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Abstract In the southwest USA, the Colorado Plateau is encircled by Late Cenozoic volcanic fields, most of which have eruptive histories that are marginally constrained. Establishing the spatiotemporal evolution of these volcanic fields is key for quantifying volcanic hazards and understanding magma genesis. The Black Rock Desert (BRD) volcanic field covers ∼700 km²of west‐central Utah. We present 46 new⁴⁰Ar/³⁹Ar ages from the BRD ranging from 3.7 Ma to 8 ka, which includes⁴⁰Ar/³⁹Ar plateau ages from olivine separates. These new ages are combined with 13 recently published⁴⁰Ar/³⁹Ar ages from the Mineral Mountains to evaluate the spatiotemporal evolution of all five BRD subfields. The oldest lavas and domes are located to the southwest, whereas the youngest lavas, which are only a few hundred years old, are located ∼30 km to the NNE. However, BRD vent migration patterns over the last 2.5 Ma are non‐uniform. They are also not consistent with North American Plate motion over a partial melt zone nor have they migrated toward the center of the Colorado Plateau. BRD eruptions are almost always coincident with mapped Quaternary faults. A shear‐velocity (Vs) model beneath the BRD indicates that the lithosphere has been thinned and that asthenospheric melt has coalesced at the lithosphere‐asthenosphere boundary, which is supported by the trace element compositions of BRD lavas that signify that they have incorporated continental lithospheric mantle. Our data and observations suggest that the asthenosphere‐lithosphere‐volcanic system in the BRD is inherently complex. 

Crystal mush systems, often referenced in the context of large silicic magma bodies, involve the reactivation of a near- solidus crystal mush by heat input from mafic injections. This model suggests that interstitial melt is extracted from the mush, leading to the generation of high-silica rhyolites and granites. Such processes have been well-documented in various tectonic settings and contribute to both large-scale eruptions and the formation of granitic plutons. However, in the Mineral Mountains, Utah, the zircon and whole rock geochemical record indicate a different scenario. The presence of sector-zoned zircons and the absence of highly evolved central domains indicative of extraction from a mush suggest rapid magma generation from partial melting of solid granitoids rather than from a long-lived crystal mush. Fractional crystallization and equilibrium partial melting models support derivation from the granitoid bodies, rather than from a common shared parental rhyolitic magma or from coeval basalts. The proposed model, presented here, for rhyolite formation in the Mineral Mountains involves episodic injections of mafic magma into the crust, leading to localized partial melting of different granitoid lithologies. Partial melting up to 30% can produce isolated, ephemeral pools of high-silica melt, which crystallize zircons rapidly and ascend to form rhyolitic domes. This process is distinct from the long-lived crystal mush model, explains the lack of intermediate compositions, and the confinement of mafic eruptions to lower elevations. By integrating geochemical data, zircon morphology, and fractionation modeling, this study provides a comprehensive framework for understanding the magmatic processes at play in the Mineral Mountains. 

The Colorado Plateau, USA, is bordered by Pleistocene continental rift volcanism in New Mexico, Arizona, and Utah. While most of the eruptions have been basaltic, rhyolitic domes, tuffs, and lavas have been produced. On the western margin, where the Colorado Plateau meets the Basin and Range extensional province, the Black Rock Desert of central Utah hosts Pleistocene-Holocene bimodal basalt-rhyolite volcanic activity. The South Twin Complex consists of six rhyolites within a single basin erupted between 2.45 and 2.40 Ma, and they precede all Pleistocene basalts of the region. In this work, we share a new rhyolite eruptive stratigraphy based on high precision 40Ar/39Ar dates and examine the zircon crystal cargo from each eruptive center. The new eruption ages allow us to examine the spatial and temporal distribution of volcanism in the South Twin Complex, whereas the zircon crystal morphology, geochemistry, and U/Pb dating allow us to assess the conditions and timescales of silicic magma processes in the subvolcanic plumbing system. Our data suggest the plumbing system beneath the region experienced punctuated influxes of magma over a brief period of thousands to tens of thousands of years. Further, the timescales and patterns of silicic magma assembly and evolution of this small anorogenic region are similar to those observed within the voluminous Yellowstone province, suggesting that the volume of magmatic flux does not control magmatic evolution in intercontinental settings. 

The origins and evolution of small-volume, high-silica intercontinental rhyolites have been attributed to numerous processes such as derivation from granitic partial melts or small melt fractions remaining from fractional crystallization. Investigations into the thermo-chemical-temporal evolution of these rhyolites has provided insights into the storage and differentiation mechanisms of small volume magmas. In the Mineral Mountains, Utah, high-silica rhyolites erupted through Miocene granitoids between ca. 0.8 and 0.5 Ma, and produced numerous domes, obsidian flows, and pyroclastic deposits. Temporally equivalent basalts erupted in the valleys north and east of the Mineral Mountains, hinting at a potential relationship between mafic and felsic volcanic activity. Here we test competing hypotheses. Are the rhyolites products of extreme fractionation of the coeval basalts? Or do they represent anatectic melts of the granitoids through which they erupted? We address these questions through modeling with new whole rock geochemical data and zircon trace element chemistry, thermometry, and U/Pb LA-ICPMS dates. We couple these data with new 40Ar/39Ar eruption ages to improve upon the volcanic stratigraphy and address the recurrence interval for the most evolved rhyolites. Geochemical data from zircon crystals extracted from six domes suggest increasing differentiation with age and eruptive location, however there is minimal evidence for recycling of earlier crystallized zircon. These data suggest that magma batches were isolated from one another and zircon nucleation and crystallization occurred close to the eruption, thus limiting the residence time of the magmas. These data also perhaps suggest that the magmas were generated in small batches within each of the granitoids rather than from a large crystal mush body underlying the region, as seen at large silicic systems. Our preliminary geochemical models and zircon petrochronology eliminate extreme fractionation and favor local anatectic melting of different granitoids as a mechanism to produce chemical signatures observed in the Quaternary rhyolites in the Mineral Mountains.