Petrology and volcanic stratigraphy of the El Sueco area, Chihuahua, Mexico

Detailed mapping of a 2000 km² area approximately midway between El Paso and Chihuahua City has delineated a greater than 1000 m section of rhyolitic, andesitic, and basaltic lava flows and rhyolitic ash-flow tuffs unconformably overlying Lower Cretaceous limestone. The area is dominated by large, NNW-trending fault-bounded mountains separated by bolsons. The volcanic section can be compositionally and texturally divided into four sequences: 1. A thick sequence of ash-flow tuffs and related volcaniclastic sediments lie over an erosional surface on Cretaceous limestone. These deposits comprise the Liebres formation, the first sequence. 2. Voluminous flows of the Rancho El Agate tholeiitic andesite, a crystal-rich unit which has a source in the map area, make up the second sequence. 3. Lava flows, plugs, flow domes, sills and minor ash-flow tuffs of the Gallego, Carneros, El Dos, and Mesteño rhyolites make up the third sequence of volcanic activity. Some of these rhyolite bodies domed the adjoining rocks. 4. Basalt and minor interlayered ash-flow tuffs comprise the fourth sequence. Chemical analyses indicate basaltic rocks of the area are metaluminous, and andesitic and rhyolitic rocks are peraluminous. A plot of alkalinity against silica indicates the rocks are gradational between the dominantly peraluminous rocks of the Sierra Madre Occidental and the metaluminous portion of the dominantly peralkaline rocks of Trans-Pecos Texas.