Choiyoi Group ( es, Grupo Choiyoi) is a
Permian
The Permian ( ) is a geologic period and 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 251.9 Mya. It is the last period of the Paleoz ...
and
Triassic-aged group of
volcano-sedimentary formation
Formation may refer to:
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* Cave formation or speleothem, a secondar ...
s in
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 ...
and
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 ...
. The group bears evidence of
bimodal-style volcanism
Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called ...
related to an ancient
subduction zone
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...
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, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...
.
[
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 in Argentina are most common in the ]Frontal Cordillera
The Frontal Cordillera ( es, Cordillera Frontal) is a mountain range in western Argentina that forms part of the Andes. It extends in a north–south direction in the provinces of La Rioja
La Rioja () is an autonomous community and province i ...
, the San Rafael Massif and the Principal Cordillera
Principal Cordillera ( es, Cordillera Principal) 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. ...
of southern Mendoza and northern Neuquén
Neuquén (; arn, Nehuenken) 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 w ...
. In parts the group reaches thicknesses of 2 to 4 km.
The plutonic equivalent of the Choiyoi Group volcanic material are mostly granitoid
A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quartz ...
s.[ The remnants of the ]magmatic arc
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
that produced much of volcanic material is now preserved as a series of batholith
A batholith () is a large mass of intrusive igneous rock (also called plutonic rock), larger than in area, that forms from cooled magma deep in Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock types, such ...
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 ( es, Cordillera de la Costa) is a mountain range that runs from north to south along the Pacific coast of South America parallel to the Andean Mountains, extending from Morro de Arica in the north to Taitao Peninsula, ...
. 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 ( es, Cuenca Cuyana) 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 ...
, 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 ...
ic 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 moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or und ...
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 mag ...
and have also other geochemical signatures indicative of an origin in subduction zone
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...
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
I ...
and westernmost Buenos Aires Province
Buenos Aires (), officially the Buenos Aires Province (''Provincia de Buenos Aires'' ), is the largest and most populous Argentine province. It takes its name from the city of Buenos Aires, the capital of the country, which used to be part of th ...
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 mineral ...
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 moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...
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 South America
Geologic formations of Argentina
Geologic formations 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