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![Generalized geologic map of Platoro caldera complex, showing ages for lavas and intrusions of the Conejos Formation. Figure is modified from Lipman et al. (1996); ages are from Table S1F (see text footnote 1).]()
![Schematic west-east section through the Platoro caldera complex. (A) Early during eruption of the Chiquito Peak Tuff (CPT) at ca. 28.8 Ma. For simplicity, ring faults are arbitrarily shown as near vertical. The inferred size and shape of the overall subcaldera magma reservoir is uncertain, and thus its depiction is schematic. (B) Section through the Platoro caldera complex post–28 Ma. Intracaldera intrusions such as the Alamosa River monzonite (ARM) intrude the Chiquito Peak Tuff and Summitville Andesite. The Cornwall Mountain resurgent block is not depicted for simplicity. The dacite of Fisher Gulch and probably the Cornwall Mountain quartz monzonite porphyry (CMP) are likely remnant magma from eruption of the Chiquito Peak Tuff. On the eastern caldera margin, the Cat Creek monzonite (CCM) intruded a volcanic edifice (volcanics of Green Ridge). West of the Platoro caldera complex, younger (ca. 27.7–26.3 Ma) amphibole-bearing intrusions such as the Lake Annella andesite porphyry (LAAP) and the Horsethief Park hornblende dacite dike (HPHD) likely originated as magmas sourced from deeper in the crust. Red dashed line indicates the schematic extent of possible intrusions; blue, remnants of the composite fill inferred from earlier caldera subsidence (Lipman et al., 1996). Note the scale change between upper and lower segments of each panel.]()
![Geologic map of the Platoro caldera complex showing locations of dated postcaldera plutons and dikes (labeled with their names) associated with the postcaldera magmatic activity at the Platoro caldera (modified from Lipman, 1974; Lipman et al., 1996). These units intrude precaldera lavas and volcaniclastic deposits of the Conejos Formation (blue), ignimbrites of the Treasure Mountain Group (green), and the caldera-filling lavas of Summitville Andesite (purple). Gray stippled patterns represent surficial deposits. HD—hornblende dacite.]()
![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).]()
![Postcollapse lavas and intrusions within Platoro caldera. View is to the north, across the margin of the 29 Ma Alamosa River pluton toward highly altered caldera-filling lavas (Summitville Andesite), overlain by unaltered 20 Ma silicic lava (rhyolite of Cropsy Mountain; Lipman, 1975). Hill caped by this lava is Lookout Mountain (3795 m). On the skyline in the distance is the 23 Ma dacite lava dome of South Mountain, host to Au-Cu mineralization at the Summitville mining district (photograph by P. Lipman, 2016).]()
![Extrusive and intrusive magmatic history of the Platoro caldera complex, Dulce dikes, 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 ages plotted, as listed in bold font). For the Dulce swarm, proximal dikes in segments D1 and 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 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; OC—Ojito Creek Tuff; RJ—Ra Jadero Tuff; RG—Rio Grande rift; SF—South Fork Tuff; vol.—volume.]()
![Photographs of radial andesite and dacite dikes west of Platoro caldera, southern Colorado. (A) Three dikes of silicic andesite, on the south ridge of Flattop 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 (Conejos 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. Lipman, 2016). (C) Large porphyritic dacite dike, ∼40 m thick (for scale, note the truck near the left side of the dike), along Stunner-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).]()
![(A) Map of the Southern Rocky Mountain volcanic field, showing erosional remnants of mid-Cenozoic volcanic rocks (in peach), ignimbrite calderas, caldera-related granitic intrusions, and mid-Cenozoic andesite and dacite dikes. The San Juan volcanic locus was active from 35 to 23 Ma and generated 23 large-volume ignimbrites from multiple calderas (Steven and Lipman, 1976; Lipman, 2007; Lipman and Bachmann, 2015). The Platoro caldera complex is denoted by the blue rectangle. Modified after Lipman et al. (2015). (B) Generalized geologic map of the Platoro caldera complex, showing preserved remnants of successive topographic walls related to eruptions of the La Jara Canyon and Chiquito Peak Tuffs. The Platoro caldera complex sourced seven major ignimbrites dated between 30.1 and 28.8 Ma including the last and largest, the Chiquito Peak Tuff. Modified from Lipman et al. (1996).]()
![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 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.]()
![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.]()
![Regional magmato-tectonic interpretation, inferring combined effects of localized uplift and radial stress field associated with the Platoro magmatic locus during the mid-Cenozoic (ca. 35–20 Ma) overlapped by an increasing component of west-directed regional extension commencing ca. 26 Ma. Most abbreviations are the same as in Figure 1. Other abbreviations: Colo—Colorado; Plat—Platoro caldera; R.G.—Rio Grande; WMT—Wall Mountain Tuff.]()
![Timing of peak growth at precursor volcanoes proximal to subsequent ignimbrite eruptions (40Ar/39Ar ages from Tables S1F and S1-G [see text footnote 1]). (A) Conejos lava and dike ages, Platoro caldera complex. Dated phases: b—biotite; g—groundmass; h—hornblende; s—sanidine. (B) Lava ages, Rawley volcanic complex, precursor to Bonanza Tuff and caldera. Gray box—best age estimate for main growth of Rawley volcanic complex.]()
![North-south section depicting inferred variations in mid-Cenozoic crustal thickness from the Platoro caldera north to Mount Princeton batholith in Colorado (USA). Data to 30 km are projected into section. BC—Blue Creek; CP—Cochetopa Park; MA—Mount Aetna. Location of section and other abbreviations are given in Figure 1.]()
![Alkali-silica variation diagram for intermediate-composition intrusions and lavas of Platoro caldera, Dulce dikes from successively southward geographic segments (Fig. 2), and eastern 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 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 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); 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), Lipman (1975); Hinsdale lavas—Lipman and Mehnert (1975), Thompson et al. (1991), Turner et al. (2019);. V.—volcano; SJ—San Juan region. (B) Comparison of Platoro dikes and granitoid intrusions with geographic segments (D1–D6) of the Dulce swarm that are progressively more distal from the Platoro locus (data from Supplemental File 1). CO—Colorado; NM—New Mexico.]()
![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), filtered to exclude picritic basalt (MgO >10 wt%), multiple analyses from the same dike site, and samples deemed anomalous due to alteration (listed in italics on table in Supplemental File 1). In parentheses, number of analyzed samples. Vertical bars show one standard deviation; inferred trend lines (dashed) are placed by visual estimate.]()
![Harker diagrams of major and trace elements versus SiO2 for rocks of the Platoro caldera complex. Intracaldera intrusions are shown in yellow, and intrusions outside the caldera in blue. Overall trends of most elements are linear for the intrusions, although K2O, Na2O, MgO, and Al2O3 show some scatter. K2O abundances define a high-K calc-alkaline trend. Data are recalculated volatile free. Symbols and data sources are as in Figure 3.]()
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Journal Article
Postcaldera intrusive magmatism at the Platoro caldera complex, Southern Rocky Mountain volcanic field, Colorado, USA
Journal: Geosphere
Publisher: Geological Society of America
Published: 02 April 2021
Geosphere (2021) 17 (3): 898–931.
...Amy K. Gilmer; Ren A. Thompson; Peter W. Lipman; Jorge A. Vazquez; A. Kate Souders Abstract The Oligocene Platoro caldera complex of the San Juan volcanic locus in Colorado (USA) features numerous exposed plutons both within the caldera and outside its margins, enabling investigation of the timing...
FIGURES
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Journal Article
Recurrent eruption and subsidence at the Platoro caldera complex, southeastern San Juan volcanic field, Colorado: New tales from old tuffs
Journal: GSA Bulletin
Publisher: Geological Society of America
Published: 01 August 1996
GSA Bulletin (1996) 108 (8): 1039–1055.
... of the multicyclic Platoro caldera complex and for caldera-forming processes generally. Masonic Park Tuff in central parts of the San Juan field, including the type area, was erupted from a concealed source at 28.6 Ma, but widespread tuff previously mapped as Masonic Park Tuff in the southeastern San Juan Mountains...
Journal Article
Magmato-tectonic links: Ignimbrite calderas, regional dike swarms, and the transition from arc to rift in the Southern Rocky Mountains
Journal: Geosphere
Publisher: Geological Society of America
Published: 30 September 2019
Geosphere (2019) 15 (6): 1893–1926.
... intrusions that radiate westward from the polycyclic Platoro caldera complex in the Southern Rocky Mountain volcanic field (southwestern United States) merge in structural trend, composition, and age with the enormous but little-studied Dulce swarm of trachybasaltic dikes that continue southwest and south...
FIGURES
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Includes: Supplemental Content
Image
Generalized geologic map of Platoro caldera complex, showing ages for lavas...
in Precursors to a continental-arc ignimbrite flare-up: Early central volcanoes of the San Juan Mountains, Colorado, USA
> Geosphere
Published: 20 December 2023
Figure 15. Generalized geologic map of Platoro caldera complex, showing ages for lavas and intrusions of the Conejos Formation. Figure is modified from Lipman et al. (1996) ; ages are from Table S1F (see text footnote 1 ).
Image
Schematic west-east section through the Platoro caldera complex. (A) Early ...
in Postcaldera intrusive magmatism at the Platoro caldera complex, Southern Rocky Mountain volcanic field, Colorado, USA
> Geosphere
Published: 02 April 2021
Figure 16. Schematic west-east section through the Platoro caldera complex. (A) Early during eruption of the Chiquito Peak Tuff (CPT) at ca. 28.8 Ma. For simplicity, ring faults are arbitrarily shown as near vertical. The inferred size and shape of the overall subcaldera magma reservoir
Image
Geologic map of the Platoro caldera complex showing locations of dated post...
in Postcaldera intrusive magmatism at the Platoro caldera complex, Southern Rocky Mountain volcanic field, Colorado, USA
> Geosphere
Published: 02 April 2021
Figure 2. Geologic map of the Platoro caldera complex showing locations of dated postcaldera plutons and dikes (labeled with their names) associated with the postcaldera magmatic activity at the Platoro caldera (modified from Lipman, 1974 ; Lipman et al., 1996 ). These units intrude precaldera
Image
Generalized geologic map of the Platoro caldera complex, showing preserved ...
in Magmato-tectonic links: Ignimbrite calderas, regional dike swarms, and the transition from arc to rift in the Southern Rocky Mountains
> Geosphere
Published: 30 September 2019
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
Image
Postcollapse lavas and intrusions within Platoro caldera. View is to the no...
in Magmato-tectonic links: Ignimbrite calderas, regional dike swarms, and the transition from arc to rift in the Southern Rocky Mountains
> Geosphere
Published: 30 September 2019
Figure 4. Postcollapse lavas and intrusions within Platoro caldera. View is to the north, across the margin of the 29 Ma Alamosa River pluton toward highly altered caldera-filling lavas (Summitville Andesite), overlain by unaltered 20 Ma silicic lava (rhyolite of Cropsy Mountain; Lipman, 1975
Image
Extrusive and intrusive magmatic history of the Platoro caldera complex, Du...
in Magmato-tectonic links: Ignimbrite calderas, regional dike swarms, and the transition from arc to rift in the Southern Rocky Mountains
> Geosphere
Published: 30 September 2019
Figure 5. Extrusive and intrusive magmatic history of the Platoro caldera complex, Dulce dikes, and basaltic lavas of the Rio Grande rift, based on new 40 Ar/ 39 Ar ages (square symbols with color fills) and ages from published sources cited in text. Data are from Table 2 (only higher
Image
Photographs of radial andesite and dacite dikes west of Platoro caldera, so...
in Magmato-tectonic links: Ignimbrite calderas, regional dike swarms, and the transition from arc to rift in the Southern Rocky Mountains
> Geosphere
Published: 30 September 2019
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 Flattop Mountain (elevation 3475 m), that trend NE across the Continental Divide (skyline) toward Platoro caldera (photograph by K
Journal Article
Genesis of ore deposits in the San Juan volcanic field, southwestern Colorado; lead isotope evidence
Journal: Economic Geology
Publisher: Society of Economic Geologists
Published: 01 February 1979
Economic Geology (1979) 74 (1): 1–26.
..., the rocks and galenas of the Platoro caldera complex, of the central San Juan caldera complex, and of the Baughman Creek center appear to contain significant components derived from 1,400- to 1,500-m.y.-old and 1,700- to 1,800 m.y.-old source materials. These also are the ages of the two main groups...
Image
(A) Map of the Southern Rocky Mountain volcanic field, showing erosional re...
in Postcaldera intrusive magmatism at the Platoro caldera complex, Southern Rocky Mountain volcanic field, Colorado, USA
> Geosphere
Published: 02 April 2021
and generated 23 large-volume ignimbrites from multiple calderas ( Steven and Lipman, 1976 ; Lipman, 2007 ; Lipman and Bachmann, 2015 ). The Platoro caldera complex is denoted by the blue rectangle. Modified after Lipman et al. (2015) . (B) Generalized geologic map of the Platoro caldera complex, showing
Image
Representative variation diagrams for the Dulce dike swarm and proximal dik...
in Magmato-tectonic links: Ignimbrite calderas, regional dike swarms, and the transition from arc to rift in the Southern Rocky Mountains
> Geosphere
Published: 30 September 2019
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 Formation, intermediate-composition lavas and intrusions of the Platoro locus, and younger Hinsdale
Image
Representative trace-element variation diagrams for Dulce dike swarm and pr...
in Magmato-tectonic links: Ignimbrite calderas, regional dike swarms, and the transition from arc to rift in the Southern Rocky Mountains
> Geosphere
Published: 30 September 2019
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
Image
Regional magmato-tectonic interpretation, inferring combined effects of loc...
in Magmato-tectonic links: Ignimbrite calderas, regional dike swarms, and the transition from arc to rift in the Southern Rocky Mountains
> Geosphere
Published: 30 September 2019
Ma. Most abbreviations are the same as in Figure 1 . Other abbreviations: Colo—Colorado; Plat—Platoro caldera; R.G.—Rio Grande; WMT—Wall Mountain Tuff.
Image
![Timing of peak growth at precursor volcanoes proximal to subsequent ignimbrite eruptions (40Ar/39Ar ages from Tables S1F and S1-G [see text footnote 1]). (A) Conejos lava and dike ages, Platoro caldera complex. Dated phases: b—biotite; g—groundmass; h—hornblende; s—sanidine. (B) Lava ages, Rawley volcanic complex, precursor to Bonanza Tuff and caldera. Gray box—best age estimate for main growth of Rawley volcanic complex.](https://gsw.silverchair-cdn.com/gsw/Content_public/Journal/geosphere/20/1/10.1130_GES02691.1/3/s_ges02691_f16.png?Expires=2147483647&Signature=w4A8yh-phvDeKO~o1E8Ldv1mCN-PwbYirems~QbuXVEnBgsdtUEDDkGQ4KQdL2QSkHB7bFbq92oH6xdm5W91ZeqFjaNmbrGxz~rqwK4rA~w0qXhMVtawdSHSgKsPwRiLA3NhuXnmMl0~ivM51n0TUPk7FmfjMF9HU7YG8t-wxS7l06vyziO3UeIZlkfBs4QT9EYms0uXznXOTL8iIjmyCiO15PTyktNqHM6em3JPVn9RVXCrii2fqeovi6zzIyikct1FFkEw9eBoW9T9yEmSrDs3vmmg90cS2rZ4GRiIgqTw8cQtCJcpzHWiYJ~Styv3GqnM1LXjQGxPglgR0tqxQg__&Key-Pair-Id=APKAIE5G5CRDK6RD3PGA)
Timing of peak growth at precursor volcanoes proximal to subsequent ignimbr...
in Precursors to a continental-arc ignimbrite flare-up: Early central volcanoes of the San Juan Mountains, Colorado, USA
> Geosphere
Published: 20 December 2023
Figure 16. Timing of peak growth at precursor volcanoes proximal to subsequent ignimbrite eruptions ( 40 Ar/ 39 Ar ages from Tables S1F and S1-G [see text footnote 1 ]). (A) Conejos lava and dike ages, Platoro caldera complex. Dated phases: b—biotite; g—groundmass; h—hornblende; s—sanidine. (B
Image
North-south section depicting inferred variations in mid-Cenozoic crustal t...
in Raising the West: Mid-Cenozoic Colorado-plano related to subvolcanic batholith assembly in the Southern Rocky Mountains (USA)?
> Geology
Published: 03 June 2021
Figure 3. North-south section depicting inferred variations in mid-Cenozoic crustal thickness from the Platoro caldera north to Mount Princeton batholith in Colorado (USA). Data to 30 km are projected into section. BC—Blue Creek; CP—Cochetopa Park; MA—Mount Aetna. Location of section and other
Image
Alkali-silica variation diagram for intermediate-composition intrusions and...
in Magmato-tectonic links: Ignimbrite calderas, regional dike swarms, and the transition from arc to rift in the Southern Rocky Mountains
> Geosphere
Published: 30 September 2019
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 eastern rift-related basaltic lavas (Hinsdale Formation). Dacitic ignimbrites erupted from the caldera
Image
Segment-averaged compositional variations among Platoro and Dulce dikes as ...
in Magmato-tectonic links: Ignimbrite calderas, regional dike swarms, and the transition from arc to rift in the Southern Rocky Mountains
> Geosphere
Published: 30 September 2019
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 ), filtered
Image
Harker diagrams of major and trace elements versus SiO 2 for rocks of the ...
in Postcaldera intrusive magmatism at the Platoro caldera complex, Southern Rocky Mountain volcanic field, Colorado, USA
> Geosphere
Published: 02 April 2021
Figure 8. Harker diagrams of major and trace elements versus SiO 2 for rocks of the Platoro caldera complex. Intracaldera intrusions are shown in yellow, and intrusions outside the caldera in blue. Overall trends of most elements are linear for the intrusions, although K 2 O, Na 2 O, MgO, and Al
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