EM - Escola de Minas
URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/6
Notícias
A Escola de Minas de Ouro Preto foi fundada pelo cientista Claude Henri Gorceix e inaugurada em 12 de outubro de 1876.
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Resultados da Pesquisa
Item Paleoproterozoic sources for Cordilleran-type Neoproterozoic granitoids from the Araçuaí orogen (SE Brazil) : constraints from Hf isotope zircon composition.(2020) Schannor, Mathias; Lana, Cristiano de Carvalho; Mazoz, Ariela Oliveira; Narduzzi, Francesco; Cutts, Kathryn Ann; Fonseca, Marco AntônioThe Araçuaí orogen of SE Brazil exposes numerous amounts of magnesian, metaluminous, calc-alkaline, and medium- to high-K granitoid batholiths that were emplaced during early convergence between the São Francisco and Congo cratons during West Gondwana assembly in the late Neoproterozoic. Previously proposed sources for these pre-collisional granitoids include the Paleoproterozoic basement and mantle-derived magma based on isotopic signatures, abundant microgranular enclaves and contemporaneous mafic intrusions. Zircons from migmatitic basement gneiss, a noritic intrusion and five granitoid intrusions were analysed for U–Pb–Hf isotopes to evaluate their magma source. Paleoproterozoic migmatites represent addition of juvenile crust in an accretionary orogenic setting at ca. 2.2–2.0 Ga evidenced by their positive zircon εHf values. The Hf isotopic composition of zircon from pre-collisional granitoids has a large range of negative εHf values between −28.8 and −1.7 indicating crustal protoliths. This range of unradiogenic Hf isotopic compositions is explained by inefficient mixing of magmas derived from felsic and mafic Paleoproterozoic sources that hybridised during incremental growth of the plutons. Zircon grains of contemporaneous mafic intrusions display slightly higher, yet enriched, Hf isotopic compositions consistent with crustal contamination of mantle magmas. The new Hf isotope data, integrated with other chemical and isotopic evidence, suggest derivation of the pre-collisional granitoids from lower crustal sources of the Paleoproterozoic basement, with only minor contributions from mantle-derived magmas. Thus, magmatism of the Araçuaí orogen related to the assembly of West Gondwana was dominantly a crustalreworking event.Item Metamorphism and exhumation of basement gneiss domes in the Quadrilátero Ferrífero : two stage dome-and-keel evolution?(2019) Cutts, Kathryn Ann; Lana, Cristiano de Carvalho; Alkmim, Fernando Flecha de; Farina, Federico; Moreira, Hugo Souza; Coelho, Viviane VianaThe presence of dome-and-keel provinces in Archean cratons has been connected with the initiation of plate tectonics on Earth as these features are most commonly observed in Archean rocks. The Quadrilátero Ferrífero in Brazil has been identified as a Paleoproterozoic dome-and-keel province for more than three decades. The prevailing model suggests that it formed during the Rhyacian Transamazonian orogeny, making it unique among dome-and-keel provinces. However, a lack of appropriate lithologies, datable minerals and the metamorphic overprint of later orogenesis has resulted in a cryptic metamorphic record for the formation of this dome-and-keel province. A clinopyroxene-bearing migmatite from the core of the Bação dome has peak PeT conditions of 5e7 kbar and 700e750 C and a published age of ca. 2730 Ma based on UePb ages of zircon from leucosomes, suggesting that this age represents the migmatisation event. A fine-grained epidote-albite-titanite assemblage overprints the coarse-grained clinopyroxene and amphibole, giving PeT conditions of 8e9 kbar and 550 C with an associated titanite age of ca. 2050 Ma. A garnet-bearing amphibolite sample also from the core of the dome has peak PeT conditions of 7e8 kbar and 650e700 C, and texturally late titanite from this sample produces an age of ca. 2060 Ma. Three additional samples were collected from the edges of the dome. A garnet-gedrite bearing felsic schist produces peak PeT conditions of 8e9 kbar and 650e700 C on a clockwise PeT evolution. This sample has a UePb zircon age of ca. 2775 Ma, which could date metamorphism or be the age of its volcaniclastic protolith. Texturally unconstrained titanite from the sample gives an age of ca. 2040 Ma. A garnet-bearing amphibolite that occurs as a boudin within the felsic schist gives both zircon and titanite ages of ca. 2050 Ma and has peak PeT conditions of 5e6 kbar and 650e700 C on a near isobaric PeT path. An amphibolite dike, observed to cross-cut the felsic schist produces a zircon UePb age of ca. 2760 Ma. Altogether this data suggests that the samples were metamorphosed in the Archean (ca. 2775e2730 Ma) and again during the Transamazonian event. The most plausible explanation for this data is that dome-and-keel formation occurred in the Archean with migmatisation and high-temperature metamorphism occurring at this time. The Paleoproterozoic event is interpreted as a reactivation of the dome-and-keel formation structures, with Paleoproterozoic keels crosscutting Archean keels and producing metamorphic aureoles. The high radiogenic heat production and the presence of dense sedimentary successions in Archean terranes make dome-and-keel provinces a uniquely Archean feature, but they are susceptible to reworking, resulting in an enigmatic record of formation.Item The complex tale of Mantiqueira and Juiz de Fora : a comment on “Eoarchean to Neoproterozoic crustal evolution of the Mantiqueira and the Juiz de Fora Complexes, SE Brazil : petrology, geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes”.(2019) Cutts, Kathryn Ann; Lana, Cristiano de CarvalhoIn a recent volume of Precambrian Research (vol. 323, pp. 82–101), Kuribara et al. (2019) published a paper on the Eoarchean to Neoproterozoic evolution of the Mantiqueira and Juiz de Fora Complexes of SE Brazil. In this work, Kuribara et al. (2019) suggest that the complexes were previously indicated to be part of the Neoproterozoic to Cambrian Brasiliano Orogeny and that the results of their work instead show that these complexes are fragments of Archaean crust that accreted during the 590–560 Ma Brasiliano orogeny. Kuribara et al. (2019) also present conventional thermobarometry for both complexes that they interpret represents the single metamorphic event that they obtain zircon U-Pb age data for from the Juiz de Fora complex. No metamorphic ages were obtained from the Mantiqueira Complex, instead they have used the metamorphic ages that many workers have obtained from the Mantiqueira Province. In this comment we show that the Archaean crustal input in the Mantiqueira complex has long been known, and the Juiz de Fora Complex has long been considered a Palaeoproterozoic ocean arc. The Mantiqueira Complex records a polymetamorphic history with high-grade metamorphism occurring at ca. 2020–1940 Ma and a later amphibolite facies Brasiliano overprint at 580–540 Ma. The Mantiqueira Province and Mantiqueira Complex are different geological entities, and while the Mantiqueira Complex is affected by the Brasiliano event and occurs within the Mantiqueira Province, the use of metamorphic ages from the Mantiqueira Province is not appropriate for the Mantiqueira Complex. Finally we show that the data presented by Kuribara et al. (2019) actually fits in with the most recent version of the model for the formation of the two complexes.Item Zircon U-Pb and Lu-Hf record from high-grade complexes within the Mantiqueira Complex : first evidence of juvenile crustal input at 2.4–2.2 Ga and implications for the Palaeoproterozoic evolution of the São Francisco Craton.(2020) Cutts, Kathryn Ann; Lana, Cristiano de Carvalho; Moreira, Hugo Souza; Alkmim, Fernando Flecha de; Peres, Guilherme GravinaThe Mantiqueira Complex together with the Mineiro Belt and Juiz de Fora Complex, characterize a long-lived system of oceanic and continental magmatic arcs. The proposed accretion of these terranes in the Rhyacian resulted in the closure of the Minas Basin, a fundamental turning point for the geology of this area. Understanding this event is critical for determining the evolution of the South American continent. Despite its importance, there is a paucity of zircon U-Pb and Lu-Hf data available for the Mantiqueira Complex and no data from two complexes within the later. U-Pb zircon data were obtained from interpreted metasediments of those two complexes, the granulite facies Acaiaca Complex and Pedra Dourada Granulite. Four targeted samples yield cores with ages between 3.5 and 2.2 Ga and a large proportion of metamorphic grains and rims at ca. 2.1–2.0 Ga. The similarity between the detrital zircon signatures of the Acaiaca Complex and Pedra Dourada Granulite, and the Archaean inheritance of the Mantiqueira Complex, indicates that these three units are related. Potentially the Acaiaca Complex and Pedra Dourada Granulite either sourced the Mantiqueira Complex or the Mantiqueira intrusive suites reworked the protoliths of the Acaiaca Complex and Pedra Dourada Granulite. Hf-in-zircon data indicate two discrete sources for both the Acaiaca Complex and Pedra Dourada Granulite. An Archaean source (model ages of 3.2–2.6 Ga), which is consistent with the suggested origin of the Mantiqueira Complex as recycled Archaean crust sourced from the São Francisco Craton and reworked during the ca. 2.1 Ga Transamazonian orogeny. A second more juvenile source (model ages of 2.4–2.2 Ga) recorded exclusively within the more mafic samples is identical in age and Hf to that found in the Juiz de Fora Complex. This could indicate a close link between the Mantiqueira and Juiz de Fora Complexes in the period 2.2 to 2.0 Ga. Alternatively, these juvenile zircons may represent an as yet unidentified juvenile portion of the Mantiqueira arc. The results of this study are consistent with the prevailing theories for formation of the Mantiqueira Complex as a continental arc with cumulative proportion curves indicating an active-margin type depositional setting.Item Metamorphic imprints on units of the southern Araçuaí belt, SE Brazil : the history of superimposed Transamazonian and Brasiliano orogenesis.(2018) Cutts, Kathryn Ann; Lana, Cristiano de Carvalho; Alkmim, Fernando Flecha de; Peres, Guilherme GravinaThe extensive Neoproterozoic Brasiliano belts that surround the São Francisco craton in southern Brazil are a consequence of tectonic processes that resulted in the formation of the supercontinent Gondwana in the Ediacaran to Cambrian. One of these Brasiliano-age orogens, the Araçuaí belt, overprints an earlier orogen developed during the so-called Transamazonian event at the transition between the Rhyacian and Orosirian periods. Archean and Paleoproterozoic basement units in the southern Araçuaí belt have been shown to have experienced both the Transamazonian and Brasiliano orogenies and have the potential to have preserved the P-T conditions of both events. This study investigates a series of granulite- and amphibolite-facies samples from the southern Araçuaí belt. Granulite-facies samples from the Acaiaca Complex and Pedra Dourada Granulite give monazite U-Pb ages in the range: 2015–1960 Ma. Both the granulite samples preserve two metamorphic textures. Coarse-grained garnet + biotite + orthopyroxene + K-feldspar + plagioclase + quartz + ilmenite + inferred melt and later, fine-grained garnet + biotite + hornblende + quartz that grows at the expense of orthopyroxene. Peak conditions for the earlier event were c. 750–800 °C and 6 kbar, whereas those of the later event are poorly constrained. A garnet-bearing amphibolite-facies sample from the Mantiqueira Complex contains monazite included within garnet, which gives U-Pb ages of c. 1940 Ma, whereas matrix monazite grains give ages of c. 590–540 Ma. Garnet core compositional zonation indicates P-T conditions of 6 kbar and 700 °C for the c. 1940 Ma metamorphism, while the matrix assemblage records a P-T path culminating in peak conditions of 8–9 kbar at 650 °C. A second amphibolite-facies sample was obtained from the Dom Silvério Group, whose depositional age post-dates the Transamazonian event. This sample gives two monazite U-Pb age populations, 597 ± 3 Ma and 547 ± 5 Ma. Garnet core zonation indicates a prograde P-T evolution with peak conditions of c. 8–9 kbar and 650 °C, although it is uncertain at this time what age population this event corresponds to. The results of this study provide better constraints on the chronology and metamorphic regimes of both the Transamazonian and Brasiliano events in the southern Araçuaí belt.Item The Archeane-Paleoproterozoic evolution of the Quadrilátero Ferrífero, Brasil : current models and open questions.(2016) Farina, Federico; Albert, Capucine; Martinez Dopico, Carmen Irene; Gil, Carmen Maria Aguilar; Moreira, Hugo Souza; Hippertt, J. P.; Cutts, Kathryn Ann; Alkmim, Fernando Flecha de; Lana, Cristiano de CarvalhoThe Quadril atero Ferrífero is a metallogenic district (Au, Fe, Mn) located at the southernmost end of the S~ao Francisco craton in eastern Brazil. In this region, a supracrustal assemblage composed of Archean greenstone and overlying NeoarcheanePaleoproterozoic sedimentary rocks occur in elongated keels bordering domal bodies of Archean gneisses and granites. The tectonomagmatic evolution of the Quadril atero Ferrífero began in the Paleoarchean with the formation of continental crust between 3500 and 3200 Ma. Although this crust is today poorly preserved, its existence is attested to by the occurrence of detrital zircon crystals with Paleoarchean age in the supracrustal rocks. Most of the crystalline basement, which is composed of banded gneisses intruded by leucogranitic dikes and weakly foliated granites, formed during three major magmatic events: Rio das Velhas I (2920e2850 Ma), Rio das Velhas II (2800e2760 Ma) and Mamona (2760e2680 Ma). The Rio das Velhas II and Mamona events represent a subduction-collision cycle, probably marking the appearance of a modern-style plate tectonic regime in the Quadril atero Ferrífero. Granitic rocks emplaced during the Rio das Velhas I and II events formed by mixing between a magma generated by partial melting of metamafic rocks with an end member derived by recycling gneissic rocks of older continental crust. After deformation and regional metamorphism at ca. 2770 Ma, a change in the composition of the granitic magmas occurred and large volumes of high-K granitoids were generated. The ca. 6000 m-thick Minas Supergroup tracks the opening and closure of a basin during the NeoarcheanePaleoproterozoic, between 2600 and 2000 Ma. The basal sequence involves continental to marine sediments deposited in a passive margin basin and contain as a marker bed the Lake Superiortype Cau^e Banded Iron Formation. The overlying sediments of the Sabar a Group mark the inversion of the basin during the Rhyacian Minas accretionary orogeny. This orogeny results from the collision between the nuclei of the present-day S~ao Francisco and Congo cratons, generated the fold-and thrust belt structure of the Quadril atero Ferrífero. Afterwards, the post- orogenic collapse resulted in the deposition of the Itacolomi Group and in the genesis of the dome-and-keel structure. In this paper, we review current knowledge about the 1500 Ma long-lasting tectonomagmatic and structural evolution of the Quadril atero Ferrífero identifying the most compelling open questions and future challenges.Item Sedimentation, metamorphism and granite generation in a back-arcregion : records from the Ediacaran Nova Venécia Complex, Araçuaí Orogen, Southeastern Brazil.(2016) Richter, Fabiana; Lana, Cristiano de Carvalho; Stevens, Gary; Buick, Ian S.; Soares, Antônio Carlos Pedrosa; Alkmim, Fernando Flecha de; Cutts, Kathryn AnnThe Nova Venécia migmatite-granulite-granite Complex (NVC) in the core of the Arac¸ uaí Orogen (AO,630–480 Ma), southeast Brazil, exposes a mid-crustal section with abundant evidence for high-grademetamorphism linked to production, extraction and emplacement of peraluminous melts. Although theAO represents the textbook example of a confined orogen, there is surprisingly a lack of detailed studieson its metamorphic evolution related to widespread granitogenesis occurring from the Neoproterozoic toearly Paleozoic. In this study, we combine U–Pb geochronology and metamorphic petrology to constrainthe evolution of the NVC migmatitic metasedimentary granulites, from deposition to high-grade meta-morphism, and to correlate the metamorphic history of the terrain with the several episodes of granitemagmatism (G1–G5) in the AO. The sedimentation of the NVC can be bracketed within a maximum 13 Myperiod, between its maximum depositional age at ca. 606 Ma and the intrusion of early syn-collisionalgranitoids at 593 Ma. Compilation of available U–Pb data shows that the bulk of the magmatic rocks in theAO (G1 + G2 rocks) crystallized contemporaneously over a period of 15 My (ca. 595–570 Ma) with a peakat ca. 575 Ma. Although it is inferred a protracted period of crustal heating in the AO (from ca. 640–480),U–Pb ages of metamorphic and magmatic zircons and monazites suggest at least two major heat pulsesat ca. 593–560 and 523–495 Ma. The timing of peak regional metamorphism is constrained from 575 to560 Ma, which temporally overlaps with the crystallization of the youngest granitoids. Phase equilibriummodeling of metasedimentary granulites from three different localities within the NVC, indicates thatall areas record similar peak P–T conditions of 750–850◦C and 5300–7500 bar. This is followed by hightemperature retrograde evolution to 640–800◦C and 4500–6000 bar. A post-collisional thermal eventlinked to the intrusion of large norite bodies (520–480 Ma) is recorded in our metagreywackes (monaziteU–Pb) and in granites (monazite and zircon U–Pb) from 523 to 495 Ma.