Uplift at many well-documented resurgent calderas started only after completion of the associated ignimbrite eruption, but arching of the large Bachelor caldera in the Southern Rocky Mountain volcanic field, Colorado (USA), began during the eruption. A well-defined arched or domical structure, initiated within thickly accumulating ignimbrite as the caldera subsided, is documented by growth of keystone faults as the caldera filled, rheomorphism and local diapiric mobilization of early-erupted tuff, decreased dips in upper welding zones, and wedging of a late-erupted dacitic phase against flanks of the growing dome. Early subsidence-induced doming may have been triggered by preferential magma draw-down along ring-fault vents, relatively impermeable vesiculation and differential magma buoyancy centrally within the caldera, peripheral loading at caldera margins by landslides and talus from caldera walls, or some combination of factors. Early inception of caldera doming has implications for models of magma withdrawal and residual compositional gradients in non-erupted magma.

Table S1: Summary of 40Ar/39Ar and U-Pb zircon age determinations, pre-ignimbrite volcanoes of the eastern San Juan region. Table S2: New and published X-ray fluorescence (XRF) rock analyses, pre-ignimbrite volcanoes of the San Juan region. Table S3 and figures: Analytical data—new 40Ar/39Ar age determinations. Table S4: Analytical data—new U-Pb zircon age determinations. Table S5: Multimineral age comparisons—single samples and multiple samples from the same locality. Table S6: Groundmass-mineral age comparisons.

Our newly acquired and recently published map, geochronologic, and compositional data for early intermediate -composition central volcanoes in the northeastern San Juan Mountains provide insights about the broad magmatic precursors to the large continental -arc ignimbrite flare-up in the mid -Cenozoic Southern Rocky Mountain volcanic field (SRMVF). Initial volcanism migrated from central Colorado to northern New Mexico ca. 40-29 Ma, as part of a more regional trend of southward -progressing mid -Cenozoic magmatism in the U.S. segment of the North American Cordillera. Within the San Juan locus, which represents the largest preserved erosional remnant of the SRMVF and site of most intense eruptive activity, new 40Ar/39Ar and U-Pb zircon ages show that eruptions at many individual edifices began nearly concurrently, at ca. 35 Ma, with peak activity at 34-32 Ma. Broadly similar precursor effusive volcanism characterizes other major loci of continental -arc ignimbrite magmatism along the western American cordilleras, but none of these sites records early volcanism as voluminous, spatially widespread, well exposed, or compositionally diverse as the San Juan locus in Colorado. Early San Juan volcanism was larger in volume than the later ignimbrites, constituting about two thirds the total erupted. Early lava and breccias are as much as 700-900 m thick where exposed along eroded flanks of the San Juan Mountains; drill holes have penetrated sections as thick as 2600 m. The early volcanoes were dominantly andesitic, with lesser dacite and minor rhyolite. Such volcanism is widely interpreted as initiated by basaltic magma from the mantle, but mid -Cenozoic basalt is almost nonexistent at the San Juan locus- an absence inferred to be due to extensive lower -crustal assimilation and fractionation. The early volcanic rocks are calc-alkaline and typical of high -K continental -arc volcanism; they become modestly more alkalic and enriched in trace elements such as light rare earth elements, Zr, Nb, and Th from the San Juan locus northeastward into central Colorado. Such variations may reflect synmagmatic crustal thickening and deeper levels of primary magma generation concurrent with assembly of upper -crustal magma bodies that could support large ignimbrite eruptions. Substantial uncertainties remain for growth histories of the early volcanoes, however, because of unexposed lower parts of edifices, eroded upper parts, and limited availability of mineral phases that could be dated reliably. Although the early volcanoes are widely distributed within the SRMVF, many are clustered at sites of subsequent ignimbrite calderas. The precursor edifices are inferred to record incubation stages in construction of overall translithospheric batholithic-scale magmatic systems. Prolonged processes of incremental magma generation, accumulation, fractionation, and solidification intermittently generated sufficient liquid to erupt large ignimbrites. Maturation of focused eruptions and intrusions was prolonged, 5 m.y. or more, prior to the culminating ignimbrite at some centers in the San Juan Mountains. Some large -volume ignimbrites and related calderas, including the similar to 5000 km3 Fish Canyon Tuff and associated La Garita caldera, formed as much as several million years later than peak growth of associated precursor volcanoes, recording a sustained interval of diminished eruptive activity as the magma reservoir increased in volume and evolved to more silicic compositions capable of supporting a subsequent large ignimbrite eruption. Dike configurations at early volcanoes that were active nearly concurrently in the SRMVF vary from symmetrically radial to more parallel trends. The contrasting dike geometries are inferred to record possible multiple fluctuations from compressive to weakly extensional regional stress, concurrent with destabilization of the prior flat -slab plate configuration that triggered mid -Cenozoic ignimbrite flare-ups along the Cordilleran margin of the North American plate. These apparent fluctuations in regional stress preceded development of substantial extensional strain in the Southern Rocky Mountain region; outflow ignimbrite sheets of the SRMVF spread across subsequent horst-and-graben structures of the Rio Grande rift without complementary thickness variations.

Geochronologic studies of ignimbrite-caldera systems in the Southern Rocky Mountains indicate significant eruptive hiatuses prior to the onset of caldera collapse-related volcanism.Detailed mapping, combined with hundreds of 40 Ar/ 39 Ar dates, in the Southern Rocky Mountain volcanic field (SRMVF), indicates that precaldera volcanic edifices were largely constructed several hundred ka to a few Ma prior to caldera collapse.For example, near complete growth of the precaldera edifice is documented to have finished ~700 to 400 ka prior to the 33.4 Ma eruption of the Bonanza caldera in the northeastern SRMVF.In the central SRMVF, the 5,000 km 3 Fish Canyon Tuff, sourced from the 28.2 Ma La Garita caldera, was preceded by precaldera andesites and dacites, exposed on the caldera rim, that yield ages from 34.5 Ma to as young as 30.5 Ma.Likewise, in the southeastern SRMVF the pre-ignimbrite volcanic edifice was constructed 3.7 to 2.4 Ma prior to the 30.2Ma onset of polycyclic collapse at the Platoro caldera complex.During this hiatus, multiple dikes were emplaced indicating continued magmatism with little preserved eruptive activity.Prior and ongoing dating of Quaternary caldera-related rocks in the Jemez Mountains volcanic field provides additional constraints on the timing of precaldera activity.Published studies indicate that precaldera volcanic activity of predominantly intermediate composition began perhaps as early as ca. 25 Ma in this field and continued to 2.93 Ma.New ages for the La Cueva Tuff, an initial ignimbrite of at least 1-4 km 3 that may have triggered a small-scale caldera collapse, are 1.90 Ma, documenting the onset of rhyolite volcanism after an apparent ca. 1 Ma pause in activity.The field appears to have sat in repose for another 300 ka before eruption of the Otowi Member of the Bandelier Tuff and collapse of the Toledo caldera at 1.61 Ma.Extensive new dating of Cerro Toledo lavas and tephras that were emplaced following the Otowi Member event indicate a ca.120 to 160 ka eruption hiatus prior to a brief 9 ka period of volcanism before collapse of Valles caldera at 1.23 Ma.In these four examples, minor eruptions may have occurred within the subsided area, but the absence of deposits on caldera rims and flanks suggest that volumes were limited compared to peak growth.These examples suggest that some volcanic fields may transition from extended periods of producing small-volume eruptions to protracted durations (e.g., 0.5 to > 2 Ma) of magma storage and incubation that culminates in large-volume ignimbrites and caldera collapse.This work supports initiatives to closely monitor Quaternary caldera systems, even those that are currently in moderate to long periods of repose.

This contribution provides in-situ LA-ICP-MS U-Pb ages and trace element determinations of zircons from dacitic to rhyolitic lavas, ignimbrites and intrusions in the Southern Rocky Mountain Volcanic Field (SRMVF) in Colorado, USA. The data record a period of intense magmatic activity in the Oligocene-early Miocene (similar to 37-22 Ma) which gave rise to some of the largest explosive ignimbrites in the geological record (e.g. the Fish Canyon Tuff). Age data are drift corrected, but not corrected for radiation dosage or Th disequilibrium, in order to allow users to apply their own algorithms. Xenocrysts (much older crystals up to 2 Ga from the Proterozoic basement) are included in this record. (C) 2022 The Author(s). Published by Elsevier Inc.

Clusters of early central volcanoes in the mid-Cenozoic Southern Rocky Mountain volcanic field (SRMVF; southwestern Colorado, USA) record sites of initial magmatic focusing that led to assembly of sizable upper-crustal magma bodies capable of generating large ignimbrites. Peak growth at precursor andesitic volcanoes was followed by extended periods (0.5 to >2 m.y.) of reduced eruptive activity during inferred prolonged incubation of the crustal reservoir prior to eruption of ignimbrites at the San Juan magmatic locus, as exemplified by the 5000 km 3 Fish Canyon Tuff and associated La Garita caldera. After a magma system became thermally mature and compositionally evolved, additional large ignimbrites could erupt more rapidly from polycyclic calderas. In contrast, incubation times for smaller ignimbrite magmas, as at Crater Lake, Oregon, were briefer than for San Juan systems. Plutonic counterparts to the temporal-compositional assembly of arc-ignimbrite magmas are exemplified by incrementally emplaced granitoid intrusions like the Mesozoic Tuolumne complex in the Sierra Nevada.

Geologic maps of the Platoro and Bonanza caldera areas, and tables summarizing ages of pre-ignimbrite volcanic rocks.

The Oligocene Platoro caldera complex of the San Juan volcanic locus in Colorado (USA) features numerous exposed plutons both within the caldera and outside its margins, enabling investigation of the timing and evolution of postcaldera magmatism. Intrusion whole-rock geochemistry and phenocryst and/or mineral trace element compositions coupled with new zircon U-Pb geochronology and zircon in situ Lu-Hf isotopes document distinct pulses of magma from beneath the caldera complex. Fourteen intrusions, the Chiquito Peak Tuff, and the dacite of Fisher Gulch were dated, showing intrusive magmatism began after the 28.8 Ma eruption of the Chiquito Peak Tuff and continued to 24 Ma. Additionally, magmatic-hydrothermal mineralization is associated with the intrusive magmatism within and around the margins of the Platoro caldera complex. After caldera collapse, three plutons were emplaced within the subsided block between ca. 28.8 and 28.6 Ma. These have broadly similar modal mineralogy and whole-rock geochemistry. Despite close temporal relations between the tuff and the intrusions, mineral textures and compositions indicate that the larger two intracaldera intrusions are discrete later pulses of magma. Intrusions outside the caldera are younger, ca. 28–26.3 Ma, and smaller in exposed area. They contain abundant glomerocrysts and show evidence of open-system processes such as magma mixing and crystal entrainment. The protracted magmatic history at the Platoro caldera complex documents the diversity of the multiple discrete magma pulses needed to generate large composite volcanic fields.

Chemical data for paleo-crustal-thickness estimates.

The Southern Rocky Mountains of Colorado, United States, have the highest regional elevation in North America, but present-day crustal thickness (similar to 42-47 km) is no greater than for the adjacent, topographically lower High Plains and Colorado Plateau. The chemistry of continental-arc rocks of the mid-Cenozoic Southern Rocky Mountain volcanic field, calibrated to compositions and Moho depths at young arcs, suggests that paleocrustal thickness may have been 20%-35% greater than at present and elevations accordingly higher. Thick mid-Cenozoic Rocky Mountain crust and high paleo-elevations, comparable to those inferred for the Nevadaplano farther west in the United States from analogous volcanic chemistry, could be consistent with otherwise-perplexing evidence for widespread rapid erosion during volcanism. Variable mid-Cenozoic crustal thickening and uplift could have resulted from composite batholith growth during volcanism, superimposed on prior crustal thickening during early Cenozoic (Laramide) compression. Alternatively, the arc-crustal thickness calibration may be inappropriate for high-potassium continental arcs, in which case other published interpretations using similar methods may also be unreliable.

The last four caldera-forming ignimbrites in the central San Juan caldera cluster, Colorado, erupted 1400 km 3 in ≤80 kyr and alternated between zoned crystal-poor rhyolite to crystal-rich dacite and unzoned, crystal-rich dacite. The zoned 150 km 3 Rat Creek Tuff (26·91 Ma), unzoned 250 km 3 Cebolla Creek Tuff, and zoned 500 km 3 Nelson Mountain Tuff (26·90 Ma) formed the nested San Luis caldera complex with slightly offset calderas, and the unzoned 500 km 3 Snowshoe Mountain Tuff (26·87 Ma) formed the Creede caldera to the south. The Rat Creek Tuff, Nelson Mountain Tuff, and Snowshoe Mountain Tuff have similar mineral assemblages of plagioclase, sanidine, quartz, biotite, hornblende, clinopyroxene, Fe–Ti oxides, and accessory zircon, titanite, and apatite. The Cebolla Creek Tuff differs from the other three ignimbrites with more abundant hornblende and a lack of quartz and sanidine. Trace element compositions of interstitial glass are unique to each ignimbrite, correlating with mineral assemblages and inferred crystallization depths. Glass, feldspar, hornblende, and clinopyroxene thermobarometry calculations provide evidence for vertically extensive crustal magma reservoirs with a common magmatic zone at ∼435–470 MPa (∼16–17 km) showing a transition into shallow pre-eruptive reservoirs between ∼110 and 340 MPa (∼4–13 km), similar to the estimated magma reservoir architecture of the Altiplano Puna Volcanic Complex. The upper portions of the eruptible parts of the magma reservoirs of the Rat Creek Tuff (215 ± 50 MPa, ∼810–820 °C), Cebolla Creek Tuff (340 ± 20 MPa, ∼860–880 °C), Nelson Mountain Tuff (215 ± 20 MPa, ∼745–800 °C), and Snowshoe Mountain Tuff (110 ± 40 MPa, 825 ± 10 °C) occupied shallow levels in the crust similar to other magma reservoirs of the central San Juan caldera cluster. Trace element modelling correlates with a deep crystallization signature in the unzoned Cebolla Creek Tuff and Snowshoe Mountain Tuff, typified by a flat trend in Ba versus Sr whole-rock and glass chemistry. The zoned Rat Creek Tuff and Nelson Mountain Tuff are typified by a steep trend in Ba versus Sr chemistry interpreted as a shallower crystallization signature. Similarly, the unzoned Cebolla Creek Tuff and Snowshoe Mountain Tuff have flatter slopes in FeO versus An space of plagioclase chemistry interpreted as more abundant deep plagioclase crystallization and a difficulty in physically mixing with Fe-rich mafic recharge magma owing to higher viscosity. The zoned Rat Creek Tuff and Nelson Mountain Tuff have higher slopes in FeO versus An space of plagioclase chemistry interpreted as more abundant shallow plagioclase crystallization and more feasible mixing with Fe-rich mafic recharge magma owing to lower viscosity. The eruption of the Rat Creek Tuff was probably triggered by mafic injection, but the other three ignimbrites lack mingling textures in pumice, suggesting that other mechanisms were important in causing such large eruptions. After a prolonged period of mantle-derived magma injection and crustal heating (∼25 000 km 3 Conejos Formation erupted during ∼35–29 Ma), the San Juan magmatic body became a robust and versatile producer of diverse eruptible magmas in short time periods during its Oligocene ignimbrite flare-up.

First posted March 23, 2020 For additional information, contact: Volcano Science Center - Menlo ParkU.S. Geological Survey345 Middlefield Road, MS 910Menlo Park, CA 94025 The San Juan Mountains in southwestern Colorado have long been known as a site of exceptionally voluminous mid-Tertiary volcanism, including at least 22 major ignimbrite sheets (each 150–5,000 km³) and associated caldera structures active at 34–23 Ma. Recent volcanologic and petrologic studies in the San Juan region have focused mainly on several ignimbrite-caldera systems: the southeastern area (Platoro complex), western calderas (Uncompahgre-Silverton-Lake City), and the central cluster (La Garita-Creede calderas).Far less studied has been the northeastern San Juan region, which occupies a transition between earlier volcanism in central Colorado and large-volume younger ignimbrite-caldera foci farther south and west. This map is based on new field coverage of volcanic rocks in thirteen 7.5' quadrangles in northeastern parts of the volcanic field, high-resolution age determinations for 130 sites, and petrologic studies involving several hundred new chemical analyses. This mapping and the accompanying lab results (1) document volcanic evolution of the deeply eroded Bonanza caldera that exposes unique features not previously described from ignimbrite calderas elsewhere, as well as the previously unstudied Marshall Pass caldera; (2) provide unique cross-sectional exposures of the steeply resurgent Bonanza caldera, from volcanic floor and underlying basement rocks through a complete 3.5-km-thick section of intracaldera ignimbrite and overlying compositionally diverse caldera-filling lavas; (3) document timing of caldera collapse concurrently with eruption of about 1,000 km3 of ignimbrite that oscillated in composition from mafic dacite to rhyolite; (4) quantify the regional time-space-volume progression from the earlier Sawatch magmatic trend southward into the San Juan region; and (5) permit more rigorous comparison between the broad mid-Tertiary magmatic belt in the western U.S. Cordillera and the type continental-margin arc volcanism of the central Andes in South America.

The San Luis caldera complex in the Southern Rocky Mountain Volcanic Field (CO, USA) consists of three overlapping calderas that overlie the sources of three large-volume mid-Cenozoic ignimbrites: the Rat Creek Tuff (RCT; zoned dacite-rhyolite, 150 km3), the Cebolla Creek Tuff (mafic dacite, 250 km3) and the Nelson Mountain Tuff (NMT; zoned dacite-rhyolite, 500 km3), which are indistinguishable in age by 40Ar/39Ar dating. In this study, we argue for a shared magmatic history for the three units on the basis of mineral trace element compositions (plagioclase, sanidine, biotite, pyroxene, amphibole, titanite and zircon), as well as zircon U-Pb geochronology in the RCT and NMT. It is postulated that these latter two are cogenetic, having occupied an elongated magma reservoir that erupted in two stages, prior to and following the eruption of the Cebolla Creek Tuff. This necessitates large-scale lateral magma transport of the NMT magma, which is corroborated by the formation of the nearby Cochetopa caldera with a paucity of intracaldera eruptive products. The implications of lateral magma transport and evolution in integrated magma chambers are discussed in the context of calculating magma fluxes, which can be redefined as an area-normalized flux to avoid inconsistencies in flux estimations.

Radial and linear dike swarms in the eroded roots of volcanoes and along rift zones are sensitive structural indicators of conduit and eruption geometry that can record regional paleostress orientations. Compositionally diverse dikes and larger intrusions that radiate westward from the polycyclic Platoro caldera complex in the Southern Rocky Mountain volcanic field (southwestern United States) merge in structural trend, composition, and age with the enormous but little-studied Dulce swarm of trachybasaltic dikes that continue southwest and south for similar to 125 km along the eastern margin of the Colorado Plateau from southern Colorado into northern New Mexico. Some Dulce dikes, though only 1-2 m thick, are traceable for 20 km. More than 200 dikes of the Platoro-Dulce swarm are depicted on regional maps, but only a few compositions and ages have been published previously, and relations to Platoro caldera have not been evaluated. Despite complications from deuteric alteration, bulk compositions of Platoro-Dulce dikes (105 new X-ray fluorescence and inductively coupled plasma mass spectrometry analyses) become more mafic and alkalic with distance from the caldera. Fifty-eight (58) new Ar-40/Ar-3(9) ages provide insight into the timing of dike emplacement in relation to evolution of Platoro caldera (source of six regional ignimbrites between 30.3 and 28.8 Ma). The majority of Dulce dikes were emplaced during a brief period (26.5-25.0 Ma) of postcaldera magmatism. Some northeast-trending dikes yield ages as old as 27.5 Ma, and the northernmost north-trending dikes have younger ages (20.1-18.6 Ma). In contrast to high-K lamprophyres farther west on the Colorado Plateau, the Dulce dikes are trachybasalts that contain only anhydrous phenocrysts (clinopyroxene, olivine). Dikes radial to Platoro caldera range from pyroxene- and hornblende-bearing andesite to sanidine dacite, mostly more silicic than trachybasalts of the Dulce swarm. Some distal andesite dikes have ages (31.2-30.4 Ma) similar to those of late precaldera lavas; ages of other proximal dikes (29.2-27.5 Ma) are akin to those of caldera-filling lavas and the oldest Dulce dikes. The largest radial dikes are dacites that have yet younger sanidine Ar-40/Ar-3(9) ages (26.5-26.4 Ma), similar to those of the main Dulce swarm. The older andesitic dikes and precaldera lavas record the inception of a long-lived upper-crustal magmatic locus at Platoro. This system peaked in magmatic output during ignimbrite eruptions but remained intermittently active for at least an additional 9 m.y. Platoro magmatism began to decline at ca. 26 Ma, concurrent with initial basaltic volcanism and regional extension along the Rio Grande rift, but no basalt is known to have erupted proximal to Platoro caldera prior to ca. 20 Ma, just as silicic activity terminated at this magmatic locus. The large numbers and lengths of the radial andesitic-dacitic dikes, in comparison to the absence of similar features at other calderas of the San Juan volcanic locus, may reflect location of the Platoro system peripheral to the main upper-crustal San Juan batholith recorded by gravity data, as well as its proximity to the axis of early rifting. Spatial, temporal, and genetic links between Platoro radial dikes and the linear Dulce swarm suggest that they represent an interconnected regional-scale magmatic suite related to prolonged assembly and solidification of an arc-related subcaldera batholith concurrently with a transition to regional extension. Emplacement of such widespread dikes during the late evolution of a subcaldera batholith could generate earthquakes and trigger dispersed small eruptions. Such events would constitute little-appreciated magmato-tectonic hazards near dormant calderas such as Valles, Long Valley, or Yellowstone (western USA).

Diverse welding, crystallization, and structural features develop when a hot ignimbrite encounters external water, depending largely on volatile-rock ratios. Such processes are spectacularly documented by a regional ignimbrite where it ponded within an older caldera in the San Juan Mountains, Colorado (USA). Interaction of hot pyroclastic flows with moist underlying sediments or standing water in a stream valley or shallow-lakeshore environment produced megascale gas-escape structures, quenched adjacent tuff, inhibited welding, and generated nonplanar crystallization zones. This site provides a context for reviewing examples of ignimbrite-water interaction elsewhere.

Many eroded calderas expose associated postcollapse plutons, but detailed fieldwork-supported studies have rarely focused on the internal structure that can contribute to understanding of emplacement dynamics. The Alamosa River monzonite pluton is a postcollapse intrusion at the Platoro caldera complex that erupted six large ignimbrites between 30.2 and 28.8 Ma in the Southern Rocky Mountains volcanic field. Magnetic fabrics in this intrusion indicate the pulsed emplacement of a vertically extensive pluton. The magmatic pulses are documented by three concentric domains of magnetic foliations elongated in NE-SW direction, corresponding to structural trends at the Platoro caldera complex and preexisting regional structures. As no evidence for deformation of wall rocks and the adjacent resurgent block has been identified, we interpret the Alamosa River pluton as a postresurgent intrusion. The space-opening process involved magmatic stoping and small-scale magma wedging. New SHRIMP-RG U/Pb zircon dates (28.98 +/- 0.18, 27.42 +/- 0.35, and 27.32 +/- 0.38 Ma) suggest a magmatic lifespan of similar to 1.7 My for the Alamosa River pluton. Our results indicate that postcaldera magmatism includes pulsed and protracted activity from large intracaldera resurgent plutons to smaller postresurgent stocks and sheeted complexes. As demonstrated by the Alamosa River pluton, some intrusions are emplaced shortly after collapse and resurgence, but postcaldera volcano-plutonic systems may remain active for several million years or more. We also suggest that subvolcanic magma bodies may be assembled incrementally and that the record of early composite magma lenses preserved as magma wedges are later obliterated by convective flowage and crystallization.