Extensive Neogene Patagonian plateau lavas (46.5¿ to 49.5 ¿ S) southeast of the modern Chile Triple Junction can be related to opening of asthenospheric slab windows associated with collisions of Chile Rise segments with the Chile Trench at ≈12 Ma and 6 Ma. Support comes from 26 new total-fusion, whole rock 40Ar/39Ar ages and geochemical data from back arc plateau lavas. In most localities, plateau lava sequences consist of voluminous, tholeiitic main-plateau flows overlain by less voluminous, 2 to 5 million year younger, alkalic postplateau flows. Northeast of where the ridge collided at ≈12 Ma, most lavas are syncollisional or postcollisional in age, with eruptions of both sequences migrating northeastward at 50 to 70 km/Ma. Plateau lavas have ages from 12 to 7 Ma in the western back arc and from 5 to 2 Ma farther to the northeast. Trace element and isotopic data indicate main-plateau lavas formed as larger percentage melts of a garnet-bearing, oceanic island basalt (OIB) -like mantle than postplateau lavas. The highest percentage melts erupted in the western and central plateaus. In a migrating slab window model, main-plateau lavas can be explained as melts that formed as upwelling, subslab asthenosphere which flowed around the trailing edge of the descending Nazca Plate and then interacted with subduction-altered asthenospheric wedge and continental lithosphere. Alkaline, postplateau lavas can be explained as melts generated by weaker upwelling of subslab asthenosphere through the open slab window. Thermal problems of high-pressure melt generation of anhydrous mantle can be explained by volatiles (H2O and CO2) introduced by the subduction process into slab window source region(s). An OIB-like, rather than a mid-ocean ridge basalt (MORB)-like source region, and the lack of magmatism northeast of where ridge collision occurred at ≈13 to 14 Ma can be explained by entrainment of weak plume(s) or regional variations in an ambient, OIB-like asthenosphere.¿ 1997 American Geophysical Union