276 D55. From Argon to Zircon: New Approaches to Understanding Crustal Magmatism

Wednesday, 7 November 2018
1:30 PM - 5:30 PM
Room 240-241

PETROCHRONOLOGY, GEOPHYSICS, AND GEOMORPHOLOGY REVEAL DYNAMICS OF AN EXTENSIVE, THERMALLY HETEROGENOUS, SHALLOW SILICIC MAGMA SYSTEM BENEATH LAGUNA DEL MAULE VOLCANIC FIELD, SOUTHERN ANDES

Paper number: 276-2
Program: Technical Sessions
Time: 1:40 PM – 1:55 PM

Authors

Abstract

To understand dynamics of the Laguna del Maule (LdM) magmatic system, that has erupted >40 km3 of rhyolite and rhyodacite in the last 20 kyr and has propelled surface inflation at >20 cm/yr since 2007, we are integrating petrochronology with geodesy, gravity, magnetotelluric, seismic, and geomorphic observations. Trace element compositions and U-Th dates of zircon reveal 160 kyr of magma emplacement and crystallization. Two compositionally distinct domains developed concurrently within the LdM magma reservoir, which produced separate episodes of rhyolitic eruptions at 23–19 and 8–2 ka. This reservoir has been imaged via seismic, gravity, and electrical resistivity methods. A 3-D shear-wave velocity model that delineates a ~450 km3 shallow magma reservoir (~2-8 km below surface) with an average melt fraction of ~5% is compatible with the gravity and geodetic observations. Apparent discrepancies between the seismic velocity model and an electrical resistivity model are reconciled by accounting for the relatively high bulk resistivity of a silicic crystal mush containing ≤5% interstitial melt. During the Holocene, a paleoshoreline was warped upward >60 m where rhyolitic eruptions, totaling ~9 km3, were concentrated. Based on a model of magma injection into an extant crystal-rich magma reservoir, we find that during the Holocene ~13 km3 of magma recharged the reservoir at ~7 km depth likely comprising several episodes similar to that driving the modern unrest. These observations imply that large, but mostly cool, systems are incubated and grow at shallow depth via localized, punctuated, high-flux injections of hot magma. The rates of magma intrusion derived from the shoreline model and the duration of zircon crystallization approach those required by thermal models to sustain a large eruptible magma body; yet, low Ti-in-zircon temperatures and plagioclase diffusion chronometry indicate that rhyolitic eruptions reflect rapid melt extraction (decades) from cool crystalline mush. On the other hand, zircon trace element zoning and modeling of diffusion-limited growth suggest melt was dominantly extracted from long-lived hot zones embedded within the much larger cool reservoir. These contrasting, coeval magma storage conditions obviate a simple hot vs cold storage dichotomy for large silicic magma systems.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-318188

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