PETROGENESIS OF HIGH-SILICA RHYOLITES OF THE MINERAL MOUNTAINS, UTAH

The Mineral Mountains, Utah, provide a unique opportunity to investigate the petrogenesis of high-silica rhyolites and their relationship to underlying granitoid lithologies and coeval basaltic volcanism. Small-volume Quaternary rhyolitic eruptions occurred through exposed granitic basement, contemporaneous with basaltic eruptions in the neighboring Cove Fort Valley. Using ⁴⁰Ar/³⁹Ar geochronology, whole rock and zircon geochemistry, and geochemical modeling, we examine the origins, timing, and evolution of these rhyolites.

Mineral Mountains rhyolites are divided into three groups based on elevation and composition: low-elevation obsidian flows, intermediate-elevation middle domes, and high-elevation high domes. Geochemical and geochronological data reveal a temporal trend of increasing chemical evolution, with less evolved obsidians erupting first (~850 ka), followed by middle domes (~750 ka), and more evolved high domes (588-483 ka). Fractional crystallization modeling shows that neither the basalts nor earlier rhyolites served as parental magmas for subsequent rhyolite eruptions, indicating independent magma sources. Zircon textures and geochemistry further suggest rapid magma generation and minimal recycling of older zircon, ruling out extraction from a long-lived crystal mush.

Partial melting models demonstrate that the Mineral Mountains rhyolites likely originated from anatectic melting of granitoid lithologies. Melting of syenite produces liquids resembling the obsidians, biotite granodiorite yields compositions similar to the middle domes, and biotite granite generates compositions matching the high domes. Contemporaneous basaltic eruptions likely supplied the heat necessary to induce partial melting of granitoids, except for the high domes, where coeval basaltic eruptions have yet to be identified. This absence may reflect a rheological barrier formed by the granitoid batholith, which inhibited basaltic ascent and focused rhyolitic melt production.

These findings suggest that the Mineral Mountains rhyolites are not products of extreme fractional crystallization of basalts but instead result from localized, small-volume anatectic melting of diverse granitoid lithologies. The interaction between mafic magma emplacement, granitoid composition, and regional extension provides a framework for understanding rhyolite genesis in similar tectonic and volcanic settings.