# Antisana volcano: A representative andesitic volcano of the eastern cordillera of Ecuador: Petrography, chemistry, tephra and glacial stratigraphy

Minard L. Hall, Patricia Mothes, Pablo Samaniego, Annemarie Militzer, Bernado Beate, Patricio Ramón, Claude Robin

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Minard L. Hall, Patricia Mothes, Pablo Samaniego, Annemarie Militzer, Bernado Beate, et al. | Antisana volcano: A representative andesitic volcano of the eastern cordillera of Ecuador: Petrography, chemistry, tephra and glacial stratigraphy. Journal of South American Earth Sciences, 2017, 73, pp.50-64. 10.1016/j.jsames.2016.11.005. hal-01456878

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1 ANTISANA VOLCANO: A REPRESENTATIVE ANDESITIC VOLCANO OF

2 THE EASTERN CORDILLERA OF ECUADOR: PETROGRAPHY,

3 CHEMISTRY, TEPHRA AND GLACIAL STRATIGRAPHY

4

5 Minard L. Hall, Patricia A. Mothes, Pablo Samaniego, Annemarie Militzer,

6 Bernardo Beate, Patricio Ramón, Claude Robin

7

8 ABSTRACT

9 Antisana volcano is representative of many active andesitic strato-volcanoes of
10 Pleistocene age in Ecuador's Eastern Cordillera. This study represents the first
11 modern geological and volcanological investigation of Antisana since the late
12 1890's; it also summarizes the present geochemical understanding of its
13 genesis. The volcano's development includes the formation and destruction of
14 two older edifices (Antisana I and II) during some 400+ ka. Antisana II suffered
15 a sector collapse about 15,000 years ago which was followed by the birth and
16 growth of Antisana III. During its short life Antisana III has generated ≥50
17 eruptions of small to medium intensity, often associated with andesitic to dacitic
18 lava flows and tephra, as well as with late Pleistocene and Holocene glacial
19 advances. Throughout its long history Antisana's lavas have been characterized
20 by a persistent mineral assemblage, consisting of 30-40 vol.% phenocrysts of
21 plagioclase, both clino- and orthopyroxene, and Fe-Ti oxides, with rare
22 occurrences of olivine or amphibole, frequently in a microcrystalline to glassy
23 matrix. This uniformity occurs despite the magma's progressive chemical
24 evolution over ≥400 ka from early basic andesites (53-58 wt.% SiO2) to
25 intermediate and Si-rich andesites (58-62% SiO2), and recently to dacites (63-
26 67% SiO2). Chemical diagrams suggest that crystal fractionation was the most
27 likely magmatic process of evolution. The exception to this slowly evolving
28 history was the short-lived emission at ~210 ka of the Cuyuja lavas from
29 Antisana II that generated a 73 km long andesitic lava flow. Contrasting with
30 Antisana's general magmatic trend, Cuyuja lava (~11 km³) is a high-Mg
31 andesite with unusually high concentrations of incompatible elements. Antisana
32 developed within the Chacana caldera complex, a large active siliceous center
33 that began ~3 Ma ago, however its lavas are chemically distinct from coeval

34 lavas of Chacana.

35 Key Words—Long-lived evolving andesitic volcanism, Antisana, Northern

36 Volcanic Zone

37 M. Hall, P. Mothes, P. Ramón

38 Instituto Geofísico, Escuela Politécnica Nacional

39 Casilla 1701-2759, Quito, Ecuador

40 P. Samaniego, C. Robin,

41 Laboratoire Magmas et Volcans, Université Blaise Pascal - CNRS - IRD,

42 Campus Universitaire des Cézeaux, 6 Avenue Blaise Pascal, 63178 Aubière,

43 France

44

45 B. Beate

46 Dept. Geología, Escuela Politécnica Nacional

47 Casilla 1701-2759, Quito, Ecuador

48

49 A. Militzer

50 Institute for Geosciences, Johannes Gutenberg Universität

51 Mainz, Germany

52

53 Corresponding Author: Minard Hall (volcan_pete@yahoo.com)

54

55

56 **1. INTRODUCTION**

57 Antisana Volcano is a large glacier-clad strato-cone in Ecuador's Eastern

58 Cordillera, 50 km SE of Quito, Ecuador's capitol city. Along with Cayambe,

59 Cotopaxi, Tungurahua, and Sangay volcanoes, Antisana is one of the most

60 important strato-volcanoes that comprise the andesitic volcanic row of this

61 cordillera (Figs. 1a, 1b) (Hall et al., 2008). The Ecuadorian volcanic arc consists

62 of two parallel mountain ranges, the Western and Eastern Cordilleras that are

63 separated by an intervening large structural depression known as the

64 InterAndean Valley. The volcanoes of the Western Cordillera represent the arc's

65 volcanic front whose eruptive centers are spaced systematically 30-35 km

66 apart. Their Late Pleistocene to present activity has been silicic andesitic to

67 dacitic in composition. The volcanic centers of the Eastern Cordillera are large

68 15-20 km-wide andesitic strato-volcanoes that are randomly distributed along
and across the breadth of that cordillera. These centers have been slightly more active during the Late Pleistocene and Holocene than those of the Western Cordillera, Sangay volcano being the most active (Monzier et al., 1999). Their youngest eruptive products are silicic andesites of normal calc-alkaline affinity (56-63% SiO). During the past 20 years numerous young rhyolitic centers have also been identified along the Eastern Cordillera (Mothes and Hall, 2008). In some volcanoes, such as Cotopaxi, rhyolitic outbursts have been interspersed with normal andesitic activity (Hall and Mothes, 2008a; Garrison et al., 2011).

Despite its location within the rhyolitic Chacana caldera and the recent bi-modal activity of nearby Cotopaxi volcano, Antisana has displayed a monotonous trend of andesitic activity during the Pleistocene, followed by a very late transition to dacitic products in the Holocene. This contribution represents the first major description of Antisana volcano, its volcanic history and chemical evolution, considered to be an excellent representative of andesitic magmatism of the Northern Andean Arc.

Antisana has built its large cone in the SE corner of the Chacana caldera complex (Fig. 2a), a large andesite and rhyolite center whose outer flanks are comprised of ignimbrites, welded tuffs, breccias, and lava flows that form a semi-circle around the 35 km-long caldera-like depression (Hall and Mothes, 2008b). Paleozoic and Mesozoic metamorphic belts comprise the underlying basement. Antisana straddles the NNE-SSW-oriented boundary that separates the metamorphic belts to the E from Quaternary volcanic rocks to the W.

Antisana along with neighboring Cayambe and Cotopaxi volcanoes sits upon the Northern Andean Block (Trenkamp et al., 2002), a small tectonic sliver that is migrating to the NE (Nocquet et al., 2014).

From Antisana's snow and ice-covered summit (5753 m elevation), drainages to the N, E, and S form the Quijos River, a tributary of the large Coca River.

Drainages to the W and SW form the Antisana River, a tributary of the larger Napo River. Both these large rivers are part of the Amazon River system. The volcano's edifice measures ~15 km in diameter and is slightly elongate in an E-W direction. Its steep (25-35°) outer flanks descend to the surrounding glaciated valleys at ~3200 m elevation. However, to the W and SW a broad detrital apron descends from 4400 m to 3900 m, and forms the volcano's base. Glaciers, ice
falls, and ice-covered rock faces characterize most flanks down to 4700-5100 m. During the 1960's glaciers covered ~26 km² and had an estimated volume of 0.8 km³, but during the past decades the ice limit has retreated upslope many hundreds of meters (Hastenrath, 1981; Cáceres et al., 2004), leaving a skirt of glacial debris around the cone's base. The surface waters from this volcanic edifice comprise one of Quito's principal water sources and it would be jeopardized by future volcanic activity.

From fissures located ~15 km to the NW and N of Antisana effusive eruptions occurred in AD 1760 and 1773, however these have more chemical similarities to lavas from Chacana than to Antisana (see below). Humboldt's 1802 sighting of smoke near Antisana remains unexplained. No Antisana ashes are found in stratigraphic sections younger than 800 aBP (this study; Ledru et al., 2013).

Reconnaissance and petrographic studies around Antisana were carried out by Humboldt (1837-38), Wolf (1892), Whymper (1892), Reiss and Stübel (1902), and Meyer (1907). Additional overviews were published by Sauer (1965), Hall (1977), Hall and Beate (1991) and Egred (2010). Volcanic hazard evaluations including a 1:50,000 scale hazards map were presented in Hall et al. (1989). The present study summarizes work that began in 1996 with a 9 day-traverse around the volcano and continued with the subsequent detailed petrographic and stratigraphic work and radiocarbon dating presented here.

122

## 2. OVERALL STRUCTURE AND MORPHOLOGY

Antisana's volcanic edifice is comprised of three prominent ridges of steep rock and ice that radiate from the young summit cone (Figs. 3, 4, 5). To the NE of the cone and forming the northern wall of a large glacier-filled valley is a short ENE-trending ridge that joins the summit cone to the Northeast Peak (~5650 m). This ridge is comprised of lavas and breccias whose layering dips steeply to the N and is thought to be a remnant of the northern flank of the older Antisana II edifice. From the summit cone a second short ridge arcs to the SW to form the South Peak (~5600 m), whose west side drops sub-vertically to the western base of the Antisana edifice. Its east side descends steeply into a cirque-like
133 depression, herein referred to as the south glacial cirque. A third knife-edge
134 ridge, bounded by near-vertical rock cliffs, runs 1.5 km eastwards from the
135 summit cone; its high point is here called the Central Peak (5670 m). This ridge
136 separates the large glacial valley to its N from the edifice's south face, which is
137 comprised of lavas and breccias that are steeply inclined to the S. This third
138 ridge continues eastwards for another kilometer before descending to its next
139 high point, here called the Southeast Peak (4565 m); it is comprised of bedded
140 breccias and lavas that dip to the SE.

141 The large glacial valley that heads on Antisana's summit descends to the E
142 between the ridges of the Northeast and Central Peaks of the old edifice. The
143 valley is ~1.1 km wide, ~2.3 km long, and up to 400 m deep (Fig. 3). Because
144 of its imposing size and central location, the valley's origin by sector collapse
145 seems likely, however a rock slide or a glacial origin cannot be discarded. The
146 lava successions that make up these two ridges dip away in opposite directions,
147 suggesting that the original vent was high above the center of this valley, just E
148 of the present summit cone.

149

150 **3. ANTISANA I**

151 Three distinct eruptive centers or edifices are recognized on Antisana. The
152 early Antisana I edifice is centered upon or near South Peak (Figs. 4, 5), whose
153 structure is that of a steep-sided cone comprised of lava and breccia sequences
154 that are inclined steeply (~35°) away to the SW. Near South Peak's summit,
155 tephra beds fill a bowl-shaped structure, ~300 m across, presumed to be
156 remnants of Antisana I's extinct crater. Immediately E of South Peak exists a
157 cirque-like depression, open to the S and less than 1 km in diameter, where a
158 ≥400 m-thick series of interbedded lavas and breccias are observed (symbol PA
159 in Fig. 4) that dip moderately to the S and SW; the thick series suggests that
160 Antisana I had a long history. Antisana I's interior is extensively altered
161 hydrothermally (Fig. 5). The eastern wall of the cirque is part of the Antisana II
162 edifice and shows no signs of alteration, implying that Antisana I is the older
163 edifice. In conclusion the Antisana I edifice had a small volume (~5.4 km³) and
164 represents the older Antisana edifice.
165

166 **4. ANTISANA II**

167 The Antisana II edifice is centered on the Central Peak or slightly northward. All
168 the N, E and S outer flanks of the present Antisana massif appear to be
169 remnant flanks of Antisana II, as their lava sequences dip away from Central
170 Peak ridge (Figs. 4, 5, 6). The Southeast Peak was a likely satellite vent of
171 Antisana II, as late lavas apparently descended from it.

172

173 **4.1 Principal lava sequences**

174 On the upper S side of Antisana II's large massif, lava flows and breccia units
175 dip steeply southward and are largely covered by glaciers and snowfields. At
176 the base of its steep cliffs a sequence of stacked lava flows and interbedded
177 breccias continues down slope for many more hundreds of meters, of which
178 only a few have been sampled and analyzed. This sequence continues around
179 the E side of the edifice (Figs. 5, 6), where it extends out from under the ice and
180 snow cover and forms cliffs up to 100 m high. Their thicknesses, inclined
181 orientation, and lower elevation would suggest that these sequences belong to
182 an early stage of Antisana II's edifice construction.

183 Antisana's NE flank, dominated by the Northeast Peak, is made up of steep
184 northward-dipping sequences of inter-bedded breccias, tephra, and 6-10 m-
185 thick lavas units. Its lower flanks are also comprised of a thick series of older
186 lava flows, petrographically similar to those observed on the E side. Many are
187 traceable northwards down slope until they end upon the metamorphic
188 basement. The flows are generally 100 to 200 m wide, 5-20 m thick, and up to 4
189 km long. Their compositions (57-60 wt.% SiO2) differ little from lavas of
190 Antisana II's east and south flanks (Figs. 6, 7). The lavas are frequently inter-
191 bedded with breccia units that carry fragments of Antisana II lavas. Late
192 Pleistocene dacitic lavas often override these older lavas in the NNW to NNE
193 quadrant.

194
## 4.2 Younger lavas of the Southeast Peak of Antisana II

On the SE flank of the old Antisana II edifice and overlying the above-described sequences extends a large, slightly-inclined surface formed by another series of lava flows and inter-bedded breccias. At least 12 individual lava flows comprise the top of this series, each about 8 to 20 m thick, accompanied by breccias that range from 5 to 8 m in thickness. This series of SE-dipping lava sheets has its apex on or near the Southeast Peak, its probable source vent. These lavas post-date the south flank succession of Antisana II, as they fill an eroded valley cut into the south flank lavas. They are petrographically similar to most Antisana II lavas.

205

## 4.3 The Cuyuja Lavas

Perhaps the most interesting event in Antisana's history corresponds to the appearance of unusually long, chemically unique, andesitic lava flows, herein called the Cuyuja lava flows. They are traceable from the SE foot of Antisana II's edifice down the Quijos River valley for 73 km (Fig. 7). Along this trajectory the flows have left numerous flat lava terraces, which today are the sites of Cuyuja, Baeza, Borja, Chaco, and Santa Rosa towns, as well as that of extensive flat agricultural land. The Cuyuja lavas are dark gray to black porphyritic andesites with phenocrysts of plag (15-25 vol.%), ol (5-8%), cpx (≤5%), opx (≤1%) in a microcrystalline to glassy matrix (55-60%) that carries parallel-oriented microlites of plag, cpx, and Fe-Ti oxides. Seven to ten distinct lava flows are recognized, apparently coeval in time, as only a trace of river sediment is occasionally observed between the individual flow units. Many have columnar jointing, implying that short cooling periods transpired between some flows. Notably these are high Mg andesites (~56% SiO₂), chemically unlike most Antisana lavas.

An Ar-Ar date of 210±30 ka was obtained for the prominent flow unit near Baeza (Opdyke et al., 2006).

The source of the Cuyuja lava flows lies near the SE corner of Antisana II's edifice, the site of their farthest up-river lava terraces on the Quijos River, where
this lava forms a 300 m-thick terrace. Unfortunately, Cuyuja lavas were not found during sampling on Antisana's upper slopes. At 8 km downstream, where the flows spilled out of the Quijos river canyon into the adjacent valley of Cuyuja town, the combined flows are 320-340 m-thick. At 25 km downstream, they are ~280 m-thick, at 34 km ~200 m-thick, at 66 km ~30 m-thick, and they end at 73 km (Fig. 8). The flows have an estimated surface area and volume of 100 km² and 11 km³, respectively. The great distance traveled by some flows is attributed in part to the relatively narrow (100-500 m-wide) valleys that the flows descended, valley gradients of 0.3% to 0.015%, the flow's notable volume, possibly to rapid emplacement, and/or to favorable rheological properties of the lava (Keszthelyi and Self, 1998).

Where the lavas entered the adjacent Papallacta River valley near Cuyuja town, they dammed that river, resulting in a small lake, which over time developed a 74 m-thick sequence of repeated beds, each 10-20 cm-thick, of lacustrine silts and sands and occasional debris flows. The sands are chiefly comprised of reworked juvenile scoria lapilli and ash that fell into the greater Antisana drainage basin and were remobilized by streams to their final resting sites. The deposit's thickness and repetitive character suggest that Antisana's eruptive activity persisted for considerable time after the lava's emplacement.

245

## 4.4 Radiometric Dates of Antisana II

Several Ar-Ar radiometric dates were provided by J-P. Eissen (IRD) (Table 1). Samples AN45 (58.4% SiO₂), sample ANT60 (58.0% SiO₂), and sample AN51 (59.5% SiO₂), all typical andesites, provide tentative ages of 378±38 ka, 280±33 ka, and 276±22 ka, respectively. These correspond to an approximate mid-way height (4400-4600 m elevation) up Antisana II's southern flank. AN45 belongs to the main core of the edifice, while AN60 and AN51 belong to younger slopes of the main edifice.

254

255
<table><caption>256 Table 1: Ar-Ar age dates of six lavas from Antisana II</caption><thead><tr><td>Sample</td><td>Approximate Location on Antisana II</td><td>Precise Location as UTM PSA 56</td><td>Ar-Ar age (ka)</td><td>Source</td></tr></thead><tbody><tr><td>AN-45</td><td>S face</td><td>82015-994418 4610 m</td><td>378±38</td><td>J-P. Eissen (IRD)</td></tr><tr><td>AN-60</td><td>SW corner</td><td>81713-99429 4460 m</td><td>280±3</td><td>J-P. Eissen (IRD)</td></tr><tr><td>AN-51</td><td>S corner</td><td>82035-994369 4380 m</td><td>276±22</td><td>J-P. Eissen (IRD)</td></tr><tr><td>AN 38</td><td>SE corner</td><td>82216-994582 4520 m</td><td>253±23</td><td>J-P. Eissen (IRD)</td></tr><tr><td>AN 43</td><td>SE flank</td><td>82132-994420 4510 m</td><td>190±23</td><td>J-P. Eissen (IRD)</td></tr><tr><td>Cuyuja Lava</td><td>Baeza</td><td>17494-994922 1880 m</td><td>210±30</td><td>Opdyke et al. 2006</td></tr></tbody></table>

257

258 Lavas that occur toward the SE corner of Antisana II's edifice and that share a similar chemistry (59% SiO₂) give Ar-Ar dates of 253±23 ka (AN38) and 190±23 ka (AN43). This youngest sample from Antisana II's edifice came from outcrops at ~4500 m elevation, about 1200 m short of the south flank's total height; this implies that Antisana II remained active for considerable time after AN43's emission. This last sample has a value similar to the 210±30 ka Ar-Ar date obtained for the Cuyuja lava flow (Opdyke et al., 2006), which had filled an already deeply-dissected valley cut into Antisana II's southern flank by 210 ka, implying that early Antisana II magmatism occurred long before 210 ka. As a consequence Antisana's total age must be considerably older. We estimate that the total Antisana II edifice had a volume of ~33 km³ and an age greater than 400 ka.

270 **4.5 Post-avalanche Lavas**

271 Earlier, in the discussion of Antisana's morphology, a sector collapse was proposed to explain the large valley that heads on the present Antisana summit. 272 This presumed sector collapse of ~0.5-1 km³ volume followed Antisana II's 273
274 formation, whose youngest lavas are less than 190 ka. Two post-collapse lava
275 flows are traceable by their lobate morphology down the amphitheater valley to
276 where they protrude from under the present valley glacier and rest in turn upon
277 older Antisana II lavas. These lavas, ANT 30 and 32, are Si and K-rich dacites
278 (67% SiO2) that represent the most siliceous rocks found on any Antisana
279 edifice. As such, their location and late arrival imply that they are related to a
280 late intrusion of silicic magma that may have led to a sector collapse.
281 Furthermore, these lavas are similar chemically to the Mauca-Machay series of
282 Antisana III's dacitic lavas, further implying that the sector collapse and the
283 dacitic magmatism occurred jointly in late Pleistocene times. Early in post-
284 collapse times the magma source shifted to the Antisana III vent near the
285 volcano's present summit.

286 The timing of the sector collapse is proposed by the glacial morphology. Paired
287 Younger Dryas (YD) lateral moraines are observed in most upper Antisana
288 valleys and their presence would suggest that a sector collapse in the
289 amphitheater valley would have had to happen prior to the YD glaciation. Late
290 Glacial Maximum (LGM) glaciers would have descended this valley earlier.
291 However, while the right lateral moraine of the LGM is clearly present in this
292 valley, as well as at its final terminal moraine farther down valley, its left lateral
293 moraine is not evident. One could argue that the missing left lateral LGM
294 moraine was carried away by the descending avalanche from the sector
295 collapse. If this happened, it would imply that the sector collapse would have
296 occurred sometime between the LGM and the YD glaciations, roughly between
297 ~15 and 11 ka (Clapperton et al., 1997). The youthful age of this scenario would
298 fit in well with the conclusion that the post-collapse lavas of the amphitheater
299 best match the youngest Antisana III Si-rich lavas, thought to be Late
300 Pleistocene in age.

301

302 **5. ANTISANA III**

303

304 **5.1 Introduction**
305 Antisana III is the youngest vent and forms the present summit (5753 m) on the west side of the well-eroded Antisana II edifice (Figs. 1a, 6). Its summit has a flattened conical form, presumably due to glacial-infilling of its crater. All Late Pleistocene and Holocene lava flows originated at this summit and flowed mainly to the N, W, and SW. Young lava flows can be traced up slope under the western and northern glaciers toward the summit, verifying that the summit cone is the present emission vent. Around the western base of the edifice, a large volcano-clastic apron has developed, comprised of thick sequences of tephra, lavas, pyroclastic flow units, debris flow deposits, and their detrital products. Based upon the regional tephra stratigraphy, Antisana III's cone formed during the Late Pleistocene and Holocene. Fumarole vents have not been identified on the edifice, however mountaineers sometimes report the odor of SO₂ gas at the higher elevations. Antisana III was the source of both andesitic and dacitic emissions at the end of the Pleistocene, as both lava types can be traced to the present summit.

320 Antisana's recent history is best recorded in the tephra and lava stratigraphy exposed on the volcano's western slopes. The W-NW side, being proximal to and downwind from the summit crater, displays the most complete tephra sequence of Antisana III, followed by the SW side which tends to have thinner deposits. Two composite stratigraphic columns from the lower NW and SW flanks of Antisana III are given in Figs. 9 and 10, respectively, which present the tephra, glacial, and lava stratigraphy in chronological order. The most complete sequences are found in the Santa Lucia and Quebradilla ravines in the NW sector. The SW composite section is based upon outcrops near the Hacienda Antisana, the Micacocha control gate, the EMAAP water intake, and downstream along the Antisana River (Fig. 4). Four successions of bedded units are recognized in each of the two columns and they are roughly intercorrelated by means of their distinguishing characteristics as well as by guide units whose ages are approximately known. Seven new radiocarbon dates are given in the text or in Figs. 9 and 10; nine others are referenced. Scarcity of datable material limited radiocarbon dating of more eruptive events.

336 In general the displayed successions in these columns are comprised of a variable series of dark andesitic scoria and lithic lapilli and ash layers,
338 occasional pumice tephra beds, lava flow units, and to a lesser degree lahar
339 and glacial-fluvial units. Pyroclastic flow deposits, comprised of small angular
340 clasts of scoria and black lithics in a rosy-colored sandy to gravelly matrix, occur
341 infrequently in the sequences. White to tan-colored distal ash beds as well as
342 gray glacial-fluvial deposits and/or green to maroon-tinted varve layers serve as
343 useful guide units in some successions.

344 Antisana's youngest lava flows are included in the columns in their respective
345 order. They are all black glassy andesites and dacites and occur only on the
346 lower SW, W, NW, and N flanks of the edifice (Figs. 4, 6). They vented from the
347 summit cone and many reached the broad detrital fan at the western foot of the
348 volcano. The lava flows are of the blocky lava type, having craggy surfaces of
349 scoria blocks and lateral levees (Fig. 1). These lavas tend to form discrete
350 lobes, 30 to 50 m-wide, 10 to 30 m-thick, and 1 to 8 km-long. Most were
351 glaciated by Late Pleistocene, Holocene, and alpine glaciations, which greatly
352 subdued their original morphology. In general these lavas share the same
353 mineralogy as that described for Antisana II.

354 The chronological order of the lava flows is based upon criteria such as their
355 location and morphology, their chemical character, tephra stratigraphy,
356 radiocarbon dates, and the Late Pleistocene to Holocene glacial limits.
357 Clapperton et al. (1997) reported that it was possible to place the lower
358 elevation limits of the Late Glacial Maximum (LGM) at ~3500-3600 m, both the
359 Sucus and Younger Dryas (YD) glaciations at ~3900-4000 m, and the Neo-
360 glacial glaciations at ~4200-4400 m in the Ecuadorian Andes. These glaciations
361 are thought to have occurred at ~33 to 15 ka, 13.5 to 12.5 ka, 11 to 10 ka, and
362 in the mid-Holocene, respectively.

363 During the LGM glaciation a contiguous ice cap covered the entire highlands
364 (Clapperton, 1987) and its terminal moraines are found 15-20 km down valley.
365 Such a massive glaciation would have greatly eroded the terrain, as observed
366 on Antisana's older edifices. YD terminal moraines and till are seen at the 3900
367 m contour on the N side of Micacocha Lake and can be traced further to the
368 NW. Neo-glacial moraines occur above the 4200-4400 m elevations on
369 Antisana's western flanks, where they are associated with early Holocene lava
flows. These elevations and the evidence of glacial erosion are also useful criteria for determining age relationships between lava flows. The well-formed, almost pristine-shape of Antisana III's present cone implies that much of its activity postdates these glaciations. Other nearby volcanic centers, such as Sincholagua, were severely eroded by pre-LGM and LGM glaciers which left only rocky ridges and scarps of their previous edifices, comparable to the erosion endured by the Antisana I and II edifices.

377

## 5.2 Stratigraphy

A brief summary of the lithology of each of four stratigraphic successions follows, from the youngest (A) to the oldest (D), as displayed in the two stratigraphic columns (Figs. 9, 10; Table 2). Ten regional tephra guide units are recognized in these successions: the 800 aBP Quilotoa ash fall bed (Mothes and Hall, 2008) and the Guagua Pichincha 1100 aBP ash fall unit (Robin et al., 2010; Ledru et al., 2013). Cotopaxi volcano was the source of the “KA” pumice lapilli fall bed (~2100 aBP) and 3 biotite-bearing rhyolitic ash layers, dated at ~5700 aBP (unit F-5), at ~5940 aBP (unit F-4), and at ~7770 aBP for unit F-2 (Hall and Mothes, 2008a). Also two distinct rhyolitic ash layers from nearby Huañuna volcano, dated at 11,710 aBP and 10,075 aBP, respectively, serve as guide units for the LGM and YD glaciations (Clapperton et al., 1997). In addition two other rhyolitic pumice lapilli guide units from nearby Urcucuy volcanic dome are dated at 6685±30 and 6450±30 aBP (this study).

Succession A, the youngest, is ~5-7 m thick and begins with the present soil (~20-25 cm thick). Overall, this succession is comprised mainly of ashy soils in its upper layers and primary to reworked blackish scoriaceous tephra fall beds toward the bottom. Ten primary scoria or ash fall units are recognized, most of which are thin beds, 6-15 cm thick, of brownish-black scoria and lithic lapilli beds, suggesting relatively small eruptions. Occasionally beige-colored pumice fall units are observed that are apparently related to the young dacitic Mauca-Machay (M-M) lavas. However, two of the youngest ash fall beds (HAL-61) are andesitic in composition (57% SiO₂), implying that andesitic magmas still
401 remained at depth. A reddish ash-rich pyroclastic flow bed also occurs in this
402 sequence.

403 Several lava emission cycles occurred in Succession A. The younger M-M lavas
404 consist of a series of associated flows that cover much of the mid-slopes of the
405 NNE to NNW flanks of Antisana III (Fig. 1). These lavas are black glassy micro-
406 porphyritic dacites (65% SiO₂) with ~20 vol.% plag, 4-7% cpx, 6-7% opx, in a
407 ~65% matrix of glass and flow-aligned feldspar microlites. The more frequent
408 presence of amphibole, albeit often altered, distinguishes these late M-M lavas.
409 Slightly older M-M lavas belong to a widespread series of thick flows that
410 descended from the summit crater and covered the cone's N flank, reaching the
411 Rio Blanco Chico valley (Fig. 4); others traveled down the W side of the edifice.
412 Dark gray to black dacites predominate, bearing 30-40 vol.% plag phenocrysts
413 plus 3-5% of both cpx and opx with traces of ol and amph. These dacites have
414 compositions slightly less siliceous (64% SiO₂) than the youngest M-M lavas.
415 The scoria tephras of these two groups of young and older M-M lavas lie
416 between the Quilotoa and Guagua Pichincha distal ash layers, indicating that
417 Antisana's last lava emissions occurred between 800 and 1100 aBP (Table 2).

418 **Succession B** includes two marker units, the Red Lahar deposit and a coeval
419 lava flow, which lie ~2.2 m below the Urcucuy marker bed. The lahar deposit is
420 generally <3 m thick, poorly sorted, unstratified, and contains red scoriaceous
421 andesitic clasts up to 1.5 m in diameter, although the average clast size is only
422 5-15 cm. The red sandy matrix makes up ~50 vol.% of the deposit. The flow
423 was somewhat viscous as implied by both the incorporation of pods of
424 unconsolidated materials and the deposit's hummocky morphology upon
425 adjacent floodplains. The presence of fresh scoriaceous lava clasts as well as
426 cauliflower-shaped scoria clots of the younger Huagra-Halina (HHJ) lava flow
427 suggest that the Red Lahar deposit was derived from the advancing near-
428 coeval HHJ lava flow which transformed into a lahar along its course, a likely
429 scenario given Antisana's large volume of snow and ice. This lahar deposit is
430 restricted to the Antisana River drainage and is traceable for ≥15 km down river.
431 At 8 km down valley it consists of an 80 cm-thick, rosy khaki-colored, fine to
432 medium sandy deposit, typically overlain by a 2-3 m-thick soil and tephra
433 accumulation. An organic soil at the lahar's base provided two nearly identical
434 $^{14}$C dates of 7400 aBP (this study).

435 The prominent HHJ lava flow is readily visible at the western foot of Antisana III
436 (Fig. 4). It is a silicic andesite (62% SiO₂) that contains small phenocrysts of
437 plag, opx, cpx, and a trace of ol in a glassy matrix. This unglaciated flow
438 impinges upon the older HHV flow, and thus is younger than that flow. A series
439 of scoria and ash fall units as well as ashy soils, probably related to the HHJ
440 eruption, underlie the HHJ lava. Reworked scoria ash beds and fluvial
441 sediments, with few primary tephra units and soils, comprise the remainder of
442 this succession. The dimensions of this and other lava flows are presented in
443 Table 2.

444 The Mud Spring Lava flow is a light gray silicic andesite (62% SiO₂) that
445 overlies the Micacocha lava flows at the east end of Micacocha Lake (Fig. 4). It
446 is a micro-porphyritic rock with few clear plag laths, plus pxo and a trace of
447 altered amph in a vitreous matrix. Given its similar appearance and chemistry to
448 the HHJ lavas, it is considered to be a related lava.

449 The age of Succession B is constrained by the overlying 6450±30 and 6685±30
450 aBP Urcucuy rhyolitic lapilli ash and the underlying Huañuna II ash dated at
451 ~10,075 aBP (Clapperton et al., 1997).

452 <u>Succession C</u> mainly concerns the deposits of the YD glacial event and consists
453 of small till deposits, glacial-fluvial beds, fluvial deposits, and interbedded tephra
454 units. The best outcrop of the YD till is found near the Micacocha control gate
455 (3900 m elevation). YD till deposits in this region are overlain by the Huañuna II
456 ash layer (10,075 aBP), while the beginning of the YD glaciation had been
457 placed at ~10,800 aBP (Clapperton et al., 1997). Often associated with these
458 glacial beds are finely-laminated greenish-grey varve deposits and Fe-rich
459 hardpan layers, as well as oxidized scoria lapilli layers.

460 The pre-YD succession is notable for its lava flows of andesitic composition.
461 The pristine-appearing Yana lava flow (58% SiO₂), which although chemically
462 different than the M-M lavas, appears to be young, unglaciated, and only
463 overlain by thin tephra layers. Other andesitic flows include those of the
464 Micacocha lavas which form two parallel flows and have their termini ~100 m
465 upslope of the present eastern edge of Micacocha lake. There, the terminus of
466 one flow is thoroughly shattered, suggesting that it was quenched upon entering
467 the lake. These two flows are twin coeval flows, being identical gray to black
468 andesites (58% SiO2) with few micro-phenocrysts of plag, cpx, opx, and ± ol.
469 Their matrices comprise ≤92 vol.% of the rock, are mainly glassy, slightly
devitrified, and contain feldspar microlites. These flows also predate the YD
470 glacial event and may be slightly younger than the HHV and Sarahuasi lavas.
471

472 An underlying pre-YD tephra sequence, ~200 cm thick, consists of at least six
473 black scoria ash and lapilli fall beds and their re-worked equivalents which
dominate the sequence in the NW stratigraphic column. These tephra units
474 appear to be related to two glaciated lava flows, the Sarahuasi flow and the
475 older 10 km-long Huagra-Halina flow (HHV) that are found at the western foot of
476 Antisana III (Fig. 4). These are andesitic lavas that share a similar mineralogy
477 (plag, cpx, opx, and ≤5% ol) and chemistry (57-58% SiO2). They mark a change
478 to an andesitic magmatism, similar but not identical to that of Antisana II. These
479 flows predate the YD glaciation and the Huañuna I ash, but postdate a ~13.9 ka
480 dacitic pumice fall (this study).
481

482 A series of older sheet-like lava flows, herein called the Santa Lucia flows,
descended the lower NNW flank of Antisana III and followed the Santa Lucia
483 stream valley for several kilometers. This series postdates the 13.9 ka dacitic
484 pumice fall but predates the Huañuna I ash (11.7 ka). They are dark gray
485 slightly porphyritic andesites (59% SiO2) with small phenocrysts of clear
486 plagioclase and green olivine in a 90 vol.% vitreous matrix (Table 2).
487

488 Toward the bottom of Succession C on the lower SW and W flanks of Antisana,
489 reworked ash beds together with varve layers are more than 3 m thick. The
490 probable glacial origin of the varve beds suggests that they may be related to
491 the small Sucus glaciation (~13.5-12.5 ka) observed in Peru (Clapperton et al.,
492 1997). In addition nine scoriaceous ash fall layers are recognized in the NW
493 sector, where they are interbedded with six beds of beige to cream-colored
494 pumiceous ash. As a consequence this succession is notable for the increase in
495 the number and thickness of primary scoria-lithic lapilli beds which suggests an
increase in the number of powerful eruptive events with short repose intervals.
As observed in Fig. 9, a minimum of 21 eruptions are registered in the 8 m-thick
sequence belonging to the ~4000 year andesitic interval between the basal
dacitic pumice bed described below and the Huañuna II ash, emphasizing the
intense eruptive activity of this period. Clearly the Santa Lucia to Yana
sequence corresponds to an andesitic period that existed from at least 14 ka to
10 ka.

The base of this succession is placed at a 1.4 m-thick coarse pumice lapilli fall
unit that rests upon a ~13,900±440 aBP peat layer formed on a regional LGM
moraine. Clasts of this beige-colored micro-vesicular dacitic pumice range up to
6 cm in size, in addition to the light grey lithic clasts that are ≤2 cm in size. The
chemistry of this amph-bearing pumice (65% SiO₂) is similar to that of the
younger M-M lavas. The fewer varve and glacial-fluvial layers found in this
succession, as compared to the underlying sequence, implies waning LGM
activity.

Succession D provides evidence of eruptive activity that is best observed at the
lower NW foot of Antisana. It consists of ~8 m of scoria and scoria-lithic lapilli
fall deposits, each 10-100 cm thick, of both air fall and reworked origin. They
are comprised of black, brownish-black, or dark grey scoria and lithic lapilli,
however scoriaceous sandy ash layers containing variable amounts of small
black lithic clasts are also prevalent. Invariably these are interbedded with
orange-colored or light gray to tan-colored clayey and silt-rich lamina (varves),
thought to represent late LGM glacial outwash. Erosional unconformities and
evidence of fluvial re-working are often present.

At the base of Succession D in the SW sector, a 2.2 m-thick sequence of
multiple scoria tephra layers overlies a black scoria bomb fall bed (Fig. 10),
which lies in turn upon a LGM tillite. This bed is comprised of bombs up to 10
cm in diameter, values which far exceed the bombs of other studied units in
Antisana's Late Pleistocene-Holocene stratigraphy, making it one of the most
explosive events of this period. Succession D occupies the time interval
between the retreat of the LGM glacier (~15 kaBP) and the 13.9 ka pumice fall
unit.
<table><caption>528 Table 2: Dimensions of the Youngest Lava Flows of Antisana III</caption><thead><tr><th>LAVA FLOWS AT ANTISANA</th><th>AGE CONTROL</th><th>LENGTH (km)</th><th>AREA (km²)</th><th>THICKNESS</th><th>VOLUME (km³)</th><th>SiO (%)</th></tr></thead><tbody><tr><td>Mauca-Machay (M-M) younger lavas</td><td rowspan="2">800 ± 50 to 1100 ± 40 aBP</td><td>7.5</td><td>8.0</td><td>0.04</td><td>0.32</td><td>65.2</td></tr><tr><td>Mauca-Machay (M-M) older lavas</td><td>3.5</td><td>3.5</td><td>0.04</td><td>0.14</td><td>63.9</td></tr><tr><td></td><td colspan="6">6450 ± 30; 6685 ± 30 aBP Rhyolitic Pumice layers from Urcucuy dome</td></tr><tr><td>Guagra-Hialina younger (HHJ) lava ≈ Red Lahar Unit</td><td rowspan="2">~7400 ± 30 aBP</td><td>4.0</td><td>3.8</td><td>0.04</td><td>0.15</td><td>62.3</td></tr><tr><td>Mud Spring Lava - HAL-51</td><td>7.5</td><td>6.0</td><td>0.03</td><td>0.18</td><td>62.0</td></tr><tr><td></td><td colspan="6">Younger Dryas Glacial Period from 10,075 ± 50 – 10,800 ± 100 aBP</td></tr><tr><td>Yana Lava</td><td rowspan="3">Early Holocene ~10,800 to 11,710 aBP</td><td>3.3</td><td>0.7</td><td>0.03</td><td>0.021</td><td>58.6</td></tr><tr><td>Micacocha Lava HAL-44</td><td>8.5</td><td>6.8</td><td>0.035</td><td>0.24</td><td>58.5</td></tr><tr><td>Micacocha Lava HAL-45</td><td>8.5</td><td>2.1</td><td>0.030</td><td>0.06</td><td>58.5</td></tr><tr><td>Sarahuasi Lava</td><td rowspan="2">≥ 11,710 ± 100 aBP</td><td>6.7</td><td>3.3</td><td>0.040</td><td>0.13</td><td>57.8</td></tr><tr><td>Guagra-Hialina older (HHV) Lava</td><td>9.5</td><td>4.2</td><td>0.02</td><td>0.08</td><td>57.2</td></tr><tr><td>Sta. Lucia lavas</td><td rowspan="3">≤ 13,900 ± 440 aBP</td><td>4.5</td><td>4.5</td><td>0.035</td><td>0.16</td><td>59.0</td></tr><tr><td>-- #3 young</td><td>6.3</td><td>9.5</td><td>0.035</td><td>0.33</td><td></td></tr><tr><td>-- #2 mid-</td><td>7.0</td><td>10.5</td><td>0.035</td><td>0.37</td><td></td></tr><tr><td>Dacite Pumice Lapilli Fall Bed HAL-62</td><td>~13,900 ± 440 aBP</td><td></td><td></td><td></td><td></td><td>64.9</td></tr></tbody></table>
532 **5.3 Summary of Antisana III activity**

533 Lava and tephra-producing eruptions dominated the last ~15 ka of Antisana
534 activity, consisting of ≥50 eruptions of small to intermediate intensity of both
535 andesitic and dacitic compositions. The last lava emission happened between
536 1100 and 800 aBP and the last important ash-generating eruptions occurred
537 before 800 aBP. Evidence of eruptive activity prior to and during the LGM
538 glaciations is lacking, undoubtedly erased by LGM glaciations or buried by their
539 deposits. Few pyroclastic flow deposits are observed in the studied sections of
540 the west flank, suggesting that very few ever formed, or have not reached the
541 volcano's lower flanks, or were transformed into lahar deposits. Debris flows
542 were generated by volcanic eruptions but also by the melting of the glacier cap,
543 as well as by glacial and morainal lake breakouts; their deposits are mainly
544 observed in stream cuts on the western fan and as far as 15 km down the
545 Antisana River valley.

546 Marked changes in magma composition distinguish this late phase of activity.
547 The 13.9 ka pumice lapilli fall layer has a dacitic composition (65% SiO2)
548 comparable to younger M-M lavas. Late Pleistocene tephras and lavas have
549 andesitic tendencies (57-59% SiO2), notably between 13.9 ka and 10.8 ka.
550 Their compositions are quite similar to late Antisana II andesites (cf. ANT 38 vs.
551 Yana lava - G-157-483). However, by 7.4 ka lava compositions had gradually
552 changed to silicic andesites (62% SiO2) and then to dacites (63% and later to
553 65% SiO2), especially during the late M-M lava stages. The most dacitic
554 compositions appear suddenly at the end of Antisana II and again during late M-
555 M eruptions. The near-contemporaneous emission of both andesite and evolved
556 dacite lavas during Antisana III times strongly implies the co-existence of two
557 active magma sources, one probably a remnant of Antisana II andesite activity,
558 the other a new more differentiated Antisana source.

559 Total volume of lava flows is estimated at 2.2 km³ for the last ~15,000 years,
560 which represents a minimum rate of production since not all flows were mapped
561 or sampled due to inaccessibility or being covered by moraines and/or later
562 lavas. The dacitic tephra fall of 13.9 ka had the greatest aerial distribution of all
563 young tephra products recognized on Antisana, and its products are found as
564 far as 15 km to the north.

565 Antisana III appears to be a very young volcano, given the apparent young age
566 of the collapse caldera event, given its small cone (0.6 km³) built upon the older
567 Antisana II massif, and given the recent and rapid transition to dacitic
568 compositions.

569

570 **6. Petrography and geochemistry of Antisana Volcano**

571 **6.1 Summary of petrographic characteristics**

572 As a whole, Antisana rocks typically have phenocryst contents of 30-50 vol.%,
573 always carrying plagioclase (plag), clinopyroxene (cpx), orthopyroxene (opx),
574 magnetite ± ilmenite (Fe-Ti oxides), and rarely olivine (ol) and amphibole
575 (amph) as accessory minerals. Distinct groupings of plagioclase are recognized
576 in all Antisana lavas, which help to characterize the various magma series. The
577 1st grouping of plagioclase often occurs as larger (1.2-5.2 mm), resorbed, melt
578 inclusion-rich phenocrysts. The 2nd group is often comprised of intermediate-
579 sized (1.6-2.6 mm) euhedral phenocrysts, with few inclusions and frequent
580 oscillatory zoning. The 3rd group refers to plagioclase microlites (≤0.1 mm) that
581 may be accompanied by pyroxene microlites in the glass-rich groundmass that
582 generally dominates the rock. Phenocryst sizes vary from 0.6-3.0 mm for cpx,
583 ≤1.0 mm for opx, and 0.4-0.6 mm for ol.

584 The andesites of Antisana I have phenocryst contents of 30-45%. Euhedral
585 plagioclase of the 2nd group is very abundant (≤30%) but without notable
586 oscillatory zoning; plagioclase is also present as microlites. 1st group
587 plagioclase is often absent. Both cpx and opx phenocrysts are present in near-
588 equal amounts, totaling 10-15%, but small anhedral crystals of pyroxene also
589 abound in the groundmass. Diamond-shaped olivine may be present but not
590 abundant (≤2%). Minor amounts of Fe-Ti oxides are always present. The glassy
591 matrix, amounting to 55-70%, contains appreciable amounts of plagioclase,
592 pyroxene, and oxides as microlites.
Antisana II lavas are typically two-pyroxene andesites with ~30% of phenocrysts of the 1st and 2nd plagioclase groups. In some cases these lavas lack the 1st groupings of larger, resorbed plagioclase or the 3rd group of plagioclase microlites. The 2nd group of plagioclase is represented by medium-sized euhedral crystals with or without oscillatory zoning. Both opx and cpx are present in approximately equal amounts that total 13-15% of the phenocrysts. The groundmass amounts to 50-55% and generally carries microlites of plag, cpx, Fe-Ti oxides with interstitial partly devitrified glass.

Antisana III lavas generally carry the same mineralogy as that of the Antisana I and II lavas, however the 1st plagioclase grouping is uncommon.

## 6.2 Geochemistry

The succession of Antisana's three edifices recognized in this study allows us to comment on Antisana's chemical evolution. Chemical analyses were published in Bourdon et al. (2002) and new major and trace element analyses are also included in the Appendix, together they comprise 82 whole-rock analyses.

Antisana rocks display a high-K calc-alkaline trend ranging from 53 to 67 wt.% SiO₂. Antisana I rocks are mostly basic andesites (53-59% SiO₂) that define a different trend when compared with Antisana II and III samples (Fig. 12). In fact, at similar silica contents, Antisana I samples show lower concentrations in MgO (and some transition elements such as Ni and Cr) and enrichment in Al₂O₃ (and some incompatible elements such as Ba). In contrast, the lavas of the subsequent Antisana II and III edifices share an overlapping trend with gradually increasing silica contents with time (57 to 67% SiO₂). In fact Antisana III lava flows are characterized by homogeneous dacitic compositions (65-66% SiO₂) that rank among the more differentiated magmas erupted by this volcano. Lastly, the Cuyuja lavas merit special treatment due to their different chemical signature. Although they are roughly contemporaneous with late Antisana II rocks, Cuyuja lavas show an important enrichment in K₂O and in other incompatible trace elements such as Rb, Sr, Ba, La, and Th. Similar compositions have been described in other volcanic systems of the Ecuadorian Eastern Cordillera, namely the Yuyos lava flows of the Chacana volcanic
complex (Chiaradia et al., 2014) or the Cono de La Virgen lava flows of Cayambe volcano (Samaniego et al., 2005).

The proximity of Antisana within the greater Chacana complex might lead us to expect some chemical inter-relationships of these two volcanic systems with time. In Fig. 12 the overall field of Chacana volcanic products is drawn together with the individual Antisana samples. Two divergent trends for most incompatible elements (i.e. K₂O, Rb, Th, La) are observed, showing higher values for differentiated samples of Antisana. In addition, Chacana has a greater diversity of calc-alkaline rock types (from andesites to rhyolites) through its 3+ Ma history, as well as with its coeval life with the younger Antisana system. If we compare these two magmatic systems over the same ~400 ka period, Chacana shows a variable spread of rock chemistry over its 56-77% SiO₂ range, while Antisana shows a more steady, gradual enrichment in SiO₂, K₂O, and other incompatible elements with time. It is interesting to note that nearby XVIII century lava flows (shown in Fig. 4) fall in the Chacana field, which strongly argues against a link between these historic lava flows and Antisana volcano. No where nor in any instance are chemically-similar rocks erupted coevally from these two neighboring systems.

In general the major oxides (excepting K₂O and Na₂O, in Fig. 12) show a negative correlation with increasing silica. Light Ion Lithophile Elements (LILE, e.g. Rb, Ba, Th), Light Rare Earth Elements (LREE, e.g. La, Ce), and some High Field Strength Elements (HFSE, e.g. Nb) show broad positive correlations. In contrast, Sr and the transition elements (e.g. Cr, Ni, V) are inversely correlated with silica. Lastly, Y and Heavy Rare Earth Elements (HREE, e.g. Dy, Yb) display scattered trends with almost no variation with silica increase. These chemical trends are easily explained by fractional crystallization processes of a cumulate comprised of the observed minerals (plagioclase, pyroxenes and some olivine and amphibole). Based on these data, together with Sr-Nd isotopic data, Bourdon et al. (2002) interpreted the overall geochemical variation of Antisana as the result of assimilation-fractional crystallization processes that have occurred during magma transfer and storage in the +50-km-thick Northern Andean crust (Garrison and Davidson, 2003; Bryant et al., 2006; Hidalgo et al., 2012; Barragan et al., 1998). Other processes might also play an important role
657 in magma genesis. For instance, the role of magma mixing in Ecuadorian 658 magmas have been stressed by Schiano et al. (2010), who studied trace 659 element behavior during differentiation and concluded that magma mixing 660 between primitive and differentiated magmas represents a first-order process in 661 arc magma genesis.

662 Bourdon et al. (2002) also discussed the adakitic signature of some Antisana 663 samples (marked by a strong depletion in Y and HREE). These authors 664 proposed that such a signature results from a heterogeneous, variably-enriched 665 mantle source, metasomatized by hydrous fluids and melts derived from the 666 subducted slab (Bourdon et al., 2002; Samaniego et al., 2005; Hidalgo et al., 667 2012). However, other models have been proposed that include deep crustal 668 processes that involve melting and/or assimilation of the thick Ecuadorian crust 669 (Garrison and Davidson, 2003; Bryant et al., 2006; Chiaradia et al., 2014). The 670 interested reader should refer to these studies for further details about the 671 proposed petrogenetic models.

672 In summary, Antisana samples define a monotonous andesitic-dacitic magmatic 673 series with little change throughout its 400 ka lifespan. This behavior contrasts 674 with those of other large long-lasting magmatic systems such as at Cayambe 675 (Samaniego et al., 2005), Mojanda-Fuya Fuya (Robin et al., 2009) or Pichincha 676 (Samaniego et al., 2010) that display notable temporal variations of magma 677 chemistry. In addition, despite its location in the Eastern Cordillera, close to 678 rhyolitic volcanic centers such as at Chacana or Cotopaxi, Antisana rocks do 679 not show a bimodal andesitic-rhyolitic behavior as is the case for the Cotopaxi 680 (Hall and Mothes, 2008a) or Chacana systems (Hall and Mothes, 2008b).

681

682 7. CONCLUSIONS

683 The Northern Volcanic Zone of the Andes encompasses the volcanoes of 684 Ecuador and Colombia. This study of Antisana volcano represents the first 685 modern geological and volcanological investigation of Antisana since the late 686 1890s; it also summarizes its present geochemical understanding and genesis. 687 While many NVZ volcanic centers have been studied to some degree, none has
688 been shown to be the most characteristic of this zone. Here, Antisana is
689 submitted as a good candidate for this distinction. The volcano's development
690 includes the formation and destruction of two older edifices (Antisana I and II)
691 during a 400+ ka period, that witnessed changes from basic andesites to
692 common andesites to Si-rich andesites, and finally during the late Pleistocene to
693 dacites (Antisana III).

694 Antisana I corresponds to a small highly eroded cone of more basic andesitic
695 composition that suffered limited hydrothermal alteration. Antisana II developed
696 contemporaneously in Antisana I's late stages, and continued, forming the
697 largest volcanic edifice by ≥378 ka, but suffered a sector collapse on its NE
698 side, possibly as late as ~15 ka ago. Subsequently Antisana III's small cone
699 developed on Antisana II's west flank and generated ≥50 eruptions of small to
700 intermediate intensity, of both andesitic and dacitic compositions, during its last
701 15,000 years. Much of Antisana III's development is well-recorded in its
702 stratigraphy, comprised mainly of interbedded tephra and lavas with late
703 Pleistocene and Holocene glacial deposits.

704 The three Antisana edifices record more than ~400 ka of magmatic history. The
705 whole rock chemistry of Antisana I is composed of basic andesites (53-58%
706 SiO₂), the rocks of Antisana II are intermediate to Si-rich andesites (58-62%
707 SiO₂), and Antisana III's lavas are comprised chiefly of dacites (63-66% SiO₂).
708 During its long history there were no major changes in its phenocryst
709 mineralogy that consisted of plagioclase, both clino- and orthopyroxene, plus
710 Fe-Ti oxides and occasional traces of olivine and amphibole, despite the
711 magma's progressive chemical evolution from basic andesites to silicic dacites.
712 Antisana's two neighboring strato-volcanoes, Cotopaxi and Cayambe, have
713 had, however, complex petrologic histories due to magma mixing and rhyolitic
714 injections (Hall and Mothes, 2008a; Garrison et al., 2011; Samaniego et al.,
715 2005). In addition, although nestled within the Chacana caldera, Antisana's
716 magmatic activity shows no close chemical relationship to that of Chacana.

717 Contrasting with the singular chemical trends of Antisana's main lavas is the
718 short-lived anomalous Cuyuja event whose lava flows (~11 km³) of
719 homogeneous andesitic lava (56 wt.% SiO₂) are chemically unlike that of other
Antisana lavas. These lava flows, one more than 73 km long, apparently had their origin on Antisana II's upper flanks about 210 ka ago. The Cuyuja lavas have similarities to the high-Mg andesites of Chacana's Yuyos lavas that contain high contents of incompatible elements (Ba, Sr, Th), as well as compatible elements (Ni, Cr), as reported by Chiaradia et al. (2014). They attributed such characteristics to young subduction-related magma genesis.

726

727 ACKNOWLEDGEMENTS

The authors wish to acknowledge the interest, contributions, and friendship of Michel Monzier and Jean-Philippe Eissen, both of the French Institut de Recherche por le Developpment (IRD), who helped to organize and participate in the traverse of Antisana volcano in 1996. Sadly both have died of medical causes in the past 10 years. In addition the authors thank IRD for chemical analyses and petrographic thin-sections, as well as Dennis Geist for many other chemical analyses and Jorge Bustillos for initiating the geologic map design. MH and PM wish to thank Mario Pallares for his endless field support in the Antisana páramos, as well as to the Chagras (Andean cowboys) for their endless help. The SENPLADES project of Ecuador has contributed several radiocarbon dates, as well as to the IGEPN's maintenance of Antisana's monitoring network. We gratefully acknowledge the help of Ings. Pedro Espin and Viviana Valverde in computation, J. Rehda in the field, and Jim Vallance (USGS) for several radiocarbon dates. William Rose and Gerardo Aguirre contributed helpful suggestions.

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849

850 **Figure and Table Captions**

851 Figs. 1a & b: a) Antisana volcano as seen from the northwest. Its summit (5753 m) and

852 crater are covered by a flat glacier. The South Peak, remnant of Antisana I, is to its

853 right. Numerous late Pleistocene and Holocene lava flows cover the northwest foot of

854 the active cone (foreground). (photo: P. Ramón, IGEPN, 2005). b) Map of the

855 volcanoes of Ecuador with Antisana shown in the Eastern Cordillera. The subduction

856 trench of the Pacific Ocean's Nazca Plate is also shown.

857 Figs. 2a & b: a) Location of Antisana volcano within the Chacana caldera complex,

858 showing the caldera limits (white dashed line), its outward-dipping flanks, and the

859 Mesozoic metamorphic basement and boundary (blue). Late Pleistocene lava flows

860 and vents shown in complex. b) Physiographic view of Antisana volcano and its

861 drainage pattern.

862 Fig. 3: In this view from the northeast, Antisana III's summit lies at the head of an

863 amphitheater, shown by a line, thought to be of sector collapse origin. The

864 amphitheater is bounded by the east ridge (ER) and the Northeast Peak (NE), both

865 being remnants of the opposing flanks of the original Antisana II edifice. (photo: P.

866 Ramón, IGEPN, 2002).

867 Fig. 4: Geologic sketch map of Antisana volcano and surrounding areas. See legend

868 and text for description of units. Letters refer to: A=Antisana III crater; S= South Peak

869 (Antisana I vent); C= Central Peak; SE= Southeast Peak; PA=Piedra Azufre site.

870 Numbers refer to sites of stratigraphic columns: 1=Sta. Lucia; 2=Quebradilla;

871 3=Hacienda Antisana; 4=Micacocha gate; 5=EMAAP water intake; 6=Rio Antisana.

872 CUY=Cuyuja lava outcrops-verified; CUY?=unverified. Points × Iv-AN and × Iv-PP refer

873 to Chacana historic lava flows. Coordinates shown as UTM Projection South America

874 1956.
875 Fig. 5: View of the south flank of the Antisana edifice with South Peak (Antisana I) at
876 far left and Antisana III's flat summit and glacier-filled crater behind. Note the glacial
877 cirque in front of the west-east ridge with its Central Peak and to its right the Southeast
878 Peak, both of Antisana II. The hydrothermally-altered core of Antisana I is being eroded
879 by glaciers, leaving yellow till deposits. (photo: P. Ramón-IGEPN, 2010)

880 Fig. 6: Map of the approximate petrologic groupings of lavas according to their SiO₂ contents. The location of Antisana III's crater and the limits of its collapse amphitheater (hachured line) are also indicated. Map shown as Universal Transverse Mercator (UTM) (Projection South America 1956). Northings UTM 9941 to 9951; Eastings UTM 813 to 826. Digital Map Base, Volcán Antisana; Instituto Geográfico Militar, Quito.

885 Fig. 7: DEM map showing a major portion in red of the 73 km-long Cuyuja lava flow, traceable from Antisana II's SE side down the Quijos river valley. Note the prominent lava terraces used as town sites and agricultural lands.

888 Fig. 8: The Cuyuja lava series, consisting of 6 to 7 distinct flows of basic andesite, strongly columnar-jointed, whose total thickness here is ~300 m. (photo: J. Rehda, 2011)

891 Fig. 9: Composite Stratigraphic Column for Antisana III's Northwest Sector

892 Fig. 10: Composite Stratigraphic Column for Antisana III's Southwest Sector

893 Fig. 11: On the extensive Sta. Lucia depositional fan of Antisana's lower northwest flank a thick sequence of scoria lapilli and ash-rich tephras are interbedded with glacial-fluvial sediments and occasionally with lavas. Photo taken at Site 1 -- Sta. Lucia on Fig. 4.

897 Fig. 12: Selected major oxides and minor elements plotted against silica. SiO vs. K₂O classification diagram modified from Peccerillo and Taylor (1976). BA=basic andesites, A=andesites, D=dacites, MK=medium K₂O levels, HK=high K₂O levels. Note that Antisana I, II, and III have similiar differentiation trends, the exception being MgO and Ba plots of Antisana I with separate, but parallel trends. Also note that the Cuyuja samples have distinct groupings, not akin to the Antisana samples. Analyses taken from Bourdon et al. (2002) and the Appendix of this study.

904

905 Table 1: Ar--Ar age dates of several lavas from Antisana II.
906 Table 2: Dimensions and approximate ages of Antisana III's youngest lava flows and

907 associated tephra units

908 Appendix: Table of Geochemical Analyses
<figure><img src="image_3.png" /><figcaption>Fig. 1a</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 1b</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 2a & 2b</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 3</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 4</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 5</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 6</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 7</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 8</figcaption></figure>
Composite Column for NW Sector, Antisana Volcano

<figure><img src="image_12.png" /><figcaption>Fig. 9</figcaption></figure>

Based upon Sections:

Quebradilla- UTM (08)126, (99)481, 4300m

Sta. Lucia-UTM (08)137, (99)504, 4280m

(See Fig. 4)
Composite Column for SW Sector, Antisana Volcano

<figure><img src="image_13.png" /></figure>

Legend for Columns

<figure><img src="image_14.png" /><figcaption>Fig. 10</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Fig. 11</figcaption></figure>
<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>