The mafic-ultramafic complex of Niquelandia is one of the three major layered intrusions occurring in the Goiàs State, Central Brasil.Previous studies on this complex [1] have revealed that the parent melts of the complex have suffered a significant crustal contamination.In a recent field work several lenses or septa of exotic metamorphic rocks were found striking parallel or sub-parallel to the complex layering: the total amount of these rocks has been estimated to represent the 13% of the volume of the whole complex.These lenses or septa were found starting from the intermediate portion of the LGZ unit upwards; however, they are mainly concentrated at the contact between LS and the US. They can be divided in three major lithotypes: quartzites, gneisses and calc-silicate rocks derived from sandstones, pelites and marls, respectively. The petrological and geochemical investigation of transects perpendicular to the complex layering provided the evidences that these rocks are the crustal contaminants of the complex parent melts: as the matter of fact they produced an enrichment in the hybrid melts of incompatible elements (i.e.: LREE, K, Ba and Rb).On the basis of their petrographic and geochemical characters, the contaminants are supposed to belong to the upper stratigraphic unit of the volcano-sedimentary sequence of Indaianopolis. This is also in agreement with geochronological data. In fact Pimentel and coworkers [2] dated one of the septa of contaminants occurring in US with the U/Pb method on zircons: they obtained an age of 1248±23 Ma and suggested that this age is referred as the US intrusion age, proposing that US and LS (their estimate of LS intrusion age is 797±10 Ma) were not coeval. This model is incongruent with the robust evidences that US and LS are cogenetic, as for instance the similare LILE patterns found in rocks of the two different units or the outcropping, in LS, of cumulus rock similar to the US ones.We present a simplified model of assimilation and fractional crystallization which tentatively supports the hypothesis that LS and US are genetically kindred. References. [1] Rivalenti, G., Correia, C.T., Girardi, V.A.V., Mazzuchelli, M., Tassinari, C.C., Bertotto, G.W. (2008): J. South Amer. Earth Sci., 25, 298-312; [2] Pimentel, M.M., Ferreira Filho, C.F., Amstrong, R.A. (2004): Precambrian Research, 132, 132-135.
PETROGENETIC ASPECTS OF THE CONTAMINATION PROCESSES IN THE MAFIC-ULTRAMAFIC COMPLEX OF NIQUELANDIA (GOIÀS, BRASIL) / Giovanardi, T.; Rivalenti, G.; Mazzucchelli, Maurizio; Girardi, V. A. V.; Correia, C. T.. - In: PLINIUS. - ISSN 1972-1366. - ELETTRONICO. - 36:(2010), pp. 293-293. (Intervento presentato al convegno L'Evoluzione del Sistema Terra dagli Atomi ai Vulcani tenutosi a Ferrara nel 13-15/09/2010).
PETROGENETIC ASPECTS OF THE CONTAMINATION PROCESSES IN THE MAFIC-ULTRAMAFIC COMPLEX OF NIQUELANDIA (GOIÀS, BRASIL).
Giovanardi T.;MAZZUCCHELLI, Maurizio;
2010
Abstract
The mafic-ultramafic complex of Niquelandia is one of the three major layered intrusions occurring in the Goiàs State, Central Brasil.Previous studies on this complex [1] have revealed that the parent melts of the complex have suffered a significant crustal contamination.In a recent field work several lenses or septa of exotic metamorphic rocks were found striking parallel or sub-parallel to the complex layering: the total amount of these rocks has been estimated to represent the 13% of the volume of the whole complex.These lenses or septa were found starting from the intermediate portion of the LGZ unit upwards; however, they are mainly concentrated at the contact between LS and the US. They can be divided in three major lithotypes: quartzites, gneisses and calc-silicate rocks derived from sandstones, pelites and marls, respectively. The petrological and geochemical investigation of transects perpendicular to the complex layering provided the evidences that these rocks are the crustal contaminants of the complex parent melts: as the matter of fact they produced an enrichment in the hybrid melts of incompatible elements (i.e.: LREE, K, Ba and Rb).On the basis of their petrographic and geochemical characters, the contaminants are supposed to belong to the upper stratigraphic unit of the volcano-sedimentary sequence of Indaianopolis. This is also in agreement with geochronological data. In fact Pimentel and coworkers [2] dated one of the septa of contaminants occurring in US with the U/Pb method on zircons: they obtained an age of 1248±23 Ma and suggested that this age is referred as the US intrusion age, proposing that US and LS (their estimate of LS intrusion age is 797±10 Ma) were not coeval. This model is incongruent with the robust evidences that US and LS are cogenetic, as for instance the similare LILE patterns found in rocks of the two different units or the outcropping, in LS, of cumulus rock similar to the US ones.We present a simplified model of assimilation and fractional crystallization which tentatively supports the hypothesis that LS and US are genetically kindred. References. [1] Rivalenti, G., Correia, C.T., Girardi, V.A.V., Mazzuchelli, M., Tassinari, C.C., Bertotto, G.W. (2008): J. South Amer. Earth Sci., 25, 298-312; [2] Pimentel, M.M., Ferreira Filho, C.F., Amstrong, R.A. (2004): Precambrian Research, 132, 132-135.
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