The isotopic signatures of Hf (zircon) and Nd (whole-rock) provided model ages (Hf-TDM; Nd-TDM) for these rocks, respectively: OG-07 (2.75 to 2.93 Ga, 2.54 Ga), OG-03 (2.70 to 2.96 Ga and 2.75 Ga), OG-05 (2.92 to 3.06 Ga, 2.96 Ga) and OG-09 (2.84 to 3.01 Ga; 2.90 Ga ) and for the granitoid G-02 (2.94 to 3.27 Ga, 2.10 Ga). The parameters ƐHf (t) and ƐNd (t) for these rocks, respectively, were OG-07 (ƐHf: +1.41 to +4.33; ƐNd: +2.81) and OG-03 (ƐHf: +3.16 to +4.79, ƐNd: -0.37) suggesting contribution of juvenile sources in the protoliths of these rocks; OG-05 (ƐHf: -0.28 to -4.11, ƐNd: -0.23) and OG-09 (ƐHf: -3.86 to -1.15; ƐNd: -5.31) suggesting reworking of Archaean crustal source. The values of the granitoid G-02 (ƐHf: -23.70 to -29.15; ƐNd: -20.10) are strongly negative, suggesting that the magma that originated these rocks was product of reworking of paleoproterozoic (Rhyacian-Siderian) and mesoarquean sources. The integrated isotopic results (U-Pb and Hf-Nd) allowed the identification of at least 4 tectono-magmatic events in the region of Granjeiro-Várzea Alegre. The age range (ca. 2.54-2.53 Ga) and juvenile signatures mark a moment of Archaean crustal growth related to the formation of the Granjeiro Complex. It was also recognized a formation of paleoproterozoic crust (ca. 2.35 Ga), correlated to the Arrojado Complex, whose negative values of ƐHf and ƐNd suggest sources derived from reworked Archaean crust. The rocks of the Granjeiro Complex would have been deformed and assimilated in an event of crustal growth ca. 2.22 Ga, that generated the rocks of the Várzea Alegre Suite. The age of 570 Ma records the last magmatic episode and marks the development of the Borborema Province shear zones that affected and deformed the archaean and paleoproterozoic rocks of this domain.
Acesso aberto (Open Access)
Geocronologia U-Pb e geoquímica isotópica Nd-Hf dos granitoides transamazônicos do domínio Lourenço, sudeste do Escudo das Guianas.
(Universidade Federal do Pará, 2019-05-16) VIANNA, Sâmia Queiroz; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
The eastern part of the Guyana Shield, northern portion of the Amazonian Craton, is located in the Maroni-Itacaiúnas Province (PMI), defined as a paleoproterozoic mobile belt added to an Archaean block during the Transamazonian event (2.26-1.95 Ga). In the state of Amapá, Northern Brazil, the Paleoproterozoic Lourenço Domain consists mainly of Rhyacian gneisses, granitoids and greenstone sequences, with some fragments of Archean rocks in its southernmost portion. To the south, the Amapá Block is defined as a continental landmass formed by Neo-Mesoarchean units (2.85-2.60 Ga) strongly reworked during the Transamazonian orogenic cycle, and by Rhyacian granitoids and supracrustal sequences. The region of Tartarugalzinho, in central-eastern sector of the state of Amapá, is located at the transition between the Paleoproterozoic Lourenço Domain and the Archean Amapá Block. Several granitoid suites were identified in this region (Flexal Intrusive Suite, Papa-Vento Tonalite and Vila Bom Jesus Granite). We performed in situ U-Pb and Lu-Hf isotope analyses on zircon byplasma induced mass spectrometry with laser ablation (LA-ICP-MS), Sm-Nd wholerock by thermal ionization mass spectrometry (TIMS), together with petrography and major and trace element geochemistry, in order to constrain the evolutionary stages of the Transamazonian orogenesis, in which these granitoids are inserted. In addition, the study aimed to contribute to insert the southern portion of the Lourenço Domain within a geodynamic models for the Southeastern Guiana Shield (SGS) and to investigate the processes of crustal growth and reworking during the Rhyacian. The U-Pb geochronology furnished cristalization ages of 2176 ± 9 Ma, 2176 ± 5 Ma e 2166 ± 15 Ma for the Flexal Intrusive Suite, 2131 ± 11 Ma for the Papa-Vento Tonalite and 2085 ± 16 Ma and 2078 ± 8 Ma for the Vila Bom Jesus Granite, that achieved the identification of the two magmatic episodes (~2.18-2.13 and ~2.08 Ga) during he Transamazonian orogeny for this sector of the Lourenço Domain. The Archean Nd-TDM (2.87-2.63 Ga) and Hf-TDM C (3.63-2.79) model ages and the negative values of εNd(t) (-2.74 to -5.43) and εHf(t) (-1.40 to -15.65) indicate the mixture of juvenile Rhyacian material with an Archean crustal component in the source of these magmas. Also, the combination of the petrographic, geochronological geochemical and isotope data allows us to recognize two distinct events. The first magmatic episode displays a calc-alkaline to high-K calcalkaline signature and peraluminous character indicating that it formed during subduction stages in a magmatic arc environment. The second magmatic episode exhibits a calc-alkaline to high-K calc-alkaline signature, metaluminous character and affinity with syn- to post-collision granites. It is still possible to correlate these magmatic events to the two main magmatic episodes that marked the Transamazonian evolution throughout the southeastern Guiana Shield; The geochronological and geochemical data suggest that this sector of the Transamazonian orogeny was built up by accretion of volcanic arcs with subsequent arc-continent collision, however the existence of a continental magmatic arc at the margin of the Archean block cannot be still totally descarded.
Acesso aberto (Open Access)
Geology, Geochemistry, and Geochronology (U-Pb) of the Rio Fortuna Gneiss - Serra do Baú Intrusive Suite - Paraguá Terrane - SW Amazonian Craton
(2014-03) FARIA, Débora Almeida; RUIZ, Amarildo Salina; MATOS, João Batista; SOUSA, Maria Zélia Aguiar de; LIMA, Gabrielle Aparecida de; MACAMBIRA, Moacir José Buenano
The Rio Fortuna Gneiss crops out in the ¬Serra Santa Barbara, near the Fortuna military headquarters, on the Brazil-Bolivia border. These orthogneisses are located in a portion of the Paraguá terrain affected by the Sunsás Orogeny (1.0-0.9 Ga.). They are classified as monzo to granodiorite orthogneisses and underwent at least three episodes of deformation. The U-Pb zircon age of 1,711 ± 13 Ma obtained by laser ablation MC-ICP-MS is interpreted as the crystallization age of this orthogneiss. Geochemically, these rocks form a sequence comprising acidic subalkaline magmatism, calc-alkalic-type high-K, and metaluminous to peraluminous.
Acesso aberto (Open Access)
Petrogenesis and U-Pb and Sm-Nd geochronology of the Taquaral granite: record of an orosirian continental magmatic arc in the region of Corumba - MS
(Universidade Federal do Pará, 2015-09) REDES, Letícia Alexandre; SOUSA, Maria Zélia Aguiar de; RUIZ, Amarildo Salina; LAFON, Jean Michel
The Taquaral Granite is located on southern Amazon Craton in the region of Corumbá, westernmost part of the Brazilian state of Mato Grosso do Sul (MS), near Brazil-Bolivia frontier. This intrusion of batholitic dimensions is partially covered by sedimentary rocks of the Urucum, Tamengo Bocaina and Pantanal formations and Alluvial Deposits. The rock types are classified as quartz-monzodiorites, granodiorites, quartz-monzonites, monzo and syenogranites. There are two groups of enclaves genetically and compositionally different: one corresponds to mafic xenoliths and the second is identified as felsic microgranular enclave. Two deformation phases are observed: one ductile (F1) and the other brittle (F2). Geochemical data indicate intermediate to acidic composition for these rocks and a medium to high-K, metaluminous to peraluminous calk-alkaline magmatism, suggesting also their emplacement into magmatic arc settings. SHRIMP zircon U-Pb geochronological data of these granites reveals a crystallization age of 1861 ± 5.3 Ma. Whole rock Sm-Nd analyses provided εNd(1,86 Ga) values of -1.48 and -1.28 and TDM model ages of 2.32 and 2.25 Ga, likely indicating a Ryacian crustal source. Here we conclude that Taquaral Granite represents a magmatic episode generated at the end of the Orosirian, as a part of the Amoguija Magmatic Arc.
Acesso aberto (Open Access)
Petrografia, geoquímica e geocronologia dos metagabros da região de Xambioá-Araguanã-TO.
(Universidade Federal do Pará, 2010-06-07) BARROS, Gleidson Santos; GORAYEB, Paulo Sérgio de Sousa; http://lattes.cnpq.br/4309934026092502
Several mafic and ultramafic bodies associated with other rocks are incased in the supracrustal sequence in the sector north of the Araguaia belt, and have been treated generically as amphibolites in literature. In the present study the bodies are focused that occur in an important area of exposition, located on Xambioá-Araguanã regions in the northwest of the Tocantins state. The bodies are emplaced in the metassedimentar rocks of the Estrondo Group and Archean gneissic rocks of the basement of the Araguaia belt. These bodies are represented by metagabbro, metadiabase and amphibolite with the indistinct presence in these two main litotypes of scapolite and hornblend as metamorphic minerals, the scapolite decreases to the measure that increases the amount of garnet. The metamorphic paragenesis identified in the metagabbros show Hb + Scp + Grt + Ti ± Qtz + (Pl An44-50 and Cpx, as original igneous minerals) and in amphibolites, Hb+ Pl ± Scp ± Grt ± Ky. Geochemical data demonstrate that the metamafic rocks of the different bodies exhibit homogeneity in their composition as far as in the most different samples. Using various discriminates geochemistry diagrams show the nature of original igneous rocks is transitional toleitic basalts series, and their geochemical signatures is comparable to P-MORB or OIB. This indicates the involvement of fertile mantle sources in the magma generation of the gabbros studied. The geochronological studies carried by U-Pb method (LA-ICP-MS) supplied an interval of 804 ± 35 to 878 ± 22 Ma, considered as minimum age of emplaced of the gabbros, in the Neoproterozoic.
Acesso aberto (Open Access)
Uranium-lead dating method at the Pará-Iso isotope geology laboratory, UFPA, Belém – Brazil
(2007-03) KRYMSKY, Robert Shamilevich; MACAMBIRA, Moacir José Buenano; LAFON, Jean Michel; ESTUMANO, Gérson dos Santos
Analytical procedures for U-Pb isotope dilution analyses at the Pará-Iso isotope geology laboratory of the Federal University of Pará (UFPA) are described in detail. The procedures are applied to zircon, titanite, rutile, apatite, columbite-tantalite and whole rock. Reagent preparation and chemical processing are done in clean-room conditions. Samples are dissolved using Teflon™ microcapsules in steel jacket Teflon™ Parr Instrument™ bomb or Teflon™ screw cap containers. U and Pb are separated using anion exchange AG 1x8 resin columns. Typical blanks for mineral sample amounts of 0.01 to 1.0 mg are less than 1 pg U and 20-30 pg Pb. Isotope analysis of the U and Pb from the same filament are carried out using a Finnigan MAT 262 mass-spectrometer in static and dynamic modes. The current analytical level is demonstrated on analyses of international standard zircon 91500 with three different 235U-205Pb and 235U-208Pb isotope tracers and whole rock standards. Results of analyses of two zircon samples are also presented.