ALBERT

Description: La Pacana caldera in the Central Andes of northern Chile is one of the largest and best exposed resurgent calderas in the world. The caldera had previously been recognised as the source of the regionally-extensive Atana ignimbrite, but additional field and stratigraphic evidence, along with new K–Ar age determinations and geochemical data have led to a revision of the geology and development of this major Andean caldera. In particular, this information allows more realistic estimates of eruptive volumes and has implications for the style of ignimbrite eruption. Two major ignimbrites appear to have originated from La Pacana caldera, based on their thickness variations, lateral distributions and stratigraphic relations: the crystal-poor, rhyolitic Toconao ignimbrite (4–5 Ma) and the crystal-rich, dacitic Atana ignimbrite (4 Ma). Following caldera collapse and formation of the resurgent Atana block, several crystal-rich dacitic–rhyolitic domes formed along the margin of the resurgent block. New K–Ar ages show that this post-caldera volcanism continued from 4 to 2 Ma, indicating that the La Pacana magmatic system was active for at least 2 Ma after the main eruption. The Atana ignimbrite extends west, south and east of La Pacana caldera. Our work shows that the ignimbrite sequence northeast of the caldera, formerly mapped as Atana outflow, represents two new units which we name the upper and lower Tara ignimbrites. The distribution of the Tara ignimbrites points to a source to the north. The upper Tara ignimbrite comprises four flow units with interbedded surge and fall deposits and a characteristic, heterogeneous pumice population. It occurs in the La Pacana moat and onlaps the resurgent block. These field relations and a new K–Ar age of 3.8 Ma show convincingly that this ignimbrite erupted after formation of La Pacana caldera. The lower Tara ignimbrite is a single extensive flow unit, and has an age of 5.6 Ma. Two outcrops of lag breccia occur adjacent to the caldera topographic margin and these are interpreted as vent areas for the Atana ignimbrite. This indicates that the structural caldera margin does not coincide with the resurgent block as previously thought, but is close to the topographic margin. Combining this redefinition of caldera geometry with the revised distribution of the Atana ignimbrite implies an overall trap-door-like configuration of caldera collapse, with a hinge to the north and maximum subsidence in the south. The new volume estimate of 2500 km3 for the caldera-forming Atana ignimbrite, including intracaldera and outflow facies, places it among the most voluminous ignimbrites of the world.