Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185
Comunicações Geológicas (2014) 101, Especial I, 157-161
IX CNG/2º CoGePLiP, Porto 2014
ISSN: 0873-948X; e-ISSN: 1647-581X
40
Ar/39Ar age of Mendanha Alkaline Intrusion, Rio de
Janeiro, Brazil
Idade 40Ar/39Ar da Intrusão Alcalina do Mendanha, Rio de
Janeiro, Brasil
C. E. Mota1*, M. C. Geraldes2, F. Jourdan3
Artigo Curto
Short Article
© 2014 LNEG – Laboratório Nacional de Geologia e Energia IP
Abstract: The Mendanha Alkaline Intrusion is one of many alkaline
occurences of the Southeastern Brazil Alkaline Province. It
comprises plutonic rocks (alkali-syenites and syenites) and
hypabyssal types (trachytes and ignibrites). In this work is presented
new 40Ar/39Ar ages for this massif. The last reported ages were about
74 My and 65 My by K/Ar method published in 1988. The obtained
results show ages of about 64 My (64.12 ± 0.40, 64.04 ± 0.42) e 58
My (58.55 ± 0.45, 57.95 ± 0.69), which are correlated to neighbor
magmatic intrusions.
Keywords:
Mendanha,
Geochronology.
Alkaline
magmatism,
Argon,
Resumo: A intrusão alcalina do Mendanha é uma das diversas
ocorrências alcalinas da Província Alcalina do Sudeste Brasileiro. As
rochas constituintes compreendem tipos plutônicos (alcali-sienitos e
sienitos) quanto hipoabissais (traquitos e ignibritos). Neste trabalho
são apresentados novas idades 40Ar/39Ar para o maciço. As últimas
idades registradas são de 73 Ma e 65 Ma, pelo método K/Ar em
1988. Os resultados obtidos apresentam idades de aproximadamente
64 Ma (64,12 ± 0,40, 64,04 ± 0,42) e 58 Ma (58,55 ± 0,45, 57,95 ±
0,69), correlatas com intrusões magmáticas vizinhas.
Palavras-chave:
Geocronologia.
Mendanha,
Magmatismo
alcalino,
Argonio,
1
Division of Geochemistry, CPRM/Geological Survey of Brazil.
Faculty of Geology, Rio de Janeiro State University, Brazil.
3
Department of Applied Geology, Curtin University, Australia.
*
Corresponding author / Autor correspondente: [email protected]
2
et al., 1997) or (3) a combination of these two models
above (Fainstein & Summerhayes, 1982).
The Mendanha Alkaline Intrusion (Klein, 1993; Mota
et al., 2012) is one of the Southeastern Brazil Alkaline
Province main occurrences and is located between the
cities of Nova Iguaçu, Mesquita and Rio de Janeiro
(22°48’S, 43°31’W). This NE-elongated ellipsoidal stock
(18 km x 7 km) is parallel with the main structural
lineaments of Ribeira Fold Belt (Heilbron et al., 2004) and
it is one of the major alkaline occurences of Rio de Janeiro
State. The Mendanha Alkaline Intrusion has maximum
altitude of 850 m, and it is surrounded by 5-45 m the
Baixada Fluminense plain areas.
2. Objectives
The objectives of this work consist in present new
Ar/39Ar thermocronological ages obtained in the
Mendanha Alkaline Intrusion. The last ages obtained for
this alkaline intrusion were 73 My and 65 My, with K/Ar
method. These ages were published by Sonoki & Garda
(1988), whose recalculates older data from Amaral et al.
(1967) and Cordani & Teixeira (1979) with decay
constants of Steiger & Jagger (1977).
40
3. Geology of Mendanha alkaline intrusion
1. Introduction
The Rio de Janeiro state has a flat area, which is known as
the Baixada Fluminense, that lies between hilly and coastal
region, where are observed some residual reliefs,
associated with Cretaceous and/or Tertiary alkaline stocks.
This alkaline magmatism comprises the Southeastern
Brazil Alkaline Province (Almeida, 1983), or either Serra
do Mar Province (Thompson et al., 1997). The generation
of Southeastern Brazil Alkaline Province’s related
intrusions is described as (1) tectono-reactivation model
(e.g. Almeida, 1983), based fundamentaly on structural
studies; (2) Mantle Plumes and/or Hot-Spots (e.g. Gibson
The Alkaline Intrusion Mendanha (Fig. 1) has mostly
trachytic to phonolitic composition, consisting of plutonic
and hypabyssal rocks. It has an elliptical shape, which is
parallel with reactivated structural lineaments of Ribeira
Fold Belt.
The contact between the intrusion and gneisses of the
Costeiro Domain (Ribeira Fold Belt) is characterized by
formation of intrusive breccias, ranging between 50 and
200m thick. The breccias have characteristic structures of
volumetric expansion due to magma pressure itself,
thermal differentiation and action of volatiles (Mota &
Geraldes, 2006). The magmatic breccias also have
compositional zoning of fragments.
158
The constituent rocks correspond to alkali-syenites and
syenites with equigranular and porphyritic textures.
Mineralogy is usually composed by sanidi---ne, orthoclase,
amphibole and/or biotite and has accessory minerals such
as pyrite and chalcopyrite and rarely, barite (Menezes &
Klein, 1973).
Trachytic rocks are observed mainly near the contact of
the intrusion, interspersed with pyroclastic breccias and in
dikes. These trachytes has colors from light to dark gray,
with aphyric texture, sometimes with disseminated
sulphides and less than 1 cm amygdales. It is also common
to find typical trachytic texture and magmatic flow
features.
The alkaline dikes generally are composed by trachytes
and rarely lamprophyres with regular NE-SW direction
with near vertical dip, parallel to the lineaments inside the
alkaline stock, which allows to associate these structures.
By relative dating it is observed that lamprophyric dikes
are younger than the trachytic ones.
To the northeast, the dyke directions exhibit a random
pattern, which can be associated with differential efforts and
in addition to the presence of pyroclastic rocks of different
Fig. 1. Geological map of Mendanha Alkaline Intrusion.
Fig. 1. Mapa geológico da Intrusão Alcalina do Mendanha.
C. E. Mota et al. / Comunicações Geológicas (2014) 101, Especial I, 157-161
granulations with approximately concentric distribution, this
area is known as the “Nova Iguaçu Volcano”.
The ignimbrites are irregularly distributed over the
massif. Generally consist of fragments of trachyte,
sometimes little amygdales filled by sulfides, carbonates,
fluorite and others. The angular fragments shows little or
no rounding, that indicates absence or low intensity of
transport, while in other outcrops, the presence of
rounded fragments may indicate underwater reworking or
abrasion. The matrix is formed by lapilli and ash in
variable proportions, the structure of these breccias can
be supported by matrix or fragments depending on the
distance between the source and site of deposition.
According (Silveira et al., 2005), the Mendanha
Alkaline Intrusion could be considered as a highly
explosive
cretaceous
volcano
and
composed
predominantly by volcanic ash and pyroclastic rocks. The
current condition of residual relief is consistent with the
exposure of chamber magma, pyroclastic conduits and
feeder dykes, verified by geophysical (e.g. Mota et al.,
2012) and volcanological studies (e.g. Motoki et al.,
2008).
40
Ar/39Ar age of Mendanha Alkaline Intrusion, Rio de Janeiro
4. Methodology
The analytical procedures were performed at Laeter John
Centre for Isotope Research, Curtin University (Australia).
K and Ar isotope abundances were obtained by mass
spectrometry. The procedures include neutron activation to
transform 40K into 39Ar and step-heating fusion of the
whole sample.
After the analytical process, two values are obtained
for ages: (1) age-plateau, using criteria of Lamphere &
Dalrymple (1976) and (2) isochron age, with isotopic data
extracted from each step of fusion of the same sample.
Correction for atmospheric Ar (40Ar/36Ar = 295.5) was also
performed. It is presented in conjunction with the plateau
ages diagram, the ratio K/Ca diagram associated with each
heating step. These indices help to indicate which mineral
phases can be degasified at certain temperatures.
The probability of success of the plateau/isocronic ages
is done by examining the Pearson’s chi-square (X2)
distribution (Wendt & Carl, 1991). This statistical method
uses as input parameters the values of MSWD and the
number of steps that define the plateau (one degree of
freedom), or the points that define the isochron age (two
degrees of freedom).
159
The age plateau defined is 64.12 ± 0.40 My (41%
probability), calculated at 95% of 39Ar released. The
MSWD calculated for the analysis is 1.03, value
considered acceptable. The K/Ca ratio obtained from each
fusion step shows that the first 65% of the sample values
indicates higher relative concentration of K. From this
point until total consumption of the sample, the ratio K /
Ca remains constant. Thus, it is possible to suggest that
there is a compositional zoning in biotite, where the center
of the mineral is depleted in K in comparison to mineral
borders.
Table 2. Analytical data for sample VNI-01-wr (whole-rock in
lamprophyre dyke).
Tabela 2. Dados analíticos para a amostra VNI-01-wr (rocha total em
dique de lamprófiro).
5. Results
Two samples were selected for dating by the 40Ar/39Ar
method by step-heating fusion. For alkali-syenite
lithofacies, it was collected a grain of biotite (VNI-bio). A
lampropryre dike sample (VNI-01-wr) was analyzed for
whole-rock. The results obtained for the construction of
diagrams are shown in table 1 and table 2.
Table 1. Analytical data for sample VNI-bio (biotite in alkali-syenite).
Tabela 1. Dados analíticos para a amostra VNI-bio (biotita em AlcaliSienito).
The values of isochronic age was calculated in 64.04 ±
0.42 My (55% probability), with MSWD of 0.90. The
initial 40Ar/36Ar ratio, calculated by method of least
squares regression, is 306.5 ± 14.0, which indicates a little
excess of 40Ar isotope, associated with presence of
atmospheric argon, but sufficient to make the obtained age
with geological significance.
Fig. 2. On the left side, age Plateau diagram, with corresponding K/Ca
ratio of the sample VNI-bio. On the right, Isochronic diagram obtained
with these steps. Analytical data available on table 1.
The sample VNI-bio (Fig. 2) shows plateau age
diagram with homogeneous steps in approximately the last
95% of the 39Ar released. Although the first 5% of fusion
was observed a slight gain of radiogenic argon, due to
excess argon in the sample, probably acquired in contact
with host rocks, which are alkaline or not.
Fig. 2. À esquerda, diagrama de idade-platô, com as correspondentes
razões K/Ca da amostra VNI-bio. À direita, diagrama isocrônico obtido
a partir dos valores de cada etapa de fusão. Dados analíticos
disponíveis na tabela 1.
The sample VNI-01-wr (Fig. 3) shows plateau age
diagram with homogeneous steps in approximately the last
160
65% of 39Ar. The first 35% of the fusion is observed a
slight gain of radiogenic argon.
The age plateau calculated is 58.55 ± 0.45 My (23%
probability), obtained at 65% of 39Ar released. The MSWD
for this analysis is 1.36, value considered acceptable. The
K/Ca ratio obtained shows that the first 11% and last 60%
of the 39Ar released, the values remains less than 1,
indicating greater relative concentration of Ca, associated
with a higher concentration of minerals such as amphibole
and plagioclase. Between 11 and 40%, the values indicate
greater relative concentration of K, related to K-feldspar
and possibly biotite.
The first 35% of the fusion process define a "plateau"
of older ages, denoting excess of radiogenic argon.
Possibly, the excess 40Ar observed could be associated
assimilation of alkaline host rocks at the intrusion moment.
The main plateau is defined in last steps and the calculated
age could have geological significance.
The isochronic age obtained was 57.95 ± 0.69 My
(20% probability), with MSWD with value of 1.30. The
initial 40Ar/36Ar ratio is 527.7 ± 91.0, which indicates
excess of 40Ar isotope, associated with the presence of
atmospheric argon. This value is considered high and age
had to be corrected.
Fig. 3. On the left side, age Plateau diagram, with corresponding K/Ca
ratio of the sample VNI-01-wr. On the right, Isochronic diagram
obtained with these steps. Analytical data available on table 2.
Fig. 3. À esquerda, diagrama de idade-platô, com as correspondentes
razões K/Ca da amostra VNI-01-wr. À direita, diagrama isocrônico
obtido a partir dos valores de cada etapa de fusão. Dados analíticos
disponíveis na tabela 2.
6. Discussions and conclusions
The age of crystallization/cooling attributed for the
Mendanha Alkaline Intrusion is measured in 64.12 ± 0.40
(plateau) and 64.04 ± 0.42 (isochronic) My, with
respective probability of 42 and 55%. These ages are
statistically considered reasonable to good, since both
have slight excesses of radiogenic argon, associated with
inhomogeneity in the distribution of argon in the sample.
The lamprophyre dyke was dated in 58.55 ± 0.45 My
(plateau) with probability of 23% and 57.95 ± 0.69 My
(isochronic), is considered with regular quality. This is
justified by the analysis be done on whole-rock sample,
which according decreases the effectiveness of
interpretative analytic techniques. Due to field
relationships and significant difference in ages of
C. E. Mota et al. / Comunicações Geológicas (2014) 101, Especial I, 157-161
intrusive rock and syenitic host rock (greater than 5 My),
this obtained age has geological significance.
In comparison with ages obtained in previous studies
and other rocks of the Southeastern Brazil Alkaline
Province and field relations, it is clear that Mendanha
Alkaline Intrusion had at least three distinct magmatic
events. In other point, some other alkaline bodies have
contemporary ages with one of these magmatic events:
(1) 73 My, age not found in this study (Rio Bonito,
Morro Redondo). (2) 65-62 My, (Tinguá, Morro de São
João) and (3) 58 My (Itaúna, Cabo Frio).
The ages obtained define an important magmatic
event in southeastern of Brazil and may be interpreted as
mantelic activity due deep crustal faults formed during
Cenozoic reactivation of South America Platform.
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