Abstract

The Meseta Buenos Aires faces the triple junction between the Nazca, Antarctic, and South American plates and consists of two olivine basaltic units separated by two sedimentary units. Seventeen whole-rock K-Ar age determinations gave a minimum age of 57 m.y. (late Paleocene) for the lower unit and ages ranging from 16 to 4.4 m.y. (early Miocene to Pliocene) for the upper unit. The Friasian Land-Mammal Stage sedimentary unit underlying the upper basaltic unit is assigned to the late Oligocene-early Miocene interval, and the marine sedimentary unit underlying the Friasian deposits and overlying the lower basaltic unit is assigned to the Eocene–early Oligocene interval. The lower basaltic unit rests unconformably on a marine fossiliferous sedimentary unit of probable Late Cretaceous to early Paleocene age.

The basalts of the Meseta Buenos Aires are alkaline and are associated with andesite flows of tholeiitic character. The lower unit formed while the Farallon plate was being subducted under the South American plate, and any relation between this unit and the triple junction is excluded. The origin of the upper unit could be related to perturbations in the subcrustal mantle caused by the subduction of segments of the Chile Rise. Geochemical considerations, in addition to the fact that both basaltic units formed under different tectonic settings, are consistent with the hypothesis that the origin of the alkali basalts is independent of the environment in which they are found.

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