Metamorphism overprint in mafic–ultramafic cumulates in the Mangabal and Americano do Brasil Complexes, Brasilia Belt, Brazil

The Americano do Brasil and Mangabal Complexes belong to a cluster of Neoproterozoic mafic–ultramafic intrusions in the southern Goiás Magmatic Arc in Brazil. Both complexes were highly deformed and metamorphosed under amphibolite facies, leaving few igneous relict domains. We have documented the mineralogy and texture of the poorly exposed rocks of the Americano do Brasil and Mangabal complexes. We use diverse analytical methods, encompassing petrography, mineral chemistry, geochemistry, and thermodynamic modelling, with the principal objective of elucidating the intricacies of secondary alteration processes and offering insights into the complex metamorphic history of the complexes. The rocks of both complexes show intense amphibolitization associated with the most metamorphosed rocks. The construction of pseudosections suggests a temperature between 670 and 710 °C and pressure up to 7.5 kbar at amphibolite facies conditions. In some amphibolite samples, we note unusual minerals such as kyanite, sillimanite, muscovite, silvialite (sulfate-rich member of calcic scapolite), and/or anhydrite. We attribute some unusual assemblages to open system behavior during metamorphism, most likely related to metasomatism surrounding diorite veins that are inferred to have released fluids rich in CO₂, Ca and SO₃. However, this open system behaviour and fluid circulation do not seem to favor the Ni–Cu mineralization.