Tectonic and Geochemical Evolution of the Accreted Talkeetna Arc, South-Central Alaska: Implications for a Type Section of Intra-oceanic Arc Crust

HR: 17:30h
AN: V44C-07 [Abstracts]
TI: Tectonic and Geochemical Evolution of the Accreted Talkeetna Arc, South-Central Alaska: Implications for a Type Section of Intra-oceanic Arc Crust
AU: * Rioux, M
EM: mrioux@umail.ucsb.edu
AF: UCSB, Geological Sciences, Santa Barbara, CA 93106
AU: Hacker, B
EM: hacker@geol.ucsb.edu
AF: UCSB, Geological Sciences, Santa Barbara, CA 93106
AU: Mattinson, J
EM: mattinson@geol.ucsb.edu
AF: UCSB, Geological Sciences, Santa Barbara, CA 93106
AU: Kelemen, P
EM: peterk@ldeo.columbia.edu
AF: LDEO, 58 Geochemistry Building, Palisades, NY 10964
AU: Blusztajn, J
EM: jblusztajn@whoi.edu
AF: WHOI, Geology & Geophysics, MS# 25, Woods Hole, MA 02543
AU: Hanghoj, K
EM: khanghoj@ldeo.columbia.edu
AF: LDEO, 58 Geochemistry Building, Palisades, NY 10964
AU: Amato, J
EM: amato@nmsu.edu
AF: NMSU, Geological Sciences P.O. Box 30001, Las Cruces, NM 88003
AB: The accreted Talkeetna arc, south-central Alaska, exposes a nearly complete cross section from the sub-arc mantle to sub-aerial volcanic rocks of a Jurassic intra-oceanic arc. Exposed arc lithologies include residual mantle harzburgite, cumulate pyroxenite and dunite, gabbroic to tonalitic plutonic rocks, and volcanic deposits, and provide a rare opportunity to directly study arc magmatic processes. As a result, the Talkeetna section has become the focus of extensive research on the development and evolution of island arcs. We present new U-Pb zircon ages and geochemical data that document the tectonic and geochemical evolution of the Talkeetna arc and justify its use as a type section of intra-oceanic arc crust. High-precision chemical abrasion-TIMS U-Pb zircon ages from the Chugach Mtns indicate that initial arc magmatism lasted from 201.5-181.4 Ma. This was followed by a northward shift in the magmatic axis and generation of an extensive and understudied plutonic suite in the Talkeetna Mtns (177.4-153.0 Ma). Samples from the youngest arc plutons, located in the western Talkeetna Mtns, contain abundant evidence for inheritance of Permian to Carboniferous zircons that reflect interaction of arc magmas with adjacent Wrangellia terrane crust. Early Jurassic (~190 Ma) plutons along the same margin contain a similar inherited component and may represent either older Talkeetna arc magmatism or previously unrecognized Wrangellia crust. The northward younging of Talkeetna magmatism in the Chugach and Talkeetna Mtns is mirrored by the progression of plutonic ages in southwestern Alaska, where preliminary LA-ICP-MS analyses from plutonic rocks on Kodiak Island record ~200 Ma ages and detailed TIMS ages from felsic plutons on the Alaska Peninsula range from 183.3-163.9 Ma. The northern contact of the Talkeetna-aged batholith on the Alaska Peninsula intrudes metasediments with Proterozoic and Phanerozoic detrital zircons. Isotopic data from the plutonic sections reflect the U-Pb zircon systematics. Samples from the Chugach and southeastern Talkeetna Mtns have a restricted array of initial isotopic ratios (ε_Nd = 5.6 to 7.2 and ε_Sr = -17.6 to -4.7) coincident with the range in epsilon values from the modern Tonga-Kermadec arc. In contrast, plutons from the northwestern Talkeetna Mtns, where there is abundant evidence for inherited older zircons, have more evolved isotopic ratios (ε_Nd = 4.0 to 5.5 and ε_Sr = -9.3 to 27.4). Samples from the Alaska Peninsula have intermediate values of ε_Nd = 5.2 to 6.6 and ε_Sr = -11.0 to -6.3. Intermediate to felsic plutons are exposed throughout the Talkeetna arc. Together with felsic lavas and volcanoclastic rocks, these provide an analogue for silicic magmatism in modern island arc settings (e.g. Izu-Bonin and Tonga arcs). Evolved plutons range from 57-77 wt% SiO₂ and define a low- to medium-K trend with typical arc trace-element ratios and flat to slightly LREE enriched patterns. These data indicate that silicic magmatism plays an important role in the development of dominantly mafic intra-oceanic arcs, but does not produce bulk arc compositions that are similar to average continental crust (i.e. high La/Yb ratios and K₂O concentrations).
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 1115 Radioisotope geochronology
DE: 8185 Volcanic arcs
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005