Influence of the substrate on maar–diatreme volcanoes — An example of a mixed setting from the Pali Aike volcanic field, Argentina

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Abstract

The morphologic parameters, pyroclastic deposits and evolution of maar–diatreme volcanoes are affected by the type of environment in which they are emplaced. End-member cases are a hard substrate (rocks) and a soft substrate (unconsolidated volcaniclastic or sedimentary deposits). In this paper, we present an example of a volcanic complex emplaced in a mixed hard–soft setting from the Pali Aike volcanic field (PAVF) near the Argentina–Chile border. The Plio-Pleistocene PAVF is an alkaline, mafic, back-arc monogenetic field which contains over 100 phreatomagmatic volcanoes. The studied volcanic complex contains two large coalescent maars overlain by scoria and spatter. The 1.4 × 1.3 km East Maar has better exposures than the shallower, 1.9 km-wide West Maar and seems to have been less modified by post-eruptive processes. The tephra rim of the East Maar was studied in detail and we infer it was produced mostly by base surges from phreatomagmatic eruption columns, with rare instances of intercalated scoria fall layers. Based on regional information, the general pre-maar stratigraphy is dominated by sedimentary and volcaniclastic rocks of the Magallanes Basin, including a thick poorly consolidated upper unit dating from the Miocene. These are overlain by Plio-Pleistocene fluvio-glacial deposits and PAVF lavas, some of which are exposed in the East Maar just below the phreatomagmatic deposits. All of these units are represented as lithic clasts in the tephra rim of the East Maar, the most abundant being the clasts from the earlier basaltic lavas and rock fragments derived from the glacial deposits. There is no specific evidence for a deep diatreme under the East Maar, and in this particular case, the mixed environment seems to have produced a maar–diatreme volcano typical of a soft substrate.

Research Highlights

► maar-diatreme volcanoes are affected by environment of emplacement: hard or soft. ► we present a two-maar complex from a mixed hard-soft setting in Argentina-Chile. ► tephra rim produced mostly by base surges from phreatomagmatic eruption. ► mixed environment seems to have produced volcano typical of a soft substrate here.

Introduction

Because monogenetic volcanoes are created by comparatively low-volume, low intensity eruptions, the environment in which they occur is an important control on the style of activity – explosive or effusive, magmatic or phreatomagmatic – and consequently on the type of volcanic edifice formed. Maar–diatreme volcanoes are formed on land when rising magma, of any composition and gas content, meets groundwater (Lorenz, 1975, Lorenz, 1986, Fisher and Schmincke, 1984). Often the groundwater supply becomes exhausted during the course of a maar-forming eruption and the activity changes to strombolian (scoria cone-forming) or to the generation of lava flows or lava lakes, depending on the properties of the magma. This pattern of phreatomagmatic activity switching more or less abruptly to magmatic activity (“drying out”) in maar volcanoes is typical all over the world. Examples include the East Eifel volcanic field of Germany (Houghton and Schmincke, 1986); the West Eifel volcanic field (Lorenz and Zimanowski, 2008, and references therein); several sites in Mexico (e.g., Aranda-Gómez et al., 1992, Ort and Carrasco-Núñez, 2009); and the Auckland volcanic field in New Zealand (Houghton et al., 1999).

Maar tephra rims always contain, apart from juvenile fragments, abundant material derived from the pre-maar rocks or unconsolidated deposits. This abundant dilution of juvenile particles by lithic clasts arises because of the excavation of the maar crater and underlying diatreme by the phreatomagmatic explosions. Lithic clasts in maar tephra rims contain information on the lithology of the substrate, the depth of the explosions and, of particular interest here, on the state of lithification of the substrate.

The two end-member types of substrate for maar–diatreme volcanism are “soft” unconsolidated deposits (sediments or volcaniclastic deposits) and “hard” rocks (Lorenz, 2003). Mixed settings, in which both rocks and unconsolidated deposits occur, are also common. The type of substrate controls the type of aquifer in which the external water sits (fracture-controlled vs. porous aquifers); the overall shape of the volcano including the diatreme; the post-eruptive lacustrine architecture in the maar crater; and some characteristics of the pyroclastic deposits of the tephra rim and diatreme (Lorenz, 2003, Auer et al., 2007, Martín-Serrano et al., 2009).

Emplacement of maars in hard substrates is relatively well known, for example in the West Eifel volcanic field (e.g., Lorenz, 1987, Büchel, 1993, Lorenz and Zimanowski, 2008), but maars emplaced in soft substrates, or mixed substrates – where hydrologic conditions may be more complex – are less well documented (although see for example White, 1991, Sohn, 1996, Sohn and Park, 2005). Specifically, there exists a shortage of contributions presenting single maars or maar complexes and their soft or mixed substrates in detail in the literature. In this paper, we describe a two-maar complex emplaced in a mixed setting from the Pali Aike volcanic field (PAVF), near the Argentina–Chile border (Fig. 1, Fig. 2). We also summarize what is currently known about maars emplaced in different types of substrates. Better understanding the influence of the external environment on eruptive processes and deposits of basaltic monogenetic volcanoes has the added benefit of potentially shedding light on kimberlite emplacement processes (e.g., Kjarsgaard, 2007; McClintock et al., 2009).

Section snippets

Tectonic setting

The Plio-Pleistocene PAVF covers ~ 4500 km2 in southern Argentina and Chile (Corbella, 2002; Fig. 1). It represents the southernmost occurrence of the Cenozoic Patagonian Plateau Lavas (e.g., Tyrrell, 1932, Corbella and Lara, 2008). Relative to the modern volcanic arc in the Andes, the Patagonian Plateau Lavas occur in a back-arc position. The PAVF eruptive products are alkaline and typically olivine-bearing, ranging from basalts to basanites, and have trace element signatures similar to those of

Location and interest

The volcanic complex described here (and briefly by Haller et al., 2006) is located in the SW portion of the PAVF at around 70.155°W, 51.995°S (Fig. 2). It is possible to drive there with a 4 × 4 vehicle using a network of private unpaved roads originating from Ruta Nacional 40, about 24 km to the NW (Fig. 3a). The complex contains some of the best maar tephra rim outcrops we have seen on the Argentinean side of the volcanic field in this area. The East Maar, in particular, represents a prime

General characteristics

The eastern maar crater has a near elliptical shape, with dimensions 1.40 km E–W by 1.28 km N–S, and a depth of 89 m (to the lake level) along profile A–A′ (Fig. 3, Fig. 4). The differential GPS survey made to establish profile A–A′ was performed approximately N–S, to avoid the interference area between the two maars which creates a high point in the NW corner of the East Maar. The crater bottom is occupied by a saline lake known informally as Laguna del Ruido.

Excluding the interference area, the

The West Maar

Dimensions for the topographic rim of the West Maar are 1.87 km N–S by 1.90 km E–W. The rim, on which phreatomagmatic deposits are locally exposed, surrounds a broad and shallow basin (Fig. 10a), rather than a typical deep maar crater. This likely reflects modification of the original crater(s) by secondary processes. The broad and shallow aspect of the basin, with the irregular contours, could also be due in part to an assemblage of coalesced craters. This interpretation is supported by the

Geochemistry

We determined the geochemical composition of twenty samples from the PAVF, including eleven from the studied volcanic complex (Fig. 11, Fig. 12; see caption of Fig. 11 for analytical methods). Weathered crusts were removed and almost all selected samples were free of visible secondary minerals (in a few cases, < 1% secondary minerals were present in vesicles). Chosen samples included five lava flows, three bombs from magmatic explosive eruptions, and twelve single cm-sized basaltic fragments

Discussion: maars in hard and soft substrates

Before discussing maars emplaced in mixed substrates such as the maar complex studied in the PAVF, it will be useful to review some background information on maars emplaced in a hard vs. soft substrates, as end-member cases (Fig. 13).

Interpretation of the studied PAVF maars

Having reviewed the end-member cases of maars emplaced in hard vs. soft substrates, we can now look at maars found in mixed settings, including the studied PAVF maar complex. The overall eruptive history of this volcanic complex is relatively straightforward: first, explosive eruptions dominated by phreatomagmatic activity, probably from two dyke segments, produced two juxtaposed maars; then, explosive magmatic eruptions produced scoria and spatter at a number of locations along the same E–W

Conclusions

The morphologic parameters, pyroclastic deposits and evolution of monogenetic volcanoes are controlled by the magma delivery system but strongly influenced by the external environment in which the magma erupts, including the nature of the substrate. In this paper, we have focused our attention on the influence of the substrate (“hard”, mixed, or “soft”) on maar–diatreme volcanoes. We have described a Quaternary two-maar complex in the Pali Aike volcanic field where explosive magma–water

Acknowledgements

We thank Bernd Zolitschka for discussions and support during field work in Argentina. Pierre Kliem and Frank Mueller helped with the differential GPS survey shown in Fig. 4. This research is partly supported by the International Continental Scientific Drilling Program (ICDP) in the framework of the Potrok Aike Maar Lake Sediment Archive Drilling Project (PASADO). Funding for PASADO is provided by the ICDP, the German Science Foundation, the Swiss National Funds, the Natural Sciences and

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