Caracterização de vulcânicas adakíticas e cálcio-alcalinas no greenstone belt do rio Itapicuru, Bahia: petrogênese e implicações geodinâmicas

Abstract

New geochemical data for andesites and dacites of the Maria Preta Unit, Rio Itapicuru greenstone belt, revealed two distinct geochemical groups: one of adakitic affinity and another calc-alkaline. The adakitic suite, localized to the west in the unit, is characterized by high values of Sr/Y, Ti/Y N >;1; it shows enrichment in LFSE (Low Field Strength Elements) relative to HFSE (High Field Strength Elements) and moderate rare earth elements fractionation. The calc-alkaline suite shows low Sr/Y ratios, lower rare earth elements fractionation and negative Eu anomalies. It also shows high abundances of incompatible elements (Y, Zr, Th) and high Cr and Ni concentrations. The most likely mechanism of formation for the calc-alkaline rocks is partial melting of mantle peridotite hydrated by fluids released from the subducting slab, followed by plagioclase fractionation. The two suites cannot be linked to each other by fractional crystallization and have probably derived from distinct magmas (different initial Nd ratios). The adakite geochemical features require partial melt of hydrous metabasalts at depths high enough to stabilize garnet+hornblende as the residual phase. This suite does not show any significant interaction with the mantle wedge as inferred from its low Cr and Ni abundances when compared to Cenozoic adakites. Sm-Nd isotope data indicated high positive εNd(T) for the adakites (+4.8 to +2.5) and slightly lower values for the calc-alkaline suite (εNd(T) +1.9 to +1.4). These data indicate a juvenile magma input to form the UMP volcanics. If the two volcanic suites are of the same age, then their geographical position suggests subduction of the oceanic slab to the west.