Feldspar Megacrysts in Alkali Basalts

Feldspar megacrysts from pyroclastic ejecta of four alkaline basaltic centers were analyzed by atomic absorption spectrophotometry. Anorthoclase is present at three localities, oligoclase at two localities, and andesine at one locality. These crystals are interpreted as phenocrysts of the basalt that crystallized under high-pressure conditions. The presence of anorthoclase in these alkali basalts is reasonable because the normative feldspar compositions from these rocks plot within the excluded portion of the Or-Ab-An diagram. This indicates that two types of feldspar, one alkali-rich and the other a more calcic plagioclase, would have crystallized from the melt. The value for D_Sr of 5.6 that we measure for the anorthoclase basalt indicates that crystallization of this mineral will significantly deplete the total strontium content of an alkaline magma.