GEOLOGIC MAPPING IN THE EL SUECO, ESTACION MOCHO, AND LOS SAUCES QUADRANGLES; LATE EOCENE TO MID-OLIGOCENE VOLCANISM, NORTH-CENTRAL CHIHUAHUA, MEXICO

These quadrangles in north-central Chihuahua, Mexico, show extensive late Eocene-early Oligocene and mid-Oligocene volcanism. Mid-Oligocene rocks include basalt and basaltic andesite lava, ashflow rhyolite, and peralkaline rhyolite. A local phenocryst-rich peralkaline ashflow rhyolite forms S. Campana and overlies a voluminous, aphyric peralkaline rhyolite erupted from a vent in lower Santa Clara Canyon. This unit caps most of the western Santa Clara Canyon drainage basin. A rhyolite welded tuff rich in sanidine and quartz phenocrysts underlies the aphyric unit and was erupted from a caldera about 40 kms to the southwest. In lower Santa Clara Canyon, a widely recognized late Eocene-early Oligocene glomerocrystic ashflow rhyolite overlies glomerocrystic andesite lavas. These rocks also cap Mesa Tinaja Lisa and the NE flank of S. Tinaja Lisa.Intracaldera ashflow rhyolites form the crest and SW flank of S. Tinaja Lisa. Granitic intrusives, rhyolite flow domes, and rhyolite lavas mark the outer limits of a caldera that is 20 kms wide and exceeds 50 kms in length. In Chontes Canyon on the SW border, a rhyolite flow dome overlies intrusive granite and is overlain by the ashflow rhyolite that caps Mesa Tinaja Lisa. Geologic relations and published age dates show that the caldera-fill rhyolites are also late Eocene-early Oligocene.Slightly younger ashflow rhyolites filled a smaller caldera that extends from S. Espinazo del Diablo northward to Gallego Canyon. A shallow glomerocrystic andesite intrusion occupies the NE corner of this caldera and probably marks a major vent for the widespread glomerocrystic ashflow rhyolite, North of Gallego Canyon in the Lomas El Gusano area, mid-Tertiary basalts from local vents and ashflow rhyolites from a caldera 30 km to the SSW near Rincon de Alamo overlie late Eocene-early Oligocene outflow ashflow rhyolites and glomerocrystic andesite lavas. Felsic and mafic magmas coexisted during both volcanic episodes and often mingled.