Cerro Overo maar and La Albondiga lava dome are two independent monogenetic volcanoes
located in the Central Volcanic Zone of the Andes in northern Chile, close to the
active Lascar and Chiliques volcanoes. Cerro Overo maar was formed <77 ka ago by explosive-effusive
eruptions, including phreatomagmatic activity, while La Albondiga lava dome (Pleistocene)
is the result of magmatic explosive-effusive activity alone. Cerro Overo and La Albondiga
are characterized by deposits composed of pyroclastic rocks and dense coherent lava
blocks. At Cerro Overo, these deposits are located around the crater rim, while at
La Albondiga, they form coherent in situ lava dome features. Cerro Overo also displays
thin ash- and lapilli-dominated deposit layers presumed to be pyroclastic surge deposits,
which include juvenile pyroclasts exhibiting cauliflower textures, numerous exotic
accidental lithics, and ballistically transported lapilli, bombs, blocks, and bomb
sags. These fragments include recycled, juvenile pyroclasts, as well as material from
older volcanic and intrusive rocks from the stratigraphic units immediately below.
These small-volume volcanoes represent two of the less silicic volcanoes (similar
to 54 wt.% SiO2) in the northern Chilean Pleistocene to Holocene volcanic provinces.
They are characterized by a fine grain size (mainly fine lapilli), olivine phenocrysts
with skeletal textures, ignimbritic, dioritic, and granitic xenoliths, and quartz
xenocrysts, with high concentrations of incompatible trace elements and light rare
earth elements. The general magmatic evolution of the Cerro Overo and La Albondiga
systems has been controlled by fast ascent (e.g., skeletal olivine phenocrysts) of
mantle-derived magma associated with mixing, fractional crystallization, and a low
degree of crustal assimilation during turbulent ascent processes. Such eruptions provide
evidence that various factors play an essential role in the transition from explosive-effusive
magmatic (Cerro Overo and La Albondiga) to phreatomagmatic (Cerro Overo) volcanic
eruption styles.