Altiplano-Puna Volcanic Complex Research Articles

The Cerro Bitiche Andesitic Field: petrological diversity and implications for magmatic evolution of mafic volcanic centers from the northern Puna

The Cerro Bitiche Andesitic Field (CBAF) is one of the two largest mafic volcanic fields in northern Puna (22–24° S) and is spatially and temporally associated with ignimbrites erupted from some central Andean Altiplano-Puna Volcanic Complex calderas. The CBAF comprises seven scoria cones and widespread high-K calcalkaline lava flows that cover an area of 200 km2. Although all erupted rocks have a relatively narrow chemical range (56–62 % SiO2, 3–6 % MgO), there is a broad diversity of mineral compositions and textures. The least evolved lavas (∼58–61 % SiO2) are high-Mg andesites with scarce (<10 %) microphenocrysts of either olivine or orthopyroxene. The small compositional range and low phenocryst content indicate evolution controlled by low percentages (<10 %) of fractional crystallization of olivine and clinopyroxene of magmas similar to the least evolved rocks from the field, accompanied by assimilation during rapid ascent through the crust. Evolved andesites (∼62 wt% SiO2), on the other hand, are porphyritic rocks with plagioclase + orthopyroxene + biotite and ubiquitous phenocryst disequilibrium textures. These magmas were likely stored in crustal reservoirs, where they experienced convection caused by mafic magma underplating, magma mixing, and/or assimilation. Trace element and mineral compositions of CBAF lavas provide evidence for complex evolution of distinct magma batches.

Geochronological imaging of an episodically constructed subvolcanic batholith: U-Pb in zircon chronochemistry of the Altiplano-Puna Volcanic Complex of the Central Andes

Zircons from 15 crystal-rich monotonous intermediate ignimbrites and 1 crystal-poor rhyolite ignimbrite erupted during the 11–1 Ma Altiplano-Puna Volcanic Complex (APVC) ignimbrite flare-up record multiscale episodicity in the magmatic history of the shallowest levels (5–10 km beneath the surface) of the Altiplano-Puna Magma Body (APMB). This record reveals the construction of a subvolcanic batholith and its magmatic and eruptive tempo. More than 750 U-Pb ages of zircon rims and interiors of polished grains determined by secondary ion mass spectrometry define complex age spectra for each ignimbrite with a dominant peak of autocrysts and subsidiary antecryst peaks. Xenocrysts are rare. Weighted averages obtained by pooling the youngest analytically indistinguishable zircon ages mostly correspond to the dominant crystallization ages for zircons in the magma. These magmatic ages are consistent with eruptive stratigraphy, and fall into four groups defining distinct pulses (from older to younger, pulses 1 through 4) of magmatism that correlate with eruptive pulses, but indicate that magmatic construction in each pulse initiated at least 1 m.y. before eruptions began. Magmatism was initially distributed diffusely on the eastern and western flanks of the APVC, but spread out over much of the APVC as activity waxed before focusing in the central part during the peak of the flare-up. Each pulse consists of spatially distinct but temporally sequenced subpulses of magma that represent the construction of pre-eruptive magma reservoirs. Three nested calderas were the main eruptive loci during the peak of the flare-up from ca. 6 to 2.5 Ma. These show broadly synchronous magmatic development but some discordance in their later eruptive histories. These relations are interpreted to indicate that eruptive tempo is controlled locally from the top down, while magmatic tempo is a more systemic, deeper, bottom-up feature. Synchroneity in magmatic history at distinct upper crustal magmatic foci implicates a shared connection deeper within the APMB. Each ignimbrite records the development of a discrete magma. Zircon age distributions of individual ignimbrites become more complex with time, reflecting the carryover of antecrysts in successively younger magmas and attesting to upper crustal assimilation in the APVC. Although present, xeno­crysts are rare, suggesting that inheritance is limited. This is attributed to basement assimilation under zircon-undersaturated conditions deeper in the APMB than the pre-eruptive levels, where antecrysts were incorporated in zircon-saturated conditions. Magmatic ages for individual ignimbrites are older than the 40 Ar/ 39 Ar eruption ages. This difference is interpreted as the average minimum Zr-saturated melt-present lifetime for APVC magmas, the magmatic duration or Δ age. The average Δ age of ca. 0.4 Ma indicates that thermochemical conditions for zircon saturation were maintained for several hundreds of thousands of years prior to eruption of APVC magmas. This is consistent with a narrow range of zircon saturation temperatures of 730–815 °C that record upper crustal conditions and Zr/Hf, Th/U, Eu/Eu*, and Ti that reveal protracted magma differentiation under secular cooling rates an order of magnitude slower than typical pluton cooling rates. In concert, these data all suggest that the pre-eruptive magma reservoirs were perched in a thermally and chemically buffered state during their long pre-eruptive lifetimes. Trace element variations suggest subtle differences in crystallinity, melt fraction, and melt composition within different zones of individual magma reservoirs. Significant volumes of plutonic rocks associated with ignimbrites are supported by geophysical data, the limited compositional range over 10 m.y., the thermal inertia of the magmatic systems, and the evidence of resurgent magmatism and uplift at the calderas and eruptive centers, the distribution of which defines a composite, episodically constructed subvolcanic batholith. The multiscale episodicity revealed by the zircon U-Pb ages of the APVC flare-up can be interpreted in the context of continental arc magmatic systems in general. The APVC ignimbrite flare-up as a whole is a secondary pulse of ∼10 m.y., with magmatic pulses 1 through 4 reflecting tertiary pulses of ∼2 m.y., and the individual ignimbrite zircon spectra defining quaternary pulses of

Recording the transition from flare-up to steady-state arc magmatism at the Purico–Chascon volcanic complex, northern Chile

The long-term evolution of continental magmatic arcs is episodic, where a few transient events of high magmatic flux or flare-ups punctuate the low-flux magmatism or “steady state” that makes up most of the arc history. How this duality manifests in terms of differences in crustal architecture, magma dynamics and chemistry, and the time scale over which transitions occur is poorly known. Herein we use multiscale geochemical and isotopic characteristics coupled with geothermobarometry at the Purico–Chascon Volcanic Complex (PCVC) in the Central Andes to identify a transition from flare-up to steady state arc magmatism over ∼800 kyr during which significant changes in upper crustal magmatic dynamics are recorded.The PCVC is one of the youngest volcanic centers related to a 10–1 Ma ignimbrite flare-up in the Altiplano–Puna Volcanic Complex of the Central Andes. Activity at the PCVC initiated 0.98±0.03 Ma with the eruption of a large 80–100 km3 crystal-rich dacite ignimbrite. High, restricted 87Sr/86Sr isotope ratios between 0.7085 and 0.7090 in the bulk rock and plagioclase crystals from the Purico ignimbrite, combined with mineral chemistry and phase relationships indicate the dacite magma accumulated and evolved at relatively low temperatures around 800–850 °C in the upper crust at 4–8 km depth. Minor andesite pumice erupted late in the ignimbrite sequence records a second higher temperature (965 °C), higher pressure environment (17–20 km), but with similar restricted radiogenic bulk rock 87Sr/86Sr = 0.7089–0.7091 to the dacites. The compositional and isotopic characteristics of the Purico ignimbrite implicate an extensive zone of upper crustal mixing, assimilation, storage and homogenization (MASH) between ∼30 and 4 km beneath the PCVC ∼1 Ma.The final eruptions at the PCVC <0.18±0.02 Ma suggest a change in the magmatic architecture beneath the PCVC. These eruptions produced three small <6 km3 crystal-rich dacite lava domes with radiogenic bulk rock 87Sr/86Sr ratios ranging from 0.7075 to 0.7081, that contain abundant basaltic-andesite inclusions with relatively low bulk rock 87Sr/86Sr ratios of 0.7057–0.7061. Plagioclase and amphibole in the host lava of Cerro Chascon, the largest of the domes, record two distinct magmatic environments; an upper crustal environment identical to that recorded in the Purico ignimbrite, and a second deeper, ∼15–20 km depth, higher temperature (∼922–1001 °C) environment. This deeper environment is recorded in textures and compositions of distinct mineral phases, and in intracrystalline isotope ratios. Plagioclase cores in the host dacite lava and mafic inclusions have in situ87Sr/86Sr isotopic compositions of 0.7083 to 0.7095, broadly similar to plagioclase from the Purico ignimbrite. In contrast, plagioclase rims and microphenocrysts in the mafic inclusions are isotopically distinct with lower 87Sr/86Sr isotope ratios (0.7057 to 0.7065 and 0.7062 to 0.7064, respectively) that overlap with the regional isotopic “baseline” compositions that are parental to the modern arc lavas.The textural and compositional characteristics of the PCVC attest to two distinct stages in its history. At ∼1 Ma the system was broadly homogeneous and dominantly dacitic recording extensive upper crustal magmatism. By ∼0.2 Ma the PCVC had transitioned to a more compositionally heterogeneous, smaller volume, mixed dacite to basaltic-andesite system, coinciding with the appearance of less-enriched “baseline” compositions. The evolution of PCVC is a microcosm of the Central Andean arc in this region where, from 10 to 1 Ma, upper crustal MASH processes resulted in the production and eruption of large volumes of homogeneous crystal-rich dacite during a regional ignimbrite flare-up. Since ∼1 Ma, decreasing explosivity, smaller eruptive volumes, increasing heterogeneity, and the emergence of less isotopically enriched basaltic-andesite to dacite composite volcanoes signal a return to steady-state arc volcanism.We posit that the transition from flare-up to steady state captured at the PCVC tracks the waning of the arc scale “thermal engine”. High magmatic fluxes during the flare-up would lead to elevated geothermal gradients and efficient crustal processing leading to a dominantly “crustal” magmatism feeding the large volume Purico ignimbrite. This upper crustal MASH zone would act as an efficient filter to any parental compositions precluding them from the eruption record. As magmatic flux and thermal energy wanes, crustal isotherms would relax leading to greater thermal contrast between parental magmas, upper crust, and remnant felsic magmas stored in the upper crust. These changes are manifested in the preservation of textural and compositional heterogeneity and the survival of less isotopically enriched magmas in the upper crust. The chemical imprint of these arc-scale changes in magma dynamics is recorded at all scales from bulk rock to intra-crystalline. The distinct magma dynamics and chemical signatures of the two modes of arc magmatism detailed here should provide a model for investigations of mature continental arc evolution through time and space.

Magnetotelluric images of magma distribution beneath Volcán Uturuncu, Bolivia: Implications for magma dynamics

The Altiplano-Puna volcanic complex in the central Andes records a history of major caldera-forming eruptions over the past 10 m.y. Geophysical and geodetic data indicate that magma is still present, and perhaps mobile, in the crust. Broadband magnetotelluric data were used to generate two-dimensional and three-dimensional electrical resistivity models of the Altiplano-Puna magma body (APMB) with a focus on the zone of inflation around Volcan Uturuncu in southern Bolivia. Low electrical resistivities ( 20%. The upper surface of the APMB is shallowest beneath Uturuncu and the geometry is consistent with geodynamic models that require the upward movement of a melt layer at this location. The shallower resistivity structure is characterized by discrete electrically conductive bodies, oriented east-west near sea level (depth of 5 km), which are interpreted as a combination of partial melt and fluids.

Seismic imaging of the magmatic underpinnings beneath the Altiplano-Puna volcanic complex from the joint inversion of surface wave dispersion and receiver functions

Located in the central Andes, the Altiplano-Puna Volcanic Complex (APVC) is the location of an 11–1 Ma silicic volcanic field, one of the largest and youngest on Earth. Yet its magmatic/plutonic underpinnings have been seismically investigated in only a few widely spaced locations. Previous studies have identified an extensive (∼60,000 km2) low-velocity zone (LVZ) below the APVC referred to as the Altiplano-Puna Magma Body (APMB); however, insufficient seismic constraints have precluded uniquely measuring its thickness, and the volume of the APMB remains mostly constrained by varying estimates of plutonic to volcanic (P:V) ratios. Here we present new 3-D seismic images of the APVC crust based on a joint inversion of Rayleigh-wave dispersion from ambient seismic noise and P-wave receiver functions from broadband seismic stations recently deployed in the area. We identify a large ∼200 km diameter and ∼11 km thick LVZ that we interpret as the plutonic complex that sourced the voluminous APVC volcanics and show that its volume is much larger than previous estimates, perhaps as much as an order of magnitude larger. The large volume (∼500,000 km3) and shallow depth (4–25 km below sea level) of the LVZ centered on the observed surface uplift below the composite volcano Uturuncu provide strong evidence linking our imaged low-velocity body (APMB) with the presence of an amalgamated plutonic complex. We suggest the APMB retains a significant percentage (up to 25%) of partial melt, most likely in a melt-crystal mush state, and is related to the source of the continued ground deformation attributed to magma ascent beneath the APVC. The seismic imaging of this plutonic complex and the well-preserved and documented volcanic deposits allow us to make one of the best-constrained calculations of a plutonic to volcanic ratio. Although this calculation is still dependent on a few critical assumptions, the large volume of the newly imaged APMB requires a much larger ratio (20–35) than often cited in the literature. This large ratio has significant implications for both petrologic and tectonic models of this portion of the Andean arc.

Experimental Constraints on Dacite Pre-eruptive Magma Storage Conditions beneath Uturuncu Volcano

Cerro Uturuncu is a long-dormant, compositionally monotonous, effusive dacitic volcano in the Altiplano–Puna Volcanic Complex (APVC) of SW Bolivia. The volcano recently gained attention following the discovery of an ∼70 km diameter Interferometric Synthetic Aperture Radar (InSAR) anomaly roughly centred on its edifice. Uturuncu dacites, erupted over the past ∼1 Myr, invariably have a phase assemblage of plagioclase–orthopyroxene–biotite–ilmenite–magnetite–apatite–zircon and rhyolite glass. To better constrain storage conditions of the dacite magmas and to help understand their relationship with the observed deformation, petrological experiments were performed in cold-seal hydrothermal vessels. Volatile-saturated (PH2O = PTOTAL and PH2O + PCO2 = PTOTAL) phase equilibria experiments were run between 50 and 250 MPa and 760 and 900°C at fO2 ∼ Ni–NiO. Two synthetic starting compositions were investigated based on a typical Uturuncu dacite whole-rock and its rhyolitic groundmass glass. Pre-eruptive magma storage conditions have been estimated by comparing results from the experiments with natural phase assemblages, modes, and mineral and glass compositions. H2O-saturated experiments constrain storage pressures to 100 ± 50 MPa, equivalent to 1·9–5·7 km below surface. In the dacite, natural phase assemblages are reproduced at 870°C, 100 MPa with both orthopyroxene and biotite stabilized concurrently. Natural glass chemistries are most closely replicated at 50 MPa at 870°C, reflecting the role of decompression crystallization prior to eruption. In H2O-saturated rhyolite experiments the natural phase assemblage is most closely replicated at 870°C, 50 MPa. Isothermal, mixed volatile dacite experiments at 870°C further constrain storage pressures to 110 ± 10 MPa. Assuming that there has been no dramatic change in the eruptive behaviour of Uturuncu in the last 270 kyr, pre-eruptive storage of dacite magmas at ∼100 MPa precludes their role in producing the large diameter deformation anomaly. If deformation is a result of magmatic intrusion, then intrusion of less evolved magmas into deeper, mid-crustal storage regions is a more probable explanation. Intrusion within the Altiplano–Puna Magma Body (APMB), the extent of which is roughly coincident with the APVC, is most likely. It is proposed that dacite magmas form from andesitic parents, via fractionation and/or assimilation, within the APMB. Dacites then rise buoyantly to shallow storage levels where they stall and crystallize prior to eruption. Microlites form during subsequent ascent from the storage region to the surface.

Neogene to Quaternary ash deposits in the Coastal Cordillera in northern Chile: Distal ashes from supereruptions in the Central Andes

Silicic volcanic ash deposits investigated at 14 localities between 22° and 25°S in the Chilean Coastal Cordillera are found to be the distal ash fall from supereruptions in the Central Andean cordillera several hundreds of kilometers to the east. Depositional textures, modal composition and granulometry of the ashes and tuffs (the latter lithified by halite and gypsum under ultra-arid conditions) allow for a distinction between primary fallout/aeolian deposits (mean 4–5 Φ, sorting 1.5–2 Φ) and secondary deposits that formed by down wash from hill slopes during local rain fall. Primary volcanic components comprise two types of glass shards (with small stretched vesicles and coarse-walled with rounded to elliptic vesicles), and biotite.Previously published studies on ash deposits in the north Chilean Coastal Cordillera reported 14 40Ar/39Ar and K/Ar ages on biotite or sanidine ranging between 6.66±0.13 and 0.6±0.4Ma. In this project, three 40Ar/39Ar ages on biotite have been determined for samples from the Cuenca del Tiburón, the northern margin of Salar de Navidad and from the Quebrada de la Chimba (3.9±0.1Ma, 4.1±0.1Ma, 6.0±0.1Ma, respectively). The range of the 17 ages coincides with the Late Miocene to Quaternary ages of the major ignimbrite-forming eruptions of the high Andes to the east such as the Altiplano Puna Volcanic Complex (APVC).Electron microprobe data of glass and biotite of the Coastal Cordillera ashes have been compared with data from major ignimbrites of the APVC, of other major Central Andean volcanic fields, and of marine ashes (ODP Leg 201). Additional new biotite microprobe data from three APVC ignimbrites (Pastos Grandes, Pujsa and Guacha) have been included in the present study. Biotite composition of the investigated Coastal Cordillera ashes is similar to those of ignimbrites from the APVC. In particular, based in Fe, Mg, Mn and Ti, distal equivalents of the 3.96±0.08Ma Atana and/or 4.09±0.02Ma Puripicar and of the 5.6±0.2Ma Pujsa and/or the 5.56±0.01Ma Guacha eruptions can be identified. In addition, based only on age relations, distal ash units of the Pastos Grandes, Tatio and Purico eruptions may be present in the Coastal Cordillera. Composition of glass is comparable to APVC ignimbrite matrix glass and to marine glass, however, significant alkali depletion and SiO2 enrichment is attributed to in situ alteration.The identification of these ashes demonstrates for the first time that the supereruptions in the southern Central Andes gave rise to voluminous ash clouds, most likely co-ignimbrite. The present outcrops represent ash dispersed by easterly winds, consistent with atmospheric models that show favorable westward-directed winds existing in the upper troposphere/stratosphere during the southern summer in the southern Central Andes. This requires that current volume estimates for the major eruptions to be considered minima with a significant augmentation likely.

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.

40Ar/39Ar chronostratigraphy of Altiplano-Puna volcanic complex ignimbrites reveals the development of a major magmatic province

The Lipez region of southwest Bolivia is the locus of a major Neogene ignimbrite flare-up, and yet it is the least studied portion of the Altiplano-Puna volcanic complex of the Central Andes. Recent mapping and laser-fusion 40 Ar/ 39 Ar dating of sanidine and biotite from 56 locations, coupled with paleomagnetic data, refine the timing and volumes of ignimbrite emplacement in Bolivia and northern Chile to reveal that monotonous intermediate volcanism was prodigious and episodic throughout the complex. The new results unravel the eruptive history of the Pastos Grandes and Guacha calderas, two large multicyclic caldera complexes located in Bolivia. These two calderas, together with the Vilama and La Pacana caldera complexes and smaller ignimbrite shields, were the dominant sources of the ignimbrite-producing eruptions during the ∼10 m.y. history of the Altiplano-Puna volcanic complex. The oldest ignimbrites erupted between 11 and 10 Ma represent relatively small volumes (approximately hundreds of km 3 ) of magma from sources distributed throughout the volcanic complex. The first major pulse was manifest at 8.41 Ma and 8.33 Ma as the Vilama and Sifon ignimbrites, respectively. During pulse 1, at least 2400 km 3 of dacitic magma was erupted over 0.08 m.y. Pulse 2 involved near-coincident eruptions from three of the major calderas resulting in the 5.60 Ma Pujsa, 5.65 Ma Guacha, and 5.45 Ma Chuhuilla ignimbrites, for a total minimum volume of 3000 km 3 of magma. Pulse 3, the largest, produced at least 3100 km 3 of magma during a 0.1 m.y. period centered at 4 Ma, with the eruption of the 4.09 Ma Puripicar, 4.00 Ma Chaxas, and 3.96 Ma Atana ignimbrites. This third pulse was followed by two more volcanic explosivity index (VEI) 8 eruptions, producing the 3.49 Ma Tara (800 km 3 DRE) and 2.89 Ma Pastos Grandes (1500 km 3 DRE) ignimbrites. In addition to these large caldera-related eruptions, new age determinations refine the timing of two little-known ignimbrite shields, the 5.23 Ma Alota and 1.98 Ma Laguna Colorada centers. Moreover, 40 Ar/ 39 Ar age determinations of 13 ignimbrites from northern Chile previously dated by the K-Ar method improve the overall temporal resolution of Altiplano-Puna volcanic complex development. Together with the updated volume estimates, the new age determinations demonstrate a distinct onset of Altiplano-Puna volcanic complex ignimbrite volcanism with modest output rates, an episodic middle phase with the highest eruption rates, followed by a decline in volcanic output. The cyclic nature of individual caldera complexes and the spatiotemporal pattern of the volcanic field as a whole are consistent with both incremental construction of plutons as well as a composite Cordilleran batholith.

Enigmatic clastogenic rhyolitic volcanism: The Corral de Coquena spatter ring, North Chile

We report on the unusual occurrence of the products of lava fountaining in a Pliocene calc-alkaline rhyolitic monogenetic center from northern Chile. Corral de Coquena is a discontinuous ring of lava located in the moat of La Pacana caldera (23°27' S, 67°23.5' W), part of the Altiplano-Puna Volcanic Complex of the Central Andes. The volcanic structure is composed of a maar-like crater, with an associated pyroclastic (possibly phreatomagmatic) unit, that is overlain by rhyolitic glassy lava ramparts, in which evidence of spatter, agglutinate and clastogenic material is found. Typical explanations for the unusual textures in a rhyolitic lava, such as peralkaline composition, high volatile content, or superheated magma are untenable in this case. We propose that the most likely explanation for this extreme style of rhyolitic volcanism is a combination of moderately high eruption rate and efficient degassing prior to eruption. In the light of reports of several other bodies of fountain-fed silicic magma from the UK, US, and Japan, we propose that Corral de Coquena is a rhyolitic spatter ring superimposed upon a maar-like crater. We further propose that pyroclastic fountaining should be considered an end-member of the spectrum of eruptive styles of calc-alkaline silicic magmas, and that Corral de Coquena is a rare example, preserved because of the hyper-arid climate in the Altiplano-Puna Volcanic Complex.

Episodic construction of batholiths: Insights from the spatiotemporal development of an ignimbrite flare-up

Using the canon that large silicic volcanic fields are the surface manifestation of batholith formation at depth as a departure, we examine what the spatiotemporal pattern of the 10–1 Ma ignimbrite flare-up of the Altiplano–Puna Volcanic Complex of the Central Volcanic Zone of the Andes reveals about the formation of its upper-crustal plutonic underpinnings. The volume–time relations of major ignimbrite eruptions reveal an episodic pattern of eruptions at rates much higher than those typical of continental arcs. At least 10,000 km 3 of dacitic magma were erupted in four pulses of eruptions at ∼ 10, 8, 6, and 4 Ma that show increasing intensity and definition with time and then catastrophic diminution after 4 Ma. Calculated rates of extrusion during the pulses (0.004 to 0.012 km 3 yr − 1 ) are up to an order of magnitude higher than the average rate for the flare-up and continental silicic volcanic provinces in general (0.001 km 3 yr − 1 ). Peak intrusion rates of 0.012 to 0.06 km 3 yr − 1 ) calculated assuming a range of 3:1 to 5:1 plutonic:volcanic ratio are consistent with calculated rates of granite pluton formation. These spatiotemporal patterns suggest that a composite upper-crustal granodiorite batholith was built episodically in pulses characterized by episodic rates of intrusion that became more intense and defined in time. This pattern can be understood in the framework of a model where the mechanical strength of the crust evolves in response to magmatism through time. We propose that the APVC flare-up records a positive feedback between the mantle input, magma production, and upward migration of the magmatic system that results in a thermomechanical evolution that promotes the accumulation of larger bodies of dacite magma in the upper crust with time. The record from long-lived nested calderas suggests that locally the sub-volcanic batholith may be constructed of plutons that themselves are composite bodies built over several million years. We find support for our ideas in the Mesozoic Sierra Nevada Batholith of California.

Igniting flare-up events in Cordilleran arcs

High-fl ux pulses of magmatism that make up most of the exposed North American Cordilleran arcs are derived primarily from upper plate lithospheric source materials, and not the mantle wedge as most models would predict, based on a compilation of thousands of previously published Sr, Nd, and O isotopic data. Mass balance calculations show that no more than 50% of that mass can be mantle-derived. Flare-ups must have fundamentally developed simultaneously with crustal/lithospheric thickening, thus implying a connection. Subduction erosion from the trench side, and retroarc shortening from the foreland side are the main tectonic shortening processes that operate in conjunction with high fl ux magmatism during subduction, and therefore are likely triggers for fl are-up events in arc. These arcs represent the sites of crustal differentiation, and thus contribute to net continental growth, only if dense residual lower crust was returned to the convective mantle. Isotopic data shown here suggest that if convective removal of batholithic roots takes place, it must be a consequence and not a cause of episodic fl are-ups. The Altiplano-Puna Volcanic Complex in South America may be the most recent continental arc segment in flmode.

Rapid ascent and contamination of magmas through the thick crust of the CVZ (Andes, Ollagüe region): Evidence from a nearly aphyric high-K andesite with skeletal olivines

Described in this study are the mineralogy and petrology of a nearly aphyric high-K andesite (herein referred to as the SC2 sample) located in the Ollagüe volcano region, characterized by microcrystalline to glassy groundmass and very few olivine and pyroxene microphenocrysts (≪ 3 vol.%). Olivine (Fo 72–82) exhibits a wide range of skeletal crystal morphologies due to the high cooling rate and geometrically-different entrapped inclusions of glass, ranging in composition from intermediate to acid (SiO 2 53–72 wt.%). Plagioclase (An 56–64), meanwhile, often shows a quench texture with holed and swallow-tailed forms. Ferroan enstatite locally occurs as subhedral microphenocrysts whereas ferroan diopsides to magnesium-rich augites are common in the groundmass. Opaque minerals (Ti–Al-magnetites and Cr-spinels) are rare and generally found as tiny euhedral inclusions in the olivines. Although SC2 magma should have contained a relatively high amount of H 2O (ca. 6 wt.%), hydrous minerals such as amphibole were never on the liquidus because a high temperature (≥ 1000 °C) was maintained by the rising magma up to the shallow crustal levels. The SC2 incompatible trace element pattern has a typical arc-like signature with Sr–Nd isotopic ratios falling within the range of the Quaternary volcanic rocks of the CVZ, straddling between the less contaminated basaltic andesites and the more contaminated intermediate to acid extrusives of the Ollagüe volcano. Pb isotopic values of the studied lava also fit very well with the overall volcanic rocks of the Southern CVZ and suggest that the SC2 magma underwent a contamination from the Paleozoic basement inferred to characterize this region of the Andean Cordillera. During decompression, the H 2O-rich mantle-derived magma reached a superheated status (as the aphyric character of the SC2 sample demand) and was contaminated to some extent by high-T, “fertile” crustal wall-rocks. The contamination process of the superheated magma probably occurred at the meso-microscale as a complex function of diffusion rates of components in the magma and gave rise to a wide compositional range of liquids, frozen within the skeletal olivines before complete homogenization. However, this does not necessarily imply the existence of geologically significant batches of magmas with compositions similar to those of the glass inclusions. Neutral to extensional stresses during a Quaternary tectonic phase of crustal relaxation of the Altiplano–Puna Volcanic Complex should have permitted the fast uprising of the SC2 magma through the thick CVZ crust without any storage and differentiation in magma chambers. The observed data indicate that in the Andean CVZ, some mantle-derived magmas, although modified by crustal contamination “en route” to the surface, were able to rise with negligible fractional crystallization, by-passing the flanks of Quaternary stratovolcanoes.

Large ignimbrite eruptions and volcano-tectonic depressions in the Central Andes: a thermomechanical perspective

AbstractThe Neogene ignimbrite flare-up of the Altiplano Puna Volcanic Complex (APVC) of the Central Andes produced one of the best-preserved large silicic volcanic fields on Earth. At least 15 000 km3 of magma erupted as regional-scale ignimbrites between 10 and 1 Ma, from large complex calderas that are typical volcano-tectonic depressions (VTD). Simple Valles-type calderas are absent. Integration of field, geochronological, petrological, geochemical and geophysical data from the APVC within the geodynamic context of the Central Andes suggests a scenario where elevated mantle power input, subsequent crustal melting and assimilation, and development of a crustal-scale intrusive complex lead to the development of APVC. These processes lead to thermal softening of the sub-APVC crust and eventual mechanical failure of the roofs above batholith-scale magma chambers to trigger the massive eruptions. The APVC ignimbrite flare-up and the resulting VTDs are thus the result of the time-integrated impact of intrusion on the mechanical strength of the crust, and should be considered tectonomagmatic phenomena, rather than purely volcanic features. This model requires a change in paradigm about how the largest explosive eruptions may operate.

Seismic Detection and Characterization of the Altiplano-Puna Magma Body, Central Andes

— The Altiplano-Puna Volcanic Complex (APVC) in the central Andes is the product of an ignimbrite “flare-up” of world class proportions (de Silva, 1989). The region has been the site of large-scale silicic magmatism since 10 Ma, producing 10 major eruptive calderas and edifices, some of which are multiple-eruption resurgent complexes as large as the Yellowstone or Long Valley caldera. Seven PASSCAL broadband seismic stations were operated in the Bolivian portion of the APVC from October 1996 to September 1997 and recorded teleseismic earthquakes and local intermediate-depth events in the subducting Nazca plate. Both teleseismic and local receiver functions were used to delineate the lateral extent of a regionally pervasive ∼20-km-deep, very low-velocity layer (VLVL) associated with the APVC. Data from several stations that sample different parts of the northern APVC show large amplitude Ps phases from a low-velocity layer with Vs ≤ 1.0 km/s and a thickness of ∼1 km. We believe the crustal VLVL is a regional sill-like magma body, named the Altiplano–Puna magma body (APMB), and is associated with the source region of the Altiplano–Puna Volcanic Complex ignimbrites (Chmielowski et al., 1999).¶Large-amplitude P–SH conversions in both the teleseismic and local data appear to originate from the top of the APMB. Using the programs of Levin and Park (1998), we computed synthetic receiver functions for several models of simple layered anisotropic media. Upper-crustal, tilted-axis anisotropy involving both Vp and Vs can generate a “split Ps” phase that, in addition to the Ps phase from the bottom of a thin isotropic VLVL, produces an interference waveform that varies with backazimuth. We have forward modeled such an interference pattern at one station with an anisotropy of 15%–20% that dips 45° within a 20-km-thick upper crust. We develop a hypothesis that the crust above the “magma body” is characterized by a strong, tilted-axis, hexagonally symmetric anisotropy. We speculate that the anisotropy is due to aligned, fluid-filled cracks induced by a “normal-faulting” extensional strain field associated with the high elevations of the Andean Puna.

Modeling of highly anisotropic crust and application to the Altiplano‐Puna volcanic complex of the central Andes

The Altiplano‐Puna volcanic complex (APVC) is located in the central Andes and covers an area over 50,000 km2. The style of volcanism is predominately caldera‐forming ignimbrite eruptions active between 10 and 1 Ma. Chmielowski et al. [1999] used teleseismic events, recorded from seven portable broadband seismic stations deployed in the APVC, to identify a 1‐km‐thick very low‐velocity zone at a depth of 19 km. Based on the correlation with surface volcanics, the extremely low shear velocities of 1 km/s or less, and the depth of the layer, the low‐velocity zone was interpreted to be a “magma body.” Using local events recorded during the same seismic deployment, we have improved the crustal model for the APVC and detected seismic anisotropy in the crust above the low‐velocity zone. Using a hexagonal symmetry anisotropy code, we computed synthetic receiver functions for many models of layered anisotropic media and found two different models consistent with the data. In both models, approximately 20–30% anisotropy is present in a 3‐km‐thick surface low‐velocity layer, and 15–20% anisotropy is present in the remaining crust above the buried low‐velocity zone. The first model has the 1‐km‐thick low‐velocity zone at 17 km, while the alternate model has the low‐velocity zone at 14‐km depth overlain by a 4‐km‐thick transition zone of intermediate velocities. This anisotropy may be due to the fracture system by which magma migrates to the surface from the midcrustal sill‐like magma body.

Boron in central Andean ignimbrites: implications for crustal boron cycles in an active continental margin

Calc-alkaline pumices and lavas from the Neogene–Pleistocene central Andean Altiplano–Puna Volcanic Complex (APVC) contain quartz phenocrysts with abundant primary melt inclusions. Melt inclusions represent the melt composition of dacitic to rhyodacitic magmas prior to eruption and they are widely protected from eruptive degassing, low-temperature vapor-phase alteration and post-depositional contamination with boron-rich dusts and aerosols. The average boron isotopic compositions of melt inclusion and matrix glasses are uniform in the units studied (overall average δ 11B=−3.8±2.8‰, 1 SD) and overlap with the range of local basement rocks ( δ 11B=−11‰ to −5‰). B is enriched in glasses (55±14 ppm, 1 SD), whereas whole-rock contents range between 10 and 40 ppm. At similar B contents, incompatible trace element ratios like B/Nb are generally higher in the ignimbrites and lavas (B/Nb=2.4 and 4.0) than in crustally derived Paleozoic to Cretaceous granitoids from NW-Argentina (B/Nb=1.0–2.7). This suggests a minor contribution of high-B/Nb arc andesites and is consistent with petrogenetic models in which APVC dacites originated from hybridization of dominantly crustally derived melts with andesitic magmas. Boron-rich hydrothermal brines and salt-lake water from within the APVC have δ 11B=−4.1 to +5.7‰. These values are close to the APVC ignimbrite values and contrast with hydrothermal brines from the eastern Puna and eastern Cordillera, which have lower δ 11B=−10‰ to −17‰ and occur in an area of little volcanic cover. This regional pattern suggests that borate enrichments resulted from the leaching of local country rocks: ignimbrites in the W Altiplano–Puna and Paleozoic basement in the eastern Puna and eastern Cordillera.

The Central Andean Altiplano‐Puna magma body

Receiver function analysis of 14 teleseismic events recorded by 6 temporary PASSCAL broadband stations within the Altiplano‐Puna volcanic complex (APVC) shows a consistent ∼2 s negative‐polarity P‐to‐S conversion for all stations for all available azimuths. Forward modeling of the largest amplitudes suggests that this conversion is produced by the top of a very low velocity zone at a depth of ∼19 km, with a Vs &lt;0.5 km/s and a thickness of 750–810 m. We interpret the characteristics of the low‐velocity zone (low Vs, areal extent, and flatness) to be consistent with a sill‐like magma body. On the basis of additional data from the German ANCORP experiment, the Altiplano‐Puna magma body appears to underlie much of the APVC, and it may therefore be the largest known active continental crustal magma body.

Crustal melting by ponding of mafic magmas: A numerical model

We examine the possibility that an abating period of mantle-related volcanic activity at continental margins triggers the production of crustal-derived melts. A vertical zone, permeated by rising mafic melts, is established below the volcanic arc if long-lasting mantle-derived magmatism occurs. This situation can be assumed to exist at continental arcs. We propose that mafic magmas release heat and water into the crustal environment, if subsequent magma ascent is stopped due to establishment of a density barrier. This process enhances intracrustal partial melting. Using a finite-element program, we simulate in a 2D model area the time-dependent change of the crustal thermal structure. The spatial distribution of potential melting zones and their degree of partial melting within the crust is computed. The amount of generated melt able to segregate is calculated and compared with volume estimates given by field investigation and geochemical constraints in a natural test case. We chose the Altiplano–Puna volcanic complex (APVC) which is part of the central Andes, as an example where the process may have happened and show, from simple approximations, that the model can explain the volumes of crustal melts represented by the felsic volcanics in this region. Although the model parameters and assumptions used are specific to this geologic context, the proposed model is generally applicable to any arc setting.

Altiplano-Puna volcanic complex of the central Andes

A model is presented accounting for many features of the Altiplano-Puna volcanic complex situated in the Central Volcanic Zone of the Andes which contains 50 recently active volcanoes. The dominant elements of the complex are several large nested caldera complexes which are the source structures for the major regionally distributed ignimbrite sheets that characterize the complex. The study of the complex reveals the importance of the intersection of subsidiary axis-oblique tectonic trends related to regional stress fields peculiar to individual oceanic ridge sections with the axis-parallel trends predominant at all spreading centers in localizing hydrothermal discharge zones.