Collaborative Research: Testing the Timescale and Geometry of Incremental Pluton Assembly Through 3-D Modeling and Thermochronology

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This research is designed to test the hypothesis that plutons record geochronologic, thermochronologic and field evidence for incremental assembly. Although suggested by theoretical considerations, geochronologic and field data in support of incremental growth (particularly over millions to tens of millions of years) are hotly debated. Hornblende, biotite and potassium-feldspar argon thermochronology of rocks in the Sierra Nevada batholith are being used to address several broad questions. First, slow incremental pluton assembly has largely been inferred from uranium-lead zircon ages, and argon-argon dates from the same samples are providing a robust test of the reliability of the uranium-lead results. Second, if large "homogeneous" plutons were assembled incrementally, then contacts between increments have been overlooked in the field, and new mapping using bulk rock magnetic susceptibility is being used to identify cryptic contacts. Finally, modeling pluton and wall rock temperature histories indicates that cooling paths are sensitive to both the rate and the geometry of incremental pluton assembly. Therefore, cooling histories based on the zircon and titanite uranium-lead systems, and hornblende, biotite and potassium-feldspar argon systems, are being compared to modeled cooling histories in order to resolve spatial arrangements of intrusive increments and the rates at which they are added. This research is addressing the physical and chemical evolution of magma chambers, including mechanisms of intrusion, development of pluton fabrics, and permissible magmatic differentiation processes. The complex thermal histories of plutons and their wall rocks suggested by our modeling bear on interpretation of hornblende argon-argon dates from plutons; the maximum temperatures and durations of contact metamorphism; the thermal, structural and petrologic evolution of early intrusions as they become the wall rocks for later intrusions; the interpretation of paleomagnetic data collected from plutons; and the extent to which formation of large-volume eruptible magma bodies is recorded by plutons. This research focuses on the rates of assembly of magma chambers that feed volcanoes at the surface of the Earth. It employs a variety of dating techniques that act as checks and balances on each other in an effort to understand how, and how fast, the magma bodies are constructed, and their potential for catastrophic eruption. The incremental assembly hypothesis predicts that growth of "super volcano" magma chambers will be profoundly different than the construction and thermal histories of magma chambers that feed other volcanoes. We should be able to use this understanding to assess the potential for catastrophic eruption of huge volcanic complexes such as Yellowstone. The research is supporting the Ph.D. research of a student from the University of North Carolina, as well as the educational activities of other M.Sc. and undergraduate students at the University of North Carolina and the University of Utah. The project includes the development of new collaborative ties between Los Alamos National Laboratory, New Mexico Tech, University of North Carolina, and the University of Utah. The research is also an ongoing part of our support of the National Parks. It is contributing to our efforts to update the geology exhibits in Yosemite National Park, and play a role in Ranger training for the Park.