Dynamics of caldera collapse during the Coranzulí eruption (6.6 Ma) (Central Andes, Argentina)

Abstract

The Coranzulí caldera (23°00′ S ? 66°15′ W) is one of the least known caldera complexes in the eastern part of the Argentinian Altiplano-Puna plateau (Central Andes). It lies at the intersection of N S, NW-SE and NE-SW fault systems and was formed about 6.6Ma ago; during the eruption, four main crystal-rich daciticignimbrites were emplaced in different directions around the caldera. Caldera collapse was not homogeneous, rather it occurred along different sectors of the ring fault as subsidence progressed. The location of co-ignimbrite lag breccias and the composition of the dominant lithic fragments within the different ignimbrite flow units reveal how the caldera collapse developed. According to the succession of deposits, in particular the lack of an initial fallout and the presence of co-ignimbrite lag breccias associated with the different ignimbriteunits, we interpret this caldera-forming eruption as a pulsating boiling-over event in which the caldera collapse developed immediately after the onset of the eruption, favored by a transtensive tectonic system. Within the central part of the caldera, there is Cerro Coranzulí, a resurgent dome that exposes a thick intracaldera ignimbrite succession covered by ~ 100-m-thick dacite lava flows that erupted at the end of caldera formation.