The Sierra Blanca igneous complex of south-central New Mexico consists of the Oligocene Sierra Blanca Volcanics which have been intruded by Oligocene monzonite to quartz syenite stocks, sills, and dikes. The Sierra Blanca Volcanics are composed of four formations: Walker Andesite Breccia, Nogal Peak Trachyte, Church Mountain Latite, and Godfrey Hills Trachyte. The Walker Andesite Breccia comprises the major volume of the eruptive rocks. It is characterized by monolithic blocks transported and frozen in a matrix of plagioclase microlites, enstatite, hornblende, and hematite. A few K-Ar dates indicate that volcanism began approximately 35 m.y. ago and continued until about 25 m.y. ago. The Walker Andesite Breccia is succeeded by flows, tuff layers, and welded ash flows of the Nogal Peak Trachyte, Church Mountain Latite, and Godfrey Hills Trachyte, which are preserved as widely separated remnants. The younger volcanic formations give evidence of a more explosive period of volcanism compared with the Walker Andesite Breccia stage.
Four hypabyssal stocks intrude the Sierra Blanca Volcanics: Rialto stock, Chaves Mountain stock, Bonito Lake stock, and Three Rivers stock. Three are multiple intrusives. Alkali enrichment is pronounced in one of the stocks. K-Ar dates of the stock range from 34.4 m.y. to 25.8 m.y.
The volcanic and intrusive rocks show progressive mineralogical and chemical changes typical of alkali-calcic series. The oldest volcanic rocks contain zoned plagioclase (An77 to An47) and enstatite, but in successive volcanic rocks plagioclase becomes more sodic (An32) and hornblende becomes the dominant ferromagnesian mineral. FeO(total Fe)/MgO ratios increased from 1.45 to >5 during magma differentiation. Fractional crystallization with removal of calcic plagioclase and enstatite during volcanism appears to have been responsible for differentiation and generation of the late-stage silicic intrusive phases.
The large volume of andesitic to trachytic volcanic rocks and intrusive syenitic rocks along with the lack of cogenetic basalt suggest that the parent magma at Sierra Blanca was andesitic. Major structural planes of weakness, which intersect in the Sierra Blanca area, would allow magma generated in the mantle to move into the crust and erupt on the surface. Similar controls appear to have been operational along the Rocky Mountain trend and parallel to the Rio Grande depression. Other andesite breccia sheets present along the trend include the Absaroka volcanic field, Wyoming, the Thirtynine Mile volcanic field, Colorado, and the Red River volcanic-intrusive complex, New Mexico. They are Oligocene in age and have many similar characteristics.