Strongly peraluminous plutons of the Old Woman-Piute Range, southeastern California, display the differentiation sequence: muscovite-biotite granite $$\rightarrow$$ muscovite-garnet-biotite granite $$\rightarrow$$ garnet-muscovite granite $$\rightarrow$$ garnet-muscovite aplite and pegmatite. The garnets show no evidence of reaction, but biotites in garnet-bearing rocks are rimmed by muscovite. Metamorphic assemblages in the country rocks suggest pressures of emplacement less than 4-5 kb. The plutons become relatively enriched in manganese with differentiation. Whole rock ratios of molar Mn/(Fe + Mg) increase from 0.01 in least felsic, garnet-free rocks to 0.3 in most felsic, garnet-rich rocks. MnO concentrations show a corresponding rise in biotites (0.4 to 1.8 wt %) and, less clearly, in garnets (9 to 25%). The differentiation sequence, textural relations, and mineral and whole rock compositions suggest that garnet paragenesis was controlled by the reaction: $$biotite + Mn-rich liquid \rightarrow garnet...