HOLOCENE MONOGENETIC SILICIC PHREATOMAGMATIC VOLCANISM ON THE ALTIPLANO OF BOLIVIA: CERRO VOLCÁN QUEMADO

Even in remote regions, explosive volcanic eruptions can have far-flung effects due to ash transport and dispersal. Despite that, the number and nature of global volcanic eruptions are under-recorded, rendering our knowledge of volcanic history incomplete. One such non-recorded example is Cerro Volcán Quemado (CVQ), a rhyolitic tuff cone-dome volcano that is located in the Altiplano behind the Central Volcanic Zone (CVZ) arc dominated by effusive andesite composite cones. The age of CVQ is not known but the youthful morphology and its superposition indicate a Holocene age. Archeological evidence indicates pre-Hispanic communities lived near CVQ only to suddenly disappear, leading us to speculate that the explosive eruption of CVQ may have been a factor in their disappearance. CVQ therefore potentially holds critical knowledge about the recent volcanic history and potential hazards in the Altiplano region. To realize this potential, we have initiated the first comprehensive study of CVQ to determine the age, origin of rhyolitic magma, and magnitude of eruption of this volcano. In this contribution, we discuss the origin of magma of Cerro Volcán Quemado.

CVQ magmas define a bimodal magma population of rhyolite and basaltic andesite, which is quite distinct from the primarily andesitic to dacitic compositions that erupted from the nearby CVZ arc cones. Using established compositional end-members for the CVZ (Blum-Oeste and Wörner, 2016, Terra Nova), CVQ trachyandesites are mixtures of the enriched basalt and the basaltic andesite end-members, whereas the rhyolites have to be derived by differentiation of the rhyodacite endmember. Further insight comes from Sr/Y which in the CVQ trachyandesites ranges from 84 to 90 ppm, whereas the rhyolites display lower Sr/Y ranges from 13 to 20 ppm. Andesites in the CVZ arc generally contain high Sr/Y values and this signature has been interpreted as indicating a deep crustal source. We propose a deep crustal source for the trachyandesites, an interpretation that is supported by P-T estimates from amphibole compositions in trachyandesites yielding pressures of equilibration between 13 - 30 km depth. The lower Sr/Y of CVQ’s rhyolites and their high-SiO2 rhyolite matrix glass compositions indicate a much shallower origin in the uppermost crust possibly through crustal melting or assimilation-fractional crystallization (AFC).