Los Gemelos-El Saladillo are monogenetic volcanoes in Argentina.
The Nazca Plate subducts beneath the South America Plate, giving rise to the volcanism in the Andean Central Volcanic Zone, including mafic back-arc volcanism which is often associated with tectonic lineaments. [1]
Los Gemelos ( 24°45′S66°10′W / 24.750°S 66.167°W ) and El Saladillo ( 24°33′23″S66°12′10″W / 24.55639°S 66.20278°W ) lie in the Calchaqui Valley, close to the towns of La Poma and El Saladillo. [2] The valley is bordered by two thrust faults, at least one (the Calchaqui fault) of which has had historical earthquakes; [3] the Los Gemelos volcanic cones were constructed along the fault. [4]
Los Gemelos consists of three individual vents, the northern two have generated scoria cones the southern features a fissure. The vents have produced about 0.45 cubic kilometres (0.11 cu mi) of lava, [5] three from the northern and two from the central vent; lava flows reach the bed of the Calchaqui River and have flowed along its bed, [6] damming the river and forming a lake. [7] El Saladillo consists of two partially eroded cones which have generated lava flows and several smaller cones generated by explosive eruptions. [6]
The two volcanoes have erupted shoshonitic rocks, [2] consisting of trachybasalt andesite. [8] They contain phenocrysts of clinopyroxene, mica, olivine and plagioclase but also xenoliths. [6]
The volcanoes were constructed during the Pleistocene. [4] The activity at Los Gemelos appears to be associated with an episode of tectonic activity, and on the basis of sediments in the lava-dammed lake the eruption occurred about 35,000 years ago. [9]
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