In the Black Hills, South Dakota, anatectic migmatites that formed above the second sillimanite isograd comprise mesosomes dominated by quartz, biotite, sillimanite, plagioclase, and sporadic poikiloblasts of K-feldspar, melanosomes dominated by biotite and sillimanite, and leucosomes. Some leucosomes have near-minimum granite composition, but many lack alkali elements and have fibrolitic sillimanite instead of feldspar. The proportion of Si/Al is the same in both leucosome types. The sillimanite-containing leucosomes include fluorapatite, which indicates high activity of F. The lack of alkali feldspar in these leucosomes is attributed to its instability relative to sillimanite + quartz in the presence of fluids with high aH+/aK+ ratios that evolved from the crystallizing leucosomes. It is suggested that quartz-aluminosilicate segregations and veins in other high-grade terranes may also represent pegmatitic melts in which alkali feldspar crystallization was inhibited by high activities of hydrogen, resulting in loss of alkali elements.