As formações Santo Antônio do Bonito e Rocinha (Grupo Vazante, Minas Gerais): sedimentação gravitacional sin a pós-glacial e fosfogênese na transição Faixa Brasília-Cráton do São Francisco

Abstract

The Santo Antônio do Bonito and Rocinha formations, basal units of the Vazante Group, comprise diamictites, sandstones, pelites and rare carbonates, originated from a subaqueous gravitational sedimentation, composing a slope-apron depositio-nal system. In the studied region, east of the city of Coromandel (Minas Gerais), in the Santo Antônio do Bonito River Val-ley, a regional stratigraphic analysis was conducted from geological mapping, sedimentary facies analysis, isotopic study and comparison between sedimentary phosphorites for regional stratigraphic analysis. Zircon U-Pb ages yield a maximum depositional age of 934 ± 8 Ma, but with subtle provenance differences between both units. Sm-Nd isotopic data show contrasting TDM model ages and εNd(t) values for the two units, also suggesting source area changes. Sm-Nd data also show great similarity between the Santo Antônio do Bonito Formation and the Jequitaí Formation (Bambuí Group), whose TDMages are Paleoproterozoic, with values between 1.8 and 2.1 Ga. Sm-Nd data also show great difference between pelites from the Rocinha Formation and pelites from the Bambuí Group (Serra de Santa Helena Formation). From east to west, the glacial sediments (Jequitaí Formation) are reworked by gravitational processes, evolving to a slope-apron system in the margin of the São Francisco pa-leocontinent (Santo Antônio do Bonito and Rocinha Formations). The phosphatic deposits of the Vazante Group, named Coromandel (basal) and Rocinha and Lagamar (upper), are thus related to a major phosphogenetic transgressive event in a post-glacial context, probably during the Ediacaran. The phosphateenrichment occurs in rhythmites (phosphoarenites rich in intraclasts with fine/medium sand granulometry, fluorapatites and cryptocritaline matrix), later enriched by supergenic processes, forming wavellite in fractures.