Faciologia da Suíte Serra dos Órgãos entre Petrópolis e Teresópolis, Região Serrana Fluminense

Abstract

The Serra dos Órgãos batholith is the dominant feature of Serra do Mar and it extends over 140 km in abundant rocky exposures that demonstrate the great continuity of this massif and its compositional variety. This variety composes the so- called Serra dos Órgãos, represented in the work area by three different facies: leucocratic granite gneiss, allanite granite gneiss and hornblende-biotite granodioritic gneiss. These facies are arranged in elongated kilometric bands, generally parallel to foliation and structural lineaments. The foliation is marked by the orientation of mafic aggregates, generally biotitic, although it contains a greater proportion of hornblende in the granodioritic types. In the study area, it was observed that this foliation most often dips to the south and south-east, unlike the known northwest dip pattern for SSO. The verification of the orientation of euhedral crystals of undeformed feldspars indicates that the foliation was generated mainly by magmatic flow. Subsequently this primary foliation would have been superimposed by small shear zones within the granitic pluton, which may represent the tension field reflections after or during the magmatic stage, imparting features of local shears and folds within the igneous bodies associated to a transpressive system. Geobarometric data calculations showed distinct crystallization pressures for leucocratic granitic gneisses (6.6kbars) and hornblende- biotite granodioritic gneisses (5.8kbars). These values allow the indication of emplacement and crystallization depths comparable to the middle and lower crust. The discriminant diagrams of the presented tectonic environments corroborate that this positioning occurred in a late tectonic context. Isotopic data suggests that the age of the magmas’ melt extraction, that originated the Serra dos Órgãos Suite, is around 1.4Ga. The protolith of this magma possibly underwent intense crustal fusion with potential Sm enrichment.