Facies architecture of an isolated long-lived, nested polygenetic silicic tuff ring erupted in a...

Journal of Volcanology and Geothermal Research 239–240 (2012) 33–48

Contents lists available at SciVerse ScienceDirect

Journal of Volcanology and Geothermal Research

j ourna l homepage: www.e lsev ie r .com/ locate / jvo lgeores

Facies architecture of an isolated long-lived, nested polygenetic silicic tuff ringerupted in a braided river system: The Los Loros volcano, Mendoza, Argentina

K. Németh a,⁎, C. Risso b, F. Nullo c, I.E.M. Smith d, Z. Pécskay e

a Volcanic Risk Solutions, Massey University, Palmerston North, New Zealandb Department of Geology, University of Buenos Aires, Argentinac CONICET, Buenos Aires, Argentinad The University of Auckland, New Zealande Hungarian Academy of Sciences, Institute of Nuclear Researches (ATOMKI), Debrecen, Hungary

⁎ Corresponding author. Tel.: +64 6 3569099x7389;E-mail address: [email protected] (K. Németh

0377-0273/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.jvolgeores.2012.06.010

a b s t r a c t
a r t i c l e i n f o
Article history:Received 18 May 2012Accepted 3 June 2012Available online 13 June 2012

Keywords:Tuff ringMaarWeldingLava domeBraided riverPhreatomagmaticSilicic

Los Loros is a small, well-preserved ~1 million year old volcanic depression. The circular ~50 m deep and~1 km broad crater is inferred to be a complex small-volume volcano; with multiple eruptive phases pro-duced by magmatic and minor phreatomagmatic explosive eruptions in two distinct eruptive episodes andconsequently produced two tuff rings separated by fluvial deposits and/or paleosols. Geochemical data,alongside a new age determination, underlies the fact that the volcano is far older than had been expectedfrom its morphology, and its composition represents a bimodal nature with eruptive products belonging toa typical intraplate basalt to phonolite and a crustal influenced intra-continental rhyolitic lineage. Tuff ring1 erupted into a braided river system that has already accumulated channelised volcaniclastic conglomeratesfrom distal sources. Tuff ring 2 formed in the same place as Tuff ring 1 and produced welded pyroclastic den-sity current deposits, a capping lava flow and a single intermediate block-and-ash flow deposit which allprevented the edifice from erosion. Los Loros is a small-volume volcano, similar to mafic tuff rings, however,its magma compositions, eruption styles, and inter-eruptive breaks suggest, that it closely resembles a volca-nic architecture commonly associated with large, composite volcanoes.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Tuff rings are small constructional volcanic landforms, that consist of abroad crater and relatively thin tephra rim that is dominated by pyroclas-tic deposits accumulated frompyroclastic density currents and fall events(Chough and Sohn, 1990; Sohn, 1996). Theymainly form by explosive in-teraction of rising magma and surface (or shallow subsurface) water(Heiken, 1971; Lorenz, 1986; White, 1991; Vespermann and Schmincke,2000). Tuff ring formation generally involvesmaficmagma such as basalt(Vespermann and Schmincke, 2000). However, an increasing number oftuff rings have been identified that were formed by the eruption ofmore silicic magma and are commonly associatedwith lava dome forma-tion (Brooker, 1988; Brooker et al., 1993; DeRosa andDellino, 1999; Riggsand Carrasco-Nunez, 2004; Cano-Cruz and Carrasco-Nunez, 2008;Zimmer et al., 2010). Such tuff rings show similar morphological featuresto those of mafic monogenetic volcanic fields in intracontinental settings(Brooker et al., 1993; Zimmer et al., 2010). Silicic tuff rings are known inassociation with composite volcanoes as proximal pumice cones in sum-mit vents that are commonly associated with long-lasting eruptive

fax: +64 6 3505632.).

rights reserved.

episodes froma central vent (Riedel et al., 2003). Silicic tuff rings are com-monly the source of silicic lava flows, coules and/or domes, that are typi-cally confined at the low rim of the tuff ring (for example, the AcigölRhyolitic Complex in Anatolia (Druitt et al., 1995)). Commonly such felsiclava dominated tuff rings form isolated volcanic complexes such as atCerro Pizzaro in Mexico (Riggs and Carrasco-Nunez, 2004). Felsic tuffrings are also known to be associated with erosion resistant enigmaticwelded lava spatter fed cones such as the Corral de Coquena rhyoliticspatter ring in northern Chile (Self et al., 2008).

Mafic tuff ring tephra rims are typically dominated by pyroclasticdeposits that are laterally restricted, thin and commonly show textur-al features characteristic of magma/water explosive interaction inshallow subsurface environments (Brand and Clarke, 2009); depositsare characteristically dominated by glassy pyroclasts and accidentallithic fragments are relatively minor (Dellino, 2000; Buttner et al.,2002). Pyroclastic deposits are cross-bedded, stratified to well bed-ded, fine grained and moderately sorted, characteristics that are typ-ical of deposition from laterally moving pyroclastic density currents(Sohn, 1996). Generally the eruptive products of tuff rings are lateral-ly restricted but there are exceptions that represent eruptive phasescapable of producing sub-Plinian to Plinian styles that form laterallyextensive and thick pyroclastic fall and flow deposits (Van derBogaard and Schmincke, 1984; Schumacher and Schmincke, 1990;

http://dx.doi.org/10.1016/j.jvolgeores.2012.06.010

mailto:[email protected]

http://dx.doi.org/10.1016/j.jvolgeores.2012.06.010

http://www.sciencedirect.com/science/journal/03770273

34 K. Németh et al. / Journal of Volcanology and Geothermal Research 239–240 (2012) 33–48

Mastrolorenzo, 1994). Such eruptions are commonly associated withmagma from complex sources (Woerner and Schmincke, 1984;Woerner and Wright, 1984) and their eruption can result in signifi-cant environmental effects similar to those from long‐lived, compos-ite volcanoes (Schmincke et al., 1999).

The influence of mafic tuff rings on the evolution of a sedimentarybasin is relatively small, and their eruptive product preservation potentialis minimal in comparison with long-lived composite volcanoes (White,1990; White, 1991; Manville et al., 2009). Mafic tuff rings commonlyshowadvance erosional features along their rim shortly after their forma-tion (Németh and Cronin, 2007). A similar record is expected from silicictuff rings, unless the eruptive sequence of such volcanoes includeswelded ignimbrite units, capping lava spatter deposits and/or the volcanoerupted in an arid environment where erosion processes act in a slowerfashion than in humid climate. However, the influence of lava dome-dominated volcanismon background sedimentation can be characteristicas recorded from a mid-Tertiary lava dome field in Arizona (Riggs et al.,1997).

In this paper we describe a small-volume silicic tuff ring that iserupted away (over 10 km) from other volcanic complexes and formsa well-preserved isolated volcanic landform. Los Loros is a tuff ring thatis typical in its geometrical parameters to other tuff rings and eruptedover a volcaniclastic succession that had accumulated in an intra-continental basin influenced by composite silicic arc-type magma. Herewe describe the sedimentary facies architecture of the Los Loros volcanoand its interplay with the background braided river sedimentation, andinfer its eruption history.

2. Geological setting

Los Loros is located in the Colorado River valley between Neuquénand Mendoza Province in western Argentina (Fig. 1). It is part of theintraplate volcanic system that has developed behind the active vol-canic arc in the Andean cordillera, a few hundred kilometres to theeast of the currently active volcanic front. The area around Los Lorosis characterised by two well distinguished morphotectonic elements,1) the Neuquén Basin and 2) Tertiary to Holocene volcanic centres(Fig. 1).

2.1. Neuquén Basin and the pre-volcanic rocks of the Neuquén Group

Sedimentation in the Neuquén Basin began in the Jurassic and wasactive through the Cenozoic. The basin formed gradually and its evo-lution stopped with the development of the Andean magmatic arc(Franzese and Spalletti, 2001). The accumulated thick and complexsedimentary succession of the basin has been intercalated with erup-tive products of the Upper Cenozoic volcanism augmenting an accu-mulation of more than 3500 m of sediments. The Los Loros eruptiveproducts described here are accumulated discordantly as one of theupper sedimentary unit of the Neuquen Group.

The Neuquén Groupwas initially described in the early 20th century(Doering, 1882; Ameghino, 1906). Its stratigraphic units and the geo-logical key to their identification developed shortly after (Roll, 1939;Groeber, 1946; Herrero Ducloux, 1946) together with its lateral exten-sion (Cazau and Uliana, 1973; Legarreta and Gulisano, 1989; Legarretaet al., 1993). The Neuquén Group has been divided into the Río Limay,Río Neuquén and Río Colorado Subgroups, characterised by alternatingsequences of sandstones, mudstones and conglomerates accumulatedfrom continental alluvial processes (Cazau and Uliana, 1973; Legarretaand Gulisano, 1989; Condat et al., 1990; Cruz, 1993). These deposits de-fine cycles of ephemeral river systems, draining a disconnectedendorheic basin from the Pacific Ocean (Legarreta and Uliana, 1999).The beginning of deposition in the basin coincides with the age of the“Intercretácica” unconformity (97±3 Ma, Early Cenomanian), whilethe upper limit of the Group is determined by the “Huantráiquica” un-conformity (74±3 Ma).

2.2. Miocene to Recent volcanism and volcanic centres in the region

The Southern Volcanic Zone of the Andes has a voluminous Quater-nary basaltic province along the retroarc (a type of back-arc basinwhichis floored by continental crust). The main sediments are fluvial, deltaic,or marine, derived from the uplifted area behind the arc. The PayeniaVolcanic Province (Fig. 1) covers an area larger than 40,000 km2 be-tween 33°30′ and 38° South latitudes, with an estimated eruptedmagma volume of about 8387 km3 producing more than 800 volcaniccentres in the last ~2 Ma (Ramos and Folguera, 2011; Llambías et al.,2010). This mainly basaltic province is subdivided into three segments:1) the northern segment has isolated minor monogenetic fields; 2) thecentral segment presents large volcanic fields with extensive lava flowscharacterised by Cerro Nevado, Llancanelo and Payún Matru volcanicfields (Fig. 1); and 3) the southern segment shows some large volcanoeswith less extensive lava flows characterised by the Tromen and AucaMahuida volcanic fields (Fig. 1) (Ramos and Folguera, 2011).

3. Volcanic framework of Los Loros tuff ring

Los Loros is located about 20 km south of a major back-arc volcaniccomplex, the Chachahuén Volcanic Complex (Fig. 1) (500 kmeast of themodern trench) a nested caldera complex belonging to the Payenia Vol-canic Complex of (Ramos and Folguera, 2011) located in the intersec-tion of a NW–SE and a NE–SW-trending fault system (Kay et al.,2006a). The Chachahuén Volcanic Complex is the only Late Miocenevolcanic centre in the central part of the Neuquén Basin. The evolutionfrom ca.7.6 Ma to 4.8 Ma of the Chachahuén Volcanic Complex (Kay etal., 2006b) involved orthopyroxene-bearing andesitic to rhyodaciticmagmas with an intraplate to arc-like chemical affinity as well as basal-tic andesite to hornblende-bearing dacite magmas with an arc-likechemistry (Kay et al., 2006a,b).

The Payún Matrú caldera is located a few tens of kilometres to thenorth of Chachahuén Complex (Fig. 1). K–Ar radiometric dating showsthat Payún Matru has been built since latest Pleistocene; ages rangefrom 280±5 ka to 7±1 ka (Germa et al., 2010). The main edifice is aHawaiian shield-like volcano that formed in a small number of explo-sive dome collapse eruptions. The original edifice of Payún Matru waspartially destroyed by an explosionwhich formed a 8×6.5 kmwide cal-dera, dated at around 168±4 ka (Germa et al., 2010), and is related tothe eruption of the widespread andesitic to trachyandesitic welded ig-nimbrite covering an area more than 2200 km2 and reaching a distanceof 60 km from the vent (Llambías, 1966).

Just 62 km to the south of Los Loros volcano a large basaltic shieldvolcano, the Auca Mahuida volcano (Fig. 1) stands about 100 km eastof the orogenic front with a well-preserved central caldera (1500 m indiameter) and an andesitic plug with coulees (Llambías et al., 2010).To the west of Los Loros the Tromen volcanic plateau (Fig. 1) was re-cently mapped in detail (Kay et al., 2006a; Galland et al., 2007;Folguera et al., 2008) (Fig. 1). Geochemical studies have shown thatmost of the sequence that forms the Tromen volcanic plateau comesfrom typical intraplate magmas, with no or very limited influence ofsubducting slab. The Tromen volcanic field developed an extensive ba-saltic lava plateau with ages ranging from 1.8 to 1.6 Ma that supportmonogenetic volcanoes and the younger Tromen volcano (elevationof 3969m asl). The intraplate lavas of the Tromen stratovolcano rangein age from 2.27±0.1 Ma to 0.04±0.04 Ma (Ar–Ar) showing a longlasting volcanic activity.

4. Los Loros tuff ring

The Los Loros tuff ring belongs to the youngest volcanoes of thePayenia Volcanic Province (Fig. 1) but it is characteristically differentfrom other volcanoes of the Payenia Volcanic Province such as the vo-luminous volcanic edifice of Auca Mahuida Volcanic Shield (Fig. 1)

Fig. 1. Simplified geological map of southern Mendoza, Argentina with distinguished volcanic zones.

35K. Németh et al. / Journal of Volcanology and Geothermal Research 239–240 (2012) 33–48

which is the most similar in age and composition to Los Loros(Llambías et al., 2010).

Los Loros is a symmetric, circular volcanic depression (Fig. 2A, Band C). The preserved crater has a steep inner wall and gently out-ward sloping rim (Figs. 2A and 3). The rim to rim crater diameter isabout 1000 m, forming a perfectly circular basin (Fig. 2A, B). Thedeepest point of the crater floor is about 870 m asl. The crater flooris relatively flat reaching to about 895 m asl in the inner crater wallmargin (Fig. 2B). The crater rim reaches about 960 m asl in thenorth, while its lowest point in the east is about 900 m asl (Figs. 2Aand 3). The crater of Los Loros has not been breached, and it has likelyfunctioned as a terrestrial sedimentary trap since its formation. Thecurrent crater dimensions can be regarded as minimum craterdepth values. It is important to note, that the inner crater wall ofLos Loros is steep in its northern sector while smooth, but still steeply

dipping inner wall inclination values are evident in the southern andwestern sectors (Figs. 2A and 3).

The top of Los Loros volcano has a characteristically smooth-surfaced proximal form while the distal parts of the volcano have adendritic gully network radiating from the crater (Figs. 2A and 3).About 150 to 400 m from the crater rim the outer gentle slopingrim is smooth with only small and shallow gullies (Figs. 2A and 3).Beyond this, up to about 1000 m from the crater rim gullies aredeeper and more mature (Figs. 2A and 3). Further away, two well-developed fluvial valleys encircle a radial gully network centred to-ward the crater (Figs. 2A and 3). A NE–SW trending fluvial networkis interrupted by the superimposed Los Loros centred gully system(Figs. 2A and 3). The NE–SW trending fluvial network coincideswith the orientation of modern stream valleys indicating relativelong term stability of the drainage pattern of the region. These

A

B

Fig. 2. Los Loros maps featuring major morphological and geological units. A) GoogleEarth satellite image of Los Loros (inset map shows the location of Los Loros) shows the locations(numbers) where measured stratigraphic columns were taken (as shown on Fig. 4). Dashed linemarks the extent of volcanic products of Los Loros bounded by active drainage network.Arrows point to a topographic step marked by slight changes in drainage pattern in the flank of the volcano. B) Geological map produced by field survey of Los Loros volcano.


contrasting drainage patterns suggest that at least the uppermost vol-canic rock units forming the Los Loros edifice and its preserved mor-phological height formed over an older NE–SW trending fluvialnetwork.

From a morphological point of view Los Loros is a unique volcanothat “mimics” a tuff ring in its morphology and geometrical parame-ters however, its eruptive sequence is more typical of eruptions asso-ciated with caldera-forming eruptions producing welded ignimbritesand moderate subsidence.

5. Volcanic stratigraphy

5.1. Basal volcaniclastic conglomerate (c1)

The Los Loros volcanic succession forms a ~100 m thick pile restingon Cretaceous continental red beds (Neuquén Group) (Fig. 2B). Thebase of the volcanic succession is exposed in deep gullies mostly in

the SW side of the volcanic edifice, about 1000 m away from the craterrim (Fig. 2B). The base of the volcaniclastic succession is exposed in onlyfew places such as the LosMuchachos Gully (Section 1 on Fig. 2B and 4),Avila Gully (Section 2 on Figs. 2B and 4) and in the East Gully (Section 3on Figs. 2B and 4). The facies relationship has been defined bymeasuredstratigraphic sections from these three main observation points (Fig. 4).

The base of the volcaniclastic succession is sharp on the NeuquenGroup red beds, at an elevation of about 830 m asl in the Los Mucha-chos Gully, 760 m asl in the Avila Gully and 730 m asl in the East Gully(Fig. 4). The elevation difference reflects a gently dipping topographicsurface on which the volcaniclastic sedimentation was initiated.

The base of the section at each location is marked by a polymictvolcaniclastic conglomerate with variable thickness ranging fromabout a metre to 5 m (Figs. 4 and 5A). The occasional cross stratifica-tion together with the undulating coarse-grained gravelly and lateral-ly discontinuous beds, indicate a channel-filling nature and an originin a braided river system (Fig. 5A). Similar textures are characteristic

Fig. 3. Aerial view of the Los Loros volcano. Avila and Los Muchachos gullies are shown on the view as the locations of the major measured stratigraphic sections.


of other identified conglomerate beds separating major volcanic units(Fig. 5B). The clasts in the basal volcaniclastic conglomerate (con-glomerate 1) are dominantly rounded gravel sized coherent interme-diate volcanic rock types, minor basalt, moderately to highly vesicularpumiceous clasts and variously altered lava rocks (Fig. 5A). The con-glomerates are fine to coarse sand, matrix supported (Fig. 5A) and lat-erally discontinuous, forming coarse grained channels as well as thinsandy volcaniclastic levee deposits. The diversity of the clasts sug-gests that they have been transported from a nearby Miocene back-arc volcanic complex, the Chachahuén Complex (~40 km), which isa lava dome dominated multiple volcano with thick silicic pyroclasticsuccessions (Kay et al., 2006b).

5.2. Lower tuff ring succession (Tuff ring 1)

The basal polymict volcaniclastic conglomerate is covered by a pu-miceous pyroclastic unit thickening toward the SSE (Fig. 2B). The baseof this unit is matrix supported, moderately to poorly sorted, pumice-rich, stratified beds forming a unit ranging from a few decimetres to2 m in thickness (Figs. 2B, 4 and 6A). These fine grained, stratifiedto cross stratified units are pumice rich and commonly exhibit scourfilling to various lithic clast trains (Figs. 4 and 6A). Their grey colouris characteristic as is their common vesicular texture. The texture ofthese pyroclastic beds indicates their density current origin which issupported by the common scour filling, clast trains and cross stratifi-cation. The vesicular nature and the presence of some ambiguousrounded fine ash balls (likely accretionary lapilli) indicate the pres-ence of condensed water in the depositional system that is typicalfor phreatomagmatic explosive eruptions.

These basal, matrix-supported beds are overlain by a pyroclasticsuccession that is composed of loosely packed rounded pumice lapilli(Fig. 6B). Chemical analyses of pumice lapilli indicate their rhyoliticcomposition (Table 1, Fig. 7). This pumiceous lapilli bed forms aunit up to 3 m in thickness and clearly demonstrates a SE-trendingdistributional pattern (Fig. 6B). These pumiceous beds are restrictedto the area SW to SE of the Los Loros edifice, suggesting its localsource. The beds have a well-sorted texture with no characteristic in-ternal stratification, indicating that they are pyroclastic fall in origin

(Fig. 6B). The bed thickness variation is inferred to reflect the paleo-wind direction toward the SE.

A thick pile of bedded pumiceous tuff (up to 20 m) overlies thebasal pumice fall unit (Fig. 6C and D). This unit is similar to those for-ming the basal pyroclastic succession and below the pumiceous fallbeds. These tuffs are sharply overlain by the pumiceous fall unitwith an apparent erosional contact. It shows a similar distributionalpattern to that of the underlying pumice fall deposits which form athicker unit along a SE-trending axis. This pumiceous pyroclastic suc-cession is stratified, cross-bedded and has erosional contacts with theunderlying pumice fall beds which suggests deposition from dilutepyroclastic density currents (Fig. 6C and D). At least three major pum-ice lapilli dominated well sorted units have been identified in this pu-miceous succession, each of them is about a few decimetres thick.Lateral correlation of these is difficult due to landslide covers and de-sert weathering features which prevent the accessibility of the clifffaces (Fig. 6C and D). Chemical analysis of large pumice lapilli indi-cates its phonolitic composition (Table 1, Fig. 7).

The particle micromorphology indicates that the pumice ash of thisunit is variable in vesicularity (Fig. 8A to H). The coarser grained fractionsare rich in tube and woody pumice ash (Fig. 8A), while the finer grainedfractions are richer in dark glassy ash and cuspate shaped glass shards(Fig. 8B, C and D). Dark, angular moderately vesicular ash is mixed withlight (commonly white) coloured, transparent highly vesicular pumiceash (Fig. 8C and D). These pyroclasts are commonly rich in tube vesicles(Fig. 8E). Cuspate shaped glassy shards are also common (Fig. 8F), manyof them exhibiting a colour transition from brown to transparent. Thepumice ash has sharp outlines, and shows limited mechanical abrasionsuggesting its primary pyroclastic origin. Blocky, darker coloured glassyfragments with moderate vesicularity and thick glass shard walls(Fig. 8G and H) indicate the limited role of external-water drivenphreatomagmatic fragmentation. The analysed pumice lapilli are moder-ately to highly vesicular, however, their colour is commonly creamy. Theirvesicles are commonly irregular in shape and their sizes vary greatly in asingle pyroclast. These pumiceousmatrix-supported tuff and fine grainedlapilli beds are juvenile pyroclast-rich, however, hydrothermally alteredexotic volcanic lithic fragments or rounded and altered pumice lapillican be up to about 20 vol.% (by visual estimates).

Fig. 4. Measured stratigraphic columns of the Los Loros volcano.


The tuff ring succession (Tuff ring 1) was accumulated over thebasal volcaniclastic conglomerate (Fig. 4). It is inferred to representa primary pyroclastic succession that is part of an original pumiceousphonolitic tuff ring that formed over a braided river network (basalconglomerate over Neuquen Group deposits). The initial cross bed-ded, matrix-supported beds indicate a potential phreatomagmatic ex-plosive eruption that opened the vent in the braided river system andwas followed by a major pumiceous pyroclastic fall event. Subse-quently, explosive eruptions producing pyroclastic density currentsand associated minor pyroclastic fall layers built a pumiceous tuffring, more or less centred around the preserved crater of Los Loros.

5.3. Intermediate paleosol and volcaniclastic conglomerate (c2)

The basal pyroclastic succession (lower tuff ring unit— Tuff ring 1)is covered by immature, but thick (dm-to-m) pelitic paleosol and/orchannel-filling volcanic conglomerates (Figs. 4 and 5B), suggesting a

significant time break (tens of thousands of years), erosion and land-scape resetting by fluvial networks after the initial pumice ring (Tuffring 1) was formed.

A volcanic conglomerate covers the eastern sector of the lowerslopes of Los Loros forming lenses over the immature paleosol. This con-glomerate is mainly composed of dark coloured moderately roundedvolcanic gravels, suggesting long-lasting alluvial deposition followingfirst stage of volcanism at Los Loros (Fig. 5B). These gravels mighthave been transported from intermittent mafic volcanic vents erodedfrom unknown sources, probably the nearby Chachahuén Complex.

5.4. Upper tuff ring succession (Tuff ring 2)

The rejuvenation of the Los Loros vent is represented by a thick(~40 m) pyroclastic deposit and lava capping unit overlying the im-mature paleosol. At least three units of pyroclastic breccia can be dis-tinguished on the basis of their welding textures and pumice-to-lithic

image of Fig.�4

A B

Fig. 5. Outcrop view of volcanic conglomerate units of the Los Loros volcano. On “A” conglomerate 1 is shown while on “B” conglomerate 2 is visible with distinct lensoid shape zonesforming characteristic strata (arrows) beneath the welded pyroclastic succession.


ratios (Fig. 9A). An increasing welding texture is apparent toward thetop of the section (Fig. 9A, B). In the basal pumiceous unit lapilli areangular to rounded and vesiculated (Fig. 9C).

In the base of this section the pumice lapilli have a rhyolitic compo-sition similar to those of the pumiceous fall deposits from the base ofthe lower (Tuff ring 1) tuff ring units (Table 1, Fig. 7). Pumiceous lapilliform a succession dominated by pumice lapilli hosted in a small volumefine ash matrix (Fig. 9B and C). The texture of the pumice lapilli showssimilarities to textures observed in the basal pumiceous fall succession.The pumice rich lapilli beds are moderately to well-sorted indicating apyroclastic fall origin likely from a local source. This pumiceous succes-sion laterally changes its thickness aswell as gradually changes laterallyinto a more matrix-supported lapilli tuff that contains large exoticvolcanic lithic as well as altered pumice lapilli. Higher in the sectionthe pumice lapilli show bed-specific flattening, that turns into an in-creasingly welded texture in the capping pyroclastic unit and formsan erosion-resistant capping unit up to 15 m in thickness, preservingthe volcanic morphology of Los Loros (Fig. 9D).

Pumice lapilli have rugged outlines and commonly show randomvariations in vesicularity in the base of the sequence (Fig. 9B and C).The outer margin of complex shaped pumice lapilli are commonly chil-led, and show fractural textures typical for contraction of hot materialupon contact with coolant and can be seen as cauliflower shaped lapilli(Fig. 9B). The matrix of the basal lapilli tuff to tuff breccia unit is finegrained and composed primarily of juvenile pyroclasts and pyrogenicminerals. Large lithic fragments commonly form clusters (Fig. 9B).Rounded gravelly volcanic lithics commonly form alignments and orlaterally discontinuous strings (Fig. 9B). The lithic clasts are rounded(Fig. 9B) and composed of various volcanic rocks ranging from basaltto intermediate compositions similar to rock types recorded from thebasal conglomerate facies deposited over the Neuquén Group. The lithicfragments are thermally slightly altered in the base of the unit, while astrong thermal effect on lithic clasts is increasingly evident up-section(Fig. 9D). Up-section, stratification becomes gradually more prominentand defined by flattened pumice lapilli (Fig. 9D). Original vesicle shapes

are flattened, but the outline of the pumice lapilli is still intact andrecognisable. Thematrix of thismiddle unit is well-packed and in placesforms hard rocks. Larger lithic fragments are apparent, and have exten-sive thermal effects on their outer rims. In the upper section, following agradual contact a more stratified, dark pyroclastic unit forms a hardcover. It is composed of stratified beds defined by dark, completelyflattened pumice clasts, which are compressed and stretched overdm-to-m scale forming typical fiamme textures. Lithic fragments canstill be recognised as light coloured fragments. Between major fiamme-bearing zones are lithic fragment-dominated units that form a well-stratified texture.

The topmost volcanic unit of Los Loros is a lava flow (5–8 m thick)of intermediate composition, which is well-preserved in the easternpart of the volcano (Figs. 2B and 10A). It is inferred that such lavaflow or at least thin lobes of it could have covered most of the easternsector of the volcano based on the drainage pattern.

The gradual and vertical transition from stratified lithic-rich pumi-ceous beds to welded pyroclastic units indicates that these units areformed from pumiceous pyroclastic density currents, e.g. small-volume intermediate ignimbrites (Freundt et al., 2000; Allen, 2004).The gradual increase in the abundance of pumiceous clasts togetherwith the increased number of large elongated fiamme indicates an in-crease of hotmagma feeding the eruption. The large number of volcaniclithics in the basal portion of the section with well-rounded shape sug-gests that they were already roundedwhen they were erupted througha vent. Their likely origin is from the underlying braided river gravelbeds that the eruption has punctured. Pumiceous clasts with chilledmargin closely resembling cauliflower bombs in the base of the sectionindicate some sort of fast cooling, potentially by external water such asstream water in the braided river and shallow ground water in thegravel beds (Vespermann and Schmincke, 2000). High magmatic fluxevidenced by the increasing signs of high temperature emplacementof pyroclasts in the upper part of the pyroclastic section indicates agradual increase of magmatic fragmentation of magma in the courseof the eruption and growth of the tuff ring 2.

Tuff ring 2 - Rhyolitic welded tuff

Tuff ring 1 - Basal phonolitic pumiceous pyroclastic rocks

Neuquen Group

Colorado River

Benmoreite lava flow

20 m

20 m

Tuff ring 1

Tuff ring 2

A B

C D

20 m

Tuff ring 2

Tuff ring 1

Neuquen Group

Fig. 6. A) Stratigraphic overview of the distal section of Los Loros volcano. Dashed line marks the contact between the underlying Neuquén Group sedimentary successions and thevolcanic edifice. Tuff rings 1 and 2 can easily be distinguished by the surface texture of the outcrops. B) Pumice fall units forming thick pile of deposits in the SW of the vent. C)Outcrop-scale view of the relationship between rock units of the Tuff rings 1 and 2. Note the hard, erosion resistant appearance of the rocks of Tuff ring 2. D) Exposed contact be-tween the underlying Neuquén Group sedimentary rocks and the Los Loros volcanic succession. Compare this image with the view shown on “C”.


The welded pyroclastic succession was subsequently covered bylava flows. The chemical composition of the capping lava flow indicatesits intermediate composition and fits into the basaltic–phonolitic intra-plate lineage (Table 1, Fig. 7). It is inferred that the initial rhyoliticmagma probably interacted with the limited amount of external wateravailable, enhancing its explosive eruption (instead of lava dome forma-tion e.g.), which subsequently quickly turned into a purely magmaticfragmentation-dominated eruption forming an eruption column pro-viding pyroclasts which formed rhyolitic pyroclastic fall deposit. Thehigh rhyolite magma flux was responsible for the further developmentof the eruption and production of the pyroclastic density currents thatformed welded small-volume ignimbrite during the peak magma dis-charge. This eruptive phase created an openvent system that subsequent-ly provided a volcanic conduit that gradually tappedmafic magma froma different (deeper) source.

5.5. Capping volcaniclastic conglomerate (c3)

The lower flank of the Los Loros volcano is covered by volcaniclasticfluvial gravels a few decimetres to a few metres thick (Fig. 2B, “c3” inReferences). The gravels are composed of a variety of volcanic lithicfragments similar to those in the basal conglomerate units. There is notextural difference, nor compositional variation between this cappingunit and the basal conglomerate units other than the presence of fewless rounded volcanic lithic gravels that were apparently derived fromthe eroding Los Loros volcano. The majority of the gravelly clasts aretexturally indistinguishable from gravels preserved in the other con-glomerate units.

Interestingly this gravel (c3) sheet andhigher in theflank of Los Lorosthe capping benmoreitic lava flow is covered by amonomict conglomer-ate/breccia (“vd” in Fig. 2B) seemingly forming a volcanic bomb, blockand lapilli sheet in the upper flank of the preserved Los Loros edificeand clearly derived from local sources. The fragments of this cappingmonomict conglomerate are sub-rounded to sub-angular, moderatelyvesicular, and benmoreitic in composition. The monomict capping unitis estimated to be not thicker than ametre on the basis of the distributionpattern of the cover. There is no gully exposing this capping monomictconglomerate to enable us to interpret its origin. The monomict natureand the variety of angularity of the deposit however could reflect a singleblock-and-ash flow episode, which disrupted a benmoreitic dome in thecrater of Los Loros. This interpretation is favoured over fluvial origin, dueto the general high elevationwhere this cover can be located and its vol-canic flank-mantling nature.

6. Volcanic facies architecture

At Los Loros, the eruptive sequence preserves an initial pumice ringformation on an active alluvial plain (Fig. 10A). The braided rivers sam-pled volcanic debris from the surrounding volcanic terrains, likely fromthe nearby Chachahuén volcanic massif. As a result a braided river net-work formedwith volcanic lithic-rich gravelly bars andfine-grainedfluvi-al volcanic sands. These rocks form the basal few metres thick section ofthe Los Loros volcano. The 3D facies architecture of Los Loros can bereconstructed on the basis of the partially exhumed surfaces and clifffaces located in its southern sector (Fig. 10B). It is clear that theunderlyingNeuquén Group continental red beds exposed beneath a volcaniclasticsuccession (Fig. 10B) allow the observation of proximal to distal volcanic

Table 1Chemical analyses of Los Loros samples.

Sample LL5 LL3-a LL3-b LL2-a LL2-d LL2-c LL2-b LL4

wt.%SiO2 52.99 58.31 58.77 63.66 64.24 66.02 66.54 73.86TiO2 1.24 0.54 0.62 0.46 0.51 0.48 0.46 0.13Al2O3 17.48 18.89 18.69 13.36 13.93 14.05 14.10 12.79Fe2O3 7.05 3.21 3.78 3.00 3.44 3.19 3.02 1.27MnO 0.13 0.14 0.14 0.07 0.08 0.07 0.07 0.02MgO 1.59 0.29 0.47 0.95 1.12 1.04 0.94 2.00CaO 6.78 1.49 1.73 4.25 3.27 2.73 2.54 0.86Na2O 5.47 6.45 6.09 3.50 3.64 3.96 3.92 1.88K2O 3.57 6.94 6.94 3.68 3.51 3.58 3.68 2.95P2O5 0.55 0.07 0.10 0.12 0.12 0.12 0.11 0.03H2O 0.99 0.76 0.46 1.17 1.68 1.38 1.15 1.09Total 99.45 99.48 99.16 97.94 98.47 99.31 99.04 99.35

ppmBe 4.68 3.92 3.20 1.89 2.06 2.21 2.06 1.87Cs 2.59 1.68 1.15 4.58 4.25 4.66 4.64 2.65Ba 430.62 69.44 292.03 532.25 495.46 526.64 543.71 437.15Rb 79.15 161.91 138.02 135.32 131.67 139.58 140.80 89.67Sr 1616.13 66.72 164.20 214.79 196.59 202.44 200.08 280.83Pb 10.34 14.43 12.22 17.17 15.81 17.48 18.95 12.05Th 11.15 24.08 19.29 18.62 17.27 18.70 19.17 16.25U 3.48 4.90 3.79 4.76 4.03 4.65 4.72 1.80Zr 379.94 607.18 500.84 195.40 199.53 199.54 201.41 108.72Nb 60.04 175.63 137.17 8.45 8.40 9.02 9.00 11.47Hf 8.10 12.35 10.23 5.56 5.69 5.54 5.67 3.45Ta 4.62 10.97 8.63 0.86 0.80 0.89 0.92 0.93Y 19.50 23.25 19.61 21.50 21.30 21.93 21.55 14.64Sc 7.27 2.15 2.59 8.70 10.09 9.29 8.59 4.49V 108 9 14 54 68 64 59 7Cr 10.36 5.57 7.11 11.96 9.91 11.00 8.98 6.56Co 15.76 23.81 15.85 15.81 36.18 36.63 33.27 10.25Ni 10.89 3.67 3.99 6.24 5.48 5.17 4.27 4.77Cu 22.43 11.78 9.45 14.42 14.24 14.89 13.63 6.72Zn 82.44 68.21 66.14 57.91 55.63 67.13 63.43 35.02Ga 21.28 22.38 20.54 13.64 13.96 13.99 14.10 13.55La 41.29 94.26 81.10 26.36 25.64 26.55 27.06 41.16Ce 71.28 153.40 129.32 51.36 50.02 52.95 53.21 75.02Pr 7.79 13.94 11.88 5.91 5.75 5.97 6.01 7.84Nd 28.78 44.87 38.93 22.33 22.30 23.27 22.70 27.63Sm 4.69 6.50 5.38 4.17 3.99 4.44 4.12 4.24Eu 1.50 1.47 1.60 0.75 0.79 0.80 0.76 0.52Gd 4.40 5.12 3.95 3.45 3.64 3.51 3.83 3.45Tb 0.66 0.73 0.63 0.59 0.56 0.61 0.59 0.49Dy 3.92 4.29 3.63 3.70 3.74 3.85 3.61 3.00Ho 0.72 0.80 0.69 0.74 0.75 0.77 0.78 0.48Er 2.03 2.39 2.30 2.32 2.34 2.31 2.47 1.55Tm 0.35 0.39 0.34 0.37 0.39 0.37 0.34 0.21Yb 2.30 2.85 2.36 2.84 2.66 2.59 2.57 1.39Lu 0.29 0.41 0.33 0.39 0.38 0.39 0.40 0.21

LOI: loss of ignition; Fe2O3=total FeO.

35 40 45 50 55 60 65 70 75

0

2

4

6

8

10

12

14

16

Dacite

AndesiteBasalticAndesite

Basalt

Trachy-Andesite

Rhyolite

Trachyte

Phonolite

Phono-tephrite

Tephrite

Basanite

Benmoreite

Mugearite

Hawaiite

PhonoliticNephelinite

Nephelinite

SiO2

(Na2

0+K

2O) Ll2 - pumice fall

Ll4 - pumicebelow welded unit

Ll3 - pumiceous middle section

LL5 - Lava cap

Fig. 7. Total alkalis vs SiO2 plot of the Los Loros samples. Symbols as follows: opensquares basal pumice unit (LL2), solid circles pumiceous tuff (LL3), solid square baseof the welded unit (LL4), open circle top lava flow. Nomenclature from Cox et al.(1979). Analyses recalculated loss free.


facies of a polygenetic small volume volcano (Fig. 10B). It is clear on theexhumed surfaces, that fluvial volcaniclastic deposits cover the NeuquénGroup terrestrial beds (Fig. 10B). It is also evident, that similar gravellybeds form laterally discontinuous, lensoid volcaniclastic units over thebasal tuff ring (Fig. 10C) indicating a long-lasting break between the erup-tion of the basal and upper tuff ring.Welded pyroclastic rocks accumulat-ed during the formation of the upper tuff ring (Fig. 10D). A theoreticalcross section has been created to illustrate the facies architecture of LosLoros (Fig. 11) in the light of the interplay of two distinct tuff ring formingevents in an active braided river system.

At the onset of the eruption, rising magma was likely influenced bythe limited available external water, that enhanced explosive magma–water interaction opening the initial vent that formed Tuff ring 1(Fig. 11).

This eruption then culminated in an event which formed a rhyolit-ic (Fig. 12) pumice fall cover (Fig. 11). Shortly after this event (hoursto days), a renewed eruption triggered by phonolitic magma (Fig. 7)formed the main section of Tuff ring 1 preserved at Los Loros(Fig. 11). In this stage, moderate magma and water interaction is

recorded in the presence of blocky, low vesicularity glass shards incross-bedded, dune-bedded tuff beds, however, the eruption primar-ily functioned as a magmatically fragmenting eruption as indicated bythe presence of mainly vesicular pumiceous ash and shards.

The significant time gap between the basal and capping primary vol-canic units as inferred from the presence of a thick paleosol and laterallydiscontinuous channelized gravelly volcanic lithic conglomerate beds(conglomerate 2) suggests a long-lasting inter-eruptive period priorto resumption of volcanic activity, forming small-volume, low aspectratio welded pyroclastic density current deposits and capping lavaflows as part of Tuff ring 2 (Fig. 11). In this stage the eruption startedas a rhyolitic pyroclastic pumice fall event that was subsequentlyinfluenced by the increasing input of hotter and larger volume ofmafic magmas that eventually formed a mafic lava flow cap in the latestage of the eruption (Fig. 11).

Los Loros is a unique volcano in the sense that it “mimics” a tuffring in its morphology and geometrical parameters; however, itseruptive sequence is more typical of those eruptions associated withlarge-volume silicic composition volcanoes with significant inter-eruptive periods.

7. Age and chemical signatures of volcanic products of Los Lorostuff ring

To establish the age of the Los Loros volcanic edifice K–Ar radio-metric age determination was performed on juvenile pyroclasts andcoherent volcanic rock samples in the Hungarian Academy of Sci-ences' Institute of Nuclear Researches (ATOMKI) laboratory in Debre-cen, Hungary. Sample preparation and analytical procedures followedthe laboratory standard routine as reported earlier in key papers(Balogh et al., 1986; Pécskay et al., 1995; Balogh et al., 1996; Baloghand Németh, 2005; Balogh et al., 2010). Whole rock analysis on indi-vidual pumiceous lapilli from the basal pumiceous rhyolite fall bedprovided inconclusive data with high error margin of 2.23+/−1.56 My.The errorwas a likely result of the hydrated nature of the samples. How-ever, two measurements on the capping intermediate composition lavaflows has provided good results of 1.16+/−0.22 and 1.14+/−0.11 My.These numbers are in good agreement with each other andwe can con-clude that Los Loros volcano's upper tuff ring is older than generallythought before. The presence of paleosols and volcaniclastic gravellybeds between the upper (Tuff ring 2) and lower (Tuff ring 1) tuff ringunits indicates that the Tuff ring 1 of Los Loros could be significantlyolder than the Tuff ring 2. This is partially supported also by the available

LL3 1 X5 4 0.5 mm

LL3 X0 1.25 2

2 mm 1 mm

LL3 X1 2.5 1

LL3 X1 5 3

0.5 mm

A

B

DC

E F

HG

500 micron

LL3 0 013

200 micron

50 micron

500 micron

LL3 0 003

LL3 2 006 LL3 1 015


20 cm

A B

C

30 cm

D

C

Fig. 9. Outcrop-scale textural features of the upper pyroclastic succession (Tuff ring 2) of the Los Loros volcano. A) Distal section of the moderately welded upper section of the LosLoros volcano. White field with “c” shows the field of view on image “C”. B) Base of the pumiceous upper tuff ring (tuff ring 2) with rounded gravels of lava rocks (arrows) in arugged, pumiceous clast-dominated succession (circle). C) Pumiceous lapilli in fine ash matrix forming the base of “Tuff ring 2” volcanic unit. Note the light and dark colour pumicelapilli with rugged surface texture and complex vesicularity. Chemical data from “Tuff ring 2” was derived from this unit. D) Welded units in the middle upper section of “Tuff ring2” volcanic unit. Dark layer-like features are fiamme, while angular to subrounded clasts are thermally strongly altered sedimentary rock fragments from the underlying NeuquénGroup sedimentary rocks.


K–Ar age data however this cannot be taken as a definitive conclusion atthis stage due to the K–Ar data's high error margin.

In table 1 we present major and trace element concentrations ineight representative samples collected from the stratigraphic unitsthatmake up Los Loros volcano. For the analytical work, clean rock frag-ments were crushed between tungsten carbide plates and a 100 g ali-quot ground to b200 mesh in a tungsten carbide ring grinder. Majorand trace element concentrationsweremeasured by X-rayfluorescence(Siemens SRS3000 spectrometer at University of Auckland) using stan-dard techniques on glass fusion discs prepared with SPECTRACHEM12–22 flux. For XRF major oxide analyses precision is generally betterthan1% (at 2 sd). Trace element datawere obtained by laser ablation in-ductively coupled plasma source mass spectrometry (LA-ICP-MS;School of Earth Sciences, Australian National University) using thefused glass discs prepared for XRF and an EXCIMER laser system, oper-ating in the ultra-violet spectrum at a wavelength of 193 nm (Eggins etal., 1998). A standard of NIST 612 was run after every 15 analyses forcalibration. Si concentrations obtained by XRFwere used as the internalstandard to correct for variation in ablation yield between samples and

Fig. 8. Microphotographs of pyroclastic deposits from the tuff ring 1 pumiceous, juvenile ash-richpumice ash (circles) (in “A”), while the fine fractions are abundant in dark, blocky glass shards (inshape shards (in “C”) or light (arrow) or dark (circle) colour blocky ones (in “D”). SEM images revdominantly blocky in shape (in “D”), exhibit abundant in surface features indicating brittle fragmeularity (in “H”) commonly with randomly grouped small number of vesicles (arrow in “H”).

calibration standards and the ‘matrix effect’ of variations betweencounts per second and ppm of different elements. During the courseof the analyses, BCR-2 glass standard was analysed (every 30 analyses)to provide an independent assessment of accuracy and precision.

The analysed samples are from the basal pumice unit (LL2— a,b,c),the pumiceous tuff (LL3 — a,b) of Tuff ring 1, the pumiceous base ofwelded unit (LL4) of Tuff ring 2 and the top lava flow (LL5). Interms of their general major element characteristics the sampledsuite falls into two distinct groups. The basal pumice unit and the pu-miceous base of the welded unit are rhyodacite and rhyolite, the pu-miceous tuff and the top lava flow are phonolite and benmoreite(Fig. 7). Each of these groups shows distinct major element character-istics and more significantly very different trace element patterns. Allare relatively depleted in compatible transition elements and the dif-ferences between the two groups are shown mainly in the abun-dances of incompatible LILE, HFSE and REE. Representative plots areshown in Fig. 12.

In terms of the REE abundances samples LL5 and LL3 show smoothnormalised patterns with light REE abundances correlates with SiO2

succession (LL3) of the Los Loros volcano. The coarse fraction of the samples is rich in tube“B”). Fine ash fractions are commonly host various glass shards such as transparent, cuspateealed fine tube vesicles on coarse pumice ash and fine lapilli (in “E” and “F”), while fine ash isntation such as step-like surface (circle in “G”), micro-cracks (arrow in “G”) and low in vesic-

A B

C Dwelded tuff brecciapart of tuff ring 2

lava flow

tuff ring 1 succession

conglomerate 2

crater

Neuquen Group

welded pyroclastic brecciaTuff ring 2

lava flow cap

conglomerate 2

conglomerate 1

west east

Neuquen Group

welded pyroclastic breccia - tuff ring 2

tuff ring 1

tuff ring 1

Los Loros crater

welded pyroclastic breccia - tuff ring 2

capping lava flow covered by monomict volcaniclastic conglomerate/brecciainferred to be generated by a final block-and-ash flow eruption

conglomerate 2

Fig. 10. Facies architecture of Los Loros volcano shown in key field views. A) Los Loros volcano looking toward the north. In this view the identified stratigraphic units are clearlyvisible. B) Neuquén Group sedimentary rocks are exhumed in the southern part of the Los Loros volcano. Proximal and distal volcanic units can be correlated due to the identifiedlayers' textural characteristics as well as their relative position. C) A major gully in the southern margin of the proximal area of the Los Loros volcano strongly controlled the accu-mulation of the tuff ring successions. The gully is also filled with channelised gravelly conglomerate as a sign that Los Loros likely erupted in an active stream valley. D) The pre-served crater of Los Loros volcano is broad and shallow, closely resembles a tuff ring crater.


content; there is no Eu anomaly. Samples LL2 and LL4 also show light/heavy fractionation patterns but LREE abundances are lower andthere is a distinct Eu anomaly that increases with SiO2 content.

There is also a contrast in trace element abundances as shown inthe lower part of Fig. 12. Samples LL2 and LL4 have a distinct negativeNb and Ta anomaly and are generally lower for most trace elements.

Neuquen Group

Erosional valley/escarpment

West

Palesol

Conglomerate 2Re-establishment of braided river system

after formation of tuff ring 1

Capping ben

TufRhyolitic welded

Conglomerate 1Basal conglomerate of

braided river system into Los Loros erupted

Tuff ring 1Rhyolitic pumice fall in base of section

Vesource o

Fig. 11. Reconstructed volcanic facies architecture of Los Loros volcano. Here we suggest to rcano erupted in an active fluvial system, that were re-established after the first volcanic ev

These are characteristics that may be attributed to magmatic systemslinked to subduction zone processes or more generally to crustal pro-cesses. This point is emphasised in Fig. 13. in which the Ta/Yb ratio ofthe LL2 and LL4 samples is markedly lower and plots in the field ofsubduction related and crustal compositions whereas the LL3 andLL5 samples plot along a mantle array in the field of within plate

Neuquen Group

Los Loros crater

East

Paleosol

Erosional valley/escarpment

moreitic lava

f ring 2pyroclastic flow units

Conglomerate 1Basal conglomerate of

braided river system into Los Loros erupted





nt 1f tuff ring 1

Vent 2source of tuff ring 2

Tuff ring 1Rhyolitic pumice fall in base of section

Volcanic debris covera product of a final block-and-ash flow

econstruct two closely spaced vents that operated a few thousand years apart. The vol-ent(s) as recorded in the presence of multiple fluvial gravel beds.

Fig. 12. Normalised trace 99 element plots for the Los Loros samples. Normalising factorsare from Sun and McDonough (1989) for REE and from McDonough and Sun (1995) forother trace elements. Symbols the same as for Fig. 7.


basalts. Fig. 13 is amodification of a discriminant diagramoriginally devel-oped for basaltic compositions (Pearce, 1982) and is therefore not strictlyapplicable here. Nonetheless the low Ta concentrations in the LL2 and LL4samples correlate with the Nb anomaly shown in Fig. 12 and clearly indi-cate an important difference in the magmatic processes that have givenrise to these samples relative to the LL3 and LL5 samples.

8. Discussion on magma petrogenesis

The number of samples analysed in this study is small and thechemical data can only be regarded as a reconnaissance. Neverthelessthe data reveal a significant finding that within this small eruptivecentre there are two distinct compositional groups. Interpretingtheir trace element abundances suggests that one group, representedby samples LL3 and LL5 have intraplate basaltic affinities althoughclearly they are evolved compositions. The second group representedby samples LL2 and LL4 show the trace element characteristics com-monly attributed tomagmas associatedwith subduction zoneprocessesalthough given their highly evolved compositions they are more likelylinked to evolution processes within the crust.

Fig. 13. Plot of Ta/Yb vs Th/Yb for the Los Loros samples. Diagram modified from thediscriminant diagram of Pearce (1982). Symbols the same as in Fig.7.

The intriguing feature of the Los Loros stratigraphy is the alterna-tion of magmatic suites of contrasting origin. The initiation of theeruption produced magma of crustal affinity; this was followed by amajor event that is logically associated with a magmatic system orig-inating as a within plate mantle derived basaltic association. In a sub-sequent major eruption event magma compositions returned to acrustal geochemical signature and the final event produced a maficcomposition linked to intraplate basaltic activity.

The alternation of magmatic styles at Los Loros is a singular featurethat casts new light onto the way that magmatic system can operatein situationswhere the crust is relatively thick. Although our data is lim-ited it does indicate the operation of two distinctmagmatic systems in asingle small scale and spatially overlapping volcanic edifice. This juxta-position indicates a causative relationship. If the ultimate heat sourcedriving the Los Loros system is represented by the intraplate suite(LL3 and LL5) then the crustal suite (LL2 and LL4) may be explainedby thermally induced crustal remobilisation. We envisage 4 stages:

(1) The intrusion of magma that is ultimately mantle related pro-motes melting of the crust to produce the basal unit.

(2) Following this highly evolved phonolitic magma stored in thecrust beneath the zone of crustal melting erupts as the pumi-ceous tuff.

(3) A second phase of crustal related magma is erupted as thewelded unit.

(4) The eruption ends with the eruption of more mafic magmarepresenting the fractionated product of the basaltic magmathat ultimately has driven the system.

The alternation of magmatic styles as shown by the contrasting geo-chemical signatures of the two eruptive suites at Los Loros is unusual.The alternation of contrasting geochemical signatures in the stratigraphicsuccession is remarkable. In this paper we have presented only a smallamount of geochemical data but the implications for how deep seatedmagmas rise and interact with the middle and upper crust are profound.It is possible that the Los Loros succession is unique, a more exciting sug-gestion is that it opens up a new chapter in our understanding of howmagmas interact with the crust in continental settings.

9. Discussion of eruption mechanisms and depositional systems

The textural characteristics of the pyroclasts in the studied volcanicunits together with their meso- and macro-scale architecture indicatesthat they were primarily formed bymagmatic explosive fragmentation.However, pyroclast textures indicate a minor role of magma–water in-teraction driven phreatomagmatic explosive events as well. This coin-cides well with the presence of volcanic conglomerates sandwichedbetween the primary volcanic units suggesting some resetting of thebraided river systems into which the Los Loros volcano erupted. Therole of phreatomagmatic explosive eruptions in the vent opening andclearing stage of small-volume silicic tuff ring-forming eruptions arewell-documented from both field studies and experimental volcanolo-gy (Austin-Erickson et al., 2008). Silicic tuff rings that producedphreatomagmatic eruptive phases are known from various geologicalsettings (Brooker et al., 1993; Brown et al., 2007; Zimmer et al., 2010).Many form clusters of silicic tuff rings and domes such as Cerro Pintoin the eastern Trans-Mexican Volcanic Belt (Zimmer et al., 2010). Suchsilicic tuff rings can be associated with maar basins such as the AcigölComplex in Central Turkey (Druitt et al., 1995; Kazanci et al., 1995).Chemical variations among individual tuff rings and associated maarssuch as the Acigöl Complex are evident inmany cases and reflect the ex-istence of various relatively small volume magma sources over thelifespan of such volcanic fields commonly associated with intra-continental settings (Druitt et al., 1995). The common eruptive volumesof small-volume silicic lava domes and associated tuff rings such as theCerro Pinto's volcanic edifices inMexico (0.04 km3 DRE) (Zimmer et al.,2010) are in the range expected inmafic monogenetic volcanoes, about

image of Fig.�13


0.05–0.001 km3DRE (White, 1991; Vespermann and Schmincke, 2000).This small eruptive volume in mafic monogenetic systems usuallyerupts within a short time, and in a single eruptive episode (Manvilleet al., 2009; Németh, 2010), however increasing evidence demonstratesthat mafic volcanoes can also behave in a far more complex way, pro-ducing eruptive products over multiple eruptive episodes (Brenna etal., 2010; Kereszturi et al., 2010; Kereszturi and Németh, 2011). Suchmultiple eruptive episodes are also common from silicic tuff ring anddome volcanoes such as Cerro Pinto (Zimmer et al., 2010). Cerro Pinto'sevolution is characterised by multiple reactivation, lava dome destruc-tion and associated sedimentation all typical for more complex polyge-netic volcanoes such as stratovolcanoes or calderas (Zimmer et al.,2010). Such complex volcanoes however are typically associated withmultiple magma pulses over the lifespan of such composite volcanoes.Cerro Pinto however has not erupted through such multiple magmapulses as indicated by its very homogeneous geochemical signaturesuggesting a small isolated magma chamber responsible for the forma-tion of this volcano (Zimmer et al., 2010). In the case of Cerro Pinto, theeruption complexity was inferred to be the result of a combination ofprocesses in the course of the eruption related to various degree ofphreatomagmatic activity and the presence of an irregularly shapedmagma chamber that was zoned with respect to volatiles (Zimmeret al., 2010). The chemical variations of the primary pyroclastic succes-sions at Los Loros in contrast indicate a complex magma source andplumbing system beneath the volcano. Such complexity is usuallyexpected to occur in large volume, composite volcanoes (Carrigan,2000; Davidson and de Silva, 2000), and rather unusual, or overlookedin a small-volume volcano such as Los Loros. However similar complex,but small-volume volcanic eruption style has been recorded in connec-tion with silicic lava dome and associated tuff ring fields such as theCerro Pizarro isolated rhyolitic dome complex in central Mexico(Riggs and Carrasco-Nunez, 2004; Carrasco-Nunez and Riggs, 2008).Cerro Pizarro, similar to Los Loros is a relatively isolated, small (edificevolume of ~1.1 km3) volcano with multiple eruptive phases includingexplosive, effusive, dome- (cryptodome-) forming and conedestructional episodes separated by long eruption repose periods all-owing the accumulation of volcaniclastic aprons during inter-eruptivetimes (Riggs and Carrasco-Nunez, 2004; Carrasco-Nunez and Riggs,2008). This eruption behaviour is typical for long lived, polygeneticcomposite volcanoes (Fisher and Smith, 1991; Smith and Lowe, 1991;Manville et al., 2009). Cerro Pizarro shows strong evidence of being asingle vent system by its morphology and stratigraphy which are simi-lar to Los Loros in spite of the recorded chemical variations in its erup-tive products indicating some degree of differentiation, crustalassimilation or their combination over the eruptive period of the volca-no (Carrasco-Nunez and Riggs, 2008). While Cerro Pizarro's chemicalvariations are characteristic they are by far not as great as have beenrecorded at Los Loros. While the simple tuff ring morphology, its geo-metrical parameters and the isolated nature of Los Loros suggest simplemagmatic and volcanic stratigraphy, the data is in contrast with theseexpectations. It is likely that Los Loros has either a single vent or closelyspaced vents as suggested by the simple morphology of the volcanohowever these vents tappedmagma fromdifferent sources. This behav-iour is typical for complex composite volcanoes that have erupted overlong time. Los Loros's complexmagmatic system and associated distincteruptive phases recorded in its volcanic architecture make it a uniquevolcano. Los Loros also differs from Cerro Pizarro, or Cerro Pinto, nothaving preserved lava domes in its edifice. There is also no evidenceto support existence of lava dome forming events that were destroyedby dome collapses that could be recorded in block and ash flow de-posits. Such deposits are lacking at Los Loros.

The presence of welded pyroclastic units in the capping successionof Los Loros also suggests, that its eruption has reached a high magmadischarge rate stage in its final phase potentially in an eruption thatcan be looked a bimodal eruption style, also more common featurein long lasting eruptions of composite volcanoes.

The presence of a capping extensive volcanic conglomerate sheetabove the topmost lava flow (conglomerate 3) suggests that Los Loros'svolcanic edificemust have been partially covered by the deposits trans-ported by the active braided river network, that were re-establishedafter the formation of the Los Loros low aspect ratio volcanic edifice. Asudden formation of a silicic tuff ring in a braided river system couldhave caused damming effect that could have raised the water level inthe drainage network partially covering the distal foothill areasof the newly formed tuff ring. The reestablishment of the modern(present day) fluvial system is evident on the basis of the strikingly dif-ferent drainage network of the cone (radial) and its surrounding areas(NE–SW trending longitudinal).

10. Conclusion

The volcanic succession of Los Loros forms a maximum of ~100 mthick pile sitting on a pre-tuff ring basaltic pahoehoe flow in its distalpart or directly on Cretaceous continental red beds. The base of the vol-canic succession is a polymict volcaniclastic conglomerate with variablebed thickness and occasional cross stratification, indicating a channel-filling nature and an origin from a braided river system. The diversityof clasts suggests that they likely have been transported from a nearbyMiocene back-arc volcanic complex, the Sierra Chachahuén (~15 km),which is a lava dome dominated multiple volcano with thick silicic py-roclastic successions.

The volcaniclastic conglomerate is covered by a rhyolitic pumi-ceous unit thickening toward the SSE. They are composed of looselypacked rounded pumice lapilli. These beds have a well-sorted texturewith no characteristic internal stratification, indicating that they arefall in origin and the bed thickness variation is inferred to reflect thepaleo-wind direction. A thick pile of phonolitic pumiceous tuff (upto 30 m) overlies the basal pumice fall unit. It is stratified, cross-bedded and having erosional contacts to the underlying pumice fallbeds suggest deposition from high particle concentration pyroclasticdensity currents from an eruption, and tapping of magma from differ-ent sources as the underlying rhyolitic pumice fall was derived. Thissuccession is inferred to represent an original pumice ring formationin a braided river network, where external surface and shallow sub-surface water were available to influence the eruption, causing slightphreatomagmatic affinity.

This initial volcaniclastic succession is covered by immature, butthick (dm-to-m) pelitic paleosoils and/or channel-filling volcanic con-glomerates, suggesting a significant time break (tens to hundreds ofthousands of years), erosion and landscape resetting by fluvial net-works after the pumice ring was formed.

The rejuvenation of the volcanic vent is represented by a thick pyro-clastic and lava capping unit. At least three units of increasingly weldedpyroclastic breccias can be separated on the basis of their welding tex-tures and pumice-to-lithic ratios. The gradual transition from stratifiedpumiceous beds to welded units indicates that these units are formedfrom laterally moving pumiceous pyroclastic density currents (e.g.small-volume ignimbrites). The topmost unit of Los Loros is an interme-diate composition lava flow, which is well-preserved in the east. Mon-omict volcanic conglomerate covers the eastern sector of the upperslopes of Los Loros, inferred to represent a single block-and-ash flowproducing eruption in the closing stage of the eruption history of LosLoros.

The eruptive sequence preserved at Los Loros indicates an initialpumice ring formation (Tuff ring 1) on an alluvial plain, collecting volca-nic debris fromdistal sources such as the ChachahuénComplex. The sig-nificant time gap recorded in channelised volcaniclastic conglomeratebeds between the basal and capping volcanic units suggests a long-lasting inter-eruptive period prior to resumption of volcanic activity,forming small-volume, low aspect ratio pyroclastic density current de-posits and capping lavaflows (Tuff ring 2). The compositional variationsrecorded in each eruptive phase (basal and capping) indicate a complex


magmatic feeding and tapping system, typical for bimodal style of poly-genetic and polymagmatic volcanism. However, the geometrical sizeand the eruptive volume of the Los Loros volcano are unique to therecorded volcaniclastic facies and rock chemical variations. Los Lorosis a unique volcano in the sense that it “mimics” a tuff ring in its mor-phology and geometrical parameters; however, its eruptive sequenceis more typical of those eruptions associated with large-volume siliciccomposition volcanoes with significant inter-eruptive periods. There-fore Los Loros is inferred to be an isolated polygenetic silicic tuff ring.Its broad, flat floored crater similar to maar craters, but the surroundingtuff rings are more typical for caldera volcanoes such as welded pumi-ceous silicic pyroclastic density current deposits and therefore couldbe referred to as a “mini caldera”.

Acknowledgements

This research was supported by the New Zealand–Argentina ISATBilateral Science and Technology Project granted to CR and KN. Theresearch is also part of the FRST–IIOF project “Facing the volcanic haz-ards challenges of Auckland volcanism”, as part of understanding thebehaviour of small-volume explosive volcanism. Part of this researchis the result of an inter-institutional cooperation between KN and PZsupported by ISAT and Massey University IVR grants. CR was partiallysupported by grants UBCyT X-102 and UBACyT 20020090100280.Valuable suggestions by Lucia Capra, Roberto Sulpizio and Mauro DiVito have elevated significantly the quality of this paper.

References

Allen, S.R., 2004. Complex spatter- and pumice-rich pyroclastic deposits from an an-desitic caldera-forming eruption: the Siwi pyroclastic sequence, Tanna, Vanuatu.Bulletin of Volcanology 67 (1), 27–41.

Ameghino, F., 1906. Les formations sedimentaires du Cretàce Supèrieur et du Tertiari de laPatagonie. Anales Museo Nacional Historia Naturales, Buenos Aires XV.

Austin-Erickson, A., Buttner, R., Dellino, P., Ort, M.H., Zimanowski, B., 2008. Phreatomagmaticexplosions of rhyolitic magma: experimental and field evidence. Journal of GeophysicalResearch— Solid Earth 113 (B11).

Balogh, K., Németh, K., 2005. Evidence for the neogene small-volume intracontinentalvolcanism in western Hungary: K/Ar geochronology of the Tihany Maar volcaniccomplex. Geologica Carpathica 56 (1), 91–99.

Balogh, K., Árva-Sós, E., Pécskay, Z., Ravasz-Baranyai, L., 1986. K/Ar dating of post-Sarmatianalkali basaltic rocks in Hungary. Acta Mineralogica et Petrographica, Szeged 28, 75–94.

Balogh, K., Konecny, V., Orlicky, O., Lexa, J., Vass, D., 1996. Method, experiences and re-sults of K–Ar dating of alkali basalt volcanism in central and southern Slovakia(west Carpathians). Acta Geologica Hungarica 39, 8–11 (Suppl.).

Balogh, K., Németh, K., Itaya, T., Molnár, F., Stewart, R., Thanh, N.X., Hyodo, H., Daróczi,L., 2010. Loss of 40Ar(rad) from leucite-bearing basanite at low temperature: im-plications on K/Ar dating. Central European Journal of Geosciences 2 (3), 385–398.

Brand, B.D., Clarke, A.B., 2009. The architecture, eruptive history, and evolution of theTable Rock Complex, Oregon: from a Surtseyan to an energetic maar eruption.Journal of Volcanology and Geothermal Research 180 (2–4), 203–224.

Brenna, M., Cronin, S.J., Smith, I.E.M., Sohn, Y.K., Németh, K., 2010. Mechanisms drivingpolymagmatic activity at a monogenetic volcano, Udo, Jeju Island, South Korea.Contributions to Mineralogy and Petrology 160, 931–950.

Brooker,M.R., 1988. Physical volcanology of the Puketarata tuff ring anddomes,Maroa vol-canic centre. In: Kamp, Peter (Eds.), Geological Society of NewZealand; Annual Confer-ence; Programme and Abstracts. : Geological Society of New Zealand MiscellaneousPublication. Geological Society of New Zealand, Christchurch, New Zealand, p. 43.

Brooker, M.R., Houghton, B.F., Wilson, C.J.N., Gamble, J.A., 1993. Pyroclastic phases of arhyolitic dome-building eruption; Puketarata tuff ring, Taupo volcanic zone, NewZealand. Bulletin of Volcanology 55 (6), 395–406.

Brown, R.J., Kokelaar, B.P., Branney, M.J., 2007. Widespread transport of pyroclasticdensity currents from a large silicic tuff ring: the Glaramara tuff, Scafell caldera, En-glish Lake District, UK. Sedimentology 54 (5), 1163–1189.

Buttner, R., Dellino, P., La Volpe, L., Lorenz, V., Zimanowski, B., 2002. Thermohydraulicexplosions in phreatomagmatic eruptions as evidenced by the comparison be-tween pyroclasts and products from molten fuel coolant interaction experiments.Journal of Geophysical Research — Solid Earth 107 (B11) (art. no.-2277).

Cano-Cruz, M., Carrasco-Nunez, G., 2008. Evolution of a rhyolitic explosion crater(maar): Hoya de Estrada, Valle de Santiago volcanic field Guanajuato, Mexico.Revista Mexicana De Ciencias Geologicas 25 (3), 549–564.

Carrasco-Nunez, G., Riggs, N.R., 2008. Polygenetic nature of a rhyolitic dome and impli-cations for hazard assessment: Cerro Pizarro volcano, Mexico. Journal of Volcanol-ogy and Geothermal Research 171 (3–4), 307–315.

Carrigan, C.R., 2000. Plumbing systems. In: Sigurdsson, H., Houghton, B.F., McNutt, S.R.,Rymer, H., Stix, J. (Eds.), Encyclopedia of Volcanoes. Academic Press, San Diego, pp.219–235.

Cazau, L., Uliana, M., 1973. El Cretácico Superior continental de la Cuenca Neuquina. 5°Congreso Geológico Argentino. Actas, 3, pp. 131–163.

Chough, S.K., Sohn, Y.K., 1990. Depositional mechanics and sequences of base surges,Songaksan tuff ring, Cheju Island, Korea. Sedimentology 37, 1115–1135.

Condat, P., Cruz, C., Kozlowski, E., Manceda, R., 1990. Ambiente deposicional de lassedimentitas del Grupo Neuquén inferior en el suroeste de Mendoza, Argentina.11º Congreso Geológico Argentino, Actas, 2, pp. 65–68.

Cox, K.G., Bell, J.D., Pankhurst, R.J., 1979. Source: Interpretation of Igneous Rocks. Allenand Unwin, London. pp. 450.

Cruz, C., 1993. Facies y estratigrafía secuencial del Cretácico Superior en la zona del RíoDiamante Provincia de Mendoza, Argentina. 12° Congreso Geológico Argentino,Actas, 1, pp. 46–54.

Davidson, J., de Silva, S., 2000. Composite volcanoes. In: Sigurdsson, H., Houghton, B.F.,McNutt, S.R., Rymer, H., Stix, J. (Eds.), Encyclopedia of Volcanoes. Academic Press,San Diego, pp. 663–682.

De Rosa, R., Dellino, P., 1999. Pyroclastic surges of the Pleistocene Monte Guardia se-quence (Lipari Island, Italy): depositional processes — discussion. Sedimentology46 (3), 577–580.

Dellino, P., 2000. Phreatomagmatic deposits: fragmentation, transportation and depo-sition mechanisms. Terra Nostra 6, 99–105.

Doering, A., 1882. Informe oficial de la Comiión Científica Agregada al Estado MayorGeneral de la expedición al río Negro. Patagonia. Entrega III. Geología. (BuenosAires).

Druitt, T.H., Brenchley, P.J., Gokten, Y.E., Francaviglia, V., 1995. Late Quaternary rhyoliticeruptions from the Acigol Complex, Central Turkey. Journal of the Geological Soci-ety 152, 655–667.

Eggins, S.M., Rudnick, R.L., McDonough, W.F., 1998. The composition of peridotites andtheir minerals: a laser-ablation ICP-MS study. Earth and Planetary Science Letters154 (1–4), 53–71.

Fisher, R.V., Smith, G.A., 1991. Volcanism, tectonics and sedimentation. In: Fisher, R.V.,Smith, G.A. (Eds.), Sedimentation in Volcanic Settings. SEPM Special Publication,Tulsa, Oklahoma USA, pp. 1–5.

Folguera, A., Bottesi, G., Zapata, T., Ramos, V.A., 2008. Crustal collapse in the Andeanback-arc since 2 Ma: Tromen volcanic plateau, Southern Central Andes (36°40′–37°30′S). Tectonophysics 459, 140–160.

Franzese, J., Spalletti, L., 2001. Late Triassic–early Jurassic continental extension insouthwestern Gondwana: tectonic segmentation and pre-break-up rifting. Journalof South American Earth Sciences 14, 257–270.

Freundt, A., Wilson, C.J.N., Carey, S.N., 2000. Ignimbrites and block-and-ash flow de-posits. In: Sigurdsson, H., Houghton, B.F., McNutt, S.R., Rymer, H., Stix, J. (Eds.), En-cyclopedia of Volcanoes. Academic Press, San Diego, pp. 581–600.

Galland, O., Hallot, E., Cobbold, P., Ruffet, G., Galland, O., Hallot, E., Cobbold, P., Ruffet, G.,d'Ars, J., 2007. Volcanism in a compressional Andean setting: a structural and geo-chronological study of Tromen volcano (Neuquén province, Argentina). Tectonics26, TC4010, http://dx.doi.org/10.1029/2006TC002011 (002024 pp.).

Germa, A., Quidelleur, X., Gillot, P.Y., Tchilinguirian, P., 2010. Volcanic evolution of theback-arc Pleistocene Payun Matru Volcanic Field (Argentina). Journal of SouthAmerican Earth Sciences 29, 717–750.

Groeber, P., 1946. Observaciones geológicas a lo largo del meridiano 70º. Hoja ChosMalal. Revista de la Asociación Geológica Argentina I (3).

Heiken, G.H., 1971. Tuff rings: examples from the Fort Rock–Christmas Lake ValleyBasin, south-central Oregon. Journal of Geophysical Research 76 (23), 5615–5626.

Herrero Ducloux, A., 1946. Contribución al conocimiento geológico del Neuquénextraandino. Boletin de Informaciones Petroleras 266.

Kay, S.M., Burns, M., Copeland, P., Mancilla, O., 2006a. Upper Cretaceous to Holocenemagmatism and evidence for transientMiocene shallowing of the Andean subductionzone under the northernNeuquén Basin. In: Kay, S.M., Ramos, V.A. (Eds.), Evolution ofan Andean Margin: A Tectonic and Magmatic View from the Andes to the NeuquénBasin (35–39°S). Geological Society of America, Boulder, Colorado, pp. 19–60.

Kay, S.M., Mancilla, O., Copeland, P., 2006b. Evolution of the late Miocene Chachahuénvolcanic complex at 37ºS over a transient shallow subduction zone under the Neu-quén Andes. In: Kay, S.M., Ramos, V.A. (Eds.), Evolution of an Andean Margin: ATectonic and Magmatic View from the Andes to the Neuquén Basin (35–39°S).Geological Society of America, Boulder, Colorado, pp. 215–246.

Kazanci, N., Gevrek, A.I., Varol, B., 1995. Facies changes and high calorific peat formation ina Quaternary maar lake, central Anatolia, Turkey — the possible role of geothermalprocesses in a closed lacustrine basin. Sedimentary Geology 94 (3–4), 255–266.

Kereszturi, G., Németh, K., 2011. Shallow-seated controls on the evolution of the UpperPliocene Kopasz-hegy nested monogenetic volcanic chain in the western Pan-nonian Basin (Hungary). Geologica Carpathica 62 (6), 535–546.

Kereszturi, G., Csillag, G., Németh, K., Sebe, K., Kadosa, B., Jáger, V., 2010. Volcanic architecture,eruptionmechanismand landformevolutionof a Plio/Pleistocene intracontinental basal-tic polycyclic monogenetic volcano from the Bakony–Balaton Highland Volcanic Field,Hungary. Central European Journal of Geosciences 2 (3), 362–384.

Legarreta, L., Gulisano, C., 1989. Análisis estratigráfico secuencial de la Cuenca Neu-quina (Triásico superior-Terciario inferior), Argentina. In: Chebli, G., Spalletti, L.(Eds.), Cuencas Sedimentarias Argentinas: Serie Correlación Geológica BuenosAires, pp. 221–243.

Legarreta, L., Uliana, M., 1999. El Jurásico y Cretácico de la Cordillera Principal y laCuenca Neuquina. In: Caminos, R. (Ed.), Geología Argentina. Subsecretaría de Min-ería de la Nación-Servicio Geológico Minero Argentino-Instituto de Geología yRecursos Minerales, Buenos Aires, pp. 399–432.

Legarreta, L., Gulisano, C.A., Uliana, M.A., 1993. Las secuencias sedimentarias jurasico-cretácicas. 12 Congreso Geológico Argentino, Relatorio, 1(9), pp. 87–114.

Llambías, E.J., 1966. Geología y petrología del volcán Payún Matrú, Mendoza. ActaGeológica Lilloana [San Miguel de Tucumán] 7, 265–310.

http://dx.doi.org/10.1029/2006TC002011


Llambías, E.J., Bertotto, G.W., Risso, C., Hernando, I., 2010. El volcanismo cuaternario enel retroarco de Payenia: una revisión. Revista de la Asociación Geológica Argentina67 (2), 278–300.

Lorenz, V., 1986. On the growth of maars and diatremes and its relevance to the forma-tion of tuff rings. Bulletin of Volcanology 48, 265–274.

Manville, V., Nemeth, K., Kano, K., 2009. Source to sink: a review of three decades ofprogress in the understanding of volcaniclastic processes, deposits, and hazards.Sedimentary Geology 220 (3–4), 136–161.

Mastrolorenzo, G., 1994. Averno tuff ring in Campi-Flegrei (south Italy). Bulletin of Vol-canology 56 (6–7), 561–572.

McDonough, W.F., Sun, S-s, 1995. The composition of the Earth. Chemical Geology 120,223–254.

Németh, K., 2010. Monogenetic volcanic fields: origin, sedimentary record, and rela-tionship with polygenetic volcanism. In: Cañón-Tapia, E., Szakács, A. (Eds.), WhatIs a Volcano? : GSA Special Papers, vol. 470. Geological Society of America, Boulder,Colorado, pp. 43–67.

Németh, K., Cronin, S.J., 2007. Syn- and post-eruptive erosion, gully formation, andmorphological evolution of a tephra ring in tropical climate erupted in 1913 inWest Ambrym, Vanuatu. Geomorphology 86, 115–130.

Pearce, J.A., 1982. Trace element characteristics of lavas from destructive plate bound-aries. In: Thorpe, R.S. (Ed.), Andesites: Orogenic Andesites and Related Rocks. JohnWiley & Sons, New York, pp. 525–554.

Pécskay, Z., Lexa, J., Szakács, A., Balogh, K., Seghedi, I., Konecny, V., Kovács, M., Márton, E.,Kaliciak,M., Széky-Fux, V., Póka, T., Gyarmati, P., Edelstein, O., Rosu, E., Zec, B., 1995. Spaceand time distribution of Neogene–Quaternary volcanism in the Carpatho–Pannonianregion. Acta Vulcanologica 7 (2), 15–28.

Ramos, V.A., Folguera, A., 2011. Payenia volcanic province in the Southern Andes: anappraisal of an exceptional Quaternary tectonic setting. Journal of Volcanologyand Geothermal Research 201 (1–4), 53–64.

Riedel, C., Ernst, G.G.J., Riley, M., 2003. Controls on the growth and geometry of pyroclas-tic constructs. Journal of Volcanology and Geothermal Research 127 (1–2), 121–152.

Riggs, N., Carrasco-Nunez, G., 2004. Evolution of a complex isolated dome system,Cerro Pizarro, central Mexico. Bulletin of Volcanology 66 (4), 322–335.

Riggs, N.R., Hurlbert, J.C., Schroeder, T.J., Ward, S.A., 1997. The interaction of volcanismand sedimentation in the proximal areas of a mid-tertiary volcanic dome field, cen-tral Arizona, USA. Journal of Sedimentary Research 67 (1), 142–153.

Roll, A., 1939. La cuenca de los estratos con dinosaurios al sur del río Neuquén. Informeinédito YPF .

Schmincke, H.U., Park, C., Harms, E., 1999. Evolution and environmental impacts of theeruption of Laacher See Volcano (Germany) 12,900 a BP. Quaternary International61, 61–72.

Schumacher, R., Schmincke, H.-U., 1990. The lateral facies of ignimbrites at Laacher Seevolcano. Bulletin of Volcanology 52, 271–285.

Self, S., de Silva, S.L., Cortes, J.A., 2008. Enigmatic clastogenic rhyolitic volcanism: theCorral de Coquena spatter ring, North Chile. Journal of Volcanology and Geother-mal Research 177 (4), 812–821.

Smith, G.A., Lowe, D.R., 1991. Lahars: volcano-hydrologic events and deposition in thedebris flow–hyperconcentrated flow continuum. In: Fisher, R.V., Smith, G.A. (Eds.),Sedimentation in Volcanic Settings. SEMP Special Publications, Tulsa, Oklahoma,USA, pp. 59–70.

Sohn, Y.K., 1996. Hydrovolcanic processes forming basaltic tuff rings and cones onCheju Island, Korea. Geological Society of America Bulletin 108 (10),1199–1211.

Sun, S-s, McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts:implications for mantle composition and processes. In: Saunders, A.D., Norry, M.J.(Eds.), Magmatism in the ocean basins. Geological Society of London. Special Pub-lications 42, 313–345.

Van der Bogaard, P., Schmincke, H.U., 1984. The eruptive center of the Laacher SeeTephra. Geologische Rundschau 73 (3), 933–980.

Vespermann, D., Schmincke, H.-U., 2000. Scoria cones and tuff rings. In: Sigurdsson, H.,Houghton, B.F., McNutt, S.R., Rymer, H., Stix, J. (Eds.), Encyclopedia of Volcanoes.Academic Press, San Diego, pp. 683–694.

White, J.D.L., 1990. Depositional architecture of a maar-pitted playa— sedimentation inthe Hopi Buttes Volcanic Field, northeastern Arizona, USA. Sedimentary Geology 67(1–2), 55–84.

White, J.D.L., 1991. The depositional record of small, monogenetic volcanoes withinterrestrial basins. In: Fisher, R.V., Smith, G.A. (Eds.), Sedimentation in Volcanic Set-tings. Society for Sedimentary Geology, Tulsa (Oklahoma), pp. 155–171.

Woerner, G., Schmincke, H.U., 1984. The Laacher See Volcano I: mineralogical andchemical zonation of the Laacher See tephra sequence. Journal of Petrology 25,805–835.

Woerner, G., Wright, T.L., 1984. Evidence for magma mixing within the Laacher Seemagma chamber (Germany). Journal of Volcanology and Geothermal Research22, 123–135.

Zimmer, B.W., Riggs, N.R., Carrasco-Nunez, G., 2010. Evolution of tuff ring-dome com-plex: the case study of Cerro Pinto, eastern Trans-Mexican Volcanic Belt. Bulletin ofVolcanology 72 (10), 1223–1240.

Facies architecture of an isolated long-lived, nested polygenetic silicic tuff ring erupted in a...

Documents

Transcript of Facies architecture of an isolated long-lived, nested polygenetic silicic tuff ring erupted in a...