El Rojo Sur

20°50′S 68°37′W^[1] El Rojo Sur is a monogenetic volcano in the Andes,^[1] in the form of a cone.^[2]

It is similar to El Rojo Norte 260 kilometres (160 mi) farther north.^[1] A mafic andesite lava flow was erupted from the 100 metres (330 ft) high cone that also features red scoria. Ages obtained by potassium-argon dating are 2.93±0.13, 3.4±0.4 and 3.23±0.12 mya.^[3]

Rojo Sur erupted undifferentiated mafic magmas. Its lead isotope composition is different from the composition of that segment of the Andes, being non-radiogenic. This isotope signature may be the effect of assimilation of deeper basement rocks during magma formation.^[2] In addition, the magmas are rich in incompatible elements.^[4]