NEOGENE PATAGONIAN PLATEAU LAVAS - CONTINENTAL MAGMAS ASSOCIATED WITHRIDGE COLLISION AT THE CHILE TRIPLE JUNCTION

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.