The superposition of two tectonic regimes (rifting and subduction) in western Mexico has led to the development of a lamprophyric volcanic front ∼70 km closer to the Middle America trench than is the main axis of andesitic volcanism. The San Sebastian cindercone field represents the northernmost extent of lamprophyric activity in western Mexico and lies 40 km trenchward of Volcán Ceboruco, a historically active, andesitic stratovolcano. The predominant country rocks are Cretaceous silicic ash flows, although marine sedimentary rocks (emplaced between 45 and 115 Ma) also occur. Minette, absarokite, and kersantite lavas were erupted between 0.48 and 0.26 Ma in the San Sebastian field in close association with flows of basaltic andesite and hornblende andesite. There is a gradual transition observed between the two intermediate lava types, basaltic andesite and kersantite, as plagioclase is suppressed and hornblende and phlogopite are stabilized in melts of intermediate composition. This transition reflects the increasing influence of water during crystallization and documents a wide range of hydrous conditions. Examination of the primary lamprophyric lava types, absarokite and minette, reveals a remarkable range in their oxidation states. If these lavas reflect their mantle source conditions, then the redox state of western Mexico's sub-arc mantle spans the interval between the Ni-NiO and magnetite-hematite buffers, a variation in fO2 of five log units. The oxidation and hydrous enrichment of Mexico's sub-arc mantle probably formed in response to subduction. The eruption of lamprophyric lavas in front of the andesitic suite of the main axis, however, may have depended upon rifting (1) to trigger melting of the modified mantle and (2) to provide an easy passage to the surface through crustal thinning and block faulting.