The eastern border of the Transition Subdomain of the Carajás Province is constituteddominantly of Archean tonalite-trondhjemite-granodiorite (TTG). Deformed monzogranites,
similar to the Planalto granite suite, and metagabbros inserted in association mafic-enderbitic
also occur. Paleoproterozoic isotropic granites and mafic dykes devoid of significant
deformation crosscut the Archean lithologies. The TTGs are exposed as blocks or as flat
outcrops in areas of low relief and commonly include quartz-diorite enclaves. The TTG rocks
display gray colour and are generally medium-grained, showing compositional banding or,
sometimes, homogeneous aspect. They show commonly a NW-SW to E-W trending foliation
with vertical to subvertical dips and were submitted to NE-SW stress. Locally, it was
identified a NE-SW foliation transposed to E-W along shear zones. In some instances, they
exhibit mylonitic to protomilonitics features, registered in the oval form of plagioclase
porphyroclasts or boudinated leucogranitics veins. Two petrographic varieties are recognized
for this association: biotite-trondjhemite and subordinate biotite-granodiorites, both have
similar mineralogical and textural aspects and are characterized by a poorly preserved igneous
texture, partially overwritten by an intense recrystallization.
EDS analyses revealed that the plagioclase is a calcic oligoclase (An27-19), with Or
ranging from 0.6 - 2.3%. The biotites are ferromagnesian, with dominance of Fe over Mg (Fe
/ [Fe + Mg] ranging from 0.54 to 0.59) and the analyzed epidote presents pistacite contents
ranging from 23 to 27.6 % and plot mostly in the range of magmatic epidotes.
The trondhjemite shows all typical characteristics of Archean TTG suites. They have
high La/Yb and Sr/Y ratios, suggesting they were derived from the partial melting of garnet
amphibolite sources at high pressures (ca. 1.5 GPa) or, at least, that their magmatic evolution
was controlled by the fractionation of garnet and possibly amphibole, without significant
influence of plagioclase. The studied TTGs show similarities with Mariazinha tonalite and
Mogno trondjemite, of the Rio Maria Domain, Colorado trondhjemite and, in at a lesser
degree, to the Rio Verde trondhjemite, of the Carajás Domain.
The granodiorites display a calc-alkaline signature and shows LILE enrichment,
specifically K²O, Rb and Ba, when compared to the trondhjemites, but still preserving some
geochemical features of the TTG. The geochemical data indicate that the trondhjemite and
granodiorite are not related by fractional crystallization. An origin of the granodiorite by
partial melting of the TTG rocks is also discarded. The granodiorite could, however, result of
contamination of TTG magmas by lithosphere metasomatism or assimilation of sediments
from subducted oceanic crust along trondhjemite liquid genesis.
In the eastern portion of the mapped area, it was identified a small, E-W trending
granite stock clearly controlled by shear zones. The rocks have mylonitic textures,
characterized by ovoid-shaped feldspar porphyroclasts, wrapped by recrystallized quartz and
mica. These granitic rocks have geochemical signatures of reduced A-type granites and are
similar to the Planalto granite suite.
Boulders of mafic rocks crop out locally in the northern portion of the area. These
rocks show a dominant granoblastic texture, and are mainly composed of amphibole and
plagioclase, with subordinate biotite and quartz.
In the northern part of the mapped area, it was identified a body of isotropic granite
without significant deformation and showing locally rapakivi textures. This granitic pluton
was correlated to the Paleoproterozoic A-type granites, represented in the Carajás Domain by
the Serra dos Carajás suite and Rio Branco Granite. These granites were not studied in detail.
The geological and geochemical aspects shown by the Archean granitoids identified in
the eastern part of the Transition Subdomain implies in the existence of significant TTG rocks
in the Transition Subdomain. This reinforces the hypothesis that the Transition Subdomain
could represent an extension of the Rio Maria Domain, but affected by crustal reworking
events in the Neoarchean.
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