The southeastern portion of the Guiana Shield is part of a large Paleoproterozoic orogenic
belt, with evolution related to the Transamazonian Orogenic Cycle (2.26 – 1.95 Ga). In this area,
previous works defined distinct tectonic domains, named Jari, Carecuru and Paru, which present
outstanding differences in terms of age, lithological content, structural pattern and geophysical
signature. The Jari Domain is constituted of a granulite-gneiss-migmatite basement assemblage
derived from Archean protoliths, and the Carecuru Domain is composed mainly of calc-alkaline
rocks and metavolcano-sedimentary sequences, developed during the Transamazonian Event. The
Paru Domain is an oval-shaped granulitic nucleous, located within the Carecuru Domain, formed
by granulitic gneisses with Archean precursors and Paleoproterozoic charnockitic plutons.
In this study, distinct geochonological methods were employed in rocks from the distinct
domains, in order to define their tectonic meaning and crustal evolution processes during
Archean and Paleoproterozoic times.
Pb-evaporation on zircon and Sm-Nd on whole rock dating were provided on magmatic
and metamorphic units from the Jari Domain, defining its long-lived evolution, marked by
several stages of crustal accretion and crustal reworking. Magmatic activity occurred mainly at
the Meso-Neoarchean transition (2.80-2.79 Ga) and during the Neoarchean (2.66-2.60 Ga). The
main period of crust formation occurred during a protracted episode at the end of Paleoarchean
and along the whole Mesoarchean (3.26-2.83 Ga). Conversely, crustal reworking processes have
dominated in Neoarchean times. During the Transamazonian Event, the main geodynamic
processes were related to reworking of older Archean crust, with minor juvenile accretion at
about 2.3 Ga, during an early orogenic phase. Transamazonian magmatism consisted of syn- to
late-orogenic granitic pulses, which were dated between 2.22 and 2.03 Ga. Most of the εNd values
and TDM model ages (2.52-2.45 Ga) indicate an origin of the Paleoproterozoic granites by mixing
of juvenile Paleoproterozoic magmas with Archean components.
The new geochronological results, added to data from previous studies, revealed that the
Jari Domain represents the southwestern part of the most expressive Archean continental
landmass of the Guiana Shield, here defined and named Amapá Block. The recognition of an
extended Archean block precludes previous statements that the Archean in the southeast of the
Guiana Shield, was restricted to isolated remnants or inliers within Paleoproterozoic terrains. In the Carecuru Domain the widespread calc-alkaline magmatism occurred at 2.19-2.18
Ga and at 2.15-2.14 Ga, and granitic magmatism was dated at 2.10 Ga. Crustal accretion was
recognized at about 2.28 Ga, in agreement with the predominantly Rhyacian crust-forming
pattern of the Guiana Shield. Nevertheless, TDM model ages (2.50-2.38 Ga), preferentially
interpreted as mixed ages, and εNd < 0, point to some participation of Archean components in the
source of the Paleoproterozoic rocks. The lithological association and the available isotopic data
registered in the Carecuru Domain, suggests a geodynamic evolution model based on the
development of a magmatic arc system during the Transamazonian Orogenic Cycle, which was
accreted to the southwest border of the Archean Amapá Block.
In the Paru Domain, Neoarchean magmatism at about 2.60 Ga was produced by
reworking of Mesoarchean crust, as registered in the Amapá Block. Crustal accretion events and
calc-alkaline magmatism were recognized at 2.32 Ga and at 2.15 Ga, respectively, as well as
charnockitic magmatism at 2.07 Ga.
U-Th-Pb chemical ages in monazites from high-grade rocks of the southwestern part of
Amapá Block, dated two main tectono-thermal events. The first one was revealed by the
monazite ages of about 2.09 Ga and marks the age of the granulite-facies metamorphism. These
data, added to petro-structural information, indicate that the granulite-facies metamorphism was
contemporaneous to the development of a thrusting system associated to the collisional stage of
the Transamazonian Orogeny. The later event was testified by monazite ages at about 2.06 Ga
and 2.04 Ga, and is consistent with a late-orogenic stage marked by granitic emplacement and
coeval migmatization of the Archean basement along strike-slip zones.
Finally, 40Ar/39Ar geochronological study on amphibole and biotite from representative
units of the Amapá Block and of the Carecuru Domain delineated contrasting cooling and
exhumation stories. In the former amphibole vary from 2.13 to 2.09 Ga, and biotite ages range
mainly between 2.10 and 2.05 Ga. In the later, amphibole and biotite ages are between 2.16 and
2.06 Ga, and 1.97 and 1.85 Ga, respectively. In the Amapá Block, fast cooling rates around 67
°C/m.y. and 40 °C/m.y indicate a tectonically controlled exhumation, related to collisional stages
of the Transamazonian Event. Conversely, in the Carecuru Domain, regional cooling rates in the
order of 3-2.3 °C/m.y. suggest slow cooling and gradual uplift, which is consistent with the
magmatic arc model, where continental growth results mainly from lateral magmatic accretion,
precluding significant tectonic crustal thickening.
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