MAGMA MIXING AND DOME FORMATION: DACITE OF EAST PASS CREEK, COLORADO

Jenna C. Streffon

A Thesis

Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements of the degree of

MASTER OF SCIENCE

August 2019

Committee:

Kurt S. Panter, Advisor

John R. Farver

Daniel F. Kelley © 2019 Jenna Streffon All Rights Reserved iii

ABSTRACT

Kurt S. Panter, Advisor

The San Juan Volcanic Field (SJVF) all together was formed during a period of cataclysmic volcanic eruptions (~23 to 37 Ma) that produced 15 known calderas including the

North Pass caldera. The formation of the North Pass caldera occurred at 32.25 ± 0.05 Ma with the eruption of the Saguache Creek tuff and was followed by the formation of post-caldera lava domes that erupted over an area of 700 km2 grouped as the Volcanics of Cochetopa Hills, which includes the Dacite of East Pass Creek (DEPC) lava domes. Two domes herein referred to as

‘Buffalo Dome’ and ‘North Dome’ in the northwest region of the DEPC eruptions contain abundant fluidal inclusions (aka. enclaves) hosted within columnar jointed lavas. The enclaves are the result of magma mingling, which has not been previously mentioned in the formation of the DEPC. Host lavas and inclusions are distinguished by mineral abundances and compositions to textural differences. Large abundances of amphibole are found within Buffalo and North dome inclusions, a rare mineral within host lavas. Instead of amphibole, host lavas contain biotite and pyroxene and are overall more evolved (e.g. higher SiO2 content) in composition than the inclusions. Major and trace element data confirm that mafic elements are in abundant within inclusions at Buffalo Dome and can be genetically related to the host lavas as a residual ‘crystal mush’. North Dome enclaves, however, are compositionally more evolved than the host and not likely genetically related to their host lavas. These intricate relationships are similar to what has been found in post-caldera domes associated with the Lake City caldera. A complex fracture network supports an environment for magma interactions such as mingling through multiple magma replenishments. The information gathered from the DEPC has important implications in our overall understanding of the life cycle of caldera systems within the SJVF and elsewhere. iv

To my family, humans and dogs included, for never backing down in your never-

ending support, and for having the confidence in me to complete this project. Many times, you had more faith in me than I had in myself. Thank you for encouraging me

every step along the way. I hope to make each of you proud in everything that I do. v

ACKNOWLEDGMENTS

Thank you to my advisor, Kurt Panter, for encouraging me to pursue geology and guiding me along every step of the way. Thank you for helping me through my frustrations throughout grad school and encouraging me to persevere and love my scientific work. I am so very appreciative for the time and effort you have spent molding me into a better scientist. It is impossible to thank you enough.

Thank you to my committee members, Dr. John Farver and Dr. Dan Kelley. Thank you, Dr.

Farver, for helping me fix every problem I caused in the thin section labs (and the rest of the department) in my seemingly never-ending years at BGSU. Dr. Kelley, thank you for always going the extra mile to stay involved with my work and for being encouraging and inquisitive throughout this process.

Finally, thank you to the Bowling Green State University Geology Department for funding and facilitating my lab and field work with the Richard D. Hoare Graduate Research Fund and through BGSU Field Camp. Thank you to Dr. Tyrone Rooney and the lab technicians at Michigan

State University for the assistance in my whole rock chemistry analysis and to Owen Neill and Dr.

Gordon Moore at the University of Michigan EMAL for making my microprobe analysis fun and easy. vi

TABLE OF CONTENTS

Page 1. INTRODUCTION ...... 1

1.1. Geological Background ...... 2

1.1.1. San Juan Volcanic Field ...... 2

1.1.2. North Pass Caldera Cycle ...... 3

1.1.3. Dacite of East Pass Creek ...... 4

1.2. Formation of Domes ...... 5

1.3. Magma Mingling ...... 6

2. METHODOLOGY AND ANALYTICAL TECHNIQUES ...... 9

2.1. Mapping Aspects and Sample Collection ...... 9

2.2. Petrography ...... 9

2.3. Analytical Methods ...... 10

2.3.1. Microprobe Analysis ...... 10

2.3.2. Whole Rock Chemistry ...... 10

3. RESULTS ...... 12

3.1. Field Results and Observations ...... 12

3.1.1. Field Observations of Buffalo and North Domes ...... 12

3.1.2. Enclaves - Field Characteristics ...... 14

3.2. Petrography ...... 15

3.2.1. Sample Descriptions ...... 15

3.2.2. Host-Inclusion Comparison ...... 16

3.2.3. Host-Inclusion Contact Boundaries ...... 17

3.3. Mineral Chemistry...... 18

3.3.1 Mineral Classification ...... 18 vii

3.3.2. Chemical Zoning in Plagioclase ...... 19

3.4. Whole Rock Chemistry ...... 20

3.4.1. Classification and Chemical Relationship ...... 20

3.4.2. Comparison with Compositions from Lake City Caldera ...... 24

4. DISCUSSION ...... 26

4.1. Characterization of Magma Mixing and Mingling ...... 27

4.2. Origins of Enclaves and Relationship to Host Lavas...... 29

4.2.1. Mineralogical Relationships ...... 29

4.2.2. Magma Genesis ...... 30

4.3. Proposal for Post-Caldera Processes of the DEPC Lava Domes ...... 34

5. CONCLUSION ...... 36

6. REFERENCES ...... 38

APPENDIX A. TABLES ...... 43

APPENDIX B. FIGURES ...... 57 1

1. INTRODUCTION

Caldera forming eruptions are highly explosive and typically produce large volumes of ash-flow tuff and pyroclastic fall deposits. With the mass evacuation of magma during these eruptions, the volcano collapses in on itself to fill in the emptied magma system beneath.

Fractures and faults created during caldera collapse then serve as pathways for sustained emissions of replenished magma and gases (Sigurdsson et al., 2015). These less-explosive eruptions in the later stages of a caldera cycle often produce post-caldera domes formed when residual magmas find their way through fractures and erupt around the exterior and center of the caldera remnants. It is within these later stage processes that magma mingling is most likely to occur because remnants of previous eruptions are not fully cleared out of the fractured system prior to the replenishment of new magmas (Perugini & Poli, 2012).

In this study, magmatic enclaves have been identified in post-caldera dome lavas of the

Dacite of East Pass Creek (DEPC) within the North Pass caldera of the San Juan Volcanic Field

(SJVF). An abundance of enclaves is found in columnar jointed lavas at two dome complexes, herein named Buffalo Dome and North Dome for this project. Their identification provides the first clue that complex magma interactions occurred during the eruptions of the DEPC lava dome system. Magmatic enclaves in a volcanic environment can provide insight on the composition of the initial magma of this system, the effects of dilution of magma by newly generated magma, and an estimate of mixing and eruption timescales (Perugini & Poli, 2012). These observations are important to understand caldera cycles within the SJVF. Specifically, the results provide significant information about the driving mechanisms of magma resurgence and recharge, magma evolution, and a glimpse into physically unobservable inner Earth processes (Kennedy et al, 2012; Bachmann & Bergantz, 2008). 2

In this thesis, post-caldera dome activity in the North Pass caldera is examined in order to understand late-stage magmatic processes which produced enclaves within DEPC lava flows.

The aim of this research is to answer the following questions: 1) are the enclaves produced by magma mingling; 2) if so, how many magmas are present in this system and what is the relationship of enclaves to the host lavas, and 3) did the addition of the enclave liquids directly cause the eruptions of the DEPC dome complex? To answer these questions, field work was undertaken to collect samples and map the volcanic geology of Buffalo Dome. Mineralogical, petrological, and geochemical data are used to propose a model of the post-caldera dome processes of the North Pass caldera cycle.

1.1. Geological Background

1.1.1. San Juan Volcanic Field

The Oligocene San Juan Volcanic Field (SJVF) is located within the southwestern Rocky

Mountains in Colorado (Figure 1) and into northern New Mexico, which were once covered in continuous volcanic fields that are now recognized by eroded remnants within the San Juan

Mountain range (Lipman & McIntosh, 2012; Lipman & McIntosh 2008). The SJVF is rich in volcanic history, which was highly influenced by differences in the tectonics and crustal structures in the Southern Rocky Mountain range. Because of these differences, the SJVF produced 15 known calderas, including the La Garita caldera which produced, volumetrically the largest eruption on Earth (~ 5,000 km3 Fish Canyon Tuff) during the Oligocene (averaging

28.201 Ma; Morgan et al., 2019). The volcanics lie between Precambrian-cored uplifts formed by the compressional stress of the Late Cretaceous to early Cenozoic low-angle subduction of the

Farallon Plate. The cessation of subduction and steepened descent (e.g. roll back) of the plate into the mantle triggered melting that lead to magmatism (Lipman et al., 1970; Steven & 3

Lipman, 1976; Lipman & McIntosh, 2008; Lipman, 2012; Keller & Morgan, 2016 and references therein). The SJVF volcanic activity began with andesite lava flows and lahar breccia

(Conejos Formation) occurring over the range of 26.9 to 36 Ma (Lipman & McIntosh, 2006).

Cataclysmic eruptions produced calderas and rhyolitic pumice fall deposits and ash flow tuffs

(ignimbrites) that occurred between ~23 and 37 Ma, beginning with the Wall Mountain Tuff

(36.85 ± 0.08 Ma; errors on ages are reported at ±2 s.d.) of the Central Colorado Volcanic Field

(Lipman & McIntosh, 2012; Lipman & McIntosh, 2008; Lipman, 2007). Following the large ignimbrite eruptions, post-caldera domes were erupted and are commonly composed of dacite and rhyolite.

Within the SJVF, there are four main clusters of calderas known as the Western, Central,

Early Eastern, and Northeastern caldera complexes (Steven & Lipman, 1976; Lipman &

McIntosh 2012). The oldest average age of eruptions occurred in the Northeastern San Juan caldera complex, beginning with the Bonanza Tuff (33.17 ± 0.06 Ma; Lipman & McIntosh,

2008), and the youngest average age of eruptions occurred in the Western San Juan caldera complex, ending with the formation of the Lake City caldera (Sunshine Peak Tuff, 23.7 ± 0.4

Ma; Lipman, 1988; Lipman & McIntosh, 2008; Lipman, 2012). The post-caldera dome processes including visible enclaves in dacite lava domes described by Kennedy et al. (2015) at

Lake City caldera are comparable to the post-caldera dome processes at the North Pass caldera, which are supplementary to this thesis and are discussed below.

1.1.2. North Pass Caldera Cycle

The North Pass caldera is grouped as part of the Northeastern caldera complex (Lipman,

2012) and produced the Saguache Creek Tuff (32.25 ± 0.05 Ma) during its formation. The age and location of the North Pass caldera defines a transition between older ignimbrite calderas

4

aligned north-south along the Sawatch Range and younger, more central caldera systems within the SJVF (Lipman, 2012).

The identification of the North Pass caldera is relatively recent (first discussed by Lipman

& McIntosh in 2006). One reason for the delay in its discovery is its small size, especially in comparison with the neighboring, topographically well-defined, Cochetopa Park caldera (26.90 ±

0.03 Ma). The erosional rim at the North Pass caldera is ~10 km in diameter, which is smaller than the diameter of the erosional rim of the neighboring Cochetopa Park caldera (~15 km).

Another factor in the late discovery is that it has been highly eroded as well as partially covered by the overlying thick post-collapse dacitic lava flows that filled the caldera. Also adding to the delay was that the Saguache Creek Tuff was originally misidentified as a distal tongue of the

Sapinero Mesa Tuff (28.27 ± 0.06 Ma). This was because the extent of the Sapinero Mesa Tuff is highly asymmetric to its source caldera (San Juan caldera) and was distributed widely, including into the Saguache valley (Lipman, 2012; Lipman & McIntosh, 2008).

1.1.3. Dacite of East Pass Creek

After caldera formation and the eruption of the Saguache Creek Tuff at 32.25 ± 0.05 Ma, intra-caldera activity consisted of the eruption of dacite and rhyolite lavas and tuffs, densely- welded to non-welded tuffs, and also grouped with these eruptions are associated conglomerate and brecciated clastic deposits. These deposits are grouped together as the Volcanics of

Cochetopa Hills (Lipman, 2012) The volcanic domes (~25 totaling a volume of about 140 km3) that are exposed occur in the northwest portion of the North Pass caldera (Figure 2) and are grouped together as the Dacite of East Pass Creek (DEPC; Lipman, 2012). The DEPC domes have not been mapped in detail and there is limited description, compositional, and chronological information. They have been described as massive flows of gray to tan crystal-rich dacite, often

5 displaying ramping columnar joints at their bases, and have individual lava flow units as thick as

200 to 500 meters based on zones of basal vitrophyre and upper carapace breccia (Lipman &

McIntosh, 2008; Lipman, 2012). Lipman (2012) presents 40Ar/39Ar ages on biotite from the main body of the unit from three separate DEPC domes that are 32.07 ± 0.17 Ma, 32.22 ± 0.12

Ma, and 32.31 ± 0.13 Ma.

1.2. Formation of Domes

Volcanic domes are often associated with calderas because they are formed after most of the magma has been evacuated by the explosive caldera-forming eruption leaving mostly degassed remnant magmas and relatively small intrusions of new magmas (replenishments) to the system. The North Pass caldera cycle reflects this scenario and the following provides some fundamental background on domes and their formation. After a large eruption, the magma complex can remain active, but not necessarily explosive. The remaining magma (typically dacite or rhyolite) continues to fill in the magma complex and can expand as the magma degasses, causing the surface of the chamber to rise as it expands (Hernandez, 2014; Sigurdsson et al., 2015). Endogenous and exogenous are two types of domes. Endogenous domes form by an internal inflammation, where magma rises and cools all beneath the surface. An exogenous dome is one that breaks through the surface and erupts lava flows and pyroclasts (Duffield et al.,

1995; Winter, 2010). Most dacitic domes are composed of mixed endogenous and exogenous origins (Duffield et al., 1995). The DEPC domes within the study area for this project are all exogenous in origin.

Dacitic domes are composed of several lithofacies types. Two of those are volcanic breccia, which are deposits of angular fragments of pre-existing rock mainly from the dome itself, and vitrophyre, which is a volcanic deposit that is largely composed of glass. Breccias can 6 be defined as basal breccias, vent breccias (within the vent or near the base of the dome, not usually visible), or carapace breccia (a ‘shell’ at the top of the dome), which are all formed by shear stress (Figure 3). Vitrophyres can be formed by the welding of pyroclastic deposits, by rapid undercooling (i.e. quenching), or by the sudden degassing of vesiculated magmas that causes decompression and rapid undercooling (Westrich et al., 1988; Winter, 2010). The vesiculation in magmas can occur by decompression that promotes the exsolution of volatiles under isothermal conditions or by isobaric crystallization where volatiles exsolve from the melt during crystallization of anhydrous minerals (“second-boiling”, Westrich et al., 1988). Because vitrophyres imply welding, quenching, or degassing, they are often found associated with dome formation (Christiansen et al., 2007; Hernandez, 2014). Vitrophyre and carapace breccia are described for Buffalo dome below.

1.3. Magma Mingling

One of the main objectives of this thesis is to understand the origin of enclaves (a term that is used interchangeably with the term inclusions throughout the text) that are found within the dome lavas. A key hypothesis is that these enclaves represent a separate magma that was mingled/mixed with the host lavas prior to eruption. Another hypothesis is that this mixing/mingling event is what initiated the eruption of these domes. Magma mingling is a pre- eruption event where a secondary magma is introduced to a host magma system that could potentially cause a volcanic eruption. The difference between a magma mixing event and a magma mingling, or comingling, event is that mixed magmas have been homogenized while magma mingling has distinct magmas separated by a compositionally distinct boundary (Lai et al., 2009; Wilcox, 1999). The addition of a second magma to a system can reside long enough to homogenize with the host (i.e. mixed). Alternatively, the magmas can interact to cause a 7 volcanic eruption and thus preserve the interaction (i.e. mingled). There are three major and interrelated ways in which magmas interact and cause eruption (Figure 4). One type of interaction is the sudden degassing of magmatic volatiles (H2O, CO2, SO2) caused by pressure release or reheating of hydrous minerals depicted in Figure 4A (devolatilization; Rutherford &

Hill, 1993). As gas exsolves from magma (vesiculation) the volume of material increases inside the system, which can over-pressurize and will drive the magmas to erupt. A second type of interaction is when two magmas have significantly different temperatures (Figure 4B).

Quenching and rapid crystallization can occur along the contact margins between the magmas

(Perugini & Poli, 2012). With cooling and crystallization there can be volatile exsolution, which again, drives volcanic eruption. A third way that magmas interact to cause eruption is if they have significantly different compositions (Figure 4C), i.e. a basaltic magma mingling with dacite. This can cause chemical or physical reactions (as described above) that can also lead to vesiculation (Kuscu & Floyd, 2001).

Alternatively, magma mingling can be associated with interactions that are not necessarily the direct cause of the eruption (Figure 5). The occurrence of this type of mingling is typically associated with a system that experiences a series of eruptive events. These magma relationships are summarized from Perugini & Poli (2012) in five often interrelated environments. On a large scale (0.1 to 10 km), magma mingling can occur by the replenishment of a magma system where two different stages of magma are introduced to one another and interact (Figure 5A) or by the assimilation of crustal contamination by melting the surrounding country rock to create a hybrid melt (Figure 5B). Magma interactions can also occur by the migration through fracture networks (Figure 5C) on a slightly smaller scale (0.1 to 10 m) and are often formed by faulting related to caldera collapse. Fractional crystallization of a magma body 8

(Figure 5D) often occurs within a large magma system (0.1 to 10 km) but can occur on much smaller scales (0.01 to 10 m) where the outer edges of a magma body begin to crystallize, leaving behind a more evolved body of magma. Finally, the partial melting of crystals (Figure

5E) can occur at a very small scale (0.1 to 10 mm) allowing concentrated crystal melts to interact with one another.

Because these systems are often complex and can be interrelated with one another, many occur as multilevel mixing events driven by the replenishment of magma overall. Remnants of a previous eruption can be ‘scooped’ up with the injection of a new, often more evolved magma.

This scenario is caused when an eruption leaves behind a portion of magma as a semi- crystallized residue (aka. mush or cumulate). An injection of a new magma through the system will incorporate the semi-solid cumulate during eruption to the surface. The types of mixing/mingling scenarios described above will be evaluated to explain the physical and chemical relationship between the host lava and enclaves. From this will be determined if magma mixing/mingling is 1) a result of mingling of discrete magma batches that prompted eruption or 2) a result of the incorporation of cumulate material by a magma that was in a state of eruption to the surface.

9

2. METHODOLOGY AND ANALYTICAL TECHNIQUES

2.1. Mapping Aspects and Sample Collection

Two separate field excursions were completed for this research, one in June of 2016 and the other in June of 2017. Both were facilitated by the Bowling Green State University Geology field camp. The objective of each trip was to map and collect samples from two of the DEPC domes of interest, informally named “Buffalo Dome” and “North Dome,” (Figure 6).

Preliminary geological background research was conducted to identify the DEPC dome before the initial trip to the Buffalo Dome in 2016. The purpose of the brief 2016 excursion was to confirm the existence of magmatic enclaves within the columnar joints at the base of the Buffalo

Dome. A select suite of samples was collected to represent the host lava, enclaves, and upper flow (labeled JCS16-#; Table 1).

The second trip for field work was completed in June of 2017 and was more extensive than the short trip in 2016. During the two-day field excursion of 2017, Buffalo Dome was systematically mapped, and samples were collected from all exposed lithologies. Aerial photographs and topographic maps assisted in navigation, and GPS was used to record waypoints for sample collection and lithological contacts. A second, more extensive suite of hand samples from both Buffalo and North domes (labeled JCS17-#) were collected, taken to Bowling Green

State University, and cut into thin and ‘thick’ sections for petrography and electron microprobe analysis for mineral chemistry, as well as powdered for whole rock geochemical analysis.

2.2. Petrography

A total of fourteen thin and thick sections from the Buffalo and North domes were prepared and examined under a petrological microscope. These samples were selected to represent different lava flows, locations, and enclaves from varied locations around the Buffalo 10 and North domes. The purpose of the petrographic evaluation was to estimate mineral type and abundance as well as textures, including vesicle amount and size, crystal shape and orientation with particular attention paid to comparing enclaves to their host lavas and characterizing the contact margins between them. During examination, thin sections were digitally imaged in preparation for electron microprobe analysis.

2.3. Analytical Methods

2.3.1. Microprobe Analysis

Thin sections were hand-prepared with a high, uniform polish in the Bowling Green State

University thin section laboratory and were then taken to the University of Michigan Electron

Microbeam Analysis Laboratory (EMAL), where they were carbon coated and analyzed for mineral chemistry. Thin sections were analyzed using the Cameca SX-100 Electron Microprobe

Analyzer with the assistance of Owen K. Neill and Gordon Moore. Microprobe analyses are used to supplement whole rock geochemical data in determining magma compositions as well as chemical differences between feldspars, amphiboles, micas, and pyroxenes present in host lavas and inclusions. Feldspars, micas, pyroxenes, and amphibole were analyzed for the major element oxides SiO2, TiO2, Al2O3, FeO, MgO, MnO, CaO, Na2O, and K2O. Major element oxides are normalized to 100% and cations were calculated based on ideal structural formulas

(Tables 2-6).

2.3.2. Whole Rock Chemistry

After a thorough petrographic examination, six samples were selected for whole rock chemical analysis of each lava flow from the Buffalo and North domes (Table 7). Samples for whole rock analysis were restricted to one sample per lava unit. Host lava and inclusions were analyzed for Buffalo and North domes and upper flow and ridge lavas from Buffalo Dome, 11 totaling four samples from the Buffalo Dome and two samples from the North Dome. These samples were chosen based on freshness (i.e. lack of alteration) and how well they represent each lava (e.g. host, inclusion, and upper flows at the Buffalo and North domes) in terms of texture and minerals present. Samples were then registered through IGSN, crushed with a jaw crusher, picked by hand, and powdered using a tungsten carbide puck mill.

Whole rock geochemical analysis included x-ray fluorescence (XRF) for major element oxides and inductively coupled plasma mass spectrometry (ICP-MS) for minor element oxides and was completed in the XRF Laboratory of the Department of Earth and Environmental

Sciences at the Michigan State University in February of 2018. Major element oxides (SiO2,

TiO2, Al2O3, MnO, MgO, CaO, Na2O, K2O, and P2O5) and trace elements (Sc, V, Cr, Co, Ni, Rb,

Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Pb, Th, and U) are all measured relative to lab standards JB1-a and BHVO-2. Based on standards, precisions of < 2% for concentrations < 50 ppm and < 1% for concentrations > 100 ppm are verified. Major element data is normalized to 100% volatile free on geochemical diagrams. 12

3. RESULTS

3.1. Field Results and Observations

3.1.1. Field Observations of Buffalo and North Domes

Prior to the field excursion made in June 2017, enclaves had only been briefly inspected at Buffalo Dome. After thoroughly mapping Buffalo Dome in 2017, nearby domes were explored to look for other occurrences of magmatic enclaves. Buffalo Dome is located along

Highway 114, west of Saguache, Colorado and is ~2 km in diameter and stratigraphically ~250 m thick. The North Dome is also located on Highway 114, about 4 km northwest of the Buffalo

Dome (Figure 6) and is ~1.25 km in diameter and ~200 m thick. It also contains enclaves.

Although visited during the second field excursion, the North Dome was not mapped and therefore observations are limited to a single lava flow that is exposed along Highway 114.

At Buffalo Dome, three main lithostratigraphic units were identified and described

(Figure 7). The basal lava flow unit (~80 m in total thickness) extends downslope from Highway

114 and exhibits well-developed columnar jointing along the road cut. This flow unit will now be referred to as ‘BD1’ at Buffalo Dome. This basal lava flow unit includes the flow that extends downslope from Highway 114, however only the top half (~45 m) of this unit, starting from the roadcut, was examined. Exposed gray to orangish-tan columnar jointed lava (~8 m thick) at

Buffalo Dome show joint orientations that ramp upwards in various directions, indicating non- uniform flow cooling directions (Figure 8). In contrast, the columnar joints at the base of North dome (~6 m thick lava (‘ND1’) are vertical and uniform with ramping flow patterns towards the top of the unit (Figure 9). Within BD1 and ND1 lavas are the enclaves. Hand samples collected from this area were chosen to represent host lava and enclaves within the BD1 and ND1 units at the Buffalo and North domes. 13

Above the columnar joints examined at the roadcut, outcrops at Buffalo Dome are dominated by pervasively flow-banded lavas (~150 m), many of them covered within forested areas. This lava flow unit ‘BD2’ is the main flow unit and is consistent with documented descriptions of DEPC domes (Lipman, 2007; Lipman & McIntosh, 2008; Lipman, 2012). It is the most widespread and thickest unit that conformably overlies BD1 and is characterized by well-developed flow banding throughout. The angle of the flow banding increases from ~10° to

~60° stratigraphically from the base to the top of this unit (Figure 10). Hand samples collected from these outcrops are gray to purple-gray porphyritic lava containing large feldspar, biotite, and occasional amphibole phenocrysts in a fine-grained groundmass (detailed in Petrography section below). Occasionally trachytic textures can be observed by the naked eye or with a hand lens (10x). Although much of the Buffalo Dome was covered in tree and plant cover, outcrops across the dome were similar in makeup.

At Buffalo Dome unit BD3 is the uppermost lava (~ 25 m) that forms a summit ridge and is characterized by the occurrence of vitrophyre (Figures 7 and 12). The upper ridge is the highest elevation of the dome. Hand samples at the ridge were composed of gray porphyritic lavas, containing large feldspar and biotite phenocrysts. Flow banding is present at the ridge but is nearly horizontal (~15 to 25°) and the very top of the ridge is rounded due to spheroidal weathering (Figure 13). The contact between units BD2 and BD3 is characterized by lenses of a brecciated carapace between flow banded lavas as well as vitrophyre. Discontinuous lenses of carapace breccia that are ~10 to 25 cm thick are found within the vitrophyre (Figure 11), but also appear at the contact between dark colored vitrophyre (~15 cm thick) and the lighter, less welded vitrophyre just beneath the BD3 lava flow. 14

3.1.2. Enclaves - Field Characteristics

Visible enclaves are present within the columnar jointed bases of the Buffalo and North domes (Figure 14). At first glance, the enclaves can be mistaken for dark patches of lichen. The overall distribution of enclaves in the exposed columnar joints at Buffalo Dome is estimated at

~25 per m2 and at North Dome at ~20 per m2. The largest of these enclaves have dimensions of

~20 x 25 cm and range in shape from elongate and ovular to spherical and rounded with a rounded or fluid boundary. The smallest observable enclaves vary from ~1 cm to ~5 mm and are found interspersed with larger inclusions. Distribution of small and large inclusions appear random. The enclaves vary in color from light yellow to dark red-brown and stand out in relief from the surrounding lavas, occasionally weathered away from their ‘pocket’, especially at North

Dome (Figure 15), which helps to highlight their occurrence. Another feature that distinguishes

Buffalo and North dome inclusions from the host lava is their texture (detailed below), which is more vesicular (~5%) than the <1% vesicularity of the host. The contact relationships between inclusion and host lavas are variable. They are typically rounded and smooth, but not perfectly spherical, and can be relatively angular and slightly crenulated, but most boundaries appear to be sharp rather than diffuse. Large inclusions (~20 to 25 cm) tend to have the most irregular shapes whereas small inclusions (~1 to 5 cm) tend to be relatively spherical in shape and have a smooth contact with the host. Enclaves rarely exist in upper flows and are less than 1 cm in diameter if they are present. Although the North Dome was not described in as much detail, many of the characteristics described for Buffalo Dome also occur here. 15

3.2. Petrography

3.2.1. Sample Descriptions

Hand samples from Buffalo Dome were taken from all three units (BD1 = columnar jointed lavas; BD2 = main flow banded lavas; and BD3 = uppermost ridge lavas) and made into thin sections for petrographic analysis. Host lavas from BD1 are porphyritic (isolated phenocrysts surrounded by groundmass) with large (~2 to 8 mm) euhedral to subhedral phenocrysts consisting of feldspars (~30%), biotite (~10%), Fe-Ti oxide (~8%), and pyroxene

(~5%) with trace amounts of amphibole (~2%) within an extremely fine trachytic groundmass

(~45%) and is poorly vesicular (<1%). All estimates are in percentage by area estimated visually. The inclusions in BD1 are fine-grained equigranular (grains ~2 to 5 mm), euhedral to subhedral, consisting of amphibole (~45%), feldspars (~30%), Fe-Ti oxide (~10%) holocrystalline groundmass or devitrified glass (~10%), and mildly vesicular (~5%). Lavas from

BD2 are porphyritic consisting of feldspar (~40%), biotite (~10%), Fe-Ti oxide (~8%), and clinopyroxene (~3%) with a trachytic groundmass (obvious flow banding in hand sample).

Ridge lavas (BD3) are red to purple, porphyritic with euhedral to subhedral phenocrysts that include large feldspars (~40%), brassy-colored biotite (~10%), Fe-Ti oxide (~8%), clinopyroxene (5%), and vesicles (<2%) in a trachytic, slightly glassy groundmass.

Samples taken from the North Dome only include host lavas and inclusions from the exposed columnar jointed road-cut section. Host lavas from North Dome are dark gray and porphyritic with euhedral to subhedral phenocrysts (~1 to 8 mm) consisting of feldspars (~38%), biotite (~9%), Fe-Ti oxide (~6%), and pyroxene (~5%) with trace amounts of amphibole (~2%) in a fine-grained trachytic, slightly glassy groundmass that is poorly vesicular (<1%). Inclusions from North Dome are yellow to orange in color and are fine-grained, nearly equigranular (~2 to 6 16 mm) euhedral to subhedral, containing amphibole (~45%), feldspars (~30%), Fe-Ti oxide

(~10%) holocrystalline groundmass or devitrified glass (~10%), and vesicles (~5%).

3.2.2. Host-Inclusion Comparison

The BD1 and ND1 host lavas and inclusions are compared for differences in texture and mineralogy, which are noted in both hand sample and thin section. Host lavas and inclusions were also observed for specific mineral and groundmass textures, paying particular attention to evidence for disequilibrium crystallization and undercooling growth textures. Because BD1 and

ND1 observations are similar, host lavas and enclaves will be described together unless otherwise indicated. General comparisons between host and inclusion magmas have been summarized in

Table 8.

Textures of the BD1 and ND1 host lavas are porphyritic and glomeroporphyritic

(phenocryst clusters surrounded by groundmass). The trachytic groundmass has the appearance of flowing around the large phenocrysts or phenocryst clusters. The long-axes of isolated phenocrysts are occasionally, but not always, aligned with the fabric of the groundmass.

Inclusions in BD1 and ND1, on the other hand, are holocrystalline with pilotaxitic arrangements of euhedral to subhedral crystals. Phenocrysts within the host lavas are similar in size (~2 to 8 mm) to the size of minerals within the enclaves (~2 to 6 mm).

Mineral assemblages in BD1 and ND1 are also described together. The North Dome lava tends to have a higher abundance of phenocrysts than the Buffalo Dome lava overall. The lavas generally contain ~45 to 65% phenocrysts, consisting of minerals in order of abundance ‒ feldspar, biotite, Fe-Ti oxide (likely magnetite and/or chromite based on shape), pyroxene, and rare amphibole. Inclusions are typically ~85 to 95% crystal-rich consisting of minerals from most abundant to least ‒ amphibole, feldspar, and Fe-Ti oxide. Feldspar is found in both host 17 and enclaves but is more abundant in the host lavas. Pyroxene and biotite are not present within inclusions except for pyroxene that is observed within one small (<1 cm) inclusion (JCS17-

012A). Amphibole is rarely present in host lavas but can occasionally be found in the host near the host-enclave border.

Disequilibrium textures are found in host lavas BD1 and ND1 and their inclusions.

Commonly, large feldspar phenocrysts within host lavas have visible resorption textures (e.g. sieve textures) within the core of the grain (Figure 16A) but occasionally grains show sieve textures between unresorbed cores and rims (Figure 16B). Microphenocrysts of feldspar do not display resorption textures. Other disequilibrium textures in feldspar phenocrysts include optically identified zoning. Figure 16C shows a concentrically zoned plagioclase phenocryst under cross-polarized light. Within the enclaves, similar sieve textures and other absorption textures (e.g. embayed and skeletal grains) are observed. For example, amphibole crystals within the enclaves are often centrally resorbed and are occasionally skeletal (Figure 18). In addition, feldspar microphenocrysts and microlites within the enclaves exhibit a unique ‘tassel- like’ or swallow tail morphology at the ends of grains (Figures 17), a texture that reflects rapid crystal growth. Another rapid growth indicator found in the enclaves is the occurrence of acicular growth patterns shown by feldspar microlites within the groundmass.

3.2.3. Host-Inclusion Contact Boundaries

The host and inclusions appear similar in composition based on petrography; however, texturally and in terms of abundance of mafic minerals, the two lithologies are quite different from one another (Table 8) and show distinct boundary textures where the host lavas and inclusions meet. Boundaries range from sharp to diffuse or gradational boundaries. Some sharp boundaries are indicated by bands of much finer-grained or glassy groundmass that occur 18 parallel to the contact between host lava and inclusions. These bands are not present along every host-inclusion boundary and they vary in extent (~100 µm to 1 mm) when present. Furthermore, the fine grain bands can occur along either the host side of the boundary (more common in North

Dome samples; Figure 19A) as well as on the inclusion side of the boundary (more common in

Buffalo Dome samples; Figure 19B). Sharp boundaries are accentuated by the presence of the larger euhedral minerals in inclusions that contrast with the finer-grained and sometimes trachytic texture in the host lava. In some samples, the trachytic texture appears to ‘flow’ around the inclusion at these boundaries (Figure 19C). Boundaries with these characteristics often show evidence of brittle deformation of weak minerals, such as biotite, in the host lava near the contact margin (Figure 19D). Other boundaries appear more diffuse but still maintain a contrast in texture that defines the contact (Figure 19E). Phenocrysts within the host at these diffuse contacts are similar in size to minerals within the inclusions (~2 to 5 mm) and neither display trachytic textures.

3.3. Mineral Chemistry

3.3.1. Mineral Classification

Because the Buffalo and North domes have a very similar mineral assemblage, they will be described and classified together but difference are noted. Feldspar, amphibole, biotite, and pyroxene are all present within the host lavas and enclaves; however, the only mineral with consistent overlap in abundance (e.g. a primary mineral in both host lavas and inclusions) is feldspar. Amphibole is primarily found within the inclusions whereas mica and pyroxenes are typically found within host lavas. Host lavas rarely contain amphibole and when found it occurs close to the host-inclusion boundary. Compositions of amphibole are restricted and range from titanian-magnesio hastingsite to titanian-magnesio hastingsitic hornblende (Table 2). There is no 19 distinction between the compositions of amphibole found in the inclusions with the rare amphibole found in the host near contact boundaries.

Enclaves do not contain appreciable amounts of biotite or pyroxenes but are occasionally found within smaller inclusions (~1 to 3 cm) or near the host-inclusion boundaries. There is no chemical distinction between biotite grains found within the inclusions and those within the host lava. Biotite are classified as magnesio-biotite and ferro-biotite (Table 3). Pyroxenes are classified as either diopside or augite, plotting along the diopside-augite boundary on the pyroxene quadrilateral (Table 4 and Figure 20). One pyroxene grain is found within a single inclusion (sample JCS17-013C) and is compositionally identical to pyroxene within the host lavas. Although minerals in lavas from both domes overlap in composition, minerals within the samples from Buffalo Dome generally fall within a higher range of silica content than minerals within samples from North Dome.

3.3.2. Chemical Zoning in Plagioclase

Feldspar is the only mineral with a high abundance found in both host lavas and inclusions (e.g. a primary mineral). Feldspar phenocrysts from the host lavas at Buffalo and

North Domes are all classified as plagioclase feldspars (Figure 21, Table 5). Core compositions of these feldspars range from bytownite to andesine with the majority found within the labradorite field (Figure 21). Host lavas and enclaves contain plagioclase that fall within the labradorite field; however, bytownite, the most mafic plagioclase type measured (i.e. highest Ca and lowest Si contents, Table 5) is only present in the inclusions. Conversely, the most sodic plagioclase andesine, occur only within host lavas. Large inclusion-free plagioclase phenocrysts within the host lavas of Buffalo and North domes were analyzed at multiple points (n = 2 to 3) to recover chemical transects from grain cores to rims in three samples (JCS17-013A, JCS16-004, 20

JCS16-006, and JCS17-013C). Plagioclase phenocrysts within the host lavas of Buffalo and

North domes typically display normal zoning patterns; Ca-rich cores and Na-rich rims (Table 5).

Two feldspar phenocrysts from North Dome host lavas and one from Buffalo Dome host lavas are reversely zoned (Na-rich to Ca-rich) and are located in proximity to their contacts with the inclusions. Plagioclase crystals within the inclusion lava were not analyzed for core-rim transects.

3.4. Whole Rock Chemistry

3.4.1. Classification and Chemical Relationship

Whole rock data from the Buffalo and North domes are plotted on the standard Total

Alkali Silica (TAS) classification diagram and are shown to range from basaltic trachyandesites to trachydacites (Figure 22, Table 6). The data shows that inclusions are more mafic in composition than their paired host lavas, which is also supported by the petrographic observations that show greater proportions of mafic minerals observed within the inclusions relative to host lava as well as the lower silica contents of minerals in the inclusions relative to minerals in the host. At Buffalo Dome, BD1 lava (JCS16-001) and an inclusion within this lava

(JCS16-002) as well as lava from the other Buffalo Dome units, BD2 (JCS16-003) and BD3

(JCS17-003), were analyzed. The selection of these samples is to evaluate compositional variation with stratigraphic height (cf. age). Overall, the lavas at Buffalo Dome show a trend toward more felsic compositions (e.g., having an increase in SiO2 content from trachyandesite to trachydacite; Figure 22) with stratigraphic height. The most mafic compositions at Buffalo

Dome are the inclusions, which are classified as basaltic trachyandesite (Figure 22). Unit BD3, which is the uppermost ridge lava, plots within the trachydacite field and is the most felsic lava sampled from either dome. At North Dome, whole rock compositions of host lavas and their 21

inclusions are comparable to those at Buffalo Dome. North Dome host lava ND1 (JCS17-013) plots within the trachyandesite field and is slightly more felsic (i.e. higher SiO2 content) than

BD1 and BD2 lavas. An inclusion from North Dome (JCS17-012) also plots within the trachyandesite field and is more mafic than its host lava, but more felsic than the inclusion at

Buffalo Dome (Figure 22).

To further investigate the relationships between lava flows and inclusions, major

t elements (MgO, Al2O3, CaO, TiO2, P2O5, and FeO ) are plotted against SiO2 content (Figure 23) to examine trends of magma differentiation. Also shown in Figure 23 are lava and enclave samples from the Lake City caldera (Kennedy et al., 2015; Kennedy et al., 2012). In general, as

t SiO2 contents increase, MgO, FeO , TiO2, Al2O3 CaO, and P2O5 concentrations decrease overall

(Figure 23). Buffalo and North dome lavas have higher SiO2 concentrations and lower concentrations in the other major elements relative to their inclusions. The exception is FeOt, which is lower in the inclusion from North Dome compared to the inclusion and lavas from

Buffalo Dome. For Buffalo Dome, trends in all major element data support that magma compositions evolved with time; BD1 → BD2 → BD3. Lake City lavas and enclaves are within the compositional range of Buffalo and North dome lavas and enclaves.

Trace elements La, Ce, Nb, Ba, Y, and Rb are plotted against Zr content (Figure 24).

Zirconium, like SiO2, increases in concentration with magma evolution for compositions within the range of basaltic andesite to trachydacite (Figure 22). The trace elements that Zr is plotted against are also considered to be incompatible (i.e. concentrating within the melt relative to minerals being crystallized) over this compositional range, with the possible exception of Y and

Ba, which are compatible in amphibole and biotite, respectively (partition coefficients in dacite equal to 11.3 and 13.9, respectively; Ewart and Griffin, 1994). Buffalo Dome shows increases in 22

La, Nb, and Ce concentrations and a decrease in Y content between the BD1 inclusion and each lava unit upwards in stratigraphic succession. Barium concentrations in Buffalo Dome lava units

BD1 → BD3 are relatively constant around 1500 ppm. Compositional differences between the two inclusions analyzed are evident with the sample from North Dome having much higher Ba,

Y, La, Nb and Ce concentrations relative to the inclusion from Buffalo Dome. However, the inclusions have similar Rb contents (Figure 24). The inclusion from Buffalo Dome has lower concentrations in trace elements than the lavas. The opposite relationship is seen at North Dome where the inclusion has much higher concentrations in Ba, Y, La and Ce than the host (ND1) with the exception of Rb. Niobium has only a slightly higher concentration in the inclusion relative to the host lava at North Dome (Figure 24).

In summary, trace element concentrations of flow units BD1, BD2, and BD3 support the observations that compositions are more evolved with increasing stratigraphic height. If the enclaves are considered as part of the petrogenetic sequence, then they represent the most mafic endmembers. Trace element data from North Dome host lavas and enclaves do not appear to agree with the major element data. For instance, if they are related by magmatic differentiation of a common parental magma, then the lower SiO2 and higher MgO content of the inclusion relative to the host (Figure 24) should also be reflected in the inclusion having lower concentrations of incompatible elements like Ba, La and Ce.

Whole rock geochemistry is also evaluated on normalized multi-element plots shown in

Figures 25 and 26. Overall, the lavas and inclusions have broadly similar concentration patterns with prominent relative depletions in Nb, Ta, P and Ti and relative enrichment in Ba and Pb

(Figures 25A&B). These are common features of calc-alkaline magmas from continental arcs worldwide (Winter, 2010). The comparison between the host lava (BD1) and inclusion at Buffalo 23

Dome show very similar patterns, although the inclusion has slightly higher abundances for many elements on the right side of the plot (e.g. Pr to Lu; Figure 25A) with the most significant differences for Nb, Ta, K, P, and Ti contents. Besides K, all of these elements display negative anomalies. Comparison of host lava and inclusion at North Dome show broadly similar patterns on multi-element plots but are much more varied than the inclusion-host comparison at Buffalo

Dome (Figure 25B). Significant differences in abundance are noted for all elements except Ba,

Nb, Ta, Zr, Yb, and Lu. The inclusion at North Dome has higher concentrations for elements from Pr to Lu relative to the host lava, however, for elements on the left side of the plot (e.g. Cs to Pb) concentrations vary being higher or lower than the host lava.

Chondrite normalized Rare Earth Element (REE) diagrams are given in Figure 25C&D and compare the concentrations of the host lavas and inclusions at Buffalo and North domes. All lavas and inclusions are enriched in light REE (LREE) relative to heavy REE (HREE) contents.

The REE contents of Buffalo Dome host lava and inclusion are similar but with slight differences in La, Ce, Eu, and Gd (Figure 25C). It is notable that the host lava exhibits a small negative spike in Eu abundance. In contrast, the REE concentrations of lava and inclusions from the North Dome show parallel LREE enriched patterns with the host being depleted in all REEs except Lu relative to the inclusion (Figure 25D). The host lava from North Dome also displays a slight negative spike in Eu abundance.

A final comparison employs the use of elements normalized to an average of upper continental crust (Figure 26). Part of the justification for using these diagrams is the likelihood that post-caldera magmas have interacted with crust during differentiation and therefore variations from unity may provide additional insight on magmatic processes. Furthermore, the samples are plotted to compare inclusions and host lavas from both domes separately. Lavas 24 from the two domes are nearly identical in their geochemical signatures with the North Dome lava having slightly higher concentrations in most elements (Figure 26A). On the other hand, the inclusions display greater compositional differences from one another in addition to having an overall greater compositional range relative to host lavas (Figure 26B). The concentration patterns of the inclusions are nearly parallel with some overlap of Pb, Co, and Ni (Figure 26B).

North Dome enclaves contain much higher REE abundances than the Buffalo Dome enclaves.

3.4.2. Comparison with Compositions from Lake City Caldera

The data are compared with compositions from another caldera within the SJVF, the

Lake City caldera, where enclaves are also found within post-caldera domes and intrusions

(Kennedy et al., 2015). When plotted on the TAS diagram, Buffalo and North dome lavas have similar compositions but are slightly more mafic than those from the Lake City caldera (Figure

22). The Lake City caldera enclaves have a much wider range in composition than those measured from Buffalo and North domes. Enclaves from Lake City range from trachyandesite to rhyolite, and their host lavas and intrusions have a much smaller variation, ranging only between trachyte and trachydacite lavas (Figure 22).

Major element oxides of the Lake City post-caldera samples show decreasing Al2O3,

t FeO , MgO, TiO2, CaO, and P2O5 with increasing SiO2, which is similar to the overall trend shown by Buffalo Dome and North Dome samples (Figure 23). Enclaves from Lake City post- caldera activity have overall higher abundance of major element oxides than their host lavas, similar to the inclusion-host relationship found at North Dome. As described above, all Buffalo and North dome samples are LREE-enriched with La/YbN ratios > 15‒24 and have slightly higher ratios than those from Lake City (ca. 16). On primitive mantle normalized plots (Figure

25A&B), post-caldera samples from Lake City have slightly higher abundances of most trace 25

elements relative to samples from Buffalo and North domes. Notable differences in trace element concentrations between Lake City post-caldera samples from Buffalo and North domes samples are the positive spikes of Cs and U, negative spike for Ba, and much higher abundances of Nb and Ta of the Lake City suite (Figure 25A&B). Also, on plots normalized to upper continental crust (Figure 26), significant excursion from Buffalo and North Dome inclusions relative to those from Lake City occur at Pb and U.

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4. DISCUSSION

Samples of lava flows and enclaves from the Buffalo and North Domes have been described, classified and compared to similar lavas and enclaves from the Lake City caldera.

These observations are now interpreted in order to understand the relationships between these magmas, how mixing occurred, and if mixing influenced the eruptions of the Buffalo and North

Domes. The post-caldera structures within the North Pass caldera, specifically the Buffalo and

North domes, exhibit substantial evidence to support that pre-eruption mixing and mingling of magmas occurred at these domes based on physical, petrographic, mineralogical, and geochemical data. On the other hand, it is less obvious as to what the physiochemical conditions

(i.e. pressure, temperature, viscosity, water/volatile content) were when these processes occurred.

Visible enclaves observed throughout the columnar jointed lavas at the base of the Buffalo and

North domes are relatively subtle, but their presence provides important evidence for magma interactions in the DEPC lava dome system. Magmatic enclaves in a volcanic environment can provide insight on the composition of the initial mafic magma, the effects of dilution of mafic magma by felsic magma, and an estimate of mixing and eruption timescales (Perugini & Poli,

2012). These observations provide a better understanding of caldera cycles, which in turn provide important clues into the driving mechanisms of magma recharge and magma evolution; a glimpse into inner Earth processes that we cannot directly observe (Bachmann & Bergantz,

2008; Kennedy et al., 2012).

In order to understand the magmatic processes of the North Pass post-caldera system, we must first determine the character of the magmas that formed the enclaves at Buffalo and North domes and the relationship of those enclaves to their host lavas. Understanding the source of the enclaves can lead to a better understanding of how the liquids interacted with one another and if 27 that interaction caused the eruptions of the DEPC lavas. Physical and geochemical data will be used to determine the characteristics of magma mingling and the origin of magmatic enclaves to propose the inner-workings of this complex magma system.

4.1. Characterization of Magma Mixing and Mingling

First and foremost, there is a significant amount of physical evidence to support the mixing and mingling occurrence in the DEPC dome complex within the North Pass caldera. The physical observations of the inclusions found in the lava at the base of Buffalo and North domes fall in line with what has been described by other studies at other locations (Wilcox, 1999; Lai et al., 2009; Perugini & Poli, 2012; Kennedy et al., 2015). Generally, inclusions are described as evenly distributed ovoidal or subrounded enclaves of what appear to be a completely different rock type and display relatively sharp contacts with the surrounding host (Wilcox, 1999; Perugini

& Poli, 2012). The contact relationships are unlike those typically found between xenoliths and host lava. Xenoliths are fragments of solid rock picked up during eruption and typically display sharp and angular contact relations with their host lavas with little to no chemical interaction between them. The designation of an inclusion as a xenolith implies that it is a piece of ‘country rock’ that is unrelated to its host magma (Winter, 2010).

Determining the compositions of the inclusions is important in determining the possibility of comingled magmas as opposed to rock fragments (i.e. xenoliths). Xenoliths may include a wide variety of rock types and compositions with no relation to one another. On the other hand, relative compositional homogeneity between inclusions within a lava or intrusive body provides support that a single magma with a different rheology was mingled (Lai et al.,

2008; Winter, 2010; Perugini & Poli, 2012). Petrographic observations of mineral abundances 28

and textures were the first evidence gathered that confirmed that the inclusions within the

Buffalo and North dome lavas, were from an individual magma type per dome.

Along with having relatively consistent mineral assemblages, the enclaves have similar textures (e.g. fine-grained, equigranular), mineral zoning and resorption textures, as well as evidence of rapid undercooling, aka. quenching (e.g. acicular crystals, radial growth and ‘carpet tassels’ on feldspar microphenocrysts and microlites, and devitrified glass; Figure 17) throughout. Quenched textures also include fine-grained, glassy bands that grades away from the contact boundaries (Figures 19A&B) and occasionally on the contact between host and inclusion magmas. Although not found in every sample, this texture varies between being found within the host or within the inclusion. Quenching is most likely to occur during eruption or when the inclusion and host lava initially come in contact with one another. Magma interaction can cause rapid under-cooling of the hotter magma and/or degassing of volatiles, both of which will produce the quenched textures. In this case, the lack of quenching throughout the host lavas aside from the inclusion-host contact boundary suggests that the quenching occurred due to the thermal-chemical interaction between the intruding, semi-molten inclusion magma and the predominately liquid host magma. Variations in quenched boundaries could suggest a complex series of temperature differences because of varied degrees of cooling within these liquids.

Typically, inclusions are quenched more than host lavas, which imply a hotter inclusion that was rapidly undercooled on host-inclusion contact. These textures are consistent in all inclusions and the disequilibrium that they reveal indicates that the mixing event upset normal crystal growth

(Lai et al., 2008).

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4.2. Origins of Enclaves and Relationship to Host Lavas

4.2.1. Mineralogical Relationships

To investigate further the environment of disequilibrium caused by these mixed magmas, minerals were probed for chemical zoning. These analyses show that there is very little chemical variation in minerals between lavas and enclaves and even less variation in their respective mineral assemblages. Plagioclase, amphibole, biotite, and pyroxene are present within host lavas and the inclusions; however, amphibole is rare within the host and the minerals biotite and pyroxene are equally rare within the inclusions. This observation paired with the lack of chemical variation, suggests that these grains were mechanically transferred between host and inclusion magmas during mingling.

Although much of this research is focused on the mingling of magmas, minor homogenization of magmas by mixing is also possible. This can occur when mingled magmas have similar rheologies and are aided by longer residence time together in a mostly liquid state prior to eruption (Perugini & Poli, 2012; Lai et al., 2008). Evidence for homogenization is supported by chemical zoning found in feldspar phenocrysts. Feldspar is found abundantly as phenocrysts and microcrysts throughout all lavas and enclaves and is therefore considered a primary mineral in this system. Chemically zoned plagioclase phenocrysts often display normal zoning (Na-rich cores to Ca-rich rims) but are occasionally reversely zoned (Ca-rich cores to Na- rich rims) or oscillatory zoned (that toggles between Na-rich to Ca-rich from core to rim). These zoned phenocrysts occur close to host-inclusion boundaries. Complex zoning is expected in a system that experiences interactions between compositionally distinct magmas because a system in chemical disequilibrium constantly works towards stability—in this case, one homogenous fluid. Feldspar grains within the enclaves were not analyzed to evaluate chemical zoning, 30

however, they are expected to be normally zoned if growth occurred soon after mixing. The occurrence of reversely zoned plagioclase phenocrysts in the host, which are in close proximity to inclusion contacts, support chemical interaction between the magmas during mixing.

4.2.2. Magma Genesis

Whole rock chemical analysis is essential in determining the origin of the enclaves seen in the DEPC domes. The allocation of elements in this system is essential in determining where the liquids came from and how the liquids relate to one another. The whole rock data is used to explore the evolution of magma from more mafic to more felsic compositions of lava flows at

Buffalo Dome and the relationship of those lavas with the inclusions they host. These data are initially used to classify and document compositional variations at Buffalo and North domes and reveal significant differences between inclusions and host lavas at both domes. This classification, again, supports the mineralogical data that the inclusions are more mafic than the host lavas.

Variations in major element oxides indicate progressive crystal fractionation from mafic

t to more felsic compositions. Concentrations of MgO, Al2O3, FeO , TiO2, CaO, and P2O5 all decrease with increasing SiO2 content indicating that they are being incorporated in the fractionating mineral assemblage (Figure 5D). In this case, the trend is occurring throughout the system, causing the lavas to become more felsic, which is emulated within the stratigraphic succession of Buffalo Dome. The removal of plagioclase, amphibole, clinopyroxene, biotite, Fe-

Ti oxides (e.g. magnetite or ilmenite), and apatite (not observed within the samples but is acknowledged here as a typical accessory phase) are likely to be involved in magma genesis.

The higher aluminum contents of the inclusions may reflect magmas that have suffered less plagioclase (Al2O3 ca. 27.95 wt.%; Table 5) fractionation than their host magma. Similarly, the

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abundance of phosphorus in the inclusions from Buffalo Dome and North Dome may indicate that they have retained or not fractionated as much apatite as the host lavas. Another compositional difference is that the enclave from North Dome is depleted in iron relative to the enclave from Buffalo dome and several lavas. The lower iron content may reflect weathering processes (e.g. oxidation) or by a compositional difference to be distinguished by trace elements.

The petrogenetic relationships between inclusions and host for both domes are evaluated by inspecting compositional patterns on normalized multi-element plots (Figures 25-26). For

Buffalo Dome, trace element concentration patterns for host lavas and inclusions are parallel to overlapping, indicating a close genetic relationship between the two lithologies (Figure 25A).

The negative anomalies at P and Ti are a result of the fractionation of apatite and Fe-Ti oxides magnetite and/or ilmenite. The other anomalies include the positive Pb spike, which likely relates to contamination of magmas by crust and the Nb-Ta negative spike, which is likely an inherited signature produced in continental volcanic arc settings (Rudnick & Fountain, 1995;

Winter, 2010). Finally, a negative Eu spike in Buffalo and North dome host lavas can be explained by the fractionation of feldspars (Rudnick & Fountain, 1995), which are possibly fractionated less within the Buffalo Dome inclusions and not at all from the North Dome inclusions (i.e. not a cumulate). The enclaves are more mafic (basaltic trachyandesite; Figure

22) in composition with respect to major elements relative to the host lava (trachyandesite;

Figure 22), however, they show nearly equivalent concentrations for many elements (Figures

25A&C). The greatest differences are in concentrations of Cs, Rb and U and REEs La and Ce.

The highly incompatible elements Cs, Rb and U are relatively fluid mobile and the lower contents of these elements in the inclusions could be the result of alteration/weathering.

Lanthanum and cerium are the most incompatible REE and their lower concentrations in

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inclusions relative to host lava is consistent with the inclusions being more mafic based on major element contents. Additionally, there are differences in middle REE (MREEs; from Sm to Er) concentrations (Figure 25C), which are in higher abundance in the inclusion relative to the host lava (BD1). The MREE are moderately compatible in amphibole relative to LREE and HREE

(Bacon & Druitt, 1988) and therefore the deviation may be explained in two ways: 1) the host lavas fractionated out amphibole, which lowered MREE concentrations relative to the inclusion, or 2) the accumulation of amphibole in the inclusions increased the concentration of MREE relative to the host lavas. Although the first explanation is reasonable, there is little evidence that the host lavas contain primary amphibole. The second explanation is favorable because it is supported by the high abundance (~45 %) of amphibole within the inclusions. In order to help quantify the possibility that the inclusions represent cumulates derived from the host magma or a similar magma composition, a simple mixing model is performed (Figure 27). The model shows that the addition of amphibole (24%) and plagioclase (24%) to BD1 (52%) can accurately predict the composition of the inclusion (BD1 enclave), thus supporting the inference made based on trace elements.

For North Dome, the inclusions are also classified as mafic (trachyandesite) in comparison to the host lavas (evolved trachyandesite; Figure 22), however trace element data suggests otherwise (Figures 25B & D). North Dome host lavas plot nearly parallel to the inclusions however; the host lavas contain lower abundances of nearly all trace elements.

Notable exceptions are the lower abundances of Cs, Rb and U for the inclusion, which are again likely to be a consequence of the mobility of these elements during alteration/weathering

(Hofmann, 1988). Significantly, the compositional discrepancy between host lava and inclusion at North Dome indicate that these two magmas are unlikely to be related to each other by simple

33

fractional crystallization or crystal accumulation within a single magma batch and therefore must represent magmas that evolved separately within the post-caldera system prior to mingling.

A direct comparison is also made between Buffalo and North dome lavas (BD1 and ND1) and between inclusions in Figure 26. The nearly identical trace element patterns displayed by the host lavas at Buffalo and North domes support that they are related and have the same origins within the post-caldera DEPC system (Lipman, 2007; Lipman & McIntosh, 2008; Lipman,

2012). The concentrations of trace elements in the North Dome lava (ND1) are slightly higher than Buffalo Dome (BD1) (Figure 26A), which agrees with the slightly more evolved composition indicated by the major element data (Figures 22 & 23). Inclusions from Buffalo and

North domes also have nearly parallel patterns; however, the North Dome inclusion has significantly higher abundances for most elements (Figure 26B). Enclaves from Buffalo Dome and North Dome are similar to one another texturally and mineralogically, but based on major differences in whole rock compositions, it is unlikely that they originated from the same magma.

A final comparison is of DEPC samples with samples from post-caldera lavas, intrusive and enclaves from Lake City. In Figures 25 & 26A, Lake City post-caldera lava compositions mimic the patterns of the DEPC lavas, indicating a similar origin overall. The main notable difference is the higher normalized concentrations of Cs, Rb, U, Nb, Ta and Co (Figure 25 & 26A). It may be reasonable to suggest that this difference relates to a greater amount of crust contaminating the Lake City magma system, however, it does not preclude possible differences in their mantle source compositions. In the comparison of enclaves (Figure 26B), the difference between Lake

City and DEPC are more pronounced and indicate diverse origins.

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4.3. Proposal for Post-Caldera Processes of the DEPC Lava Domes

The evidence outlined above leads to the confirmation of the following hypotheses for

Buffalo Dome: 1) magma mingling occurred between a homogeneous magma composition (host lava) and subordinate magma type (enclave), and 2) the enclaves are petrogenetically related to the host lava. The third original hypothesis that remains unsupported is that the addition of the enclave liquids to a resident magma system directly caused the eruptions of the DEPC dome complex. Instead, the preferred model is that the eruption of Buffalo Dome was caused by the recharge of magmas into a fracture system (Figure 5C) where fractional crystallization (Figure

5D) and minor crustal contamination (Figure 5B) of a residual magma was actively occurring.

This model is similar to the Lake City caldera complex as described by Kennedy et al. (2012). A systematic model (Figure 28) has been created to explain the proposed events for the eruption of

Buffalo Dome.

This proposed model suggests that following the collapse of the North Pass caldera at

32.25 ± 0.25 Ma, a complex sub-surface fracture network was established within the caldera

(Figure 28A). These fractures serve as pathways for the late-stage gas-charged magmas to move easily through the caldera fracture system as they ascend to the surface. Post-caldera magmas then fill the fracture system and erupt, occasionally leaving small portions of the liquid behind.

Residual liquids crystallize within the fracture network and minerals accumulate (e.g. amphibole). It is also likely that with the crystallization of these liquids some of the surrounding country rock is assimilated (Figure 28B). Recharge of a more voluminous and gas-enriched magma occurs, which is then able to pick up the residual crystal mush and carry it to the surface during the eruption and formation of exogenous lava domes (Figure 28C). The low amounts of vesiculation recorded in inclusions and host lava indicate that it is unlikely that the interactions 35

between these magmatic liquids were explosive. It is more likely that refilling of the fracture system and overall increase in volume forced magmas to the surface rather than increased volume caused by exsolution of volatiles alone. Often times the recharge of magma can be from the same source as the previously emplaced residual liquid that crystallized. At Buffalo Dome, this is likely the case. Enclaves are similar to host magmas aside from being more mafic but contain a high proportion of amphibole, which is likely a cumulate phase.

The results from North Dome provide additional insight on the complexity of this post- caldera magma system and fracture network. The magma source for the North Dome enclaves cannot be explained in the same way as enclaves from Buffalo Dome; that is, their compositions do not support a residual of the host. The enclaves at North Dome are likely a separate magma that evolved along a parallel but significantly different liquid line of descent. In this case, the fracture network provides a complex space for multiple magmas to differentiate and rise within the post-caldera system.

36

5. CONCLUSION

Visible enclaves found abundantly throughout the columnar jointed lavas of the Buffalo and North domes within the DEPC dome complex are the first clue that complex magma interactions occurred prior to the eruptions of the DEPC lava domes. Physical, mineralogical, petrological, and geochemical data were used to determine that the enclaves produced by magma mingling are of multiple magmas within this system and can be genetically related to the host lavas or produced by separate generations of magmas. It is also suggested that the addition of the enclave liquids did not directly cause the eruptions of either Buffalo or North Dome. It is more likely that the collapse of the North Pass caldera formed fracture systems, which allowed for the transport of less-explosive volatile-depleted magmas to erupt to form dacitic lava domes.

These lava domes show evidence of a complex, interlinked magmatic system for multiple compositions of different liquids to interact with one another prior to eruption. Major and trace element data are key to determining the relationships of these enclaves to their host lavas. Trace element abundances link Buffalo Dome enclaves to host lavas genetically and can be explained by a residual amphibole-rich crystal mush. These same data indicate that the enclaves at North

Dome are not as closely related but are likely separate magmas formed within the same system.

Cycles of magma regeneration within this fracture network allowed for the evacuation of multiple magmatic compositions occasionally leaving behind remnants of previous eruptions

(aka residual magmas). Replenishing magmas served as a transportation device for these residual magmas (e.g. amphibole-rich crystal mush) to erupt at the surface.

Future works are encouraged for further examination of the DEPC lava domes. Field work in search of more magmatic enclaves throughout the DEPC would determine the extent of magma mixing/mingling processes and if they were occurring throughout the dome field. 37

Second, a larger sample size for the determination of the recovered lava and enclave compositions in comparison to those defined in this study and more chemical analysis of enclaves (e.g. larger sample size) defined within this study is encouraged to confirm the homogeneity of the inclusion compositions. Finally, isotopes of Sr, Nd, and Pb could be analyzed to constrain origin and possible crustal contamination of the DEPC lavas. This new information could be used to test the idea that magma systems are varied, systematically, in composition across the SJVF in space and time.

38

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43

APPENDIX A. TABLES

Table 1. Summary of field and petrographic observations, Buffalo and North Domes, Colorado, USA.

(1) Buffalo Dome: BD; North Dome: ND. BD1: Buffalo Dome basal columnar jointed lavas and inclusions, BD2: main flow unit with flow banding, BD3: uppermost summit ridge lava and vitrophyre, ND1: North Dome basal columnar jointed lava and inclusions, n.d.: not determined (2) Lithofacies: refer to Figures 6&7; †dacitic conglomerate (Tsd) as defined by Lipman (2012) (3) Pl = plagioclase, Bi = biotite, CPX = clinopyroxene, Amph = amphibole, Mag = magnetite (4) Hyc = hypocrystalline, Hc = holocrystalline, Glm = glomeroporphyritic, Tr = trachytic, Grn = equigranular (5) Df = diffuse, Tr = trachytic, Dev-e = devitrified glass in enclave, Dev-H = devitrified glass in host, n/a: not applicable 44

Table 2. Amphibole chemistry from electron microprobe analysis. 17- 17- Sample 013A_1 17-013A_2 013A_3 17-013A_4 16-004_1 16-004_2 16-004_3 16-004_4 Location ND ND ND ND BD BD BD BD encl/bound enclave encl/bound enclave enclave host/bound encl/bound enclave Prefix titanian- titanian- titanian- titanian- titanian- titanian- titanian- magnesian ferroan Modifier magnesio- magnesio- magnesio magnesio hastingsitic magnesio- magnesio- pargastic Name hastingsite hastingsite hastingsite hastingsite hornblende hastingsite hastingsite hornblende SiO2 42.20 41.50 41.90 42.20 43.40 41.90 41.60 42.50 TiO2 2.09 2.96 2.64 2.84 2.58 2.93 2.98 3.18 Al2O3 12.60 11.80 12.10 12.10 10.70 12.40 11.60 12.00 FeO 12.02 11.85 11.67 12.51 14.32 11.56 11.16 11.02 MgO 14.13 13.71 13.70 12.93 12.45 14.06 14.01 11.60 MnO 0.17 0.18 0.21 0.20 0.41 0.21 0.20 0.15 CaO 11.35 11.57 11.52 11.35 11.48 11.50 11.32 11.55 Na2O 2.65 2.27 2.18 2.17 1.95 2.20 2.15 2.17 K2O 0.91 0.99 0.98 1.00 1.22 1.01 0.99 0.98 Total 98.12 96.83 96.90 97.30 98.51 97.77 96.01 98.15 Structural formulae on the basis of 23 oxygen* Si 6.130 6.133 6.170 6.209 6.362 6.104 6.171 6.150 Ti 0.228 0.329 0.292 0.314 0.284 0.321 0.332 0.346 Altotal 2.158 2.056 2.100 2.099 1.849 2.130 2.029 2.047 Al-iv 1.870 1.867 1.830 1.791 1.638 1.896 1.829 1.850 Al-vi 0.288 0.189 0.270 0.308 0.211 0.234 0.199 1.970 Mn2+ 0.021 0.023 0.026 0.025 0.051 0.026 0.025 0.018 Fe2+ 0.839 0.972 0.935 1.092 1.308 0.832 0.862 0.788 Mg 3.059 3.020 3.006 2.835 2.720 3.053 3.097 3.149 Na 0.177 0.143 0.150 0.189 0.175 0.162 0.162 0.165 K 0.169 0.187 0.184 0.188 0.228 0.188 0.187 0.181 Total Cation 15.738 15.694 15.656 15.618 15.607 15.647 15.644 15.624 45

Table 2. (Continued) Sample 16-004_5 16-004_6 16-006_1 16-006_2 16-006_3 16-006_4 16-006_5 16-006_6 Location BD BD BD BD BD BD BD BD encl/bound host/bound enclave enclave enclave host enclave boundary Prefix titanian- titanian- titanian- titanian- titanian- titanian- titanian- titanian- magnesio- Modifier magnesio- magnesio- magnesio- magnesio- magnesio- hastingsitic magnesio- magnesio- Name hastingsite hastingsite hastingsite hastingsite hastingsite hornblende hastingsite hastingsite SiO2 42.20 42.70 40.90 41.50 41.50 43.00 40.50 43.30 TiO2 2.88 2.63 3.04 2.91 3.18 2.82 2.92 2.52 Al2O3 12.00 11.60 11.60 12.30 12.20 11.00 12.00 11.00 FeO 11.89 12.31 11.39 11.44 11.53 11.75 11.58 12.39 MgO 13.89 13.77 13.73 14.12 14.33 14.05 13.68 14.36 MnO 0.23 0.33 0.18 0.22 0.18 0.25 0.19 0.36 CaO 11.27 11.30 11.33 11.38 11.37 11.24 11.41 11.27 Na2O 2.13 2.21 2.07 2.26 2.22 2.13 2.11 2.24 K2O 1.00 0.99 0.99 1.00 0.96 1.05 1.06 1.00 Total 97.49 97.84 95.23 97.13 97.47 97.29 95.45 98.44 Structural formulae on the basis of 23 oxygen* Si 6.166 6.230 6.126 6.084 6.056 6.295 6.059 6.269 Ti 0.316 0.289 0.342 0.321 0.349 0.310 0.329 0.274 Altotal 2.067 1.995 2.048 2.126 2.099 1.898 2.116 1.878 Al iv 1.834 1.770 1.874 1.916 1.944 1.705 1.941 1.731 Al vi 0.233 0.225 0.175 0.210 0.155 0.194 0.175 0.147 Mn2+ 0.028 0.041 0.023 0.027 0.022 0.031 0.024 0.044 Fe2+ 0.858 0.927 0.871 0.790 0.731 0.914 0.839 0.841 Mg 3.025 2.994 3.065 3.085 3.117 3.065 3.050 3.099 Na 0.181 0.183 0.150 0.166 0.171 0.198 0.144 0.187 K 0.186 0.184 0.189 0.187 0.179 0.196 0.202 0.185 Total Cation 15.609 15.626 15.640 15.663 15.636 15.603 15.670 15.627 Sample numbers exclude ‘JCS’, but include sample year and number (17-013A, 16-004, and 16-006) and the number in sequence (1, 2, 3, etc) Location of collection: ND - North Dome, BD - Buffalo Dome; Location of probed mineral within sample: encl - enclave, bound - boundary; *Structural formula of element calculations from Esawi (2004) 46

Table 3. Mica chemistry from electron microprobe analysis. 17- 17- 17- 16- 16- Sample 013A_1 013A_2 013A_3 004_1 16-004_2 16-004_3 006_1 16-006_2 Location ND ND ND BD BD BD BD BD host host host host/bound host enclave host host/bound Prefix magnesio- ferro- ferro- magnesio- magnesio- magnesio- ferro- magnesio- Name biotite biotite biotite biotite biotite biotite biotite biotite

SiO2 36.90 37.30 37.00 37.00 37.30 36.50 36.60 37.00

TiO2 4.98 5.04 4.94 5.21 5.20 4.45 5.42 5.42

Al2O3 14.10 14.30 14.10 14.20 14.50 14.90 14.50 14.00 FeO 13.72 14.84 14.85 13.83 13.27 13.27 14.15 13.63 MnO 0.18 0.23 0.26 0.21 0.17 0.22 0.19 0.18 MgO 15.42 14.86 15.02 15.54 16.15 15.51 15.02 15.39 CaO 0.05 0.01 0.06 0.02 0.03 0.13 0.16 0.06 Na2O 0.74 0.68 0.70 0.70 0.82 0.76 0.67 0.68 K2O 9.20 8.86 9.12 9.07 8.81 8.67 8.90 9.00

Li2O 1.04 1.14 1.07 1.08 1.16 0.91 0.95 1.07

H2O 4.08 4.11 4.09 4.10 4.15 4.06 4.08 4.09 Total 100.38 101.43 101.21 100.95 101.63 99.76 100.69 100.53 Structural formulae on the basis of 4 oxygen Si 5.428 5.434 5.423 5.414 5.394 5.386 5.374 5.429 Al iv 2.443 2.456 2.436 2.439 2.472 2.589 2.514 2.418 Ti 0.551 0.553 0.545 0.573 0.565 0.495 0.598 0.597 Fe 1.688 1.809 1.820 1.691 1.603 1.696 1.738 1.671 Mn 0.023 0.028 0.032 0.027 0.021 0.028 0.023 0.023 Mg 3.382 3.231 3.282 3.386 3.478 3.416 3.287 3.363 Li 0.613 0.670 0.629 0.633 0.675 0.541 0.564 0.633 Ca 0.009 0.018 0.009 0.004 0.005 0.020 0.026 0.010 Na 0.210 0.192 0.199 0.198 0.229 0.219 0.191 0.194 K 1.726 1.648 1.705 1.692 1.625 1.633 1.666 1.682

Total Al 2.443 2.456 2.436 2.439 2.472 2.589 2.514 2.418 Fe/Fe+Mg 0.333 0.359 0.357 0.333 0.316 0.332 0.346 0.332 47

Table 3. (Continued) 16- 17- 17- 17- 17- 17- 17- Sample 16-006_3 006_4 013C_1 013C_2 013C_3 013C_4 013C_5 013C_6 Location BD BD ND ND ND ND ND ND host/bound host host host host host enclave encl/bound Prefix ferro- ferro- ferro- magnesio- magnesio- ferro- magnesio- ferro- Name biotite biotite biotite biotite biotite biotite biotite biotite

SiO2 36.70 36.40 36.10 36.00 36.50 36.50 36.80 36.00

TiO2 5.31 5.36 4.95 4.80 4.88 4.90 5.01 4.69

Al2O3 14.10 14.10 14.10 13.80 14.10 14.10 14.60 13.70 FeO 14.74 14.77 14.60 14.00 13.87 15.20 11.48 15.15 MnO 0.19 0.24 0.23 0.21 0.22 0.21 0.11 0.20 MgO 14.69 14.64 14.76 14.94 15.45 14.66 16.54 14.68 CaO 0.15 0.17 0.11 0.00 0.08 0.10 0.12 0.14

Na2O 0.61 0.64 0.65 0.70 0.55 0.65 0.64 0.63

K2O 9.08 9.05 9.05 9.15 9.34 9.19 9.19 9.24

Li2O 0.97 0.88 0.80 0.78 0.93 0.93 1.00 0.79

H2O 4.06 4.04 4.00 3.98 4.05 4.05 4.08 3.99 Total 100.52 100.23 99.26 98.39 100.01 100.45 99.51 99.19 Structural formulae on the basis of 4 oxygen Si 5.411 5.390 5.399 5.428 5.405 5.410 5.398 5.422 Al iv 2.446 2.459 2.480 2.458 2.467 2.456 2.526 2.421 Ti 0.590 0.598 0.557 0.545 0.543 0.546 0.553 0.530 Fe 1.820 1.831 1.828 1.765 1.718 1.884 1.409 1.906 Mn 0.024 0.030 0.029 0.027 0.027 0.026 0.013 0.026 Mg 3.233 3.235 3.295 3.357 3.409 3.240 3.620 3.292 Li 0.575 0.526 0.479 0.474 0.551 0.552 0.589 0.478 Ca 0.024 0.027 0.017 0.000 0.013 0.016 0.018 0.023 Na 0.175 0.185 0.188 0.205 0.158 0.187 0.183 0.185 K 1.709 1.712 1.729 1.759 1.763 1.737 1.722 1.774

Total Al 2.446 2.459 2.480 2.458 2.467 2.456 2.526 2.421 Fe/Fe+Mg 0.360 0.361 0.357 0.345 0.335 0.368 0.280 0.367 Sample numbers exclude ‘JCS’, but include sample collection year and number (17-013A, 16-004, 16-006, and 17-013C) and the number in sequence (1, 2, 3, etc) Location of collection: ND - North Dome, BD - Buffalo Dome; Location of probed mineral within sample: encl - enclave, bound - boundary 48

Table 4. Pyroxene chemistry from electron microprobe analysis. 16- 16- 16- 16- Sample 17-013A_1 17-013A_2 17-013A_3 004_1 004_2 004_3 006_1 16-006_2 Location ND ND ND BD BD BD BD BD host host host host host host host host Name diopside diopside diopside augite diopside augite augite diopside SiO2 52.60 52.20 52.00 52.40 52.00 53.10 52.70 52.90 TiO2 0.30 0.25 0.17 0.30 0.29 0.28 0.31 0.19 Al2O3 1.70 1.30 0.80 1.50 1.60 1.50 1.40 1.20 Fe2O3 2.27 1.76 2.38 0.31 0.88 0.00 1.34 1.11 FeO 6.30 6.74 6.52 7.91 7.87 8.46 7.14 7.38 MnO 0.65 0.70 0.84 0.66 0.66 0.70 0.74 0.84 MgO 14.48 12.73 13.82 14.16 13.22 14.05 14.39 14.16 CaO 21.92 22.31 22.08 20.98 21.94 20.81 21.49 21.79 Na2O 0.50 0.46 0.44 0.48 0.48 0.62 0.48 0.44 Total 100.72 99.46 99.05 98.70 98.94 99.52 99.98 100.01 Structural formulae on the basis of 6 oxygen* Si 1.941 1.956 1.960 1.971 1.961 1.982 1.959 1.969 Ti 0.008 0.007 0.005 0.008 0.008 0.008 0.009 0.005 Al 0.074 0.057 0.036 0.067 0.071 0.066 0.061 0.053 Fe3 0.063 0.050 0.068 0.009 0.025 0.000 0.037 0.031 Fe2 0.194 0.211 0.205 0.249 0.248 0.264 0.222 0.230 Mn 0.020 0.220 0.027 0.021 0.021 0.022 0.023 0.026 Mg 0.797 0.767 0.776 0.794 0.743 0.782 0.798 0.786 Ca 0.867 0.896 0.892 0.846 0.887 0.832 0.856 0.869 Na 0.036 0.033 0.032 0.035 0.035 0.045 0.035 0.032 Tot. cat 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 Tot. oxy 6.000 6.000 6.000 6.000 6.000 6.000 6.000 6.000

End members Wo 45.120 46.560 45.930 44.570 46.590 44.310 44.750 45.360 En 41.480 39.870 40.000 41.850 39.060 41.630 41.690 41.020 Fs 13.400 13.570 14.060 13.580 14.350 14.060 13.560 13.620 49

Table 4. (Continued) 16- 16- 16- 17- 17- 17- 17- 17- Sample 006_3 006_4 006_5 013C_1 013C_2 013C_3 013C_4 013C_5 Location BD BD BD ND ND ND ND ND host host host host host enclave host host Name augite diopside diopside diopside diopside augite diopside diopside SiO2 52.90 51.80 52.00 52.40 51.00 52.70 51.80 51.50 TiO2 0.27 0.24 0.35 0.24 0.39 0.20 0.13 0.24 Al2O3 1.40 1.20 1.70 1.40 2.10 1.20 0.80 1.50 Fe2O3 1.94 2.28 1.11 0.62 2.51 1.66 2.95 1.77 FeO 6.28 6.75 7.73 7.63 7.71 6.90 5.33 6.86 MnO 0.65 0.63 0.66 0.62 0.71 0.65 1.17 0.67 MgO 15.10 13.59 13.20 13.88 11.98 12.39 13.98 13.67 CaO 21.51 22.28 22.05 21.87 22.13 21.85 22.13 21.70 Na2O 0.45 0.43 0.50 0.40 0.69 0.43 0.49 0.45 Total 100.49 99.21 99.30 99.06 99.22 99.98 99.78 98.37 Structural formulae on the basis of 6 oxygen* Si 1.951 1.950 1.955 1.968 1.932 1.960 1.941 1.951 Ti 0.007 0.007 0.010 0.007 0.011 0.006 0.004 0.007 Al 0.061 0.053 0.075 0.062 0.094 0.053 0.035 0.067 Fe3 0.054 0.065 0.032 0.018 0.072 0.046 0.111 0.051 Fe2 0.194 0.213 0.243 0.240 0.244 0.215 0.167 0.217 Mn 0.020 0.020 0.021 0.020 0.023 0.020 0.037 0.021 Mg 0.830 0.763 0.740 0.777 0.676 0.798 0.781 0.772 Ca 0.850 0.899 0.888 0.880 0.898 0.871 0.888 0.881 Na 0.032 0.031 0.036 0.029 0.051 0.031 0.036 0.033 Tot. cat 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 Tot. oxy 6.000 6.000 6.000 6.000 6.000 6.000 6.000 6.000

End members Wo 44.100 46.350 46.690 45.970 47.510 45.130 45.620 45.850 En 43.070 39.340 38.890 40.590 35.780 41.350 40.100 40.190 Fs 12.830 14.310 14.430 13.440 16.710 13.520 14.290 13.950 Sample numbers exclude ‘JCS’, but include sample collection year and number (17-013A, 16- 004, 16-006, and 17-013C) and the number in sequence (1, 2, 3, etc) Location of collection: ND = North Dome, BD = Buffalo Dome; Location of probed mineral within sample: encl. = enclave, bound = boundary Wo: wollastonite, En: enstatite, Fs: Ferrosilite *Structural formula of elements and end member calculations from Mineral Formulae Recalculation. John Brady, Smith College and Dexter Perkins, University of North Dakota https://serc.carleton.edu/research_education/equilibria/mineralformulaerecalculation.html 50

Table 5. Feldspar chemistry from electron microbeam analysis. 17- 17- 17- 17- 17- 17- 17- 17- Sample 13A_1 13A_2 13A_3 13A_4 13A_5 13A_6 13A_7 13A_8 Location ND ND ND ND ND ND ND ND host host host boundary boundary enclave host host core mid rim rim rim rim core rim Transect 1 1 1 ------2 2 Name lbd and and lbd lbd lbd and and

SiO2 54.60 57.70 58.30 55.50 53.50 54.90 57.30 59.90

TiO2 0.03 0.00 0.03 0.05 0.05 0.04 0.00 0.00 Al2O3 27.40 25.90 26.00 28.30 29.10 28.00 26.90 25.10 FeO 0.36 0.36 0.41 0.37 0.77 0.78 0.30 0.37 MgO 0.00 0.03 0.02 0.03 0.07 0.10 0.00 0.00 CaO 9.65 8.01 7.79 9.92 11.34 10.22 8.52 6.43

Na2O 5.30 6.16 6.25 5.44 4.45 4.96 5.94 6.94

K2O 0.51 0.71 0.71 0.45 0.35 0.48 0.64 0.98 Total 97.85 98.87 99.51 100.06 99.63 99.48 99.60 99.72

Structural formulae on the basis of 8 oxygen* Si 2.517 2.623 2.633 2.500 2.436 2.497 2.588 2.689 Ti 0.001 0.000 0.001 0.002 0.002 0.001 0.000 0.000 Al 1.488 1.387 1.384 1.502 1.562 1.501 1.432 1.328 Fe2+ 0.014 0.014 0.015 0.014 0.029 0.030 0.011 0.014 Mg 0.000 0.002 0.001 0.002 0.005 0.007 0.000 0.000 Ca 0.477 0.390 0.377 0.479 0.553 0.498 0.412 0.309 Na 0.474 0.543 0.547 0.475 0.393 0.437 0.520 0.604 K 0.030 0.041 0.041 0.026 0.020 0.028 0.037 0.056

Total cations 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 Total oxygen 8.010 8.024 8.032 8.030 8.012 8.017 8.025 8.023 End Members An 49 40 39 49 57 52 43 32 Ab 48 56 57 48 41 45 54 62 Or 3 4 4 3 2 3 4 6 51

Table 5. (Continued) 17- 17- 17- 17- 17- Sample 13A_9 13A_10 13A_11 13A_12 13A_13 16-004_1 16-004_2 Location ND ND ND ND ND BD BD host host host host enclave host/boun host/boun core mid rim rim core mid rim Transect 3 3 3 4 4 5 5 Name and and and lbd lbd lbd lbd

SiO2 58.50 59.10 59.00 52.10 54.40 52.00 53.90

TiO2 0.00 0.00 0.00 0.00 0.05 0.04 0.03

Al2O3 25.50 25.30 26.80 29.90 28.40 28.00 29.80 FeO 0.37 0.36 0.36 0.59 0.71 0.54 0.51 MgO 0.03 0.02 0.04 0.05 0.08 0.07 0.09 CaO 7.12 6.69 8.02 12.16 10.65 10.93 11.60

Na2O 6.46 6.67 6.19 3.81 4.92 4.29 4.26

K2O 0.83 0.91 0.73 0.38 0.46 0.54 0.36 Total 98.81 99.05 101.14 98.99 99.67 96.41 100.55

Structural formulae on the basis of 8 oxygen* Si 2.656 2.674 2.623 2.394 2.469 2.446 2.433 Ti 0.000 0.000 0.000 0.000 0.002 0.001 0.001 Al 1.365 1.349 1.404 1.619 1.519 1.552 1.586 Fe2+ 0.014 0.014 0.013 0.023 0.027 0.021 0.019 Mg 0.002 0.001 0.003 0.003 0.005 0.005 0.006 Ca 0.346 0.324 0.382 0.599 0.518 0.551 0.561 Na 0.569 0.585 0.534 0.339 0.433 0.391 0.373 K 0.048 0.053 0.041 0.022 0.027 0.032 0.021

Total cations 5.000 5.000 5.000 5.000 5.000 5.000 5.000 Total oxygen 8.030 8.030 8.037 8.023 8.001 8.012 8.030

End Members An 36 34 40 62 53 57 59 Ab 59 61 56 35 44 40 39 Or 5 5 4 2 3 3 2 52

Table 5. (Continued) 16- 16- 16- 16- 16- 16- 16- 16- Sample 004_3 004_4 006_1 006_2 006_3 006_4 006_5 006_6 Location BD BD BD BD BD BD BD BD host host host host host host boundary enclave mid mid core mid rim core core core Transect -- -- 6 6 7 7 -- -- Name and and lbd lbd lbd lbd and lbd

SiO2 56.80 56.80 54.70 55.10 54.90 54.00 58.80 52.20

TiO2 0.00 0.00 0.05 0.03 0.00 0.00 0.04 0.05

Al2O3 24.50 27.40 28.90 27.90 28.80 29.20 26.20 28.60 FeO 0.34 0.30 0.51 0.54 0.33 0.42 0.33 0.49 MgO 0.07 0.05 0.06 0.04 0.00 0.05 0.03 0.09 CaO 6.90 8.96 10.67 9.87 10.49 11.20 7.65 11.52

Na2O 6.10 5.59 4.75 5.24 4.96 4.63 6.44 4.18

K2O 0.87 0.62 0.51 0.57 0.40 0.42 0.84 0.40 95.58 99.72 100.15 99.29 99.88 99.92 100.33 97.53 Structural formulae on the basis of 8 oxygen* Si 2.669 2.567 2.473 2.505 2.485 2.447 2.630 2.429 Ti 0.000 0.000 0.050 0.001 0.000 0.000 0.001 0.002 Al 1.357 1.459 1.540 1.495 1.536 1.559 1.381 1.569 Fe2+ 0.013 0.011 0.019 0.021 0.012 0.016 0.012 0.019 Mg 0.005 0.003 0.004 0.003 0.000 0.003 0.002 0.006 Ca 0.347 0.434 0.517 0.481 0.509 0.544 0.037 0.574 Na 0.556 0.490 0.416 0.462 0.435 0.407 0.559 0.377 K 0.052 0.036 0.029 0.033 0.023 0.024 0.048 0.024

Total cations 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 Total oxygen 8.044 8.034 0.021 8.006 8.023 8.011 8.019 8.015

End Members An 36 45 54 49 53 56 38 59 Ab 58 51 43 47 45 42 57 39 Or 5 4 3 3 2 2 5 2 53

Table 5. (Continued) 16- 16- 16- 16- 16- 17- 17- Sample 006_7 006_8 006_9 006_10 006_11 013C_1 013C_2 Location BD BD BD BD BD ND ND host host host encl/boun enclave host host rim mid core core rim core rim Transect 8 8 8 -- -- 9 9 Name lbd by by by lbd and and

SiO2 53.80 50.20 49.80 50.40 52.40 56.00 58.10

TiO2 0.00 0.03 0.04 0.00 0.00 0.03 0.00

Al2O3 29.30 31.50 31.80 31.70 29.80 27.60 26.30 FeO 0.52 0.59 0.65 0.67 0.57 0.37 0.41 MgO 0.07 0.06 0.09 0.07 0.06 0.02 0.02 CaO 11.32 14.01 14.53 14.33 12.20 9.44 8.07

Na2O 4.50 3.11 2.76 2.98 4.00 5.45 6.15

K2O 0.41 0.22 0.23 0.19 0.32 0.55 0.67 Total 99.92 99.72 99.90 100.36 99.35 99.46 99.72 Structural formulae on the basis of 8 oxygen* Si 2.440 2.297 2.280 2.295 2.397 2.539 2.620 Ti 0.000 0.001 0.001 0.000 0.000 0.001 0.000 Al 1.566 1.699 1.716 1.701 1.606 1.475 1.398 Fe2+ 0.020 0.023 0.025 0.260 0.220 0.014 0.015 Mg 0.005 0.004 0.006 0.005 0.004 0.001 0.001 Ca 0.550 0.687 0.713 0.699 0.598 0.459 0.390 Na 0.396 0.276 0.245 0.263 0.355 0.479 0.538 K 0.024 0.013 0.013 0.011 0.019 0.032 0.039

Total cations 5.000 5.000 5.000 5.000 5.000 5.000 5.000 Total oxygen 8.013 8.004 8.010 8.008 8.013 8.022 8.030 End Members An 57 70 73 72 62 47 40 Ab 41 28 25 27 37 49 56 Or 2 1 1 1 2 3 4 54

Table 5. (Continued) Sample 17-013C_3 17-013C_4 17-013C_7 17-013C6 17-013C_7 Location ND ND ND ND ND host host enclave enclave host core rim core core mid Transect 10 10 ------Name and and lbd lbd and

SiO2 57.30 57.00 54.50 53.40 59.10

TiO2 0.00 0.00 0.03 0.04 0.00

Al2O3 26.70 27.70 29.30 29.10 25.40 FeO 0.33 0.41 0.44 0.31 0.32 MgO 0.05 0.04 0.10 0.03 0.00 CaO 8.29 9.01 10.92 11.10 6.65

Na2O 5.81 5.86 4.53 4.74 6.80

K2O 0.65 0.53 0.42 0.43 0.96 Total 99.13 100.55 100.24 99.43 99.23 Structural formulae on the basis of 8 oxygen* Si 2.602 2.551 2.464 2.430 2.666 Ti 0.000 0.000 0.001 0.001 0.000 Al 1.429 1.461 1.561 1.561 1.350 Fe2+ 0.013 0.015 0.017 0.012 0.012 Mg 0.003 0.003 0.007 0.002 0.000 Ca 0.403 0.432 0.529 0.541 0.321 Na 0.512 0.508 0.397 0.418 0.595 K 0.038 0.030 0.024 0.250 0.055

Total cations 5.000 5.000 5.000 5.000 5.000 Total oxygen 8.042 8.012 8.035 7.995 8.016

End Members An 42 45 56 55 33 Ab 54 52 42 42 61 Or 4 3 3 3 6 Sample numbers include sample probed (JCS17-013A, JCS16-004, and JCS16-006) and the number in sequence (1, 2, 3, etc.) Location of collection: ND - North Dome, BD - Buffalo Dome; Location of probed mineral within sample: encl - enclave, bound - boundary; location of point within the mineral: core - central point of mineral, mid - between core and rim, rim - near the edge; transects numbered (1, 2, 3 etc.) lbd: labradorite, and: andesine, by: bytownite; An: anorthite, Ab: albite, Or: orthoclase *Structural formula of elements and end member calculations from Mineral Formulae Recalculation. John Brady, Smith College and Dexter Perkins, University of North Dakota https://serc.carleton.edu/research_education/equilibria/mineralformulaerecalculation.html 55

Table 6. Whole rock major and trace element compositions. Flow unit (1) BD1 BD1-encl BD2 BD3 ND1 ND1-encl Sample JCS16-001 JCS16-002 JCS16-003 JCS17-003 JCS17-012 JCS17-013 SiO2 60.41 53.02 61.19 63.58 55.93 61.66 TiO2 0.73 1.01 0.69 0.59 1.16 0.7 Al2O3 16.72 18.09 16.98 16.43 18.78 16.96 Fe2O3 5.58 8.59 5.2 4.44 4.35 4.385 MnO 0.1 0.1 0.08 0.07 0.09 0.06 MgO 1.93 3.37 1.67 1.16 2.54 1.455 CaO 4.31 6.56 4.22 3.01 6.34 3.96

Na2O 3.75 3.53 4.03 4.29 4.54 3.925

K2O 4.18 2.38 3.95 4.37 2.99 3.995 P2O5 0.37 0.55 0.34 0.3 0.93 0.41 LOI 1.62 2.48 1.35 1.47 1.92 2.17 Sum 98.08 97.2 98.35 98.24 97.65 97.51 Trace element compositions (ppm) Sc 11.06 17.25 10.45 8.20 16.02 10.09 V 98.24 137.44 91.63 69.62 140.01 88.78 Cr 3.73 2.79 2.68 3.37 3.12 3.44 Co 65.75 46.25 62.08 31.38 28.49 41.92 Ni 5.64 6.48 5.31 4.18 3.20 3.66 Rb 89.23 33.06 77.68 88.98 38.74 80.24 Sr 837.01 1101.69 849.24 743.49 1648.45 906.14 Y 27.86 29.63 27.37 26.56 34.28 28.78 Zr 287.34 253.01 291.02 304.35 311.62 308.62 Nb 17.15 13.56 17.46 21.09 23.91 21.44 Cs 1.55 0.46 1.24 1.10 0.22 1.29 Ba 1496.71 1446.94 1538.89 1522.50 1890.17 1571.76 La 64.42 53.12 64.44 70.71 94.53 74.09 Ce 124.40 103.76 120.52 131.63 192.96 143.39 Pr 14.28 13.16 13.99 14.87 22.45 16.21 Nd 53.02 53.11 51.93 53.51 85.59 59.29 Sm 8.79 9.47 8.45 8.51 14.26 9.51 Eu 2.08 2.62 2.00 1.96 3.61 2.21 Gd 6.75 7.77 6.65 6.36 10.24 7.13 Tb 0.91 1.02 0.89 0.84 1.31 0.95 Dy 4.87 5.51 4.76 4.62 6.62 5.03 Ho 0.98 1.09 0.99 0.94 1.27 1.01 Er 2.76 2.90 2.64 2.67 3.31 2.83 Tm 0.40 0.39 0.38 0.39 0.45 0.41 Yb 2.56 2.49 2.55 2.57 2.82 2.68 Lu 0.41 0.37 0.39 0.40 0.42 0.41 Hf 7.06 6.30 7.24 7.55 7.66 7.59 Ta 0.95 0.69 1.00 1.20 1.05 1.15 Pb 22.49 18.40 23.06 21.07 17.44 23.43 Th 10.97 8.02 10.99 12.63 9.64 12.11 U 2.87 1.37 2.81 3.21 1.77 3.09 (1) BD1: Buffalo Dome basal columnar jointed lavas, BD1-encl: Buffalo Dome enclave, BD2: main flow unit with flow banding, BD3: uppermost summit ridge lava and vitrophyre, ND1: North Dome basal columnar jointed lava and ND1-encl: North Dome enclaves 56

Table 7. Comparison summary for host vs. inclusion lavas based on mineralogic, petrographic, and chemical analysis.

Host Inclusion Minerals (Abundance Plagioclase Feldspar, Biotite, Clinopyroxene, Fe- Amphibole, Plagioclase Feldspar, Fe-Ti Oxide, high to low) Ti Oxide, Amphibole (trace) Biotite (trace)

Textural Observations Porphyritic with fine-grained, trachytic Euhedral to subhedral equigranular growth. groundmass (flows around phenocrysts and inclusion boundaries).

Resorption textures and zoning throughout Acicular and skeletal amphibole growth, tassel-like feldspar phenocrysts, occasional shearing along growth extending from the ends of feldspars, zoned host/inclusion boundary growth, and radial growth of feldspars.

Evidence of devitrified glass. Mineral Chemistry Feldspars – Andesine and labradorite Feldspars – Labradorite compositions compositions Amphibole – (<5%) titanian- Hastingsite and Amphibole – titanian- Hastingsite and hornblende hornblende compositions compositions

Mica - Biotite Mica – (<5%) Biotite Pyroxene – Diopside and augite Pyroxene – Not present 57

APPENDIX B. FIGURES

Figure 1. Map of the San Juan Volcanic Field (SJVF) adapted from Lipman (2007) and Bachmann et al. (2002). There are four main caldera clusters within the SJVF: Western, Central, Northeastern, and Early Eastern, all of which are indicated by a different pattern. The North Pass Caldera (NP) is the most recently discovered caldera because of its heavily weathered remnants. The North Pass caldera sits on the continental divide and defines a transition between older and younger caldera systems (more central) along the Sawatch range (north-south; Lipman 2012). The Lake City Caldera (LC) is similar to North Pass in post-caldera activity and is supplementary to this study (Kennedy, 2015). 58

Figure 2. Topographic map of the northeastern quadrant of the Cochetopa Pass caldera (outlined in blue dots) and the northern half of the North Pass caldera (outlined in red alternating dashes and dots). The general extent of DEPC dome group (~25 domes) is outlined in orange in the northwestern quadrant of the North Pass caldera based on the North Pass caldera and Cochetopa Park caldera map by Lipman (2012). The main study area for this project are the informally named Buffalo and North domes, which is outlined in the northeastern portion of the North Pass caldera and is indicated in Figure 6. 59

Figure 3. Idealized cross-section of a volcanic dome by Hernandez (2014). Most dacitic domes are composed of mixed endogenous and exogenous dome eruption types (Duffield et al., 1995). Here, the idealized dome is representative of an exogenous rhyolitic dome eruption similar to the dacitic lava domes which compose the DEPC complex. The eruption of the lava creates friction along the superficial country rock, creating basal, vent, and carapace breccias. Vitrophyres are formed between breccia and flow foliated lava flows, which often increase in flow angle towards the top of the dome. Flow foliated lavas can form columnar joints near the glassy base where cooling occurs rapidly against cool country rock. 60

Figure 4. There are three magma mixing types which cause volatile release to produce an explosive volcanic eruption: A) The crystallization of an anhydrous mineral phase such as olivine and pyroxene cause volatiles (i.e. H2O, CO2) to exsolve, vesiculating the magma. In turn, this causes and increase in pressure within the magmatic system; B) Hot and cold magmas mix together to cause rapid crystallization, rapid undercooling, which in turn creates a sudden exsolution of volatiles between hot and cold boundaries, which over-pressurize the system; and C) the mixing of magmas that are chemically different from one another can react with a sudden degassing of volatiles, which will also over-pressurize the magmatic system. 61

Figure 5. Occurrence of magma mixing model modified from Perugini & Poli (2012). There are five main ways magma mixing and mingling can occur: A) Replenishment of the magma chamber by a new injection of a magma, which is often but not always related to the pre-existing magma; B) Assimilation of colder crustal contamination by melting bedrock surrounding the hot magma melt; C) Migration of magma through fracture networks allows multiple liquids follow a common path and interact with one another; D) Fractional crystallization which often occurs along the cooler walls of the chamber and creates a compositional gradient towards the center of the melt; and E) Partial melting creates enriched melts of incompatible elements causing large compositional gradients at a very small scale, which can easily trigger mixing process. 62

Figure 6. Aerial photo of the area outlined by the black box in Figure 3. The informally named Buffalo Dome (outlined in red) is located along Highway 114, west of Saguache, Colorado and is ~2 km in diameter and stratigraphically ~250 m thick. The North Dome (outlined in blue) is located 4 km northwest along Highway 114 and is slightly smaller (~1.25 km in diameter and ~200 m thick) than Buffalo Dome. Columnar jointed outcrops containing magmatic enclaves are present at both domes, which are indicated with a star at each dome. 63

Figure 7. Map of Buffalo dome based on field observations of lithologies. BD1 represents the basal flow unit, which is characterized by its columnar jointing. Enclaves are abundantly present within the columnar jointed portion of the BD1 flow unit. BD2 represents the main flow unit characterized by well-developed flow-banding. BD3 represents the summit ridge lava flows defined by vitrophyre and carapace breccia. Patterns which indicate columnar jointing, flow banding, breccia, and vitrophyre are only mapped where they were physically observed in the field. Labeled numbers (8, 10A-D, 11-14, & 16) represent the location of corresponding Figures where photos were taken. 64

Figure 8. A photo stitch of the ramping columnar joints at Buffalo Dome (BD1), showing an irregular flow pattern (photo location indicated on Figure 7). Arrows are aligned to the direction of the cooling pattern. Columnar joints cool perpendicularly to the flow of the lava. Non-uniform flow patterns indicate an irregular cooling front or barrier such as a narrow valley. 65

Figure 9. Columnar joints at North Dome are more uniform in size and orientation relative to those at Buffalo Dome. There is also less exposure of this outcrop relative to Buffalo Dome. The base of this outcrop is composed of nearly vertical columnar joints followed by ramping columnar joints immediately above them. This flow pattern likely indicates that the North Dome eruption had an easier flow path than the Buffalo Dome lavas and the cooling was mostly vertical. 66

Figure 10. Well-developed flow banding is a major feature of flow unit BD2 at Buffalo Dome and is the most widespread unit of this dome (locations indicated on Figure 7 as ‘10A-D’). The angle of flow banding increases (~10° to ~60°) towards the top of the unit. Flow banding at the outcrops appear to be oriented away from the center of the dome, which likely indicate a concentric flow pattern. 67

Figure 11. Discontinuous lenses of carapace breccia (location indicated on Figure 7) are found within the vitrophyre layers near the BD2 and BD3 contact. The appearance of carapace breccias are indicative of friction between lava flows along the upper portion of the dome. 68

Figure 12. A thin layer of vitrophyre (~15 cm) is found between lava flow units BD2 and BD3, (indicated on Figure 7) often associated with discontinuous lenses of carapace breccia. Vitrophyres in a lava dome system can most commonly be formed by the welding of pyroclastic deposits or by rapid undercooling and degassing of magmas. The degassing of magmas can occur when the loss of volatile solubility decrease pressure (e.g. isothermal decompression) or by the loss of volatiles due to phase change during crystallization (e.g. isobaric crystallization; Westrich et al., 1988). 69

Figure 13. The summit of Buffalo Dome (indicated on Figure 7) is composed of the flow unit, BD3, which is characterized by discontinuous carapace breccia and vitrophyre. Outcrops along the spine of Buffalo Dome display rounded weathering patterns which look like, but are not, pillow basalt texture. Flow banding angles at ridge are at a much lower angle than the lower flows (~15 to 25°). 70

Figure 14. The Buffalo Dome columnar joints contain randomly distributed magmatic enclaves (~25 per m2) that range from ~1 x 0.5 cm to ~20 x 25 cm in size (photo location indicated in Figure 7). Many inclusions at Buffalo Dome are dark brown to red-orange in color but can occasionally be light yellow in color. Host lava to enclave contacts are distinct and range from smooth and rounded to slightly crenulated but are all relatively rounded in shape. Host-enclave contacts are also occasionally emphasized by high-relief surfaces where the two compositions meet (bottom right photo). This is a feature formed by differential weathering. 71

Figure 15. Enclaves at North Dome are typically light yellow or cream in color and are highly weathered at the host-enclave boundaries. Boundary distinctions can be easily made at this location because enclaves are highlighted by their rounded shape and their weathering patterns. Occasionally, entire enclaves are missing from their ‘pocket’ because of this weathering feature. 72

Figure 16. Images of disequilibrium textures seen in thin section samples under plane polarized (left) and cross-polarized light (right). These images are taken of feldspars within the host lavas of Buffalo and North Domes. Images A) show skeletal resorption textures that are commonly found at the core of feldspar phenocrysts within the BD1 and ND1 host lavas and images B) show that these resorption textures can also occur around the center (excluding the center) and near the rim of feldspar phenocrysts. C) Feldspar phenocrysts also commonly host concentric zoning patterns, which are seen under cross-polarized light. 73

Figure 17. Optical photomicrographs under plane (left) and cross-polarized (right) light of plagioclase feldspar (Pl). Crystals within the inclusions at Buffalo and North domes exhibit acicular crystal shape on the ends of microphenocrysts which blend into the acicular plagioclase microlites which make up the groundmass. These textures are likely produced by rapid undercooling. A) Plagioclase grains can include concentrically zoned rims paired with acicular growth along the ends of these same grains. B) Plagioclase grains can also produce this same acicular growth along the ends of the grain with no visible zoning, and C) plagioclase crystals can also support a ‘carpet tassel’ texture of acicular growth along the full end of the grain, this specific grain also shows minor sieve textures in the center of the grain. Also present within these images are amphibole (Amph) and vesicles (Ves). 74

Figure 18. Optical photomicrographs under plane (left) and cross-polarized (right) light of amphibole crystals with central resorption textures. This feature is more common along the host-enclave boundary. 75

Figure 19. Images of host-inclusion boundaries (designated by red dashed lines) are presented under plane polarized (left) and cross- polarized (right) light for five different samples. A) North Dome samples occasionally show a dark band grading into the host lavas away from the host-inclusion boundary. Conversely, B) Buffalo Dome samples often display the opposing direction of this band grading into the inclusion and away from the host-inclusion boundary. C) Commonly observed at Buffalo and North domes, trachytic flow patterns that make up the host lava groundmasses flow around the inclusions and can also occur D) where brittle deformation of biotite is present along the host-inclusion boundary. Finally, E) diffuse boundaries also occur at both Buffalo and North domes where separate compositions are distinct but are classified as gradational changes rather than frictional relationships. 76

Figure 20. Pyroxene plotted on a standard quadrilateral classification diagram. Pyroxene occurs along the boundary between diopside and augite. All but one of the pyroxene samples are from Buffalo and North dome host lavas. The single grain within the inclusion of JCS17-013C plots near average in comparison to the compositions within the host lavas.

Figure 21. All feldspars from Buffalo and North Dome plot as a plagioclase composition within the bytownite to andesine range. Bytownite compositions are only found within enclaves and andesine compositions are only found within host lavas. Host lavas and enclaves all include labradorite compositions. 77

Figure 22. Total Alkali versus silica (TAS) classification diagram normalized to 100% volatile- free (LeBas et al., 1986) for Buffalo and North Dome lavas and enclaves. Comparisons are made with lava groups from Lake City caldera (Kennedy et al., 2015). 78

Figure 23. Major element oxides in weight %. Buffalo Dome and North Dome lavas and enclaves are compared to Lake City lavas and enclaves from Kennedy et al. (2015). All major oxides are normalized to 100% volatile-free; FeO is expressed as FeO total (FeOt). 79

Figure 24. Select trace elements in ppm against Zr for Buffalo and North domes. Lake City comparisons are excluded because of their high Zr content (~280 to ~500 ppm) that would significantly modify the scale for presentation. 80

Figure 25. Primitive mantle normalized multi-element plot from Sun & McDonough (1989) and chondrite normalized rare earth element (REE) plot from Sun and McDonough (1989) showing Buffalo and North dome lavas and enclaves against the Lake City suite (Kennedy, 2015). In general, the Lake City Suite patterns follow host lava abundances from Buffalo and North Domes. A) Buffalo Dome host and enclave trace element compositions show very similar patterns to each other and to Lake City. Host lavas contain a higher abundance of incompatible trace elements Cs through Pb and nearly overlapping compositions for host and enclaves occur from Pr through Lu. B) North Dome host and enclave trace elements show an overall similar pattern but vary much more than Buffalo Dome. Enclaves generally contain higher abundances of trace elements than North Dome lavas except Cs, Rb, Th, Y, Ta, K, and Pb. Lake City compositions encompass the North Dome host and enclave trace element compositions. C) REE contents at Buffalo Dome are similar with slight differences in La, Ce, and Eu through Er. D) North Dome REE contents are parallel to one another with much higher abundances of REEs within the enclaves. 81

Figure 26. Compositions of host lava and enclave from Buffalo and North domes normalized to upper continental crust (Taylor and McLennan, 1985; Rudnick and Fountain, 1995). A) Host comparisons are nearly identical with a slightly more evolved composition (i.e. higher trace element abundances) of North Dome lavas. B) Enclave comparisons have a large variance from one another with much more evolved compositions of North Dome enclave compositions. Co and Ni are in higher abundances at Buffalo Dome than at North Dome. 82

Figure 27. Mix modeling of BD1 major element oxides and average amphibole and plagioclase compositions. Increasing amphibole (wt. %) and plagioclase (wt. %) mixing percentages approach BD1-encl compositions. The best fit mixing compositions are 24% amphibole + 24% plagioclase mixed with 52% BD1. 83

Figure 28. Proposed systematic model for magma mingling of the post-caldera dome formation (DEPC) from the North Pass caldera. A) Faulting from caldera collapse creates a fracture network allowing easy transportation for sustained emissions of magma and gases to ascend to the surface. Beneath the fracture network lies a series of larger fractures (i.e. dikes and sills) for the replenishment of magma into the system. Within this fracture network, incomplete eruptions leave behind residual magmas. B) Residual magmas begin to crystallize within the fracture networks to create a ‘crystal mush’. During this crystallization period, assimilation of crustal contamination also occurs due to contact melting. C) Voluminous replenishments of magma with parallel evolution (e.g. similar to those which left the cumulate) fill the fracture network and remobilizes the crystal mush in its ascent. During this ascent, mixing of magmas, quenching, and minor vesiculation may occur along the boundaries of the residual and replenished magmas.