U-Pb in Zircon Chronochemistry of the Altiplano-P

Research Paper THEMED ISSUE: PLUTONS: Investigating the Relationship between Pluton Growth and Volcanism in the Central Andes

GEOSPHERE Geochronological imaging of an episodically constructed

GEOSPHERE; v. 12, no. 4 subvolcanic batholith: U-Pb in zircon chronochemistry of the

doi:10.1130/GES01258.1 Altiplano-Puna Volcanic Complex of the Central Andes

1 1 2, 1, 1, 2, 11 figures; 3 tables; 4 supplemental files Jamie M. Kern , Shanaka L. de Silva , Axel K. Schmitt *, Jason F. Kaiser *, A. Rodrigo Iriarte *, and Rita Economos * 1College of Earth, Ocean, and Atmospheric Science, Oregon State University, Corvallis, Oregon 97331, USA 2 CORRESPONDENCE: desilvas@geo.oregonstate Department of Earth and Space Sciences, University of California, Los Angeles, California 90021, USA .edu

CITATION: Kern, J.M., de Silva, S.L., Schmitt, A.K., Kaiser, J.F., Iriarte, A.R., and Economos, R., ABSTRACT magmatic history at distinct upper crustal magmatic foci implicates a shared 2016, Geochronological imaging of an episodically connection deeper within the APMB. constructed subvolcanic batholith: U-Pb in zircon Zircons from 15 crystal-rich monotonous intermediate ignimbrites and 1 Each ignimbrite records the development of a discrete magma. Zircon chronochemistry of the Altiplano-Puna Volcanic Complex of the Central Andes: Geosphere, v. 12, crystal-poor rhyolite ignimbrite erupted during the 11–1 Ma Altiplano-Puna age distributions of individual ignimbrites become more complex with time, no. 4, p. 1054–1077, doi:10.1130/GES01258.1. Volcanic Complex (APVC) ignimbrite flare-up record multiscale episodicity in reflecting the carryover of antecrysts in successively younger magmas and the magmatic history of the shallowest levels (5–10 km beneath the surface) of attesting to upper crustal assimilation in the APVC. Although present, xeno Received 1 September 2015 the Altiplano-Puna Magma Body (APMB). This record reveals the construction crysts are rare, suggesting that inheritance is limited. This is attributed to Revision received 4 April 2016 of a subvolcanic batholith and its magmatic and eruptive tempo. basement assimilation under zircon-undersaturated conditions deeper in the Accepted 4 May 2016 Published online 27 May 2016 More than 750 U-Pb ages of zircon rims and interiors of polished grains APMB than the pre-eruptive levels, where antecrysts were incorporated in zir- determined by secondary ion mass spectrometry define complex age spec- con-saturated conditions. tra for each ignimbrite with a dominant peak of autocrysts and subsidiary Magmatic ages for individual ignimbrites are older than the 40Ar/39Ar erup- antecryst peaks. Xenocrysts are rare. Weighted averages obtained by pooling tion ages. This difference is interpreted as the average minimum Zr-saturated the youngest analytically indistinguishable zircon ages mostly correspond to melt-present lifetime for APVC magmas, the magmatic duration or Δ age. The the dominant crystallization ages for zircons in the magma. These magmatic average Δ age of ca. 0.4 Ma indicates that thermochemical conditions for ages are consistent with eruptive stratigraphy, and fall into four groups defin- zircon saturation were maintained for several hundreds of thousands of years ing distinct pulses (from older to younger, pulses 1 through 4) of magmatism prior to eruption of APVC magmas. This is consistent with a narrow range of that correlate with eruptive pulses, but indicate that magmatic construction zircon saturation temperatures of 730–815 °C that record upper crustal condi- in each pulse initiated at least 1 m.y. before eruptions began. Magmatism was tions and Zr/Hf, Th/U, Eu/Eu*, and Ti that reveal protracted magma differen- initially distributed diffusely on the eastern and western flanks of the APVC, tiation under secular cooling rates an order of magnitude slower than typical but spread out over much of the APVC as activity waxed before focusing in pluton cooling rates. In concert, these data all suggest that the pre-eruptive the central part during the peak of the flare-up. Each pulse consists of spatially magma reservoirs were perched in a thermally and chemically buffered state OLD G distinct but temporally sequenced subpulses of magma that represent the during their long pre-eruptive lifetimes. Trace element variations suggest sub- construction of pre-eruptive magma reservoirs. Three nested calderas were tle differences in crystallinity, melt fraction, and melt composition within dif- the main eruptive loci during the peak of the flare-up from ca. 6 to 2.5 Ma. ferent zones of individual magma reservoirs. Significant volumes of plutonic These show broadly synchronous magmatic development but some discor- rocks associated with ignimbrites are supported by geophysical data, the lim- OPEN ACCESS dance in their later eruptive histories. These relations are interpreted to indi- ited compositional range over 10 m.y., the thermal inertia of the magmatic cate that eruptive tempo is controlled locally from the top down, while mag- systems, and the evidence of resurgent magmatism and uplift at the calderas matic tempo is a more systemic, deeper, bottom-up feature. Synchroneity in and eruptive centers, the distribution of which defines a composite, episodically constructed subvolcanic batholith. *Current addresses: (Schmitt) Institut für Geowissenschaften, Universität Heidelberg, Im Neuen- The multiscale episodicity revealed by the zircon U-Pb ages of the APVC heimer Feld 234-236, 69120 Heidelberg, Germany; (Kaiser) Geosciences, Southern Utah Univer- flare-up can be interpreted in the context of continental arc magmatic sys- sity, 351 W. University Blvd., Cedar City, Utah 84720, USA; (Iriarte) Facultad de Ciencas, Universi This paper is published under the terms of the dad Major San Andres, La Paz, Bolivia; (Economos) Roy M. Huffington Department of Earth tems in general. The APVC ignimbrite flare-up as a whole is a secondary CC‑BY license. Sciences, Southern Methodist University, PO Box 750395, Dallas, Texas, USA. pulse of ~10 m.y., with magmatic pulses 1 through 4 reflecting tertiary pulses

GEOSPHERE | Volume 12 | Number 4 Kern et al. | U-Pb in zircon chronochemistry of the Altiplano-Puna Volcanic Complex Downloaded from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/4/1054/4178070/1054.pdf 1054 by guest on 24 October 2019 Research Paper

of ~2 m.y., and the individual ignimbrite zircon spectra defining quaternary Andes (de Silva, 1989a; de Silva et al., 2006a). The APVC is the result of one of pulses of <1 m.y. This hierarchy of pulses is thought to reflect how a magmatic the youngest ignimbrite flare-ups on Earth, erupting >15,000 km3 of magma front, driven by the primary mantle power input, propagates through the crust during multiple supereruptions from 11 to 1 Ma that define an episodic vol with individual magmatic events occurring over sequentially smaller spatial canic history (Fig. 1). Here we present an extensive data set of high-spatial- and faster temporal scales in the upper crust of the Central Andes from ~30 km resolution U-Pb zircon ages that define zircon crystallization histories over the to the surface. ~10 m.y. spatial and temporal span of the APVC ignimbrite flare-up and use this to complement and extend our understanding of the development of continental magmatic systems. The U-Pb data are complemented by reconnais- INTRODUCTION sance zircon trace element data that targeted sample subsets representative of the diversity of composition and eruptive style in the APVC. In particular, The eruptive histories of ignimbrite flare-ups are useful as a proxy for plu- we explore the implications of these data for the development of the plutonic ton formation at depth (Elston, 1984; Lipman, 2007; Best et al., 2013; Lipman system underlying large silicic volcanic fields. and Bachmann, 2015; Christiansen et al., 2016). However, such eruptive histories provide an incomplete picture as magma could be emplaced but not erupted. Identifying the pre-eruptive history of the magmas is critical to ef- GEOLOGIC BACKGROUND forts to better understand the volcanic-plutonic connection. At the forefront of these efforts are recent studies that take advantage of advancements in The APVC in the Central Andes occupies an arid, high-elevation plateau at high-spatial-resolution U-Pb and U-Th zircon dating in the past 15 years that the political triple junction between Argentina, Chile, and Bolivia (Fig. 1A). The allow for identification of zircon crystal populations that significantly predate dry climate preserves multiple large caldera centers and their accompanying eruption ages (Reid et al., 1997; Brown and Fletcher, 1999; Reid and Coath, ignimbrites, providing an unparalleled view of some of the largest explosive 2000; Vazquez and Reid, 2002; Schmitt et al., 2002, 2003; Charlier et al., 2005; silicic eruptions in the world. The ignimbrite plateau of the APVC developed Bryan et al., 2007). These studies have successfully used pre-eruption zircon as part of a regional ignimbrite flare-up during the Neogene development of crystallization histories to infer the behavior of magmas prior to eruption. In a the Central Volcanic Zone of the Andes (de Silva et al., 2006; Salisbury et al., silicic magma, zircon begins to crystallize once a magma of appropriate com- 2011; Freymuth et al., 2015). The APVC is the most intense and youngest locus position cools below the zircon saturation temperature determined by the melt of the central Andean flare-up, where over a 10 m.y. period multiple large ig- composition (Watson and Harrison, 1983; Boehnke et al., 2013). Because the nimbrite sheets from several large caldera sources were emplaced over an zircon saturation temperature is often reached prior to eruption, zircons can area of 70,000 km2 (de Silva, 1989a, 1989b). Today, the APVC is broadly studied record an extended magma history between crystallization and eruption. In from physical volcanological, geochemical, and geophysical viewpoints to bet- tandem with eruption ages, the crystallization ages determined by U-Pb or ter understand the development of large silicic volcanic fields and the potential U-Th in zircon can provide critical insight into the magmatic history of the sys- for future APVC eruptions. tem being studied. The APVC is situated on the Altiplano-Puna plateau, which is rivaled only Cooling ages determined with the 40Ar-39Ar technique are the most com- by the Tibetan Plateau in height and extent. The Altiplano-Puna plateau is a mon data interpreted as eruption ages for volcanic systems older than a few major element of the Central Andes, resulting from convergence of the Nazca thousand years (Spell and Harrison, 1993; Gansecki et al., 1996; Renne et al., and South American plates. Significant crustal shortening and magmatic addi 1997; McDougall and Harrison, 1999). U-Pb and U-Th zircon dating of young tion over the Cenozoic produced a 70-km-thick crust and an average elevation (younger than 10 Ma) silicic systems compared to 40Ar/39Ar eruption ages of 4000 m, with some stratovolcanoes on the plateau reaching 6000 m (All- shows that zircon can crystallize as much as hundreds of thousands of years mendinger et al., 1997). Since the mid-1990s several geophysical studies have prior to eruption (Bachmann et al., 2007a, 2007b; Simon et al., 2008; Costa, identified a thick zone of ultralow seismic velocities, negative gravity signa- 2008, Schmitt, 2011; Cooper, 2015). The difference between zircon crystalliza- ture, and high conductivities with a similar areal extent to the APVC that has tion ages and eruption ages represents the duration of zircon crystallization been called the Altiplano-Puna Magma Body (APMB; Chmielowski et al., 1999; and therefore melt presence if zircon crystallization can be shown to be con- Zandt et al., 2003; Götze and Krause, 2002). The most recent work identifies a tinuous until the time of eruption (Reid et al., 1997). As such U-Pb (and U-Th) coincidence between the APVC, a negative Bouguer anomaly of 400–500 mGal zircon ages can be used to delineate the pre-eruption magmatic history as a (Prezzi et al., 2009), and a 500,000 km3 volume of anomalous seismic (shear) complement to the eruptive records and in doing so provide a more complete velocities <2.9 km/s between 9 and 31 km below the surface (Ward et al., 2014) view of magmatic history. (Fig. 1). Estimates of the volume of partial melt in this zone are >20% (Comeau This study explores the zircon crystallization histories recorded in ig- et al., 2015), and plutonic:volcanic ratios of ~20:1 have been calculated (Ward nimbrites from the Altiplano-Puna Volcanic Complex (APVC) of the Central et al., 2014), supporting the contention that the APMB is the intrusive equiva

A B 68° 67° 20°S 21 °

Chile Bolivia 22 ° Chuhuilla, 5.45 Ma

Pastos Grandes, 2.89 Ma Carcote (5.65 Ma) Vilama, 8.41 Ma

Lower Rio San Guacha, 5.65 Ma 22°S Pedro ca. 9.4 Ma Sifon, 8.3 Ma ° Figure 1. Eruptive record of the Altiplano Toconce Panizos, 6.79 Ma 23 6.6–5.6 Ma Puna Volcanic Complex (APVC). (A) Map of Calama Granada ca. 9.8 Ma the APVC in the Central Andes defined by

Artola (ca. 9.8 Ma) the approximate outlines of the mapped Puripicar, 4.09 Ma Pairique 050 100 extent of each of the major ignimbrites. Purico, 1.03 Ma ca. 11 Ma Tara, 3.49 Ma km Solid lines and normal font are for those Pampa Chamaca 2.2 Ma Coranzuli, 6.6 Ma 24° focused on in this study. Dashed lines and Pujsa 5.6 Ma Atana, 3.96 Ma 2.9 km/s velocity italics are for other eruptions in the re- 24°S gion that are not focused on in this study. Stage 4 contour at 10 km depth Ages are the latest 40Ar-39Ar or K-Ar ages Stage 3 (Ward et al., 2014) reported in the literature (Table 1). Colors are propogated throughout the manu- Stage 2 Area of –400 mGal script and reflect different stages of the Argentina Salta Stage 1 Bouguer anomaly (Prezzi development of the APVC. Some erup- 70°W 68°W 66°W tions (Divisoco, San Antonio, Toconao, et al., 2009) and Alota ignimbrites) are not shown for clarity. Data sources are given in the text. C (B) Locations of the known eruption sites 4 3 2 1 for each of the ignimbrites are color keyed to their stage of formation (given in C). 1600 Sizes and shapes are based on mapping

) and image interpretation. Map extent of 3 seismic low-velocity zone (Ward et al., Atana 2014) and negative gravity Bouguer anom- 1400 a aly (Prezzi et al., 2009) approximates the ch present extent of the Altiplano-Puna Magma Body. (C) Age and eruptive vol-

Gua 1200 lamaVi umes of the ignimbrites focused on in this study. DRE is dense rock equivalent. Four Chuhuilla stages of eruptions are identified. Width

Sifon 1000 of the vertical bars is a measure of the age uncertainty. Small numbered arrows on the x axis are for eruptions that are not featured in this study. Numbers refer 800 to the following ignimbrites. 1—Tatio; 2—Pampa Chamaca; 3—Alota; 4—Pujsa;

Panizos 5—Carcote; 6—Coranzuli; 7—Lower Rio

Pastos Grandes 600 San Pedro; 8—Granada; 9—Divisoco; 10—

a Coyaguama.

Puripicar

ra Ta

Erupted Volume (DRE km 400

conc

l 200 To

Antoni

Purico

Laguna Colorad

Arto

conao

San

Puripica Chico

To 1 2 3 4 5 6 7 8 10 0 2 4 6 8 910 12 Age (Ma)

lent of the voluminous APVC volcanics (de Silva et al., 2006). This upper crustal on Fe-Ti oxide thermometry (Lindsay et al., 2001a; Schmitt et al., 2001; Burns

MASH (melting, assimilation, storage, homogenization; after Hildreth and et al., 2015; Grocke et al., 2016). Using these temperatures, TiO2 activities, aTiO2 Moorbath, 1988) zone (Burns et al., 2015), represents the largest known conti- (relative to rutile saturation), of ~0.8 are estimated from the dominant trend for nental crustal magmatic zone (Zandt et al., 2003; Ward et al., 2014). Continued arc-related volcanic rocks in Ghiorso and Gualda (2013). magmatic activity in the APMB is inferred from satellite interferometry studies The ignimbrites display clear arc affinities in their geochemistry with (Pritchard and Simons, 2002; Fialko and Pearse, 2012) that have identified a characteristic trace element ratios like high Ba/Nb and Ba/La (de Silva et al., zone of surface uplift centered on the composite volcano Uturuncu with an 2006; Kay et al., 2010; Folkes et al., 2011; Freymuth et al., 2015). Isotopic and outer ring of subsidence that extends the surface deformation to ~150 km in geochemical studies constrain the magmas as a mixture of mantle-derived diameter (Fig. 1). melts and crustal melts in crustal to mantle ratios from 1:1 to 7:3 (de Silva, Further correlations include the strong spatial coincidence between the 1989a; Coira et al., 1993; Ort et al., 1996; Schmitt et al., 2001; Lindsay et al., APVC elevated asthenospheric depths of 60–80 km and high-heat-flow values 2001a; Schilling et al., 2006; Kay et al., 2010; Folkes et al., 2013), depending on of >100 mW/m2 (Prezzi et al., 2009). These support the long-standing geo the composition of the mantle source and contaminant chosen (see McLeod dynamic model that major changes in the geometry of the subduction zone et al., 2012, for further discussion). Peraluminous compositions in the eastern conspired to produce voluminous mafic melt that in turn triggered crustal APVC have the most crustal Sr and Nd isotopes (87Sr/86Sr > 0.712; 143Nd/144Nd melting and ignited the ignimbrite flare-up in the APVC from ca. 11 to 1 Ma (de ~ 0.512200) and other characteristics attesting to the influence of a metapelitic Silva, 1989a). The onset of volcanism in the APVC is correlated in time with a basement in the east (Caffe et al., 2012). large degree of steepening of the subducting Nazca plate from nearly flat-slab The petrological, geochemical, and geophysical data thus indicate a crustal subduction prior to 16 Ma to an ~30° dip today (Barazangi and Isacks, 1976; magmatic system that had three distinct but linked levels that become succes- Allmendinger et al., 1997). The onset of steepening (slab roll-back) is thought sively more silicic upward through the crust (de Silva et al., 2006): (1) a lower to have induced decompression melting in the mantle and facilitated delami crustal MASH zone where mafic magmas pond and mix with crustal melts to nation of the base of the continental lithosphere (Allmendinger et al., 1997; Kay produce basaltic andesite compositions (e.g., Hildreth and Moorbath, 1988) that and Coira, 2009). then (2) accumulate at ~15–30 km (the APMB). Further differentiation through recharge, assimilation, and fractional crystallization of this basaltic andesite magma produces andesitic and dacitic melts that then separate and (3) accu- GENERAL PETROLOGY AND GEOCHEMICAL BACKGROUND mulate and differentiate further in the upper crust (220–110 MPa; Schmitt, 2001; Muir et al., 2014a, 2014b; Grocke, 2014) between 10 and 5 km to produce the The magmas erupted during the APVC ignimbrite flare-up are broadly crystal-rich dacites and their rhyolitic derivative magmas that eventually erupt similar in their petrological and geochemical characteristics. The ignimbrites in the caldera-forming eruptions. Early peraluminous rhyolites may have been are typical of the large monotonous intermediate genre (e.g., Hildreth, 1981) generated through contamination of calc-alkaline intermediate compositions and evince that ~95% of the >15,000 km3 of magma erupted is crystal-rich, with metapelite at depths ~20 km within the APMB (Caffe et al., 2012). high-K dacite to rhyodacite in bulk composition. The magmas were domi- nantly calc-alkaline, although ~1000 km3 of peraluminous dacites were erupted in the eastern APVC (Ort et al., 1996; Caffe et al., 2002). Rhyolitic bulk com- SURFACE RECORD OF VOLCANISM IN THE APVC positions are rare. Andesitic compositions are only found as rare individual pumices in some ignimbrites or more commonly as bands and inclusions The surface volcanic record of the APVC is dominated by large-volume ig- in pumices in most ignimbrites (de Silva, 1991; Lindsay et al., 2001a, 2001b; nimbrite eruptions composed primarily of high-K dacites and rhyodacites with Schmitt et al., 2001; Burns et al., 2015; Wright et al., 2011). Although these are minor rhyolites. Ignimbrite eruptions are sourced at large, multicyclic, nested volumetrically insignificant, they provide evidence for the role of intermediate calderas and smaller ignimbrite shields (see de Silva et al., 2006; de Silva and magmas in the evolution of the dacitic magmas, representing the thermal and Gosnold, 2007). Seven eruptions from the La Pacana, Guacha, Pastos Grandes, material input from the APMB into the pre-eruptive reservoirs (Burns et al., and Vilama calderas exceed 1000 km3 of magma, with the 2400 km3 erup- 2015). The mineralogy of the pumice demonstrates that the dacite magmas tion of the Atana ignimbrite at 3.98 Ma from the La Pacana caldera being the crystallized a low-pressure assemblage with quartz, plagioclase, amphibole, largest yet recognized in the region. The space-time-volume record of APVC biotite, and Fe-Ti oxides as the major mineral phases. Sanidine is found in a ignimbrites suggests that peaks of intense eruptive activity ca. 8.4, 5.5, and few ignimbrites but is absent in others; when present, it is always subordinate 4.0 Ma are bracketed by a period of waxing activity from ca. 11 to 8.4 Ma and a to plagioclase. Ubiquitous accessory minerals include apatite, zircon, allanite, waning stage from ca. 2 Ma to recent (Fig. 1; Table 1). These were organized into and titanite. Monazite is found in some heavy mineral separates. Temperatures four clusters of eruptions or eruptive stages in de Silva et al. (2015). We detail

of equilibrium range from 700 to 800 °C with f O2 of Ni-NiO (NNO) + 1 based these eruptive stages in the following as a prelude to presenting the zircon data.

TABLE 1. SUMMARY OF IGNIMBRITES USED IN THIS STUDY buried fissure source; a vent location beneath the Piedras Grandes and Copa- Erupted volume coya domes on the western edge of the APVC near El Tatio is the most prob- Eruption age† (magma) able source (de Silva, 1989b). The 1400 km3 (DRE), crystal-rich, pumice-poor Ignimbrite Source caldera (Ma ± 2σ) (km3) Vilama ignimbrite erupted from the Vilama caldera on the Bolivia-Argentina Purico Purico 0.98 ± 0.03 100 border (Soler et al., 2007). A single exposure of an unnamed 8.35 ± 0.03 Ma Puripica Chico Guacha 1.70 ± 0.02 10 rhyolite ignimbrite was found on the western flanks of Cerro Chaxas (de Silva, Laguna Colorada Laguna Colorado 1.98 ± 0.03 60 1989b; K-Ar age on biotite). The ignimbrites of this stage are the lowest in the Pastos Grandes Pastos Grandes II 2.89 ± 0.01 1500 sequence with basal contacts with Paleogene to lower Miocene and Paleo- Tara Guacha II 3.49 ± 0.01 800 Atana La Pacana 3.96 ± 0.02 2400 zoic basement throughout the APVC (de Silva, 1989b; Ort, 1993; Lindsay et al., Toconao La Pacana 4.00 ± 0.12 300 2001; Soler et al., 2007; Caffe et al., 2008, 2012). A minimum total volume of Puripicar Chaxas 4.09 ± 0.02 1500 ~3000 km3 erupted during this first stage. Chuhuilla Pastos Grandes I 5.45 ± 0.02 1200 Eruptive stage 2 of the APVC flare-up began ca. 6.7 Ma and extended until Guacha Guacha 5.65 ± 0.01 1300 ca. 5.2 Ma (pulse 3 of de Silva and Gosnold, 2007). Early eruptions in this stage Toconce Loma Lucero 6.52 ± 0.19 100 are found in the east of the APVC, where the 6.79 ± 0.02 Ma Panizos ignimbrite Panizos Panizos 6.79 ± 0.02 650 (600 km3 DRE; Ort, 1993) and the Coranzuli ignimbrite (6.6 ± 0.15 Ma K-Ar age Sifon Piedras Grandes 8.33 ± 0.06 1000 on biotite; Seggiaro, 1994; 650 km3 deposit volume) erupted from their epony- Vilama Vilama 8.41 ± 0.02 1400 mous sources. Several relatively small (~100 km3) eruptions from the vicinity Artola Unknown buried 9.40 ± 0.03 >100 San Antonio Cerro San Antonio 10.33 ± 0.64 >10 of the Piedras Grandes–Loma Lucero region of Chile deposited the Toconce Formation between 6.52 ± 0.19 and 6.33 ± 0.12 Ma (40Ar/39Ar ages on biotite; Note: Eruption ages and volumes were reported in Salisbury et al. (2011) and de 3 Silva and Gosnold (2007), respectively. Volumes are reported as dense rock (magma) Salisbury et al., 2011). Stage 2 culminated with >3000 km of magma erupting equivalent. in <250 k.y., marked by the eruptions of the >500 km3 Pujsa (5.60 ± 0.02 Ma; †Ar-Ar data in Salisbury et al. (2011) were calculated relative Taylor Creek rhyolite K-Ar age on biotite; de Silva, 1989b), 5.65 ± 0.01 Ma Guacha (1300 km3) and the sanidine at 28.34 Ma, which is equivalent to Fish Canyon Sanidine at 28.02Ma (Renne 5.45 ± 0.02 Ma Chuhuilla (1200 km3) ignimbrites. The Guacha ignimbrite, which et al., 1998). covers >5800 km2, is sourced from the Guacha caldera in southern Bolivia near the Argentinian border. The Chuhuilla ignimbrite is the first eruption from the Pastos Grandes caldera in the northern part of the APVC (Fig. 1; Salisbury et al., The waxing of the flare-up is designated eruptive stage 1. This initiated 2011). The Pujsa ignimbrite is the oldest eruption known from the La Pacana ca. 11 Ma and culminated with two supereruptions ca. 8.4 Ma (this com- caldera (Lindsay et al., 2001b). A distinct high-Si rhyolite eruption from the bines pulses 1 and 2 of de Silva and Gosnold, 2007). The earliest eruptions at vicinity of the Salar de Carcote in Chile at 5.4 ± 0.4 Ma (K-Ar age in biotite; 11–9 Ma occurred at separate, distant centers along the periphery of the APVC Baker and Francis, 1978) deposited the extensive but poorly studied Carcote ig- and produced small-volume ignimbrites. These include the Artola ignimbrite nimbrite. Together these eruptions define a distinct 250-km-long north-north- (9.40 ± 0.03 Ma; all average age uncertainties reported 2s) erupted from a bur- west–south-southeast trend (335°) from La Pacana to the Salar de Carcote. The ied source in the San Bartolo–Rio Grande area, and the San Antonio ignimbrite final eruption from this stage is the >10 km3 Alota ignimbrite (5.23 ± 0.01 Ma; (10.33 ± 0.64 Ma) that crops out sparsely near Volcan Uturuncu (Fig. 1). Other 40Ar/39Ar on sanidine) that forms an ignimbrite shield capped by the Juvina poorly exposed ignimbrites of this age range include the Lower Rio San Pedro dome on the northern flank of the Pastos Grandes caldera. At least 4000 km3 of (10.71 ± 0.14 Ma; 40Ar/39Ar age on biotite; Salisbury et al., 2011) and the Divisoco magma erupted during this second stage. ignimbrite (10.18 ± 0.15 Ma; 40Ar/39Ar age on biotite; Salisbury et al., 2011) in Stage 3 of the flare-up extends from 4.09 to 2.89 Ma (pulse 4 of de Silva the westernmost APVC (de Silva, 1989b) around Volcan San Pedro. Of simi- and Gosnold, 2007). The period initiated with ~3500 km3 of magma erupted lar age are the Granada ignimbrite (K-Ar age on biotite 10.1 ± 0.4 Ma; Caffe in ~100 k.y. At 4.09 ± 0.02 Ma the 800 km3 Puripicar ignimbrite erupted from et al., 2008) and Pairique volcanics, including the 11.28 ± 0.03 Ma Coyaguayma the area of the Chaxas dome complex, while soon after the massive 2700 km3 ignimbrite (40Ar/39Ar age on biotite; Caffe et al., 2012). These early eruptions Atana-Toconao eruption at 3.96 ± 0.02 and 4.0 ± 0.01 Ma, respectively, followed suggest that an extensive magmatic system was already developing beneath to form the La Pacana caldera (Lindsay et al., 2001b; Salisbury et al., 2011). At the APVC by at least 10 Ma (de Silva and Gosnold, 2007; Salisbury et al., 2011). 3.49 ± 0.01 Ma the 800 km3 Tara ignimbrite erupted from the Guacha caldera Two large-volume eruptions that deposited the 8.33 ± 0.06 Ma Sifon and the and terminated an intense period of eruptions from sources within 50 km of 8.41 ± 0.02 Vilama ignimbrites define the climax of this first waxing stage 1 each other. The final major eruption of stage 3 is the 2.89 ± 0.01 Ma eruption of of the APVC flare-up (de Silva, 1989a; Soler et al., 2007; Salisbury et al., 2011). the Pastos Grandes caldera that deposited 1200 km3 of magma as the Pastos The ~1000 km3 (dense-rock equivalent, DRE) Sifon ignimbrite erupted from a Grandes ignimbrite. Note that each of the Atana, Tara, and Pastos Grandes

eruptions are the second eruptions from the La Pacana, Guacha, and Pastos maybe co-Plinian ash. These considerations notwithstanding, the 15,000 km3 Grandes calderas, respectively, that all erupted in stage 2. likely reflects only a small portion of the entire volume of the magma system The waning of the ignimbrite flare-up is marked by a significant reduction that fed the APVC, an assertion that is consistent with the significant plutonic in eruptive volumes in the past 2.5 m.y.; we designate this as eruptive stage 4. volumes estimated based on the geophysical studies mentioned above (e.g., The most extensive of these young ignimbrites are the largely unstudied Patao Ward et al., 2014). The APVC eruptions discussed here tapped the uppermost ignimbrite (2.52 ± 0.06 Ma; 40Ar/39Ar on plagioclase; Barquero-Molina, 2003) reaches of a crustal-scale magmatic zone where the pre-eruptive magmas and the spatially and temporally proximal Talabre–Pampa Chamaca eruption staged, differentiated, and equilibrated. Preliminary work by Schmitt et al. of southern La Pacana at 2.42 ± 0.06 Ma (40Ar/39Ar on plagioclase; Barquero- (2002) and Folkes et al. (2011) suggests that each major eruption taps magma Molina, 2003). Collectively these cover significant areas in the southernmost, that resided and crystallized at pre-eruptive levels for at least a few hundred least well studied area of the APVC. Our reconnaissance mapping and the thousand years before eruption. Here we explore this pre-eruptive magmatic work of Gardeweg and Ramírez (1987) suggest several hundreds of cubic history further to reveal the magmatic development of the APVC through the kilometers of ignimbrite extending beneath the Punta Negra volcanic complex lens of zircon geochronology. and into the Socaire area. Several small to intermediate volume ignimbrites erupted from sources in the center of the APVC, including the Laguna Colorada Zircon as a Geochronometer (60 km3), Puripica Chico (10 km3), and Purico (100 km3) ignimbrites (Salisbury et al., 2011). The Laguna Colorada ignimbrite forms the radiating flanks of the Zircon has long been recognized as an excellent geochronometer because Laguna Colorada ignimbrite shield, erupted between the Pastos Grandes and of its high partition coefficients for uranium and thorium radionuclides used in Guacha calderas. Puripica Chico is generally associated with the Guacha cal- radiometric dating and its low affinity for Pb (e.g., Mahood and Hildreth, 1983; dera, although its source is a small lava dome on the western edge of Guacha Blundy and Wood, 2003). Extremely slow diffusion rates of tetravalent cations (Salisbury et al., 2011). The Purico ignimbrite erupted from the Cerro Purico enable zircon to retain geochemical and isotopic information through multi- ignimbrite shield complex near the Chile-Bolivia border that also includes ple melting events (Cherniak et al., 1997). In addition, zircon is a common ac- a compositionally identical lava dome (dome D; Schmitt et al., 2001). The cessory phase in silicic magmas, making it an abundant and readily obtained ~50 km3 Tatio ignimbrite (0.7 ± 0.01 Ma; 40Ar/39Ar on biotite; Salisbury et al., phase for U-Pb geochronology in crustal rocks (Harrison and Watson, 1983). 2011), the Filo Delgado ignimbrite (younger than 1 Ma based on stratigraphy; Zircon crystallization begins when a silicic melt cools below its zircon satu Lindsay et al., 2001b), and the southern locally exposed Tuyajto ignimbrite ration temperature, which has been constrained experimentally for a wide (0.53 ± 0.17 Ma; 40Ar/39Ar on biotite; Barquero-Molina, 2003) are the youngest range of metaluminous to peraluminous melt compositions and zirconium ignimbrites so far identified. A total erupted magma volume of 500 km3 is a concentrations (e.g., Watson and Harrison, 1983; Boehnke et al., 2013). Zircon conservative estimate for this stage 4. The youngest eruptions in the APVC are with dimensions typically analyzed (~100 µm) can crystallize over time scales effusive eruptions younger than 200 ka that occur around the western margins of 1000–10,000 yr once saturation conditions are met, and it can be resorbed of the volcanic province, interspersed with the arc composite cones, some of in a zircon-undersaturated melt over equivalent durations (Watson, 1996), with which locally share temporal and chemical affinity with this waning stage of larger zircon crystals surviving preferentially over smaller zircon crystals. When the APVC (Godoy et al., 2014). The transition to effusive eruptions might sug- zircon crystallization conditions are reached anew in a magma undergoing gest that the waning of the flare-up signaled the death of the volcano-plutonic cycles of recharge and cooling, surviving zircon crystals are expected to pro- system as a whole. However, a cluster of five lava domes (studied by de Silva vide nucleation sites for additional zircon growth. In this way, zircon can record et al., 1994) exhibit concordant eruption and zircon age and chemistry char- multiple magmatic events, often with hiatuses corresponding to episodes of acteristics that require a shared thermal history within a magmatic system of resorption or crystal residence at subsolidus conditions (e.g., Storm et al., 2011). superuption scale (Tierney et al., 2015). Thus even though the erupted volumes Reid et al. (1997) pioneered the use of high spatial resolution, in situ U-Th only total <50 km3, a vigorous plutonic system of supervolcanic proportions zircon geochronology to understand the time scales of magma storage based appears to have underpinned these for several hundred thousand years prior on the principles that (1) zircons are frequently found as inclusions within to eruption (Tierney et al., 2015). major phenocryst phases, suggesting that they formed early during the crys- Overall, a total of >15,000 km3 of magma erupted over ~10 m.y. has been tallization process and (2) zircon crystallizes once a magma cools below its estimated (de Silva et al., 2006; de Silva and Gosnold, 2007; Salisbury et al., zircon saturation temperature, which is higher than eruption temperatures. 2011). This estimate includes room for the unknown volumes of intracaldera Based on these observations, which are backed by many recent high-precision ignimbrite from several large ignimbrites including the Pujsa, Puripicar, and U-series and U-Pb zircon crystallization ages with accompanying high-preci- Sifon, the sources of which have not been studied in detail. Moreover, distal ash sion 40Ar/39Ar eruption ages (see following examples), zircon dating can be an deposits from APVC eruptions are only recently being recognized (Breitkreuz effective way to constrain the pre-eruption history of a magma in the shallow et al., 2014), attesting to significant unaccounted volumes of co-ignimbrite and crust (Simon et al., 2008; Schmitt, 2011; Cooper, 2015).

METHODS

Sample Collection

Pumice samples from 16 ignimbrites representative of the temporal and spatial span of the APVC were chosen for this study (Table 1). The focus of this study is the eruptions that define the peak ignimbrite flare-up. In this strategy we sam- pled 9 of the 10 main eruptions of stages 2 and 3. The only large ignimbrite not studied was the ca. 5.6 Ma Pujsa ignimbrite (Gardeweg and Ramirez, 1987; de Silva, 1989b) erupted from La Pacana (extractable pumice blocks were not available from this unit). In addition to these nine main units, four eruptions from stage 1, the waxing stage, and three eruptions from stage 4, the waning stage, were studied. Pumice was selected as opposed to bulk ignimbrite as representative of the juvenile magma to avoid any zircons foreign to the magma being entrained during eruption or emplacement (Schmitt et al., 2002). New samples collected for this study in 2007 and 2010 from the Guacha, Puripica Chico, Pastos Grandes, Chuhuilla, and Laguna Colorada ignimbrites in southwest Bolivia were supplemented with samples from around the APVC collected by our group over 25 years of field work and mapping in the APVC. Selected samples were fist- sized or larger and had minimal visible alteration. Another part of our strategy was to use, as far as possible, the same samples (the same mineral separates) processed for Ar-Ar analysis (Salisbury et al., 2011), but several samples post- date that study. Another key strategy was to focus on the dominant pumice type. Figure 2. Cathodoluminescence (CL) images of representative zircon crystals. Circles repre- We have found that each ignimbrite contains one dominant pumice type with sent single spots analyses with corresponding ages. Zircons are from the Pastos Grandes ig- limited bulk and mineral chemical variation in each ignimbrite (de Silva and nimbrite [on left (upper) B07027 grain 3 and (lower) B07027 grain 9; Supplementary Table 1] and Chuhuilla ignimbrite [on right (upper) 6050 grain 9 and (lower) grain 6; Supplementary Table 1]. Francis, 1989; Ort et al., 1996; Lindsay et al., 2001a; Schmitt et al., 2001). We chose See Supplementary Figure 1 for our complete database of images. large single or multiple smaller pumice blocks from the dominant pumice type in each ignimbrite. In a few cases two or more samples from widely separated locations from the same eruption were processed and analyzed separately to electron microscope at the University of California, Los Angeles (UCLA). U-Pb check for intraunit and intersample variability. zircon ages were obtained using the UCLA CAMECA IMS 1270 ion probe using analytical conditions similar to those applied by Schmitt et al. (2003). A 10– 20 nA 16O– beam was focused on a 25–30 mm spot with a total depth resolution Mineral Separation of ~0.5 mm for individual analysis spots. Secondary ions were extracted at 10 kV using an energy band pass of 50 eV. The mass spectrometer was tuned to a Zircons were separated using standard separation techniques. For dating, mass resolution of ~5000 (measured at 10% peak height) to resolve molecular a minimum of 20 but more often 50–60 representative zircon crystals were interference in the mass range analyzed. The relative sensitivity between Pb hand-picked for each ignimbrite. Grains were cast in 2.54-cm-diameter epoxy and U was calibrated to allow U concentrations for unknown analyses to be disks and ground to the approximate mid-sections of grains, and polished to calculated based on a value of 81.2 ppm for zircon standard 91500 (Gehrels a 1 µm finish. et al., 2008). U-Pb ages were calibrated to zircon age standard AS3 (1099.1 Ma; Paces and Miller, 1993), which was measured throughout the analytical session. All 206Pb/238U ages are reported with a correction for 230Th disequilibrium. The 1Supplemental Figures 1 and 2. Cathodoluminescence correction was based on the ratio of zircon-melt partitioning values for U and images of zircons (Supplemental Fig. 1) and rank or- U-Pb Secondary Ionization Mass Spectrometry Analysis der plots (ROP) of rim and interior ages with proba- Th (Schärer, 1984) using an average whole-rock Th/U = 2 for aphyric APVC rhyo- bility density function curves for the Pastos Grandes, High-spatial-resolution secondary ionization mass spectrometry (SIMS) has lites, which better represent melt compositions compared to crystal-rich dacites Guacha, Tara, and Chuhuilla ignimbrites (Supple- proven to be a highly effective tool for U-Pb zircon dating, in particular for com- (Lindsay et al., 2001). The resulting uncertainty from reasonable variations in mental Fig. 2). Please visit http://dx.doi .org /10 .1130 /GES01258.S1 or the full-text article on www.gsapubs plexly zoned grains. Catholodoluminescence (CL) images (Fig. 2; Supplemen- melt Th/U is negligible relative to other sources of analytical uncertainty which .org to view the Supplemental Figures 1 and 2. tary Fig. 11) were taken of each zircon mount using a Leo 1430 VP secondary for young zircon are dominated by the effects of common Pb.

Both interior and rims on polished grains were analyzed for a minimum elements overlapped with or were as close as possible within the same CL of 20 total analyses per sample, with an ideal goal of 50 analyses per sample. zone as the U-Pb analytical pit. Analyses were conducted using the UCLA Rims were preferentially measured in order to examine more closely the last CAMECA IMS 1270 following analytical procedures in Schmitt and Vazquez stages of zircon crystallization. Interior measurements composed ~15%–20% (2006). Relative sensitivity factors in relation to Si (assumed to be present in of analyses per sample. stoichiometric abundances in zircon) were calibrated using NIST SRM 610 All data are presented in Supplementary Table 12 and summary data are glass (Pearce et al., 1997) and SL13 (6.32 ppm Ti; Harrison and Schmitt, 2007); in Table 2. For each unit, U-Pb ages were calculated as weighted mean ages. zircon standard 91500 was analyzed as a secondary reference. Significant ma- All errors in the text and Table 2 are quoted as 2s standard error, while the trix effects between glass and zircon were only detected for Ti (~–11%), and full data set in Supplementary Table 1 is given as 1s standard errors. Data are consequently SL13 was used as the primary standard for Ti, and NIST SRM 610 presented as rank order plots of individual analyses and probability density for all other elements. Additional elements (Mg, Mn, and Fe) were monitored

Supplementary table 1: APVC ignimbrite U-Pb zircon results Correlation of TW 1 1 Concordia 1 204Pb/ 206 206 Pb/ 238U 206Pb/ 238 U sample grainspotCore/Rim 238 U/ 206Pb 1 238 U/ 206Pb 207 Pb/ 206Pb 207Pb/ 206 Pb Ellipses 204 Pb/ 206Pb Pb % 206 Pb* age [Ma] age [Ma] U (ppm)U/ThUO/UComments Artola Artola function curves generated using Isoplot (Ludwig, 2012). Weighted averages to detect potential beam overlap onto non-zircon phases (e.g., adherent glass) 2011_08_22Aug\ [email protected] 11R6.13E+023.20E+019.52E-02 9.05E-03 -0.23 5.00E-03 2.24E-03 93.7 9.93 0.57 2692.777.4 2011_08_22Aug\ [email protected] 21R6.70E+023.01E+016.30E-02 2.27E-030.033.24E-036.96E-0497.99.470.43 2045 1.03 7.4 2011_08_22Aug\ [email protected] 31R6.46E+023.76E+018.03E-02 7.27E-03 -0.22 6.08E-03 3.51E-03 95.6 9.62 0.59 1502.787.4 2011_08_22Aug\ [email protected] 41R6.19E+023.33E+017.00E-02 4.62E-03 -0.11 2.80E-03 1.54E-03 97.0 10.2 0.6342 2.97 7.2 2011_08_22Aug\ [email protected] 51R6.98E+023.14E+015.92E-02 5.06E-030.023.49E-031.56E-0398.49.170.42426 3.00 7.5 2011_08_22Aug\ [email protected] 61R6.69E+023.54E+017.18E-02 7.99E-03 -0.08 3.11E-03 1.71E-03 96.7 9.40 0.52 3092.657.4 2011_08_22Aug\ [email protected] 71R6.30E+023.43E+017.48E-02 6.51E-030.125.49E-032.65E-0396.49.940.56224 1.73 7.3 2011_08_22Aug\ [email protected] 81R6.14E+023.40E+018.22E-02 4.44E-03 -0.11 3.31E-03 1.72E-03 95.4 10.1 0.6315 3.50 7.3 206 were obtained by pooling the youngest analytically indistinguishable zircon or inclusions, but not quantified due to a lack of certified standards for these 2011_08_22Aug\ [email protected] 91R5.13E+022.27E+012.61E-01 2.22E-02 -0.16 1.49E-02 3.16E-03 72.6 9.20 0.69 3293.227.7 Pb* <80% 2011_08_22Aug\ [email protected] 83002101R6.76E+022.96E+017.39E-02 7.48E-030.096.18E-032.85E-0396.59.290.43214 2.62 7.5 2011_08_22Aug\ [email protected] 11 1R5.65E+022.57E+011.81E-01 7.44E-03 -0.01 1.56E-02 3.39E-03 82.8 9.53 0.54 3432.147.4 2011_08_22Aug\ [email protected] 83002121R5.91E+023.47E+011.27E-01 7.24E-03 -0.05 1.08E-02 3.72E-03 89.7 9.84 0.65 2161.547.2 2011_08_22Aug\ [email protected] 83002131R5.93E+022.95E+011.55E-01 1.67E-020.133.09E-028.20E-0386.19.450.59124 3.61 7.3 2011_08_22Aug\ [email protected] 83002141R6.56E+023.26E+018.02E-02 5.50E-03 -0.03 5.60E-03 1.77E-03 95.7 9.49 0.49 5053.317.4 206 206 2011_08_22Aug\ [email protected] 83002151R2.25E+021.14E+015.76E-01 9.84E-030.063.64E-024.00E-0332.39.331.75244 3.15 7.4 Pb* <80% 2011_08_22Aug\ [email protected] 83002161R3.73E+012.11E+007.92E-01 7.00E-030.035.12E-022.81E-034.7 8.21 13.31199 2.65 7.1 206 Pb* <80% 2011_08_22Aug\ [email protected] 83002171R6.72E+023.69E+017.29E-02 8.69E-03 -0.14 3.38E-03 2.05E-03 96.6 9.36 0.54 2482.737.4 ages. Only ages with radiogenic Pb > 80% were used in age calculations. elements in zircon. Analyses were discarded where these monitoring elements 2011_08_22Aug\ [email protected] 83002181R6.14E+023.32E+019.37E-02 7.18E-030.035.54E-033.05E-0393.99.940.58166 1.70 7.3 2011_08_22Aug\ [email protected] 83002191R6.84E+023.24E+015.80E-02 3.91E-03 -0.05 2.66E-03 1.29E-03 98.5 9.36 0.45 4882.277.3 2011_08_22Aug\ [email protected] 83002201R6.65E+022.86E+015.01E-02 1.82E-03 -0.03 2.45E-03 6.19E-04 99.5 9.69 0.42 1812 0.87 7.5 2011_08_22Aug\ [email protected] 83002211R1.24E+028.43E+006.49E-01 1.05E-02 -0.08 4.78E-02 2.89E-03 22.9 11.9 4.7489 2.31 7.1 206 Pb* <80% 2011_08_22Aug\ [email protected] 83002221R6.74E+023.16E+017.55E-02 4.58E-03 -0.10 6.55E-03 2.58E-03 96.3 9.28 0.45 3532.177.5 2011_08_22Aug\ [email protected] 83002231R5.15E+022.50E+012.36E-01 1.52E-02 -0.20 1.59E-02 3.94E-03 75.7 9.55 0.69 2531.967.4 206 Pb* <80% 2011_08_22Aug\ [email protected] 83002241R4.86E+022.74E+012.44E-01 6.85E-030.041.55E-023.35E-0374.79.980.78304 1.94 7.3 206 Pb* <80% 2011_08_22Aug\ [email protected] 83002251R6.80E+022.96E+016.38E-02 5.12E-03 -0.36 2.30E-03 1.63E-03 97.8 9.32 0.42 2601.028.2 206 were significantly elevated relative to the zircon references. 2011_08_22Aug\ [email protected] 83002261R4.02E+021.66E+013.33E-01 9.34E-030.002.03E-024.00E-0363.310.20.7 2413.057.9 Pb* <80% 2011_08_22Aug\ [email protected] 83002271R6.47E+022.83E+019.31E-02 8.56E-03 -0.27 5.31E-03 2.93E-03 94.0 9.44 0.45 1731.928.2 2011_08_22Aug\ [email protected] 83002281R6.56E+022.54E+016.39E-02 6.61E-030.014.27E-031.84E-0397.79.680.39384 1.62 8.0 2011_08_22Aug\ [email protected] 83002291R6.58E+022.67E+016.81E-02 4.91E-030.194.11E-031.79E-0397.29.610.40414 2.86 7.9 2011_08_22Aug\ [email protected] 83002301R6.52E+022.70E+016.87E-02 6.31E-03 -0.11 7.01E-03 2.71E-03 97.1 9.66 0.42 2761.168.1 2011_08_22Aug\ [email protected] 83002311R5.82E+022.31E+011.35E-01 1.01E-02 -0.22 8.59E-03 3.04E-03 88.7 9.88 0.47 2461.378.0 2011_08_22Aug\ [email protected] 83002321R5.71E+022.34E+011.56E-01 6.17E-030.098.45E-032.44E-0386.09.790.48373 2.78 8.0 Zircon Trace Element Analysis by SIMS 2Supplemental Table 1. Complete zircon U-Pb data RESULTS for this study. Please visit http://dx.doi.org/10 .1130 A subset of zircons from selected samples analyzed for U-Pb ages was also /GES01258.S2 or the full-text article on www.gsapubs analyzed for trace elements including Y, rare earth elements (La, Ce, Pr, Nd, Sm, Zircon Trace Elements .org to view Supplemental Table 1. Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), P, Ti, Fe, HfO2, Th, and U. Three units were chosen; two crystal-rich dacites (3.96 Ma Atana ignimbrite and the 1 Ma Purico Trace elements in zircons from three representative units in the APVC, the

Supplementary table 2: APVC ignimbrite trace element zircon results

Reference spot La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu YPTi Fe HfO2 Th U ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm wt.% ppm ppm ignimbrite, representative for the peak and waning stages of the flare-up, re- Atana, Toconao and Purico ignimbrites, are typical of trace elements from zir- 91500 10.03132.910.0128 0.298 0.425 0.206 2.16 0.846 11.0 5.03 25.9 6.62 65.4 15.7 140 33.75.57 88.00.704 32.9 101 91500 20.03212.900.0147 0.160 0.528 0.255 2.00 0.898 11.1 5.06 25.2 6.88 66.2 15.4 146 32.05.07 108 0.697 31.5 99.4 91500 30.01952.900.0191 0.08500.410 0.192 2.42 0.921 11.8 5.22 26.5 6.95 67.6 15.9 148 33.99.66 102 0.691 31.3 99.4 91500 40.04883.220.0161 0.251 0.326 0.268 2.57 0.910 10.9 5.52 28.9 7.42 75.8 17.1 160 32.55.09 90.40.756 36.4111 91500 50.02652.920.0129 0.130 0.405 0.188 1.95 0.853 11.5 4.73 26.1 6.92 67.4 15.7 146 31.25.42 89.30.698 34.5 101 91500 60.02402.960.0225 0.255 0.514 0.219 2.10 0.932 11.1 4.89 26.5 6.90 69.0 16.3 146 29.85.73 96.10.705 32.2 97.2 3 91500 70.07442.700.0318 0.223 0.328 0.203 1.78 0.771 9.25 4.63 22.8 5.99 60.0 13.4 143 34.75.386.870.724 24.3 81.4 91500 80.05982.640.0251 0.259 0.289 0.226 1.69 0.810 9.82 4.51 22.2 5.86 56.3 13.1 139 38.44.608.430.707 25.5 79.0 spectively) and the aphyric Toconao rhyolite (4.0 Ma). SIMS spots for trace con in the continental crust (Fig. 3; Supplementary Table 2 ). The Zr/Hf can be 91500 90.05642.770.0231 0.142 0.4110.215 1.82 0.795 10.2 4.65 22.3 6.10 59.1 12.9 139 39.04.99b.d.0.724 24.1 78.1 91500 10 0.06722.790.0143 0.156 0.308 0.178 1.86 0.796 10.1 4.37 23.6 6.02 59.0 13.7 140 37.04.697.770.707 24.8 79.4 91500 11 0.03193.080.0259 0.222 0.432 0.180 2.04 0.901 11.3 5.19 26.3 6.92 69.1 15.2 163 47.05.513.200.731 26.9 94.5

3Supplemental Table 2. Complete zircon trace element 206 238 data for this study. Please visit http://dx.doi .org /10 TABLE 2. SUMMARY OF ZIRCON Pb/ U IN ZIRCON AGE DATA FOR THE ALTIPLANO-PUNA VOLCANIC COMPLEX IGNIMBRITES USED IN THIS STUDY .1130/GES01258.S3 or the full-text article on www Magmatic age .gsapubs.org to view the Supplemental Table 2. Weighted mean and 2σ uncertainty MSWD N Magmatic duration ∆ age (m.y.) Ignimbrite Dominant peak and 2σ uncertainty Table S3 - Magmatic ages - difference between Weighted Purico 1.27 ± 0.16 4.4 24 of 45 0.29 ± 0.08 mean U-Pb zircon crystallization age in Table 2 and Weighted mean Ar-Ar eruption age inTable 1 Puripica Chico 2.05 ± 0.10 7.4 40 of 46 0.35 ± 0.05 ERROR calculated as the square of the sum of the squares Laguna Colorada 2.38 ± 0.22 3.1 18 of 19 0.40 ± 0.11 of the errors of the two ages differenced. Pastos Grandes 3.33 ± 0.14 4.2 51 of 58 0.44 ± 0.07 ∆ age Tara 3.65 ± 0.10 1.6 38 of 46 0.16 ± 0.05 Purico 0.29 0.08 0.29 ± 0.08 Atana 4.17 ± 0.06 0.77 49 of 62 0.21 ± 0.03 Puripica Chico 0.35 0.05 0.35 ± 0.05 Laguna Colorada0.4 0.11 0.40 ± 0.11 Toconao 4.50 ± 0.16 1.07 22 of 27 0.50 ± 0.10 Pastos Grandes0.440.070.44 ± 0.07 Puripicar 4.49 ± 0.10 2.2 45 of 62 0.40 ± 0.05 Tara 0.16 0.05 0.16 ± 0.05 Atana 0.21 0.03 0.21 ± 0.03 Chuhuilla 6.01 ± 0.22 1.5 34 of 41 0.56 ± 0.11 Toconao0.5 0.10 0.50 ± 0.10 Guacha 5.95 ± 0.16 2.1 46 of 52 0.30 ± 0.08 Puripicar 0.40.050.40 ± 0.05 Toconce 6.67 ± 0.30 2.4 17 of 19 0.15 ± 0.17 Chuhuilla 0.56 0.11 0.56 ± 0.11 Guacha 0.30.080.30 ± 0.08 Panizos 7.15 ± 0.32 0.47 15 of 31 0.36 ± 0.16 Toconce0.150.170.15 ± 0.17 Sifon 8.67 ± 0.14 1.6 40 of 53 0.34 ± 0.08 Panizos 0.36 0.16 0.36 ± 0.16 Sifon0.340.080.34 ± 0.08 Vilama 8.78 ± 0.24 0.76 29 of 31 0.37 ± 0.12 Vilama 0.37 0.12 0.37 ± 0.12 Artola 9.59 ± 0.36 0.3 21 of 29 0.19 ± 0.18 Artola 0.19 0.18 0.19 ± 0.18 San Antonio 0.44 0.34 0.44 ± 0.34 San Antonio 9.53 ± 0.32 0.83 25 of 34 0.44 ± 0.34* Note: The weighted mean age is the magmatic age of the ignimbrite. The ∆ age is the estimate of the magmatic duration calculated using the difference between the magmatic age and the eruption age given by the 40Ar/39Ar ages reported in Table 1. See text for further details. The 1σ of the ∆ age was computed using the square root of the sum of the 4Supplemental Table 3. Magmatic duration or D age squares of the 1 uncertainties of the two ages differenced. MSWD—mean square of weighted deviates. See Supplementary Table 34. N—number of zircon analyses used in the calculations. Please visit http://dx .doi.org/10 .1130 σ magmatic age calculation. /GES01258.S4 or the full-text article on www.gsapubs *Determined using U-Pb zircon ages as the difference between 9.09 ± 0.60 Ma (youngest surface age) and the weighted mean U-Pb zircon magmatic age of 9.53 ± 0.32 Ma. .org to view the Supplemental Table 3.

2.0 Ti‑in-zircon implies that zircon crystallized, and the magma differentiated ex- A Purico tensively, over a very narrow range in temperature; ~150 °C for aTiO2 of 0.8 ± 1.5 Atana 0.1 (Fig. 3). In this framework the decrease in U/Th and Eu/Eu* with Zr/Hf sug- Toconao gests that plagioclase and possibly allanite fractionation accompanied frac- Xenocryst 1.0 tionation of zircon in the APVC systems.

Th/U 0.5 U-Pb Zircon Ages

0.0 T

i Crystal size varies greatly between ~50 and ~300 mm, and cathodolumines- B n

- cence images reveal zoned zircon crystals in all ignimbrites (Fig. 2; Supple- 15 z

800 i r mentary Fig. 1). Two morphological populations of zircon crystals are present

10 o in each ignimbrite: equant, subhedral crystals and elongated, prismatic crys- 750 n tals. There is no significant difference in the rim ages between zircon popula-

T (ppm ) tions (Supplementary Fig. 2). CL distinct interiors are often present in zircon 5 700 ( Ti

° 650 C crystals; however, these usually have parallel or subparallel boundaries with a(TiO2) = 0.8 0 ) apparent rim overgrowths and little evidence for resorption. C Probability density function (PDF) curves from all of the APVC ignimbrites 0.4 show dominant peaks in zircon age density a few hundreds of thousands of 0.3 years prior to the eruption age (Fig. 4). These peaks approximate a range of strongly overlapping zircon ages with no significant age gaps and indicate 0.2 the highest density of zircon ages. Most ignimbrites also show small inflections off the dominant peak or smaller peaks slightly older than but overlapping with

Eu/Eu* continental crust 0.1 (80% of data in ages constituting the dominant peak. These represent subpopulations within Carley et al., 2014) 0.0 the overlapping zircon age spectrum. In some cases, interiors yield ages out- 30 35 40 45 50 55 60 65 side of the APVC age span, but only 24 zircon ages (~3% of the total population) plot outside the 40Ar/39Ar age span of the APVC (Fig. 5). Zr/Hf In most cases, significant differences between rim and interior analyses are

Figure 3. Secondary ionization mass spectrometry–determined trace elements in zircon from absent, and they appear to be randomly distributed along the continuous age three ignimbrites from the Altiplano-Puna Volcanic Complex. Directly temperature-dependent spectra of individual samples (Supplementary Fig. 1). Interior and rim analyses trace elements (Ti) and trace element ratios that track differentiation in the magma (Th/U, on the same crystal are also often analytically indistinguishable, but interior Eu/Eu*) are plotted against Zr/Hf, which measures melt evolution. Ti-in-zircon temperatures ages predating the rim ages by several hundred thousand years have been were calculated using the calibration by Ferry and Watson (2007) and aTiO2 = 0.8 (see text). Field (dashed outline) demarks the composition of zircons of continental crustal provenance with the detected in some crystals (Fig. 2; Supplementary Table 1). outline demarcating ~80% of the data in the compilation by Carley et al. (2014). The youngest zircon ages in each age population overlap, within uncertainty, the eruption age for each ignimbrite inferred from 40Ar/39Ar ages of sani dine and in most cases the 40Ar/39Ar ages of biotite. The amount of overlap be- used as an index of differentiation; because Zr is more compatible than Hf in tween zircon and 40Ar/39Ar eruption ages varies between ignimbrites; in some zircon and Zr/Hf ratios in zircon are much higher than in the melt, progressive cases, the youngest zircon ages overlap the eruption age (i.e., Guacha, Tara), zircon fractionation results in decrease of Zr/Hf in the melt and therefore in and in other ignimbrites overlap is within error of only the youngest zircon later crystallizing zircon crystals. The Th/U, Ti, and Eu/Eu* all decrease with ages (i.e., Pastos Grandes). In all cases it is clear that zircon crystallizes over a decreasing Zr/Hf. The covariability of all these parameters is compatible with significant age range from a few hundred thousand years before eruption until crystal fractionation operating during zircon growth. Because Ti partitioning eruption, and the youngest zircons are often concordant with the 40Ar/39Ar age into zircon is temperature dependent and correlates with indirectly tempera- (see also Bachmann et al., 2010). ture-dependent compositional parameters (e.g., Zr/Hf), the decrease in Ti is There is one anomaly, the oldest ignimbrite in the APVC, the San Antonio a useful proxy for decreasing temperature. Although the exact temperature ignimbrite. A 40Ar/39Ar biotite age of 10.33 ± 0.66 Ma was obtained (Salisbury remains difficult to constrain from Ti-in-zircon data alone because of uncer- et al., 2011; weighted mean age of 9 of 12 single crystal fusions) for this ig- tainties in Ti activities and the experimental calibration, the limited range of nimbrite, which is stratigraphically consistent. However, the weighted mean

Purico 0.8 0.98 ± 0.03 Ma 0.15 800 0.6 Puripica Chico 3000 1.70 ± 0.02 Ma 700 0.4 Laguna Colorada 2000 1.98 ± 0.03 Ma 600 0.10 0.2 U 1000 Pastos Grandes 500 23 8 2.89 ± 0.01 Ma 0.0 0510 15 20 25 400

Tara Pb*/ Atana 1 3.49 ± 0.01 Ma

20 6 300 Guacha 1 0.05 Panizos 13 Atana Pastos Grandes 1 3.96 ± 0.02 Ma 200 Puripicar 1 SanAntonio 4 Toconao 100 Sifon 1 age in Ma 4.00 ± 0.12 Ma Toconao 1 2eσ rrors 0.00 Puripicar 0.00.5 1.01.5 2.02.5 4.09 ± 0.02 Ma 207Pb*/235U Chuhuilla 5.45 ± 0.02 Ma Figure 5. Concordia diagram for zircon ages outside of the Altiplano-Puna Volcanic Complex (APVC) time span (>10 m.y.) interpreted as inherited xenocrysts from the basement. Post-analy- Relative Probability Guacha sis imaging suggests that discordant zircons result from beam overlap between xenocrystic and 5.65 ± 0.01 Ma APVC-aged domains. Table inset indicates number of xenocrystic zircons per unit.

Toconce 6.52 ± 0.19 Ma U-Pb age of the zircon population is 9.53 ± 0.32 Ma (2s; mean square of Panizos 6.79 ± 0.02 Ma weighted deviates, MSWD 0.83; Table 2), significantly younger than the biotite 40Ar/39Ar age. Previous work in the APVC has noted that some biotite 40Ar/39Ar Sifon ages can be compromised (Hora et al., 2010; Salisbury et al., 2011); these works 8.33 ± 0.06 Ma reported that 40Ar/39Ar ages from sanidine are often younger than those from Vilama coeval biotite crystals, and the discordance was interpreted as the result of 8.41 ± 0.02 Ma extraneous 40Ar being retained in biotite, requiring that apparent ages from biotite be treated with caution. Similar concerns with biotite ages from other Artola ignimbrites were reported by Smith et al. (2008), Lipman and McIntosh (2008), 9.4 ± 0.03 Ma and Bachmann et al. (2010), among others, and 39Ar recoil, inheritance, or excess 40Ar are posited as the likely causes. In particular, like our case of the San San Antonio 40 39 10.33 ± 0.64 Ma Antonio ignimbrite, Bachmann et al. (2010) found that biotite Ar/ Ar ages of 0 1 2 3 4 5 6 7 8 9 10 11 12 the units in the Kos-Nisyros volcanic complex are older than the youngest zir- Age (Ma) con U-Pb ages; they argued that in this context the youngest zircon U-Pb ages are a better approximation of the maximum eruption age than the 40Ar/39Ar Figure 4. Eruptive history of the Altiplano-Puna Volcanic Complex represented by a rank order biotite age. We believe that the discordance in the zircon U-Pb age and the ing of the 16 ignimbrites that are the subject of this study. Ignimbrites are ranked based on 40 39 39Ar/40Ar eruption ages (Table 1) represented by the bold vertical lines with 2σ errors in the Ar/ Ar biotite age of the San Antonio ignimbrite is real and due to the bio- dashed lines when discernible. Corresponding probability density function plots of U-Pb in tites being compromised. Therefore we suggest that the youngest autocryst zircon ages for each ignimbrite are superimposed in colors that correspond to Figure 1. Group- age for the San Antonio ignimbrite of 9.09 ± 0.60 Ma (2s; BOL06033 grain 30; ings of zircon age spectra with overlapping distributions into four magmatic pulses are indicated by the vertical master index color panels. Details of U-Pb in zircon data for each group Supplementary Table 1) is the best estimate of the maximum eruption age for and the constituent ignimbrites are presented in Figures 6 through 9 and associated text. the San Antonio ignimbrite.

Xenocrysts lytical uncertainties (cf. Mahon, 1996). For each ignimbrite, this population of zircon rim, and some interior, ages that predate the eruption forms the domi Zircon xenocrysts in the APVC ignimbrites have U-Pb ages older than nant peak in each of the zircon spectra (Fig. 4). Because there are zircon auto 12 Ma (maximum age defined by the autocrysts and antecrysts; Fig. 5). Most cryst ages that are distinctly older than the eruption age, and the autocryst data samples yielded at least a single xenocryst age (24 in total); however, this num- show large age dispersion, this implies that zircon was growing well before ber may be biased due to spot selection typically targeting interiors of polished eruption. The weighted mean age of this population is the most common crys- crystals clearly visible in CL, whereas we avoided analyzing highly irregular tallization age for autocrysts prior to eruption, and using this we estimate a interiors. Only the Panizos and San Antonio ignimbrites from the eastern edge magmatic age (Table 2). of the APVC yielded multiple zircon xenocrysts, the Panizos xenocryst popu- These model magmatic ages are all consistent with stratigraphy and the lation comprising almost half of the analyses from that sample. The 206Pb/238U Ar-Ar ages of the units, giving us confidence that these mean ages are a geo- ages of zircon xenocrysts include 2 early Miocene zircons, 1 Oligocene, and logically reasonable estimate of when zircon-saturated melt was present in 20 xenocrysts spanning the rest of the Phanerozoic, and a single Precambrian the system. Accordingly, we calculate a minimum pre-eruptive crystallization xenocryst (Fig. 5). Many of these ages, however, are discordant as a result of history, the D age (Table 2), as the difference between the 40Ar/39Ar eruption beam overlap onto different age domains. age for each ignimbrite and its zircon magmatic age. From these it is clear that minimum pre-eruption crystallization histories of several hundreds of DISCUSSION thousands of years are recorded in these zircons (longer durations being possibly indicated by antecrystic zircon). With one exception, the Toconce ig- In the following, we interpret an individual zircon U-Pb age to indicate that nimbrite, model durations range from ~150 to 500 k.y., with a weighted mean that zircon-saturated melt was present at that time. The fact that individual zir- of ~400 k.y. (Table 2). Because these model magmatic ages are only slightly con rim analyses define a continuous population to within error of the eruption older than eruption compared to the dispersion of ages, this implies significant age supports this interpretation over one where the U-Pb zircon age represents crystallization just prior to and continuing until eruption. the age of magma and zircon cooling and later rejuvenation. If all the zircons in While we are mindful of the potential problems of biotite versus sanidine a magma share a crystallization history over time scales equivalent to or faster 40Ar/39Ar eruption age for these ignimbrites, we note that if biotites give older than the analytical resolution, the weighted mean age of the population should eruption ages than sanidine (Hora et al., 2010), this would reduce the difference reveal the prevalent zircon crystallization age. However, zircon spectra are no- with zircon magmatic ages. We find no systematic differences in the durations toriously complex and record multiple origins of zircon crystals that need to calculated using eruption ages derived from sanidine or biotite; units with pre- be recognized and those populations deconvolved (Miller and Wooden, 2004; ferred eruption ages from biotite or sanidine both give magmatic durations Miller et al., 2007; Folkes et al., 2011; Storm et al., 2011). Informed by previous <400 k.y. and >400 k.y. This, and the fact that the dominant zircon peak ex- work (e.g., Bacon and Lowenstern, 2005; Charlier et al., 2005; Miller et al., 2007; tends further back in time, suggesting a much longer magmatic prehistory, Walker et al., 2010), we use the following framework. Zircon autocrysts crys- gives us confidence that average minimum pre-eruption magmatic histories of tallize in the magma that accumulates prior to an eruption and are used to ~400 k.y. are recorded in the zircons of the APVC ignimbrites. Long pre-erup- constrain the final pre-eruptive crystallization history of a magma. Zircon ante- tive magmatic histories, complex zircon population distributions, and excess crysts (Hildreth, cited in Charlier et al., 2005) are incorporated from material left scatter recorded as high MSWDs are thought to be the natural consequence of from older episodes of magmatic activity at the same center indicating that zir- the long thermal lifetimes and complex magmatic architecture of these large con-saturated magma was present earlier in the history of the magmatic sys- mature silicic magmatic systems (e.g., Simon et al., 2008; de Silva and Gregg, tem. An erupted record of this history may or may not be present. Xenocrysts 2014; Lipman and Bachmann, 2015). are incorporated from surrounding country rock and have no genetic relationship to any magma pulses associated with autocryst or antecryst production Spatiotemporal Development of the APVC Magmatic System (e.g., Brown and Smith, 2004). Within this framework we interrogate the zircon ages recorded in the APVC ignimbrites. Magmatic Groups Define Pulses of Magmatism within the APVC Flare-Up U-Pb Ages of Autocrysts Reveal the Magmatic Age of APVC Ignimbrites and Model Duration of Zircon Crystallization Based on qualitative peak matching, ignimbrites with overlapping age spectra can be grouped into four magmatic age groups (Fig. 3 color groups) We note that only 6 of the 16 zircon populations for individual units yield with only insignificant overlap in age spectra between the groups. The four MSWD values that plot within the 95% confidence interval for the given n magmatic age groups are defined by the age ranges 11–8.3 Ma, 8–5.5 Ma, 5.2– value, meaning that most of the ages show dispersion beyond assigned ana- 2.9 Ma, and 1.7–1.0 Ma leading into and merging with the four eruptive stages

8.33 ± 0.6 Ma Sifon described herein. In each group, each successive eruption contains a zircon 8.67 ± 0.14 Ma N = 40 of 46 age spectrum that overlaps the previous eruption in the group. These quali- MSWD = 1.6 tative groupings can be assessed statistically using the Kolmogorov–Smirnov (KS) test to quantitatively determine whether zircon populations comprising the overlapping age spectra sample the same distribution (Press et al., 1992). The KS test measures the maximum distance between the cumulative probability function of two populations to determine the probability (P) that the populations come from the same distribution, with the null hypothesis being that they come from the same distribution. The null hypothesis is rejected if P < 0.05; P values > 0.05 do not indicate higher confidence for similarity between populations as P approaches 1. This is a conservative approach because KS 8.41 ± 0.02 Ma Vilama tests tend to overemphasize heterogeneity when in fact two populations are 8.78 ± 0.24 Ma identical (Saylor and Sundell, 2016). With this caveat, we use the KS test as N = 29 of 31 a first-order statistical tool to assess age homogeneity versus heterogeneity MSWD = 0.76 within APVC magmatic groups. Magmatic group 1. This group (11–8.3 Ma) consists of overlapping U-Pb age distributions of the San Antonio, Artola, Vilama, and Sifon ignimbrites Figure 6. Rank order plots (ROP) and (Fig. 6). This magmatic age group correlates with eruptive stage 1, the wax- zircon 206Pb/238U ages as probability density function (PDF) curves for ing stage of the APVC flare-up. We have reassigned the eruption age of the

y magmatic group 1 ignimbrites. Ig- San Antonio ignimbrites to 9.09 ± 0.60 Ma, the age of youngest zircon in the nimbrites are rank ordered from old- dominant peak. This notwithstanding, group 1 contains the earliest record of est to youngest from bottom to top, with PDF curves from previous erup- continuous zircon crystallization in the APVC, beginning ca. 11 Ma with the tions for reference. Each shaded PDF oldest zircons in the San Antonio ignimbrite and continuing until the eruption 9.4 ± 0.03 Ma Artola curve represents the distribution for of the Sifon ignimbrite ca. 8.2 Ma. 9.59 ± 0.36 Ma the ROP displayed. Ages stated are N = 26 of 29 the weighted mean and 2σ error for The San Antonio U-Pb in zircon age spectrum consists of a dominant peak

MSWD = 0.3 the dominant peak in the distribution centered at 9.5 Ma and a smaller peak at 10.8 Ma. Artola yields a single domi Relative Probabilit Relative (Table 2). Colors of curves as in Fig- nant peak. Because the same relative uncertainties in U-Pb dating translate into ures 1 and 4. Red lines and text represent the 40Ar/39Ar eruption ages with larger absolute age errors for older ignimbrites compared to younger units, this 2σ errors for each ignimbrite. MSWD may obscure some intrapopulation heterogeneities in San Antonio and Artola is mean square of weighted deviates. zircons. Overall, the San Antonio and Artola ignimbrites show nearly simultaneous zircon crystallization peaks for their dominant peaks (Fig. 6). Comparing the distributions of the zircon ages composing the prominent peaks using a KS test yields an acceptable probability of relationship (P = 0.715) between Artola and San Antonio zircon age distributions (Table 3; Fig. 6), suggesting that the San Antonio 10.33 ± 0.64 Ma magmatic development of these systems was contemporaneous, despite the 9.53 ± 0.32 Ma N = 15 of 25 fact that their sources are >100 km apart (Fig. 3). MSWD = 0.83 The PDF peaks of the U-Pb age spectra for both the Vilama and Sifon ignimbrites overlap almost entirely. Zircon ages from the Vilama ignimbrite

TABLE 3. SUMMARY OF KOLMOGOROV–SMIRNOV STATISTICS FOR CONTEMPORANEOUS ZIRCON AGE SPECTRA Ignimbrites Probability Artola–San Antonio0.715 Vilama-Sifon0.213 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 Chuhuilla–Panizos, Toconce, Guacha 0.242 Age (Ma) Puripicar-Toconce 0.779

5.45 ± 0.02 Ma Chuhuilla define a single peak while those from the Sifon show some small inflections 6.01 ± 0.22 Ma and irregularities (Fig. 6). Autocryst populations composing the prominent N = 34 of 41 MSWD = 1.5 peaks from Sifon and Vilama are statistically indistinguishable (P = 0.213), suggesting contemporaneity of zircon crystallization despite the distance between the two sources (separated by >100 km). Their 40Ar/39Ar ages also overlap within error. Both these younger ignimbrites contain antecrysts that correlate in age with the Artola and San Antonio peaks (Fig. 6), suggesting that the earlier phase of magmatism represented by the Artola and San Antonio ignimbrites is also recorded in the magmatic history of the Vilama and Sifon ignimbrites. Magmatic group 2. This group (8–5.5 Ma) is defined by overlapping zircon 5.65 ± 0.01 Ma age spectra from the Panizos, Toconce, Guacha, and Chuhuilla ignimbrites Guacha 5.95 ± 0.16 Ma (Fig. 7). There is no significant overlap in zircon ages between the youngest zir- N = 46 of 52 con crystals from Sifon and Vilama ignimbrites of magmatic stage 1 and the MSWD = 2.1 oldest antecrysts in this second group, those in the Panizos ignimbrite. Thus this second period of continuous zircon crystallization is defined from the onset of

Figure 7. Rank order plots (ROP) and Panizos crystallization ca. 8 Ma until the eruption of the Chuhuilla ignimbrite ca. zircon 206Pb/238U ages as probabil- 5.45 Ma. This magmatic stage covers eruption ages ranging from 6.5 to 5.45 Ma. ity density function (PDF) curves The Panizos age spectrum defines a single peak ca. 7.1 Ma. Several younger for magmatic group 2 ignimbrites. Ignimbrites are rank ordered from ignimbrites have zircon ages that correlate with Panizos. An antecrystic peak oldest to youngest from bottom to from Guacha is within the spread of the Panizos PDF and inflections off the top, with PDF curves from previous prominent peaks in both Chuhuilla and Toconce correlate well with the Panizos eruptions for reference. Each shaded 6.52 ± 0.19 Ma peak (Fig. 7). The Guacha PDF curve consists of a dominant peak ca. 5.8 Ma Toconce PDF curve represents the distribution 6.67 ± 0.30 Ma for the ROP displayed. Ages stated with a slight inflection ca. 6.3 Ma represented by a kink in the slope of the N = 17 of 19 are the weighted mean and 2σ error rank order plot of zircon ages (Fig. 7). This correlates with Chuhuilla zircon

MSWD = 2.4 for the dominant peak in the distri- ages. The spread of the Chuhuilla PDF is noticeably skewed toward older ages y Relative Probabilit Relative bution (Table 2). Colors of curves as in Figures 1 and 4. Red lines and text with a large proportion of older crystals relative to the other ignimbrites in this represent the 40Ar/39Ar eruption ages period, with a significant number of crystals predating the eruption by almost with 2σ errors for each ignimbrite. 2 m.y. (Fig. 8). Rank order plots show a slight stepped pattern compared to the MSWD is mean square of weighted deviates. continuous slopes seen in Guacha and Panizos. The oldest Chuhuilla zircons are well within the range of Panizos ages, while the youngest overlap with the youngest Guacha zircons, suggesting that Chuhuilla zircons record crystallization over nearly the entire span of the second period. A KS test comparing the Chuhuilla zircon distribution to the combined 6.79 ± 0.02 Ma Panizos zircon distributions from Guacha, Toconce, and Panizos suggests age equiv- 7.15 ± 0.32 Ma N = 15 of 31 alence at P = 0.242. This suggests that zircon crystallization recorded in the MSWD = 0.47 Chuhuilla coincides with that in at least three different ignimbrites erupted from separate centers over 2 m.y. The distance between each of these centers spans nearly the entire spatial distribution of the APVC (Fig. 1). Panizos is the easternmost eruption in the APVC, Toconce erupted from a buried source on the western edge, the Guacha caldera is located to the south, and Chuhuilla erupted from the northernmost caldera, Pastos Grandes. This magmatic age group correlates with eruptive stage 2, part of the peak of the APVC flare-up. Magmatic group 3. The third set of overlapping age spectra defines magmatic group 3 and includes the Puripicar, Toconao, Atana, Tara, and Pastos 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 Grandes ignimbrites, delineating a magmatic age range of 5.2–2.9 Ma (Fig. 8). Age (Ma) A few isolated zircon ages from Puripicar and Toconao overlap in age with the

2.89 ±0.01 Ma Pastos Grandes 3.33 ± 0.14 Ma N = 51 of 58 zircon histories from Panizos, Chuhuilla, and Guacha spectra in the preceding MSWD = 4.2 period, but most of the age spectra are clearly distinct. Zircon crystallization over this period began ca. 5.2 Ma and continued until the Pastos Grandes eruption at 2.89 Ma. Eruption ages range from 4.09 to 2.89 Ma. The Puripicar and Toconao zircon spectra overlap considerably and have eruption ages that are indistinguishable within error (Fig. 8). While Toconao has a uniform peak, the Puripicar PDF shows some irregularities in the peak that do not correlate with any other zircon population. A KS test also shows an acceptable probability of fit (P = 0.779) between the Puripicar and Toconao 3.49 ± 0.01 Ma Tara 3.65 ± 0.10 Ma zircon crystallization histories, suggesting that they developed contemporane- N = 38 of 46 ously (Fig. 8). Their eruption source vents are ~50 km apart. The Atana ignim MSWD = 1.6 brite zircon age spectrum is the narrowest in the APVC, despite the large number of crystals (n = 61) analyzed (Fig. 8). The shoulder and minor peak at ages older than the dominant peak for the Atana ignimbrite consist of five zircon ages that overlap with the Toconao and Puripicar zircon ages. The two youngest eruptions in stage 3, the 3.49 Ma Tara and 2.89 Ma Pastos Grandes ignimbrites, display continuous zircon crystallization spectra that overlap within error ca. 3.8 Ma (Fig. 8). We interpret this as indicating that 3.96 ± 0.01 Ma Figure 8. Rank order plots (ROP) and zircon Atana 206Pb/238U ages as probability density func- both ignimbrites had synchronous zircon crystallization histories until the Tara 4.17 ± 0.06 Ma tion (PDF) curves for magmatic group 3 eruption at 3.49 Ma; zircon crystallization continued at Pastos Grandes until y N = 49 of 62 MSWD = 0.77 ignimbrites. Ignimbrites are rank ordered eruption at 2.89 Ma. Zircon ages responsible for the inflection ca. 3.6 Ma in the from oldest to youngest from bottom to top, with PDF curves from previous erup- Pastos Grandes peak overlap with the dominant peak of the Tara eruption. The tions for reference. Each shaded PDF curve Tara peak is largely uniform, whereas the Pastos Grandes peak has a shoulder represents the distribution for the ROP ~500 k.y. older than the dominant younger peak. There is very little overlap displayed. Ages stated are the weighted mean and 2σ error for the dominant peak between the dominant peak from the Pastos Grandes and Tara ignimbrites

Relative Probabilit Relative in the distribution (Table 2). Colors of and the older Puripicar, Toconao, and Atana age spectra (Fig. 9). However, curves as in Figures 1 and 4. Red lines and the older tail in the Pastos Grandes PDF and obvious antecrysts, one of which text represent the 40Ar/39Ar eruption ages 4.00 ± 0.12 Ma is also present in the Tara ignimbrite, indicates a synchronous magmatic his- Toconao with 2σ errors for each ignimbrite. MSWD 4.50 ± 0.16 Ma is mean square of weighted deviates. tory with the three older systems located almost 200 km south of the Pastos N = 22 of 27 MSWD = 1.07 Grandes caldera. This magmatic age group correlates with eruptive stage 3, the peak of the APVC flare-up. Magmatic group 4. This group is defined by a period of overlapping age spectra from the Laguna Colorada, Puripica Chico, and Purico ignimbrites (Fig. 9), and zircon ages are continuous between ca. 2.7 Ma until the eruption of the Purico ignimbrite at 0.98 Ma. The oldest zircon age from the Laguna Colorada ignimbrite overlaps within error the Pastos Grandes eruption age. Laguna Colorada and Puripica Chico are unusual because their age distribu- 4.09 ± 0.02 Ma Puripicar tions show a minor, younger population that forms a shoulder offset from the 4.49 ± 0.10 Ma N = 45 of 62 dominant peak by ~300 k.y. Purico and Puripica Chico both contain many minor MSWD = 2.2 peaks consisting of multiple zircon ages that overlap the Laguna Colorada age spectrum. The Purico ignimbrite is the only APVC ignimbrite lacking a single dominant pre-eruption peak. Instead, Purico contains several distinct zircon age populations, the oldest of which overlaps significantly with the Laguna Colorada age distribution. Overall, Purico contains a nearly continuous zircon record extending over 1.5 m.y.; a similar age range is present in Dome D lavas (Supplementary Table 1). This magmatic age group correlates with eruptive 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 stage 4, the waning of the APVC flare-up. Age (Ma)

One small eruption that belongs to eruptive stage 4 but has a magmatic 0.98 ± 0.03 Ma Purico 1.27 ± 0.16 Ma history extending back to magmatic stage 3 is Cerro Bola, the youngest dome N = 18 of 36 from the La Pacana caldera. This small high-Si rhyolite dome has an eruption MSWD = 4.4 age of 2.7 ± 0.2 Ma (40Ar-39Ar biotite; Lindsay et al., 2001b) that would make it concordant with the eruption of the Pastos Grandes ignimbrite eruption ~100 km away. The autocryst age population of Cerro Bola yields a weighted mean age of 2.57 ± 0.12 Ma within error of the 40Ar-39Ar biotite age, although the youngest zircons of Bola extend to ca. 2.4 Ma. Thus part of the autocryst history correlates with stage 4, overlapping the early zircon history of Laguna Colorada. However, a second distinct peak at 3.02 ± 0.04 Ma overlaps with the peak of the Pastos Grandes ignimbrite. These correlations suggest synchro neity between centers over a 150 km north-south distance.

1.70 ± 0.02 Ma Puripica Chico 2.05 ± 0.10 Ma Spatiotemporal Development of Magmatism in the APVC N = 40 of 46 MSWD = 7.4 and the Development of Magmatic Foci

The spatiotemporal distribution of eruptions in the APVC is interpreted to have evolved from diffuse to focused over the four eruptive stages (de Silva and Gosnold, 2007; Salisbury et al., 2011). The magmatic record of zircon crys-

tallization histories enhances this view with contemporaneous locations of y Relative Probabilit Relative magmatism (as recorded by zircon age data) without eruptions. By combining these records with petrological and geochemical constraints we can provide some constraints on the crustal depths where zircon crystallized. The crustal compositions of the zircons (Fig. 3) and available zircon d18O 1.98 ± 0.03 Ma Laguna Colorada data (Folkes et al., 2013) indicate that zircons grew in magmas that were iso- 2.38 ± 0.22 Ma topically enriched and well homogenized. Geobarometry (Al-in-amphibole N = 18 of 19 phase equilibria, melt inclusions; Schmitt, 2001; Grocke et al., 2012), geother- MSWD = 3.1 mometry (Fe-Ti oxides phase equilibria, Al-in-amphibole phase equilibria, and zircon saturation), experimental phase equilibria (Muir et al., 2014a), and geochemical modeling of trace element and isotopic compositions (de Silva, 1989; Lindsay et al., 2001; Schmitt et al., 2001; Kay et al., 2010; Muir et al., 2014b; Burns et al., 2015; Freymuth et al., 2015) all indicate that APVC eruptions evac- uated upper crustal magma reservoirs that stalled, crystallized, and acquired their trace element and isotopic compositions in the shallow crust. In con- trast, magmas that are interpreted to be derived from deeper in the APMB (10–30 km) are isotopically less enriched, hotter, andesitic, and zircon under- 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 saturated, obviating the APMB as a source of zircons. These considerations Age (Ma) and U-Pb zircon rim ages that suggest zircon crystallization continued right Figure 9. Rank order plots (ROP) and zircon 206Pb/238U ages as until eruption convince us that the U-Pb zircon ages extend our understanding probability density function (PDF) curves for magmatic group 4 of the spatiotemporal evolution of the volcano-plutonic system of the APVC to ignimbrites. Ignimbrites are rank ordered from oldest to youngest shallow crustal depths of the pre-eruptive magma chamber ~5–10 km beneath from bottom to top, with PDF curves from previous eruptions for reference. Each shaded PDF curve represents the distribution for the surface, the uppermost reaches of the APMB (Fig. 10). the ROP displayed. Ages stated are the weighted mean and 2σ Each of the time slices in Figure 10 portrays distinct ~2–3 m.y. pulses of error for the dominant peak in the distribution (Table 2). Colors magmatism during which spatially distinct but magmatically contemporane- of curves as in Figures 1 and 4. Red lines and text represent the 40Ar/39Ar eruption ages with 2σ errors for each ignimbrite. MSWD ous magma reservoirs developed. The spatial footprint of the shallow magma is mean square of weighted deviates. system varied with time, and some of the constituent magma bodies erupted.

68° 67° 68° 67° 21° 21° Stage 1 Stage 2 11> >8 Ma 8> >5 Ma Salar de AscotaA n 22° 22°

Lag. Colorado

23° 23°

Salar 68° 67° de 21° Atacama 050100 050 100

km km 24° Salar 24° de Figure 10. Spatiotemporal development of Ascotan 21° 21° the pre-eruptive Altiplano-Puna Volcanic Stage 3 Stage 4 Complex (APVC) magma system through 5> >3 Ma 3> >0.5 Ma time. Each frame represents a single time slice corresponding to each of the mag- 22° matic pulses. Magma presence in the subvolcanic system between 5 and 10 km 22° 22° is indicated by the presence of zircon U-Pb Lag. ages of appropriate age for each time slice. Colorado Colored shapes represent the approximate distribution of the discrete subvolcanic magma reservoirs as estimated by their location and approximate size. Color 23° scheme is consistent with Figures 1, 4, and 23° 23° 6–9. The outline of the Bouguer anomaly and low velocity zone from Figure 1B are Salar shown. Star is Uturuncu volcano. Loca- de tions of key salars and lakes are shown. 050100 050 100 APMB—Altiplano-Puna Magma Body. Atacama km km Note: Interactive PDF (see figure at left) 050 100 24° 24° allows the reader to toggle through the 68° 67° 68° 67° magmatic stages of the APVC. km 21° 24° Composite All stages Magma reservoir that erupted Note: Interactive PDF allows the reader to toggle through the magmatic stages of the APVC. Reader should enable layers in Adobe Acrobat. Legend (zircon autocrysts present) is that for Figure 10. 22° Inferred magma reservoir (eruption known, but no zircon data available) Magma reservoir, no eruption (zircon antecrysts in younger 23° eruptions)

>10 km depth footprint of the

050100 magmatic stage within the APMB

km 24° 68° 67°

Other magma bodies had delayed eruptions, but their continued magmatic Waning Stage Ignimbrite Flare-up Waxing Stage history is revealed in the zircon age spectra. The time slices clearly support the 45 impression from the eruptive record that magmatism focused and intensified 4 321 A with time from the earliest pulse to the third with diminishing breadth and 40 intensity by the fourth pulse, corresponding to the waxing, climax, and waning of the flare-up. 35 Within the magmatic record is evidence that three magmatic foci developed during the peak of the flare-up and account for the bulk of the volume 30 of magmatism. The surface manifestations of these are the major resurgent calderas of La Pacana, Guacha, and Pastos Grandes. All three appear to have 25 been initiated during magmatic pulse 2 as the flare-up peaked, and were active through pulse 3. During this time each was the source of two sequential erup-

Frequenc y 20 tions, i.e., Pujsa-Toconao–Atana, Guacha-Tara, and Chuhuilla–Pastos Grandes. The older eruptions were broadly contemporaneous ca. 5.6 ± 0.2 Ma. These 15 were followed by younger eruptions of the Atana (4.09 Ma), Tara (3.49 Ma), and Pastos Grandes (2.89 Ma) ignimbrites that reveal significant departure 10 from contemporaneity compared to the older eruptions. However, their zircon spectra evince a more robust correlation of their magmatic histories (Figs. 5 7 and 8). Thus, contemporaneous magmatism at these centers separated by large distances (50–200 km) is not always correlated with contemporaneous 0 eruptions. This appears to be true throughout the history of the APVC. During 4 3 2 1 B the waxing pulse, the San Antonio and Artola ignimbrites and the Vilama and 1600 Sifon ignimbrites were erupted from widely separated sources, and show strong correlation in magmatic history; however, the former pair represents

Atana 1400 relatively small localized eruptions, while the latter are two supereruptions that occurred almost a million years later. This suggests that while spatially, and

ilama

Guacha

V e 1200 thus physically, separated magma systems may develop in parallel, interrup- tion of the magmatic evolution by eruption is unique to each center. Some may

Chuhuilla

Sifon 1000 Pastos Grandes erupt while others may continue to develop and erupt at a later time. In these large mature magmatic systems eruption is thought to reflect the breach of a 800 thermomechanical threshold (e.g., Gregg et al., 2012). Thus the different histo- ra Ta ries of otherwise similar contemporaneous systems may reflect differences in

Panizos 600 local magmatic flux and consequent thermomechanical evolution of the upper Erupted Volum Puripicar crust in which the magma reservoirs developed. 400

200 Antonio Integrating Magmatic and Eruptive Histories: Episodic Development

Purico

conce

Laguna Colorada Artola

conao

San

To of the APVC Magmatic System

Puripica Chico

To 0 12 2 4 6 8 10 Correlations between the four pulses of magmatism and eruptive stages Age (Ma) are revealed by pulses in zircon crystallization with distinct breaks between magmatic pulses on the regional scale (Fig. 11). All four eruptive stages have Figure 11. Zircon U-Pb age based magmatic pulses compared to eruption ages. (A) The magmatic record is represented by histograms and associated probability density function curve for all magmatic zir- overlapping zircon ages, and dominant peaks in the younger pulses are pre- con ages determined for Altiplano-Puna Volcanic Complex ignimbrites. (B) Eruption ages and volumes ceded by distinct peaks in zircon crystallization that are interpreted to reflect (from Fig. 1; Table 1). separate events of magma accumulation in the upper crust. Available U-Pb ages indicate that these precursor events initiated 0.5–1 m.y. prior to erup-

tions, with possibly continuous presence of zircon saturated magma until each Zircon Insights into Magmatic Processes and Magma Dynamics eruption. Thus, each of the four magmatic pulses consists of spatially distinct, temporally sequenced (with overlap) subpulses or magmatic events. There- The presence of abundant zircons in the pumices of the APVC ignimbrites fore, an episodicity at multiple scales is defined and led de Silva et al. (2015) to and their age spectra indicate that the thermochemical conditions for zircon characterize the pattern as fractal (sensu latu). Looking at flare-ups in continen saturation were maintained for several hundreds of thousands of years for tal arcs as a whole, they suggested a hierarchy of pulses with each scale re- each system. Zircon saturation temperatures based on the most recent experi flecting the time scale of processes occurring at different levels in the arc crust. mental studies by Boehnke et al. (2013) range from ~730 °C to 815 °C, broadly This tempo of continental arc magmatism is interpreted to reflect modulation concordant with the 750–850 °C estimated from Fe-Ti oxide phase equilibria of the mantle power input as it is progressively filtered through the continental and Al-in-amphibole (de Silva, 1991; Lindsay et al., 2001; Schmitt et al., 2001; crust. In this framework, the APVC ignimbrite flare-up as a whole is a sec Grocke, 2014). Trace element data from zircons suggest magma differentiation ondary pulse with magmatic pulses 1 through 4 reflecting tertiary pulses, and over a temperature range of ~150–200 °C derived from Ti-in-zircon (Fig. 3). the individual ignimbrite zircon spectra defining quaternary pulses. In sum- Taking a conservative 400 k.y. as the duration over which zircon saturation mary, the secondary pulse of the 10 Ma ignimbrite flare-up of the APVC con- was maintained, the temperature estimates suggest extremely low secular sists of four tertiary pulses of intrusion and/or eruption with a periodicity of cooling rates of 4–5 × 10–4 °C/yr. Such cooling rates are an order of magnitude ~2 m.y. The tertiary pulses consist of three or more distinct quaternary pulses, slower than typical conductive cooling rates of plutons (~4 × 10–3 °C/yr) that each of <1 m.y. duration. consider the temperature dependence of thermal diffusivity (a) and heat ca-

The tertiary pulses in the APVC indicate processes that not only divide the pacity (CP) (Whittington et al., 2009; Nabelek et al., 2012; Gelman et al., 2013) secondary pulse into shorter pulses (~2 m.y. magmatic events in the APVC or realistic mature thermal gradients in which such systems might develop that culminated in eruptions), but produce the characteristic space-time-vol- (de Silva and Gosnold, 2007; de Silva and Gregg, 2014) suggesting that the ume pattern of waxing, climax, and waning of the flare-up. The tempo of these pre-eruptive magma reservoirs were thermally buffered. Such conditions are tertiary pulses may ultimately reflect the influence of a combination ofmantle, typical of development of the pre-eruptive magma reservoirs in the elevated crustal, and upper plate tectonic modulation (or other nonmagmatic upper geothermal gradients (~50 °C/km) typical of the upper crust during the APVC crustal process). The waxing-climax-waning pattern seen in the tertiary pulses flare-up (de Silva and Gosnold, 2007). in the APVC appears to be a characteristic pattern in ignimbrite flare-ups (de Silva et al., 2006b; Bachmann et al., 2007c; Lipman, 2007) and may reflect the Carryover of Zircon Antecrysts: Evidence for Upper Crustal Assimilation evolution of crustal melt production with time, thereby mimicking the mantle pulse topology (e.g., Elston, 1984; Best and Christiansen, 1991). Alternatively A consequence of the thermal longevity of the pre-eruptive magma res- (or in addition), the pattern may reflect a progressive evolution of crustal rheol- ervoirs is that assimilation-fractional crystallization was a dominant process ogy resulting from the thermal signal from the mantle progressing through the during the evolution of the APVC ignimbrite magmas (e.g., see Kay et al., 2010; crust by intrusion and advection (e.g., Best and Christiansen, 1991; Gans et al., Folkes et al., 2013, and references therein). During this process restite of as- 1989; de Silva and Gosnold, 2007; de Silva and Gregg, 2014). This progres- similated country rock in the form of antecrysts or xenocrysts should be re- sion may result in the formation of a middle to upper crustal MASH zone and corded. Zircon is quite robust during magmatic recycling and reworking, and magma staging area and an elevation of the brittle-ductile transition to shallow carryover or scavenging of antecrystic zircons is frequently observed in stud- levels in the crust. Over time, these processes combine to allow successively ies assessing the magmatic development of volcanic systems (Watson, 1996; larger magma bodies to be built in the uppermost crust before they erupt (e.g., Charlier et al., 2005; Bacon and Lowenstern, 2005; Walker et al., 2010; Folkes de Silva and Gosnold, 2007). et al., 2011). In these examples, younger eruptions incorporate antecrysts The quaternary pulses have time scales of ~1 m.y. and define the mag- from a previous magmatic cycle. Xenocrysts may also be incorporated during matic evolution of the upper crustal pre-eruptive reservoirs and the thermo- crustal assimilation. The relative proportions of antecrysts versus xenocrysts mechanical evolution of the host rock–reservoir system. As magma accumu- in the APVC ignimbrites may therefore offer some insight into the pre-eruptive lates in the upper crust and evolves to zircon saturation, feedbacks between magmatic evolution. temperature, host-rock mechanics, and chamber pressurization result in A characteristic of the zircon age spectra for the ignimbrites (Figs. 3 and ductile host-rock rheologies that promote storage and growth over eruption 6–9) is that they become more complex with time. The oldest ignimbrites show and eventual eruption is due to crossing a mechanical threshold (Gregg largely unimodal zircon peaks with few inflections or smaller peaks (i.e., Artola, et al., 2012). The broad correlation of magmatic and eruptive tempo of the Vilama), although analytical uncertainties may mask variability that is visible in APVC supports a strong coupling between the magma dynamics and thermo younger ignimbrites. Progressively younger ignimbrites show more complex mechanics of periodically constructed long-lived upper crustal magma reser- zircon crystallization histories with frequent satellite peaks off the prominent voirs that form the shallowest part (5–10 km) of the APMB. peak (i.e., Guacha, Toconao), isolated older peaks consisting of single zircon

ages, as well as satellite peaks (Puripicar, Tara); the youngest ignimbrites yield considering that 13 of 24 xenocrysts are from a single unit (Panizos). Moreover, the most complex and variable PDF curves (i.e., Pastos Grandes, Puripica obvious CL patterns suggesting the presence of inherited crystal interiors are Chico, Purico). For these units, the precision of individual U-Pb zircon ages scarce, and if they were present, they were targeted by interior-rim pairs of can resolve differences in magmatic ages of zircon crystals from individual analyses. We therefore conclude that scarcity of zircon inheritance (with the eruptions, and we note that even in the oldest magmatic pulse, evidence for notable exception of Panizos) is a true characteristic of the APVC. This con- zircons with U-Pb ages outlying the main peak is found. In all but the two trasts with the whole-rock isotopic (Sr, Nd, O) evidence for a significant crustal oldest eruptions, these antecrystic ages correlate with older eruptions, the contribution (as much as 70%) in the APVC magmas. These observations can antecrysts in the Artola and San Antonio representing the earliest magmatic be reconciled if crustal zircons became resorbed in the APMB, which is the history of the APVC for which there is apparently no volcanic equivalent (see parental reservoir that supplied magmas to the shallow pre-eruptive magma also Schmitt et al., 2002). We suggest that the change in complexity of the reservoirs where most zircon crystallized. The preservation of xenocrysts in zircon populations over the history of the APVC likely reflects the development the Panizos ignimbrite magma may be due to the peraluminous character of of a multistage, multicyclic plutonic system in the shallow crust (~5–10 km). the magmas. Caffe et al. (2012) demonstrated that these peraluminous mag- Each magma is built by incremental intrusion from the APMB into the shallow mas form through contamination of calc-alkaline dacitic magmas, similar crust and the final stages of the pre-eruption accumulation history are partly to typical APVC ignimbrites by metapelite at APMB depths (≥18 km depth). recorded in the zircon autocrysts. Zircon antecrysts record earlier history of the Caffe et al. (2012) further determined that crystallization of the rhyolite magma magma system, with or without eruption. Younger ignimbrites contain more started at ~5 kbar and 800 °C, and continued almost isobarically to 720 °C. antecrysts because the upper crustal magmas feeding these eruptions are em- These conditions are much cooler than the typical APMB conditions, allowing placed in a more mature magmatic system with a longer history made up of the possibility that along the eastern flanks of the APMB, where peraluminous a multicyclic mush; there is simply more of a history to be scavenged by later magmas were being produced, conditions were more conducive to preserv- magmas. It is interesting that the Purico ignimbrite, erupted from the Purico ing inheritance than the rest of the APMB, where andesitic compositions with ignimbrite shield located just to west of the Guacha and La Pacana calderas temperatures of 900–1000 °C were typical (e.g., Burns et al., 2015). The lim- and to the south of the Chaxas complex from which the Puripicar ignimbrite ited evidence for true inheritance in the APVC magmas is then attributed to was erupted, has the most complex U-Pb zircon age spectrum that we have basement assimilation occurring primarily in the zircon-undersaturated envi- found. However, there is no hint of any antecrysts of these three older systems, ronment of the APMB at deeper levels than pre-eruption levels. This is consis- suggesting a very focused relatively small magmatic system that was estab- tent with the consensus that the APMB is the major upper crustal MASH zone lished between the giant systems beginning ca. 2 Ma. Similar nonsystematic in which the APVC baseline chemical and isotopic compositions signature is behavior is mirrored by the somewhat older Cerro Bola dome at La Pacana. developed. The pre-eruptive magma reservoirs in which the autocryst and The peaks in 206Pb/238U ages of autocrysts and antecrystic zircon interiors are antecryst record develops are fed from the deeper parts of the APMB in which both significantly younger than the caldera-forming Atana ignimbrite at La few xenocrysts survived except in the east. This also implies that for the main Pacana. However, as noted here, the autocryst and antecryst peaks of Cerro volume of the APVC, zircon crystallization only started after emplacement in Bola overlap with the early history of the Laguna Colorada ignimbrite shield the shallow (~5–10 km) magma reservoirs. ~100 km away and the peak of activity at the Pastos Grandes caldera ~150 km to the north, respectively, suggesting contemporaneity of magmatism, but not eruption, at these three centers. Zircon-Based Perspective for APVC Magmatism Despite the strategy of preferentially measuring near-rim domains over interiors, zircon antecrysts were found to be a ubiquitous but minor component As the APVC flare-up developed, the synchroneity of crystallization histories of the zircon populations of APVC ignimbrites. These reveal unequivocal evi- of APVC eruptive centers separated by >100 km suggests a significant level of dence for carryover of antecrysts in all of the investigated APVC ignimbrites. In temporal connectivity, and it might be tempting to look at the time slices in association with the evidence presented here for the upper crustal evolution of Figure 10 and imagine APVC-wide connectivity of magma at the pre-eruptive the APVC magmas, we interpret these antecrysts as attesting to upper crustal levels. This might be supported by the fact that the general chemical, petrologi cannibalization of remnants of progenitor magmas. cal, and isotopic compositions are concordant in all these magmas, including their zircon trace element compositions (Fig. 3) that suggest a close kinship. The Crustal Zircon Inheritance significant spatial separation of the eruptive sites might give one pause, but this could be rationalized as local control of the eruption sites above a broad True xenocrysts, defined here as pre-APVC aged zircon inherited from APVC wide sill at 5–10 km below the surface. However, in detail the U-Pb zircon regional basement rocks, make up a scarce 3% of the analyzed populations age spectra and subtle petrochemical details suggest that each erupted magma (Fig. 5). This overall proportion of xenocrystic zircon is further reduced when body developed uniquely. For example, if the Vilama and Sifon ignimbrites were

erupted from vents separated by >100 km above a shared magma reservoir, the CONCLUSIONS younger centers that subsequently developed between the respective eruptive sites should record the presence of antecrysts of Sifon and Vilama ages. How- U-Pb ages of zircon in magmas erupted during the 11–1 Ma APVC ignimbrite ever, none of the younger ignimbrites (Guacha, Chuhuilla, Tara Pastos Grandes, flare-up were determined by SIMS. These data reveal complex age spectra and Laguna Colorada) record any antecrysts of appropriate age. Similarly, with a dominant peak of autocrysts and subsidiary antecryst peaks. Xenocrysts while the Puripicar and Toconao share very similar U-Pb age characteristics, the are rare, except for a peraluminous ignimbrite from the eastern margin of the Toconao rhyolite is volcanologically and petrochemically linked to the Atana APVC. Model magmatic ages, calculated as the weighted mean of the young- ignimbrite dacite magma, which is chemically distinct from the Puripicar dacite est population of zircons with overlapping ages, are consistent with eruptive magma (de Silva and Francis, 1989; Lindsay et al., 2001a, 2001b). The Panizos is stratigraphy. In combination with pressure-temperature estimates of the mag- clearly quite chemically distinct from its pulse 2 ignimbrite cohort. Thus the cor- mas, the U-Pb in zircon ages record multiscale episodicity in the magmatic relations of U-Pb age spectra point to synchroneity, not physical connectivity; history of the shallowest levels (5–10 km beneath the surface) of the APMB. and each eruption is interpreted to have tapped a unique pluton of a developing The ages fall into four groups defining distinct pulses of magmatism that composite upper crustal batholith. correlate with eruptive pulses but indicate that magmatic construction often Although isolated at shallow levels (between 5 and 10 km), it is very proba- initiated ~1 m.y. before eruptions began. Magmatism was initially (11–8 Ma) ble that separate systems were interconnected at a deeper level, likely deeper distributed diffusely on the eastern and western flanks of the APVC, but spread within the APMB (Fig. 10). The extent of overlap of zircon age spectra sug- out over much of the APVC as activity waxed before focusing in the central gests that shallow crustal magma reservoirs separated by tens to hundreds part during the peak. Each pulse consists of spatially distinct but temporally- of kilometers were undergoing simultaneous thermal or material input from sequenced subpulses of magma that represent the construction of pre-erup- deeper levels, particularly once the flare-up reached peak intensity during tive magma reservoirs. pulse 2 and 3 when the magmatic foci of the three major calderas developed. Three long-lived calderas are the eruptive outlets for the main magmatic foci With each successive input of material, magma accumulates in the shal- during the peak of the flare-up and show broadly synchronous magmatic devel- low crust and each coexisting magma records a very similar zircon history. opment but some discordance in their later eruptive histories. In the context of Magmas either develop and erupt nearly synchronously, as in the case of the recent models of caldera mechanics, this suggests that eruptive tempo is con- Vilama and Sifon, Toconce and Panizos, the Pujsa, Guacha, and Chuhuilla, and trolled locally from the top down, while magmatic tempo is a more systemic and the Puripicar and Toconao-Atana ignimbrites, or they share a common his- deeper bottom-up feature. Synchroneity in magmatic history at distinct upper tory until one erupts while the other continues to crystallize, as inferred from crustal magmatic foci implicates a shared connection deeper within the APMB. the zircon crystallization histories of the Tara and Pastos Grandes ignimbrites. Model magmatic ages are on average ~400 k.y. older than the eruption A significant volume of nonerupted magma drove resurgent uplift and post- ages, revealing minimum pre-eruptive magmatic durations of APVC ignimbrite caldera volcanism, and eventually matured into uneruptible mush or solidified magmas. These indicate that thermochemical conditions for zircon saturation into plutons to be recycled in later episodes of magmatism. were maintained for extended (several hundreds of thousand year) periods. As the flare-up progressed, thermal maturation of the APVC upper crust Zircon saturation temperatures record upper crustal conditions and trace ele- led to an elevated thermal gradient (e.g., de Silva and Gosnold, 2007) in which ments reveal protracted magma differentiation under secular cooling rates an the upper crustal pre-eruptive chambers were thermally buffered and zircon order of magnitude slower than typical pluton cooling rates, suggesting that saturated, leading to comparatively long durations of zircon crystallization. pre-eruptive magma reservoirs were thermally buffered, consistent with the Assimilation in the upper crust results in carryover of antecrysts that form the extended minimum magmatic time scales >400 k.y. we have estimated. nucleation sites for autocrystic zircon growth recorded in rim ages. Zircons Zircon spectra become more complex with time, reflecting the carryover crystallized in serial and in parallel over protracted time periods in different of antecrysts in successively younger magmas and attesting to upper crustal zones within larger magma reservoirs with slightly different cooling histories assimilation in the APVC. Although xenocrysts are present, they are rare, sug- (e.g., de Silva and Gregg, 2014), but were well mixed either due to pre-erup- gesting that inheritance is limited. This is attributed to basement assimilation tive stirring (e.g., Huber et al., 2009) or eruptive mixing (e.g., Kennedy et al., occurring under zircon-undersaturated conditions deep in the APMB in con- 2008), accounting for the nonanalytical dispersion of ages (indicated by ele- trast to the pre-eruptive levels where antecrysts were incorporated under zir- vated MSWD values > 1) in the dominant autocryst population. At present it con-saturated conditions. appears that the shallowest parts of the APMB MASH zone, from 5 to 10 km, The multiscale episodicity revealed by the zircon U-Pb ages of the APVC has matured into a composite batholith with some melt-rich zones that may flare-up can be interpreted in the context of continental arc magmatic systems lead to local diapiric intrusion and rapid surface uplift, as observed at Uturuncu in general. The APVC ignimbrite flare-up as a whole is a secondary pulse, with (Pritchard and Simons, 2002; Fialko and Pearse, 2012; del Potro et al., 2013; magmatic pulses 1 through 4 reflecting tertiary pulses, and the individual ig- Muir et al., 2014a, 2014b). nimbrite zircon spectra defining quaternary pulses. This hierarchy of pulses is

thought to reflect how a magmatic front, driven by the primary mantle-power Blundy, J., and Wood, B., 2003, Partitioning of trace elements between crystals and melts: Earth input, propagates through the crust, producing sequentially smaller spatial and Planetary Science Letters, v. 210, p. 383–397, doi:10.1016 /S0012 -821X (03)00129-8. Boehnke, P., Watson, E.B., Trail, D., Harrison, T.M., and Schmitt, A.K., 2013, Zircon saturation and faster temporal scales in the upper crust of the Central Andes from ~30 km re-revisited: Chemical Geology, v. 351, p. 324–334, doi:10.1016/j.chemgeo.2013.05.028. to the surface. Breitkreuz, C., de Silva, S.L., Wilke, H.G., Pfänder, J.A., and Renno, A.D., 2014, Neogene to Qua- ternary ash deposits in the Coastal Cordillera in northern Chile: Distal ashes from super eruptions in the Central Andes: Journal of Volcanology and Geothermal Research, v. 269, ACKNOWLEDGMENTS p. 68–82, doi:10.1016/j.jvolgeores.2013.11.001. Brown, S.J.A., and Fletcher, I.R., 1999, SHRIMP U-Pb dating of the pre-eruption growth history Funding by the National Science Foundation (grant EAR-0838536 to de Silva and Schmitt and of zircons from the 340 ka Whakamaru Ignimbrite, New Zealand: Evidence for > 250 k.y. grants EAR-0710545 and EAR-0908324 to de Silva) is gratefully acknowledged, as are Geological magma residence times: Geology, v. 27, p. 1035–1038, doi:10.1130 /0091 -7613(1999)027 <1035: Society of America Graduate Research grants to Kern, Kaiser, and Iriarte. Support in the field by SUPDOT>2.3.CO;2. Nestor Jimenez, Mayel Sunagua, Benigno Godoy, Dale Burns, Casey Tierney, Stephanie Grocke, Brown, S.J.A., and Smith, R.T., 2004, Crystallisation history and crustal inheritance in a large Michael Ort, Morgan Salisbury, and various PLUTONS Project (funded by the National Science silicic magma system: 206Pb/238U ion probe dating of zircons from the 1.2 Ma Ongatiti ig- Foundation Continental Dynamics Program and the Natural Environment Research Council) col- nimbrite, Taupo Volcanic Zone: Journal of Volcanology and Geothermal Research, v. 135, leagues helped realize the mapping, sampling, and other studies that facilitated this study. We p. 247–257, doi:10.1016/j.jvolgeores.2004.03.004. thank Matt Coble and Jonathan Miller for thorough reviews and Gary Michelfelder and Ray Russo Bryan, S.E., Ferrari, L., Reiners, P.W., Allen, C.M., Petrone, C.M., Ramos-Rosique, A., and Camp- for editorial handling, all of which improved this work in clarity, focus, and impact. The ion micro bell, I.H., 2007, New insights into crustal contributions to large-volume rhyolite generation probe facility at University of California, Los Angeles, is partly supported by a grant from the in the mid-Tertiary Sierra Madre Occidental Province, Mexico, revealed by U-Pb geochronol- Instrumentation and Facilities Program, Division of Earth Sciences, National Science Foundation. ogy: Journal of Petrology, v. 49, p. 47–77, doi:10.1093 /petrology /egm070. Burns, D.H., de Silva, S.L., Tepley, F., III, Schmitt, A.K., and Loewen, M.W., 2015, Recording the transition from flare-up to steady-state arc magmatism at the Purico-Chascon volcanic com- REFERENCES CITED plex, northern Chile: Earth and Planetary Science Letters, v. 422, p. 75–86, doi:10.1016 /j .epsl Allmendinger, R.W., Jordan, T.E., Kay, S.M., and Isacks, B.L., 1997, The evolution of the Altiplano- .2015.04.002. Puna plateau of the central Andes: Annual Review of Earth and Planetary Sciences, v. 25, Caffe, P., Soler, M., Coira, B., Onoe, A., and Cordani, U., 2008, The Granada ignimbrite: A com- p. 139–174, doi:10.1146/annurev.earth.25.1.139. pound pyroclastic unit and its relationship with upper Miocene caldera volcanism in the Bachmann, O., Oberli, F., Dungan, M., Meier, M., Mundil, R., and Fischer, H., 2007a, 40Ar/39Ar northern Puna: Journal of South American Earth Sciences, v. 25, p. 464–484, doi:10.1016 /j and U-Pb dating of the Fish Canyon magmatic system, San Juan Volcanic field, Colorado: .jsames.2007.10.004. Evidence for an extended crystallization history: Chemical Geology, v. 236, p. 134–166, doi: Caffe, P.J., Trumbull, R.B., Coira, B.L., and Romer, R.L., 2002, Petrogenesis of early Neogene 10.1016/j.chemgeo.2006.09.005. magmatism in the Northern Puna; implications for magma genesis and crustal processes Bachmann, O., Charlier, B.L.A., and Lowenstern, J.B., 2007b, Zircon crystallization and recycling in the Central Andean Plateau: Journal of Petrology, v. 43, p. 907–942, doi:10.1093 /petrology in the magma chamber of the rhyolitic Kos Plateau Tuff (Aegean arc): Geology, v. 35, p. 73, /43.5.907. doi:10.1130/G23151A.1. Caffe, P.J., Trumbull, R.B., and Siebel, W., 2012, Petrology of the Coyaguayma ignimbrite, north-

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