Abstract
The Star Mountain rhyolite covers 500 square miles in the Davis Mountains, 100 miles east of El Paso, Texas. It appears to be the largest known rhyolitic mass to have been emplaced as a liquid flow rather than as a « froth flow » or nuée ardente. That it was emplaced as a liquid is shown by the lineation of phenocrysts, the vesicular and autobrecciated zones, baking of underlying tuffs, and by the complete absence of shard structures and evidence for welding. Three flow units with an aggregate thickness of 800 feet can be distinguished on the basis of field, chemical, and petrographic criteria. With the exception of some vesicular zones the rock is either finely crystalline and massive, or glassy and brecciated. Minor structures indicate that the flow of each unit was highly irregular.
Anorthoclase feldspar is the only significant phenocryst species in the unit. Phenocryst-groundmass compositional relationships cannot be explained on the basis of conventional granite system phase relationships, nor do recently proposed pantellerite relationships fit this case. An experimental investigation of this unusual case is under way.
A high temperature of extrusion of the rhyolite is indicated by the lack of measurable triclinicity in the feldspar. A low water vapor pressure is demonstrated by the presence of apparently primary magnetite and the scarcity of vesicular zones. Despite the low water content, high temperature caused the low viscosity necessary for the melt to spread over the 500 square mile outcrop area, presumably with approximately the same fluidity as a typical basalt flow.
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Gibbon, D.L. Origin and development of the Star Mountain rhyolite. Bull Volcanol 33, 438–474 (1969). https://doi.org/10.1007/BF02596519
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DOI: https://doi.org/10.1007/BF02596519