Bulletin of Volcanology

, Volume 71, Issue 8, pp 933–952

Palaeoenvironment reconstruction, volcanic evolution and geochronology of the Cerro Blanco subcomplex, Nevados de Chillán volcanic complex, central Chile

Authors

    • Lancaster Environment Centre, Faculty of Science and TechnologyLancaster University
    • British Geological Survey
  • Jennie S. Gilbert
    • Lancaster Environment Centre, Faculty of Science and TechnologyLancaster University
  • David W. McGarvie
    • Department of Earth and Environmental SciencesThe Open University
  • José A. Naranjo
    • Servicio Nacional de Geología y Minería
  • Malcolm S. Pringle
    • Department of Earth, Atmospheric and Planetary SciencesMassachusetts Institute of Technology
Research Article

DOI: 10.1007/s00445-009-0277-7

Cite this article as:
Mee, K., Gilbert, J.S., McGarvie, D.W. et al. Bull Volcanol (2009) 71: 933. doi:10.1007/s00445-009-0277-7

Abstract

Nevados de Chillán Volcanic Complex, central Chile, has been active for at least 640 ka—a period spanning a number of glacial and interglacial periods. Geologic mapping, radiometric dating and geochemical analysis have identified six new volcanic units and produced four new 40Ar/39Ar ages for Cerro Blanco, the northern subcomplex of Nevados de Chillán volcano. Compositions range from dacite to basaltic-andesite and a new geologic map is presented. Examination of lava fracture structures on both newly mapped lavas and those mapped during previous studies has enabled interpretations of former eruptive environments. Palaeoenvironment reconstructions, combined with 40Ar/39Ar ages and comparison with the marine oxygen isotope record, show that at least three phases of volcanic activity have occurred during the evolution of Cerro Blanco: (1) a constructive, pre-caldera collapse period; (2) a period of caldera formation and collapse; and (3) a constructive period of dome growth forming the modern day volcanic centre. This style of volcanic evolution, whereby large-scale caldera collapse is followed by growth of a new stratocone is common at Andean volcanoes.

Keywords

Volcano–ice interactionNevados de ChillánCerro BlancoPalaeoenvironment reconstructionSnow-contactCaldera-collapse40Ar/39Ar dating

Introduction

Nevados de Chillán Volcanic Complex (NCVC) is situated at 36° S, 71° W in the Southern Volcanic Zone (SVZ) of the Andean cordillera, central Chile. It is a large stratovolcano composed of two volcanic centres situated 6 km apart along a NNW–SSE trending axis (Fig. 1) (Dixon et al. 1999).
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Fig. 1

Location map of the northwestern region of Nevados de Chillán Volcanic Complex, showing the locations around the Cerro Blanco subcomplex referred to throughout this paper. Base map: Nevados de Chillán Sheet 3645-7115, Instituto Geográphico Militar de Chile. Inset shows location of Nevados de Chillán within Chile

The two subcomplexes of Nevados de Chillán, Cerro Blanco and Las Termas, lie at the NW and SE ends of the axis, respectively (Fig. 2). The Las Termas subcomplex is predominantly dacitic, whilst the Cerro Blanco subcomplex is predominantly basaltic-andesite to andesite in composition (Dixon et al. 1999). Volcanic products include subaerial, subglacial and snow-contact lavas, pyroclastic flow deposits, tephras, lahar deposits and scoria cones (Dixon et al. 1999; Mee et al. 2006). High precision 40Ar/39Ar and 14C analysis reveals that the volcano has been active since at least 640 ka, with the eruption of the subglacial Los Pincheira lavas, which have an estimated remaining volume of 10 km3 (Dixon et al. 1999). Activity continues to the present day, with recent eruptions occurring during September 2003 (Naranjo and Lara 2004) and then in January 2009 (http://www.volcanolive.com/chillan.html) both from the Las Termas subcomplex.
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Fig. 2

Nevados de Chillán Volcanic Complex, with the Cerro Blanco subcomplex (NW) at left and Las Termas subcomplex (SE) at right (viewed looking east). The two subcomplexes are approximately 6 km apart

Cerro Blanco is composed of several volcanic cones, each with their associated lavas. The most recent of these lavas, the Santa Gertrudis scoria cone and lava, was erupted between 1861 and 1865 (Dixon et al. 1999; Mee et al. 2006) and dominates the northwestern flank of Cerro Blanco. To the north and west of Cerro Blanco are curved cliff sections interpreted by Déreulle and Déreulle (1974) as remnants of two caldera walls (Fig. 1). Although this theory is now generally accepted, evidence is restricted to the morphological structure of the cliff escarpment, clearly visible in the field and on aerial photographs. The inner cliff sections are thought to represent a younger caldera approximately 5 km in diameter, with the outer cliffs suggesting an older caldera of approximately 10 km in diameter (Fig. 1).

Geologic mapping by Dixon et al. in 1999 concentrated on the cone and flanks of the Cerro Blanco and Las Termas subcomplexes and the surrounding valleys, with volcanic products being assigned to the appropriate subcomplex on the basis of geochemistry. The highly complex caldera wall areas were thus grouped together as CB1 (the Lanalhue Lavas of Dixon et al. 1999), covering an area of ~15 km2. Since much of the area is dominated by steep-sided and scree covered slopes, interpretation of stratigraphic relationships are mostly confined to the caldera wall cliffs, hence field mapping for this project was concentrated in these localities. Detailed mapping during 12 weeks of fieldwork in 2001 and 2002, have allowed us to identify three new volcanic units along the outer caldera wall (OCW) and upper Santa Gertrudis valley and three new volcanic units along the inner caldera wall (ICW) and these are presented in a new geologic map (Fig. 3). Volcanic facies from each of these units are described in two sections: (1) the outer caldera wall and upper Santa Gertrudis valley (SGV) units and facies, and (2) the inner caldera wall units and facies.
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Fig. 3

Geologic map of the Cerro Blanco subcomplex

We interpret the palaeoenvironment at the time of emplacement for each of these facies. These interpretations are combined with geochemical data, 40Ar/39Ar dates and evidence from the marine oxygen isotope record (MOIR) to evaluate the volcanic evolution of Cerro Blanco. It must be noted that accessibility to most of the exposed rock around the outer caldera wall and Santa Gertrudis valley is severely limited due to the extreme topography, and although access can be gained via the foot of the Santa Gertrudis valley to the north, this was beyond the scope of this project. Geochemical and radiometric analyses are therefore limited for these units but observations are included for completeness.

Field observations and petrography

Field observations for each of the newly mapped units are described below. Table 1 summarises the field observations and Table 2 gives a summary of the geochemistry and petrology for each newly mapped unit. Further geochemical data are provided with the online version of this article. The distributions of newly mapped and existing units at Cerro Blanco are provided in Fig. 3.
Table 1

Summary of field observations for newly mapped units at Cerro Blanco

Unit

Unit name/facies

Geomorphology/structure

Texture/fracture types

SGV1

North Santa Gertrudis Valley (SGV) lavas

150–200 m high cliffs—upper regions comprise: 10–20 cm wide columns, <1 m length, variable orientation

Reddish-brown, strongly altered, massive lava (NB: Poor accessibility meant only upper parts of cliffs could be seen through binoculars)

SGV2

South Santa Gertrudis Valley (SGV) lavas

150 m high cliffs comprising zones of massive/weathered lava, columnar jointed lava and fine-grained, bedded deposits (see Fig. 4)

Huge complexity in textures and fracture types—detailed observations limited due to access

OC1

Outer caldera wall lavas

50–100 m high cliffs, 2.5 km long; 10 lava flow deposits, 5–10 m thick, inter-bedded with lens-shaped patches of red oxidised scoria

Smooth/striated upper surfaces at western end; broad vertical joints towards eastern end; holocrystalline, devitrified

Outer caldera wall breccias

20 m v-shaped patch at base of OCW lavas to west of SGV; 50 m v-shaped patch on east side of SGV (observations through binoculars only)

No detailed observations due to limited access

IC1

Inner caldera wall lavas

6 lava flow deposits (see Fig. 7), each 3–8 m thick, extend laterally 10–100m; dip 5–10° W away from Cerro Blanco summit

Lavas 1, 2, 5 and 6: porphyritic, broad columnar jointing, scoriaceous upper margins. Lavas 3 and 4: some bulbous lobes, glassy/hackly-jointed exterior. Lavas separated by irregular patches of breccia, clasts up to 40 cm diameter in fine-grained matrix (Fig. 7)

IC2

Lanalhue vent facies: welded breccia and spatter

Multiple beds to south of vent, increasing dip from near horizontal at base to 45° N at top of succession. Beds are 0.25–3 m thickness

See Fig. 9

Lanalhue vent facies: massive andesite body

8 m thick, andesite intrusion, exposed at base and to north side of centre of Lanalhue Vent

Massive, crystal-rich, highly vesicular: vesicles sub-millimeter to >45 cm diameter accumulations. Gradational contact between intrusion and breccia (see Fig. 9)

North ridge vent facies: north ridge lavas

Several lava flow deposits, form bedded semi-circle at southern end of North Ridge (2,400 m elevation); lava strongly eroded/polished with 2 sets of striae to N

Lavas have red scoriaceous tops, platy centre; to north (2,350 m elevation), lava shows greater degree of welding and no scoriaceous top

North ridge vent facies: upper Santa Gertrudis Valley (USGV) lavas (see Mee et al. 2006)

3 geomorphologic zones: Zone 1 (2,300–2,260 m elevation) elongate lobes 8–13 m long, 4–6 m wide, 2–3 m high; Zone 2 (2,250 m) Several interconnected bulbous lobes on plateau; Zone 3 (2,250–220 m) Steep-sided lava

Zone 1: Autobreccia and pseudopillow fractures; Zone 2: Glassy outer carapace, crystalline interiors, hyaloclastite around base; Zone 3: Glassy outer carapace, narrow columnar joints orthogonal to outer surface and hyaloclastite

IC3

Platy sills and dykes

1–3 m thick; sills found along basal margins of vent intrusion (IC2); one sill breached ground surface at ~2,325 m and formed lave flow deposit; dykes cut through Lanalhue vent, one dyke cuts through IC1 lavas;

Sill that breached ground surface produced platy lava flow deposit with red, scoriaceous top and glassy base with pseudopillow fractures—extends only 1.5 m but huge volume of scree suggests was once much more extensive

Glassy lava lobes

2 bulbous lobes, 3–5 m high, 4–6 m wide plus several (~1 × 1 m) smaller, interconnected lobes exposed in scree. Larger lobes have near-vertical, down-slope surfaces; elevation of lobes ~2,275 m

~25 cm thick glassy, hackly-jointed exterior; hackly joints are 8–15 cm diameter, 4–8 sides. Interior is holocrystalline, 10–30 cm thick concentric plating, which break around phenocrysts. Irregular patches of hyaloclastite surround lobes

Table 2

Selected representative geochemical and petrologic analyses of newly mapped units at Cerro Blanco

Unit name

SGV1

OC1

(IC1) Lanalhue caldera wall lavas

(IC2) ICW vent facies

(IC3) Glassy lava lobes

Facies and sub-facies names

North SGV lavas

Outer caldera wall lavas

Lava 1

Lava 2

Lava 3

Lava 4

Lava 5

Lava 6

Welded breccia/spatter

Vesicular intrusion

USGV lava

Lava lobes

Platy sills/dykes

Sample no.

CB53

CB59

CB23

CB24

CB48

CB26

CB27

CB30

CB35

CB74

CB55

CB02

CB43

SiO2

60.96

60.17

55.15

54.07

59.12

58.17

62.94

65.33

59.03

58.92

58.37

57.52

60.58

TiO2

1.36

1.64

1.00

0.99

1.09

1.25

0.94

0.92

1.39

1.42

1.07

1.28

1.44

Al2O3

16.33

15.20

18.01

17.57

17.34

16.17

16.33

15.71

16.53

16.02

17.11

17.00

16.85

Fe2O3

7.13

8.00

8.22

8.21

6.61

7.63

5.97

5.23

7.21

8.02

7.15

7.92

6.72

MnO

0.13

0.14

0.13

0.14

0.12

0.13

0.10

0.09

0.14

0.14

0.11

0.14

0.13

MgO

2.09

2.40

4.42

4.93

3.06

3.42

2.27

1.70

2.26

2.62

3.32

3.24

1.93

CaO

4.66

4.85

8.66

8.65

5.96

6.66

4.76

3.81

4.37

5.70

6.67

6.75

4.12

Na2O

5.00

4.74

3.86

3.71

4.55

4.06

4.45

4.48

5.58

4.71

4.19

4.36

5.54

K2O

2.15

1.75

0.68

0.71

1.78

1.42

1.91

2.35

1.95

1.44

1.43

1.17

1.94

P2O5

0.35

0.54

0.19

0.19

0.23

0.27

0.20

0.20

0.43

0.36

0.25

0.39

0.47

SO3

0.01

0.00

0.00

0.00

0.00

0.01

0.00

0.00

0.01

0.00

0.00

0.00

0.01

Rb

64.50

36.70

68.91

96.84

54.38

41.13

45.49

67.17

54.68

68.11

50.26

42.33

54.50

Sr

395.2

535.3

360.02

333.63

451.09

509.68

476.24

378.69

395.77

385.40

433.82

471.25

388.70

Y

30.5

18.9

35.78

40.31

22.68

26.34

30.45

32.61

38.51

31.50

30.57

27.64

35.90

Zr

242.7

129.7

270.08

299.61

182.75

169.82

189.02

246.59

221.61

246.81

215.86

169.61

239.20

Rock type

A

A

BA

BA

A

A

A

D

A

A

A

B–A

A

Phenocrysts (%)

7

10

5–10

5–10

5–10

5–10

5–10

5–10

<5

n/a

<5

5

15

Phenocryst assemblage

f+cpx+opx+ores+ap

f+cpx+opx+ol+ores

f+cpx+opx+ol+ores

f+cpx+opx+ol+ores

f+cpx+opx+ores

f+cpx+opx+amp+ores+ap

f+cpx+opx+ores+ap

f+cpx+opx+ores+qtz+ap

f+cpx+opx+ores

f+cpx+opx+ol+ores

f+cpx+opx+ol+ores

f+cpx+opx+ol+ores

f+opx+cpx+ores

All samples from Cerr Blanco collected for this project between February 2001 and April 2002 and analysed at the XRF Facility at the University of Leicester. Samples were numbered in order of collection and prefixed “CB” for Cerro Blanco

A andesite, BA basaltic-andesite, f feldspar, cpx clinopyroxene, opx orthopyroxene, ol olivine, amp amphibole, ap apatite, qtz quartz, ores Fi–Ti oxides

Santa Gertrudis valley and the outer caldera wall

The upper reaches of the Santa Gertrudis valley (SGV) begin ~2.5 km to the north of the Cerro Blanco summit, where several tributaries collect runoff from the north and western flanks of Cerro Blanco, the ICW and the OCW before deflecting north into the Río Santa Gertrudis and the Santa Gertrudis valley proper. From Cerro Blanco, only the tops of the cliffs can be accessed. Here, the valley is ~30 m wide with near vertical cliff faces of 100–200 m in height. The north and south sides of the valley appear to be comprised of two entirely different lithologic units (SGV1 and SGV2, respectively), with the outer caldera wall unit (OC1) making up the third newly mapped unit in this locality.

SGV1—North Santa Gertrudis valley lava

Despite earlier mapping by Dixon et al. (1999) identifying the lavas in this area as the 81.5 ka old CB1 Lanalhue Lavas, it is clear from the much greater degree of weathering and alteration, that the lava on the north side of the Santa Gertrudis valley is much older and must be considered as a separate unit (SGV1). The SGV1 lava exhibits a mixture of chaotic, hackly joints and irregular columnar joints. The latter are ~10–20 cm in width and sometimes up to 1 m in length, although are generally much less than this. Since the cliff face was inaccessible, detailed measurements of column orientation and geometry was not possible. The lava itself is dark orange–brown and heavily weathered in appearance and geochemical analysis reveals this unit to be andesitic in composition (69 wt.% SiO2) (Table 2).

SGV2—South Santa Gertrudis valley sequence

The SGV2 unit on the south side of the valley is, by contrast, much more complex with four separate facies identified (Fig. 4). It is likely that each of these facies represents a separate geologic unit but since detailed studies of the cliffs were prevented by poor accessibility the facies have been grouped together for simplicity. The base of the cliffs is comprised of ~100 m thickness of massive, slightly weathered lava with no obvious fracture patterns or bedding (Zone 1). Above this, lies a 10–70 m thick section of pale grey, fine-grained and bedded material (Zone 2). This becomes thinner towards the west where it comes into contact with a 40 m thick lens of columnar jointed lava (Zone 3). The succession is completed by the IC2 lava lobes (discussed later), which is the only part of the cliff face that can be accessed from the cliff tops and hence no samples of the SGV2 unit could be collected for geochemical or radiometric analysis.
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Fig. 4

The SGV2 unit on the southern side of the Upper Santa Gertrudis valley showing a the SGV2 unit split into three zones on the lower part of the cliffs and overlain by the IC2 lavas, b the IC2 lava lobes, and c a schematic diagram of the cliffs showing the entire succession

OC1—outer caldera wall lavas and breccias

Overlying the SGV1 lava on the north side of the valley is the outer caldera wall (Fig. 1). It is ~2.5 km long and comprises at least 10 separate andesite (60 wt.% SiO2) lavas flow deposits, each 5–10 m thick and inter-bedded with irregular layers (<3 m thick) of red scoriaceous material (Fig. 5). Towards the western end of the OCW where the upper elevation is ~50 m lower than the eastern end, the lava is extremely polished and exhibits two sets of striae with variable orientations. The first set of striae is broad and deep and has the same weathered surfaces of the surrounding rocks. The second set, which cuts through the first, is much sharper and narrower, with fresher grey surfaces. On both the west and east sides of the Santa Gertrudis valley, small patches of breccia are banked up against the OC1 lavas. The east side of the valley was inaccessible but observations through binoculars reveal that the breccia is bedded, dips gently (<10°) to the south and contacts unconformably with the adjacent OC1 lava.
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Fig. 5

The inter-bedded lava and scoria layers that comprise the outer caldera wall cliffs (OC1); viewed from the North Ridge (northern end of the inner caldera wall) and looking N

Inner caldera wall units and facies

The inner caldera wall (ICW) is also ~2.5 km long and curves around the northwestern flank of Cerro Blanco (Figs. 1 and 3). It is characterised by a topographic high at each end: Lanalhue at the southern extremity and the North Ridge at the northern extremity (Fig. 1). Three different volcanic units have been identified along the ICW, each with characteristic facies: the inner caldera wall lavas (IC1), the inner caldera wall vent facies (IC2) and a series of lava lobes, sills and dykes (IC3).

IC1—inner caldera wall lavas

The inner caldera wall lavas are best exposed at the southern end of the ICW, where at least six individual lava flow deposits can be observed in the south-facing cliffs (Figs. 6 and 7). The near vertical caldera wall scarp cuts through the IC1 lavas to the east and a 25 m think section of the lava is exposed half-way up the cliff face, with scree obscuring the lower 20–30 m and the IC2 Lanalhue vent topping the sequence (Fig. 6). Each layer is 3–8 m thick and extends westwards (away from Cerro Blanco) for 10–100 m. Whilst the two upper and two lower lava flows dip gently westwards (~5°), the two central lavas are much less uniform and in places the lava has draped over the underlying lavas forming several bulbous and hackly jointed lobes (Fig. 7). The two upper and two lower lavas exhibit broad columnar jointing and scoriaceous upper margins, whilst the two central lavas are bulbous in places, with glassy texture and hackly or pseudopillow fractures (Mee et al. 2006). Individual lava flows are separated by irregular patches of breccia, which is comprised of 5–40 cm angular clasts of lava in a pale grey and fine-grained matrix. The geochemical composition of the lava flows range from basaltic-andesite (54 wt.% SiO2) to dacite (65 wt.% SiO2).
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Fig. 6

Southern section of the inner caldera wall showing the distribution of the newly mapped IC1, IC2 and IC3 units. The view is looking SW from the top of the North Ridge and the field of view is approximately 1 km wide

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Fig. 7

Schematic diagram of the IC1 inner caldera wall lavas at the southern end of the inner caldera wall. At least six individual lava flow deposits are observed and can be traced laterally for ~100 m, although irregularities on the cliff face make it difficult to trace all of the flows along the entire length of the cliff section (~500 m). Not to scale

IC2—inner caldera wall vent facies

The topographic highpoints at either end of the ICW (Lanalhue and the North Ridge) are characterised by remnants of volcanic vents. At the southern Lanalhue end, a 50 m thick cross-section through the vent facies is exposed towards the top of the inner caldera wall cliffs (Fig. 8). Several facies can be identified at the Lanalhue and North Ridge vent locations, ranging in composition from basaltic andesite (55 wt.% SiO2) to andesite (59 wt.% SiO2). The main constituent, present at both localities, is a series of bedded and welded breccia and spatter deposits. Secondly is a vesicular, andesitic body, which is exposed in the Lanalhue vent cross-section at the southern end of the inner caldera wall. Thirdly, is the Upper Santa Gertrudis valley (USGV) lava, which first crops out on the eastern flanks of the North Ridge and is exposed eastwards to the top of the Santa Gertrudis valley. These facies are described in more detail below and in Tables 1 and 2, with the exception of the USGV lava, which is the subject of a more detailed study by Mee et al. (2006) and hence is only summarised here.
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Fig. 8

Relationship between the IC1, IC2 and IC3 units in the volcano-facing cliff face at the southern end of the inner caldera wall. The view is looking W away from the summit of Cerro Blanco and the field of view is approximately 200 m wide (Ar40/Ar39 age is from Dixon et al. 1999)

Welded breccia and spatter

A cross-section through the Lanalhue vent exposes an ~20 m think sequence of andesitic, bedded breccia and spatter, displaying varying degrees of welding. Figure 9 summarises the typical structure and composition of the vent. The beds can be traced southwards for up to 70 m, where they form a fanned array with the thickness of the beds increasing from 25 to 3 m with distance from the centre of the vent. The dip of the beds increases upwards through the succession from near horizontal to 45° S (i.e. back towards the centre of the vent). The structure of each bed becomes more chaotic towards the centre of the vent and, in places, exhibits substantial brecciation, slumping and faulting (Fig. 9). Contacts between the beds are often unconformable, particularly close to the centre of the vent. Clast-supported, polymict breccia containing angular to heavily smeared glassy lithics and scoria bombs, fills the centre of the vent (Fig. 9). A grey, platy dyke from the IC3 unit (described below) cuts through this breccia at an angle of ~85° S. Small, 2–3 cm clasts of the dyke have been incorporated into the adjacent breccia (Fig. 9).
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Fig. 9

Schematic diagram showing the structure of the IC2 vent at the southern end of the inner caldera wall. Inset 1 shows the typical structure of an individual bed and inset 2 shows the contact between the vent-fill breccia and the underlying andesitic intrusion

To the north, any beds that were deposited are now obscured by a 50 m wide and up to 15 m high andesitic body. The texture is massive and vesicular with no bedding or fracturing. Vesicles are roughly spherical and range in size from 0.1 to 2 cm, with large vesicle accumulations of >45 cm in diameter also present. The contact between the andesite body and the surrounding breccia is gradational over ~30 cm, with clasts of breccia being incorporated into the andesite.

To the north, the highest point along the ICW reaches 2,456 m at the top of the North Ridge. Here, a small plateau of ~0.5 km2 that slopes gently to the NW (away from the summit of Cerro Blanco) also exhibits a series of welded breccia and spatter beds (Fig. 10). The beds form a curved, semi-circular array at the southern end of the North Ridge (nearest to the Cerro Blanco summit) and exhibit similar features and structures as the welded breccia and spatter beds of the Lanalhue vent (Fig. 9). As the North Ridge slopes away from Cerro Blanco, dropping in elevation from 2,456 to 2,375 m, the vent facies become increasingly polished and eroded and is marked by one set of striations.
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Fig. 10

Photograph and sketch of the North Ridge at the northern end of the inner caldera wall: a showing the dimensions of the North Ridge, with inset showing part of the IC3 subglacial lobes; b showing the distribution of different facies on the North Ridge. View is looking S, from the outer caldera wall

Upper Santa Gertrudis valley (USGV) lava

The andesitic, USGV lava can be divided into three textural and geomorphological zones, each occurring at different elevations (Table 1): autobrecciated and elongate lava lobes with pseudopillow fractures at the upper elevations; interconnected, hackly-jointed lava lobes and hyaloclastite at the mid-elevations; and glassy, columnar-jointed lava at the lower elevations. Detailed descriptions and palaeoenvironment interpretations for these facies and the Santa Gertrudis lava are given in Mee et al. (2006).

IC3—subglacial lobes, sills and dykes

This unit can be divided into two facies: (1) a series of grey platy sills and dykes cutting through the inner caldera wall vent sequence and in places cropping out onto the current topographic surface; and (2) glassy lava lobes situated on the col to the south of the Lanalhue vent sequence (Fig. 6) and on the north and eastern flanks of the North Ridge (Fig. 10).

Platy sills and dyke

The sills and dykes are 1–3 m thick, with narrow (>1–2 cm) platy fractures aligned parallel to the flow surfaces. Several platy sills cut below the IC2 andesite intrusion and in places have breached the ground surface on and above the col to the north of the Lanalhue sequence (Fig. 8). In one case, a sill has breached the ground surface at an elevation of ~2,375 m in the col region to the north of the Lanalhue vent (Fig. 11a). The lava can be followed laterally for less than 2 m before being reduced to scree and has a 10–15 cm thick, red scoriaceous upper surface (Fig. 11). The scree is estimated to cover an area of at least 100 m2.
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Fig. 11

The IC3 unit at the southern end of the inner caldera wall: a distribution of the subglacial lava lobes on the col region of the inner caldera wall and the sills that breached the ground surface above the col (inset); b the two larger subglacial lobes on the col; c detailed view of one of the lobes, showing a partial cross-section through the centre of the lobe; and d annotated sketch of the lobe in c, showing the general structure of the subglacial lobes

A 2 m wide dyke also cuts through the IC1 cliffs and is exposed in the volcano-facing cliffs (Fig. 8). It appears to feed a small laccolith at its top, which is exposed below the southern end of the IC2 welded breccia and spatter beds. Poor accessibility meant that detailed observations of this structure were limited. However, a sample collected by Dixon et al. (1999) during their 1996 expedition, is now known to have come from base of this dyke (Jennie Gilbert, oral communication, 2006).

Glassy lava lobes

Several bulbous and glassy lava lobes crop out on the col that separates the Lanalhue ridge from the central regions of the ICW and on the north and eastern flanks of the North Ridge. The best examples of these lobes are two steep-sided lava lobes on the col to the north of the Lanalhue vent (Figs. 6 and 11). The more easterly of the two lobes is 7 m high and 14 m wide, whilst the more westerly of the two lobes is 12 m high and 8 m wide. Both are bulbous in shape and have steep (>45° to near vertical) outer margins, particularly those facing down-slope. In between these two larger lobes are several smaller (~1 m diameter), interconnected lobes (Fig. 11). The outer margins of the lobes are 5–30 cm thick and dominated by narrow (8–15 cm in diameter with four to eight sides), glassy hackly joints. The interior of the lobes is, by contrast, holocrystalline with 10–30 cm thick concentric fractures that break around phenocrysts. Irregular patches of yellow, clast-supported and unbedded breccia is observed at the bases of the two larger lobes and surrounding the smaller lobes in the scree (Fig. 11). In places, finger-like structures of the breccia curve upwards into the overlying lava lobe (Fig. 11).

40Ar/39Ar analysis

By determining the ages of volcanic units that have been interpreted as having erupted under certain conditions (e.g. subglacially, in contact with snow/ice or subaerially), we are able to: (1) confirm their stratigraphic position at Cerro Blanco and, (2) compare ages with the record of global temperature (marine oxygen isotope record) in order to confirm that the interpreted conditions would have been likely at the time of eruption. Alternatively, if field observations suggest that a lava was cooled under a substantial thickness of ice, yet radiometric dates suggest that the volcano should not have been covered by snow or ice at the time of eruption, then this would trigger further study of either the volcanic fracture patterns and stratigraphic relationships or radiometric dates to ensure that the most confident interpretations were reached.

Sample selection

After establishing stratigraphic relationships in the field, eight samples were selected for 40Ar/39Ar analysis. All of these samples came from the inner caldera wall units, since poor accessibility to the outer caldera wall and Santa Gertrudis valley made it impossible to obtain samples suitable for dating. Samples were chosen on the basis of:
  1. 1.

    Palaeoclimatic importance—a mixture of subaerial, subglacial and snow-contact lavas were chosen to enable correlation of the inferred eruptive environment with the marine oxygen isotope record. It was anticipated that the ages of these samples would span a glacial/interglacial transition.

     
  2. 2.

    Stratigraphic importance—i.e. those units that displayed key stratigraphic relationships with other units not chosen for analysis; and

     
  3. 3.

    Compositional/petrographic suitability for 40Ar/39Ar dating—thin sections of all samples were studied to select samples with the least amount of chemical alteration or other possible sources of contamination.

     
Following petrographic analysis, it was obvious that many of the more glassy rocks (and therefore those most likely to have erupted during subglacial conditions) would be unsuitable for 40Ar/39Ar analysis and hence fewer of these units were analysed than was originally intended. Whole rock and groundmass separates were analysed at the Scottish Universities Research and Reactor Centre (SURRC), East Kilbride, Scotland, using the same 40Ar/39Ar incremental-heating procedures as Dixon et al. (1999), which are more fully described in Singer and Pringle (1996) and Harford et al. (2002). To deem these dates acceptable, the following criteria were used (Pringle 1993; Singer and Pringle 1996):
  1. 1.

    At least three contiguous high-temperature steps representing 50% of the 39Ar must be used in the age calculation.

     
  2. 2.

    These steps must be below the F-variate critical value (Pringle 1993; Taylor 1982; Harford et al. 2002; York 1969).

     
  3. 3.

    The plateau and isochron ages (Pringle 1993; Singer and Pringle 1996) must be concordant with one another (i.e. their errors should overlap).

     
  4. 4.

    The isochron 40Ar/36Ar intercept should not be significantly different from the atmospheric argon value of 295.

     

Results

In total, eight samples from Cerro Blanco were selected for analysis, each representing different volcanic facies. Two samples (from IC1 and OC1) did not produce a full data set to enable age calculations and two others (from IC1 and IC3) produced high errors and should, at best, be considered reconnaissance age estimates. The remaining four samples have yielded acceptable ages, although they represent only two of the newly mapped units (IC2 and IC3). Tables 3 and 4 give a full summary of both the new and published (Dixon et al. 1999) 40Ar/39Ar analyses for Cerro Blanco and the complete data repository is available from the journal’s electronic archive.
Table 3

Summary of new 40Ar/39Ar analyses from Cerro Blanco

New 40Ar/39Ar analyses

Total gas age

Age spectrum

Isochron analysis

Sample no.

Unit

Sample type

K/Ca

Age ± 2 SD (ka)

No. of stepsa

Furnace power

39Ar (%)

Age ± 2 SD (ka)

MSWDb

Age ± 2 SD (ka)

MSWDb

40Ar/36Ar intercept

CB14c

IC3

wr core

0.55

75.6 ± 0.7

7 of 15

8.0–11.0

71.8

80.0 ± 0.6

1.75

80.9 ± 1.4

2.06

291.7 ± 4.7

CB14d

IC3

wr core

0.54

73.9 ± 1.0

8 of 14

7.0–12.0

91.3

79.9 ± 0.6

1.20

79.6 ± 1.2

1.35

297.2 ± 4.2

CB74

IC2

gr mass

0.53

103.0 ± 3.5

8 of 17

6.6–8.7

62.4

92.4 ± 3.5

2.38

95.6 ± 16.6

3.86

294.5 ± 4.5

CB55

IC2

wr core

0.32

76.3 ± 2.3

7 of 16

8.0–10.5

55.8

90.0 ± 0.6

0.57

90.7 ± 1.2

0.54

293.7 ± 2.5

CB83

IC2

wr core

0.33

87.0 ± 1.5

10 of 14

6.2–13.5

83.0

88.9 ± 1.0

1.04

88.5 ± 1.5

1.17

295.6 ± 0.9

Analytical methods and data reduction are summarised in Singer and Pringle (1996). All ages relative to 28.34 Ma for Taylor Creek Rhyolite sanidine TCR-2a (85G0033)

aNumber of increments used in regression

bF variate critical value i.e. the MSWD value should be less than this number for the age to be accurate at the 99% confidence level

cFirst split of sample

dSecond split of sample

Table 4

Summary of existing 40Ar/39Ar analyses from Cerro Blanco, after Dixon et al. 1999

Published 40Ar/39Ar analyses (Dixon et al. 1999)

Total gas age

Age spectrum

Isochron analysis

Sample no.

Unit

Sample type

K/Ca

Age ± 2 SD (ka)

No. of stepsa

Increments used (°C)

39Ar (%)

Age ± 2 SD (ka)

MSWDb

Age ± 2 SD (ka)

MSWDb

40Ar/36Ar intercept

C-9

CB2b

wr core

0.940

19.0 ± 2.6

10 of 12

600–1,150

86.5

14.9 ± 0.9

0.78

14.2 ± 1.1

0.71

296.5 ± 1.3

C-27

CB2a

wr core

0.312

53.1 ± 5.1

7 of 11

525–1,350

86.8

26.2 ± 2.5

0.73

23.9 ± 2.7

0.74

296.8 ± 0.8

C-90

CB1c

wr core

0.614

93.3 ± 1.7

4 of 9

825–1,100

53.8

82.9 ± 1.8

0.37

81.5 ± 4

0.56

296.8 ± 3.2

97-01

LP

wr core

0.279

803.3 ± 18

6 of 11

700–1,100

45.0

640 ± 14

5.3

641 ± 20

4.3

295 ± 1.5

Analytical methods and data reduction are summarised in Singer and Pringle (1996). All ages relative to 28.34 Ma for Taylor Creek Rhyolite sanidine TCR-2a (85G0033)

aNumber of increments used in regression

bF variate critical value i.e. the MSWD value should be less than this number for the age to be accurate at the 99% confidence level

cField mapping during this study has confirmed this sample to be from the IC3 unit

The IC2 unit has yielded three acceptable ages: 92.4 ± 3.5 ka for the andesite intrusion located to the north of the Lanalhue vent; 90.0 ± 0.6 ka for the upper Santa Gertrudis Valley lava; and 88.9 ± 1.0 ka for the welded breccia and spatter beds of the North Ridge. A sample of the IC3 lava lobes, sills and dykes has produced two acceptable ages from different splits of the same sample. Specifically, this sample comes from the sill that has breached the ground surface above the col region at the southern end of the ICW. This unit has been dated at 80.0 ± 0.6 and 79.9 ± 0.6 ka.

Discussion

In recent years Cerro Blanco has had a permanent icecap and an even larger distribution of seasonal snow (Fig. 2). Fracture structures associated with snow and/or ice contact appear to be common features of the volcanic units on Cerro Blanco. The distribution and extent of these fractures is vital for interpretation of the local climatic conditions at the time of eruption.

The following sections use field observations and stratigraphic relationships to help interpret the eruptive settings of volcanic units at Cerro Blanco, particularly in terms of the presence or absence of snow and ice at the time of eruption. We consider the various phases of volcanic evolution of Cerro Blanco and draw comparisons with other Andean volcanoes. Finally, where 40Ar/39Ar analysis has been successful, the ages are compared with the marine oxygen isotope record (MOIR) to confirm that field and stratigraphic observations of each unit are concordant with climate conditions at the time of eruption, i.e. if a lava is interpreted as having erupted under a large thickness of ice, this should be supported by global ice volumes according to the MOIR.

Palaeoenvironment interpretations

The Santa Gertrudis valley and outer caldera wall

At depths of nearly 200 m, the Santa Gertrudis valley is likely to have been an important runoff channel for several hundred thousands of years. The presence of narrow, hackly and chaotic columnar joints on the SGV1 lavas on the north side of the valley supports the presence of water during their emplacement, although the lack of pillow structures suggests that this was not entirely subaqueous. The chaotic nature of the columns is therefore interpreted as the upper, entablature zone of a two-tiered lava flow (Degraff et al. 1989). This is similar in structure to the upper zone of the Los Pincheira lavas (Dixon et al. 1999), which was interpreted by Dixon et al. to have cooled against glacial ice with meltwater causing the fine-scale penetrative jointing of the entablature zone (Lescinsky and Sisson 1998). Whether the entablature zone of the SGV1 lava was caused by glacial meltwater or runoff from the volcano is unclear since the lower portions of the cliffs were inaccessible.

The outer caldera wall cliffs, which lie stratigraphically above the SGV1 lavas, have been interpreted by Déreulle and Déreulle (1974) as remnants of an ancient caldera wall on account of the abrupt cliff faces separating the recent volcanic cones at the centre of the complex from the older, shallow-dipping lavas to the west and north. The broad, columnar joints, holocrystalline texture and scoriaceous upper surfaces of the lava flows are indicative of slow cooling and hence the OC1 lavas are inferred to be subaerial (Cas and Wright 1987). Deep glacial striae and polishing to the upper surfaces of the cliffs at the western end of the OCW, suggest that the cliffs have been subjected to repeated glaciations. By contrast, the eastern end of the OCW, where elevation is 50–100 m higher, does not exhibit the same degree of polishing, suggesting that snow, ice and water in this area were diverted into the Santa Gertrudis valley.

The complex mixture of facies on the south side of the Santa Gertrudis valley (SGV2) are clearly not part of the SGV1 or OC1 units on the north side of the valley. Since detailed observations of the cliffs could not be made, it is not possible to make meaningful interpretations of the different facies that make up SGV2. It is possible, however, to make inferences about relative ages of the units despite the lack of 40Ar/39Ar ages, since the high levels of weathering to the north side of the valley (SGV1 and OC1), suggest they are much older than the SGV2 unit on the south side of the valley. This adds further support to the interpretation of OC1 as caldera wall cliffs, with the SGV2 unit representing the downthrown side of a caldera wall fault.

The inner caldera wall

The northern extremity of the inner caldera wall is ‘overlapped’ by the outer caldera wall cliffs, which form an amphitheatre around the north and western flanks of the North Ridge. This, along with the presence of two sets of glacial striae and a greater degree of weathering on the outer caldera wall lavas compared to one set of glacial striae on the inner caldera wall units, leads us to believe that the inner caldera wall cliffs and their associated geologic units are younger than those of the outer caldera wall and the Santa Gertrudis valley.

The IC1 lavas are the most extensive along the inner caldera wall, covering an area of at least 1 km2, and it is likely that they extend up to a further 2 km north, through the central region of the inner caldera wall (Figs. 3 and 4). It is also likely that the IC1 lavas were, at one time, more extensive to the east but have since been cut by the inner caldera wall fault with any remnants now obscured by scree and the more recent Colcura cone and lavas (Dixon et al. 1999).

Each layer of the IC1 unit is thought to represent a separate pulse of lava that flowed west away from the current Cerro Blanco stratocone. The two upper and two lower lava flows, with their crystalline interiors and scoriaceous tops, exhibit characteristics typical of subaerial lavas (Cas and Wright 1987). Although the overall dip of the lavas is west, away from Cerro Blanco, in places the two central lavas appear to have followed topographic depressions causing the morphology to be bulbous in places (Figs. 5 and 6). Hackly and pseudopillow fractures on the lava in these locations, along with their glassy texture, suggest that these lobes were cooled rapidly, either by steam, water or snow (Lescinsky and Fink 2000; Mee et al. 2006). The lack of fragmental material (and the local distribution of ice-contact features) indicates that water did not accumulate in any great volume hence it is unlikely that a thick ice sheet was present during the time of eruption (Lescinsky and Fink 2000; Lescinsky and Sisson 1998). Since snow and ice are persistent throughout the year at elevations of ~2,300 m and upwards at Cerro Blanco (i.e. the approximate elevation at which the IC1 lavas are exposed) it is more likely that portions of the lava flowed over or into snow and ice that had accumulated in topographic depressions, with steam and water from the melting snow causing hackly and pseudopillow fractures.

The IC2 vent facies is best exposed at the southern end of the ICW with the cross-section through the Lanalhue vent. Several beds of welded breccia and spatter represent successive pulses of a small (<0.25 km2) eruption. The increasing dip of the beds towards the centre of the vent, suggests that material tended to accumulate at distances of 50–70 m from the vent. The presence of unconformable contacts, brecciation and faulting within the deposits at the mouth of the vent suggests a greater degree of instability with proximity to the vent. Slumping within the beds has produced rheomorphic structures indicating that material flowed back towards the vent after initial deposition (Sumner and Branney 2002). Finally, the small andesitic body was intruded to the north side of the vent. It is suggested that the breccia that now fills the vent was part of the lava intrusion and that the high level of oxidation is due to contact with either snow or water during intrusion (Naranjo et al. 1992). Since the dyke was well insulated from the breccia, its high temperature was preserved, enabling the formation of the platy, welded and sheared structure during the high-pressure intrusion (Fig. 9). The final facies associated with the IC2 unit is the Upper Santa Gertrudis valley (USGV) lava, which is exposed on the eastern flanks of the North Ridge and in the upper reaches of the Santa Gertrudis valley. The gradation from one textural zone to another has led to the interpretation of a multi-phase cooling history of the lava, consisting of subaerial, snow-contact and then ice-constraint cooling environments at successively lower elevations, caused by the presence of a valley glacier in the Santa Gertrudis valley at the time of emplacement (Mee et al. 2006).

The youngest unit associated with the inner caldera wall is IC3, a series of platy sills and dykes and glassy lava lobes. Where one of the dykes has breached the ground surface, the scoriaceous and vesicular top suggest it cooled subaerially (Cas and Wright 1987) yet the glassy texture and polygonal, pseudopillow fractures on the base are indicative of snow-contact (Mee et al. 2006). Although the preserved lava only persists for 2 m, the large volume of scree (~0.1 km2) suggests that, at one time, the lava was much more extensive. It is likely that melting snow at the base of the lava combined with the heavily fractured and extremely brittle lava interior caused the lava to collapse.

Several pieces of evidence suggest that the glassy lava lobes, which are exposed at a lower elevation in the col to the north of the Lanalhue vent, were glacially cooled. Firstly, the glassy and hackly jointed outer carapaces, which protect the more crystalline interior, imply quenching of the lava and fracture by steam penetration (Lescinsky and Fink 2000). Since the lobe interiors are relatively crystalline compared to their glassy outer carapaces, quenching and solidification of the carapace must have allowed the fluid lava of the interior to cool more slowly and develop a larger crystal size (Lescinsky and Fink 2000). Secondly, cooling fractures can be used to infer the palaeo-cooling surface of a fractured lava body since they form perpendicular to the cooling surface (Lescinsky and Sisson 1998). Although the chaotic nature of hackly joints means that measurement of individual fracture surfaces would be unlikely to give true orientations of any former cooling surfaces, the presence of the quenched carapaces surrounding the remaining body of lava suggests they were entirely enveloped by the cooling surface at the time of emplacement.

The presence of hyaloclastite breccia around the base of the lobes would suggest that some degree of interaction with water occurred (Tuffen et al. 2001, 2002a, b). However, since there is a lack of any ash-grade fragmental material (produced by phreatomagmatic activity) it seems unlikely that a large amount of water was present. In addition to this, there are several factors that suggest that the palaeotopography was similar to that of today and hence that water would have constantly drained down slope. Firstly, part of the breccia beneath the more westerly of the two lobes has been folded upwards (Fig. 11c, d) into the base of the lobe, suggesting that some degree of post-emplacement gravitational slumping occurred. Secondly, the steep, sub-vertical front surfaces of the lobes suggest that they were constrained down-slope by a near vertical, robust cooling surface, such as ice (Fig. 11). Finally, the more easterly of the two IC3 lobes drapes either side of the existing col (Fig. 11a, b), suggesting that a similar topography to that of today was present during the time of emplacement. All these factors favour down-slope drainage of water away from the lava lobes, thus making it virtually impossible for significant volumes of water to have accumulated. It is, therefore, more likely that the surrounding breccia was caused by intrusion of the sills through a waterlogged substrate (Tuffen et al. 2001, 2002a, b) followed by disaggregation and brecciation. Breccia at the base of the lobes is most likely to have been caused by further quenching and brecciation of the lava as it slumped downwards.

Since the base of one of the sills that breached the ground surface appears to have been cooled by snow-contact (Mee et al. 2006) rather than subglacially, it may be used in conjunction with the subglacial lava lobes at a lower elevation on the col to estimate the approximate upper level of ice at the time of emplacement (Fig. 12). For example, we know that the sill penetrated the ground surface above the Lanalhue col at the current altitude of ~2,325 m. The glassy, quenched texture on the base of the lava suggests that it was rapidly cooled, most likely due to snow-contact given the presence of pseudopillow fractures (Mee et al. 2006). It is thought that as the ice and snow retreated, any lava that had flowed over the snow and cooled would have collapsed, resulting in the large volume of scree that remains in the vicinity today. In this case, 2,325 m would have been the approximate upper limit of ice (Fig. 12). Sills lower down in the col region (at ~2,250 m elevation) were emplaced subglacially causing constraint of the lava, which enabled quenching and solidification of the outer carapace and resulted in the steep front surfaces observed on the lobes today. In addition, the subglacial lobes on the north and eastern flanks of the North Ridge also appear to have been emplaced subglacially at 2,275 m, further supporting the inference of the upper level of ice between 2,275 and 2,325 m. Figure 12 shows our interpretation of the palaeoenvironment into which the IC3 sills and lobes were emplaced. From our interpretation we think that a valley glacier was present on Cerro Blanco approximately 80 ka ago and that the IC3 lobes were ice-constrained.
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Fig. 12

Palaeoenvironment reconstruction showing the approximate level of ice at the time of emplacement of the IC3 sills, dykes and subglacial lobes

Volcanic evolution and caldera formation

Since detailed geochemical analyses have not been conducted during this study, the magmatic evolution of Cerro Blanco is not discussed and to do so would require further investigation. Instead, we focus on the chronology of eruptive events at Cerro Blanco and discuss caldera formation.

Volcanic units at Cerro Blanco appear to fall into two categories: (1) those that erupted from a central vent (or vents) producing voluminous (up to 10 km3) lava flows that dip and radiate away from the current volcanic centre; and (2) those which erupted from smaller vents along the caldera wall faults and tend to be localised and less voluminous (<1 km3). The oldest known units to be associated with Nevados de Chillán volcano are the Los Pincheira lavas at 640 ka old, which are thought to be part of the early development of the volcanic complex due to their radial distribution around the volcano before either caldera was formed (Dixon et al. 1999). The next oldest units are those that make up the Santa Gertrudis valley and caldera wall lavas (SGV1, SGV2, OC1 and IC1). These all fall into the former category of lavas erupted from a central vent and we believe these to be part of an older complex for the following reasons:
  1. 1.

    The lavas all dip away from the caldera wall and therefore appear to have originated from much higher up the volcano, towards the current summit of Cerro Blanco. Although their exact sources are unknown, they show no evidence of having erupted along a caldera wall fault.

     
  2. 2.

    They are estimated to be at least twice the (preserved) volume of the IC2 and IC3 units, which extend for only a short distance (20–200 m) away from the caldera wall. By contrast, the inner and outer caldera wall lavas are 50–150 m in thickness and extend up to 1 km away from the volcano. The Santa Gertrudis valley sequences are 100–200 m in thickness and occupy the entire Santa Gertrudis valley. All appear to be much more weathered and altered than the IC2 and IC3 units.

     
The age constraints for this older complex are rather loose (640–90 ka old) but they are dissected by at least two caldera-forming events. A lack of detailed study of the outer caldera wall makes it impossible to infer the timing of caldera formation, unlike the inner caldera where ages of both pre- and post-caldera collapse units provide a relatively tight timescale for caldera formation. The cross-section through the Lanalhue vent at the southern end of the inner caldera wall is clear evidence that the vent facies (IC2) were emplaced prior to caldera formation, since the caldera fault cuts through this unit. By contrast, the IC3 unit crops out on the col region to the south of the Lanalhue vent where it has draped down the caldera wall fault, suggesting that the palaeotopography was similar to present day and hence suggesting IC3 was emplaced post-caldera formation. This would put the caldera collapse event that produced the inner caldera wall at between 90 and 80 ka BP, which agrees with the Pleistocene age interpreted by Naranjo et al. (1994) and Dixon et al. (1999).

Several volcanic facies have been identified along the inner caldera wall, all of which have been assigned to the IC2 vent unit. Since three of these ages (92.4 ± 3.5, 90.0 ± 0.6 and 88.9 ± 1.0 ka) are statistically indistinguishable, the exact order of eruptions at either end of the caldera wall is uncertain, although it is possible that the eruptions were simultaneous. The strong lithological and chronological correlation between the vent sequences at each end of the inner caldera wall facies suggests that volcanic activity was occurring along the entire length of the inner caldera wall at ca. 90 ka. It is quite possible that eruption of the IC2 vent facies created the initial fracture lines along which caldera collapse ultimately occurred, with subsequent eruptions over the following millennia (i.e. of the IC3 unit) continuing to exploit these structural weaknesses.

Sometime after eruption along the caldera fault, volcanism switched to the modern Cerro Blanco and Las Termas subcomplexes with lavas being erupted from several vents located around either stratocone (Dixon et al. 1999). At the latest, this would have occurred at 23.9 ka ago given the age of the oldest known lava to have erupted from the modern Cerro Blanco cone (i.e. CB2a of Dixon et al. 1999). This stratocone has been active as recently as 1865 with the eruption of the Santa Gertrudis lava (Dixon et al. 1999; Mee et al. 2006).

The combination of extreme topography and large accumulations of snow and ice make access to all remnants of the caldera walls (inner and outer) very difficult. Since this project has focussed primarily on the Cerro Blanco subcomplex it is not possible to make any accurate interpretations of caldera forming mechanisms, which of course would apply to the entire Nevados de Chillán Volcanic Complex and not just the Cerro Blanco subcomplex. It is unclear by what mechanism the calderas at Cerro Blanco (and hence Nevados de Chillán) formed but there are many models that have been applied to Andean volcanoes. Through the Central Volcanic Zone (CVZ) and the northern sector of the Southern Volcanic Zone (SVZ), caldera collapse is often attributed to large-magnitude explosive volcanism, characterised by plinian fall deposits and ignimbrites (Gilbert et al. 1996). Such deposits do occur at Nevados de Chillán, particularly to the south and west of the Las Termas subcomplex (Dixon et al. 1999), but these have not been studied or mapped in detail and thus it is unknown whether they are related to any caldera-forming process. By contrast, caldera formation at the predominantly mafic volcanoes of the southern SVZ, show less evidence to suggest they formed by explosive volcanism (Gilbert et al. 1996).

At Sollipulli, Gilbert et al. (1996) describe how the caldera was filled with a large body of ice (volume >6 km3) soon after caldera collapse and that subsequent volcanic eruptions occurred along the caldera margins rather than under the greatest thickness of ice (i.e. towards the centre of the caldera). This is, at least in part, similar to the eruption of the IC3 subglacial lobes, dykes and sills along the inner caldera wall after caldera collapse, and it is certain that the caldera was at least partially filled with ice (up to ~2,325 m; Fig. 12) during this time. However, it must be noted that if any volcanism had occurred towards the centre of the caldera, the modern Cerro Blanco complex would now cover these units.

Volcanic units erupted from the Cerro Blanco subcomplex can be assigned to three periods of volcanic activity: (1) the older, pre-caldera collapse complex; (2) caldera formation and eruption along the inner caldera wall fault and (3) the modern volcanic complex. This type of evolution whereby caldera collapse is followed by growth of a new stratocone is common at many Andean volcanoes. At Nevado del Tolima, Colombia, Thouret et al. (1995) describe four periods of eruptive activity. The early period produced Quaternary age basement lava flows, followed by the formation of an ancient stratovolcano, which has since been cut by caldera formation. A second stratovolcano was subsequently formed and is now overlain by the modern Tolima complex. The early constructive phase of volcanism at Puyehue, Chile, is estimated to comprise 80% of the total volume and is characterised by two broad shields (Singer et al. 2008). The modern Puyehue stratovolcano has since been built on the more southerly of these two shields (Singer et al. 2008). Clavero et al. (2004) have identified four stages of volcanic evolution at Parinacota volcano, Chile: (1) the early formation of dome complexes and associated pyroclastic deposits; (2) formation of a steep-sided stratocone on top of this dome complex; (3) partial collapse of the volcanic edifice; and (4) formation of a new steep-sided stratocone and several flank cones.

At Cerro Blanco, the early, constructive phase of volcanism is characterised by the eruption of multiple, shallow-dipping lava flows that make up the Santa Gertrudis valley and both the outer and inner caldera wall units (i.e. SGV1, SGV2, OC1 and IC1). The more localised eruptions along the inner caldera wall, that produced the vent facies (IC2) and subglacial lava lobes (IC3), coincide with a period of caldera formation and collapse when volcanism was restricted to the caldera wall fault. Finally, volcanism switched to the centre of the complex with the production of the modern Cerro Blanco stratocone.

Comparison with the marine oxygen isotope record

Figure 13 shows the chronology of eruptions at Cerro Blanco over the past ca. 120 ka, alongside the marine oxygen isotope record (MOIR) where oxygen isotope variations are used as a proxy for global ice volumes. The lack of 40Ar/39Ar ages for the older Cerro Blanco subcomplex makes it difficult to assign them to a particular stage of the MOIR and although we know a minimum age of 92.4 ka, they are likely to be considerably older. Prior to 92.4 ka, early volcanic activity at Cerro Blanco produced the inner and outer caldera wall lavas (OC1 and IC1), the platform on which the modern volcanic subcomplex is built. A large amount of volcanic activity occurred between ca. 80–92.4 ka during the second half of marine oxygen isotope stage (MOIS) 5, when eruptions were apparently confined to the edges of the caldera. The eruption of the IC2 vent facies at ca. 90 ka is thought to coincide with the initial formation of the inner caldera wall fault, and subaerial textures on the vent facies suggest that, locally at least, ice was lower than 2,300 m. By ca. 80 ka, when the IC3 dykes, sills and subglacial lobes were emplaced, the inner caldera was formed and was likely to have been filled with ice. The upper level of the ice can be constrained to between 2,275 and 2,325 m (the approximate elevation of the caldera rim) given the presence of subaerial, snow-contact and subglacial features on the same unit (Fig. 12) (Mee et al. 2006). It is likely that a combination of an ice-filled caldera and the presence of the inner caldera wall fault caused the eruption of the IC3 unit to be confined to the rim of the caldera (Gilbert et al. 1996).
https://static-content.springer.com/image/art%3A10.1007%2Fs00445-009-0277-7/MediaObjects/445_2009_277_Fig13_HTML.gif
Fig. 13

Chronology of volcanic activity at Cerro Blanco compared with oxygen isotope variations over the past 120 ka. Isotopically light values (warmer conditions) to the left and isotopically heavier values (cooler conditions) to the right. Marine oxygen isotope stages (MOIS) are given on the left. The isotopic signal is interpreted as a proxy for extent of global palaeoglaciation. The vertical line represents the isotope ratio that corresponds to the limited ice cover typical of the Holocene. Modified after Broecker and Denton (1990) and Lowe and Walker (1997)

An apparent hiatus in volcanic activity at Cerro Blanco occurred between 80 and 25 ka, although eruptions were taking place elsewhere on Nevados de Chillán (Dixon et al. 1999). When volcanism returned to Cerro Blanco, focus had shifted to the centre of the complex with the eruption of the CB2 Colcura and eastern units, which Dixon et al. (1999) suggest are remnants of a probable post-caldera collapse shield volcano, overlain by remnants of a steep-sided andesitic stratocone. Ages for this unit suggest it would have been erupted during the last glacial maxima in the area, which occurred between 33.5 and 14 ka according to Lowell et al. (1995). It is unknown, however, if this unit exhibits any subglacial or ice-contact features that would be consistent with widespread glaciation. Subsequent eruptions at Cerro Blanco, up to and including the eruption of the Santa Gertrudis (CB4) lava between 1861 and 1865 (Dixon et al. 1999) have continued to build the modern stratocone.

Conclusions

  1. 1.

    Identification of volcanic facies characteristic of subaerial, subaqueous, subglacial, ice-constraint and snow-contact activity is important for establishing past eruptive environments and particularly for establishing palaeoclimate conditions at the time of an eruption. Ages of glacially-cooled lavas can be compared with the marine oxygen isotope record to confirm whether or not large amounts of snow and ice were likely to have been present on the volcano at the time of an eruption. Volcanic facies are, therefore, powerful tools for palaeoclimate reconstruction as well as for interpreting past eruptive environments.

     
  2. 2.

    Six new volcanic units, each displaying a wide range of volcanic facies, have been identified along the inner caldera wall, the outer caldera wall and in the upper Santa Gertrudis valley of Cerro Blanco and a new geologic map has been produced.

     
  3. 3.

    The SGV1, SGV2, OC1 and IC1 units are thought to represent an older volcanic complex, characterised by voluminous (>10 km3), constructional volcanism; a feature which is typical of many Andean volcanoes.

     
  4. 4.

    The inner caldera wall fault was an important location for volcanism between 100 and 80 ka BP, during which time it is thought that the inner caldera wall was formed. At ca. 90 ka BP, volcanism was occurring at both ends of the inner caldera walls producing welded breccia and spatter beds on the ridge tops and subglacial lavas (USGV lava) in the valleys, suggesting that a small valley glacier was present at the time of eruption. At ca. 80 ka BP, after the inner caldera wall had been formed, a series of sills and dykes was intruded along the inner caldera wall, some of which were intruded into ice below a level of ~2,325 m, producing subglacial lobes.

     
  5. 5.

    Volcanic evolution at Cerro Blanco can be divided into three periods of eruptive activity: (1) constructive, pre-caldera collapse volcanism; (2) caldera formation and collapse; and (3) formation of the modern Cerro Blanco stratocone. This volcanic cycle is typical at many Andean volcanoes.

     

Acknowledgements

The authors would like to thank Hugh Tuffen and Mike James for their valuable contributions in the field, along with Holly Frey and Charles Stern for their thorough reviews, all of which have considerably improved this manuscript. KM was funded by a NERC Studentship; JSG acknowledges receipt of a 2001 Lancaster University Small Grant; DM received support from the OU Science Staff Tutor research fund; and JAN would like to thank Sernageomin’s PRV and Fondecyt Project No. 1960186.

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© Springer-Verlag 2009