Andean Central Volcanic Zone Research Articles

Magma storage conditions of Lascar andesites, central volcanic zone, Chile

Abstract. Lascar volcano, located in northern Chile, is among the most active volcanoes of the Andean Central Volcanic Zone (CVZ). Its activity culminated in the last major explosive eruption in April 1993. Lascar andesites which erupted in April 1993 have a phase assemblage composed of plagioclase, clinopyroxene, orthopyroxene, Fe–Ti oxides, and rhyolitic glass. To better constrain storage conditions and mechanisms of magmatic differentiation for andesitic magmas in a thick continental crust, crystallization experiments were performed in internally heated pressure vessels at 300 and 500 MPa, in the temperature (T) range of 900–1050 °C, at various water activities (aH2O) and oxygen fugacities (logfO2 between QFM+1.5 and QFM+3.3 at aH2O =1; QFM is quartz–fayalite–magnetite oxygen buffer). The comparison of experimental products with natural phase assemblages, phase compositions, and whole-rock compositions was used to estimate magma storage conditions and to reconstruct the magma plumbing system. We estimate that Lascar two-pyroxene andesitic magmas were stored at 975±25 °C, 300±50 MPa, and logfO2 of QFM+1.5±0.5, under H2O-undersaturated conditions with 2.5 wt % to 4.5 wt % H2O in the melt. The geochemical characteristics of the entire suite of Lascar volcanics indicates that a fractionating magmatic system located at a depth of 10–13 km is periodically replenished with less evolved magma. Some eruptive stages were dominated by volcanic products resulting most probably from the mixing of a mafic andesitic magma with a felsic component, whereas compositional variations in other eruptive stages are better explained by crystal fractionation processes. The relative importance of these two mechanisms (mixing vs. crystal fractionation) may be related to the amount and frequency of magma recharge in the reservoir.

Petrological and geochemical constraints on the magmatic evolution at the Ampato-Sabancaya compound volcano (Peru)

In order to gain insights into continental arc magmatic processes, we have conducted a petrological and geochemical study of major and trace elements and Sr, Nd, and Pb isotopes of the Ampato-Sabancaya compound volcano, which belongs to the Andean Central Volcanic Zone (CVZ). Whole-rock compositions for Ampato and Sabancaya range from andesites to dacites (56.7–69.3 wt% SiO2) and both belong to a medium- to high-K calk-alkaline magmatic series. Ampato-Sabancaya samples are characterized by high contents of large-ion lithophile elements (LILE; e.g., K, Rb, Ba, Th), low concentrations of high field strength elements (HFSE; e.g., Nb, Zr) and heavy rare earth elements (HREE; e.g., Yb), with consequently high La/Yb and Sr/Y ratios. An increase in these ratios is usually interpreted as a result of magmatic differentiation in the presence of garnet in the deep crust. A detailed analysis reveals that the rocks of Ampato-Sabancaya display three different compositional groups. (1) The first, composed mainly of andesites (56.7–59.8 wt% SiO2), corresponds to lavas from the early stage of the Ampato Basal edifice, as well as pyroclastic deposits from the Ampato Upper edifice. (2) The second group corresponds to andesitic and dacitic compositions (60.0–67.3 wt% SiO2) from the Ampato Basal edifice (Moldepampa stage), the Ampato Upper edifice, and the Sabancaya edifice. (3) The third group corresponds to dacitic compositions (65.0–69.3 wt% SiO2) associated with the Corinta Plinian fallout and pyroclastic flow deposits from the Ampato Upper edifice. This last group of dacites, erupted during the Ampato Upper edifice stage, have drastically different compositions from the other groups with Sr/Y (<27) and Sm/Yb (<4.7) ratios lower than other lavas and lacking evidence of amphibole and/or garnet fractionation during their genesis. As a whole, Sr, Nd, Pd isotopic ratios suggest that mantle-derived magmas are significantly affected by assimilation processes during their evolution, due to the thick (65–70 km) continental crust beneath the CVZ in southern Peru. In summary, the magmatic evolution of group 1 and 2 can be explained by a two-step model in which primitive magmas evolved in the deep crust in the so-called melting-assimilation-storage-homogenization (MASH)-type reservoirs by assimilation-fractional crystallization (AFC) processes involving garnet and/or amphibole. Then, amphibole-dominated upper crustal AFC processes and magma mixing are responsible for the geochemical diversity of the main ASCV trend. In contrats, the group 3 dacites followed an upper crustal AFC process (without amphibole) from a different primitive magma, which did not suffer the high pressure, garnet-dominated AFC processes. This evolution highlights the complexities associated to magma genesis and differentiation at continental arcs contructed on a thick crust.

Oxygen and radiogenic isotopes require different contamination paths in the Azufre volcano, Central Andes, northern Chile

Azufre Volcano (21°47′S, 68°15′W), located in the Altiplano-Puna Volcanic Complex of the Andean Central Volcanic Zone (CVZ), is formed by two coalescent edifices (Northern and Southern) constructed between 120 and 1300 kaExtensive hydrothermal alteration occurred within a host sequence of andesites/dacites (120–700 ka in age) in the Northern edifice. The silica content of the erupted lavas of this volcano range from ∼60% m/m (andesite) to ∼67% m/m (dacite). These lavas have 87Sr/87Sr ratios ranging from 0.7066 to 0.7074, 143Nd/144Nd ratios ranging from 0.51229 to 0.51239, and quartz δ18O values ranging from 7.9 to 9.8‰. The original magmas had δ18O values from 7.4 to 9.4‰ (average ∼8.0‰) assuming that Δquartz-melt = 0.5‰.A three-stage magmatic evolution is proposed for Azufre. In the first stage, a mantle-derived magma (87Sr/86Sr = 0.70376; δ18O ∼5.7‰) ascended to plagioclase-dominated felsic crustal levels (25–50 km depth), assimilating up to 21% of crustal material. This generated a sub-arc parental melt with 87Sr/86Sr ∼0.705 and δ18O ∼6.5‰. The second stage involves contamination of the parental magma by assimilation of up to 25% of crust with higher 87Sr/86Sr ratios (0.714) and δ18O (6.4–11.8‰) at upper crustal levels (10–25 km depth). This upper crust corresponds to partially molten zones which coincides with the location of the Altiplano-Puna Magma Body. The generated melts have 87Sr/86Sr ratios up to ∼0.7075 and δ18Omelt 8.5–9.4‰, which fractionates high-δ18O quartz (9.0–9.8‰). The final stage occurred at shallow crustal levels (<10 km depth) where the magmas underwent fractional crystallization during assimilation of crustal material, which due to high-temperature hydrothermal alteration had low δ18O value (δ18Omelt 7.4–8.5‰). During this stage, high-δ18O quartz of the previous stage remains as antecrysts, coexisting with the low-δ18O quartz (δ18Oquartz 7.9–9.0‰) fractionated at shallow depths.

Electrical resistivity structure in the Tocomar geothermal system obtained from 3-D inversion of audio-magnetotelluric data (Central Puna, NW Argentina)

The Tocomar Geothermal System is located in the Puna Plateau (NW Argentina), within the Central Puna Energy Hub, and is considered one of the most promising places to harness potential alternative of power and heat sources in the Central Andean Volcanic Zone (16-28 °S). It is related to the Calama-Olacapato-Toro lineament and to the Quaternary Tocomar volcanic centre. Moreover, it is surrounded by active and fossil geothermal manifestations, like hot-springs, travertines and siliceous sinter deposits. Despite some geological studies in the area, no geophysical investigations have targeted the geothermal fields along the Central Puna.In this work we present a 3-D inversion of audio-magnetotelluric data around the Tocomar Geothermal System. These data was obtained in the frequency range of 1000-0.1 Hz to map the main elements of the geothermal system (clay cap and potential reservoir) at depths of approximately 1000 m. To achieve this goal, previous geoelectrical studies, the local geology and the trend of the main structures were also considered. For the 3-D inversion process the ModEM code was used.The model shows a low-resistivity layer (less than 10 Ωm) at least 300 m thick, at a depth of about 200–500 m, aligned with both the strike of the Calama-Olacapato-Toro lineament and the local superficial geothermal manifestations (hot-springs and hydrothermally altered rocks). This low-resistivity layer is linked with the clay cap at the shallow depth of the geothermal reservoir. At depths greater than 800 m, a gradual increase in resistivity is observed related to a potential reservoir within the fractured Ordovician basement. The final 3-D resistivity model highly correlates with the conceptual models of high-temperature volcanic geothermal systems.

The Hydrothermal Alteration of the Cordón de Inacaliri Volcanic Complex in the Framework of the Hidden Geothermal Systems within the Pabelloncito Graben (Northern Chile)

Detailed mineralogical analyses in areas with surface hydrothermal alteration zones associated with recent volcanism (&lt;1 Ma) in the Central Andean Volcanic Zone could provide key information to unravel the presence of hidden geothermal systems. In the Cordón de Inacaliri Volcanic Complex, a geothermal field with an estimated potential of ~1.08 MWe·km−2 has been recently discovered. In this work, we focus on the hydrothermal alteration zones and discharge products of this area, with the aim to reconstruct the geological processes responsible for the space-time evolution leading to the geothermal records. We identified (1) discharge products associated with acid fluids that could be related to: (i) acid-sulfate alteration with alunite + kaolinite + opal CT + anatase, indicating the presence of a steam-heated blanket with massive fine-grained silica (opal-CT), likely accumulated in mud pots where the intersection of the paleowater table with the surface occurred; (ii) argillic alteration with kaolinite + hematite + halloysite + smectite + I/S + illite in the surrounding of the acid-sulfate alteration; and (2) discharge products associated with neutral-alkaline fluids such as: (i) discontinuous pinnacle-like silica and silica deposits with laterally developed coarse stratification which, together with remaining microorganisms, emphasize a sinter deposit associated with alkaline/freshwater/brackish alkaline-chlorine water bodies and laterally associated with (ii) calcite + aragonite deriving from bicarbonate waters. The scarce presence of relics of sinter deposits, with high degree crystallinity phases and diatom remnants, in addition to alunite + kaolinite + opal CT + anatase assemblages, is consistent with a superimposition of a steam-heated environment to a previous sinter deposit. These characters are also a distinguishing feature of paleosurface deposits associated with the geothermal system of the Cordón de Inacaliri Volcanic Complex. The presence of diatoms in heated freshwater bodies at 5100 m a.s.l. in the Atacama Desert environment could be related with the last documented deglaciation in the area (~20–10 ka), an important factor in the recharge of the hidden geothermal systems of the Pabelloncito graben.

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.

Long-Term Temperature Cycling in a Shallow Magma Reservoir: Insights from Sanidine Megacrysts at Taápaca Volcano, Central Andes

Abstract Hybrid dacite magmas from Taápaca volcano in the Central Andean Volcanic Zone (18°S, northern Chile) contain sanidine crystals of unusual size (1–12 cm) and abundant mafic enclaves of variable composition throughout the entire eruptive history (1·5 Ma to recent) of the volcano. They are rich in mineral inclusions and strongly zoned in Ba with distinct growth bands separated by resorption interfaces. Resorption is followed by a sudden increase in Ba with compositional contrasts up to 2·3 wt% BaO. We argue that resorption and the sharp jumps in Ba concentration reflect distinct heating and melting events, suggesting that different growth zones formed at different temperatures. Amphibole–plagioclase thermobarometry based on mineral inclusions gives variable temperatures of ∼720–820 °C at shallow pressures (0·1–0·3 GPa) for individual growth zones. Using these temperatures for diffusion modelling, Ba profiles from X-ray scanning profiles and grey-scale gradients based on accumulated back-scattered electron images across these interfaces allow us to estimate crystal residence and reactivation times prior to eruption. This temperature control allowed the application of a ‘non-isothermal’ diffusion algorithm to obtain diffusion times for individual diffusive boundaries that range from 0·4 to 490 kyr and add up to total residence times of 9–499 kyr for different crystals from different stages of eruption. A combination of temperatures, pressure, diffusion times and R-MELTS modelling of the parent rhyodacite suggests storage conditions for the Taápaca reservoir at near eutectic composition at shallow depth (4–10 km). Temperatures never fell below the magma solidus but frequently cycled between 720 °C and 820 °C (i.e. between eruptible and non-eruptible state with crystallinity circling around ∼40–50 vol%) for tens to hundreds of thousands of years. We define this as ‘long-term transitional temperature cycling’ or LTTC storage. Frequent recharge events of basaltic andesite magma, as represented by abundant mafic enclaves, orchestrated the temperature cycling, resulted in multiple heating events that caused frequent resorptions and interrupted crystal growth, and kept the reservoir thermally ‘alive’. Recharge events became more frequent only ∼3–11 kyr before the eventual eruption that carried a particular set of sanidine megacrysts to the surface. Thus, after many earlier recharge events that did not result in eruption, a final event involved mixing at a critical recharge rate to mobilize, entrain, and erupt a particular set of megacrysts from the resident rhyodacite in a hybrid dacite host. This process, happening not more than a few centuries before an eruption, has been repeated at similar time-scales at different stratigraphic stages throughout the 1·5 Myr history of Taápaca volcano. The observed mineral zonation patterns and size of sanidine crystals from the resident magma reservoir below Taápaca volcano are identical to those observed in the megacrysts from granite intrusions that also show typical age ranges of zircon crystallization that are comparable with the residence times extracted here from Ba zonation. Taápaca sanidines thus may represent an erupted equivalent and provide ‘smoking gun’ evidence of temperature cycling during the formation of such K-feldspar megacrysts in granites.

Microbial Communities of High-Elevation Fumaroles, Penitentes, and Dry Tephra "Soils" of the Puna de Atacama Volcanic Zone.

The aim of this study was to understand the spatial distribution of microbial communities (18S and 16S rRNA genes) across one of the harshest terrestrial landscapes on Earth. We carried out Illumina sequencing using samples from two expeditions to the high slopes (up to 6050m.a.s.l.) of Volcán Socompa and Llullaillaco to describe the microbial communities associated with the extremely dry tephra compared to areas that receive water from fumaroles and ice fields made up of nieves penitentes. There were strong spatial patterns relative to these landscape features with the most diverse (alpha diversity) communities being associated with fumaroles. Penitentes did not significantly increase alpha diversity compared to dry tephra at the same elevation (5825m.a.s.l.) on Volcán Socompa, but the structure of the 18S community (beta diversity) was significantly affected by the presence of penitentes on both Socompa and Llullaillaco. In addition, the 18S community was significantly different in tephra wetted by penitentes versus dry tephra sites across many elevations on Llullaillaco. Traditional phototrophs (algae and cyanobacteria) were abundant in wetter tephra associated with fumaroles, and algae (but not cyanobacteria) were common in tephra associated with penitentes. Dry tephra had neither algae nor cyanobacteria but did host potential phototrophs in the Rhodospirillales on Volcán Llullaillaco, but not on Socompa. These results provide new insights into the distribution of microbes across one of the most extreme terrestrial environments on Earth and provide the first ever glimpse of life associated with nieves penitentes, spire-shaped ice structures that are widespread across the mostly unexplored high-elevation Andean Central Volcanic Zone.

El Último Máximo Glaciar local y la deglaciación de la Zona Volcánica Central Andina: El caso del volcán HualcaHualca y del altiplano de Patapampa (Sur de Perú)

El objetivo de este trabajo es conocer cuándo comenzó el retroceso de los glaciares desde el Último Máximo Glaciar Local tanto en el volcán HualcaHualca y el Altiplano de Patapampa, ambos localizados al sur de Perú, como en la Zona Volcánica Centroandina. Para ello se presentan 9 edades de exposición a la radiación cósmica procedentes de morrenas y umbrales rocosos pulidos y estriados. Dichas edades indican que los glaciares del HualcaHualca alcanzaron su máxima extensión hace ~17-16 ka en sincronía con el Heinrich 1 y la formación del paleolago Tauca. Desde entonces los glaciares empezaron a retroceder hasta ~12 ka cuando experimentaron un reavance o una fase de estabilización. La deglaciación fue constante desde hace ~11.5 ka en el HualcaHualca, coincidiendo con la desaparición del casquete de Hielo de Patapampa. Esta evolución de los glaciares del área de estudio no corrobora un Último Máximo Glaciar Local más antiguo que el Último Máximo Glaciar global pero sí indica su elevada sensibilidad a los cambios en la precipitación. De acuerdo con el análisis de las edades de exposición a la radiación cósmica de los volcanes HualcaHualca, Sajama y Tunupa, se infiere que el inicio de la deglaciación en la Zona Volcánica Centroandina tuvo lugar al finalizar el evento Heinrich 1 y la fase lacustre Tauca. Sin embargo, el retroceso no fue continuo ya que se registra al menos un reavance o fase de estabilización en la mayoría de los volcanes estudiados aunque la inconsistencia entre sus edades no permite relacionar con claridad dicha fase glaciar con los eventos climáticos fríos Younger Dryas y Antarctic Cold Reversal. Después, las masas de hielo de la Zona Volcánica Centroandina experimentaron un marcado retroceso, interrumpido temporalmente por al menos tres reavances o periodos de estabilización de menor entidad que evidencian que el clima durante el Holoceno no fue continuamente cálido y/o seco. Por último cabe destacar que a pesar de que las edades de exposición a la radiación cósmica proporcionan una información cronológica valiosa, de momento no permiten reconstruir de forma sólida la historia de los glaciares y establecer conexiones claras con los principales eventos fríos debido a las limitaciones que presentan las tasas de producción y el uso de diferentes modelos de escala. Con el objeto de reducir la incertidumbre derivada de estas limitaciones, sería necesario determinar una tasa de producción precisa de cada isótopo cosmogénico en los Andes Centrales, un modelo de escala de referencia y recalcular las edades publicadas.

Spatial–temporal distribution of explosive volcanism in the 25–28°S segment of the Andean Central Volcanic Zone

The present contribution describes and analyses the spatial–temporal distribution and main features of the volcanic rocks, with emphasis in pyroclastic deposits, located in the 25–28°S segment of the Central Andes. We built a new database, based on our own field studies and compilation from the literature, where we discriminate the different pyroclastic units. We find that the pyroclastic deposits cover an area of at least 7770km2 and are distributed mainly along N–S to NNE–SSW, NW–SE to WNW–ESE and NE–SW trends. The spatial distribution of these deposits has varied with time: those >14.5Ma are concentrated west of 69°W; during 14.5–11.5Ma explosive magmatism shifted east of 67°W; the deposits <11.5Ma became more dispersed. The main explosive activity occurred during the 26–18Ma, 14.5–11.5Ma and ≤5.3Ma intervals. For pyroclastic deposits, dacitic compositions dominate during the Miocene, whereas rhyodacitic and rhyolitic compositions are more abundant since the Pliocene. In the 25–28°S segment of the CVZ there is no clear migration pattern of the magmatism, except for the well-known eastward migration occurred at ca. 26Ma.Moreover, we find that a) basement lithology or composition or mechanical features influenced distribution of volcanic activity; b) the NE–SW alignment of <2.5Ma caldera complexes coincides with the maximum depth of the Moho discontinuity and may be a further indicator of delamination processes in the Puna–Eastern Cordillera/Pampean Ranges border; and c) intense explosive volcanism concentrated during the 14–11.5Ma interval along the eastern Puna border, which coincides in time with a peak in deformation and in space with the contact between basement units with different mechanical properties. No relationship between magmatic activity and azimuth or convergence rate between South American and Nazca plates are found, except for a peak in convergence rate at ~26Ma.

Persistent uplift of the Lazufre volcanic complex (Central Andes): New insights from PCAIM inversion of InSAR time series and GPS data

We reanalyzed the surface displacements observed at the Lazufre volcanic complex in the Southern Andean Central Volcanic Zone using GPS measurements made between 2006 and 2008 and a large InSAR data set. We performed a detailed spatiotemporal analysis of the displacements using a principal component analysis inversion method (PCAIM). The PCAIM reveals a source with no significant changes in shape and dimension and with a remarkably linear strength increase over the whole period of observation (i.e., 2003–2010). Then we used a three-dimensional mixed boundary element method (MBEM) to invert the first component of surface displacement as obtained from PCAIM. We explored a continuum of geometries from a shallow elliptic crack to a deep massive truncated elliptical cone that could represent a sill or a large magma chamber, respectively. The best models indicate a large flat-topped source with a roof area between 40 and 670 km2 and a depth of between 2 and 14 km below ground surface. Lastly, on the basis of the limited data available for the thermomechanical structure of the crust in the Southern Andean Central Volcanic Zone, we consider some possible scenarios to explain the spatial and temporal pattern of displacements at Lazufre.

Natural and anthropogenic variations in atmospheric mercury deposition during the Holocene near Quelccaya Ice Cap, Peru.

Mercury (Hg) is a toxic metal that is transported globally through the atmosphere. The emission of Hg from mineral reservoirs and subsequent recycling in surface reservoirs (i.e., soil/biomass, ocean, and atmosphere) are fundamental to the modern global Hg cycle, yet past emissions from anthropogenic and natural sources are not fully constrained. We use a sediment core from Yanacocha, a headwater lake in southeastern Peru, to study the anthropogenic and natural controls on atmospheric Hg deposition during the Holocene. From 12.3 to 3.5 ka, Hg fluxes in the record are relatively constant (mean ± 1σ: 1.4 ± 0.6 μg m-2 a-1, n = 189). Past Hg deposition does not correlate with changes in regional temperature and precipitation, inferred from nearby paleoclimate records, or with most large volcanic events that occurred regionally, in the Andean Central Volcanic Zone (~300-400 km from Yanacocha), and globally. In B.C. 1450 (3.4 ka), Hg fluxes abruptly increased and reached the Holocene-maximum flux (6.7 μg m-2 a-1) in B.C. 1200, concurrent with a ~100-year peak in Fe and chalcophile metals (As, Ag, Tl) and the presence of framboidal pyrite. Continuously elevated Hg fluxes from B.C. 1200-500 suggest a protracted mining-dust source near Yanacocha that is identical in timing to documented pre-Incan cinnabar mining in central Peru. During Incan and Colonial time (A.D. 1450-1650), Hg deposition remains elevated relative to background levels but lower relative to other Hg records from sediment cores in central Peru, indicating a limited spatial extent of preindustrial Hg emissions. Hg fluxes from A.D. 1980 to 2011 (4.0 ± 1.0 μg m-2 a-1, n = 5) are 3.0 ± 1.5 times greater than pre-anthropogenic fluxes and are similar to modern fluxes documented in remote lakes around the world.

Low-pressure evolution of arc magmas in thickened crust: The San Pedro–Linzor volcanic chain, Central Andes, Northern Chile

Magmatism at Andean Central Volcanic Zone (CVZ), or Central Andes, is strongly influenced by differentiation and assimilation at high pressures that occurred at lower levels of the thick continental crust. This is typically shown by high light to heavy rare earth element ratios (LREE/HREE) of the erupted lavas at this volcanic zone. Increase of these ratios with time is interpreted as a change to magma evolution in the presence of garnet during evolution of Central Andes. Such geochemical signals could be introduced into the magmas be high-pressure fractionation with garnet on the liquidus and/or assimilation from crustal rocks with a garnet-bearing residue. However, lavas erupted at San Pedro–Linzor volcanic chain show no evidence of garnet fractionation in their trace element patterns. This volcanic chain is located in the active volcanic arc, between 22°00′S and 22°30′S, over a continental crust ∼70 km thick. Sampled lavas show Sr/Y and Sm/Yb ratios <40 and <4.0, respectively, which is significantly lower than for most other lavas of recent volcanoes in the Central Andes. In addition, 87Sr/86Sr ratios from San Pedro–Linzor lava flows vary between 0.7063 and 0.7094. This is at the upper range, and even higher than those observed at other recent Central Andean volcanic rocks (<0.708). The area in which the San Pedro–Linzor volcanic chain is located is constituted by a felsic, Proterozoic upper crust, and a thin mafic lower crustal section (<25 km). Also, the NW–SE orientation of the volcanic chain is distinctive with respect to the N–S orientation of Central Andean volcanic front in northern Chile. We relate our geochemical observations to shallow crustal evolution of primitive magmas involving a high degree of assimilation of upper continental crust. We emphasize that low pressure AFC- (Assimilation Fractional Crystallization) type evolution of the San Pedro–Linzor volcanic chain reflects storage, fractionation, and contamination of mantle-derived magmas at the upper felsic crust (<40 km depth). The ascent of mantle-derived magmas to mid-crustal levels is related with the extensional regime that has existed in this zone of arc-front offset since Late-Miocene age, and the relatively thin portion of mafic lower crust observed below the volcanic chain.

Historia eruptiva de los volcanes monogenéticos de El Toro (23º05’S-66º42’W), Puna norte, Argentina

The monogenetic volcanism from the El Toro region (23&ordm;05&rsquo;S-66&ordm;42&rsquo;W) in NW Argentina comprises a group of low-volume (&lt;5 km2) mafic volcanic rocks erupted during the Late Miocene-Pliocene in the northern Puna (Andean Central Volcanic Zone). The activity of at least four eruptive centers resulted in the discharge of several lava flows from a cluster of cinder cones, whose architecture coincides with the typical facies model defined for edifices constructed during Strombolian style eruptions. In some cases, volcanoes recorded brief phases of transitional Hawaiian activity that resulted in small and localized fire-fountaining (occasionally with oblique development), responsible for the generation of agglutinated deposits as spatter and clastogenic lava. These highly welded products, still hot and ductile, may have acted as zones of weakness from which the edifices collapsed gravitationally during lava effusions (rafting process). The latter suggests that effusive and pyroclastic eruptions sometimes may have been coeval. Volcanic rocks are geochemically classified as arc-like, high-K calc-alkaline basalts, basaltic andesites and andesites. Mobile/inmobile trace element ratios (e.g., Ba/Nb) suggest that the El Toro rocks have arc-like signatures, transitional between frontal arc andesites (Le&oacute;n Muerto) and mafic to intermediate Andean rocks with intraplate affinities (Las Burras). The latter confirms a near-arc backarc position for the western northern Puna during the Upper Miocene to Pliocene. Although geochemically indistinguishable, rocks from El Toro volcanoes have significantly different petrographic characteristics (phenocryst assemblage, % of crystals). This suggests that some volcanic rocks from El Toro represent the eruption of very similar magma batches that show strong differences in temperature and degree of crystallization, i.e., batches evolving at different crustal depths and/or with different magma chamber residence times. On the other hand, magmas of different chemical compositions were erupted almost simultaneously from the same vents. This intriguing compositional pattern demonstrates that short-lived monogenetic volcanoes may have a complex polymagmatic character.

The Volcanic Evolution of Cerro Uturuncu: A High-K, Composite Volcano in the Back-Arc of the Central Andes of SW Bolivia

Cerro Uturuncu, southwest Bolivia, is a high-K, calc-alkaline, composite volcano constructed upon extremely thick continental crust approximately 125 km behind the arc-front of the Andean Central Volcanic Zone (CVZ). Eruptive activity occurred between 890 - 271 ka in a single phase of volcanism lasting ~620,000 years. The edifice consists of a central cone and several flank vents where dacitic and andesitic lava flows and domes erupted. Volumes of individual eruptive units range from 0.1 to ~10 km3; the composite volume of Uturuncu is ~89 km3. In this paper, we present new field, petrographic, and geochemical data in an effort to understand the volcanic and magmatic evolution of Uturuncu. Lava flows and domes have a restricted range in whole rock compositions ranging from 61 wt% - 67 wt% SiO2; magmatic inclusions contained within these units have a larger range from 53 wt% - 64 wt% SiO2. Typical phenocryst assemblages are plagioclase > orthopyroxene > biotite >> quartz and Fe-Ti oxides. Pb isotope ratios are characteristic of the southern CVZ by containing high 207Pb/204Pb and 206Pb/204Pb and moderate to high 208Pb/204Pb. Sr and Nd isotope ratios indicate that Uturuncu magmas were modified by high 87Sr/86Sr and low 143Nd/144Nd felsic basement lithology during magma migration and differentiation. In all eruptive units, there is petrographic and geochemical evidence for magma mixing and mingling. In this regard, magma mixing and mingling is considered to be responsible for the small range in lava flow and dome compositions throughout the eruptive history of the center.

Modification of the Continental Crust by Subduction Zone Magmatism and Vice-Versa: Across-Strike Geochemical Variations of Silicic Lavas from Individual Eruptive Centers in the Andean Central Volcanic Zone

To better understand the origin of across-strike K2O enrichments in silicic volcanic rocks from the Andean Central Volcanic Zone, we compare geochemical data for Quaternary volcanic rocks erupted from three well-characterized composite volcanoes situated along a southeast striking transect between 21° and 22° S latitude (Aucanquilcha, Ollagüe, and Uturuncu). At a given SiO2 content, lavas erupted with increasing distance from the arc front display systematically higher K2O, Rb, Th, Y, REE and HFSE contents; Rb/Sr ratios; and Sr isotopic ratios. In contrast, the lavas display systematically lower Al2O3, Na2O, Sr, and Ba contents; Ba/La, Ba/Zr, K/Rb, and Sr/Y ratios; Nd isotopic ratios; and more negative Eu anomalies toward the east. We suggest that silicic magmas along the arc front reflect melting of relatively young, mafic composition amphibolitic source rocks and that the mid- to deep-crust becomes increasingly older with a more felsic bulk composition in which residual mineralogies are progressively more feldspar-rich toward the east. Collectively, these data suggest the continental crust becomes strongly hybridized beneath frontal arc localities due to protracted intrusion of primary, mantle-derived basaltic magmas with a diminishing effect behind the arc front because of smaller degrees of mantle partial melting and primary melt generation.

Andean Adakite-like high-Mg Andesites on the Northern Margin of the Chilean–Pampean Flat-slab (27–28·5°S) Associated with Frontal Arc Migration and Fore-arc Subduction Erosion

Glassy, plagioclase phenocryst-free, ∼7–3 Ma, andesitic lavas erupted at the southern end of the Andean Central Volcanic Zone (CVZ) at 27–28·5°S are argued to contain a component from continental crust that was incorporated into the sub-arc mantle in a major pulse of fore-arc subduction erosion that removed ∼50 km of fore-arc. The 7–3 Ma amphibole-bearing Pircas Negras (54–64% SiO2) and ∼7·7 Ma pyroxene-bearing Dos Hermanos lavas erupted as the frontal arc was displaced some 50 km eastward over a developing bend in the Wadati–Benioff zone at the northern margin of the Chilean–Pampean flat-slab region. Their chemistry differs from that of older Miocene and younger CVZ arc lavas in the region in having more pronounced high field strength element (La/Ta = 40–100; Ba/Ta = 800–2400) and heavy rare earth element (Sm/Yb ∼ 4–9) depletions and an extreme adakitic-like character (∼600–1400 ppm Sr; Sr/Yb ∼ 400–1350). They also differ from older Miocene lavas in having higher 87Sr/86Sr and lower 143Nd/143Nd at the same wt % SiO2, while still showing trends with increasing wt % SiO2 that are best attributed to contamination in the ∼65–70 km thick crust. Overall, the trace element and isotopic features of the Pircas Negras lavas are consistent with modeling that attributes differences from older and younger lavas to enrichment of the arc mantle wedge by partial melts of tectonically eroded fore-arc crust, with the modeled eroded component being an outcrop and geophysically constrained mixture of 80–90% Jurassic mafic magmatic rocks and 10–20% silicic Paleozoic crust. An adakitic-type partial melt of this eroded crust generated at >2 GPa, reacted with mantle peridotite, can explain the high Mg# values (50–61) and Cr (100–350 ppm) and Ni (40–70 ppm) contents in some 5–3 Ma Pircas Negras lavas. Pre-eruption temperatures over 1060°C, calculated from mineral thermometry and the MELTS program, permit these magmas to subsequently melt and react with overlying eclogitic crust. Magma storage in this thick crust then led to amphibole crystallization and suppression of plagioclase fractionation. Quartz and feldspar xenocrysts were acquired from locally pooled mid-crustal magmas and the silicic upper crustal basement during final rapid ascent to the surface. Given a constant arc–trench gap of 300 km over the last 10 Myr, ∼198 km3 Myr–1 km–1 of fore-arc crust needs to be removed by subduction erosion at 8–3 Ma to account for the material balance of the fore-arc crust.

Geology, geochemistry, geochronology, and economic potential of Neogene volcanic rocks in the Laguna Pedernal and Salar de Aguas Calientes segments of the Archibarca lineament, northwest Argentina

This study presents new geochemical, geochronological, isotopic, and mineralogical data, combined with new geological mapping for a 2400km2 area of Neogene volcanic rocks in northwestern Argentina near the border with Chile, between 25°10′S and 25°45′S. The area covers the zone of intersection between the main axis of the Cordillera Occidental and a set of NW–SE-trending structures that form part of the transverse Archibarca lineament. This lineament has localized major ore deposits in Chile (e.g., the late Eocene La Escondida porphyry Cu deposit) and large volcanic centers such as the active Llullaillaco and Lastarría volcanoes on the border between Chile and Argentina, and the Neogene Archibarca, Antofalla, and Cerro Galán volcanoes in Argentina.Neogene volcanic rocks in the Laguna Pedernal and Salar de Aguas Calientes areas are mostly high-K calc-alkaline in composition, and range from basaltic andesites, through andesites and dacites, to rhyolites. Magmatic temperatures and oxidation states, estimated from mineral compositions, range from ~1000°C and ∆FMQ≈1.0–1.5 in andesites, to ~850°C and ∆FMQ≈1.5–2.0 in dacites and rhyolites.The oldest rocks consist of early–middle Miocene andesite–dacite plagioclase–pyroxene-phyric lava flows and ignimbrites, with 40Ar/39Ar ages ranging from 17.14±0.10Ma to 11.76±0.27Ma. Their major and trace element compositions are typical of the Andean Central Volcanic Zone, and show strong crustal contamination trends for highly incompatible elements such as Cs, Rb, Th, and U. These rocks are geochemically grouped as sub-suite 1.This widespread intermediate composition volcanism was followed in the middle–late Miocene by a period of more focused rhyodacitic flow–dome complex formation. These felsic rocks are characterized by less extreme enrichments in highly incompatible elements, and increasing depletion of heavy rare earth elements. These rocks are geochemically grouped as sub-suite 2. The youngest rocks in this sub-suite show the highest La/Yb ratios, and are characterized by abundant hornblende phenocrysts (not commonly seen in other rocks from the area).In the Pliocene–Pleistocene, there was a return to more typical andesite–dacite volcanism, with geochemical characteristics similar to the early–middle Miocene lavas, and are also grouped in sub-suite 1.Finally, extensional tectonics in the Quaternary led to localized outpouring of mafic (basaltic andesitic to andesitic) monogenetic lava flows and cones. One particularly large flow, the Vega Aguas Calientes lava flow, covers approximately 90km2, and samples form two groupings, with affinities similar to the least-evolved samples from sub-suites 1 and 2 (sub-groups BA1 and BA2, respectively).Nd and Sr isotopic compositions indicate moderate to strong crustal contamination, especially in more felsic rocks, and extend from 87Sr/86Sr (0.706) and εNd (−2.4), values typical of Central Volcanic Zone rocks, to more evolved compositions (0.709 and −6.8, respectively) typical of large-volume ignimbrites of the Altiplano–Puna Volcanic Complex and Cerro Galán. The latter compositions are thought to be derived by extensive interaction between mantle-derived arc magmas and Paleozoic granitoid rocks that form much of the crustal column in this region.The distinctive mineralogy and geochemistry of the sub-suite 2 middle–late Miocene rhyodacitic flow–dome complexes indicate that these magmas had higher water content than both the earlier and later sub-suite 1 andesites–dacites. They were erupted during a period of tectonic quiescence following the Quechua orogenic phase, and geophysical evidence suggests that they were proximally derived from a large upper crustal magma chamber which partially collapsed to form a trap-door caldera. Strong fumarolic alteration associated with the youngest of these felsic volcanoes, Cerro Abra Grande, suggests the potential for the existence of epithermal-type mineralization within the volcanic edifice, or porphyry-type mineralization at depth.

Erosion rates and erosion patterns of Neogene to Quaternary stratovolcanoes in the Western Cordillera of the Central Andes: An SRTM DEM based analysis

Erosion patterns and rates of 33 stratovolcanoes in the arid to hyperarid Central Andean Volcanic Zone (14°S to 27°S) have been constrained by morphometric modelling. All selected volcanoes belong to the short-lived, symmetrical, circular andesitic stratocone type, with ages spanning 14 Ma to recent. Starting from the initial, youthful volcano morphology of this type, represented in our study by Parinacota volcano, and comparing reconstructed volumes of progressively eroded volcanoes, such a time span allows us to infer long-term erosion rates. Typical erosion rates of < 10 to 20 m/Ma have been obtained for the Altiplano–Puna Plateau. Lowest erosion rates typify the hyperarid Puna plateau (7–9 m/Ma), while somewhat higher values (13–22 m/Ma) are recorded for volcanoes in the more humid South Peru, suggesting climatic control on differences in erosion rates. By contrast, much higher short-term erosion rates of 112 to 66 m/Ma, decreasing with age, are found for young (Late Quaternary) volcanoes, which indicates that juvenile volcanoes erode more rapidly due to their unconsolidated cover and steeper slopes; surface denudation slows down to approximately one tenth of this after a few Ma. An inverse correlation is observed between the degree of denudation (defined as volume removed by erosion/original volume) and edifice height from base to top after erosion. This relationship is independent of climate and original edifice elevation. The degree of denudation vs. volcano age provides a rough morphometric tool to constrain the time elapsed since the extinction of volcanic activity. This method can, however, only be applied to the volcanoes of the Altiplano (i.e. under uniform, long-term arid climate) with an uncertainty of ~ 1 Ma. Finally, an erosional pathway is suggested for volcanoes of the Altiplano–Puna preserving a peculiar “edelweiss” valley pattern related to glaciations. This pattern may have overprinted previous drainages and resulted in a discontinuous height reduction of the degrading stratovolcanoes.

The geochemistry of a dying continental arc: the Incapillo Caldera and Dome Complex of the southernmost Central Andean Volcanic Zone (~28°S)

The Pleistocene Incapillo Caldera and Dome Complex (5,570 m) marks the southernmost siliceous center of the Andean Central Volcanic Zone (~28°S), where the steeply dipping (~30°) segment of the subducting Nazca plate transitions into the Chilean “flatslab” to the south. The eruption of the Incapillo Caldera and Dome Complex began with a 3–1 Ma effusive phase characterized by ~40 rhyodacitic dome eruptions. This effusive phase was terminated by an explosive “caldera-forming” event at 0.51 Ma that produced the 14 km3 Incapillo ignimbrite. Distinctive and virtually identical chemical signatures of the domes and ignimbrites (SiO2 = 67–72 wt%; La/Yb = 37–56; Ba/La = 16–28; La/Ta = 30–50; 87Sr/86Sr = 0.70638–0.70669; e Nd = −4.2 to −4.6) indicate that all erupted lavas originated from the same magma chamber and that differentiation effects between units were minor. The strong HREE depletion (Sm/Yb = 6–8) that distinguishes Incapillo magmas from most of the large ignimbrites of the Altiplano–Puna plateau can be explained by the extent and degree of partial melting at lower crustal depths (>40 km) in the presence of garnet. At upper crustal depths, this high-pressure residual geochemical signature, also common to adjacent late Miocene/Pliocene Pircas Negras andesites, was partially overprinted by shallow-level assimilation and fractional crystallization processes. Energy-constrained AFC modeling suggests that incorporation of anatectic upper crustal melts into a fractionated “adakite-like” dacitic host best explains the petrogenesis of Incapillo magmas. The diminution of the sub-arc asthenospheric wedge during Nazca plate shallowing left the Incapillo magma chamber unreplenished by both mafic mantle-derived and lower crustal melts and thus stranded at shallow depths within the Andean crust. Based on its small size and distinctive high-pressure chemical signature, the Incapillo Caldera and Dome Complex provides an endmember model for an Andean caldera erupting within a waning magmatic arc over a shallowing subduction zone.