Petrogênese das suites alcalinas da porção oriental do Complexo Vulcânico de Nova Iguaçu, RJ

Abstract

Trachytes and syenites are the commonest rocks outcroping in the eastern portion of the Nova Iguaçu Volcanic Complex. These lithotypes occur in the area along with pyroclastic rocks such as breccias, agglomerate and lapilli. Trachytes and syenites are rather variable in texture in the study area. Trachytes are represented by three porphyritic facies whereas syenites are represented by three equigranular and three inequigranular facies. Field relationships point to a volcanic origin for the porphyritic trachytes in the area. Lithogeochemical data were used to discriminate undersaturated and saturated/oversaturated, trachytic and syenitic suites. Alkaline basalts that occur in the area as enclaves have rare earth element compositions capable of generating the undersaturated trachytes by crystal-liquid equilibria processes. Nevertheless, these data have shown that the trachytes and syenites are not cogenetic and also that undersaturated types cannot evolve to saturated/oversaturated types under fractional crystallisation process. In general, the data indicate that plutonism and volcanism in the area are non-coeval processes. This implies that magmatic processes may have been operating in a local scale for as long as approximately 30 Ma. Geodynamic models focused on cold continental lithosphere translation over fixed hotspots seem not to apply to explain the reincidence of magmatic activity in the studied area. Alternative models involving the reactivation of anomously hot subcontinental lithospheric mantle over a prolonged period of time seem more adequate to explain the magmatism and volcanism in the Nova Iguaçu Complex.