Petrogênese de ultramafitos do Complexo Passo Feio, noroeste de Caçapava do Sul, RS

Abstract

The ultramafics of Passo Feio Complex (PFC), are composed of serpentinites, associated withsteatites, chlorites and carbonate minerals. They occupy an area of 3.5 hectares located in the central-southern portion of Rio Grande do Sul, northeast of Caçapava do Sul. These rocks present unique characteristics to the others in the PFC, corroborating their origin prior to the formation of the PFC. The petrographic analyses define mineralogical relations and indicate the separation of mineral phases of antigorite (atg1 and atg2), magnetite (mag1 and mag2), carbonate (cb1and cb2), talc, chlorite and brucite, as a result of metamorphic events and metasomatic processes. Geochemical analyses configure basalts affinities to andesitic basalts of the toleitic series, with intraplate basalts and subordinate calcium-alkaline basalts. The serpentinites and steatites have low Al2O3 values and the chloritites show contrasting behavior of Al2O3 associated with Fe2O3. The protolith identification occurs by classification diagrams and suggests a harzburgitic-dunitic protolith under serpentinization and carbonation processes observed by the enrichment of the MgOSiO2-H2O-CO2 system. Geochemical analyzes configure affinities of tholeiitic basalts, with intraplate basalts of the N-MORB type.