Peridotite xenoliths from the Quaternary Cerro del Fraile basalts, southernmost South America, sample the mantle less than 25 km east of the Andean Austral Volcanic Zone (AVZ), an arc segment characterized by melting of a young, ‘hot’, subducted slab and the eruption of adakites. Many of these peridotite xenoliths are modified by either modal and/or cryptic Na-rich metasomatism, which produced elevated Sr/Y, La/Yb and La/Nb ratios typical of slab melts. Some of the metasomatized xenoliths, derived from a relatively deep and hot portion of the mantle, contain an interconnected network along mineral grain boundaries of high-Mg#, low-Y andesitic glass with major and trace element composition similar to the high-Mg adakites erupted in the AVZ. We interpret this adakitic glass to be a quenched slab melt that has infiltrated the mantle wedge from below. The texture and chemistry of this quenched melt and surrounding mantle minerals suggest that selective assimilation of predominately mantle clinopyroxene, some spinel and minor olivine is an important process in producing high-Mg adakites from primary low-Mg slab melts.