Miocene basalt plugs in Goseong contain a large variety of crustal and mantle xenoliths and xenocrysts. One of basalt plugs, Unbongsan, are derived from 160 km depth. Whole-rock geochemistry and pressure and temperature conditions of mineral phases indicate that Unbongsan volcanic rocks are alkali basalts and the source magma of the alkali basalts was generated from about $0.2{sim}2%$ partial melting of depleted garnet peridotite. Crystallization pressures and temperatures of mineral phases within ascending magma of Unbongsan alkali basalt indicate that olivines, clinopyroxenes, and plagioclases were crystallized at $75{sim}110km,;40{sim}52km,;37{sim}54km$ depth, respectively. The ascending magma of Unbongsan alkali basalts enclosed mantle xenoliths at about $57{sim}67km$ depth.