Eric H. Christiansen

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.

Contrasting origins of Cenozoic silicic volcanic rocks from the western Cordillera of the United States

Two fundamentally different types of silicic volcanic rocks formed during the Cenozoic of the western Cordillera of the United States. Large volumes of dacite and rhyolite, mostly ignimbrites, erupted in the Oligocene in what is now the Great Basin and contrast with rhyolites erupted along the Snake River Plain during the Late Cenozoic. The Great Basin dacites and rhyolites are generally calc-alkaline, magnesian, oxidized, wet, cool (<850°C), Sr-and Al-rich, and Fe-poor. These silicic rocks are interpreted to have been derived from mafic parent magmas generated by dehydration of oceanic lithosphere and melting in the mantle wedge above a subduction zone. Plagioclase fractionation was minimized by the high water fugacity and oxide precipitation was enhanced by high oxygen fugacity. This resulted in the formation of Si-, Al-, and Sr-rich differentiates with low Fe/Mg ratios, relatively low temperatures, and declining densities. Magma mixing, large proportions of crustal assimilation, and polybaric crystal fractionation were all important processes in generating this Oligocene suite. In contrast, most of the rhyolites of the Snake River Plain are alkaline to calc-alkaline, ferroan, reduced, dry, hot (830–1,050°C), Sr-and Al-poor, and Nb-and Fe-rich. They are part of a distinctly bimodal sequence with tholeiitic basalt. These characteristics were largely imposed by their derivation from parental basalt (with low fH2O and low fO2) which formed by partial melting in or above a mantle plume. The differences in intensive parameters caused early precipitation of plagioclase and retarded crystallization of Fe–Ti oxides. Fractionation led to higher density magmas and mid-crustal entrapment. Renewed intrusion of mafic magma caused partial melting of the intrusive complex. Varying degrees of partial melting, fractionation, and minor assimilation of older crust led to the array of rhyolite compositions. Only very small volumes of distinctive rhyolite were derived by fractional crystallization of Fe-rich intermediate magmas like those of the Craters of the Moon-Cedar Butte trend.

The Oligocene Lund Tuff, Great Basin, USA: a very large volume monotonous intermediate

Abstract Unusual monotonous intermediate ignimbrites consist of phenocryst-rich dacite that occurs as very large volume (>1000 km 3 ) deposits that lack systematic compositional zonation, comagmatic rhyolite precursors, and underlying plinian beds. They are distinct from countless, usually smaller volume, zoned rhyolite–dacite–andesite deposits that are conventionally believed to have erupted from magma chambers in which thermal and compositional gradients were established because of sidewall crystallization and associated convective fractionation. Despite their great volume, or because of it, monotonous intermediates have received little attention. Documentation of the stratigraphy, composition, and geologic setting of the Lund Tuff – one of four monotonous intermediate tuffs in the middle-Tertiary Great Basin ignimbrite province – provides insight into its unusual origin and, by implication, the origin of other similar monotonous intermediates. The Lund Tuff is a single cooling unit with normal magnetic polarity whose volume likely exceeded 3000 km 3 . It was emplaced 29.02±0.04 Ma in and around the coeval White Rock caldera which has an unextended north–south diameter of about 50 km. The tuff is monotonous in that its phenocryst assemblage is virtually uniform throughout the deposit: plagioclase>quartz≈hornblende>biotite>Fe–Ti oxides≈sanidine>titanite, zircon, and apatite. However, ratios of phenocrysts vary by as much as an order of magnitude in a manner consistent with progressive crystallization in the pre-eruption chamber. A significant range in whole-rock chemical composition (e.g., 63–71 wt% SiO 2 ) is poorly correlated with phenocryst abundance. These compositional attributes cannot have been caused wholly by winnowing of glass from phenocrysts during eruption, as has been suggested for the monotonous intermediate Fish Canyon Tuff. Pumice fragments are also crystal-rich, and chemically and mineralogically indistinguishable from bulk tuff. We postulate that convective mixing in a sill-like magma chamber precluded development of a zoned chamber with a rhyolitic top or of a zoned pyroclastic deposit. Chemical variations in the Lund Tuff are consistent with equilibrium crystallization of a parental dacitic magma followed by eruptive mixing of compositionally diverse crystals and high-silica rhyolite vitroclasts during evacuation and emplacement. This model contrasts with the more systematic withdrawal from a bottle-shaped chamber in which sidewall crystallization creates a marked vertical compositional gradient and a substantial volume of capping-evolved rhyolite magma. Eruption at exceptionally high discharge rates precluded development of an underlying plinian deposit. The generation of the monotonous intermediate Lund magma and others like it in the middle Tertiary of the western USA reflects an unusually high flux of mantle-derived mafic magma into unusually thick and warm crust above a subducting slab of oceanic lithosphere.

Origin of broken phenocrysts in ash-flow tuffs

Surprisingly little attention has been devoted to the textural nature of phenocrysts of feldspar and quartz in tuff. Although many geologists have briefly alluded to “broken” phenocrysts, none have addressed their origin in any detail. Petrographic study of 117 cooling units in the middle Tertiary ash-flow province of the Great Basin, United States, provides a basis for characterization of the shapes and for interpretation of the origin of felsic phenocrysts in ash-flow tuffs. Although not proven to be wholly ineffective, breakage of phenocrysts by mutual impact in the erupting magma and pyroclastic flow is doubtful for at least four reasons. First, the statistical probability of mutual collision between phenocrysts diminishes exponentially as their proportion to vitroclasts diminishes (e.g., only 1% probability for 10% phenocrysts); collision is less likely if pyroclasts move by laminar rather than turbulent flow. Second, the coating of glass and/or melt on the phenocrysts provides a cushion that absorbs the impact force. Third, plagioclases broken by impact in the laboratory have unusual shapes unlike those seen in Great Basin tuffs. Fourth, euhedral phenocrysts of feldspar are commonplace in many Great Basin tuffs, and in some they constitute a significant proportion of the phenocrysts, indicating that mutual impact does not modify all intratelluric crystals during explosive eruption. The two most populated categories of phenocryst shape in Great Basin tuffs probably correspond to what has been previously called “broken” phenocrysts. Somewhat less than half of the plagioclase and many sanidine phenocrysts are subhedral to anhedral. These are similar in shape, size, and composition to grains in polycrystalline aggregates within the same thin section. Kindred aggregates and discrete phenocrysts could have been derived from holocrystalline to partly crystalline material in the magma chamber that was disaggregated to varying extents during explosive eruption. More than half of the plagioclase and all of the quartz phenocrysts in Great Basin tuffs consist of irregularly shaped fragments with cuspate, embayed outlines, resembling pieces of a jigsaw puzzle, which we call phenoclasts. Inclusions of glass are common and are especially evident in larger, more or less whole crystals. Textural features of some phenocrysts in cognate pumice clasts in the tuffs reveal that they broke apart while still in the vesiculating but unfragmented magma. As the erupting magma decompressed, vesiculation of the melt that was entrapped at higher pressures as inclusions within the phenocrysts blew them apart, forming the phenoclasts. Shapes of felsic phenocrysts in volcanic rocks provide insight into their mode of emplacement. Euhedral phenocrysts are common in ash-flow tuffs as well as lava flows. Phenoclasts, however, are diagnostic of ash-flow tuffs, because they do not occur in Plinian ash-fall deposits and are rare in lava flows. These textural contrasts are useful for interpretation of generally older, but in any case altered and recrystallized, volcanic rocks. In such rocks, critical groundmass features and field relations that could provide clues to their origin have been obscured, but the shapes of relict phenocrysts are commonly well preserved.

Contrasting processes in silicic magma chambers: evidence from very large volume ignimbrites

Very large volume (> 1000 km 3 of magma) crystal-rich dacitic ignimbrites that lack pronounced evidence of fractional crystallization or vertical zonation erupt in some continental magmatic arcs (e.g. the Lund Tuff of the Great Basin and the Fish Canyon Tuff of Colorado in western USA). Apparently, their magma chambers were only modestly heterogeneous and not systematically zoned from top to bottom. These ignimbrites have 40 to 50 % phenocrysts set in a high-silica rhyolite glass. Mineral assemblages and mineral compositions suggest pre-eruption temperatures were 730 to 820 °C and water and oxygen fugacities were relatively high. We have speculated that these very large volume ignimbrites are unzoned because crystallization and convection in slab-shaped magma chambers inhibited separation of crystals from liquids and resulted in a chamber filled with compositionally heterogeneous magma that lacked systematic chemical zonation or strong fractionation. However, many other very large volume silicic ignimbrites are strongly fractionated and may be vertically zoned (e.g. tuffs related to the Yellowstone hotspot). These rhyolitic tuffs typically have few phenocrysts, anhydrous mineral assemblages, low oxygen fugacities, crystallization temperatures of 830 to 1050 °C, and a strong imprint of fractional crystallization. Yet these Yellowstone-type rhyolites are derived from chambers 40 to 70 km across which have sill-like shapes (depth/diameter ratios much less than 1). Thus, factors other than chamber shape must be important for establishing the degree of evolution and nature of zonation in silicic magma chambers. Here, the role of crystallinity-dependent viscosity on the evolution of these two types of contrasting magmas is explored. Calculated magma viscosities for the hot, dry, crystal-poor rhyolites are significantly lower than for the cooler, wetter, crystal-rich dacites. Perhaps these hot rhyolites had low enough crystal contents and viscosities to allow efficient crystal–liquid separation, probably by a combination of unhindered crystal-settling, floor crystallization (including compaction), and crystallization on the walls of large chambers. Clean separation of melt from residual solids at their sources may have been promoted by their high temperatures and low viscosities ( 4.5 Pa s). In contrast, monotonous dacitic magmas may never have been crystal-free near-liquidus magmas. Their large magma chambers may have developed by progressive growth at a shallow level with repeated input of intermediate to silicic magma. Crystallization of the water-enriched dacitic magmas occurred at lower temperatures ( 10 6.5 Pa s) were significantly higher. These characteristics inhibited all forms of crystal–liquid separation, hindered development of systematic vertical zonation, and promoted quasi-equilibrium crystallization in small domains within large heterogeneous magma chambers. Eruptions of these crystal-rich dacites may only occur if the roof fails over a growing magma chamber that is becoming increasingly molten.

Lahars in the Elysium region of Mars

Photogeological studies of the Elysium volcanic province, Mars, show that its sinuous channels are part of a large deposit that was probably emplaced as a series of huge lahars. Some flows extend 1000 km from their sources. The deposits are thought to be lahars on the basis of evidence that they were (1) gravity-driven mass-flow deposits (lobate outlines, steep snouts, smooth medial channels, and rough lateral deposits; deposits narrow and widen in accord with topography, and extend downslope); (2) wet (channeled surfaces, draining features); and (3) associated with volcanism (the deposits and channels extend from a system of fractures which also fed lava flows). Heat associated with magmatism probably melted ground ice below the Elysium volcanoes and formed a muddy slurry that issued out of regional fractures and spread over the adjoining plain. The identification of these lahars adds to the evidence that Mars has a substantial volatile-element endowment.

The Great Basin Altiplano during the middle Cenozoic ignimbrite flareup: insights from volcanic rocks

Uncertainty surrounds the fate of the orogenic plateau in what is now the Great Basin in western Utah and Nevada, which resulted from the Mesozoic and earliest Cenozoic contractile deformations and crustal thickening. Although there is some consensus regarding the gravitational collapse of the plateau by extensional faulting and consequent crustal thinning, whether or not the plateau existed during the middle Cenozoic Great Basin ignimbrite flareup – one of the grandest expressions of continental volcanism in the geologic record – had remained in doubt. We use compositions of contemporaneous calc-alkaline lava flows as well as configurations of the ignimbrite sheets to show that the Great Basin area during the middle Cenozoic was a relatively smooth plateau underlain by unusually thick crust. We compare analyses of 376 intermediate-composition lava flows in the Great Basin that were extruded at 42–17 Ma with compositions of >6000 analyses of the late Cenozoic lava flows in continental volcanic arcs that correlate roughly with known crustal thickness. This comparison indicates that the middle Cenozoic Great Basin crust was much thicker than the present ca. 30 km thickness, likely as much as 60–70 km. If isostatic equilibrium prevailed, this unusually thick continental crust must have supported high topography. This high terrain in SE Nevada and SW Utah was progressively smoothed as successive ignimbrite outflow sheets were emplaced over areas currently as much as tens of thousands of square kilometres to aggregate thicknesses of as much as hundreds of metres. The generally small between-site variations in the palaeomagnetic directions of individual sheets lend further support for a relatively smooth landscape over which the sheets were draped. We conclude that during the middle Cenozoic, especially towards the close of the ignimbrite flareup, this Great Basin area was a relatively flat plateau, and because it was also high in elevation, we refer to it as an Altiplano. It was not unlike the present-day Altiplano-Puna in the tectonically similar central Andes, where an ignimbrite flareup comparable to that in the Great Basin occurred at ca. 9–3 Ma. Outflow ignimbrite sheets that were deposited from 35 to 23 Ma on the progressively smoothed Altiplano in south-eastern Nevada were derived from source calderas to the west. Of the 12 major sheets from seven sources, nine are distributed unevenly east of their sources while the remaining three sheets are spread about as far east as west of their sources. This eccentricity of sources near the western margin of 75% of the sheets indicates the existence of a NS-trending topographic barrier in central Nevada that restricted westward dispersal of ash flows. In a symmetric manner, eastward dispersal of ash flows from sources farther west seemed to have been impeded by this same topographic barrier. The westward dispersal was controlled in part by westward-draining stream valleys incised in the sloping flank of the Great Basin Altiplano in western Nevada and adjacent California; at least one of these ash flows travelled as far west as the western foothills of the Sierra Nevada. The nature and origin of the implied topographic barrier are uncertain. It is possible that heavy orographic precipitation on the western slope of the Altiplano and consequent focused denudation and isostatic uplift created a NS-trending topographic high at the crest of the western slope and facing the smoothed Altiplano to the east. The barrier also lies near and essentially parallel to the buried western edge of the Precambrian basement and to a zone of thermal-diapiric domes that were spawned in thickened crust as the basement edge was overrun by late Palaeozoic–Mesozoic thrust sheets.

Introduction: The 36–18 Ma southern Great Basin, USA, ignimbrite province and flareup: Swarms of subduction-related supervolcanoes

During the middle Cenozoic, from 36 to 18 Ma, one of the greatest global expressions of long-lived, explosive silicic volcanism affected a large segment of southwestern North America, including central Nevada and southwestern Utah in the southern Great Basin. The southern Great Basin ignimbrite province, resulting from this flareup, harbors several tens of thousands of cubic kilometers of ash-flow deposits. They were created by more than two hundred explosive eruptions, at least thirty of which were super-eruptions of more than 1000 km 3 . Forty-two exposed calderas are as much as 60 km in diameter. As in other parts of southwestern North America affected by the ignimbrite flareup, rhyolite ash-flow tuffs are widespread throughout the southern Great Basin ignimbrite province. However, the province differs in two significant respects. First, extrusions of contemporaneous andesitic lavas were minimal. Their volume is only about 10% of the ignimbrite volume. Unlike other contemporaneous volcanic fields in southwestern North America, only a few major composite (strato-) volcanoes predated and developed during the flareup. Second, the central sector and especially the eastern sector of the province experienced super-eruptions of relatively uniform, crystal-rich dacite magmas; resulting deposits of these monotonous intermediates measure on the order of 16,000 km 3 . Following this 4 m.y. event, very large volumes of unusually hot and dry trachydacitic magmas were erupted. These two types of magmas and their erupted volumes are apparently without parallel in the middle Cenozoic of southwestern North America. A fundamental goal of this themed issue is to present basic stratigraphic, compositional, chronologic, and paleomagnetic data on the unusually plentiful and voluminous ignimbrites in the southern Great Basin ignimbrite province. These data permit rigorous correlations of the vast outflow sheets that span between mountain-range exposures across intervening valleys as well as correlation of the sheets with often-dissimilar accumulations of tuff within dismembered source calderas. Well-exposed collar zones of larger calderas reveal complex wall-collapse breccias. Calculated ignimbrite dimensions in concert with precise 40 Ar/ 39 Ar ages provide insights on the growth and longevity of the colossal crustal magma systems. Exactly how these subduction-related magma systems were sustained for millions of years to create multicyclic super-eruptions at a particular focus remains largely unanswered. What factors created eruptive episodes lasting millions of years separated by shorter intervals of inactivity? What might have been the role played by tears in the subducting plate focusing a high rate of mantle magma flux into the crust? What role might have been played by an unusually thick and still-warm crust inherited from earlier orogenies? Are the numerous super-eruptions, especially of the unusual monotonous intermediates and succeeding trachydacitic eruptions, during the Great Basin ignimbrite flareup simply a result of the coupling effect of high mantle-magma flux and a thick crust, or did other factors play a role?

The 36–18 Ma Indian Peak–Caliente ignimbrite field and calderas, southeastern Great Basin, USA: Multicyclic super-eruptions

The Indian Peak–Caliente caldera complex and its surrounding ignimbrite field were a major focus of explosive silicic activity in the eastern sector of the subduction-related southern Great Basin ignimbrite province during the middle Cenozoic (36–18 Ma) ignimbrite flareup. Caldera-forming activity migrated southward through time in response to rollback of the subducting lithosphere. Nine partly exposed, separate to partly overlapping source calderas and an equal number of concealed sources compose the Indian Peak–Caliente caldera complex. Calderas have diameters to as much as 60 km and are filled with as much as 5000 m of intracaldera tuff and wall-collapse breccias. More than 50 ignimbrite cooling units, including 22 of regional (>100 km 3 ) extent, are distinguished on the basis of stratigraphic position, chemical and modal composition, 40 Ar/ 39 Ar age, and paleomagnetic direction. The most voluminous ash flows spread as far as 150 km from the caldera complex across a high plateau of limited relief—the Great Basin altiplano, which was created by late Paleozoic through Mesozoic orogenic deformation and crustal thickening. The resulting ignimbrite field covers a present area of ∼60,000 km 2 in east-central Nevada and southwestern Utah. Before post-volcanic extension, ignimbrites had an estimated aggregate volume of ∼33,000 km 3 . At least seven of the largest cooling units were produced by super-eruptions of more than 1000 km 3 . The largest, at 5900 km 3 , originally covered an area of 32,000 km 2 to outflow depths of hundreds of meters. Outflow ignimbrite sequences comprise as many as several cooling units from different sources with an aggregate thickness locally reaching a kilometer; sequences are almost everywhere conformable and lack substantial intervening erosional debris and angular discordances, thus manifesting a lack of synvolcanic crustal extension. Fallout ash in the mid-continent is associated with two of the super-eruptions. Ignimbrites are mostly calc-alkalic and high-K, a reflection of the unusually thick crust in which the magmas were created. They have a typical arc chemical signature and define a spectrum of compositions that ranges from high-silica (78 wt%) rhyolite to andesite (61 wt% silica). Rhyolite magmas were erupted in relatively small volumes more or less throughout the history of activity, but in a much larger volume after 24 Ma in the southern part of the caldera complex, creating ∼10,000 km 3 of ignimbrite. The field has some rhyolite ignimbrites, the largest of which are in the south and were emplaced after 24 Ma. But the most distinctive attributes of the Indian Peak–Caliente field are two distinct classes of ignimbrite: 1. Super-eruptive monotonous intermediates. More or less uniform and unzoned deposits of dacitic ignimbrite that are phenocryst rich (to as much as ∼50%) with plagioclase > biotite ≈ quartz ≈ hornblende > Fe-Ti oxides ± sanidine, pyroxene, and titanite; apatite and zircon are ubiquitous accessory phases. These tuffs were deposited at 31.13, 30.06, and 29.20 Ma in volumes of 2000, 5900, and 4400 km 3 , respectively, from overlapping, multicyclic calderas. A unique, and possibly kindred, phenocryst-rich latite-andesite ignimbrite with an outflow volume of 1100 km 3 was erupted at 22.56 Ma from a concealed source caldera to the south. 2. Trachydacitic Isom-type tuffs. Also relatively uniform but phenocryst poor ( > clinopyroxene ≈ orthopyroxene ≈ Fe-Ti oxides >> apatite. These alkali-calcic tuffs are enriched in TiO 2 , K 2 O, P 2 O 5 , Ba, Nb, and Zr and depleted in CaO, MgO, Ni, and Cr, and have an arc chemical signature. Magmas were erupted from a concealed source immediately after and just to the southeast of the multicyclic monotonous intermediates. Most of their aggregate outflow volume of 1800 km 3 was erupted from 27.90 to 27.25 Ma. Nothing like this couplet of distinct ignimbrites, in such volumes, have been documented in other middle Cenozoic volcanic fields in the southwestern U.S. where the ignimbrite flareup is manifest. Magmas were created in unusually thick crust (as thick as 70 km) where large-scale inputs of mantle-derived basaltic magma powered partial melting, assimilation, mixing, and differentiation processes. Dacite and some rhyolite ignimbrites were derived from relatively low-temperature (700–800 °C), water-rich magmas that were a couple of log units more oxidized than the quartz-fayalite-magnetite (QFM) oxygen buffer at depths of ∼8–12 km. In contrast to these “main-trend” magmas, trachydacitic Isom-type magmas were derived from drier and hotter (∼950 °C) magmas originating deeper in the crust (to as deep as 30 km) by fractionation processes in andesitic differentiates of the mantle magma. “Off-trend” rhyolitic magmas that are both younger and older than the Isom type but possessed some of their same chemical characteristics possibly reflect an ancestry involving Isom-type magmas as well as main-trend rhyolitic magmas. Andesitic lavas extruded during the flare up but mostly after 25 Ma constitute a roughly estimated 12% of the volume of silicic ignimbrite, in contrast to major volcanic fields to the east, e.g., the Southern Rocky Mountain field, where the volume of intermediate-composition lavas exceeds that of silicic ignimbrites.

The record of Middle Jurassic volcanism in the Carmel and Temple Cap Formations of southwestern Utah

Altered volcanic ash beds in the Middle Jurassic Temple Cap and Carmel Formations in southwestern Utah record a pulse of active arc-related volcanism between 166 and 171 Ma. A second pulse between 148 and 155 Ma has previously been documented in the Upper Jurassic Morrison Formation. Volcanic and volcaniclastic rocks of these same ages have also been identified closer to or within the arc in California in the Inyo Mountains, the Cowhole Mountains, the Palen Mountains, and the central Mojave Desert. The upper part of the volcaniclastic Mount Wrightson Formation and the strata of Cobre Ridge in southern Arizona are ca. 170 Ma in age and appear to be time correlative with the Middle Jurassic formations in southwestern Utah. The altered ash beds found in the Temple Cap and Carmel Formations typically contain phenocrysts of sanidine, quartz, biotite, apatite, zircon, and titanite. Plagioclase was likely present originally in all of the ashes, but was removed by alteration and is now found only in the Temple Cap red beds. Quartz and sanidine are absent in two crystal-poor ash beds that contain two pyroxenes, hornblende, and biotite. Although major and trace element concentrations in the ash beds have been substantially modified, compositions of relict phenocrysts reveal that the magmas were calc-alkaline rhyolites to andesites. Two-pyroxene, two-feldspar, biotite, and biotite-apatite thermometers suggest that crystallization occurred at temperatures ranging from 740 to 910 8C. Hornblende geobarometry yields pressures of 1‐2 kilobars for the two ash beds that contain the appropriate buffer assemblage. The mafic silicates all have moderately high Mg/Fe ratios. This fact, combined with the presence of hornblende, biotite, and titanite, suggests that the phenocrysts crystallized at high oxygen fugacities similar to those of the granites of the batholiths of California. The ash probably erupted from a low-lying arc cut by strikeslip faults in what is now southern California and western Nevada. Major Jurassic unconformities occur near or within the ash-bearing formations in southwestern Utah. Laser-fusion singlecrystal 40 Ar/ 39 Ar measurements have defined the ages of the unconformities and the associated volcanism. The age of the J-1 unconformity, found at the base of the Temple Cap Formation in southwestern Utah, is older than ca. 170.5 Ma. The J-2 unconformity, which lies between the Temple Cap and Carmel Formations, formed between ca. 169 and 168 Ma. The origin of these unconformities is still unclear, but may be related to the Middle Jurassic pulse of magmatism and the oblique plate convergence along the western margin of North America. The age range of ash beds in the Carmel Formation between 166.3 and 168.0 6 ;0.5 Ma is consistent with a Bajocian-Bathonian boundary of ca. 166 Ma.