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.