Monotonous Intermediates Research Articles

Pre‐Eruptive Evolution and Timescales of Silicic Volcanism in the Tarim Large Igneous Province

AbstractThe widespread (∼48,000 km2) coexisting crystal‐rich monotonous intermediates (trachydacites) and crystal‐poor high‐silica rocks (rhyolites, ignimbrites) in the Tarim Large Igneous Province (T‐LIP) offer a rare opportunity to explore pre‐eruptive timescales and evolutionary patterns of different silicic magmas in LIPs. We combined information from petrography, geochemistry, crystal kinetics, and diffusion chronometry of felsic rocks from nine boreholes to investigate their pre‐eruptive history. The complex reverse Ti zoning patterns in quartz corresponding to temperature variations over 100°C, lognormal crystal size distributions, high‐Ti rims with faster growth rates, variable pressures, and Nd isotopic modeling, indicate silicic magmas in T‐LIP might have witnessed a universal evolutionary pattern of periodic recharge and rejuvenation, coarsening, and cooling and crystallization in magma chambers at different crustal levels, which we term here as RRCCC model. The uniform composition and crystallinity of trachydacites might result from latent heat buffering, and their eruption was triggered by melt‐induced fracturing. The chemical and thermal gradients preserved in rhyolites were generated by thermally variable magma inputs and extensive fractionation at different pressures. Co‐existence of these two types of rocks was controlled by interaction between mantle plume impingement and re‐melting of hydrous crust built up by early subduction. The best‐fitting Ti‐in‐quartz diffusion curves obtained by Neural Networks indicate that magma residence time lasted for ∼106 years, and eruption was triggered within ∼105 years after the last recharge event.

The monotonous intermediate magma system of the Permian Wurzen caldera, Germany: Magma dynamics and petrogenetic constraints for a supereruption

The Lower Permian Wurzen caldera formed during a VEI 8 supereruption that tapped monotonous intermediate magmas in a continental rift. The well-preserved crystal-rich intracaldera ignimbrite (<58 vol%) and its related plutonic facies allow for compositional and textural studies on juvenile fragments and mineral phases, providing a unique opportunity to unravel the caldera's pre-eruptive evolution.The climactic Wurzen ignimbrite contains different fiamme of fine-grained mafic, fine-grained rhyolitic (71.8 to 76.6 wt% SiO2), porphyritic (trachy-) dacitic (64.5 to 67.6 wt% SiO2), and coarse porphyritic rhyodacitic composition (64.7 to 74.3 wt% SiO2), which point to a permanent interplay between rejuvenation, differentiation, and cooling in the Wurzen magma body. The similarity of their trace element concentrations suggests derivation from a shallow magma chamber (Nb and Ta at 22 and 1.6 ppm, respectively). The picture becomes more differentiated when considering mineral composition and thermobarometric estimations. Resorption textures in anorthitic plagioclase (An >45 mol%) are presumably formed by excess heat from underplating magmas. Coarse sieve textures in plagioclase indicate a rapid ascent of magma into the shallow magma body. In contrast, oscillatory zonation and rapakivi texture in quartz, albitic plagioclase, and sanidine indicate convection during crystal growth. The spatially discrete crystallization of augite and pigeonite indicates tapping of anhydrous Ca-saturated and -undersaturated magma. The presence of pigeonite indicates eruption from a superheated anhydrous (trachy-) dacitic magma batch at temperatures of 1010 °C, whereas the occurrence of annite implies simultaneous tapping of hydrous shallow magma chambers at low temperatures (~750 °C). By applied barometers on clinopyroxene and biotite, it suggests a deep-seated magma chamber at depths of 25 to 15 km and a shallow magma chamber at ca. 11 km, respectively.Mineral assemblage of the intracaldera Wurzen ignimbrite (quartz, sanidine, plagioclase, calcic and calcic-sodic clinopyroxene, pigeonite, annite) is more typical for crystal-poor, hot rhyolites like in the Snake River Plain (Idaho, USA) or Messum igneous complex (Namibia), than that of monotonous intermediates at active continental margins.

Evolution of the Lower Permian Rochlitz volcanic system, Eastern Germany: reconstruction of an intra-continental supereruption

Extensional tectonics in the Late Paleozoic Central Europe was accompanied by rift magmatism that triggered voluminous intracontinental caldera-forming eruptions. Among these, the Lower Permian Rochlitz Volcanic System (RVS) in the North Saxon Volcanic Complex (Eastern Germany, Saxony) represents a supereruption (VEI 8, estimated volume of 1056 km3) of monotonous rhyolites followed by monotonous intermediates. Mapping, petrography, whole-rock geochemistry along with mineral chemistry and oxygen isotopes in zircon display its complex eruption history and magma evolution. Crystal-rich (> 35 vol%), rhyolitic Rochlitz-α Ignimbrite with strong to moderate welding compaction erupted in the climactic stage after reheating of the magma by basaltic injections. Due to magma mixing, low-volume trachydacitic-to-rhyolitic Rochlitz-β Ignimbrite succeeded, characterized by high Ti and Zr-values and zircon with mantle δ18O. Randomly oriented, sub-horizontally bedded fiamme, and NW–SE striking subvolcanic bodies and faults suggest pyroclastic fountaining along NW–SE-oriented fissures as the dominant eruption style. Intrusion of the Leisnig and the Grimma Laccoliths caused resurgence of the Rochlitz caldera forming several peripheral subbasins. In the post-climactic stage, these were filled with lava complexes, ignimbrites and alluvial to lacustrine sediments. Significant Nb and Ta anomalies and high Nb/Ta ratios (11.8–17.9) display a high degree of crustal contamination for the melts of the RVS. Based on homogenous petrographic and geochemical composition along with a narrow range of δ18O in zircon Rochlitz-α Ignimbrite were classified as monotonous rhyolites. For the Rochlitz-β Ignimbrites, underplating and mixing with basic melts are indicated by Mg-rich annite–siderophyllite and δ18O < 6.0 in zircon. The wide spectrum of δ18O on zircon suggests an incomplete mixing process during the formation of monotonous intermediates in the RVS.

Voluminous and crystal-rich igneous rocks of the Permian Wurzen volcanic system, northern Saxony, Germany: physical volcanology and geochemical characterization

The North Saxon Volcanic Complex (NSVC) is a nested caldera edifice dominated by the c. 295 Ma Rochlitz Volcanic System and the c. 289 Ma Wurzen Volcanic System (WVS). The climactic activity of the WVS resembled a VEI ≥ 7 fissure ‘supereruption’ resulting in voluminous and crystal-rich caldera-fill ignimbrites (minimum volume c. 199 km3); caldera outflow facies is not known sofar. Precursory to the WVS ‘monotonous intermediates’, rhyolitic and rhyodacitic volcanic activity led to deposition of the low-volume Wermsdorf and Cannewitz ignimbrites. Modal analysis of the WVS pyroclastic units reveals an inhomogeneous crystal population (≤ 58 vol%) comprising k-feldspar, plagioclase, quartz, ortho- and clinopyroxene and minor amounts of biotite. The Wurzen caldera fill ignimbrites feature three types of fiamme: (1) felsic fiamme; (2) mafic fiamme; and (3) granite-porphyry fiamme. This, the modal variation, and the common presence of clinopyroxene and biotite indicate a strong magma mingling component in the WVS—characteristics which have not been observed in the precursory, Wermsdorf and Cannewitz ignimbrites. The caldera fill ignimbrites feature a large compositional variation from (basaltic) trachyandesite to rhyolite caused by basaltic injection and magma mingling. It is proposed that magmatic underplating led to reheating crystal mush and finally to convection processes within the WVS magma chamber. The predominance of either pyroxene or biotite as mafic mineral in the (trachy-) dacitic to rhyolitic ignimbrites indicates eruption of crystal mush from different magma batches. Prominent negative Nb and Ta anomalies of the Wurzen caldera fill ignimbrites, porphyries, and mafic dykes indicate enhanced melt–crust interaction or contamination of mantle melt. In the aftermath of the WVS caldera eruption, basaltic, trachyandesitic, andesitic and rhyolitic melts ascended puncturing the Wurzen-α and β ignimbrites leading to an array of NW–SE-trending dykes, subvolcanic bodies, and lava domes. Among these, voluminuous granite-to-syenite porphyries emplaced. The deeply eroded WVS caldera allows insight into one of the major magmatic processes that governed the post-collisional phase of the Variscan orogeny in Europe. The study of the deeply eroded supervolcano caldera will lead to the understanding of the connection between a monotonous intermediate ignimbrite and related post-eruptive intrusions.

Slab-rollback ignimbrite flareups in the southern Great Basin and other Cenozoic American arcs: A distinct style of arc volcanism

In continental-margin subduction zones, basalt magmas spawned in the mantle interact with the crust to produce a broad spectrum of volcanic arc associations. A distinct style of very voluminous arc volcanism develops far inland on thick crust over periods of 10–20 m.y. and involves relatively infrequent caldera-forming explosive eruptions of dominantly calc-alkaline rhyolite, dacite, and trachydacite with repose times of 104–106 yr. Volumes of individual eruptions are large (102–103 km3), and nested super-eruptions of thousands of cubic kilometers are common. Calderas are as much as 60–75 km in diameter, and surrounding individual ignimbrite outflow sheets extend outward as much as 150 km, blanketing upwards of 105 km2. Little or no basalt is extruded, whereas andesitic differentiates coeval with silicic ignimbrites range from ­minor to dominant in relative volume. A common feature in these flareups is essentially nonextending, thick, inland crust overlying a subducting oceanic plate with transverse tears that rolled back to a steeper dip from a previously flat configuration. Lithospheric delamination is locally possible. Large volumes of basalt that provide heat and mass for silicic magma generation in the crust form by fluid fluxing of the growing mantle wedge overlying the steepening dehydrating slab and from asthenospheric decompression. Variations in the mantle input, together with variations in crustal thickness, temperature, and composition, modulate the expression of the flareups. As a consequence of the high flux of mantle-derived magma into the thick crust, geotherms become elevated, and the brittle-ductile transition can rise to depths as shallow as 7 km. At this transition, diapirically rising magmas from a melting, assimilation, storage, and homogenization (MASH) zone are blocked and spread laterally into discoid chambers that grow until a thermomechanical threshold is attained, triggering climactic eruption and caldera collapse. This ignimbrite flareup style of continental arc volcanism is exemplified by the mid-Cenozoic southern Great Basin ignimbrite province; other examples include the contemporaneous Southern Rocky Mountain, Mogollon-Datil, vast Sierra Madre Occidental volcanic fields, and the late Cenozoic Altiplano-­Puna volcanic complex in the Central Andes. Rhyolitic and trachydacitic ignimbrites typically have erupted, but where the crust was predominantly felsic, prewarmed, and orogenically thickened, well-developed MASH zones have spawned multiple super-eruptions of phenocryst-rich dacite, or monotonous intermediates, and smaller volumes of calc-alkaline rhyolite ignimbrite. In the Great Basin, eruptions of dry, hot trachydacite magma followed the monotonous intermediates. Partial melting in thinner crust with a major mafic component yielded more alkalic rhyolite and related trachydacite.

Experimental petrology of monotonous intermediate magmas

Abstract Large-volume, high-crystallinity, chemically homogeneous ignimbrites, dubbed ‘monotonous intermediates’, provide a unique opportunity to investigate the evolution of crustal magmatic reservoirs. We present the results of hydrothermal experiments on a dacite from Fish Canyon Tuff (FCT) in Colorado (USA), a classic example of a monotonous intermediate deposit, in order to characterize the variations in chemical and physical properties of hydrous dacite magmas as a function of temperature. The experiments (200 MPa, 720–1100°C) span the inferred pre-eruptive conditions of FCT magmas, and are shown to provide the best match to the chemical and physical properties of the erupted magmas at 790±10°C under conditions at or close to water-saturation. The results show the important effect of water content in controlling the chemical and physical evolution of magma, and the contrasted behaviour of water-saturated v. water-undersaturated magmas. In both cases, however, there is a broad interval of temperature (200°C) over which crystal fraction changes little. By recasting this behaviour in terms of enthalpy, rather than temperature, as the independent variable we show that this interval corresponds to a minimum in the change in crystallinity per unit of energy added or subtracted from the system, such that small perturbations to the heat content of the system (e.g. by cooling or new magma injections) results in very little change in magma properties. The crystal content in this interval is 55–65 wt%, which is close to the phenocryst content (40–55 wt%) of monotonous intermediates. We propose that crystal-rich magmas tend to settle in this ‘petrological trap’, changing little in physical and chemical properties over time as the system grows. Petrological trapping enables very large volumes of intermediate magma to accumulate in the shallow crust until such time as the net buoyancy force of these crystal-rich magma is sufficient to overcome the strength of the roof rocks, leading to a potentially very large eruption.

Zircon U‐Pb geochronology of the Mount Givens Granodiorite: Implications for the genesis of large volumes of eruptible magma

AbstractThe Mount Givens Granodiorite, a large pluton in the central Sierra Nevada batholith, California, is similar in area to zoned intrusive suites yet is comparatively chemically and texturally homogenous. New zircon U‐Pb geochronology indicates that the pluton was constructed over at least 7 Ma from 97.92 ± 0.06 Ma to 90.87 ± 0.05 Ma. Combining the new geochronology with the exposed volume of the pluton yields an estimated magma flux of &lt;0.001 km3/a. The geochronologic data are at odds with the previously speculated links between plutons such as the Mount Givens Granodiorite and large‐volume homogeneous ignimbrites (often termed monotonous intermediates). Existing data indicate that large plutons accumulate at rates of ≤0.001 km3/a, 1–2 orders of magnitude less than fluxes calculated for dated monotonous intermediates. If monotonous intermediates are remobilized, erupted plutons accumulated at rates comparable to dated examples, they should preserve a record of zircon growth of up to 10 Ma. Alternatively, the long history of zircon growth recorded in plutons may be erased during the processes of reheating and remobilization that precede supervolcano eruption. However, zircon dissolution modeling, based on hypothetical temperature‐time histories for preeruptive monotonous intermediates, indicates that rejuvenation events would not sufficiently dissolve zircon. We suggest that eruptions of monotonous intermediates occur during high magmatic flux events, leaving little behind in the intrusive rock record, whereas low fluxes favor pluton accumulation.

Development, mobilisation and eruption of a large crystal-rich rhyolite: The Ongatiti ignimbrite, New Zealand

The Ongatiti ignimbrite (>~500km3 DRE) was erupted at 1.21±0.04Ma from the Mangakino volcanic centre (Taupo Volcanic Zone, New Zealand). The ignimbrite is crystal rich (20–30%), of rhyodacite to low-silica rhyolite composition and lacks any consistent compositional zonation. Pumice whole-rock and glass compositions and crystal rim chemistries imply that the final erupted magma body was, to a first order, homogeneous, with the only variations reflecting crystal:glass proportions in analysed clasts. Crystals from pumices can be divided into two populations based on textural and chemical signatures: those that are inferred to have grown within the final erupted magma body (82% of plagioclase, 88% of orthopyroxene, 17% of amphibole), and those that originated in a chemically heterogeneous mush zone (18% of plagioclase, 12% of orthopyroxene, 83% of amphibole). Crystal-rich microcrystalline clasts, and clots within pumices, provide direct samples of parts of this heterogeneous source region. Amphibole model temperatures and pressures, coupled with in-situ trace element concentrations suggest that the mush region extended to ~15km depth, near the base of the quartzofeldspathic crust and was rich in amphibole. Amphibole plus subordinate plagioclase and orthopyroxene, as well as antecrystic zircon were extracted from the crystal mush and ascended to a final storage region. Model temperatures and pressures from amphibole rims and Fe–Ti oxide model temperatures imply that the final erupted magma body was stored between 770 and 840°C at 4–6km depths. Homogenisation of the magma body occurred through convective stirring accompanying gradually rising temperatures induced by less-evolved magma(s) emplaced at deeper levels. Plagioclase records a steady core-to-rim increase in An content, implying that a gradual heating and/or increase in H2O in the final erupted magma body occurred over some significant period prior to the eruption. No signals of a rapid defrosting or rejuvenation event that could be considered as an eruption trigger are recorded by the crystal phases. The Ongatiti ignimbrite has features both common to, and distinct from crystal rich ‘monotonous intermediates’ and crystal-poor compositionally zoned rhyolites. As such, the Ongatiti ignimbrite demonstrates that ‘monotonous rhyolites’ can also be developed on a large scale.

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

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

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

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 km3. 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 km3. 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.

The limitations of melting on the reactivation of silicic mushes

High crystallinity silicic ignimbrites (such as the Monotonous Intermediates) typically erupt magma with an average crystallinity ranging from 40 to 50%. This average crystallinity is believed to be just under the threshold at which magma behaves as a solid (50–60% crystals), i.e. the locking point crystallinity, where convection is suppressed and large eruptions are unlikely. These magmas often display textural features which suggest that their average crystallinity was once higher and decreased before the eruption as a result of reheating through the injection of new magma. In this study, we use a theoretical 1D heat conduction model with phase change to test the ability of different melting scenarios of crystal mushes to meet the 40 to 50% crystallinity constraint observed in the field. Our heat conduction and melting models allow us to derive analytical solutions for the average crystallinity in the magma body (initially a crystal mush). We focus on the propagation of the melting front coinciding with the locking point crystallinity for different crystallinity–temperature relationships and various choices of temperature boundary conditions. We develop another analytical model based on stagnant-lid convection scaling to assess the role of convection on the expected average crystallinity of the magma subjected to wholesale steady-state convection. We find that, for all realistic melting scenarios, the average crystallinity of a silicic magma body that passed through the rheological transition is always substantially lower than what is observed in the field. We further show with a simple energy balance that the thermal energy needed to unlock/remobilize these magma bodies requires the intrusion of about an order of magnitude of more magma than the mush. Based on these results we argue that, although melting is a key process in the thermal reactivation of high crystallinity magma bodies, another coupled process is required in order to reactivate large volumes of crystal mushes. Moreover this additional process has to (1) be more efficient energetically and (2) lead to a smaller overall crystallinity reduction than melting alone.

Two Competing Effects of Volatiles on Heat Transfer in Crystal-rich Magmas: Thermal Insulation vs Defrosting

The build-up and eruption of crystal-rich ignimbrites, commonly referred to as ‘Monotonous Intermediates’, has been attributed to the incremental addition of new magma batches and the episodic partial melting and reactivation of rheologically locked crystal-rich magma bodies (mushes). In this study, we explore the role of volatiles exsolved from a hot intrusion underplating a crystal mush on the thermal evolution of the coupled mush–intrusion system. We solve the enthalpy conservation equation for the mush and the intrusion and investigate the exsolution of volatiles from the intrusion and their transport through the mush in one dimension. Our calculations span a range of pressures (from 1 to 4 kbar), mush composition (andesite to rhyolite) and initial water contents (from 1·5 to 6 wt %). The mobility of volatiles in the mush is controlled by the volume fraction of the pore space they occupy, and, as a consequence, by the amount of melting and the injection rate of volatiles from the intrusion to the mush. We find that volatiles affect the melting of the mush (or defrosting) in two opposite ways depending on pressure and the initial water content of the intrusion. When the intrusion volatile content is high and pressure relatively low (>4 wt % H2O at 2 kbar), the mass transfer of volatiles from the intrusion to the mush carries enthalpy beyond the melting front and can thus enhance defrosting and possibly remobilization of the mush. For lower initial volatile contents (< 4 wt % H2O) in the intrusion and/or higher pressure (3–4 kbar), volatiles stall at the interface between the two magma bodies and prevent defrosting as they thermally insulate the mush from the intrusion. We propose that the dual role played by volatiles during the thermal evolution of crystal-rich magmas reheated by an underplating intrusion can explain the presence of crystal-rich ignimbrites in arcs and their absence in drier hotspots or extensional regions.

The Upper Crustal Evolution of a Large Silicic Magma Body: Evidence from Crystal-scale Rb–Sr Isotopic Heterogeneities in the Fish Canyon Magmatic System, Colorado

Batholith-sized bodies of crystal-rich magmatic 'mush' are widely inferred to represent the hidden sources of many large-volume high-silica rhyolite eruptive units. Occasionally these mush bodies are ejected along with their trapped interstitial liquid, forming the distinctive crystal-rich ignimbrites known as 'monotonous intermediates'. These ignimbrites are notable for their combination of high crystal contents (35-55%), dacitic bulk compositions with interstitial high-silica rhyolitic glass, and general lack of compositional zonation. The 5000 km 3 Fish Canyon Tuff is an archetypal eruption deposit of this type, and is the largest known silicic eruption on Earth. Ejecta from the Fish Canyon magmatic system are notable for the limited compositional variation that they define on the basis of whole-rock chemistry, whereas similar to 45 vol.% crystals in a matrix of high-silica rhyolite glass together span a large range of mineral-scale isotopic variability ( microns to millimetres). Rb/Sr isotopic analyses of single crystals (sanidine, plagioclase, biotite, hornblende, apatite, titanite) and sampling by micromilling of selected zones within glass plus sanidine and plagioclase crystals document widespread isotopic disequilibrium at many scales. High and variable 87 Sr/ 86 Sr i values for euhedral biotite grains cannot be explained by any model involving closed-system radiogenic ingrowth, and they are difficult to rationalize unless much of this radiogenic Sr has been introduced at a late stage via assimilation of local Proterozoic crust. Hornblende is the only phase that approaches isotopic equilibrium with the surrounding melt, but the melt ( glass) was isotopically heterogeneous at the millimetre scale, and was therefore apparently contaminated with radiogenic Sr shortly prior to eruption. The other mineral phases (plagioclase, sanidine, titanite, and apatite) have significantly lower 87 Sr/ 86 Sr i values than whole-rock values ( as much as - 0.0005). Such isotopic disequilibrium implies that feldspars, titanite and apatite are antecrysts that crystallized from less radiogenic melt compositions at earlier stages of magma evolution, whereas highly radiogenic biotite xenocrysts and the development of isotopic heterogeneity in matrix melt glass appear to coincide with the final stage of the evolution of the Fish Canyon magma body in the upper crust. Integrated petrographic and geochemical evidence is consistent with pre- eruptive thermal rejuvenation of a near-solidus mineral assemblage from similar to 720 to 760 oC (i.e. partial dissolution of feldspars+quartz while hornblende+titanite+biotite were crystallizing). Assimilation and blending of phenocrysts, antecrysts and xenocrysts reflects chamber-wide, low Reynolds number convection that occurred within the last ~10 000 years before eruption.

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

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.