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Geología del volcán Villarrica - Regiones de La Araucanía y Los Lagos (MAP) Villarrica volcano geology map, Chile Mapa geológico del volcán Villarrica, Chile
Abstract
The Villarrica volcano geology map 1:50.000 scale.
El volcán Villarrica es un centro eruptivo del Pleistoceno Medio a Holoceno ubicado en los Andes del Sur de Chile
(39,5°S). Durante su etapa temprana de evolución (Unidad Villarrica 1, Pleistoceno Medio a Superior) un estratovolcán
ancestral fue construido esencialmente a través de la emisión de lavas basálticas a andesítico-basálticas, lahares, flujos y
caída de piroclásticos. Hace ca. 100 ka este edificio ancestral colapsó parcialmente y generó una caldera de forma elíptica
de 6,5 por 4,2 km de diámetros (Caldera 1). Con posterioridad (95-14 ka) y durante la Glaciación Llanquihue, fueron emitidas
tanto lavas andesítico basálticas subglaciales como extensos flujos piroclásticos y caída de piroclastos, de composiciones
andesítico-basálticas y dacíticas (y mezcladas), así como un conjunto de domos y diques dacíticos, aunque no hay evidencias
claras de que durante este período se haya vuelto a construir un estratovolcán. Un segundo colapso, anidado en
el anterior, ocurrió hace ca. 14 ka (Caldera 2), con la generación de una serie de flujos piroclásticos de gran envergadura,
incluyendo la voluminosa Ignimbrita Licán (ca. 10 km3), lo que marcó un aumento en la explosividad eruptiva. Un nuevo
estratovolcán comenzó a edificarse en el borde noroccidental de las calderas 1 y 2, a través de sucesivas erupciones
efusivas y explosivas (Unidad Villarrica 2, 13.850-3.700 años AP). A los 3.700 años AP, la parte superior de este edificio
colapsó, y formó una caldera somital, circular, de 2,2 km de diámetro (Caldera 3), asociada a la erupción de la Ignimbrita
Pucón (ca. 5 km3), que marcó el término de la Unidad Villarrica 2. Poco después del colapso, un nuevo cono comenzó a
edificarse dentro de la caldera somital. Este cono se ha construido, y continua construyéndose, a través de una sucesión
de erupciones efusivas y explosivas (Unidad Villarrica 3, 3.700 años AP al Presente). El último evento importante explosivo
generador de flujos piroclásticos ocurrió hace ca. 530 años AP, con la generación de un pequeño flujo piroclástico dirigido
hacia el flanco norte del volcán. La actividad histórica del volcán Villarrica, sin embargo, se ha caracterizado esencialmente
por episodios de baja explosividad (hawaiiano a estromboliano), con la ocurrencia en el interior del cráter de un pequeño
lago de lava casi permanente, de profundidad variable, con actividad fumarólica continua.
We report whole-rock chemistry, mineral chemistry, and volatile content from Villarrica volcano’s major recent paroxysms and background activity. Composition of the volcanic products are basalt to basaltic andesite with whole-rock SiO 2 content between 50 and 56 wt%, and a mineralogy dominated by olivine (Fo 71-80 ), clinopyroxene (Mg# ~ 50) and plagioclase (An 60–80 ). Volatile contents in melt inclusions are up to 1.5 wt% H 2 O, 500 ppm CO 2 , 1230 ppm sulfur and 580 ppm chlorine. Regardless of the type of activity, there are no substantial variations in whole-rock composition or the volatile content when the activity switches from background activity to a major paroxysm, strongly suggesting that this shift does not just depend on the arrival of new magma in the shallow magmatic system. Geothermobarometry constrains crystallization of the major mineral phases at various depths between 3 and 12.7 km, suggesting that degassing of a volatile-rich recharge magma occurs deeper than 12 km, producing efficient mixing throughout the whole system, and sustaining the lava lake activity in Villarrica’s summit crater. The occurrence of a permanent lava lake also suggests that the magma recharge must be close to continuous and therefore sudden changes between background and paroxysmal volcanic activity are likely controlled by relatively small changes in the rate of recharge and/or the volatile release rate in the magmatic system. This has important implications for the understanding of eruption triggers and the forecasting of volcanic eruptions.
Graphical abstract
A key method to investigate magma dynamics is the analysis of the crystal cargoes carried by erupted magmas. These cargoes may comprise crystals that crystallize in different parts of the magmatic system (throughout the crust) and/or at different times. While an individual eruption likely provides a partial view of the sub-volcanic plumbing system, compiling data from multiple eruptions can build a picture of a whole magmatic system. In this study we use machine learning techniques to analyze a large (>2000) compilation of mineral compositions from a highly active arc volcano: Villarrica, Chile. Villarrica’s post-glacial eruptive activity (14 ka–present) displays large variation in eruptive style (mafic ignimbrites to Hawaiian style effusive eruptions) yet its eruptive products have a near constant basalt-basaltic andesite bulk-rock composition. What therefore, is driving explosive eruptions at Villarrica and can differences in storage dynamics be related to eruptive style? Here we use hierarchical cluster analysis to detect previously unseen structure in the composition of olivine, plagioclase and clinopyroxene crystals erupted at Villarrica, revealing the presence of compositionally distinct clusters within each crystal population. Using rhyolite-MELTS thermodynamic modeling we related these clusters to intensive magmatic variables: temperature, pressure, water content and oxygen fugacity. Our results provide evidence for the existence of multiple discrete (spatial and temporal) magma reservoirs beneath Villarrica where melts differentiate and mix with incoming more primitive magma. The compositional diversity within an erupted crystal cargo strongly correlates with eruptive intensity, and we postulate that mixing between primitive and differentiated magma drives explosive activity at Villarrica.
Villarrica (Chile) is a basaltic stratovolcano, currently in an open-conduit condition. It now has relatively frequent Strombolian and effusive eruptions, but it had large explosive eruptions in prehistoric times. Among them, the most recent eruption was Chaimilla, which occurred about 3100 years ago and produced deposits that indicate complex, multiphase eruptive dynamics. Significant differences in mineralogy and glass compositions of the erupted scoria suggest the eruption was fed by two distinct magma batches with similar bulk compositions but distinct crystallization and degassing histories. The lower sequence scoria has a complex crystal assemblage with several crystal populations produced by mixing between a relatively degassed magma containing Fo 75-79 olivine, normally or reversely zoned plagioclase (An 70-94) and augite (type 1 magma), and a subordinate volume of more-primitive and more volatile-rich magma rising from depth (type 2 magma) and carrying normally zoned pla-gioclase and higher-Mg (Fo 81-85) olivine crystals. Type 2 magma was the main component emitted during the larger and more explosive eruptive phase that deposited the upper sequence. The Chaimilla eruption occurred under closed-vent conditions and was fed by water-rich magmas. When compared with the petrological features of the magma currently erupted at Villarrica, which has slightly more-evolved bulk compositions, lower crystal content and lower water content, these results suggest that the evolution in eruptive style of the volcano from highly explosive to a lava lake/Strombolian activity corresponds to significant changes in the shallow plumbing system (which is now at much shallower depths); these plumbing-system changes were not associated with significant changes in the parental magma compositions.
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