VOLCANOLOGY AND PETROLOGY OF AN ENIGMATIC BACKARC VOLCANO ON THE BOLIVIAN ALTIPLANO: IMPLICATIONS FOR NON-SUBDUCTION RELATED VOLCANISM

Volcán Tunupa is a Pliocene to Pleistocene volcano in the Bolivian backarc region of the central Altiplano at ~19.8ºS. The volcano is situated ~110 km east of the main arc of the Western Cordillera and ~115 km west of the of the Eastern Cordillera and the Los Frailes ignimbrite complex. Tunupa is ~175 km above the downgoing Nazca plate and is the easternmost expression of a Miocene to Pleistocene silicic volcanic field that extends orthogonally into the Altiplano region. Previous seismic studies have identified a thick lithospheric lid (~100-150 km) beneath the central Altiplano that is largely absent beneath the Western and Eastern Cordilleras and disparate melting mechanisms are thought to occur beneath the two Cordilleras. Whereas a fluid component from a typical subduction zone results in Western Cordillera volcanism, non-arc related melting, perhaps from delamination, may be responsible for the volcanism of the Eastern Cordillera. In this study, we provide the first detailed temporal and geochemical investigation into silicic volcanism in the central Altiplano and evaluate possible sources and melting mechanisms through Ar-Ar age dating, whole-rock geochemical, and mineral analysis. Volcanism at Tunupa is separated into two distinct phases: 1) lavas of trachyandesite to trachydacite composition (SiO₂ = 59.3 – 64.6 wt%) constructed the main edifice in the Pliocene to Pleistocene, which was followed by 2) an east-west trending series of more silicic trachydacitic domes (SiO₂ = ~66 wt%) along the eastern flank of the volcano. A subdued subduction component during both phases of volcanism is exemplified by relatively low Ba/Nb ratios (~50) compared to the main arc (~120). A similarly low Ba/Nb value of a mid-Miocene flow < 5 km east of the domes indicates that this signature is long-lived. These values are generally supportive of non-arc related melting (i.e. delamination) although alternative hypotheses are also considered such as backarc decompression due to corner flow, or a reduced subduction-related fluid flux in the backarc region. The relationships of backarc Andean volcanism with crustal thickening, plateau uplift, and lithospheric dynamics may be revealed through continued investigation into the timing and geochemistry of central Altiplano lavas.