Plio-Pleistocene (3.4-0.125 Ma) post-plateau magmatism in the Meseta del Lago Buenos Aires (MLBA; 46.7°S) in southern Patagonia is linked with the formation of asthenospheric slab windows due to ridge collision along the Andean margin ε1 6 Ma ago. MLBA post-plateau lavas are highly alkaline (43-49% SiO2; 5-8% Na2O+K2O), relatively primitive (6-10% MgO) mafic volcanics that have strong OIB-like geochemical signatures. Their relatively enriched Sr-Nd isotope ratios (87Sr/86Sr=0.7041-0. 7049; 143Nd=0.51264-0.51279), low 206Pb/204Pb(18.13-18.45), steep REE patterns (La/Yb=11-54), and low LILE/LREE and LILE/HFSE ratios (Ba/La<15, La/Ta<15, Ba/Ta<180; Sr/La=15-22; Th/La<0.13; Ce/Pb>15) are distinctive from most other Neogene Patagonian slab window lavas. These data are interpreted to indicate contamination of OIB-like asthenosphere- derived slab window magmas with an EM1-type component derived from the Patagonian continental lithospheric mantle (CLM). The EM1-type signature in Patagonian slab window lavas are geographically associated with the Deseado Massif and indicate important regional differences in lithospheric mantle chemistry beneath southern Patagonia. We propose that hot, upwelling subslab asthenosphere in slab window tectonic settings can cause significant thermo-mechanical erosion and thinning of the continental lithospheric mantle and, thus, may be an important process in slab window magma petrogenesis.