Caracterização petrográfica e geoquímica do Complexo Alcalino do Morro de São João, Casimiro de Abreu-RJ

Abstract

The Morro de São João Alkaline Complex is located at Barra de São João, district of Casimiro de Abreu-RJ, eastern portion of the Rio de Janeiro state. It is composed of alkaline rocks, being the only one in the state where occurs the interaction of felsic, mafic and ultramafic rocks relating each other and exhibiting textures that results of interaction between felsic and mafic magmatic magmas. This work presents petrographic data including studies of textures of magmatic mixture and assimilation that are observed throughout the massif and lithogeochemical data to generate a detailed analysis of the petrogenetic processes and magmatic evolution that generated this massif. Petrographic characterization was based on detailed descriptions, in the macro and microscopic scales, added to elementary mineral chemistry with electron probe analysis on feldspar, amphibole, pyroxene and garnet crystals. The JEOL JXA-8230 electronic probe micro-analyzer was used for this analytical method. The lithogeochemistry was based on the analysis of the major and trace elements in a classificatory and discriminatory levels added to the geochemical modeling of trace elements to determine the percentage of fractional crystallization and magmatic mixture. The whole rock analysis of the major elements was done by the ICP-AES method and for the trace elements ICP-MS method. The main lithotypes mapped were classified as gabbro hornblende, syenite hornblende with nepheline, syenite, syenite with nepheline, nepheline syenite and phonolite. From these classifications, three facies were defined according to similar petrographic characteristics of the mapped lithotypes: Melanocratic Facies, represented by gabbro hornblende, Mesocratic Facies, syenite hornblende with nepheline and Leucocratic Facies, represented mainly by syenite nepheline. The main mineralogy consists in nepheline, alkaline feldspar (albite), plagioclase (bytownite), amphiboles (Fe-kaersutite, Fe-pargasite and sadanagaite), clinopyroxene, titanite, magnetite, garnet (grossular), apatite and zircon. With the exception of the aphanitics phonolites, all rocks in question are phaneritic, equigranular and medium granulometry. They are potassic, metaluminous and form two distinct geochemical series: the first one is silica-saturated with normative hyperistene + olivine and the second one is moderate to strongly silica-unsaturated with nepheline + diopside (sometimes olivine). The ratios CaO/Al2O3, (La/Zr)N and (Ti/Eu)N corroborate with the hypothesis that the mafic and ultramafic rocks of Morro de São João are products of a partial fusion of a metasomatized peridotitic mantle with CO2 rich fluids. Felsic rocks are interpreted as a pulse of a more evolved liquid. By means of the calculations of geochemical modeling of trace elements, the main hypothesis if that the Morro de São João Alkaline Complex is a result of an interaction between two magmatic chambers with different evolutionary histories. Both mafic and felsic rocks, including phonolites, contain signs of assimilation of the siliceous basement and of another alkaline rock with a distinct geochemical character.