Central Volcanic Zone Research Articles

The Helium and Carbon Isotope Characteristics of the Andean Convergent Margin

Subduction zones represent the interface between Earth’s interior (crust and mantle) and exterior (atmosphere and oceans), where carbon and other volatile elements are actively cycled between Earth reservoirs by plate tectonics. Helium is a sensitive tracer of volatile sources and can be used to deconvolute mantle and crustal sources in arcs; however it is not thought to be recycled into the mantle by subduction processes. In contrast, carbon is readily recycled, mostly in the form of carbon-rich sediments, and can thus be used to understand volatile delivery via subduction. Further, carbon is chemically-reactive and isotope fractionation can be used to determine the main processes controlling volatile movements within arc systems. Here, we report helium isotope and abundance data for 42 deeply-sourced fluid and gas samples from the Central Volcanic Zone (CVZ) and Southern Volcanic Zone (SVZ) of the Andean Convergent Margin (ACM). Data are used to assess the influence of subduction parameters (e.g., crustal thickness, subduction inputs, and convergence rate) on the composition of volatiles in surface volcanic fluid and gas emissions. He isotopes from the CVZ backarc range from 0.1 to 2.6 RA (n = 23), with the highest values in the Puna and the lowest in the Sub-Andean foreland fold-and-thrust belt. Atmosphere-corrected He isotopes from the SVZ range from 0.7 to 5.0 RA (n = 19). Taken together, these data reveal a clear southeastward increase in 3He/4He, with the highest values (in the SVZ) falling below the nominal range associated with pure upper mantle helium (8 ± 1 RA), approaching the mean He isotope value for arc gases of (5.4 ± 1.9 RA). Notably, the lowest values are found in the CVZ, suggesting more significant crustal inputs (i.e., assimilation of 4He) to the helium budget. The crustal thickness in the CVZ (up to 70 km) is significantly larger than in the SVZ, where it is just ∼40 km. We suggest that crustal thickness exerts a primary control on the extent of fluid-crust interaction, as helium and other volatiles rise through the upper plate in the ACM. We also report carbon isotopes from (n = 11) sites in the CVZ, where δ13C varies between −15.3‰ and −1.2‰ [vs. Vienna Pee Dee Belemnite (VPDB)] and CO2/3He values that vary by over two orders of magnitude (6.9 × 108–1.7 × 1011). In the SVZ, carbon isotope ratios are also reported from (n = 13) sites and vary between −17.2‰ and −4.1‰. CO2/3He values vary by over four orders of magnitude (4.7 × 107–1.7 × 1012). Low δ13C and CO2/3He values are consistent with CO2 removal (e.g., calcite precipitation and gas dissolution) in shallow hydrothermal systems. Carbon isotope fractionation modeling suggests that calcite precipitation occurs at temperatures coincident with the upper temperature limit for life (122°C), suggesting that biology may play a role in C-He systematics of arc-related volcanic fluid and gas emissions.

Physical and chemical evolution of the largest monogenetic lava field in the Central Andes: El Negrillar Volcanic Field, Chile

El Negrillar volcanic field is one of the most voluminous monogenetic volcanic fields so far identified in the Central Volcanic Zone (CVZ) of the Andes: its lava field comprises 84 eruptive phases from 35 small eruptive centers. The study of this volcanic field offers a unique opportunity to assess the volumetric, morphometric, rheological, and compositional evolution of the effusive activity in an entire monogenetic cluster; the great extent and exposure of these small eruptive centers is difficult to observe in other monogenetic volcanoes from the Central Volcanic Zone, as they normally appear as isolated vents or in small clusters (< 3 eruptive centers) with less associated eruptive phases. Our methodology utilizes GIS mapping tools to outline the different eruptive phases of El Negrillar to create a detailed lava flow map, allowing us to estimate the volume emitted by each eruptive center. This analysis was also possible through the paleoreconstruction of the buried lava flows, allowing the determination of the variation in the magma supply. This yielded a total bulk volume of ~7.6 km3 (6.8 km3 DRE, considering the low vesicle abundance of the eruptive products), which exceeds by more than one order of magnitude the volume emitted by other monogenetic centers from the Salar de Atacama region (e.g., El País lava flow, Tilocálar Norte, Tilocálar Sur, La Albondiga and Cerro Overo). We also determined the composition of the entire effusive activity of El Negrillar; andesitic magmas represent more than 46 vol% of the lava field. We identify three main clusters (i.e., Northern El Negrillar (NEN), Central El Negrillar (CEN), and Southern El Negrillar (SEN)), that revealed a compositional variation together with a change in the calculated morphometrical and rheological parameters, implying the existence of a local trend within the monogenetic lava field. The NEN cluster represents the most differentiated magmatism in the field, characterized by amphibole-rich lavas with almost absent olivine, and the highest crystallinity and vesicle content, which led to an effusive activity sustained for more than 3 years, with lava accumulation predominating over its transport, leading to the shortest and thickest flows of the field. The presence of “cauliflower shape bombs” in an early stage of one of its eruptive centers indicates that the NEN cluster may have been influenced by environmental factors leading to an early explosive episode during its activity. On the contrary, the SEN cluster represents the less differentiated magmatic activity of the field, characterized by olivine-rich lavas with almost absent amphibole, low crystallinity, and poor vesicles content, which along with a large magma supply led to effusive activity sustained for approximately two years where the lava flows achieved the longest distances in the field. The CEN cluster is compositionally closer to the SEN, but morphometrical and rheologically resembling to the NEN, suggesting that it represents an intermediate behavior between the other two clusters.

Structural and temporal relationships between volcanic activity, hydrothermal alteration, epithermal Ag–Pb–Zn mineralization and regional stress regime in the Quevar Volcanic Complex (Puna plateau, Salta Province, NW Argentina)

In the Cenozoic Central Volcanic Zone of the Andes, the genetic relationships between regional deformation, volcanic activity, and epithermal mineralization are not fully established. In order to shed some light on the matter, we present new macro and mesoscopic structural data, fault-slip data inversion and 40Ar-39Ar geochronology for volcanic rocks, hydrothermally altered equivalents and epithermal Ag–Pb–Zn mineralization of the Quevar Volcanic Complex, which is located in the Puna plateau of NE Argentina.These data allow us to establish a first evolutionary stage with the eruption of the lavas and pyroclastic deposits of the Quevar Dacite in the Tortonian (9.9–8.7 Ma), when the volcanic activity was controlled by the NW-trending Calama-Olacapato-Toro lineament (COT). A second stage included the implantation of a hydrothermal alteration system and the related formation of the epithermal mineralization in the Messinian (7.7–6.3 Ma). These processes were contemporaneous with an ENE to E-directed shortening regime, which produced the reactivation of the COT and the formation of a left-lateral strike-slip stepover between WNW-trending regional faults. The third stage included the cessation of the hydrothermal activity and the erosion of the volcanic edifice. During this stage, mineralized veins and strike-slip faults were reactivated as normal faults by a NNW-directed extensional regime.

Central vs. lateral growth and evolution of the < 100 ka Peinado composite volcano, southern Central Volcanic Zone of the Andes

Composite volcanoes often display shifts in activity between central and lateral vents that are related to the local stress field and the underlying magmatic plumbing system. The Quaternary Peinado composite volcano in the southern Central Volcanic Zone (CVZ) of the Andes exhibits a striking symmetrical configuration of lateral vents around a main central cone and vent. In this contribution, we carry out detailed mapping, morphometric analysis and unspiked K-Ar dating, together with petrographic and whole-rock major oxides, trace element and Sr-Nd-Pb isotopic analyses, to constrain the growth history of the volcano and the interplay between central and lateral activity. Ten new unspiked K-Ar ages ranging between 36.8 ± 3.8 and 72.6 ± 8.0 ka highlight that Peinado is a young potentially active volcano. A main central cone-building stage ≥70–60 ka of mostly andesitic composition formed the bulk of the edifice. During this stage, vents around the cone base erupted basaltic andesite lava flows that are among the most mafic compositions of CVZ composite volcanoes, possibly related to the location of Peinado on an alignment of mafic monogenetic centers. At ~50–40 ka, the locus of activity shifted to mid-flank vents emplaced around the central vent, with eruption of andesite > basaltic andesite lava flows of similar composition to the central cone. Minor coeval-to-younger activity from the central vent was progressively more felsic, with eruption of dacitic lavas and block and ash flows, and possibly minor fallout. The shift of the bulk of activity from the central to the lateral mid-flank vents was likely related to the effects of the edifice load and magma compositions, with more mafic and dense magmas erupting preferentially through the flank vents and more differentiated magmas from the central vent. Although activity has been dominantly mafic and effusive, an evolution towards more felsic products from the central vent suggests that future activity may be somewhat explosive. The particular configuration of the lateral vents and structures around the central cone, together with other features, point towards a possible future collapse of the edifice, either a sector collapse or a caldera collapse.

Inferring the biogeography of New Zealand’s only endemic holoparasitic plant, the threatened Dactylanthus taylorii (Mystropetalaceae)

ABSTRACT Despite a rich history of botanical research, little is known about the distribution of genetic variation within New Zealand plant species. With a native vascular flora that consists of ∼80% endemics, nearly one-half of which are threatened, there is a dire need for a better understanding of intraspecific diversity for individual plant species and of biogeographic patterns for the wider New Zealand flora. To these ends, we have investigated population genetic structure in the endemic root holoparasite, Dactylanthus taylorii (pua o te Rēinga, Mystropetalaceae). A total of 235 individuals from 31 locations across the North Island of New Zealand (Te Ika-a-Māui, Aotearoa) were genotyped for a suite of nuclear microsatellite loci. Genotype data were analysed at the individual and population levels to assess genetic diversity within and among populations and to investigate the geographic distribution of genetic variation within the species. Population structure was assessed using Bayesian clustering, discriminant analysis of principal components, and a Neighbor-Net analysis. A hierarchical pattern of genetic diversity was revealed, with evidence for three broad geographic regional groupings (West, East, and Central Volcanic Plateau (CVP)). Upon further analysis, each of the three geographic groups further subdivide hierarchically into two or three genetic subgroups. Genetic diversity was found to be relatively high in this endangered species but varies considerably across populations. The distribution of genetic groups appears to have been influenced by past volcanic activity as the CVP group is largely coincident with a distinct tephra isopach from the 1.8 ka Taupō eruption and the West and East groups may have diverged due to recurring volcanic activity in the Central Volcanic Zone. The genetic structure resolved here provides important information for the management of D. taylorii specifically and for our understanding of New Zealand biogeography generally.

Advances in scientific understanding of the Central Volcanic Zone of the Andes: a review of contributing factors

Advances in scientific understanding of the Central Volcanic Zone of the Andes: a review of contributing factors

Evolution of the Azufre volcano (northern Chile): Implications for the Cerro Pabellón Geothermal Field as inferred from long lasting eruptive activity

Azufre volcano (21°47′S, 68°15′W) is part of the Pleistocene NW-SE trending Azufre-Inacaliri Volcanic Chain, located in the Chilean Central Volcanic Zone of the Andes, near the currently exploited Cerro Pabellón Geothermal Field. Geochronological data and geomorphological features indicate that Azufre was constructed between ~1300 and ~ 120 ka through four magmatic Stages (namely Stage I, II, III and IV), with eruptions from different vents that generated the Northern (Stages II and IV) and Southern (Stages I and III) edifices. Whole rock geochemical data indicates that lavas erupted in Stage II (700–500 ka) and Stage IV (300–120 ka) mainly show lower Al2O3, Na2O and Sr concentrations, and higher FeO, MgO and Ti2O concentrations at similar SiO2 contents than those of Stage I (1,300–700 ka) and Stage III (500–300 ka). These differences suggest dominant plagioclase and Fe–Mg rich mineral phases (i.e. olivine and orthopyroxene) fractionations in the Northern edifice with respect to the Southern one. This is in agreement with petrographic characters that reveal more olivine and orthopyroxene contents in less evolved (<60 wt% SiO2) lavas of the Northern edifice whilst those of the Southern edifice shows higher amphibole content at similar SiO2 compositions. Finally, mineral chemistry of samples of the Northern edifice was used to establish a preliminary estimation of the physical conditions of the magmas below Azufre volcano. Two-pyroxene thermobarometry gave temperature values spanning from 1000 °C up to 1060 °C (±39 °C), and fractionation pressures between 350 and 470 (±370) MPa, whereas amphibole thermobarometry indicated pressures of magma fractionation between 140 and 220 (±11.5%) MPa, with temperature ranging from 810 °C to 890 °C (±23.5 °C). These thermobarometric conditions are similar to those previously published of the nearby Apacheta-Aguilucho Volcanic Complex (~1050–600 ka) and the young (< 150 ka) dacitic domes of Chanka, Chac-Inca and Pabellón, therefore suggesting common petrogenetic systems. Evolution of Azufre volcano fill the time gap of the volcanic activity between the growth of the adjacent Apacheta-Aguilucho Volcanic Complex and the emplacement of the young dacitic domes surrounding the Cerro Pabellón Geothermal Field. This possibly unravels a long lasting (more than 1 Ma) active magmatic system in the northeastern-most sectors of the Azufre-Inacaliri Volcanic Chain and new constraints on the longevity of the heat source regions of the high-enthalpy Cerro Pabellón Geothermal Field, possibly opening new perspectives in the geothermal exploration of this area.

Composition and Potential Functions of Rhizobacterial Communities in a Pioneer Plant from Andean Altiplano

Plant microbiota that associate with pioneer plants are essential to their growth and adaptation to harsh conditions found in the Central Volcanic Zone of the Andes. In this sense, the rhizosphere of pioneer species represents a unique opportunity to examine how bacterial communities are recruited and support the growth of plants under abiotic stress conditions, such low nutrient availability, high solar irradiation, water scarcity, soil salinity, etc. In this study, we explored the community composition and potential functions of rhizobacteria obtained from specimens of Parastrephia quadrangularis (Meyen) Cabrera, commonly called Tola, grown on the slopes of the Guallatiri, Isluga, and Lascar volcanoes in the Atacama Desert of Chile by using 16S rRNA amplicon sequencing. Sequence analysis showed that the Actinobacteria, Proteobacteria, Acidobacteria, and Bacteroidetes were the most abundant phyla of the rhizobacterial communities examined. A similar diversity, richness, and abundance of OTUs were also observed in rhizosphere samples obtained from different plants. However, most of OTUs were not shared, suggesting that each plant recruits a specific rhizobacterial communities independently of volcanoes slope. Analyses of predicted functional activity indicated that the functions were mostly attributed to chemoheterotrophy and aerobic chemoheterotrophy, followed by nitrogen cycling (nitrate reduction and denitrification), and animal parasites or symbionts. In addition, co-occurrence analysis revealed that complex rhizobacterial interactions occur in P. quadrangularis rhizosphere and that members of the Patulibacteraceae comprise a keystone taxon. This study extends our understanding on the composition and functions of the rhizobiome, which is pivotal for the adaptability and colonization of pioneer plant to harsh conditions of the Atacama Desert, widely recognized as the driest place on planet Earth.

Geomorphological, morphometric, and spatial distribution analysis of the scoria cones in the Negros de Aras monogenetic volcanic field, northern Chile

The Negros de Aras monogenetic volcanic field in the Central Volcanic Zone of the Andes displays the highest concentration of scoria cones in northern Chile (0.23 per km2). It contains 66 vents, 22 of which are emission points of lava flows without an associated recognizable volcanic edifice, whereas 44 are typical scoria cones. Most vents are associated with basaltic andesite and andesite lava flows of up to 4.6 km in length. A few scoria cones show signs of either initial or final phreatomagmatic eruptions recorded in their pyroclastic successions. A geomorphological, morphometric, and spatial distribution analysis of the scoria cones of this volcanic field was carried out with the purpose of identifying the factors and processes that governed their emplacement, growth, and final morphology. The morphometric analysis was performed on 16 well-delimited scoria cones applying the MORVOLC algorithm using the TanDEM-X 12 m resolution digital elevation model. Analysis of the distribution and elongation of the craters of the scoria cones defines four alignments inferred to be related to the local and regional structural elements of the area. Volcanic activity seems to have been related to a N-S fault system with an E-W maximum stress orientation, favoring magma ascent and scoria cone emplacement following inherited fractures in these preferential orientations. The scoria cones show a wide range of morphologies and morphometries that can be related to different syn- and post-eruptive processes. Cones with large craters and high crater width / basal width ratios show clear evidence of phreatomagmatic eruptions (within their pyroclastic successions), whereas horseshoe-type cones are related to cone breaching or coeval lava flow effusion, and more pristine (i.e., relatively younger) cones are generally steeper.

Effusion rate estimation based on solidified lava flows: Implications for volcanic hazard assessment in the Negros de Aras monogenetic volcanic field, northern Chile

Effusion rate is the instantaneous lava flow output by a vent. It is one of the most important factors that govern the emplacement and dynamics of lava flows and can be determined by direct measurements or estimated through modeling. The Negros de Aras monogenetic volcanic field, located in the Central Volcanic Zone of the Andes, is the largest volcanic field in northern Chile. It is situated north of the active Socompa volcano and south of the Salar de Atacama basin, consisting of scoria cones and lava flows. Here, we estimate the effusion rates of three overlapping and representative solidified lava flows from this volcanic field applying an iterative procedure using the Q-LavHA GIS plugin, where the effusion rate is treated as an unknown parameter and the solution is reached by comparing the simulated results with the real extension of the flows. The pre-eruptive surface was reconstructed using the 12 m resolution TanDEM-X digital elevation model. Other required input parameters for the modeling (e.g., channel ratio, viscosity, temperature) were estimated through geomorphometric, petrographic, and geochemical analysis. The estimated effusion rates vary between 14 and 113 m3/s, comparable with rates measured elsewhere. The method was validated by applying it to two directly measured (Nyamuragira 2006; Mount Etna 2001) and one calculated effusion rate (Lentiscal 2450 BP, Canary Islands). The estimated effusion rates gave an average underestimation of 10%. Considering this percentage of adjustment, the effusion rates for the three studied lava flows from the Negros de Aras monogenetic volcanic field vary from 15 m3/s to 124 m3/s. Our results provide valuable information for mitigating volcanic risk in the event of future lava flow eruptions, which could affect nearby villages and mining operations.

Geoelectrical characterization of Socompa lagoon area in the Andean Central Volcanic Zone from 3-D audiomagnetotelluric inversion

The Socompa stratovolcano is located in the Andean Central Volcanic Zone, where active volcanism turns it into an area of interest for geothermal resources. In the surroundings of the volcano, the Socompa Geothermal Prospect was carried out looking for thermal manifestations related to an active geothermal system. The results of these preliminary studies are not conclusive.Here we present a geoelectrical model from a 3-D inversion of audiomagnetotelluric data acquired in an area of 16 km2. It covers the topographic low where the Socompa lagoon and the thermal springs are located, in order to determine the reservoir of circulating geothermal waters. The most distinctive feature determined by the 3-D model is a high conductive layer (less than 10 Ωm) at a depth of about 400–500 m and at least 200 m thick. Due to the environment and the depth to which the conductive layer is found, there is more than one possibility to explain its presence. It could be due to a deep saline aquifer that would be hosting the circulating waters. But since the low topography could coincide with a cryptic caldera of Miocene - Pliocene(?) age, the high conductivity layer could also be explained by the presence of a clay cap developed during the period of activity. From the inversion of the audiomagnetotelluric data it was possible to carry out a geoelectrical characterization of the upper units present in the study area, as well as to delimit, in a first approximation, the depth to which the hydrothermal reservoir would be found.

Origin of andradite in the Quaternary volcanic Andahua Group, Central Volcanic Zone, Peruvian Andes

Euhedral andradite crystals were found in trachyandesitic (latitic) lavas of the volcanic Andahua Group (AG) in the Central Andes. The AG comprises around 150 volcanic centers, most of wich are monogenetic. The studied andradite is complexly zoned (enriched in Ca and Al in its core and mantle, and in Fe in this compositionally homogenous rim). The core-mantle regions contain inclusions of anhydrite, halite, S- and Cl-bearing silicate glass, quartz, anorthite, wollastonite magnetite and clinopyroxene. The chemical compositions of the garnet and its inclusions suggest their contact metamorphic to pyrometamorphic origin. The observed zoning pattern and changes in the type and abundance of inclusions are indicative of an abrupt change in temperature and subsequent devolatilization of sulfates and halides during the garnet growth. This process is interpreted to have taken place entirely within a captured xenolith of evaporite-bearing wall rock in the host trachyandesitic magma. The devolitilization of sediments, especially sulfur-bearing phases, may have resulted in occasional but voluminous emissions of gases and may be regarded as a potential hazard associated with the AG volcanism.

New ages, morphometric and geochemical data on recent shoshonitic volcanism of the Puna, Central Volcanic Zone of Andes: San Jerónimo and Negro de Chorrillos volcanoes

Shoshonitic volcanic centres are scarce within the back-arc mafic volcanism of the Puna Plateau in the Central Volcanic Zone of the Andes. San Jerónimo and Negro de Chorrillos volcanoes are two well preserved shoshonitic monogenetic edifices that developed mainly Strombolian activity and lava flows, which extended more than 9 and 4 km from the vent, producing a volume DRE (Dense Rock Equivalent) of 0.091 and 0.020 km3, respectively. The new unspiked K–Ar (groundmass) ages of San Jerónimo (144 ± 3 ka) and Negro de Chorrillos (51 ± 2 ka) eruptions presented in this work are both younger than those previously published. Petrographic and compositional features allow distinguishing between potassic and ultrapotassic suites in volcanic products of both eruptions. Phlogopite occurrence and low glass content characterize the ultrapotassic suite. Based on the crustal thickness of this part of the Puna and the experimental constraint, these magmas last equilibrated at Moho depths at around 50 km (the highest depth of spinel lherzolite stability field) for temperatures of 1100–1200 °C. Once in the crust, the K-rich magma underwent a minor interaction with a trachytic-trachydacitic melt, which is evidenced petrographically and geochemically, e.g., mismatch of mineral assemblages (co-occurrence of olivine and quartz), disequilibrium crystal textures (dusty zones in plagioclase, embayments in sanidine), and a wide range of compositional variation in the glasses. Ultrapotassic (i.e., phlogopite-bearing) and potassic lavas in the same eruption are interpreted as pulses of the same magma but with a longer mixing time with the trachytic-trachydacitic melt for the resulting ultrapotassic rocks within an eruptive time frame of hours or days. The singular shoshonitic character of these magmas could be related to their origin along an old terrane boundary. Afterwards, as occurs for other mafic magmas of the Puna, major NW-SE fault systems would favour their ascent to the surface.

A window on mantle-derived magmas within the Central Andes: eruption style transitions at Cerro Overo maar and La Albóndiga lava dome, northern Chile

Cerro Overo maar and La Albondiga lava dome are two independent monogenetic volcanoes located in the Central Volcanic Zone of the Andes in northern Chile, close to the active Lascar and Chiliques volcanoes. Cerro Overo maar was formed <77 ka ago by explosive-effusive eruptions, including phreatomagmatic activity, while La Albondiga lava dome (Pleistocene) is the result of magmatic explosive-effusive activity alone. Cerro Overo and La Albondiga are characterized by deposits composed of pyroclastic rocks and dense coherent lava blocks. At Cerro Overo, these deposits are located around the crater rim, while at La Albondiga, they form coherent in situ lava dome features. Cerro Overo also displays thin ash- and lapilli-dominated deposit layers presumed to be pyroclastic surge deposits, which include juvenile pyroclasts exhibiting cauliflower textures, numerous exotic accidental lithics, and ballistically transported lapilli, bombs, blocks, and bomb sags. These fragments include recycled, juvenile pyroclasts, as well as material from older volcanic and intrusive rocks from the stratigraphic units immediately below. These small-volume volcanoes represent two of the less silicic volcanoes (~54 wt.% SiO2) in the northern Chilean Pleistocene to Holocene volcanic provinces. They are characterized by a fine grain size (mainly fine lapilli), olivine phenocrysts with skeletal textures, ignimbritic, dioritic, and granitic xenoliths, and quartz xenocrysts, with high concentrations of incompatible trace elements and light rare earth elements. The general magmatic evolution of the Cerro Overo and La Albondiga systems has been controlled by fast ascent (e.g., skeletal olivine phenocrysts) of mantle-derived magma associated with mixing, fractional crystallization, and a low degree of crustal assimilation during turbulent ascent processes. Such eruptions provide evidence that various factors play an essential role in the transition from explosive-effusive magmatic (Cerro Overo and La Albondiga) to phreatomagmatic (Cerro Overo) volcanic eruption styles.

The Tutupaca volcanic complex (Southern Peru): Eruptive chronology and successive destabilization of a dacitic dome complex

Several processes have been proposed as triggering mechanisms for the large sector collapses that affect most volcanoes, and which may occur several times in the volcano's lifetime. Here we present and discuss the case of Tutupaca volcano, located in southern Peru and part of the Central Volcanic Zone of the Andes. Tutupaca is composed of an old, hydrothermally altered and highly eroded Basal edifice, as well as younger twin peaks located in the northern part of the complex (the Western and Eastern Tutupaca). The youngest Eastern edifice of Tutupaca is composed of at least seven coalescing lava domes and associated deposits, including block-and-ash flow and debris avalanche deposits. We identified two debris avalanche deposits. An older unit (Azufre debris avalanche deposit) was channeled in the valleys located to the E and SE of the basal volcano, reaching up to 3.5 km from its source region. Four cosmogenic nuclide exposure dates (10Be/feldspar) were obtained from boulders of this debris avalanche deposit and ranged between 6.0 ± 0.7 and 7.8 ± 1.5 ka. The younger unit (Paipatja deposit) was associated with the sector collapse of the edifice reconstructed just after the first debris avalanche (domes IV to VIII). The sector collapse produced a debris avalanche deposit that outcrops immediately to the NE of the amphitheater and was associated with a large pyroclastic density current deposit that was previously dated by radiocarbon at 218 ± 14 a BP (Samaniego et al., 2015). Both debris avalanche deposits have two contrasting sub-units: (1) the main subunit, hereafter called hydrothermal-altered debris avalanche deposit, is a whitish-yellow volcanic breccia with heterolithic and heterometric blocks, which originated from the Basal edifice, and (2) a dome-rich debris avalanche deposit, composed by non-altered dome blocks from Eastern Tutupaca. In proximal areas, the dome-rich unit overlaps the hydrothermally-altered unit while in distal areas, these two units are mixed forming a hummocky and/or ridged topography. In addition to the similarity of both debris avalanches, we propose that the triggering mechanism for these debris avalanches was similar. The dacitic dome growth, coupled with a substrate of older, hydrothermally-altered rock, induced the destabilization of the edifice, producing the debris avalanches and the related pyroclastic density currents.

Sr isotopes and geochemistry of Cerro Overo maar provide a unique window into arc magma genesis in the Central Volcanic Zone of the Andes

Cerro Overo maar is the most mafic lava (54 wt% SiO2, 7.4 wt% MgO) found within the modern subduction arc of the Central Andes. Structural weaknesses in the crust allowed magma to rapidly ascend while avoiding extensive crustal contamination, interactions with the mid-crustal Altiplano-Puna Magma Body, or storage in magma chambers. Cerro Overo lava thus preserves chemical and mineralogical signals from deep-crustal or upper-mantle magmatism which are otherwise absent in the modern Andes Central Volcanic zone (CVZ). Radiogenic isotope ratios of CVZ rocks are characteristically enriched, and researchers have debated for decades whether this enrichment is derived from an enriched mantle source or the result of crustal processes. Phenocrysts of high-forsterite olivine (Fo86) from Cerro Overo contain three populations of silicate melt inclusions: 1. basaltic andesite inclusions chemically identical to the host lava, 2. rhyodacite inclusions representing assimilated upper crust, and 3. basaltic melt inclusions (SiO2 = 49.0 wt%) of minimally-altered magma, reflecting an early-stage melt which may be common to the active arc of the Altiplano-Puna region. Melt inclusion 87Sr/86Sr (0.70376 to 0.70720) also form three clusters, including the most primitive Sr isotopic ratios reported for the modern CVZ arc: direct evidence that the sub-arc mantle source retains non-radiogenic Sr signatures. The least-evolved Olivine-hosted melt inclusions from Cerro Overo match the composition of postulated arc endmembers and provide direct evidence that the overall radiogenic isotopic characteristic of the modern CVZ results from lithospheric processes and not inheritance from a mantle source with an initially radiogenic 87Sr/86Sr signature.

Geological evolution of the Guallatiri volcano, Arica y Parinacota Region, northern Chile

Guallatiri volcano is an active stratovolcano located in the Arica y Parinacota Region of northern Chile. It belongs to the Central Volcanic Zone (CVZ) of the Andean range and is the youngest and southernmost volcano of the Nevados de Quimsachata volcanic chain. Guallatiri is an ice-capped volcano considered the third-highest risk center in northern Chile with a current activity characterized by two fumarolic fields with strong and persistent gas emissions. This work presents the first detailed geological mapping of the volcano based on stratigraphic, petrographic, morphological, and geochemical data. The volcano has evolved into seven units, whit the initial stages characterized by the widespread and effusive emission of several thick lava flows around the volcanic edifice. The later evolution units involved the extrusion of lava domes at the summit, with a Holocene plinian eruption accompanied by pyroclastic density currents, tephra fallout, and lahars. Furthermore, two parasitic Holocene domes, Tinto and South domes were nested on the volcano's southwestern flank. High eruptive rates (0.36 km3/ka) indicate that Guallatiri volcano has been one of the most active volcanoes during the Late Pleistocene-Holocene. The volcanic products from Guallatiri mainly correspond to trachyandesites, trachydacites, and dacites rich in amphibole and biotite, with minor clinopyroxene and olivine. They belong to the High K2O Calc-Alkaline Series, similar to other active volcanoes of the region. The construction of detailed geological maps of active volcanoes is used as the basis for the generation of hazard and risk maps, which enable the mitigation of associated volcanic risks. In the case of Guallatiri, the occurrence of lahars, pyroclastic flows, and tephra fallout are considered the main volcanic hazards, which could affect the villages of Guallatire and Ancuta, in addition to small hamlets in Bolivia.

Features That Favor the Prediction of the Emplacement Location of Maar Volcanoes: A Case Study in the Central Andes, Northern Chile

Maar volcanoes are monogenetic landforms whose craters cut below the pre-eruptive surface and are surrounded by a tephra ring. Both the maar crater and the surrounding tephra rim deposits are typically formed due to magma–water explosive interactions. Northern Chile is located in the Central Volcanic Zone of the Andes where, in literature, 14 maars have been recognized as parasite (6) and individual (8) volcanoes. Amongst these individual maars, 3 of them, namely the Tilocálar Sur, Cerro Tujle, and Cerro Overo volcanoes, are not related to calderas and were emplaced &lt;1 Ma ago by magmatic explosive-effusive and phreatomagmatic eruptions. Based on the evolution and control of the volcanic eruptive styles of these three maars, which have been determined in previous research through fieldwork, stratigraphic, morphometric, textural (density and vesicularity), petrographic, and geochemical analyses, a set of key features that favor a prediction of the emplacement location of maar volcanoes in Central Andes, northern Chile are proposed. The set of features that permit and favor the growth mechanisms for maar formations corresponds to (i) a compressive tectonic setting (e.g., ridge structures), (ii) groundwater recharge (e.g., snowmelt and seasonal rainfall), (iii) the lithological setting (e.g., layers of low permeability), (iv) the presence of aquifer and/or endorheic basins (e.g., lakes or salars), and (v) a period of stress relaxation that permits magma ascent to the surface in volcanically active areas. Considering these characteristics, it is possible to identify places where phreatomagmatic eruption can occur. If the magma ascent flux is lower than the groundwater flux, this can lead to a phreatomagmatic eruption because of groundwater coming into contact with the magma. These eruptive features evidence internal—and external—factors that play an essential role in the transition from explosive-effusive magmatic to phreatomagmatic volcanic eruption styles during the same eruptive period that is one of the biggest challenges in volcanic hazard evaluations. Although, in this contribution, a set of features that permit and favor the growth mechanisms for a prediction of the emplacement location of maars in northern Chile is proposed, these considerations could also be applied to identify potential locations in other parts of the world where magma–water interaction eruption could occur. Therefore, this approach could be useful in the prediction of hydromagmatic volcanic eruptions and, thus, in mitigating the impact of volcanic hazard for the inhabitants of the surrounding areas.

Geology, geochemistry and geochronology of Lindero porphyry gold deposit in the Southern Puna plateau, Argentina

The Lindero gold deposit is located in the Southern Puna plateau, northwest Argentina. The deposit is centered in a cluster of six subvolcanic intrusions emplaced at the margin of the Arizaro Basin. Three alteration types were recognized: (i) Ca–Na silicate (clinopyroxene + magnetite + K-feldspar + quartz + calcite ± plagioclase), (ii) K-silicate (K-feldspar + quartz + magnetite ± biotite ± anhydrite) and (iii) chlorite-calcite alteration. The highest ore grades are linked to the K-silicate alteration. The proven plus probable reserves of Lindero are 84,226 t with average grades of 0.63 g/t Au and 0.11% Cu. A previous study assigned Lindero to the iron oxide copper-gold (IOCG) deposit type but many features of Lindero suggest that it is a porphyry gold deposit, including (i) the temporal and spatial link between alteration and the intrusive bodies, (ii) the alteration distribution pattern, particularly the small volume of rock affected by Ca–Na silicate alteration, (iii) the Au-rich and Cu-poor mineralization style.The magmatic complex at Lindero comprises an early-mineral unit (FPD), four inter-mineral units (CPD1, PBFD, CPD2 and DDP) and one post-mineral unit (PMI). In-situ U/Pb SIMS dating of the oldest (FPD), an intermediate (PBFD) and the youngest (DDP) intrusive units, confirms a middle Miocene age. The weighted mean ages of the oldest and youngest units are indistinguishable, with 15.36 ± 0.13 Ma (n = 21) and 15.47 ± 0.11 Ma (n = 16), respectively. Individual ages from each unit range by ~1 m.y. and the overall spread of zircon ages is 15.92 ± 0.23 to 14.44 ± 0.33 Ma. We suggest that emplacement of the subvolcanic stocks took place within this span time, likely at the lower end of this range (15.0–14.4 Ma). Two 40Ar/39Ar ages of hydrothermal biotites from the K-silicate alteration (14.99 ± 0.16 Ma and 14.93 ± 0.12 Ma) indicate that hydrothermal alteration began practically simultaneously with the emplacement of the porphyry units.All of the intrusive units are similar compositionally. They show a fine- to medium-grained porphyric texture (1–4 mm) comprising plagioclase, amphibole, clinopyroxene and scarce quartz phenocrysts (40–55 vol % of phenocrysts) in a K-feldspar ± quartz microcrystalline (0.02–0.07 mm) groundmass, except in the post-mineral unit which has a cryptocrystalline groundmass. Whole-rock analyses reveal a narrow range of dioritic composition (58.6–61.9 wt % SiO2) and high-K calc-alkaline character for all units. Trace element features (low Ba/Nb ratios, high Nb) of the Lindero magmas indicate a back-arc affinity, similar to those from the Southern Puna and distinct from the Central Volcanic Zone (CVZ) frontal arc. The Sr and Nd isotope ratios (87Sr/86Sr = 0.706042 to 0.706607; 143Nd/144Nd = 0.512501 to 0.512582) from Lindero intrusives are also similar to Southern Puna back-arc volcanic rocks. The Pb isotope ratios of Lindero (206Pb/204Pb = 18.79 to 18.83; 207Pb/204Pb = 15.60 to 15.63; 208Pb/204Pb = 38.66 to 38.74) overlap with both back-arc and arc magmas in the CVZ. The narrow age range, spatial association and uniform chemical and isotopic composition of Lindero porphyry units suggest that were derived from a common magma source, which underwent fractionation and/or crustal assimilation before emplacement as suggest by the low concentrations of Mg, Cr, Ni and Sr. The Lindero porphyry units show chemical and isotopic similarities with those from porphyry gold deposits in the Maricunga belt, Chile, and with the porphyry copper deposits of Argentina located in a back-arc setting; however, they differ from porphyry copper deposits in the frontal-arc setting of Chile, notably by the lack of an adakite-like signature (high Sr/Y ratio).

Paleomagnetism and 40Ar/39Ar chronology of ignimbrites and lava flows, Central Volcanic Zone, Northern Chile

New 40Ar/39Ar ages and paleomagnetic data are presented from a wide range of ignimbrites and lava flows from the region east of the Atacama Desert in the northern Chilean part of Central Volcanic Zone of the Andes. Eight dacitic to rhyolitic ignimbrites, 12 lava flows and one shallow intrusion were dated by the 40Ar/39Ar incremental heating method, and are the first 40Ar/39Ar ages reported for these units. Ignimbrites range in age from 4.2 Ma to 0.5 Ma, with the ~100 km3 Puripicar ignimbrite dated at 4.18 ± 0.02 Ma, the ~2500 km3 Atana ignimbrite, one of the largest ignimbrites in the world, at 3.99 ± 0.02 Ma, and the Purico ignimbrite at 0.99 ± 0.03 Ma. The new 40Ar/39Ar ages fall within the uncertainty limits of published K-Ar dates for the Purico, Patao and Puripicar tuffs but are different at the 95% confidence level for Atana and Talabre tuffs. Isochron ages from the 12 lava flows range from 3.5 Ma to 0.1 Ma, with 11 of the flows younger than 2.5 Ma. One lava sample and the shallow intrusion are from the Plio-Pleistocene magnetite-rich El Laco volcanic complex, and give ages of 2.18 ± 0.03 Ma and 1.83 ± 0.09 Ma respectively. Paleomagnetic directions from 49 sites (23 ignimbrites and 26 lava flows) provide a mean direction of I = −43.8°, D = 352.7° with α95 = 10.0°, and a corresponding mean pole position of −81.1° and 179.1°. Normal directions are predominantly from lava flows and have steeper than expected inclinations (ΔI = 15.3°) whereas reverse sites are mostly ignimbrites and yield directions close to the geo-axial dipole field (ΔI = −3.6°). Paleosecular variation studies on a selection of 39 sites yield an angular standard deviation of site poles about the mean pole of 18.5°, considerably larger than predicted by current models of magnetic field variation with latitude.