PAMPEAN PLEISTOCENE ASH DEPOSITS FROM ANDEAN VOLCANOES - ARGENTINA

Abundant vitroclastic components are present in loessic sediments of the Argentine pampean region. Near Mar del Plata city (37°56'S, 57°35'W) and along 20 km of the Atlantic coast the vitroclasts constitute true tuff deposits. They outcrop as white lenses up to 15 cm thick interbedded in late Pleistocene sandy silt lacustrine sediments assigned to the Facies Camet Norte of the Santa Clara Formation. Two samples of ash shards, dated (⁴⁰Ar/³⁹Ar) by the U.S.G.S (Menlo Park), gave coincident values of 20.9 ±2.5 and 20.9 ±2.6 Ka BP. The composition of these tuffs, principally glass shards, corresponds to a K-rich rhyolite, with abundant quartz and reduced normative hypersthene and K₂O/Na₂O=1,05. Few crystalloclasts of Ti-biotite, pargasitic hornblende, hedenbergite, augite, Na-Ca plagioclase, scarce quartz and zircon are present. The Fe/Ca ratios of the analyzed glass shards plot in the field of glasses from volcanoes built-up in areas where the crust was subjected to extension. According to the atmospheric circulation, the provenance of the ashes should be from the SW. Between 34°S and 39°S, the Pleistocene-Holocene Andean volcanic arc is divided into a N-NNE volcanic front located on the Cordillera Principal and volcanic centers east of the front located on blocks separated by grabens produced by extension. These last volcanoes have alkaline character due to their high content of alkalis, and subalkaline affinities because of the orthopyroxene presence. Copahue and Pino Hachado volcanoes 1100 km from Mar del Plata, with resembling silicic lithologies, are suspected of being the source. The importance of this event is emphasized if the thickness of these deposits is compared with that of the ash deposits registered during the last southern South American big eruptions and allows to speculate about the consequences that an eruption of similar magnitude would have at the present time.