Summary: Previous interpretations relating to precipitation of anorthoclase megacrysts from alkaline basaltic magmas under high-pressure conditions are contradicted. It is proposed that these feldspars were crystallized above the solvus from hydrous melts at shallow depths, under low-pressure equilibrium conditions. These melts had been considerably enriched with volatiles of sodium, strontium, and barium, and to some extent also of rubidium.