Andean Central Volcanic Zone Research Articles

The Incapillo Caldera and Dome Complex (∼ 28° S, Central Andes): A stranded magma chamber over a dying arc

The Plio-Pleistocene Incapillo Caldera and Dome Complex (5570 m) is situated at the southernmost termination of the Andean Central Volcanic Zone (∼ 28° S), where the more steeply dipping segment of the subducting Nazca plate transitions into the Chilean–Pampean “flat-slab” to the south. The ∼ 250 m high walls of the elliptical caldera (6 × 5 km) are composed of a homogeneous pumice-rich rhyodacitic ignimbrite that radiates ∼ 15 km from the caldera rim. New biotite 40Ar/ 39Ar ages along with previous K–Ar ages reveal that the final eruption of the Incapillo Caldera occurred at ∼ 0.51 ± 0.04 Ma. Flanking the caldera are ∼ 40 steep-sided rhyodacitic domes that predate the main “caldera-forming” ignimbrite. The morphology of these domes reflects the non-explosive effusive eruption of a crystal-rich gas-poor rhyodacitic magma. A volumetrically-minor debris flow composed of abundant pumice, dacites, and lithic clasts is topographically confined to a glacial valley south of the caldera and unconformably overlies the ignimbrite. Mixed pumices at the distal end of the ignimbrite reflect the convective physical condition within the magma chamber immediately prior to the pyroclastic eruption. Including the domes and ignimbrite, the total estimated volume of erupted Incapillo magma is ∼ 37 km 3. Based on its age, small size, and effusive eruptive style, 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.

Geochemistry, chronology and stratigraphy of Neogene tuffs of the Central Andean region

Tuff layers are vital stratigraphic tools that allow correlations to be made between widely dispersed exposures. Despite their widespread occurrence in the central Andes, tuffs from both natural exposures and sedimentary cores extracted from the region's extensive salars (salt pans) are relatively unstudied. Here we lay the foundation for a tephrostratigraphic framework in the central Andes (14–28°S) by chemically and morphologically characterizing ash shards, and in some cases dating 36 Neogene distal tuffs. These tuffs occur in lacustrine and alluvial deposits from the southern Bolivian Altiplano and adjacent Atacama Desert. All tuffs are calc-akaline rhyolites, consistent with their setting in the Central Andean Volcanic Zone. Five of the older tuffs were 40Ar/ 39Ar dated and yield an age range of 6.63–0.75 Ma. Organic material associated with tuffs deposited into paleolake sediments, paleowetland deposits, or urine-encrusted rodent middens provide constraints on the age of several Late Pleistocene and Holocene tuffs. These tuffs provide key stratigraphic markers and ages for lake cycles and archeological sites on the Bolivian Altiplano and for assessing rates of surficial processes and archeology in both the Atacama and Altiplano. While modern climate, and consequently questions about geomorphic processes and climate change, differs in the hyperarid Atacama and the semi-arid Altiplano, the most extensive air-fall tuffs covered both regions, placing the Atacama and the Bolivian Altiplano in the same tephrostratigraphic province. For example, the Escara B tuff (∼1.85 Ma), can be securely identified in both the Altiplano and Atacama. On the Altiplano, dates from the Escara B and E tuffs securely establish the age of the Escara Formation—representing the oldest expansive lake documented on the Bolivian Altiplano. By contrast, the presence of the Escara B tuff below ∼6 m of alluvial sediment at the Blanco Encalado site in the Atacama desert yields information about sedimentation rates in this hyperarid region. Indeed, most tuffs from the Atacama Desert are older than 600,000 years, even though they occur within fluvial terraces immediately adjacent to the alluvial fans that are still active. Most of these geomorphic surfaces in the Atacama also possess well-developed saline soils that, when combined with the radiometric ages of the distal tuffs, suggest slow rates of geomorphic change and exceptional landscape stability for this area during the Quaternary. In contrast, younger tuffs are more abundant in the more recent lake records of the Altiplano. The Chita tuff was deposited at ∼15,650 cal yr B.P., during the regressive phase of the region's deepest late Quaternary lake cycle—the “Tauca lake cycle”—which spanned 18.1–14.1 cal yr B.P. Two Holocene tuffs, the Sajsi tuff and the Cruzani Cocha tuff, are widespread. The Sajsi tuff was deposited just before 1700 cal yr B.P., whereas the Cruzani Cocha tuff appears to be mid-Holocene in age and shows some chemical affinities to a Holocene tuff (202B) deposited between 4420 and 5460 cal yr B.P. in a urine-encased rodent midden in the Atacama Desert.

Faulting-assisted lateral collapses and influence on shallow magma feeding system at Ollagüe volcano (Central Volcanic Zone, Chile-Bolivia Andes)

The role played by the regional tectonics and relationships between volcanic structures and their shallow magmatic feeding system are of paramount importance in understanding multiple lateral collapse events and, thus, for hazard assessment in active volcanoes. Ollagüe (21°18′S, 68°11′W, Andean Central Volcanic Zone, Chile-Bolivia border) is an active, andesite-dacite, composite volcano developed on a regional NW-striking extensional fault system that bisects the volcanic edifice. Geological and stratigraphic studies on the summit part of the cone, together with new 40Ar/ 39Ar dating and petrographic and geochemical (major-trace elements) analyses, have allowed us to delineate the volcanic history of Ollagüe and recognize multiple deformation and lateral collapse events. The evolution of Ollagüe appears to be characterized by four stages of volcano building, separated by three main events of deformation of the cone. These deformation events developed progressively by downthrow of the SW sector of the volcano (∼ 800 ka), along the NW-striking normal fault propagating from the substrate, to a final catastrophic failure (∼ 300 ka) of the SW sector and debris avalanche emplacement. Both NW-trending regional tectonics and weakness of the edifice caused by the successive deformation and collapse events are responsible for both the geometric variations of the shallow magma feeding system and the coexistence of summit and parasitic dacitic vents. Central and lateral vents shifted in a SW-trending direction, i.e. perpendicular to the tectonic trend and along the axis of the collapse movement, and developed along a NW-trending alignment, i.e. parallel to the main tectonic trend and collapse scarps.

Twenty-five million years of silicic volcanism in the southern central volcanic zone of the Andes: Geochemistry and magma genesis of ignimbrites from 25 to 27 °S, 67 to 72 °W

Silicic volcanism in the Andean Central Volcanic Zone (CVZ) produced one of the world's largest Neogene ignimbrite provinces. The largest and best-known CVZ ignimbrites are located on the Altiplano-Puna plateau north of 24 °S. Their compositions and huge erupted volumes suggest an origin by large-scale crustal melting, and present-day geophysical anomalies in this region suggest still active zones of partial melting in the middle crust. Farther south in the CVZ, the Cerro Galán complex erupted ignimbrites in the late Miocene and Pliocene that are quite similar in volume and composition to those from north of 24 °S and they have a similar origin. However, there are a great many other, smaller ignimbrites in the southern CVZ whose compositions and geodynamic significance are poorly known. These are the subject of this paper. We present a geochemical study of 28 ignimbrite units from the southern CVZ at 25 °S to 27 °S, whose ages cover the full span of arc activity in this area, from about 25 Ma to 1 Ma. The small to medium volume ignimbrites (< 10 km 3) form valley-fill or sheet-like deposits, many of which are chemically zoned. Notwithstanding individual differences, there are several common characteristic features within the group of southern CVZ ignimbrites. By far the dominant composition is metaluminous, crystal-poor rhyolite (mean values from 236 samples: SiO 2 = 73.5 wt.%, A/CNK = 1.02, K 2O/Na 2O = 1.4). Regular major and trace element differentiation trends suggest fractionation from intermediate arc magmas and this is supported by radiogenic isotopic ratios of Sr, Nd and Pb, which show complete overlap between the silicic ignimbrites and andesite–dacites from contemporary stratovolcanoes. There are no major changes in composition of the silicic ignimbrites over the 25 Ma span of activity. We attribute minor but significant differences in isotopic composition and Nb–Ta concentration according to location in the West Cordillera (Chile) or the southern Puna plateau (Argentina) to variations in local basement compositions. The major conclusion of this study is that silicic magmas of crustal origin are rare in the CVZ south of 24 °S, and the only known example, Cerro Galán, is considerably younger than the ignimbrite “flare-up” in the Altiplano-Puna plateau region. The thermal conditions for widespread crustal melting in the Altiplano-Puna north of 24 °S are related to increased mantle heat flux caused by slab steepening and/or delamination at the base of overthickened lithosphere. These conditions have not (yet) been achieved in the southern CVZ because of the southward shallowing of slab dip and younging of compressive deformation and plateau growth.

Volcano evolution and eruptive flux on the thick crust of the Andean Central Volcanic Zone: 40Ar/39Ar constraints from Volcan Parinacota, Chile

The 163 k.y. history as well as the chemi- cal and 46 km 3 volumetric evolution of Vol- can Parinacota are described in detail by new mapping, stratigraphy, and 57 40 Ar/ 39 Ar ages determined from groundmass or sani- dine crystals in basaltic andesitic to rhyolitic lavas. A more precise chronology of eruptions and associated eruptive volumes of this cen- tral Andean volcano, which was built upon 70-km-thick crust, provides a more com- plete view of how quickly volcanic edifi ces are built in this setting and how their mag- matic systems evolve during their lifetime. Development of the complex involved initial eruption of andesitic lava fl ows (163-117 ka) followed by a rhyodacite dome plateau (47- 40 ka) synchronous with the onset of the building of a stratocone (52-20 ka), which was later destroyed by a debris avalanche ~3 times larger than that at Mount St. Helens in 1980. Dome plateau emplacement occurred faster and later than has previously been published, implying a compressed duration of cone building and introducing a preced- ing 65 k.y. hiatus. Debris avalanche timing is refi ned here to be older than 10 but younger than 20 ka. Rapid postcollapse rebuilding of the volcanic edifi ce is delineated by 16 groundmass and whole-rock 40 Ar/ 39 Ar ages, which include some of the youngest lava fl ows dated by this method. Increase in cone- building rate and a continued trend toward more mafi c compositions following collapse imply an inter-relationship between the pres- ence of the edifi ce and fl ux of magma from the feeding reservoir. Cone-building rates at Parinacota are similar to those at other well- dated volcanoes on thinner crust; however, the distributed basaltic volcanism prevalent in those other arcs is virtually absent both at Parinacota and elsewhere in the Central Volcanic Zone. This suggests that while the hydrous, calc-alkaline magmas that make up the central volcanoes are not signifi cantly retarded by thick crust, primitive, dry basalts might be.

New chemical and original isotopic data on waters from El Tatio geothermal field, northern Chile

The El Tatio geothermal field is located at an height of 4200-4300 m on the Cordillera de los Andes (Altiplano). Geysers, hot pools and mudpots in the geothermal field and local meteoric waters were sampled in April 2002 and analyzed for major and trace elements, δ2H, δ18O and 3H of water, δ34S and δ18O of dissolved sulfate, δ13C of dissolved total carbonate, and 87Sr/86Sr ratio of aqueous strontium. There are two different types of thermal springs throughout the field, that are chloride-rich water and sulfate-rich water. The chemical composition of chloride springs is controlled by magma degassing and by water-rock interaction processes. Sulfate springs are fed by shallow meteoric water heated by ascending gases. In keeping with the geodynamic setting and nature of the reservoir rocks, chloride water is rich in As, B, Cs, Li; on the other hand, sulfate water is enriched only in B relative to local meteoric water. Alternatively to a merely meteoric model, chloride waters can be interpreted as admixtures of meteoric and magmatic (circa andesitic) water, which moderately exchanges oxygen isotopes with rocks at a chemical Na/K temperature of about 270°C in the main reservoir, and then undergoes loss of vapor (and eventually mixing with shallow water) and related isotopic effects during ascent to the surface. These chloride waters do not present tritium and can be classified as sub-modern (pre-1952). A chloride content of 5,400 mg/l is estimated in the main reservoir, for which δ2H and δ18O values, respectively of -78‰ and -6.9‰, are calculated applying the multistage-steam separation isotopic effects between liquid and vapor. From these data, the meteoric recharge (Cl≈0 mg/l) of the main reservoir should approach a composition of -107‰ in δ2H and -14.6‰ in δ18O, when a magmatic water of δ2H = -20‰, δ18O = +10‰ and Cl = 17,500 mg/l is assumed. The 87Sr/86Sr ratios of the hot springs are quite uniform (0.70876 to 0.70896), with values within the range observed for dacites of the Andean central volcanic zone. A water δ18O-87Sr/86 Sr model was developed for the main geothermal reservoir, by which a meteoric-magmatic composition of the fluids is not excluded. The uniform δ34S(SO42-) values of +1.4 to +2.6‰ in the chloride waters agree with a major deep-seated source for sulfur, possibly via hydrolysis in the geothermal reservoir of sulfur dioxide provided by magma degassing, followed by isotopic exchange between sulfate and sulfide in the main reservoir. This interpretation is supported by the largely negative δ34S(SO42-) value in steam-heated water sulfate (-9.8‰) and mass-balance calculation, which exclude leaching at depth of igneous iron-sulfides with δ34S near zero per mill. All the δ13C values of total carbonate in the chloride waters are negative, with variable values from -9.2 to -20.1‰, pointing to an important proportion of biogenic carbon in the fluids. The interpretation of these data is problematic, and a number of alternative explanations are reported in the text.

The Llullaillaco volcano, northwest Argentina: construction by Pleistocene volcanism and destruction by sector collapse

Llullaillaco is one of a chain of Quaternary stratovolcanoes that defines the present Andean Central Volcanic Zone (CVZ), and marks the border between Chile and Argentina/Bolivia. The current edifice is constructed from a series of thick dacitic lava flows, forming the second tallest active volcano in the world (6739 m). K–Ar and new biotite laser 40Ar/ 39Ar step-heating dates indicate that the volcano was constructed during the Pleistocene (≤1.5 Ma), with a youngest date of 0.048±0.012 Ma being recorded for a fresh dacite flow that descends the southern flank. Additional 40Ar/ 39Ar measurements for andesitic and dacitic lava flows from the surrounding volcanic terrain yield dates of between 11.94±0.13 Ma and 5.48±0.07 Ma, corresponding to an extended period of Miocene volcanism which defines much of the landscape in this region. Major- and trace-element compositions of lavas from Llullaillaco are typical of Miocene–Pleistocene volcanic rocks from the western margin of the CVZ, and are related to relatively shallow-dipping subduction of the Nazca plate beneath northern Chile and Argentina. Oversteepening of the edifice by stacking of thick, viscous, dacitic lava flows resulted in collapse of its southeastern flank to form a large volcanic debris avalanche. Biotite 40Ar/ 39Ar dating of lava blocks from the avalanche deposit indicate that collapse occurred at or after ∼0.15 Ma, and may have been triggered by extrusion of a dacitic flow similar to the one dated at 0.048±0.012 Ma. The avalanche deposits are exceptionally well preserved due to the arid climate, and prominent levées, longitudinal ridges, and megablocks up to 20-m diameter are observed. The avalanche descended ∼2.8 km vertically, and bifurcated around an older volcano, Cerro Rosado, before debouching onto the salt flats of Salina de Llullaillaco. The north and south limbs of the avalanche traveled 25 and 23 km, respectively, and together cover an area of approximately 165 km 2. Estimates of deposit volume are hampered by a lack of thickness information except at the edges, but it is likely to be between 1 and 2 km 3. Equivalent coefficients of friction of 0.11 and 0.12, and excess travel distances of 20.5 and 18.5 km, are calculated for the north and south limbs, respectively. The avalanche ascended ∼400 m where it broke against the western flank of Cerro Rosado, and a minimum flow velocity of ∼90 m s −1 can be calculated at this point; lower velocities of ∼45 m s −1 are calculated where distal toes ascend ∼200 m slopes. It is suggested that the remaining precipitous edifice has a high probability for further avalanche collapse in the event of renewed volcanism.

Stable isotopes and amphibole chemistry on hydrothermally altered granitoids in the North Chilean Precordillera: a limited role for meteoric water?

Whole rock and mineral stable isotope and microprobe analyses are presented from granitoids of the North Chilean Precordillera. The Cretaceous to Tertiary plutonic rocks contain important ore deposits and frequently display compositional and textural evidence of hydrothermal alteration even in barren rocks. Deuteric alteration includes replacement of biotite and amphibole by chlorite and epidote, sericitization and saussuritization of feldspars, and uralitization of clinopyroxene and/or amphibole. While whole rock compositions are not significantly affected, compositional variations in amphiboles suggest two types of hydrothermal alteration. Hornblende with actinolitic patches and rims and tight compositional trends from hornblende to Mg-rich actinolite indicate increasing oxygen fugacity from magmatic to hydrothermal conditions. Uralitic amphiboles exhibiting irregular Mg-Fe distribution and variable Al content are interpreted as reflecting subsolidus hydration reactions at low temperatures. The δD values of hydrous silicates vary from −63 to −105‰. Most δ18O values of whole rocks are in the range of 5.7 to 7.7‰ and are considered normal for igneous rocks in the Andes. These δ18O values also coincide well with the oxygen isotope composition of geochemically similar recent volcanics from the Central Andean Volcanic Zone (δ18O = 7.0–7.4‰). Only one sample in this study (δ18O = 3.0‰) appears to be depleted by isotope exchange with light meteoric water at high temperatures. The formation of secondary minerals in all other intrusions is mainly the product of deuteric alteration. This also holds true for the sample from El Abra, the only pluton associated with mineralization. This indicates the dominant role of a magmatic rather than a meteoric fluid in the alteration of the Cretaceous and Tertiary granitoids in northern Chile.

Effusive eruption of viscous silicic magma triggered and driven by recharge: a case study of the Cerro Chascon-Runtu Jarita Dome Complex in Southwest Bolivia

The Cerro Chascon-Runtu Jarita Complex is a group of ten Late Pleistocene (∼85 ka) lava domes located in the Andean Central Volcanic Zone of Bolivia. These domes display considerable macroscopic and microscopic evidence of magma mixing. Two groups of domes are defined chemically and geographically. A northern group, the Chascon, consists of four lava bodies of dominantly rhyodacite composition. These bodies contain 43–48% phenocrysts of plagioclase, quartz, sanidine, biotite, and amphibole in a microlite-poor, rhyolitic glass. Rare mafic enclaves and selvages are present. Mineral equilibria yield temperatures from 640 to 750 °C and log ƒO2 of –16. Geochemical data indicate that the pre-eruption magma chamber was zoned from a dominant volume of 68% to minor amounts of 76% SiO2. This zonation is best explained by fractional crystallization and some mixing between rhyodacite and more evolved compositions. The mafic enclaves represent magma that intruded but did not chemically interact much with the evolved magmas. A southern group, the Runtu Jarita, is a linear chain of six small domes (<1 km3 total volume) that probably is the surface expression of a dike. The five most northerly domes are composites of dacitic and rhyolitic compositions. The southernmost dome is dominantly rhyolite with rare mafic enclaves. The composite domes have lower flanks of porphyritic dacite with ∼35 vol.% phenocrysts of plagioclase, orthopyroxene, and hornblende in a microlite-rich, rhyodacitic glass. Sieve-textured plagioclase, mixed populations of disequilibrium plagioclase compositions, xenocrystic quartz, and sanidine with ternary composition reaction rims indicate that the dacite is a hybrid. The central cores of the composite domes are rhyolitic and contain up to 48 vol.% phenocrysts of plagioclase, quartz, sanidine, biotite, and amphibole. This is separated from the dacitic flanks by a banded zone of mingled lava. Macroscopic, microscopic, and petrologic evidence suggest scavenging of phenocrysts from the silicic lava. Mineral equilibria yield temperatures of 625–727 °C and log ƒO2 of –16 for the rhyolite and 926–1000 °C and log ƒO2 of –9.5 for the dacite. The rhyolite is zoned from 73 to 76% SiO2, and fractionation within the rhyolite composition produced this variation. Most of the 63–73% SiO2 compositional range of the lava in this group is the result of mixing between the hybrid dacite and the rhyolite. Eruption of both groups of lavas apparently was triggered by mafic recharge. A paucity of explosive activity suggests that volatile and thermal exchanges between reservoir and recharge magmas were less important than volume increase and the lubricating effects of recharge by mafic magmas. For the Runtu Jarita group, the eruption is best explained by intrusion of a dike of dacite into a chamber of crystal-rich rhyolite close to its solidus. The rhyolite was encapsulated and transported to the surface by the less-viscous dacite magma, which also acted as a lubricant. Simultaneous effusion of the lavas produced the composite domes, and their zonation reflects the subsurface zonation. The role of recharge by hotter, more fluid mafic magma appears to be critical to the eruption of some highly viscous silicic magmas.

Formula omitted] isotope geochemistry of silicic lava flows erupted from Volcán Ollagüe, Andean Central Volcanic Zone

Twenty-one new oxygen isotope analyses have been obtained from basaltic andesite to dacitic lava flows erupted at Volcán Ollagüe in the Andean Central Volcanic Zone. Variation of δ 18O values (7.1–8.1‰ relative to SMOW) for the entire lava suite is small and the data as a whole exhibit no simple correlation with any parameter of compositional evolution, except that values for basaltic andesite lavas are lower than those of andesitic and dacitic lavas. Within comagmatic suites of the andesitic and dacitic lavas, however, δ 18O values are negatively correlated with major and trace element contents and Sr-isotope ratios. Furthermore, the rate of decrease in δ 18O as a function of compositional evolution is greater for young andesitic lavas erupted from the summit relative to older flows erupted low on the flanks of the volcano. The oxygen isotope compositions of Ollagüe lavas are explained by a two-stage intracrustal contamination model. In the first stage, parental basalt and basaltic andesite magmas assimilate large amounts of high- 18 O 16 O lower continental crust. In the second stage, the andesitic and dacitic magmas melt and assimilate low- 18 O 16 O hydrothermally altered wall rocks during differentiation in shallow crustal magma chambers. Modeling of magma evolution trends for δ 18O and 87 Sr 86 Sr suggests that the upper crustal contaminant for the young andesitic flows erupted from the summit probably had δ 18O ≈ −4‰ and Sr-isotope ratios (0.707) identical to lavas erupted during the initial cone-building phase of Ollagüe. In contrast, older lavas erupted from vents low on the flanks require a crustal source with δ 18O between 2‰ and −1‰, and Sr-isotope compositions (∼ 0.711) similar to the Miocene ignimbrites upon which the volcano is constructed. The differences in the isotopic compositions of the crustal contaminants can be explained by increasing degrees of hydrothermal alteration and hybridization of the shallow crust with time or toward the center of the volcano, or both. Although the δ 18O values for the Ollagüe lavas are not low, this model for upper crustal contamination is similar to models advanced for the generation of low-δ 18O magmas at other continental volcanic centers. We explain the high δ 18O values at Ollagüe as resulting from contamination of parental basaltic andesite magmas during the earlier episode of differentiation in high-δ 18O lower crust. This process may have general application in explaining why eruption of low- δ 18O silicic magmas is a relatively rate phenomenon.

Intracrustal Derivation of Na-Rich Andesitic and Dacitic Magmas: An Example from Volcán Ollagüe, Andean Central Volcanic Zone

Andesitic and dacitic lavas erupted from Quaternary stratovolcanoes in the Central Volcanic Zone (CVZ) of the Andes have major and trace element compositions similar to proposed melts of subducted mid-ocean ridge basalt and Na-rich trondjhemites, tonalites, and granodiorites (TTG) in Early Archean high-grade gneiss terrains. Suites of lavas from individual volcanoes have average La/Yb of 25-45, Sr/Y of 30-65, Yb contents of 1.5-1.1 ppm, and $$Na_{2}O$$ contents > 3.8 wt %, with lavas erupted on the volcanic front displaying the most affinity to the Early Archean suites. Because slab-melting is not involved in the derivation of the CVZ magmas, these data indicate that rocks with Early Archean TTG compositions can also be produced through intracrustal differentiation processes. Isotopic, major element, and REE data from Volcan Ollague are consistent with a model incorporating contamination of mantle-derived basaltic magmas in garnet-bearing deep continental crust to produce basaltic andesite magma, followed...

Young mafic back arc volcanic rocks as indicators of continental lithospheric delamination beneath the Argentine Puna Plateau, central Andes

The spatial distribution of some major and trace element and isotopic characteristics of backarc Plio‐Quaternary basaltic to high‐Mg andesitic (51% to 58% SiO2) lavas in the southern Puna (24°S to 27°S) of the Central Andean Volcanic Zone (CVZ) reflect varying continental lithospheric thickness and the thermal state of the underlying mantle wedge and subducting plate. These lavas erupted from small cones and fissures associated with faults related to a change in the regional stress system in the southern Puna at ≈ 2 to 3 Ma. Three geochemical groups are recognized: (1) a relatively high volume intraplate group (high K; La/Ta ratio &lt;25) that occurs over a thin continental lithosphere above a gap in the modern seismic zone and represents the highest percentage of mantle partial melt, (2) an intermediate volume, high‐K calc‐alkaline group ( La/Ta ratio &gt;25) that occurs over intermediate thickness lithosphere on the margins of the seismic gap and behind the main CVZ and represents an intermediate percentage of mantle partial melt, and (3) a small‐volume shoshonitic group (very high K) that occurs over relatively thick continental lithosphere in the northeast Puna and Altiplano and represents a very small percentage of mantle partial melt. Mantle‐generated characteristics of these lavas are partially overprinted by mixing with melts of the overlying thickened crust as shown by the presence of quartz and feldspar xenocrysts, negative Eu anomalies (Eu/Eu &lt; 0.90; most &lt; 0.80), and radiogenic Sr (&gt; 0.7055) and Pb and nonradiogenic Nd ( εNd &lt; −0.4) isotopic ratios. Mixing calculations show that the lavas generally contain more than 20% to 25% crustal melt. The eruption of the intraplate group mafic lavas, the change in regional stress orientation, and the high elevation of the southern Puna are suggested to be the result of the late Pliocene mechanical delamination of a block (or blocks) of continental lithosphere (mantle and possibly lowermost crust). The loss of this lithosphere resulted in an influx of asthenosphere that caused heating of the subducting slab and yielded intraplate basic magmas that produced extensive melting at the base of the thickened crust. Heating of the subducting slab led to formation of the seismic gap and trenchward depletion of the slab component. Backarc calc‐alkaline group lavas erupted on the margins of this delaminated block, whereas shoshonitic group lavas erupted over a zone of relatively thick nondelaminated lithosphere to the north.

Petrology of Calc-Alkaline Lavas at Volc n Ollag e and the Origin of Compositional Diversity at Central Andean Stratovolcanoes

Volcán Ollagüe (21°17'S) is a large stratovolcano that lies slightly east of the main axis of Quaternary Volcanoes in the Andean Central Volcanic Zone (CVZ). Euptive products range from basaltic andesite to dacite and define a high-K, calc-alkaline suite. This compositional range is similar to the collective compositional range of the other stratovolcanoes in the CVZ, and it provides a record of both early and late-stage differentiation processes operating at the stratovolcanoes. The volumetrically dominant andesitic and dacitic lavas are divided into four eruptive series on the basis of vent locations and petrography. In ascending stratigraphic order they are: the Vinta Loma, Chasca Orkho, post-collapse, and La Celosa series. Whole-rock compositions of the lavas are remarkably similar regardless of eruptive series. Variations in phenocryst assemblages and magmatic fo2 however, suggest differences in subliquidus volatile contents for magma chambers developed beneath the summit of the volcano versus those developed beneath the flanks. Basaltic andesite magmas are principally preserved as quenched inclusions within the andesitic and dacitie lava flows. Large ranges in isotopic ratios over a narrow compositional range indicate that the basaltic andesites were derived by crystal fractionation coupled with large amounts of crustal assimilation. Increasing Ce/Yb ratios with decreasing Yb contents further suggest that this initial stage of differentiation occurred at deep crustal levels where garnet was stable. Additional supporting evidence for differentiation in the deep crust includes isotopic and trace element compositions that indicate assimilation by the basaltic andesite magmas of a crust different from upper-crustal rocks exposed at present in the region. Whole-rock major and trace element trends of the dacitic lavas can be simulated largely by fractional crystallization of parental andesitic magma. The fractionating assemblages for the different eruptive series are consistent with the observed modes of the parent magmas. Small increases in Sr isotope ratios with increasing Rb contents indicate that the fractionating magmas also assimilated small amounts of wall rocks similar in composition to the upper-crustal basement to the volcano. Consideration of the chemical trends, mineral compositions, and eruptive history of Ollagüe rocks permits construction of a model for the evolution of shallow crustal magma chambers beneath the stratovolcanoes in the CVZ. At a relatively mature stage, the magma chambers may be compositionally, thermally, and density stratified. Temperatures estimated from Fe-Ti oxide and pyroxene thermometry for the chambers beneath Ollagüe range from 1000 to 790°C with increasing SiO2 from 59 to 67 wt.% in the upper reaches, and from 1150 to 1020°C with increasing SiO2 from 53 to 59 wt.% in the lower reaches. The occurrence of basaltic andesite magmatic inclusions within the intermediate lavas and the repeated eruption of monotonous composition andesitic magmas indicate that the shallow chambers are periodically replenished with parental basaltic andesite magmas. Ubiquitous, reversely zoned plagioclase and pyroxene phenocrysts in the lavas at Ollagüe suggest that convective cooling of the basaltic andesite releases buoyant derivative liquid that mixes with the overlying intermediate-composition body of the chambers. Further crystallization and differentiation of the intermediate magmas may take place in solidification zones at the boundaries of the magma chambers. If so, the return of residual liquid from the crystallizing margins and mixing with the interior are highly efficient such that magma differentiation can be modeled as a simple, homogeneous, fractional crystallization process.

Effusive silicic volcanism in the Central Andes: The Chao dacite and other young lavas of the Altiplano‐Puna Volcanic Complex

The largest known Quaternary silicic lava body in the world is Cerro Chao in north Chile, a 14‐km‐long coulée with a volume of at least 26 km3. It is the largest of a group of several closely similar dacitic lavas erupted during a recent (&lt; 100,000 year old) magmatic episode in the Altiplano‐Puna Volcanic Complex (APVC; 21‐24°S) of the Central Andean Volcanic Zone. The eruption of Chao proceeded in three phases. Phase 1 was explosive and produced ∼1 km3 of coarse, nonwelded dacitic pumice deposits and later block and ash flows that form an apron in front of the main lava body. Phase 2 was dominantly effusive and erupted ∼22.5 km3 of magma in the form of a composite couleé covering ∼53 km2 with a 400‐m‐high flow front and a small cone of poorly expanded pumice around the vent. The lava is homogeneous with rare flow banding and vesicular tops and selvages. Ogives (flow ridges) reaching heights of 30 m form prominent features on its surface. Phase 3 produced a 6‐km‐long, 3‐km‐wide flow that emanated from a collapsed dome. Ogives are subdued, and the lava is glassier than that produced in previous phases. All the Chao products are crystal‐rich high‐K dacites and rhyodacites with phenocrysts of plagioclase, quartz, hornblende, biotite, sphene, rare sanidine, and oxides. Phenocryst contents reach 40–60 vol % (vesicle free) in the main phase 2 lavas but are lower in the phase 1 (20–25%) and phase 3 (∼40%) lavas. Ovoid andesitic inclusions with vesicular interiors and chilled margins up to 10 cm are found in the later stages of phase 2 and compose up to 5% of the phase 3 lava. There is little evidence for preemptive zonation of the magma body in composition, temperature (∼840°C), ƒO2 (10−11), or water content, so we propose that eruption of the Chao complex was driven by intrusion of fresh, hot andesitic magma into a crystallizing and largely homogeneous body of dacitic magma. Morphological measurments suggest that the Chao lavas had internal plastic viscosities of 1010 to 1012Pa s, apparent viscosities of 109 Pa s, surface viscosities of 1015 to 1024 Pa s, and a yield strength of 8×105 Pa. These estimates indicate that Chao would have exhibited largely similar rheological properties to other silicic lava extrusions, notwithstanding its high phenocryst content. We suggest that Chao's anomalous size is a function of both the relatively steep local slope (20° to 3°) and the available volume of magma. The eruption duration for Chao's emplacement is thought to have been about 100 to 150 years, with maximum effusion rates of about 25 m3 s−1 for short periods. Four other lavas in the vicinity with volumes of ∼5 km3 closely resemble Chao and are probably comagmatic. The suite as a whole shares a petrologic and chemical similarity with the voluminous regional Tertiary to Pleistocene ignimbrites of the APVC and may be derived from a zone of silicic magmatism that is thought to have been active since the late Tertiary. Chao and the other young lavas may represent either the waning of this system or a new episode fueled by intrusions of mafic magma.

Volcanological and petrological evolution of Volcan Tata Sabaya, SW Bolivia

Tata Sabaya (19°08'S 68° 32'W; 5430 m a.s.l) is located on the northern shores of Salar de Coipasa on the Altiplano of Bolivia. It is a 1700-m-high symmetrical composite cone; one of two Holocene centers in a chain of edifices trending E-W from the main N-S-trending volcanic arc of the Andean Central Volcanic Zone (CVZ). Four main stages are envisaged for the evolution of the edifice. Stage 1: formation of a pyroclastic “shield” possibly around a central dome complex. Stage 2: formation of an andesitic composite cone on top of the early pyroclastic sequence. Stage 3: collapse of a 120° sector of the southern flank of the volcano which produced a prominent debris avalanche deposit covering ∼300 km 2 of the Salar de Coipasa to the south. Collapse was induced by inflation of the edifice by an input of fresh magma into the sub-volcanic magma chamber. Stage 4: flows and domes which have since healed the edifice. Tata Sabaya shows a restricted range of whole-rock compositions, most units fall in the range 60–62.5% SiO 2 with a general evolution to the more silicic stage 4 lavas through time. The typical phenocryst assemblage is plagioclase > hornblende > hypersthene > augite > oxide. Biotite is found in the stage 4 lavas and is dominant over pyroxene. Hornblende and biotite are thought to be a late-stage sub-liquidus products. By far the most mafic compositions at Tata Sabaya are the hornblende and plagioclase inclusions in two of the stage 4 domes (∼ 52% SiO 2). Macroscopic, microscopic, and chemical evidence for magma mixing (mixed phenocryst and matrix populations, resorption features, mixed glass compositions, contraindicative temperature indicators etc.,) are found throughout Tata Sabaya. The whole-rock chemistry bears out that magma mixing was a dominant process and that it was responsible for buffering the composition of the magma in an open magmatic system. Despite being located 35km to the east of the main arc front the rocks of Tata Sabaya are compositionally similar to those from other centers in the northern (17–19°S) segment of the CVZ. The high K 2O, calc-alkaline suite is characterised by LREE and extreme LILE enrichments (up to 1800 ppm Ba and Sr). 87Sr/ 86Sr (0.7056–0.7068) and 143Nd/ 144Nd (0.51217–0.51227) are similar to other CVZ lavas; this is regarded as a baseline signature derived during deep crustal contamination. This is corroborated by Pb-isotope data which indicates influence of the low 206Pb/ 204Pb Precambrian basement that is thought to underlie the northern portion of the CVZ. Modification of the isotope ratios during open system processes at shallow level is also indicated. The most extreme incompatible element enrichments are found in the quenched mafic inclusions. These characteristics have been reported from other Holocene mafic lavas elsewhere in the northern CVZ, and may represent a distinct recent, magmatic episode.

The volcanic and magmatic evolution of Volcán Ollagüe, a high-K, late quaternary stratovolcano in the Andean Central Volcanic Zone

Volcán Ollagüe is a high-K, calc-alkaline composite volcano constructed upon extremely thick crust in the Andean Central Volcanic Zone. Volcanic activity commenced with the construction of an andesitic to dacitic composite cone composed of numerous lava flows and pyroclastic deposits of the Vinta Loma series and an overlying coalescing dome and coulée sequence of the Chasca Orkho series. Following cone construction, the upper western flank of Ollagüe collapsed toward the west leaving a collapse-amphitheater about 3.5 km in diameter and a debris avalanche deposit on the lower western flank of the volcano. The deposit is similar to the debris avalanche deposit produced during the May 18, 1980 eruption of Mount St. Helens, U.S.A., and was probably formed in a similar manner. It presently covers an area of 100 km 2 and extends 16 km from the summit. Subsequent to the collapse event, the upper western flank was reformed via eruption of several small andesitic lava flows from vents located near the western summit and growth of an andesitic dome within the collapse-amphitheater. Additional post-collapse activity included construction of a dacitic dome and coulée of the La Celosa series on the northwest flank. Field relations indicate that vents for the Vinta Loma and post-collapse series were located at or near the summit of the cone. The Vinta Loma series is characterized by an anhydrous, two-pyroxene assemblage. Vents for the La Celosa and Chasca Orkho series are located on the flanks and strike N55 W, radial to the volcano. The pattern of flank eruptions coincides with the distribution in the abundance of amphibole and biotite as the main mafic phenocryst phases in the rocks. A possible explanation for this coincidence is that an unexposed fracture or fault beneath the volcano served as a conduit for both magma ascent and groundwater circulation. In addition to the lava flows at Ollagüe, magmas are also present as blobs of vesiculated basaltic andesite and mafic andesite that occur as inclusions in nearly all of the lavas. All eruptive activity at Ollagüe predates the last glacial episode (∼ 11.000 a B.P.), because post-collapse lava flows are overlain by moraine and are incised by glacial valleys. Present activity is restricted to emission of a persistent, 100-m-high fumarolic steam plume from a vent located within the summit andesite dome. Sr and Nd isotope ratios for the basaltic andesite and mafic andesite inclusions and lavas suggest that they have assimilated large amounts of crust during crystal fractionation. In contrast, narrow ranges in 143Nd/ 144Nd and 87Sr/ 86Sr in the andesitic and dacitic lavas are enigmatic with respect to crustal contamination.

Implications of Quaternary volcanism at Cerro Tuzgle for crustal and mantle evolution of the Puna Plateau, Central Andes, Argentina

The high-K Tuzgle volcanic center, (24° S, 66.5° W) along with several small shoshonitic centers, developed along extensional Quaternary faults of the El Toro lineament on the east-central Puna plateau, ≈275 km east of the main front of the Andean Central Volcanic Zone (CVZ). These magmas formed by complex mixing processes in the mantle and thickened crust (>50 km) above a ∼200 km deep scismic zone. Tuzgle magmas are differentiated from shoshonitic series magmas by their more intraplate-like Ti group element characteristics, lower incompatible element concentrations, and lower 87Sr/86Sr ratios at a given eNd. Underlying Mio-Pliocene volcanic rocks erupted in a compressional stress regime and have back-arc like calc-alkaline chemical characteristics. The Tuzgle rocks can be divided into two sequences with different mantle precursors: a) an older, more voluminous rhyodacitic (ignimbrite) to mafic andestitic (56% to 71% SiO2) sequence with La/Yb ratios 35. La/Yb ratios are controlled by the mafic components: low ratios result from larger mantle melt percentages than high ratios. Shoshonitic series lavas (52% to 62% SiO2) contain small percentage melts of more isotopically “enriched” arc-like mantle sources. Some young Tuzgle lavas have a shoshonitic-like component. Variable thermal conditions and complex stress system are required to produce the Tuzgle and shoshonitic series magmas in the same vicinity. These conditions are consistent with the underlying mantle being in transition from the thick mantle lithosphere which produced rare shoshonitic flows in the Altiplano to the thinner mantle lithosphere that produced back-are calc-alkaline and intraplate-type flows in the southern Puna. Substantial upper crustal type contamination in Tuzgle lavas is indicated by decreasing eNd (-2.5 to-6.7) with increasing 87Sr/86Sr (0.7063 to 0.7099) ratios and SiO2 concentrations, and by negative Eu anomalies (Eu/Eu* <0.78) in lavas that lack plagioclase phenocrysts. Trace element arguments indicate that the bulk contaminant was more silicic than the Tuzgle ignimbrite and left a residue with a high pressure mineralogy. Crustal shortening processes transported upper crustal contaminants to depths where melting occurred. These contaminants mixed with mafic magmas that were fractionating mafic phases at high pressure. Silicic melts formed at depth by these processes accumulated at a mid to upper crustal discontinuity (decollement). The Tuzgle ignimbrite erupted from this level when melting rates were highest. Subsequent lavas are mixtures of contaminated mafic magmas and ponded silicic melts. Feldspar and quartz phenocrysts in the lavas are phenocrysts from the ponded silicic magmas.