Date of Award:
Master of Science (MS)
Donald W. Fiesinger
Lava flows of Tertiary-Quaternary age occur in Enoch Valley, Upper Valley, and Slug Valley in southeastern Idaho. The basalts in Upper Valley and Enoch Valley contain olivine (Fo69 to Fo37), plagioclase (An62 to An39), augite and Fe-Ti oxides. The lava in Slug Valley lacks plagioclase, but contains sanidine (Or70 to Or56) with a trace of biotite and amphibole, and thus, has been termed alkali trachyte.
Black Mountain, on the eastern side of Bear Lake, northeastern Utah, is capped by basalt. Minerals present include olivine (Fo83 to Fo72), plagioclase (An71 to An53 J, augite, and magnetite.
Chemically, the basalt of Enoch Valley is comparable to olivine tholeiite of the Snake River Plain, as it contains olivine and hypersthene in the norm. The basalt of Upper Valley contains a greater amount of Si02 and K2O and less MgO than tholeiite of the Snake River Plain. This basalt contains normative quartz and hypersthene and is classified as tholeiite. The presence of nepheline and olivine in the norm of the basalt from Black Mountain indicates that it is an alkali-olivine basalt. The lava from Slug Valley contains high K and Mg, moderate Si, and low Al and Na. It is similar to lamproites of orenditic affinity.
The temperatures of crystallization calculated from co-existing olivine and pyroxene, range from 1,015 degrees C to 996 degrees C for the valley basalts, and range from 1,021 degrees C to 1,002 degrees C for the alkali trachyte. The temperature calculated for the basalt sample from Black Mountain is 1,015 degrees C. The temperatures estimated using coexisting magnetite and ilmenite range from 1,021 degrees C to 978 degrees C for the valley basalts.
The proposed origin of the Enoch Valley basalt is that it is a direct product of partial melting of a mantle of pyrolite composition. Fractionation, during ascent of the magma, could possibly have produced the Upper Valley lava. The basalt on Black Mountain was possibly derived as the result of partial melting of a pyrolitic mantle as well, but due to differences in mineralogy and normative constituents, it seems to be unrelated to the valley basalts. The origin of the Slug Valley alkali trachyte is uncertain. This lava may have been generated from a mica peridotite mantle and is possibly related to the Leucite Hills lava in Wyoming.
Puchy, Barbara J., "Mineralogy and Petrology of Lava Flows (Tertiary-Quaternary) In Southeastern Idaho and at Black Mountain, Rich County, Utah" (1981). All Graduate Theses and Dissertations. 3817.
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