Ignimbrite Calderas, Regional Dike Swarms, and the Transition from Arc to Rift in the Southern Rocky Mountains

Home , Dike swarm, Ignimbrite, San Luis Hills

Research Paper

GEOSPHERE Magmato-tectonic links: Ignimbrite calderas, regional dike swarms, and the transition from arc to rift in the Southern Rocky Mountains

1, 2, GEOSPHERE, v. 15, no. 6 Peter W. Lipman * and Matthew J. Zimmerer * 1U.S. Geological Survey, Menlo Park, California 94025, USA 2New Mexico Bureau of Geology and Mineral Resources, Socorro, New Mexico 87801, USA https://doi.org/10.1130/GES02068.1

18 figures; 3 tables; 1 set of supplemental files ABSTRACT 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 CORRESPONDENCE: [email protected] Radial and linear dike swarms in the eroded roots of volcanoes and along along the Rio Grande rift, but no basalt is known to have erupted proximal rift zones are sensitive structural indicators of conduit and eruption geometry to Platoro caldera prior to ca. 20 Ma, just as silicic activity terminated at this CITATION: Lipman, P.W., and Zimmerer, M.J., 2019, Magmato-tectonic links: Ignimbrite calderas, regional that can record regional paleostress orientations. Compositionally diverse magmatic locus. The large numbers and lengths of the radial andesitic-dacitic dike swarms, and the transition from arc to rift in the dikes and larger intrusions that radiate westward from the polycyclic Platoro dikes, in comparison to the absence of similar features at other calderas of the Southern Rocky Mountains: Geosphere, v. 15, no. 6, caldera complex in the Southern Rocky Mountain volcanic field (southwest- San Juan volcanic locus, may reflect location of the Platoro system peripheral p. 1893–1926, https://doi.org/10.1130/GES02068.1. ern United States) merge in structural trend, composition, and age with the to the main upper-crustal San Juan batholith recorded by gravity data, as enormous but little-studied Dulce swarm of trachybasaltic dikes that con- well as its proximity to the axis of early rifting. Spatial, temporal, and genetic Science Editor: Shanaka de Silva Associate Editor: Valerio Acocella tinue southwest and south for ~125 km along the eastern margin of the links between Platoro radial dikes and the linear Dulce swarm suggest that Colorado Plateau from southern Colorado into northern New Mexico. Some they represent an interconnected regional-scale magmatic suite related to Received 23 September 2018 Dulce dikes, though only 1–2 m thick, are traceable for 20 km. More than 200 prolonged assembly and solidification of an arc-related subcaldera batholith Revision received 16 April 2019 dikes of the Platoro-Dulce swarm are depicted on regional maps, but only a concurrently with a transition to regional extension. Emplacement of such Accepted 18 July 2019 few compositions and ages have been published previously, and relations to widespread dikes during the late evolution of a subcaldera batholith could Platoro caldera have not been evaluated. Despite complications from deuteric generate earthquakes and trigger dispersed small eruptions. Such events Published online 30 September 2019 alteration, bulk compositions of Platoro-Dulce dikes (105 new X-ray fluores- would constitute little-appreciated magmato-tectonic hazards near dormant cence and inductively coupled plasma mass spectrometry analyses) become calderas such as Valles, Long Valley, or Yellowstone (western USA). more mafic and alkalic with distance from the caldera. Fifty-eight (58) new 40Ar/39Ar ages provide insight into the timing of dike emplacement in relation to evolution of Platoro caldera (source of six regional ignimbrites between ■■ INTRODUCTION 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 Radial and linear dike swarms in the eroded roots of central volcanoes and dikes yield ages as old as 27.5 Ma, and the northernmost north-trending dikes along rift zones have long been recognized as structures that document the have younger ages (20.1–18.6 Ma). In contrast to high-K lamprophyres farther geometry of conduits for eruptions and provide records of paleostress geom- west on the Colorado Plateau, the Dulce dikes are trachybasalts that contain etry (Nakamura, 1977; Aldrich et al., 1986; Acocella, 2014). In comparison, dike only anhydrous phenocrysts (clinopyroxene, olivine). Dikes radial to Platoro swarms appear to be relatively uncommon at large ignimbrite calderas (Smith caldera range from pyroxene- and hornblende-bearing andesite to sanidine and Bailey, 1968; Cole et al., 2005), perhaps because growth of batholithic-scale dacite, mostly more silicic than trachybasalts of the Dulce swarm. Some distal magma bodies beneath calderas decouples the overlying crust from regional andesite dikes have ages (31.2–30.4 Ma) similar to those of late precaldera stress geometry (Christiansen et al., 1965; Steven and Lipman, 1976). Even lavas; ages of other proximal dikes (29.2–27.5 Ma) are akin to those of caldera- ignimbrite systems that erupted during regional extension, such as Valles, filling lavas and the oldest Dulce dikes. The largest radial dikes are dacites Yellowstone, and Long Valley calderas in the western USA, typically host only that have yet younger sanidine 40Ar/39Ar ages (26.5–26.4 Ma), similar to those sparse fissure-controlled magmatism (the Mono-Inyo chain north of Long of the main Dulce swarm. Valley being an exception). The older andesitic dikes and precaldera lavas record the inception of a In contrast, an enormous system of long-recognized but little-studied long-lived upper-crustal magmatic locus at Platoro. This system peaked in dikes in the southwestern USA, which radiate from the Oligocene Platoro magmatic output during ignimbrite eruptions but remained intermittently caldera in Colorado (Fig. 1) to merge with the Dulce dike swarm that contin- This paper is published under the terms of the ues ~125 km into New Mexico (Fig. 2), provides exceptional opportunities to CC‑BY-NC license. *E-mail: [email protected]; [email protected] explore magmato-tectonic–temporal links between dike emplacement and a

GEOSPHERE | Volume 15 | Number 6 Lipman and Zimmerer | Magmato-tectonic links Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/6/1893/4876434/1893.pdf 1893 by guest on 30 September 2021 Research Paper

108°W 106° 104° 40° N 0 60 mi

0 100 km Denver F r N o n t

R a n g S e a GP w South a Park t BP Colorado c x Figure 1. Map of the Southern Rocky Mountain vol- h Springs 39 Mile canic field (SRMVF) and Rio Grande rift in southern volcanic Colorado and northern New Mexico (USA). Large MP area rectangle, location of study area for the Platoro-Dulce

MA l dike swarm (Fig. 2); smaller rectangle, Platoro caldera Approx. original limit l West Elk l l R map area (Fig. 3). Also shown are other ignimbrite of volcanic locusrocks Gunnison g calderas of the SRMVF, major erosional remnants e x and inferred original extent of mid-Cenozoic volcanic cover (Steven, 1975), caldera-related granitic M Bz W intrusions (Tweto, 1979; Lipman, 2007), and later e sedimentary fill in asymmetric grabens of the Rio C t

NP S Rosita Grande rift. Rift graben asymmetry and bound- a x M ary-fault geometry reverse from east-dipping in Saguache n 38° SL g t the San Luis Valley segment to west-dipping in r s e . the Sawatch Range–upper Arkansas River valley LGn S San B San Luis Valley segment, segment to the north. Blue-dashed lines, major Juan SC bounding faults of asymmetrical rift grabens. Arrows Cr volcanic Del x indicate the trend of Late Cretaceous–early Cenozoic locus SR Norte Rio Grande rift (Laramide) intrusions of the Colorado Mineral Belt. d Calderas: B—Bachelor; Bz—Bonanza; C—Cochetopa e LGs Fig. 3 Park; Cr—Creede; GP—Grizzly Peak; LGn—La Gar- Approximate original limit ita, north segment; LGs—La Garita, south segment; M—Marshall; MA—Mount Aetna; NP—North Pass; of volcanic rocks Pl Pl—Platoro; S—Silverton; SL—San Luis complex;

SR—South River. Other features: BP—Buffalo Peak; C Spanish

Line of SC—Summer Coon volcano. CO—Colorado; NM—

section, r

i Peaks New Mexico. Modified from McIntosh and Chapin

Fig. 16 s

t Colorado o (2004). Rge—Range; Mts.—Mountains. New Mexico T u s a Questa-Latir s volcanic M M locus t t s s Fig. 2 . .

Explanation

Granitoid intrusion MP Mount Princeton batholith Mid-Cenozoic volcanic areas Trend of Colorado Mineral Belt Sedimentary fill of Rio Grande rift Caldera Late-rift mafic lavas Regional structural attitude

Approximate inferred extent of ignimbrites erupted from 26.4 Platoro caldera complex (Treasure Mountain Group) 21.3 26.5 20.9 26.9 26.2 PLATORO 20.4 *27.1 CALDERA 27.4 26.2 *29.0 27.7 * 25.1 * 29.0 AR * Platoro: * JM 25.1 CRETACEOUS * distal 27.5 dikes (PD) ca. 29.2 SEDIMENTARY ROCKS, 31 NE trend, CO-N SAN JUAN BASIN 31.4 (D1) 26.8 29.2 26.5 x Platoro: 19.1 Pagosa Springs 27.5 BB Platoro map (Lipman, 1974) proximal 26.5 dikes (PP) 18.6 27.3 25.3 N trend, CO-N 18.6 25.9 30.2 30.7 (D2A) ~27.5 31.2 VOLCANIC ROCKS, SAN JUAN MOUNTAINS 27.3 24.5 ~21 V- 25.1 NE trend, CO-S Mtn (D2B) 24.6 N trend, CO-S 25.0 (D3)

21.5 20.1 25.5 25.5

37° AM AM Colorado N *15.5 New Mexico 26.4 XRF chem 25.6 25.3 sample sites x Dulce 25.4 Dacite

N trend, NM-N Andesite (D4) Trachybasalt 26.0

25.5 Ar/Ar ages Blue, DG (2015); red, no success

U-Pb* ages* Blue, DG (2015); black, AG (2018)

N trend, NM-mid (D5) Granitoid intrusions

N Line of projected chemical plots (Fig. 13) N trend, NM-S (D6) Geologic base maps: 25.0 CO: Steven et al. (1974) NM: NMBGMR (2003) Platoro: Lipman (1974) 20 kilometers

107°W Figure 2. Generalized map (modified from Tweto, 1979; New Mexico Bureau of Geology and Mineral Resources, 2003) showing geometric relations between Platoro caldera and the Dulce dike swarm, locations of newly analyzed samples (XRF chem—X-ray fluorescence chemical analysis), and isotopic ages (in Ma, generalized from Table 2). Dashed black lines mark boundaries between geographic segments of the Dulce dike swarm in relation to distance from the Platoro locus (segment identifier in parentheses), as used in chemical plots (Figs. 6, 11–13). Intrusion locations: Am—Archuleta Mesa sill; AR—Alamosa River pluton; BB—Blanco Basin laccolith; JM— Jackson Mountain pluton; V-Mtn—V Mountain sill. Reference abbreviations: DG 2015—Gonzales (2015); AG 2018—Gilmer et al. (2018). CO—Colorado; NM—New Mexico.

major ignimbrite center. Questions motivating our study include: (1) when Laramide-age basement uplifts (Fig. 1), perhaps related to westward retreat or are dikes emplaced during a caldera cycle, (2) how far can magma migrate rollback of the foundering Farallon slab (Coney and Reynolds, 1977; Ricketts et outward from caldera systems, and (3) how do dike compositions vary in al., 2016). Shallow granitoid intrusions are exposed at near-roof levels within relation to age and distance from a caldera locus? The Platoro-Dulce dikes are or adjacent to many of the calderas. The widespread distribution and relatively interpreted here as providing a unique regional-scale record of uplift during uniform thickness of the outflow ignimbrites demonstrate that these eruptions prolonged emplacement and solidification of an arc-related subcaldera batho- predated inception of major extensional faulting along the Rio Grande rift. lith concurrently with a transition to regional extension. Although available geologic maps depict distribution of the Platoro-Dulce dikes fairly reliably (Fig. 2), the overall dike swarm has not previously been ■■ PLATORO-DULCE GEOLOGIC SUMMARY studied in detail. Several publications include compositions and radiogenic ages of variable precision for a few dikes near the Colorado–New Mexico bor- Igneous suites associated with the multicylcic Platoro caldera (Lipman, 1975) der, but these were only parts of data sets used primarily to interpret regional include precaldera lavas, ignimbrites, postcollapse lavas and granitoid intru- paleostress geometry and mafic-magma petrogenesis (Aldrich et al., 1986; Gib- sions, dikes of the Platoro-Dulce swarm, and Miocene basaltic lavas (Hinsdale son et al., 1993; Gonzales, 2015; Gonzales and Lake, 2017). This paper presents Formation). Notably, lavas as mafic as basalt are nearly absent among Oligo- new petrologic data (compositions for 105 widely distributed samples) and 58 cene eruptions of the SRMVF, suggesting that mantle-derived components new 40Ar/39Ar ages for the overall areal extent of the Platoro-Dulce dike swarm mingled efficiently with crustal melts (Lipman et al., 1978; Riciputi et al., 1995). in relation to concurrent regional magmato-tectonic evolution (Tables 1–2; Basalts began to erupt only at ca. 26 Ma, concurrent with initial prominent exten- Supplemental Files 1–51). sion along the Rio Grande rift (Lipman and Mehnert, 1975; Turner et al., 2019).

■■ REGIONAL MAGMATO-TECTONIC FRAMEWORK Precaldera Lavas

SUPPLEMENTAL FILE 1. PETROGRAPHIC SUMMARIES AND BULK-SAMPLE XRF CHEMICAL ANALYSES, PLATORO-DULCE DIKES, LAVAS, AND ASSOCIATED ROCKS Segment, [Bold: least-altered minerals] Anomalous, or atypical for segment Italics: excluded from average (anomalous, or ambiguous location) Hi MgO, Ni, Cr (olivine-rich) Mid-point Latitude N Longitude W Phenocrysts Groundmass SUM/w Total # distance* Sample Rock type Location Deg Min Deg Min [Pseudomorph] Mafics Plagioclase SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 Sum LOI % SO3 Cl volatiles alkalis Ni Cr Sc V Ba Rb Sr Zr Y Nb Ga Cu Zn Pb La Ce Th Nd U Platoro: proximal granitoid intrusions SRM-23 Resurgent intrusion Cornwall Mountain 37 21.03 106 30.16 Plag, bio phenocrysts F.g. matrix 64.12 0.58 16.28 4.03 0.22 1.36 4.52 3.93 4.04 0.13 99.21 3.33 0.00 99.21 7.97 3 3 7 70 986 85 573 214 20 13 18 13 66 15 36 66 10 29 3 SRM-22 Alamosa River pluton Telluride Mountain 37 23.07 106 32.79 Med. grain plag, bio, cpx 62.10 0.70 15.19 6.30 0.28 2.93 5.04 3.29 4.38 0.16 100.38 0.80 0.04 100.42 7.67 21 35 15 151 770 109 566 192 25 12 18 198 97 16 31 69 10 33 4 SRM-24 Alamosa River pluton N of Stunner Pass 37 22.67 106 33.94 Med. grain plag, bio, cpx 60.24 0.94 15.85 6.45 0.31 3.50 5.38 3.40 3.75 0.12 99.92 1.51 0.00 99.92 7.15 33 67 15 146 783 103 601 243 26 14 19 58 82 13 39 84 12 36 3 SRM-25 Granodiorite Jasper 37 25.09 106 28.54 Med. grain plag, bio, cpx 62.20 0.86 15.42 5.95 0.27 2.62 4.11 3.70 4.28 0.11 99.53 0.70 0.02 99.55 7.98 22 40 14 125 747 131 563 296 27 16 19 33 87 17 43 89 17 39 4 SRM-26 Granodiorite Cat Creek 37 24.31 106 18.35 Fine- grain plag, bio, cpx 60.28 0.97 16.39 6.76 0.36 2.54 4.90 4.02 3.32 0.09 99.64 0.71 0.01 99.65 7.34 26 40 13 147 954 81 764 188 22 10 21 40 89 13 31 68 7 31 2 SRM-33 F.g. granodiorite porphyry Crater Creek 37 24.99 106 42.59 Plag, bio phenocrysts F.g. matrix 62.56 0.75 16.32 5.48 0.11 2.27 4.34 4.01 3.67 0.31 99.82 2.06 0.06 99.88 7.68 8 10 10 103 1069 80 678 179 22 13 18 19 77 19 39 79 12 36 3 AVERAGE: 61.92 0.80 15.91 5.83 0.26 2.53 4.72 3.73 3.91 0.15 99.75 1.52 7.63 19 32 12 124 885 98 624 219 24 13 19 60 83 16 37 76 11 34 3 PP Platoro: proximal dikes and lavas Interpretation of the Platoro-Dulce dikes in relation to Cenozoic development Early lavas in the vicinity of Platoro caldera (Conejos Formation, 35–30 Ma; 5 16L-36 Andesite dike, NNE-trend (post-Platoro) At Rito Gato 37 20.43 106 34.48 F.g. interlocking plag; altered mafics 57.50 1.04 16.98 8.00 0.14 3.49 5.00 3.65 3.37 0.36 99.53 4.86 99.53 7.02 13 4 17 166 739 76 1076 193 25 11 18 44 97 14 34 71 8 32 4 16L-38 Andesite dike, N-trend (post-Platoro) At lake shore 37 19.97 106 34.64 Bladed plag (to 1 mm), alt. mafic phenos 53.89 1.20 17.26 9.07 0.20 2.65 9.67 3.09 2.06 0.41 99.49 6.04 99.49 5.15 14 0 19 206 618 32 810 153 27 8 19 60 96 10 29 64 5 33 1 16L-42 Andesite dike, N trend (post-Platoro) N of Rito Gato 37 20.46 106 34.42 No TX 55.06 1.08 17.02 8.24 0.13 4.02 6.97 3.25 3.36 0.38 99.51 7.91 99.51 6.61 22 11 18 178 794 70 993 163 22 10 19 86 87 11 32 68 7 31 2 16L-28 Andesite dike, NNW-trend Cat Cr road junction 37 19.59 106 35.28 Plag (to 2 mm), [cpx?] F.g. matrix 58.48 0.98 15.84 7.91 0.14 3.92 5.82 2.93 3.00 0.30 99.33 5.40 0.14 99.47 5.93 25 28 18 168 639 66 435 203 26 11 19 64 89 11 31 73 9 33 3 16L-30 Andesite dike S of Adams Fork 37 19.04 106 35.69 Microphenos of blocky plag F.g. matrix 58.07 0.90 17.61 9.22 0.06 2.80 4.81 2.48 3.22 0.32 99.47 4.46 99.47 5.69 11 3 18 145 901 30 469 156 29 8 19 44 70 10 27 58 3 27 2 16L-31 Andesite dike, smaller northern Three Forks road 37 18.88 106 35.70 Microphenos & bladed gm plag; green hbl 57.52 0.83 17.63 7.46 0.19 2.49 7.74 2.75 2.35 0.34 99.32 2.78 99.32 5.10 5 2 18 119 657 34 611 152 33 7 19 13 115 8 25 57 4 29 2 16L-33 Andesite dike, small Below Three Forks road 37 18.73 106 35.63 Microphenos of altered plag (& mafics?) 51.82 1.18 18.16 9.72 0.22 2.43 10.72 3.22 1.65 0.35 99.47 7.90 99.47 4.87 19 9 26 242 613 21 671 124 31 6 18 91 104 6 26 52 2 30 0 16L-34 Andesite dike, small Below Three Forks road 37 18.76 106 35.65 Microphenos of partly altered plag & cpx 58.22 0.78 17.65 7.22 0.16 1.97 7.33 3.21 2.57 0.35 99.44 3.33 99.44 5.78 6 5 17 106 687 47 596 156 32 8 19 15 99 11 25 58 4 31 2 16L-32 Hbl-andesite dike, NNE-trend (Conejos) Below trail 37 17.96 106 36.37 Green hbl to 5 mm, small plag F.g. matrix 61.17 0.65 16.72 6.16 0.13 2.00 5.19 3.03 3.97 0.28 99.29 2.98 99.29 7.01 3 1 11 86 854 64 463 158 25 8 17 11 91 11 29 61 5 28 1 16L-35 Hbl andesite dike, NE-trend (Conejos) Adams Fork, along trail 37 19.12 106 37.28 Sparse hbl (0.5-1 mm), small plag & alt cpx 58.38 0.73 17.94 7.00 0.23 3.11 5.52 3.15 3.24 0.35 99.65 3.94 99.65 6.39 4 0 13 96 763 54 572 151 29 8 18 7 98 10 26 55 6 30 1 Lavas 16L-40 Andesite lava, Summitville Andesite Hillman Park road 37 19.66 106 35.93 Small phenos of plag & cpx, f.g. matrix 57.82 1.05 16.89 7.27 0.09 3.21 5.57 3.56 3.47 0.37 99.30 1.36 99.30 7.04 25 9 16 169 758 86 719 222 25 14 20 62 87 14 30 77 10 36 3 of the Southern Rocky Mountains depends on a broad framework of magma- Table 2) are basaltic andesite to dacite (>54% SiO ; Colucci et al., 1991), more 16L-41 Andesite lava, Summitville Andesite Hillman Park road 37 19.51 106 35.66 Small phenos of plag & cpx, f.g. matrix 56.15 1.07 17.10 8.11 0.13 3.57 6.44 3.54 2.94 0.38 99.43 1.07 99.43 6.48 24 11 19 193 718 66 787 175 24 10 19 91 92 12 30 70 7 35 3 2 AVERAGE: 57.01 0.96 17.23 7.95 0.15 2.97 6.73 3.15 2.93 0.35 99.43 4.34 0.14 99.45 6.09 14 7 18 156 728 54 683 167 27 9 19 49 94 11 29 64 6 31 2 PD Platoro: distal dikes 20 15L-61 Sanidine dacite dike Rio Blanco trail 37 16.40 106 44.52 Bio, san (to 2 cm), plag 66.14 0.58 14.69 3.77 0.08 1.59 3.43 3.62 4.45 0.23 98.58 1.26 98.58 8.07 16 26 8 72 992 99 486 176 13 11 19 23 62 16 39 74 13 29 4 17L-13 Sanidine dacite sill Blanco Basin 37 14.81 106 44.89 Bio, san (to 3 cm), plag 67.18 0.59 15.34 3.89 0.07 1.50 2.74 3.83 3.87 0.23 99.26 0.74 7.70 16 27 6 75 884 96 540 170 14 11 19 20 59 17 40 71 14 31 4

16L-1 Andesite, f.gr. NE-trend (Conejos) Blanco Basin road 37 12.48 106 49.94 - - Altered F.gr. felty 58.45 1.07 16.82 7.41 0.13 2.60 6.36 3.57 2.68 0.43 99.52 4.72 99.52 6.25 5 1 17 149 721 47 573 217 29 11 20 53 89 12 34 73 8 35 3 16L-22 Plag andesite dike, along road (Conejos) Sparks Creek 37 16.18 106 51.44 Plag to 8 mm, cpx no mica 55.93 1.12 16.97 8.40 0.10 3.57 6.52 3.48 2.80 0.38 99.27 2.55 99.27 6.28 26 19 18 188 667 70 682 219 28 13 19 98 91 11 33 75 9 35 3 17L-6 Plag andesite dike (Conejos?) Rito Blanco road 37 17.80 106 48.61 Plag, cpx [oliv?] no mica To 6 mm 56.39 1.09 17.56 8.08 0.15 2.88 6.44 3.37 3.00 0.38 99.35 3.71 6.37 21 15 16 174 862 70 693 207 24 11 19 93 92 11 34 67 9 34 3 16KA-1 Fine-gr andesite, NE-trend (Conejos?) Flattop Mtn, S ridge 37 12.01 106 43.82 Cpx [opx, oliv?] no mica To 2 mm 59.36 0.95 16.00 7.06 0.13 3.04 5.70 3.24 3.46 0.29 99.23 2.48 99.23 3.75 21 25 17 156 744 89 605 226 28 12 19 57 87 14 36 75 11 35 5 16KA-2 Hbl andesite, NE-trend dike (Conejos) Flattop Mtn, S ridge 37 11.67 106 43.74 Hbl, plag no mica F.gr. felty 60.43 0.74 17.99 5.58 0.09 1.84 5.32 3.83 3.03 0.38 99.24 1.49 99.24 3.42 8 10 12 97 831 57 695 178 24 9 19 33 87 12 29 66 5 32 1 AVERAGE: 58.03 0.98 17.13 7.28 0.12 2.83 6.00 3.48 3.08 0.36 99.27 2.56 99.27 4.96 19 17 16 153 776 72 669 207 26 12 19 70 90 12 33 71 8 34 3 to-tectonic events in the Cordilleran USA. As the Mesozoic subduction system silicic than most Dulce dikes. A cluster of Conejos volcanoes is inferred to D1 Dulce NE-trending (E of Pagosa Springs, CO): NW to SE 35 15L-37 Basalt, f.g. dike Mill Creek 37 15.71 106 56.03 - - Gr-br mic F.gr. felty 49.77 1.43 16.47 11.58 0.10 4.29 10.64 2.95 1.70 0.42 99.34 8.97 0.32 99.34 4.65 61 120 27 265 1019 22 851 121 24 8 18 111 96 7 27 61 5 33 4 16L-70 Basalt dike (same as 15L-37) Willow Draw (Mill Creek) 37 16.37 106 55.24 - - Green mic F.gr. felty 48.76 1.43 17.01 10.80 0.10 3.72 12.37 3.13 1.53 0.45 99.30 9.98 99.30 4.66 59 114 29 260 806 17 836 126 25 7 18 110 98 8 32 64 5 32 1 15L-36 Basalt, f.g. dike Mill Creek 37 15.55 106 55.42 - [oliv?] Green mic F.gr. felty 51.06 1.43 16.73 11.36 0.14 5.57 8.14 3.91 0.32 0.42 99.07 10.08 0.16 99.23 4.23 67 155 32 264 546 4 482 118 24 8 17 93 148 5 26 57 3 32 2 16L-21 Basalt dike, along rd (same as 15L-36) Mill Creek 37 16.51 106 54.16 [cpx?] [oliv?] Mica(f.g.) Bladed 50.14 1.40 16.21 12.07 0.14 4.69 10.11 2.87 1.53 0.40 99.56 9.52 99.56 4.40 70 143 31 265 709 19 830 112 21 7 18 96 99 9 30 56 5 31 1 16L-24 Basalt, f.g. dike River Forest Drive 37 13.33 106 59.10 [cpx?] - - F.gr. bladed 50.17 1.40 16.34 11.66 0.17 4.76 10.37 3.80 0.26 0.41 99.33 11.24 99.33 4.06 57 117 29 260 352 4 738 112 23 7 18 92 97 7 25 53 4 29 1 16L-23 Basalt, f.g. dike Skyline Drive 37 13.55 106 58.59 No TX 50.46 1.42 16.70 10.04 0.09 5.45 10.76 3.32 0.66 0.42 99.32 10.09 99.32 3.98 60 123 30 264 546 9 814 116 24 8 19 99 91 8 26 59 4 32 3 17L-1 Basalt, f.g. dike Spruce Canyon 37 14.89 106 56.22 Resorbed qtz F.gr mica F.gr. bladed 49.84 1.39 16.28 10.79 0.17 5.39 10.81 2.74 1.63 0.42 99.44 10.31 99.44 4.37 58 122 29 260 776 20 805 115 24 7 17 95 93 7 27 60 5 32 3 16L-52 Trachybasalt (NE end, 15L-39, in D3?) S of Echo Ditch 37 13.66 106 54.54 - - Br mica Med. gr. 50.15 2.21 15.14 9.46 0.13 4.94 8.22 5.43 0.34 1.32 97.33 9.59 0.20 97.53 5.77 33 65 14 208 10970 8 999 158 18 14 20 59 135 11 63 138 4 74 1 15L-60 Trachybasalt, coarse (same as 15L-38?) Blanco Basin road 37 12.57 106 52.72 Cpx [oliv] Mic/amp Bladed 51.30 1.91 14.36 9.60 0.12 6.42 9.36 3.08 2.01 0.40 98.57 4.44 0.51 98.57 5.09 155 300 24 214 917 34 1002 168 19 20 19 58 116 6 28 62 3 30 2 15L-38 Trachybasalt, coarse (same as 15L-60?) Blue Creek 36 11.21 106 53.80 Cpx [oliv] Br mica Bladed 50.45 1.87 14.55 10.09 0.16 7.70 7.99 3.82 2.03 0.66 99.34 7.26 0.23 99.57 5.86 116 201 21 206 1585 41 610 170 23 20 19 57 103 6 42 89 5 44 2 AVERAGE: 50.22 1.52 16.07 10.89 0.13 5.33 10.06 3.29 1.30 0.45 99.25 9.10 0.20 0.42 99.30 4.59 78 155 28 251 806 19 774 129 23 10 18 90 105 7 29 62 4 33 2 between the North American and eastern Pacific (Farallon) plates flattened have grown within the area now occupied by Platoro caldera, as documented D2A Dulce N-trending (WSW of Pagosa Springs, CO): W to E 45 16L-60 Trachybasalt dike Hwy 160, W of Pagosa 37 14.32 107 9.57 - - F.gr mica F.gr. bladed 47.80 2.10 17.71 11.67 0.16 6.40 7.68 4.27 0.81 0.78 99.38 7.17 0.17 5.08 92 20 18 254 831 16 821 162 20 16 21 73 128 7 38 82 4 45 2 16L-61 Basalt dike, float along ridge E of Summit Trail 37 13.89 107 8.73 - - Br mica Bladed 49.46 2.01 17.60 10.42 0.11 4.57 9.11 4.19 0.48 0.69 98.62 7.94 0.15 99.15 4.67 73 7 16 230 900 9 791 180 20 18 21 65 87 6 41 89 3 47 1 16L-62 Basalt dike (same dike as 16L-60?) Hollow Drive, S of Hwy 160 37 14.14 107 9.40 - - Br mica Bladed 49.24 2.01 17.70 11.46 0.11 5.36 7.89 3.27 0.96 0.71 98.70 5.99 0.17 99.10 4.23 76 9 16 233 883 17 1155 185 20 18 21 67 118 10 46 93 4 43 3 17L-5 Basalt, f.g. dike CO 129 (near Dyke) 37 13.06 107 9.06 - - F.gr mica F.gr. bladed 49.61 1.97 17.40 11.04 0.12 6.23 8.14 3.75 0.48 0.70 99.45 7.07 0.18 4.23 77 8 15 231 595 9 833 176 20 17 22 68 113 5 40 86 2 43 2 16L-64 Basaltic andesite dike N of Burns Canyon 37 9.11 107 3.16 - - F.gr mica F.gr. bladed 53.59 1.64 15.43 10.36 0.11 4.77 9.41 3.03 0.92 0.29 99.55 6.50 3.95 88 169 18 175 482 10 768 108 19 8 19 59 104 4 19 43 3 22 0 16L-65 Basaltic andesite dike (same as 16L-64?) Columbia Court 37 12.30 107 3.20 - - F.gr mica F.gr. bladed 53.03 1.58 14.97 9.95 0.14 5.22 10.61 2.98 0.60 0.28 99.38 8.05 0.19 3.59 88 171 18 162 525 6 693 103 18 8 19 59 100 2 16 39 6 22 1 17L-8 Basaltic andesite dike (same as 16L-65) Columbia Ct, Buttress Ave 37 12.30 107 3.16 - - F.gr mica F.gr. bladed 52.88 1.71 15.28 10.60 0.12 4.35 9.92 3.68 0.40 0.29 99.22 8.20 0.20 0.11 4.08 83 160 19 171 363 4 690 108 19 7 18 57 110 4 20 44 4 23 1 16L-66 trachyandesite dike Taylor Canyon 37 11.41 107 3.22 - - Br mica Bladed 57.58 1.48 15.18 7.30 0.11 4.22 5.58 5.55 1.39 0.45 98.84 6.33 0.21 99.10 6.94 44 126 12 146 380 23 337 190 16 12 20 49 87 8 37 81 4 38 0 16L-67 Trachybasalt dike CO-119; Mouth, Mill Creek 37 14.64 107 0.49 - - Br mica Bladed 49.26 2.09 17.80 10.73 0.15 3.77 9.33 4.02 0.91 0.65 98.71 10.01 0.25 99.10 4.93 78 15 14 202 656 15 1192 157 19 18 19 63 89 8 32 69 4 35 1 AVERAGE: 51.38 1.84 16.57 10.39 0.13 4.99 8.63 3.86 0.77 0.54 99.10 7.47 0.19 99.18 4.60 78 76 16 200 624 12 809 152 19 14 20 62 104 6 32 70 4 35 1 D2B Dulce NE-trending (farther SW, CO): NW to SE during the Late Cretaceous and early Cenozoic (Lipman et al., 1972; Coney and by outward-dipping flanks preserved along caldera margins (Lipman, 1975). 45 16L-68 Trachybasalt, coarse Baver Place 37 12.09 106 56.93 Cpx [oliv] Br mica F.gr. bladed 46.97 1.19 11.49 9.00 0.16 11.24 12.40 2.24 3.05 0.79 98.54 10.67 0.11 98.65 5.29 366 823 26 180 1035 62 719 224 29 10 12 72 78 4 65 146 13 78 4 17L-2 Trachybasalt, f.g. dike (same as 16L-52?) Catchpole Creek pass 37 12.22 106 56.13 - - Mica, amph F.gr. bladed 47.71 2.17 15.93 9.64 0.09 4.19 11.86 4.77 0.30 1.32 97.97 9.24 0.71 98.68 5.06 38 63 15 206 4790 6 1571 172 19 17 22 61 125 10 66 149 6 79 3 15L-39 Trachybasalt (same as 16L-52?) Hwy, 64, N of Turkey Mtn 37 11.38 106 57.06 [cpx?] [oliv] - F.gr. 44.93 2.07 15.56 10.81 0.15 5.97 12.35 3.58 2.65 0.89 98.96 13.38 0.43 0.41 99.39 6.23 102 168 19 205 1242 44 1163 216 22 32 20 50 104 7 46 98 4 54 2 17L-7 Fine-grain trachyandesite dike SW ridge, Serviceberry Mtn 37 9.16 107 56.21 [cpx?] - Br mica F.gr., bladed 57.42 1.61 14.04 9.15 0.09 4.33 8.27 3.32 0.48 0.55 99.26 7.42 0.18 99.44 3.80 162 284 19 182 607 8 749 161 18 14 18 59 96 6 38 74 5 36 2 17L-3A Coarse biotite, dike interior Halfway Canyon 37 8.47 106 55.50 [cpx?] - Br mica Coarse blades 49.12 2.29 14.63 10.27 0.13 6.40 10.43 4.49 0.58 1.05 99.39 9.97 0.23 99.62 5.07 61 145 19 226 910 12 436 192 21 19 19 57 105 7 52 116 5 62 2 17L-11 Fine-grain dike Valle Seco 37 7.38 106 58.27 - - Br mica F.gr., bladed 48.19 1.81 13.96 10.29 0.16 7.88 11.17 2.63 2.19 0.71 99.00 12.09 0.23 99.23 4.82 152 367 22 208 1185 36 904 169 25 14 17 72 133 7 47 104 5 55 3 16L-27 Trachybasalt, f.gr. W of Halfway Canyon 37 7.64 106 57.38 [cpx?] - Mica, amph F.gr., bladed 51.48 2.05 14.13 10.24 0.14 5.42 9.19 3.64 1.65 0.92 98.86 8.03 0.22 99.08 5.28 43 229 16 202 4428 29 1409 197 19 18 19 35 109 8 53 113 3 60 2 17L-9A Coarse biotite, interior (same as 16L-27?) Valle Seco 37 7.31 106 57.77 Br mica Bladed 49.48 2.00 13.74 11.04 0.18 6.57 10.54 2.91 1.55 0.90 98.93 9.17 0.37 99.30 4.46 43 217 16 198 1348 25 1182 187 18 17 18 36 124 7 49 112 3 57 2 17L-9B Fine-grain margin of dike Valle Seco 37 7.31 106 57.77 - - Chloritic Felty 48.76 2.13 15.44 9.97 0.10 6.35 9.76 4.26 0.84 0.95 98.57 8.60 0.49 99.06 5.11 44 221 18 204 2963 16 1414 212 19 20 21 34 110 8 59 123 3 63 3 16L-26 Trachybasalt, f.gr. W of Halfway Canyon 37 7.78 106 56.77 [cpx?] - Altered F.gr. 51.61 1.64 14.25 8.50 0.18 5.50 11.88 3.27 1.64 0.56 99.03 11.61 0.03 99.06 4.91 161 317 19 182 677 18 787 156 17 13 18 53 99 5 38 72 4 35 1 17L-10 Fine-grain dike Valle Seco 37 6.89 106 52.59 - - Br mica F.gr., bladed 47.96 2.23 14.50 9.62 0.15 5.91 11.54 3.06 3.03 0.91 98.91 8.21 0.23 99.14 6.09 78 121 20 222 1869 51 1331 207 22 29 19 47 104 6 42 97 5 51 2 DU-16 Same dike as 17L-10)? Valle Seco 37 6.87 106 57.63 47.51 2.25 15.02 10.50 0.13 5.55 10.07 3.42 2.92 0.99 98.36 9.00 98.36 6.34 70 130 1739 55 1305 260 22 35 52 112 3 58 Reynolds, 1977), crustal compression generated basement-cored north-trending Such an interpretation is consistent with the convergence of newly identified 16L-25 Trachybasalt, f.g. W of Halfway Canyon 37 7.86 106 55.90 [cpx?] ? - F.gr. 49.59 2.06 14.65 10.51 0.17 6.66 9.43 3.25 1.88 0.91 99.11 8.44 0.18 99.29 5.13 73 156 19 217 1355 30 1309 198 21 22 18 54 136 9 47 105 5 59 1 16L-2 Trachybasalt, f.g. W of Spence Reservoir 37 6.64 106 52.30 - - Br mica Bladed 49.35 2.54 13.96 12.26 0.12 5.06 8.89 3.70 1.79 1.20 98.87 8.94 98.87 5.49 36 94 17 240 1740 25 1422 176 20 15 21 46 172 7 53 120 4 67 3 17L-4 Fine-grain dike W of lower Coyote Creek 37 5.42 106 52.76 [cpx?] - Altered F.gr. 50.73 2.53 14.55 12.03 0.11 4.16 7.60 3.82 2.61 1.08 99.23 7.39 0.25 99.48 6.43 33 83 17 249 1512 30 1223 188 20 15 21 43 125 7 56 133 5 69 3 16L-4 SW end, same dike as 16L-2? Along Hwy 64 37 4.72 106 53.06 - - Br mica F.gr. bladed 53.10 2.53 14.65 10.54 0.10 3.75 7.29 5.94 0.22 1.11 99.21 8.17 0.33 99.54 6.16 31 83 17 229 503 4 390 184 20 15 20 42 203 7 59 132 2 71 1 16L-3 Basaltic trachyandesite sill, capping hill W of Spence Reservoir 37 7.66 106 52.11 - - Br mica Bladed 55.24 1.91 15.44 7.71 0.07 4.71 6.90 4.29 1.40 1.08 98.76 6.57 0.16 98.92 5.69 20 68 14 171 1814 21 1337 207 20 17 20 42 132 10 64 138 4 72 2 AVERAGE: 49.95 2.06 14.47 10.12 0.13 5.86 9.97 3.68 1.69 0.94 98.88 9.23 0.28 0.41 99.12 5.38 89 210 18 208 1748 28 1097 194 21 19 19 50 122 7 52 114 5 60 2 D3 Dulce N-trending: S CO (W to E) 55 15L-47 Trachybasalt Lower Gomez Canyon 37 2.15 107 10.60 - - Cpx Bladed 50.53 1.98 17.49 10.87 0.12 5.39 4.93 5.74 0.81 0.70 98.57 3.52 0.27 0.17 98.84 6.55 78 11 15 243 1279 19 1722 186 20 19 21 68 116 7 44 92 3 43 2 15L-48 Vesicular trachybasalt Upper Gomez Canyon 37 4.545 107 8.25 - - Gr-br mic F.gr. bladed 47.68 2.42 17.49 12.67 0.39 4.92 7.69 4.86 0.36 0.79 99.27 6.45 0.41 99.68 5.22 50 0 15 237 510 4 845 183 21 20 20 52 120 8 43 92 3 46 1 16L-63 Basaltic trachyandesite dike Head Burns Canyon 37 9.13 107 7.09 - - Br mica Bladed 53.50 1.85 14.97 8.96 0.11 3.42 8.65 5.84 0.13 0.97 98.39 8.42 0.18 99.18 5.97 57 110 14 169 4251 3 592 209 20 19 19 48 113 4 55 112 3 55 2 15L-46 Mica trachybasalt, f.g. lt tan (oxidized) E of Juanita 37 1.28 107 7.02 - - Br mica Coarse blades 51.75 2.97 14.43 11.52 0.20 3.70 7.66 6.03 0.04 1.27 99.57 5.53 0.11 99.68 6.07 36 86 17 258 188 3 279 213 22 18 20 53 143 10 61 139 3 77 2 15L-45 Amph trachbyasalt coarse crumbly San Juan River 37 3.50 107 0.80 Cpx [oliv] Br amph Med. gr. 47.76 2.06 12.87 10.85 0.32 10.68 9.08 3.29 1.28 0.60 98.78 5.14 98.78 4.57 272 482 25 238 956 25 882 183 20 36 18 73 100 5 37 77 6 40 4 uplifts that initially defined the Southern Rocky Mountains (Tweto, 1975; Cather, Conejos-age dikes toward a locus within the caldera (Fig. 2). Conejos andes- 15L-44 Mica trachbyasalt f.g. (Hail 41?) Archuleta road 37 3.38 107 1.78 - - Mic/amp Med. gr. 50.76 2.07 14.23 10.49 0.15 4.54 9.46 3.74 2.15 0.99 98.57 5.46 0.16 0.21 98.74 5.89 44 141 19 210 2037 26 1770 199 20 18 19 66 115 12 68 136 4 73 3 DU-10 Same location as 15L-44? Archuleta road 37 3.33 107 1.72 50.11 1.99 13.90 10.33 0.14 5.31 8.53 3.36 2.44 0.96 97.06 97.06 5.79 40 160 2263 34 1705 214 23 26 70 149 3 69 1/31/19 PW LIPMAN 2:Dulce-Platoro ms:Dulce-Platoro tables :Suplemental Files:Supple.1 File:Supple F.1 Platoro-Dulce chemistry.xlsx 2004). The eastward migration of compressional tectonics was accompanied by ites in the Platoro area vary from aphyric to highly porphyritic, including both scattered volcanic eruptions and associated intrusions (Mutschler et al., 1987). anhydrous (plagioclase-pyroxene) and hydrous (hornblende) types, but olivine 1 Supplemental Files. Five files of chemical and geo- A renewed flare-up of continental-arc magmatism in the eastern Cordillera andesites have not been recognized. Although basalt is rare among precal- chronological analytical data on which the discussion and interpretation of results are based. File 1. was transgressive in time and space, beginning at ca. 55 Ma in the northern dera Conejos lavas regionally, informative comparisons with Dulce dikes and Bulk-sample XRF chemical analyses and petrographic Rockies, migrating southward, and reaching Colorado at ca. 40 Ma (Lipman, 1980; Hinsdale basalts are provided by olivine-bearing rocks from the 33–32-Ma summaries: Platoro-Dulce dikes, lavas, and associ- McIntosh and Chapin, 2004). Large ignimbrite eruptions were associated with Summer Coon volcano just north of Platoro (Fig. 1; Lipman, 1968; Parker et ated rocks. File 2. Inductively coupled plasma mass intermediate-composition lavas (andesite-dacite) and upper-crustal granitoid al., 2005; Lake and Farmer, 2015). spectrometry (ICP-MS) analyses: Platoro-Dulce dikes, lavas, and associated rocks. File 3. Previously unpub- intrusions, constituting a typical high-K continental-margin arc suite, similar to lished chemical analyses of Dulce dikes in southern concurrent magmatism farther west in Nevada and Utah, and comparable to Colorado and northern New Mexico (E. Landis and the younger Altiplano Volcanic Complex of the Andes (Best et al., 2016). In all Platoro Caldera and Associated Intrusions W. Hail, Jr., ca. 1972, personal commun.). Analyses by U.S. Geological Survey “rapid-rock” analytical of these areas, the ignimbrite flare-ups occurred during destabilization of the methods (Shapiro, 1967). File 4. Leaching of carbonate low-angle subduction geometry, just preceding a transition to regional extension. Platoro caldera (Fig. 3), the most southerly and oldest ignimbrite center in with dilute HCl; effects on bulk-sample compositions. In the Southern Rocky Mountain volcanic field (SRMVF), initial intermediate- the San Juan locus of the SRMVF (Lipman, 1975; Dungan et al., 1989; Lipman A. Dulce Dikes: XRF bulk-sample vs. HCl-leached analyses. B. Dulce dikes: ICP-MS bulk sample versus HCl- composition lavas (Conejos Formation) erupted from clusters of central et al., 1996), is unique in the number of large ignimbrites erupted from a com- leached analyses. File 5. Summary of 40Ar/39Ar age volcanoes, many later becoming loci for ignimbrite eruptions (Lipman et al., mon site: six dacitic tuffs (~75–1000 km3) between 30.3 and 28.8 Ma. Andesitic data, analytical methods, and instrumentation. Please 1978; Lipman and Bachmann, 2015). At least 28 ignimbrites of high-K calc-al- lavas continued to erupt between outflow ignimbrites and ponded within the visit https://doi.org/10.1130/GES02068.S1 or access 3 the full-text article on www.gsapubs.org to view the kaline type with individual volumes of 100–5000 km erupted between 37 and caldera. Shallow plutons of fine-grained equigranular to porphyritic monzon- Supplemental Files. 27 Ma. Most source calderas are on the western flank of the broad crest of ite within and adjacent to Platoro caldera (Lipman, 1975) have crystallization

TABLE 1. REPRESENTATIVE PETROGRAPHIC SMMARIES AND -RAY LORESCENCE CHEMICAL ANALYSES, PLATORO-DLCE DIES, LAVAS, AND ASSOCIATED ROCS Latitude Longitude Maor oides Trace elements Groundmass N W Phenocrysts wt ppm Sample Rock type Deg. Min. Deg. Min. [pseudomorph] LOI Mafics Plagioclase SiO TiO Al O eO MnO MgO CaO Na O O P O Sum Ni Cr Sc V Ba Rb Sr Zr Y Nb Ga Cu Zn Pb La Ce Th Nd ′ ′ 2 2 2 3 2 2 2 5

DLCE DIES

NE trending: CO-N segment D1 15L-36 Basalt, f.gr. dike 3 15.55 106 55.2 [Oliv?] Green mica .gr. felty 51.06 1.3 16.3 11.36 0.1 5.5 8.1 3.91 0.32 0.2 99.0 10.08 6 155 32 26 56 82 118 2 8 1 93 18 5 26 5 3 32 16L-23 Basalt, f.gr. dike 3 13.55 106 58.59 No t 50.6 1.2 16.0 10.0 0.09 5.5 10.6 3.32 0.66 0.2 99.32 10.09 60 123 30 26 56 9 81 116 2 8 19 99 91 8 26 59 32 N trending: CO-N segment D2A 16L-6 Basaltic andesite dike 3 9.11 10 3.16 .gr. mica .gr. bladed 53.59 1.6 15.3 10.36 0.11 . 9.1 3.03 0.92 0.29 99.55 6.50 88 169 18 15 82 10 68 108 19 8 19 59 10 19 3 3 22 16L-6 Trachybasalt dike 3 1.6 10 0.9 Br. mica Bladed 9.26 2.09 1.80 10.3 0.15 3. 9.33 .02 0.91 0.65 98.1 10.01 8 15 1 202 656 15 1192 15 19 18 19 63 89 8 32 69 35 NE trending: CO-S segment D2B 16L-2 Trachybasalt, f.gr. 3 6.6 106 52.30 Br. mica Bladed 9.35 2.5 13.96 12.26 0.12 5.06 8.89 3.0 1.9 1.20 98.8 8.9 36 9 1 20 10 25 122 16 20 15 21 6 12 53 120 6 1L-9A Coarse biotite, interior 3 .31 106 5. Br. mica Bladed 9.8 2.00 13. 11.0 0.18 6.5 10.5 2.91 1.55 0.90 98.93 9.1 3 21 16 198 138 25 1182 18 18 1 18 36 12 9 112 3 5 N trending: CO-S segment D3 15L-3 Mica trachybasalt, f.gr. 3 3.60 106 59.3 Br. mica .gr. bladed 8.29 2.3 16.13 8. 0.12 .3 11.88 3.99 2.26 1.02 98.81 9.3 6 119 21 232 118 0 891 228 22 31 19 9 108 3 92 50 15L- Mica trachybasalt, f.gr. 3 3.38 10 1.8 Mica/amph M.gr. 50.6 2.0 1.23 10.9 0.15 .5 9.6 3. 2.15 0.99 98.5 5.6 11 19 210 203 26 10 199 20 18 19 66 115 12 68 136 3 N trending: NM-N segment D 15L-53 Trachybasalt, f.gr. 36 55.21 10 .58 Cp Acic. amph .gr. bladed 9.88 2.8 1.0 11.25 0.32 .6 .06 3.10 1.8 0.98 98.5 .55 6 162 19 252 18 26 133 190 21 22 21 55 128 8 55 118 62 16L-8 Trachybasalt, Dulce dike north 36 5.86 106 59.0 [Cpx?] [Oliv?] Br. mica Bladed 9.53 1.69 13.65 9.68 0.1 5.95 12.28 2.62 3.12 0. 99.3 9.93 131 390 21 188 1095 9 996 201 22 13 15 9 98 5 3 8 6 N trending: NM-mid segment D5 15L-58 .gr. dike, dark fresh 36 8.8 10 6.20 Interst. amph .gr. felty 8.8 2.66 1.2 10.89 0.2 .3 10.2 .20 1.5 1.00 98.0 .1 56 128 19 26 1390 18 1313 193 23 1 21 51 138 8 106 3 62 16L-13 Trachybasalt, westernmost dike 36 6.8 10 9. [Oliv?] .gr. mica .gr. bladed 9.22 1.88 15.2 9.85 0.30 6.12 9.8 5.39 0.52 0.9 99.38 8.3 10 19 19 192 932 10 823 250 22 26 21 8 102 8 51 108 3 5 N trending: NM-S segment D6 15L-5 Trachybasalt, southern dike 36 33. 10 .0 Acic. amph .gr. bladed 50.33 2.8 1.5 9.62 0.22 5.98 .33 .08 2. 1.15 98.2 3.12 68 151 16 219 1581 0 1639 25 21 30 21 3 133 11 63 10 5 1 16L-11 Trachybasalt, southern dike 36 30. 10 .8 .gr. amph .gr. 9.2 2.5 1.65 10.25 0.20 5.98 8.6 3.6 2.95 1.20 99.25 .9 15 1 21 181 0 18 20 22 28 21 3 135 9 66 13 5 3 DISTAL GRANITOID INTRSIONS 15L-59 Granodiorite f.gr., Jackson Mountain 3 20.33 106 56.3 -feld, t eno. .gr. plag, cp alt., bio 5.88 1.2 15.5 .02 0.11 3.08 6.33 .1 3.00 0.2 99.2 .86 23 38 12 1 153 63 11 191 16 16 20 5 101 11 51 10 9 52 15L-62 Granodiorite f.gr., Blanco Basin 3 15.81 106 5.62 Plag to 2 mm Plag, cp, alt op 61.29 0.65 16.69 5.2 0.1 2.2 5.20 3.9 2.61 0.29 98. 1.1 13 2 13 81 800 52 69 19 25 9 1 20 8 11 31 62 30 16L-5 Porphyritic granodiorite, V Mountain 3 .95 106 .55 Plag, cp to mm .gr., same as 15L-62 5.3 0.9 15.38 8.1 0.13 .56 6.52 3.12 2.8 0.32 99.23 1.6 51 100 21 16 08 0 583 221 2 11 18 83 89 11 0 5 9 3 16L-6 Andesite porphyry, base Archuleta Mesa 36 58.83 106 58.31 Plag, cp to mm .gr. biotite .gr. 5.3 0.95 1.80 6.95 0.12 2.59 6.26 3.1 3.1 0.38 99.29 1.83 1 12 15 13 12 80 1 219 26 12 19 63 82 13 36 2 9 36 PLATORO CALDERA

Proimal granitoid intrusions SRM-22 Granodiorite, Alamosa River pluton 3 23.0 106 32.9 M.gr. plag, bio, cp 62.10 0.0 15.19 6.30 0.28 2.93 5.0 3.29 .38 0.16 100.38 0.80 21 35 15 151 0 109 566 192 25 12 18 198 9 16 31 69 10 33 SRM-23 Resurgent intrusion, porphyritic dacite 3 21.03 106 30.16 Plag, bio phenos. .gr. matri 6.12 0.58 16.28 .03 0.22 1.36 .52 3.93 .0 0.13 99.21 3.33 3 3 0 986 85 53 21 20 13 18 13 66 15 36 66 10 29 SRM-25 Granodiorite, Jasper pluton 3 25.09 106 28.5 M.gr. plag, bio, cp 62.20 0.86 15.2 5.95 0.2 2.62 .11 3.0 .28 0.11 99.53 0.0 22 0 1 125 131 563 296 2 16 19 33 8 1 3 89 1 39 SRM-26 Granodiorite, Cat Creek pluton 3 2.31 106 18.35 .gr. plag, bio, cp 60.28 0.9 16.39 6.6 0.36 2.5 .90 .02 3.32 0.09 99.6 0.1 26 0 13 1 95 81 6 188 22 10 21 0 89 13 31 68 31 SRM-33 .gr. granodiorite porphyry, Elwood Creek 3 2.99 106 2.59 Plag, bio phenos. .gr. matri 62.56 0.5 16.32 5.8 0.11 2.2 .3 .01 3.6 0.31 99.82 2.06 8 10 10 103 1069 80 68 19 22 13 18 19 19 39 9 12 36 Proimal dikes and lavas segment PP 16L-35 Hbl andesite dike, NE trending Coneos m. 3 19.12 106 3.28 Sparse hbl 0.5-1 mm, small plag 58.38 0.3 1.9 .00 0.23 3.11 5.52 3.15 3.2 0.35 99.65 3.9 0 13 96 63 5 52 151 29 8 18 98 10 26 55 6 30 and alt. cp 16L-36 Andesite dike, NNE trending post-Platoro 3 20.3 106 3.8 .gr. interlocking plag; alt. mafics 5.50 1.0 16.98 8.00 0.1 3.9 5.00 3.65 3.3 0.36 99.53 .86 13 1 166 39 6 106 193 25 11 18 9 1 3 1 8 32 16L-0 Andesite lava, Summitville Andesite 3 19.66 106 35.93 Small phenos. of plag and cp .gr. matri 5.82 1.05 16.89 .2 0.09 3.21 5.5 3.56 3. 0.3 99.30 1.36 25 9 16 169 58 86 19 222 25 1 20 62 8 1 30 10 36 Distal radial dikes segment PD 15L-61 Sanidine dacite dike 3 16.0 106 .52 Bio, sanidine to 2 cm, plag 66.1 0.58 1.69 3. 0.08 1.59 3.3 3.62 .5 0.23 98.58 1.26 16 26 8 2 992 99 86 16 13 11 19 23 62 16 39 13 29 16L-1 Trachyandesite, f.gr. NE trending Coneos m. 3 12.8 106 9.9 Alt. .gr. felty 58.5 1.0 16.82 .1 0.13 2.60 6.36 3.5 2.68 0.3 99.52 .2 5 1 1 19 21 53 21 29 11 20 53 89 12 3 3 8 35 16L-22 Plag andesite dike, along road Coneos m. 3 16.18 106 51. Plag to 8 mm, cp .gr. matri 55.93 1.12 16.9 8.0 0.10 3.5 6.52 3.8 2.80 0.38 99.2 2.55 26 19 18 188 66 0 682 219 28 13 19 98 91 11 33 5 9 35 16A-2 Hbl andesite, NE trending dike Coneos m. 3 11.6 106 3. Hbl, plag .gr. felty 60.3 0. 1.99 5.58 0.09 1.8 5.32 3.83 3.03 0.38 99.2 1.9 8 10 12 9 831 5 695 18 2 9 19 33 8 12 29 66 5 32 Notes: Complete data set is in Supplemental ile 1 see tet footnote 1. Analyses were performed at Washington State niversity Geoanalytical Lab Pullman, Washington, SA in 2015–201 by Scott Boroughs. Maor oides are normalied, volatile free, to reported sums. Dash indicates that data is not present. Abbreviations: acic.acicular; alt.altered; amphamphibole; biobiotite; br.brown; COColorado; cpclinopyroene; f.gr.fine grained; m.ormation; hblhornblende; interst.interstitial; -feld-feldspar; LOIloss on ignition; m.gr.medium grained; NMNew Meico; olivolivine; oporthopyroene; phenos.phenocrysts; plagplagioclase; t uart; tthin section; eno.enocryst. Two columns for Dulce dikes cp, oliv.

TABLE 2. SMMARY O AGE DETERMINATIONS, PLATORO-DLCE DIES, LAVAS, AND ASSOCIATED ROCS Latitude N Longitude W Age calculation MSWD Age Sample nit and location Mineral 2 Comments and/or data source Deg. Min. Deg. Min. method n Ma σ ′ ′

DLCE DIES

NE trending, CO-N segment D1 15L-3 Dense fine-grained dike: N of Mill Creek 3 15.1 106 56.03 Groundmass orced plateau 15.88 5 25.5 0.0 Barely missed plateau criteria 15L-36 Dense fine-grained micaceous: Mill Creek 3 15.55 106 55.2 Groundmass Inverse isochron 1.2 8 Barely contains ecess Ar 0Ar/36Ar 29.3 0.8 16L-23 Dense fine-grained dike: Skyline Drive 3 13.55 106 58.59 Groundmass orced plateau 13.20 5 15L-60 Coarsely crystalline plagioclase-mica: Blanco Basin Road 3 12.5 106 52.2 Groundmass Inverse isochron 5.93 25.85 0.05 1L-2 Dense fine-grained dike: Catchpole Creek pass 3 12.22 106 56.13 Groundmass Integrated age 25.38 0.52 Low radiogenic yield. Segment D3 compositional type N- and NW-trending trachybasalts, CO-N segment D2A 16L-60 resh dense fine-grained dike: .S. Highway 160, roadcut 3 1.32 10 9.5 Groundmass Inverse isochron 3.55 16L-62 resh dense fine-grained dike: Hollow Drive 3 1.1 10 9.0 Groundmass Inverse isochron 11.8 9 Dated with Argus VI mass spectrometer 16L-62 resh dense fine-grained dike: Hollow Drive 3 1.1 10 9.0 Groundmass Inverse isochron 8.02 5 Replicate analyses with Heli MC Plus; indistinguishable from Argus VI age 16L-66 Relatively coarsely crystallied: Taylor Canyon 3 11.1 10 3.22 Groundmass No age determined 16L-6 resh dense fine-grained: CO Highway 119, mouth of Mill Creek 3 1.6 10 0.9 Groundmass Inverse isochron 19.30 6 Tetbook outcrop, ust south of Pagosa Springs 1L-5 Dense fine-grained dike: CO Highway 129 near Dyke 3 13.06 10 9.06 Groundmass No age determined 1L-8 Dense fine-grained dike: Columbia Court, Buttress Avenue 3 12.30 10 3.16 Groundmass Inverse isochron 26.91 8 2.55 0.29 -shaped spectrum typical of ecess Ar, but poor fit on isochron 16L-6 Trachybasalt dike: N of Burns Canyon 3 9.11 10 3.16 Groundmass Plateau 1.22 Age more like segment D1 dikes NE-trending trachybasalts, CO-S segment D2B 1L-9A Coarse dike interior: Valle Seco 3 .31 106 5. Groundmass orced plateau 6.1 Adacent dike may be same dike as sample PS16 Gonales, 2015 PS16 Mafic dike: S of Rio Blanco 3 6.8 106 5.63 Mica Plateau 9.1 9 Gonales 2015 1L-3A Coarse-mica dike: Halfway Canyon 3 8. 106 55.50 Mica Plateau 0.61 Discordance in the low-temperature steps. 1L-3A Coarse-mica dike: Halfway Canyon 3 8. 10 55.50 Groundmass Inverse isochron 11.62 26.63 0.22 Classic -shaped spectrum indicative of ecess Ar 16L-2 Easternmost dike: W of Spence Reservoir 3 12.22 106 56.13 Groundmass Plateau 1.8 5 N-trending trachybasalts, CO-S segment D3 15L-0A Relatively coarse: Archuleta Road 3 3.6 106 56.98 Groundmass Inverse isochron 1.0 5 15L-5 Western dike, coarse: San Juan River 3 3.50 10 0.80 Amphibole Plateau 0.6 25.9 0.38 Low radiogenic yields PS10 Mafic dike: Archuleta Road 3 3.33 10 1.2 Groundmass Plateau 0.52 3 Gonales 2015 15L- Crystallied trachybasalt dike: lower Gome Canyon 3 2.15 106 10.60 Groundmass No age determined. Etreme Ar loss 15L-8 Dark trachybasalt dike: upper Gome Canyon 3 .5 106 8.25 Groundmass Plateau 0.55 5 Low radiogenic yields, but plateau included most gas released 16L-63 Trachybasalt dike: head Burns Canyon 3 9.13 106 .09 Groundmass Integrated age 21. 1.30 Very discordant spectrum, but likely young N-trending trachybasalts, NM-N segment D 15L-55 ine-grained dike: head of Dike Canyon 36 55.62 10 12.80 Groundmass Inverse isochron .35 8 25. 0.6 Low radiogenic yields 60; low confidence in age 15L-56 Coarse-mica dike: Chimissosa Canyon 36 55.69 10 09.06 Mica Inverse isochron .00 10 16L-8 Coarse-mica dike: Dulce, north end of dike 36 5.86 106 59.0 Groundmass orced plateau 18.51 3 15L-50 Easternmost dike: Lumberton 36 55.90 106 56.0 Groundmass orced plateau 59.9 3 25.5 0.29 N-trending trachybasalts, NM-mid and NM-S segments D5–D6 16L-11 Dense fine-grained, southernmost: S of NM Highway 53 36 30. 10 0.8 Groundmass orced plateau 12.91 9 Southernmost sampled dike 15L-58 ine-grained dike, dark fresh: .S. Highway 6, Burns Canyon 36 8.8 10 06.20 Groundmass orced plateau 16.59 5 25.56 0.18 16L-9B Coarse-mica dike: .S. Highway 6, S of Dulce 36 9.91 10 01.52 Mica orced plateau 6.32 2

DISTAL SOTHWESTERN GRANITOID INTRSIONS ARM1 Diorite porphyry: Archuleta Mesa sill 3 0.565 10 0.9 Zircon -Pb Gonales 2015 JM1 uart mononite: Jackson Mountain 3 20.33 106 56.50 Zircon -Pb Gonales 2015 JM2 uart mononite: Jackson Mountain 3 21. 106 5.52 Zircon -Pb Gonales 2015

PLATORO CALDERA SYSTEM

Distal postcaldera intrusions segment PD 15L-33 Dacite, northwest: Silver Creek 3 25.53 106 5.2 Sanidine Inverse Isochron 1.19 15L-61 Dacite, west: Rio Blanco trail 3 16.0 106 .52 Sanidine SCL wt. mean 2.11 11 15L-61 Dacite, west: Rio Blanco trail 3 16.0 106 .52 Biotite Inverse isochron 3.6 10 26.8 0.03 Slightly older age compared to sanidine, maybe slow cooling of large dike 16L-5 Dacite, east: Oito Creek 3 13.26 106 16.16 Sanidine Wt. mean of plat. 0. 3 Eight single crystals step-heated. Weighted mean of three plateaus 16L-69 Dacite, northwest 3 2.38 106 6.08 Sanidine Wt. mean of plat. 3. 3 Eight single-crystals step-heated. Weighted mean of three plateaus 1L-13 Thick dacite sill, southwest: Blanco Basin 3 1.81 106 .89 Sanidine Wt. mean of plat. 1.18 6 ifteen single-crystals step-heated. Weighted mean of si plateaus 1L-13 Thick dacite sill, southwest: Blanco Basin 3 1.81 106 .89 Biotite Plateau 1.00 6 Very flat spectrum 16L-50 Distal plagioclase-andesite dike: East ork San Juan River 3 2.89 106 5.19 Groundmass Plateau 3.2 3 Proimal postcaldera dikes and porphyritic intrusions PP 11L-22 Dacite dike, South Mountain type: W slope, South Mountain 3 25.6 10 36.96 Sanidine SCL wt. mean 6.02 13 Prior -Ar age on the South Mountain dome is 23 Ma Mehnert et al., 193. Some scatter in individual ages 11L-19 Dacite of North Mountain: Park Creek road 3 26.1 106 38.2 Sanidine SCL wt. mean 9.20 1 Some scatter in individual ages 11L-23 Sanidine dacite: Schinel Meadow 3 23.88 106 38.9 Sanidine Inverse isochron 1.18 16 Large dike ig. 10C; same dike as sample -132 -132 Sanidine dacite: Schinel Meadow 3 23.88 106 38.9 Sanidine Mean Lipman et al. 1996 16L-36 Dense dark trachyandesite: Rito Gato 3 20.3 106 3.8 Groundmass Inverse isochron 10.92 6 Dense aphyric dike, intrudes intracaldera ignimbrite and sediment 16L-38 Dense dark trachyandesite: Platoro Reservoir inlet 3 19.9 106 3.6 Groundmass orced plateau 20.12 6 Dense aphyric dike, intrudes intracaldera ignimbrite and sediment continued

TABLE 2. SMMARY O AGE DETERMINATIONS, PLATORO-DLCE DIES, LAVAS, AND ASSOCIATED ROCS continued Latitude N Longitude W Age calculation MSWD Age Sample nit and location Mineral 2 Comments and/or data source Deg. Min. Deg. Min. method n Ma σ ′ ′

Granitoid intrusions on proimal flanks of the caldera SRM-33 Porphyritic granodiorite: Elwood Creek 3 2.99 106 2.59 Biotite Inverse Isochron 8.6 8 Satellitic dike SRM-33 Porphyritic granodiorite: Elwood Creek 3 2.99 106 2.59 Zircon -Pb SHRIMP age by Gilmer et al. 2018 SRM-26 Granodiorite: Cat Creek pluton 3 2.31 106 18.35 Zircon -Pb SHRIMP age by Gilmer et al. 2018 Granitoid intrusions within the caldera DC-2 Lookout Mountain Porphyry: Alum Creek (drill core) ~37 23.3 ~106 33.3 Hornblende K-Ar — 26.6 ± 1.0 Drill core (3393–3398 ft), Inspiration Development Company; unpublished age by H. Mehnert, 1975 DC-2 Lookout Mountain Porphyry: Alum Creek (drill core) ~37 23.4 ~106 33.4 K-feldspar K-Ar 23.4 ± 0.6 Drill core (3393–3398 ft), Inspiration Development Company; unpublished age by H. Mehnert, 1976 11L-25 Alamosa River pluton: mouth, Bitter Creek 3 23.0 106 33.06 Biotite Inverse isochron 2.1 10 67L-113 Alamosa River pluton: W slope, Telluride Mountain ~37 23.1 ~106 32.8 Biotite K-Ar 29.10 ± 1.20 Lipman et al. (1970) SRM-22 Alamosa River pluton:W slope, Telluride Mountain 3 23.0 106 32.9 Zircon -Pb SHRIMP age by Gilmer et al. 2018 SRM-25 Granodiorite: Jasper pluton 3 25.09 106 28.5 Zircon -Pb SHRIMP age by Gilmer et al. 2018 SRM-23 Resurgent granodiorite: Cornwall Mountain 3 21.03 106 30.16 Zircon -Pb SHRIMP age by Gilmer et al. 2018 Postcaldera lavas Ds29c Rhyolite of Cropsy Mountain: Cropsy Mountain ~37 24.75 ~106 35.95 Sanidine K-Ar 19.5 ± 0.8 Steven et al. (1967), Lipman et al. (1970) Ds29c Rhyolite of Cropsy Mountain: Cropsy Mountain ~37 24.75 ~106 35.95 Biotite K-Ar 20.6 ± 0.8 Steven et al. (1967), Lipman et al. (1970) Ds29c Rhyolite of Cropsy Mountain: Cropsy Mountain ~37 24.75 ~106 35.95 Hornblende K-Ar 20.6 ± 0.8 Steven et al. (1967), Lipman et al. (1970) 11L-20 Rhyolite lava, or shallow intrusion or dome: Grayback Mountain 3 2.6 106 33.2 Sanidine SCL wt. mean 2.0 10 Single population 65L-161A Rhyolite lava (Hinsdale Formation): Beaver Creek 37 30.8 106 38.1 Obsidian K-Ar 22.2 ± 0.9 Lipman et al. (1970) 71L-49 Alteration alunite: South Mountain ~37 25.2 ~106 35.7 Alunite K-Ar 22.7 ± 0.5 Mehnert et al. (1973) 70L-134 Dacite of South Mountain: South Mountain ~37 24.5 ~106 35.2 Sanidine K-Ar 23.1 ± 0.6 Mehnert et al. (1973) 66L-101B Dacite of Cat Creek volcano, upper: Green Ridge ~37 24 ~106 15 Biotite K-Ar 29.1 ± 1.1 Lipman et al. (1970) 67L-109 Dacite of Cat Creek volcano, lower: Green Ridge ~37 28 ~106 13 Biotite K-Ar 28.9 ± 1.2 Lipman et al. (1970) -58 Intracaldera dacite of isher Gulch 3 20.3 106 28.8 Sanidine Mean Lipman et al. 1996. Lowest caldera-fill lava Ignimbrites of the Treasure Mountain Group MD-8a Chiuito Peak Tuff, intracaldera: Platoro Reservoir 3 19.11 106 35.6 Sanidine Mean 28.81 0.0 Lipman et al. 1996 11L-2 Chiuito Peak Tuff, outflow: The Canyon 3 23.1 106 15.55 Sanidine SCL wt. mean 2.5 28. 0.03 MD-192 Chiuito Peak Tuff, outflow: N of Bishop Rock 3 29.8 106 16.9 Sanidine Mean 28. 0.0 Lipman et al. 1996 05L-33 South ork Tuff: Alder Creek 3 0.88 106 3.56 Sanidine SCL wt. mean 3.13 28.86 0.1 Alternatively, local upper one of Ra Jadero Tuff MD-16 South ork Tuff: E side, Bennett Peak 3 29.2 106 2.6 Sanidine Mean 28.92 0.0 Lipman et al. 1996 11L-26 Ra Jadero Tuff: E of Terrace Reservoir 3 22.1 106 16.21 Sanidine SCL wt. mean 2.9 13 29.12 0.0 MD-165 Ra Jadero Tuff: E side, Bennett Peak 3 2.6 106 29.2 Sanidine Mean 29.12 0.0 Lipman et al. 1996 MD-3 Ra Jadero Tuff: Dog Mountain 3 36. 106 21.2 Sanidine Mean 29.15 0.0 Lipman et al. 1996 MTM Middle Tuff, detrital tuff bed: Alamosa Creek 3 21.1 106 15.9 Sanidine SCL wt. mean 15.56 29.93 0.03 Collected by M. Dungan and M. Myers, 2013 11L-B Black Mountain Tuff, upper vitrophyre: Rock Creek 3 28.50 106 18.9 Hornblende Plateau 0.91 6 30.19 0.16

CONEJOS ORMATION, SOTHEASTERN SAN JAN MONTAINS REGION

Dikes radiating from the Platoro locus 16L-22 Plagioclase-phyric dike, in Cretaceous: Sparks Creek 3 16.18 106 51. Groundmass Inverse isochron 1.58 9 Syn-caldera Ra Jadero eruption or Coneos dike with Ar loss 16L-1 Distal andesite, intrudes Cretaceous: Blanco Basin Road 3 12.8 106 9.9 Groundmass Plateau 6.93 Dense, fine grained, Platoro-like composition 16L-32 Andesite dike: Platoro Reservoir 3 1.96 106 36.3 Hornblende Inverse isochron 3.99 8 16L-32 Andesite dike: Platoro Reservoir 38 1.96 10 36.3 Biotite Inverse isochron 3.88 5 31.31 0.08 Scattered data, but clearly Coneos age 16L-35 Andesite dike: Adams ork 3 19.12 106 3.28 Hornblende Inverse isochron 69.86 5 30.99 0.09 Scattered data, but clearly Coneos age 16L-51 Distal hornblende andesite, SW of Platoro: ish Creek 3 13.02 106 2.29 Hornblende Inverse isochron 6.20 8 30.2 0.06 Part of radial dike swarm 16A-02 Distal hornblende andesite, SW of Platoro: lattop Mountain 3 11.6 106 3. Hornblende Inverse isochron 19.93 10 Part of radial dike swarm 16L-55 Distal dacite dike, SE of Platoro: Jim Creek 3 15.9 106 2.90 Sanidine Wt. mean of plat. 5.99 3 Eight single crystals step-heated. Weighted mean of three plateaus Lava and volcaniclastic units 15L-31 Cobble, conglomerate: S of Wolf Creek Pass, .S. Highway 160 3 28.08 106 52.51 Plagioclase Inverse isochron 3.35 9 Discordant spectra; isochron within error of atmosphere 15L-32 Basal dacitic breccia: Treasure alls, .S. Highway 160 3 26.63 106 52.66 Biotite Inverse isochron 6.20 Agrees with hornblende isochron age 15L-32 Basal dacitic breccia: Treasure alls, .S. Highway 160 3 26.63 106 52.66 Hornblende Inverse isochron 0.81 8 Agrees with biotite isochron age Notes: Ages in italics are early-determined -Ar ages 1960s–190s. Segment locations shown on igure 2. Abbreviations: C.D.Continental Divide; COColorado; NMNew Meico; SHRIMPsensitive high-resolution ion microprobe. Groundmassgroundmass concentrate phenocrysts were removed prior to dating. SCL wt. meanweighted mean of single-crystal laser-fusion analyses. Wt. mean of platstep-heated single crystals; weighted mean of those that yielded plateaus. Plateauages calculated using the plateau criteria of leck et al. 19. orced plateauweighted mean of incremental step-heating data that did not meet the criteria of leck et al. 19. Inverse isochronage calculated not assuming that the trapped 0Ar/36Ar component is atmospheric; used primarily for samples with ecess 0Ar. MSWDmean suare weighted deviates; n refers to the number of analyses e.g., single-crystal laser-fusion analyses, steps in the plateau or steps in the step-heat analyses used on the isochron to calculate the final age. MSWD and n are reported only for new ages in this study. New and previously determined 0Ar/39Ar ages calculated relative to neutron flu monitor C-2 eual to 28.201 Ma uiper et al., 2008. Two-sigma 2σ errors for new 0Ar/39Ar ages include analytical errors, interfering reactions, and J uncertainties. New 0Ar/39Ar ages in bold are interpreted to be high-uality ages and include those with MSWD values indicative of a single population, samples where different phases yield indistinguishable ages, new ages that are indistinguishable from published ages, or ages with elevated MSWD values but the ages are relatively insensitive to which individual data points i.e., steps or single-crystal ages are used in the age calculation.

106°40 ' W 106°20 ' W P

a 3100 r k 3700 3100 37°30' C N re 3400 ek 3700 3100 th Fork Rock Creek 3100 3400 or Bennett N 3400 2800 Peak k e re 3100 C k c o 3400 rk R 3700 Ja South Fo Green 3100 Ridge 3400 Summitville 3400 Silver Mtn

3100 3100

3700 Jasper 3400

3700 ? 3100 Cat t

fault l

Alamosa River u Cr a

f 2800 RESURGENTBLOCK

3700

Cornwall h 2800 BC ? c l A R Cornwall u 3400 G 3400 Mtn 3100 2800 r 3700 e 3100 3100 sh Fi 3400 Summit Pk Red Mtn 3700 Platoro x 3100 3400 3400 37°20' 3400 N 3400 3100 x 3100 3400 Willow Mtn 3700 3100 er v 3400 i R 3700 s 3100 o j 3400 0 5 km e Conejos Pk 3700 n 3700 o C 3700

MAP UNITS CALDERA MARGINS Miocene basalt and rhyolite flows Long dashed where approximately located, short dashed where concealed Late caldera lavas Tuffs and lavas of the central caldera cluster Granitoid intrusion Topographic wall, Chiquito Peak caldera Summitville Andesite, upper member Chiquito Peak Tuff Ojito Creek, Ra Jadero, South Fork Tuffs Summitville Andesite, lower member Inferred ring fault, Summitville caldera La Jara Canyon Tuff Conejos Formation, tuff of Rock Creek Conejos Formation Topographic wall, La Jara Canyon caldera remnant

Figure 3. Generalized geologic map of the Platoro caldera complex, showing preserved remnants of topographic walls related to eruption of the La Jara Canyon and Chiquito Peak Tuffs, caldera-filling lavas, and major granitoid plutons (AR—Alamosa River; BC—Bear Creek; Cat Cr—Cat Creek; Ja—Jasper). Surficial deposits and most faults omitted. Topography contour interval, 300 m. Modified from Lipman et al. (1996).

ages close to that of the culminating ignimbrite eruption (Chiquito Peak Tuff), Rhyolite of Cropsy Mountain, filling of the caldera by ponded lavas, and intrusion-driven resurgent uplift of Dacite of South Mountain, post-alteration (20 Ma) a triangular hinged block of caldera floor (Table 2; Gilmer et al., 2018). Dikes Summitville mine (23 Ma) that radiate westward from Platoro caldera (Fig. 2) vary from aphyric andesites Altered caldera- ll andesite, along north margin of Alamosa River pluton that appear similar to the caldera-filling lavas, to coarsely porphyritic silicic (ca. 29 Ma) dacites. The most distal andesitic dikes with Platoro affinities extend south- westward beyond the preserved volcanic cover and merge in areal extent with trachybasaltic dikes of the Dulce swarm (Fig. 2). Platoro caldera is unusual in its record of prolonged postcollapse volcanic and intrusive activity, intermittently from 28.8 to ca. 20 Ma and ranging from andesitic to silicic rhyolitic (Fig. 4; Table 2). In contrast, eruptive activity waned within a few hundred thousand years after ignimbrite eruptions at most other caldera systems in the SRMVF (Lipman, 2007). The postcollapse magmatic history at Platoro (Fig. 5) overlaps in time with the regional transition to extension along the northern Rio Grande rift at ca. 26 Ma (Lipman et al., 1970; Thompson et al., 1991; Gibson et al., 1993). The oldest cooling (uplift) ages along the rift at ca. 25 Ma (Ricketts et al., 2016) coincide with that of a broad magmatic flare-up along the Colorado–New Mexico border (i.e., Navajo volcanic field, Dulce dikes, Figure 4. Postcollapse lavas and intrusions within Platoro caldera. View is to the north, across the Questa caldera, Spanish Peaks; Gonzales, 2015; Zimmerer and McIntosh, 2012a; margin of the 29 Ma Alamosa River pluton toward highly altered caldera-filling lavas (Summitville Penn and Lindsey, 1996). Concurrently, the petrologic assemblage in the San Andesite), overlain by unaltered 20 Ma silicic lava (rhyolite of Cropsy Mountain; Lipman, 1975). Juan region became broadly bimodal (Hinsdale Formation), including trachy- Hill caped by this lava is Lookout Mountain (3795 m). On the skyline in the distance is the 23 Ma basalt lavas and high-silica rhyolites. Space-time-compositional variations dacite lava dome of South Mountain, host to Au-Cu mineralization at the Summitville mining district (photograph by P. Lipman, 2016). among dikes of the Platoro-Dulce system provide additional perspectives on the regional transition from continental-arc to extensional magmatism. anhydrous mafic phenocrysts (olivine, clinopyroxene). Mica and amphibole are present in many Dulce samples, but only in late-crystallized groundmass. Dulce Dike Swarm

The Dulce dikes are defined here as the arcuate linear swarm of trachybasalt Early-Rift Lavas and basaltic trachyandesite dikes that intrude Mesozoic sedimentary strata along the east margin of the Colorado Plateau for ~100 km from southern Important rocks for comparison with Dulce dikes are trachybasaltic to Colorado into northern New Mexico (Fig. 2). Some Dulce dikes, though only trachyandesitic lavas of the Hinsdale Formation that spread widely in the south- 1–2 m thick, are traceable for 20 km, and their regional extent was recognized eastern San Juan region and adjacent Rio Grande rift (Larsen and Cross, 1956; at least as early as the mid–20th century (Dane, 1948; Wood et al. 1948). The Lipman and Mehnert, 1975). Recent data suggest pulses at 26–25 Ma and at Dulce swarm is diffuse, as much as 25 km wide near the Colorado–New Mex- 21–19 Ma (Turner et al., 2019). The earliest basalts, including subalkaline olivine ico state line where as many as 20 dikes have been depicted along east-west tholeiites, probably came from vents within the present-day Rio Grande rift transects. The Dulce dikes form linear rather than radial trends, are thinner (Thompson et al., 1991), but no basalt appears to have erupted within or proxi- but more laterally continuous than the Platoro dikes, and are intrusive into mal to Platoro caldera before ca. 21 Ma. Like the Dulce dikes, many of these lavas Cretaceous strata along the eastern margin of the Colorado Plateau. contain olivine, but they lack hydrous groundmass minerals. They constitute a Prior to our work, dikes of the Dulce swarm had been considered distinct proximal rift-related magmatic suite, interpreted as counterparts in composition in composition, age, geometry, and geologic setting from the Platoro cal- and age to the more distal Dulce dikes across the Continental Divide. dera locus. Published regional studies interpreted Dulce dikes as a high-K lamprophyric suite, including minette, vogesite, and kersantite characterized by phenocrysts of amphibole and phlogopite (Gibson et al., 1993; Gonzales ■■ SAMPLING AND ANALYTICAL STRATEGIES and Lake, 2017), but our petrographic and chemical data (Supplemental File 1 [footnote 1]) show that although the Dulce dikes are more mafic than cal- Sampling of the Platoro-Dulce dike system was designed to test for age dera-related San Juan rocks, they are not highly potassic and contain only and compositional variations longitudinally with distance from the caldera,

as well as along lateral transects (Supplemental Files 1–5 [footnote 1]). No regional-scale remapping of the lengthy Dulce swarm was attempted; focus was on representative sampling, guided by regional maps (Dane, 1948; Wood et al., 1948; Steven et al., 1974). Many dikes depicted on these maps were read- Postcollapse magmatism D2 dikes ily located, but some sizable ones were not found. We also checked several mapped round or elliptical sites, hoping to locate vents. One site (sample 16L-3; Lavas 37°7.66′N, 106°52.11′W) is a hill-capping sill of Dulce-type basaltic trachyandes- 20 Andesite ite; another hill-forming site (37°18.10′N, 106°53.45′W) is an erosional outlier of Dacite Rhyolite older Conejos andesitic lava and breccia. One possible vent root is a wide and (WEST)

coarsely crystalized exposure of trachyandesite (~10 m, 57.6% SiO2) in Taylor Dulce dikes Canyon (sample 16L-48; 37°11.41′N, 106°3.22′W) within an 8-km-long dike that (trachybasalt, San Juan Basin) is elsewhere only 1–2 m wide and lower in SiO2 (~53%). Distal intrusions: Granodiorite Dacite

Rock Chemistry 25 JM+ New X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS) analyses for the dikes (Table 1; Supplemental Files 1–2 Large-vol. (EAST) [footnote 1]; 105 total), along with prior data, document sizable variations that caldera- ll + Hinsdale basalt

AGE (Ma) AGE lavas (SE San Juan Mtns; are inferred to record tectonic setting and processes of magma generation. + NE dikes CP inception, RG rift) Evaluation of primary composition is problematic, however, especially for RJ (D1) Dulce dikes. These commonly contain deuterically altered mafic minerals, SF ++ OC Proximal calcite in vesicles and replacing groundmass, erratic alkali ratios, and high 30 LJ intrusions: volatile contents compared to lavas. Values for loss on ignition (LOI) are BM Granodiorite typically >5%, some >10%. Previously unpublished analyses obtained by Large ignimbrites Andesite (Treasure Mtn Group) Dacite Rhyolite others in the 1960s have values of CO2 as high as 7.8% (Supplemental Files 3–4). Accordingly, all analyses have been calculated volatile free to facilitate PLATORO CALDERA comparisons. (SE San Juan) Comparisons with samples collected from the same apparent dike site during prior studies agree fairly well (Supplemental File 1 [footnote 1]), espe- Ar/Ar age, all colors (this study) cially in light of diverse methods at different labs and ambiguity about some 35 K/Ar age (multiple sources) locations. Agreement is also good for most samples along strike of a single Precursor volcanism + U-Pb zircon age (Gonzales, 2015; mapped dike, although contrasts between paired samples (15L-39, 16L-52) from (Conejos Fm) Gilmer et al., 2018) one NE-trending dike, depicted as continuous for 6 km, suggest presence of Stratigraphic control a composite dike or two closely aligned ones. Comparisons between fine- GENERAL STRATIGRAPHIC SEQUENCE (younger) grained margin and coarser interior of dikes were inconsistent. The margin and interior of one dike (samples 15L-40A, 15L-40B) agree closely. In contrast, Figure 5. Extrusive and intrusive magmatic history of the Platoro caldera complex, Dulce dikes, similarly paired samples from another (16L-9A, 16L-9B) differ substantially in and basaltic lavas of the Rio Grande rift, based on new 40Ar/39Ar ages (square symbols with color fills) and ages from published sources cited in text. Data are from Table 2 (only higher-precision major oxides, probably due to abundant calcite, high LOI values, and other ages plotted, as listed in bold font). For the Dulce swarm, proximal dikes in segments D1 and alteration; trace elements agree more closely. D2 are plotted as light-gray squares; segments D3–D6 are solid black (see Fig. 2 for delineation of dike-swarm segments). For the Conejos Formation, dikes are solid-blue boxes, lavas are light blue. Proximal and distal Platoro intrusions correspond to areas delimited on Figure 2. Analytical uncertainty for K-Ar ages is indicated by a vertical line; uncertainties for 40Ar/39Ar and most U-Pb Age Determinations ages are smaller than the symbol size. Abbreviations: BM—Black Mountain Tuff; CP—Chiquito Peak Tuff; Fm—Formation; JM—Jackson Mountain pluton; LJ—La Jara Canyon Tuff; Mtns—Mountains; New 40Ar/39Ar ages for Platoro-Dulce dikes and related rocks of Platoro cal- OC—Ojito Creek Tuff; RJ—Ra Jadero Tuff; RG—Rio Grande rift; SF—South Fork Tuff; vol.—volume. dera (58 total; Table 2; Supplemental File 5 [footnote 1]) were determined at the New Mexico Geochronology Research Laboratory (Socorro, New Mexico)

by methods similar to our prior work in the SRMVF (e.g., Lipman et al., 2015). more typical of neighboring ones. Additionally, some lengthy dikes cross The 40Ar/39Ar ages are calculated relative to the 28.201 Ma FC-2 interlaboratory segment boundaries, and assignment becomes arbitrary. standard (Kuiper et al., 2008), in part because this monitor age seems better intercalibrated with U-Pb zircon dating (e.g., Wotzlaw et al., 2013). Table 2 sum- marizes ages for the Platoro-Dulce dikes and related rocks, and indicates the ■■ DIKES AND INTRUSIONS RADIAL TO PLATORO CALDERA highest-quality ages based on Ar systematics. Two-sigma (2σ) uncertainties are reported for individual ages in the text and tables, whereas generalized ages for Numerous dikes that range in composition from andesite to silicic dacite groups of samples are listed without uncertainties for simplicity. Mineral and and rare rhyolite radiate from several of the granitoid plutons in the vicinity groundmass preparation techniques, full analytical methods, data tables, and of Platoro caldera (Fig. 2). Especially conspicuous are the dikes that radiate plots (ideograms, spectra, and inverse isochrons) are in Supplemental File 5. northwest to southwest for 20–25 km from a locus roughly coincident with the Accuracy and reproducibility of the 40Ar/39Ar ages were tested by dating Alamosa River pluton, the largest intrusion at Platoro (Fig. 3). Some additional mineral pairs in the same sample when available (e.g., sample 15L-32, biotite dikes (not discussed here) are crudely radial to the Cat Creek pluton, just east of and hornblende), by comparing the new 40Ar/39Ar ages to published ages for Platoro caldera. The distribution of these dikes was delineated in conjunction the same site (e.g., sample 11L-27 versus MD-8a), or dating the same sample with regional mapping (Steven et al., 1974) and associated study of Platoro using different mass spectrometers (e.g., sample 16L-62). In most tests, the caldera (Lipman, 1974, 1975). More than 100 dikes, including andesite and dac- compared ages are indistinguishable at 2σ uncertainties. Despite the large ite with diverse phenocryst assemblages, were depicted at a scale of 1:48,000, number of exposed dikes, no intersecting pairs were found; thus, 40Ar/39Ar ages as was their trend toward the Dulce swarm, beyond the preserved extent of could not be compared to crosscutting relations. However, the dated dikes and volcanic rocks (Lipman, 1975, p. 87). The radial dikes west of Platoro caldera larger intrusions within Platoro caldera or intruded into older Conejos lavas had not been previously studied in any detail; compositions were estimated yielded younger ages than their wall rocks. largely from hand-sample comparisons with chemically analyzed lavas. Only two analyses of dikes (both dacite) had been published (Patton, 1917, p. 31; Lipman, 1975, his table 10); no analyses were obtained for any andesitic dikes. Areal Distribution of Dike Samples Additional petrologic studies at Platoro in the 1980s were summarized in a field guide (Dungan et al., 1989), but published analytical data are only for Sample sites for the Platoro-Dulce swarm have been subdivided into nine precaldera lavas (Colucci et al., 1991). semi-equal geographic segments (Fig. 2; Table 3), based largely on dike ori- Dikes analyzed for the present study (Supplemental File 1 [footnote 1]), entation and access limitations, in order to evaluate variations as a function along with published lava compositions, show that Platoro andesites and dac-

of distance from the Platoro locus and orthogonally across the dike swarm. ites form a coherent high-K calc-alkaline suite (54%–61% SiO2), similar to other Chemical compositions, phenocrysts, and textures of dominant dike types tend intermediate-composition rocks of the SRMVF, that straddles the boundary with to differ from those in adjacent segments, especially for the more proximal trachytic compositions (Fig. 6). All of the Platoro rocks are described here as and distal areas, although a few dikes within each segment have compositions “andesite” and “dacite”, rather than trachyandesite or trachydacite, for brevity

TABLE 3. GENERAL EATRES O SEGMENT SBDIVISIONS, PLATORO-DLCE DIE SWARM LOCATIONS ON IG. 2 Segment Name Distance Rock type phenocrysts Notes km PP Platoro proimal, radial 0–10 A and BA pl, cp or hb, D pl, bi, sn Many postcaldera, intrude caldera-fill rocks PD Platoro distal, radial 10–30 A and BA pl, cp or hb, D pl, bi, sn Both early Coneos ormation and postcaldera ages D1 NE trending, CO-N 30–0 B, SB, and TB sparse altered mafics Several andesite dikes of Platoro affinities D2A N trending, CO-N 0–50 B, TB, and BTA no phenocrysts Coarsely bladed groundmass plagioclase D2B NE trending, CO-S 0–50 TB and BTA cp, altered ol Variably altered clinopyroene phenocrysts especially common D3 N trending, CO-S 50–60 TB and BTA cp, altered ol Many samples with coarse groundmass mica, some with amphibole D N trending, NM-N 60–80 TB, diverse tetures and compositions Many samples with phenocrysts of little-altered clinopyroene D5 N trending, NM-mid 80–110 TB, diverse tetures and compositions Teturally and compositionally diverse, some with groundmass amphibole D6 N trending, NM-S 110–130 TB no phenocrysts Groundmass acicular amphibole, without mica Arcuate distance from western Platoro caldera margin see ig. 2. COColorado; NMNew Meico. Rock abbreviations: Aandesite; Bbasalt; BAbasaltic andesite; BTAbasaltic trachyandesite; Ddacite; SBsubalkaline basalt; TAtrachyandesite; TBtrachybasalt. Phenocryst abbreviations: bibiotite; cpclinopyroene; hbhornblende; plplagioclase; ololivine; snsanidine.

9 Figure 6. Alkali-silica variation diagram for intermediate-composition intrusions and lavas of Platoro caldera, Dulce dikes from successively southward geographic segments (Fig. 2), and east- SE SAN JUAN - DULCE DIKES ern rift-related basaltic lavas (Hinsdale Formation). Dacitic ignimbrites erupted from the caldera complex would extend Platoro trends to higher values of silica. Even though alkali ratios vary 8 widely, total alkalis appear to retain near-magmatic values, as indicated by correlation with other elements. For example, the low total alkalis that characterize many Dulce samples in segments D1 and D2A are paralleled by low values of these elements, such as Ti, P, Zr, and La; low values of 7 these elements are consistent with weakly subalkaline basaltic compositions. (A) Comparison of

Dulce dikes with older continental-arc suites: Conejos Formation (precaldera lavas of the Platoro locus and Summer Coon volcano), rocks of the Platoro locus, and younger Hinsdale lavas. Data sources: Summer Coon volcano—Lipman (1968), Parker et al. (2005), Lake and Farmer (2015); 6 other Conejos Formation—Lipman (1975), Colucci et al. (1991); intermediate-composition lavas and intrusions of the Platoro locus and Dulce dikes—Supplemental File 1 (see text footnote 1), alkalis (wt %) Lipman (1975); Hinsdale lavas—Lipman and Mehnert (1975), Thompson et al. (1991), Turner et Total 5 al. (2019);. V.—volcano; SJ—San Juan region. (B) Comparison of Platoro dikes and granitoid Summer Coon V intrusions with geographic segments (D1–D6) of the Dulce swarm that are progressively more Conejos Fm SE SJ distal from the Platoro locus (data from Supplemental File 1). CO—Colorado; NM—New Mexico. 4 Platoro locus Hinsdale SE SJ Dulce dikes and in part to contrast with the more alkalic character of many Dulce dikes. 3 The Platoro-area radial dikes form better-defined linear arrays on alkali-silica 44 48 52 56 60 64 SiO (wt %) and other compositional-variation diagrams than the Dulce dikes (Fig. 6). Other 2 9 than slightly higher SiO2 contents (58%–61%) of dated hornblende-phyric dikes, no compositional distinction is evident between the relatively few Platoro-area PLATORO - DULCE LOCUS dikes that can be demonstrated by stratigraphy or geochronology to be precal- Phono- 8 tephrite dera (Conejos) versus those of postcaldera or uncertain ages. More detailed Basaltic Trachyandesite field, petrologic, and age data for the Platoro-area dikes would be desirable, Tephrite trachyandesite especially for less-studied areas adjacent to and west of the Continental Divide 7

and to distinguish more clearly between precaldera versus postcaldera dikes. %) (wt 6

akalis Platoro granitoids Andesite Platoro dikes Total 5 D1 NE trend, CO-N Basaltic Dikes mapped as andesite (Lipman, 1974) include dark-gray aphyric and Andesite D2A N trend, CO-N andesite D2B NE trend, CO-S texturally diverse porphyritic rocks. The porphyritic andesites typically contain D3 N trend, CO-S 4 phenocrystic plagioclase and augite, or plagioclase and hornblende, but lack Basalt D4 N trend, NM-N biotite. No olivine-bearing dikes (or lavas) of basaltic composition were recog- D5 N trend, NM-Mid D6 N trend, NM-S nized in the Platoro area other than the younger Miocene basaltic lavas of the 3 Hinsdale Formation. Andesitic dikes are the most numerous and widespread 44 48 52 56 60 64 SiO2 (wt %) type in the Platoro area, and they are traceable farther outward than more- silicic dikes. Several dikes with Platoro petrologic characteristics (Supplemental File 1 [footnote 1]; samples 16L-1, 16L-22) are present to the southwest beyond proximal dikes of the western radial group were emplaced concurrently with the erosionally preserved margins of San Juan volcanic rocks. Most andesitic postcaldera andesitic volcanism, as documented both by stratigraphy and ages, dikes are narrow, rarely more than a few meters thick, and traceable along but several large radial dikes are precaldera, associated with Conejos volcanoes. strike for only short distances. Additional dikes of this group undoubtedly Confirmed postcollapse dikes at Platoro include south-trending andesites were missed during mapping, in contrast to the thick dacites, most of which that intrude caldera-fill lavas and volcaniclastic rocks within the southwestern likely have been identified. caldera margin. Several have yielded groundmass 40Ar/39Ar ages between 29 The andesitic dikes that radiate westward from the Alamosa River pluton are and 27 Ma (Table 2), consistent with timing for postcaldera lava eruptions. of several ages that have been challenging to evaluate. Many of the andesitic Without age determinations, however, many andesitic dikes that radiate west- dikes lack K-bearing phenocrysts or unaltered groundmass suitable for 40Ar/39Ar ward from Platoro are difficult to distinguish confidently as postcaldera versus measurement, and only a few ages have been successfully determined. Some precaldera. Relatively few dikes of the Platoro suite have been sampled for age

or petrologic study, and only reconnaissance mapping is available for large Some may be low-silica rhyolite. Carlsbad-twinned sanidine megacrysts are as areas west of Platoro, especially west of the Continental Divide. As a result, much as 5 cm across in some dikes. These form the largest and most spectacular the westward extent of postcaldera andesite dikes remains poorly constrained. dikes and plugs in the region; they are commonly >10 m thick (locally as much Dikes containing hornblende or large tabular plagioclase phenocrysts are as 50 m), and some are continuously exposed for several kilometers. Many of especially likely to be of Conejos age, because these mineral assemblages the larger sanidine-dacite intrusions are distant from the caldera complex, but are uncommon in postcaldera lavas within and near Platoro. Two proximal in contrast to some andesite dikes, none extends beyond the preserved volcanic hornblende-phyric dikes that intrude volcaniclastic rocks just beyond the south- rocks. Many of the sanidine-dacite dikes have marginal vitrophyres, a feature western caldera margin yielded variable-quality ages of 32–31 Ma (Table 2, rarely observed in more mafic dikes. A few dacite dikes display marginal grooves samples 16L-32, 16L-35), clearly recording their relation to precaldera volcanoes. along contacts that plunge eastward toward the Alamosa River pluton (Fig. 7D). Two SW-trending hornblende-phyric dikes (samples 16L-51, 16KA-02), ~20 Except for the dike of late Conejos age at Jim Creek mentioned previously, km from the margin of Platoro caldera (Fig. 7A), have similar late-precaldera all other dated intrusions of sanidine-bearing dacite have yielded ages ~2.5 (Conejos) ages at ca. 31 Ma, as has an even-more-distal aphyric andesite dike m.y. younger than the last ignimbrite erupted from Platoro (Chiquito Peak Tuff of Platoro compositional affinity that intrudes Cretaceous sedimentary strata at 28.8 Ma). New 40Ar/39Ar sanidine ages for four SW- to NW-trending dacite beyond the preserved volcanic rocks (sample 16L-1). An isolated south-trending dikes of the radial system west of Platoro caldera cluster between 26.25 ± 0.04 dike of mafic dacite along Jim Creek 6 km southeast of the caldera (sample and 26.49 ± 0.06 Ma (Table 2, PP and PD dike segments; Fig. 5). A dacite plug 16L-55), even though atypical for the Conejos Formation in containing biotite just beyond the southeastern caldera margin at Ojito Creek yielded a similar and sparse small phenocrysts of sanidine, yielded a sanidine age of 30.36 age, as did a thick dacite sill in Blanco Basin at the southwestern margin of ± 0.03 Ma, further confirming early development of widespread radial dikes preserved volcanic rocks (Fig. 2). Scattered dikes of petrographically similar at the Platoro locus prior to ignimbrite eruptions. sanidine dacite that intrude postcollapse lavas within Platoro caldera are too In contrast, another SW-trending plagioclase-phyric andesite dike of Conejos altered to date by 40Ar/39Ar methods, but are deemed likely to be similar in age. petrologic affinity (sample 16L-22), also intrusive into Cretaceous strata within The ages for these silicic intrusions of the Platoro-Dulce swarm are markedly dike-swarm segment D1, yielded a relatively young groundmass age of 29.18 younger than the caldera-filling andesitic lavas, are not associated with pre- ± 0.06 Ma. This age is indistinguishable from that for the Ra Jadero Tuff from served lavas, and even postdate the 27.0 Ma Snowshoe Mountain Tuff, the final Platoro caldera, suggesting that this distal dike may be syncaldera, fed by deeper major ignimbrite eruption from the central caldera complex (Lipman, 2007). and more mafic parts of the magma system than for the dacitic ignimbrites. Other shallow proximal intrusions of silicic dacite and rhyolite have yielded Alternatively, a few groundmass determinations elsewhere in the SRMVF have younger ages. A dike and plug of silicic dacite within the northwestern margin documented Ar loss from fine-grained intermediate rocks (e.g., lavas that yielded of the caldera have sanidine ages of 20.44 ± 0.02 and 20.87 ± 0.02 Ma, and a ages younger than the intruding dikes; Lipman et al., 2015, their supplemental nearby rhyolite intrusion or lava dome has an age of 21.32 ± 0.02 Ma (Table 2). file 1). Thus, this Conejos-like dike may have experienced Ar loss, possibly Emplaced temporally between these and earlier radial dacite dikes at 26.5 related to a thermal event such as emplacement of a nearby but unexposed dike. Ma is the large mineralized dacite dome at Summitville (ca. 23 Ma by K-Ar: Mehnert et al., 1973). Highly altered rocks of the Summitville dome, in turn, are overlain unconformably by erosional remnants of silicic lava (rhyolite Dacite of Cropsy Mountain), ca. 20 Ma by early K-Ar analyses (Steven et al., 1967). Thus, Platoro caldera has a history of postcollapse intermediate to silicic volca- Two groups of light-gray biotite-bearing dacite dikes, with larger and more nism and dike emplacement, continuing for at least 9 m.y. after its last ignimbrite abundant phenocrysts than in the andesite suite, were distinguished (Lipman, eruption. The only other calderas in the SRMVF with exposed records of lengthy 1974). Most of these are probably postcaldera, because similar compositions postcaldera magmatism (all as granitoid plutons) are Questa (7 m.y.), following are rare in precaldera lavas near Platoro. eruption of the Amalia Tuff at 25.4 Ma concurrently with early development of

One dike group is low-silica dacite (~61%–64% SiO2), based on comparison the Rio Grande rift (Lipman, 1988; Zimmerer and McIntosh, 2012a), and Aetna with similar postcaldera lavas (dacite of Park Creek). These dikes (previously (4 m.y.), after the Badger Creek Tuff at 34.3 Ma (Zimmerer and McIntosh, 2012b). rhyodacite; Lipman, 1974, 1975) are thicker than andesitic ones (commonly 5 m or more), better exposed, and more continuous along strike. Several dikes at the head of the Alamosa River are traceable for a kilometer or more. Because Granitoid Intrusions these dikes lack sanidine, 40Ar/39Ar dating was not attempted. The other dacite group (quartz latite in the above-cited publications) is more Granitoid plutons are aligned roughly east-west across Platoro caldera, coarsely porphyritic—many with large phenocrysts of sanidine, some with including the Alamosa River and Jasper intrusions within the caldera, Cat

quartz—and has more silicic compositions, mostly in the range 65%–68% SiO2. Creek just to the east, and Bear Creek to the west (Fig. 3). Most are fine- to

Intracaldera Chiquito Peak Tu Figure 7. Photographs of radial andesite and dacite dikes west of Platoro caldera, southern Colorado. (A) Three dikes of silicic andesite, on the south ridge of Flat- top Mountain (elevation 3475 m), that trend NE across the Continental Divide (skyline) toward Platoro caldera (photograph by K. Anderson, 2016). Dike in the foreground (60.4% SiO2) is ~2 m thick, has a hornblende age of 31.14 ± 0.03 Ma (site 16KA-02, Table 2), indicating emplacement related to late-precaldera volcanoes (Cone- jos Formation). (B) North-trending dike of

aphyric andesite (53.9% SiO2) along the shoreline of Platoro Reservoir (sample 16L-38). This dike intrudes intracaldera Chiquito Peak Tuff, demonstrating postcaldera age; the groundmass age of the dike (29.21 ± 0.07 Ma, Table 2) is not considered significantly different from age of the 28.8 Ma host ignimbrite (photograph by P. Lip- man, 2016). (C) Large porphyritic dacite dike, ~40 m thick (for scale, note the truck near the left side of the dike), along Stun- ner-Summitville road, at south fork of Iron Creek in Schinzel Meadows (photograph by P. Lipman in 1971). Poorly exposed wall rock is Summitville Andesite that overlies Chiquito Peak Tuff near the west wall of Platoro caldera. Sanidine from this dike has a 40Ar/39Ar age of 26.25 ± 0.04 Ma (Table 2, sample 11L-23). (D) Wall of large dacite porphyry dike displaying low-angle flow grooves. The grooves are 5–10 cm deep and ~25–50 cm apart; the plunge of the grooves and the strike of the dike are to the northeast, toward the Alamosa River pluton. Outcrop of the dike wall is ~50 m across, at ~3840 m elevation on the west slope of the 3921 m (12,866 ft) peak on the Continental Divide at the head of Prospect Creek (photograph by P. Lipman, 1966).

medium-grained equigranular monzonite, some phases containing plagioclase ignimbrite sheets in the adjacent San Juan Mountains. Farther south in New phenocrysts. In places, near-aphanitic phases are best described as andesite Mexico, the thickness of volcanic and other mid-Cenozoic rocks above present or dacite. These high-level plutons have been interpreted as roots of postcol- exposures may have been even less. Similar estimates for emplacement depths lapse volcanoes that were active soon after the ignimbrite eruptions (Lipman, are constrained by paleogeomorphic reconstruction of the Chuska sand sea (erg) 1975). The east-west zone of intrusions trends anomalously orthogonal to and by apatite fission-track dating in the San Juan Basin (Cather et al., 2008, 2012). the basement uplifts of the Southern Rocky Mountains (Figs. 1, 3), perhaps Dike outcrops tend to be geometrically simple, but some features provide related to location along the south margin of the large Bouguer gravity low insight into emplacement processes. Widths of individual dikes are typically no that defines a subvolcanic batholith below the central San Juan region (Plouff more than a few meters; dike walls tend to be planar. Gently plunging fluting and Pakiser, 1972; Drenth et al., 2012). or grooves in rare exposures indicate local flow directions (Fig. 10A), and a Published K-Ar and new 40Ar/39Ar and U-Pb zircon ages from granitoid intru- collaborative study of magnetic anisotropy indicates that subhorizontal magma sions at Platoro suggest two main pulses of crystallization at ca. 29 and 27–28 flow is widely characteristic (Johnson et al., 2016). Development of spherulitic Ma for texturally contrasting phases (Table 2). The main intracaldera intrusions textures and near-glass interstices at some dike margins (Figs. 10B–10C) indi- (Alamosa River, Jasper plutons) of relatively coarse equigranular and porphy- cates rapid quenching, while the lengths of dikes (as much as 20 km) require ritic monzonite that intrude the resurgent block have zircon ages at ca. 29 Ma rapid emplacement during lateral magma flow in thin dikes. Selvages of sed- (Gilmer et al., 2018), within analytical uncertainty of the 28.8 Ma 40Ar/39Ar erup- imentary wall rocks are thermally darkened and indurated, but for only tens tion age of the caldera-related Chiquito Peak Tuff. In contrast, the intrusion at of centimeters adjacent to contacts. Hydrothermal alteration appears largely Cat Creek just east of Platoro caldera, which is flanked by andesite and dacite absent adjacent to dikes, but small-scale brecciation of dike margins at a few lava that have been interpreted as eroded remnants of a large postcaldera sites may have resulted from local flashing of connate fluids. No large-scale volcano (Lipman, 1975), has a U-Pb age of 28.00 ± 0.19 Ma (Gilmer et al., 2018), brecciation or volcaniclastic textures that might record roots of a vent were distinctly younger than the intracaldera granitoid plutons. Another sizable gran- recognized, despite apparently shallow emplacement. Many dikes are dense itoid body, along Bear and Elwood Creeks west of the caldera, appears to be rock, but some interiors contain 5%–10% small vesicles (typically 2–3 mm). even younger, with ages at 26.61 ± 0.01 Ma (40Ar/39Ar, biotite, sample SRM-33) The first chemical analyses for dikes of the Dulce swarm, obtained in the and 27.06 ± 0.18 Ma (U-Pb, zircon [same sample]). A previously unpublished 1960s for the U.S. Geological Survey by E. Landis and by W.J. Hail, documented hornblende K-Ar age of 26.6 ± 1.0 Ma (Table 2, sample DC-2) for a porphyritic trachybasaltic to trachyandesitic compositions characterized by variable alkalis phase of the Alamosa River pluton also suggests delayed cooling or crystalliza- and high carbonate and water contents (Supplemental File 3 [footnote 1]; data tion at this large composite intrusion within Platoro caldera. Available chemical not previously published). Major-oxide analyses and K-Ar ages (23.5 ± 0.9, 27.2 analyses (Supplemental File 1 [footnote 1]; Lipman, 1975, his table 10) show ± 1.1 Ma) were published by Aldrich et al. (1986) for two Dulce dikes in northern that these intrusions tend to be slightly more silicic than the bulk of the calde- New Mexico, in conjunction with a regional paleostress survey. As part of a ra-filling lavas (Summitville Andesite) but have overlapping chemical trends. broad petrologic study, Gibson et al. (1993) obtained major- and trace-element analyses, K-Ar ages (23.5 ± 0.6, 24.6 ± 0.5 Ma), and the first radiogenic isotopic data (Sr, Nd) for several dikes near the Colorado–New Mexico border. Additional ■■ TRACHYBASALT–BASALTIC TRACHYANDESITE DIKES OF THE chemical analyses (five samples), Sr and Nd isotopic data (four samples), and DULCE SWARM two 40Ar/39Ar ages (24.97 ± 0.06, 20.10 ± 0.06 Ma) were reported for Dulce dikes in southern Colorado (Gonzales, 2015; Gonzales and Lake, 2017). In addition to Dikes of the Dulce swarm tend to form linear ridges because they are more confirming mafic alkalic compositions, these results suggested a prolonged resistant to erosion than the sedimentary wall rocks. Many dike ridges are time span (20–26 Ma) for dike emplacement, and the isotopic data indicate conspicuous on aerial images such as in Google Earth (Fig. 8), some traceable near-mantle magma compositions. Sample sites for these studies were mainly for 20 km. Some dikes rise 10 m or more above wall rocks of the surrounding along two east-west road transects near the Colorado–New Mexico state line, terrain, especially in arid areas of northern New Mexico (Fig. 9). Exposures extending only ~20 km north-south, and locations of some samples are uncertain. are less apparent on the more vegetated slopes farther north in Colorado, but individual ridge-forming dikes are mappable for comparable distances (Wood et al., 1948). Some sites include multiple dikes separated by only a few meters Mineralogy and Texture of intervening sedimentary rock (Fig. 9C). Outcrop elevations rise from as low as 2000 m at the southern end of the Throughout their 100 km extent, Dulce dikes have a similar megascopic swarm to 2700 m in Colorado. Emplacement depths appear to have been shallow, appearance: medium to dark gray, fine grained, and without conspicuous perhaps only ~1 km. This depth estimate is based on the projected thickness of internal boundaries or structures at outcrop scale. Phenocryst assemblages volcanic cover and on levels of the late Oligocene land surface, as defined by and textures overlap widely along the trend of the dikes, but modest provincial

37° N

0 10 20 Km

107° W

Figure 8. Google Earth image showing morphologic expression of ridge-forming Dulce dikes in northern New Mexico and southern Colorado. Compare with Figure 2, for dike locations; area covered is approximately the same, except extends less far north or east. Reference points on both figures are towns of Pagosa Springs, Colorado, and Dulce, New Mexico.

Figure 9. Outcrops of Dulce dikes (photographs by P. Lipman, 2015). (A) Northern part of the southernmost dike; view is to the north from New Mexico State Highway 537 (sample site 15L-57). In the left-center of the image, a large mass of light-colored Cretaceous sandstone is exposed in contact with the dike wall. (B) North-trending dike (~2 m thick) in the cliffs on the north side of Amargo Canyon west of Dulce, New Mexico (sample site 16L-18). (C) Three closely spaced dikes (yellow arrows) along Montezuma Creek, southern Colorado. Note the person for scale at the leftmost dike. Large central dike is sample site 15L-44.

differences are evident. Many dikes are aphyric, especially in dike-swarm segments D2A and D6 (Supplemental File 1 [footnote 1]). About half contain sparse small equant phenocrysts (as much as ~5%–10%, to 3 mm across) of variably altered clinopyroxene or dark mineral clots, mainly green mica, that are pseudomorphic replacements of mafic phenocrysts. Inference of former olivine is confirmed chemically by high MgO, Ni, and Cr in samples that contain abundant pseudomorphs (Supplemental File 1). The olivine and clinopyroxene phenocrysts identify these dikes as alkalic basalt (trachybasalt), in contrast to high-K lamprophyres characterized by hydrous mafic phenocrysts as reported from the western San Juan Mountains and the adjacent Navajo province (Lake and Farmer, 2015; Gonzales and Lake, 2017). No trachybasaltic dikes of Dulce type contain phenocrystic plagioclase; two plagioclase-phyric dikes in segment D1 of the Dulce swarm (samples 16L-1, 16L-22) are andesite with chemical and age affinities to proximal rocks of the Platoro locus. Groundmass textures of the Dulce dikes vary from microcrystalline to fine grained. The most crystalline textures, from dike interiors, are dominated by intergrowth of bladed plagioclase (up to 0.5 mm), with finely granular clinopyroxene, opaque oxides, and alteration products filling interstices. Groundmass plagioclase in well-crystallized samples is relatively fresh, but pyroxene is typically variably altered and primary olivine is completely replaced by secondary minerals. Many samples contain groundmass flakes of black mica (brown in thin section), as coarse as 1.5 mm across in the interiors of some dikes (e.g., samples 15L-40A, 15L-56, 16L-8, 16L-9B, 16L-18, 17L-3A, 17L-9A). Electron microprobe analyses for one trachybasalt (sample 16L-9B) show that the groundmass mica and phenocrystic clinopyroxene have higher MgO/FeO ratios (1.5–2.0, 2.3–2.6, respectively) than these phases in San Juan andesites and dacites (~1.1–1.5, 1.8–2.1; Colucci et al., 1991; Riciputi, 1991), as expectable for a basaltic composition. Because of its black color and modest MgO/FeO ratios, the mica seems appropriately described as biotite rather than phlogopite. A few north-trending dikes of the D3 segment and farther south contain acicular groundmass crystals of brown amphibole (Mg-rich hornblende?) that are typically <0.5 mm long. The hydrous mafic minerals in the groundmass of Dulce samples contrast conspicuously with their anhydrous phenocrysts. The proximal northeasterly dikes (segment D1) tend to be fine grained and contain clinopyroxene and olivine as phenocrysts and pseudomorphs. In contrast, the northerly north-trending dikes (segment D2A) have well-crystallized bladed plagioclase in the groundmass but lack phenocrysts. Several dike groups farther south have diverse textures: granular to bladed and variably porphyritic, some aphyric but others containing fresh clinopyroxene and/or

Figure 10. Margin of a Dulce dike in a roadcut along U.S. Highway 64 west of John Mills Lake, New Mexico (sample site 16L-14; photographs by P. Lip- man, 2015). (A) Subhorizontal grooves, indicative of lateral magma flow. (B) Spherulitic crystallization texture at the dike margin, recording rapid quenching against the wall rock of Cretaceous sandstone. (C) Closer view of the spherulitic texture.

olivine pseudomorphs. The southernmost sampled dike (segment D6) has a were avoided for 40Ar/39Ar dating. A potentially informative topic for further well-crystallized groundmass of finely bladed plagioclase and acicular amphi- study could be stable-isotopic analysis (O, H, S) of variations among preserved bole, but lacks phenocrysts. Phenocrystic plagioclase or hornblende are absent phenocrysts (mainly clinopyroxene) in comparison to hydrous groundmass throughout the Dulce swarm, in contrast to the andesitic dikes and lavas more minerals (mica, amphibole) to evaluate roles of magmatic versus meteoric proximal to Platoro, while the Platoro dikes lack hydrous groundmass minerals. fluids during dike solidification and deuteric recrystallization. Dulce samples contain variable amounts of late-formed calcite, filling vesicles and cracks, partially replacing mafic phenocrysts and groundmass plagioclase, and in interstices between magmatic crystals. No megascopically Age Determinations consistent outcrop-scale variations were recognized in carbonate content or degree of groundmass alteration in relation to dike thickness, distance longi- Twenty-six (26) new 40Ar/39Ar ages for representative Dulce dikes (Table 2; tudinally along the Dulce swarm, or location in lateral transects. Some dike Fig. 5) along with two published ages (Gonzales, 2015) indicate emplacement rocks yield a distinct hydrocarbon aroma when freshly broken, contain pyrite, during two pulses: mainly 25–27 Ma, with a smaller pulse at 19–21 Ma. The two

and yield detectable SO3 and/or Cl by XRF analysis (Supplemental File 1 [foot- oldest ages, 27.51 ± 0.14 (sample 15L-36) and 27.32 ± 0.16 Ma (sample 16L-23), note 1]), indicating likely introduction of fluids from wall rocks. These dikes are from the NE-trending proximal end of the Dulce swarm (segment D1) that is

also distinctive in the subalkaline chemistry of many samples. The youngest age bodies of substantial size (Jackson Mountain, Blanco Basin, V Mountain, from this segment (sample 17L-2; rather imprecise, at 25.38 ± 0.52 Ma) is from a Archuleta Mesa) within or adjacent to the Dulce swarm (Fig. 2) have chemical dike that continues southwest across the segment D1–segment D2B boundary and age affinities to the Dulce and Platoro dikes. These intrusions appear to be and is compositionally similar to other D2B dikes. Fourteen (14) dates from more large laccoliths or sills. They are more silicic than most Dulce dikes and composi- southerly segments cluster at 25–26.5 Ma, similar to the sanidine-dacite dikes tionally broadly similar to the proximal Platoro plutons, but are variably younger.

that radiate more proximally from Platoro caldera. These ages also coincide The Jackson Mountain intrusion of distinctive mafic monzonite (~58% SiO2) with those of initial trachybasaltic to tholeiitic lavas within the Rio Grande rift to appears laccolithic, although thus far mapped only at 1:250,000 scale (Steven the east (Lipman and Mehnert, 1975; Thompson et al., 1991; Turner et al., 2019). et al., 1974). Two samples yielded identical U-Pb zircon ages at 25.1 ± 0.5 Ma In contrast, four samples from the western segment of north-trending dikes (Gonzales, 2015), broadly similar in age to the younger suite of granitoids and (segment D2A), which form a petrologically distinctive suite in their absence of dacite dikes more proximal to Platoro, but differing chemically in containing phenocrysts and relatively coarse groundmass textures, have yielded closely higher Ti, P, La, and Ce than otherwise similar intrusions in the southeastern grouped ages at 19.1–18.6 Ma. Attempts to date samples from two additional San Juan region (Supplemental File 1 [footnote 1]). In these respects, Jackson north-trending dikes of similar petrography within this segment (Table 2; samples Mountain has closer affinities to trachyandesite dikes of the Dulce swarm. Dis- 16L-66, 17L-5) were unsuccessful. The southward extent of this younger suite of tinctively, this intrusion contains megacrysts of variably resorbed K-feldspar dikes remains incompletely constrained. To the southwest within the adjoining to 4 cm in length and rounded quartz pellets, which are in disequilibrium with segment D3, a western dike (sample 15L-48) has a relatively young age of 21.48 the relatively mafic bulk composition. These minerals also form clustered clots, ± 0.33 Ma, and another dike sample on trend farther north (sample 16L-63), as much as 15 cm across, that suggest mingling with mushy granitic magma. although characterized by a discordant spectrum with low radiogenic yields and Closely grouped ages of zircons and the absence of xenocrystic zircons from low precision, has an integrated age of ca. 21 Ma that suggests that it is likely Proterozoic basement (Gonzales, 2015) indicate that any mingled granitic com- part of the younger suite. Along with these results, a whole-rock 40Ar/39Ar age ponent crystallized at about the same time as the overall intrusion.

of 20.10 ± 0.06 Ma from a dike farther east, along the Montezuma road within A laccolith or thick sill of fine-grained equigranular monzonite (61% SiO2), segment D3 (sample PS10; Gonzales, 2015), indicates that scattered dikes of well exposed along the north side of Blanco Basin, is as much as 300 m thick, the young group continue south nearly to the state line (Fig. 2). This age, which emplaced within sedimentary strata just below the basal volcanic rocks. No differs from new ages of 25.7 and 25.9 Ma from dikes only a few kilometers to age is available, although a large sill of sanidine dacite across Blanco Basin the east and west in southern Colorado (Table 2; samples 15L‑40A, 15L-45), doc- to the south has a 40Ar/39Ar age of 26.53 ± 0.02 Ma (Table 2, sample 17L-13). uments southward interfingering of the young dike suite with older parts of the A thick sill (to 150 m), forming Archuleta Mesa, that straddles the Colorado– swarm along its western flank. An age of 27.28 ± 0.13 Ma for a north-trending dike New Mexico state line (also home to Dulce Base, the alleged joint human-alien within segment D2A (sample 16L-64) suggests similar interfingering farther north. collaborative facility: https://en.wikipedia.org/wiki/Dulce_Base), consists of

Overall, we consider that clear age groupings and geographic trends have fine-grained monzonite (58% SiO2) with small phenocrysts of plagioclase and been documented for the Dulce swarm, despite an incomplete sample suite clinopyroxene. It is the youngest known granitoid intrusion in the region, hav- and variable quality of the dated samples. Ages cluster into distinct groups: ing a U-Pb zircon age of 15.5 ± 0.3 Ma (Gonzales, 2015). Another thick sill 21.5–18.6 Ma, 26.5–25.0 Ma, and a few dikes as old as 27.5 Ma at the northeastern (150–175 m), intruded between Cretaceous strata and basal Cenozoic volcanic end of the swarm. Groundmass-concentrate ages, in particular, yielded the most rocks at V Mountain (Larsen and Cross, 1956, p. 104) consists of finely porphy- complex age spectra, commonly characterized by low radiogenic yields related ritic monzonite that is petrographically and chemically similar to the Archuleta to the variable alteration of the samples. Ages for coarse groundmass mica from Mesa sill 20 km to the southwest (Supplemental File 1 [footnote 1]). Permissively, dike interiors (four samples) appear especially robust (25.0–26.4 Ma), and similar these two localities could be erosional remnants of a single vast sill. V-Mountain groundmass-concentrate ages from nearby dikes, combined with the clustering lacks phases suitable for 40Ar/39Ar analysis, but future zircon dating could test of ages, add confidence to the trends within the Dulce swarm. However, some relations to Archuleta Mesa and other distal Platoro intrusions. determinations, especially for bulk groundmass, are likely to be geologically less significant than implied by the high analytical precision because of variable deuteric alteration, secondary carbonate, and other issues of sample quality. ■■ CHEMICAL COMPARISONS

The new chemical analyses of Dulce dikes scatter substantially on variation ■■ DISTAL SOUTHWESTERN GRANITOID INTRUSIONS diagrams, markedly more than for andesitic rocks of the Platoro region such as precaldera Conejos rocks, caldera-related lavas (Summitville Andesite), and In addition to proximal granitoid intrusions adjacent to Platoro caldera (Cat granitoid intrusions, or postcaldera rift-related basalts of the Hinsdale Forma- Creek to the east, Bear Creek to the west), several fine-grained to porphyritic tion (Figs. 6, 11–12). To facilitate interpretation, parallel variation-diagram plots

are used: (1) to compare Dulce dikes broadly with older continental-arc suites by correlation with other elements and with location. Most samples that have (Conejos Formation, rocks of the Platoro locus) and younger Hinsdale lavas, low total alkalis (<5%), plotting within the subalkaline basalt field, also have low and (2) to compare geographic segments of the Dulce swarm as a function values of elements such as Ti, P, Zr, and La. These are from the northernmost of distance from the Platoro locus. The scatter among Dulce samples, involv- dikes (segments D1, D2A) nearest to the San Juan Mountain front and merging ing most major oxides and many trace elements, is interpreted as reflecting geometrically with the dikes radial to Platoro caldera. The highest alkali values at substantial variations in magmatic values, variably modified by deuteric and low silica are from the southern parts of the Dulce swarm (segments D4 to D6). other secondary processes. These include alteration of mafic phenocryst and Some Dulce dikes with anomalous alkali contents, accompanied by highly

groundmass minerals, deposition of calcite in vesicles and in altered mafic variable Na2O/K2O and Rb/K2O ratios (0.1–46, 11–61 respectively) among sam- minerals, and probable alkali exchange. ples that are otherwise chemically similar (Supplemental File 1 [footnote 1]), High volatiles in most dikes (LOI: 3%–14% for Dulce samples, 2.5%–8% for are interpreted as reflecting alkali mobility during deuteric recrystallization. Platoro radial dikes; Supplemental File 1 [footnote 1]) also accompany modi- Many samples with anomalous alkali ratios also have high CaO and LOI indic- fication from magmatic values. Comparisons between Platoro andesite dikes ative of abundant calcite, fine-grained groundmass, and otherwise extreme

and related caldera-filling lavas are pertinent; compositions are closely similar bulk compositions. These include many of the low-SiO2 samples (16L-12, 16L- for most elements, but LOI values are much higher for the dikes (3%–6%) than 16, 16L-17, 17L-12) that plot deep in the tephrite field (Fig. 6). Unexpectedly, for lavas (1%–1.5%; Supplemental File 1; Lipman, 1975, his table 9). Even higher the apparent alkali exchange in some Dulce samples involves increased Na LOI values characterize many sediment-hosted western San Juan and Navajo relative to K, in contrast to the “potassium metasomatism” documented in province dikes (to 14%; Gonzales and Lake, 2017), suggesting that introduction rhyolites (Ratté and Steven, 1967; Chapin and Lindley, 1986). of external volatiles is especially likely during cooling and deuteric recrystal- In comparison, lavas, proximal dikes, and granitoid intrusions of the Platoro lization of dikes intruded into permeable strata. In contrast, primary basalt locus are more silicic than most Dulce samples but define alkali-silica trends magma appears likely to contain no more than a few percent volatiles; for that project into lower-alkali parts of the Dulce data. Precaldera Conejos lavas example, alkalic-basalt and tephrite glass quenched at deep-ocean pressure and dikes (including those from Summer Coon volcano) and younger rift-re-

has only ~1% H2O (Coombs et al., 2006). lated basaltic lavas of the Hinsdale Formation also scatter considerably but Despite the deuteric and other alteration, primary magmatic trends are iden- follow trends similar to those of the Platoro rocks, extending into the Dulce data tified for the Platoro-Dulce swarm by sizable variations in elements inferred to (Fig. 11). Many rift-related Hinsdale samples are more alkalic than continen-

have been relatively immobile during alteration (e.g., TiO2, P2O5, Zr, rare earth ele- tal-arc rocks of the Platoro locus, overlapping more fully with the Dulce swarm.

ments [REEs]). Notably, the compositions inferred to record magmatic processes Values of TiO2 and P2O5, which may be less subject to deuteric and second-

become more alkaline with distance from Platoro caldera, as illustrated by aver- ary alteration, vary substantially for Dulce samples of similar SiO2 contents, aged compositions for midpoints of progressively more southerly dike segments many samples having values higher than trends projected from analyses of (Fig. 13). As a whole, the Dulce swarm forms a continuum on these plots with precaldera Conejos lavas, Platoro intrusions, or the rift-related basaltic lavas

the Platoro radial dikes, with elements such as TiO2, P2O5, Zr, and La increasing of the Hinsdale Formation (Fig. 11). Although alteration effects have likely with distance from the caldera. Differences are especially striking between dike- modified contents of other elements, substantial portions of the scatter in swarm segment averages for the NE-trending Dulce dikes (segments D1 to D2B), these major-element compositions on silica variation diagrams appear to be and between the northernmost and southernmost north-trending dikes (seg- geographically related, with dike-swarm segment–averaged values increasing

ments D2A to D6). In contrast, compositions overlap among the north-trending with distance from Platoro caldera (Fig. 13). Low contents of TiO2 and P2O5 are dike segments at intermediate distances (segments D3 to D5). typical for the northernmost north- and NE-trending trachybasalt Dulce dikes closest to Platoro that are also low in total alkalis (segments D1, D2A), while the highest values characterize the long southernmost dike in New Mexico (seg- Major Elements ment D6). Dikes at intermediate distances in southern Colorado and northern New Mexico (segments D2B to D4) display overlapping compositional scatter Dulce dikes scatter on an alkali-silica diagram but dominantly are trachy- for these elements. Dikes containing groundmass amphibole become more basalts and basaltic trachyandesites (Fig. 6). The Dulce swarm is not a K-rich common southward from the Colorado–New Mexico border, concurrently

lamprophyric suite (K2O/Na2O >1), contrary to interpretation by Gonzales and with bulk-composition increases in TiO2 and P2O5. Amphibole-bearing dikes

Lake (2017, p. 206). Only 10 of the 75 analyzed Dulce dikes have K2O/Na2O >1, differ only modestly in chemistry from dikes in these segments that contain

these due more to erratically low Na than to exceptionally high K. Average groundmass mica, mainly having higher average values of TiO2, Sr, and Ba.

alkali ratios for dike segments are only 0.2–0.7, tending to increase southward. Variation in SiO2-FeOTOT and SiO2-MgO among Dulce samples (Supplemen-

Alkali ratios vary widely among samples, many involving high Na2O and tal File 1, Figs. S1A, S1B [footnote 1]) plot near the semi-linear magmatic trends

low K2O, but total alkalis appear to retain near-magmatic values, as indicated defined by Hinsdale trachybasalts, especially allowing for the variable former

SE SAN JUAN - DULCE DIKES PLATORO - DULCE LOCUS Platoro granitoids Summer Coon V 3.0 3.0 Platoro dikes Conejos Fm SE SJ D1 NE trend, CO-N Platoro locus D2A N trend, CO-N Hinsdale SE SJ D2B NE trend, CO-S

Dulce dikes D3 N trend, CO-S 2.0 2.0 D4 N trend, NM-N D5 N trend, NM-Mid D6 N trend, NM-S (wt %) (wt 2 TiO

1.0 1.0

0.0 0.0 44 48 52 56 60 64 44 48 52 56 60 64 1.5 1.5

1.0 1.0 (wt %) %) (wt 5 O 2 P 0.5 0.5

0.0 0.0 44 48 52 56 60 64 44 48 52 56 60 64 SiO2 (wt %) SiO 2 (wt %)

Figure 11. Representative variation diagrams for the Dulce dike swarm and proximal dikes radial to Platoro caldera. (A) Comparison of Dulce dikes with older continental-arc suites (Conejos Forma tion, intermediate-composition lavas and intrusions of the Platoro locus, and younger Hinsdale Formation lavas in the southeastern San Juan Mountains). (B) Comparison of Platoro dikes and granitoid intrusions with geographic segments (D1–D6) of the Dulce dike swarm that is progressively more distal from the Platoro caldera locus. Data sources are the same as for Figure 6. V.—volcano; SJ—San Juan region; CO—Colorado; NM—New Mexico.

300 300 SE SAN JUAN - DULCE DIKES PLATORO - DULCE LOCUS

250 250

200 200 Zr (ppm) Zr (ppm) 150 150 Platoro granitoids Summer Coon V Platoro dikes Conejos Fm SE SJ D1 NE trend, CO-N 100 Platoro locus 100 D2A N trend, CO-N D2B NE trend, CO-S Hinsdale SE SJ D3 N trend, CO-S D4 Dulce dikes N trend, NM-N D5 N 50 50 44 48 52 56 60 64 45 50 55 60 trend, NM-65Mid D6 100 N trend, NM-S 100

80 80

60 60

La (ppm) 40 40

20 20

0 0 44 48 52 56 60 64 SiO (wt %) 45 50 55 60 65 SiO2 (wt %)

Figure 12. Representative trace-element variation diagrams for Dulce dike swarm and proximal dikes radial to Platoro caldera. (A) Comparison of Dulce dikes with older continental-arc suites (Conejos Formation, intermediate-composition lavas and intrusions of the Platoro locus, and younger Hinsdale Formation lavas in the southeastern San Juan Mountains). (B) Comparison of Platoro dikes and granitoid intrusions with geographic segments (D1–D6) of the Dulce dike swarm that is progressively more distal from the Platoro caldera locus. Data sources are the same as for Figure 6. V.—volcano; SJ—San Juan region; CO—Colorado; NM—New Mexico.

Figure 13. Segment-averaged compositional variations among Platoro and Dulce dikes as a function of dike-segment midpoint distance from Platoro caldera. See Figure 2 for delineation of dike-swarm segments (PP, PD, D1–D6). Data are from Supplemental File 1 (see text footnote 1), 2.5 D3 filtered to exclude picritic basalt (MgO >10 wt%), multiple analyses from the same dike site, and D2B D6 (15) (18) N trend (3) samples deemed anomalous due to alteration (listed in italics on table in Supplemental File 1). D5 In parentheses, number of analyzed samples. Vertical bars show one standard deviation; inferred (6) trend lines (dashed) are placed by visual estimate. D4 NE trend (15) , wt %

2 1.5 D2A D1 (7) TiO (9) Platoro presence of olivine. Values of MgO also correlate closely with those of Ni and Cr, PP PD (12) (5) indicating substantial preservation of magmatic compositions despite alteration 0.5 0 40 80 120 of olivine. Variations in Al2O3 are more complex (Supplemental File 1, Fig. S1C), the Dulce samples broadly defining a diffuse arcuate trend that increases with 1.5 SiO2, as is typical for alkalic-basalt suites elsewhere in the region (e.g., Gibson et al., 1993; Leat et al., 1988; Lake and Farmer, 2015; Gonzales and Lake, 2017).

High values of Al O at low SiO contents, which are especially conspicuous for D6 2 3 2 D2B D3 (3) the northern Dulce dikes (segments D1 and D2A), are interpreted as geographic 1.0 (15) (18) N trend variations, with lower values to the south. Similarly high Al O also character- D5 2 3 (6) , wt % izes Platoro postcaldera lavas and dikes, in comparison to basaltic lavas of the 5 O D4 2

P (15) Hinsdale Formation that define a nearly linear array at values similar to many NE trend

of the more southerly Dulce dikes. In addition, Al O in Dulce samples tends to 0.5 PD 2 3 PP Platoro (5) D2A correlate inversely with K2O, TiO2, and P2O5. The high-Al2O3 end of the array is (12) D1 (7) (9) anchored by the northern dikes (segments D1 and D2A), consistent with their 0 40 80 120 relatively low K2O, TiO2, and P2O5 values, along with low trace elements such

as Zr, Nb, and the light REEs (LREEs) that suggest affinities toward subalka- 250 line (tholeiitic) compositions. Most of these samples plot low in, or below, the D6 trachybasalt field on an alkali-silica diagram (Fig. 6). (3) N trend As anticipated by abundant calcite in most Dulce dikes, CaO scatters widely D3 D2B (18) D5 on SiO2 variation diagrams (Supplemental File 4, Figs. S4-1A–S4-1B [foot- 200 PD (15) (6) Platoro D4 note 1]), and many samples have substantially higher CaO than trachybasaltic (5) (15) lavas in the San Juan region (Hinsdale Formation) or the most mafic dikes

from the precaldera Summer Coon volcano. Alternatively, a crude positive Zr (ppm) PP NE trend correlation of CaO with LOI in Dulce samples is consistent with presence of 150 (12) D2A D1 (7) much of this element in the secondary carbonate that is abundant in these (9) dikes (Supplemental File 4, Fig. S4-1D). Previously unpublished analyses of 15 0 40 80 120 Dulce dikes, from samples collected by W. Hail, Jr., and E. Landis (U.S. Geolog-

ical Survey) in the late 1960s, directly determined as much as 7.8% CO2, with

eight samples containing >3% CO2 (Supplemental File 3). The weight percent 60 D6 of calcite in these rocks would be about double that of the CO2, thus indicating (3) as much as 15% calcite in some samples. N trend A related issue for Dulce dikes concerns the proportion of introduced versus D2B D3 recycled magmatic calcium in the carbonate. Early during the present study, La (ppm) (15) (18) D4 some Dulce samples were leached in dilute HCl (15 mol%) prior to analysis 40 (15) D5 NE trend (6) (Supplemental File 4 [footnote 1]) in a failed attempt to optimize comparisons Platoro PD D2A with magmatic compositions. The acid leaching lowered CaO contents by (7) PP (5) D1 as much as 5 wt% (40% of the total for sample 15L-39). Many of the leached (12) (9) 20 trachybasalts yielded anomalously low CaO values (some <5%), only about 0 40 80 120 two-thirds that expected from projected magmatic trends. Thus, substantial Distance (dike-segment midpoint) from Alamosa River pluton, km

calcium in the carbonate appears to have been derived from magmatic minerals (mainly clinopyroxene?), rather than from external sources. An inverse

correlation of LOI with SiO2 in Dulce samples (Supplemental File 4, Fig. S4-1C) also supports interpretation that much of the CaO in secondary carbonate came from alteration of clinopyroxene, a phase which would have been abundant

in more mafic dikes. Correlation of high CaO and CO2 with low SiO2 contents in these analyses (Supplemental File 4, Fig. S4-1B) further suggests that much calcium in the carbonate was derived from alteration of pyroxene. In addition, many leached samples have low values for other elements of

interest such as P and the LREE (La, Ce, Nd), in comparison to unleached splits Sample / chondrite (Supplemental File 4 [footnote 1]). Accordingly, all analyses in Supplemental File 1 and plotted in text figures are for unleached samples. Although these analyses have been recalculated to reported totals with the LOI contents excluded, some samples with high CaO presumably have a component of introduced calcium.

Trace Elements

Trace elements also define compositional variations among the Dulce swarm that in part correlate with distance from Platoro caldera (Fig. 12). As for the major oxides, Dulce samples scatter more on trace-element diagrams than do rocks of the Platoro locus or Hinsdale lavas. Elements such as Zr and

La are markedly higher on SiO2 diagrams than for arc-related rocks of the Platoro locus, with Hinsdale lavas tending to plot at more overlapping values. Variations in some elements, such as Ni and Cr, appear to reflect magmatic processes such as presence of olivine as already noted; nonsystematic large Sample / chondrite variations in elements such as Ba and Sr are probably mainly related to alteration. Despite such data scatter, dike-swarm segment–averaged values for elements such as Zr and the LREEs increase southward (Fig. 13), consistent with

trends for TiO2 and P2O5 that suggest increasingly alkalic compositions with distance from the Platoro locus. Segment D6 contains only a single long dike; three sites sampled along 7 km of dike length have similar compositions that are the most alkalic of any segment, based on high Ti, P, Zr, and LREE content. Chondrite-normalized REEs also show sizable changes southward in the Dulce swarm. Normalized light-heavy fractionation increases (Fig. 14), with La/

YbN ratios increasing from 9 for Dulce segment D1 southward to 29 in segment D6. In comparison, proximal caldera-related lavas, dikes, and granitoid plu-

tons at Platoro are tightly grouped, with La/YbN ratios of 9–12. Only the 25 Ma

Jackson Mountain pluton, far west of Platoro, has a La/YbN (31) comparable to that of the more southerly Dulce dikes. In contrast, 19–21 Ma Hinsdale lavas close to Platoro caldera (at Red Mountain, Green Ridge) have variable REE Sample / chondrite

Figure 14. Chondrite-normalized rare-earth element (REE) plots. Dulce and Platoro data are from Supplemental File 2 (see text footnote 1); Hinsdale Formation basalts, from Turner et al. (2019); identifiers in parentheses are sample numbers. See Figure 2 for delineation of Dulce dike segments (D1–D6). Location of Green Ridge and Red Mountain shown on Figure 3. Analyses are normalized to the recommended composition for carbonaceous chondrites (McDonough and Sun, 1995). La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb

compositions, including a greater range in La (23–70 ppm, versus 28–63 ppm all indicative of large mantle-derived components (Fig. 15). Although including

for the Dulce swarm) and nearly as large REE fractionations (La/YbN, 9–23). more mafic compositions, the Dulce dikes are broadly aligned in isotopic compositions with early-rift lavas of the Hinsdale Formation southeast of Platoro. Precaldera intermediate-composition lavas of the Conejos Formation are more

Additional Areal Variations silicic and more radiogenic in εNd (Colucci et al., 1991), probably recording larger lower-crustal components. Mafic alkalic rocks of the Navajo volcanic field,

While dike-swarm segment–averaged dike compositions change southward farther west within Colorado, are more mantle like (highest in εNd). The Dulce along the Dulce swarm in multiple major and trace elements, a few dikes are dikes are thus transitional between Hinsdale lavas and the lamprophyric Navajo anomalous relative to the dominant compositions within a segment. Some outliers are likely related to the arbitrary segment boundaries that divide the continuum of the dike swarm. In addition, the great length of some dikes is likely 0.7065 Dulce dikes to have produced interfingering of contrasting compositions along traverses NVF orthogonal to the strike of the swarm. For example, sample 16L-52 from seg- Hinsdale Basalt

ment D1 has atypically high values of TiO2, P2O5, Ba, Zr, and the LREEs. In these 0.7060 Conejos Formation respects, this site is more similar to dike compositions in segment D2B, and indeed the 16L-52 site has been mapped as the northeastern end of a dike that 0.7055 continues into this segment, where two additional samples (15L-39, 17L-2) have

similar compositions (Supplemental File 1 [footnote 1]). Analogously within seg- Sr*i ment D2B, two of the 17 analyses (16L-26, 17L-7) have some characteristics (low 0.7050 P, Ti, Zr, LREEs) more closely resembling the dominant compositions of dikes in segment D1. Dikes in other segments, especially at mid distances (segments 0.7045 D3 to D5), display substantial variability but less extreme outlier compositions. Some variations may be due to incorrectly distinguishing the extent of individual dikes or to local fractionation effects. For example, the trachyandesite 0.7040 45 47 49 51 53 55 57 59 61 63 65 site 16L-66 is mapped as part of a long dike within segment D2A (Wood et SiO (wt %) al., 1948); samples farther north at three additional sites (16L-64, 16L-65, 17L-8) 2 yielded more mafic compositions with markedly lower Zr and LREE contents. 2 Some transverse variations are also present within individual segments. Sam- ples are listed in Supplemental File 1 (footnote 1) in approximate sequence from 0 west to east, orthogonal to dike trends for each segment. Within segment D1, NVF Dulce dikes

several dikes on the southeastern side of the swarm are higher in Ti and Zr than -2 Hinsdale Basalt

those to the northwest, and are transitional to compositions in segment D2B; these Nd Conejos Formation dikes also have younger isotopic ages (24.5–25.9 Ma) than dikes to the northwest, 144 -4

which are the oldest (27.5 Ma) from anywhere in the Dulce swarm. In segment D2A, Nd/ the central dikes are higher in Si and lower in Ti, P, Zr, and LREEs than flanking 143 -6

dikes. Dikes in segment D2B have a large range in SiO2 (45%–55%), tending to become more silicic to the southeast. Analyses from segments D3 and D4 also -8

have sizable ranges in SiO2 and other elements, but lack obvious transverse trends. Dikes in segment D5 are especially mafic; seven of the eight analyses have <50% -10 45 50 55 60 65 SiO2 calculated volatile-free, and silica content decreases eastward. SiO2 (wt %)

Figure 15. Isotopic comparisons (εNd, Sr*I [initial Sr isotope ratio]) for precaldera lavas (Conejos Formation) of the Platoro locus and Summer Coon Isotopic Data volcano, Dulce dikes, rift-related basaltic lavas (Hinsdale Formation), and alkalic rocks of the Navajo volcanic field (NVF). Data sources: Dulce dikes— Gibson et al. (1993), Gonzales and Lake (2017); Hinsdale Basalt—Lipman et Sparse published isotope data (εNd, Sr*I [initial Sr isotope ratio]; Gibson et al. (1978), Thompson et al. (1991); Conejos Formation—Colucci et al. (1991), al., 1993; Gonzales and Lake, 2017) show that the Dulce dikes are generally sim- Riciputi et al. (1995), Parker et al. (2005), Lake and Farmer (2015); Navajo prov- ilar to other middle Cenozoic igneous rocks in the Southern Rocky Mountains, ince—compilation from Lake and Farmer (2015), Gonzales and Lake (2017).

rocks. Unfortunately, isotopic data for the Dulce swarm are available only for the large Platoro ignimbrites are crystal-rich dacite, containing hydrous mafic dikes close to the Colorado–New Mexico line (segments D3, D4); no isotopic phenocrysts (biotite, sparse hornblende), that are compositionally similar to data exist for subalkaline NE-trending dikes farther northeast in Colorado, for ignimbrites of continental-arc affinity erupted widely in the SRMVF, elsewhere more-alkalic dikes farther south in New Mexico, or for any andesite dikes of in the western USA during the middle Cenozoic, and farther north and south Platoro type. Somewhat surprisingly, the single isotopic analysis (sample along the American Cordillera (Best et al., 2016). Consistent with their arc-type

DU-10; Gonzales, 2015, for a Miocene (20 Ma) dike is low in εNd, more similar mineralogy and geochemistry, such ignimbrites were able to spread widely to values for Conejos lavas than for the Hinsdale basalts. and with relatively uniform thickness across areas where later extension generated horsts and grabens in the Southern Rocky Mountains and elsewhere. Monzonitic plutons (Alamosa River, Jasper), which intruded caldera-filling ■■ DISCUSSION: TRANSITION FROM ARC TO RIFT andesitic lavas and are interpreted to have formed in the cores of postcaldera volcanoes at Platoro, and a porphyry intrusion into the resurgent caldera The Platoro-Dulce magmatic suite provides an exceptional space-time-com- block yielded U-Pb zircon ages analytically indistinguishable from that of the positional record for the volcanic-intrusive-tectonic evolution of an unusually last-erupted ignimbrite, the 28.8 Ma Chiquito Peak Tuff (Table 2; Gilmer et long-lived large igneous system associated with recurrent ignimbrite eruptions al., 2018). These plutons are interpreted to be shallow portions of a larger during the transition from continental-arc to extensional tectonics (Fig. 16). and vertically extensive composite batholith, underlying Platoro caldera, that Large-volume mantle-derived mafic inputs generated only intermediate-com- consisted dominantly of residual near-solidus crystal mush that accumulated position to silicic eruptions in the Platoro area during the arc stage (ca. 32–27 during the prolonged duration of large-volume andesitic-dacitic volcanism Ma) due to interaction with continental crust at this intense magmatic locus and associated intrusions of continental-arc affinity. No true basalt reached (Lipman et al., 1978; Dungan et al., 1989). Basaltic magmas were unable to the surface during this peak period of magmatism at Platoro. penetrate the upper crust until input to the Platoro locus waned, starting at Available age data indicate that the generation, rise, and crystallization of ca. 26 Ma, and regional extension overprinted the arc setting. shallow intermediate-composition magma continued episodically at declin- ing rates for several million years after the last ignimbrite eruption at Platoro, probably recording recurrent mafic recharge and enlargement of the under- Growth of the Platoro Locus lying subvolcanic batholith. A porphyritic phase of the Alamosa River pluton, the eastern Cat Creek pluton, and the western Bear Creek granitoid intrusion As elsewhere in Oligocene SRMVF, initial development of an intense have all yielded 40Ar/39Ar and U-Pb zircon ages of 28–26 Ma, modestly younger magmatic locus at Platoro was recorded by clustered growth of several inter- than peak caldera magmatism. mediate-composition volcanoes, dominantly composed of diverse high-K Injection of andesite and dacite dikes radiating from a locus approximately andesitic lavas of continental-arc type (Conejos Formation), preparatory to coincident with the Alamosa River pluton, which had commenced during the multistage ignimbrite flare-up and associated caldera collapses. Dikes of construction of precaldera Conejos volcanoes, continued after cessation of hornblende and pyroxene andesite, which radiated as much as 30 km outward ignimbrite eruptions. Radial postcaldera andesite dikes are well documented from the core of Platoro magmatic locus during this early stage, have yielded proximally along the southwestern margin of Platoro caldera; their distal extent ages of 32–31 Ma, just prior to the flare-up. The radial distribution of these remains poorly constrained. Thick sanidine-phyric dikes of silicic dacite, com- large dikes, emplaced prior to the initial caldera-related ignimbrite, is inter- parable in composition to the Platoro ignimbrites and traceable as much as preted to record stresses generated by initial upward and outward inflation 20 km to the northwest and southwest from the Platoro locus, have yielded associated with a large-scale upper-crustal locus of magma accumulation and closely clustered ages at 26.5 Ma. These appear to record a discrete late pulse growth of the clustered precaldera volcanoes at Platoro. Such precursor dike of evolved melt from a long-lived composite body of mushy magma that emplacement, while rarely documented elsewhere at large caldera systems, is remained at the Platoro locus after cessation of ignimbrite eruptions. The consistent with some recent models for the origin of caldera-forming magma radial distribution of the dikes is interpreted to reflect the continued influence bodies (Jellinek and DePaolo, 2003; Karlstrom et al., 2012). of stresses generated by intermittent upward and outward inflation of the Recurrent large eruptions of crystal-rich dacite ignimbrite in rapid succes- sub-Platoro batholith, >2 m.y. after the final caldera-related ignimbrite. sion, with a collective volume >3000 km3, repeated concurrent caldera collapse, and associated continued outpouring of intermediate-composition lavas, define the peak stage of magmatism at the Platoro locus from 30.3 to 28.8 Ma. The Initial Rift-Basalt and Flanking Magmatism first major ignimbrite (Black Mountain Tuff, at 30.3 Ma) and inferred collapse of an associated caldera record initial full development of an integrated upper- Broadly concurrently with emplacement of the latest granitoids east and crustal reservoir, within which voluminous eruptible magma accumulated. All of west of Platoro and intrusion of radial dacite dikes, basaltic lavas were first

10 Mac

Silicic A M Intermediate

Generalized compositions 20 Dulce N Dikes, lava Hinsdale

Dulce S J M S P Hinsdale Dulce NE Lavas, dikes, granitoid plutons (mainly W and E of caldera) Age (Ma) Age 60 km

Lavas, granitoid plutons 30 Ignimbrites, T M Group

Dikes Concealed Andesitic-dacitic lavas, volcaniclastic rocks, and dikes only (within (Conejos Formation) R G rift)

Figure 16. Diagrams summarizing time-space- compositional variations of the Platoro magmatic locus, Dulce dike swarm, and associated intrusions in the Southern Rocky Mountains. (A) Generalized age-compositional variations along an arcuate line SW 0 (CO-NM line) 25 50 75 100 Km East of section from the Colorado–New Mexico (CO-NM) border northeastward to the San Luis Valley seg- N-trend dikes West ank, East ank, RG rift ll NE-trend Platoro caldera ment of the Rio Grande rift (see Fig. 1 for line of San Juan Basin volcanic rocks volc. rocks (San Luis Valley) section). Abbreviations: AM—Archuleta Mesa; JM— Jackson Mountain; RG—Rio Grande; SP—Spanish Peaks; Su—South Mountain lava dome at Summit- 1500 ville; TM—Treasure Mountain Group. (B) Schematic

3 interpretation of episodes of lava eruption and intrusion during prolonged post-ignimbrite decline of calc-alkaline magmatism at the Platoro locus, concurrently with pulses of basaltic volcanism at ca. 26 and 21 Ma during initial regional extension.

1000 Ignimbrite are-upIgnimbrite

500

PLATORO LOCUS Hi-K calc-alkaline (radial stress eld) Extension-related basaltic Schematic rate of extrusive and intrusive magmatism, Km /m.y. magmatism, and intrusive of extrusive Schematic rate 35 30 25 20 15 Age (Ma)

erupted in the southeastern San Juan region at ca. 26 Ma. These are now pre- Other than the large dacite lava dome at Summitville at 23 Ma, little igne- served as erosional remnants of the Hinsdale Formation (Lipman and Mehnert, ous activity of any composition, intrusive or extrusive, has been documented 1975; Thompson et al., 1991; Turner et al., 2019) and as dikes of the Dulce swarm in the southeastern San Juan region between 25 and 21 Ma. Then, mafic and farther southwest in Colorado and northern New Mexico. The early Hinsdale silicic activity at Platoro, Hinsdale basaltic lavas, and late Dulce dikes define basalts, which include both tholeiitic and trachybasaltic compositions, appear a final regional episode. Magma as mafic as basalt initially intruded shallow to have erupted as far-traveled lavas, mainly from vents within the present-day crust or erupted proximally in the Platoro area beginning at ca. 20.5 Ma, con- Rio Grande rift (Turner et al., 2019); no basaltic lavas erupted proximal to Pla- currently with a late pulse of silicic magmatism at 21.3–20.4 Ma. This pulse toro caldera have been recognized prior to ca. 21 Ma. included eruption of rhyolite as flows and domes and intrusion of dikes of Initial intrusion of the Dulce swarm may have commenced as early as silicic crystal-rich dacite along the northern margin of the caldera, forming 27.5 Ma, as suggested by ages from several subalkaline dikes in dike-swarm a bimodal suite that contrasts with the prior caldera-related magmatism of segment D1 and trachybasalt dikes within segment D2A, but the majority dominantly intermediate compositions. Nearly concurrently, a late group of of dikes have ages of 26.5–25 Ma (Figs. 5, 16). Thus, the bulk of the Dulce north-south dikes was emplaced along the western margin of the northern swarm is closely concordant with inception of tholeiitic and trachybasaltic Dulce swarm (segments D2A, D4) at 21.4–18.6 Ma. The north-south–striking lavas of the Hinsdale Formation erupted from vents far to the east, within the dikes of segment D2A, in contrast to NE trends of older dikes at the northern Rio Grande rift. This concordance in age and magma compositions, along end of the Dulce swarm (segments D1, D2B), and the changing proportions north-south trends on both the west flank of the Laramide-age uplifts of the of magma compositions that transition to a more bimodal assemblage, are Southern Rocky Mountains and axially along the broad uplift crest, is inter- interpreted to record waning influence of the Platoro magmatic locus on the preted to mark initial impacts of the transition from magmatic arc to regional regional stress field, along with increasing impacts of extensional tectonics. crustal extension. Deflection to NE trends along northern segments of the The lengthy magmatic history of the Platoro locus (from >32 to 19 Ma), Dulce swarm (segments D1, D2B) and geometric continuity with the radial involving precaldera Conejos lavas, voluminous ignimbrites, postcaldera lavas dacite dikes of similar age shows that inflation at the Platoro locus remained and intrusion, and later extension-related Dulce dikes, constitutes an exception- an active influence on a regional stress field that was increasingly affected by ally long-lived and recurrently active magmatic system that involved the entire westward-directed extension. Variations in dike compositions, becoming gen- lithospheric section and probably deeper. Prolonged survival of an upper- erally more alkalic southward, are inferred to record decreasing proportions crustal body of mushy near-solidus magma beneath Platoro, which could be of mantle melting and/or decreasing interaction with lower crust in response recurrently reactivated by intermittent arrival of mafic mantle melts to generate to distance from the Platoro locus. Subhorizontal magma flow for distances surface eruptions and additional intrusions at shallow depth, seems likely to of 10 km or more within individual dikes may account for intermingling of exemplify general processes in formation of large-volume continental silicic subtly differing compositional types when sampled transversely across the systems (Lipman and Bachmann, 2015; Best et al., 2016; Cashman, et al., 2017). Dulce swarm. Emplacement of the satellitic Jackson Mountain granitoid body at 25 Ma was concurrent with that of the youngest dikes of the main Dulce swarm, Geometry of Rift Extension, Fault Trends, and Regional Asymmetries although ~10 km to the northwest. This intrusion is also spatially aligned with the east-west zone of granitoid plutons within and proximal to Platoro caldera Age relations of the Dulce swarm and the inception of the Rio Grande rift to (Fig. 3), but it is distinct in petrography and chemistry, having closer chemical regional NW-trending normal faults that are the dominant Cenozoic structures affinities with trachyandesite dikes of the Dulce swarm. Presence of several along the eastern margin of the San Juan Basin in southern Colorado and north- other large laccoliths and sills farther south along the western mountain front ern New Mexico (Fig. 17) remains incompletely understood. The dike swarm at Blanco Basin and V Mountain, emplaced along the contact zone between is nearly orthogonal to these NW-trending faults, especially along its north- Cretaceous sedimentary strata and overlying basal volcanic deposits, suggests eastern segments (D1, D2B), recording contrasting directions of extensional that waning of magmatic intensity at the Platoro locus and increasing impacts strain. Existing geologic maps (Wood et al., 1948; Dane, 1948) are insufficiently of regional extension may have led to a structural environment conducive to detailed to indicate reliably whether any Dulce dikes are cut by faults of this shallow-crustal magma injection laterally outward from the long-lived Platoro trend, but aligned faults with similar trends in the mountains to the east displace locus. Regionally, a broad east-west zone of magmatism was active at 27–25 Ma rocks similar in age to or younger than the dike swarm. In the eastern Tusas (Figs. 1–3), discontinuously from the Navajo province on the Colorado Plateau Mountains just south of the Colorado–New Mexico state line, basaltic lavas of (Gonzales and Lake, 2017) eastward through Jackson Mountain, Platoro caldera, the Hinsdale Formation at least as young as 25.6 Ma (Turner et al., 2019, their and Cat Creek volcano and its central intrusion, to the Questa-Latir locus east figure 3-23) are cut by a parallel system of NW-trending faults. Another large of the Rio Grande rift (Lipman, 1988; Zimmerer and McIntosh, 2012a), and on fault system, which trends NW from Platoro to Wolf Creek Pass in Colorado to the High Plains at Spanish Peaks (Penn and Lindsey, 1996). (Fig. 17), displaces the 23 Ma dacite dome at Summitville and probably the

PLATORO CALDERA

JM CRETACEOUS SEDIMENTARY ROCKS, SAN JUAN BASIN Platoro: proximal BB Platoro map (Lipman, 1974) dikes

VOLCANIC ROCKS, SAN JUAN MOUNTAINS V- Mtn

o 37 N AM Colorado New Mexico Dike sample sites

Dacite

Andesite

Trachybasalt

Normal fault

Granitoid intrusions

Geologic N base maps: CO: Steven et al. (1974)

NM: NMBGMR (2003) 20 kilometers

107 o W

Figure 17. Geometric relations of Dulce dikes and regional fault trends. Mid-Cenozoic displacement ages of at least some faults require southwest-directed extension, in contrast to a westward direction during intrusion of the north-trending Dulce dikes. Abbreviations and dike-swarm segments are the same as in Figure 2. CO—Colorado; NM—New Mexico.

crystal-rich dacite-rhyolite of North Mountain, for which one dated phase is Another regional asymmetry in the SRMVF is the more areally extensive as young as 20.7 Ma (sample 11L-19). And to the east, in the Questa-Latir area, and voluminous mid-Cenozoic volcanism and intrusion on the western flanks of many NW-trending dikes are younger than the 23 Ma Rio Hondo pluton (Lip- the Laramide uplift than to the east (Fig. 1). Western centers include the Mount man, 1988), with at least one as young as 16.6 Ma (Zimmerer and McIntosh, Princeton batholith and ignimbrite calderas along the present-day Sawatch 2012a). Within the eastern San Juan Basin near the state line, a NW-trending Range, continuing southward into the San Juan region and to the northern fault displaces the 15.5 Ma Archuleta Mesa sill by ~100 m, yet 20 Ma and 25 Tusas Mountains. Eruptive centers to the east, including the Latir-Questa locus Ma dikes nearby to the east and west trend due north. These relations suggest (the only eastern ignimbrite source) and intermediate-composition volcanoes that episodes of SW-directed extension have alternated with periods of west-di- to the north (Rosita, 39 Mile, Buffalo Butte), are notably smaller in volume and rected strain during early rifting in the Southern Rocky Mountains. Additional extent. The eastern volcanic rocks, especially in the 39 Mile area and farther detailed mapping could improve understanding of the temporal and geometric east, consist largely of ignimbrites from calderas in the Sawatch Range to relations between emplacement of Dulce dikes and regional faulting. the west. The location of the lengthy Dulce swarm, intruded into little-deformed No arc-type caldera of the SRMVF other than Platoro, including those closer Mesozoic strata along the eastern margin of the Colorado Plateau, across the in age to inception of regional extension at ca. 26 Ma, is closely associated with Continental Divide from the present-day Rio Grande rift, is unique in the region a comparably developed radial dike system or other prolonged postcaldera and somewhat puzzling. Perhaps initial extension along Dulce and Rio Grande mafic volcanism. The numerous dikes associated with granitoid plutons south axes was influenced by geometry of the broad Laramide-age uplift in the of the early-rift Questa caldera are rhyolitic, trend NW, and are geometrically Southern Rocky Mountains (Fig. 18). This uplift is well defined by Proterozoic related to regional extension, largely unaffected by the adjacent caldera (Lip- basement rocks exposed along the Sangre de Cristo Range on the east and the man, 1988). Erosional remnants of Miocene trachybasalt lavas and rare mafic Sawatch Range and Tusas Mountains to the west of the axial rift basins (San dikes farther north and west in the San Juan region (Lipman and Mehnert, Luis and upper Arkansas River valleys; Fig. 1). A broadly monoclinal crustal 1975; Gonzales, 2015) likely represent less-focused responses to late extension, hinge along the western margin of the regional Laramide uplift, as defined in without clear age or geometric links to nearby calderas. part by the Paleogene depositional axis of the San Juan Basin (Cather et al., Available models for caldera resurgence do not predict dike emplacement or 2018), may have become a locus of weak extension, influencing intrusion of other deformation beyond the area of caldera subsidence (Marsh, 1984; Roche the Dulce dikes. In contrast, at least the main initial Hinsdale vents (at ca. 26 et al., 2000; Galetto et al., 2017). However, these models do not include possible Ma: Turner et al., 2019) were along the crest of Laramide uplift, which became effects from upper-crustal magma bodies that may be substantially larger than the dominant axis of extension for present-day rift geometry. the associated caldera, or possible linkages between magma emplacement An additional geometric complexity of the magmato-tectonic transition is and regional tectonic stress. Perhaps the location of the Platoro system, along the more intense radial diking on the western side of Platoro caldera than to the or just beyond the south margin of the large gravity low that is interpreted east. Scattered dikes of andesite and dacite and several small laccoliths radiate to record the presence of a subvolcanic batholith (Plouff and Pakiser, 1972; outward for a few kilometers from the eastern Cat Creek pluton (Lipman, 1974), Drenth et al., 2012), permitted relatively efficient interconnection with the but these are modest in number and extent compared to the western dikes changing regional stress field, in contrast to the calderas more centrally above that project outward from the locus near the Alamosa River pluton. Basaltic a vertically and laterally extensive batholith. Whatever the ultimate controls dikes, more comparable to the Dulce swarm, may underlie rift fill axially in on the voluminous magmatism at the Platoro locus, it provides an exceptional the San Luis Valley, but despite less rugged topographic relief, the Cenozoic site for evaluation of processes at a long-lived (in this case, >10 m.y.) crustal volcanic rocks east of Platoro are sufficiently exposed to preclude any connect- system of batholithic scale. ing dike system on a scale comparable to those west of Platoro. More likely, the contrasting scales of dike emplacement reflect influence of the regional tectonic framework: the west flank of the broad Laramide-age uplift may have ■■ MAGMATO-TECTONIC HAZARD IMPLICATIONS FOR CALDERA formed an eastern buttress and barrier to dike emplacement. At the Platoro SYSTEMS locus situated roughly along the hinge zone along this flank of the Laramide uplift (Fig. 18), dikes propagated preferentially westward toward the Colorado The implied link between tectonic and magmatic regimes, and associated Plateau. Such a structural interpretation of dike geometry at Platoro would impacts of abrupt shifts over geologically short time scales, has implications for be analogous to that previously advanced for counterpart asymmetry at the continental structural evolution and long-term hazards associated with distal Spanish Peaks intrusive cluster (Fig. 18), where central granitoid intrusions caldera magmatism. Rapid emplacement of dikes comparable in scale to the and the adjacent asymmetrical dike system were localized along the eastern Platoro-Dulce swarm, in conjunction with late evolution of a caldera-related flank of the Rocky Mountains uplift, mainly from 26 to 21 Ma (Odé, 1957; Penn batholith, could have generated extension-related earthquakes and triggered and Lindsey, 1996). dispersed volcanism. Such events would constitute previously little-recognized

108°W 106° 104° 40° N 0 60 mi

0 100 km Denver F r N o n LARAMIDEt

BASEMENTR a n WMT

UPLIFTS g

l S e l

COLORADO l a South l GP l l w Park l l l a t PLATEAU c Colorado h BP 39 Mile Springs volcanic

MP area MA l

l West Elk l l locus R Gunnison g x e HIGH

l Bz

l l

l W Figure 18. Regional magmato-tectonic interpreta-

l PLAINS

l e l l C l t tion, inferring combined effects of localized uplift l l

NP l

l l S

l l and radial stress field associated with the Platoro l l

l l l

l l

l l l l

l l

l a

l l

l M

l l

l Saguache n magmatic locus during the mid-Cenozoic (ca. 35– l l

l l

l l Structural

l t l

38° l l l g

l SL s

l l

l l l 20 Ma) overlapped by an increasing component l

l l l l r

l l l boundary . l

l l l l l l e

l l l l l

l l

l of west-directed regional extension commencing

l LGn

l l

l San Luis Valley segment,

l l

S l l B l l

l San l l

l l

l l ca. 26 Ma. Most abbreviations are the same as l l

l l l l

l l

l Juan l l SC l l l in Figure 1. Other abbreviations: Colo—Colorado;

l Cr

volcanic l l

l l l l Del Plat—Platoro caldera; R.G.—Rio Grande; WMT—Wall

l x

l Rio Grande rift

locus l l l SR l

l Norte Mountain Tuff. l

l l l l l l d l l

l l e

l l

LGsl l l

l l l l l

l l

Approx. original limit l l l Plat l l Pl l of volcanic rocks l l l

Spanish

r Peaks

s t Colorado

SAN o New Mexico T

u l l

JUAN s l

a l Questa-Latir s l volcanic l M BASIN M l l l l locus t t s s . .

Explanation

Granitic intrusion MP Mount Princeton batholith

Mid-Cenozoic volcanic areas Trend of Colorado Mineral Belt

l l Major San Juan Sedimentary fill of Rio Grande Rift l l l Caldera Bouguer l R.G. rift Late-rift mafic lavas Regional structural attitude gravity low faults

magmato-tectonic hazards that could occur near active and dormant calderas precaldera (31.2–30.4 Ma) to as young as 20.3 Ma, but many or all of the dacite in the western USA and elsewhere. For example, emplacement of long-traveled dikes were emplaced concurrently with the main Dulce swarm. mafic dikes southward from Valles caldera in New Mexico, along extensional Dikes of the Platoro-Dulce swarm have large variations in textures, phe- faults of the Rio Grande rift and comparable in scale to the 125 km Dulce nocrysts, and elemental compositions that correlate broadly with distance from swarm, could reach Albuquerque (New Mexico) or farther south. Such dikes Platoro caldera, becoming more mafic and alkalic outward. The dike system would be too young to have been exposed by erosion, but perhaps the basaltic is associated with protracted postcaldera volcanic and large-volume intrusive vents that are aligned north-south on San Felipe Mesa and farther south at activity at Platoro, recording interactions between caldera-related and regional the Albuquerque volcanoes (a basaltic fissure eruption as young as 220 ka; stress fields and appearing to document varying magma sources in cratonic Peate et al., 1996; Singer et al., 2008) constitute a younger counterpart to the lithosphere as a function of distance from the caldera locus. Proximal radial Dulce swarm. These vents extend 80 km or more from Valles caldera, which dikes are inferred to record the effects of magma inflation and uplift associated last erupted a large ignimbrite at 1.26 Ma but has continued to be active in with prolonged assembly and solidification of a composite batholith beneath smaller volumes as recently as 68 ka (Wolff et al., 2011; Zimmerer et al., 2016). Platoro caldera. Concurrently with late Platoro magmatism, linear dikes of the Zircons from the 68 ka eruption yield a spectrum of U-Th ages back to secular Dulce swarm were emplaced during weak extension along the eastern margin equilibrium (ca. 350 ka), documenting a long-lived system beneath the caldera of the Colorado Plateau, satellitic to the early Rio Grande rift and associated from which deep mafic magma could have intermittently propagated as dikes. basaltic volcanism farther east. More broadly, these features are interpreted to Similarly, north of Long Valley caldera, which erupted a major ignimbrite at record a regional transition from arc- to rift-related magmatism in a continental 760 ka, the Inyo-Mono chain of rhyolite domes has been active as recently as setting, with implications for potential hazards near active calderas elsewhere. 650 yr B.P., in association with northward-propagating dikes (Hildreth, 2004). Any basaltic dikes intruded along this trend for distances comparable to that of the Dulce swarm could come close to the Nevada state capital at Carson ACKNOWLEDGMENTS City. At the smaller Crater Lake caldera in Oregon, proximal basaltic vents We especially thank Kyle Tator and Todd Asmera for arranging access within the Jicarilla Apache form radial trends for at least 12 km from rims of this 7700 yr B.P. ignimbrite Nation Reservation. William McIntosh, Kyle Anderson, and Amy Gilmer assisted with some field- work. Also much appreciated are diverse data on southeast San Juan igneous rocks provided center (Bacon and Lanphere, 2006), while more distal mafic vents are parallel by other researchers. Previously unpublished major-oxide chemical analyses of Dulce samples to regional extensional faults that trend north-south. On older, larger scales, (Supplemental File 3 [footnote 1]), shared with Lipman more than 40 years ago by W.J. Hail, Jr., could the Miocene dikes associated with basaltic eruptions on the Columbia and E. Landis of the U.S. Geological Survey, constituted an early stimulus for the present study. Amy Gilmer determined U-Pb zircon ages on five samples of granitoid intrusions we had collected Plateau in Oregon-Washington and mafic vents along the Northern Nevada in the Platoro area (Gilmer et al., 2018) and also provided electron-microprobe analyses of mafic rift zone to the south (Camp et al., 2015) constitute distal mafic components, minerals in a Dulce dike. Samuel Johnson shared preliminary results of his magnetic-anisotropy broadly analogous to the Dulce dike swarm, that project at least 200 km north studies that help constrain dike-emplacement processes. Amy Gilmer, Kathryn Watts, David Gonzales, an anonymous reviewer, and Geosphere associate editor Valerio Acocella provided and south from concurrent loci of intense ignimbrite-caldera magmatism at helpful comments on the manuscript. Any use of trade, firm, or product names is for descriptive 16–14 Ma during inception of the Yellowstone hot spot? Regardless of whether purposes only and does not imply endorsement by the U.S. Government. these calderas are genetically linked to smaller eruptions by long dikes, the Platoro-Dulce dikes demonstrate magma emplacement at great distances from a source caldera. Such relationships suggest that the regional extent of REFERENCES CITED magmato-tectonic hazards associated with calderas may be underestimated. Acocella, V., 2014, Structural control on magmatism along divergent and convergent plate boundaries: Overview, model, problems: Earth-Science Reviews, v. 136, p. 226–288, https://doi.org /10.1016/j.earscirev.2014.05.006. Aldrich, M.J., Chapin, C.E., and Laughlin, A.W., 1986, Stress history and tectonic development of ■■ SUMMARY the Rio Grande rift, New Mexico: Journal of Geophysical Research, v. 91, p. 6199–6211, https:// doi.org/10.1029/JB091iB06p06199. Bacon, C.R., and Lanphere, M.A., 2006, Eruptive history and geochronology of Mount Mazama and The sweeping continuity of dikes that radiate westward from the Platoro the Crater Lake region, Oregon: Geological Society of America Bulletin, v. 118, p. 1331–1359, caldera locus to merge with the arcuate Dulce swarm, in conjunction with https://doi.org/10.1130/B25906.1. overlapping ages and partial convergence in chemical arrays, suggest that Best, M.G., Christiansen, E.H., de Silva, S., and Lipman, P.W., 2016, Slab-rollback ignimbrite flareups they constitute a long-lived integrated dike system of regional extent. Dikes in the southern Great Basin and other Cenozoic American arcs: A distinct style of volcanism: Geosphere, v. 12, p. 1097–1135, https://doi.org/10.1130/GES01285.1. of the Dulce swarm have a broad range of trachybasaltic to trachyandesitic Camp, V.E., Pierce, K.L., and Morgan, L.A., 2015, Yellowstone plume trigger for Basin and Range

compositions (46%–56% SiO2) and were emplaced during two pulses (mainly extension, and coeval emplacement of the Nevada–Columbia Basin magmatic belt: Geosphere, 26.5–25.0 Ma, with a smaller pulse at 21.4–18.7 Ma). Proximal dikes that radi- v. 11, p. 203–225, https://doi.org/10.1130/GES01051.1. Cashman, K.V., Sparks, R.S.J., and Blundy, J.D., 2017, Vertically extensive and unstable magmatic ate westward from Platoro caldera to merge with the Dulce swarm are more systems: A unified view of igneous processes: Science, v. 355, eaag3055, https://doi .org/10

silicic (andesite-dacite; mostly 54%–67% SiO2) and range widely in age, from .1126/science.aag3055.

Cather, S.M., 2004, Laramide orogeny in central and northern New Mexico and southern Colorado, Jellinek, A.M., and DePaolo, D.J., 2003, A model for the origin of large silicic magma chambers: in Mack, G.H., and Giles, K.A., eds., The Geology of New Mexico: A Geological History: New Precursors of caldera-forming eruptions: Bulletin of Volcanology, v. 65, p. 363–381, https:// Mexico Geological Society Special Publication 11, p. 203–248. doi.org/10.1007/s00445-003-0277-y. Cather, S.M., Connell, S.D., Chamberlin, R.M., McIntosh, W.C., Jones, G.E., Potochnik, A.R., Lucas, Johnson, S.P., Geissman, J.W., Zimmerer, M.J., and Lipman, P.W., 2016, Evaluation of magma flow S.G., and Johnson, P.S., 2008, The Chuska erg: Paleogeomorphic and paleoclimatic impli- and emplacement mechanisms of the mafic Dulce-Platoro dike swarm, NW New Mexico and cations of an Oligocene sand sea on the Colorado Plateau: Geological Society of America SW Colorado using magnetic susceptibility fabrics: Geological Society of America Abstracts Bulletin, v. 120, p. 13–33, https://doi.org/10.1130/B26081.1. with Programs, v. 48, no. 7, https://doi.org/10.1130/abs/2016AM-287759. Cather, S.M., Chapin, C.E., and Kelley, S.A., 2012, Diachronous episodes of Cenozoic erosion in Karlstrom, L., Rudolph, M.L., and Manga, M., 2012, Caldera size modulated by the yield stress southwestern North America and their relationship to surface uplift, paleoclimate, paleodrain- within a crystal-rich magma reservoir: Nature Geoscience, v. 5, p. 402–405, https://doi.org ages, and paleoaltimetry: Geosphere, v. 8, p. 1177–1206, https://doi.org/10.1130/GES00801.1. /10.1038/ngeo1453. Cather, S.M., Heizler, M.T., and Williamson, T.E., 2018, Detrital sanidine and paleocurrent constraints Kuiper, K.F., Deino, A., Hilgen, F.J., Krijgsman, W., Renne, P.R., and Wijbrans, A.J., 2008, Syn- on deposition of the Paleocene Nacimiento and Animas Formations, San Juan Basin, New chronizing rock clocks of Earth history: Science, v. 320, p. 500–504, https://doi .org/10.1126 Mexico: Geological Society of America Abstracts with Programs, v. 50, no. 5, https://doi.org /science.1154339. /10.1130/abs/2018RM-314150. Lake, E.T., and Farmer, G.L., 2015, Oligo-Miocene mafic intrusions of the San Juan Volcanic Field, Chapin, C.E., and Lindley, J.F., 1986, Potassium metasomatism of igneous and sedimentary rocks southwestern Colorado, and their relationship to voluminous, caldera-forming magmas: in detachment terranes and other sedimentary basins: Economic implications, in Beatty, B., Geochimica et Cosmochimica Acta, v. 157, p. 86–108, https://doi .org /10 .1016 /j .gca .2015 .02 .020. and Wilkinson, P.A.K., eds., Frontiers in Geology and Ore Deposits of Arizona and the South- Larsen, E.S., Jr., and Cross, C.W., 1956, Geology and petrology of the San Juan region, southwest: Arizona Geological Society Digest 16, p. 118–126. western Colorado: U.S. Geological Survey Professional Paper 258, 303 p., https://doi.org/10 Christiansen, R.L., Lipman, P.W., Orkild, P.P., and Byers, F.M., Jr., 1965, Structure of the Timber .3133/pp258. Mountain caldera, southern Nevada, and its relation to Basin-Range structure, in Geological Leat, P.T., Thompson, R.N., Morrison, M.A, Hendry, G.L., and Dickin, A.P., 1988, Compositional- Survey Research 1965, Chapter B: U.S. Geological Survey Professional Paper 525-B, p. B43–B48. ly-diverse Miocene–Recent rift-related magmatism in northwest Colorado: Partial melting, Cole, J.W., Miller, D.M., and Spinks, K.D., 2005, Calderas and caldera structures: A review: Earth-Sci- and mixing of magmas from 3 different asthenospheric and lithospheric mantle sources: ence Reviews, v. 69, p. 1–26, https://doi.org/10.1016/j.earscirev.2004.06.004. Journal of Petrology, Special Volume, no. 1, p. 351–377, https://doi.org/10.1093/petrology Colucci, M.T., Dungan, M.T., Ferguson, K.M., Lipman, P.W., and Moorbath, S., 1991, Precaldera /Special_Volume.1.351. lavas of the southeast San Juan volcanic field: Parent magmas and crustal interactions: Lipman, P.W., 1968, Geology of Summer Coon volcanic center, eastern San Juan Mountains, Journal of Geophysical Research, v. 96, p. 13,413–13,434, https://doi.org/10.1029/91JB00282. Colorado: Colorado School of Mines Quarterly, v. 63, p. 211–236. Coney, P.J., and Reynolds, S.J., 1977, Flattening of the Farallon slab: Nature, v. 270, p. 403–406, Lipman, P.W., 1974, Geologic map of the Platoro caldera area, southeastern San Juan Moun- https://doi.org/10.1038/270403a0. tains, Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map I-828, scale Coombs, M.L., Sisson, T.W., and Lipman, P.W., 2006, Growth history of Kilauea inferred from vol- 1:48,000. atile concentrations in submarine-collected basalts: Journal of Volcanology and Geothermal Lipman, P.W., 1975, Evolution of the Platoro caldera complex and related volcanic rocks, south- Research, v. 151, p. 19–49, https://doi.org/10.1016/j.jvolgeores.2005.07.037. eastern San Juan Mountains, Colorado: U.S. Geological Survey Professional Paper 852, 128 Dane, C.H., 1948, Geologic map of part of eastern San Juan Basin, Rio Arriba County, New Mexico: p., https://doi.org/10.3133/pp852. U.S. Geological Survey Oil and Gas Preliminary Map 78, scale 1:63,360. Lipman, P.W., 1980, Cenozoic volcanism in the western United States: Implications for continental Drenth, B.J., Keller, G.R., and Thompson, R.A., 2012, Geophysical study of the San Juan batholith tectonics, in Burchfiel, B.D., Oliver, J.E., and Silver, L.T., eds., Continental Tectonics: Wash- complex, southwestern Colorado: Geosphere, v. 8, p. 669–684, https://doi .org /10 .1130 /GES00723 .1. ington, D.C., National Research Council, p. 161–174. Dungan, M.A., Lipman, P.W., Colucci, M., Ferguson, K., and Balsley, S., 1989, Southeastern (Platoro) Lipman, P.W., 1988, Evolution of silicic magma in the upper crust: The mid-Tertiary Latir volca- caldera complex, in Lipman, P.W., ed., IAVCEI Fieldtrip Guide: Oligocene-Miocene San Juan nic field and its cogenetic granitic batholith, northern New Mexico, U.S.A.: Transactions of Volcanic Field, Colorado (Field Excursions to Volcanic Terranes in the Western United States, the Royal Society of Edinburgh: Earth Sciences, v. 79, p. 265–288, https://doi .org/10.1017 Volume 1: Southern Rocky Mountain Region): New Mexico Bureau of Mines and Mineral /S0263593300014279. Resources Memoir 46, p. 305–329. Lipman, P.W., 2007, Incremental assembly and prolonged consolidation of Cordilleran magma Fleck, R.J., Sutter, J.F., and Elliot, D.H., 1977, Interpretation of discordant 40Ar/39Ar age-spectra chambers: Evidence from the Southern Rocky Mountain volcanic field: Geosphere, v. 3, of Mesozoic tholeiites from Antarctica: Geochimica et Cosmochimica Acta, v. 41, p. 15–32. p. 42–70, https://doi.org/10.1130/GES00061.1. Galetto, F., Acocella, V., and Caricchi, L., 2017, Caldera resurgence driven by magma viscosity Lipman, P.W., and Bachmann, O., 2015, Connecting ignimbrite to batholith in the Southern Rocky contrasts: Nature Geoscience, v. 8, 1750, https://doi.org/10.1038/s41467-017-01632-y. Mountains: Integrated perspectives from geological, geophysical, and geochronological data: Gibson, S.A., Thompson, R.N., Leat, P.T., Morrison, M.A., Hendry, G.L., Dickin, A.P., and Mitchell, Geosphere, v. 11, p. 705–743, https://doi.org/10.1130/GES01091.1. J.G., 1993, Ultrapotassic magmas along the flanks of the Oligo-Miocene Rio Grande rift, USA: Lipman, P.W., and Mehnert, H.H., 1975, Late Cenozoic basaltic volcanism and development of the Monitors of the zone of lithospheric mantle extension and thinning beneath a continental rift: Rio Grande depression in the Southern Rocky Mountains, in Curtis, B.F., ed., Cenozoic History Journal of Petrology, v. 34, p. 187–228, https://doi.org/10.1093/petrology/34.1.187. of the Southern Rocky Mountains: Geological Society of America Memoir 144, p. 119–154, Gilmer, A.K., Thompson, R.A., Lipman, P.W., and Vazquez, J.A., 2018, Connecting volcanic and https://doi.org/10.1130/MEM144-p119. plutonic activity at the Platoro caldera complex, Southern Rocky Mountains Volcanic Field, Lipman, P.W., Steven, T.A., and Mehnert, H.A., 1970, Volcanic history of the San Juan Mountains, Colorado: Abstract V33D-0264 presented at American Geophysical Union Fall Meeting, Wash- Colorado, as indicated by potassium-argon dating: Geological Society of America Bulletin, ington, D.C., 10–14 December. v. 81, p. 2329–2352, https://doi.org/10.1130/0016-7606(1970)81[2329:VHOTSJ]2.0.CO;2. Gonzales, D.A., 2015, New U-Pb zircon and 40Ar/39Ar age constraints of the Cenozoic plutonic Lipman, P.W., Prostka, H.J., and Christiansen, R.L., 1972, Cenozoic volcanism and plate-tectonic record, southwestern Colorado: Implications for regional magmatic-tectonic evolution: The evolution of the Western United States: I. Early and middle Cenozoic: Philosophical Transac- Mountain Geologist, v. 52, no. 2, p. 5–32. tions of the Royal Society of London, Series A: Mathematical and Physical Sciences, v. 271, Gonzales, D.A., and Lake, E.T., 2017, Geochemical constraints on mantle-melt sources for Oligocene p. 217–248, https://doi.org/10.1098/rsta.1972.0008. to Pleistocene mafic rocks in the Four Corners region, USA: Geosphere, v. 13, p. 201–226, Lipman, P.W., Doe, B.R., Hedge, C.E., and Steven, T.A., 1978, Petrologic evolution of the San Juan https://doi.org/10.1130/GES01314.1. volcanic field, southwestern Colorado: Pb and Sr isotope evidence: Geological Society of Hildreth, W., 2004, Volcanological perspectives on Long Valley, Mammoth Mountain, and Mono America Bulletin, v. 89, p. 59–82, https://doi .org /10 .1130 /0016 -7606 (1978)89 <59: PEOTSJ>2 .0 .CO;2. Craters: Several contiguous but discrete systems: Journal of Volcanology and Geothermal Lipman, P.W., Dungan, M.A., Brown, L.D., and Deino, A., 1996, Recurrent eruption and subsidence Research, v. 136, p. 169–198, https://doi.org/10.1016/j.jvolgeores.2004.05.019. at the Platoro caldera complex, southeastern San Juan volcanic field, Colorado: New tales

from old tuffs: Geological Society of America Bulletin, v. 108, p. 1039–1055, https://doi .org/10 Roche, O., Druitt, T.H., and Merle, O., 2000, Experimental study of caldera formation: Journal of .1130/0016-7606(1996)108<1039:REASAT>2.3.CO;2. Geophysical Research, v. 105, p. 395–416, https://doi.org/10.1029/1999JB900298. Lipman, P.W., Zimmerer, M.J., and McIntosh, W.C., 2015, An ignimbrite caldera from the bottom up: Shapiro, L., 1967, Rapid analysis of rocks and minerals by a single-solution method, in Geo- Erosionally exhumed floor and fill of the resurgent Bonanza caldera, Southern Rocky Mountain logical Survey Research 1967, Chapter B: U.S. Geological Survey Professional Paper 575-B, volcanic field, Colorado: Geosphere, v. 11, p. 1902–1947, https://doi.org/10.1130/GES01184.1. p. B187–B191. Marsh, B.D., 1984, On the mechanics of caldera resurgence: Journal of Geophysical Research, Singer, B.S., Jicha, B.R., Kirby, B.T., Geissman, J.W., and Herrero-Bervera, E., 2008, 40Ar/39Ar dating v. 89, p. 8245–8251, https://doi.org/10.1029/JB089iB10p08245. links Albuquerque Volcanoes to the Pringle Falls excursion and the Geomagnetic Instability McDonough, W.F., and Sun, S.-s., 1995, The composition of the Earth: Chemical Geology, v. 120, Time Scale: Earth and Planetary Science Letters, v. 267, p. 584–595, https://doi .org/10.1016/j p. 223–253, https://doi.org/10.1016/0009-2541(94)00140-4. .epsl.2007.12.009. McIntosh, W.C., and Chapin, C.E., 2004, Geochronology of the central Colorado volcanic field,in Smith, R.L., and Bailey, R.A., 1968, Resurgent cauldrons, in Coates, R.R., Hay, R.L., and Anderson, Cather, S.M., McIntosh, W.C., and Kelley, S.A., eds., Tectonics, Geochronology, and Volcanism C.A., eds., Studies in Volcanology: Geological Society of America Memoir 116, p. 613–662, in the Southern Rocky Mountains and Rio Grande Rift: New Mexico Bureau of Geology and https://doi.org/10.1130/MEM116-p613. Mineral Resources Bulletin 160, p. 205–237. Steven, T.A., 1975, Middle Tertiary volcanic field in the Southern Rocky Mountains,in Curtis, B.F., Mehnert, H.H., Lipman, P.W., and Steven, T.A., 1973, Age of mineralization at Summitville, Colorado, ed., Cenozoic History of the Southern Rocky Mountains: Geological Society of America Memoir as indicated by K-Ar dating of alunite: Economic Geology and the Bulletin of the Society of 144, p. 75–94, https://doi.org/10.1130/MEM144-p75. Economic Geologists, v. 68, p. 399–401, https://doi.org/10.2113/gsecongeo.68.3.399. Steven, T.A., and Lipman, P.W., 1976, Calderas of the San Juan volcanic field, southwestern Col- Mutschler, F.E., Larson, E.E., and Bruce, R.M., 1987, Laramide and younger magmatism in Colo- orado: U.S. Geological Survey Professional Paper 958, 35 p., https://doi.org/10.3133/pp958. rado—New petrologic and tectonic variations on old themes, in Drexler, J.W., and Larson, Steven, T.A., Mehnert, H.H., and Obradovich, J.D., 1967, Age of volcanic activity in the San Juan E.E., eds., Cenozoic Volcanism in the Southern Rocky Mountains Updated: A Tribute to Rudy Mountains, Colorado, in Geological Survey Research 1967, Chapter D: U.S. Geological Survey C. Epis—Part I: Colorado School of Mines Quarterly, v. 82, no. 4, p. 1–47. Professional Paper 575-D, p. D47–D55. Nakamura, K., 1977, Volcanoes as possible indicators of tectonic stress orientation—Principle and Steven, T.A., Lipman, P.W., Hail, W.J., Jr., Barker, F., and Luedke, R.G., 1974, Geologic map of the proposal: Journal of Volcanology and Geothermal Research, v. 2, p. 1–16, https://doi.org/10 Durango quadrangle, southwestern Colorado: U.S. Geological Survey Miscellaneous Inves- .1016/0377-0273(77)90012-9. tigations Map I-764, scale 1:250,000, https://doi.org/10.3133/i764. New Mexico Bureau of Geology and Mineral Resources (NMBGMR), 2003, Geologic map of New Thompson, R.A., Johnson, C.M., and Mehnert, H.H., 1991, Oligocene basaltic volcanism of the Mexico: scale 1:500,000. Northern Rio Grande Rift: San Luis Hills, Colorado: Journal of Geophysical Research, v. 96, Odé, H., 1957, Mechanical analysis of the dike pattern of the Spanish Peaks area, Colorado: Geo- p. 13,577–13,592, https://doi.org/10.1029/91JB00068. logical Society of America Bulletin, v. 68, p. 567–576, https://doi .org /10 .1130 /0016 -7606 (1957)68 Turner, K.J., Thompson, R.A., Chan, C.F., Cosca, M.A., Morgan, L.E., and Drenth, B.J., 2019, Taos [567:MAOTDP]2.0.CO;2. Plateau volcanic field and central San Luis Basin: in Geologic Field-Trip Guide to Continental Parker, D.F., Ghosh, A., Price, C.W., Rinard, B.D., Culler, R.L., and Ren, M., 2005, Origin of rhyolite Arc to Rift Volcanism of the Southern Rocky Mountains—Southern Rocky Mountain, Taos by crustal melting and the nature of parental magmas in the Oligocene Conejos Formation, Plateau, and Jemez Volcanic Fields of Southern Colorado and Northern New Mexico: U.S. San Juan Mountains, Colorado, USA: Journal of Volcanology and Geothermal Research, Geological Survey Scientific Investigations Report 2017-5022-R (in press). v. 139, p. 185–210, https://doi.org/10.1016/j.jvolgeores.2004.08.015. Tweto, O., 1975, Laramide (Late Cretaceous–early Tertiary) orogeny in the Southern Rocky Moun- Patton, H.B., 1917, Geology and ore deposits of the Platoro-Summitville mining district, Colorado: tains, in Curtis, B.F., ed., Cenozoic History of the Southern Rocky Mountains: Geological Colorado Geological Survey Bulletin 13, 122 p. Society of America Memoir 144, p. 1–44, https://doi.org/10.1130/MEM144-p1. Peate, D.W., Chen, J.H., Wasserburg, G.J., and Papanastassiou, D.A., 1996, 238U-230Th dating of a Tweto, O., 1979, compiler, Geologic map of Colorado: Denver, Colorado, U.S. Geological Survey, geomagnetic excursion in Quaternary basalts of the Albuquerque Volcanoes field, New Mexico scale 1:500,000. (USA): Geophysical Research Letters, v. 23, p. 2271–2274, https://doi.org/10.1029/96GL02064. Wolff, J.A., Brunstad, K.A., and Gardner, J.N., 2011, Reconstruction of the most recent volcanic Penn, B.S., and Lindsey, D.A., 1996, Cenozoic igneous rocks and Laramide structure and stratig- eruptions from the Valles caldera, New Mexico: Journal of Volcanology and Geothermal raphy of the Spanish Peaks region, south-central Colorado: Road log and descriptions from Research, v. 199, p. 53–68, https://doi.org/10.1016/j.jvolgeores.2010.10.008. Walsenberg to La Veta (first day) and La Veta to Aguilar (second day): Colorado Geological Wood, G.H., Kelley, V.C., and MacAlpin, A.J., 1948, Geology of the southern part of Archuleta Survey Open-File Report 96-04-28D, 21 p. County, Colorado: U.S. Geological Survey Oil and Gas Investigations Preliminary Map OM-81, Plouff, D., and Pakiser, L.C., 1972, Gravity study of the San Juan Mountains, Colorado, in Geological scale 1:63,360, https://doi.org/10.3133/om81. Survey Research 1972, Chapter B: U.S. Geological Survey Professional Paper 800-B, p. 183–190. Wotzlaw, J.-F., Schaltegger, U., Frick, D.A., Dungan, M.A., Gerdes, A., and Günther, D., 2013, Track- Ratté, J.C., and Steven, T.A., 1967, Ash flows and related volcanic rocks associated with the Creede ing the evolution of large-volume silicic magma reservoirs from assembly to supereruption: caldera, San Juan Mountains, Colorado: U.S. Geological Survey Professional Paper 524-H, Geology, v. 41, p. 867–870, https://doi.org/10.1130/G34366.1. 58 p., https://doi.org/10.3133/pp524H. Zimmerer, M.J., and McIntosh, W.C., 2012a, The geochronology of volcanic and plutonic rocks at Ricketts, J.W., Kelley, S.A., Karlstrom, K.E., Schmandt, B., Donahue, M.S., and van Wilk, J., 2016, the Questa caldera: Constraints on the origin of caldera-related silicic magmas: Geological Synchronous opening of the Rio Grande rift along its entire length at 25–10 Ma supported by Society of America Bulletin, v. 124, p. 1394–1408, https://doi.org/10.1130/B30544.1. apatite (U-Th)/He and fission-track thermochronology, and evolution of possible driving mecha- Zimmerer, M.J., and McIntosh, W.C., 2012b, An investigation of caldera-forming magma chambers nisms: Geological Society of America Bulletin, v. 128, p. 397–424, https://doi.org /10 .1130 /B31223 .1. using the timing of ignimbrite eruptions and pluton emplacement at the Mt. Aetna caldera Riciputi, L.R., 1991, Petrology and Nd, Sr, and Pb isotopes of the central San Juan caldera cluster, complex: Journal of Volcanology and Geothermal Research, v. 245–246, p. 128–148, https:// Colorado [Ph.D. thesis]: Madison, University of Wisconsin–Madison, 802 p. doi.org/10.1016/j.jvolgeores.2012.08.007. Riciputi, L.R., Johnson, C.M., Sawyer, D.A., and Lipman, P.W., 1995, Crustal and magmatic evo- Zimmerer, M.J., Laffety, J., and Coble, M.A., 2016, The eruptive and magmatic history of the lution in a large multicyclic caldera complex: Isotopic evidence from the central San Juan youngest pulse of volcanism at the Valles caldera: Implications for successfully dating late volcanic field: Journal of Volcanology and Geothermal Research, v. 67, p. 1–28, https://doi Quaternary eruptions: Journal of Volcanology and Geothermal Research, v. 310, p. 50–57, .org/10.1016/0377-0273(94)00097-Z. https://doi.org/10.1016/j.jvolgeores.2015.11.021.