Feldspar-liquid equilibria in peralkaline acid liquids; an experimental study

Abstract

The liquidus surfaces of four pseudoternary joins have been determined. A plot of the molecular ratio (Na ~2~ O + K ~2~ O)/Al ~2~ O ~3~ (peralkalinity) versus Na ~2~ O/(Na ~2~ O + K ~2~ O) (sodicity) for peralkaline acid volcanic and plutonic rocks shows a distribution of points closely corresponding to that predicted from the experimental results. This gives support to the hypothesis that crystal-liquid processes have played important parts in the genesis of these rocks. Fractional crystallization or partial melting of any slightly alumina-undersaturated, silica-saturated trachyte or syenite, or of a similarly alumina-deficient rhyolite or granite, would be likely to give rise to a sodic peralkaline acid liquid.