Intellectual Merit. Continental magmatic arcs are the site of massive additions of mantle derived magmas which serve to drive melting, thickening, and reorganization of the crust. Much of what we know about Cretaceous and older continental magmatic arcs results from studies of the physics and chemistry of plutonic rocks, and the derivative data strongly influence models of crustal growth and evolution. In contrast to modern arcs where single eruptive units may be analyzed, plutons record a much richer, albeit complex, history and represent a much greater percentage of the total magmatic flux than volcanic rocks alone. Critical questions concern how and at what rate intermediate composition magmas move through the crustal column, and how the crust accommodates these potentially large fluxes? Key to answering these questions is to understand how large masses of crystallized magmas or plutons and batholiths are constructed. To address these questions, collaborative research is proposed to better understand sub-arc magmatic plumbing systems. This project focuses on the roots of a continental magmatic arc in the North Cascades that comprises three well-exposed intrusive bodies emplaced at different depths in the crust. Integrated field mapping, structural analysis, mineral geochemistry, high-precision geochronology and coupled isotopic studies will be employed to [1] determine the degree to which plutons represent single magma batches or complex mixtures of partially crystallized magmas of different origins, [2] estimate magma flux rates and timescales for melt generation, transport, and eruption of intermediate composition magmas, and [3] evaluate potential linkages between regional tectonic strain and magma formation. This multidisciplinary collaborative research is expected to contribute significantly to understanding of magma plumbing systems and the formation of plutons in continental magmatic arcs.
Broader Impacts. This research will support Ph.D. students at MIT and USC, and at least 2 M.S. students at SJSU, and multiple undergraduate student assistants at all 4 institutions. Bpth SJSU PIs have supervised 2 B.S. research projects supported by their most recent grants, and Paterson teaches a formal research class at USC that has involved >20 undergraduates in NSF-supported research. Graduate students from SJSU and USC will visit Kent's lab at OSU to conduct their own analyses, and the MIT graduate student will visit SJSU and assist in SHRIMP analyses. Four M.S. students at SJSU have gone to MIT to carry out U-Pb dating under the tutelage of PI Bowring and his students, and this type of interaction is expected to continue. All of the PIs routinely incorporate their research in class lectures and use research samples in lab courses. Most importantly, The PI's are demonstrating to a large cross section of new students the power of strongly interdisciplinary and collaborative work.
The goal of this project was to examine the temporal pattern of the non-steady state, coupled processes of subduction, orogeny, magmatism, exhumation, and erosion in a continental margin orogen and associated magmatic arc at 5-30 km in the Cascades core, Washington. We were also particularly keen to examine the geochemical histories of various minerals at different crustal levels to examine the histories recorded by these minerals in rising and interacting magma pulses. To reach these goals a large number of geochronologically constrained databases is needed and we continue to work on the establishment and synthesis of these databases as listed below: 1. We have completed LA-ICPMS detrital zircon geochronology from a number of host rock units in this region including the ~120 Ma Chiwaukum Formation, the ~192 to >220 Ma Cascades-Holden unit, the ~210 Napeequa Formation, and the ~73 Ma Swakane Biotite Gneiss. These host rock ages, when combined with the previously obtained structural data and plutonic ages, provides us we a nice summary of the ages of deformation and tectonic amalgamation of units in this orogenic belt. This is allowing us to examine questions about the episodicity of deformation as well as magmatism. 2. We continue to collect new, and synthesize pre-existing whole rock elemental and isotopic geochemical data across the Cascades core. Results are varied but typically give fairly primitive "oceanic" values and clearly indicate a regional relationship between age and isotopic value both regionally and in individual plutons. These data are providing insights into the incremental evolution of single magmatic systems, plus how the entire arc evolved through time. 3. We continue to collect and synthesize regional structural data and link these to ages and pressure-temperature estimates from both metamorphic assemblages and intruded plutons. These help constrain temporally evolving deformation histories and vertical motion of crust during this deformation. 4. We began, but have not yet completed our single mineral geochemical studies of ~90-95 Ma plutons and are now vigorously pursuing these. Our Oregon State colleagues have obtained excellent mineral data from the ~30 km deep Tenpeak pluton and we are now analyzing minerals from surface volcanics (Midnight Peak unit), the 3-7 km deep, tilted Black Peak Batholith, the ~6-10 km deep Mt. Stuart batholith, the ~18-23 km deep Seven Fingered Jack pluton, and the 20-30 km deep Dirty Face pluton. Once complete we can compare preserved geochemical histories from 0-30 km depth in this former arc. Broader Impacts: Three USC Ph.D. students received extensive training in conducting research, project management, proposal writing and in teaching undergraduate researchers, partly while working in the Cascades. Several undergraduate Earth Sciences majors from USC and other international institutions received training in the planning, initiation, and completing research projects in the Cascades core. They also got to interact with a larger international group and thus experienced useful collaborative research skills. Summary Our Cascades studies of the episodic nature of both tectonism and magmatism is allowing us to better understand how these magma-rich, orogenic belts form and evolve over time. This Cascade study is also just one part of our regional studies of Mesozoic magmatism and orogenic episodicity along the entire North and South American margin. We are linking these episodic tectono-magmatic processes and associated intense volcanism to the long-term evolution of mountains, changes in paleo-climate and plate motions as well as exploring why this episodicity occurs (e.g., internal feedback cycles or external forcing events). A magmatism-tectonics-biosphere lineage certainly exists. Intense magmatic flare-ups are of relatively short duration (<20-30 m.y.) separated by magmatic lulls (40-50 m.y.). Similar cyclicity is recognized for the continental deformation in these belts with periods of higher and lower deformation rates. Our studies of single mineral chemistries at different crustal depths is still at a preliminary stage, in part because the mineral geochemistry has proved to be highly varied from mineral core to rim and between adjacent minerals. We thus are still learning a great deal about how to read and interpret the different chemical signals in different zoned minerals. We certainly can already state that crystal mixing resulting in diverse crystal populations is widespread in these magma columns. As we expand our understanding of these crystal records we can better track processes (e.g., source melting, mixing, fractionation, eruptions) in evolving magmatic systems.
