Eruptive processes and caldera formation in a nested downsagcollapse caldera: Cerro Panizos, central Andes Mountains

Abstract

Abstract The Cerro Panizos ignimbrite center, in the central Andes Mountains, produced two ignimbrite sheets and many lava flows. The ignimbrite of Quebrada Cienago was erupted at 7.9 Ma, and effusive eruptions continued until the two cooling units of the crystal-rich dacitic Cerro Panizos Ignimbrite were emplaced at 6.7 Ma. The lower unit has no laterally continuous flow breaks and was erupted from a single vent or small cluster of vents with limited fluctuation in discharge. Lithic fragments reach significant concentrations (>5%) only in the uppermost two meters of this cooling unit, where they document vent-wall collapse or the opening of a new vent. The upper cooling unit contains many flow units with variations in welding, thicknesses, and lithic fragment concentrations, implying an unsteady eruption column, the opening of many vents, and probable caldera collapse. Triangulation of anisotropy of magnetic susceptibility flow-direction measurements locate a single vent for the lower cooling unit, whereas the upper cooling unit had many vents within the present dome cluster. A 15-km diameter topographic depression, marked by inward dips of 4–8 at the cooling unit contact, is centered on the vent area of the lower cooling unit. The depression is interpreted as a downsag caldera formed during emplacement of the lower cooling unit. Collapse began late in the eruption of the lower cooling unit and continued through the emplacement of the upper cooling unit. Resurgent magmatism occurred as lava flows that mundated the caldera area. A ring of dacite domes, erupted until at least 6.1 Ma, in the northern half of the downsag caldera traces the margin of a collapse caldera associated with the upper cooling unit. Maximum caldera subsidence (353 km 3 ) is not enough to account for the erupted volume (652 km 3 DRE minimum). The ash-poor, crystal-rich nature (up to 50% crystals in the pumice. 75% in the matrix) of the Cerro Panizos pyroclastic flows resulted in poor retention of gases, and there is little evidence of fluidization. The high crystal content, however, favored modified grain-flow processes, in which several particle support mechanisms combine with grain-dispersive forces. It is postulated that the flows were initially partially fluidized, but rapidly lost their gases and grain-flow processes grew in relative importance. Grain-flow processes destroyed evidence of fluidization before deposition.