This research is primarily concerned with the tectonic evolution of an Archaean
crustal segment of the Amazonian Craton which is thought to encompass two distinct
Archaean terranes: the Rio Maria Granite-Greenstone Terrain, to the south, and the
Itacaiúnas Belt, to the north. These two terranes show similar lithologies and a short
range variation in geochronological ages, but their structural pattern differs significantly.
The rocks exposed in the domains of the Rio Maria Granite-Greenstone Terrain
comprise mainly TTG series (2.9 – 2.86 Ga), alkaline plutons (2.86 Ga) and greenstone
belts (2.9 Ga), which display ages older than those found on the Itacaiúnas Belt. A
compositional layering can be seen in the tonalites, trondhjemites and granodiorites, and
a magmatic foliation is locally observed in the monzogranites and syenogranites. These
fabrics strike NW-SE to E-W with medium to shallow dips to NE and SW. The linear
fabrics associated with these structures are only locally developed, and generally show
shallow plunges to SE. In the Itacaiúnas Belt the main structural feature is a coarse
mylonitic foliation present in the tonalites, granodiorites and migmatites associated with
the Xingu Complex (2.7 Ga), and also in the syenogranites and alkali-feldspar granites
of the Plaquê Suite (2.7 Ga). This foliation is steeply dipping to N and S, trending E-W to
NW-SE. The stretching lineation present in the mylonitc foliation planes shows shallow
dips to NE, SE, NW and SW. Five structural domains were defined based on strain
facies analysis method. From north to south, these facies show partition of displacement
into compressional and strike-slip components and also distinct fabric generation
patterns. Facies V and IV, located in the southern portion of the studied area, are
characterized by a dominant compressional component of frontal to oblique nature
associated with structures generated by magmatic to submagmatic flow. The central
portion of the study area is marked by facies III, which represents a junction between
different structural patterns and fabric generation processes. This facies shows interplay
between compressional and strike-slip displacements, as well as fabrics developed
either by magmatic flow and solid-state flow. The northern portion of the studied area
comprises facies II and I, which contain fabrics derived mainly from plastic strain
associated with a high degree of planar transposition. The overall kinematic pattern is mainly sinistral, with dextral displacements observed where the compressional
component of partitioned transpression is stronger. The two northern facies display
sinistral displacement, while facies III and facies IV show a combination of
compressional and transcurrent motion. Local shear sense indicators suggest sinistral
displacement for facies V. The geological evolution of the studied area involved two
main stages: i) a first stage of emplacement of TTG granitoids in an Archaean protocrust
mainly located at the region of Rio Maria Granite Greenstone Terrain, and ii) a second
stage marked by shear zone nucleation, magma generation and pluton emplacement in
a partitioned transpressional setting. The partitioned transpression was responsible for
nucleation of a regional scale set of shear zones which partitioned the deformation
across the area. In the northern facies, the deformation pattern is mainly characterized
by strike-slip transposition of planar fabrics formed under solid state conditions. To the
south, magmatic flow is dominant and compressional components of transpression are
evident. The regional shear zones also worked as conduits for magma ascension and
emplacement at higher levels of the crust. These magmas were derived from partial
melting of the TTG series and are mainly alkaline and/or migmatitic in composition. The
structural pattern observed in the area between the Rio Maria Granite-Greenstone
Terrain and the Itacaiúnas Belt is interpreted as a result of progressive deformation in a
sinistral partitioned transpressional setting. This strain pattern comprised geometric and
kinematic partitioning in zones of strike-slip displacement and compressional
components, and also distinct fabric generation patterns ranging from magmatic to solidstate flow. These characteristics suggest that these two terranes are involved in the
same transpressional event and, as such, the Itacaiúnas Belt can be interpreted as a
crustal fragment associated with the Rio Maria Granite-Greenstone Terrain, which
evolution is marked by progressive events of tectonic reworking and granite
emplacement.
|
