DEGEO - Departamento de Geologia
URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/8
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Item Metallogenetic Mn-model of the Rhyacian-aged Buritirama Formation, Carajás domain (Amazon Craton).(2021) Salgado, Silas Santos; Caxito, Fabrício de Andrade; Silva, Rosaline Cristina Figueiredo e; Uhlein, Gabriel Jubé; Nogueira, Leonardo Brandão; Nalini Júnior, Hermínio Arias; Aranda, Ramon de OliveiraThe Mn-bearing Buritirama Formation is composed of clastic and chemical metasedimentary rocks representing a Rhyacian platformal basin (ca. 2.18–2.06 Ga) that limits the northern border of the Archean-Paleoproterozoic Caraj ́ as domain with the Paleoproterozoic Bacaj ́ a domain in the southern Amazon Craton, northern Brazil. The Buritirama Formation is divided into three main stratigraphic units. The Lower and Upper units consist of a basal quartzite followed by carbonate–silicate rocks (marls), dolomitic marbles (dolostone) and minor biotite schist while the Intermediate Unit consists of Mn-carbonate rocks topped by a supergene Mn-oxide deposit (Cenozoic age). These rocks were deformed and metamorphosed during the Transamazonian event (ca. 2.06 Ga). Geochemical data from dolomitic marbles (Lower Unit) exhibit negative Ce anomalies, high Y/Ho ratios (ca. 48) and seawater-like REE + Y patterns that reinforce an open platform setting for the Buritirama basin. Positive δ13C values (+3.2‰ to +5.0‰ V-PDB) for dolomitic marbles are considered to be an expression of the Loma- gundi Carbon Isotopic Excursion. The metallogenesis of Mn-carbonate rocks is evaluated based on redox- sensitive trace elements (e.g. Zn and Ni), PAAS-normalized REE + Y patterns and δ13C values (− 2.56‰ to +0.15‰ V-PDB) and point out to a classical multi-stage redox-controlled Mn-model involving: i) a primary hydrothermal fluid source related to ocean vents and/or normal faults; ii) microbially-mediated reduction of Mn- oxides particles at the ocean bottom; and iii) reactions between Mn2+ and CO3 2− (organic matter and seawater related), giving rise to diagenetic Mn-carbonates. The Rhyacian age, Mn deposits, and presence of Lomagundi- like δ13C allows to correlate the Buritirama Formation with the Aguas ́ Claras Formation within the Caraj ́ as domain further south, thus establishing an important metalliferous event that was up to now largely overlooked within the southern Amazon Craton.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.