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Geoquímica e geocronologia do granitóide Barra Alegre, faixa móvel Ribeira, Rio de Janeiro

2007; Sociedade Brasileira de Geologia; Volume: 37; Issue: 01 Linguagem: Inglês

10.25249/0375-7536.2007371101113

2177-4382

Júlio Cézar Mendes, Patrícia Duffles Teixeira, Gabriel Corrêa de Matos, Isabel Pereira Ludka, Felipe Fonseca Fernandes de Medeiros, Ciro Alexandre Ávila,

Geography and Environmental Studies

Orthoderived rocks of the Rio Negro Complex and Serra dos Órgãos batholith are the dominant rocks of the mountain region of Rio de Janeiro State. They are locally associated with paraderived gneisses and belong to the Neoproterozoic Central Ribeira Belt. Between the cities of Bom Jardim and Trajano de Moraes, the NE-SW elongated Barra Alegre granitoid body occurs enveloped by migmatitic gneisses of the Rio Negro Complex and paraderived Imbé Unit. The granitoid is monzogranitic to quartz monzonitic in composition and shows a porphyritic texture. The size and shape of the microcline phenocrysts made possible the definition of two lithofacies in the granitoid body: 1. the Barra Alegre facies presents euhedral to subhedral megacrysts and weak schistossity; 2. the Carijó facies is characterized by augen texture and well developed schistossity. Biotite is the dominant mafic phase, while opaque minerals, zircon, apatite, and allanite are the main accessories minerals. Stretching quartz grains, fold biotite and plagioclase lamellae, and phenocrysts as well matrix recrystalization are the main deformation features. Based on field and petrographic evidences, the Carijó lithofacie is interpreted as a result of heterogeneous deformation from the Barra Alegre lithofacie. The geochemical characteristics along with petrographic and field data, point to a high-K calc-alkaline I-type, mildly peraluminous, character for these rocks. The restrict SiO2 variation indicates a moderately differentiated sequence. The rare earth element patterns of different rocks are similar, displaying LREE enrichment and a smooth Eu negative anomaly. 207Pb/206Pb single zircon evaporation dating yielded a minimum crystallization age of 579 ± 3 Ma for the Barra Alegre granitoid. Such age is concordant with the syn-collisional episode established for the central segment of the Ribeira Belt.