Geoquímica e condições paleoambientais de deposição das rochas carbonáticas da Formação Sete Lagoas na região da Área de Proteção Ambiental Carste de Lagoa Santa, MG

LUISA COSTA MARTINS VIEIRA, TÂNIA MARA DUSSIN, LEILA NUNES MENEGASSE VELÁSQUEZ

Abstract


Major and trace elements in Neoproterozoic carbonate rocks of Sete Lagoas Formation outcrop in region including Environmental Protection Area of Lagoa Santa Karst, north of Belo Horizonte, were analyzed for obtain depositional and paleo-environmental conditions information. Pedro Leopoldo is the basal member of Sete Lagoas Formation, with pelitic-carbonate composition, characterized by variable contents of Ca (14,6-39,4%) and Si (0,23-20,12%) and low content of Mg (0,05-1,98%) e Al (0,08-3,9%). Th, U and Zr contents are high and their PAAS-normalized (Post Archean Australian Shale) REE+Y patterns exhibit light enrichment of heavy REE, positive La and negative Ce anomalies. Pedro Leopoldo member is overlaid for purer and darker Lagoa Santa carbonate rocks. These marbles show high concentration of Ca (38,6 a 40,5%) and lower contents of Si (0,02-1%), of Mg (0,08-0,23%) and of Al (0,01-0,27%). They are characterized by lower concentrations of Th, U and Zr relatively by inferior sequence; negative La and positive Ce and Y anomalies, and REE+Y patterns showing wide variation of heavy and light REE fractionation. The petrographic and geochemical characteristics suggest the sedimentation began in marine environment, under deep, anoxic waters and with a marked terrigenous contribution. The chemical characteristics of Lagoa Santa member indicate in this period the deposition environment changed, evolving to more proximal and oxic.


Keywords


Sete Lagoas Formation; Geochemistry; Limestones; Depositional environment.



DOI: http://dx.doi.org/10.21715/GB2358-2812.2018321001

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