Petrological and geochemical characteristics of the mafic–ultramafic Americano do Brasil Complex, central Brazil, and the implications for its genesis

Abstract

The Americano do Brasil Complex occurs in the Neoproterozoic Goias Magmatic Arc, central Brazil. It is composed of two mafic–ultramafic cumulate sequences, intruded into granodioritic gneisses. Although deformed and partially recrystallized by a regional metamorphic overprint, the complex still preserves relict igneous features, such as adcumulate to heteradcumulate textures. The Northern sequence is mostly composed of olivine and olivine-clinopyroxene cumulates, whereas the Southern consists mainly of two-pyroxene cumulate rocks, with plagioclase and olivine cumulates occurring in lesser amounts. The complex has three main orebodies, with textures that range from disseminated to massive sulfide breccias with durchbewegung texture. Thermodynamic modeling using a single picrite parental magma composition can predict cumulate rock compositions and mineral modes similar to all of the observed cumulate rock compositions of the Americano do Brasil Complex. Equilibrium crystallization of the liquid and assimilation-batch-crystallization involving up to 45 % of the host gneisses in the upper crust produces solids similar to the cumulates described in the Northern and Southern sequences, respectively. Modeled pressure–temperature emplacement conditions of the magma were c.a. 2.5 kbar and 1310 °C. Both sequences have similar incompatible trace element patterns which, together with the results of the modeling, imply a broadly comagmatic origin.