Ultramafic flows of the Crixas greenstone belt, Goias — Brazil

Abstract

Abstract The Crixas greenstone belt, together with the Guarinos and Pilar belts, has a basal, intermediate and an upper unit consisting of extrusive and intrusive ultramafic, mafic and sedimentary rocks, respectively. The Crixas belt, the main object of this study, has a basal unit formed of peridotitic and pyroxenitic lava flows intercalated with iron formation, chert, graphite schist, mafic and acid tuffs, and intrusive bodies of pyroxenite and gabbro. The ultramafic unit becomes thicker towards the south end of this belt and it is there that the flows are best exposed, particularly along the Alagadinho creek, where detailed mapping was carried out. Three types of ultramafic flows were recognized: flows with olivine spinifex texture in the central zone, flows with polyhedral jointing at the base, clinopyroxene spinifex texture towards the top and flows without spinifex, but showing polyhedral jointing throughout. The flows are composed of olivine, clinopyroxene, glass and opaques. Greenschistfacies metamorphism, together with deformation, have obliterated part of the primary volcanic textures and structures in these rocks, and partially changed olivine to serpentine and chlorite, clinopyroxene to tremolite, and glass to a very fine chlorite. Preliminary chemical work suggests that the ultramafic rocks are komatiites, characterised by MgO > 20%, CaO/Al 2 O 3 > 1 and low TiO 2 , Na 2 O and K 2 O; these compositions coupled with the presence of spinifex texture indicate an origin from an ultramafic liquid. The flows from Crixas represent the first recognition of spinifex-textured komatiites in the central-western part of Brazil.