Lanín volcano (39.5°S), Southern Andes: geology and morphostructural evolution

Lanín volcano is a compound stratocone, mainly effusive, made up by four units defined through morphological criteria. The first unit represents an ancient volcano; the youngest three units form the present stratocone built since the Middle-Late Pleistocene. Compositionally, volcanic rocks from Lanín Volcano are mainly basalts/basaltic andesites and dacites with scarce intermediate types. Postglacial pyroclastic deposits are also silicic and confirm a sharp bimodality of the magmas. Major oxides and REE patterns suggest a low-pressure magmatic evolution dominated by fractional crystallization of plagioclase and orthopyroxene with extraction of olivine, clinopyroxene and magnetite without complex interactions. The effusive eruptive cycles would be controlled by a short residence in a shallow magma chamber with rapid and coeval evacuation of dacites and basalts. In recent eruptions, viscous magma would have sealed the central conduit inducing the lateral drainage of basalts and, possibly, the partial collapse of the upper part of the cone. Nevertheless, the most active degradational processes are those related to the ice-cover condition of the present stratocone. The singular evolution of Lanín volcano, geochemically and morphologically intermediate between the monogenetic cones and the stratovolcanoes of the Villarrica-Lanín chain, could be related to its distance to the trench which causes low degree of partial melting in the source and the ascent of small batches of magma that would be stored in an ephemeral magma chamber El volcán Lanín (39,5°), Andes del sur: geología y evolución morfoestructural. El volcán Lanín es un estratovolcán compuesto, predominantemente efusivo, para el que se han definido cuatro unidades según criterios geomorfológicos. La primera de ellas representa restos de una estructura ancestral; las tres siguientes constituyen el edificio volcánico actual construido esencialmente desde el Pleistoceno Medio-Superior. Composicionalmente, las rocas volcánicas del volcán Lanín corresponden, principalmente, a basaltos/andesitas basálticas y dacitas subordinadas con escasas variedades intermedias. Los depósitos piroclásticos post glaciales muestran también composiciones silíceas y confirman una estricta bimodalidad composicional de los magmas. Los patrones de tierras raras y elementos mayores indican una evolución magmática de baja presión controlada por fraccionamiento de plagioclasa y ortopiroxeno con extracción de olivino, clinopiroxeno y magnetita, sin interacciones complejas. Los ciclos volcánicos efusivos serían controlados por un red