HR: 11:00h
AN: V52A-03    [Abstracts]
TI: Geochemistry of Oligocene volcaniclastic sediments from DSDP Site 296, Palau Kyushu Ridge
AU: * Pages, M
AF: Department of Earth Sciences, Florida International University, Miami, FL 33199 United States
AU: Rebenack, C
AF: Department of Earth Sciences, Florida International University, Miami, FL 33199 United States
AU: White, M
AF: Department of Earth Sciences, Florida International University, Miami, FL 33199 United States
AU: Williams, D
AF: Department of Earth Sciences, Florida International University, Miami, FL 33199 United States
AU: Hickey-Vargas, R
EM: hickey@fiu.edu
AF: Department of Earth Sciences, Florida International University, Miami, FL 33199 United States
AB: DSDP Site 296 is located on the northern Palau-Kyushu ridge (PKR), which is a remnant of the early Izu-Bonin-Mariana (IBM) arc. At about 30 Ma, early IBM arc activity ceased, and the arc was rifted lengthwise into remnant (PKR) and trenchward sections (the active IBM arc basement), separated by the Shikoku and Parece Vela back arc basins. Site 296 is the northernmost location on the PKR where early IBM arc volcanics have been sampled. Lapilli tuffs, volcanic sandstones and siltstones comprise a 634 meter thick, early to late Oligocene section that extends to 1087 meters below seafloor. A biostratigraphic age of early Oligocene was deduced for the oldest unit, but an Ar/Ar age of 47.5 Ma (middle Eocene) was reported for lapilli tuff near the bottom of the section, suggesting that that older, reworked material was incorporated into the sediment pile. Large clasts from a sandstone unit were isolated and analyzed for major and trace elements. All are andesite (58-60% SiO₂), with distinct low-K and high-K features. High-K andesites are strongly incompatible element enriched, with Ba and Th abundances of 100 X primitive mantle values and normalized LREE/HREE = 7.5-8.0. Glass fragments and pyroxene grains were separated from 17 lapilli tuff and sandstone units. Glass compositions range from basalt (47% SiO₂) to rhyolite (73% SiO₂). Most glasses form a low to intermediate-K tholeiitic series, with 0.8 to 1.2% TiO₂ in basaltic glasses, but a few units have clearly high-K characteristics. All units have both orthopyroxene (En80 to En50), and augite (Wo44:En52:Fs4 to Wo40:En38:Fs22). Based on the absence of En-rich orthopyroxene and the composition of volcanic glass, it appears that magnesium-rich boninites are not present in the section. The predominantly low-K character of the volcaniclasts is comparable to the depleted nature of present-day Izu-Bonin arc volcanics. The occurrence of high-K rocks and glass in the section indicates that more enriched magma sources were also present beneath the Izu-Bonin arc during the Oligocene.
DE: 1020 Composition of the crust
DE: 1050 Marine geochemistry (4835, 4850)
DE: 3675 Sedimentary petrology
DE: 8105 Continental margins and sedimentary basins
DE: 8404 Ash deposits
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly