Petrography and preliminary geochemical data of the syenitic and trachytic alkaline suites northwards the Nova Iguaçu Volcanic Complex, Rio de Janeiro

Abstract

Distinctive syenites, trachytes and subordinately also pyroclastic breccia outcrop within the Vigné Quarry mining area located northwards the Nova Iguaçu Volcanic Complex in the State of Rio de Janeiro. This paper presents field and petrographic data for these rocks as well as the results of lithogeochemical analysis for the syenites and trachytes only. The detailed geological mapping discriminated three trachytic facies, undiferentiated syenites and polimythic breccia in the study area, besides tabular intrusions of trachytes and lamprophyres. The petrography allowed the discrimination of two distinctive groups of syenites included in the undifferentiated syenites unit as such: the inequigranular porphyritic group represented by porphyritic alkaline feldspar syenite, porphyry alkaline feldspar syenite and fine-grained porphyritic syenite; and the equigranular group represented by coarse-grained biotite syenite, medium-grained syenite and coarse-grained alkaline feldspar syenite. The trachytic rocks by their side were grouped in three distinctive units all with porphyritic lithotypes: the light-grey porphyritic trachyte unit, the purple porphyritic trachyte unit and the porphyry trachyte unit. The lithogeochemical data showed that syenites and porphyritic trachytes that outcrop in the Vigné Quarry area represent an alkaline miaskitic, predomiantly sodic series although ultrapotassic trachytes also occur subordinately. Syenites and trachytes are metaluminous and peraluminous rocks as corroborated by the absence of normative acmite. Syenitic and trachytic suites can be discriminated on the basis of silica saturation degrees. In general, these suites are likely to have evolved by fractional crystallisation or AFC as opposed to previously proposed petrogenetic models involving magma mixing in subvolcanic, zoned magma chambers.