Jurassic volcanic rocks in northeastern Mexico: A possible remnant of a Cordilleran magmatic arc

Pre-Oxfordian Mesozoic subaerial volcanogenic rocks occur in a band extending northwest from Ciudad Victoria, Tamaulipas, to Santa Maria del Oro, Durango. These strata include Nazas, Rodeo, and Caopas Formations in Durango, Coahuila, and Zacatecas; La Boca Formation and its underlying volcanic basement at Canon de Huizachal, Tamaulipas; and volcanic units below La Joya Formation at Real de Catorce and Charcas, San Luis Potosi. Rocks at these localities have similar lithologies, stratigraphic positions, and paleontologic and isotopic ages. Field mapping in the Caopas-Pico de Teyra area, northern Zacatecas, and ancillary research provide insight into the nature of this suite. At least 3 km of abundant airfall tuff, tuffaceous siltstone, and uncommon ashflow tuff are present near Pico de Teyra; this sequence appears to belong to a more distal facies than the flows, breccias, and laharic conglomerates of the Nazas and Rodeo Formations exposed 25 km to the north. Porphyritic rhyolite (Caopas Formation) occurs within these volcanogenic rocks as a fault-bounded block and is interpreted as a cogenetic, subvolcanic pluton. A relatively undeformed portion of the Caopas has yielded a zircon U-Pb age of 158 ± 4 Ma. Petrographic and limited chemical data from these formations show that calc-alkaline andesite, dacite, and rhyolite are the most common compositions. The volcanogenic rocks in northeastern Mexico are south of the inferred trace of the Mojave-Sonora megashear. Their large volume, their lithologic and chemical characteristics, and their age suggest that these rocks may be a component of the Jurassic arc of western North America that was translated southeastward along the megashear