Altiplano–Puna volcanic complex

The Andes mountain chain originated from the subduction of the Nazca Plate below the South American Plate and was accompanied by extensive volcanism. Between 14° S and 28° S lies one volcanic area with over fifty recently active systems, the Central Volcanic Zone (CVZ). Since the late Miocene between 21° S and 24° S a major ignimbrite province formed over thick crust, the Altiplano–Puna volcanic complex, between the Atacama and the Altiplano. The Toba volcanic system in Indonesia and Taupō in New Zealand are analogous to the province. The APVC is located in the southern Altiplano-Puna plateau, a surface plateau wide and long at an altitude of , and lies east of the volcanic front of the Andes. Deformational belts limit it in the east. and some individual volcanic centres reach altitudes of more than . The Pastos Grandes-Lipez-Coranzuli lineament forms the northern boundary of the Altiplano-Puna volcanic complex. ==Geology==

lava dome The APVC is generated by the subduction of the Nazca Plate beneath the South American Plate at an angle of nearly 30°. Delamination of the crust has occurred beneath the northern Puna and southern Altiplano. Below depth, seismic data indicate the presence of melts in a layer called the Altiplano–Puna low velocity zone or Altiplano Puna magma body. Regional variations of activity north and south of 24°S have been attributed to the southwards moving subduction of the Juan Fernández Ridge. This southwards migration results in a steepening of the subducting plate behind the ridge, causing decompression melting Ignimbrites deposited during eruptions of APVC volcanoes are formed by "boiling over" eruptions, where magma chambers containing viscous crystal-rich volatile-poor magmas partially empty in tranquil, non-explosive fashion. As a result, the deposits are massive and homogeneous and show few size segregation or fluidization features. Such eruptions have been argued to require external triggers to occur. Petrologically, ignimbrites are derived from dacitic–rhyodacitic magmas. Phenocrysts include biotite, Fe–Ti-oxides, plagioclase and quartz with minor apatite and titanite. Northern Puna ignimbrites also contain amphibole, and clinopyroxene and orthopyroxene occur in low-Si magmas, while higher Si magmas also contain sanidine. These magmas have temperatures of and originate in depths of . Eruptions are affected by the local conditions, resulting in high altitude eruption columns that are sorted by westerly stratospheric winds. Coarse deposits are deposited close to the vents, while fine ash is carried to the Chaco and eastern cordillera. The highest volcanoes in the world are located here, including high Ojos del Salado and high Llullaillaco. Some volcanoes have undergone flank collapses covering as much as . ==Scientific investigation==

The area's calderas are poorly understood and some may yet be undiscovered. Some calderas were subject to comprehensive research. Neodym, lead and boron isotope analysis has been used to determine the origin of eruption products. The dry climate and high altitude of the Atacama Desert has protected the deposits of APVC volcanism from erosion, but limited erosion also reduces the exposure of buried layers and structures. Evidence of volcanic activity and cyclic variation has been obtained from remote fallout deposits as well. ==Geologic history==

The APVC area before the upper Miocene was largely formed from sedimentary layers of Ordovician to Miocene age and deformed during previous stages of Andean orogeny, with low volume volcanics. During this flare-up it erupted primarily dacites with subordinate amounts of rhyolites and andesites. This triggered the formation of evaporite basins containing halite, boron and sulfate The sudden increase is explained by a sudden steepening of the subducting plate, similar to the Mid-Tertiary ignimbrite flare-up. Other active centres include the El Tatio and Sol de Mañana geothermal fields and the fields within Cerro Guacha and Pastos Grandes calderas. The latter also contains <10 ka rhyolitic flows and domes. ==Extent==

The APVC erupted over an area of from ten major systems, some active over millions of years and comparable to Yellowstone Caldera and Long Valley Caldera in the United States. forming a batholith. Alternatively, the body revealed by seismic studies is the remnant mush of the magma accumulation zone. Deposits from the volcanoes cover a surface area of more than . La Pacana is the largest single complex in the APVC with dimensions , including the caldera. Magma generation rates during the pulses are about , based on the assumption that for each of arc there is one caldera. These rates are substantially higher than the average for the Central Volcanic Zone, . During the three strong pulses, extrusion was even higher at . Intrusion rates range from and resulted in plutons of volume beneath the calderas. ==Source of magmas==

Modelling indicates a system where andesitic melts coming from the mantle rise through the crust and generate a zone of mafic volcanism. Another model requires the intrusion of basaltic melts into an amphibole crust, resulting in the formation of hybrid magmas. Partial melting of the crust and of hydrous basalt generates andesitic–dacitic melts that escape upwards. A residual forms composed from garnet pyroxenite at a depth of . This residual is denser than the mantle peridotite and can cause delamination of the lower crust containing the residual. Until that point, differentiation and crystallization of rising mafic magmas had mostly produced andesitic magmas. The change in plate movements and increased melt generation caused an overturn and anatexis of the melt generating zone, forming a density barrier for mafic melts which subsequently ponded below the melt generating zone. Dacitic melts escaped from this zone, forming diapirs and the magma chambers that generated APVC ignimbrite volcanism. The formation of silicic magmas in the crust hinders the ascent of other denser magmas, which resume erupting only after large explosions have removed the silicic magma. The magmas beneath the APVC are noticeably rich in water derived from the subduction of water-rich rocks. A volume ratio of about 10-20% of water has been invoked to explain the pattern of electrical conductivity at a depth of . The total amount of water has been estimated to be , comparable to large lakes on Earth. ==Tomographic studies==

Seismic tomography is a technique that uses seismic waves produced by earthquakes to gather information on the composition of the crust and mantle below a volcanic system. Different layers and structures in the Earth have different propagation speeds of seismic waves and attenuate them differently, resulting in different arrival times and strengths of waves travelling in a certain direction. From various measurements 3D models of the geological structures can be inferred. Results of such research indicate that a highly hydrated slab derived from the Nazca Plate – a major source of melts in a collisional volcanism system – underlies the Western Cordillera. Below the Altiplano, low-velocity zones indicate the presence of large amounts of partial melts that correlate with volcanic zones south of 21° S, whereas north of 21° S thicker lithospheric layers may prevent the formation of melts. Next to the Eastern Cordillera, low-velocity zones extend farther north to 18.5° S. A thermally weakened zone, evidenced by strong attenuation, in the crust is associated with the APVC. This indicates the presence of melts in the crust. A layer of low velocity (shear speed of ) thick is assumed to host the APVC magma body. and a temperature of about . ==Subsystems==

• Aguas Calientes caldera () • Cerro Chascon () • Cerro Panizos () • Chipas caldera • Kapina caldera () • Negra Muerta volcanic complex () • Pocitos () • Rincon volcanic complex () • Tastil volcano () • TulTul () • Uturuncu () • Vallecito caldera () • Vilama () ==Ignimbrites==

• Abra Grande Ignimbrite, 6.8 mya. • Blanco Ignimbrite, . • Caspana Ignimbrite, 4.59–4.18 mya. • Cerro Blanco Ignimbrite, 0.5–0.2 mya. • Cerro Colorado, 9.5–9.8 mya. • Cerro Lucho lavas, 10.6 mya. • Cerro Panizos Ignimbrite, 6.7–6.8 mya. • Chuhuilla Ignimbrite, 5.45 mya. • Cienago Ignimbrite, 7.9 mya. • Cueva Negra/Leon Muerto Ignimbrites, 3.8–4.25 mya. • Cusi Cusi Ignimbrite, >10 mya. • Galan Ignimbrite, 2.1 mya. • Granada/Orosmayo/Pampa Barreno Ignimbrite, 10-10.5 mya. • Grenada Ignimbrite, 9.8 mya. • Guacha Ignimbrite, 5.6–5.7 mya. • Guaitiquina Ignimbrite, 5.07 mya. • Laguna Amarga Ignimbrite, 3.7–4.0, 5.0 mya. • Laguna Colorada Ignimbrite, 1.98 mya. • Laguna Verde Ignimbrite, 3.7–4.0 mya. • Las Termas Ignimbrite 1 and 2, 6.45 mya. • Los Colorados Ignimbrite, 7.5–7.9 mya. • Merihuaca Ignimbrites, 5.49–6.39 mya. • Morro I Ignimbrite, 12 mya. • Morro II Ignimbrite, 6 mya. • Pairique Chico block and ash, 10.4 mya. • Pampa Chamaca, 2.52 mya. • Pitas/Vega Real Grande Ignimbrites, 4.51–4.84 mya. • Potrero Grande Ignimbrite, 9.8–9 mya. • Potreros Ignimbrite, 6.6 mya. • Purico Ignimbrite, 1.3 mya. • Puripicar Ignimbrite, 4.2 mya. • Rachaite volcanic complex, 7.2–8.4 mya. • Rosada Ignimbrite, 6.3–8.1 mya. • Sifon Ignimbrite, 8.3 mya. • Tajamar/Chorrillos Ignimbrite, 10.5–10.1 mya. • Tamberia Ignimbrite, 10.7–9.5 mya. • Tara Ignimbrite, 3.6 mya. • Tatio Ignimbrite, 0.703 mya. • Toba 1 Ignimbrite, 7.6 mya. • Toconao pumice, 4.65 mya. • Vallecito Ignimbrite, 3.6 mya. • Verde Ignimbrite, 17.2 mya. • Vilama Ignimbrite, 8.4–8.5 mya. • Vizcayayoc Ignimbrite, 13 mya. ==References==