Bull Volcanol (1987) 49:547--566 Volia ology © Springer-Vertag 1987

La Pacana caldera and the Atana Ignimbrite - - A major ash-flow and resurgent caldera complex in the Andes of northern Chile

Moyra Gardeweg and Carlos F Ramirez

Servicio Nacional de Geologia y Mineria, Casilla 10465, Santiago, Chile

Abstract, The recently discovered La Pacana cal- dera, 60× 35 km, is the largest caldera yet de- scribed in South America. This resurgent caldera of Pliocene age developed in a continental plate- margin environment in a major province of ignim- brite volcanism in the Central Andes of northern Chile at about 23 ° S latitude. Collapse of La Pa- cana caldera was initiated by the eruption of about 900 km 3 of the rhyodacitic Atana Ignim- brite. The Atana Ignimbrite was erupted from a composite ring fracture system and formed at least four major ash-flow tuff units that are sepa- rated locally by thin air-fall and surge deposits; all four sheets were emplaced in rapid succession about 4.1 _+ 0.4 Ma ago. Caldera collapse was fol- lowed closely by resurgent doming of the caldera floor, accompanied by early postcaldera erup- tions of dacitic to rhyolitic lava domes along the ring fractures. The resurgent dome is an elongated, asymmetrical uplift, 48.5 x 12 km, which is broken by a complex system of normal faults locally forming a narrow discontinuous apical graben. Later, postcaldera eruptions produced large ande- sitic and dacitic stratocones along the caldera margins and dacitic domes on the resurgent dome beginning about 3.5 Ma ago and persisting into the Quaternary. Hydrothermally altered rocks oc- cur in the eroded cores of precaldera and postcal- dera stratovolcanoes and along fractures in the re- surgent dome, but no ore deposits are known. A few warm springs located in salars within the cal- dera moat appear to be vestiges of the caldera geothermal system.

Offprint requests to: M Gardeweg

Introduction

The La Pacana caldera and the Atana Ignimbrite lie in the Cordillera de los Andes in the Chilean region of Antofagasta (Fig. 1). The center of this volcanic complex is located at about 23 ° 10' S and 67°25'W in the easternmost part of Chilean territory. The caldera floor lies at 4200--4500 m, and its highest walls and central uplift are at 5200m. Several volcanic edifices nearby are hundred of meters higher, and Volcan Acamara- chi (6046 m) is the highest peak in the district (Fig. 2).

The studied area (Fig. 1) is located in an ex- ceedingly arid, high mountainous region (Altipla- no). Vegetation is exceedingly sparse, but the typ- ical fauna of the Altiplano, such as vicufias, rheas, chululos (Andean mole), vizcachas (South Amer- ican rodent), and several varieties of flamingos, geese, and ducks around the lakes and salars (salt- flat playa) are commonly seen. Although most of the area is uninhabited, about 1500 people live in small villages near the eastern border of the Salar de Atacama where the climate is milder and deep gorges dissecting the ignimbrite plateau of the A1- tiplano carry shallow fresh-water streams.

We present here the first results of our studies on the well-preserved resurgent La Pacana cal- dera, source of the voluminous and extensive Atana Ignimbrite of Pliocene age (about 4 Ma). This work is a result of six years (1980--1985) of regional mapping on a scale of 1:250000 for the Toconao (Ramirez and Gardeweg 1982) and Rio Zapaleri (Gardeweg and Ramirez 1985) sheets. Some detailed local mapping was also carried out specifically for the La Pacana caldera, although systematic detailed mapping is yet to be done. The mapping was supported by aerial photo- graphs (scale of 1:50000). K--Ar dating, and 35

548 Gardeweg and Ramirez: La Pacana caldera and the Atana Ignimbrite

~3

70 ~ 69 ~ 68 ~ 67 °

Fig. 1. Location map showing area studied (stippled), topogra- phic rim of La Pacana caldera (hatched), and resurgent dome of caldera (shaded)

thin sections and 15 chemical analysis of the Atana Ignimbrite, in addition to the study of many other rocks related to the caldera.

The discovery of La Pacana caldera resulted largely from the field mapping, which showed that a large ash-flow field called the Guaitiquina Ignimbrite by Francis and Baker (1978) actually consists of at least three different major ignim- brites. The uppermost and most widespread is the Atana Ignimbrite, which we traced to La Pacana caldera. The lower units correlate with a 5.1-Ma tuff that underlies the Atana Ignimbrite in Paso Guaitiquina near the Chilean-Argentinian border, for which we retained the name, Guaitiquina Ig- nimbrite (Gardeweg and Ramirez 1985), and with an unnamed 7.6-Ma Miocene tuff that underlies the Atana Ignimbrite near the Chilean-Bolivian frontier (Gardeweg and Ramirez 1985; Fig. 2).

In the last two decades, many ash-flow fields in the Central Andes have been mapped at differ- ent scales, and various other ignimbrites have been described (Guest 1968, 1969; Hollingworth and Rutland 1968; Salas et al. 1966; Baker 1981; Marinovic and Lahsen 1984). Few sources have been identified, mainly because of the large size of such structures, the lack of detailed mapping, and extensive mantling by younger stratovolca- noes. Lately, Landsat imagery enabled Kussmaul et al. (1977), Francis and Baker (1978), and Baker (1981) to identify a few major calderas in the Cen- tral Andes, but only a little subsequent geological work has been carried out on these structures. In

addition to the La Pacana caldera, several others have recently been described. They include the following: the Cerro Galen caldera, 200km southwest of La Pacana in northwestern Argen- tina (Francis et al. 1978; Sparks et al. 1985), the Karl Kari caldera in Bolivia, about 430 km north- east of La Pacana (Francis et al. 1981), the Cerro Guacha caldera, identified by Landsat imagery by Francis and Baker (1978) in Bolivia near the border with Chile, and possible sources of some of the Miocene tufts that underlie the Atana Ig- nimbrite in this area, and a few calderas in Cen- tral Peril like Cailloma (D~tvila 1981), Pampa Gal- eras (Noble et al. 1979), and Nevado Portugueza (Noble and McKee 1982). Recently, near La Pa- cana, Francis et al. (1984) described the Cerro Pu- rico shield complex, source of the Quaternary Ca- j6n Ignimbrite (Fig. 2). The geology of the region has been summarized by Ramirez and Gardeweg (1982), Marinovic and Lahsen (1984), and Garde- weg and Ramirez (1985).

La Pacana caldera

Geologic and tectonic setting

La Pacana caldera lies in the Central Andes within the Central Volcanic Zone (16°--28 ° S), one of the three segments of Upper Cenozoic vol- canism within the Cordillera de los Andes. It forms a major province of ignimbrite volcanism

Gardeweg and Ramirez: La Pacana caldera and the Atana Ignimbrite

[

549

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v ~ v i v

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B 0 L V I A

I 67o501 67000 '

v 5,747 v

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v v ,~ v ~ ~ g

v v ~ ~ v v ~ v v v v v

v ~ ~ v v v

LGkes

I f A l l u v i a l deposi ts

Sot t deposi ts

Andes i tJc loves

Ign imbr i t es and labors

P y r o c l a s t i c deposits

Dec i t [c and rhyo l i t i c domes end, subci rcu lar cra ter

Atanr l Ignirnbr l fe

A n d e s l t i c to d~c l t i c lavas and dac i t i c po rphyr ies

Daci f ic domes

Ign i rnb r i t es

Prevorcanic rocks

v v

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POST- CALDERA V O L C A N I C S

P R E I C A L D E R A V O L C A N I C S

Based on Rem{rez and Gardeweg ( 1 9 8 2 )

Mar inovfc and Lohsen [ 1984.) and Gerdeweg and Rarn[rez ( 1 9 8 5 ) . I

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ha H y d r o t h e r m a l a l t e r a t i o n 0 5 I0 15 20 krn

V o l c a n i c v e n t s

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• K I A r rc ld iornetr ic ages {mi l l ion of yea rs )

- - Road Note : Contacts between un i ts of same pa t t e rn separates ou ts tand ing

f(~'J~ Flow d i rec t ions vo lcan ic centers .

ARGENTINA

Fig. 2. Geological map of La Pacana caldera complex and adjacent terrain. Abbreviations for key locations: A, Arenoso; C, Chamaca; CB, Cerro Bola; CC, Corral de Coquena; CG, Cerros de Guayaques; CIN, Cerro Incaguasi Norte; CLP, Cord6n La Pacana, resurgent dome; CM, Chivato Muerto; CP, Cerros de Pill; CPN, Cord6n Puntas Negras; CPU, Cerro Purifican; CRN, Cerros de Rio Negro; G, Guaitiquina pass; LT, Laguna Trinchera; MN, Morro Negro; PC, Pampa Chamaca Ignimbrite; RP, Rio de Pill; RS, Rio Salado; S, Socaire; SA, Salar de Atacama; SAC, Salar Aguas Calientes Central; SAN, Salar Aguas Calientes Norte; SP, Salar de Pujsa; SQ, Salar de Quisquiro; ST, Salar de Tara; T, Toconao; TA, Talabre; VA, Volc/m Acamarachi; VC, Volc/m Colachi; VII, Volcfin Hualitas; VL, Volcfin Lfiscar; VP, Volc~n Purico

550 Gardeweg and Ramirez: La Pacana caldera and the Atana Ignimbrite

(Brtiggen 1950; Lahsen 1982), where voluminous ash-flow sheets of Early Miocene (22 Ma) to Qua- ternary age are extensively exposed and notably well preserved (Guest 1969; Hollingworth and Rutland 1968; Salas et al. 1966; Baker 1981; Ramirez and Gardeweg 1982; Marinovic and Lahsen 1984). These sheets are interbedded mainly with the products of dacite-andesite stra- tocones and dome complexes and with smaller proportions of basaltic and rhyolitic lavas and py- roclastics (Ramirez and Gardeweg 1982; Garde- weg and Ramirez 1985). They are locally interbed- ded with gravels, the thickness and abundance of which increase toward the west away from the Andean axis (Salas et al. 1966; Galli and Ding- man 1962), where La Pacana complex is located.

La Pacana caldera developed in a continental plate-margin environment where the Nazca oceanic plate underthrusts the South American plate along a zone where the crust is at present 70 km thick (Lomnitz 1962). The Andean volcanic chain in this area lies some 130--160km above the Benioff zone, which dips 25°--30° E (Bara- zangi and Isacks 1976). The area studied is lo- cated where the Andean volcanic chain deviates 50 km eastward from its general trend and where the distance between the chain and the trench axis reaches a maximum of 370 km (Barazangi and Is- acks 1976). In this area, known as the Altiplano or Puna, volcanic centers have been superimposed since mid-Miocene time with no obvious spatial migration; they rest upon pre-Cenozoic basement rocks at elevations of about 4000 m, largely con- cealing the older geological record of the area.

The La Pacana caldera complex is in a region dominated structurally by N-S, NW-SE, and NE- SW fault systems controlling the spatial distribu- tion of most volcanic vents of northern Chile (Lahsen 1982). Among them, three regional linea- ments or faults, conspicuous on satelite images, may have controlled the location and form of this caldera. They are the Miscanti Lineament, a 180- kin-long N-S-trending fault located west of the caldera which also controls the location of large Pliocene-Quaternary pre- and postcaldera strato- cones and domes and of small lakes; the NE-SW Socompa Lineament, a 200-km-long regional structure that runs parallel to the southeastern wall of the caldera and controls the vent locations of some young lava flows (e.g., Punta Negra) and of a few large volcanoes like Socompa (Deru611e 1978); and the Quisquiro Lineament, a poorly de- fined structure that runs parallel to the northeas- tern wall of the caldera and can be traced discon- tinuously for 110 km.

Precaldera rocks

The oldest rocks of the prevolcanic basement are a series of folded marine sediments of Ordovician age which in the east, in Argentina, rest on Pre- cambrian metamorphic basement (Schwab 1973). Cretaceous and Tertiary sedimenta~ rocks crop out in erosional windows through the younger volcanic rocks, mainly at the periphery of the mapped area (Fig. 2).

Cenozoic volcanic activity began elsewhere in the Central andes 22 Ma ago (Lahsen 1982), but in the La Pacana region the oldest recognized pre- caldera volcanic rocks are dacitic porphyritic in- trusives and andesitic stratocones about 7.5-- 11 Ma old (Tables 1, 2; Gardeweg and Ramirez 1985). These older volcanic rocks crop out in the walls of La Pacana caldera, where they commonly underlie the Atana Ignirnbrite (Fig. 3).

Also underlying the Atana Ignimbrite in the western wall of the caldera (Fig. 3) are two ash- flow tuff units, the widespread Pujsa Ignimbrite (5.8 Ma, Table 1) and the Pliocene Toconao Ig- nimbrite. The Pujsa Ignimbrite is very similar in distribution and composition to the Atana Ignim- brite. It contains pink euhedral quartz as large as 5 ram, dark-brown or reddish biotite, sparse red and green hornblende, and small amounts of or- thopyroxene phenocrysts in addition to titano- magnetite, sphene, and apatite in a brownish ma- trix of fine glass shards and dust, partially devi- trified and locally spherulitic. Abundant white aphyric pumice clasts are conspicous, whereas li- thic clasts of various tuffs and oxidized andesites are scarce and small.

The Toconao Ignimbrite (Guest 1969; Rami- rez and Gardeweg 1982) conformably overlies the Pujsa Ignimbrite in the western wall of the cal- dera. It is a distinctive ash-flow tuff that has only been mapped west of the caldera, where it also underlies the Patao Ignimbrite (Ramirez and Gar- deweg 1982). A new radiometric age of 4.0+0.9 Ma for the Toconao Ignimbrite (Tables l, 2) is compatible with its stratigraphic position beneath the Atana Ignimbrite, and thus exceeds the whole-rock radiometric age of 1.3 ___ 0.7 Ma re- ported earlier by Francis et al. (1984). The To- conao Ignimbrite is a partially welded, rhyolitic ash-flow tuff with characteristic columnar joint- ing in its more densely welded, upper portion. It is white to grayish with silky almost aphyric, fili- form pumice clasts, which in vapor-phase zones are brown and crumbly. Crystals are scarce and small, mainly of plagioctase, quartz, and biotite,

Gardeweg and Ramirez: La Pacana caldera and the Atana Ignimbrite

Table 1. Eruptive units related to the La Pacana caldera

551

Unit Locality Description Age (Table 2)

Precaldera rocks

Ceja Alta and Quilapana E wall Precaldera dacitic porphyries, 8.0 + 2.0 Ma porphyries slightly altered 10.9 _+ 0.4 Ma

Andesites, dacites, and S and SE wall In folded stratified sequences and 7.5 + 0.6 Ma tufts eroded stratocones 10.2_+ 1.7 Ma

Cerro Aguas Calientes E wall Andesitic eroded stratocone 9.1 _+ 0.4 Ma

Cerro Gigantes W wall Precaldera stratocone and dome 7.8 + 0.6 Ma

Pujsa Ignimbrite W wall Precaldera ash-flow tuff, very similar 5.8 + 0.1 Ma to the Atana Ignimbrite 5.7_+ 0.4 Ma

Toconao Ignimbfite W wall Precaldera highly distinctive aphyric 4.0+0.9 Ma rhyolitic ash-flow tuff

Chamaca, Chivato Muerto, S end of the re- Precaldera dacitic domes 4.8 +0.2 Ma and Arenoso domes surgent dome

Atana Ignimbrite Resurgent dome Intracaldera unit 4.5 _+ 0.4 Ma

4.2 + 0.2 Ma East, west, and Outflow unit 4.1 _+ 0.4 to south of the caldera 3.8 + 0.3 Ma

Morro Negro dome

Corral de Coquena

Pyroclastic deposit of the Corral de Coquena

Huailitas volcano

Filo Delgado Ignimbrite

Cerro Incaguasi

Cerros de Pili

Cerros Negros

Cerro Bola

Purifican

Pampa Chamaca Ignimbrite

Cerros de Guayaques

E of the resur- gent dome, W of Salar de Quisquiro

SE moat

SE moat, surrounding the Corral de Coquena structure

NE moat and wall

NE of the caldera

N of the resur- gent dome

Central W moat

SW moat

Central W resur- gent dome

Central W resur- gent dome

SE end of the moat E of La Pacana caldera

N end of the resur- gent dome

Postcaldera rocks

Early postcaldera dacitic dome

Early postcaldera rhyolitic subcircular crater

Ash, pumice and rhyolitic blocks related to the explosive destruction of the former dome of Corral de Coquena

Postcaldera andesitic stratovolcano, source of the Filo Delgado Ignimbrite

Postcaldera eutaxitic ash-flow tuff

Postcaldera (?) eroded stratocone with hydrothermally altered core

Postcaldera andesitic stratocones

Postcaldera stratocones

Postcaldera dacitic dome

Postcaldera dacitic flow dome complex

Postcaldera ash-flow tuff that fills the moat

Postcaldera dacitic domes and cones

4.4±0.7 Ma

4.4+0.3 Ma

Pliocene (between 4.0 and 2.4 Ma)

3.5 +_ 0.7 Ma

Pliocene (post 3.5 Ma) 9

Pliocene

3.0+0.2 Ma

2.7+0.2 Ma

Pliocene

2.4_+0.4 Ma

Plio.--Quat

in a light-brown to colorless matrix of fine dust and abundant glass shards.

Precaldera rocks (Table 1) also incluce three small dacitic domes that have a N-S alignment (Chamaca, Chivato Muerto, and Arenoso; Garde- weg and Ramirez 1985; Figs. 2, 3, 4). They are lo- cated immediately south of the resurgent dome where they are covered by just a few meters of in- tracaldera Atana Ignimbrite and probably repre- sent premonitory volcanism immediately preced-

ing the major caldera-forming eruption of the Atana Ignimbrite. The radiometric age of one of the dacites (4.8 Ma, Table 2) indicates that their emplacement preceded the climactic eruption of the Atana Ignimbrite by 0.3--1.0 Ma.

Eruption of Atana Ignimbrite and caldera collapse

La Pacana caldera subsided in response to erup- tion of the Atana Ignimbrite. The contrast be-

552 Gardeweg and Ramirez: La Pacana caldera and the Atana Ignimbrite

Table 2. K - - A r dates of the Atana Ignimbrite and related rocks a

Sample Rock type Mineral K (%) Atmosphe- Volume Ar 40 Age and no. ric Ar 4° radiogenic error b

(%) (nl/g) (Ma)

GMC-15 Dacitic porphyry Whole rock 3.359 36 1.425 10.9-+0.4 GMC-27 Pyroxene andesite Plagioclase 0.514 92 0.205 10.2___ 1.7 PG-274 Pyroxene andesite Plagioclase 0.433 96 0.153 9.1 -+4.3 GMC- 12 Dacitic porphyry Plagioclase 0.465 95 0.145 8.0 -+ 2.0 To-181-2 Andesite Whole rock 1.883 74 0.553 7.5 + 0.6 To-310 Pyroxene andesite Whole rock 2.557 74 6276 7.8_+0.6 To-301-B Pujsa Ignimbrite Biotite 7.468 19 i.677 5.8 + 0.1 To-183 Dacitic dome Biotite 7.284 36 t.370 4.8 _+ 0.2 To-704 Toconao Ignimbrite (pumice) Whole rock 3.665 91 0.571 4.0_+0.9

GMC-22 Atana Ignimbrite, intracaldera Biotite 7.358 83 1.295 4.5 +_ 0.4 To-219 Atana Ignimbrite, intracaldera Biotite 7.003 54 1.133 4.2 + 0.2 GMC-5 Atana Ignimbrite, out-flow Biotite 7.024 83 I. 115 4.1 __ 0.4 PG-223 Atana Ignimbrite, out-flow Biotite 6.844 63 L 101 4.0 _+ 0.2 M-88 Atana Ignimbrite, out-flow Biotite 7.242 73 1.128 4.0_+ 0.3 P-99 Atana Ignimbrite, out-flow Biotite 7.135 75 l. 107 4.0 +_ 0.3 PG-218 Atana Ignimbrite, out-flow Biotite 7.345 71 1.143 4.0 __ 0.3 To-301-D Atana Ignimbrite, out-flow Biotite 7.564 61 1.186 4.0±0.2 PG-279 Atana Ignimbrite, out-flow Biotite 7.342 80 1.093 3.8 +_ 0.5 PG-278 Atana Ignimbrite, out-flow (pumice) Biotite 7.115 77 1.049 3.8 -+ 0.3

GMC-28 Spherulitic rhyolite, Corral Biotite 6.827 76 1.166 4.4__+0.3 de Coquena subcircular crater

PG-205 Dacitic dome, Morro Negro Biotite 7.395 73 1.255 4.4+ 0.7 PG-221 Pyroxene andesite, Huailitas volcano Whole rock 2.272 93 0.313 3.5 _ 0.7 To-167 Biotite-hornblende dacite Biotite 7.561 59 0.883 3.05:0.2 GMC-1 Dacitic dome, Cerro Bola Biotite 6.731 78 0.707 2.7+__0.2 PG-260 Pampa Chamaca Ignimbrite Biotite 6.521 84 0.609 2.4+_0.4

a From the Toconao (Ramirez and Gardeweg 1982) and Rio Zapaleri (Gardeweg and Ramirez 1985)sheets b Determined in the Servicio Nacional de Geologia y Mineria, Chile, and calculated with Xl~=4.962x10-~ayr -~,

X~ = 0.581 x 10-~o yr-1, and K / K = 0.01167%. All error quoted at the 2 ~ level

I • I

Andesitic and dacitic "o o

volcenics •

[gnimbrites

Docitio porphyries

Dacitic domes

[ ~ Prevelcenic rocks

Solers

Inferred position of • • • • • ring foutt

t l" Rim of topographic well

- . - - ' - - - Faults

0 25km I I I I I [

67o45 ' I

°e %o e°°°e o

• •

o* • *

Fig. 3. Distribution of prevolcanic and precaldera volcanic rocks and postcaldera salars

25o50'- -

Gardeweg and Ramirez: La Pacana caldera and the Atana Ignimbrite 553

Fig. 4. Aerial view from the east of the southwestern part of the caldera moat. The subcircular crater Corral de Coquena (CC) resulted from the explosion of a postcaldera dome and is surrounded by pyroclastic deposits (PY). Also shown are the postcaldera Pampa Chamaca Ignimbrite (PC) and the late-precaldera Chamaca (C), Chivato Muerto (CM), and Are- noso (A) domes. In the distance, the Salar Aguas Calientes Central (SAC), the western wall (W), and the young volcanoes of the Cord6n Puntas Ne- gras (CPN), Aguas Calientes Central (ACC), and L~scar (LI0

Fig. 5. Landsat image of the La Pacana caldera. Resurgent dome, Cord6n La Pacana (CLP), shows pattern of faults. A, Atana out- flow unit; SA, Salar de Atacama. (The line in the western part of the photo is a photo splice, not a fault.)

554 Gardeweg and Ramirez: La Pacana caldera and the Atana Ignimbrite

tween relatively thin outflow tuff and generally thick intracaldera tuff indicates that subsidence began early in the eruption and continued as the deepening caldera filled with Atana tuff.

At the most conservative estimate, more than 900 km 3 of ash-flow tuff was erupted and era- placed as outflow sheets and intracaldera tuff. This voluminous eruption must have rapidly drained a significant part of the magma chamber, removing support from the chamber roof and per- mitting collapse of an inner cauldron block bounded by the main ring faults. The collapse re- sulted in a steep-walled depression more than 1 km deep. The subsidence was probably differen- tial during collapse, being much deeper in the central part than at the northern and southern ends. This is deduced from the present features of the caldera wall, which remains very steep in the central part but is quite subdued at the northern and southern ends of the caldera. Also at the southern end of the resurgent dome the surficial positions of the precaldera domes Chamaca, Chi- vato Muerto, and Arenoso (Figs. 2, 4), and the thin intracaldera Atana Ignimbrite that mantles them, support a lesser subsidence of this end of the caldera.

La Pacana caldera, as defined by its topogra- phic rim, is elliptical in shape and about 60 x 35 km in size (Fig. 5) with its longest axis trending north-south, parallel to one of the main fault systems of the Central Andes (Lahsen 1982). The principal topographic elements of the caldera are its (1) topographic wall and rim, (2) central uplift, and (3) moatlike floor. The floor lies at 4200--4500 m and its highest rim and central uplift are at 5200 m (Fig. 2). The wall is spectacu- larly exposed and can be traced almost contin- uously, with gaps only along the northern and western borders where younger dacitic-andesitic stratovolcanoes buried it (Fig. 7). The wall has variable height and arcuate reentrants, the largest of which is located on the southeastern side where Corral de Coquena, a postcaldera dome, was era- placed along an outer ring fault (Fig. 4). The pres- ent-day rim or topographic border, which has 1000 m of maximum relief above the moat, is gen- erally a receding cliff of extracaldera Atana Ig- nimbrite but includes older andesites, porphyries, and ignimbrites which crop out along the welt-de- fined topographic walls (Fig. 8). This wall is not parallel to what has been inferred as the main ring fracture zone. This zone is not well exposed or de-

67~45 '

Fig, 6. Distribution of Atana Ignimbrite outcrops

Gardeweg and Ramlrez: La Pacana caldera and the Atana Ignimbrite 555

E I

Andesitic 0nd dacitic loves

File Delgodo ignimbrite

[C%--?'1 Pampa Chomaca ignimbrite

Pyroclastic deposits CIN

Doeitic and rhyolitic domes ~ 25000,

Alluvial fill of the moat ~ %o.

~ v _ ° " " % :'Z'o... v

~ Selers ". : . O o ~ o ~ ~

,.-'o , , " : , ° ~ - ! i S o ~ Hot spring clusters ~-o o . .,o ~.i.7::~o o.~

o o °o oo o

. / ° o • - ~ : . o o o V0tcanlc vents I - °o o ° : .o o o ~ : ~

I -o o o ° o : -~ o o o ~ . ~ . . ® °* "* Inferred ring fracture ~. ° o ~ o ~ o °,,®o o ~

Caldera topographic ~/-~.~ ~ °_li!'('Vo A~ '̂~ °~ '~ ' [¢? f .

Flow directions ~ v ~ ~ ~0 :~: , : ~ MN \b ~ ~ v v v v v[^ ° t, ~ CPU .v.v~..~::::::/

)~ v v-X- v v °o o

k' v ~{ v v v v i o o o :

~ o"-~o ~ °o N ~ o d o k : 4Oo :~.~o: ; O~oO; 7 ,v-o o o: ( % ~ oo:

/!<f

67045 ' 67050 ' [ I

Fig. 7. Distribution of postcaldera volcanics and sediments. Abbreviations for key locations: CB, Cerro Bola; CC, Corral de Coquena; CIN, Cerro Incaguasi Norte; CP, Cerros de Pili; MN, Morro Negro; CPU, Cerro Purifican; CRN, Certes de Rio Negro; PC, Pampa Cbamaca; VA, Volc{in Acamarachi; VC, Volc~n Colachi; VH, Volc{tn Huailitas

fined but its position is inferred to mainly sur- round the La Pacana resurgent dome, where younger domes and volcanoes have been con- structed (Fig. 8). Some of the outer faults of this system promoted erosional recession of the cal- dera wall, and a few are exposed along the south- eastern and southern rim, where they parallel the wall and cut the outflow unit and precaldera rocks (Fig. 9). The faults are normal and displace- ment is commonly downward toward the moat. Younger faults far outside the topographic rim

(Fig. 9) also parallel the Socompa Lineament, but seemingly have no relation with recession of the rim.

The central uplift is a complex structural dome that raised the center of the caldera floor to heights as great as the present rim (5200 m): the La Pacana structure is thus a "resurgent caldera" (Smith and Bailey 1968).

The moat is an irregular, 2- to 10-km-wide ba- sin surrounding the resurgent dome on its west- ern, southern, and eastern flanks. It is now par-

556 Gardeweg and Ramirez: La Pacana caldera and the Atana Ignimbrite

Fig. 8. The moat and western wall (W) of La Pacana caldera. The moat is filled by the Salar de Aguas Calientes Central (SAC). In the background, at the right the Aguas Calientes Central (ACC) and LAscar (VL) volcanoes, at the left the Cerro Corona dome (CC), and in foreground the Pampa Chamaca (PC) Ignimbfite

tially filled by alluvial, lacustrine, and evaporitic sediments related to the five salars now present in its deeper portions (Fig. 7).

Resurgence of Cord6n La Pacana resurgent dome

The Cord6n La Pacana is an asymmetrical resur- gent structure within La Pacana caldera (Figs. 1,

2, 5, 11). The elongate dome covers about 350 km 2

or about 25% of the area within the topographic depression. Its long axis (about 48.5 km) trends NNE in the southern part of the caldera, but turns toward a NW direction in the northern part of the caldera. The dome is about 12 km wide in its cen- tral part. Maximum topographic relief above the moat is 1 kin, virtually equal to the relief of the topographic rim.

-23030 '

I

L'Ti ,oooo,o0,,~r,m ¢"- ~!l ~, ~\~,, ,o,e,~. ~os,,,oo o, ~. "'. \ , .* \ " ~ • ""'" ,~o.io~.oo,o,e ' ' ' < " "" ~'>"- ~ J / , L

i s ide ) •

S e l o r s

:',II 7.:~ ' t ~ ..." i11.5"

o , , , L "

6 7 o 4 5 ~ 6 7 0 3 0 ' / I I 1

Fig. 9. Main structural features related to collapse and resurgence including eroded topographic rim, inferred position of the ring fracture, and faults in the resurgent dome and outflow

Gardeweg and Ramirez: La Pacana caldera and the Atana Ignimbrite 557

Evidence for structural doming consists of uplifted caldera-fill and lacustrine sediments and Atana ignimbrite in the resurgent block. The uplifted ash-flow tufts on the flanks of the dome dip 15°--30 ° away from its axis, shallowing to 7 ° near the top. The southern part of the dome is a faulted anticline, and the tuff is nearly horizontal across the crest of Cord6n La Pacana where the strike of the dome and faults changes from north- west to south-southwest.

The resurgent dome is broken by numerous NW- and NE-striking normal faults (Fig. 9) and by closely spaced vertical rectilinear joint sets. The faults form a complex graben system parallel to the long axis of the dome and a narrow discon- tinuous and poorly defined apical graben (0.5 km wide) occupies the central part of the uplift.

Although the time of resurgent doming cannot be precisely established, it must have followed caldera collapse very closely, i.e., probably within a few hundred thousands of years. The Morro Negro dacitic lava dome which cuts the Atana Ig- nimbrite on the resurgent dome and the rhyolitic dome Corral de Coquena, both of which may rep- resent leakage of resurgent magma during uplift, are indistinguishable in K - - A t age from the Atana Ignimbrite (Tables 1, 2).

The Atana Ignimbrite

Definition, stratigraphic relations, and distribution

The Atana Ignimbrite (Ramirez and Gardeweg 1982) is a voluminous rhyodacitic ash-flow tuff composite sheet that consists of at least four flow units, forming a single cooling unit, which were erupted almost simultaneously at a time only loosely constrained by the radiometric age data to lie between about 3.8 and 4.5 Ma ago (Table 2). No evidence of erosion or sedimentary deposits has been observed between flows. Within the study area (Fig. 1) the Atana outflow tufts are spectacularly exposed on all flanks of La Pacana caldera, except on the northern flank (Fig. 6), where they are covered by younger volcanic rocks and alluvium (Figs. 2, 7). The intracaldera Atana Ignimbrite crops out continuously and is notably well preserved mainly on the Cord6n La Pacana resurgent dome, but small patches are also ex- posed in the caldera moat (Fig. 11). East of the caldera, the Atana tufts overlie deformed Meso- zoic sedimentary rocks and older late Tertiary ig- nimbrites, from which they are locally separated

by surge and air-fall deposits. West of the caldera, Atana tuffs overlie the Toconao Ignimbrite (4.0+0.9 Ma, Table 2) and are overlain by the Pleistocene Caj6n Ignimbrite (Ramirez and Gar- deweg 1982). Northwest of the caldera (Fig. 2) the Atana is largely covered by younger lavas and tuffs, but 80 km northwest of the topographic rim, the Puripicar Ignimbrite (Guest 1969; Marinovic and Lahsen 1984) of similar age and composition may correlate with the Atana Ignimbrite.

Volume and age

In assessing the total volume of the Atana Ignim- brite, we must take in account both the intracal- dera and outflow facies. The present-day area of the caldera floor is 1425 km 2. Given that more than 50% of it might be a product of erosional en- largement, 700 km 2 is a more realistic estimate of the original surface area of the cauldron block. The thickness of the intracaldera tuff that crops out mainly in the Cord6n La Pacana resurgent dome is uncertain because its base is not exposed, but the stratigraphic thickness of the uplifted block is commonly more than 1 km, except at the southern end where it is just a few meters thick. Given an average thickness of 900 m the mini- mum volume of the intracaldera tuff is 630 km3. ~ The outflow sheet crops out radially and almost continuously, dipping outward at 2.50--4.0 ° from the topographic rim of the caldera and at present is exposed over an area of 6500 km 2. Undoubt- edly, it originally extended further because 4 Ma of erosion must have removed large quantities of the deposits, but considering the low erosion rate of this area we estimate that the original surface was approximately 20% larger, exceeding 7700 km z. The thickness of the ignimbrite varies in its eastern outcrops from 60 m, 15 km northeast of the rim, to 10 m at its southeastern erosional front, 28 km from the rim. The western outcrops are 40--50 m thick, except at the eroded edges where it is only a few meters thick. Given an aver- age thickness of 35 m, the outflow sheet would have a conservatively estimated volume of 270 km 3. Given these volumes of intracaldera and outflow ignimbrite, the original total volume of the Atana must have exceeded 900 km 3. If we add to these value the volume of the Puripicar Ignim- brite that, according to Marinovic and Lahsen (1984), has a surface area of 600 km with an aver- age thickness of 20 m, the volume of ignimbrite related to the La Pacana caldera exceeds 910 km 3.

558 Gardeweg and Ramirez: La Pacana caldera and the Atana Ignimbrite

Fig. 10. irregular ground-surge deposit between two flows of the Atana Ignimbrite south of Cerro Huailitas. Note cross-bedding and layers rich in pumice gran- ules

K--Ar determinations on biotites of seven bulk samples and one pumice (Table 2) of differ- ent units of the outflow Atana Ignimbrite show its age to be between 3.8+0.3 Ma and 4.1+0.4 Ma, and thus is analytically indistinguishable from K--Ar ages of intracaldera Atana tufts (4.2_+0.2 Ma and 4.5_+0.4 Ma). The youngest val- ues (3.8 Ma) were obtained in tufts that partially fill the northern end of the moat (Fig. 11) and may represent the last tufts erupted by La Pacana caldera.

Lithologic description and petrography

The Atana Ignimbrite, although chemically and mineralogically nearly homogeneous within a narrow range of variation, displays notable physi- cal differences between its outflow and intracal- dera facies. The outflow Atana is an assemblage of at least four different flow units separated by thin (2--30 cm) pumiceous air-fall deposits and local irregular surge deposits (Fig. 11). These bed- ded deposits generally thicken and become

Fig. 11. Aerial view of the La Pa- cana resurgent dome (PRD) look- ing westward showing the well-de- veloped longitudinal faults. In the foreground, the moat with alluvial deposits (AD), an isolated outcrop of the Atana ignimbrite (14), and the northern end of the Salar de Aguas Calientes Norte (SAN). Be- hind the dome, the Salar de Pujsa (SP) and the western wall (W) of the caldera

Gardeweg and Ramirez: La Pacana caldera and the Atana Ignimbrite 559

Fig. 12. Cliff of the Atana lgnim- brite nearly 60 m high showing co- lumnar jointing in the upper mem- ber north of Salar de Tara. The lower member is formed by three nonwelded flows separated by thin ash falls

coarser grained toward the caldera. They also show rapid fluctuations in thickness (Fig. 10) and often contain low-angle cross-bedding.

The outflow Atana Ignimbrite also differs be- tween eastern and western outcrops. The eastern outcrops clearly show two major members; a lower member formed by 1--3 nonwelded flows, each 1--15 m thick, rich in pumice clasts, intercal- ated with air-fall deposits, and a thick (10--45 m) upper member, whose intercalations, if any, are obscured by welding (Fig. 12). This upper mem- ber has a nonwelded, fine-grained base poor in li- thics and pumice clasts that grades into a more

welded zone, still poor in lithics but with large pu- mice, which shows columnar-jointed cliffs in the zone of maximum welding (Fig. 12). Different zones of welding commonly exhibit distinctive colors or shades of the same color, i.e., from white in the nonwelded zone to shades of brownish- pink in the more consolidated parts. Locally pres- ent is a densely welded, grayish-pink, thin (1-- 3 cm) basal vitrophyre, which by sorting during emplacement concentrated crystals resulting in a glass-depleted, crystal-rich tuff. The base of this vitrophyre concentrates well-sorted, rounded li- thic clasts (1--5 cm). Two flow units are known to

Fig. 13. Thin section of the Atana Ignimbrite from an outflow unit in plane light showing a 0.6-ram- long sphene crystal at the center, plagioclase phenocrysts, and an irregular opaque crystal at right, all in a slightly welded matrix of fine dust and glass shards

560 Gardeweg and Ramirez: La Pacana caldera and the Atana Ignimbrite

be present on the western side of the caldera, in- tercalated with a fine-grained ash-flow deposit with cross-bedding and bands enriched in biotite; these intercalations are locally emphasized by weathering. At these outcrops, no intercalations between flow units within each of the units are distinguishable, mainly due to welding.

The Puripicar Ignimbrite, which is thought to correlate with the Atana Ignimbrite, also consists of four flow units which, according to Marinovic and Lahsen (1984), were emplaced in close suces- sion forming a single cooling unit.

The intracaldera Atana Ignimbrite consists of a thick (nearly 1 km) cooling unit of welded ash- flow tuff in which no intercalations of air-fall de- posits or other material have yet been observed between flows. It differs spectacularly from the cogenetic outflow, particularly in degree of weld- ing, jointing, abundance of phenocrysts, devitrifi- cation, and the presence of associated breccias. These differences made correlation difficult and initially led Ramirez and Gardeweg (1982) to con- sider them discrete units. Closely spaced vertical and rectilinear joints are distinctive, in contrast to the typical columnar joints of the associated out- flow unit. Locally, the tuff shows intense welding and secondary flowage, exhibiting extreme flat- tening of pumice and eutaxitic texture; it tends to be dark pinkish gray in contrast with the light pink or white of the outflow sheet. Brecciated, unaltered welded tuff with clasts and matrix of similar composition are also characteristic of the intracaldera tuff, although such exposures are

sparse and their relation with fault zones is still unknown.

The Atana Ignimbrite is a crystal-rich and commonly pumice-rich tuff poor in lithic frag- ments. The intracaldera tuff is typically denser than most of the cogenetic outflow tuff, owing to more thorough welding and devitrification. It is also more altered with slight general silicification and local hydrothermal alteration related to the faults of the resurgent dome. The bulk of the ig- nimbrite contains abundant phenocrysts of biotite (5% 7%), plagioclase (22%--33%), and large, characteristically pink euhedral quartz (3% 9%). Red or green hornblende (0%--5%) and small cli- nopyroxene (0% 2%) occur only sparsely and are not always present. Euhedral sphene (Fig. 13) and opaque minerals are characteristic, whereas small apatite and zircon crystals are sparse.

Most of the ignimbrite has white rhyolitic (the most abundant nearly 90%), dark-gray andesitic, and banded pumice fragments. The largest, up to 30 cm in diameter, are in the lowermost flows, where they stand out in relief on weathered sur- faces of poorly welded tuff. Some pumice frag- ments are yellowish and crumbly owing to vapor- phase alteration. The white pumice is mineralogi- cally similar to the bulk of the ignimbrite, with 30% large (0.5--5 mm) phenocrysts of pink quartz and plagioclase showing under the microscope sieved and resortion textures. Brown biotite, brownish-green hornblende, and sparse small clinopyroxene are also present. Large (0.4 0.8 mm) euhedral sphene is also characteristic. In

Fig. 14. Thin section of welded portion of the Atana intracaldera tuff in plane light showing abun- dant quartz and plagioclase phe- nocrysts in a matrix of welded glass shards and pumice clasts, devitrified to brown spherulites. The spherulite pumice lenticle is 3.3 mm long

Gardeweg and Ramirez: La Pacana caldera and the Atana Ignimbrite

Table 3. Chemical analyses of the Atana Ignimbrite and a postcaldera rhyolite dome a

561

Sample no. GMC-5 M-88 PG-223 PG-99 GMC-22 PG-259 u PG-220-4 PG-220-5 PG-282

SiO2 61.61 66.69 66.49 70.33 66.88 68.50 57.50 69.75 66.75 TiOe 1.15 0.57 0.65 0.37 0.66 0.55 2.42 0.38 0.62 A1203 16.85 15.91 16.21 15.04 15.38 14.34 16.47 14.43 15.30 Fe203 4.04 3.18 4.06 1.87 3.61 2.90 4.73 1.67 2.87 FeO 0.84 0.69 0.17 0.41 1.11 1.17 0.21 0.14 0.88 MnO 0.09 0.05 0.08 0.04 0.06 0.06 0.10 0.04 0.15 MgO 1.61 0.83 1.16 0.50 1.19 1.06 2.45 0.60 1.23 CaO 5.15 3.36 3.60 2.15 3.31 5.36 3.75 2.62 3.32 Na20 3.23 3.61 3.55 2.94 3.28 3.01 2.90 2.80 3.32 K20 3.03 3.52 3.36 4.23 3.53 1.18 2.45 3.92 3.74 P205 0.25 0.16 0.14 0.09 0.14 0.11 0.47 0.11 0.16 H20 0.86 0.64 0.32 1.67 1.01 1.34 6.26 3.12 1.66 H20 0.38 0.59 0.22 0.39 0.36 . . . . CO2 - - 0.07 0.12 - - - - 0.14 0.05 0.05 0.07 S 0.27 - - - - - - 0.01 0.03 0.16 0.26 <0.01 C 0.07 0.02 0.04 0.01 0.02 -- 0.02 -- 0.08 Total 99.43 99.89 100.07 100.10 100.55 99.75 99.94 99.97 100.14

SiO2 63.06 67.74 67.04 71.79 67.55 69.79 61.58 72.31 67.93 TiO2 1.18 0.58 0.66 0.38 0.67 0.56 2.59 0.39 0.63 A1203 17.25 16.16 16.34 15.35 15.54 14.61 17.64 14.96 15.57 Fe203 2.71 2.10 2.17 1.91 2.18 2.09 4.20 1.73 2.61 FeO 2.14 1.71 1.90 0.42 2.44 1.97 1.00 0.15 1.58 MnO 0.09 0.05 0.08 0.04 0.06 0.06 0.11 0.04 0.15 MgO 1.65 0.84 1.07 0.51 1.20 1.08 2.62 0.62 1.25 CaO 5.27 3.41 3.63 2.19 3.34 5.46 4.02 2.72 3.38 Na20 3.31 3.67 3.58 3.00 3.31 3.07 3.11 2.90 3.38 K20 3.10 3.58 3.39 4.32 3.57 1.20 2.62 4.06 3.81 P205 0.26 0.16 0.14 0.09 0.14 0.11 0.50 0.11 0.16

A 50.71 61.94 58.59 73.44 55.10 46.40 43.62 74.98 60.07 F 36.25 30.85 32.42 21.44 35.27 41.86 36.41 18.33 29.46 M 13.04 7.21 8.99 5.12 9.63 11.74 19.97 6.69 10.47

GMC-5, Whole rock of northern outcrops of the outflow Atana Ignimbrite M-88--PG-223, Whole rock of eastern outcrops of the outflow Atana Ignimbrite P-99, Whole rock in an outflow Atana Ignimbrite with vapor-phase alteration GMC-22, Whole rock of intracaldera Atana Ignimbrite PG-259, Basal vitrophyre of outflow Atana Ignimbrite PG-220-4, Dark pumice of outflow Atana Ignimbrite PG-220-5, White pumice of outflow Atana Ignimbrite PG-280, Morro Negro postcaldera dome

~ Chemical Laboratory of the Servicio Nacional de Geologia y Mineria, Chile b Analysis after drying the samples at l l 0 ° C

Note, The Fe20~ content has been corrected if it was over 1.5% of the sample. The amount over 1.5% is transformed in FeO multiplying it by the factor 0.8998

welded zones the pumice shows spherulitic (Fig. 14) and axiolitic devitrification textures. The dark-gray pumice contains small (0.1--0.5 mm), abundant (60%) crystal fragments of plagioclase, green hornblende, and brown biotite; also present are apatite and opaque minerals. In addition to these principal pumice types and some banded mixtures of them, small aphyric pumice fragments with pumiceous or filiform textures, were ob- served under the microscope.

Microscope study of the basal vitrophyre re- veals about 55% phenocrysts including plagio-

clase (37%) with primary glass inclusions, quartz (8%), deformed biotite (7%), hornblende (2%), and large euhedral sphene, zircon, and apatite in a matrix (45%) of colorless perlitic glass or inci- piently devitrified to irregular spherulites. At this stratigraphic level, pumice clasts are almost ab- sent and only visible under the microscope.

Lithic fragments are scarce except in the basal vitrophyre and are small, generally less than 5 cm. They are usually homogeneously distributed in the different flows of the ignimbrite, but they are not always present and locally increase upward in

562 Gardeweg and Ramirez: La Pacana caldera and the Atana Ignimbrite

size and variety. The fragments are primarily of spherulitic and microcrystalline rhyolites with a lesser proportion of oxidized brown welded tufts, porphyritic dacites, andesites, and oxidized sand- stones. In the basal vitrophyre fragments of ande- sites and welded tufts were recognized.

The intracaldera tufts are so strongly devitrif- ied that shard textures are obliterated and recog- nition of their pyroclastic character is difficult. Phenocrysts tend to be more abundant than in the outflow sheet, although of the same types and characteristics. Pumice clasts are strongly flat- tened, commonly dark gray, and mostly devitrif- ied.

Chemical Composition

Chemical analyses of bulk samples of the Atana Ignimbrite reveal that they are calc-alkaline rocks and have a wide range of compositions, similar to those of the pumice clasts. They range from nearly andesitic (S iO2 = 63.1%) to rhyolitic (SIO2=71.9%), although they are mainly of da- citic composition. Samples with vapor-phase al- teration have the highest SiO2 and K20 and the lowest Na20 (Table 3). The chemical composition of the intracaldera unit is similar to that of the cogenetic outflow (Table 3).

The basal vitrophyre, rhyodacitic in bulk com- position (SiO2 = 69.8%), has a very low potassium content (K20= 1.2%) and the highest calcium content (CAO=5.5%) because of enrichment in plagioclase crystals. Chemical analysis of separate pumice clasts shows the composition of the white fragments to be rhyolitic (SIO2=72.3%), whereas the dark-gray pumice is andesitic (SIO2=61.6%, Table 3). Between these two extreme composi- tions no dacitic pumices have been registered, ex- cept in the banded pumice where the intermediate composition results from physical mixing. More detailed geochemical study of the Atana Ignim- brite and its components is needed to understand this compositional gap.

Postcaldera rocks

Postcaldera volcanic rocks

Postcaldera volcanism took place along the ring- fracture zone and faults related to the resurgent dome (Fig. 7) and during or following uplift of the caldera floor. Pyroclastic and lava eruptions shortly after caldera subsidence formed the

Morro Negro and Corral de Coquena domes (Ta- ble 1). These were followed by the formation of other domes and of large andesitic and dacitic stratocones of Pliocene and Quaternary age (Ta- ble 1).

Among such events, the oldest was eruption of the Morro Negro dome (4.4+0.7 Ma, Table 1) which was emplaced along the main ring fault southeast of the resurgent dome (Fig. 2). Because this dome is slightly offset by faults related to the resurgent dome, it may have been emplaced dur- ing the uplift. It is a crystal-rich (56%) dacite (SIO2=67.9%, Table 3) with phenocryst content similar to that of the Atana Ignimbrite, having 30% plagioclase, brownish-red biotite (7%), and scarce oxidized hornblende (1%) as well as con- spicuous quartz phenocrysts (5%) and tiny opaque oxides and pyroxenes (1%) in a groundmass of colorless glass and brown spherulites that contain opaque needles and biotites.

The Corral de Coquena is a subcircular crater in the southeastern corner of the caldera moat (Figs. 2, 4). The crater is 3 km wide and has a rim 200 m above a nearly flat floor. The crater formed by explosive eruption of a glassy rhyolite dome (4.4+_0.3 Ma, Table 2) that is petrographically similar to Morro Negro dome, but with a lower proportion of phenocrysts (18%). Accompanying explosive destruction of the dome, a pumiceous pyroclastic eruption deposited up to 10 m of ash and pumice in subhorizontal beds that surround the Corral de Coquena (Figs. 4, 7), filling approx- imately 50 km 2 of the moat of La Pacana caldera and overlying the Atana Ignimbrite. These pyro- clastic deposits consist of crudely stratified beds of fine ash and white angular pumice pyroclasts (mostly < 2 cm). At the top of the deposit is a bed rich in angular fragments (up to 40 cm) of the rhyolitic vitrophyre that forms the Corral de Co- quena crater walls.

Other major postcollapse volcanic events started 3.5 Ma ago with eruption of glassy ande- sitic lavas of the Huailitas volcano that partially covered the northeastern end of the caldera (Ta- ble 1, Figs. 2, 7). West of this cone, a north-south array of undated dacitic domes and cones (Cerros de Guayaques) covers the northern end of the Cord6n La Pacana resurgent dome; some of them have large hydrothermally altered cores (Ramirez and Gardeweg 1982: Marinovic and Lahsen 1984). Other dacitic edifices, which partly cover the western margin of the caldera, include the large Quaternary stratovolcanoes Acamarachi (6046 m) and Colachi (5631 m; Figs. 2, 7). Within the caldera, filling the western moat, there are two

Gardeweg and Ramirez: La Pacana caldera and the Atana Ignimbrite 563

groups of andesitic cones probably of Pliocene age, i.e., the Cerros de Pili and Cerros de Rio Ne- gro located west and southwest of the resurgent dome. No younger volcanic activity occurred in the moat. In contrast, the eastern rim has no post- collapse volcanic activity except for the Corral de Coquena (Fig. 7).

Along the main ring fracture surroundings the La Pacana resurgent dome, besides the Morro Negro located on the western flank (Fig. 2), two other volcanic bodies have been emplaced. They are the Purifican rhyodacitic f low/dome complex and the Cerro Bola, a dacitic vitrophyric dome that yielded a K - - A r age of 2.7 + 0.2 Ma (Tables 1, 2).

Two postcaldera ash-flow tuffs that appear to be related to the ring fracture zone of La Pacana caldera are the Pampa Chamaca Ignimbrite at the southern margin of the caldera and Filo Delgado Ignimbrite at its northern end (Figs. 2, 7). Both directly overlie the Atana Ignimbrite with no air- fall or sedimentary deposits between them. The larger, the Pampa Chamaca Ignimbrite (Garde- weg and Ramirez 1985), is a rhyolitic tuff that yielded a K - - A r age of 2.4 + 0.4 Ma (Table 2). It was probably erupted from a source presently covered by the young lavas of the Cord6n Puntas Negras (Figs. 2, 4). The Pampa Chamaca is found as a thin, dark-gray, welded sheet on top of the Atana outflow, and also on the caldera floor where it is up to 10 m thick overlying the pyro- clastic deposits of the Corral de Coquena (Fig. 4). Although it traveled up to 20 km from its proba- ble vent, its present surface area is only about 330 km 2, its average thickness about 1.5 m, and its estimated volume only 0.5 km 3.

The Filo Deigado Ignimbrite erupted from the crater of the Huailitas volcano and covered its northern and eastern slopes (Fig. 7). It is a dacitic, eutaxitic tuff (Gardeweg and Ramirez 1985) with an estimated volume of 0.1 km 3 and an average thickness of about 1 m.

Moat deposits

The caldera moat extends as a 2- to 10-kin-wide depression between the topographic rim of the caldera and Cord6n La Pacana resurgent dome. The moat covers an area of about 920 km 2, or nearly two-thirds of the caldera floor. The lowest parts of the moat are now occupied by several sal- ars (Figs. 7, 8), where a few seeps, warm springs, and occasional streams combine to form small lakes. High evaporation rates increase the salt

contents of the water in these closed basins to form oversaturated saline solutions, or brines, from which the crusts are precipitated. Salars and lakes cover about 200 km 2 of the moat (Fig. 2).

The moat is partly filled with postcaldera vol- canic rocks and thick sedimentary deposits de- rived mainly from erosion of the resurgent dome and the caldera walls; lake sediments surround the present day salars and lakes. The Huailitas volcano, a 3.5-Ma andesitic stratovolcano which is thought to be the source of the Filo Delgado Ignimbrite (Table 1), occupies the northeastern part of the moat, and the southwestern moat is filled by andesitic lavas of the Cerros de Pill and Cerros de Rio Negro (Figs. 2, 7). The northern end of the moat is blocked by undated lavas of the Incaguasi Norte, which appear to overlie the Atana Ignimbrite, but this relation is still uncer- tain.

Hydrothermal alteration and geothermal springs

Hydrothermal alteration in the La Pacana region is associated largely with pre- and postcaldera stratovolcanoes and to a lesser extent with frac- ture zones on the Cord6n La Pacana resurgent dome. Precaldera hydrothermally altered rocks are evident mainly in the caldera walls, where the altered cores of stratovolcanoes were truncated during caldera collapse. Similarly altered rocks in the cores of postcaldera stratovolcanoes are rep- resented mainly by alteration zones in Cerros de Pili, the group of andesitic volcanoes filling the western caldera moat (Fig. 7). Extensive areas of altered rocks also occur at several levels in the eroded dacitic cone of Incaguasi Norte. Other in- dications of hydrothermal activity in the region are a few warm springs located in the salars, pre- sumably vestiges of the La Pacana caldera geo- thermal system.

Hydrothermal alteration related to pre- and postcaldera stratovolcanoes is generally of the solfataric type and is characterized by replace- ment of feldspar phenocrysts by clays and alunite and by pervasive silicification accompanied by lo- cal pyrite and chalcopyrite mineralization and abundant supergene limonite. On the resurgent dome, where hydrothermal alteration is confined to local patches and narrow bands along faults, the hydrothermal alteration is of the same type. In addition, a weakly pervasive silicification has al- tered much of the Atana Ignimbrite on the resur- gent dome. No economic mineralization has been found associated with the La Pacana caldera.

564 Gardeweg and Ramirez: La Pacana caldera and the Atana Ignimbrite

Several clusters of warm springs are distri- buted in the two Aguas Calientes salars and along faults at the base of the caldera wall. The temper- ature of these springs is lower than 25°C (CORFO 1978). Other important springs are those that form the small lakes Ojos del Rio Salado, La- guna Chivato Muerto, and Laguna Trinchera, and the streams Rio de Pili and Rio Salado (Fig. 2), all of them located in the moat and low in tempera- ture. Cold-water seeps are also located along faults of La Pacana resurgent dome. The low tem- perature of these springs and seeps suggest that there is no longer any important geothermal sys- tem related to La Pacana caldera.

Discussion

The large number of voluminous ignimbrite sheets characterizing the Central Andes indicates the existence of numerous ring-structure calderas, few of which have yet been identified. As in the Andes, the numerous, large Cenozoic pyroclastic units of the western United States, where ash-flow tufts have been widely studied (Smith 1960; Hil- dreth 1979; Christiansen 1979; Lipman 1964; Lip- man 1984, among many others) suggest that 250-- 500 associated caldera structures should be pres- ent (Smith 1979) of which less than 100 have been identified (Lipman 1984).

The small number of calderas discovered solar in the Central Andes led Francis et al. (1974) to suggest that few calderas exist there. Williams (1941), however, demonstrated that most, if not all, large-volume ignimbrites can be associated with caldera sources, and it is now generally ac- cepted that most large silicic eruptions (of more than 25--50 km 3) are associated with collapse of roof rocks over a shallow magma chamber and the violent eruption of large volumes of pumice and ash. Among such structures few other cal- deras in the world attain the dimensions of La Pa- cana, the Toba caldera in northern Sumatra being the largest known with a maximum dimension near 100 km (Van Bemmelen 1939).

In the Central Andes, most of the described calderas have tectonic settings similar to that of La Pacana caldera, their longest axes extending north-south parallel to the regional trend of the Andes. Each is usually also associated with a line of contemporaneous composite volcanoes. In Cerro Galfin, two N-S faults clearly coincide with the western and eastern margins of the caldera (Sparks et al. 1985), but in La Pacana the tectonic setting is also influenced by a pair of transverse

regional faults or lineaments of NE and N W di- rections. La Pacana caldera developed within a preexisting composite volcanic field active for about 7 Ma prior to caldera collapse, which is in- termediate in composition, as in the case of many other calderas in the world (Lipman 1984).

The evolution of La Pacana caldera can be generally related to the seven-stage cycle de- scribed by Smith and Bailey (t968), who used the Valles Caldera of New Mexico as a model. Pre- monitory activity (stage I) would be represented by the precaldera domes of Arenoso, Chamaca, and Chivato Muerto (Figs. 3, 4). Their petrogra- phic similarities and brief time span (0.3--1.0 Ma) prior to the eruption of the Atana Ignimbrite sug- gest leaks from the evolving magma reservoir. No evidence of premonitory deformation has been observed. The eruption of nearly 900kin 3 of Atana Ignimbrite from the ring fracture zone would represent stage lI, accompanied by col- lapse along the ring fractures to produce an initial caldera depression (stage III). Stage IV with mi- nor pyroclastic and lava eruptions and caldera fill is not well documented. Lack of exposed caldera- wall slide breccias certainly is due to the undis- sected cover of moat sediments and volcanics, owing to the ineffectiveness of erosion in this ex- tremely arid area.

The resurgent doming (stage V) produces an asymmetrical uplift, arcuate in plan (Figs. 2, 5) and differing greatly from the resurgent dome of the Valles Caldera. In other areas a great diversity of uplifts related to caldera evolution have been documented, including the asymetrical San Luis caldera in the San Juan volcanic field of Colorado (Steven and Lipman 1976), and the Calabozos cab- dera in central Chile (Hildreth et al. 1984). The maximum relief of the Cord6n La Pacana resur- gent dome is 1 km, but the total structural uplift remains unknown because the uneroded moat fill covers the caldera floor.

According to Smith and Bailey (1968), most resurgent domes should uplift through lakes, which then would be an integral part of the post- collapse history of calderas. At La Pacana the high evaporation rate and low precipitation of this arid area never allowed accumulation of large amounts of water, as such a climate has been characteristic of northern Chile at least since Mio- cene time (Naranjo and Paskoff 1980).

Postcaldera volcanism (stage VI) formed in the Valles caldera a discontinuous ring of rhyol- itic domes and flows constructed along the ring fractures, peripheral to the central uplift. Postcal- dera volcanism related to the La Pacana caldera is

Gardeweg and Ramirez: La Pacana caldera and the Atana Ignimbrite 565

represented by large stratovolcanoes constructed along the western and northern caldera walls and moat, but by only four domes, three of them on the resurgent dome and one at the southeastern margin of the caldera (Fig. 7). Whereas the post- caldera dacitic and rhyolitic domes of Morro Ne- gro and Corral de Coquena (Table 1) probably leaked from the same magma reservoir as the Atana Ignimbrite, the other domes and the large andesitic stratocones that formed later are proba- bly not related to the same reservoir or magmatic cycle as the Atana tufts.

As at La Pacana, the earliest P0stcaldera lavas at La Primavera, Long Valley, Valles, and Yellow- stone calderas yield K--Ar ages older than the caldera-related ash-flow tufts that preceeded them. Mahood and Drake (1982) attributed the presence of excess of extraneous argon in the lavas that immediately followed caldera collapse to the inhibited outgassing of all the argon dis- solved in the magmas coming from deeper levels in the magma chambers. Thus, the apparent in- compatibility of K--Ar dates for the dacitic and rhyolite domes (4.4 Ma) with their stratigraphic position above the Atana Ignimbrite (3.8-- 4.5 Ma) may be a problem of extraneous argon (Mahood and Drake 1982). Better resolution of the ages of the lavas and the tufts may eventually provide the answer.

The terminal stage (stage VII) of solfataric and hot-spring activity is weakly present in the La Pa- cana caldera in small patches of hydrothermal al- teration on the resurgent dome, in some postcal- dera stratovolcanoes, and in the low-temperature springs, suggesting that the magmatic-hydrother- real system waned, apparently without forming near-surface ore deposits.

The spectacular preservation of La Pacana caldera suggests that many other sources of ash- flow tufts on the Altiplano should also be well preserved. We expect that future field mapping and K- -At dating will help us to solve the prob- lem of correlation of the ignimbrites on the Alti- plano and the location and multiplicity of their sources.

Acknowledgements. This work was carried out while mapping the Rio Zapaleri and Toconao sheets, which are part of the Chilean Geological Map sponsored by the Servicio Nacional de Geologia y Mineria (SNGM), Chile. We are particularly grateful to W. Hildreth who advised and encouraged us in many ways to finish this paper and whose reviews substan- tially improved the manuscript, and to J. Skarmeta for com- ments on a first draft. The final version was improved by val- uable comments and suggestions from Jim Ratt6. Fruitful dis- cussions in the field with W. Hildreth, R.L. Smith, G. Ericks- en, and P. Francis are gratefully appreciated. A. Diaz, S. Pal-

ma, and S. Mfinquez, drivers for SNGM, and P. Cruz, guide, provided enthusiastic and skillful support under difficult con- ditions in the back country.

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Received February 10, 1986/Accepted December !8, 1986