Mogollon-Datil Volcanic Field Research Articles

Tracing magmatic genesis and evolution through single zircon crystals from successive supereruptions from the Socorro Caldera Complex, USA

Large volume rhyolitic ignimbrite volcanism is a significant contributor to the evolving crust. The introduction of high-silica material into the upper crust, differentiation within the middle crust, and partial melting in the lower crust contributes to geochemical and isotopic evolution of the crust. Developing accurate models for the genetic evolution of these events is dependent upon geochronology to determine rates of magmatic processes as model constraints. We present new zircon high-precision CA-ID-TIMS U-Pb geochronology and MC-ICPMS Hf isotope geochemistry for four ignimbrites from the nested caldera complex near Socorro, New Mexico (USA), within the Mogollon-Datil volcanic field. In agreement with past 40Ar-39Ar data, interpretations of new U-Pb data indicate eruptions from the Socorro caldera cluster were pulsed. These pulses were intermittently spaced, and a volcanic hiatus following the Hells Mesa Tuff at 33.442 ± 0.015 Ma was interrupted by four successive eruptions, beginning with the La Jencia Tuff at 29.158 ± 0.025 Ma and finishing with the South Canyon Tuff at 28.066 ± 0.021 Ma. Zircon age spectra became more protracted with each eruption, exhibiting age dispersions ranging from 0.347 Myr in the Hells Mesa Tuff to 4.502 Myr in the South Canyon Tuff. The increased dispersion is paralleled by an increase in the proportion of normally discordant grains, indicative of xenocryst incorporation. These protracted age spectra are not necessarily a function of thermal maturation in the middle to upper crust due to long-lived magma chambers. Rather, they are likely the result of increased melting of zircon-bearing lower crust due to deep thermal maturation from repeated juvenile magma injections based on the incorporation of zircon material at the melt source. In contrast, the Hf isotope record is volumetrically dominated by autocrystic zircon domains and becomes more radiogenic through time, recording juvenile replenishment of the lower crust during progressive melting. Together, these data record the protracted evolution of the lower crust sampled by ignimbrites, lend insight into that evolution, and emphasize the need for detailed interpretation of high-precision datasets to advance volcanic models.

Discovering hidden geothermal signatures using non-negative matrix factorization with customized k-means clustering

Discovery of hidden geothermal resources is challenging. It requires the mining of large datasets with diverse data attributes representing subsurface hydrogeological and geothermal conditions. The commonly used play fairway analysis approach typically incorporates subject-matter expertise to analyze regional data to estimate geothermal characteristics and favorability. We demonstrate an alternative approach based on machine learning (ML) to process a geothermal dataset from southwest New Mexico (SWNM). The study region includes low- and medium-temperature hydrothermal systems. Several of these systems are not well characterized because of insufficient existing data and limited past explorative work. This study discovers hidden patterns and relations in the SWNM geothermal dataset to improve our understanding of the regional hydrothermal conditions and energy-production favorability. This understanding is obtained by applying an unsupervised ML algorithm based on non-negative matrix factorization coupled with customized k-means clustering (NMFk). NMFk can automatically identify (1) hidden signatures characterizing analyzed datasets, (2) the optimal number of these signatures, (3) the dominant data attributes associated with each signature, and (4) the spatial distribution of the extracted signatures. Here, NMFk is applied to analyze 18 geological, geophysical, hydrogeological, and geothermal attributes at 44 locations in SWNM. Using NMFk, we find data patterns and identify the spatial associations of hydrothermal signatures within two physiographic provinces (Colorado Plateau and Basin and Range) and two sub-regions of these provinces (the Mogollon-Datil volcanic field and the Rio Grande rift) in SWNM. The ML algorithm extracted five hydrothermal signatures in the SWNM datasets that differentiate between low (<90°C) and medium (90-150°C)-temperature hydrothermal systems. The algorithm also suggests that the Rio Grande rift and northern Mogollon-Datil volcanic field are the most favorable regions for future geothermal resource discovery. NMFk also identified critical attributes to identify medium-temperature hydrothermal systems in the study area. The resulting NMFk model can be applied to predict geothermal conditions and their uncertainties at new SWNM locations based on limited data from unexplored regions. The code to execute the performed analyses as well as the corresponding data can be found at https://github.com/SmartTensors/GeoThermalCloud.jl.

Neogene drainage reversal and Colorado Plateau uplift in the Salt River area, Arizona, USA

UPb detrital zircon and 40Ar39Ar detrital sanidine dating of paleoriver deposits refines the timing of the mid-Cenozoic drainage reversal from NE- to SW-flowing rivers across the southern edge of the Colorado Plateau. NE-flowing paleorivers of the Mogollon Rim Formation were of multiple ages: ≤59.38 Ma for the Flying V outcrop and ≤37–33 Ma for other outcrops. These NE-flowing paleorivers were defeated by construction of the 38–23 Ma Mogollon–Datil volcanic field and extensional collapse of the Mogollon Highlands. The 37.6–21.8 Ma Whitetail Conglomerate of the Salt River paleocanyon records the transition from NE-flowing rivers to internal drainage that persisted from 30 to 14.67 Ma during continued extensional collapse. The SW-flowing proto-Salt River was established by about 12 Ma and flowed into Tonto Basin ~7 Ma. The Salt River extended its length to near Phoenix via basin spillover from the Tonto and Verde basins about 2.8 Ma and became established in its present path only in the past ~500 ka. Between 3.0 and 0.52 Ma, incision rates of the Salt River's headwaters have been steady at 95 m/Ma as calculated using the age and height of four far-traveled basalt runouts from the Springerville volcanic field. These incision rates contrast with 10 m/Ma rates for downstream areas near the Sentinel-Arlington volcanic field on the Gila River over the past 2.37 Ma. This 85 m/Ma of differential incision was a geomorphic response of headwater drainages to changes in base level (base level fall plus headwater uplift) that, at least in part, was a consequence of top-down integration of the Salt and Verde river systems via spillover. Neogene headwater uplift is proposed to have set the stage for and perhaps driven this downward integration by increasing river gradients, and progressive uplift in the past 3 Ma may help explain post-integration steady headwater incision. Surface uplift components include construction of the Springerville and San Francisco volcanic fields and related mantle-driven epeirogenic uplift of the southern rim of the Colorado Plateau. Long-term differential incision rates for the Salt River (85 m/Ma) are less than for the Colorado River system (140 m/Ma across Grand Canyon) suggesting west-up neotectonic tilting of the Colorado Plateau.

Maturation and rejuvenation of a silicic magma reservoir: High-resolution chronology of the Kneeling Nun Tuff

Knowledge of the conditions of magma storage prior to volcanic eruptions is key to their forecasting, yet little is known about how melt compositions, crystallinity and intensive parameters within individual magma reservoirs evolve over time. To address this, we studied the Kneeling Nun Tuff, a voluminous (>900 km3) deposit of an Eocene caldera-forming eruption from the Mogollon–Datil volcanic field in New Mexico, USA. Whole-rock, feldspar and amphibole compositions were combined with zircon trace-element geochemistry and precise isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) U–Pb zircon crystallisation ages to arrive at a detailed, time-resolved record of chemical and physical changes within the voluminous, upper-crustal (∼2.2 kbar) magma reservoir. Chemical compositions and zircon ages from the Kneeling Nun Tuff and from co-magmatic clasts hosted within it reveal prolonged (>1.5 million years) growth and maturation of the magma reservoir that was heterogeneous in terms of temperature, melt composition and crystallinity. This protracted storage at a dominant crystallinity in excess of 50% culminated in a period of ca. 50 ky of increase in recharge heat supply and related homogenisation, decrease in crystallinity to 40–50%, and potential increase in average melt temperature, leading up to eruption at 35.299 ± 0.039 Ma. Sampling of co-magmatic lithic clasts derived from early-cooled domains of the reservoir shows that the long, million year-scale maturation time is shared across all erupted domains of the magmatic system, irrespective of their final cooling history. This study provides key observations from a natural system against which thermal and mechanical models of upper-crustal magma reservoir construction can be validated.

Oligocene Laramide deformation in southern New Mexico and its implications for Farallon plate geodynamics

The Silver City Range in southwest New Mexico contains Proterozoic basement rocks that are overlain by a sequence of Paleozoic, Mesozoic, and Paleogene strata. These rocks are folded in a broad, NW-SE–trending, east-facing monocline that lies structurally above an east-directed thrust fault. The youngest rocks folded in the Silver City monocline are similar to other late Eocene and early Oligocene volcanic rocks of the Mogollon-Datil volcanic field; an ash-flow tuff near the bottom of the volcanic sequence gives an 40 Ar/ 39 Ar age on sanidine of 34.9 ± 0.4 Ma (2σ), and another tuff near the top of the section contains zircons that yield a weighted 206 Pb/ 238 U age of 34.6 ± 0.6 Ma (2σ). We interpret similar structures in the Little Burro Mountains, Lone Mountain, and Bayard area, immediately east and west of the Silver City monocline, to all be genetically related to a system of basement-involved thrust faults. Modeling of these structures from the Mangas Valley in the southwest to the Mimbres Valley in the northeast, suggests ∼17% total shortening. We conclude that Laramide shortening was active in southwest New Mexico generally, and the Silver City region in particular, from the Cretaceous until the earliest phase of Mogollon-Datil volcanism beginning at ∼37 Ma, during which time the earliest extension in the southern Rio Grande rift was initiated. The final stage of Laramide shortening, recorded in the Silver City monocline, took place during a lull of volcanism (and extension) from ∼31.5 to ∼29.3 Ma. We explain the contemporaneity of shortening, significant ignimbrite eruptions, and crustal extension as the consequence of intermittent slab breakoff and renewed underthrusting of the downgoing Farallon plate.

The Chuska erg: Paleogeomorphic and paleoclimatic implications of an Oligocene sand sea on the Colorado Plateau

Great thicknesses of eolian dune deposits of early Oligocene age crop out in the Chuska Mountains of northwestern New Mexico-Arizona (as much as 535 m thick) and in the Mogollon-Datil volcanic field of western New Mexico-Arizona (as much as 300 m thick). 40 Ar/ 39 Ar ages of intercalated volcanic rocks indicate eolian deposition in these areas was approximately synchronous, with eolian accumulation beginning regionally at ca. 33.5 Ma and ending at ca. 27 Ma. Probable eolian sandstone of Oligocene age 483 m thick is also present in the subsurface of the Albuquerque Basin of the Rio Grande rift. The beginning of eolian deposition on the Colorado Plateau corresponds closely to the beginning of eolian (loessic) deposition in the White River Group of the Great Plains and major Oi1 glaciation in Antarctica, suggesting possible global paleoclimatic control. Successions of Oligocene eolian sandstone on the Colorado Plateau are thicker than all of the better known Upper Paleozoic-Mesozoic eolianites in the region, except the Jurassic Navajo Sandstone. We suggest that the widely separated Oligocene eolianites in the Colorado Plateau region were probably originally continuous, and thus are erosional remnants of an extensive (∼140,000 km 2 ), regional sand sea (the Chuska erg). This interpretation is based on: (1) comparison with thickness trends of older eolianites in the Colorado Plateau region, (2) evaluation of regional topographic gradients of modern ergs, and (3) hydrologic modeling of a 300- to 400-m–thick zone of saturation that existed during eolian deposition in the Chuska Mountains. The Chuska erg represents the final episode of Paleogene aggradation on the central and southern Colorado Plateau. Aggradation was driven primarily by trapping of fluvial sediments on the plateau by development of major volcanic fields along the eastern plateau margin. These volcanic fields blocked earlier Laramide drainages that had previously transported sediments eastward off the plateau. Following a shift to widespread eolian deposition at ca. 33.5 Ma, constructional volcanic topography induced eolian accumulation upwind of developing volcanic fields. Stratal accumulation rates (not decompacted) of eolian deposits were ∼28–82 m/m.y. The reconstructed top of the Chuska erg would lie at a present-day elevation of ∼3000 m or more, and provides a datum for assessing subsequent erosion on the Colorado Plateau. Major exhumation (≥1230 m) occurred during the late Oligocene and early Miocene, following the end of Chuska deposition and prior to the onset of Bidahochi Formation deposition at ca. 16 Ma on the south-central part of the plateau. The Bidahochi Formation attained a thickness of ∼250 m by ca. 6 Ma, followed by ∼520 m of late Miocene and younger erosion in the valley of the Little Colorado River. The depth of late Oligocene-early Miocene (ca. 26–16 Ma) exhumation of the central and southern Colorado Plateau thus was more than twice that of the late Miocene-Holocene (ca. 6–0 Ma). The timing of initial deep erosion in the Colorado Plateau-Southern Rocky Mountains region suggests the beginning of major epeirogenic rock uplift occurred during post-Laramide magmatism.

The evolution of the Eagle Peak volcano — a distinctive phase of middle miocene volcanism in the western Mogollon-Datil volcanic field, New Mexico

The andesitic to dacitic Eagle Peak volcano represents a distinctive phase of Middle Miocene, post-caldera volcanism in the western part of the Mogollon-Datil volcanic field in southwestern New Mexico. Erupted during Basin and Range extensional tectonism, rocks of the Eagle Peak volcano are chemically and isotopically distinct from the bimodal suite, extension-related basalt and rhyolite that also erupted in this area from the Early Miocene to the Pleistocene. Instead, they have close petrogenetic affinities to the early post-caldera (~ 27–23 Ma) calc-alkaline, Bearwallow Mountain Andesite erupted from shield volcanos aligned along prominent Basin and Range fault structures. Geologic mapping and detailed petrographic and chemical studies of the Eagle Peak volcano has enabled the distinction of five different flow units, a central plug and a feeder dike. The flows were erupted from a central vent and two subsidiary “satellitic” centers on the western and southwestern flanks of the volcano. 40Ar 39Ar age-spectrum and paleomagnetic studies indicate that the Eagle Peak volcano was active between 12.1 and 11.4 Ma; its activity spanned at least one magnetic polarity reversal. With exception of late satellitic eruptions on the northwestern margin of the volcano, central vent and satellitic flows were erupted in rapid succession and have an average age of 11.7 Ma. The central plug yielded a plateau age of 11.4 Ma, which is a minimum of 90,000 years (2σ) younger than the 11.7 Ma average age of the central vent and satellitic flows. Major-oxide, trace-element and isotope geochemistry define two distinct magmatic series: a central vent and a satellitic series. Rocks of the satellitic series, although similar in modal mineralogy and rare earth element patterns, are slightly more alkaline and relatively enriched in the high field strength elements Nb, Ta, P and Ti compared to the central vent eruptives. Sr and Nd isotopes further demonstrate these differences; a sample of the satellitic flows exhibits lower ( 87Sr 86Sr ) i (0.7084) and higher ϵ Nd values ( ϵ Nd = −4.8) relative to an upper flow of the central vent series [ ( 87Sr 86Sr ) i = 0.7096), ϵ Nd = −8.5 ]. Major- and trace-element data support petrogenetic models based on periodic tapping of the central vent and satellitic series magmas, which both evolved by crystal fractionation. Central vent magmas evolved mainly by a modified process of filter pressing that accompanied the transfer of magma from a deep into a higher-level reservoir. Thus, a portion of the melt with some of the original crystals was extracted during this transer changing the resulting bulk magma chemistry but not affecting the major phenocryst compositions. In contrast, crystal fractionation within the satellitic magmas accompanied a progressive evolution in both phenocryst composition and bulk magma chemistry. Although temporally associated with bimodal basalt-rhyolite volcanism (< 21 Ma to Holocene), Eagle Peak rocks differ markedly from the suite of mafic bimodal rocks in trace-element and Sr and Nd isotope chemistry. Mafic rocks of the bimodal suite have Sr and Nd isotopic initial ratios that range from depleted mantle to bulk earth values [ ( 87Sr 86Sr ) i = 0.7030-0.7057, ϵ Nd = 0 to +9.12 ], and are substantially more primitive than Eagle Peak rocks [ ( 87Sr 86Sr ) i = 0.70839-0.70958, ϵ Nd = − 8.4 to − 4.9 ]. In contrast, Eagle Peak volcanics are geochemically more similar to the 27-23 Ma post-caldera Bearwallow Mountain Andesite, which is characterized by ( 87Sr 86Sr ) i = 0.7070–0.7102 and ϵ Nd = −8.15 to −5.95. The Eagle Peak volcanics, like the Bearwallow Mountain Andesite, assimilated a significant component of crustal material and were both likely derived from a similar lithospheric mantle source beneath the western Mogollon-Datil volcanic field.

Calc‐alkaline magmatism, lithospheric thinning and extension in the Basin and Range

Most of the volcanic rocks in the Colorado River Trough (CRT) and the Mogollon‐Datil Volcanic Field (MDVF) in the Basin and Range exhibit calc‐alkaline major element trends and relatively low high field strength element abundances, similar to those erupted from the volcanoes of Aso and Towada in Japan. Such features are widely regarded as characteristic of subduction‐related magmatism, and yet the rocks in the Basin and Range were generated in response to lithospheric extension. The preextensional to synextensional rocks of the CRT and the MDVF have higher Na2O, K2O, and TiO2, in the range 47–55% SiO2, and relatively low Al2O3, and overall, they tend to have higher Sr contents and Zr/Y and La/Nb ratios than those from Aso and Towada. In addition, the basalts in the Basin and Range tend to be more aphyric than those in Japan, consistent with more rapid movement of magma through the crust during extension in the Basin and Range, and the rate of melt generation appears to have been significantly less in the Basin and Range than along recent destructive plate margins. The geochemical differences are attributed to smaller degrees of partial melting in the Basin and Range and to source regions that had been enriched in incompatible elements since the Proterozoic, resulting in parental magmas with higher alkali contents than those commonly observed in subduction‐related calc‐alkaline suites. Within the CRT the subsequent calc‐alkaline trend was due at least in part to mixing with crustal derived melts, whereas in the MDVF such trends reflect both crustal contamination and fractional crystallization involving magnetite and amphibole. The small volumes of magma with minor and trace element features similar to oceanic basalts indicate that relatively little melt was generated in underlying asthenosphere. Thus it is inferred that magmatism in the Basin and Range was not associated with a significant increase in temperature, such as might be attributed to a mantle plume, but rather it was in response to lithospheric extension. Calculations are presented which demonstrate that the magma volumes and inferred source regions, extension, present‐day heat flow, and topography are consistent with a model of convective lithospheric thinning after thickening in the Laramide and Sevier orogenies.

Geochemical and tectonic transitions in the evolution of the Mogollon-Datil Volcanic Field, New Mexico, U.S.A.

The mafic to intermediate rocks of the Mogollon-Datil Volcanic Field have been subdivided petrologically and geochemically into three groups: Pre-30 Ma, 30-20 Ma and Post-20 Ma. The Pre-30 and 30-20-Ma rocks are broadly calc-alkaline, with high LILE/HFSE ratios and enriched radiogenic isotope compositions. Their Pb isotope ratios define a linear array on a PbPb diagram with a slope corresponding to an age of ∼ 1.7 Ga and they appear to have been derived largely from within the mantle lithosphere. The Post-20-Ma lavas are small-volume alkali olivine basalts, that typically have OIB-like isotope and trace-element ratios and are therefore inferred to have been derived from the underlying asthenosphere. Most of the igneous rocks of the Mogollon-Datil Volcanic Field were generated in a period of extensional tectonics. The peak of extension occurred at ∼ 28.5 Ma and, although there was a small contribution from asthenosphere-derived magmas in the 29-20-Ma rocks, such magmas only became dominant (≥75%) at ∼ 14 Ma, i.e. 15 Ma after the peak of extension. Published models are used to evaluate the temperature of the upper mantle at the time of magmatism and in particular whether the presence of OIB-like magmas should be regarded as evidence for a mantle plume. Partial melting in the mantle lithosphere is thought to have taken place in the presence of small amounts of H 2O and CO 2, whereas the underlying asthenosphere is assumed to have been anhydrous. For mechanical boundary layer thicknesses in the range 80–100 km and β<2, partial melting in the asthenosphere is initiated at the relatively low potential temperatures of ∼ 1320°C. This is consistent with the low eruption rates in the Mogollon-Datil Volcanic Field and it is concluded that magmatism took place in response to lithospheric extension over mantle that was only slightly hotter than that beneath mid-oceanic ridges.

Early calc-alkaline magmatism in the Mogollon-Data Volcanic Field, New Mexico, USA

The Mogollon-Datil Volcanic Field in southwest New Mexico was active between 40 and 20 Ma. It is situated c. 500 km from the western North American plate margin, and yet the early pre-30 Ma volcanic rocks comprise a high-K calc-alkaline suite ranging from basaltic andesites to dacites. All the samples are characterized by high and variable LILE and LREE contents, and LILE/HFSE ratios, and variable Nd and Sr isotope ratios of 0.51231–0.51250 and 0.7055–0.7065 respectively. In detail, three groups are recognized: Group 1 rocks have the lower SiO 2 , incompatible element abundances and 87 Sr/ 86 Sr ratios; Group 2 preserves the clearest evidence for open system differentiation, and Group 3 exhibits little variation in 87 Sr/ 86 Sr and the most coherent major and trace element arrays. The Group 2 rocks have been modelled in terms of simple binary mixing between selected Group 1 basaltic andesites and a higher silica crustal endmember, which appears to have been derived from the mid-lower crust in the presence of residual garnet. The Group 3 rocks, by contrast, are consistent with near closed system differentiation, modelled in three stages and involving Plag-Aug-O1-Mag and subsequently hornblende and K-feldspar. Published experimental data indicate that the differentiation of the parental basaltic magmas took place at elevated pressures of c . 4-5 kbar, and water contents, c . 2wt% H 2 O. It is argued that the parental magmas to these calc-alkaline suites were generated in a period of lithospheric extension, and that they were derived from within the continental mantle lithosphere, as inferred for mid- to Late Tertiary rocks elsewhere in the western US, with no significant contribution from contemporaneous subduction processes.

The petrogenesis of 30?20 Ma basic and intermediate volcanics from the Mogollon-Datil Volcanic Field, New Mexico, USA

In the western USA calcalkaline magmas were generated hundreds of kilometres from the nearest destructive plate margin, and in some areas during regional extension several Ma after the cessation of subduction. The Mogollon-Datil Volcanic Field (MDVF) in southern New Mexico was a centre of active magmatism in the mid- to late-Tertiary, and a detailed field, petrographic and geochemical study has been undertaken to evaluate the relations between extensional tectonics and calcalkaline magmatism in the period 30–20 Ma. The rocks comprise alkalic to high-K calcalkaline lavas, ranging from basalt to high silica andesitc. Most of the basaltic rocks have relatively low HFSE abundances, elevated 87Sr/86Sr and low 143Nd/144Nd, similar to many Tertiary basalts across the western USA, and they are inferred to have been derived from the continental mantle lithosphere. Two differentiation trends are recognised, with the older magmas having evolved to more calcalkaline compositions by magma mixing between alkalic basaltic andesites and silicic crustal melts, and the younger rocks having undergone 30–40% fractional crystallisation to more alkalic derivatives. The younger basalts also exhibit a shift to relatively higher HSFE abundances, with lower 87Sr/86Sr and higher 143Nd/144Nd, and these have been modelled as mixtures between an average post-5 Ma Basin and Range basalt and the older MDVF lithosphere-derived basalts. It is argued that the presence of subduction-related geochemical signatures and the development of calcalkaline andesites in the 30–20 Ma lavas from the MDVF are not related to the magmatic effects of Tertiary subduction. Rather, basic magmas were generated by partial melting of the lithospheric mantle which had been modified during a previous subduction event. Since these basalts were generated at the time of maximum extension in the upper crust it is inferred that magma generation was in response to lithospheric extension. The association of the 30–20 Ma calcalkaline andesites with the apparently ‘anorogenic’ tectonism of late mid-Tertiary extension, is the result of crustal contamination, in that fractionated, mildly alkaline, basaltic andesite magmas were mixed with silicic crustal melts, generating hybrid andesite lavas with calcalkaline affinities.

Experimental melting of crustal xenoliths from Kilbourne Hole, New Mexico and implications for the contamination and genesis of magmas

Experiments (P=6.9 kb; T=900–1000°C) on four crustal xenoliths from Kilbourne Hole demonstrate the varying melting behavior of relatively dry crustal lithologies in the region. Granodioritic gneisses (samples KH-8 and KH-11) yield little melt ( 900 ppm Ba in high-temperature melts coexisting with a K-feldspar-free restite. Although REE were not measured in either feldspar or melt, the high Kspar/melt Kds for Eu suggests that the melts coexisting with K-feldspar will have strong negative Eu anomalies. Isotopic and trace element models for magma contamination need to take into account the melting behavior of isotopic reservoirs. For example, the most radiogenic (and incompatible element-rich) sample examined here (the pelitic granulite,87Sr/86Sr=0.757) is refractory, while samples with far less radiogenic Sr (87Sr/86Sr=0.708-0.732) produced substantial melt. This suggests that, in this area, the isotopic signature of contamination may be more subtle than expected. The experimental results can be used to model the petrogenesis of Oligocene volcanic rocks exposed 150 km to the NW of Kilbourne Hole, in the Black Range in the Mogollon-Datil volcanic field. The experimental results suggest that a crustal melting origin for the Kneeling Nun and Caballo Blanco Tuffs is unlikely, even though such an interpretation is permitted by Sr isotopes. Curstal contamination of a mantle-derived magma best explains the chemical and isotopic characteristics of these tuffs. Both experimental and geochemical data suggest that the rhyolites of Moccasin John Canyon and Diamond Creek could represent direct melts of granodiorite basement similar, but not identical, to the Kilbourne Hole granodiorites, perhaps slightly modified by crystal fractionation. The absence of volcanic rocks having87Sr/86Sr>0.74 in the region is consistent with the refractory character of the pelitic granulite.

Basic and intermediate volcanism of the Mogollon-Datil volcanic field: implications for mid-Tertiary tectonic transitions in southwestern New Mexico, USA

Basic and intermediate volcanism of the Mogollon-Datil volcanic field: implications for mid-Tertiary tectonic transitions in southwestern New Mexico, USA

Time-stratigraphic framework for the Eocene-Oligocene Mogollon-Datil volcanic field, southwest New Mexico

A time-stratigraphic framework for discontinuously exposed regional ignimbrites in the Eocene-Oligocene Mogollon-Datil volcanic field has been established using correlations aided by 40Ar/39Ar age determinations and paleomagnetic analyses. 40Ar/39Ar age spectra from sanidine separates (25 regional ignimbrites, 85 samples, 97 spectra) yield well-defined plateau ages that are precise (within-sample and within-unit 1σ < ± 0.5%) and agree closely with independently established stratigraphic order. Paleomagnetic remanence directions (404 sites) from individual ignimbrite outflow sheets are generally vertically and horizontally uniform throughout facies ranging from thick (100-500 m), densely welded, proximal ignimbrites to thin (1.5-30 m), unwelded, distal fringes. Between-unit differences in paleomagnetic directions provide useful correlation criteria, particularly for units having ages too close to be resolved using 40Ar/39Ar dating. The Mogollon-Datil time-stratigraphic framework clarifies ignimbrite history and provides improved age control for intercalated lavas and sedimentary rocks. Ignimbrite activity was strongly episodic; outflow sheets were primarily erupted in four discrete pulses representing synchronized activity of two separate cauldron complexes. Activity in the southern complex began at 36.2 Ma near Las Cruces, New Mexico, and subsequently migrated 220 km northwest, culminating in the 28.0 Ma Bursum cauldron. Activity in the northern complex, located west of Socorro, New Mexico, underwent a less defined and more modest 40-km westward migration over its 32.0 to 24.3 Ma life span. The four pulses of ignimbrite activity were (1) 36.2-24.3 Ma, 12 major units, >1,500 km3 total volume; (2) 32.0-31.4 Ma, three major units, >1,500 km3 volume; (3) 29.1-27.4 Ma, nine major units, >6,000 km3; and (4) 24.3 Ma, one major unit. The third and largest ignimbrite pulse was accompanied by extensive rhyolitic dome and flow eruptions in the area between the two main cauldron complexes.

Center-to-center analysis and flow fabric characterization in ash-flow tuffs

The center-to-center method of strain analysis can be used to estimate flow lineation in high-silica ash-flow tuffs. It can be used as an alternative or supplement to other techniques for flow lineation identification, such as the examination of flow textures in thin sections and the measurement of anisotropy of magnetic susceptibility. The center-to-center method is a modification of a technique described by Fry (1979) and by Ramsay and Huber (1983) for determination of finite strain based on the spacing of particles within a deformed rock. In the present study, application of the method to an anticlustered array of phenocrysts in the flattening plane of an ash-flow tuff produces an ellipse with center to edge distances representative of the minimum distance between centers of phenocrysts in all directions within the flattening plane. The long axis of the ellipse corresponds to the maximum axis of finite strain; this direction is suggested to correspond to the flow lineation. The orientation of the stretching lineation was chosen both by eye and by least-squares analysis from center-to-center plots. The calculated orientation of the long ellipse axis can be varied by choice of a maximum distance between digitized objects which are included in the calculation. Comparison is made between late-stage flow lineations identified using the center-to-center method, and the AMS (anisotropy of magnetic susceptibility) method on samples of the high-silica Oligocene Bloodgood Canyon Tuff from the Mogollon-Datil volcanic field of southwestern New Mexico. Flow lineations based on center-to-center analyses of flattening plane-parallel rock slabs and thin sections agree well with AMS-derived flow lineations on most samples from which high-quality AMS lineations were obtained. Center-to-center analuses from flattening plane-perpendicular, lineation-parallel planes of ash-flow tuff produce ellipses inclined from 15° to 85° to the flattening plane, despite compaction of the ash, which should cause angles of inclination to be very low. The inclined ellipses may result from heterogeneities in grain size and distribution of phenocrysts in vertical sections of tuff, or from fragmentation of phenocrysts which occurred during the final stages of emplacement and compression. Center-to-center analyses on rock slabs rather than thin sections helps to avoid the effects of either textural heterogeneity and fragmentation of phenocrysts. With flow lineation identified by center-to-center analysis, petrographic examination of thin sections cut perpendicular to the flattening plane and parallel to the flow lineation allow for the identification of flow direction.

Strontium and neodymium isotopic study of the western Mogollon-Datil volcanic region, New Mexico, USA

The Sr- and Nd-isotopic compositions of large mid-Cenozoic caldera-forming eruptions, and related rocks, from the western portion of the Mogollon-Datil volcanic field have been determined. The average initial 87Sr/86Sr ratios of 27 samples from felsic flows range from 0.70629 to 0.72872; however, all but two flows are 0.71337 or less. Ten analyses of intermediate and mafic rocks showed a tendency towards lower initial 87Sr/86Sr ranging from 0.70363 to 0.70968. Initial 143Nd/144Nd ratios of II felsic and intermediate rocks range from 0.51216 to 0.51231. Two basalts analyzed for 143Nd/144Nd have ratios of 0.51250 and 0.51291. During the course of the volcanic activity from 34 Ma to the present, there was a shift towards lower initial 87Sr/86Sr ratios, and lower SiO2 contents. A number of models of crustal melting, fractionation, mixing, and assimilation and fractional crystallization (AFC), using a variety of possible endmembers, were tested, to see if they could explain the isotopic and geochemical characteristics of the Mogollon-Datil volcanic rocks. The best fit was an AFC model using two components, one a mantle-sourced primary magma, with isotopic ratios of the Kilbourne Hole, N. M., basanite, and the other an upper crust with average continental isotopic ratios, and Sr and Nd abundances similar to the Texas Canyon pluton of Arizona.

Magnetic fabrics of the Bloodgood Canyon and Shelley Peak Tuffs, southwestern New Mexico: implications for emplacement and alteration processes

Anisotropy of magnetic susceptibility (AMS) of the middle Tertiary Bloodgood Canyon and Shelley Peak Tuffs of the Mogollon-Datil volcanic field has been used to (1) evaluate the ability of AMS to constrain flow lineations in low-susceptibility ash-flow tuffs; (2) establish a correlation between magnetic fabric, magnetic mineralogy, tuff facies, and characteristics of the depositional setting; and (3) constrain source locations of the tuffs. The tuffs are associated with the overlapping Bursum caldera and Gila Cliff Dwellings basin. The high-silica Bloodgood Canyon Tuff fills the Gila Cliff Dwellings basin and occurs as thin outcrops outside of the basin. The older Shelley Peak Tuff occurs as thin outcrops both along the boundary between the two structures, and outside of the complex. AMS data were collected from 16 sites of Bloodgood Canyon Tuff basin fill, 19 sites of Bloodgood Canyon Tuff outflow, and 11 sites of Shelley Peak Tuff. Sites were classified on the basis of within-site clustering of orientations of principal susceptibility axes, based on the categories of Knight et al. (1986). Most microscopically visible oxide minerals in the Bloodgood Canyon Tuff outflow and basin fill, and in the Shelley Peak Tuff are members of the hematite-ilmenite solid solution series. However, IRM acquisition data indicate that Bloodgood Canyon Tuff basin fill and Shelley Peak Tuff have magnetic mineralogy dominated by single- or pseudo-single-domain magnetite, and that the magnetic mineralogy of the Bloodgood Canyon Tuff outflow is dominated by hematite. Hematite in Bloodgood Canyon Tuff outflow is likely to be the result of deuteric and/or low-temperature alteration of magnetite and iron silicate minerals. Bulk magnetic susceptibility is higher in magnetite-dominated ash-flow tuff (Bloodgood Canyon Tuff basin fill and Shelley Peak Tuff) than it is in hematite-dominated ash-flow tuff (Bloodgood Canyon Tuff outflow). Bloodgood Canyon Tuff outflow has the highest total anisotropy (H) of the three units, followed by Shelley Peak Tuff and Bloodgood Canyon Tuff basin fill. All three ash-flow tuffs are genearlly characterized by oblate susceptibility ellipsoids, with those of the Bloodgood Canyon Tuff basin fill nearest to spherical. At high values of total anisotropy, Shelley Peak Tuff susceptibility ellipsoids attain a prolate shape; those of Bloodgood Canyon Tuff outflow attain an increasingly oblate shape. Three factors may influence differences in total anisotropy and susceptibility ellipsoid shape: (1) ash which travelled the greatest distance before deposition may show the best development of magnetic fabric, particularly of magnetic lineation; (2) deposition of ash in a closed basin may inhibit laminar flow throughout the sheet and the resulting development of flow textures; and (3) replacement of magnetite and iron silicates preferentially oriented within the foliation plane by hematite with strong crystalline anisotropy may enhance the magnetic susceptibility within that plane. Scatter in AMS axis orientation within sites may result from: (1) greater orientation inaccuracy in block-sampled than in fielddrilled samples; (2) rheomorphism; and (3) low accuracy of AMS measurement in low-susceptibility ashflow tuffs. Evaluation of flow lineation based on AMS of sites with well-clustered K1 axes indicates that (1) Bloodgood Canyon Tuff basin fill flowed along a generally northwest-southeast azimuth; (2) Shelley Peak Tuff located on the boundary of the Bursum caldera and the Gila Cliff Dwellings basin flowed along a nearly east-west azimuth; and (3) Bloodgood Canyon Tuff outflow sites have K1 susceptibility axes generally radial to the Bursum-Gila Cliff Dwellings complex, but within-site scatter of K1 orientations is generally too large to draw conclusions about flow lineation orientation. Limited petrographic work on pilot thin sections adds flow direction information to AMS-derived flow lineation information.

Stress and volcanism in the northern Mogollon-Datil volcanic field, New Mexico: Effects of the post-Laramide tectonic transition

The tectonic transition between Laramide crustal shortening and mid-Tertiary extension occurred during Datil Group (upper Eocene-lower Oligocene) volcanism in the northern Mogollon-Datil field, New Mexico. Radiometrically constrained stratigraphic evidence for (1) displacement reversal across intrabasin fault zones and (2) initial foundering and burial of Laramide uplifts that would later form basins of the Rio Grande rift indicates that the tectonic transition occurred about 36 Ma. The change from Laramide shortening to mid-Tertiary extension coincides temporally and spatially with the end of early intermediate- composition volcanism (lower Datil Group) and the beginning of bimodal volcanism (upper Datil Group and younger rocks) in the northern Mogollon-Datil field. Lithospheric stress regimes provided a common link between tectonism and volcanism during Datil time. Tectonic compression during Laramide crustal shortening prevented dike propagation on a regional scale. Diapirism was the only major mechanism available for ascent of early Datil magmas. Because diapirs derive buoyancy from density contrast over a short depth interval, basaltic melts must evolve to at least andesitic composition to penetrate low-density continental crust. Tectonic tension began about 36 Ma in west-central New Mexico, allowing widespread dike propagation. Upper Datil basaltic andesite melts, which were more dense than parts of the continental crust, were able to ascend via dike propagation because dikes have sufficient vertical extent to derive buoyancy from roots in the dense upper mantle or lower crust. Crustal melting caused by widespread intrusion of mafic dikes may have produced the silicic mode of late Datil volcanism.

High-precision 40Ar/39Ar sanidine geochronology of ignimbrites in the Mogollon-Datil volcanic field, southwestern New Mexico

40Ar/39Ar age spectra have been obtained from 85 sanidine separates from 36 ignimbrites and one rhyolitic lava in the latest Eocene-Oligocene Mogollon-Datil volcanic field of southwestern New Mexico. Of the 97 measured age spectra, 94 yield weighted-mean plateau ages each giving single-spectrum 1σ precision of±0.25%–0.4% (±0.07–0.14 Ma). Replicate plateau age determinations for eight different samples show within-sample 1σ precisions averaging ±0.25%. Plateau ages from multiple (n=3–8) samples of individual ignimbrites show 1σ within-unit precision of ±0.1%–0.4% (±0.04–0.13 Ma). This within-unit precision represents a several-fold improvement over published K-Ar data for the same ignimbrites, and is similar to the range of precisions reported from single-crystal laser fusion studies. A further indication of the high precision of unit-mean 40Ar/30Ar ages is their close agreement with independently established stratigraphic order. Two samples failed to meet plateau criteria, apparently due to geologic contamination by older feldspars. Effects of minor contamination are shown by six other samples, which yielded slightly anomalous plateau ages. 40Ar/39Ar plateau ages permit resolution of units differing in age by 0.5% (0.15 Ma) or less. This high resolution, combined with paleomagnetic studies, has helped to correlate ignimbrites among isolated ranges and has allowed development of an integrated timestratigraphic framework for the volcanic field. Mogollon-Datil ignimbrites range in age from 36.2 to 24.3 Ma. Ignimbrite activity was strongly episodic, being confined to four brief (<2.6 m.y.) eruptive episodes separated by 1–3 m.y. gaps. Ignimbrite activity generally tended to migrate from the southeast toward the north and west.

Rb, Sr, Rb-Sr, and isotopic Sr values for volcanic rocks from the southwestern part of the Mogollon-Datil volcanic field

Rb, Sr, Rb-Sr, and isotopic Sr values for volcanic rocks from the southwestern part of the Mogollon-Datil volcanic field