FORMAÇÃO METAMÓRFICA DE CORONAS EM ROCHAS DOS COMPLEXOS MÁFICO-ULTRAMÁFICOS DE MANGABAL l E II, GOIÁS

Abstract

Coronas of orthopyroxene, pargasitic amphibole and spinel occur between olivine and plagioclase in metaperidotites, feldspathic metaharzburgites and meta-olivine gabbronorites of the two small ultramafic complexes Mangabal I e Mangabal II Geological, textural and phase petrological evidence indicate that corona formation was contemporaneous with a thermal event producing more hydrated paragenesis in other parts of the complex pointing to a metamorphic overprint under high grade conditions. The different stages of corona formation and the development of the more hydrated paragenesis (clinochlore, orthopyroxene, magnesium hornblende) could be explained by changes in the composition of the fluid phase under virtually similar P-T conditions. Thermochemical calculations for chemically closed systems and isochores of CO 2 , as determined from fluid inclusions, point to about 750°C and 6 to 7 kbar as P-T conditions of metamorphism.

Petrogenesis of Mafic-Ultramafic Intrusions in the Southern Goiás Magmatic Arc, Central Brazil

“…These objections to metamorphic origins of coronas cannot be raised at Mangabal, where deformation and addition of metamorphic amphibole are widespread. Our observations and reasoning are in agreement with the suggestion by Candia et al (1991) that the coronas from Mangabal Complex were metamorphic in origin, and the same reasoning applies also to corona textures at Americano do Brasil.…”

“…The body was defined as a lopolith that has an ultramafic sequence below a mafic sequence composed of norite and gabbro. Pfrimer et al (1981), Candia (1983), Candia and Girardi (1985), Candia et al (1991) and Candia and Gaspar (1997) The Americano do Brasil complex was the object of detailed geological surveys, mineral exploration, including drilling and petrographic study of cumulate rocks and associated Ni-Cu sulfide ore (Nilson, 1981(Nilson, , 1984Nilson and Santos, 1982;. It was first drilled by METAGO in 1975, who defined a small Ni-Cu deposit…”