Black Mountain

The building of the Black Mountain volcano involved the eruption of a series of voluminous ash-flow tuffs, multiple caldera subsidence, and partial or complete filling of the caldera by lavas. The rocks range from subalkaline basalts, mugearites, trachytes and rhyolites, to peralkaline trachytic rhyolites, comendites and pantellerites. The Thirsty Canyon tuff is the most extensive of the tuff deposits and comprises five distinct ash-flow sheets, two of which, the Spearhead and Labyrinth Canyon members, are comenditic and a third, the Gold Flat member, pantelleritic. The Gold Flat Member is densely to partially welded and contains 10-25% phenocrysts of alkali feldspar, quartz, sodic plagioclase, fayalitic olivine, clinopyroxene, barkevikitic amphibole and opaques with rare biotite, zircon and chevkinite, in a dense groundmass (Noble, 1965). Chemical analyses are given by Noble and Parker (1975, Table 1), REE data by Noble et al. (1979, Table 1) and an account of the general geological setting of Black Mountain by Ekren et al. (1971).