CERRO VOLCÁN QUEMADO: AN ENIGMATIC HOLOCENE MONOGENETIC PHREATOMAGMATIC VOLCANO IN THE ALTIPLANO OF BOLIVIA

Cerro Volcán Quemado (CVQ) is a rhyolitic tuff cone-dome complex consisting of several nested craters in the Altiplano of Bolivia. As such it is unique in composition and character when compared to the surrounding andesitic volcanoes in the hyper-arid Altiplano region. The age of Cerro Volcán Quemado is unknown but we observed fumarolic activity there and a late Pleistocene to Holocene age is suggested by the climate history of the region. This enigmatic young volcano is therefore a critical part of the recent volcanic history of the Altiplano and records crucial information about the potential hazard to the region. We have initiated the first comprehensive study of the enigmatic volcano to determine the age and origin of this volcano. Beyond the obvious question of its apparently very youthful age, the rhyolitic composition raises questions about the origin of the magma and development of a significant phreatomagmatic center. In this contribution, we report the first petrological data on Cerro Volcán Quemado.

The CVQ lithologies are predominantly variably vesicular, banded rhyolite with volumetrically insignificant but ubiquitous andesitic enclaves. These are part of a high-K suite typical of the Central Volcanic Zone volcanic arc with typical LILE and LREE enrichments and Nb,Ta depletions but with Sr, P, Ti, and Eu depletions in the rhyolites. The rhyolites are crystal-poor (4 to 5% by volume) with phenocrysts of plagioclase, biotite, hornblende and accessory minerals Fe-Ti oxides, zircon, and apatite in a vesicular, often sheared glassy matrix. Enclaves vary from sheared bands to discrete “cauliflowered” blebs. Plagioclase microcrysts are predominantly normally zoned oligoclase (An_20-22), whereas andesine and labradorite (An₄₇ to An₆₂) zones and rims are found in reversely zoned variants. Most of the amphibole of the CVQ are tschermakitic pargasite and, in minor proportion, low calcium amphibole. Preliminary P-T estimates based on amphibole thermobarometry indicate temperatures between 820 to 910 °C and pressure of equilibration between 6 and 15 km depth.

These data suggest an upper crustal rhyolitic magma was recharged by andesitic magma initiating ascent that culminated in the interaction with surface water to trigger an extensive episodic phreatomagmatic eruption.