New Mexico Mineral Symposium — Abstracts

A variety of mantle and crustal xenoliths are found in the base-surge and air-fall deposits of the five Pleistocene maars of Doña Ana County, south-central New Mexico. The mantle xenoliths are ultramafic in composition and include lherzolite, spinel peridotite, dunite, and pyroxenite. These xenoliths represent a portion of the spinel peridotite mantle found beneath the southern Rio Grande rift and record chemistry and petrology of the mantle at 27 to 65 km depth. The maars are located in the Potrillo volcanic field and are typical of other maars located in the southwestern United States. These maars appear to be confined to mafic alkaline volcanic fields rather than to tholeiitic volcanic fields.

A maar is a large volcanic crater that is cut into the country rock and possesses a low rim composed of pyroclastic debris. The pyroclastic material is ejected into the air and forms base-surge and air-fall deposits near the vent. Three types of maar are recognized based on changes in the geometry and amount of pyroclastic material. An explosion crater is formed and the conduit is partially filled in with pyroclastic material and later by slump and often lacustrine material. Tuff-ring maars form with a basal breccia composed of the mobilized surface material and a base-surge deposit. The depression forms within the tuff ring and is usually above the level of the surrounding country¬side. Tuff-cone maars form with a basal breccia composed of the mobilized surface material, a base-surge deposit, and a thick air-fall tuff.

Kilbourne Hole is the largest and most popular maar in the group. It is approximately 1,600 m long by 2,000 m wide and has the largest array of mantle and crustal xenoliths exposed. Hunt's Hole is smaller, approximately 1,500 m wide, and is 17,000 years old (W. Williams, J. Poths, and E. Anthony, personal communication 1992). It is 3 km south of Kilbourne Hole. Hunt's Hole may be synchronous with Kilbourne Hole. Both mantle and crustal varieties of xenoliths have been reported to occur at Hunt's Hole. Potrillo maar is located 17 km to the south and straddles the international border. It is 4,000 m north-south, and 3,000 m east-west, and is 55,000 years old. A vast array of mantle and large crustal xenoliths are present at this maar. All three maars are explosion craters.

Malpais maar is a tuff-ring or tuff-cone maar and is 20 km west of Potrillo maar in the southern part of the West Potrillo volcanic field. It is 1,400 m in diameter and stands about 400 m above the surrounding area. It is one of the oldest maars in the area, about 232,000 years old. Anorthoclase megacrysts and olivine xenocrysts have been reported in the pyroclastic deposits. Riley maar is a less-known tuff-cone or tuff-ring maar and is approximately 14 km north of Malpais maar. It measures approximately 1,000 m in diameter. Mantle xenoliths of spinel peridotite have been reported from this locality, as well as pyroxene and anorthoclase megacryst.