Igneous charnockites in the southeastern transition zone between the São Francisco craton and the Costeiro mobile belt, Brazil.

Abstract

Orthopyroxene-bearing felsic and mafic rocks are important components of the transition zone between the Archean Sao Francisco Craton and the Late Proterozoic Costeiro Mobile Belt in southeastern Brazil. The Pedra Dourada Charnockite is a felsic rock of the charnockite-enderbite and the granite-tonalite series which intruded into mafic pyribolites of a gabbroic composition. Besides the clearly intrusive contacts between the felsic and the mafic rocks, transitional, migmatite-type contacts also occur. The charnockites have relict magmatic textures which contrast with the metamorphic textures of the pyribolites. Thermobarometric calculations based on several exchange thermometers and one barometer suggest temperatures between 605 and 716°C and a pressure of 7,3kbar (~27km depth) for the generation of the Pedra Dourada Charnockite. Considering the high grade parageneses of these rocks, calculated temperatures are too low, possibly due to reequilibration of mineral chemical compositions during subsequent uplift and deformation. Field evidence, supplemented by mineralogical, textural, and chemical data of the Pedra Dourada Charnockite suggests an origin by partial melting of deep continental crust under the dry conditions of granulite facies. The felsic magma crystallized as rocks of the Charnockite and the granite series. The mafic pyribolites are likely to be restites. This study shows that charnockitic rocks of igneous origin may be much more common in basement terranes around the world than has so far been assumed.