Southwestern Nevada Volcanic Field Research Articles

The 36–18 Ma Central Nevada ignimbrite field and calderas, Great Basin, USA: Multicyclic super-eruptions

The 36–18 Ma Central Nevada ignimbrite field and calderas, Great Basin, USA: Multicyclic super-eruptions

Melt in the mantle beneath the amagmatic zone, southern Nevada

Surface wave tomography of the western United States reveals the presence of an unusually slow shear wave velocity anomaly beneath a region in the Basin and Range Province, sometimes termed the amagmatic zone, which is renowned for its lack of volcanic activity. We pre­sent a model of the three-dimensional shear wave velocity structure beneath the southwestern United States constructed from the inversion of fundamental mode Rayleigh wave dispersion. The abundance of data from the deployment of the USArray (a component of the EarthScope project) provides unprecedented resolution. There is an excellent correlation between the location of slow shear wave velocity anomalies and recent (younger than 1 Ma) volcanism. This correlation and the unusually low values of absolute shear velocity beneath the magmatic gap lead us to conclude that there is melt present beneath this region. Previous studies have proposed that the preservation of cold continental lithosphere beneath the magmatic gap accounted for the lack of volcanism; however, this hypothesis is inconsistent with anomalously slow shear wave velocities in the 50–100 km depth range. Perhaps the prevalence of low-angle normal faulting in this region has made it difficult for the melt to escape the mantle. Beneath an immediately adjacent region, the southwestern Nevada volcanic field, where volcanism was abundant 10–15 m.y. ago but has since waned, significantly higher shear velocities suggest that the melt has been extracted from the mantle.

Tectonically controlled, time-predictable basaltic volcanism from a lithospheric mantle source (central Basin and Range Province, USA)

Understanding the evolution of basaltic volcanic fields is critical to our concepts of basaltic magmatism and to volcanic risk assessment. We summarize physical volcanological, geochemical, and time–volume characteristics of the Plio-Pleistocene part of the Southwestern Nevada Volcanic Field (SNVF) as an example of an extremely low volume-flux end member of basaltic fields. The SNVF has produced 17 volcanoes of dominantly trachybasaltic composition over the past 5 Myr with a total volume of slightly less than 6 km 3. Eruptive fissure lengths, volumes, and inferred lava effusion rates decreased between Pliocene- and Pleistocene-age volcanoes. Major element data suggest that most of the magmas underwent similar degrees of fractionation during ascent, and trace element compositions indicate a decrease in the degree of partial melting of the lithospheric mantle source since ∼ 3 Ma. Isotopic data support an interpretation wherein magmas ascended relatively quickly from their source regions with little if any interaction with crustal rocks. Relationships between age and cumulative erupted volume indicate that the repose interval between eruptive episodes is determined by the volumes of prior episodes and, since ∼ 3 Ma, an average eruption rate of ∼ 0.5 km 3/Myr, i.e., the field is time-predictable. All of these features support a model wherein magmatism is a passive result of regional tectonic strain. Partial melt resides in pockets of lithospheric mantle that are relatively enriched in hydrous minerals; slow deformation focuses melt, occasionally resulting in sufficiently high melt pressure to drive dikes upward and feed eruptive episodes. Larger source volumes result in larger eruptive volumes and wider dikes that relieve relatively more strain in the crust than smaller volume events, and therefore are followed by longer repose intervals required for recovery of crustal stresses. We suggest that time-predictability may be a fundamental property of tectonically controlled basaltic fields, where melt accumulation and ascent are controlled by tectonic strain rate. This behavior contrasts with magmatically controlled fields where magma flux is sufficiently high to overwhelm local tectonic strain, eruptions are primarily caused by magmatic processes that build pressure in reservoirs, and the systems are more likely to be volume-predictable.

Eruptive styles and inferences about plumbing systems at Hidden Cone and Little Black Peak scoria cone volcanoes (Nevada, U.S.A.)

We describe two small scoria cone volcanoes, Hidden Cone and Little Black Peak (ages between ~320–390 ka), in the Southwestern Nevada Volcanic Field and discuss their eruption mechanisms and inferences about their plumbing systems. Cone-forming pyroclastic deposits are consistent with eruptive styles ranging from Strombolian to violent Strombolian, and lavas emanated from near the bases of the cones. The volcanoes are monogenetic (rather than polycyclic, as allowed by previous geomorphic interpretations). Vents at each volcano appear to coincide with pre-existing normal faults, consistent with observations at older, deeply eroded volcanoes in the region. The existence of these two volcanoes on a topographically high area (particularly Hidden Cone) provides evidence for short feeder dike lengths (~500 m at the surface). We infer that this short length reflects the small length scale of the mantle source region that was tapped to feed each volcano.

Decreasing magmatic footprints of individual volcanoes in a waning basaltic field

The distribution and characteristics of individual basaltic volcanoes in the waning Southwestern Nevada Volcanic Field provide insight into the changing physical nature of magmatism and the controls on volcano location. During Pliocene‐Pleistocene times the volumes of individual volcanoes have decreased by more than one order of magnitude, as have fissure lengths and inferred lava effusion rates. Eruptions evolved from Hawaiian‐style eruptions with extensive lavas to eruptions characterized by small pulses of lava and Strombolian to violent Strombolian mechanisms. These trends indicate progressively decreasing partial melting and length scales, or magmatic footprints, of mantle source zones for individual volcanoes. The location of each volcano is determined by the location of its magmatic footprint at depth, and only by shallow structural and topographic features that are within that footprint. The locations of future volcanoes in a waning system are less likely to be determined by large‐scale topography or structures than were older, larger volume volcanoes.

Rapid generation of both high- and low-δ18O, large-volume silicic magmas at the Timber Mountain/Oasis Valley caldera complex, Nevada

var googletag = googletag || {}; googletag.cmd = googletag.cmd || []; googletag.cmd.push(function () { googletag.pubads().disableInitialLoad(); });

Sequence, age, and source of silicic fallout tuffs in middle to late Miocene basins of the northern Basin and Range province

The latest Cenozoic (<6 Ma) ash beds in the western United States have been intensively studied for several decades. The more widespread of these ash beds are well-documented event horizons that are of great value in studies of the timing and pace of geological, climatological, and biological events throughout the region. Because explosive volcanism was not restricted to latest Neogene time in this region, many older ash beds are likely to prove as useful as younger beds as event horizons, once they are located, characterized, and dated. As a first step in developing a useful chronology of older Cenozoic ash beds in the western United States, we have sampled and analyzed silicic fallout tuffs in middle to late Miocene sedimentary basins across the northern Basin and Range province. The northern Basin and Range basins, ideally situated in the vicinity of major coeval silicic volcanic centers, contain numerous relatively unaltered, silicic fallout tuffs. We have correlated tuffs between all sampled sections on the basis of glass shard composition. The composite stratigraphic sequence established by the correlations contains more than 200 individual tuffs, including 59 widely distributed tuffs termed correlative tuffs. The tuffs vary widely in composition, but most are in one of two compositional groups: gray metaluminous vitric tuffs (Gm tuffs) or white metaluminous vitric tuffs (Wm tuffs). Distribution patterns, compositional characteristics, and correlation with ash-flow tuffs show that the source for most Gm tuffs was the Snake River Plain volcanic province along the northern edge of the northern Basin and Range, and the source for most Wm tuffs was the southwestern Nevada volcanic field in the southern part of the northern Basin and Range. The northern Basin and Range tuffs range in age from ca. 16–6 Ma. The ages of individual tuffs are determined variously by direct isotopic dating, by correlation to previously dated fallout and ash-flow tuffs, or by interpolation age estimation. Ages for most tuffs are known to within 0.25 m.y. (1σ) or less and for many tuffs to within 0.1 m.y. or less. The sequence and ages of tuffs established in this study provide insights into the evolution of the northern Basin and Range basins and patterns of explosive volcanism in coeval volcanic centers, and contribute to the development of a high-resolution stratigraphy and chronology of coeval sedimentary deposits throughout the western United States.

Magma batches in the Timber Mountain magmatic system, Southwestern Nevada Volcanic Field, Nevada, USA

The common occurrence of compositionally and mineralogically zoned ash flow sheets, such as those of the Timber Mountain Group, provides evidence that the source magma bodies were chemically and thermally zoned. The Rainier Mesa and Ammonia Tanks tuffs of the Timber Mountain Group are both large volume (1200 and 900 km 3, respectively) chemically zoned (57–78 wt.% SiO 2) ash flow sheets. Evidence of distinct magma batches in the Timber Mountain system are based on: (1) major- and trace-element variations of whole pumice fragments; (2) major-element variations in phenocrysts; (3) major-element variations in glass matrix; and (4) emplacement temperatures calculated from Fe-Ti oxides and feldspars. There are three distinct groups of pumice fragments in the Rainier Mesa Tuff: a low-silica group and two high-silica groups (a low-Th and a high-Th group). These groups cannot be related by crystal fractionation. The low-silica portion of the Rainier Mesa Tuff is distinct from the low-silica portion of the overlying Ammonia Tanks Tuff, even though the age difference is less than 200,000 years. Three distinct groups occur in the Ammonia Tanks Tuff: a low-silica, intermediate-silica and a high-silica group. Part of the high-silica group may be due to mixing of the two high-silica Rainier Mesa groups. The intermediate-silica group may be due to mixing of the low- and high-silica Ammonia Tanks groups. Three distinct emplacement temperatures occur in the Rainier Mesa Tuff (869, 804, 723 °C) that correspond to the low-silica, high-Th and low-Th magma batches, respectively. These temperature differences could not have been maintained for any length of time in the magma chamber (cf. Turner, J.S., Campbell, I.H., 1986. Convection and mixing in magma chambers. Earth-Sci. Rev. 23, 255–352; Martin, D., Griffiths, R.W., Campbell, I.H., 1987. Compositional and thermal convection in magma chambers. Contrib. Mineral. Petrol. 96, 465–475) and therefore eruption must have occurred soon after emplacement of the magma batches into the chamber. Emplacement temperatures of the pumice fragments from the Ammonia Tanks Tuff show a continuous gradient of temperatures with composition. This continuous temperature gradient is consistent with the model of storage of magma batches in the Ammonia Tanks group that have undergone both thermal and chemical diffusion.

Strike-slip fault system in Amargosa Valley and Yucca Mountain, Nevada

The Amargosa Desert fault system, part of a 250-km-long system of dextral strike-slip faults east of the Death Valley region, is a throughgoing, long-lived, NW-striking, dextral strike-slip shear system that lies beneath Crater Flat and Yucca Mountain. Evidence for the fault system includes gravity, seismic, structural, stratigraphic, and paleomagnetic data, the distribution of springs and basaltic volcanic centers, and patterns of late Quaternary surface faulting. Where the Amargosa Desert fault system passes beneath Miocene ash-flow tuffs of the Southwestern Nevada Volcanic Field, we infer that the fault system changes into a series of detachments near the basement/cover contact and is expressed as a system of rotational normal faults in the ash-flow tuffs. Displaced geologic features indicate that total dextral displacement on the Amargosa Desert fault system exceeds 30 km. Vertical-axis rotations in ash-flow tuffs suggest that about 25 km of dextral displacement may have occurred since about 12.75 Ma. This fault system has important implications for the proposed high-level nuclear waste repository at Yucca Mountain, since it has experienced Quaternary and possibly Holocene displacements. Both normal faults and basaltic volcanic cones near the proposed repository may be controlled by the evolution of this regional strike-slip fault system.

Episodic caldera volcanism in the Miocene southwestern Nevada volcanic field: Revised stratigraphic framework, 40Ar/39Ar geochronology, and implications for magmatism and extension

The middle Miocene southwestern Nevada volcanic field (SWNVF) is a classic example of a silicic multicaldera volcanic field in the Great Basin. More than six major calderas formed between >15 and 7.5 Ma. The central SWNVF caldera cluster consists of the overlapping Silent Canyon caldera complex, the Claim Canyon caldera, and the Timber Mountain caldera complex, active from 14 to 11.5 Ma and centered on topographic Timber Mountain. Locations of calderas older than the Claim Canyon caldera source of the Tiva Canyon Tuff are uncertain except where verified by drilling. Younger peralkaline calderas (Black Mountain and Stonewall Mountain) formed northwest of the central SWNVF caldera cluster. We summarize major revisions of the SWNVF stratigraphy that provide for correlation of lava flows and small-volume tuffs with the widespread outflow sheets of the SWNVF. New laser fusion 40 Ar/ 39 Ar isotopic ages are used to refine and revise the timing of eruptive activity in the SWNVF. The use of high-sensitivity mass spectrometry allowed analysis of submilligram-sized samples with analytical uncertainties of ∼0.3% (1σ), permitting resolution of age differences as small as 0.07 Ma. These results confirm the revised stratigraphic succession and document a pattern of episodic volcanism in the SWNVF. Major caldera episodes (Belted Range, Crater Flat, Paintbrush, Timber Mountain, and Thirsty Canyon Groups) erupted widespread ash-flow sheets within 100-300 k.y. time spans, and pre- and post-caldera lavas erupted within 100-300 k.y. of the associated ash flows. Peak volcanism in the SWNVF occurred during eruption of the Paintbrush and Timber Mountain Groups, when over 4500 km 3 of metaluminous magma was erupted in two episodes within 1.35 m.y., separated by a 750 k.y. magmatic gap. Peralkaline and metaluminous magmatism in the SWNVF overlapped in time and space. The peralkaline Tub Spring and Grouse Canyon Tuffs erupted early, and the peralkaline Thirsty Canyon Group tuffs and Stonewall Flat Tuff erupted late in the history of the SWNVF, flanking the central, volumetrically dominant peak of metaluminous volcanism. Magma chemistry transitional between peralkaline and metaluminous magmas is indicated by petrographic and chemical data, particularly in the overlapping Grouse Canyon and Area 20 calderas of the Silent Canyon caldera complex. Volcanism in the SWNVF coincided with the Miocene peak of extensional deformation in adjoining parts of the Great Basin. Although regional extension was concurrent with volcanism, it was at a minimum in the central area of the SWNVF, where synvolcanic faulting was dominated by intra-caldera deformation. Significant stratal tilting and paleomagnetically determined dextral shear affected the southwestern margin of the SWNVF between the Paint-brush and Timber Mountain caldera episodes. Larger magnitude detachment faulting in the Bullfrog Hills, southwest of the central SWNVF caldera cluster, followed the climactic Timber Mountain caldera episode. Postvolcanic normal faulting was substantial to the north, east, and south of the central SWNVF caldera cluster, but the central area of peak volcanic activity remained relatively unextended in postvolcanic time. Volcanism and extension in the SWNVF area were broadly concurrent, but SWNVF area were broadly concurrent, but in detail they were episodic in time and not coincident in space

Paleomagnetism and rotation constraints for the middle Miocene southwestern Nevada volcanic field

Middle Miocene rocks of the southwestern Nevada volcanic field (SWNVF) lie across the projection of the Walker Lane belt within the Basin and Range province and thus provide an interesting opportunity to test for late Cenozoic vertical‐axis rotation. Paleomagnetic data from individual ash flow sheets document no significant relative vertical‐axis rotation among localities within central SWNVF, an area of relatively low stratal tilts and widely spaced faults. A time‐averaged mean paleomagnetic direction (D = 351.4°, I = 52.7°, α95 = 4.5°) calculated from data from numerous separate rock units suggests that the central SWNVF underwent minimal counterclockwise vertical‐axis rotation (R = −7.1° ± 6.6°) with respect to the North American craton. No clockwise vertical‐axis rotation is found to support projection of dextral faults of the Walker Lane beneath the central SWNVF. Clockwise rotation of variable magnitude is common at numerous sites from southern and western margins of the field. These clockwise rotations probably reflect dextral shear strain developed at the interface between the little extended central SWNVF block and more strongly extended areas to the south and southwest of the field. Negligible rotation of 11.45‐Ma to 13.25‐Ma tuffs relative to the central SWNVF was found at the southeast margin of the field where 90° clockwise rotation at the northwest termination of the Las Vegas Valley shear zone had been postulated. Any clockwise rotation in this area must predate 13.25 Ma, and thus dextral shear within this part of the Walker Lane belt was not synchronous or connected across the southern margin of the field. Small counterclockwise vertical‐axis rotation relative to the craton, as found for the central SWNVF block, might be a regional feature in the western Great Basin.

Pyritic ash-flow tuff, Yucca Mountain, Nevada

The Yucca Mountain site is underlain by a 1,500-m-thick Miocene volcanic sequence that comprises part of the southwestern Nevada volcanic field. Rocks of this sequence, which consists mainly of ash-flow tuff sheets with minor flows and bedded tuff, host precious metal mineralization in several areas as near as 10 km from the site. In two such areas, the Bullfrog and Bare Mountain mining districts, production and reserves total over 60 t gold and 150 t silver. Evidence of similar precious metal mineralization at the Yucca Mountain site may lead to mining or exploratory drilling in the future, compromising the security of the repository. The authors believe that most of the pyrite encountered by drilling at Yucca Mountain was introduced as pyroclastic ejecta, rather than by in situ hydrothermal activity. Pyritic ejecta in ash-flow tuff are not reported in the literature, but there is no reason to believe that the Yucca Mountain occurrence is unique. The pyritic ejecta are considered by us to be part of a preexisting hydrothermal system that was partially or wholly destroyed during eruption of the tuff units. Because it was introduced as ejecta in tuff units that occur at depths of about 1,000 m, such pyritemore » does not constitute evidence of shallow mineralization at the proposed repository site; however, the pyrite may be evidence for mineralization deep beneath Yucca Mountain or as much as tens of kilometers from it.« less

Contrasting styles of epithermal precious-metal mineralization in the southwestern Nevada volcanic field, USA

The southwestern Nevada volcanic field contains epithermal precious-metal deposits hosted by Miocene volcanic rocks and pre-Tertiary sedimentary rocks with production+reserves greater than 60 t of gold and 150 t of silver. The volcanic rocks consist predominantly of ash-flow tuffs erupted between 15 and 7 Ma during three major magmatic stages: the main stage (ca. 15-13 Ma); the Timber Mountain stage (ca. 13-9 Ma); and the late stage (ca. 9-7 Ma). Hydrothermal activity and precious-metal mineralization in the southern part of the field took place between ca. 13 and 8.5 Ma, coinciding with portions of all three magmatic stages. Regional extension during this period produced imbricate normal and detachment faulting that provided structural control for some of the mineralization. Contrasts in the style and geochemistry of mineralization, together with stratigraphic and radiometric age data and differences in geologic setting reflect the variable nature of hydrothermal activity during development of the southwestern Nevada volcanic field. During the main magmatic stage, silver-rich vein mineralization of the adularia-sericite type occurred in an intermediate volcanic center at Wahmonie. Secondary high-salinity fluid inclusions in felsic subvolcanic intrusions, a trace element suite that includes bismuth and tellurium, and geophysical data support the presence of a buried porphyry-type magmatic system at Wahmonie. Hydrothermal activity at Bare Mountain took place during the main magmatic stage, and may have continued into the Timber Mountain magmatic stage. Bare Mountain contains gold-rich, disseminated Carlin-type deposits with high arsenic, antimony, mercury and fluorine in sedimentary and igneous rocks. In northern and eastern Bare Mountain, mineralization is associated with felsic porphyry dikes that contain secondary high-salinity fluid inclusions. A genetic relationship between porphyry magmatism and shallow Carlin-type gold deposits seems likely at Bare Mountain. Sedimentary-rock-hosted mineralization at Mine Mountain is spatially associated with a thrust and was apparently deposited, in part, by a hydrothermal system active during the Timber Mountain magmatic stage. The silver:gold ratio is high and base-metal, arsenic, antimony, mercury and selenium contents are very high. Mine Mountain mineralization shares features with vein and disseminated silver deposits at Candelaria, Nevada. Gold-silver deposits in the areally extensive Bullfrog district comprise the largest known precious-metal resource in the volcanic field. They are mainly quartz-carbonate±adularia veins with alteration and mineralization styles similar to other adularia-sericite-type deposits in the Great Basin. Deposits in the Rhyolite area and at the Gold Bar mine have very low contents of arsenic and mercury compared to other epithermal deposits in the Great Basin, although copper and antimony are locally elevated. Similarities in mineralization style and assemblages, which include two occurrences of the rare gold-silver sulfide uytenbogaardtite, indicate deposition under similar conditions in different parts of the district. Hydrothermal activity in the Bullfrog district was coeval with extensional tectonism and may have continued from the Timber Mountain stage into the late magmatic stage. Mineralization at some deposits in the Bullfrog and Bare Mountain districts is spatially associated with, and, in part, structurally controlled by a regional detachment fault system. However, significant differences in age, mineralization style and geochemistry indicate that mineralization in the two districts is unrelated.

Nd isotopic gradients in upper crustal magma chambers: Evidence for in situ magma-wall-rock interaction

Multiple Nd isotopic analyses were obtained for one metaluminous and two peralkaline Tertiary rhyolitic ash-flow tuffs in the Great Basin to determine whether upper crustal silici magmas chemically evolve under closed- or open-system conditions. All the ash-flow tuffs analyzed show significant internal Nd isotopic variations. The largest variations occur within the peralkaline Double-H Mountains Tuff ({epsilon}{sub Nd} = +2.0 to +6.4) at the McDermitt volcanic field in north-central Nevada, and the smallest within the metaluminous Topopah Spring Tuff ({epsilon}{sub Nd} = {minus}10.6 to {minus}11.7) at the southwestern Nevada volcanic field. In all cases the isotopic variation are correlated with magmatic Nd contents, even though the Nd concentrations decreased roofward for the metaluminous rhyolite and increased for the peralkaline rhyolites. The consistent positive correlation between [Nd] and {epsilon}{sub Nd} provides strong evidence for in situ open-system addition of low {epsilon}{sub Nd} wall-rock material to the silicic magmas during their residence in the upper crust. The proportion of wall-rock Nd required to produce the isotopic zonations is small (1 to 15 mol%) for both the peralkaline and metaluminous rhyolites. All levels of the parental magmas sampled by the ash-flow tuffs, and not just magma occupying the roof zone, were open to wall-rockmore » interaction. These results suggest that upper crustal silicic magma bodies evolve under open-system conditions and the effects of such processes should be addressed in models for their chemical differentiation.« less

Introduction to the Special Section on the Southwestern Nevada Volcanic Field

The southwestern Nevada volcanic field (SWNVF) is a classic area for studying the relationships among ash flow sheets, lava flows, calderas, and high‐silica magma evolution. Ash flow sheets are fundamental to our understanding of the evolution of high‐level silicic bodies because they represent a sample of a large portion of a magma body at an instant in time. This special section is a report of research conducted on four caldera complexes that form a large part of the southwestern Nevada volcanic field. These complexes erupted over a period of 16–6.5 m.y. ago, and their eruptive products cover an area about 160 km long by 110 km wide.

Volcanic centers of southwestern Nevada: Evolution of understanding, 1960–1988

Since about 1960, geologists of the U.S. Geological Survey and, more recently, those of Los Alamos and Lawrence Livermore national laboratories, supported largely by the U.S. Department of Energy (DOE) and its predecessors, have been unraveling a complex series of ash flow sheets, lavas, and related calderas in the southwestern Nevada volcanic field in and near the Nevada Test Site (NTS). Extensive detailed geologic mapping aided in delineation of four major calderas: Silent Canyon (∼14 Ma), Timber Mountain‐Oasis Valley (∼11.5 Ma), Black Mountain (∼7.5 Ma), and Stonewall Mountain (∼6 Ma). In the 1960s, key concepts that contributed to the understanding of volcanology were the recognition of vertical compositional zonation within ash flow sheets, the significance of caldera rim and moat lavas, the relation between caldera collapse and intracaldera breccias and ash flow facies, and the correlation of intracaldera and outflow‐sheet facies. Deep drill holes within Silent Canyon and Timber Mountain calderas provided vital information on caldera geometry and intracaldera facies. Radiometric dating has produced nearly 100 dates that define the age of the field between about 16 and 6 Ma. During the middle part of that period a major ash flow eruption occurred once in about every half million years. Continuing support by the DOE for earth science at the NTS during the 1970s and 1980s has permitted a unique longevity of studies and provided opportunities to restudy mapped areas, revise some incorrect relationships, and work out important details of caldera history and structure that otherwise would not have come to light. Petrochemical and isotopic studies contributed to the understanding of the PT environment of the magma bodies that generated the major ash flow sheets. In the last decade, specialized work has continued on stratigraphic and petrologic problems, resulting in understanding of petrochemical cycles, in wider and more accurate correlation of certain units, and in understanding the time and spatial relationship between petrochemically very similar ash flow sheets from the Black Mountain and Stonewall Mountain calderas. Drilling and detailed Earth science studies in connection with preliminary characterization of a proposed nuclear waste repository at Yucca Mountain have greatly advanced knowledge of the older and younger parts of the volcanic sequence, including the basalts. A newly defined volcano‐tectonic collapse area in Crater Flat, probably a caldera, is strikingly similar in structure, location, and geophysical expression to the Silent Canyon caldera to the north. Rhyolite lavas peripheral to the Timber Mountain‐Oasis Valley caldera complex are intercalated with the major ash flow sheets from the complex, and analyses of the lavas fit well on compositional trends determined by the ash flow sheets. Renewed studies since 1981 include eruptive dynamics and magma chemistry of the major ash flow sheets and sources of mafic and intermediate volcanism. Hydrothermal alteration and mineralization occur after major magmatic pulses. New data on the age, structure, and distribution of the volcanic rocks have resulted in revised structural models involving volcano‐tectonic and detachment faulting processes.

Petrologic evolution of divergent peralkaline magmas from the Silent Canyon Caldera Complex, Southwestern Nevada Volcanic Field

The Silent Canyon volcanic center consists of a buried Miocene peralkaline caldera complex and outlying peralkaline lava domes. Its location has been corroborated by geophysical data and more than 50 drill holes. Two widespread ash flow sheets, the Tub Spring and overlying Grouse Canyon members of the Miocene Belted Range Tuff, were erupted from the caldera complex and have volumes of 60–100 km3 and 200 km3, respectively. Eruption of the ash flows was preceded by widespread extrusion of precaldera comendite domes and was followed by extrusion of postcollapse peralkaline lavas and tuffs within and outside the caldera complex. Lava flows and tuffs were also deposited between the two major ash flow sheets. Rocks of the Silent Canyon center vary significantly in silica content and peralkalinity. The most mafic rocks are precollapse and postcollapse trachytes (65–69% SiO2). Low‐silica comendites (69–73% SiO2) were erupted as the mafic upper part of the chemically zoned Grouse Canyon Member and as postcollapse lavas. The lower part of the Grouse Canyon Member and the underlying rhyolite of Split Ridge are moderately peralkaline comendite (PI is molar ratio Na + K/Al is 1.17–1.26). These comendites have major element characteristics and trace element enrichments approaching those of pantellerites. The Tub Spring Member, by contrast, is a weakly peralkaline chemically unzoned silicic comendite (75–76% SiO2) ash flow tuff. Weakly peralkaline silicic comendites (PI 1.0–1.1) are the most abundant precaldera lavas. Postcollapse lavas range from trachyte to silicic comendite; some have anomalous light rare earth element (LREE) enrichments. Silent Canyon rocks follow a common petrologic evolution from trachyte to low‐silica comendite; above 73% SiO2, compositions of the moderately peralkaline comendites diverge from those of the weakly peralkaline silicic comendites. These contrasting differentiation paths are shown in the behavior of Fe and other transition metals, Al, Na, K; the trace elements Ba, Zr, Nb; and probably F and Cl. Weakly peralkaline silicic comendites show a LREE/heavy REE crossover in early erupted/late erupted rocks; moderately peralkaline comendites are enriched in all REE. The development of divergent peralkaline magmas, toward both pantelleritic and weakly peralkaline compositions, is unusual in a single volcanic center.