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INSIGHTS FROM ZIRCON GEOCHRONOLOGY AND TRACE ELEMENT GEOCHEMISTRY ON

THE INTERPLAY OF PLUTONISM AND VOLCANISM WITHIN THE MOUNT PRINCETON

MAGMATIC COMPLEX OF CENTRAL COLORADO

Benjamin James Thyer,

A thesis submitted to the faculty at the University of North Carolina at Chapel Hill in partial fulfillment

of the requirements for the degree of Bachelor of Science in the Department of Geological Science.

Chapel Hill

2020

APPROVED BY

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ABSTRACT

Combining high-precision CA-ID-TIMS zircon U/Pb geochronology with multivariate analysis of trace element data provides insight into the evolution of crustal magmatic systems. The Mt. Princeton magmatic center (MPMC) contains the largest Cenozoic batholith in Colorado (450 km2) as well as volcanic rocks erupted from within or adjacent to the batholith boundary. Over a kilometer of vertical relief at the MPMC, caused by Rio Grande rifting and the collapse of the Mt. Aetna caldera, exposes a variety of volcanic and plutonic rocks, making the MPMC a well-suited locality to study the relationship between plutonism and volcanism. Here we present new U/Pb zircon

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INTRODUCTION

Understanding the relationship between plutonism and volcanism is critical in assessing geohazard risk in volcanic systems,

including systems known to be capable of catastrophic “supereruptions” (referring to eruptions with volumes of ~500 km3 or

more), like Yellowstone National Park in the USA (Mark and Elis, 2013; Calvin and Mark, 2008). Models of the depths, size,

and inputs of the shallow crustal magma chambers that feed rare supereruptions determine how the immediate risk of

supereruptions in places like Yellowstone are evaluated (Wotzlaw et al., 2015). Additionally, caldera-forming eruptions

might be linked to the mineralization of economic ores including copper and molybdenum (Rosera et al., 2013; Lipman,

2007). Similarly, modelling the geochemistry of the crust requires an accurate understanding of the volcanic and plutonic

processes that drive its chemical evolution (Rudnick and Fountain, 1995; Keller et al. 2015). Our understandings of these

processes therefore influence our understanding of the chemical evolution of the crust throughout deep time and into the

present.

Refinement in isotope dilution, thermal annealing, and chemical abrasion procedures (Mundil et al., 2004; Mattinson, 2005;

Schoene, 2014) have greatly increased the resolution of ID-TIMS U/Pb geochronology; revealing increasingly complex

crystallization histories of magmatic systems. Samperton et al. (2015) found that 50% of individual zircon crystals in a

mid-crustal intrusive suite crystalized over a minimum of 150 kyr, with one sample recording a crystallization history of at least

857 ± 35 kyr. This observation suggests that many (>10) concordant, single-grain and/or sub-grain analyses are required to

adequately represent the zircon systematics of a sample (Samperton et al., 2015). Zircons (single or subgrain fractions) from

six samples are presented here to supplement existing zircon U/Pb data (Mills and Coleman, 2013; Hallman, 2014) in order

to investigate magmatic evolution of the Mount Princeton magmatic complex (MPMC). In addition, statistical analysis of

trace element geochemistry indicates coupling of geochemical behavior with time at the MPMC.

Extant literature generally agrees that calderas are formed by the collapse and outflow of shallow, ephemeral, large-volume

magma chambers (Mark and Elis, 2013; Calvin and Mark, 2008; Mills and Coleman, 2013; Simon et al., 2013; Glazner et al.,

2004), but the relationship of ignimbrites to plutons is debated. In broad strokes, two models have been favored to explain the

generation of large-volumes (>10 km3) of eruptible rhyolitic magma. The first model postulates that large-volume ignimbrite

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granodioritic magma chambers (Bachmann and Bergantz, 2004; Hildreth, 2007; Karakas et al., 2019). The second model

predicts that large-volume ignimbrites are nearly completely evacuated from ephemeral magma chambers, leaving behind

little plutonic record (Coleman et al. 2004; Tappa et al. 2011; Mills and Coleman, 2013). Each model makes testable

predictions about the temporal and geochemical relationship between plutons and spatiotemporally proximal caldera systems.

Temporal predictions of these models are tested by interpreting newly presented ID-TIMS zircon U/Pb geochronology

results, while petrochemical predictions are tested by statistically analyzing preexisting whole-rock trace element data.

Results from principal component analysis (PCA), a method shown to characterize the latent petrologic controls on a

complicated multivariate data set, are presented to rigorously characterize the internal petrochemical variation of samples

from the MPMC in order to define transitions in magma generation and differentiation processes (Ragland et al., 1997; Zieg

and Wallrich, 2018; Davis, 2002).

GEOLOGIC SETTING

TECTONOMAGMATIC SETTING

The Mount Princeton magmatic complex (MPMC) in central Colorado is part of the late Cretaceous-Eocene Colorado

Mineral Belt, a 500 km long southwest-northeast trending belt of plutons and world-class hydrothermally-emplaced ores

(Chapin, 2012). It also records some of the earliest magmatic activity in the northern portion of the Southern Rocky

Mountain Volcanic Field (SRMVF), which is a southwesterly migrating trend of waxing and waning volcanic loci erupting

large-volume ignimbrites (>400 km3), often preceded by andesitic flows and followed by resurgent, comparatively

low-volume plutonism (Lipman, 2007; Lipman and McIntosh, 2008; McIntosh and Chapin 2004). Previous workers have posited

that the southwestern migration of the SRMVF volcanic loci was controlled by the rollback of the flat-slab subducting

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km3 Wall Mt. Tuff at 37.25 Ma (Zimmerer and McIntosh, 2012) and terminated after the eruption of the Amalia Tuff in the

Latir Volcanic Field at 25.52 Ma (Tappa et al. 2011; Lipman, 2007). Ignimbrite volcanism peaked with the eruption of the

28.02 Ma Fish Canyon Tuff (Renne et al., 1998), which at 5000 km3 is one of the largest volcanic eruptions known on Earth

(Lipman et al., 1997).

GEOLOGY AND ASSEMBLY OF THE MOUNT PRINCETON BATHOLITH

The post-Laramide Mount Princeton magmatic complex (MPMC) consists of the epizonal Mt. Princeton batholith, the largest

exposed tertiary pluton in Colorado (Shannon, 1988; Toulmin and Hammarstom, 1990), earlier andesitic volcanic flows (this

study), the Mt. Aetna cauldron collapse structure nested within the Mt. Princeton batholith, and the later intruding,

petrochemically evolved leucogranites (Shannon, 1988; Toulmin and Hammarstom, 1990; Mills and Coleman, 2013;

Zimmerer and McIntosh, 2012). Vertical displacement caused by Rio Grande rifting and the collapse of the Mt. Aetna

cauldron have exposed the earliest advances of the Mt. Princeton batholith, revealing pre-plutonic andesitic flows, wall rocks,

and early-intruding roof rocks within the center of the Mt. Princeton batholith that offer insight into the construction of

epizonal plutons (Mills and Coleman, 2013; this study).

Shannon (1988) and Toulmin and Hammarstom (1990) mapped the Mt. Princeton batholith as a subcircular pluton 25-35 km

across composed of an equigranular interior unit bounded by and including texturally and chemically diverse border units,

herein collectively referred to as the Mt. Princeton Quartz Monzonite. The chemical and textural heterogeneity of the bulk

batholith, gradational contacts, and imprecise Th/Pb geochronology led Shannon (1988) to conclude that the Mt. Princeton

batholith was emplaced and chemically differentiated as a cohesive magma chamber, but this interpretation is at odds with

Toulmin and Hammarstom’s observation (1990) that there was very little contact metamorphism of the country rock intruded

by the batholith. Alternatively, studies of incrementally assembled Mesozoic plutons in the Sierra Nevada have demonstrated

that chemical zonation of incrementally assembled plutons can be explained by changes in input magma composition over

time (Coleman et al., 2004; Glazner et al., 2004) and the lack of discrete contacts between successive intrusions can be

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The epidotized, chloritized, and texturally variable Mt. Pomeroy Quartz Monzonite sits at the roof of the Mt. Princeton

batholith, preserved today due to its subsidence during the formation of the Mt. Aetna cauldron (Shannon, 1988). Shannon

(1988) recommended that the Mt. Pomeroy Quartz Monzonite be included within the texturally and compositionally

heterogeneous border unit of the Mt. Princeton quartz monzonite. However, Toulmin and Hammarstrom (1990) suggested

that the Mt. Pomeroy Quartz Monzonite was emplaced earlier in the intrusive history of the Mt. Princeton batholith due to its

position at the structural roof of the Mt. Princeton batholith. We elect to maintain the name of the Mt. Pomeroy Quartz

Monzonite within this study because it was emplaced earlier than previously dated border units of the Mt. Princeton Quartz

Monzonite (this study; Mills and Coleman, 2013).

The paleovalley outcrop of Wall Mt. Tuff radiating outwardly from the geographic area now containing the Mt. Princeton

batholith (Epis and Chapin, 1975) led Shannon (1988) and Lipman (2007) to interpret the Wall Mt. Tuff as the evacuated

felsic cap of the comparatively mafic Mt. Princeton batholith, a process predicted to occur in upper-crustal plutons that

chemically differentiated in situ by fractional crystallization (Bachmann and Bergantz, 2004; Hildreth, 2007). However,

Ar/Ar and U/Pb chronology clearly preclude the possibility that the 37.25 Ma Wall Mt. Tuff (Ar/Ar, Zimmerer and

McIntosh, 2012) and 36.27  0.09 - 35.37  0.10 Ma Mt. Princeton batholith are coeval, confirming earlier skepticism based on their distinct Srinitial isotopic compositions (Campbell, 1994; U/Pb weighted mean ages: this study and Mills and Coleman,

2013).

THE INTERPLAY OF PLUTONISM AND VOLCANISM WITHIN THE MOUNT AENTA VOLCANIC CENTER

The Mt. Aetna cauldron is a caldera structure that collapsed after the evacuation of the 34.35  0.08 Ma Badger Creek (U/Pb weighted mean age: this study) and Antero dacite tuffs, erupting collectively ~250 km3 to the NE, now preserved in the

Thirtynine Mile Volcanic Field and within the cauldron as the intracaldera Badger Creek Tuff (McIntosh and Chapin, 2004;

Lipman and McIntosh, 2008; Zimmerer and McIntosh, 2012).

Evidence supporting the interpretation of the Mt. Aetna Cauldron as a caldera include mapped megabreccia and “rubble

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caldera and the contemporaneous yet relatively distal “tuff dike”, with similar composition and age to the Badger Creek Tuff

(Shannon, 1988; Toulmin and Hammarstom, 1990; Zimmerer and McIntosh, 2012; Mills and Coleman, 2012). Additionally,

the locally high degree of preservation of structurally high Mt. Princeton border units, roof rocks, wall rocks and older

volcanic rocks within the mapped cauldron structure can be explained by the subsidence of the overlying caldera structural

block into an elevation that was comparatively erosionally stable.

Field mapping and structural data favor the interpretation of the main body of the plutonic and porphyritic Mt. Aetna Quartz

Monzonite as a resurgent pluton, emplaced after the eruption of the Badger Creek Tuff (Shannon, 1988; Toulmin and

Hammarstom, 1990; Lipman and McIntosh, 2008). This study is largely motivated by the apparent disagreement of this

interpretation with high resolution ID-TIMS U/Pb geochronology, which instead suggests that parts of the Mt. Aetna pluton

are older than the Badger Creek Tuff (Mills and Coleman, 2013).

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METHODS

FIELD METHODS

Ten samples were collected from the Sawatch Range of Chaffee county in Central Colorado in August of 2019. Eight

samples, BT-1901 through BT-1906, BT-1908 and BT-1909, were collected within a mile of the North Fork Reservoir off

county road 240 along the ridges of Sewanee Peak and Calico Mt. BT-1907 was collected near the disused “Pride of the

West” mine on the slope of Mt. Pomeroy. BT-1910 was collected at Chalk Creek pass on the slope below Monumental Peak.

All dated rocks were sampled in-situ from outcrops and therefore yield georeferenced data.

The units on the Mt. Aetna volcanic complex are spatially clustered and clearly distinguished on the scale of 10’s of meters;

however, extensive faulting and rubble zones precluded meaningful interpretation of field relationships between major units

in most cases.

Shannon (1980) claimed that the Mt. Aetna Quartz Monzonite (referred to as the Mt. Aetna 'resurgent intrusion’) cut the

Badger Creek Tuff (referred to as the ‘intracaldera tuff’). Field examination of the contact was highly limited by pervasive

brecciation of the area surrounding the contact. Abundant local fault breccia suggested an active history of faulting, but a

single contact was nonetheless located between the Mt. Aetna Quartz Monzonite and Badger Creek Tuff and a sample

(BT-1910) was taken for later imaging. The contact exposure was too limited to meaningfully interpret the nature of the contact.

ZIRCON GEOCHRONOLOGY METHODS

Cobble-sized whole-rock samples (BT-1905, BT-1907, BT-1908) were pulverized by a jaw crusher and disc mill, yielding

sand and silt-sized grains. Zircon were isolated from the whole rock-sample using standard density methods, including water

table separation and methylene iodide heavy liquid separation; and magnetic separation techniques, including hand-held

magnet removal of native iron and magnetite. Decreasingly magnetic mineral separates were extracted using a Franz

magnetic separator to yield a zircon-rich mineral separate. A representative population of zircon grains was selected under

binocular microscope, then thermally annealed for 48 hours at 900 ˚C and then chemically abraded for 14 hours in 29M

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affected by lead loss caused by radiation damage (Mundil et al. 2004; Mattinson 2005; Schoene 2014). Individual grains of

zircon of varying sizes and morphologies were selected, cleaned, spiked, and dissolved in individual microcaps using 29M

hydrofluoric acid in pressure-aided acid digestion vessels at 220˚C for 48 hours.

Fractions were spiked using a 205Pb-233U-236U tracer after Parrish and Krogh (1987). Uranium and lead were then isolated

using water and HCl through anion exchange resin after Krogh (1973). Ages for fractions were determined using isotope

ratios of U and Pb that were collected using Isotope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS) on a

PhoeniX62 mass spectrometer at the University of North Carolina at Chapel Hill. Lead metal and uranium oxide were loaded

on separate Re filaments with silica gel. Data were collected using the Daly ion-counting photomultiplier system on single

collection mode. Data processing and age calculations for individual fractions were completed using ET Redux and Tripoli

programs by CIRDLES. U238/Pb206 ages were corrected for thorium disequilibrium using whole-rock U/Th after Mattinson

(1973). Ages of eruption for MPRM-30 (Badger Creek Tuff) and BT-1905 and BT-1908 (Calico Mt. Andesite flow and tuff)

were calculated using the Zircon Bayesian Eruption Age Estimator of Keller et al. (2018), which fits discrete zircon U/Pb

ages to a predicted crystallization history in an erupted magma in order to calculate the eruption age without underestimating

error. The weighted mean ages and errors of BT-1907 (Mt. Pomeroy Quartz Monzonite), and MPRM-21 and MPRM-DSC

(Mt. Aetna Quartz Monzonite) were similarly calculated by repeatedly generating and resampling from simulated large

populations of zircon following normal distributions about real 238U/206Pb ages and standard deviations with weightings

inversely proportioned to 2 errors (R Core Team, 2018).

PRINCIPAL COMPONENT ANALYSIS METHODS

Here we employ principal component analysis (PCA), a numerical procedure that distills otherwise complicated variance

within a dataset to a series of scores along principle components. The variables included in our analysis are trace element

concentrations shown to exhibit sensitivity to petrogenetic processes from which each studied sample had usable, available

data: Ta, U, Rb, Th, Yb, Zr, Ba, La, Ce, Sm, Nd, Eu, Tb, Hf, and Sr. Whole rock data for three samples of the Mt. Princeton

Quartz Monzonite (including one border unit), the Mt. Aetna Quartz Monzonite, the ring dike, the ~30 Ma leucogranites, the

Wall Mt. Tuff, and the Badger Creek Tuff are taken from Mills and Coleman (2013), while five samples of the Mt.Princeton

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Hammarstrom (1990). Data for the chemically distinct leucogranites are included to test the effectiveness of PCA and to

increase the overall variance of the trace element data, and data from the Wall Mt.Tuff are included to evaluate the

hypothesis that is related to magmatism within the MPMC.

In conventional analysis of trace element data, each trace element represents an axis along which variation between samples

can be observed, but variance of trace element concentration often exhibits complex systematics. The primary advantage of

PCA is that it is able to reduce the dimensionality of datasets exhibiting meaningful variation in many dimensions (fifteen in

this case) into two or three meaningful dimensions, condensing complex multivariate datasets into a more digestible form.

Before PCA is applied to our dataset, the data for each trace element are recast in terms of standard deviations about the trace

element mean in order to alleviate the uneven weighting of varyingly abundant trace elements. All computations were

performed in R (R Core Team, 2018).

The complete matrix of standardized trace element data is decomposed into eigenvalues and eigenvectors, such that further

numerical processing of eigenvectors yields principal component sample scores and variable loadings associated with

eigenvalues, which define the relative variance explained by each principal component (PC). PCA generates a matrix with

the same dimensionality as the original dataset, but with the variance concentrated within the first few PC’s, allowing the

other comparatively meaningless PC’s to be ignored in order to reduce the dimensionality of the data. PCA variable loadings

define the weight and direction (positive or negative) that each trace element contributes to a sample’s orientation, or score,

along that PC. In this way, PC’s can be conceptualized as abstract variables describing the relative extent and nature of latent

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PCA sample scores are the linear combinations of input variables (standardized trace element concentrations) and variable

loadings (coefficients that define the orientation of the PC with respect to the trace element data) such that for a given sample

(i):

PC1Si = L1Ta•XiTa + L1Nd•XiNd + L1Rb•XiRb …

PC2Si = L2Ta•XiTa + L2Nd•XiNd + L2Rb•XiRb …

……

PCnSi = LnTa•XiTa + LnNd•XiNd + LnRb•XiRb …

where “PCnSi” refers to the score of sample i of the nth principal component (PC), and LnTa•XiTa refers to the product of the

loading coefficient (L) of the nth principal component for the trace element tantalum (Ta) and the input variable (XiTa): the

standardized concentration of Ta in sample i.

For a more complete detailing of the numerical procedures involved in PCA, see Davis (2002, pgs. 509-525).

RESULTS

U/Pb ZIRCON GEOCHRONOLOGY

Calico Mountain Andesite flow and welded tuff

BT-1908 (Calico Mt. Andesite) was sampled in situ on the southern flank of Calico Mountain. BT-1905 (Calico Mt. welded

tuff) was sampled in situ on a structurally resistant ridge adjoining Island Lake on the eastern slope of Sewanee Peak and the

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Figure 3: Data compiled from Mills and Coleman (2013), Hallman (2014), and the unpublished work of Mills (2014) are presented in a zircon U/Pb rank-order plot, where samples are grouped and ordered from left to right by increasing age and association, where 30, 39, and 4 are interpreted as relics of caldera collapse; MA13-01, MPRM-DSC, MPRM-20, and MPRM-21 are texturally variable phases of the Mt. Aetna resurgent pluton; MPRM-17, MPRM-6, MPRM-33, and 1907 are interior units, border units, and the roof of the Mt. Princeton batholith; and 1908 and BT-1905 are surficial, pre-plutonic andesitic volcanism. The estimated age of the hydrothermal mineralization of the Tomchi complex (conterminous with the southernmost limits of the MPMC) is given by a chartreuse line at 36 Ma. Observations that samples older than 36 Ma are more hydrothermally altered allow for the interpolation of the same hydrothermal event occurring in the MPMC, perhaps due to expelled fluids during the emplacement of the majority of the underlying Mt. Princeton batholith.

Figure 4:Principal Component Analysis distills broad trends from complex and noisy whole-rock trace element data, used in this case to differentiate samples from the Mt. Princeton magmatic complex into four distinct populations. Dashed, grey arrows show which trace elements are controlling the variance in the plot, where positive correlation of sample scores and trace element loadings means generally higher abundances of those trace elements in those samples. With trace element data alone, Q-mode principal component analysis differentiates samples into four populations of unique ages, suggesting that PCA reveals latent trends in magmatic source and/or differentiation in the MPMC. Clustering of PCA scores corresponding to a distinct time and magmatic fate implies correlation between magmatic fate, time, and chemistry, suggesting that melt-generating processes

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Mount Pomeroy Quartz Monzonite

BT-1907 (Mt. Pomeroy Quartz Monzonite) was sampled in situ by the abandoned Pride of the West mine on the eastern flank

of Pomeroy Mountain. BT-1907 yields 11 concordant analyses ranging from 35.82  0.33 Ma to 37.54  0.45 Ma.

Mount Aetna Quartz Monzonite

MPRM-21 (Mt. Aetna Quartz Monzonite) was previously sampled by Mills and Coleman (2013) and MPRM-DSC was first

dated by Hallman (2014). The 10 concordant analyses of MPRM-21 within this study yield ages from 34.40  0.11 Ma to 35.402  0.084 Ma. The 7 concordant analyses of MPRM-DSC within this study yield ages from 36.1  0.6 Ma to 34.48 

0.16 Ma.

Badger Creek Tuff

MPRM-30 (outflow Badger Creek Tuff) was previously sampled by Mills and Coleman (2013). The 9 concordant,

Eocene-age analyses of this study yield Eocene-ages from 34.28  0.14 Ma to 34.76  0.11 Ma, with 1 discordant analysis that yields an upper-intercept discordia array age of ~1.4 Ga.

PRINCIPAL COMPONENT ANALYSIS (PCA)

Trace element results are presented in the form of Euclidean distances between sample scores in 3-dimensional principal

component space (65.3 % of variance: Figure 5) and in sample scores and trace element loadings plotted in 2-dimensional

principal component space (54.7 % of variance: Figure 4).

DISCUSSION

INTERPRETATION OF ZIRCON AGE SPECTRA

Single crystal ID-TIMS U/Pb zircon dates presented here reveal simple, albeit high-uncertainty, ages for the previously

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BT-1905 (F-5 and F-8 of the Calico Mt. welded tuff: Table 2) yield ages that are indistinguishable within error from the

calculated eruption age, but the fractions that do not agree within uncertainty are especially high-error analyses ( >1 Ma) and therefore are likely unreliable. This eruption age is further corroborated by all three low-error ( 0.20 Ma) analyses of BT-1905 (F-3, F-10, and F-11 of the Calico Mt. welded tuff: Table 2). All fractions of BT-1908 (Calico Mt. Andesite flow)

yield high uncertainties because of low uranium concentrations; however, they are all indistinguishable within error from the

calculated eruption age of BT-1905 (Calico Mt. welded tuff). Zircon U/Pb data for both rock types from the Calico Mt.

volcanic unit are consistent with the eruption age (calculated from the tuff) of 37.48 ± 0.18 Ma. Thus, within this discussion

we refer to the Calico Mt. volcanic rocks as one unit, consistent with geologic mapping by Toulmin and Hammarstrom

(1990).

The geochronologic results for the Calico Mt. volcanic rocks are inconsistent with earlier field work: Shannon (1988)

mapped the Calico Andesites as a member of the intracaldera Badger Creek Tuff and Toulmin and Hammarstrom (1990)

posited that the Calico volcanic rocks are younger than the Mt. Pomeroy Quartz Monzonite (36.27  0.09 Ma). Notably, these authors remarked on the poor exposure of contacts between the Calico Mt. volcanic rocks and surrounding rock units. The

37.48 ± 0.18 U/Pb eruption age of the Calico Mt. rocks (Table 2, Figure 3) is at least 2 Ma older than any portion of the Mt.

Aetna/Badger Creek units (aside from F-33 of MPRM-DSC, a likely inherited zircon discussed below: Table 1, Figure 3) and

so is likely not associated with the 34.35 ± 0.08 Ma Badger Creek magmatism, as Shannon (1988) had speculated. Regarding

the relative age of the Calico Mt. volcanic rocks and the Mt Pomeroy Quartz Monzonite, we suggest that the geochronologic

results here support a model in which the Calico Mt. volcanism predates the crystallization of the 36.27  0.09 Mt. Pomeroy Quartz Monzonite, in contrast to the relative field dating of Toulmin and Hammarstom (1990). At 37.48  0.18 Ma, the Calico Mountain volcanic rocks are the oldest Cenozoic rocks preserved in the MPMC, showing that magmatism within the

MPMC began as early as 37.48 Ma, around ~1.5 Ma older than previously understood (Zimmerer and McIntosh, 2012; Mills

and Coleman, 2013).

BT-1907, The Mt. Pomeroy Quartz Monzonite, records a more complex history of zircon crystallization compared to the

Calico Mt. Andesite. The oldest two fractions of the Mt. Pomeroy Quartz Monzonite are interpreted to be inherited from the

adjacent Calico Mt. Andesite because the oldest zircon fractions of the Mt. Pomeroy Quartz Monzonite have similar

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Andesite. A stochastically resampled weighted mean calculation yields an emplacement age of 36.27  0.09 Ma, but it is possible that this age is skewed by additional undetectable partial inheritance of ~37.5 Ma zircon.

New zircon U-Pb ages for samples MPRM-21 and MPRM-DSC (Mt. Aetna Quartz Monzonite) and MPRM-30 (Badger Cr.

Tuff) expand the age range (Figure 3) reported for those two samples by Mills and Coleman (2013) and Hallman (2014).

F-33 of MPRM-DSC (Mt. Aetna Quartz Monzonite: table 1; figure 3), is interpreted to be inherited from the Mt. Pomeroy

Quartz Monzonite or Calico Mt. Andesite due to its low uranium content and age. Otherwise, the protracted zircon age

spectra are difficult to interpret because of the potential for Pb-loss or inheritance. Pb-loss occurs when the zircon system is

opened and radiogenic Pb is lost from the system, thus producing a spuriously young age. Thermal annealing and chemical

abrasion techniques (Mattinson, 2005) greatly mitigate the Pb-loss problem by removing metamict zones of the zircon where

damage to the crystal lattice results in pathways for Pb escape. However, Pb-loss likely persists to some degree in even the

most calibrated systems (Mundil et al., 2004; Widmann et al., 2019). Conversely, incorporation of zircon from older rocks

(i.e. inheritance) produces isotopic mixing of the U-Pb systems. The detectability of inheritance in the U-Pb isotopic data

highly depends on the magnitude of the age difference between the inherited zircon xenocryst or antecryst and the autocrystic

zircon, where native autocrysts date the cooling of the host magma body, xenocrystic zircon are inherited from much older

magmatism, and antecrystic zircon are inherited from earlier magmatism within the same magmatic system (Miller et al.,

2007). When the age difference is small (< 1 Ma) and the inheritance is sourced from earlier portions of the same magmatic

system, it is difficult to distinguish whether the older zircon record the protracted thermal life of the host magma, or the

cooling of a different magma. Because of the uncertainty in tracing Pb-loss and partial inheritance in systems exhibiting

complex zircon U/Pb systematics, we interpret the U/Pb ages cautiously.

It is possible that very few data presented here are affected by Pb-loss and/or inheritance. Previous work on the assembly of

plutons (Glazner et al., 2004, Coleman et al., 2004; Stearns and Bartley, 2014) and the periodicity of caldera-forming

volcanic systems (Lipman, 2007; Lipman et al., 2015; Wotzlaw et al., 2015; Bachmann et al., 2007) have demonstrated that

magmatism in the continental crust can be protracted over millions of years by continued inputs from deeper sources within

the crust or mantle. It is plausible that perhaps most data presented here are from autocrystic zircon, which record

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Zircon U/Pb geochronology is most powerful when used in tandem with U/Pb, Ar/Ar, and U-Th/He thermochronology of

other minerals. Thermochronology of zircon, titanite, hornblende, biotite, sanidine, and apatite date the cooling of the system

to each mineral’s respective closure temperature in order to reconstruct the thermal history of a sample (Shuster and Farley,

2005; Coleman et al., 2016). Since zircon’s closure temperature is >1000˚C and its saturation temperature is ~800˚C, even

completely original autocrystic zircon may cease crystalizing long before the end of the magmatic life of the igneous system

(Cherniak et al., 2004), therefore it is important to interpret zircon U/Pb ages in concert with other available

thermochronological constraints whenever possible.

New high precision ages for the Calico Mt. Andesite flows and tuffs and the Mt. Pomeroy Quartz Monzonite indicate that the

MPMC was active intermittently at least from 37.48  0.18 to 33.93  0.09 Ma (Ar/Ar age: Zimmerer and McIntosh, 2012) before the onset of chemically distinct bimodal rift magmatism at around 30 Ma (Mills and Coleman, 2013), recording a

long-lived span of magmatic activity comparable to large, mid-crustal plutons (Coleman et al., 2004). Aside from grains

interpreted to be inherited (figure 3), high precision U/Pb geochronology reveals a temporal disconnect between modes of

magmatism (figure 6). Beyond the errors of calculated weighted means and eruption ages, the zircon of the MPMC record a

systematic progression from volcanism (“Calico-mode”) to plutonism (“Princeton-mode”), and finally to ignimbrite-related

magmatism (“Aetna-mode”), each drawing from independent, temporally-distinct populations of autocrystic zircon growth

(figure 6). The notable absence of distinct modes of magmatism occurring contemporaneously within the MPMC suggests

that melt-generating processes govern the fate of a magma, because models that emphasize the roles of upper-crustal

differentiation would predict that complimentary, distinct modes of magmatism would be active contemporaneously,

although it is possible that evidence of upper-crustal differentiation has been eroded away or lies beneath current exposures.

This is in agreement with Mills and Coleman (2013), who suggest that regional ignimbrite quiescence within the SRMVF

during the assembly of the Mt. Princeton batholith favors the interpretation that melt-generating variables such as magmatic

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ISOLATING SHIFTS IN CHARACTER OF A MAGMATIC SYSTEM THROUGH INTEGRATED ZIRCON U/Pb

GEOCRHONOLOGY AND TRACE ELEMENT STATISTICAL GEOCHEMISTRY

Tight clustering of like-samples in principal component space serves as assurance that PC1 and PC2 effectively discriminate

latency in trace element systematics (figure 4). Additionally, the distinct PCA scores of the 30 Ma leucogranites (a product

of an entirely different tectonomagmatic setting: Mills and Coleman, 2013) and the 37.25 Ma Wall Mt. Tuff (a sample of

unknown pedigree within the northern SRMVF: Zimmerer and McIntosh, 2012) demonstrate that PCA effectively unveils

shifts in tectonomagmatic controls of the trace element geochemistry of a system (figure 5). Notably, the whole rock trace

element data from the Wall Mt. Tuff and the Badger Creek Tuff were from pumice-like lithic-free fragments (Mills and

Coleman, 2013), but the accidental inclusion of small lithics could have skewed the results.

Integrated high-precision U/Pb zircon geochronology and trace element PCA reveals that temporally distinct modes of

magmatism also bear distinct principal component score-signatures. Figure 4 schematically illustrates the systematic

evolution of principal component score-signatures over time. Each mode of magmatism corresponds to a distinct chemical

signature, except for the earliest stages of Princeton-mode plutonism, which record the transition from Calico-mode chemical

character to the chemical character of the interior units of the Mt. Princeton Quartz Monzonite. This implies that the

tectonomagmatic processes that control the trace element chemical signature of a magma might also control its ultimate

depositional fate within the crust or on the surface, since shifts from large-scale volcanism to plutonism correspond to

temporal and chemical disconnects. Distinct character of trace-element principal component scores within different modes of

magmatism reinforces that upper-crustal differentiation likely played a minimal role in determining the chemical character of

magmas within the MPMC at current erosional levels, since models emphasizing the role of upper-crustal differentiation

would predict contemporaneous, distinct, and complimentary trace element systematics (Glazner et al., 2008).

MOUNT POMEROY QUARTZ MONZONITE: THE BIRTH OF A BATHOLITH

The age spectra for BT-1907 (Mt. Pomeroy Quartz Monzonite: figure 3) overlaps with and bridges the difference between the

spatially adjacent Calico Mt. volcanic rocks (BT-1908, BT-1905) and the Mt. Princeton Quartz Monzonite border units

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(MPRM-33: Mills and Coleman, 2013). If the oldest Zircon U/Pb dates of the Mt. Pomeroy Quartz Monzonite are inherited,

then the Mt Pomeroy intrusion is temporally distinct from the Calico Mt. Andesite and is much closer in age to the assembly

of the Mt. Princeton batholith. Previous field studies observed: 1) gradational contacts between the 35.80 ± 0.09 Ma border units of the Mt. Princeton Quartz Monzonite and the 36.27  0.09Mt. Pomeroy Quartz Monzonite and 2) chilled margins in the later intrusions of the 35.37  0.10 Ma porphyritic facies of the Mt. Princeton Quartz Monzonite with the Mt. Pomeroy Quartz Monzonite (Shannon, 1988; Toulmin and Hammarstrom, 1990), in agreement with the interpretation that the Mt.

Pomeroy Quartz Monzonite represented the earliest pulses in the incremental assembly of the Mt. Princeton batholith.

Nearly any interpretation of the mixing behavior in the zircon U/Pb systematics (discussed in “Interpreting Zircon Age

Spectra”) of the Mt. Pomeroy Quartz Monzonite would agree that the Mt. Pomeroy Quartz Monzonite represents a shift from

Calico-mode volcanism to Princeton-mode plutonism, and that the intrusion of the Mt. Pomeroy Quartz Monzonite occurred

between the two.

Results from the principal component analysis (figure 4, figure 5) reveal that the plutonic Mt. Pomeroy Quartz Monzonite

and the slightly younger border units of the Mt. Princeton Quartz Monzonite record trace element systematics that are

intermediate with respect to the older Calico Mt. Andesite flow and tuff and the younger interior units of the Mt. Princeton

Quartz Monzonite. Two potential interpretations for this clustering are: 1) the trace element signature in the Mt. Pomeroy Quartz Monzonite records an intermediate step in a transition of magmatic source or differentiation processes from

Calico-mode volcanism to Princeton-Calico-mode plutonism, or 2) hydrothermal alteration following the nearby emplacement of the massive Mt. Princeton batholith affected the trace element signature of the previously divergent Calico Mt. rocks and early

Mt. Princeton-Pomeroy rocks similarly, causing an apparent convergence due to secondary alteration.

MOUNT AETNA QUARTZ MONZONITE: RESURGENT PLUTON OR INTRUSIVE EQUIVALENT FOR THE BADGER

CREEK TUFF?

Perhaps one of the most intriguing problems of the MPMC is the apparent disagreement of the U/Pb zircon geochronology

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ranked some samples of the Mt. Aetna Quartz Monzonite as older than the Badger Creek Tuff, which is at odds with the

previous literature that interpreted the Mt. Aetna Quartz Monzonite as a younger pluton, resurgent into the intracaldera

Badger Creek Tuff (McIntosh and Chapin, 2004; Lipman and McIntosh, 2008; Shannon, 1988; Toulmin and Hammarstrom

1990). Mills and Coleman (2013) attempted to reconcile this apparent paradox by tentatively interpreting the Mt. Aetna

Quartz Monzonite as an incrementally assembled pluton that recorded both pre- and post-caldera plutonism. Hallman (2014)

collected additional high precision U/Pb ID-TIMS data on another sample of the Mt. Aetna Quartz Monzonite that was

adjacent to the contact with the intracaldera Badger Creek Tuff, concluding that the Mt. Aetna Quartz Monzonite recorded

pre- and post-caldera plutonism distinct from the Badger Creek Tuff, in the absence of observed syn-caldera zircon growth.

Subsequent work presented here, including this study and analyses by Mills (unpublished), have filled that discontinuity

(Figure 3).

Single-grained fractions of U/Pb ID-TIMS analyses of zircon presented here show a wide range of ages (outside of

uncertainties) for each sample of the Mt. Aetna Quartz Monzonite. In spite of this expansion of the age spectra, individual

samples of comparable textures (like the indistinguishable MPRM-21 and MPRM-DSC) yield distinct age-populations of

zircon, suggesting the Mt. Aetna resurgent pluton was either: 1) assembled incrementally like the cordilleran plutons of the Sierras (Coleman et al., 2004; Glanzer et al., 2004), or 2) systematicallyaffected by inheritance (±Pb-loss) heterogeneously throughout the pluton.

In light of inferences that large plutons are assembled incrementally (Glazner et al., 2004; Colemen et al., 2004), several

workers have posited that large-volume silicic ignimbrites can be generated through crystal fractionation in the upper crust,

so long as the hot, near-solidus crystal mush is episodically recharged by magma flux to allow for the extraction of evolved

interstitial melts (Bachman et al., 2007; Lipman, 2007). Observed protracted crystallization of primary titanite in near-solidus

or sub-solidus upper crustal crystal mushes and hot rocks suggests that ignimbrite-producing magmatic systems can have a

long thermal life in between episodes of eruption, occasionally producing small amounts of non-eruptible interstitial melt

(Szymanowski et al., 2017). Thus, the protracted zircon U/Pb age spectra of the Mt. Aetna Quartz Monzonite could be

explained by a long history of episodic thermal recharge of the crystal mush that later solidified as the Mt. Aetna resurgent

pluton. However, the major element (Mills and Coleman, 2013) and radiogenic isotope (Mills, 2012) chemistry of the Mt.

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similar trace element behavior as evidenced by their proximity in 3-dimensional principal component space (figure 5). This

suggests that upper crustal differentiation within the Mt. Aetna volcanic center is undetectable at modern erosional levels.

Thermal models predict that a pluton orders of magnitude more voluminous than the Mt. Aetna Quartz Monzonite must cool

to sub-solidus temperatures within a few hundred thousand years if emplaced in a single pulse (Harrison and Clarke, 1979);

however, the time difference recorded by Ar/Ar K-feldspar geochronology (33.93  0.09: Zimmerer and McIntosh, 2012) and the weighted mean age of MPRM-21 (Mt. Aetna Quartz Monzonite: 34.97  0.09 Ma) is about 1 Ma, at least an order of magnitude more than expected. In addition to reconciling the paradoxically old zircon U/Pb ages of the Mt. Aetna pluton,

proposed models must explain why the zircon crystallization history of the Mt. Aetna pluton is protracted, or alternatively

why unwanted artefacts of zircon systematics like inheritance or Pb-loss would affect the system heterogeneously. Two

interpretations are presented here to explain the zircon U/Pb systematics within the Mt. Aetna volcanic center:

1. The Mt. Aetna pluton is a purely resurgent pluton, emplaced shortly after the eruption of the Badger Creek Tuff,

similar to the resurgent plutons in the nearby Grizzly Peak caldera and Bonanza caldera (Frazer, 2017; Lipman et al.

2015). The voluminous pulse of Badger Creek magma remobilized and partially melted Mt. Princeton material,

freeing zircon to be captured by trailing magma. As the residual heat of the system decreased, entrained Mt.

Princeton zircon were subjected to decreasing degrees of Pb-loss and re-equilibration to the Badger Creek magma,

and fewer zircon were available to be captured (inherited). This explanation plausibly explains variation on the

pluton scale, as the zircon emplaced there have systematically different degrees of inheritance and Pb-loss than

nearby rocks with similar textures, depending on when they were emplaced. Variable inheritance and Pb-loss also

explains the spread of U/Pb ages on the handsample scale because pervasive inheritance would be overprinted by

primary zircon growth varying with crystal geometry. The younger population of zircon within the Badger Creek

Tuff (figure 3) could be explained by the high magmatic flux related to the ignimbrite; any signal of zircon

inheritance from the Mt. Princeton Quartz Monzonite would be more likely to equilibrate with the voluminous

Badger Creek magma, and grains still affected by this inheritance would be more likely to be a minority in the

population of primary zircon, and subsequently less likely to be sampled for analysis. The trace element systematics

(figure 4) of the Mt. Aetna resurgent pluton places it as intermediate with respect to the Badger Creek Tuff and the

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the hypothesis that remobilized Mt. Princeton Quartz Monzonite was integrated into a magma with a similar

composition to the Badger Creek Tuff.

2. The Mt. Aetna pluton was incrementally assembled before, after, and perhaps during the eruption of the Badger

Creek Tuff from ~35–33.93 Ma, with relatively high episodic heat inputs that allowed for the ephemeral generation

of small proportions of interstitial melt on the pluton scale, supporting protracted zircon growth. In this model, the

Mount Aetna pluton was largely remobilized after the eruption of the cogenetic Badger Creek Tuff, which was

produced by a high-volume volcanic ‘burp’ of similar composition. Following remobilization, the magmas intruded

the overlying collapsed roof of the caldera into the intracaldera Badger Creek Tuff. The megacrystic, porphyrytic

texture of the Mt. Aetna Quartz Monzonite could be explained by this thermal cycling (Mills et al., 2011). The

distinct populations of zircons observed in massive rocks of similar textures could be explained by a model of

incremental assembly by crack-seal veins, annealing contacts through sub-solidus crystal growth (Stearns and

Bartley, 2014). Zimmerer and McIntosh’s Ar/Ar age of 33.93  0.09 Ma (2012) would be interpreted to represent the final cooling of the pluton after a complex, ~1 Ma magmatic life.

CALICO MOUNTAIN FLOWS: REGIONALLY CONTEXTUALIZING EARLY VOLCANISM

The 1000 km3 eruption of the rhyolitic Wall Mt. Tuff marked the onset of ignimbrite volcanism within the Southern Rocky

Mountain Volcanic Field at 37.25 Ma, but no associated caldera structures have been documented at present erosional levels

(Zimmerer and McIntosh, 2012; Lipman and McIntosh, 2008). Some researchers have posited that the interpreted

convergence of paleovalleys infilled with the Wall Mt. Tuff about the MPMC (Epis and Chapin, 1975) suggests that a caldera

structure and/or residual frozen magma chamber could have existed structurally above the modern exposure of the MPMC

(Lipman and McIntosh, 2008; Mills and Coleman, 2013). While data presented here show that the Mt. Princeton batholith

began assembling at 36.27  0.09Ma with the Mt. Pomeroy Quartz Monzonite, there still remains a ~1 Ma temporal disconnect between the Ar/Ar age of the Wall Mt. Tuff and the ID-TIMS U/Pb ages of the Mt. Princeton batholith of this

study, excluding the possibility that the Mt. Princeton batholith was contemporaneous with the Wall Mt. Tuff. The newly

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This does not conclusively link the Wall Mt Tuff to the MPMC, but if Calico Mt. volcanism was part of the Wall Mt.

magmatic system then several additional details need further discussion. Lipman et al. (2015) and Wotzlaw et al. (2015)

found that caldera systems may erupt multiple times in a single location, often overprinting older magmatic events. Lipman et

al. (2015) also documented extensive laccolithic deformation from resurgent plutonism in the nearby Bonanza caldera,

suggesting that similar deformation from the assembly of the Mt. Princeton batholith could have raised caldera structures

hypothetically left by the Wall Mt. Tuff to be particularly susceptible to erosion. The absence of extant structural evidence

confirming the Wall Mt. Tuff’s origin above the modern exposure of the Mt. Princeton batholith does not preclude the

possibility. While the Calico Mt. Andesite flows and tuffs are clearly not a unit of the rhyolitic Wall Mt. Tuff, they could

represent the andesitic, pre-ignimbrite precursors to the Wall Mt. Tuff, characteristic of many caldera systems in in the

SRMFV (McIntosh and Chapin, 2004; Lipman, 2007, Lipman and McIntosh, 2008). It is just as likely, however, that the

Calico Mt. Andesite flows and tuffs record preplutonic volcanism heralding the onset of magmatic activity within the

MPMC, or are analogues within the MPMC to the ~38 Ma andesitic flows and breccias that represent the very first volcanic

activity in the SRMVF (McIntosh and Chapin, 2004; Lipman and McIntosh, 2008).

PCA of the Wall Mt. Tuff demonstrates that it is largely distinct with respect to the rocks emplaced within the MPMC;

however, Figures 3 and 4 show that it is the least dissimilar to the Calico Mt. Andesite in principal component space. These

results nonetheless are inconclusive because the generally distinct character of the trace element systematics of the Wall Mt.

Tuff and Calico Mt. Andesite could be explained by their vastly different major element chemistry (Toulmin and

Hammarstrom, 1990; Mills and Coleman 2013), and yet the comparative proximity of the two units in principal component

space can also be explained by shared regional tectonomagmatic controls on the broadly contemporaneous volcanic rocks.

While the Wall Mt. Tuff is least dissimilar to the Calico Mt. Andesite, the andesites are generally more similar to other rocks

occurring in the MPMC, especially the earliest intrusions of the Mt. Princeton batholith (figures 4 and 5). This proximity in

PCA scores may be explained by either a similar petrogenesis or by a similar history of hydrothermal alteration that caused

convergence of trace element systematics between the units.

The history of hydrothermal alteration recorded in the Mt. Pomeroy Quartz Monzonite and the Calico Mt. Andesite flows and

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Ma, following the emplacement of the Mt. Pomeroy Quartz Monzonite and before the emplacement of the Mt. Princeton

border units. Interestingly, the adjacent intrusive suite bordering the MPMC to the South, the granitic Tomchi complex, also

records hydrothermal alteration associated with deposits of copper and molybdenum ore at around 36 Ma (Gray and Sim,

2017). Following observations of clustering of some ores proximal to caldera rims, Rosera et al. (2013) modelled the timing

and geometries of the emplacement of some forms of economic mineralization following ignimbrite eruptions and resurgent

plutonism. It is possible then for the hydrothermal event recorded in the Calico Mt. Andesite flows and tuffs and the Mt.

Pomeroy Quartz Monzonite to be related to the mineralization of the Tomchi complex. This model would suggest that the

emplacement of the Mt. Princeton batholith could have carried fluids remobilized from depth by an older high-volume

ignimbrite eruption (the Wall Mt. Tuff?), or that other hydrothermal events nearby may provide insights in the search for the

source of the Wall Mt. Tuff or additional examples of economic mineralization. Existing ages of the Tomchi complex are

relative ages based on field relationships (Gray and Sim, 2017), and so further data are needed to test these hypotheses and to

continue to model the association of high volume ignimbrite eruptions and economic mineralization.

CONCLUSIONS

New zircon U/Pb ages show that the Mount Princeton Magmatic Complex was intermittently active for ~4 Ma, beginning at

37.48  0.18 Ma with the Calico Mountain Andesite flows and tuffs.

The 37.48  0.18 Ma Calico Mountain Andesite flows and tuffs may be broadly contemporaneous with the Wall Mountain Tuff, providing an opportunity for further studies to test hypotheses on the origins of the Wall Mountain Tuff and/or to

contextualize the early magmatism of the Southern Rocky Mountain Volcanic Field at the time of the eruption of the Wall

Mountain Tuff.

Statistical methods like principal component analysis are useful for elucidating latent controls on complex multivariate

chemical datasets of magmatic systems. High precision U/Pb zircon geochronology and trace element statistical

geochemistry of Cenozoic volcanism and plutonism in the Mount Princeton magmatic complex demonstrate that distinct

modes of magmatism are temporally and chemically exclusive, suggesting that melt-generating processes are responsible for

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The disagreement of zircon U/Pb geochronology with relative dating by field relationships within the Mount Aetna volcanic

center may suggest that inherited zircon can nearly completely eclipse signals from primary autocrystic zircon in shallow

volcanic systems (within the spatial resolution of ID-TIMS) or that resurgent plutons may represent the remobilized magma

reservoirs that feed ignimbrite eruptions.

Hydrothermal alteration of the Mount Princeton magmatic complex occurring at ~36 Ma may represent the same

hydrothermal event that mineralized copper and molybdenum deposits in the conterminous Tomchi complex to the south of

the Mount Princeton magmatic complex. Further geochronological and geochemical data are needed to test this hypothesis

and to build higher resolution models of caldera-related mineralization of valuable-metal ores.

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