Geologic Setting

The Black Rock Desert volcanic field is situated near the boundary between the Colorado Plateau and the Basin and Range physiographic provinces. The predominant volcanic products of the past 2.5 million years include high-silica rhyolites and subalkaline basalts, with minor intermediate rocks. Five subfields have been defined; from north to south, they are (1) Fumarole Butte, (2) Ice Springs, (3) Beaver Ridge, (4) Twin Peaks, and (5) Cove Fort (Nash, 1986). The Cove Fort subfield includes the high-silica rhyolites of the Mineral Mountains and the 1.3 to 0.3 Ma basalt to andesite flows in the valley east of the Mineral Mountains (fig. 1). This valley is informally known as the Cove Fort valley.

The predominant rock types of the Mineral Mountains are Oligocene to Miocene granitoids composing the Mineral Mountains batholith, with an exposed area of about 250 square kilometers. The batholith intruded into 1.725 giga-annum (Ga) banded gneiss, quartzite, and sillimanite schist (in the west and north) and Paleozoic and Mesozoic carbonates, quartzites, and other sedimentary rocks (south, north, and east sides of the range; Sibbett and Nielson, 1980; Coleman and Walker, 1992). Zircon U-Pb dates indicate much of the batholith was emplaced between 18 and 16 Ma with subsequent dike emplacement at approximately 9 Ma (Coleman and Walker, 1992; Coleman and others, 2001). Intrusions became more silicic with time and contain hornblende and biotite along with accessory titanite, apatite, allanite, and zircon (Coleman and Walker, 1992). Exhumation of the batholith has been estimated to have occurred between 10 and 8 Ma (Nielson and others, 1986; Coleman and others, 2001).

Pleistocene volcanism above the batholith produced two obsidian flows, a pyroclastic deposit consisting of ash-fall and ash-flow tuffs, and numerous monogenetic porphyritic rhyolite domes that vented mostly along the ridge crest (fig. 1), totaling approximately 15 cubic kilometers (km³) (Nash, 1986). Previous K-Ar dating constrained eruptions between 860 and 540 kilo-annum (ka) (Lipman and others, 1978; Nash, 1986), and to the east of the range, the mafic flows of the Cove Fort valley are inferred to have erupted contemporaneously (Condie and Barsky, 1972; Best and others, 1980; Nash, 1986). Mafic crustal injections associated with the basalts of Cove Fort valley may have provided the necessary heat for rhyolite production in the Mineral Mountains; quenched basalt xenoliths present within some of the rhyolites indicate direct injection of mafic material into the silicic magma bodies (Nash, 1986). However, the large uncertainties of the K-Ar ages do not allow for temporal relations to be established among the rhyolites or between the rhyolites and adjacent mafic lavas.

Structural control may be responsible for the distribution of the pluton, rhyolite domes, and geothermal activity (Sibbett and Nielson, 1980). Pluton emplacement was oriented in a north-south direction, likely a result of east-west Basin and Range Province extension (Sibbett and Nielson, 1980). Structural control of the rhyolites is less defined, but the vents are distributed north-south along the ridge crest and are located at or near the contact between two types of granitoids within the batholith (Sibbett and Nielson, 1980). In the central Mineral Mountains, the Roosevelt Hot Springs Known Geothermal Resource Area hosts a 28-megawatt geothermal power plant that utilizes high conductive heat at relatively shallow depth (Nielson and others, 1978, 1986). Geothermal activity at Roosevelt Hot Springs is structurally controlled in that the west side of the range is bounded by the Opal Mound north-striking normal fault. High heat flow is well known in the region, with an average of 90 milliwatts per square meter (Chapman and others, 1978). It is likely that the heat at Roosevelt Hot Springs is derived from the igneous material that fed the Pleistocene rhyolite eruptions, which may also be linked to the mafic activity in the Cove Fort valley (Lipman and others, 1978). Two low-velocity zones, one as shallow as 5 kilometers (km), have been recognized and interpreted to be magma bodies (Ward and others, 1978; Robinson and Iyer, 1979); however, gravity and magnetic surveys do not indicate a low-density partial melt as the source of heat for the geothermal system (Ward and others, 1978).

Description of Units and Previous Geochronology

Nine Pleistocene lava domes have been identified across the Mineral Mountains, most of them at the highest elevations along the ridge crest. Physical characteristics and outcrop patterns are detailed by Lipman and others (1978) and Nash (1976, 1986) and are briefly summarized here. Mineral assemblages across all domes are similar, consisting of phenocrysts of quartz, plagioclase, and sanidine. In general, the domes at the highest elevations have 1–2 percent phenocrysts, whereas domes in the middle of the mountain range have 10–12 percent crystals, contain minor amounts of biotite (<1 percent), and trace amounts of titanite and zircon. Compositional zoning is common in feldspars and quartz and all domes contain two feldspars, with approximate compositions of 65 percent orthoclase (Or₆₅) and 80 percent albite (Ab₈₀) (Evans and Nash, 1978; Nash, 1986).

Six of the youngest domes, herein referred to as the high domes, span more than 10 km along the Mineral Mountains range crest. Three domes are identified in Ranch Canyon: North Twin Flat Mountain, South Twin Flat Mountain, and Ranch Canyon dome. Based on the degree of dissection, North and South Twin Flat Mountain domes were assumed to be among the oldest erupted in this canyon, with a K-Ar age of 500±70 ka (all uncertainties are reported at 2 standard deviations [2σ]) for South Twin Flat Mountain (Lipman and others, 1978). Nash (1986) reported ages of 690±80, 630±60, and 620±80 ka for pumice from South Twin Flat Mountain and ages of 730±120 and 680±60 ka for North Twin Flat Mountain. Ranch Canyon dome produced anomalously old ages of 7.39±0.78 and 6.55±0.46 Ma (Nash, 1986).

In Wildhorse Canyon, Bearskin Mountain (750±100 and 600±120 ka) and Little Bearskin Mountain (610±50 ka) domes were thought to be the youngest lavas based on degree of dissection. Nash (1986) reported ages of 620±220 ka for Bearskin Mountain and 740±940 ka for Little Bearskin Mountain, which are similar to those of Lipman and others (1978). The 800±80 ka (Nash, 1986) Big Cedar Cove dome is located down-canyon (west) from Bearskin and Little Bearskin Mountains. North Wildhorse Canyon dome and south Wildhorse Canyon dome are two smaller domes located south of Big Cedar Cove dome (fig. 1). We designate these central domes (Big Cedar Cove dome, north Wildhorse Canyon dome, and south Wildhorse Canyon dome) into a group called the “middle domes.” Prior to this study, no isotopic ages had been determined for either Wildhorse Canyon domes.

The obsidian flows of Bailey Ridge and Wildhorse Canyon are high-silica rhyolite flows located along the western front of the Mineral Mountains (Lipman and others, 1978; Nash, 1986). Vents for these two flows have not been identified, but the extension of the Wildhorse Canyon flow up-canyon toward the middle domes indicates either the vent is buried by a younger flow or that the source of one of those domes also produced the flow (Nash, 1976). The Bailey Ridge obsidian flow is considered to be the oldest of the Pleistocene lavas, with a reversed magnetic polarity and K-Ar age of 860±140 ka (weighted mean of two analyses by Lipman and others, 1978, and Nash, 1986). No previous geochronology exists for the Wildhorse Canyon obsidian flow, but the reversed polarity and chemical similarity to the Bailey Ridge obsidian flow led Lipman and others (1978) to assume a similar age.

One pyroclastic deposit (0.4 km³; Nash, 1976, 1986) is exposed within Ranch Canyon (fig. 1). The tuff of Ranch Canyon is characterized by a basal proximal fallout deposit with large (>10 centimeter) pumice blocks and a variety of lithic clasts, including glassy and snowflake obsidian, rhyolite, and granite. Above the fallout deposit is a sequence of normally graded pumice lapilli, capped by a series of cross-bedded surge deposits. Lipman and others (1978) obtained a K-Ar age of 700±40 ka from an obsidian block hosted within the basal tuff, placing a maximum age on the deposit.

Whereas the majority of the Mineral Mountains is of felsic composition, the Cove Fort subfield of the Black Rock Desert volcanic field produced several mafic cinder cones and lava flows at the north end of the range (fig. 1). A tholeiitic basaltic lava flow in the western part of the Cove Fort subfield, referred to informally as the Black Rock flow by Crecraft and others (1981), has dominant olivine and plagioclase and intergranular clinopyroxene. Referred to herein as the Black Rock lava flow, this lava flowed northwest about 25 km along the ancestral Cove Creek (Nash, 1986). Two K-Ar ages of 1.32±0.18 Ma and 920±520 ka were obtained from two different samples of this lava (Nash, 1986). To the east of the Mineral Mountains, the Cove Fort subfield is characterized by basaltic andesites and trachyandesites (55 to 62 weight percent SiO₂; Nash, 1986). The oldest of these erupted around 1.1 Ma, with additional eruptions at 500 and 300 ka (Best and others, 1980; Nash, 1986).

Whole-Rock Major, Trace, and Rare Earth Element Geochemistry

All felsic samples analyzed in this study are high-silica rhyolites, with normalized volatile-free concentrations that are in a narrow range between 76.8 and 78.0 weight percent SiO₂, 0 and 0.2 weight percent MgO, and 8.2 and 9.2 weight percent total alkalis (Na₂O + K₂O) (fig. 2A, table 2). Despite the limited range in SiO₂ and MgO concentration, other major element oxides vary. The high domes have higher Al₂O₃ concentrations but lower TiO₂ and total iron as FeO (FeO^T) than the obsidian flows and middle domes. The obsidian flows have the highest TiO₂ concentrations. The distinction of the three groups of rhyolites is also apparent in trace and rare earth element concentrations. Incompatible elements, such as Rb and Nb, vary widely among the rhyolites, such that the obsidian flows have the lowest Nb concentrations (23–24 parts per million [ppm]) and the middle dome lavas (27–28 ppm) and high dome lavas (36–44 ppm) have higher concentrations. This pattern is also observed for Rb (fig. 2C), but Eu systematically decreases from obsidian to the middle dome lavas and finally to the high dome lavas (fig. 2G). Trace element ratios, such as Y/Yb and Ba/Sr, also demonstrate the chemical distinction between the three groups.

Chondrite-normalized rare earth element (REE) patterns additionally demonstrate the chemical distinction between the obsidian flows, middle domes, and high dome lavas and tuff (fig. 3; table 3). The obsidian flows are nearly indistinguishable from each other, are more enriched in light REE, and have a less negative Eu anomaly relative to lavas of the other groups. The high dome lavas have a strong negative Eu anomaly, and the middle REEs form a saddle where heavy REEs are more enriched than middle REEs. The REE patterns of the middle dome lavas are nearly identical, and the saddle shape is present but not as prominent.

⁴⁰Ar/³⁹Ar Geochronology

We obtained new ⁴⁰Ar/³⁹Ar ages for thirteen samples from rhyolites of the Mineral Mountains and mafic lavas from the adjacent Cove Fort valley. None of the samples show evidence of trapped excess argon and thus the plateau and weighted mean ages are used hereafter. Two incremental heating experiments of plagioclase from the Black Rock lava flow resulted in a weighted mean age of 865±30 ka. Groundmass analyses of two intermediate lavas, Crater Knoll and Cunningham Wash from the Cove Fort valley, produced plateau ages of 754±14 and 748±14 ka, respectively. Incremental heating of trachyandesite groundmass from the Cove Fort andesite resulted in an age of 417±23 ka (fig. 4).

Incremental heating of purified obsidian glass resulted in ages of 851±3 and 786±5 ka for the Bailey Ridge and Wildhorse Canyon obsidian flows, respectively (fig. 5). Plateaus include >75 percent of the ³⁹Ar_K released and isochron intercepts are within uncertainty of the atmospheric value. Three single feldspar grains from north Wildhorse Canyon dome produced a plateau that included >85 percent of the ³⁹Ar_K with ages between 768±5 and 765±7 ka (fig. 6). A weighted mean of these grains yields an age of 766±3 ka (mean square of weighted deviates [MSWD] = 0.33). Eight single-crystal fusion experiments from south Wildhorse Canyon dome yield a weighted mean age of 758±6 ka (MSWD = 0.86). Five of the high domes were dated for this study (fig. 6). Three grains from Little Bearskin Mountain produced plateau ages ranging from 593±13 to 580±15 ka. These result in a weighted mean age of 588±7 ka (MSWD = 0.88). Five grains from Bearskin Mountain produced plateau ages ranging from 546±12 to 537±6 ka, resulting in a weighted mean age of 539±4 ka (MSWD = 0.48). Eight feldspar crystals from North dome were individually step heated. Each grain produced a plateau with >90 percent of the ³⁹Ar_K. Plateau ages range from 523±24 to 512±22 ka. A weighted mean of all grains results in an age of 520±4 ka (MSWD = 0.18). Single feldspar crystals from Ranch Canyon dome were analyzed via total fusion. The distribution of ages ranges from 548±8 to 489±26 ka. A weighted mean age for grains defining the youngest, dominant mode is 501±4 ka (MSWD = 1.53; number of dates in weighted mean calculation per total number of single crystal fusion dates [n/N] = 14/20; table 1). Seven grains from South Twin Flat Mountain produced plateau ages ranging from 490±12 to 480±12 ka, yielding a weighted mean age of 483±4 ka (MSWD = 0.58). Three additional units were analyzed (Big Cedar Cove dome, North Twin Flat Mountain, and pumice from the tuff of Ranch Canyon), but all produced inconclusive results. Ages are summarized in table 1.

Estimated Eruption Volumes

Eruption volumes for the high-silica rhyolites of the Mineral Mountains range from <0.1 to 0.55 km³ (table 4; fig. 7). The largest is the 851 ka Bailey Ridge obsidian flow. The obsidian flow of Wildhorse Canyon is smaller, with an estimated volume of 0.23 km³. Individual middle domes have eruptive volumes of approximately 0.1 km³, for a collective volume of 0.29 km³. The average eruptive volume of the high domes is 0.23 km³; the largest are the 483 ka South Twin Flat Mountain and the undated North Twin Flat Mountain, both located in Ranch Canyon. The volume of the pyroclastic deposit, also in Ranch Canyon, was not determined because of limited exposure but was previously estimated to be 0.4 km³ of material (Nash, 1976). The total eruptive volume for the high-silica rhyolite lavas and domes is about 2.4 km³, considerably less than the total 15 km³ estimated by Nash (1976).

Regional Geochemical and Geochronologic Context

Rhyolitic volcanism in the Basin and Range Province extends back to the Oligocene and was largely associated with an ignimbrite flare-up in Nevada. In southwestern Utah, topaz rhyolites erupted between 22 and 18 Ma and again between 13 and 11 Ma (Christiansen and others, 1984, 2007). Located 150 km north of the Mineral Mountains, the Topaz Mountain Rhyolite in the Thomas Range and the rhyolite of the Hogback at Fumarole Butte erupted 7–6 Ma and were followed by the eruption of the rhyolite of Smelter Knolls in the northern Sevier Desert at 3.4 Ma (Peterson and Nash, 1980; Turley and Nash, 1980). Notably, the Thomas Range hosts high-silica topaz rhyolites that are distinctly enriched in fluorine (Christiansen and others, 2007).

The Twin Peaks subfield of the Black Rock Desert volcanic field (2.1–2.7 Ma) is located about 40 km north of the Mineral Mountains and hosts numerous basaltic lavas and at least five distinct rhyolites. The prominent approximately 12 km³ South Twin Peak dome and Cudahy Mine pumice and obsidian deposits are high-silica rhyolites that do not bear topaz (fig. 8; Crecraft and others, 1981; Johnsen and others, 2014). The youngest rhyolitic volcanism in Utah occurred approximately 55 km to the northeast of the Mineral Mountains. The 400 ka (Nash, 1986) White Mountain dome is an isolated rhyolitic lava characterized by a lithophysae-rich obsidian and a xenolith-bearing stony rhyolite. Most xenoliths are basalts, which neighbor and underlie White Mountain.

Whereas the Mineral Mountains lavas are temporally correlative to the rhyolite of White Mountain, they are chemically most similar to the 2.7–2.4 Ma high-silica rhyolites (>76 weight percent SiO₂; ≤1 weight percent CaO) of the Twin Peaks volcanic subfield, and in particular, the Cudahy Mine deposits. Johnsen and others (2014) used whole-rock Sr, Nd, and Pb isotopes to show that the range of compositions within the Twin Peaks subfield could be generated through mixing of an enriched mantle type I basalt (Zindler and Hart, 1986) with an evolved rhyolite, which could represent a crustal composition. This model could be applied to the mafic and felsic lavas of the Mineral Mountains and Cove Fort volcanic subfield; however, petrogenetic modeling is not the focus of this study.

Spatial and Temporal Evolution of Volcanism

New ⁴⁰Ar/³⁹Ar geochronology places better temporal constraints on the eruptive history of the Pleistocene rhyolites of the Mineral Mountains and neighboring mafic-intermediate lavas of the Cove Fort valley. Although results are similar to those obtained via the K-Ar technique (Lipman and others, 1978; Best and others, 1980; Nash, 1986), the increased precision on individual eruption ages helps to define eruptive periods and recurrence intervals. Additionally, this work provides the first eruption ages for several of the lavas. Pleistocene rhyolitic volcanism within the Mineral Mountains persisted for ≤400 thousand years (k.y.), and mafic volcanism of the Cove Fort valley may have had a similar duration. Volcanic activity began in the northern part of the range with the eruption of the Black Rock lava flow at 865 ka and the Bailey Ridge obsidian flow at 851 ka (fig. 7 and 9A). Uncertainties on these eruption ages allow that the two lavas could have erupted simultaneously on either side of the Mineral Mountains or in sequence separated in time by only a few hundred to thousands of years (fig. 7). Thus, there may be a petrogenetic connection between these two lavas, or another mafic injection beneath Bailey Ridge may have triggered the rhyolitic eruption.

After approximately 65 k.y. of repose, volcanism within the Mineral Mountains migrated south into Wildhorse Canyon with the eruption of the obsidian flow of Wildhorse Canyon (786 ka) and the middle domes (Big Cedar Cove dome, north Wildhorse Canyon dome, and south Wildhorse Canyon dome) at 770–760 ka (fig. 9B). Previous workers concluded that the obsidian flows of Bailey Ridge and Wildhorse Canyon were the same age based on the nearly identical whole-rock major and trace element geochemistry and the reversed polarity of both units (Nash, 1986). However, new ⁴⁰Ar/³⁹Ar dating indicates that these two flows are distinct, despite their similar chemistry, and erupted approximately 65 k.y. apart. Although a vent has not been identified, it seems probable that the obsidian flow of Wildhorse Canyon vented from one of the Wildhorse Canyon domes. An eruption age of 786 ka for the Wildhorse Canyon obsidian flow is consistent with field relations that suggest the obsidian dammed the canyon, producing a local basin that subsequently captured the approximately 765 ka Bishop Tuff fallout (Nash, 1976; Andersen and others, 2017). The mineralogical and whole-rock geochemical similarities among the three middle domes may indicate that they are all sourced from the same large magma body. Using feldspar-liquid equilibria and trace element concentrations, Evans and Nash (1978) demonstrated that the more evolved lavas could be produced via fractional crystallization from the magma that produced the obsidian flows. To the east, the intermediate lavas of Cunningham Wash and Crater Knoll erupted around the same time as the eruption of the middle domes (fig. 7), suggesting that the influx of mafic material at this time may have provided the heat needed to generate the rhyolitic melts in the Mineral Mountains (Valentine and others, 2021).

The eruptions of the middle domes were followed by approximately 180 k.y. of inactivity. Volcanism next occurred up-canyon at higher elevations at 588 ka with the eruption of the Little Bearskin Mountain dome, followed by Bearskin Mountain dome at 539 ka (fig. 9C). The rhyolites of Little Bearskin and Bearskin Mountains erupted through Miocene granitoids and appear to be structurally controlled by the contact between granitoid rock types (Kirby, 2019). Subsequent volcanism was constrained to higher elevations, producing the high domes of North dome (520 ka), North Twin Flat Mountain (undated), Ranch Canyon dome (501 ka), and South Twin Flat Mountain (483 ka; fig. 9D). The high domes are compositionally more evolved than the older eruptions, as indicated by their large negative Eu anomalies (fig. 3), greater concentrations of Rb and Nb, and more varied Ba/Sr ratios (fig. 2). The apparent recurrence interval for these high domes is approximately 20 k.y., and eruptions appear to young toward the south. Overall, the eruptive trend within the mountain range is that less evolved compositions erupted early and closest to the valley floor, whereas progressively more differentiated lavas erupted at higher elevations over time. The temporal, spatial, and compositional trends may be explained through progressive differentiation of a shared magma body, as suggested by Evans and Nash (1978).

Recurrence Intervals within the Mineral Mountains

Recent work by Valentine and others (2021) estimated that Quaternary basaltic monogenetic volcanism of the broader American Southwest has a recurrence interval of <3 k.y.; they further note that eruptions in space and time are intrinsically irregular. However, the recurrence interval for individual Quaternary volcanic fields ranges from 3.6 to 27 k.y. with an average of 15 k.y. (table 5). Whereas the duration of rhyolitic volcanism in the Mineral Mountains is much shorter (<400 k.y.) relative to the basaltic fields of the American Southwest, we calculate a recurrence interval of 33 k.y. (table 5) over the lifetime of the field, from about 850 to 480 ka. If we subdivide the Mineral Mountains rhyolites into two main phases of volcanism, an early approximately 93 k.y. phase consisting of the obsidian flows and middle domes and a later approximately 105 k.y. phase consisting of the high domes (fig. 7), the recurrence interval of the early phase becomes 23 k.y. and the later phase has a recurrence interval of 18 k.y. (table 5), consistent with averages for individual volcanic fields across the region. Although recurrence intervals vary from one volcanic field to another, our work in the Mineral Mountains suggests that basalts and rhyolites erupt in similar long-term patterns on the order of tens of thousands of years.

Although recurrence intervals for recent small-volume silicic eruptions have received less attention from modern high-precision ⁴⁰Ar/³⁹Ar geochronology studies, numerous studies have investigated the timing of pre-, syn-, and post-caldera rhyolitic activity in areas such as Yellowstone Caldera, Valles Caldera, and Long Valley Caldera systems. In the Yellowstone Caldera system, small-volume rhyolites erupted within several thousand years of the 1.3 Ma caldera-forming Mesa Falls Tuff (Rivera and others, 2016; Troch and others, 2017; Stelten and others, 2018). In the Valles Caldera system, frequent eruptions of post-caldera, small-volume rhyolites began immediately after caldera formation and continued for more than 100 k.y. (Nasholds and Zimmerer, 2022). As part of their work, Nasholds and Zimmerer (2022) divided the 34 post-caldera units into 17 eruptive events as either pulses, phases, or episodes. They define single events separated by distinct repose periods as pulses, two or more units with indistinguishable ages as phases, and three or more events that have statistically indistinguishable oldest and youngest ages with no discernable repose period between as eruptive episodes. Using their nomenclature, the rhyolites of the Mineral Mountains are eruptive pulses. Nasholds and Zimmerer (2022) calculate recurrence intervals for the post-caldera eruptions at Valles Caldera according to different groupings of pulses, phases, and events; however, if each of the 34 units is considered an eruptive pulse, like we define the Mineral Mountains eruptions, then the recurrence interval for post-caldera resurgence at Valles Caldera over approximately 100 k.y. is 34.2 k.y.

The Coso volcanic field in California is located at the junction of the western Basin and Range Province and the Sierra Nevada, similar in tectonic setting to the Mineral Mountains and Cove Fort valley. Thirty-eight high-silica (>77 weight percent SiO₂, volatile-free), nearly aphyric rhyolites erupted between 1.04 and 0.079 Ma (Bacon, 1982; Burgess and others, 2021), and 14 basalts erupted contemporaneously (1.08 to 0.039 Ma) to the south of the rhyolite domes and lava flows (Bacon and others, 1980; Bacon, 1982). The estimated volume of erupted rhyolites in the Coso volcanic field is 2.4 km³ (Lanphere and others, 1975), which is identical to the volume calculated in this study for the Mineral Mountains rhyolites. Bacon and others (1981) divided the rhyolites into seven chemical groups that have distinct eruptive periods separated by as little as 5 k.y. to more than 400 k.y. (Burgess and others, 2021). These would be eruptive phases according to the terminology of Nasholds and Zimmerer (2022). Using a duration of 961 k.y. for rhyolitic activity based on existing obsidian glass K-Ar ages (Bacon, 1982) and zircon ²³⁸U–²³⁰Th ages (Burgess and others, 2021), the recurrence interval for the 38 rhyolitic eruptions of the Coso volcanic field is approximately 25 k.y. However, as Bacon (1982) noted, eruption intervals may be controlled by the volume of material erupted during the previous eruptive phase; thus, smaller eruptions occur more frequently and larger eruptions less often. Our first-order comparison to both extension and caldera-related, small-volume rhyolites indicates that the frequency of eruptions from small- and large-volume silicic systems may be remarkably similar and warrants further investigation to determine if there is a fundamental control on silicic eruptive recurrence rates.

Magmatic Flux of Small-Volume High-Silica Rhyolites

New high precision ⁴⁰Ar/³⁹Ar dating of the Mineral Mountains rhyolites has allowed us to identify two periods of volcanism. Excluding the 865–417 ka mafic-intermediate lavas of the Cove Fort volcanic subfield, we define the early phase (860–750 ka) as the obsidian flows and middle domes, and the later phase (590–480 ka) as the more chemically evolved high domes. The eruptive flux calculated for each approximately 100-k.y.-long period of felsic activity is approximately 0.01 km³/k.y., which is greater than the rhyolitic flux in the Coso volcanic field (0.003 km³/k.y.) but markedly less than the syn-resurgent dome (0.05 km³/k.y.) or post-caldera (2.66 km³/k.y.) eruptive fluxes from the Valles Caldera (Nasholds and Zimmerer, 2022).

The amount of nonerupted silicic material is largely unknown in the Mineral Mountains despite the excellent exposures of underlying plutons. In numerous localities globally, it has been suggested that silicic magmas may form a viscous rheologic barrier that may block eruption of subsequent magmas, causing them to stagnate within the crust (for example, Hildreth, 1981; Streck and others, 2015; Galetto and others, 2017; Burgess and others, 2021). It has also been suggested that some small-volume eruptions may occur along the periphery of the rheological barrier or within the barrier in areas of regional extension. Using these models for the Mineral Mountains, the rheological barrier may be the Oligocene to Miocene granitic batholith that underlies the high-silica rhyolites. The granitic plutons may suppress the eruption of basalt, forcing voluminous basalts to erupt in the adjacent Cove Fort valley (that is, along the periphery of the batholith), whereas small-volume rhyolites may be generated through anatectic melting of the granitoids (Nash, 1986) and ascend to the surface via zones of local extension. The rheological barrier may also explain the absence of a basaltic eruption preceding the high domes at approximately 588 ka. Rather than erupting the basalt, the heat may have been transferred to the granitoids to generate the high domes.

This model is similar to others proposed for volcanic fields of the American Southwest that host significant (>30 percent) evolved magmatism—the input of high-flux basalts in the upper crust provides the heat necessary to sustain long-lived mush zones, leading to the construction of polygenetic volcanoes (Cashman and others, 2017). However, volcanic fields in the American Southwest that have minor evolved magmatism may reflect limited storage and diversification to produce monogenetic volcanoes (Valentine and others, 2021). The contemporaneous eruption of basalt and small-volume rhyolite, along with the lack of basalt within the mountain range, indicates that the input of basaltic magma, at least during the first (early) cycle, may have facilitated eruption of the monogenetic rhyolites of the Mineral Mountains and mafic eruptions along the periphery of the rheological barrier in the adjacent volcanic field. A zircon petrochronology study could aid in resolving timescales of magmatic extraction, diversification, and recharge, as well as relations between the host granites and erupted rhyolites.