Choiyoi Group () is a

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...

and Triassic-aged group of volcano-sedimentary formations in

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

and

Chile Chile, officially the Republic of Chile, is a country in western South America. It is the southernmost country in the world and the closest to Antarctica, stretching along a narrow strip of land between the Andes, Andes Mountains and the Paci ...

. The group bears evidence of bimodal-style

volcanism Volcanism, vulcanism, volcanicity, or volcanic activity is the phenomenon where solids, liquids, gases, and their mixtures erupt to the surface of a solid-surface astronomical body such as a planet or a moon. It is caused by the presence of a he ...

related to an ancient

subduction zone Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...

that existed along the western margin of the

supercontinent In geology, a supercontinent is the assembly of most or all of Earth's continent, continental blocks or cratons to form a single large landmass. However, some geologists use a different definition, "a grouping of formerly dispersed continents", ...

Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...

. The Choiyoi Group has a large areal extent through western Argentina and parts of Chile, covering at least , but probably around . While the subsurface extent of the group is large,

exposures Exposures can refer to: *'' Andy Warhol's Exposures'' *'' Exposures – In Retrospect and Denial'' *The Exposures, a stage name of German electronic musician Jan Jelinek Jan Jelinek is a German electronic musician who also operates under the nam ...

in Argentina are most common in the Frontal Cordillera, the San Rafael Massif and the

Principal Cordillera Principal Cordillera () is the Andean mountain range that makes up the boundary between Central Chile and neighbouring areas of Argentina. It is also a continental divide between the Atlantic and the Pacific watersheds. It extends in a north–s ...

of southern Mendoza and northern

Neuquén Neuquén (; ) is the capital city of the Argentine province of Neuquén and of the Confluencia Department, located in the east of the province. It occupies a strip of land west of the confluence of the Limay and Neuquén rivers which form t ...

. In parts the group reaches thicknesses of 2 to 4 km. Pangaea physical map

The plutonic equivalent of the Choiyoi Group volcanic material are mostly

granitoid A granitoid is a broad term referring to a diverse group of coarse-grained igneous rocks that are widely distributed across the globe, covering a significant portion of the Earth's exposed surface and constituting a large part of the continental ...

s. The remnants of the

magmatic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc lo ...

that produced much of volcanic material is now preserved as a series of

batholith A batholith () is a large mass of intrusive rock, intrusive igneous rock (also called plutonic rock), larger than in area, that forms from cooled magma deep in the Earth's crust. Batholiths are almost always made mostly of felsic or intermediate ...

s, including the

Coastal Batholith of central Chile The Coastal Batholith of central Chile is a group of plutons in the Chilean Coast Range of Central Chile appearing contiguously from 33° S to 38° S. At a latitude of 40° S an outlying group of plutons of the batholith appear in a more eastward ...

in the

Chilean Coast Range The Chilean Coastal Range () is a mountain range that runs from north to south along the Pacific Ocean, Pacific coast of South America parallel to the Andes, Andean Mountains, extending from Morro de Arica in the north to Taitao Peninsula, where ...

. During the Permian, the zone of arc magmatism moved inland from the Chilean Coast Range, reaching San Rafael about 280 million years ago. Many Triassic basins in southern South America, including

Cuyo Basin Cuyo Basin () is a sedimentary basin in Mendoza Province, western Argentina. The Cuyo Basin has a NNW-SSE elongated shape and is limited to the west by the Sierra Pintada System and to the east by the Pampean pericraton. To the north the basin re ...

, have their lowermost sections made up of Choiyoi Group formations.

Stratigraphy

The Choiyoi Group can be stratigraphically subdivided into a Lower Choiyoi Group and an Upper Choiyoi Group. In the Lower Choiyoi Group

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

lava flow Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

s, andesites and breccias are common. Volcanic rocks and sediments in the Lower Choiyoi Group belong to the

calc-alkaline magma series The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the evolution of a mafic ...

and have also other geochemical signatures indicative of an origin in

volcanism. Volcanic rocks of the Lower Choiyoi in

La Pampa La Pampa () is a sparsely populated province of Argentina, located in the Pampas in the center of the country. Neighboring provinces are from the north clockwise San Luis, Córdoba, Buenos Aires, Río Negro, Neuquén and Mendoza. History In ...

and westernmost

Buenos Aires Province Buenos Aires, officially the Buenos Aires Province, is the largest and most populous Provinces of Argentina, Argentine province. It takes its name from the city of Buenos Aires, the capital of the country, which used to be part of the province an ...

have shoshonitic and syenitic features. Part of the Lower Choiyoi Group were erupted in association with the San Rafael orogeny while some Choiyoi Group sediments were deformed by the same orogeny.

Rhyolitic Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The miner ...

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 surrou ...

s and

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

s are more common in the Upper Choiyoi Group. The Choiyoi Group include the following formations: * El Palque, San Juan Province * Horcajos, San Juan Province, Mendoza Province * Portezuelo del Cenizo, San Juan Province, Mendoza Province * Tambillos, Mendoza Province. This formation contains '' Sphenophyte'' fossils that are thought to have grown near an ancient lake. * Vega de Los Machos, San Juan Province

Notes

References

{{Major South American geological formations Volcanic groups Geologic groups of Argentina Geologic groups of Chile Permian System of South America Permian Argentina Permian Chile Permian volcanism Triassic System of South America Triassic Argentina Triassic Chile Triassic volcanism Formations Andesite groups