The Southern Volcanic Complex III is form ed by basalts, a lesser am o u n t o f trachybasalts and some scarce trachytes, a ll o f them m ild ly alkaline. Crystal fra ction ation w a s in an early stage, controle d by crystallisation and extraction of olivine (60%) and clinopyroxene (40%). The crystallisation path is, then, ruled by dominant clynopyroxene and lesser proportions of plagioclase and Ti-magnetite. Later extraction of plagioclase and clinopyroxene and some other differentation processes, could account for the presence of trachytes.The Angostura Formation representing the most recent activity within the entire Southern V o lc a n ic C om ple x, consists o f s tr o n g ly a lk a lin e an d u ltra a lk a lin e rocks. These rocks are scarcely differentiated, though certain fractionation tendences dominated by olivine extraction are observed