Geochemistry and isotope systematics of small- to medium-volume Neogene–Quaternary ignimbrites in the southern central Andes: evidence for derivation from andesitic magma sources

The geochemical and isotopic compositions of small- to medium-volume (<10 km3, <50 km3) Neogene–Quaternary ignimbrite sheets of the southern central Andes (25°00′–26°45′S, 67°30′–69°30′W) have been analysed in order to assess the petrogenesis of this type of calc-alkaline magma. The ignimbrites are derived from the active volcanic zone around Salar de la Isla (SIS) in Chile and from behind the active arc in the area of the Salar de Antofalla (SAF), Argentina. The ignimbrites in both areas are characterized by 87Sr/86Sr(t) ratios (0.7066–0.7090) that are lower and εNd(t) values (−2.4 to −5.1) that are higher than in the large-volume, caldera-sourced ignimbrites of Cerro Galán, 100 km further east and those of ignimbrites from other megacalderas of the central Andes. This difference reflects smaller amounts of crustal material in the SIS and SAF ignimbrites rather than a change in basement composition as indicated by Pb provinciality. The isotope signature of the SIS and SAF ignimbrites is similar to that of late- to post-Miocene composite calc-alkalic andesite/dacite stratovolcanoes of the frontal arc in this region. This finding, combined with major and trace element modelling, lends support to a petrogenetic model for the SIS and SAF ignimbrites involving fractional crystallization from a LIL-depleted andesitic parental magma, which was beforehand modified by crustal contamination.