GEOCHEMICAL EVOLUTION OF THE FELSIC ALKALINE ROCKS OF TANGUÁ AND RIO BONITO INTRUSIVE BODIES, STATE OF RIO DE JANEIRO, BRAZIL

This article presents chemical composition and magmatic evolution processes for the felsic alkaline rocks of Tangua and Rio Bonito intrusive complex rock bodies, State of Rio de Janeiro, Brazil, and shows also the preliminary Rb-Sr age for Tangua intrusive body. Most of the analysed rocks are undersaturated in silica with moderate (Na+K)/Al and high K2O/(Na2O+K2O) ratios, being classified to be potassic nepheline syenite. The variation diagrams strongly affirm the crystallisation of titanite, ilmenite, apatite, and clinopyroxene or amphibole and moderately nepheline and sodic alkaline feldspar. The geochemical evolution of the nepheline syenite magma occurs in three stages: K2O/(Na2O+K2O) reduction by leucita fractionation; The same by potash feldspar; Compositional transition from silica undersaturated to oversaturated field crossing over the thermal divide by means of assimilation of country rock of the continental crust. The fractionation crystallisation took place under the H2O pressure of about 0.7kb, which corresponds to a depth of 3km. The crustal assimilation is relevant in the Soarinho body, moderate in the Tangua and Rio Bonito complexes.. To cross over the thermal divide, super-reheating of the nepheline syenite magma or injection of fluid-rich magma is required. The preliminary Rb-Sr dating for the rocks of Tangua body shows the intrusive age of 66.8Ma with the Sr initial ratio of 0.7062