Jemez Mountains Volcanic Field Research Articles

Trace Element and Isotope Geochemistry of Tschicoma Formation Intermediate Composition Dome Complexes, Jemez Mountains Volcanic Field, New Mexico, USA

Abstract Repeated intrusions of mafic magma have long been known to be a driver of long-lived magmatic systems. Although the importance of mafic recharge of silicic magma systems is well-documented in igneous petrology, the origin of this recharge magma is sometimes obscure. By examining the pre-caldera intermediate dome complexes of the Tschicoma Formation and their relationship to a dacitic recharge event into the Tshirege Member of the Bandelier Tuff, we aim to better understand the origin of mafic recharge events into the Bandelier magma chamber of Valles caldera, and the relationship between different stages of volcanic activity within the broader Jemez Mountains volcanic field (JMVF). Based on major, trace element and radiogenic isotopic data, we divide the Tschicoma Formation into three geochemical groups with similar petrologic evolutionary paths. The Cerro Grande, Cerro Rubio and Pajarito Mountain volcanic dome complexes form group A and have assimilated various amounts of a granitoid crustal component with low εNd, εHf and radiogenic Pb. Group B consists of the Sawyer Dome, Rendija Canyon and Caballo Mountain dome complexes, which have principally evolved through different degrees of fractional crystallization of the same parent magma, itself a result of complex interactions of a mafic mantle-derived magma with the crust. The dacite domes and flows around Tschicoma Peak and the newly described Cañada Bonita dacite form group C and are the result of mixing of Rendija Canyon magma with mafic recharge magma which is preserved as distinct mafic enclaves. At a later stage of the JMVF, during the eruption of the Tshirege Member, distinctive hornblende–dacite pumices formed as a result of the influx of more mafic recharge magma into the system, which mobilized a pre-existing dacite intrusion and injected it into the Tshirege rhyolite ( Stimac, 1996; Boroet al.,2020). Based on trace element and isotopic compositions, we propose that dacite which was injected into the Tshirege magma chamber was related to the earlier-erupted Tschicoma Formation and itself represents a mixing product of Tshirege rhyolite and a precursor to the Tschicoma dacites. This implies that the Tschicoma magmatic system was long-lived yet dormant during the eruption of the Otowi Member of the Bandelier Tuff, then was reactivated shortly before the Tshirege eruption, temporarily co-existing and interacting with the Bandelier system as it erupted.

Ten Thousand Years of Magma Storage Preceding the Last Caldera-Forming Eruption of the Bandelier Magmatic System, New Mexico, USA

Abstract In this study, we present new evidence for changes in magma storage conditions that preceded the 1232 ka caldera-forming eruption of the Bandelier magmatic system in the Jemez Mountains Volcanic Field. Using high precision 40Ar/39Ar sanidine dating we determine that at least eight rhyolites erupted within 8.6 ± 3.4 kyr of the ~400 km3 eruption that formed Valles caldera. Some of those rhyolites contain fayalite with or without biotite, others contain only biotite. An eruption of fayalite-bearing rhyolite at 1240.5 ± 2.1 ka ended an eruption hiatus of at least 100 kyr. Following that first post-hiatus episode of volcanism, at least four more eruptions of fayalite-bearing rhyolite and three eruptions of biotite-bearing rhyolite occurred prior to the caldera-forming eruption. We use phase equilibrium experiments and geothermobarometry to infer the storage conditions and processes that led to these differing crystal cargos and ultimately generated ~400 km3 of predominantly fayalite rhyolite ignimbrite (Tshirege Member of the Bandelier Tuff). We find that biotite-bearing rhyolites were stored at 695–750°C, 75–160 MPa, and at an oxygen fugacity more oxidizing than the quartz-fayalite-magnetite (QFM) buffer reaction. Fayalite-bearing rhyolites were similarly stored at 695–745°C and 70–190 MPa, but at more reducing conditions (${f}_{O_2}$≤ QFM). We suggest that the reduced, fayalite-bearing rhyolite was most likely produced via interaction of crystal-poor rhyolitic magma with a reducing, potentially Cl-bearing, and H2O-rich supercritical fluid phase. This fluid flux event was a key component of the substantial magmatic rejuvenation that enabled the mobilization of ~400 km3 of mostly fayalite-bearing rhyolite prior to not only the Tshirege event, but the older Otowi event as well.

Long-lived dacitic magmatic systems and recharge dynamics in the Jemez Mountains volcanic field, western USA

We examine the connected history of dacite-dominant volcanic rocks of the Tschicoma Formation, erupted between 5.5 and 2 Ma from the Jemez Mountains volcanic field, western USA. Zircon samples from two separate eruptions have continuous SHRIMP U–Pb age spectra spanning 0.84–1.08 Myr duration (3.12–3.96 Ma and 3.50–4.58 Ma, respectively), following an episode of zircon crystallization 0.28–0.50 Myr earlier (at 4.46 Ma and 4.86 Ma, respectively). Zircon chemical variations, as well as ubiquitous resorption textures that commonly show large core-rim age differences (up to 720–740 kyr), suggest that they grew in separate melt lenses. Zircons were likely stored at near-solidus or even sub-solidus conditions after crystallization, but may have been reactivated in response to at least four major magma recharge events every 300–400 kyr and smaller events in between. A cycle of zircon dissolution (from heating), recrystallization (during cooling), and storage repeated in different locations in the Tschicoma mush system throughout its lifespan; each recharge-induced heating stage may last for several hundred to more than a thousand years based on calculations of zircon dissolution. We envisage the melt lenses to be distributed in a crystal mush zone, coalescing into a single magma batch as magma recharge occurs shortly before eruption. Once active, increasing magma supply rates may trigger large-scale partial melting of the pre-existing mush and caldera-forming eruptions.

Crustal evolution leading to successive rhyolitic supereruptions in the Jemez Mountains volcanic field, New Mexico, USA

The magmatic systems that feed supereruptions result from high magma supply from depth that sustains large regions of partial melt in the shallow crust. However, many questions remain around the processes and timescales over which super-sized magmatic systems develop. We present new zircon age, trace elemental and isotopic data from the Jemez Mountains volcanic field (JMVF) to reveal contrasting magmatic processes that led to successive rhyolitic supereruptions at 1.60 and 1.25 Ma. Before the supereruptions, zircons from lavas erupted over ~8.4 Myr have predominantly unimodal age distributions, close to their respective eruption ages. This highlights that distinct magma domains gradually formed in the JMVF crust. Zircon crystal inheritance shows that magma from the first supereruption at 1.60 Ma included at least three of these precursor plutons and basement rock that were partially molten by enhanced magma supply from ~2 Ma. In contrast, absence of inherited zircon and a change in zircon chemistry in ignimbrite from the second supereruption at 1.25 Ma indicates a thermal resetting of the remnants of the old magma reservoir and rapid construction of a new super-sized magma body. The sudden change in eruptive behaviour in the young JMVF reflects both thermal and chemical maturation of the crust, as well as elevated magma supply and partial melting of precursor plutons. Our study highlights that although crustal conditioning may occur during gradual magmatism over millions of years, the assembly of super-sized magma bodies occurs over much faster timescales and may occur in succession due to thermal adjustment following evacuation.

Mineralogical Evidence of Pre-caldera Magma Petrogenesis in the Jemez Mountains Volcanic Field, New Mexico, USA

Abstract The Jemez Mountains volcanic field (JMVF) is the site of the two voluminous, caldera-forming members of the Bandelier Tuff, erupted at 1·60 and 1·25 Ma, following a long and continuous pre-caldera volcanic history (∼10 Myr) in this region. Previous investigations utilizing whole-rock geochemistry identified complex magmatic processes in the two major pulses of pre-caldera magmatism including assimilation–fractional crystallization (AFC) and magma mixing. Here we extend the petrological investigation of the pre-caldera volcanic rocks into the micro-realm and use mineral chemistry and textural information to refine magma evolution models. The results show an increasing diversity of mineral populations as the volcanic field evolved. A range of plagioclase textures (e.g. sieved cores and rims) indicate disequilibrium conditions in almost all pre-caldera magmas ranging from andesite to rhyolite, reflecting plagioclase dissolution and regrowth. Coarsely sieved or dissolved plagioclase cores are explained by resorption via water-undersaturated decompression during upward migration from a deep melting, assimilation, storage and homogenization (MASH) zone. Plagioclase crystals with sieved rims are almost ubiquitous in dacite-dominated magmatism (La Grulla Plateau andesite and dacite erupted at ∼8–7 Ma, as well as Tschicoma Formation andesite, dacite and rhyolite at ∼5–2 Ma), reflecting heating induced by magma mixing. These plagioclase crystals often have An-poor cores that are chemically distinct from their An-rich rims. The existence of different plagioclase populations is consistent with two distinct amphibole groups that co-crystallized with plagioclase: a low-Al, low-temperature, high-fO2 group, and a high-Al, high-temperature, low-fO2 group. Calculation of melt Sr, Ba, La, and Ce concentrations from plagioclase core and rim compositions suggests that these chemical variations are largely produced by magma mixing. Multiple mafic endmembers were identified that may be connected by AFC processes in the MASH zone in the middle to lower crust. The silicic component in an early andesite-dominated magmatic system (Paliza Canyon andesite, dacite and rhyolite, 10–7 Ma) is represented by contemporaneous early rhyolite (Canovas Canyon Rhyolite). A silicic mush zone in the shallow crust is inferred as both the silicic endmember involved in the dacite-dominant magmatic systems and source of the late low-temperature rhyolite (Bearhead Rhyolite, 7–6 Ma). Recharging of the silicic mush by mafic melts can explain observed diversity in both mineral disequilibrium textures and compositions in the dacitic magmas. Overall, the pre-caldera JMVF magmatic system evolved towards cooler and more oxidized conditions with time, indicating gradual thermal maturation of local crust, building up to a transcrustal magmatic system, which culminated in ‘super-scale’ silicic volcanism. Such conditioning of crust with heat and mass by early magmatism might be common in other long-lived volcanic fields.

Quaternary tephra from the Valles caldera in the volcanic field of the Jemez Mountains of New Mexico identified in western Canada

AbstractA fine-grained, up to 3-m-thick tephra bed in southwestern Saskatchewan, herein named Duncairn tephra (Dt), is derived from an early Pleistocene eruption in the Jemez Mountains volcanic field of New Mexico, requiring a trajectory of northward tephra dispersal of ~1500 km. An unusually low CaO content in its glass shards denies a source in the closer Yellowstone and Heise volcanic fields, whereas a Pleistocene tephra bed (LSMt) in the La Sal Mountains of Utah has a very similar glass chemistry to that of the Dt, supporting a more southerly source. Comprehensive characterization of these two distal tephra beds along with samples collected near the Valles caldera in New Mexico, including grain size, mineral assemblage, major- and trace-element composition of glass and minerals, paleomagnetism, and fission-track dating, justify this correlation. Two glass populations each exist in the Dt and LSMt. The proximal correlative of Dt1 is the plinian Tsankawi Pumice and co-ignimbritic ash of the first ignimbrite (Qbt1g) of the 1.24 Ma Tshirege Member of the Bandelier Tuff. The correlative of Dt2 and LSMt is the co-ignimbritic ash of Qbt2. Mixing of Dt1 and Dt2 probably occurred during northward transport in a jet stream.

Volcanism and sedimentation along the western margin of the Rio Grande rift between caldera-forming eruptions of the Jemez Mountains volcanic field, north-central New Mexico, USA

The Cerro Toledo Formation (CTF), a series of intracaldera rhyolitic dome complexes and their associated extracaldera tephras and epiclastic sedimentary deposits, records the dynamic interplay between volcanic, tectonic, and geomorphic processes that were occurring along the western margin of the Rio Grande rift between major caldera-forming eruptions of the Bandelier Tuff 1.65–1.26Ma. The Alamo Canyon and Pueblo Canyon Members differ significantly despite deposition within a few kilometers of each other on the Pajarito Plateau. These differences highlight spatial distinctions in vent sources, eruptive styles, and depositional environments along the eastern side of the Jemez Mountains volcanic field during this ca. 400,000year interval. Intercalated pyroclastic fall deposits and sandstones of the Pueblo Canyon Member reflect deposition with a basin. Thick Alamo Canyon Member deposits of block-and-ash-flow tuff and pyroclastic fall deposits fill a paleovalley carved into coarse grained sedimentary units reflecting deposition along the mountain front. Chemistry and ages of glass from fall deposits together with clast lithologies of sedimentary units, allow correlation of outcrops, subsurface units, and sources. Dates on pyroclastic fall deposits from Alamo Canyon record deep incision into the underlying Otowi Member in the southern part of the Pajarito Plateau within 100k.y. of the Toledo caldera-forming eruption. Reconstruction of the CTF surface shows that this period of rapid incision was followed by aggradation where sediments largely filled pre-existing paleocanyons. Complex sequences within the upper portion of the Otowi Member in outcrop and in the subsurface record changes in the style of eruptive activity during the waning stages of the Toledo caldera-forming eruption.

A GIS-based volcanic hazard and risk assessment of eruptions sourced within Valles Caldera, New Mexico

The objective of this study is to evaluate the spatial extent of a possible future eruption using a GIS-based volcanic hazard tool designed to simulate pyroclastic fallout and density currents (PDCs) as well as lava flows and to assess the social and economic vulnerabilities of the area at risk. Simulated pyroclastic fallout deposits originating from the El Cajete crater within the Valles Caldera, Jemez Mountains volcanic field, New Mexico, are calibrated to isopach and lithic isopleth maps of the Lower and Upper El Cajete as constructed by Wolff et al. (2011). The change in the axial orientation of fallout deposits between the Lower and Upper El Cajete is best matched using seasonal variations in wind speed and direction based on modern atmospheric records. The calibration of PDCs is based on the distribution and run-out of the Battleship Rock Ignimbrite. Once calibrated, hazards are simulated at a second vent location determined from probability distributions of structural features. The resulting hazard simulation maps show the potential distribution of pyroclastic fallout, PDCs and lava flows, indicating areas to the S/SE of Valles Caldera to be at greatest risk.To assess hazard preparedness, social vulnerability is evaluated for all census-designated places (CDPs) within the study site. Based on methods by the Hazards and Vulnerability Research Institute, twenty-four variables were selected as proxies of social vulnerability and a principal component analysis was used to generate eight components, which account for 64% of the total variance. The eight component scores are summed into a final score for each CDP, and the standard deviations from the mean of the scores are mapped with the CDPs, allowing for an easy visualization of areas that are considered more socially vulnerable.Economic vulnerability is evaluated through a multi-criteria evaluation of population, infrastructure, road types and land use. Each variable is categorized and assigned a value representing relative vulnerability based on cost and importance. The variables are assigned weights relative to one another through a pairwise comparison and summed together into a final map showing the distribution of economic vulnerability, which is useful when used with the hazard simulation maps for targeting areas for mitigation to reduce economic loss.In order to evaluate the overall risk, the hazard simulation maps and vulnerability assessments are aggregated through weighted linear combination and pairwise comparison matrices, creating a total of three risk maps. Although the actual maps provide greater detail, overall, based on the criteria chosen, the risk maps show that pyroclastic fallout has the greatest impact, effecting areas up to 50km S/SE of the Valles Caldera, including highly vulnerable cities, such as Los Alamos and White Rock. The PDC and lava flow hazards, however, impact significantly smaller areas, primarily disturbing forested land. The methodology presented in this paper allows for a robust analysis of the risks posed by eruptions sourced from the Valles Caldera and is especially useful in focusing mitigation strategies to reduce the loss from such hazardous events.

Petrogenesis of Pre-caldera Mafic Lavas, Jemez Mountains Volcanic Field (New Mexico, USA)

The Miocene–Quaternary Jemez Mountains volcanic field ( JMVF), the site of the Valles caldera, lies at the intersection of the Jemez lineament, a Proterozoic suture, and the Cenozoic Rio Grande rift. Parental magmas are of two types: K-depleted silicaundersaturated, derived from the partial melting of lithospheric mantle with residual amphibole, and tholeiitic, derived from either asthenospheric or lithospheric mantle. Variability in silica-undersaturated basalts reflects contributions of melts derived from lherzolitic and pyroxenitic mantle, representing heterogeneous lithosphere associated with the suture. The K depletion is inherited by fractionated, crustally contaminated derivatives (hawaiites and mugearites), leading to distinctive incompatible trace element signatures, with Th/ (Nb,Ta) and La/(Nb,Ta) greater than, but K/(Nb,Ta) similar to, Bulk Silicate Earth. These compositions dominate the mafic and intermediate lavas, and the JMVF is therefore derived largely, and perhaps entirely, from melting of fertile continental Jemez lineament lithosphere during rift-related extension. Significant variations in Pb and Nd isotope ratios (Pb/Pb 1⁄4 17 20–18 93; Nd/ Nd 1⁄4 0 51244–0 51272) result from crustal contamination, whereas Sr/Sr is low and relatively uniform (0 7040–0 7048). We compare the effects of contamination by low-Sr/Sr crust with assimilation of high-Sr/Sr granitoid by partial melting, with Sr retained in a feldspathic residue. Both models satisfactorily reproduce the isotopic features of the rocks, but the lack of a measurable Eu anomaly in most JMVF mafic lavas is difficult to reconcile with a major role for residual plagioclase during petrogenesis.

Hybrid origin of Rio Grande rift hawaiites

Weakly alkaline lavas erupted in the Pliocene Cerros del Rio and El Alto volcanic fields in the Espanola basin of the Rio Grande rift have the unusual trace element signature of approximately chondritic K/Nb combined with high La/Nb and Th/Nb. These lavas have previously been interpreted as the products of modified arc-source–like mantle, ultimately related to subduction of the Farallon plate beneath North America during the early Cenozoic. We show that the chemical signature can be produced by contamination of strongly silica undersaturated K-depleted magmas by continental crust, and that there is no need to invoke a subduction component in Rio Grande rift magma genesis. The same signature appears among voluminous Miocene lavas of the Jemez Mountains volcanic field. Hence, while nephelinite and basanite magmas resembling oceanic island basalts have been recognized as trace components in Rio Grande rift magmatism, our results may require a drastic upward revision of their volumetric significance in the northern rift through late Miocene-Pliocene time.

Sr isotope disequilibrium during differentiation of the Bandelier Tuff: Constraints on the crystallization of a large rhyolitic magma chamber

The 1.61 Ma Otowi Member of the Bandelier Tuff (Jemez Mountains volcanic field, New Mexico) is a caldera-forming high-silica rhyolitic ignimbrite with a precursor fallout deposit. The Otowi Member has high Rb/Sr ratios; the 87 Rb/ 86 Sr ratios from sanidines and glasses range from 14 to 570. Most sanidines from glomerocrysts in the ignimbrite have 87 Sr/ 86 Sr I = 0.7052–0.7056, whereas the ratios from glasses from glomerocrysts range from 0.7052 to 0.7079. Quartz phenocrysts containing glass inclusions from both the initial fallout and the ignimbrite are markedly more radiogenic, at 87 Sr/ 86 Sr I = 0.7105–0.7113, despite having much lower Rb/Sr ratios than glomerocryst glasses. These relations require that the inclusion glasses are more contaminated with Proterozoic country rock than are glomerocryst glasses. Textural, isotopic, and trace element data support a model in which crystals grow in a boundary layer with the most inclusion-rich quartz grains closest to the magma–country rock contact. Phenocrysts and glomerocrysts represent fragments from different zones of the chamber9s crystalline carapace, disseminated throughout the magma prior to eruption. An important implication of these results is that glass inclusions do not necessarily represent precursor magma compositions; hence extrapolation of measured volatile contents of inclusion glasses to the entire volume of an erupted magma should be approached with caution.

The behavior of light lithophile and halogen elements in felsic magma: geochemistry of the post-caldera Valles Rhyolites, Jemez Mountains Volcanic Field, New Mexico

Post-collapse rhyolite lava domes, lava flows and pyroclastic rocks from Valles caldera (1140 ka), erupted from 1133 ka to approximately 520-60 ka, have been sampled to study variations of light lithophile (Li, Be, B) and halogen (F, Cl) elements. Our principal objectives were: (1) to examine the mobility of these elements during post-eruptive devitrification and hydration; and (2) to study their behavior during magma differentiation. Compared to fresh glassy samples, devitrified rocks from the same dome are depleted in B, Li, F and Cl, but not in Be. During devitrification, Be was immobile while the other elements were progressively more mobile in the order B < Li < Cl < F, fluorine being the most mobile element. Considering only fresh glassy samples, Li, Be and B were enriched in residual liquids and behaved incompatibly during differentiation of successive magma batches at 973-787 ka and 557-521 ka. The rhyolites have low B/Be ratios of 2–3 which decrease slightly with increasing Be; these values suggest a small fractionation of B from Be during evolution of the magmas. While F behaves like the light lithophile elements, Cl shows (1) much smaller temporal enrichment during differentiation at 973-787 ka and (2) depletion with time from 557 to 521 ka. At the same time, the Cl/Be ratio declines progressively from ~250 in the oldest rhyolites to ~ 100 in the youngest rhyolites. These data suggest that (1) a magmatic fluid phase continuously extracted Cl from fluid-saturated magmas and (2) some of the magmatic Cl lost could have been incorporated into the Valles hydrothermal system.

Is the Valles caldera entering a new cycle of activity?

The Valles caldera formed during two major rhyolitic ignimbrite eruptive episodes (the Bandelier Tuff) at 1.61 and 1.22 Ma, after some 12 m.y. of activity in the Jemez Mountains volcanic field, New Mexico. Several subsequent eruptions between 1.22 and 0.52 Ma produced dominantly high-silica rhyolite lava domes and tephras within the caldera. These were followed by a dormancy of 0.46 m.y. prior to the most recent intracaldera activity, the longest hiatus since the inception of the Bandelier magma system at ∼1.8 Ma. The youngest volcanic activity at ∼ 60 ka produced the SW moat rhyolites, a series of lavas and tuffs that display abundant petrologic evidence of being newly generated melts. Petrographic textures conform closely to published predictions for silicic magmas generated by intrusion of basaltic magma into continental crust. The Valles caldera may currently be the site of renewed silicic magma generation, induced by intrusion of mafic magma at depth. Recent seismic investigations revealed the presence of a large low-velocity anomaly in the lower crust beneath the caldera. The generally aseismic character of the caldera, despite abundant regional seismicity, may be attributed to a heated crustal column, the local effect of 13 m.y. of magmatism and emplacement of mid-crustal plutons. Seismic signals of magma movement in the deep to mid-crust may therefore be masked, and clear seismic indications of intrusion may only be generated within a few kilometres of the surface. We therefore encourage the establishment of a local dedicated volcanic monitoring system.

The influence of ephemeral processes on pyroclastic sedimentation in a rift-basin, volcaniclastic-alluvial sequence, Española basin, New Mexico

The Plio-Pleistocene Puye Formation, north-central New Mexico, is a coarse-grained conglomeratic wedge of primary and reworked volcanic deposits that prograded eastward into an actively aggrading floodbasin of the central Rio Grande rift. This alluvial system grew in response to the degradation of dacitic and rhyolitic lava domes in the northeastern part of the Jemez Mountains volcanic field. Analysis of Puye deposits shows that fan sedimentation was governed by the complex interaction of explosive volcanism, rift-basin tectonics, and the highly variable localized ephemeral processes common to arid-region alluvial systems. Preserved in this well exposed sequence are complex associations between primary pyroclastic facies related to explosive eruptions from lava domes, and reworked tuffs that record the rapid emplacement of poorly consolidated pumice and ash. The sporadic availability of water in this semi-arid region, and the episodic flood frequency and runoff that accompanied individual eruptions, largely controlled Puye facies characteristics. Important among the combined influences of explosive eruptions and ephemeral processes were: (1) fluvial transport and reworking of pumice sequences, (2) remobilization of loosely consolidated volcanic talus and tuffs by mass flow, and (3) marked downslope changes in mass-flow deposits and fallout tephra emplaced into lakes.

Discrimination of eolian and pyroclastic-surge processes in the generation of cross-bedded tuffs, Jemez Mountains volcanic field, New Mexico

Discrimination of pyroclastic deposits emplaced by volcanic processes and reworked pyroclastic sediments is a primary objective of studies in volcaniclastic strata. Cross-bedded tuffs of both eolian and pyroclastic-surge origins are identified within the late Miocene Peralta Tuff Member of the Bearhead Rhyolite, in north-central New Mexico. Distribution, paleoflow directions, grain-size characteristics, and depositional structures permit discrimination of these two similar facies. Eolian tuffs exhibit structures diagnostic of eolian sands, are relatively well sorted, and lack both the fine ash and coarse lapilli and bombs typically found in surge deposits. Discrimination between eolian reworked and primary surge pyroclastic deposits is critical for volcanic hazard studies near modern volcanoes and for correct interpretation of proximity to vents in ancient volcaniclastic sequences.

40Ar/39Ar dating of the Bandelier Tuff and San Diego Canyon ignimbrites, Jemez Mountains, New Mexico: Temporal constraints on magmatic evolution

Abstract The Jemez Mountains volcanic field (JMVF), located in north-central New Mexico, has been a site of basaltic to rhyolitic volcanism since the mid-Miocene with major caldera forming eruptions occurring in the Pleistocene. Eruption of the upper Bandelier Tuff (UBT) is associated with collapse of the Valles Caldera, whereas eruption of the lower Bandelier Tuff (LBT) resulted in formation of the Toledo Caldera. These events were previously dated by K-Ar at 1.12 ± 0.03 Ma and 1.45 ± 0.06 Ma, respectively. Pre-Bandelier explosive eruptions produced the San Diego Canyon (SDC) ignimbrites. SDC ignimbrite “B” has been dated at 2.84 ± 0.07 Ma, whereas SDC ignimbrite “A”, which underlies “B”, has been dated at 3.64 ± 1.64 Ma. Both of these dates are based on single K-Ar analyses. 40Ar/39Ar dating of single sanidine crystals from these units indicates revision of the previously reported dates. Isochron analysis of 26 crystals from the UBT gives a common trapped 40Ar/36Ar component of 304.5, indicating the presence of excess 40Ar in this unit, and defines an age of 1.14 ± 0.02 Ma. Isochron analysis of 26 crystals from the LBT indicates an atmospheric trapped component and an age of 1.51 ± 0.03 Ma. An age of 1.78 ± 0.04 Ma, based on the weighted mean of 5 individual analyses, is indicated for SDC ignimbrite “B”, whereas 3 analyses from SDC ignimbrite “A” give a weighted mean age of 1.78 ± 0.07 Ma. Evidence for xenocrystic contamination in the SDC ignimbrites comes from analyses of a correlative air-fall pumice unit in the Puye Formation alluvial fan giving ages of 1.75 ± 0.08 and 3.50 ± 0.09 Ma. The presence of xenocrysts in bulk separates used for the original K-Ar analyses could account for the significantly older ages reported. Geochemical data indicate that SDC ignimbrites are early eruptions from the magma chamber which evolved to produce the LBT, as compositions of SDC ignimbrite “B” are virtually identical to least evolved LBT samples. Differentiation during the 270-ka interval between eruption of SDC ignimbrite “B” and the LBT produced an array of high-silica rhyolite compositions which were erupted to form the LBT. Mixed pumices associated with eruption of the LBT indicated an influx of more mafic magma into the system which produced shifts in some incompatible trace-element ratios. Lavas and tephras of the Cerro Toledo Rhyolite record the geochemical evolution of the Bandelier magma system during the 370-ka interval between eruption of the LBT and the UBT. The combined geochronologic and geochemical data place the establishment and evolution of the Bandelier silicic magma system within a precise temporal framework, beginning with eruption of the SDC ignimbrites at 1.78 Ma, and define a periodicity of 270–370 ka to ash-flow eruptions in the JMVF. These intervals are comparable to those in other multicyclic caldera complexes and are a measure of the timescales over which substantial fractionation of large silicic magma bodies occur.

Petrogenesis of Valle Grande Member rhyolites, Valles Caldera, New Mexico: Implications for evolution of the Jemez Mountains Mgmatic System

The Valles Caldera, located near the center of the Jemez Mountains Volcanic Field in north‐central New Mexico, was formed at ∼1.12 Ma upon eruption of the upper Bandelier Tuff. The Valle Grande Member (VGM) consists of 8 rhyolite domes or dome complexes erupted from vents which define the ring fracture within the caldera. They range in age from the time of caldera formation to ∼0.45 Ma and are volumetrically and temporally the dominant post‐caldera eruptives. VGM rhyolites are high‐silica (&gt;75% SiO2 anhydrous), high‐K, are characterized by low abundances of Fe, Mg, Mn, Ti, and Ca, and are metaluminous to weakly peraluminous. Phenocryst assemblages include sanidine (Or44–61) + quartz + plagioclase (An7–20) + biotite + hornblende + Fe‐Ti oxides ± zircon ± allanite ± apatite ± clinopyroxene ± orthopyroxene. Many phenocryst phases show progressive changes in composition, size, and abundance with decreasing age. Whole rock chemistry, phenocryst chemistry, and isotopic dating indicate that VGM rhyolites can be divided into 3 groups: group 1 ranging in age from 1.18 to 0.71 Ma, group 2 from 0.55 to 0.51 Ma, and group 3 at 0.45 Ma. Groups 1 and 2 define differentiation trends in which trace elements such as Rb, Cs, Y, Nb, HREE, Ta, Th, and U increase with decreasing age, whereas Sr, Ba, Zr, LREE, and Eu decrease. Abundances of trace elements may increase or decrease 2–3 fold within eruptive groups; major elements remain relatively constant. The observed differentiation trends can be accounted for by crystal‐liquid fractionation at a minimum or eutectic using modal phenocryst phases. Fractionation may have occurred by roof and side wall crystallization with the collection of bouyant differentiated liquids at the roof of the magma chamber. Two‐feldspar geothermometry, geobarometry using hornblende rim Al contents, and plots of normative compositions on the Q‐Ab‐Or diagram indicate that fractionation took place at depths of 2.5–7.5 km with pre‐eruptive temperatures ranging from ∼720–810°C. Rhyolites erupted in the Jemez Mountains over the last 2–3 m.y. appear to be the products of many separate magma batches and were not derived from one large long‐lived magma chamber. VGM rhyolites represent 3 separate magma batches (groups 1, 2, and 3), two of which remained closed systems for sufficient periods of time to generate highly evolved differentiates (groups 1 and 2).

Lava-dome growth and explosive volcanism in the Jemez Mountains, New Mexico: Evidence from the plio-pleistocene puye alluvial fan

Abstract The Plio-Pleistocene Puye Formation, north-central New Mexico, is a 200-km2 volcanogenic alluvial fan shed eastward from the Tschicoma volcanic center, part of the Jemez Mountains volcanic field. The fan contains > 15km3 of volcaniclastic material derived from closely spaced lava domes in the northeastern portion of the Tschicoma center. Interbedded in the fan sediments are at least 25 primary pyroclastic units from explosive eruptions of dacitic and rhyolitic lava domes. Tephra occur mainly as pumice falls but include several pumiceous ignimbrites and two thick proximal block-and-ash pyroclastic flows. The upper part of the fan also contains rhyolitic plinian deposits erupted from sources in the central portion of the Jemez field and basaltic ash derived from the central Rio Grande rift. Fanglomeratic (pyroclastic and epiclastic) facies exhibit considerable lateral variation. Primary tephra deposits, however, provide a stratigraphic framework for reconstruction of the growth of individual lava domes. While only volcanic domes and lava flows are exposed in the Tschicoma center, Puye tephra layers show that vulcanian, subplinian and plinian activity, and block-and-ash pyroclastic flows accompanied many dome-forming events. Degradation of these lava domes produced coarse-grained debris flows dominated by lava and dome carapace clasts. From these epiclasts it is possible to identify lithologies of lavas that are either currently buried or have been obliterated by erosion or by collapse of the Valles calderas. The preservation of Puye deposits reflects very high rates of aggradation and gradual down-faulting in the Espanola basin of the central Rio Grande rift. This record of pyroclastic and epiclastic deposition allows detailed interpretation of the evolution of a volcanic center from its pyroclastic and erosion products, and is the first example of a volcanogenic alluvial fan in an intracontinental rift setting to be described in detail. This study emphasizes that interpretation of any ancient volcanic terrain based solely upon data derived from mapping, radiometric dating, and chemical analysis of lavas or existing denuded edifices is likely to greatly underestimate both the role of explosive volcanism in the build-up of volcanic centers and subsequent estimates of the volumes of material produced.

Microthermometry of fluid inclusions from the VC‐1 core hole in Valles Caldera, New Mexico

Fluid inclusions in vein quartz and calcite from core samples of the VC‐1 hole were studied with microscope heating/freezing and crushing stages. All samples originate from hydrothermally altered Paleozoic rocks predating formation of the Jemez Mountains volcanic field and Valles caldera. Most homogenization temperatures (Th) of the liquid‐rich inclusions are above the present well temperature, but some Th of primary inclusions from 515 m and those of secondary inclusions from 723 m fit the present well temperature curve measured 10 months after completion of the well. The maximum temperature recorded by the primary inclusions is 275°C from hydrothermal quartz in the Sandia Formation at 811‐m depth. The total range of Th for samples from several depths (90°C) indicates cooling from the maximum temperature. The salinity of fluid inclusions in hydrothermal quartz and calcite is generally low, &lt;1 wt % NaCl eq. High‐salinity fluid, up to 5 wt % NaCl eq, has been found in several calcite veins from the lower part of the Madera Limestone. The salinity decreases with decrease of Th of the secondary inclusions, and that with lowest Th at the lower part of the Madera Limestone is similar to those from the other depths. These data show that early hot water circulation system involved several types of fluid, whereas the later one was a homogeneous fluid. The salinity of fluid inclusions in detrital quartz (presumably inherited inclusions) is higher than that in hydrothermal minerals. Some of these inclusions show extraordinary low temperatures of final melting point of ice (about −40°C), suggesting that a CaCl2 component is present. CO2 contents in fluid inclusions were estimated by the bubble behavior on crushing. Crushing results indicate that CO2 content of the early fluid is ≧0.35 wt %, and that of the later fluid is ∼0.2–0.3 wt %. Geothermal fluid trapped in the fluid inclusions representing the present temperature regime is comparable in composition to those from the Baca geothermal field inside the caldera and to those from hot springs in San Diego Canyon.

San Diego Canyon ignimbrites: Pre‐Bandelier tuff explosive rhyolitic volcanism in the Jemez Mountains, New Mexico

Ignimbrites up to 80 m thick underlie the lower Bandelier Tuff in the southwestern part of the Jemez Mountains volcanic field, New Mexico. They were formed during silicic explosive eruptions that preceded the climactic events of the Bandelier Tuffs and collapse of the Valles I (Toledo) and Valles II calderas. Source vents for these units, herein called the San Diego Canyon ignimbrites, probably underlie the southwestern portion of the Valles caldera. Small‐scale caldera collapse may have been associated with the eruption of these ignimbrites, but evidence of an early caldera, or any related vent structure, has since been obliterated. This late Pliocene (about 2.5 Ma), high‐silica rhyolite activity was contemporaneous with eruptions from the predominantly dacitic Tschicoma volcanic center in the northeastern part of the Jemez Mountains and suggests a significantly different history for the Plio‐Pleistocene development of the Jemez Mountains volcanic field than has previously been considered.