San Jerónimo and Negro de Chorrillos are two shoshonitic monogenetic volcanoes placed in the Pluna plateau, in the Central Volcanic Zone of the Andes, in the north of Argentina. They are younger than previously thought. They erupted 144.00 and 51.000 years ago respectively, according to a new study that has dated and characterized the materials erupted from these volcanoes by using new methods The paper has been recently published in the "Journal of South American Earth Sciences".

The study, which aimed to gain insight into the origin of the shoshonitic volcanoes in a zone where they are not common, also suggests that an old continental suture zone generated hundreds of millions of years ago is still driving the present vulcanism distribution. The term shoshonitic was first used in 1895 to refer to an igneous rock with high potassic content.

The Central Volcanic Zones of the Andes is an area with a high volcanic activity due to the subduction of the Nazca plate underneath the South American continental plate. There are hundreds of volcanoes in the central volcanic arc and also in the vast area of the Altiplano-Puna plateau, placed in the back-arc region. However, only a few of the volcanoes of this region are shoshonitic. San Jeronimo and Negro de los Chorrillos were selected by the researchers because they are well-preserved edifices and also because” they have erupted shoshonitic lavas with a wide variety of mineral contents and textures”.

"These volcanoes have the particular feature of having been formed from shoshonitic magmas, which are scarce and with a primitive composition”, said José Luis Fernández-Turiel, researcher at GEO3BCN-CSIC and first author of the study. According to Fernández-Turiel, the analysis of this type of magma “can provide insights regarding the mantle composition and mantle-crust interactions of discrete lithospheric mantle domains at continental subduction zones”. It is also an opportunity to “comprehend what is going on beneath one of the thickest continental crust on Earth, reaching up to 80 km in some areas”.

The researchers mapped and sampled San Jerónimo and Negro de los Chorrillos volcanoes. “We selected six lava samples from each volcano to perform petrography and modal mineralogy by conventional optical microscopy. analysis and geochemical studies by using high-resolution mass spectrometry and X-ray fluorescence spectrometry.”, said Francisco Pérez Torrado from the University of Las Palmas de Gran Canaria and co-author of study. By doing so, the authors of the study could finally select the two best samples for age determination by using K-Ar method.

By doing so, the researchers were able to determine the eruption ages of the two volcanoes. “The new ages are 144.000 and 51.000 years respectively, significantly younger than those published before”, said Fernánez-Turiel. Furthermore, the researchers determined the depth at which these magmas were generated. “These shoshonitic melts were generated at Moho depths around 50 km and interacted with other more evolved, silicic magmas, during the fast ascent to the surface, which we estimate in took only a few hours or days”, said Fernández-Turiel.

The study also provides a possible answer to the existence of shoshonitic volcanoes in the Argentinian Puna plateau, an area where this type of volcanism is scarce. “The oddity in the Andean context would be related with the magma generation in a specific geotectonical setting, related with old terrain boundaries of the Gondwana supercontinent”, notes Alejandro Rodríguez-González, co-author of the study and researcher from University of Las Palmas de Gran Canaria.

“The shoshonitic centers are approximately located in the intersection of the N-S suture between the terranes of Antofalla and Pampia with the major NW-SE fault systems of Lípez-Conranzulí”, the authors wrote. The Antofalla-Pampia suture zone, parallel to the modern Andean chain in the central Andes, is a significant weakness zone developed in Paleozoic times along the southwestern margin of Gondwana which existed between 550 and 270 million years ago. According to the article, this zone would represent the mechanism that favors the “development of small volumes of potassic melts (precursors of shoshonitic rocks)”.

The study is also signed by researchers from IRNASA-CSIC, Institute of Environmental Studies and Natural Resources (i-UNAT) of the University of Las Palmas de Gran Canaria, Laboratory of the Sciences for the Climate and the Environment from the University Paris-Saclay (CEA-CNRS-UVSQ), and the Faculty of Earth Sciences from the University of Barcelona.

Reference article

Fernandez-Turiel JL, Saavedra J, Perez-Torrado FJ, et al. New ages, morphometric and geochemical data on recent shoshonitic volcanism of the Puna, Central Volcanic Zone of Andes: San Jerónimo and Negro de Chorrillos volcanoes. J South Am Earth Sci. 2021:103270. doi: https://doi.org/10.1016/j.jsames.2021.103270

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