Paleoproterozoic metamorphism of the Mineiro belt: petrographic considerations from the Brumado and Rio Grande Diorites

Abstract

The Brumado and Rio Grande diorites are located in the southernmost portion of São Francisco craton and are composed mainly of hornblende and calcic plagioclase. The rocks of these bodies are represented by diorites, quartz-diorites, and tonalites. Brumado diorite was subdivided into four facies (course, medium, fine/medium, and microporphyritic seriate), while primary features were not present in Rio Grande diorite. Brumado and Rio Grande diorites display xenoliths of amphibolites and metaultramafic rocks of Rio das Mortes greenstone belt. Dykes and apophysis of Ritápolis granitoid (2121 ± 7 Ma) and Itumirim granitoid (2101 ± 8 Ma) cut randomly the dioritic bodies. The minimum ages of crystallization of Rio Grande (2155 ± 3 Ma) and Brumado (2131 ± 4 Ma) diorites relate them to the Mineiro belt Paleoproterozoic evolution. The primary mineralogy of both bodies was modificated by a green schist metamorphic facies characterized by actinolita+albite+epidote+sphene paragenesis. The presence of pale blue hornblende in the Rio Grande diorite indicates conditions of green schist or epidote amphibolite. The minimum age of regional metamorphic event that occurred in the Lavras–São João del Rei cities vary between 2121–2101 Ma and is significantly older than the reported metamorphic event in the Quadrilátero Ferrífero region (2059-2041 Ma). The metamorphic mineralogy of Brumado diorite was locally modified by a potassic metassomatic event related to the Ritápolis granitoid.