SEDIMENTATION, METAMORPHISM AND GRANITE
GENERATION IN A BACK-ARC REGION: THE CRUSTAL
PROCESSES RECORDED IN THE EDIACARAN NOVA
VENÉCIA COMPLEX (ARAÇUAÍ OROGEN, SOUTHEAST
FUNDAÇÃO UNIVERSIDADE FEDERAL DE OURO PRETO
Reitor
Marcone Jamilson Freitas Souza
Vice-Reitor
Célia Maria Fernandes Nunes
Pró-Reitor de Pesquisa e Pós-Graduação
Valdei Lopes de Araújo
ESCOLA DE MINAS
Diretor
Issamu Endo
Vice-Diretor
José Geraldo Arantes de Azevedo Brito
DEPARTAMENTO DE GEOLOGIA
Chefe
CONTRIBUIÇÕES ÀS CIÊNCIAS DA TERRA
–
VOL 74
DISSERTAÇÃO DE MESTRADO
Nº 326
SEDIMENTATION, METAMORPHISM AND GRANITE GENERATION
IN A BACK-ARC REGION: THE CRUSTAL PROCESSES RECORDED
IN THE EDIACARAN NOVA VENÉCIA COMPLEX (ARAÇUAÍ
OROGEN, SOUTHEAST BRAZIL)
Fabiana Richter
Orientador
Cristiano de Carvalho Lana
Co-orientador
Gary Stevens
Dissertação apresentada ao Programa de Pós-Graduação em Evolução Crustal e Recursos Naturais do Departamento de Geologia da Escola de Minas da Universidade Federal de Ouro Preto
como requisito parcial à obtenção do Título de Mestre Ciência Naturais, Área de Concentração: Tectônica, Petrogênese e Recursos Minerais
OURO PRETO
2014
Campus Morro do Cruzeiro s/n - Bauxita 35.400-000 Ouro Preto, Minas Gerais
Tel. (31) 3559-1600, Fax: (31) 3559-1606 e-mail: pgrad@degeo.ufop.br
Os direitos de tradução e reprodução reservados.
Nenhuma parte desta publicação poderá ser gravada, armazenada em sistemas eletrônicos, fotocopiada ou reproduzida por meios mecânicos ou eletrônicos ou utilizada sem a observância das normas de direito autoral.
ISSN
Depósito Legal na Biblioteca Nacional Edição 1ª
Catalogação elaborada pela Biblioteca Prof. Luciano Jacques de Moraes do Sistema de Bibliotecas e Informação - SISBIN - Universidade Federal de Ouro Preto
A meus pais, Karin e Roberto, ao meu irmão, Rafael, aos meus avós, Marlene e Humberto, e ao Maurício,
Que mesmo longe, sempre estavam perto.
À Ouro Preto e à República Virada pra Lua,
Agradeço sinceramente a todos que me apoiaram, me ajudaram e me incentivaram durante estes anos. .
Agradeço ao meu orientador, Cristiano, por ter me proporcionado tantas e tantas oportunidades de crescimento. Obrigada por ter acreditado em mim, de graça, desde a graduação. Por ter me mandado pra Stellenbosch. Por ter sido meu orientador desde o TCC. Por estar sempre disponível, mesmo tão ocupado. Por tornar as coisas simples, quando elas pareciam tão complexas. Você foi determinante.
Agradeço à minha família. Aos meus amados pais, Roberto e Karin, que sempre me apoiaram de longe e nunca falharam em me ajudar em tudo que precisei. Não teria chegado aqui sem vocês. Aos meus avós, Marlene e Humberto, grandes exemplos. Tudo o que vocês fizeram, e por tudo o que lutaram, se reflete na minha vida. Ao meu irmão Rafael. Sempre que penso em você, fico feliz. À minha madrinha Cláudia e minha tia Kiti, por tanto amor. Ao meu tio Jorge, pelo apoio. Aos meus tios e primos tão queridos, pelo carinho. Ao Maurício, por ter aberto meu coração para o amor e minha mente para a Ciência.
Agradeço à Universidade Federal de Ouro Preto pelo ensino gratuito e de qualidade. Agradeço aos professores do DEGEO pelos anos de aprendizagem e pela dedicação. Agradeço à SICEG (ah, se aquela sala falasse!). Ao pessoal do AIR Group, por tanta geologia e colaboração. Esse grupo vai longe.
Agradeço aos meus colegas de pós-graduação. Ao meu amigo Marcha (professor Gonçalves): mesmo com toda a nossa identificação, você sempre torna as coisas mais leves! Muito obrigada pelo companheirismo. Aos meus colegas de sala, Francesco e Capuccine, pelo grande apoio. Ao Hugo, Kathryn, Federico, Leo, Marilane, Carmen, Carmen (2) e Gautier. E tantos outros que estiveram por perto!
Agradeço ao Calota pelo incentivo, pela atenção, pelos campos e pela amizade. Ao professor Alkmim, por compartilhar seu grande conhecimento. O trabalho de vocês, durante todos estes anos, enriquece imensamente a geologia no Brasil.
Agradeço ao pessoal de Stellenbosch, por ter me proporcionado tanto amadurecimento acadêmico. Ao meu co-orientador Gary Stevens, que me inseriu no ambiente científico e me mostrou como ele funciona. Obrigada por ter dedicado seu tempo e seu conhecimento a mim. Ao professor Ian Buick pela ajuda no manuscript e pela grande atenção.
Se quiser ir rápido, vá sozinho
Se quiser ir longe, vá junto.
SUMMARY ... XI LIST OF FIGURES ... XV LIST OF FIGURES - MANUSCRIPT (CHAPTER 6) ... XVI LIST OF TABLES ...XVII LIST OF TABLES - MANUSCRIPT (CHAPTER 6) ...XVII CHAPTER 1
INTRODUCTION ... 5
3.1 - MAIN GOALS ... 6
3.2 - RATIONALE ... 7
3.3 - LOCALITY AND MAIN ACCESS ROADS ... 7
3.4 - METODOLOGY ... 8
Field work and sampling ... 8
Microscopic petrography, whole-rock and trace element chemistry, mineral chemistry analysis, geochronology and metamorphic modeling ... 10
Microscopic petrography 10 Mineral chemistry analysis 10 Whole-rock and trace element chemistry 12 U-Pb LA-Q-ICP-MS Geochronology 13 U-Pb LA-ICP-MS in-situ monazite dating 14 Metamorphic Modelling 15 CHAPTER 2 GENERAL ASPECTS OF THE ARAÇUAÍ OROGEN ... 17
2.1 - STRATIGRAPHY ... 18
The basement ... 18
Rift-related rock-types ... 19
Macaúbas Group and correlative units ... 20
Accretionary wedge ... 20
Granitoids ... 20
Jequitinhonha and Nova Venécia Complex ... 21
Salinas Formation ... 22
2.2 - ANATOMY OF THE ARAÇUAÍ-WEST CONGO OROGEN ... 22
The Paramirim aulacogen ... 22
The Guanhães Basement Block ... 23
The Dom Silvério shear zone and adjacent structures ... 23
The Itapebi shear zone ... 23
The High grade internal zone ... 23
The West Congo Belt ... 24
2.3 - THE HIGH GRADE ANATECTIC AND CRYSTALLINE CORE OF THE OROGEN ... 24
The Rio Doce Group ... 25
The Nova Venécia Complex ... 26
Granitoid Supersuites ... 28
The G1 Supersuite 28 The G2 Supersuite 31 The G3 Supersuite 32 The G4 Supersuite 32 The G5 Supersuite 35 2.4 - KINEMATIC EVOLUTION OF THE ARAÇUAÍ OROGEN ... 37
CHAPTER 3 GEOCHRONOLOGY ... 41
3.1 - Introduction ... 41
3.2 - Zircon in igneous rocks ... 43
Textural characterization of igneous zircon ... 43
External morphology of igneous zircon 43 Internal morphology of igneous zircon 45 3.3 - Xenocrystic zircon cores ... 45
3.4 - Zircon in metamorphic rocks ... 46
Textural characterization of metamorphic zircon ... 47
External morphology of metamorphic zircon 47 Internal morphology of metamorphic zircon 48 3.5 - Zircon and metamorphic petrogenesis ... 50
The growth of new zircon during metamorphism ... 51
... 54
... 54
... 54
3.6 - The law of radioactive decay ... 54
Lead isotopes ... 56
3.7 - Using ages: how to evaluate and filter U-Pb ages ... 58
How to filter data in a detrital zircon study ... 61
Evaluating the interpretations of discordant U-Pb ages ... 62
Metamictization and recrystallization 65 Conditions for Pb-loss and examples 66 Causes for discordance 67 Interpretation from magmatic zircons 68 Interpretation from zircons in metamorphic rocks 68 CHAPTER 4 MELTING OF THE CONTINENTAL CRUST – A SUMMARY ... 71
CHAPTER 5 WHOLE ROCK CHEMISTRY ... 75
CHAPTER 6 SEDIMENTATION, METAMORPHISM AND GRANITE GENERATION IN A BACK-ARC REGION: RECORDS FROM THE EDIACARAN NOVA VENÉCIA COMPLEX (ARAÇUAÍ OROGEN, SOUTHEASTERN BRAZIL) ... 83
6.1 - Introduction ... 84
6.2 - Geological setting ... 84
6.3 - Previous studies on the NVC and related rocks ... 89
6.4 - Methodology ... 89
6.5 - Anatectic features and lithotypes of the NVC and intrusive granitoids ... 91
Outcrops 10 and 11 ... 92
Outcrop 7 ... 94
Outcrops 3 and FMC10 ... 94
6.6 - Petrography, mineral chemistry and whole-rock chemistry ... 95
Biotite – garnet ± cordierite metagreywacke (BGCM-101 and BGM-114) ... 95
Biotite – garnet – orthopyroxene metagreywacke (BGOM-7A1B) ... 96
Biotite-garnet-cordierite metagreywacke (BGCM-101) ... 101
P-T path and P-T conditions of stable assemblages 102 Biotite-garnet-orthopyroxene metagreywacke (BGOM-7A1B) ... 102
P-T path and P-T conditions of stable assemblages 103 Cordierite granulite (CG-3A1) ... 106
P-T path and P-T conditions of stable assemblages 106 6.8 - Zircon and monazite U-Pb geochronology ... 108
Peraluminous granodiorite (PG-7D) ... 108
Granite (G-7B) ... 109
Biotite-garnet-orthopyroxene metagreywacke (BGOM-7A1B) ... 110
Biotite-garnet-cordierite metagreywacke (BGCM-101) ... 110
Cordierite granulite (CG-3A1) ... 111
Biotite-garnet metagreywacke (BGM-114) ... 111
6.9 - Discussion and concluding remarks ... 114
Magmatism in the Araçuaí Orogen ... 114
Nature and provenance of the Nova Venécia Complex ... 114
Metamorphism in the Nova Venécia Complex ... 115
Metamorphic history of the Araçuaí Orogen as recorded by the Nova Venécia Complex ... 119
CHAPTER 7 CONCLUSIONS ... 123
REFERENCES ... 125
APPENDICES... 135
Figure 1- Map showing main access roads and the main localities within the study area. ... 8 Figure 2- The study area and the main units in this segment of the Araçuaí Orogen core (Modified from
Gradim 2013). In detail, localities (outcrops) from where samples were collected. ... 9 Figure 3 - Four of the (five) thin sections used for mineral chemistry and the spots where analysis were
made. ... 12 Figure 4- The Araçuaí-West Congo Orogen and the adjacent São Franciso-Congo craton in the context
of West Gondwana. South America-Africa fit after De Wit (1988). V= Vitória; S=Salvador; L=Luanda; C=Cabinda (From Alkmim et al. 2006). ... 17
Figure 5- Regional map of the Araçuaí Orogen (modified from Pedrosa-Soares et al. 2007). ... 18
Figure 6- The main structural domains in the Araçuaí-West Congo Orogen ... 24 Figure 7- Paleogeographic profile reconstruction of the Rio Doce magmatic arc (Modified from Vieira
2007). ... 26 Figure 8- Simplified kinematic evolution of the Brasiliano-Pan African orogenic system, with emphasis
in the Araçuaí Orogen and its proposed evolution following five stages (the Nutcracker Tectonics, Alkmim et al. 2006). ... 39
Figure 9 - Zircon external morphology variation (from Corfu et al. 2003 and references therein).. ... 44
Figure 10 - Variation in growth zoning in magmatic zircon (from Corfu 2003 and references therein). ... 49 Figure 11 - Variable appearance of xenocrystic cores in magmatic and high-grade metamorphic rocks
(from Corfu 2003 and references therein). ... 50 Figure 12 - Zircon crystallization textures (from Hoskin & Schaltegger 2003 and references
therein).. ... 53 Figure 13 - Isotope and trace-element characteristics of zircon from a meta-granitoid gneiss, northern
Queensland, Australia (from Hoskin and Schaltegger 2003 and references therein). ... 54 Figure 14 - Pb/U Concordia diagram, showing the three chronometers that are commonly used (from
Geherels 2012).. ... 60 Figure 15 - Example showing the orientation and magnitude of uncertainty ellipses for Phanerozoic,
Proterozoic, and Archean ages (from Geherels 2011). ... 60 Figure 16 - Diagram showing that, for a mid-Proterozoic analysis with Pb loss, the 206Pb*=207Pb* age is
more accurate than 206Pb*=238U and 207Pb*=235U ages. ... 62 Figure 17 - Concordia diagram showing strategy of using clustering and discordance filters to evaluate
detrital zircon ages. (from Gehrels 2011 and references therein).. ... 63 Figure 18 - Example of the power of using multiple small-volume analysis on a single grain to resolve
a complex history of zircon growth. (from Gehrels 2011 and references therein). ... 63 Figure 19 - Examples of analysis showing discordant U-Pb ages (modified from Mezger & Krogstad
1997). ... 64 Figure 20 - Interpretation of zircons that became strongly discordant as a result of a high grade
metamorphic overprint can be particularly difficult. ... 70 Figure 21 - Geochemical Al2O3–Fe2O3(T)–MgO and CaO–K2O–Na2O ternary plots for relevant
Figure 1 - Map of the Araçuaí Orogen and the study area (modified from Pedrosa-Soares et al.
2007). ... 85
Figure 2 - Simplified evolution of the Brasiliano-Pan African orogenic system, with emphasis in the Araçuaí Orogen (Alkmim et al. 2006).. ... 87
Figure 3 - Map of the study area. The outcrops from where samples were collected are: outcrops 3 and FMC10 (samples 3A1, FMC10, 3A4, 3A2, 3A3, 3A5); outcrop 4 (4A1, 4A2); outcrop 5 (5A, 5B); outcrop 7 (7A1B, 7B, 7D, 7A1A, 7A2A, 7A2B, 7C1, 7C2); Outcrop 8 (8A, 8B); outcrop 10 (101, 102, 103); outcrop 11 (114, 111, 112, 113, 116). ... 90
Figure 4 - Anatectic features of the high-grade rocks in outcrop-scale. ... 93
Figure 5 - Petrography of samples observed in thin sections. ... 98
Figure 6 - P-T pseudosections calculated for BGCM-101. ... 104
Figure 7 - P-T pseudosections calculated for BGOM-7A1B. ... 105
Figure 8 - P-T pseudosections calculated for CG-3A1. ... 107
Figure 9 - Density concordia diagrams constructed by cumulatively adding the normalized probability density functions (PDF's) of each individual analysis according to their calculated mean values and 2σ uncertainties. ... 113
Figure 10 - Probability density plots for zircons and monazites analyzed from metasedimentary and granitic rocks in this study and from compiled data. ... 117
Figure 11 - Suggested and simplified metamorphic evolution for all samples related to granulite and amphibolite conditions of metamorphism and to temperature intervals where biotite and muscovite breakdown most likely occur (Vielzeuf and Holloway, 1988; Patiño Douce and Johnston, 1991; Montel and Vielzeuf, 1997; Stevens et al., 1997; Pickering and Johnston, 1998). ... 119
Table 1- Distance between the main cities in the studied area and from Ouro Preto – MG. (Source:
https://maps.google.com/, em 21 de fevereiro de 2013). ... 7
Table 2- Sampling sites coordinates ... 9
Table 3- Methods and analysis used to study each of the samples. Abbreviations are: Rock-type (RT); Microscopic petrography (MP); Whole-rock and trace element chemistry (WR and TEC); Mineral Chemistry (MC); Isotopic analysis via LA-Q-ICP-MS (IA); In-situ monazite dating (ISMD); Metamorphic modeling (MM). ... 11
Table 4- Temperature and pressure metamorphic conditions in which Nova Venécia rocks must have equilibrated according to literature. ... 27
Table 5- Main characteristics of the G1-G5 Supersuites. 1-Roncato 2009; 2-Pedrosa-Soares et al 2007; 3-Pedrosa-Soares et al 2011, *and references therein; 4- Gradim et al. 2014; 5-Martins et al. 2004. 6-Nalini 1997. ... 29
Table 6- Ages of the G1 Supersuite obtained from literature. ... 30
Table 7- Ages of the G2 Supersuite obtained from literature. ... 33
Table 8 – Ages of the G3 Supersuite obtained from literature. ... 34
Table 9 - Ages of the G4 Supersuite obtained from literature. ... 35
Table 10 - Ages of the G5 Supersuite obtained from literature. ... 36
Table 11 – Ultimate parent-daughter pairs of uranium and thorium. ... 57
Table 12 – Compositions from samples used for the construction of diagrams in Figure 21 ... 77
List of Tables - Manuscript (Chapter 6)
Table 1 - Main characteristics of the G1-G5 Supersuites. 1-Roncato 2009; 2-Pedrosa-Soares et al 2007; 3-Pedrosa-Soares et al 2011, *and references therein; 4- Gradim et al. 2014; 5-Martins et al. 2004. ... 88Table 2 - Samples collected in this study and their coordinates... 91
Table 3 - Representative mineral chemistry of samples 101, 7A1B and 3A1. The number of ions was calculated on the basis of 12 oxygens for garnet, 22 oxygens for biotite and muscovite, 8 oxygens for feldspar, and 18 oxygens for cordierite. XAlm = Fe2+/(Fe2+ + Mn) XSpss = Mn/(Fe2+ + Mn + Mg + Ca), XGrs = Ca/(Fe2+ + Mn + Mg + Ca), Mg# = 100 × Mg/(Mg + Fe2+), XAb = Na/(Ca + Na + K), XAn = Ca/(Ca + Na + K), XSan = K/(Ca + Na + K), XFeCrd = Fe2+ /(Mg + Fe2+), XMgCrd = Mg/(Mg + Fe2+)... 99
Table 4 - Bulk-rock compositions used for modeling. The H2O content was modified according to values coherent with the modeling (see text) and normalized to 100%. ... 101
Table 5 - Main populations that compose the zircon detrital dataset and calculated maximum sedimentation ages. ... 115
Resumo
O Complexo migmatítico-graniulítico-granítico Nova Venécia (CNV), localizado no núcleo do Orógeno Araçuaí (OA, 630-480 Ma), sudeste do Brasil, registra processos crustais anatéticos ocorridos no norte da Província Mantiqueira durante a amalgamação Brasiliana-Pan Africana de Gondwana Ocidental. O núcleo do OA compreende abundantes e volumosos granitoides tipo-S e –I (Supersuítes G1 a G5), que são espacialmente e temporalmente associados a eventos metamórficos de alto grau no NVC. Este estudo integra observações de campo, análises de química mineral, petrografia, geocronologia U-Pb LA-ICP-MS de zircões e monazitas e modelagem termodinâmica, a fim de definir a evolução dos migmatitos-granulitos do CNV, desde sua deposição até o metamorfismo de alto grau, e correlacionar a história metamórfica com os vários episódios de magmatismo granítico (G1-G5). Sete populações compõe a base de dados de zircões detríticos. A gama mais significativa de zircões detríticos concordantes zircão são representados pelas duas populações mais jovens, variando 650-610 Ma. Isso indica que a principal fonte do CNV é provavelmente o Arco Rio Doce, com contribuições menores de fontes contemporâneas ao Arco Rio Negro. Populações mais velhas sugerem proveniência dos primeiros registros do arco Rio Negro e de segmentos do OA relacionados a riftes de idades Criogeniana e Toniana. O período de sedimentação do CNV é limitado entre a idade máxima de sedimentação em ca. 606 Ma e a intrusão dos primeiros granitóides sin-colisionais (ca. 593 Ma), ou seja, durante ca. 13 Ma. Compilação dos dados disponíveis de U-Pb em zircão mostra que a maior parte dos granitoides G1 e G2 se cristalizaram contemporaneamente ao longo de um período de 15 Ma (595-570 Ma, com um pico a 575 Ma), interpretado como o período sin-colisional no OA. O período de pico metamórfico regional no OA é limitado em 575-560 Ma, o que pode ser uma consequência de magma underplating G1 + G2.
Petrografia detalhada e análises de química mineral mostram diferentes assembléias de pico metamórfico (regional) que contêm quantidades variáveis de granada, ortopiroxênio e cordierita peritéticos e cordierite retrógrada. Sugerimos que essas diferenças são principalmente devidas a parâmetros de composição dos protólitos, e não devidas a diferentes evoluções de P-T entre as amostras. A química de rocha total neste estudo sugere que os protólitos do CNV eram grauvacas peraluminosas contendo diferentes quantidades de componentes de matriz (isto é, porções pelíticas) e que as rochas de alto-grau do CNV devem ter perdido melt para terem se tornado caracteristicamente restíticas. Isto é
corroborado pelo nosso conjunto de dados de zircões detríticos, que mostram diferentes contribuições percentuais entre as 7 populações que compõem as amostras. Além disso, a modelagem termodinâmica indica que todas as amostras modeladas registram um caminho P-T semelhante, desde condições PT de metamorfismo regional de pico a 750-850 ° C e 5300-7500 bares (granulito, profundidades de ~ 25 km) a condições de estabilidade das assembléias preservadas a 640- 800 ° C e 4500-6000 bares (transição entre amfibolito superior a granulito, profundidades de ~ 18 km). Infere-se que o metamorfismo regional de alto grau (575-560 Ma) deve ter afetado ambos os metassedimentos e granitos pré-existentes, corroborado pelo fato de que ambos mostram feições anatéticas datadas em ca. 571 Ma. Os produtos da fusão parcial em todo o OA poderia ser, pelo menos, parte dos granitóides contemporâneos àqueles formados durante os períodos G2 (570-540 Ma) e G3 + G4 (540-525 Ma). O evento térmico pós-colisional G5 (520-480 Ma), relacionado ao colapso tectônico do OA, é registrado em metagrauvacas (monazita U-Pb) e em granitos (monazita e zircão U-Pb) entre 507 e 495 Ma. Sugerimos que, a essa altura, as metagrauvacas já haviam sido submetidas a alguma descompressão e arrefecimento, com base em modelagem metamórfica, observações de campo e datação de um dique tardio não deformado que intrude rochas do CNV (518 Ma). Infere-se que o evento termal pós-colisional G5, registrado por abundantes intrusões de granitoides tipo-I em todo o OA, causou um segundo período de metamorfismo de alto-grau a ca. 500 Ma. A principal característica deste evento em rochas metassedimentares é, além das idades U-Pb em monazitas, um overprint parcial de Baixa Pressão-Alta Temperatura em assembléias
Abstract
CHAPTER 1
INTRODUCTION
The P-T-t evolution of high-grade orogenic cores are recorded in migmatites, granulites and associated granites, which account for processes of melt/magma production, extraction and emplacement that ultimately lead to crustal differentiation. Migmatitic and granulitic terranes in orogenic belts that record thickening before maximum temperatures are achieved typically display clockwise P-T paths (Brown 1994). Within this path, metamorphic peak is achieved during prograde heating, which is followed by near-isothermal decompression and a posterior close to isobaric cooling (Yakymchuck and Brown 2014). Reactions occuring during prograde heating generate melt, the loss of which produce high-grade mineral assemblages (Brown and Korhonen 2009). Residual compositions and preserved anhydrous high-grade mineral assemblages with minor evidence of retrogression are consistent with loss or isolation of melt from these middle to lower crustal domains (Brown 2004). As H2O strongly partitions into the melt, melt isolation and loss creates a system where no free H2O-rich volatile phase attends high-grade metamorphism (Brown and Korhonen 2009).
Many studies suggest that, at high-T conditions (T > 800°C), fluid-absent reactions dominate during most crustal melting and production of mobile, large-volume granitic magmas (e.g. Stevens and Clemens 1993, Clemens and Watkins 2001). The major types of fluid-absent reactions depend on the protolith mineralogy and temperature. Notably, major crustal melting in peraluminous metasedimentary protoliths involves muscovite then biotite breakdown producing high-grade assemblages that commonly bear peritectic sillimanite, garnet, cordierite and orthopyroxene (e.g., Brown 2007, Taylor et al. 2014).
These assemblages and the proportion of melt produced by those reactions depend on P-T conditions and protolith compositions, where different protholiths will produce different melt volumes (Brown 2007, Korhonen et al. 2010). Predictive modeling of protolith fertility (Clemens and Vielzeuf 1987,
Many terrains that expose high-grade orogenic cores provide insights on crustal processes that occured in deep crustal levels of old large-scale orogenic belts. Some examples are: the Fosdick migmatite granite complex in West Antartica (e.g., Korhonen et al. 2010, 2012); the Round Hill, Broken
Hill in southeastern Australia (e.g., White et al. 2004, White et al. 2005); the Ivrea Zone, Italy (Barboza
and Bergantz 2000, Barboza et al. 1999, Redler et al. 2012); and the Southern Marginal Zone of the
Limpopo Belt (e.g., Stevens & Van Reenen 1992, Nicoli et al. 2014, Taylor et al. 2014). In such terrains,
determining the processes governing crustal anatexis, the P-T conditions and the timing of magmatic and metamorphic events may help elucidate the tectonic evolution of orogenic crust. This ultimately
leads to a better understanding of Earth’s crustal evolution through time.
One important segment in South America that still lacks detailed investigation is the core of the Araçuaí Orogen (AO), in Brazil, which formed as part of the Brasiliano-Pan African Orogenic System during the assembly of West Gondwana. The metamorphic core of the AO exposes a nearly continuous crustal section from mid-crustal anatectic migmatites and granulites in the southern part of the orogen to progressively shallow-level granitoid plutons and pegmatites towards the north (Pedrosa Soares et al.
2011). Migmatites and granulites of the Nova Venecia Complex (NVC) are exposed through an extensive area of the AO and record different high-grade preserved mineral assemblages with little evidence for retrogression, which are exposed in close association with voluminous S-type and I-type granitoids. The sediments that formed the protoliths of the NVC are believed to have been eroded from a magmatic arc constructed in a pre-collisional stage of the orogen (ca. 630-585 Ma) and subsequently deposited in a back-arc region (Noce et al. 2004, Gradim et al. 2014). In a syn-collisional stage of the
orogen (ca. 585-560 Ma), the rocks are believed to have been metamorphosed under high-grade conditions and their anatectic features are believed to be associated with the genesis of syn-collisional S-type granitoids, which have ages spanning from 585-560 Ma (e.g. Pedrosa-Soares et al. 2011, Roncato
2009, Gradim 2013). Post-collisional I-type granitoids related to the tectonic collapse of the AO have been emplaced within all of these units, recording ages of ca. 500 Ma (Gradim et al. 2014). These late
intrusions might have a link with late- to post-collisional fault zones that inverted and disrupted units across the orogen (Oyhantçabal 2011), causing the last (G5) thermal event recorded in the AO core (e.g. Alkmim et al. 2006, Roncato 2009).
3.1 -
MAIN GOALS
bodies occurring in close association with the sampled high-grade rocks.
3.2 -
RATIONALE
Details of the metamorphic reactions that affected these rocks as well as the timing and extent of partial melting remain largely unknown. Specifically, it is not clear whether the age of peak metamorphic conditions coincide with main crystallization ages of granitoids emplaced in the high-grade anatectic zone and very little has been documented about the effect caused by the post-collisional granitoids on all units.
3.3 -
LOCALITY AND MAIN ACCESS ROADS
The Nova Venécia Complex lies approximately southwards from latitude 18°00’S, encompassing both the States of Minas Gerais and Espírito Santo. The field work in this study took place mostly in the region between the cities of Central de Minas, Nova Venécia, Baixo Guandú and Colatina. Table 1 shows the distance between the main cities within the studied area. Figure 1 shows the main roads from Ouro Preto - MG to the studied area and the main access roads in the area.
Table 1- Distance between the main cities in the studied area and from Ouro Preto – MG. (Source:
https://maps.google.com/, em 21 de fevereiro de 2013).
From To Distance (km)
Ouro Preto Nova Venécia 571
Ouro Preto Central de Minas 434
Nova Venécia São Mateus 66.1
Nova Venécia São Gabriel da Palha 43.6
Nova Venécia Colatina 119
Figure 1- Map showing main access roads and the main localities within the study area.
3.4 -
METODOLOGY
Field work and sampling
Table 2- Sampling sitescoordinates
Outcrop X Y Sample Rock type Locality
3 355606 7928182 3A1, 3A2, 3A3,
3A4, 3A5 Cordierite granulite Nova Venécia
FMC10
354665 7929744 FMC10 Cordierite-garnet granulite Nova Venécia
4 354099 7928733 4A1, 4A2 Biotite-garnet
metagreywacke Nova Venécia
5 352870 7928844 5A, 5B G2 and G3 granite Nova Venécia
(Quary)
7
348076 7905779 7A1B, 7A2B, 7B, 7A1A, 7A2A, 7C1, 7C2, 7D, 7E
Biotite-garnet-orthopyroxe metagreywacke
and G2 granite
Vila Fartura (Road ES428)
8 344854 7905130 8A, 8B G2 and G3 granite Road ES428
10
328442 7838497 101, 103, 104 Biotite-garnet-cordierite
metagreywacke Colatina
11 332399 7840820 111, 112, 113,
114, 116 metagreywacke Biotite-garnet Colatina (Quary)
Figure 2- The study area and the main units in this segment of the Araçuaí Orogen core (Modified from Gradim
Microscopic petrography, whole-rock and trace element chemistry, mineral
chemistry analysis, geochronology and metamorphic modeling
Table 3 summarizes the methods used to study each of the samples. Whole rock chemistry was obtained for samples 3A1, 3A5, 3A4, 3A3 ,3A2, 7A2A, 7A1A, 7A1B, 7A2B, 113, 114, 112, 116, 111, 4A2, 4A1, 102, 104, 103, 101, 7C1, 7C2, FMC10. Samples 101, 7A1B, 3A1, FMC10 were chosen for detailed study according to their petrographic aspects determined from thin sections and mineral assemblages recorded in each of them. Three samples (101, 7A1B and 3A1) were chosen that have good potential for metamorphic modelling based on their whole rock chemistry, mineral chemistry and observed microstuctures and textures. Samples chosen for geochronology are: 3A1, 101, 7A1B, 7B, 7B (zircon and monazite U-Pb LA-Q-ICP-MS) and sample 114 (U-Pb LA-ICP-MS in situ monazite analysis).
Microscopic petrography
Twenty-seven thin sections were made in the LAMIN (Universidade Federal de Ouro Preto). The thin sections were inspected for mineral paragenesis and textures in the rocks.
Mineral chemistry analysis
Mineral chemistry analysis was carried out at Stellenbosch University. Prior to imaging and analysis, the thin sections were carbon-coated. Samples were analyzed by a wavelength-dispersive X-ray spectroscopy (EDS) using a Zeiss EVO MA 15 Scanning Electron Microscope (SEM) fitted with an Oxford Instrument Wave Dispersive X-ray Spectrometer and Oxford INCA Software. Beam conditions during the analysis were 20.00 kV and approximately 1.0 A, with a working distance of 8.5 mm and a specimen beam current of -20.00 nA. Analysis were quantified using natural mineral standards. Later on, mineral compositions were recalculated to mineral stoichiometries to obtain mineral structural formulae using the Software Excel 2010.
Table 3- Methods andanalysis used to study each of the samples. Abbreviations are: Rock-type (RT); Microscopic petrography (MP); Whole-rock and trace element chemistry (WR and TEC); Mineral Chemistry (MC); Isotopic analysis via LA-Q-ICP-MS (IA); In-situ monazite dating (ISMD); Metamorphic modeling (MM).
RT MP WR and
TEC MC IA ISMD MM
3A1 CG X X X X X
3A2 CG X X
3A3 CG X X
3A4 CG X X X
3A5 CG X X
FMC10 CGG X
4A1 BGM X X
4A2 BGM X X
5A G2 granite
5B G3 granite
7A1A BGOM X X
7A1B BGOM X X X X X
7A2A BGOM X X
7A2B BGOM X X
7B granite X
7C1 BGOM X X
7C2 BGOM X X
7D granite X
7E granite
8A G2 granite
8B G3 granite
101 BGCM X X X X X
102 BGCM X X
103 BGCM X X
111 BGM X X
112 BGM X X
113 BGM X X
114 BGM X X X X
Figure 3 - Four of the (five) thin sections used for mineral chemistry and the spots where analysis were made.
Whole-rock and trace element chemistry
Stellenbosch University, South Africa. Samples were crushed to a fine powder using a jaw crusher and swing mill, and glass disks prepared for XRF analysis using 1.5 g of high purity trace element and REE element free flux (LiBO2 = 80%, Li2B4O7 = 20%) mixed with 0.28 g of the rock sample. Whole-rock major element compositions were determined by XRF spectrometry on a Philips 1404Wavelength Dispersive spectrometer. The spectrometer is fitted with an Rh tube, analyzing crystals LIF200, LIF220, LIF420, PE, TLAP and PX1. The instrument is fitted with a gas-flow proportional counter, a scintillation detector. The gas-flow proportional counter uses a 90% Argon, 10% methane gas mixture. Major elements were analyzed on a fused glass disk at 50 kV and 50mA tube operating conditions. Matrix effects in the samples were corrected for by applying theoretical alpha factors and measured line overlap factors to the raw intensities measured with the SuperQ Philips software. Control standards that were used in the calibration procedures for major element analyses were AGV (Andesite from the United States Geological Survey, Reston), NIM-G (Granite from the Council for Mineral Technology, South Africa) and BHVO-1 (Basalt from the United States Geological Survey, Reston).
Trace element chemistry was performed at the Central Analytical Facilities (CAF), Department of Earth Sciences, Stellenbosch University, South Africa. Trace element in bulk rock samples were analyzed on polished mounts prepared from XRF fusion disks. For laser ablation work, a Resonetics 193nm Excimer laser was connected to an Agilent 7500 ICP-MS for the trace element analysis.
U-Pb LA-Q-ICP-MS Geochronology
Samples were prepared in the “Laboratório de Preparação de Amostras para Geoquímica e
Geocronologia” (LOPAG), in the Department of Geology of the University of Ouro Preto. Zircons and monazites were separated from the chosen samples (3A1, 7A1B, 7B, 7D, 101) by rock crushing, concentration of heavy minerals (heavy liquid separation), magnetic separation and individual selection and handpicking of grains with the use of a petrographic binocular. The grains were subsequently mounted on an acrylic plate covered with a double-face tape where collected grains were disposed in lines, so each of the lines represented one distinct sample. A mixture of resin and a hardening solution was added to a mold that was put over the tape, which produced mounts (2.5x1 cm). The mounts were polished until zircon and monazite grains were exposed to half thickness. Cathodoluminescence images were produced in the Department of Geology from the University of São Paulo.
from all samples, as well as some ages obtained on the indicated spots.
U-Pb LA-ICP-MS in-situ monazite dating
Monazite grains were dated in thin section in the Stellenbosch University Central analytical facility (CAF) ICP-MS unit using an Agilent 7500ce quadrupole ICP-MS coupled to a 213 nm New Wave laser. Prior to analysis monazite grains were imaged in BSE (back-scattered electron) mode using a ZEISS EVO MA15VP SEM housed in the Central Analytical Facility (CAF) electron micro-beam unit. Operating conditions for monazite are described in Buick et al. (2011) and were optimised to
provide maximum sensitivity for the high masses (Pb–U) while inhibiting oxide formation (ThO+/Th+<0.5%). Ablations occurred in a custom-built small-volume, teardrop-shaped sample cell (cf. Horstwood et al. 2003) in a He carrier gas, and the resulting aerosol was mixed with Ar prior to
introduction into the ICP-MS via a signal-smoothing manifold. Initial data reduction used the Glitter software package (van Achterbergh et al. 2001) to calculate the relevant isotopic ratios (207Pb/206Pb,
208Pb/206Pb, 208Pb/232Th, 206Pb/238U and 207Pb/235U). 235U was calculated from 238U counts via the natural abundance ratio 235U = 238U/137.88 (Jackson et al. 2004). Individual isotopic ratios were displayed in time-resolved mode. Isotopic ratios generated during the first 5-10s of each analysis were not used, and from the remainder of each analysis the integration window was chosen so as to maximise concordance (Jackson et al. 2004). Ablation depth-dependent elemental fractionation was corrected for by tying the
integration window for the unknown monazite to the identical integration window of the standard (Jackson et al. 2004).
Instrumental drift was corrected against the monazite standard using linear interpolative fits. The U-Pb data were plotted on a Wetherill concordia diagram using the software Isoplot (Ludwig 2003). Uncertainties in these Tables were propagated assuming a 1% uncertainty on the age of the standards. 204Pb-based common Pb corrections were not applied because of the well-known contamination of the carrier gases by 204Hg (e.g. Jackson et al. 2004).
The analytical runs involved repeated analysis cycles of the USGS 46609 standard (424.9 Ma; Aleinikoff et al. 2006), used as the primary standard, Thompson Mine monazite (1766 Ma; Williams et al. 1996), used as a control/secondary standard, and 6-8 analyses of monazite from the thin section.
Integration times for U/Pb age determinations were 15 ms for 206Pb, 40 ms for 207Pb, and 10 ms for 208Pb, 204Pb, 232Th and 238U. LA-ICP-MS acquisitions consisted of a 60 second measurement of the gas blank, followed by 40 seconds of measurement of U, Th and Pb signals during ablation. Laser ablations were performed at a frequency of 4 Hz and an energy density of ~4.5-5 J/cm2, and produced 20 nm diameter wide pits. As noted by Buick et al. (2011), a problem with U/Pb analysis of monazite at Stellenbosch
isotope analyses of the old monazite are reversely discordant, even when independent TIMS analyses show that this monazite is concordant. This was the case in the present study where analyses of the nominally concordant high-Th Thompson Mine secondary standard were 7-13% reversely discordant. However, as noted by Buick et al. (2011) the weighted mean 207Pb/206Pb age of the monazite is
unaffected. This was found to be the case in the, present study, where the weighted mean 207Pb/206Pb age of 9 analyses of the Thompson Mine monazite was 1753±17 Ma (95% confidence level, or c.l.; MSWD = 0.57), within error of the accepted value.
From DF20912-1 (sample 114), 16 LA-ICP-MS analyses were made in thin section. Unfortunately, 7 of these were apatite inclusions, rather than monazite. Of the remaining 9 analyses all were concordant (98-100% based on the agreement between 206Pb/238U and 207Pb/235U ages), but not all were the same age. Two concordant analyses had apparent spot 206Pb/238U ages of c. 550 Ma (analyses MNZ_6 and _9). The remaining seven analyses were younger and could be combined together to yield a concordia age of 502.1 ± 6.3 Ma (2sigma; MSWD of concordance and equivalence = 0.51)
Metamorphic Modelling
Constraining metamorphic conditions and evaluating the relation between mineral assemblage stability and melting is crucial to determine the nature of the two metapelites regarding to one another and to the granites in the studied area. The use of equilibrium phase diagrams constructed within the confines of a specific bulk rock composition, i.e. pseudosections (Powell et al. 1998), is a powerful
approach to evaluate this relationship. The metamorphic conditions in peak and preserved assemblages in three samples of the NVC were investigated using numerous quantitative T-MFe2O3, T-MH2O, T- MMelt, P- MMelt and P-T pseudosections. Calculations were undertaken in the chemical system Na2O–CaO–
K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) using THERIAK-DOMINO software (de Capitani & Petrakakis 2010), in combination with the up-dated Holland & Powell (1998) dataset. The modelling uses the a-x relationships of White et al. (2007) for silicate melt; White et al.
(2002) for orthopyroxene, spinel and magnetite; Holland & Powell (2003) for K-feldspar and plagioclase; White et al. (2000, 2005) for ilmenite; White et al. (2005) for garnet and biotite; Holland
CHAPTER 2
GENERAL ASPECTS OF THE ARAÇUAÍ OROGEN
The Brasiliano-Pan African amalgamation of West Gondwana generated a complex and large orogenic system limited by the São Francisco, Paranapanema and Rio de la Plata cratons to the west and the Congo and Kalahari cratons to the east. To the south of the São Francisco and Congo crations, the complementary orogenic segments that are exposed today in east South America and west Africa are: the Araçuaí Orogen, the Ribeira Fold Belt (RFB) and the Dom Feliciano Belt (DFB) in Brazil and Uruguay (Mantiqueira Province); and the West-Congo Belt, the Kaoko Belt, the Damara Belt and the Gariep Belt in Africa. These orogenic segments were amalgamated in the Neoproterozoic-Lower Paleozoic during the closure of the Adamastor Ocean (Pedrosa-Soares et al. 1998) and Khomas Ocean
(Gray et al. 2008) and posteriorly separated by the Cretaceous South Atlantic rifting. Prior to this rifting,
it is suggested that the São Francisco and Congo Cratons had remained connected since the Amazonian-Eburnian Cycle (2300-1900 Ma) by the so-called Bahia-Gabão Bridge (Porada 1989). This indicates that the Araçuaí-West Congo segment was formed as a confined orogenic system inside a gulf of the Adamastor Ocean, limited by the São Francisco-Congo craton to the north, east and west (Pedrosa-Soares et al. 2001). Figure 4 shows the location of the Araçuaí-West Congo Orogen, which is today
exposed in Brazil and Africa, limited by the São Francisco and Congo Cratons.
Figure 4- The Araçuaí-West Congo Orogen and the adjacent São Franciso-Congo craton in the context of West
2.1 -
STRATIGRAPHY
Figure 5 shows a regional map of the Araçuaí Orogen. The evolution of the AO is recorded by major lithostratigraphic units that compose this system, from the basement and rift events to the plutonism and metamorphism that mark the orogenic Brasiliano-Pan African event.
Figure 5- Regional map of the Araçuaí Orogen (modified from Pedrosa-Soares et al. 2007) and the study area.
The basement
Essentially, the basement encompasses the Guanhães, Gouveia, Porteirinha, Mantiqueira and Juiz de Fora Complexes, as defined by Noce et al. (2007). They constitute segments of a Rhyacian
Guanhães, Gouveia, Porteirinha, Mantiqueira are autochthonous to para-autochthonous segments while the Juiz de Fora Complex is juxtaposed to the Mantiqueira Complex by an extense shear zone (the Abre Campo Fault; Noce et al. 2007).
The Guanhães, Porteirinha and Gouveia include tonalite–trondhjemite–granodiorite (TTG) gneisses and migmatites, granitic plutons and greenstone belt sequences that are similar to the Quadrilátero Ferrífero basement, although there is a mismatch among their ages (Noce et al. 2007). In
the Guanhães Complex, TTG gneisses and migmatites yield ages of ca. 2.867 and 2711 Ma and a granitic intrusion yield an age of 2710 Ma (Silva et al. 2002). In the Gouveia Complex, a greenstone belt
sequence and a granitic pluton yield ages of 2971 and 2839 Ma (Machado et al. 1989), respectively.
U-Pb data for the Porteirinha Complex is currently unavailable. In the Mantiqueira Complex, the characteristic orthogneisses yield magmatic crystallization ages of 2180 to 2041 Ma (Silva et al. 2002,
Noce et al. 2007). Sr and Nd data suggest melting of an ancient crustal segment, which is infered to
represent a magmatic arc(s) developed around an Archean paleocontinent margin (Noce et al. 2007).
On the other hand, the Juiz de Fora Complex seems to record the development of an intra-ocean magmatic arc, with isotopic signature that suggests melting of a juvenile crust and magmatic crystallization at ca. 2134 and 2084 Ma (e.g. Heilbron et al. 2001, Noce et al. 2007).
Rift-related rock-types
According to Pedrosa-Soares & Alkmim et al. (2011), the Araçuaí-West Congo orogeny was
preceded by at least six events of rifting and-or anorogenic magmatism, namely: the Statherian E1 (1.77-1.7 Ga), Calymmian E2 (1.57-1.5 Ga), Early Stenian E3 (1.18 - ? Ga), Stenian-Tonian E4 (ca. 1 Ga), Tonian E5 (930-850 Ma) and Cryogenian E6 (750-670 Ga) events. Until now, the records of events E1, E2, and E3 were only found in the Araçuaí Orogen. The E1 event is recorded in the Paleo/Mesoproterozoic plutonic, volcanic and sedimentary basal section of the Espinhaço Supergroup and in the Borrachudos (Guanhães Complex) and Lagoa Real Suites. The Borrachudos Suite have an age of 1.740±8 a 1.670±32 Ga (Silva et al. 2002). The E2 event is recorded in the northern Espinhaço
and Chapada Diamantina by sediments and volcanics of the middle portion of the Espinhaço Supergroup. The E3 event is recorded in the syn-rift deposition of the Sopa-Brumadinho Formation, in the Espinhaço Supergroup. The E4 event is represented by the anorogenic Noqui granites (ca. 999 ± 7 Ma; Africa) and related volcanic rocks, which can be correlated to to mafic dykes in southern Bahia State (Brazil). The E5 (Tonian rift) event is representd by: the thick bimodal volcanic pile of The Zadinian and Mayumbian groups in the West Congo Belt; and by the A-type Salto da Divisa (Silva et al. 2008), Tonian mafic dykes (e.g. Pedro Lessa Suite) and the pre-glacial formations of the Macaúbas
alkaline Province (ca. 735-675 Ma, Brazil, Rosa et. al 2007), the deposition of diamictites formations of the Macaúbas Group, and possibly the La Louilla felsic volcanism in SW Gabon. All these rifting events were unsuccessful in splitting the São Francisco-Congo craton. This cratonic segment remained connected since the Amazonian-Eburnian Cycle (2300-1900 Ma) by the so-called Bahia-Gabão Bridge (Porada 1989) and was only split by the Cretaceous Atlantic rifting.
Macaúbas Group and correlative units
The Macaúbas Basin is the precursor Neoproterozoic basin of the Araçuaí Orogen, which evolved mainly from a Cryogenian continental rift (E6 event) to an inland-sea basin (Pedrosa-Soares & Akmim 2011). The basin, which was partially floored by a Cryogenian oceanic crust, was inverted during the Neoproterozoic Brasiliano-Pan African orogeny. The rocks within this basin sector include the remnants of Neoproterozoic oceanic lithosphere (660 Ma, Queiroga et al. 2007) and the thick
Macaúbas Group that includes rift, transitional and passive margin sedimentary, volcanic and intrusive rocks with late Tonian to Cryogenian ages (Silva et al. 2008, Pedrosa-Soares et al. 2008, 2011).
Accretionary wedge
A suture related accretionary wedge was characterized in the region of São José da Safira, west of the Rio Doce magmatic arc, by Peixoto et al. (2015). The suture zone consist of a 30km-wide and
100 km-long complex schist belt. It includes pelitic schist intercalated with quartzites, metaultramafic schists and diopsidites, intruded by collisional granites. The belt is limited to the west by the Guanhães Block and to the east by the Rio Doce arc. It presents the architecture of an asymmetric flower structure developed in a transpressional regime and a Barrovian-type metamorphic zoning. Detrital zircon data from a lower succession point to maximum depositional ages similar to passive deposits of the precursor basin (Macaúbas; ca. 819 Ma). Detrital ages obtained from an upper succession indicate main provenance from the Rio Doce arc (ca. 600 Ma). A metamorphic age of 560 ± 20 Ma was obtained for the sheared top of the basement and a crystallization age of 544 ± 10 was obtained for the intrusive Santa Rosa granite, which are both interpreted to represent the syn-collisional stage of the Brasiliano orogeny. This led to the suggestion by Peixoto et al. (2015) that the basin closure during the assembly of Western
Gondwanaland lasted close to the Ediacaran-Cambrian boundary, i.e. approximately 20 Ma later than previously suggested.
Granitoids
age from 630-580 Ma (Pedrosa-Soares et al. 2007, 2011). The G2 Supersuite comprises S-type
granitoids that crystallized and were deformed during a syn-collisional stage of the Brasiliano orogeny that have ages of 585-560 Ma (Pedrosa-Soares et al. 2007, 2011). S-Type G3 granitoids were formed in
a late to post-collisional stage of the Araçuaí orogeny (545-520 Ma; Pedrosa-Soares et al. 2007, 2011).
S-type G4 granitoids (535-500 Ma) and I-type G5 granitoids (520-480 Ma) were formed during a post-collisional stage of the orogeny and represent the lateral scape of the southern segment and the gravitational collapse of the AO (Alkmim et al. 2006, Pedrosa-Soares et al. 2007, 2011).
Jequitinhonha and Nova Venécia Complex
The Jequitinhonha and Nova Venécia are paragneiss complexes that border the eastern limits of the Araçuaí Orogen from north to south, respectively. It is believed that the protoliths of the Nova Venécia Complex have been deposited in a barc-arc basin, whereas the Jequitinhonha Complex records a distal passive margin sedimentation that would be correlative to post-glacial Macaúbas sequences (Noce 2004, Gradim 2013, Vieira 2007, Gonçalves-Dias et al. 2011).
The Jequitinhonha Complex is located in the northeast portion of the Minas Gerais State, Brazil. It consists of peraluminous kinzigitic migmatitic paragneisses intercalated with quartzite, graphite gneiss and calcsilicate rocks. Detrital zircon ages indicate provenance from: the São Francisco-Congo Craton basement; the Espinhaço-Chapada Diamantina; and the rift-related rocks from Noqui-Zadinian-Mayumbian-Salto da Divisa rift system. The Jequitinhonha Complex is interpreted as a passive margin deposit of the precursor basin of the Araçuaí Orogen, younger than the glaciation recorded in the Macaúbas Group (Gonçalves-Dias et al. 2011).
Silva et al. (1987) first included the Nova Venécia Complex as part of a larger complex, known
as Paraíba do Sul, which was interpreted as a volcano-sedimentary migmatitic and granitic Proterozoic sequence (Féboli 1993a, 1993b). Tuller (1993) divided this complex into a proximal, an intermediary and a distal marine domain. The first one (to the west) would comprise aluminous gneisses, quartzites and calc-silicate rocks. The second one comprised marble and amphibolitic sequences, which was interpreted to represent a relative proximity to a basic magmatic source (Signorelli 1993). The third one (to the east) consisted of major aluminous gneisses and calc-silicate rocks, with minor quartzite and amphibolites. This distal domain was denominated Nova Venécia Complex by Pedrosa-Soares et al.
(2006). The NVC comprises migmatitic and granulitic paragneisses rich in biotite, garnet, cordierite and/or sillimanite, and calc-silicate rocks (Roncato 2009). Detrital zircons in these rocks yield ages of 631±19 Ma (Noce et al. 2004), 608 ±18 Ma (Pedrosa-Soares et al. 2008) and ca. 590 Ma (Gradim
Salinas Formation
The Salinas Formation is a metasedimentary unit comprising meta-arenites, metapelites and metaconglomerates metamorphosed under greenschist to amphibolite conditions. This unit lies discordantly onto the Macaúbas Group, to the west, and is intruded by Neoproterozoic and Cambrian granites, to the east. It records the development of a syn-orogenic basin (flysch) between a passive margin and an orogenic front (Santos et al. 2009).
2.2 -
ANATOMY OF THE ARAÇUAÍ-WEST CONGO OROGEN
There are nine main structural domains in the Araçuai West Congo Orogen that differ from each other in terms of style, orientation, deformation history and shear sense (Figure 6). They have been described by Alkmim et al. (2006), based on a synthesis from numerous individual studies, and represent
an image of the general kinematic that took place during the Brasiliano Pan-African Orogeny.
The Serra do Espinhaço Fold-Thrust Belt
It encompasses the Archean basement and units from the Espinhaço Supergroup and Macaúbas Group. This domains consists of a 700 km west verging fold-thrust belt bordering the eastern limits of the São Francisco Craton.
The Chapada Acauã shear zone
Chapada Acauã is a 50 km wide x 150 km long plateau that lies on the east side of the Serra do Espinhaço. The rocks that underlie this plateau comprise the hanging wall of a regional east-dipping shear zone, the Chapada Acauã Shear Zone (CASZ; Marshak et al. 2006).
The Minas Novas corridor
This domain is characterized as a 30 km wide and 150 km long dextral strike-slip shear zone that cuts Macaúbas Group metapelites.
The Rio Pardo salient
This salient was traced out by the internal trend-lines of the Serra do Espinhaço fold-thrust belt in a regional-scale convex-to-the-foreland curve (Cruz and Alkmim 2006). It involves rocks within the Espinhaço Supergroup and the Macaúbas Group.
This domain presents a NNW trend marking the limit between the northeast and the southeast lobe of the São Francisco Craton. The structure was initially formed by the Espinhaço rifting event and reactivated during the Macaúbas rifting event. Subsequently, the segment was partially inverted during the Brasiliano Pan African Orogeny (Danderfer e Dardenne 2002).
The Guanhães Basement Block
The Guanhães Block is exposed southeast of the Serra do Espinhaço fold-thrust belt. It consists of Archean TTG gneisses, migmatites that have ages ranging from 2.8 – 2.7 Ma (Silva et al. 2002).
Northwards, a shallowly dipping shear zone juxtaposes Macaúbas Group strata onto the Guanhães basement, recording reverse-sinistral and normal-dextral distinct phase motions (Peres et al. 2004). The
west margin of the Block is a foreland-(west)-verging thrust, where the Guanhães basement lies onto the Espinhaço Supergroup strata. In the southeastern margin of the Guanhães Block, the Dom Silvério shear zone juxtaposes Rio Doce metapelites to the east against the basement to the west.
The Dom Silvério shear zone and adjacent structures
The Dom Silvério shear zone is a 100 km long, 4 km wide, steeply dipping belt of mylonitic rock that terminates against the dextral strike-slip Abre Campo shear zone at its northern end. The shear zone has a NNE-trending and a dominantly sinistral motion (Peres et al. 2004).
The Itapebi shear zone
The Itapebi is a NW-trending shear zone that is characteristic of an overall dextral transpressive system at the northern edge of the Araçuaí Orogen. It affects the Archean basement, the Salto da Divisa Suite and the Jequitinhonha Complex.
The High grade internal zone
The high grade internal zone comprises the granite Supersuites (G1 to G5) and the paragneisses complexes (Jequitinhonha and Nova Venécia complexes). This internal zone consists of two distinct sub-domains, from a structural standpoint. The northern one contains west- and east- verging thrust sense shear zones, whereas the southern subdomain contains a system of dextral-transpressional shear zones. One of the most representative of these shear zones is known as Abre Campo shear zone, a major geophysical and structural discontinuity that can be traced for 300 km. It is interpreted to represent either a Paleoproterozoic suture (Cunningham et al. 1998; Brueckner et al. 2000) or a Brasiliano suture
developed during closure of the oceanic portion of the Macaúbas basin (e.g. Peixoto et al. 2015). The
basement: the Archean Mantiqueira Complex and the Paleoproterozoic Juiz de Fora Complex, metamorphosed in amphibolite and granulite facies, respectively. Support for the second proposal comes from the fact that the Neoproterozoic Rio Doce arc occurs just to the east of the Abre Campo shear zone, south of 18°S latitude.
The West Congo Belt
The Cretaceous South Atlantic rifting split the AWCO into the Araçuaí Orogen in Brazil and its complimentary part in West Africa (Gabon, Congo and Angola), the West Congo Belt. The West Congo Belt is a single NW-trending and ENE-verging fold-thrust belt that borders the west margin of the Congo craton (Alkmim et al. 2006). It includes thrust slices of Archean and Paleoproterozoic basement, as well
as a thick pile of Neoproterozoic metavolcanic and metasedimentary units (Alkmim et al. 2006).
Figure 6- The main structural domains in the Araçuaí-West Congo Orogen: 1) the Serra do Espinhaço Fold-Thrust
Belt; 2) the Chapada Acauã shear zone; 3) the Minas Novas Corridor; 4) the Rio Pardo Salient; 5) the Guanhães Basement Block; 6) the Dom Silvério shear zone; 7) the Itabepi shear zone; 8) the high-grade internal zone; and 9) the West Congo Belt. Modified from Alkmim et al. (2007).
2.3 -
THE HIGH GRADE ANATECTIC AND CRYSTALLINE CORE OF THE
OROGEN
The study area within the NVC is inserted in the high grade zone of the Araçuaí Orogen, comprising metasedimentary and arc-related rocks, including pre- to post-collisional granitoids. The relevant units for this study are: (1) the Rio Doce Group, comprising an arc-related metavolcano-sedimentary sequency (Vieira 2007, Peixoto et el. 2015); (2) the Nova Venécia Complex, which consists of migmatitic and granulitic metasedimentary rocks whose protoliths have been deposited in back-arc basin (Gradim et al. 2014); and (3) Granitoid Supersuites G1 (arc-related pre-collisional granitogenesis),
The Rio Doce Group
The Rio Doce Group records components of a volcano-plutonic-sedimentary system, accreted to an active continental margin during a pre-collisional stage of the Araçuaí West-Congo Orogen (ca. 630-585 Ma; Vieira 2007). According to Vieira (2007), field and geographic relations and geochemical similarities between the metavolcanic rocks of the Rio Doce Group and the G1 Supersuite indicate that the former unit represents the upper-crustal section of the Araçuaí Orogen and the latter the plutonic segment of the AO magmatic arc. One of the basins related to this arc would have evolved to form the Nova Venécia Complex. Figure 9 shows a model of a paleogeographic profile reconstruction of these units. From base to top, the Rio Doce Group comprises the Tumiritinga (mica-schist, gneiss and volcanoclastic rocks), São Tomé (metagraywacke, mica-schist and meta-dacite), Palmital do Sul (mica-schist and gneiss) and the João Pinto (quartzite) Formations. The following description of these units is based on Vieira (2007).
The Palmital do Sul Formation is composed by schists and gneisses containing minor lenses of quartzite and calc-silicate rocks. The first metavolcanic rock ever observed in the Rio Doce Group lies intercalated with schists within this formation (Vieira 2007). From compositions and textural observations, this rock is classified as a dacitic tuff (metapyroclastic rock). The pyroclastic rocks yield ages of ca. 585 Ma.
The Tumiritinga Formation consists of schists intercaled with calc-silicate rocks and of metavolcanoclastic rocks. P-T quantitative estimations in two samples using THERMOCALC show values of T = 468 ± 50 ºC; P = 4,96 ± 1 kb and T = 638 ± 76 ºC; P =4,56 ± 1 kb. That constrains metamorphic conditions from green-schist/amphibolite transition to upper amphibolite facies (Castañeda et al. 2007). The protoliths of the schists are interpreted to be graywacke pelites, which
suggests a source with granodiorite-tonalite-diorite composition. It is suggested that the metavolcanoclastic rocks represents a marine distal deposition in relation to the metapyroclastic rock in the Palmital do Sul Formation. Yet, both formations would be correlatives, as their rocks yield approximately the same ages (ca. 585 Ma).
The São Tomé Formation are non-anatectic rocks consisting of schists with variable amounts of quartz, mica and plagioclase. The protoliths of this rocks have been deposited in a marine environment (deep continental shelf to slope), with deposition controlled by the magmatic arc. U-Pb TIMS detrital zircon analysis yield ages of ca. 594 Ma in arenites within this unit. This age differs from the maximum age of deposition for the Nova Venécia protoliths (next section) suggested by Noce et al. (2004) at 631
Tomé Formation.
The João Pinto Formation consists of pure, micaceuous and/or feldspatic quartzite. Its protoliths are interpreted to be quartz-arenites with small clay fractions. The mature sedimentary strata from this formation covers the arc-related volcanic-sedimentary strata from all other formations in the Rio Doce Group.
Figure 7- Paleogeographic profile reconstruction of the Rio Doce magmatic arc (Modified from Vieira 2007).
The Nova Venécia Complex
The NVC consists essentially of peraluminous migmatites, granulites and of calc-silicate rocks (Pedrosa-Soares et al. 2006). The main rock-types in the NVC are cordierite-rich granulites and biotite
+ garnet ± cordierite ± orthopyroxene ± sillimanite migmatitic paragneisses that record a foliation developed during the main ductile deformation (Roncato 2009, Gradim 2013). Three main field relations between the NVC and other units within the AO core are reported: (1) A transitional contact observed between biotite-garnet gneisses (NVC) and the Ataléia granitic Suite (G2 Supersuite) is interpreted to reveal anatexis increase in biotite-garnet gnaisses from granite-free sites to granitic sites (i.e. Ataléia Suite) bearing calc-silicate and gneiss xenoliths (Roncato 2009); (2) The NVC is interpreted to represent a roof-pendant in relation to the Carlos Chagas Batolith (G2 Supersuite) northwest from the area in his study (Roncato 2009); and (3) I-type post-collisional plutons (G5 Supersuite) intrude the NVC and all other units in the study area (e.g., Gradim et al. 2014).
The NVC high-grade rocks have a foliation parallel to the migmatitic layers, being syn-cinematic to the main ductile regional deformation (Pedrosa-Soares et al. 2006). Most commonly,
leucossomes. Mesossome and melanossome layers/schlieren are composed of biotite, feldspar, quartz, garnet, cordierite, sillimanite and/or hercynite. Restitic rocks are considered to be sillimanite-cordierite-garnet-biotite gneisses, spinel-sillimanite-cordierite-biotite gneisses, cordierite granulite and calc-silicate rocks (interpreted to truly represent the paleossome; Roncato 2009). All anatectic processes are believed to relate to syn-collisional granitogenesis and generation of the G2 Supersuite (e.g. Ataléia Suite) (Pedrosa-Soares et al. 2007, Gradim 2013).
Altough there is a lack of more detailed studies on the NVC metamorphism, some studies report that the paragneisses were metamorphosed under high-grade conditions, reaching high amphibolite facies to granulite facies (Table 4; e.g. Munhá et al. 2005, Gradim 2013). Qualitative estimations from
the paragenesis Qtz+Pl+Kfs+Bt+Sil+Grt+Crd±Opx, Qtz+Pl+Kfs+Bt+Sil+Grt+Crd±Opx, Qtz+Pl+Kfs+Bt+Crd±Sp, Qtz+Pl+Kfs+Bt+Sil+Grt+Crd+Sp±Opx (for biotite-garnet gneisses) and Qtz+Kfs+Pl+Crd+Bt±Sil±Sp (for cordierite granulites) point to metamorphism under amphibolite to granulite facies transition. P-T conditions estimated from semi-quantitative thermobarometry using those assemblages are 5 to 6.1 kbar and 712º to 930º C (Gradim 2013). Munhá et al. (2005) suggests
that the maximum P-T metamorphic conditions were 6.5 ± 0.5 kbar, 820 ± 30 ºC. The suggested age of peak metamorphism in the NVC is 575 Ma, according to Söllner et al. (2000), de Campos et al. (2004)
and Gradim et al. (2014). In the northern Rio de Janeiro State (RFB), Bento dos Santos et al. (2011)
suggested peak metamorphism for high-grade rocks at 850 ± 50◦C and 8 ± 1 kbar, occurring simultaneously to the formation of high-temperature deep thrusts and shear zones, followed by clockwise P-T-t retrograde path with cooling and decompression to ca. 500°C and 5 kbar.
Table 4- Temperature and pressure metamorphic conditions in which Nova Venécia rocks must have equilibrated
according to literature.
Rock-type Locality Temperature (ºC) Pressure (kBar) References
Paragneiss Between Colatina and Santa Tereza 820 ± 20 6.5 ± 0.5 Munhá et
al. (2005)
Paragneiss Minas and Mantena Between Central de 725 ± 35 4.43 ± 0.46 (2007) Vieira
Paragneiss Between Ecoporanga and Guarapari 712 to 930 5 to 6.1 Gradim (2013)
Gradim (2013) and Gradim et al. (2014) suggest that the NVC protoliths were greywacke
sediments enriched with a peraluminous pelitic fraction and plagioclase. Geochemical signatures reported in studies from Vieira (2007), Pedrosa Soares et al. (2008) and Roncato (2009) indicate that
those protoliths have been deposited in a backarc region. U-Pb detrital zircon ages from the NVC corroborates that proposition, considering that the pre-collisional stage of the AO (630 to 585 Ma) encompasses the presumably arc-related G1 plutonism (e.g., Pedrosa-Soares et al. 2006). Noce et al.
sedimentation age) and 774 ± 13 Ma, 2104 ± 12 Ma (other sources). Gradim et al. (2014) constrains the
maximum depositional age range for the NVC from 590 to 641 Ma and other sources at 733-810 Ma, 901 Ma and 2086-2124 Ma.
Granitoid Supersuites
Several studies present relevant data on the petrology, structural geology, geochemistry and geochronology of the granitic magmatism in the Araçuaí Orogen, bringing many new insights on the evolution of the orogeny. Inicially, plutonism in the orogen was divided into six main suites: G1, G2, G3S, G3I, G4 e G5 (Pedrosa-Soares & Wiedemann-Leonardos 2000; Pedrosa-Soares et al. 1999, 2001;
Silva et al. 2005). De Campos et al. (2004) and Pedrosa-Soares et al. (2006) regrouped those suites into
G1, G2, G3, G4 e G5. Pedrosa-Soares et al. (2011) suggested a substitution from the term Suite to
Supersuite according to the specific geotectonic significance of each group of granitoids. According to Pedrosa-Soares et al. (2011), this would avoid confusion arising from the innumerous names given
locally to batoliths and small granitic bodies. The main characteristics of the granitic magmatism of the Araçuaí orogen and its genesis is synthesized in Table 5.
The G1 Supersuite
Pedrosa-Soares et al. (2007) suggest that the magmatic arc of the Araçuaí-West Congo Orogen
is represented by the volcanic rocks within the Rio Doce Group and by the plutonic G1 Supersuite. Therefore, the G1 Supersuite is interpreted to have formed during a pre-collisional stage of the Araçuaí orogeny, from 630-585 Ma (Table 6; Pedrosa-Sares et al 2007, 2011). It consists of calc-alkaline, metaluminous to slightly peraluminous, I-type granitoids that evolved during the pre-collisional stage of the Araçuaí Orogen (Noce et al. 2000a). Litogeochemistry and isotopic data (Nd epsilon between -5
and -13, and TDM between 1,2 e 2,2 Ga) suggest that G1 is an expanded calc-alkaline suite, representing a magmatic arc in an active continental margin, with hybrid geochemical signature and predominant contribution from crust over mantle (Nalini 2000, Noce et al. 2000b, Pedrosa-Soares &
Wiedemann-Leonardos 2000). The G1 batholiths and stocks are mainly tonalities and granodiorites, with minor dioritic and mafic facies bearing xenoliths of metassedimentary rocks. They record a regional foliation (many times mylonitic) imprinted in a collisional stage of the orogeny.
The G1 Supersuite encompasses numerous plutons locally known as: Alto Capim, Brasilândia, Cuité Velho, Derribadinha, Divino, Estrela-Muniz Freire, Galiléia, Guarataia, Manhuaçu, Mascarenhas-Baixo Guandu, Muriaé, São Vitor, Teófilo Otoni, Valentim, Rancho Alegre e Topázio (Pedrosa-Soares & Wiedemann-Leonardos 2000; Pedrosa-Soares et al. 2001, 2011; Vieira 2007; Novo 2013).
