CONSANGUINITY OF METASOMATITES, PEGMATITES, AND GRANITES

Abstract

Fields of granitic pegmatites containing rare metals in industrially important Precambrian zones are accompanied by regional zones of hydrothermally-altered rocks containing high concentrations of hydromagmatophilic elements typical of the crystallization residue of granitoid magmas. The constant restriction of these metasomatic formations to pegmatite fields, their geochemical specialization in common with the pegmatites, and unidirectional mineral-forming processes indicate the development of metasomatites, pegmatites and granites during the evolution of a common magmatic system. Hence, in constructing a model of pegmatite genesis, it is necessary to analyze the system as a whole, including granitoids, pegmatites and metasomatic alteration of their host rocks. Such an approach was used in constructing a quantitative geochemical model, based on the behavior of rubidium and cesium in pegmatite genesis. The model was developed from the example of a Precambrian pegmatite field in which granitoid magmatism evolved in two stages: a granite stage proper (microcline granite-migmatite - two-mica granite - pegmatoid granite) and a pegmatite stage (microcline-albite dikes - microcline-albite dikes with muscovite - spodumene-feldspar dikes).