The São João Granite (SJG) is a subcircular anorogenic batholith with approximately
200km² of area, outcropping between Água Azul do Norte and Bannach cities,
southeastern of the Amazonian Craton, Carajás Province. It is intrusive in
trondhjemitic and leucogranitic Archean units of the Rio Maria Granite-Greenstone
Terrain. The SJG consists mainly of monzogranite and syenogranite isotropic rocks,
with pink to reddish colors, poor in mafic minerals, and formed essentially by quartz,
alkali feldspar and plagioclase. Amphibole and biotite are the main ferromagnesian
minerals, and zircon, apatite, allanite and minerals opaques the dominant accessory
phases. The SJG is dominantly medium to coarse-grained, locally fine-grained,
outcropping as small hills, big blocks and flooring often fractured. Petrographic
studies, based on macroscopic, microscopic and SEM/EDS analysis allowed the
identification of four different petrographic facies: amphibole-biotite monzogranite
(ABMG), biotite monzogranite (BMG), amphibole-biotite syenogranite (ABSG) and
biotite syenogranite (BSG). Among the monzogranite rocks, the BMG is the more
abundant facies. These rocks are concentrated in the central west part of the body,
with some occurrences in the north and southeast. Rocks of the ABMG facies are
widely disperse in the east, southwest and west-central portions of the body. On the
other hand, the syenogranite rocks (ABSG and BSG) occur preferentially in the
border areas of the pluton, with the BSG rocks being restricted to the western border
and the ABSG to the eastern border, with some occurrences in the north and
southeast. A representative sample of BMG was selected for dating by the Pb on
zircon evaporation method. Among the zircon analyzed crystals, five showed an
average age of 1890±2 Ma, interpreted as crystallization age of the SJG. This age is
similar to that obtained for the Seringa Granite (1895±1 Ma). Due to petrographic,
mineral chemistry (EDS/SEM) and geochronological similarities with rocks of the
Seringa Granite, located immediately to the west, and granites of the Serra dos
Carajás suite, the São João Granite is included in a first approach, in that important
granitoide suite.
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