Evidências de Eventos de Oxigenação a partir da Geoquímica de Formações Ferríferas Bandadas do Greenstone Belt Pitangui (Meso-Neoarqueano), porção sul do Cráton São Francisco

Nicollas Ferreira de Oliveira Santos, Calvin Ferreira, Mariana Brando Soares, Kathelyn Nunes da Silva Santos, Lorrana Roriz Faria, Atlas Vasconcelos Corrêa Neto, Pedro Henrique Montenegro da Silva, Ricardo Oliveira de Araújo Mabub

Abstract


The Greenstone Belt Pitangui, located northwest of the Quadrilátero Ferrífero in Minas Gerais, is a terrain of meso-neoarquean age. Its stratigraphy is composed of intercalations of metavulcanic rocks that are records of volcanic activity, and metastatic chemical and clastic rocks (metamorphosed in green shale to low amphibolite facies) such as metarenites and banded iron formations (BIFs), rocks commonly observed in oceanic greenstone belts. The biochemical conditions of the oceans and the atmosphere that gave rise to BIFs (restricted to the Pre-Cambrian) differ considerably from those prevailing today. These rocks are the result of the bio-induced chemical precipitation of Fe and Si oxide, resulting from unique conditions exclusive to Earth's past. The deposition of BIFs in the Greenstone Belt Pitangui predominated in stages of starved basin and cessation of volcanism, these being intercalated with metarenites resulting from occasional turbiditic flows. The geochemistry of these BIFs indicates the incorporation of deposited clastic material during short interruptions in biochemical precipitation. This factor can be identified by observing patterns in larger elements (Fe, Ti, Al, Mn) in four groups of samples of different stratigraphic levels (Pimentão, Biquinho, Aparição and São Francisco). The proportions of ETRs and Y suggest the classification of these BIFs as the Algoma type. Samples of the Pimentão level have negative anomalies of Ce, which indicates the existence of free oxygen in the Archaean seas, even locally. This corroborates in ways consistent with the negative anomalies of Ce previously described for BIFs of the Biquinho level (superior to Pimentão level). The occurrence of these anomalies in subsequent stratigraphic intervals evidences the temporal continuity of the oldest oxygenation event already registered in Brazil (2.86- 2.78 Ga), which is prior to the proposed Great Oxygenation Event (GEO - 2.4 Ga) in literature.


Keywords


Geochemistry of BIF's; Greenstone Belt Pitangui; Iron Quadrangle; Great Oxidation Event.



DOI: http://dx.doi.org/10.21715/GB2358-2812.2019331028

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