Postcaldera Intrusive Magmatism at the Platoro Caldera Complex, Southern Rocky Mountain Volcanic Field, Colorado, USA GEOSPHERE, V

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GEOSPHERE Postcaldera intrusive magmatism at the Platoro caldera complex, Southern Rocky Mountain volcanic field, Colorado, USA GEOSPHERE, v. 17, no. 3 Amy K. Gilmer1, Ren A. Thompson1, Peter W. Lipman2, Jorge A. Vazquez2, and A. Kate Souders1 1U.S. Geological Survey, Denver, Colorado 80225, USA https://doi.org/10.1130/GES02242.1 2U.S. Geological Survey, Menlo Park, California 94025, USA

16 figures; 3 tables; 1 set of supplemental files ABSTRACT related plutonic and volcanic components may not be exposed or are difficult CORRESPONDENCE: [email protected] to discern. Shallow postcaldera intrusions are either later pulses of magmas The Oligocene Platoro caldera complex of the San Juan volcanic locus in or residue left behind during an eruption (e.g., Bachmann et al., 2007; Mills CITATION: Gilmer, A.K., Thompson, R.A., Lipman, P.W., Vazquez, J.A., and Souders, A.K., 2021, Post‑ Colorado (USA) features numerous exposed plutons both within the caldera and Coleman, 2013; Bacon et al., 2014; Bachmann and Huber, 2016; Watts et caldera intrusive magmatism at the Platoro caldera and outside its margins, enabling investigation of the timing and evolution of al., 2016). Both can be possibilities in any given magmatic system (e.g., Tappa complex, Southern Rocky Mountain volcanic field, postcaldera magmatism. Intrusion whole-rock geochemistry and phenocryst et al., 2011; Zimmerer and McIntosh, 2012; Colgan et al., 2018). Postcaldera Colorado, USA: Geosphere, v. 17, no. 3, p. 898–931, https://doi.org/10.1130/GES02242.1. and/or mineral trace element compositions coupled with new zircon U-Pb geo- intrusions vary in geometry from ring dikes to laccoliths to larger plutons and chronology and zircon in situ Lu-Hf isotopes document distinct pulses of magma are commonly associated with mineralization and hydrothermal alteration Science Editor: Shanaka de Silva from beneath the caldera complex. Fourteen intrusions, the Chiquito Peak Tuff, (e.g., Smith, 1960; Lipman, 1984; Kennedy et al., 2012; Zimmerer and McIntosh, and the dacite of Fisher Gulch were dated, showing intrusive magmatism began 2012; Colgan et al., 2018; Tomek et al., 2017). Received 6 February 2020 after the 28.8 Ma eruption of the Chiquito Peak Tuff and continued to 24 Ma. Several Oligocene calderas of the Southern Rocky Mountain volcanic field Revision received 1 December 2020 Additionally, magmatic-hydrothermal mineralization is associated with the intru- (SRMVF) (Fig. 1) include exposed intrusions inferred to reflect the subvolcanic Accepted 11 February 2021 sive magmatism within and around the margins of the Platoro caldera complex. plutonic parts of caldera-forming magmatic systems (Lipman, 2007), offering

Published online 2 April 2021 After caldera collapse, three plutons were emplaced within the subsided excellent opportunities to track the evolution of a magmatic system to its end- block between ca. 28.8 and 28.6 Ma. These have broadly similar modal miner- point. For example, the majority of plutons in the Questa-Latir volcanic locus alogy and whole-rock geochemistry. Despite close temporal relations between (New Mexico, USA) postdate the caldera-forming Amalia Tuff, with only a modest the tuff and the intrusions, mineral textures and compositions indicate that component of unerupted residual magma (Lipman, 1988; Johnson et al., 1989; the larger two intracaldera intrusions are discrete later pulses of magma. Tappa et al., 2011; Zimmerer and McIntosh, 2012). Likewise, in the northeastern Intrusions outside the caldera are younger, ca. 28–26.3 Ma, and smaller in part of the SRMVF, the Mount Princeton pluton is interpreted to reflect a period exposed area. They contain abundant glomerocrysts and show evidence of of low magma flux that occurred substantially after the ignimbrite eruption (Mills open-system processes such as magma mixing and crystal entrainment. The and Coleman, 2013). Elsewhere in the SRMVF, the ca. 23 Ma Lake City caldera protracted magmatic history at the Platoro caldera complex documents the contains postcaldera syenite intrusions that represent mushy magma that did diversity of the multiple discrete magma pulses needed to generate large not erupt, while monzonite intrusions represent chemically distinct, less-evolved composite volcanic fields. magma emplaced after caldera collapse (Hon, 1987; Kennedy et al., 2012, 2016). The Platoro caldera complex of the San Juan volcanic locus (SJVL) in the southeastern San Juan Mountains erupted five large-volume, crystal-rich dac- ■■ INTRODUCTION ite ignimbrites over 1.5 m.y., a period considerably longer than that associated with most other multicycle calderas in the SJVL. Postcaldera magmatism is Constraining conditions of magmatic storage and differentiation processes preserved in compositionally diverse volcanic deposits ranging from andesite that resulted in either eruption or pluton emplacement can help constrain the to dacite and as hypabyssal intrusions of diorite to quartz monzonite. Post- larger architecture of magmatic systems (e.g., Buddington, 1959; Hamilton and caldera intrusions occur both within the caldera, including one inferred to Myers, 1967, 1974; Smith, 1979; de Silva, 1991; Lipman, 1984, 2007; Cashman have caused late-stage caldera resurgence, and external to the caldera mar- and Giordano, 2014). However, understanding the waning stage of a magmatic gins. Previous geochronological studies focused primarily on eruptive ages system, after large-volume caldera-forming eruptions, commonly poses sig- of volcanic deposits (Lipman, et al., 1970, 1996; Lipman and Zimmerer, 2019). nificant challenges because the temporally, spatially, and/or petrogenetically Emplacement ages for intrusions were inferred mainly from field observations This paper is published under the terms of the of crosscutting relations and stratigraphic position (Lipman, 1974, 1975), limit- CC‑BY-NC license. Amy Gilmer https://orcid.org/0000-0001-5038-8136 ing rigorous determination of temporal and magmatic associations between

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39° 107°W 106°W 105°W 106°40'W 106°20'W P Front Range a 30'N 37° r k r 30'N C k C Area of A re Bennett Peak rth Fork Roc ek o Sawatch N r South So C Park u ck t o Colorado h R Springs Fork Green 39 Mile Silver Mountain volcanic Summitville Ridge area Jasper

West Elk Range locus Gunnison ? Wet Mountains

Resurgentblock ? Alamosa River

Telluride Cornwall

Mountain Mountain Saguache S a 38°N San Juan n Platoro fault g Summit volcanic re Peak locus 37° San Luis

20'N Willow Valley d e Mountain

segment of

Platoro Rio Grande C Cone r jo i s River rift s

t Conejos Peak

o Spanish

Area of B u

n Peaks 0 5 10 km

Colorado a

n New Mexico s Questa-Latir Tusas Miocene basalt and rhyolite flows Caldera margins Mts volcanic locus Late caldera lavas Long dashed where 36° Tuffs and lavas of the central caldera cluster approximately located, short 30'N Granitoid intrusion dash where concealed 0 50 100 km Summitville Andesite, upper member Chiquito Peak Tuff Topographic wall, Ojito Creek, Ra Jadero, South Fork Tuffs Chiquito Peak caldera Caldera Intrusion Summitville Andesite, lower member La Jara Canyon Tuff Fault Dike Inferred ring fault, Conejos Formation, tuff of Rock Creek Summitville caldera Conejos Formation

Topographic wall, La Jara Canyon caldera remnant

Figure 1. (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 de- noted 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).

postcaldera intrusions and the caldera-forming magmatic system. However, high-resolution ion microprobe (SHRIMP) dates and trace element data in exceptional exposures and documented field relations of postcaldera volcanic combination with other mineral compositions, we address the timing of post- and intrusive rocks (Lipman, 1974) enable geochronologic determination of collapse pluton emplacement at Platoro in relation to compositions of the intrusion ages and assessment of their petrologic affinities to eruptions from ignimbrite magma, later effusive volcanism, magma generation, and attendant the Platoro caldera complex as monitors of the evolution of that magmatic shallow pluton assembly. system from explosive, silicic ignimbrite eruption to intrusion emplacement. In this study, we examine the temporal and spatial constraints on the emplacement history of postcaldera intrusions at the Platoro caldera com- ■■ GEOLOGICAL SETTING plex and assess the genetic relationship between the postcaldera intrusions, caldera-filling lavas, and the Chiquito Peak Tuff, the last major ignimbrite The SRMVF (Fig. 1A) is one of North America’s largest mid-Cenozoic vol- sourced from the Platoro caldera complex. Using new zircon U-Pb sensitive canic fields. As part of the larger Cordilleran ignimbrite flareup, the SRMVF

is an eastern manifestation of volcanism related to subduction of the Faral- Precaldera rocks in the Platoro area (Figs. 1, 2), as well as generally in the lon plate beneath North America. Farallon slab removal in the mid-Cenozoic San Juan Mountains, are andesitic to dacitic lavas and volcaniclastic deposits likely caused asthenospheric mantle melting, resulting in the ignimbrite fla- of the Conejos Formation (Lipman, 1975; Colucci et al., 1991). These deposits, reup (Coney and Reynolds, 1977; Humphreys et al., 2003; Farmer et al., 2008). volumetrically the largest component of the SJVL, were erupted from multi- The composite SRMVF sourced 26 large-volume (>100 km3) ignimbrites from ple centers between 35 and 30 Ma (Lipman et al., 1970). Eruptive centers for multiple calderas between 37 and 23 Ma, depositing >17,000 km3 of crys- Conejos lavas in the Platoro area were interpreted as being located mainly tal-rich, high-silica dacite and crystal-poor, low-silica rhyolite across a region of within the area of subsequent caldera subsidence; flanks of several strato ~100,000 km2 (Lipman, 2007; Lipman and Bachmann, 2015). Estimates of total volcanoes are preserved along caldera rims (Lipman, 1975). erupted volume for the SRMVF, including precursor lava flows, are approxi- The Platoro caldera complex sourced seven named ignimbrite units of the mately a factor of four greater than that of the ignimbrites, and the dominantly Treasure Mountain Group between 30.1 and 28.8 Ma. These ignimbrites are silicic batholithic roots of the volcanic field may constitute as much as an intercalated with andesitic lava eruptions and predate intrusion of monzonites additional 300,000 km3 (Farmer et al., 2008; Lipman and Bachmann, 2015). (Fig. 2) (Lipman, 1975; Lipman et al., 1996; Tomek et al., 2019). The five largest Current crustal thickness in the region is estimated to vary from ~41 to 49 km ignimbrites associated with the Platoro caldera complex, the Black Mountain, (Hansen et al., 2013). La Jara Canyon, Ojito Creek, Ra Jadero, and Chiquito Peak Tuffs, have volumes Stratigraphic and structural exposure levels within individual calderas are between 100 and 1000 km3. Ignimbrites of the Treasure Mountain Group con- variable across the SRMVF owing to diverse volcanogenic controls, including sist dominantly of compositionally crystal-rich dacites (Lipman et al., 1996). syncollapse structural juxtaposition of intracaldera and extracaldera deposits, The Black Mountain Tuff, a densely welded dacite tuff with a volume of postcollapse magmatic resurgence, and presence or absence of postcaldera ~300–400 km3, has an 40Ar/39Ar hornblende date of 30.19 ± 0.16 Ma (Lipman and volcanism. More importantly, regional uplift and extension associated with Zimmerer, 2019). Dates for underlying and overlying units indicate that the next the northern Rio Grande rift, and opening of the Upper Arkansas and San Luis ignimbrite of the Treasure Mountain Group, the La Jara Canyon Tuff, erupted valleys, have resulted in extensive dip-slip and oblique-slip faulting, structur- between 30.1 and 29.9 Ma (Lipman et al., 1996; Lipman and Zimmerer, 2019). ally exposing deeper levels of magmatic systems adjacent to basin-bounding This crystal-rich dacitic ignimbrite, which contains plagioclase, biotite, and systems of north- and northwest-trending faults. Coupled with extensive flu- augite phenocrysts, is widespread, thick, and has a volume >1000 km3 (Lipman, vial dissection driven by tectonic and climate influences, exposures vary from 1975). Subsequent to eruption of the La Jara Canyon Tuff, the resulting col- dominantly plutonic remnants of volcanic loci, such as exposed in the Sawatch lapsed caldera was partially filled by andesite lavas of the lower member of the Range north of the SJVL, to surface preservation of primary caldera morphol- Summitville Andesite (Lipman, 1974; Lipman et al., 1996). The next erupted unit, ogy, as in the minimally dissected Creede and Cochetopa Park calderas of the middle tuff, consists of 10–15 separate, small-volume ignimbrite sheets. the central SJVL (Fig. 1A). Most calderas of the SJVL are enclosed by a large The Ojito Creek, Ra Jadero, and South Fork Tuffs are also widespread dacitic negative Bouguer gravity anomaly, interpreted as the geophysical expression ignimbrites that may have been erupted from the Summitville caldera, a ten- of a composite upper-crustal batholith (Plouff and Pakiser, 1972; Steven and uously postulated structure within the larger Platoro caldera complex (Fig. 1) Lipman, 1976; Drenth et al., 2012). The Platoro caldera complex lies along the (Lipman, 1975). Sanidine from the Ra Jadero and South Fork Tuffs yielded southeastern margin of the gravity low. 40Ar/39Ar dates of 29.12 ± 0.07 Ma and 28.86 ± 0.14 Ma, respectively (Lipman The products of postcaldera magmatism in the SJVL include intermediate and Zimmerer, 2019). lavas, andesitic to dacitic dikes, and granodiorite to granite plutons (Lipman, The outflow sheet of the last major ignimbrite erupted from the Platoro 2007). The subvolcanic intrusions (Fig. 1) are typically more mafic than the caldera complex, the Chiquito Peak Tuff, was originally mapped as parts of associated ignimbrites but overlap the compositions of the more primitive the Masonic Park and La Jara Canyon Tuffs (Lipman, 1974) but was later rec- parts of some compositionally zoned ignimbrites (e.g., San Luis caldera–Nel- ognized as a separate ignimbrite based on the presence of sparse sanidine, son Mountain Tuff) (Lipman, 2007). Among the few published intrusion ages, differences in rock and mineral chemistry, and paleomagnetic data (Lipman et some are within error of the ages of the associated ignimbrites; others are al., 1996). Sanidine for the Chiquito Peak Tuff yielded an 40Ar/39Ar date of 28.77 demonstrably younger. Intrusion textures range from equigranular and nearly ± 0.03 Ma (Lipman and Zimmerer, 2019). The tuff is a lithic-rich, crystal-rich aphanitic to porphyritic, and many of the intrusions have been interpreted dacite with plagioclase, biotite, augite, and sanidine phenocrysts. Lipman et as the cores of volcanic edifices. Within the SJVL, small- to moderate-scale al. (1996) concluded that eruption of the Chiquito Peak Tuff created the major- hypabyssal plutons are exposed at the Bonanza, South River, San Luis, Sil- ity of exposed caldera features, including the thick ponding of intracaldera verton, Lake City, Uncompahgre–San Juan, San Juan, and Platoro calderas ignimbrite (Fig. 1). The total estimated volume of ignimbrites erupted from (Steven and Lipman, 1976; Lipman, 2007; Lipman and Bachmann, 2015). Pub- the Platoro caldera complex is >2600 km3 (Lipman et al., 1996). lished intrusion ages are limited, and only one caldera intrusion, the Sultan Andesitic lavas of the upper Summitville Andesite (Lipman, 1974), locally Mountain pluton, has a published U-Pb zircon date (Gonzales, 2015). >500 m thick, filled the Platoro caldera after the eruption of the Chiquito Peak

106°45'W 106°15'W 37°30'N TsmTwp Ttr Thb Thb Tcv Tlp LIST OF MAP UNITS Tcr Tsu Ttl Thu Tcv Thm Tcr Tcv Tcu Regional lavas Tfc Tcr Tpd Tto Tma • •Tqp Ttc Hinsdale Formation - basalt • Trc Thb Ttr Tpd Thr Tcr Thb Tcu

Ttr • • Tsu Tcu Thm Hinsdale Formation - mixed lavas Ttm Tpd • Tfc Ttsf Hinsdale Formation - rhyolite • Thr • Tcv Ttm Tcv Tlp Ttl Tcv Ttj • Ttr Thu Huerto Formation - andesite • Tlp Tcq Tma • Tfc Ttl

• Tlp Tcv • • Tsu Ttj Regional ignimbrites Tsu Andesite dike Tgd Tsu Tsu Tpd • Tsm Ttl • Snowshoe Mountain Tuff • l • • Thr Tga Ttm Andesite dike Thb Twp Wason Park Tuff • Tcc Tqp • Tfc Tpd • Tcr Carpenter Ridge Tuff

• Tcv • Tcv Tcr • Ttj Tcq Tsu Ttr Tfc Fish Canyon Tuff TKa Jasper Creek • • • Tgd Tpd Tcq Jasper Tto • Tpd Tma Kl • Tpd Tsu monzonite Tsu Tcu Green Ridge Masonic Park Tuff

Tcv • Trc Tga • Summitville Tmp Ttj Tlp Lavas and volcaniclastics • • Summitville Andesite

• • associated with the Platoro caldera complex Ttl • Tga Tsu • • quartz monzonite • Tcv • Tsq Burnt Creek Thb Tcq Rhyolite of Cropsy Mountain • • Tsu (sampled core) Silver Tcc • Mtn

Tfc Tsu • Tsq Dacite of South Mountain Jasper Creek • • •

Tcc • • • • • • HD dike Crater Creek area • Tm Tpd Dacite of Park Creek • • Tsq Tga

• Ttj Tpd • Tdp • South •

•

• Andesite of Green Ridge • Tga • Mtn

TKa • • Tsu • • Tto Ttm Tcu • • • Ranger Creek • Dacite of Green Ridge

Tsu • Tlp • Tgd

• Tpd • •

• • • Thr HD dike

• Tsu Ttsf Tsu Tcc Tgr Rhyolite of Green Ridge • •

• Cropsy

Elwood Creek • • •

Mtn • Ttsf

• Tpd Alamosa River • • Tgr monzonite Tm • Ttc Ttc Ttj Tlp Tsu Summitville Andesite— upper member • Tdp

• • • • • • Crater Creek

• • • •

• Tsu Tsu Tsl Summitville Andesite— lower member

• •

•

• •

monzonite• Cat Creek

•

• Sanidine •

•

• Tto Ttr • monzonite Dacite of Fisher Gulch

• dacite dike Tf

• • Alamosa River Tmp •

• •

•

• •

• Tsu •

• •

•

Ttj • •

Ttj •

• • •

•

• Tcv •

• • Tsu• Tsl Intrusions associated with Platoro

Bear Creek• • Tmp • • • Tsu Tcv Ttc caldera complex • • Tm monzonite Tsu • Thm

• • Ttl •

• Tm • •

• • Ttj Tcc Lake Annella Tmp • Tqp Quartz dacite porphyry

• • • • • • • Ttj

• Tcc

• • andesite porphyry Tsu • • Tsu • • Chiquito Peak

• Tdp Dacite porphyry Tcv • •

Tcc • Cataract Alum Creek • Thb Tuff

• • Tdp • •

Ttj porphyry • Tcv Ttr Tap Andesite porphyry Creek •

• Ttj Ttc • • • •

Tmp • • Tcv •

monzonite • • Monzonite porphyry • Tmp • Tga

Ttc Tsl • Tcv • TKa • • • • Ttj Monzonite Ttl Horsethief Park Tm Tm • Tm

• Ttj •

• • • • • • • Cornwall Mountain • HD dike • • • •

• Treasure Mountain ignimbrites • •

Ttj Tcv • quartz monzonite • • Tdp •

•

Tcc Alamosa River •

• • • •

Tcv • • porphyry Ttc Chiquito Peak Tuff

• • • Tsu • Tcc monzonite •

• Ttc• Tcc

Tsu • Platoro Ttsf South Fork Tuff

• • • • •

• Ttc

• Tcc • •

• Ttr Ra Jadero Tuff • • •

r • Ttc Tsu Ttr • i Ttr o • Thb

• Tcv Tsu rv Ttj • • e • • Tsu s Ttr Tto Ojito Creek Tuff Ttsf • Tsu •

• e • • • • Ttr Ttj R • • o • Ttj

Tcv r • Co • Ttc Ttm Middle tuff

o •

• Tap n • t la • e Tto Ttr

j • P • o

•

• Ttj La Jara Canyon Tuff s

• Thb

•

Tcv R Tga • Tto

• Tf • i Lower tuff • Ttl

Ttj Ttj v Thb Tcu

Ttc e

• • • r • • • Tcc Tf Tcc Conejos Formation

Tqp Tcu Upper lava unit

• Tsu

Dacite of Fisher L Thb • Trc Tuff of Rock Creek Tap Ttc Tcc Tcb Gulch Tcb Cone breccias • Tcv Tcc Volcaniclastics

• Tto Tcv Vent facies Ttm Tcv Tcc Ttj Thb Ttj • Ttm Prevolcanic rocks

• • Ttj TKa Animas Formation Conejos River Tcc • Ttl Kl Lewis Shale Tcv Tcv Tto Tcc • Tm •

Tcc ReservoirLa Jara Tcc Tto Ttm Tcv Contact Tcv Tcv Ttj • Ttj Fault Tcv Dike

• Tcc Tcv Thb Ttm Tto 37°15'N Tto 0 5 10 km

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 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.

Tuff (Figs. 1, 2). Stratigraphically overlying these andesites are dacite and minor Cross, 1956; Lipman, 1974). With the exception of a K-Ar date of 29.1 ± 1.2 Ma rhyolite lavas of Park Creek, Green Ridge, Silver Mountain, South Mountain, on biotite from the Alamosa River monzonite (Lipman et al., 1970), there have and Cropsy Mountain. The andesites and dacites at Green Ridge and Silver been no prior radiometric dates on these plutons. Mountain (Fig. 2) have been interpreted as eroded flanks of a stratovolcano Mineralization in the Summitville, Platoro, Stunner, and Jasper districts on the eastern margin of the Platoro caldera complex in the Cat Creek area has been interpreted as being related to postcaldera structures and postcal- (Lipman, 1974, 1975). dera magmatic activity of the Platoro caldera complex (Patton, 1917; Steven Intrusions associated with the Platoro caldera complex (Table 1) include and Ratté, 1960; Mehnert et al., 1973; Lipman, 1975; Neuerburg, 1978; Bethke, quartz monzonite to diorite plutons and stocks as well as andesitic to dacitic 2011). The largest deposit, the Summitville Au-Ag-Cu deposit, is a 22.5 ± 0.5 Ma dikes (Patton, 1917; Larsen and Cross, 1956; Lipman, 1974). These intrude high-sulfidation, epithermal deposit associated with the quartz dacite volcanic volcanic and volcaniclastic deposits of the Conejos Formation, ignimbrites dome at South Mountain and underlying quartz monzonite porphyry intrusion of the Treasure Mountain Group, and the Summitville Andesite. In general, (Bethke et al., 2005). The deposit is situated at the intersection of the caldera the porphyritic intrusions crosscut the equigranular monzonites (Larsen and margin and a northwest-trending fault zone that extends from southeast of

TABLE 1. SUMMARY OF THE PETROGRAPHIC CHARACTERISTICS OF THE PLATORO POSTCALDERA INTRUSIONS, CHIUITO PEAK TUFF, AND POSTCALDERA LAVAS Unit Texture Mineralogy

Intracaldera Alamosa River monzonite Equigranular to porphyritic 30–0 pl; 2–40 af; 10–20 qtz; 2– bt; 2–6 aug; accessory: ap, mag, ilm, zrn Cornall Mountain quartz monzonite porphyry Porphyritic 60–70 microcrystalline gm; phenocrysts: 20–2 pl; –8 aug; 2 bt; matrix/accessory: qtz, pl, af, ap, mag, ilm, ttn, zrn Jasper monzonite Equigranular to porphyritic 0–60 pl; 20–30 af; 20–2 qtz; bt; 2 aug; accessory: ap, mag, ilm, zrn Alum Creek porphyry Porphyritic 60–70 gm; phenocrysts: 20–2 pl; –8 aug; 2– bt; matrix/accessory: qtz, ap, mag, ilm, zrn Summitville quartz monzonite Equigranular 0–60 pl; 20–30 af; 20–2 qtz; 2– bt; accessory: ap, mag, ilm, zrn Summitville Andesite upper member Equigranular to porphyritic 80–90 gm; phenocrysts: 60–7 pl; 10–1 aug; 8–10 opx; 1–6 mag; accessory: ap, mag, ilm Dacite of Fisher Gulch Porphyritic 30 crystals; phenocrysts: 20–2 pl; 2–6 bt; 1–2 aug; 1 san; accessory: ap, mag, ilm, zrn Extracaldera Cat Creek monzonite Equigranular to porphyritic 0–60 pl; 20–30 af; 20–2 qtz; bt; 2 aug; 0–1 opx; accessory: qtz, ap, mag, ilm, zrn Lake Annella andesite porphyry Porphyritic 40 gm; phenocrysts: 8 pl; 10 bt; 3 mag; 2 hb; matrix/accessory: pl, af, qtz, aug, mag, ilm, ap Bear Creek monzonite Equigranular to porphyritic 0–60 pl; 20–30 af; 20–2 qtz; bt; 2–4 hb; 0–2 aug; 1 opx; matrix/ accessory: ap, mag, ilm, zrn Elood Creek monzonite Equigranular to porphyritic 0–60 pl; 20 af; 20–2 bt;10 qtz; 4 bt; 2 aug; accessory: ap, mag, ilm, zrn Cataract Creek monzonite Equigranular to porphyritic 0–60 pl; 20–30 af; 20–2 qtz; bt; 2– hb; accessory: ap, mag, ilm, zrn Sanidine dacite dikes Porphyritic 40 gm; phenocrysts: 8 pl; 3– bt; 2–4 hb; 3 mag; 2 san; 1 qtz; matrix/accessory: pl, af, qtz, aug, mag, ilm, ap Intracaldera and extracaldera Hornblende dacite‑andesite dikes Porphyritic 40–4 gm; phenocrysts: 68–80 pl; 10–16 bt; –20 hb; 0– aug; matrix/ accessory: pl, af, qtz, mag, ilm, ttn, ap Andesite dikes Equigranular 6–80 gm; phenocrysts: 30–0 pl; –10 aug; 2– opx; accessory: ap, mag, ilm Chiquito Peak Tuff Welded to unelded 3–4 crystals; phenocrysts: 30 pl; 0–3 san; 6 bt; 2 aug; accessory: ap, mag, ilm, zrn Note: afalkali feldspar; apapatite; augaugite; btbiotite; gmgroundmass; hbhornblende; ilmilmenite; magmagnetite; opxorthopyroxene; plplagioclase; qtz quartz; sansanidine; ttntitanite; zrnzircon. Units occur both ithin and outside of the caldera. Observed in porphyritic phase only.

Platoro village northwest to Wolf Creek Pass. Largely subeconomic mineral- 12 Tephri- Trachyte ization in the other districts has been minimally studied, and relationships to phonolite 10 Trachy- postcaldera magmatic activity are not well established; however, mineralization Phono- Trachy- dacite in the Jasper district occurs within a postcaldera pluton at the intersection of Tephrite andesite two caldera-related faults (Lipman, 1974, 1975). Molybdenite vein stockwork 8 Rhyolite Basaltic and copper-lead-zinc vein mineralization (Neuerburg, 1978) and alteration trachy- O (wt%) in the Crater Creek area are localized around plutons and along northwest- 2 6 Trachy- andesite Dacite basalt trending faults and dikes.

O + K Andesite 2 4 Basaltic Basalt andesite Na ■■ INTRUSIONS, CHIQUITO PEAK TUFF, AND POSTCALDERA LAVAS 2

0 Plutons (Table 1) associated with the postcaldera magmatism at the Pla- 45 50 55 60 65 70 75 toro caldera complex range from diorite to quartz monzonite and are locally SiO2 (wt%) intruded by small aplitic veins (Figs. 3, 4A). The dikes define a similar com- Chiquito Peak Tuff (this study) positional range as the plutons but are texturally more variable, commonly Extracaldera intrusions Chiquito Peak Tuff (Lipman et al.,1996) porphyritic (Fig. 4B), and contain mafic enclaves locally (Fig. 4C). We distinguish Lake Annella andesite Summitville Andesite two groups of the intrusions: (1) those that are within and (2) those that are porphyry Dacite of Fisher Gulch Cat Creek monzonite outside the Platoro caldera complex. Plutons within the caldera (Fig. 2) were Intracaldera intrusions Cataract Creek monzonite emplaced along structures related to subsidence and/or resurgence (Lipman, Alamosa River monzonite Elwood Creek monzonite 1975). Plutons outside the Platoro caldera are less related to specific structures, Alum Creek porphyry Bear Creek monzonite Jasper monzonite Sanidine dacite dikes but many of the dikes are radial to the western caldera boundary. In the Cat Cornwall Mountain quartz monzonite Hornblende andesite-dacite Creek area, east of the caldera, dikes are approximately radial to the Cat Creek porphyry Hornblende dacite dikes dikes pluton. Field, age, and compositional relations suggest that the postcaldera Mafic (andesite) dikes lavas, the dacite of Fisher Gulch and the Summitville Andesite, are also genetically associated with Platoro caldera magmatism. Figure 3. Total alkali versus silica diagram (Le Maitre, 2002) for Platoro whole-rock samples. Intracaldera intrusions (in yellow) are displaced to higher total alkalis when compared to samples from intrusions outside the caldera (in blue). Details of analyses are in Table 2, Table S1 Intracaldera Plutons (footnote 1), and Lipman et al. (1996), recalculated volatile free.

Supplementary material Table 1. Whole rock compositions for the Platoro Intrusions and Chiquito Peak Tuff, Colorado

Sample 17AG05 17AG58 SRM24 SRM22 17AG52 17AG54 17AG45 DC1 17AG31 SRM25 SRM23 17AG68 17AG12 SRM26 17AG24 17AG26 17AG21 17AG06 U281H 17AG83 17AG87 17AG86 SRM33 17AG39 17AG49 17AG28 17AG46 U281J 17AG27 17AG22 17AG50 17AG51 17AG90 18AG35 17AG29 18AG33 19AG03 18AG58 17AG01 17AG04 19AG05 19AG02 19AG09 17AG73 17AG11 18AG38 17AG32 17AG64 18AG43 Horsethief Jasper Creek Meadow Ranger Creek Alamosa River Alamosa River Alamosa River Alamosa River Alamosa River Alamosa River Alum Creek Alum Creek Cornwall Cornwall Cornwall Cat Creek Cat Creek Cat Creek Lake Annella Lake Annella Cataract Creek Bear Creek Elwood Creek hornblende hornblende hornblende Hornblende Hornblende Hornblende Hornblende Sanidine Sanidine Sanidine Chiquito Peak Chiquito Peak Chiquito Peak Chiquito Peak Chiquito Peak Chiquito Peak Chiquito Peak Dacite of Dacite of Summitville Summitville Summitville Summitville Unit M M M M M M MP QMP Jasper M Jasper M Mountain QMP Mountain QMP Mountain QMP Cat Creek M QMP QMP QMP AP AP M Bear Creek M MP M dacite dike dacite dike dacite dike andesite dike andesite dike dacite dike dacite dike dacite dike dacite dike dacite dike Andesite dike Andesite dike Andesite dike Tuff Tuff Tuff Tuff Tuff Tuff Tuff Fisher Gulch Fisher Gulch Andesite Andesite Andesite Andesite Latitude 37.39507 37.37229 37.3778 37.3845 37.37484 37.3851 37.37865 37.3879 37.41556 37.41817 37.3505 37.35801 37.35391 37.40517 37.40267 37.4009 37.41478 37.37428 37.3733 37.37269 37.38926 37.38917 37.4165 37.4278 37.37116 37.40986 37.37592 37.3733 37.37482 37.41675 37.39782 37.37388 37.40033 37.46337 37.41687 37.47058 37.57151 37.33311 37.46889 37.40162 37.74713 37.61981 37.61439 37.32533 37.32916 37.44651 37.41668 37.43151741 37.9699 Longitude -106.55122 -106.56744 -106.5657 -106.5465 -106.59252 -106.56629 -106.57536 -106.5735 -106.47938 -106.47567 -106.50267 -106.50532 -106.52386 -106.30583 -106.3284 -106.30451 -106.33153 -106.62424 -106.6278 -106.67149 -106.69889 -106.69751 -106.70983 -106.48197 -106.64262 -106.38153 -106.60976 -106.6278 -106.32856 -106.31923 -106.64988 -106.62524 -106.65839 -106.45359 -106.39299 -106.57774 -106.32456 -106.57798 -106.2533 -106.44555 -106.31577 -106.3453 -106.56319 -106.51927 -106.47264 -106.56671 -106.485124 -106.5670001 -106.64965

SiO2 58.63 59.65 60.28 61.87 62.57 63.72 58.24 65.24 61.59 62.50 64.63 65.26 66.47 60.50 65.61 66.84 67.03 57.27 60.69 58.56 55.64 58.60 62.67 62.75 63.01 64.77 60.34 60.50 66.72 67.84 67.79 69.67 69.97 61.79 62.04 62.16 63.12 63.83 64.09 64.49 64.59 65.47 64.17 62.84 63.60 57.45 59.16 59.46 59.72

TiO2 0.99 0.96 0.94 0.70 0.78 0.81 0.90 0.61 0.78 0.86 0.58 0.59 0.52 0.97 0.52 0.47 0.54 0.88 0.85 0.84 1.03 0.79 0.75 0.79 0.68 0.71 0.77 0.82 0.43 0.39 0.54 0.48 0.47 0.87 0.68 0.84 0.64 0.54 0.61 0.49 0.63 0.59 0.65 0.73 0.69 0.97 0.89 0.88 1.03

Al2O3 15.64 16.08 15.86 15.13 16.08 15.75 16.77 15.51 15.81 15.49 16.41 16.52 16.31 16.45 16.87 16.48 15.94 17.64 17.29 17.27 17.72 17.72 16.35 15.71 16.45 15.54 17.08 16.95 16.02 16.25 15.44 15.11 14.25 16.21 17.00 16.23 17.45 18.03 17.63 17.74 16.66 16.85 17.61 16.48 17.27 17.76 16.42 17.54 17.33

Fe2O3 7.93 7.83 nd nd 6.02 5.83 7.66 4.90 6.58 nd nd 4.28 3.66 nd 4.60 3.97 3.93 8.22 7.16 8.01 9.45 7.84 nd 6.08 5.85 4.93 7.08 6.99 3.84 3.49 4.16 3.15 3.58 6.32 5.92 6.94 4.81 3.98 4.37 3.70 5.00 4.18 4.58 5.70 4.46 7.84 7.41 6.78 7.86 FeO nd nd 6.45 6.28 nd nd nd nd nd 5.98 4.06 nd nd 6.79 nd nd nd nd nd nd nd nd 5.49 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd MnO 0.13 0.15 0.12 0.16 0.10 0.10 0.23 0.11 0.11 0.11 0.13 0.11 0.09 0.09 0.10 0.08 0.08 0.18 0.18 0.18 0.17 0.16 0.11 0.09 0.10 0.08 0.12 0.15 0.11 0.07 0.10 0.06 0.07 0.12 0.09 0.29 0.16 0.13 0.07 0.11 0.11 0.08 0.09 0.11 0.08 0.14 0.14 0.12 0.17 MgO 3.62 3.55 3.50 2.92 2.38 1.89 3.21 1.87 3.06 2.63 1.37 1.15 1.27 2.55 1.33 1.14 1.50 3.08 2.92 3.20 3.32 2.51 2.27 2.47 2.03 2.07 2.49 2.52 1.28 0.98 1.58 0.85 1.59 2.32 1.79 1.56 1.07 1.55 0.76 1.34 1.25 0.90 1.04 1.76 1.53 2.37 2.99 2.70 2.40 CaO 5.64 4.08 5.38 5.02 3.57 2.89 4.86 3.71 3.86 4.13 4.56 3.36 3.29 4.92 2.76 2.84 2.73 5.91 4.45 5.15 6.61 6.28 4.35 4.31 4.47 3.74 4.42 5.13 3.95 2.88 2.69 2.18 2.84 4.55 4.71 4.23 4.04 3.80 3.65 3.49 3.62 3.32 3.16 4.21 3.94 7.41 4.99 5.49 5.12

Na2O 3.40 3.68 3.40 3.28 4.12 3.76 4.29 3.78 3.64 3.72 3.96 3.53 4.08 4.03 4.26 4.31 3.72 3.92 3.50 3.37 3.54 3.62 4.02 3.75 3.67 3.98 4.37 3.70 3.94 4.04 3.18 4.15 1.67 3.46 3.85 3.66 4.48 4.22 4.48 4.71 3.83 4.04 4.18 3.51 4.13 3.25 3.41 3.62 3.33

K2O 3.72 3.72 3.75 4.36 4.07 5.01 3.50 4.03 4.26 4.30 4.07 4.99 4.12 3.33 3.73 3.67 4.33 2.54 2.59 3.08 2.13 2.09 3.68 3.78 3.47 3.85 3.01 2.89 3.55 3.89 4.27 4.13 5.34 4.05 3.54 3.81 3.97 3.64 4.00 3.70 4.07 4.34 4.25 4.35 4.05 2.39 4.24 3.09 2.69 P2O5 0.30 0.30 0.31 0.28 0.30 0.24 0.34 0.25 0.31 0.27 0.22 0.22 0.18 0.36 0.23 0.18 0.20 0.35 0.37 0.34 0.39 0.38 0.31 0.26 0.27 0.32 0.32 0.35 0.17 0.16 0.24 0.21 0.21 0.32 0.37 0.30 0.26 0.29 0.33 0.23 0.25 0.22 0.25 0.30 0.23 0.42 0.35 0.32 0.36 Alamosa River Monzonite LOI 0.75 2.3 1.51 0.80 3.57 2.29 2.38 0.93 1.37 0.70 3.33 4.52 2.55 0.71 2.44 0.74 2.96 2.31 4.64 3.53 1.25 0.39 2.06 1.29 3.08 1.06 2.73 4.55 3.38 2.08 2.55 0.93 4.07 1.49 2.08 1.55 1.77 2.46 1.42 1.97 1.49 1.62 2.25 3.44 1.49 4.2 4.11 2.01 2.74 Unnormalized Total 100.53 101.21 99.92 100.38 100.58 98.18 100.77 100.87 99.43 99.53 99.21 99.53 100.64 99.64 99.68 100.23 100.23 100.97 97.74 98.48 97.76 99.71 99.82 99.94 100.36 100.82 99.35 98.93 101.40 101.13 100.35 98.24 98.82 99.57 99.12 99.53 99.20 100.07 100.66 99.50 99.34 99.53 99.33 100.51 99.92 99.93 99.11 100.06 99.70

V 181 177 146 151 118 104 147 89 128 125 70 69 57 147 67 63 64 117 122 154 154 97 103 125 98 92 104 118 52 47 66 48 59 117 80 133 77 56 74 31 78 71 65 115 80 148 153 131 178 Cr 47 36 67 35 21 12 44 22 55 40 3 <10 <10 40 12 12 <10 <10 <10 <10 <10 <10 10 15 13 42 <10 <10 <10 <10 17 19 13 16 20 13 <10 <10 <10 <10 <10 <10 <10 11 <10 <10 25 <10 11 Co 24.3 22.9 nd nd 16 12.9 19 8.9 19 nd nd 8.2 6.3 nd 10 7.3 9.5 15.6 22 22.4 21.3 16.3 nd 18.1 14.1 14.3 15.6 15.3 8.4 6.8 9.5 7.1 9.2 14.6 12.1 21.1 9 8.2 8.3 6.8 11.2 8.7 7.5 14.8 9.9 21.7 20.6 16.2 21.8 Ni 32 26 33 21 18 15 29 15 38 22 3 7 5 26 20 16 8 6 12 17 13 10 8 16 11 24 9 11 10 11 11 30 16 15 37 18 <5 6 <5 10 7 6 <5 17 8 15 25 9 16 Cu 83 101 58 198 48 71 38 28 55 33 13 11 7 40 6 9 21 12 52 38 29 12 19 53 29 28 18 52 9 6 13 18 14 53 27 70 9 11 14 6 12 12 7 32 27 51 90 13 46 Zn 105 82 82 97 91 63 216 65 86 87 66 65 51 89 150 83 63 94 126 93 94 85 77 75 89 82 65 95 65 56 62 72 49 83 82 92 68 75 82 76 67 65 66 86 76 88 107 85 104 Ga 21 21 19 18 21 18.2 21 19 19.6 19 18 18 18 21 18.8 20 20 21 20 20.1 18.5 18.3 18 21 19 21 20 20 18 19 18 20.7 15.1 20.1 19 20.3 20.1 19.5 21 19.2 19.7 21.3 21.1 20.3 20 20 19 21 21.7 Rb 115 109 103 109 106 160 83 85 112 131 85 120 90 81 75.5 79 110 49 62 66.2 37 40.2 80 98 85 92 69 66 71 73 103 100 125 122 95.6 101 74.2 67.8 81 76.7 96.7 95.9 85.9 135 91 42.2 99 58 57.9 Sr 629 614 601 566 558 465 706 590 577 563 573 636 595 764 569 621 494 744 563 636 694 615 678 686 648 719 801 673 519 647 441 476 803 656 682 656 719 767 733 711 584 619 640 764 645 864 734 838 680 Ba 667 687 783 770 932 742 752 833 868 747 986 1200 986 954 1280 1270 856 652 510 920 658 617 1069 882 649 1170 703 753 1290 1230 993 1240 1070 783 899 816 1260 1060 1190 1060 1110 1010 1050 1030 920 711 851 875 743 Y 27 28 26 25 19 29 21 18 23.4 27 20 20 19 22 16.7 17 19 28 21 23.3 28.1 28.6 22 21 21 16 27 22 16 15 15 8.6 13.8 23.8 24.3 22.1 77 19.5 21 17.9 78 71 65 17.5 22 21.1 21.2 23 22.1 Zr 277 255 243 192 226 397 185 163 314 296 214 230 236 188 231 211 242 154 157 179 171 169 179 212 188 190 184 158 202 201 154 212 149 273 231 202 256 250 293 232 221 254 287 192 217 166 185 182 184 Nb 14 14 14 12 12 16.3 9 11 11.7 16 13 13 13 10 10.8 11 12 8 10 8.7 7.7 7.9 13 12 11 13 10 10 10 11 10 12.9 10.5 11.1 11.6 7.5 10.3 9.8 12 10.5 14.7 13.4 11.6 9.5 14 5.5 9.9 10 5.8 given that these commonly occur in clusters. These mineral phases all con- Pb 16 13 14 17 13 11 24 17 16 17 14 13 14 13 11 27 20 6 36 12 8 5 19 16 15 18 13 32 18 46 70 16 20 15 12 15 42 15 17 14 14 17 16 13 19 12 14 10 10 Th 13 14 12 8 12 21.3 8 11 17.1 17 9 9 9 7 8.5 8 13 5 7 8 3.9 3.2 12 10 11 12 7 7 8 9 15 9.2 17.8 15.7 11 11.9 6.8 7.4 7 7.6 9.2 9.2 8.5 6.7 9 7.1 9.8 6 7 U 4.5 3.2 3.5 2.4 3.7 5.24 2.6 3.8 4.18 3.9 2.5 2.3 2.9 1.2 2.09 1.9 3.6 1.5 2.2 2.35 1.11 1.04 3.5 3.2 3.6 3.7 2.4 2.4 2.1 2.3 4.7 3.34 4.96 4.48 3.26 3.39 1.74 2.08 2.2 2.21 2.24 3.2 2.56 2 2.9 1.83 3.01 1.9 2.06 La 38 40 39 33 35 51.4 31 34 42.4 45 36 36 34 35 40.2 35 39 29 33 31.4 28 25.9 42 36 35 42 32 32 33 36 38 45.2 35.8 42.2 42.4 37.2 36.9 38.2 35 36.2 43.7 41.4 40.6 27.7 36 32 34.1 31 32 Ce 77 81 80 66 70 90.7 66 66 85.6 90 69 70 65 70 74 66 76 60 66 64.6 59.4 56.1 79 72 70 77 65 65 62 66 66 82.1 68.7 87.7 86.3 75.8 70.5 76.7 68 67.8 84.2 77.7 73.6 56.8 72 65.8 67.7 63 68.3 Pr 9.6 9.8 9.7 8.1 8.4 12.4 8 7.5 10.3 10.8 8 8.1 7.4 8.6 8.59 7.4 8.8 7.7 8 7.97 7.55 7.14 9.2 8.7 8.4 9 8 7.8 6.8 7.3 7.6 8.86 7.11 10.3 10.8 9 8.42 8.95 8 8.03 10.1 9.36 9.54 6.86 8.7 8.05 8.5 8 8.28 Nd 40 42.3 37 31.1 35.8 48.7 35.6 30.9 41.3 40 29.9 33.5 30.4 33.8 32.5 28.3 35.5 35.4 34.8 32.3 32.2 30.2 34.4 35.9 34.6 35.5 34.4 33.4 26 28.2 29.5 34.3 25.8 37.7 44.3 34.2 30.7 33.7 34.2 33 35.4 32.5 35.3 28.1 36.1 31.1 35.6 33.9 32.7 Sm 7.1 7.7 7.12 6.19 6.3 8.7 6.5 5.6 7.5 7.51 5.55 5.7 5 6.56 5.4 4.6 5.8 6.6 6.2 6.2 6.7 6.3 6.44 6.6 6 5.8 6.5 6.2 4.4 4.2 4.8 5.1 4.4 6.9 8.3 6.5 5.5 6 5.7 5.8 6.3 5.9 6.2 5.3 6.3 6.2 6.7 6.3 6.2 Eu 1.6 1.66 1.67 1.55 1.45 1.59 1.79 1.34 1.54 1.58 1.43 1.4 1.3 1.78 1.48 1.3 1.28 1.99 1.6 1.75 1.9 1.8 1.74 1.62 1.52 1.59 1.75 1.61 1.14 1.19 1.32 1.24 1.13 1.57 1.88 1.52 1.59 1.73 1.59 1.64 1.48 1.51 1.57 1.54 1.64 1.66 1.64 1.68 1.69 Gd 6.48 6.47 5.9 5.1 5.11 7.89 5.7 4.5 6.69 6.02 4.4 4.99 4.2 5.37 4.72 3.89 4.45 6.27 5.37 6.09 6.73 6.38 5.1 5.49 5.16 4.75 5.93 5.32 3.49 3.58 3.98 3.98 4 6.11 6.94 5.49 4.81 5.19 5 5.05 5.41 5.5 5.59 4.87 5.19 5.76 6.15 5.58 5.74 Tb 0.82 0.86 0.86 0.78 0.64 1.01 0.71 0.6 0.88 0.89 0.65 0.64 0.57 0.78 0.62 0.52 0.59 0.87 0.73 0.84 0.96 0.92 0.73 0.67 0.7 0.57 0.77 0.68 0.48 0.5 0.46 0.45 0.53 0.81 0.93 0.75 0.67 0.7 0.66 0.67 0.8 0.76 0.73 0.68 0.68 0.73 0.8 0.71 0.76 Dy 5.19 5.35 4.96 4.49 3.69 5.55 3.96 3.4 4.71 5.11 3.76 3.73 3.55 4.28 3.28 3.03 3.72 5.21 4.17 4.4 5.35 5.13 4.08 3.84 4.16 3.16 4.93 4.14 2.76 2.79 3.01 1.96 2.55 4.57 5.12 4.31 3.66 3.75 4.01 3.53 4.23 4.32 4.04 3.4 4.18 4.26 4.3 4.41 4.37 Ho 0.88 0.93 0.97 0.9 0.65 1.04 0.71 0.6 0.89 1.01 0.72 0.66 0.57 0.8 0.61 0.55 0.62 0.92 0.74 0.82 1.04 1.02 0.78 0.7 0.74 0.52 0.89 0.73 0.47 0.49 0.52 0.34 0.49 0.88 0.97 0.79 0.68 0.71 0.68 0.66 0.8 0.82 0.74 0.62 0.74 0.79 0.81 0.79 0.84 Er 2.72 2.85 2.6 2.46 1.98 3.27 1.98 1.88 2.8 2.72 2.06 2.04 1.85 2.11 1.96 1.71 1.79 2.83 2.11 2.41 2.96 3 2.11 2.17 2.18 1.51 2.63 2.17 1.41 1.38 1.61 0.9 1.43 2.68 3.04 2.47 1.96 2.01 2 2.05 2.57 2.35 2.12 1.76 2.18 2.34 2.45 2.22 2.59 Tm 0.37 0.37 0.37 0.37 0.26 0.44 0.26 0.26 0.38 0.4 0.31 0.31 0.25 0.3 0.26 0.22 0.27 0.37 0.28 0.34 0.43 0.44 0.3 0.3 0.29 0.2 0.35 0.28 0.23 0.22 0.22 0.11 0.21 0.36 0.4 0.32 0.26 0.29 0.28 0.28 0.33 0.32 0.28 0.26 0.32 0.3 0.32 0.32 0.33 Lu 0.38 0.37 0.38 0.39 0.25 0.44 0.26 0.29 0.38 0.4 0.31 0.3 0.29 0.29 0.28 0.24 0.26 0.37 0.3 0.37 0.45 0.49 0.3 0.25 0.33 0.2 0.35 0.32 0.25 0.23 0.21 0.11 0.24 0.38 0.41 0.36 0.28 0.3 0.29 0.28 0.38 0.37 0.29 0.27 0.3 0.32 0.31 0.31 0.33 The Alamosa River pluton, the largest postcaldera intrusion (~3 × 7 km), tain apatite inclusions, suggesting apatite saturation near the liquidus. Zircon Yb 2.7 2.6 2.4 2.4 1.8 2.7 1.8 1.9 2.3 2.6 2 2.1 1.9 1.9 1.7 1.6 1.9 2.7 2 2.3 2.9 3 1.9 1.9 2.3 1.4 2.6 2.2 1.6 1.5 1.7 0.7 1.4 2.6 2.5 2.5 2 2 2.2 1.8 2.5 2.5 2.1 1.7 2.3 2.3 1.9 2.2 2.3 W 1 2 nd nd 1 2 2 1 1 nd nd <1 <1 nd <1 <1 <1 <1 <1 <1 <1 <1 nd 1 1 <1 <1 <1 <1 <1 2 1 2 1 1 1 1 <1 <1 <1 1 <1 <1 <1 <1 <1 1 <1 <1 Te <0.5 <0.5 nd nd <0.5 <0.5 <0.5 <0.5 <0.5 nd nd <0.5 <0.5 nd <0.5 <0.5 <0.5 <0.5 0.6 <0.5 <0.5 <0.5 nd <0.5 <0.5 <0.5 <0.5 0.6 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 Tl 1 0.6 nd nd 0.6 1 0.9 1.1 <0.5 nd nd <0.5 <0.5 nd 0.6 1.6 1.3 0.6 1.4 <0.5 <0.5 <0.5 nd 0.8 0.7 1 1 2.2 1.1 1.2 0.9 <0.5 0.9 <0.5 <0.5 <0.5 <0.5 <0.5 0.9 <0.5 <0.5 0.5 <0.5 3.8 1.3 <0.5 <0.5 <0.5 <0.5 Ta 0.9 0.9 0.9 0.7 0.8 1 0.6 0.9 0.7 1.1 0.9 0.9 0.8 0.6 0.7 0.7 0.8 0.5 0.7 0.6 <0.5 <0.5 0.9 0.8 0.8 0.8 0.6 0.7 0.7 0.7 0.8 0.7 0.8 0.8 0.7 0.6 <0.5 <0.5 0.8 0.7 <0.5 <0.5 <0.5 0.6 0.9 <0.5 0.6 0.6 <0.5 Cs 3.4 2 0.8 1.4 0.6 4.6 3.6 1.9 1.5 1.6 1.1 1.4 6.3 1.3 1.1 2.3 1.1 0.5 3.2 0.8 0.7 1 1.2 1.5 1.1 2.4 0.9 3.5 1.7 1 1.5 0.9 1.1 2.1 1.5 1.6 0.3 1.1 1.3 1 0.4 4 0.8 8.6 1.8 0.6 1.1 0.4 0.7 Mo 3 3 nd nd <2 4 3 2 <2 nd nd <2 <2 nd <2 <2 <2 <2 <2 2 <2 4 nd <2 <2 <2 <2 2 <2 <2 <2 <2 <2 3 2 2 <2 <2 <2 <2 <2 <2 <2 3 3 <2 <2 <2 <2 Sb 0.3 0.2 nd nd 0.5 0.6 0.8 0.3 0.2 nd nd 0.2 0.3 nd 0.7 0.1 0.2 <0.1 0.6 0.4 0.4 0.3 nd 0.3 0.2 0.2 0.2 0.3 0.1 0.2 0.2 0.2 0.8 0.3 0.2 0.2 0.2 0.2 0.3 1.1 0.2 0.3 0.1 1.6 0.3 0.2 0.2 0.2 0.1 Sc 18 17 15 15 10 12 13 8 14 14 7 7 5 13 7 6 6 14 11 15 17 15 10 11 10 8 12 10 <5 <5 6 <5 6 12 10 13 8 9 7 7 8 7 7 9 9 15 17 14 17 Sn 2 2 nd nd 2 2 2 1 2 nd nd <1 <1 nd <1 <1 1 1 1 1 <1 <1 nd 1 1 1 1 1 <1 <1 1 1 <1 2 2 1 <1 2 1 1 <1 <1 1 1 1 2 1 1 1 Se <5 <5 nd nd <5 <5 <5 <5 <5 nd nd <5 <5 nd <5 <5 <5 <5 <5 <5 <5 <5 nd <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 Mn 1010 1160 nd nd 771 873 1700 819 875 nd nd 765 682 nd 835 648 577 1350 1280 1520 1280 1210 nd 730 768 595 931 1060 810 534 772 467 582 902 804 2120 1180 954 559 901 829 581 600 906 627 984 1070 943 1170 Li 19 16 nd nd 10 11 <10 <10 <10 nd nd 18 30 nd 22 14 12 12 17 23 14 14 nd 19 13 25 11 13 17 15 19 23 32 19 15 10 19 19 13 21 12 12 15 48 29 <10 <10 14 15 In <0.2 <0.2 nd nd <0.2 <0.2 <0.2 <0.2 <0.2 nd nd <0.2 <0.2 nd <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 nd <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 Hf 7 7 6 5 6 10 5 5 8 8 5 6 6 5 6 5 6 4 4 5 4 4 5 6 5 5 5 4 5 5 4 6 4 7 7 6 6 7 7 6 6 7 7 5 6 5 5 5 5 Ge 1 2 nd nd 1 1 1 1 1 nd nd <1 2 nd 2 1 1 2 1 2 2 1 nd 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 was emplaced into Summitville Andesite (upper member) and the Chiquito occurs both as inclusions in biotite and more commonly in interstitial quartz Cd <0.2 <0.2 nd nd <0.2 <0.2 0.7 0.2 <0.2 nd nd <0.2 <0.2 nd <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 nd <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 0.3 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 Bi <0.1 <0.1 nd nd <0.1 0.2 <0.1 0.4 <0.1 nd nd <0.1 <0.1 nd <0.1 <0.1 <0.1 <0.1 0.2 <0.1 0.3 <0.1 nd <0.1 <0.1 <0.1 <0.1 <0.1 0.2 0.2 0.1 <0.1 0.4 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 0.1 <0.1 <0.1 Be <5 <5 nd nd <5 <5 <5 <5 <5 nd nd <5 <5 nd <5 <5 <5 <5 <5 <5 <5 <5 nd <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 B 22 14 nd nd 10 17 14 <10 19 nd nd <10 <10 nd 21 <10 <10 14 16 26 31 22 nd 12 <10 <10 22 13 <10 <10 <10 <10 <10 17 21 22 <10 <10 16 <10 <10 <10 <10 21 11 16 20 11 15 As <5 <5 nd nd <5 <5 <5 <5 <5 nd nd <5 <5 nd 6 <5 <5 <5 19 <5 <5 <5 nd <5 <5 <5 <5 10 <5 14 <5 <5 <5 <5 <5 <5 <5 <5 <5 9 <5 <5 <5 <5 <5 <5 <5 <5 <5 Ag 2 <1 nd nd <1 <1 <1 <1 <1 nd nd <1 <1 nd <1 <1 <1 <1 <1 <1 <1 <1 nd <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 3 <1 <1 <1 <1 Note: MP - monzonite porphyry; M - monzonite; QMP - quartz monzonite porphyry; AP - andesite porphyry Units are in wt% for major oxides and ppm for trace elements. nd - not detemined LOI - loss on ignition Coordinates in WGS84 XRF data are normalized to 100% volatile-free compositions. Analytical methods- For SRM samples from Lipman and Zimmerer (2019): https://environment.wsu.edu/facilities/geoanalytical-lab/technical-notes/xrf-method For all other samples: https://www.usgs.gov/media/files/contract-chemistry-method-summaries Peak Tuff (Fig. 4D) at the northwestern margin of the hinged Cornwall Mountain and orthoclase. All equigranular and several porphyritic samples include gra- block, which has been interpreted as a resurgent structure within the Platoro nophyric intergrowths of quartz and alkali feldspar (Fig. 5B). 1 Supplemental Material. Table S1: Whole-rock com- caldera complex. Lipman (1975) suggested that the Alamosa River pluton is Alteration within the intrusion is localized along the northwestern margin positions of analyzed samples. Table S2: Major and the intrusive core of the volcano that sourced the compositionally similar Sum- of the intrusion and is most pervasive near the contact with the Alum Creek trace element geochemistry of feldspar. Table S3: mitville Andesite. Magnetic fabrics in this intrusion suggest pulsed magma porphyry. The dominant alteration is propylitic; chlorite and actinolite replace Major and trace element geochemistry of pyroxene. Table S4: Major and trace element geochemistry of emplacement of a vertically extensive pluton (Tomek et al., 2019). biotite, augite, and orthopyroxene, but localized advanced argillic alteration biotite. Table S5: Major and trace element geochemis- The pluton ranges in composition from monzonite to local quartz monzonite, was also observed. try of amphibole. Table S6: Zircon geochronology and 59–64 wt% SiO (Table 2; Table S1 in the Supplemental Material1; Fig. 3) and trace element geochemistry. Table S7: Lutetium and 2 hafnium isotopic compositions of zircon. Table S8: is equigranular fine to medium grained and locally porphyritic (Fig. 4A). The Amphibole-plagioclase thermometry. Table S9: Sam- equigranular and porphyritic phases of the Alamosa River monzonite contain Alum Creek Porphyry ple locations and lithologies. Please visit https://doi plagioclase, augite, orthoclase, quartz, biotite, Fe-Ti oxides, and minor ortho- .org/10.1130/GEOS.S.13929935 to access the supplemental material, and contact [email protected] pyroxene and accessory titanite, apatite, and zircon (Table 1; Fig. 5A). Textural The Alum Creek porphyry intrudes the Alamosa River monzonite north of with any questions. relationships suggest co-crystallization of plagioclase, augite, and Fe-Ti oxides, the Alamosa River (Fig. 2) and consists of fine- to medium-grained porphyritic

Figure 4. Photos of Platoro intrusions and the Chiquito Peak Tuff. (A) Alamosa River monzonite with aplite veins. Hammer length is 33 cm. (B) Hornblende dacite dike at Ranger Creek with CPT large plagioclase phenocrysts. Scale bar on pencil denotes 1 cm increments. (C) Mafic enclave within a hornblende dacite dike in Horsethief ARM Park. Scale bar on pencil denotes 1 cm increments. (D) Alamosa River monzonite (ARM) intrusive into the Chiquito Peak Tuff (CPT) at Telluride Mountain. Scale bar on pencil denotes 1 cm increments.

to coarse-grained monzonite to quartz monzonite. Analyzed and dated sam- the Jasper monzonite might be no younger than 28 Ma based on alteration ples are from an outcrop and a drill hole along Alum Creek. The Alum Creek and crosscutting relations between the adjacent Summitville Andesite, which

porphyry ranges from 58 to 65 wt% SiO2 (Table 2; Table S1 [footnote 1]; Fig. 3). it intrudes, and the overlying Green Ridge andesite lavas. Groundmass quartz and orthoclase enclose phenocrysts of plagioclase, biotite, and augite (Table 1; Fig. 5C). Accessory minerals include Fe-Ti oxides, titanite, apatite, and zircon. Cornwall Mountain Quartz Monzonite Porphyry Lipman (1974, 1975) previously considered this intrusion to be a porphyritic phase of the Alamosa River pluton, but crosscutting contacts between the The Cornwall Mountain quartz monzonite porphyry (Fig. 2) intrudes intr- Alum Creek porphyry and Alamosa River monzonite are sharp, and the Alum acaldera Chiquito Peak Tuff within the resurgent block of the Platoro caldera

Creek porphyry contains inclusions of equigranular Alamosa River monzonite (Lipman, 1974, 1975). It contains 65–66 wt% SiO2 (Table 2; Table S1 [footnote 1]; (Calkin, 1967, 1971; Tomek et al., 2019). Fig. 3) and has 35%–40% phenocrysts of plagioclase, augite, and biotite in a Along Alum Creek, quartz-sericite-pyrite alteration is dominant but transi- groundmass of quartz, plagioclase, and orthoclase (Table 1; Fig. 5E). Acces- tions to pervasive argillic alteration in places. In drill-core samples from the sory minerals include Fe-Ti oxides, titanite, apatite, and zircon. Most of the Alum Creek porphyry, plagioclase phenocrysts are variably sericitized and porphyry is propylitically altered, with biotite and augite replaced by chlorite chlorite and actinolite replace biotite and augite. Locally, disseminated pyrite and carbonate. Plagioclase phenocrysts are replaced by sericite. is spatially associated with Fe-Ti oxides, and calcite is disseminated in veinlets.

Summitville Quartz Monzonite Jasper Monzonite The Summitville quartz monzonite does not crop out but was sampled The Jasper monzonite intrudes the Summitville Andesite along Burnt and from drill core. This intrusion is spatially associated with the Summitville Jasper Creeks and along the Alamosa River (Fig. 2). The texture of the Jasper epithermal Au-Ag deposit and mine (Fig. 2). Sample U449, from a depth of monzonite is heterogeneous, ranging from porphyritic to fine- to medium- 1260–1263 m, is equigranular, contains extensive quartz-sericite-pyrite hydro- grained equigranular. Phenocrysts consist of plagioclase, augite, and biotite, thermal alteration, and is cut by quartz-pyrite veins. Primary minerals include within groundmass orthoclase, Fe-Ti oxides, and quartz (Table 1; Fig. 5D). The plagioclase, quartz, alkali feldspar, and minor biotite (Table 1). Bethke et al.

SiO2 content ranges from 62 to 63 wt% (Table 2; Table S1 [footnote 1]; Fig. 3). (2005) suggested that mineralization at Summitville is genetically related to Lipman (1975) suggested that much of the exposed Jasper intrusion is fine this intrusion. Although the Summitville quartz monzonite has not previously grained near its margins, but pervasive hydrothermal argillic alteration has been dated, sanidine and biotite from the mineralization host, the dacite of obscured textures within much of this intrusion. Where less altered, mainly South Mountain, yielded 40Ar/39Ar biotite and sanidine dates of 23.0 ± 0.1 Ma in its interior, the Jasper intrusion is chemically, mineralogically, and textur- and 23.1 ± 0.1 Ma, respectively (Getahun, 1994), indistinguishable from san- ally similar to Alamosa River monzonite that crops out ~5.5 km upstream. idine, biotite, and alunite-alteration ages determined using the K-Ar method Although the intrusion was previously undated, Lipman (1975) concluded that (Mehnert et al., 1973). Additionally, Getahun (1994) reported total-gas and

TABLE 2. REPRESENTATIVE BULK‑ROCK CHEMICAL ANALYSES OF THE PLATORO INTRUSIONS, CHIUITO PEAK TUFF, AND POSTCALDERA LAVAS Sample number 17AG0 SRM22 17AG4 DC1 SRM2 SRM23 SRM26 17AG26 17AG06 17AG83 17AG87 17AG86 SRM33 17AG28 17AG39 17AG49 17AG1 18AG3 17AG04 19AG09 17AG11 17AG64 Unit Alamosa Alamosa Alum Creek Alum Creek Jasper M Cornall Cat Creek M Cat Creek Lake Annella Cataract Bear Creek M Bear Creek Elood Ranger Creek Jasper Creek Horsethief Meado Sanidine Andesite dike Chiquito Chiquito Dacite of Summitville River M River M MP MP Mountain MP MP AP Creek M MP Creek M hornblende hornblende Park hornblende dacite dike Peak Tuff Peak Tuff Fisher Gulch Andesite dacite dike dacite dike dacite dike Location IC IC IC IC IC IC EC EC EC EC EC EC EC IC IC EC EC IC IC EC IC IC Maor elements

SiO2 8.63 61.87 8.24 6.24 62.0 64.63 60.0 66.84 7.27 8.6 .64 8.60 62.67 64.77 62.7 63.01 69.67 61.79 64.49 64.17 63.60 9.46

TiO2 0.99 0.70 0.90 0.61 0.86 0.8 0.97 0.47 0.88 0.84 1.03 0.79 0.7 0.71 0.79 0.68 0.48 0.87 0.49 0.6 0.69 0.88

Al2O3 1.64 1.13 16.77 1.1 1.49 16.41 16.4 16.48 17.64 17.27 17.72 17.72 16.3 1.4 1.71 16.4 1.11 16.21 17.74 17.61 17.27 17.4

Fe2O3 7.93 nd 7.66 4.90 nd nd nd 3.97 8.22 8.01 9.4 7.84 nd 4.93 6.08 .8 3.1 6.32 3.70 4.8 4.46 6.78 FeO nd 6.28 nd nd .98 4.06 6.79 nd nd nd nd nd .49 nd nd nd nd nd nd nd nd nd MnO 0.13 0.16 0.23 0.11 0.11 0.13 0.09 0.08 0.18 0.18 0.17 0.16 0.11 0.08 0.09 0.10 0.06 0.12 0.11 0.09 0.08 0.12 MgO 3.62 2.92 3.21 1.87 2.63 1.37 2. 1.14 3.08 3.20 3.32 2.1 2.27 2.07 2.47 2.03 0.8 2.32 1.34 1.04 1.3 2.70 CaO .64 .02 4.86 3.71 4.13 4.6 4.92 2.84 .91 .1 6.61 6.28 4.3 3.74 4.31 4.47 2.18 4. 3.49 3.16 3.94 .49

Na2O 3.40 3.28 4.29 3.78 3.72 3.96 4.03 4.31 3.92 3.37 3.4 3.62 4.02 3.98 3.7 3.67 4.1 3.46 4.71 4.18 4.13 3.62

K2O 3.72 4.36 3.0 4.03 4.30 4.07 3.33 3.67 2.4 3.08 2.13 2.09 3.68 3.8 3.78 3.47 4.13 4.0 3.70 4.2 4.0 3.09

P2O 0.30 0.28 0.34 0.2 0.27 0.22 0.36 0.18 0.3 0.34 0.39 0.38 0.31 0.32 0.26 0.27 0.21 0.32 0.23 0.2 0.23 0.32 LOI 0.7 0.80 2.38 0.93 0.70 3.33 0.71 0.74 2.31 3.3 1.2 0.39 2.06 1.06 1.29 3.08 0.93 1.49 1.97 2.2 1.49 2.01 Unnormalized total 100.3 100.38 100.77 100.87 99.3 99.21 99.64 100.23 100.97 98.48 97.76 99.71 99.82 100.82 99.94 100.36 98.24 99.7 99.0 99.33 99.92 100.06 Trace elements V 181 11 147 89 12 70 147 63 117 14 14 97 103 92 12 98 48 117 31 6 80 131 Cr 47 3 44 22 40 3 40 12 10 10 10 10 10 42 1 13 19 16 10 10 10 10 Co 24.3 nd 19 8.9 nd nd nd 7.3 1.6 22.4 21.3 16.3 nd 14.3 18.1 14.1 7.1 14.6 6.8 7. 9.9 16.2 Ni 32 21 29 1 22 3 26 16 6 17 13 10 8 24 16 11 30 1 10 8 9 Cu 83 198 38 28 33 13 40 9 12 38 29 12 19 28 3 29 18 3 6 7 27 13 n 10 97 216 6 87 66 89 83 94 93 94 8 77 82 7 89 72 83 76 66 76 8 Ga 21 18 21 19 19 18 21 20 21 20.1 18. 18.3 18 21 21 19 20.7 20.1 19.2 21.1 20 21 Rb 11 109 83 8 131 8 81 79 49 66.2 37 40.2 80 92 98 8 100 122 76.7 8.9 91 8 Sr 629 66 706 90 63 73 764 621 744 636 694 61 678 719 686 648 476 66 711 640 64 838 Ba 667 770 72 833 747 986 94 1270 62 920 68 617 1069 1170 882 649 1240 783 1060 100 920 87 Y 27 2 21 18 27 20 22 17 28 23.3 28.1 28.6 22 16 21 21 8.6 23.8 17.9 6 22 23 r 277 192 18 163 296 214 188 211 14 179 171 169 179 190 212 188 212 273 232 287 217 182 Nb 14 12 9 11 16 13 10 11 8 8.7 7.7 7.9 13 13 12 11 12.9 11.1 10. 11.6 14 10 Pb 16 17 24 17 17 14 13 27 6 12 8 19 18 16 1 16 1 14 16 19 10 Th 13 8 8 11 17 9 7 8 8 3.9 3.2 12 12 10 11 9.2 1.7 7.6 8. 9 6 U 4. 2.4 2.6 3.8 3.9 2. 1.2 1.9 1. 2.3 1.11 1.04 3. 3.7 3.2 3.6 3.34 4.48 2.21 2.6 2.9 1.9 La 38 33 31 34 4 36 3 3 29 31.4 28 2.9 42 42 36 3 4.2 42.2 36.2 40.6 36 31 Ce 77 66 66 66 90 69 70 66 60 64.6 9.4 6.1 79 77 72 70 82.1 87.7 67.8 73.6 72 63 Pr 9.6 8.1 8 7. 10.8 8 8.6 7.4 7.7 7.97 7. 7.14 9.2 9 8.7 8.4 8.86 10.3 8.03 9.4 8.7 8 Nd 40 31.1 3.6 30.9 40 29.9 33.8 28.3 3.4 32.3 32.2 30.2 34.4 3. 3.9 34.6 34.3 37.7 33 3.3 36.1 33.9 Sm 7.1 6.19 6. .6 7.1 . 6.6 4.6 6.6 6.2 6.7 6.3 6.44 .8 6.6 6 .1 6.9 .8 6.2 6.3 6.3 Eu 1.6 1. 1.79 1.34 1.8 1.43 1.78 1.3 1.99 1.7 1.9 1.8 1.74 1.9 1.62 1.2 1.24 1.7 1.64 1.7 1.64 1.68 Gd 6.48 .1 .7 4. 6.02 4.4 .37 3.89 6.27 6.09 6.73 6.38 .1 4.7 .49 .16 3.98 6.11 .0 .9 .19 .8 Tb 0.82 0.78 0.71 0.6 0.89 0.6 0.78 0.2 0.87 0.84 0.96 0.92 0.73 0.7 0.67 0.7 0.4 0.81 0.67 0.73 0.68 0.71 Dy .19 4.49 3.96 3.4 .11 3.76 4.28 3.03 .21 4.4 .3 .13 4.08 3.16 3.84 4.16 1.96 4.7 3.3 4.04 4.18 4.41 Ho 0.88 0.9 0.71 0.6 1.01 0.72 0.8 0. 0.92 0.82 1.04 1.02 0.78 0.2 0.7 0.74 0.34 0.88 0.66 0.74 0.74 0.79 Er 2.72 2.46 1.98 1.88 2.72 2.06 2.11 1.71 2.83 2.41 2.96 3 2.11 1.1 2.17 2.18 0.9 2.68 2.0 2.12 2.18 2.22 Tm 0.37 0.37 0.26 0.26 0.4 0.31 0.3 0.22 0.37 0.34 0.43 0.44 0.3 0.2 0.3 0.29 0.11 0.36 0.28 0.28 0.32 0.32 Lu 0.38 0.39 0.26 0.29 0.4 0.31 0.29 0.24 0.37 0.37 0.4 0.49 0.3 0.2 0.2 0.33 0.11 0.38 0.28 0.29 0.3 0.31 Yb 2.7 2.4 1.8 1.9 2.6 2 1.9 1.6 2.7 2.3 2.9 3 1.9 1.4 1.9 2.3 0.7 2.6 1.8 2.1 2.3 2.2 W 1 nd 2 1 nd nd nd 1 1 1 1 1 nd 1 1 1 1 1 1 1 1 1 Te 0. nd 0. 0. nd nd nd 0. 0. 0. 0. 0. nd 0. 0. 0. 0. 0. 0. 0. 0. 0. Tl 1 nd 0.9 1.1 nd nd nd 1.6 0.6 0. 0. 0. nd 1 0.8 0.7 0. 0. 0. 0. 1.3 0. Ta 0.9 0.7 0.6 0.9 1.1 0.9 0.6 0.7 0. 0.6 0. 0. 0.9 0.8 0.8 0.8 0.7 0.8 0.7 0. 0.9 0.6 Cs 3.4 1.4 3.6 1.9 1.6 1.1 1.3 2.3 0. 0.8 0.7 1 1.2 2.4 1. 1.1 0.9 2.1 1 0.8 1.8 0.4 Mo 3 nd 3 2 nd nd nd 2 2 2 2 4 nd 2 2 2 2 3 2 2 3 2 Sb 0.3 nd 0.8 0.3 nd nd nd 0.1 0.1 0.4 0.4 0.3 nd 0.2 0.3 0.2 0.2 0.3 1.1 0.1 0.3 0.2 Sc 18 1 13 8 14 7 13 6 14 1 17 1 10 8 11 10 12 7 7 9 14 Sn 2 nd 2 1 nd nd nd 1 1 1 1 1 nd 1 1 1 1 2 1 1 1 1 Se nd nd nd nd nd Mn 1010 nd 1700 819 nd nd nd 648 130 120 1280 1210 nd 9 730 768 467 902 901 600 627 943 Li 19 nd 10 10 nd nd nd 14 12 23 14 14 nd 2 19 13 23 19 21 1 29 14 In 0.2 nd 0.2 0.2 nd nd nd 0.2 0.2 0.2 0.2 0.2 nd 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Hf 7 8 4 4 4 6 6 7 6 7 6 Ge 1 nd 1 1 nd nd nd 1 2 2 2 1 nd 1 2 1 1 1 1 1 1 1 Cd 0.2 nd 0.7 0.2 nd nd nd 0.2 0.2 0.2 0.2 0.2 nd 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Bi 0.1 nd 0.1 0.4 nd nd nd 0.1 0.1 0.1 0.3 0.1 nd 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Be nd nd nd nd nd B 22 nd 14 10 nd nd nd 10 14 26 31 22 nd 10 12 10 10 17 10 10 11 11 As nd nd nd nd nd 9 Ag 2 nd 1 1 nd nd nd 1 1 1 1 1 nd 1 1 1 1 1 1 1 3 1 Notes: ‑ray fluorescence data are normalized to 100 volatile‑free compositions. Units are eight percent t for maor oxides, and parts per million ppm for trace elements. Mmonzonite; MPmonzonite porphyry; MPquartz monzonite porphyry; APandesite porphyry. Locations for all samples in Table S9 in the Supplemental Material see text footnote 1. Location: ICintracaldera; ECextracaldera. LOIloss on ignition. ndnot determined.

qtz pl pl

pl pl

aug aug or or

qtz qtz 100 µm 200 µm 100 µm E

pl pl bt

Figure 5. Photomicrographs (cross-polarized light) of pl Platoro intrusions and the Chiquito Peak Tuff showing representative textures. Latitude and longitude of each sample bt in Table S9 in Supplemental Material (text footnote 1). (A) Alamosa River monzonite (porphyritic phase) (sample 17AG52). (B) Equigranular Alamosa River monzonite with granophyric texture; right panel is shown in cathodo 100 µm 100 µm 100 µm luminescence (SRM22). (C) Alum Creek monzonite porphyry (DC1). (D) Jasper monzonite (SRM25). (E) Cornwall Moun- G H bt pl tain quartz monzonite porphyry (SRM23). (F) Lake Annella andesite porphyry (17AG06). (G) Bear Creek monzonite (17AG87). (H) Elwood Creek monzonite (SRM33). (I) Cata- ract Creek monzonite (17AG83). (J) Hornblende dacite dike, hb Ranger Creek (17AG28). (K) Sanidine dacite dike, Crater Lake trail (17AG90). (L) Chiquito Peak Tuff (17AG13). Mineral abbreviations: aug—augite; bt—biotite; hb—hornblende; aug or—orthoclase; pl—plagioclase; qtz—quartz; san—sanidine.

pl pl bt

100 µm 100 µm 100 µm san pl qtz pl

qtz

pl hb aug 100 µm bt 100 µm 1000 µm

plateau 40Ar/39Ar dates on sericite associated with wall-rock alteration of 22.5 equigranular. Accessory minerals in these intrusions include Fe-Ti oxides, ± 0.1 Ma and 22.6 ± 0.6 Ma, respectively. titanite, apatite, and zircon. The Bear Creek monzonite, the largest of the intrusions in the Crater Creek area, intrudes lava and volcaniclastic deposits of the Conejos Formation and Extracaldera Plutons the La Jara Canyon Tuff. The Bear Creek intrusion includes both porphyritic and equigranular phases. The porphyritic phase contains phenocrysts of Cat Creek Monzonite plagioclase, amphibole, and augite in a groundmass of quartz, plagioclase, orthoclase, and minor biotite and orthopyroxene (Table 1). The equigranular The Cat Creek monzonite intrusion, east of the Platoro caldera complex phase has a similar composition but contains less amphibole (Fig. 5G). No (Fig. 2), has been interpreted as the core of a volcano that sourced the volca- exposed contact between the two phases was observed. nics of Green Ridge (Lipman, 1975). This unit intrudes lavas and volcaniclastic The Crater Creek and Elwood Creek monzonites intrude the Conejos For- deposits of the Oligocene Conejos Formation and consists of equigranular mation, Summitville Andesite, and La Jara Canyon Tuff. The Elwood Creek and porphyritic phases. The fine-grained monzonite equigranular phase is 61 monzonite is coarsely porphyritic and contains phenocrysts of plagioclase,

wt% SiO2, whereas the porphyritic phase is quartz monzonite, ranging from augite, and biotite (Table 1, Fig. 5H). The Cataract Creek monzonite is only

66 to 67 wt% SiO2 (Table 2; Table S1 [footnote 1]; Fig. 3). Both phases contain ~0.5 km across and is the smallest of the Crater Creek plutons. It intrudes the plagioclase, orthoclase, augite, and biotite; the equigranular phase contains La Jara Canyon Tuff, and intermediate composition dikes intrude the monzonite. minor quartz, and also rare orthopyroxene largely replaced by chlorite (Table 1). Though dominantly porphyritic, it is equigranular and fine grained in places; Accessory phases include Fe-Ti oxides, titanite, apatite, and zircon. Alteration phenocrysts include plagioclase, hornblende, biotite, and augite (Table 1, Fig. 5I). is pervasive in the porphyritic phase, with extensive sericite overprinting of Only the Elwood Creek intrusion has been previously dated. However, its plagioclase, zones of intense silicification, and local replacement of augite by biotite 40Ar/39Ar inverse isochron date of 26.61 ± 0.01 Ma has a mean square calcite and chlorite. weighted deviation (MSWD) value of 8.76 (Lipman and Zimmerer, 2019), which suggests that this is not a statistically robust date. The large MSWD suggests an open system or substantial underestimation of the error associated with Lake Annella Andesite Porphyry the determined date. Patton (1917), Steven and Ratté (1960), Neuerburg et al. (1978), and The Lake Annella andesite porphyry is a small elongate east-west–trending Neuerburg (1978) documented porphyry-epithermal–style mineralization in stock that intrudes the Conejos Formation and the lower Summitville Andesite the Crater Creek area. Alteration and mineralization are localized, and all the along the western margin of the caldera (Fig. 2). It ranges from 57 to 61 wt% intrusions have unaltered domains.

SiO2 (Table 2; Table S1 [footnote 1]; Fig. 3) and contains plagioclase, amphibole, and biotite phenocrysts in a finer-grained matrix of plagioclase, orthoclase, biotite, quartz, and magnetite (Table 1; Fig. 5F). Accessory minerals include Dikes Fe-Ti oxides, titanite, apatite, and zircon. This intrusion, unlike most of the Platoro intrusions, lacks augite. The Lake Annella andesite porphyry con- Andesite Dikes tains numerous mafic enclaves that are as much as 10 cm long, contain rare plagioclase phenocrysts, and have a finer-grained groundmass than the host. Andesite dikes are widely radial to the western and southwestern margins The Lake Annella andesite porphyry is propylitized locally; actinolite, epi- of the caldera and locally to the Cat Creek monzonite. They predominantly dote, and calcite replace biotite and amphibole. The intrusion was drilled during intrude Conejos Formation lavas and volcanoclastic deposits, but some also exploration for porphyry Cu-Au mineralization. One of the resulting drill holes, cut lavas of the Summitville Andesite. Widths of these dikes range from ~0.5 m sampled during this study, intercepted zones of quartz-sericite-pyrite alteration. to 1.5 m. These dikes have equigranular to porphyritic textures and contain phenocrysts of plagioclase and augite or plagioclase and hornblende (Table 1). Some dikes have minor orthopyroxene. Fe-Ti oxides and apatite are the prin- Plutons of the Crater Creek Area cipal accessory phases. The andesite dikes proximal to Platoro together with more distal trachy- The Bear Creek, Crater Creek, Elwood Creek, and Cataract Creek intrusions, basalt dikes have been described as the Platoro-Dulce dike swarm. 40Ar/39Ar ~5 km west of the Platoro caldera margin (Fig. 2), range from diorite to monzon- dates of the andesite dikes range from 27.41 ± 0.03 Ma to 31.27 ± 0.05 Ma;

ite and contain 56–63 wt% SiO2 (Table 2; Table S1 [footnote 1]; Fig. 3). Textures consequently, they both predate and postdate caldera formation (Lipman of these intrusions vary from coarsely porphyritic to coarse- to fine-grained and Zimmerer, 2019). The hornblende-bearing dikes and dikes with large

tabular plagioclase phenocrysts are likely related to the precaldera Conejos Tuff, Alum Creek monzonite porphyry, and Lake Annella andesite porphyry Formation, whereas at least many of the equigranular, pyroxene-bearing dikes (Lipman, 1974), range from 10 to 100 m thick, and in some cases extend

postdate caldera formation (Lipman, 1975; Lipman and Zimmerer, 2019). SiO2 for several kilometers. These dikes range from 68 to 70 wt% SiO2 (Table 2; is 61.8–62.2 wt% in three fine-grained, plagioclase-pyroxene andesite dikes Table S1 [footnote 1]; Fig. 3). The dikes are coarsely porphyritic with sanidine (Table 2; Table S1 [footnote 1]; Fig. 3) that were emplaced during or after phenocrysts as large as 3 cm and also contain plagioclase, biotite, quartz, and caldera activity, one intruding the Ra Jadero Tuff and the other intruding the hornblende phenocrysts (Table 1; Fig. 5K). Quartz phenocrysts are rounded Summitville Andesite. and resorbed. Accessory minerals include Fe-Ti oxides, titanite, apatite, and zircon. Sanidine from a sanidine dacite dike near Schinzel Meadows yielded an 40Ar/39Ar date of 26.25 ± 0.04 Ma (Lipman and Zimmerer, 2019). Hornblende Andesite-Dacite Dikes

Hornblende andesite-dacite dikes are radial to the Platoro caldera complex Chiquito Peak Tuff on its western side but also are exposed adjacent to the Cat Creek monzonite to the east of the caldera. Two hornblende dacite dikes were sampled within the Although the Platoro caldera complex sourced multiple major ignimbrites, caldera along Jasper and Ranger Creeks where they crosscut Summitville and we compare the Chiquito Peak Tuff with the postcaldera intrusions because Sheep Mountain Andesites, respectively. Another hornblende dacite dike was this tuff was the last major ignimbrite and its eruption was responsible for sampled just west of the caldera in Horsethief Park. Hornblende andesite-dac- most exposed deposits and structures of the caldera complex. This ignimbrite, ite dike widths range from a meter to 30 m wide, and individual dikes may having an estimated volume of ~1000 km3, is one of the most voluminous of extend for several kilometers, particularly on the western side of the caldera. the Platoro-associated tuffs (Lipman et al., 1996). The Chiquito Peak Tuff is a

Compositions of the hornblende andesite-dacite dikes range from andesite crystal-rich dacite that contains 63–65 wt% SiO2 (Table 2; Table S1 [footnote 1];

to dacite, contain 60–68 wt% SiO2 (Table 2; Table S1 [footnote 1]; Fig. 3), and are Fig. 3) and as much as 45% phenocrysts as large as 2–3 mm, where unbro- all porphyritic in texture. Plagioclase, amphibole, and biotite are the dominant ken, of plagioclase, biotite, augite, and minor sanidine (Table 1; Fig. 5L). Bulk phenocrysts in these dikes; sanidine and quartz are absent (Table 1). Augite is chemistry for this ignimbrite suggests it is unzoned, most variation resulting present only as phenocryst and/or glomerocryst phase in the intracaldera dike from variable winnowing of ash during eruption (Lipman et al., 1996). It is at Jasper Creek (Fig. 5J). Phenocryst abundance and size vary among dikes locally rich in lithic clasts, mainly andesite, as much as 3 cm in size. Acces- (Figs. 4B–4C) and also may vary considerably along strike within any particular sory minerals include Fe-Ti oxides, titanite, zircon, and apatite. Intracaldera dike. Plagioclase phenocrysts are as large as 3 cm. Many hornblende andes- Chiquito Peak Tuff is densely welded, >800 m thick without any exposed base, ite-dacite dikes contain plagioclase-biotite-magnetite ± amphibole ± augite and widely propylitically altered; a vitrophyre is exposed only at one small glomerocrysts. These crystal clusters may be as large as 0.5 cm. Accessory site high along the southwestern caldera margin. Outflow Chiquito Peak Tuff phases include Fe-Ti oxides, titanite, apatite, and zircon. The texture and com- is less welded and has a maximum thickness of ~100 m (Lipman et al., 1996). position of these dikes, particularly those west of the Platoro caldera complex, Intracaldera Chiquito Peak Tuff contains dense fiamme, whereas pumice blocks are similar to those of the Lake Annella andesite porphyry. A hornblende andes- in outflow tuff are vesicular and as much as 15 cm across. Sanidine from the ite-dacite dike sampled in Horsethief Park contains numerous mafic enclaves Chiquito Peak Tuff yielded an 40Ar/39Ar date of Ma 28.77 ± 0.03 Ma (Lipman like those within the Lake Annella andesite porphyry. Enclaves are as much and Zimmerman, 2019). as 10 cm across. The enclaves have a finer-grained groundmass than their host and contain rare plagioclase phenocrysts (Fig. 4C). Enclave margins are commonly irregular, and some plagioclase phenocrysts straddle the boundary Dacite of Fisher Gulch between the enclave and host porphyry. The dacite of Fisher Gulch is a 350-m-thick lava dome at the southeastern boundary of the Platoro caldera complex (Fig. 2). It overlies the intracaldera Sanidine Dacite Dikes Chiquito Peak Tuff, banks against lavas of the Conejos Formation on the caldera wall, and is in turn overlain by Summitville Andesite (Lipman, 1974). The dacite

Sanidine-quartz rhyolite to silicic dacite porphyritic dikes crop out in and contains 63–64 wt% SiO2 (Table 2; Table S1 [footnote 1]; Fig. 3) and as much around the Alamosa River monzonite and the Alum Creek monzonite porphyry as 20% phenocrysts, including plagioclase, biotite, augite, and minor sanidine but are most common west of the caldera on its western side near Crater (Table 1; Lipman, 1975). Accessory minerals include Fe-Ti oxides, apatite, and and Elwood Creeks. These dikes intrude the Conejos Formation, Summit- zircon. It is similar in chemistry and petrography to the immediately under- ville Andesite, Alamosa River monzonite, dacite of Park Creek, Fish Canyon lying intracaldera ignimbrite (Tables 1–2; Table S1), as described previously

(Lipman, 1975). Sanidine from the dacite of Fisher Gulch yielded an age of (SEM). For electron probe microanalyzer analyses, a 20 kV, 10 nA beam setup 28.74 ± 0.09 Ma, analytically indistinguishable from that of the Chiquito Peak with a 1 µm spot size was used to measure Si, Al, Ca, Na, K, Fe, Ti, Mg, Mn, Tuff (Lipman et al., 1996; as recalculated to Fish Canyon Tuff standard age of and Cl abundances in the crystal phases (Tables S2–S4 [footnote 1]). Counting 28.2 Ma by Lipman and Zimmerer [2019]). times ranged from 15 to 60 s, with Na measured first and with a shorter count time to minimize Na mobility. Additionally, Ba abundances were determined in most plagioclase and sanidine crystals using a counting time between 60 and Summitville Andesite 120 s. Diopside and amphibole standards were run three times as unknowns at the beginning and end of each session. Lavas of Summitville Andesite that fill the Platoro caldera are >500 m thick in places (Lipman, 1974, 1975). These lavas comprise two members intercalated within ignimbrites of the Treasure Mountain Group. The lower member Zircon U-Pb Geochronology and Trace Element Analysis consists of lavas and volcaniclastic sedimentary rocks deposited after the eruption of the La Jara Canyon Tuff, now exposed only along small sectors Zircon U-Pb dating and trace element (Ti, Fe, Y, rare earth elements [REEs], at the eastern and western margins of the caldera complex (Fig. 2); the more U, and Th) analyses were acquired from zircon separates from 17 samples widespread upper member consists of lavas emplaced following the eruption using the SHRIMP–reverse geometry (SHRIMP-RG) instrument at the Stan-

of the Chiquito Peak Tuff. Summitville Andesite ranges from 57 to 60 wt% SiO2 ford–U.S. Geological Survey SHRIMP-RG lab (Stanford, California, USA). (Table 2; Table S1 [footnote 1]; Fig. 3) and is aphanitic to porphyritic with as Analyses were made in three sessions over one year and followed the ana- much as 20% phenocrysts, including plagioclase, augite, and orthopyroxene lytical protocol described in Matthews et al. (2015) for combined U-Pb and − (Table 1). Fe-Ti oxide clots and apatite needles are also present. Lipman (1975) trace element analyses using an O2 primary beam. Zircons were separated interpreted the lavas to represent the lower flanks of an intracaldera strato- from bulk-rock samples using standard heavy-liquid and magnetic-separation volcano cored by the Alamosa River monzonite. techniques and were handpicked, mounted in epoxy, and polished to expose the interiors of the grains. Grains chosen for analysis were imaged by cathodoluminescence techniques on a JEOL 5600 SEM to assess the complexity of ■■ ANALYTICAL METHODS zircon growth histories and to choose analytical spot locations. Analysis spot dimensions were ~20–25 µm × ~2 µm depth. Complete U-Pb results and trace Whole-Rock Geochemistry element data are given in Table S5 (footnote 1). The Temora-2 zircon standard was analyzed after every four unknowns. Trace element concentrations were Whole-rock major and trace element analyses were determined by X-ray determined using the MAD-559 standard and the reported mass fractions in fluorescence and inductively coupled–plasma mass spectrometry (ICP-MS) Coble et al. (2018). Raw data were reduced with Squid 2.51 software (Ludwig, for 49 representative samples of intrusions and volcanics from the Platoro 2009) using the Temora-2 zircon standard (206Pb/238U date = 416.8 Ma; Black caldera complex at AGAT Laboratories in Mississauga, Ontario, Canada et al., 2004) and a Pb/U–UO/U calibration; ratios were derived from weighted (https://www.usgs.gov/media/files/contract-chemistry-method-summaries), averages of within-spot scans. Individual dates and weighted means were or at the Washington State University GeoAnalytical Laboratory (samples calculated using Isoplot 3.76 software (Ludwig, 2012). Reported 206Pb/238U dates whose names begin with SRM) in Pullman, Washington, USA, following the were corrected for common Pb using a 207Pb correction (Ludwig, 2012) with methods of Johnson et al. (1999). Results are summarized in Table 2 and 207Pb/206Pb values derived from the Stacey and Kramers (1975) evolution model. Table S1 (footnote 1). All 206Pb/238U zircon dates are reported in millions of years (Ma); 2σ uncertainties are noted in the form of ± x/y, where x is the analytical uncertainty suitable only for intramethod comparison, and y is the total uncertainty, including Scanning Electron Microscope and Electron Probe Micro-Analysis the decay constant and standard (Jaffey et al., 1971) suitable for intermethod comparison of dates. Representative major and trace element compositions were determined for To compare U-Pb dates generated in this study with previously published plagioclase, sanidine, biotite, augite, amphibole, and Fe-Ti oxides using a JEOL 40Ar/39Ar dates, systematic decay constant uncertainties of both systems and JXA-8230 electron microprobe at the University of Colorado Electron Micro- uncertainties in the standards used must be propagated into the calculated probe Laboratory (Boulder, Colorado) and at the Louisiana State University dates (e.g., Schoene et al., 2013). Changes in the accepted ages of primary Shared Instrumentation Facility (Baton Rouge, Louisiana, USA). Prior to anal- standards can affect the interpreted age of an unknown determined relative ysis, backscattered electron images were obtained to evaluate compositional to that standard (Mercer and Hodges, 2016). Consequently, it is desirable to zoning of major phases using the JEOL 5800LV scanning electron microscope recalculate these dates based on the current accepted value for the standard

and to report uncertainties that incorporate not only analytical uncertainty but oscillatory and sector growth zoning patterns, suggestive of a single period also systematic uncertainties in the decay constants. Unfortunately, given the of zircon crystallization (Fig. 7). different vintages of data and laboratories, it was not possible to incorporate Emplacement ages are interpreted from the weighted mean of 206Pb/238U the systematic uncertainties in the published 40Ar/39Ar dates, and thus cited dates for these samples (Fig. 6; Table 3; Table S2 [footnote 1]). Five samples 40Ar/39Ar errors in the paper do not include this external uncertainty, making each contained a single xenocryst (Alamosa River, Jasper, Bear Creek, and intramethod comparisons more challenging. Cataract Creek monzonites and the Chiquito Peak Tuff); four Proterozoic (1138– 1709 Ma) and one Mesozoic (158 Ma) in age. Data for the xenocrysts are not included in Figure 6 but are presented in Table S2. The 206Pb/238U dates of some Zircon Lu-Hf Analysis by LA-MC-ICP-MS samples scatter over several million years, exceeding analytical uncertainty. This spread may be the result of magmatic processes such as protracted crys- Hafnium (Hf) isotopic compositions of zircon grains were determined by tal growth prior to emplacement or eruption, the inheritance of antecrysts, or laser–ablation multicollector ICP-MS (LA-MC-ICP-MS) using a Nu Plasma II possible Pb loss. MC-ICP-MS coupled to an ESI NWR 193 excimer laser ablation system in the The new 206Pb/238U dates indicate periodic emplacement and crystalliza- Mineral Isotope Laser Laboratory at Texas Tech University (Lubbock, Texas, tion of most plutons and dikes from 28.98 to 26.92 Ma. The oldest intrusions USA). A 30 µm laser spot was focused on each zircon grain directly on top of are within the mapped Chiquito Peak caldera: the Alamosa River monzonite the U-Pb analysis location, firing at a frequency of 10 Hz with an energy den- at (28.98 ± 0.18/0.26 Ma, data included here and in Tomek et al. [2019]), the sity of 5 J/cm2 for 300 pulses. 171Yb, 172Yb, 173Yb, 174(Hf + Yb), 175Lu, 176(Hf + Yb + Cornwall Mountain quartz monzonite porphyry (28.97 ± 0.25/0.28 Ma), and Lu), 177Lu, 178Hf, 179Hf, and 180Hf were all measured simultaneously in 10 Faraday the Jasper monzonite (28.81 ± 0.19/0.19 Ma). The dates for these intrusions collectors. Each analysis consisted of 15 s of gas background measurement are all within error of each other and with the two dated samples of Chiquito followed by 30 s of laser ablation and ending with 15 s of monitoring wash Peak Tuff (28.92 ± 0.38/0.50 Ma and 28.31 ± 0.55/0.57 Ma) (Fig. 6; Table 3) and out. Isotope measurements were acquired using a 0.2 s on-peak integration its 40Ar/39Ar date (28.77 ± 0.03 Ma; Lipman and Zimmerer, 2019). They are also time. Raw data were processed offline using an in-house spreadsheet based within error of the 206Pb/238U and 40Ar/39Ar dates for the dacite of Fisher Gulch on calculations and corrections described by Souders et al. (2013). (28.89 ± 0.31/0.48 Ma and 28.74 ± 0.09 Ma [Lipman et al., 1996, recalculated to For unknown zircon grains, the initial 176Hf/177Hf ratios were calculated using the Fish Canyon Tuff standard age of 28.2 Ma]), respectively. However, field the measured 176Lu/177Hf, the 176Lu decay constant (λ = 1.865 × 10− 11 /yr) of Söder- relations indicate these intrusions are younger than the Chiquito Peak Tuff

lund et al. (2004), and the SHRIMP U-Pb date for each sample. Epsilon Hf (εHf) (Lipman, 1974, 1975; Tomek et al., 2019; this study). values were calculated using the present-day 176Hf/177Hf and 176Lu/177Hf values Zircons from two samples of Alum Creek monzonite porphyry, which of 0.282785 and 0.0336, respectively for present-day chondritic uniform res- intrudes the Alamosa River monzonite, yielded 206Pb/238U dates of 27.42 ervoir (CHUR) (Bouvier et al., 2008). The depleted mantle model of Griffin et ± 0.35/0.57 Ma (sample DC1) and 27.32 ± 0.38/0.48 Ma (17AG45), significantly al. (2000), modified to the176 Lu decay constant of Söderlund et al. (2004) and younger than the Alamosa River monzonite (data included here and in Tomek et present-day CHUR Lu-Hf composition of Bouvier et al. (2008), was used as a al. [2019]). The four analyzed zircons with significantly older ages may represent 176 177 reference. This model has a present-day Hf/ Hf value of 0.28325 (εHf = +16.4) antecrysts associated with the Alamosa River monzonite (Fig. 6). Intrusions at 176Lu/177Hf = 0.0388, similar to modern-day mid-ocean-ridge basalt (MORB). outside the caldera yielded dates ca. 28 Ma or younger. Zircons from the The results for all unknown zircon grain analyses and for all reference materials Cat Creek monzonite yielded a date of 28.00 ± 0.19/0.19 Ma, while the Lake run during the analytical sessions are presented in Table S7. Annella andesite porphyry yielded a date of 27.71 ± 0.31/0.46 Ma. Three intrusions on the west side of the caldera in the Crater Creek area—the Cataract Creek monzonite, the Elwood Creek monzonite, and the Bear Creek monzon- ■■ RESULTS ite (equigranular phase)—have indistinguishable dates within error (27.29 ± 0.21/0.36 Ma, 27.06 ± 0.22/0.25 Ma, and 26.92 ± 0.43/0.52 Ma, respectively). Zircon U-Pb Dates Hornblende andesite-dacite dikes within the caldera along Ranger Creek (sample 17AG28) and Jasper Creek (17AG39) yield analytically indistinguish- Zircons from ten plutons, four dikes, the Chiquito Peak Tuff, and the dacite able dates of 28.27 ± 0.34/0.47 Ma and 28.25 ± 0.30/0.45 Ma. Likewise, zircons of Fisher Gulch were dated by U-Pb geochronology (Fig. 6; Table 3). Zircons from dikes in Horsethief Park (17AG49) and along Crater Lake trail (17AG90) from the intrusions, the Chiquito Peak Tuff, and the dacite of Fisher Gulch are yielded dates of 27.62 ± 0.35/0.49 Ma and 27.45 ± 0.33/0.46 Ma that are ana- predominantly euhedral prismatic crystals with the exception of most zircons lytically indistinguishable from the dates of the intrusions of the Crater Lake in the Alamosa River monzonite, which are subhedral to anhedral. Cathodolu- area. The youngest intrusion is the Summitville quartz monzonite (24.07 minescence images of the zircons in most samples have predominantly simple ± 0.25/0.36 Ma).

Intracaldera intrusions 32 Dacite of Fisher Gulch Jasper monzonite Alum Creek Hornblende (17AG11) (SRM25) porphyry dacite dike, 31 28.89 ± 0.31 Ma 28.81 ± 0.19 Ma (17AG45) Jasper Creek n = 20/20 n = 13/16 27.32 ± 0.38 Ma (17AG39) 30 MSWD=0.9 MSWD = 1.6 n = 14/17 28.25 ± 0.30 Ma MSWD = 1.6 n = 25/25 MSWD = 1.1 29

28 Summitville quartz monzonite 27 (U449) 24.07 ± 0.25 Ma Age (Ma) n = 15/16 26 MSWD = 1.3 Chiquito Peak Tuff Chiquito Peak Tuff Alamosa River Cornwall Mountain Alum Creek Hornblende 25 intra-caldera outflow monzonite quartz monzonite porphyry dacite dike, (17AG4) (15SJ20) (SRM22) porphyry (DC1) Ranger Creek 24 28.92 ± 0.38 Ma 28.31 ± 0.55 Ma 28.98 ± 0.18 Ma (SRM23) 27.42 ± 0.35 Ma (17AG28) n = 17/17 n = 11/13 n = 13/16 28.97 ± 0.25 Ma n = 12/12 28.27 ± 0.34 Ma MSWD = 1.6 MSWD = 1.2 MSWD = 1.6 n = 16/18 MSWD = 0.9 n = 15/17 23 MSWD = 0.9 MSWD = 1.6

22 32 Extracaldera intrusions Elwood Creek Bear Creek 31 monzonite monzonite (SRM33) (17AG87) 27.06 ± 0.22 Ma 26.92 ± 0.43 Ma 30 n = 20/21 n = 13/16 MSWD = 1.8 MSWD = 0.8 29

27 Age (Ma) 26

25 Cat Creek Cataract Creek Lake Annella Hornblende Sanidine dacite dike, 24 monzonite monzonite andesite porphyry dacite dike, Crater Lake trail (SRM26) (17AG83) (U281) Horsethief Park (17AG90) 23 28.00 ± 0.19 Ma 27.29 ± 0.21 Ma 27.71 ± 0.31 Ma (17AG49) 27.45 ± 0.33 Ma n = 17/18 n = 14/17 n = 22/22 27.62 ± 0.35 Ma n = 17/17 MSWD = 1.2 MSWD = 2.4 MSWD = 0.7 n = 25/25 MSWD = 0.7 22 MSWD = 1.2

Figure 6. Summary of 206Pb/238U zircon crystallization dates. Each data point represents an analysis of a single spot; error bars in rank order plots are 1σ errors for single U-Pb analyses. The black horizontal line and colored box represent the calculated weighted mean date and 2σ uncertainty, respectively. MSWD—mean square weighted deviates; n = #/#—number of grains of total number analyzed used in calculation. Dates for samples SRM22, DC1, and 17AG45 are from Tomek et al. (2019). Full dataset is in Table S6 (footnote 1).

TABLE 3. IRCON U‑Pb SHRIMP AGE DETERMINATIONS FOR PLATORO INTRUSIONS, CHIUITO PEAK TUFF, AND DACITE OF FISHER GULCH Sample Unit Latitude Longitude Age MSWD n/n Data source N W Ma 2σ

Intracaldera units SRM22 Alamosa River monzonite 37.384 106.46 28.98 0.18 1.6 13/16 Tomek et al. 2019 SRM23 Cornall Mountain quartz monzonite porphyry 37.30 106.027 28.97 0.2 0.9 16/18 This study SRM2 Jasper monzonite 37.4182 106.477 28.81 0.19 1.6 13/16 This study DC1 Alum Creek porphyry 37.3879 106.73 27.42 0.3 0.9 12/12 Tomek et al. 2019 17AG4 Alum Creek porphyry 37.3787 106.74 27.32 0.38 1.6 14/17 Tomek et al. 2019 17AG39 Jasper Creek hornblende dacite dike 37.4278 106.4820 28.2 0.30 1.1 2/2 This study 17AG28 Ranger Creek hornblende dacite dike 37.4099 106.381 28.27 0.34 1.6 1/17 This study 17AG04 Chiquito Peak Tuff 37.4016 106.446 28.92 0.38 1.6 17/17 This study 17AG11 Dacite of Fisher Gulch 37.3292 106.4726 28.89 0.31 0.9 20/20 This study U449 Summitville quartz monzonite 37.4224 106.600 24.07 0.2 1.3 1/16 This study

Extracaldera units SRM26 Cat Creek monzonite 37.402 106.308 28.00 0.19 1.2 17/18 This study SRM33 Elood Creek monzonite 37.416 106.7098 27.06 0.22 1.8 20/21 This study 17AG87 Bear Creek monzonite 37.3893 106.6989 26.92 0.43 0.8 13/16 This study 17AG49 Horsethief Park hornblende dacite dike 37.3712 106.6426 27.62 0.3 1.2 2/2 This study 17AG90 Sanidine dacite dike 37.4003 106.684 27.4 0.33 0.7 17/17 This study U281 Lake Annella andesite porphyry 37.3733 106.6278 27.71 0.31 0.7 22/22 This study 17AG83 Cataract Creek monzonite 37.3727 106.671 27.29 0.21 2.4 14/17 This study 1SJ20 Chiquito Peak Tuff 37.387 106.292 28.31 0. 1.2 11/13 This study Notes: SHRIMPsensitive high‑resolution ion microprobe. Locations for all samples in Table S9 in the Supplemental Material see text footnote 1. MSWDmean square eighted deviation; n/n refers to the number of zircons used to calculate age versus number of zircons analyzed.

Whole-Rock Geochemistry Cornwall Mountain quartz monzonite porphyry Silica (SiO2) contents of the Platoro intrusions vary from 56 to 70 wt% (Table 2; Table S1 [footnote 1]; Fig. 3). These analyses are consistent with previously published data for some of the intrusions and for the Chiquito Elwood Creek monzonite Peak Tuff (Lipman, 1975; Lipman et al., 1996). Variation diagrams display some

compositional scatter for K2O, Na2O, MgO, and Al2O3, but for the other major elements, overall trends are linear for the intrusions (Fig. 8). Lithic fragments were removed from Chiquito Peak Tuff samples prior to analysis, although minor contamination by small lithic clasts is possible. FeOT (FeOT is total Fe

Chiquito Peak Tuff as FeO), MgO, P2O5, CaO, and TiO2 concentrations decrease with increasing

Alamosa River monzonite SiO2 (Fig. 8). Incompatible element concentrations (e.g., Na, K, Ba) correlate

positively with SiO2, and K2O abundances define a high-K calc-alkaline trend

(Fig. 8). Sr and Al2O3 concentrations correlate negatively, though somewhat

irregularly, with increasing SiO2 (Fig. 8), with intracaldera intrusions being less 100 µm enriched compared with the Chiquito Peak Tuff and the postcaldera lavas. The Hornblende dacite dike, Ranger Creek Chiquito Peak Tuff data are displaced slightly from the overall trends for MgO, FeOT, and Al O . Most elemental compositions for the Chiquito Peak Tuff are Figure 7. Representative zircon cathodoluminescence images for zircons from the Pla- 2 3 toro plutons, dikes, and the Chiquito Peak Tuff. Most zircon grains have simple igneous more evolved than those of the largest postcaldera intrusion, the Alamosa River oscillatory and sector zoning associated with magmatic crystal growth. monzonite, but are similar to those of the Cornwall Mountain quartz monzonite

porphyry. The Chiquito Peak Tuff has higher Sr and Ba concentrations, at a Mountain quartz monzonite porphyry (Fig. 11C). The Sr content of plagioclase

given SiO2 content, than the Alamosa River monzonite. Overall, the intracal- in the upper member of the Summitville Andesite is 1200–2300 ppm, between dera intrusions have higher K contents than the intrusions outside the caldera. that of the Chiquito Peak Tuff and the intracaldera intrusions. Plagioclase Platoro-related samples have chondrite-normalized REE patterns that are compositions (both anorthite and Sr contents) for the dacite of Fisher Gulch enriched in light REEs relative to heavy REEs (Fig. 9A). These REE patterns are overlap those of the Chiquito Peak Tuff, Summitville Andesite, and intracal- slightly concave up, suggestive of a clinopyroxene-amphibole fractionation dera intrusions. (Tatsumi, 1989; Pearce and Peate, 1995; Davidson et al., 2013). Sanidine dacite dikes have the most concave patterns, suggesting even greater clinopyroxene-amphibole influence on REE pattern shape (Fig. 9A) (Davidson et al., 2013; Potassium Feldspar Smith, 2014). Only the Alamosa River and Jasper monzonite REE patterns have small negative Eu anomalies (Fig. 9A). La/Yb increases with increasing Sanidine phenocrysts are as much as 3 mm long in the Chiquito Peak Tuff silica, whereas Dy/Yb decreases with silica in the Platoro samples (Fig. 9B). and 7 mm long in the sanidine dacite dikes. A single highly resorbed and embayed sanidine phenocryst (0.2 mm) was identified in the Ranger Creek hornblende dacite dike, but none were recognized in the other hornblende Mineral Compositions andesite-dacite dikes. In all other units, potassium feldspar occurs either in the groundmass, as granophyric intergrowths with quartz, or as rims on Plagioclase plagioclase.

Sanidine compositions range in orthosclase content from Or69–64 in the In the equigranular Alamosa River, Jasper, Cat Creek and Elwood Creek Chiquito Peak Tuff, a range similar to that reported by Lipman et al. (1996). monzonites, euhedral plagioclase is commonly surrounded by granophyric Zoning in these phenocrysts is less well developed than that characteristic of quartz-orthoclase intergrowths as much as 300 µm across or by euhedral plagioclase in the Chiquito Peak Tuff. This phase in a sanidine dacite dike is

to subhedral orthoclase rims as much as 200 µm (e.g., Fig. 5B). Plagioclase zoned with Or72–79. The single sanidine phenocryst in the Ranger Creek horn-

grains in these units are normally zoned to unzoned with minor oscillatory blende dacite dike ranged Or64–68. Sanidines in the Chiquito Peak Tuff contain zoning in some samples. Anorthite content for these equigranular monzon- ~5900–17,000 ppm Ba. Rare Ba-zoned sanidines include sharp resorption sur- ites shows a consistent range between units (Fig. 10; Table S2 [footnote 1]). faces, across which Ba concentrations are ~9100 ppm in the core to as much as Plagioclase phenocrysts in the Alamosa River monzonite, for example, show 15,700 ppm in the rim. Ba concentrations in sanidines from the sanidine dacite

limited zoning with anorthite (An) content ranging from An52–33 in the cores dike range from ~2600 to 17,100 ppm. Two types of Ba zoning were identified:

to An47–27 in the rims (Fig. 11A). By contrast, compositions of plagioclase in (1) oscillatory zoning, in which Ba concentrations in rims are high (10,000–

the Chiquito Peak Tuff range more broadly; core compositions range An75–21 17,100 ppm), and (2) step-function zoning in which resorbed low-Ba cores are

and rim compositions An36–16. Its anorthite-rich cores are typically patchy and surrounded by high-Ba rims (10,000–15,000 ppm). The single resorbed grain

resorbed with outermost rims of An~40–30 (Fig. 11B). These cores may be either in the Ranger Creek hornblende dacite dike preserves the latter type of zoning, entrained crystals or relict cores in disequilibrium with subsequent melt from with a core containing 4000 ppm Ba and a rim with 15,000 ppm Ba. which the rims grew. Hornblende andesite-dacite dikes and the Lake Annella andesite porphyry contain euhedral, normally zoned plagioclase microphenocrysts as much as Quartz 500 µm long, but some crystals have resorbed cores and sharp oscillatory zoning (Fig. 10). These textures are consistent with crystallization from more Quartz occurs predominately as anhedral groundmass grains in the large mafic magma that cooled resulting in normally zoned domains, followed by intrusions, although the sanidine-quartz dacite dikes contain resorbed and interaction with either more mafic and/or hotter pulses of magma that pro- rounded phenocrysts (as much as 4 mm in diameter), many with embayed duced sharp jumps in anorthite content. Of the intrusions west of the Platoro margins, consistent with decompression or mixing with a hotter magma. caldera complex, plagioclase in the Bear Creek monzonite is the most calcic;

core compositions range An80–36 and rim compositions An69–30 (Fig. 10). Strontium concentrations in plagioclase from Platoro caldera complex Pyroxene rocks range from below detection limit (~150 ppm) to 3720 ppm (Table S2 [footnote 1]). The intracaldera monzonite intrusions have, in general, the low- Augite is present in all the large Platoro intrusions and in one of the horn- est concentrations, and the Chiquito Peak Tuff has the highest concentrations, blende andesite-dacite dikes. In a few samples of the Alamosa River and much higher than the intracaldera Alamosa River monzonite and Cornwall Jasper monzonites, fine exsolution textures in augite, a consequence of

8 7 Shoshonite series 6 3 5 High - K calc-alkaline series 4 2 O (wt%)

2 3 K Calc-alkaline series Mg O (wt%) 2 1 1

7 1.2 6 1.0 5 0.8 4 (wt%) 2 0.6 3 Ca O (wt%) TiO 2 0.4 1 0.2

8 7 0.4 6 0.3 5 (wt%) (wt%) T 4 5 O

2 0.2 Fe O 3 P

2 0.1 1

18 4 (wt%)

3 16 O (wt%) 2 O 2 Na Al 2 14

1200 800

1000 600 Ba (ppm)

Sr (ppm) 800

400 600

50 55 60 65 70 75 80 50 55 60 65 70 75 80

SiO2 (wt%) SiO2 (wt%)

Figure 8. 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.

40 Intracaldera

100 30

10 Alamosa River monzonite 20

Jasper monzonite La/Yb

Rock/chondrite Chiquito Peak Tuff Dacite of Fisher Gulch 1 Garnet 10 La Yb Intracaldera Amphibole Differentiation 100 0

Cornwall Mountain 2.5 10 quartz monzonite Figure 9. (A) Whole-rock rare earth ele- Alum Creek porphyry ment (REE) concentrations normalized to

Rock/chondrite average chondrite (Sun and McDonough, Summitville Andesite 1989) for the rocks of the Platoro caldera complex. Patterns show light REE enrich- 1 2.0 ment relative to heavy REEs. Only the Alamosa River and Jasper monzonites

Extracaldera Dy/Yb have negative Eu anomalies. (B) Ratios 100 of La/Yb and Dy/Yb versus silica, as in- dicators of mineral fractionation paths

1.5 Garnet (after Davidson et al., 2007).

Dy Yb Amphibole 10 Cat Creek monzonite Differentiation Lake Annella andesite porphyry Cataract Creek monzonite 1.0 Rock/chondrite Elwood monzonite 55 60 65 70 75 Bear Creek monzonite SiO (wt%) 1 2

Chiquito Peak Tuff Extracaldera intrusions Lake Annella andesite Summitville Andesite 100 porphyry Dacite of Fisher Gulch Cat Creek monzonite Intracaldera intrusions Cataract Creek monzonite Alamosa River monzonite Elwood Creek monzonite Alum Creek porphyry Bear Creek monzonite Jasper monzonite 10 Sanidine dacite dikes Hornblende Cornwall Mountain quartz monzonite andesite-dacite dikes porphyry Hornblende andesite-dacite Hornblende dacite dikes dikes Rock/chondrite Sanidine dacite dikes Mafic (andesite) dikes Andesite dikes 1 La Pr Pm Eu Tb Ho Tm Lu Ce Nd Sm Gd Dy Er Yb

20 Chiquito Peak Tuff Jasper Creek Bear Creek monzonite hornblende dacite dike 20 6

4 10 10

Number of analyses 2

0 0 0 14 Dacite of 15 Ranger Creek Cataract Creek monzonite Fisher Gulch hornblende dacite dike 12 16

10 10 12 8

6 8 5 4 Number of analyses 4 2 Figure 10. Histograms of mole percent 0 0 0 anorthite (An) content for plagioclase Summitville Andesite Alum Creek Elwood Creek monzonite 12 crystals for various units of the Platoro (upper member) 8 porphyry 6 caldera complex; y-axis represents the 10 number of analyses. Core anorthite con- 6 8 tents are displayed as solid-color bars, 4 and rim contents as uncolored bars. 6 4 Shades of yellow indicate intracaldera intrusions; shades of blue, extracaldera 4 2 Number of analyses 2 intrusions. The intracaldera monzonites 2 have similar ranges in anorthite content, while intrusions outside the caldera are 0 0 0 more variable. The Chiquito Peak Tuff 12 15 Andesite dike Lake Annella Alamosa River monzonite has a broad range in anorthite content, andesite porphyry 10 15 with anorthite-rich cores.

8 10 10 6

4 5 5 Number of analyses 2

0 0 0 Cornwall Mountain Cat Creek monzonite 15 Sanidine quartz monzonite porphyry 6 dacite dike 8 5 10 4 6

3 4 5 2

Number of analyses 2 1

0 0 0 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 An (mol%) An (mol%) An (mol%)

the Bear Creek monzonite has the broadest range (Mg# 60–76). Augite in the hornblende dacite dike 17AG39 has a more restricted compositional range

(Mg# 66–76). TiO2 concentrations in the augite range from below detection limit (0.01 wt%) to 1.5 wt%. Relative to augite phenocrysts in the other intru- An 39 sions, those in the hornblende dacite dike at Jasper Creek have the highest An41 mean TiO2 concentration (0.65%) and define the largest compositional range An51 An72 (0.33%–1.41%). Augite TiO2 concentration correlates inversely with Mg# in this dike (Fig. 12) but is uncorrelated in the other intrusions. Augite from the Ala- mosa River monzonite and the Bear Creek monzonite have indistinguishable

mean TiO2 contents (0.51 wt%), whereas augite from the Cat Creek monzonite 100 µm 100 µm has a lower mean TiO2 concentration (0.33 wt%). The mean TiO2 content of augite from the fine-grained andesite dike 17AG80 is 0.48 wt%. Augite from 4000 the remaining units, including the Chiquito Peak Tuff, contains <0.32 wt% Chiquito Peak Tuff Alamosa River monzonite TiO2 (Fig. 12). MnO concentrations of the analyzed augites vary considerably. Cornwall Mountain quartz monzonite porphyry Augite in the Alamosa River monzonite, Alum Creek monzonite porphyry, Cat Creek monzonite, equigranular Bear Creek monzonite, andesite dike 17AG80, 3000 Dacite of Fisher Gulch Summitville Andesite Summitville Andesite (upper member), and hornblende dacite dike on Jasper Creek contains 0.11–0.64 wt% MnO, whereas that in the sanidine dacite dike 17AG90, porphyritic Bear Creek monzonite, Cataract Creek monzonite, and 2000 Chiquito Peak Tuff contain >0.8 wt% (Fig. 12). Augite in the Chiquito Peak Tuff has the greatest MnO contents, as much as 1.52 wt% (Fig. 12). Sr (ppm) Unaltered orthopyroxene was identified only in the Bear Creek monzonite and an andesite dike (sample 17AG80), although Lipman (1975) reported minor 1000 hypersthene in the Alamosa River, Cat Creek, and Jasper monzonites. Domains of serpentine ± chlorite in these units may constitute altered orthopyroxene. All analyzed orthopyroxene crystals have lower Mg# than the analyzed augite crystals. For example, augite from the Bear Creek monzonite has mean Mg# 0 0 10 20 30 40 50 60 70 80 90 100 of 68, whereas that for the orthopyroxene is 53. An (mol%)

Figure 11. Plagioclase images and compositions. (A) Backscattered electron (BSE) image of plagioclase from the Alamosa River monzonite showing little zoning from core to rim. Biotite (B) BSE image of plagioclase from the Chiquito Peak Tuff showing a resorbed, high-anorthite core. Red circles indicate anorthite content for core and rim analyses. (C) Strontium Biotite is present in all Platoro-associated units except the fine-grained concentration versus anorthite content of plagioclase crystals in the Chiquito Peak Tuff, andesite dikes; it is most abundant in the intrusions and porphyritic dikes Alamosa River monzonite, Cornwall Mountain quartz monzonite porphyry, Summitville west of the caldera. Biotite is anhedral and interstitial to plagioclase in the Andesite, and dacite of Fisher Gulch. Concentrations of Sr in plagioclase are much higher in the Chiquito Peak Tuff and dacite of Fisher Gulch than in the intracaldera intrusions. equigranular monzonites (Fig. 5C), but in the porphyritic units, phenocrysts An—mole percent anorthite. are euhedral to subhedral and form glomerocrysts with plagioclase + Fe-Ti oxides ± augite ± amphibole (Fig. 5I). Biotite commonly contains inclusions of apatite and may be intergrown with Fe-Ti oxides. Chloritized biotite in several porphyritic units could not be analyzed. augite-pigeonite-hypersthene immiscibility, are indicative of rapid cooling. Although the biotite grains have variable Mg#, they are unzoned and Altered pyroxenes in the Cornwall Mountain quartz monzonite porphyry were have compositions that are somewhat unique to each of the units (Table S4 not analyzed. [footnote 1]; Figs. 12C–12D). Biotite grains in the Bear Creek monzonite gener- Augite in the intrusions has a wide range of Mg# [Mg# = 100 × Mg2+/(Mg2+ ally have the most iron-enriched Mg# of the analyzed biotites and the lowest 2+ + Fe )], Ti, and Mn (Figs. 12A–12B; Table S3 [footnote 1]). Mg# for augite TiO2 concentrations (<4 wt%). Biotite in the other units contains 1.8–7.1 wt%

phenocrysts and groundmass crystals ranges from 60 to 84; augite from the TiO2. Biotite grains in the Alamosa River and Cat Creek plutons are much more

Cat Creek monzonite is the most magnesian (Mg# 69–84), whereas that in magnesian than those in the Chiquito Peak Tuff. TiO2 content is relatively

1.5 8 Clinopyroxene Biotite

6 1.0 (wt%) (wt%) 2 2 4 TiO TiO

0.5 2

0.0 0 2.0 0.6 Clinopyroxene Biotite Figure 12. Plots showing compositions of mafic 1.5 mineral phases. (A–B) Augite compositions 0.4 show overlap within the intrusions and broad Mg# (Mg# = 100 × Mg/Mg + FeT), whereas the Chiquito Peak Tuff shows a narrow range of 1.0 Mg# and the highest MnO content. (C–D) Bi- otite compositions show distinct groupings in MnO (wt%) MnO (wt%) TiO2 and MnO content, with little to no overlap 0.2 in composition with biotites in the Chiquito 0.5 Peak Tuff. (E) Total alkali contents of amphibole phenocrysts, within each unit and for the entire group of analyzed grains, increase with increas-

ing AlTOT (total aluminum). Two subgroups are: 0.0 0.0 (1) a high-AlTOT subgroup, and (2) a low-AlTOT 60 70 80 90 50 60 70 80 subgroup. Mg# Mg# 2.5 Chiquito Peak Tuff Hornblende dacite dike (17AG28) E Lake Annella Hornblende dacite dike (17AG39) andesite porphyry 2.0 Cat Creek monzonite Alum Creek porphyry Alamosa River monzonite Cataract Creek monzonite Summitville Andesite TOT Elwood Creek monzonite 1.5 Andesite dike Al Bear Creek monzonite Dacite of Fisher Gulch Sanidine dacite dike (17AG90) 1.0

Amphibole

0.5 0.3 0.4 0.5 0.6 0.7 0.8 Na + K

uniform within samples, except for samples of the Alamosa River monzonite. are weakly positively correlated with increasing Hf abundances (Figs. 13A– MnO concentrations among the analyzed biotites decrease with increasing 13B). Zircon from the Alamosa River monzonite also contains the highest Th Mg#, and biotite has higher MnO concentrations in the Chiquito Peak Tuff contents. U and Th contents of zircon from the Alamosa River monzonite are than in the monzonite and quartz monzonite intrusions. significantly greater than those of the Chiquito Peak Tuff (U, mean 170 ppm; Th, mean 135 ppm). Among zircon from intrusions younger than 28 Ma, most U contents range from 100 to 1000 ppm, but are uncorrelated with Hf content. Amphibole Chondrite-normalized zircon REE patterns for all samples are similar to well-established magmatic REE patterns and include characteristic positive Ce The Lake Annella andesite porphyry, Cataract Creek monzonite, Bear Creek anomalies (Trail et al., 2012). The Alamosa River and Jasper monzonites also monzonite, and hornblende andesite-dacite and sanidine dacite dikes contain contain significantly greater total REE abundances than the Chiquito Peak Tuff euhedral to subhedral amphibole phenocrysts; amphibole grains are also glom- (Table S6 [footnote 1]). Although the magnitude of negative Eu anomalies in erocryst constituents. Amphibole phenocrysts contain inclusions of apatite, zircon is commonly negatively correlated with Hf abundance, which is indic- plagioclase, and Fe-Ti oxides. Amphibole is absent in the intracaldera plutons ative of feldspar crystallization (e.g., Hoskin and Schaltegger, 2003; Cooper et and in the Cat Creek intrusion. The amphiboles are Ca and Mg rich and are al., 2012; Watts et al., 2016), no such correlation is evident among zircons from composed of magnesio-hornblende, magnesio-hastingsite, and pargasite the Platoro intrusions or the Chiquito Peak Tuff (Figs. 13C–13D). Importantly, Eu (Hawthorne et al., 2012) (Table S5 [footnote 1]). anomalies in zircon may also be influenced by the redox state of the magma Total alkali contents of amphibole phenocrysts within each unit, and for all (Dilles et al., 2015). At any given Hf content, the intracaldera monzonites have

analyzed grains, increase with increasing AlTOT (total aluminum). Compositions lower Eu/Eu* where Eu* is √(SmN × GdN) than the Chiquito Peak Tuff. The Bear of amphibole from the Platoro intrusions form two groups (Fig. 12E): (1) a Creek monzonite also has low Eu/Eu* (<0.2). The hornblende andesite-dacite

high-Al group (Al2O3 = 9.4–13.3 wt%) of predominantly magnesio-hastingsite dikes, sanidine dacite dike 17AG90, Elwood Creek monzonite, and the Sum-

and pargasite, and (2) a low-Al group (Al2O3 = 5.9–8.4 wt%) of magnesio-horn- mitville quartz monzonite all have Eu/Eu* >0.4. blende. Amphibole phenocrysts from the Lake Annella andesite porphyry, Zircon Hf isotopic composition is commonly used as a geochemical tracer Bear Creek monzonite, and sanidine dacite dike constitute the high-Al group of a host magma’s origin, in a similar manner to the way in which whole-rock (although a few analyses from the sanidine dacite dike are consistent with the Nd isotopes are used. Additionally, the robust nature of zircon, with regard to low-Al group), whereas amphiboles from the Cataract Creek monzonite and Lu and Hf, means that this system is less vulnerable to isotopic disturbances. the hornblende dacite dike of Ranger Creek define the low-Al group (Fig. 12E). Lu and Hf isotopic data for zircon from the Platoro caldera complex define

εHf isotopic compositions from −9.2 (±3.2) to 1.4 (±1.7); most are generally uniform within a given unit (Fig. 14A; Table S7 [footnote 1]). Single zircons in Zircon the Cataract Creek monzonite and Alamosa River monzonite have cores with outlier values of −28.3 (±1.4) and −30.4 (±3.2), respectively. These analyses

Abundances of trace elements in zircon from Platoro rocks vary consider- may represent inherited cores and are not included in Figure 14A. Initial εHf

ably (Fig. 13; Table S6 [footnote 1]). Hf concentrations in these zircons range values and SiO2 abundances are uncorrelated, although the younger intrusions from 6170 to 12,400 ppm (Fig. 13). Higher Hf concentrations are commonly appear to be characterized by progressively more negative values. Relative to

interpreted as indicative of more evolved melts (Hoskin and Schaltegger, 2003), crustal Lu/Hf evolution lines, zircon εHf values for Platoro-associated rocks are and whole-rock silica contents of Platoro intrusions correlate positively with consistent with derivation from a lithospheric source with a similar age and Hf concentrations. The analyzed grains are unzoned with respect to trace composition as the 1.1–1.7 Ga crust of the southern Rocky Mountains (Fig. 14B). element abundances. Zircon in the Summitville quartz monzonite contains has the highest Hf (9400–12,400 ppm). However, zircon in the Alamosa River monzonite varies most broadly and includes similarly elevated abundances Intensive Parameter Estimates (~7770–11,950 ppm). By contrast, zircon in the Bear Creek monzonite (58 wt%

SiO2) has the lowest abundances and narrowest range of Hf abundances Temperature (6630–8500 ppm). Although compatible in zircon, U and Th are incompatible in silicic melts, Titanium-in-zircon thermometry estimates the temperature of zircon crys-

and thus their abundances are positively correlated with increasing Hf in zir- tallization in magma, provided that SiO2 and TiO2 activities (aSiO2 and aTiO2) con (Claiborne et al., 2010). Zircons in the Alamosa River monzonite define the are known (Watson and Harrison, 2005; Watson and Harrison, 2006; Ferry

broadest compositional range for U among all analyzed samples, include the and Watson, 2007). We assume an aSiO2 of 1, as indicated by the presence of

greatest U values (110–6220 ppm, mean 1660 ppm), and have U values that quartz in the Platoro-associated rocks, and an aTiO2 of 0.7, as indicated by the

10,000 Chiquito Peak Tuff Intrusions younger than 28 Ma Intrusions older than 28 Ma Dacite of Fisher Gulch Extracaldera Intrusions Lake Annella andesite porphyry

1000 Cat Creek monzonite

Cataract Creek monzonite U (ppm) Elwood Creek monzonite

Bear Creek monzonite 100 Sanidine dacite dike (17AG90)

Hornblende dacite dike (17AG49)

Intracaldera Intrusions 10 Hornblende dacite dike (17AG28) 1.0 Hornblende dacite dike (17AG39) Alum Creek porphyry Cornwall Mountain quartz monzonite 0.8 porphyry Jasper monzonite

Alamosa River monzonite 0.6 Summitville quartz monzonite Eu/Eu* 0.4

0.2

0 1100 E

1000

900

800

Ti-in-zircon temperature ( ° C) 700

600 6000 7000 8000 9000 10,000 11,000 12,000 13,000 6000 7000 8000 9000 10,000 11,000 12,000 13,000 Hf (ppm) Hf (ppm)

Figure 13. Trace element compositional diagrams for zircons. Shades of yellow indicate intracaldera units, and shades of blue indicate extracaldera units. (A–B) U versus Hf. (C–D)

Eu/Eu* where Eu* is √(SmN × GdN)versus Hf. (E–F) Temperatures plotted versus Hf for aTiO2 (TiO2 activity) of 0.7, using the Ti-in-zircon thermometer of Ferry and Watson (2007).

20 10 18 Chiquito Peak Tuff DM 16 5 Extracaldera Intrusions Lake Annella andesite porphyry 14 1.1 Ga zircon CHUR Cat Creek monzonite 12 0 Platoro range Cataract Creek monzonite Figure 14. (A) Zircon εHf(t) versus crystal- εHf(t) lization age for individual analyses of 10 1.4 Ga -5 Elwood Creek monzonite zircon from the Platoro caldera com- 8 Bear Creek monzonite plex units. Error bar indicates 2σ errors. 1.7 Ga (B) Plot of zircon εHf(t) range versus age 6 -10 Sanidine dacite dike (17AG90) showing crustal evolution lines. Zir- 0 50 100 con ε appears to decrease slightly Age (Ma) Hornblende dacite dike (17AG49) Hf(t) 4 through time, while the range for all Zirocn ε Hf(t) Intracaldera Intrusions units is consistent with derivation from 2 Hornblende dacite dike (17AG28) a lithospheric source with a similar age CHUR and composition as the 1.1–1.7 Ga crust 0 Hornblende dacite dike (17AG39) hosting the samples. DM—depleted mantle; CHUR—chondritic uniform -2 Alum Creek porphyry reservoir. Data are listed in Table S6 Cornwall Mountain quartz monzonite (footnote 1). -4 porphyry

-6 Alamosa River monzonite Summitville quartz monzonite -8

-10 24.0 27.2 27.6 28.0 28.4 28.8 29.2 Age (Ma)

presence of titanite in most of these rocks. Varying assumed aTiO2 between Applied to in-contact amphibole-plagioclase pairs, the uncertainty associated 0.6 and 0.8 results in calculated temperature variations of as much as 30 °C with this method is ±40 °C (2σ). Calculated amphibole-plagioclase tempera- (Ferry and Watson, 2007). Data for the zircons from the largest intrusions tures for Platoro system rocks range from ~820 to 970 °C (Fig. 15; Table S7 (the Alamosa River, Jasper, Cat Creek, and Bear Creek monzonites) all sug- [footnote 1]). Temperatures range from 822 to 829 °C for the intracaldera gest decreasing temperature with increasing Hf (Figs. 13E–13F, 15; Table S8 hornblende dacite dike on Ranger Creek, 925–966 °C for the Lake Annella [footnote 1]), which is typical of zircon behavior in granitic rocks (e.g., Claiborne andesite porphyry, 849–951 °C for the Bear Creek monzonite, and 817–877 °C et al., 2010; Cooper et al., 2014; Watts et al., 2016; Colgan et al., 2018). Other for the Cataract Creek monzonite. These calculated temperatures extended to Platoro intrusions exhibit smaller temperature variations during crystallization, higher values than determined by the Ti-in-zircon thermometer for the same consistent with more rapid cooling. Alamosa River monzonite zircons have unit, confirming earlier crystallization of these phases compared with zircon. the highest and greatest range of Ti concentrations (5–40 ppm) and consequently exhibit the most extensive range of zircon crystallization temperatures (719–950 °C). Zircon from the Jasper, Cat Creek, and Bear Creek monzonites Pressure reflect similar crystallization temperatures (mostly >800 °C; Fig. 13). Zircon thermometry from the porphyritic intrusive units yields cooler temperatures, The validity of pressure estimations based on amphibole barometry has

<800 °C in some cases, extending almost to the H2O-saturated granite solidus been increasingly questioned due to the sensitivity of amphiboles to melt (Fig. 15). Zircon thermometry from the Chiquito Peak Tuff defines temperatures composition and temperature in addition to pressure (e.g., Erdman et al., 2014; between 719 and 822 °C, except for one zircon that yields an anomalously high Putirka, 2016). Most amphibole barometers require buffering assemblages, temperature of 888 °C. and many are calibrated for temperatures <800 °C (e.g., Anderson and Smith, Crystallization temperatures for amphibole-bearing units were calculated 1995; Mutch et al., 2016). The amphibole-bearing units at Platoro contain the using the plagioclase-amphibole thermometer of Holland and Blundy (1994). necessary mineral assemblage; however, some units, most notably dikes,

exhibit open-system petrologic characteristics including presence of mafic Ti-in-zircon thermometer Hbl-Pl therm. 1050 enclaves and glomerocrysts. Additionally, because most plagioclase-amphibole temperature estimates for these units are >800 °C, we do not estimate 1000 crystallization pressures for these units. 950

■■ DISCUSSION 900

We integrate new U-Pb zircon ages with observed stratigraphic constraints, 850 mineral assemblages, chemistry, and texture as well as whole-rock chemistry 800 for Platoro intrusions to better constrain the magmatic history of the terminal emperature (ºC) T postcaldera magmatic system, interpreted here as the waning stages of the Platoro caldera complex. All U-Pb zircon dates are interpreted as magma intru- 750 sion ages within limits of reported two-sigma errors. All intrusions postdate the 700 terminal erupted ignimbrite from the Platoro caldera system, the Chiquito Peak Tuff, as supported by either new geochronology or mapped field relations. As 650 such, these units may reflect a range of petrogenetic affinities associated with the youngest erupted ignimbrite, or the magmatic underpinnings of postcaldera 600 effusive volcanism to which some intrusions are spatially associated. Alterna- tively, new data are also considered in light of a hypothesis whereby most, if 550

not all, postcaldera intrusions simply reflect largely unrelated pulses of magma uff introduced into the upper crust from magma reservoirs at mid- to lower-crustal levels that experienced rapid rise and emplacement in the upper crust and min-

imum residence times at shallow levels. This would imply most intrusions need Chiquito PeakJasper T monzonite Cat Creek monzonite not be derived from a large integrated upper-crustal magma chamber but are Alum Creek porphyry Bear Creek monzoniteBear Creek monzonite rail sanidine daciteElwood dike Creek monzonite potentially derived from a shared petrogenetic history at deeper crustal levels. Alamosa River monzonite Annella andesite porphyryCataract Creek monzoniteCataract CreekAnnella monzonite andesite porphyry Summitville quartz monzonite Specifically, we evaluate petrogenetic relationships between the intracaldera Lake Lake

JasperRanger Creek Creek hornblende hornblende dacite dacite dike dike Ranger Creek hornblende dacite dike intrusions, Chiquito Peak Tuff, Summitville Andesite, and postcaldera dacite of HorsethiefCrater Park Creek hornblende t dacite dike Fisher Gulch. Although not necessarily definitively, we use new data to better Cornwall Mountain quartz monzonite porphyry constrain timing of ore-deposit mineralization and hydrothermal alteration. Figure 15. Box-and-whisker plot of Calculated temperatures based on the Ti-in-zircon

thermometer of Ferry and Watson (2007) using values of aTiO2 (TiO2 activity) = 0.7 and

aSiO2 (SiO2 activity) = 1.0, and calculated hornblende-plagioclase (Hbl-Pl) temperatures Temporal Constraints on Postcaldera Intrusive Magmatism and based on the thermometer of Holland and Blundy (1994). Each box represents the lower Mineralization 25% to upper 75% of values for each sample and the whiskers extend to minimum and maximum values. The horizontal black line in each box shows the median value. Gray bar represents the H O-saturated granite solidus at pressures between 200 and Intrusion emplacement ages for the Platoro caldera complex are spatially 2 500 MPa from Johannes and Holtz (1996). Shades of yellow indicate units that are associated with proximity to mapped caldera margins of the ca. 29 Ma Chiq- intracaldera intrusions; shades of blue, extracaldera intrusions. The larger intracaldera uito Peak Tuff eruption. With two exceptions, intracaldera intrusions yield monzonites show higher Ti-in-zircon temperatures than the other intrusions. Horn- crystallization ages ranging from 28.98 ± 0.18 Ma to 28.25 ± 0.30 Ma and are blende-plagioclase temperatures are higher overall than Ti-in-zircon temperatures. indistinguishable, within analytical uncertainty, from previously reported eruption ages for the Chiquito Peak Tuff (Fig. 6; Table 3; Lipman et al., 1996). Notable exceptions include the Alum Creek monzonite porphyry (27.32 ± 0.38 Ma) and 26.3 Ma. Younger volcanic activity in the Platoro area is volumetrically minor, the Summitville quartz monzonite (24.07 ± 0.25 Ma). These younger intrusions bimodal, and associated with late Oligocene to Miocene volcanism more typ- either host or are inferred to have been causative to important hydrothermal ically associated with early stages of Rio Grande rift extension (Lipman, 1974, mineralization. New ages on these intrusions provide additional constraints on 1975; Thompson and Machette, 1989; Thompson et al., 1991). post-emplacement alteration. Extracaldera intrusions yield crystallizations ages The resolution of the new U-Pb zircon geochronology is insufficient to ranging from ca. 28.0 to 26.3 Ma with a magmatic hiatus between ca. 27.3 and establish discrete temporal breaks in intrusive emplacement beyond the age

ranges delineated above. Consequently, quantifying the duration of pulsed that hosts it. However, if four anomalously old zircons (possible antecrysts, activity or periods of quiescence or quantitatively estimating magma flux over with uncertainties beyond the weighted mean) are excluded, the monzonite the life span of a single or even multiple intrusion(s) is beyond the scope of date becomes analytically indistinguishable from that for the mineralized dacite. this study. High-precision chemical abrasion isotope dilution thermal ionization Therefore, the Summitville quartz monzonite could have been the source of mass spectrometry (CA-ID-TIMS) U-Pb zircon investigation may better resolve fluids responsible for mineralization and wall-rock alteration at Summitville, both the duration of emplacement for individual plutons and the spatial and as previously inferred by Bethke et al. (2005). Most importantly, mineralization temporal evolution between mapped intrusions. and hydrothermal alteration spatially associated with intrusions of the Platoro All the exposed granitoid plutons are too young to have been associated caldera complex occurred recurrently from ca. 29 to 23 Ma, all postdating with any of the pre–Chiquito Peak ignimbrites of the Platoro caldera complex. ignimbrite eruptions and associated caldera collapse. The absence of older caldera-related intrusions suggests a possible change It is notable that ages of the exposed extracaldera Platoro plutons (28.0– in magma-chamber evolution during the terminal cycle of the Platoro system. 26.3 Ma) closely parallel the time span of major younger ignimbrite eruptions Previous eruptive cycles may have been characterized by higher magma fluxes, from the central San Juan caldera cluster (ca. 28.2–27.05 Ma) (Lipman, 2007). facilitating development of focused shallow chambers subject to ignimbrite This reflects the migration of magmatic activity to the northwest, with fluxes eruption, rather than smaller, isolated intrusions beyond caldera margins. decreasing in the Platoro area as they increased in the area of the central San It is commonly accepted that intrusions form over protracted spans of Juan caldera cluster. time as a result of incremental growth from repeated pulses of magma (e.g., Coleman et al., 2004; Glazner et al. 2004; Leuthold et al., 2012; Miller et al., 2011; Frazer et al., 2014; Annen et al., 2015). The Alamosa River monzonite Magma Storage and Differentiation is the largest intrusive body exposed within the caldera (Lipman, 1974), and magnetic fabrics delineated in the Alamosa River monzonite document three Compositions of the Platoro postcaldera rocks range from andesite to domains within the intrusion that are interpreted as discrete intrusive pulses rhyolite and plot along a high-K calc-alkaline trend (Figs. 3, 8A), as is charac- of magma emplaced over a short, but unknown, time period relative to the teristic for the Southern Rocky Mountain volcanic field (SRMVF). Although the total intrusion emplacement interval (Tomek et al., 2019). As such, it may have ca. 24 Ma Summitville quartz monzonite is more evolved, the long time span had a more protracted emplacement history than that of small intrusions. of intrusion and absence of systematic compositional evolution through time Hydrothermal alteration and mineralization are spatially associated with shows that the associated magmas cannot be related by any closed-system several postcaldera intrusions of the Platoro caldera magmatic system, most fractional crystallization process. Consequently, multiple batches of magma notably the Alum Creek, Alamosa River, and Summitville intrusions. Previous were likely responsible for the compositional range observed in the Platoro studies (Steven and Ratté, 1960; Mehnert et al., 1973; Lipman, 1975; Neuerburg, system. Increasing La/Yb and decreasing Dy/Yb with increasing silica in the 1978; Bethke et al., 2005) asserted causative and/or temporal association of min- Platoro magmas, indicative of amphibole (and possibly titanite) fractionation eralization with individual plutons in area mineralization districts. However, the (Davidson et al., 2007), support differentiation at even deeper crustal levels lack of geochronologic data limits determination of both spatial and temporal for some of these magmas (Fig. 9B) prior to emplacement in shallow crust. interpretations of genetic links to intrusion emplacement. Our new intrusion The variable mineralogy within the restricted bulk compositional range emplacement ages further constrain the timing of hydrothermal alteration of the larger Platoro intrusions (monzonite to quartz monzonite) suggests and mineralization in the Platoro area. In the Stunner district, alteration and cotectic or peritectic conditions with the magmas in a low variance state. The mineralization are superposed on the 27.32 ± 0.38 Ma Alum Creek monzonite reaction-boundary crystallization buffers the bulk chemical composition, as porphyry. Steven and Ratté (1960) recognized two periods of hydrothermal demonstrated in reactive flow models (Jackson et al., 2018) and in phase-equi- activity in this area: (1) an older one they interpreted as associated with the libria experiments on Mount St. Helens dacite (Washington State, USA) (Blatter northern side of the Alamosa River pluton in the area of the Alum Creek mon- et al., 2017). Phase relations within magmas of similar compositions vary with zonite porphyry, and (2) a younger one associated with the dacite lava dome at pressure, thus differentiation of magmas at different depths leads to varia- Summitville. The new geochronologic data cannot resolve multiple episodes of tions in mineral assemblages and compositions, as evidenced in the Platoro alteration but do support an interpretation that it is associated with the Alum intrusions. For example, larger monzonite intrusions contain augite but no Creek monzonite porphyry. Alteration and mineralization in the Crater Creek amphibole, whereas chemically similar intrusions west of the caldera contain area (Fig. 2) locally overprint all intrusions, including the sanidine dacite dike abundant amphibole. 17AG90, and consequently must be younger than ca. 26.3 Ma. In the smaller intrusions and dikes, mafic enclaves and abundant glom- The U-Pb zircon date for the Summitville quartz monzonite (24.07 erocrysts suggest open-system behavior, likely magma mixing, particularly ± 0.25/0.36 Ma; Fig. 6) is older than the ca. 23 Ma date for alteration in the Sum- within the <28 Ma intrusions west of the caldera. Phenocrysts with distinct rim mitville epithermal gold deposit and for the quartz-sanidine dacite lava dome compositions, well-developed resorption surfaces, and compositionally diverse

plagioclase crystals in single samples of the hornblende dacite dikes and Lake Consequently, magma represented by the Chiquito Peak Tuff likely equilibrated Annella andesite porphyry suggest that magma mixing played a prominent role in a reservoir in which the prevailing pressure was ~200 MPa. In addition, the in the petrogenesis of these younger intrusions. In addition, these units contain lack of hornblende and the coexistence of sanidine and augite suggest that

abundant glomerocrysts that are consistent with crystal entrainment where the the H2O content of the Chiquito Peak magma was likely less than that of the

transporting melt scavenged crystals and crystal clusters from mushy domains Fish Canyon Tuff magma (<4 wt% H2O) (e.g., Johnson and Rutherford, 1989). within the magmatic system. The larger intracaldera intrusions, by contrast, For the amphibole-bearing intrusions of the Platoro magmatic system— lack enclaves and glomerocrysts, and their plagioclase phenocrysts have more the Bear Creek monzonite, Cataract Creek monzonite, Lake Annella andesite uniform compositions, suggesting either that reservoirs associated with the porphyry, hornblende dacite dikes, and sanidine dacite dikes—amphibole is earliest intrusive magmatism were less affected by open-system behavior or commonly euhedral and commonly intergrown with plagioclase, suggesting

that entrained crystals were completely resorbed by larger melt volumes pos- it crystallized early. The H2O contents of these magmas were likely >4–6 wt%, sibly associated with higher early-postcaldera magma flux rates. as required to stabilize amphibole (Naney, 1983; Richards, 2011; Richards et al., Crystals record magma-reservoir compositional evolution through time. 2012; Blatter et al., 2017). Plagioclase-amphibole thermometry for these units In the Platoro magmatic system, phenocrysts in intrusions constrain how the constrains the temperature range for their magma storage to be between 960 unerupted parts of associated magma reservoirs evolved. Within the >28 Ma and 850 °C for the Lake Annella and Bear Creek intrusions and between 880 monzonite plutons, there are consistent mineral assemblages of plagioclase, and 820 °C for the Cataract Creek monzonite and Ranger Creek hornblende augite, biotite, potassium feldspar, and quartz, and in the analyzed phases, dacite dike.

compositions are broadly similar. Among these older units, plagioclase compo- All zircon εHf values for the Platoro intrusions and Chiquito Peak Tuff plot sitions are relatively uniform (Fig. 10), which suggests compositional evolution below those for depleted mantle and chondritic uniform reservoir (CHUR) without significant mixing of crystal cargo. Negligible Eu anomalies (Eu/Eu* (Fig. 14A), which helps constrain likely sources for these magmas. The data = 0.74–0.95) characteristic of most of these intrusions, excluding the Alamosa indicate a similar source but with inadequate resolution to evaluate varia- River and Jasper monzonites (Eu/Eu* = 0.59–0.93), suggest early suppression of tion in the amounts of assimilated crust. The lack of a correlation between

plagioclase at high pH2O, probably during deep differentiation (Moore and Car- zircon εHf and SiO2 content suggests that the isotopic signature was imparted michael, 1998; Müntener et al., 2001). Euhedral augite and plagioclase grains from parental melts during differentiation with some assimilation, which is in the larger monzonite intrusions suggest these phases crystallized early in supported by the presence of zircon xenocrysts in the Alamosa River, Jasper, the crystallization sequence, with augite, or plagioclase in some instances, Bear Creek, and Cataract Creek monzonites and the Chiquito Peak Tuff, as

likely being liquidus places. well as the two extremely negative εHf values interpreted as inherited cores

Granophyric textures, characteristic of the Alamosa River, Jasper, Cat Creek, (Table S7 [footnote 1]). εHf data for the Platoro zircons are similar to those for and Elwood Creek monzonites (Figs. 5B, 11A), are evidence of undercooling and zircon from Paleogene intrusions in New Mexico and Texas (Chapman et al.,

rapid solidification (Barker, 1970; Morgan and London, 2012), consistent with 2018). The range of zircon εHf values for Platoro plots on crustal Lu/Hf evolution depressurization and volatile loss during emplacement into the shallow crust lines (Fig. 14B) between 1.1 and 1.7 Ga, and most are consistent with 1.4 Ga, from deeper reservoirs or even associated with venting during volcanic eruptions. an observation in accord with recognition that most of the San Juan volcanic Experimental petrology studies focused on magmatic storage conditions locus (SJVL) is underlain by 1.4 Ga plutons and 1.7–1.6 Ga crust of the Mazatzal pertinent to the Fish Canyon Tuff (Johnson and Rutherford, 1989; Caricchi and province (Reed et al., 1993; Shaw and Karlstrom, 1999; Whitmeyer and Karl- Blundy, 2015), erupted from the nearby La Garita caldera in the central San strom, 2007; Bickford et al., 2015). The zircon Hf isotopic data are consistent Juan Mountains (Fig. 1A), help constrain the intensive parameters prevailing with those of other whole-rock isotopic (Sr, Nd, and Pb) studies on pre- and for the Chiquito Peak magmatic reservoir. We compare the experimental run postcaldera Platoro rocks indicating these magmas have a lithospheric mantle products from Caricchi and Blundy (2015) and Johnson and Rutherford (1989) origin with lower-crustal contamination (Lipman et al., 1978; Doe et al., 1979; with the modal mineralogy of the Chiquito Peak Tuff. Despite a similar bulk Colucci et al., 1991; Balsley and Gregory, 1998) as well as studies in other cal- composition to the Fish Canyon Tuff, the Chiquito Peak Tuff contains no horn- dera systems and volcanics in the SRMVF (Johnson et al., 1990; Riciputi et al., blende or quartz and has only minor sanidine. Johnson and Rutherford (1989) 1995; Lake and Farmer, 2015). reported experimental results for a range of pressures between 200 and 500 The time interval spanned by the eruption of Chiquito Peak Tuff, the intracal- MPa, whereas all experiments of Caricchi and Blundy (2015) were performed dera intrusions, and the youngest intrusion, the Summitville quartz monzonite, at 200 MPa. Both sets of experiments showed that augite (a major mafic phe- is ~5 m.y., an extensive amount of time for postcaldera magmatism. Heat-trans- nocryst mineral in the Chiquito Peak Tuff) disappears in the range of 800–820 °C fer models for magma delivered in pulses to the upper crust separated by long for runs at 200 MPa. Experiments by Caricchi and Blundy (2015) indicated that pauses show that magma pulses solidify in the interim (Annen, 2009; Schöpa hornblende is stable only below 800 °C in water-saturated runs, and below and Annen, 2013; Barboni et al., 2015). Consequently, emplacement rate con-

850 °C for runs that contained 4 wt% H2O. Sanidine is stable below 775 °C only. trols whether large-scale magma chambers form (Hardee, 1982; Gelman et al.,

2013; Annen et al., 2015; Blundy and Annen, 2016). At Platoro, the volumes of Chiquito Peak Tuff but do overlap somewhat with that of the dacite of Fisher magma erupted or emplaced decreased through time, suggesting a declining Gulch (Fig. 11B).

magma supply rate. These temporal relations in addition to variable mineral Augite compositions have similar ranges in Mg#, TiO2, and MnO for the assemblages and whole-rock and mineral chemistry indicate that the Platoro intracaldera intrusions, Summitville Andesite, and dacite of Fisher Gulch. The

postcaldera magmas consisted of multiple discrete batches emplaced into Chiquito Peak Tuff has much higher MnO and much lower TiO2 than the other the shallow crust. units. This likely reflects earlier saturation in Fe-Ti oxides in the Chiquito Peak magma versus in the other units, with the decrease in Ti in augite the result of less Ti in the liquid due to its incorporation in ilmenite and titanomagnetite Postcollapse Evolution of Intracaldera Magmas (e.g., Walker et al., 1973). Biotite compositions show more distinct groupings for these units (Fig. 12). Field relations indicate that all Platoro intracaldera intrusions are younger Zircon trace element compositions for the Alamosa River and Jasper mon- than the Chiquito Peak Tuff (Fig. 4D), but the resolution of U-Pb SHRIMP zircon zonites are distinct from those of the Chiquito Peak Tuff and dacite of Fisher dates preclude temporal distinction between ignimbrite eruption and emplace- Gulch, as demonstrated by U content, Eu/Eu*, and Ti-in-zircon temperatures ment of intracaldera intrusions. Consequently, any petrogenetic relationship (Fig. 13). By contrast, zircon trace element compositions of and Ti-in-zircon between Chiquito Peak Tuff magma and associated residuum, postcaldera temperatures for the Cornwall Mountain quartz monzonite porphyry overlap lavas (dacite of Fisher Gulch and Summitville Andesite), and intracaldera completely with those for the Chiquito Peak Tuff and dacite of Fisher Gulch intrusions can only be evaluated using petrologic constraints based on bulk (Fig. 13). Distinctive zircon trace element characteristics, such as those dis- and mineral compositions. This does not presume the possibility of a direct played by Chiquito Peak Tuff and the Alamosa River and Jasper monzonites, parental relationship between post-ignimbrite residuum and subsequent vol- suggest crystallization from discrete, separate magma reservoirs (e.g., Till canic eruptions or intrusion emplacement, but does constrain the probability, et al., 2019). or lack thereof, of related petrogenetic evolution paths within the broader Thus, we conclude that the Alamosa River and Jasper monzonites are not Platoro magmatic system. the unerupted part of the Chiquito Peak upper-crustal magma reservoir, but Whole-rock major oxides show similar abundances for a given silica con- rather that their petrogenetic ancestries likely diverged prior to final differenti-

tent among the units, but the main intracaldera intrusions (Alamosa River ation, emplacement, or eruption. Their εHf values overlap, as do those for most

and Jasper monzonites) are lower in Al2O3 at a given silica content than the of the postcaldera intrusions; all may have differentiated from a similar paren- postcaldera lavas and the Chiquito Peak Tuff (Fig. 8). Trace elements such as Sr tal magma but at different depths in discrete reservoirs within the crust. The and Ba are also lower in these intrusions than in the tuff and postcaldera lavas whole-rock compositional affinity, modal mineralogy, and mineral chemistries

for a given silica (Fig. 8). This, along with the lower Al2O3, is opposite of what for the Cornwall Mountain quartz monzonite porphyry, dacite of Fisher Gulch, would be expected if the intrusions represented crystal residue left behind in and Chiquito Peak Tuff do support an upper-crustal, cogenetic differentiation. associated eruptions because these elements are preferentially incorporated The Alamosa River and Jasper monzonites are similar in whole-rock com- into plagioclase and alkali feldspar, principal contents of the putative crystal position to the postcollapse Summitville Andesite but extend to more evolved residue (Glazner et al., 2015). By contrast, the Cornwall Mountain quartz mon- whole-rock and plagioclase compositions. These intrusions were postulated to zonite porphyry has Sr and Ba abundances similar to those of the Chiquito have cored volcanic edifice(s) related to eruption of the Summitville Andesite Peak Tuff and dacite of Fisher Gulch. because they intrude this unit (Lipman, 1974, 1975; Lipman et al., 1996). The Mineral assemblages are similar among the units, except for the lack of Summitville Andesite likely represents a large recharge component without biotite and potassium feldspar in the less-evolved Summitville Andesite. How- prolonged upper-crustal storage and fractionation, similar to the precaldera ever, the Chiquito Peak Tuff contains larger and more abundant plagioclase andesitic lavas of the Conejos Formation. The Alamosa River and Jasper mon- and biotite phenocrysts (as much as ~3 mm) than the Alamosa River or Jasper zonites may be related to the Summitville Andesite, although the intrusions

monzonites, which are mostly fine grained and equigranular. Mineral compo- have lower Al2O3 and Sr for given silica content, likely due to plagioclase sitions among units show more variation. Plagioclase compositions for the fractionation. These intrusions are likely distinct pulses of magma emplaced Chiquito Peak Tuff and the postcaldera lavas extend to higher anorthite content into the volcano that had sourced the Summitville Andesite. than do those of the younger intracaldera intrusions (Fig. 10). For example,

the Chiquito Peak Tuff contains high-anorthite (An>60) plagioclase cores, absent from intracaldera-intrusion plagioclase cores but overlapping with the range Crustal Geometry of Intracaldera Intrusions of anorthite content observed in the dacite of Fisher Gulch and Summitville Andesite (Fig. 10). Additionally, the Sr contents in plagioclase in the intracal- Present-day exposures of the intracaldera intrusions at Platoro are at lev- dera intrusions is distinctly lower than that of the Summitville Andesite and els close to the Oligocene paleo–land surface (Fig. 16), too shallow to provide

W E W E

Approximate present-day Summitville Chiquito Peak Underlying erosional level Andesite Dacite of Tuff volcanic rocks Fisher Gulch 0 Caldera 0 floor HPHD ? ARM CMP CCM CPT LAAP CPT Magma Prevolcanic Older intracaldera crystal −5 upper crust −5 Ring Older mush (?) ignimbrites fault intracaldera ignimbrites (?) Eruptible ? Subcaldera magma plutons (?) −10 −10

? ? −20 −20 Depth (km)

Lower crust

−40 −40

Upper mantle

0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 Distance (km) Distance (km)

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 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.

direct constraints on the geometry of caldera intrusions deeper in the crust. et al., 1996), processes well documented at other calderas (e.g., Bacon, 1983; Geologic mapping and the new ages demonstrate that no sizable exposed Lipman, 1984; Hildreth and Mahood, 1986). intrusions are older than the Chiquito Peak Tuff. If intrusions associated with Eruptions of large-volume ignimbrites have been inferred by some to result the earlier ignimbrite eruptions exist, they must lie deeper in the composite from near-complete emptying of magma bodies assembled at high flux rates caldera block. The lack of granitoid lithics in the Chiquito Peak Tuff provides no without concurrent formation of sizable deeper granitoid plutons, while batho- information about such intrusions at depth. Other lithics such as Precambrian lithic-scale plutons are proposed to have been assembled under contrasting basement, Mesozoic sediment, and earlier ignimbrites are also rare or absent. conditions of incremental magma accumulation at lower fluxes without major The Chiquito Peak Tuff contains only andesitic lithics, likely derived from ear- associated volcanism (e.g., Annen, 2009; Tappa et al., 2011; Zimmerer and McIn- lier caldera-filling lavas or the precaldera Conejos Formation by shallow vent tosh, 2012; Mills and Coleman, 2013; Caricchi et al., 2014; Frazer et al., 2014; enlargement or in-sliding of adjacent oversteepened caldera walls (Lipman Schaltegger et al., 2019). Alternatively, geologic map relations for seemingly

cogenetic volcano-plutonic suites at crustal levels deeper than those at the a crystal-rich low-silica rhyolite in north-central Nevada, by contrast, show SJVL and modeling of gravity, seismic, and other geophysical data document evidence of being genetically equivalent to the least-evolved part of the tuff, the presence of vertically extensive batholithic-scale granitoid magma bodies representing residual cumulate material (Watts et al., 2016). or solidified intrusions in the upper crust beneath caldera sources for large Within the SRMVF, studies of the volcanic-plutonic connection have shown ignimbrites (Hamilton and Myers, 1967, 1974; Lipman, 1984, 2007; Bachmann that most shallow-crustal intrusive activity is the result of distinct magmatic et al., 2007; de Silva and Gosnold, 2007; Ward et al., 2014; Best et al., 2016). pulses and not the solidification of residual ignimbrite magma (Tappa et al., A large negative gravity anomaly suggests the presence of such a com- 2011; Zimmerer and McIntosh, 2012; Mills and Coleman, 2013). For example, posite upper-crustal batholith beneath much of the SJVL (Plouff and Pakiser, the Questa-Latir magmatic system has a similarly protracted magmatic history 1972; Steven and Lipman, 1976; Drenth et al., 2012; Lipman and Bachmann, (~7 m.y.) and number of exposed plutons as the Platoro system. Numerous 2015), although the timing of its assembly is unconstrained. Similar gravity intrusions associated with the Questa-Latir volcanic system also postdate the anomalies are associated with ignimbrite calderas along the Sawatch trend caldera-forming eruption of the ca. 25.4 Ma Amalia Tuff; only the peralkaline farther north in the SRMVF (Fig. 1) and at the Questa locus farther south (Cordell granite pluton of Virgin Canyon and small-volume dikes represent possible et al., 1985; Lipman, 2007). The Platoro caldera complex is located along the remnants of Amalia magma (Lipman, 1988; Johnson et al., 1989; Tappa et al., southeastern margin of the inferred SJVL batholith, however, posing uncer- 2011; Zimmerer and McIntosh, 2012). tainty about the possible presence of larger intrusions at greater depth than the exposed intracaldera plutons. Ignimbrite events probably largely drain eruptible parts of a magma body, ■■ CONCLUSIONS but the erupted magma may be a relatively thin upper zone of a vertically extensive and variably crystallized caldera-wide (perhaps composite) pluton. The evolution of the magmatic system that sourced the Chiquito Peak Tuff, Depressurization accompanying the eruption would tend to promote crystal- postcaldera intrusions, and lavas of the Platoro caldera complex is summa- lization in remaining portions, further decreasing the eruptibility of remaining rized schematically in Figure 16. Isotopic studies (Lipman et al., 1978; Riciputi magma. Figure 16 depicts the geometry of post-ignimbrite intrusions at Platoro and Johnson, 1990; Riciputi et al., 1995; this study) support an origin from caldera; the schematic extent of a possible caldera-wide composite intrusion mantle-derived basaltic melts that assimilated lower crust for the magmas is indicated by a red dashed line. Alternatively, if little or no intrusive rem- in the Platoro magmatic system. The bulk compositional similarities among nants of the ignimbrite magmas remain as deeper intrusions and the exposed the larger Platoro intrusions are consistent with the chemical character of the postcollapse intrusions do not widen with depth, the lower structure of the Platoro system having been established in the lower to mid-crust, as also caldera complex would be dominated by the successive intracaldera accu- inferred for other continental-arc systems (Hildreth and Moorbath, 1988; Annen mulations of the six large Treasure Mountain ignimbrites, with a composite et al., 2006, 2015; Jagoutz, 2010; Clemens et al., 2010; Solano et al., 2012), while thickness previously estimated at 10 km or more (Lipman et al., 1996). Com- the diverse textures, mineralogy, and mineral compositions of the magmas parable composite fill thicknesses should initially have been present at other reflect the crystallization paths of distinct magma batches in the upper crust. large multicyclic ignimbrite calderas, but at well-documented sections through Eruption of the Chiquito Peak magma (Fig. 16A), ca. 28.8 Ma, was followed volcano-plutonic assemblages elsewhere, thick caldera-related volcanic strata by eruption of the dacite of Fisher Gulch and Summitville Andesite. Emplace- are truncated by underlying batholithic-scale intrusions (Lipman, 1984, 2007). ment of the Cornwall Mountain quartz monzonite porphyry, the Alamosa River and Jasper monzonites, and the intracaldera hornblende andesite-dacite dikes into the Chiquito Peak Tuff and Summitville Andesite (Fig. 16B) followed within Comparison with Other Postcaldera Intrusions analytical uncertainty of the date of the tuff eruption. Intrusive magmatism on the eastern side of the caldera is documented by the ca. 28 Ma Cat Creek Volcano-plutonic relations at Platoro are similar to those of several other monzonite, which cored the stratovolcano that sourced the volcanics of Green mid-Cenozoic magmatic systems, based on limited available data. As at the Ridge. On the western side of the caldera, intrusions are ca. 27.7–26.3 Ma and Platoro caldera complex, plutons associated with the mid-Cenozoic Stillwa- show evidence of open-system processes. Although pressure constraints ter–Clan Alpine calderas in central Nevada (USA) postdate caldera-forming are not available for these intrusions, characteristics of the Platoro rocks are eruptions. Most of the intrusions have been interpreted to be subsequent consistent with open-system, polybaric fractionation of the associated mag- magmatic pulses rather than the solidified remnants of ignimbrite magma mas, the younger intrusions west of the caldera being derived from wetter chambers (Colgan et al., 2018). However, two small intrusions exhibit charac- parts of the system. The ca. 27.4 Ma Alum Creek monzonite porphyry was teristics of residual magma based on zircon trace element chemistry, much emplaced into the Alamosa River monzonite. The youngest granitoid intru- like the Cornwall Mountain quartz monzonite porphyry and the Chiquito Peak sion by far, the ca. 24 Ma Summitville quartz monzonite as well as the dacite Tuff in the Platoro system. The intrusions associated with the Caetano Tuff, of South Mountain, document the protracted nature of magmatic activity

and magmatic-hydrothermal mineralization at Platoro. In fact, all magmatic- Best, M.G., Christiansen, E.H., de Silva, S., and Lipman, P.W., 2016, Slab-rollback ignimbrite flareups in the southern Great Basin and other Cenozoic American arcs: A distinct style of volcanism: hydrothermal mineralization associated with the Platoro caldera complex Geosphere, v. 12, p. 1097–1135, https://doi.org/10.1130/GES01285.1. intrusions postdates ignimbrite eruptions and caldera collapse. Bethke, P.M., 2011, Mineralization in the eastern San Juan Mountains, in Blair, R., and Bracksieck, Map relations indicate that the intracaldera intrusions postdate the G., eds., The Eastern San Juan Mountains: Their Geology, Ecology, and Human History: Boulder, University Press of Colorado, p. 39–60. last-erupted ignimbrite, while our geochronologic data document a multi- Bethke, P.M., Rye, R.O., Stoffregen, R.E., and Vikre, P.G., 2005, Evolution of the magmatic-hydro- million-year emplacement history but cannot quantify the intervals between thermal acid-sulfate system at Summitville, Colorado: Integration of geological, stable-isotope, eruption and pluton emplacement or between successive intrusions. With the and fluid-inclusion evidence: Chemical Geology, v. 215, p. 281–315, https://doi.org/10.1016 possible exception of the Cornwall Mountain quartz monzonite porphyry and the /j.chemgeo.2004.06.041. Bickford, M.E., Van Schmus, W.R., Karlstrom, K.E., Mueller, P.A., and Kamenov, G.D., 2015, dacite of Fisher Gulch, the intrusions are not remnants of the magma reservoir Mesoproterozoic-trans-Laurentian magmatism: A synthesis of continent-wide age that sourced the Chiquito Peak Tuff but likely constitute distinct post-ignimbrite distributions, new SIMS U-Pb ages, zircon saturation temperatures, and Hf and Nd iso- pulses of magma derived from the waning Platoro magmatic system. topic compositions: Precambrian Research, v. 265, p. 286–312, https://doi.org/10.1016/j .precamres.2014 .11 .024 . Black, L.P., Kamo, S.L., Allen, C.M., Davis, D.W., Aleinikoff, J.N., Valley, J.W., Mundil, R., Campbell, I.H., Korsch, R.J., Williams, I.S., and Foudoulis, C., 2004, Improved 206Pb/238U microprobe ACKNOWLEDGMENTS geochronology by the monitoring of a trace-element-related matrix effect; SHRIMP, ID-TIMS, We thank Julien Allaz and Peter Horvath for help with the microprobe data acquisition and Heather ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards: Chemical Lowers and David Adams for assistance with SEM analysis. Jeremy Havens is thanked for his Geology, v. 205, p. 115–140, https://doi.org/10.1016/j.chemgeo.2004.01.003. assistance in drafting the figures. We also thank Ryan Mills, Edward du Bray, Drew Downs, Chris- Blatter, D.L., Sisson, T.W., and Hankins, W.B., 2017, Voluminous arc dacites as amphibole reac- tine Chan, two anonymous reviewers, and Science Editor Shanaka de Silva for constructive tion-boundary liquids: Contributions to Mineralogy and Petrology, v. 172, 27, https://doi.org reviews of this manuscript. Amy Gilmer, Ren Thompson, and Peter Lipman were supported by /10.1007/s00410-017-1340-6. the U.S. Geological Survey’s National Cooperative Geologic Mapping Program, and Amy Gilmer Blundy, J.D., and Annen, C.J., 2016, Crustal magmatic systems from the perspective of heat trans- was also supported by the U.S. Geological Survey’s Mineral Resources Program. 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