Petrography and geochemistry of the Aimorés Charnockitic Complex: an example of post-orogenic plutonism of the Araçuaí/Ribeira Belt

Abstract

The Aimorés Complex is an inversely zoned multistage ring-like structure which crops out over about 150 km² and was intruded approximately 500 Ma years ago, during the post-collision event related to Araçuai-Ribeira Fold Belt. This complex consists of a monzodioritic core with hypersthene, and a charnockitic intermediary ring encircled by granodiorites. The contact between core and rim is marked by a mingled/mixed zone, where small occurrences of garnet-granite were also found. Abundant enclaves of different shapes and sizes occur showing circular, diffuse and sharp contacts with the groundmass. The contact with the country rocks is sharp and sub-vertical, varying from concordant to discordant and presenting dips toward the intrusion. The enclosing rocks are orthogneiss, which have experienced high-amphibolite to granulite-facies metamorphism. Most of the facies of the complex are metaluminous, except for the large enclaves embedded in the inner portion, which are peraluminous. The aluminium saturation index is approximately 1 and the agpaitic index ((Na+K)/Al) ranges from 0.45 to 0.77. The negative correlation of CaO, MgO, Fe2O3, TiO2 e P2O5 in the Harker diagram can be explained by pyroxene and feldspar fractionation. The MgO/TiO2 ratio close to 1 and the tectonic discrimination diagrams are compatible with post-collisional granitoids, as well as the sum of Zr, Nb, Ce and Y (500 - 1.000 ppm). The Ba/Sr, Sr/Rb, Ba/Rb and Zn/MgO ratios indicate that there was initially fractionation by pyroxene and plagioclase, followed later by fractionation by amphibole, biotite and K-feldspar. Geochemical data probably reflects the combination of crystal fractionation and/or crustal contamination and magma mingling/mixing processes.