EVOLUTION OF A LARGE, HOT, RESTLESS RHYOLITIC MAGMA SYSTEM AT LAGUNA DEL MAULE, CHILE

The Laguna del Maule Volcanic Field (LdM) is the foremost example of post-glacial rhyolitic volcanism in the Southern Andes. New 40Ar/39Ar ages indicate that silicic eruptions began at 25 ka, flared-up with deglaciation at 19 ka, and persisted until <2 ka. These rhyolitic and rhyodacitic domes and coulees total 6.5 km3 and encircle the 23 by 16 km lake basin. This is not only the greatest concentration of post-glacial rhyolite in the Andes, but there has been no comparable rhyolite flare up this recently anywhere else on Earth. Colinear major and trace element variation suggests these lavas share a common evolution (Hildreth et al., 2010). Moreover, InSAR and cGPSgeodesy indicate that LdM is inflating at a rate of 30 cm per year since 2007. Modeling predicts that an expanding magma body at 5 km depth is driving inflation (Feigl et al., in press). The distribution of high-silica rhyolite lavas erupted on opposite sides of the lake basin within a few kyr of each other, magnetotelluric data showing a zone of low resistivity at 5 km depth, and numerous local volcano-tectonic and long-period earthquakes shallower than 5 km, suggest that magma intrusion is impacting only a portion of a much larger rhyolitic body, potentially of caldera-forming dimensions. Significant andesitic and dacitic volcanism is absent from the LdM basin since the early post-glacial silicic flare up began suggesting that a large body of low density rhyolite has blocked mafic magmas from reaching the surface. The most evolved lava was erupted early in the post-glacial period followed by slightly lower-silica rhyolites. Two oxide thermometry reveals that the rhyolites tapped highly oxidized, hot melt stored at 760 to 850 oC and fO2 at log QFM+2. EC-AFC modeling reproduces the high-silica compositions from a basaltic parent magma. The preferred model predicts 73% crystallization (plag+cpx+mt+opx+ilm+ap+bio+zir) while cooling from 1190 to 830 oC at a depth of 5 km, resulting in 4-6 wt. % H2O in the residual melt. The evolution of rhyolite is complicated by basaltic recharge, assimilation of wall rock and cumulate melts, and trace element diffusion. The magmatic reservoir is currently experiencing a recharge event that is driving the extraordinary surface inflation, raising the potential for either a modest explosive eruption, or a caldera-forming super-eruption.