HR: 0830h
AN: V31C-0949    [PDF]
TI: The Evolution of Silicic Magmatism in the Accreted Talkeetna Arc, South-Central Alaska: Implications for the Growth of Continents
AU: * Rioux, M
EM: mattrioux@umail.ucsb.edu
AF: University of California, Santa Barbara, Geological Sciences, Santa Barbara, CA 93106-9630 United States
AU: Hacker, B
EM: hacker@geol.ucsb.edu
AF: University of California, Santa Barbara, Geological Sciences, Santa Barbara, CA 93106-9630 United States
AU: Mattinson, J
EM: mattinson@geol.ucsb.edu
AF: University of California, Santa Barbara, Geological Sciences, Santa Barbara, CA 93106-9630 United States
AU: Kelemen, P
EM: peterk@whoi.edu
AF: Woods Hole Oceanographic Institution, Geology and Geophysics, Woods Hole, MA 02543 United States
AU: Plank, T
EM: tplank@bu.edu
AF: Boston University, Department of Earth Sciences 685 Commonwealth Avenue, Boston, MA 02215 United States
AU: Reiners, P
EM: peter.reiners@yale.edu
AF: Yale University, Department of Geology and Geophysics PO Box 208109, New Haven, CT 06520-8109 United States
AB: The accreted Talkeetna arc, south-central Alaska, presents a rare opportunity to study both the intrusive and extrusive record of an intraoceanic arc. The exposed arc section includes residual mantle harzburgites, mafic to ultramafic cumulates, mafic to felsic plutonic rocks, and related volcanic units. The plutonic section of the Talkeetna arc consists of $\sim$20-30%, km-scale, intermediate to felsic plutons. Evolved Talkeetna arc plutons have 57-77 wt% SiO$_{2}$ 0-1.4 wt% K$_{2}$O, and flat to slightly LREE-enriched REE patterns (Ce/Yb = 5-12). Our TIMS and SIMS U/Pb zircon analyses from arc plutonic rocks indicate crystallization ages of 181.3 $\pm$ 0.3 Ma, 181.4 $\pm$ 0.2 Ma, 184.0 $\pm$ 0.3 Ma, 184.9 $\pm$ 0.3 Ma, 186.0 $\pm$ 0.2 Ma, 186.1 $\pm$ 0.2 Ma, 190.4 $\pm$ 2.3 Ma, 192.4 $\pm$ 2.9 Ma, 193.2 $\pm$ 0.2 Ma, 193.8 $\pm$ 0.2 Ma, 198.6 $\pm$ 0.4 Ma, and 201.4 $\pm$ 0.3 Ma. The new ages suggest a relatively short arc lifespan of 20 Myr, similar to many modern arcs. New and existing $^{40}$Ar/$^{39}$Ar hornblende ages from the arc range from 181-175 Ma and five new replicated single-grain zircon U-Th/He analyses (T$_{c}$ $\sim$$180\deg$C) yielded cooling ages of 59 $\pm$ 5 Ma, 98 $\pm$ 8 Ma, 103 $\pm$ 8 Ma, 107 $\pm$ 9 Ma, and 137 $\pm$ 11 Ma. The $^{40}$Ar/$^{39}$Ar and He ages suggest that rapid post-magmatic cooling was followed by slow final exhumation through depths of 6-9 km between 137-60 Ma. In combination, the U/Pb zircon data and the geochemical analyses show that silicic plutons were intruded throughout the life of the arc and that there was no change in K$_{2}$O content with arc maturity. Several factors thus indicate that intraoceanic arcs have a more felsic bulk composition than commonly considered and may help explain the apparent discrepancy between bulk arc compositions and average continental crust: i) intermediate to felsic plutons comprise a significant fraction of the Talkeetna arc and were intruded throughout its history; ii) these plutons are similar in composition to the Tanzawa complex, Japan, which has been interpreted to be exhumed middle crust from the Izu-Bonin-Mariana arc (Kawate and Arima, 1998); and iii) we calculate P-wave speeds of 6.0-6.3 km/s for the intermediate to felsic Talkeetna arc plutons, similar to mid-crustal P-wave speeds measured in the active Izu-Bonin-Mariana arc (Suyehiro et al., 1996).
DE: 1020 Composition of the crust
DE: 1035 Geochronology
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting