Associação de Fácies, Litoquímica e Geologia Isotópica da Formação Lagamar (Grupo Vazante, Minas Gerais): uma Plataforma Carbonática no Mesoproterozoico

Abstract

The Lagamar Formation represents a platform deposited in a probable extensional period where carbonates pre-cipitated in a structural high in an open sea, along the margin of the São Francisco paleocontinent. This platform is relat-ed to a reef environment: fore reef (consisting of carbonate reworking in slope), reef (representing structures of isolated reefs) and back reef (favored by the shallow environment, with high salinity, protected by the reef barrier). Lithochemical and isotopic analyzes show predominance of siliciclastic contamination masking the primary seawater signal, with more REE enriched in fore reef than the restricted environment in back reef, which has higher Mg abundances due to dolo-mitization. When normalized to PAAS, samples do not follow the typical pattern of marine carbonates, showing LREE/HREE ≈ 1.0, strong positive La anomaly, absence of Ce and Gd anomalies and low Y/Ho ratios at 29-39. The isotopic data showed δ13C values ranging from -0.14 and 2.20 ‰, and δ18O from -9.82 to -3.18 ‰ showing a homogeneity of values co-incident with the tectonic quiescence period of Mesoproterozoic and, 87Sr/86Sr a rou nd 0.7068 where the primal signature was affected by post-diagenetic processes.