Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185 Comunicações Geológicas (2014) 101, Especial I, 81-84 IX CNG/2º CoGePLiP, Porto 2014 ISSN: 0873-948X; e-ISSN: 1647-581X

Chemical composition and genesis of the clinohumites from marbles of Itaoca-Gironda, Espírito Santo State, Brazil Composição química e gênese das clinohumitas dos mármores de Itaoca-Gironda, Estado do Espírito Santo, Brasil M. L. S. Fernandes1, A. Oliveira Chaves1*

© 2014 LNEG – Laboratório Nacional de Geologia e Energia IP

Abstract: Humite-bearing 650-500 Ma high-grade marbles in southernmost India, South Madagascar, Sri Lanka, East Antarctica, Argentina, and Brazil have been reported in geological literature. These occurrences suggest an intercontinental 'humite-epoch' in Gondwanaland coeval with the waning stages of Pan-African/Brasiliano tectonothermal event. With a chemical formula 4[Mg2SiO4][(Mg,Ti,Fe)(OH,F)2], Brazilian hydroxylclinohumites were formed at the expense of forsterite during retrograde amphibolite facies metamorphism, when fluorinated water-rich fluid activity took place. Considering that fluorine is responsible by clinohumite genesis and stability and assuming a pressure around 2 kbar, it is possible to estimate that such mineral crystallized around 600-650ºC.

Keywords: Clinohumite, Pan-African/Brasiliano, Gondwana. Resumo: Mármores de alto-grau metamórfico de 650-500 Ma contendo humitas têm sido relatados na literatura geológica em várias localidades como na Índia meridional, ao sul de Madagascar, Sri Lanka, leste da Antártida, Argentina e Brasil. Estas ocorrências sugerem uma “época da humita” intercontinental no Gondwana contemporânea com os estágios finais do evento tectonotermal Pan-African/Brasiliano. Com uma fórmula química 4[Mg2SiO4][(Mg,Ti,Fe)(OH,F)2], as hidroxiclinohumitas brasileiras se formaram às custas da forsterita durante metamorfismo retrógrado de fácies anfibolito, a partir da atividade de um fluido fluorado e rico em água. Considerando que o flúor é responsável pela formação e estabilidade da clinohumita e estimando uma pressão da ordem de 2 kilobars, é possível aproximar sua temperatura de cristalização em torno de 650°C.

Palavras-chave: Clinohumita, Pan-Africano/Brasiliano, Gondwana. 1Centro de Pesquisas Professor Manoel Teixeira da Costa (CPMTC), Instituto de Geociências, Universidade Federal de Minas Gerais (IGC-UFMG) – Brazil. *Corresponding author / Autor correspondente: alochaves@yahoo.com.br

1. Introduction

The minerals of the humite group have the general formula n(M2SiO4).M1−xTix(OH,F)2−2xO2x, where M includes the octahedrally coordinated cations Mg, Fe, Mn, Ca, Zn in decreasing order of abundance and x< 1. The members of the group are distinguished by the value of n in the formula, where n = 1 for norbergite; 2 for chondrodite; 3 for humite and 4 for clinohumite (Jones et al., 1969). They are extremely

rare yellow to dark orange nesosilicates and form only under high partial pressure of fluorine, in the presence of water-rich fluids (Young & Morrison, 1992).

There are several occurrences of 650-500 Ma humite-bearing marbles in the Kerala Khondalite Belt of southernmost India, South Madagascar, Sri Lanka, Lutzow-Holm Complex and Central Dronning Moud Land of East Antarctica (Pradeepkumar & Krishnanath, 2000; Piazolo & Markl, 1999). In South America they also can be found at San Carlos terrain (Córdoba-Argentina, Guereschi & Martino, 1999) and at Araçuaí-Ribeira Belt (southeastern Brazil, Chaves & Fernandes, 2012), suggesting an intercontinental 'humite-epoch' that resulted of the Pan African-Brasiliano convergence and collision of paleocontinental (“cratonic”) blocks during the Neoproterozoic to Cambrian times, leading to the amalgamation of the West-Gondwana supercontinent (Fig. 1).

This manuscript aims to present the chemical composition of the Brazilian clinohumites from marbles of Itaoca-Gironda region (Espírito Santo State – Brazil), as well as to discuss their formation.

2. Geological setting

The Itaoca-Gironda terrains belong to the Mantiqueira Tectonic Province. According to Valeriano et al. (2011), the Mantiqueira Province is a 3000 km-long orogen that extends in roughly a NE-SW direction along the Atlantic coast of southeastern Brazil and Uruguay as a result of the “Brasiliano”-Pan African (650–500 Ma) tectonothermal event.

The Araçuaí/Ribeira Belt make up the northern portion of the Mantiqueira Province (Fig. 2), which is part of Western Gondwana and continues in Africa as the West-Congo Belt. A complex collision between the São Francisco Craton, now in Brazil, and the Congo/Angolan Craton, now in Africa, drove the evolution of this belt (Pedrosa-Soares & Wiedemann-Leonardos, 2000).

The post-Transamazonian supracrustal rock sequence in the Araçuaí-Ribeira Belt has been attributed to the Paraíba do Sul Complex, which is marked by the presence

Artigo Curto Short Article

82 M. L. S. Fernandes, A. Oliveira Chaves / Comunicações Geológicas (2014) 101, Especial I, 81-84

of abundant banded gneisses, partially migmatized, metamorphosed in the amphibolite to granulite facies transition. It represents sedimentary marine sequences formed in two marine environments: a proximal environment, probably a shallow shelf which received terrigenous siliciclastic material to produce common sandy rocks (presently represented by graywacke-derived gneisses and sillimanite-quartzites) interlayered with thick carbonate layers; and a distal pelite-rich environment with minor carbonate intercalations. Distal pelites gave rise to extensive kinzigitic gneisses with thin calc-silicate lenses (Pedrosa-Soares & Wiedemann-Leonardos, 2000). Small igneous intrusions, consisting of metamorphosed gabbro, pyroxenite, diorite and biotite-andesine granitoids are also found in these terrains.

In this context, Itaoca-Gironda marbles are found as a subvertical NE-SW lens, 10 km in length, inside a NE-SW shear zone which have been active during Pan-African/Brasiliano orogeny. Tupinambá et al. (2007) described them as white marbles, containing calcite and dolomite, with granoblastic texture and showing incipient foliation. These rocks present decimetric dark layers containing silicate minerals that constitutes the subject of the present paper. According to Chaves & Fernandes (2012), micropetrography and X-ray diffraction studies revealed that, besides calcite and dolomite, the marbles contain clinochlore, forsterite, spinel, serpentine (antigorite), diopside, tremolite, and (titan)clinohumite. Forsterite (Fo), spinel (Spl), and diopside (Di) possibly have been formed during granulite facies prograde metamorphism, by the reaction between dolomite (Dol) and clinochlore (Clc): Dol + 2 Clc = 4 Fo + 2 Spl + Di + 2 CO2 + 8 H2O [1] Clinohumite (Chu), serpentine (Srp), and tremolite (Tr) have been formed from forsterite during amphibolite facies retrograde metamorphism, when fluorinated water-rich

fluid activity took place. Micropetrography reveals that clinohumite always occurs replacing forsterite in Itaoca-Gironda marbles (Chaves & Fernandes, 2012). The main retrograde reactions were: 4 Fo + Dol + H2O + Fluorine = Chu + Cal + CO2 [2] (formation of clinohumite) 8 Fo + 13 Cal + 9 CO2 + H2O = Tr + Dol [3] (formation of tremolite) 2 Fo + Cal + CO2 + 2 H2O = Srp + Dol [4] (formation of serpentine)

3. Analytical methods and results

Field work has been done at Itaoca-Gironda in order to collect regional white marble samples, but only those with orange mineral macroscopic incrustations or veins. In order to get full chemical composition of (titan)clinohumite from Itaoca, a polished thin-section was prepared.

Microprobe analyses were performed on a JEOL 8900R in WDS mode at Microanalysis Laboratory of the Universidade Federal de Minas Gerais, Brazil. Element concentrations were calibrated by using natural and synthetic standards (main element line and crystal used, in parentheses): Mg – MgO (Kα TAP), Fe – Mn-Hortonolite (Kα, LIF), Ca – anorthite (Kα, PETJ), Al – YAG (Kα, TAP), Ti – rutile (Kα, PETJ), Si – quartz (Kα, TAP), Mn – rhodonite (Kα, PETJ), and F – fluorite (Kα, TAP). Measuring conditions were 20 nA and 15 kV, measuring times were 10 seconds for all elements except for Mg, Si and F, which were measured for 5 seconds. According to Jones et al. (1969), ferric iron is reported in nearly all of the bulk chemical analyses of humites, but, since its presence cannot be confirmed by microprobe analysis, all iron was considered as FeO. Table 1 shows the average values of the chemical analysis results recalculated on basis of 18 oxygen atoms and 13 cations.

Fig. 1. Gondwana assembly reconstruction, showing location of Pan-African/Brasiliano events and occurrence of humite bearing marbles (adapted from Grunow, 1995). Fig. 1. Reconstrução do supercontinente Gondwana, indicando a localização dos eventos Pan-Africano/Brasiliano e a ocorrência de mármores contendo humita (adaptado de Grunow, 1995).

Clinohumites from Itaoca-Gironda marbles, Brasil 83

Fig. 2. (A): Geological map of the southern Araçuaí-Ribeira Belt and cratonic surroundings, highlighting the Neoproterozoic units (modified after De Campos et al., 2005). 1. Achaean meta-sediments; 2. TTG complexes, greenstone belts and metasedimentary units; Late Paleoproterozoic and Mesoproterozoic: 3. Paleoproterozoic granitoid suite; 4. metavolcanosedimentary unit; 5. Juiz de Fora Complex; 6. Rio Doce Group; 7. Granulite facies domain of Paraíba do Sul Kinzigitic Complex; Late Neoproterozoic to Cambrian granitoid suites: 8. I-type G3-I; 9. S-type G3 and G2 suite. Late Cambrian to Ordovician granitoid suites: 10. I-type G5, including charnockites; 11. High-amphibolite facies domain of Paraíba do Sul Kinzigitic Complex; 12. Phanerozoic covers; 13. Oblique to strike-slip faults or ductile shear zones; 14. Thrust and detachment faults or ductile shear zones. 15. Itaoca-Gironda marbles. Cities: GV - Governador Valadares, PN - Ponte Nova. (B): Simplified tectonic map of Brazil. 1. Phanerozoic basins; 2. Bambuí Group; 3. Neoproterozoic orogens; 4. Cratons (A - Amazon, B - São Luis, C - São Francisco, D - Luis Alves, E - Rio de la Plata). Fig. 2. (A): Mapa Geológico da porção sul da Faixa Araçuaí-Ribeira e arredores cratônicos, destacando as unidades neoproterozoicas (modificado a partir de De Campos et al., 2005). 1. Metassedimentos arqueanos; 2. complexos TTG, greenstone belts e unidades metassedimentares. Tardi-Paleoproterozoico e Mesoproterozoico: 3. Granitoides paleoproterozoicos; 4. unidade metavulcano-sedimentar; 5. Complexo Juiz de Fora; 6. Grupo Rio Doce; 7. Domínio de fácies granulítico do Complexo Kinzigítico Paraíba do Sul. Suítes granitoides tardi- neoproterozoicas a cambrianas: 8. Suite G3 tipo- I; 9. Suites G3 e G2 tipo-S. Suítes granitoides tardi-Cambrianas a ordovicianas 10. Suite G5 tipo-I, incluindo charnockitos; 11. Domínio de fácies anfibolito-alto do Complexo Kinzigítico Paraíba do Sul. 12. Coberturas fanerozoicas; 13. Falhas oblíquas ou zonas de cisalhamento dúctil; 14. Falhas reversas ou zonas de cisalhamento dúctil. 15. Mármores de Itaoca-Gironda. Cidades: GV - Governador Valadares, PN - Ponte Nova. (B): Mapa tectônico simplificado do Brasil. 1. Bacias fanerozoicas; 2. Grupo Bambuí; 3. Orógenos neoproterozoicos; 4. Crátons (A - Amazonas, B - São Luis, C - São Francisco, D - Luis Alves, E - Rio de la Plata).

For the studied clinohumites, SiO2 contents are between 38.63 to 39.72 wt%; TiO2, from 1.69 to 2.48 wt%; MgO, from 54.85 to 57.57 wt%; FeO, from 0.56 to 0.79 wt%; and F, from 1.37 to 2.78 wt%. Al2O3, CaO and MnO are below detection limits.

Mean values for the number of atoms per formula unit are in the right side of Table 1. Si occupies the tetrahedral site (4.00 a.p.f.u) and Mg is assumed to fill the octahedral (M) site in the “forsterite” part of the formula. In the second part of the formula, site M is occupied mainly by Mg-excess (0.741 a.p.f.u.), but also by Ti (0.169 a.p.f.u.) and Fe (0.059 a.p.f.u.), reaching a total of roughly one in this octahedrally coordinated site, which corresponds to the nearly ideal chemical formula 4[Mg2SiO4][(Mg,Ti,Fe)(OH,F)2]. According to Jones et al. (1969), titanium apparently plays a special role in the humite minerals and occupies only the octahedral sites in the (OH, F) region of the clinohumite structure. Although H2O contents were not analyzed, calculated values point to a predominance of OH over F, which leads to classify them as hydroxylclinohumites.

Chemical composition of clinohumites from marbles of Dronning Maud Land (Antarctica) were investigated by

Piazolo & Markl (1999). Compared with clinohumites from Itaoca-Gironda, the Antartic ones contain much more fluorine and iron but they are poorer in titanium and magnesium.

Table 1. Average composition and chemical formula of clinohumite from Itaoca, Brazil.

Tabela 1. Composição química média e fórmula química de clinohumitas

de Itaoca-Gironda, Brasil.

* a.p.f.u. = atoms per formula unit ** H2O content back-calculated from the calculated OH content *** Al, Mn and Ca below detection limits

84 M. L. S. Fernandes, A. Oliveira Chaves / Comunicações Geológicas (2014) 101, Especial I, 81-84

4. Conclusions

Taking into account that fluorine is responsible by clinohumite genesis and stability through the reaction [2] and estimating a pressure around 2 kbar, from the diagram of the figure 3 it is possible to suggest that such mineral crystallized around 600-650ºC.

Fig. 3. Isobaric (2 kbar) temperature versus fluid composition diagram for equilibria in the system CaO-MgO-SiO2-CO2-H2O-HF (Rice, 1980). Mole fraction of F in clinohumite fixed at 0.7. Shaded area marks the stability field for clinohumite (Chu). Fig. 3. Diagrama envolvendo temperatura isobárica (2 kbar) versus a composição da fase fluida para o equilíbrio no sistema CaO-MgO-SiO2-CO2-H2O-HF (Rice, 1980). Fração molar de F na clinohumita fixada em 0,7. A área sombreada marca o campo de estabilidade para a clinohumita (Chu).

In Itaoca-Gironda region, a sedimentary sequence composed of pelites and carbonates underwent high-grade metamorphism which produced partial melting in pelites and metamorphic reactions in the carbonates. Dolomite marbles were produced and behaved like resisters in such environment. Forsterite, spinel, and diopside have been formed during granulite facies prograde metamorphism at the expenses of clinochlore and dolomite. During amphibolite facies metamorphic retrogression, fluorine-rich hydrothermal fluxes from anatexis of pelites attained marbles and promoted the formation of the mineral assemblage composed of serpentine (antigorite), tremolite, and (titan)clinohumite from forsterite. This mineral assemblage is practically the same in all regions over West-Gondwana and the occurrence of humite-group minerals allows the conclusion that Pan-African/Brasiliano

Orogeny was characterized not only by widespread charnockite and granite genesis, but also by concomitant intense fluorinated, water-rich fluid activity, which was transcontinental through West-Gondwana.

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