HR: 0800h
AN: V51D-1522    [Abstracts]
TI: Crystallization of Arc Lower Crust in the Jurassic Talkeetna Island Arc
AU: * Greene, A R
EM: agreene@eos.ubc.ca
AF: UBC, Earth and Ocean Sciences, Vancouver, BC V6T 1Z4 Canada
AU: DeBari, S
EM: debari@geol.wwu.edu
AF: WWU, Geology Dept, Bellingham, WA 98225 United States
AU: Kelemen, P
EM: peterk@ldeo.columbia.edu
AF: LDEO, Dept of Earth and Environnmental Sciences, Palisades, NY 10964 United States
AU: Clift, P
EM: pclift@abdn.ac.uk
AF: Univ. Aberdeen, Dept of Geology and Petroleum Geology, Aberdeen, AB24 3UE United Kingdom
AU: Blusztajn, J
EM: jblusztajn@whoi.edu
AF: WHOI, Dept of Geology and Geophysics, Woods Hole, MA 02543 United States
AU: Hanghoj, K
EM: khanghoj@ldeo.columbia.edu
AF: LDEO, Dept of Earth and Environnmental Sciences, Palisades, NY 10964 United States
AB: The Jurassic Talkeetna arc section offers a rare opportunity to directly compare volcanic and plutonic rocks from an island arc. The section includes, in order of decreasing depth: (1) residual mantle harzburgite and dunite; (2) pyroxenite; (3) basal gabbronorite; (4) lower crustal gabbronorite; (5) mid-crustal plutonic rocks; and (6) volcanic and volcaniclastic rocks. The pyroxenites overlie residual mantle, with some interfingering along the contact. The basal gabbronorite overlies pyroxenite, again with some interfingering. Lower crustal gabbronorite (<10 km thick) is marked by well developed modal layering. Mid-crustal plutonic rocks include gabbroic rocks, diorite to tonalite (30-40% area), mafic dikes, and chilled mafic inclusions. Volcanic and both primary and reworked volcaniclastic rocks span basalt to rhyolite compositions. Consistent Nd isotope ratios plus mineral and whole-rock chemistry throughout the section suggest a simple co-magmatic differentiation sequence. Mid-crustal intermediate to felsic plutonic rocks and lavas have similar major- and trace-element concentrations. Rare earth (REE) and high field strength elements increase and Mg# decreases with differentiation, while lower crustal ``cumulate'' gabbronorites have systematically different major-element compositions, but comparable trace-element characteristics. Chondrite-normalized REE patterns are parallel and increase in abundance systematically from the basal gabbronorite and lower crustal gabbronorite (1-10 times chondrite) through the volcanic upper crust (8-38 times chondrite). Calculated parental liquids from ion probe analyses of clinopyroxene (cpx) in lower crustal gabbronorites indicate that cpx formed in equilibrium with liquids like the lavas. Whole-rock Ti and Eu enrichments in lower crustal gabbronorites are mirrored by Ti and Eu depletions in evolved volcanic rocks. The compositional variation of the main series of lavas and chilled mafic inclusions can be modeled through fractionation of observed phase compositions and phase proportions in lower crustal gabbronorite. Mantle-derived melts in the Talkeetna arc underwent fractionation of pyroxenite at the base of the crust. More than 25 wt percent of the primary melts crystallized as pyroxenite, in contrast to the observed proportion of pyroxenites (less than 5 percent of the section). This discrepancy may result from gravitational instability, with dense pyroxenite cumulates, probably together with garnet granulites, foundering into the mantle while the Talkeetna arc was active, or during cooling and sub-solidus garnet growth after magmatism ceased.
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3640 Igneous petrology
DE: 8185 Volcanic arcs
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005