Navegando por Assunto "Geocronologia U-Pb em zircão"

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Acesso aberto (Open Access)
Geologia Isotópica U-Pb e Lu-Hf no tonalito Cândido Mendes e arenito da formação igarapé de areia: aplicação em proveniência sedimentar e implicações para evolução do cráton São Luís
(Universidade Federal do Pará, 2023-10-16) FERREIRA, Fernanda Batista; MOURA, Candido Augusto Veloso; http://lattes.cnpq.br/1035254156384979
Using the Lu-Hf isotopic signature in zircon of known U-Pb age allows the characterization of the magma that crystallizes this mineral. Thus, it is a potential tool for sedimentary provenance investigation. The application of this method in provenance studies was tested using detrital zircons from sandstones of the Igarapé de Areia Formation, occurring in the São Luís Cratonic Fragment (SLCF) and the Gurupi Belt (GB) in the Gurupi region, NE of Pará an NW Maranhão. Most zircon grains of the sandstone have Rhyacian age, which allows for evaluating the applicability of the Lu-Hf isotopic system for provenance studies. The morphological analyses of these zircon grains indicate a magmatic origin. The U-Pb data yield Ryancian (2051 to 2256 Ma) and Orosirian (1912 to 2049 Ma) ages. These ages are compatible with the timing of the orogenic event that formed the SLCF. The Bom Jesus Granodiorite, a large granitoid from the Intrusive Tromaí Suite of the SLCF, is representative of the Paleoproterozoic calc-alkaline magmatic event and has a U-Pb zircon concordant age of 2149±5.4 Ma. The Lu-Hf isotopic signatures of the zircons of both tonalite and sandstone are similar, with ƐHf(t) ranging from 0.65 to 6.82 and from 1.84 to 6.07, respectively. Furthermore, the zircon model ages show similar results, with Hf-TDMC from 2.36 to 2.53 Ga for the Igarapé de Areia Formation and from 2.28 to 2.67 Ga for Intrusive Tromaí Suite, indicating a source area for the sandstone made up by juvenile Paleoproterozoic crust. These results support the hypothesis that the sedimentary sources of the sandstone were the rocks located in the SLCF and GB and their Birimian equivalents in the West African Craton (WAC). The lithological and geochronological similarity of the Paleoproterozoic rocks of the Gurupi region with rock units from the Baoulé-Mossi Domain from WAC is well established. In turn, Neoarchean Hf TDMC model ages may indicate a small contribution from the Archean crust, which is recorded more clearly in the rocks of the Baoulé-Mossi Domain. This contamination process may have occurred during the subduction and melting of sediments from the Archean crust or by incorporating a Paleoproterozoic Island arc into Archean terranes. The Lu-Hf isotope system has proved to be a handy tool for investigating sedimentary provenance.
Acesso aberto (Open Access)
Petrografia, geoquímica e geocronologia de rochas vulcano- plutônicas orosirianas do SE do Cráton Amazônico: um estudo da fronteira dos domínios Tapajós e Iriri-Xingu
(Universidade Federal do Pará, 2020-06-16) SILVA, Amanda Suany Marinho da; MACAMBIRA, Moacir José Buenano; http://lattes.cnpq.br/8489178778254136
The central part of the Amazonian Craton encompasses the Tapajós (TJD) and Iriri-Xingu (IXD) tectonic domains, which record volcano-plutonic events from the Orosirian times. The boundary between both domains was proposed from Nd isotopes and is marked by the predominance of juvenile paleoproterozoic sources or with little participation of the archaean crust for the TJD, whereas the rocks of the IXD present archaean crustal sources. However, the boundary between these domains is still subject of discussion due to the lack of isotopic data, the homogeneity of the lithotypes and the absence of tectonic structures that could outline this frontier. The study area is in the municipality of Trairão, southwestern Pará state, the probable boundary region between TJD and IXD, where there are occurrences of paleoproterozoic volcano-plutonic associations. We performed in situ U-Pb on zircon by plasmainduced mass spectrometry with laser ablation (LA-ICP-MS), Sm-Nd whole-rock by thermal ionization mass spectrometry (TIMS), together with geochemical analyses and petrographic data aiming to reconstruct the tectonic and chronological emplacement of the magmatic rocks, as well as to better establish the boundary between TJD and IXD. In the study area, the volcanic sequences are represented by the Moraes Almeida, Salustiano and Aruri formations, and the plutonic units correspond to the Creporizão, Parauari (granodioritic and granitic units) and Maloquinha suites. The effusive and volcanoclastic rocks are rhyolites, dacites, andesites, rhyolitic ignimbrites, and tufts. The plutonic units are granodiorites, quartz monzonites, monzogranites, and syenogranites. The geochemical results showed that the rocks of the Creporizão and Parauari intrusive suites (granodioritic and granitic units) have high-K calcalkaline to shoshonitic signature, a meta to peraluminous character, with enrichment in LILE (K, Rb, Ba, and Sr), moderate fractionation of heavy ETR and weak negative Eu* anomalies. Distinctively, the granitoids of the Maloquinha Intrusive Suite and the Salustiano and Moraes Almeida formations have a high-K calc-alkaline signature, from peraluminous to peralkaline character. These rocks present enrichment in HFSE (Zr, Hf, and Th), high ETR contents and pronounced negative Eu* anomalies. In tectonic environment classification diagrams, the granitoids of the Creporizão and Parauari (granitic and granodioritic units) intrusive suites and, Salustiano and Aruri formations showed geochemical affinities with volcanic arc granites (VAG), related to magmatic arc, while the rocks of the Maloquinha Intrusive Suite and the Moraes Almeida Formation are related to intraplate anarogenic environment. The U-Pb zircon dating allowed to identify the oldest plutonic and volcanic rocks, belonging, respectively, to the Intrusive Creporizão Suite, with crystallization age of 1980±6 Ma, and to the unit mapped xi as Salustiano Formation, aged of 1975±11 Ma. The rocks of the Maloquinha Intrusive Suite were generated at 1880±9 Ma, with a volcanic correspondent of the Moraes Almeida Formation of 1877±14 Ma, followed by the plutonic rocks of the Parauari Intrusive Suite (granodioritic unit, aged of 1876±9 Ma, and the granitic unit, of 1867±15 Ma), and the volcanic rocks of the Aruri Formation, generated at 1867±7 Ma. The geochemical characteristics combined with the geochronological data allowed to define a geodynamic evolution involving an context related to magmatic arc environment at ca. 1.98 Ga, which favored the formation of the rocks of the Intrusive Suite Creporizão and Salustiano Formation, followed by an intraplate extensional environment (ca. 1.88 Ga), marked by the coexistence of the types A and I rocks of the Maloquinha and Parauari (granodioritic and granitic units) suites, and Moraes Almeida and Aruri formations. Model ages Nd-TDM (2.31-2.64 Ga), with slightly to strongly negative values of εNd(t) (-1.39 to -7.11), indicate magmas derived from the melting of crustal sources of the Paleoproterozoic and Archaean. These results allowed to draw an NW-SE line, where mainly rocks of paleoproterozoic (<2.5 Ga) and archaean (> 2.5 Ga) model outcrop, belonging, respectively, to the TJD and IXD.