Caracterização Microestrutural e da alteração hidrotermal dos granitoides arqueanos próximos de vila Planalto, município de Canaã dos Carajás, Província Carajás

Abstract

The Carajás subdomain, northern of the Carajás Province, consists of a wide variety of granitoids and mafic rocks of Archean age, intruded by granites type-A Paleoproterozoic. Petrographic, geochemical and geochronological on granitoids of Canaã dos Carajás area, allowed a considerable advance in knowledge of the crustal evolution of the Carajás Province. However, there was need for better characterization of deformation / metamorphism (?) and hydrothermal alteration of granites along the Itacaiúnas shear zone that affected the near Planalto village units, north of the town of Canaan. The petrographic, geochemical and geochronological on granitoids of Canaan area of Carajás, allowed a considerable advance in knowledge of the crustal evolution of the Carajás Province. Therefore, the main objective of this work is the microstructural characterization (deformation and / or metamorphism) and hydrothermal alteration of granitic Archean near Planalto village. The three study target units are: (1) Rio Verde trondhjemite; (2) Bom Jesus granitic gneiss; and (3) Serra Dourada granite. The information about microcrystalline deformation show that the microstructures present in rocks is directly related to shear zones that cross the study area, working under different conditions of deformation and temperature. Thus, it was noticed that the closer to the shear zones, due to the increase in pressure and temperature, primary textures are modified and deformation structures are printed, and there is the formation of secondary minerals. Therefore, in this work the Rio Verde trondhjemite and the Serra Dourada Granite are both metamorphosed and deformed igneous rocks just along shear zones. While Bom Jesus Gneiss is regarded as metamorphic rock of igneous origin, formed in amphibolite facies conditions. In addition, the following developments of mineral felsic deformation of the three units are represented by the following microstructures: (1) undulant extinction; (2) sub-grains; (3) new grains; (4) undulant extinction of new grains; (5) subgrains formation in new grains; and (6) fractures. As well as the evolutionary following the deformation mechanisms associated with microstructures is: (1) dislocation migration (undulant extinction); (2) recovery (subgrains formation); (3) recrystallization (new grain formation); (4) dislocation migration in new grains (wavy extinction); (5) recovery in new grains (subgrains formation); and (6) intercrystalline fracture (fractures). Based on mineralogical description of secondary minerals and their associations were possible to correlate them with four types of hydrothermal alteration: sodium alteration, sodium-calcium alteration, Potassic alteration I and II. In Rio Verde trondhjemite is common sodium-calcium alteration, actinolite + epidote represented by, and Potassic alteration II evidenced by abundant formation of sericite. In Serra Dourada granite contacted the presence of sodium alteration (albite + scapolite) and Potassic alteration I (biotite + magnetite). The semiquantitative analysis by EDS allowed characterize the action of hydrothermal alteration. In this analysis we observed the presence of green biotite, impoverished in Ti by hydrothermal processes, as well as training and actinolite epidote crystals from the plagioclase alteration. Finally, crystals of oxidized magnetite and ilmenite presence are subordinate. The sequence of hydrothermal alteration stages is given by: (1) albitization; (2) scapolitization; (3) Formation of iron oxides; (4) Potassic I; (5) calcium-Sodium; and (6) Potassic II. Thus, it is concluded that the hydrothermal alteration in the Canaã area was performed by percolation of hydrothermal fluids fractures generated by shear zones and resemble those described in the type IOCG deposits of Carajás, dated between the Mesorchean and Neoarchean.