ORIGIN OF MYRMEKITE IN THE LIGHT OF PHASE RELATIONS IN THE QUARTZ-FELDSPAR SYSTEM

摘要

Myrmekite (a quartz-feldspar intergrowth) is widespread within two-feldspar granites and geneisses, occasionally in occurs in anorthosites. The study of granitic rocks from the Urals and a search through literature sources have shown that the typical features of myrmekite are very stable, i.e. indicating the universal mechanism of its formation. According to the proposed model, the principal cause of myrmekite origination relates to the metastable melting and crystallization of plagioclase. Natural silicate melts contain the more calcium plagioclase, whereas the conjugated solid phases, on the contrary, are poorer in anorthite as compared with the thermodynamic equilibrium. Crystallization of myrmekite replacing the potassic feldspar in granites and gneisses, or the andesine-labrador in anorthosites, shifts the phase relations towards the equilibrium. Myrmekite is formed both at the final stage of crystallization and during the ultrametamorphism process as a result of interaction between the previously formed feldspars and a water fluid coexisting with a silicate melt under the close-to-solidus temperature.