The largest holocene eruption of the Central Andes found

Abstract

We present new data and interpretation about a major eruption -spreading 110 km3 ashes over 440.000 km2- long thought to have occurred around 4200 years ago in the Cerro Blanco Volcanic Complex (CBVC) in NW Argentina. This eruption may be the biggest during the past five millennia in the Central Volcanic Zone of the Andes, and possibly one of the largest Holocene eruptions in the world. The environmental effects of this voluminous eruption are still noticeable, as evidenced by the high content of arsenic and other trace elements in the groundwaters of the Chacopampean Plain. The recognition of this significant volcanic event may shed new light on interpretations of critical changes observed in the mid-Holocene paleontological and archaeological records, and offers researchers an excellent, extensive regional chronostratigraphic marker for reconstructing mid-Holocene geological history over a wide geographical area of South America. More than 100 ashes were sampled in Argentina, Chile and Uruguay during different field campaigns. Ash samples were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), grain size distributions laser diffraction, and geochemically by electron microprobe (EMPA) and laser ablation-HR-ICP-MS. New and published 14C ages were calibrated to calendar years BP. The age of the most recent CBVC eruption is 4407-4093 cal y BP, indirectly dated by 14C of associated organic sediment within the lower part of a proximal fall deposit of this event (26°53'16.05"S-67°44'48.68"W). This is the youngest record of a major volcanic event in the Southern Puna. This age is consistent with other radiocarbon dates of organic matter in palaeosols underlying or overlying distal ash fall deposits. Based on their products, all of rhyolitic composition, we have distinguished 8 main episodes during the evolution of the most recent CBVC eruption: 1) the eruption began with a white rhyolite lava dome extrusion; 2) followed by a Plinian proximal and distal dispersal of purely fallout (~110 km3, bulk volume); 3) the eruptive column collapsed, producing white co-ignimbrite lag breccia, ignimbrite flow deposits, and associated surge and ash cloud deposits (~1 km3); 4) a resurgent white rhyolite lava dome was extruded that 5) collapsed to produce several lateral blasts directed into the Cerro Blanco caldera that emplaced lithic-rich block-and-ash flow deposits; 6) a new pinkish rhyolite lava dome extruded and 7) also laterally collapsed forming new lithic-rich block-and-ash flow deposits within the same caldera; finally, 8) the development of a post-eruption geothermal field that produced white sinter deposits within the Cerro Blanco caldera. Financial support was provided by the QUECA Project (MINECO, CGL2011-23307).