DEGEO - Departamento de Geologia
URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/8
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Resultados da Pesquisa
Item Petrogenetic processes at the tipping point of plate tectonics : Hf-O isotope ternary modelling of Earth’s last TTG to sanukitoid transition.(2020) Moreira, Hugo Souza; Storey, Craig Darryl; Fowler, Mike; Seixas, Luís Antônio Rosa; Dunlop, JosephModern style plate tectonics is characterized by one-sided subduction and continental margin basaltandesite-dacite-rhyolite (BADR) magmatism, whereas continental magmatic rocks in the Archaean record had tonalite-trondjhemite-granodiorite (TTG) composition. Their main difference is the absence (in the modern style) and the presence (in the ancient style) of abundant juvenile, basalt-derived felsic magmatism. The diversity of modern continental magmas depends partly on metasomatic processes in the mantle wedge providing sediment input (melt/fluid). Contrasting scenarios in the rock record therefore are the presence or absence of (1) basalt-derived melt (TTG) and (2) sedimentary input to the magmas. A late, “Archaean-style” tectonic regime is recorded in the Palaeoproterozoic Mineiro Belt (Brazil) using whole-rock geochemistry of its plutons coupled to zircon Hf and O isotopes from these rocks and from detrital grains from the local (meta)sediments. Increasing δ18O with decreasing εHf(t) values in zircon indicate oceanic crust recycling and input of sediments to the mantle wedge during the Palaeoproterozoic. Since some form of early subduction including the formation of a mantle wedge has occurred since the Mesoarchaean, it seems that the onset of subduction and the final establishment of modern plate tectonics at the global scale are temporally distinct. The latter did not occur until the Palaeoproterozoic. Prior to this time (“ante-plate tectonics”), punctuated subcretion/subduction marks a gestational stage that took c. 800 Myr to transition diachronously to a truly global mechanism. The TTGsanukitoid transition, which signals the opening of a mantle wedge, occurred in the Mineiro Belt during the early Proterozoic “magmatic lull” and thus represents the last gasp of the ante-plate tectonics regime.Item Evolution of Siderian juvenile crust to Rhyacian high Ba-Sr magmatism in the Mineiro Belt, southern São Francisco Craton.(2018) Moreira, Hugo Souza; Seixas, Luís Antônio Rosa; Storey, Craig Darryl; Fowler, Mike; Lasalle, Stephanie; Stevenson, Ross; Lana, Cristiano de CarvalhoPlutonic rocks from the Mineiro Belt, Brazil record a delayed onset of the transition from TTG to sanukitoid-type magmatism (high Ba-Sr), starting during the Siderian magmatic lull when little juvenile magma was added to the continental crust. Rocks mostly belong to the calc-alkaline series, meta- to peraluminous and originally “I-type”, meaning that oxidized magmas were formed by partial melting of subducted material. The temporal distribution and apparent secular changes of the magmas are consistent with the onset of subduction-driven plate tectonics due to an increase of the subduction angle and opening of the mantle wedge. New isotopic analyses (Sm-Nd whole rock and Lu-Hf in zircon) corroborate the restricted juvenile nature of the Mineiro Belt and confirm the genetic link between the Lagoa Dourada Suite, a rare ca. 2350 Ma high-Al tonalite-trondhjemite magmatic event, and the sanukitoid-type ca. 2130 Ma Alto Maranhão Suite. U-Pb dating of zircon and titanite constrain the crystallisation history of plutonic bodies; coupled with major and trace element analyses of the host rocks, they distinguish evolutionary trends in the Mineiro Belt. Several plutons in the region have ages close to 2130 Ma but are distinguished by the lower concentration of compatible elements in the juvenile high Ba-Sr suite.Item Petrology of the high-Mg tonalites and dioritic enclaves of the ca. 2130Ma Alto Maranhão suite: evidence for a major juvenile crustal addition event during the Rhyacian orogenesis, Mineiro Belt, southeast Brazil.(2013) Seixas, Luís Antônio Rosa; Bardintzeff, Jacques-Marie; Stevenson, Ross; Bonin, BernardCombined field observations, petrography, mineral chemistry, geochemistry, Nd isotopes and U–Pb zir-con data are presented for the ca. 2130 Ma Alto Maranhão suite. The suite, composed by allanite-bearingbiotite hornblende tonalites and commingled dioritic mafic magmatic enclaves (MME), occupies an esti-mated area of >300 km2in the southern Quadrilátero Ferrífero region, Minas Gerais, Brazil. Tonalitesand commingled MME are medium-K rocks, with high-Mg (Mg-number ≥ 0.46) and high-Cr (≥55 ppm)contents. They are also enriched in large-ion lithophile elements (LILE, Sr, Ba), some high-field-strengthelements (HFSE, Th, U), and light rare earth elements (REE), but depleted in Nb, Ta, and heavy REE. TDMmodel ages for tonalites and MME are similar and range from 2.3 to 2.4 Ga. The εNd(t) values are groupedaround chondritic values (=0 ± 1.0). Field relations and geochemical data indicate that tonalites and com-mingled dioritic MME crystallized as synchronous independent magma pulses, with limited mixing. Bothmagmas are compositionally equivalent to high-Mg andesites in modern subduction zones, implying thatthe suite was issued from the partial melting of a mantle wedge metasomatised to different degrees byslab-derived melts. Geobarometry (aluminium-in-hornblende) indicates crystallization at mid-crustaldepths (i.e. 0.59–0.42 GPa). The igneous suite evolved by fractional crystallization of hornblende, biotiteand accessory minerals (zircon, apatite, allanite and Fe–Ti oxide). The ubiquitous occurrence of horn-blende and allanite in tonalites and dioritic MME suggests that this mineral pair was largely responsiblefor the observed changes of light REE/heavy REE and Eu/Eu* ratios within the members of the suite. Thegeographical extent (>1000 km2) and Paleoproterozoic ages of the Alto Maranhão suite (ca. 2130 Ma) andassociated granitoids of the Mineiro Belt (2.1–2.2 Ga) constitute a major event of juvenile addition to thecontinental crust in the context of the southern São Francisco craton, and the ca. 2.2–2.1 Ga Rhyacianorogenesis of the South American platform. The age and general geochemical characteristics of the AltoMaranhão suite and its emplacement after a ca. 2.35 Ga juvenile, tholeiitic-source derived high-Al TTGsuite, indicate a tectonic setting akin to that of Late-Archaean high-Mg granitoids.Item Química mineral, cristalização e deformação de granitos sintectônicos brasilianos da região de Arrozal, SW do Rio de Janeiro.(2007) Nummer, Alexis Rosa; Seixas, Luís Antônio Rosa; Machado, RômuloEste trabalho apresenta dados de análises de química mineral de feldspatos (feldspato alcalino e plagioclásio) e biotita de granitos sintectônicos da região de Arrozal, porção sudoeste do Estado do Rio de Janeiro, envolvendo fácies deformadas e não deformadas dos Maciços Graníticos Arrozal (MGA) e Getulândia (MGG) e amostra da rocha encaixante. São maciços graníticos alongados na direção ENE-WSW e associados a zonas de cisalhamento de alto ângulo do vale do Rio Paraíba do Sul. Possuem foliação magmática na parte central e de deformação no estado sólido nas bordas. Do MGA foram analisadas três faciologias: (i) Porfirítica (central), (ii) Foliada (borda), (iii) Leucogranítica. Do MGG foram analisadas duas faciologias: (i) Inequigranular e (ii) Equigranular foliada. A composição do feldspato alcalino situa-se próximo ao pólo do ortoclásio no diagrama Or – Ab – Na; a do plagioclásio é albítico (fácies leucogranítica) e andesina (fácies porfirítica, foliada e enclave). A biotita do MGA é mais férrica (Fe # entre 0,5 a 0,8) e aluminosa (17 a 19% de Al2O3), situando-se entre os pólos da annita e siderofilita; a biotita do MGG é mais magnesiana (Fe # ~0,5) e menos aluminosa (14 a 15% de Al2O3), e situa-se entre os pólos da flogopita e eastonita (Fe # entre 0,5 e 0,8). As biotitas estudadas são preferencialmente primárias e foram pouco afetadas pela deformação no estado sólido. As biotitas da fácies foliada do MGA, em relação às das demais fácies do maciço, possuem conteúdos semelhantes em Mg0 e Ti02, porém mais elevados em Al2O3.Item Geochemistry, Nd isotopes and U–Pb geochronology of a 2350 Ma TTG suite, Minas Gerais, Brazil : implications for the crustal evolution of the southern São Francisco craton.(2012) Seixas, Luís Antônio Rosa; David, Jean; Stevenson, RossThe Paleoproterozoic is considered the main period of crustal growth in the South American continent, with voluminous production of granitoid suites emplaced during the onset of the ca. 2.2–2.1 Ga Trans- Amazonian orogeny. This work documents and interprets mineral chemistry, whole rock geochemistry, Nd isotopes and U–Pb geochronology data for the high-aluminium Lagoa Dourada TTG suite. This suite constitutes a hitherto unknown episode of Paleoproterozoic (ca. 2350 Ma) juvenile felsic plutonism in the southern São Francisco craton, Minas Gerais, Brazil. It is also a rare example of juvenile continental crust production in the global crustal age-gap between 2.45 and 2.2 Ga. The suite is composed of metaluminous to slightly peraluminous, low-Mg#, low-K and high-Ca, biotite-hornblende to hornblende-biotite (± garnet-magnetite) tonalites and biotite trondhjemites, varying from ∼62 to 73 wt.% SiO2. The trace element geochemistry of the suite is characterized by mildly depleted Nd isotope compositions [εNd(t) = +1.0 to +2.1], low large-ion-lithophile element (LILE, i.e., Rb, Ba, and including the highly incompatible Th) and heavy Rare Earth element (REE, Yb < 1.00 ppm) contents. The suite is also characterized by high Sr/Y ratios (≥41 up to 81), high (La/Yb)N ratios (≥12 up to 46), and positive Eu/Eu* anomalies. These data are consistent with the derivation of the suite from a tholeiitic metabasaltic source, which had a short crustal residence time prior to melting. The chemical diversity of the suite is attributed mainly to the fractional crystallization of hornblende and accessory minerals, coupled with plagioclase accumulation in the tonalitic parental melt. Emplacement of new batches of magma could explain samples which diverge from the proposed liquid line of descent of the suite. The composition and age of the Lagoa Dourada suite suggest a link with the accretion of greenstone belt packages in an intra-oceanic tectonic setting.Item Estruturas primárias em turbiditos do greenstone belt Rio das Velhas, neoarqueano do Cráton São Francisco meridional.(2007) Seixas, Luís Antônio Rosa; Silva, Rômulo César; Gomes, Luciano; Gomes, Caroline Janette SouzaA preservação das estruturas sedimentares primárias no greenstone belt Rio das Velhas (~ 2,7 Ga - 2,8 Ga) é limitada a uns poucos locais nos quais as rochas metassedimentares foram poupadas de deformação tectônica penetrativa intensa. Entretanto, diferentes pesquisadores consideram que uma parte importante desse cinturão é constituída de rochas metassedimentares de origem turbidítica, como parte de uma associação de litofácies clásticas ressedimentadas. Este trabalho documenta e interpreta estruturas sedimentares primárias em rochas dessa associação, aflorantes no setor sudoeste da Serra do Curral, região oeste do Quadrilátero Ferrífero. A seção foi estudada em um corte de estrada de cerca de 150 m de comprimento, e consiste da delgada interestratificação de camadas de metarenitos e metapelitos, dobrados e com alto a moderado ângulo de mergulho. As estruturas sedimentares identificadas compreendem acamamento gradacional e maciço em metarenitos, laminações paralelas, cruzadas, climbing e convolutas em metarenitos finos, metargilitos maciços ou com laminação plano-paralela, e estruturas em chama, clastos de argilitos do tipo rip-up mud clasts, e deformação sin-sedimentar em precursores inconsolidados. O predomínio de contatos basais planos das camadas de metarenitos sobre os metapelitos, as sequências verticais de estruturas sedimentares que reproduzem os intervalos A a D, ou B ou C a D da sequência de Bouma, alguns intervalos com misturas de metarenitos com clastos de metapelitos ou clastos de metapelitos em metarenitos (rip-up mud clasts), assim como a ausência de metassedimentos mais grossos conglomeráticos, sugerem que a seção estudada representa diferentes fácies de sedimentação turbidítica de leque submarino intermediário a distal.