CHEMICAL AFFINITIES OF LOW-GRADE METAMORPHIC FORMATIONS OF THE CONTENDAS-MIRANTE PROTEROZOIC COMPLEX (BAHIA, BRAZIL)

PIERRE SABATÉ, MOACYR M. MARINHO

Resumo


The formations of the Lower Proterozoic Contendas-Mirante Complex are formed by low-grade metamorphic rocks among which magmatic components interbedded in greywackes are recognized. The origin of undifferentiated metamorphites is established by the behaviour of Al, Na, K. Ultramafic and mafic rocks are shown to be of picritic composition. Chemical affinities are evidenced. AI, Fe + Ti, Mg behaviour permits the separation of two magmatic suites, tholeiitic and transitional calc-alkaline, and their respective anorthositic and ultramafic cumulares. The demonstrated hygromagmatophile character of Nb, Y and Zr permits the following of the magmatic evolution. This is the result of fractional crystallization of liquids fonned by various degrees of partial fusion of two mantle sources of different composition. The fractionation modes are shown by the behaviour of Cr and Ni which also permits the evaluation of the nature of the solid source. High PH2O must be present to explain the production of transitional series and ultramafic rocks and could also explain the formation of the spilitic rocks. Hydrothermal post-magmatic phenomenon is shown by Ba and Rb contents and behaviour. These affinities suggest a complex geotectonic environment. The whole volcanosedimentary sequence is affected by folding and metamorphism of Transamazonian age.


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