Constraining Upper Crustal Magmas in the N. Chilean Andes

Chuck Lewis College of Earth, Ocean, and Atmospheric Sciences Oregon State University

Senior thesis

June 3rd 2020

Advisor: Professor Shanaka de Silva

Abstract The 4.59-4.18 Ma Caspana , found within the Altiplano-Puna Volcanic Complex

(APVC), is the result of an ~8km3 eruption that took place during the Neogene Ignimbrite Flare- up in the N. Chilean Andes. While most of the eruptions during this flare up are large, monogenetic eruptions, the Caspana that is characterized by a compositional gap of ~16% SiO2 between the parental and daughter . The rhyolitic plinian fallout is distinct in composition from the majority rhyolite but is also the result of fractionation from andesite. The andesitic magma in the Caspana system are shown to represent some of the most mafic magma that was recharging the upper crustal chambers in the APVC, though all the seem to share a similar signature. Depth estimates and spatial locality place the chamber at the edge of the large thermal anomaly known as the Altiplano-Puna Magma Body (APMB), where cooling would allow for dense solidification during the waning stages of mantle flux. During an initial period of cooling, the system rapidly fractionated along a shallow cotectic and produced the first of two . The system was remobilized following recharge and renewed convection produced the second of two rhyolites and fractionation models are shown for both crystallization sequences. The bulk rhyolitic pumice contains fayalite, amongst other Fe-rich minerals, that indicate low Fe3+/Fe2+. Oxidation state in the rhyolite was likely due to the crystallization of orthopyroxene in the presence of magnetite in the parental andesite. With these two phases crystallizing together without clinopyroxene, the latter of which is common in other APVC magmas, the oxidation state in the daughter can be lowered without another method of reduction.

1 1. Introduction

Explosive volcanic eruptions are an intrinsic threat to nearby communities, with continental arc settings having a concentration of this geologic hazard. The assembly of granodioritic batholiths and long-lived volcanic provinces within arcs can occur by periodic high mantle flux into the upper crust, known as an ignimbrite flare-up (de Silva, 2008). containing super evolved, explosive compositions tend to have a less silicic pumice overlying the basal deposits, and are commonly interpreted as an inverted magma chamber with the parental magma following the fractionate during eruption (Hildreth, 1981). The processes capable of producing rhyolitic magma have been thoroughly investigated and attributed to various causes such as incremental recharge, assimilation, rapid compositional change during fractionation, and gas percolation through dense cumulate structures (Depaolo, 1981; Grove and Donnelly-Nolan,

1986; Bachmann and Bergantz, 2006; Coleman et al., 2012). Many of the dynamics occurring in a magma chamber during evolution, inferred through phase chemistry and whole rock data alike, include the partial melting of cumulates, storage in a rigid mush, recharge, and behavior of magmatic volatiles (Eichelberger et al., 2000; Waters and Lange, 2015; Wolff et al., 2015;

Shamloo and Till, 2019). Though it appears there is a non-unique solution to producing rhyolitic compositions, some of these processes certainly prevail over others for a given magmatic system.

The Caspana ignimbrite, found within the Altiplano-Puna Volcanic Complex (APVC) of the North

Chilean Andes, is an archetypal occurrence of bimodal compositions seen in continental arc settings (de Silva, 1991). The spatiotemporal characteristics of the APVC make the presence of bimodal compositions somewhat of an anomaly, as most of the eruptions in the region are monotonous intermediates erupted from large calderas (de Silva and Kay, 2018). This ignimbrite signifies that high silica, low fragility (explosive) magma can be produced in the upper crust even in regions where astronomically high mantle input causes the geotherm to raise,

2 promoting long term storage and generally homogenous batholiths. The compositional gap between andesite and rhyolite bears with it a phase assemblage that is not common in the

APVC. While most andesites in the APVC have two pyroxenes, the andesite in the Caspana has only orthopyroxene. The bulk of the rhyolite that constitutes the ~8km3 of erupted material contains a particularly iron rich assemblage that is undocumented in the APVC: fayalite, allanite, biotite, and oxides. The purpose of this paper is therefore two-fold: 1) we will test the original hypothesis of de Silva (1991) to find if the rhyolite indeed fractionated from the andesite and 2) investigate how these phases were stabilized. Both of these questions are linked to the overarching goals of constraining development of explosive compositions that pose a threat, the construction of the continental (granodioritic) crust, and answering the geologically interesting questions of why here, why now?

2. Background

2.1 The APVC and the Rhyolites Therein

The Altiplano-Puna Volcanic Complex (APVC) in the Central Volcanic Zone (CVZ) of the N.

Chilean Andes is an expression of long-lived subduction zone magmatism (Figure 1). The plateau where the APVC resides is associated with the Central Andean Neogene Ignimbrite province and was constructed from approximately 10-1 Ma with four distinct pulses of high mantle flux (Folkes et al., 2011a; Kern et al., 2016). Delamination of the Nazca plate is thought to produce a MASH (Melting, Assimilation, Storage, and Homogenization) zone at the base of the crust where partial mantle melts stall due to density differences (Hildreth and Moorbath,

1988). At the base of the crust, they undergo MASH processes until their buoyancy allows them to ascend into the mid and upper crust. High flux into the region led to the construction of an arc-scale batholith beneath the APVC, as represented by a large low velocity zone (Ward et al.,

2014) and was aptly named the Altiplano-Puna Magma Body (APMB). Periods of high mantle flux into the mid-crustal mash increase eruption rates (i.e. a flare up) by allowing silicic magmas

3 to rise into upper crustal chambers (de Silva et al., 2006). Conversely, periods of waning lead to more heterogeneity and generally more effusive eruptions as the thermal engine cools off and allows geotherms to relax (Burns et al., 2015). Recent work has shown that thermal erosion along the base of the crust is characterized by higher ƒO2 along the arc than the back arc, and that heat transfer through the crust is relatively efficient (Burns et al., 2020). This allows production of high-Mg andesites near the base of the crust, owing to the coupling of Si and Mg with increasing ƒO2. Additionally, it appears that clinopyroxene plays a significant role as magma leaves the lower crust to undergo further AFC processes.

The subducting Nazca plate experienced changes in dip angle through time, resulting in an

Eastward migration of the arc as the slab flattened. A subsequent steepening caused the arc to migrate back West, resulting in a crustal shortening that generated the thickest continental crust on Earth of ~70km (Allmendinger et al., 1997). More recent work shows that steepening of the slab followed the passing of the Juan Fernandez ridge. As the slab steepened and mantle melting increased, the delamination of the SCLM allowed for lower crustal mash and recycling of crustal material, raising the baseline isotopic compositions. These architectural changes facilitated the raising of the geotherm, promoting episodic high-mantle power input to the mid and upper crust. The mid-crust experienced substantial thermal and rheological changes as a result, promoting the formation of the Alti-Plano Puna Magma Body (APMB) (de Silva and

Gosnold, 2007), which is now proposed as the mid-crustal source of the large volume dacitic eruptions (de Silva, 1989; de Silva and Francis, 1989). Amongst the large volume and their parental andesites, several rhyolites have been found within the APVC (Lindsay et al.,

2001b; Schmitt et al., 2001; Folkes et al., 2011b; Grocke et al., 2017b) though none seem to share an explicit petrogenetic origin. All of these rhyolites seem to be derived from within the upper crust by different manners. Large crystalline mushes that undergo periodic influx from the source and closed system fractionation, with or without large degassing events, may form some

4 of these rhyolites. Conversely, these mushes can be remobilized if recharge is substantial enough to melt the base of the mush, promoting convection and thereby renewed fractionation as the mush begins to ‘unzip’ (Burgisser and Bergantz, 2011). Tiered systems or cupolas forming a chemically distinct cap are hypothesized to occur in the APVC among other volcanic centers These rhyolites can continue to undergo internal fractionation, especially if a portion of the eruptible magma ascends during eruption but remains in the chamber. We will assess which differentiation processes can produce the observed rhyolites in the APVC and tease out which of these processes can produce the compositions and assemblages preserved in the Caspana ignimbrite.

1.2 Large Scale Beauty: Lithologies and Field Constraints of the Caspana Ignimbrite

The Caspana ignimbrite (4.59-4.18 ma; de Silva, 1989) provides a unique glimpse into a highly evolved system, expressed by bimodal compositions found together in outcrop (Figure 2). The plinian deposit is located more proximal to the inferred source, which is inferred to be buried under the modern stratocones of Toconce and Cerro del Leon, along with the Chao . A co-plinian ash indicates that the eruption plume had time to settle before the larger pyroclastic density currents (PDC’s) deposited on top (Figure 2). More distal deposits are characterized by a thin reworked layer near the base. The larger outcrops in this area contain both andesitic and rhyolitic pumices within the matrix (Figure 2, 3). Additionally, some outcrops show lenses of lithics, which also provide evidence for an eruption with multiple hiatuses. An atypical ignimbrite of the APVC, the Caspana is characterized by a relatively small volume eruption (~8km3; de

Silva, 1991) with 4 distinct pumices identifiable amongst two compositions. A description of individual samples can be found in table 1. The pumice comprising the plinian fallout is typically pea to marble sized with some pumices reaching up to 6 cm and deteriorates with pressure between the thumb and forefinger. Rhyolitic pumices in the unit above the plinian fallout are

5 somewhat larger (~6-10cm), contain some feldspar phenocrysts, and are more resistant.

Vesicles in these pumices are typically sheared.

The flow unit above the reworked layer in the more distal deposit contains the large rhyolitic and andesitic pumices, reaching up to 25cm. Rhyolites in this flow unit and the one above are typically oxidized and all are crystal poor. The crystals in this rhyolite are biotite, fayalite, allanite, and plagioclase. Fresh surfaces show a gleamy white to gray, glassy matrix that may or may not be sheared. There are two distinct andesitic pumices. The first is highly vesiculated, dark gray to black, glomeroporphyritic with plagioclase, orthopyroxene, and Fe-Ti oxides making up the glomerocrysts. All of these minerals can be found as phenocrysts and crystallinities come to around 40%. This pumice is found in the flow unit with large pumices or closer to the top of the upper flow unit in the distal deposits and is generally on the order of 15-25cm. The second is highly oxidized, generally smaller than the first andesitic pumice at 8-15cm, and contains far less glomerocrysts with lower crystallinity (20-30%).

1.3 Art at the Micron Scale: Petrography

The andesitic pumice is glomeroporphyritic to porphyritic and contains 20-30% crystal content of hypersthene, ilmenite, plagioclase, magnetite, and apatite as an accessory phase. There are also abundant xenoliths that appear to be comprised of feldspar, biotite, Fe-Ti oxides that may or may not be secondary, and chlorite. Glomerocrysts are comprised of either a combination of the aforementioned minerals or are strictly plagioclase (Figure 4), the former of which are described here as a micron scale gabbronorite. These micron scale gabbronorite sometimes display cumulate like textures though they are sometimes broken apart, apparently from the inside out. Amphibole has been identified as an inclusion within hypersthene via EDS, as a reactant, and as a phenocryst. Accessory phases have not yet been positively identified but are likely to be epidote or titanite. Optically, multiple plagioclase populations exist within the andesite. Those found in glomerocrysts are typically more lath like or lamellar, though

6 sometimes tabular. Many phenocrysts also display this shape and are inferred to be the same population. A second population is larger (~700-900um diameter), euhedral, and tabular. These typically display oscillatory zoning and many have pronounced signs of disequilibrium (i.e. abundant melt inclusions or convolute regions) along a zoning boundary approximately 200-

300um from their core. A third population is more oblate in shape, intermediate in size (300-

500um), subhedral, and have oscillatory zoning. The last (fourth) population, are entirely sieved or convolute, displaying a spongy texture. These are typically large (400-600 um) but can also be much smaller (100-300 um). Like plagioclase, orthopyroxene displays a variety of textures.

One group is euhedral to subhedral and may or may not be embayed; the group of andesites with more glomerocrysts typically has a higher abundance of embayed phenocrysts. The euhedral phenocrysts have the smallest amount of inclusions and may be found with euhedral apatite and ilmenite. Another group is highly resorbed at the edges and may or may not appear to be part of a glomerocryst that was broken apart and resorbed. These occur in most thin sections and may result from a dense crystalline mush that trapped gas as it exsolved

(Okumura et al., 2019). Ilmenite is typically 100-250um but can be large, occurring as a phenocryst up to ~400um across or as inclusions. Magnetite is present but clearly resorbed and much smaller than ilmenite (25-100um), occurring in glomerocrysts or as inclusions.

The bulk rhyolitic pumice contains phenocrysts of biotite, fayalite, allanite, and plagioclase, with

Fe-Ti oxides, apatite, and zircon as accessories. Biotite is the dominant phase and typically occurs with zircon. Owing to its stability, plagioclase is by far the largest phenocryst. Optically, there are two populations of plagioclase in the rhyolite. The first is larger (~500-600um), oscillatory zoned, and subhedral. The second is smaller (~200-400um), slightly more lamellar, subhedral, and has no zoning. Fayalite always contains inclusions of apatite and zircon, often together at the center of the crystal. The one sample collected above the plinian fallout is

7 notably different in thin section. It is highly sheared with lath like feldspar microphenocrysts

(~100um). The dominant crystal in this pumice is by and large, and quite surprisingly, zircon.

The plinian fallout has a distinct glass that is reminiscent of thick smoke and is nearly aphyric.

Plagioclase is present but small (~50-150um) and 4 crystals of either clinopyroxene or amphibole were found in mineral separates. Fe-Ti oxides, zircon, and apatite are accessories with zircon being the most abundant of those three. Titanite was found in mineral separates of the 150-250um fraction and displays the characteristic bright green color and appears to be euhedral. Poorly formed muscovite was also found in thin section but has not been positively identified by microprobe at this stage and was also found in mineral separates.

Unfortunately, a highly birefringent carbonate is apparent in the thin sections. It is most prominent in the andesite though it is sometimes found in the rhyolite.

2. Analytical Methods

2.1 X-Ray Fluorescence (XRF)

Samples with little to no oxidation were selected for analysis. Those that contained oxidized surfaces were sawed or chipped off at Oregon State University to expose the inner most fresh face possible. Supplementary whole rock data and glass data measured by XRF is taken from de Silva (1991). These samples are denoted by ‘SDS’ following the sample number in table 1.

Whole rock samples were crushed using steel plates in the jaw-crusher and grinded to a fine powder at Oregon State University. Samples were processed at Washington State University using a ThermoARL AdvantXP machine following the method of Johnson et al. (1999). The method requires powdered samples to be combined with a lithium flux in a 2:1 ratio and subsequently fused into a bead, which is then polished at the surface. One sample was run twice to ensure reproducibility. Loss on ignition (LOI) are particularly high for some samples, owing to carbonate contamination. This problem is pronounced for the andesites which

8 abundant secondary minerals (specifically carbonates) were found in thin section, while little to no carbonates are found in the rhyolite or plinian magma. It is recognized that this may cause erroneous calcium values, which would subsequently drive down other major and minor element values. Trace elements, however, are considered to be mostly unaffected. Trace element values for Ba, Ni, Cr, V, Cu, Zn, and Ga were measured via XRF.

2.2 Inductively Coupled Plasma - Mass Spectrometry (ICP-MS)

Samples were crushed and powdered as for XRF and brought to Washington State University using the Finnigan Element2 following the procedure of Johnson et al. (1999). All REE and trace elements, except those listed above, were measured via LA-ICP-MS. A repeat analysis was done to ensure reproducibility.

2.3 Electron Probe Micro-Analysis (EPMA)

Supplementary plagioclase and ilmenite data from de Silva (1991) and de Silva (1987) are reported. All other EPMA analysis were carried out at Stanford University with a JEOL JXA-

8230 SuperProbe. To minimize alkali loss in glass K, Ca, Si, Al, Na, and Mg were run together.

P, Fe, Mn, Ti, Cl, and S were run separately and treated as minors. Only those glass analyses with totals >95% were kept and renormalized to 100%.

2.4 Instrumental Neutron Activation Analysis (INAA)

Supplementary trace element data for the ‘SDS’ samples from de Silva (1991) and de Silva

(1987) were run via INAA. Linear regression of LA-ICP-MS and INAA shows that values for

LREE and actinide elements are generally accurate by this method (with the exception of Cs) but generally get progressively worse through the HREE. Calibrating these data can only be done with two points, which is statistically invalid, so the values are left as reported in the aforementioned publications.

9

3. Results

3.1 Whole Rock Geochemistry

The Caspana pumices are defined by a large compositional gap (~16 wt% SiO2) along a calcium-alkaline trend (Figure 5). Representative whole rock and glass analyses can be found in table 2. The plinian pumice has the lower silica content of the two rhyolitic pumices by approximately 1% and the majority rhyolite has higher alkali content. The andesite is particularly mafic, nearing the field. The gap between the two compositions spans nearly the entire range of the APVC dataset with only a few crystal rich dacites and cognates from the

Cerro Purico Volcanic Complex approaching the most primitive andesite in the APVC. There are few rhyolites with higher silica content; namely the Toconao, which has been attributed as a fractionate from the Atana dacite by geochemical and field investigations (Lindsay et al., 2001a,

2001b).

Here, two distinct groups of andesites are noted and described by noting the differences of the first (Figure 6). The more crystalline andesites are deemed group 1 and are more mafic in composition using SiO2 and Rb as indices, the latter of the two being the most incompatible trace element in the system. Additionally, these andesites share a distinct chemical trend in trace element space (Figure 6). Mg numbers appear to be conspicuously low and the cause of this is brought on by two possibilities (Table 2): 1) Carbonate crystals are driving down other major element abundances, or 2) these andesites had relatively low fO2 during their rise from the lower crust (Burns et al., 2020). Almost all major and minor element analysis are erroneous and lie off the APVC trend. Thus, the particularly low Mg numbers for the andesites are attributed to the former of the two possibilities.

10 It should be noted that the plinian pumice is chemically distinct from the rhyolitic pumice, except for the rhyolitic sample CH19C003-CL, which is more identical to the plinian pumice. The plinian pumice is higher in A/NK (the inverse of which being the agpaitic index) than the bulk rhyolitic pumice and is more mafic in major element space, with the exception of iron and sodium (Table

2, Figure 6). FeO/(FeO+CaO) values are up to 10% higher in the majority rhyolite than the plinian, and this is inversely correlated with the Ca number (CaO/(CaO+MgO)). These indices are inferred as a proxy for melt polymerization and the Fe3+/Fe2+ (Mysen et al., 1982, 1984,

1985) When A/NK is close to one, the melt is more polymerized as aluminum can be charge balanced in tetrahedral coordination. Melts with this characteristic have less non-bonding oxygens, correlating with lower Fe3+/Fe2+, less Ca with respect to Fe, and lower Fe2+/Mg. The decrease in network modifiers and increase in ferrous iron connotes a lower oxidation state, irrespective of water content.

C1 normalized REE patterns using values from McDonough and Sun (1995) are somewhat steep with La/Yb values ranging from ~7 to 15 (Figure 7). The lowest values are from the bulk rhyolitic pumice, which also has the most pronounced europium anomaly. The highest values are from the plinian magma, which are LREE enriched with progressive depletion in higher atomic numbers. The andesites are intermediate at La/Yb ~8, with group 1 andesites generally being less evolved (shallower slope for this particular system) than group 2.

3.2 Phase Chemistry

3.2.1 Matrix Glass

Major oxide glass analyses for the andesites are rhyodacitic in composition and lie in the middle of the compositional gap between andesite and rhyolite whole rock data (Figure 5). P2O5 and Zr values are elevated in andesite glass relative to the andesite but lower for rhyolitic pumice

(Table 2, Figure 6), indicating late stage apatite and zircon crystallization at higher silica

11 content. Supporting the rise and fall of these oxides is the presence of these accessory phases often adhering to one another in the rhyolitic magma, should the compositional gap be from differentiation across ~16% SiO2. Major elements for the rhyolitic glass are more evolved and reflect the general phase assemblage of biotite, fayalite, and andesine. REE are generally depleted in the rhyolitic glass with respect to the whole rock, reflecting the partition coefficients found in the literature for these phases and is consistent with the general acceptance that partition coefficients increase with silica content for most elements. However, these values could also be caused from mixing with a more mafic magma. The plinian glass overlaps the plinian pumice values using SiO2 as an index but trace elements show no consistency that is indicative of more or less evolution. MgO and MnO are approximately the same as the pumice while FeO and Al2O3 are depleted, and alkalis have opposing trends. One anhedral plagioclase crystal was found in thin section which could create the observed depletion in aluminum.

However, Na is higher for glass indicating that either the plagioclase in the plinian is relatively far from pure albite, or that mixing may be important. Additionally, the depletions in FeO and enrichment in CaO cannot be reconciled with the observed phase assemblage in the plinian magma.

3.2.2 Plagioclase

Plagioclase analysis presented here are from the phenocryst analysis of de Silva (1991).

Chemically, most of these plagioclases are normally zoned with higher An numbers at the core though some are reversely zoned. These analysis span a relatively close range of An number

(An78-81) but vary much more in their iron content (Table 3) though the data show no immediate correlation.

Plagioclase in the rhyolite varies core to rim, with a rim composition falling between core compositions and has relatively high An values for a rhyolite with this SiO2 content. The analysis

12 show far wider variation in the rhyolite than the andesite (An28-42), with lower An content corresponding to lower FeO.

3.2.3 Fayalite

Fayalite is only found in the majority rhyolitic pumice and is quite distinguishable. Olivine compositions are displayed on the pyroxene quadrilateral (figure 9) and representative analysis can be found in table 4. Core and rim spot analysis are remarkably consistent (Fa88-89), though a transect across three grains shows that fayalite may in fact record changes in magma composition (Figure 8). While MgO values towards the cores of the fayalite grains have no correlations between one another, the compositions towards the rim begin to develop the same profile.

3.2.4 Biotite

Representative biotite analysis can be found in table 5 and, like fayalite, are nearly consistent in core and rim analysis. Recalculation of biotite was done using 11 oxygens, and the recalculated composition places the biotite well into the annite field. Additionally, Cl content for the biotite is relatively high in comparison to other biotite found in the APVC (Grocke et al., 2017a, 2017b), as is TiO2. Fe/(Fe+Mg) is remarkably high at ~0.8 and varies by less than a tenth of a percent.

3.2.5 Fe-Ti oxides

Magnetite is oxidized in the andesite, preventing estimation of P-T and fO2 by two-oxide oxybarometry. The presence of Ti-magnetite in the majority rhyolite is confirmed from EDS analysis. Fe and Ti are inversely correlated, indicating decreasing temperature with increased iron content (Table 6), based on the inference that temperature calculations are mainly a function of Ti content. As expected, the lowest Ti values for ilmenite are found within the rhyolite, indicating that the rhyolite was cooler than the andesite (Buddington and Lindsley,

1964).

13 3.2.6 Orthopyroxene

There are two modes for the rim compositions in MgO space (19.5 wt% and 21.75 wt%) and core compositions span a range of ~ 16.5 – 22 wt% MgO (Figure 10).Four distinct groups of orthopyroxenes are present among the andesites and chemical heterogeneity is used in addition to the textures discussed in section 1.3. Group one is subhedral to euhedral with some resorption at the rims and are more Fe rich at the rim. Group two is habited by a single phenocryst that is highly resorbed at the rim. Core and rim compositions alike are Mg rich and an amphibole inclusion has been identified via EDS but has not been analyzed via WDS. Group three are subhedral and contain the highest Fe in the core, with rims becoming progressively

Mg rich. These crystals are typically embayed and when they occur with Fe-Ti oxides or accessories they are all generally resorbed. Group four are from micron scale gabbronorite and compositions lie between the two extremes in Mg-Fe space. Data suggest that the group four

Opx compositions represent the chemical equilibrium that pyroxenes are attempting to equilibrate at. Pyroxenes within these tiny gabbronorites also record an initially low Ca content in the andesite where the glomerocrysts grew (Figure 10). This is significant, as this seems to be a mirror reflection of the high FeO indices in the rhyolite (Figure 5). All four populations are found in the most mafic andesite, CH12020(2), while Opx grains in sample 83069 (de Silva,

1991) have compositions overlapping those of group 1.

4. Modeled Intensive Parameters

4.1 fO2 and Dissolved Water Content

As noted, ƒO2 cannot be estimated due to dissolved Fe-Ti oxides and lack of WDS data for amphibole. An estimation of ƒO2 is inferred and discussed in section 5.3. Accurate estimations of water content have been intensely debated (Humphreys et al., 2016) and, as this study shows, with good reason. Calculated water content was estimated using the hygrometers of

14 Putirka (2008) and Lange et al. (2009) (Table 7). The Putirka plag-glass estimated that plagioclase is out of equilibrium for both the andesite and rhyolite via Ca-Na exchange.

Estimated values for P-T-H2O estimated by this thermometer appear erroneous with H2O content at <1% and >7% for andesite and rhyolite and give negative pressure estimates, thus this hygrobarometer is disregarded for this study. The plag-glass hygrometer of Lange et al.

(2009), which places its validity in Ab-An exchange on a experiments at equilibrium, estimates

~4.6 and 5.5 wt% H2O for andesite and rhyolite, respectively. As this is in agreeance with typical arc magmas in the APVC, these values are accepted as relatively accurate. Interestingly, these estimations place raw glass totals near or at 100% with Cl and SO3 measured as well.

4.2 P-T

The Opx-Glass thermobarometer of Putirka (2008) is employed for this study and estimated P-T conditions were used with the melt water contents described above. Most pyroxenes are in equilibrium, as governed by Fe-Mg exchange (Roeder and Emslie, 1970) with any given glass composition and displayed graphically in the Rhodes diagram (Rhodes et al., 1979; Figure 11).

Average estimated values using pyroxene rim compositions in equilibrium are 271 +/- 0.26 MPa and 979 +/- 48 °C (Table 7). Assuming an average crustal density and pressure of 2.8 x 103 kg*m-3 and 27 MPa/km places the chamber at ~10km depth, a reconcilable depth for upper crustal APVC magmas.

The presence of fayalite provides a method of estimating temperature for the rhyolitic magma.

Fayalite in equilibrium with the glass via Fe-Mg exchange gives an estimated temperature of

845 °C (Table 7, Figure 11). Few olivine-glass pairs fall within equilibrium and the majority falls to the right of the equilibrium envelope on the Rhodes diagram (i.e. higher liquid Mg number).

This is somewhat expected, as the highest Mg numbers in from all pumices are in the majority rhyolite. This thermometer is used with extreme caution and minimal confidence, as silica content lies outside the calibration range by ~2 wt. % SiO2.

15 4.3 Density and Viscosity

Density, viscosity, and relevant physical constraints on magma rheology are reported in Table 7.

Density estimates were generated using DensityX (Iacovino and Till, 2019),using dissolved water content and P-T estimates described in sections 4.1 and 4.2. This method employs the partial molar volume, thermal expansion, and compressibility of individual oxides species.

Fe3+/Fe2+ was calculated using the ratio 0.8998 and the observed difference from the total is considered Fe2O3. Though this is not ideal, the redox state of the magma cannot be accurately determined and the differences in calculation parameters (i.e. compressibility and thermal expansion) between Fe2O3 and FeO are substantial. Thus, inferring a typical redox state of arc magmas throughout most of the magmatic evolution and generating an estimate of Fe2O3 is more accurate than disregarding the redox state entirely. Estimated fractional density for group

1 andesites are higher than group 2 andesites, even with the same melt water content.

Depending on assumed water content (CO2 is disregarded) plinian magmas are either higher or lower than the rhyolitic magma, with overlapping values occurring at ~5.2% water content.

Melt viscosity was estimated using the model of Giordano et al. (2008). The largest errors for this model are assumed to come from the empirical constants used in the model itself. The rhyolitic magma is more viscous and less fragile (i.e. explosive) than the plinian and, of course, the andesite (Figure 12), assuming the plinian magma is hotter. Water content from 5-5.5 wt%

H2O for the plinian magma gives an estimate of fragility that is less than that of the rhyolite, and temperature does not affect the fragility estimate. Assuming temperature and water content equivalent to the rhyolite causes plinian values for melt viscosity to overlap the rhyolite and higher temperature give lower melt viscosity values. This supports the proxies for higher melt polymerization in the bulk rhyolitic magma and, considering volatile contents, gives way to the models of Giordano et al. (2008) who propose that adding volatiles to a magma with relatively more network modifiers may in fact reduce fragility. This is in stark contrast to the common

16 thought that dissolved water content increases NBO/T and thus polymerization. A thorough review of analytical problems and inferences supporting this, along with the potential temperature dependence of reactions involving volatiles within a silicate melt can be found in

Kohn (2000). This is an important consideration for the Caspana ignimbrite, due to the hydrous mineral assemblage amongst other parameters that are discussed in later sections.

Nonetheless, Giordano et al. (2008) also show that adding hydrous rhyolite to more mafic melts may increase its viscosity beyond that of the rhyolite itself at the temperatures relevant to this study. This facilitates the classic fluid dynamical models for preferential extraction (Blake and

Ivey, 1986a, 1986b) and provides evidence for the interpreted interaction between silicic and mafic melt, which will be discussed in section 5.

Melt viscosity models were then used to calculate magma viscosity models using the equation:

5 훷 − 휂푚푎푔푚푎 = 휂푚푒푙푡(1 − ) 2 훷0

Where Φ are the crystallinities shown in table 1 and Φ0 is the closest packing of crystals (i.e. critical crystallinity: 60%) (Spera, 2000). When magma viscosity is taken into account, the plinian magma is clearly less viscous and would be subject to a more laminar flow than the rhyolitic magma and would thus flow through virtually any magma within the reservoir.

5. Discussion

The evolution of the Caspana ignimbrite through pre- and syn- eruptive processes is complex and multiple upper crustal processes are invoked to describe its development. The two andesites, identifiable by their chemical, petrological, and macroscopic characteristics appear to be the direct parent of both the majority rhyolite and the plinian (Figure 13). Yet the two rhyolites vary in their compositions, with notable differences in trace element space. The Caspana ignimbrite provides an opportunity to investigate the mixing of intermediate to mafic magmas

17 that are generally homogenous and only cryptically mixed in the upper crust. This also allows for a look into how these magmas can forgo intermediates between themselves and their fractionates; in this case with two separate daughters. One of the key observations that has been made for extreme compositional zoning is that of a rapid change in compositional space without a significant loss in temperature (Grove and Donnelly-Nolan 1986; de Silva, 1991). The most relevant parameter of this theory is that the fractionating assemblage within the parental magma must be able to deplete major elements such that the residual liquid is able to stabilize the observed mineral assemblage. Our one glass inclusion analysis from glomerocrysts that rendered acceptable totals is consistent with the enrichment of iron and depletion of other compatible major elements (Table 2).

Another important consideration is that of magma chamber geometry. Magma chambers with low aspect ratios are considered to promote sidewall fractionation on short timescales (de Silva and Wolff, 1996). A long lived inference that eruptive products are but a minor fraction of the subsurface melt has stood the test of time in the interpretations used in studies of volcanic systems (Spera and Crisp, 1981). The small eruptive volume in the Caspana (for the APVC), crystallization timescales predicted by de Silva and Wolff (1996), and rapid compositional change lead to the inference of a low aspect ratio chamber that can assist in facilitating the compositional gaps. A theory that has recently gained attention throughout the community is that of a rigid crystalline mush, for which evidence from large scale processes down to the crystal scale have shown this to be a robust theory for how magma chambers generally reside in the crust (Cashman et al., 2017; Wieser et al., 2020). However, some textural features in the glomerocrysts such as lath like plagioclase concentrated at the edges of the glomerocrysts, with their laths protruding into melt, and abundant portions of trapped melt forgo efficient micro- settling that should take place in the mush (Holness, 2018). These disparities are recognized as confounding evidence for the long-term storage conditions but resolving these issues is not the

18 focus here, though they are important to note as chamber geometry is considered in these interpretations. The processes at play regarding rapid crystallization and long-term storage are likely to play directly into one another throughout the evolution of highly zoned systems. The advent of the rigid mush model has brought on debate about how volcanic systems behave in both physical and chemical space (i.e. temperature and spatial distribution), and the extent to which crystal and/or melt chemistry is a product of physical constraints, including mechanisms such as roof collapse and interaction with hydrothermal systems (Bindeman, 2008; Kent et al.,

2010; Cooper and Kent, 2014; Ellis et al., 2014; Wolff et al., 2015; Troch et al., 2017). The distinct compositions found within the Caspana ignimbrite allow us to tease out which processes may have occurred while building up to eruption.

5.1 Foundations: Mixing and Potential Sources for the Andesites

The two groups of andesites display two distinct trends that indicate varying history between them (Figure 6,13), though constraints on their REE patterns show that they are in fact genetically related (Figure 14). Dy/Dy* values show overlapping values between both groups, indicating that mixing has smeared out which of the two groups experienced more amphibole fractionation from their parental magmas (Davidson et al., 2007, 2013). Interestingly, CeN/NdN show that group one andesites may be more closely related to their parental mantle derived magmas (i.e. less fractionated), like that seen in New Zealand magmas on the Southern Island

(Burns et al., unpublished). The andesites in the Caspana have the lowest CeN/NdN of all andesite pumices in the APVC, but lower MgO (likely due to carbonate contamination).

Compounding on this evidence, the lowest Rb and SmN/YbN values for the APVC pumices in our dataset are found in the group one andesites, with group two andesites either overlapping or being close to group one values. This evidence shows that the andesites of the Caspana are amongst the most mafic that erupted explosively of those in our dataset. If P-T estimates are correct, it follows that these andesites would cool relatively quickly and fractionate densely, as

19 they would not be buffered by the large intermediate chambers that characterize most of the

APVC.

Opx data also supports the interpretation of an andesite with a more mafic parental assemblage. The low Ca values in the cores of Opx within glomerocrysts may signify that these andesites were in equilibrium with clinopyroxene prior to their emplacement in the upper crust.

While most APVC magmas contain two pyroxenes, Cpx is conspicuously absent in the Caspana andesites. Another simpler interpretation, follows that thermal cooling at the sidewall pushed

Opx compositions towards more calcic based on the stability fields of (Martel et al., 1999; see their stability fields). Neither of these interpretations can be positively confirmed but what is clear is that the upper crustal crystallization sequence did not consist of Cpx. It is important to note that Martel et al. (1999) address the point that Cpx crystallization seems to be somewhat unpredictable, and largely relates to oxidation state.

The Caspana andesites appear somewhat cryptically mixed in major element spaced, though trace elements show more heterogeneity (Figure 6, 13). The most likely reason for preservation of the heterogeneity is the physical barrier (i.e. a dense, rigid mush) that would be established when the andesite fractionated the first of two rhyolites, both of which require large degrees of fractionation. This effectively forms a pluton that bars the mafic root upon intrusion, though gas filter pressing from the mush and gas sparging from the recharging magma due to decompression was likely an important process, as supported by Holness (2018). This would enrich lead, give biotite relatively high Cl values, and give the glass the observed volatile contents (Cl ~ 1000pm, SO3 ~200ppm); all of which show that volatile transfer from andesite to rhyolite was efficient. Yet the rhyolites themselves are expected to have low fO2 due to the stabilization of abundant Fe2+ bearing minerals, and further discussion of this matter can be found in section 5.3.

20 Most of the andesites that erupted onto the APVC during the Neogene were forced to rise through the large thermal anomaly that makes up the mid-crustal MASH zone. Recent isotopic data has shown that at the edges of the hypothesized APMB mafic magma can escape to the edges of the APMB due to buoyancy differences (González-Maurel et al., 2019). Isotopic data published by de Silva (1991) for the Caspana is limited and values may be erroneous due to carbonate contamination, though the Caspana andesites do have the highest Sr values in the dataset. The inference is thus made, with great caution, that group one of the andesites may be one of those that eluded the APMB. Supporting evidence for this is minimal, lying primarily in the chemistry (Figure 13) and what may prove to be forsterite inclusions in orthopyroxene with further work (Figure 15).

5.2 Efficiency: Production of High Silica Rhyolite from Andesite

The wide gaps in major elements between andesite and the bulk rhyolitic composition is inextricably large. The development of such a large gap was initially ascribed to the fractionating assemblage of Opx + Plagioclase + Ilmenite (de Silva, 1991). Conceptually, this assemblage has the bulk composition to deplete the major elements given enough fractionation and can deplete the glass inclusion in all but iron, as discussed. The plinian magma does not display gaps nearly as large as the bulk rhyolitic magma and is more related to the andesite in major element composition, indicating its genesis is different than the majority rhyolite. To test fractionation, major elements were modelled using least squares residuals with the program

PetroGraph (Petrelli et al., 2005). This program calculates the modal amount of the mineral assemblage and outputs the degree of crystallization (F). Ideal compositions (i.e. ideal stoichiometry) were used for fractionation of apatite, titanite, and magnetite within andesite, and a pargasite and magnetite from Schmitt et al. (2001) are supplemented as well. The required modal abundance that must be removed from the liquid were then used to test Rayleigh fractionation models using partition coefficients from various sources to calculate a bulk D. LSQ

21 results, publications for which partition coefficients are used, and data for partitioning values pulled from the literature can be found in table 8. The most important part of the assumptions employed here are that the observations (i.e. petrography and textural constraints) agree with one another. The assemblages modelled by major elements do generally align, with the notable exception being amphibole due to its finnicky nature.

Fractionation from andesite to bulk rhyolite is carried out in two steps: 1). Andesite to its rhyodacitic glass and 2) rhyodacitic glass to rhyolite. LSQ results of the first step show that fractionation of the observed glomerocryst assemblage, Plag + Opx + Ilmenite, can create the andesitic glass (F: 45.13%, R2: 0.3917). The second step of this takes solid solution and the glass composition into consideration, yielding a satisfactory LSQ result (F: 30.37%, R2: 04952).

For the second step, the most mafic plagioclase core (highest An content) within the rhyolite is used for the LSQ along with Fe-Ti oxides, Opx, and apatite. Noting that the glass is truly rhyodacitic in composition and that partition coefficients are never truly constant along a fractionation path, the applied partition coefficients are for dacite or low silica rhyolite.

The plinian magma is hypothesized to fractionate directly from the andesite, with the influence of amphibole and accessories in addition to the glomerocryst assemblage, also rendering satisfactory LSQ values (F: 48.51%, R2: 0.0069). This assemblage gives the most satisfactory result for LSQ from all that were modelled, suggesting that amphibole has an important role in the development of the Caspana ignimbrite. This is justified by the Dy/Dy* values that shows the most mafic andesites in the APVC dataset are intrinsically related with only small variations. In addition to amphibole, titanite appears to play a significant role for the observed depletions in the plinian magma. Without both titanite and amphibole, the plinian would not have the observed depletions. Unsurprisingly, amphibole appears to be relatively elusive in the Caspana as well as within other volcanic systems (Grove and Donnelly-Nolan, 1985; Davidson et al.,

2007), owing to its sensitivity to the environment.

22 Sr and Eu/Eu* values show that plagioclase had a dominant influence during fractionation and confirms that the bulk rhyolitic pumice underwent substantially more plagioclase fractionation

(Figure 13) than the plinian. We used the most incompatible element, Rb, as an index to test chemical differentiation processes. No combination of assemblages can create both the plinian and rhyolitic magma on the same liquid line of descent using Rb as an index in Rayleigh- fractionation models. However, both proposed paths here are in agreement with the variations in Sr and Ba for creating these magmas from the andesite. We leveraged the dominant fraction of plagioclase to investigate if Y values are consistent with the observed depletions in HFSE within the plinian magma. Rayleigh fractionation paths suggest that the two-step model proposed for the rhyolite and fractionation of the plinian directly from andesite can create this difference. This indicates that the two rhyolites coexisted in the chamber prior to the eruption of the plinian fallout. Compounding on this evidence is the sample collected above the plinian base lies closest to the majority rhyolite.

Many published works have been done with significantly less granular data, using spidergrams and REE arrays to test fractional crystallization. We have thus supplemented a REE array for fractionation to both pumices directly from andesite in 1D space as opposed to 2D space

(Figure 16). The difference for the bulk rhyolitic pumice is that there is only one step and apatite is used in the model, as neither the plinian nor the rhyolite could be produced without removing this phase during fractionation. It should be noted that there is no mixing required here and that fractionation models are a conceptualization with non-unique solutions but instead provide insight into the processes that were likely to occur. Bulk D values for these models are also presented in Table 9 and data for these models are in the supplementary tables 1 & 2.

Fe-Ti oxides, which strongly partition niobium, are highly sensitive to their environment and can often be used to fingerprint various magmatic processes (Huang et al., 2019). In trace element space, Nb is one of the few trace elements that is consistently higher in the plinian than the

23 rhyolite. This suggests that ilmenite did not play nearly as significant of a role in creating the plinian magma, which is also consistent with TiO2, vanadium values and tantalum values. It is also important to consider the various phases that can affect this ratio during AFC processes if this argument is to be made here. Fractionation of Nb from Ta can occur during interaction with basement amphibolite within the region (Lucassen et al., 2001) or fractionation of ‘cryptic’, high

Mg amphibole (if present) (Davidson, 2007), as high Mg amphibole will partition Ta more than

Nb, thereby raising Nb/Ta. The opposite control on this ratio is exhibited by the two pyroxenes commonly found in the APVC, with Cpx having a marked effect. Though there is significant scatter in the data, it appears that Cpx fractionation dominates this ratio until mid-crustal MASH processes (Burns et al., 2015) begins raising this ratio again for the large monotonous dacites by partial melting of amphibolite or cryptic amphibole (Figure 17). If these inferences are correct, it appears that the genesis of the plinian magma requires less of this AFC style and resided in the crust for a shorter period of time. To reiterate, the plinian has more Ta but less Rb, showing its time in the upper crust was short lived and didn’t undergo as much ilmenite fractionation; but also has a higher Nb/Ta ratio, likely due to direct fractionation of amphibole as opposed to partial melting of basement rock.

5.3 Inferred ƒO2 and Stabilizing Fayalite in the APVC

One major question of this project is to investigate how a magma chamber in the APVC can stabilize a phase that is generally only found in continental settings, with few exceptions (Bacon et al., 1981; Mahood, 1981; Novak and Mahood, 1986; Streck and Grunder, 1999; Troch et al.,

2017). The high FeO indices in the rhyolite follows with a synthesis of rhyolites that contain fayalite as a primary phase (Warshaw and Smith, 1988 and references therein). Warshaw and

Smith (1988) show that rhyolites containing fayalite are characterized by high iron with respect to calcium and aluminum (i.e. a decrease in NBO/T and melt polymerization due to network modifiers) allowing Fe to be stabilized in 2+ coordination. While many of these rhyolites are

24 generated at hotspots and spreading centers, the Sierra la Primavera (Mahood, 1981) is also found in a continental arc setting. Calculated oxygen fugacity for the Primavera rhyolite places the rhyolites slightly below the FMQ buffer. and the presence of fayalite and magnetite together in the Caspana rhyolite also indicates the magma is likely close to FMQ. However, the Caspana is likely significantly different than the Primavera magmas, as it appears to be more closely related to its parent (i.e. they occur in outcrop together). Also, the Caspana contains a hydrous mineral assemblage (biotite) but lacks ferrohedenbergite, indicating that the ratio of Fe2+/Ca was significantly lower in Primavera rhyolites, and the Primavera magmas are presumed to be relatively dry (Mahood, 1981). Estimation for water content in the Caspana also appear to be substantial, supported by the high An plagioclase (Waters and Lange, 2015) and pressure estimates are not significantly different from other APVC upper crustal chambers.

Further indication of non-anomalous fugacity in the parental andesite is the presence of amphibole. Thus, the inferred ƒO2 of the Caspana andesites are inferred to be that of typical arc magmas (∆NNO ~+0.1 – +0.7) throughout its evolution. Yet the presence of fayalite in addition to annite are a clear indication of low fO2 in the rhyolite. We propose that the presence of fayalite is due simply to the crystallization sequence in the parent. With hypersthene as the only pyroxene in the presence of magnetite, the residual liquid would have high Fe2+/Mg and low fugacity. Thus, it appears that fugacity is not a direct function of water content, consistent with work done in this arc and a large well known volcanic center (Grocke et al., 2016; Jolles and

Lange, 2019). An interesting point here is that calculated models suggest that apatite crystallization mainly occurred during later stages of evolution, EDS data suggest these apatites are high in halogen content (like biotite) and would further reduce the magma. It is duly noted that major element data for these rocks is likely affected by carbonate contamination and may provide unreliable data. However, the contamination of carbonate would only pull down

FeO/CaO values, indicating that this data is robust. Further evidence for the control of

25 crystallization on fO2 is that the enrichment of iron specifically, as opposed to other generally compatible major elements, is consistent with the crystallization of hypersthene, high An plag, and ilmenite with magnetite (Figure 6). This is consistent with fayalite being found generally in continental settings only, where hot basalts are fractionating forsterite and abundant orthopyroxene. It appears that the phase, and thus oxidation state of upper crustal differentiated magmas, may be purely due to the parental assemblage (disregarding crustal melts).

We must also consider the possibility of significant crustal contamination. Sr/Y and Sm/Yb are often used as indicators to the phases that were important during petrogenesis (Kay et al.,

2010), with higher Sm/Yb indicating a transition from Cpx, to amphibole, to garnet with increasing crustal depths. Sr/Y decreases for upper crustal compositions that have undergone significant plagioclase fractionation. Relative to other APVC andesites in the supplementary dataset used here and compositions reported by other workers (Godoy et al., 2019), the

Caspana andesite seems to have a relatively felsic signature (Figure 18). While this may seem to be relatively obvious from the whole rock and phase chemistry that has been discussed so far, a critical point is that this andesite appears to bear its own unique signature. It is proposed that this is due extensive interaction with a felsic crustal composition such as a tonalite or granodiorite, consistent with the xenoliths that are found in thin section. Dehydration melting experiments of these compositions also produced abundant orthopyroxene, andesine, biotite, and ilmenite with a rhyolitic residual liquid at mid to shallow crustal pressures (Patiño Douce,

1997). Orthopyroxene has also been shown to be more dominant, along with increasing plagioclase, as melts become lower in fO2 which would occur during crystallization of these glomerocrysts (Martel et al., 1999; Sisson et al., 2005). It is important to note that even if a biotite (and/or amphibole) bearing granitoid were assimilated into the andesite, this would most likely not significantly effect the fO2 due to the arguments above and the work of Grocke et al.

(2016). Again, preliminary isotopic data is inconclusive, but suggests that the reservoir may

26 have interacted with a granodioritc crust. Without further evidence it is inconclusive on the exact fO2 conditions that characterized this reservoir, though it is clear that the production of granitic magmas similar to the Caspana is likely a product of fractionation combined with assimilation.

5.4 Mobility: Disturbing the Mush and Mixing of Rhyolite

Magmatic degassing and second boiling have been thoroughly investigated yet remain poorly understood (Cashman and Mangan, 1994; Blower, 2001). At the macroscopic and micro scale, there is textural evidence that the andesites resided in two different environments (Figure 3,19).

The more mafic, glomerocryst bearing samples generally have more convolute vesicles in both hand sample and thin section, along with a higher proportion of fractured and embayed crystals.

These observations are consistent with work that shows dense crystalline mushes will act as a gas trap until sufficient overpressure develops to disrupt the rigid framework and cause eruption

(Ruth et al., 2016; Okumura et al., 2019). The dense mush that would be present as indicated by large degrees of crystallization likely became remobilized when the chamber experienced an overpressure, causing trapped gas bubbles to expand. Yet, the absence of abundant microlites shows that decompression and extensive undercooling likely didn’t occur rapidly enough to promote nucleation rather than growth (Hammer and Rutherford, 2002).

Meanwhile, the slightly more differentiated group 2 andesites have a lower proportion of unfractured crystals, contain 2-5% microlites (as opposed to virtually none), and slightly less convolute vesicles (Figure 19). Following the logic above, it appears that these pumices experienced somewhat rapid decompression and slightly more volatile loss prior to their extrusion from the chamber. This begs the question of how two andesites with similar rheologic properties (i.e. they are subject to mixing thoroughly enough to obscure any chemical distinction) can be brought to the surface near syn-eruptively with preserved chemical and textural distinctions.

27 Fluid dynamical models predict that the onset of mush mobilization can be established by intrusion of more mafic magma with timescales of magma input overlapping those estimated for the APVC flare-up (de Silva and Gosnold, 2007; Burgisser and Bergantz, 2011). The result of this input is to establish a relatively mobile layer underlying the crystalline mush due to conductive growth. When Rayleigh-Taylor instabilities are reached and convection is reestablished, the mush effectively begins ‘unzipping’ and the chamber is free to reestablish fractionation at the base. The authors note that this process is not necessarily an eruption triggering effect, but this is an important inference for this chamber specifically. The renewal of convection at the base will inherently cause crystallization to continue, as the magma is no longer stagnant, thereby creating another residual liquid. A residual liquid rising through the mush due to buoyancy differences will likely assist in remobilizing the mush and any interstitial liquid, thereby allowing mafic magma from different portions of the mush to be primed for eruption. While this mechanism will not necessarily trigger an eruption, the gas sparging and filter pressing can enhance this effect and assist in triggering eruption. This mechanism could have produced either of the two rhyolites, with the plinian magma being the more likely of the two in this scenario based on the calculated rheology presented here. It is also possible that when the andesite was emplaced into the upper crust, it may have flash cooled to produce the plinian, allowing the majority rhyolite to form beneath.

While most of the large chambers in the APVC are proposed to be sourced towards the center of the APMB, the Caspana source is likely to be where the seismic velocities begin to speed up

(Ward et al., 2014; Figure 20). The presumably cooler country rock in this area is not nearly as primed for the long-term storage, as this basement likely has higher thermal conductivity and heat loss to the country rock was likely efficient, compared to the reservoirs that hosted the monotonous intermediates (de Silva and Gregg, 2014). Combined with the fluid dynamical models, rheology, and two-phase flow through the mush described above, the spatiotemporal

28 characteristics of the Caspana ignimbrite clearly promote large volumes of rhyolitic magma and dense fractionation of parental andesite. This evidence and the models provided here support the original adaptation of a shallow cotectic for the fractionation of these rhyolites, though the model is slightly adjusted to the specifics described herein (Figure 21; (Grove and Donnelly-

Nolan, 1986; de Silva, 1991).

6. Conclusions and Further Work

Combined whole rock and phase data provides key insights into the evolution of the anomalous

Caspana ignimbrite. When viewed through the lens of the existing phases, the broader scale evolution that took place within the upper crust becomes more clarified. There are two distinct rhyolites of the Caspana, both of which were produced by fractionation. The two-step process fractionation scheme for the majority rhyolite is also in line with observations made by other researchers who first proposed the ‘crustal ladder’ within mush regions that are primarily vertical

(Cashman et al., 2017). Additionally, crystallization of the observed assemblage can promote low fugacity in the daughter with typical water contents of rhyolites associated with volcanic arcs.

Crystallizing an andesite rapidly would inherently develop a rigid crystalline mush along the extent of the chamber. Magma recharge rates were higher than average in the APVC during the flare-up (0.012 – 0.06 km3/yr ; de Silva & Gosnold, 2007) and available plagioclase data, along with their texture, suggests the chamber experienced relatively frequent recharge and/or thermal disturbances by underplating magmas. This would thereby disrupt the crystalline mush given enough time, but not without the re-establishment of a convecting layer at the base of the mush. Once this layer begins to convect, magma is free to begin the differentiation process again. As discussed, chemical and rheologic data both show that there are two distinct andesites with one being more mafic than the other. It is proposed here that because of the similarities between resident and intruding magma, a similar phase assemblage, itself likely a

29 function of granodiorite or tonalite assimilation, was able to fractionate at the base of the crystalline mush to produce a second rhyolite: the plinian magma. Once this low viscosity, low density magma accumulated enough mass by fractionation processes it would begin to rise, assisting in the remobilization of the rigid crystalline mush and it’s interstitial liquid. During this remobilization, magmatic gasses could begin to exsolve on the abundant oxides which have the appropriate angle to promote exsolution (Gualda & Ghoriso, 2007) and any trapped gas would release from the mush, driving high F apatite and high Cl biotite. The intruding andesite, be it group 1 or 2, was likely the driver of eruption and boiling, as Fe-Ti oxides are out of equilibrium and display exsolution laminae; which are known to re-equilibrate relatively quickly following a mixing event (Venezky and Rutherford, 1999). One rhyolitic pumice in particular (CH19C003-

CL) is nearly identical to the plinian magma. This is attributed to mixing that would have occurred during an eruption consisting of multiple hiatuses, as interpreted by field relationships and models of Trial and Spera (1992).

We propose that the rapid fractionation was assisted by a large thermal contrast between wall rock and andesite at the edge of the APVC. The source of the andesites in the Caspana may be from either the APMB, escaped around the base of the APMB (González-Maurel et al., 2019), or both. The age of the Caspana supports these inferences if the thermal constraints that characterize the Neogene flare-up are considered. As the prior flare up began to wane, the edges of the APVC would begin to cool and solidify first. As the geotherm raised again, the edges of the APMB are the most likely to contain densely solidified plutons that would be remobilized when mantle flux turned back on. It is possible that this eruption records the expansion of the APVC during the beginning stages of the flare-up.

Again, it is proposed that this rapid differentiation process occurred due to rapid crystallization of a shallow andesite intrusion at a depth equivalent to the large monotonous dacitic chambers in the region. There are a few lingering questions that this work has manifested. 1) Why is this

30 andesite with the same chemical signature seen elsewhere in the region concentrated in this eruption? 2) Is the isotopic data from de Silva (1991) indeed accurate and if so, is the phase assemblage due to interaction with a basement granitoid? 3) Are the two groups of andesites recharging the upper crustal chamber from the same mid- to upper crustal source? Answering these questions will require isotopic data, governed by the hypothesis that the chamber was fed from two distinct mid crustal sources. The depth and locality of the eruption are in line with the interpretation that one of these andesites may be derived from mafic input that could not filter into the Alti-Plano Puna Magma Body (APMB) (González-Maurel et al., 2019). Yet the second andesite may be derived from the APMB itself. If this hypothesis proves to be correct, it may be that the Caspana ignimbrite is recording a distinct pulse of the flare-up and expansion of the

APMB. In doing so, this eruption would show that the flare up remobilized and erased its own granodioritic pluton during thermal expansion of upper crustal bodies (de Silva and Gosnold,

2007; de Silva and Gregg, 2014).Thus, isotopic data will likely show extreme assimilation, along with a more mantle like signature in one group of the andesites. In addition to these data, we also seek to derive partition coefficients for this ignimbrite by LA-ICP-MS.

Acknowledgements

A portion of this work was supported by the REU at OSU, which is funded by the National

Science Foundation (NSF OCE-1758000). I would like to thank the VIPER group at OSU for their vast pool of knowledge. Special thanks to Dale Burns for his dedication. I would also like to thank my advisors Shan de Silva and Deron Carter who have been exemplary examples

(academically and otherwise), Andy Barth for his insights, Chris Russo for all the work he has put into him, and Sarah Lapinski for her friendship. Conversations with Jordan Lubbers were instrumental in the solidification of the ideas presented here - I look forward to the work we will do together in the coming years.

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38

Figures – In order of mention in text

B

Figure 1: A. Map showing approximate extent of the APVC, where large ignimbrite sheets were erupted on the plateau during the Neogene Ignimbrite flare up. Inset shows approximate extent in South America. B. Hillshade of the Caspana and the nearby modern stratocones of Cerro de Leon and Toconce, along with the Chao dacite.

A

39

Figure 2: Stratigraphic sections of the Caspana ignimbrite. Image shows the plinian fallout in the more proximal deposits. The dashed line represents the co-plinian ash that separates the fallout from the next flow unit. More proximal deposits are characterized by reworked layers and lithic lenses that represent multiple hiatuses. The fraction of andesitic pumice in the upper flow unit in these deposits increases stratigraphically up, as does pumice size. The largest pumices are found in the ~1m flow unit near the base and contains both rhyolite and andesite.

40

Figure 3: Whole rock samples. A. Group 1 andesite. B. Group 2 andesite. Note the difference in color and crystallinity between group one and two. C. Rhyolite with sheared vesicles. 4. Plinian pumice.

41 Zirc + Apa Fe-Ti oxide Zir Bio

Fay Opx

All Plag

Plag Amph

Figure 4: Petrographic features in the Caspana ignimbrite. A. fayalite with zircon and apatite. B. A micron scale gabbronorite (see text). C. Biotite with touching allanite and zircon. D. Photomicrograph of unknown accessory phase in andesite. Likely to be titanite or epidote. E. A glomerocryst composed of strictly plagioclase. F. Amphibole. G. A large oscillatory zoned plagioclase phenocryst. H. A sieved plagioclase. I. Orthopyroxene with plagioclase showing a cumulate like texture. The annotation refers to a plagioclase crystal that is not part of the convolute textured plagioclase in the center of the pyroxene.

42

Table 1: Sample list including brief descriptions Rock Long Crystallinity Sample Lat (°S) Sample Description Assemblage Type (°W) (%) CH12022- Plinian - - White-gray chalky; aphiric <1 Plag light CH12022- White-gray to pale orange, chalky; Plinian - - <1 Plag dark aphiric CH19C002- Large white, highly sheared plinian; Plinian 22.2588 68.1946 <1 Plag CL aphiric CH19C009- Plinian 22.2588 68.1946 V. small white to gray plinian; aphiric <1 Plag CL 84014- Bio + Fay + Plag + Ilm Rhyolite - - Oxidized xl-poor glassy pumice 3 Caspana + All Bio + Fay + Plag + Ilm 83070 Rhyolite - - Oxidized xl-poor glassy pumice 4 + All Bio + Fay + Plag + Ilm CH12021 Rhyolite - - Oxidized xl-poor glassy pumice 5 + All CH19C003- Rhyolite 22.2588 68.1946 Chalky white-gray pumice, xl-poor 1 Bio + Plag CL Bio + Fay + Plag + Ilm 84014 SDS Rhyolite - - Oxidized xl-poor glassy pumice 3 + All Bio + Fay + Plag + Ilm 83070 SDS Rhyolite - - Oxidized xl-poor glassy pumice 4 + All 84015-Dark Andesite - - Chocolate milk brown Glassy Pumice 25 Plag + Opx + Ilm 84015-Light Andesite - - Gray glassy pumice 25 Plag + Opx + Ilm 84015- Andesite - - Chocolate milk brown Glassy Pumice 25 Plag + Opx + Ilm Dark® CH12020 Andesite - - Chocolate milk brown Glassy Pumice 30 Plag + Opx + Ilm + Hbl (2) CH19C006- Andesite 22.3446 68.2269 Gray glassy pumice, abundant lithics 35-40 Plag + Opx + Ilm CL CH19C007- Fresh gray to black, glassy Andesite 22.345 68.2256 25 Plag + Opx + Ilm CL glomeroporhpyritic 83069 SDS Andesite - - - 30 Plag + Opx + Ilm 84015 SDS Andesite - - Gray glassy pumice - -

43 Table 2: Representative Whole rock and glass analysis Sample: CH12022-light 83070 CH12020(2) CH12020(2) 83069 Lithology: Plinian Rhyolite Andesite Andesite Andesite Whole Whole Whole Glom Analysis: Glass Glass Glass Glass Rock Rock Rock Inclusion XRF

SiO2 73.80 70.08 74.67 73.30 57.62 65.59 66.30 TiO2 0.20 0.17 0.09 0.07 0.58 0.53 0.34

Al2O3 15.70 14.53 13.13 12.96 19.18 15.37 15.61 FeO* 1.24 1.07 1.56 1.32 3.43 3.68 3.99

MnO 0.08 0.08 0.04 0.03 0.07 0.04 0.09 MgO 0.29 0.27 0.35 0.06 2.03 0.95 0.47

CaO 1.41 1.38 1.26 0.84 12.34 3.19 3.07 Na2O 3.11 3.46 3.17 2.44 2.52 2.97 3.04 K2O 4.12 3.81 5.70 5.89 2.03 3.47 2.92 P2O5 0.05 0.09 0.02 0.03 0.19 0.40 0.31 Cl 0.11 0.10 0.05 0.05 SO3 0.03 0.01 0.01 0.01 Total 100.00 94.93 100.00 96.93 100.00 96.20 96.14 A/CNK 1.29 1.18 0.96 1.09 0.67 1.07 1.14 A/NK 1.64 1.48 1.15 1.25 3.02 1.78 1.91 Mg# 31.66 33.13 30.98 8.77 53.91 33.92 19.04 LA-ICP- MS La 40.42 40.44 37 24.08 37 Ce 75.15 83.20 63 48.06 74 Pr 8.89 9.82 5.88 Nd 31.30 37.26 22.72 Sm 5.62 8.15 4.62 Eu 1.13 0.76 1.41 Gd 4.31 7.51 4.18 Tb 0.68 1.27 0.66 Dy 3.95 7.95 3.87 Ho 0.74 1.53 0.77 Er 1.93 4.14 2.03 Tm 0.30 0.60 0.30 Yb 1.83 3.60 1.75 Lu 0.28 0.56 0.26 Ba 964.84 1085.57 1094 622.53 834 Nb 15.27 12.55 14 8.55 15 Y 20.12 40.86 42 19.80 34 Ta 1.14 1.06 0.64 Pb 22.77 27.04 24 13.58 24 Rb 128.28 180.28 168 69.41 123 Sr 210.69 146.61 158 486.54 356 Zr 147.23 126.71 128 125.10 238 Ga 16.86 16.83 19.25

44

Figure 5: TAS diagram of the samples in this study. Note that the glass falls along the continuum between andesite and both rhyolites. The andesites of the Caspana appear to be particularly mafic, abundant, and are found relatively easily in the field compared to other eruptions. The plinian and bulk rhyolitic pumice are distinct, as reflected by their mineral assemblages.

45

Figure 6: Iron indices show that the bulk rhyolitic pumice is enriched in iron regardless that it was more polymerized (i.e. low Fe3+/Fe2+). Low TiO2 values indicate ilmenite had a less dominant influence on the plinian magma. FeO/MgO values for the two rhyolitic melts are similar, highlight the importance that it is truly the ratio of Fe2+/Mg that is important for stabilizing the observed phase assemblage. High field strength elements distinguish some differences between the rhyolitic magmas and the rest of the APVC. Zirconium values can be explained by fractionation of zircon for the bulk rhyolitic pumice, but not the plinian magma. Yttrium values appear to be inexplicably high in both the andesitic glass and bulk rhyolitic pumice, though the plinian appears substantially off-trend. Niobium values indicate more fractionation of Fe-Ti oxides for the rhyolitic magma than the plinian.

46

Figure 7: Whole rock REE patterns normalized to C1 chondrite after McDonough & Sun (1995). Andesite values are typical for andesites in the APVC. Plinian pumice has the highest La/Yb with a break in slope occurring in the MREE while the bulk rhyolitic pumice is enriched through HREE. All La/Yb values displayed here are averages.

Table 3: Representative Plagioclase analysis Lithology Andesite Andesite Rhyolite Rhyolite Sample 83069 83069 83070 83070 Position Core Rim Core Rim SiO2 46.38 46.48 55.87 58.27 Al2O3 32.25 32.29 25.86 24.08 FeO 0.31 0.30 0.20 0.17 CaO 16.13 16.08 8.49 6.43 Na2O 2.09 2.30 6.16 7.13 K2O 0.08 0.08 0.44 0.65 Total 97.24 97.53 97.02 96.73 An 80.62 79.07 42.11 28.70 Ab 18.90 20.47 55.29 66.90 Or 0.48 0.47 2.60 4.40 Oxides in wt %

47

Figure 8: Transects through 3 fayalite grains in 5-micron steps. MgO values begin to develop the same profile towards the rim of the fayalite grains. The spikes and dips appear to be more conformable to one another towards the edges of the grains, suggesting the grains were exposed to the same environment leading up to eruption.

Table 4: Representative fayalite analysis Sample: 83070 83070 CH12021 CH12021 Crystal: Fay_1_2 Fay_1_1 Fay_1_1 Fay_1_5 Position: Core Rim Core Rim SiO2 30.0953 29.891 30.1354 30.8303 TiO2 n.d. n.d. 0.0049 n.d. Al2O3 n.d. n.d. 0.0041 0.1302 FeOt 64.5809 64.6693 64.4701 63.8103 MnO 1.3757 1.4321 1.3121 1.3119 MgO 3.5857 3.5292 3.5518 3.5148 CaO 0.0718 0.0535 0.0785 0.1002 Na2O n.d. n.d. n.d. n.d. K2O n.d. n.d. n.d. n.d. Cr2O3 n.d. 0.0052 n.d. n.d. P2O5 0.0432 n.d. n.d. n.d. Total 99.7526 99.5803 99.5569 99.6977 Fo 0.088339395 0.086894 0.0877901 0.0877588 Fa 0.892406733 0.893076 0.8937862 0.893633 Tp 0.019253872 0.020031 0.0184237 0.0186082 Oxides in wt %

48

Table 5: Representative biotite analysis from the Caspana rhyolite Sample: CH12021 CH12021 CH12021 CH12021 CH12021 CH12021 Crystal: 1.2 1.4 2.3 2.4 3.2 3.3 SiO2 33.43 33.7961 33.4388 33.6031 33.8381 33.6347 TiO2 5.2854 5.2606 5.2836 5.3481 5.3559 5.2566 Al2O3 14.2118 14.4242 14.2352 14.2363 14.2861 14.1583 FeO 29.4414 29.2818 29.372 29.7929 29.7588 29.348 MnO 0.1625 0.1577 0.1605 0.1439 0.1365 0.1537 MgO 4.2862 4.1994 4.2996 4.173 4.2881 4.2122 CaO 0.0182 0.0078 0.0185 0.0437 0.0053 0.0396 Na2O 0.5701 0.5778 0.6015 0.588 0.6234 0.5345 K2O 9.0696 9.1459 8.9559 8.915 8.9835 8.9816 Cl 0.265 0.257 0.2615 0.2554 0.2556 0.2428 Total 96.7402 97.1083 96.6271 97.0994 97.5313 96.562 Fe/(Fe+Mg) 0.794775423 0.797214972 0.793879914 0.801005134 0.796447064 0.797088008 Recalculated on the basis of 11 oxygens, assuming all Fe is in the m site

Table 6: Ilmenite analysis Lithology Andesite Rhyolite Sample 83069 83070 SiO2 0.1 1.86 TiO2 51.1 49.07 Al2O3 0.17 0.42 FeO 42.84 42.49 MnO 0.51 0.69 MgO 3.54 0.35 Total 98.26 94.88

Oxides in wt %

49 Figure 9: Ternary diagrams showing composition of plagioclase in both endmembers along with fayalite and pyroxenes. There is a lack of pyroxenes bearing primarily iron in the andesite while fayalite is stabilized in the bulk rhyolitic pumice. Core and rim compositions on plagioclase, from de Silva (1991), overlay one another in the andesite and rhyolite.

A B Cores Rims

Figure 10: Data for orthopyroxene in the andesite. Group 1, 2, and 3 are phenocrysts; group 4 are glomerocrysts. Filled dots C are cores and crosses are rims. A) The orthopyroxene cores essentially span the range of all compositions, indicating that they may have nucleated under different conditions. There is a large spike in rims, indicating that many pyroxenes attempted to equilibrate in isothermal and isobaric conditions. However, some rim compositions also show the most MgO rich composition, indicating disequilibria. B) We can see that after defining the grains texturally, they also correspond with one another and amongst groups chemically. Almost all grains appear to be equilibrating to the conditions that the glomerocrysts are in as they continued to grow from core to rim. MgO rich rims are from the groups where growth initiated in a more mafic melt. C) Calcium versus Mg# shows that thermal cooling likely occurred rapidly as the glomerocrysts began to crystallize. See text for details

50 Table 7: Calculated intensive parameters Andesite Rhyolite Plinian Reference Method Opx-Glass Ol - Glass P (MPa) 271 ± 0.26 Putirka 2008 T (°C) 979.67 ± 48 845.73 ± 0.36 Putirka 2008 Method Plag-Glass Plag-Glass H2O Wt % 4.62 ± 0.32 5.48 ± 0.32 Lange et al. 2009 Magma Density (kg*m^-3) 2484.25 2155.83 2147.69 Wohletz 2002 log Melt Visc. (Pa*s) 2.71 4.39 4.14 Giordano et al. 2008 log Magma Visc (Pa*s) 3.32 4.46 4.16 Spera 2000 Fragility 28.15 17.6 18.55 Giordano et al. 2008 T (Glass Transition) 714.21 621.53 645.93 Giordano et al. 2008

Figure 11: Representative Rhodes diagrams (Rhodes et al., 1979) for fayalite and orthopyroxene. Almost all glass analysis give the same equilibrium estimate for the orthopyroxenes. Most of the fayalite-glass pairs fall to the right of what is displayed here.

51

Figure 12: Rheology calculations for the magmas in the Caspana. The plinian is assumed to have equivalent temperature to the rhyolite or higher in these cases. If it is indeed higher temperature, it would likely behave independently of the majority rhyolite.

52

Figure 13: A,B) Mixing of two different andesites seen in major element space. The dominant andesite (group 2) likely recharged the upper crustal reservoir periodically. This would be followed by extensive fractionation prior to another recharge event, thereby enriching iron in the bulk rhyolitic pumice relative to the plinian magma. Meanwhile, lower titanium shows that extensive fractionation of the critical phase ilmenite took place in production of the bulk rhyolitic magma. C) As expected, plagioclase fractionation is ubiquitous and is clearly seen to be extensive in the rhyolite, consistent with the observations of titanium depletion. D-F) Selected trace elements showing that the andesites can produce the observed rhyolites by crystal fractionation. Group 1 andesites are capable of producing the plinian, while group 2 andesites appear to have produced the majority of the silicic magma in the system.

53 Figure 14: Constrains on the andesites within the Caspana. They display particularly mafic signatures with respect to the APVC. The Dy/Dy* chart puts most andesites from the APVC just below the rhyolitic pumice, and they lie to the right (~100ppm Rb) in Sm/Yb space. Ce/Nd ratios show that the plinian also shows influence of amphibole (after Burns et al., unpublished). See text for details.

54

50 um

Figure 15: Forsterite (?) inclusion in an orthopyroxene. Scale is 50um.

55 Table 8: LSQ results and fractionation models for the scenarios described in the text. Model 1 is andesite to andesitic glass. Model 2 is andesitic glass to rhyolite. Model 3 is andesite to plinian. Values below show less granular models for the REE arrays

Model 1 Parent Daughter Fractionated Phase (%) F ΣR2 CH12020- 45.1 Andesite Glass Plag 83069 Rim (1st) Ilmenite 83069 1 0.3917 2opx1_rim3 3 CH12020- Bulk CH19C006-CL 81.77 0.58 17.65 2_glass4¥ D Rb 70.9 123 0.161 0.013 0.013 0.13 Sr 524.09 356 2.22 0.013 0.013 1.8 Ba 554.84 834 0.273 0.013 0.13 0.24 Y 20.47 27 0.0661 0.000454 0.461 0.14 Model 2 Parent Daughter Fractionated Phase (Modal %) F ΣR2 83070 Ilmenite 83069 Avg. CH12020- 30.3 Glass Rhyolite PEDTimag± 0.4952 plag core 1° Apa.° 2opx1_rim3 7 CH12020- Bulk 83070 82.07 1.27 0.84 10.4 5.43 2_glass4¥ D Rb 123 180.28 0.465 0 0.48 0.053 0.011 0.39 Sr 356 146.61 2.846 0.4151 24.339 0.027510 0.0771 2.55 0.0097 Ba 834 1050.84 0.75 0 0.002710 0 0.57 9 Y 27 47 0.387 0.3151 309 0 0 0.57 Model 3 Parent Daughter Fractionated Phase (%) F ΣR2 Plag CH12020- Avg. Avg Avg 48.5 Andesite Glass 83069 Rim CHA973parg± 0.0069 2opx6_rim2 Mag Apa Titanite 1 (1st) CH19C002- Bulk CH19C007-CL¥ 72.07 5.09 2.75 16.65 0.91 2.53 CL D Rb 79.4 131.6 0.161 0.0621 0.151 0.43 0 0 0.19 1.35 Sr 502 213 2.4111 0.15511 0.111 0.5113 2716 2.55 14 0.31 Ba 581 968 0.273 0.13 0.2612 0.09966 0 0.22 5 Y 22.42 20.18 0.0661 0.461 0.641 1.4713 0 3016 1.09

¥: Trace Elements from de Silva (1991); ± Composition from Schmitt et al. (2001); ° D values from low silica rhyolite; 1. Ewart & Griffin (1994); 2. Dunn & Senn (1994); 3. Bacon & Druitt (1988); 4. Nielsen et al. (1992); 5. Dudas et al. (1971); 6. Philpotts & Schnetzler (1971); 7. Schmitt et al. (2001); 8. Mahood & Stimac (1980); 9. Brophy et al. (2011); 10. Nagasawa & Schnetzler (1971); 11. Okamoto (1979); 12. Luhr & Carmichael (1980); 13. Sisson (1994); 14. Watson & Green (1981); 15 Luhr et al. (1981); 16. Olin & Wolff (2012)

56

Figure 16: Less granular, more typical models for testing fractionation are displayed to the left on the REE diagram. Gray fields show range of modelled values, as derived from all andesites.

57 Table 9: Bulk D values for the LSQ REE model La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Andesite to Rhyolite 0.49 0.42 0.55 0.53 0.44 1.90 0.48 0.42 0.26 0.26 0.35 0.28 0.32 0.22 Andesite to 1.52 1.09 1.05 1.10 1.06 0.97 1.09 1.16 1.02 Plinian 0.40 0.48 0.55 0.70 0.87

Values for the model are as in table 9, with the addition of Tiepolo et al. (2002) and Prowatke & Klemme (2004) for titanite partition coefficients. LSQ tables are supplementary and are presented at the end of the document.

Figure 17: Nb/Ta normalized to C1 chondrite. The data suggest that the ratio decreases, likely due to fractionaton of Cpx as magmas undergo their rise from a lower crustal MASH zone into the mid and upper crust. There is a stark kink in the chart where the ratio begins to raise again, likely due to amphibole influence in the mid-crust, by either interaction with basement amphibolite or fractionation of high Mg amphibole.

58

Figure 38: The Caspana andesites display an upper crustal, felsic signature here. Note the plinian only retains a higher pressure signature due to amphibole influence.

59 Figure 19: Upper left – Phenocryst from the group one andesites, which are typically more embayed compared to phenocrysts in the group two andesites (above). Lower left – an example of a thin section scan from group one andesites, which typically displays large, convolute vesicles that sometimes appear to have broken apart micron scale gabbronorites.

60

Figure 20: Velocity models from Ward et al. (2014), showing that the likely source of the Caspana is in the region where seismic velocity beings to speed up. Depth constraints place the chamber above and near the edge of the APMB, where cooling and fractionation would be particularly intense during waning stages. Pin shows where the plinian outcrops near the town of Caspana.

61

Figure 21: Adapated from de Silva (1991) and Grove & Donnelly-Nolan (1986). Amphibole and accessories have a significant influence on a shallow cotectic and the key to causing this in the Andes appears to be the occurrence of plag, 1 pyroxene, and ilmenite. Temperature estimates are used tentatively here.

62

Supplementary Tables

Supplementary Table 1: Tables for the bulk D values on the REE chart. Andesite to bulk rhyolite

Model 1 Parent Daughter Fractionated Phase (%) F ΣR2 Plag 83069 Rim CH12020- Ilmenite Avg. Avg. Andesite Rhyolite (1st) 2opx6_rim2glom 83069 1 Apa. Mag 53.73 0.3691

CH19C006-CL 84014 80.17 16.9 1.86 1.01 0.06

La 25.09123559 40.75 0.28 0.26 0.01 14.5 0.335

Ce 50.3399767 82.76 0.1785 0.31 0.01 21.1 0.27

Pr 6.124337827 9.79 0.17 0.0048 41.33

Nd 23.78399009 37.54 0.199 0.049 0.01 32.8 0.4

Sm 4.841122779 8.22 0.06 0.46 0.01 31.4 0.42

Eu 1.578558786 0.79 1.12545 0.245 0.01 25.5 0.32

Gd 4.318732922 7.63 0.0415 0.0388 0.0077 43.9 0.0055

Tb 0.690484724 1.29 0.0505 0.024 0.01 34 0.52

Dy 4.060357957 7.91 0.06 0.5485 11.65 0.51

Ho 0.803017357 1.57 0.063 0.41 0.13 13.3 0.0079

Er 2.129997909 4.21 0.034 0.548 22.7

Tm 0.294704232 0.62 0.0855 1.09 0.1 3

Yb 1.80696613 3.71 0.16 1.56 0.0255 15.4 0.49

Lu 0.271455108 0.56 0.1 1.9 0.453 6.05 0.38

63 Supplementary Table 2: Tables for the bulk D values on the REE chart. Andesite to plinian.

Model 1 Parent Daughter Fractionated Phase (%) F ΣR2 Plag 83069 CH12020- Avg. Avg. Avg. Andesite Plinian Rim (1st) 2opx6_rim2glom Mag CHA973parg Apa. Titanite 48.51 0.0069 CH19C006- CH19C002-

CL CL 72.07 5.09 2.75 16.65 0.91 2.53

La 25.09123559 39.61 0.2171 0.26 0.335 0.165 14.5 2.39

Ce 50.3399767 74.80 0.1785 0.31 0.27 0.22 21.1 4.01

Pr 6.124337827 8.79 0.17 0.0048 0.3 41.33 2.07

Nd 23.78399009 30.92 0.09 0.049 0.4 0.825 32.8 7.48

Sm 4.841122779 5.51 0.1145 0.46 0.42 1.45 31.4 8.89

Eu 1.578558786 1.08 1.12545 0.245 0.32 1.7 25.5 6.85

Gd 4.318732922 4.25 0.0454 0.4725 0.0055 2.0165 43.9 11.9

Tb 0.690484724 0.66 0.15 0.69 0.52 2 34 10

Dy 4.060357957 3.83 0.1515 0.5485 0.51 2.1 34.8 11.3

Ho 0.803017357 0.73 0.063 0.41 0.0079 2.2 13.3 20

Er 2.129997909 1.95 0.034 0.548 2.34 22.7 6

Tm 0.294704232 0.29 0.0855 1.09 1.125 3 30

Yb 1.80696613 1.83 0.16 1.56 0.49 2.29 15.4 17 Lu 0.271455108 0.28 0.1 1.9 0.38 2.3 13.8 13.2

Key

Okamoto 1979 Luhr et al 1984

Schnetzler & Philpotts 1970 Brophy et al. 2011

McKenzie & O'Nions 1991 Watson & Green 1981

Bacon and Druit 1988 McKenzie & O'Nions 1992

Fujimaki 1986 Brenan et al 1995

Dunn & Senn 1994 Irving & Frey 1984

Luhr & Carmichael 1980 Nicholls & Harris 1980

Aignertorres et al. 2007 Sisson 1994

Reid 1983 Marks et al. 2004

Fujimaki et al. 1984 Tiepolo et al. 2002

Onuma et al. 1968 Prowatke & Klemme 2004

Nielsen et al. 1992 Olin & Wolff 2012

Paster et al. 1974

64