The Sierra el Virulento, located in the Maxican Basin and Range province of eastern Chihuahua, is composed of over 700 m of siliceous high-K calc-alkalic ash flow tuff ranging in age from 33.7¿0.3 to 29.5¿0.5 m.y. At least 70% of phenocrysts in the tuffs are zoned feldspar crystals that have andesine cores and anorthoclase rims. Subordinate phenocrysts include clinopyroxene, biotite, ilmenite, magnetite, sanidine, hornblende, quartz, and orthopyroxene. Pyroxenes are Mg-Rich and have a restricted range of Mg-Fe ratios. Fe-Ti oxide minerals equilibrated at high temperatures (880¿--1055 ¿C) and oxygen fugacities (-log fO2 = 8.3--11.0). The tuffs are strongly enriched in light rare earth elements (LREE) and have moderate negative Eu anomalies. Initial 87Sr/86Sr ratios range from 0.7043 to 0.7065 and show a rough correlation with SiO2, Sr, and Rb/Sr. The Sr-isotopic compositions suggest that the rocks formed from an isotopically heterogeneous source material and were modified by later crustal contamination. Dacite tuffs may have originated by either fractional crystallization of more mafic magmas or by partial melting of crustal material. The series dacite to rhyolite seems to have evolved by a combination of low-pressure fractionation in shallow magma chambers and crustal interaction. If variations from dacite to rhyolite are the result of crystal fractionation, the mineralogy and major-element data suggest that anorthoclase was the dominant fractionating phase.