Non-explosive magma water interaction in a continental setting: Miocene examples from the Eastern Cordillera (central Andes; NW Argentina)

Transcription

1 Bull Volcanol (2009) 71: DOI /s RESEARCH ARTICLE Non-explosive magma water interaction in a continental setting: Miocene examples from the Eastern Cordillera (central Andes; NW Argentina) Luigina Vezzoli & Massimo Matteini & Natalia Hauser & Ricardo Omarini & Roberto Mazzuoli & Valerio Acocella Received: 3 May 2007 / Accepted: 30 July 2008 / Published online: 29 August 2008 # Springer-Verlag 2008 Abstract The Middle-Upper Miocene Las Burras Almagro- El Toro (BAT) igneous complex within the Eastern Cordillera of the central Andes ( 24 S; NW Argentina) has revealed evidence of non-explosive interaction of andesitic magma with water or wet clastic sediments in a continental setting, including peperite generation. We describe and interpret lithofacies and emplacement mechanisms in three case studies. The Las Cuevas member (11.8 Ma) comprises facies related to: (i) andesite extruded in a subaqueous setting and generating lobe-hyaloclastite lava; and (ii) marginal parts of subaerial andesite lava dome(s) in contact with surface water, comprising fluidal lava lobes, hyaloclastite, and juvenile clasts with glassy rims. The Lampazar member (7.8 Ma) is represented by a syn-volcanic andesite intrusion and related peperite that Editorial responsibility: J. McPhie L. Vezzoli (*) Dipartimento Scienze Chimiche e Ambientali, Università dell Insubria, via Valleggio 11, Como, Italy luigina.vezzoli@uninsubria.it M. Matteini Instituto de Geociencias, Universidade de Brasilia, Brasilia, Brazil N. Hauser : R. Omarini Facultad de Ciencias Naturales, Universidad Nacional de Salta-CONICET, Salta, Argentina R. Mazzuoli Dipartimento Scienze della Terra, Università di Pisa, Pisa, Italy V. Acocella Dipartimento di Scienze Geologiche, Università di Roma Tre, Rome, Italy formed within unconsolidated, water-saturated, coarse-grained volcaniclastic conglomerate and breccia. The andesite intrusion is finger-shaped and grades into intrusive pillows. Pillows are up to 2 m wide, tightly packed near the intrusion fingers, and gradually become dispersed in the host sediment 50 m from the parent intrusion. The Almagro A member (7.2 Ma) shows evidence of mingling between water-saturated, coarsegrained, volcaniclastic alluvial breccia and intruding andesite magma. The resulting intrusive pillows are characterized by ellipsoidal and tubular shape and concentric structure. The high-level penetration of magma in this coarse sediment was unconfined and irregular. Magma was detached in apophyses and lobes with sharp contacts and fluidal shapes, and without quench fragmentation and formation of a hyaloclastite envelope. The presence of peperite and magma water contact facies in the BAT volcanic sequence indicates the possible availability of water in the system between 11 7 Ma and suggests a depositional setting in this part of the foreland basin of the central Andes characterized by an overall topographically low coastal floodplain that included extensive wetlands. Keywords Magma water interaction. Volcaniclastic succession. Peperite. Hyaloclastite. Syn-volcanic intrusion. Miocene volcanism. Central Andes Introduction Magma water interaction in continental settings produces a well-known variety of phreatomagmatic explosive eruptions and volcanic structures as maars and tuff cones (e.g., White 1991, 1996; Elliot and Hanson 2001). Records of non-explosive magma water interaction in continental settings are somewhat less common, although examples have been described from sub-glacial (e.g. Lescinsky and

2 510 Bull Volcanol (2009) 71: Fink 2000; Smellie 2000, and references therein), and lacustrine (e.g. Cas et al. 2001; Brown and Bell 2007) environments. In a continental setting, the recognition and study of products and processes controlled by magma water interaction are fundamental tools in the interpretation of the temporal and spatial evolution of contemporaneous magmatism, sedimentation and tectonism. Moreover, study of the products of magma water interactions can furnish useful information on the regional paleogeography and paleoclimatic context. In this paper we describe and interpret three case studies of non-explosive magma water interaction developed in continental fluvio-lacustrine environments in the Eastern Cordillera of the central Andes of NW Argentina. These examples occur at three stratigraphic levels of the Miocene (14 6 Ma) Las Burras Almagro-El Toro igneous complex (Matteini et al. 2005; Mazzuoli et al. 2008). In the volcanic sequences of this igneous complex there are several andesitic volcaniclastic deposits that display evidence of subaqueous deposition and associated non-explosive magma fragmentation. The volcaniclastic deposits are associated with andesite to dacite lavas and lava domes extruded at the margins of and into the El Toro sedimentary basin (Marrett and Strecker 2000; Hilley and Strecker 2005). This evidence of interaction with external water during andesitic Miocene magmatism in the central Andes is important as it provides useful information on paleoenvironmental conditions, chronology, and tectonic reconstruction of the Eastern Cordillera. One of the studied deposits has been interpreted as peperite and is one of the first occurrences of this kind of facies in the Miocene of the central Andes (e.g., Harris et al. 2006). This study also provides opportunities for analysis of the poorly known deposits produced by non-explosive magma water interaction in andesitic rocks (e.g., Brooks et al. 1982; Kano 1989; Hanson 1991). Moreover, the described examples improve our knowledge of the behaviour of magma that interacts with coarse-grained, poorly sorted, and highly permeable sediments (e.g., Squire and McPhie 2002). Geological setting In the Eastern Cordillera of the central Andes at S, W, the Middle-Upper Miocene Las Burras Almagro-El Toro (BAT) igneous complex (Fig. 1; Matteini et al. 2005; Mazzuoli et al. 2008) formed about 300 km to the E of the Miocene-Quaternary volcanic arc (Western Cordillera; Fig. 1a and b) along the NW-striking Calama-Olacapato- El Toro transverse fault system (Allmendinger et al. 1983; Riller et al. 2001; Matteini et al. 2002a, b; Riller and Oncken 2003; Acocella et al. 2007). At this latitude, the Eastern Cordillera consists of a W- dipping basement-involved thrust system that migrated eastward during Miocene-Quaternary times, controlled by main WNW ESE and WSW ENE directions of compression (Marrett et al. 1994; Marrett and Strecker 2000). In the study area (Fig. 1c), the Eastern Cordillera basement is represented by the low-grade metasedimentary Puncoviscana Formation (black siltstone and sandstone, Precambrian Eocambrian) intruded by the Santa Rosa de Tastil pluton (grey granodiorite and red granite, Lower Cambrian). The basement is unconformably overlain by Paleozoic to Paleogene sedimentary formations: quartzite of the Upper Cambrian Meson Group, sandstone and siltstone of the Lower Ordovician Santa Victoria Group, and reddish sandstone and conglomerate of the Cretaceous-Paleogene Salta Group. Tertiary- Quaternary continental conglomerate and sandstone filled the El Toro basin (Marrett et al. 1994; Marrett and Strecker 2000; Hilley and Strecker 2005). At present, the El Toro basin is an intramontane N S elongate basin, bounded by two opposite-verging thrust faults (Fig. 1c): the San Bernardo fault located along the Almagro range to the W, and the Golgota fault located along the Sierra Pascha to the E. The BAT igneous complex consists of the monzogabbroic to monzogranitic (SiO wt.% recalculated anhydrous) Las Burras laccolith and small andesitic (SiO wt.%) and dacitic (SiO wt.%) lava centers (Figs. 1 and 2). Seven distinctive lithostratigraphic units have been mapped (Fig. 2) and can be assigned to two magmatic phases in the geologic history of the BAT igneous complex (Matteini et al. 2005; Mazzuoli et al. 2008). The older magmatic phase (14 12 Ma) comprises the Las Burras intrusion (Las Burras member, 14.2 Ma) and the oldest volcanic unit (Puerta Tastil member, 12.8 Ma). The younger phase (11 6 Ma) comprises five volcanic units, represented essentially by andesite to dacite lavas and lava domes and by extensive, genetically related, coarse volcaniclastic deposits (from the oldest: Las Cuevas, Lampazar, Almagro A, Almagro B and Almagro C members). These lavas and volcaniclastic deposits rest directly on Precambrian Lower Cambrian basement of the Almagro range (average elevation 3,700 m a.s.l.) in the northwestern sector of the study area, and are widely interbedded with the sedimentary succession of the El Toro basin (average elevation 3,000 m a.s.l.) in the southeastern sector (Figs. 1 and 2). The BAT igneous complex and the sediments of the El Toro basin were deformed during the uplift of the Eastern Cordillera (Marrett et al. 1994; Acocella et al. 2005; Acocella et al. 2007). Case study 1: Magma water contact at the margin of andesitic lava domes and lava flows (Las Cuevas member) The Las Cuevas member is exposed on both sides of the Rio Las Cuevas valley, near the Alfarcito village (24 29

3 Bull Volcanol (2009) 71: Fig. 1 a Location map of the study region in the central Andes. CVZ: Central Volcanic Zone. b The Miocene Las Burras Almagro-El Toro igneous complex lies in the easternmost part (Box C) of the volcanic chain related to the NW-trending Calama-Olacapato-El Toro fault system, intersecting the Miocene-Quaternary volcanic arc at 24 S. c Simplified geologic map of the Las Burras Almagro-El Toro igneous complex and locations of the sites described in this work (1 Las Cuevas member; 2 Lampazar member; 3 Almagro A member). SBF San Bernardo fault, GF Golgota fault, E Estaciόn (railway station), Q Quebrada (valley) 10 S, W, 2,860 m a.s.l.; Fig. 1c), and is composed of stratified andesitic volcanic breccia interbedded with subordinate fine and coarse tuff (Fig. 3). Its overall thickness is ~70 m, and the K/Ar age is 11.12±0.17 Ma (Mazzuoli et al. 2008). Lava and pumice clasts show a homogeneous andesitic (SiO wt.%) composition and phenocrysts population (amphibole and plagioclase). The Las Cuevas member (Fig. 3) nonconformably overlies the Santa Rosa de Tastil grey granodiorite, and is unconformably covered by non-volcanic conglomerate and sandstone, referred to as Alfarcito conglomerate (Marrett and Strecker 2000; Hilley and Strecker 2005). The Las Cuevas member dips to the E, and is tilted and folded along the San Bernardo fault (Marrett et al. 1994; Marrett and Strecker 2000), which thrusted the Santa Rosa de Tastil grey granodiorite over the Miocene deposits of the El Toro basin (Fig. 1c). Minor normal faults deform the contacts among the Las Cuevas member, the Alfarcito conglomerate, and the grey granodiorite (Fig. 3). The Las Cuevas member includes five types of volcanic breccia facies interbedded with stratified fine and coarse tuff and massive pumice tuff-breccia facies (Figs. 3, 4, 5 and 6). Neither coherent lava facies nor vent structures are preserved. The principal lithofacies of the Las Cuevas member are described and interpreted in the following sections, and summarized in Table 1. Stratified fine and coarse tuff This lithofacies is composed of fine and coarse tuff layers. Beds are 5 20 cm thick. The lower contacts with the volcanic breccia facies are sharp and planar, whereas the upper contacts are irregular and erosive (Fig. 4a, b). Glass grains are composed of irregularly cuspate and platy, bubble-wall shards and poorly vesicular, equant micropumices. Crystal fragments are mainly plagioclase. Impact sags under andesitic, breadcrusted lava blocks (Fig. 6a, c and d) and massive, poorly sorted tuff beds are also present. Massive pumice tuff-breccia The massive pumice tuff-breccia facies constitutes ungraded and disorganized beds cm thick (Fig. 4c, d). Andesitic pumice clasts contain phenocrysts of amphibole

4 512 Bull Volcanol (2009) 71: Fig. 2 Sketch of the stratigraphic and geometric relationships of the BAT volcanic succession at 7 6 Ma along a NW SE transect between the Almagro range and the El Toro basin. Ages are from Mazzuoli et al. (2008) and plagioclase and are moderately vesicular (vesicularity vol.%). A few pumice clasts show tube vesicles. Several pumice clasts are elongate and long axes are aligned parallel to the basal contact (Fig. 4d inset), and show ragged, wispy terminations. Subordinate, smaller subangular clasts consist of non-juvenile andesitic lava. The matrix is composed of glass shards, micropumice and moderately vesicular vitric fragments. Monomictic amphibole-phyric andesite breccia This volcanic breccia (Figs. 4a and 5a) is interbedded with the stratified fine and coarse tuff facies and is composed of juvenile amphibole-phyric andesite clasts, showing the same composition and phenocryst population (amphibole and plagioclase) and three different groundmass textures: (a) Pale grey or pinkish grey vitrophyric lava (75 vol.%), varying from micro-vesicular to coarsely vesicular and partly pumiceous; (b) black or dark grey lava (20 vol.%) with non- to poorly vesicular glassy groundmass; and (c) red and black flow-banded lava (5 vol.%) with parallel, centimetre-wide flow foliation, defined by variations in colour and in vesicularity. Type-a clasts are sub-rounded, whereas type-b and type-c clasts are sub-angular. Basement clasts and hydrothermally altered igneous rocks are not present. Single clasts are mostly 5 40 cm in size; up to a maximum size of m. The matrix is composed of the same lava types as the clasts. This breccia is moderately to poorly sorted, varying from matrix-supported (60% matrix) to clast-supported (30% matrix); in some cases it shows crude coarse-tail normal grading. Some beds exhibit trains of coarser clasts (Fig. 5a). Contacts between the breccia beds are sharp, locally erosive, or gradational and amalgamated. A fine-grained, roughly planar laminated, 5 20 cm thick layer is commonly present at the base of the breccia beds (Fig. 4a). Monomictic andesitic lobe breccia The monomictic andesitic lobe breccia facies forms a single, massive bed 3 4 m in maximum thickness, and is composed of large (0.5 3 m across) pods and lobes of pale grey andesite enclosed within a breccia of identical composition (Fig. 5b). The andesite contains amphibole and plagioclase phenocrysts and scattered microphenocrysts in a glassy groundmass and is poorly to non-vesicular. Lava lobes show jagged and fingered shapes. Lobe boundaries are both sharp, with glassy rims showing tiny normal joints, and gradational into the enclosing breccia. In the latter case, the coherent lobe core is increasingly fractured and fragmented outward and grades into a zone of in situ breccia about 70 cm wide. At the margins of the lobes, andesite fragments exhibit jigsaw-fit textures. The breccia is poorly sorted and comprises polyhedral andesite clasts, centimetre- to decimetre-sized, suspended in a cogenetic, finer grained matrix consisting of angular glassy andesite

5 Bull Volcanol (2009) 71: Fig. 3 a General view of the Las Cuevas member at the northern side of the Alfarcito outcrop (Fig. 1c). The member lies between the Santa Rosa de Tastil batholith and the nonvolcanic coarse sediments of the El Toro basin. The contacts are either stratigraphic (dashed lines) or faults (solid lines). Dotted line outlines the sharp separation between the lower part of the Las Cuevas member dominated by subaerial primary volcanic facies, and the upper part showing evidence of waterrelated fragmentation and deposition. Box indicates the location of Fig. 5b. b Schematic sketch illustrating the stratigraphic relationships between some lithofacies. The massive pumice tuff-breccia and polymictic volcanic breccia facies are not exposed in this outcrop fragments of the same composition up to 10 mm across. The monomictic andesitic lobe breccia bed shows irregular upper and lower contacts (Fig. 5b). Along the lower contact, the underlying stratified fine and coarse tuff shows truncated and deformed strata. The overlying matrix-rich polymictic volcanic breccia contains a few fragments of the andesite lobes at its base (Fig. 5b). Monomictic olivine-phyric andesite breccia This lithofacies forms a single bed defined by a train of aligned olivine-phyric andesite clasts (Fig. 5c) enclosed within a granule- and sand-sized matrix. The andesite is black, with a cryptocrystalline groundmass and scattered olivine phenocrysts 1 3 mm across. Coarser (maximum size m) olivine-phyric andesite fragments are polyhedral, defined by convex, curviplanar, and smooth surfaces. Smaller clasts are blocky and angular, with concave conchoidal margins, and are commonly grouped in jigsaw-fit domains (Fig. 5c). The matrix is composed of fragments derived from the same olivine-phyric andesite. Stratified matrix-rich polymictic volcanic breccia This coarse-grained volcanic breccia (Fig. 6a) is composed of centimetre- to metre-sized andesite clasts, supported by granule- and sand-sized, vitric matrix. The andesite clasts consist of: (a) Pale grey lava (containing amphibole and plagioclase phenocrysts in a microcrystalline groundmass with interstitial glass) in sub-rounded, fluidal pods and bulbs showing internal radial joints, complete glassy margins, and contractional cracks on the surface (Fig. 6a);

6 514 Bull Volcanol (2009) 71: Fig. 4 a The stratified fine and coarse tuff facies is interbedded with the monomictic amphibolephyric andesite breccia facies. Note (by the white arrow) the finer and diffusely laminated base of the upper breccia bed. b Detail of the irregular basal contact (dashed line) of the monomictic amphibole-phyric andesite breccia facies (black arrows) that erodes the stratified tuff. c The massive pumice tuffbreccia facies is interbedded with the polymicitc matrix-rich breccia facies at the southern side of the Alfarcito outcrop. Box indicates the location of the d. d The massive pumice tuffbreccia facies. The inset shows the fluidally stretched and aligned pumice clasts. The hammer is 30 cm long (b) angular blocks of vitrophyric lava (containing amphibole and plagioclase phenocrysts), in some cases with complete glassy margins and contractional cracks (Fig. 6a, e); (c) armoured, fluidal fragments of lava coated by vitric shards that are different from the fragments composing the host matrix; (d) blocky clasts of monomictic breccia composed of vitrophyric fragments (Fig. 7); (e) angular fragments of black obsidian, commonly flowfoliated; (f) glassy clasts with contractional cracks (Fig. 6b); and (g) vitrophyric, blocky clasts with internal perlitic textures. Some lava clasts show jigsaw-fit textures. The type-d monomictic breccia clasts are composed of pale grey amphibole-phyric andesite fragments immersed in a reddish matrix consisting in fine-grained, formerly glassy particles and abundant crystals of plagioclase (Fig. 7). The larger andesite fragments (1 10 cm) within these breccia clasts have fluidal shape and show a porphyritic core and a well-developed, 1 5 mm wide, crenulated, chilled margin of black glass (Fig. 7). The smaller andesite fragments ( mm) consist of: (i) clasts with subangular shape, porphyritic texture, and partially black glassy margins; and (ii) clasts with amoeboid shape, vitrophyric texture, and lobate to crenulate margins (Fig. 7). These smaller fragments show jigsaw-fit textures (Fig. 7).

7 Bull Volcanol (2009) 71: Fig. 5 The principal lithofacies of the Las Cuevas member. a Monomictic amphibole-phyric andesite breccia, interpreted as blockand-ash flow deposits from subaerial lava domes. Arrows outline a train of coarser clasts. b Monomictic andesitic lobe-hyaloclastite breccia, showing a 3 m-long ragged lava lobe (dashed white line) of amphibole-phyric andesite with radial and polyhedral joints (dotted white line), set within a monomictic hyaloclastite breccia (dashed black line) of the same composition. Dotted black lines trace the downward deflection of the basal contact and crosscut stratification of the underlying tuffs. Location is in Fig. 3. c Monomictic olivinephyric andesite breccia is composed of angular lava blocks showing jigsaw-fit textures and curviplanar quench fractures. d Polymictic volcanic breccia with angular clasts of black siltstone of the Puncoviscana Formation (P) Polymictic volcanic breccia This lithofacies is represented by a coarse-grained volcanic breccia (Fig. 5d) with clasts of different types of nonjuvenile lavas and basement rocks, up to several metres in length, within a sand and granule matrix of the same composition. Clast composition includes: (a) Pale grey porphyritic (amphibole and plagioclase) lava with glassy micro-vesicular groundmass; (b) dark grey vitrophyric or microporphyritic lava with non-vesicular groundmass and very small phenocrysts of plagioclase; (c) sub-rounded pale grey glassy lava; and (d) angular clasts, decimetre-sized, of black siltstone derived from the Puncoviscana Formation. Interpretation The glass shards of the stratified fine and coarse tuff facies and the pumice clasts of the massive pumice tuff-breccia facies can be considered as juvenile components derived from pyroclastic fragmentation processes. The pumice clasts with phenocrysts of amphibole and plagioclase have similar petrographic characters to the juvenile andesite clasts occurring in the volcanic breccias, suggesting they are genetically related. Planar- and cross-laminations (Figs. 4a, b and 6c) and textures of the vitric grains suggest that the stratified fine and coarse tuff facies represents primary pyroclastic surge deposits. The massive and matrix-supported fabric and the elongate and ragged pumice lapilli (Fig. 4d) of the massive pumice tuff-breccia facies suggest emplacement as a pyroclastic flow deposit. Both these facies are inferred to record explosive activity contemporaneous with emplacement of the volcanic breccias. The presence of non-juvenile lava and basementderived angular clasts dispersed in the stratified tuff beds may suggest conduit disintegration and erosion during eruption. The presence of breadcrusted blocks (Fig. 6a, c) suggests vulcanian (Wright et al. 2007) or possibly hydromagmatic explosive behaviour. The andesite clasts of the monomictic amphibole-phyric andesite breccia facies show similar phenocryst assemblages but different groundmass textures, suggesting their derivation from different parts of single parent lava. The clast types are referable to the typical outward textural zonation recorded in subaerial lavas and lava domes (Huppert et al. 1982; McPhie et al. 1993; Maeno and Taniguchi 2006), which comprises, from the lava or dome core: (i) Coherent porphyritic massive lava with crystalline groundmass; (ii) coherent flow-banded lava; (iii) finely

8 516 Bull Volcanol (2009) 71: Fig. 6 The stratified matrix-rich polymictic volcanic breccia facies in the Las Cuevas member. a Massive beds of very coarse matrix-rich polymictic volcanic breccias are interbedded with stratified fine and coarse tuff (fct) interpreted as pyroclastic surge deposits. Clasts of the coarse matrix-rich polymictic breccia consist of: (1) fluidal lobes of amphibole-phyric andesite, showing curvilinear contacts with the host breccia, well-preserved radial joints, and 2-mm-wide chilled margins; and (2) angular blocks of amphibole-phyric andesite with glassy groundmass. An impact sag under a lava block is also shown (see also c and d). b Clast of amphibole-phyric andesite with cooling contraction cracks on the surface. c Picture and d schematic sketch of the impact sag under an amphibolephyric andesite block (in black) within the stratified fine and coarse tuff that appears disrupted and faulted (dashed lines). Location is shown in a. e A large, blocky, amphibole-phyric andesite clast vesicular coherent glassy lava; (iv) coarsely vesicular coherent glassy lava; and (v) glassy, pumiceous autoclastic breccia. The monomictic amphibole-phyric andesite breccia facies is a poorly sorted, massive mixture of dense juvenile blocks (decimetre- to metre-sized) within a lapilli-sized matrix that contains little fine ash (Fig. 5a). Moreover, this facies shows erosional contacts (Fig. 4a, b), fine-grained basal layers (Fig. 4a), beds <4 m thick, and trains of coarser clasts (Fig. 5a). These depositional structures typically characterize the high-density basal avalanche deposit of a block-and-ash flow which is produced by the collapse of the steep-sided and unstable marginal parts of subaerial lavas or domes (Druitt 1998; Ui et al. 1999; Schwarzkopf et al. 2005). Absence of hydrothermally altered clasts may

9 Bull Volcanol (2009) 71: Table 1 Las Cuevas member: summary of the lithofacies, their characteristics and interpretation Lithofacies Characteristics Interpretation Components, texture and composition Bedforms, sedimentary structures, and facies architecture Clast forming processes Transport and deposition processes and environment Stratified fine and coarse tuff (Figs. 4a, b and 6c) Massive pumice tuffbreccia (Fig. 4c, d) Monomictic amphibolephyric andesite breccia (Fig. 5a) Monomictic andesitic lobe-hyaloclastite breccia (Fig. 5b) Monomictic olivinephyric andesite breccia (Fig. 5c) Stratified matrix-rich polymictic volcanic breccia (Fig. 6) Polymictic volcanic breccia (Fig. 5d) Glass- and crystal-ash; sparse single clasts or trains of angular clasts (up to several cm across) of non-juvenile lava and red granite Subrounded to flattened, white pumice (1 10 cm); subangular, non-juvenile lava clasts (0.3 2 cm); vitric coarse ash matrix Subrounded, vesicular to pumiceous, porphyritic clasts; blocky, angular, flow-banded, vitrophyric clasts; cogenetic granular matrix Ragged lava lobes (0.5 3 m); radial and polyhedral joints; glassy margins; hyaloclastite granular matrix Blocky and polyhedral clasts (0.3 2 m); curviplanar surfaces; cogenetic granular-sandy matrix Texturally diverse andesite lava clasts; subrounded pillows with radial joints, glassy margins, and breadcrust surfaces; angular polyhedral glassy lava with perlitic textures; foliated obsidian; hyaloclastite breccia clasts; vitric matrix Texturally diverse non-juvenile andesite lava clasts; basement clasts; sandy and granular matrix Planar- and low-angle cross-laminated; impact sags; interstratified with the monomictic amphibole-andesite breccia and stratified matrix-rich polymictic volcanic breccia Massive, 1 m thick, matrixsupported, poorly sorted; interstratified with the stratified matrix-rich polymictic volcanic breccia Very thick (2 4 m), wedge shaped, internally massive or normal graded beds; poorly sorted; clast- to matrix-supported Non-stratified, poorly sorted, 3 4 m thick, massive single bed; jigsaw-fit texture Cluster of aligned clasts in a non-stratified, poorly sorted, m thick, massive single bed; jigsaw-fit texture Very thick (2 4 m), internally massive beds; poorly sorted; matrix-supported; local jigsaw-fit texture Stratified; massive and disorganized beds, 1 3 m thick; clast- to matrix-supported; very poorly sorted Explosive fragmentation, possibly hydromagmatic Explosive fragmentation Autoclastic fragmentation of the outer margin of subaerial lava dome Autoclastic quench fragmentation Autoclastic quench fragmentation Pyroclastic surge; subaerial or shallow subaqueous Pyroclastic flow; subaerial or shallow subaqueous Subaerial block and ash flow, from gravitational collapse of subaerial lava dome Lava flow and in-situ hyaloclastite; subaqueous or subaerial lava flowing into water Subaerial? lava flowing into water Quench fragmentation Gravity-driven failure of unstable autoclastic breccia from probably subaqueous margins of lava domes; negligible clast modification during transport; subaerial or subaqueous deposition from sediment gravity flows Autoclastic, pyroclastic and epiclastic fragmentation Subaerial or subaqueous transport and deposition from sediment gravity flows

10 518 Bull Volcanol (2009) 71: Fig. 7 Polished slab (a) and schematic sketch (b) of a breccia clast comprising amphibolephyric andesite fragments set in a reddish glassy matrix. The smaller lava fragments are subangular to amoeboid in shape, with lobate and crenulate margins. The larger lava fragment has well-developed glassy chilled margins. The oxidized glassy matrix shows abundant sparse crystals (plagioclase). Many fragments show jigsaw-fit textures suggest that block-and-ash flows were generated from the collapse of active domes rather than from older dome remnants. Therefore, we interpret the monomictic amphibolephyric andesite breccia facies as block-and-ash flow deposits produced by the collapse of lava domes. Interbedded stratified coarse and fine tuff layers and erosional contacts define the boundaries between successive block-and-ash-flow deposits. In the monomictic andesitic lobe breccia facies, the joint pattern (Fig. 5b) is interpreted to define hot magma lobes that have been quenched (De Rosen-Spence et al. 1980; Yamagishi and Dimroth 1985; Kano 1996). The increasing outward fracturing, jigsaw-fit textures, and the envelope of massive monomictic breccia indicate clast production by insitu quench fragmentation (Cas et al. 2001). On the basis of these textures, we interpret this facies to represent a lava flow that was incompletely quenched by contact with water, in a subaqueous setting, generating lobes and hyaloclastite (McPhie et al. 1993; Yamagishi 1987, 1991; Kano et al. 1991). The irregular basal contact with the stratified tuff facies (Fig. 5b) may be interpreted as: (i) a load structure developed below the largest lava lobes and deforming the finer and probably unconsolidated underlying pyroclastic layer; or (ii) an erosive surface produced by the flowing lava. The upper irregular contact may be considered as the original upper surface of the bed from which the largest lava lobes protruded above the finer deposit. The exposed upper surface of lava lobes was probably subsequently eroded because the overlying matrix-rich polymictic volcanic breccia contains a few fragments of the lava lobes at its base (Fig. 5b). Polyhedral blocky clasts defined by curviplanar fractures in the monomictic olivine-phyric andesite breccia (Fig. 5c) suggest quench fragmentation of lava that flowed into water (Yamagishi 1987; Kano et al. 1991) or extruded underwater. Stratified matrix-rich polymictic volcanic breccia can be interpreted as a resedimented syn-eruptive facies on the basis of the following observations: (a) The entire deposit is stratified and the sedimentation units are relatively thin (1 3 m; Fig. 6a); (b) the clast population includes fresh, possibly juvenile, volcanic clasts with different textures; (c) primary clast shapes are largely unmodified; and (d) this facies is intercalated with compositionally similar intrabasinal primary volcanic facies (stratified fine and coarse tuff and massive pumice tuff-breccia). The massive, poorlysorted, and disorganized nature of the beds indicates massflow transport processes. Glass splinters on the surface of the armoured fluidal lava lobes may represent glass fragments derived from spalling of the carapace of the lava dome and welded on the surface of hot portions of lava that broke the dome crust. This texture suggests that the redeposition occurred without significant clast abrasion and lapse of time after the eruption. Clasts grouped in jigsaw-fit domains may have been incorporated in the mass-flow as: (a) Hot magma pods that broke in situ by cooling contraction granulation during transport (Doyle and McPhie 2000); or (b) cold and fractured, dense juvenile clasts that progressively disintegrated by dilation during transport (Davies et al. 1999). Andesite clasts (Fig. 6) retain shapes (fluidal lobes and polyhedral blocks), internal textures (radial fractures, breadcrust surfaces, glassy margins, and contractional cracks), and petrographic characteristics (perlitic, glassy and pumiceous groundmass) which suggest chilling and brecciation of hot lava probably in the

11 Bull Volcanol (2009) 71: presence of water or wet sediments. The clasts of monomictic breccia composed of vitrophyric fragments (Fig. 7) can be interpreted as hyaloclastite intraclasts. Similar shape, textural, and petrographic characteristics were partly described in subaqueous lava domes (Cas et al. 1990; De Rita et al. 2001; Goto and Tsuchiya 2004), and at the margin of subaerial domes that came in contact with water (Kokelaar 1986; Maeno and Taniguchi 2006). In the case of the stratified matrix-rich polymictic volcanic breccia, we interpret the juvenile components to be derived from a lava dome(s), the marginal parts of which interacted with surface water. Quenching of the dome margins generated abundant glassy, perlitic, fractured coherent and breccia facies subject to syn-eruptive collapse. Gravity-driven failures at the margins of the dome and deposits derived from it produced resedimented sediment gravity flow deposits. The polymictic volcanic breccia facies (Fig. 5d), which contains basement and non-juvenile lava clasts, can be interpreted as volcanogenic sedimentary deposits. Basement clasts were produced by the erosion of the incipiently uplifted and exposed Eastern Cordillera basement, W of El Toro basin. Massive and poorly sorted textures indicate the likely transport and deposition processes were mass flow. The limited exposure and the absence of near-vent facies prevent discrimination between subaerial and subaqueous environments for the extrusion of the lava flows and lava domes that generated the volcaniclastic rocks of the Las Cuevas member. Considering the paleogeographic scenario, it is more likely that the lava flows and lava domes originated in subaerial environment and then flowed into and interacted with standing bodies of water. The internal variation of lithofacies from the base to the top of the Las Cuevas member (Fig. 3) records a transition from primary volcanic facies in a likely subaerial environment to resedimented volcaniclastic facies in a water-dominated setting. The regional context (Reynolds et al. 2000; Hernandez et al. 2005) constrains the setting to be continental, with the depositional environment probably ranging from shallowwater lacustrine, to swampy and alluvial. Despite the absence of coherent lava exposures and nearvent structures, the distinctive lithofacies and composition of the primary volcanic and dome-derived syn-eruptive resedimented facies demonstrate that the Las Cuevas member represents the product of a volcanic centre, nearby but outside the present outcrop area. quebrada Carachi ( S, W, 2,856 m a.s. l.; Figs. 1c and 8). The Lampazar member is composed of stratified polymictic pumice-matrix breccia and sandstone intruded by coherent andesite (SiO wt.%). A K/Ar age of 7.87±0.12 Ma has been determined on a juvenile pillow-lobe of andesite within the member (Mazzuoli et al. 2008). Volcaniclastic sediments dip to the SE and show a stratigraphic thickness of about 100 m. The base of the Lampazar member is not exposed. The overlying unit consists of weakly stratified reddish non-volcanic conglomerate and sandstone (Figs. 2 and 8) forming part of the Alfarcito conglomerate (Marrett and Strecker 2000; Hilley and Strecker 2005). The contact is erosive with an angular unconformity. In fact, the Alfarcito conglomerate dips to the N and the basal portion is represented by a coarse layer of cobbles and boulders of basement rocks that truncates the stratification of the Lampazar member. The Case study 2: Shallow-level andesite intrusion in coarse-grained, wet volcaniclastic sediments (Lampazar member) The studied stratigraphic section crops out in the southernmost tributary valley on the hydrographic left bank of the Fig. 8 General view (a) and schematic sketch (b) of the Lampazar member in the quebrada Carachi section. Box indicates the location of Fig. 9

12 520 Bull Volcanol (2009) 71: volcaniclastic deposits of the Almagro A member cap the stratigraphic section (Fig. 8). The Lampazar member dominantly comprises four volcanic lithofacies that are described and interpreted in the following sections, and summarized in Table 2. Stratified polymictic pumice-matrix breccia and conglomerate This lithofacies is composed of interstratified breccia, conglomerate and sandstone; it is crudely stratified, and occurs in massive beds 1 3 m thick. The conglomerate is composed of well-rounded pebbles and cobbles consisting of basement rocks (>50%) and subordinate different types of lavas (amphibole-phyric andesite, clinopyroxene-phyric andesite). The breccia is composed of sub-rounded to angular clasts of pumice, different types of lava and scoria, and subordinate basement rocks. The matrix of both the conglomerate and breccia comprises sandstone and granuleconglomerate with lithic and pumice fragments of the same composition as the clasts in the breccia. Conglomerate and breccia are present in subequal proportions. Sandstone and granule-conglomerate also form separate beds interlayered with the breccia. Coherent amphibole-phyric andesite Coherent amphibole-phyric andesite forms a fingered sheet intrusion (e.g. Pollard et al. 1975) composed of numerous closely spaced fingers, 0.5 to 2 m thick and 3 5 m in visible length (Fig. 9a, b). The andesite is massive, pale grey in colour, and micro-vesicular, with phenocrysts of amphibole and plagioclase in a hyalopilitic groundmass. Fingers dip 70 to the SE and are broadly conformable with the stratification of the host volcaniclastic sediments (Fig. 8). Fingers show two systems of joints: (a) parallel, and (b) perpendicular to the finger elongation. Some of these joints are intersecting conchoidal fractures that define polyhedral fracture-bounded domains (Fig. 9a, b). Fingers are packed together or are separated by stringers and pockets ( m wide) of a non-stratified sediment-matrix conglomerate. The contacts between the coherent andesite and this conglomerate are characterized by: (a) single pebbles or cobbles or pockets of pebbles and cobbles incorporated into the coherent andesite (Fig. 9b); (b) transition from fractured coherent andesite to groups of tightly packed jigsaw-fit blocky andesite clasts; and (c) sharp, smooth, curviplanar and glassy surfaces. Pillowed amphibole-phyric andesite within non-stratified sediment-matrix conglomerate The amphibole-phyric andesite fingered sheet appears to be partly interconnected with fluidal tongues and apophyses forming round, irregular or amoeboid pillow lobes, one metre to several metres in size, packed near the coherent amphibolephyric andesite finger edges and enclosed within a nonstratified sediment-matrix breccia (Fig. 9a, b). The pillows are composed of amphibole-phyric andesite of the same composition and phenocryst assemblage as the coherent amphibole-phyric andesite facies. The pillows are dark grey to black in colour, aphyric or sparsely porphyritic (amphibole and plagioclase) in the core and perlitic at the margins. The pillow interiors show well developed concentric and radial joints, which in some cases are curviplanar (Fig. 9). A few pillows have a vesicular core and well developed external rind with radial columnar joints (Fig. 9c, d). A few pillows are red or brown as a result of oxidation. In some cases, margins of pillows are fractured and consist of polyhedral blocky clasts that show jigsaw-fit textures. The non-stratified sediment-matrix conglomerate comprises polymictic, well rounded, sub-spherical or ellipsoidal in shape, pebbles and cobbles (Fig. 9) in a sand-sized matrix of the same composition. Clasts are composed of basement rocks (siltstone, grey granodiorite) and subordinate non-juvenile lavas. The non-stratified sediment-matrix conglomerate also penetrates between the fingers of the coherent andesite (Fig. 9) and into the fractures of the pillows. Pillow-bearing non-stratified polymictic pumice-matrix breccia This very coarse breccia facies, about 30 m thick, is composed of both fluidal pillows and angular blocky amphibole-phyric andesite clasts isolated within massive matrix comprising fine sand- to cobble-sized pumice and lithic fragments (Fig. 10a). Pumice clasts are sub-rounded to sub-angular, with surfaces that intersect the vesicle walls and vesicles filled by the finer fragments. The lithic clasts are composed of different types of lavas and basement rocks. The amount of the amphibole-phyric andesite clasts ranges between 50% in volume near the coherent amphibole-phyric andesite facies and 10% at a distance up to m from the coherent amphibole-phyric andesite facies. The components of the pillow-bearing non-stratified polymictic pumice-matrix breccia are very similar to those of the stratified polymictic pumice-matrix breccia facies. The contacts between these two breccia facies are very subtle and gradational (Fig. 10c). The amphibole-phyric andesite pillows have well-developed radial and concentric joints, generally filled with the nonstratified polymictic pumice-matrix breccia (Fig. 10b, c). In many cases, they show porphyritic texture at the core and rims up to 8 mm thick consisting of perlitic glass (Figs. 11 and 12). The glassy rim is locally fragmented, and fragments are dispersed in the matrix or occur in

13 Bull Volcanol (2009) 71: Table 2 Lampazar member: summary of the lithofacies, their characteristics and interpretation Lithofacies Characteristics Interpretation Components, texture and composition Bedforms, sedimentary structures, and facies architecture Clast-forming processes Coherent amphibolephyric andesite (Fig. 9) Pillowed amphibolephyric andesite (Fig. 9) Non-stratified sedimentmatrix breccia (Fig. 9) Pillow-bearing nonstratified polymictic pumice-matrix breccia (Fig. 10) Stratified polymictic pumice-matrix breccia and sandstone Massive, evenly porphyritic with plagioclase and amphibole phenocrysts; microvesicular and hyalopilitic groundmass Vitrophyric, aphyric or sparsely porphyritic texture with plagioclase and amphibole phenocrysts; radial and concentric joints; chilled glassy margins; vesicular core Well-rounded pebbles and cobbles of basement rocks and non-juvenile lavas in a matrix of coarse sandstone and fine conglomerate (i) Fluidal globular juvenile clasts, glassy rim, interstitial perlitic glass; radial and concentric joints filled with pumiceous sandy matrix; (ii) blocky angular juvenile clasts, with sharp, smooth and rectilinear surfaces; fine sand- to pebble-sized matrix composed of pumice, different types of lavas, and basement rocks Pebble and cobble-sized clasts of variable proportion of basement rocks, non-juvenile lavas, pumice and scoria; sandy to granular matrix of the same composition as the clasts Finger-like protusions, m thick; polyhedral joints; local jigsaw-fit texture; local curviplanar and glassy margins; incorporating conglomerate pebbles and cobbles Pillows, m in size; jigsaw-fit textures; interconnected with the coherent amphibole-andesite; enclosed within the non-stratified sediment-matrix breccia Massive; clast- to matrix-supported; along the margins of the coherent amphibole-andesite fingers and among the pillows Matrix-supported, massive, and poorly sorted; jigsawfit texture at the margin of juvenile clasts Interstratified breccia, conglomerate and sandstone; massive to diffusely planar layered beds, 1 3 m thick; unsorted to poorly sorted; matrix-supported Quench fragmentation Quench fragmentation Quench fragmentation Autoclastic, pyroclastic and epiclastic fragmentation Transport and deposition processes and environment Coherent facies of synvolcanic, shallow-level fingered intrusion in wet sediments Pillowed lobes from the edge of the fingered intrusion in wet sediments Packed peperite in wet sediments around the fingered sheet intrusion Peperite in wet volcaniclastic sediments Resedimented volcaniclastic materials in alluvial and lacustrine setting

14 522 Bull Volcanol (2009) 71: Fig. 9 Photograph (a) and schematic sketch (b) showing interfingering contact between the lithofacies of the Lampazar member. The coherent pyroxene-phyric andesite of a fingered intrusion grades into pillowed pyroxene-phyric andesite, and both are enveloped by nonstratified sediment-matrix breccia. Well-rounded conglomeratederived pebbles and cobbles are incorporated within coherent andesite fingers and andesite pillows. We interpret this mingling between sediment and juvenile andesite as peperite. c, d Detail of a pillow showing the interconnection with a coherent pyroxene-phyric andesite finger, the pillow vesicular core and the pillow external rind with radial columnar joints. The non-stratified sediment-matrix breccia penetrates between the coherent andesite fingers groups with jigsaw-fit texture (Fig. 11). Ovoid pillows have vesicular cores. At the margin of some pillows are groups of curved, concentric andesite splinters (Fig. 10b) showing a jigsaw-fit texture and separated by the nonstratified polymictic pumice-matrix breccia. The angular blocky amphibole-phyric andesite clasts (Fig. 10a, d) are composed of vitrophyric to sparsely porphyritic (amphibole and plagioclase) andesite with polyhedral fractures and irregular vesicularity. These clasts are bounded by sharp, smooth surfaces, or by brecciated zones with jigsaw-fit textures (Fig. 10d). The fractures between jigsaw-fit fragments are filled by the non-stratified polymictic pumice-matrix breccia. Moreover, the angular blocky clasts are characterized by well-developed glassy margins, up to 10 mm wide, with irregular fiamme-like lenses of black glass along the margin (Fig. 10d). These obsidian fiamme-like lenses are defined by a different degree of vesiculation of the glassy rim. In some cases, clusters of blocky amphibolephyric andesite clasts form jigsaw-fit aggregates. Interpretation Interfingering contacts between the coherent amphibolephyric andesite, pillowed andesite, and pillow-bearing non-

15 Bull Volcanol (2009) 71: Fig. 10 Pillow-bearing non-stratified polymictic pumice-matrix breccia lithofacies of the Lampazar member. a Non-stratified, very coarse, polymictic, pumice-matrix breccia containing both angular juvenile clasts and pillow-like juvenile pods. b Pillow lobe showing chilled margins, radial joints along the margin, and internal concentric joints. Marginal fractures are filled with pumiceous sand-sized matrix, and the pillow is progressively fragmented outward along the curved joints into jigsaw-fit fragments. c Fractured pillow lobe and concentric pillows dispersed within the non-stratified polymictic pumice-matrix breccia facies. Irregular contact (dashed white line) with stratified polymictic pumice-matrix breccia is also shown. d Blocky andesite juvenile clast showing both well developed, sharp, and chilled margin, and gradational, fragmented contact into the enclosing non-stratified polymictic pumice-matrix breccia. Polyhedral fractures are filled by polymictic pumice-matrix breccia stratified polymictic pumice-matrix breccia suggest that the coherent amphibole-phyric andesite facies represents a shallow-level, syn-volcanic, fingered sheet-like intrusion (Pollard et al. 1975; Squire and McPhie 2002). The geometry and extent of the intrusion are uncertain due to limited lateral exposure and cover by colluvium. The present high-angle dip of the andesite fingers (Fig. 8) is due to tectonic folding of the El Toro basin sequence. Fluidal shapes, contractional joints and complete glassy rims in the pillows, and obsidian fiamme-like lenses and complete glassy rims in the angular blocks suggest that these two types of fresh amphibole-phyric andesite clasts Fig. 11 Polished slab (a) and schematic sketch (b) showing the contact between fluidal juvenile clasts and the pumiceous sand-sized matrix in the Lampazar member. Juvenile component is represented by amphibole-phyric andesite with internal porphyritic texture, and perlitic glass along the margin. The chilled glassy margin is locally fragmented and dispersed as hyaloclastite in the pumiceous matrix

16 524 Bull Volcanol (2009) 71: Fig. 12 Photomicrograph of the margin of an andesite pillow in the peperite domain of the Lampazar member. The rim of perlitic glass is inferred to have formed by quenching of magma at the contact with wet volcaniclastic sediment. a, c In planepolarized light; b, d are same views in crossed-polarized light. Pl plagioclase, Hbl hornblende within the pillow-bearing non-stratified polymictic pumicematrix breccia are juvenile clasts. On the basis of facies relationships and textures, we interpret the pillow-bearing non-stratified polymictic pumice-matrix breccia and nonstratified sediment-matrix breccia facies to be peperite (Brooks et al. 1982; White et al. 2000; Skilling et al. 2002) generated as a mingling between an andesite sheet intrusion and coarse volcaniclastic sediments represented by the stratified polymictic pumice-matrix breccia and conglomerate facies. Our interpretation of peperitic mingling of molten andesite with unconsolidated or poorly consolidated, water-saturated, volcaniclastic deposits is supported by: (a) The gradational contacts between the parent intrusion of coherent andesite and pillow-like fluidal andesite clasts (Fig. 9); (b) the jigsaw-fit texture of many groups of blocky andesite clasts (Fig. 10b,d); (c) the presence of thick and complete glassy rims on both fluidal and blocky andesite clasts (Figs. 10c, d, 11 and 12); (d) the presence of the host sediments within fractures penetrating the andesite and between jigsaw-fit clasts (Figs. 9 and 10b d); and (e) the destruction of bedding in the host sediments around the andesite clasts (Fig. 10c). Due to rapid quenching and mixing with unconsolidated coarse volcaniclastic sediments, the andesite did not spread far from its conduit. The leading edge was finger-shaped and graded into a pillowed intrusion (Hanson 1991; Kano 1991). Pillows are tightly packed near the intrusion fingers and gradually become dispersed in the host sediment as far as 50 m from the parent intrusion. The destruction of bedding in the host around the pillows and the sedimentfilled fractures suggests that lava quenching was accompanied by fluidization of the host sediment (Kokelaar 1982; Kano 1989; Goto and McPhie 1996; Hanson and Hargrove 1999). In situ fragmentation of andesite clasts occurred at macro- and micro-scale. Micro-scale peperite was formed by millimetre-scale mingling between intruding andesite magma and fine-grained volcaniclastic sediments (Fig. 11). The syn-volcanic amphibole-phyric andesite intrudes a volcano-sedimentary succession of stratified polymictic pumice-matrix conglomerate and breccia interbedded with vitric-crystal sandstone. Volcanic activity coeval with or preceding the intrusion is recorded by pumice and lava clasts in these host sediments. In contrast, the Alfarcito conglomerate overlying the Lampazar member is lacking in volcanogenic clasts (Fig. 8). Pumice within the polymictic pumice-matrix breccia facies is inferred to record explosive volcanic activity during the Lampazar member deposition. The host sediments are interpreted as volcanogenic sedimentary deposits. Bed-forms and textures within the host sedimentary unit are consistent with deposition in a continental alluvial environment. The coherent andesite intrusion, together with volcanogenic clasts in the host sediments to the peperite probably record the existence of a volcanic centre located near the present exposures of the Lampazar member in the quebrada Carachi.

17 Bull Volcanol (2009) 71: Case study 3: Intrusive pillows in coarse-grained, wet volcaniclastic sediments (Almagro A member) The Almagro A member occurs in a wide area comprising both the Almagro range and the El Toro basin (Figs. 1 and 2). This volcanic member has K/Ar age of 7.20±0.11 Ma (Mazzuoli et al. 2008), and is generally composed of volcanic breccias interbedded with pyroclastic flow deposits and fluvial volcaniclastic conglomerates. In the Almagro range, the Almagro A member unconformably overlies the Puncoviscana Formation, is conformably overlain by the volcaniclastic deposits of the Almagro B member, and is intruded by the feeder dykes, domes and cryptodomes of the Almagro B and C members (Fig. 2). In the El Toro Basin, the Almagro A member overlies the Alfarcito conglomerate (Marrett and Strecker 2000; Hilley and Strecker 2005), and is unconformably overlain by the volcaniclastic deposits of the Almagro B member (Fig. 2). The studied stratigraphic section is exposed along the banks of the quebrada Lagunillas flood plain, near the Diego de Almagro railway station (Figs. 1c and 13; S, W; 3,320 m a.s.l.). At this locality, the volcaniclastic sequence was deformed by a NW-striking and sub-vertical transtensional fault belonging to the Calama-Olacapato-El Toro fault system (Acocella et al. 2007; Mazzuoli et al. 2008); consequently the Almagro A member dips to the W. The stratigraphic thickness at the studied locality is 200 m in a 600-m-long exposure. Two principal lithofacies were recognized in this outcrop (Fig. 13). They are described in the following sections and in Table 3. Stratified polymictic breccia Stratification in the polymictic breccia is defined by variations in clast size and matrix/clast ratio, alignment of coarser clasts, and intercalations of coarse sandstone with the same clast composition as the breccia (Fig. 13). Beds are massive, irregular and often discontinuous. Bed contacts are gradational and amalgamated. Few beds show Fig. 13 a A general view of the Almagro A member in the section near the Diego de Almagro railway station. Pyroxene-phyric andesite bodies (white lines) are dispersed in the stratified polymictic breccia interbedded with sandstone (yellow lines). The pumice lapilli-tuff bed intercalated in the upper part of the breccia is not visible in this picture. Boxes indicate the location of the c, d and 14c. b Schematic sketch illustrating the stratigraphic relationships between the lithofacies. c, d Details of stratigraphic relations between pyroxene-phyric andesite bodies and host sediments in the Almagro A member. Pyroxene-phyric andesite bodies crosscut the sediment stratification and are enveloped by massive sandstone, suggesting their emplacement as intrusive pillows and destruction of primary lamination by local fluidization. White arrows point to sets of angular clasts into the stratified polymictic breccia that appear to have been oriented parallel to the deformed sandstone boundary

18 526 Bull Volcanol (2009) 71: Table 3 Almagro A member: summary of the lithofacies, their characteristics and interpretation Lithofacies Characteristics Interpretation Components, texture and composition Bedforms, sedimentary structures, and facies architecture Clast forming processes Transport and deposition processes and environment Pyroxenephyric andesite bodies (Figs. 13 and 14) Stratified polymictic breccia (Fig. 13) Globular or tubular fluidal clasts (0.5 3 m); microcrystalline, massive core; concentric rinds; laminated concentric foliation; glassy quenched rim with radial columnar joints; microcrystalline or glassy groundmass, phenocrysts of plagioclase and clinopyroxene Pebbles to boulders (1 3 m)of basement rocks and amphibolephyric andesite lithic and crystal sandy to granular matrix Discordant with bedding in the stratified polymictic breccia; lamination in the host sandstone beds is deformed or not preserved Weakly stratified in massive and irregular beds (0.5 7 m); unsorted to poorly sorted; clast- to matrixsupported; normal or inverse graded; interbedded with massive whitish sandstone and laminated sandstone and siltstone Epiclastic fragmentation Intrusive pillows at the apophysis of a feeder dyke; intrusion into fluvial and lacustrine sediments Syn-volcanic debris-flow, in a braided fluvial plain with lacustrine episodes normal or inverse grading. Clasts comprise basement rocks (black siltstone of the Puncoviscana Formation, grey granodiorite and red granite of the Santa Rosa de Tastil batholith) and non-juvenile lava (pink-grey microvesicular andesite with phenocrysts of amphibole and plagioclase). Clasts are angular to sub-rounded. The matrix is sand- and granule-sized, composed of lithic fragments (siltstone, granite, granodiorite, and altered lavas) and crystal grains (plagioclase, quartz). The lower part of this facies is matrix-poor, roughly stratified in massive beds, 1 7 m thick (Fig. 13), and dominated by clasts of basement rocks ( vol.%). The upper part is matrix-rich, diffusely stratified in irregular beds, m thick, and characterized by the increase in abundance of Fig. 14 The pyroxene-phyric andesite bodies within the Almagro A member. The hammer is 30 cm long. a Picture and b schematic sketch of a pyroxene-phyric andesite body, showing two concentric rinds and indentation against clasts in the host breccia. c Picture and d schematic sketch of a tubular pyroxene-phyric andesite body showing concentrically zoned textures and elliptical cross section. The attitude of the tubular body is oblique with respect to the breccia stratification and outcrop face. The andesite body deforms the planar stratification of the host sandstone

19 Bull Volcanol (2009) 71: amphibole-phyric andesite clasts (50 60 vol.%) and of andesitic vitric fragments in the matrix. Beds of massive and planar laminated, whitish, vitric fine sandstone and siltstone, m thick, are interlayered within the polymictic breccia (Fig. 13). A bed of massive lapilli-tuff, 1 m thick, composed of vitric ash matrix (80%) and white, poorly-vesicular andesite pumice and pink amphibole-phyric andesite clasts, 1 2 cm sized, is intercalated with the upper part of the breccia. Intrusive pyroxene-phyric andesite pillows Pyroxene-phyric andesite forms discrete, oblate to irregular bodies (Figs. 13 and 14) that are non-uniformly distributed and apparently isolated within the stratified polymictic breccia over a stratigraphic thickness of about 20 m. The andesite is dark grey, with a microcrystalline or glassy groundmass, and scattered phenocrysts of plagioclase, 2 5 mm in size, and subordinate smaller clinopyroxene. This lithology is not present in the normal clast population of the stratified polymictic breccia. The pyroxene-phyric andesite bodies are discordant relative to bedding in the stratified polymictic breccia (Fig. 13c, d). They show sharp contacts with the enveloping sediment. Within the contact zone, generally <20 cm across, the lamination in the sandstone beds is truncated, disrupted or contorted. The pyroxenephyric andesite bodies are surrounded by an aureole, 1 30 cm thick, with massive texture, composed of the finer components of the host sediments (sandstone, siltstone, and sand-sized matrix of the polymictic breccia). This aureole crosscuts the sedimentary bedding (Fig. 13c, d). Several centimetre- to decimetre-sized clasts of the polymictic breccia are also rearranged parallel to the margin of this aureole (Fig. 13c, d). In some cases, pyroxene-phyric andesite bodies deform planar lamination in the sandstone and siltstone layers along both the lower and upper margins of the clasts (Fig. 14c, d). The pyroxene-phyric andesite bodies are defined by continuously curving outer surfaces and are decimetre- to metre-sized. In one case, an elongate tubular body up to 3 m in length, cm wide, and elliptical in cross section, is exposed (Fig. 14c, d). The margins of the pyroxene-phyric andesite bodies are plastically deformed and fluidally indented against adjacent clasts of the host breccia (Fig. 14a, b) (c.f. Brooks et al. 1982). The pyroxene-phyric andesite bodies are internally texturally zoned (Fig. 14). Their core is microcrystalline, massive and coherent, commonly with planar flow bands parallel to body margins, irregular vesicularity, and polyhedral joints. The margins of the pyroxene-phyric andesite bodies show two distinct concentric rinds with different textures (Fig. 14). The glassy to microcrystalline, inner rind contains closely spaced concentric foliations, and is oxidized to a yellow-orange color. The foliation planes follow concentric bands parallel to the outer surface, and consist of alternating vesicular and non-vesicular bands on a millimetre to centimetre scale. Vesicles are commonly aligned parallel to the foliation bands (c.f. Kano et al. 1991). The outer rind consists of a continuous glassy andesite rim, 1 5 cm thick, with joints perpendicular to the outer surface (Fig. 14). The surfaces of the pyroxene-phyric andesite bodies are smooth, with polyhedral contractional cracks. In a few clasts, marginal radial fractures are filled with the fine sandstone matrix of the stratified polymictic breccia. Interpretation In the lower part of the stratified polymictic breccia, clast textures and types suggest a basement-derived provenance, epiclastic fragmentation and transport processes, and deposition by debris flow, in a distal position with regard to the source of the volcanic clasts. The common intercalations of stratified sandstone and laminated fine sandstone and siltstone in the upper part of this facies suggest a fluvial braid plain setting with periodic lacustrine deposition. The proportion of lava clasts and vitric matrix components increases upward, suggesting an increase in coeval volcanic activity. The interpretation of the pyroxene-phyric andesite bodies is not entirely clear. In fact, the observations on their characteristics can be explained by three models: (1) they represent spheroidal clasts resulting from weathering; (2) they are resedimented from a coeval, possible active andesitic lava or dome nearby; or (3) they are part of a dismembered younger intrusion. Spheroidal weathering is a common feature of basic lavas (e.g. Patino et al. 2003), but concentric arrangement of textures is also described in pillow lavas (e.g. Walker 1992), at the marginal parts of lava domes (Maeno and Taniguchi, 2006), and in intrusive pillows (Yamagishi 1987; 1991). The following observations support an interpretation of the pyroxene-phyric andesite bodies in terms of in situ weathered clasts: (1) they are characterized by a central unaltered core and an external concentric onion-like foliation; and (2) their inner rind is oxidized. The second model is suggested by: (1) the absence of a hyaloclastite envelope; (2) the dispersion of the pyroxene-phyric andesite bodies in the host sediments; and (3) the absence of the evidence, in the two-dimensional outcrop surface, that these bodies are elongate, interconnected tubes. The third model considers the pyroxene-phyric andesite bodies as intrusive lobes of new magma that may represent a younger igneous intrusion mingled with an older sedimentary deposit. The following observations and considerations argue in favor of this hypothesis. (1) There are systematic lithological differences between the pyroxene-phyric andesite bodies and the volcanic components of

20 528 Bull Volcanol (2009) 71: the hosting stratified polymictic breccia of the Almagro A member. Moreover, the pyroxene-phyric andesite bodies match the petrographic composition of the lavas of the younger Almagro B member. (2) The pyroxene-phyric andesite bodies exhibit a distinct glassy outer rind with joints perpendicular to the outer surface (Fig. 14) suggesting that these clasts were hot lobes of molten lava that chilled surrounded by cold, host sediments. Moreover, this glassy margin does not show textures or an alteration mineral assemblage attributable to weathering. (3) The pyroxene-phyric andesite bodies show alternating vesicular and non-vesicular bands concentric and parallel to the outer surface (e.g. Templeton and Hanson 2003). (4) The pyroxene-phyric andesite bodies show a smooth, continuously curved outer surface that in some cases was plastically deformed near angular clasts of the stratified polymictic breccia (Fig. 14a, b) (e.g. Brooks et al. 1982). (5) Thin tendrils of sandstone occupy the joints along the margin of the pyroxene-phyric andesite bodies and an aureole of homogenized sediments occur around these clasts (Fig. 13c, d), suggesting that quenching was accompanied by fluidization of the host sediment (Kokelaar 1982; Goto and McPhie 1996). (6) The bedding in the host sediment was deformed upward and downward (Figs. 13c, d and 14d) and abruptly truncated (Fig. 13c, d) (e.g. Hanson 1991) by the pyroxene-phyric andesite bodies. (7) The amphibole-phyric andesite clasts of the stratified polymictic breccia do not show concentric foliation and textural or mineralogical evidence of alteration related to weathering. The different mineral paragenesis of the two andesite clast types enclosed into the same polymictic breccia deposit does not support a different degree of in situ weathering. The above described arguments favor the model of intrusive lobes of new magma. As a consequence, we interpret the pyroxene-phyric andesite bodies as intrusive pillows resulting from intrusion of andesite magma, related to the younger Almagro B member, into the coarse-grained volcaniclastic sediments of the Almagro A member. Fluidization processes are inferred to have affected textures in the host sediments, suggesting further that the intrusion was emplaced in a wet or water-saturated environment. The pyroxene-phyric andesite intrusive pillows are generally localized at the contact between fine and coarse breccia beds, and the deformation of the bedding of the host Fig. 15 The inferred structural setting during the emplacement of the BAT volcaniclastic deposits showing the paleoenvironmental conditions of the magma water interaction. Not to scale and no relative timing is indicated. a Magma water contact in lacustrine environment during lava flow and lava dome emplacement. b Shallow intrusion in coarse volcaniclastic alluvial deposits with development of widely dispersed peperite domain. c Feeder dike intrusion in coarse volcaniclastic breccia with detached concentric pillow lava lobes. In the models a and c, the proximity to a volcanic feeder structure is speculative