CONCURRENT GROWTH OF TWO CHEMICALLY DISTINCT SILICIC MAGMA RESERVOIRS BENEATH THE LAGUNA DEL MAULE VOLCANIC FIELD, CENTRAL CHILE

Coupled zircon crystallization ages and compositions are powerful tracers of the growth and evolution of silicic magma reservoirs. Compositionally and isotopically zoned crystals can be derived from the recycling of previously emplaced silicic magma or assimilation of the continental crust, the chemical evolution of a magma system through time, or the development of heterogeneous magma reservoirs within a larger magmatic province. Varied silicic magma batches stored coevally within the middle to upper crust can then be either erupted successively through time or amalgamated prior to a large, caldera-forming eruption.

The Laguna del Maule volcanic field (LdM), central Chile, is an exceptional example of post-glacial rhyolite volcanism and sustained volcanic unrest reflecting the presence of a large, active silicic magma system in the shallow crust. New ion probe ²³⁰Th-²³⁸U crystallization ages and trace element compositions of LdM zircon reveal that the assembly of the currently restless silicic magma system began approximately 40,000 years ago. It has produced two periods of concentrated rhyolite volcanism within overlapping spatial extents at c. 23–19 ka and 14.5–<1.8 ka. Zircon compositions show these eruptive episodes are derived from physically and chemically discrete magma reservoirs. Despite producing eruptive flare-ups at different times, overlapping zircon age spectra indicate these reservoirs grew concurrently.

Most LdM zircons are characterized by overgrowths, dark in cathodoluminescence (CL) images, mantling lighter CL cores. The compositions of the volumetrically dominant dark CL crystal domains are enriched in trace elements suggesting rapid crystal growth and disequalibrium mineral-melt partitioning. Accordingly, the relatively continuous distribution of zircon ages belies crystal growth dominated by discrete episodes of rapid crystallization rather than gradual down-temperature evolution. These episodes are likely catalyzed by repeated shallow magma emplacement during the late Pleistocene and Holocene that resulted in the voluminous system beneath LdM that remains active to this day.