GRANITES OF PEDRA DE GUARATIBA, RJ: FACIOLOGICAL VARIATIONS AND FIELD EVIDENCES OF MEGACRYSTS ACCUMULATION BY MECHANICAL SEGREGATION IN ZONE OF ENCLAVES

Abstract

The Serra da Capoeira Grande and  the Morro do Silvério in Pedra de Guaratiba constitute an southwest extension  of the Massif da Pedra Branca. The dominant rock is granite with NE-SW oriented crystals of tabular microcline of intense pink color. Faciological diversification occurs in metric to centimeter scale as a result of changes in grain size, phenocryst/matrix ratio or lineage intensity. The  contacts between facies are always transitional  The enclaves have lenticular contours, with different degrees of assimilation and stretch orientation concordant with tabular microcline and igneous banding.   Outcrop patterns suggest processes of mechanical segregation as the origin of subtypes, with fine-grained equigranular granite occurring as  product of segregation between phenocrysts and interstitial material. Enclaves of metassediments and of ortho-derivative rocks occur along a linear NE-SW direction. Spatial arrangement between enclaves, equigranular granite and microcline clusters indicate that enclaves acted as filters during magmatic flow. Allanite concentrates occur along the enclave zone, presuming an origin of this mineral by fluid mobilization of rare earth elements in a structurally more permeable zone.