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′S 66°10′W) and El Saladillo (24°33′23″S 66°12′10″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]