Journal of Volcanology and Geothermal Research 177 (2008) 812–821

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Journal of Volcanology and Geothermal Research

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Enigmatic clastogenic rhyolitic volcanism: The Corral de Coquena spatterring, North Chile

Stephen Self a,⁎, Shanaka L. de Silva b, Joaquín A. Cortés c

a Volcano Dynamics Group, Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UKb Department of Geosciences, Oregon State University, Corvallis, Oregon 97333-5506, USAc Department of Geology, University of Buffalo, The State University of New York, 876 Natural Sciences Complex, Buffalo, NY 14260-3050, USA

⁎ Corresponding author.E-mail addresses: stephen.self@open.ac.uk (S. Self), d

(S.L. de Silva), caco@buffalo.edu (J.A. Cortés).

0377-0273/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.jvolgeores.2008.01.047

a b s t r a c t

a r t i c l e i n f o

Article history:

We report on the unusual o Accepted 25 January 2008Available online 7 June 2008

Keywords:rhyolitemagmaspatter ringlava fountainingmonogenetic volcanoChile

ccurrence of the products of lava fountaining in a Pliocene calc-alkaline rhyoliticmonogenetic center from northern Chile. Corral de Coquena is a discontinuous ring of lava located in themoat ofLa Pacana caldera (23°27' S, 67°23.5' W), part of the Altiplano-Puna Volcanic Complex of the Central Andes. Thevolcanic structure is composed of a maar-like crater, with an associated pyroclastic (possibly phreatomagmatic)unit, that is overlain by rhyolitic glassy lava ramparts, in which evidence of spatter, agglutinate and clastogenicmaterial is found. Typical explanations for theunusual textures ina rhyolitic lava, suchasperalkaline composition,high volatile content, or superheated magma are untenable in this case. We propose that the most likelyexplanation for this extreme style of rhyolitic volcanism is a combination of moderately high eruption rate andefficient degassing prior to eruption. In the light of reports of several other bodies of fountain-fed silicic magmafrom the UK, US, and Japan, we propose that Corral de Coquena is a rhyolitic spatter ring superimposed upon amaar-like crater. We further propose that pyroclastic fountaining should be considered an end-member of thespectrum of eruptive styles of calc-alkaline silicic magmas, and that Corral de Coquena is a rare example,preserved because of the hyper-arid climate in the Altiplano-Puna Volcanic Complex.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Volcanic edifices composed of spatter, agglutinate, and clastogenic(or fountain-fed) lavas, and related structures, are usually formed bythe eruption of mafic or peralkaline silicic magmas (e.g., Head andWilson, 1989; Stevenson et al., 1993; Stevenson and Wilson, 1997).They are the product of fire-fountains where fallback is sufficientlyhot, poorly fragmented, and of low-enough viscosity to weld uponimpact and form piles of agglutinated spatter around the vent. In somecases thematerial is hot enough to flow after deposition and feeds lavaponds or rootless flows (Wilson et al., 1995; Sumner et al., 2005).Formation of such structures by explosive eruption of calc-alkalinesilicic magma is reported much more rarely as such magmas usuallyexplosively disrupt in the conduit and vent, and the fragmentalfallback is not usually coarse or hot enough, nor is deposited rapidlyenough, to form spatter accumulations. Proximal welded facies ofcalc-alkaline silicic fall deposits have been occasionally recognized(e.g., Sparks and Wright, 1979) but do not usually form constructsaround the vents, whereas welded fall deposits and clastogenic lavaflows are common in peralkaline volcanic systems due to the lowviscosity of the magmas (e.g., Stevenson et al., 1993).

esilvas@geo.oregonstate.edu

l rights reserved.

Calc-alkaline rhyolitic spatter deposits have, however, been reportedfrom sequences with awide range of ages. An ancient example is from aSilurian caldera structure in the Lake District volcanic province, UK(Branney et al., 1992). Extensive Oligocene clastogenic lavas have beenreported from Colorado, USA (Bachmann et al., 2000). Several examplesof Miocene-age fountain-fed rhyolitic lava flows come from the BlackRange of New Mexico, USA (Duffield, 1990; Duffield and Dalrymple,1990) and fromspatter rings surrounding silicic domes in SWIdaho,USA(Godchaux and Bonnischen, 2003). These occurrences, plus therecognition of Quaternary-age spatter deposits in Kyushu, Japan(Furukawa and Kamata, 2004) and the present contribution, suggestthat this eruptive style may be quite commonplace during eruptions ofmetaluminous calc-alkaline silicic magma.

Amongst a wide spectrum of volcanic structures present in theAltiplano-Puna Volcanic Complex of the Central Andes (22° to 24°S; deSilva, 1987, 1989; de Silva et al., 2006) are several that appear to resultfrom rapid accumulation and agglutination of explosively erupted calc-alkaline silicic magma. These include clastogenic lavas and severalunusual silicic monogenetic centers (de Silva et al., 1988; Francis et al.,1989). We here describe one of these, Corral de Coquena (23°27'S,67°23.5'W), which we propose is a rhyolitic spatter ring with minorflows of clastogenic lava superimposed upon a maar-like structure.Further, since formation by a fountainingmechanism clearly requires arestrictive set of physical magmatic conditions, we present a basicanalysis of these. They include magma temperature, viscosity, and the

813S. Self et al. / Journal of Volcanology and Geothermal Research 177 (2008) 812–821

ability for volatiles to permeate freely through the rising magma, incombination with moderate ejection velocities, moderately high orfluctuating eruption rates, and localized, very high accumulation rates.

2. Regional setting and description

Corral de Coquena (Coquena, for short) is a monogenetic volcanolocated on the Tropic of Capricorn in the SE moat of the large(60×35 km), c. 4-Ma-old, La Pacana caldera complex, shown on Fig. 1(Lindsay et al., 2001a,b). Coquenawas first described by Gardeweg andRamirez (1985) and Gardeweg and Ramirez (1987) who suggestedthat it was an exploded lava dome located on an outer caldera ringfault, and who also recognized a surrounding apron of pyroclasticdeposits. The structure does appear to be situated above one of themajor caldera-collapse faults near the base of the topographic rim(Figs. 2 and 3).

Coquena has the form of a slightly oval, semi-continuous ridge ofrhyolite lava-like rock that forms the rim of a shallow, 2.5×2.7-km-widesediment-filled crater (Fig. 3a). The longer axis is oriented NE–SW,parallel to the structural trend of that part of the caldera floor. The rimofCoquena is 4572 m above sea level, while the lowest point on the craterfloor is 4363m. Inner rim heights range from 30 to 210 m, of which thelava-dominatedpart, extending to theouter rampart of lava, ranges from40 to 125 m in thickness, and the depth of the crater relative to thesurrounding surface varies from30 to 80m (Fig. 3b). Coquena is thus notonly formed by a construct of lava-like rock around the edge but also hasa crater in the center that was excavated into the substrate (the localcaldera moat floor) during its formation. In total, the volume of magmaerupted during the formation of Coquena is b1 km3.

Fig. 1. Geological map of the La Pacana area, simplified from Lindsay et al. (2001a), showing tPacana, and some of the intracaldera and pre-caldera lava bodies, including Corral de Coquenblock of the caldera. Villages are shown by dots.

The whole Coquena structure appears to sit atop a localtopographic high in the moat, an uplift ~5 km in diameter fromwhich the underlying strata dip gently in all directions except SE-wards. The general level of the La Pacana caldera floor in this area isapproximately 4250 m while Coquena appears to have formed uponthe uplifted surface at about 4450 m, which is the lowest elevation ofits rim deposits. The crater is believed to have considerable infillthrough mass wasting and was therefore originally deeper than thepresent maximum of 80 m below the local caldera floor (Fig. 3b).

3. Age

Coquena is one of the youngest andmost obvious features in the SWmoat of La Pacana (Fig. 2). The lava body clearly post-dates the maincaldera-forming event and thus the Atana ignimbrite, dated by the K-Armethod at 4.0–4.5±0.5 Ma. It was interpreted to be the result of post-caldera activity (Gardeweg and Ramírez, 1985). A 4.4±0.3 Ma age by theK-ArmethodonCoquenabiotitewas reported byGardeweg andRamirez(1985, 1987) suggesting that its formation followed caldera formationquite quickly. Newer K-Ar ages in the range 3.8–4.2±0.1–0.2 Ma byLindsay et al. (2001a) on biotites fromAtana ignimbrite pumices (but notCoquena) perhaps suggest a slightly younger caldera-forming event, andthus Coquena may be less than 4 Ma in age. On the other hand, it is notclear from field relations whether the later Pampa Chamaca ignimbrite(2.4–2.5±0.4Ma; K-Ar on biotite from the non-welded glassy basal unit;Gardeweg and Ramírez,1985), erupted from the Salar de Aguas Calientesarea in the southern part of La Pacana caldera (Fig. 2), overlies orunderlies the Coquena lava and ash units. It seems likely that if thePampa Chamaca eruption is younger than Coquena it would have filled

opographic rim of La Pacana caldera, present distribution of ignimbrites derived from Laa (see also Fig. 2). The boot-shaped outcrop of Atana ignimbrite (centre) is the resurgent

Fig. 2. Sub-scene of a Thematic Mapper satellite image (false colour composite of bands 7, 4,and 2) showing an ~15×15 km region of La Pacana caldera with Corral de Coquena incenter. Prominent peak to SE of Coquena is Cerro Coquena, a pre-caldera volcanic center partly exhumed by erosion (Gardeweg and Ramirez, 1987). Also labeled are other featuresmentioned in the text. Dashed area delimits that of geological map presented in Fig. 3a. Bold dotted lines are interpreted faults.

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the craterwith the thinwelded ignimbrite veneer that is draped over thesurrounding caldera floor and up the topographic rim in the area (Fig. 2).

4. Stratigraphy and field relations

The stratigraphy of Coquena is apparently simple (Fig. 4) but apartfrom the inwardly draping crater wall lava exposures there are fewvertical incisions through the strata exposing the interior of thestructure due to inefficient erosion in this hyper-arid climate.Following Gardeweg and Ramirez (1987) we recognize two maincomponents: an early pyroclastic deposit from an explosive phase thatformed the crater and a main lava structure that forms the raised rimand lava rampart. However, we interpret an apron of heterolithologicclastic material, which extends 2–3 km around the northern andwestern sides of the crater (Fig. 2), as reworked secondary materialbecause it consists of angular clasts of Coquena rhyolite and sub-rounded clasts derived from the Atana ignimbrite (both ignimbriteand lithic fragments). This deposit was previously thought to be ofprimary pyroclastic origin (Gardeweg and Ramirez,1985,1987) but webelieve it to be stratigraphically above the Coquena pyroclastic layer,described below. There is a possibility that some of the clastic materialwas derived from early phreatomagmatic explosions, and has beenreworked into its present stratigraphic position. This could explain thepresence of Atana ignimbrite clasts in the surficial sediments in thecaldera moat at this location, mixed with Coquena-like rhyolite clasts.

The Coquena pyroclastic deposit is poorly exposed in severaldiscontinuous outcrops. It consists of (possibly) several thin (10–30 cm)poorly sorted ash beds seen on thewestern andnorthern side of Coquenaat a distance of ~5 km from the center of the structure (Fig. 3a). At oneexposure, the bed is composed offine-grained ash at its base but contains

larger, cm-sized juvenile clasts up-section. There is also a concomitantchange in clast character up section from vesicular, moderate density,light-coloured crystal-poor pumice to a dense, poorly vesicular crystal-rich rhyolite typical of the Coquena deposits. The deposit is mostlycomposed of ash-grade material; one 30-cm-thick bed examined hasweak cross-stratification and may be a surge deposit. In summary, thisdeposit has some of the characteristics of silicic phreatomagmatic ash(e.g., Self, 1983) and is thought to be of phreatomagmatic origin.

The main edifice of Coquena consists of two low-profile crescenticrhyolite lavabodies, up to800mwide, around theoval crater, separatedbybreaches to theNNEandS. The lava-like rock is light-coloured, crystal-rich,massive, and variably vesicular with domains of abundant lithophysae.More vesicular zones occur in bands defining crude “bedding” and thevesicles are stretched parallel to the bedding. Dips of bedding vary fromhorizontal to subvertical and are generally inwards on the inner rim andhorizontal or outwards on the outer rim. These structures are especiallywell-preserved on the western and southern rims of the main lava body(Fig. 5). There, particularly around the highest (W) part of the rim, theuppermost lava mantles the rim, showing crude layering and forminglobes down the inner rimwith steep (60–80°) dips into the crater (Fig. 5a,b, f). The outer wall is made of a series (up to 9) of subtle, low-angleramparts consisting of terraces and steps (Fig. 5c), which appear to be theproduct of downslope viscous flow of lava. On the image in Fig. 2, surfaceflow ridges (ogives) can be seen that are the expression of these steps.

On the northern side of the main lava body the outer wall is moredissected than the rest, exposing the upper internal parts in rare places.Here lava with variable dips can be seen in sections through parts oframp structures (Fig. 5d). The rhyolite lava is more massive and glassierin the core of these structures (the “vitrophyre” referred to byGardewegand Ramírez, 1987), and shows abundant large lithophysae and flow

Fig. 3. (a) Geological sketch map of the area around Corral de Coquena. (b) Geological section along the line A–A'. Comment “not exposed” inside crater is because the lava/substratecontact is covered by talus.

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banding. A particularly striking, although not pervasive, characteristic ofthe rhyolite is its clastic appearance in some places. In such locations, ithas the appearance of being formedbyaccumulation of large “flattened”sub-rounded clasts of poorly to non-vesicular lava (Fig. 5e) that haveagglutinated by impact-induced pressure (Wolff and Sumner, 2000).

5. Corral de Coquena rhyolite

The rhyolite lava of Coquena is variably phyric and contains up to20 vol.% phenocrysts, dominantly plagioclase with biotite, minorembayed and resorbed quartz (Fig. 6a), and sparse magnetite andilmenite (Fig. 6b). Vesicularity of the slightly devitrified glass isvariable but the lava typically has a “fibrous” texture, which weinterpret was being produced by extreme flattening and elongation of

vesicles during impact and post-agglutination flowage (Fig. 6b). Thistexture is pervasive in the glass and wraps around the phenocrysts, onthe margins of which undeformed vesicles remain in pressureshadows. Non-juvenile clasts are absent from the lava.

Newwhole-rock chemistry determined using XRF at the Universityof Hawaii is consistent with published data (Lindsay, 1999). Electronmicroprobe was used to analyse the composition of glass in somesamples (J.D. Webster, personal communication, 2002) and also tomeasure volatiles that could affect the viscosity of the melt (e.g. F, Cl).Results (Table 1) show values within normal ranges in a rhyolite. Basedon the TAS classification of Le Bas et al. (1986), the Coquena lava isrhyolitic in composition (Fig. 7). The alkali content is typical of thehigh K calc-alkaline suite defined by the evolved rocks in the CentralAndes: K2O between 4 and 5 wt.%, and Na2O between 3 and 4.5 wt.%.

Fig. 4. Schematic composite section through Coquena units (this sequence is notexposed in the field); total thickness of pyroclastic deposit exposed at 5 km from centreof crater is about 1 m. Top of Coquena lava is not shown at this scale and base is not seenin direct contact with the pyroclastic layer (see comment on Fig. 3b).

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Analyses and thin sections (Fig. 6a, b) show that the samples are ratherfresh, and glass and phenocrysts lack alteration. It is therefore possiblethat the variability in the amount of volatiles (i.e. loss-on-ignition inthe analysis, Table 1) could be a primary feature.

The composition of Coquena rhyolite is typical of the CVZ and the LaPacana suite. Coquena magma was distinctly more evolved than thedominantly dacitic Atana ignimbrite magma and that of theMorro Negropost-La Pacana caldera lava dome located to the north of Coquena(Gardeweg and Ramirez, 1987; Lindsay, 1999). Although Coquena'scomposition is broadly similar to the rhyolitic PampaChamaca ignimbrite,there are clear differences beyond analytical precision (Fig. 7); Coquenarhyolite is slightly more evolved, being distinctly poorer in TiO2, Fe2O3,MgO, Na2O, and P2O5 than Pampa Chamaca ignimbrite pumices. Thepresence of fresh biotite and, perhaps, the high (N3 wt.%) LOI in thefreshest samples (CC2; Table 1), might suggest that the initial watercontent of the magma was ~3–4 wt%. This is not unusually high for arhyolite, and is consistent with the low total of electron microprobeanalysis of glass inclusions (see Table 1; sample 89-CC2gi). Magmatictemperatures derived from ilmenite–magnetite pairs (3 determinationsfrom sample 89-CC2) are in the range the 800±50 °C.

Pumice and poorly vesicular clasts from the upper part of theassociated pyroclastic deposit have an identical mineral assemblage tothat of the lava but an overall lower crystal content. Vesicles in thepumice are generally sub-spherical and donot showsigns offlattening,lending further weight to our interpretation that extreme flattening ofvesicles in the lava was a result of impact and rapid accumulation.Pumice clasts are too small and altered to obtain useful compositionaldata, but, by analogy with other rhyolitic lava-dominated centers inthis region such as the Cerro Chason–Runtu Jarita complex in Bolivia,these early pyroclastic deposits are likely to be more evolved.

6. Discussion

6.1. Evaluation of possible origins

The following features need to be taken into account whenevaluating the origin of Corral de Coquena:

1) There is a crater excavated below the level of the local surface. Inthis regard, Coquena is maar-like and has a gently concave floorfilled with colluvium. Other youngmonogenetic volcanoes occur inthe region, for example, the small basaltic maar of Cerro Overo(Thorpe et al., 1984; de Silva and Francis, 1991) and the HoloceneNekhe Khota and Jayu Khota maars in Bolivia (de Silva and Francis,1991; Davidson and de Silva, 1995). Such features provide evidenceof phreatomagmatic activity despite the hyper-arid climate thathas persisted in the region since the beginning of the Miocene

(Alpers and Brimhall, 1988). Assuming that the groundwater tablehas not changed substantially in the last few million years, andbased on the elevation of nearby water bodies in La Pacana calderamoat (e.g., Salar de Aguas Calientes, Salar de Quisquiro), the top ofthe local water table would have lain at ~150 m below Coquena atthe time of its eruption. Thus, crater-formation by phreatomag-matic explosions could well have occurred, accompanied bydeposition of the bedded pyroclastic layers found to the north.

2) The semi-circular, lava-like pile of vesicular rhyolite that surroundsthe crater shows prominent terraces or ramparts on the outerslopes and part of the upper pile drapes the rim and rests at steepdips down the inner crater wall. Relationships suggest that crater-formation preceeded the generation of the lava pile, because thereis no trace of a cover of pyroclastic material on the lava, and theinward-dipping lava drapes the inner crater wall, suggesting atransition from more explosive eruptive behaviour to lavafountaining.

3) Macro- and microscopic textures in the rhyolite support aclastogenic, agglutinate origin for the lava pile.

If Coquena formed by explosive disruption of a post-caldera dome(Gardeweg and Ramirez, 1987), the remnant “dome” lava might beexpected to have a cover of explosion breccia and the lava would notdrape the rim of the crater. Field evidence suggests an alternativeorigin. We earlier noted that Coquena sat atop a high area in thecaldera moat and one possibility is that this area was pushed up by acryptodome, part of which intersected the water table, thus triggeringventing to the surface and formation of a crater. Another possibility isthat during surface venting from the cryptodome the arcuate lava pilewas formed by effusive eruption along a curved fissure. However, suchan eruptionmechanism does not account for the excavation of a crater,nor do textures and structures in the lava suggest eruption from theaxis of the arcuate pile. Rather, the lava appears to be clastogenic,draping the crater rim, and accumulated in an arc around a centralvent.

Based on the above observations, we suggest that Coquena is amonogenetic feature formed when magma, possibly rising in acryptodome, encountered groundwater, leading to an initial phreato-magmatic explosive phase which created a maar-like crater, followedby a period of lava fountaining. The lava pile most probablyaccumulated by spatter agglutination of dominantly coarse rhyoliticclasts from a large fire-fountain, withmost flowing outwards to form aset of ramparts and some of the later lava flowing and slumping intothe pre-existing crater. This is a common sequence in eruptions ofmafic magma (and more evolved types) where crater-forming,groundwater-magma explosive interaction precedes a dryer phase ofHawaiian, Strombolian, or effusive activity (White and Houghton,2000). Because the field data is limited due to a lack of incision of theCoquena deposits, we do not attempt more elaborate interpretationsof the eruption mechanism, but below we try to constrain themagmatic and environmental conditions that may have led to theformation of Coquena.

6.2. Silicic magmas that produce spatter

Volcanic structures produced by the accumulation and agglutina-tion of spatter are commonly described from eruptions that formbasaltic and intermediate lavas and cones, and from those ofperalkaline silicic magma. Spatter edifices are generally about 1 kmor less in diameter but may be longer if arranged along a fissure vent(e.g., The Sproul, San Francisco volcanic field, USA (Finnemore and Self,1991). Peralkaline silicic magmas display the ability to erupt by fire-fountaining mechanisms and create thick lava flows (Stevenson et al.,1993; Stevenson and Wilson 1997) and spatter deposits (e.g., onTenerife in the Canary Islands: Soriano et al., 2002; Gottsmann andDingwell 2001; Giordano et al., 2000).

Fig. 5. Field characteristics of Corral de Coquena: (a) View of western part of crater capped by steeply dipping lobes of lava on the inner rim, here ~200 m high; (b) closer-up view ofhighest point on rim in (a) showing inward-dipping lava drape; (c) view towards SW from outer slope of Coquena, looking downs across several rampart steps (person for scale inright middle distance). In middle distance is moat of La Pacana caldera and behind, the resurgent block (centre and right/west); Cordon Puntas Negras volcanic chain is in far distance(left/east); (d) bedded Coquena rhyolite lava near to northernmost exposure of lava (by dip symbol on Fig. 3a), where lava dips towards crater at end of ramp structure (beddingdashed on photo); (e) crudely bedded, slightly flattened, and variably vesicular rhyolite spatter clasts at the crest of the northern rim. These show variable flattening ratios with thoseat base beingmore flattened that those at top; (f) view down rim from near highest point (see b) showing strike (dashed lines) of steeply inward-dipping lava layers; (g) view of outerform of Coquena looking NE from below the structure, showing lava feature sitting on top of bulge and step-like form of rampart structures (best displayed on left side). Maximumheight of lava structure is about 200 m.

817S. Self et al. / Journal of Volcanology and Geothermal Research 177 (2008) 812–821

High-K calc-alkaline silicicmagmas are not usually thought to be ableto produce spatter upon disruption due to their high viscosity and highwater content, which promote extreme fragmentation. However, rare

examples of non-peralkaline eruptions producing spatter and associatederuptionshavebeendescribed in the literature (Sparks andWright,1979;Duffield,1990; FurukawaandKamata, 2004) suggesting that under some

Fig. 7. Plot of total alkali vs. silica in wt.% (Le Bas et al., 1986) for Coquena rhyolitesamples (Table 1). Data for whole-rock juvenile clasts from the Atana and PampaChamaca ignimbrites shown for comparison (after Lindsay et al., 2001b); dominantAtana pumice type forms central data point cluster with two outlying groups beingfrom subordinate pumice types.

Fig. 6. Photomicrographs (plane polarized light, 25×) of thin sections of Corral deCoquena rhyolite: (a) Example of a quartz crystal with resorbed rims in a fluidal,stretched vesicle-rich glassy matrix. (b) Example of texture showing extreme flatteningof vesicles. Note wrapping of vesicular texture around crystals of biotite (bt), quartz(qtz), and plagioclase (pl) and development of pressure shadows (ps); sample 89CC-2.

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conditions eruptions of non-peralkaline silicic magma can producespatter, agglutinate, and clastogenic lavas. A spectacular example of calc-alkalinedacitemagma that erupted in anunusual style is thePagosa PeakDacite, which was the preliminary phase of the huge 5000 km3 FishCanyon Tuff eruption from La Garita Caldera in the Central San Juan

Table 1Analyses of Cerro de Coquena rhyolite lava

Sample SiO2 TiO2 Al2O3 Fe2O3 MgO CaO MnO Na2O K2O P2O5 L

89-CC2A 72.25 0.22 13.51 1.21 0.21 1.18 0.07 3.09 4.29 0.06 389-CC2B 72.54 0.22 13.55 1.21 0.20 1.19 0.07 3.17 4.33 0.06 389CC4 73.80 0.12 14.10 0.76 0.05 0.33 0.07 3.00 5.16 bdl 0COQU-96h-1 73.70 0.23 14.20 1.25 0.27 1.15 0.07 4.19 4.26 0.05 0COQU-96h-2 73.60 0.22 13.90 1.24 0.27 1.23 0.06 4.25 4.11 0.05 1COQU-96h-3 74.00 0.23 13.90 1.28 0.26 1.23 0.06 4.31 4.08 0.06 0COQU-96h-4 72.30 0.22 13.60 1.22 0.26 1.17 0.07 3.92 4.29 0.05 289-CC2⁎ 70.20 0.21 13.11 1.17 0.19 1.15 0.07 3.07 4.19 0.06 389-CC2⁎ rec 75.14 0.22 14.03 1.25 0.20 1.23 0.07 3.29 4.49 0.06 089-CC2A (gi) 75.18 0.23 14.05 1.26 0.22 1.23 0.07 3.22 4.47 0.06 •

89-CC2B (gi) 75.13 0.23 14.03 1.26 0.21 1.24 0.07 3.28 4.49 0.06 •

Analysis made by: (ref.)(1) XRF at University of Hawaii, 1996 (T.Hulsebosch, analyst); bdl=below detection limit. • n(2) XRF performed by J.M. Lindsay (pers. comm., 2001).(3) XRF S.L. de Silva, 1999. ⁎same sample as CC2A analysed in different lab. 89-CCR⁎ (rec) re(4) Electron microprobe analysis of glass inclusions in samples 89-CC2A and 89-CC2B by J.DSamples: CC2, Light to dark grey, blobby-appearing lava from N end of spatter-ring-like struCC4, Most layered-appearing lava from NW edge of spatter-ring-like structure in part of a rCOQU-96h-1, Whitish rhyolite lava; lowest sample collected in structure.COQU-96h-2, Grey, medium coarsely crystallized lava with dark matrix.COQU-96h-3, Pinkish, medium coarsely crystallized lava, ? altered and silicified.COQU-96h-4, White, vesicular lava with large biotites, collected from top of lava structure.

Mountain Volcanic Field of Colorado (Bachmann et al., 2000). There,300 km3 of poorly vesicular, crystal-rich dacite forms thick denselywelded deposits proximal to the vents. The mass shows evidence ofrheomorphic flow under the influence of gravity that has resulted in amacroscopic appearance similar to flow-layered silicic lavas.

We have also observed several examples of such deposits andstructures in different settings in the Altiplano-Puna VolcanicComplex of the Central Andes. They vary in size from local beds, tomounds and ramparts of agglutinate around vents of plinian eruptions(La Pacana caldera, Lindsay et al., 2001a; Huaynaputina volcano,Adams et al., 2001; S. L. de Silva, unpublished data), or domes (CerroRuntu Jarita; Watts et al., 1999), and silicic monogenetic features suchas Cerro Coquena described here, and Cerro Jarellon to the north (deSilva et al., 1988; Francis et al., 1989). Cerro Bola on the other side ofthe La Pacana resurgent dome also has welded pyroclastic deposits atits base. We suggest that these features may be more common thanpreviously thought but are rarely preserved well-enough for positiverecognition — the excellent preservation and exposure in the Central

OI Total Nb V Cr Ni Y Sr Zr Ba ref.

.12 99.21 • • • • • • • • 1

.12 99.66 • • • • • • • • 1

.24 97.63 • • • • • • • • 1

.48 99.85 18.00 13.00 10.00 10.00 22.00 126.00 162.00 776.00 2.09 100.00 18.00 14.00 10.00 10.00 24.00 126.00 164.00 765.00 2.26 99.67 18.00 12.00 10.00 10.00 22.00 138.00 163.00 777.00 2.58 99.68 18.00 11.00 10.00 10.00 23.00 121.00 153.00 733.00 2.12 96.54 19.26 5.25 6.15 5.00 27.60 122.00 154.00 765.00 3.00 100.00 19.26 5.25 6.15 5.00 27.60 122.00 154.00 765.00 3

99.99 • • • • • • • • 4100.00 • • • • • • • • 4

ot analysed.

calculated volatile-free.. Webster (pers. comm., 2000).cture ; A lighter coloured matrix patch; B darker coloured matrix patch.amp feature.

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Andes clearly displays the volcanic textures and structures.We believethat this form of eruption is an important end-member style in thespectrum for calc-alkaline silicic magmas.

6.3. Conditions and viscosity of magma

Empirical methods have been developed in order to calculate theviscosity of a silicate liquid depending on its chemical compositionand temperature (Bottinga and Weil, 1972; Shaw, 1963, 1972;Pinkerton and Stephenson, 1992). In particular, using the Bottingaand Weil (1972) or Pinkerton and Stephenson (1992) methods, at aneruption temperature of 800 °C and the composition of sample 89-CC2A (Table 1), the estimated viscosity of the Coquena melt is of theorder of 106–7 Pa s. This value is at the low end of the spectrum forrhyolites and rhyodacites. Glass transition temperatures that maycontrol agglutination and welding can be quite low (600–700 °C) ineven slightly hydrous silicic melts (Giordano et al., 2005), which mayalso help explain the continued flow of Coquena rhyolite after impactand coalescence, assuming minimal loss of temperature in the lavafountain.

Further insight into conditions required for agglutination comesfrom Wolff and Sumner (2000) and Sumner et al. (2005). This workshows that for a given accumulation rate, agglutination is a functionprimarily of clast size implying that heat retention is the primarycontrol. The following relationship provides the limiting case foragglutination:

Log r ¼ 0:673 log η−4:44

where r is the clast diameter in meters, and η is clast viscosity in Pa s.Sumner et al. (2005) suggest that below viscosities of 105 Pa s(possible for Coquena rhyolite), clasts N10 cm across will flatten anddeform when hitting the ground at terminal velocity. Using theviscosity estimated above, we see that at clast sizes above 0.5 magglutination should be readily attained. It is interesting to note thatthis is about the dimension of the "clasts" that make up parts of theCoquena lava.

6.4. Conditions for fountaining

The rhyolite of Corral de Coquena shows no unusual petrologiccharacteristics that would suggest a rheology that is atypical of thenormal calc-alkaline felsic lavas in this region (cf. Francis et al., 1989;de Silva, 1987; de Silva and Francis, 1991; de Silva et al., 1994; Feeleyand Davidson,1994). There is no evidence for superheating or elevated

Fig. 8. Schematic representation of formation of Corral de Coquena volcano (not to scale). Prethat formed during an early phreatomagmatic phase; a low tuff ringmay also have formed into west by an angled or non-central vent and /or due to prevailing winds. Spatter accumulatethere; on inner rim accumulation produced steeply inward-dipping spatter accumulations.

halogen contents and therefore any unusual conditions in theCoquena magma, other than a slightly low viscosity. We thereforebelieve that the enigmatic character of Coquena is the result oferuptive conditions that promote the formation of spatter agglutinatethrough fountaining rather than vesiculated pumice and pyroclasticflows fed by a sustained eruption column.

Asmentioned above, a possible analogue for the Coquena eruption,albeit on amuch larger scale and displaying only the proximal outflowbut not the vent area, is the Pagosa Peak Dacite of the La Garita Calderain Colorado, USA (Bachmann et al., 2000). Eruption by low-columnpyroclastic fountaining with relatively low emission velocities for agiven discharge rate is thought to have led to rapid accumulation offall-out that resulted in dense welding and rheomorphic flow. Theexplosive potential of the Pagosa Peak dacite magma may have beenreduced by degassing during ascent through wide fissure-likeconduits. Eruption by fountaining has therefore been proposed as atenable mechanism for normal silicic magmas.

We do not attempt a detailed analysis of the conditions that led tofountaining behavior at Coquena as the lava-like deposit is notamenable for detailed grain size measurements. However, we attemptto define some general conditions. In order to agglutinate upon impact,clasts must have a sufficiently low viscosity to undergo plasticdeformation and sintering. Rapid accumulation rate also aids furtherwelding by hindering loss of volatiles (Sparks andWright,1979; Sparkset al., 1999) and aids heat retention, required for sintering to adjacentclasts. We point out that evidence of welding in rhyolitic pyroclasts isabundant in welded ignimbrites, where ultra-rapid accumulation andheat and volatile retention are perhaps the key parameters, and also inwelded silicic fall deposits (e.g., Santorini: Sparks and Wright, 1979;and Asjka, Iceland: Sparks and Wilson, 1977).

We suggest that field evidence at Coquena is most consistent withan origin as an accumulation of agglutinated spatter, possibly from aslightly elongate vent or a pair of aligned vents. A mechanism forproducing such a feature would have the properties of moderateejection velocities and moderate to high eruption and accumulationrates, which we estimate as follows:

a) The furthest distance for fountain clasts to be ejected is ~1350 m(center of crater to high point on furthest rim), with probablelocations of the vents nearer to west side than the east.

b) The ejection angle cannot be precisely determined, and may havevaried considerably during the course of the eruption but it waslikely to have been steep (at least late in the event) as the highestarcuate rim accumulation is quite narrow. Applying a ballisticejection equation (but ignoring the reduced atmospheric pressure

sence of possible underlying cryptodome not shown here. Crater is a maar-like structurethis early stage. This was followed by fountaining that may have been directed primarilyd on outer rim, agglutinated, and flowed downslope to produce ogives and ramps seen

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at 4 km altitude), the terminal velocity of coarse clasts is on theorder of 115 m/s, in agreement with values of 90–130 m/s fromStevenson andWilson (1997) at a 45° ejection and deposition angle.

c) Assuming a terminal velocity of 115 m/s the fountain height wouldbe a maximum of 670 m. The greater accumulation on the westside than the east suggests an angled fountain (or a vent nearer thewest wall). This makes any height estimate very approximate.

d) A model proposed by Capaccioni and Cuccoli (2005) shows that atfountain heights of ~500 m and 45° ejection, the range is justN1 km with ejection velocities of 100 m/s (consistent with thedistribution of Coquena deposits). Moreover, large particles do notlose much heat through transport in a fountain (e.g., Thomas andSparks, 1992) and accumulation rates of N0.2 m/hr will promotewelding; however, it may be possible to attain conditions foragglutination at lower rates if the particles are larger (N50 cm).

In summary, a fire-fountaining mechanism seems the most likelyexplanation of all the features observed in the upper lava-like Coquenadeposits (Fig. 8). Post-eruption erosion of the deposits, consistent withthe local erosion observed in the welded Pampa Chamaca ignimbrite(Fig. 2), or the sedimentation of younger deposits may have removedor buried any ash deposits related to the fire-fountaining.We note thatthe scale of the Coquena construct, 2–3 km across, is similar to thatreported for the extent of the Yamakogawa Rhyolite in Japan(Furukawa and Kamata, 2004). Perhaps, if there was deeper dissectionof Coquena deposits, a more complex proximal stratigraphy would bedisplayed, as seen in the Japanese example.

7. Concluding remarks

We suggest that Corral de Coquena, in the moat of La Pacanacaldera, Chile, is a rare example of a well-preserved spatter ringproduced by clastogenic/fountaining eruption of typical, calc-alkalinerhyolite lava. It may be the surface manifestation of a partly degassedcryptodome, present under the bulge in the caldera floor surroundingCoquena, that was disrupted by intersectionwith the local water table.This caused explosions in an early, partly phreatomagmatic ‘wet’phase of the eruption, and formation of the maar-like crater, afterwhich the aquifer ran dry. The early phase was followed by ‘dry’ lavafountaining in the second phase of the event. The Coquena lava-likedeposit may represent a case where partly degassed silicic magmafragmented under the influence of external water, perhaps accom-panied by a sudden increase in eruption rate brought on by ventwidening, but the clasts yielded plastically and could still undergoagglutination upon deposition because of a high accumulation rate.Deposition took place along a relatively narrow fall-out zone, fed bythe fountain, and in this zone high accumulation rates prevailed,promoting agglutination. This combination of eruption mechanismsresulted in a volcanic construct that is unusual and enigmatic but is,unfortunately, not well-enough exposed in its interior to permit amore detailed examination of the deposits, and of eruptiveconditions. Corral de Coquena may be a rare, example of the productof an end-member style of eruption in calc-alkaline silicic magmaticactivity.

Acknowledgements

We dedicate this paper to the honor of Professor Michael F.Sheridan and to the memory of the late Peter Francis, with whom SSand SdeS spent many windy and cold days working in this region.The authors acknowledge Jan Lindsay and Marti Godchaux for theirthorough and incisive reviews of an earlier version of this manu-script. Jim Webster is thanked for the glass inclusion analysis todetermine F and Cl. John Bailey, Janet Sumner, and John Wolff areacknowledged for fruitful discussions of preliminary versions of thismanuscript.

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