Ignimbrites to Batholiths Ignimbrites to Batholiths: Integrating Perspectives from Geological, Geophysical, and Geochronological Data

Home , Amalia Tuff, Charlier (lunar crater), Icy Peak, La Garita Caldera, Magma supply rate, Ruth Mountain

Ignimbrites to batholiths Ignimbrites to batholiths: Integrating perspectives from geological, geophysical, and geochronological data

Peter W. Lipman1,* and Olivier Bachmann2 1U.S. Geological Survey, Mail Stop 910, Menlo Park, California 94028, USA 2Institute of Geochemistry and Petrology, ETH Zurich, CH-8092 Zürich, Switzerland

ABSTRACT related intrusions cooled and solidified soon shorter. Magma-supply estimates (from ages after zircon crystallization, as magma sup- and volcano-plutonic volumes) yield focused Multistage histories of incremental accu- ply waned. Some researchers interpret these intrusion-assembly rates sufficient to gener- mulation, fractionation, and solidification results as recording pluton assembly in small ate ignimbrite-scale volumes of eruptible during construction of large subvolcanic increments that crystallized rapidly, leading magma, based on published thermal models. magma bodies that remained sufficiently to temporal disconnects between ignimbrite Mid-Tertiary processes of batholith assembly liquid to erupt are recorded by Tertiary eruption and intrusion growth. Alternatively, associated with the SRMVF caused drastic ignimbrites, source calderas, and granitoid crystallization ages of the granitic rocks chemical and physical reconstruction of the intrusions associated with large gravity lows are here inferred to record late solidifica- entire lithosphere, probably accompanied by at the Southern Rocky Mountain volcanic tion, after protracted open-system evolution asthenospheric input. field (SRMVF). Geophysical data combined involving voluminous mantle input, lengthy with geological constraints and comparisons residence (105–106 yr) as near-solidus crystal INTRODUCTION with tilted plutons and magmatic-arc sections mush, and intermittent separation of liquid elsewhere are consistent with the presence of to supply volcanic eruptions. The composi- Recent geochronologic and petrologic stud- vertically extensive (>20 km) intermediate tions of the least-evolved ignimbrite magmas ies have convincingly demonstrated that large to silicic batholiths (with intrusive:extrusive tend to merge with those of caldera-related magma bodies, which form granitoid crustal ratios of 10:1 or greater) beneath the major plutons, suggesting that the plutons record plutons in Cordilleran-arc settings, were assem- SRMVF volcanic loci (Sawatch, San Juan, nonerupted parts of long-lived cogenetic bled incrementally and crystallize over 105–106 Questa-Latir). Isotopic data require involve- magmatic systems, variably modified prior yr intervals (Coleman et al., 2004; Matzel et al., ment of voluminous mantle-derived mafic to final solidification. Precambrian-source 2006a; Memeti et al., 2010; Frazer et al., 2014). magmas on a scale equal to or greater than zircons are scarce in caldera plutons, in Building on twentieth-century discussions that of the intermediate to silicic volcanic and contrast to their abundance in some periph- (such as Daly, 1914; Kennedy and Anderson, plutonic rocks. Early waxing-stage intrusions eral waning-stage intrusions of the SRMVF, 1938; Buddington, 1959; Smith, 1979; Lip- (35–30 Ma) that fed intermediate-composi- implying dissolution of inherited crustal man, 1984; Macdonald and Smith, 1988), these tion central volcanoes of the San Juan locus zircon during lengthy magma assembly for results have promoted renewed controversy are more widespread than the geophysi- the ignimbrite eruptions and construction concerning connections between volcanic and cally defined batholith; these likely heated of a subvolcanic batholith. Broad age spans intrusive processes, especially how magma res- and processed the crust, preparatory for of zircons (to several million years) from ervoirs for large ignimbrite eruptions are related ignimbrite volcanism (32–27 Ma) and large- individual samples of some ignimbrites and to the emplacement of granitic plutons and the scale upper-crustal batholith growth. Age intrusions, commonly averaged and inter- crustal depths at which silicic compositions and compositional similarities indicate that preted as “intrusion-emplacement age,” are generated (Glazner et al., 2004; Metcalf, SRMVF ignimbrites and granitic intrusions alternatively provide an incomplete record 2005; Bachmann et al., 2007b; Lipman, 2007; are closely related, but the extent to which the of intermittent crystallization during pro- Miller et al., 2011; Davis et al., 2012; de Silva plutons record remnants of former magma tracted incremental magma-body assembly, and Gregg, 2014; Frazer et al., 2014; among reservoirs that lost melt to volcanic eruptions with final solidification only when the system others). For example, geochronologic data and has been controversial. Published Ar/Ar- began to wane. Analyses of whole zircons chemical patterns of volcanic and shallow plu- feldspar and U-Pb-zircon ages for plutons cannot resolve late stages of crystal growth, tonic rocks have been interpreted by some as spatially associated with ignimbrite calderas and early growth in a long-lived magmatic indicating melt generation in the deep crust with document final crystallization of granitoid system may be poorly recorded due to peri- minimal differentiation at shallow crustal levels, intrusions at times indistinguishable from ods of zircon dissolution. Overall, construc- leading to pluton assembly in small increments the tuff to ages several million years younger. tion of a batholith can take longer than that crystallized rapidly, with temporal and geo- These ages also show that SRMVF caldera- recorded by zircon-crystallization ages, while metric disconnects between ignimbrite eruption the time interval for separation and shallow and intrusion growth (Glazner et al., 2004; Bart- *[email protected] assembly of eruptible magma may be much ley et al., 2005; Annen, 2009; Coleman et al.,

Geosphere; June 2015; v. 11; no. 3; p. 705–743; doi:10.1130/GES01091.1; 14 figures; 7 tables. Received 18 June 2014 ♦ Revision received 30 December 2014 ♦ Accepted 19 February 2015 ♦ Published online 2 April 2015

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2012; Zimmerer and McIntosh, 2012a; Mills associated SRMVF volcanic and granitoid rocks Tertiary rocks in the northern San Juan Basin and Coleman, 2013). Alternatively, others have are interpreted to record late solidification after (Gonzales et al., 2010; Lake and Farmer, 2015; concluded that rhyolitic magmas are commonly prolonged histories of open-system evolution Gonzales and Pecha, 2015), but such composi- generated at low pressure (e.g., Tuttle and in the shallow crust, involving recurrent mag- tions have not been identified centrally within Bowen, 1958; Lipman, 1966; Fowler and Spera, matic recharge, lengthy (105–106 yr) residence loci of large-volume San Juan volcanism. Vol- 2010; Gualda and Ghiorso, 2013) by crystal- as near-solidus crystal mush, and intermittent canic centers tended to migrate from north to liquid separation in the upper crust, comple- upper-crustal separation of liquid to supply south in the SRMVF, both intermediate-compo- mented by voluminous underlying cumulates eruptions. Ignimbrite eruptions in the SRMVF sition lava eruptions and ignimbrites (Fig. 1B, (e.g., Hildreth, 1981, 2004; Bacon and Druitt, are inferred to record relatively brief episodes Table 1; Lipman, 2007); the general southward 1988; Vazquez and Reid, 2002; Bachmann and of increased mantle-magma recharge and con- migration is parallel to that long documented Bergantz, 2004; Deering et al., 2011). These and current upper-crustal pluton construction at for eruptive centers in the Basin-Range region, other recent studies (e.g., Wilson and Charlier, focused sites within the broader areal extent of probably related to disruption of the subducted 2009; Allan et al., 2013; Pamukcu et al., 2013; the volcanic field. The eruptible magmas that Farallon plate (Stewart et al., 1977; Lipman, Wotzlaw et al., 2014; Cashman and Giordano, sourced SRMVF ignimbrites are considered to 1980; Henry and John, 2013). 2014) have inferred broadly lenticular shapes, be short-lived and volumetrically minor, shal- Structural unroofing, associated with later rapid assembly rates, and brief life spans for low portions of much longer-lived and verti- extension along the Rio Grande rift and deep the shallow magma bodies that erupt as large cally extensive underlying plutonic systems erosion of this high-standing region, has ignimbrites. From complementary perspectives, dominated by near-solidus mushy magma. Such exposed broadly synvolcanic batholithic intru- this paper evaluates the vertical extent and tem- temporal and volumetric features of volcanic- sions associated with the ignimbrite centers. The poral construction of the overall crust-mantle plutonic evolution at the SRMVF are suggested earliest well-documented regional ignimbrite, magmatic system inferred to have developed to be representative of continental-arc magma- erupted from a caldera source in the SRMVF, concurrently with ignimbrite eruptions. tism worldwide. was the far-traveled Wall Mountain Tuff at 37 Although available analytical methods and Ma (Chapin and Lowell, 1979; Zimmerer and resulting data have thus far been only partly SOUTHERN ROCKY MOUNTAIN McIntosh, 2012a), erupted from the Princeton successful in addressing such issues, recent vol- VOLCANIC FIELD AND ITS batholith area in the Sawatch Range (Fig. 1); canologic, geophysical, petrologic, geochrono- BATHOLITH the southernmost, and among the youngest logic, and modeling results for the Middle Ter- ignimbrites, was eruption of the Amalia Tuff tiary Southern Rocky Mountain volcanic field The mid-Tertiary SRMVF, site of 25 large from the Questa caldera at 25 Ma in northern (SRMVF) are consistent with prior proposals ignimbrites (mainly 37–27 Ma) with related New Mexico (Lipman, 1988; Tappa et al., 2011; that large long-lived silicic volcanic fields are calderas and subvolcanic intrusions (Fig. 1; Zimmerer and McIntosh, 2012b). Farther north surface expressions of composite upper-crustal Table 1), provides an exceptional laboratory for in Colorado, the mid-Tertiary magmatism is magma bodies comparable to the Sierra Nevada processes of Cordilleran magmatism (Steven, marked by scattered shallow intrusions and or Boulder batholiths (Smith 1960; Hamilton and 1975; McIntosh and Chapin, 2004; Lipman, sparse small erosional remnants of lava and tuff. Myers, 1967; Lipman, 1984, 2007; Bachmann 2007). In places, virtually pristine volcanic mor- A thick section of welded tuff associated with an et al., 2007b). Such volcanism offers sequential phology has been exhumed by recent erosion; intrusive complex in the Never Summer Moun- snapshots of processes during early magmatic elsewhere, rugged topography and structural tains may record remnants of a small isolated cal- evolution in continental arcs, while upper-crustal tilting expose multikilometer volcanic sections, dera system active at 28–29 Ma (O’Neill, 1981; plutons provide a composite record of lengthy down into upper levels of subvolcanic intru- Jacob et al., 2011, 2015), concurrent with peak assembly and later crystallization. sions. Small granitoid plutons, many spatially activity in the San Juan Mountains to the south. Building on a prior review of incremental and temporally associated with ignimbrite cal- Geographically and temporally between the assembly and prolonged consolidation in Cor- deras, are exposed at near-roof level (Table 2), early and late centers, the San Juan region con- dilleran magma chambers (Lipman, 2007), new while geometry and composition of a vast tains the largest preserved erosional remnant of data for the SRMVF, summarized here within a composite batholith that is vertically extensive the composite Oligocene volcanic field (Larsen framework of global perspectives on continen- beneath the volcanic locus are constrained by and Cross, 1956; Steven et al., 1974). The San tal-margin arc magmatism, permit more quan- geophysical and geochemical modeling. Juan locus is notable for the large number of titative assessment of alternative geometric and As summarized more fully previously (Lip- high-volume, compositionally diverse ignim- genetic models for large silicic magma bodies, man, 2007, and references), dominantly inter- brites (cumulatively, ~15,000 km3) and associ- relations of mineral-crystallization ages to plu- mediate-composition lavas and associated ated caldera collapses, at least 18 in the 3 m.y. ton-assembly processes, and resulting implica- breccias (andesite, dacite) were voluminous interval 30.1–26.9 Ma (Table 1). Unzoned uni- tions for magma-supply rates during evolution precursors to most ignimbrite eruptions, and form crystal-poor rhyolite, crystal-rich dacite of the SRMVF and comparable Cordilleran-arc eruption of similar lavas continued concurrently (“monotonous intermediates”), and ignimbrites systems. Geological and geophysical data are with the major ignimbrites, commonly filling that grade from initially erupted rhyolite upward used to estimate vertically extensive geom- caldera depressions (Steven and Lipman, 1976). into dacite are present in subequal numbers. etry and volumes of magmatic systems at the At the San Juan locus, the central volcanoes and Sizable precursor Plinian-fall deposits have not SRMVF, where mantle basalt recurrently mixed their clastic aprons (~25,000 km3) constitute been recognized beneath any of these ignim- with lower-crustal melts to erupt thick sections almost two thirds of total volcanic volume (Lip- brite types, contrary to some recent inferences of lavas and ignimbrites, underlain by largely man et al., 1970, 1978). Basalt is nearly absent, (e.g., Gregg et al., 2012; Cashman and Gior- coeval upper-crustal granitic batholiths with despite repeated searches for primitive compo- dano, 2014). deep roots of more mafic residua. Petrologic sitions. Mafic alkalic dikes (lamprophyres) with The composite SRMVF, now widely erosion- features and crystallization ages of spatially mantle isotopic signatures locally intrude pre- ally dissected, was one site of discontinuous

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108° 106° 104°W 40° N 060 mi

0100 km Denver F r o N n t

R a n

s g

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l a P l South l w l l l l a Park t c Colorado h 39-Mile Springs volcanic area

pprox. original limit MP l

A l l West Elk l of volcanic rock R locus Gunnison g x e

l B

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l l e l l P l t l

l l l S

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Explanation Granitic intrusion MP, Mount Princeton batholith Mid-Tertiary volcanic areas

Trend of Colorado Mineral Belt

l l

Sedimentary fill of Rio Grande Rift l l l l l Caldera A Late rift basalts Regional structural attitude Bounding fault, Rio Grande rift

Figure 1 (on this and following page). Generalized maps showing preserved remnants of composite Southern Rocky Mountain volcanic field, modified from McIntosh and Chapin (2004) and Lipman (2007); inferred original limit of volcanic rocks is from Steven (1975); intrusions are from Tweto (1979) and Lipman et al. (2013). (A) Ignimbrite calderas, associated granitic intrusions, inferred original extent of once nearly continuous mid-Tertiary volcanic cover, and later Tertiary sedimentary fill of the Rio Grande rift zone. Calderas and associated intrusions: B—Bachelor; Bz—Bonanza; Cr—Creede; C—Cochetopa Park; GP—Grizzly Peak; LG—La Garita; LGn—La Garita north; LGs—La Garita south; M—Marshall; MP—Mount Princeton–Aetna; NP—North Pass; Pl—Platoro; S—Silverton; SL—San Luis; SR—South River. Arrows indicate trend of Late Cretaceous–Early Tertiary (Laramide) intrusions of the Colorado mineral belt.

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108° W 106° critical parameters for interpreting geochrono- Sawatch trend logic and petrologic data on magma-crystalli- NE San Juan SOUTH zation history, duration of magma-body assem- 34.3 SE San Juan PARK bly, and magma-supply rates during volcanism. Central San Juan Upper-crustal exposures document that many Western San Juan Cordilleran plutons in the western United States Latir-Questa 36.7 reached shallow levels, some intruding broadly 34 cogenetic volcanic deposits (Buddington, 1959; Salida Smith, 1960; Hamilton and Myers, 1967; Lip- 30 man, 1984). Such granitic bodies have been 34 increasingly recognized as emplaced incremen- Gunnison 33.1 tally during protracted intervals (e.g., Pitcher, 26.9 32.2 1979; Wiebe and Collins, 1998; Coleman et al., 28.4 38° SAN LUIS 2004; Matzel et al., 2006a; Miller et al., 2011; N 23 26.9 28.0 Davis et al., 2012; Mills and Coleman, 2013). 27.6 28.5 27.5 Many recent models infer assembly and solidi- 26.9 fication of silicic plutons as successions of 28.7 V 27.4 ALLE stacked sill-like bodies with overall broadly

28.0 Y lenticular shapes (Cruden, 1998; Petford et al., 30.0– 2000; Glazner et al., 2004; Bartley et al., 2008; SAN JUAN 28.3 Annen, 2009). Estimates of pluton thickness BASIN Alamosa Pagosa have varied widely, in part because of limited Springs topographic relief in even the most rugged ter- COLORADO 37° rains, and many large granitic plutons have been NEW MEXICO inferred to be 10 km or less thick (Hamilton and Myers, 1967, 1974; Cruden et al., 1999; 0 50 100 km 25.1 McNulty et al., 2000; Annen, 2009). However, geologic-mapping and geobarometric data for individual plutons, deep-crustal exposures Volcanic rocks Sediments, Rio Grande rift zone through tilted magmatic arcs, and geophysical B data for crustal structure beneath large volcanic Figure 1 (continued). (B) Ignimbrite calderas and eruption ages, fields increasingly provide robust evidence for documenting general southward progression and focusing of Oligo- vertically extensive subvolcanic batholiths and cene ignimbrite eruptions in central San Juan region at 28–27 Ma. their component plutons (Saleeby et al., 2003; Ages are from Table 1; calderas are color-coded by regional loca- Ducea et al., 20010; DeBari and Green, 2011; tion, as guide to Tables 1–2. Jagoutz and Schmidt, 2012). Such bodies are here interpreted to occupy much of the crust and to be associated with large-scale compo- Middle Tertiary Cordilleran magmatism, con- served but eroded to depths that expose shallow sitional modification of the underlying litho- tinuing southward through the Mogollon-Datil parts of contemporaneous granitoid intrusions. spheric mantle. region in New Mexico (Elston, 1984; Ratté (2) Detailed regional volcanic stratigraphy and et al., 1984; McIntosh et al., 1992), into Trans- abundant petrologic, geochemical, and geo San Juan Batholith: Geophysical Pecos, Texas (Henry and Price, 1984), and the chronologic data provide a comprehensive Expression vast Sierra Madre Occidental of northern Mex- record of eruptive history. (3) Rocks of the ico (McDowell and Clabaugh, 1979; Ferrari San Juan locus were emplaced mainly onto the Deep Bouguer gravity lows (to –340 mGal) et al., 2007; McDowell and McIntosh, 2012). structurally simple Colorado Plateau block that in the Southern Rocky Mountains (Fig. 2A), The SRMVF was originally comparable in size, has been broadly stable since craton formation, which have long been interpreted as expression composition, and magmatic duration to large thereby permitting well-constrained gravity and of upper-crustal Tertiary granitic intrusions, young ignimbrite terranes such as the well- seismic modeling of subsurface intrusion geom- coincide spatially with major clusters of ignim- documented Altiplano-Puna volcanic complex etry. (4) Isotopic contrasts between Precambrian brite calderas in the San Juan region (Plouff and (APVC) of the central Andes (de Silva, 1989; crust and underlying mantle provide robust geo- Pakiser, 1972; Drenth et al., 2012), with sev- Lindsay et al., 2001; Schmitt et al., 2002; de chemical tracers for evaluating magma-genera- eral older calderas and subvolcanic intrusions Silva and Gosnold, 2007; Salisbury et al., 2011; tion processes. along the Sawatch Range (Fig. 2B; Isaacson and del Potro et al., 2013). Smithson, 1976; Case and Sikora, 1984; McCoy Within the SRMVF, the San Juan magmatic BATHOLITH GEOMETRY AND et al., 2005), and with the Questa caldera and locus provides an exceptional natural labora- COMPOSITION multiple associated granitic plutons exposed tory for evaluating the geometry, composition, in the Latir Range (Cordell et al., 1985; Lip- and emplacement history of intrusive bodies The vertical extents and volumes of large man, 1988). in relation to broadly associated surface vol subvolcanic batholiths and their component plu- The gravity anomaly associated with the San canism: (1) Volcanic rocks are widely pre- tons, even if only broadly determined, provide Juan volcanic locus has been documented and

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TABLE 1. REGIONAL IGNIMBRITES AND CALDERAS OF THE SOUTHERN ROCKY MOUNTAINS VOLCANIC FIELD Ignimbrite Caldera 3 Site Name% SiO2 Rock, phenocrystsVolume (km ) Age (Ma) Name Area (km) West San Juan Sunshine Peak Zoned, 76–68 Qtz, sodic san 400 22.9 Lake City 15 × 18 Latir Mountains, NM Amalia 76–77 Peralk: qtz, sodic san 500 25.1 Questa 14 × >15 Central San Juan Snowshoe Mtn 62–66 Xl-rich dacite 500 26.85 Creede 20 × 25 Nelson Mountain Zoned 74–63 Zoned: xp rhy - xl dacite 500 26.90 San Luis-Cochetopa9 × 9, 20 × 25 Cebolla Creek 61–64 Xl dacite, hbl, no san 30026.90 San Luis complex 14 × 16 Rat Creek Zoned 74–65 Zoned: xp rhy - xl dacite 150 26.9 San Luis complex9 × 12 Wason Park Zoned 72–63 Xl rhyolite - xl dacite500 27.35South River 20 × 20 Blue Creek 64–68 Xl dacite, no san 250 27.4 [concealed] — Carpenter Ridge Zoned, 74–66 Zoned: xp rhy - xl dacite 1000 27.55 Bachelor 25 × 30 West San Juan Crystal Lake 72–74 Xp rhyolite 75 27.6 Silverton 20 × 20 Central San Juan Fish Canyon 66–68 Xl dacite, san,hbl,qtz 5000 28.02 La Garita 35 × 75 West San Juan Sapinero Mesa 72–75 Xp rhyolite100028.35 Uncompahgre-San Juan 20 × 45 Dillon Mesa 72–75 Xp rhyolite 75 28.4 Uncompahgre? 20 × 20 Blue Mesa 72–74 Xp rhyolite 350 28.5 Lost Lakes (buried) 10 × 10 Ute Ridge 66–68 Xl dacite, sanidine 350 28.6 Ute Creek 8 × 8 Southeast San Juan Chiquito Peak 64–67 Xl dacite, sanidine 500 28.6 Platoro 18 × 22 Central San Juan Masonic Park 62–66 Xl dacite, no san 500 28.7 [concealed] — Southeast San Juan South Fork 68–70 Xl dacite, sanidine7528.8 Platoro/ Summitville?8 × 12? Ra Jadero 64–66 Xl dacite, sanidine150 28.8 Summitville?8 × 12 Ojito Creek 67–70 Xl dacite, no san100 n.d. Summitville?8 × 12 La Jara Canyon 66–68 Xl dacite, no san 1000 29.9 Platoro 20 × 24 Black Mountain 67–69 Xl dacite, no san 350 30.1 Platoro — Northeast San Juan (Barret Creek) 65–73 (Xl dacite-rhy lavas) 29.8 [failed?] — Saguache Creek 73–75 Alkali rhyolite, no bio350 32.25North Pass 15 × 17 North-south Sawatch Range trend Bonanza§ 74–63 Zoned: dac-rhy-dac 1000 33.15 Bonanza-Gribbles 15 × 20 Thorn Ranch† 77–66 Zoned: xl rhyolite-dacite500 33.9Marshall Creek 15 × 15 ? Badger Creek 69–70 Xl dacite 500 34.0 Mount Aetna 10 × 15 Grizzly Peak Zoned: 77–57 Xl rhy-dacite-andesite500 34.3 Grizzly Peak 17 × 23 Wall Mountain* 70–73 Xl rhyolite 1000 36.7 Mount Princeton 15 × 30?

Cumulative volume: 17,475 Notes: Modiﬁed from Lipman (2007); main changes are fewer (but larger-volume) ignimbrites erupted from Sawatch Range trend. Color-coded by location (Fig. 1B); compiled from diverse sources, cited in text. Abbreviations: rhy—rhyolite; dac—dacite; san—sanidine; qtz—quartz; hbl—hornblende; xl—crystal-rich; lp—crystal-poor. SJ—San Juan; NM—New Mexico. *Includes Stirrup Ranch Tuff of Epis and Chapin (1974), now recognized as a breccia sheet derived from the Wall Mountain Tuff, probably as a break-out ﬂood deposit similar to that described by Henry (2008). †Includes East Gulch Tuff of Epis and Chapin (1974), now recognized as partly welded lower parts of the Thorn Ranch Tuff. §Includes Gribbles Park Tuff of Epis and Chapin (1974), now recognized as an eastern rhyolitic facies of Bonanza Tuff (Lipman et al., 2013).

modeled in particular detail; it has steep mar- area of ~8200 km2 beneath central parts of the Drenth et al. (2012) would imply an average

ginal gradients and a subdued interior structure; volcanic cover (Drenth et al., 2012). Using a composition (~75% SiO2) of silicic granite (Fig. most individual calderas lack expression, and preferred density contrast of 80 kg/m3 between 3C). Few reliable density measurements exist the lowest gravity values are within a central assumed densities for the Tertiary batholith for exposed Tertiary intrusions in the San Juan area where calderas are absent. Such features (2620 kg/m3) and upper-crustal Precambrian region, but the largest bodies and most common led to the interpretation that the bulk of the basement (2700 kg/m3), these authors obtained compositions are granodiorite (~62%–68%

anomaly is due to the presence of a large com- an average batholith thickness of 13 km and a SiO2). Intrusive rocks with >70% SiO2 are posite batholith: ~10,000 km2 in area, with an volume of 82,000–130,000 km3 (Figs. 3A–3B). rare, relatively small, mainly peripheral to the average density contrast to country rock of 100 Compositions and densities of exposed San geophysically defined batholith, and mostly kg/m3, its top widely occurring at a few kilome- Juan granitoid intrusions suggest, however, emplaced late during growth of the SRMVF ters below present-day surface, and extending to that the batholith volume and thickness may (fig. 8in Lipman, 2007; Gonzales and Pecha, depths greater than 19 km (Plouff and Pakiser, be even larger. By comparison with measured 2015). Granodiorite to quartz monzonite aver- 1972). As modeled in elegant detail, combin- densities for granitic rocks elsewhere such as age compositions and accompanying higher ing more abundant gravity data with seismic- from the Sierra Nevada batholith (Oliver et al., densities are also typical for exposed levels of velocity profiles and adjusting for low densities 1993; Moore, 2000, his fig. 6.20), the relatively Cordilleran plutons and batholiths elsewhere of near-surface rocks, the San Juan batholith low assumed density (2620 kg/m3) modeled for in the western United States: e.g., 2690 kg/m3,

has more recently been inferred to underlie an the batholithic rocks in the San Juan region by 68% SiO2 for the Sierra Nevada (Oliver et al.,

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, 3

, 1993), and 2660 kg/m for the Boulder batholith r e r

e (Biehler and Bonini, 1969; Vejmelek and Smith- m

3 son, 1995). The average upper crust is also m i 1 4 Z 0 98 1 4 .

2 granodioritic (Taylor and McClennan, 1981; 0 1 , M 2 0 n ; , 2 1 Rudnick and Gao, 2003). a . 1 , n . 0 m u n 0 ; e If the average composition of the San Juan l 2 8 m m ; o , 8 8 . m m l Coleman, 201 3 C 9 batholith were 68% SiO , with a density of 2660 8 2 o o a 1 9 d c c t

, 3 3 1 n 1975; e

n kg/m (Fig. 3) and a lesser contrast of 60 kg/m n n , a o e e n and McIntosh, 200 8 e n t t s n t t l v a i i l with slightly denser country rocks, the average i n r r o appa et al., 201 m a w w M B Rosera et al., 2013 Zimmerer and McIntosh, 2012b; writtten commun., 2014 Czamanske et al., 1990; T Lipman et al., 1996; M. Zimmerer Mills and p on and Lipman, 1989; h hannon, 1988; i ipman et al., 2013 batholith thickness would be 20 km or more. S L Data sources Lipman and McIntosh, 2008 Lipman, 2000, 200 6 Lipman et al., 2013 Lipman, 2000, 2006; M. Zimmerer Lipman. Lipm a Bove et al., 2001 Fridrich et al., 1991, 19 Bove et al., 2001

) The overall batholith could be even thicker if l l l l l the average batholith composition were less l l l l l ? a a a a a 1 6 1? 7S 5H 1.6 silicic (as suggested by the exposed intrusions), m m m m m mall ~ ~ ~1? ~1? ~1? S S S S S Small Small Small Small Small Smal lL Small? Small? Small? 2–5?

undated undated or if the density contrast at roof level decreased Postcaldera

duration (m. y. to near zero as granitoids became more mafic AIN VOLCANIC FIEL D at depth. Alternatively, the country rocks around 7 * 3 the San Juan batholith could be more dense 0 7 1 5 . . . . 4 3 6 4 5 3 8 3 3 2 2 3 2 1 32. 2S 25 24. 5~ 27 * 32.1 24.5 25.9* 29. 6~ 19 27.9 34.35 32.1 26.6 34.8±1.1 26. 7~ (perhaps 2750 kg/m ), while maintaining the 20–23 undated undated age (Ma) inferred contrast of 80 kg/m3 (Fig. 3B). A dense 36.6?–~35.5 Crystalization upper crust may be inconsistent, however, with 3 6 6 2 7 6 6 6 7 72 6 6 – – – the regionally high elevations and near-con- 6 6 7 59 74 76 6 0 4 SiO 6 6 6 64–66 73–77 70–76 60–66 58–63 56–66 68–77 62–76 61–65 65–69 56–65 58–60 73–77 64–66 60–70 (wt %) stant Moho depths (Hansen et al., 2013) in the Southern Rocky Mountains beyond the batholith area. More complex geometric models that r r a l

a could fit the gravity data include a multilayered l u u n n

a batholith, becoming more dense downward, as r a r c c6 g i i t g i i expected from petrology and exposed crustal u r u q y c c i i q e t h t i i

e sections (see following sections). r r p n r i y y n i o a h h r

a In either case, if the San Juan batholith is less p p p r g r r - g n o o i n d

i silicic in average composition than implied by p p a d e r a e r n n g i i m - g

m the density model of Drenth et al. (2012) and/or a a r r o m e t o g g t u s - i - - r becomes more mafic at depth, the geophysically e d e e e a n e n n edium grain equigranular n edium-grain porphyriti o orphryritic-aphanitic i i i i ine to medium grain exture modeled thickness of granitic rocks beneath F M Med.-grain miarioliti c7 Med-grain equigran to porphyritic F Fine- to med-grain equigranular Fine-grain porphyritic F F C Fine- to med-grain equigranular Fine-grain equigranular to aphanitic Fine- to med-grain equigranular Med-grain, equigran to porphyritic Med-grain porphyritic Fine-grain porphyritic Fine-med-grain equigran to porphyritic Fine-grain equigranular to aphanitic Fine-med-grain equigran to porphyritic Coarse grain to aplitic Medium-grain equigranular Porphyritic-aphanitic the San Juan volcanic locus would necessarily AND SILICIC VOLCANIC CENTERS, SOUTHERN ROCK Y MOUNT e

t be greater. More mafic compositions have been i r o i inferred to occur commonly at deeper levels in d o e e e t n t t i subvolcanic magma bodies, based on eruptions i i a c e n n r t a i o o g r d

- that record compositional gradients (e.g., Hil- z z - o i e n n t e i d t e o o i t

l dreth, 1981; Bachmann and Bergantz, 2004). n e o i t M M i o e n n c y y a a z z r t r t acite porphyry a h

acite Consistent with such interpretations from the e plite - granite Granite Qtz monzonite - granite Granodiorite D G Granodiorite to anesite Q Granodiorit eM S R G Granodiorite Q Granodiorite Granodiorite to andesite Qtz Monzonite Biotite leucogranite Evolved granit eF Quartz monzonite Dominant compositio nT Granodiorite-qtz monz Evolved rhyolit eP Granodiorite Granodiorite Granodiorit eM volcanic record, some Cordilleran granitic TED WITH CALDERAS l l l l 4 8 6 3 5 a a plutons expose mafic lower zones (Best, 1963; ble 1. × × × × × m m × 4 × 2 × 2D × 3 × 12 × 3 × 8 × 8 × 5A × 8 Size (km) s s small Ta 3 × 2 4 3 × 7 3 1 5 2 × 11 5 × 10 5 × 8 2 × 3D 1 × 2.5 4 × 6 Coleman et al., 1995; Sisson et al., 1996; Wiebe 25 × 35 et al., 2002; Miller et al., 2011; Putirka et al., ASSOCIA .3 2014). Many discussions of magma geom- e r etry and intrusion depth note that geophysical o ows.

eld c ﬂ t n o anomalies provide few constraints on extent e n g a r

c below depths where density and seismic-veloc- l h u t i s o l v e o

r ity contrasts diminish between intrusion and - h - t k a k

e wall rocks. b e e t t r ABLE 2. INTRUSIONS e s s S r n n T e e Several early San Juan calderas, notably the C - e e C k k l i i g g o a g r r d d r r Platoro complex (30.1–28.6 Ma) to the south- n u u e e i g g s s r c n n n i ochetopa Pass - ring intrusio n1 e e i i p urquoise Mine - resurgent u east and the Bonanza-Marshall cluster (34– Rito del Medio - resurgen t2 Pinabete Pk -resurgen t1 Cat Creek - proximal Cabresto Lak e3 Cataract Cr - sector grabe n2 M S T R Mt Princeton - subcaldera pluton Moly Mine - S caldera rin g2 R R L Piedra-Goose Cr - ring intrusions Spring Creek - resurgen t3 R Antero - late intrusion Rio Hondo - S batholit h8 Rough Creek - volcano core Sultan Mtn - bulbous ring intrusio n3 Alamosa River - bulbous ring intr Capital City - ring intrusion Early core, silicic dome ﬁ Lincoln Gulch - piston resurgent Intrusion name, structural setting Nellie Creek -arcuate plugs 33 Ma) to the northeast (Fig. 2A), lie beyond the batholith margins as modeled by Drenth et al. (2012), hinting at additional interpretive complexities. Bonanza, where resurgent intru-

Intrusions are color coded by location (Fig. 1B), as in sions range from mafic granodiorite to high- a t a s n

e silica aplitic granite (Lipman et al., 2013), has all Mtn” t *Intrusion age estimated from associated lava Note: u e North Pas sC South River Bonanza A “W Q San Luis Silverton Platoro Needle Cr Uncompahgre Caldera, silicic center Grizzly Peak Lake City modest gravity expression and was included as

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Figure 2. Relations among regional gravity lows, Middle Tertiary calderas, and associated intrusions of the Southern Rocky Mountain volcanic 108°W 106° field (SRMVF). (A) Regional Bouguer gravity 40° N 060 mi map (Behrendt and Bajwa, 1974; volcanic rocks, calderas, and associated intrusions from Lipman, 0 100 km A 2007) for central parts of the SRMVF, showing locations of calderas and intrusions. Contour N interval = 5 mGal. Two large negative gravity anomalies (blue colors) aligned along the axis of the SRMVF, both much larger than any individual ignimbrite caldera, are interpreted to image Explanation subvolcanic batholiths, approximately delimited

l Rio Grande rift fill

by the –300 mGal contour (at boundary between GP l a l w l l green and blue colors). A southern steep-sided, l l a 39° t Granitic intrusion c flat-floored, 50 mGal gravity low that trends east- h west coincides with most calderas of the San Juan Middle Tertiary

volcanic locus (Plouff and Pakiser, 1972; Drenth RG Rif volcanic rocks

l l

et al., 2012). A northern low (Isaacson and Smith- Approx.ertiary original volcanic limit rocks MP l l l Caldera

l l l l son, 1976; Case and Sikora, 1984) encloses earlier West Elk l l R of T locus t calderas of the Sawatch Range trend (Princeton, g e

Aetna, Grizzly Peak) and trends northeast along . l the Colorado mineral belt (arrows). Most indi- l l M l

l l Bz

l l

vidual calderas have little or no gravity expres- l l

l l sion, probably because any shallow low-density l CP l l NP l l

l l l l

l l l l l

fill has been largely removed by erosion. Exposed l l

l l

l l l

l SL l caldera-related intrusions are small, except Mount Ul l l 38° l l San l

l l

l l l

l l

l l l l

l Juan

Princeton, which underlies the inferred source of l l l l LG l

l l

l l l

SJ l

l volcanic

l l

B l l l

S l N

l l

the 37 Ma Wall Mountain Tuff. Small intrusive l

l l

l l l

ll l l l l l l

l l locus LL l l

l l l l cores to many of the early intermediate-composi- l l l l l

C l l l l

l l l

tion volcanoes of the San Juan locus (not plotted) l l

l l

l l l SR l l lie outside the southern gravity low, as do Oligo- l l l

l l l l l l l cene laccolithic intrusions in the West Elk Moun- l LG l l l l P tains. The San Juan batholith largely intrudes the northeastern flank of the Colorado Plateau structural block, while the Sawatch Range batholith was emplaced into uplifted crust of the Southern Rocky Mountains. Calderas and associated intru- B sions: B—Bachelor; Bz—Bonanza; C—Creede; CP—Cochetopa Park; GP—Grizzly Peak; LG— La Garita; LGN—La Garita north; LL—Lost Lakes; M—Marshall; MP—Mount Princeton– Aetna; NP—North Pass; P—Platoro; S—Silver- ton; SJ—San Juan; SL—San Luis; SR—South River; U—Uncompahgre. (B) Sawatch Range and mid-Tertiary Princeton batholith, the largest exposed mid-Tertiary intrusion in the eastern Cordillera, as viewed from the east toward the western fault-scarp boundary of the Rio Grande rift zone. Exposed granodiorite and quartz monzonite are 2 km thick, from floor of Upper Arkan- sas Valley (2340 m at Nathrop, Colorado) to sum- mit of Mount Princeton (4327 m). The Princeton batholith is interpreted to underlie the now-eroded caldera source of the 37 Ma Wall Mountain Tuff, the initial large regional ignimbrite erupted from the SRMVF, but interpretation of timing of pluton crystallization in relation to the ignimbrite eruption has been controversial (Zimmerer and McIntosh, 2012a; Mills and Coleman, 2013).

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Figure 3. Geophysical model and interpretation of San Juan batholith beneath part of the Southern Rocky Mountain volcanic field (SRMVF). (A) East-west gravity profile just south of the 38°N Parallel (Drenth et al., 2012; Fig. 11). Green line (labeled “regional field”) indicates the gravitational effect of inferred broad zone of low density within the (Drenth et al., 2012) A upper mantle. (B) East-west crustal-section model (Drenth et al., 2012; Fig. 11) infers low-density batholith layers, cumulatively 10–18 km thick (avg. 13 km) and partitioned by a high-seismic-velocity zone (higher density) at ~8 km depth below sea level. Yellow region indicates rocks above 2286 m (7500 ft) elevation, roughly equivalent to volcanic strata of the SRMVF. Abbreviations: CC— central caldera cluster (mainly La Garita (Drenth et al., 2012) and Creede in line of profile); WC—western B calderas (San Juan, Silverton); V.E.—vertical exaggeration; D—density (in kg/m3). (C) Density of granitic rocks vs. composition (black squares), Sierra Nevada batholith (modified from Moore, 2000), and implications for interpretation of the San Juan (S.J.) C gravity anomaly as related to a subvolcanic batholith. Most exposed San Juan granitic intrusions are granodiorite (commonly S.J. crustal density ~62%–67% SiO2), which would be more mafic/dense than the average intrusion value of 2620 kg/m3 (appropriate for silicic gran- S.J. model intrusion density

ite, ~75% SiO2) used in the gravity-anomaly Common model of Drenth et al. (2012). A more mafic intrusion and dense intrusion average, appropriate for compositions the granodiorite commonly exposed in San Juan intrusions, would have a reduced den- Sierra Nevada granite data (black), sity contrast with crustal wall rocks, yielding from Moore (2000) a thicker batholith than the 13 km average modeled by Drenth et al. (2012).

an outlier of the batholith as modeled by Plouff Based on these interpretations of the San Juan crustal levels. Because gravity and seismic data and Pakiser (1972). The Platoro complex is geo- geophysical data, comparisons with other Cor- are mainly sensitive to crustal layering, geophysically obscure despite having sourced five dilleran plutons and with exposed crustal sec- physical models have limited capacity to distin- large-volume dacitic ignimbrites, each followed tions through volcanic arcs (reviewed in a fol- guish aggregates of steeply dipping plutons as by eruptions of andesitic lavas, and associated lowing section), the granitoid San Juan batholith the building blocks of crustal batholiths (e.g., with widely scattered intrusions mainly of mafic is here inferred to have its roof ~2–5 km below fig. 4 in Saleeby et al., 2003; fig. 3 in Paterson granodiorite (Lipman, 1975; Dungan et al., the present-day surface, an average thickness et al., 2011). 1989; Lipman et al., 1996). The locations of of at least 20 km (more likely 25–30 km), and these calderas, peripheral to the geophysically thereby occupying much of the Southern Rocky Sources and Age of the San Juan defined batholith, along with relatively mafic Mountains crust (thickness 40–45 km; Prodhel Geophysical Anomaly associated ignimbrites, caldera-filling lavas, and and Lipman, 1989; Hansen et al., 2013). The late granitic intrusions, suggest that any associ- companion batholith beneath the Sawatch The correlation between the gravity anomaly ated upper-crustal pluton lacks significant den- Range to the north (Isaacson and Smithson, and caldera sites of large-scale silicic eruptions sity contrast with host rocks. These relations 1976; Case and Sikora, 1984; McCoy et al., (Fig. 2A) suggests that the volumetric bulk of further point to the probability that the overall 2005) and smaller bodies associated with the San Juan batholith was assembled mainly San Juan batholith, including such mafic com- the Questa caldera in northern New Mexico during the ignimbrite eruptions and associated ponents, is likely to be substantially larger than (Cordell et al., 1985; Lipman, 1988) appear to caldera formation: at 28.8–26.8 Ma (Table 1), that imaged and modeled from available gravity have broadly similar density structure and verti- or more broadly ~34–27 Ma if the Marshall and and other geophysical data. cal dimensions; these also likely extend to deep Bonanza caldera cycles that lie just outside the

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intrusion modeled by Drenth et al. (2012) are to batholith growth. Such intrusions are typically rhyolitic (Lipman et al., 1997; Bachmann et al., included. Granitoid intrusions of varying size small, many as dikes, sills, and modestly larger 2002). In contrast, most erupted rhyolites are and composition are exposed within eroded cal- laccoliths. Most are exposed peripherally to the crystal poor, and phenocrysts are in near-equi- deras (Fig. 2A; Table 2), but the absence of geo- geophysically defined batholith and associated librium with groundmass (absence of intricate physical expression for individual plutons or the calderas, with major concentrations west and zoning or resorption textures). Bulk chemis- overlying calderas indicates that granitic rocks south of erosionally preserved volcanic rocks try, groundmass compositions, and phenocryst are widespread beneath the clustered calderas (Gonzales and Pecha, 2015). Many of these are assemblages (sodic sanidine, absence of quartz) and intervening areas. silicic representatives of the bimodal magmatic are indicative of low-pressure fractionation in No sizable contribution to the gravity anom- suite associated with diffuse regional extension the upper crust (Lipman et al., 1978; Bachmann aly seems likely from the granitic rocks of the and opening of the Rio Grande rift zone (Chris- and Bergantz, 2004; Huber et al., 2012; Gualda Proterozoic basement; such rocks are distrib- tiansen and Lipman, 1972; Chapin, 1979), in and Ghiorso, 2013), rather than being inher- uted widely in southwestern Colorado (Tweto, contrast to the dominantly intermediate-compo- ited from melting of lower-crustal sources as 1979), far beyond margins of the San Juan sitions and arc-type petrology of the Oligocene proposed by others (Annen et al., 2006; Tappa gravity anomaly without obvious geophysical magmatism. The few caldera-related loci of et al., 2011; Coleman et al., 2012). Some large expression (Drenth et al., 2012, p. 674). Late waning magmatism are peripheral to or beyond crystal-poor ignimbrites are uniform rhyolite; Cretaceous to Paleocene intrusions (75–60 Ma; the geophysically defined San Juan batholith. others are zoned upward into more crystal-rich “Laramide”) associated with the northeast- Small intrusions of silicic rhyolite intruded ring dacite (Table 1), suggesting transition to addi- trending Colorado mineral belt (Cunningham fractures of the 28.3 Ma Uncompahgre caldera tional crystalline residua deeper in a vertically et al., 1994; Stein and Crock, 1990; Chapin, at 19 Ma in the Lake City area; small 20 to extensive source body. For large ignimbrites 2012) also appear to have been unimportant: The 22 Ma rhyolites adjacent to the Platoro caldera in the SRMVF and elsewhere, caldera areas San Juan gravity anomaly is elongate east-west, complex are similarly millions of years younger and subsidence depths indicate that the erupted at a high angle to the mineral belt trend (Fig. and compositionally more evolved than caldera- magmas were laterally extensive bodies only a 2A), and the small intrusions of Laramide age forming events there. few kilometers thick. Probably most available southwest of the San Juan region (e.g., La Plata, crystal-poor silicic magma was erupted, down Rico) lack large-scale geophysical expression. SRMVF Magma Generation and Volume to a “viscosity barrier” (Smith, 1979; Karlstrom Small stocks and laccoliths associated with et al., 2012), but late-erupted scoria of crystal- the central volcanoes of dominantly intermedi- Diverse petrologic and geochemical stud- rich mafic dacite and andesite provide direct ate compositions (ca. 34–30 Ma) that preceded ies on processes of magma generation for the samples of mafic cumulates in compositionally the ignimbrite eruptions also seem unlikely to SRMVF and similar Cordilleran igneous prov- zoned ignimbrite eruptions such as the Bonanza record major mid- to upper-crustal batholith inces have led to broad consensus: The dominant and Carpenter Ridge Tuffs (Lipman et al., 2013; growth. These volcanoes are scattered widely andesitic to rhyolitic magmas were generated by Bachmann et al., 2014). Geochemical signa- beyond the area of the main gravity anomaly rise of voluminous mantle-derived basalt that tures of crystal accumulation complementary to (Steven et al., 1974; Tweto, 1979) but lack geo- provided heat to assimilate variable amounts rhyolite fractionation probably are obscured in physical expression indicative of sizable silicic of lower crust, as the evolving magmas crystal- many late-erupted ignimbrite dacites (and asso- intrusions. Any larger intrusions at depth, asso- lized and fractionated (e.g., Lipman et al., 1978; ciated plutons) by high volumetric ratios of the ciated with preserved central volcanoes, would DePaolo, 1981; DePaolo et al., 1992; Hildreth source magma mush relative to erupted evolved have to be relatively mafic (diorite?) and/or and Moorbath, 1988; Johnson, 1991; Riciputi liquid (Bachmann and Bergantz, 2008; Deering relatively deep to minimize density contrast. et al., 1995; Farmer et al., 2008; Kay et al., 2010; et al., 2011; Gelman et al., 2014). The early magma fluxes would still have been Jacob et al., 2015). The much-discussed poten- Because the >60,000 km3 volume of eruptions high, because the composite volume of cen- tial for mantle-generated mafic input to rejuve- in the SRMVF (Lipman, 2007) was accom- tral volcanoes was large (greater than that of nate and prolong the life spans of upper-crustal panied by much greater (but less constrained) ignimbrites at the San Juan locus; Lipman et al., magmatic systems (e.g., Smith, 1979; Hildreth, volumes of associated granitoid intrusions, 1970). Additionally, intrusive fluxes may have 1981; Mahood, 1990; Bachmann et al., 2007b; perhaps on the order of 250,000–300,000 km3 been higher and intrusions larger at early central Cooper and Kent, 2014; de Silva and Gregg, (Table 3; Fig. 4), the volume of associated volcanoes within the area of the gravity anom- 2014) has been explicitly proposed to explain mantle-generated mafic magma becomes criti- aly (though exposed remnants of such proxi- complex petrography and mineral chemistry of cal for models of crustal and lithospheric evo- mal volcanoes are not obviously larger or more large ignimbrites in the SRMVF and elsewhere lution. Several attempts to estimate proportions silicic than peripheral ones). Such waxing-stage (e.g., Lipman et al., 1997; Bachmann et al., and total volumes of required mantle basalt, intrusion and volcanism (Colucci et al., 1991) 2002; Bachmann and Bergantz, 2004; de Silva based on compositional and isotopic mass-bal- likely warmed the upper crust, providing needed and Gosnold, 2007; Huber et al., 2012). ance calculations, suggest 1:1 to >2:1 ratios of thermal preparation for the ignimbrite eruptions The volumetrically dominant intermediate- mafic magma to the intermediate to silicic mag- and associated assembly of multiple upper- composition lavas (andesite-dacite) of the mas that reached upper-crustal levels (Riciputi crustal plutons into a composite batholith (Lip- SRMVF contain diverse crystal cargos indica- and Johnson, 1990; Perry et al., 1993; Farmer man et al., 1978; Jellinek and DePaolo, 2003; tive of crystallization at variable depths and et al., 2008; Kay et al., 2010; Lake, 2013). de Silva and Gosnold, 2007; Gregg et al., 2012; volatile contents in the upper crust (clino For just the San Juan magmatic locus, such Gelman et al., 2013). pyroxene vs. amphibole assemblages; Colucci geochemical modeling suggests that 200,000– Scattered granitic bodies were also intruded et al., 1991; Parat et al. 2005). Dacites are 300,000 km3 of mantle basalt hybridized the widely in the San Juan region during protracted typically crystal rich; disequilibrium pheno- preexisting crust to generate the Middle Ter- waning magmatism (ca. 26–10 Ma) but again cryst textures (complex zoning and resorption) tiary volcanism and underlying batholith; total seem unlikely to have contributed significantly are common; and groundmass compositions are magma volume for the San Juan locus would be

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a 3 , ~400,000–500,000 km (Table 3; Fig. 4). Vol m u f canic deposits and the batholith are estimated at a m o i t n i o about half of this volume, with mantle magma o m r ; p

y and crustal melt in subequal amounts, and the l e n e o

d remaining half of total volume present as deeper s t u u s

c residue (cumulates, restite) from generation of o o h l t i the crustal magmas. From such a perspective, n w a , u

e the ignimbrite “supereruptions” are only a few J m n g based on isotopic dat a u complex); 20-km thick pluton; a l percent of the total magmatic system. ACF—assimilation-crystallization c granitoid or any mantle input o

S v , h t m i l u

o Batholith Growth and Crustal Structure m h i t n a i b m f ; o The prodigious inputs of basalt involved in y l l Abbreviations: e n × (volc+gran), conservative gran/volc ratio (4:1) d o generating the SRMVF and associated batho- o s m u

c granitoid, or any mantle inpu t lith might suggest substantial thickening of the l c

a o l c i Rocky Mountain crust (Riciputi and Johnson, n s , based on isotope modelin a y u h 1990), possibilities that can be evaluated by J p o n lcanic volume, adjusted for San Juan locu without deep root of more-maﬁ mantle-magma input esimate from isotope data more-maﬁ antle vol = 1.5 e

a geophysical data for the present-day crustal G Notes San Juan locus; mantle-magma input estimate from isotope dat Proposed for San Juan locus (but volcanic volume is too large) S Includes outlying central volcanoes Preferred model for entire SRMV F, Vo Clustered caldera sources for 9 ignimbrites, in 2 m.y Preferred model for San Juan locus, based on isotopic dat San Juan locus; mantle-magma input estimate from isotope data Low values: granitic batholith and ratio to volcanic rocks Preferred model for San Juan locus, based on isotopic data Central SJ caldera cluster (w/o SL Unlikely Single caldera-forming cycle structure. Somewhat surprisingly, the crustal 0

7 thickness in the Southern Rocky Mountains 0 1 4 05 13 – otal 620 408 1 670 587 46 0m 220 447 58 5 392 519 880 0

T (~40–45 km) is no greater than that of the adja- 2 >500 volume magma 1 cent High Plains (Prodehl and Lipman, 1989). TIA RY IGNEOUS ROCKS OF SRMVF lcanic ﬁeld (SRMVF) in dark red. Elevation even tends to correlate inversely with Moho depth, without the presence of any deep h volume (could be 25–30 km), and mantle input (compare Riciputi). crustal root to support the mountain topog- .0 .9 .0 .0 .5 .2 .0 1.0 1.4 0.9 1.9 1.3 2. 42 1.4 raphy (the “rootless Rockies”: Hansen et al., Ratio: TIOS, MID-TER granitic 2013). Rather, the high elevations are gravita-

mantle/volcanic+ tionally supported by atypically low densities (low seismic velocities) within the middle to lower crust and in the upper mantle. As a result, if the inferred granitoid thickness (20–30 km) for the San Juan batholith and volume of asso- 91 00 22 11 21 92 11 – – 11 14 10 14 20 14 ONIC/VOLCANIC RA

Ratio: ciated mafic input associated with the SRMVF volcanic are valid, space in the lower crust is inadequate granitic+mantle/ to accommodate the volume and thickness of

AND PLUT the dense (high-velocity) mafic residue (cumu- ) 3 lates, restite from partial melting) that would be 5 . KM 3 3 5 .1 .0 .0 . .0 .0 .0

– complementary to generation of the San Juan 4. 29 5. 01 4.0 4.0 3. 21 2 4. 01 0 . 10. 02 10. 02 Ratio:

2 and other batholith loci beneath then SRMVF. Because seismic velocities in the lower crust granitic/volcanic beneath the SRMVF are relatively slow, much

.6 of the expected mafic residue must lie beneath 0 10 10 48 – – 11 20 44 34 75 20 75 260 385 27 12 14 55 52 05 the seismic Moho and/or have delaminated (see TED VOLUMES (10 >300 volume later discussion). volcanic+plutonic) Mantle input ×

0 Comparisons with Other Cordilleran 3 66 0 1 64 00 00 00 80 50 0 00 – Volcanic Regions 25 03 30 03 13 03 160 160 160 1 0 volume 8 Granitic batholith ABLE 3. ESTIMA T The inferred vertically extensive geometry of

.6 the composite SRMVF batholith is suggested to 0 01 02 02 01 0 0 03 60 4 60 41 40 40 41 10 4

olcanic be a well-constrained representative example of volume V volcano-plutonic connections. Analogous corre-

)4 lations between upper-crustal gravity lows and caldera-forming volcanism in other Cordilleran )

2 volcanic regions have been similarly interpreted ACF 1 0 )

2 as evidence for shallow batholithic intrusive ( 7 (20-km thick plutons, mantle volume = 1.0 . 0 l complexes in diverse structural settings. Exam- 0 a 2 t ( Preferred estimates: Central San Juan (SJ) caldera cluster in green; locus blue; entire Southern Rocky Mountain vo e ples include the Latir and Mogollon-Datil fields n h t a

n farther southwest in New Mexico (Cordell et al., m Entire SRMVF San Juan locu s4 Adjusted: mantle-derive d4 Adjusted: mantle+crus t4 Maximum estimate Central SJ caldera cluste r1 Mantle+crust, Minimum estimate Minimum estimate Creede calder a0 Entirely mantle-derive d6 Maximum estimate e Note: p r i fractionation (DePaolo, 1981); SL—San Luis caldera complex. Conservative estimates are used for: pluton thickness and batholit Riciputi & Johnson (1990 Farmer et al. (2008) D Riciputi et al. (1995) Sources L Lake and Farmer (2015) This report 1985; McIntosh et al., 1992; Schneider and

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ENTIRE SRMVF Thickness of Mid- to Upper-Crustal Plutons

Although relatively rarely exposed, par- 600 tial sections through several tilted plutons and batholiths display original thicknesses of 10 km SAN JUAN LOCUS or more, providing direct evidence for the thick- batholith geometry and dominantly granodioritic compositions (becoming more mafic at depth) inferred from roof-zone intrusions in

3 the SRMVF. Volcanic

km 400 rocks Along the Nevada-Arizona boundary (Colo- 3 (ignimbrite, rado River extensional corridor), several tilted , 10 light blue) Miocene plutons (Spirit Mountain, Searchlight, Silicic Aztec Wash) provide partial sections of litho- olume batholith

v logically diverse granitic rocks, 5–10 km thick,

ve including roof contacts (Miller and Miller, 2002; Walker et al., 2007; Miller et al., 2011). These

mulati 200 plutons are compositionally diverse, most con-

Cu taining relatively homogeneous granite that overlies and merges laterally and with depth into Mantle basalt less-evolved granodiorite; heterogeneous zones of alternating mafic and felsic sheets provide evidence for recurrent open-system mafic recharge. At the Wooley Creek pluton in northern Cali- fornia, geologic and petrologic evidence indi- 0 Riciputi & Farmer Lake & Minimum Farmer Minimum cates that erosion has exposed a sequence of Johnson, et al., Farmer, estimate, et al., estimate, 1990 2008 2015 this report, Table 3 2008 this report, Table 3 cogenetic granitic rocks at least 12 km thick. No base is exposed for these rocks, which crystal- Figure 4. Magmatic-volume estimates for extrusive and intrusive components, SRMVF. Total lized at 159–158 Ma and are zoned downward volumes are dominated by the crustal granitic batholith and mafic magma from the mantle; from granite to tonalite (Barnes et al., 1986; the ignimbrite “supereruptions” are only a few percent of the overall magmatic system. Coint et al., 2013). In the Coastal batholith of the Northern Cas- cades and southern British Columbia, elongate Keller, 1994), the Marysvale volcanic field in (–50 mGal) has been modeled as evidence of (“tadpole”) plutons dominantly of granodiorite, Utah (Steven et al., 1984), Elkhorn Mountains an extensive batholith from near the surface to separated by metamorphic septa, display tex- volcanic field and underlying Boulder batholith 30 km depth or more (del Potro et al., 2013). The tures indicative of depth zones from mesozonal in Montana (Robinson et al., 1968; Hamilton mush zone has been interpreted to be as much as to hypabyssal and subvolcanic (Cater, 1982). and Myers, 1974; Biehler and Bonini, 1969; 200 km across with a volume of ~300,000 km3, Intrusion depths are estimated to range from Vejmelek and Smithson, 1995), and the Yellow- suggesting a ratio of batholith to erupted magma ~25–30 km to as shallow as ~7 km, with some stone caldera cluster (Eaton et al., 1975; Chris- (~15,000 km3) of 20:1–35:1 (Ward et al., 2014), individual plutons exposed for >8 km vertically tiansen, 2001). i.e., notably higher than often-cited estimates of (Miller et al., 2009). A particularly informative comparison with 10:1 or less (Crisp, 1984; White et al., 2006) or The spectacularly outcropping Bergell intru- the San Juan region is the APVC of the central ratios calculated here for the SRMVF (Table 3). sion in northeast Italy is reported to preserve a Andes (de Silva, 1989; de Silva and Gosnold, 10–12 km crustal transect, with no indication 2007; Kay et al., 2010). The APVC is a younger PLUTON GEOMETRY FROM TILTED that deepest parts of the intrusion are exposed (1–10 Ma) but otherwise striking analog to the CRUSTAL SECTIONS (Rosenberg et al., 1995; Berger et al., 1996; SRMVF, in terms of volcanic compositions, vol- Samperton et al., 2013). umes, eruptive processes, and magmatic dura- Because the batholith geometry inferred In northwest Italy, granite and granodiorite, tion (table 4 in Lipman and McIntosh, 2008). beneath the SRMVF (mosaic of vertically exten- exposed to paleodepths of ~12 km, intrude the No granitic intrusions are exposed at the little- sive intrusions, collectively 20–30 km thick, contemporaneous ignimbrite fill of the large eroded APVC, but combined seismic, gravity, likely becoming more mafic downward; residue Sesia caldera (Quick et al., 2009; Sbisà, 2010). and deformation data (Chmielowski et al., 1999; of mafic cumulate gabbro and restite at depth) This upper-crustal assemblage overlies the Zandt et al., 2003; Fialko and Pearce, 2012; comes mainly from geophysical and petrologic well-known lower-crustal mafic complex of the Ward et al., 2014) document a large subvolcanic data that are open to alternative interpretations, Ivrea-Verbano zone, both with similar Permian batholith. Seismic attenuation in the middle to relations observable more directly from exposed ages (ca. 290 Ma). Thus, a subcaldera magmatic upper crust is inferred to define a zone of mushy tilted plutons and sections through deep parts plumbing system is exposed to ~25 km depth, magma at depths of 4–25 km below sea level of magmatic arcs elsewhere provide additional directly linking intrusion of mantle-derived that is coextensive with the volcanic field (Ward evidence for thick subvolcanic Cordilleran basalt into the deep crust with large-scale silicic et al., 2014). An associated gravity anomaly batholiths. volcanism.

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Crustal Sections through Magmatic Arcs Sesia Sierra Coastal Famatinian Talkeetna SRMVF caldera- Nevada, Kohistan complex, arc, arc, (proposed, Tectonically tilted crustal sections through Ivrea zone N-S arc WA-BC Argentina Alaska this study) 0 several continental-margin and oceanic arcs ii (Fig. 5) provide further documentation, on ?? even larger scales, that subvolcanic Cordilleran - ? - - ? - batholithic bodies occupy much of the crust, with geochronologic implications for duration of pluton assembly and crystallization, total - ? - - ? - ?? magma-body volume, and evidence for removal of their deep-crustal mafic roots (DeBari and 20 Greene, 2011). The Kohistan arc of northeast Pakistan con- tains an exceptional crustal section, from shal- Moho? low volcanic and sedimentary deposits intruded by granites of the Kohistan batholith down to ?? ??

paleodepths of 30 km (Jagoutz and Schmidt, DEPTH, km Volcanic (i, ignimbrite) - ? - - ? - Moho 2012, 2013). Mafic cumulates as residue from ?? the shallower felsic magmatism are interpreted 40 Felsic granitoid to dominate the lower crust down to the Moho at 50 km depth, but petrologic modeling sug- Ma c granitoid Moho gests that the preserved cumulates are volumetrically inadequate by a factor of 2–3 to have Metamorphic generated the exposed upper-crustal batholith. Moho Large-scale foundering of lower-crustal residue Gabbro cumulate has been inferred to account for the volumetric Moho? - - ? - - discrepancies. Mantle Structural discontinuity Structural and paleobarometric data indicate 60 that exposed levels of the Sierra Nevada batho- Figure 5. Generalized thickness estimates and rock compositions, tilted crustal sections of lith deepen southward, from dominant grano- magmatic arcs: Sesia caldera-Ivrea zone, Italy; north-south in the Sierra Nevada, Califor- diorite and quartz monzonite at upper-crustal nia; Kohistan arc, Pakistan; North Cascades–Coastal batholith in Washington (WA) and levels preserving cogenetic Cretaceous volcanic British Columbia (BC); Famatinian arc, Argentina; Talkeetna arc, southeast Alaska. Also rocks near Yosemite National Park (Fiske and proposed section for crust of the SRMVF (see Fig. 14). Felsic-plutonic sections are com- Tobisch, 1994; Schweikert and Lahren, 1999), monly ~20–30 km, similar to thickness inferred from geophysical data for the SRMVF to tonalite–quartz diorite at paleodepths as great batholith. In the Coastal batholith of Washington, metamorphic wall rocks separate verti- as 35–40 km in southern areas (Saleeby et al., cally extensive “tadpole” plutons (Cater, 1982; Miller et al., 2009), as shown schematically. 1990, 2003). Mafic residue (restites, cumu- Steeply dipping metamorphic septa and screens are common in other silicic batholiths (e.g., lates) from batholith generation should have fig. 4in Saleeby et al., 2003) but are not depicted in these simplified crystal sections. Thick 1–2 times the volume of the granitic rocks, cumulate gabbros are exposed at deep crustal levels in the Ivrea and Kohistan arc sections; but seismic data, xenolith studies, and buoy- in these and elsewhere, additional cumulate and restite residua likely underlie the seismic ancy considerations indicate that much of this Moho. Data are from references cited in the text. residue has detached from beneath the batholith (Ducea, 2001; Saleeby et al., 2003; Jones et al., 2004; Zandt et al., 2004). In the Coastal 465 Ma) expose an intact ~25 km mid- to lower- Miller et al., 2007; Simon et al., 2008; Schmitt, plutonic complex of British Columbia and its crustal section of granodiorite-tonalite-gabbro 2011), because applying such data to the inter- southern extension into the North Cascades, (Ducea et al., 2010; Otamendi et al., 2012). pretation of magma crystallization, supply rates, Cretaceous to Paleogene (96–45 Ma) plutons and duration of pluton assembly remains chal- of tonalite and lesser volumes of diorite record CRYSTALLIZATION AGES AND lenging. Various studies have attempted to test paleodepths of ~7–35 km (Armstrong, 1988; ASSEMBLY PROCESSES IN whether exposed subvolcanic plutons represent Miller et al., 2009). SUBVOLCANIC MAGMA BODIES direct samples of a magma body that fed an Exhumed and tilted sections in the Jurassic eruption (Glazner et al., 2008; Tappa et al., 2011; Talkeetna island arc expose upper-crustal vol Additional perspectives on magma-assembly Zimmerer and McIntosh, 2012a, 2012b), but no canic sequences, 5–7 km thick, intruded by tonal- duration come from recent age determinations a priori reasons exist for such a magma body to ites and quartz diorites and underlain by middle- on SRMVF igneous rocks and other subvolcanic undergo final crystallization and cooling with- and lower-crustal gabbros that are ~15 km thick. intrusions by 40Ar/39Ar and U-Pb methods that out further modification, especially if associated The Talkeetna arc section overlies residual mantle have much-improved analytical precision com- with recurrent mafic recharge and multiple erup- that includes ultramafic cumulates (DeBari and pared to earlier K/Ar and fission-track analyses. tions in a long-lived volcanic field. Many, prob- Greene, 2011, and references). Similarly, in the Before summarizing results for the SRMVF, ably most, subvolcanic magma bodies are likely Famatinian region of Argentina, tilted portions these geochronologic techniques are reviewed to undergo compositional evolution over varied of an Ordovician continental-margin arc (485– briefly (building on excellent discussions by time intervals, with final crystallizing and cool-

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ing below solidus temperatures only as magma (LA-ICP-MS), tend to yield partly complemen- gests that weighted-mean TIMS ages are likely supply wanes. Alternatively, at some intrusions, tary but partly conflicting results that have led to underestimate duration of zircon crystalli- decompression-induced volatile release may to diverse interpretive approaches (see summary zation, especially if the number of analyses is induce rapid crystallization in the nonerupted in Schmitt, 2011). Determining chronology small or analyses are made on multiple zircon upper-crustal mush immediately after an erup- of magmatic events by U-Pb dating of zircon crystals. tion (Bachmann et al., 2012). Such a process seems somewhat analogous to dating the growth Also, as averages for a crystal that may be could generate the common porphyritic textures history of a forest by 14C measurements on wood zoned, TIMS ages will tend to underestimate in subvolcanic plutons. Perhaps more generally, fragments (resorbed zones in zircons are even duration of crystallization. Many recent TIMS waning of magma supply, which would disrupt mimicked by overgrown fire scars in old trees). studies interpret weighted-mean TIMS zircon quasi-equilibrium between long-term volatile (1) Dates for whole zircon crystals (by TIMS, dates as the crystallization age (e.g., Tappa release and magmatic recharge, would promote with high precision) or dates on whole cross- et al., 2011; Mills and Coleman, 2013), while increased crystallization in subvolcanic intru- sectional slabs of a tree trunk provide average others use the younger zircon fractions as a sions at diverse time intervals after an eruption. ages for overall growth history of crystal or tree, record of late magmatic growth (e.g., Memeti Single-crystal laser 40Ar/39Ar determinations respectively. (2) Dates on multiple fragments of et al., 2010; Schoene et al., 2012). Because can provide high-resolution measures of erup- a zircon crystal, or on broken chunks of a tree much of total volume is in the outer zones of the tion age, especially for sanidine (with analyti- slab, provide a partial record of growth dura- crystal, the presence of an antecrystic core that cal uncertainties commonly now only 20–30 tion but inevitably underestimate the total time grew a few million years earlier during evolu- k.y. at 2s for mid-Tertiary volcanic rocks), but span. (3) Zircon core, rim, and surface ages (by tion of a long-lived Tertiary magmatic system they offer few insights concerning pre-eruption SIMS or LA-ICP-MS, but with lower precision would be difficult to detect by even the highest- magma-chamber processes (Costa, 2008). Simi- than TIMS), or ages for inner and outermost tree resolution TIMS analysis. For example, almost larly, application of 40Ar/39Ar methods to min- growth rings, provide the best age approxima- half the total volume is in the outer 20% of the erals from granitic intrusions yields much evi- tions for initial and last growth event. However, radius in a simple spherical crystal, while less dence on rates of postcrystallization cooling but none of these methods provides reliable infor- than 1% of the volume resides in the inner- little information for duration of magma assem- mation on earliest growth history: for forests most 20%. Even a xenocrystic core of this size, bly or inception of crystallization (Memeti where long-dead trees have decomposed by bio- derived from Proterozoic SRMVF basement et al., 2010; Zimmerer and McIntosh, 2012a, genic processes, or for long-lived magma bodies (1800 Ma), would increase the whole-crystal 2012b). Despite the high analytical precision of that resorbed early-formed zircon when cycled age of a 35 Ma zircon growth by only 15 m.y.; 40Ar/39Ar methods, calibration of absolute age through contrasting Zr-saturation environments. a similar-size antecrystic core 3 m.y. older than remains controversial (ages in this paper are Recent analytical innovations, including the rim would increase the whole-crystal age by normalized to Fish Canyon Tuff at 28.02 Ma, chemical-abrasion (CA) pretreatment (Mat- a nonresolvable 0.025 m.y. (at constant U con- for consistency with recent SRMVF reports; tinson, 2005), now provide remarkably precise tent). As a further complication, metamict parts Lipman and McIntosh, 2008; Lipman, 2012). ages by TIMS methods (some to 0.1% or less) of zircons that are preferentially removed by CA In contrast to the eruption ages for volcanic for individual crystals of mid-Tertiary age, while pretreatment are likely to be high-U zones that deposits, U-Pb determinations for granitic plu- reducing complications of Pb loss (Schaltegger grew from chemically evolved silicic phases, tons, mainly on zircon, are likely to provide only et al., 2009; Schoene et al., 2012). High-preci- and this stage of growth history is thus likely to partial records of potentially lengthy magmatic sion CA-TIMS zircon ages for mid-Tertiary and be poorly recorded by a bulk-crystal TIMS age. crystallization of this refractory mineral, and the older systems have documented lengthy crystal- Many researchers in TIMS laboratories are cur- timing of zircon growth relative to other mineral lization histories for some ignimbrite magmas rently exploring methods for improving resolu- phases can vary greatly as a function of magma (Fig. 6A; Wotzlaw et al., 2013) and large plu- tion and precision by techniques such as com- composition, liquidus-solidus temperature, tons (Coleman et al., 2004; Memeti et al., 2010; bining ages with trace-element data on the same and zircon saturation (Watson, 1996; Miller Frazer et al., 2014). Zircons from some individ- zircons (Schoene et al., 2012); dating multiple et al., 2007). Complex zoning of igneous zir- ual granitic samples yield prolonged semicon- fragments from individual zircons (e.g., Sam- cons, some with internal resorption boundaries, tinuous age spectra (105–106 yr; Matzel et al., perton et al., 2013); or comparing matrix zircons likely records variable durations of fluctuating 2005, 2006b; Crowley et al., 2006; Schaltegger with those enclosed in host crystals (Barboni magma-chamber pressures and temperatures et al., 2009; Memeti et al., 2010), complicating and Schoene, 2014). during assembly of subvolcanic magma bod- interpretive distinction between autocrystic and In contrast, much smaller portions of a crys- ies (Robinson and Miller, 1999; Miller et al., antecrystic growth. tal can be analyzed by SIMS methods, with 2007; Schmitt, 2011; Erdmann et al., 2013). TIMS analyses are time-intensive, however, analysis-spot diameters commonly ~25 mm. Complexities include open-system recharge by and many ages are reported as weighted means Such spatial resolution has the potential to mantle-derived magma, mingling between con- of relatively few individual zircon analyses, an resolve core-to-rim differences in magmatic trasting compositional batches, and assimilation approach that can obscure geologically infor- growth history (e.g., Brown and Fletcher, 1999), and mixing with partial melts of adjacent rocks, mative information about duration of magma but typical precision for an individual analysis both earlier-solidified phases of the same broad crystallization (von Quadt et al., 2011). Unsur- is at least an order of magnitude lower than for magmatic system (antecrysts) and older crustal prisingly, the range in apparent ages tends to TIMS determinations on mid-Tertiary rocks. country rocks (xenocryts). increase with the number of individual crystals Despite such limitations, SIMS zircon studies The analytical methods in most common use analyzed, especially for silicic (cool-wet) calc- have begun to provide important insights about for zircon age determinations, thermal ioniza- alkaline magmas such as the SRMVF (Fig. duration of zircon growth. Core-rim zircon tion mass spectrometry (TIMS), secondary ion 7A), which may never have been strongly zir- growth durations of as long as ~5 × 105 m.y. mass spectrometry (SIMS), and laser-ablation– con undersaturated. This positive correlation have been documented for several Pleistocene inductively coupled plasma–mass spectrometry between age range and number of analyses sug- volcanic units analyzed by in situ U-series and

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increasingly have been reported for Tertiary magmatic systems (some summarized here). Zircon U/Pb ages thus can document times of zircon crystallization in the long-lived, incrementally assembled magmatic systems that are inferred here to generate vertically extensive plutons and batholiths beneath Cordilleran and Andean arcs, but contrary to some interpretations, such zircon ages provide only incomplete constraints on processes or overall duration of magma-body assembly. Early growth of a long- lived magmatic system may be poorly recorded due to periods of zircon dissolution, and the A more-precise SIMS analyses of whole crystals may not resolve late stages. The construction of c Wotzlaw et al. 2013 a batholith can take longer than recorded by zircon-crystallization ages, while the time interval for separation and shallow assembly of eruptible magma may be much shorter.

San Juan Locus

The San Juan volcanic region preserves the most numerous and diverse ignimbrites and source calderas in the composite SRMVF (Table 1); small granitic plutons are exposed at more than half the calderas, commonly intrusive into volcanic host rocks (Table 2). High-precision ages are available for all the ignimbrites and increasingly for the associated intrusions. Compositional, structural, and age relations B for several of these systems provide special perspectives on processes and rates of magma supply and pluton assembly.

Figure 6. Examples of prolonged zircon crystallization in large calc-alkalic arc ignimbrites, Fish Canyon Tuff, La Garita Caldera illustrating challenges in constraining magma-assembly duration by zircon geochronology. The Fish Canyon Tuff, erupted at 28 Ma from CA-TIMS analyses provide high-precision for whole crystals but have only limited capabil- the La Garita caldera, is among the best known ity to resolve presence of antecrystic cores. In contrast, SIMS and LA-ICP-MS methods ignimbrites in the SRMVF and worldwide can measure zircon cores in comparison to crystal rims and surfaces, but precision is lower. because of its exceptional volume (>5000 km3) (A) Multiple TIMS ages on whole crystals of Fish Canyon Tuff (FCT), erupted at 28.0 Ma, and widespread use of its crystal cargo as geo- indicate a minimum 450 k.y. duration (ca. 28.2–28.7 Ma) of zircon crystallization (modified chronologic reference standards. No associated from Wotzlaw et al., 2013). NCD (Nutras Creek Dacite) is an intracaldera lava flow indis- granitic intrusions are exposed, but the tuff con- tinguishable in composition or age from the ignimbrite. An even longer duration for FCT tains granodiorite fragments with zircon U/Pb zircon growth (<1.1 m.y.) is suggested by SIMS core-surface ages (Coble, 2013). (B) Younger ages indistinguishable from those of pumices Toba Tuff, erupted at 70 ka: depth-drilled ages determined by SIMS on a zircon crystal have (Bachmann et al., 2007c). Numerous recent age a 500 k.y. range, becoming older toward the core (Reid, 2008). Age range within this single determinations on Fish Canyon sanidine and crystal is as great as that discernible among multiple TIMS analyses on whole zircons from zircon provide critical insights about a complex the Fish Canyon system. magmatic history (see Phillips and Matchan, 2013, including references). Fish Canyon sanidine ages, determined by U-Pb methods (Fig. 6B), especially calc-alkaline is greater than by SIMS methods. A counter- 40Ar/39Ar, have proved highly consistent from arc rocks (Brown and Fletcher, 1999; Vazquez balancing advantage of LA-ICP-MS methods crystal to crystal, despite complex resorption and Reid, 2004; Bachmann et al., 2007a; Reid, for some problems is rapid analysis, permitting textures and compositional zoning in this phase 2008; Simon et al., 2008; Claiborne et al., 2010; generation of large data sets. Despite greater (Bachmann et al., 2002) and common isotopic Schmitt et al., 2010). uncertainties for U-Pb SIMS and LA-ICP-MS disequilibrium between crystal phases and the Analyses by LA-ICP-MS also permit deter- determinations on older rocks in comparison melt (Charlier et al., 2007). U-Pb determinations minations on subareas of crystals with similar to TIMS, antecrystic zircon cores as much as on Fish Canyon zircons have yielded ages mod- precision (Guillong et al., 2014), although spot a million years or more older than the time of estly older than those for sanidine by 40Ar/39Ar size is somewhat larger, and excavation depth eruption, or final crystallization in intrusions, methods, and recent high-precision studies have

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0.8 Princeton batholith (Mills and Coleman, 2013) Mt Princeton quartz monzonite Aetna quartz monzonite

Questa-Latir batholith (Tappa et al., 2011; Rosera et al., 2013) Cabresto Lake pluton South ring intrusions 0.6 Rio Hondo quartz monzonite

Adamello batholith, Italy (Schaltegger et al., 2009) Re di Castello pluton or single sample

PRINCETON BATHOLITH, border phase (MP 33) LA-ICPMS zircon ages (Zimmerer and McIntosh, 2012a) sts ? 0.4 ry

TIMS analyses f Antec Wall Mtn Tu * = 37.15 Ma (Ar/Ar)

.: multiple TIMS* = 35.80 ± 0.09 Ma , m.y 0.2 *Mills and Coleman, 2013 *Zimmerer and McIntosh, 2012 ge range

A Pb loss ?

B 0 2 46810 12 A Number of individual TIMS analyses per sample

Figure 7. Challenges in measuring the duration of crystallization in subvolcanic intrusions by U-Pb zircon age determinations. (A) Positive correlation between total number of individual analyses and range in ages from multiple U-Pb zircon determinations by CA-TIMS for calc- alkaline mid-Tertiary intrusions (Southern Rocky Mountain volcanic field [SRMVF] subvolcanic intrusions, Adamello batholith in Italy). The positive correlation suggests that weighted-mean TIMS ages for a sample are likely to underestimate duration of zircon crystallization, especially if the number of analyses is small or analyses are made on multiple zircon crystals. With sufficient analyses, the age span among zircons in a sample will reach a plateau, of course, but this plot suggests that 10 or more determinations would be desirable for samples from calc-alkaline systems such as the SRMVF (e.g., Wotzlaw et al., 2013). (B) Alternate interpretations of U-Pb zircon ages for the Princeton batholith and the associated ignimbrite, comparing a high-precision CA-TIMS age with LA-ICP-MS analyses for the same sample. The weighted-mean TIMS age was interpreted as recording rapid emplacement and zircon crystallization of the intrusion ~1.3 m.y. later than eruption of the Wall Mountain Tuff (Mills and Coleman, 2013). In contrast, the less-precise LA-ICP-MS results span a multimillion-year range (Zimmerer and McIntosh, 2012a), extending back to ages as old as the tuff, suggesting a prolonged history of zircon crystallization. MSWD—mean square of weighted deviates.

documented analytically significant durations of must have been later than the youngest whole- U-Pb study reported whole zircon crystals of zircon crystallization (Fig. 6A). crystal dates (ca. 28. 2 Ma), and similarly earli- Proterozoic age that were petrographically Pre-eruption spans of zircon crystallization est growth in zircon cores must have been earlier distinct in a bulk sample of Fish Canyon Tuff; on the order of 450 k.y. in Fish Canyon magma than the oldest dates (ca. 28.65 Ma). Consistent Lanphere and Baadsgaard, 2001). Thus, the have been documented by single-crystal CA- with such inferences, a detailed SIMS study of observed range in Fish Canyon zircon ages TIMS analyses for large zircon populations Fish Canyon zircons yielded a weighted-mean seems unlikely to be due to variable presence of (19 crystals—Bachmann et al., 2007c; 58 crys- age of 28.0 Ma from unpolished zircon surfaces, small xenocrystic cores; the absence of ages that tals with particularly high precision—Wotzlaw while core ages from sectioned crystals were as step back significantly toward that of the base- et al., 2013). These age spans must be minimal, much as 1.1 m.y. older (Coble, 2013). Although ment (1400–1800 Ma) suggests that assimi however, because even the most precise whole- less precise than the TIMS results, the differ- lated crustal zircons were completely resorbed crystal TIMS analyses provide no information ences between the surface and core zircon ages during early stages of magma-body assembly. on core-rim variation. The sizable whole-crystal suggest a substantially longer duration of zircon The extended durations of zircon crystalliza- age ranges require that the total duration of Fish crystallization and magma-body assembly. tion by both TIMS and SIMS methods, back Canyon zircon growth and magma assembly Neither zircon study detected xenocrystic in time at least to that of the preceding ignim- was significantly longer; final rim crystallization cores with basement ages (although an earlier brite eruption from the same area (Masonic

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Park Tuff at ca. 28.7 Ma), is consistent with the lion years younger than the associated ignimbrite the Wall Mountain Tuff had erupted. Ages inference of lengthy pre-eruption evolution, as (Bove et al., 2001), similar to the situation at Sil- on hornblende and biotite from this pluton previously developed from petrologic and iso- verton. A resurgent intracaldera granodiorite in document relatively rapid cooling after zircon topic evidence of disequilibria in Fish Canyon Alpine Gulch at Lake City has 40Ar/39Ar ages growth (Zimmerer and McIntosh, 2012a) but magma (Lipman et al., 1997; Bachmann et al., (biotite, K-feldspar) within analytical uncertain- offer no direct information on timing of magma 2002; Bachmann and Bergantz, 2003; Charlier ties of the associated ignimbrite (M. Zimmerer, assembly. et al., 2007). 2014, written commun.). Intrusions at South In contrast to results for the deeply eroded River and San Luis were emplaced along caldera Princeton intrusion, two of three granitic sam- Bonanza Caldera structures and are likely closely related in age but ples from the Aetna caldera yielded CA-TIMS This source of the complexly zoned Bonanza have not been dated directly. zircon ages indistinguishable from that of Tuff, erupted at 33.15 Ma, preserves exception- the associated Badger Creek Tuff at 34.5 Ma ally diverse exposures: from topographic rim, Sawatch Range (Mills and Coleman, 2013). Zircons from a thick intracaldera ignimbrite and overlying lava Calderas aligned north-south along the crest third sample have a distinctly older weighted- fill, to caldera floor and granitic intrusions in of the Sawatch Range (Fig. 1) were sources mean age (34.95 ± 0.04 Ma), even though this its steep resurgent dome (Lipman et al., 2013). of the earliest ignimbrite eruptions from the body as presently mapped intrudes the intra- The resurgent intrusions vary widely in compo- SRMVF. These include the 37 Ma Wall Moun- caldera Badger Creek Tuff. The dated sample sition and texture, from mafic granodiorite and tain Tuff, erupted from an erosionally removed has been suggested to be from an unmapped andesite to silicic granite and aplite (56%–77% caldera above the Mount Princeton batholith, older intrusion (Coleman et al., 2013), but this

SiO2), a compositional range similar to that in and the 34 Ma Badger Creek Tuff, erupted from interpretation would require that the older body the caldera-filling lavas. K-feldspar 40Ar/39Ar the Mount Aetna caldera fragment just to the was erosionally exposed extraordinarily rapidly, ages on lavas and resurgent intrusions are close south (Table 1). In many respects, the 33 Ma prior to deposition of the Badger Creek Tuff. to those from the tuff, to at most a few hundred Bonanza caldera within the NE San Juan Moun- Alternatively, the 34.95 Ma weighted-mean thousand years younger, indicating emplace- tains is a transitional southern continuation of age could reflect antecrystic zircon growth not ment and cooling of at least upper portions of the Sawatch structural trend. detectable by whole-crystal TIMS analyses, the caldera-related magma body soon after the Recent detailed geochronologic studies of the perhaps extending back in time to assembly of ignimbrite eruption. Mount Princeton and Aetna intrusions by U-Pb the Princeton Quartz Monzonite. and 40Ar/39Ar methods have provided impres- In addition to the 40Ar/39Ar age and CA-TIMS Platoro Caldera Complex sively precise mineral ages that document com- zircon determinations for Princeton and Aetna After eruption of five large dacite ignimbrites plex relations between timing of the ignimbrite intrusions, a few of the same samples were from Platoro between 30.1 and 28.6 Ma, ande eruptions versus crystallization and cooling of dated by LA-ICP-MS (Zimmerer and McIntosh, sitic lavas filling the composite caldera were subvolcanic intrusions (Fig. 7). Weighted-mean 2012a). Analysis of 25–35 zircons per sample intruded by the 3 × 8 km Alamosa River stock. U-Pb zircon ages by CA-TIMS from three yielded weighted-mean dates similar to those by A 40Ar/39Ar biotite age of 27.98 ± 0.11 Ma samples of Mount Princeton Quartz Monzo- TIMS methods, but with a much broader spec- from typical granodiorite (M. Zimmerer and W. nite (Fig. 7B) are 1–1.5 m.y. younger than the trum of individual ages (Fig. 7B). Ages statis- McIntosh, 2013, written commun.) suggests 37 Ma eruption age (40Ar/39Ar, sanidine) of tically younger than the weighted mean were final crystallization ~0.5 m.y. younger than the the Wall Mountain Tuff (Mills and Coleman, inferred by Zimmerer and McIntosh to reflect last-erupted ignimbrite from Platoro and about 2013; Zimmerer and McIntosh, 2012a), lead- Pb loss, a potential complexity also noticed in concurrent with eruption of Fish Canyon Tuff ing these authors to infer that emplacement of other mid-Tertiary granitic rocks (DuBray et al., from the central caldera complex. the Mount Princeton Quartz Monzonite was 2011; Colgan et al., 2012; von Quadt et al., unrelated to the ignimbrite magma. Differ- 2014). Ages that were statistically older than Silverton Caldera ences of 0.1–0.3 m.y. in weighted-mean U-Pb the weighted mean, some as old as 38 Ma and At the deeply eroded Silverton caldera, ages among the three analyzed samples were similar to the eruption age of the Wall Moun- source of the 27.6 Ma Crystal Lake Tuff, the 4 × interpreted to record discrete intrusion events tain Tuff, were inferred to record early zircon 8 km Sultan Mountain stock (among the largest and rapid cooling of separate small magma growth, well before final solidification of this exposed intrusions in the San Juan Mountains) batches during incremental batholith assembly. intrusion. Such interpretations were tempered, intrudes older volcanic rocks along the south- Interpretations are complicated, however, by however, by the relatively large uncertainties of ern caldera margin. A 40Ar/39Ar age of 26.6 ± the small numbers of samples (3) and analyses the LA-ICP-MS technique. 0.03 Ma on biotite (Bove et al., 2001) and a per sample (4–7), some individual analyses on similar LA-ICP-MS age on zircon (Gonzales multiple grains, lack of dates from the earliest- Questa-Latir Area and Pecha, 2015) from granodiorite indicated crystallized margin of the batholith where in that, in contrast to results from Bonanza, final contact with Precambrian country rock (the Recent U-Pb and 40Ar/39Ar age determina- crystallization of the Sultan Mountain stock was fine-grained texturally heterogeneous Pomeroy tions from the Latir volcanic field, the Questa about a million years after the ignimbrite erup- phase of Toulmin and Hammarstrom, 1990), caldera source of the 25.5 Ma Amalia Tuff, and tion and caldera formation at Silverton. and a 700 k.y. spread among the analyses with associated intrusions have yielded notably pre- a continuum of overlapping ages rather than cise and coherent results (Tappa et al., 2011; Other Caldera-Related Intrusions discrete groupings for each sample (Mills and Zimmerer and McIntosh, 2012b; Rosera et al., Silicic plutons are also exposed at Uncom- Coleman, 2013, their fig. 4). Alternatively, as 2013), with broad similarities to those from the pahgre, Lake City, South River, and San Luis discussed in a later section, these results may Sawatch Range. This younger ignimbrite-cal- calderas (Table 2). The Capitol City intrusion at record late zircon crystallization in a vertically dera system preserves a more complete proxi- Uncompahgre has a 40Ar/39Ar age about a mil- extensive long-lived magma body from which mal volcanic sequence and compositionally

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diverse associated granitic intrusions. The intru- parable to those at the Bonanza and Aetna cal- tion of the associated ignimbrite (Colgan et al., sions are exposed progressively deeper south- deras but contrast with the more deeply eroded 2012). Mid-Tertiary ignimbrites and shallow ward from the caldera, but gravity data show Mount Princeton batholith. caldera-related plutons have yielded similar age that the isolated exposures are upper parts of a concordances in the Stillwater and Clan Alpine large composite batholith (Cordell et al., 1985; Basin and Range, Nevada and Utah Mountains (John et al., 2014). Lipman, 1988). At the Caetano caldera, Nevada, rotational At the mineralized porphyry in Bingham The shallowest intracaldera resurgent intru- basin-range faulting exposes exceptionally Canyon, Utah, the oldest and youngest intrusion (peralkaline granite of Virgin Canyon) has thick (~5 km) intracaldera rhyolitic ignim- sive phases have yielded zircon ages within the a composition and age similar to the Amalia brite, intruded by granite that has an identical range 38.1–37.8 Ma, but in each phase, several Tuff, and the slightly deeper metaluminous Rito 40Ar/39Ar age (34.0 ± 0.5 Ma) within analyti- concordant zircons are significantly older, rang- del Medio and Canada Pinabete granitic plutons cal uncertainties (John et al., 2008; Henry and ing back to at least as far as 38.5 Ma and pos- yield CA-TIMS crystallization ages for zircon John, 2013). Zircon SIMS determinations on sibly to ca. 40.5 Ma (von Quadt et al., 2011). that are at most a few hundred thousand years both ignimbrite and granite also have peak ages These variations were interpreted as a mini- younger (Tappa et al., 2011). Two samples from at 34 Ma, but include younger and older outlier mum lifetime of the magma reservoir of at least the deeper and less-evolved granodiorite of values (John et al., 2009). Pretreatment of zir- 0.7 m.y., possibly more than 2 m.y. The Caetano Cabresto Lake have slightly younger crystal- cons from Caetano ignimbrite and granite by and Bingham results are thus comparable to the lization ages (25.1–25.0 Ma), and mineralized chemical abrasion reduced numbers of anoma- documented antecrysts in the Fish Canyon Tuff granitic intrusions along the southern ring-fault lously young ages while preserving and aug- (Wotzlaw et al., 2013; Coble, 2013) and to less- zone are variably still younger, 25.2–24.5 Ma menting proportions of statistically significant robust data suggesting the presence of antecrys- (Rosera et al., 2013). South of Questa caldera, determinations that indicate presence of ante- tic zircons at Mount Princeton (Fig. 7B; Zim- four samples from the large Rio Hondo pluton, crystic zircon as much as 2 m.y. older than erup- merer and McIntosh, 2012a). which has its roof zone entirely within Precam- brian basement, yielded weighted-mean TIMS ages from 23.0 to 22.6 Ma, but the individual ages define a continuum without analytically Figure 8. (on folloowing page) Variable apparent durations of U-Pb zircon crystalliza- significant gaps between samples (Fig. 8A, tion: within single samples, among multiple samples from same igneous unit, and com- inset; Tappa et al., 2011). parisons between shallow-subvolcanic intrusions (Southern Rocky Mountain volcanic field These authors interpreted the progression of [SRMVF]) and deeper plutons (Tuolumne igneous suite). (A) Shallow subvolcanic intru- zircon ages, which get younger southward from sions of the SRMVF, showing relatively small U-Pb age ranges for individual zircon analyses the caldera area, as recording incremental top- within a sample or among weighted-mean ages for multiple samples from a pluton (data down emplacement and rapid crystallization from Tappa et al., 2011; Mills and Coleman, 2013). Apices of V-shape tie lines (points on of separate small magma batches, analogous the x axis) indicate total reported age range among multiple samples from a single pluton; to the interpretations of TIMS zircon results height of tie lines (scale on y axis) indicates age range among multiple zircon analyses from from the Sawatch Range by Mills and Coleman a single sample (in black, number of individual U-Pb analyses per sample). CL—Cabresto (2013). Alternatively, could at least parts of the Lake pluton (Questa); MA—Mount Aetna pluton (Sawatch); MP—Mount Princeton batho- 2–3 m.y. crystallization span of the Questa-Latir lith (Sawatch); RH—Rio Hondo pluton (Questa). Inset: Zircon ages for four samples from intrusive suite record protracted assembly of Rio Hondo pluton (fig. 2 in Tappa et al., 2011); range of weighted-mean ages between sam- an intermittently reactivated crustal-scale mag- ples is no greater than that among individual zircon analyses within a sample (although matic system? most analyses are for multiple crystals), and no distinct gaps in single-crystal ages are present between samples. Data for two samples of the Rio Hondo pluton, MZQ-33 and MZQ-9 Other Cordilleran-Arc Systems (Tappa et al., 2011), are linked to their V-shaped tie lines to illustrate data source. MSWD— mean square of weighted deviates. (B) Fish Canyon Tuff (same y-axis scale as A): CA-TIMS Recent studies of volcanic areas and asso- ages on individual zircons document a minimum crystallization duration of ~450 k.y. (24 ciated upper-crustal batholiths elsewhere in analyses, modified from fig. 5 in Wotzlaw et al., 2013), while SIMS age ranges between crys- the Cordillera provide additional perspectives tal surface and polished interior indicate a crystallization duration >1.1 m.y. in the magma that augment data available from the SRMVF. source of a single ignimbrite (Coble, 2013). These durations, as long as or longer than those Results from a few especially pertinent areas are observed among multiple samples from individual SRMVF intrusions (plotted in A), raise summarized briefly here. questions concerning the significance of weighted-mean zircon ages in interpreting assembly durations of plutons (see text discussion). (C) Deeper plutons: Tuolumne igneous com- Organ Caldera and Batholith, New Mexico plex of Sierra Nevada batholith (data from Memeti et al., 2010). Age ranges among crystals The Organ Mountains, farther south in the within a single sample and between samples from an intrusive unit are much longer than Rocky Mountains of New Mexico, expose a for the SRMVF subvolcanic intrusions plotted in A. (Note 4× change in scale.) Do deeper composite batholith, from surface levels to intrusions record longer crystallization histories (e.g., to 2.5 m.y. duration among 11 zircon >6 km depth, that intrudes a thick intracaldera crystals in a single sample from equigranular Half Dome Quartz Monzonite)? Older zircon ignimbrite (Seager and McCurry, 1988). Com- ages (light-color rectangles) were interpreted as antecrysts by Memeti et al., but continuity bined 40Ar/39Ar sanidine and sparse U-Pb zir- in ages and overlap in analytical uncertainty (Inset: from fig. 10in Memeti et al., 2010) in con data show that ignimbrite and intrusions this and other Tuolumne samples alternatively suggest interpretation as prolonged crystal- are closely similar in composition and age, at lization in a long-lived body, recurrently recharged by hot mafic magma. Abbreviations: 36–35 Ma (Zimmerer and McIntosh, 2013; EHD—equigranular Half Dome Quartz Monzonite; PHD—porphyritic Half Dome Quartz Rioux et al., 2010). These relations are com Monzonite; KCL—Kuna Crest lobe; CPL—Cathedral Peak lobe.

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Sierra Madre Occidental, Mexico SOUTHERN ROCKY MOUNTAIN VOLCANIC FIELD Several large-volume rhyolitic ignimbrites SUBCALDERA PLUTONS from this vast Cordilleran-arc province have Surface-interior yielded U-Pb zircon populations 1–4 m.y. older 1.0 age range, than the eruption age as determined by K/Ar

. (1.1 m.y.) 40 39 SIMS, N =39 and Ar/ Ar; the range of ages was interpreted

, m.y (Coble, 2013) as indicating derivation of much of the zircon by remobilization of partially molten to solidi-

) fied rocks formed during preceding phases of Sierra Madre volcanism (Bryan et al., 2008). In ystals

or a sample the volcanic province of Sierra Madre del Sur, field relations and LA-ICP-MS zircon ages from the Tilzpotla caldera and batholith suggest pro- 0.5 Multiple crystals longed semicontinuous assembly of a volcano- (0.45 m.y.) plutonic system starting ca. 39.5 Ma, which Questa-Latir Princeton-Aetna CA-TIMS, N =24 climaxed with eruption of the Tilzpotla ignim- CA-TIMS CA-TIMS (Wotzlaw et al, (Tappa et al., 2011) (Mills & Coleman, 2013) 2013) brite and associated caldera at 34.3 Ma. Cores of , multiple analyses f (some analyses: 2-3 cr RH MA MP individual zircons have ages as much as 2 m.y. 6 6 4 older than rims or K-Ar ages for the same unit (Martiny et al., 2013, their table 3).

ge range 6 A CL 44 4 5 Altiplano-Puna Volcanic Complex, Andes The well-documented APVC of the central 0 0.5 FISH CANYON TUFF Andes (de Silva, 1989; Schmitt et al., 2002; de Age range (wt. mean, m.y.), (age range, single sample) 2-4 samples from same pluton Silva et al., 2006; Salisbury et al., 2011), other ABthan its younger age (10–1 Ma), is closely com- TUOLUMNE BATHOLITH, CA parable to the SRMVF (table 4 in Lipman and McIntosh, 2008). At Cerro Galan caldera, SIMS MARGINAL LOBES zircon ages from a sequence of nine large silicic (CA-TIMS: Memeti et al., 2010) 4 EHDL 889 ignimbrites erupted between 2.0 and 5.6 Ma (Note di erent scale: 4x) provide direct evidence of prolonged crystallization (Folkes et al., 2011). Interiors of zircons commonly crystallized up to several hundred , m. y. thousand years prior to eruption, and zircons 6 from many ignimbrites contain antecrystic 3 KCL cores from a previous cycle in the Cerro Galan sequence. At the long-lived, lava-dominated EHD Aucanquilcha volcanic cluster, semicontinu- 11 ous zircon age spectra from individual samples CPL extend to as much as 2 m.y. older than eruption 8 ages, durations consistent with widespread dis- 2 10 solution of antecrystic zircon during the thermal Number of peak of magmatism (Walker et al., 2010). 9 crystals 6 analyzed Elkhorn Mountain Volcanics and Boulder Batholith The 50 × 100 km Butte Quartz Monzonite multiple zircons within a sample and associated smaller plutons of the Boulder 1 batholith were intruded to shallow crustal PHD levels, roofed largely by ignimbrites of the 6 closely related Elkhorn Mountain volcanics

ge rang e, 6

A (Robinson et al., 1968; Tilling, 1974; Hamilton and Myers, 1974). Recent SIMS determinations document zircon crystallization in plutons 0 123 from 81 to 73 Ma, an 8 m.y. span that is com- Age range (wt. mean, m.y.), parable to the duration of ignimbrite eruptions in continental-margin arcs such as the SRMVF 2 samples from same lobe C and APVC. Internal resorption boundaries and core to crystal-surface differences of as much as Figure 8. several million years within individual zircons

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have been interpreted to record complex pro- events during potentially prolonged assembly analyzed zircons characterized by simple mor- cesses, including multiple episodes of crystal- intervals. Such relations also bring into question phology and texture, while others have analyzed lization and magma-reservoir replenishment whether reliable information concerning dura- more diverse crystal suites. that spanned several million years during zircon tion of magma assembly and pluton construc- Despite such limitations, existing data sug- growth (Aleinikoff et al., 2012). tion can be obtained from weighted-mean ages gest that basement zircon xenocrysts are rare from multiple analyses of whole-zircon crystals in SRMVF intrusions that are closely related Adamello Batholith, Northern Italy (Miller et al., 2007; Schmitt, 2011; de Silva and to ignimbrite calderas within areas of the geo- Recent CA-TIMS zircon studies at the 43 to Gregg, 2014). physically defined subvolcanic batholith, while 33 Ma Adamello batholith have provided per- at least some outlying Tertiary intrusions, espe- haps the most precise data thus far for any com- CRUSTAL XENOCRYSTS IN cially those of relatively small size, have more parably complex granitic system (Schaltegger THE SRMVF AND ASSOCIATED diverse zircon populations (Table 4; Fig. 9). et al., 2009; Schoene et al., 2012). Zircon ages INTRUSIONS Particularly informative are relatively large from the composite Re di Castello pluton at the LA-ICP-MS data sets that document at most southern end of the batholith have been inter- Because geochemical evidence is compelling a few percent of basement-derived zircons in preted to indicate 1.5 m.y. of crystallization, that generation of SRMVF and other Cordi caldera-related intrusions: Princeton batholith with zircons from individual samples spanning llera-arc magmas involved voluminous melting (3 of 238 analyses: 8 samples), Aetna (2 of as much as 700 k.y. (Schaltegger et al., 2009). and assimilation of crustal basement, provid- 65 analyses: 2 samples), and Organ (1 of 185 Because the analyses are from whole zircons, ing as much as 50% of erupted magma (e.g.,, analyses: 7 samples), as reported by Zimmerer the measured durations of growth likely are DePaolo, 1981; DePaolo et al., 1992; Johnson, and McIntosh (2012a, 2013). In contrast, some minima for assembly and crystallization. 1991; Perry et al., 1993; Ducea and Barton, outlying western San Juan intrusions peripheral 2007; Farmer et al., 2008; Kay et al., 2010), the to the geophysically defined batholith contain Sierra Nevada Batholith presence or absence of basement-derived zircon abundant basement-age zircons; e.g., more than Documentation of an ~8 m.y. span of zircon provides information on processes of magma 50% of analyses from the Ophir and Wilson ages (94–86 Ma) from granitic units of the con- evolution and pluton assembly. The presence of stocks (Table 4; Gonzales and Pecha, 2015). centrically zoned Tuolumne igneous suite (Fleck even small relict zircon cores from Proterozoic Zircon age populations are likely to be more and Kistler, 1994; Coleman et al., 2004), fol- rocks (1400–1800 Ma) beneath the SRMVF complex and difficult to interpret in ignimbrites, lowed by similar results for other Sierran intru- should be recognizable; a relict core only 1% by because eruption and transport processes may sions (Davis et al., 2012; Frazer et al., 2014), volume would increase apparent age of a mid- incorporate xenocrysts from the vent and/or has stimulated much controversy concerning Tertiary zircon by ~15 m.y. ground surface. For example, 10 zircon deter- assembly duration and crystallization history Survival of basement xenocrysts could imply minations for an APVC ignimbrite pumice of Cordilleran plutons. The multimillion-year either (1) a low-temperature history of magma yielded a tightly clustered array defining an age durations of zircon crystallization in these plu- generation and pluton assembly, which appears of 4.65 ± 0.13 Ma, in agreement with the K/Ar tons are impressively longer those obtained inconsistent with compositions of many SRMVF biotite age for the same sample, but 13 zircons by similar methods for shallower subvolcanic intrusions (relatively mafic granodiorite), from a bulk sample of this ignimbrite yielded intrusions such as those associated with the or, more likely, (2) rapid magma generation, a spectrum of older APVC ages (9–13 Ma), as SRMVF. The large spans of Sierran zircon ages emplacement, and crystallization. Alternatively, well as some Paleozoic basement ages (Schmitt have been interpreted as requiring incremental sparseness or absence of zircon xenocrysts in et al., 2002). Nevertheless, several detailed zir- intrusion in small sheet-like batches, followed SRMVF magmas would imply initial melting/ con studies of the Fish Canyon Tuff detected no by rapid solidification that precluded existence assimilation of basement rocks in a Zr-under- surviving Precambrian component in this large of eruptible magma comparable in volume to saturated environment and/or dissolution during SRMVF ignimbrite (Bachmann et al., 2007c; large ignimbrites. Processes of pluton-batho- episodes of later magmatic recharge at tem- Wotzlaw et al., 2013; Coble, 2013). For the lith construction were thus interpreted as dis- peratures above zircon saturation. The absence 34 Ma Badger Creek Tuff, erupted from Mount connected from generation of magma bodies of xenocrysts is unlikely to have resulted from Aetna caldera, only one of 31 zircons in a bulk capable of large-volume explosive volcanism separation of crustal melts from a restite that sample analyzed by LA-ICP-M has a Protero- (Glazner et al., 2004, 2008; Bartley et al., 2005; retained zircon crystals, because andesitic to zoic age (Zimmerer and McIntosh, 2012a). Both Mills and Coleman, 2013; Frazer et al., 2014). dacitic SRMVF magmas have ordinary to rela- these crystal-rich dacitic ignimbrites contain Other results, especially for CA-pretreated tively high Zr contents for calc-alkaline systems complexly resorbed crystal cargos, indicative of single crystals, have documented additional (mostly in range 175–250 ppm; e.g., CD-ROM recharge and mixing with mafic magmas, yet, complexities in the zircon geochronology of tables in Lipman, 2006, 2012). like associated SRMVF intrusions, they retain Sierran plutons (Matzel et al., 2006b; Crowley Evaluation of xenocryst distribution as func- little record of crustal melting and assimilation et al., 2006; Memeti et al., 2010; Frazer et al., tions of diverse variables (pluton composition, during magma generation. Precambrian zircon 2014), broadly similar to those just summa- age, size, relation to ignimbrite eruption) is lim- ages were modestly more abundant in a bulk rized for ignimbrites such as the Fish Canyon ited by sparse data for SRMVF plutons, as well sample of Wall Mountain Tuff (7 of 40 LA-ICP- Tuff (Fig. 8B). Multiple crystals from some as uncertainties resulting from diverse sample MS spot analyses; Zimmerer and McIntosh, individual samples define a continuum of ages processing. Typically, only small numbers of 2012a), perhaps representing crystals from vent spanning as much as several million years (Fig. zircons have been analyzed in TIMS studies, walls and the ground surface during eruption 8C). As evaluated here, these ages provide and many of the larger SIMS and LA-ICP-MS and emplacement of this first large ignimbrite in insights into the duration of late crystallization data sets are dominated by rim and surface the SRMVF. Contrary to the inference that zir- as pluton construction waned, but they may analyses in effort to date final crystallization. con dissolution should be slower than magma- preserve only an incomplete record of earlier Also, some studies may have preferentially body assembly in silicic magmas (Frazer et al.,

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ertiary 32 26 36 12 5 5 6 7 13 1 0 0 0 3 4 3 0 0 4 00 61 16 1 1 2 18 Number pre-T

Y INTRUSIONS 40 0% 40 0% 00 0% 90 0% 90 0% 00 0% 0 7 5 8 9 7 0 4 5 52 3.1% 0 300 100 100 900 –– 50 50 40 31 3 2 8 5 5 1 33 2 2 1 27 52 51 5 5 700 18 170 18 71 1 238 Zircon TIAR VERAGE: VERAGE: VERAGE: analyses A A A 13 7 19 16 23 11 26 15 1 2 7 2 2 3 8 2 1 3 26 1 12 52 64 41 samples Number of YSTS IN MID-TER ) ) 9 4 . . 6 4 3 3 ( ( 4 3 4 6 7 7 5 5 . . – . . . . 5 4 0.16 71 6 6 6 3 4 7 4 5 4 3 12 – (Ma)* laser ablation-inductively coupled plasma mass spectrometry; SIMS 3 3 38–20 18. 8– 30–35 33. 84 27.55 3 28.02 28.02 28.02 28. 51 31.9? 36. 91 34. 41 2 2 3 3 26. 71 6 3 2 68. 11 26. 12 3 35. 82 3 35. 21 25.5 - - 5.2–24.5 6.6 (27.6) – – – ~ 8 9 23.0–22.5 . . 5 4 3 3 Main zircon crystal age 4 5 2 7 7 62 4 6 3 8 8 3 53 62 62 – – 58 72 6 66 6 6 65 6 7 2 2 1–66 SiO ~75 ~74 ~68 74–76 70–76 70–77 66–70 66–69 66–68 64–67 67–69 7 7 69–71 68–76 66–74 (wt %) ABLE 4. BASEMEN T ZIRCON XENOCR T o c) c i AZ s x s e l l k a i V M a s k P M N , k e c M , e N k c a n o l P a N o c o t , t n o o z t r t c , s a n a o a i f t n u d e r n r a p a b k i s t l c a e a a a n m ll Mountai n~ c h e e a l n t n g z R Coxcatlan i C Buenavist a6 Resurgent S Ring fault zon e7 Rio Hondo a l p r u a o e l r e o i Kos Plateau, Greece Sierra Madre, Mexico Peach Spring, Caetano, NV Nelson Mountai n6 Carpenter Ridge Fish Canyon Fish Canyon Fish Canyon Chiquito Peak Black Mountain Badger Creek O Squaretop (ma ﬁ Intrusion Wa C Bernardo Peak B Jackson Mountai n6 O P C H Hermosa Peak T A B Late granite s7 Silverton/Sulta n6 Q TIMS—thermal ionization mass spectrometry; LA-ICPMS— *Age of associated ignimbrite given in parentheses. t Distal intrusions, W and S San Juan (mainly laccoliths): Large ignimbrites: SRMVF caldera-related: Other caldera cycles:

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PRECAMBRIAN ZIRCON XENOCRYSTS IN MID-TERTIARY INTRUSIONS, tight cluster of zircon ages that record autocryst SOUTHERN ROCKY MOUNTAINS & ELSEWHERE growth during rapid cooling of the intrusive 100 batch, rather than the overlapping broad spec- Caldera intrusions, SRMVF Ophir, tra of ages (105–106 yr) commonly observed by Sawatch Range to 100% Princeton TIMS analyses for subcaldera and deeper plu- Aetna tons (Figs. 7–8). 80 San Juan Squaretop San Luis AGE-DEPTH RELATIONS: Silverton

tiary zircons INTRUSIONS ASSOCIATED WITH

er Questa, NM T IGNIMBRITE CALDERAS Resurgent Ophir 60 S ring zone Rio Hondo Further insights concerning magma assembly

ian/mid-

r Wilson and crystallization come from age and depth Distal intrusions, SRMVF relations between caldera-related intrusions and San Juan recamb associated ignimbrites (Fig. 10). Data from the P Other caldera intrusions SRMVF and elsewhere suggest that shallow

atio:

r Organ, NM caldera plutons, especially ones that resurgently Caetano, NV intrude caldera-filling tuff, tend to have crystal- Tilzapotla, Mexico entage lization ages similar to those of the associated 20 Blackface ignimbrite (Table 5; Fig. 10A). In contrast, intru-

rcPe sions at deeper levels, beneath or adjacent to an ignimbrite caldera, tend to be variably younger. Jackson For example, the diverse intrusions associated with the Questa caldera decrease in age from 40 35 30 25 AGE, Ma 25.5 to ca. 23 Ma (Tappa et al., 2011; Rosera et al., 2013) as exposure depth deepens south- Figure 9. Presence of Precambrian zircon xenocrysts in mid-Tertiary granitic intrusions, ward (Lipman, 1988). Deeper intrusions such SRMVF and elsewhere (data from Table 4). Xenocrysts are near-absent in caldera plutons as Rio Hondo, south of the caldera, have even but common in satellitic bodies that are peripheral to the San Juan batholith: This is inter- younger crystallization ages (23.1–22.6 Ma; preted as evidence for prolonged zircon-undersaturated/long-mush-stage assembly of cal- Tappa et al., 2011), raising interpretive questions dera intrusions, and more rapid emplacement and crystallization at smaller satellitic intru- about genetic relation to the ignimbrite magma. sions. NM—New Mexico, NV—Nevada. A schematic interpretation, based on zircon- crystallization ages in subcaldera SRMVF intrusions (Fig. 10B), infers large variations in upper 2014), basement zircon xenocrysts are sparse mining conditions under which preexisting zir- portions of vertically extensive subcaldera plu- to absent in many other major ignimbrites that con crystals could be resorbed during recharge tons but overlapping ages at greater depths. have been well characterized: the 161 ka Kos events (Watson, 1996; Frazer et al., 2014) The age-depth correlations are variable due in Plateau Tuff in Greece (Bachmann et al., 2007a; likely include melt composition, pressure, tem- part to limits of analytical resolution, but they Guillong et al., 2014), the mid-Tertiary ignim- perature, and volatile content, duration of such also reflect geologic variability. Some intrusion brites of the Sierra Madre Occidental (Bryan Zr-undersaturated intervals, and stage during ages plotted in Figure 10A have large uncertain- et al., 2008; Martiny et al., 2013), the Late Ter- construction of the magma body. A related com- ties, especially LA-ICP-MS results, and some tiary La Pacana and Cerro Galan ignimbrites of plexity is that a growing system may become U-Pb crystallization ages may be anomalously the APVC (Schmitt et al., 2002; Folkes et al., increasingly armored to contamination by old old because of presence of antecrysts. Contin- 2011), the 35 Ma Caetano Tuff and associated basement as earlier-formed intrusions comprise ued intrusive activity preferentially along cal- granitic intrusions in Nevada (John et al., 2009; the host rock for later magmatic increments, dera structures would be expected at long-lived Colgan et al., 2012), the 18.8 Ma Peach Springs but even the earliest-erupted ignimbrites in the magmatic systems for varied intervals after Tuff in Arizona (McDowell et al., 2014), and the SRMVF and elsewhere in the Cordillera seem an ignimbrite eruption. Magma-body assem- Oligocene Black Mountain, Chiquito Peak, Car- generally to lack zircon xenocrysts. bly that leads to a large ignimbrite eruption penter Ridge, and Nelson Mountain Tuffs of the The contrasts in xenocryst populations would involve major disruption of the upper SRMVF (M. Zimmerer and M. Verdon, 2014, between small distal versus caldera-related crust, generating zones of structural weakness written commun.). intrusions (Fig. 9) also suggest that plutons that could facilitate continued emplacement of Thus, the apparent paucity of xenocrystic and batholiths constructed by multiple small younger intrusions. In addition, a recurrently basement zircons both in large silicic ignim- intrusive batches that crystallized rapidly, as replenished long-lived magmatic system need brites and in caldera-related intrusions (Table proposed for SRMVF intrusions (Tappa et al., not wane at the time of any single large ignim- 4) favors interpretations of prolonged magma- 2011; Mills and Coleman, 2013) and for Sierra brite eruption. For example, in the SRMVF, body assembly accompanied by intermittent Nevada intrusive suites (Glazner et al., 2004; multiple large ignimbrites were erupted dur- crystal dissolution, involving temperature Davis et al., 2012; Frazer et al., 2014), should ing prolonged spans from within the same site, cycling above/below zircon saturation during contain bimodal zircon populations. At least such as the five ignimbrites from Platoro caldera mafic-recharge events, as also proposed in some some samples would be predicted to preserve complex at 30.1–28.6 Ma, or the nine tuffs from reports cited earlier. Critical variables in deter- abundant basement xenocrysts along with a the central cluster at 28.7–26.9 Ma (Table 1).

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Figure 10. Observed and interpreted age-depth relations in ignimbrites and subcaldera intrusions. (A) Observed age-depth relations: major ignimbrites and subcaldera intrusions (data from Table 5). Crystallization ages for many shallow-caldera intrusions are indistinguishable, within analytical uncertainty, from associated ignimbrite. Intrusions that are younger than associated ignimbrite tend to be deeper in subvolcanic crust, interpreted to record delayed late-stage crystallization as the system wanes in a ther- mally mature crust. Intrusion-depth index of wall rocks (varied by ±0.2, for plot visibility): 1—caldera-fill or wall; 2—caldera-floor volcanic rocks; 3—uppermost basement; 4—deeper basement. Magmatic duration: time interval between ignimbrite eruption and intrusion-crystallization age. Small negative values in part reflect sizable analytical uncertainties, especially for mid-Tertiary intrusions dated by U-Pb LA-ICP-MS or SIMS methods. SRMVF—Southern Rocky Moun- A tain volcanic field; NM—New Mexico; NV—Nevada. (B) A schematic interpretation of zircon-crystallization ages Ignimbrite eruption in a subcaldera pluton that was assembled incrementally during an ~5 m.y. interval, in relation to time of ignimbrite eruption and fluctuating intensity 1. Zircon ages, ignimbrite of mafic recharge. In detail, spectrum & pluton roof of zircon ages at any level and their progression with depth are likely to 0 differ substantially from pluton to pluton, as a function of varied intensities

of mafic recharge, thermal histories, km 2. Zircon ages, upper-crustal pluton zone state of zircon saturation in the domi- (5–10 km, granodiorite) nant magma volume, and other factors: 10 (1) Within the ignimbrite and at the pluton roof, bulk of zircons are crystallized within ~0.5 m.y. prior to erup- 3. Zircon ages, deep-pluton zone

Depth in pluton, (15–20 km, tonalite) tion, while preserving sparse antecrysts 20 from intermittent precursor magmatism (dashed line) or completely lacking them (e.g., Fish Canyon Tuff; Fig. 4. Intensity, ma c recharge 6A). (2) At upper crustal levels within the granodioritic phase of the pluton, peak time of zircon crystallization may B be delayed to 1–1.5 m.y. younger than 5 4 3 2 1 0 associated ignimbrite, due to continued Time, m.y. mafic input and associated heating that promotes resorption of many earlier-crystallized zircons and delays final solidification (e.g., Princeton batholith, in relation to Wall Moun- tain Tuff; Fig. 7B; also Mills and Coleman, 2013). (3) At deep crustal levels, final solidification and associated zircon growth may be nearly concurrent with crystallization at upper levels within the pluton (Coint et al., 2013), or further delayed until termination of mafic recharge. (4) Intensity of recharge by mafic mantle-derived magma fluctuates over a 4.5 m.y. interval, peaking shortly before ignimbrite eruption. Dashed and solid lines portray possibly variable wavelengths of recharge intensity; data are largely lacking to constrain such alternatives.

726 Geosphere, June 2015

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/3/705/3334629/705.pdf by guest on 26 September 2021 Ignimbrites to batholiths ) n n l continued ( eruption esurgent intrusio n esurgent intrusio n esurgent intrusio n esurgent intrusion esurgent intrusio n hallow resurgent intrusio n Southern pluton, deeper leve Southern pluton, roof zone Central resurgent intrusion Related to postcollapse lavas Southern caldera ring intrusio Resurgent intrusio n Resurgent intrusion Resurgent intrusion Southern caldera ring intrusio Shallow resurgent intrusion Uncertain relation to caldera Southern ring intrusio n Post-resurgent dacite dom e Southern caldera ring intrusion Notes )S )R 1) 1) 1) 1, 2015) AND ELSEWHERE ppa et al. (201 (2012a) (2012a) (2012a) written commun. ) written commun. ) (2012b) (2012b) (2012b) ove et al. (2001) ove et al. (2001) ove et al. (2001) immerer and McIntosh ipman et al. (2013 )R ipman et al. (2013 )R ipman et al. (2013 )R ipman et al. (2013 )R appa et al. (201 acob et al. (2015) appa et al. (201 acob et al. (201 appa et al. (20 11 appa et al. (20 11 Zimmerer and McIntosh Zimmerer and McIntosh Ta Rosera et al. (2013) Mills and Coleman (2013) Rosera et al. (2013) Mills and Coleman (2013) Mills and Coleman (2013) M. Zimmerer (2014, Rosera et al. (2013) Mills and Coleman (2013) M. Zimmerer (2014, Data source Zimmerer and McIntosh T Zimmerer and McIntosh Zimmerer and McIntosh 3.9 1. 2L 3. 8T 3.9 1Z 3.2 2.8 1. 2L 1 1.8 3.9 3J 1 1 1.2 3.9 2 1J 2B 2L 2B 1B 2. 2T 2L 0. 9T 1.8 1.8 2.2 0.9 index* Depth ADJACEN T REGIONS, .)

1.75 2.93 2.25 2.54 2.04 0.18 1.61 1.88 0.32 1.45 0.41 0.04 1.80 0.96 0.43 0.25 0.01 0.17 0.44 0.01 (m.y –0.19 –0.13 –0.48 –0.24 –0.07 –0.18 –0.12 Duration , , A-TIMS LA-ICP LA-ICP Ar-Ar Ar-Ar Ar-Ar CA-TIMS CA-TIMS Ar-Ar CA-TIMS CA-TIMS Ar-Ar CA-TIMS CA-TIMS CA-TIMS TIMS CA-TIMS Ar-Ar Ar-Ar Ar-Ar Method CA-TIMS Ar-Ar TIMS Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar CA-TIMS Ar-Ar CA-TIMS 1 .08 .09 .02 .07 .09 .08 .13 .04 .10 .04 .04 .0 8C .1 .10 .15 .07 .03 .04 .04 .17 0.3 0.2 0.04 0.10 0.01 0.09 (avg 5) ±0 ±0 ±0 ±0 ±0 ±0 ±0 ±0 ±0 ±0 ±0 ± ±0 ±0 ±0 ±0 ±0 ±0 ±0 ±0 ±0 Age (Ma) 5.35 4.95 35. 5± 33.38 22.59 24.48 35. 0± 24.91 22.98 33.01 34.60 25.20 35.37 28.975–28.742 34.95 23.17 25.38 25.64 35. 8± 27.98 28.01 5± 26.39 32.94 33.26 26.66 23. 11 25.3 8± 25.09 IGNIMBRITE ERUPTIONS: SRMV F, TO IONS Granodiorite Granodiorite Granodiorite Granite (BG) Granodiorite Granite (SG) Granite Granodiorite Qtz monzonite Granite (RR) Granod., lower Granodiorite Qtz monzonite Granodiorite Peralk. granite Peralk. granite Granod., upper Granodiorite Syenogranite Granodiorite Aplitic granite Granodiorite Granodiorite Dacite Granite Granit e2 Granodiorite Granit e2 Composition ., west ., mid ., east rquoise Mine irgin Canyon irgin Canyon Princeton Eagle Gulch Rio Hondo-deep Princeton S ring intr Rio Hondo-top Rawley Gulch Aetna ring, f.g. S ring intr Princeton Mt Richthofen Aetna ring intrus. Alpine Gulch S ring intr Princeton Alamosa River Mount Cumulus Capitol City Tu Spring Creek Sultan Mountain Red Mountain Pinabete Pinabete Cabresto Lake Rito del Medio V Intrusion V OIDS: DEPTH-AGE REL AT Ar-Ar Ar-Ar CA-TIMS CA-TIMS Ar-Ar Ar-Ar CA-TIMS Ar-Ar CA-TIMS CA-TIM S CA-TIMS CA-TIMS Ar-Ar CA-TIMS Ar-Ar CA-TIMS CA-TIMS Ar-Ar CA-TIMS Method CA-TIMS Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar .08 .05 .06 .08 .06 .05 .05 .08 .05 .05 .08 .02 .04 .09 .03 .05 .07 0.06 0.06 0.06 0.06 0.04 0.06 0.06 0.04 0.06 TED GRANIT ±0 ±0 ±0 ± ±0 ±0 ± ±0 ±0 ± ±0 ± ±0 ±0 ± ± ± ± ± ±0 ±0 ±0 ±0 ±0 ±0 ±0 Age (Ma) 37.25 33.19 25.52 25.52 37.25 34.26 26.52 33.19 34.47 26.52 37.25 25.52 33.19 34.47 25.39 25.52 25.52 25.39 25.52 37.25 22.93 25.39 28.39 28.05 28.19 33.19 27.62 eld, Colorado

ll Mountain ABLE 5. CALDERA-RELA all Mountain T adger Creek apinero Mesa onanza Wa Sunshine Pk Ignimbrite Amalia Chiquito Peak [unnamed] Crystal Lake eS nW aB Aetn aB Southern Rocky Mountain volcanic ﬁ Uncompahgr Mt Never Summer Lake City Caldera Questa - Latir magmatic system, Northern New Mexico Questa Platoro Silverton Bonanz Mt Princeto

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Such recurrent large eruptions, within time spans that are relatively brief in comparison to t durations of zircon crystallization documented for some plutons (Figs. 7–8), provide direct evidence for long-lived recurrently reactivated

) magma bodies. In contrast, some relatively small upper-crustal intrusions record rapid cooling and crystalliza- esurgent intrusio n ntruded along ring faul continued Resurgent intrusion Notes Resurgent intrusio n tion over sizable depth ranges. Near-surface and deep samples from a 4-km-deep drill hole into the geologically young Eldzhurtinsky Granite (Tirniauz, Russia), which may be associated with an ignimbrite eruption (Gurbanov et al., 2004), have yielded analytically indistinguish-

AND ELSEWHERE ( able weighted-mean SIMS-zircon ages of 2.04 ± John et al. (2014) John et al. (2014) John et al. (2014) (2013) (2013) enry and John (2013); enry and John (2013); 0.03 Ma; slightly greater dispersion of ages in acon et al. (2014) ucker et al. (2007) ucker et al. (2007) Data source du Bray et al. (2004) McDowell et al. (2014) Henry and John (2013); T T Ferguson et al. (2013) John et al. (2008, 2009) Lipman (1993 )I Gazis et al. (1995 )R Zimmerer and McIntosh Rioux et al. (2010) Zimmerer and McIntosh Rioux et al. (2010) the deeper sample likely reflects slower cooling (Grun et al., 1999). Biotite 40Ar/39Ar ages from REGIONS, 1.1 1.2 1 1.1 1.1 1H 1.2 1.1 1H 1.2 1.1 1B 0.8 0.8 0.8 0.8 index* Depth the drill hole decrease with depth from 1.9 to

.) 1.5 Ma and are also interpreted as recording cool- 0.04 0.15 0.32 0.30 0.36 0.5 0.01 0.27 0.1 0.0 0.88 0.53

(m.y ing history (Gazis et al., 1995). Multiple sheets at –0.03 –0.01 –0.47 –0.12 Duration ADJACENT

, the shallow Torres del Paine laccolith of Miocene trusion; BG—Bear Gulch intrusion.

, , age (12.5 Ma) in southern Chile are interpreted from TIMS zircon ages as recording assembly Method LA-ICP LA-ICP TIMS Ar-Ar U-Pb SIMS CA-TIMS CA-TIMS SIMS Ar-Ar U-Pb Ar-Ar Ar-Ar TIMS Ar-Ar SIMS Ar-Ar SIMS sement rocks; 4—deeper basement. within 90 ± 40 k.y. (Michel et al., 2008). Similar .09 .05 .03 processes and durations likely characterize other 0.08 0.04 0.2 0.4 0.3 0.1 0.50 0.50 0.2 1.2 relatively small tabular intrusions. ± ± ±0 ±0 ±0 Age (Ma) In addition to age-depth variations for large .3 6± .4 2± 8.81 3.78 2.84 caldera-related intrusions, deeper intrusions 18.6 3± 26. 9± 28. 9± 24. 8± 36.5 35.15 35.5 36.15 25. 0± 73.0 70. 0± appear to have longer time spans of zircon crystallization, based on CA-TIMS data among crystals from individual samples. In the SRMVF IGNIMBRITE ERUPTIONS: SRMVF

TO (Fig. 8), zircon age spans from single samples Felsite Dacite porphry Granit e3 Granodiorite Granite Composition Quartz diorit e3 Granite Granite Syenite Syenite Granodiorit e1 Granit e3 Granodiorite Granodiorite Granodiorite Gran. porphyry of relatively shallow caldera-related plutons are typically in the range 0.2 m.y. or less at Questa TIONS (Tappa et al., 2011; Rosera et al., 2013) and at Princeton-Aetna (Mills and Coleman, 2013). In contrast, individual samples from granitic plu- pluton tons at deeper intrusion levels have commonly imes porphyry Resurg intrusion Nooksack Cirque T Chalk Mountain Intrusion Icy Peak IXL Organ Needle-top Organ Needle-top Organ N-deep Organ N-deep Moss porphyry Carico Lake Freeman Creek Amole Central intrusion Middle Fork yielded longer spans: 0.7 m.y. at Adamello (Schaltegger et al., 2009), as much as 1.2 m.y. in r r S A A the North Cascades (Matzel et al., 2006a), and - - M r r I OIDS: DEPTH-AGE RELA A A S Ar-Ar Ar-Ar Ar-Ar Method Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar 0.5–2.5 m.y. at the Tuolumne complex (Memeti 5 2 0 0 0 5 et al., 2010). At the Bergell intrusion (Italy), age . .12 . . .01 .05 0 0 0 0.16 0.02 0.16 0.02 0.52 spans within individual samples are about as ± ±0 ± ±0 ± ± ±0 great as the span of weighted-mean ages from 7 3 TED GRANIT 2 4 8 Age (Ma) . . . .7 2± 5 9 2 the entire pluton (Samperton et al., 2013). The 2 26.94 25.12 2 36.03 73.1 36.0 3± 18.7 8± 33.79 69. 9± longer spans in some deeper intrusions might be due to presence of antecrysts unrelated to the active magma-assembly event (Matzel et al., 2005; Miller et al., 2007; Memeti et al., 2010), but the common absence of analytically detect- hyolite Canyon Ruth Mountain Cat Mountain Ignimbrite Hannegan Pea k3 Squaw Mountain Peach Springs Caetano Middle Fork able time gaps among such zircon age popula- ABLE 5. CALDERA-RELA

T tions (Fig. 8C, inset) permits interpretation that these semicontinuous age spectra provide a ) V a i V N s

, partial record of earlier assembly and crystalli- N s n , u o n zation during long-lived construction in deeper y R o ( n , y a n

m parts of a mushy pluton (Fig. 10B), involving a C e C g o sustained magma supply, slow cooling, and e c cson Mtn b *Intrusion-depth index (varied by ±0.2, for plot visibility): 1—caldera-ﬁll/wall; 2—caldera-ﬂoor volcanic rocks; 3—uppermost ba Abbreviations: gran.—granite; granod.—granodiorite; qtz—quartz; peralk.—peralkaline; RR—Red River intrusion; SG—Sulfur Gulch in h o urkey Cree kR o T Organ Mountains magmatic system, southern New Mexico Elevenmile Canyon, NV Caldera Hannegan Silver Creek P Middle Fork Ignimbrite-caldera systems elsewhere Organ Caetano, NV J Tu C intermittent zircon crystallization.

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Perhaps distinction should be made between Figure 11. Intrusion ages and antecryst populations for which ages suggest a compositions, in relation to Basalt-gabbro Granodiorite-Monzonite Dacite Rhyolite-granite continuum of crystallization (e.g., Fish Canyon distance radially from west 40 Tuff—Fig. 6A; Kos Plateau Tuff—Bachmann margins of the geophysically et al., 2007a; Guillong et al., 2014; Aucanquil- defined San Juan batholith cha—Walker et al., 2010; and granitic plutons (300 mGal contour of Drenth 30 San Juan Ignimbrite CC noted earlier herein) versus those where analyti- et al., 2012). More distal intru- SM are-up (32–26.9 Ma) cally significant gaps exist within crystallization- sions tend to be younger and age spectra (e.g., Taupo—Brown and Fletcher, more bimodal in composition. Ma

1999; Charlier et al., 2005; Crater Lake—Bacon Larger intrusions (granodi- e, 20 and Lowenstern, 2005; Mount St. Helens—Clai- orite-monzonite) tend to be Ag borne et al., 2010; Tarawerea—Storm et al., within or near the batholith 2012). Such apparent gaps in crystallization his- margin and close in age to 10 tory need not imply subsolidus cooling. They waning of ignimbrite erup- may result from periods of slow growth (e.g., tion. Caldera-related granitic (Within batholith)(Beyond) Watson, 1996), armoring within other mineral intrusions (Table 2): CC— 0 phases that interrupted growth, or prolonged stor- Capital City (Uncompahgre), –20 –10 0 10 20 30 age of melt and crystals that interacted intermit- and SM—Sultan Mountain Distance from batholith margin (300 mGal contour), km tently with hotter mafic influxes before erupting (Silverton). Many small dacite (e.g., Miller et al., 2007; Schmitt et al., 2010; de and rhyolite intrusions (22–14 Ma) are associated with the margins of the Silverton cal- Silva and Gregg, 2014). Gaps in antecryst popu- dera. Main data sources: Bove et al. (2001); Cunningham et al. (1994); Gonzales and Pecha lations seem more likely to survive and record (2015); Lipman et al. (1976); Naeser et al. (1980). significant magmatic events at central volcanoes that erupt relatively small volumes than during prolonged crystallization in larger long-lived ca. 26–5 Ma), the late intrusions include more- canic magma bodies (Annen, 2009; Paterson systems that lead to ignimbrite supereruptions. evolved silicic compositions than in the Oligo et al., 2011; Tappa et al., 2011; Gelman et al., In such large systems, zircon ages are inferred cene caldera-related magmatism; they have 2013). Some recent geochronologic and model- to preferentially record younger episodes, inter- long been interpreted as components of bimodal ing studies have tried to use variations in zircon- rupted by periods of thermal and compositional magmatism associated with regional extension crystallization ages among surface samples of rejuvenation, and provide an incomplete record and opening of the Rio Grande rift zone (Lip- a pluton to compute duration of magma-body of early magma assembly. man et al., 1970; Lipman and Mehnert, 1975; assembly. Combined with estimates of exposed Thompson et al., 1991). intrusion volume, such age ranges have been REGIONAL AGE DISTRIBUTION OF In contrast, no silicic intrusions or lavas of interpreted to indicate incremental tabular SHALLOW INTRUSIONS Miocene age have been identified centrally assembly of upper-crustal granites and “pluton- within the geophysically defined batholith, and fill” rates (0.002 km3/yr or less) that would be While most of the dated shallow caldera- timing of the shallow batholith-related magma- too low to accumulate magma capable of sup- related intrusions in the San Juan region are sim- tism appears broadly antithetic to that of the distal plying large ignimbrite eruptions (e.g., Glazner ilar in age to the associated ignimbrite (Table 2), intrusions (Fig. 11). Only volumetrically minor et al., 2004; Bartley et al., 2005; Annen, 2009; some intrusions near the margins of the geophys- mafic Hinsdale lavas (silicic basalt and basaltic Tappa et al., 2011; Davis et al., 2012; Mills ically defined batholith and most of the more- andesite), erupted from dikes or pipes, seemingly and Coleman, 2013; Schöpa and Annen, 2013; distal ones have younger ages (ca. 26.5–5 Ma). were able to penetrate central parts of the Oligo Frazer et al., 2014). As discussed previously, Major concentrations of such batholith-margin cene batholith (Lipman and Mehnert, 1975). however, the sample-to-sample variations intrusions include 23 to 20 Ma dikes and subvol- Major additions to the batholith during the Mio- in weighted-mean ages are open to alterna- canic plugs of dacite and rhyolite along the north cene, after waning of the ignimbrite eruptions, tive interpretations, and none of these studies side of the Platoro caldera complex (Lipman, seem improbable without surface volcanism. documented systematic age-depth progres- 1975; Lipman, W. McIntosh, and M. Zimmerer, Rather, the regionally contrasting space-time- sions within a pluton. Additionally, low pluton- 2013, written commun.) and small intrusions of compositional relations suggest that only small filling rates, obtained from the elevation range similar composition within and along the west volumes of silicic magma were generated dur- among surface exposures (e.g., ~1 m.y./km at margin of the Silverton caldera that have yielded ing the Miocene. These magmas were unable to Mount Princeton and Rio Hondo plutons in the ages of 26–10 Ma (Lipman et al., 1976; Bove penetrate central parts of the batholith, perhaps SRMVF; as much as ~4–7 m.y./km for Sierran et al., 2001). Distal sills and laccoliths of monzo- because still-warm subsolidus granitic rocks that intrusions such as Half Dome or Mount Givens; nite to granite and porphyritic rhyolite, including occupied much of the subvolcanic crust impeded Mills and Coleman, 2013; Frazer et al., 2014), bodies near Ophir and Rico west and southwest the rise of intrusions more silicic than basalt. would imply improbably lengthy assembly of Silverton, have ages from 26 to 5 Ma (Naeser durations (~100 m.y.!) for vertically extensive et al., 1980; Cunningham et al., 1994; Gonzales MAGMA-SUPPLY AND PLUTON- plutons (20 km or more) as inferred here for and Pecha, 2015). ASSEMBLY RATES SRMVF intrusions and those in tilted crustal These widely distributed post-ignimbrite sections (Fig. 5). intrusions of Miocene age record the waning of Especially challenging goals for volcano- In contrast to the large differences in U-Pb San Juan magmatism (Fig. 11). Concurrent with plutonic studies have been determination of the zircon ages within many samples (Fig. 8) and mafic lavas of the Hinsdale Formation (also supply rates and assembly durations of subvol- in relation to depth below associated volcanic

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rocks (Fig. 10A), some plutons exposed over multi-kilometer ranges have little detected variation in zircon age with depth: The top and /yr**) 3 0.05 0.16 0.02 0.04 0.18 0.13 0.06 0.05 0.13 0.10 bottom of the 4 km drill-hole section into the 0.24 Eldzhurtinsky granite are indistinguishable at (km otal magma supply

2.04 ± 0.03 Ma (Grun et al., 1999); the bulk T of the 10-km-thick Spirit Mountain pluton is within analytical uncertainty at 16.0–15.7 Ma

(Walker et al., 2006; Miller et al., 2011); ages /yr) 3 .050 0.122 0.07 9 0.066 0.009 0.021 0.092 0.063 0.030 0.026 from the roof zone and deepest exposed level in 0.02 6 (km

the ~12 km tilted section through the Jurassic olc+granitoid V magma supply Wooley Creek pluton differ by only ~0.5 m.y. (Coint et al., 2013); dates from the 12 to 15 km depth of exposures through the Bergell intru- 1 /yr) 3 .004 .005 .005 .004 .005 sion vary by ~2 m.y. (32–30 Ma, only modestly .005 0.01 0.00 50 0.007 0.006 0.001 greater than age ranges within single samples; (km vg. volcanic A Samperton et al., 2013). Such limited variations magma supply

in age with depth suggest widespread crystal- AIN VOLCANIC FIEL D lization deep in plutons mainly late during assembly as magma supply waned (Fig. 10B). .) MOUNT .7 .1 .5 l sections elsewhere. 1.10 0.45 6. 30 1. 90 1. 90 The “fill-rate” approach, based on averaged 6. 30 8. 00 (m.y whole-crystal zircon ages, is likely to mainly 12. 00 ( 38–26 Ma ) (26.9–26.8 Ma ) ( 37–35.5 Ma ) ( 33–26.8 Ma ) (28.45–28.0 Ma ) ( 29.1–28.0 Ma ) ( 28.7–28.0 Ma ) ( 28.7–26.8 Ma ) date late crystallization, incompletely recording ( 34.5–26.5 Ma ) Assembly duration early stages of long-duration pluton construc- )

tion. Also, interpretations made solely from sur- 3 face exposures are likely to underestimate the total magmatic volume and thermal budget in 0,00 0 0,00 0 10,00 00 13,10 00 27,00 01 TES, SOUTHERN ROCKY 11 11 270,000 348,000 240,000 378,000 408,000 vertically extensive magma bodies generated by 620,000 otal magma T volume** (km voluminous mantle input. RA ) Assembly of the SRMVF Batholith 3 (km § 1972; Drenth et al., 2012). – r, ,550 55,000 55,000 55,00 01 Alternatively, duration of batholith assembly 13,500 135,000 174,000 120,000 189,000 204,000 310,000 Mantle-basalt and time-space fluctuations in focused magma volume supply may be inferred from eruptive history, and Pakise AND MAGMA-SUPP LY

while overall volume of the magmatic system ff . 0 ) 3

can be estimated from geochemical and isotopic 0 0 , 0 6,00 06 data in conjunction with geophysical constraints (km 50,000 5 50,000 12,000 10,000 10,000 164,000 164,000 164,000 250,000

on batholith area and vertical extent. Using such ASSEMB LY . an approach, age and volume estimates from Granitoid volume*

the SRMVF may provide useful broad insights ONIC

concerning long-term magma-supply rates and ] ff ) 0 3 ] Tu pluton-assembly processes (Table 6; Fig. 12A), 0 ff 550 0 , (km 5,000 5,000 5 1,500 even though such rates and processes were Tu 10,000 25,00 01 10,00 01 25,000 40,000 60,000

likely modulated by shorter-term fluctuations olcanic volume and areal magmatic focusing. Varied time and V volume approximations are explored, from the ) 0 2 0 scale of individual ignimbrite-caldera cycles 00 ABLE 6. VOLCANO-PLUT 300 5 T , (km 2,500 2,500 8,200 8,200 8,200 8,200 2 (Creede, La Garita), to the composite San Juan AIN VOLCANIC FIELD: 25,000 batholith, and the entire magmatic system of 100,000 Exposed area the SRMVF. Magma volumes and supply rates are estimated separately for the volcanic rocks, MOUNT

the crustal granitoid batholith, and the overall † magmatic system, including mafic input from # # ff mantle sources (Table 6). /6 Tu THOLITH/REGION

Alternate assembly durations for construc- 20-km vertical extent of batholith (more maﬁc downward), based on gravity data (Drenth et al., 2012), and anlogies with crusta ≥ ) are-up only) ﬂ tion of the San Juan batholith and associated 3 1

3 3 0 2

volcanic ejecta (~400–500 × 10 km ; Table 3) ( tal magma volume and supply = volcanic + granitic mantle basalt ll Mountain e l tzlaw et al. (2013) Three alternative assembly durations, Fish Canyon magmatic system.

could be 2–8 m.y., depending whether peak Ratio could double, if mantle-underplate volume (cumulate plus restite) is included Alternative, minimum average batholith thickness: ~13 km: from gravity models (Plou b [duration since prior ignimbrite eruption, Masonic Park Wa ** To [duration since prior ignimbrite eruption, Nelson Mountain [zircon age span] [zircon age span] † § # *Preferred o SAN JUAN BA (ignimbrite San Juan batholith Entire SJ region OTHER SOUTHERN ROCKY Creede caldera & tuff La Garita caldera & tuff (calderas within gravity low) (entire volc duration) (area from Lipman, 2007; volume Farmer et al., 2008) System SAN JUAN CALDERAS/IGNIMBRITES San Juan batholith Princeton batholith Entire SRMVF C magma supply was focused during the caldera- Wo

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Figure 12. Estimated average magma supply and assembly rates for overall construction of plutons, and potential for development of eruptible magma bodies. (A) Estimated magma supply for subvol 106 canic caldera-related plutons of SRMVF (data from Table 6). Vol- A umes and supply rates for upper-crustal evolved magma (V+G: L G volcanic plus granitoid plutons) are only modestly lower than those 105 modeled for total magma supply (total M.S.: volcanic + granitoid ) pluton + mantle input). In all cases, the magma supply is sufficiently 3 /yr

3 (log Caldera, total M.S. high (>0.005–0.01 km /yr, from recent thermal models; Annen, 2009; 3 0.1 km 4 Caldera, V+G

Schöpa and Annen, 2013; Gelman et al., 2013) to support generation , km 10 C SJ Bath & Region, total M.S. of large eruptible magma bodies and long-lived mushy cumulate sys- 3 /yr km lume 3 /yr SJ Bath & Region, V+G tems as sources for SRMVF ignimbrites. The high modeled supply 0.01 Vo Other SRMVF, total M.S. rates in part reflect the large areal extent of the San Juan batholith 0.005 km 103 Other SRMVF, V+G (SJ Bath) and the whole SRMVF region, but high supply also char- 3 /yr acterizes models for individual calderas such as Creede (C) or La Minimum supply rate, large eruptible magma body: 0.001 km 0.01 km3/yr, Annen (2009), Garita (LG, three alternative durations plotted). Even higher supply 0.005 km3/yr, Gelman et al. (2013) and pluton-assembly rates would be likely when adjusted for volu- 102 metric areal and temporal fluctuations and magmatic focusing on the 0.1 1 10 100 many scales recorded by the evolution of volcanic activity. Magmatic 106 focusing in space and time likely provided high supply for ignimbrite B magma bodies, complemented by much lower assembly rates within adjacent parts of the batholith that never supported ignimbrite erup- Sierra Nevada: 105 tions and during several prolonged time gaps (m.y.) between ignim- thick plutons, brite eruptions. Further discussion in text. (B) Comparison with ) published 3 /yr

published results for western Cordilleran plutons (Sierra Nevada, (log 3 0.1 km North Cascades: North Cascades; data from Table 7). Most published estimates of 4 km 10 magma supply, based largely on volumes of surfaces exposures in thick plutons, 3 /yr published

lum e, r comparison to age ranges of analyzed samples, are too low to have km 3 /y

Vo 0.01 been compatible with generation of ignimbrite-scale magma bodies. 0.005 km Caldera-related: When adjusted for vertically extensive pluton geometry (thickness 103 thick plutons, ~20 km), however, these plutons plot within ranges of magma supply 3 /yr published 3 (>0.005 km /yr; Gelman et al., 2013) that could sustain large ignim- 0.001 km brite eruptions. Tie lines with arrows (shown for only a few data 102 points, to minimize figure clutter) show shift in volume, from pub- 0.1 1 10 100 lished values to thick-pluton model. For two caldera-related plutons Duration, Ma (log) (Princeton, Rio Hondo), alternative interpreted assembly durations (Table 7) yield sloping tie lines.

forming eruptions (28.7–26.8 Ma) or spread area. However, the widely distributed interme- et al., 2004; Davis et al., 2012; Mills and Cole- more uniformly over the entire time of vol diate-composition stratovolcanoes that preceded man, 2013; Frazer et al., 2014). Magma supply canism (34–26 Ma). For the entire span, result- the ignimbrite eruptions were fed by small cen- for such plutons would become an order of mag- ing total magma-supply rate for the batholith is tral conduits and radiating dike systems, many nitude or more higher, however, if modeled as 0.04–0.06 km3/yr (Table 6), at the high end of of which lie beyond the main gravity low, lack vertically extensive (~20 km thick) and involv- supply rates for crustal systems (Crisp, 1984; discernible geophysical expression, and seem ing subequal volumes of mafic residue (Table 7; White et al., 2006). If the bulk of the batholith less likely to have been associated with high Fig. 12B). As a further complexity, the thermal was assembled during the ignimbrite eruptions, rates of pluton construction. models published to date do not consider the (all San Juan calderas except the early Platoro These San Juan and SRMVF magma-sup- potentially substantial effects from voluminous system lie above the gravity-defined batholith), ply rates are 10–20 times the threshold values mafic mantle input into the crust. the total magma supply could have been much required to grow and maintain crustal magma The San Juan and SRMVF supply rates are higher during that interval (0.18 km3/yr; Table 6). reservoirs sufficiently large to feed supererup- for the batholithic-scale area, however, which For magma-supply focused at areas of individual tions, as estimated from caldera-scaled thermal are much larger than plausible sizes of individual caldera cycles, such as La Garita or Creede dur- models (~0.01 km3/yr—Annen, 2009; or as caldera-related plutons such as the 10 km pluton ing the interval since the prior ignimbrite erup- low as ~0.005 km3/yr allowing for nonlinear radius explored by the thermal models of Annen tion, estimated supply rates are similarly high. crystallization and temperature-dependent wall- and Gelman. Average supply rates per unit area An analogous calculation for construction rock conductivity—Gelman et al., 2013). The of the entire SRMVF would be lower, ~0.005 of the entire SRMVF and associated intru- SRMVF values are also 30–100 times greater km3/yr for an area comparable to the pluton sions (38–26 Ma, total estimated magma vol- than representative pluton-fill rates (Table 7) geometry modeled by Annen (2009) or Gelman ume ~620,000 km3) still yields an average total estimated from age variations among surface et al. (2013), i.e., at the low end of supply val- magma-supply of 0.05 km3/yr, but across a vast samples of other Cordilleran plutons (Glazner ues to support generation of a large ignimbrite

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TABLE 7. PLUTON ASSEMBLY AND MAGMA SUPPLY; COMPARISONS WITH OTHER AREAS Batholith volume* Exposed area Exposed/estimated Granitoid Mantle-basalt Total magma Assembly Total magma System (km2) pluton thickness (km) (km3) (km3) volume* (km3) duration (m.y.) supply† (km3/yr) SIERRA NEVADA BATHOLITH Tuolumne complex 1200 5 6,000 6,000 10 0.0006 (Coleman et al., 2004) 1200 20 24,000 24,000 48,000 10 0.0048 Half Dome pluton only 600 20 12,000 12,000 24,000 40.0060 Entire Muir Suite 1,700 8 12,750 12,750 12 0.0011 Lamarck pluton 600 8 4,500 4,500 3 0.0015 (Davis et al., 2012) Entire Muir Suite 1,700 20 34,000 34,000 68,000 12 0.0057 Lamarck pluton 600 20 12,000 12,000 24,000 3 0.0080 Mt Givens pluton 1,500 3 4,500 4,500 4 0.0011 (Frazer et al., 2014) Mt Givens pluton 1,500 20 30,000 30,000 60,000 4 0.0150 Bass Lake pluton (3 pulses) 2,000 2.5 5,000 5,000 15 0.0003 (Lakey et al., 2012; as interpreted by Mills and Coleman, 2013) Bass Lake pluton 2,000 20 40,000 40,000 40,000 15 0.0053 NORTH CASCADES (Matzel et al., 2006a): Mount Stuart pluton 480 2.5 1,200 1,200 5.5 0.0002 480 20 9,6009,600 19,200 60.0035 Youngest domaine 208 2.5 520 520 0.17 0.0031 208 20 4,1604,160 8,3200.170.0489 Tenpeak pluton 197 2.0 394 394 2.6 0.0002 197 20 3,9403,940 7,880 2.6 0.0030 CALDERA RELATED Princeton batholith (Mills and Coleman, 2013) Surface exposure only 600 1.5 900 900 0.43 0.0021 Princeton batholith/ 600 20 12,000 12,000 24,000 1.5 0.0160 Wall Mountain Tuff (37–35.5 Ma) Questa-Latir, NM (Tappa et al., 2011; Rosera et al., 2013) Rio Hondo pluton 90 1.2 108 108 0.4 0.0003 (Alternative: area from gravity data, thick batholith (Lipman, 1988)) Rio Hondo pluton 200 20 4,000 4,000 8,000 0.4 0.0200 Caldera plutons 300 20 6,000 6,000 6,000 0.55 0.0218 Yellowstone caldera (modiﬁ ed from Lowenstern and Hurwitz, 2008) basalt 3x granite 1,800 8.35 15,030 45,090 60,120 1.0 0.0601 Thicker pluton, 1x basalt 1,800 20.00 36,000 36,000 72,000 1.00.0720 Note: Numbers in black are based on exposed/estimated thickness and range of crystallization ages. Numbers in red are based on inference of vertically extensive composite batholith. *Basis for alternative volumes: ≥ 20-km vertical extent of batholith (more maﬁc downward, analogies with crustal sections elsewhere). †Total magma supply = volcanic + granitic + mantle basalt.

eruption. No model of constant magma supply ciated intrusions have long been recognized as sites of ignimbrite eruptions and associated cal- seems plausible or necessary throughout the major controls on eruptive processes and pluton dera collapse, leading to sustained maintenance assembly time and across the overall area of a assembly (e.g., Smith, 1979; Hildreth, 1981; of mushy granodioritic magma bodies in near- large composite batholith, however, and sizable de Silva, 1989). Such fluctuation and focusing solidus environments, permits an interpretive fluctuations in magma supply rate, areal focus- would be consistent with growth of the San Juan crustal model of pluton assembly that is consis- ing of pluton assembly, and wall-rock struc- magmatic locus and the space-time variations tent with the volcanic eruptive history, regional tural response seem inevitable (Paterson et al., in ignimbrite eruptions (Fig. 13): Sizable areas geophysical data, geochronologic results, and 2011). In-progress thermal modeling by S. Gel- centrally within the geophysical batholith lack petrologic evidence (Fig. 14). Early waxing man and O. Bachmann is showing that the esti- calderas, and multimillion-year pauses between magma supply fed eruptions at widely scat- mated magma flux for the San Juan batholith, ignimbrite eruptions alternate with successive tered sites in the upper crust, generating large sustained for several million years, could have eruptions that are too brief to resolve (e.g., three central volcanoes dominated by intermediate generated ignimbrite-size volumes of eruptible large ignimbrites in <0.1 m.y. from the San Luis compositions and only limited pluton volume magma from chambers with horizontal dimen- caldera complex; Lipman and McIntosh, 2008). in the upper crust. After several million years sions on the scale of SRMVF calderas. Such While the record of magmatic focusing in space of dispersed magmatism, increasing magma large reservoirs buffer temperature and gener- and time seems clear in the volcanic record and supply, and warming of the upper crust, larger ate ductile wall-rock halos, promoting survival probably in associated shallow intrusions, long- intrusive bodies could accumulate at shallow of long-lived chambers that undergo complex term deep regional flux may be more nearly levels, focused at sites of closely clustered older crystallization at near-zircon-saturation tem- constant, as suggested by the lengthy crystal- volcanoes. Resulting caldera-scale granodio- peratures and provide recurrent opportunities to lization recorded by deeper intrusions in the ritic magma bodies could generate crystal-poor recycle, resorb, and reset geochronologic clocks Sierra Nevada (Coleman et al., 2004; Memeti caps by low-pressure fractionation in the upper (Bachmann et al., 2007b; Lipman, 2007; Gel- et al., 2010; Frazer et al., 2014). crust within a few hundred thousand years or man et al., 2013; Gregg et al., 2012). Inferred protracted construction of a verti- less, leading to eruption of large ignimbrites. Volumetric and compositional variations in cally extensive San Juan batholith, where peak Continuation of such processes produced the magma flux within volcanic systems and asso- magma supply was intermittently focused at overall composite batholith, 20–30 km thick,

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Figure 13. Conceptual thermal and magma- SAWATCH RANGE CENTERS T

T SAN JUAN LOCUS s

supply models for growth of SRMVF and J Tu T 0.1 underlying composite batholiths. (A) Large o st S J ntral S anch Bonanza

temporal fluctuations and magmatic focus- m /yr (log) all Mtn Plator We Ce n R Saguache Cr W ing are inferred for assembly of shallow , k Thor magma bodies that were direct sources of Badger Cr / Grizzly Lake City e large ignimbrite eruptions. Voluminous o ols co co Early andesites Ea Hotter eruptible magma bodies formed at loci of at h rinceton rinc batholith Large magma bodies P intrusions

high magma supply; lower flux at other Late satellit La 0.01 times and to other parts of the SRMVF Avg. zircon batholiths produced slower incremental saturation pluton-fill rates, feeding growth of ande sitic central volcanoes without development

of large eruptible magma bodies. Zircon- Princeton ooler zircon ages saturation temperatures varied in response C

Aetna volcanic + granitoid magma supply to magma state (composition, temperature, erage temperature in upper batholith Main growth, 0.001 zircon ages g. Av S J batholith volatile content). Peaks in the magma- (>(> BBC C TuTu) Av supply plot provide approximate proxies segment for major times of recharge by mafic magma. Intersections of these peaks with 40 35 30 25 20 A AGE, Ma the zircon-saturation curve qualitatively indicate favorable environments for zir- 100% con dissolution, but the high-temperature excursions resulting from mafic recharge CUMULATIVE would have been briefer and more fre- VOLCANIC VOLUME quent than can be depicted in a generalized diagram. Long-term deep regional flux may have been more constant. Deeper a. intrusions record more prolonged crystal- 0% lization; thus, zircon ages for the Princeton 100% batholith indicate final crystallization later than eruption of the Wall Mountain Tuff, Waxing Waning while some ages from the shallow Mount CUMULATIVE Aetna intrusion preserve antecrystic zircon GRANITOID VOLUME (UPPER-CRUST)

crystallization in this magma body prior to are-up eruption of the Badger Creek Tuff (BCT). b. 0% S J—San Juan; T—Tuff. (B) Interpretation 10–20 of cumulative volumes and intrusive/extrusive ratios during growth of the San Juan INTRUSIVE/EXTRUSIVE magmatic locus. Widespread distribution RATIO and the large composite volume of erup- (UPPER-CRUST) tions from the scattered early-central volcanoes (35–30 Ma) indicate that the waxing Ignimbrite c. stage generated a major fraction (25%– 1–2 100% 50%) of total magmatic output (a), even as upper-crustal intrusive/extrusive ratios (1) remained relatively low (1–2) and waxing CUMULATIVE pluton growth was modest in the shallow MAFIC INPUT crust (b) until the transition to ignimbrite (2) (LOWER CRUST) eruptions. In contrast, (c) petrologic and geophysical evidence suggests much higher d. 0% intrusive/extrusive ratios (10–20) and 40 30 20 Ma major batholith growth in the upper crust B during ignimbrite eruptions (30–27 Ma). Volcanic output diminished greatly as magmatism waned, but the intrusive/extrusive ratio may have remained high, as most of the magma supply solidified within the upper crust without erupting. Mafic input and AFC and MASH processes (DePaolo, 1981; Hildreth and Moor- bath, 1988) in the lower crust likely preceded upper-crustal magmatism by poorly known intervals (d). Possible alternatives might be dominant mafic input into the lower crust during the waxing stage (d-1), largely preceding the ignimbrite eruptions; or more likely, mafic input increasing during the waxing stage (d-2), reaching peak rates early during the ignimbrite eruptions, with a trend similar to the cumulative volcanic output. In detail, all the curves should be “wiggly,” reflecting short-duration fluctuations in magma supply.

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Figure 14. Schematic inferred crustal-sec- W C C C Colorado Plateau SRMVF SJ S tion model of the San Juan batholith beneath Rio Gr Rift part of the SRMVF. Section is east-west, just 0 south of the 38°N Parallel, along axis of the Silverton, large gravity anomaly as mapped by Drenth UPPER CRUST San Juan Creede La et al. (2012). Volcanic rocks in the central (Precambrian) Garita D = 2700 and western San Juan Mountains overlie relatively simple structures of the Colorado Silicic Mush (granodiorite-granite) Plateau, but the eastern edge of the section D = 2620–2700 is impacted by extension and downfaulting along the San Luis Basin segment of the 20 Intermediate Mush (tonalite-granodiorite) Rio Grande (Gr) rift zone. Densities are in D = ~2700–2900 kg/m3. Abbreviations: CC—central caldera DEPTH, km cluster (mainly La Garita and Creede in line of profile); WC—western calderas (San Juan, Silverton); SJ S—San Juan sag, locally DEEP CRUST preserving thick Cretaceous strata between D = ~2900 Tertiary volcanic rocks and underlying Pre- 40 D = ~ 2900–3200 cambrian metamorphic and granitic units; Moho V.E.—vertical exaggeration. (A) Interpre- tive model based on combined volcanologic, petrologic, and geophysical data. A verti- Seismically indistinguishable from mantle? SHALLOW MANTLE cally extensive composite granitic batholith Or gravitationally unstable sinker? D = ~3200 extends through the upper and middle crust, becoming more mafic and dense downward. 60 V.E. = 3 A Shallow cupolas of caldera-related plutons 0 100 200 locally intrude broadly cogenetic volcanic DISTANCE, km ejecta. The crustal-scale compositional gradients detected by geophysical methods 0 overprint and obscure a smaller-scale intrusion architecture of vertically elongate plutons, separated by screens of country rocks km (e.g., fig. 4 in Saleeby et al., 2003; fig. 3 in Paterson et al., 2011). Possible shapes for upper parts of individual caldera-related DEPTH, plutons are indicated by dotted lines. During No V.E. B the protracted volcanic activity, the batho- 60 lith and component plutons consist mainly of crystal mush, containing variable amounts of interstitial melt, with localized short-lived shallow bodies of eruptible ignimbrite magma generated recurrently at upper levels (indicated by yellow lenses, beneath calderas). The total volume of cumulate and restite residual from extraction of more silicic melt would project into the upper mantle; deep portions of this residue below the Moho are either seismically indistinguishable from adjacent lithospheric mantle or have detached and sunk to greater depth under gravitational forces (Arndt and Goldstein, 1989). The geophysical model of a thinner batholith (10–18 km, avg. 13 km) from Drenth et al. (2012), previously depicted in Figure 3A, is superposed in subdued outline, for comparison. (B) Same crustal cross section as in A., but without vertical exaggeration.

becoming more mafic downward. Geophysical tained pluton assembly at lower rates without batholithic rock, perhaps shallow cumulates that definition of the ignimbrite-related batholith, developing large volumes of eruptible magma. are complementary to erupted rhyolitic ignim- but not for the earlier central volcanoes of the Deeper intrusive rocks, mainly granodiorite and brites (Deering et al., 2011; Bachmann et al., SRMVF, is closely analogous to the APVC in tonalite similar to those exposed at midcrustal 2014; Gelman et al., 2014). Alternatively, it relation to the active volcanic zone farther west levels elsewhere in the Cordillera (southern might record a zone of mafic injection, similar in the Andes. Sierra Nevada, Coastal batholith of British to that inferred from seismic and deformation Eruptible magma capable of sourcing ignim- Columbia), are likely at approximate density data, at slightly greater depth near Socorro, New brite eruptions accumulated only in the upper- equivalence to wall rocks, thereby generating no Mexico (Sanford, 1983; Fialko and Simons, most few kilometers (Fig. 14A), at local sites detectible geophysical signature. 2001), or seen deep in upper-crustal Cordilleran of focused high-mantle-magma supply. Other The seismically defined upper-crustal zone plutons (e.g., Best, 1963; Coleman et al., 1995; thick central parts of the San Juan batholith that of high-velocity material (8–10 km depth; Fig. Miller et al., 2011). Because seismic model- lack associated calderas, such as the deep grav- 3B), inferred from reprocessing of vintage ing would be unable to resolve steep contacts ity low between the central and western caldera seismic data (Drenth et al., 2012), is of unclear between plutons and wall rocks, other interpre- clusters (Fig. 2), probably record sites of sus- significance. It may represent pods of mafic tive complexities may exist.

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Granitic rocks of the San Juan batholith tabular “megalaccolithic” geometry due to great individual samples from the Sierra Nevada are inferred to overlie an approximately equal lateral extent (Fig. 14B). and other batholiths. Two-dimensional thermal thickness and volume of dense residua, includ- Direct evidence for large volumes and pro- models for Sierran plutons are also consistent ing restite from widespread partial melting of longed life spans of batholith-scale mushy with assembly over intervals as long as several lower crust along with cumulates from crystal- magma beneath loci of ignimbrite volcanism million years (Paterson et al., 2011). lization of voluminous mantle-derived mafic comes from the APVC in the central Andes The presence of laterally extensive bodies magma (assimilation-fractional crystalliza- (Ward et al., 2014). Anomalously slow seismic- of interconnected melt high in such long-lived tion [AFC] and melting-assimilation-storage- shear velocities indicate that partial melt is cur- mushy plutons is recorded by the eruption of homogenization [MASH] processes; DePaolo, rently present at 10–30 km depths beneath the large calc-alkaline ignimbrites (102–103 km3; 1981; Hildreth and Moorbath, 1988). The mod- entire surface footprint of the APVC (~200 especially crystal-poor rhyolites and those that eled residua (Farmer et al., 2008) are too volu- km diameter). The slow velocities (<2.9 km/s, grade into late-erupted dacite with more com- minous to be accommodated above the seismic locally to 1.9 km/s) would not be possible from plex crystal cargos; Hildreth, 1981; Bachmann Moho, within the present-day crust (which is even extreme variations in rock composition or and Bergantz, 2004), triggering multikilometer relatively thin and low density; Prodehl and temperature; a low-end estimate is 5% partial caldera collapse bounded by ring faults. Field Lipman, 1989; Hansen et al., 2013). Either melt, locally possibly as much as 25%. The total data document common subsidence depths of lower portions have delaminated or otherwise volume of the mush zone beneath the APVC, 2–5 km at such calderas (Lipman, 1984; John separated from the crust (as widely proposed estimated at 300,000 km3, cannot represent a et al., 2008; Best et al., 2013), confirming large for roots of Cordilleran-type magmatic systems; single magma body; it must be a composite of integrated volumes of eruptible magma at shal- e.g., Arndt and Goldstein, 1989; Kay and Kay, multiple smaller zones of partial melt that amal- low depth; ignimbrite and subsidence volumes 1993; Saleeby et al., 2003; Zandt et al., 2004; gamated incrementally. Although individual are broadly proportional (Smith, 1979; Spera Jones et al., 2004; Jagoutz and Schmidt, 2012), calderas have no resolvable velocity expression, and Crisp, 1981; Gregg et al., 2012; Geshi et al., or perhaps they have just become so dense that substantial lateral and vertical variations in melt 2014). Analogue models show that the geometry no geophysical distinction is possible between percentage are likely as functions of volcano of caldera subsidence changes from ring-fault the residua and adjacent older mantle. Whatever age, magma composition, and presence of wall- to downsag depression as thickness of the roof the fate of these deep residua, the mid-Tertiary rock septa between plutons. above the magma increases (Marti et al., 1994; magmatic processes must have caused major Maintenance or reinvigoration of such a large Roche et al., 2000; Acocella et al., 2000; Ken- chemical and physical reconstruction of the mush zone must depend on voluminous sus- nedy et al., 2004), but large downsag calderas lithospheric column, probably accompanied tained recharge. Most plausibly, this involves that should result from draining of deep magma by asthenospheric input as well (e.g., Farmer processes at multiple crustal levels. Mafic man- rarely accompany voluminous ignimbrite erup- et al., 2008). tle melts interact with the lower crust by AFC/ tions (Lipman, 1997; Cole et al., 2005). Other MASH processes to generate intermediate-com- silicic magma bodies that are sufficiently liquid Assembly of Cordilleran Plutons position magmas with compositions much like to erupt, especially as sources for smaller ignim- the andesite and dacite that commonly erupt dur- brites with disequilibrium crystal assemblages, Processes of incremental pluton assembly, ing early stages of an ignimbrite cycle. Despite may develop as multiple poorly connected melt by recurrent addition of subhorizontal magma clear isotopic and other petrologic evidence for pockets at more varied depths, which become lenses, are well documented for some lacco- involvement of enormous volumes of mantle- interconnected and mixed shortly before or dur- liths and isolated plutons of modest size that are generated basaltic magma in the SRMVF and ing the eruption (e.g., review by Cashman and exposed at optimum crustal levels (Wiebe et al., other Cordilleran systems, little if any basaltic Giordano, 2014). 2002; Miller and Miller, 2002; Michel et al., magma reached the surface centrally within Some large ignimbrites record much briefer 2008; Horsman et al., 2009; Rocchi et al., 2010; areas of high mid-Tertiary magmatic fluxes zircon-crystallization histories than the pro- Miller et al., 2011). Such structural and com- marked by ignimbrite calderas and geophysi- longed magmatic evolution inferred here from positional evidence of incremental assembly is cally constrained subvolcanic intrusions. the U/Pb age spectra in Cordilleran arcs. For unlikely to survive, however, in longer-lived and Rising basaltic and derivative intermediate- example, recent TIMS analyses from several larger intrusive systems constructed by recurrent composition melts would mix efficiently with relatively dry, high-temperature rhyolite ignim- open-system recharge at high magma-supply residual mush of an upper-crustal magma body brites of the Yellowstone–Snake River Plain rates, such as inferred for the San Juan batholith. that differs only modestly in composition and region (Huckleberry Ridge, Kilgore Tuffs) Extensive upward (and downward?) flow late density, thereby increasing temperatures and indicate that most zircons in these intraplate during pluton emplacement (or by convection in melt proportion in the mush zone. Such pro- magmas crystallized within 5000–10,000 yr the reservoir; Gutierrez et al., 2013), as recorded cesses would permit assembly of an eruptible prior to their eruption, as limited by 40Ar/39/Ar by widespread steep mineral foliations in eroded melt-rich zone at the top of the mush body (as ages (Rivera et al., 2014; Wotzlaw et al., 2014; Cordilleran plutons, would have disrupted and commonly diagrammed for individual calderas: Bindeman and Simakin, 2014). The tight group- largely obliterated the early history of tabular e.g., fig. 7in Hildreth, 2004; fig. 6in de Silva ing of zircon ages for these ignimbrites, without magma assembly. Large individual plutons and et al., 2006), along with smaller-volume melt xenocrysts recording Archean basement and composite batholith-size bodies, even if incre- lenses at greater depth (Cashman and Giordano, near absence of antecrysts from earlier volcanic mentally constructed by magma batches having 2014). The continued presence of voluminous activity, strongly suggests that these magma lenticular aspect ratios, aggregate on scales that mushy magma beneath the long-lived APVC batches remained mostly zircon-undersaturated eventually occupy much of the crust. Never- (10 m.y.) implies prolonged cycles of pluton and able to resorb assimilated zircon crystals theless, although vertically extensive, typical assembly and crystallization, which should have until late in their assembly. Any antecrystic composite Cordilleran batholiths, such as that counterparts in the durations (to 106 m.y.) of zir- zircons inherited from early stages in magma beneath the San Juan region, retain an overall con ages within single plutons, and even within generation and assembly, involving voluminous

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assimilation of hydrothermally altered country obscured by high volumetric ratios of verti- before the next intrusive pulse. Near-absence rocks as inferred from low 18O values of the cally extensive intrusions relative to extracted of Proterozoic xenocrysts in SRMVF caldera- Kilgore magmas (Bindeman et al., 2007; Wotz evolved liquid and the intrinsic inefficiency of related intrusions, in contrast to their abundance law et al., 2014), are poorly recorded by the extracting highly viscous melt from crystalline in some peripheral intrusions (Fig. 9), suggests dominant zircon ages. In contrast, low 18O val- residue. The compositions of some voluminous lengthier assembly histories for the caldera ues are largely or entirely absent in the colder- eruptions in the SRMVF varied widely within intrusions, including intervals of zircon under- wetter, calc-alkaline ignimbrite magmas of the time spans too brief to resolve by available geo- saturation and dissolution. Varied crystallization SRMVF and other mid-Tertiary ignimbrites of chronologic methods (e.g., the San Luis caldera ages laterally and vertically across a pluton are the Cordilleran arc (Larson and Taylor, 1986), complex; Lipman and McIntosh, 2008). In con- inferred to reflect existence of discontinuous indicating limited involvement of altered upper trast to the protracted evolution of the overall domains where magma composition and pres- crust in their generation. Because early-formed volcano-plutonic system, eruptible ignimbrite sure-temperature conditions were locally con- antecrystic zircons in continental-arc ignim- magmas with high melt proportions must have ducive to zircon saturation and crystal growth at brites would be less soluble than in the hot-dry been generated rapidly, with only brief lifetimes different times. Large intra- and intergrain varia- silicic magmas of the Snake River Plain (e.g., before venting. The bulk of eruptible magma tions in concentrations of U, Hf, and other trace Watson and Harrison, 1983; Miller et al., 2003), reached the surface as volcanic deposits, but elements (to order-of-magnitude) from grain to they preserve lengthier records of magma-body the more voluminous underlying magma mush grain and zone to zone in zircons and within a assembly: e.g., 500–1000 k.y. for the Fish Can- continued to evolve and to be recharged by single intrusion sample demonstrate variable yon Tuff (Wotzlaw et al., 2013; Coble, 2013), sustained mantle input, finally solidifying only growth environments at many scales within and 250–500 k.y. for the younger Taupo, Toba, when deep magma supply waned. The volume small magma batches (Schaltegger et al., 2009; and Kos Plateau ignimbrites (Brown and of the overall mantle input likely was 1–2 times Claiborne et al., 2010; Zimmerer and McIntosh, Fletcher, 1999; Charlier et al., 2005; Reid, 2008; that of the volcanic ejecta and underlying crustal 2012a, their supplemental data; Erdmann et al., Guillong et al., 2014). batholiths (Fig. 4). 2013). Ephemeral local compositional gradients Recently published CA-TIMS zircon ages for must have developed during crystallization and CONCLUSIONS SRMVF intrusions (Figs. 7–8) define average late-stage mixing. times of crystallization as the magmatic system Subtle compositional, textural, and structural Geologic, geophysical, and geochronologi- cooled but do not document the total duration of discontinuities within lithologically mappable cal data are consistent with prior proposals that pluton assembly. Mineral ages (mainly biotite, pluton phases also suggest the existence of spa- large, long-lived, silicic volcanic fields such as hornblende) determined by 40Ar39Ar methods tially discontinuous domains during prolonged the SRMVF are surface expressions of compos- are only slightly younger than the zircon ages, assembly and sustained maintenance of crystal ite upper-crustal magma bodies comparable to indicating that late crystallization was accompa- mush through much of an evolving pluton. Indi- the Boulder or Sierra Nevada batholiths. Recent nied by rapid cooling and solidification. Crys- vidual or small groups of rising magma pulses, data summarized here permit improved quanti- tallization ages for caldera-related intrusions which cool to form discrete plutons in the shal- tative assessment of batholith geometry, dura- vary from indistinguishable from the associated low crust, merge and coalesce with adjacent tion of magma assembly and crystallization, ignimbrite to ~2 m.y. younger, and age differ- residual crystal mush at deeper levels to form and rates of magma supply during evolution of ences tend to increase with depth of pluton larger granitoid bodies in which boundaries the SRMVF and comparable Cordilleran mag- emplacement (Fig. 10). Inferences that such between magma packages become gradational matic systems. age variations date times of intermittent magma or otherwise obscure. Extraction of interstitial The San Juan volcanic locus, associated assembly are not supported by any documented melt, heterogeneous flowage, and mixing within batholith, and the broader SRMVF record age-depth correlation within a single pluton. remaining crystal mush could further compli- multimillion-year incremental construction by More plausibly, deeper levels of plutons that cate the distribution of apparent ages within and prolonged open-system processes at high aver- cooled slowly preserve a lengthier record of between such domains in a pluton. age rates of magma supply, involving volumi- ascent and crystal growth in sequential magma Accordingly, contrary to some interpreta- nous mafic-mantle inputs, large-scale crustal pulses, punctuated by periods of zircon under- tions, zircon U/Pb ages provide few constraints assimilation, and concurrent generation of dense saturation and crystal resorption (Fig. 10B). on processes or durations of magma-body residua (cumulate, restite) that now lie mostly Age variations among separate crystals from assembly in granitoid plutons, especially at beneath the seismic Moho. Recurrent genera- a single sample and among multiple samples times of high magma supply. Prolonged duration of melt-dominated silicic magma lenses at from deeper intrusions in the Sierra Nevada are tion of pluton construction and mixing of crystal times of peak magma supply, above vertically much larger than for the subvolcanic plutons in cargo late during assembly are documented by extensive bodies of near-solidus crystal mush the Southern Rocky Mountains (Fig. 8), a rela- SIMS age ranges of >105 yr within zoned single in the upper crust and underlain by voluminous tion consistent with more protracted pluton con- zircon crystals, similar spans among CA-TIMS input from mafic mantle-derived magma, led to struction and crystallization at greater depth. single-crystal zircon ages from individual sam- repeated ignimbrite supereruptions and caldera Proximal intrusions of focused magmatism ples of caldera-related intrusion, and antecryst formation in the SRMVF. Erupted SRMVF in the SRMVF were vertically extensive and zircon ages that predate eruption age of the asso- rhyolites have liquid compositions indicative of capable of supporting large ignimbrite erup- ciated ignimbrite. Close agreement among auto- low-pressure fractionation; most dacites carry tions, as high magma supply mixed and variably cryst zircon and titanite ages, and only slightly disequilibrium crystal-rich cargos but have a homogenized successively emplaced incremen- younger 40Ar/39Ar hornblende dates suggest low-pressure rhyolitic groundmass. Comple- tal inputs. In contrast, more-distal plutons tend major zircon growth late during assembly of mentary crystal accumulations resulting from to be smaller and laccolithic in shape, and they granitic plutons as the magma supply wanes and rhyolite fractionation have been identified in may more closely record assembly from succes- cooling accelerates. During earlier stages, com- some systems, although they are commonly sive magma increments that largely crystallized positionally contrasting intrusive phases within

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a composite pluton complex can have coexisted Annen, C., 2009, From plutons to magma chambers: Ther- Barker, S.J., Wilson, C.J.N., Smith, E.G.C., Charlier, B.L.A., mal constraints on the accumulation of eruptible silicic Wooden, J.L., Hiess, J., and Ireland, T.R., 2014, Post- for lengthy periods as largely crystallized bodies magma in the upper crust: Earth and Planetary Science supereruption magmatic reconstruction of Taupo vol- with residual interstitial liquid, subject to suc- Letters, v. 284, p. 409–416, doi:10.1016/j.epsl.2009.05 cano (New Zealand), as reflected in zircon ages and cessive additional magma pulses that could slow .006. trace elements: Journal of Petrology, v. 55, p. 1511– Annen, C., Blundy, J.D., and Sparks, R.S.J., 2006, The gen- 1533, doi:10.1093/petrology/egu032. or reverse cooling and affect zircon stability. esis of intermediate and silicic magmas in deep crustal Barnes, C.G., Rice, J.M., and Gribble, R.F., 1986, Tilted Based on vertically extensive intrusion geom- hot zones: Journal of Petrology, v. 47, p. 505–539, doi: plutons in the Klamath Mountains of California and etry, total magma supply for many Cordilleran 10.1093/petrology/egi084. Oregon: Journal of Geophysical Research, v. 91, Armstrong, R.L.A., 1988, Mesozoic and early Cenozoic p. 6059–6071, doi:10.1029/JB091iB06p06059. batholiths (>0.005 km3/yr for the overall intru- magmatic evolution of the Canadian Cordillera, in Bartley, J.M., Glazner, A.F., and Coleman, D.S., 2005, Do sion assembly at the San Juan locus) would have Clark, S.P., Jr., Burchfiel, B.C., and Suppe, J., eds., large silicic eruptions leave behind even larger plutons: Processes in Continental Lithospheric Deformation: Eos, v. 86, no. 18, p. 58. been sufficient to maintain large volumes of Geological Society of America Special Paper 218, Bartley, J.M., Coleman, D.S., and Glazner, A.F., 2008, Incre- crystal mush above the solidus for multimillion- p. 55–92, doi:10.1130/SPE218-p55. mental pluton emplacement by magmatic crack seal: year durations in the upper crust (Fig. 12). Plu- Arndt, N.T., and Goldstein, S.L., 1989, An open boundary Proceedings of the Royal Society, Earth and Environ- between lower continental crust and mantle; its role mental Sciences, v. 97, p. 383–396. ton-fill rates in such composite batholiths likely in crust formation and crustal recycling; growth of the Behrendt, J.C., and Bajwa, L.Y., 1974, Bouguer Gravity and fluctuated substantially both in time and space; continental crust: Tectonophysics, v. 161, p. 201–212, Generalized Elevation Maps of Colorado: U.S. Geo- times of high supply (likely >0.01–0.05 km3/yr) doi:10.1016/0040-1951(89)90154-6. logical Survey Map GP-896, scale 1:1,000,000. Bachmann, O., and Bergantz, G.W., 2003, Rejuvenation of Berger, A., Rosenberg, C.L., and Schmid, S.M., 1996, focused at localized areas would generate large- the Fish Canyon magma body: A window into the evo- Ascent, emplacement and exhumation of the Bergell volume (102–103 km3) capping lenses of erupti lution of large-volume silicic magma systems: Geol- Pluton within the Southern steep belt of the Central ogy, v. 31, p. 789–792, doi:10.1130/G19764.1. Alps: Schweizerische Mineralogische und Petrogra- ble magma to form ignimbrite eruptions and Bachmann, O., and Bergantz, G.W., 2004, On the origin of phische Mitteilungen, v. 76, p. 357–382. associated calderas. The inferred vertically crystal-poor rhyolites: Extracted from batholithic crys- Best, M.G., 1963, Petrology of the Guadalupe igneous com- extensive magma-body geometry, interaction tal mushes: Journal of Petrology, v. 45, p. 1565–1582, plex, south-western Sierra Nevada Foothills, Califor- doi:10.1093/petrology/egh019. nia: Journal of Petrology, v. 4, p. 223–259, doi:10 .1093 of voluminous mafic magma from the mantle Bachmann, O., and Bergantz, G.W., 2008, Rhyolites and /petrology/4.2.223. with crustal melts, prolonged durations of plu- their source mushes across tectonic settings: Journal of Best, M.G., Christiansen, E.H., Deino, A.L., Gromme, S., ton assembly and crystallization, and volumi- Petrology, v. 49, p. 2277–2285, doi:10.1093/petrology Hart, L.G., and Tinge, D.G., 2013, The 36–18 Ma /egn068. Indian Peak–Caliente ignimbrite field and calderas, nous silicic volcanism occurring concurrently Bachmann, O., Dungan, M.A., and Lipman, P.W., 2002, southeastern Great Basin, USA: Multicyclic super- with batholith construction as recorded by the The Fish Canyon magma body, San Juan volcanic eruptions: Geosphere, v. 9, p. 864–960, doi:10.1130 field, Colorado: Rejuvenation and eruption of an upper /GES00902.1. SRMVF are suggested to typify continental- crustal batholith: Journal of Petrology, v. 43, p. 1469– Biehler, S., and Bonini, W.E., 1969, A regional gravity margin arc magmatism globally. 1503, doi:10.1093/petrology/43.8.1469. study of the Boulder batholith, Montana, in Larsen, Bachmann, O., Charlier, B.L.A., and Lowenstern, J.B., 2007a, L.H., Prinz, M., and Manson, V., eds., Igneous and ACKNOWLEDGMENTS Zircon crystallization and recycling in the magma cham- Metamorphic Geology: Geological Society of Amer- ber of the rhyolitic Kos Plateau Tuff (Aegean Arc): ica Memoir 115, p. 401–422, doi:10.1130/MEM115 A stimulating Geological Society of America Geology, v. 35, p. 73–76, doi:10.1130/G23151A.1. -p401. (GSA) Field Forum on the Sierra Nevada batholith, Bachmann, O., Miller, C.F., and de Silva, S.L., 2007b, The Bindeman, I.N., and Simakin, A.G., 2014, Rhyolites—Hard volcanic-plutonic connection as a stage for understand- to produce, but easy to recycle and sequester: Integrat- organized in 2005 by Drew Coleman, Allen Glazner, ing crustal magmatism: Journal of Volcanology and ing microgeochemical observations and numerical and John Bartley, re-energized Lipman’s interest in Geothermal Research, v. 167, p. 1–23, doi:10 .1016/j models: Geosphere, v. 10, p. 930–957, doi:10.1130 issues of pluton-crystallization ages and duration of .jvolgeores.2007.08.002. /GES00969.1. magma-body assembly. Subsequent studies on subvol- Bachmann, O., Oberli, F., Dungan, M.A., and Fischer, H., Bindeman, I.N., Watts, K.E., Schmitt, A.K., Morgan, L.A., canic plutons in the SRMVF (by Coleman and students 2007c, 40Ar/39Ar and U-Pb dating of the Fish Can- and Shanks, P.W.C., 2007, Voluminous low-18O mag- at the University of North Carolina, Matt Zimmerer yon magmatic system, San Juan volcanic field, Colo- mas in the late Miocene Heise volcanic field, Idaho: and Bill McIntosh at New Mexico Tech, and David rado: Evidence of an extended crystallization history: Implications for the fate of Yellowstone hotspot cal- Gonzales at Fort Lewis College) generated enjoy- Chemical Geology, v. 236, p. 134–166, doi:10 .1016/j deras: Geology, v. 35, p. 1019–1022, doi:10.1130 .chemgeo.2006.09.005. /G24141A.1. able field work, thought-provoking data, and some Bachmann, O., Deering, C.D., Ruprecht, J.S., Huber, C., Bove, D.J., Hon, K., Budding, K.E., Slack, J.F., Snee, L.W., divergent interpretations. We especially thank David Skopelitis, A., and Schnyder, C., 2012, Evolution of and Yeoman, R.A., 2001, Geochronology and Geology Gonzales and Kathryn Watts for sharing data on zir- silicic magmas in the Kos-Nisyros volcanic center, of Late Oligocene through Miocene Volcanism and con populations in western San Juan intrusions and the Greece: Cycles associated with caldera collapse: Con- Mineralization in the Western San Juan Mountains, Caetano caldera system. Jake Lowenstern and Kathyrn tributions to Mineralogy and Petrology, v. 163, p. 151– Colorado, with Special Emphasis on the Lake City Watts of the U.S. Geological Survey, and Geosphere 166, doi:10.1007/s00410-011-0663-y. Caldera Area: U.S. Geological Survey Professional reviewers and editors (Ben Drenth, Jonathan Miller, Bachmann, O., Deering, C.D., Lipman, P.W., and Plummer, Paper 1642, 30 p. Lang Farmer, and Shan de Silva) provided exception- C., 2014, Building zoned ignimbrites and recycling Brown, S.J.A., and Fletcher, I.R., 1999, SHRIMP U-Pb dat- silicic cumulates: Insight from the 1000 km3 Carpen- ing of the pre-eruptive growth history of zircons from ally helpful comments on the manuscript. This work ter Ridge Tuff, CO: Contributions to Mineralogy and the 340 ka Whakamaru Ignimbrite, New Zealand: Evi- was partly supported by Schweizerischer National- Petrology, v. 167, p. 1025, doi:10.1007/s00410-014 dence for >250 k.y. magma residence times: Geology, fonds (SNF) fund 200021_146268 to Bachmann. -1025-3. v. 27, p. 1035–1038, doi:10.1130/0091-7613(1999)027 Bacon, C.R., and Druitt, T.H., 1988, Compositional evo- <1035:SUPDOT>2.3.CO;2. 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