Abstract

In the Andean Range, south of 39°S, there is a suite of eroded volcanic centres that includes remains of vents or proximal, intermediate or distal volcanic facies. New geochronological K-Ar data show that their ages are in the Late Pliocene-Middle Pleistocene interval, and these eroded centres are equivalent to those located on the eastern side of the Andes. Geochemical patterns of pliocene-pleistocene volcanic centres are similar to those of the Quaternary volcanoes of this segment with a gradual variation toward the east in the La/Sm, Ba/La ratios and K even reaching back-arc signature of the volcanoes far from the trench. Previous works, under the assumption that all volcanic rocks from the western side of the Andes were quaternary in age, have proposed a western migration of the volcanic arc to the west after Pilocene. Instead, our geochronological and geochemical data show that the initial wide Late Pliocene-Middle Pleistocene arc narrows in the Quaternary time preserving the volcanic front. An important variation of the convergence velocity was recognized at the end of Pliocene and the authors propose that it is a first order factor on the geometry and location of the volcanic arc south of the 39°S.