PRECAMBRIAN MAFIC AND ULTRAMAFIC ROCKS OF THE CANA BRAVA COMPLEX, BRAZIL - MINERAL COMPOSITIONS AND EVOLUTION

Abstract

The Cana Brava mafic-ultramafic complex, Goias State, Brazil, is an Early Precambrian differentiated massif formed by intrusion and differentiation of a presumably basaltic parental material. Rock types cover a continuous sequence ranging from harzburgites to pyroxenites to ferronorites and ferrogabbros, as indicated by changes in mineral associations and mineral compositions. After the emplacement, the mafic-ultramafic suite was affected by several sub-solidus reequilibrations. The first one took place at approximately 900°C and pressures of 6-7 kb. Textural features as well as mineral distribution of certain elements show that complete equilibrium was not achieved during this event. A successive partial recrystallization converted some of the rocks on an high-grade amphibolite assemblage. A late low-temperature event caused the formation of serpentinites, rodingites and tale schists. P-T estimates for the high-temperature reequilibration at Cana Brava coincide broadly with results obtained from other Early Precambrian Brazilian mafic-ultramafic complexes. The values thus estimated were plotted to obtain geothermal gradients to be compared with those presented in the literature for other Precambrian occurrences.