Chemical composition and genesis of the clinohumites from

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*
Artigo Curto
Short Article
© 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 PanAfrican/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.
1
Centro de Pesquisas Professor Manoel Teixeira da Costa (CPMTC), Instituto
de Geociências, Universidade Federal de Minas Gerais (IGC-UFMG) – Brazil.
*
Corresponding author / Autor correspondente: [email protected]
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 humitebearing marbles in the Kerala Khondalite Belt of
southernmost India, South Madagascar, Sri Lanka, LutzowHolm 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 WestCongo 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
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 PanAfrican/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 ItaocaGironda marbles (Chaves & Fernandes, 2012). The main
retrograde reactions were:
4 Fo + Dol + H2O + Fluorine = Chu + Cal + CO2
(formation of clinohumite)
8 Fo + 13 Cal + 9 CO2 + H2O = Tr + Dol
(formation of tremolite)
2 Fo + Cal + CO2 + 2 H2O = Srp + Dol
(formation of serpentine)
[2]
[3]
[4]
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-SiO2CO2-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, fluorinerich 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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