SILICIC VOLCANISM IN PLIOCENE AND QUATERNARY MAFIC VOLCANIC FIELDS IN UTAH, NEVADA AND CALIFORNIA

Although the Miocene was a time of widespread, large volume bimodal basalt-rhyolite volcanism, Pliocene and Quaternary volcanism in the Basin and Range Province (BRP) is generally thought to be mostly basaltic in nature with few, if any, rhyolite eruptions (e.g., Christiansen and Lipman, 1972). However, basalt-rhyolite associations persisted in several Pliocene to Recent volcanic fields. The question becomes: What mechanism is responsible for producing silicic volcanism in a dominantly mafic volcanic field? Our study deals with the petrogenesis of coeval basalt and rhyolite eruptions in 4 Pliocene and younger “mafic” volcanic fields (Black Rock Desert (BRD), western Utah; Markagunt Plateau, southern Utah; Death Valley, southern California; and the Reveille-Lunar Crater belt, central Nevada) to understand the mechanisms involved in creating silicic eruptions in highly extended terrain. We found that several mechanisms account for the eruption of rhyolite in each volcanic field. For example:

The BRD field has experienced coeval basalt and rhyolite eruptions since ~3.4 Ma, with the youngest basalt flow erupting 660 y.b.p. and the youngest rhyolite dome ~400 ka. The BRD field encompasses several smaller volcanic fields spanning 125 km in western Utah, including: Twin Peaks, Beaver Ridge, Ice Springs, and Cove Fort. There are multiple mechanisms for creating rhyolite. At Twin Peaks (ca. 2.5 Ma), basalt and rhyolite show significant and similar enrichment in light rare-earth element abundance (LREE) relative to Ocean Island Basalt, indicating that they are independent partial melts (PM) and not related by fractional crystallization (FC). At Beaver Ridge (ca. 1 Ma-400 ka), basalt and dacite erupted at 1 Ma show similar trends, with dacite being enriched in all REEs relative to basalt, suggesting FC as the differentiation mechanism. Rhyolite erupted between 400 and 500 ka at Beaver Ridge has depleted LREEs and enriched heavy rare-earth concentrations relative to coeval basalt, suggesting not only that the rhyolite is an independent PM but that it was also heavily fractionated before reaching the surface. In summary, independent PM, FC, and/or a combination of processes can account for co-eruption of basalt and rhyolite in a given volcanic field. It is, therefore, dangerous to assume one mechanism of differentiation.