The Incapillo Caldera and Dome Complex (∼ 28° S, Central Andes): A stranded magma chamber over a dying arc

Journal of Volcanology and Geothermal Research 184 (2009) 389–404

Contents lists available at ScienceDirect

Journal of Volcanology and Geothermal Research j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s

The Incapillo Caldera and Dome Complex (∼ 28° S, Central Andes): A stranded magma chamber over a dying arc A.R. Goss a,⁎, S.M. Kay b, C. Mpodozis c, B.S. Singer d a

Department of Geological Sciences, University of Florida, Gainesville, FL 32611, USA Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14853, USA Antofagasta Minerals, Apoquindo 4001, Piso 18, Santiago, Chile d Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, WI 53706, USA b c

a r t i c l e

i n f o

Article history: Received 30 January 2009 Accepted 6 May 2009 Available online 14 May 2009 Keywords: Central Andes caldera domes Incapillo ignimbrite

a b s t r a c t The Plio-Pleistocene Incapillo Caldera and Dome Complex (5570 m) is situated at the southernmost termination of the Andean Central Volcanic Zone (∼ 28° S), where the more steeply dipping segment of the subducting Nazca plate transitions into the Chilean–Pampean “flat-slab” to the south. The ∼ 250 m high walls of the elliptical caldera (6 × 5 km) are composed of a homogeneous pumice-rich rhyodacitic ignimbrite that radiates ∼ 15 km from the caldera rim. New biotite 40Ar/39Ar ages along with previous K–Ar ages reveal that the final eruption of the Incapillo Caldera occurred at ∼ 0.51 ± 0.04 Ma. Flanking the caldera are ∼ 40 steepsided rhyodacitic domes that predate the main “caldera-forming” ignimbrite. The morphology of these domes reflects the non-explosive effusive eruption of a crystal-rich gas-poor rhyodacitic magma. A volumetrically-minor debris flow composed of abundant pumice, dacites, and lithic clasts is topographically confined to a glacial valley south of the caldera and unconformably overlies the ignimbrite. Mixed pumices at the distal end of the ignimbrite reflect the convective physical condition within the magma chamber immediately prior to the pyroclastic eruption. Including the domes and ignimbrite, the total estimated volume of erupted Incapillo magma is ∼ 37 km3. Based on its age, small size, and effusive eruptive style, the Incapillo Caldera and Dome Complex provides an endmember model for an Andean caldera erupting within a waning magmatic arc over a shallowing subduction zone. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The Central Andes (15°–28° S) is generally considered as the type example of a continental-margin volcanic arc associated with a noncollisional convergent tectonic setting. The eruptive style, size, and composition of explosive silicic volcanism varies along the margin and these differences can to a first order be explained by spatial and temporal differences in tectonic setting and variations in the composition and rheology of the Andean continental crust (Kay et al., 1999). During the late Miocene to Pliocene, explosive silicic volcanism occurred within the amalgamated 11 to 4 Ma Altiplano– Puna Volcanic Complex (APVC — Fig. 1; de Silva, 1989; Coira et al., 1993; Kay et al., 1999; de Silva and Gosnold, 2007) that straddles the border between Argentina, Chile, and Bolivia. Additionally, the massive ∼6.4 to 2.2 Ma Cerro Galán caldera of northwest Argentina erupted in the backarc near 26° S east of the southern Central Volcanic Zone (CVZ)

⁎ Corresponding author. Tel.: +1 352 392 5873; fax: +1 352 392 9294. E-mail addresses: argoss@ufl.edu (A.R. Goss), [email protected] (S.M. Kay), [email protected] (C. Mpodozis), [email protected] (B.S. Singer). 0377-0273/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2009.05.005

(Sparks et al., 1985; Francis et al., 1989; Kay et al., 2008b). Large-scale (N1000 km3) Miocene to Pliocene silicic volcanism within the Altiplano and northern Puna has been attributed to an ignimbrite flare up caused by a rapidly steepening Nazca slab, expanding asthenospheric wedge, piecemeal delamination, and massive crustal melting by intruded mantle-derived basalt (Kay et al., 1999, 2008a). In contrast, smaller (b50 km3) mid-late Miocene ignimbrites exposed near the Salar de Antofalla (SAF — Fig. 1) at 25°–26° S have been explained as fractionated andesitic melts that subsequently assimilated upper crust (Siebel et al., 2001; Schnurr et al., 2007). Approximately 100 km further south along the CVZ frontal arc, the Incapillo Caldera and Dome Complex is the southernmost ignimbritic caldera of the Central Andes (Fig. 1). Its small size, high-pressure geochemical signatures, association with steepsided rhyodacitic domes, and position along the volcanic front of a waning magmatic arc testify to the different processes responsible for siliceous magmatism at this latitude compared to the larger centers to the north (Kay and Mpodozis, 2000; Goss et al., 2006). In this paper, we investigate the volcanology and geochronology of Incapillo magmatic units with respect to other siliceous volcanic centers of the Central Andes and provide petrographic descriptions and representative major element abundances of

390

A.R. Goss et al. / Journal of Volcanology and Geothermal Research 184 (2009) 389–404

Fig. 1. SRTM digital elevation model of the Central Andes showing main morphotectonic features. Nazca plate convergence direction is the long-term integrated NUVEL-1 rate from DeMets et al. (1990). Dark solid lines show contours of the subducting Nazca plate (Cahill and Isacks, 1992) with depths (km) to the Wadati–Benioff zone. Thick grey lines show extents of the Central (CVZ) and Southern (SVZ) Volcanic Zone volcanic fronts with the flat-slab amagmatic gap marked by a thick dashed line. Tan box shows the study area of the Incapillo Caldera and Dome Complex shown in detail in Fig. 2 and a black open rectangle gives the position of the early to late Miocene Maricunga Belt volcanic arc (Kay et al., 1994; Mpodozis et al., 1995). Shaded area shows the position of the Altiplano–Puna Volcanic Complex (APVC; de Silva, 1989; de Silva and Gosnold, 2007) with inset of major silicic centers. APVC volcanic center abbreviations and eruptive ages are: Pu = Purico; Pz = Panizos (7.9–6.7 Ma; Ort, 1993); PG = Pastos Grandes; G = Guacha (4.2 Ma; Francis and Baker, 1978; de Silva and Francis, 1991); V = Vilama-Coruto (8.5–6.0 Ma; Coira et al., 1996; Soler et al., 2007); C = Coranzuli (6.7–6.4 Ma; Seggiaro, 1994); Pr = Pairique (11–10 Ma; Coira et al., 1996; Caffe et al., 2007); P = La Pacana (5.5–4 Ma; Gardeweg and Ramírez, 1987; Lindsay et al., 2001). Other centers mentioned in the text include Cerro Galán, Ojos del Salado (OS), Tres Cruces (T), the ignimbrites from the Salar de Antofalla region (SAF; Siebel et al., 2001; Schnurr et al., 2007), Cordon de San Buenaventura (SB), Cerro Tupungatito (T), 11–8 Ma Volcán Copiapó (Cp), the 16–9 Ma Cerro de los Tórtolas–Tambo Formation (CdT), and the 6–5 Ma Vallecito–Vacas Heladas (V-VH) ignimbrite (Kay and Mpodozis, 2002; Kay et al., 2008b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

correlative volcanic units from the Incapillo Caldera and Dome Complex. In addition, we calculate erupted volumes of the Incapillo ignimbrite and dome material and present a series of new 40Ar/ 39 Ar fusion ages that complement previously-reported K–Ar ages (Kay and Mpodozis, 2000) and a comprehensive eruption model for the volcanic complex.

2. Regional setting The latest Pliocene to Pleistocene (Tables 1–3) Incapillo Caldera and Dome Complex (28° S, 68.8° W) is situated within the smooth northern transition zone between the steep-slab segment of the Central Andes (north of 27° S) and the Chilean–Pampean flat-slab

A.R. Goss et al. / Journal of Volcanology and Geothermal Research 184 (2009) 389–404

391

Table 1 List and locations of sampled Incapillo volcanic units. Ar/Ar fusion ages are reported in Table 3, K–Ar ages obtained at SERNAGEOMIN and are presented in Table 2, R = rhyolite (SiO2 N 70 wt.%); RD = rhyodacite (SiO2 = 67–70 wt.%); D = dacite (SiO2 = 63–67 wt.%), A = andesite (SiO2 = 60–63 wt. %). Sample Extrusive — domes and lava flows Pre-caldera unit (proto-cone) CO 516 Incapillo caldera domes CO 136 CO 137 CO 317 CO 318 CO 319 CO 139 Eastern Incapillo domes CO 323 CO 517 Oxidized dacitic domes CO 322 CO 140 CO 518 CO 135 CO 525 CO 522 CO 142 CO 151 CO 157 Western (Rio Salado) domes CO 184 CO 314 CO 316 CO 505 CO 506 CO 508 CO 509 CO 510 CO 511 CO 513 Ignimbrites and debris flows Incapillo ignimbrite CO 138 CO 320 CO 514 CO 141 CO 325 CO 524 CO 143 CO 527 CO 183 Veladero debris flow CO 515 CO 523 CO 526 Late Miocene–Pliocene CVZ arc Volcan Pissis CO 37 CO 38 CO 39 CO 180 CO 182 Sierras de Veladero CO 152 CO 153 CO 154 Bonete Chico CO 147 CO 148 CO 149 CO 150 CO 321

Type

Lat

Lon

Elev

Age

(m)

(Ma)

Location

Intracaldera dacite

RD

27.902

68.788

5438

0.873 ± 0.08

Incapillo Incapillo Incapillo Incapillo Incapillo Incapillo

R RD RD RD D RD

27.904 27.901 27.904 27.906 27.897 27.964

68.850 68.858 68.850 68.861 68.861 68.822

5570 5498 5533 5473 5476 5234

1.1 ± 0.4 2.9 ± 0.4

Eastern Incapillo dome Eastern Incapillo dome

R R

27.915 27.902

68.769 68.788

5605 5420

1.337 ± 0.129

Eastern Incapillo S. Incapillo dome S. Incapillo dome S. Incapillo dome S. Incapillo dome S. Incapillo dome S. Incapillo dome Fandango dome Fandango dome

RD D RD D RD RD D D D

27.935 27.964 27.971 27.922 28.020 28.021 28.038 28.148 28.184

68.760 68.822 68.801 68.815 68.837 68.825 68.848 68.845 68.838

5760 5186 5198 5367 4976 4983 4863 4589 4694

Rio Salado Rio Salado Rio Salado Rio Salado Rio Salado Rio Salado Rio Salado Rio Salado Rio Salado Rio Salado

D D D RD R R RD RD RD R

27.831 27.916 27.911 27.920 27.938 27.938 27.940 27.890 27.934 27.909

68.975 69.088 69.101 69.009 69.034 69.055 69.054 69.008 69.002 68.993

4818 4615 4596 4739 4725 4990 4766 4680 4838 4817

Incapillo pumice Incapillo pumice Incapillo pumice Incapillo pumice Incapillo pumice Incapillo pumice Incapillo pumice Incapillo pumice W. Incapillo pumice

RD R RD RD RD R RD R RD

27.916 27.935 27.950 27.939 28.015 28.024 28.038 28.038 27.802

68.864 68.851 68.821 68.860 68.829 68.836 68.848 68.848 69.005

5484 5410 5200 5186 5044 4967 4863 4863 4668

Veladero debris flow (whole rock) Veladero debris flow (whole rock) Veladero debris flow (pumice)

– – –

27.969 28.009 28.026

68.807 68.824 68.855

5156 4967 4859

Lava flow Lava flow Lava flow Lava flow Lava flow

D A A D D

27.730 27.710 27.690 27.756 27.800

68.760 68.770 68.780 69.029 68.998

5186 5028 4959 4631 4749

4.1 ± 0.4 6.2 ± 0.5 4.2 ± 0.2 3.2 ± 0.3 4.6 ± 0.7

North face of Pissis North face of Pissis North face of Pissis West face of Pissis West face of Pissis

Veladero dome Lava flow Lava flow

D D A

28.109 28.109 28.090

68.944 68.944 68.934

4853 4853 5016

4.2 ± 0.4 5.6 ± 1.0

Sierras de Veladero Southeast Veladero Southeast Veladero

Lava flow Lava flow Lava flow Lava flow Lava flow

D D D A D

28.140 28.124 28.080 28.130 27.935

68.672 68.743 68.754 68.787 68.760

4512 4954 5088 4890 5760

2.5 ± 0.4 4.0 ± 0.2 4.2 ± 0.3 – –

Southern Bonete Chico South face of Bonete Chico South face of Bonete Chico South face of Bonete Chico North face of Bonete Chico

dome dome dome dome dome dome

dome dome dome dome dome dome dome dome dome dome

region (27°–33° S; Ramos et al., 2002) where the subducting Nazca plate attains a near-horizontal dip at 100 km depths (Fig. 1; Cahill and Isacks, 1992). Nearly-orthogonal subduction of the Nazca plate below

1.9 ± 0.7

4.6 ± 0.2 4.7 ± 0.5 3.6 ± 0.5

2.558 ± 0.030

0.521 ± 0.029 1.6 ± 0.5 0.509 ± 0.036

Lower north caldera wall West rim of caldera NE-flank of Veladero West rim of caldera West rim of caldera West rim of caldera South of caldera along stream NW flank of Bonete Chico East rim of caldera NW flank of Bonete Chico NE-flank of Veladero Q. Veladero — south of Incapillo Q. Veladero — south of Incapillo Q. Veladero — below ignimbrite Q. Veladero — south of Incapillo Q. Veladero — below ignimbrite Q. Veladero — Fandango Q. Veladero — Fandango Headwaters of Rio Salado West side of Rio Salado West side of Rio Salado East side of Rio Salado East side of Rio Salado East side of Rio Salado East side of Rio Salado East side of Rio Salado East side of Rio Salado East side of Rio Salado

Caldera — southwest rim Caldera — south rim Caldera — south rim Q. Veladero — proximal Q. Veladero — medial Q. Veladero — medial Q. Veladero — distal Q. Veladero distal Headwaters of Rio Salado Q. Veladero Q. Veladero Q. Veladero

South America (075° N) at an average velocity of 84 mm/a is responsible for the continuous chain of active arc volcanism spanning from Sara Sara volcano in southern Peru (15° S) to Ojos del Salado in

392

A.R. Goss et al. / Journal of Volcanology and Geothermal Research 184 (2009) 389–404

Table 2 K–Ar ages for Incapillo and late Miocene/Pliocene CVZ arc centers. Ages obtained at the SERNAGEOMIN geochronology lab (Santiago, Chile); WR = whole rock analysis. %K

Ar40

% Ar atm

Age

Incapillo Caldera domes CO 136 Rhyodacite CO 137 Dacite

7.14 7.1

0.808 0.31

84 92

2.9 1.1

Incapillo ignimbrite CO 141 Pumice

7.05

0.44

91

Oxidized dacitic domes CO 140 Dacite CO 140 Dacite CO 142 Dacite CO 151 Dacite CO 157 Dacite

2.3 2.3 2.69 2.57 2.61

0.16 0.17 0.48 0.47 0.370

L. Miocene/Pliocene CVZ arc Pissis CO 37 Dacite CO 38 Andesite CO 39 Andesite CO 180 Dacite CO 182 Dacite

7.240 7.01 2.96 2.440 2.47

Sierras de Veladero CO 152 Dacite CO 153 Dacite Bonete Chico CO 147 Dacite CO 148 Dacite CO 149 Dacite

Sample

Type

±

Type

± ±

0.4 0.4

Biotite Biotite

1.6

±

0.5

Biotite

98 96 45 81 84

– 1.9 4.6 4.7 3.6

± ± ± ±

– 0.7 0.2 0.5 0.5

WR WRa WR WR WR

1.16 1.83 0.49 0.31 0.44

73 71 36 76 87

4.1 6.2 4.2 3.2 4.6

± ± ± ± ±

0.4 0.5 0.2 0.3 0.7

Biotite Biotite WR WR WR

7.26 2.189

1.07 0.476

93 89

3.8 5.6

± ±

1.0 1.0

Biotite WR

2.62 2.79 2.06

0.26 0.38 0.334

83 35 51

2.5 3.5 4.2

± ± ±

0.4 0.1 0.3

WR WR WR

a

Average of two repeat analyses.

Chile (26° S). South of the Incapillo Caldera, the Chilean–Pampean flat-slab region is characterized by a 500 km wide zone of increased intracrustal seismicity in the backarc (Smalley and Isacks, 1990; Pardo et al., 2002), Laramide-style thick skinned deformation (Jordan et al., 1983), and an amagmatic gap that extends ∼ 700 km south to Volcán Tupungatito (33.5° S), the northernmost active volcano in the Southern Volcanic Zone (SVZ). This gap in active volcanism has been explained by Barazangi and Isacks (1976) as resulting from the loss of asthenospheric wedge volume due to the shallowing of the Nazca plate that began at ∼18 Ma and culminated between 10 and 5 Ma (Kay et al., 1987, 1991). Crustal thicknesses below the Incapillo Caldera and Dome Complex are among the thickest for any volcanic region on Earth. Using seismic depth phase precursors, McGlashan et al. (2008) reported a 70 km MOHO depth directly below the Incapillo Caldera at 28° S. This crustal thickness is similar to the 65–75 km thick crust estimated for the Altiplano and somewhat thicker than the 42–67 km thick crust of the Puna and of the central Chilean–Pampean flat-slab regions (Fig. 1; Beck et al., 1996; Beck and Zandt, 2002; Yuan et al., 2002; Fromm et al., 2004; McGlashan et al., 2008).

The late Oligocene to Pliocene magmatic history of the northern flat-slab transition zone is characterized by regional tectonic instability and frontal arc migration in tandem with backarc slab shallowing and an increased rate of forearc subduction erosion along the margin (Kay and Mpodozis, 2000, 2002; Goss and Kay, 2009). During the late Oligocene to late Miocene, magmatic activity in the region was concentrated within the southern Maricunga Belt ∼ 50 km west of the Incapillo Caldera (Fig. 1; Kay et al., 1994; Mpodozis et al., 1995). Major silicic centers of the Maricunga Belt outlined by these authors include 27–21 Ma small-volume dacitic ignimbrites and dome complexes (Refugio, Pantanillo), 17–14 Ma andesitic stratovolcanoes and ignimbrites (Santa Rosa, Pastillos, Cadillal, and the Valle Ancho ignimbrite), the 11–7 Ma Copiapó andesite to dacite complex, and the ∼6 Ma Nevado de Jotabeche rhyodacitic caldera. The final ignimbritic eruption of Nevado de Jotabeche (Kay et al., 1994; McKee et al., 1994; Mpodozis et al., 1995) marked the termination of volcanism in the Maricunga belt and was contemporaneous with (1) the eruption of regionally-scattered small-volume mafic lavas (Pircas Negras andesites, see Goss and Kay, 2009) and (2) the migration of the volcanic front ∼ 50 km to the east. The Miocene–Pliocene (6–2 Ma) dacitic eruptions of Monte Pissis, Sierra de Veladero, and Cerro Bonete Chico define the main axis of the latest Miocene–Pliocene CVZ arc that largely predates the eruption of the Incapillo complex (Figs. 1 and 2, Kay and Mpodozis, 2000). By the late Pliocene, siliceous magmatism persisted along the CVZ to the north near Ojos del Salado (Mpodozis et al., 1996; Gardeweg et al., 2000; Kay et al., 2008b) while at 28° S, large-scale dacitic volcanism waned. The Incapillo Caldera and Dome Complex erupted during this post-migration period between 3–0.5 Ma as the last volcanic event within the currently amagmatic arc segment (Kay and Mpodozis, 2000). Most of the Incapillo domes and the entire ignimbrite are contained within a topographic low between the high peaks of Monte Pissis (6793 m) to the north, Cerro Bonete Chico (6759 m) to the east, and Sierra de Veladero (6335 m) to the west (Fig. 2A). 3. Caldera morphology With a rim altitude of 5500 m, the Incapillo Caldera (translated from Quechua as “Crown of the Inca”) is the highest explosive caldera in the world, although it was first described by Baker (1981) and later by de Silva and Francis (1991) from satellite photos as “Cerro Bonete”. The well exposed 200–350 m high walls of the 6 × 5 km elliptical caldera are dominantly composed of a homogeneous pumice-rich layered outflow ignimbrite here referred to as the Incapillo ignimbrite (Figs. 2 and 3). This ignimbrite unconformably overlies a series of altered dacitic domes that are exposed along the north wall of the caldera (Fig. 3A). Based on their lower stratigraphic position and general northward dip, these flows reflect the edifice of a pre-caldera dome complex. Isolated rhyodacitic domes on the north and south rims of the caldera show evidence for collapse of the caldera walls, which is likely due to erosional processes associated with the posteruptional widening of the caldera. Based on early remote sensing

Table 3 40 Ar/39Ar single-crystal laser fusion analyses on mineral separates from Incapillo dome lavas and ignimbritic pumices. Complete 40Ar/39Ar methods are outlined in Appendix A. MSWD — mean square of weighted deviations. All ages calculated using the decay constants of Steiger and Jäger (1977) (λ40K = 5.543 × 10− 10 yr− 1). J-values calculated relative to 1.194 Ma Alder Creek sanidine (Renne et al., 1998). Sample CO-323 CO-508b CO-514b CO-524b C0-516b CO-507 a b

Type Eastern dome Western dome Incapillo Ign. Incapillo Ign. Pre-caldera flow Pircas Negras

Mineral Bio Bio Bio Bio Bio Hbde

Na 8 of 8 16 of 18 12 of 12 10 of 11 9 of 9 12 of 12

Weighted mean ages

Isochron ages

Age (Ma) ± 2σ

MSWD

K/Ca

40

1.355 ± 0.022 2.553 ± 0.014 0.510 ± 0.017 0.515 ± 0.022 0.881 ± 0.016 3.898 ± 0.085

0.08 1.00 0.55 0.32 0.11 0.40

10.3 10.5 12.2 12.0 19.0 0.1

297.7 ± 15.7 294.7 ± 4.3 294.3 ± 2.6 296.2 ± 3.4 297.0 ± 15.0 292.8 ± 6.6

Number of single-crystal fusions included in the age statistics. Indicates that sample has been subjected to a clean-up step prior to fusion.

Ar/36Ari ± 2σ

MSWD

Age (Ma) ± 2σ

0.08 1.06 0.53 0.34 0.12 0.38

1.337 ± 0.129 2.558 ± 0.030 0.521 ± 0.029 0.509 ± 0.036 0.873 ± 0.077 3.968 ± 0.192

A.R. Goss et al. / Journal of Volcanology and Geothermal Research 184 (2009) 389–404

studies, these lava domes were originally thought to be flat-topped “mesetas” formed by eroded ignimbrite (Baker, 1981; de Silva and Francis, 1991). A similar, but younger, extrusive lava dome within the center of the caldera exhibits extensive hydrothermal alteration (Fig. 3A). Adjacent to this magmatic dome is the Laguna Corona del Inca, a 2 × 1 km crater lake that is likely the source of lacustrine and evaporite deposits that blanket the Incapillo ignimbrite on the caldera floor. Based on thermal infrared imaging, the temperature of the crater lake is estimated at ∼13 °C and thought to be fed by a weakly active hydrothermal system (Markham and Barker, 1986). 4. Description of principle eruptive units The major eruptive units of the Incapillo Caldera and Dome Complex are mapped in Fig. 2 and include: (1) the caldera-forming Incapillo ignimbrite, (2) the stratigraphically younger and topographicallyconfined Veladero debris flow, (3) more than 40 flat-topped rhyodacitic domes, (4) a series of oxidized dacitic domes, and (5) post-explosion intracaldera domes. Exact sample locations are shown on Fig. 2B and

393

are given in Table 1. Major element analyses of representative samples from Incapillo ignimbrite and dome units are given in Table 4. 4.1. Incapillo ignimbrite The Incapillo ignimbrite is the most obvious Incapillo unit visible on satellite imagery (Fig. 2A). This lithic-poor pumice-rich ignimbrite composes the cliff-like walls of the caldera (Fig. 3A) and radiates up to 15 km in all directions from the caldera rim. The deposits are generally topographically confined to the Quebrada de Veladero to the south and the valley of an east-flowing ephemeral river on the southern flank of Monte Pissis. However, a pumice-rich unit from near the headwaters of the Rio Salado may also be part of the Incapillo ignimbrite (sample CO 183). Ignimbrite thicknesses taper from a maximum of ∼250 m at the southern edge of the caldera to ∼10 m at the distal end within the Quebrada de Veladero. At the base of the ignimbrite is a ∼ 5 cm thick ground surge deposit composed of fine grained ash with angular lithics (up to 2 cm) entrained from fluvial gravels below. This surge deposit is visible where the ignimbrite is

Fig. 2. (A) Composite ASTER digital image (15 m resolution) of the 3–0.5 Ma Incapillo Caldera and Dome Complex and latest Miocene/Pliocene dacitic centers of the northern flat-slab transition zone (Cerro Bonete Chico, Monte Pissis, Sierra de Veladero, and Cerro Bonete Grande). (B) Geological map of the Incapillo Caldera and Dome Complex adapted from Kay et al. (2008b), Mpodozis et al. (1997), Rubiolo et al. (2002), Caminos and Fauqué (2000), and Fauqué (2000). Principle late Miocene to Pleistocene volcanic units as well as exposures of the Permo-Triassic Choiyoi granite–rhyolite province and quartzite metamorphic basement are shown. Sample locations (Table 1) are shown with available K–Ar (Table 2) and 40 Ar/39Ar ages (Table 3). Small numbers correspond to domes used in volume calculations (Table 5).

394

A.R. Goss et al. / Journal of Volcanology and Geothermal Research 184 (2009) 389–404

Fig. 2 (continued).

undercut by fluvial erosion and unconformably caps late Miocene dacitic flows from the Sierra de Veladero (Fig. 4A,B). In medial and distal exposures of the Incapillo ignimbrite (Fig. 4A, C,E), the lowermost sections are extremely lithic-poor (b0.5%) and composed of 0.5–1.0 meter thick bands of crystal-rich (40–50%, mainly quartz and plagioclase) and ash-dominated pyroclastic flow deposits (∼10% pumice) that alternate with uniform matrix-supported layers of white pumice (N50% pumice). Massive deposits characterize the upper sections of the ignimbrite. Within 1 km of the caldera however, the ignimbrite becomes completely massive and matrix dominated (Fig. 4C). All of the Incapillo ignimbrite units within the Quebrada de Veladero and along the caldera rim are entirely unwelded. Pumice clasts are generally 5–20 cm in diameter, dominated by 50–60% translucent devitrified glass, and have porosities near 30–60%. Pumices contain crystals (b1 mm, 5–10% of total volume) of plagioclase (25–30%), quartz (20–25%), sanidine (20– 25%), biotite (10–15%), and hornblende (10–15%) with minor Fe–Ti oxides (∼ 5%), apatite, and titanite. Typical Incapillo ignimbrite pumices have high-K rhyodacitic compositions (SiO2 up to 72 wt.%) with low MgO contents (b1.3 wt.%). The largest pumice clasts in the Incapillo ignimbrite occur at the far distal end of the flow in the Quebrada de Veladero. Here, ∼2 m thick rafts

composed of “football-sized” mixed pumices (5–30 cm in diameter) are interlayered with 1–2 m thick internally-laminated ash flow deposits (see Fig. 4E). At least 3 separate pumice raft sequences are apparent at the distal toe of the ignimbrite. These pumices are distinct from the white homogenous pumices that dominate the Incapillo ignimbrite as they clearly result from mixtures of two magmas: a less-viscous biotitebearing dacite (SiO2 = 67%; FeO = 3.88%) and a more viscous and crystal-rich (sanidine and quartz) rhyolite (SiO2 = 71.11%; FeO = 2.05%, see Fig. 4F and Table 4) that is similar to the pumice in the bulk of the ignimbrite. 4.2. Veladero debris flow The Veladero debris flow, which conformably overlies the Incapillo ignimbrite, is spatially confined to the Quebrada de Veladero (Figs. 2– 4A,B) and is not exposed at the rim or within the walls of the Incapillo Caldera. The deposit is 15–25 m thick at its northernmost extent some ∼ 5 km south of the caldera rim and thins to 10–15 m at its southernmost extent within the Quebrada de Veladero. In the field and on satellite imagery (ASTER and Landsat TM), the Veladero debris flow can be distinguished from the underlying and larger Incapillo ignimbrite by its oxidized yellow-orange color, marginal levees, and

A.R. Goss et al. / Journal of Volcanology and Geothermal Research 184 (2009) 389–404

395

Fig. 3. Top: Photograph of the north wall of the 6 × 5 km Incapillo Caldera showing principle volcano-structural units. Thin dashed black line marks the stratigraphic contact between the 0.87 ± 0.08 Ma pre-caldera dome dacite (sample CO 516) and the overlying 0.51 ± 0.04 Ma Incapillo ignimbrite. Bottom: Photograph looking west across the Quebrada de Veladero from Cerro Bonete Chico showing main pyroclastic deposits associated with the Incapillo Caldera. Thin dashed black line shows the contact between the Incapillo ignimbrite and the lithic-rich overlying Veladero debris flow. Dashed arrows show flow directions of the caldera-forming Incapillo pyroclastic flow.

highly-eroded hummocky topography (Fig. 2). Since the deposition of the Veladero debris flow, the initial topography has been sculpted by the prevailing winds into regular NNW–SSE trending hummocky ridges. The composition and texture of the Veladero flow is distinct from the Incapillo ignimbrite (Fig. 4D) as it is comprised of clasts of red and grey biotite-bearing dacite (15–20%), glassy brown pumice (10–15%), highly vesiculated white pumice similar to the Incapillo ignimbrite pumice (∼5–10%), abundant angular and rounded lithic fragments (5–10%), and euhedral crystals of quartz (1–5 mm; 5–10%), biotite (0.1–1 mm; ∼5%), and plagioclase (1–5 mm; ∼ 5%). The lithics are generally clasts of late Miocene Sierra de Veladero or Cerro Bonete Chico dacites or fragments of nearby Pircas Negras andesitic flows (see Goss and Kay, 2009). All clasts are supported by a light to moderately cemented ash- and clay-rich matrix (∼25–30%). Matrix cementation permits the near-vertical erosional cliffs of the Veladero debris flow in comparison to the shallowly sloping erosional edges of the Incapillo ignimbrite (Fig. 4A). Except for near the basal contact at the distal end of the flow, where larger pumices and dacites (10–20 cm) form a weakly laminated and sheared stratigraphy, the Veladero debris flow is massive, poorly-sorted, and non-graded. Based on its restricted

spatial extent, cemented texture, heterogeneous lithic-rich composition, and massive bedding, the Veladero unit likely formed from an unconsolidated debris flow or lahar. 4.3. Incapillo dome field Approximately 40 steep-sided rhyodacitic domes, which are locally referred to as “tortas” (cake-shaped), surround the Incapillo Caldera. These domes erupted in two main clusters (see Figs. 2 and 5): a western group within the Sierra de Veladero along the upper Rio Salado and an eastern group bounded by Cerro Bonete Chico and Monte Pissis. The western domes unconformably cap outcrops of Pircas Negras andesite along the length of the Rio Salado. A few smaller domes erupted where the western and northern rims of the Incapillo Caldera are now located. One such pre-caldera dome (CO 516, Fig. 3A) is exposed the within the northern wall of the caldera and is overlain by thick deposits of Incapillo ignimbrite. The domes range from 100 to 600 m in vertical relief and many have multi-lobate morphologies with a ∼ 1 km wide basal apron of eroded dome material (Fig. 5B). However, no basal ash or tephra layer is evident. Field observation and ASTER imagery reveal numerous

Table 4 Major element analyses for representative Incapillo eruptive units. Major element data from electron microprobe at Cornell University are given in wt.% oxide (H2O-free and normalized to 100% totals). All Fe given as FeOt. Sample CO 527m and CO 527f represent the mafic (m) and felsic (f) endmembers of the mixed Incapillo ignimbrite pumices shown in Fig. 4E,F. Pre-caldera

Incapillo dome field

Incapillo ignimbrite

Oxidized dacitic domes

Sample

CO 516

CO 514

CO 141

CO 524

CO 527m

CO 527f

CO 136

CO 137

CO 323

CO 508

CO 510

CO 140

CO 151

CO 157

SiO2 TiO2 Al2O3 FeOt MnO MgO CaO Na2O K2O P2O5 Total Al/(K + Na + Ca)

69.97 0.40 14.99 2.14 0.05 0.89 2.95 4.70 3.77 0.14 100.00 0.87

69.91 0.40 14.66 2.03 0.04 0.79 4.42 3.79 3.77 0.18 100.00 0.80

69.13 0.38 15.80 2.12 0.04 0.84 3.07 4.42 4.02 0.17 100.00 0.92

71.51 0.38 14.79 2.00 0.08 0.70 2.55 4.08 3.80 0.11 100.00 0.96

67.23 0.52 16.14 3.88 0.07 1.29 3.74 3.82 3.08 0.21 100.00 0.98

71.11 0.39 15.02 2.05 0.04 0.70 2.46 4.21 3.90 0.12 100.00 0.96

70.49 0.30 15.48 1.90 0.04 0.66 2.30 4.41 4.28 0.13 100.00 0.96

69.46 0.42 14.80 2.59 0.05 1.21 2.93 4.14 4.22 0.17 100.00 0.89

71.20 0.41 15.28 2.09 0.03 0.96 1.79 4.03 4.07 0.14 100.00 1.07

70.41 0.43 14.83 2.40 0.04 1.00 2.53 4.05 4.21 0.11 100.00 0.94

68.90 0.49 15.22 2.38 0.06 1.38 3.56 4.19 3.66 0.17 100.00 0.88

63.82 0.71 16.61 3.91 0.06 2.24 4.33 4.56 3.50 0.26 100.00 0.87

64.68 0.75 16.49 4.02 0.07 2.10 4.13 4.50 3.05 0.21 100.00 0.91

64.92 0.75 16.80 4.02 0.07 1.88 3.94 4.32 3.10 0.21 100.00 0.95

396

A.R. Goss et al. / Journal of Volcanology and Geothermal Research 184 (2009) 389–404

Fig. 4. Photographs of pyroclastic deposits associated with the Incapillo Caldera. (A and B) — Contacts between the Incapillo ignimbrite and the overlying Veladero debris flow within the Quebrada de Veladero, (C) — non-welded and homogeneous pumice-rich Incapillo ignimbrite, (D) — cemented lithic- and dacite-rich Veladero debris flow, (E) — 2 m thick pumice rafts at the distal end of the Incapillo ignimbrite within the Quebrada de Veladero, and (F) — mixed pumice clast from the upper pumice raft shown in Fig. 4E.

water-filled pit craters 20 m in diameter on the summits of the largest of the western domes (Fig. 5B–C). These explosion craters are surrounded by 10 m high lava spires composed of blocky dacite.

Incapillo dome lavas are crystal-rich (20–40%) with large plagioclase (0.5–12 mm; 40–50%), quartz (1–8 mm; 30–40%), biotite (1–3 mm; 10– 15%), amphibole (0.2–3 mm; 5–10%), and titanite (0.2–0.5 mm; 1–3%)

A.R. Goss et al. / Journal of Volcanology and Geothermal Research 184 (2009) 389–404

397

Fig. 5. Images of rhyodacitic domes from the Incapillo Caldera and Dome Complex. (A) — ASTER satellite image of the caldera and eastern dome field. 5–2 Ma oxidized domes associated with the Pliocene dacitic centers (i.e., Cerro Bonete Chico, Monte Pissis, and Sierra de Veladero) are overlain by the younger steep-sided Incapillo domes. (B) — Western dome field along the upper Rio Salado on the west side of Sierra de Veladero. Explosion craters/pits dot the summit of the largest and southernmost dome. (C) — Photograph of an explosion crater surrounded by ∼10 m high rhyodacitic lava spines.

phenocrysts with minor abundances of oxides, apatite, and zircon suspended in a translucent cryptocrystalline groundmass. Some alkali feldspar is present (b5%) in a few dome lavas. This phenocrystic assemblage is ubiquitous in the dome lavas associated with the Incapillo magmatic system, including those that are 20–25 km west of the caldera along the upper Rio Salado. Incapillo dome lavas have higher MgO contents (0.5–2.0 wt.%) than ignimbrite pumices (b1.3 wt.%) at similar SiO2 concentrations (69–71 wt.%; see Table 4) possibly due to greater crystallinity. Major element concentrations are generally like those of evolved lavas and pumices from the Puna with Al2O3 = 14–17 wt.%, CaO = 2–5 wt.%, and K2O N 2.5 wt.%., TiO2 = 0.28–0.72 wt.% (Table 4). Chemically, the 0.87 ± 0.03 Ma pre-caldera dome from the north caldera wall (CO 516, Table 3) is similar to the Incapillo domes. 4.4. Oxidized dacitic domes A suite of generally older domes erupted on the flanks of Cerro Bonete Chico and Sierra de Veladero within the Quebrada de Veladero. These domes, which are identifiable in satellite imagery by their reddish oxidized color (Figs. 2A,B and 5A), also occur north and east of

the Incapillo Caldera. Many of the oxidized domes are partially covered by younger Incapillo domes and ignimbrite. The petrographic features of the older dome lavas are on the whole similar to the Incapillo domes (plagioclase, quartz, biotite, amphibole, titanite, Fe–Ti oxides). In detail, the older domes contain more amphibole (25–50%), less quartz (0–25%), and similar abundances of plagioclase (40–70%) compared to the younger domes. Despite an oxidized appearance in the field and on satellite imagery, only negligible alteration is observed in thin section. Unlike the younger Incapillo units, the 4.7–1.9 Ma oxidized domes are dominantly dacites (SiO2 = 64–65 wt.%; Table 4) and represent the least evolved of all sampled dome lavas. Chemically, these lavas are similar to late Miocene to Pliocene dacites from Monte Pissis, Cerro Bonete Chico, and Sierra de Veladero (Kay and Mpodozis, 2000). 4.5. Intracaldera domes The small domes (b200 m) in the center of the Incapillo Caldera differ from those around the caldera rim by their strongly hydrothermally

398

A.R. Goss et al. / Journal of Volcanology and Geothermal Research 184 (2009) 389–404

Fig. 6. 40Ar/39Ar isochron diagrams for selected Incapillo dome and ignimbritic units. Complete 40Ar/39Ar data is shown in Table 3. Grey ovals show each laser fusion analysis with 2σ error ellipse. Also shown is an isochron diagram for a Pircas Negras andesite (CO 507) that directly underlies Incapillo dome lava (CO 508). MSWD = mean square of weighted deviations.

altered condition. These domes likely reflect a post-collapse secondary eruptive event as opposed to a remnant of the pre-caldera topography (i.e., caldera floor). This interpretation is favored by the (1) apparent lack

of preserved ignimbritic deposits above these domes and (2) the characteristic dome-like morphology of these eruptive centers given their location within the middle of an explosive caldera.

A.R. Goss et al. / Journal of Volcanology and Geothermal Research 184 (2009) 389–404

5. Geochronology (K–Ar and

40

Ar/39Ar dating)

5.1. Methods Whole rock and biotite K–Ar ages for Incapillo ignimbrite pumices, rhyodacitic dome lavas, and dacitic stratovolcanoes were obtained by the Chilean Geological Survey (SERNAGEOMIN). Analytical data for these ages, which were discussed by Kay and Mpodozis (2000), are presented in Table 2. K–Ar analytical methods and data reduction procedures are discussed in Mpodozis (1995). 40Ar/39Ar laser fusion ages from selected ignimbrite and dome lavas were determined at the University of Wisconsin-Madison Rare Gas Chronology Laboratory following methods outlined in Appendix A. These new 40Ar/39Ar ages provide a tighter control on the eruptive chronology of the Incapillo Caldera and Dome Complex and refine the magmatic evolution of the southernmost CVZ arc. 5.2. Results The results of laser fusion 40Ar/39Ar analyses from five Incapillo samples are summarized in Table 3 with calculated isochrons shown in Fig. 6. Dated minerals include (1) biotite from rhyodacitic domes in both the eastern (CO 323) and western (CO 508) dome fields, (2) hornblende from a Pircas Negras andesitic flow from the upper Rio Salado (CO 507; Goss and Kay, 2009) that is directly overlain by the Incapillo domes, (3) biotite from two Incapillo ignimbritic pumices (CO 514 and CO 524) and, (4) biotite from the pre-caldera dacitic dome from the north wall of the Incapillo Caldera (CO 516). Isochron ages are preferred over the weighted mean ages, as they make no assumption as to the composition of the trapped argon component (e.g., Singer et al., 2004). 5.2.1. Incapillo domes and pre-caldera units A 40Ar/39Ar biotite age of 1.337 ± 0.129 Ma for the eastern Incapillo dome (CO 323) is consistent with biotite K–Ar ages of 2.9 ± 0.4 Ma to 1.1 ± 0.4 Ma for similar domes along and just south of the Incapillo Caldera (CO 136 and CO 137, see Table 2). The western Incapillo dome has a 40Ar/39Ar biotite age of 2.558 ± 0.030 Ma (CO 508) and is younger than a Pircas Negras andesite (CO 507) directly below that yielded a 40Ar/39Ar hornblende age of 3.968 ± 0.192 Ma. These ages show that Incapillo dome eruptions began simultaneously at ∼3.0– 2.5 Ma on both flanks of the Sierra de Veladero and that silicic domeforming eruptions along the upper Rio Salado commenced during the waning stages of Pircas Negras magmatism between 4 and 2 Ma (Goss and Kay, 2009). The pre-caldera dome unit exposed at the base of the north caldera wall yielded a 40Ar/39Ar biotite age of 0.873 ± 0.077 Ma (CO 516) and thus is the last known pre-caldera effusive event. 5.2.2. Incapillo ignimbrite and caldera units Two Incapillo ignimbrite pumices dated at 0.521 ± 0.029 Ma (CO 514, proximal) and 0.509 ± 0.036 Ma (CO 524, distal) mark the age of the explosive formation of the Incapillo Caldera and are 0.35–0.40 Ma younger than the last dated effusive event at 0.873 Ma. These 40Ar/39Ar ages are younger than the less-precise 1.6 ± 0.5 Ma K–Ar biotite age for the Incapillo ignimbrite (CO 141; Table 2) and reveal that the Incapillo Caldera and Dome Complex erupted nearly contemporaneous with the Ojos del Salado (1.53 ± 0.13 to 0.34± 0.19 Ma; Mpodozis et al., 1996; Gardeweg et al., 1998) and Tres Cruces (2.1 ± 0.3 to 0.028 ± 0.011 Ma; Gardeweg et al., 2000) complexes in the southernmost CVZ [∼27° S, see Kay et al. (2008b) for summary of Pliocene to Recent magmatism in the southernmost Puna plateau]. Based on these ages, the ∼0.51 Ma explosive Incapillo eruption marks the youngest known volcanic event between 27.5° and 33° S in the Central Andes. 5.2.3. Oxidized dacitic domes and late Miocene dacitic stratovolcanoes Whole rock and biotite K–Ar ages from oxidized domes on the flanks of Cerro Bonete Chico and Sierra de Veladero range from 4.7 ±

399

0.5 Ma to 1.9 ± 0.5 Ma (Table 2) and largely predate the dome eruptions associated with the Incapillo magmatic system. These domes erupted simultaneously with the larger dacitic volcanic centers including Monte Pissis (6.2 ± 0.5 to 3.2 ± 0.3 Ma), Sierra de Veladero (5.6 ± 1.0 to 4.2 ± 0.4), and Cerro Bonete Chico (4.2 ± 0.6 to 2.5 ± 0.4 Ma) that defined the frontal arc during the latest Miocene and Pliocene (Table 2). 6. Calculation of erupted volume In order to constrain the magnitude of the Incapillo eruptive event, surface-integrated volumes of eruptive units were calculated using the 90 meter resolution SRTM digital elevation model (DEM) for the Central Andes imported into the terrain and surface-modeling Surfer® program. With a known upper and/or lower surface elevation, topographically-integrated erupted volumes are more precise than binning units into perfect geometric shapes (i.e., approximating a dome as a cylinder). For example, the volume of a typical Incapillo dome (e.g., dome #10; Fig. 2B) is substantially overestimated by geometric-binning [where V = basal area (km2) ⁎ height (m)] compared to topographic-integration (3.68 km3 vs. 2.42 km3). Table 5 gives calculated volumes and masses for Incapillo domes, ignimbrites, and the Veladero debris flow. Erupted volumes for the largest b 3 Ma Incapillo domes were calculated by defining a lower surface specific to each dome and using the digital topography as the upper surface. No correction was made for sloping lower surfaces or for dense rock equivalent (DRE). The largest domes have volumes that range from 1.2–7.0 km3 with a total volume of erupted material of ∼16 km3. Converting this volume into mass using an average density for rhyodacite (2650 kg/m3) yields 4.2 × 1013 kg of erupted Incapillo dome lava. The topographically-integrated volume of the Incapillo Caldera was calculated by defining the upper surface as the average caldera rim Table 5 Estimates of erupted volume for the Incapillo Caldera and Dome Complex. Dome # is shown on Fig. 2B. Only volumes of the largest domes are reported. Volume

Density

DREa

Mass

(km3)

(kg/m3)

(km3)

(kg)

–

–

–

Topo-integrated

1675 1675 1675 1950

14.1 6.3 20.4 0.5

3.7E+13 1.1E+13 4.7E+13 1.3E+12

Geometric binning Geometric binning Geometric binning Topo-integrated

DREa

Mass

Method

Caldera Pyroclastic deposits Incapillo ignimbrite Outflowa Inflowa Total Veladero debris flowb

4.3

21.9 9.8 31.7 0.7

Dome #

Volume 3

Domes 1 2–3 5 10 22 24–27 28–29 30 31 32–34 36 40 Total a

Density 3

(km )

(kg/m )

(km3)

(kg)

2.16 0.58 1.77 2.42 0.73 0.11 0.11 0.06 0.02 0.20 0.08 7.80 16.03

– – – – – – – – – – – – 2650

– – – – – – – – – – – – –

– – – – – – – – – – – 4.2E+13

Method

Topo-integrated Topo-integrated Topo-integrated Topo-integrated Topo-integrated Topo-integrated Topo-integrated Topo-integrated Topo-integrated Topo-integrated Topo-integrated Topo-integrated Topo-integrated

Average density reflects an ignimbrite composition of 60% pumice with an averaged measured density (δ) of 1022 kg/m3 and 40% crystalline matrix comprised of quartz (67%; δ = 2620 kg/m3), albite (20%; δ = 2620 kg/m3), sanidine (5%; δ = 2520 kg/m3), biotite (5%; δ = 3090 kg/m3), and hornblende (3%; δ = 3270 kg/m3) consistent with the observed mineral abundances in the ignimbrite. b Density of the Veladero debris flow is calculated from cut and weighed blocks of cemented debris flow material.

400

A.R. Goss et al. / Journal of Volcanology and Geothermal Research 184 (2009) 389–404

elevation (5400 m) and the lower surface as the DEM topography of the caldera floor, which defines the unfilled collar height of the caldera. The resultant volume of the caldera is 4.3 km3 and is similar to other intermediate-sized collapse calderas such as Katmai in Alaska (∼5 km3; Hildreth,1991). This is a maximum estimate of the initial caldera volume as post-eruptive erosion has likely widened the caldera over the last 0.5 Ma. Due to its gradually tapering thickness and unknown basal topography, the topographically-integrated volume of the Incapillo ignimbrite could not be effectively calculated using the DEM. Instead, the ignimbrite outflow volume was calculated by approximating the unit as a 250 m high cone of siliceous material that progressively thins to 10 m at a radial distance of 10 km. This approach gives a total volume of 26.2 km3. After subtracting out 4.3 km3 of missing caldera material and correcting for vesicularity, the total outflow volume of the Incapillo ignimbrite is ∼ 14.1 km3 (DRE). Dense rock equivalent (DRE) is based on a calculated density of 1675 kg/m3 for the Incapillo ignimbrite, which corresponds to a rhyolitic magma with 35% vesicularity and is consistent with estimated density and vesicularity for the Atana ignimbrite from La Pacana caldera in the APVC (Fig. 1; Lindsay et al., 2001). For the Incapillo Caldera, the volume of intracaldera tuff (i.e., inflow) is difficult to estimate due to the lack of exposed caldera ring faults or exposures of sub-caldera basement that could provide some constraints on the basement topography and subsidence depth. Assuming a realistic subsidence depth of 0.5 km, the total ignimbrite inflow volume is estimated at 6.3 km3 DRE. Combined with the outflow, the total volume of the Incapillo ignimbrite is ∼20 km3 DRE and is a minimum estimate as co-ignimbrite ash is not included in the volume total. Lastly, the lithic-rich Veladero debris flow has a topographicallyintegrated volume of ∼ 0.7 km3 or ∼0.5 km3 DRE and a mass of 0.13 × 1013 kg (average density = 1959 kg/m3 measured from cemented debris flow blocks). This is an order of magnitude smaller than the Incapillo ignimbrite and is consistent with its origin as a topographically-confined debris flow. The total erupted volume in DRE for all Incapillo ignimbrites (∼20 km3) is comparable to the 1912 eruption of Katmai, which produced a total of 12 km3 of ignimbrite (Hildreth, 1991). Including the rhyodacitic domes, the total calculated volume of all erupted Incapillo magmas is ∼37 km3 and is far smaller than other Central Andean calderas (Fig. 1) such as Cerro Galán (∼2000 km3; Sparks et al., 1985) in the southern Puna and APVC centers [Vilama (1400 km3; Soler et al., 2007), Coranzuli (650 km3; Seggiaro, 1994), Granada (100 km3; Caffe et al., 2008), Panizos (∼650 km3; Ort, 1993), La Pacana (2400 km3; Lindsay et al., 2001), Purico (∼100 km3; Schmitt et al., 2001)]. The Incapillo ignimbrite is similar in size to numerous ignimbrites in the southern Puna with volumes ranging from 5–50 km3 (Siebel et al., 2001). 7. Discussion The Incapillo Caldera and Dome Complex is an example of a b3 Ma evolved Andean center that erupted during a period of waning arc magmatism following a major late Miocene tectonic event. This event was characterized by rapid eastward migration of the arc front (7– 3 Ma), enhanced forearc subduction erosion, and shallowing of the subducting slab beneath the backarc (Kay and Mpodozis, 2002; Goss and Kay, 2009; Kay and Coira, in press). These spatial and temporal changes in tectonic setting controlled the dominantly-effusive eruptive style, volume, and geochemical evolution of the Incapillo Caldera and Dome Complex, which contrasts with the large ignimbritic megacalderas of the Central Andean APVC associated with the inception of large-scale siliceous magmatism along the arc front and across the backarc between 11 and 4 Ma (Kay et al., 1999; de Silva and Gosnold, 2007; Kay and Coira, in press). Here, we discuss the eruptive sequence of the Incapillo Caldera and Dome Complex

using the K–Ar and 40Ar/39Ar ages and present a comprehensive eruption model based on petrography and mapping from both lithostratigraphy and digital imagery. 7.1. Eruptive sequence and model Both field relationships and 40Ar/39Ar biotite ages in Table 3 clearly show that the pumice-rich Incapillo ignimbrite was ejected in the final major eruptive event associated with the Incapillo magmatic system at ∼ 0.51 Ma. The Incapillo eruptive sequence is thus divided into the three stages shown in Fig. 7: (1) an initial period of regional effusive eruptions of rhyodacitic domes; (2) an episode of intense pyroclastic eruptions followed by caldera collapse; and (3) a terminal period of post-eruptive localized debris flows and intracaldera dome eruptions. 7.1.1. Stage 1 — Effusive dome eruptions Siliceous magmatism associated with the Incapillo complex began at ∼3 Ma with the effusive eruption of steep-sided domes and coulées along the slopes of Cerro Bonete Chico and Sierra de Veladero. Based on the breached pre-caldera rhyodacitic dome that is exposed within the north wall of the caldera (Fig. 3A), this phase persisted at least until 0.87 Ma. The chemical (Table 4) and petrographic similarity of the domes irrespective of their distance from the caldera suggests that all erupted dome magmas were generated in a common magma chamber located either directly below the caldera or in multiple shallowchambers fed by a larger and deeper primary chamber. This gas-poor viscous rhyodacitic magma likely migrated to the surface laterally via now obscured E–W trending faults that parallel a larger E–W regional fault corridor that extends westward into the southernmost Maricunga Belt in Chile (Fig. 1; Mpodozis et al., 1997) or through a preexisting network of magma conduits formed during the older eruptions of Sierra de Veladero and Cerro Bonete Chico. Similarly, Baldwin and Marrett (2004) proposed that the eruption of non-explosive dacitic domes within the Cordillera de San Buenaventura (27° S; Fig. 1) was controlled by the opening of a E–W releasing bend between a system of regionalscale N–S strike-slip faults. With regard to eruption dynamics of these early domes, degassing and lava extrusion occurred contemporaneously via gas-permeable conduits that resulted in the characteristic pit craters and lava spines observed on the summits of the largest domes (Fig. 5B–C). Eichelberger et al. (1986) modeled tephra-filled “vent funnels” within the Mono-Inyo domes (California, USA) as a means to rapidly devolatilize and effusively erupt a highly siliceous magma. However, the initial “open-system” ash and tephra layer reported for Mono-Inyo domes and observed at Cerro Chao and Tocopuri at ∼22° S in the Central Andes (de Silva et al., 1994) is not apparent at the base of the Incapillo domes or within the pit craters. Instead, the Incapillo domes directly overlay late Miocene to Pliocene andesitic Pircas Negras and dacitic Sierra de Veladero and Cerro Bonete Chico lavas flows (Table 1). This observation supports a purely nonexplosive initial magmatic stage for the Incapillo Caldera and Dome Complex. 7.1.2. Stage 2 — Pyroclastic eruption and edifice collapse The Incapillo ignimbrite was deposited during the ∼ 0.51 ± 0.04 Ma pyroclastic eruption that formed the Incapillo Caldera and terminated ∼2 million years of effusive volcanism. Given the paucity of lithics in the Incapillo ignimbrite, the pre-caldera edifice must have been small and may have been morphologically similar to the b3 Ma Incapillo domes that surround the caldera. Proximal finely-laminated deposits at the base of the ignimbrite indicate that the initial blast generated a ground surge that was immediately followed by a series of pumice-rich pyroclastic flows. With the exception of its unwelded nature, the Incapillo ignimbrite shares many of the physical characteristics of peralkaline ignimbrites (Freundt, 1998) that have thicknesses of 10–30 m, volumes of ∼5–20 km3, extend ∼10–40 km from calderas 3–10 km in diameter,

A.R. Goss et al. / Journal of Volcanology and Geothermal Research 184 (2009) 389–404

401

Fig. 7. Three stage eruptive model for the formation of the Incapillo Caldera and Dome Complex. Stage one (3–1 Ma) is characterized by effusive eruption of compositionally homogeneous rhyodacitic domes. Stage 2 (0.51 ± 0.04 Ma) represents the pyroclastic eruption of the Incapillo magma chamber contemporaneous with caldera subsidence and deposition of the Incapillo ignimbrite. Stage 3 (b 0.51 Ma) is characterized by the intracaldera dome eruption of residual magma, deposition of the Veladero debris flow, and erosional widening of the caldera.

and generally lack air fall deposits (Schmincke, 1974; Mahood, 1984). These types of ignimbrites are thought to originate within extensional tectonic settings from caldera subsidence-related pyroclastic eruptions that could not support high convecting Plinian eruption columns. Like these ignimbrites, the lack of associated ash fall deposits covering the 6– 2 Ma dacitic centers or any of the b3 Ma Incapillo domes suggests that the explosive ∼0.51 Ma Incapillo eruption produced an initial series of pyroclastic density currents (e.g., Freundt, 1999; Allen, 2001) rather than a sustained Plinian cloud that reached stratospheric altitudes (N10 km).

Ground-hugging pyroclastic flows from an oscillatory collapsing fountain traveled around older volcanic structures via topographic lows and formed the laminated lower section of the Incapillo ignimbrite. As the eruption progressed, a more-stable collapsing fountain built-up the massive proximal upper-section of the ignimbrite to a thickness of N200 m around the central vent. This eruption model is consistent with the presence of local fault-bounded grabens to the north and west (Fig. 2; Mpodozis et al., 1997) that suggest the Incapillo eruption may have occurred within a localized extensional setting.

402

A.R. Goss et al. / Journal of Volcanology and Geothermal Research 184 (2009) 389–404

The similar chemical characteristics between the ignimbrite pumices and 3–1 Ma dome lavas strongly suggest that all Incapillo eruptive units originated from the same source (Table 4). The reasons as to why the Incapillo magma chamber suddenly exploded at ∼0.51 Ma rather than forming another non-explosive dome may lie in the basal pumice rafts at the distal end of the ignimbrite. These earliest and most energetic pyroclastic flows reflect the physical condition of the magma at the start of the eruption. The mixing textures evident in the distal pumice rafts suggest that the explosive eruption that generated the Incapillo ignimbrite was catalyzed by the mixing of a dacitic dome-like magma with a crystal-rich rhyolitic cap (Fig. 4F and Table 4). Intrusion of a possibly hotter more-basic magma into the base of a cooling partiallycrystalline acidic magma chamber would lead to vigorous thermal convection and overpressurization of the magma chamber, which may have ultimately triggered the explosive eruption (Sparks et al., 1977; Folch and Marti, 1998). Alternatively, contractive closure (e.g., McNulty et al., 1998; Schmitt, 2001) of the magmatic conduits that permitted effusive eruption and passive degassing of Incapillo dome lavas could also potentially explain the sudden change to explosive volcanism. However, this model does not explain the clear mixing textures in the pumices nor the lack of structural evidence to support a change to a more-contractional setting after 1 Ma. Like other calderas formed by overpressurization of a magma chamber under tensional stress (see Martí et al., 2009), caldera collapse probably occurred synchronous with evolving pyroclastic eruptions that eventually exposed the pre-caldera dome unit and the 200 meter thick ignimbritic outflow units that now comprise the walls of the caldera. Though no clear ring faults are identifiable within the interior of the caldera, its symmetrical shape, uniform depth, and steeply sloped walls suggest a piston-type subsidence mechanism (Lipman, 1997). The steep edges of the caldera, particularly along its northern rim, may reflect near vertical ring faults that controlled the foundering of the caldera floor and may have been eroded back into a collapse collar by mass wasting induced scarp-retreat and/or buried by intracaldera fill. 7.1.3. Stage 3 — Post-caldera debris flows and magmatic domes The third and final eruptive stage of the Incapillo Caldera and Dome Complex is marked by the deposition of the Veladero debris flow and the eruption of domes within the newly-formed caldera. The geographical extent of the debris flow is limited, as it is only found within the Quebrada de Veladero south of the caldera. Moderate matrix cementation of the deposit with clay-sized ash (b5 µm) suggests the involvement of water and transport of the debris flow as a fluidized slurry (e.g., Rodine and Johnson, 1976). The source of the water could be from melting of a Pleistocene glacial cap or by expulsion from a paleocrater lake. The non-conformable contact between the Incapillo ignimbrite and the Veladero debris flow (Fig. 4A–B) indicates that this depositional event occurred after the pyroclastic eruption terminated. Continued dome activity within the Incapillo Caldera followed the ∼0.51 Ma pyroclastic eruption, although the lag time between caldera collapse and the final dome eruption is unknown as there are no radiometric ages for these units. Given that no known post-ignimbrite domes occur outside of the caldera walls, these magmas could have ascended to the surface via the main explosive conduit that produced the Incapillo ignimbrite. The elevated temperatures of the Incapillo crater lake waters (Markham and Barker, 1986) may also indicate the presence of an active hydrothermal system below the Incapillo Caldera. These observations preclude a classification as a completely extinct Andean volcanic center. 8. Conclusions The Incapillo Caldera and Dome Complex (28° S, 68.8° W), situated within the transition zone between the Central Andean steep-slab and Chilean–Pampean flat-slab segments of the Andes, is the southernmost Quaternary explosive caldera of the Central Andean Volcanic

Zone (CVZ). This study marks the first investigation of the morphology and volcanology of this center. The main conclusions are: (1) The dominant eruptive mechanism of the Incapillo magmatic system was the effusive non-explosive eruption of more than 40 crystal-rich steep-sided rhyodacitic domes. The alignments of the domes suggest that magmas ascended along preexisting structural weaknesses in the crust that parallel regional faults. K–Ar and 40Ar/39Ar biotite ages of the dome lavas indicate effusive eruptions began by 5 Ma during the 7–3 Ma migration of the Andean arc front at this latitude and continued until about 0.8 Ma. (2) The ∼250 m high walls of the elliptical Incapillo Caldera (5 × 6 km) are principally composed of a homogeneous pumice-rich and lithic-poor ignimbrite that radiates up to 15 km in all directions from the caldera. Both proximal and distal outflow pumices of the Incapillo ignimbrite yield 40Ar/39Ar biotite ages of ∼0.51 Ma, which date the caldera-forming explosive eruption. The lack of Plinian fall deposits covering any preexisting domes or overlying Incapillo ignimbrite suggests an over-pressurized style of eruption, where topographically-confined pyroclastic density currents followed by massive ignimbrites produced by a stable collapsing fountain, generated the Incapillo Caldera. (3) Evidence for magma mixing in the Incapillo ignimbrite comes from pumice rafts at the distal end of the ignimbrite. Large (b40 cm) compositionally mixed pumices record the mixing of a dacitic magma (SiO2 = 67 wt.%) into a partially-crystalline rhyolitic host (SiO2 = 71 wt.%). Vigorous thermal convection and overpressurization caused by mixing catalyzed the explosive caldera-forming eruption. (4) The final eruptive stage was marked by the deposition of a lithicrich debris flow that unconformably overlays the Incapillo ignimbrite. The debris flow likely formed from a lahar following the caldera-forming eruption. Lastly, a suite of intracaldera domes erupted within the floor of the Incapillo Caldera. A potentially active hydrothermal system below the Incapillo Caldera is evidenced by highly-altered intracaldera domes and a crater lake with elevated temperatures compared to local nonvolcanic lakes (Markham and Barker, 1986). (5) Volume estimates indicate over 16 km3 of Incapillo dome material effusively erupted before the formation of the Incapillo Caldera, which itself has a volume of ∼4.3 km3. The total volume of Incapillo ignimbrite inflow and outflow units converted to DRE is estimated at ∼20 km3. The debris flow has a DRE volume of ∼0.5 km3. The total volume for the Incapillo Caldera and Dome Complex is ∼37 km3, which is similar to the small- to medium-sized ignimbrites (b50 km3) from the southern Puna (Siebel et al., 2001), but far smaller than the giant Altiplano–Puna megacalderas with erupted volumes ranging from 650–2500 km3. (6) In sum, the Incapillo Caldera and Dome Complex represents the model of a caldera erupting during the waning stages of a dying central Andean arc and contrasts with the “ignimbrite flare-up” model for the 11–4 Ma Altiplano–Puna calderas that erupted in a waxing arc over a steepening slab. Acknowledgements We particularly thank Beatriz Coira, Gabriela Depine, Daniel Rubiolo, and Andrés Folguera for their contributions in the field and Brian Jicha for his assistance with the Ar–Ar geochronology. We also need to recognize the high alpine 4 × 4 driving ability of Jorge Lemp and Antonio Diaz (SERNAGEOMIN) that greatly aided in sample collection and field mapping. Financial support for this research came from the Chilean CONICYT (National Commission for Technological and Scientific Research, Chile, Grant # 1920149), National Science

A.R. Goss et al. / Journal of Volcanology and Geothermal Research 184 (2009) 389–404

Foundation Grant (0126000), and a NASA Earth Systems Science Graduate Research Fellowship. This manuscript was improved by the constructive reviews of Gerhard Wörner and Pablo Caffe. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jvolgeores.2009.05.005. References Allen, S.R., 2001. Reconstruction of a major forming eruption from pyroclastic deposit characteristics: Kos Plateau Tuff, eastern Aegean Sea. Journal of Volcanology and Geothermal Research 105, 141–162. Baker, M.C.W., 1981. The nature and distribution of upper Cenozoic ignimbrite centres in the Central Andes. Journal of Volcanology and Geothermal Research 11, 293–315. Baldwin, A.K. and Marrett, R.A., 2004. Evidence for a releasing bend at the southern margin of the Puna plateau, Argentine Andes, GSA Annual Meeting, abstract, Denver, CO. Barazangi, M., Isacks, B.L., 1976. Spatial distribution of earthquakes and subduction of the Nazca Plate beneath South America. Geology 4 (11), 686–692. Beck, S.L., Zandt, G., 2002. The nature of orogenic crust in the Central Andes. Journal of Geophysical Research 107 (B10). doi:10.1029/2000jb000124(B10). Beck, S.L., et al., 1996. Crustal-thickness variations in the Central Andes. Geology 24 (5), 407–410. Caffe, P.J., Soler, M.M., Coira, B., Cabrera, A., Flores, P.I., 2007. Estratigrafía y centros eruptivos de la región de Pairique, Puna Jujeña. Revista de la Asociación Geológica Argentina 62 (2), 242–256. Caffe, P.J., Soler, M.M., Coira, B.L., Onoe, A.T., Cordani, U.G., 2008. The Granada ignimbrite: a compound pyroclastic unit and its relationship with Upper Miocene caldera volcanism in the northern Puna. Journal of South American Earth Sciences 25, 464–484. Cahill, T.A., Isacks, B.L., 1992. Seismicity and shape of the subducted Nazca Plate. Journal of Geophysical Research 97 (B12), 17,503–17,529. Caminos, R. and Fauqué, L., 2000. Carta Geológica de la Republica Argentina, Tinogasta, 2969-II. SEGEMAR Instituto de Geología y Recursos Minerales. Coira, B., Kay, S.M., Viramonte, J.G., 1993. Upper Cenozoic magmatic evolution of the Argentine Puna — a model for changing subduction geometry. International Geology Review 35, 677–720. Coira, B., Caffe, P.J., Kay, S.M., Diaz, A. and Ramirez, A., 1996. Complejo volcanico Vilamasistema caldérico del Cenozoico superior en Puna, Jujuy, 13° Congreso Geológico Argentino, Buenos Aires, Argentina, pp. 603–620. DeMets, C., Gordon, R.G., Argus, D.F., Stein, S., 1990. Current plate motions. Geophysical Journal International 101 (2), 425–478. de Silva, S.L., 1989. Altiplano–Puna volcanic complex of the Central Andes. Geology 17 (12), 1102–1106. de Silva, S.L., Francis, P.W., 1991. Volcanoes of the Central Andes. Springer-Verlag, Berlin. 216 pp. de Silva, S.L., Gosnold, W.D., 2007. Episodic construction of batholiths: insights from the spatiotemporal development of an ignimbrite flare-up. Journal of Volcanology and Geothermal Research 167, 320–335. doi:10.1016/j.jvolgeores.2007.07.015. de Silva, S.L., Self, S., Francis, P.W., Drake, R.E., Ramirez, C.R., 1994. Effusive siliceous volcanism in the Central Andes: the Chao dacite and other young lavas of the Altiplano–Puna volcanic complex. Journal of Geophysical Research 99 (B9), 17805–17825. Eichelberger, J.C., Carrigan, C.R., Westrich, H.R., Price, R.H., 1986. Non-explosive silicic volcanism. Nature 323, 598–602. Fauqué, L., 2000. Carta Geológica de la Republica Argentina, Pastillos, 2969-I. SEGEMAR Instituto de Geología y Recursos Minerales. Folch, A., Marti, J., 1998. The generation of overpressure in felsic magma chambers by replenishment. Earth and Planetary Science Letters 163 (1–4), 301–314. Francis, P.W., Baker, M.C.W., 1978. Sources of two large ignimbrites in the central Andes: some Landsat evidence. Journal of Volcanology and Geothermal Research 4 (1–2), 81–87. Francis, P.W., et al., 1989. Petrology and geochemistry of volcanic rocks of the Cerro Galán Caldera, Northwest Argentina. Geological Magazine 126 (5), 515–547. Freundt, A., 1998. The formation of high-grade ignimbrites, I: experiments on high- and low-concentration transport systems containing sticky particles. Bulletin of Volcanology 59 (6), 414–435. Freundt, A., 1999. Formation of high-grade ignimbrites, II: a pyroclastic suspension current model with implications also for low-grade ignimbrites. Bulletin of Volcanology 60 (7), 545–567. Fromm, R., Zandt, G., Beck, S.L., 2004. Crustal thickness beneath the Andes and Sierras Pampeanas at 30 °S inferred from Pn apparent phase velocities. Geophysical Research Letters 31 (6). doi:10.1029/2003GL019231(6). Gardeweg, M., Ramírez, C.F., 1987. La Pacana caldera and the Atana ignimbrite — a major ash-flow and resurgent caldera complex in the Andes of northern Chile. Bulletin of Volcanology 49, 547–566. Gardeweg, M., Mpodozis, C., Clavero, J., Cuitiño, L., 1998. The Ojos del Salado complex: the highest active volcano in the world. Volcanoes and Their Roots — IAVCEI, Abstracts, Cape Town, South Africa, p. 21. Gardeweg, M., Clavero, J., Mpodozis, C., Pérez de Arce, C., Villeneuve, M., 2000. El Macizo Tres Cruces: Un complejo volcánico longevo y potencialmente activo en la Alta

403

Cordillera de Copiapó, Chile. 9° Congreso Geológico Chileno, Actas, Puerto Varas, Chile, pp. 191–195. Goss, A.R., Kay, S.M., 2009. Extreme high field strength element (HFSE) depletion and near-chondritic Nb/Ta ratios in Central Andean adakite-like lavas (∼28° S, 68° W). Earth and Planetary Science Letters 279 (1–2), 97–109. Goss, A.R., Kay, S.M., Mpodozis, C. and Depine, G., 2006. The Incapillo Caldera: A stranded magma chamber over a dying Andean arc, EOS Transactions, Fall Meeting. Hildreth, W., 1991. The timing of caldera collapse at Mount Katmai in response to magma withdrawal toward Novarupta. Geophysical Research Letters 18 (8), 1541–1544. Jordan, T.E., et al., 1983. Andean tectonics related to geometry of subducted Nazca Plate. Geological Society of America Bulletin 94 (3), 341–361. Kay, S.M. and Coira, B.L., in press. Shallowing and steepening subduction zones, continental lithospheric loss, magmatism and crustal flow under the Central Andean Altiplano–Puna plateau. In: S.M. Kay, V.A. Ramos and W.R. Dickinson (Editors), Backbone of the Americas: Shallow Subduction, Plateau Uplift, and Ridge and Terrane Collision. Geological Society of America, Memoir 204. Boulder, Colorado, pp. 261–292. Kay, S.M., Mpodozis, C., 2000. Chemical signatures from magmas at the southern termination of the central Andean Volcanic Zone: the Incapillo/Bonete and surrounding regions. 9° Congreso Geológico Chileno, Puerto Varas, Chile, pp. 626–629. Kay, S.M., Mpodozis, C., 2002. Magmatism as a probe to the Neogene shallowing of the Nazca Plate beneath the modern Chilean flat-slab. Journal of South American Earth Sciences 15 (1), 39–57. Kay, S.M., Maksaev, V., Moscoso, R., Mpodozis, C., Nasi, C.,1987. Probing the evolving Andean lithosphere: Mid-late Tertiary magmatism in Chile (29°–30° 30′) over the modern zone of subhorizontal subduction. Journal of Geophysical Research 92 (B7), 6173–6189. Kay, S.M., Mpodozis, C., Ramos, V.A., Munizaga, F., 1991. Magma source variations for midlate Tertiary magmatic rocks associated with a shallowing subduction zone and a thickening crust in the Central Andes (28 to 33° S). In: Harmon, R.S., Rapela, C.W. (Eds.), Andean Magmatism and its Tectonic Setting: Geological Society of America Special Paper 265, Boulder, Colorado, pp. 113–137. Kay, S.M., Mpodozis, C., Tittler, A., Cornejo, P., 1994. Tertiary magmatic evolution of the Maricunga mineral belt in Chile. International Geology Review 36 (12), 1079–1112. Kay, S.M., Mpodozis, C., Coira, B., 1999. Neogene magmatism, tectonism, and mineral deposits of the Central Andes (22° to 33°S latitude). In: Skinner, B.J. (Ed.), Geology and Ore Deposits of the Central Andes, pp. 27–59. Kay, S.M., Coira, B., Caffe, P.J., 2008a. Geoquímica, fuentes y evolución del magmatismo neógeno de la Puna norte., Geología y Recursos Naturales de la Provincia de Jujuy. Relatorio del 17° Congreso Geológico Argentino, San Salvador de Jujuy, Argentina, pp. 322–334. Kay, S.M., Coira, B., Mpodozis, C., 2008b. Field trip guide: Neogene evolution of the central Andean Puna plateau and southern Central Volcanic Zone. In: Kay, S.M., Ramos, V.A. (Eds.), Field Trip Guides to the Backbone of the Americas in the Southern and Central Andes: Ridge Collision, Shallow Subduction, and Plateau Uplift: Geological Society of America Field Guide, vol. 13, pp. 119–183. doi:10.1130/2008.0013(05). Lindsay, J.M., de Silva, S.L., Trumbull, R., Emmermann, R., Wemmer, K., 2001. La Pacana caldera, N. Chile: a re-evaluation of the stratigraphy and volcanology of one of the world's largest resurgent calderas. Journal of Volcanology and Geothermal Research 106, 145–173. Lipman, P.W., 1997. Subsidence of ash-flow calderas: relation to caldera size and magma-chamber geometry. Bulletin of Volcanology 59, 198–218. Mahood, G.A., 1984. Pyroclastic rocks and calderas associated with strongly peralkaline magmatism. Journal of Geophysical Research 89, 8540–8552. Markham, B.L., Barker, J.L., 1986. Landsat MSS and TM post-calibration dynamic ranges, exoatmospheric reflectances and at-satellite temperatures. EOSAT Technical Notes 1, 3–8. Martí, J., Geyer, A., Folch, A., 2009. A genetic classification of collapse calderas based on field studies, and analogue and theoretical modeling. In: Thordarson, T., Self, S. (Eds.), Volcanology, the Legacy of George Walker: Geological Society of London Special Publication. McGlashan, N., Brown, L.D., Kay, S.M., 2008. Crustal thickness in the central Andes from teleseismically recorded pmP depth phase precursors. Geophysical Journal International 175, 1013–1022. McKee, E., Robinson, A.C., Rybuta, J.J., Cuitiño, L., Moscoso, R., 1994. Age and Sr-isotopic composition of volcanic rocks in the Maricunga Belt, Chile: implications for magma sources. Journal of South American Earth Sciences 7, 167–177. McNulty, B.A., Farber, D.L., Wallace, G.S., Lopez, R., Palacios, O., 1998. Role of plate kinematics and plate-slip-vector partitioning in continental magmatic arcs: evidence from the Cordillera Blanca, Peru. Geology 26 (9), 827–830. Mpodozis, C., 1995. Evolución magmática terciaria de la región de transición entre la CVZ y la zona de subducción subhorizontal de los Andes Chilenos. Servicio Nacional de Geología y Minería, Open File Report, Santiago, Chile. Mpodozis, C., Kay, S., Cornejo, P., Tittler, A., 1995. La Franja de Maricunga: síntesis de la evolución del frente volcánico oligoceno-mioceno de la zona sur de los Andes Centrales. Revista Geológica de Chile 22 (2), 273–313. Mpodozis, C., Kay, S.M., Gardeweg, M.P., Coira, B., 1996. Geología de la region de Ojos del Salado (Andes centrales, 27° S): Implicancias de la migración hacia el este del frente volcánico Cenozoico superior. 13° Congreso Geológico Argentino, Buenos Aires, Argentina, pp. 539–548. Mpodozis, C., Kay, S.M., Gardeweg, M., Coira, B., 1997. Geología de la region de ValleAncho-Laguna Verde (Catamarca, Argentina): Una ventana al basamento del extremo sur de la zona volcánica de los Andes Centrales. 8° Congreso Geológico Chileno, Actas, Antofagasta, Chile, pp. 1689–1693. Ort, M.H., 1993. Eruptive processes and caldera formation in a nested downsag-collapse caldera: Cerro Panizos, central Andes. Journal of Volcanology and Geothermal Research 56 (3), 221–252.

404

A.R. Goss et al. / Journal of Volcanology and Geothermal Research 184 (2009) 389–404

Pardo, M., Comte, D., Monfret, T., 2002. Seismotectonic and stress distribution in the central Chile subduction zone. Journal of South American Earth Sciences 15 (1), 11–22. Ramos, V.A., Cristallini, E.O., Perez, D.J., 2002. The Pampean flat-slab of the Central Andes. Journal of South American Earth Sciences 15 (1), 59–78. Renne, P.R., et al., 1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chemical Geology 145 (1–2), 117–152. Rodine, J.D., Johnson, A.M., 1976. Ability of debris, heavily freighted with coarse clastic materials, to flow on gentle slopes. Sedimentology 23 (2), 213–234. Rubiolo, D., Zappettini, E., Lizuain, A., Hickson, C., 2002. Regional aspects of the southern end of the central volcanic zone (between 27° and 28° S), Argentina. 5th International Symposium on Andean Geodynamics, Toulouse, France, p. 560-557. Schmincke, H.U., 1974. Volcanological aspects of peralkaline silicic welded ash-flow tuffs. Bulletin of Volcanology 38, 594–636. Schmitt, A.K., 2001. Gas-saturated crystallization and degassing in large-volume, crystal-rich dacitic magmas from the Altiplano–Puna, northern Chile. Journal of Geophysical Research 106 (B12), 30,561–30,578. Schmitt, A.K., de Silva, S.L., Trumbull, R.B., Emmermann, R., 2001. Magma evolution in the Purico ignimbrite complex, northern Chile: evidence for zoning of a dacitic magma by injection of rhyolitic melts following mafic recharge. Contributions to Mineralogy and Petrology 140 (6), 680–700. Schnurr, W.B.W., et al., 2007. Twenty-five million years of silicic volcanism in the southern central volcanic zone of the Andes: geochemistry and magma genesis of ignimbrites from 25 to 27 °S, 67 to 72 °W. Journal of Volcanology and Geothermal Research 166, 17–46.

Seggiaro, R.E., 1994. Petrología, geoquímica y mecanismos de erupción del complejo volcánico Coranzulí, Ph.D. thesis, Universidad Nacional de Salta, Salta, Argentina, 230 pp. Siebel, W., et al., 2001. Geochemistry and isotope systematics of small- to mediumvolume Neogene–Quaternary ignimbrites in the southern central Andes: evidence for derivation from andesitic magma sources. Chemical Geology 171, 213–237. Singer, B.S., Ackert, R.P., Guilllou, H., 2004. 40Ar/39Ar and K–Ar chronology of Pleistocene glaciations in Patagonia. Geological Society of America Bulletin 116 (3–4), 434–450. Smalley Jr., R., Isacks, B.L., 1990. Seismotectonics of thin- and thick-skinned deformation in the Andean foreland from local network data: evidence for a seismogenic lower crust. Journal of Geophysical Research 95 (B8), 12,487–12,498. Soler, M.M., Caffe, P.J., Coira, B.L., Onoe, A.T., Kay, S.M., 2007. Geology of the Vilama caldera: a new interpretation of a large-scale explosive event in the Central Andean plateau during the Upper Miocene. Journal of Volcanology and Geothermal Research 164, 27–53. Sparks, R.S.J., Sigurdsson, H., Wilson, L., 1977. Magma mixing: a mechanism for triggering acid explosive eruptions. Nature 267, 315–318. Sparks, R.S.J., et al., 1985. Ignimbrites of the Cerro Galán Caldera, NW Argentina. Journal of Volcanology and Geothermal Research 24 (3–4), 205–248. Steiger, R.H., Jäger, E., 1977. Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters 36 (3), 359–362. Yuan, X., Sobolev, S.V., Kind, R., 2002. Moho topography in the Central Andes and its geodynamic implications. Earth and Planetary Science Letters 199 (3–4), 389–402.