Geologia, petrografia e geoquimica de duas suites TTG do dominio norte do complexo campos gerais, sudoeste de Minas Gerais

AUTOR(ES)
DATA DE PUBLICAÇÃO

2004

RESUMO

South of Passos and Alpinópolis, in the southwestem region of Minas Gerais, there are at least two tonalitetrondhjemite-granodiorite suites (TIG) associated with the Morro do Ferro Greenstone Belt that is a part of the northern domain of the Campos Gerais Complex (CCG). The two TTG suítes studied here belong to the Serra do Dondó and Córrego das Almas belts that stretch in a NW/SE - WNW/ESE direction. The rocks of these two suites are distinct in their geological, petrographic, and geochemical characteristics. The contacts between the two belts, and also with the metaultrabasic rocks of the volcanic sequence which only occur in the Serra do Dondó (FSD) belt, are always tectonic and fault bounded. The FSD shows a greater variation of rock types, with porphyritic tonalites, trondhjemitos, granodiorites, granodioritic gneisses, protomylonites and mylonites.The second suite, Córrego das Almas (FCA), south of FSD, consists of non-porphyritic tonalites, followed by migmatites, subordinate granites and mylonites. The two TIG suites present distinct features and intensities of deformation: the rocks of the FSD are more intensely deformed in relation to the rocks of the FCA. Whereas the rocks of the FSD were affected by intense mylonitization that reoriented an older foliation, the FCA rocks have incipient foliation and preserved igneous features. The FSD consists of porphyritic tonalites, with phenocrysts of oligoclase and euhedra1 titanite, and trondhjemites, both without magnetism; the rocks in the FCA are nonporphyritic tonalites, slight1y magnetic, with subhedra1 titanites. Protomylonites and mylonites with segregations of quartz and feldspar are frequent in the FSD, forming some of the granodiorites and granodioritic gneisses that occur in stream beds. The FCA has migmatites that are stromatic and folded, and granites with schlieren structure. Geochemically, the studied rocks are classified as TIG suites, calc-alka1ine to calcic, meta1uminous to peraluminous, in the medium potassium field. Harker diagrams bring out good correlations between silica and the major and minor elements; however, for trace elements the correlations are not so satisfactory. An expanded range of compositional variation is also observed for the porphyritic tonalites and granodiorites of the FSD. This group shows higher concentrations of Fe#, Th, Ce, Nb (except sample 102), Rb, Y; and less of Mg#, Mr. and Ga compared to FCA. For some of the rocks of the FSD, for the same silica content, Fe2O3, MnO, P2Os, Zr, V, Zn , La and, in some samples, K2O, tend to be higher, and Al2O3, Na20 and Pb, lower, in relation to the non-porphyritic tonalites of the FCA. REE distribution pattems show strong light REE enrichment and fractionated heavy REE, although this is less intense in the samples of the FSD. The latter also show negative anomalies of Eu and Sr. On the other hand, the non-porphyritic tonalites of the FCA do not present anomalies of Eu and Sr, and their "hockey stick" distribution pattern is typical for Archaean tonalites in general. In clear contrast to the rocks of FCA, the porphyritic texture in the tonalites, the larger variation of the chemical elements, and the negative anomalies of Eu and Sr of the FSD are indicative of igneous differentiation. It should also be mentioned that the behaviour of the elements of the FSD is to some degree due to the tectono-metamorphic modifications that affected these rocks more intensely than those of the FCA.

ASSUNTO(S)

geoquimica geologia - minas gerais

ACESSO AO ARTIGO

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