In the Carajás basin, northern Mineral Province of Carajás, sequences of volcanosedimentary
rocks with ages between 2.76 and 2.73 Ga make up the Itacaiúnas
Supergroup. This supergroup encompasses the Igarapé Salobo (2.76 Ga), Grão Pará
(2.76 Ga), Igarapé Bahia (2.74 Ga) and Igarapé Pojuca (2.73 Ga) groups. The proximity
of ages and the lithological affinity of these units indicate that possibly these rocks have
been formed under the same geological context, being different expressions of the same
event. The Grão Pará Group consists of two formations: a thick sequence of volcanic
rocks (Parauapebas Formation; 2758 ± 2 Ma, U-Pb in zircon), and jaspilites with iron
ore (Carajás Formation). In view of the great difficulty of understanding the geographic
and geochronological limits of the Grão Pará Group, it was carried on petrographic,
geochronological (Pb-evaporation in zircon) and typological study of zircon crystals
from of samples associated with the Carajás and Parauapebas formations. Rocks from
the Grão Pará Group, as well as the majority of the archaean rocks of the Carajás basin,
are hydrothermally altered in different degrees of intensity, irrespective of their
lithological type. However, a great number of primary textural features were preserved
and petrographic studies allowed the classification of the igneous rocks into two distinct
groups: the basalt of dark gray to greenish coloration, fine granulation, mainly
intergranular texture, from the Parauapebas Formation; and gabbro with intergranular
texture, subofitic to micrographic intensively hydrothermalized (mainly serecitization)
from dikes which cut the whole sequence. For the Serra Sul of the Carajás basin, two
samples of saprolite of volcanic rocks interbeded with the iron formation and collected
in cores from drill holes, were analyzed in this work. Due to the advanced weathering
that affected these samples, a typological study of zircon was used in an attempt to
identify the previous rocks, which allowed to classify the lithotypes as monzogranitealkaline
granite affinity. Pb-evaporation method on zircon crystals, in two samples of
volcanic rock saprolite interspersed with the iron formations belonging to the Carajás
Formation indicated, indicated ages of 2745 ± 2 Ma for sample FS11D-161 and 2746 ±
2 Ma for sample FS11D-122, understood as time of crystallization of the analyzed
crystals and the formation of volcanic rock which contains them. In addition to the high
accuracy of these data, they are in perfect agreement with each other and within the gap
established for the Itacaiúnas Supergroup. Moreover, because the dated samples come
from rocks interbeded with the iron formation, they can be coeval (lava flow) or
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younger (intrusive), and establish a minimum limit for the deposition age of this
formation. On the other hand, the age of the Parauapebas Formation, button of the Grão
Pará Group, establishes the maximum age for Carajás Formation as 2759 ± 2 Ma.
Therefore, the Carajás Formation was deposited in about 15 million of years, which is
coherent for the estimated thickness of 400 m. In addition, the Grão Pará Group is
covered by the Igarapé Bahia Group, whose volcanic rocks from the button are 2745 ± 1
million years old, which perfectly coincides with the ages obtained in this work. This
age corroborates the the gap proposed for the deposition of the Carajás Formation, that
is, a maximum of 15 million years. Additionally, the insignificant age difference
proposed here for the Carajás Formation and for the volcanic from the Igarapé Bahia
Group suggests that all these rocks are part of a same sequence.
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