Ignimbrites of the Cerro Galan caldera, NW Argentina

Journal o f Volcanology and Geothermal Research, 24 (1985) 205--248

205

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

IGNIMBRITES

OF THE CERRO GALAN CALDERA,

NW ARGENTINA

R.S.J. SPARKS 1, P.W. FRANCIS ~, R.D. HAMER 3, R.J. PANKHURST4, L.O. O'CALLAGHAN2, R.S. THORPE 2 and R. PAGE s

1Department o f Earth Sciences, University o f Cambridge, Cambridge, CB2 3EQ England 2Department o f Earth Sciences, Open University, Milton Keynes, Bucks, M K 7 6AA, England 3British Antarctic Survey (Natural Environment Research Council), Madingley Road, Cambridge CB3 0ET, England "British Antarctic Survey at Institute o f Geological Sciences, 64--78 Grays Inn Road, London WCIX 8NG, England sServicio Geologico Nacional, Av. Santa Fe 1548, 1060 Buenos Aires, Argentina (Received December 21, 1983; revised and accepted November 27, 1984)

ABSTRACT Sparks, R.S.J., Francis, P.W., Hamer, R.D., Pankhurst, R.J., O'Callaghan, L.O., Thorpe, R.S. and Page, R., 1985. Ignimbrites of the Cerro Gal~in caldera, NW Argentina. J. Volcanol. Geotherm. Res., 24: 205--248. The 35 x 20 km Cerro Galen resurgent caldera is the largest post-Miocene caldera so far identified in the Andes. The Cerro G a l ~ complex developed on a late pre-Cambrian to late Palaeozoic basement of gneisses, amphibolites, mica schists and deformed phyllites and quartzites. The basement was uplifted in the early Miocene along large north--south reverse faults, producing a horst-and-graben topography. Volcanism began in the area prior to 15 Ma with the formation of several andesite to dacite composite volcanoes. The Cerro Gal&n complex developed along two prominent north--south regional faults about 20 km apart. Dacitic to rhyodacitic magma ascended along these faults and caused at least nine ignimbrite eruptions in the period 7--4 Ma (K-Ar determinations). These ignimbrites are named the Toconquis Ignimbrite Formation. They are characterised by the presence of basal plinian deposits, many individual flow units and proximal coignimbrite lag breccias. The ignimbrites also have moderate to high macroscopic pumice and lithic contents and moderate to low crystal contents. Compositionally banded pumice occurs near the top of some units. Many of the Toconquis eruptions occurred from vents along a north--south line on the western rim of the young caldera. However, two of the ignimbrites erupted from vents on the eastern margin. Lava extrusions occurred contemporaneously along these north--south lines. The total D.R.E. volume of Toconquis ignimbrite exceeds 500 km 3. Following a 2-Ma dormant period a single major eruption of rhyodacitic magma formed the 1000-km 3 Cerro Galen ignimbrite and the caldera. The ignimbrite (age 2.1 Ma on Rb-Sr determination) forms a 30--200-m-thick outflow sheet extending up to 100 km in all directions from the caldera rim. At least 1.4 km of welded intracaldera ignimbrite also accumulated. The ignimbrite is a pumice-poor, crystal-rich deposit which contains few lithic clasts. No basal plinian deposit has been identified and proximal lag breccias are absent. The composition of pumice clasts is a very uniform rhyodacite which has

0377-0273/85/$03.30

© 1985 Elsevier Science Publishers B.V.

206 a higher SiO2 content but a lower K20 content than the Toconquis ignimbrites. Preliminary data indicate no evidence for compositional zonation in the magma chamber. The eruption is considered to have been caused by the catastrophic foundering of a cauldron block into the magma chamber. Post-caldera extrusions occurred shortly after eruption along both the northern extension of the eastern boundary fault and the western caldera margin. Resurgence also occurred, doming up the intracaldera ignimbrite and sedimentary fill to form the central mountain range. Resurgent doming was centred along the eastern fault and resulted in radial tilting of the ignimbrite and overlying take sediments.

INTRODUCTION T h e Cerro Galen caldera, NW A r g e n t i n a is the largest p o s t - M i o c e n e caldera s t r u c t u r e so far identified in the Andes, m e a s u r i n g a p p r o x i m a t e l y 35 × 20 km. T h e s t r u c t u r e occurs in a r e m o t e p a r t o f the A n d e s and was o n l y identified in 1 9 7 5 ( F r i e d m a n and Heiken, 1 9 7 7 ; Francis a n d Baker, 1978). R e c o n n a i s s a n c e field studies of the w e s t e r n flanks of the volcano, t o g e t h e r w i t h i n t e r p r e t a t i o n o f the satellite images, i n d i c a t e d t h a t the flanks were c o v e r e d by a t h i c k a p r o n o f i g n i m b r i t e and t h a t a large r e s u r g e n t d o m e o c c u p i e d the centre of the caldera (Francis et al., 1978). B o t h pre-caldera and p o s t - c a l d e r a lava d o m e s were also identified. In D e c e m b e r 1981 a n d J a n u a r y 1 9 8 2 an A n g l o - A r g e n t i n e Scientific E x p e d i t i o n carried o u t a geological investigation o f the caldera (Francis et al., 1 9 8 3 b ) . This p a p e r d o c u m e n t s t h e geological a n d stratigraphical relationships o f the Cerro Galen volcanic r o c k s w i t h p a r t i c u l a r r e f e r e n c e to the ignimbrites. N e w g e o c h r o n o l o g i c a l data, using K - A r and Rb-Sr m e t h ods, are p r e s e n t e d f o r b o t h the b a s e m e n t r o c k s a n d the y o u n g volcanic rocks. We consider h o w the i g n i m b r i t e v o l c a n i s m relates to the e v o l u t i o n o f the caldera and the resurgent centre. TECTONIC SETTING AND REGIONAL GEOLOGY T h e Cerro Galfin is l o c a t e d at 67~W, 26°S in the n o r t h w e s t c o r n e r of A r g e n t i n a in C a t a m a r c a Province (Fig. 1). T h e s t r u c t u r e occurs astride the high A n d e a n Plateau (Puna or A l t i p l a n o ) w i t h the f l o o r o f the caldera being a p p r o x i m a t e l y 4.5 k m a b o v e sea level. T h e s u m m i t o f the r e s u r g e n t c e n t r e e x c e e d s 6 0 0 0 m. T h e m o d e m t e c t o n i c setting o f this region has been d e s c r i b e d by J o r d a n et al. ( 1 9 8 3 ) and b y Coira et al. ( 1 9 8 2 ) w h o s h o w t h a t the Cerro GalOre area o c c u r s at a t r a n s i t i o n b e t w e e n t w o distinct s e g m e n t s o f the A n d e a n s u b d u c t i o n zone. N o r t h o f 24°S, t h e seismic z o n e has a dip o f 30 °, reflecting s u b d u c t i o n o f the N a z c a plate. S o u t h o f 28 °S, the seismic z o n e is nearly h o r i z o n t a l b e n e a t h the A n d e s and coincides w i t h a region of u p l i f t of the late p r e - C a m b r i a n to P a l a e o z o i c P a m p e a n massif and w i t h a zone n o t a b l e f o r the a b s e n c e o f Q u a t e r n a r y volcanism. A line o f Q u a t e r n a r y calc-alkaline c o m p o s i t e v o l c a n o e s runs a b o u t 200

207 km east of the trench along the Bolivia--Chile and Argentina--Chile frontiers as far south as 27°S. A separate discontinuous zone of large silicic volcanic centres, dating from 20 Ma to the present (Grant et al., 1979), occurs approximately 150 km east of the line of andesitic stratovolcanoes. This zone extends from Bolivia, where it is most extensively developed, southwards into Argentina. The centres are characterised by abundant silicic ignimbrites and some caldera structures have been identified (Baker, 1981; Francis et al., 1981; Francis et al., 1983a). Cerro Galen lies at the southern end of this zone. 74° I

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Fig. 1. Location of Cerro Gal~in caldera on map of western South America. The distribution of Miocene to Recent volcanic rocks is indicated. The Cerro Galen volcanic complex is formed within and upon a complex basement composed of greenschist to amphibolite facies metagreywackes, metapelites, amphibolites and granitoid augen-gneiss unconformably overlain by Palaeozoic marine sediments (Figs. 2 and 3). The intrusive granitoid gneisses include intermediate and acid varieties. Rb-Sr and K-Ar geochronology of the basement rocks in the region (22 to 26°S) of the Andes indicates metamorphic ages from late pre-Cambrian to late Palaeozoic (Halpern and Latorre, 1973; Rapela et al., 1982; Coira et al., 1982). Biostratigraphic data (Acenolaza et al., 1976) confirm that some of the metasedimentary rocks in the Cerro Galen area are of early Palaeozoic age. Dating of granitoid rocks from the Pampean Ranges indicates at least two episodes of plutonism: 600--500 Ma and ca. 475 Ma (McBride et al., 1976; Rapela et al., 1982). The basement rocks collected by us from the caldera area were dated

208 by K-Ar m e t h o d s (Table 1). An a m p h i b o l i t e ( L 0 9 9 ) sampled from the western wall records a late pre-Cambrian (Pampean Cycle) age while a foliated diorite and a granitic augen-gneiss sample indicate that basement o f late Palaeozoic age is also present. Our age data and palaeontologieal studies s h o w that the basement rocks of Cerro Galen contain types from

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209 both the Pampean and Hercynic cycles which have been identified as major magmatic and tectonic events in this part of the Andes (Coira et al., 1982). The region surrounding the Cerro Galen caldera is characterised by major north--south faults which have disrupted the basement into large blocks (Fig. 3). Geological studies (Turner, 1972, 1973) and fault-plane solutions (Jordan et al., 1983) indicate that these are reverse faults with moderate to high angle dips. Limited evidence indicates that faulting and uplift (the Quechua tectonic movements) commenced before 10 Ma (Schwab and Lipholt, 1974). Figure 2 shows the general geology of the Cerro Galen region based on the reconnaissance geological survey carried out during the expedition and on interpretation of digitally processed satellite imagery (Fig. 3). The local basement consists of metamorphic rocks (mica schists, amphibolites and granitoid augen-gneisses) and zones of isoclinically folded Ordovician quartzites, slates and phyllites. Miocene continental sediments rest unconformably on the basement and are often tilted up to 30 ° . Several large, denuded, dacite to andesite stratovolcanoes of late Miocene age (10--15 Ma) occur

Fig. 3. Satellite image of Cerro Gal~in caldera. Pale toned gullied formations are ignimbrites. Digitally processed image extracted from LANDSAT scene 2418-13393, 15 March 1976.

Locality

SW caldera rim

E caldera

rim

SW h o t ' springs at

C o l c h a in caldera

S a m p l e No.

L036

SS159

SS159

PWF32

PWF32

Mineral a n a l y s e d

granite augengneiss

foliated diorite

biotite

muscovite

biotite

muscovite

amphibolite hornblende

Rock type

7.810

7.810

7.899

8.828

0.235

K, %

K-At d e t e r m i n a t i o n s o f b a s e m e n t r o c k s in Cerro Gal,'in caldera

TABLE 1

122.92

116.137

145.949

160.959

6.500

Vol. R a d i o g e n i c A F °, nl/g

46.07

10.80

18.37

24.87

29.29

Atmospheric Ar ~°, %

365.8 ±

347.0 ±

422.0 ±

417.0 ±

9.0

8.2

9.8

9.8

599.8 ± 13.0

Age, Ma

211 to the west o f the caldera and were attributed to the Beltran F o r m a t i o n of Acenolaza et al. (1976). Basalt and andesite lavas occur to the west of Cerro Galen ( T hor pe et al., 1984). Although some of these mafic lavas are buried by the Cerro Galen ignimbrites, some y o u t h f u l basaltic cinder cones and lavas also rest u p o n the youngest ignimbrite. The crystalline and metamo r p hi c basement and the overlying Miocene sedimentary/volcanic succession form a rugged, fault-controlled t o p o g r a p h y with a relief of several hundred metres blanketed by the Cerro Galen ignimbrites. Figure 4 shows the distribution of major topographic features, t he location of expedition camps and locality numbers mentioned in this paper. 1

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Fig. 4. Geographical map of Cerro Galen region showing distribution of rivers, settlements, major peaks, lake and expedition camps, Localities, where detailed stratigraphic sections are presented, are indicated. IGNIMBRITE STRATIGRAPHY The volcanic products of the Cerro Galen com pl ex are dominated by dacitic to rhyodacitic ignimbrites which form a dissected plateau up to 400 m thick on the flanks surrounding the caldera (Figs. 2 and 3). Several individual ignimbrites can be recognised, but only the upper ignimbrite,

212

the Cerro Gahin ignimbrite, can be related to the present caldera. The older ignimbrite units were probably associated with caldera collapse, but evidence for this has been obscured by the formation of the young caldera. Lava flows and domes of high silica andesite to rhyolite composition are also present, being exposed principally around and just within the caldera rim. The oldest rocks of the Cerro GaMn complex are lavas concentrated along the western flanks of the present caldera. This precaldera lava sequence is not clearly distinguishable from the Miocene Beltran Formation. There are also few stratigraphic contacts between ignimbrites and lavas, although the presence of abundant lithic clasts of such lavas in the ignimbrites suggests that lava activity preceded and perhaps accompanied the major episodes of explosive volcanism. Since the ignimbrites make up the dominant volume of the younger volcanics and can be related to one another stratigraphically, this paper is largely concerned with their relationships and characteristics. Figures 5 and 6 show some selected stratigraphic sections on the western and eastern flanks of the caldera. The localities chosen are marked on West

East

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Fig. 5. Generalised s t r a t i g r a p h y o f t h e ignimbrites o n the w e s t e r n flanks along east-west transect (see Fig. 3 for localities). CG = Cerro Galen i g n i m b r i t e ; R G = Real G r a n d e i g n i m b r i t e ; Mu, M m , Ml = Merihuaca ignimbrites; B = Blanco ignimbrite. The a l t i t u d e o f each s e c t i o n is i n d i c a t e d at t h e base o f each column. Distance m e a s u r e d f r o m pass across caldera rim at 5300 m ( m a r k e d in Fig. 3). The base o f c o l u m n s CG10 and CG30 consists o f Palaeozoic b a s e m e n t . The base o f c o l u m n CG16 consists o f T e r t i a r y sediments.

213

Fig. 4, On the western flanks (Fig. 5) detailed stratigraphy is presented for an east--west transect down the Quebrada Vega Real Grande and the lower parts of the Quebrada Merihuaca*. However, studies in the Quebrada Toconquis and upper reaches of the Quebrada Merihuaca showed that the same ignimbrites were present to the north of this transect, confirming that the relationships shown in Fig. 5 are representative of the western flanks. On the eastern flanks the only detailed information comes from a transect to the east of Camp D along the Rio Leon Muerto (Fig. 6). A reconnaissance visit indicated the same units were present on the northeast flanks, about 15 km east of Camp C. West

East

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Fig. 6. Generalised stratigraphy o f the ignimbrites on the eastern flanks along an east-w e s t transect (see Fig. 3 for localities). C G = Cerro Galhn ignimbrite; C N = Cueva Negra ignimbrite; L M u = Upper L e o n Muerto ignimbrite; L M I = L o w e r L e o n Muerto ignimbrite; H n = h o r n b l e n d i c w e l d e d tuff. Distances measured from caldera rim 4.5 km w e s t o f Camp D.

On both flanks an older group of ignimbrites can be distinguished from the young Cerro Galen ignimbrite. This group is named here the Toconquis Ignimbrite Formation (Table 2). A large erosional unconformity exists between the Cerro Galfin ignimbrite and the Toconquis ignimbrite. The older ignimbrites on the eastern flanks rest unconformably on a petrographically distinct welded ignimbrite (Hn in Fig. 6) which contains abundant hornblende and sphene. Within the caldera, the prominent resurgent centre (Figs. 2 and 3) is largely composed of a single welded, crystal-rich ignimbrite, mineralogically identical to the young Cerro Galen ignimbrite. The ignimbrite is interpreted as the intra caldera facies of the Cerro Galen ignimbrite. *Quebrada means c a n y o n .

214 TABLE 2 Stratigraphy of ignimbrites

~9

West flanks

East flanks

Cerro Gal~in ignimbrite

Cerro Galen ignimbrite

Unconformity

Unconformity

Real Grande Ignimbrite Member

Cueva Negra ignimbrite member

Upper Merihuaca ignimbrite member

Upper Leon Muerto ignimbrite member

Middle Merihuaca ignimbrite member

Lower Leon Muerto ignimbrite member

Lower Merihuaca ignimbrite member O C9 O

Blanco ignimbrite member

AGE DETERMINATIONS Francis et al, (1983b) presented some preliminary K-Ar age determinations on separated biotites from some of the ignimbrites. A more comprehensive study has been completed using bot h the conventional K-Ar m e t h o d and the Rb-Sr methods. In m os t cases biotite was separated from pumice clasts for K-Ar determination. However, some samples of welded t u f f contained no pumice and, in these cases, biotite was separated from whole-rock samples. K-Ar determinations on sanidine and hornbl ende were also carried out on suitable samples. Rb-Sr age determinations were completed on four samples using plagioclase, biotite and sanidine/glass mixtures to establish isochrons. Tables 3 and 4 list the geochronological data. The geochronological study revealed significant and systematic discrepancies between the Rb-Sr and K-Ar ages, the form er being 0.5--1.4 Ma younger (Table 4). However, both sets of age data are broadly consistent with the observed stratigraphy. A major discrepancy in the K-Ar ages relates to the age of the Cerro Gal~in ignimbrite and the resurgent centre. Whereas K-Ar ages o f a whole-rock sample from the resurgent centre give, for example, 3.80 + 0.49 Ma for separated biotite and 3.92 -+ 0.16 Ma for separated sanidine, the Cerro Gal~in ignimbrite exposed on the flanks gives ages for separated biotite between 2.46 and 2.72 Ma. Rb-Sr isochrons, however, give ages close to agreement for the t w o units: 2.03 + 0.07 Ma for the Cerro Gal~in ignimbrite and 2.39 _+ 0.15 Ma for the resurgent centre. We

215 are confident on the basis of available field, petrological and geochemical data that the two units were formed from the same eruption. There is also no evidence of any major hydrothermal or heating event which might cause Sr mobility and misleading Rb-Sr ages, since all the ignimbrites are fresh with no signs of alteration of phenocrysts. We consider that the Rb-Sr determinations represent the true ages and that these rocks must contain excess argon. This is a preliminary conclusion and further work is in progress to resolve the apparent discrepancy. We note that the K-Ar ages are largely in agreement with observed stratigraphy. If t h e y contain excess argon then the available evidence indicates that the effect has been systematic throughout the sequence. We thus consider that the age data are meaningful, even though a full interpretation must await further analysis. THE TOCONQUIS IGNIMBRITE FORMATION

Western flanks The older group of ignimbrites has been divided into several members in the west and east (Table 2). Each member is considered to be the product of a single major eruption separated in time from adjacent ignimbrite members by substantial periods of inactivity. On the western flanks (Fig. 5) the older ignimbrites have been traced out to approximately 22 km from the rim of the caldera. The total thickness of these ignimbrites ranges from 200 to 300 m except in the most distal exposures where t h e y are affected by erosion. It is clear that distal parts of these ignimbrites were eroded away prior to deposition of the Cerro Galen ignimbrite. Five major ignimbrites have been recognised (Table 2), of which the uppermost Real Grande ignimbrite is the thickest and most extensive. In the following text MP and ML refer to the mean of the five largest pumice and lithic clasts respectively, measured at a single exposure. (1) Blanco ignimbrite member. This ignimbrite is only exposed in the Quebrada around Camp 1 (Figs. 4 and 5). It is a white, non-welded ignimbrite characterised by abundant large sub-angular, orange and grey, andesitic and dacitic lithic clasts (ML ~ 11 cm) and smaller pumice clasts (MP ~ 2.5 cm). No basement clasts were observed. The matrix is crystal-rich (~ 30%) with a b u n d a n t biotite, feldspar and bipyramidal quartz crystals. In the Quebrada close to Camp 1 (15 km west of the caldera rim) there is a pronounced displacement of the ignimbrite across the valley. The upper surface of the ignimbrite occurs at 4800 m altitude on the southern side of the valley and has a thickness of 20 m where it rests on older lavas of the Miocene Cerro Colorado composite volcano. On the northern side of the valley the ignimbrite upper surface stands at 4520 m and is juxtaposed against river gravels by an east--west fault.

1004

1022

1002

1050

1003

SS216

SS216

SS162

LO152

SS40

----

A78-1" A78-16" A79-31

994

998

SS169

SS216

995

PWF15

1047

1049

RST29

SS223

Analysis No

Sample No.

Leon Muerto valley, east flanks Leon Muerto (?), NE flanks caldera rim on west flanks

resurgent centre

resurgent centre

resurgent centre

resurgent centre

western flanks western flanks western flanks

eastern flanks

western flanks

A q u a Caliente

Locality

ign. ign. ign.

ign.

ign.

pumice from ignimbrite whole-rock welded tuff Real Grande lava

t o p of intra caldera ign. beneath lake sediments (pure) whole-rock welded tuff whole-rock welded tuff whole-rock welded tuff

Cerro Galen (pumice) Cerro Galen (pumice) Cerro Galen Cerro Galen Cerro Gal~in

Post-caldera d o m e

Rock type

K-Ar age determinations of Cerro Galen ignimbrites

TABLE 3

biotite

biotite

biotite

sanidine

biotite

biotite

biotite

biotite biotite biotite

biotite

biotite

biotite

Mineral

7.025

7.087

6.275

10.523

7.462

7.462

7.453

7.537 7.713

7.336

7.064

7.400

K,%

-

1.329849

1.156656

1.036813

1.604584

1.102213

1.05905

0.88099

-

0.7301 0.7393

0.732177

0.747757

0.605491

V o l u m e * A r 4°, nl/g

72.94

81.92

82.36

75.99

92.59

90.58

79.27

80.13 68.70 82.36

84.33

80.30

89.13

Atmosperic Ar 4°, %

4.86 (± 0.19)

4.19 (+ 0.26)

4.25 (± 0.24)

3.92 (± 0.16)

3.80 (+ 0.49)

3.65 (÷ 0.45)

3.04 (_+ 0.15)

2.49 (± 0.12 2.46 (± 0.12 2.58 (_+ 0.13

2.57 (± 0.16

2.72 (± 0.15

2.10 (± 0.28

Age (20) (Ma) (errors)

b.a

western flanks

1025

992

1006

RST-02

SS168

SS168

biotite

biotite

biotite

biotite

Mineral

old hornblende welded tuff old hornblende welded tuff

Lower Merihuaca (pumice)

biotite

biotite

biotite

Upper Merihuaca ( ? ) biotite (pumice)

Upper Merihuaca (pumice)

Real Grande ign. (pumice) Real Grande ign. (pumice) Real Grande ign. (pumice)

Rock type

6.350

6.350

6.268

6.518

6.675

6.454

7.489

7.581

K, %

3.656301

3.924616

1.559594

1.365

1.334384

1.206

1.420

1.516152

Volume *Ar 4°, nl/g

46.98

53.07

89.09

82.25

75.09

78.47

88.94

81.22

14.75 (± 0.40)

15.83 (± 0.44)

6.39 (± 0.57)

5.38 (± 0.26)

5.14 (± 0.21 )

4.80 (÷ 0.24)

4.87 (± 0.39)

5.14 (± 0.29)

Atmo- Age (2a) speric (Ma) (errors) Ar 4°, %

Asterisks ( * ) denote analyses presented by Francis et al. (1973). K-Ar determinations by R.D. Hamer at Institute of Geological Sciences. Decay constants from Steiger and Jhger (1977).

eastern flanks

eastern flanks

western flanks

A78-20"

western flanks

LO98

996

western flanks

A78-51"

western flanks

Locality

western flanks

999

Analysis No.

A78-14"

LO99

Sample No.

TABLE 3 (continued)

to

eastern flanks in Quebrada Leon Muerto

resurgent centre

Real Grande lava

eastern flanks

SS169

SS216

SS40

SS168

hornblendic welded tuff

dacite lave

whole-rock welded tuff

pumice from Cerro Galen ign.

Rock type

plagioclase

biotite

plagioclase

biotite

sanidine

plagioclase

biotite

sanidine plus glass

plagioclase

biotite

Minerals

4.5

540.7

20.3

606

186

9.9

754

495

8.4

753

Rb ppm

884.5

32.3

830

26.9

405

424

10.0

352

556

12.0

Sr ppm

0.0146

48.526

0.0708

65.636

1.332

0.070

216.544

4.071

0.044

180.782

S~Rb/S6Sr

0.70802

0.71782

0.71108

0.71480

0.71169

0.71164

0.71898

0.71173

0.71155

0.71679

(_+0.0001)

0.70802

0.71108 (÷0.0001)

0.71164 (+0.0001)

0.71158 (+0.0001)

sTSr/S6Sr sTSr/S6Sri

3.79 (average of 3)

2.56 (average of 4)

K-Ar age, Ma

14.22 15.29 (+0.33) (average of 2)

4.00 4.86 (+-0.22) (1 analysis)

2.39 (+ 0.15)

2.03 (+0.07)

Age

K-Ar ages are shown for comparison. Analyses by R.J. Pankhurst and R.D. Hamer at Institute of Geological Sciences.

Locality

Sample no.

Rb-Sr age determinations of Cerro Galfin ignimbrites

TABLE 4

O0

to

219 (2) Lower Merihuaca ignimbrite member. This ignimbrite (6.39 + 0.57 Ma, Table 3) is only exposed in the Quebrada Vega Real Grande, east of Camp 1. Two main units are apparent, separated by a prominent crystal-rich ground layer (0--15 cm thick) which shows typical pinch and swell structure and dune-bedding. The lower flow unit is a stratified unit (3--5 m thick), showing reverse grading of pumice, and is internally bedded due to variations of pumice size, like the ignimbrite veneer facies of Wilson and Walker (1982). A massive pumice flow deposit (45--50 m thick) overlies the ground layer, showing slight reverse grading of pumice. The ignimbrite has a moderate crystal content (~ 15%) of biotite, bipyramidal quartz and plagioclase. Both units have MP ~ 4.5--5.5 cm and ML ~ 3--5 cm. Lithic clasts are mostly lava fragments but there are a few metamorphic (amphibolite and semi-pelite) clasts. (3) Middle Merihuaca ignimbrite member. This ignimbrite varies from 15 to 30 m thick and is a white non-welded pumice flow deposit (MP 9.0 cm and ML ~ 4.2 cm at CG4) and overlies a basal plinian pumice-fall deposit (15 cm thick with MP ~ 2.3 cm and ML ~ 1.8 cm at section CG4). The ignimbrite contains pumice with biotite, plagioclase and quartz crystals (~ 15%). The uppermost parts of the ignimbrite contain crystal-rich pumice, banded grey and white dacitic pumice, vesicular microgranitic clasts and clasts of orange, vesiculated welded tuff. (4) Upper Merihuaca ignimbrite member. The Upper Merihuaca ignimbrite is well exposed to over 22 km from the caldera rim and varies from 80 to at least 150 m thick. Two K-Ar age determinations on biotite yield ages of 5.14 -+ 0.21 Ma and 5.38 + 0.26 Ma. The contact with the underlying Middle Merihuaca ignimbrite is only exposed in a few places and consists of a generally flat surface cut by small erosional gullies which never exceed a few metres depth and which contain immature volcanoclastic fluvial gravels. The time break between the eruptions could not have been great ( ~ 10 s years). The fluvial gravels in the Quebrada Real Grande contain abundant welded t u f f fragments derived from the underlying ignimbrite. They also show striking recumbent folds suggestive of strong shear stresses acting on the wet-sediment surface. The lowermost unit of the Upper Merihuaca ignimbrite is a plinian pumice fall deposit which can be traced over a wide area, and is overlain in places by a thin crystal-rich ground layer (Fig. 7). The main Upper Merihuaca ignimbrite is divisible into three flow units which can be identified in most localities. A type section is shown in Fig. 7. The lowest unit is a nonwelded grey ignimbrite containing some welded t u f f and banded pumice clasts. The middle flow unit is consistently the thickest and is a yellow nonwelded to incipiently welded ignimbrite. The deposit is characterised by a high lithic content (20--30 wt.%) with slight normal grading of lithic

220 °

!

REAL GRANDE IGNIMBRITE

.

120

REAL GRANDE PLINIAN DEPOSIT FLUVIAL GULLY INFILLS MP 4"6 ML 4"9

110

UPPER COLUMNAR JOINTED FLOW UNIT

100

90

80

~

MP 123 PUMICEFLOTATIONZONE(MIXED ML 3"0 BANDED PUMICE)

70

Z

~o

< ~9 <

a ~a.

LITHIC-RICH FLOW UNIT

~9

/~

~: 5o

ul L~ e~

40

30 MP 7 5 ML 5'2 20 LOWER ASH-RICH FLOW UNIT

~

10

1

GROUNDLAYER THINASH FLOWDEPOSIT PEINIANPUMICEDEPOSIT(MP2.5 c m ) FLUVIAL GULLY INFILL MIDDLE MERIHUACA IGNIMBRITE

SECTION

THROUGH

IGNIMBRITE

AT

CG4

Fig. 7. Detailed lithological section of Upper Merihuaca ignimbrite at locality CG4. clasts. The top of this unit shows a pumice flotation zone with bimodal grain-size distribution. Two contrasted pumice types can be identified in this zone. One type, which is the main magmatic constituent of the whole ignimbrite, is a white rhyodacitic pumice containing 10--15% phenocrysts of biotite, plagioclase and quartz. The other is a grey, denser pumice which contains a similar phenocryst assemblage. However, the phenocrysts are about one third the size of those in the rhyodacitic pumice and it is very crystal-rich (~ 40 wt.%) with abundant biotite, giving the clasts the appearance of being sprinkled with pepper. The two types are found intimately mixed together in single pumice clasts. Middle unit lithics are orange and grey volcanic clasts and fragile clasts of marl. No basement clasts are present. The unit shows some horizontal alignment of fibrous, low-density pumice clasts and the rock shows in places lithification by vapour-phase recrystallisation.

221

Fig. 8. Vertical gas escape pipes containing concentrations of coarse lithic clasts in Upper Merihuaca ignimbrite exposed in Quebrada Seca, 5 km S of Camp I.

The u p p e r m o s t flow unit is somewhat finer-grained and n o t so lithicrich. It is massive, incipiently welded and has developed a zone of large vertical columnar joints, each column having a width of 3--4 m. To the south in the Quebrada Seca the ignimbrite shows m any individual flow units. The middle units are lithic-rich and the u p p e r m o s t pumice flow units at this locality contain the two pumice varieties described above. The ignimbrite in this locality contains spectacular fluidisation features with lithic breccia lenses and vertical breccia pipes (Fig. 8). The pipes are anomolously coarse-grained (compare Walker, 1971), containing lithic clasts up to 30 cm in diameter. Many of the lithics are derived locally. The spectacular d e v e l o p m e n t of fluidisation structures could be due to two factors. First, the flows may have cascaded over steep cliffs into a deep palaeovalley, trapping air beneath them. Second, the flows rest on river gravels which may have been water-saturated at the time of emplacement and the pipes could represent local steam vents.

222

Fig. 9. Coarse co-ignimbrite lag breccia horizon in proximal breccia facies of Upper Merihuaca ignimbrite at closest point to caldera in traverse up Quebrada Merihuaca.

In the Q u e b r a d a M e r i h u a c a a m a j o r 5 - m - t h i c k breccia h o r i z o n is d e v e l o p e d at the t o p o f the u n i t (Fig. 9). T h e average size o f 7 breccia clasts was 40.6 cm. T h e breccia h o r i z o n f o r m s a p r o m i n e n t erosional b e n c h , which can be t r a c e d f o r m a n y k i l o m e t r e s t o t h e n o r t h a n d east. Clasts in the breccia are well r o u n d e d , are d o m i n a n t l y andesites and dacites w h e r e a s b a s e m e n t clasts are rare. T h e thickness o f the h o r i z o n a n d size of clasts in it decrease w e s t w a r d s . We c o n s i d e r t h a t the n a t u r e and e x t e n t o f the h o r i z o n indicate a p r o x i m a l c h a r a c t e r for the d e p o s i t w h i c h we i n t e r p r e t as a coi g n i m b r i t e lag breccia ( D r u i t t and Sparks, 1982). T h e f a c t t h a t a similar d e p o s i t is n o t d e v e l o p e d at t h e s a m e s t r a t i g r a p h i c level in the Q u e b r a d a Real G r a n d e suggests t h a t the s o u r c e o f the U p p e r M e r i h u a c a i g n i m b r i t e m a y have been n o r t h and east o f the h e a d o f t h e Q u e b r a d a Merihuaca.

17

lop NO] t×POSti)

5

......

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ff.oog~o

ooo

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mtt~

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o $

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I

17

v

CALDERA

PLINIAN D E P O S I i

o

RIM

Fti~l~Y f I O l & FION Z O N E

FROM

Mp 1 5 M L O-9

MI S X

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oo

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21

~ ~: ~.

o

°o?

0. q . ~ ~ : p

0•

....

CGI0

°°

o o o ~

o~

% °

,

c

o

27

T:.?_ "d,

"i

MPzs0 Mr21

MP19~ ~L21

o

18

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oaoq,

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'o%%

o

• • :j

CG3

PLMiC[ FIOl~r~o~ZON[ ~ ] T H '41X[ b 8 ~ N D E D

~ i .R,~ ~,ALA'~, I~,~lMBml~

CGI6

M,, ,:

(kin)

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20

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DISIANCE [ROM CALDERA RIM (kml

4

%

/

•

I

o\,e

p[ MI( i F[ {)] & I H ~ N Z()Ni RI( H IN B A N i ) I D Pt ~ 4 K L

24

J

_

Fig. 10. Detailed lithological s e c t i o n s t h r o u g h t h e Real G r a n d e ignimbrite s h o w i n g progressive facies changes a w a y f r o m caldera rim. Inset illustrates d e c r e a s e in m a x i m u m lithic d i a m e t e r a w a y f r o m caldera rim. Figures at t h e base o f e a c h c o l u m n are t h e d i s t a n c e f r o m t h e caldera rim in k i l o m e t e r s .

50

100

150

50

I O0

150

CG6

b0 bO C,O

224

(5) Real Grande ignimbrite. This unit is the most extensive and thickest of the Toconquis ignimbrites. Three age determinations on the biotite separates yield ages of 5.14 + 0.29, 4.87 -+ 0.39 and 4.80 + 0.24 Ma. In the traverse away from the caldera rim it maintains a thickness of about 150 m to 15 km away and thins somewhat to 80 m in the most distal localities (Fig. 10). The ignimbrite overlies the Upper Merihuaca ignimbrite, the upper surface of which is characterised by a thin (1--2 cm) orange weathering horizon (Fig. 11). Occasional gullies up to 2 m in depth are found containing fluvial volcanoclastic gravels, but in general the boundary between the two ignimbrites is a remarkably regular surface (Fig. 1 1 ) w h i c h slopes away from the caldera rim at an angle of 2.5 ° (Fig. 12). A short time interval between the two eruptions is indicated. The K-Ar age determinations cited above confirm this, since the standard errors of the age determinations overlap and indicate a gap of no more than 0.3 Ma. In ternal s tra tigraphy The lowermost layer in the Real Grande ignimbrite is a plinian pumice fall deposit (Fig, l l b ) . The deposit thins and becomes finer-grained away from the caldera (Fig. 13), but there is not enough geographical distribution of data to define a dispersal axis. Near the caldera rim, the plinian deposit is divided into three units separated by several intraformational ignimbrite flow units (Fig. 13). Traced away from the caldera rim these flow units thin rapidly and eventually can be recognised as three thin ash-fall beds (Fig. 13). The pumice fall deposit is lithic-poor, but contains Ordovician phyllite and lava clasts. In proximal areas crystal-rich grey pumice clasts are found with peppered appearance due to abundant fine biotite. The ignimbrite shows major vertical changes in lithology. Most of the deposit consists of lithic-poor (~ 5%), non- to incipiently welded ignimbrite containing pink rhyodacitic tube pumice. The ellipsoidal pumice clasts often show horizontal alignment of their long axes (Fig. 14a). The crystal content of the pumice is moderate {10%) with biotite, plagioclase and sparse quartz phenocrysts. Towards the top of each section abundant clasts of banded pumice occur, consisting of grey crystal-poor pumice interbanded with the rhyodacitic pumice. Above this horizon the deposit becomes much more lithic-rich (20%) and the maximum lithic diameter nearly doubles. Dark pumice is also abundant. Although basement clasts (Ordovician phyllites) occur occasionally beneath the mixed pumice horizon, most of the lithics are volcanic. However, at and above this horizon basement clasts become much more abundant and include augen-gneiss and amphibolite. In addition glassy welded tuff and prismatically jointed lava blocks occur. There are some outcrops on the western flanks where an erosional disconformity occurs between the lithic-rich upper units and units beneath.

225

Fig. 11. a. Real Grande ignimbrite overlies Upper Merihuaca ignimbrite along knifesharp contact in Quebrada Real Grande. b. Detail of contact between Real Grande and Upper Merihuaca ignimbrites showing basal plinian deposit of Real Grande being sampled by geologist. Contact can be traced in distant cliffs, and is visible over large areas.

226

5200

/

5000 4800

~4600 F-

4400

/

42OO

4000 3800

S

1 _ .~t.. / + ,oL+~+o+,+,++j, 30 20 10 DISTANCEFROMCALDERARIM(kin)

Fig. 12. Altitudes of the upper surfaces o f the Cerro Galen ignimbrite (+) and Upper Merihuaca ignimbrite (0) as functions of distance from caldera rim. Section is along Quebrada Real Grande and Quebrada Merihuaca.

+lf+:

B 10

• + +

I10

20

[ ) I S ] ANCI-:(km)

CG6

I

llO D I S I A N C E (kin)

~

=.= ++°

* ,,

,, 7,=)

0

1

I ~x

P

-_

IiI

~ _ _

CO4

,, CG3

5 lO 16 I)ISTANC[ FROM CALDERA RIM (kin

19

Fig. 13. Thickness, l i t h o l o g y and grain-size data o n basal plinian unit of Real Grande ignimbrite. P marks plinian p u m i c e layers and I marks intraformational ignimbrite.

227

Fig. 14. a. P u m i c e c o n c e n t r a t i o n zone at t o p of flow u n i t s h o w i n g a l i g n m e n t of p u m i c e clasts, b. Large isolated lithic clast in p r o x i m a l p a r t of Real G r a n d e i g n i m b r i t e . Such lithic clasts o c c u r in well-defined h o r i z o n s a n d c a n be t r a c e d laterally i n t o c o - i g n i m b r i t e lag breccia layers.

228 Facies variations The Real Grande ignimbrite displays striking facies variations found from proximal to distal localities (Fig. 10). In the most proximal localities at the head of the Quebrada Real Grande the ignimbrite is predominantly stratified (CG6, Fig. 10). The stratification varies from successions of thin (50--300 cm) reversely graded pumice flow units to an alternation of pumice-rich and pumice-poor layers with no individual flow unit boundaries distinguishable. The difference in grain size is due to variation in abundance of coarse clasts. The deposits are poorly sorted with a fine matrix. The deposits are similar to the ignimbrite veneer facies of Wilson and Walker (1982). A low-angle cross-stratification (1--3 °) is sometimes evident. The proximal deposits are also characterised by coarse bimodal pumice swarms and pods, in which close-packed assemblages of large pumice blocks are set in a fine ignimbrite matrix (Fig. 14a). These are pumice flotation zones (Sparks, 1976) since they often occur at tops of flow units. The most conspicuous feature of the proximal facies is the presence of coarse horizons of lithic breccia (up to 3 m thick). These horizons include exceedingly coarse breccias which can be both clast-supported and matrix-supported. In one breccia layer a block of dacite lava measured 2.6 X 2.1 X 1.3 m and blocks predominantly range from 5 to 100 cm. The breccia layers become more abundant towards the caldera rim and, traced away from the caldera the breccia horizons thin and change from clast-supported to matrix-supported. Eventually the breccia horizons pass laterally into horizons of individual large lithics, set in an ignimbrite matrix (Fig. 14b). The breccia horizons in the Real Grande member closely resemble those in the Upper Merihuaca member (Fig. 9). Good evidence is found for the formation of these layers by fall-out. Impact structures occur in the bedding of stratified ignimbrite facies and in one case a large phyllite clast (40 cm diameter) had shattered on impact into more than a dozen pieces. Breccia horizons could not be traced further than 6 km from the caldera rim. These breccias are interpreted as co-ignimbrite lag breccias (Wright and Walker, 1977; Druitt and Sparks, 1982). Traced away from these proximal facies the stratified character of the ignimbrite becomes less noticeable. Occasional bimodal pumice swarms (Fig. 14b) and a thin stratified (veneer facies) horizon occur (CG5 and CG2, Fig. 10) and the lowermost few metres often show thin reversely graded units. However, the ignimbrite is homogeneous and massive throughout most of its thickness, with flow unit boundaries hard to detect (CG2, Fig. 10). Beyond 15 km from the caldera rim bimodal pumice flotation zones become thicker and more abundant showing that the ignimbrite is divisible into several units (CG10, 3 and 30, Fig. 10). By 16 km from the rim the deposit becomes much richer in pumice and at least seven reversely graded units were observed (CG3, Fig. 10). In the most distal locality many of the flow units entirely consist of bimodal pumice deposits with thin basal layers (CG16, Fig. 10). The maximum lithic diameter decreases away from the source (inset in Fig. 10).

229

Volumes of Toconquis ignimbrites The limited geographical spread of data in this report makes volume estimates rather speculative. All the ignimbrites studied u n d o u b t e d l y extended much further originally. The total thickness of the ignimbrites is approximately 250 m to a distance of 22 km from the caldera rim. About one-half to two-thirds of the thickness is made up of Real Grande ignimbrite. Assuming that these same ignimbrites form a continuous outcrop to the north (30 km), then the conservative volume of the old ignimbrites on the western flanks is 150 km 3 (100 km 3 for the Real Grande ignimbrite). Given that we know there to be large volumes of co-ignimbrite ash fall in such eruptions (Sparks and Walker, 1977), that substantial erosion has taken place and that some ignimbrite was certainly deposited in the area now occupied by the young caldera, a volume of 400--500 km 3 is probably a more realistic estimate. O. Gonzalez (pers. commun., 1984) has found a succession of ignimbrites 100 km north of the caldera rim at Ojo Ratones. An ignimbrite immediately underlying the Cerro Galen ignimbrite at this locality has a K-Ar age of 5.3 Ma (S. de Silva, unpublished analysis). If these ignimbrites prove to correlate with the Toconquis Ignimbrite Formation, then the area covered and volume of these ignimbrites would be considerably greater.

Western flank stratigraphy: summary The members of the Toconquis Ignimbrite Formation represent a succession of non- to incipiently welded ignimbrites erupted over a restricted period of time (6.39--4.80 Ma on K-Ar ages). There are only minor erosional breaks between the members and the deposits can be regarded as the products of a single magmatic cycle. The thickness and distribution data suggest that the eruptions increased in magnitude with time, culminating in the Real Grande ignimbrite, which makes up two-thirds of the total preserved volume. Most eruptions began with a plinian phase and three of the eruptions ejected banded mixed pumice towards the end. The commonest accidential lithic types are dacite and andesite lava, which may come from disruption of lava dome complexes extensively developed along the western margin of the young caldera or from earlier Miocene centres. Basement clasts (Ordovician phyllites and mica schists, augen-gneiss and amphibolite) and vitrophyric welded t u f f clasts also occur and are c o m m o n in some horizons. Such rock-types occur within and around the caldera wall. The proximal character of the Real Grande and Upper Merihuaca ignimbrites, together with the accidental lithic types, indicates that the vents were located on the western side of the present caldera.

Eastern flanks The stratigraphic and geological relationships were examined in detail on the eastern flanks in one area: the Rio Leon Muerto which is a drainage

230 system off the eastern margin and dissects the ignimbrite apron (Fig. 6). A reconnaissance geological map was made based o n several detailed sections and interpretation o f aerial photographs (Fig. 15). The area consists of an irregular rounded topography of basement rocks (a strongly foliated biotiterich quartz diorite, rich in mafic xenoliths). The Toconquis Ignimbrite Formation rests on top of this basement and a densely welded ignimbrite. This older unit is a massive red-coloured, vitrophyric welded ignimbrite at least 100 m thick. The rock is very crystalrich (60%) and contains an assemblage of hornblende-biotite-alkali feldsparplagioclase-sphene and quartz. Similar rocks occur within the caldera rim near Camp 2, and in the north of the caldera 3 km south of Aguas Calientes. K-Ar ages on two biotite separates are 15.83 + 0.44 Ma and 14.75 _+ 0.4 Ma. A Rb-Sr age determination yielded an age of 14.22 Ma. These data show that this unit is n o t part of the Cerro Galen system and must represent part of the earlier Miocene volcanism. The Toconquis Ignimbrite Formation consists in the east of three members (Table 2 and Fig. 7) and is separated from the overlying Cerro Galen ignimbrite by a large erosional unconformity.

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MAP OF PHOTO 2767-205-1 :-:.,. ...,,. - . . . . . . ....:.:-:.:.:.:.:,:.:.:. •..,..........,........ +

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Leon Muerto

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Crystal rich Ignimbrite Foliated diorite basement

+

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Stratigraphic Sections

km

Fig. 15. Reconnaissance geological map of area around Camp D and in Rio Leon Muerto on eastern flanks.

231 (1) Lower Leon Muerto ignimbrite member. This ignimbrite is only exposed completely in one locality where it rests on river gravels. The ignimbrite has a basal plinian pumice fall deposit (20 cm thick) and contains a b u n d a n t foliated diorite clasts identical to the local basement rocks. This indicates that the source vent must have been on the eastern margins of the present caldera. The ignimbrite (12 m thick) consists of grey non-welded, crystal-rich (50%) ignimbrite containing sparse pumice. A ground layer occurs at the base above the plinian deposit. The crystals include biotite, sanidine, quartz and plagioclase. (2) Upper Leon Muerto ignimbrite member. The ignimbrite varies from 50 to 30 m in thickness. The basal unit consists of a lithic-rich (20--30%) plinian pumice fall deposit, 40 cm thick in a locality 6 km from the caldera rim. Most of the lithics are of a uniform, pale grey hornblende dacite, but occasional clasts of the local foliated diorite basement occur, again indicating proximity to the vent. The lowermost few metres of the ignimbrite consist of stratified material showing well-developed bedding and thin, reversely graded units. This facies can be traced onto surrounding ridges and is considered to be a veneer. The stratified facies is characterised by several horizons of large angular lithic blocks (ML ~ 65 cm). The blocks were emplaced ballistically as they display good impact structures in the ignimbrite bedding as well as impact breakage. The lithic clasts are pale grey hornblende-bearing dacite and foliated diorite of the local basement. The abundance of the diorite and the large size of the lithic blocks indicate that the source vent must have been close to the eastern margin of the caldera. The stratified ignimbrite grades up into more homogeneous white pumice flow deposits. A pumice flotation zone 8 m above the base contains grey and white banded mixed pumice clasts. The main pumice is a white fibrous rhyodacite material containing biotite, plagioclase and quartz. The upper parts of the ignimbrite become lithic-rich (20--30%) and contain augengneiss and lava clasts. (3) Cueva Negra ignimbrite member. The Cueva Negra ignimbrite overlies the smooth flat upper surface o f the Upper Leon Muerto ignimbrite. Erosional channels are observed (up to 20 m) which are filled with Cueva Negra ignimbrite, but there is no evidence for a large time-gap between the eruptions. A K-Ar age determination of biotite yielded an age of 4.25 + 0.24 Ma. A petrographically similar welded t u f f from the northeast flanks yielded an age of 4.19 _+ 0.26 Ma and is believed to be the same unit. The member has a thin basal plinian pumice fall deposit. The thickness of the ignimbrite varies from 95 to 121 m. The ignimbrite is welded, with the degree of welding increasing towards the top. The original ignimbrite is estimated to have been 130--150 m thick prior to compaction. Internally the ignimbrite is composed of several flow units as

232 deduced from the occurrence of pumice-rich zones and basal layers (Sparks, 1976). Where non-welded, the ignimbrite contains coarse pink pumice rhyodacitic clasts which are fibrous and have a high density. They contain biotite, plagioclase and quartz and have a similar character to the pumice clasts in the Real Grande ignimbrite in the west. The ignimbrite also contains occasional banded grey/white pumice with folds, which are similar to those found in the Real Grande ignimbrite. When welded, the matrix is pink to grey and contains fiamme. The maximum lithic clast diameters range from 3 to 6 cm and there are no lag breccias, suggesting that the outcrops are n o t proximal.

Eastern flank stratigraphy: Summary There are several broad similarities between the older Toconquis ignimbrites in the east and in the west, which allows us to be confident that they both belong to the same general episode of volcanism. The eruptions become larger with time in the east as judged by the increasing thickness of successive ignimbrites. The field evidence points to short time periods between eruptions (no major unconformities). The mineralogy of the eastern ignimbrites is similar to the western ignimbrites. In the case of the Cueva Negra and Real Grande ignimbrites, the similarities in the texture, density and mineralogy of pumice clasts, thickness, presence of banded pumice and stratigraphic position are sufficiently great that a correlation is conceivable. However, the K-Ar ages of the two units differ by about 0.7 Ma (Table 3). In view of other anomalies in K-Ar ages, we do not consider this age difference sufficient to rule out a correlation. The vents for the Real Grande eruption were located on the western rim of the volcano, as indicated by the proximal character of the deposits beneath the rim on the west. The Cueva Negra ignimbrite is not proximal even close to the eastern rim and the source is deduced to be within the caldera area or even on the western rim. A direct correlation between the Leon Muerto and Merihuaca ignimbrites is less attractive. It is clear from the lithology of lithic clasts and grain size characteristics that both Leon Muerto ignimbrites were derived from vents located in the foliated diorite terrain which characterised the eastern rim of the caldera. The Merihuaca ignimbrites have a lithic population dominated by dacite lava and some augen-gneiss found on the western rim. There are also three white pumiceous ignimbrites in the west and only one (the upper Leon Muerto ignimbrite) in the east. The lower Leon Muerto ignimbrite is too crystal-rich and pumice-poor to correlate with known deposits in the west. Although the Leon Muerto and Merihuaca ignimbrites are broadly contemporaneous, they originated from different source vents. The earliest period of the Cerro Galen system consisted of eruptions located along both the western and eastern sides of the pre-caldera volcano. The location of vents is considered to be controlled by regional north-south faults. Lava dome complexes were constructed on the faults with

233 most of this extensive activity being developed on the western flank. The eruptions became larger in magnitude with time, culminating in the Real Grande ignimbrites in the west and the Cueva Negra ignimbrites in the east. These two ignimbrites were probably formed in the same event. If so, the total volume may have been between 250 and 500 km 3. This eruption must have been associated with the formation of a caldera, but its morphology appears to have been obliterated by the younger structure. CERRO GAL.~N IGNIMBRITE The most prominent and extensive ignimbrite of the Cerro Galen system is the youngest unit, which forms an extensive outflow in all directions to distances reaching 100 km from the caldera rim. The same ignimbrite also forms almost the entire resurgent centre of the caldera. This correlation is based on the pumice-poor and crystal-rich character of both outcrops and on their similarity in chemical composition and mineralogy. Both units contain sanidine together with quartz, biotite and plagioclase, while hornblende is rarely observed. Detailed studies of the mineral chemistry (to be presented elsewhere) show no variations in feldspar or biotite compositions between the outflow and intracaldera facies. The Toconquis ignimbrites are all poorer in crystals and rich in pumice. They contain little or no sanidine, but minor hornblende. Eruption of the Cerro Galen ignimbrite was responsible for the formation of the caldera. K-Ar age determination on five biotite separates from different localities of the outflow sheet yielded dates of 2.72 + 0.15, 2.57 + 0.16, 2.49 + 0.12, 2.58 + 0.13 and 2.46 + 0.12 Ma. A Rb-Sr age determination on the outflow sheet, using separated biotite, plagioclase and a sanidine/glass mixture, yield a date of 2.03 + 0.07 Ma. This date is preferred to the K-Ar ages. There is a marked discrepancy between the K-At age of the resurgent centre ignimbrite (~ 3.8 Ma) and the Rb-Sr age of 2.39 + 0.15 Ma. Since the field, geochemical and petrological evidence suggest that the resurgent centre and Cerro Galen ignimbrite are the same unit, the Rb-Sr age is preferred. The alternative interpretation, that the resurgent centre is significantly older, is hard to rationalise with the stratigraphy of the flank regions, which shows no evidence of any ignimbrite between the Cerro Galen ignimbrite and the Toconquis ignimbrites. OUTFLOW SHEET

Pre-eruption topography The erosional configuration of the Toconquis ignimbrites is an important factor in determining some of the thickness variations in the Cerro Galen ignimbrite. The Toconquis ignimbrites originally formed a smooth apron with an upper surface which dips t o d a y at 2.5 ° in the west and 2° in the

234 east a w a y f r o m t h e ealdera (Fig. 12). M u c h o f t h e r o u g h t o p o g r a p h y o f t h e b a s e m e n t was buried. In the 2.3 Ma d o r m a n t p e r i o d t h e a p r o n was e r o d e d b a c k t o a b o u t 2 0 - - 2 2 k m f r o m t h e caldera r i m in t h e w e s t a n d a b o u t 15 k m in t h e east. E a s t - - w e s t - t r e n d i n g q u e b r a d a s , similar t o t h o s e draining t h e flanks t o d a y , c u t d o w n into t h e a p r o n , f o r m i n g p a l a e o v a l l e y s u p to 200 m deep. When t h e Cerro G a l e n e r u p t i o n o c c u r r e d t h e n e a r - s o u r c e ( < 15 k m ) flanks o f the v o l c a n o f o r m e d a s m o o t h surface. Palaeovalleys cut d o w n into t h e a p r o n , b e c o m i n g w i d e r a n d d e e p e r t o w a r d s t h e edge w h e r e p r o m i n e n t ( 1 0 0 - - 2 0 0 m ) cliffs o c c u r r e d . In the west a r o u g h series o f n o r t h - s o u t h ridges o c c u r r e d at 1 5 - - 3 0 k m f r o m t h e source, c o n t r o l l e d b y n o r t h - s o u t h b l o c k faulting o f the b a s e m e n t (Fig. 16). T h e t o p o g r a p h y n e a r and within the caldera area was also i m p o r t a n t in c o n t r o l l i n g d i s t r i b u t i o n . T h e w e s t e r n caldera rim was a s u b s t a n t i a l ridge w h e r e a s t h e r i m to t h e n o r t h , east a n d s o u t h was m o r e s u b d u e d .

Fig. 16. Outflow sheet of Cerro Galgn ignimbrite (30 m thick) is incipiently welded and shows columnar jointing. The ignimbrite rests on a rough topography of isoclinically folded phyllites. Volcanics of Miocene Beltran Formation from the mountain on horizon. Picture taken from west 30 km west of caldera rim.

Thickness On t h e s m o o t h flanks n e a r the w e s t e r n r i m the Cerro Gal~in i g n i m b r i t e is a l m o s t e n t i r e l y r e m o v e d b y erosion, e x c e p t in isolated outliers t y p i c a l l y 1 0 - - 1 5 m thick. N e a r t h e e r o d e d edge o f t h e old i g n i m b r i t e p l a t e a u in b o t h the west a n d east t h e i g n i m b r i t e t h i c k e n s t o b e t w e e n 70 and 180 m and is well preserved. T h e i g n i m b r i t e fills p a l a e o v a l l e y s u p to 180 m d e e p a n d is t h i c k e s t in areas i m m e d i a t e l y in f r o n t o f t h e e r o d e d f r o n t o f the old i g n i m b r i t e a p r o n and in the lee o f high b a s e m e n t ridges. T o b o t h east and

235 west the ignimbrite thins across the smoother valleys further from the source. For example, the sheet is about 12--15 m thick at Antofagasta de la Sierra in the west. The intriguing interplay of pre-existing topography and emplacement of the ignimbrite has resulted in the Cerro Galen ignimbrite being more completely preserved well beyond the caldera rim: exposures are rare closer than 10 km from the western rim.

Lithology The Cerro Galen ignimbrite is crystal-rich ( - 55%), containing abundant plagioclase, sanidine, bipyramidal quartz, Fe-Ti oxides and biotite. Rare hornblende and clinopyroxene are found together with accessory apatite and ellanite. The deposit, except for rare horizons, is pumice poor and pumice clasts larger than 2 cm are generally hard to find. Where present the pumice is highly vesicular. The deposit is fine grained on the thin sheet overlying the Toconquis ignimbrite apron and tends to become somewhat coarser in the palaeovalley fills. The distal localities also become finer grained. The ignimbrite is very poor in lithics (~ 0.1 wt.%) in most localities, except near the base. Maximum lithic diameters in both the west and east are in the restricted range 1.5--3.5 cm and show no variation from caldera rim to the most distal localities. However, the base of the ignimbrite often contains somewhat larger lithic clasts (Fig. 11) with ML often double that in the overlying main part of the ignimbrite. These lithics include older andesite and dacite fragments and are all derived locally by erosion of underlying material. A striking sedimentological feature is the occurrence of flame structures at the base of the ignimbrite. These are well-developed where the ignimbrite rests on loose river gravels. The flames form from the gravels and occur as thin sheets of lithic material which diverge at angles of 1--2 ~ from the base of the ignimbrite and can be traced distances of m a n y metres into the ignimbrite. They always point away from the caldera rim. When followed along into the ignimbrite, the flames thin and gradually change into trains of lithic clasts in the ignimbrite. The basal few metres of the ignimbrite becomes lithic-rich and is stratified due to the repetition of many lithicrich beds, each bed being traceable to a flame. In localities where the ignimbrite has crossed rough basement ridges {Fig. 16) fragments of Ordovician phyllites and quartzites can be found throughout the flow thickness. These clasts have been picked up and mixed turbulently with the flow. The outflow shows several features indicating high-temperature emplacement. In the west the ignimbrite becomes densely welded towards its base in thick palaeovalley fills {> 150 m), but is more often non-welded or incipiently welded. Zones of vapour-phase crystallisation are evident in m a n y areas with devitrified pumice clasts. Spectacular columnar jointing is developed in some localities (Fig. 16). In the east and north dense welding is more fully developed with the ignimbrite being largely welded even where

236

it is less than 30 m thick. The ignimbrite is coloured grey where partially or densely welded and the non-welded parts of the ignimbrite are cream to pink coloured. Internal divisions within the ignimbrite are generally n o t discernible. In some localities two or three flow-unit boundaries are evident, but they generally do n o t extend over long distances (Fig. 17). Pumice concentration zones are sparse and usually confined to the uppermost levels. A prominent horizon of compositionally banded pumice clasts occurs in a stratified horizon which varies from 1 to 5 metres in thickness in the western sheet {Fig. 17). The pumices consist o f crystal-poor rhyolite glass, often interbanded with grey mafic pumice and some display highly vesiculated darkened centres. A similar horizon is found near the base in eastern o u t f l o w sheet. 80-

70 M L 2"4

MASSIVE A S H F[ OW

ML

MASSIVE I N C I P I E N T L Y t~E[ D | D ASH-FLOW

60

~

50

1"5

_

E ~0

~o 4 0 lid Z n..

P U M I C E - B E A r I N G A S H El OW

(9 •~ [..,

30 -

ML 2,0

FINE MASSIVE A S H F L O W

H O R I Z O N RICH IN B A N D E D P U M I ( t: STRATIFIED IGNIMBRITE

20o ooe,

i

Q a

!0.,,o-o,

10-,ll

o-

ML 12

P U M I C E - R I C H ZONE:

M L 4.0

L I T H I C - R I C H A S H F L O W ( I o l al Lilhil sl

~ ~..o°o

r,veR ~RAVEL~FUL~ OF OROOV,C,A~

Fig. 17. Detailed lithological s e c t i o n through the Cerro Galfin ignimbrite in Quebrada Merihuaca 30 k m w e s t o f caldera rim ( l o c a l i t y CG16).

Resurgent centre The Cerro Galfin rises as a large, frost-shattered mountain-mass reaching approximately 6300 m altitude (Fig. 18). Most o f the exposed parts of the

237

Fig. 18. View of resurgent centre from flanks of Cerro Toconquis on western caldera rim. Middle distance is composed of gravel terraces from caldera walls. resurgent massif are composed of densely to partially welded Cerro Galen ignimbrite. Petrographic examination indicates that there is little change in mineralogy t h r o u g h o u t the entire thickness. Some isolated inliers of basement o ccu r in the s o u t h e r n m o s t parts of the dome, suggesting that in this area the original caldera floor had a highly irregular relief. Most parts of the d o m e visited were com pos ed of the ignimbrite, although the ignimbrite was observed to locally overlie andesite and dacite lava in the s o u t h e r n m o s t part. The structure of the d o m e can be elucidated from the dips of the eutaxitic layering and boundaries between zones of welding (Fig. 19). The highest part o f the range and much of the eastern margin have horizontal layering, whereas in o t h e r parts of the centre the layering dips radially away from the central eastern area. The dips of layering reach 70°S in the southern part and 15--20°W in the western part. In a traverse across the centre from east to west, the basement rocks can be traced right up to the f o o t of the

238 resurgent centre mountains where a major fault occurs. On the inner side of the fault the ignimbrite base is not exposed and the stratigraphic thickness of welded intracaldera ignimbrite is at least 1400 m. On the western margin the non-welded top of the ignimbrite is overlain by pumiceous fluvial gravels and finer-grained lake sediments dipping at 20 ° to the west.

DIPDA|AON RE SL : RO E N r C E N T R E

aldera margin ~tlelllllllll

Fig. 19. Dips of primary compaction foliation in Cerro Galen ignimbrite within resurgent centre. Crosses indicate horizontal foliation. The intracaldera ignimbrite is, for the most part, welded with several zones of vitrophyric densely welded tuff. The lithic clasts are somewhat larger than those observed in the outflow sheet (ML 3--5 cm) although the ignimbrite is generally lithic poor. The eastern part contains augengneiss, Ordovician slate and mica-schist clasts. Breccia horizons containing coarse augen-gneiss and amphibolite blocks (ML ~ 50 cm) set in ignimbrite matrix were found at the stratigraphically highest parts of the ignimbrite in the south.

Volume o f ignimbrite In assessing the total volume of the eruption, three factors must be taken into account: the volume of the caldera facies; the outflow facies; and the dispersed, fine-grained "co-ignimbrite ash". Given that the area of the caldera floor is 450 km 2, the minimum volume is of the order 450

239

km 3 D.R.E. (Dense R ock Equivalent). The outflow sheet is exposed at present over an area of 7500 km 2, with a volume of 280 km 3 (D.R.E.) being a conservative estimate. K-At dating of ash horizons in fossil-bearing late Tertiary continental sediments from several localities in NW Argentina suggest that some are derived from Cerro Galfin. In particular the i m p o r t a n t " t o b a dacitica" at Uquia (L-Ar age of 2.78 -+ 0.09 Ma), 270 km NE of Cerro Galen, may correlate with the Cerro Galen ignimbrite (Marshall and Patterson, 1981). We suspect that the volume of co-ignimbrite ash may make the total erupted magma volume in excess of 1000 km 3. LAVA EXTRUSIONS

Dacitic to rhyodacitic lava flows and domes are present around the margins of the Cerro Galen caldera. T hey are extensively developed on the western caldera margin, where many are strongly affected by hydrothermal alteration. These lavas are interpreted as c o n t e m p o r a n e o u s with the Toconquis Ignimbrite Formation. A very prom i nent lava, with wellpreserved surface features, occurs at the head of the Quebrada Real Grande on the southwestern caldera margin. This lava has a K-Ar age on biotite o f 4.86 -+ 0.19 Ma and rests on top of the Real Grande ignimbrite. A Rb-Sr age determination on separated biotite and plagioclase yielded an age of 4.00 + 0.22 Ma, the preferred age. Young dacitic lava domes also occur in the n o r th at Aguas Calientes (Figs. 2 and 3). One sample gave a K-Ar age on separated biotite of 2.10 -+ 0.28 Ma, confirming that this represents a post-caldera extrusion. RIVER TERRACES

A distinctive morphological feature of the Cerro Galen volcano is the occurrence of well-defined river terraces. On the western flanks and to the n or th of the caldera two major terraces occur (Fig. 20). There may also be an older less well-developed terrace (marked 3 on Fig. 20). Incision up to 10 m into the earlier well-preserved terrace (2) had occurred before the eruption of t he Cerro Galfin ignimbrite which fills palaeovalleys cut NOR 1 tt

SOt T I t

loo

[]

/ ......................

0

\ I

1

0

500

I

i

1000

1500

DISIANCE

Fig. 20. S e c t i o n across Q u e b r a d a Merihuaca a b o u t 30 k m w e s t o f caldera rim s h o w i n g river terraces. Locality close to CG16 (Fig. 4).

5

100.28

283 21 272

Total

Rb(ppm) Nb(ppm) Ba(ppm)

68.90 14.57 0.62 3.36 0.06 1.18 2.59 3.09 4.54 0.19 n.d. 1.50

267 21 342

100.32

2 68.77 14.21 0.57 3.13 0.05 1.10 2.44 3.37 4.42 0.18 n.d. 2.10

285 22 330

100.35

3 66.77 14.67 0.53 3.11 0.04 1.19 2.57 3.17 4.56 0.21 0.09 3.50

252 15 532

100.27

4

99.21

65.96 14.50 0.51 3.02 0.05 1.14 2.58 3.46 4.46 0.19 0.15 3.19

258 16 479

5 64.77 14.49 0.51 2.88 0.04 1.39 2.54 4.40 4.51 0.19 0.18 4.46

264 15 456

100.36

6

99.2-~"

66.56 14.89 0.56 3.23 0.05 1.24 2.60 2.91 4.53 0.21 0.01 2.47

278 16 581

7

99.14

62.96 14.24 0.54 2.93 0.04 1.11 3.25 4.43 4.07 0.20 0.33 5.04

228 16 443

8 66.65 15.85 0.58 3.26 0.05 1.18 3.21 3.31 4.21 0.21 n.d. 1.83

232 15 545

100.32

9

241 15 404

100.26

67.76 15.17 0.53 3.10 0.05 1.11 2.69 3.42 4.40 0.13 n.d. 1.93

10

192 12 504

99.93

65.40 16.29 0.37 2.19 0.04 0.88 3.79 3.76 3.63 0.14 n.d. 3.43

11

302 18 516

i00.~

0.28 n.d. 2.21

4.77

66.76 15.08 0.63 3.43 0.05 1.35 2.92 2.84

12

Key: A n a l y s e s ( 1 ) ~ 3 ) : Cerro G a l e n i g n i m b r i t e ; A n a l y s e s ( 4 ) - - ( 7 ) : Real G r a n d e i g n i m b r i t e ; A n a l y s e s (8) a n d ( 9 ) : U p p e r M e r i h u a c a i g n i m b r i t e ; A n a l y s i s (10) : Middle M e r i h u a c a i g n i m b r i t e ; A n a l y s i s (11 ) : L o w e r M e r i h u a c a i g n i m b r i t e ; Analysis ( 1 2 ) ; C u e v a Negra i g n i m b r i t e .

69.17 14.62 0.60 3.27 0.06 1.14 2.56 3.24 4.16 0.23 0.02 1.21

1

Si% AI20~ TiO2 Fe20~ MnO MgO CaO Na~O K~O P:O~ S L.O.I.

Wt. %

R e p r e s e n t a t i v e m a j o r - e l e m e n t a n d selected t r a c e - e l e m e n t a n a l y s e s of p u m i c e clasts f r o m C e r r o Gal~in i g n i m b r i t e s . A n a l y s e s were carried o u t b y X-ray f l u o r e s c e n c e t e c h n i q u e s

TABLE O

b~

241 in this terrace (Fig. 20). A second period of incisement occurred (up to 30 m) following the eruption. To the north of the caldera two prominent terraces dip northwards at 5° whereas those on the west dip westwards. An attractive but speculative explanation for the episodes of rejuvenation and incisement is that uplift occurred associated with the eruption of the Cerro Galfin ignimbrite and subsequent resurgence. The earlier uplift could represent the pre-eruption tumescence associated with increasing magma pressure. The second incisement could represent uplift associated with resurgence. Within the caldera, in the moat area rimming the resurgent centre, some excellent examples of reversely sloping river terraces are found. The terraces entirely consist of gravels composed of basement augen-gneiss, schists, pegmatite and amphibolites together with dacitic lava clasts. No ignimbrite clasts occur and the material is manifestly derived from the caldera wall. However, the terraces now dip at 2--5 ° radially outwards from the resurgent centre towards the caldera wall and are partly covered by young fluvial outwash fans derived from the resurgent centre and entirely composed of Cerro Galen ignimbrite clasts. The terraces must have originally been part of outwash fans sloping away from the caldera walls into the caldera depression. Resurgence in the centre has tilted them up and reversed their slope. Striking river gravel terraces are also found on top of the resurgent centre in the eastern central part resting on the ignimbrite. They are entirely composed of clasts known to form the immediately adjacent eastern caldera wall (foliated diorite and mica schists). The highest parts of the caldera wall are 1000 m below these terraces and hence these deposits are considered to be outwash gravels rafted on top of the resurgent centre. GEOCHEMISTRY Representative major-element and selected trace-element analyses of individual pumice clasts from the major ignimbrite units are reported in Table 5. A detailed account of the petrology and geochemistry of the ignimbrites is to be presented separately. The samples have either rhyodacitic or dacitic compositions, using the simple criteria that dacites have waterfree SiO2 contents between 63% and 68% and rhyodacites have SiO2 contents between 68% and 72%. These analyses, together with analyses of intermediate and mafic lavas from the Cerro Galen region (Thorpe et al., 1984), show that the rocks are calcalkaline and fall in the high potassium field (Gill, 1981). The Toconquis ignimbrites range from high-silica dacite to rhyodacite (Fig. 21). There is some evidence of compositional zoning in the Toconquis ignimbrites with a scatter of compositions present in individual units shown by covariation of SiO2 with MgO, Fe203, CaO and K20. The upper parts of several units contain banded pumice clasts, the darker bands of which have more mafic dacitic compositions.

242 Pumices collected from the Cerro Galen ignimbrite are very uniform in all major and trace elements (Fig. 21). They are rhyodacites with high silica content and lower K20 content than the Toconquis ignimbrites. They are also richer in Nb and Ba but poorer in Rb. No evidence of compositional diversity has yet emerged, except for the rare mixed pumice clasts. Initial STSr/S6Sr data from three samples generated for the geochronological study (Table 4) show values of 0.71108--0.71164. These values suggest that the silicic magmas are either derived from melting of continental crust or by assimilation -- fractional crystallisation (Francis et al., 1980). i

o

I •

• •

° •

• •

K20

D~ 0 0 QO

+ ++ +

4

3

o

65

6'6

68

¢0

Si02

Fig. 21. K~O versus SiO: plot for pumice clasts from the Cerro Galen ignimbrite. Solid circles = Toconquis ignimbrite; crosses = Cerro Cal~n ignimbrite.

DISCUSSION The evolution of large calderas provides evidence of the ascent and emplacement of voluminous silicic magma bodies in the upper crust. Much of the current knowledge on large resurgent calderas has been gained from studies of Tertiary and Quaternary calderas in the United States (Smith and Bailey, 1968; Lipman, 1984). Some of these well-documented calderas, such as the Valles caldera, New Mexico, occur in extensional environments and are characterised by bimodal volcanism and generation of high-silica rhyolites. The volcanic field of the San Juan Mountains, Colorado (Lipman, 1984) consists of seventeen overlapping caldera complexes composed of intermediate to silicic lavas and voluminous ignimbrites of general calc-alkaline affinity. These caldera complexes, many of which are resurgent, are closer in character to the Cerro Galen than the Valles caldera, but their tectonic setting is still incompletely understood. The Cerro Galen

243 provides an o p p o r t u n i t y to examine the evolution of a large resurgent caldera in an Andean tectonic setting and to contrast its development and geology with the well-known U.S. calderas. Lipman (1984) has proposed a three-stage cycle for caldera development based on studies in the San Juan Field. The cycle commences with voluminous outpourings of intermediate lava from a cluster of composite volcanoes. Large-volume silicic ignimbrites, caldera form at i on and resurgence follow later in a cycle, although andesite and dacitic lava can be erupted throughout the life of a cycle. Geochronological studies indicate that such a cycle for an individual caldera extends over time periods of 3--5 Ma. Volcanism was initiated in t he Cerro Galfin region with the form at i on of several overlapping calcalkaline stratovolcanoes (the Beltran Formation). The volcanics range f r om basalt through rhyolite, but are dom i nat ed by andesite and dacite. Geochemical studies suggest that the magmas were derived from b ot h mantle and continental crustal sources (Francis et al., 1980; T h o r p e et al., 1984). There are two problems with envisaging these volcanics as the early intermediate lava stage of the Cerro Galfin, as docum en ted in the San Juan systems. First, some centres formed well to the west of the Cerro Galfin (e.g. Cerro Beltran, Fig. 1) and do not appear to have provided the focus for later silicic volcanism. Second, geochronological studies (Francis et al., 1978, 1980) give ages of 10--15 Ma for these centres, which appears to be t oo great for them to belong to the same magmatic cycle as the Cerro Galen. The rims of the caldera do expose some dacite and andesite volcanics which may represent the precursory lava stage, but even near source the ignimbrites often rest directly on basement. The available evidence suggests that a substantial volume of early intermediate volcanic lava did n o t develop in the Cerro Galfin. The Toconquis ignimbrites represent the first certain activity of the Cerro Galfin system. This formation was erupted over a 2.4-Ma period and involved the eruption of at least 500 km 3 of dacite and rhyodacitic magmas. The limited evidence suggests that most of the activity was concentrated towards the end of this period with eruption of the Real Grande and Cueva Negra ignimbrites representing a substantial p o r p o r t i o n of the volume. Fresh lava fragments of similar compositions and general petrography are ab u n d an t in m a ny of the ignimbrites and indicate that lava extrusions were f o r med in association with eruption. Three lines o f evidence indicate that the source vents for the ignimbrites on the western flanks were located along the western margin of the caldera. First, the western caldera wall and rim partly consists of a complex of dacitic and rhyodacitic lava extrusions. These rocks have the same general compositional and petrographic features as the ignimbrites. Fragments o f fresh lava are a bunda nt in the ignimbrites and indicate that lava prod uctio n was coeval with the ignimbrites. One of the youngest lavas on the southwestern flanks (Table 3) rests on t o p of the Toconquis ignimbrites. Second, the Toconquis ignimbrites on the western flanks show proximal

244 facies, in particular coarse co-ignimbrite lag breccias, b o m b sags and coarse plinian deposits, in localities close to the caldera wall. The presence of blocks up to 2 m in diameter in the lag breccias can only be explained if the vents were on the western rim. Third, basement fragments in the ignimbrites coincide with lithologies observed in the western caldera wall. On the eastern flanks two of the ignimbrites (Leon Muerto ignimbrites) contain abundant dioritic basement gneiss only found on the eastern flanks and they show proximal facies. The age and distribution of the Toconquis ignimbrites are interpreted in the following way. A large pluton of silicic magma ascended through the crust. As the magma approached the surface two major north--south faults, about 20--25 km apart, provided convenient lines of weakness, allowing silicic magma to penetrate to the surface at about 6.35 Ma. The two regional faults coincide with the western and eastern margins of the young caldera. Both these margins can be traced on satellite photographs into large regional faults to the north and south of Cerro Galen. The early eruptions produced relatively small volume valley-ponding eruptions with vents located sometimes on the eastern fault (Leon Muerto ignimbrites) but more often on the western flanks, judging from the greater accumulation of lavas and evidence that many of the Toconquis ignimbrites (including the Cueva Negra ignimbrite) were erupted from the west. The largest eruptions occurred between 5 and 4 Ma, presumably as the magma b o d y got closer to the surface and exploited the fault-bounded conduits. These eruptions were of sufficient magnitude to have formed calderas, but there is no evidence preserved to demonstrate their existence. There is no direct evidence for pre-caldera ring fractures. The ignimbrites of the pre-caldera Toconquis Formation are typical of many pyroclastic deposits in other parts of the world. They exhibit the familiar associations of plinian deposits, surge horizons, gas escape pipes, proximal lag breccias and multiple-graded flow units. They contain moderate to abundant pumice contents, abundant lithics and have poor to moderate crystal contents. Whole-rock compositions and the presence of banded dacitic pumice in the uppermost flow units of some members suggest that they erupted from weakly zoned magma chambers. The initial plinian deposits suggest volatile-rich caps were present and the presence of many internal divisions reflect the pulsatory character of the explosive activity. They are interpreted as forming from column collapse at vents located along major regional faults. A substantial period of quiescence (~ 2 Ma) separated the Toconquis ignimbrites from the Cerro Galen ignimbrite and caldera formation. Formation of river terraces before eruption could represent regional swelling prior to eruption as magma pressure increased (Smith and Bailey, 1968). The eruption is estimated to have produced 1000 km 3 of rhyodacitic ejecta. A 30--200-m-thick outflow sheet was formed extending up to 100 km in all directions, and at least 1.4 km of intracaldera ignimbrite was ponded

245 in the new caldera. The ignimbrite is thus a good example of an event where caldera collapse was initiated during the eruption (Lipman, 1984). The caldera was clearly influenced by the eastern and western regional faults which are thought to have controlled its width. The Cerro Galen ignimbrite contrasts markedly in its character with the Toconquis ignimbrites. The ignimbrite is remarkably homogeneous with a high crystal content and low pumice content. The rare pumice clasts are also crystal-rich and highly vesicular. In many outcrops only a single flow unit is descernible, showing that the eruption was a rapid event, unlike the earlier ignimbrites which are characteristically accumulations of many flow units. In the northeast three flow units were recognised in proximal localities, but these divisions could not be traced far. There is also no basal plinian deposit, which implies that the eruption developed extremely quickly to massive proportions. There was apparently no initial period of sufficiently low discharge rate, where a convecting column could be sustained. A high magma eruption rate is also indicated by the presence of ignimbrite on ridges several hundred metres above the valleys on the western distal areas of the sheet. Despite the evident intensity of the eruption, however, the ignimbrite is exceedingly poor in lithics ( ~ 0.5%). Even in proximal localities near the caldera rim, lithic diameters rarely exceed 5 cm and lag breccias are not present. Lithics up to 30 cm diameter occur in some horizons in the intracaldera ignimbrite. Pumice clasts analysed so far show remarkable compositional uniformity. There is no evidence for a crystalpoor, volatile-rich cap to the magma chamber. The eruption, on present evidence, occurred from a crystal-rich homogeneous chamber where zoning was either weak or absent (compare Smith, 1979; Hildreth, 1981). The striking contrasts with the older ignimbrites suggest a fundamentally different mechanism of eruption for the ignimbrite. We note that m a n y of the largest volume ignimbrites of the Central Andes resemble the Cerro Galen ignimbrite in field characteristics and must share a c o m m o n origin. There are two main factors which might account for the differences. First the magma chamber was homogeneous, crystal-rich and lacked a volatilerich, crystal-poor cap. The physical properties of the magma would thus have been very different from zoned magma bodies. For example, high crystal content would greatly increase viscosity. We are, however, uncertain what the actual effects of these differences from zoned systems on eruption style would be. An appraisal of the physical properties and volatile contents of the magma must await detailed analysis of the mineral chemistry. The second factor is the character of the eruption itself. We propose that the eruption was caused by the catastrophic foundering of a coherent cauldron block into the magma chamber. Many of the field features are consistent with the idea that sudden and rapid subsidence initiated the eruption. The subsidence caused a very high discharge rate as magma was forced up the bounding ring fracture which immediately created conditions for column collapse. High discharge rates would also be required to create

246 a single thick extensive outflow sheet. The scarcity of lithic clasts can also be interpreted in such a model. First, if the ring fractures are outward dipping then the fracture will widen automatically as the cauldron block subsides, leading to increasing discharge rates (Druitt and Sparks, 1984). No conduit erosion is required to widen the vent, as is often presumed to be the case in central vent eruptions (Wilson et al., 1980). Second, as the cauldron block subsides, the explosions will be confined to the footwall of the caldera wall. If subsidence of hundreds or thousands of metres takes place, only fine-grained ejected may be capable of flowing over the rim of the depression into the outflow. Coarse lithics would be confined to the intracaldera facies. This model does not completely account for the lack of coarse lithic clasts since the early flows would not be screened by the caldera walls and other evidence suggests highly energetic eruption. One possibility is that an old caldera depression already existed from the Toconquis ignimbrites prior to the Cerro Galen eruption. If the eruption began in such a pre-existing depression only fine-grained material might be able to escape to form the outflow sheet. The relationship between the Cerro Galen and Toconquis magmas is not yet clear. A 2-Ma time gap is sufficiently great to interpret the magmas as separate batches. However, the geochemistry of the two groups of ignimbrites is similar in broad terms. The difference between the Cerro Galen magma, which is more evolved, and the Toconquis magma could be accounted for by sanidine fractionation which would increase SiO~ but lower K20 and Ba, as is observed. The samples of Cerro Galen and Real Grande pumice have the same STSr/S6Sr ratios. If the magma chamber was sufficiently large (comparable in diameter to the caldera) then 2 Ma may well have been too short a period for the Toconquis chamber to have completely solidified. After the Cerro Galen eruption a lake formed in the caldera on top of the ignimbrite. Talus aprons and fluvial fans formed, sloping into the depression as the caldera walls eroded. Resurgence involved uplift along the eastern regional fault, causing radial dips in the ignimbrite. Shortly after eruption ( - 2 Ma) rhyodacite lavas were extruded along the northern extension of the eastern fault. We suggest that there may be a close link between resurgence and post-caldera volcanism. A simple explanation of the resurgence is that magma ascended up the eastern fault. In the caldera the magma could n o t penetrate to the surface through the low-density intracaldera ignimbrite, but instead formed a shallow intrusion which pushed up the resurgent dome. In the north, however, thick ignimbrite was not present and so magma reached the surface. Resurgence also tilted up lake sediments and reversed the slopes on talus slopes and fluvial fans. We emphasise the importance of regional faults, which have dominated the development of the Cerro GalOre. Ring fracture volcanism, except for the main eruption, is not evident in either the pre- or post-caldera periods. The regional faults have controlled the position of the caldera. The post-

247 caldera lavas are also l o c a t e d along a regional fault r a t h e r t h a n t h e ring fracture. In detail, t h e Cerro Galen raises several new q u e s t i o n s o n caldera m a g m a t i s m . D o the t w o episodes o f ignimbrite volcanism r e p r e s e n t separate p l u t o n s e m p l a c e d r a p i d l y after o n e a n o t h e r or can t h e y be linked t o processes o p e r a t i n g in t h e same c h a m b e r ? W h y are the geological and petrological characteristics o f the t w o episodes s o d i f f e r e n t in closely c o m p a r a b l e m a g m a t y p e s ? Much a t t e n t i o n has been given in r e c e n t studies to z o n e d ignimbrites like t h o s e o f t h e T o c o n q u i s f o r m a t i o n (Hildreth, 1981). However, h o m o g e n e o u s , crystal-rich ignimbrites, like t h e Cerro Gal~in ignimbrite are a b u n d a n t in t h e A n d e s a n d deserve detailed s t u d y . ACKNOWLEDGEMENTS The Cerro Galfin e x p e d i t i o n was s p o n s o r e d by m a n y organisations, all o f w h o m are a c k n o w l e d g e d . The R o y a l Society, R o y a l G e o g r a p h i c a l S o c i e t y and the British A r m y provided substantial financial and logistic s u p p o r t . We are particularly grateful to o u r colleagues f r o m the R o y a l Electrical and Mechanical Engineers w h o enabled us to d e v o t e o u r time exclusively to g e o l o g y in a r e m o t e and inhospitable terrain. T h e field studies were carried o u t with A r g e n t i n e colleagues f r o m the A r g e n t i n e S u r v e y and Servicios Mineras. We a c k n o w l e d g e the c o n t r i b u t i o n o f R.E. de Barrio, G. Gillou and O.E. G o n z a l e z to field studies. The w o r k was carried o u t while P.W.F. was a visiting scientist at the L u n a r a n d P l a n e t a r y I n s t i t u t e , H o u s t o n , Texas. A review b y C.J.N. Wilson substantially i m p r o v e d t h e paper. RSJS is c u r r e n t l y s u p p o r t e d b y the B.P. V e n t u r e Research Unit. REFERENCES Acenolaza, F.G., Toselli, A.J. and Gonzalez, O., 1976. Geologia de la region comprendida entre el Salar del Hombre Muerto y Antofagasta de la Sierra, Provincia de Catamarca. Rev. Asoc. Geol. Argent., 31 : 127--315. Baker, M.C.W., 1981. Nature and distribution of Upper Cenozoic ignimbrite centres in the Central Andes. J. Volcanol. Geotherm. Res., 11 : 293--315. Coira, B., Davidson, J., Mpodozis, F. and Ramos, V., 1982. Tectonic and magmatic evolution of the Andes in Northern Argentina and Chile. Earth°Sci. Rev., 18: 303-332. Druitt, T.H. and Sparks, R.S.J., 1982. A proximal ignimbrite breccia facies on Santorini, Greece. J. Volcanol. Geotherm. Res., 13: 147--171. Druitt, T.H. and Sparks, R.S.J., 1984. Dynamics of caldera collapse in silicic volcanic centres. Nature, 310: 679--681. Francis, P.W. and Baker, M.C.W., 1978. Sources of two large ignimbrites in the Central Andes: some Landsat evidence. J. Volcanol. Geotherm. Res., 4: 81--87. Francis, P.W., Hammill, M., Kretzschmar, G.A. and Thorpe, R.S., 1978. The Cerro Galan Caldera, North-west Argentina and its tectonic setting. Nature, 274 : 749--751. Francis, P.W., Thorpe, R.S., Moorbath, S., Kretzschmar, G.A. and Hammill, M., 1980. Strontium isotope evidence for crustal contamination of calc-alkaline volcanic rocks from Cerro Galen, northwest Argentina. Earth Planet. Sci. Lett., 48 : 257--267. Francis, P.W., Baker, M.C.W. and Halls, C., 1981. The Kari Kari caldera, Bolivia, and the Cerro Rico Stock. J. Volcanol. Geotherm. Res., 10: 113--124. Francis, P.W., O'Callaghan, L., Kretzschmar, G.A., Thorpe, R.S., Sparks, R.S.J., Page, R.N., de Barrio, R.E., Gillou, G. and Gonzalez, O.E., 1983a. The Cerro Galan Ignimbrite. Nature, 301: 51--53.

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