Aguas Calientes is a major

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

in Salta Province,

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...

. It is in the Central Volcanic Zone of the

Andes The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...

, a zone of volcanism covering southern

Peru , image_flag = Flag of Peru.svg , image_coat = Escudo nacional del Perú.svg , other_symbol = Great Seal of the State , other_symbol_type = National seal , national_motto = "Firm and Happy f ...

, Bolivia, northwest Argentina and northern

Chile Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...

. This zone contains stratovolcanoes and

s. The activity of the Central Volcanic Zone is linked to the subduction of the

Farallon Plate The Farallon Plate was an ancient oceanic plate. It formed one of the three main plates of Panthalassa, alongside the Phoenix Plate and Izanagi Plate, which were connected by a triple junction. The Farallon Plate began subducting under the west ...

and later its splinter, the Nazca Plate, below the South American Plate. The Aguas Calientes caldera is located on a Precambrian basement that was

thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that sys ...

over more recent (

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

and younger) layers of

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...

. Aguas Calientes caldera was the source of two major

ignimbrite Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surro ...

s; the Tajamar Ignimbrite (including the Chorrillos Ignimbrite inside the caldera) and the Verde Ignimbrite. The first was erupted 10.5–10.1 Ma ago and is a body of ignimbrite of about . The second was erupted 17.2 Ma ago and has a volume of .

Geography and structure

Aguas Calientes caldera lies in the northwestern Salta Province of Argentina, in the

San Antonio de los Cobres San Antonio de los Cobres is a small town of population 5,482 (per the 2001 INDEC census) in northwestern Argentina. It is the capital of the Los Andes Department of the Salta Province. Geography The town is known for its high elevation of app ...

district to the southeast of the town of the same name. Aguas Calientes caldera is part of the Central Volcanic Zone (CVC), which is located in southern Peru, northern Chile, southwestern Bolivia and northwestern Argentina in highlands over high. At least six potentially active calderas, 44 active major and 18 active minor volcanoes lie in this area, of which

Lascar A lascar was a sailor or militiaman from the Indian subcontinent, Southeast Asia, the Arab world, British Somaliland, or other land east of the Cape of Good Hope, who was employed on European ships from the 16th century until the middle of the ...

volcano is the most active. The largest eruption in historical times in the CVZ occurred in 1600 on

Huaynaputina Huaynaputina ( ; ) is a volcano in a volcanic high plateau in southern Peru. Lying in the Central Volcanic Zone of the Andes, it was formed by the subduction of the oceanic Nazca Plate under the continental South American Plate. Huaynaputina is ...

volcano in Peru. Aguas Calientes caldera is a roughly circular caldera limited on the western and eastern side by north–south trending fault systems. Two superimposed calderas form the volcanic system. Cerro Verde inside the caldera is a dome formed by the uplifting of the Verde ignimbrites in the caldera after their deposition and cooling. Cerro Aguas Calientes was formed in the same way from the Tajmar ignimbrites, conclusions based in both cases on the outward dip of the ignimbrites contained in the domes.

Geology

The Central Volcanic Zone (CVZ), active mainly since the

, is the volcanically active area of the

between 16 and 28°S. Its activity is dependent on the subduction of the Nazca Plate, previously the

, below the South American Plate. This subduction process is responsible for the formation of the

and the volcanic activity on the eastern margin of the South American continent. Estimates of the volumes erupted and surface areas covered by the CVZ volcanoes in the Neogene vary. with a surface area for the latitudes 18–28°S and with a surface area for the latitudes north of 25°S have been calculated. In the southern segment of the CVZ, volcanic activity has varied over time. Before 26 Ma, volcanic activity was limited to a belt from the

Peru–Chile Trench The Peru–Chile Trench, also known as the Atacama Trench, is an oceanic trench in the eastern Pacific Ocean, about off the coast of Peru and Chile. It reaches a maximum depth of below sea level in Richards Deep () and is approximately long; ...

. An increased subduction rate around 26 Ma, possibly related to the breakup of the

and change in the subduction direction, resulted in increasing activity and eastward migration of the activity. 14.5 to 5.3 Ma the volcanic activity expanded laterally and reached a maximum 14.5 to 11.5 Ma with a width and a maximum trench-volcano distance of . During this phase volcanism in the western part of the CVZ was effusive. 11.5–8.3 Ma the volcanism moved back westwards and after 5.3 Ma it was restricted to a narrow belt from the trench. The currently active belt is located east of the trench.

Local

Aguas Calientes caldera is located on the Late Neoproterozoic to

Early Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...

Puncoviscana Formation with turbiditic

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

that was affected by metamorphism. A volcano-sedimentary

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

sequence of marine origin is the only other

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

structure in the area. The caldera itself is located on the Precambrian-Ordovician basement. The volcano is linked to the major Calama–Olacapato–El Toro fault system that cuts the Andean chain in a northwest–southeast direction. This system also affects the Negra Muerta volcanic complex,

Cerro Tuzgle Cerro Tuzgle is a dormant stratovolcano in the Susques Department of Jujuy Province in northwestern Argentina. Tuzgle is a prominent volcano of the Back-arc basin, back-arc of the Andes and lies about east of the main volcanic arc. Part of the ...

, Chimpa volcanoes and several smaller volcanic systems. The formation of this fault system and other thrust faults parallel to the Andes chain is linked to the compression and orogeny of the Andean chain since the

Geologic record

Aguas Calientes caldera was between 11 and 10 Ma the source of large scale

sheets. Aguas Calientes caldera is considered to be an overpressure caldera, where eruption is triggered by intrusions from the volcano sill weaken the host rocks and cause the formation of fissures.

Composition

Minerals in the Tajamar ignimbrite include biotite,

hornblende Hornblende is a complex inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common in igneous and metamorphic rock ...

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

and some augite. The eruption products are uniformly

dacitic Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyol ...

in nature.

Hydrothermal alteration Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical co ...

s have generated Sb- Au and Pb- Ag- Zn deposits that may be of economic importance. These deposits occur in the form of mineralized

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...

and

. The La Poma-Incachule

mining Mining is the extraction of valuable minerals or other geological materials from the Earth, usually from an ore body, lode, vein, seam, reef, or placer deposit. The exploitation of these deposits for raw material is based on the economic ...

district northeast of the caldera is part of the volcanic system and contains

argentiferous Silver mining is the extraction of silver from minerals, starting with mining. Because silver is often found in intimate combination with other metals, its extraction requires elaborate technologies. In 2008, ca.25,900 metric tons were consumed ...

galena, sphalerite, antimonite and

arsenopyrite Arsenopyrite ( IMA symbol: Apy) is an iron arsenic sulfide (FeAsS). It is a hard ( Mohs 5.5-6) metallic, opaque, steel grey to silver white mineral with a relatively high specific gravity of 6.1. When dissolved in nitric acid, it releases elem ...

. These deposits formed through

hydrothermal Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...

, deuteric alteration and supergene processes and were influenced during their formation by local fault systems.

Eruptive history

Aguas Calientes caldera erupted twice

ignimbrites. was the source of two major ignimbrites; the Tajamar Ignimbrite (including the Chorrillos Ignimbrite inside the caldera), and the Verde Ignimbrite. The first was erupted 10.5–10.1 Ma and is a body of

of about . The second was erupted 17.2 Ma and has a volume of . The Abra de Gallo ignimbrite was formerly considered the third and erupted 10.0-10.5 Ma; Petrinovic ''et al.'' consider it part of the Tajamar ignimbrite. Both eruptions probably resulted from a disruption of the magma chamber integrity by vertical faults created through the activity of horizontal fault systems in the region. There is no evidence of plinian

eruption column An eruption column or eruption plume is a cloud of super-heated Volcanic ash, ash and tephra suspended in volcanic gas, gases emitted during an explosive volcanic eruption. The volcanic materials form a vertical column or Plume (fluid dynamics), ...

s in the eruption deposits and the chemistry of the deposits indicates they originated in a homogeneous magma chamber. The first eruption, which took place 17.15 Ma, was the first caldera forming episode and the eruption occurred through a central vent. Its deposit, the Verde Ignimbrite, is

pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicular v ...

rich and green in colour. The deposits in thickness range from more than on Cerro Verde to in the southern outcrops. It covers a surface area of about . After the eruption, the Verde ignimbrites were tectonically deformed and partly buried by the Tajamar ignimbrites. The second eruption was more extensive and covered at a minimum . Starting from the caldera rim, it is clearly distinguishable in two units; the intracaldera Chorrillos and the extracaldera Tajamar units which were formerly considered to be separate events. The Chorrilos deposits are gray coloured and have pumice fragments of less than 5 cm diameter; they also contain

lenses. The Tajmar deposits overlie the Chorrillos deposits and have a red to pale pink colour. The ash flows from the eruption flowed out of the caldera, forming flow structures similar to large lava flows. The deposits underwent some

devitrification Devitrification is the process of crystallization in a formerly crystal-free (amorphous) glass. The term is derived from the Latin ''vitreus'', meaning ''glassy'' and '' transparent''. Devitrification in glass art Devitrification occurs in glass a ...

due to vapour action. This ignimbrite has a maximum thickness of on Cerro Aguas Calientes in the caldera centre and thins to less than away from the volcano. Post-caldera activity included

– geothermal activity and may have migrated westward to the neighbouring Quevar volcanic centre. Subsequent to the caldera formation, the caldera floor was uplifted for . Geothermal activity still occurs at the Incachule field.

References

{{reflist Supervolcanoes VEI-7 volcanoes Volcanoes of Salta Province Calderas of Argentina Miocene calderas