HR: 0800h
AN: V51D-1514    [Abstracts]
TI: Revised ⁴⁰Ar/³⁹Ar age of Aleutian Island arc formation implies high rate of magma production
AU: * Jicha, B R
EM: bjicha@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison 1215 W. Dayton St., Madison, WI 53706 United States
AU: Scholl, D W
EM: dscholl@pangea.stanford.edu
AF: Moss Landing Marine Laboratories, California State University System, Moss Landing, CA 95039 United States
AU: Singer, B S
EM: bsinger@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison 1215 W. Dayton St., Madison, WI 53706 United States
AU: Yogodzinski, G M
EM: gyogodzin@geol.sc.edu
AF: Department of Geological Sciences, University of South Carolina 701 Sumter St., Columbia, SC 29208 United States
AU: Kay, S M
EM: smk16@cornell.edu
AF: Department of Geological Sciences, Cornell University Snee Hall, Ithaca, NY 14853
AB: ⁴⁰Ar/³⁹Ar incremental heating data from subaerial and submarine volcanic and plutonic rocks in the Aleutian Island arc provide insight into the timing of arc formation in the late Eocene. Groundmass and plagioclase separates from the Finger Bay volcanics, the oldest exposed rocks in the arc, gave a weighted mean isochron age of 37.4 ± 0.6 Ma, that is 12-17 m.y. younger than a widely cited age of 55-50 Ma. Twenty-six ⁴⁰Ar/³⁹Ar ages agree with existing K-Ar ages and constrain the duration of arc magmatism to the last 40 m.y. The initiation of magmatism at this time is in agreement with the late Eocene to early Oligocene ages for the fossiliferous sequence of sedimentary deposits on northern Adak Island that overlie 37-38 Ma units, yet it is at odds with all of the existing models of Aleutian arc formation because no major tectonic events in the north Pacific occurred at that time. We have also identified three main pulses of arc-wide plutonism and volcanism at 38-29, 16-11, and 6-0 Ma. The geochronology--in concert with new-generation transverse and arc-parallel seismic constraints on the composition and structure of the Aleutian Island arc and volumetric estimates of crust generated and eroded over the last 40 m.y.--leads to astonishingly high time-averaged magma production rates of 110-205 km³/km/m.y. for the entire arc. This exceeds magma production rates based on older geophysical and petrologic paradigms for the Aleutian arc by almost an order of magnitude. Because the majority of crustal growth likely occurred during the first few m.y. of the arc's history, magma productivity may have been as high as that of mid-ocean ridge spreading centers or continental batholiths (e.g., near 1000 km³/km/m.y.). Rapid Eocene arc growth has recently been proposed for both the Izu Bonin-Mariana and Tonga island arcs in the western Pacific Ocean. Determining whether extraordinarily high rates of island arc magma production in the Eocene reflects increased plate velocities and subduction fluxes into the mantle, availability of exceptionally fertile mantle below these arcs, or other factors, is critical to understanding the past and present geodynamics of the entire Pacific region.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1115 Radioisotope geochronology
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005