Caracterização petrografia de rochas máficas e ultramáficas com textura cumulática aflorantes na região de Xinguara, domínio Rio Maria

Abstract

The mafic-ultramafic cumulate rocks from the Rio Maria Domain, located in the Xinguara area, in the southern portion of the Carajás Province, are xenoliths of varying sizes, locally with an area of 30 km2, included in Xinguara granite and forming contacts with the latter. In this work, five facies of cumulate rocks were described: Equigranular Medium-grained Leucogranodiorite (Lem), Inequigranular Medium- to coarse-grained Pyroxene Hornblendite (Phimg), Seriated Inequegranular Pyroxene Hornblendite (Phis), Inequigranular Fine- to coarse-grained Hornblende Gabro (Hgifg) and Amphibolite. The primary structure is not evident in these rocks like compositional and textural layering, occurring only one faciologic variation that can be seen in the field, which was used to help guide the detailed mapping work. These cumulate rocks were formed through the accumulation of amphibole crystals, except for the facies with dominant plagioclase and intercumulus phases as pyroxene, amphibole, accessory and secondary phases. In most facies, the amphibole accumulation occurred in moderate to high proportions, with a minor proportion of the interstitial fluid rich in pyroxene, amphibole and scarce plagioclase. Samples of the Lem facies show a major accumulation of plagioclase with intercumulus pyroxene. All these facies were affected by subsolidus deformation, evidenced by features observed in the field, such as fracturing, foliation and hydrothermal products, and in microscopic scale, such as microfaults and filled fractures, kink band, granoblastic texture, etc.