The Square Peak volcanic series, norhtern Quitman Mountains, Hudspeth County, Texas
— Thomas M. C. Hobbs and Jerry M. Hoffer

A number of Tertiary volcanic centers occur along the Rio Grande in West Texas. This zone of aligned volcanic centers is approximately coincident with the hingeline that separates the Diablo and Coahuila platforms from the Chihuahua Trough (McAnulty, 1976).

The volcanic rocks of the northern Quitman Mountains represent the westernmost volcanic center in West Texas (fig. 1). The igneous complex has been termed a cauldron and is located at the northeastern limit of thrust faults of the Chihuahua Tectonic Belt (McAnulty, 1976). The volcanic rocks of the range crop out in a northerly elongate mass approximately 7.5 km long by 3.5 km wide, just west of the town of Sierra Blanca.

Both extrusive and intrusive igneous rocks constitute the bulk of the northern Quitman Mountains. The Square Peak Volcanic Series, named by Huffington (1943), forms the core of range and occurs as a roughly layered pile with an aggregate thickness exceeding 1000 m. The volcanic units consist of welded tuffs ranging in composition from trachyte to rhyolite. The volcanic units rest with angular unconformity on folded Cretaceous sedimentary rocks and are unconformably overlain by younger colluvium and alluvium.

Intrusive rocks of the Quitman pluton crop out in an almost continuous ovate body surrounding the volcanic rocks. The ringshaped outcrop is continuous at the surface except on the east side of the range, where it is covered by alluvium (Albritton and Smith, 1965). The Quitman pluton is interpreted as a ring dike (Huffington, 1943).