Bull Volcanol (2011) 73:1427–1454 DOI 10.1007/s00445-011-0459-y RESEARCH ARTICLE A re-appraisal of the stratigraphy and volcanology of the Cerro Galán volcanic system, NW Argentina Chris B. Folkes & Heather M. Wright & Raymond A. F. Cas & Shanaka L. de Silva & Chiara Lesti & Jose G. Viramonte Received: 15 February 2009 /Accepted: 31 January 2011 /Published online: 6 May 2011 # Springer-Verlag 2011 Abstract From detailed fieldwork and biotite 40Ar/39Ar the Toconquis Group (the Pitas and Vega Ignimbrites). dating correlated with paleomagnetic analyses of lithic clasts, Toconquis Group Ignimbrites (~5.60–4.51 Ma biotite ages) we present a revision of the stratigraphy, areal extent and have been discovered to the southwest and north of the volume estimates of ignimbrites in the Cerro Galán volcanic caldera, increasing their spatial extents from previous complex. We find evidence for nine distinct outflow estimates. Previously thought to be contemporaneous, we ignimbrites, including two newly identified ignimbrites in distinguish the Real Grande Ignimbrite (4.68±0.07 Ma biotite age) from the Cueva Negra Ignimbrite (3.77± Editorial responsibility: K. Cashman 0.08 Ma biotite age). The form and collapse processes of This paper constitutes part of a special issue: the Cerro Galán caldera are also reassessed. Based on re- interpretation of the margins of the caldera, we find evidence Cas RAF, Cashman K (eds) The Cerro Galan Ignimbrite and Caldera: for a fault-bounded trapdoor collapse hinged along a regional characteristics and origins of a very large volume ignimbrite and its N-S fault on the eastern side of the caldera and accommo- magma system. dated on a N-S fault on the western caldera margin. The Electronic supplementary material The online version of this article collapsed area defines a roughly isosceles trapezoid shape (doi:10.1007/s00445-011-0459-y) contains supplementary material, which is available to authorized users. elongated E-W and with maximum dimensions 27×16 km. The Cerro Galán Ignimbrite (CGI; 2.08±0.02 Ma sanidine * : : C. B. Folkes ( ) H. M. Wright R. A. F. Cas age) outflow sheet extends to 40 km in all directions from School of Geosciences, Monash University, Building 28, the inferred structural margins, with a maximum runout Victoria 3800, Australia distance of ~80 km to the north of the caldera. New deposit e-mail: [email protected] volume estimates confirm an increase in eruptive volume through time, wherein the Toconquis Group Ignimbrites H. M. Wright 3 US Geological Survey MS 910, increase in volume from the ~10 km Lower Merihuaca Menlo Park, CA 94025, USA Ignimbrite to a maximum of ~390 km3 (Dense Rock Equivalent; DRE) with the Real Grande Ignimbrite. The S. L. de Silva climactic CGI has a revised volume of ~630 km3 (DRE), Department of Geosciences, Oregon State University, Corvallis, OR 97331, USA approximately two thirds of the commonly quoted value. C. Lesti Keywords Cerro Galán . 40Ar/39Ar . Ignimbrite volumes . Universitá Roma Tre, Caldera collapse . Trapdoor Largo S. Leonardo Murialdo, 1-00146 Roma, Italy J. G. Viramonte Introduction Univesidad Nacional de Salta, Instituto GEONORTE and CONICET, Av. Bolivia 5150, The Cerro Galán caldera, located in northwestern Argentina Salta 4400, Argentina in the Central Volcanic Zone (CVZ) of the Andean 1428 Bull Volcanol (2011) 73:1427–1454 continental margin volcanic arc of South America, is often The Cerro Galán volcanic system, which lies ~200 km cited as the product of one of the largest caldera-forming south of the APVC (Altiplano-Puna Volcanic Complex; ‘super-eruptions’ on Earth (e.g., Mason et al. 2004).No Fig. 1), is a classic example of a long-lived silicic super-eruptions have been witnessed in human history and magmatic system (e.g., Mason et al. 2004). The cataclys- little is known about their frequency, eruption mechanisms mic supervolcanic eruption of the ~2.1 Ma Cerro Galán or the emplacement processes of erupted material. Previous Ignimbrite (CGI) has previously been proposed to have work has shown that at least one VEI 8 eruption occurred at been of the same order of magnitude as the eruption that Cerro Galán since 2.5 Ma (Francis et al. 1983). In addition, produced the Fish Canyon Tuff (e.g., Sparks et al. 1985), a sequence of large volume pyroclastic eruptions preceded currently thought to have been the largest explosive this cataclysmic eruption (Sparks et al. 1985). Therefore, eruptiononEarth(Masonetal.2004). Furthermore, the the time transgressive sequence of pyroclastic deposits from total volume of pre-2.1 Ma pyroclastic eruption was Cerro Galán provides information about the long term thought to total half of the CGI volume (Sparks et al. evolution of large-volume magma chambers. 1985). No modern, systematic and detailed investigation The CVZ has been the site of numerous large-volume into the stratigraphic and age relationships or eruption silicic ignimbrite eruptions since ~10 Ma (Guest 1969; mechanisms of the eruption sequence has, however, been Sparks et al. 1985; Gardeweg and Ramirez 1987; Ort 1993; completed despite significantly better access since the first Coira et al. 1996; Kay et al. 1999; Soler et al. 2007). The seminal work on the system by Francis, Sparks and co- highest concentration of these large-volume ignimbrites in workers in the late 1970’s and early 1980’s. We have the CVZ is in the Altiplano-Puna Volcanic Complex revisited this classic system to further investigate the (APVC; Fig. 1), covering ~70,000 km2 and comprising eruption mechanisms and emplacement of large volume >10,000 km3 of crystal-rich dacitic ignimbrites emplaced ignimbrites and the development of long-lived, large during late Miocene to Pleistocene time (de Silva and volume silicic systems. In this paper we focus on the Francis 1989; de Silva 1989a, b; Lindsay et al. 2001). In the stratigraphy of the ignimbrite succession associated with APVC, magma supply rates are high, and large chamber the Cerro Galán volcanic system. Using new field volumes are inferred to have developed over hundreds of mapping and stratigraphic evidence, coupled with thousands of years (de Silva and Gosnold 2007). 40Ar/39Ar age determinations and paleomagnetic evidence, Fig. 1 Regional setting of the o o Cerro Galán caldera and its 70 W Salar de 66 W associated ignimbrites. The N Uyuni Altiplano-Puna Volcanic Complex (APVC), regional BOLIVIA strike-slip lineaments and the location of Fig. 2 are also shown o Calama-Olacopato- 22 S El Toro lineament Pacific Ocean APVC Antofagasta 24oS ARGENTINA CHILE Salar del Hombre Muerto Salta Salar de Antofalla Fig. 2 26oS Antofagasta de La Sierra Archibarca lineament Approximate extent of the Cerro Galan Ignimbrite Caldera Town 28oS 0 100 200 km Major stratovolcano Major salar Bull Volcanol (2011) 73:1427–1454 1429 we present an updated stratigraphy, a more complete set of characterized the ignimbrite as a crystal-rich but pumice and ignimbrite distribution maps and more-accurate estimations of lithic-poor pyroclastic flow deposit comprising one massive distributions and volumes for each ignimbrite. Using new flow unit, up to 120 m thick. A more detailed investigation of 40Ar/39Ar age determinations, we have identified new the Galán stratigraphy was performed by Sparks et al. (1985), ignimbrites (the Pitas and Vega Ignimbrites) and separated who described an older sequence of ignimbrites. Rb-Sr age others that were previously thought to have erupted determinations on the CGI indicated an eruption age of concurrently (e.g., the Real Grande and Cueva Negra 2.1 Ma (Table 1). Outflow deposits 20–300 m thick were Ignimbrites). A reinterpretation of the caldera collapse and estimated to reach up to 100 km from the caldera rim. Six structure is presented, and we quantify and improve the ignimbrites were documented underlying the CGI, with a assumptions that are involved in calculating realistic volume total thickness in excess of 250 m. Collectively, these pre- estimates for the original erupted ignimbrites. In aggregate, CGI ignimbrites were designated as the Toconquis Forma- these data elucidate the temporal evolution of the Cerro tion. They include the Blanco, Lower Merihuaca, Middle Galán magmatic system, allowing it to be put in context with Merihuaca, Upper Merihuaca, Real Grande and Cueva other large silicic magmatic systems. Negra Ignimbrites, from oldest to youngest. The Toconquis Formation Ignimbrites were dated from ~6.4 to 4.8 Ma (using combined K-Ar and Rb-Sr age dates; Table 1). This Cerro Galán Caldera—previous work sequence of eruptions culminated with the largest volume Real Grande Ignimbrite on the western flanks and the Cueva The Cerro Galán caldera was first identified in the late Negra Ignimbrite on the eastern flanks, which were 1970’s using satellite images from the Central Andes interpreted as correlative deposits of the same eruptive (Friedman and Heiken 1977; Francis and Baker 1978). episode. The caldera (25°55′S 66°52′W) is located in the Puna Initial volume estimates of these deposits were based on region of the southern CVZ of the South American Andes several assumptions. Sparks et al. (1985) extrapolated the (Fig. 1), approximately 200 km south of the southern thickness (250 m) of ignimbrites belonging to the Tocon- margin of the Altiplano-Puna Volcanic complex (APVC). quis Formation to uncharted areas north of the caldera. A The caldera complex is situated at the SE end of the combined volume of 400-500 km3 for the whole formation Archibarca Lineament—a NW-SE striking regional strike- was proposed based on mapped exposure, deposit thickness slip fault that is part of a sub-parallel fault series thought to and the assumption that significant deposition occurred in have been formed via transtensional activity related to the area now occupied by the modern caldera. This estimate overthickening of the local lithosphere in this part of the was considered a minimum value, based on the inference Puna plateau (Viramonte et al.