A COMMON MAGMA SOURCE FOR CLOSELY ASSOCIATED IGNIMBRITES AND LAVAS FROM SOUTH-CENTRAL ANDES

摘要

Ignimbrites from the South Central Andes (22°20' to 24°30' S) and calc-alkaline stratovolcanoes erupted during Plio-Quaternary. Ignimbrites are (high-silica) andesites, dacites and rhyolites, whereas lavas are basaltic andesites, andesites and dacites. Within the same range of siiica contents, phenocrysts (plagioclase, hornblende, biotite, orthopyroxene and Fe-Ti oxides) show similar distribution and eomposition in ignimbrites and lavas. Chemical compositions (major and trace elements) of ignimbrites and lavas follow the same differentiation trends. Ignimbrite Sr-isotope ratios range between 0.7062 and 0.7096, similar to lavas from the same area (0.7056-0.7089). Lavas associated with stratovolcanoes and ignimbrites that erupted in their immediate vicinity have very similar Sr-isotope ratios, suggesting that parental magma sources should be locally similar for lavas and ignimbrites.