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Polycyclic scoria cones of the Antofagasta de la Sierra basin, Southern Puna plateau, Argentina

WALTER BA ´ EZ 1*, GERARDO CARRASCO NUN ˜ EZ 2, GUIDO GIORDANO 3,4 , JOSE´ G. VIRAMONTE 1 & AGOSTINA CHIODI 1 1Unidad de recursos geolo´gicos y geote´rmicos INENCO (UNSa – CONICET), Av. Bolivia 5150, A4400FVY Salta, Argentina 2Centro de Geociencias, UNAM Campus Juriquilla, Blvr. Juriquilla 3001 Quere´taro, Qro. 72300 Mexico 3Department of Sciences, University Roma Tre, 00146 Rome, Italy 4CNR-IDPA Institute for Dynamics of Environmental Processes, Via M. Bianco, 20131 Milan, Italy *Correspondence: [email protected]

Abstract: Despite a number of published papers focusing on the geodynamic implications of the recent Southern Puna mafic magmatism, there have been fewer studies of the volcanology and stratigraphy of this outstanding volcanism. This paper presents a detailed map of two well- preserved Quaternary scoria cones showing their complex stratigraphy. Complementary morpho- metric, morpho-structural, petrographic and geochemical data were used to reconstruct the evolu- tion of both volcanoes. The occurrence of more than one eruption at each volcano was inferred by the recognition of temporal hiatuses using morpho-stratigraphic criteria. The polycyclic nature of both scoria cones could be related to a combination of a high input magma in response to litho- spheric delamination, a favourable regional stress field and the interaction of rising magma with pre-existing faults. The youngest eruptions in both volcanoes were complex, with shifts in the erup- tive style from violent strombolian to hawaiian /strombolian phases, and probably lasted for a few years. The explosive activity was accompanied by the emission of lava flows from lateral vents. Phreatomagmatic activity was triggered during the waning stages of the eruptions. The occurrence of more than one eruption in a single scoria cone and the changes in the eruptive style during long- lasting eruptions are important topics for volcanic hazard assessment in the Southern Puna.

Scoria cones are small monogenetic volcanoes extrusion rates and long-lived volcanic activity produced during eruptions with a broad spectrum (kyr to myr) consisting of periods of quiescence of eruptive styles, including strombolian, violent and erosion (Walker 2000). However, some studies strombolian, hawaiian and phreatomagmatic. Mono- carried out in different geological settings suggest genetic scoria cones are characterized by eruptions that many scoria cones and maars may actually lasting from days and months to several years, involve multiple eruptions separated by tens to involving discrete, small-volume magma batches hundreds of thousands of years with geochemical and associated with dispersed plumbing networks variations (e.g. Turrin & Renne 1987; Renault et al. (Vespermann & Schmincke 2000). Scoria cones 1988; Wells et al. 1990; Bradshaw & Smith 1994; typically occur as fields of small volcanoes together McKnight & Williams 1997; De Benedetti et al. with other monogenetic volcanic structures, such 2008; Sheth et al. 2009; Kereszturi et al. 2010; as maars, tuff rings and lava domes (Connor & Needham et al. 2011; Sheth 2014). In an attempt Conway 2000; Ne´meth 2010) and are usually con- to clarify this paradoxical situation, Ne´meth & trolled by the regional tectonic setting (e.g. Lesti Kereszturi (2015) proposed a new definition for a et al. 2008; Pardo et al. 2009). In contrast, poly- monogenetic volcano: a volcanic edifice with a genetic volcanoes are characterized by more com- small cumulative volume (typically ≤1 km 3) that plex plumbing systems, including the development has been built up by one continuous, or many dis- of relatively large magma chambers in the upper continuous, small eruptions occurring over a short crust. Thus polygenetic volcanoes show complex timescale (typically ≤10 years) and fed from one patterns of compositional variations, higher magma or multiple magma batches through a relatively

From: N e´meth, K., C arrasco-N u´n˜ez , G., A randa-G o´mez, J. J. & S mith, I. E. M. (eds) Monogenetic Volcanism. Geological Society, London, Special Publications, 446, http: // doi.org/10.1144/SP446.3 # 2016 The Author(s). Published by The Geological Society of London. All rights reserved. For permissions: http:// www.geolsoc.org.uk/permissions. Publishing disclaimer: www.geolsoc.org.uk /pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

W. BA ´ EZ ET AL. simple, closely spaced feeder dyke (and sill) Geological setting system with no associated well-developed magma chamber. In addition, Ne´meth & Kereszturi (2015) The Miocene–Quaternary volcanic activity in the indicated the existence of departures from the Altiplano-Puna plateau originated the Andean monogenetic volcanoes sensu stricto that create a CVZ (ACVZ) (Fig. 1). A north–south volcanic arc nearly continuous spectrum to the polygenetic has developed since Neogene times, initially along volcanoes sensu stricto. One type of ‘transitional’ the Maricunga belts and finally stabilized 50 km to volcano is the polycyclic monogenetic volcano the west in the present day volcanic arc (Western (Kereszturi et al. 2010; Ne´meth & Kereszturi Cordillera) (Kay & Coira 2009; Guzma´n et al. 2015). Such volcanoes are formed by multiple and 2014). The eastwards broadening of arc magmatism complex eruptive episodes with small volumes of along regional NW–SE vertical strike-slip fault magma and separated by time gaps from thousands systems can be explained by shallowing of the to millions of years. Thus the resultant volcanic subducting slab (Viramonte et al. 1984; Viramonte structures show a complex facies architecture, but & Petrinovic 1990; Petrinovic et al. 1999; Riller morphologically resemble the small-volume volca- et al. 2001; Trumbull et al. 2006). The orogen- noes traditionally defined as monogenetic (Ne´meth parallel thrust faults also played an important part & Kereszturi 2015). in focusing the arrest of hydrofractures, the forma- The Southern Puna plateau of Argentina differs tion of magma chambers and the emplacement of from the rest of the Andean Central Volcanic polygenetic volcanoes in the back-arc region (Nor- Zone (CVZ) as a result of the abundance of late ini et al. 2013). Major changes in the magmatic Miocene and younger small mafic (basaltic andes- and deformational style in the Southern Puna back- ite) back-arc volcanic centres (Viramonte et al. arc region began at 7 Ma (Marrett et al. 1994; Kay & 1984; Kay et al. 1994; Risse et al. 2008; Viramonte Coira 2009; Montero Lo´pez et al. 2010; Guzma´n et al. 2010). The presence of this mafic volcanism in et al. 2014). A shift from a purely contractional to the Southern Puna plateau is one of the main a mixed stress regime with normal, strike-slip and arguments supporting late Cenozoic lithospheric thrust faults, and the eruption of both mafic (basaltic delamination beneath this region (Kay et al. 1994; andesite) lavas and the dacitic ignimbrites from Whitman et al. 1996). Despite the publication of Cerro Gala´n Caldera are considered as superficial several papers focused on both the petrogenesis evidence supporting the delamination model pro- and geodynamic implications of this outstanding posed by Kay et al. (1994) for this portion of the magmatism (Knox et al. 1989; Kay et al. 1994; Central Andes. There are currently two different Risse et al. 2008, 2013; Drew et al. 2009; Ducea models that are used to explain the lithospheric et al. 2013; Murray et al. 2015), the stratigraphy delamination below the Southern Puna using the of each volcanic centre and its physical volcanology geochemical features of mafic lavas. The first remains poorly studied. The main aim of this paper model suggests that the mafic magmas were gener- is to contribute to a better volcanological knowledge ated from a relatively homogeneous asthenospheric of the mafic volcanism in the back-arc region of mantle peridotite that upwelled adiabatically and the Southern Puna plateau by performing a detailed melted in the wake of a large foundering block of study of the stratigraphy and geological evolution lower lithosphere (Kay et al. 1994; Risse et al. of two Quaternary scoria cones located in the 2013). The second model proposes that it is not nec- Antofagasta de la Sierra basin: the Alumbrera and essary to catastrophically founder large volumes of De La Laguna (also called Antofagasta) volcanoes. lithosphere to generate the Puna mafic magmas, The stratigraphic criterion in the field survey and but that the small-scale dripping and melting of het- geological mapping was the identification of litho- erogeneous dense lower lithospheric blocks (pyrox- stratigraphic units, including their distribution and enites) are the source of these magmas (Drew et al. stratigraphic relationships. Although no absolute 2009; Ducea et al. 2013; Murray et al. 2015). age is available, some hiatuses in the stratigraphic The Southern Puna back-arc mafic volcanism succession were recognized by significant variations consists of numerous scoria cones, lava domes, in the degree of erosion of each mapped unit. lava flows, tuff rings and maars grouped in small Complementary morphometric, morpho-structural, volcanic fields, one of them located in the Antofa- petrographic and geochemical data were used to gasta de la Sierra Basin (ASB) (Fig. 1). The ASB is reconstruct the evolution of the two volcanoes. a north–south-trending, thrust fault-bounded, intra- The polycyclic nature of both volcanoes and the fac- montane depression. This tectonic depression is tors controlling the variations in the eruptive style closed to the north by the Neogene volcanic chain during a single eruption are discussed and compared Archibarca-Gala´n (Viramonte et al. 1984; Kay & with other mafic volcanic centres of the Southern Coira 2009) and has a floor that dips gently south- Puna plateau and similar well-known historical wards. The sedimentary basin infill is represented eruptions worldwide. by Neogene–Quaternary continental sequences Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

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Fig. 1. Geological map of the Antofagasta de la Sierra Basin. Inset box shows location within the Central Volcanic Zone of the Andean cordillera. and Miocene ignimbrites from the Cerro Gala´n degradation and relative age (Wood 1980a, b; Hase- Caldera (Folkes et al. 2011 and references cited naka & Carmichael 1985; Martin & Ne´meth 2006; therein). The onset of mafic volcanism within the Inbar et al . 2011) and the geometry of the magma- ASB occurred at c. 7 Ma (Risse et al. 2008) and feeding fractures (e.g. Tibaldi 1995; Corazzato & has probably been active until Holocene times (De Tibaldi 2006; Lesti et al. 2008). A morphometric Silva & Francis 1991). The youngest scoria cones analysis was performed in a geographical infor- in the region are located in the middle of the ASB: mation system environment by integrating data the Alumbrera and De La Laguna volcanoes. There extracted from high-resolution optical images is only one radiometric age available for these vol- from Google Earth (cone area, cone major diameter, canoes, determined in a lava flow associated with crater major diameter, maximum crater elongation the De La Laguna Volcano (0.34 + 0.06 Ma, azimuth, maximum cone elongation azimuth, cone groundmass 40 Ar /39 Ar age; Risse et al. 2008) and and lava flow fields areas), direct field measure- there is no report of an historical eruption. ments (cone height, crater depth, lava flow thick- ness), trigonometric equations (cone slope) and simple geometrical equations (cone and lava flow Interplay of morphometry and volumes). In addition, the degree of crater ellipticity volcanotectonics and degree of cone ellipticity were calculated using the spatial statistics tools of ArcGIS 9.3 software. Morphological and morphometric studies of Morphometric parameters and the estimated vol- scoria cones provide insights into cone growth, umes of erupted products of the Alumbrera and De Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

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La Laguna volcanoes are shown in Table 1. The vol- of the Alumbrera Volcano, the morphometric fea- ume of the tephra blanket was not estimated and a tures suggest a NW–SE-trending magma-feeding DRE correction was not performed. Thus the vol- system. The lava flows associated with the De La ume estimate for each volcano is a minimum bulk Laguna cone occupy an area of 6.8 km 2 and have volume. A more reliable estimation of the magma a bulk volume of 0.12 km 3. volumes issued from both volcanoes requires the Manual and semi-automatic regional morpho- use of more accurate methods, such as DSM / structural analyses were performed (using ArcGIS DTM/DEM-based volumetric estimates that 9.3 and PCI Geomatics software, respectively) on include pre-eruptive surface constraint and DRE shadowed images with different lighting directions correction (e.g. Kereszturi et al. 2013). (hillshades extracted from SRTM DEMs) to recog- The Alumbrera cone rises 164 m above the nize the major tectonic features in the study area floor of the ASB and has an elliptical morphology and their relationship with the magma-feeding frac- in plan view (degree of cone ellipticity 1.4), with tures of the Alumbrera and De La Laguna volca- the maximum elongation oriented N1468 and a max- noes. As shown in Figure 3a, b and c, the tectonic imum cone diameter of 1.3 km (Fig. 2a, c). The lineaments in the ASB are dominantly oriented Alumbrera cone has a volume of 0.12 km 3 and an NE–SW , different from the inferred NW–SE- asymmetrical profile, which is possibly associated trending magma-feeding systems of the Alumbrera with the direction of the prevailing winds during and De La Laguna volcanoes. The longest and the eruption (Fig. 2c). The crater has an elliptical most well-defined lineaments in the study area are morphology (degree of crater ellipticity 1.2) with a related to the north–south principal thrust, whereas maximum elongation oriented N1438 (Fig. 2a) and lineaments parallel to the Archibarca-Gala´n Volca- is 50 m deep. The crater border exhibits an irregular nic Chain are shorter and more scarce. topography with an average altitude of c. 3575 m a.s.l. and the highest elevations in the SE sector (3603 m a.s.l.) (Fig. 2a). In addition to the principal Morpho-stratigraphy crater, three NW–SE-trending eruptive fissures were mapped (Fig. 2a, e). The orientation of the The exceptional exposure and lack of vegetation in eruptive fissures plus the directions of the maximum the high-altitude hyper-arid Puna region allowed crater and cone elongation axes suggest that the precise field mapping at the 1:10 000 scale of the magma-feeding fractures are oriented NW–SE. At Alumbrera and De La Laguna scoria cones. The the summit of the volcano there is a series of sub- resulting geological map (Fig. 4) covers an area of vertical inwards- and outwards-dipping arched c. 50 km 2 and displays the spatial distribution of faults related to the gravitational instability of the all the lithostratigraphic units described and defined cone. A collapse scarp affecting only the external in the field. Temporal hiatuses into the sequence are layers of the cone was identified on the NW flank identified by significant variations in the degree of of the volcano (Fig. 2f). This morphology is not a wind erosion (Figs 5–7) using a relative scale partial rebuild of a previous horseshoe-shaped col- based on direct field observations, the validity of lapse (e.g. Ne´meth et al . 2011). The internal surface which for relative dating is discussed later in this exposed by the collapse shows a high degree of paper. This qualitative scale includes three levels hydrothermal alteration. The lava flows associated of erosion (high, moderate and incipient) with with the Alumbrera scoria cone cover an area of respect to ventifact development. In this paper, the 41.3 km 2, have a bulk volume of 0.29 km 3 and term ventifact is used according to the definition expand southwards along the regional slope. proposed by Knight (2008), meaning clasts or bed- The De La Laguna cone rises 153 m above the rock surfaces that have been abraded by the action floor of the ASB and has an elliptical morphology of wind-blown particles. (degree of cone ellipticity 1.3), with the maximum The highly eroded units have a homogeneous, elongation oriented N1318 and a maximum cone light grey, smooth polished surface with well- diameter of 1.1 km (Fig. 2b). The De La Laguna developed wind-parallel grooves (Figs 5a, b, 6c, f cone has a volume of 0.12 km 3 and an asymmetrical & 7b) regardless of their original surface morphol- profile, which, in a similar manner to the Alumbrera ogy (e.g. toothpaste and aa lava flows, or scoria Volcano, can be related to wind dispersion during and bomb deposits). Some bombs present several eruption (Fig. 2d). The crater is rather circular opposing facets. (degree of crater ellipticity 1.1) and is 23.5 m deep. The moderately eroded units have a dark grey The average altitude of the crater is c. 3533 m a.s.l., colour and preserve much of the original surface with the highest elevations in the SE sector (3544 m morphology. However, on close inspection, surface a.s.l.) (Fig. 2b). In addition to the principal crater, polishing with a light-reflecting shine is evidenced, one NW–SE-trending fissure was identified on the particularly in the less vesiculated sector of each NW flank of the volcano (Fig. 2b). As in the case unit (Figs 6a, b & 7a). In contrast with the highly Downloaded from from Downloaded http://sp.lyellcollection.org/

Table 1. Morphometric parameters of the Alumbrera and De La Laguna volcanoes CONES SCORIA POLYCYCLIC

Cone Lava flows

Volcano Aco Sco Wbco Hco Wbcr Hcr DCrE DCoE MCrEA MCoEA V A V (km 2) (8) (km) (m) (km) (m) (8) (8) (km 3) (km 2) (km 3)

Alumbrera 0.9788117681 32 1.31 164 0.41 49.5 1.2 1.4 143 146 0.1256072228 41.35 0.2894188654 de La 0.7966363322 29 1.07 153 0.33 23.5 1.1 1.3 n.d 131 0.0774486561 6.80 0.1087264688 27,2016 onSeptember byguest Laguna

A, area; Aco, cone area; DCoE, degree of cone ellipticity; DCrE, degree of crater ellipticity; Hco, cone height; Hcr, crater depth; MCrEA, maximum crater elongation azimuth; MCoEA, maximum cone elongation azimuth; Sco, cone slope; V, volume; Wbco, cone major diameter; Wbcr, crater major diameter. Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

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Fig. 2. Photographs showing principal morphological and morphometric features of the ( a) Alumbrera and ( b) De La Laguna volcanoes. Ellipses, calculated theoretical ellipticity of the cones and craters; solid fine lines, direction of maximum elongation of the cones and craters; solid thick lines, eruptive fissure and inferred eruptive fissure; dotted line, collapse scarp. (c, d ) SE–NW profiles of both volcanoes showing their asymmetry. ( e) NW flank collapse of the Alumbrera Volcano. ( f) Dyke intruding along a SW-trending eruptive fissure.

eroded units, no wind-parallel groove or facet is small spines on the top of the toothpaste lava present. flows (Fig. 5d). An incipient surface polishing is The incipiently to uneroded units have a black present only in sectors highly exposed to wind action colour and preserve all the original surface morphol- and characterized by a non-vesicular lithology (e.g. ogy (Fig. 5c), including delicate features such as lava flow edges and bombs along the crater border). Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

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Fig. 3. Morpho-structural analysis of the study area: ( a) manual analysis; ( b) semi-automatic analysis; and ( c) rose diagrams showing the main directions of the major tectonic features in the study area and their relationship with the inferred magma-feeding system of the Alumbrera and De La Laguna volcanoes.

Alumbrera morpho-stratigraphy the degree of erosion in the PAV units suggest tem- porary hiatuses between them, but the qualitative Following these geomorphological criteria, the criteria used in this paper cannot definitely resolve stratigraphic sequence of the Alumbrera Volcano the length of the temporal gaps in the sequence. has been subdivided in older and younger units. The youngest units that are incipient to uneroded The oldest units, which present high to moderate are grouped into the Neo-Alumbrera Volcano levels of wind erosion, are grouped into the Palaeo (NAV) and were probably formed without signifi- Alumbrera Volcano (PAV) units. Differences in cant temporal gaps between them. Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

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Fig. 4. Geological map and stratigraphy of the Alumbrera and De La Laguna volcanoes.

De La Laguna morpho-stratigraphy La Laguna Volcano (NDLV) grouping the youn- gest, moderately eroded units. As in the PAV, dif- In the same way, the stratigraphic sequence of the ferences in the degree of erosion in the PDLV De La Laguna Volcano has been divided into the units could be related to temporal hiatuses that Palaeo De La Laguna Volcano (PDLV) grouping are not possible to define with the methodology the oldest, highly eroded units and the Neo De applied here. Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

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Fig. 5. Photographs showing similarities in morphology and texture (toothpaste lava flow) for the Alumbrera Volcano units. (a, b ) AL1 unit showing higher degree of ventifact formation compared with ( c, d ) AL11 unit.

Lithostratigraphy and lithofacies porphyritic texture with olivine (c. 15%) and plagio- architecture clase (c. 15%) phenocrysts up to 0.5–1 mm in size in a hyalopilitic groundmass of olivine ( c. 10%) We present here a systematic description of all the and plagioclase microlites (c. 30%), opaque oxides lithostratigraphic units mapped during fieldwork. (c. 2%) and glass (c. 25%). A brief geochemical and petrographic characteriza- tion of each unit is also given (Fig. 8a–f). The vol- AL2 unit. The AL1 unit is covered by a series of canic products were classified following the criteria moderately eroded aa lava flows, which form the used by Murray et al. (2015) for the geochemical AL2 unit. These aa lavas were extruded from the classification of the Southern Puna mafic lavas NE eruptive fissure and are restricted to the eastern (Fig. 8a, b, Table 2). The percentages of phenocrysts sector of the study area (Fig. 6). These aa lava flows and matrix components are visual estimates aided are reddish brown in colour, 2–6 m thick and by abundance charts. preserve typical rough surfaces and morphologies such as channels limited by levees and pressure Palaeo Alumbrera Volcano ridges. The AL2 lavas are subalkaline basaltic tra- chyandesites with Mg# . 60 and MgO , 8 wt% AL1 unit. The stratigraphic sequence of the Alum- (primitive, Fig. 8a, b). Microscopically, they have brera Volcano starts with a series of highly eroded a porphyritic texture with olivine ( c. 25%) and cli- compound toothpaste lava flows outcropping north- nopyroxene (c. 5%) phenocrysts up to 1–2.5 mm eastwards and southeastwards from the cone. These in size in an intergranular groundmass of clinopyr- lava flows are grey in colour and are 2–3 m thick oxene (c. 15%), olivine (c. 10%), plagioclase micro- (Fig. 5a, b). The AL1 lava flows are alkaline basaltic lites (c. 35%) and opaque oxides (c. 10%). The trachyandesites with Mg# , 60 and MgO , 7 wt% groundmass commonly shows fluidal structures (evolved, Fig. 8a, b). In thin section they have a around the phenocrysts. Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

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Fig. 6. Photographs showing (a, b ) moderately eroded lava flows and ( c–f ) highly eroded lava flows in the De La Laguna Volcano. Note the development of wind-parallel grooves in the highly eroded lava flows (DLL1 and DLL3).

AL3 unit. At the margin of the NE eruptive fissures, compound lava flow field erupted from the NE erup- a poorly sorted, moderately eroded, non-welded tive fissure (Fig. 4) and shows incipient wind ero- fluidal bomb and scoria lapilli deposit crops out sion features. The AL4 lavas are black in colour, (AL3 unit, Fig. 4). The pyroclasts of the AL3 unit have thicknesses ranging from 3 to 20 m and are have similar petrographic features to the AL2 unit. formed of primary lava lobes and innumerable secondary toothpaste lava tongues. Many of them Neo-Alumbrera Volcano consist of ‘slab pahoehoe’ derived from mobiliza- tion and break-up of toothpaste lava tongues (Peter- AL4 unit. The NAV stratigraphic sequence starts son & Tilling 1980; Rowland & Walker 1987). with the AL4 unit, which constitutes a toothpaste The AL4 lava flow field is partially covered by a Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

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Fig. 7. Photographs showing (a) moderately eroded bomb and ( b) highly eroded bomb in the De La Laguna Volcano. Note the development of wind-parallel grooves in the highly eroded bomb (DLL5). fallout blanket (AL6 unit) and by dilute pyroclastic 0.5 mm in size in a highly vesiculated ( c. 80%) density current deposits (AL10 unit). The AL4 hyalopilitic groundmass of small ( ,0.03 mm) oliv- lava flows have similar petrographic features to ine (c. 1%) and plagioclase microlites (c. 4%), and the AL1 unit. glass (c. 10%) (Fig. 8f).

AL5 unit. This unit represents the external portion AL7 unit. A deposit with a hummocky morphology of the scoria cone and is formed by beds, typically and a thickness of 10–15 m was mapped at the NW 0.1–0.5 m thick, composed of non-welded rela- base of the cone (Fig. 9c). Internally, the deposit tively well-sorted, highly vesicular scoria lapilli preserves a weak subvertical stratification and is to coarse ash and minor fluidal bomb deposits formed by scoria lapilli and minor fluidal bombs (Fig. 9a). The AL5 unit is interpreted as proximal similar to the AL5 unit. The AL7 unit is poorly fallout deposits. The pyroclasts of the AL5 unit are exposed as it is overlain by the youngest lava alkaline basaltic trachyandesites with Mg# , 60 flows (AL11 unit) and it is very likely that it covers and MgO , 7 wt% (evolved, Fig. 8a, b). Under the most of the original deposit, which is interpreted as a microscope they have a porphyritic texture with debris avalanche associated with the collapse scarp olivine (c. 7%) and plagioclase (c. 3%) phenocrysts on the NW flank of the Alumbrera Volcano. The up to 0.5–1.5 mm in size in a highly vesiculated ( c. pyroclasts of the AL7 unit have similar macroscopic 70%) hyalopilitic groundmass of olivine ( c. 3%) and and petrographic features to the AL5 unit. plagioclase microlites (c. 6%), opaque oxides (c. 1%) and glass (c. 10%) (Fig. 8d). AL8 unit. The stratigraphic sequence continues with a partly welded deposit consisting mainly of spatter AL6 unit. A well-sorted, clast-supported, open and minor fluidal bombs. This unit covers part of the framework, massive to planar-stratified scoria lapilli inner walls of the crater and shows a crude bedding to ash deposit (Fig. 9b) was mapped beyond the cone dipping inwards towards the crater (Fig. 9d). The (as much as 15 km from the vent towards the east). AL8 unit is interpreted as a proximal fall deposit The top of the deposit shows the incipient develop- associated with a lava fountain. The spatters and ment of a mega-ripple surface morphology as a bombs of the AL8 unit are alkaline basaltic tra- result of reworking by wind. The AL6 unit is inter- chyandesites with Mg## , 60 and MgO , 7 wt% nally stratified with alternations in grain size (evolved, Fig. 8a, b). Microscopically, they have a throughout the deposit and individual layers ranging porphyritic texture with olivine (c. 20%) and plagio- from 0.03 to 0.12 m in thickness. The thickness of clase (c. 10%) phenocrysts up to 1–2 mm in size the whole deposit ranges from 1.6 m (1.5 km from in a hyalopilitic groundmass of olivine ( c. 10%) the vent) to 0.4 m (3.5 km from the vent). In the and plagioclase microlites (c. 25%), opaque oxides more distal zones the deposit is fully reworked. (c. 5%) and glass (c. 30%) (Fig. 8c). The AL6 unit is interpreted as a fallout deposit. The pyroclasts of the AL6 unit are subalkaline tra- AL9 unit. Around the crater rim, a great quantity of chybasalts with Mg# . 60 and MgO . 8 wt% (high vesicle-poor fluidal bombs up to 3 m in maximum Mg, Fig. 8a, b). In thin section, they have a porphy- diameter overlie the AL8 unit (Fig. 9e). Some coarse ritic texture with olivine phenocrysts (c. 5%) up to bombs rolled down the cone slopes and were Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

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Fig. 8. (a) Total alkalis vs SiO 2 (wt%) diagram showing rock classification fields after Le Bas et al. (1986). (b) Geochemical classification of samples from this study based on wt% MgO and magnesium number, after Murray et al. (2015). NAV, Neo-Alumbrera Volcano; NDLV, Neo De La Laguna Volcano; PAV, Palaeo Alumbrera Volcano; PDLV, Palaeo De La Laguna Volcano. ( c–f ) Representative microphotographs showing typical rock textures of the ( c, d ) evolved, ( e) primitive and ( f) high magnesium geochemical groups. Cpx, clinopyroxene; Ol, olivine; Pl, plagioclase; V, vesicle. Note the similar microlite percentage in ( c) the poorly vesiculated spatter deposit from the AL8 unit and in the highly vesiculated scorias from the ( d) AL5 and ( f) AL6 units. deposited in the cone flanks and base, including the deposit emplaced following ballistic trajectories. surface exposed by the partial collapse scarp. The The bombs of the AL9 unit have similar petro- AL9 unit is interpreted as a proximal strombolian graphic features to the AL8 unit. Table 2. Whole-rock composition of samples from the Alumbrera and De La Laguna volcanoes from Downloaded

Sample Unit Location Type* SiO 2 TiO 2 Al 2O3 Fe 2O3t MnO MgO CaO Na 2O K 2O P 2O5 LOI Total

ANT-06 DLL5 26.133694 8 S 67.410333 8 W Primitive 53.83 1.51 15.74 8.69 0.14 7.12 6.85 3.43 2.33 0.37 0.06 99.92 ANT-07 DLL3 26.129389 8 S 67.417444 8 W Primitive 53.92 1.39 16.14 8.63 0.13 7.22 7.04 3.31 2.14 0.32 0.08 100.18

ANT-36 DLL1 26.131738 8 S 67.421698 8 W Evolved 52.19 2.09 16.06 9.20 0.13 6.22 6.74 3.66 2.64 0.50 0.68 99.96 http://sp.lyellcollection.org/ ANT-41B DLL2 26.139536 8 S 67.419429 8 W Primitive 53.58 1.40 16.08 8.43 0.13 6.84 6.96 3.41 2.23 0.32 0.69 99.92

ANT-41C DLL3 26.139345 8 S 67.418706 8 W Primitive 54.08 1.42 16.23 8.50 0.13 6.94 6.90 3.39 2.21 0.32 0.13 100.12 CONES SCORIA POLYCYCLIC ANT-01 DLL6 26.130056 8 S 67.413611 8 W Primitive 54.07 1.49 15.90 8.55 0.13 6.99 6.82 3.36 2.32 0.36 0.07 99.93 ANT-02 DLL8 26.129667 8 S 67.410306 8 W Primitive 54.38 1.49 15.98 8.54 0.14 6.87 6.77 3.32 2.35 0.36 0.01 100.19 ANT-03 DLL9 26.129222 8 S 67.409611 8 W Primitive 53.66 1.49 15.83 8.49 0.14 7.00 6.94 3.36 2.28 0.36 0.39 99.94 ANT-08 DLL12 26.129389 8 S 67.417444 8 W Primitive 54.33 1.49 15.79 8.54 0.14 6.84 6.71 3.43 2.35 0.36 0.05 100.04 ANT-23 DLL11 26.127972 8 S 67.428361 8 W Primitive 53.42 1.47 15.80 8.38 0.13 6.75 7.09 3.43 2.28 0.36 1.04 100.15 ANT-24 DLL12 26.135389 8 S 67.421889 8 W Primitive 52.97 1.49 15.62 8.45 0.14 7.11 7.45 3.38 2.19 0.36 1.01 100.18 ANT-41D DLL12 26.139198 8 S 67.417710 8 W Primitive 52.51 1.48 15.54 8.38 0.14 7.04 7.81 3.41 2.19 0.36 1.00 99.86

ANT-34 DLL4 26.116639 8 S 67.420139 8 W High Mg 50.79 1.42 14.91 9.32 0.15 10.20 7.34 3.11 1.94 0.33 0.35 99.85 27,2016 onSeptember byguest ANT-18 AL2 26.141972 8 S 67.383222 8 W Primitive 54.48 1.48 15.87 8.43 0.13 6.77 6.81 3.39 2.36 0.36 0.08 100.15 ANT-22 AL1 26.133028 8 S 67.378778 8 W Evolved 52.87 2.05 16.58 9.13 0.14 6.13 6.56 3.60 2.57 0.50 0.25 100.39 ANT-28A AL2 26.153972 8 S 67.379056 8 W Primitive 53.85 1.52 15.92 8.61 0.14 7.25 6.92 3.30 2.25 0.36 0.04 100.13 ANT-28B AL1 26.153972 8 S 67.379056 8 W Evolved 52.93 2.08 15.99 9.14 0.14 6.06 6.64 3.73 2.63 0.49 0.07 99.91 ANT-12 AL5 26.146861 8 S 67.383472 8 W Evolved 52.48 1.97 15.70 9.18 0.14 6.69 6.89 4.01 2.54 0.54 0.11 100.25 ANT-19 AL11 26.140253 8 S 67.382824 8 W Evolved 52.99 1.87 15.91 9.05 0.14 6.43 6.81 3.64 2.57 0.56 0.10 100.06 ANT-26 AL11 26.181806 8 S 67.418444 8 W Evolved 53.02 2.08 16.21 9.21 0.14 6.23 6.66 3.59 2.62 0.50 0.10 100.37 ANT-10A AL6 26.131056 8 S 67.382861 8 W High Mg 50.60 1.40 14.34 9.46 0.15 11.47 7.26 3.05 1.92 0.33 0.07 100.04 ANT-38 AL6 26.162917 8 S 67.370167 8 W High Mg 51.33 1.49 15.68 9.22 0.15 8.95 7.98 3.21 1.97 0.34 0.14 100.44 ANT-15 AL8 26.149667 8 S 67.383167 8 W Evolved 52.69 2.04 16.12 9.20 0.13 5.67 6.51 3.07 2.46 0.50 1.75 100.13

Oxides in wt% by X-ray fluorescence spectrometry. *Classification for the Southern Puna mafic lavas proposed by Murray et al. (2015). Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

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Fig. 9. (a) Detail of a high vesicular scoria that forming the AL5 unit. ( b) Stratified fallout deposit of the AL6 unit. (c) Debris avalanche with surface hummocky morphology preserving its original stratification (AL7 unit). (d) Spatter deposits covering part of the inner walls of the crater (AL8 unit). ( e) Vesicle-poor fluidal bombs around the crater rim of the Alumbrera Volcano (AL9 unit). ( f) General view of the pyroclastic density current deposits of the AL10 unit.

AL10 unit. In the summit region of the Alumbrera a yellowish colour. The AL10 unit reaches 0.5 km Volcano, a 4 m thick, non-welded, well-sorted, pla- beyond the cone and covers the fluidal bombs of nar to cross-stratified lapilli and minor ash deposit the AL9 unit. In the distal facies, the AL10 unit is crops out (Fig. 9f). U-shaped erosional channels a reddish, well-sorted, ash–lapilli deposit with pla- within the sequence and bombs (10–30 cm) without nar to low-angle cross-stratification. The AL10 unit basal sag structures are present. The deposit shows a is interpreted as a product of dilute pyroclastic den- high degree of hydrothermal alteration, which gives sity currents. The high degree of hydrothermal Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

POLYCYCLIC SCORIA CONES alteration in the AL10 unit hindered its geochemical lava flows are alkaline basaltic trachyandesites and petrographic characterization. with Mg# , 60 and MgO , 7 wt% (evolved, Fig. 8a, b). In thin section they exhibit a porphyritic tex- AL11 unit. The youngest unit of the NAV stratigra- ture with olivine (c. 20%) and plagioclase (c. 7%) phy is a compound toothpaste lava flow field with phenocrysts up to 0.7–1 mm in size in a hyalopilitic incipient wind erosion features (Fig. 5c, d). The groundmass of olivine ( c. 10%) and plagioclase AL11 unit was issued from many vents along the microlites (c. 30%), opaque oxides (c. 3%) and SW and central eruptive fissures and it constitutes glass (c. 30%). the biggest lava flow field in the study area. The AL11 lava flows are black in colour and have thick- DLL2 unit. A highly eroded aa lava flow crops nesses ranging from 3 to 40 m. The AL11 lava flow out 1.4 km south of the cone (Fig. 4); its strati- field was divided into two morphological domains. graphic relationship with the DLL1 unit is unknown. In the western sector the lava flow field fails to Despite its field similarities with the DLL1 unit, spread out as a result of lateral topographic confine- the DLL2 lava flow consists of subalkaline basal- ment. Consequently, the lava flows exhibit a ‘slab tic trachyandesites with Mg# . 60 and MgO , pahoehoe’ morphology and develop very large elon- 8 wt% (primitive, Fig. 8a, b). Microscopically, gate tumuli (e.g. Glaze et al. 2005; Orr et al. 2015) they have a porphyritic texture with olivine ( c. reaching 800 m in length and 200 m in width (e.g. 15%) and clinopyroxene (c. 10%) phenocrysts up 26.1624088 W 67.413064 8 S). By contrast, the east- to 3–0.5 mm in size in an intergranular groundmass ern sector of the lava flow field expanded to the of clinopiroxene ( c. 15%), olivine (c. 10%) and pla- south along the gentle regional slope. As a result, gioclase microlites (c. 45%) and opaque oxides the lava flows have smooth surfaces and preserve (c. 5%). The groundmass commonly flows around the primary toothpaste lava lobe morphology, reach- the phenocrysts. ing 600 m in length and 50 m in width (e.g. 26.1851148 W 67.385285 8 S) and innumerable sec- DLL3 unit. The DLL1 and DLL2 units are covered ondary toothpaste lava tongues. Lava tubes and by a series of heavily eroded blocky lava flows associated pit craters are also present. The AL11 outcropping south of the cone. These lava flows unit has complex stratigraphic relationships that are grey in colour with thicknesses ranging from vary from one sector to another, suggesting that its 5 to 15 m (Fig. 6e, f). The frontal lava flow lobes formation occurred during most of the eruption. In have developed axial cracks 1–3 m wide and the SE sector the fallout blanket (AL6 unit) partially 20–150 m long. The DLL3 lava flow are subal- covers the AL11 unit. On the western flank of the kaline basaltic trachyandesites with Mg# . 60 and cone the AL11 unit overlies the collapse deposits MgO , 8 wt% (primitive, Fig. 8a, b). Microscopi- (AL7 unit). The presence of large fluidal bombs cally, they exhibit a porphyritic texture with olivine emplaced above the surface of some of the AL11 (c. 25%) and clinopyroxene (c. 5%) phenocrysts up lava flows suggests that part of its emission occur- to 1–2 mm in size in an intergranular groundmass of red simultaneously with the strombolian activity clinopyroxene (c. 5%), olivine (c. 20%) and plagio- of the central crater (AL9 unit). In the NW sector, clase microlites (c. 45%). the AL11 unit covers the AL10 deposit, indicat- ing that the AL11 compound toothpaste lava flow DLL4 unit. A highly eroded blocky lava flow field field was active until the end of the eruption. The was mapped north of the cone (Fig. 4). Despite its AL11 lava flows are alkaline basaltic trachyande- field similarities with the DLL3 unit, the DLL4 sites with Mg# , 60 and MgO , 7 wt% (evolved, lava flow field consists of subalkaline trachybasalts Fig. 8a, b). In thin section they have a porphyritic with Mg# . 60 and MgO . 8 wt% (high Mg, texture with olivine (c. 15%) and plagioclase (c. Fig. 8a, b). In thin section they exhibit a porphyritic 10%) phenocrysts up to 1–1.5 mm in size in a hya- texture with olivine phenocrysts (c. 20%) up to 1– lopilitic groundmass of olivine ( c. 5%) and plagio- 1.5 mm in size in a hyalopilitic groundmass of oliv- clase microlites (c. 35%), opaque oxides (c. 5%) ine (c. 5%) and plagioclase microlites (c. 15%) and and glass (c. 30%). glass (c. 60%) (Fig. 8e).

Palaeo De La Laguna Volcano DLL5 unit. A highly eroded deposit formed by flu- idal massive bombs and angular scoria crops out DLL1 unit. The stratigraphic sequence of the De La at the base of the cone and overlies the DLL2 Laguna Volcano starts with a series of highly eroded unit (Fig. 7b). This deposit represents a marginal aa lava flows (Fig. 6c, d) outcropping in two small facies of an older cone covered by the growth of areas located 1.6 km north and 1 km SW from the the currently visible cone. The pyroclasts of the cone. These lava flows are grey in colour and have AL3 unit have similar petrographic features to the thicknesses ranging from 2.5 to 4 m. The DLL1 AL2 unit. Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

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Neo De La Laguna Volcano petrographic features to the AL6 unit (Alumbrera Volcano). DLL6 unit. The external part of the cone is formed by a non-welded, relatively well-sorted, open DLL8 unit. The crater is largely filled by a partly framework, highly vesicular scoria lapilli with welded spatter and minor fluid bomb deposit that minor fluidal bombs (Fig. 10a, b). The DLL6 unit shows a crude bedding dipping inwards towards is interpreted as a fallout deposit. The DLL6 pyro- the crater (Fig. 10d). The DLL8 unit is interpreted clasts are subalkaline basaltic trachyandesites with as a hawaiian lava fountain deposit. The DLL8 Mg# . 60 and MgO , 8 wt% (primitive, Fig. 8a, spatters are subalkaline basaltic trachyandesites b). Microscopically, they exhibit a porphyritic tex- with Mg# . 60 and MgO , 8 wt% (primitive, ture with olivine (c. 7%) and clinopyroxene (c. Fig. 8a, b). Microscopically, they exhibit a porphy- 3%) phenocrysts up to 0.7–2 mm in size in a highly ritic texture with olivine (c. 20%) and clinopyroxene vesiculated (c. 65%) intergranular groundmass of (c. 10%) phenocrysts up to 0.5–1.5 mm in size in clinopyroxene (c. 4%), olivine (c. 1%) and plagio- an intergranular groundmass of clinopyroxene clase microlites (c. 15%). (c. 20%), olivine (c. 5%) and plagioclase microlites (c. 45%). DLL7 unit. A well-sorted, open framework, massive to planar-stratified scoria lapilli to ash deposit was DLL9 unit. Around the crater rim, a great quantity mapped 1.5 km beyond the cone towards the NE of vesicle-poor fluidal bombs up to 4.5 m in maxi- (Fig. 10c) and is interpreted as a fallout deposit. mum diameter overlie the DLL8 unit (Fig. 7a). The youngest lava flow issued from the De La DLL9 coarse bombs rolled down the cone slopes Laguna Volcano (DLL11) and Alumbrera Volcano and were deposited to form a ring around the (AL10b unit) covers the DLL7 fallout deposit. base. The DLL9 unit is interpreted as a proximal The pyroclasts of the DLL7 unit have similar strombolian deposit emplaced following ballistic

Fig. 10. (a) Well-sorted, open framework, highly vesicular scoria lapilli and minor fluidal bomb deposit of the DLL6 unit. (b) Detail of the DLL6 deposits showing the well-sorted and open framework internal fabric. ( c) Stratified fallout deposit of the DLL7 unit. ( d) De La Laguna Volcano crater filled by a partly welded spatter deposit and minor fluid bomb deposit (DLL8 unit). Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

POLYCYCLIC SCORIA CONES trajectories. The bombs of the DLL9 unit have sim- has questioned the use of the stage of degradation ilar petrographic features to the DLL8 unit. of the scoria cones in the Southern Puna region to obtain information about their relative ages (Risse DLL10 unit. Two small outcrops of a 0.4 m thick, et al. 2008). Nevertheless, considering that the non-welded, well-sorted planar to cross-stratified semi- to hyper-arid climatic conditions of the South- lapilli and minor ash deposit were mapped at the ern Puna have persisted for several million years western and eastern crater rim, respectively (Fig. (Alonso et al. 2006), it is reasonable to suggest 4). The DLL10 unit covers the DLL8 and DLL9 that morphological parameters, such as gully den- units. The DLL10 unit is interpreted as the product sity and the surface morphology of lava flows, can of dilute pyroclastic density currents and its high be used to obtain a relative age for mafic volcanic degree of hydrothermal alteration hindered the geo- centres (Hasenaka & Carmichael 1985; Dohrenwend chemical and petrographic characterization. et al. 1986; Corazzato & Tibaldi 2006). The forma- tion of ventifacts and the abrasion rate depend on DLL11 unit. To the north and west of the cone, an the position of the rock relative to the direction of aa lava flow field with a moderate degree of venti- maximum wind velocity (Bridges et al. 2004). Dur- fact development was mapped. The DLL11 lavas ing fieldwork, systematic variations in the degree of show rough surfaces and morphologies such as wind erosion for each unit were observed regardless channels limited by levees and pressure ridges its position respect to the main wind direction. (Fig. 6a, b). The DLL11 unit was issued from an On the other hand, the original rock shape affects eruptive fissure located on the NW cone flank and ventifact formation, with steeper faces abrading overlies the DLL1, DLL3, DLL4 and DLL5 units. faster than shallower faces and rocks with irregular The DLL11 unit is covered by the youngest lava features, such as pits or grooves, abrading at greater flows issued from the Alumbrera Volcano (AL11 rates than rocks with smooth surfaces (Bridges et al. unit). The DLL11 lava flows are dark grey in colour 2004). In the study area, the presence of units with and have thicknesses ranging from 3 to 12 m. a similar morphology and texture, but different Compositionally, they are subalkaline basaltic tra- degrees of ventifact formation (Figs 5, 6 & 7), sug- . , chyandesites with Mg# 60 and MgO 8 wt% gests different ages of emplacement. In addition, the (primitive, Fig. 8a, b). In thin section they have a initial conditions of the lava flow surface, such as porphyritic texture with olivine (c. 20%) and clino- the presence or absence of a fallout tephra blanket, pyroxene (c. 15%) phenocrysts up to 0.4–2.5 mm are important in avoiding an erroneous interpreta- in size in an intergranular groundmass of clinopyr- tion of the relative ages of lava fields (Valentine & oxene (c. 15%), olivine (c. 10%) and plagioclase Harrington 2006; Valentine et al. 2007). However, (c. 40%) microlites. lava flow fields in the study area with and without a fallout tephra blanket have the same degree of ven- DLL12 unit. A moderately eroded aa lava flow tifact formation (e.g. the AL4 and AL11 units). In field was mapped south of the cone (Fig. 4). This addition, active dunes partially cover some of the lava flow field was issued from a secondary vent lava flows in the study area. However, these aeo- located 0.5 km from the cone and overlying the lian deposits are not related to any particular unit DLL1, DLL2 and DLL3 units. The DLL11 unit is within the stratigraphy of the two volcanoes. Thus covered by the youngest lava flows issued from the very youthful-looking ventifacts of the Alum- the Alumbrera Volcano (AL11 unit). The DLL12 brera Volcano and the absence of ventifacts in the lava flows are subalkaline basaltic trachyandesites . , NAV suggest a probable Holocene age for its youn- with Mg# 60 and MgO 8 wt% and have simi- gest activity. In contrast, the De La Laguna Volcano lar petrographic features to the DLL11 unit. presents a greater degree of erosion, which is consistent with the only available radiometric age Discussion (0.34 + 0.06 Ma, Risse et al. 2008). Despite the existence of temporal hiatuses in the evolution of Polycyclic nature of the Alumbrera and De both the Alumbrera and De La Laguna volcanoes, La Laguna volcanoes the determination of their durations remains a major challenge and needs high-resolution geochro- The stratigraphic reconstruction of the Alumbrera nology. Experimental work in a wind tunnel proved and De La Laguna volcanoes reveals a previously that ventifacts in Antarctica can be formed over a unreported high complexity in the evolution of few decades to centuries (Miotke 1982). However, some Southern Puna scoria cones. The variations the lack of historical reports and archaeological evi- in the degree of ventifact development along the dence of volcanic activity in the study area suggests stratigraphic sequences suggest that the Alumbrera an age of at least a few thousand years for the youn- and De La Laguna volcanoes could have been gest lava flows without ventifacts. This means that formed by more than one eruption. Previous work the time required for the full development of Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

W. BA ´ EZ ET AL. ventifacts on the surface of a lava field in the Puna ascent of magma through the upper crust, which is environment is probably much greater than that sug- reflected by the more primitive composition of the gested by Miotke (1982). Therefore temporal gaps mafic lavas from the Arizaro and Pasto Ventura between different eruptions of decades or centuries basins (Murray et al. 2015). In addition, these clus- cannot be identified in the Southern Puna environ- ters are related to a low rate of magma input from the ment by geomorphological methods. Even though source due to either their marginal position with the absolute age of the different units is not known respect to a large delaminated lithospheric block with certainty, the scoria cones studied in this (Kay et al. 1994; Risse et al. 2013) or their position work are considered to be polycyclic (Kereszturi above a small delaminated lithospheric block (Drew et al. 2010; Ne´meth & Kereszturi 2015) and possi- et al. 2009; Ducea et al. 2013; Murray et al. 2015). bly evolved over thousands of years. In addition, In contrast, the Antofagasta de la Sierra Cluster has along the stratigraphic sequence of both volcanoes, relatively large volcanoes with an irregular spatial units with different geochemical and petrographic distribution. Some scoria cones are aligned along features are present, including the three lava types regional north–south faults, whereas others, includ- proposed by Murray et al. (2015) for the Southern ing the Alumbrera and De La Laguna scoria cones, Puna mafic lavas (evolved, primitive and high mag- are aligned NW–SE without an apparent relation- nesium; Fig. 8). The emission of successive magma ship with the regional faults. The relatively high vol- batches with geochemical changes over time in a ume of each volcanic centre and the total amount of single scoria cone suggests that both volcanoes are erupted magma in the ASB suggest a high rate of also polymagmatic (sensuBrenna et al. 2010). The magma input from the source as a result of its posi- geochemical changes could reflect variations in tion above a relatively large delaminated litho- the source and/or crustal storage conditions. spheric block (Kay et al. 1994; Risse et al. 2013; Murray et al. 2015) (Fig. 11). In this context, the dykes are larger and develop a high magma pressure Factors controlling the occurrence of and thus are able to reactivate pre-existing fractures polycyclic scoria cones in the Southern of any orientation relative to the prevailing stress Puna region field (Alaniz-Alvarez et al. 1998; Le Corvec et al. 2013a). An example of this situation is the mafic Despite the ongoing convergence of the Nazca plate volcanism located along north–south regional faults beneath the South American plate, the Southern (sub-parallel to the minimum principal compressive Puna regional stress field is characterized by sub- stress) in the ASB (Fig. 11). The geometry of the horizontal extension with a low deformation rate north–south thrust fault plains in the Southern since late Miocene times (Montero Lo´pez et al. Puna region becomes horizontal in a detachment 2010; Zhou et al. 2013). This horizontal non- level located at around 11 km depth (Seggiaro lithostatic tension favours ascent through the thick, et al. 2000). This crustal level with unfavourable less dense continental crust and the eruption of geometry should promote the arrest of dyke propa- small mafic volcanoes (Marrett & Emerman 1992). gation and sill formation (e.g. Gudmundsson However, the magma pathway is governed near 2011) (Fig. 11). However, if the propagating dyke the surface by the influence of pre-existing fractures has enough overpressure and intercepts the regional rather than the regional stress field (Alaniz-Alvarez detachment level near the root of a thrust fault, then et al. 1998; Valentine & Krogh 2006; Lesti et al. the dyke may reach the surface (Fig. 11). Moreover, 2008; Le Corvec et al. 2013a, b). The mafic volca- when the magma supply from a deep source is rela- nism of the Southern Puna region forms clusters tively large and sustained over time, it is plausible with different spatial distributions, total volumes that the magma will begin to be stored in the detach- of erupted magma and size of individual volcanoes ment level in the upper crust (Gudmundsson 2011) (Viramonte et al. 2010). Some clusters have a low (Fig. 11). Despite the low buoyancy of the mafic density of small and simple scoria cones, lava magmas in the upper crust, the magma pressure domes, tuff rings and maars, aligned along regional may exceed the lithostatic pressure and any poten- NE–SW-trending faults (e.g. the Arizaro Basin, tial differential crustal stress when the accumulation Viramonte et al. 1984; the Pasto Ventura Basin, of magma is large enough; the generation of Filipovich et al. 2014). In these examples, each indi- magma-filled tensile cracks is then feasible (Naka- vidual volcano is related to a discrete small magma mura 1977) (Fig. 11). These tensile cracks will batch, which propagates as a dyke with low magma open perpendicular to the minimum principal com- pressure. Thus these dykes are only able to exploit pressive regional stress (s3 ¼ north–south /NE– pre-existing faults oriented nearly perpendicular to SW in the case of the Southern Puna region). the minimum principal compressive stress (s3 ¼ This mechanism in the Southern Puna region is north–south /NE–SW in the case of the Southern aided by the current sub-horizontal extension and Puna region). Such a fault geometry aids the fast could explain the mismatch between the regional Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

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Fig. 11. Conceptual model explaining the occurrence of polycyclic scoria cones in the Antofagasta de la Sierra Basin. The model proposes that the presence of polycyclic volcanoes results from the formation of small upper crustal magma reservoirs in response to (1) the high and continuous input of magma related to the delamination of a large lithospheric block, (2) the favourable regional stress field and (3) the interaction of the rising magma with pre-existing crustal faults. structures and the inferred magma-feeding system The deposits forming the external part of the in the Alumbrera and De La Laguna volcanoes. cone (AL5 unit) have textural features akin to mate- The development of upper crustal mafic magma rial deposited by fallout from a sustained column reservoirs can explain the polycyclic character of rather than deposition by direct ballistic emplace- both volcanoes. Another possibility is that a buried ment (e.g. Valentine et al. 2005). The tephra fallout step-over structure between possible subsurface blanket (AL6 unit) that covers some of the terrain faulting could be a preferential means of ascent of around the volcano probably resulted from the successive magma batches (Calhoun 2010). Thus same sustained column phase that formed the AL5 the presence of polycyclic scoria cones would only unit. The geochemical and petrographic characteri- be related to a continuous magma supply in a narrow zation of the AL5 unit was carried out from a sample region during the delamination processes. More located at the base of the deposit. Hence the mis- detailed structural and petrological studies are ne- match in the geochemical and petrographic features eded to fully understand the plumbing system that between AL5 and AL6 units could be related to geo- fed the Alumbrera and De La Laguna volcanoes. chemical changes during eruption (e.g. Johnson et al. 2008; Brenna et al. 2010; Erlund et al. 2010; Eruptive dynamic shifting Presta & Caffe 2014). A more detailed geochemical stratigraphy of the fallout blanket (AL6 unit) is The youngest units in the stratigraphic sequence of required to fully assess this topic. The AL11 tooth- the Alumbrera (NAV) and De La Laguna (NDLV) paste lava flow field is partially covered by the fall- volcanoes were formed without significant temporal out blanket, suggesting that it was initially issued hiatuses between them and are interpreted as prod- simultaneously with the development of a sustained ucts of single eruptions that developed in different column phase. The size, internal complexities and phases (Fig. 12). stratigraphic relationships of the AL11 lava flow field suggest that it was active during most of the Neo-Alumbrera eruption. The AL4 lava flow field eruption. The formation of scoria cones from sus- covered by the AL6 fallout unit is interpreted as tained columns, with the coeval formation of a the oldest unit of the Neo-Alumbrera eruption. Thus tephra blanket beyond the cone, and the duality in the eruption probably started by the reactivation of the eruption dynamics (with simultaneous explosive the NE eruptive fissure with an effusive activity. activity at the central vent and effusive activity from Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

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Fig. 12. Conceptual model for the Neo-Alumbrera eruption explaining the occurrence of the shift in the eruptive dynamics in response to internal factors such as a decline in the mass eruption rate and external factors such as magma–water interactions. the lateral vents) are common features of typical Paricutı´n Volcano, where the occurrence of violent violent strombolian eruptions (Valentine et al. strombolian eruptions requires that magma from 2005; Pioli et al. 2008, 2009). The absence of natu- the feeder conduit splits into vertical gas-rich and ral cuts or quarries that could reveal the core of the lateral gas-poor branches somewhere near the base Alumbrera scoria cone limits the inferences about of the previously formed and stabilized scoria the initial cone formation. Pioli et al. (2009) pro- cone. In this sense, a phase of mild explosive strom- posed a model for the historical eruption of the bolian activity responsible for constructing the Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

POLYCYCLIC SCORIA CONES internal portion of the scoria cone is indirectly of the volcano due to its rapid growth (over- inferred, which was followed by a high-explosive, steeping) and high internal pressure at the end of violent strombolian phase that formed the external the violent strombolian phase. The collapse direc- part of the cone (AL5 unit). By comparing similar tion in the Alumbrera Volcano is consistent with historical eruptions, the initial strombolian phase a magma-feeding system oriented NW–SE, as probably lasted for several days or a few weeks, dur- expected for a scoria cone grown on a horizontal ing which time possibly coeval lava flows (currently or sub-horizontal substrate topography ( ,98) (Cor- covered) were issued from the same vent to form a azzato & Tibaldi 2006). In the same way as simi- horseshoe-shaped cone (e.g. Paricutı´n, Gonza´lez & lar historical eruptions (e.g. Paricutı´n, Pioli et al. Foshag 1946; Navidad cone, Moreno & Gardeweg 2008), the violent strombolian phase was strongly 1989). The temporal relationship between this pulsatory, resulting in a fine stratification of the inferred mild explosive strombolian activity and tephra blanket (AL6 unit). the effusive activity in the NE eruptive fissure is The progressive decline in the mass eruption rate unknown. If the AL4 lava flow field development (,10 3 kg s 21) favoured passive magma degassing, was coeval with the inferred initial central vent producing both lava effusion and low explosive activity, it is plausible that the NE eruptive fissure activity. At the central conduit violent strombolian was connected to the main feeder conduit at depth activity changed to a hawaiian style, with fountain- and had a low mass eruption rate ( ,10 3 kg s 21), ing events forming spatter deposits (AL8 unit). In which allowed the passive degassing and domi- response to a further decline in the mass eruption nantly effusive emission of the magma. rate, a gas-poor magma lake with sporadic strom- The violent strombolian eruptions have been bolian bursts was established. The strombolian linked to volatile-rich magmas ( c. 4 wt%) and activity was caused by the periodic rise of conduit- mass eruption rates between 10 3 and 10 5 kg s 21 filling gas bubbles (Taylor bubbles) within the con- (Pioli et al. 2008, 2009). The published olivine- duit, resulting in metre-scale fluidal bombs with hosted melt inclusion data from the Southern Puna low vesicularity (AL9 unit). The final phase of the mafic lavas are scarce and suggest that the water Neo-Alumbrera eruption was characterized by the contents of some of them are low ( ,1 wt%) com- generation of diluted pyroclastic density currents, pared with arc values (Risse et al. 2013). However, which travelled at least 0.5 km beyond the cone the high vesicularity of the juvenile clasts forming base (AL10 unit). The occurrence of pyroclastic the AL5 and AL6 units suggests that the violent density currents associated with scoria cones can strombolian phase involved volatile-rich magma. be related to the collapse of the eruptive column dur- The relatively low percentage of microlites in the ing violent strombolian eruptions (Taddeucci et al. juvenile clasts forming the AL5 unit (Fig. 8d) also 2004; Valentine et al. 2007; Valentine & Gregg suggests that brittle magma fragmentation due to 2008; Kereszturi & Ne´meth 2012) or short-lived an increase in viscosity and gas overpressure in phreatomagmatic activity (Valentine et al. 2007; response to microlite crystallization during magma Di Traglia et al. 2009). The stratigraphic record ascent was not the dominant process controlling suggests a progressive decrease in the magma sup- the occurrence of the violent strombolian phase ply rate through the development of the Neo- (e.g. Valentine et al. 2005; Houghton & Gonner- Alumbrera eruption. The high degree of hydrother- mann 2008; Vona et al. 2011). The high mass erup- mal alteration of the AL10 unit and its association tion rate for the violent strombolian phase is inferred with the waning stage of the eruption suggest that indirectly by the partial gravitational collapse of this unit was formed by phreatomagmatic activity the Alumbrera Volcano. Flank instabilities in scoria due to sudden groundwater access to the volcanic cones can be caused by the stress applied by magma conduit in response to a low magma supply rate bulging or fracture propagation (Corazzato & (e.g. Martin & Ne´meth 2006; Di Traglia et al. Tibaldi 2006), the weakest zone in the cone (Coraz- 2009). As a result of this highly explosive final zato & Tibaldi 2006), gradual erosion and lowering phase, the crater of the Alumbrera Volcano acquired of the cone flank due to a lava flow pouring out from its final morphology. the crater or lateral vents at the base of the cone The AL11 lava flow field was issued from many (Tibaldi 1995), the inclination of the pre-eruptive vents located on the western flank of the cone and terrain (Corazzato & Tibaldi 2006; Kereszturi was formed along the entire eruption. However, et al. 2012) or the prevailing wind direction (Inbar most of its growth occurred simultaneously with & Risso 2001). Taking into account the gently dip- the low explosive activity in the central crater. ping floor of the ASB and the absence of both a truly Toothpaste lava flows are related to lavas with a horseshoe-shaped morphology in the cone (the scarp similar viscosity to those forming aa flows, but only affects the external layers of the cone) and require slower effusion rates (Rowland & Walker pyroclastic rafts in the lava flows, the partial col- 1987). In addition, the occurrence of complex com- lapse was possibly related to gravitational instability pound lava flow fields with the development of lava Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

W. BA ´ EZ ET AL. tubes is associated with long-lived eruptions (a few crustal magma reservoirs in response to a high weeks to a few years) with relatively low and steady input of magma related to the delamination of a mass eruption rates (Harris & Rowland 2009). large lithospheric block, a favourable regional stress Despite the low mass eruption rate, the large aver- field and the interaction of the rising magma with age volume erupted and thermally well-insulated pre-existing crustal faults. However, more detailed flow allowed the lava to travel up to 7.3 km from structural and petrological studies are needed to the vent. Based on an analogy with historical erup- fully understand the occurrence of polycyclic scoria tions that produced similar volumes and types of cones in the Southern Puna plateau. pyroclastic deposits and lava flows (e.g. the Paricu- The youngest activity of both volcanoes was tı´n and Jorullo volcanoes in the Michoaca´n–Guana- characterized by long-lasting eruptions (lasting for juato monogenetic volcano field, Mexico, Rowland a few years by comparison with modern analogues), et al. 2009; the Madinah eruption, Saudi Arabia, with shifts in the eruptive dynamics in response Camp et al. 1987), it follows that the eruption to a decrease in the mass eruption rate from could have lasted from a few weeks to a few years. high-explosive, violent strombolian to hawaiian / strombolian phases. The explosive activity was Neo-De La Laguna eruption. The evolution of the accompanied by the emission of lava flows from youngest eruption in the De La Laguna Volcano lateral vents, which occurred mostly during the was similar to the previously described Neo- hawaiian/strombolian phases. During the waning Alumbrera eruption. The external part of the cone stage of the eruptions, intermittent phreatomagmatic (DLL6 unit) and the fallout deposit outcropping activity developed as a result of the sudden access beyond the cone (DLL7 unit) were formed during of groundwater into the volcanic conduit. A gravi- a high-explosive, violent strombolian phase devel- tational flank collapse of the scoria cone occurred oped just after a mildly explosive strombolian during the Neo-Alumbrera eruption. The develop- phase inferred indirectly. In response to the decline ment of long-lasting, mafic, high-explosive erup- in the mass eruption rate, the eruptive activity tions and the occurrence of more than one becomes less explosive, with a hawaiian eruptive eruption in each individual scoria cone are impor- style first and then a strombolian eruptive style, tant topics in the assessment of volcanic hazard in forming the DDL8 and DLL9 units, respectively. the Southern Puna plateau. The occurrence of vio- In the same way as in the AL10 unit, the high degree lent strombolian eruptions could cause extensive of hydrothermal alteration of the DLL10 unit and economic and social disruption as far as hundreds its association with the waning stage of the eruption of kilometres from the vent as a result of ash disper- suggest that this unit was formed by phreatomag- sion. In addition, the occurrence of a high-explosive matic activity. However, the limited distribution phreatomagmatic phase generating pyroclastic den- of the DLL10 unit indicates that phreatomag- sity currents during the waning stage of the eruption matic pulses during the final stage of the eruption is a very hazardous feature of this type of volcanism. were scarce. The weak phreatomagmatic activity allowed the preservation of the spatter deposits We thank Karoly Ne´meth, Gabor Kereszturi and an anon- that infill the crater. Simultaneous with the explo- ymous reviewer for their constructive comments that sive activity in the central conduit, two aa lava helped us to improve the manuscript. This work was sup- flow fields were issued from the base of the cone ported by a IPGH-GEOF 04 2014 grant (Estudio compara- tivo del vulcanismo bimodal reciente en las regiones (DLL11 unit) and from a vent located 0.5 km from oriental del Cinturo´n Volca´nico Mexicano y la Puna Aus- the cone (DLL12 unit). The latter was probably gen- tral Argentina, ge´nesis e implicaciones de riesgo volca´- erated in the same way as the AL4 unit in the Alum- nico) and is under the cooperation agreement between brera Volcano. INENCO-CONICET and the Secretarı´a de Ambiente de la Provincia de Catamarca. The geochemical analysis was supported by the Laboratorio de Fluorescencia de Conclusions Rayos X, Centro de Geociencias, Juriquilla, UNAM. This paper presents the first detailed geological map of individual scoria cones (Alumbrera and De La References Laguna volcanoes) in the Southern Puna plateau ´ and shows their relatively complex stratigraphy. Alaniz-Alvarez , S.A., Nieto-Samaniego, A .F. & Fer- The occurrence of more than one eruption at both rari, L. 1998. Effect of strain rate in the distribution of monogenetic and polygenetic volcanism in the volcanoes was reconstructed by the recognition of Transmexican volcanic belt. Geology, 26 , 591–594, temporal hiatuses along their stratigraphy using http: // doi.org /10.1130/0091-7613(1998)026 ,0591: morpho-stratigraphic criteria. The presence of rela- EOSRIT.2.3.CO;2 tively large polycyclic scoria cones in the ASB is Alonso, R.N., Bookhagen, B. et al . 2006. Tectonics, inferred to result from the development of upper climate, and landscape evolution of the southern Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2016

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