Date of Award


Document Type


Degree Name

Master of Science (MS)



First Advisor

F.R. Karner


The purpose of this study was to map and study the characteristics and origin of the Cenozoic phonolites and related rocks of the Houston Creek area, located on the southwestern flank of the Bear Lodge Mountains, 10 km northwest of Sundance, Wyoming. Mapping was conducted on a 2 1:10,000 scale over a 16 km area. Microprobe, optical, and x-ray diffraction procedures were employed to observe and describe petrographic and chemical characteristics of the rocks.

The Cenozoic igneous rocks of the Houston Creek area are classified as phonolite, phonolite porphyry, trachyte porphyry, sodalite-bearing phonolite and altered felsite. These hypabyssal rock types are characterized by porphyritic volcanic textures and trachytic to pilotaxitic fabrics. They contain phenocrysts of sanidine, aegirine-augite, hauyne, sodalite, orthoclase, sphene, and melanite, with analcime, natrolite, calcite, and clays(?) occurring as major alteration products. Field relationships indicate that phonolite and trachyte porphyry were emplaced as sills within the Minnelusa Formation and phonolite porphyry was emplaced as a sill at the Deadwood-Whitewood Formation contact. Sodalite-bearing phonolite appears to have been injected as dikes along fracture zones developed in altered felsite, which comprises the intrusive core of the Bear Lodge Mountains.

Major element chemistry shows that the phonolites are sodic in character and are higher in total alkalis and lower in s10 2 than trachyte porphyry. Chemical and petrographic trends indicate that these rocks resemble those of a typical alkali olivine basalt series and that the primary magma was phonolitic in composition. Granitic xenoliths, present in slightly Si02 -enriched phonolites, suggest that granitic crustal contamination may have been a major cause of igneous variation in the Houston Creek area.

The magma was partly crystallized at the time of emplacement and is 0 interpreted to have had a temperature of somewhat less than 715 C. Present evidence indicates the possibility that the phonolitic magma was generated by partial melting of alkali olivine basaltic material in the lower crust. The partial melting may have been initiated by Laramide induced uplift and relief of pressure at depth, which produced an influx of alkali-rich volatiles from the underlying mantle. These volatiles served to focus beat, reduce crustal melting temperatures, and provide the magma with its alkaline character.

O' Toole (70231 kB)

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