Cerro Toledo Rhyolite Research Articles

El Paso Phase Obsidian Procurement in Southern New Mexico: Implications for Jornada Mogollon Regional Interaction and Exchange

Archaeologists rarely discuss obsidian procurement for the Jornada Mogollon region, but studies indicate groups overwhelmingly used obsidian that can be collected from Rio Grande gravels in southern New Mexico. Obsidian artifacts from two El Paso phase sites, Cottonwood Spring Pueblo and Madera Quemada Pueblo, were sourced using EDXRF spectrometry to determine: (1) is there evidence for non-Rio Grande gravel obsidian use, (2) is there a difference between the sites, and (3) how do these sourcing results compare with contemporaneous sites in the Casas Grandes area? This study has implications for regional interaction and exchange as the results validate the high frequency of Cerro Toledo Rhyolite obsidian in assemblages, but non-Rio Grande gravel sources were also used including Mule Creek projectile points. People at both sites used similar sources, with a few noted exceptions. Furthermore, groups in the Jornada Mogollon and Casas Grandes regions utilized dramatically different sources.

Obsidian Distribution: Evidence of West to East Movements in the Southern Plains Prior to the Plains Village Tradition

Eleven artifacts visually similar to obsidian were recovered from secure subsurface contexts in a Late Archaic component at the Pipeline site (41PT185/C) near Amarillo, Texas. X-ray florescence analysis of nine pieces yielded chemical results that indicate that eight obsidian pieces came from three separate sources in the Jemez Mountains of north central New Mexico. Two pieces were from El Rechuelos, three pieces were identified as Cerro Toledo Rhyolite, and three pieces were identified as Valles Rhyolite. One piece was chemically similar to modern glass. The radiocarbon dating of this Late Archaic component documents earlier west to east movement of obsidian onto the High Plains, the western section of the southern Plains, prior to the more extensive trading networks envisioned for the subsequent Plains Village tradition (Middle Ceramic period).

Thermal History of the Bandelier Magmatic System: Evidence for Magmatic Injection and Recharge at 1.61 Ma as Revealed by Cathodoluminescence and Titanium Geothermometry

The rhyolitic Valles caldera complex, New Mexico, is one of the type examples of resurgent calderas and has experienced two well‐studied caldera‐forming eruptions. The first formed the Lower Bandelier Tuff (LBT) at 1.61 Ma, and the second emplaced the Upper Bandelier Tuff (UBT) at 1.22–1.26 Ma. During the time between the LBT and the UBT, the much smaller‐scale Cerro Toledo Rhyolite (CTR) was sporadically erupted. Quartz crystals from these stages of activity were imaged with cathodoluminescence microscopy, and growth zones in certain quartzes, due to varying Ti content, were revealed. Crystallization temperatures were obtained with a titanium‐in‐quartz geothermometer. The LBT quartzes are unzoned, with temperatures clustering between 660° and 715°C when a calculated $$a_{\mathrm{TiO}\,_{2}}$$ of 0.4 is applied to the system. These near‐solidus temperatures imply that the LBT magma chamber was highly crystalline at one point. However, the low crystal content and the widespread presence of resorption features in LBT crystals require that pervasive melting affected the LBT magma chamber at some point before eruption. This melting is hypothesized to result from a hot magmatic injection into the system, with the injection also being a likely trigger of the cataclysmic LBT volcanism. The earliest‐erupted CTR units contain many zoned quartz crystals. Inner zones are usually rounded and invariably reveal cold (∼660°–700°C) cores and hot (∼750°–825°C) rims. We interpret these results as thermal evidence of magmatic recharge, whereby new magma mixed vigorously with leftover magma and high‐temperature rims crystallized around low‐temperature restitic quartz cores. Thermal data for the rest of the CTR record the continuing cooling and evolution of this mixture of magma, while results for the culminating UBT reveal generally unzoned quartz crystals with a roughly constant temperature of 685°–725°C. Altogether, these results present an unprecedented glimpse into the thermal history of the Bandelier magma system, as well as strong evidence concerning the timing and overall importance of magmatic injections in silicic magma systems.

Cerro Toledo Rhyolite, Jemez Volcanic Field, New Mexico: 40Ar/39Ar geochronology of eruptions between two caldera-forming events

The Cerro Toledo Rhyolite comprises a group of domes and tephra which were erupted during the interval between two caldera-forming ignimbrites, the Tshirege Member and Otowi Member of the Bandelier Tuff, in the Jemez Volcanic Field, New Mexico. To provide a chronologic framework for geochemical and isotopic studies on these rhyolites, which record the evolution of the Bandelier magma system during this interval, a 40 Ar/ 39 Ar geochronology study was undertaken. Pumice from major pyroclastic fall deposits within the rhyolite tephra and samples from the rhyolite domes were dated as well as the stratigraphically bracketing Bandelier Tuff. Analyzed crystal populations range from being fairly homogeneous juvenile material to very heterogeneous mixed juvenile and xenocrystic assemblages. In most cases dominant groups of juvenile sanidine crystals define 40 Ar/ 39 Ar ages which agree with stratigraphic constraints. Plagioclase analyses are distinctly more scattered and do not typically define reasonable ages. The 40 Ar/ 39 Ar ages for the two members of the Bandelier Tuff yield an interval of 380 ± 20 k.y. between these caldera-forming eruptions. During this interval nine major pyroclastic pumice units were deposited in the sections studied, for which six yield isochron ages, one a weighted mean age, one a maximum age, and one no reliable age due to lack of sanidine. 40 Ar/ 39 Ar dates on pumice fall units within the Cerro Toledo Rhyolite tephra indicate that eruptive activity occurred at >1.59, 1.54, 1.48, 1.37 and 1.22 Ma. 40 Ar/ 39 Ar dating of Cerro Toledo Rhyolite domes indicates these were erupted within the caldera at 1.54, 1.45, 1.38–1.34, and 1.27 Ma. The dates obtained indicate that eruptive activity occurred throughout the 380 k.y. interval between the two members of the Bandelier Tuff, but suggest that eruptions producing both tephra and domes occurred during discrete intervals at ca. 1.54, 1.48, and 1.38–1.34 Ma. The interval from 1.34 to 1.38 Ma was particularly active; 7 of 18 units dated are these ages.

Gas saturation and evolution of volatile and light lithophile elements in the Bandelier magma chamber between two caldera‐forming eruptions

We have studied the distribution of light and volatile elements (Li, Be, B, H2O, F, Cl) in matrix glasses and melt inclusions in quartz and sanidine phenocrysts from tephras of the Cerro Toledo Rhyolite, which were erupted from the Bandelier magma chamber between two caldera‐forming eruptions at 1.608 Ma and 1.225 Ma. Since the small‐volume tephras of the Cerro Toledo Rhyolite record chemical evolution of the upper parts of the magma chamber during this 383‐kyr period, we can track volatile evolution in the magma during its differentiation, specifically, the development of a separate fluid phase from the magma, the partitioning of volatile and light elements between this fluid and the magma, and the timing of the second caldera eruption at 1.225 Ma. The melt inclusion data reveal that Li, Be, and B were progressively enriched with time at the top of the magma chamber, showing a similar three fold enrichment. By contrast, F, Cl, and F2O show more variable behavior. F was enriched 3.6 times in the magma and Cl increased 2.5 times, while water concentrations remained constant. Evidence for a separate fluid phase includes (1) comparatively small magmatic enrichment factors for Cl and H2O, elements which are known to partition into fluid, (2) decline of the Cl/Be and Cl/B ratios in melt inclusions with time, indicating preferential loss of Cl, and (3) maximum Cl contents of 2900–3000 ppm in melt inclusions, which are concentrations at or close to the saturation limit for Cl in a two‐phase gas‐saturated magma. The lack of temporal H2O enrichment in the magma suggests that it became gas saturated at an early stage. The Cl fluid‐magma partition coefficient was initially low, due to the presence of a low‐salinity single‐phase vapor. The fluid is calculated to have increased its salinity from 0.76 wt % equivalent NaCl to 3.1 wt % NaCl by crystallization and Cl enrichment in the magma. Consequently, the fluid was transformed to two immiscible phases. As a result, the Cl partition coefficient increased, and the Cl content in the magma was buffered at 2900–3000 ppm. Overpressures in the magma chamber have been modeled as a function of crystallization, total pressure and depth, H2O solubility in rhyolite, resurgence, crystallization contraction, and initial fluid mass. For the large amounts of crystallization (40–70%) required by the geochemical relations, unrealistically large overpressures (>75 MPa) are indicated, unless (1) the magma existed as a foam or (2) the chamber was able to degas passively, and thus buffer the buildup of pressure. To maintain realistic overpressures in the magma chamber (<50 MPa), an equilibrium was required between gas buildup by crystallization and volatile loss by passive degassing. The change from a one‐phase vapor to a two‐phase immiscible fluid before the Upper Bandelier Tuff eruption may have promoted partial or complete sealing of the magmatic‐hydrothermal system, permitting significant overpressures to develop. The result could have been a cataclysmic caldera‐forming eruption.

Plio-pleistocene pumice floods in the ancestral Rio Grande, southern Rio Grande rift, USA

At least four times during the late Pliocene and early Pleistocene pyroclastic eruptions in the Jemez volcanic field, northern Rio Grande rift, flooded the ancestral Rio Grande with gravel-sized pumice. Following as much as 400 km of fluvial transport, the pumice was deposited in beds 0.2 to 2.0 m thick in the Camp Rice Formation of the southern Rio Grande rift. A combination of reversal magnetostratigraphy and single-crystal sanidine 40Ar/39Ar dating constrains the ages of pumice-clast conglomerates at 3.1, ∼2.0, 1.6, and 1.3 Ma. The coarsest pumice beds (cobbles, boulders) were deposited as antidune-like bedforms in a fluvial channel and as a crevasse-splay sheet. Granule and pebble-sized pumice was deposited as dune bedforms in fluvial channels and as ripple bedforms on the floodplain. The abundance of pumice clasts in the gravel fraction (60–100%) suggests very rapid transport downriver, probably in a few days or weeks. The two older pumice-clast conglomerates correlate with the Puye Formation in the Jemez volcanic field, whereas the younger two are coeval to the Lower Bandelier Tuff and Cerro Toledo Rhyolite.

Replenishment and crystallization in epicontinental silicic magma chambers: evidence from the Bandelier magmatic system

The Cerro Toledo Rhyolite (Jemez volcanic field, New Mexico) records the volcanism that occurred after the eruption of the Lower Bandelier Tuff ignimbrite, which formed Toledo caldera at 1.51 ± 0.03 Ma. Pumice samples from the top of this ignimbrite and from the base of the Cerro Toledo Rhyolite tephra sequence are stratigraphically and geochemically distinct from younger Cerro Toledo Rhyolite tephras that overlie this basal suite. All samples exhibit decreasing La/Lu with increasing Lu concentrations. For a given Lu content, however, the Lower Bandelier Tuff and basal member Cerro Toledo Rhyolites have higher La/Lu compared to overlying Cerro Toledo Rhyolites. This difference can be explained either by (1) differentiation of a single batch of magma in which trace mineral solubilities increased with time due to higher volatile contents, or (2) replenishment of the magma chamber by an input of silicic magma after caldera collapse at 1.51 Ma. Large-scale replenishment of epicontinental silicic magma chambers may be restricted to timescales of 10 5 years.

Variations in trace element partition coefficients in sanidine in the Cerro Toledo Rhyolite, Jemez Mountains, New Mexico: Effects of composition, temperature, and volatiles

Trace element partition coefficients have been measured for one plagioclase and five sanidine mineral separates from the Cerro Toledo Rhyolite, New Mexico. Sanidine partition coefficients vary substantially and systematically within the Cerro Toledo Rhyolite. Partition coefficients for Ca, Sr, Zn, La, and Eu are lowest in the most evolved rhyolites, whereas Sm and HREE partition coefficients are highest. Rubidium partition coefficients remain constant, while those for Ba, Ce, and Th are variable. Variations of the Sr, Zn, La, and Eu partition coefficients are correlated with the Ca contents and partition coefficients of the sanidines. Calcium may have controlled the distribution of these elements in the sanidine by modifying the feldspar structure. The low Ca partition coefficients in sanidines for the most evolved rhyolites may be the consequence of modification of the melt structure, possibly due to increased volatile contents at the top of the magma chamber(s) during evolution of the Cerro Toledo Rhyolite. The Zn and La partition coefficients between sanidine and melt also may have been controlled by this change in melt structure. Modelling using major elements and the constant Rb partition coefficient for sanidine indicates 70% crystallization of magma during Cerro Toledo Rhyolite time by removal of 68% sanidine and 32% quartz. Estimates of the volume (1) of initial parental magma and (2) of the magma that crystallized during this period are 11,670 km 3 and 8170 km 3, respectively. The average intrusion rate of silicic magma during Cerro Toledo Rhyolite activity was 35 × 10 −3 km 3/a.

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.

Restoration of compositional zonation in the Bandelier silicic magma chamber between two caldera‐forming eruptions: Geochemistry and origin of the Cerro Toledo Rhyolite, Jemez Mountains, New Mexico

The Cerro Toledo Rhyolite is a group of high‐silica rhyolite domes and tephras that range in age from 1.45 to 1.12 Ma. The unit crops out in the Jemez Mountains of northern New Mexico and lies stratigraphically between the compositionally zoned upper and lower members of the Bandelier Tuff. The Cerro Toledo Rhyolite provides an exceptional opportunity to study the origin of compositional zonation in silicic magma chambers because it allows us to follow the restoration of the zonation with time between these two large caldera‐forming ignimbrite eruptions. Based upon stratigraphic, geochronologic, and geochemical evidence, we have correlated different Cerro Toledo Rhyolite tephra units with groups of domes. Early Cerro Toledo Rhyolite domes appear to be located generally along the Toledo caldera ring fracture, whereas younger domes tend to cluster in the Toledo embayment. The Cerro Toledo Rhyolite appears to have tapped the most fractionated liquids at or near the top of the Bandelier magma chamber and records the restoration of compositional gradients over a period of 0.33 m.y. after the Lower Bandelier ignimbrite eruption. Cl, Rb, Cs, heavy rare earth elements, Y, Nb, Th, and U increase in concentration in progressively younger Cerro Toledo Rhyolite rocks. Because of depletions in K/Cs, Sr, Zr/Nb, and La/Yb over the same interval, we favor crystal fractionation of essentially quartz, alkali feldspar, zircon, and a light rare earth element enriched phase (probably allanite) as the primary mechanism by which the compositional gradients were reestablished. We believe diffusive processes did not play an important role because (1) certain cations of widely different valencies and diffusivities are not fractionated with respect to each other and (2) the observed chemical gradients conflict with those predicted by recent experimental Soret studies.

Intracaldera volcanic activity, Toledo Caldera and Embayment, Jemez Mountains, New Mexico

The Toledo caldera was formed at 1.47±0.06 Ma during the catastrophic eruption of the lower member, Bandelier Tuff. The caldera was obscured at 1.12±0.03 Ma during eruption of the equally voluminous upper member of the Bandelier Tuff that led to formation of the Valles caldera. Earlier workers interpreted a 9‐km‐diameter embayment, located NE of the Valles caldera (Toledo embayment), to be a remnant of the Toledo caldera. Drill hole data and new K‐Ar dates of Toledo intracaldera domes redefine the position of Toledo caldera, nearly coincident with and of the same dimensions as the younger Valles caldera. The Toledo embayment may be of tectonic origin or a small Tschicoma volcanic center caldera. This interpretation is consistent with distribution of the lower member of the Bandelier Tuff and with several other field and drilling‐related observations. Explosive activity associated with Cerro Toledo Rhyolite domes is recorded in tuff deposits located between the lower and upper members of the Bandelier Tuff on the northeast flank of the Jemez Mountains. Recorded in the tuff deposits are seven cycles of explosive activity. Most cycles consist of phreatomagmatic tuffs that grade upward into Plinian pumice beds. A separate deposit, of the same age and consisting of pyroclastic surges and flows, is associated with Rabbit Mountain, located on the southeast rim of the Valles‐Toledo caldera complex. These are the surface expression of what may be a thicker, more voluminous intracaldera tuff sequence. The combined deposits of the lower and upper members of the Bandelier Tuff, Toledo and Valles intracaldera sediments, tuffs, and dome lavas form what we interpret to be a wedge‐shaped caldera fill. This sequence is confirmed by deep drill holes and gravity surveys. This fill accumulated in depressions formed during precaldera rifting and episodes of caldera collapse. We interpret the Toledo‐Valles caldera complex to be a pair of nearly coincident trapdoor calderas, with the hinge on the west side and thick caldera fill in the east.

126. The K-Ar ages of the Cerro Toledo Rhyolite from Jemez Mountains, New Mexico, U.S.A.(Abstracts of Papers Presented at the Spring Meeting of the Society)

126. The K-Ar ages of the Cerro Toledo Rhyolite from Jemez Mountains, New Mexico, U.S.A.(Abstracts of Papers Presented at the Spring Meeting of the Society)

Contents volume 10, 1981

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.