José A. Naranjo

High velocity debris avalanche at Lastarria volcano in the north Chilean Andes

A pre-historic collapse of the southeast flank of Lastarria volcano (∼ 5700 m) in the north Chilean Andes (25° 10′ S), produced a fluidized volcanic debris avalanche whose morphology and surface structures are exceptionally well preserved. The avalanche travelled to the east-south-east, covering an area of 9.3 km2, and came to rest after climbing and over-riding a 125 m high older scoria cone. The 0.091 km3 avalanche has an apparent coefficient of friction (H/L) of 0.15 and an excessive travel distance index (Le) of 5.1 km, indicating high emplacement velocity, perhaps of the order of 80 m s−1. An important cause of the high mobility may have been the predominance of low-density, poorly cohesive scoriaceous and pumiceous layers in the source region. The flow may have had properties similar to those of a small ignimbrite.

The geology of Nevados de Chillán volcano, Chile

Nevados de Chillan volcano is a large composite stratovolcanic complex in the Southern Volcanic Zone of the Chilean Andes. It is one of the highest-risk volcanoes in Chile due to high levels of historic activity and rapid development of economic activity in the area. High precision 40 Ar/ 39 Ar and 14 C geochronology, geochemistry and petrology have been employed in addition to photogeology and field mapping to elucidate the evolution of this volcano and assess its hazards. Nevados de Chillan has been active since at least 640 ka when a large group of subglacial andesite flows were erupted. Since 100 ka, sequences of andesite and dacite lavas have been erupted into both subaerial and subglacial environments. Ignimbrites were erupted at around 40 ka and may have been associated with caldera collapses. Two separate eruptive centres have evolved since 40 ka: the Cerro Blanco and Las Termas subcomplexes. The two centres are 6 km apart, yet have contemporaneously erupted geochemically distinct magmas. Subglacial lavas have been identified on the high flanks of the volcano and 40 Ar/ 39 Ar dating has confirmed their eruption during recent glaciations (isotope stages 4 and 2). Tephra fall deposits have been dated by 14 C analysis of interstratified organic material and indicate that no proximal tephra fallout deposits older than 9 ka remain. Tephra dispersal indicates that Holocene activity has involved vulcanian to subplinian eruptions. At least, 3 pyroclastic flow eruptions have occurred during the Holocene and lahar deposits are common in the valleys around the volcano. Historically, the Santa Gertrudis vent erupted during 1861-1865 and the dacite lava cone complexes Nuevo and Arrau were constructed during 1906-1943 and 1973-1986, respectively. Historic records indicate that lahars and landslides are major hazards to economic developments on the lower flanks and valleys

Geochemistry of Nevado de Longaví Volcano (36.2°S): a compositionally atypical arc volcano in the Southern Volcanic Zone of the Andes

The Quaternary Nevado de Longavi volcano of the Andean Southern Volcanic Zone (SVZ) has erupted magmas that range in composition from basalt to low-silica dacite, although andesites are the dominant erupted magma type. Amphibole is a common phenocryst phase in andesites throughout the volcano, and it is the dominant mafic phase in Holocene dacites and their included mafic enclaves. Compositions of magmas erupted at Longavi volcano define arrays that diverge from trends delineated by neighboring frontal-arc volcanoes. Although mafic compositions at Longavi are broadly similar to basalts at other SVZ centers, Longavi intermediate and evolved magmas have systematically lower abundances of incompatible major (K 2 O, P 2 O 5 ) and trace elements (Rb, Zr, Nb, REE, Th, etc), as well as high Ba/Th, Sr/Y, and La/Yb ratios. Longavi volcano magmas define two differentiation series with regard to enrichments of Rb (and other incompatible elements) with increasing silica. A high-Rb series that includes the oldest units of the volcano comprises basalts to andesites dominated by anhydrous mineral assemblages with chemical compositions similar to other SVZ magmatic series. The series with low Rb, on the other hand, includes the Holocene units that evolved from basaltic andesites to dacites by means of fractional crystallization wherein amphibole and calcic plagioclase dominate the mineral assemblage. Magmas parental to low-Rb series are interpreted to be high-degree mantle melts, highly hydrous and oxidized, formed as a response to high fluid inputs into the subarc mantle. Enhanced water transport to the subarc mantle is a plausible effect of the subduction of the oceanic Mocha Fracture Zone that projects beneath Nevado de Longavi. Volcanoes located over oceanic fracture zones further south along the SVZ have erupted hornblende-bearing magmas that share some chemical similarities with Longavi volcano magmas.

Subvolcanic contact metasomatism at El Laco Volcanic Complex, Central Andes

Studies of drill cores from the Pasos Blancos area at El Laco in the central Andes, northern Chile, give evidence of an intense and extensive subvolcanic contact-metasomatic process. This process resulted from shallow-level emplacement of very volatile-rich iron-oxide magma, with discharge of volatiles that resulted in extensive fracturing of overlying volcanic rocks. The brecciated rocks were altered (mainly extensive scapolitization and formation of pyroxene) by hot magmatic fluids emitted from the cooling intrusion, and accompanied by magnetite deposition. With time and decreasing temperature, the metasomatic fluids evolved to fluids of hydrothermal character, and a final recent geothermal event took place that deposited superficial gypsum over a large part of the El Laco Volcanic Complex.

The magmatic and eruptive response of arc volcanoes to deglaciation: Insights from southern Chile

In tectonic settings where decompression melting drives magmatism, there is compelling evidence that changes in ice loading or water loading across glacial-interglacial cycles modulate volcanic activity. In contrast, the response of subduction-related volcanoes remains unclear. A high-resolution postglacial eruption record from a large Chilean stratovolcano, Mocho-Choshuenco, provides new insight into the arc magmatic response to ice-load removal. Following deglaciation, we identify three distinct phases of activity characterized by different eruptive fluxes, sizes, and magma compositions. Phase 1 (13–8.2 ka) was dominated by large dacitic and rhyolitic explosive eruptions. During phase 2 (7.3–2.9 ka), eruptive fluxes were lower and dominated by moderate-scale basaltic andesite eruptions. Since 2.4 ka (phase 3), eruptive fluxes have been elevated and of more intermediate magmas. We suggest that this time-varying behavior reflects changes in magma storage time scales, modulated by the changing crustal stress field. During glaciation, magma stalls and differentiates to form large, evolved crustal reservoirs. Following glacial unloading, much of the stored magma erupts (phase 1). Subsequently, less-differentiated magma infiltrates the shallow crust (phase 2). As storage time scales increase, volcanism returns to more evolved compositions (phase 3). Data from other Chilean volcanoes show a similar tripartite pattern of evacuation, relaxation, and recovery, suggesting that this could be a general feature of previously glaciated arc volcanoes.

Erupciones holocenas principalmente explosivas del volcán Planchón, Andes del sur (35°15'S)

RESUMEN La reducida actividad eruptiva holocena del Planchon se ha desarrollado a traves de 5 crateres, en su mayoria de explosion, fuente de los depositos piroclasticos distribuidos sobre el sector oriental. La primera erupcion registrada consto de tres etapas sucesivas cuyas facies son representativas de los respectivos regimenes eruptivos. La Oleada Piroclastica Valenzuela (OPV) fue una oleada de base humeda originada por el colapso de una columna de erupcion freatomagmatica, en presencia de un casquete glacial mucho mas extenso y grueso que el actual. La disminucion de la razon agua/magma durante la erupcion, habria incrementado la densidad de la columna eruptiva, generando una fase culminante de flujo piroclastico con mayor concentracion de bombas escoriaceas (Flujo Piroclastico Los Ciegos, FPLC). Dataciones AMS indican una edad holocena ~7000 AP para la erupcion. Una erupcion subpliniana ocurrida entre 1400 y 1050 AP origino el deposito de pomez de caida Los Banos (PLB) que evidencia la presencia de magmas daciticos en un sistema dominado por la generacion de basaltos a andesitas. Esta erupcion constituye un proceso unico dentro de la evolucion del volcan Planchon, tras un periodo prolongado de diferenciacion en una camara magmatica pequena. La diferencia de edad de varios miles de anos entre las erupciones explosivas holocenas mas destacadas (OPV+FPLC y PLB), asi lo justifica. Las erupciones menores de febrero de 1991 y noviembre de 1998 se produjeron por la interaccion del agua freatica con niveles rocosos sobrecalentados encima de la camara magmatica del Planchon y/o con los niveles mas elevados de esta. La camara magmatica de este volcan muestra un notable grado de diferenciacion con un volumen reducido de magma dacitico en su parte alta. Este ha sido segregado en un periodo de a lo menos 150 anos, considerando la erupcion magmatica, unica emision de lava andesitica post-glacial del volcan, ocurrida en febrero de 1837

Lanín volcano (39.5°S), Southern Andes: geology and morphostructural evolution

El volcan Lanin es un estratovolcan compuesto, predominantemente efusivo, para el que se han definido cuatro unidades segun criterios geomorfologicos. La primera de ellas representa restos de una estructura ancestral; las tres siguientes constituyen el edificio volcanico actual construido esencialmente desde el Pleistoceno Medio-Superior. Composicionalmente, las rocas volcanicas del volcan Lanin corresponden, principalmente, a basaltos/andesitas basalticas y dacitas subordinadas con escasas variedades intermedias. Los depositos piroclasticos post glaciales muestran tambien composiciones siliceas y confirman una estricta bimodalidad composicional de los magmas. Los patrones de tierras raras y elementos mayores indican una evolucion magmatica de baja presion controlada por fraccionamiento de plagioclasa y ortopiroxeno con extraccion de olivino, clinopiroxeno y magnetita, sin interacciones complejas. Los ciclos volcanicos efusivos serian controlados por un reducido tiempo de residencia en una camara magmatica superficial con evacuacion rapida y simultanea de dacitas y basaltos. En erupciones recientes, el sellamiento del conducto central con magma viscoso induciria el drenaje lateral de los basaltos y, eventualmente, el colapso parcial de la zona apical del cono. Sin embargo, los procesos de degradacion mas activos de la estructura volcanica actual se relacionan mas bien con procesos exogenos ligados a la cobertura de hielo. La singular evolucion del volcan Lanin, composicional y morfologicamente intermedio entre los conos monogenicos y los estratovolcanes de la cadena Villarrica-Lanin, se asociaria a su posicion distal de la fosa que origina bajos grados de fusion en la astenosfera y reducidos pulsos de magma ascendente que se almacenarian en una camara de actividad efimera.

Magnetite spherules in pyroclastic iron ore at El Laco, Chile

Abstract The El Laco iron deposits in northern Chile consist of magnetite (or martite) and minor hematite, pyroxene, and apatite. The orebodies are situated on a volcanic complex and resemble lavas and pyroclastic deposits, but a magmatic origin is rejected by some geologists who regard the ores as products of hydrothermal replacement of volcanic rocks. This study describes spherules of magnetite in the ore at Laco Sur and outlines a previously unrecognized crystallization process for the formation of spherical magnetite crystal aggregates during volcanic eruption. Mining at Laco Sur, the second largest deposit at El Laco, shows that most of the ore is friable and resembles pyroclastic material; hard ore with vesicle-like cavities occurs subordinately. The friable ore is a porous aggregate of 0.01–0.2 mm magnetite octahedra with only a local stratification defned by millimeter-thin strata of apatite. Films of iron phosphate are common on magnetite crystals, and vertical pipes called gas escape tubes are abundant in the ore. A SEM study reveals that magnetite spherules in the range 0.05–0.2 mm occur in most samples of friable ore from the central-lower part of the deposit. The proportion of spherules in a sample varies from high to nil, but overall the spherule content is low in the ore. The spherules are aggregates of octahedral crystals, or single octahedra, that have been rounded by stepwise, subparallel growth of magnetite with a systematic slight shift in orientation of successive steps. The shape of the spherules demonstrates that they formed unattached to any surface. Growth from hot magmatic gas saturated in iron in a volcanic plume and deposition as ash fall can account for the features of the spherule-bearing friable ore.

Corrigendum Motoki et al, 2006 (vol. 33 n1)

A new shallow multichannel seismic survey was carried out in the Llancanelo Lake region (Southern Mendoza Province, Argentina) in order to depict the major Neogene sedimentary-volcanic sequences and regional evolution of a tectonic-volcanic basin located in a key region at the eastern foot of the Andes cordillera. This research is settled on early studies that comprised seismic works reaching depths of 600/700 m as well as geoelectric and electromagnetic surveys reaching the uppermost 80-100 m of the sequences. The processing of the reflection seismic data enabled to obtain a stacked section useful for a preliminary geological interpretation. The obtained results indicated the presence of three major sedimentary units with increasing volcanic (basaltic layers) intercalations with depth that accommodate to the geometry of the depocenter. The entire sequence encompasses most of the Neogene. The previous research set the methodological basis for performing more detailed shallow seismic surveys and allowed defining the strategies for obtaining a better constrained geological model. Seismic transects located north of the lake were densified by adding new profiles. Deep seismic and borehole data from the industry were included in this study in order to provide a regional stratigraphic structure of the Cenozoic sequences and to contribute to the knowledge of the final stages of evolution of the Llancanelo basin.The Cuyania stratigraphic record of the Pygodus serra Zone, and the E. robustus and E. lindstroemi subzones, is discussed in this contribution. Three classical sections have been sampled for conodonts in Precordillera and the San Rafael Block. The studied successions are composed mainly by clastic rocks with variable amount of carbonate. The Ponon Trehue and La Cantera formations start with conglomerates and represent the input of coarse clastic deposits to the Cuyania basin. The Los Azules Formation, in Los Amarillitos section, exhibits a massive sandstone bed with carbonate nodules between the black shale. The key conodonts recovered indicate a late Darriwillian age for the bearing strata, these conodonts specimens from the Ponon Trehue region are compare with those from the Precordillera. Finally, the studied clastic sedimentary successions that record this biozone have been compared, concluding that eustacy did not control the sedimentary change in Cuyania for this time interval.In this contribution we present new specimens of Litopterna recovered during the last decade in Quebrada Fiera (Mendoza Province, Argentina), whose fossiliferous sediments, currently recognized as the base of Agua de la Piedra Formation, are assignable to Late Oligocene (Deseadan South American Land Mammal Age). Two remains mentioned in the first publication on this locality were neither detailed nor described, and they have not been located in the corresponding repository. The new material consists of postcranial fragmentary remains (astragali, calcaneum, and metapodials) of Macraucheniidae (Cramaucheniinae) and an incomplete upper molar (M3) of Proterotheriidae (Proterotheriinae). These few remains of litopterns contrast with the abundance of notoungulates at Quebrada Fiera. A comparative study was carried out with material from Patagonia (Argentina) and taxa recorded in Bolivia and Peru for the same temporal interval. The specimens of Cramaucheniinae are assigned to Coniopternium andinum and the molar of Proterotheriinae to cf. Lambdaconus suinus. This contribution allows us to extend the geographical range of Coniopternium, filling the gap between the Patagonian and lower latitude localities (Bolivia and Peru) in which this genus was found. The record of L. suinus in Quebrada Fiera expands the geographical range of this species outside from Patagonia.ABSTRACT. The aquatic sloth, Thalassocnus, is one of the most intriguing linage of mammal knew from the southern pacific coast of South America during the late Neogene. It was initially recognized in Peru, but recent paleontological surveys also demonstrate its presence in Chile. However, the determination of how many species of Thalassocnus were in Chile remains as an open question. Here, we provide a detailed morphological description of an isolated distal fragment of humerus recovered at the Mina Fosforita member (7-7.5 Ma), Bahia Inglesa Formation (Atacama Region, northern Chile), which shows affinities with Thalassocnus. Morphological comparisons with others forms from Chile and Peru allow us to attribute the CPUC/C/557 specimen to T. natans, though some degree of intraspecific variation is acknowledged. The assessment of the stratigraphic provenance of the materials with previously assigned to Thalassocnus from the Bahia Inglesa, Horcon and Coquimbo formations, permits us to propose that the taxonomic diversity of Thalassocnus in Chile is unequivocal conformed by T. carolomartini, and T. natans. We also admitted the likely presence of T. antiquus and a younger species than T. natans (probably T. yaucensis). Futures discoveries of more complete specimens, and/or the detailed analysis of undescribed specimens from Chile will undoubtedly contribute to illustrate the evolutionary history of this extraordinary and enigmatic clade of organisms.The uppermost Cretaceous continental deposits of the Neuquen Basin (west-central Argentina) have yielded a high diversity vertebrate assemblage, including numerous dinosaur species. The Neuquen Group is the richest dinosaur-bearing unit of the basin and comprises the Rio Limay, the Rio Neuquen and the Rio Colorado subgroups. In this group, there is abundance of isolated, disarticulated or partially articulated sauropods and theropods. However, little is known about the taphonomic history of fossil assemblages. In this study, dinosaur remains from the Rio Neuquen Subgroup (Plottier Formation) found in the Cerro Guillermo area in southern Mendoza are examined. The investigation of fossil occurrences within the study area revealed the existence of different taphonomic modes, from isolated bones until partially articulated skeletons preserved in floodplain and channel settings. SEM-EDS analysis showed the substitution of hydroxyapatite by francolite in the bone microstructure. The presence of fluorine –in one of the cases– suggested a link between the elemental composition and depositional environments: floodplain and fluvial channel. The survey of the vertebrate accumulation types and their sedimentary context allowed documenting a wide range of processes responsible for their genesis, operating within a fluvial-dominated environment. This contribution represents an holistic approach about taphonomic history of Cretaceous dinosaurs for assessing the differential preservation of fossil assemblages in fluvial environments.

Mainly explosive Holocene eruptions of the Planchón volcano, southern Andes (35°15'S).

La reducida actividad eruptiva holocena del Planchon se ha desarrollado a traves de 5 crateres, en su mayoria de explosion, fuente de los depositos piroclasticos distribuidos sobre el sector oriental. La primera erupcion registrada consto de tres etapas sucesivas cuyas facies son representativas de los respectivos regimenes eruptivos. La Oleada Piroclastica Valenzuela (OPV) fue una oleada de base humeda originada por el colapso de una columna de erupcion freatomagmatica, en presencia de un casquete glacial mucho mas extenso y grueso que el actual. La disminucion de la razon agua/magma durante la erupcion, habria incrementado la densidad de la columna eruptiva, generando una fase culminante de flujo piroclastico con mayor concentracion de bombas escoriaceas (Flujo Piroclastico Los Ciegos, FPLC). Dataciones AMS indican una edad holocena ~7000 AP para la erupcion. Una erupcion subpliniana ocurrida entre 1400 y 1050 AP origino el deposito de pomez de caida Los Banos (PLB) que evidencia la presencia de magmas daciticos en un sistema dominado por la generacion de basaltos a andesitas. Esta erupcion constituye un proceso unico dentro de la evolucion del volcan Planchon, tras un periodo prolongado de diferenciacion en una camara magmatica pequena. La diferencia de edad de varios miles de anos entre las erupciones explosivas holocenas mas destacadas (OPV+FPLC y PLB), asi lo justifica. Las erupciones menores de febrero de 1991 y noviembre de 1998 se produjeron por la interaccion del agua freatica con niveles rocosos sobrecalentados encima de la camara magmatica del Planchon y/o con los niveles mas elevados de esta. La camara magmatica de este volcan muestra un notable grado de diferenciacion con un volumen reducido de magma dacitico en su parte alta. Este ha sido segregado en un periodo de a lo menos 150 anos, considerando la erupcion magmatica, unica emision de lava andesitica post-glacial del volcan, ocurrida en febrero de 1837.