Brian S. Penn Pan-American Center for Earth & Environmental Studies Dept. of Geological Sciences University of Texas at El Paso El Paso, TX 79968
Abstract of paper presented at Rocky Mountain 1995 AGU meeting
Huerfano Butte is located 10 km north of Walsenburg, CO and is conspicuous on the eastside of I-25. This conical feature rises approximately 100 m above the surrounding plain. Metamorphism of the surrounding Pierre Shale to argillite extends radially about 80 m from the butte. The medium- to fine-grained size of the alkali-grabbro and lack of adjacent ejecta deposits do not support the hypothesis that Huerfano Butte represents a volcanic edifice. It is more likely that Huerfano Butte is a hypabyssal plug.
Compositionally, Huerfano Butte is a biotite olivine alkali-gabbro cut by two east-west trending dikes of different compositions. The larger felsic dike, visible from the west side and running from the base up to the cleft in the summit, is a 10 m thick east-west trending biotite monzonite dike. The smaller mafic dike is only visible from the east side and appears to be a heavily weathered alkali-lamprophyre. This conclusion is based on a number of observations of similarly weathered alkali-lamprophyre dikes found in the Spanish Peaks region.
40Ar/39Ar data for both the biotite gabbro (25.2 +/- 0.8 Ma) and monzonite (25.2 +/- 0.18 Ma) yield late-Oligocene ages, which are synchronous with the alkaline intrusive rocks of the Spanish Peaks found 50 km to the southwest. The concordont age spectra for both the dike and the surrounding plug are identical; implying that during the intrusion of one of the dikes the age of the plug was reset. Consequently, the age of the alkali-gabbro plug is probably greater than 25.2 Ma. The lack of cross-cutting relationships between the alkali-lamprophyre and the monzonite dikes make it difficult to ascertain their order of intrusion. But, based on compositionaly similarities, it is likely that the alkali-lamprophyre and alkali-gabbro intruded around the same time. In addition to the temporal similarities, major element analysis (Al2O3, K2O, CaO, MgO, TiO2, and SiO2) suggests that the alkali-gabbro is geochemically associated with the earliest alkaline intrusive rocks of the Spanish Peaks region.