Evolution of Rhyolite at Laguna del Maule, a Rapidly Inflating Volcanic Field in the Southern Andes

The Laguna del Maule Volcanic Field (LdM) is host to both the foremost example of post-glacial rhyolitic volcanism in the southern Andes and rapid, ongoing crustal deformation. The flare-up of high-silica eruptions was coeval with deglaciation at 24 ka. Rhyolite and rhyodacite domes and coulees totaling 6.5 km3 form a 20 km ring around the central lake basin. This spatial and temporal concentration of rhyolite is unprecedented in the history of the volcanic field. Colinear major and trace element variation suggests these lavas share a common evolutionary history (Hildreth et al., 2010). Moreover, geodetic observations (InSAR & GPS) have identified rapid inflation centered in the western side of the rhyolite dome ring at a rate of 17 cm/year for five years, which has accelerated to 30 cm/yr since April 2012. The best fit to the geodetic data is an expanding magma body located at 5 km depth (Fournier et al., 2010; Le Mevel, 2012). The distribution of high-silica volcanism, most notably geochemically similar high-silica rhyolite lavas erupted 12 km apart of opposite sides of the lake within a few kyr of each other, raises the possibility that the shallow magma intrusion represents only a portion of a larger rhyolitic body, potentially of caldera forming dimensions. We aim to combine petrologic models with a precise geochronology to formulate a model of the evolution of the LdM magma system to its current state. New 40Ar/39Ar age determinations show rhyolitic volcanism beginning at 23 ka with the eruption of the Espejos rhyolite, followed by the Cari Launa Rhyolite at 14.5 ka, two flows of the Barrancas complex at 6.4 and 3.9 ka, and the Divisoria rhyolite at 2.2 ka. In contrast, significant andesitic and dacitic volcanism is largely absent from the central basin of LdM since the early post-glacial period suggesting a coincident basin-wide evolution from andesite to dacite to rhyolite and is consistent with a shallow body of low-density rhyolite blocking the eruption of less evolved magma. Temporal trends in the major element compositions of the rhyolite domes show the most evolved was erupted early in the post-glacial period followed by slightly lower-silica rhyolites. Major element fractional crystallization modeling using the rhyolite calibration of the MELTS algorithm (Gualda et al., 2012) largely reproduces the high-silica compositions from a basaltic parental composition. The preferred model predicts 86% crystallization while cooling from 1290° to 800° C at a depth of 5-8 km resulting in a water content of 4-6 wt. % in the residual high-silica magma. Trace element assimilation fractional crystallization modeling predicts only moderate assimilation of anatectic melts and fractionation of zircon and apatite controlling trace element compositions at high silica contents. The suite of recent LdM lavas lies on a single evolutionary pathway supporting a cogenetic source; furthermore, the model parameters are consistent with a shallow magma chamber with the potential to fuel an explosive, caldera-forming eruption.