Downloaded from gsabulletin.gsapubs.org on February 6, 2012 Geological Society of America Bulletin 40Ar/39Ar chronostratigraphy of Altiplano-Puna volcanic complex ignimbrites reveals the development of a major magmatic province Morgan J. Salisbury, Brian R. Jicha, Shanaka L. de Silva, Brad S. Singer, Néstor C. Jiménez and Michael H. Ort Geological Society of America Bulletin 2011;123, no. 5-6;821-840 doi: 10.1130/B30280.1 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Bulletin Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Notes © 2011 Geological Society of America Downloaded from gsabulletin.gsapubs.org on February 6, 2012 40Ar/39Ar chronostratigraphy of Altiplano-Puna volcanic complex ignimbrites reveals the development of a major magmatic province Morgan J. Salisbury1,†, Brian R. Jicha2, Shanaka L. de Silva1, Brad S. Singer2, Néstor C. Jiménez3, and Michael H. Ort4 1Department of Geosciences, Oregon State University, Corvallis, Oregon 97331, USA 2Department of Geoscience, University of Wisconsin–Madison, Madison, Wisconsin 53706, USA 3Instituto de Investigaciones Geológicas y del Medio Ambiente, Universidad Mayor de San Andrés, La Paz, Bolivia 4School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, Arizona 86011, USA ABSTRACT with the eruption of the 4.09 Ma Puripicar, tablishment of a regional stratigraphy based 4.00 Ma Chaxas, and 3.96 Ma Atana ignim- on physical characteristics of successive ig- The Lípez region of southwest Bolivia is brites. This third pulse was followed by two nimbrites. Although this region includes some the locus of a major Neogene ignimbrite fl are- more vol canic explosivity index (VEI) 8 erup- of the world’s largest documented ignimbrite up, and yet it is the least studied portion of tions, producing the 3.49 Ma Tara (800 km3 eruptions (Guest, 1969; de Silva, 1989b) includ- the Altiplano-Puna volcanic complex of the dense rock equivalent [DRE]) and 2.89 Ma ing the Atana (Gardeweg and Ramírez, 1987; Central Andes. Recent mapping and laser- Pastos Grandes (1500 km3 DRE) ignim- Lindsay et al., 2001a), Puripicar (de Silva and fusion 40Ar/39Ar dating of sanidine and bio- brites. In addition to these large caldera- Francis, 1989), and Vilama (Soler et al., 2007) tite from 56 locations, coupled with paleo- related eruptions, new age determinations ignimbrites, many fundamental stratigraphic magnetic data, refi ne the timing and volumes refi ne the timing of two little-known ig- details of the volcanic fi eld remain to be under- of ignimbrite emplacement in Bolivia and nimbrite shields, the 5.23 Ma Alota and stood. A conspicuous gap in the knowledge of northern Chile to reveal that monotonous 1.98 Ma Laguna Colorada centers. Moreover, the regional ignimbrites lies within the Lípez intermediate volcanism was prodigious and 40Ar/39Ar age determinations of 13 ignim- region of southwest Bolivia, in the heart of the episodic throughout the complex. The new brites from northern Chile previously dated Altiplano-Puna volcanic complex, where we results unravel the eruptive history of the by the K-Ar method improve the overall have concentrated our study and suffi cient expo- Pastos Grandes and Guacha calderas, two temporal resolution of Altiplano-Puna vol- sure of basement allows us to defi ne the onset large multicyclic caldera complexes located canic complex development. Together with of the major ignimbrite volcanism. We report in Bolivia. These two calderas, together with the updated volume estimates, the new age de- new geologic mapping, precise 40Ar/39Ar age the Vilama and La Pacana caldera com- terminations demonstrate a distinct onset of determinations from 56 sites in Bolivia and plexes and smaller ignimbrite shields, were Altiplano-Puna volcanic complex ignimbrite Chile, geochemical analyses, and volume cal- the dominant sources of the ignimbrite- volcanism with modest output rates, an epi- culations of, until now, little-studied ignimbrites producing eruptions during the ~10 m.y. his- sodic middle phase with the highest eruption and eruptive centers including the large Pastos tory of the Altiplano-Puna volcanic complex. rates, followed by a decline in volcanic output. Grandes and Guacha caldera complexes. The The oldest ignimbrites erupted between 11 The cyclic nature of individual caldera com- 13 40Ar/39Ar sample sites in northern Chile were and 10 Ma represent relatively small vol- plexes and the spatiotemporal pattern of the previously described and K-Ar dated by de umes (approximately hundreds of km3) of volcanic fi eld as a whole are consistent with Silva (1989b). Additionally, we incorporate magma from sources distributed throughout both incremental construction of plutons as 40Ar/39Ar ages determined for fi ve Altiplano- the volcanic complex. The fi rst major pulse well as a composite Cordilleran batholith. Puna volcanic complex ignimbrites in Chile re- was manifest at 8.41 Ma and 8.33 Ma as the ported by Barquero-Molina (2003). Using these Vilama and Sifon ignimbrites, respectively. INTRODUCTION data, we reveal in detail the development of During pulse 1, at least 2400 km3 of dacitic one of the world’s youngest and best-preserved magma was erupted over 0.08 m.y. Pulse The Altiplano-Puna volcanic complex (de large silicic volcanic fi elds. This contribution 2 involved near-coincident eruptions from Silva, 1989a) of the Central Andes is one of to Altiplano-Puna volcanic complex volcanism three of the major calderas resulting in the the world’s premier settings for studying large includes new estimates of (1) the spatial extent 5.60 Ma Pujsa , 5.65 Ma Guacha, and 5.45 Ma silicic volcanism and its relation to plutonic of individual ignimbrites, (2) rates of volcanic Chuhuilla ignimbrites, for a total minimum underpinnings. The hyperarid climate of the fl uxes and pulses, and (3) the longevity and re- volume of 3000 km3 of magma. Pulse 3, the Altiplano-Puna region during the past several pose periods of multicyclic nested caldera sys- largest, produced at least 3100 km3 of magma million years has resulted in excellent preser- tems. We in turn examine the underlying causes during a 0.1 m.y. period centered at 4 Ma, vation of Altiplano-Puna volcanic complex of the Altiplano-Puna volcanic complex ignim- ignimbrites, but it has also limited erosion and brite fl are-up and the assembly of individual †E-mail: [email protected] vertical exposure, thereby hampering the es- plutonic centers and Cordilleran batholiths. GSA Bulletin; May/June 2011; v. 123; no. 5/6; p. 821–840; doi: 10.1130/B30280.1; 10 fi gures; 6 tables; Data Repository item 2011016. For permission to copy, contact [email protected] 821 © 2011 Geological Society of America Downloaded from gsabulletin.gsapubs.org on February 6, 2012 Salisbury et al. GEOLOGICAL CONTEXT— 74° 70° 66°W THE ALTIPLANO-PUNA VOLCANIC COMPLEX 1 2 3 4 The Altiplano-Puna plateau of the Cen- Figure 1. Location map of the 16°S tral Andes is second only to the Tibetan Pla- Altiplano-Puna volcanic com- Peru teau in terms of areal extent, elevation, and plex (APVC) and known Neo- Bolivia crustal thickness. Unlike Tibet, however, the gene ignimbrite eruptive centers 5 Altiplano-Puna plateau was formed at an active of the Central Andes. Major Peru-Chile Trench 7 6 continental margin characterized by subduc- calderas within the extent of 8 tion of a dense oceanic plate (Nazca) beneath Altiplano-Puna volcanic com- Altiplano old, continental lithosphere (Brazilian craton of plex ignimbrites are outlined. Ig- 20° the South American plate). Thus, the plateau nimbrite centers: 1—Cailloma, 10 9 is the only modern region of its size and charac- 2—Macusani, 3—Arequipa, 4— ter to also involve subduction-zone magmatism. Salas, 5—Mauri-Lauca, 6— APVC Subduction zone volcanism has been active Curahuara, 7—Soledad, 8— in the Central Andes since the Jurassic, and Morococala, 9—Frailes, 10— signifi cant tectonic shortening has occurred Empexa, 11—Galán. Inset shows Chile throughout the Cenozoic with major uplift of location of map along the west- Puna 24° the plateau in the Late Cenozoic. Crustal short- ern edge of South America. Argentina ening resulted in the presently thick continental crust (up to 70 km) beneath the Altiplano-Puna 11 Plateau. After a hiatus related to fl at-slab sub- 200 km duction, volcanism in the Altiplano-Puna pla- teau was abruptly widespread at ca. 27 Ma and may be related to delamination of the lower lithosphere (see Kay and Coira, 2009). The large crustal thicknesses have resulted magmas (de Silva, 1989a; Ort et al., 1996; Lind- herent stratigraphic framework throughout the in severe contamination of nearly all Central say et al., 2001b; Schmitt et al., 2001). Large region. Our work in the La Pacana caldera to Andean arc magmas that postdate crustal short- dacitic ignimbrite sheets are typically associated the south (Lindsay et al., 2001a), Panizos on ening (Davidson et al., 1991; Davidson and with equally impressive caldera dimensions, the eastern edge of the Altiplano-Puna volcanic de Silva, 1992; Wörner et al., 1994).