HR: 1340h
AN: V43D-1646    [Abstracts]
TI: A Trace-Element and Radiogenic-Isotopic Pattern of Oceanic Arc Inception, Maturity, Demise, and Rejuvenation: Viti Levu, Fiji
AU: * Todd, E
EM: etodd@pmc.ucsc.edu
AF: Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, United States
AU: Gill, J
EM: jgill@pmc.ucsc.edu
AF: Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, United States
AU: Pearce, J
EM: pearceja@cardif.ac.uk
AF: School of Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff, CF10 3YE, United Kingdom
AU: Kempton, P
EM: p.kempton@nigl.nerc.ac.uk
AF: NIGL, British Geological Survey, Kingsley Dinham Centre, Keyworth, Nottingham, NG12 5GG, United Kingdom
AB: Viti Levu, Fiji and \textquoteleftEua, Tonga have the oldest subaerial exposures of the remnant Vitiaz Arc which formed when subduction initiated (≥45Ma) as the result of a Pacific Plate motion change. We present new high-precision ICP-MS trace element and MC-ICP-MS isotope data for newly collected Fiji rocks and previously analyzed (XRF,TIMS) Viti Levu basalts and \textquoteleftEua gabbro that represent proto- to mature-arc, syn- rifting prior to opening of the South Fiji Basin (SFB), and post-SFB magmatic stages of early Vitiaz arc evolution. Earliest Fiji arc rocks make up the Yavuna formation in southwestern Viti Levu and are mostly basalt to basaltic andesite pillows, dikes, and flows. Most analyzed \textquoteleftEua and Yavuna rocks have chondritic Sm/Hf, but both have populations with negative Hf anomalies. \textquoteleftEua is LREE depleted whereas Yavuna ranges broadly from LREE-depleted to LREE-enriched (La_N/Yb_N from 0.2 to ~4). Both include boninitic lavas with HREE & HFSE concentrations only about twice those of primitive mantle. Both show similar ranges for ¹⁴³Nd/¹⁴⁴Nd, but Yavuna is slightly more radiogenic and less variable in ¹⁷⁶Hf/¹⁷⁷Hf. Both are "Pacific" in isotopic character. The demise of the early Yavuna arc (~20-30 Ma) resulted from a shift from arc volcanism to rifting and backarc spreading and, eventually, opening of the SFB. Volcanism associated with rifting is preserved as a bi- modal suite of dacitic (edifice) and basaltic (basin) rocks (Kalaka and Dakadaka formations, respectively) that crop out west and south of Yavuna and adjacent to SFB crust. Edifice rocks are flat to slightly LREE-depleted and basinal rocks are all LREE-enriched. Both are sub-chondritic to Hf-enriched with respect to REE. Post-SFB volcanism (Wainimala Group) is bi-modal basalt plus dacite and included at least one major plutonic event. These rocks comprise most of modern Viti Levu basement. Wainimala rocks have a range of Sm/Hf similar to Yavuna but are generally more LREE-depleted. This is consistent with mantle depletion during SFB formation. Higher ¹⁷⁶Hf/¹⁷⁷Hf ratios indicate a more depleted source mantle than existed prior to spreading. The early arc was characterized by extreme variations in HFSE, reflecting large variations in percent melting, extending to very high degree melting necessary to generate boninitic trace-element concentrations. Rift stages are indicated by synchronous arc- and basin-like volcanism, and more typical LREE-depleted arc tholeiites appear only after opening of the SFB with rejuvenation of arc volcanism.
DE: 1037 Magma genesis and partial melting (3619)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 8185 Volcanic arcs
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting