Plagioclase phenocrysts, from 0.3 to 4 mm long, are the most abundant phase in Cerro Quemado lavas, making up to 40 volume percent. Plagioclase commonly exhibits complex zoning, including normals, reverse, patchy and oscillatory zoning, and cellular or sieve-like zones. Two phenocryst populations are observed, An35-45 and An-70-85. Plagioclase microlites (An40-70) are the most common groundmass phase. Rare, subhedral quartz phenocrysts, fractured, embayed and charged with fine needles and clinopyroxene (to 0.1 mm in length) also occur in Cerro Quemado lavas.
The petrographic evidence, summarized in the following table, strongly suggests an origin by magma mixing for the Cerro Quemado lavas.