Short Papers – IV South American Symposium on Isotope Geology
NEW ISOTOPE EVIDENCES CONFIRMING THE EXISTENCE OF A PALEOPROTEROZOIC
TERRAIN IN THE REGION AT THE NORTH OF THE CARAJAS MINERAL PROVINCE
Macambira, M.J.B.1; Silva, D.C.C.2; Barros, C.E.M.3 and Scheller, T.1
1. Laboratório Para-Iso/Universidade Federal do Pará. [email protected]
2. Curso de Geologia/Universidade Federal do Pará. [email protected]
3. Grupo de Petrologia de Granitóides/Universidade Federal do Pará. [email protected]
Keywords: Amazonian Craton, geochronological provinces, Nd isotopes, zircon dating
INTRODUCTION
The geotectonic model of evolution for the
Amazonian craton admitting a sequence of Proterozoic
provinces surrounding an Archean nucleus was first
proposed in the seventies (Cordani et al., 1979), and has
been established as the best appropriated model to explain
the main geological features of the craton. Since then, this
model has been updated and followed, but it is not yet a
consensus (e. g. Lima, 1984; Teixeira et al., 1989;
Tassinari & Macambira, 1999; Santos et al., 2000;
Dall´Agnol et al., 2000).
The limits of these provinces encompass important
changes in timing, tectonic regime and products, and are
key-regions for the understanding of the evolution of the
craton as a whole. However, generally, the geology of
these regions is poorly known, specially the nature and
geographical location of the boundaries. One of these
regions is the boundary between the Archean Carajás
Mineral Province and the Paleoproterozoic domain to the
north (Maroni-Itacaiunas or Transamazonian Province).
Cordani et al. (1984), based on K-Ar and Rb-Sr data,
suggested that it is located nearby the 5° S parallel.
Further mappings, as those improved by CPRM (Oliveira
et al., 1994), show that in this region, the rocks are similar
to those of the Carajás Province, and are, probably,
Archean. Tassinari et al. (2000) keep this limit but they
have introduced some incertitude about its geographical
location. On the other hand, some Archean ages obtained
on granulites and granitoids from the basement of the
Amapá State, as well as on detrital zircons from a
quartzite of the French Guyana (e. g. Lafon et al., 1998)
induced Santos et al. (2000) to propose the boundary of
the Archean domain as located at the central Amapá. For
these last authors, the central-southern part of the Amapá
State and the Carajás region make part of a single
province (Carajás-Imataca), encompassing the Imataca
region, in Venezuela.
This work presents new isotope data for the rocks
outcropping in the region at the north of the Carajás
Mineral Province, Pará State, in order to characterize the
nature of these rocks and provide addicional information
for the establishment of boundary between the Archean
and Paleoproterozoic domains of the southeastern part of
the Amazonian craton.
GEOLOGICAL SETTING
The study region represents a 250 km NW band
linking the Novo Repartimento and Belo Monte (Xingu
river margin) towns, crossed by the Transamazonian
route. The rocks of the region are mainly composed by
granitoids covered, to the north and east, by sedimentary
sequences of the Phanerozoic Amazon and Parnaíba
basins, respectively. To the west, the granitoids seem to
be associated with a set of metamorphic rocks yielding
Rb-Sr ages between 1.9 and 2.0 Ga, and covered by the
volcanic rocks and crosscut by the plutonic rocks of the
Uatumã Supergroup (Santos et al., 1988). As noted
above, the Archean rocks of the Carajás Province occur to
the south, but the location of the limit and the nature of
the relationships between these two domains are not yet
established.
The geology of the study region is relatively
monotonous and composed by granitoids previously
reported by Reis et al. (1974). They are mainly mediumgrained, pink to grey, and monzogranitic to granodioritc
in composition. Locally they show heterogranular and
porphyritic textures, and aplitic veins. Macambira et al.
(2001) reported additional descriptions and propose a
syntectonic regime for the emplacement of these
granitoids based on their structural and textural features.
These authors also reported an age obtained by zircon
evaporation method of 2076 ± 6 Ma (2 sigmas), for a gray
granodiorite sampled 9 km west from Novo Repartimento
town.
ZIRCON GEOCRONOLOGY
Two additional samples of granitoids were analyzed
by the zircon evaporation method in the Para-Iso
Laboratory of the Federal University of Para (Belém,
Brazil) using a Finnigan MAT 262 mass spectrometer.
The first one was a pink monzogranite collected 6 km
west from Novo Repartimento town. Three zircon grains
yielded a confident age of 2075 ± 3 Ma (2 sigmas),
confirming the previous age reported by Macambira et al.
(2001). As pointed out by these authors, this age is
correlated with that of the Transamazonian Cycle, which
is supported by the syntectonic signature of the granitoids.
The other sample was collected in the extreme west of the
region. It represents a pink banded granitoid outcropping
on the Xingu river margin. Five crystals from this sample
yielded an age of 2154 ± 4 Ma (2 sigmas) and probably
express another moment of the Transamazonian Cycle.
Nd ISOTOPES
Four granitoids, including the three dated samples
mentioned above, were selected to a Sm-Nd study in the
Para-Iso Laboratory using a Finnigan MAT 262 mass
spectrometer. The fourth sample is a monzogranite
- 205 -
Short Papers – IV South American Symposium on Isotope Geology
collected similarly nearby Novo Repartimento town. This
sample showed higher Sm and Nd contents (25 and 171
ppm, respectively) than the other samples (average of 3
and 25 ppm, respectively). The Nd model ages are
comprised between 2.25 and 2.35 Ga, while the εNd(t)
values range from +0.83 to -0.6, except the fourth sample,
which yielded a model age of 2.57 Ga and an εNd(t) value
of -4.12. The behavior of the fourth sample is not well
understood, but is probably related to the more evolved
character of this sample. In spite of its 147Sm/144Nd
(0.08903) similar to those of other samples, the model age
of this sample (2.57 Ga) is quite different and it was
neglected in these preliminary considerations.
DISCUSSION AND CONCLUSIONS
Taking into account our field survey, the study rocks
express a domain of coeval granitoids emplaced in a
syntectonic
regime
probably
related
to
the
Transamazonian Cycle, 2.07 B.y. ago. The sample from
the Xingu river is somewhat older (2.15 Ga), but it could
represent an early magmatism in the same cycle. The Nd
isotopes indicate that these rocks were extracted from the
mantle just before their emplacement and are mainly
composed by material with restricted crustal contribution.
The comparison of the rocks from the study region
with similar rocks occurring in the Maroni-Itacaiunas or
Transamazonian provinces, in French Guyana, suggests a
similar evolution in both regions. It could include crustal
shortening, anatexis and emplacement of granitic plutons
along shear zones, as the modern orogenies, such as
proposed by Vanderhaeghe et al. (1998) for the Guyana
shield. For the study region, it lacks petrological and
structural investigations to propose a more consistent
model. Anyway, the volume of rocks is expressive and
this region certainly represents a distinct tectonic domain
from those of Carajás Province and central-southern part
of the Amapá State, as previously proposed by others (e.
g. Tassinari & Macambira, 1999). Alternatively, it could
represent a magmatic arc which had blended the Archean
domains occurring to the north and south.
ACKNOWLEDGMENTS
We are greathful to M. A. Galarza and R. Florença for
the isotope analyses at the Para-Iso laboratory. This work
is a Núcleo PRONEX/UFPA contribution.
REFERENCES
Cordani U.G., Tassinari C.C.G., Teixeira W., Basei M.A.S.,
Kawashita, K.1979. Evolução tectônica da Amazônia com
base nos dados geocronológicos. In: Cong. Geol. Chileno,
2, Arica, Actas, 4:137-148.
Cordani U.G., Tassinari C.C.G., Kawashita, K.1984. A Serra
dos Carajás como região limítrofe entre províncias
tectônicas. Ciências da Terra, 9:6-11.
Dall’Agnol, R., Lafon, J.M., Fraga, L.M., Scandolara, J.E.,
Barros, C.E.M. 2000. The Precambrian evolution of the
Amazonian Craton. In: Intern. Geol. Cong., 31, Rio de
Janeiro, CD-Rom.
Lafon, J.M., Rossi, P.H., Delor, C., Avelar, V.G., Faraco
M.T.L. 1998. Novos testemunhos de relíquias arqueanas na
crosta continental paleoproterozóica da Província MaroniItacaiunas (sudeste do Escudo da Guianas). In: Cong. Bras.
Geol., 40, B. Horizonte, Anais, p.64.
Lima, M.I.C. 1984. Províncias geológicas do Cráton
Amazônico em território brasileiro. In: Symp. Amaz. , 2,
Manaus, Anais, p. 9-23.
Macambira, M.J.B., Barros, C.E.M., Silva, D.C.C., Santos,
M.C.C. 2001. Novos dados geocronológicos para a região
ao norte da Província de Carajás: evidências para o
estabelecimento do limite arqueano-paleoproterozóico no
sudeste do Cráton Amazônico. In: Simp. Geol. Amaz., 7,
Belém, CD-Rom.
Oliveira, J.R., Silva Neto, C.S., Costa, E.J.S. 1994. Programa
Levantamentos Geológicos Básicos do Brasil. Programa
Grande Carajás. Folha SB.22-X-C Serra Pelada. Belém,
CPRM .
Reis R.M., Silva G.H., Lobato, T.A.M. 1974. Projeto
Transamazônica;
trecho
Estreito-Itaituba.
Belém,
MME/CPRM. Relatório Final. 106p.
Santos, J.O.S., Hartmann, L.A., Gaudette, H.E., Groves, D.I.,
McNaughton, N.J., Fletcher, I.R. 2000. A new
understanding of the provinces of the Amazon Craton based
on integration of field mapping and U-Pb and Sm-Nd
geochronology. Gondwaana Res., 3:453-488.
Santos M.V., Tassinari C.C.G. Souza Filho E.E., Teixeira W.,
Ribeiro A.C.O., Payolla B.L., Vasconi A.V. 1988.
Litoestratigrafia da rochas pré-cambrianas na bacia do
médio rio Xingu – Altamira - PA. In: Cong. Latino-Amer.
de Geol., 7, Belém, Anais, p.363-377.
Tassinari,C.C.G. & Macambira, M.J.B. 1999. Geochronological
provinces of the Amazonian Craton. Episodes, 22:174-182.
Tassinari, C.C.G., Bettencourt, J.S. Geraldes, M.C. Macambira,
M.J.B. Lafon, J.M. 2000. The Amazonian Craton. In:
Cordani, U.G., Milani, E.J., Thomaz Filho, A., Campos,
D.A. (eds.) 2000. Tectonic Evolution of South America. Ac.
Bras. Cienc. etc. Rio de Janeiro. p.41-96.
Teixeira, W., Tassinari C.C.G., Cordani U.G., Kawashita, K.
1989. A review of geochronology of the Amazonian Craton;
tectonic implications. Precambrian Res., 42:213-227.
Vanderhaeghe, O., Ledru, P., Thiéblemont., Egal, E., Cocherie,
A., Tegyey., M., Milési, J.P. 1998. Contrasting mechanism
of crustal growth: geodynamic evolution of the
Paleoproterozoic granite-greenstone belts of French Guiana.
Precambrian Res., 92:165-193.
- 206 -