Huerfano Butte: A Hidden Gem In The Spanish Peaks Region of Colorado

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2023-08-12に共有

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. The medium- to fine-grained size of the alkali-grabbro and lack of adjacent ejecta or breccia deposits do not support the hypothesis that Huerfano Butte represents a volcanic edifice. It is more likely that Huerfano Butte is a hypabyssal plug. Metamorphism of the surrounding Pierre Shale to argillite extends radially about 80 m from the butte.

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, probably a camptonite. 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 20 km to the southwest.