Sector collapse and rapid rebuilding of Parinacota Volcano: extending 40Ar/39Ar dating of lava flows into the Holocene

Catastrophic debris avalanches have been recognized as a relatively common occurrence in the life cycle of arc stratovolcanoes. In many cases, for instance at Mt. St. Helens, debris avalanches reflect major changes or additions of new magma into the subvolcanic reservoir. In only a few cases, however, are we able to compare the state of the magmatic system that preceded a catastrophic cone collapse with magmatic processes accompanying the complete re-growth of the volcano. Parinacota is a 46 km3 stratovolcano in the Andean Central Volcanic zone that collapsed to produce a 6 km3 debris avalanche (3 times the size of the collapse at St. Helens), yet has regrown a post-collapse cone of 18 km3. This entire complex is extraordinarily well-preserved owing to the hyper-arid climate and low erosion rates. Parinacota has erupted a variety of high-K calc-alkaline basaltic andesite through rhyolite and is therefore an Ideal candidate to study changes in the magmatic system both before and after a major debris avalanche. We have combined field mapping and stratigraphy with a quantitative and comprehensive geochronology effort in order to understand the pre- and post-collapse history of the magmatic system. New 40Ar/39Ar ages from groundmass and whole-rock samples of 16 lava flows representing the most recent parts of cone-regrowth extend the useful range of this technique well into the Holocene. Previously, precise 40Ar/39Ar dating had been obtained only from sanidine in this age range. Since the greatest source of uncertainty in dating young samples with ~2-3 wt% K2O is low radiogenic 40Ar yield, analytical blanks and instrumental mass fractionation must be well-constrained to obtain precise and accurate ages. Immediately preceding the debris avalanche, andesitic and dacitic (58-64% SiO2) lava flows include abundant petrographic and geochemical evidence for a dynamic magma system, which was likely intermittently compartmentalized, and experienced periodic mixing. The youngest pre-collapse sample is a 20.0 ± 4.0 ka andesite (59% SiO2), constraining the upper limit of debris avalanche timing. Since only the outermost part of the modern cone is accessible by sampling, and these dates range from 8.1 ± 1.8 to 3.6 ± 1.1 ka, the majority of cone re-building must have occurred prior to about 8 ka. This indicates post-collapse eruption rates that averaged in excess of 1.2 km3/ky that were twice the rate of cone growth prior to the collapse, but comparable to peak eruption rates of other large stratovolcanoes studied in similar detail.