Caracterização petrográfica e mineralógica dos granulitos da área de Cruzeiro do Sul, Domínio Bacajá, Província Transamazonas

Abstract

The Bacajá Domain, south of the Transamazonas Province, is located in the central-eastern portion of the Pará state. It comprises a high-grade metamorphic terrain with rocks (gneisses, granulites, migmatites, granitoids and charnockite rocks), paleoproterozoic and Archean in age, reworked during the Transamazonic Cycle (2,26-1,95 Ga). The Novolândia Granulitic Complex, in the Vila Cruzeiro do Sul area, southern portion of the Bacajá Domain, is formed by four main groups: (1) Felsic granulite; (2) Mafic granulite; (3) Aluminosous granulite (Kinzigite) and migmatite. The granulite bodies vary from little to very deformed, arranged in NW-SE direction, sustaining hills or in the form of blocks in situ. Felsic granulites are dominant among granulite varieties. Its predominant mineral composition is represented by> 30% felsic minerals, formed by plagioclase + quartz + alkali feldspar + biotite ± orthopyroxene ± garnet. The mafic granulites have> 30% of mafic minerals, with mineral association formed by plagioclase + orthopyroxene + clinopyroxene + quartz + biotite + amphibole ± garnet. The Aluminosous granulites are found in the northernmost portion of the area and were thus classified due to the occurrence of aluminous minerals, mainly cordierite, which are typical of metamorphic rocks of sedimentary origin. The mineralogy is composed of plagioclase + quartz + biotite + cordierite + garnet. Microdeformational features were analyzed allowing to define that these rocks were subjected to intracrystalline plastic deformational regime under influence of recovery and recrystallization processes that led to evolutionary sequencing (1) displacement formation (undulating extinction) (2) recovery (subgrade formation) recrystallization (formation of new grains) (4) formation of displacements in new grains (undulating extinction). The ductile deformation regime with fluid phase participation caused slowing reactions by reaction or softening of crystal, forming recurrent changes of saussuritization and sericitization in the plagioclase and pyroxenes of the granulite varieties studied. Mineral chemistry and deformational textures suggests that granulites were generated under temperature conditions above 700° C under influence of intermediate to high pressure.