GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 198-15
Presentation Time: 9:00 AM-6:30 PM


SCHAEN, Allen, Geoscience, University of Wisconsin-Madison, 1215 W. Dayton St, Madison, WI 53706, SCHOENE, Blair, Department of Geosciences, Princeton University, Guyot Hall, Princeton, NJ 08544, SINGER, Brad S., Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, DUFEK, Josef, Department of Earth Sciences, University of Oregon, 311 Ferst Drive, Eugene, OR 97403-1272, EDDY, Michael P., Department of Geosciences, Princeton University, Princeton, NJ 08544, JICHA, Brian R., Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton St., Madison, WI 53706 and COTTLE, John M., Department of Earth Science, University of California-Santa Barbara, 2028 Webb Hall, Santa Barbara, CA 93106-9630

High-silica rhyolite is thought to originate in the upper crust via extraction of melt from crystal-rich magma reservoirs, a process that has been implicated in some of the most catastrophic eruptions in Earth history. Shallow plutons that record high-silica melt segregation provide important connections to subvolcanic rhyolitic plumbing systems, complementing models which are dominated by studies of erupted volcanic products. We have previously reported bulk-rock geochemical, mineralogical, textural and in-situ petrochronologic evidence that the 150 km3 late Miocene (7.2–6.2 Ma) Risco Bayo-Huemul plutonic complex (36°S, Chilean Andes) preserves a record of high-silica melt segregation and complementary residual silicic cumulate formation. Here we present new bulk zircon trace element analyses (TEA) from the same aliquots as U-Pb CA-ID-TIMS dates, permitting the use of zircon crystallization through time as a tracer of magmatic processes. Huemul zircon display trace element trends consistent with fractional crystallization over ~190 kyr from 6.4 to 6.2 Ma. Silicic cumulate zircons record the older end of this population with lower Lu/Hf and higher Eu/Eu* than zircon from the high-silica granite that caps the complex and is in contact with exposed roof rocks. The youngest, most chemically evolved zircon are from two hand samples of high-silica granite 14 km apart which display indistinguishable TEA signatures. Synthetic bootstrapping of the U-Pb zircon dates from these hand samples suggest coeval zircon crystallization and melt extraction of ~15 km3 of rhyolite melt in ≤130 kyr, comparable to a volcanic timescale. We use a multiphase dynamics approach coupled with rhyolite-MELTS thermodynamics calculations to examine thermally viable scenarios for melt reservoir evolution and melt extraction. Combining this with a zircon saturation model, we examine the spatio-temporal distribution of extracted melt flux that has achieved zircon saturation.