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

The geochemical and isotopic compositions of small- to medium-volume (< 10 km(3), < 50 km(3)) Neogene-Quaternary ignimbrite sheets of the southern cen tral Andes (25 degrees 00'-26 degrees 45'S, 67 degrees 30'-69 degrees 30'W) have been analysed in order to assess the petrogenesis of this type of cal c-alkaline magma. The ignimbrites are derived from the active volcanic zone around Salar de la Isla (SIS) in Chile and from behind the active are in t he area of the Salar de Antofalla (SAF), Argentina. The ignimbrites in both areas are characterized by Sr-87/Sr-86(t) ratios (0.7066-0.7090) that are lower and epsilon Nd(t) values (-2.4 to -5.1) that are higher than in the l arge-volume, caldera-sourced ignimbrites of Cerro Galan, 100 km further eas t and those of ignimbrites from other megacalderas of the central Andes. Th is difference reflects smaller amounts of crustal material in the SIS and S AF 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/da cite stratovolcanoes of the frontal are in this region. This finding, combi ned 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 modifie d by crustal contamination. (C) 2001 Elsevier Science B.V. All rights reser ved.