Geochronology and stratigraphy of the Boa Vista de Minas Group: An example of a Meso-to Neoarchean greenstone-belt in amphibolite-to granulite-facies

Abstract

The Boa Vista de Minas Group is a suite of metamorphic rocks ranging from amphibolite to granulite facies. It appears as disrupted, folded, and keel-shaped bands amidst the metagranite-migmatite domes of the Divinópolis Complex. It is challenging to define its lithostratigraphic architecture and thickness due to intense weathering and tectonic overprints. The internal organization of the complex is characterized by three main Formations. The basal level, known as the Nova Serrana Formation, primarily consists of amphibolites and mafic granulites (metabasites), meta-ultramafic rocks, cummingtonite schists, and cordierite-anthophyllite schists (immature volcanogenic metagraywackes). Zircon U-Pb analysis (LA-ICP-MS) from a sample of cordierite-anthophyllite schists from the Nova Serrana Formation yielded a maximum deposition age of ca. 2.82 Ga. Orthopyroxene-bearing iron formations and ferruginous meta-cherts define interbedded layers within the basal sections, collectively referred to as the Córrego do Cedro Formation. Above these units lies the Igaratinga Formation, which consists of calc-silicate rocks interspersed with lenses of amphibolites and granulites, sillimanite gneisses, and meta-ultramafic rocks, including common lenses of biotite-orthopyroxene granulite (immature volcanogenic metagraywackes). A sample of biotite- orthopyroxene granulite from this Formation provided a maximum deposition age of ca. 2.72 Ga (U-Pb, LA-ICP-MS). Additionally, zircon rims showing light luminescence and newly formed zircon yielded a Concordia age of approximately 2.0 Ga, interpreted as reflecting a high-grade metamorphic overprint. The patterns of LREE enrichment, Nb-Ta-Ti depletions, and variable Th/Nb ratios observed in the metabasic rocks of both the Nova Serrana and Igaratinga Formations are similar to those formed under interactions between LREE-enriched plumes and arcs. The similarities in lithology, lithogeochemistry, and depositional ages of the rocks in the Boa Vista de Minas Group suggest that it represents a high-grade counterpart to the Pitangui Greenstone Belt. This implies that the tectonic-thermal metamorphic event associated with the final stages of the Minas-Bahia Orogeny extended to the northern limit of the Divinópolis Complex and is more extensive than previously anticipated.