Petrography and Geochemistry of the Aimorés Charnockitic Complex: An Example of Post-orogenic Plutonism of the Araçuaí/Ribeira Belt

• Main Authors: Fernando Machado de Mello, Rômulo Machado, Essaid Bilal
• Format: Article
• Language: English
• Published: Universidade de São Paulo 2011-04-01
• Series: Geologia USP. Série Científica
• Subjects: Charnockite; Geochemistry; Petrography; Magma mingling/mixing; Post-collisional
• Online Access: http://ppegeo-local.igc.usp.br/pdf/guspsc/v11n1/03.pdf

The Aimorés Complex is an inversely zoned multistage ring-like structure which crops out over about 150 km2 and wasintruded approximately 500 Ma years ago, during the post-collision event related to Araçuai-Ribeira Fold Belt. This complexconsists of a monzodioritic core with hypersthene, and a charnockitic intermediary ring encircled by granodiorites. Thecontact 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. Thecontact with the country rocks is sharp and sub-vertical, varying from concordant to discordant and presenting dips toward theintrusion. 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 areperaluminous. The aluminium saturation index is approximately 1 and the agpaitic index ((Na+K)/Al) ranges from 0.45 to0.77. The negative correlation of CaO, MgO, Fe2O3, TiO2 e P2O5 in the Harker diagram can be explained by pyroxene andfeldspar fractionation. The MgO/TiO2 ratio close to 1 and the tectonic discrimination diagrams are compatible with postcollisionalgranitoids, 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 ratiosindicate that there was initially fractionation by pyroxene and plagioclase, followed later by fractionation by amphibole, biotiteand K-feldspar. Geochemical data probably reflects the combination of crystal fractionation and/or crustal contamination andmagma mingling/mixing processes.