Abstract: Proterozoic amphibolites from Sweetwater Wash, in the Old Woman Mountains of southeastern California, contain a variety of mineral assemblages dominated by the low-Ca amphiboles anthophyllite, gedrite, and cummingtonite. Their Mg- and Al-rich, Ca-poor bulk compositions suggest that the amphibolites represent basalts that were altered prior to metamorphism. In addition to amphiboles, mineral assemblages include cordierite, biotite, garnet, Ca-rich plagioclase, and ilmenite ± rutile. Corundum, staurolite, and spinel occur locally in Al-rich enclaves associated with cordierite. Orthoamphiboles commonly exhibit complex microstructures, including twinning, intergrowths, and apparent exsolution; spot analyses show an unusually large range of chemical compositions, even within a single thin section, in several cases extending across the crest of the solvus in the orthoamphibole system. Ionic substitution within the orthoamphibole series was dominated by the Mg-tschermakitic and edenitic exchange reactions. The amphibolites are thought to have been subjected to metamorphic temperatures above the orthoamphibole solvus during both the Proterozoic and the Cretaceous. Cretaceous metamorphism was followed closely by rapid uplift and denudation, which may be responsible for the exsolution and range of compositions of the orthoamphiboles.