In the Amazon Basin located in the region of Monte Alegre, western of the Pará state
occur sills and dykes of dolerites intruded in the paleozoic megasequences rocks,
with emphasis on the central-southern area, where these rocks form the prominent
dome-type structure Monte Alegre. These rocks know as Penatecaua Dolerite are
evidences of an expressive basic magmatism that occurred in Lower Jurassic related
to the disruption of Pangea and opening of the Central Atlantic. Associated with basic
rocks of similar age that occur in other continents in the vicinity of that ocean, the
rocks of the study area are within the Central Atlantic Magmatic Province (CAMP).
The objectives of this work was the detailed characterization of the petrographic and
geochemistry of these rocks. Based on mineralogical and textural features were
individualized three sets that are dolerite, olivine-dolerite and olivine-basalt, where
the first two are medium and coarse grained and a wide textural variation which,
however, is mainly sub-ophitic, while the olivine basalts are porphyritics and have
devitrified material. The three groups are composed essentially of labradorite and
pigeonite, and opaque minerals and olivine (when present) are varietal and apatite is
an accessory. The geochemical aspects in classification diagrams these rocks follow
a trend of basaltic andesite to basalt of tholeiitic affinity and geochemistry signatures
typical of intracontinental environment, probably derived from an enriched mantle
source with limited or no crustal contamination. Whereas the FeOt/MgO reason as
index of differentiation with respect to TiO2, most of the studied rocks can be
correlated to the group of high-Ti tholeiites of the CAMP, necessitating an even large
number of chemical analyses, for a more precise correlation.
|
