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Petrology And Mineralogy Of Quaternary Basalts, Gem Valley And Adjacent Bear River Range, Southeastern Idaho, William D. Perkins

All Graduate Theses and Dissertations, Spring 1920 to Summer 2023

Quaternary basalts of Gem Valley, Idaho, are present as valley fill (Group 1) and as well defined flows in the Bear River Range (Group 2) east of Gem Valley. Minerals present in both groups of basalts include olivine (Fo₇₃ -Fo₃₉), augite (Wo₄₁ En₃₉ Fs₂₀), plagioclase (An₇₅ -An₄₀), and Fe-Ti oxides. Coexisting pairs of magnetite and ilmenite, and olivine and clinopyroxene in several samples indicate temperatures of crystallization from 958°c to 1167°c. The Group 2 basalts exhibit a cumulate texture with abundant large (2 cm) phenocrysts of plagioclase.

Chemically, the Gem …