Altiplano-Puna Volcanic Complex Research Articles

Rare metal concentration features at a caldera type deposit in the Miocene-Quaternary boron-lithium province of North America. Search for analogues

The fact that lithium has a wide range of applications in many fields including the production of lithium-ion batteries determines an increased interest in lithium mining. The most common types of lithium raw material sources are underground brines, saline lakes (“salars”), and ore minerals. In 2021, the first and unique deposit of lithium clays was discovered in the McDermitt caldera (Nevada, USA). Its resources are estimated at 13.7 million tons of lithium carbonate with the lithium concentration of 2231 mg/l. The uniqueness of this deposit raises the interest in the formation of model ideas about lithiumclay genesis to search for analogous deposits and explore them. The purpose of the article is to provide an overview of the geological structure and describe the main development periods of the McDermitt caldera. The authors also characterize the potential sources of lithium (felsic igneous rocks and hydrothermal fluids), migration paths of lithium-bearing brines as well as the formation mechanism of clays with a high lithium content (hectorite, illite and smectite). A generalized formation model of this type of deposits is proposed. Particular attention is paid to the role of hydrothermal fluids as a potential additional source of lithium “supply” to the caldera basin. Key criteria characteristic of industrial accumulations of lithium of this type have been formed in order to explore and identify analogous deposits. In conclusion, the authors put forward a hypothesis about the presence of deposits that are analogous to the Thacker Pass in the McDermitt caldera in the lithium province on the Altiplano-Puna plateau in one of the calderas of the Altiplano-Puna volcanic complex, and in Eastern Kamchatka.

The Cerro Guacha Caldera complex, SW Bolivia: A long-lived, multicyclic, resurgent caldera complex in the Altiplano-Puna Volcanic Complex of the Central Andes

Measuring ∼50 × 30 km, the Cerro Guacha Caldera Complex (CGCC) is a polycyclic, nested caldera complex within the Altiplano Puna Volcanic Complex (APVC) of the Central Andes. Previous work had established that CGCC was built in three stages. Catastrophic supereruptions of the 1300 km3 (DRE - Dense Rock Equivalent) Guacha ignimbrite at ca. 5.65 Ma and the 800 km3 (DRE) Tara ignimbrite at ca. 3.49 Ma resulted in two nested caldera collapses that define the first two stages (G1C and G2C) of the CGCC. The third stage, G3C, initiated ∼1.8 Ma with the eruption of the Puripica Chico ignimbrite and associated lavas. The only previous detailed systematic study of the CGCC was of the G2C (Grocke et al., 2017a, 2017b). In this work we provide new field, petrochemical and geochronological data on the first and third cycles that now allow us to present the first comprehensive account of the eruption history, as well as the magmatic and volcanological evolution of the CGCC as a long-lived caldera complex.Erupted compositions during the G1C and G3C cycles are dominantly high-K, calc-alkaline dacites and minor andesites and these fall within the andesite to rhyolite compositional array previously defined for the G2C. In the CGCC as a whole, dacitic compositions account for over 90% of the ∼2200 km3 of magma erupted. These data reveal a consistency of magma composition, storage and evolution throughout the >4 Ma history of the CGCC.New UPb in zircon ages reveal distinct zircon age distributions that indicate that each caldera cycle represented new magmatic cycles that accumulated 10's to 100's of thousands of years prior to eruption. We find zircon antecrysts in all eruptive stages which we interpret as assimilation of non-erupted progenitors of earlier magmatic cycles. Occassional zircon xenocrysts and cores are interpreted as assimilation of upper crustal basement lithologies, supporting previous interpretations of upper crustal open system magmatic evolution for the G2C to the entire CGCC.Both the G1C and G2C calderas are west-hinged trap-door style collapses. Of these, the 5.65 Ma G1C caldera measures ∼50 km × 30 km, whereas the 3.49 Ma G2C is 30 × 20 km and is nested within the G1C. Resurgent uplift in both the calderas reveals at least a kilometer thickness of intracaldera ignimbrite. Outflow ignimbrites from the CGCC extend 100 km N-S and 40 km E-W around CGCC and were topographically limited. Intracaldera ignimbrite volume dominates outflow volumes by a ratio of ∼5:1. Repose periods between successive stages are now calculated to be 2.1 (G1C to G2C) and 1.8 (G2C to G3C) Myr respectively.The evolution of the CGCC parallels other APVC calderas and is closely linked to the history of the APVC flare-up. The two supereruptions at 5.65 Ma (Guacha Ignimbrite) and 3.49 Ma (Tara Ignimbrite) coincide with the peak of the flare-up and its eruptive and magmatic tempo correlate with the regional tempo. The eruptive and magmatic histories connote a composite granodioritic subcaldera pluton of significant extent accumulated in the upper crust beneath the CGCC.

Amphibole recycling and polybaric crystallization in rhyolitic lava domes from melt inclusion geochemistry at Cerro La Torta, Altiplano-Puna Volcanic Complex

La Torta is a rhyolitic lava dome located in the El Tatio volcanic complex, Región de Antofagasta (northern Chile), to the west of the Cerros de Tocorpuri volcanoes. La Torta is considered to be an important part of the central zone of the Altiplano Puna Magma Body (APMB) based on chemical and petrological signatures that distinguish it from other highly silicic volcanic systems in the area (i.e., Chao dacite). The petrological and geochemical characteristics, along with parental composition and magma evolution, were determined by analyzing amphibole and plagioclase-hosted melt inclusions using electron microprobe (EMP) and laser ablation ICP-MS (LA-ICP-MS). Our results show that La Torta dome was formed from a rhyolitic Ca- and K-rich, calc-alkaline magma, with an average SiO2 content of 76.8 wt % and H2O content of about 5.4 wt %, which was emplaced in the upper continental crust between 3.9 and 7.6 km below the paleosurface. The parental source of La Torta dome was a highly hydrous mush of andesitic composition, with plagioclase + amphibole ± quartz ± pyroxene crystal assemblage.Heating by an influx of magmas ascending from deeper sources to upper crustal depths (i.e., Altiplano-Puna Magma Body) caused the partial melting of amphibole and pyroxene in the source, evidenced by amphibole dissolution textures, enriching the melt in Cr, Ni, HFSE and HREE. As the system starts to cool and ascend, new amphibole crystallized from this mush, trapping the enriched residual melt as enriched melt inclusions in its cores, along with crystallization of biotite and plagioclase. We refer to this process as amphibole recycling. At the same time, the new melt drags the plagioclase and quartz inherited from the parental mush, forming the magma that emplaced at shallower levels in the crust to eventually erupt as La Torta.

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.

Subduction and Hydrogen Release: The Case of Bolivian Altiplano

Natural hydrogen is known to be generated in the crust by water/rock interactions, especially the oxidation of iron-rich rock or radiolysis. However, other sources, especially deeper ones, exist. In the context of subduction, the dehydration of the slab, the destabilization of the NH4, and the hydration of the mantle wedge above the subducting lithosphere may generate H2. We present here a compilation of the known gases in the central part of the Pacific subduction and the results of a first field acquisition dedicated to H2 measurements in Bolivia between La Paz and South Lipez. Various zones have been studied: the emerging thrust faults of the western borders of the Eastern Cordillera, the Sajama area that corresponds to the western volcanic zone near the Chile border northward from the Uyuni Salar, and finally, the Altiplano-Puna Volcanic Complex in South Lipez. Soil gas measurement within and around the Salar itself was not fully conclusive. North of the Uyuni Salar, the gases are very rich in CO2, enriched in N2 and poor in H2. On the opposite, southward, all the samples contain some H2; the major gas is nitrogen, which may overpass 90% after air correction, and the CO2 content is very limited. On the western border of the Cordillera, the δC13 isotope varies between −5 and −13‰, and it is not surprisingly compatible with volcanic gas, as well as with asthenospheric CO2. The methane content is close to 0, and only a few points reach 1%. The isotopes (−1‰) indicate an abiotic origin, and it is thus related to deep H2 presence. The high steam flow in the geothermal area of South Lipez combined with the H2 content in the water results in at least 1 ton of H2 currently released per day from each well and may deserve an evaluation of its economic value. The nitrogen content, as in other subduction or paleo-subduction areas, questions the slab alteration.

Rhyolitic melt production in the midst of a continental arc flare-up—The heterogeneous Caspana ignimbrite of the Altiplano-Puna volcanic complex of the Central Andes

Abstract The ~5 km3, 4.54–4.09 Ma Caspana ignimbrite of the Altiplano-Puna volcanic complex (APVC) of the Central Andes records the eruption of an andesite and two distinct rhyolitic magmas. It provides a unique opportunity to investigate the production of silicic magmas in a continental arc flare-up, where small volumes of magma rarely survive homogenization into the regional magmatic system that is dominated by supereruptions of monotonous dacitic ignimbrites. The fall deposit and thin flow unit that record the first stage of the eruption (Phase 1) tapped a crystal-poor peraluminous rhyolite. The petrological and geochemical characteristics of Phase 1 are best explained by partial melting of or reheating and melt extraction from a granodioritic intrusion. Phase 2 of the eruption records the emplacement of a more extensive flow unit with a crystal-poor, fayalite-bearing rhyolite and a porphyritic to glomeroporphyritic andesite containing abundant plagioclase-orthopyroxene-Fe-Ti oxide (norite) glomerocrysts. The isotopic composition of Phase 2 is significantly more “crustal” than Phase 1, indicating a separate petrogenetic path. The mineral assemblage of the noritic glomerocrysts and the observed trend between andesite and Phase 2 rhyolite are reproduced by rhyolite-MELTS–based models. Pressure-temperature-water (P-T-H2O) estimates indicate that the main (Phase 2) reservoir resided between 400 and 200 MPa, with the andesite recording the deeper pressures and a temperature range of 920–1060 °C. Rhyolite phase equilibria predict an estimated temperature of ~775 °C and ~5 wt% H2O. Pressures derived from phase equilibria indicate that the rhyolite was extracted directly from the noritic cumulate at ~340 MPa and stored at slightly shallower pressures (200–300 MPa) prior to eruption. The rhyolite-MELTS models reveal that latent-heat buffering during the extraction and storage process results in a shallow liquidus during the extensive crystallization that produced a noritic cumulate in equilibrium with a rhyodacitic residual liquid. Spikes in latent heat facilitated the segregation of the residual liquid, creating the pre-eruptive compositional gap of ~16 wt% SiO2 between the andesite and the Phase 2 rhyolite. Unlike typical Altiplano-Puna volcanic complex (APVC) magmas, low fO2 conditions in the andesite promoted co-crystallization of orthopyroxene and ilmenite in lieu of clinopyroxene and magnetite. This resulted in relatively high Fe concentrations in the rhyodacite and Phase 2 rhyolite. Combined with the co-crystallization of plagioclase, this low oxidation state forced high Fe2+/Mg and Fe/Ca in the Phase 2 rhyolite, which promoted fayalite stability. The dominance of low Fe3+/FeTot and Fe-Ti oxide equilibria indicates low fO2 (ΔFMQ 0 − ΔFMQ − 1) conditions in the rhyolite were inherited from the andesite. We propose that the serendipitous location on the periphery of the regional thermal anomaly of the Altiplano-Puna magma body (APMB) permitted the small-volume magma reservoir that fed the Caspana ignimbrite eruption to retain its heterogeneous character. This resulted in the record of rhyolitic liquids with disparate origins that evaded assimilation into the large dacite supereruption-feeding APMB. As such, the Caspana ignimbrite provides a unique window into the multi scale processes that build longlived continental silicic magma systems.

Pliocene-Pleistocene ash-fall tuff deposits in the intermountain Humahuaca and Casa Grande basins, northwestern Argentina: tracers in chronostratigraphic reconstructions and key to identify their volcanic sources

Ash-fall tuffs of the Pliocene-Pleistocene deposits of Humahuaca and Casa Grande intermountain basins, northwestern Argentina, have been differentiated into two groups based on new geochemical and geochronological data which correspond to the tuffs of the Pliocene-Lower Pleistocene alluvial fan deposits dominated by debris flow, deep sandy gravel braided, and shallow ephemeral lake deposits (Uquia and Mal Paso formations), and those recorded in Pleistocene alluvial fans sheet flood deposits. The two clusters of ages recognized: 4.3 to 2.6 Ma, and 2.2-pre 0.8 Ma, corresponding to these tuff groups, are in agreement with pulses of ignimbrite eruptions in the Altiplano Puna Volcanic Complex (APVC), and Southerm Puna calderas located west of the study region. The ash-fall tuffs of both groups are mainly vitreous to phenocryst-poor of rhyodacite-dacite composition with minor andesites to trachyandesites, characterized by 58-69% SiO2 contents, A/CNK 1-1.4, FeO/MgO (0.8-2.8), which plot in the calc-alkaline range. They can be differentiated based on its immobile trace elements ratios as indicated by a slight enrichment in LREE, higher arc affiliation and somewhat higher Sm/Yb ratios in the case of the younger group, although in both Sm/Yb ratios are indicative of sequestration of HREE in residual hornblende. The new geochemical and geochronological data of those ash fall tuffs point to these as chrono-stratigraphic tracers of the Humahuaca and Casa Grande intermountain basins stratigraphy, during the Pliocene-Pleistocene, also as the key to identify their volcanic sources.

Remote Sensing Survey of Altiplano-Puna Volcanic Complex Rocks and Minerals for Planetary Analog Use

The Altiplano-Puna Volcanic Complex (APVC) of the Central Andes is an arid region with extensive volcanism, possessing various geological features comparable to those of other solar system objects. The unique features of the APVC, e.g., hydrothermal fields and evaporite salars, have been used as planetary analogs before, but the complexity of the APVC presents a wealth of opportunities for more analog studies that have not been exploited previously. Motivated by the potential of using the APVC as an analog of the volcanic terrains of solar system objects, we mapped the mineralogy and silica content of the APVC up to ~100,000 km2 in northern Chile based on a combination of remote sensing data resembling those of the Moon and Mars. The band ratio indices of Landsat 8 Operational Land Imager multispectral images and mineral classifications based on spectral hourglass approach using Earth Observing-1 Hyperion hyperspectral images (both in the visible to shortwave infrared wavelengths) were used to map iron-bearing and alteration minerals. We also used Hyperion imagery to detect feldspar spectral signatures and demonstrated that feldspar minerals can be detected on non-anorthosites, which may influence interpretations of feldspar spectral signatures on Mars. From the Terra Advanced Spaceborne Thermal Emission and Reflection Radiometer Global Emissivity Dataset, we derived the silica percentage of non-evaporite rocks within errors of approximately 2–3 wt.% SiO2 for those in the 60–70 wt.% range (about 8 wt.% errors for the 50–60 wt.% range). Based on an integrated assessment of the three datasets, we highlighted three regions of particular interest worthy of further field investigation. We also evaluated the benefits and limitations of all three remote sensing methods for mapping key minerals and capturing rock diversity, based on available samples and existing geological maps.

Rayleigh wave group velocity maps at periods of 10–150 s beneath South America

SUMMARY Based on new data from permanent and temporary networks, we present fundamental mode Rayleigh wave group velocity maps at periods of 10–150 s related to the lithosphere beneath South America. We analyse waveform data from 1043 earthquakes, from 2002 to 2019, which were recorded by 282 stations. To isolate fundamental mode Rayleigh waves, a phase-matched filter is applied, and the measurements of group velocity are obtained from multiple filter analysis techniques. Thus, we obtain 17 838 paths, covering most of the South American continent, which reach their maximum at the period of 30 s and decrease for both shorter and longer periods. We calculate average dispersion curves and probability density distribution of all measured curves to check the consistency of our data set. Then, regionalized group velocity maps are obtained by iteratively combining the fast marching method and the subspace inversion method. The resolution of our models is assessed by checkerboard tests, which show that the synthetic group velocities are well recovered, despite some amplitude and smearing effects in some portions of the model, probably owing to regularization and uneven ray path coverage. Compared to previous group velocity studies for South America, our models present better resolution, mainly for shorter periods. Our maps of 10 and 20 s, for example, show an excellent correlation with the sedimentary thickness (CRUST1.0) and topography density (UNB$\_$TopoDens). Regions of exposed basement and high-density are related to fast group velocities, while sedimentary basins and low-densities are observed as areas of slow group velocities. We identify small-scale fast group velocity heterogeneities that may be linked to the Rio Apa and Rio Tebicuary cratons as well as to the geochronological provinces of the Amazonian Craton. The most striking feature of our map at 40 s is a fast group velocity structure with the same NE trend of the Transbrasiliano lineament, a Neoproterozoic megashear fault that crosses a large part of the South American continent. Our long-period maps sample lithospheric depths, revealing that cratonic areas of South America, such as the Amazonian and São Francisco cratons, correlate well with fast group velocities. Another interesting feature is the presence of a strong group velocity gradient between the Paraná and Chaco-Paraná basins, which nearly coincides with the location of the Western Paraná Suture, a continental-scale gravity discontinuity. From our group velocity maps, we estimate 1-D S-wave velocity depth profiles at 10 locations in South America: Chaco-Tarija Basin, Borborema Province (BP), Amazonian Craton, Paraná Basin, Tocantins Province, Acre Basin (AcB), Altiplano-Puna Volcanic Complex, Mantiqueira Province (MP), Parnaíba Basin and São Francisco Craton. Most of our inverted S-wave velocity profiles show good agreement with the SL2013sv model at lithospheric depths, except the BP, AcB and MP profiles. Particularly for the BP, a low shear wave velocity, from about 75 to 150 km depth, is a feature that is not present in the SL2013sv model and was probably resolved in our model because of our denser ray path coverage. This decreased S-wave velocity may be due to a lithospheric thinning beneath the BP, as already pointed out by previous studies.

Repeated extraction of aphyric melts in a rhyolitic system revealed by zircon age and composition: The Ramadas Volcanic Centre (Puna plateau), NW Argentina

Rhyolitic volcanism can provide important information about the mechanisms by which highly-evolved crystal-poor melts can be extracted from silicic crystal-mush reservoirs. In the Altiplano-Puna plateau (Central Andes), rhyolites are volumetrically less abundant than the high-volume, crystal-rich intermediate products emitted during the ignimbritic flare-up (ca. 10–1 Ma), and their geological and temporal relation with the widespread, upper crustal, dacitic mush systems is not well constrained. We studied the isotopic (UPb ages), trace (U, Th, Hf, Y, Ti, P) and rare earth element compositions of zircon contained in the rhyolitic products of the Ramadas Volcanic Centre (late Miocene), which erupted extremely-aphyric, garnet-bearing tubular pumice during a single Plinian event (Corte Blanco Tuff; northern Puna plateau). Results reveal a complex pre-eruptive magmatic history characterized by variable crystallization conditions existing at different times within an upper-crustal crystal-mush reservoir. The unmixing model applied to magmatic zircon sharing textural and geochemical features (oscillatory texture, Th/U = 0.2–0.6; Eu/Eu* = 0.1–0.7) indicates the existence of at least two mush-related crystallization events, which are separated by a protracted hiatus (ca. 2 Ma), and are supported by independent isotopic ages. An episode of zircon crystallization (average disequilibrium-corrected 206Pb/238U age of 9.06 ± 0.19 Ma) coincides with the ages determined for accessory phases associated with garnet in pumice samples (9.163 ± 0.037 Ma, UPb age determination on zircon; 8.70 ± 0.23 Ma, U-Th-Pb age determination on monazite). A further zircon crystallization event is recorded at ca. 6.64 ± 0.12 Ma, which is concordant with published radiometric ages dating the eruption at 6.3 ± 0.3 Ma (average 40Ar/39Ar age of glass shards from distal locations) and at 6.63 ± 0.28 Ma (fission track age of proximal obsidian). The existence of a late-stage to hydrothermal crystallization event is evident from another zircon population with low Th/U ratios (< 0.1) and a 206Pb/238U age of 6.514 ± 0.058 Ma, which crystallized in cold and highly-evolved parts of the reservoir resulting in negative Eu/Eu* and Ti depletion in zircon before these crystals were recycled into the erupting magma. The extreme aphyric character of the rhyolitic products and the nearly-complete lack of phenocrysts and glomerocrysts despite their evolved composition indicate that the Plinian eruption was preceded by effective extraction of crystal-poor melts from the mush zone, during which only micrometric antecrystic minerals (≤ 200 μm) were incorporated. Gas filter-pressing, combined with a compressional local stress field, likely contributed to melt-crystal separation, which was favoured by high volatile contents (H2Omelt 3–5 wt%) and shallow emplacement levels (< 10 km). Finally, the distribution of the U-Pb ages of zircon antecrysts suggests a correlation between the evolution of the Ramadas magmatic system and the fluctuating pattern characterizing the Altiplano-Puna Volcanic Complex flare-up activity, highlighting a possible geological and temporal connection between rhyolitic volcanism and the widespread dacitic mush systems in this sector of the Puna plateau.

Magmatic Evolution of Zoned and Unzoned Ignimbrites: Evidence for a Complex Crustal Architecture Feeding four Rapid-sequence, Caldera-forming Eruptions in the San Juan Mountains, Colorado

Abstract The last four caldera-forming ignimbrites in the central San Juan caldera cluster, Colorado, erupted 1400 km3 in ≤80 kyr and alternated between zoned crystal-poor rhyolite to crystal-rich dacite and unzoned, crystal-rich dacite. The zoned 150 km3 Rat Creek Tuff (26·91 Ma), unzoned 250 km3 Cebolla Creek Tuff, and zoned 500 km3 Nelson Mountain Tuff (26·90 Ma) formed the nested San Luis caldera complex with slightly offset calderas, and the unzoned 500 km3 Snowshoe Mountain Tuff (26·87 Ma) formed the Creede caldera to the south. The Rat Creek Tuff, Nelson Mountain Tuff, and Snowshoe Mountain Tuff have similar mineral assemblages of plagioclase, sanidine, quartz, biotite, hornblende, clinopyroxene, Fe–Ti oxides, and accessory zircon, titanite, and apatite. The Cebolla Creek Tuff differs from the other three ignimbrites with more abundant hornblende and a lack of quartz and sanidine. Trace element compositions of interstitial glass are unique to each ignimbrite, correlating with mineral assemblages and inferred crystallization depths. Glass, feldspar, hornblende, and clinopyroxene thermobarometry calculations provide evidence for vertically extensive crustal magma reservoirs with a common magmatic zone at ∼435–470 MPa (∼16–17 km) showing a transition into shallow pre-eruptive reservoirs between ∼110 and 340 MPa (∼4–13 km), similar to the estimated magma reservoir architecture of the Altiplano Puna Volcanic Complex. The upper portions of the eruptible parts of the magma reservoirs of the Rat Creek Tuff (215 ± 50 MPa, ∼810–820 °C), Cebolla Creek Tuff (340 ± 20 MPa, ∼860–880 °C), Nelson Mountain Tuff (215 ± 20 MPa, ∼745–800 °C), and Snowshoe Mountain Tuff (110 ± 40 MPa, 825 ± 10 °C) occupied shallow levels in the crust similar to other magma reservoirs of the central San Juan caldera cluster. Trace element modelling correlates with a deep crystallization signature in the unzoned Cebolla Creek Tuff and Snowshoe Mountain Tuff, typified by a flat trend in Ba versus Sr whole-rock and glass chemistry. The zoned Rat Creek Tuff and Nelson Mountain Tuff are typified by a steep trend in Ba versus Sr chemistry interpreted as a shallower crystallization signature. Similarly, the unzoned Cebolla Creek Tuff and Snowshoe Mountain Tuff have flatter slopes in FeO versus An space of plagioclase chemistry interpreted as more abundant deep plagioclase crystallization and a difficulty in physically mixing with Fe-rich mafic recharge magma owing to higher viscosity. The zoned Rat Creek Tuff and Nelson Mountain Tuff have higher slopes in FeO versus An space of plagioclase chemistry interpreted as more abundant shallow plagioclase crystallization and more feasible mixing with Fe-rich mafic recharge magma owing to lower viscosity. The eruption of the Rat Creek Tuff was probably triggered by mafic injection, but the other three ignimbrites lack mingling textures in pumice, suggesting that other mechanisms were important in causing such large eruptions. After a prolonged period of mantle-derived magma injection and crustal heating (∼25 000 km3 Conejos Formation erupted during ∼35–29 Ma), the San Juan magmatic body became a robust and versatile producer of diverse eruptible magmas in short time periods during its Oligocene ignimbrite flare-up.

Characteristics and emplacement mechanisms of the Coranzulí ignimbrites (Central Andes)

We present a detailed stratigraphy of the Coranzulí caldera-forming deposits. This caldera, located in the Altiplano-Puna Volcanic Complex (Central Andes), generated four ignimbrite deposits with similar field characteristics and facies that differ from each other in, above all, the nature of the lithic fragments they contain. Three different lithofacies (fine-grained cross-stratified facies, massive lithic breccia facies and massive ignimbrite facies) are found in all the ignimbrite deposits, which occasionally also contain a lenticular lithic-rich facies and/or a pumice-rich facies. These field characteristics and, in particular, local deposit thicknesses were used to develop a theoretical model of the dynamics and emplacement mode of the Coranzulí pyroclastic flows. Our results show that these ignimbrites were emplaced by dense pyroclastic density currents subjected to high accumulation rates and velocities, thereby indicating rapid en masse emplacement that was also influenced by local paleotopography as deduced from facies analysis.

Constraining the sub-arc, parental magma composition for the giant Altiplano-Puna Volcanic Complex, northern Chile

The Andean continental arc is built upon the thickest crust on Earth, whose eruption products reflect varying degrees of crustal assimilation. In order to robustly model magma evolution and assimilation at subduction zones such as the Andes, the compositions of parental magmas feeding crustal magma reservoirs need to be defined. Here we present new olivine and clinopyroxene oxygen isotope data from rare mafic volcanic rocks erupted at the margins of the giant Altiplano-Puna Magma Body (APMB) of the Altiplano-Puna Volcanic Complex, Central Andes. Existing olivine and pyroxene δ18O values for the Central Andes are highly variable and potentially not representative of sub-arc parental compositions. However, new olivine (n = 6) and clinopyroxene (n = 12) δ18O values of six Central Andean volcanoes presented here display a narrow range, with averages at 6.0‰ ± 0.2 (2σ S.D.) and 6.7‰ ± 0.3 (2σ S.D.), consistent with a common history for the investigated minerals. These data allow us to estimate the δ18O values of sub-arc, parental melts to ca. 7.0‰ ± 0.2 (2σ S.D.). Parental melts feeding the APMB and associated volcanic centres are postulated to form in the felsic continental crust following assimilation of up to 28% high-δ18O basement rocks by mantle-derived magmas.

The great escape: Petrogenesis of low-silica volcanism of Pliocene to Quaternary age associated with the Altiplano-Puna Volcanic Complex of northern Chile (21°10′-22°50′S)

The Pliocene to Quaternary volcanic arc of the Central Andes formed on 70–74 km thick continental crust. Physical interaction between mafic and acid magmas for this arc are therefore difficult to recognize due to the differentiation of mantle-derived magma during ascent through the thickened crust and a corresponding lack of erupted primitive lavas. However, a rare concentration of less evolved rocks is located marginal to the partially molten Altiplano-Puna Magma Body (APMB) in the Altiplano-Puna Volcanic Complex of northern Chile, between 21°10′S and 22°50′S. To unravel the relationship between this less evolved magmatism and the APMB, we present major and trace element data, and Sr and Nd isotope ratios of fourteen volcanoes. Whole-rock compositional and Sr and Nd isotope data reveal a large degree for compositional heterogeneity, e.g., SiO2 = 53.2 to 63.2 wt%, MgO = 1.74 to 6.08 wt%, Cr = 2 to 382 ppm, Sr = 304 to 885 ppm, 87Sr/86Sr = 0.7055 to 0.7088, and 143Nd/144Nd = 0.5122 to 0.5125. The combined dataset points to magma spatial compositional changes resulting from magma mixing, fractional crystallization and crustal assimilation. The least evolved products erupted along the periphery of the APMB and are likely equivalent to the replenishing magmas that thermally sustain the large APMB system. We suggest that the mafic to intermediate eruptives we have investigated reflect mafic melt injections that underplate the APMB and escape along the side of the large felsic body to avoid significant compositional modifications during ascent, which helps to assess the evolution of the APMB through space and time.

Multiscale Analysis of DInSAR Measurements for Multi-Source Investigation at Uturuncu Volcano (Bolivia)

Uturuncu volcano (southwestern Bolivia) is localized within one of the largest updoming volcanic zones, the Altiplano Puna Volcanic Complex (APVC). In several geodetic studies the observed uplift phenomenon is analyzed and modeled by considering a deep source, related to the Altiplano Puna Magma Body (APMB). In this framework, we perform a multiscale analysis on the 2003–2010 ENVISAT satellite data to investigate the existence of a multi-source scenario for this region. The proposed analysis is based on Cross-correlation and Multiridge method, pointing out the spatial and temporal multiscale properties of the deformation field. In particular, we analyze the vertical component of ground deformation during two time interval: within the 2005–2008 time interval an inflating source at 18.7 km depth beneath the central zone of the APVC is retrieved; this result is in good agreement with those proposed by several authors for the APMB. Between August 2006 and February 2007, we identify a further inflating source at 4.5 km depth, beneath Uturuncu volcano; the existence of this latter, located just below the 2009–2010 seismic swarm, is supported by petrological, geochemical, and geophysical evidence, indicating as a possible interpretative scenario the action of shallow, temporarily trapped fluids.

The upper crustal magma plumbing system of the Pleistocene Apacheta-Aguilucho Volcanic Complex area (Altiplano-Puna, northern Chile) as inferred from the erupted lavas and their enclaves

The evolution of the magma plumbing system of the Pleistocene Apacheta-Aguilucho Volcanic Complex area (Altiplano-Puna Volcanic Complex, northern Chile) was investigated through petrographic, geochemical and isotopic studies of representative lavas and related enclaves. Updated available dates of these products, both from the Apacheta and Aguilucho stratovolcanoes and nearby domes, allow us to define the activity during the last 1 Ma. This investigation shows that the andesitic magmas were affected by processes of Assimilation plus Fractional Crystallization (AFC, with a significant role played by amphibole fractionation) during their ascent through the upper crust, presumably by the interaction with the Altiplano-Puna Magma Body (15–20 km). These andesitic magmas were erupted with no or minor additional contamination at shallower levels, or experienced plagioclase-dominated Fractional Crystallization (FC) to dacite within shallower crustal magma chambers (4–8 km depth). The constructional phase of the Apacheta and Aguilucho stratovolcanoes (≥1 to ca. 0.6 Ma) reflect a transition from high-flux (i.e. flare-ups) to steady state magmatism, as also documented in other Pleistocene volcanic complexes of the Altiplano-Puna Volcanic Complex. During this stage the mafic magma recharge was high enough to permit a large spectrum of hybridization of the resident magmas in the upper crust to form the abundant andesites and dacites lavas. In contrast, at ∼150–100 ka, the magmatism turned to a new stage of recharge (waning stage?) and the episodic intrusion of small-volumes of andesitic magmas permitted the remobilization of the crystal-rich dacites, triggering the extrusions of the Chanka, Chac-Inca and Cerro Pabellón domes. The andesitic enclaves in the domes studied here represent a snapshot of the magmatic processes of interaction that occurred in the shallower reservoir at the interface of the resident dacite with the ascending andesitic magma. Nevertheless, as there are no dated volcanic products from the area between 0.6 and 0.1 Ma, the youngest dacitic domes could also be interpreted as the beginning of a new magmatic pulse of the Altiplano-Puna Volcanic Complex. Independently from the significance of the mafic recharge at 150–100 ka (waning stage vs. new pulse) the youngest investigated domes share similar geochemical features and crustal depth constraints (4–8 km) with the ignimbrites of the Altiplano-Puna Volcanic Complex, therefore suggesting that the remobilized magmas erupted as the domes are possibly remains of older plumbing systems left over from the last magmatic flare-up of the Altiplano-Puna Volcanic Complex.

Thermomechanical modeling of the Altiplano-Puna deformation anomaly: Multiparameter insights into magma mush reorganization

A 150-km-wide ground deformation anomaly in the Altiplano-Puna vol­canic complex (APVC) of the Central Andes, with uplift centered on Uturuncu volcano and peripheral subsidence, alludes to complex subsurface stress changes. In particular, the role of a large, geophysically anomalous and partially molten reservoir (the Altiplano-Puna magma body, APMB), located ∼ 20 km beneath the deforming surface, is still poorly understood. To explain the observed spatiotemporal ground deformation pattern, we integrate geophysical and petrological data and develop a numerical model that accounts for a mechanically heterogeneous and viscoelastic crust. Best-fit models ­imply subsurface stress changes due to the episodic reorganization of an interconnected vertically extended mid-crustal plumbing system composed of the APMB and a domed bulge and column structure. Measured gravity-height gradient data point toward low-density fluid migration as the dominant process behind these stress changes. We calculate a mean annual flux of ∼ 2 × 10 7 m 3 of water-rich andesitic melt and/or magmatic water from the APMB into the bulge and column structure accompanied by modest pressure changes of 100 yr) residual deformation at Uturuncu. Episodic mush reorganization may be a ubiquitous characteristic of the magmatic evolution of the APVC.

Neogene monogenetic volcanism from the Northern Puna region: products and eruptive styles

Abstract The Neogene mafic volcanism of the Northern Puna region in the Central Andes is represented by scoria cones and lava flows dispersed over a wide region ( c. 9150 km 2 ) as isolated or poorly clustered centres. Although all the products are basaltic andesites to andesites, the behaviour of these magmatic systems resembles that seen in basaltic monogenetic fields. These centres were studied with the aim of defining the main volcanic lithofacies and evaluating the eruptive styles. The results suggest that the eruptions developed under a dry strombolian dynamic, with brief periods of lava fountaining and hydrovolcanism, the latter usually restricted to the early stages of cone construction. Changes in eruptive style are thought to be caused by variations in both the internal (e.g. magma ascent) and external (e.g. surficial water availability) conditions. The transitions do not reflect compositional changes, as evidenced by the small chemical differences observed among the products of the studied eruptive centres. Stratigraphic analysis, in addition to a few pre-existing radiometric dates, suggests that this volcanic activity occurred during the Late Miocene to Early Pliocene. This information supports the inference that these eruptions occurred before the peak of Southern Puna mafic volcanism and that they were coeval with eruptions of some of the most important silicic calderas of the Altiplano-Puna Volcanic Complex. The good preservation of volcanic edifices reveals that erosion rates were extremely low, in agreement with the high aridity conditions that have prevailed in the Puna region since the Mid- to Late Miocene.

Voluminous plutonism during volcanic quiescence revealed by thermochemical modeling of zircon

Abstract Five late Pleistocene lava domes with a combined eruptive volume of ∼40 km3 distributed over an area of ∼2000 km2 represent the waning stages of the 10–1 Ma ignimbrite flare-up in the Altiplano Puna Volcanic Complex (APVC) of the Central Andes. Zircon crystal face (on unsectioned rims) and interior (on sectioned crystals) ages (U-Th and U-Pb, respectively) for a total of 252 crystals indicate remarkably consistent zircon crystallization histories: the youngest zircon surface ages (ca. 104–83 ka) are near 40Ar/39Ar eruption ages from sanidine and biotite (ca. 120–87 ka), but a significant population of surface ages predates eruption, ranging to secular equilibrium (with U-Pb interior ages to 3.5 Ma). The essentially continuous zircon crystallization history implies protracted magma presence, which agrees with temporally invariant Ti-in-zircon model temperatures, backed by the homogeneity of indirectly temperature-dependent compositional parameters. Zircon age spectra modeled using a finite-difference thermal and mass-balance model for open-system magma evolution indicate protracted zircon production in the magma reservoirs that require time-integrated recharge rates of ∼1 × 10−3 km3/yr, corresponding to high intrusive to extrusive ratios of 75: 1. This rate is below the ∼5 × 10−3 km3/yr threshold proposed in the literature for incubating the supereruptions defining the flare-up. When accounting for the shorter durations of high versus low recharge episodes over the ∼10 m.y. lifetime of the APVC flare-up, the contributions to composite batholith formation in the shallow crust of the APVC remained broadly constant during peaks and lulls in eruptive activity. This connotes that eruptive fluxes are a poor measure for intrusive fluxes. A corollary of this interpretation is that commonly applied intrusive to extrusive ratios will severely underestimate pluton formation rates during periods of low eruptive flux.

Compositional variations of ignimbrite magmas in the Central Andes over the past 26 Ma — A multivariate statistical perspective

Multivariate statistical and geospatial analyses based on a compilation of 890 geochemical and ~1200 geochronological data for 194 mapped ignimbrites from the Central Andes document the compositional and temporal patterns of large-volume ignimbrites (so-called “ignimbrite flare-ups”) during Neogene times. Rapid advances in computational science during the past decade led to a growing pool of algorithms for multivariate statistics for large datasets with many predictor variables. This study applies cluster analysis (CA) and linear discriminant analysis (LDA) on log-ratio transformed data with the aim of (1) testing a tool for ignimbrite correlation and (2) distinguishing compositional groups that reflect different processes and sources of ignimbrite magmatism during the geodynamic evolution of the Central Andes. CA on major and trace elements allows grouping of ignimbrites according to their geochemical characteristics into rhyolitic and dacitic “end-members” and to differentiate characteristic trace element signatures with respect to Eu anomaly, depletions in middle and heavy rare earth elements (REE) and variable enrichments in light REE. To highlight these distinct compositional signatures, we applied LDA to selected ignimbrites for which comprehensive datasets were available. In comparison to traditional geochemical parameters we found that the advantage of multivariate statistics is their capability of dealing with large datasets and many variables (elements) and to take advantage of this n-dimensional space to detect subtle compositional differences contained in the data.The most important predictors for discriminating ignimbrites are La, Yb, Eu, Al2O3, K2O, P2O5, MgO, FeOt, and TiO2. However, other REE such as Gd, Pr, Tm, Sm, Dy and Er also contribute to the discrimination functions.Significant compositional differences were found between (1) the older (>13Ma) large-volume plateau-forming ignimbrites in northernmost Chile and southern Peru and (2) the younger (<10Ma) Altiplano–Puna–Volcanic-Complex (APVC) ignimbrites that are of similar volumes. Older ignimbrites are less depleted in HREE and less radiogenic in Sr isotopes, indicating smaller crustal contributions during evolution in a thinner and thermally less evolved crust. These compositional variations indicate a relation to crustal thickening with a “transition” from plagioclase to amphibole and garnet residual mineralogy between 13 and 9Ma. Compositional and volumetric variations correlate to the N–S passage of the Juan-Fernandéz-Ridge, crustal shortening and thickening, and increased average crustal temperatures during the past 26Ma.