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Petrografia e Geoquímica do Complexo Charnockítico de Aimorés: Um Exemplo de Plutonismo Pós-orogênico do Cinturão Araçuaí/Ribeira =Petrography and Geochemistry of the Aimorés Charnockitic Complex: An Example of Post-orogenic Plutonism of the Araçuaí/Ribeira Belt

Abstract : O Complexo Aimorés corresponde a um corpo multi-intrusivo anelar com cerca de 150 km2 de exposição. Possui características pós-orogênicas e exibe zonas de misturas de magma no contato e no interior do corpo. O complexo é constituído de monzodioritos no centro, com um anel intermediário de charnockitos, circundado por granodioritos. Há ocorrências menores de leucogranitos com granada. São abundantes enclaves básicos e intermediários, de diferentes tamanhos e formas. O contato com a encaixante é de alto ângulo e mostra-se concordante e discordante. Nas rochas encaixantes predominam gnaisses de fácies anfi bolito a granulito. As diversas fácies do complexo são meta-aluminosas, exceto os leucogranitos e os megaenclaves que são peraluminosos. O índice de saturação em alumina é ao redor de 1 e o índice de agpaicidade ((Na+K)/Al) situa-se entre 0,45 e 0,77. Nos diagramas de Harker, a correlação negativa de CaO, MgO, Fe2O3, TiO2 e P2O5 pode ser explicada principalmente pelo fracionamento de piroxênio e feldspato. A razão MgO/TiO2 próxima a 1 é compatível com granitoide tardi a pós- -colisional. As razões Ba/Sr, Zn/MgO, Sr/Rb e Ba/Rb indicam fracionamento importante promovido inicialmente por piroxênio e plagioclásio e depois por anfi bólio, biotita e feldspato de potássio. Os dados geoquímicos parecem refl etir a combinação de processos de cristalização fracionada e/ou de contaminação crustal e de mistura de magmas. =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 postcollisional 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 refl ects the combination of crystal fractionation and/or crustal contamination and magma mingling/mixing processes.
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Fernando Machado de Mello, Rômulo Machado, Essaïd Bilal. Petrografia e Geoquímica do Complexo Charnockítico de Aimorés: Um Exemplo de Plutonismo Pós-orogênico do Cinturão Araçuaí/Ribeira =Petrography and Geochemistry of the Aimorés Charnockitic Complex: An Example of Post-orogenic Plutonism of the Araçuaí/Ribeira Belt. Geologia USP Série Cientifica, 2011, 11 (1), pp.33-57. ⟨emse-00611733⟩

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