CERRO BARRANCAS, LAGUNA DEL MAULE VOLCANIC FIELD, CHILE: unraveling its eruptive history to constrain hazard assessment

Abstract:

The postglacial record of the Laguna del Maule Volcanic Field (LMVF) (36° 10´S, 70° 30’ W) includes rhyolite and rhyodacite lava flows, domes, tephra falls and pyroclastic flows that issued from ~40 vents. Ongoing uplift of >20 cm/year since 2007 makes unraveling its eruptive history imperative in a hazard assessment. The Barrancas center in the southeastern part of LMVF has been an especially productive and long-lived vent complex. Much of its ~6 km³ postglacial eruptive volume is preserved in the headwaters of Río Barrancas, Argentina. Detailed mapping, combined with petrographic, grain-size, textural and geochemical analyses and new Ar/Ar dating reveal a complex eruptive stratigraphy. An early episode of multi-stage dome building (75.7% to 72% SiO₂) constructed a western complex dated between ~14.5 ka and ~11.4 ka. Partial dome collapse produced block-and-ash flows that traveled ~13 km from source, filling the Puente de Tierra, La Parva and Curamilio valleys with deposits ~60 m thick. Based on geomorphology and hydrothermal alteration, at least 2 main domes have been distinguished. Dome 1 is highly affected by advanced argillic alteration with a mineral assemblage dominated by alunite, jarosite, opal and montmorillonite. Dome 2 shows cm-wide veins of diopside-andradite with associated albite, calcite and quartz and disseminated magnetite-hematite. This silicatic type of alteration is indicative of high heat flux (>300°C). Hydrothermal alteration affects both the dome and the block-and-ash deposits suggesting that it may have triggered the explosive event. A younger, eastern volcanic sequence (73% to 75% SiO₂) overlaps the western one and includes 8 obsidian flows, 2 of which have been dated at 5.6 ka and 1.85 ka and several tephra falls and pyroclastic flows as old as ~11 ka, related to activity of a partially preserved pumice cone. In case of reactivation, Cerro Barrancas could have widespread effects in Chile and Argentina, particularly in southern Mendoza and northern Neuquen provinces. Due to its remote location, hazard assessment is focused on tephra fall and pyroclastic flow distal impact. Particularly, Laguna Fea, which has been dammed by the block-and-ash deposits, is highlighted as a major threat to the Barrancas valley.