La Pacana caldera, N. Chile: a re-evaluation of the stratigraphy and volcanology of one of the world's largest resurgent calderas

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Abstract

La Pacana caldera in the Central Andes of northern Chile is one of the largest and best exposed resurgent calderas in the world. The caldera had previously been recognised as the source of the regionally-extensive Atana ignimbrite, but additional field and stratigraphic evidence, along with new K–Ar age determinations and geochemical data have led to a revision of the geology and development of this major Andean caldera. In particular, this information allows more realistic estimates of eruptive volumes and has implications for the style of ignimbrite eruption.

Two major ignimbrites appear to have originated from La Pacana caldera, based on their thickness variations, lateral distributions and stratigraphic relations: the crystal-poor, rhyolitic Toconao ignimbrite (4–5 Ma) and the crystal-rich, dacitic Atana ignimbrite (4 Ma). Following caldera collapse and formation of the resurgent Atana block, several crystal-rich dacitic–rhyolitic domes formed along the margin of the resurgent block. New K–Ar ages show that this post-caldera volcanism continued from 4 to 2 Ma, indicating that the La Pacana magmatic system was active for at least 2 Ma after the main eruption.

The Atana ignimbrite extends west, south and east of La Pacana caldera. Our work shows that the ignimbrite sequence northeast of the caldera, formerly mapped as Atana outflow, represents two new units which we name the upper and lower Tara ignimbrites. The distribution of the Tara ignimbrites points to a source to the north. The upper Tara ignimbrite comprises four flow units with interbedded surge and fall deposits and a characteristic, heterogeneous pumice population. It occurs in the La Pacana moat and onlaps the resurgent block. These field relations and a new K–Ar age of 3.8 Ma show convincingly that this ignimbrite erupted after formation of La Pacana caldera. The lower Tara ignimbrite is a single extensive flow unit, and has an age of 5.6 Ma.

Two outcrops of lag breccia occur adjacent to the caldera topographic margin and these are interpreted as vent areas for the Atana ignimbrite. This indicates that the structural caldera margin does not coincide with the resurgent block as previously thought, but is close to the topographic margin. Combining this redefinition of caldera geometry with the revised distribution of the Atana ignimbrite implies an overall trap-door-like configuration of caldera collapse, with a hinge to the north and maximum subsidence in the south. The new volume estimate of 2500 km3 for the caldera-forming Atana ignimbrite, including intracaldera and outflow facies, places it among the most voluminous ignimbrites of the world.

Introduction

The Central Andes are generally accepted as a prime example of a volcanic province associated with active continental margin tectonics. In particular, during the Late Tertiary to Recent, voluminous explosive eruptions produced one of the most extensive and well-exposed ignimbrite provinces in the world (Guest, 1969, Pichler and Zeil, 1972, de Silva, 1989a). However, while many ignimbrites in the Central Andes have been identified and mapped at different scales, few source calderas have been identified due to the large size of such structures, the lack of detailed field mapping in the logistically and physically demanding terrain, paucity of dissection, and mantling by younger stratovolcanoes. With the application of Landsat imagery in the 1970s and 1980s, a number of potential calderas were recognised (Francis and Baker, 1978, Baker, 1981, Kussmaul et al., 1977) and subsequently described (Fig. 1). These include the Cerro Galán caldera in NW Argentina (Francis et al., 1978, Sparks et al., 1985, Francis et al., 1989), the Cerro Guacha and Pastos Grandes calderas in southern Bolivia (Francis and Baker, 1978, de Silva and Francis, 1991), the Cerro Panizos caldera straddling the border to Bolivia and Argentina (Ort, 1993), the Kari Kari caldera in Bolivia (Francis et al., 1981) and La Pacana caldera in North Chile (Gardeweg and Ramı́rez, 1987). More recently, the Vilama–Coruto, Pairique and Coranzuli calderas in northwest Argentina have also been recognised and described (Coira et al., 1996, Seggiaro, 1994). Nevertheless, many more ignimbrites have been identified than have source calderas, and in only relatively few cases can it be said with conviction that a given ignimbrite originated from a particular caldera.

The calderas are typically located ≤200 km east of the modern arc, with the greatest concentration of large calderas and ignimbrites in the Central Andes identified as the Altiplano–Puna Volcanic Complex (APVC) (de Silva, 1989a). Located between ∼21°S and 24°S (Fig. 1), the APVC covers some 70,000 km2 and is one of the most voluminous concentrations (∼30,000 km3) of late Miocene to Pleistocene ignimbrites on earth. Several regionally extensive, crystal-rich dacitic ignimbrites with volumes commonly in excess of 1000 km3 dominate the stratigraphy (e.g. de Silva, 1989b, de Silva and Francis, 1989). The general family resemblance of these ‘monotonous intermediate’ ignimbrites to those in other provinces around the world (e.g. San Juan volcanic field, Colorado, USA; Great Basin, western USA; Sierra Madre Occidental, Mexico; Lachlan Fold Belt, Australia) suggest a consistency of volcanotectonic processes that remains to be fully explored. The youth and excellent preservation and exposure of the APVC make it a compelling focus for studies of these large, dacitic ignimbrites and their caldera systems.

One of the main problems associated with detailed studies of the calderas in the APVC is that ignimbrite correlation is incomplete, mainly because the large-volume ignimbrites all have similar field characteristics. The consequence of this is that one map unit may represent two or more ignimbrites, and conversely, a single ignimbrite may be represented by two or more map units (e.g. de Silva and Francis, 1989). In order to better understand magmatic and volcanic processes in the APVC as a whole, it is therefore necessary to look in detail at individual calderas, and determine which of the numerous ignimbrites in the region they are likely to have produced. Here we focus on one of the largest and best exposed calderas in this region, La Pacana Caldera, first recognised by Gardeweg and Ramı́rez (1987). This contribution presents new field and stratigraphic evidence, K–Ar age determinations, and geochemical data that allow us to revise the ignimbrite stratigraphy around, and related to, the caldera. Based on this, we redefine the caldera geometry and the distribution of its ignimbrites, provide new and improved volume estimates, and present a more comprehensive assessment of caldera development. This work forms the stratigraphic and volcanologic framework for detailed geochemical and petrologic studies of the La Pacana magmatic system which are presented in a separate paper (Lindsay et al., 2000).

Section snippets

La Pacana caldera — previous work

The general geology of the La Pacana region has been described in RamÍrez and Gardeweg, 1982, Marinoviç and Lahsen, 1984 and age determinations from these and other studies are given in Table 1. Ignimbrites associated with La Pacana caldera were first identified on Landsat images by Francis and Baker (1978), who at that time thought that these extensive outflow ignimbrites were most likely sourced from the Cerro Guacha caldera in southern Bolivia (Fig. 2). It was later recognised by Gardeweg

Redefinition of the Atana ignimbrite and recognition of new units — the Tara ignimbrites

The units in the northeastern part of the study area near the borders with Argentina and Bolivia were mapped by Gardeweg and Ramı́rez (1987) as the Atana ignimbrite. To differentiate these from the Atana ignimbrite proper, we refer to these units as the Tara ignimbrites after their type locality in the cliff sections on the northern shore of Salar de Tara (22°58.55′S, 67°17.63′W).

The key features of the Tara ignimbrites and the differences between them and the rest of the Atana outflow are

Revised geology of La Pacana caldera

Our new observations are summarised on a revised geological map of La Pacana caldera and a schematic stratigraphic correlation diagram (Fig. 11, Fig. 12, respectively).

Discussion

La Pacana caldera has been accepted as one of the largest resurgent calderas in the world based on the work by Gardeweg and Ramı́rez (1987), who estimated a minimum volume of the Atana ignimbrite as 900 km3, and considered that caldera collapse occurred over an area approximated by the resurgent block of Cordón La Pacana. Our new mapping and stratigraphic correlations have resulted in revision of the distribution of the Atana ignimbrite outflow. This, combined with new information about vent

Conclusions

La Pacana caldera in northern Chile is the largest of a group of late Miocene to Pleistocene calderas in the Central Andes which constitute the APVC (after de Silva, 1989a), one of the most voluminous concentrations of Neogene ignimbrites on earth (ca. 30,000 km3). Detailed information from individual caldera systems is needed in order to understand the petrologic and volcanologic processes behind such extreme development of felsic magmatism, and this was the motivation for the re-evaluation of

Acknowledgements

This study was carried out as part of the interdisciplinary research project SFB 267, ‘Deformation processes in the Andes’, funded by the Deutsche Forschungsgemeinschaft (DFG). We would like to thank the staff at the Universidad Católica del Norte in Antofagasta for logistical support in the field, and Moyra Gardeweg of Sernageomin, Chile, for much helpful advice. J.M.L. thanks Axel Schmitt for valuable assistance in the field, and S.deS. gratefully acknowledges the collaboration and support of

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