Ash-flow Sheets Research Articles

Geology of the Deer Butte Formation, Malheur county, Oregon: faulting, sedimentation and volcanism in a post-caldera setting

The Deer Butte Formation accumulated during the middle Miocene in fault-controlled basins in an extensional setting. The basins developed as regional faults asserted influence after eruption of ash-flow sheets and collapse of calderas of the Lake Owyhee volcanic field. The sequences of Hurley Flat, Dry Creek, and Oxbow Basin contain a lower basalt tephradominated unit formed by basalt hydrovolcanism overlain by fine-grained fluvial and lacustrine volcaniclastic sedimentary units. The sequence of Freezeout Creek was deposited in an erosional valley that was incised into older units and cut across the concurrently active Wall Rock Ridge fault zone. The sequence of Hurley Flat and Dry Creek contain alkaline tholeiitic basalt flows and tephra deposits, whereas the sequences of Freezeout Creek and Oxbow Basin contain subalkaline calcalkaline basaltic andesite. The compositional change occurred after local uplift due to faulting along the Wall Rock Ridge fault zone. The youngest unit, well-sorted, medium-grained, muscovite-bearing arkose of the arkose of Dry Creek Buttes, was deposited in a large river that drained westward from source areas in western Idaho. The Deer Butte Formation was deposited between approximately 15 and 12.6 Ma, while basin and range-type faulting dominated regional structural patterns.

Paleomagnetism of three upper Jurassic ash‐flow sheets in southeastern Arizona: Implications for regional deformation

A sequence of three petrologically distinct Upper Jurassic ash‐flow tuffs has been sampled for paleomagnetic analysis in the Huachuca Mountains, Canelo Hills, and Mustang Mountains of southeastern Arizona. Site‐mean paleomagnetic directions for these units indicate significant vertical‐axis rotations between sampling localities (≃15°–40°), and a comparison of these and previously published data with reference directions from the Colorado Plateau implies that the Jurassic rocks in southeastern Arizona have been rotated in a clockwise sense with respect to stable North America. These new data are consistent with paleomagnetic results showing clockwise rotation of Upper Cretaceous rocks in south central Arizona, and further indicate that studies of North American apparent polar wander based on rocks from southern Arizona could be subject to error. The clockwise rotations in southern Arizona are likely related to Late Cretaceous and early Tertiary strike‐slip movement on northwest‐trending high‐angle faults in the region, and this movement may be linked to an increase in oblique Farallon‐North American plate convergence at that time.

Limited extension during peak Tertiary volcanism, Great Basin of Nevada and Utah

The relative timing and magnitude of middle Tertiary extension and volcanism in the Great Basin (northern Basin and Range province) of the western United States remain controversial. To constrain the timing, we present 31 stratigraphic sections from the central part of the province, together with data from other studies in the Great Basin. Especially significant in this record of regional paleogeographic and associated tectonic conditions are thick sections of many well‐dated ash flow sheets emplaced during the period of the most voluminous, or peak, volcanic activity about 31–20 Ma. From these data we make the following conclusions: (1) Extension prior to the period of peak volcanism was apparently localized. (2) Extension during peak volcanism (the ignimbrite flareup) was minor and in places possibly related to magmatic processes in the shallow crust, rather than to regional tectonic processes. Angular unconformities and interbedded epiclastic deposits within sequences of volcanic rocks from 31 to about 22–20 Ma that would manifest synvolcanic faulting, tilting, and erosion are limited. (3) In the Great Basin as a whole, major extension and peak volcanism correlate poorly in space as well as time. (4) Essentially dip‐slip faults cutting the entire conformable volcanic sequence are common in the Great Basin and indicate a widespread episode of extension after peak volcanism. Southward sweeping Tertiary volcanism in the Great Basin reflects migration of the mantle magma supply that powered crustal magma systems. We suspect this migration was related to progressive southward foundering and steepening of dip of a subducting oceanic plate (after an earliest Tertiary near‐horizontal configuration beneath the continental lithosphere) and consequent backflow of asthenospheric mantle into the widening wedge between the plates. In the northern Great Basin, where the sweep was rapid, we postulate that relatively small volumes of mantle‐derived magma were inserted as dikes into the lower, locally extending crust which was unusually warm because of Mesozoic compressional thickening; crustal magma systems so powered were repeatedly tapped to feed modest volume eruptions of chiefly intermediate composition lava and minor silicic ash flow tuff. As the sweep stagnated in the central southern Great Basin, copious volumes of mafic magma were inserted into the crust, apparently mostly as extensive horizontal sheets, or sills, in a nonextending, uplifting crust in a state of nearly isotropic horizontal stress. These sills and the high mantle power input optimized crustal magma generation, creating huge volumes of silicic magma that vented as large volume ash flows, chiefly about 31–20 Ma. After about 22–20 Ma, the volcanic‐capped plateau collapsed in a widespread network of north striking extensional faults as plate boundary compressive forces were overcome by spreading forces within the uplift. Eruption of lava again became the dominant mode of volcanism.

Magnetism and magnetic mineralogy of ash flow tuffs from Yucca Mountain, Nevada

The magnetic susceptibility (χ) and remanent magnetization of an ash flow sheet are profoundly influenced by cooling history after emplacement. Spatial variations in these properties, therefore, provide a potentially useful tool for geological studies. Maxima and minima in χ measured along profiles at Yucca Mountain, Nevada, identify persistent magnetic marker horizons within vitric portions of the Tiva Canyon and underlying Topopah Spring Members of the Paintbrush Tuff. The correlatable variations of χ with vertical position in these units exist over the 8 km of strike examined. The observed stratigraphic changes in magnetic properties reflect variations in amounts and mineralogy of Fe‐Ti oxide phenocrysts, and the presence, shape, size, and mineralogy of magnetic Fe‐oxide microcrystals that precipitated at high temperature after emplacement of each sheet. Vitrophyre near the top of the Topopah Spring Member has a distinctive maximum in χ, due to titanomagnetite phenocrysts. Susceptibility in the lower portion of the overlying Tiva Canyon Member reaches maximum values of about 1.3×10−2 (SI) near the base of the member, beneath the columnarly jointed vitrophyre, due to the presence of abundant superparamagnetic cubic Fe‐oxide. These elongate microcrystals are several tens of nm long, and are typically deformed and tapered. Up section in the Tiva Canyon Member, the susceptibility decreases, going to a minimum within the columnar vitrophyre. At this stratigraphic level the precipitated Fe‐oxides are larger and carry intense NRM of up to 30 A/m; χ is only 3×10−3. The whisker‐like microcrystals of Fe‐oxide, which typically nucleated on what appears to be chain silicate, grew as needles up to 25 μm in length. The size variations of the precipitated Fe‐oxides, which were established using transmission electron microscopy (TEM) and petrographic observation, are consistent both with variations in magnetic susceptibility measured at the outcrop and with variations in the intensity of remanent magnetization. Several interpretations of the shape anisotropy of the precipitated Feoxide are possible, including growth by a dislocation mechanism. Additionally, the observed elongation of precipitated microcrystals is consistent with theoretical predictions for growth in a uniaxial stress field. Susceptibility variations as established at the outcrop, as well as in the borehole, offer a potentially useful tool for stratigraphic correlation of ash flow sheets.

Fossil and active fumaroles in the 1912 eruptive deposits, Valley of ten thousand smokes, Alaska

Fumaroles in the ash-flow sheet emplaced during the 1912 eruption of Novarupta were intensely active throughout the Valley of Ten Thousand Smokes (VTTS) when first studied in 1917. Fumarole temperatures recorded in 1919 were as hot as 645°C. Influx of surface waters into the hot ash-flow sheet provided the fluid flow to sustain the fumaroles but also enhanced cooling so that by the mid-1930's vigorous activity survived only in the vent region. Configuration and distribution of high-temperature fissure fumaroles tens of meters long, that are prevalent in the middle and upper VTTS, were controlled largely by sintering and degree of welding, which in turn controlled fracturing and permeability of the ash-flow tuff. One fracture type developed parallel to the enclosing valley walls during compaction of the ash-flow sheet. Another type extends across the VTTS nearly perpendicular to the flow direction. A third type of randomly oriented fractures developed as cooling contraction cracks during vapor-phase devitrification. In distal parts of the ash-flow sheet where the tuff is nonwelded, prominent fumaroles have irregular funnel-shaped morphologies. Fumarole distribution in the nonwelded part of the ash-flow sheet is concentrated above pre-emplacement river channels. The hottest, longest-lived fumaroles occurred in the upper VTTS near the 1912 vent where the ash-flow sheet is thicker, more indurated, and on average more mafic (richer in dacite and andesite) in contrast to the thinner, nonwelded rhyolitic tuff in the distal part of the sheet. Fumarolic activity was less intense in the distal part of the tuff because of lower emplacement temperatures, more diffuse fumarole conduits in the nonwelded tuff, and the thinness of the ash-flow sheet. Chemical leaching of ash-flow tuff by hot rising fluids took place adjacent to fumarolic conduits in deep parts of the fumaroles. Deposition of incrustation minerals, the components of which were carried upward by fumarolic gases, took place in the upper part of the ejecta, mostly in the fallout layers. The permeability difference between the ash-flow tuff and the overlying coarse dacite fallout was a critical factor in promoting the abrupt gradients in temperature, pressure, and fO 2 that resulted in deposition of minerals from the fumarolic gases. The permeability difference between nonwelded ash-flow tuff and overlying fine-grained fall layers in the lower VTTS is less pronounced. The total mass of fumarolically deposited minerals appears large at first glance owing to the conspicuous coloration by Fe minerals; the mass is appreciably less than is apparent, however, because most incrustations are composed largely of ejecta coated or cemented by fine-grained fumarolic minerals. A large mass of unstable incrustation minerals, mainly chlorides and sulfates, reported during the 1917–1919 studies have since been removed by dissolution and weathering. In the vent region, argillic alteration that followed high-temperature degassing is localized along arcuate subsidence fractures in fallback ejecta. At widely scattered residual orifices, fumarolic gases presently are near-neutral steam, and temperatures are as hot as 90°C.

Paleomagnetic constraints on the geometry and timing of deformation at Yucca Mountain, Nevada

Paleomagnetic data from Miocene ash flow sheets indicate that southern Yucca Mountain has undergone about 30° of clockwise vertical‐axis rotation since emplacement of the Tiva Canyon Member of the Paintbrush Tuff at about 13 Ma. Declinations of remanent magnetization from 32 sites in the reversely magnetized Tiva Canyon Member display a systematic southward increase over the 25 km north–south extent of Yucca Mountain. Probable errors in estimating structural attitudes are a major source of uncertainty in determining the amount of rotation between any two sites. Analysis of tilt‐corrected directions of remanent magnetization from the Tiva Canyon sites indicates that structural attitudes determined from outcrop patterns on geologic maps are generally more accurate than field measurements of attitudes of compaction foliation or contacts. Attitudes determined from map patterns apparently minimize effects of (1) initial dips, (2) poorly defined compaction foliation, and (3) tilting that occurred after compaction but before blocking of remanence; however, such attitudes cannot accurately represent tilting related to unmapped local structures. Rotations implied by data from three older ash flow sheets, the Topopah Spring Member of the Paintbrush Tuff (seven sites), and the Prow Pass (four sites) and Bullfrog (three sites) Members of the Crater Flat Tuff are generally consistent with the amount and sense of rotation documented by data from the Tiva Canyon Member. Uncertainties in the declinations of tilt‐corrected site mean directions from these older units preclude evaluating whether older units have undergone more rotation than the Tiva Canyon Member.

Flow directions in ash-flow tuffs: a comparison of geological and magnetic susceptibility measurements, Tshirege member (upper Bandelier Tuff), Valles caldera, New Mexico, USA

This study concludes that the elongation axis (K 1) of the ellipsoid of anisotropic magnetic susceptibility (AMS) is a suitable proxy for flow axis in ashflow tuffs. 153 oriented samples (176 specimens) were studied from 18 sites in the 1.1 Ma Tshirege member of the Bandelier Tuff. These sites are distributed around the Valles caldera at distances of 5–25 km outside of the rim.K 1 axes correlate well with postulated radial flow axes at 13 sites.K 1 also agrees with measured geological flow indicators, mainly imbricated larger clasts, at 7 sites. At 2 of the 5 sites where significant disagreement is seen between theoretical radial flow directions and measuredK 1 axes, theK 1 axes correspond well with geological flow indicators, indicating that the divergence of flow from the predicted radial flow pattern is real. Two major topographic buttresses are suggested as the cause of flow divergence for the Tshirege ash flows: the San Pedro buttress northwest of the caldera, and the San Miguel buttress in the southeast. In situK 1 axes plunge about 7° toward the source at two-thirds of the sites; therefore the plunge ofK 1 is a plausible in situ indicator for thedirection of flow. Multiple flow zones in sections of several meters thickness indicate changes of flow direction that are both rapid and large during ash-flow emplacement. These observations raisre the question of how best to represent ‘mean’ flow directions in ash-flow sheets: by eigenvector methods, by vector-sum methods, or by modes. A method for measuring imbrication of larger clasts using apparent dips in vertical joints is outlined. Imbrication, determined in this way at one-third of the sites, dips toward the source, i.e., up-flow. The minimum (K 3) axis of the AMS ellipsoid correlates with the flow foliation rather than with the larger clast imbrication. The flow axes of ash flows correspond with theK 1 axes, not with the declination ofK 3 axes as suggested by some authors. Initial dip of the sampled ash flows is not large and does not affect the paleomagnetic remanence direction, which is reversed with a mean ofD=173.5°,I=-38.4°, α95=3.4°N=18. This mean is not different at the 95% confidence level from that of earlier workers. The mean pole, at 098.0°E, 74.8°N,A 95=3.3°,N=18, is about 15° far-sided relative to the expected time-averaged geomagnetic pole, suggesting a history of emplacement too short to adequately average secular variation.

Cenozoic stratigraphy, magmatic activity, compressive deformation, and uplift in northern Peru

Research Article| August 01, 1990 Cenozoic stratigraphy, magmatic activity, compressive deformation, and uplift in northern Peru DONALD C. NOBLE; DONALD C. NOBLE 1Mackay School of Mines, University of Nevada-Reno, Reno, Nevada 89557 Search for other works by this author on: GSW Google Scholar EDWIN H. McKEE; EDWIN H. McKEE 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar THOMAS MOURIER; THOMAS MOURIER 3Laboratoire de Géologie Historique, Université de Paris-Sud, 91405 Orsay Cedex, France Search for other works by this author on: GSW Google Scholar FRANÇOIS MÉGARD FRANÇOIS MÉGARD 4Centre National de la Recherche Scientifique, Centre Géologique et Géophysique, U.S.T.L., Place E. Bataillon, 34060 Montpellier Cedex, France Search for other works by this author on: GSW Google Scholar Author and Article Information DONALD C. NOBLE 1Mackay School of Mines, University of Nevada-Reno, Reno, Nevada 89557 EDWIN H. McKEE 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 THOMAS MOURIER 3Laboratoire de Géologie Historique, Université de Paris-Sud, 91405 Orsay Cedex, France FRANÇOIS MÉGARD 4Centre National de la Recherche Scientifique, Centre Géologique et Géophysique, U.S.T.L., Place E. Bataillon, 34060 Montpellier Cedex, France Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1990) 102 (8): 1105–1113. https://doi.org/10.1130/0016-7606(1990)102<1105:CSMACD>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation DONALD C. NOBLE, EDWIN H. McKEE, THOMAS MOURIER, FRANÇOIS MÉGARD; Cenozoic stratigraphy, magmatic activity, compressive deformation, and uplift in northern Peru. GSA Bulletin 1990;; 102 (8): 1105–1113. doi: https://doi.org/10.1130/0016-7606(1990)102<1105:CSMACD>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Integrated field study and radiometric dating in the Llama-Bambamarca area of the Western Cordillera of northern Peru have resulted in major revisions in the stratigraphic and structural relations and ages of thick sequences of Tertiary volcanic rocks and constrain the timing of compressive deformation, magmatic activity, and uplift. Rocks of the upper part of the Llama Formation yield K-Ar mineral dates of 54.8 ± 1.8 and 44.2 ± 1.2 Ma; altered volcanic rocks composing the lower part of the formation in the western part of the area may be older. Eocene K-Ar dates have been reported for several granitic plutons in the region, and other plutons of probable similar age intrude the Llama Formation and are overlain by upper Eocene strata. The pervasive alteration that affects much of the Llama Formation may be the result of large hydrothermal systems driven by a coeval, and presumably cogenetic, batholith. The Chota Formation, which is mostly volcaniclastic, has yielded ages of about 50 and 44 Ma. The Chota Formation thus does not predate the Llama Formation but rather is an eastern distal-facies equivalent of rhyolitic ash-flow sheets and dacitic volcanic rocks of the Llama Formation.The major unconformity at the base of the Llama Formation reflects deformation during Late Cretaceous (Peruvian tectonic phase) and/or Paleocene (Incaic I) time. The time of Incaic II deformation is bracketed between the 44.2 ± 1.2 Ma date on the Llama Formation and the age of about 39 Ma of a thick ash-flow sheet that comprises the lower part of the unconformably overlying Huambos Formation. Dates on rocks postdating Incaic II tectonism in central Peru suggest that deformation had ceased by about 41 Ma. Incaic II tectonism in northern and central Peru appears to have been a short but intense compressive event that peaked about 43 m.y. ago. The Incaic II event is coeval with formation of the Hawaiian-Emperor bend and with a period of rapid convergence of the Nazca and South American plates and therefore may reflect a major plate change in lithospheric plate movement patterns.A largely volcaniclastic sequence of early Miocene age shows that the early Neogene pulse of volcanic activity recognized throughout the Central Andes is represented in northern Peru. Beds of conglomerate within this sequence are probably the result of Quechua I tectonism, although the 23.2 ± 1.5 Ma date obtained on tuff from the unit appears somewhat older than the time of about 19 Ma recognized for Quechua I tectonism in central and southern Peru. Units of partly welded and unaltered ash-flow tuff that fill deeply incised paleovalleys have ages of about 8.2 and 11.4 Ma, showing that the Western Cordillera of northern Peru was uplifted before late Miocene time. This content is PDF only. Please click on the PDF icon to access. 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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.

Evidence for dynamic withdrawal from a layered magma body: The Topopah Spring Tuff, southwestern Nevada

The Topopah Spring Tuff is a classic example of a compositionally zoned ash flow sheet resulting from eruption of a compositionally zoned magma body. Geochemical and petrographic analyses of whole rock tuff samples indicate that the base of the ash flow sheet and the predominant volume of erupted material consist of crystal‐poor high‐silica rhyolite, with a gradational transition into overlying crystal‐rich quartz latite. However, major and trace element analyses of glassy pumice lumps and microprobe analyses of their silicate and oxide phenocrysts provide closer approximations of the chemical and thermal gradients within the magma body. The gradients inferred from these data indicate that the transition from high‐silica rhyolitic to quartz latitic magma within the chamber was abrupt, rather than gradational, with a distinct liquid‐liquid interface separating the contrasting magma layers. Compositionally and texturally distinct pumice lumps are present throughout the ash flow sheet. The degree of heterogeneity within and among pumice lumps increases with stratigraphic height, becoming most pronounced in the uppermost quartz latite, where the chemical variability among pumice lumps is as great as that of the entire ash flow sheet. These observations are consistent with fluid dynamic models in which the velocity field developed near the entrance region of the vent(s) results in simultaneous withdrawal of magma from all points of a continuously expanding lateral and vertical region within the chamber. The abrupt transition to chemically bimodal pumice types near the top of the ash flow sheet, dominated by those of quartz latitic composition, implies that the interface between the magma layers remained relatively stable until draw‐down breached the interface and preferentially erupted hotter, more mafic magma along with lesser amounts of the remaining high‐silica rhyolitic magma.

Chemical evolution of a magmatic system: The Paintbrush Tuff, Southwest Nevada Volcanic Field

The chemical and thermal evolution of a single magmatic system is recorded in the Paintbrush Tuff, a series of four ash flow sheets, that were erupted from the same caldera within 0.6 Ma. Glassy pumice samples were collected from the tops and bottoms of the ash flow sheets. Because these samples closely approximate magma compositions, their chemical and mineralogical compositions were used to quantitatively evaluate magma mixing and fractional crystallization as potential differentiation mechanisms. Prior to eruption of the first ash flow sheet, the Topopah Spring Member (&gt;1200 km3), an abrupt compositional interface existed in the magma chamber between high‐silica rhyolite and quartz latite magmas. The subsequent Pah Canyon Member (&lt;40 km3) records mixing of the contrasting magma types represented in the Topopah Spring Member. The magma mixing was most likely due to disruption of the compositional interface during eruption of the Topopah Spring Member. The next unit, the Yucca Mountain Member (&lt;20 km3), represents the reestablishment of high‐silica rhyolite magma in the subcaldera chamber. The origin of this high‐silica rhyolite is best modeled by 15% to 24% fractional crystallization from the Pah Canyon Member. The Tiva Canyon Member is the fourth ash flow sheet (&gt;1000 km3) to be erupted, and it consists of three compositional modes, a higher‐silica rhyolite, rhyolite, and quartz latite. The high‐silica rhyolite of the Yucca Mountain Member is an early eruptive phase of the higher‐silica rhyolite of the Tiva Canyon Member, and their origins are the same. The rhyolite most likely formed by a combination of fractional crystallization and magma mixing of the Pah Canyon magma and the quartz latite of the Tiva Canyon Member. Alternatively, the rhyolite may have formed by fractional crystallization and magma mixing of higher‐silica rhyolite and quartz latite of the Tiva Canyon Member, or by fractional crystallization from the Tiva Canyon Member. The origin of the quartz latite has not been determined. Quantitative modeling indicates that all of the chemical variation within the Pah Canyon, Yucca Mountain, and Tiva Canyon members can be accounted for by fractional crystallization or magma mixing operating alone or in conjunction; no other processes need be invoked. Volume estimates of the magma bodies based on these quantitative models predict much larger volumes than the sizes of the ash flow sheets would indicate. A recurring theme in the evolution of the Paintbrush Tuff is the interaction of quartz latitic magmas with high‐silica magmas in conjunction with crystal fractionation.

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.

Evolution of a Chemically Zoned Magma Body: Black Mountain Volcanic Center, southwestern Nevada

Rocks of the Black Mountain volcanic center consist of four ash flow sheets and units of lava that underlie, interfinger with, and overlie the sheets. Rocks from the center represent three magma types. Magma type c was present through the history of the center, whereas types a and b were available after the eruption of the Rocket Wash Member, during the eruptions of the Pahute Mesa and Trail Ridge members. The magma types are defined by trace element ratios; for example, magma types a, b, and c have La/Th values of 1.0–3.5, &gt;7.5, and 3.5–7.5. Silica contents in the magma types a, b, and c range from 71.5 to 74.1, from 65.8 to 69.2, and from 55.6 to 73.8 wt %, respectively. The stratigraphic distribution of chemically distinct pumice fragments within the ash flow sheets is used to show that magma type a was located in the uppermost part of the chamber and was underlain successively by magma types b and c. Because pumice fragments that belong to all three magma types occur in individual cooling units, a zoned magma body must have existed during this period. Magma mixing is indicated by the disequilibrium phenocrysts which are common in pumice fragments from all magma types; however, this mixing did not destroy the original zoning of the upper part of the magma body. Most of the chemical variation of magma type c is consistent with fractionation of feldspar, olivine, and pyroxene, but abundant disequilibrium, mafic phenocrysts indicate that magma replenishment and mixing were common. Magma type b had much higher La/Th and light rare earth element (LREE)/heavy rare earth element values and must have originated independently from magma type c. Most likely the two types were derived from different source material. The low La/Th values of magma type a can be explained by separation of a phenocryst assemblage containing both a LREE‐bearing phase and zircon from either magma types b or c, or possibly by the partial melting of source material containing these phases.

Phenocryst abundances and glass and phenocryst compositions as indicators of magmatic environments of large‐volume ash flow sheets in southwestern Nevada

The Topopah Spring, Tiva Canyon, Rainier Mesa, and Ammonia Tanks tuffs are large‐volume, silicic ash flow sheets that provide samples of four magmatic systems in southwestern Nevada. Successively erupted within a span of 2 m.y. from the same source area, they allow comparison of the sequential evolution of large, mature Cordilleran magmatic systems. Each large‐volume sheet has a rhyolitic lower zone and quartz latitic upper zone. Coeval basaltic andesite and basalt show petrochemical continuity with these sheets and may represent mantle contributions that triggered eruptions of the midcrustal silicic portions. Abundances of phenocrysts and accessory phases increase upward with whole rock Fe (FeOt) from the base of all four sheets to maximum values unique for each system. Although maximum abundances of each mineral are unique for each sheet, each maximum occupies the same relative position within each sheet. High‐temperature minerals such as plagioclase increase in abundance continuously with FeOt in each system, showing a decrease with FeOt only within basaltic andesite at the base of the Rainier Mesa system. Late crystallizing minerals such as quartz and sphene show maximum abundances at much lower FeOt, at or near the top of the rhyolitic zone. Minerals that normally form at intermediate stages of crystallization, such as sanidine, show maxima at intermediate FeOt for each sheet. A continuum of glass and phenocryst compositions occurs within the Topopah Spring and Rainier Mesa sheets. Variations in phenocryst compositions with FeOt are generally consistent with those expected for crystallization within magma reservoirs characterized by vertical thermal and compositional gradients. However, simple fractional crystallization does not adequately explain the close relationship in each sheet among the mineral chemistry, glass (magma) chemistry, and phase assemblages, which indicate a close approach to equilibrium within each magma system. Nor does fractional crystallization account for resorption textures and maximum abundances for phenocrysts, which demonstrate dissolution rather than crystal growth within the deepest parts of each magma system. We hypothesize that phenocrysts reequilibrate as they gravitationally settle into higher‐temperature magma at greater depth or dissolve if the phase becomes unstable. In either case, lower‐density liquids are produced that should buoyantly migrate upward. Their compositions suggest that glasses and minerals with the lowest FeOt, in the base of each sheet, equilibrated at 3.5 kbar water pressure and ∼660°C. Phase assemblages and mineral chemistries indicate progressively higher temperatures with increasing FeOt within each sheet. Maximum equilibrium temperatures, ∼900°C, are obtained for the most Fe‐rich assemblage, basaltic andesite, also found at the base of the Rainier Mesa sheet. The phase assemblages indicate water saturation or near saturation for the entire magmatic system and thus indicate depths of ∼10 km to the top of each magmatic system. Considering similarities in magmatic environments of the four sheets, the distinctive petrochemical differences among rhyolitic zones probably reflect chemical differences inherited from their source regions.

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.

Stonewall Mountain Volcanic Center, southern Nevada: Stratigraphic, structural, and facies relations of outflow sheets, near‐vent tuffs, and intracaldera units

Directly south and southeast of Stonewall Mountain, Nevada, a depression and north facing caldera scarp were formed during and(or) after eruption of the Spearhead Member of the late Miocene Stonewall Flat Tuff. Abundant large lithic and juvenile blocks are present in the Spearhead Member within 0.5 km of this topographic margin but absent elsewhere in the ash‐flow sheet, consistent with eruption from vents in the Stonewall Mountain area. Within about 100,000 years, comendite tuff of the overlying Civet Cat Canyon Member of the Stonewall Flat Tuff buried the depression and associated scarp. The Civet Cat Canyon Member is traceable continuously to the north from an outflow sheet capping northwestern Pahute Mesa, into near‐vent tuff on the southeastern flank of Stonewall Mountain. Proximal outflow‐sheet tuff locally exhibits strong rheomorphic disruption and is overlain without a cooling break by surge, flow, and fall deposits of trachytic composition. Much of Stonewall Mountain is composed of welded tuff and megabreccia interpreted as intracaldera tuff of the Civet Cat Canyon Member, strongly suggesting that the vent area of the member was largely within Stonewall Mountain. Welded tuff of trachytic composition comprises an important part of the intracaldera Civet Cat Canyon Member, which was intruded by dikes and plugs of trachyte and rhyolite. Juvenile inclusions of basalt dispersed in near‐vent facies trachyte tuff provide direct evidence for the high‐level involvement of basaltic magma in the evolution of the highly potassic Stonewall Mountain center. Complex discordant compaction foliations and the widespread presence of megabreccia within the intracaldera tuff suggest, following Foley (1978), cauldron subsidence by piecemeal collapse during eruption of the Civet Cat Canyon Member. The elevation of intracaldera tuff and intrusions in Stonewall Mountain above the surrounding ashflow sheet suggests a significant amount of magmatic uplift, perhaps involving the emplacement of plugs, dikes, and small stocks within the intracaldera tuff prism.

Magma mixing due to disruption of a layered magma body

Abstract Pyroclastic eruptions provide a mechanism for the disruption of a compositionally zoned magma body and can result in the formation of homogeneous mixed magmas. The large volume (> 1200 km3) Topopah Spring ash-flow sheet, represents an eruption from a magma body that contained an abrupt compositional interface between a high-silica rhyolite and a quartz latite. The smaller volume ( km 3 ) Pah Canyon ash-flow sheet overlies the Topopah Spring Member, and represents an eruption of a relatively homogeneous magma that is intermediate in composition to the contrasting magmas of the Topopah Spring Member. We propose a model in which the Pah Canyon magma formed by mixing of the high-silica rhyolite and the quartz latite magma that remained in the magma chamber after the eruption of the Topopah Spring ash-flow sheet. Evaluation of this model, using variation diagrams of the major and trace elements, ratio-ratio plots, and least-squares multiple linear regression indicates that the Pah Canyon magma can be derived by mixing of approximately equal volumes of the high-silica rhyolite and the quartz latite end members. Phenocrysts in the mixed magma have reequilibrated and reflect the new composition, temperature and oxygen fugacity conditions.

Oligocene caldera complex and calc-alkaline tuffs and lavas of the Indian Peak volcanic field, Nevada and Utah

The Indian Peak volcanic field is representative of the more than 50,000 km 3 of ash-flow tuff and tens of calderas in the Great Basin that formed during the Oligocene-early Miocene ignimbrite flareup in southwestern North America. The field formed about 32 to 27 Ma in the southeastern Great Basin and consists of the centrally positioned Indian Peak caldera complex and a surrounding blanket of related ash-flow sheets distributed over an area of about 55,000 km 2 . The field has a volume on the order of 10,000 km 3 . A cluster of two obscure source areas and four calderas comprise the ∼80 x 120 km caldera complex. Only minor volumes of rhyolite and two pyroxene andesite lavas were extruded episodically throughout the lifetime of the magma system that formed the field, chiefly during its youth and old age. Six ash-flow sequences alternate between rhyolite and dacite in a volume ratio of about 1:8, and a culminating seventh is trachytic. The first, fourth, and sixth tuff units are of rhyolite that contains sparse to modest amounts of phenocrysts, chiefly plagioclase and biotite, and abundant lithic and pumice lapilli; these deposits are confined within the caldera complex and form multiple and compound cooling units that are normally zoned with respect to bulk chemical composition and crystal type, content, and size. The second, third, and fifth tuff sequences are of crystal-rich dacite that forms extensive simple cooling-unit outflow sheets and partial caldera fillings of compound cooling units. Each dacite unit contains similar amounts of plagioclase, biotite, hornblende, quartz, two pyroxenes, and Fe-Ti oxides; trace amounts of sanidine and titanite also occur in the youngest. Cognate inclusions in the dacites show only slight intra- and inter-unit differences in bulk chemical composition. The seventh eruptive sequence consists of several cooling units of trachydacite tuff containing small to modest amounts of plagioclase and two pyroxenes. These dominantly high-K calc-alkaline rocks are a record of the birth, maturation, and death of a large, open, continental magma system that was probably initiated and sustained by influx of mafic magma derived from a southward-migrating locus of magma production in the mantle. The small volumes of chemically diverse andesitic rocks were derived from separately evolving magma bodies but are modified representatives of the mantle power supply. Recurrent production of very large batches (some greater than 3,000 km 3 ) of quite uniform dacite magmas appears to have required combination of andesite magma and crustal silicic material in vigorously convecting chambers. Compositional data indicate that rhyolites are polygenetic. As the main locus of mantle magma production shifted southward, trachydacite magma could have been produced by fractionation of andesitic magma within the crust.

Geology and petrology of the Woods Mountains Volcanic Center, southeastern California: Implications for the genesis of peralkaline rhyolite ash flow tuffs

The Woods Mountains Volcanic Center is a middle Miocene silicic caldera complex located at the transition from the northern to the southern Basin and Range provinces of the western United States. It consists of a trachyte‐trachydacite‐rhyolite‐peralkaline rhyolite association of lava flows, domes, plugs, pyroclastic rocks, and epiclastic breccia. Volcanism began at about 16.4 Ma, near the end of a local resurgence of felsic to intermediate magmatism and associated crustal extension. Numerous metaluminous high‐K trachyte, trachydacite, and rhyolite lava flows, domes, and pyroclastic deposits accumulated from vents scattered over an area of 200 km2 forming a broad volcanic field with an initial volume of about 10 km3. At 15.8 Ma, about 80 km3 of metaluminous to mildly peralkaline high‐K rhyolite ash flows were erupted from vents in the western part of fhe field in three closely spaced pulses, resulting in the formation of a trap door caldera 10 km in diameter. The ash flows formed the Wild Horse Mesa Tuff, a compositionally zoned ash flow sheet that originally covered an area of about 600 km2 to a maximum thickness of at least 320 m. High‐K trachyte pumice lapilli, some of which are intimately banded with rhyolite, were produced late in the two later eruptions, Intracaldera volcanism from widely distributed vents rapidly filled the caldera with about 10 km3 of high‐K, mildly peralkaline, high‐silica rhyolite lava flows and pyroclastic deposits. These are interlayered with breccia derived from the caldera scarp. They are intruded by numerous compositionally similar plugs, some of which structurally uplifted and fractured the center of the caldera. The center evolved above a high‐K trachyte magma chamber about 10 km in diameter that had developed and differentiated within the upper crust at about 15.8 Ma. Petrological, geochemical, and geophysical data are consistent with the idea that a cap of peralkaline rhyolite magma formed within the trachyte chamber as a result of fractional crystallization within chemical boundary layers.

Physical and Chemical Models of Zoned Silicic Magmas: The Loma Seca Tuff and Calabozos Caldera, Southern Andes

The late Pleistocene Calabozos ash-flow and caldera complex lies in central Chile in a section of the Andean cordillera that is transitional between dominantly andesitic-to-rhyolitic volcanism to the north and mafic andesitic and high-alumina basaltic volcanism to the south. The Calabozos rocks range in composition from basaltic andesite to rhyodacite and define a high-K calcalkalic suite. They contain 2–25% phenocrysts of plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides, and apatite, ± minor biotite or amphibole. More than 1000km3 of rhyodacitic to dacitic magma erupted at the Calabozos caldera complex as three major compositionally zoned ash-flow sheets, Unit L (0·8 Ma), Unit V (0· 30Ma), and Unit S (0·15 Ma) of the Loma Seca Tuff. Phenocryst modes, trace-element contents, inferred magmatic volatile contents, and oxygen fugacities vary systematically with major-element composition in the tuffs. In the cases of Units V and S, it is possible to reconstruct compositional, thermal, and volatile gradients that existed in density-stratified magma chambers shortly prior to their eruption. The magma graded from crystal-poor, water-rich, and biotite-bearing rhyodacite in the upper reaches of the chamber to more crystal-rich, water-poor, and amphibole-bearing dacite at deeper levels. Fe-Ti oxide equilibration temperatures are ∼800 to 900°C for rhyodacite and ∼900 to 950°C for dacite. Magma that erupted as Unit S was slightly hotter and more oxidized than magma that gave rise to Unit V. More mafic magmas were associated with the voluminous rhyodacitic to dacitic magma reservoir, as indicated by the presence of andesite and basaltic andesite lava flows and by scoria inclusions in Unit V. The compositional suite from basaltic andesite to rhyodacite can be simulated satisfactorily by crystal-fractionation calculations based on major-element phenocryst and rock compositions, and is consistent with modes of the Calabozos rocks. Rhyodacites of Units V and S, however, are enriched in elements such as Rb, Ba, and Zr relative to trace-element contents predicted by crystal-fractionation models. The enrichment can be achieved by assimilation of wall rock or a partial melt of the wall rock. The latter requires that the ratio of assimilation rates to crystallization rates be between ∼0·1 and 0·3. Rates of assimilation versus crystallization were greater for Unit S than for Unit V, which is consistent with the lower Fe-Ti oxide temperatures and less oxidized state of the latter. The Loma Seca Tuff is similar in bulk composition to sanidine-bearing ash-flow sheets erupted on ‘mature’ continental crust, but it is mineralogically akin to ash-flow tuffs erupted on ‘immature’ crust. The difference is attributed, in part, to the effect of the density of the crust on the rate of magma ascent at shallow levels. The ascent of large bodies of silicic magma is slower in silicic (less dense) crust than in mafic crust, causing the magmas to be erupted at a later point in the crystallization history.