Abstract
Sodic sanidine and anorthoclase phenocrysts from three specimens of comendite ash-flow tuff and four lavas of comendite composition contain appreciably less calcium than do the enclosing glassy groundmass materials. Since crystallization of such feldspars from melts having the composition of the associated glasses would tend to raise the calcium contents of the residual liquids, slightly peralkaline silicic magmas will contain significant amounts of calcium regardless of the degree of fractional crystallization that they have undergone. Late-stage intratelluric crystallization of small amounts of calcium-rich mafic minerals probably is responsible for the glasses being poorer in calcium than the low-temperature compositions which would have obtained through feldspar crystallization. Strontium, which is preferentially incorporated in alkali feldspars relative to glass, and which is excluded from pyroxenes and amphiboles, provides a more meaningful parameter by which to infer the degree of crystal fractionation that a silicic melt has undergone.
