The chemistry of Jotabeche ignimbrites is distinct from other Tertiary volcanic rocks in the Andes. The rocks of Jotabeche have La, Sr and Y concentrations which fit an eclogitic trend, similar to other Andean volcanoes Pircas Negras, Tortolas and Vallecito. More generally, the geochemistry is indicative of the volcanic activity being influenced both by a thick crust and other factors of crustal contamination. Other composition data are indicative of a drop in mantle water content above the shallowing slab. The crust above the Jotabeche ignimbrite source region was thick enough to permit the stability of garnet in said source region.