13 results on '"olivine"'
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2. Petrogênese do Banco Davis, Cadeia Vitória-Trindade, Atlântico Sul: o Papel de Voláteis (H2O e CO2) na Evolução Magmática do Banco Davis.
- Author
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Mendes de Jesus, João Vitor, Costa dos Santos, Anderson, Cezar Mendes, Júlio, Holanda dos Santos, Werlem, Queiroz Rego, Caio Assumpção, and Cesar Geraldes, Mauro
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MODAL analysis ,PLAGIOCLASE ,MINERAL analysis ,AMPHIBOLES ,OLIVINE ,SAMARIUM - Abstract
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- Published
- 2019
- Full Text
- View/download PDF
3. Mineralogical and geochemical controls of nickel in the ultramafic rocks of Sul-Riograndense Shield
- Author
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Thamy Lara de Souza, Marcus Vinícius Dorneles Remus, and Norberto Dani
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Olivine ,Serpentinization ,Cerro Mantiqueiras ,Electron microprobe ,Geology ,QE1-996.5 - Abstract
This work investigates the processes controlling the mobility and concentration of nickel in serpentinized ultramafic rocks of the Sul-Riograndense Shield (ESrg). Four ultramafic units were selected: Serrinha Complex, Cerro Mantiqueira Sequence, Pedras Pretas Massive and Cambaizinho Complex. Conventional techniques allow to chemically classify the rocks and to relate rock texture with the intensity and degree of the different events that contributed to changes in the mineralogy of the protholiths. Electron microprobe analysis of olivine and serpentine allowed determining the concentrations of nickel and its variations in all of the investigated units. The nickel-bearing minerals were identified, and established the relationship between geological events, element mobility and element concentration. The olivine of Pedras Pretas Massive contain lower levels of nickel with a range of 0.08 to 0.29%, and an average value of 0.17%, while olivines of Cerro Mantiqueiras Sequence contain higher NiO levels (maximum 0,44%), with an average value of 0.35%. The olivine composition of Pedras Pretas Massive and Cerro Mantiqueiras Sequence are homogeneous, while those of Cambaizinho Complex have a slightly higher range of variation with composition ranging between Fo92 and Fo84, but with NiO amounts between 0.17 e 0.30%. In Serrinha Complex, the olivine was not identified probably due to the intense serpentinization. Cambaizinho Complex and Cerro Mantiqueiras Sequence have serpentine minerals with a tendency to higher concentrations of nickel in relation to values obtained in the olivines, but some serpentines have lower values in the last sequence. By comparing the concentrations of nickel in each ultramafic unit, the olivines of Cerro Mantiqueiras Sequence have the highest NiO values, while Pedras Pretas Massive has the lowest values among the investigated ultramafic rocks. It is important to mention that the NiO concentration in the studied ultramafic units show equivalent values when compared to the other ultramafics around the world.
- Published
- 2014
- Full Text
- View/download PDF
4. Rochas ultramáficas plutônicas do greenstone belt Rio das Velhas na porção central do Quadrilátero Ferrífero, Minas Gerais, Brasil.
- Author
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da Fonseca, Gabriela Magalhães and Evangelista, Hanna Jordt
- Subjects
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ULTRABASIC rocks , *SERPENTINE , *CHLORITE minerals , *OLIVINE , *PYROXENE , *ROCK-forming minerals - Abstract
In Amarantina district of Ouro Preto (State of Minas Gerais, Brazil), ultramafic rock exposures are found along two areas of about 500 m² each. The rocks crop out in the Bação complex, which is the gneissic basement of the Rio das Velhas greenstone belt in the central portion of the Quadrilátero Ferrífero (QF). The interest in a petrogenetic study of the ultramafic rocks is the partial preservation of igneous minerals, which are not observed in most of the completely metamorphosed ultramafic rocks in the QF. Among them, the steatites and the serpentinites are the best studied because of their economic importance. The ultramafic rocks from Amaranima are classified as metaperidotites due to the equigranular texture characteristic of plutonic origin. The rocks are made up of large grains of olivine, pyroxene, and spinel preserved from de original magmatic rock, which are distributed in a fine grained mass with talc, serpentine, chlorite, amphibole, and opaque minerals. Scarce arite (NiSbAs) and breithauptite (NiSb) were generated after pentlandite during hydrothermal metamorphism. Comparison of the chemical composition with a metakomatiite with spinifex texture from de QF as well as with known komatiitic rocks from other parts of the world reveals that the metaperidotites are chemically similar to non Al-depleted komatiites. Therefore it is probable that the studied ultramafic rocks correspond to the plutonic portion of the komatiitic magmatism of the Nova Lima group, at the base of the Rio das Velhas greenstone belt. [ABSTRACT FROM AUTHOR]
- Published
- 2013
- Full Text
- View/download PDF
5. Exumação das Rochas Mantélicas no Arquipélago de São Pedro e São Paulo, Oceano Atlântico Equatorial, e sua Implicação na Possível Geração de Hidrocarbonetos Abiogenéticos por Serpentinização
- Author
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Aurélio Kasakewitch Ribeiro, José Antônio Baptista Neto, Isa Brehme, Kenji Freire Motoki, Akihisa Motoki, Thais Vargas, Peter Szatmari, Susanna Eleonora Sichel, and Alcides Nobrega Sial
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Olivine ,Geography, Planning and Development ,Transform fault ,Geology ,Exumação do manto ,Serpentinização ,Hidrocarboneto abiogenético ,Zona de falhas transformantes de São Paulo ,Cadeia de transpressão ,Development ,Pressure ridge ,engineering.material ,Mantle (geology) ,Mantle exhumation ,Serpentinisation ,Abiogenic hydrocarbon ,Saint Paul transform fault zone ,Pressure-ridge ,Tectonics ,Ultramafic rock ,engineering ,Economic Geology ,Petrology ,Seismology ,Sea level ,General Environmental Science ,Mylonite - Abstract
This paper presents the mantle rock exhumation at Saint Peter and Saint Paul Islets, Equatorial Atlantic Ocean, and its implication to possible generation of abiogenic hydrocarbons by serpentinization. In Saint Paul transform fault zone, there are two contrasting tectonic processes for mantle exhumation: Extension at amagmatic spreading centre and compression along a pressure ridge. On the ocean bottom, the ultramafic rocks of exhumed mantle react chemically with sea-water generating heat and abiogenic hydrocarbons, the phenomenon called serpentinisation. The amagmatic spreading with megamullion formation occurs along inter-transform ridge segments. The ultramafic rocks are originated from uppermost mantle and have advanced serpentinisation and little expressive plastic deformation. The olivine porphyroblasts show brittle fracturing. On the other hand, the pressure-ridge occurs along the transform fault span where the fault directions and relative plate motion are oblique. Due to the direction discordance, the strike-slip movement of the transform fault generates compression stress perpendicular to the fault. This stress squeezes out the underlying deep mantle from the ductile deformation depth up to the Earth's surface. The pressure ridge at Saint Peter and Saint Paulo Islet, namely Brachiosaurus Ridge, is the only confirmed example of mantle exhumation above sea level in Atlantic Ocean. The mantle rocks have little expressive serpentinisation and extremely developed plastic deformation, showing mylonitic texture. The porphyroclasts and matrix show either brittle fracturing or plastic deformation. Este trabalho apresenta a exumação das rochas mantélicas no Arquipélago de São Pedro e São Paulo, Oceano Atlântico Equatorial, e sua implicação na possível geração de hidrocarbonetos abiogenéticos por serpentinização. Na zona de falhas transformantes de São Paulo, observam-se duas condições tectônicas contrastadas para exumação do manto: Distensão no centro de espalhamento amagmático e compressão ao longo da cadeia de transpressão. No fundo do oceano, as rochas ultramáficas do manto exumado reagem quimicamente com a água do mar gerando energia térmica e hidrocarbonetos abiogenéticos, o fenômeno denominado serpentinização. O espalhamento amagmático com a formação de megamullion ocorre ao longo dos segmentos inter-transformantes da cadeia meso-oceânica. As rochas ultramáficas são originadas no manto superficial e possuem serpentinização avançada e deformação plástica pouco expressiva. Os porfiroblastos de olivina apresentam fraturamento rúptil. Por outro lado, a cadeia de transpressão ocorre ao longo do trecho da falha transformante em que a direção da falha e o movimento relativo das placas são oblíquos. Devido à discordância direcional, o movimento transcorrente gera o esforço de compressão perpendicular à falha. Este esforço levanta o manto profundo subjacente a partir da profundidade de deformação dúctil até a superfície da Terra. A cadeia de transpressão no Arquipélago de São Pedro e São Paulo, denominada Cadeia de Brachiosaurus, é o único exemplo confirmado exumação do manto acima do nível do mar no Oceano Atlântico. As rochas do manto têm serpentinização pouco expressiva e deformação plástica extremamente desenvolvida, apresentando textura milonítica. Os porfiroclastos e a matriz apresentam tanto o fraturamento rúptil quanto a deformação plástica.
- Published
- 2017
6. Fe-Ti-V ASSOCIADO AO GABRO DE LÍDICE, ESTADO DO RIO DE JANEIRO
- Author
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Ronaldo Mello Pereira, Marcelo dos Santos Salomão, Paulo Vicente Guimarães, Enrico Pedroso, and Reiner Neumann
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Pharmacology ,Ti oxides ,Olivine ,Gabbro ,Gabro, titanomagnetita, ilmenita, titânio, vanádio ,Rare earth ,Analytical chemistry ,Palavras chave ,Mineralogy ,Vanadium ,chemistry.chemical_element ,Pyroxene ,engineering.material ,chemistry ,engineering ,Ilmenite - Abstract
Resumo O Gabro de Lidice corresponde a um corpo tabular, com cerca de 4,0 quilometros de extensao e espessura entre 100 e 200 metros, constituido por hiperstenio, diopsidio, olivina, plagioclasio e minerais opacos (oxidos de Fe e Ti). Assinaturas petrografica e geoquimica indicam carater toleitico, entre os campos alcalino e subalcalino. O enriquecimento em V e de 2 a 3 vezes maior do que o clarck esperado para tal tipo de rocha. O principal oxido e a titanomagnetita, com inclusoes e exsolucoes de ilmenita. O vanadio concentra-se principalmente na titanomagnetita, variando os teores entre 0,68% e 2,07% de V2O5. Analises ICP-MS em pre-concentrado de titanomagnetita forneceram teores de 50,86% de Fe2O3t, 13,56% de TiO2 e 3.721 ppm de vanadio (~ 0,66% de V2O5). Quimicamente a ilmenita tem conteudo em Fe2O3t (46,75% a 48,80%) e TiO2 (49,99% a 51,36%). Amostras enriquecidas em titanomagnetita exibem valores dos elementos terras raras e traco 30 vezes inferiores aos observados nas demais amostras. Ao longo de todo o corpo, a susceptibilidade magnetica varia em escala centimetrica a decimetrica e esta associada a concentracoes anomalas de titanomagnetita na rocha, o que pode representar um bandamento magmatico. Os dados apresentados sugerem uma origem associada a ambiente intraplaca. Palavras Chave: Gabro, titanomagnetita, ilmenita, titânio, vanadio. Abstract Fe-Ti-V ASSOCIATED WITH LIDICE GABBRO, RIO DE JANEIRO STATE. The Gabbro of Lidice has about 4.0 km in length and thickness between 100 and 200 meters, consisting of pyroxene, olivine, plagioclase and opaque minerals (Fe and Ti oxides). Petrographic and geochemical signatures indicate toleitic character, between the alkaline and subalcalino fields. Vanadium enrichment is from 2 to 3 times higher than the expected clarck for such rock. The main oxide is titanomagnetite, with inclusions and exsolutions of ilmenite. Vanadium (0,68% to 2,07% V2O5) is mainly concentrated in titanomagnetite. ICP-MS analyses in pre-concentrated of titanomagnetite provided levels of 50.86% of Fe2O3t, 13.56% of TiO2 and 3,721 ppm of vanadium (~ 0.66% de V2O5). Chemical analyses of ilmenite show content in Fe2O3t (from 46.75% to 48.80%) and TiO2 (from 49.99% to 51.36%). Samples enriched with titanomagnetite display values of trace and rare earth elements 30 times lower than those observed in other samples. Throughout the gabbro, the magnetic susceptibility varies from centimeter to decimeter scales and is associated with abnormal concentrations of titanomagnetite, which may represent magmatic banding. The data presented may suggest an origin associated with intraplate environment. Keywords: Gabbro, titanomagnetite, ilmenite, titanium, vanadium.
- Published
- 2016
7. The chromium and copper-nickel mineralized Várzea do Macaco body, Jacurici Mafic-ultramafic Complex, São Francisco Craton, Bahia, Brazil
- Author
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José Carlos Frantz, Juliana Charão Marques, Ronei Osório Giusti, João Rodrigo Vargas Pilla Dias, and Waldemir José Alves de Queiroz
- Subjects
Olivine ,sulfide ,lcsh:QE1-996.5 ,Geochemistry ,Metamorphism ,petrografia ,engineering.material ,metalogênese ,petrography ,lcsh:Geology ,Ultramafic rock ,engineering ,metallogeny ,General Earth and Planetary Sciences ,Chromitite ,Chromite ,Metasomatism ,Mafic ,sulfeto ,Amphibole ,Geology - Abstract
O Complexo Máfico-ultramáfico Jacurici, localizado na porção nordeste do Cráton São Francisco, é formado por diversas intrusões estratificadas orientadas N-S e hospeda o maior depósito de cromita do Brasil. O corpo Várzea do Macaco ocorre na parte norte e também hospeda uma mineralização de Cu-Ni sulfetada. Este estudo descreve esse corpo e as mineralizações, e os compara com intrusões mais ao sul (Ipueira-Medrado), onde uma evolução petrológica já foi estabelecida previamente. O corpo Várzea do Macaco está invertido estratigraficamente e desmembrado em cinco blocos, deslocados lateralmente por falhas. É constituído por dunito, lherzolito, ol-websterito, cromitito e gabronorito com intensidades variáveis de serpentinização. O minério sulfetado (Po ± Pn ± Cpy) está concentrado nas proximidades da espessa camada de cromitito e ocorre como: primário magmático com sulfetos intersticiais associados à olivina e ao piroxênio; e mineralização remobilizada, com sulfetos associados a veios e lentes metassomatizadas que interceptam a estratificação primária. O intervalo onde a mineralização magmática ocorre é caracterizado pela presença de anfibólio magmático que possivelmente favoreceu as transformações metamórficas e metassomáticas, mais intensas nesse intervalo. Nos sulfetos remobilizados, a calcopirita é mais abundante, evidenciando um aumento na razão Cu/Ni. O corpo Várzea do Macaco pode ser subdividido em uma Zona Ultramáfica e uma Zona Máfica, as mesmas propostas para Ipueira-Medrado. Comparativamente, o corpo Várzea do Macaco é enriquecido em clinopiroxênio. Possivelmente ambos os corpos pertencemao mesmo sistema intrusivo, caracterizado por magma primitivo rico em Mg e Ni. Contaminação crustal é considerada como gatilho para a formação da mineralização de cromita. Em Várzea do Macaco se atingiu a saturação de enxofre. The Jacurici Mafic-ultramafic Complex, located in the northeastern portion of the São Francisco craton, consists of several N-S oriented layered bodies that host the largest chromite deposit in Brazil. The Várzea do Macaco body is at the northern part and also host a Ni-Cu sulfide mineralization. This study describes this body and its mineralization and compare with the southern intrusions (Ipueira-Medrado) where a petrological evolution was previous established. The Várzea do Macaco is stratigraphically inverted and disrupted in five blocks, laterally dislocated by late faults. It is constituted by dunite, lherzolite, ol-webesterite, chromitite and gabbronorite with variable serpentinization. The sulfide ore (Po ± Pn ± Cpy) is concentrated close to the main thick chromitite layer and occurs as: primary magmatic with interstitial sulfides associated with olivine and pyroxene; and as a remobilized ore, with sulfides associated to metasomatic veinlets or lenses that crosscut the primary layering. The interval where magmatic ores occur is characterized by the presence of magmatic amphibole that possible favoured late metamorphism and metasomatism transformation, stronger in this interval. The remobilized sulfide is enriched in chalcopyrite showing an increase in the Cu/Ni ratio. Comparing to Ipueira-Medrado, the Várzea do Macaco is enriched in clinopyroxene, but it can be subdivided in the same Ultramafic and Mafic zones. Possibly, both bodies are part of a single intrusive system characterized by a primitive magma with high Mg and Ni contents. The chromite mineralization is considered to be triggered by crustal contamination. At Várzea do Macaco, sulfur saturation was reached.
- Published
- 2014
8. Rochas ultramáficas plutônicas do greenstone belt Rio das Velhas na porção central do Quadrilátero Ferrífero, Minas Gerais, Brasil
- Author
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Gabriela Magalhães da Fonseca and Hanna Jordt Evangelista
- Subjects
Ultramafic rock ,Geochemistry ,Metamorphism ,Rocha ultramáfica ,peridotito komatiitico ,Pyroxene ,Greenstone belt ,engineering.material ,komatiitic peridotite ,Geochemistry and Petrology ,General Materials Science ,Rio das Velhas greenstone belt ,Waste Management and Disposal ,Amphibole ,geochemistry ,Olivine ,Geology ,geoquímica ,greenstone belt Rio das Velhas ,Igneous rock ,Basement (geology) ,engineering ,Economic Geology ,Minas Gerais ,Quadrilátero Ferrífero - Abstract
Em Amarantina, distrito de Ouro Preto, encontram-se rochas ultramáficas expostas em duas áreas com cerca de 500 m² cada. As rochas afloram no Complexo do Bação, que é o embasamento gnáissico do greenstone belt Rio das Velhas, na porção central do Quadrilátero Ferrífero (QF). O interesse no estudo petrogenético desses corpos deve-se à preservação parcial de minerais ígneos, ausentes na maior parte das rochas ultramáficas totalmente metamorfizadas do QF. Entre essas rochas, destacam-se os esteatitos e os serpentinitos, devido a sua importância econômica. As rochas ultramáficas de Amarantina possuem textura equigranular, fato que as caracteriza como tendo origem plutônica, isto é, trata-se de metaperidotitos. Possuem grãos maiores de olivina, piroxênio e espinélio da rocha ígnea original distribuídos em matriz metamórfica fina com talco, serpentinas, cloritas, anfibólios e minerais opacos. Escassas arita (NiSbAs) e breithauptita (NiSb) foram formadas a partir de pentlandita durante o metamorfismo associado a hidrotermalismo. A comparação da composição química com a de um metakomatiito com textura spinifex do QF, bem como com rochas komatitiiticas de outras partes do mundo, mostra que os metaperidotitos são, quimicamente, semelhantes aos komatiitos não-desfalcados em alumínio. Portanto é provável que as rochas ultramáficas estudadas correspondam à porção plutônica do magmatismo komatitiitico do Grupo Nova Lima, que é a unidade basal do greenstone belt Rio das Velhas. In Amarantina, district of Ouro Preto (State of Minas Gerais, Brazil), ultramafic rock exposures are found along two areas of about 500 m² each. The rocks crop out in the Bação complex, which is the gneissic basement of the Rio das Velhas greenstone belt in the central portion of the Quadrilátero Ferrífero (QF). The interest in a petrogenetic study of the ultramafic rocks is the partial preservation of igneous minerals, which are not observed in most of the completely metamorphosed ultramafic rocks in the QF. Among them, the steatites and the serpentinites are the best studied because of their economic importance. The ultramafic rocks from Amarantina are classified as metaperidotites due to the equigranular texture characteristic of plutonic origin. The rocks are made up of large grains of olivine, pyroxene, and spinel preserved from de original magmatic rock, which are distributed in a fine grained mass with talc, serpentine, chlorite, amphibole, and opaque minerals. Scarce arite (NiSbAs) and breithauptite (NiSb) were generated after pentlandite during hydrothermal metamorphism. Comparison of the chemical composition with a metakomatiite with spinifex texture from de QF as well as with known komatiitic rocks from other parts of the world reveals that the metaperidotites are chemically similar to non Al-depleted komatiites. Therefore it is probable that the studied ultramafic rocks correspond to the plutonic portion of the komatiitic magmatism of the Nova Lima group, at the base of the Rio das Velhas greenstone belt.
- Published
- 2013
9. Caracterização Química de Cromitas nos Maciços de Cromínia e Mairipotaba, Goiás, Brasil
- Author
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Antenor Zanardo, Silvio Roberto Farias Vlach, Nelson Angeli, Guillermo Rafael Beltran Navarro, Universidade Estadual Paulista (Unesp), and Universidade de São Paulo (USP)
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Araxá group ,Grupo Araxá ,paragenesis ,Greenschist ,Metamorphic rock ,orthopyroxene ,Geochemistry ,Cromita ,engineering.material ,Brasilia Belt ,Ophiolitic melange ,source rock ,parent body ,greenschist facies ,chemical composition ,QUÍMICA ,Paragenesis ,Parent rock ,Mineral chemistry ,olivine ,Metamorphic facies ,geochemistry ,Goias ,Faixa Brasília ,Olivine ,concentration (composition) ,melange ,lcsh:QE1-996.5 ,Química mineral ,Chromite ,Geology ,chromitite ,lcsh:Geology ,Mélange ofiolítica ,ophiolite ,engineering ,Chromitite ,Brazil - Abstract
Submitted by Vitor Silverio Rodrigues (vitorsrodrigues@reitoria.unesp.br) on 2014-05-27T11:24:38Z No. of bitstreams: 0Bitstream added on 2014-05-27T14:35:44Z : No. of bitstreams: 1 2-s2.0-77951069166.pdf: 718441 bytes, checksum: 8eb48304099737179873b54a458b7597 (MD5) Made available in DSpace on 2014-05-27T11:24:38Z (GMT). No. of bitstreams: 0 Previous issue date: 2010-03-01 Metaultramafic bodies tectonically emplaced within the metasedimentary sequence of the Araxá Group are associated with an ophiolitic melange in southeast Goiás. In the region of Crominia - Mairipotaba, they occur as lenticular bodies aligned E-W. Cumulate textures and geochemical data indicate that the parent rocks had harzburgitic to dunitic compositions. Relicts of primary crystals of olivine and orthopyroxene are suggestive of amphibolite facies metamorphic re-equilibration fabrics, even though the paragenesis and mineral associations of these metaultramafic rocks are typical of greenschist facies (T < 550°C and P = 5.5 kbar). The chromitites exhibit massive to breccioid structure and pull-apart texture, with chromite crystals around 0.5 mm in size. Chromite concentrations in the chromitite levels reach 70 to 85% by volume of the rock. The crystals are dispersed in the matrix, which is composed essentially of serpentine, and subordinately of chlorite and talc. The textures and geochemical data (Cr 2O 3 x TiO 2 and Mg x Cr ratios present in the chromitite) are similar to those observed in ophiolitic complexes. Hence, they correspond to allochthonous bodies (Alpine type) associated with an ophiolitic mélange. Departamento de Petrologia e Metalogenia UNESP, Av. 24-A 1.515, CEP 13506-900, Rio Claro, SP Pós-graduação em Geologia Regional Instituto de Geociências e Ciências Exatas UNESP, Rio Claro, SP Departamento de Mineralogia e Geotectônica Instituto de Geociências USP, São Paulo, SP Departamento de Petrologia e Metalogenia UNESP, Av. 24-A 1.515, CEP 13506-900, Rio Claro, SP Pós-graduação em Geologia Regional Instituto de Geociências e Ciências Exatas UNESP, Rio Claro, SP
- Published
- 2010
10. Geologia e geocronologia do Maciço Alcalino Máfico-Ultramáfico Ponte Nova (SP-MG)
- Author
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Gaston Eduardo Enrich Rojas, Celso B. Gomes, Rogério Guitarrari Azzone, and Excelso Ruberti
- Subjects
geography ,Olivine ,geography.geographical_feature_category ,Gabbro ,lcsh:QE1-996.5 ,Geochemistry ,Província Serra do Mar ,Mineralogy ,Geology ,Massif ,engineering.material ,Cumulatos máfico-ultramáficos ,lcsh:Geology ,Porphyritic ,MAGNETISMO ,Equigranular ,Ultramafic rock ,Geocronologia ,Geochronology ,Breccia ,engineering ,Magmatismo alcalino - Abstract
O maciço alcalino máfico-ultramáfico Ponte Nova, localizado no setor norte da Província Serra do Mar, junto à porção oriental da Serra da Mantiqueira, na região Sudeste do Brasil, encontra-se encaixado em granitoides e gnaisses migmatíticos Pré-Cambrianos. O maciço apresenta duas áreas distintas de exposição, estando ambas separadas por rochas do embasamento: uma maior (~5,5 km²), de forma elíptica e com grande variedade de litotipos; outra menor (~1 km²), de forma irregular e petrograficamente menos complexa, e situada a sul da primeira. É predominantemente composto por uma associação gabroide alcalina, gerada por sucessivos pulsos magmáticos. As rochas mais abundantes são cumulatos máficos e ultramáficos (e.g., olivina clinopiroxenitos e melagabros com olivina) presentes nos níveis inferiores do maciço, ao lado de rochas gábricas e monzogábricas porfiríticas, equigranulares e bandadas nos níveis superiores, todos representativos da evolução magmática dos principais pulsos. Um corpo de brecha magmática e uma suíte de diques (lamprófiros a tefrifonólitos) interceptam todas as rochas intrusivas. Análises geocronológicas por K/Ar para diferentes rochas indicam uma idade média de 87,6 Ma (Cretáceo Superior), que é similar às obtidas para as demais ocorrências do setor norte da referida província. A incompatibilidade das idades torna difícil sustentar a hipótese de sua associação genética com a pluma mantélica de Trindade
- Published
- 2009
11. Rochas metaultramáficas de Lamim, sul do Quadrilátero Ferrífero, MG: contribuição ao conhecimento do protólito da pedra-sabão
- Author
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Hanna Jordt-Evangelista and Maria Elizabeth da Silva
- Subjects
pedra-sabão ,metaultramafic rock ,Greenschist ,Geochemistry ,Greenstone belt ,Pyroxene ,engineering.material ,protolith ,serpentinite ,Geochemistry and Petrology ,rocha metaultramáfica ,General Materials Science ,Waste Management and Disposal ,serpentinito ,Amphibole ,Metamorphic facies ,geochemistry ,Peridotite ,Olivine ,soapstone ,Geology ,petrogênese ,petrogenesis ,komatiite ,engineering ,Economic Geology ,protólito ,Minas Gerais ,Quadrilátero Ferrífero ,Protolith ,komatiito - Abstract
A região de Lamim, sudeste do Quadrilátero Ferrífero (QF), é constituída de gnaisses e rochas metamáficas e metaultramáficas, estas do Grupo Nova Lima, base do greenstone belt Rio das Velhas. As principais rochas metaultramáficas são pedra-sabão, amplamente explotada no QF, e serpentinito. A região distingue-se por rochas com olivina preservada, interpretadas como o protólito magmático das metaultramáficas. São olivina-anfibólio fels com textura semelhante a cumulus, com inclusão de vários grãos de olivina em anfibólio de porte maior. O teor relativamente alto de ferro da olivina (Fo75Fa25) indica a sua origem magmática. Há dois tipos de anfibólio, cristais centimétricos de tremolita, envolvendo vários grãos de olivina, interpretados como pseudomorfoses, substituindo o piroxênio intercumulus original, e antofilita acicular. Os teores de serpentina, clorita, dolomita e talco variam. A composição química e as texturas sugerem que o provável protólito das rochas metaultramáficas são komatiitos peridotíticos plutônicos e não komatiitos vulcânicos encontrados em outras regiões do QF. O peridotito foi variavelmente metamorfizado em condições de fácies xisto verde alto a anfibolito baixo durante a infiltração de fluidos aquosos que acompanharam o processo metamórfico-metassomático. The region of Lamim, in the southeastern portion of the Quadrilátero Ferrífero (QF), is composed of gnaisses, metamafic and metaultramafic rocks. The last ones belong to the Nova Lima group, base of the Archean Rio das Velhas greenstone belt. The main metaultramafic rocks are soapstone, largely exploited in the QF, and sepentinite. Rocks preserving olivine, interpreted as the magmatic protolith of the metaultramafic rocks, distinguish the region of Lamim. These rocks are olivine-amphibole fels with cumulus-like textures characterized by the inclusion of several grains of olivine within larger amphibole crystals. The relatively high Fe-content of olivine (Fo75Fa25) is an indication of its magmatic origin. Two types of amphibole were identified, centimetric tremolite crystals involving olivine grains, interpreted as pseudomorphs replacing the original intercumulus pyroxene, and acicular anthophyllite. The amounts of serpentine, chlorite, dolomite and talc vary. Rock chemistry and textures suggest that the most probable protolith of the metaultramafic rocks are plutonic komatiitic peridotites and not volcanic komatiites found elsewhere in the QF. The peridotite was variably metamorphosed under conditions of higher greenschist to lower amphibolite facies during infiltration of aqueous fluids accompanying the metamorphic-metassomatic process.
- Published
- 2005
12. Geoquímica das rochas máficas toleíticas da suíte pós-colisional Paulo Lopes, Neoproterozóico do Sul do Brasil
- Author
-
Lauro Valentim Stoll Nardi, Leandro Menezes Betiollo, Luana Moreira Florisbal, and Maria de Fátima Bitencourt
- Subjects
Tholeiitic gabbros ,Olivine ,Gabbro ,Basic rocks ,Geochemistry ,Garopaba ,engineering.material ,Baddeleyite ,Augite ,Equigranular ,Pigeonite ,engineering ,General Earth and Planetary Sciences ,Geoquímica ,Mafic ,Rochas máficas ,Post-collisional magmatism ,Geology ,Post-collisional gabbros ,Hornblende - Abstract
The Paulo Lopes Suite (SPL), composed of the Paulo Lopes Granite (GPL), Garopaba Granitoids (GG) and Silveira Gabbro (GS), is an association of contemporaneous acid and basic rocks, comprising monzo and syenogranites associated to basic dikes, with abundant mafic microgranular enclaves, interpreted as co-mingling products. The contacts between the granitoids and the basic rocks are evidences of coeval and interactive magmas. The Silveira Gabbro occurs in the study area as a main body and several narrow dikes of NNE orientation. Field relations define a chronological order of magmatic events, where the GPL is the first magmatic pulse and, while steel partially crystallized, was intruded by mingled magmas, represented by the GG and the GS. The basic components are medium-grained, equigranular rocks, with subophitic and ophitic textures. The centre of the main body contains medium- to coarse-grained, equigranular rocks, where agglomerates of early-formed clinopyroxene and plagioclase crystals are found. In the chilled margins, they are microporphyritc rocks of aphanitic groundmass, indicative of rapid crystallization. The Silveira Gabbro rocks are composed of labradorite-andesine, orthopyroxene, augite, pigeonite, olivine (occasionally serpentinized), Fe-hornblende and magnesian hornblende, red biotite, magnetite, ilmenite, apatite, and baddeleyite. The composition is tholeiitic, similar to the high-Ti-P basalts of the Serra Geral Formation. Their high contents of K, Rb, Sr and Ba, as well as negative anomalies of Nb and Ta in multielemental diagrams are similar to the ones observed in magmatic rocks from mature arcs or post-collisional environments. The Neoproterozoic basic rocks may be discriminated from the ones belonging to the Cretaceous Serra Geral Formation by their higher contents of alcalis, Cs, U, Th, and by their fractionated REEpatterns, expressed in the La/LuN ratio. The associated granitoids are structural and compositionally compatible with the ones found in post collisional settings, which indicates that the SPL magmatism developed in such environment.
- Published
- 2005
13. Geoquimica e petrologia das rochas máficas e ultramáficas do complexo estratiforme de Cana Brava-GO, e das suas encaixantes
- Author
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Ciro Teixeira Correia and Vicente Antonio Vitorio Girardi
- Subjects
Basalt ,geography ,geography.geographical_feature_category ,Olivine ,Metamorphic rock ,Geochemistry ,Massif ,Fractionation ,engineering.material ,ROCHAS ULTRAMÁFICAS ,Mantle (geology) ,engineering ,General Earth and Planetary Sciences ,Geology ,General Environmental Science - Abstract
The Cana Brava body is a 2.0 Ga, anorogenic, stratiform complex whose layers dip from 30o to 50o NW. The massif is made up of five units, composed of layers containing several associations of cumulus phases and variable amounts of inter-cumulus minerais. Transitions between units are characterized by changes in the composition and/or in the relative abundance of these phases. Stratigraphically, the lowest sequence is composed of amphibolites (PICB1), overlain by serpentinites (PICB2), metawebsterites (PICB3) and metagabbroic rocks (PICB4 and PICB5) at the top. This sequence was originally formed by microgabbros, peridotites, websterites and gabbroic rocks. The rock sequence of the complex can be attributed to a single magmatic fractionation. There is no evidence of multiple injections. Metamorphic events affected the different units heterogeneously. The gabbroic rocks (PICB4 and PICB5) were the least transformed; hence their chemical compositions are close to the original ones. Units PICB1 to PICB3 were the most affected by post-magmatic processes. Geochemical evidences suggest that PICB1 may be the chilled border of the massif. However, due to post-magmatic phenomena, the composition of the parent magma was calculated using the chemical compositions and inferred volumes of units PICB2 to PICB5. For PICB2 and PICB3 the compositions of mineral phases were utilized. The possible Cana Brava parent magma is olivine-tholeiite, similar to the majority of the complexes utilized for comparison. Its amount of normative olivine is within the range for liquids derived from 25 to 35% of mantle melting. Phase equilibria condictions point to pressures under 6-7 Kbar during crystallization. The possible parental magma of the Cana Brava complex is compared with the alcali-olivine basaltic magma of the Palmeiropolis metavolcano-sedimentary sequence, and with the possible parental magma of the Serra da Bota satellite body.
- Published
- 1998
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