Linking Plutons to Volcanism
High silica magmas cause the most explosive and potentially destructive volcanic eruptions on Earth. Studies of volcanic rocks have shed light on the processes that occur in magma chambers prior to explosive eruptions. However, many of these processes have not been fully documented in intrusive magmatic rocks, some of which represent fossil magma chambers. My research is focused on identifying the plutons, or parts of plutons, that represent coherent fossil magma chambers and using these natural laboratories to study the processes that ultimately lead to eruption.
Meter-scale granodioritic enclaves (dark rock) within the Golden Horn batholith. The enclaves represent hotter, less evolved magma that intruded into a crystal-rich granitic magma and cooled quickly. If you look closely you can see crystals that were entrained in the enclaves during intrusion.
Golden Horn Batholith, WA
I have an ongoing project documenting the history of the Eocene Golden Horn batholith in Washington's North Cascades with Bob Miller (San Jose State University), Jeff Tepper (University of Puget Sound), and Akshay Mehra (Princeton). My geologic mapping has shown that the batholith is composed of a series of sills ranging in composition from granite to granodiorite. High-precision U-Pb zircon geochronology from these sills show that the batholith was intruded rapidly over 739 ± 34 kyr, with the most voluminous sill (> 424 km3 intruding over 26 ± 25 kyr. The implied emplacement rate is compatible with the rates needed to build large, eruptible reservoirs of silicic magma in thermal models and it represents the first documented instance of a large silicic pluton being assembled at these rates (Eddy et al., 2016). Ongoing work is focused on linking chemical and textural variation within this sheet to the process of crystal-liquid separation, as well as using isotopic data to determine the source of this unique intrusion.
Geologic map and cross section of the Golden Horn batholith reproduced from Eddy et al. (2016). A: Location of the batholith (star) relative to other Mesozoic and Paleogene granitic intrusions in the North American Cordillera. B: Geologic map and sample locations. C: Cross section through the batholith from the NW to the SE.
Exposure of the Golden Horn batholith's roof on Mt. Azurite. Seen as dark metasedimentary rocks overlying lighter colored granite. The extreme topographic relief made it possible to calculate robust volumes for each intrusive phase of the batholith.
The largest and most rapidly assembled phase of the batholith is exposed over >1.5 km of vertical relief on Mt. Hardy. Using samples collected in 2016, we are currently assessing whether there is vertical variation in texture and geochemistry in this area.
Searchlight Pluton, NV
I have recently started working on the Searchlight pluton in southern Nevada with Blair Schoene and Ayla Pamukcu at Princeton University. This Miocene intrusion is exposed in a steeply tilted fault block that contains includes rocks from 0-12 km paleodepth and an overlying section of cogenetic volcanic rocks. Our goal is to expand on the previously published petrologic studies of the Searchlight pluton by producing new geochemical and high-precision U-Pb zircon geochronologic data. We will use these analyses to assess the timing and extent of magmatic differentiation in any potential large, coherent magma body that formed during the pluton's construction. A unique aspect of this study is that we will be able to compare changes in eruptive composition and style to changes within the pluton at the <10 kyr scale.
A thick package of rhyolitic tuffs overlying the Searchlight pluton. The paired intrusive and extrusive record of this magmatic system may help shed light on the processes that lead to eruption and the timescales over which they occur.