TEXTURAS INDICATIVAS DE METASSOMATISMO POTÁSSICO NOS AUGEN-GNAISSES DE NITERÓI, RJ

JOÃO FERNANDO MARTINS HIPPERTT

Resumo


The augen gnaisses o f Niterói are coarse grained rocks, with porphyroblasts of microcline and recrystallized tonalitic matrix. In this petrographic study, many replacement textures are discussed in detail , as well as some nonsubstitutive features are presented as product of blastic process too. In the K-metassomatism activity , megaporphyroblasts of microcline are originated by solid state cationic exchange in the former tonalitic matrix or plagioclase megacrystals. Transition textures and inherited features may be identified in the substitutive K-fe1dspar. The isostructural K-metassomatic transformation of plagioclase occurs with cell volume spread, resulting in granulometric reduction in the nonmetassomatized matrix. Finally, in the evolutive textural model suggested, myrmekite texture is interpreted as associated with K-metassomatism.

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