Exposed around the margins of Mono basin and the periphery of Long Valley caldera are ∼100 km3 of potassic, basic to intermediate lavas that record high water and oxygen fugacities. Most eruptions occurred between 4 and 2 Ma, although sporadic potassic volcanism continued into the Quaternary with a pulse between 0.1 and 0.5 Ma, ∼15 km northeast of Mono basin. The lava types include absarokite, minette, hornblende lamprophyre, trachybasalt, and trachyandesite. Estimated water contents for lavas without hydrous phenocrysts range between 2 and 3 wt% considerably more than mid-ocean ridge, oceanic island, or back-arc magmas. Calculated fO2, values fall between -0.4 and +1.2 log units of the Ni-NiO buffer. Although this potassic suite was erupted in an extensional tectonic setting at the western margin of the Basin and Range province, its high K2O/TiO2 and low Zr/Ba ratios suggest a subduction-modified mantle source. If subduction was the process that enriched the lithospheric mantle in large-ion lithophile elements (K, Ba, etc.), it was also a mechanism for mantle oxidation and hydration.