HR: 16:00h
AN: V44C-01 INVITED    [Abstracts]
TI: Zircon U-Pb Ages Chronicle 3 Myr of Episodic Crystallization in the Composite Miocene Tatoosh Pluton, Mount Rainier National Park, Washington Cascades
AU: * Bacon, C R
EM: cbacon@usgs.gov
AF: USGS, 345 Middlefield Rd, Menlo Park, CA 94025, United States
AU: du Bray, E A
EM: edubray@usgs.gov
AF: USGS, Denver Federal Center, Denver, CO 80225, United States
AU: Wooden, J L
EM: jwooden@usgs.gov
AF: USGS, USGS-Stanford Ion Probe Lab, Stanford, CA 94025, United States
AU: Mazdab, F K
EM: fmazdab@usgs.gov
AF: USGS, USGS-Stanford Ion Probe Lab, Stanford, CA 94025, United States
AB: Zircon geochronology of upper crustal plutons can constrain longevities of intermediate to silicic magmatic systems. As part of a larger study of the geochemistry and metallogeny of Tertiary Cascades magmatic arc rocks, we used the USGS-Stanford SHRIMP RG to determine 20 to 28 ²³⁸U-²⁰⁶Pb ages for zircons from each of 6 quartz monzodiorite (qmd), quartz monzonite (qm), or granodiorite (grd) samples representative of the Tatoosh pluton, and one grd from the nearby Carbon River stock. The 7x12 km composite Tatoosh pluton, discontinuously exposed on the south flank of Mount Rainier, consists of at least 4 petrographic/compositional phases, here termed Pyramid Peak, Nisqually, Reflection Lake, and Tatoosh. These collectively intrude gently folded and weakly metamorphosed basaltic andesite flows and volcaniclastic rocks of the Eocene Ohanapecosh Formation, silicic ignimbrites and sedimentary rocks of the Oligocene Stevens Ridge Formation, and basaltic to intermediate volcanic rocks of the Miocene Fifes Peak Formation. Histograms and relative probability plots of U- Pb ages indicate 2 to 4 age populations within each sample. The weighted mean age of each of the youngest populations (all ±2σ) is interpreted as the time of final solidification: Pyramid Peak qmd (58.5% SiO₂) 17.4±0.2 Ma, Nisqually grd (in Paradise Valley; 65.4% SiO₂) 16.7±0.2 Ma, Nisqually grd (at Christine Falls; 66.4% SiO₂) 17.3±0.2 Ma, Reflection Lake qm (along Pinnacle Peak trail; 66.6% SiO₂) 17.1±0.2 Ma, Tatoosh grd (in Stevens Canyon; 67.8% SiO₂) 18.2±0.2 Ma, Tatoosh grd (south of Louise Lake; 69.3% SiO₂) 19.3±0.1 Ma, and Carbon River grd (68.0% SiO₂) 17.4±0.3 Ma. The older Nisqually grd age is indistinguishable from a TIMS zircon age of 17.5±0.1 Ma reported by Mattinson (GSA Bulletin 88:1509-1514, 1977) for grd from a nearby locality. None of the 164 SHRIMP-RG U-Pb ages, including cores, is older than 21 Ma. The relatively small, high-level pluton likely was emplaced and solidified in pulses; zircons with ages significantly greater than the final solidification age of a given sample are considered to be antecrysts recycled from earlier crystallization episodes within the larger magmatic system. Although interpretation of the age populations is subjective, we have identified 17.4-Ma antecrysts in the 16.7-Ma Nisqually grd and antecrysts of 1-3 ages averaging 18.1, 18.8, 19.3, and 20.0 Ma in the other samples. Notably, age populations are separated by 0.5-0.7 Myr, intervals similar to lifetimes of large arc volcanoes. The total duration of pluton assembly is ~2.6 Myr, or as much as ~3.6 Myr if the oldest antecrysts are considered. The oldest, most differentiated rocks are on the east side of the pluton (Tatoosh phase) and the youngest, least differentiated on the west (Pyramid Peak phase). Two samples each of the Nisqually and Tatoosh phases yield apparent crystallization age differences of 0.7- and 1.1-Myr, respectively, for petrographically and chemically similar rocks. The Tatoosh U-Pb data appear to chronicle repeated waxing and waning of a long-lived igneous system as recorded by crystal-rich magma that periodically solidified near the system's roof at the close of well-defined crystallization episodes.
DE: 1100 GEOCHRONOLOGY
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3618 Magma chamber processes (1036)
DE: 3640 Igneous petrology
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting