The alkaline activity in the Cabo Frio region is made up mainly of intermediate and felsic differentiate rocks emplaced about 55 Ma ago into the crystalline basement as sills, plugs and dykes. Two magma suites can be distinguished: (a) a strongly silica-undersaturated tephriphonolite to phonolite serie; (b) a weakly silica-undersaturated to satured trachyandesite-trachyte series. Petrography, mineral chemistry, whole-rock chemistry trends and isotopic data strongly support a genetic link among the lithotypes of each suites consistent with fractional crystallization processes dominated by the observed phases. The same data, however appear to exclude a link between the two groups of rocks, supporting a petrogenesis by prolonged differentiation processes starting from two distinct parental magma with a slight difference in the SiO2 saturation. The scarce basanites and trachybasalts reported in the same area could represent the most probable compositions for the parental magmas of the two suites. The initial 87Sr/86Sr (0.70401-0.70458) and 143Nd/144Nd (0.51239-0.51247) isotopic ratios and the significant potassic component indicate derivation of these rocks from an enriched lithospheric mantle source
