Pegmatitos: evolução dos conceitos e sistemas de classificação

Pegmatites: evolution of concepts and classification systems

Abstract

Pegmatites are granitic igneous rocks that are distinguished from others by their coarse to very coarse grain size and wide granulometric heterogeneity. This textural complexity is generally accompanied by mineralogical variations that, in conjunction, characterize an internal pegmatitic structure formed by primary zones of contact, wall, intermediate and core, in addition to the possible secondary replacement zones. According to the most accepted petrogenetic models, pegmatitic melts originate from magmatic fractionation of parental granitic plutons or from anatexis. The most comprehensive and widely used pegmatite classification systems are based on temperature and pressure of pegmatite formation, as well as their mineralogical and chemical compositions. These classification systems reveal important insights about the genesis of pegmatitic melts and are therefore essential for petrogenetic investigation of these rocks. Based on the strong textural, mineralogical, structural and petrogenetic complexity of pegmatites, the objective of this paper is to provide a synthesis of the key-concepts for an initial understanding of these rocks, which includes definition, petrogenetic models, internal structure and main classification systems. Reading this material is strongly recommended before first contact with pegmatites in order to elucidate the main features that should be observed for any petrographic and/or petrogenetic study of pegmatites.