The Bacajá Domain is located in the centrar-eastern part of the Pará State, in the southern part of the Maroni-Itacaiúnas Province, which is characterized by a large exposure of Paleoproterozoic and Archean rocks reworked during the Transamazonian Cicle Orogeny. The study area is situated in the Novo Repartimento town, to the south of the Maracajá village, where monzogranites, granodiorites and tonalites, as well as quartzites, granulites and gneisses are exposed. Amphibolites occur as xenoliths in the granitoids. Diabasie dikes are found locally. Aerogeophysical data obtained has improved the cartography of the mapped area. The magnetic structures, mainly determined from the horizontal (x, y) and vertical derivatives (z), evidence a major set of lineaments trending WNW-ENE, consistent with the regional structures. NE-SW lineaments comprise mafic dikes. Maps of potassium, uranium and thorium channels, total count and ternary (RGB, CMY) has allowed the individualization of ten gamma-spectrometric domains. High radiometric values correspond to biotite-granites and gneisses. Leucomonzogranites, hornblenda granites and quartzites show moderate to low radiometric values. A penetrative foliation is recognized at regional scale in the granitoids and displays NW-SE and WNW-ESE strikes, generally with subvertical dips. Primary layering showing gentle to steep dip evolves to a secondary foliation and to mylonitic zones, both concordant. The mineral lineation is weak and have gentle plunges. Microstructural features indicate a transition from magmatic flow to solid-state deformation during the development of foliations in granitoids. Structural analysis of the granitoids suggests that the regional deformation and the emplacement of great volume of intermediate to felsic magma were contemporaneous. Pervasive foliation at the regional scale and homogeneous foliation at the outcrop scale are very common features in syntectonic plutonic belts, and generally generated in a magmatic arc setting. Fourteen petrolographic facies are recognized in the granitoids, so they can represent several plutons. These rocks are metaluminous, medium-to high K calcalkaline and distinguished by high light REE and low heavy REE contents. Eu negative anomalies are very weak or absent. Petrological, geochemical and structural features of the granitoids suggest a magmatic arc environment for the studied region, and that granitoid emplacement is contemporaneous with a compressive tectonic event.
|
