Textural evidence for liquid immiscibility in the kamafugite-carbonatite association: the alkaline-carbonatite complex of Catalão I, Alto Paranaíba Igneous Province.

RIBEIRO, C. C.; BROD, J. A.; GASPAR, J. C.; PETRINOVIC, I. A.; GRASSO, C. B.; JUNQUEIRA-BROD, T. C.,

Cabo Frío

Congreso; III SIMPÓSIO SOBRE Vulcanismo e Ambentes Associados; 2005

GSB

                Abstract – Catalão I and II are the northernmost alkaline-carbonatite complexes in the Alto Paranaíba Igneous Province (APIP). Catalão I comprises rocks of the bebedourite, carbonatite and phoscorite series, intruding the Late-Proterozoic Brasília Mobile Belt. Petrographic types include include pyroxenite, bebedourite, dunite, peridotite, carbonatite, phoscorite, nelsonite, apatitite, magnetitite, monazitite. Most ultramafic rocks are converted to phlogopitite through carbonatitic metasomatism. Fine-grained phlogopite-picrites form thin dykes, locally locally evolving to polymitic conduit breccias containing fragments of most rock-types in the complex. The breccia matrix is coherent, petrographically similar to the phlogopite-picrites occurring in all APIP complexes. Textural evidence for liquid immiscibility in the breccia domains can be found over a range of scales. Immiscibility starts by the development of millimeter-sized carbonate globules, which coalesce to form centimeter- to decimeter-sized pockets. These small carbonatite masses are collected in a stockwork of dykes and veins and, eventually, form massive bodies ranging from meters to tens of meters. The carbonate in the globules is dominantely calcite, but as the immiscible carbonatite evolves it becomes progressively more dolomitic. This is interpreted as slower Mg diffusion rate compared with Ca, which would imply longer equilibrium periods to produce dolomite carbonatite from ultramafic carbonate magmas.