Petrologia de rochas metamáficas e metaultramáficas da Sequência de Itaguara (sul do Cráton do São Francisco-Minas Gerais): vestígios de litosfera oceânica paleoproterozoica?

Abstract

Ophiolitic sequences are important evidence of ancient oceanic lithospheres, contributing to the understanding of terrestrial tectonic evolution. Metamafic- metaultramafic rocks are found between the cities of Itaguara and Crucilândia (Minas Gerais) in the south of the São Francisco Craton, and together with quartzites, metacherts, banded iron formations, mica-quartz schist and Córrego do Peixoto granite comprise the paleoproterozoic Itaguara Sequence (SI). Such rocks show characteristics similar to those of ophiolitic sequences, which provide a new geological interpretation for this locality. Through field geologal data, petrography, geochemical, it was pursued to understand the metamorphic conditions under which metamafic-metaultramafic rocks of the Itaguara Sequence were submitted. The metamafic rocks are amphibolites with E- MORB geochemical signature, mineralogically composed of hornblende, plagioclase, quartz and ortho- and clinopyroxene, and having problably gabbro as protolith. The ortopyroene reveals that amphibolitc rock has reached granulite facies, and subsequently amphibolite facies retrograde metamorphism. The metaultramafic rocks show metamorphism and hydration in varying degrees of intensity, with lithotypes that reveal the spinel wehrlite protolith little modified and other lithotypes with the primary mineralogy of olivine, clinopyroxene and spinel completely transformed to serpentine, chlorite, tremolite, talc and opaque minerals. Binary Harker diagrams suggest cogeneticity between SI metamafic and metaultramafic rocks and both were possibly part of an oceanic lithosphere that was subducted and involved in a Paleoproterozoic collisional event between the Divinópolis and Campo Belo/Bonfim Complexes. This oceanic lithosphere appears to be preserved as an ophiolite in this geological context.