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

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 6.6 Ma ago; during the eruption, four main crystal-rich dacitic ignimbrites 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 ignimbrite units, 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.