Magmatic evolution of the Puyehue Cordón Caulle Volcanic Complex (40° S), Southern Andean Volcanic Zone: From shield to unusual rhyolitic fissure volcanism
Magmas erupted from Quaternary volcanoes of Southern Andes between 37° and 46° S latitude are mainly basaltic to andesitic. However, PCCVC (40° S) shows a singular magmatic evolution due to the abnormal evacuation of rhyolites, especially in the last 100 ka. In addition, PCCVC is the result of juxtaposing products from the NW-trending alignment of Cordillera Nevada caldera, Cordón Caulle fissure volcano and the Puyehue stratocone. Using 40Ar/ 39Ar and 14C geochronology it can be established that they evolved since ca. 500 ka as coeval but separated vents with a first stage of shield volcanism, followed by repeated collapses that formed an internal NW-elongated graben. From ca. 100 ka, volcanic activity occurred in both a fissure system (Cordón Caulle) and a central volcano (Puyehue). Holocene explosive eruptions, mainly in the Puyehue crater, accompanied the dome growing along a NW-trending fissure system. Last historical eruptions were in 1921 and 1960 when NW fissures of Cordón Caulle fed rhyodacitic lava flows. In 1960, the fissure eruption was triggered by a remote Mw: 9.5 thrust earthquake. Cordillera Nevada caldera presents a reduced compositional range (52-63% SiO 2) and geochemical features of low-pressure magma mixing and assimilation. Instead, Cordón Caulle and Puyehue volcanoes have a wide silica range (48-71% SiO 2) and an outstanding affinity, which can be modelled with initial high-pressure fractional crystallization, moderate magma mixing and subsequent low-pressure fractional crystallization from a common parental source. The exceptional magmatic evolution and eruptive style of PCCVC in Southern Andes could be related with the physics of the plumbing system, which in turn can be controlled by external factors as the structure of the continental crust and the ongoing stress regime.