Date of Award:


Document Type:


Degree Name:

Master of Science (MS)




Donald C. Feisinger


Basalt flows occur in the Snowville area of north-central Utah and the Table Mountain and Holbrook areas of south-central Idaho. All basalt flows are aphanitic in groundmass, and contain olivine, plagioclase, augite, and opaque oxides. They can be distinguished by texture. Snowville basalt has predominantly subophitic to intergranular textures. Table Mountain basalt is fine grained, with stumpy groundmass plagioclase and equant ilmenite crystals. Holbrook basalt has pilotaxitic to intergranular textures, with the presence of plagioclase phenocrysts and characteristic exsolution lamellae in Fe-Ti oxides. The olivine grains in Holbrook area are intensely oxidized to Fe-Ti oxides.

Snowville basalt contains olivine phenocrysts (Fo88 -Fo44 ) in a groundmass of olivine (Fo63 -Fo47), augite (Wo42 -Wo36), and plagioclase (An77-An52). The lower flow unit of Table Mountain basalt contains olivine phenocrysts (Fo88-?) in a groundmass of augite (Wo44 En44 Fs17), and plagioclase (An58-An48). The upper flow unit of Table Mountain basalt has olivine phenocrysts (Fo82-Fo65), plagioclase phenocrysts (An73-An67), and plagioclase groundmass (An64-An55). The Holbrook basalt is composed of olivine phenocrysts (Fo67-Fo57)and plagioclase phenocrysts (An68-An43 ) in a groundmass of olivine (Fo59Fos53) augite (Wo39 En44 Fs17), and plagioclase (An67-An35). The basalts of the Snowville and Holbrook areas, represent petrographic, mineralogical, and chemical characteristics of both olivine-tholeiitic basalt and alkali-olivine basalt, whereas Table Mountain upper and lower flow units show their affinity with alkali-olivine basalt. Chemically, basalts from these three areas are consistently high in silica, magnesium, and alkali content. The Snowville basalt has a high Ba content and high strontium isotope ratio.

Fractional crystallization models indicate that the basalt flows from the three different areas are genetically unrelated. The testing also suggests that the upper and lower flow units of the Table Mountain area are not genetically related. The basalts of the three areas also can not be evolved from the basalts found at Kelton, the Rozel Hills or Black Mountain. Basalts of the Snowville area have consistently higher magnesium and silica contents than Snake River basalt, Kelton area basalt, and Rozel Hills and Black Mountain basalt, indicating that they may represent what was initially a very primitive basaltic lava. High Ba content and strontium isotope ratio indicate that the Snowville basalt was contaminated by crustal material.

Table Mountain and Holbrook basalt may have formed as a result of partial melting from a pyrolite or garnet peridotite mantle.

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