GEOCRONOLOGIA E GEOQUÍMICA DE ORTOGNAISSES DA REGIÃO ENTRE IPORÁ E FIRMINÓPOLIS: IMPLICAÇÕES PARA A EVOLUÇÃO DO ARCO MAGMÁTICO DE GOIÁS

Abstract

In this study we report new Sm-Nd and Rb-Sr isotopic data and preliminary geochemical results for orhtogneissic and metavolcanic rocks units exposed roughly between the towns of Iporá and Firminópolis, southwestern Goiás. Samples from orthogneisses adjacent to the Iporá volcano-sedimentary sequence yielded a Rb-Sr isochron age of 673 ± 75 Ma, with initial 87 Sr/ 86 Sr ratio of 0.7048 ± 0.0004. ε Nd (T) values for these rocks are between ca. +0.4 and +1.9 and T DM ages are between 1.0 and 1.1 Ga, indicating the juvenile character of the parental magma. Trace element results also indicate the arc affinity of the magma. One felsic metavolcanic rock from the Iporá volcano-sedimentary sequence yielded an U-Pb zircon age of 636 ± 6 Ma and T DM model ages between 0.76 and 1.0 Ga. It represents the youngest volcanic example in the Goiás Magmatic Arc. In the Fazenda Nova area, samples from tonalitic and dacitic units display juvenile Nd isotopic characteristics, with T DM model ages of 0.95 and 0.88 Ga, respectively. Further to the south, in the southern end of the Moiporá-Novo Brasil mylonitic zone, biotite gneisses and mylonites have substantially older T DM ages, ranging from ca. 1.7 to 3.2 Ga, indicating the presence of an older sialic fragment in that area. The gneisses in the Moiporá area together with the Ribeirão gneisses in the vicinity of Arenópolis and displaying Archaean model ages, represent the only two exposures of older terranes known in the Goiás Magmatic Arc. The Firminópolis hornblende gneisses, in the eastern end of the area investigated, show arc-type geochemical signature and Nd isotopic characteristics (Sm-Nd whole-rock "errorchron" age of ca. 630 Ma and T DM ages of 1.1 -1.4 Ga) which indicate that the tonalitic protoliths were still part of the Neoproterozoic arc system. The results indicate that the juvenile rock units belonging to the Goiás Magmatic Arc extend to the east as far as the Firminópolis area where gneisses with very primitive characteristics, similar to the Arenópolis, Matrinxã and Sanclerlândia orthogneisses, are exposed.