Abstract
The formation of quartzo-feldspathic rocks-principally pegmatites-is discussed from the viewpoint of thermodynamic stability of minerals and mineral associations. Most quartzo-feldspathic rock bodies, migmatites, and pegmatitic veins encountered in folded chains exhibit mineral associations which indicate a formation at temperatures below which silicate melts cannot exist. Data from a granulite facies area in western Greenland show that, even at the relatively high P-T conditions that prevailed there, the granitic component in the system was unable to form melts. Furthermore, pegmatites and quartzo-feldspathic massifs usually possess mineral associations in harmony with the degree of metamorphism prevailing in the surrounding region. In high-grade areas the pegmatites-independent of their bulk chemical composition-show high-temperature mineral assemblages; in low-grade areas low-temperature assemblages occur in the pegmatites. Because the melting curve of a magma is definitely determined by the bulk composition of the system (at constant pressure), the logical conclusion must be that most pegmatites, veined gneisses, migmatites, and several acid massifs were formed by some metamorphic-metasomatic processes in which silicate melts were unable to co-operate.
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