Extensive Neogene Patagonian plateau lavas (46.5 degrees to 49.5 degre
es 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 approximate to 12 Ma a
nd 6 Ma. Support comes from 26 new total-fusion, whole rock Ar-40/Ar-3
9 ages and geochemical data from back are plateau lavas. In most local
ities, plateau lava sequences consist of voluminous, tholeiitic main-p
lateau flows overlain by less voluminous, 2 to 5 million year younger,
alkalic postplateau flows. Northeast of where the ridge collided at a
pproximate to 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 b
ack are 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 t
han postplateau lavas. The highest percentage melts erupted in the wes
tern and central plateaus. In a migrating slab window model, main-plat
eau 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 asthenosp
here through the open slab window. Thermal problems of high-pressure m
elt 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) -l
ike source region, and the lack of magmatism northeast of where ridge
collision occurred at approximate to 13 to 14 Ma can be explained by e
ntrainment of ''weak'' plume(s) or regional variations in an ambient,
OIB-like asthenosphere.