The intra-back arc region of the Andean Southern Volcanic Zone (34-38ºS) is characterized by more than 800 monogenetic cones erupting alkalic basalts ranging in composition from volcanic-front to ocean-island type basalts. The origin of this latter group is debated. We suggest the isotopic and trace element variations reflect the contribution of the metasomatized subcontinental lithospheric mantle (SCLM) in the Payenia source. We present a simple forward model of cumulate formation and metasomatism and subsequent melting within the SCLM that predicts the observed Payenia lava compositions.  Variations in 143Nd/144Nd and 176Hf/177Hf suggest that the age of the SCLM cumulates is 100-150 Ma, which coincides with the development of the proto-Pacific Andean arc and the breakup of Gondwana. Variations in δ18Ooli­ values from modeled cumulate-derived melts indicate that differentiation and melting within the SCLM represents a process that can fractionate oxygen isotopes even when the melt forming the cumulate has MORB-like δ18O values, explaining the observations of low-δ18O signatures of Payenia.