Petrogênese e geocronologia das intrusões alcalinas de Morro Redondo, Mendanha e Morro de São João: caracterização do magmatismo alcalino no Estado do Rio de Janeiro e implicações geodinâmicas

Abstract

The models for formation of alkaline plutons of the Southeastern Brazil Alkaline Province or Poços de Caldas-Cabo Frio Magmatic Lineament, which genetic modeling associates crust reactivations or mantle plumes, with definition of a hot spot track. The objective of this work is to report new data and interpretations to contribute to a better understanding and discussion about the model of alkaline rock generation. The studies involved geological mapping, petrography, litogeochemistry, Sr-Nd-Pb isotopes and 40Ar/39Ar geochronology. The selected alkaline complexes are the Morro Redondo, Mendanha and Morro de São João, located at Rio de Janeiro State. These intrusions are well-distributed along the Poços de Caldas-Cabo Frio Magmatic Lineament. The Morro Redondo alkaline intrusion is composed mainly by nepheline syenites and nepheline-bearing syenites and mafic rocks are rare. It was defined as a sodic silica-undersaturated alkaline suite, with metaluminous to peralkaline characteristics. The intrusion was dated at 74 Ma (40Ar/39Ar plateau age). The Mendanha alkaline intrusion is compose by various types of syenitic rocks, breccias and subvulcanic structures, as pyroclastic rocks and dikes. It was defined by a sodic silica-saturated alkaline suite with metaluminous characterisics. The intrusion presented two distinct 40Ar/39Ar ages for the magmatism: 64 Ma for Mendanha rocks and 54 Ma to lamprophyre dike, which illustrates a polycyclic magmatism. The Morro do Marapicu 40Ar/39Ar age yielded 80 Ma. The Morro de São João alkaline intrusion has a large variety of silica-undersaturated to silica-saturated rocks, as syenites, alkali-syenites and monzosyenites (some pseudoleucite-bearing), with melanocratic varieties, as malignites and ferguites. These rocks defined distinct alkaline silica-undersaturated suggenting sodic and potassic types. There was found an alkaline silica-saturated suite, defined by alkaline gabbros and shonkinites. The petrogenesis of these intrusions corresponds to the fractional crystallization, with assimilation of host rocks, and the crustal contamination is indicated by high variability of Sr isotope ratios. For Morro de São João origin is suggested a K-Na bimodal magma. These intrusions were generated from enriched mantle-derived magmas, possible associated to ancient subduction zone of Ribeira orogen. In terms of the new 40Ar/39Ar data, the hot spot model is not plausible, because the Morro do Marapicu is older than the other studied intrusions. Some models projected mantle plumes with 1000 Km size, what may explain the reason for Mendanha and Morro de São João have the nearly the same age. The obtained isotopic signatures for these intrusions were not associated to Trindade signature and, if the mantle plume s model is correct, the plume that has the most similar signature is Tristão da Cunha.