HR: 17:00h
AN: V44C-05    [Abstracts]
TI: Crustal Genesis and Dynamics in the Jurassic Talkeetna Arc
AU: * Kelemen, P
EM: Peterk@ldeo.columbia.edu
AF: Lamont, LDEO, Palisades, NY 10964
AU: Amato, J
EM:
AF: NMS, Univ, Las Cruces, NM 88003
AU: Behn, M
EM:
AF: GG, WHOI, Woods Hole, MA 02543
AU: Blusztajn, J
EM:
AF: GG, WHOI, Woods Hole, MA 02543
AU: Christensen, N
EM:
AF: UW, Geol, Madison, WI 53706
AU: Clift, P
EM:
AF: Aberdeen, Univ, Aberdeen, AB24 3UE United Kingdom
AU: DeBari, S
EM:
AF: WW, Univ, Bellingham, WA 98225
AU: Draut, A
EM:
AF: USGS, Pacific, Santa Cruz, CA 95060
AU: Greene, A
EM:
AF: UBC, Geol, Vancouver, V6T 1Z4 Canada
AU: Hacker, B
EM:
AF: UCSB, Geol, Santa Barbara, CA 93106
AU: Hanghoj, K
EM:
AF: Lamont, LDEO, Palisades, NY 10964
AU: Hart, S
EM:
AF: GG, WHOI, Woods Hole, MA 02543
AU: Hirth, G
EM:
AF: GG, WHOI, Woods Hole, MA 02543
AU: Mattinson, J
EM:
AF: UCSB, Geol, Santa Barbara, CA 93106
AU: Mehl, L
EM:
AF: GG, WHOI, Woods Hole, MA 02543
AU: Pavlis, T
EM:
AF: UNO, Geol, New Orleans, LA 70148
AU: Rioux, M
EM:
AF: UCSB, Geol, Santa Barbara, CA 93106
AU: Trop, J
EM:
AF: Brucknell, Univ, Lewisburg, PA 17837
AB: We summarize studies of the accreted Talkeetna arc section in Alaska, focusing on the role of arc crust in continental genesis and arc geodynamics. The 200 to 175 Ma section extends from residual mantle peridotite to volcanics, including - in order of decreasing depth - a few 100 m of pyroxenite, rare garnet granulites, cumulate gabbronorites, and felsic plutonic rocks. Lack of inheritance in zircon and relatively constant, high Nd and low Sr isotope ratios indicate that to first order the crustal rocks differentiated from a common parental melt composition and that magmas did not incorporate older crustal material. Amphibolite formed from arc lava. Metasediment had pelagic protoliths, except on the Alaska Peninsula. The arc probably formed new crust during extension, rather than intruding older rocks. South to north younging of zircon and hornblende ages indicates that the arc migrated northward, so the entire section is not a single, tilted vertical column though the Chugach region alone could be. Total crustal thickness was 25 to 35 km, with 6 to 8 km of volcanics. Mid to lower crust is mainly cumulate gabbronorite formed at NNO+2. The thickness of lavas/cumulates is consistent with crystal fractionation to form average lavas from the most primitive observed melt compositions. The bulk composition of the crust and the most primitive melts has ~ 49 wt% SiO2, 19 wt% Al2O3, 8 wt% MgO, and Mg# ~ 60. Derivation of these melts from magmas in equilibrium with residual mantle (melt Mg# > 70) requires more than 25% crystal fractionation of cumulates with cpx Mg# between 85 and 93, found only in primitive pyroxenite along the Moho, ~ 1% of the crustal thickness. This discrepancy is likely due to foundering of dense pyroxenites (and garnet granulites) into underlying mantle. Flat REE patterns, low K, low Th/La, and low Mg# at a given SiO2 distinguish Talkeetna most samples from continental crust, which is Th-enriched, LREE enriched, HREE depleted, K-rich, high Mg# andesite. Even if Talkeetna mid and lower crust were removed, remaining felsic plutonic and volcanic rocks would be distinct from continental crust. Thus, Talkeetna is similar to Izu-Bonin-Marianas and Tonga, and different from western Aleutian and Kurile island arcs where lavas are similar to continental crust. Quartz diorites and tonalites in a klippe south of the main Talkeetna section have igneous garnet formed from LREE enriched, HREE depleted, high Th/La, felsic melts. Mixing of such melts with primitive basalt, combined with removal of the garnet-bearing residue, could produce crust similar to continental crust. However, such melts are rare in the Talkeetna lava stratigraphy to the north. Olivine fabrics in residual peridotites and ultramafic plutons indicate arc parallel a-axis lineation and arc parallel viscous flow, consistent with seismic anisotropy in many modern arcs. Such fabrics in plutons suggest viscous coupling of lower crust and mantle at some times. But, Moho pyroxenites and lower crustal gabbronorites are almost undeformed. Metamorphic conditions recorded by garnet-bearing rocks are ~ 1000 C and 1 GPa near the Moho and 700 and 0.7 GPa in the klippe, consistent with mantle temperatures of ~1300 C at 45 km beneath this and other arcs.
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8178 Tectonics and magmatism
DE: 8185 Volcanic arcs
DE: 8412 Reactions and phase equilibria (1012, 3612)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005