HR: 1340h
AN: V13B-1482    [Abstracts]
TI: Magmatic Differentiation in the Accreted Talkeetna Arc, South-Central Alaska
AU: * Rioux, M
EM: mrioux@umail.ucsb.edu
AF: University of California, Santa Barbara, Geological Sciences, Santa Barbara, CA 93106 United States
AU: Kelemen, P
AF: Lamont Doherty Earth Observatory, 58 Geochemistry Building, Palisades, NY 10964 United States
AU: Mattinson, J
AF: University of California, Santa Barbara, Geological Sciences, Santa Barbara, CA 93106 United States
AU: Hacker, B
AF: University of California, Santa Barbara, Geological Sciences, Santa Barbara, CA 93106 United States
AU: Blusztajn, J
AF: Woods Hole Oceanographic Institution, Geology and Geophysics, Woods Hole, MA 02543 United States
AB: Geochemical studies of island-arc magmatism have principally focused on the volcanic carapace of arcs because of the limited exposures of intrusive rocks in active arc settings. As a result, our understanding of how mantle-derived magmas evolve within island arcs is largely based on geochemical trends of arc volcanic suites. The accreted Talkeetna arc, however, exposes a nearly complete crustal section and provides an opportunity to compare both the plutonic and volcanic trends from a Jurassic intraoceanic arc. Our ongoing research on the Talkeetna arc includes U/Pb zircon dating and geochemical analyses that provide the basis for modeling magmatic differentiation within the arc. U/Pb zircon ages from the Chugach Mountains record active Talkeetna arc magmatism from 201-180 Ma, which is consonant with biochronology that constrains arc-related volcanism to Late Triassic to early Bajocian (206-169 Ma). Plutonic ages from the Alaska Peninsula (183-164 Ma) and the Talkeetna Mountains (177-156 Ma) suggest a northward shift in arc magmatism at $\sim$180 Ma, with continued plutonism into the Late Jurassic. Radiogenic isotope data from the Chugach Mountains record limited isotopic variation of $^{87}$Sr/$^{86}$Sr$_{initial}$ = 0.7032-0.7037 and $^{143}$Nd/$^{144}$Nd$_{initial}$ = 0.512707-0.512743. The restricted range in the isotopic ratios is similar to primitive modern arcs such as the Izu-Bonin-Mariana arc and demonstrates the juvenile nature of the main section of Talkeetna plutonic rocks. Isotopic ratios from the Talkeetna Mountains define a wider range of $^{87}$Sr/$^{86}$Sr$_{initial}$ = 0.7035-0.7053 and $^{143}$Nd/$^{144}$Nd$_{initial}$ = 0.512627-0.512735. The geographic distribution of the data suggests that the more evolved ratios may reflect assimilation of an older crustal component. The primitive isotopic ratios within the Talkeetna arc are consistent with an oceanic origin, while the slightly enriched isotopic ratios in the Talkeetna Mountains may reflect either a collisional event or a pre-existing crustal boundary that was intruded by arc magmas. The rare exposures in the Talkeetna arc provide direct constraints on models of magmatic differentiation. Greene et al. (2002) used least-squares calculations to reproduce Talkeetna arc lava compositions with 48-57 wt% SiO$_{2}$ by fractional crystallization of mineral compositions found in arc gabbronorites. We will use similar least-squares models to determine whether more evolved lava and plutonic compositions (57-77 wt% SiO$_{2}$) can also be explained by simple crystal fractionation of observed cumulate phases, or whether these compositions require multistage differentiation.
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting