Caracterização geoquímica e geocronológica da Suíte São Bento dos Torres na região de Ibertioga – MG: implicações geotectônicas

Abstract

The diachronic accretion of archaean crustal fragments led to the amalgamation of the São Francisco Paleocontinent in the Minas Orogeny. In this context of interactions between magmatic arcs and microplates, our data suggests that the slab pull forces of the oceanic lithosphere in the subduction with the Mantiqueira Complex (2.2-2.1 Ga) to the east, associated with westward amalgamation at the margin of the São Francisco Paleocontinent (2.1 Ga), led to an extensional tectonic in Piedade Block with the generation of an intraplate bimodal magmatism represented by the São Bento dos Torres Suite (2.08 Ga). The petrographic data investigated from this Suite shows two groups of metabasic rocks, typified by metagabbro and amphibolites, associated with metadiorite, biotite gnaisse and hornblende-biotite gnaisse. The metagabros have preserved igneous textures with reliquiar pyroxene cores and replacement reactions indicating a progressive superimposed metamorphism from the edges. Anpybolite has epidote-amphibolite paragenesis with acicular crystals of actinolite, hornblende with bluish green pleochroism and coronas of epidote. The metagranitoides paragenesis also indicates mineralogy and textures of amphibolite facies, with mafic phases represented by biotite and amphibole, and pyroxene absent. Geochemical analyses show a toleitic affinity for metagrabros and metadiorites, being a group with NMORB signature and two EMORB-signed groups, the latter differing in the degree of fractionation and content of light rare earths. Compositional variability suggests the partial melts of distinct domains, with the participation of an astenospheric component and the enriched sublithospheric mantle, driving by distensional processes that led to the rise of the asthenosphere. The metagranitoids were separated into two groups, both with high peraluminousness and signatures suggesting partial melt of crustal lithologies, with a contribution of a sedimentary protolith to Group 4 and an igneous component to Group 5. They present strongly negative εNd(t) (-8 to -11) and high 87Sr/86Sr(i) ratios (0.71375 to 0.73620), compatible with the isotopic evolution of the Piedade Block. Possibly the placement of the basic magmas at the base of the crust generated conditions for partial melt of the crust and generation of the granitoids studied here. U-Pb geochronological analyses on zircon show ages of 2078±4Ma and 2080±4.9Ma for the crystallization of the toleitic lithotypes. The tectonic model now proposed represents the final stages of the collision between the Paleocontinent São Francisco the Siderian-Riacian magmatic arcs recorded in the Mineiro Belt and the Piedade Block, estimated at 2.1Ga, while representing the pre-collisional stage of the Mantiqueira Complex, with collision estimated at 2.05 Ga.