The southern part of Central Amazonia Province (Iriri-Xingu Domain) located in Amazonian
Craton is one of the geologically less known region in Brazil. The few available
geochronological data for this domain are scarce for the precise delimitation of the boundaries
of the geotectonic provinces which were defined mainly based in these data. The domain is
dominated by paleoproterozoic volcanic rocks and granitoids, not metamorphosed and locally
covered by sedimentary sequences, and the regional basement rarely crops out. The inclusion
of this domain in the Central Amazonia Province is based in a supposed “pre-transamazonian”
age (>2.3 Ga) of the basement proposed according to some >2.5 Ga Sm-Nd model-ages (TDM)
and negatives (< -6) εNd values showing significative crustal contribution to the rocks of this
domain, which contrasts with the paleoproterozoic TDM from the rocks of the western adjacent
Ventuari-Tapajos Province. The general aim of this work is the characterization of crustal
evolution and the southern boundary between Central Amazonia and Ventuari-Tapajos
provinces of the Iriri and Curuá rivers area, through petrographic and isotope studies (Pb
evaporation in single zircon crystal and Sm-Nd on whole rock methods). The petrographic
studies and field features enable to determine three rocks units: A-type Granites
(monzogranites and quartz-monzonites with biotite and amphibole), I-type Granites (quartzmonzonites
with pyroxene to biotite-monzogranites) and Iriri Group (rhyolites and dacites).
The Pb-evaporation ages in zircon yielded 1992 ± 3 Ma and 1988 ± 2 Ma, and around 1880
Ma obtained, respectively, for the A-type Granites, volcanic of the Iriri Group and I-type
Granites. The 1988 ± 2 Ma rock (Iriri Group) provided Sm-Nd TDM model-age of 2.61 and
3.13 Ga, and εNd of -10 and -5 what allow frame the study region in the Central Amazonia
Province. These results show that a 1.99 Ga magmatic event, which is common in Tapajós
Domain (Ventuari-Tapajós Province), also occur in Iriri-Xingu Domain, but withstands for its
typical archean crustal source. Additionally, the data point out volcanic rocks older than 1.88
Ga accepted belonging to Iriri Group. However, because the dated volcanic rocks come from
a area close to the Iriri Group type area, it is proposed to adopt the 1988 ± 2 Ma age for this
unit. The younger volcanic rocks (1.88 Ga) must belong to another lithostratigraphic unit.
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