Geochemical Models for the Petrogenesis of Komatiites from the Hidrolina Greenstone Belt, CentraI Goias, Brazil

The Archaean greenstone belt of Hidrolina (Centrai Goiás, Brazil) is a severely tectonized and metamorphosed volcano-sedimentary sequence. It consists of a lower sequence (LS) with ultramafic and mafic flows (compositionally corresponding to komatiites) and an upper scquence (US) with intercalated mafic and felsic flows (compositionally corresponding to tholeiites, dacites, and rhyolites). Practically no primary structure or texture is preserved. Geochemical major and trace element modelling allows distinction between liquid compositions and cumulates. Crustal contamination does not seem to have been effective in controlling the geochemical variations. Batch partial melting models and CMAS plot indicate that: (a) the komatiites probably resulted from moderate (28-30%) melting of a spinel peridotite rather than a garnet one; (b) the komatiites were formed by the mixing of a liquid of basaltic composition with peridotite source material; (c) the mantle source was heterogeneous, the flows higher in the stratigraphy coming from a more depleted source than the lower ones; and (d) US tholeiites resulted from lower degree melting of the same depleted source that produced the upper LS flows.